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NATIONAL LIBRARY OF MEDICINE




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Jack H.  U.  Brown,  Ph.D, 
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       CARPENTER'S





ELEMENTS OF PHYSIOLOGY. 
 
ELEMENTS
                       OF






          PHYSIOLOGY,




                     INCLUDING





PHYSIOLOGICAL  ANATOMY.





                       BY



     WILLIAM B. CARPENTER, M.D., F.R.S., F.G.S.,

   EXAMINER IN PHYSIOLOGY AND COMPARATIVE ANATOMY IN THE UNIVERSITY OF LONDON: AND AUTHOR OF
        " THE PRINCIPLES OF HUMAN PHYSIOLOGY," AND '; THE PRINCIPLES OF GENERAL
                AND COMPARATIVE PHYSIOLOGY," ETC.




                  SECOND AMERICAN,


         FRO 31 A NEW AND KEVISKD  LONDON EDITION.



     WITH ONE HUNDRED AND NINETY ILLUSTRATIONS.
       PHILADELPHIA:

BLANCHARD  AND LEA.

             1851. 

C.  SHERMAN,   PRINTER. 
PREFACE.
  The present volume owes its  origin to a desire on  the part of the
Publisher, that an elementary treatise on Physiology should be added
to the series of admirable Students'  Manuals, on the  various depart-
ments of Medical Science, which he had previously issued.
  In  carrying  this desire into execution, the Author  has endeavored
to avoid inflicting upon the class for whose use the Treatise is especially
intended, the injury of placing in their  hands such a superficial and
imperfect sketch of the science, as, whilst affording them but a limited
amount of knowledge of  its facts, should leave  them very ill-informed
as to its general doctrines.  His  object has rather been to convey to
the Student as clear an idea as possible of those Principles of Physiology
which are based on the broadest and  most satisfactory  foundation, and
to point out the mode in which  these principles are applied to the
explanation of the  phenomena presented by the living actions of the
Human body.   In  this manner has the Author desired to prepare him
for that more detailed study of the latter,  which becomes necessary when
Physiology is pursued (as it ought  to be)  in connexion with the changes
produced  in the  living body by Morbific and Remedial Agents,  and is
thus taken as a guide in the study of  the causes, prevention, and treat-
ment of Disease—which  should be the primary object of attention with
every one who undertakes the  practice of his Profession.
  Although this Manual combines  in some degree the scope of the
Author's  "Principles  of Physiology, General  and Comparative," and
of his "Principles of  Human Physiology," yet it cannot be regarded
as a mere abridgment of  them, having been written for the  most part
with very little reference to them, and  with every  desire to make it
complete in itself.  As the matter of which these volumes are composed
is itself condensed to the utmost practicable degree,  it is  manifestly
impossible that the present Manual  should  contain more than a mere
outline' of  the  subjects of which  they treat.  To them, therefore, he 
Vlii                          PREFACE.
 would refer such of its readers as may desire further information upon
 various topics which are here only slightly touched upon; and in them,
 also, will be found references to various original authorities, the intro-
 duction  of  which would  be incompatible with the limited  scope of a
 treatise like the present.
   The Author has only to add, that he feels most grateful for the kind
 appreciation which this Manual has experienced; and that, in the prepa-
 ration of the present Edition, he has used his best endeavors to render
 it still more worthy of a favorable reception.   The whole treatise has
 been subjected to a  most  careful revision;  many statements which the
 advance  of  science has shown to  be doubtful or erroneous, have  been
 omitted  or  corrected; and a considerable amount of new matter has
 been introduced.   Of the First, Eleventh, and Twelfth Chapters, more
 especially, a considerable  proportion  has  been entirely rewritten; and
 the Author  ventures to believe  that  the  doctrines which  they contain
 will enable such as may master them to obtain a clearer comprehension
 of the  facts of Physiological  Science than  they could  previously have
acquired.
Regent's Park, London,
   September, 1851. 
TABLE  OE  CONTENTS.
                                 BOOK I.


                          GENERAL  PHYSIOLOGY.

CHAPTER.                                                                  PAGE.
    I. On the Nature and Objects of the Science of Physiology   .      .     17
          1.  General Characters of Organized Structures  ...        18
                                                                          25
                                                                          44
                                                                          53
                                                                          56
       2. Digestive Characters of Vital Actions
       3. Of the Forces concerned in the production of Vital Phenomena
       4. Of Degeneration and Death .....
       5. General Summary      ....

II. Of the External Conditions of Vital Activity .
       1. Of Light,  as a Condition of Vital Activity
       2. Of Heat, as a Condition of Vital Activity  .
       3. Of Electricity, as a Condition of Vital Activity,
       4. Of Moisture, as a Condition of Vital Activity
58
61
71
94
97
III. Of the Elementary Parts of Animal Structures   .      .      .      105
        1. Of the Primary Components of the Animal Fabric .       .      .   107
        2. Of the Simple Fibrous Tissues  .      .      .      .      .      119
        3. Of the Basement or Primary Membrane    ....   127
        4. Of Simple Isolated Cells, employed in the Organic Functions  .      130
        5. Of Cells connected together, as permanent constituents of the Tissues  154
        6. Of Cells coalesced into Tubes, with Secondary Deposit.      .      196
                              BOOK II.


                        SPECIAL  PHYSIOLOGY.


IV. Of Food, and the Digestive Process    .....   236
       1. Sources of the Demand for Aliment   ...      •       .      236
       2. Of the Digestive Apparatus, and its Actions in general    .      .   252
       3. Of the Movements of the Alimentary Canal    .      .       .      257
       4. Of the Secretions poured into the Alimentary Canal, and of the
           Changes -which they effect in its contents  ....   264
       5. Of Hunger, Satiety, and Thirst .....      274

 V. Of Absorption and Sanguification      .....   277
       1. Of Absorption from the Digestive Cavity       .      .       .      277
       2. Of the Passage of Chyle along the Lacteals, and its  admixture
           with the Lymph collected from the General System      .      .281
       3. Of the Spleen, and other Glandular Appendages to the Lymphatic
           System ........      286
       4. Of the Composition and Properties of the Chyle and Lymph      .   292
       5. Of Absorption from the External and Pulmonary  Surfaces    .      296
       6. Of the Composition and Properties of the Blood    .       .      .   297 
X
TABLE  OF  CONTENTS.
CHAPTER.
    VI. Or the Circulation of the Blood    ....
          1. Nature and Objects of the Circulation of Nutrient Fluid
          2. Different forms of the Circulating Apparatus
          3. Action of the Heart .....
          4. Movement of the Blood in the Arteries .
          5. Movement of the Blood in the Capillaries
          6. Movement of Blood in the Veins

   VII. Of Nutrition     ......
          1. Selecting Power of Individual Parts
          2. Varying Activity of the Nutritive Processes
          3. Of Death, or Cessation of Nutrition
          4. Disordered Conditions of the Nutritive Processes   .

  VIII. Of Respiration      ......
          1. Essential Nature and Conditions of the Respiratory Process
          2. Different  forms of the Respiratory Apparatus in the lower Animals
          3. Mechanism of Respiration in Mammalia, and in Man  .
          4. Chemical Phenomena of Respiration
          5. Effects of Insufficiency, or Suspension, of the Aerating Process

   IX. Of Secretion   ........
          1. Of the Secreting Process in general, and of the  Instruments by
              which it is effected .......
          2. Of the Liver, and the Bile      .....
          3. Of the Kidneys, and the  Urine      .....
          4. Of the Cutaneous and Intestinal Glandulas
          5. General Summary of the Excreting Processes

    X. Of the Development of Light, Heat, and Electricity, in the Animal
        Body  .........

   XI. Generation and Development
          1. General View of the Nature of the Process
•          2. Action of the Male  .      .    .   .
          3.  Action of the Female

  XII. Of the Nervous System

          1.  General view of the  operations, of which the Nervous System is
              the Instrument .......
          2.  Comparative Structure and Actions of the Nervous  System
          3.  Functions of the Spinal Cord and its Nerves
          4.  Functions of the Medulla Oblongata
          5.  Functions of the Sensory Ganglia
          6.  Functions of the Cerebellum
          7.  Functions of the Cerebrum
          8.  Functions of the Sympathetic System

 XIII. Of Sensation, General and Special  .
          1.  Of Sensation in general
          2.  Of the Sense of Touch  .
          3.  Of the Sense of Taste
          4.  Of the Sense of Smell   .
          5.  Of the Sense of Hearing
          6.  Of the Sense of Sight   .
PIOE.
 304
 304
 305
 320
 327
 331
343
343
345
350
352

357
357
363
375
382
389

393

393
400
40G
416
422
425

434
434
442
445

473

473
479
496
504
508
514
516
527

529
529
534
536
538
540
544
XIV. Of the Voice and Speech .
553 
LIST  OE   WOOD   ENGRAVINGS.
FIO.
 1. Simple isolated Cells, containing reproductive molecules
 2. Fibrous structure of exudation-membrane ; after Gerber        .      .
 3. Fibrous membrane lining egg-shell (original) ....
 4. "White fibrous tissue of areolar tissue and tendon; after Gerber .
 5. White fibrous tissue of ligament;  after Gerber
 6. Yellow fibrous tissue of ligamentum nuchse ; after Gerber
 7. Development of fibres from cells;  after Lebert
 8. Ideal Section of a Joint    .......
 9. Capillary vessels of Skin; after Berres       ....
10. Capillary vessels of Intestinal villi; after Berres ....
11. Capillary vessels around orifices of Mucous follicles ;  after Gerber  .
12. Capillary vessels around follicles of Parotid Gland ; ditto
13. Distribution of Sensory nerves in  Skin; after Gerber
14. Primary membrane, with germinal spots ;  after Goodsir .
15. Primary membrane, showing component cells ;  after Goodsir
16. Cells from Chorda Dorsalis of Lamprey;  after  Quekett .
17. Multiplication of  Cartilage-cells by duplication; after Leidy
18. Parent-cells, with contained  secondary cells, of cancerous structure; after
      Lebert    .       .      .       .       .      .            f .
19. Cells from fluid of Herpes;  after Addison    ....
20. Oblique section of Epidermis ; after Henle       ....
21. Epidermic cells from Conjunctiva; after Gerber
22. Portion of Choroid-coat, showing pigment cells ; ditto
23. Separate Pigment-cells; after Mandl .....
24. Detached epithelium-cells from mucous membrane of  mouth ;  after Lebert
25. Pavement-epithelium from bronchial tubes ; after Lebert
26. Layer of cylindrical epithelium, with  cilia; after Henle  .
27. Follicles from liver of Crab, with  contained secreting cells; after Goodsir
28. Follicles of Mammary gland,  with contained secreting cells ;  after Lebert
29. Secreting Cells of Human Liver      .....
30. Formation of Spermatozoa within cells;  after Wagner
31. Diagram of Intestinal  Mucous membrane, in  intervals of digestion; after
      Goodsir      ........
32. Extremity of Placental villus; after Goodsir     ....
33. Progressive stages of cell-growth, in Shell-membrane (original)
34. Progressive stages of coalescence of cells, in Shell-membrane (original)
35. Fusiform tissue of plastic exudations; after Lebert
36. Areolar and Adipose tissue;  after Mandl    ....
37. Capillary network around Fat-cells;  after Berres
38. Cartilage of Mouse's ear; after Quekett     ....
39. Section of Cartilage;  after Schwann"      .....
40. Distribution of vessels on surface of Cartilage; after Toynbee
41. Nutrient vessels of Cornea;  after Toynbee      ....
42. Shell of Echinus  (original)     ......
43. Sections of  Shell of Pinna (original)       .....
44. Tubular shell-structure from  Anomia (original)
45. Cancellated structure at extremity of Femur;  after Toynbee 
Xll
LIST OF WOOD  ENGRAVINGS.
 FIG.
 46.
 47.
 48.
 49.
 50.
 51.
 52.
 53.
 54.
 55.
 56.
 57.
 58.
 59.
 60.
 61.
 62.
 63.
 64.
 65.
 66.
 67.
 68.
 69.
 70.
 71.
 72.
 73.
 74.
 75.

 76.
 77.
 78.
 79.
 80.
81.
 82.
83.
84.
85.
86.
87.
 90.
 91.
 92.
 93.
 94.
 95.
 96.
 97.
 98.
 99.
100.
101.
102.
103.
104.
105.
106.
107.
108.
 Lacunae of Osseous substance ; after Mandl  .
 Section of Bony Scale of Lepidosteus (original) .
 Network of Haversian canals, from vertical section of Tibia; after Mandl
 Transverse section of long bone; after Wilson
 Section of Cartilage, near seat of Ossification ; after Wilson
 Section of Cartilage, at the seat of Ossification;  after Wilson
 Vessels of Dental Papilla; after Berres
 Oblique Section of Dentine ;  after Owen .
 Vertical Section of human molar tooth ;  after Nasmyth
 Development of Teeth ; after Goodsir
 Structure of Hair of Musk-deer and Sable (original)
 Structure of the  Human Hair; after Wilson
 Fasciculus of striated Muscular fibre; after Mandl  .
 Non-striated Muscular fibres; after Bowman
 Striated Muscular fibre separating into fibrillre
 Muscular fibre cleaving into disks;  after Bowman
 Transverse section of muscular fibres ; after Bowman
 Structure of ultimate fibrillre of striated fibre (original)
 Nucleated fibres from non-striated muscle;  after Wilson
 Fusiform contractile cells ; after  Kolliker
 Nuclei in striated muscular fibres of foetus;  after Bowman .
 Capillaries of Muscle ; after Berres
 Distribution of nerves in Muscle; after Burda,ch
 Components of gray substance of brain;  after Purkinje
 Capillaries of Nervous centres ;  after Berres
 Structure of Ganglion of  Sympathetic ;  after Valentin  .
 Distribution of Sensory nerves in lip ; after Gerber  .
 Capillaries at margin of lips; after Berres
 Section of Human Stomach  .....
 Mucous coat of small intestine, showing Villi, and orifices of follicles ;  afte
  Boehm   ........
 Peyerian glandula;  after Boehm    ....
 Stomach of  Sheep .......
 Section of Stomach of Sheep, showing demi-canal; after Flourens
 Lobule of Parotid Gland ; after Wagner  ....
 Gastric glanduloe ; after Wagner     ....
 Orifices of Gastric tubuli; after Boyd    ....
 Distribution of Capillaries in Intestinal Villus; after Berres
 Commencement of Lacteal in Intestinal Villus ; after Krause   .
 Diagram of  Lymphatic gland ; after Goodsir
Epithelial cells of intra-glandular Lymphatic;  after Goodsir
 Course of Thoracic duct     .....
 Appearance of inflamed Blood; after Addison   .
 Vascular area of Fowl's egg; after Wagner  .
 Diagram of  the Circulation in Fish
 Diagram of the Circulation in Reptile
 Diagram of  complete Double Circulation .
 Anatomy of Human Heart and Lungs
 Capillaries of Nervous centres; after Berres
 Capillaries of Glandular follicles; after Berres
Capillaries of Conjunctival membrane; after Berres
 Capillaries of Choroid coat;  after Berre3
 Capillaries around orifices of mucous follicles;  after Berres
Capillaries in Skin of finger ; after Berres
Capillaries in fungiform papilla of Tongue; after Berres
Doris, showing branchial tufts ; after Alder and Hancock    .
One of the arborescent processes of gills of Doris; ditto
 Respiratory apparatus of Insects    ....
 Diagram of  different forms of Respiratory Apparatus (original)
 Capillaries of Gill of Eel (original)   .        .      .       .
 Section of Lung of Turtle ;  after Bojanus
 Capillaries of Human Lung (original)
 Simple glandular follicles; after Miiller  .
Embryonic development of Liver; after Miiller 
LIST OF WOOD  ENGRAVINGS.
Xlll
FIG.
109.
110.
111.
112.
113.
114.
115.
116.
117.
118.
119.
120.
121.
122.
123.
124.
125.
126.
127.
128.
129.
130.
131.
132.
133.
134.

135.
136.
137.
138.
139.
140.
141.
142.
143.

144.
 145.

 146.

147.
148.
149.
150.
151.
152.
153.
 154.

 155.

 156.

157.
158.
159.
 160.
 161.
162.
163.
164.
165.
Rudimentary Pancreas, from Cod; after Miiller .
Mammary Gland of Ornithorhyncus ;  after Miiller  .
Meibomian Glands; after Miiller .
Portion of Cowper's Gland ;  after Miiller
Lobule of Lachrymal  Gland; after Miiller
Hepatic Follicles from Crab ; after Goodsir  .
Ultimate follicles from Mammary gland; after Lebert
Surface of Lobule of  Liver  of Squilla; after Miiller
Interior of Lobule of  Liver  of Squilla; after Miiller
Liver of Tadpole; after Miiller       .....
Distribution of Blood-vessels in Lobules of Liver;  after Kiernan
Connexion of Lobules of Liver with Hepatic vein ; ditto
Distribution of Hepatic ducts around Lobules of Liver; after Kiernan
Secreting Cells of Liver      ......
Development of Kidney in embryo of Lizard;  after Miiller
Kidney of  foetal Boa; after Miiller   .....
Portion of Kidney of Coluber; after  Miiller
Fasciculus of tubuli uriniferi of Bird;  after Miiller .
Section of Kidney .......
Section of portions of Kidney, slightly magnified; after Wagner
Distribution of vessels in  Kidney ; after Bowman
Vertical section of Skin ;  after Wilson
Hepatic Cells gorged  with Fat;  after Bowman
Various stages of development of HjEmatococcus binalis; after Hassal
Successive stages of development of simpler Algae; after  Kutzing
Diagram representing the three  principal forms of the Generative process in
  Plants (original)     .......
Successive stages of Segmentation of the vitellus of Ascaris;  after Bagge
Anatomy of the Testis       ......
The Uterus and its appendages   ......
Successive stages of  Segmentation of Mammalian vitellus;  after Coste
Formation of the Mulberry mass ; after Coste   ....
Plan of  Early Uterine Ovum; after Wagner .      .
Germ and surrounding parts; after Coste       ....
Vascular Area of Fowl's  Egg ; after Wagner
Diagram of Ovum, at commencement of separation of digestive cavity; afte
   Wagner     ........
Diagram of Ovum, showing the  formation of the Amnion ; after Wagner
 Diagram of Human Ovum, in second month, showing the Allantois; after
   Wagner .........
Diagram of the Circulation in the Ovum at the  commencement of the forma
   tion of the placenta;  after Coste
Extremity of Placental Villus ;  after Goodsir
External membrane and cells of placental villus;  after Goodsir
Diagram illustrating the arrangement of the placental decidua ; after Goodsir
Plan of  the Foetal Circulation   ......
Termination of portion of milk-duct in follicles; after Sir A. Cooper
Portion  of the Ganglionic tract  of Polydesmus; after Newport
Human embryo at sixth week;  after  Wagner
Dissection of the Medulla Oblongata, showing the connexions of its
   tracts; after Solly (altered)       ....
Diagram of the relations of the Cerebrum to the Sensory Ganglia, as
   horizontal section (original)       ....
Diagram of  the relations of the several parts of the Encephalon, as
   vertical section (original)  .....
Capillary network at margin of Lips ;  after Berres
Distribution of tactile nerves in Skin;  after Gerber .
Capillaries of fungiform papilla of Tongue; after Berres
Distribution of Olfactory nerve; after Wilson
Diagram of  the Auditory apparatus ;  after Wilson
Refraction of rays of light through convex lens
Formation of images in eye      ....
Capillary network of Retina ; after Berres  .
Structure of the Larynx;  after Willis
several 
EXPLANATION  OE  PLATE  I.
  The Figures in this Plate represent the Cells floating in the various animal fluids ; and
they are all, with the exception of Figs. 4 and 5, copied from the representations given
by M. Donne" in his "Atlas de l'Anatomie Microscopique."  These representations are
transcripts of Daguerreotype pictures, obtained from the objects, by a solar microscope,
with a magnifying power of 400 diameters.

Fig. 1.  Red Corpuscles of Human Blood, viewed by their flattened surfaces ($ 215).
Fig. 2.  Red  Corpuscles of Human Blood, adherent by their  flattened surfaces, so as to
         form rolls;—at a, the entire surfaces are adherent; at b, their surfaces adhere
         only in part.
Fig. 3.  Red  Corpuscles of Human Blood, exhibiting the granulated appearance which
         they frequently present, a short time after being withdrawn from the vessels.
Fig. 4.  Colorless Corpuscles of Human Blood (§ 214).
Fig. 5.  The same, enlarged by imbibition of water.
Fig.  6. Red Corpuscles of Frog's Blood (§ 215).
Fig. 7.  The same, treated with dilute acetic acid;'the first effect of which is to render
         the nucleus  more distinct, as at b;  after which  the outer vesicle becomes
         more transparent, and its solution commences, as at a.
Tig. 8.  The same, treated with water; at a is seen a corpuscle nearly unaltered, except
         in having the nucleus more sharply defined; at b, others which have become
         more spherical, under the more prolonged action of water; at c, the nucleus
         is quitting the centre, and approaching the circumference, of the disk; at d
         it is  almost freeing  itself from the envelope;  and at e it has completely
         escaped.
Fig.  9. Globules of Mucus, newly secreted (g 237).
Fig.  10. The same, acted on by acetic acid.
Fig.  11. Globules of Pus, from a phlegmonous abscess ($ 637).
Fig.  12. The same, acted on by acetic acid. 
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EXPLANATION   OE   PLATE  II.
  The Figures in this Plate represent the principal forms of the Nervous centres in diffe-
rent classes of animals.  The 1st is copied from a Memoir by M. Blanchard; the 2d,
3d,  and 4th, from Mr. Newport's delineations; the 5th to the 13th from the work of M.
Guillot on the  Comparative Anatomy of the Encephalon in the different classes of Verte-
brata;  and the last two from the work of M. Leuret on the same subject.

Fig. 1.  Nervous System of Solen; a, a, cephalic ganglia, connected together by a trans-
          verse band passing over the CEsophagus, and connected with the other ganglia
          by cords of  communication;  b, pedal ganglion, the branches of which  are
          distributed to the powerful muscular foot; c, branchial ganglion, the branches
          of which proceed to the gills d, d, the siphons e, e, and other parts.   On some
          of these branches, minute ganglia  are seen;  as also at/, /, on the trunks that
          pass forwards from the cephalic ganglia (§ 852).
Fig. 2.  Nervous System of the Larva  of Sphinx ligustri; a,  cephalic  ganglia;  1-12,
          ganglia of the ventral cord (§ 856).
Fig. 3.  Thoracic portion of the Nervous System of the Pupa of Sphinx ligustri;  a, b, c,
          three ganglia of the ventral  cord; d,  d, their connecting trunks; e, e, respi-
          ratory ganglia (g 862).
Fig. 4. Anterior portion of the Nervous  System of the  Imago of Sphinx ligustri; a, cepha-
          lic ganglia; b, b, eyes ; c, anterior median ganglion, and d, d, posterior lateral
          ganglia of stomato-gastric  system ; e, f, large ganglionic masses in the thorax,
          giving origin to the nerves of the legs  and wings (§ 863).
Fig. 5. Brain of the Perch, seen from above (§ 869).
Fig. 6. The same, as seen from below.
Fig. 7. Interior of the  same, as displayed by a vertical section.

   The following references are common to the three  preceding, and to the succeeding
figures.

            a, a, Olfactory lobes or ganglia.
            b, b, Cerebral ganglia or Hemispheres.
            c, c, Optic lobes.
              d, Cerebellum.
               e, Spinal Cord.
              /, Pineal gland.
               g, Lobi inferiores (their precise character not determined).
              h, Pituitary body.
               i,  Optic Nerves. 
xvi
EXPLANATION OF PLATE  II.
Fig. 8. Brain of the Common Lizard, seen from above (\ 871).
Fig. 9. The same, as seen from below.
Fig. 10. The same, as displayed by a vertical section.
Fig. 11. Brain of the Common Goose, as seen from above (g 872).
Fig. 12. The same, as seen from below.
Fig. 13. The same, as displayed by a vertical section.
Fig. 14. Brain of the Sheep, viewed sideways (g 873).
Fig. 15. The same, as displayed by a vertical section.

  In addition  to the parts indicated  by the  preceding  references,  we have here to
notice;—k, the corpus callosum ;  I, the septum lucidum ;  and m, the Pons Varolii. 
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BOOK  I.
GENERAL  PHYSIOLOGY.
                          CHAPTER  I.

  ON THE  NATURE AND OBJECTS OF THE SCIENCE OF  PHYSIOLOGY.

  1. The general distribution of the objects presented to us by external
nature, into three kingdoms—the Animal, the Vegetable, and the Mine-
ral,—is familiar to every one; and not less familiar is the general distinc-
tion between living bodies, and dead inert matter.   True it is, that we can-
not always clearly assign the limits which separate these distinct classes
of objects.   Even the professed Naturalist is constantly subject  to per-
plexity as to the exact boundary between the Animal and the Vegetable
kingdoms;  and the distinction  between Animal and Vegetable  struc-
tures on the one  hand, and Mineral masses on the other,—or between
living bodies,  and  aggregations of inert matter,—is by  no  means  so
obvious in  every  case, as to be  at once  perceptible to  the unscientific
observer.   Thus, a  mass of Coral, if its growing portion be kept out-of
view, or a solid Nullipore attached  to the surface of a rock, might be
easily confounded with the mineral bodies to which  they bear so close  a
resemblance; and a minute examination might be required to  detect the
difference.   Nevertheless, a well-marked distinction does exist, between
the  organized structures of  Plants  and  Animals,  and the  inorganic
aggregations of Mineral matter; as well as  between the condition of
a living  being, whether Animal or  Plant, and that  of dead or  inert
Mineral bodies.   It is upon these distinctions, which are usually obvious
enough, that the sciences of Anatomy  and Physiology* are founded;
these sciences  taking cognizance,—the former, of those structures which
are termed organized,—and the  latter, of the actions which are peculiar
to those  structures, and which are distinguished  by the  term vital. It
will be desirable to  consider,  in a somewhat systematic  order, the  prin-:
cipal  ideas which we attach to these terms; as we shall be thus led most
directly to  the distinct comprehension of the nature and objects of Phy-
siological science.
                                 2 
18       NATURE AND  OBJECTS OF PHYSIOLOGICAL SCIENCE.
             1.  General Characters of Organized Structures.

  2. Organized  structures are  characterized, in the first  place,^ by the
peculiarities of their form.—Wherever a definite form is  exhibited  by
Mineral substances, it is bounded by straight lines  and angles,  and is
the effect of the process  termed crystallization.   This process results
from the tendency which  evidently exists in particles of  matter, espe-
cially when passing gradually from the fluid to the solid state, to arrange
themselves in a regular and conformable manner in regard to one ano-
ther.   There is, perhaps, no inorganic element or combination,  which is
not capable of assuming such a  form, if placed in circumstances adapted
to the manifestation  of this tendency among its* particles; but if these
conditions  should be wanting, and the simple  cohesive attraction is ex-
ercised in  bringing them together, without any general  control over
their direction,  an indefinite or shapeless figure is the result.—Neither
of these  conditions finds a parallel in the  Organized creation.  From
the highest to the lowest, we find the  shape  presenting a determinate
character  for each species or race, with a certain  limited amount  of
variation amongst individuals ; and this shape is such, that, instead  of
being circumscribed  within plane surfaces,  straight  lines, and angles,
organized bodies are bounded by convex surfaces, and present rounded
outlines.   We may usually gather, moreover, from their external form,
that they are composed of a number of dissimilar  parts,  or  organs;
which  are  combined together in  the one individual body,  and are cha-
racteristic of it.  Thus in the Vertebrated or  Articulated animal, we  at
once distinguish the head and extremities from the trunk, which  consti-
tutes the principal mass;  and where there exist no  external organs  of
such distinctness, as in some  Molluscs, the rounded character  of the
general form is sufficiently characteristic.  The very simplest grades of
animal and vegetable  life  present themselves under a shape, which ap-
proaches more or less closely to  the globular.   It is among the  lower
tribes of both kingdoms, that we  find  the greatest tendency to irregular
departures from the typical form of the species ;  and thus is presented
an  approach, on the  one hand,  to that indefiniteness which is characte-
ristic of uncrystalline mineral masses  ; and,  on the other, to  that variety
of crystalline forms which  the same mineral body may present, accord-
ing to the  circumstances which influence its crystallization.
   3.  With regard to size, again nearly the  same remarks  apply.  The
magnitude of Inorganic masses is entirely indeterminate, being  altoge-
ther dependent upon  the  number  of particles which can  be  brought
together to constitute them.  On the  other hand, the size of Organized
structures is restrained, like their form, within tolerably definite limits,
which may nevertheless vary to a certain extent among the individuals
of  the same  species.   These limits are  least obvious in vegetables, and
in  the lower classes  of animals.  A forest-tree may go  on extending
itself to an almost indefinite extent;  certain species of sea-weed attain
a length of many hundred feet, and their growth does not appear to be
restrained by any limit;  and the same may be said of those enormous
masses of  coral, which compose  so many islands and reefs in the Poly- 
OF ORGANIZED  STRUCTURES IN GENERAL.
19
nesian Archipelago, or of which the debris seem  to  have constituted
many of the calcareous rocks of ancient  formation.  But in these cases,
the increase is produced  by the  multiplication of similar parts, which,
when once completely evolved, have but little dependence upon one ano-
ther,  and might  be almost considered  as distinct  individuals.   Thus,
each bud of a tree,  if placed under  favorable circumstances,  can main-
tain its life by itself,  and can  perform all the actions proper to the
species.  Each polype of the coral  mass, in like manner, at first pro-
duced by a process  of budding from the original stock, comes in  time to
be completely independent of it,  and  of those with which it is associated.
And in the sea-weed, each portion of the frond is an almost precise repe-
tition of every other, and grows for and by itself;  neither receiving
from nor communicating to, any other part, the materials of its organic
structure.   Thus among Plants and the lower  Animals, we  find  an
indefiniteness in point of size, depending upon the tendency to multipli-
cation of similar parts, which has been designated as vegetative repetition.
   4.  It is, however, in the internal arrangement or aggregation of the
particles,  respectively composing Organized structures  and Inorganic
masses, that we find  the difference between  the  two  most strongly
marked.—Every  particle of a Mineral body (in which there  has not
been a mixture  of  ingredients)  exhibits the same  properties as those
possessed by the whole;  so that  the chemist, in experimenting with any
substance, cares not, except as a matter of convenience merely, whether
a grain or a ton be  the subject of his researches.   The  minutest 'atom of
carbonate of lime, for instance, has all the properties of a crystal of this
substance, were it as large as a mountain.   Hence  we are to regard a
mineral body as made up of an indefinite number of constituent particles,
similar to it and to  each other in properties, and having no  further re-
lation among themselves than that which they derive  from their juxta-
position.  Each particle, then, may be considered as possessing a sepa-
rate individuality ; since we can predicate of its properties all that can
be said of the  largest mass.—The  Organized structure, on the other
hand, receives its designation from being made up of a  number of dis-
tinct parts or organs, each of which has  a texture or consistence peculiar
to itself; and it derives its character from the whole of these collectively.
Every one of these, as we shall hereafter see, is  the  instrument of a
certain action or  function, which it performs under  certain conditions;
and the concurrence of all these actions is required  for the maintenance
of the structure in its normal or regular  state, and  for  the  prevention
or the reparation of those changes, which chemical  and  physical forces
would otherwise speedily produce in it, from causes hereafter to be ex-
plained.  Hence there is a relation of mutual  dependence among the
parts of an Organized structure; which is quite distinct  from that of
mere proximity.  Thus,  the perfect plant, which has roots, stem, leaves,
and flowers, is an example of an organized structure, in which the rela-
tion of the  different parts to the integrity  of the whole is  sufficiently
obvious ; since, when entirely deprived of either set of  these organs, the
race must perish, unless  the plant have within itself  the  power of re-
placing them.
   5.  It is not only in Zoophytes and other aggregate  Animals,  that we 
 20       NATURE AND OBJECTS OF  PHYSIOLOGICAL  SCIENCE.

 notice the tendency to " vegetative repetition ;" for  it may be observed
 in many animals which can be divided without the destruction of  their
 lives,—especially among the Radiated, and the lower Articulated tribes.
 Where such a repetition exists, some of the organs may be removed  with-
 out permanent  injury  to the structure; their function being performed
 by those that remain.   Thus it is not uncommon to meet with specimens
 of the common five-rayed Starfish, in  which not only one or two, but even
 three or  four, of the arms have been  lost without the destruction of the
 animal's life ; and this is the  more  remarkable, as  the arms  are not
 simply organs of locomotion  or prehension, but contain prolongations of
 the stomach.  In the bodies of the higher animals, however, where there
 are few or no such repetitions (save  on the two sides of the body), and
 where there  is consequently a greater diversity in character and function
 between the different organs, the mutual dependence of their actions  upon
 one another is much  greater,  and the  loss of a single part is much  more
 likely to  endanger the existence of the whole.   Such  structures are said
 to be more highly organized than those of the lower classes ; not because
 the whole number of parts is greater,—for it is frequently much  less;
 but because  the number of dissimilar parts, and  the consequent adap
 tation to a variety of purposes, is much greater,—the principle of divi-
 sion  of labor, in  fact, being  carried much  further, a  much larger  class
 of objects being attained, and a  much greater perfection in the accom-
 plishment of them being thus provided for.
  6. Keeping in view, then, what  has just been stated in  regard to the
 divisibility of a Tree  or a Zoophyte into a number of parts, each capable
 of maintaining its own  existence, we may trace a certain  gradation  from
 the condition of the Mineral body to that of  the highest Animal,  in
 regard to the character in question.   Thus, the individuality of a Mi-
 neral substance may  be said to reside in each molecule ;  that of a Plant
 or Zoophyte, in each complete member; and that of one  of the higher
 Animals, in  the sum of all the organs.  The distinction is much greater,
 however, between the  lowest organized fabric  and any mineral body,
 than it is between the highest and the lowest organized structures ; for, as
 we shall hereafter see, the highest and most complicated may be regarded
 as made up  of an assemblage of the lowest and simplest; whose structure
 and actions  have been so  modified as to render them mutually depen-
 dent ;  but  which yet  retain  a separate individuality, such as enables
 them to continue performing their functions when separated from the
 mass, so  long as the proper conditions are  supplied.
   7.  Between the  very simplest Organized fabric, and every form  of
 Mineral matter, there is a marked  difference in regard to intimate struc-
 ture and.consistence.   Inorganic substances can scarcely be regarded as
 possessing a structure;  since (if there be no admixture of components)
 they are  uniform and homogeneous throughout, whether existing in the
 solid, the liquid, or the gaseous form;  being composed of  similar parti-
 cles, held together by  attractions which affect  all alike.   Far different
 is the character of Organized structures;  for in the minutest  parts of
 these may be detected a heterogeneous composition,—a mixture of  solid
and fluid elements, which are so  intimately combined and arranged, as
to impart such peculiarities to the tissues, even in regard to their physi- 
OF ORGANIZED STRUCTURES IN GENERAL.
21
        cal properties, as we never encounter amongst Mineral bodies.   In the
        latter, solidity or hardness may be looked upon as the characteristic con-
        dition ; whilst in Organized structures, softness (resulting from the large
        proportion of fluid components) may be considered the distinctive quality,
        being most obvious in the parts that are most actively concerned in vital
        operations.   This softness is evidently connected with the roundness of
        form  characteristic of Organized fabrics, which is most  evident when
        the tissues  contain the greatest  proportion of fluid; whilst the  plane
        surfaces  and angular  contours of Mineral bodies  are evidently due  to
p-       the mode in which the solid particles are aggregated together, without
        any intervening spaces.
           8.  The greatest solidity exhibited by Organized fabrics, is found where
        it is desired to impart to them the simple physical property of resistance;
        and this is attained by the disposition of solid particles, often of a mineral
        character, in tissues that were originally soft and yielding.  It is in this
        manner that the  almost jelly-like substance, in  which all the organs of
        animals  originate, becomes condensed into cartilage,  and that the carti-
        lage is afterwards converted into bone; it is in the same manner,  also,
        that the  stones of fruit, and the heart-wood of timber-trees, are formed
        out of softer tissues.   But, as we shall hereafter see, this  kind of con-
        version,  whilst it renders the  tissue more solid and durable, cuts it off
        from any active participation in the vital operations ; and thence reduces
        it to  a state much more  nearly analogous to  that of mineral bodies.
        This resemblance is rendered  more close by the  fact, that  the earthy
        deposits  frequently  retain a distinctly crystalline  condition; so that,
        when they are present in large proportion, they impart a more or less
        crystalline aspect to the mass, and especially a crystalline mode of frac-
        ture,  which is  evident enough in many shells.   It must not  be hence
i        concluded, however, that such substances are of an inorganic nature ;
        all that  is shown by their crystalline structure  being, that the animal
        basis  exists in comparatively small amount, and that the mode in which
        the mineral matter  was deposited has not  interfered with  its crystalline
        aggregation.
           9. It  is not to be disputed  that a certain degree of homogeneity is
        apparently to be found in  the minutest  elements, into which certain
        Organized tissues are to  be resolved.  Thus, in the membranes which
        form the walls of Animal and Vegetable cells, the highest powers of the
        microscope fail in detecting any such distinction of fluid and solid com-
        ponents,  as that which has been described  as characteristic of  organized
        structures.  Nevertheless it is indubitable that such distinct components
        must  exist; and this especially from the properties of these membranes
        in regard to water.   For it is  one of the most remarkable facts in  the
        whole  range of science, that  a membrane,  in which not  the  slightest
        appearance of a pore can be discovered under the highest powers of the
        microscope, should  be  capable of allowing  water  to pass through it; and
        that,  too, with no  inconsiderable rapidity.  The  change which  these
        membranes undergo in drying, is  another proof that they are  not so
        homogeneous as they appear, and that water is an element of their struc-
        ture, not merely chemically, but mechanically.  The same may be said
        in regard to the fibres, which form the apparently  ultimate elements of 
22
NATURE AND OBJECTS  OF PHYSIOLOGICAL SCIENCE.
the simple fibrous tissues in Animals, and which are also met with in the
interior of certain cells and vessels in Plants.  These fibres would appear
to be of perfectly simple structure; yet we know from the loss of fluid,
and the change of properties which they undergo in drying, that water
must have formed part of  their substance.—It may be  remarked, how-
ever,  in regard to both these elementary forms of Organized tissue, that
the simplicity of their function is in complete conformity with the appa-
rent homogeneousness of their structure; for the cell-membrane is chiefly
destined to act, like the porous septum in  certain forms of the voltaic
battery, as a  boundary-wall  to the contained fluid, without altogether
interfering with its  passage  elsewhere;  the forces which produce  its
imbibition or expulsion being probably situated, not in this pervious wall,
but in the cavity which it bounds.   And,  in the same manner, the func-
tion of the fibrous tissues, to w7hich allusion was just now made, is of an
entirely physical character;  being simply to resist strain  or pressure,
and yet to allow of a certain degree of yielding by their elasticity.
   10. In all cases in which active vital operations  are going on,  we can
make a very  obvious  distinction of the structures subservient to them,
into liquid and solid parts;  and it  is, indeed by the continual reaction
which is taking place between these, that the fabric is maintained in its
normal condition.  For, as we shall  hereafter see, it  is liable to a constant
decomposition or separation into its ultimate elements;  and it  is conse-
quently necessary that the matters which have  undergone that  disinte-
gration should be carried  off, and that they should be replaced by new
particles.  These processes of removal and replacement, with the  various
actions subservient  to  them, make up  a large proportion of the life of
all Organized beings.  Now as all the alimentary matter must be reduced
to the liquid form, in order that it may be conveyed to the situations in
which it is required, and as all the decomposed or disintegrated matter
must be reduced to  the  same form  in order  to be  carried off, the inter-
mingling or mutual penetration of solids and liquids in the minutest parts
of the body is at once accounted for.  We shall hereafter see that a cell
or closed vesicle,  formed of  a membranous  wall, and containing fluid,
may be regarded as the simplest form of a living body, and the simplest
independent part or instrument of the more  complex fabrics (§ 30).
   11. Organized structures are further distinguished  from Inorganic
masses, by the peculiarity of their chemical constitution.   This pecu-
liarity does not consist, however, in the presence of any  elementary sub-
stances which are not found elsewhere; for  all the elements, of which
organized bodies are composed, exist abundantly in the world  around.
It might have been suppose^ that beings endowed with such remarkable
powers as those of Animals and Plants,—powers which depend, as  we
shall  hereafter see,  upon  the exercise of properties  to which  we  find
nothing analogous in the  Mineral  world,—would  have  had an entirely
different material constitution; but a little  reflection will show,  that the
identity of the ultimate elements of Organized structures  with  those of
the Inorganic  world, is a necessary consequence of the mode  in which
the former are built up.   For  that  which the parent communicates, in
giving origin to a new being, is not so much the  structure itself as the
power of forming that structure from the surrounding elements; and it 
OF ORGANIZED  STRUCTURES IN GENERAL.
23
is  by gradually drawing  to itself certain  of these elements, that the
germ becomes developed into the complete fabric.  Now, of the sixty-
two simple or elementary substances, which are known  to occur  in the
Mineral world, only about eighteen or nineteen are found in Plants and
Animals; and many of these in extremely minute proportion.   Some of
these appear to be merely  introduced, to answer certain chemical or
mechanical purposes; and the composition of the parts which possess
the highest vital endowments, is for the  most part  simple and more
uniform.
   12.  The actual tissues of Plants, when entirely freed from the sub-
stances they may  contain, have been found  to possess  a very uniform
composition,  and to agree in their chemical properties.   The substance
which  forms the principal part of the thickness of the walls of the cells,
vessels, &c, of which the  Vegetable  organism is  composed,  is identical
with Starch in the  proportion of its  components ;  but as these are in a
different state of aggregation, it  is distinguished as  Cellulose.   It con-
sists of 12 Carbon, 10 Hydrogen, and 10 Oxygen; or, in other  words,
of Carbon united to the elements of water, in the proportion  of eight
of the  former to seven of the latter.  It  may be very easily converted
into gum or sugar,  by chemical  processes, whieh effect the  removal or
the  addition of the elements of water.  Now  there  is no compound
known to exist in the Inorganic world, which  bears the remotest analogy
to this;  and we have no reason  to believe that it could be produced in
any other way, than by that peculiar combination of force which exists
in  the growing Plant.   But  although  Cellulose is the predominating
component of the Vegetable fabric, yet it is not the most essential; for
late researches have shown, that within what has been ordinarily consi-
dered  as the cell-wall, is a delicate membrane, termed  the " primordial
utricle," which is really the original  cell-wall, from the exterior of which
the layer of cellulose is  secreted.   And it  is a very interesting fact,
that the   composition of this  membrane corresponds with  that  of the
proper cell-walls of the  Animal tissues ;  it  being, in fact, a proleine
compound (§ 13).   Hence every act of Vegetable growth  involves the
production of this substance  also, which is  still more removed in its
composition from ordinary Inorganic compounds.
   13.  The  composition of the  Animal tissues,  when  freed from the
fluids  they may contain, or  from the solid matters which may have been
deposited within  them,  is nearly  as uniform.   We may distinguish
among them two chief proximate principles, which appear under  various
modifications in a  great  variety of dissimilar  parts,  and  which seem
capable  of conversion into other principles by the addition or subtrac-
tion of some of their constituents.   The first  and most important  of
these, named Proteine,*  consists of  40 Carbon, 31 Hydrogen, 5 Nitro-
gen, and 12 Oxygen ;  and although its  composition  is  so  complex, it
appears  to act like a simple body, in uniting with Oxygen, Acids, &c,
in definite proportions, as well as with  Sulphur  and  Phosphorus ; with

   * Although it may be doubted whether Proteine has ever been actually obtained in a
separate state, yet the term may be conveniently applied to that composite base, which
is united with different equivalents of sulphur and phosphorus in the albuminous com-
pounds. 
 24       NATURE AND OBJECTS OF PHYSIOLOGICAL SCIENCE.

 which last, indeed, it is always found combined, in the Albumen, Fibrine,
 &c, that  are  commonly regarded  as  the  organic constituents of the
 Animal tissues.—The second of the chief proximate principles, termed
 Gelatine, is largely diffused  through the Animal  body; but the tissues
 which are composed of it possess a  simple fibrous structure, and a purely
 mechanical function;  and no vital  action seems to take  place in them,
 subsequently to  their first production.  It consists of 13 Carbon, 10
 Hydrogen, 2 Nitrogen, and  5 Oxygen; and it is principally characte-
 rized by its solubility in  hot water, and by the insolubility of its com-
 pound with tannic acid.
   14. We shall hereafter dwell more in detail  upon the Chemical Con-
 stitution of the Animal tissues  and products (chap, hi.)  These  sub-
 stances  are only noticed here, in illustration of the general statement,
 that the " proximate principles" of Animal and Vegtable bodies  (that
 is,  the  simplest  forms to which  their component structures  can be
 reduced, without altogether  separating them  into their  ultimate  ele-
 ments),  are of extremely peculiar constitution ; being  made up of three
 or four  elements, of which the  atoms do not seem to be  united two by
 two, or  by the  method  of  binary composition, but of which a large
 number are brought together to form one compound atom  of ternary
 or quaternary composition.   This  compound atom, like Cyanogen, and
 many others derived from Organic products, acts like  a  simple or ele-
 mentary one in its combinations with other substances.—It is worthy
 of remark, however, that,  in this"respect as in others,  the Vegetable
 kingdom is intermediate  between  the Animal and  the  Mineral.   For
 whilst Proteine and Gelatine are  remarkable,  not only for  containing
four elements, but for the very large number of atoms of these compo-
 nents which enter into the single  compound atom of  each ; the Cellu-
 lose of Plants is  much simpler in its composition, since it includes only
 three elements, and the numbers which represent their proportions are
 smaller.   And further, the proportions of the  components of Cellulose
 are themselves such as suggest the  idea of simplicity in their method of
 combination,—the union of  water  and carbon in the common binary
 method;—an idea which is confirmed by the mode of its  original  pro-
 duction, which indicates a direct union of carbon with water; as well as
 by the fact, that  the  chemical  difference between  Cellulose and nume-
 rous other substances found in Plants, may be represented by the simple
 addition or subtraction of a  certain number of  atoms of water, and  that
 the  chemist can effect an actual conversion of the former into certain of
 the latter, by means which are  calculated to effect such an addition or
subtraction.
  15. We shall hereafter see that Vegetables are  intermediate between
 the Animal kingdom' and the Inorganic world in another most important
 particular—the nature of  the chemical operations they effect; for it is
 their function to combine the oxygen, hydrogen, carbon, and nitrogen,
of the Inorganic world into Organic Compounds ; which not only serve as
the materials of their own growth, but also as the food of Animals, whose
existence is entirely dependent  upon them, since  they possess no such
combining power.  It is from the Water, Carbonic Acid, and Ammonia,
supplied by the atmosphere and by the soil in which they are fixed, that 
OF VITAL  ACTIONS IN GENERAL.
25
Plants derive these elements.   On the other hand, the Animal, making
use of the ternary and quaternary compounds which  have been  elabo-
rated by Plants, is continually restoring their elements to the Inorganic
world, in the very forms which they originally possessed ; for, as we
shall hereafter see, the excretion of  Water, Carbonic acid, and Ammo-
nia is constantly taking place in the Animal body during life,  as the
result of those  changes in  which its peculiar activity consists.   And
thus is sustained that balance betwreen Animal and Vegetable nutrition,
which  is found  to be the more  wonderful  and complete, the more  care-
fully it is scrutinized.

                2. Distinctive Characters of Vital Actions.

   16.  We are now arrived at the second head of our inquiry,—namely,
the nature of those actions which distinguish  living beings  from masses
of inert matter, and which are designated as  Vital, to mark their dis-
tinctness from  Physical and Chemical phenomena.   There can  be no
doubt whatever, that, of the many changes which take place during the
life, or state of  vital activity, of an Organized  being, and  which  inter-
vene between its first  development and its final decay, a large propor-
tion are effected by the direct  agency of those forces, which operate in
the Inorganic world;  and there is no necessity whatever for the suppo-
sition, that these forces have any other operation in  the  living  body,
than they would have out of it under similar  circumstances.  Thus the
propulsion of the blood by the heart  through the large  vessels, is a phe-
nomenon precisely analogous  to the propulsion of  any  other  liquid
through a system of pipes by  means of a forcing pump ; and if the ar-
rangement of the  tubes, the elasticity of their walls, the contractile
power of the heart, and the physical properties of the fluid, could  be
precisely imitated, the artificial apparatus would give us an exact repre-
sentation of the  actions of the real one.  The motor force of the muscles
upon  the bones,  again, operates in a mode that might be precisely repre-
sented by an arrangement of cords and levers; the peculiarity here, as
in the  former case, being solely in the mode in which  the force is first
generated.  So, again, the digestive operations which take  place  in the
stomach are capable of being  closely imitated in the laboratory of the
chemist;  when the same solvent fluid is employed, and the same agencies
of heat, motion, &c, are brought into play.   Moreover  we shall here-
after see reason  to  believe, that the  peculiar form of Capillary Attrac-
tion, to which the term "endosmose" is applied, performs an important
part in the changes which are continually taking place in the living body.
   17.  But after every possible  allowance  has been made for the opera-
tion of Physical and Chemical forces, in the living Organism, there still
remain a large  number of phenomena, which cannot be in  the least ex-
plained by them, and which we can only investigate with success, when
we regard them  as resulting from the agency of forces as distinct from
those of Physics and Chemistry, as they are from each other.   It is to
these phenomena  that we give  the  name of  Vital; the forces  from
whose  operation we assume them to result, are termed vital forces ; and
the properties, which  we must  attribute  to the substances manifesting 
26       NATURE  AND  OBJECTS OF  PHYSIOLOGICAL SCIENCE.

those forces, are termed vital properties.  Thus we say that the act of
contraction in a muscle is  a vital phenomenon ; because its character
appears totally distinct from  that of a  Physical  or Chemical  action ;
and  because it is dependent upon other vital changes  in the muscular
substance.   The act is the manifestation of a certain force, the posses-
sion of which is peculiar to the muscular structure, and which is named
the Contractile force.   But that force is only exerted under certain con-
ditions, and these may only recur at long intervals, through the capacity
for exerting it be  always present in the  organized tissue ;_ this capacity
is termed & property ; and thus we regard it as  the essential peculiarity
of living Muscular tissue, that it possesses the vital property  of  Con-
tractility.  Or, to reverse  the order, the muscle  is said to possess the
property of Contractility;  the property, when the appropriate conditions
are supplied, gives rise to the Contractile Force ; and the force produces,
if its operation be  unopposed, the act of Contraction.
  18.  It may be said  that the  distinction here made  is a  verbal  one;
and that a very simple  thing  is thus made complex; but it  will be pre-
sently seen  that it is  necessary, in order to enable us to take  correct
views of the  nature of  Vital phenomena, and to understand their  rela-
tions to those of the Inorganic world.-  And in fact the distinction
between the property, the force, and the action, become apparent  upon
a little consideration.   Of the property we are altogether unconscious,
so long as it is not called into exercise ; Ave could  not, for example,  de-
termine by the  simple exercise  of any of  our senses, whether a certain
piece of muscle retained, or had lost, its contractility.  When the pro-
perty is called into action by its appropriate stimulus,  we may convince
ourselves that a force is  generated, even if no sensible action is pre-
sented ;  thus, if Ave were to  hold  the  tAVO extremities of  a muscle  so
firmly, as to prevent them from approximating  in the least degree  when
its contractility was excited, Ave should be conscious of a poAverful force
tending to draAV our hands together; and Ave might measure the amount
of  that force, by mechanical means adapted  to determine the Aveight it
would sustain.   And lastly, if  no  obstacle be interposed to the act of
 contraction, it then becomes obvious to our senses, by the change in  the
shape of the muscle, and by the approximation  of its two extremities, as
well as of the bodies to which they are attached.
   19. The advantage of this method of viewing the phenomena of Life
 is best shoAvn, by turning  our  attention for a moment to  the mode of
investigation practised in Physics and  Chemistry.  Thus, when a stone
 falls towards  the earth,  Ave say that  this is  an act or phenomenon of
 Gravitation.   The force with Avhich the stone tends to fall to the ground,
 Avhether it actually falls or not, is called the force  of  Gravitation;  and
 we  speak of the tendency Avhich every substance has to act in this  mode,
 as the property of Gravitation.  Now  from observation of the Moon's
 motion, it is shown that she too is drawn towards  the  Earth;  her ellip-
 tical path around it being the result of the combined action of the cen-
 trifugal or tangential, and of the centripetal or gravitative forces.   And
 it is further established, that not only does the Moon  gravitate towards
 the Earth, but the Earth gravitates towards the  Moon ;  so that if the
 t\vo bodies Avere  entirely free from  the action of all other forces, they 
OF VITAL ACTIONS IN  GENERAL.
27
would fall  towards each other (the distance  traversed by each being in
proportion to the size  of the  other), and Avould meet in their common
centre of  Gravity.   Hence  it  is  evident that  the  attractive  force is
similar in both bodies ; and our idea of the property of Gravitation must
be extended, therefore, from  the  Earth to the Moon.   Again, we find
ample reason to believe  that the same  force acts between the Sun and the
Planets,—between the Planets and the Sun,—and amongst the Planets
themselves ; and further, careful experiment shows that masses of matter
upon the Earth's surface are not  only attracted  by it, and attract  it in
their turn, but that they attract and are attracted  by each other.   Hence
Ave arrive at the idea of the universality of this  property of mutual at-
traction ; and we  percieve  that, in spite of varieties in the actions it
produces, and of  differences in the  amounts of the forces  to which it
gives rise, the property is the same throughout;  so that we can predict
all these actions, and anticipate the forces Avhich will be developed, from
the simple general expression of the property of Mutual  Attraction, and
of the conditions under which it is manifested,—constituting the Law
of Gravitation or Mutual Attraction.
  20. Now in this case of Mutual Attraction, we  have no opportunity
of Avitnessing the dormant condition of the  property  in  any mass of
matter ;  for, as nothing is wanting but the presence of another mass to
call  this property  into operation, it is always  generating force, and
giving rise to  actions.  If we  could  conceive of  the  existence of but a
single mass of matter in the universe, we shall at once see that though
possessed of the property of mutual  attraction, it would not be able to
exercise it, so as to generate an attractive force, or to produce a move-
ment.
  21. But we will turn to another  case; in which  there is  a closer
analogy  to the condition of living  beings; and by which, therefore, the
view here  put forth may be more clearly illustrated.   When a magnet
(itself a bar of iron, having no peculiarity of appearance) draws towards
it a  piece of iron,  we say that a Magnetic action or phenomenon takes
place; further, we speak of the poAver which produced  the  movement,
as the Magnetic force ; and wre attribute this force to a certain property
inherent in the Magnet, by virtue of which it draAvs toAvards itself all
pieces of iron  that are within  the  sphere of its operation, and we speak
of the iron bar  as endowed Avith  the property of Magnetic attraction.
Now we cannot ascertain the presence or absence  of this property in a
certain bar of iron, by any difference in its aspect, its specific gravity,
its chemical properties, nor, in fact, by any other mode than the putting
it in circumstances adapted to  call the Magnetic property into action  if
it really exist:—thus Ave dip it into iron filings,  and judge by their ad-
hesion whether it is capable of  attracting them ;  or, as a still more deli-
cate test, we  ascertain whether it is  capable of  exerting any repulsive
pow7er on a delicately-suspended needle  already  magnetized.  Again,  a
needle, or bar of iron, which exhibits  this magnetic power of attracting
other portions of  the same metal, exhibits another power, which would
seem at  first sight totally distinct; namely, that of constantly turning
one of its extremities towards the north, and the other toAvards the south,
Avhen it is so supported as  to  be  free  to do so.  And yet there is no 
28        NATURE AND OBJECTS OF  PHYSIOLOGICAL  SCIENCE.

doubt Avhatever, that this directing power is only another manifestation
of the same magnetic attractiveness ; depending on the relation betAveen
the magnetized  bar, or needle, and the Earth, which must itself be re-
garded as a great magnet.  Hence the idea of a peculiar kind of mutual
attractiveness,—existing in only a limited class of bodies, capable of being
excited in one by a certain agency on the part of the other,*—and re-
quiring for its exercise or manifestation a certain set  of conditions, Avith-
out Avhich no phenomenon  results,—is that  Avhich we regard as funda-
mental in the Science of Magnetism.
  22. We may noAV turn from these departments of Physical Science to
Chemistry; and here Ave shall find that the investigation is carried on
upon the  very  same plan.   In fact, the Avhole science of Chemistry is
founded upon the idea of a certain  attractiveness  or affinity existing
among the ultimate particles or molecules of the  different elementary
substances ; and therefore entirely distinct from the homogeneous attrac-
tion, which holds together  the particles of the same mass, or from the
gravitative attraction, Avhich operates alike upon all masses, whatever be
their composition.   Thus we say that Sulphuric acid  and Potash have an
affinity  for each other; because  they unite when they are  brought
together and  form a new compound.   This is a Chemical  action or phe-
nomenon.   Now we know that they tend to unite with a certain force;
a force, hoAvever, Avhich cannot be  measured mechanically, and  which
can only  be  expressed by comparing  it wTith some other force of the
same kind.   Thus Ave say that the mutual affinity of Sulphuric acid and
Potash is  greater than that of Nitric acid  and Potash ;  because, if we
pour Sulphuric  acid upon Nitrate of Potash, the Sulphuric acid detaches
(as it Avere) the Potash from its connexion with the Nitric acid, forms a
new compound  with it,  and sets the  Nitric  acid free.   Hence Ave say
that it  is a property of Sulphuric acid to have a very strong affinity for
Potash.  This  property exists in  every particle of Sulphuric  acid  that
exists, whether  free or combined; but it does not manifest itself, except
when called  into operation by the contact of Potash; and it then de-
velopes a force, which  may completely change the combinations  previ-
ously existing, and give rise to neAv ones.
  23. Now of this property Ave  are  not informed by any of the other
properties of  Sulphuric acid;  and Ave only recognise its existence  by the
action which  is  the result  of its exercise.   If a neAV element  or com-
pound be discovered, the chemist is totally unable to predict its force of
affinity for this  or that substance; and he can  only guess by analogy,
Avhat Avill  be  its behavior under various circumstances.  Thus if it have
the external  properties of a Metal, he presumes that it Avill correspond
with the Metals in possessing a strong affinity for oxygen, sulphur, &c.;
whilst if it seem analogous to Iodine, Chlorine, &c, he infers that it will
be a supporter of combustion, that it will form an acid with  hydrogen,
and so  on. But, even though such guesses may be made  with a certain
amount of probability, nothing  but  experience can show the positive
degrees of affinity which the substance may have for others of  different
kinds ;  and the  experimental determination can only be made, by observ-
  * As when one magnet is made by another; or when iron rails, pokers, &c, become
magnetic by the influence of the earth. 
OF VITAL ACTIONS IN  GENERAL.
29
ing the actions of the body when placed in different circumstances, from
which Ave judge of  its properties, and of the forces to which these proper-
ties give rise when they are called into operation.
  24.  It is hoped that the propriety of the  distinct  use of the terms
 Vital Action, Vital Force, and Vital Property, will  noAV be  evident;
and that the  student will be prepared to attach distinct ideas to each of
them.   It is  t-he business of  the Physiologist to study those actions or
phenomena, Avhich are peculiar to living  beings, and  which are hence
termed vital:—he endeavors to trace them  to the  operation of specific
forces  acting  through  organized  structures, just  as the Astronomer
traces  all the movements,  regular and perturbed, of  the heavenly bodies,
to the  mutual attraction of their masses, acting concurrently with their
force of onward rectilinear movement; or as the Chemist attributes  the
different acts of combination  or separation, Avhich  it  is his province to
study,  to the  mutual affinities of  the  substances concerned :—and  the
physiologist, like the astronomer or the chemist, seeks to determine  the
laws according to which these forces act, or, in other  Avords, to express
the precise conditions under Avhich they are  called into play, and  the
actions which they then produce.  It is  only in this manner, that Phy-
siology can be rightly studied and  brought to the level of other sciences.
There  can be no doubt  that  its progress has been greatly retarded by
the assumption, that its  phenomena  Avere all to  be attributed  to  the
operation of  some general controlling  agency, or Vital  Principle ; and
that the  laws expressing  the  conditions of those phenomena,  must be
sought for by methods of investigation entirely  distinct  from  those
which  are employed in other sciences.   But a better spirit is now abroad;
and the student cannot be too strongly  urged to discard any ideas of
this kind as absolutely untenable;  and to keep steadfastly in view, that
 the laws of Vital Action are to be  attained in the same manner as those
 of Physics or Chemistry,—that is, by the  careful  collection and com-
parison of vital phenomena, and by applying to them the same method
 of reasoning,  as that which is used in determining the forces  and pro-
perties on  which  other phenomena depend.   True it is, that Ave  can
scarcely yet  hope to reach the same degree of simplification, as that of
which  other sciences are capable ;  and this on account of the very com-
plex nature of the phenomena  themselves, and the  difficulty of satisfac-
torily determining their conditions.   The uncertainty of the results of
Physiological experiments is  almost proverbial: that uncertainty does
 not result,  however,  from any Avant  of fixity in the  conditions under
 which  the vital forces operate, but merely from the influence of diffe-
 rences in those conditions, apparently so slight  as  to elude observation,
 and yet  sufficiently powerful to produce an  entire  change in the result.
 And, owing to that mutual dependence of the different actions of the orga-
 nized structure, to which reference has been already made (§5), we cannot
 seriously derange one class of these actions, without also deranging, or
 even suspending others:—a circumstance Avhich obviously renders vital
 phenomena much more difficult of investigation than those of inorganic
 matter.
   25.  All sciences have their "ultimate facts;" that is, facts for which
 no  other cause can be assigned than the Will of the Creator.   Thus, in 
30        NATURE AND  OBJECTS OF PHYSIOLOGICAL SCIENCE.

Physics, we cannot ascend above the fact of Attraction (which operates
according to a simple and universal law) between all masses of matter;
and in Chemistry, we cannot rise beyond the fact of Affinity (limited by
certain conditions  which are not yet well  understood) betAveen the par-
ticles of different kinds of matter.   When we say that Ave have explained
any phenomenon,  we merely imply that we have traced its origin to
properties Avith which we were previously acquainted, and shown that  it
takes place in  accordance with the knoAvn laws of their operation.  Of
the existence of the properties, and the determination of the conditions
of their action, we can  give no other account, than that the  Creator
willed them so to be; and, in looking at the vast variety of  phenomena
to Avhich they give rise,  we cannot avoid being  struck with the general
harmony that  exists  amongst them,  and the mutual dependence  and
adaptation that may  be  traced between them, when  they are considered
as portions of the general economy of Nature.  There is no difference
in this respect between  Physiology and other sciences ;  except that the
number of these (apparently)  ultimate facts is at  present  greater in
physiology than it is in other departments, because we are not at present
able to include them  all under any more general expression.   But, as
will presently appear, a considerable degree of simplification appears
practicable in our view  of them ; and although  Ave may not be able to
say why the structure called Muscular should possess contractility,  and
why the structure  called Nervous should be capable of  generating  and
conveying the  force which excites  that contractility to  action, we may
draw, from the study of the conditions under which they respectively
manifest themselves,  some indications of the existence of a common tie,
such as that which binds together  the planetary  masses, at the  same
time that it  Aveighs doAvn the bodies on the surface of the earth towards
its centre.
  26. In the study of any branch of science, it is most desirable to com-
mence with definite views of the nature of  the phenomena with which  it
is  concerned;  and such  are best gained by the  examination of these
phenomena under their simplest aspects.  This course is  most especially
necessary in Physiology: since the  complexity of the conditions under
Avhich its phenomena usually  present themselves, often  tends to mask
their real character,  causing that to  be  regarded  as essential  which  is
only accidental  or  contingent, and  vice versd.  It is extremly difficult,
however, and frequently impossible, for the Physiologist  to isolate these
several conditions, and to study them separately, in the way that  the
Chemical or Physical investigator Avould do; and his best course is to
take advantage  of those "experiments  ready prepared by Nature,"
which he finds in  the variety of forms of living organized beings, Avith
which the globe is so richly peopled.  Now it is in the simplest forms of
Cryptogamic Vegetation, that the phenomena of Life present themselves
under their least complicated aspect; for we shall find in the operations
of each of the simple cells of which  such  Plants  are  composed  (all of
them resembling one another in structure and actions), an epitome, as it
were, of those of the  highest and most complex Plant; whilst those of
the higher  Plants bear a close correspondence with those which  are
immediately concerned in the Nutrition and Reproduction of the Animal 
OF VITAL ACTIONS  IN GENERAL.
31
body.  And Avhen Ave come to consider the proper Animal functions, we
shall find that they are not so far removed in their essential nature from
those of Plants, but that they may be ranked under the same category,
and  regarded as different manifestations of the same  original forces.
A Cell, then, in Physiological language,  is a closed
vesicle  or minute bag,  formed by a membrane in
which no definite structure can be discerned,  and
having a cavity which may contain matters of varia-
ble consistence.  Such a cell constitutes the Avhole
organism of such  simple plants as the Protococcus
nivalus ('red snow'), or Palmella cruenta ('gory
deAv');  for  although  the patches  of  this  kind of §£°g<
vegetation, which attract our notice, are made up
of vast aggregations  of such cells, yet  they have
no  dependence  upon  one another, and the  actions tain\ngle reproducUv^'moie-
of each are an exact repetition  of those of the rest.cules-
In such a cell, every organized fabric, however com-
plex,  originates.   The  vast tree, almost  a  forest in  itself, and  the
feeling, thinking, intelligent man, spring  from a germ, that differs in no
obvious particular from the permanent condition of  one of these lowly
beings.  But  whilst the powers of the latter are restricted, as  Ave shall
see,  to  the continual multiplication of new and distinct  individuals like
itself, those of the former enable it to produce neAV cells that remain in
closer connexion with each  other; and  these are  gradually converted,
by various transformations of their own,  into the diversified elements of
a complex fabric.   The  most highly-organized being, hoAvever, will be
shown to  consist in great part of cells that have undergone no such trans-
formation, amongst  Avhich the different functions performed by the indi-
vidual in the  case just cited, are distributed, so  to speak; so that each
cell  has its particular object in  the general economy, whilst the history
of its own life is essentially the  same as if it Avere maintaining a  separate
existence.
   27.  We shall now  examine, then, the  history of the solitary cell of
one  of the simplest  Cryptogamic Plants,  from  its first  development to
its final decay: in other words, we shall note those  Vital Phenomena,
which are as distinct from those of any inorganic body, as is its  orga-
nized structure (simple as it appears) from the mere aggregation of par-
ticles in a mineral  mass.   In  the first place,  the cell  takes its origin
from a  germ, which may  be a minute  molecule, that  cannot  be  seen
without a  microscope of high power.*   This molecule  appears, in its
earliest condition, to  be a  simple homogeneous particle,  of spherical
form; but  it  gradually increases in size;  and a distinction  becomes
apparent  between  its  transparent  exterior and its colored  interior.
Thus we have the first  indication of the cell-ivall, and  the  cavity.   As
the enlargement proceeds,  the distinction becomes  more obvious;  the
cell-wall is seen to be of extreme tenuity  and perfectly transparent, and
to be homogeneous  in its texture;  whilst the contents of the cavity are

  * The modes in which new cells may be generated are various ; but the above exam-
ple is purposely drawn from one of those simple Algse, whose usual mode of multipli-
cation is by "zoospores."  (Seethe Author's "Principles of Physiology," §§139-144.) 
32       NATURE AND OBJECTS  OF PHYSIOLOGICAL SCIENCE.
distinguished by their  color, which  is very commonly either green  or
crimson.   At  first they, too, appear to be homogeneous; but a finely-
granular appearance  is then perceptible ; and a change gradually takes
place, which seems to consist in the aggregation of the minute granules
into molecules of more distinguishable size and form.   These molecules,
which are the germs of new  cells, seem  to be at first attached to  the
wall  of  the  parent-cell; afterwards they separate from  it, and  move
about in its cavity;  and at a  later period, the parent-cell bursts and
sets them free.  Now, this is  the termination of the life of the  parent-
cell ; but the commencement of the life of a new brood; since every one
of these germs may become  developed into a cell, after precisely  the
foregoing manner, and will then  in its  turn multiply its kind by a simi-
lar process.
   28. By reasoning upon the foregoing history, we may arrive  at cer-
tain  conclusions,  Avhich will be  found  equally applicable to  all  living
beings.  In the first place, the cell originates in a germ or reproductive
body, which has been prepared by another similar  cell that previously
existed.  There is no  sufficient  reason to believe that there is  any ex-
ception to this rule.   So far as we at present know, every Plant and
every Animal is the offspring of a parent, to Avhich it bears a resemblance
in all essential particulars; and  the same may be said of the individual
cells, of Avhich the Animal and Vegetable fabrics are composed.  But
how does this germ, this apparently  homogeneous molecule, become a
cell ?   The answer to this is only to  be found in its peculiar property,
of draAving materials to itself from the  elements around, and of incorpo-
rating these with its  own substance.   The Vegetable cell may grow
Avherever it can obtain  a supply  of Avater, carbonic acid, and ammonia;
for these compounds supply it Avith oxygen, hydrogen,  carbon, and nitro-
gen, in the state most adapted for the  exercise of the  combining power,
by Avhich it converts them into those new compounds, whose properties
adapt them to become part of the groAving organized fabric.  Here, then,
we have  two distinct operations ;—the union  of these elementary sub-
stances into that  composite protoplasma, which seems to be the imme-
diate pabulum of the Vegetable  tissues;—and the incorporation of that
product with the  substance of  the germ itself.
   29. The first of these changes may be, and probably is, of a purely
Chemical nature; and analogous cases are not wanting,  in the  pheno-
mena of Inorganic Chemistry, in which one body, A, exerts an influence
upon two other bodies, b and c, so as to occasion their separation or
their union, without  itself  undergoing any change.   Thus platinum, in
a finely divided state, Avill cause the union of  oxygen and  hydrogen at
ordinary temperatures; and finely-powdered glass will do  the  same at
the temperature of  572°.   This kind of action is  called catalysis.  A
closer resemblance, perhaps, is presented by the act of fermentation; in
which a new arrangement of particles takes place in a  certain compound,
by the presence of another body which is itself undergoing change, but
Avhich does  not  communicate  any of its  elements to  the neAv products.
Thus, if a small portion of animal membrane, in a  certain stage of  de-
composition, be placed in  a solution of  sugar, it  will occasion  a new
arrangement  of its  elements,  which will generate two  new products. 
OF VITAL ACTIONS IN  GENERAL.
33
alcohol and carbonic acid.   If the decomposition of the membrane have
proceeded further, a different  product will result; for instead of alcohol,
lactic acid will be generated.  And in a further stage of decomposition,
the ferment is the means of  producing butyric acid (the fatty acid  of
butter).  There appears no improbability, then, in  the  idea, that the
influence exerted by the germinal molecule is of an analogous nature;
and that it operates  upon  the elements of the surrounding water and
carbonic acid,  according  to purely  chemical  laws, uniting the carbon
with the elements  of water, and setting free the oxygen.   This result of
the nutritive operations of the simple cellular  plants may be easily veri-
fied experimentally, by  exposing the  green  scum, which floats  upon
ponds, ditches, &c, and  which consists of the  cells of a minute Crypto-
gamic Plant, to the influence of light and warmth beneath a receiver;
it is found  that oxygen is then  liberated, by the decomposition  of the
carbonic acid contained in the Avater.   We shall presently have to return
to the consideration of the Chemical phenomena of living beings ; and
shall  pass on, therefore, to consider those to which no such  explanation
applies.
   30.  The second stage in the nutritive process consists  in the  appro-
priation of the new products  thus generated  to the enlargement of the
living  cell-structure;   a  phenomenon  obviously distinct  from the pre-
ceding.  It is well to  obsenre, that  this process, which constitutes the
act of Organization, may be clearly distinguished in the higher  Plants
and Animals, as consisting of two stages;—the first of these being that
of assimilation, Avhich  consists in the further  preparation or elaboration
of the fluid matter, by certain alterations whose nature is not yet clear,
so as to render it plastic  or organizable;—the second being the  act  of
formation,  or the  conArersion  of  the organizable matter  into the solid
texture, in which  process  the properties that distinguish that texture
come to manifest  themselves.  Thus,  for example, we do not  find that
a solution of dextrin (of starch-gum) is capable of being at once applied
to the development of  Vegetable tissue, although it is identical in com-
position with cellulose; for it  must first pass through a stage, in which
it possesses a peculiar glutinous  character, and exhibits a tendency  to
spontaneous  coagulation, that seems like an attempt  at the production
of organic forms.  And in like manner, the albumen  of Animals is evi-
dently not  capable of being applied to the nutrition of the fabric, until
it has been first converted into fibrin;  which also is distinguished by its
tenacious character, by its  spontaneous coagulability, and by the fibrous
structure of its clot.   Now, in both these cases, there is  probably some
slight modification in chemical composition, that is, in the proportions
of the ultimate  elements ;  but the principal alteration is  evidently that
which is effected by the rearrangement of the  constituent particles;  so
that, without any considerable change in their proportions, a compound
of a very  different nature is generated.   Of the possibility of such
changes we have  abundant illustrations in ordinary Chemical pheno-
mena ; for there is a large class of substances, termed isomeric, which,
with an identical composition, possess chemical and physical  properties
of a most diverse character.
  31. But  we cannot attribute the production of Fibrin from Albumen, 
 34        NATURE AND  OBJECTS OF  PHYSIOLOGICAL  SCIENCE.

 the organizable from the unorganizable material, to the simple operation
 of the same agencies  as those  Avhich determine the production  of  the
 different  isomeric  compounds;  for the  properties of Fibrin  are  much
 more vitally distinct from those of Albumen, than they are either che-
 mically or physically; that is, we  find in the one an incipient manifes-
 tation of Life, of which the other shoAvs no indications.   The  spontane-
 ous coagulation  of fibrin, which takes place very soon after it has been
 AvithdraAvn from the vessels of the living  body, is a phenomenon to which
 nothing analogous can be found  elsewhere; for it has  been clearly shoAvn
 not to be occasioned by any mere physical  or chemical change  in its
 constitution; and it takes place in  a manner Avhich indicates that a new
 arrangement of particles has been  effected in  it, preparatory to its being
 converted into a living solid.   For this  coagulation is not the mere  ho-
 mogeneous setting, which takes place in a solution of gelatine in cooling;
 nor is it the aggregation  of particles in a mere granular state (closely
 resembling that of a chemical precipitate),  which takes place in the
 coagulation  of albumen:  it is the  actual production  of  a simple fibrous
 tissue, by the union of the particles  of  fibrin in a  determinate manner,
 bearing a close resemblance  to the  similar process  in the living body
 (§ 188).   We say, then, that the coagulation  of Fibrin, and the produc-
 tion of a fibrous tissue, are  the  manifestation of its  vital properties,
 rather than the direct result of chemical or physical agencies; because
 no substance is known to perform any such actions, without having been
 subjected to the  influence of  a living body ; and because the  actions
 themselves are  altogether different from  any which Ave AYitness else-
 where. "*^
   32. The act of Formation  seems to consist of a continuation  of  the
 same kind of change,—that is, a new arrangement of the particles, pro-
 ducing substances which differ both as to structure and properties from
 the materials employed, but Avhich may be so closely allied to them  in
. chemical  composition, that the difference cannot  be detected.   Thus,
 from the  "protoplasma"* of Plants are generated, in the process of cell-
 development, the membranes Avhich  constitute  the walls of the cells :
 chemically speaking, there seems to  be no essential distinction between
 these  substances; and yet between the living, growing, reproducing cell,
 and the  gelatinous, semifluid matter in which  they  are  imbedded, how
 wide the  difference!   So in the Animal body, Ave find that the composi-
 tion of the proper muscular tissue scarcely differs,  in regard to the pro-
 portion of its elements, from the fibrin, or even from the albumen,  of
 the blood; and yet what  an entire rearrangement must  take place in
 the particles of either, before  a tissue so complex in structure, and  so
 peculiar  in properties, as muscular fibre, can  be generated !
    33.  Both in the Plant and the Animal, we find that tissues presenting
 great diversities both in structure  and properties, may take their origin
 in the same organizable material;  but in  every  case  (at least  in  the
 ordinary processes of growth and reparation)  the neAv tissue of each
 kind  is formed in  continuity with that previously  existing.   Thus in

   * This term is now commonly employed to designate that combination of starchy and
 albuminous matters, in which all newly-forming cells appear  to  originate.   See § 28. 
OF VITAL ACTIONS  IN GENERAL.
35
the stem of a growing  tree, from the very same glutinous sap or cam-
bium, intervening between the wood and the bark, the wood generates,
in contact with its last-formed layer, a new  cylinder of wood ; whilst
the bark produces in contact with its last-formed layer, a neAY cylinder
of bark ; the woody cylinder being characterized by the predominance
of ligneous fibre  and ducts, and the cortical by the predominance of
cellular tissue.   In like manner we  find that, in animals,  muscle pro-
duces muscle, bone generates bone, nerve developes nerve, in continuity
Avith itself, all at the expense of the materials supplied by the very same
blood.
   34.  The Nutrition of tissues, by  the  organization of the materials
contained in the  nutrient fluid with  which they are supplied,  may be
superficially compared, therefore, to  the  act  of crystallization, when it
takes place in a mixed  solution of tAvo or more salts.   If in such a solu-
tion Ave place small crystals  of the salts it contains,  these  crystals  will
progressively increase  by their  attraction for the other particles of the
same kind,  Avhich were  previously dissolved ;  each crystal attracting the
particles of its own  salt, and exerting no influence over the rest.  And
it is curious that if either of the crystals be broken, the  neAV  deposit
will take place upon it in such a mode as gradually to reproduce its
characteristic form.   But it  must be borne in mind, that such a resem-
blance  goes no further than the surface ; for  the growth of a crystal
cannot  be  really regarded as in  the least analogous to that of a  cell.
The crystal progressively increases  by the deposit of particles upon its
exterior;  the interior undergoes no  change;  and Avhatever may be the
size it ultimately attains, its properties remain precisely the  same as
those of the original nucleus.  On the other hand, the cell grows from
its original  germ by a  process of interstitial deposit; the  substance of
which its wall is  composed  extends itself in  every part; and the  new
matter  is completely incorporated with the old.
   35.  Moreover, as the increase proceeds, Ave see an evident distinction
between the cell-wall and its cavity; and Ave observe that  the cavity is
occupied by a peculiar  matter, different from the substance of the  cell-
wall, though obviously introduced through it.   Of the essential diffe-
rence which may exist in composition, between the cell-Avail  and the  con-
tents of the cavity, we  have a  remarkable example in the case of the
simple Cryptogamic plant, which constitutes Yeast, and which differs in
no essential part  of the history of its growth from the examples already
referred to.  The principal component of its cell-walls is nearly identical
with  ordinary  cellulose ; whilst the contents  of the cells are  closely
allied in composition to proteine.  Again, in the fat-cells  of Animals,
the cell-wall is formed  from  a proteine compound ;  whilst the oily con-
tents agree, in the absence of nitrogen, and  in their general chemical
relations, with  the materials of  the  tissues  of Plants.   It is  evident,
then, that one of the inherent powers of the cell, is that by which it not
only combines the surrounding materials into a substance adapted for
the extension of its wall, but that which enables it to exercise a similar
combining power on other materials derived from the same source,  and
to form a compound,—-of an entirely different  character, it may be,—
which occupies its cavity.  Now this process is  as essential to our idea
 
36       NATURE  AND  OBJECTS OF PHYSIOLOGICAL  SCIENCE.

of a living cell, as is the growth of its wall ;  and it must never be left
out of view,  when considering the history of its development.
  36. Every kind of  cell has its own specific  endowments; and gene-
rates  in its interior a  compound  peculiar to  itself.   The nature of this
compound is much  less  dependent upon  the nutrient  materials which
are supplied to the  cell, than upon  the original  inherent powers of the
cell itself, derived from its germ.   Thus we  find that  the  "red snow"
and "gory deAv" invariably form a peculiar red secretion ; and that they
Avill only groAv where they can obtain, from the air and moisture around,
the elements of that  secretion.   Again,  the "yeast-plant" invariably
forms a secretion analogous to animal proteine; and it will only grow
in a fluid Avhich supplies it with  the materials of that substance. Hence
the "red snow" would not grow in a fermentable saccharine fluid; nor
would the "yeast plant" vegetate on  damp cold surfaces.   Yet there is
little  difference, if any, between their cell-Avails, in  regard to  chemical
composition.—So, also, we shall find  hereafter, that one set of cells in
the Animal  body will  draAV into themselves, during the process of growth,
the elements  of bile;  another, the elements  of milk; another,  fatty
matter ; and so on : the peculiar endowments  of each being  derived from
their  several  germs,  Avhich seem to have an attraction for these  sub-
stances  respectively,  and which thus draw  them  together; whilst the
cell-wall appears to have a uniform composition in all instances.
  37. The term Secretion, or setting  apart, is commonly applied to this
operation, to distinguish it from Nutrition or growth;  but it is obvious
from  what has now been stated, that the  act of secretion  is in reality
the increase of growth of the cell-contents, just as the process  of en-
largement is the  increase or growth of the  cell-Avail;  and that the two
together make up the whole process of Nutrition,  which cannot be pro-
perly understood, unless both are taken into account.  It is to be remem-
bered, however,  that the contents of the cell  may  not be destined  to
undergo organization ; indeed we shall find hereafter, that the  main use
of certain cells is to draw off from the circulating  fluid such materials
as are incapable of organization; and the operation  may be so  far attri-
buted, therefore, to the agency  of Chemical forces.  But  we shall find
that,  in other instances, the cell-contents are destined to undergo orga-
nization, and this either within  the parent-cell, or  after they  leave it;
here, then,  we must recognise  a distinct vitalizing agency, as exerted
by the cell  upon its contents.
   38. This organizing or vitalizing influence  must be exerted upon a
 certain portion of the contents  of every cell that is capable  of repro-
 ducing itself; for it is in this manner that those germs are  produced, in
 which all the wonderful properties are inherent, that are destined to
 manifest themselves,  when they are set free from the parent-cell.   This
 power of Reproduction is one  of those, which most remarkably distin-
 guishes the living being; and we shall find that, in  the highest Animal,
 as in the  humblest Plant, it essentially consists in the preparation of a
 cell-germ, which,  when set free, gradually developes itself into a struc-
 ture  like  that from Avhich it sprang.  The  reproductive  molecules or
 cell-germs are formed, like the tissue  and  the contents of the parent-cell,
 from the nutrient materials which it has the  poAver of bringing together 
OF VITAL ACTIONS  IN GENERAL.
37
and combining ; and  in  their  turn  they pass  through a corresponding
series of changes ;  and at length produce a new  generation  of  similar
molecules, by Avhich  the race is destined  to  be  continued.   Notwith-
standing the mystery Avhich has  been supposed to  attach itself to this
process,  it is  obvious  that there  is nothing in reality more difficult to
understand in the fact, that the  parent-cell organizes and vitalizes the
protoplasma Avhich it  has elaborated, so that it should  form the germ of
a new individual  possessing  similar properties with itself; than in its
incorporating the  same material Avith its oivn  structure, and causing it
to take a share in its  oayh actions.
   39.  Finally, the parent-cell having arrived at its full  development,
having passed through the whole series of changes which is character-
istic of the species, and having  prepared the  germs by which the race
is to be  propagated,  dies and decays ;—that is, all  those  operations,
Avhich  distinguish  living organized  structures from  inert matter, cease
to be performed;  and it is given up to the influence of chemical forces
only, Avhich  speedily occasion  a  separation of its elements, and cause
them to  return to their original forms, namely, water, carbonic  acid,
and ammonia.  It is not, hoAvever, in the dead organism alone that this
decomposition occurs; for it is certain that interstitial death and decay
are incessantly taking place during the Avhole life of the being ; and that
the maintenance of its healthy or normal condition depends upon the
constant removal of the products  of that decay, and upon their continual
replacement.   If, on  the one hand, those  products be retained, they act
in the  manner of poisons; being  quite as injurious to the Avelfare of the
body, as the most deleterious substances introduced  from without.   On
the other hand, if they  be  duly  carried off, but  be not  replaced, the
conditions essential to vital action are not fulfilled, and the  death of the
organism must be the result.
   40.  Now it is to be observed that, as Plants obtain the chief materials
of their growth from  Avater and carbonic acid, taking the carbon  from
the latter and setting free the  oxygen, so do they require, as the condi-
tion for their decay, the presence of oxygen, Avhich may reunite with the
carbon that is to be given back  to  the  atmosphere.  If secluded  from
this, the  vegetable tissues may  be  preserved  for  a long  time Avithout
decomposition.  Generally speaking, indeed, they are not prone to rapid
decay, except at a high temperature ; and hence it is that we have so
little evidence, in  Plants, of  the  constant  interstitial  change, of wdiich
mention  has just been made.   Its existence, however (at least in all the
softer  portions of the  structure), is made evident by the fact, that a con-
tinual  extrication  of  carbonic acid takes  place,  to an  amount Avhich
sometimes nearly equals that  of the  carbonic acid  decomposed, and of
the oxygen set free, in the act of  Nutrition (§ 28).   The latter operation
is only effected under the stimulus of sunlight; the former is constantly
going  on,  by day and by night,  in sunshine and in  shade; and  if  it be
impeded  or  prevented by Avant  of  a due supply  of oxygen, the plant
speedily becomes  unhealthy.   Now this extrication of the products of
interstitial decay  is termed Excretion.  It is usually confined in Plants
to the  formation of carbonic acid  and water, by the union of the particles
of their tissues with  the oxygen of the air,—a process  identical  with 
 38       NATURE  AND  OBJECTS OF PHYSIOLOGICAL SCIENCE.

 that which occurs after the death of the entire structure.  But in Ani-
 mals it is much more complicated; owing to the larger number of  con-
 stituents in their fabric,  and to the much  greater variety in the propor-
 tions in Avhich these  are  combined; hence the  products  of interstitial
 decomposition are much  more numerous and varied, and several distinct
 modes  are devised for getting  rid of them.  Moreover, as the animal
 tissues  are much further removed than the Vegetable from the composi-
 tion of Inorganic bodies, they are subject to much more rapid and  con-
 stant decay;  and Ave shall  find that this  decay is so considerable  in
 amount, as to require on  the one hand a  very complex excretory appa-
 ratus to carry off the disintegrated matter, and  on the other a large
 supply  of nutrient materials to replace it.
   41. The preceding history may be  thus summed  up.   I.  The Vege-
 table cell-germ or  reproductive  molecule draAvs to itself, and combines
 together, certain inorganic elements ;  and thereby produces a new and
 peculiar compound.   This compound, however, exhibits no properties  that
 distinguish it  from others,  in Avhich ordinary Chemical agencies have
 been  concerned ; and we may, therefore, regard the first act of the cell-
 germ as of a purely chemical nature.   We shall presently see that  che-
 mical agencies are undoubtedly  concerned in it, to a very considerable
 degree.  The Animal cell-germ does not possess the  same  Chemical
 powrer ; it is not capable of decomposing  the Avater, carbonic acid,  and
 ammonia, Avhich include  the elements  of  its tissues; and it is entirely
 dependent for its growth, upon the supply of nutriment previously pre-
 pared for it by the  agency of the vegetable kingdom, many species  of
 which possess the poAver of generating a large amount of proteine-com-
 pounds  Avithin  their cells, though they do not organize them.  n. The
 cell-germ then exerts an  Assimilating agency upon  the pabulum thus
 prepared; by which  a neAV  arrangement  of its  particles is  produced.
 This  new arrangement gives new and  peculiar  qualities to the fluid,
 Avhich shoAV that it is something  more than a mere chemical compound,
 and that it is in the act of undergoing the  process of organization,   in.
 The Formation of this elaborated pabulum into tissue then takes place;
 its materials are withdrawn  from the  fluid, and incorporated with  the
 solid texture;  and in thus becoming part of the organized fabric, they
 are caused to  exhibit its own peculiar properties,   iv. At the same
time,  another portion  of this  pabulum is gradually prepared to serve as
 the germ of a neAV cell, or set of  cells, by which the same properties are
 to be exhibited in another generation.   V. By an operation resembling
 that concerned in the first preparation of the pabulum, certain products,
 more  or less differing from it in  character, but not destined to undergo
 organization, are formed in the cavity of the cell.  VI.  A decomposition
 or disintegration of the organized structure is continually going on, by
 the separation of its elements into simpler forms, under the influence of
 purely Chemical agencies; and the setting free of these products by an
 act of excretion, is thus incessantly restoring to the Inorganic Avorld a
 portion  of the elements that have been AvithdraAvn from it.   vn. When
the term of life of the parent-cell has expired, and  its  reproductive
molecules are  prepared to continue the  race, the actions of nutrition
cease; those of decomposition go on unchecked; and the death of  the 
OF VITAL ACTIONS  IN GENERAL.
39
structure, or the loss of its distinguishing vital properties, is the result.
By the decomposition, Avhich then takes place with increased rapidity, its
elements are restored to the Inorganic Avorld ; presenting the very same
properties as they did when first withdrawn from  it; and becoming capa-
ble of being again employed, by any successive numbers of living beings,
to go through the  same series of operations.
   42.  Thus, then, we  see that our fundamental  idea of the properties
of the simplest Living  being consists in this;—that it has the capability
of draAving into its own  substance certain of the elements furnished by
the inorganic  world :—that it forms these 'into neAV combinations (which
the chemist may find out methods of imitating);—that it rearranges the
particles of these combinations, in that peculiar mode Avhich we call or-
ganization ;—that in producing this neAV arrangement, it  renders them
capable of exhibiting a neAV  set of properties, Avhich Ave call vital,  and
which are manifested  by them, either as  connected Avith the parent or-
ganism,  or as appertaining to the germs of new structures, according to
the mode in Avhich the materials are applied ;—that, notwithstanding its
peculiar condition, it remains subject to the ordinary laAVS of Chemistry,
and that decomposition of its  structure is continually  taking  place;—
and finally, that the duration of its vital activity is limited; the changes
which the organic structure undergoes, in exhibiting  its  peculiar actions,
being such  as to  render it (after  a longer  or  shorter continuance of
them) incapable of any longer performing them.
   43. There  is abundant evidence,  that the duration  of the  Life, or
state of  Vital Activity, of an organized  structure,  is inversely propor-
tioned to the  degree  of that  activity; and consequently^ that Life  is
shortened by  an  increased or abnormal activity;  whilst  it may often
be prolonged  by influences which diminish that activity.   Thus, we shall
hereafter find reason to believe, that the duration of life in the Muscular
and Nervous  tissues of Animals is  entirely dependent upon the degree
in Avhich they are exercised;  every call upon their  activity causing the
death and disintegration of a certain part; whilst if they be alloAved to
remain in repose for a time, only that amount of decomposition  will  take
place, to which their chemical character renders them liable.   Again,
w© may  trace the connexion  between the vital activity of a part and the
duration of its life, by comparing the transitory existence of the leaves
of a Plant, which are its active organs of  nutrition, with the comparative
permanence of its woody stem, the parts of which, Avhen once completely
formed,  undergo  very little subsequent change.    The most  striking
manifestation of this connexion, hoAvever, is afforded by that condition,
in which, without any appreciable amount of vital  activity or change,
an organized  structure  may  remain unaltered for  centuries; not  only
presenting  at the end  of that time its original structure, but being pre-
pared to go through its regular series of vital operations, as if these had
never been interrupted.  This state  may be  designated as Dormant
 Vitality.   It differs,  on the one hand, from  Life ; because  Life  is a
 state of Activity.  On the other  hand, it differs from  Death; because
 Death implies not merely a suspension of activity, but a total loss of
vital properties.   Now in the state of Dormant vitality, the  vital  pro-
perties are retained;  but they are  prevented  from manifesting them- 
40        NATURE AND OBJECTS OF PHYSIOLOGICAL SCIENCE.

selves, by the want of the necessary conditions.  When these conditions
are supplied, the state  of vital activity is resumed, and all the functions
of life go on with energy.
  44. Of this Dormant Vitality it may be Avell  to  adduce some exam-
ples ; which may assist in impressing on the mind  of the student the
general views here put forth.  This condition  is manifested in the most
remarkable manner by the seeds  and germs of plants;  many of which
are adapted to remain, for  an unlimited period, in a state  of  perfect
repose,  and yet to vegetate  with the  greatest  activity,  as soon  as ever
they meet with  the necessary conditions.  Thus  the  sporules of the
Fungi, which can only develope themselves in  decaying organized mat-
ter,  seem  universally diffused through  the  atmosphere, and ready to
vegetate Avith the most extraordinary rapidity,  Avhenever a fitting oppor-
tunity presents itself.   This at least appears to be the only feasible
mode of explaining  their appearance, in  the forms  of Mould, Mildew,
&c, on all moist decaying substances; and that  there is no improbabi-
lity  in the supposition itself, is shown by the excessive multiplication of
these germs,  a single individual producing not less  than ten millions of
them, so minute as  when  collected to be scarcely visible to the naked
eye, rather resembling  thin smoke,  and so light as  to be wafted by
every movement of the atmosphere; so that, in fact, it  is difficult to
imagine any place from which they  can be excluded.—Moreover, it is
certain  that an  equally tenacious vitality exists  in  the  seeds of  higher
plants.   Those   of  most species  inhabiting  temperate  climates  are
adapted to remain  dormant during the winter;  and may be easily pre-
served,  in  dry air, and at  a  moderate  temperature, for  many  years.
Some of those which had  been kept   in  the Herbarium  of Tournefort
during upwards of a century, were found  to have preserved their ferti-
lity.  Cases are of no unfrcquent occurrence, in  which ground that has
been turned  up, spontaneously produces plants dissimilar to any in
their neighborhood.  There is no doubt that in some of these cases, the
seed is conveyed by the wind, and becomes developed  in  spots which
afford congenial soil, as already remarked in  the  case  of the  Fungi.
Thus it is  commonly observed that clover makes  its  appearance on soils
which have been rendered alkaline by lime, by strewed wood-ashes, »or
by the burning of weeds.  But there are many  authentic  facts, which
can only be explained upon the supposition, that the seeds  of the  newly-
appearing plants have4ain  for a long period  imbedded in  the  soil, at
such a distance from the  surface as  to  prevent  the access of air and
moisture;  and that, retaining their vitality  under these circumstances,
they have been excited to germination when at last exposed to  the re-
quisite conditions.   Thus Professor Lindley states  as a  fact, that he
has raised three raspberry-plants from seeds taken from the stomach of a
man, whose  skeleton was found thirty  feet beloAv the surface  of  the
earth, at the bottom of a barrow which was opened near  Dorchester ; as
his body had  been buried with some  coins of the Emperior Hadrian,
there could be no doubt that the  seeds  were  1G00  or 1700 years old.
Again, there  are  undoubted instances of  the germination  of grains of
wheat, Avhich  were enclosed in the  wrappers  of Egyptian  mummies 
OF VITAL  ACTIONS IN GENERAL.
41
 perhaps twice that number of years ago;  the wheat being different in
 some of its characters from that now growing in the country.
   45. These  facts make it evident that there is really no limit to the
 duration of this condition;  and that Avhen a seed  has been  thus pre-
 served for ten years,  it may be for a hundred, a thousand, or ten  thou-
 sand, provided that no change of circumstances either exposes it to de-
 cay, or calls its  vital properties  into activity.   Hence in cases where
 seeds have been buried deep in the earth, not by human agency, but by
 some geological  change,  it  is impossible  to  say how long  anteriorly to
 the creation of man they  may have been produced and buried ; as in the
 following very curious instance.—Some Avell-diggers in  a tOAvn on the
 Penobscot River, in  the  State of Maine  (New England), about  forty
 miles from the sea, came  at the depth of about twenty feet upon a stra-
 tum of sand ; this strongly excited curiosity and interest, from  the cir-
 cumstance that no similar sand was to be found anywhere  in the neigh-
 borhood, and that none like it was nearer than the  sea-beach.  As it
 was drawn up from the well, it was placed in a pile by itself;  an umvil-
 lingness having been  felt to  mix  it with  the stones and  gravel which
 were also drawn up.   But Avhen the work was about to be  finished, and
 the pile  of stones and  gravel to  be removed,  it was necessary also to
 remove the sand-heap.  This, therefore, was scattered about the  spot
 on which it had been formed, and was  for some  time scarcely remem-
 bered.   In a year or two, hoAvever, it was perceived that  a number of
 small trees had sprung from the  ground over which the  heap  of  sand
 had been strewn.   These trees became in  their  turn objects of strong
 interest, and care was taken that no injury should come to them.   At
 length it was ascertained  that they were  Beach-Plum trees ;  and  they
 actually bore the  Beach-Plum,  which  had never  before been  seen,
 except immediately upon the sea-shore.   The trees had therefore sprung
 from  seeds, which Avere'in  the stratum of sea-sand, that had been pierced
 by the Avell-diggers.  By  what convulsion  they had  been thrown there,
 or how long they had quietly slept beneath the surface, cannot possibly
 be determined with exactness ; but the enormous length   of  time that
 must  have elapsed since the stratum  in Avhich the seeds  were buried
 formed part of the sea-shore, is  evident from  the  accumulation of no
 less than twenty feet  of vegetable  mould upon it.
  46. Numerous  instances will be related in the succeeding Chapter, of
 the occurrence of a similar condition in fully-developed Plants, and  even
 in Animals of high organization.   In some of these it is a  regular part
 of the history of their lives,  coming on periodically like sleep ; whilst in
 others it is capable of being induced at any time, by a withdrawal of
 some of the conditions essential to vital  activity.   In regard  to all  of
 them, hoAvever, it may be observed, that the vitality can only be retained,
 when  the ,organized structure itself is secluded from such influences as
would produce its decay.   Thus, the hard dry tissue of the seed is but
little liable to decomposition;  and all that is usually required for the
prevention of change  in its  structure, is seclusion from the free acces3
of air and from  moisture, and a  steady low or moderate temperature.
If a seed be  exposed to air  and moisture, but the temperature be not
high enough to occasion its germination,  it will gradually undergo decay, 
42        NATURE AND OBJECTS  OF PHYSIOLOGICAL SCIENCE.

and will  consequently lose  its vitality.  The animal  tissues are more
liable, as already mentioned, to spontaneous  decomposition; and  the
only instances in Avhich they can retain  their vitality  for a  lengthened
period without any nutritive actions, are  those in which all  decomposi-
tion is prevented, either by the action of cold, or by the complete depri-
vation of air or of moisture,—as Avhen  Frogs, Snakes, &c, have been
preserved for years in  an ice-house, or  Wheel-Animalcules have been
dried upon a slip of glass.
  47.  The class of phenomena last brought  under notice, serves to ex-
hibit  in a very remarkable manner the dependence of all Vital Action
upon  certain  other conditions, than those furnished  by the organized
structure alone.  Thus a seed does not germinate of itself; it requires
the influence  of certain  external  agencies, namely,  Avarmth, air, and
moisture ; and  it can  no more  produce a plant without the operation of
these,  than Avarmth, air, and moisture could produce  it Avithout  a germ
prepared  by a  pre-existing organism.  Now when we come to study
these conditions, Ave find that they may be arranged under two  catego-
ries, the material and the dynamical.  Thus, a seed cannot germinate
without sufficient water to bring  the  contents of its cells into a state in
which  their chemico-vital reactions can take place; and it must be sur-
rounded with an atmosphere  containing oxygen, since Avithout the pre-
sence of this element  the  necessary chemical transformations cannot go
on.  Thus oxygen and  water are the material  conditions  required  by
the germinating seed  ;  in almost every other case, alimentary matters
are required in  addition ;  but these the seed possesses within itself.
Even if supplied, however, Avith an  unlimited amount of Avater and oxy-
gen, the seed cannot  germinate,  unless it be acted upon by Heat; and
this, in fact, may be considered with great probability, as supplying the
force, of Avhich  not merely the chemical transformations, but the growth
and development of the tissues of the Plant, are the manifestation.  This
view will be more fully developed hereafter (Sect. 3).
   48.  This dependence of Vital actions upon certain external Agencies,
as Avell as upon the properties of the organism Avhich  manifest them, is
no greater than the dependence of any of the phenomena exhibited by
an Inorganic substance, upon some other agency external to itself.   In
fact, no  change whatever can be said to be truly spontaneous ;  that is,
no property can manifest itself, unless it be called into action by some
stimulus fitted to excite it.  Thus, Avhen spontaneous decomposition  (as
it is commonly termed) occurs in an organized or an inorganic substance,
it is due to the forces generated  by the mutual  attraction between cer-
tain elements of the substance, and the oxygen of the atmosphere ;  and
this  attraction is  sufficient to  overcome  that which tends to hold toge-
ther the particles in their original state.   If the air be totally excluded,
decay will not take place ;* because  no neAV force comes into operation,
to cause a separation of  the components from  their  original modes of
union.   The influence of  the  Dynamical  conditions which  are essential

  * On this principle meats, vegetables, and even liquid soups,  are  now largely pre-
served, for the use of persons undertaking long voyages; by enclosing them in tin cases,
carefully soldered up.  There is no limit to the time, during  which decomposition may
thus be prevented. 
OF VITAL ACTIONS  IN GENERAL.                43
to Vital activity, will be fully explained in  the  next Chapter;  and at
present it Avill be sufficient to remark, that the degree in which they are
supplied possesses a well-marked influence upon  the  amount of activity
and energy manifested in the actions of the organized structure; that
there is a limitation  in  the case of  each of them, as to the degree in
which it can operate beneficially, the limitation  being usually narrower
and more precise, according to  the elevation of  the being in  the scale;
that an excessive supply may be destructive to  the  vital  properties of
the organism, by over-stimulating it,  and thus causing it to live too fast,
or by  more directly producing some  physical or chemical change in  its
condition ; and  that a deficiency will keep  doAvn or suspend all  vital
activity, leaving the  structure to  the unrestrained  operation of those
agencies which are always tending to its disintegration, and consequently
occasioning a speedy loss of the vital properties,—save in  those cases
in which they may be preserved in a dormant condition, and Avhich are
exceptions to the general rule, that the  death or departure of the vital
properties follows closely upon the cessation  of vital actions.
   49.  Our fundamental idea of Life, then, is that of a state of constant
change or action; this change being manifested  in at least two sets of
operations;—the  continual  withdrawal  of certain elements from  the
inorganic world  ;—and the incorporation  of these with the peculiar struc-
tures termed organized, or the  production from them of the germs that
are hereafter to accomplish this.  As the conditions of this continual
change, Ave recognise the necessity of an organized structure on the one
hand, or of a germ which is capable of becoming  so;  whilst we also per-
ceive the necessity of a supply of certain kinds of matter from the  inor-
ganic world, capable of being combined into the materials of that struc-
ture, which may be designated as the  alimentary substances ; and, fur-
ther, Ave see that the organism can exert no influence  upon these, except
with the assistance of certain  dynamical agencies, such as light, heat,
&c, that supply the forces or powers without  which no change can occur.
And to these forces, acting under the conditions which the Organized
body alone can supply, may be attributed (as will hereafter appear) the
phenomena which we distinguish as Vital.
  50.  But just  as we find among Inorganic  bodies,  that various kinds
are to be distinguished by their different properties,  whilst all agree in
the general or essential  properties of matter, so  do we  find that living
organized substances are distinguished by a variety of properties inherent
in themselves, whilst  they all agree in the foregoing general or essential
characters.   In many instances, the  difference of their  properties is as
obviously coincident with differences in their  structure and composition,
as it usually is among the bodies of the Mineral world: thus we find the
property of Contractility on the application of a stimulus, to be for the'
most part restricted to a certain form of organized tissue, the Muscular;
and Ave find that the property by which that stimulus is capable of being
generated and conveyed to  a distance, seems to be restricted to another
kind of tissue, the Nervous.   In a great number  of cases, hoAvever, very
obvious differences in properties manifest themselves, Avhen no perceptible
variations exist, either in structure  or  composition;  thus  it Avould  be
impossible to distinguish the germ-cell of a Zoophyte from that of Man, 
44        NATURE AND  OBJECTS OF PHYSIOLOGICAL SCIENCE.
by any difference in  its aspect or composition; yet neither can be deve-
loped into any other form than that of the parent species, and they must
be regarded, therefore, as essentially different in  properties.  In the
same manner we shall find that,  in the same organized fabric, there are
very great varieties in the actions of its  component cells, which indicate
a similiar variety in their properties ; and yet they are to all appearance
identical.  But there can be no reasonable doubt, that differences really
exist in such cases ; though  our means  of observation are not such  as
to enable us to take  cognizance of these, by the direct impressions they
make upon  our  senses.   Analogous instances  are  not wanting  in the
Mineral  world;  for  the  Chemist is familiar with a class of compounds
designated isomorphous, in which, with perfect similarity in external
form  and physical properties, there is  a difference, more or  less com-
plete, in  chemical composition, and consequently in the effects of re-
agents.
   51.  Whatever  may be  the peculiar vital properties  possessed by  an
organized tissue,  we find that they are always dependent upon the main-
tenance of its characteristic structure and composition,  by the nutritive
operations of  Avhich Ave have spoken; and that their existence forms a
part,  as  it Avere, of the  more general phenomena of its Life.   They
manifest themselves  with  the first  complete development of the tissue;
they are retained and  exhibited so long as active nutritive changes are
taking place in it; their manifestation is  weakened or suspended if the
nutritive operations  be  feebly  exerted;  and  they depart altogether,
whenever,  by the cessation  of  those actions,  and the  uncompensated
influence of ordinary Chemical forces, the structure begins to lose that
normal composition  and arrangement of parts, Avhich constitues its state
of organization.   Hence we may regard these peculiar properties  as con-
formable, in all  the essential conditions of their existence, with those
more general properties, Avhich have  been  previously dwelt upon  as
characterizing a  living  organized structure.

     3. Of the Forces concerned in the Production of  Vital Phenomena.

   52. In prosecuting  his inquiry into  the causes of those phenomena
of Living organisms, Avhich, being of a totally different order from those
of Inorganic  matter, are distinguished as Vital, the  Physiologist must
take  as  his guide those  methods  of investigation, which have proved
successful in  other  departments of scientific  research.   If he  turn,
then,  to the  sciences  of Mechanics,  Optics,  Thermotics, Electricity,
 Magnetism,  or  Chemistry, he  finds that the  phenomena which they
 respectively comprise  are referable  to the operation of  certain forces,
 and that Avhat are termed the laws of  those sciences, are  nothing else
 than expressions of the conditions of action of those forces.   Thus in
 Mechanics we have  principally to do with the motion of masses  of mat-
 ter, and our idea of force is chiefly derived from our  OAvn experience of
 the exertion  of a, power  in  producing or resisting motion;  Avhilst  the
 ' laws' of Mechanics are nothing else than expressions of the  conditions,
 under which  the forces  or  powers that produce  motion operate upon
 matter.   So in Optics, we have  to do with the force Avhich we term light; 
OF VITAL  FORCE IN  GENERAL.
45
and the laAvs of Optics are expressions of the conditions  under Avhich
that force is propagated, and of its action  on material  substances.  In
Thermotics, again, Ave have to do with the force of Heat; and its laws
are expressions of the circumstances under which heat  is propagated,
and of the changes which it occasions in the substances it affects.   So in
the sciences of Electricity and Magnetism, Ave have to do with the forces
knoAvn under those names;  and with the laAvs expressive of their action
upon matter.  And the scientific Chemist refers all  the phenomena with
which he is concerned to the operations of Chemical Affinity, and  endea-
vors to deduce from observation of the phenomena  the  laAvs  of the
operation of this force.—So the Physiologist will be justified in assuming
a  Vital Force (or Forces) as the power which operates in  producing
Vital phenomena;  and  will most legitimately pursue his science, in
inquiring into the conditions under which that force operates.
   53.  The analogy  of the Physical  Sciences may be advantageously
pursued further.—Although we are  accustomed to  speak  of the power
that produces Mechanical Motion, of Light,  of Heat, of Electricity, of
Magnetism, and of Chemical  Affinity, as distinct forces, yet it has gra-
dually become apparent that very intimate  relations  subsist between
them, and that they are, in fact, mutually convertible; so that one force
(a) operating upon a certain form of  matter, ceases  to manifest itself,
but developes another force (b), in its  stead; whilst, in  its turn, the
second force (b) may be reconverted into the first (a), or into some other
(c), which, again, may reproduce either the first  (a), or second (b), or
some other (d or e).—It  was in the case of Electricity and Magnetism,
that this reciprocal relation, which is designated as  'correlation,' was
first clearly apprehended.  If an electric current be  passed round a piece
of soft iron, that iron becomes magnetic, and remains  so as long as the
current is circulating : on the other hand, from a magnet put in motion,
an electric current may be obtained.   Hence Ave  are accustomed  to con-
nect those tAvo forces under the term Electro-Magnetism;  but they can
be easily shown to be quite distinct in their modes of operation on matter;
and their relation is not really more intimate than  that of other  forces.
For Heat may be developed by Electricity ; as when a galvanic current,
sent through a thin platinum wire, heats it to ignition, or  even fuses it.
Conversely, Electricity  may  be  developed by Heat;  as when heat is
applied to bars composed of dissimilar metals in contact with each other.
Again, if Mechanical Motion be retarded, as  in friction, we immediately
have a development  either of Heat or of Electricity;  heat alone being
developed, when the two rubbing surfaces are composed of precisely the
same substance; and electricity being produced, when these  substances
are different.   And it is for the most part through  the medium of Heat
or Electricity, that the  force of Mechanical Motion is ' correlated' to
Light, Magnetism, and Chemical Affinity.
   54.  The idea of correlation also involves that of a  certain definite ratio,
or relation of equivalence, between the two forces  thus mutually inter-
changeable ; so that the measure of force B, which is excited by a certain
exertion of force A, shall, in its turn, give rise to the same measure of
force A, as that originally in operation.  Thus, when an electric current
is set in motion  by galvanic action, we  have a conversion of chemical 
46        NATURE AND OBJECTS OF PHYSIOLOGICAL SCIENCE.

force (Avhich has manifested itself in the decomposition of the Avater and
the oxidation of the zinc) into electrical; but the electrical current may,
in its turn,  be made to produce chemical decomposition ; and the amount
of this kind  of  change Avhich it will effect, bears a precise correspon-
dence {coeteris paribus) Avith the amount of zinc which  has undergone
oxidation in the galvanic cell.   In like manner, when water at 212° is
converted into steam, the heat which it receives is no longer manifested
as heat,  but mechanical  force is developed in  its stead, and this in a
certain definite  ratio, so that the 'mechanical equivalent' of heat is capa-
ble of being exactly determined: so soon, however, as the steam, losing
its  elasticity by condensation, returns to the  condition  of  Avater, the
original  equivalent of heat is again developed, its mechanical force being
no longer manifested.*
  55. Now  in  every  case  in which one  force is thus  converted  into
another, the change is effected through the medium of a certain form of
matter, or material substratum.   This may be,  in some cases, of almost
any  description whatever;  as when Heat is produced by the friction or
retarded motion of solids, liquids, or even gases; or when  Motion (as
shown in expansion) is produced by the application of heat to any kind
of material  substance.  But in  other  cases, the  change  can only be
effected through some  special form of matter;  or if several substances
may serve as its medium, there is some one which is greatly superior to
all the rest, in  the readiness with which a certain force manifests itself
through  it.   Thus iron is the only substance through Avhich Electricity
can  be converted  into Magnetism; and the development of magnetic
force, therefore, can  only take place through  this  medium.   So, Heat
is more readily  converted into Electricity through a combination of bis-
muth and antimony,  than through any other metals;  and the affection
of Light by magnetic force (discovered a feAV years since by Prof. Fara-
day), though producible through  any transparent substance, can be made
much more  obvious when the magnetism  is made to act upon a peculiar
glass composed  of vitrified borate of lead, than  through the medium of
any  other substance yet  known.  This  speciality in the action of diffe-
rent substances, Avhen subjected  to the same forces, is a fact of funda-
mental importance; and  it  is on  it, indeed, that our  notion of  their
several properties  depends.
  56. Now as  the properties of every kind of matter require  certain
conditions for their manifestation, our acquaintance Avith them  entirely
depends  upon whether the conditions of their action have been afforded.
Thus, to go  back  to  a former illustration, supposing  a new chemical

  * The above statement is an expression of the simple facts of the  case, which, when
thus understood, render the hypothesis of " latent heat" altogether unnecessary.  This
hypothesis, however  ingenious, will doubtless share the fate of many other  such
attempts to substitute  a form  of words for realities. It supposed the 966 degrees of
heat expanded in converting a certain  amount of water at 212° into steam at  212°, to
become altogether inactive or latent;  and gave no account whatever of the mechanical
force which is produced in that act of conversion.  The idea of an inactive force, in fact,
is one that cannot  be  entertained;  for if  a force ceases to be active, it is no longer
'force.' And it cannot be imagined that force, any more than matter, should cease to
exist; it must manifest itself under some other aspect.—For a complete  exposition of
the mutual relations existing among  the above-named agents, see Prof. Grove's treatise
" On the Correlation of the Physical Forces." 
OF VITAL  FORCE IN  GENERAL.
47
element to be discovered, we could not know its  properties in regard to
heat, electricity, or magnetism, the mode of its  combination with other
elements, the nature and  properties of the compounds produced,  their
reactions with the other compounds, &c, until we have tried a complete
series of experiments  upon,—that is, until we have placed it  in all the
circumstances or conditions requisite to manifest the properties, with
Avhich we seek to become  acquainted, or whose  absence we seek to de-
termine if they do not exist.  Now we might have made all the experi-
ments we could devise  upon such a body ; and yet Ave might have failed
in detecting some remarkable aud distinguishing property  inherent in it,
simply because Ave had not placed it in the requisite circumstances for
the manifestation of this peculiarity.   Further,  even in the elements or
compounds with  Avhich Ave are best acquainted,  it is very possible that
properties exist, of which  we as yet know nothing, simply  because they
have not yet been called  into  action by the requisite combination of
conditions.   For example,  no one could have thought it possible, a few
years since, that water could be frozen in a red-hot metallic vessel ; and
yet  this is now known to  be effected  with ease and  certainty, in the
proper combination of  conditions.
   57.  Again, the properties of a compound  substance are,  in general at
least, altogether different from those which present themselves in either
of its components; so that we could not in the least degree judge of the
former from the  latter, or of the latter from the former.   What more
different,  for example, than the physical  and  chemical  properties of
Water, from those of  either the  Oxygen  or the Hydrogen that  enter
into its composition ?   Or what more  different than the properties  of a
neutral salt, from those of the acid and alkali by Avhose union  it is pro-
duced ?—Further, the  properties of a  substance may be completely
changed, by an alteration in its condition as regards  Heat or any other
of the forces, already  mentioned.  For example, the particles  of Avater
have so strong an attraction for each other, at a low temperature, as to
become aggregated in  a crystalline form, and to produce  a dense solid
mass;  at somewhat a higher temperature, their mutual attractions are so
slight, that a very small amount of mechanical force is sufficient to sepa-
rate them, and they move  upon each  other  with the utmost  freedom ;
whilst at a  still higher temperature,  they manifest  a poAver of mutual
repulsion, Avhich increases with the greatest rapidity with every augmen-
tation of temperature.   Yet when the temperature of the substance is
loAvered to its former standards, Ave observe that it first returns to  the
liquid, and then to the  solid form; and that, in those  states, it manifests
all the properties Avhich before characterized it.   Not merely  the phy-
sical, but the chemical  properties of bodies may be affected  by a change
in their mechanical condition.  Thus, it is Avell known that oxygen and
iron, at  ordinary temperatures, have a  mutual  affinity, which is  only
sufficient to produce a  slow combination between them ; whilst at high
temperatures, that affinity is such as to  cause their rapid and energetic
union.  Now if iron, in a state of very minute division, such  as it pos-
sesses when set free from the state of oxide by means of hydrogen, at the
loAvest possible temperature, be brought into contact Avith oxygen or even
with atmospheric air, at ordinary temperatures, it immediately becomes 
 48       NATURE AND OBJECTS OF PHYSIOLOGICAL SCIENCE.

 red-hot, and is converted into an oxide.   The minuteness of the division,
 predisposing to  chemical union, appears to be the occasion of our power
 of causing  many substances to combine, Avhen one or both are in the
 nascent state (that is, when just set free from some other combination),
 Avhich could not be made to unite in any more direct manner; thus, Avhen
 a quantity, however minute, of any preparation of arsenic is dissolved
 in fluid in which hydrogen is being generated, the hydrogen will detach
 the metal from  its previous combination,  and Avill pass forth in union
 with it, as arseniuretted hydrogen, a compound which cannot be formed
 by the direct union of the elements.  In like manner, in that mechanical
 mixture of three finely-divided substances,  which  we call gunpowder, the
 rapidity with  Avhich combustion is propagated through the largest col-
 lection of it, is  entirely dependent upon the  minute  subdivision of its
 components, and the very close approximation of their particles.   Hence
 it may be very correctly said, that the true chemical properties of the
 substances  are  not manifested, except when they are in a state of very
 minute division; and that these are in fact obscured, by the aggregation
 of the particles  into masses.  Thus, then, we  are at  no loss to discover
 examples, in the Inorganic world, of an alteration  in  the  sensible  pro-
 perties, both Chemical  and Physical, of the bodies composing  it,  by a
 change in the conditions in which they are  placed.  And it  may be
 stated as  a general fact, that we never Avitness the manifestation of new
 properties in a substance, unless it has undergone some  change in its
 own condition, of which altered state these properties  are the necessary
 attendants.
   58.  Now if we apply the same methods to the phenomena of Life, we
 shall see that they Avill lead to  a mode of viewing them, which will  con-
 siderably tend to the simplification of Physiological science.  In the
 first place we have to look  at these phenomena as the  results of certain
forces, acting through those forms of matter which  we term Organized;
 and these forces we shall provisionally designate as Vital.   Thus in the
 growth of the simple  Vegetable cell as already described (§§  26-41),
 we trace  the  operation of  a force closely allied to  ordinary chemical
 affinity, but so far different  that it can only be exerted through  a living
 organism; of  a  force of assimilation or vital transformation ; and  of a
 force of organization and complete vitalization.  Now  although we  may
 provisionally designate these as distinct forces, on account  of the diver-
 sity of  their manifestations, it is  impossible not to see that they are
 mutually  dependent, and that  they form the  successive elements  of a
 continuous series of phenomena belonging to the same category, that of
 cell-life;  and further, we  observe  that  they  operate under the same
 conditions, namely, the presence of a cell-germ and of the materials of
 its growth, and the action of light and heat.   Again, in the multiplica-
 tion of  the original cell, by  whatever method performed, we cannot but
 trace the continued action of forces of the same  character; since  this
 operation takes  place as a continuation  of  the process of  groAvth,  and
 under  precisely  the same  influences.   Further,  we  occasionally meet
 with examples, even among the simplest forms of Vegetation, of every
 active movement; thus the filaments or elongated cells of the Oscillato-
 rio3 are continually bending themselves backwards and forwards, with a 
OF VITAL FORCE  IN GENERAL.
49
 regular  rhythmical undulation; and the "zoospores" of the Confervese
 are propelled through the water by the rapid vibration of the cilia with
 which they are furnished (§  234).   Noav that  such a production of a
 purely physical change is a manifestation of vital force, is obvious from
 this,—that it  takes  place only Avhile  the vitality of the  organism
 endures, and that it is dependent upon the very same conditions as the
 other vital operations require ; and it is further interesting to remark
 in  the case of the "zoospores," that it seems  to take the  place of the
 operations of growth, for these do not commence until the movement of
 the spore has ceased.   The spiral filaments,  again, Avhich have been
 discovered in most  of the higher Cryptogamia, and which  seem to per-
 form the same function with  the spermatozoa of Animals (§ 240), have
 a similar  spontaneous  movement, Avhich  must be looked upon as an ex-
 pression  of their vital force.  Many cases of motion produced by a
 change of form of  certain contractile  cells, might be cited from among
 the higher tribes of the Vegetable kingdom ;  these movements being
 sometimes rhythmical and spontaneous, as in the Hedysarum gyrans,—
 sometimes taking place  only in respondence  to stimulation,  as in the
 Dionoea muscipula (Venus's fly-trap),—and sometimes occurring as part
 of  the series of ordinary vital phenomena, although producible also by
 stimulation, as in the Mimosa pudica (sensitive plant), Avhich regularly
 closes its leaves at night, but will do so at  any  time when they are
 touched  or otherwise irritated.*  These movements only  take place
 during the life of the Plant;  and it is particularly observable in the
 last-named species,  that the facility with Avhich they may be excited in
 any individual is closely related to the  activity of its vegetating pro-
 cesses.  Thus even  in the Plant, we see that the Vital forces manifest
 themselves, not merely in growth, but  in movement.
   59. When we examine the  structure of one of the higher Plants, we
 find that, although  the principal  part of its fabric is still  made up of
 unmetamorphosed cells, yet that certain portions of it have undergone
 histological transformation ;  that is, its primordial  cells have  lost their
 original  character,  having been changed  into  other kinds of tissue.
 This transformation  takes place to a much greater extent in  the Animal
 body ; in Avhich the  variety of actions to  be performed is  much larger,
 and in which we  accordingly find a much greater variety of tissues de-
 veloped as their instruments.   But  hoAvever widely these tissues  may
 depart from  their original  character,  Ave find that the process of trans-
 formation takes place under the same  conditions as  that of groAvth, and
 must be  regarded  as a  continuation  of  it; being,  in fact,  the  special
 manifestation of  vital force in one  set  of cells, as multiplication is  in
 another, or as motion in another.   And we shall find, that, in proportion
 as this transformation takes place, do the tissues lose their proper vital
 endoAvments; for it may be stated as a general fact, that even in the most
 complicated and elaborate Animal organism, all  the most active vital ope-
 rations are performed by  tissues which retain their original cellular con-
stitution  with little or no change.—Further, it is to be observed, that as it
is the peculiar character of such organisms that each of their parts should

  *  For a fuller analysis of these phenomena, see  the Author's "Principles of Physio-
logy, General and Comparative," chap. xix. 
50       NATURE AND  OBJECTS OF PHYSIOLOGICAL SCIENCE.

be appropriated to some  distinct office Avhich it is specially adapted to per-
form, so do Ave find that the cells which become the instruments of some
one particular kind of operation seems to lose their other endoAvraents ;
as if the expenditure of the vital force of each cell upon  any one pur-
pose, unfitted  it  for any other  agency.  Of this  we shall  meet  with
numerous examples hereafter ; it  will be sufficient here to refer to two
of the most characteristic.  It is necessary for every act of Secretion,
that a set of cells should be formed Avithin the ultimate follicles of the
Gland which is the instrument of  the  function (§ 238); and these ulti-
mate follicles  are really to be  regarded as parent-cells, which produce
the true secreting cells in proportion as  the materials of their growth
are supplied by the blood.  Now these parent-cells themselves possess
no  secreting  power, their vital forces being entirely expended  in the
production of  the true  secreting cells.  On the  other hand, the  true
secreting cells possess no reproductive  power, but die and are cast off
when they have reached  their maturity;  as if their Avhole vital  force
were expended in the secreting process, which is nothing else on their
parts than an  act of growth.   So,  again, the cells which constitute the
fibrillse of Muscular fibre, and of whose change of form the contraction
of the muscle is the  result (§336), exercise no power of chemical transfor-
mation, undergo no histological change, and appear to be entirely desti-
tute of the power of self-multiplication; the expenditure of their  vital
force in the act of muscular contraction involves their death and disinte-
gration ; and their renewal appears to be accomplished by a production of
neAV cells from the nucleus of  the Myolemma (§ 338), Avhich, itself pos-
sessing no contractile power, retains its reproductive capacity.
  60.  Hence,  then, we  have  reason to  believe, that all the truly  Vital
phenomena, however diversified, are but  results of the operation of one
and the same Force, whose particular manifestations are determined by
the nature of the material substratum through which it acts : the same
fundamental agency producing simple growth in one case, transformation
in another, multiplication in a  third, mechanical movement in a fourth,
whilst in a fifth  it  developes nervous power, which may itself operate
in a variety of different modes.  Such a view seems fully justified by
the  consideration,  (1)  that all these  forces are  exerted, even in the
most highly-organized living being, through a common instrumentality,
the simple cell; (2) that the  entire assemblage of cells making up the
totality of any organism,  have all a  common parentage,  being lineally
descended from the single primordial cell in which it originated; and
(3) that they are  manifested in connexion  with  each other, as parts of
the life of each individual cell, in those simple organisms  which are the
lowest members of the two kingdoms respectively, and  in which there
is no separation or specialization of function.
  61.  The question next arises,—Avhat is the source of the Vital Force,
of which the phenomena of Life are the manifestations ; and  under the
guidance of the  ideas derived from Physical Science, we  shall have no
difficulty in referring it to the operation of those external agencies, the
influence of Avhich has long been known to be essential to Vital action,
and  which  have  been  usually designated  by the  term Vital Stimuli.
Thus,  the  growing Vegetable  cell cannot decompose  carbonic  acid, 
OF VITAL  FORCE IN  GENERAL.
51
except when acted upon by  Light; and  the  amount  of this change
which it effects, is in strict ratio (eceteris paribus), with the illuminating
poAver of the rays which it receives (§86).  So, again, neither Plants
nor Animals can  maintain  their activity,  except  under the continual
influence of a certain measure of Heat; and the amount of that activity
will be  shown to  bear  a constant ratio, in all  those tribes which have
no independent power of sustaining it, to the quantity which they receive
from  external  sources (chap. n. Sect. 2);  this  being  true, not merely
of the general rate of the Vegetative actions of growth and development,
but also of  those  manifestations of  vital power which  are peculiar to
Animals.   Thus  Ave may say, that Light and Heat  acting  upon the
organic germ, become transformed into Vital force, in the same manner
as Heat acting upon a certain combination of metals becomes Electricity,
or as Electricity acting upon iron developes itself as Magnetism;  and
we shall find that this view is in complete  harmony with all the pheno-
mena of Vital action.   Moreover, the Vital force thus engendered fre-
quently manifests itself in producing Physical or Chemical phenomena ;
thus  completing that mutual relationship, or correlation, which has been
shown to exist among the Physical and Chemical forces themselves (§§ 53,
54).  Of this  we have already seen  an  instance in the movements pro-
duced by muscular contraction and by ciliary vibration.   The production
of heat  by certain Plants and by  warm-blooded Animals, is another oppo-
site exemplification of the  same principle.   But  the most remarkable
illustration  is undoubtedly derived from the Nerve-force; which, whilst
itself a  peculiar form of the general Vital force,-and capable of affecting
all the  other  manifestations  of the  same force  (as in  the modifications
which it produces in the processes of Nutrition and  Secretion, as well as
in exciting Muscular Contraction), is capable of  developing Electricity as
well as  Light  and Heat, and is also  capable of being called forth by the
action of Light,  Heat,  Electricity, Chemical Affinity, or  even Mecha-
nical Motion, on the Nervous  tissue.  It is  a most remarkable confirma-
tion of  the  views here  advanced, that  the nerve-force, Avhich must be
accounted, in  its relations to  Mind, as the highest of all  the forms of
Vital force, should yet be the one wThich is  most directly and intimately
related  to the Physical forces,—the " correlation" even of Electricity
and  Magnetism not being more complete, than the "correlation" of
Electricity and Nerve-force may be  shown  to be (§ 396).
   62. Thus, then, not only are the materials draAvn from the Inorganic
world by vital agencies, given back  to it again  by the disintegration of
the living structures of  Avhich  they form a part; but all the forces which
are operative in  producing the  phenomena of  Life, being first  derived
from the Inorganic universe, are returned to it again under some form
or other.   The Plant forms those organic compounds, at the expense of
Avhich Animal life (as well as its OAvn), is  sustained, by  the decomposi-
tion  of carbonic acid, Avater,  and  ammonia; and  the light, by whose
agency  alone  this process can be effected,  may be  considered as meta-
morphosed into the  peculiar  affinity, by Avhich the elements  of these
compounds  are held together.   The heat which Plants  receive, acting
through their  organized structures as Vital force, serves to augment
these structures to an almost unlimited extent, and thus  to supply new 
52        NATURE AND OBJECTS OF  PHYSIOLOGICAL  SCIENCE.

instruments for the  agency of light and for the  production of organic
compounds.  The whole nisus of Vegetable life may be considered as
manifested in this production; and, in effecting it, each organism is not
only draAving material, but force,  from the universe around it.   Sup-
posing that no Animals existed to  consume these organic compounds,
they Avould be all at last restored to the inorganic condition by sponta-
neous decay, which Avould reproduce the carbonic acid, water, and am-
monia, from which they Avere generated.  In this decay, hoAvever slow,
heat and light are given out, in the same amount as when more evidently
produced  in the ordinary combustive process ; and this sometimes occurs
even during the life  of the plant, Avhose vital movements, also, may be
considered as restoring to  the Inorganic universe a  certain measure of
the force they have derived from it under other forms.  Thus in making
use of the stores of Coal Avhich have been prepared for his Avants by the
luxuriant Flora of past ages, Man is not only restoring to  the  atmo-
sphere the carbonic acid, the water, and the ammonia, of the Carboni-
ferous period, but is actually reproducing  and  applying  to his own
purposes,  the  Light and  Heat which  Avere  operating to produce the
growth of vegetation at that remote period in the Earth's history.
  63. But the organic compounds  which the agency of Light and Heat
upon the Vegetable structures has produced, are designed for a much
higher purpose, than that of being merely given back to the Inorganic
universe by decay or combustion; and the forces which hold together
their elements have a much more  exalted destiny.   In serving  as the
food of Animals, a part of them become the materials of their organized
tissues, and the instruments through which the nervous and muscular
forces are  developed:  Avhilst  another  part  are applied to sustain the
combustive process,  by Avhich  the heat of the higher classes is  main-
tained quite independently of the external  supply of that  force.  The
greater part of the  Animal kingdom, however, is dependent, like the
Vegetable, upon the Inorganic Universe, for the  Heat  Avhich serves as
its organizing force; and it is only under the constant influence of this
agent, that the  operations  of  growth, development,  and  maintenance
can take place.   The Animal is not dependent like the Plant upon Light;
and this is obviously because that agent is  chiefly  concerned  in that
preliminary operation, by  which the organic compounds are generated
as the pabulum of the growing tissues;  in fact, the embryo Avithin the
germinating seed, which,  like  the animal,  is  nourished  upon  matter
previously  prepared  for it, is most rapidly  developed in  the absence of
light, up to the time when, its  store being  exhausted, its  further sup-
plies must be obtained by its own instrumentality.—The Vital activity
of Animals, then, may be considered as chiefly sustained by the  Che-
mical forces subsisting in their food, Avhich are set  free when  the ele-
ments are reconverted to their original state;  and by the Heat  which
they derive from external  sources, or from  the combustion of a part of
their food.   These forces may be considered as in a state of continual
restoration to the Inorganic Universe, during the whole life of Animals,
in the heat, light, electricity, still more  in the motion, which they deve-
lope; and, after their death, in the production of  heat and light  during
the processes of decay.  During Animal life, there is a continual resto- 
OF DEGENERATION AND DEATH.                 53
ration to the mineral world, of the  carbonic acid, Avater, and ammonia,
which have been appropriated by Plants;  and it  will hereafter appear,
that the amount thus given  off  by  the animal organism bears a close
correspondence, on the  one  hand, Avith its degree of vital  activity, as
shown in the amount of heat and motion which it  generates,  and, on the
other, with the amount of the organic  compounds Avhich it consumes as
food.  So  that, on the whole, there is strong reason to  believe that the
entire amount of force (as of materials), received by an animal  during
a given period, is given back by  it during  that period, provided that its
condition at the end of the term is  the same as it was at first; and fur-
ther, thaUall the force (like the  material), Avhich  has been expended in
the building up of the organism,  is given back by its decay after death.*

                    4. Of Degeneration and Death.

   64. We have seen that the general history of the phenomena of Life
is fully conformable with the view, that the Vital properties of a tissue
(that is, the properties in virtue of Avhich  the  forces that  act upon it
are caused to manifest themselves in Vital action),  are  dependent upon
that state of combination and arrangement, which  is termed Organiza-
tion.   As long as each tissue retains its normal or regular constitution,
renovated  by the actions  of absorption and  deposition through  Avhich
that constitution is preserved, and surrounded by  those other conditions
which a living  system alone can afford, so long,  we have reason to be-
lieve,4t will retain its vital properties,—and  no  longer.  And just as
we have no evidence  of  the existence of vital properties in any other
form of matter than that Avhich Ave call organized, so have Ave no reason
to believe that  organized matter  can retain  its regular constitution, and
be subjected to the appropriate forces, without  exhibiting vital actions.
The advance of pathological science renders it every day more probable
(indeed the probability may  noAV be said to amount to  almost positive
certainty), that derangement in function,—in other Avords, an imperfect
or irregular action,—always results, either  from some change of  struc-
ture or composition  in  the  tissue   itself, or from some corresponding
change in the forces  by which the  properties  of  the organ are  called
into  action.  Thus, when a Muscle has been  long disused, it can scarcely
be excited  to contraction by the usual stimulus, or may  even be altoge-
ther poAverless;  and minute examination of its  structure shows it to have
undergone a change, which is obvious to the microscope, though it may
not be perceptible to  the unaided eye, and which results from imperfect
nutrition.  Or,  again, convulsive or irregular actions  of the Nervous
system may be  produced, not by any change in  its own composition,
but by the presence  of various  stimulating  substances in the  blood,
although their amount be so small that they can scarcely be recognised.
  65.  As there is a constant tendency, in the Animal tissues more espe-
cially, to spontaneous decay, so  must the maintenance of the vital pro-

  * The whole of this subject is more fully  developed in the Author's "Principles of
Physiology, General and Comparative," chaps, hi. and v.; and in a Paper on "The
Mutual Relations  of the Vital and Physical Forces," contained in the  Philosophical
Transactions for 1850. 
54       NATURE AND  OBJECTS OF  PHYSIOLOGICAL  SCIENCE.

perties depend upon their continual regeneration by the nutritive opera-
tions.  Hence Ave have no difficulty in accounting  for the Death of the
whole system, on the cessation or serious disturbance of any one  impor-
tant function; for any such check or change must suspend or disorder
the nutrient processes, in such a degree that they can  no longer main-
tain the normal constitution of the several tissues.  But as there is  a great
variety in the rapidity of the decomposition of the tissues, whenthe
act of nutrition is suspended, so do Ave Avitness a corresponding variety
in the duration of their vital properties, after that permanent severance
of the chain of functions, Avhich is distinguished as somatic death,—i. e.,
the death of the body as a whole.  It is by the Circulation of the Blood,
that the connexion  of the different functions is essentially maintained;
that fluid being not only the material for the nutrition of the tissues, but
in many cases supplying also the stimulus to their activity.   Hence
with the permanent cessation of the Circulation, somatic  death must be
regarded as taking  place.
  QQ. Yet  after this, we observe that vitality lingers in  the tissues; and
that it departs from them only as they lose their proper composition.
Thus we find that, although the Nervous centres  cannot originate the
stimulus necessary to  produce Muscular contraction, after the Circula-
tion has ceased,—yet the Nervous fibres can convey such a stimulus, long
after somatic death ; so that contractions may be  excited in muscles by
the application of galvanism, or of mechanical or chemical stimulants,
to the trunks that supply them.   The molecular death of the  Nervous
tissue,  therefore,  has not yet taken place.   After a time, hoAvever, this
power is lost; the tissue no longer  exhibits its distinguishing vital pro-
perties ; and  incipient  decomposition and change  of structure manifest
themselves.   Yet for some time after this, the Muscular tissue, especially
in a cold-blooded animal, continues to possess its peculiar, contractility;
for contractions may be excited in it, by stimuli directly  applied to itself,
long after the nerves have ceased to convey their influence.  Sometimes,
indeed, the contractility of muscle endures, until changes in its struc-
ture and composition become evident to the senses ;  thus the heart of a
Sturgeon, removed  from the body, and hung up to dry,  has been known
to continue alternately contracting and dilating, until the movement pro-
duced a crackling noise, in  consequence of  the dryness of the texture,
Again, there  is evidence, that various processes of nutrition and secre-
tion may go on, for some  time after somatic death, and  even after the
removal of the organs from the body, provided a sufficient quantity of
blood remains in them; and the blood itself retains its  vitality, so as
not to coagulate,  Avhilst contained in the vessels of tissues still living.
  67. Hence it is, that parts which have been completely separated from
the body may often be reunited Avith it, if they  were  previously in a
healthy state, and too much time have not elapsed ; thus, there are many
eases on record, in  which fingers, toes, noses, or  ears,  that have been
accidentally chopped off, have been made to adhere and grow as  before,
by bringing the cut surfaces into contact,  even  some hours  after  their
severance.  It is evident, then, that  the parts so severed cannot  haAre
lost  their vitality; since  no treatment could  produce  union  between
a dead mass and  a living body.  And Ave are fully justified in assuming, 
OF DEGENERATION AND DEATH.
55
that, in cases Avhere attempts at such reunion have not been successful,
the death of the separated part has  resulted from the too  prolonged
interruption of its  regular nutritive operations, whereby chemical  and
physical changes have taken place in it, and destroyed the peculiar struc-
ture and composition of its several parts.  The ordinary phenomena of
Death, therefore, as Avell as those of Life, bear out the views which have
been here advanced.
   68. But it has  been maintained by those who consider Vitality as
something  superadded to an Organized  Structure, essentially indepen-
dent of it, and capable of being subtracted from it, that Death frequently
takes place under circumstances, Avhich leave the organism as it was ; so
that " the dead  body may have  all the organization it ever had whilst
alive."   For such an assumption, there is not the least foundation.   In
nearly all cases  in which death takes place as a result  of disease, the
connexion  betAveen changes of structure and  composition,  either in the
tissues or in the blood, and such a loss of the vital properties of some
part or organ as is sufficient to bring the Circulation to a stand, is so
palpable as to  require no proof; and in  by far the greater majority of
cases in which  it is not at once obvious, a more careful  scrutiny will
reveal it.  It must be confessed on both  sides,  that our means of inves-
tigation, and our knowledge of the normal structure and composition of
the tissues and the blood, are not yet sufficient to enable us to detect
minute shades of alteration, nor to assert  Avhat extent of change is incon-
sistent with the continuance of life.  But as no one has yet shown, by the
careful and exact microscopical  and chemical  examination  of the solids
and fluids of a dead body, that it has all the organization  it had whilst
alive, the assertion above quoted is totally unwarranted by experience,
and is  contradicted  by  all our  positive knowledge  of  the  matter.
(See § 187.)
   69.  But it has been urged, that  Death may result from the  sudden
operation of some agency of an immaterial  character, which leaves no
trace behind it—such as a powerful  electric shock, or a violent mental
emotion.   Here, too, the argument entirely fails.   It is  impossible that
a powerful electric shock could be transmitted through a mass like the
animal body, composed of elements in such a loose state  of combination
that they are always undergoing decomposition, without producing impor-
tant chemical changes in it; and its imperfect conducting power renders
it equally liable to physical  disturbances.  As a  matter  of fact  it has
been noticed, that the bodies of animals killed by electricity pass into
decomposition with unusual rapidity; showing that the ordinary chemical
affinities of their components have received a powerful  stimulus;  and it
has also been  ascertained, that when eggs in process of  development
have had their vitality destroyed by an Electric shock, the minute vessels
of the vascular area (§ 551) have been ruptured.—Nor is  it  more difficult
to explain the immediate cause of death, as a result of Mental  emotion.
In some cases,  an  obvious physical change has been  produced, by the
too violent action of the heart, the movements of Avhich are  stimulated
by the emotion; thus, even in a healthy  person, rupture of the heart or
aorta has been  known  to take place,—an  occurrence  to Avhich  those
affected by previous disease of that organ are much more liable.   Where 
56        NATURE AND  OBJECTS OF  PHYSIOLOGICAL  SCIENCE.

there is any disorder in the  heart's action, resulting  from thickened
vahres, narrowed orifices, &c, the  physical influence of mental emotion
can be easily accounted for.   But  it must be admitted that cases have
occurred,  in Avhich no such explanation  can be offered ;  sudden death
having taken place without any perceptible structural cause.   We are
not obliged, hoAvever, to have recourse to any hypothesis, for an explana-
tion of even these cases, which is not borne out by ample analogy.   For
it is Avell known  that mental emotions, acting through the  nervous force,
exert a poAverful influence over the composition of the fluids of the body,
and are capable of instantaneously  altering these.  Thus in many human
beings and still  more  in  the lower  animals,  alarm or agitation Avill
occasion the immediate disengagement of powerfully odorous secretions,
Avhich must have  resulted from new combinations suddenly formed; and
a fit of passion  may immediately occasion such  a change  in the milk of
a nurse as renders it a rank poison  to the  infant.   There is no reason
to doubt, therefore, that the  blood itself may undergo changes  of analo-
gous character  from the same cause; and that it may become  a violent
poison to the individual  himself, instead of being the source of whole-
some nutriment, or the stimulus  to vital activity.—But the effect of
Electricity, of Mental  Emotion, or  even  of Mechanical  force, may be
exerted more dynamically than organically ; destroying the vital poAvers,
by antagonizing the  forces  that produce them, Avithout occasioning any
perceptible material change.   This,  in fact, we see in the state of pros-
tration or 'shock,' induced by sudden and violent impressions of almost
any description.

                        5. General Summary.

  70. To conclude,  then ;—we only knoAv of Life, as  exhibited by an
Organized structure, when  subjected to the operations of certain forces
Avhich call it into activity;  and we only know of Vitality, or  the state
or endowment of the being which exhibits that action, as conjoined with
that particular aggregation and composition Avhich Ave term Organiza-
tion.  We have  seen that the act  of Organization, and the consequent
development of peculiar properties in the tissues which are produced by
it, can only be attributed to the vital force of a pre-existing organism;
and hence it is, that whilst the operation of Physical forces  upon an
organized body gives rise to vital phenomena,  no such phenomena can
be  manifested as the result  of their action upon any kind of inorganic
matter.  It is in fact, the speciality  of the material instrument thus fur-
nishing the medium of the  change  in their modus operandi, Avhich es-
tablishes and must ever maintain,  a well-marked boundary-line betAveen
the Physical  and the Vital  forces.   According to the views here pro-
pounded,  the Vital force is  as different from Heat or Electricity, as they
are from each other ; but just as  Heat, acting  under  certain peculiar
conditions, is capable of transformation into Electricity, whilst Electri-
city is capable, under certain other conditions, of being metamorphosed
into Heat, so may either of these forces, acting  under conditions Avhich
an Organized  fabric alone  can supply, be  converted into Vital force
whilst, in  their turn, they may be  generated by  Vital Force. 
GENERAL SUMMARY.
57
  71. Starting, then, Avith the abstract notion of one general Force, we
might say that this Power, operating through Inorganic matter, mani-
fests  itself in  those phenomena Avhich  Ave call electrical,  magnetical,
chemical, thermical, optical, or mechanical; the agents immediately con-
cerned in these being so connected by the relation of reciprocal agency,
or "correlation," that Ave must regard them as fundamentally the same.
But the very same  Force or PoAver, when  directed  through Organized
structures, effects the operations of groAvth,  development, metamorphosis,
and the like;  and is further transformed,  through the instrumentality
of the structures  thus generated, into  nervous agency and  muscular
poAver.   If Ave only kneAV of Heat, for example, as it  acts upon the or-
ganized creation, the peculiarities of its operation upon inorganic matter
Avould seem  no less strange to the  physiologist, than  the  effects here
attributed to it may appear to those Avho are only  accustomed to con-
template the physical phenomena to Avhich it gives rise.   Of the exis-
tence of Force or Power, Ave can give no other account than by referring
it, as we are led by our  OAvn  consciousness to do, to the exertion of a
Will;  and  this unity among the Forces of Nature is  the  strongest
possible  indication of the Unity of the Will of  Avhich they are the  ex-
pressions.  And further, the constancy of the actions which result from
them, Avhen  the conditions are  the same,—that is,  their conformity to a
fixed  plan, or (in the language commonly employed) their subordination
to laws,—indicates the constancy and unchangeableness  of the Divine
Will,  as well as the Infinity of that  Wisdom, by which the  plan was at
first arranged with  such perfection, as to require no departure from it,
in order  to produce the most complete harmony in its results.
   72. So also, if we endeavor to  assign a  cause for the existence of a
cell-germ, Ave are led  at first to fix upon the  vital  operations of  the  pa-
rental organism by Avhich it was produced ; and for these we can assign
no  other cause than the peculiar endowments of  its original  germ,
brought into activity by the forces Avhich have operated upon it.   Thus
we are obliged to go backwards in  idea from one generation  to another;
and when at last brought to a  stand  by  the  origin of the race, Ave are
obliged to rest  in the Divine Will as the source of those Avonderful pro-
perties, by Avhich the first germ  developed the  first organism of that
race from material previously  unorganized, this organism producing a
second germ, the second germ a  second organism, and so  on without
limit, by the uniform repetition of the same processes.  Yet Ave are not
to suppose that the continuation of  the race is really in any  way less
dependent upon the Will  of the Creator, than the  origin  of it.  For
Avhilst Science  leads us to discard  the idea  that the Deity is continually
interfering,  to change  the working of the system He has made,—since
it everywhere presents us with the  idea of uniformity in the plan, and
of constancy in the execution of it,—it equally discourages the notion
entertained  by some, that the creation of matter,  endowed with certain
properties, and therefore subject to  certain actions, was the final act of
the Deity, as far as the  present system of things is concerned,  instead
of being  the mere commencement of  his operations.   If it be admitted
that matter  OAves its origin  and properties to the  Deity,  or, in  other
words, that its first existence was but an  expression of the Divine  Will, 
r
58           EXTERNAL CONDITIONS OF VITAL  ACTIVITY.

Avhat is its continued existence, but  a continued operation  of the same
Will ?   To suppose that it could  continue to exist, and to perform its
various actions, by itself, is at once to assume the property of self-exist-
ence as belonging to matter, and thus to do  away with the necessity of
a Creator altogether;—a conclusion  to which it may be safely  affirmed
that no  ordinarily  constituted Man can arrive, Avho reasons upon  the
indications  of Mind in the phenomena of Nature, in the same way as he
does in regard to the creations of Human Art.
                          CHAPTER II.

         OF  THE EXTERNAL CONDITIONS OF  VITAL ACTIVITY.

  73. It has been shown in the preceding Chapter, that the most general
conditions of Vital phenomena are twofold;—one set being supplied by
the organized structure, which is endoAved (in virtue of its organization)
with certain peculiar properties, but Avhich is inert so long as it is alto-
gether secluded from the influence of external agents ;—whilst the other
is derived from external sources, and consists in a supply of those ma-
terials of which the organized structure is built up,  and in  the operation
of those forces by which the organism is made to appropriate those ma-
terials, which are the sources  of its peculiar powers.  We might thus,
in a rough and rude way it is  true,  compare the living body to a set of
machinery adopted to convert  cotton from the raw material into a Avoven
fabric.  Each portion of the  machinery does its own special work, in
virtue of its peculiar construction ;  e. g. one part  cards, another spins,
and a third weaves;  but their actions are closely related  and even mu-
tually dependent.  Further, their operations all result from one and the
same force or power; and their products may consequently be regarded
as the expressions or manifestations of that Force, Avhich  acts through
the different portions of the mechanism, each in its own peculiar mode.
Noav such a machine can produce no result, without  the concurrence of
these conditions; namely, the perfectly constructed organism (for so in
the wide sense of the term it  may be designated), a supply of the raw
material on which it  is to operate, and an adequate moving power. And
it is to be observed,  that the amount of its product  Avill depend rather
upon the power, than upon the material supplied; for whilst its activity
cannot be increased by any augmentation in the quantity of the material,
beyond that amount  which  it  has poAver to employ, it can be promoted
by  a more energetic  application of  the force, as well as retarded by its
diminution; the amount of material appropriated being increased or
diminished accordingly.
  74.  In like manner, it is requisite to distinguish, among the  external
conditions Avhose concurrence is necessary to produce a Living Organism,
between those which furnish the materials  requisite for its construction
and maintenance, and the forces or powers on which its  operations are
dependent; in other  words, between the Material and Dynamical con- 
EXTERNAL  CONDITIONS OF VITAL ACTIVITY.           59
ditions of Vital Activity.  Under the former group must be comprised,
not merely the Alimentary substances Avhich are capable of being con-
verted into portions of the solid fabric, but also those which are used
(among the warm-blooded animals)  for the maintenance of the  bodily
heat by the combustive process.   In addition, we have to include  the
Water which is requisite to maintain the due proportion of liquid in the
organized fabric; and the Oxygen, Avhose presence in  the  surrounding
medium is essential in various modes to  the maintenance of its Vital
activity.   The dependence of Vital Activity upon Food and Oxygen
will  be fully considered  hereafter  (chaps, iv. and vin.); and  in  the
present Chapter it will be only necessary to take account of the demand
for Moisture (Sect. 4).
   75.  The Forces to Avhose  operation  we can most clearly trace  the
phenomena of Life, are Light and Heat, of which the  latter is the one
whose agency is  the  most  universal, and most  immediately connected
with the  acts of growth and development.   The agency of Light is
indispensable for the first production of organic compounds by the in-
strumentality of the Vegetable fabric; but it  Avould possess no efficacy
whatever, without the simultaneous operation  of Heat; and when these
compounds have  been generated, we  find that they can  be applied to
the purposes of Vegetable nutrition, no less  than to  the nutrition of
Animals, without the aid of  Light; as is seen in the fact, that the ger-
mination of seeds takes place in  darkness, and  that the  formation of
neAV wood in a stem takes place beneath a thick covering of bark.   A
very large proportion of the vital operations of Animals have no direct
dependence upon Light; yet it is entirely through its operation upon
Plants, that  they derive  the  materials of their nutriment; so that  La-
voisier Avas fully justified in the assertion that "without Light, nature
Avere without life and without soul; and a beneficent God, in  shedding
light over creation, strewed the surface of the earth with organization,
Avith  sensation, and with thought."  As an example of the very direct
relation which subsists between the amount of Light and  Heat acting
on an  organism,  and  the  amount of  vital  change produced, it may be
well to advert to  the statement of Boussingault, that the same annual
plant, in arriving at its full development, and going through the process
of flowering and of the maturation of its seed, everywhere requires the
same amount of Light and Heat, Avhether it be grown at the equator or
in the temperate zone; the whole time occupied being inversely to the
intensity of these forces, and the  rate of  growth having  a relation of
direct  equivalence to it.—We have little certain knowledge of the degree
of the ordinary dependence of Vital Activity upon Electricity ; although
there can be no  doubt that it  is capable of exerting a most  important
influence upon the living organism.
   76.  In regard  to all these Forces it  may be observed,  that the de-
pendence of Vital Action upon  their  constant influence is  greater in
proportion to the high organization of their structure, and vice versd ;
so that beings of simple organization are capable of enduring a depri-
vation of them, which would be fatal to those higher in  the scale.   This
will be partly understood, when it  is borne in  mind that the higher the
development of the living being, the more  complete is the distribution 
60           EXTERNAL  CONDITIONS OF VITAL ACTIVITY.
of its different actions amongst separate organs,—the more close, there-
fore, is their mutual dependence,—and the more readily, in consequence,
are they all brought  to a  close by the interruption of any  one.   But
there is no doubt, that the  actions of  even the individual parts of the
higher organism require for their excitement a greater supply of  these
poAvers, than the similar actions of the corresponding parts in the loAver:
whilst if these forces be exerted upon the lower Avith the intensity that
is required for the higher, they destroy the vital properties of the tissues
altogether, by the  excess of their action.   This distinction is  most ob-
vious in regard to  the relative influence of Heat,  upon Avarm-blooded
and cold-blooded animals ;  of Avhicbve-xamples will be given hereafter.
   77. It may also be observed of the influence  of  these, as  of that of
other forces Avhose agency  is less -general, that it is rather  relative than
absolute ;  being frequently  dependent upon the degree of change, rather
than upon the  measure of the actual amount.  This constitutes a marked
difference betAveen the influence  of  these forces on  mere chemical com-
pounds, and their  operation on  bodies  endowed Avith vitality.  In the
former  case their  action is always uniform;  thus  the same amount of
heat, the some exposure to light, the same charge  of electricity, Avould
be required to produce a given  Chemical  effect, hoAV often  soever the
action might be  repeated.   But this is  not the case with living bodies;
since an increase or diminution in the intensity of Heat, which, if made
suddenly, would be  scarcely  compatible with the  continuance of Life,
may be so brought about, as to produce no marked change  in  its  phe-
nomena,—the organism possessing a certain power  of adapting itself to
conditions which are habitual  to it, and thus  allowing great changes in
these conditions  to be  gradually effected, without  any serious  disturb-
ance.—Thus of  tAVo  individuals of the  same species, one may  become
torpid at a temperature of 60°, because it  has  been accustomed  to  a
temperature of 70° , whilst another, habituated to a temperature of 60°,
would require to be cooled doAvn to 50°, in order  to induce torpidity;
the influence of temperature upon the vital  condition being proportioned,
more  to the variation  from  the usual standard, than  to the actual degree
of heat or cold in operation.   Yet the first of these  individuals might
be gradually habituated to live in the same temperature with the second;
and to require the  same amount of  further depression for the induction
of torpidity.   (See § 132.)
  78. It is a very  curious  fact, that, whilst the lower classes of living
beings are more capable than  the higher of bearing the deprivation of
these Vital stimuli, they  are at the same time more liable to alterations
in their own structure and development, in consequence of variations
in the degree of their agency, or from  other causes external to them-
selves.  Thus the forms of the lower tribes of Plants and Animals are
liable to be greatly affected by the conditions under which they grow;
and  these especially modify their degree of development.   It seems as
if  the formative power Avere  less vigorous in the loAver,  than in the
higher classes;  so  that the mode in which  it manifests  itself in the
former is more dependent upon external influences; whilst in the latter
it either predominates over them, causing the regular actions to be per-
formed, or gives way  altogether.—The same principle applies to the 
            LIGHT, AS  A CONDITION OF VITAL  ACTIVITY.           61

early condition  of the  higher  organisms;  their embryos, like those
beings of permanently-low type which they resemble in degree of deve-
lopment, being liable  to be affected by  modifying causes,  Avhich the
perfect beings of the same kind are able  to  resist.  It is in  this way
that we  are  to explain  the  influence which the female parent exerts
upon the embryo, during the period through Avhich it is dependent upon
her for the materials for its  development.

              1.  Of Light, as a Condition of Vital Activity.

   79.  The importance of this agent, not  only to the Vegetable but to
the Animal World, is not in general  sufficiently estimated.   Under  its
influence alone can that first  process be  accomplished, by which Inor-
ganic matter is transformed  into an Organic compound, adapted by its
nature and properties  to form part of  the organized fabric.  The fol-
lowing is an example of the simplest phenomena of this kind; and  it
demonstrates the influence of Light the more clearly on account of that
simplicity.   "If Ave expose  some spring-water to the  sunshine,  though
it may have  been clear and transparent at first, it presently begins to
assume a greenish tint; and, after a Avhile flocks  of green matter col-
lect on the sides of the vessel in which it is contained.   On these flocks,
whenever the sun is shining, bubbles of gas may be seen, which, if col-
lected, prove to be a  mixture of oxygen  and nitrogen, the proportion
of the two  being variable.  Meanwhile the green matter rapidly grows;
its new parts, as they are developed, being all  day long covered  with
air-bells, which disappear as soon as the sun has set.   If these observa-
tions be made upon a stream of water, the current of Avhich runs sloAvly,
it will  be discovered that the green matter serves as food for thousands
of  aquatic  Insects, Avhich make their habitations in it.  These  insects
are  endowed with  poAvers of rapid locomotion, and possess a  highly-
organized structure ; in their turn they  fall a prey to the Fishes Avhich
frequent such streams."*  Such is the  general  succession of nutritive
actions in  the  Organized  Creation.   The highest Animal is either
directly dependent  upon the Vegetable Kingdom for  the  materials of
its fabric, or it is furnished with these by  some other Animal, this again
(it  may be) by another, and so on ;  the last in the  series  being  always
necessitated  to  find its  support  in the Vegetable kingdom, since the
Animal does  not possess the poAver  of causing the Inorganic elements
to unite into  even the simplest Organic compound.   This power is pos-
sessed in a high degree by Plants ;  but it can only be exercised under
the  influence of  Light. We shall now examine, more  in  detail, the
conditions  of this influence, both  in the instance  just quoted  and in
others drawn from the actions of the higher Vegetable organisms.
   80.  The "green matter  of Priestly," (as it is commonly  called),
which  makes its appearance  when water of average purity is submitted
to the  action of the Sun's light, and which also presents  itself on the
surface of  walls  and rocks that are constantly kept  damp, is now known
by Botanists to consist  of cells in various stages of development,—the

   * Prof. Draper, on the Forces which produce the Organization of Plants ; p. 15. 
62           EXTERNAL  CONDITIONS OF VITAL ACTIVITY.
early forms, it may be, of several different species of Confervse.   That
these cells all originate from  germs, and not merely from  a combina-
tion of inorganic elements, appears not only from general considera-
tions, but also from the fact that, if measures be taken to free the Avater
entirely from any possible infusion of organic matter, and to admit into
contact Avith it such air alone as has undergone a  similar purification,
no green flocks  make their appearance, under the  prolonged influence
of the strongest  sunlight.   We find, then, that the  presence  of a  germ
is one of the conditions indispensable to the chemical transformation in
question.   It may  be  asked hoAv  it can be certainly ascertained  that
light, and not heat, is the  essential condition of this process ; seeing
that the  two agents are  combined in  the solar beam.  To this  it  may
be replied,  that a certain moderate amount of heat is undoubtedly ne-
cessary ; but that  no  degree of heat without light will be effectual in
producing the change, as  is easily proved by  exposing  the water to -
Avarmth in a dark place.  Moreover, when a certain measure of  light is
afforded, variations in the  amount of heat make very little  difference;
but Ave shall  presently see that under the same  degree of  heat, the
amount of the change  is  directly proportional to  the intensity of the
light.  Although,  therefore, heat  furnishes an  essential condition, it
cannot  be  questioned that light is the  chief agent in the  process, by
Avhich the germ brings into union the  elements  to be employed in the
development of its OAvn fabric.
  81.  The  next question is,—What are these elements, and Avhence are
they obtained?  All water  that is  long exposed to the atmosphere ab-
sorbs from it a certain amount of its constituent  gases ; but these do
not enter it in the proportions in which they are contained in the
atmosphere itself; their relative quantities, in a given measure of water,
being proportional  to the facility with  which they are respectively ab-
sorbed by the liquid.  Thus, carbonic acid is most readily absorbable;
oxygen next,  and nitrogen least so.    From the experiments of  Prof.
Draper, it  Avould appear that, notAvithstanding  the very small  propor-
tion  of  carbonic acid contained in the atmosphere  (usually not  more
than  l-2000th part), it  forms  as much as 29 per cent, of the Avhole
amount of air expelled  from Avater by boiling.  Of the residue, one-third
consists of oxygen, and the remaining  tAvo-thirds of nitrogen;  so that
the proportion of the oxygen to the nitrogen is  as one to two, instead
of being one to four, as in atmospheric air.  The  absolute quantity of
this water-gas, contained in any measure of water, is subject to varia-
tion with the temperature; the quantity being diminished as the tem-
perature rises.   Now when water thus impregnated wTith carbonic acid,
oxygen, and nitrogen, and  containing  the germs  of aquatic plants, is
exposed to the sun's light,  a development of vegetable structure takes
place, indicated by the green flocculent appearance, as already men-
tioned.   If  the changes,  which are  noAv occurring  in the  water, be ex-
amined, we find  that the carbonic acid is diminishing in amount; and
that oxygen is being evolved.   The growing mass  increases  in volume
and weight; and after a time exhausts the whole  carbonic  acid origi-
nally contained in the water.   If it be then prevented  from receiving
an  additional supply,  the  process stops; but as conducted naturally, 
INFLUENCE OF  LIGHT ON  PLANTS.               63
there is a free exposure to the atmosphere, through which carbonic acid
is diffused;  and hence, as fast as it is removed by decomposition, it is
restored by absorption.
  82.  Here then are the conditions and materials ;  what is the result'(
As a consequence of the conjoint action of light and of a vegetable cell-
germ, with a moderate degree of heat, upon carbonic acid  and Avater,
we find a vegetable structure produced, Avhose  fabric chiefly consists of
carbon, united with the elements of Avater.  Whether this union is really
as simple and direct as is implied by this expression, or whether  the
same proportions of ogygen, hydrogen, and carbon are united in a dif-
ferent form, is not a matter of consequence to the present inquiry ;  the
general fact being, that by the decomposition of the carbonic acid, oxy-
gen  is set free, and carbon is made to unite with the elements  of Avater;
so as to form an organic compound, Avhich is appropriated by the Vege-
table organism as the material for its groAvth.—How far Light is also
concerned in the production of  the proteine-compounds which  are gene-
rated by Plants, not merely for the use of Animals, but also as  part
of the material of their OAvn growth, has not been yet ascertained;  but
it is  probable that these are not the less dependent  upon its agency for
their formation  since they are formed  under the same circumstances
with the preceding.
  83.  The process whose conditions we have thus examined, is carried
on in the individual cells, that compose the highest and most  complex
Plants, precisely as in those which constitute the entire organisms of the
lowest.   Thus if a few garden-seeds of any kind be sown in a flower-pot
and  be caused to germinate in  a dark room, it will soon  be perceived
that  although they can grow for a time without the influence of light, that
time is  limited; the weight of their solid contents diminishes, although
their bulk may increase by the absorption of water; their young leaves,
if any should be put  forth, are of a yellow or gray-white color, and
they soon fade away and die.  But if  these plants are brought out suffi-
ciently soon into the bright  sunlight,  they speedily begin to turn green
they unfold their leaves, and evolve their different  parts  in  a natural
way  ; and the proportion of their solid contents goes on increasing from
day  to day.  If the fabric be then subjected to chemical analysis, it is
found to contain oxygen, hydrogen, carbon,  and nitrogen ;  united in
various proportions, so  as to form  compounds that differ in the various
species, though  some,—such as gum,  starch, cellulose, and albuminous
matter,—are the same in all.  If the plants be made to grow in closed
glass vessels,  under such circumstances  that  an  examination -can  be
accurately made  as to the changes they are  impressing on the atmo-
sphere, it is discovered that they are constantly decomposing its carbonic
acid,—appropriating its carbon, and  setting free its  oxygen,—so long
as they are exposed to the  influence  of sunshine or bright  daylight.
They also appropriate a part of the minute quantity of ammonia which
is diffused  through the  atmosphere; extracting its nitrogen to employ
it in the production of their azotized compounds.  It is capable of being
demonstrated by experiment that these changes are confined to  the green
surfaces of plants, and therefore to the leaves or leaf-like organs, to the
young shoots, and to the stems of herbaceous plants, or of those in which 
64           EXTERNAL  CONDITIONS OF  VITAL ACTIVITY.

(as in the Cactus tribe) the leaves are wanting and the enlarged succu-
lent stem supplies their place.   When these surfaces cease to  become
green, the decomposing  action  also ceases;  carbon is no longer fixed
and oxygen set free ; but, on the contrary, carbonic acid is exhaled:
this is the case when the leaves  change  color,  previously to their  fall,
in the autumn.  Thp  compounds Avhich are thus generated in the green
surfaces  are conveyed to the remote parts of  the fabric, by the circu-
lation of the sap, and become the materials of their nutrition; and  thus
the green cells of the leaves have exactly the  same function, in minis-
tering to  the  growth  of  the fabric of  the largest tree, which the green
cells of the humble  Conferva perform in regard to themselves alone.
  84.  It  has  been  already mentioned, that the  decay Avhich is always
taking place in the  softer vegetable structures, gives rise to a continual
production of  carbonic acid, even in the living plant; this process, which
must  be  regarded as  a true Respiration, is effected, as in Animals, by
the union of the carbon of the Plant with oxygen derived from the atmo-
sphere ; and it is carried on, not by the green parts only, but also,  per-
haps  chiefly, by  the  darker surfaces.  Being antagonized  during the
day by the converse change just described, it can only be made sensible,
by placing the plants for a time in an  atmosphere in Avhich no carbonic
acid previously existed;  and  it  will then  be found that, even in full
daylight a certain amount  of that gas is exhaled.  The fact, however,
becomes  much more obvious at night, or  in darkness ;  since  the decom-
position of the surrounding carbonic acid  by the green surfaces is then
completely at  a stand, and the  full effect of the  respiratory process is
seen.   Moreover, when a plant  becomes unhealthy, from too long  con-
finement in a limited atmosphere, it begins to exhale more carbonic  acid
than  it decomposes ; and the same is the case, as just  noAV stated,  in
regard to leaves that have merely reached the term of their lives.   It
does not  admit of question, hoAvever, that, under ordinary circumstances,
nearly the Avhole carbon of  a sloAV-growing plant is derived from  the car-
bonic  acid  of the  atmosphere; either directly through  the leaves,  or
indirectly by absorption  through  the roots ; and  that there must  be a
vast surplus, therefore, of the carbonic acid decomposed, over that which
is exhaled, during the Avhole life of the tree,—that surplus being in fact
represented by the  total amount of carbon contained in its tissues.
  85. It is probable that the minute amount of Carbonic Acid at present
contained in the atmosphere, is as much as could be beneficially supplied
to Plants, under the average  amount of light to which they are  sub-
jected, over the whole globe, and throughout  the year.  It has been
clearly shown, that, under  the  influence of strong  sunlight, an atmo-
sphere containing as much as 7 or 8 per cent, of carbonic acid may  be
not merely tolerated by Plants, but may be positively beneficial to them,
producing a great acceleration in their growth ; but as soon as the light
is Avithdrawn,  it acts upon them  most injuriously, causing them speedily
to become unhealthy, and altogether destroying  their vitality, if  they
are long subjected  to  it.  Under more cloudless skies than ours, how-
ever,  the continual  supply  of a  larger quantity of carbonic acid,  than
our atmosphere contains, is found to  be  quite  compatible Avith healthy
vegetation; especially in the case of Cryptogamic plants, which (as will 
INFLUENCE OF  LIGHT ON  PLANTS.
65
be presently shown)  require a less amount of this agent than those of a
higher kind.   Thus  in  the  lake  Solfatara in Italy, an unusual supply
of carbonic acid is  afforded by the constant escape of that gas from
fissures in the bed of the lake, with a violence that gives to the water
an  appearance of ebullition ;  and on  its surface  there  are numerous
floating islands,  which consist  almost  entirely of Confervse and other
simple cellular plants, growing most luxuriantly on this rich pabulum.
And  it has been  remarked, that  the vegetation around the springs in
the valley of Gottingen, which abound in carbonic acid, is very rich and
luxuriant; appearing several Aveeks earlier in the spring, and continuing
much later in the autumn, than  at other spots in the same  district.
Many circumstances  lead to the  belief, that  at former  epochs in  the
Earth's history, the atmosphere  was much more  highly  charged with
carbonic acid than  at present;  and that to this circumstance, in con-
junction with a more intense and constant influence of light  and heat, we
are to attribute  that extraordinary luxuriance of the vegetation of those
periods, of which we have most abundant evidence, in the vast beds of
disintegrated  Aregetable matter—Coal—that are of such value  to Man,
and in the remains which have been  more perfectly preserved to us, and
which indicate that  not only the general forest mass, but many of the
individual  forms, attained a degree of  development which cannot noAv
be paralleled even between the Tropics.
   86. Various experiments have  been recently made, with the view of
determining more precisely the conditions  under  which Light  acts, in
producing  the chemical changes that have  been now discussed.   These
experiments for  the most part agree in the very interesting result, that
the amount of carbonic acid decomposed by plants  subjected to the dif-
ferently-colored rays of the  solar spectrum, but otherwise placed in
similar circumstances, varies with the  illuminating power of  the rays,
and not with their heating or their chemical power.   The method adopted
by Prof. Draper, Avhich seems altogether the most satisfactory, consisted
in exposing leaves of grass, in tubes  filled with water which had been
saturated Avith carbonic acid (after the expulsion of the previously dis-
solved air by boiling), to the influence of the different rays of the solar
spectrum, dispersed  by a prism ;  these were kept  motionless upon the
tubes for a sufficient length  of time to  produce an active decomposition
of the gas in the tubes which were most favorably influenced by the
solar beams ;  and the relative quantities of the oxygen set free were
then  measured.    It was then evident  that the  action had been almost
entirely confined to  two of  the tubes,  one  of them being placed in the
red and orange  part of the spectrum, and the other in the yellow and
green.   The quantity of carbonic acid  decomposed by the plant in the
latter of these, was  to that decomposed in the former, in the ratio of
nine  to five; the quantity found in the tube  that had been placed in the
green and blue portion of the spectrum, would not amount, in the same
proportion, to one;  and in  the  other  tubes, it was either  absolutely
nothing, or extremely minute.  Hence it is obvious that the yellow ray,
verging into orange on one  side, and into  green  on the other, is the
situation of the  greatest exciting power  possessed by light on this most
important function of plants; and as this coincides with the seat of the 
QQ           EXTERNAL  CONDITIONS OF  VITAL ACTIVITY.

greatest illuminating power of the spectrum, it can scarcely be doubted
that light is the agent here concerned; more especially as the place of
greatest heat is in the red ray, and that of greatest  chemical power is
in the blue, both of which  rays were found to be quite inert in the  ex-
periment just quoted.  It must not be supposed  from this experiment,
however, that the yelloAV ray, and those immediately adjoining  it,  are
the only sources of this power in the  Solar spectrum; since  it proves
no more than that, when the leaves were exposed to a highly carbonated
atmosphere, they could only decompose it under the influence of these
rays.  It  is certain, from  other experiments, that plants will grow, in
an  ordinary atmosphere, under  rays of different colors; and it appears
that the amount of carbon they severally fix, bears a constant  proportion
to the illuminating powers of the respective  rays.
  87. Although this fixation of carbon by the decomposition of carbonic
acid, is the most universally dependent, of all the processes of the Vege-
table economy, upon the influence of  Light, yet it is not the only one,
especially among the higher Plants, in which  that  agent becomes  an
important condition.  Of the whole quantity of moisture imbibed by the
roots, and contained in the ascending sap, a large proportion is exhaled
again by the leaves; a small part only being retained (together with
the  substances previously  dissolved in  the Avhole)  to form part of the
fabric.  Now upon the rapidity of this Exhalation depends the rapidity
of the absorption ; for the roots will not continue to take up  more than
a very limited amount of fluid, when it is not discharged again from the
opposite extremity (so to speak)  of the stem.   The loss of fluid by the
leaves appears to be a simple process of evaporation, depending in great
part upon the temperature and  dryness of the  surrounding air; this
evaporation, however, does not take place solely, or even chiefly, from
the external surface of the leaves, but from the walls of the passages
which are channeled-out in their interior.   Into this complex labyrinth,
the outer  air finds its way through orifices in the  cuticle,  which are
termed stomata;  and through these it comes forth  again, charged with
a large amount of vapor communicated  to  it by the  extensive moist
surface, with which it comes in  contact in the interior of the leaf.  Now
the stomata are bounded by two or more cells, in such a manner that they
can be opened or closed by changes in the form of these; and this alte-
ration is regulated by the amount of Light, to Avhich the leaves are sub-
jected.  When  the stomata are  opened under the influence of light, the
external air  is freely admitted to the extended  surface of moist tissue
within the leaf, and a rapid loss  of fluid is  the result; more especially
if the temperature be high, and the atmosphere in a dry state.   On the
other hand, if the stomata be closed, the only loss of fluid that can take
place from the internal tissue of the leaves, is through  the cuticle; the
organization of which seems destined to enable it to resist evaporation,
so that the exhalation  is  almost entirely checked.   The influence of
light upon this important function is easily shown by experiment.  If
a plant, which is actively transpiring and absorbing under a strong sun-
shine, be  carried  into a  dark  room, both these operations are almost
immediately checked, even though the surrounding temperature be higher
than tbat  to which the plant was previously exposed. 
INFLUENCE OP  LIGHT ON  PLANTS.
67
  88.  The effect of the complete and  continued  withdrawal of Light
from a  growing plant, is to produce an etiolation or blanching of its
green surfaces: a loss of Aveight of  the solid parts, owing to  the conti-
nued disengagement of carbon from its tissues, unbalanced by the  fixa-
tion of that element from the atmosphere;  a dropsical distension of the
tissues, in  consequence of the continued absorption  of water, Avhich is
not got rid of by exhalation; a want of  power to form its peculiar secre-
tions,  or even to  generate new  tissues, after  the  materials previously
stored up have been exhausted; in fine, a cessation of all the operations
most  necessary to  the preservation of  the vitality of the  structure,  of
which  cessation its death is the inevitable result.   A partial withdrawal
of the  influence of  light, hoAvever, is frequently used by the Cultivator,
as a means of giving an  esculent character to certain  Plants, which
would  be otherwise altogether uneatable;  for in this manner  their  tis-
sues are rendered more succulent and less "stringy, " whilst their pecu-
liar secretions are  formed in  diminished amount, and communicate an
agreeable flavor instead of an  unwholesome rankness of taste.
   89.  There is one period in  the life of the Flowering Plant, however,
in which the influence of Light is rather injurious  than beneficial;  this
is during the first part of the  process of germination of seeds, which is
decidedly retarded  by its agency.  This forms no exception, however,
to the  general rule; since the  decomposition of the carbonic acid of the
atmosphere, and the  fixation of carbon  in the tissues, do not  constitute
a part of the operation.   On the contrary, the  embryo being nourished,
like an animal, by organic compounds previously elaborated and stored
up in the seed, the  chemical changes which take place in  them involve
the opposite action,—the extrication of carbon, which is converted  into
carbonic acid by uniting  with the oxygen of  the  atmosphere.  It is
obvious, then, why light should not only be useless,  but even prejudicial,
to this  process; since it tends to fix the carbon in  the tissues, which
ought  to be thrown off.   As soon, however, as the cotyledons or seed-
leaves  are unfolded, the influence of light upon them  becomes as impor-
tant, as it is on the ordinary leaves at a subsequent time; their surfaces
become green, and  the fixation of carbon  from the atmosphere  com-
mences.  Up to that point, the young plant is diminishing day by day
(like a plant that is  undergoing etiolation), in the weight of its solid
contents; although its bulk has increased  by the absorption  of water.
From the time,  however, that its cotyledons begin to act upon  the air,
under the influence of light, the  quantity of solid matter  begins to in-
crease ; and its augmentation  subsequently takes place, at a rate  pro-
portional to the amount  of green  surface  exposed,  and the  degree of
light to which it is  subjected.
  90.  The influence  of Light upon  the direction of  the growing parts
of Plants,  upon the  opening  and closing  of flowers, &c, is  probably
due to  its share in the operations  already detailed.   Thus the green
parts of Plants, or those  which effect  the decomposition of carbonic
acid (such as the leaves and stems), have a tendency to grow towards
the light; whilst the  roots, through whose dark surfaces carbonic acid
is  thrown  out  by  respiration,  have an  equal tendency  to  avoid  it.
That the first direction of the stems and roots of plants is very much 
68
EXTERNAL  CONDITIONS OF  VITAL ACTIVITY.
influenced  in  this manner, appears from  the fact, that, by  reflecting
light upon germinating  seeds, in  such a manner as that  it shall only
fall upon  them from below, the stems are  caused to direct themselves
dowmvards, whilst the roots grow upwards.—There  can be no  doubt,
however, that  Light has also a more direct  influence on  the  develop-
ment of particular organs in certain Vegetables.  Thus  when the gem-
mules* of the Marchantia polymorpha (one  of  the Hepaticce or Liver-
worts), are in  process  of development, it has been  shown by repeated
experiments, that stomata  are formed on the side exposed to the light
and that roots grow  from the lower surface;  and that it is a matter of
indifference which side of the little disk  is  at first turned upAyards,
since each has the power of developing stomata, or roots, according to
the influence it receives.   After the tendency to the formation of these
organs has once been  given, however, by the sufficiently prolonged in-
fluence of light upon one side, and of darkness and  moisture  upon the
other, any attempt to alter  it is found to be vain;  for if the surfaces
be then inverted, they are  soon restored to their original aspect by the
twisting groAvth of the plant.
  91. The same  amount of this agent is  not requisite  or  desirable for
all Plants ; and we find in the different habitats Avhich are characteristic
of different species, even amongst our  native plants, that the amount
congenial to each varies considerably.  Generally speaking, the succu-
lent thick-leaved  Plants  require the  largest amount: their stomata  are
few in  number; and the full influence  of light is requisite to  induce
sufficient  activity in the exhaling  process;  accordingly we find them
growing, for the most part, in exposed situations, where there is nothing
to interfere with  the full influence  of the  solar rays.  On  the other
hand, plants  with thinner and more delicate leaves, in which the ex-
haling  process  is easily  excited to an excessive amount, evidently find
a congenial home is  more  sheltered situations; and there are some
which can only develope themselves in full luxuriance in  the deep shades
of a plantation or a  forest.   By a further adaptation of the same kind,
some species of Plants are enabled to live and acqufre their green color
under an  amount of deprivation Avhich would be fatal to  most others;
thus in the mines of Freyburg, in which the  quantity of light admitted
must be  almost  infinitesimally small, Humboldt met  with Flowering
Plants of various species ;  and Mustard and  Cress  have been raised in
the dark abysses  of the collieries of this country.
   92.  Generally  speaking, however, the Cryptogamia  Avould seem to
be better  adapted than  Flowering Plants to  carry on their vegetating
processes under a low or very moderate amount of  this  agency.   Thus
Humboldt found  a  species  of  sea-weed near the  Canaries, which pos-
sessed a bright grass-green  hue, although  it had groAvn at a depth of
190 feet  in  the  sea, where, according to computation, it could  have
received  only l-1500th  part of  the solar rays that would have fallen
upon it at the surface of the ocean.   Many Ferns, Mosses, and Lichens

  * These gemmules are analogous to the buds of higher plants; and they consist of
little collections  of cells, arranged in the form of flat  disks; which are at first attached
by footstalks to the  parent plant, but afterwards fall off, and are developed  into new
individuals. 
INFLUENCE OF  LIGHT ON  PLANTS.
69
seem as if they avoided the light, choosing the northern rather than the
southern  sides of hedges, buildings, &c,  for their  residence ; so that
the former often present a luxuriant growth of Cryptogamic vegetation,
whilst the latter are comparatively  bare.   It  must not be supposed,
however,  that they avoid light altogether, but only Avhat is to them an
excessive  degree of it.   The avoidance  of  light seems to  be much
stronger  in  the  Fungi,  which  groAV most luxuriantly  in very  dark
situations ; and the reason of this is probably to be found in the fact
that, like  the germinating seed (§ 89), they form  rather  than  decom-
pose carbonic acid;  their food being supplied to them  from the decay-
ing  substance on which they grow; and the rapid changes in  their
tissues giving rise to a high amount of  Respiration,—a change exactly
the  converse of that, on Avhich, as we have seen, Light exerts such a  re-
markable poAver.
   93. In regard to the agency of Light upon the functions of Animals,
comparatively little is certainly known.   It is evident that the influence
it exerts on those chemical processes Avhich constitute the first stage of
Vegetable nutrition, can have scarcely any place in Animals; because they
do not perform any  such acts of combination, but make use of the pro-
ducts already prepared  for them  by Plants.  Hence,  Ave do not find
that the surface of Animals  undergoes  that  extension,  for the purpose
of being exposed to the  solar rays, which is so characteristic  a  feature
in the Vegetable fabric, and so important in its economy.   Still  there
can  be no doubt, that  the degree  of  exposure to light  has  a  great
influence  upon the colors of the  Animal  surface; and  here Ave seem to
have a manifestation of Chemical agency, analogous to that Avhich  gives
color to the  Vegetable  surface.   Thus  it  is a matter of familiar  expe-
rience, that the influence of light upon the skin of many persons, causes
it to become  spotted with brown freckles; these freckles being aggre-
gations of brown pigment cells, which either owed their development to
the  agency of light, or were enabled by that agency to perform a che-
mical  transformation which  they  could  not otherwise  effect.  In like
manner, the  swarthy hue, which  many  persons acquire  in  warm  cli-
mates, is  due to a development of dark pigment-cells  diffused through
the  epidermis (§ 229);  and an increased development of the same kind
ogives rise to  the blackness of  the  Negro-skin.   There can be no doubt
that the prolonged influence of light upon one generation  after another,
tends to give a permanent character to  this variety of  hue ;  Avhich will
probably  be more easily acquired, in proportion  to the  previously-exist-
ing  tendency to that change.  Thus  it  is well known that a  colony of
Portuguese Jews, which settled at  Tranquebar  about three centuries
ago, and  which has kept itself distinct from the surrounding tribes, can-
not  noAV be distinguished .as  to color from the native Hindoos.  But it
is probable that a similar colony  of fair-skinned Saxons would not, in
the  same time, have acquired anything like  the  same depth of color in
their skins.
   94. It  can scarcely be questioned, that the brilliancy of color which
is characteristic of many tribes of animals in tropical climates, especially
Birds and Insects, is in great part dependent, like the brightness of  the
foliage and fruit  of the same countries,  upon the brightness of the light 
70
EXTERNAL CONDITIONS OF VITAL  ACTIVITY.
to which their surfaces are exposed.  When  birds of warm  climates,
distinguished by the  splendor of their plumage, are  reared under  an
artificial temperature in our OAvn country, it is uniformly observed that
they are much longer  in acquiring the hues characteristic of the adult;
and that these are never  so  bright as Avhen they have  been produced
by the influence  of the tropical sun.   And it  has been also remarked,
that if certain  Insects  (the  Cockroach for  example),  which naturally
inhabit dark  places, be  reared in an entire seclusion from light, they
groAv up almost as colorless as Plants that are made to vegetate under
similar circumstances. *
  95.  There is reason to believe that Light exercises an important in-
fluence on certain processes  of development in Animals, as well as in
Plants.  Thus, the appearance of Animalcules in infusions of  decaying
organic matter is much retarded, if the vessel be altogether  secluded
from it.  The rapidity with which the small Entomostracous Crustacea
(Water-Fleas, &c.) of our  pools, undergo their transformations, has been
found to be much influenced by the amount of light  to  Avhich they are
exposed.  And it has been ascertained that, if equal  numbers of Silk-
worm's eggs be preserved  in a dark room, and  exposed to common day-
light, a much larger proportion  of larvse are  hatched from the latter
than from the former.   The most striking proof of the influence of Light
on Animal development, however, is afforded by the experiments of Dr.
Edwards.   He  has  shown that if  Tadpoles  be nourished with  proper
food, and be  exposed  to the  constantly renewed contact  of water (so
that their respiration  may be freely carried on, whilst  they remain in
their fish-like condition), but  be entirely deprived of light, their growth
continues, but their metamorphosis  into the condition  of air-breathing
animals is arrested, and they remain in the condition  of gigantic tad-
poles.   It is  interesting to remark, that the Proteus anguineus,  an ani-
mal  which closely corresponds  in  its fully-developed  form  with  the
transition stage betAveen the  Tadpole and the Frog, finds  a congenial
abode  in the  dark lakes of the caverns of Styria and Carniola, and in
the underground caverns that connect them ; thus shoAving its adapta-
tion to a condition, which  keeps down to the same standard  the develop-
ment of an animal, that is empowered under other circumstances to ad-
vance  beyond it.  Numerous  facts, collected from different sources, lead
to the  belief that the healthy  development of the Human body, and the
rapidity  of  its recovery from disease,  are  greatly influenced by  the
amount of light to which  it has  been exposed.   It has  been observed,
on  the one  hand, that a remarkable freedom  from deformity  exists
amongst nations who wear very little  clothing; whilst, on  the other, it
appears certain that an unusual tendency to deformity is  to  be found
among persons  brought up in cellars or mines, or in dark and narrow
streets.  Part of this  difference is doubtless owing to the relative purity
of the  atmosphere in the former case, and the want of ventilation in the
latter ; but other instances might be quoted, in  which a marked variation
presented itself,  under circumstance  otherwise the same.   Thus, it has
been stated by Sir A. Wylie  (who  was long at the head of the medical
staff in the Russian army), that the cases of disease on the dark side of
an extensive  barrack at St. Petersburgh, have been uniformly, for many 
INFLUENCE OF HEAT  ON PLANTS.
71
years, in the proportion of three to one, to those on the side exposed to
strong light.  And in one of the  London Hospitals, with a long range
of frontage  looking  nearly due north  and south, it has  been observed
that residence in the south wards is much more conducive  to the welfare
of the patients than  in those on the north side of the building.
   96. These facts being  kept in view, it is easy to perceive that there
must be differences among the  various species of  Animals, as among
those of  Plants, in regard to the  degree of  light which is congenial to
them.  Among the  lowest tribes, in  which no special  organs of vision
exist, there  is evidently a susceptibility to the influence of light, which
appears  scarcely to  deserve  the  name of sensibility, but which seems
rather analogous to  that which is manifested by Plants ;  thus among
those Polypes Avhich  are not fixed  to particular spots, and amongst Ani-
malcules, there are some  species which seek  the light, and others which
shun it.   And it appears from various  observations upon the depths at
which marine animals are found, especially from the extensive series of
facts collected by Prof. E. Forbes,*  that there are a series of zones, so
to  speak, to be met with  in  descending from the  surface  towards  the
bottom of the ocean,  each of  which is  characterized by  certain species
of animals peculiar to itself,  whilst other species have a range  through
two or more of the zones;—the extent of the range of depth, in each
species bearing a close correspondence with the extent of its geographical
distribution.  Noav there  can be no doubt  that  the restriction of par-
ticular species to particular  zones is due in  great part to the degree of
pressure of  the  surrounding medium  ;  but. there can be as little doubt,
that the  variation  in the  degree of Light also exerts a most important
influence, the solar rays in their passage through sea water being subject
to a loss  of one half for every seventeen feet.  From the results of Prof.
Forbes's researches, it appears that no species of Invertebrated animals
habitually live at a greater depth than 300 fathoms ; and although Fishes
haA'e been captured at a depth of from 500 to 600 fathoms, it is probable
that they had strayed from their usual abodes.

              2. Of Heat,  as a Condition of Vital Activity.

   97. The most perfectly-organized  body, supplied with all the other
conditions requisite  for its activity, must remain completely inert, if it
do  not receive a sufficient amount of Heat.   The  influence which this
agent exerts upon living  beings, is far more remarkable  than its effects
upon inorganic matter ; although  the  latter are usually  more  obvious.
We are all  familiar with  its power of producing expansion,—with  the
liquefaction which is the consequence of  its application to solids,—with
the evaporation which it  occasions in  liquids,—and with the enormous
repulsive force which it generates  among the particles and vapors ;  but
it is not  until we  look  deeper  than  the surface that  we perceive how
immediate is the dependence of every action of Life upon  this myste-
rious agent.  The temporary or permanent loss of vitality, in parts of
the body subjected to extreme cold, is a " glaring instance" of the effect

  * Report on the Invertebrata of the iEgean Sea, in Transactions of British Associa-
tion, 1843. 
72           EXTERNAL CONDITIONS OF  VITAL ACTIVITY.

of its withdrawal.  This change, however, is  not  immediate.  Its first
step is a mere depression of the vitality of the part, involving a partial
stagnation of the  capillary circulation, diminution of sensibility, and
want of muscular power.   But the continued action of cold on the sur-
face, not compensated by a sufficient generation of  heat within, causes
the circulation of the part  to be completely suspended, its small vessels
contract so that they become almost emptied of blood, its sensibility and
poAver of movement are destroyed,—in a word, its vital activity is com-
pletely  suspended.    In such a  state, a timely but cautious application
of warmth may produce the gradual renewal of the circulation, and the
restoration of the other properties of the part which are dependent upon
that function ; but any abrupt change Avould complete the mischief which
the cold has begun ; and would altogether destroy, by the  violence of
the reaction, the vitality which was only suspended, causing the  actual
death of the part.   Hence,  when the extremities are frost-bitten, nothing
can be more  injurious than to bring them near a fire;  whilst no treat-
ment has been found so safe and effectual as the rubbing them with snow.
   98. The influence of Heat upon Vital activity, is attested on a larger
scale, by the striking contrast between the dreary barrenness of Polar
regions, and the luxuriant  richness of Tropical countries, where almost
every spot to which moisture is supplied teems with  Animal and  Vege-
table life.  And  the  alteration  of Winter and Summer  in temperate
climates, may be almost  said to bring under our own view the opposite
conditions of those two extreme cases.   The effect of the Avithdrawal of
Heat is most obvious  in the Vegetable kingdom; since all its operations
are dependent upon a certain supply of  that agent;  and in no case are
Plants possessed of the power of generating  that  supply within  them-
selves,—excepting in certain organs which do not impart it to the rest
of the structure.  When the temperature of the air falls to the freezing-
point, therefore, wre  find all the operations of the Vegetable economy
undergoing a complete suspension ; yet a very trifling  rise will produce
a reneAval of  them.    It is  not  only in Evergreens, that the vital  pro-
cesses continue to be performed to a certain extent during the winter ;
for there is abundant evidence  that, even in the trunk and branches of
trees unclothed Avith leaves, a circulation of sap takes place, whenever
there is even a slight return of  warmth.   In  this manner, the leaf-buds
are gradually prepared during  the milder days of winter, so as to be
ready to  start forth into full development, with  the returning steady
warmth of spring.
  99. The influence of Heat upon  Vegetation is easily made apparent
by experiment; in fact experimental illustrations of it,  on a large scale,
are daily in progress. _ For the Gardener, by artificial warmth, is not
only enabled to rear with success the plants of tropical climates,  whose
constitution would not bear the chilling influence of our winter ;—but
he can also, in some degree, invert the order of the seasons, and produce
both blossom and fruit  from the plants of our own country, when all
around  seems dead.  This  process  of forcing, however, is unfavorable
to the health and  prolonged  existence of the plants  subjected  to it ;
since the period  of repose, which is natural to them, is interrupted; and
they are caused, as  it were, to live too  fast.   The  same result occurs, 
INFLUENCE OF  HEAT  ON PLANTS.
73
when a plant or tree of temperate climates is transported to the tropics.
Within a very short period after one crop of leaves has fallen off, a new
one makes its appearance.  This goes through all its changes of develop-
ment and decay more rapidly than it Avould do in its native clime; and
in its turn falls off, and is speedily succeeded by another.  Hence the
fruit-trees of this country, transported to the East or West Indies, bear
abundant crops of leaves,—three, perhaps, in one year, or five in two
years,—but little or no fruit; and the  period of their existence is  much
shortened.
   100. As Plants are almost wholly dependent upon the temperature of
the surrounding  medium for the supply of  Heat necessary for  their
growth, many regions must have been devoid of Vegetable life altogether;
if there were  not a remarkable adaptation, in the wants of different
species, to the various degrees of temperature of the  habitations prepared
for them.  Thus we see the Cacti and Euphorbise attaching themselves
to the surface of the most arid rocks of tropical regions, luxuriating, as
it would seem,  in the full glare of the vertical  sun, and laying up a store
of moisture from the periodical rains, of which even  a long-continued
drought is not sufficient to deprive them.   The Orchideous tribe, on the
other hand, whose greatest development occurs  in  the same zone, find
their congenial habitation in the depths of the  tangled forests, where,
Avith scarcely an inferior amount of heat, they have the adA^antage of a
moister atmosphere, caused by the exhalations  of the trees on Avhich they
cling.   The majestic Tree-Fern, again, reaches  its full development in
insular situations;  where, with a moist atmosphere,  it  can secure  a
greater equability of temperature than is to be met with in the interior
of  the vast  tropical continents.  None of these  races  can   develope
themselves elsewhere to their full extent at least,  unless  their natural
conditions of growth are imitated  as far as possible ;  and in proportion
as this imitation can be made complete, in that proportion may the  plant
of the tropics be successfully reared in temperate regions.
   101. There are some  examples of the adaptation of particular forms
of Vegetable life to extremes of temperature,  which are interesting as
showing the extent to Avhich this adaptation  may  be  carried.   In hot
springs near a river of Louisiana, of the temperature of from 122° to
145°, there have  been seen to grow, not merely Confervse and herbaceous
plants, but shrubs  and trees; and a hot-spring in  the Manilla Islands,
Avhich raises the thermometer to 187°,  has  plants flourishing in it, and
on its borders.   A species of Chara has been  found growing and repro-
ducing itself in one of the hot-springs of Iceland, which boiled an egg in
four minutes; various Confervse, &c, have  been observed in the boiling-
springs of Arabia and the Cape of Good Hope  ; and at  the island of New
Amsterdam, there is a  mud-spring, which, though  hotter  than boiling-
water, gives birth to a species of  Liverwort.   On the other hand,  there
are some forms of Vegetation, which seem to luxuriate in degrees of cold,
that are fatal  to most  others.  Thus  the  Lichen, which serves as the
winter food of the Reindeer, spreads itself over the ground whilst thickly
covered with snow; and the beautiful little Protococcus nivalis, or Red
Snow, reddens extensive tracts in the Arctic regions, where the perpetual 
74
EXTERNAL  CONDITIONS OF VITAL  ACTIVITY.
frost of the surface scarcely yields to the influence of the solar rays  at
Midsummer.
   102.  It is, for the most part, among  the Cryptogamic tribes,—the
Ferns,  Mosses,  Liverworts,  Fungi, and Lichens,—that the greatest
power of growing under a low temperature  exists; and we  accordingly
find that the proportion  of these to the Phanerogamia, or Flowering
Plants,  increases as we  proceed from the Equator towards the  Poles.
It has been estimated by Humboldt, that, in Tropical regions, the num-
ber of species of Cryptogamia is only about one-tenth that of the Flow-
ering  Plants;  in the part of the  Temperate zone which lies between
lat. 45° and  52°, the  proportion rises  to one-half;  and the relative
amount gradually increases as we proceed towards the  Poles, until, be-
tween lat. 67° and 70°,  the number of species  of Cryptogamia  equals
that of the  Phanerogamia.  Among the Flowering Plants,  moreover,
the greatest endurance of cold is to be found in those,  which approach
most nearly to the Cryptogamia in the low degree of their development;
thus the Glumaceous group of Endogens,  including the Grasses, Rushes,
and Sedges, which  forms about  one-eleventh of  the whole  amount  of
Phanerogamic vegetation in the Tropics, constitutes one-fourth of  it  in
the Temperate regions, and one-third in the Polar; and the ratio of the
Gymnospermic group of Exogens, which chiefly consists of the Pine
and Fir tribe, increases  in like manner.  Still the influence  of a high
temperature is evident even upon the Cryptogamia and their allies; for
it is only under the influence of the light  and warmth of tropical climes,
that the Ferns,—the highest among the former,—can develope a  woody
stem, and assume the character  of trees; and it is only there that the
tall Sugar-Canes, and the gigantic Bamboos, which are but  Grasses on
a large  scale, can flourish.
   103.  It appears, then, that to every species of Vegetable there is a
temperature which  is most  congenial,  from  its  producing the most
favorable influence on its general  vital actions.   There is a considera-
ble difference between the power of growing and of flourishing,  at a
given temperature.   We may lower the heat of a plant to such a degree,
as to allow it to  continue  to live;  yet its condition will be  unhealthy.
It absorbs food from the earth and air, but cannot  assimilate  and con-
vert it.   Its tissue grows, but becomes distended  with water, instead  of
being  rendered firm  by solid deposits.   The usual secretions are not
formed; flavor,  sweetness, and  nutritive matter,  are each  diminished;
and the power of flowering and producing fruit is lost.   We see a diffe-
rence in the amount of heat required for  the vegetating processes, even
in the various species indigenous to our own climate; thus the common
Chickweed and Groundsel evidently grow readily at a temperature but
little above the freezing  point, Avhilst the Nettles, Mallows, and  other
weeds around them remain torpid.  But the difference is  much more
strongly marked in  the  vegetation of different climates;  showing an
evident  adaptation  of the tribes  indigenous to  each, to that range of
temperature which they will there experience.  Instead of being scantily
supplied with such of the tropical  plants as could  support a  stunted and
precarious life in ungenial climates, the temperate regions  are stocked
with a multitude of vegetables which appear to be constructed expressly 
INFLUENCE OF  HEAT ON PLANTS.
75
for them ; inasmuch as these species  can no more flourish at the Equa-
tor, than the equatorial species can in these Temperate regions.  And
such new supplies,  adapted  to  new conditions, recur perpetually as we
advance towards the  apparently frozen and untenantable regions in the
neighborhood of the Pole.  Every zone has  its peculiar  vegetables:
and while Ave miss some, we find others making their appearance, as if
to replace those which are absent.
   104. Thus in  the  countries  lying  near  the Equator, the vegetation
consists in great part of  dense forests of leafy evergreen trees, Palms,
Bamboos, and Tree-Ferns, bound  together  by  clustering  Orchidge  and
strong creepers  of various kinds.   There are no verdant meadows, such
as form  the chief beauty of  our temperate regions; and the lower orders
of  Vegetation are  extremely rare.  It is  only in this torrid zone that
Dates,  Coffee,  Cocoa, Bread-fruit, Bananas, Cinnamon,  Cloves,  Nut-
megs, Pepper,  Myrrh,  Indigo, Ebony, Logwood, Teak, Sandal-wood
and many others of the vegetable  products most highly  valued for their
flavor,  their odor, their  color, or their  density, come to full  perfec-
tion.  As we recede from  the  Equator, we find the leafy evergreens
giving  place  to  trees with deciduous  leaves; rich  meadows  appear,
abounding with  tender herbs;  the  Orchidae no longer find in the atmo-
sphere, and on the surface  of the trees over Avhich they cluster, a suffi-
ciency of moisture for their support,  and the  parasitic species are re-
placed by others which  grow from  fleshy roots implanted in the  soil;
but aged trunks, are now clothed with Mosses :  decayed vegetables are
covered Avith parasitical  Fungi; and the waters abound with Confervse.
In  the  Avarmer  parts of  the  temperate regions, the Apricot,  Citron,
Orange, Lemon, Peach,  Fig, Vine, Olive, and Pomegranate, the Myrtle,
Cedar,  Cypress, and Dwarf Palm, find their congenial abode.  These
give place as we pass  northwards, to  the  Apple,  the Plum, and the
Cherry,  the Chestnut, the Oak, the Elm, and the Beech.  Going fur-
ther still, we find that the fruit trees are unable to flourish, but the tim-
ber-trees maintain  their  ground.   Where these last fail,  we meet with
extensive forests  of the various  species of Firs ; the Dwarf Birches and
WilloAV  replace the  larger species  of the same kind ;  and even near or
within the  Arctic circle we find large flowers of great  beauty,—the
Mezereon,  the yellow and  white  Water-Lily, and  the  Globe-flower.
Where  none of  these can  flourish, where trees  wholly disappear, and
scarcely any floAvering-plants are to be met with, an humbler Cryptoga-
mic  vegetation still raises its head, in  proof that no part of the Globe is
altogether  unfit for  the residence of human beings, and  that the empire
of Flora has no limit.
  105.  But distance from the Equator is by no means the only  element
in the determination of the mean temperature of a particular spot, and
of the vegetation which  is congenial to it.  Its  height  above the  level
of the sea is equally important;  for  this produces  a variation in the
amount of heat derived  from the  Sun,  at least as  great as that occa-
sioned by difference of latitude.  Thus it  is  not alone on the summits
of Hecla, Mount Blanc, and other mountains  of Arctic or  temperate
regions,  that we find a coating of perpetual  snoAV;  we find a  similar
covering on  the  lofty summits of the  Himalayan chain, which extends 
76           EXTERNAL CONDITIONS OF VITAL ACTIVITY.

to within a few degrees of the Tropic of Cancer ; and even on the higher
peaks of that part of the ridge of the Andes, Avhich lies  immediately
beneath the Equator.  The height of the snow-line beneath the Equator
is betAveen 15,000 and 16,000 feet above the level  of the sea;  on the
south side of the Himalayan ridge it is about  15,500 feet, but on the
north side  it rises to 18,500  feet ;  and in the Swiss Alps it is about
8000 feet.   Its position is  very much affected, however, by  local cir-
cumstances, such as  the neighborhood of a large expanse of land or of
sea; hence  the small  quantity  of land in the Southern Hemisphere,
renders its climate generally so much colder than that of the Northern,
that in SandAvich Land (which is in lat. 59° S.,  or the same parallel as
the  north  of Scotland) the Avhole  country, from  the summits of the
mountains  down  to  the  very  brink of  the sea-cliffs, is  coArered many
fathoms  thick  with  everlasting  snow ;  and in  the  Island  of Georgia
(which is in lat. 54°  S., or in the same parallel as Yorkshire), the limit of
perpetual snow descends  to  the  level of the ocean, the partial melting
in summer only disclosing a feAV rocks, scantily covered with moss and
tufts of grass.   Yet the  highest  mountains of  Scotland, which ascend
to an elevation of nearly 5000 feet, and are four degrees more  distant
from the equator, do not attain the limit of  perpetual  snow; this is
reached, however, by mountains in Norway, at  no greater elevation.
  106.  If, then, Temperature  exert  such an influence on Vegetation as
has  been stated, we ought  to find on the sides of lofty mountains in
tropical regions, the same progressive alterations  in the characters of
the  Plants  that cover them, as Ave  encounter in  journeying from the
equatorial towards the polar regions.   This is actually the case.  The
proportion of Cryptogamia to Flowering Plants, for example, is no more
than one-fifteenth on the plains of the Equatorial region ; whilst it is as
much as one-fifth on  the mountains.   In  ascending the  Peak of Tene-
riffe, Humboldt remarked as  many as  five distinct  zones, wdiich Avere
respectively  marked by the  products  which characterize  different
climates.    Thus at the  base the  vegetation  is  altogether tropical;
the  Date-Palm, Plantain, Sugar-cane, Banyan, the succulent Euphor-
bia, the Dracaena, and other trees and  plants of the torrid zone  there
flourish.  A little higher grow the  Olive, the  Vine, and  other  fruit-
trees of Southern  Europe;  there  Wheat flourishes;   and  there the
ground is  covered with  grassy herbage.   Above  this  is  the Avoody
region, in which are found the Oak, Laurel, Arbutus, and  other  beau-
ful  hardy evergreens.  Next above is the region of Pines ; characterized
by  a vast forest of  trees resembling the Scottish  Fir,  intermixed with
Juniper.  This gives place  to a tract  remarkable for the abundance of
Broom ; and at last  the  scenery is terminated by Scrofularia, Viola, a
few Grasses, and Cryptogamic plants, which  extend to  the borders of
the perpetual snow that caps the summit of the mountain.
  107. The effects of  Temperature  on Vegetation  are not  only seen in
its  influence upon the Geographical distribution of Plants, that  is, in
the limitation of particular species to  particular climates;  for they are
shoAvn, perhaps even more remarkably, in the variation in the size of
individuals of  the same species ;  when that species possesses the power
of adapting itself to widely different conditions, which is the case with 
INFLUENCE OF  HEAT  ON PLANTS.
77
some.   Thus the Cerasus  Virginiana groAvs in the Southern States  of
North America as a noble tree, attaining  one  hundred feet in height;
in the sandy plains of the Saskatchawan, it does not exceed twenty feet;
and at its  northern  limit,  the  Great Slave Lake, in lat.  62°, it is re-
duced to a shrub of five  feet.  Another curious effect of heat  is'shown
in its  influence  on the sexes of  certain Monoecious flowers; thus  Mr.
Knight mentions that Cucumber and  Melon plants  will produce none
but male or staminiferous floAvers, if their  vegetation be accelerated by
heat;  and all female or pistilliferous, if its  progress be retarded by cold.
   108. The injurious  influence of excessive  Heat can be, to a certain
extent, resisted by Plants, through the cooling process  kept up by the
continual evaporation  of moisture from their surface.   But the  power
of maintaining this cooling process entirely depends upon the supply of
fluid, with which the plant is furnished.   If the supply be adequate  to
the demand, the effect of  heat will be to stimulate all the vital  opera-
tions of the plant, and to  cause  them to be performed with increased
energy; though, as we have already seen, this energy may be such  as
to occasion a premature exhaustion in its powers, by the excessive luxu-
riance which it occasions.   But if the supply of water  be deficient, the
plant is  burnt  up by the continuance of heat in a dry atmosphere;  and
it^ either withers or  dies, or its tissues become dense and contracted,
without losing  their vitality.  Thus it has been remarked, that shrubs
groAving among the  sandy deserts of the  East,  have as stunted an ap-
pearance as those attempting to  vegetate  in the Arctic  regions; their
leaves being converted into prickles, and their leaf-buds prolonged  into
thorns instead of branches.—The influence of excessive heat in destroy-
ing life, can sometimes be traced through the direct physical changes
which it occasions in the  vegetable tissues.   Thus it has been  ascer-
tained that grains of corn Avill  vegetate  after exposure  to Avater or
vapor possessing a considerable degree of  heat; provided that heat do
not amount to  144° in the case  of water, and 167° in that of vapor.
At these temperatures, the structure  of the seed undergoes a disorga-
nizing change,  by the  rupture of the  vesicles  of  starch which form a
large part of it;  and the loss of  its  power of germinating is therefore
readily accounted for.   The highest temperature which  the soil usually
possesses in tropical  climates, is about 126°, though Humboldt has once
observed the thermometer rise to  140°.  Seeds imbedded in such  a  soil,
therefore, may  not lose their vitality, although they will not germinate
in such temperatures.   The temperature most favorable to germination
probably varies in different species, and is one of the  conditions  that
produces their  adaptation  to different climates.  Thus  it  appears  that
Corn will not  germinate in water at a higher  temperature than  95°,
whilst Maize wHl germinate in water at 113°; and,  as  is  well known,
Maize will flourish in countries in which Corn cannot be grown.
  109. We must not confound the power which Plants possess of vege-
tating, or exhibiting  vital activity,  under widely-different  degrees of
temperature,  with the power of  retaining their vitality in  a dormant
condition, which many of them  possess in a very remarkable degree.
When  the external temperature  is much below  the freezing-point,  it is
impossible  that any vegetating processes  can go  on;  since the  Plant 
78
EXTERNAL  CONDITIONS  OF VITAL ACTIVITY.
does not possess the power of generating heat within itself.  Now such
a complete cessation of activity is quite compatible, in  many instances,
with the preservation of the organized structure in a condition perfectly
unchanged, and, in consequence, with the continuance of its peculiar
properties; so that these properties may be again called into operation,
when the temperature  shall have risen.  But in  other  cases, the plant
may be killed by the intensity of the cold ;  that is, the return of warmth
will  not excite it  to activity.   We have  occasion to notice, in every
severe winter, the  difference in this  respect  amongst the  plants which
are cultivated in our own  climate; some of them  being  killed by a hard
frost, the effects of Avhich  are resisted by others, even though their situa-
tion be more exposed.   In general it will be  found, that the cold  acts
most powerfully (as might be expected) upon plants which are not indi-
genous to our country, but Avhich have been introduced and naturalized
from some warmer regions.   But it is worthy of note, amongst other pe-
culiarities in the relation of Heat and Vegetation, that  many plants are
readily killed by a low temperature, which  yet flourish Avell under a very
moderate amount  of warmth ; so that they will grow in  situations where
the mean temperature of the year is low and the summers cool, provided
the  winters are not severe ; whilst  they cannot  be preserved  without
special protection, in situations where the Avinters are colder, even though
the summers should be much hotter, and the mean  temperature of the
whole year should be considerably higher.   Thus there  are shrubs grow-
ing in the Botanic Garden of Edinburgh, which cannot be safely left in
the open air in the neighborhood of London, and which would  be most
certainly killed by the winter-cold of Central France.
   110. It does not admit of doubt, that  the destructive influence  of a
very low temperature upon the Vitality of Plants, is immediately exerted
through its chemical and  physical effects upon the tissues and their con-
tents.  Thus it will produce congelation of their fluids; and the expan-
sion which takes place in  freezing will injure  the walls of the containing
cells,—distending, lacerating, or even bursting them.  The same cause
will probably occasion the expulsion of air from some parts which ought
to contain it; and the  introduction of it into other parts which ought to
be filled with  fluid.  And  a separation will take place, in the act of
freezing, between  the  constituent parts of the vegetable juices; Avhich
will render them unfit for discharging  their  functions, when returning
warmth would  otherwise call them into activity.   Hence we are enabled
in some degree to account for the differences in the power of resisting
cold, which the various species of Plants, and even  the various parts of
the same  individual,  are  found to possess.  For, other things being
equal, the power of each  plant, and of each part  of the plant, to resist a
low temperature,  will  be in the inverse ratio of the quantity of water
contained in the tissue; thus a succulent herbaceous plant suffers  more
than one  with a  hardy Avoody  stem and  dense  secretions; and young
shoots are destroyed by a  degree of cold, which does not affect old shoots
and branches of the same shrub or tree.   Again, the viscidity of the
fluids of some  plants is an  obstacle to their  congelation, and  therefore
enables them to resist cold; thus it is, that  the resinous Pines are,  of
all trees, those Avhich  can endure the lowest temperature.  The dimen- 
INFLUENCE OF  HEAT  ON PLANTS.                79
sions of the cells, too, of which the tissue is composed, appear to have
an influence; the liability to  freeze being diminished by a very minute
subdivision of the fluids.   And when the roots  are  implanted deep in
the soil, where the'temperature does not fall by many degrees so  low as
that of the surface, the fluidity of the sap may be maintained, in spite
of an extremely cold state of the atmosphere.
   111. It is in Cryptogamic plants, that the greatest power of sustain-
ing Cold exists ; as might be inferred from what has been already stated
in regard to their geographical distribution.  The little Fungus (Torula
Cerevisice) which is one of the principal constituents of Yeast, does not
lose its vitality by exposure to a temperature of 76° below zero ; though
it  requires a somewhat  elevated temperature for its active growth.  It
would  appear  that Seeds are enabled to sustain  a degree of cold, without
the loss of their vitality, which would be fatal  to growing plants  of the
same species ; thus grains of corn, of various kinds, will germinate after
being exposed  for a quarter  of an  hour to a temperature equal  to that
of frozen mercury.   It is not difficult  to account for this, when the close-
ness of their texture, and the small quantity of fluid  which it includes,
are kept in view.  The act of Germination, however, will only take place
under  a rather elevated  temperature; and we  find  in the Chemical
changes  Avhich it involves, a provision for maintaining this, when the
process has  once commenced.
   112.  The influence of Heat upon the vital activity of Animals, is quite
as strongly marked as we have seen it to  be in the case of Plants; but
the mode in which it is  exerted is in many instances very different.  In
those animals  which are endowed  with great energy of muscular move-
ment, and  in  which, for the  maintenance of that energy, the nutritive
functions are kept in constant activity, we  find  that a provision exists
for the development of  heat from within, so as  to keep the temperature
of the  body at a certain uniform standard, Avhatever may be the climate
in which they  live.   Their energy and activity are, in  fact, so dependent
upon the steady maintenance of a high temperature in their bodies, that,
if this  be not kept up nearly to its regular standard,  a diminution  or
even a complete cessation  of vital action  takes place, and even a total
loss of vitality may result.   In these warm-blooded animals, as they are
termed, we do not so evidently trace the effects of Heat, because they
are constantly  being  exerted, and  because  external changes have but
little influence upon them,  unless these changes are of an extreme kind.
But if those internal operations, on Avhich the   maintenance of the tem-
perature is dependent, are from any  cause  retarded or suspended, the
effect is  immediately visible, in  the  depressed activity of the  whole
system.  In the class of  Birds, whose muscular energy, and whose ge-
neral functional activity, are greater and more constant than those of
any other animals, the temperature is pretty steadily maintained at from
108° to 112°;  and we shall presently see, that  a depression of the heat
of the body to  about 80° is fatal.   Among Mammalia, the temperature
is usually maintained at from 98° to 102°; and  it seems that in them
too a depression of about  thirty degrees is ordinarily fatal.
  113. In the different tribes of Birds and Mammals, we find a very
diversified poA\Ter of generating heat;  and on this depends their adapta- 
80           EXTERNAL CONDITIONS OF VITAL ACTIVITY.
tion to various climates.  Where  the  usual  temperature of the atmo-
sphere is but little below the  normal  standard of the body, a small
amount of the internal  calorifying power is required; and accordingly
we find that animals Avhich naturally inhabit the torrid zone, cannot be
kept alive elseAvhere, except, like the Plants of the same regions, by ex-
ternal heat.   On  the other hand,  the animals of the  colder temperate
and frigid  climes are endowed with a much greater internal calorifying
power;  and their  covering is adapted  to  keep in the  heat which they
generate.  Such animals (the Polar  Bear for example) cannot be kept
in health, in  the  summer of our own  country, unless  means are taken
for their refrigeration.   The constitution of Man seems to acquire, by
habitation to a particular set  of conditions through successive genera-
tions, an adaptation  to differences  of climate, of which  that of few other
animals is susceptible ; and thus we find different races of human beings
inhabiting countries, which are subject to the extremes  of heat and cold.
The Hindoo or the Negro, suddenly transported to Labrador or Siberia
during the depth of  winter, would probably sink in the course of a few
days, from want of poAver to generate within his body a sufficient amount
of heat,  to resist  the depressing influence of  the external  cold; whilst
on the  other hand, the Esquimaux, suddenly conveyed to the  hottest
parts of India or Africa, would speedily become the subject of disease,
which would probably terminate his life  in a short time.   It is in the
inhabitant of temperate climates,  who  is naturally exposed during the
seasonal changes of  his year, to a wide range of external temperature,
that we find the greatest power of sustaining the extremes of either cold
or heat; and yet, even in such, the continued exposure to either extreme,
during a long series of years, will so much influence the heat-producing
power, that the constitution does not adapt itself readily to a change of
conditions.
   114. We see, then, that  the  variations observable between different
races in  this respect, are only exaggerations (so to speak)  of the alter-
nations which an individual may undergo in the course of  a few years;
and it is easy to understand how such an  adaptation may take place to
an increased  extent  in  successive  generations;—this being the regular
law, not  merely in regard to Man,  but in regard to other animals placed
under new conditions, to which they have a certain,  but limited, power
of adapting themselves.  Thus  we find  that a European, who has lived
for several years in  the East or West Indies,  suffers considerably from
the cold, wThen he first returns to winter in his native country; his con-
stitution having for a time, lost some of its power of generating heat.
After a few years' residence, however, this power is commonly recovered
to its original extent,  unless the  age  of  the  individual be too far ad-
vanced ;  but his children, if they have been not  only born,  but brought
up, in the hotter climate, experience much greater difficulty in adapting
themselves to the colder one.
   115.  The conditions on which the power of maintaining the heat of
the body, in despite of external cold, is dependent, will become the sub-
ject of inquiry hereafter (chap. x.).   It is sufficient here to state, that
this power is the result of numerous  Chemical changes going on within
the body ; and especially of a process analogous to combustion, in which 
INFLUENCE OF  HEAT  ON ANIMALS.
81
 carbon and hydrogen, taken in as food are made  to  unite with oxygen
 derived  from the  atmosphere.    It  is dependent, therefore, as to its
 amount, upon the due supply of the  combustible  material  on the one
 hand, and of  atmospheric air on the other.  If the former be not fur-
 nished either by the food, or by the fatty matter of the body (which
 acts as a kind of reserved store laid  up against the  time of need), the
 heat cannot be maintained; and it is in part for want of power to digest
 and assimilate a sufficient amount of this kind of  aliment, that animals
 of  warm climates cannot maintain their temperature in colder regions.
 On the other  hand, if the supply of oxygen be deficient, as it is when
 the respiration is impeded by  diseased conditions of various kinds, there
 is a similar depression of temperature.
   116. Noav if, from either of these causes, the temperature of the body
 of a  Bird or Mammal (except in the case of the hybernating  species of
 the  latter, to be presently noticed) be loAvered to about  30° below its
 usual standard,  not only is there a cessation of vital activity, but a total
 loss  of vital  properties; in other words, the death  of the animal is a
 necessary result.   This occurrence is preceded by a gradually-increasing
 torpidity; which shoAvs the depressing influence of the cooling  process
 upon the functions  in general.   The temperature of the superficial parts
 of the body is, of course, first affected ; the circulation is at first retarded,
 causing lividity  of  the skin; but as the temperature becomes lower, the
 blood is almost entirely expelled from  the surface  by the contraction of
 the vessels, and paleness succeeds.  At the same time there is a gradu-
 ally-increasing torpor of the nervous and muscular systems, which first
 manifests itself in an indisposition to exertion of any kind, and  then in
 an almost irresistible tendency to sleep.  At the same  time the respi-
 ratory movements become slower, from  the want  of  the  stimulus that
 should be given by the warm current of blood to the Medulla Oblongata,
 which is the centre of those movements; and the  loss of heat goes on,
 therefore, with increased rapidity, until the temperature of the Avhole
 body is so depressed, that its vitality is altogether  destroyed.
   117. But when there is a deficiency of the proper animal heat, the
 vital activity of  the system may be maintained by caloric  applied by
 external sources.  This fact is of high scientific value, as giving the most
 complete demonstration of the immediate dependence of the vital func-
 tions  of Avarm-blooded animals  upon a sustained  temperature;  and its
 practical importance can  scarcely be overrated.  It  rests  chiefly upon
 the recent experiments of Chossat, who had in view to determine  the
 circumstances attending  death by Inanition, or starvation.  He found
 that,  Avhen Pigeons were entirely deprived of food  and water, their
 average temperature underwent a tolerably regular diminution  from day
 to day; so that after several days (the  exact number varying with their
 previous condition), it Avas about 4|° lower  than at  first.  Up  to this
 time,  it seems  that  the  store  of fat laid up in the  body supplies the
 requisite material for the combustive process ;  so that no very injurious
 depression of  temperature occurs.  But, as soon as  this is exhausted,
 the temperature falls rapidly from hour to hour;  and as soon  as the
total depression has reached 29J° or 30°, death supervenes.  ^Yet it was
found  by M.  Chossat, that when  animals  thus  reduced by starvation, 
82
EXTERNAL CONDITIONS OF VITAL ACTIVITY.
whose death seemed impending (death actually taking place  in many
instances whilst the preliminary processes of weighing, the application
of the thermometer, &c, were being performed), were subjected to artifi-
cial heat, they were almost uniformly restored, from a state of insensi-
bility and want of muscular poAver, to a condition of comparative activity.
Their temperature rose, their muscular  power returned, they took food
when it Avas presented to them, and their secretions Avere reneAved ; and, if
this artificial assistance Avas sufficiently prolonged, and they were supplied
with food, they recovered.    If the heat was withdrawn, howrever, before
the time when the digested food was  ready  in  sufficient amount, to
supply the combustive  process, they still sank for want of it.
   118. Various important  practical hints may be derived from the con-
sideration of these facts.  There can be no doubt that, in many diseases
of exhaustion, the want of  power to sustain the requisite  temperature,
is the immediate cause of death ; the whole combustible  material of the
body  having been exhausted and the digestive apparatus not being able
to supply Avhat is required.  Now Avhere  this  is the  case, there is  no
doubt that  life may be  prolonged, and  that recovery may be  favored,
by the judicious sustentation of the temperature of the body.  Thi3 may
be effected either by internal or by external means.   Of the internal, the
most  efficient is  undoubtedly the administration  of  Alcoholic fluids ;
which, for  reasons hereafter to be given (§ 495), will  be absorbed into
the circulating system, Avhen no other alimentary substance can be taken
in ; and which, moreover, exert a favorable influence by their specific
stimulating effect upon the  nervous system.  It is a matter of familiar
experience, that, in such conditions  of the body, the quantity of alcohol
which may be administered with positive and evident benefit, is such as
would in  ordinary circumstances be  productive of the most  injurious
results and this is fully accounted for by the reflection, that it is burnt
off as fast as it is taken in.  But a most  important adjunct in all such
cases,—and in many instances a substitute  for alcohol when the latter
would be inadmissible,—will be found in the application of external heat;
and especially in the subjection of the whole surface to its  influence, by
means of the hot-air bath.  This is  a valuable portion of the treatment,
in  the recovering of persons Avho have been  reduced to  insensibility by
suffocation  of  any kind; and especially in cases of drowning, since the
heat of the body is rapidly withdraAvn by the conducting power of the
water.  Indeed it may be stated as a general rule, that, where the tem-
perature of the body is lowered from any cause, external  heat may be
advantageously applied ; and much evidence has lately been produced to
shoAV, that the reparative processes by which extensive Avounds are healed
go on more favorably  under the contact of warm  dry air, than with
any other application.
   119. On the other hand  where the object is to keep down a tendency
to a  too violent action, the local application of moderate cold is found
to be of the greatest value ; all surgeons of eminence being now agreed
upon  the efficacy of water-dressing in restraining the inflammatory pro-
cess, especially in cases of  wounds of the joints in  Avhich this  action  is
most to be apprehended.  The general application of cold to the surface,
by means of continued  exposure to cool air,  or by a short  immersion  in 
INFLUENCE  OF HEAT  ON ANIMALS.               83
cold Avater, is frequently in the highest degree beneficial, by imparting
tone to the  system, i. e., by producing a firmer condition  in the solids
which were previously relaxed, and more especially by calling into action
the tonicity of the walls of the blood-vessels, Avhich imparts to them an
increased resistance,  and thus  favors  the  regular and vigorous circu-
lation of blood, upon  principles Avhich will  be hereafter stated (§ 609).
But so far from producing any permanent depression in the temperature
of the body, this measure has a tendency to elevate it, by the increased
vigor it  produces in  the circulation; hence the  gloAv Avhich  is experi-
enced after the use of the cold bath.  If this effect be not produced, and
a chilling of the body, instead of  an invigorating warmth, be  the result
of the use of cold, it is evident that this cannot be beneficial.  The inju-
rious results of the too-prolonged application of even a  moderate degree
of cold, are  seen in the depression of temperature, without a correspond-
ing reaction, Avhich is the consequence  of an immersion in water of 50°
or 55° prolonged for several hours ; and still more in that chilling of the
whole  surface, frequently productive of the most  serious consequences,
which arises from the  evaporation of fluid from garments that have been
moistened, either by perspiration  from Avithin, or by the fall  of rain or
deAV upon their  exterior.   There is no doubt that the obstruction to the
continuance  of  the perspiration, presented by a covering already satu-
rated Avith moisture, is one cause of the injurious results  that so com-
monly follow such an  occurrence; but there is as little doubt that  the
chilling influence of the external  evaporation has a large share in pro-
ducing  them.  For experience  shows that, if the evaporation  be pre-
vented by an impenetrable covering, the contact of a garment thoroughly
saturated with moisture is not productive of the same  injurious conse-
quences.
   120.  The practical  importance of the due comprehension of the prin-
ciples, upon  which Heat and Cold should be employed,  in the  treatment
of disease and the preservation of health,  has required this digression.
We noAv proceed to consider the influence of temperature upon a certain
group of warm-blooded animals; which offers a remarkable peculiarity
in this respect,—their poAver of generating heat being for a time greatly
diminished or almost completely suspended;  the temperature of their
bodies following that of the air around, so that it may be brought down
nearly to the freezing-point;  their general vital actions being carried
on with such feebleness as to be scarcely perceptible;  and yet the vital
properties of the tissues being retained, so  that, when  the temperature
of the body is again raised, the usual activity returns.   This state, which
is called hybernation,  appears to be  as natural to certain animals,  as
sleep is to all, and it corresponds with sleep in its tendency to  periodical
return.
  121.  No account can be given of the causes to which it is due;  but
the condition of the animals presenting it offers several  points of much
interest.  There are some, as the Lagomys, in which it appears to differ
but little from deep ordinary sleep; they retire into situations which
favor the retention of their warmth; and  they occasionally  wake up,
and apply themselves to some of the store of food, which they have pro-
vided in the autumn.   In  other cases, a great accumulation of fat takes 
84           EXTERNAL CONDITIONS OF VITAL  ACTIVITY.

place within  the  body in autumn,  favored by  the oily nature  of the
seeds, nuts,  &c, on Avhich the  animals then feed;  and this servesthe
purpose of maintaining the temperature for a sufficient length of time,
not indeed to the usual  standard, but to one  not  far below it.  The
state of torpor in these animals is more profound than that of deep sleep,
but it is not such as to  prevent them  from  being easily aroused; and
their respiratory movements, though diminished in frequency, are still
performed without interruption.   But in the Marmot, and in animals
Avhich like it, hybernate completely, the temperature of the body (owing
to the want of internal  power to generate heat) and the  general  vital
activity, are proportionally depressed; the respiratory movements fall
from 500 to 14 per hour,  and are performed Avithout any  considerable
enlargement of chest; the pulse sinks from 150  to 15 beats per minute;
the state of torpidity  is so profound, that the animal is Avith difficulty
aroused from it; and the heat of the body is almost  entirely dependent
upon  the  temperature of the surrounding air,  not being  usually more
than a degree or  two  above it.  When the  thermometer  in the air is
somewhat below the freezing-point, that placed  Avithin the  body falls to
about 35°;  at this point it may  remain for  some  time,   Avithout any
apparent injury to the animal, Avhich revives when subjected to a higher
temperature.   When, however,  the body is exposed  to a  more  intense
degree of cold, the animal functions undergo a  temporary reneAval; for
the cold seems to act like  any  other stimulus in  arousing them.  The
respiratory movements and the  circulation increase in activity, so  as to
generate an increased amount of heat;  but this  amount is  insufficient to
keep up the  temperature of the  body, which is at last depressed  to a
degree inconsistent with the maintenance of life; and not  only the sus-
pension of activity, but the total loss of vital properties, is the result.
   122.  Now  the condition of a hybernating Mammal closely resembles
that of a cold-blooded animal, in regard to the  dependence of its bodily
temperature upon external  conditions.   There is  this important  diffe-
rence, however;—that the reduction of the  temperature  of the former
to 60° or 50° is incompatible with a state of activity, Avhich is only exhi-
bited when the temperature rises to nearly the usual Mammalian standard;
—whilst a permanently low or  moderate temperature is natural to the
bodies of most cold-blooded animals, whose functions could  not  be Avell
carried  on under  a higher temperature.   Thus all the muscles of a  Frog
are thrown into  a state  of permanent  and rigid contraction, by the
immersion of its body in water no warmer that the blood Avhich naturally
bathes those  of the Bird; and we find, accordingly, that cold-blooded
animals which cannot sustain a high temperature, are provided writh a
frigorifying  rather than  with  a  calorifying apparatus.   Although  we
are accustomed to rank all animals, save Birds and Mammals, under the
general term cold-blooded, yet there exist among them considerable diver-
sities as to the power of generating heat  within themselves, and of thus
rendering themselves independent of external  variations.   Thus among
Reptiles, it appears that there are some which can sustain  a temperature
several degrees above that of the atmosphere, especially when the latter
is sinking; and among  Fishes, it is certain that there are  species,—the
Shunny and Bonito  for  example,—which  are almost  entitled to the 
INFLUENCE OF HEAT  ON ANIMALS.
85
name of Avarm-blooded animals, their temperature being kept up to nearly
100°, when that of the sea is about 80°.  It is uncertain, hoAvever, to
Avhat extent it Avould be depressed, by a lowering of that of the sur-
rounding  medium.   The  greatest  power  of  developing  heat in cold-
blooded animals appears to exist, Avhen their bodies are reduced nearly
to the freezing-point;  and Avhen that of the surrounding air or water is
much below it.  Thus Frogs have  been found alive in the midst of ice
whose temperature Avas as Ioav as 9°, the heat of their own bodies being
33° ;  and it has been observed that even Animalcules contained in water
that is being frozen, are not at once destroyed, but that each lives for
a time in  a small uncongealed space, where the fluid  seems to  be kept
from solidifying, by the caloric liberated from the  Animalcule.
   123. The  peculiar condition of the class of Insects, in regard to its
heat-producing poAver, exhibits in a very striking manner the  connexion
between  an  elevated  temperature and vital activity.  In the  Larva
state of Insects, the temperature of the animal follows closely that  of
the  surrounding  air, as in the cold-blooded classes generally  ; but it is
usually from J to 4° above it.   In  the Pupa  condition, which is one of
absolute rest  in  most  insects that undergo a complete metamorphosis*
the  temperature  scarcely rises above  that of the  surrounding medium;
except at nearly the  close of the period, when it is about to burst its
envelopes  and come  forth  as the perfect Insect.   The  temperature
which different Insects  possess in  their Lmago state, varies in part ac-
cording to the species, and in part Avith the condition of the individual
in regard  to rest or activity;  but  the same principle is evidently ope-
rating in  both  cases,  since  the variation existing  amongst different
species, in regard to their  heat-producing power, is closely  connected
with  the  amount of activity natural to them.  The highest  amount is
to be found in the industrious Hive-Bee and its allies, and in the  elegant
and sportive Butterflies, which are almost constantly on  the Aving in
search of food; next  to these come the Beetles of active  flight; and
lastly  those  which seldom  or never raise  themselves  upon the wing,
but pursue their labors on the ground.  The temperature of individual
Bees has been found to be about 4° above that of the atmosphere, when
they are in a state of repose ;  but it rises  to 10° or 15°, Avhen they are
excited to activity.  When  they are  aggregated  together in clusters,
however, the temperature Avhich they possess is often as much  as 40°
above  that of the atmosphere.   When reduced  to  torpidity by cold,
they still  generate heat enough to keep them from being frozen,'unless
the cold be very severe ; and they may be aroused by moderate excitement
to  a state of activity, in which  the temperature rises to a very con-
siderable  elevation.  Noav although the increased  production of  heat is
in these cases, as in hybernating Mammals similarly aroused, the conse-
quence of  the increased activity, there can be no  question that it  is a
condition  necessary to the  continuance of  that activity;  since we  find
that, if the temperature of the body be again reduced by external cold,
the activity cannot be long maintained.
  124. Whilst the foregoing facts exhibit the  connexion between an
elevated temperature,  and the  most  active condition of the  muscular
and  nervous  systems,  in  cold-blooded animals,  there  is abundant evi- 
86           EXTERNAL CONDITIONS OF VITAL  ACTIVITY.
dence of the same kind in regard to the influence of Heat upon the pro-
cesses  of nutrition  and  development.   Thus  the  time of emersion of
Insect-larvae from their eggs,—or in other words, the rate at which the
previous formative processes go on, is entirely dependent upon the tem-
perature.  In the case of the Bird, we find  that, if the temperature be
not sufficient  to  develope the  egg, chemical changes soon take place,
which involve the loss of its vitality;  or if the temperature be reduced,
so low  as to prevent the occurrence of those changes, the loss of  heat
is in itself  destructive of life.   But this is not the  case in regard to the
eggs of  cold-blooded animals in  general;  for, like the beings they are
destined to produce, they may be reduced to a  state of complete inac-
tion by a depression of the external temperature ;  Avhilst a slight eleva-
tion of this renews  their vital operations, at a rate corresponding to the
warmth supplied.   Hence the  production  of larvae from the eggs of
Insects may be accelerated or retarded at pleasure; and this is, in  fact,
practised in  the  rearing of Silk-worms, in order to adapt the time of
their emersion from the  egg to  the supply of  food which is ready for
them.   The  same may be said in  regard to  the  eggs of other cold-
blooded  animals ;  those,  for example,  of the minute Entomostracous
Crustacea (Water-Fleas,  &c), which people our  ditches and ponds.   In
many of these, the  race is continued solely by the eggs, which remain
dormant through the winter; all the parents being destroyed by the cold.
The common Daphnia pulex produces two kinds of eggs; from one, the
young  are very speedily hatched; but the others, which are produced in
the autumn, and enveloped in a peculiar covering, do not give birth to
the contained young until the succeeding spring.  They may be at any
time hatched, however, by artificial warmth.
  125.  We sometimes find special provisions for imparting to the  eggs
a temperature beyond that which is natural to the bodies of the parents;
thus it has been shown that in Serpents, the temperature of  the poste-
rior part of the body rises considerably, when the eggs are lying in the
oviduct, preparatory to being discharged,—evidencing  a  special heat-
producing power  in the surrounding parts at this period, which is obvi-
ously for the purpose of aiding  the  maturity of  the  eggs.   The Viper,
whose eggs are frequently hatched in the maternal oviduct, so that the
young  are brought  forth alive,  is occasionally seen basking  in the sun,
in such a position as to receive its strongest heat on the parts that cover
the oviduct.  Certain Birds have recourse to substitutes for the usual
method of incubation.   The Talleyalla of NeAV  Holland is directed by
its remarkable instinct, not to sit upon  its eggs, but  to bring them to
maturity by depositing them in a sort of hot-bed, which it constructs of
decaying vegetable  matter.  The Ostrich is believed to sit upon its eggs,
when the temperature falls beloAv a certain  standard, but to leave them
to the influence of the  solar heat Avhen this is sufficient to bring them
to  maturity;  and this statement derives confirmation from a similar
fact observed in a Fly-catcher, which built in a hot-house during several
successive years,—the bird quitting its eggs when  the  temperature was
high, and resuming its  place when it  fell.  In all  these cases,  as in
many more which might be enumerated, Ave observe the influence of an
elevated  temperature upon the processes of development; and the pro- 
INFLUENCE OF HEAT ON  ANIMALS.
87
visions made by Nature, in the physical  or  mental constitution of ani-
mals, for affording that influence.  The development of heat around the
oviduct of the  Serpent is a process  over which the individual has no
control, being entirely dependent upon certain organic changes ; whilst
the imparting of Avarmth  to its eggs by the Bird, either from its own
body or  through artificial means, is  committed to the guidance of its
Instinct,—which same instinct leads  it to suspend  the process when it
is not  necessary.
  126. Phenomena of an equally interesting and  instructive character
may be observed in the history of the Pupa-state of Insects; which, in
those that undergo a complete metamorphosis, may be almost charac-
terized as a re-entrance into the egg.   In fact we shall obtain the most
correct idea of the nature of that metamorphosis, by considering  the
Larva as an embryo, Avhich comes forth from the egg in a very early and
undeveloped condition, for the sake of obtaining materials for its continued
development, which the egg does not supply in sufficient amount.  When
these have been digested and stored-up in the body, the animal becomes
completely inactive,  so far as regards its  external manifestations of life;
and it forms some kind of envelope for its protection, which may not be
unaptly compared to the  shell or  horny covering  of the egg.  Within
this are gradually developed the wings, legs, and other parts which are
peculiar to  the perfect Insect; whilst even those  organs, which it pos-
sesses in  common with the Larva, are  for the most part completely
altered in character.  When  this process of development is completed
the Insect emerges from its Pupa case, just as the Bird comes forth from
the  egg ;  then only  does its  Insect  life begin, its  previous condition
having been that of a Worm ;  and the alteration of its character is just
as  evident in its instinctive  propensities, as it is in its locomotive and
sensorial powers.
   127. Now this process  of  development is remarkably influenced by
external temperature; being accelerated by genial warmth, and retarded
by  cold.   There are many Larvae,  Avhich naturally pass  into the Pupa
state during the  autumn, remain in  it  during  the entire winter, and
emerge as perfect Insects with the return of spring.  It Avas  found by
Reaumur, that Pupae, Avhich  Avould not  naturally have  been  disclosed
until May, might be caused to undergo their metamorphosis during the
depth of winter, by the influence of artificial heat; whilst, on  the other
hand, their  change might be delayed a whole year beyond its usual
time, by the prolonged influence of  a cold atmosphere.  In order to
hasten the development of the pupae of the Social Bees, a very curious
provision is  made.  There is a certain set, to which the  name of Nurse-
bees has been  given, whose duty it is to  cluster over the cells in wyhich
the Nymphs or Pupae are  lying, and to  communicate the heat to them,
Avhich is developed by the energetic movements of their OAvn bodies,  and
especially by respiratory  actions of extreme rapidity.   The nurse-bees
begin to crowd upon the cells  of the nymphs, about ten or twelve hours
before these last  come forth as perfect Bees.  The incubation (for so it
may be called) is  very assiduously persevered in during this period by
the  Nurse-bees; when one quits its cell, another takes  its place; and
the rapidity of the respiratory movements increases, until they rise to 
88           EXTERNAL  CONDITIONS OF VITAL ACTIVITY.

130 or 140 per minute, so as to generate the greatest  amount of heat
just before the young bees are  liberated from the combs.   In one  in-
stance, the thermometer  introduced among seven nursing-bees stood at
92|° ; the temperature of the  external air being 70°.   We observe in
this  curious propensity a manifest  provision for accelerating the deve-
lopment of the perfect Insect, which requires (as already pointed out) a
higher temperature than  the larva, in virtue of its greater activity.  The
Nurse-bees do not station  themselves over the cells which  are occupied
by the larvae; nor do they incubate the nymph-cells with any degree of
constancy and regularity, until the  process of development is approach-
ing its highest point.
  128.  The influence  of variations in the Heat of the body upon  its
vital activity, is further manifested  by the very remarkable experiments
of Dr. Edwards; who has shown that Cold-blooded animals  live much
faster (so to speak) at high temperatures, than at low ;  so that they die
much sooner, when deprived of other vital  stimuli.   Thus  when Frogs
were  confined in a limited quantity  of water, and Avere not permitted to
come to the surface to breathe, it was found that the duration of their
lives Avas inversely  proportional to the degree of heat of  the fluid.   Thus
when it was cooled  down to the freezing-point, the frogs immersed in it
lived  during from 367 to 498 minutes.   At the temperature of 50°, the
duration of theirTives was from 350  to 375 minutes ; at 72°, it was from
90 to 35 minutes; at 90°, from 12 to  32  minutes; and at 108° death
was almost instantaneous.  The prolongation of life at the lower tempe-
ratures Avas not due to torpidity, for the animals perform the functions of
voluntary motion, and enjoy the use of  their senses ; but it is occasioned
by their diminished activity, which  occasions a less demand for air.   On
the other hand, the elevation  of temperature increases the demand for
air, and causes speedier  death  when it is withheld; by increasing the
general agility.   The natural habits of these animals are in correspon-
dence Avith these facts.  During the winter, the  influence of a sufficient
amount of aerated water upon  their exterior serves to  maintain the re-
quired amount of  respiration through  the  skin, so that they are not
obliged to come to the surface to take in  air by the mouth.   As the
season advances, however,  their activity increases, a  larger amount of
respiration is required, and the animals are obliged to  come frequently
to the surface to breathe.  During summer, the yet higher temperature
calls forth an increased  energy and activity in  all  the  vital functions;
the respiration must be proportionably  increased; the action of the air
upon the cutaneous surface, as well as upon the  lungs, is required; and
if the animals are prevented from quitting the Avater to  obtain this, they
die, as soon as the Avarmth of  the  season  becomes considerable.   The
result of experiments on Fishes, in regard to the deprivation or limited
supply of the air contained in the Avater in Avhich they are immersed, is
exactly similar; the duration of life being inversely as the temperature.
And precisely the same has been ascertained Avith respect  to hyberna-
ting Mammals ; Avhich, as already remarked, are for a time reduced, in
all such conditions, to the level of cold-blooded animals.
  129.  The energy of the reparative actions of  Animals is much influ-
enced by temperature, as might be inferred from what has been just 
INFLUENCE OF  HEAT ON ANIMALS.
89
 said of their nutritive and developmental  operations.   Thus the rate at
 which regeneration of  lost parts, like that  of the ordinary process of
 budding, takes place in the common Hydra (Fresh-water Polype), is in
 close  accordance with the  temperature  in which it lives ; and in  like
 manner, the healing of wounds  in  Frogs takes place  more rapidly in
 summer than  in Avinter.  In many of the higher animals,  indeed, it
 appears that the complete regeneration of parts requires a higher tem-
 perature than is necessary to sustain the  ordinary vital activity.   Thus
 it has been found that the common Triton (Avater-neAvt) can reproduce a
 limb  that has been cut off, if it be kept at a temperature of from 58° to
 75°;  but cannot do  so if a less amount of heat be afforded to it.  And
 in like manner, the snail can regenerate its head, if it be kept in a Avarm
 atmosphere, but not at a low temperature.  Noav it has been justly re-
 marked by Mr. Paget,  that the process of development seems to require
 a higher amount of vital force than simple growth ; and we see that  the
 relation already pointed out betAveen  Heat and Vital force, here holds
 good in such a marked degree, as to afford a strong confirmation of the
 idea  of their mutual relationship.
   130. It is quite  conformable  to the same  principle, that we should
 find Cold-blooded animals able to sustain  the deprivation of food during
 a much longer  period,  at cold temperatures, than  at warm.  The case
 is precisely the reverse, hoAvever, in regard to most Warm-blooded  ani-
 mals  ; since in them a  due supply of  food is a condition absolutely  ne-
 cessary (as Ave have already seen) for the maintenance of  that amount
 of bodily heat, whose loss is fatal to them ;  and exposure to a low tem-
 perature will of course more speedily bring about that crisis.   Hence it
 is that Cold and starvation combined  are  so destructive to'life.   But in
 this respect also, the hybernating Mammals correspond with  the (fold-
 blooded classes ; their  power of abstinence being inversely as the tem-
 perature of their bodies.
   131. We haAre seen that the animals termed cold-blooded are greatly
 influenced  as to the  temperature of their  bodies, by the temperature of
 the surrounding medium ; although  many  of them are endowed  with the
 power of keeping themselves a certain number of degrees above it.   Noav
 the consequence of this is, that  all  of them which are subject to any
 considerable and prolonged  amount of cold, pass into  a state of more or
 less complete inactivity  during its continuance ; Avhich state bears a close
 correspondence with the hybernation of certain Mammalia.  Among the
 Reptiles of cold and temperate countries; this torpid  state  uniformly
 occupies a  considerable  part of the year; as it does also Avith Insects,
 terrestrial  Molluscs, and other Invertebrated animals, which are subject
 to the influence of the cold.   On the other hand, Fishes, Crustacea, and
 other marine animals, do not usually appear to pass into a state of tor-
 pidity ; the temperature of the medium they  inhabit never undergoing
 nearly so great a degree of depression, as  that of the atmosphere.  The
 amount of  change necessary to produce this effect, or  on the other hand
 to call the  animals from a state of torpidity to one of active energy,
 differs for different  species ;  and  there is  probably a  considerable diffe-
rence even among individuals of the same  species, according to the tem-
perature under which they habitually live.  Thus one animal may remain 
90           EXTERNAL  CONDITIONS OF  VITAL ACTIVITY.

torpid  under a  degree of warmth which  will be  sufficient to arouse
another of the same kind, accustomed  to a somewhat colder climate ;
because the stimulus is relatively greater  to the latter.
  132.  It Avas observed by Mr. Darwin,  that at Bahia Blanca in South
America, the first appearance of activity in animal and vegetable life, a
feAV days before the  vernal equinox, presented itself under a mean tem-
perature of 58°, the range of the thermometer in the middle of the day
being between 60° and 70°.  The plains  were ornamented by the flowers
of a pink wood-sorrel,  wild  peas, evening primroses, and  geraniums;
the birds began to lay their eggs ;  numerous beetles Avere craAvling about;
and lizards, the constant inhabitants of a sandy soil, were darting about
in every direction.   Yet a few days previously,  it seemed as  if nature
had scarcely granted a living creature  to this dry and  arid country;
and it was only by  digging in the ground that  their existence had been
discoA^ered,—several insects, large spiders, and lizards, having been found
in a half-torpid  state.   Noav at Monte Video, four  degrees nearer the
Equator, the mean  temperature had been above 58° for some time pre-
viously, and the thermometer rose occasionally during the middle of the
day to 69° or 70°;  yet Avith this elevated temperature, almost equivalent
to the full summer heat of our OAvn country, almost every beetle, several
genera of spiders, snails, and land-shells, toads and lizards, were still
lying torpid beneath stones.  We have seen that at Bahia Blanca, whose
climate  is but a little colder,  this same temperature, with a rather  less
extreme heat, was sufficient to aAvake all  orders of animated beings ;—
showing hoAV nicely the required  degree of stimulus is adapted  to the
general climate of the place,  and how little it depends on absolute tem-
perature.
  133.  AVe may learn much from the Geographical distribution of the
different species of  cold-blooded  animals in regard  to  the influence of
temperature on Animal life.  No general  inferences of this  kind  can
be found  upon the distribution  of  Avarm-blooded animals ; since their
OAvn heat-evolving poAvers make them in great degree independent of
external warmth.    And it  is probably from the  distribution of the
marine  tribes,  whose extension is less influenced  by local  peculiarities,
that the  most satisfactory deductions  are  to be drawn.   In  regard to
the class of Crustacea, which is  the one  that has  been most fully inves-
tigated  in this respect, the folloAving  principles have been  pointed out
by M. Milne Edwards; and they are probably more  or  less applicable
to most others.
  I. The varieties of form  and organization manifest themselves more,
in proportion as we pass the Polar Seas toAvards the Equator.
  n.  The differences of form and organization are not only more nume-
rous and more characteristic  in the Avarm than in the  cold regions of
the globe; they are also more important.
  in. Not only are those  Crustacea, which  are  most elevated in the
scale, deficient in the Polar regions ; but their relative number increases
rapidly  as Ave pass from the Pole toAvards the Equator.
  iv. When Ave compare together the Crustacea of different  parts of
the Avorld, Ave  observe that the  average size of these animals is con- 
INFLUENCE OF  HEAT  ON ANIMALS.               91
siderably  greater in tropical  regions  than in  the  temperate or frigid
climes.
  v. It is Avhere the  species are most numerous and varied and  where
they attain the  greatest size,—in other words, where the temperature
is most elevated,—that the peculiarity of  structure which characterize
the several groups,  are most strongly manifested.
  vi. Lastly, there is a remarkable coincidence between the tempera-
ture of different regions, and  the prevalence of certain forms of Crus-
tacea.
  134. Noav although, as  appears from  the  foregoing general state-
ments, the number of species of Crustacea  inhabiting the  colder seas
bears a very small  proportion to that which is  found within the tropics,
and although the species formed to inhabit cold climates are so far  in-
ferior both as to size,  and  as  to  perfection of  development, yet  it does
not follow that the same  proportion  exists in regard to the relative
amount of Crustacean life  in the tAvo regions; for this depends upon
the multiplication of individuals.  In fact it  may be questioned whether
there  is any inferiority in  this  respect; so  abundant are some  of  the
smaller species in the Arctic and Antarctic, as  well as in the Temperate
seas.  Thus  Ave see that a  low range of temperature  is as well adapted
to sustain their  life, as a higher range is to  call  forth those larger and
more fully-developed forms which abound  in the tropical ocean.   There
is an obvious reason why the  seas of the frigid zones should be much
more abundantly peopled than the land ; the mean temperature  of  the
former being much higher.  And  it Avould almost seem as if  Nature
had intended to compensate for the dreariness and desolation of the one,
by  the profuseness  of  life which she has fitted  the other to support.
   135. The  influence of Temperature  in  producing  a variation  in  the
size of individual Animals  of any one species, is not so strongly marked
as  it is in the case of Plants; for this reason, perhaps, that an amount of
continued depression or elevation, Avhich might be sustained by a Plant,
but Avhich Avould exert a modifying influence upon its groAvth, would be
fatal to an Animal formed to exist in  the  same climate.   Instances  are
not Avanting, however, in which such a modifying influence is evident;
and these, as might be anticipated, are to be met with  chiefly  among
the cold-blooded tribes.  Thus the Bulimus  rosaceus, a terrestrial mol-
lusc, is found on the mountains of Chili of  so much less a size than that
which  it  attains on the coast, as to have  been described as a distinct
species.   And the Littorina petrcea  found on the south side of Plymouth
Breakwater, acquires, from its superior  exposure  to light  and heat
(though  perhaps also  from the greater supply of nutriment which it
obtains),  twice the  size common to individuals living on the north side
within the harbor.—The  following circumstance shoAvs  the  favorable
influence  of  an  elevated temperature in producing an unusual prolific-
ness in Fish ;  Avhich  must be  connected  with general vital activity.
Three pairs of Gold-fish were placed,  some years'  since, in one of  the
engine-dams or ponds common  in  the manufacturing  districts,  into
which  the water from the  engine is conveyed  for the purpose of being
cooled ; the  average temperature of such dams is about 80°.   At  the
end of three years, the progeny of these Fish, which were accidentally 
92           EXTERNAL  CONDITIONS  OF VITAL ACTIVITY.

poisoned by verdigris  mixed with the  refuse tallow from the  engine,
were  taken out  by wheelbarrowfuls.  It is not improbable that  the
unusual supply of aliment,  furnished by the  refuse  grease  that  floats
upon  these ponds (which would impede the  cooling of the water, if it
were not consumed by  the Fish), contributed Avith the high temperature
to this unusual fecundity.
   136.  Although a very low temperature is positively inconsistent with
the continuance  of vital activity, in Animals  as  in Plants, yet  Ave find
that even  very  severe cold  is not necessarily destructive of the vital
properties  of organized tissues; so that, on  a restoration of the proper
amount of heat,  their functions may continue as before.  Of  this  Ave
have already noticed an example, in the case of  frost-bitten limbs;  but
the fact is much more remarkable, Avhen considered in reference to  the
Avhole body of an animal, and the complete suspension of all its func-
tions.   Yet it is unquestionably true, not only of the  lowest and sim-
plest  members of  the Animal kingdom, but also of Fishes and Reptiles.
In  one of Captain Ross's Arctic  Voyages, several  Caterpillars of  the
Laria Rossii having  been exposed to a temperature of 40° below zero,
froze  so completely,  that, when throAvn into a  tumbler,  they  chinked
like lumps of ice.  When thaAved, they resumed  their movements, took
food,  and underwent their transformation into the Chrysalis state.  One
of them, which  had been frozen and thawed four times, subsequently
became a Moth.   The eggs  of  the Slug have  been exposed to a similar
degree of cold, without the  loss of their fertility.  It is not uncommon
to meet in the  ice of rivers, lakes,  and seas, with Fishes which have
been  completely frozen,  so as  to become quite brittle; and Avhich  yet
revive Avhen thaAved.   The same thing  has been observed in regard to
Frogs, Newts, &c.; and the experiment of freezing and subsequently
thawing them, has been frequently put in practice.   Spallanzani kept
Frogs and Snakes in an ice-house for three years; at the end of Avhich
period they revived on being subjected  to warmth.
   137. It does not appear,  hoAvever,  that the same capability exists, in
the case of any  Avarm-blooded  animals; since if  a total suspension* of
vital activity take place in the body of a Bird or Mammal for any length
of time, in consequence of the prolonged application  of severe cold, re-
covery is found to be  impossible.  The  power Avhich exists in these ani-
mals, however, of generating a large amount of heat within their bodies,
acts as a compensation for the want of the faculty possessed by  the
cold-blooded  tribes; since they can resist, for a great length of time (if
in their healthy  or normal condition), the depressing influence of a tem-
perature, sufficiently Ioav to produce a complete suspension in the acti-
vity of the latter.
   138. It only remains to say a feAv words regarding the degree of heat
Avhich certain Animals can sustain Avithout  prejudice, and Avhich even
appears  to be genial  to them.  Among the higher classes, this  range
seems  to  be  capable of great extension.   Thus  many  instances are on
record, of  a heat of from 250° to  280° being endured, in dry air, for a
considerable  length of time, Avithout much inconvenience; and  persons

  * In the case of hybernating Mammals, the suspension is not total; and if  it be  ren-
dered such, the same result follows as in other instances. 
                  INFLUENCE OF HEAT  ON ANIMALS.               93

who have become habituated to this kind of exposure, can (with proper
precautions) sustain a temperature of from 350° to 500°.   In all such
cases, hoAvever, the  real heat  of the  body undergoes very little eleva-
tion ;  for, by means of the copious  evaporation from its surface,  the  ex-
ternal heat is  prevented from acting upon it.   But if this evaporation
be  prevented, either  by an insufficiency in the supply of  fluid from
Avithin, or by  the saturation  of the surrounding air Avith moisture,  the
temperature of the body begins to rise; and it  is then found, that it
cannot undergo an elevation of more than a few degrees, without fatal
consequences.   Thus in several experiments which have been tried on
different species of Avarm-blooded  animals, for the purpose of ascertain-
ing the highest temperature  to which the body could be raised  Avithout
the destruction of life, it Avas found that as soon as the heat of the body
had been increased, by continued immersion in a limited quantity of hot
air (Avhich Avould soon become  charged Avith moisture), to from 9°-13°
above the natural standard, the  animals died.  In general Mammals
die, Avhen the temperature of their bodies is  raised to about 1110 ;  the
heat which is  natural to the bodies of Birds.   The  latter  are killed by
an  equal  amount of elevation of bodily heat above their natural standard.
   139.  Hence Ave see that the actual range of  temperature, within
which vital activity can be maintained in warm-blooded animals, is ex-
tremely  limited;  a temporary  elevation of the bodily heat  13° above
the natural standard,  or a depression  to 30° below it, being positively
inconsistent, not merely with  the  continuance of vital operations,  but
also with the preservation of vital properties: and a continued departure
from  that standard, to the extent  of  only a very few'degrees above or
below it,  being very injurious.  The provisions Avith which these animals
are endowed,  for generating heat  in  their interior, so as to supply the
external deficiency, and for generating cold (so to speak), when the ex-
ternal temperature is too high, are therefore in no  respect superfluous:
but are positively necessary  for the maintenance of the life of such ani-
mals, in  any climate,  save one  Avhose mean should be  comformable to
their  standard, and Avhose extremes should never vary more than a very
feAv degrees above or  below  it.  Such a climate does not exist on the
surface of the earth.
   140.  The range of external temperature, within which  cold-blooded
animals can sustain their activity, is much more limited, as well in regard
to its highest  as to its loAvest point;  notwithstanding that the  range of
bodily heat, which  is consistent with the maintenance of their life,  is so
much greater.  In those which, like  the Frog, have a soft  moist skin,
which permits a copious evaporation from  the  surface, a considerable
amount of heat may be resisted, provided the air be dry, and the supply
of fluid from  within be  maintained.*  But immersion in Avater of the
temperature of  108°, is almost immediately fatal.  In many other cold-
blooded  animals, elevation of the temperature  induces a state  of  tor-
  * The  Frog has a remarkable provision for this purpose; in a bladder, which  is
ttructurally analogous  to our Urinary bladder, but which has for its chief function  to
contain a store of fluids for the exhaling process. It has been noticed that, when this
store is exhausted by continued exposure of the animal to a warm dry atmosphere, the
bladder becomes full again, when  the animal is placed in a moist situation, even though
it takes in no liquid by its mouth. 
 94           EXTERNAL CONDITIONS OF VITAL ACTIVITY.

 pidity, analogous to that  which is  produced by its  depression.  Thus
 the Helix pomatia (Edible Snail) has  been found to become torpid and
 motionless in water  at 112°;  but to recover its energy when placed in
 a colder situation.   It Avould seem to be  partly from this cause,  but
 partly also from the deprivation of moisture, that the hottest part of the
 tropical year brings about.a cessation of activity in many tribes of cold-
 blooded animals, as complete as that which takes place during the winter
 of temperate climates.
   141. The  highest limit of temperature  compatible  with the life of
 Fishes has not been certainly ascertained: and it appears probable that
 there are considerable variations in this respect amongst  different species.
 Thus it is certain that there  are some which are killed* by immersion in
 water at 104° ; whilst it is  also certain that others cannot  only exist, but
 can find a congenial habitation, in Avater of 113°, or even of 120°; and
 examples of the existence of Fishes  in thermal springs of  a much higher
 temperature  than this, have been put  on record.  Various fresh water
 Mollusca have been  found  in thermal springs, the heat  of which is from
 100°, to 145°.   Rotifera and other  animalcules have been met  Avith  in
 Avater at 112°.   Larvae of Tipulae have been found in hot springs  of
 205°; and small black beetles, which died Avhen placed  in cold water, in
 the hot sulphur  baths of  Albano.  Entozoa inhabiting  the  bodies  of
 Mammalia and of Birds must of course be adapted to  a  constant tem-
 perature of from 98° to 110° : and they become torpid when exposed to
 a cool atmosphere.  These lowly organized animals seem more capable
 of resisting the effects of extreme heat, than any others;—at least if we
 are to credit  the statement, that the Entozoa inhabiting  the intestines
 of the Carp have been found  alive, when the Fish was brought  to table
 after  being boiled.   In all  such cases,  it is  to be remembered,  that  the
 heat of the animal body must  correspond Avith that  of the fluid in which ■
 it is immersed ; and we have here, therefore, evident proof of the com-
 patibility of vital activity,  in certain cases,  Avith a very elevated tempe-
 rature.   Additional  and more exact observations, however, are much
 wanting on this subject.

            3. Of Electricity, as  a Condition of Vital Activity.        *

   142.  Much less is certainly knoAvn with respect to the  ordinary influ-
 ence of this agent, than in  regard to either  of the tAvo  preceding ; and
yet there can be little doubt, from the effects we observe when it is pow-
erfully applied, as well as from our knowledge  of its connexion  with all
 Chemical phenomena, that  it is in constant though imperceptible opera-
tion.   Electricity differs from  both Light and  Heat in this respect;—
that no manifestation of it takes place so long as it is uniformly diffused,
or is in a state of equilibrium ;  but in proportion as this  equilibrium is
disturbed by a change in the  electric  condition of one body, which is
prevented, by its  partial or  complete insulation, from communicating
itself  to others-, in that  proportion  is  a force produced which exerts
itself  in various ways according to its degree.   The mechanical effects
 of a powerful charge, when passed through a  substance  that is a  bad
conductor of Electricity, are well known ; on the other hand, the chemical 
        OF  ELECTRICITY AS  A CONDITION  OF VITAL ACTIVITY.      95

effects of even the feeblest current are equally obvious.   The agency of
Electricity in producing Chemical change is the more powerful, in pro-
portion as  there  is already a predisposition to  that change ; thus, the
largest collection of oxygen  and  hydrogen gases, or of hydrogen and
chlorine mingled together, may be caused to unite by the minutest elec-
tric  spark,  Avhich brings  into the condition  required  for their  active
exercise, and mutual affinities that were previously dormant.  Hence it
cannot but be inferred, that its agency in the Chemical phenomena of
living bodies must be of an important character; but this may probably
be exerted rather in the way of aiding decomposition, than of producing
neAV combinations, to Avhich (as we have seen) Light appears  to be the
most effectual stimulus.  Thus it has been shown that  pieces of meat,
that have been electrified  for some hours,  pass much more rapidly into
decomposition than similar pieces placed under the same circumstances,
but not electrified.   And in like manner, the bodies  of animals that have
been killed by electric shocks, have been observed to putrefy much more
readily than those of similar animals killed by an  injury to the brain.
It  is  Avell known, moreover, that in  thundery Aveather,  in which the
electric state of  the atmosphere  is much  disturbed, A'arious fluids con-
taining organic compounds, such  as milk, broth, &c, are peculiarly dis-
posed to torn sour ; and that saccharine fluids, such as the wort of brewers
are extremely apt to pass into the acetous fermentation.
   143. The actual amount of influence, however, Avhich Electricity exerts
over a growing Plant or Animal, can  scarcely be estimated.  It would,
perhaps, be the most correct to say, that the state of Electric  equilibrium
is that which is generally most favorable;  and Ave  find  that there is a
provision in the structure of most living beings, for maintaining such an
equilibrium,—not only between the  different parts of their  own bodies,
but also between  their  own fabrics, and the surrounding  medium.   Thus
a charge given to any part  of a Plant, or Animal, is immediately diffused
through its Avhole mass ;  and though Organized bodies are not sufficiently
good conductors to transmit very poAverful  shocks without being  them-
selves affected, yet a discharge of any moderate quantity may be effected
through them, without any permanent injury,—and this  more especially
if it be made to take place slowly.  Noav the points on the surfaces of
Plants appear particularly adapted to  effect this transmission;  thus it
has been found that a Leyden jar might be discharged by holding a blade
of grass near it, in one-third of the time required to produce the same
effect by means of a metallic point; and an Electroscope furnished with
Vegetable points  has been found to give more delicate indications of the
electric state of the atmosphere, than  any  other.  Plants designed for
a rapid growth have generally a strong pubescence or downy covering;
and  it does not seem improbable  that one purpose of  this  may be to
maintain that equilibrium between themselves and the atmosphere, which
would otherwise  be  disturbed by  the various  operations of  vegetation,
and  especially by the  process of .evaporation, which takes  place  with
such activity from the  surface of the  leaves.
  144. There appears to  be sufficient evidence that, during a  highly
electrical state of the  atmosphere, the growth of the young shoots of
certain plants is increased  in rapidity; but it would be Avrong thence to 
96
EXTERNAL  CONDITIONS OF  VITAL ACTIVITY.
infer that this excitement is useful to the process of Vegetation in gene-
ral, or that the same kind of electric excitement universally operates to
the benefit  or injury of the Plant.  From some experiments recently
made it  would appear, that potatoes,  mustard, and  cress, cinerarias,
fuchsias, and  other plants, have their  development, and, in some in-
stances, their  productiveness, increased by being made to grow between
a copper and a zinc plate,  connected  by a  conducting wire ; while, on
the other hand, geraniums and  balsams are destroyed by the same in-
fluence.   The transmission of a series of moderate sparks through plants
in like manner, has been found to accelerate the growth of some, and to
be evidently injurious to  others.   It  is not  unreasonable tosuppose,
that as a  great variety of chemical processes are constantly taking place
in the groAving plant,  an electric disturbance Avhich acts as a stimulus
to some, may  positively retard others ; and that its good or evil results
may thus depend  upon  the balance between these individual  effects.
This  would  seem  more  likely  from  the circumstance, that,  in  the
process of Germination, the chemical changes concerned in which are of
a similar  character, Electricity seems to have a more decided and uni-
form influence.  The  conversion of the starch of the seed into sugar,
which is an essential  part  of this change,  involves the liberation of a
large quantity of carbonic, and of some acetic acid.   Now as all acids
are negative, and as like electricities repel  each other it maybe inferred
that the seed  is at that time in an electro-negative  condition; and it  is
accordingly found that the process of germination may be quickened by
connexion of  the seed with the negative pole of a feeble galvanic appa-
ratus whilst it is retarded by a similar  connexion with the positive pole.
A similar acceleration may be produced by the contact of feeble alkaline
solutions, Avhich favor the liberation of the acids ; whilst^ on the same
principle, a very small admixture  of acid  in the  fluid with which the
seed is moistened, is found to produce  a decided retardation.
   145. It is well known that  Trees and Plants may be easily killed by
poAverful  electric shocks; and that, when the charge  is strong  enough
(as is the case of a stroke of lightning), violent mechanical effects,—as
the  rending  of trunks, or  even the splitting or scattering  of  minute
fragments,—are produced by it.   But it has also been ascertained, that
charges Avhich produce no perceptible influence of this kind, may  destroy
the  life of plants ; though  the effect is  not always immediate.   In par-
ticular it has been noticed, that  slips and grafts  are  prevented from
taking root and budding.  There can  be little  doubt that, in these  in-
stances, a change is effected in the chemical state of the solids or fluids ;
although no structural alteration is perceptible.
   146. In regard to the influence of Electricity upon the Organic func-
tions of Animals,  still less is certainly known; but there is evidence that
 it may act as a powerful stimulant in certain disordered  states of them.
 Thus in  Amenorrhoea, a series  of slight  but rapidly-repeated  electric
 shocks will often bring on the catamenial flow; and  it is certain that
 chronic tumors have been dispersed,  and dropsies relieved by the ex-
 citement of the  absorbent process, through similar agency.  In fact,
 there is strong reason to believe, that Electricity may be advantageously 
          OF MOISTURE  AS A CONDITION OF VITAL ACTIVITY.       97

 employed remedially in  many states of  disordered nutrition; in virtue
 of its power of modifying the operations of the Vital forces.
   147. The closest relations of Electricity, however, are Avith the proper
 Animal functions ;  for  these,  as  will  be shown  hereafter, are more
 directly and obviously subject to  its influence, than  are the Organic.
 Thus Electricity, when  transmitted along a Nerve, whether sensory or
 motor, a nerve of " special" or one of "common"  sensation, is capable
 of calling forth all the actions of which that nerve is the instrument;
 and, when brought to bear on  a Muscle,  it immediately excites a con-
 tractile movement.   It  is probably through the  influence of this agent
 upon the Nervous system, that electric states of the atmosphere induce
 in certain individuals a degree of langour  and depression, which cannot
 be accounted for in any  other way.  An instance is on record, in which
 the  atmosphere Avas in such an extraordinary state of electric disturbance,
 that all pointed bodies within its influence exhibited a distinct luminosity;
 and it was noticed, that  all the persons who Avere exposed to the agency
 of this highly electrified air, experienced spasms in the limbs and an ex-
 treme state of lassitude.
   148. Animals, like Plants, are liable to be killed by shocks of Elec-
 tricity ; even when these are not sufficiently powerful to occasion any
 obvious physical  change  in their structure.   But, as formerly mentioned
 (§ 69),  there can be no doubt that  minute changes may be produced in
 their delicate parts, which are quite sufficient to account for the  destruc-
 tion of their vitality, even though these can only be discerned with  the
 microscope.  The production of changes in the Chemical arrangement
 of their elements, is, however,  a much more  palpable  cause  of death;
 since it may be fully anticipated beforehand, and  can easily be rendered
 evident.  To take one instance only;—it is  well known, that albumen
 is made to  coagulate, i.  e., is changed from its soluble to  its insoluble
 form, under the influence of an electric current; and it cannot be  doubted
 that the production of this change in the fluids of the living body (almost
 every one of which contains albumen), even to a very  limited extent, is
 quite a sufficient cause  of death,  even  in animals that  are othenvise
 most tenacious of life.   "I once discharged a battery of considerable
 size," says Dr. Ilodgkin, "through a common Earth-worm, which would
 in all probability have shoAvn signs of life long  after  minute division.
 Its death  Avas as  sudden as the shock; and  the semi-transparent sub-
 stance of the animal was changed like Albumen which has been  exposed
 to heat."

             4.  Of Moisture, as a Condition of Vital Activity.

  149.  Independently of the utility of Water as  an article of food, and
 of the part it performs in the Chemical  operations  of the living body,
 by supplying tAvo of their most  important materials (oxygen and hydro-
 gen), there can be no doubt that a certain supply of moisture is requi-
site,  as  one of the conditions  without which  no vital action can go  on.
It has  been already  remarked, indeed, that one of the  distinguishing
peculiarities of Organized structures, is the presence in all of them  of
solid and liquid component parts; and this in the minutest portions  of 
98           EXTERNAL  CONDITIONS OF  VITAL ACTIVITY.

the organism, as well as in the aggregate mass.   And in all the vital,
as well as in the chemical actions, to Avhich  these structures are subser-
vient, the presence of  liquid is essential.   All nutrient  materials must
be reduced to the  liquid form, before  they can be  assimilated by the
solids; and, again, the solid  matters which are destined to  be carried
off by excretion, must be again reduced to the liquid state, before they
can be thus Avithdrawn from the body.   The tissues  in which  the most
active changes of a purely vital character are performed,—namely, the
Nervous and Muscular,—naturally contain a very large proportion  of
water; the former as much as 80 and  the latter 77 per cent.   On the
other hand, in tissues Avhose function is of a purely mechanical nature,
such as Bone, the  amount of liquid is as small as is consistent  with the
maintenance of a certain amount of nutrient action in its interior.  By
the long-continued application of dry heat  to  a  dead body, its Aveight
was found to be reduced from 120 pounds to no more thau 12; so that,
taking the average of the whole, the amount of water,  not  chemically
combined, but simply interstitial, might  be  reckoned at as much as 90
per cent.   It is certain, however, that  much decomposition  and loss  of
solid matter must have taken place in this procedure ; and we shall pro-
bably estimate the proportion more accurately, if  we regard the weight
of the fluids of the human body as exceeding that of the solids by six
or seven times.
  150.  There is a great variation  in this respect, however,  among dif-
ferent tribes of living beings.   There are probably no highly organized
Animals,  whose texture contains  less  liquid than that  of Vertebrata
(unless it may be,  certain Beetles); but there  can be  no question  that,
among some of the Zoophytes, the proportion of solids to liquids is just
the other way.   In those massive coral-forming animals, which seem  to
have been expressly created for the purpose of uprearing islands and
even continents  from the depth of the  ocean, we find the soft tissues
confined to the surface, and all  within of a rocky hardness,   It is not
however,  correct to say (as is commonly done), that the coral-polypes
" build up" these stony structures as habitations for themselves ; for the
stony matter is deposited, by an act of nutrition, in the living  tissue  of
these animals, just as much, as it is in the bones of Man.  But the parts
once consolidated henceforth remains dead, so far as the animal is con-
cerned ;  they are not connected with the  living tissues  by any vessels,
nerves, &c, their density prevents them from undergoing any but a very
sIoav disintegrating change, so that they require and receive no nutrient
materials; and they might be altogether removed, by accident  or decay,
without any direct injury to  the still-active, because yet unconsolidated
portions of the polype structure.
   151.  There is a close correspondence, in this  respect, between the
condition of the stony  or horny stem of a Coral, and the heart-wood  of
the trunk of a Tree ; for the  latter, becoming consolidated by  internal
deposit,  for the purpose of affording mechanical  support, is thenceforth
totally unconnected with the vegetative operations of the tree, and might
be removed (as it frequently is by  natural decay) Avithout effecting them.
•in all the parts, in which the nutrient processes are actively going  on,
do we observe that the tissue contains a  large proportion of water; and 
          OF MOISTURE AS A  CONDITION  OF VITAL ACTIVITY.       99

that, if the succulent portions be dried up, their vital properties are de-
stroyed.  Thus it is in the soft tissue at the extremities of the radicles
or root fibres, that the function of absorption  takes  place with  the
greatest activity; so that these  parts  have received the name of spon-
gioles : it is in the cells which form the soft parenchyma of the leaves,
that the elaboration of the sap takes place, the fixation  of carbon from
the  atmosphere, and the preparation of the peculiar secretions  of  the
plant:  and it is in the space between the bark and the Avood, Avhich is
occupied (at the season of most active growth) by a saccharine glutinous
fluid, that the formation of the new layers of Avood and bark takes place.
Noav, as soon as these parts become consolidated, they cease to perform
any active vital operations.  The spongioles, by the lengthening of the
root-fibres,  become converted into a portion of those fibres, and remain
subservient merely to the transmission of the fluids absorbed; the leaves
gradually become choked by the saline and earthy particles contained
in the ascending sap, which they have  had no poAver of excreting, and
they Avithcr, die, and fall off; and the new layers of wood and  bark,
Avhen once  formed, undergo but little further  change, and are subser-
vient to little else than the transmission of the ascending and descending
sap  to the parts where they are  to be respectively appropriated.
   152. There are  some  remarkable instances in both the Animal and
Vegetable kingdoms, of an immense preponderance in  the amount of
the fluids over that of the solids of the  structure.  This is characteristic
of the whole class of Acalephsea or Jelly-Fish, giving to their tissues that
softness from which their common name is derived;  these Animals, in
consequence, are unable  to live out of Avater; for when they are removed
from it, a  drain  of their  fluids  commences, which soon reduces their
weight  to a degree that destroys their lives,—a Medusa weighing fifty
pounds being thus dried doAvn to a weight of as many grains.  The most
remarkable instances of a parallel kind among Plants, are to be found
in the tribe of  Fungi; certain members of which  are  distinguished by
an almost equally small proportion of solid materials in  their textures,
presenting  a most delicate gossamer-like  appearance  to the eye, and
possessing such little durability, that they come to maturity and undergo
decay in the course of a few hours.   These are not inhabitants of  the
Avater, but will vegetate only in a very damp atmosphere.
  153.  As  avc find Ararious Plants and Animals very differently con-
structed, in  regard to the amount of fluid contained in their tissues, so
do we also find them dependent in very different  degrees upon a con-
stant supply of  external moisture.   There is  no relation,  however,  be-
tween the succulence of a plant, and the degree of its dependence upon
water;  in fact, Ave commonly find the most succulent plants growing in
the driest situations; whilst the plants, which are  adapted to localities
where they can obtain a constant  supply of fluid, are  not usually re-
markable for  the  amount of water in their own structure.   This, hoAV-
ever, is easily  explained.  We find the most  succulent plants,—such
as the Sedums  or Stone-crops of our OAvn country, and the Cacti and
Euphorbiae  of the  tropics,—in dry exposed situations, where they seem
as if they would  be utterly destitute of nutriment.  The fact is,  how-
ever,  that they lose their fluid by exhalation very sloAvly,  in consequence 
100          EXTERNAL CONDITIONS OF VITAL ACTIVITY.
of their small number of stomata; whilst, on the other hand, they absorb
with great readiness during rainy weather, and are enabled,  by the
fleshiness  of their substance, to store  up a large quantity of moisture
until it is required.   In some  parts of Mexico, the heat is so intense,
and  the soil  and atmosphere so dry,  during a large part of  the year,
that no vegetation is found at certain seasons, save a species of Cactus;
this affords a wholesome and refreshing article  of food, on which travel-
lers  have  been able to subsist  for many days together,  and without
AA'hich these tracts Avould form impassable barriers.  On the other hand,
the plants of damp situations usually exhale  moisture almost as fast as
they imbibe it;  and consequently, if their usual  supply be  cut off or
diminished, they soon wither and die.   Plants  that usually live entirely
submerged, are destitute of the  cuticle or thin skin, which covers the
surface in other cases; in consequence of this, they very rapidly  lose
their fluid, when they are  removed from the water; and they are hence
dependent upon  constant  immersion  in  it for a  continuance of their
lives, although  their tissues may not be remarkable for the amount of
fluid which they contain.
   154.  There are some Plants which are capable of adapting themselves
to a great variety of situations, differing Avidely as to  the amount of
moisture which their inhabitants  can  derive from the soil and atmo-
sphere ; and we may generally notice a marked difference in the mode
of groAvth, when we compare individuals  that have grown under oppo-
site circumstances.  Thus a plant from  a dry  exposed situation,  shall
be stunted and hairy, whilst another of the same species, but developed
in a damp sheltered situation,  shall be  rank  and glabrous  (smooth).
But in general there is a certain quantity of  moisture congenial to each
species; and the excess or deficiency  of this  condition has,  in conse-
quence, as great an influence in determining the geographical distribu-
tion  of Plants,  as the  amount  of light  and heat.   Thus, as already
remarked, the Orchideae and Tree Ferns of the tropics grow best in an
atmosphere loaded with dampness; whilst the Cactus tribe, for the most
part, flourishes best in dry situations.   The former become stunted and
inactive, if limited in their supply of aerial moisture; whilst the latter,
if too  copiously nourished, become dropsical and  liable to rot.  Among
the plants of our own  country, we find a similar limitation; a moist
boggy situation being indispensable to  the growth of some, whilst a dry
exposed elevation is  equally essential  to the  healthy  development of
others.   There is a beautiful species of exotic Fern,  the  Trichomanes
speciosum ;  the rearing of Avhich has been frequently attempted in this
country and elsewhere, without success; but  which only requires an
atmosphere saturated with dampness, for its healthy development, being
easily reared in one of Mr. Ward's closed glass-cases.   In this, as in
similar examples,  it  is only necessary  to  imitate  as closely as possible
the conditions under which the species  naturally groAVS; and sometimes
this can only be  accomplished,  by surrounding  the plant with small
trees and shrubs, so as to give it a moister atmosphere than it could
otherwise attain.  Professor Royle mentions  the  growth, under such
circumstances,  of a fine specimen  of  the Xanthochymus dulcis, one of
the Guttiferce or Gamboge-trees, in  the garden of the King of Delhi"; 
         OF  MOISTURE  AS A CONDITION OF  VITAL ACTIVITY.      101

this tree is naturally found only in the southern parts of India; and the
success of its cultivation  in this northerly situation is  entirely due  to
its being sheltered by the numerous buildings  within the lofty palace
wall, surrounded by almost  a forest of trees, and receiving the benefit
of perpetual  irrigation from a branch  of the canal which Aoavs through
the garden.
   155. In regard to  the  influence  of external moisture upon Animal
life, there is  much  less to be said;  since the mode in which  fluid is re-
ceived into the  system  is so entirely  different.  It may be  remarked,
however, that Animals habitually  living beneath  the water, like sub-
merged Plants,  are usually incapable  of sustaining life  for any length
of time when removed from it, in consequence of the rapid loss of fluid
which  they undergo from their surface.  It is,  however, by the  desic-
cation of the respiratory  surface,  preventing the due  aeration  of the
blood, that the  final result  is for the most part occasioned;  since we
find that when there  is a special provision  to  prevent  this, as  in the
case of certain Fishes and  Crustacea, the animals  can quit the water
for a great length of time.  There  can be no doubt that the amount  of
Atmospheric moisture is one of those conditions, Avhich  are collectively
termed climate, and  which influence the  geographical  distribution  of
Animals, no  less than that of  Plants.  But it is difficult to say how far
the variations in moisture act alone.   There can be no doubt, however,
of their operation ; for  every one  is conscious of the effect, upon his
health and spirits, of such variations as take  place in  the  climate  he
may inhabit.  The two  principal modes in  which these will operate, will
be by accelerating or checking the exhalation of fluid from the skin and
from the pulmonary surface;  for Avhen  the  air is  already loaded with
dampness,  the exhaled moisture cannot  be  carried off  with the same
readiness as  Avhen it is in a condition  of greater dryness ; and  it will
consequently either remain within the system, or it will accumulate and
form sensible perspiration.
   156. Now each of  these states may be salutary, being  the  one best
adapted to particular constitutions, or  to  different states of the same
individual.    A  cold drying wind shall  be  felt as invigorating to the
relaxed frame as it is chilling to one that has no warmth or moisture  to
spare;  on the other hand, a Avarm damp atmosphere,  which  is refresh-
ing to  the latter, shall be most depressing to  the former.   All who have
tried the effect of  closely-fitting garments, impervious  to moisture, are
well aAvare how  oppressive they soon  become; this feeling being de-
pendent upon the  obstruction they occasion  to the act of  perspiration,
by causing the  included  air to be  speedily saturated  with moisture.
When  the fluids  of the system  have been diminished in amount,  either
by the suspension of a due supply of  Avater, or by an  increase  in the
excretions, there is a peculiar refreshment in a soft damp atmosphere,
or in a Avarm bath, Avhich alloAvs the loss to  be replaced by absorption
through the  general cutaneous surface.   The reality of such absorption
has been placed  beyond all  doubt, by observations upon men, who had
been exposed to  a hot dry air for some  time, and aftenvards placed  in
a warm bath; for it Avas found that the system would by this unusual 
 102   .      EXTERNAL  CONDITIONS  OF VITAL ACTIVITY.

 means supply the deficiency, which had been created by the  previous
 increase in the transpiration.
   157. The effect of a moist or dry atmosphere, then, upon the Animal
 body,  cannot  be by any means unimportant;  although,  as  we shall
 hereafter see, there exists in it a series of the most remarkable  provi-
 sions for regulating  the amount of its fluids.   The influence of atmo-
 spheric moisture, however, is most obvious in disordered  states  of the
 system.   Thus in persons who are subject  to the form of Dyspepsia
 called atonic, which is  usually connected with a generally-relaxed con-
 dition of the system, a very perceptible  influence is  experienced from
 changes in the quantity of atmospheric moisture; the digestive power,
 as well as the general functions of the body, being invigorated by dry-
 ness, and depressed  by damp.   Again there is no doubt that, where a
 predisposition exists to the Tuberculous  Cachexia, it is greatly favored
 by habitual exposure to a damp atmosphere, especially when accom-
 panied by cold; indeed it would appear, from the influence of cold damp
 situations upon  animals brought  from Avarmer  climates,  that these two
 causes may induce the disease, in individuals previously healthy.  On
 the other hand, there are some forms of pulmonary complaints,  in which
 an irritable state of  the mucous membrane of the  bronchial tubes has a
 large share; when this irritation presents itself in  the dry form, a warm
 moist atmosphere is  found most soothing to it; whilst a drier and more
 bracing air is  much more beneficial, when the irritation is  accompanied
 by a too copious secretion.
   158. Although, as already stated, no vital actions can go on without
 a reaction between the solids and fluids of the body, yet there may be
 an entire loss of the latter, in certain cases, without necessarily  destroy-
 ing life; the structure being reduced to  a  state of dormant vitality, in
 which  it may remain unchanged for an unlimited period ; and yet being
 capable of renewing  all its  actions, Avhen moisture  is again  supplied.  Of
 this we find numerous examples among both the Vegetable and  the Ani-
 mal kingdoms.   Thus the Mosses  and Liverworts, which inhabit situa-
tions Avhere  they are liable to occasional drought, do  not  suffer from
being,  to all appearance,  completely dried up ; but revive and  vegetate
 actively, as soon  as  they have  been thoroughly moistened.  Instances
 are recorded, in which Mosses that have been for many  years  dried up
in an Herbarium, have been restored by moisture to active life.   There
is  a Lycopodium (Club-Moss) inhabiting Peru, Avhich, when dried up for
 want of moisture, folds its  leaves and contracts into a ball; and in this
 state, apparently quite devoid of animation,  it is blown hither and thither
along the surface by  the wind.  As soon, hoAvever, as it reaches a moist
 situation, it sends down its roots into the soil, and unfolds to the atmo-
sphere its  leaves, which, from  a dingy brown, speedily  change  to the
bright green of active vegetation.   The Anastatica (Rose of Jericho) is
the subject of similar  transformations;  contracting into  a ball, when
dried up by the  burning sun and  parching air; being  detached by the
wind from the spot where its slender roots  had fixed it, and rolled over
the plains to indefinite distances;  and then, when  exposed  to moisture,
unfolding its leaves,  and  opening its rose-like flower, as  if roused from
sleep.   A blue Water-Lily abounds in several of the canals of  Alexan- 
         OF  MOISTURE  AS A CONDITION OF  VITAL ACTIVITY.      103

dria, which at certain seasons become so dry, that their beds  are burnt
as hard as bricks by the action of the sun, so as to be fit for use as car-
riage roads:  yet the plants do  not thereby lose their vitality; for Avhen
the Avater is  again admitted, they resume their groAvth Avith redoubled
vigor.
   159.  Among the lower Animals, Ave find several of considerable com-
plexity  of structure, which are able to sustain the most complete desic-
cation.   This is most remarkably the case in the common Wheel-Ani-
malcule ; which may be reduced to a state of most  complete dryness,
and kept in this  condition  for  any length of time, and which will yet
revive immediately on being moistened.   The same  individuals  may be
treated in this manner, over and over  again. Experiments have been
carried  still further wTith the allied tribe of Tardigrades ;  individuals  of
which have been kept in a vacuum for  thirty days, with sulphuric  acid
and Chloride of Calcium  (thus  suffering the  most complete desiccation
the Chemist  can effect), and yet have not lost their vitality.  It is sin-
gular that in this desiccated condition, they may be heated  to  a tem-
perature of  250°, without the  destruction of their vitality;  although,
Avhen in full  activity, they will not sustain a  temperature  of more than
from 112°  to 115°.   Some of  the minute Entomostracous Crustacea,
which are nearly allied  to the Rotifera, appear to partake with them  in
this curious  faculty.  Many instances  are on record  in  which Snails
and other terrestrial Mollusca  have  revived, after what appeared to be
complete desiccation;  and  the  eggs of the Slug, when dried  up by the
sun or by artificial heat, and reduced to minute points only visible with
the Microscope, are found not to have lost their fertility, when they are
moistened by a shoAver of rain, or by immersion in water, Avhich restores
them to their former plumpness.  Even after being treated eight times
in  this  manner, the eggs were hatched Avhen placed in  favorable cir-
cumstances ;  and even eggs in which the embryo  was distinctly formed,
survived such treatment without damage.—That  such capability should
exist in the animals and eggs just mentioned, shows a remarkable adap-
tation to the  circumstances in which  they are destined to exist; since
Avere it  not for their poAver of surviving desiccation, the races of Wheel-
Animalcules  and Entomostraca must speedily become  extinct, through
the periodical drying up  of the small  collections of water which they
inhabit; and  a  season  of prolonged  drought must be equally fatal  to
the terrestrial Mollusca.
   160.  It would seem that many  cold-blooded animals  are reduced, by
a moderate deficiency of fluid, to a state of torpidity  closely resembling
that induced by cold; and hence it is, that during the hottest and driest
part of the tropical year,  there is  almost as complete an inactivity as  in
the Avinter of temperate regions.  The common Snail, if put into a box
without food, constructs a thin operculum or  partition across the orifice
of the shell,  and attaches itself to the  side of the box: in this state it
may remain dormant for years,  without being affected by any ordinary
changes of temperature:  but it  Avill speedily revive if plunged in Avater.
Even in their natural  haunts,  the terrestrial Mollusca of our own cli-
mates are often found in this state during the summer, when  there is a
continued drought; but  with  the first shower they revive and move 
104          EXTERNAL CONDITIONS OF VITAL ACTIVITY.
about.   In like manner it is observed that the rainy season, between
the tropics, brings  forth the hosts of insects, which the drought  had
caused to remain inactiA'e in their hiding places.  Animals thus rendered
torpid  seem to have a tendency to bury themselves  in  the ground,  like
those which are driven to winter  quarters by cold.   Mr. Danvin men-
tions that he  observed with some surprise at Rio de Janeiro, that, a few
days after some little depressions had been changed into pools of Avater
by the  rain, they Avere peopled by numerous full-grown shells and beetles.
  161. This  torpidity  consequent upon drought is not confined to In-
vertebrated animals.   There are  several Fish, inhabiting fresh Avater,
which  bury themselves  in the mud when their streams  or pools are dried
up, and which remain  there  in  a torpid condition until they are again
moistened.  This is the case with the curious Lepidosiren, which forms
so remarkable a connecting link between Fishes and the Batrachian Rep-
tiles ;  it is an inhabitant of the upper parts of the river Gambia, Avhich
are liable to be dried up during much more than half the year; and the
whole  of this period is  spent by it in  a hollow which it excavates for
itself deep in the mud, where it lies  coiled up in  a  completely torpid
condition,—whence it is called by  the natives  the sleeping-fish.   When
the return  of the rainy season causes the streams to be again filled, so
that the water finds its way down to the hiding-place of the Lepidosiren,
it comes forth again for its brief period of activity;  and Avith the ap-
proach of  drought, it  again works its way down into the mud, which
speedily hardens around it into a solid mass.  In the  same manner, the
Proteus, an inhabitant of certain lakes in the  Tyrol, which are liable to
be periodically dried up, retires at these periods to the underground pas-
sages that  connect them, where it  is believed to remain in a torpid con-
dition ; and it thence  emerges  into the lakes, as  soon as  they again
become filled with water.  The Lizards and Serpents,  too, of tropical
climates appear to be  subject to  the same kind of torpidity, in conse-
quence of drought,  as  that which affects  those of temperate  regions
during the cold of winter.  Thus Humboldt has related the strange acci-
dent of a hovel having been built over a spot where a young Crocodile
lay buried alive, though torpid, in the hardened mud; and he mentions
that the Indians  often find enormous Boas in the same lethargic state;
and that these revive when irritated or wetted with  water.   All these
examples show the  necessity of a fixed amount of fluid, in the animal
structure, for the  maintenance of vital activity: whilst they also demon-
strate, that the preservation of the vital properties of that  structure is
not always incompatible with the partial, or even the  complete abstrac-
tion of that fluid; the solid  portions being then much less liable to de-
composition than  by heat, or by other agencies, than  they are in their
ordinary condition. 
ELEMENTARY PARTS OF ANIMAL STRUCTURES.         105
                          CHAPTER  III.

          OF THE ELEMENTARY PARTS OF ANIMAL STRUCTURES.

   162. In the investigation of the operations of a complex piece of Me-
 chanism, and in the study of the forces which combine to produce the
 general result,  experience shows the advantage of first examining  the
 component  parts of the Machine,—its springs, wheels, levers, cords,
 pulleys, &c,—determining the  properties of  their  materials, and ascer-
 taining  their individual  actions.  When  these  have been  completely
 mastered, the attention may be  directed to their combined actions: and
 the bearing of these combinations upon each other, so as to produce the
 general result, would be the last object of study.
   163.  This seems  the plan Avhich the Student of Physiology may most
 advantageously pursue, in the difficult task of making himself acquainted
 Avith the operations  of the living Organism, and Avith the mode in which
 they concur in  the  maintenance of Life.   He should first examine the
 properties of the component materials of the structure, in their simplest
 form :  these he Avill  find in its nutrient fluids.   He may next proceed to
 the simplest forms of organized tissue, Avhich result from the mere solidi-
 fication of those materials, and whose properties are chiefly of a mechani-
 cal nature.   From these he will pass to the consideration of the struc-
 ture and actions of those tissues that consist chiefly of  cells;  and Avill
 investigate  the  share  they take in  the  strictly vital operations  of  the
 economy.   Next his attention Avill be engaged by the tissues produced
 by the transformation of cells ;  of which some  are destined chiefly for
 affording mechanical support to the fabric, and others for peculiar vital
 operations.   And he will be then prepared to understand the part Avhich
 these elementary tissues severally perforin in  the more complex organs.
 A due knoAvledge of  these elementary parts, and of their physical, chemi-
 cal, and vital properties, is essential to every one Avho aims  at a scientific
 knoAvledge of Physiology.  True it is, that Ave may study the results of
 their operations, without  acquaintance with them ;  but we should know
 nothing more of the workings of the machine, than we should know of
 a cotton-mill, into Avhich  we saAV cotton-Avool  entering, and from Avhich
 Ave saAV woven fabrics issuing forth ; or of a paper-making machine, which
 we saAV fed  at one end with rags, and discharging hot-pressed paper, cut
 into sheets, at the other.   The study of these results affords, of  course,
 a very important part of the knowledge we have to acquire respecting
 the  operations of the machine ; but we could learn from them very little
.of  the  nature of the separate processes effected by it; still less should
 we be prepared, by  any disorder or  irregularity in  the general results,
 to seek for, and rectify, the cause of that disturbance in  the Avorking of
 the  machine, by Avhich  the abnormal result Avas occasioned.
   164. Noav just as in a cotton-mill, there are machines of several dif-
 ferent kinds, adapted to effect different steps of the  process by which
 the raAv material  is  converted  into the woven fabric, so do we find that
 in the complex Animal fabric there is a great variety of organs for per- 
106         ELEMENTARY PARTS OF ANIMAL STRUCTURES.

forming the several changes, by Avhich the fabric itself  is built  up and
maintained in a condition fit for the performance of  its peculiar opera-
tions.  These operations are the phenomena of sensation, of spontaneous
motion, and of mental action.  They are  the great  objects of Animal
existence; just as the combination of elements  into organic substances,
that are to furnish the materials of the Animal fabric, seems  to be the
great purpose of  Vegetative Life.  The  vital  phenomena which are
peculiar to Animals, are manifestations of the properties of certain forms
of organized matter,—the Nervous and Muscular tissues,—which are
restricted to themselves; just  as those which  are common  to Animals
and Plants,  are effected by organized  structures which  are found  alike
in both kingdoms.  Here, then,  we have one essential  distinction between
these kingdoms;—namely, the  presence in Animals of a peculiar appa-
ratus, and  the consequent  possession by them  of peculiar endoAvments,
which are totally wanting in Plants.  There are, it is  true, many species,
indeed Avhole tribes, in which it is impossible to say with certainty, how
far  sensibility and spontaneity  of action may be justly inferred from the
movements  they exhibit; so that,  their structure being so simple as to
afford  no distinctive characters, our assignment of them to the Animal
or to the Vegetable kingdom must be determined entirely by  the mode
in Avhich they obtain the  materials of their nutrition  (§§ 62, 63).
  165. All the operations, then,  which are common to Animals and
Plants, are concerned in  the  building up of  the organized fabric, in the
maintenance of its integrity, and in the preparation of the germs of new
structures, to compensate for the loss of the parent by death.   These
operations, as formerly explained (§ 41), involve a series of very distinct
processes; which, though all performed by the simple  cell of the humblest
plant,  are distributed  in  more complex structures through  a number of
parts or organs,  whose several actions are almost as separate as those
of the dissimilar machines of the cotton-mill,—although, like them, sus-
tained by the same powers, and so far mutually dependent, that neither
of them can be suspended without  in a short time putting a stop to the
rest.   Now just  as in each of  the  machines of the cotton-mill Ave  may
have similar elements,—such as Avheels, levers, pulleys, bands, &c, put
together in different methods, and consequently  adapted for different pur-
poses, as carding, spinning, weaving, &c, so shall Ave find in the animal
body, that these different organs are composed of very similar  elements,
and that the individual actions  of these elementary parts are the same;
but that the difference of result is the consequence of the variety in their
arrangement.   Thus Ave shall find that the groAvth of cells, their absorp-
tion of certain matters from the surrounding fluids, and their subsequent
liberation of these  by the  bursting or liquefying of the cell-wall Avhen
their term of life is come to an end, are means employed in one part of
the body to introduce nutrient  materials  into the current  of  the circu-
lation, whilst in another  the same processes are used as means to with-
draw,  from  that  very same  current, certain substances of which  it  is
necessary to get  rid.   Noav  certain combinations of elementary struc-
ture, adapted to the performance of a set of actions tending to one pur-
pose, and thus resembling one of the machines of a cotton-mill, is termed
an organ ; and the  sum-total of its actions is termed its function.  Thus 
ALBUMINOUS COMPOUNDS.
107
 we haA'e in the function of  Respiration, which essentially consists of an
 interchange of oxygen and  carbonic acid betAveen the air and the blood,
 a multitude of distinct changes, some of them of a character apparently
 not in the least related to it, but all  necessary, in the  higher and more
 complex fabric, to bring the blood and the air into the necessary relation.
 The sum-total of these changes constitutes the function of Respiration;
 and the structures by Avhich they are effected are organs of Respiration.
   166. The entire Organized structure, then, may be regarded as made
 up of distinct organs, having  their several and  (to a  certain extent)
 independent purposes ; and these  organs may be  resolved, in like man-
 ner, into simple  elementary parts, whose structure and composition are
 the same, in Avhatever part  of  the fabric they occur.   And in like man-
 ner, the phenomena of Life, considered as a  whole, may be arranged
 under several groups or functions, according  to the immediate purpose
 to which they are directed; and  yet in every one of  these  groups, we
 shall find repeated the  same  elementary changes which are concerned
 in the rest.   Thus in the act of Respiration, the same kind of muscular
 movements, the  same sort of nervous  agency,  are  concerned, as  con-
 tribute to the ingestion of the food;  together with a  circulation  of
 blood, similar to  that which supplies  the materials for the nutrition of
 the tissues.  Hence Ave see  the  propriety of applying  ourselves first to
 the consideration of the elementary parts of the living structure,  and of
 the properties by which they effect the  changes, that are characteristic
 of its several organs.

          1. Of the Primary Components of the Animal Fabric.

   167. As we can best  study  the  primary components  of the Animal
 fabric, by investigating their properties before the process  of Organiza-
 tion begins, or whilst it is taking place, we must have recourse for this
 purpose to the nutrient fluid,—the Blood,—in which  these are contained
 in the state most  completely prepared for the reception of the vitalizing
 influence.  The same substances  may be found, in  an earlier stage of
 preparation, in the Chyle and  Lymph;  and also in  the Eggs of  ovipa-
 rous animals.   The circumstances attending the development of the
 latter afford, indeed, the most satisfactory proof of the convertibility of
 the simple chemical product, Albumen, with certain inorganic substances,
 into every form of organized structure.   For  the  white of the egg con-
 sists of nothing else than albumen, combined  Avith phosphate of lime ;
 whilst the yolk is chiefly composed of the same substance, mingled Avith
 oily matter, and  a minute  quantity  of sulphur, iron, and some  other
 inorganic bodies.   Yet  this  albumen and  fatty matter  are  converted,
 after the  lapse of a feAv days, under  the agency of  an elevated tempe-
 rature upon the germ, into  a complex fabric,  composed of bones, mus-
 cles,  nerves, tendons, ligaments,  cartilages,  fibrous  membranes, fat,
 cellular tissue, &c, and  endowed  with  the  properties characteristic of
 all these substances, which,  when brought into  consentaneous activity,
 manifest themselves in the life  of the chick.—In tracing these Avonder-
ful' transformations, therefore, we  should rightly commence  with Albu-
 men. 
108         CHEMICAL COMPOSITION OF ANIMAL TISSUES.

  168.  Albumen exists, not merely in  the Avhite and yolk of the egg,
but also in the various liquids Avhich supply the materials for the nutri-
tion of the Animal tissues.   Thus it is found in the  Chyme, or product
of the digestive operation,  Avhenever Animal  flesh,  or  any Vegetable
substance corresponding with it in composition, has been taken in as
food.  And it is  absorbed from  the alimentary canal  into the  Chyle
and  Blood, of whose solid constituents it forms a very large proportion.
In its soluble state,  Albumen is  always combined with a small quantity
of free soda,  Avith which it seems to be  united as an acid with its base;
and  to  this state of combination, its solubility is  regarded  by  most
Chemists as being due.   When the fluid in which it  is dissolved is eva-
porated at  a low temperature (not  exceeding  126°), the  Albumen, or
rather Albuminate of Soda, may be  dried, without losing its solubility;
when dried, it may be exposed to a temperature of 212°, without under-
going change ;  and it forms, when  again  dissolved in water, the  same
glairy, colorless, and nearly tasteless fluid  as  before.   When a higher
temperature is employed,  however, the  Albumen passes into the insolu-
ble  form ; and presents itself either as a cloudy or flocculent precipitate,
or as a firm consistent coagulum, according to the strength of the origi-
nal solution.  The same condition regulates the amount of heat requisite
for the purpose ;  thus if the quantity of albumen be so  great that the
liquid has a slimy aspect,  a temperature of 145° or 150°  is sufficient for
the purpose, and the whole becomes solid, white, and opaque; but in a
very dilute condition, boiling is required, and  the albumen then  sepa-
rates in the form  of white finely-divided flocks.   In either case the soda
and  other  soluble salts are  separated  from the albumen, and remain
dissolved in the water.  When  the coagulation of Albumen takes place
rapidly, the coherent mass seems quite homogeneous, and shows no trace
of anything like  definite  arrangement; but when the process is more
gradual, minute granules  present themselves, which do not, hoAvever,
exhibit a tendency towards  any higher form of structure.   The insolu-
ble  coagulum,  or pure Albumen, dries up to  a  yellow,  transparent,
horny substance; which, Avhen  macerated in water,  resumes its former
whiteness and opacity.—Pure Albumen may also be obtained  from the
solid mass which  remains  Avhen an albuminous fluid is  dried at  a low
temperature, by reducing  it to a fine poAvder, and then  washing it with
cold water  on a filter;  common salt, with sulphate,  phosphate, and car-
bonate of soda,  are dissolved out; and a soft swollen  mass remains upon
the filter, Avhich has  all the  character of Albumen obtained by precipi-
tation, except that it is readily soluble in a solution of nitrate of potash,
which Avill not dissolve the latter substance.
  169.  Albumen may be thrown doAvn from its solution, in  a coagulated
state, not merely  by heat, but by Alcohol and  Creasote,  and by most
Acids when added in excess, so as to do more than neutralize the alkali.
Nitric acid is particularly efficacious in  occasioning its coagulation ;  on
the other hand, Hydrochloric and Acetic acids, and common or tribasio
Phosphoric acid, do not precipitate it, these acids having  the property
of dissolving pure Albumen.  When albumen is dissolved in hydrochlo-
ric acid, a pinkish hue is  at first seen ; the liquid then  becomes of -an
intense purple color, and, on applying heat a little  longer, an intense 
ALBUMINOUS  COMPOUNDS.                  109
blue; after  standing for some time, it again assumes its former pink or
claret  color.—In  the  precipitation  of Albumen by an  acid, definite
compounds are formed between the two;  in which the Albumen acts the
part of a base.  On the other hand, it serves as an acid in its combina-
tions with the caustic alkalies, and is held in  solution by them.  Most
of the  metallic  salts,  as those of Copper, Lead, Mercury, &c, form
insoluble compounds with albumen, and thus give precipitates with its
solution; hence the value of white of egg as an antidote, in cases of
poisoning with Corrosive Sublimate.   The simplest method of detecting
the presence of soluble Albumen  in very small  quantity, is to boil  the
liquid, and add  nitric acid;  if  turbidity is then produced, the existence
of albumen may be inferred.   A more delicate  test of the  presence of
Albumen, however, is the precipitate Avhich is given  by the addition of
the ferro-prussiate of potash to the liquid, Avhen this  has been first
acidulated  Avith  acetic acid.
   170. Albumen is readily  decomposed by the action of the fixed alka-
lies ;  a disengagement of ammonia being  occasioned by the addition of
caustic potass to even a very weak solution of albumen.  This may be
made evident by adding a solution of sulphate of copper, a deep purple
color being produced  by the action of the liberated ammonia upon the
metal;  and thus the addition  of liquor potassae and a solution of  sul-
phate of copper, forms a very delicate test for the presence of albumen.
If Albumen,  or any albuminous  compound,  be heated  Avith  caustic
potash, it  is  completely decomposed; not, however, being  resolved at
once into its ultimate  constituents, or altogether into  simple  combina-
tions of them, but in great part into other organic compounds.   One of
these, termed Leucin, is a crystalline substance, which forms colorless
scales,  destitute of taste and odor ; it is soluble  in water and  alcohol,
and sublimes unchanged.   It  consists of 12  Carbon, 12 Hydrogen,  1
Nitrogen, and 4 Oxygen.  There is not at present any evidence that it
is produced in the living body; and the chief interest which attaches to
it arises from the  fact, that it may be procured from Gelatine  as well
as from proteine;  Avhich indicates a certain relationship  between these
two substances.  Another compound  obtained by the same  reaction, is
called  Tyrosin ; it crystallizes in brilliant needles;  and its composition
is 16 C, 9  H,  1 N,  5 0.—Besides these substances, Ammonia,  with
Formic and  Carbonic  Acids,  are produced; the acids unite with  the
potash  employed to effect  the  decomposition; and the ammonia is set
free.—The action of caustic potash upon Albumen has also the effect
of liberating sulphur, which unites with the potash, and  remains in the
solution, where its presence  may be recognised by the black precipitate
formed on the addition of a  solution of acetate of lead.  The existence
of unoxidized Sulphur in albumen is  shoAvn by the  familiar fact of the
blackening of a  silver spoon by a boiled egg,  which is due to the forma-
tion of  an  alkaline sulphuret during  coagulation.  Albumen  may also
be shown to contain Phosphorus in an unoxidized state.   In its soluble
state, Albumen is very commonly united with Phosphate of Lime, about
tAvo per cent, of which  may be taken up by it; and  it is chiefly in this
mode, that  this very important substance is introduced into the Animal
body. 
110         CHEMICAL COMPOSITION OF ANIMAL TISSUES.
  171.  The ultimate  composition of  the Albumen of  the blood may be
stated as folloAVS :—
              Carbon,    -------   548
              Hydrogen,     ------    71
              Oxygen,.......212
              Nitrogen,      ------    159
              Sulphur,   -------    7
              Phosphorus,    ------     3

                                                  1000
  It has been maintained by Mulder, that the sulphur and phosphorus
may be completely separated from the substance formed  by the  union
of the first four elements; and  to this substance  he  gave the name of
Proteine, believing it to be the base of the whole series of albuminous
compounds, which are supposed to consist of proteine, united with  sul-
phur and phosphorus in varying proportions.   It does not appear, how-
ever, that proteine has ever been really obtained in  an isolated  form;
and its existence  must at present be  considered  as  hypothetical.   It
may be comrenient, however, to use the phrase " proteine-compounds,"
to designate  the  whole  series of Animal  and Vegetable substances,
which  are  capable of being converted into Albumen in  the digestive
process: and in this sense alone will  it be here employed.—The atomic
constitution of Proteine is  considered by Liebig  to be  represented by
the formula—
              49 Carbon, 3G Hydrogen, 14 Oxygen, 6 Nitrogen,
whilst by Mulder  the following formula is adopted—
              40 Carbon, 31 Hydrogen, 12 Oxygen, 5 Nitrogen.
Both  these formulae are sufficiently conformable to the relative propor-
tions of the components; but it has not yet been determined Avhich best
represents the combining equivalent of the substance.
  172.  Nearly allied to  Albumen is  the  substance  termed Caseine,
which replaces it in milk;  and this is specially Avorthy of notice here,
because it  is  the  sole form in which  the young Mammal receives Pro-
teine into its body, during the period of lactation.  Like Albumen, this
substance may exist in two forms, the soluble, and the insoluble or coa-
gulated ; and it further agrees with  it, in requiring, as a condition of
its solubility, the  presence  of a free alkali,  of which, hoAvever, a very
small quantity suffices for the purpose.  It differs from Albumen, how-
ever, in this:  that it does not coagulate by heat, and  that it  is precipi-
tated from its solution  by Acetic acid.  Caseine is further remarkable
for  the  facility with which its coagulation  is  effected  by the  contact of
certain animal membranes, as in the ordinary process  of cheese-making.
This change is considered by some Chemists to be due, however,  not to
any direct action of the membrane upon the caseine, but to its influence
in converting some of the milk-sugar into lactic acid,  which, separating
the alkali of  the caseine, will occasion the precipitation of the  latter.
The only difference which can be detected between Albumen  and Case-
ine, in regard to  the proportions  of their elements, consists in the  ab-
sence of Phosphorus, and the smaller proportion of Sulphur, in  the
latter; but this can scarcely be the cause of the foregoing  differences in 
ALBUMINOUS  COMPOUNDS.                   HI
their properties.   Caseine appears even to surpass Albumen in its power
of combining Avith the phosphates of lime and magnesia, and of render-
ing them soluble.
   173.  Albumen and Caseine, then,  may be  regarded as constituting
the raw materials, at the expense of which the organized tissues of the
Animal fabric are built up ; and we have sufficient  evidence, in the de-
velopment of the Chick from the egg, and of the young Mammal  from
milk, that they may be transformed into any of the  proteine-compounds
Avhich are to be found in the Animal body.   There is good  ground to
believe, however, that, for  the formation of all the Animal tissues, the
presence of fatty matter, as well as of an albuminous compound, is es-
sential ; and it is a circumstance worthy of note,  that in both the  fore-
going cases, fatty matter is mingled with the  albumen, in the  aliment
destined for the development of the young animal.   This subject, how-
ever, Avill be more fully discussed hereafter.
   171.  The Animal derives the materials of its nutrition, however, not
only from the Albuminous compounds furnished by flesh, eggs, and milk,
but also from those Avhich are supplied by the Vegetable kingdom. Every
groAving Plant, as already mentioned (§ 12), forms albuminous compounds
by the  combination of inorganic elements,  as  the  pabulum of its own
tissues; but many Plants generate them in much  larger proportion, and
store them  up in their cells;  and it is from such, that Animals derive
their largest supply of nutritive material.  Thus the gluten of Avheat, or
the tenacious mass which is left after the removal  of the starch by wash-
ing, is principally composed  of a  substance AA'hich  is closely allied in
composition and properties to the Albumen of animals ; being moderately
soluble by water, coagulated by heat, and precipitated by acids (except
the phosphoric and acetic) and by metallic salts;  and this is designated
Vegetable Albumen.  From the seeds of Leguminous plants a substance
termed Legumin may be separated, which corresponds Avith Caseine in
not being precipitated by heat, and also in the  absence of phosphorus;
hence it is sometimes designated as Vegetable Caseine.   Various other
modifications of the albuminous principle are  found in Plants;  but the
foregoing are the  most important in their relations  to the  nutrition of
Animals.   Like flesh, cheese,  &c, they are reduced by the digestive
process  to  the state of soluble Albumen ; and in  this form they are
taken up, and carried into the circulation.
   175.  Next in importance to the Albuminous compounds as a consti-
tuent of the Animal fabric, is Gelatine ; which is obtained by the action
of boiling Avater on White Fibrous tissue, and on the various membranes,
&c, of Avhich this is the chief component.  The composition of Gelatine
is much simpler than that  of the Proteine-compounds, so far, at least,
as regards the number of atoms of  its several elements; for it consists
of 13 Carbon,  10 Hydrogen, 5  Oxygen, 2 Nitrogen.  This composition is
the same, Avhether the Gelatine be obtained from  isinglass, from fibrous
membranes, or from bones.  The distinctive characters of Gelatine are
its solubility in Avarm water, its coagulation  on cooling into a uniform
jelly, and its formation of  a peculiar insoluble compound with Tannic
acid.   Gelatine is  very sparingly  soluble in cold water; though pro-
longed  contact with  it will  cause  the Gelatine to swell  up and soften. 
112         CHEMICAL  COMPOSITION  OF ANIMAL TISSUES.

Its poAver of forming a jelly on  cooling is  such, that a solution of one
part in 100 of Avater Avill become a consistent solid.   And  its reaction
with Tannic acid is so distinct, that the presence of one part of Gelatine
in 5000 of water is at once detected by infusion of Galls.   There can
be no doubt that Gelatine does not  exist exactly as such in the Fibrous
tissues ; since none can be dissolved out of them by the continued action
of cold  water, and it usually requires the prolonged action of hot Avater,
to occasion their complete conversion.   There are some substances, how-
ever, in which this is not  requisite; and from which the gelatine may
be extracted within a shorter time.   This is the case, for example, with
the air-bladder of  the Cod and other  fish; which when cut into shreds
and dried, is known as Isinglass.  It is the case also with the substance
of bones, from which the calcareous matter  has been removed.   In both
instances it would  seem that the state of organization is very imperfect;
the fibrous structure being by no means well-marked.   When the fibrous
arrangement is more  complete,  the solubility of  the tissue is much di-
minished.  Hence it would seem that the  particles have a  different ar-
rangement in the  tissues, from  that Avhich they  have in  the  product
obtained by boiling.  Their ultimate composition,  however,  is the same;
for when any serous membrane, or other tissue principally  composed of
the white fibrous element, is  analysed by combustion,  the elements are
found to have the same  proportion to each other as Gelatine, allow-
ance being made  for the small  admixture of other substances.  The
action of Tannic acid, too, is the same on the  organized  tissue, as it is
on the gelatine extracted from it; and hence results its utility in pro-
ducing an insoluble  compound, not liable to undergo  decomposition, in
the substance of the Skin, converting it into leather.
  176.  It is not yet known how Gelatine is produced in the Animal body.
There can be no doubt that  it may be elaborated from Albumen;  since
we find  a very large amount of Gelatine in  the tissues of young animals,
which are entirely formed  from albuminous matter; and  also in the
tissues of herbivorous animals, which cannot receive it in their food, as
Plants yield no substance resembling gelatine.  Carnivorous animals,
however, Avill receive it ready^formed, as part of their aliment.  There
is no reason to believe that it is capable of  being  converted into Albu-
men ; and consequently it can never be applied to the nutrition of the
albuminous tissues. If Gelatine be boiled for some time in caustic potash,
it is decomposed, with an escape  of ammonia; and two new compounds,
leucine, and glycocoll or gelatine-sugar, are generated.  The production
of leucine from Gelatine, by the action of the same reagent as that
which caused its generation from Albumen, is a fact of much importance;
as showing that,  notwithstanding their difference of composition and
characters, a certain similarity in  the arrangement  of their ultimate
elements still subsists between these two bodies.  Glycocoll is an organic
base of  great interest from its relations to  other substances;  as will be
shown hereafter.   It has a  strong SAveet  taste, and is very soluble in
water, from which it may be crystallized like ordinary sugar.  Its compo-
sition is  comparatively simple; being 4 Carbon, 4 Hydrogen, 1 Nitrogen,
3 Oxygen.
  177.  A peculiar modification  of Gelatine,  which presents itself in 
ALBUMINOUS COMPOUNDS.
113
Cartilage,  is distinguished as Chondrine.   This requires longer boiling
than gelatine for its solution in water;  but the solution fixes into a jelly
in cooling, and dries by evaporation  into a glue that  cannot  be distin-
guished from that of gelatine.  Like gelatine, it is thrown down from its
solution by alcohol, creasote, tannic acid, and bichloride of mercury; but
it is also precipitated by acetic acid, alum, acetate of  lead, and prosul-
phate of iron, which  do not  disturb a solution of gelatine.—It is curious
that, in proportion as  Cartilages become fibrous,  their Chondrine gives
place to  Gelatine ; and during the progress of ossification, the Chondrine
seems to be  entirely  replaced by  Gelatine, of which the fibrous basis of
the bony tissue is composed.
   178.  The foregoing may be considered as the chief among the "raw
materials" of the Animal fabric  ; and  it is noAV to be shown, that while
the Gelatinous components  merely become subservient to the  formation
of tissues, Avhose structure  is the simplest  possible, and whose function
is purely mechanical, the destination of the Albuminous  compounds is
much higher; it being at the expense of the latter that those tissues are
generated, Avhich are the instruments of the purely vital operations of
the Animal  economy.  In their progress towards the  state of complete
organization, however, we find  that they pass through an intermediate
condition,  which is one that requires  special  consideration.  The fluids
that are formed at the expense of the  Albuminous matters which have
been digested and absorbed, contain a substance, which is so closely related
to Albumen in its ultimate  Chemical composition, as  not to  be distin-
guishable from it Avith any  degree of certainty,* but which, though still
fluid Avhilst circulating in the living vessels, exhibits a  decided tendency
to assume the organized form, and  manifests properties which  are so
different from those of inorganic matter, that they must be regarded as
vital.  This substance is Fibrine.  It  is found in  the Chyle, or crude
blood, soon after  this  is taken up from the food; it  presents itself in
gradually increasing proportion,  as the Chyle sloAvly  passes  along the
Lacteal vessels, and through the  Mesenteric glands, towards the termi-
nation of the Absorbent system  in the Venous; and it is also found in
the fluid contents of that other  division of the Absorbent system, the
Lymphatics, Avhich is distributed  through the  body at  large, and which
seems to have for its chief  office  to take up, and to reintroduce into the
circulating current, such particles contained in the fluids of the tissues,
as do not require to  be at once cast out of the body, but may be again
employed in the process of Nutrition.   But it is found, above  all, in the
Blood; the fluid Avhose ceaseless  and rapid course through the body sup-
plies to every element of the structure  the materials of  its groAvth and

  * According to some analyses, Fibrine differs slightly from Albumen in ultimate com-
position, the proportions  of its several constituents in 1000 parts being as follows:—
Carbon 546, Hydrogen 70, Oxygen 220, Nitrogen 157, Sulphur 4, and Phosphorus 3.
The difference in their vital relations, however,  is far greater than any such difference
in their chemical composition can  account for, and can only be justly attributed to  the
forces brought to act upon the fibrine during its circulation  in the vessels of the living
body.  It has been recently maintained, that Fibrine is not to be regarded (as there
represented) as Albumen in the transition-stage of incipient organization, but that it is
a product of disintegration of the tissues, only received back into the blood in order
that it may be carried out of the system through the excretory channels. For a discus-
sion of this hypothesis, see the Brit, and For. Med. Chir. Review, vol. vii. pp. 153, 473.
                                   8 
114         CHEMICAL  COMPOSITION OF ANIMAL TISSUES.

deA'elopment;  and the varying proportions  in which it  presents itself
there are evidently closely connected with the formative poAvers of that
fluid.  It is also a principal  element of certain colorless exudations,
which are put forth from Avounded  or inflamed  surfaces, or Avhich are
deposited in the interstices of inflamed tissues; these exudations, when
possessed of a high formative property (that is, a readiness  to produce
an organized tissue), are said  to be composed of coagulable or  organi-
zable lymph, Avhich is nothing more  than the fibrinous  element of the
blood, in  an unusually concentrated  state.   We  shall first notice  the
Chemical properties of Fibrine; and  shall then inquire into those, Avhich
present the first dawnings or indications of Vitality.
  179.  Like  the other  Proteine-compounds,  Fibrine  may exist  in
solution,  or in  an insoluble  form;  but there   is this  important  dif-
ference,—that its soluble form  is not  a permanent one, and cannot
be maintained in any fibrinous fluid that  has  been drawn  from the
living vessels, Avithout the influence  of reagents, Avhich  totally  destroy
its peculiar properties.  All investigations of a Chemical nature, there-
fore, must be made upon insoluble Fibrine;  and this  may be obtained in
its purest state, by whipping fresh  blood Avith  a bundle of  twigs, by
which operation it will be caused, in coagulating,  to adhere to the twigs
in the form of long, white, elastic filaments,  Avith scarcely an admixture
of foreign matter.  When dried in vacuo, or at a  gentle heat, it becomes
translucent and horny;  and in this condition, it closely resembles coagu-
lated albumen.  It further resembles that substance,  in being soluble in
very dilute  caustic alkali, and in phosphoric acid;  and  the  solutions
exhibit many  of the properties of the similar  solutions of  albumen.
When the Fibrine of venous blood is  triturated in a mortar Avith a solu-
tion  of nitrate of potash,  and the mixture is left for twenty-four hours
or more, at a temperature of from  100°  to 120°, it becomes  gelatinous,
slimy, and eventually entirely liquid.  In this condition it  exhibits all
the properties of a solution of Albumen which has been  neutralized by
acetic acid.  It coagulates by heat;  it is precipitated by alcohol, corro-
sive  sublimate, &c.; and, when largely diluted,  it deposits  a flocculent
substance, not to be distinguished from  insoluble albumen.   The close
Chemical relation of Fibrine and Albumen is further proved by the ready
conversion of the former into the latter in the act of  digestion;  Animal
flesh, which consists of Fibrine, being reduced to the form of Albumen
with the same facility, as  the Vegetable compounds which resemble the
latter much more closely  in the first  instance.   The  Fibrine of  arterial
blood, however, cannot  be reduced to the fluid  form by solution with
nitre;  and this appears to be due to its oxidized condition; for in a solu-
tion  of Venous fibrine in  nitre, contained in  a deep cylindrical jar, and
having its surface freely exposed to the air,  a fine flocculent precipitate
is gradually seen to form  ; and this, when collected, is found to have the
properties of arterial fibrine.   The Fibrine of Animal flesh  agrees with
that  of venous, rather than with that of arterial blood.  Fibrine,  like
Albumen, unites with acids as a base, forming definite compounds;  and
with bases as an acid.  It also possesses the property of uniting with
the earthy phosphates; of which from 0-7 to 2-5 per  cent, are found in
the ash that is left after its combustion. 
FIBRINE.
115
  180. We see, then, that when considered in its simply-Chemical rela-
tions, Fibrine does not differ in any essential particular from Albumen ;
and that the chief point of obvious variation, is the spontaneous coagu-
lation of the former, when it is removed from the  living body.  There
is, however, in  the structure of the coagulum itself, a  most important
difference ; for instead of  consisting of a homogeneous structureless
mass, or of a simple aggregation of minute granules, it is found by  the
Microscope to possess  a definite fibrous arrangement, the fibres crossing
one  another in every direction.   In the ordinary coagulum or clot of
Blood, these fibres do  not present any great degree of firmness: they
may be  hardened, however, by  boiling;  and their  arrangement then
becomes more definite.   They may be seen much more clearly, however,
in the "buffy coat" of Inflammatory blood; in which there is not only
an increased proportion of Fibrine, but the Fibrine itself seems to have
undergone a higher elaboration, that is, to have proceeded still further
in the change towards regular organization.   In this state, the process
of coagulation  is unusually slow;  the clot formed  by the fibrous tissue
is much more  solid; and it continues for some hours, or even days, to
increase in solidity, by the mutual attraction of the particles composing
the fibres, which cause them to contract and to expel the fluid contained
in their interstices.
   181.   The  most perfect fibrous structure originating in the simple
coagulation of fibrine, is to be  found, however,  in those  exudations,
which take place  either from inflammation, or from a  peculiar forma-
tive  action, destined to repair  an old tissue or to produce  a  new one.
                   Fig. 2.                        Fig. 3.
Fibrous structure of inflammatory exudation     Fibrous membrane, lining the egg-shell,
        from peritoneum.             and forming the animal basis of the shell
                                itself.
Thus in Fig. 2 is shown  the  fibrous structure  of  a false membrane,
formed by the consolidation of a fibrinous exudation from the surface of
an inflamed peritoneum.  And in Fig. 3 is displayed a similar fibrous
structure  (in which, however, the fibres have more of a reticulated ar-
rangement),  Avhich incloses the fluid contents of the egg, and enters
into the composition of the shell itself.   As the  ovum (Avhich, at  the
time of its  quitting the ovarium, consists  of the yolk-bag only) passes
along the oviduct of the parent, it  receives its  coating of albuminous
matter, of which layer after layer is thrown out by the vessels of  the
oviduct.   When a sufficient supply has thus been furnished, it appears 
116          CHEMICAL  COMPOSITION  OF ANIMAL TISSUES.

that fibrinous instead of albuminous matter is poured forth;  and this,
in coagulating, forms a very thin layer of fibrous tissue, which enve-
lopes the albumen.   Layer after  layer is gradually added ;  and at last,
by the superposition  of these layers, that firm tenacious membrane is
formed, Avhich is afterwards found lining the egg-shell.  The process is
then continued, with  this  variation, that carbonate of lime is also  se-
creted from the blood in  a chalky state, and  its  particles lie  in the in-
terstices of  the fibrous  network, and  give it that  solidity  Avhich  is
characteristic of the  shell.   If they  be removed by the  agency of a
weak acid, or if the bird be not sufficiently supplied with lime at the
time of laying, the outer membrane  has the same consistence as the
inner: and either may be separated, after prolonged  maceration,  by
dexterous manipulation, into a series of layers of  a fibrous matting, like
that represented in Fig. 3.
  182.  It is scarcely possible to  deny to such a  tissue  the designation
of an organized structure, even though  it contains no vessels, and may
not participate in any further Vital  phenomena.  We  shall  hereafter
find, that a tissue presenting very similar characters forms a large part
of the Animal fabric; and that  the vessels Avith  which  it  is  copiously
supplied, have for their  object nothing else than  the removal of its dis-
integrated  or decaying portions, and the deposition of new matter in a
similar  form (§ 194).  In the production of new parts,  we find  this
simple fibrous tissue performing the important function  of serving as a
matrix or bed for the support of  the  vessels;  and as, by the more gra-
dual transformation of the nutritive materials they bring, neAV  and more
permanent tissues are formed, the original one gradually undergoes dis-
integration, and all traces of it are in time lost.  This would  appear to
be the history of  the Chorion of the Mammalian ovum;  which is at
first nothing else than a fibrous unvascular bag, formed round the ovum
in its passage through the Fallopian tube, precisely after the manner of
the shell-membrane  of  the Bird's egg; but which is afterwards pene-
trated by vessels proceeding from the embryo, and in time acquires a
new structure (chap,  xi.)
  183.  The completeness of the production of such a  fibrous tissue
depends in  part,  as we  have seen,  upon the  degree of  elaboration
which  the  Fibrine has undergone;  but  in great  part also upon the  na-
ture of the surface, on which the  coagulation takes place.  Thus we
never find  so perfect a membrane formed by the consolidation  of  the
Fibrine out  of the living  body,—on  a slip of glass for  example,—as
when it takes place on the surface of a living membrane, or in the in-
terstices of a living tissue.  This may perhaps be accounted for by the
fact, that the coagulation takes  place much more  slowly in the latter
case than  in the former, and that the particles may thus haAre more
time  to arrange themselves in the definite fibrillation, which seems to
be  their characteristic  mode of aggregation:  just as crystallization
takes place best when the action is  slow; and  as  a substance, whose
particles would remain in an amorphous or disunited form if too rapidly
precipitated from  a solution, may present a most regular arrangement
when they are separated from it  more  slowly.   Of this view it would
seem to be a confirmation, that the most perfect fibrillation out of the 
COAGULATION  OF FIBRINE.                  117
body is usually seen in those cases,  in  which coagulation  takes  place
least rapidly.
  184. The conditions  under  Avhich the  spontaneous coagulation of
Fibrine takes place, are best known  from the  observation of that pro-
cess as it occurs  in the Blood ; and although this fluid, as we shall here-
after see, is of a very complex nature, yet  as the Fibrine alone is con-
cerned in its coagulation, and as that act appears to take place  in the
same manner as if no  other substance was present, there appears to be
no objection to the employment of the phenomena of Blood-coagulation
as the  basis of our account of the properties of Fibrine.—There can be
no doubt, from Microscopial observation of the circulating Blood, that
Fibrine is in a state of perfect solution in the fluid; and in  this  condi-
tion it remains, so long  as it is in motion in the  living body.  That  its
fluidity,  however, does not depend only  upon its  movement, is evident
from two facts;—first, that no kind  of  motion seems effectual in pre-
venting the coagulation of the blood, after  it has been drawrn  from the
 vessels;—and  second, that a state of rest  within the living body does
not immediately  produce  coagulation; a portion  of blood, included be-
tween two ligatures in a living vessel, remaining fluid for a long  time ;
and blood that has been reduced to a state of complete stagnation  by
 inflammatory action,  being often found in a fluid state  after some days.
 On  the  other  hand, it seems certain that  the state  of vitality of the
parts  with Avhich the  blood is in contact, has a great influence in pre-
serving its fluidity; thus  it has been found that, if the  brain and  spinal
cord of an animal be broken doAvn, and  by  this measure the vitality of
the body at large be lowered, clots of blood are  formed in  their  trunks
within a  feAv  minutes.   Nevertheless,  a  mass  of blood effused  into a
cavity of the living body, undergoes  coagulation almost as soon  as it
would  in a dead vessel; but this may be  accounted for by the very small
surface which is  in contact with the blood, as compared Avith the mass
of the  latter.   It must be remembered that the circulating blood is con-
tinually being subdivided into countless  streams; and that each of these
passes through the living tissue, in such a manner that all its particles
are in  close relation with the living  surface.  Moreover it  is probable
that the  form of  matter which Ave term Fibrine, never  remains long in
that condition  in the  ordinary state  of  the system ; being  continually
AvithdraAvn by the nutritive processes, and as continually re-formed from
the  Albumen, by an elaborating action hereafter  to be  considered.
Hence we may regard the state of motion through  living vessels, as
essential  to the permanent continuance of fibrine in the  fluid form.
   185. The length of time, hoAvever  during Avhich Fibrine  may remain
uncoagulated, after it has been  withdraAvn  from  the living  body,  varies
according to various conditions; some of which are not Avell understood.
In the first place, as already remarked,  the more elaborated and  more
concentrated the condition of  the  Fibrine, the more  slowly does it
usually coagulate.  Thus Avhen a large quantity of blood  is draAvn, at
one bleeding, into several vessels, that Avhich Aoavs first takes the longest
time to coagulate, and forms the firmest clot;  whilst that which  is last
drawn coagulates most rapidly, and with the least tenacity.   The coagu-
lation  is accelerated by moderate heat,  and retarded by cold; but it is 
118          CHEMICAL COMPOSITION OF ANIMAL TISSUES.

not prevented even by extreme cold ; for if blood be frozen immediately
that it is drawn, it will coagulate on being thawed,—thus preserving its
vitality in spite of the freezing process, like the organized structures  of
many of the lower  animals.   Again, the  coagulation is accelerated by
exposure to air; but it is not prevented, though it is retarded, by com-
plete exclusion from it.  Various Chemical  agents retard the coagula-
tion, without preventing it; this is the  case especially with solutions of
the  neutral  salts.   The  coagulation is not  so firm, however, or the
fibrillation so perfect, after the use of these ; and there can be no doubt
that they  modify the properties of the  fibrine, by acting chemically
upon it.
  186.  After remaining in this condition for a certain length of time,
the Fibrine undergoes a further change, which is evidently the result of
decomposition; the coagulum becomes soft, and exhibits appearances  of
putrefaction.  This takes place the  more rapidly, as the first coagula-
tion was less complete.  Thus in the imperfectly-elaborated Fibrine  of
the Chyle, the coagulum is sometimes  so incomplete, that it does not
separate itself from the serum, and liquefies again in half an hour.  In
certain  states of  disease, the  solidifying properties  of  the  Fibrine are
very much  impaired;  so that it soon liquefies and decomposes.   In these
cases, there is scarcely any trace of the characteristic  fibrous arrange-
ment of the particles.  On the other hand,  the fibrinous coagulum  of
inflamed blood, as it is more solid, is also more persistent than that of
ordinary blood ; and the greatest persistency of all  is seen in the fibrous
network formed by exudation, as  in the cases just now mentioned.
  187.  The coagulating power of Fibrine,—in other words, its peculiar
vital property,—may be destroyed by various causes operating  within
the living body;  so  that the  blood  remains fluid  after death.   These
may be classed under three heads.   In the first place, the vitality of
the fibrine  may be destroyed  by  substances  introduced into  the blood
from without; which have the power of acting in the manner of ferments,
and Avhich occasion an obvious chemical change in its condition.   Such
is the case  in those severe forms of Fever which are termed malignant;
and especially those which  result  from the contact of putrescent matter,
as Glanders, Pustule  maligne, &c.   Secondly, it may be impaired  or
altogether destroyed by morbid actions  originating  in the system itself,
and depending upon irregular  nutrition  or imperfect excretion; thus the
blood has been found fluid after  death in severe cases of Scurvy and
Purpura, also in  cases of Asphyxia  (consequent upon  the  retention of
carbonic acid in  the  blood), and in the bodies of  overdriven animals.
The same result may  follow, Thirdly, from violent shocks or impressions,
which suddenly destroy the vitality of the whole system at once ; these
may be such as are obviously capable of producing a chemical or me-
chanical change, as in the  case of death by Lightning or by a violent
Electrical  discharge;  or they may act  through the nervous system, in
a manner not yet clearly understood, as Avhen  death results  from con-
cussion of the bjjain, from a  blow upon the  epigastrium, from violent
mental  emotion, or from a coup de soleil.—It is not to be supposed that
the non-coagulability of the Blood is a  phenomenon by any means inva-
riable under  the  foregoing circumstances; but it has been  occasionally 
SIMPLE  FIBROUS  TISSUES.
119
observed in all of them.   We must not mistake for the absence of coagu-
lating power, the remarkable retardation of the act of coagulation which
sometimes occurs.   Thus the blood is occasionally found in a fluid con-
dition in the bodies of persons that have been dead for some days ; and
yet Avhen withdraAvn from the vessels, it coagulates.

                   2. Of the Simple Fibrous  Tissues.

   188.  A large part of the Animal fabric, especially among the higher
classes in Avhich the parts have the greatest amount of motion upon one
another, is composed of tissues,  which  seem  as if they consisted  of
nothing  else  than fibres, of the simple  character already described,
woven together in  various  Avays, according to  the  purposes  they are
destined  to serve.   These  fibres  are altogether different from  those
hereafter to be described as  constituting the Muscular and Nervous tis-
sues, and must not be confounded with  them.    The former are  solid,
and possess none but physical properties ; the latter are tubular, and
are distinguished by their peculiar vital endoAvments, which seem chiefly,
if not entirely, to reside  in the contents of the  tubular fibre.   The Sim-
ple Fibrous tissues, of which we have now to treat, appear to have it for
their sole office in  the animal body to bind together the other elementary
parts into  one whole, without uniting them so closely as to render them
immovable ; and we find the same elements  arranged in very different
modes, according to  the  purposes they are destined to fulfil.  Thus  in
the Tendons, by which the  muscles are  connected with the  bones, and
impart motion to them, the  only property required is  that of  resisting
strain or tension in one direction ;  and in these we find the fibres dis-
posed in a  parallel  arrangement, passing continuously in straight line
betAveen the points of attachment.   In  the Ligaments which connect
the bones together, and  which also have for  their  purpose to afford
resistance  to strain, but which are liable to tension in a  greater variety
of directions, Ave find bundles of  fibres crossing each other according to
these directions ; and in some  instances we find the ligaments endowed
also with a certain degree of elasticity.   The  structure  of  the  strong
                                Fig. 4.
             Simple fibrous tissue; a, fibres of areolar tissue; 6, tendinous fibres.

Fibrous Membranes, which form the envelopes to different organs and
bind together the contained parts, is very similar ; each  of these mem-
branes being composed of several layers of a dense network, formed by 
120      STRUCTURE  AND  ENDOWMENTS OF ANIMAL TISSUES.
the interAveaving of bundles of fibres in  different  directions.   In the
Fibro-Cartilages, w?e find  a mixture of the characteristic structure of
Ligament  with that of Cartilage ; bundles  of fibres, similar to those
which constitute the former, being disposed  among the cells which are
the chief organized  constituents of the latter.   In certain Fibro-Carti-
lages, hoAvever, these fibres are endowed Avith a high degree of elasticity.
   189.  These  tAvo  qualities,—that of resistance to tension without any
yielding—and  that of resistance combined with elasticity,—are charac-
teristic of two distinct forms of Fibrous tissue, the White and  the Yellow.
The White Fibrous tissue presents itself under various forms,  being some-
times composed of fibres  so minute as to be scarcely distinguishable;
and sometimes presenting itself under the aspect of bands, usually of a
flattened form and  attaining the breadth of l-500th of an inch.   These
bands are  marked by*numerous longitudinal streaks, but they cannot be
torn up into minute fibres of determinate size; hence they  must be re-
garded as  made up of an aggregation of the same  elements as those
which may become developed  into separate fibres.   The fibres and bands
are occasionally somewhat wavy in their direction.   The tissue, which is
perfectly inelastic, is easily distinguished from the  other by the effect of
Acetic acid, which  swells  it  up and renders it transparent,  at the same
time bringing into vieAv certain  oval corpuscles, which are supposed to
be the nuclei of the cells that were concerned in  the formation of the
tissue.
   190. The Yellow Fibrous  tissue exists in  the form of long,  single,
elastic, branched filaments, with a dark  decided border, which are dis-
posed to  curl when not put on the stretch.   They are for the most part
between l-5000th and l-10,000th of an inch in diameter; but they are
often met with  both larger and smaller.    They frequently anastomose,
so as to form a network, as shown in Fig. 6.   This tissue does not un-
Fig. 5.
Fig. 6.
 Fasciculus of fibres of white fibrous tissue
from lateral ligament of knee joint.
  Yellow fibrous tissue from ligamentum
nuchas; a, the fibres drawn apart, to show
their reticulate arrangement; b, the fibres
in situ.
dergo any change, when treated with acetic acid.   It exists alone (that
is, without any mixture of the white) in  parts which require a peculiar
elasticity, such as the middle coat of the  Arteries, the Chordae Vecales,
the Ligamentum Nuchse (of  Quadrupeds) and the Ligamenta subflava ;
it enters largely into the  composition of  certain parts,  which are com- 
SIMPLE FIBROUS  TISSUES.
121
Fig. 7.
monly regarded as Cartilaginous, such as the external ear; and it is also
a principal component of other tissues to be presently described.
  191. These tissues are very different in Chemical composition.   Those
which are composed  of  the White fibrous element,—namely, Tendons,
Ligament, &c.—are almost entirely resolved by long boiling into Gela-
tine;  and this substance  is  also largely obtained from  the  Skin, and
from Mucous and  Serous membranes, of which, as  Ave shall presently
see, that  element is a principal component; whilst it is also yielded in
great quantity by Bones, whose animal basis is almost entirely gelatinous.
  192. The composition of the Yellow fibrous tissue appears to be alto-
gether dissimilar.  It scarcely  undergoes any change by prolonged boil-
ing; it is unaffected also  by the Aveaker acids;  and it preserves its
elasticity, if kept moist, for an almost unlimited period.  According to
Scherer,  it consists of 48 Carbon, 38  Hydrogen, 6 Nitrogen, and 16
Oxygen; and he considers it to  be composed  of  an atom  of Proteine
with two  atoms of water.  (See § 171.)
  193. The simple Fibrous tissues appear  to be
very little susceptible of change in the living body;
and we  find them  very  sparingly supplied with
blood-vessels.   In  the solid  Tendons, the  bundles
of  straight  parallel fibres  are a little  separated
from each other by the intervention of the Areolar
tissue to be presently noticed;  and  this  permits
the sparing access  of vessels to their  interior.  In
the  Fibrous  Membranes  and  Ligaments, this is
 found in somewhat larger amount; and the vascu-
larity of these tissues is rather greater.   Tavo  dif-
 ferent views of their mode  of  development have
 been taken by those Avho have studied it.   By some
 it has been  maintained  that the White  fibres  are
first developed as cells, which progressively  be- ^7^1!™%]^"^
come elongated and solidified;  their nuclei at  the coming pointed; c, the same
      ,••,.       .        mi     i   ■      •   become fusiform,  the nuclei
 same time disappearing,  until  brought into vieAv being sun  apparent; d,  the
by  acetic acid (Fig. 7);  and the Yellow fibres have KiSKffg&SpSa
been supposed to Love a similar origin.   By others
it has been considered that  the White fibres  are produced by the direct
fibrillation of a blastema or plastic exudation (§ 181), and that the Yellow
proceed from the muscular particles Avhich this contains ; no development
of cells being requisite for the production of either.    The recent inqui-
ries of Mr. Paget and others tends to shoAv that both these methods are
adopted  in the production of the fibrous tissue Avhich is  developed for
the repair of injuries in the adult body; the former being  seen in the
reparation of external wounds to Avhich air has access; the latter in the
organization of " coagulable lymph" effused into internal cavities,  or
into the interstices of tissue, altogether secluded from  it.
  194. The great  use of the foregoing  tissues  appears to be, to afford a
firm resistance to tension ; by which they may either communicate motion,
as in the case of Tendons; or restrain it, as in the case of  Ligaments;
or altogether prevent it,  as in the case of  Aponeuroses and Fibrous
Membranes.  With this firm resistance, a considerable amount of  elas-
Development of fibres from 
122      STRUCTURE  AND  ENDOAVMENTS OF ANIMAL TISSUES.

ticity may be combined.   But we  have now to notice a tissue, in Avhich
a very different  arrangement of the same elements presents itself; and
the object of this is, to bind together the elements of the different fabrics
of the body, and at the same time to endoAv them Avith a greater or less
degree of freedom of movement upon  one another.   The  tissue, Avhich
is called the Areolar, consists of a netAvork of  minute fibres and bands,
which are  interwoven in every direction, so as to leave innumerable
areolae or little spaces, which communicate freely with one another.  Of
these fibres, some are  of the yellow or elastic  kind;  but the  majority
are composed of the white fibrous tissue, and, as  in  that  form of ele-
mentary structure, they frequently present the form of broad  flattened
bands, or membranous shreds, in Avhich no distinct fibrous arrangement
is  visible.   The interstices are filled  during life  with  a fluid, which
resembles very dilute serum of the blood; consisting chiefly of Avater,
but containing a sensible quantity of common salt and albumen.  This
tissue (Avhich has been frequently but  erroneously termed Cellular) is
very  extensible  in all  directions, and  very elastic, from the structural
arrangement of  its  elements.  It  cannot be said to possess any dis-
tinctly vital endoAvments; for although it has a certain amount of sensi-
bility, this merely depends upon the presence of nerves which it is con-
veying to other  parts ; and the small  amount of contractility which it
shows, depends rather upon the muscular tissue of the vessels that tra-
verse it.
  195.  As already mentioned, we find this tissue in almost every part
of the body; thus it binds together the ultimate fibres of the Muscles
into minute fasciculi, unites these fasciculi into larger ones, these again
into larger  ones  which are  obvious to  the eye, and these into the entire
muscle.  Again  it forms the membranous septa between distinct mus-
cles, or between  muscles and fibrous aponeuroses.  In like manner it
unites the elements of nerves, glands, &c.; binds together the  fat-cells
into minute bags, these into larger ones, and so on ; and in this manner
penetrates and forms a considerable part of all the softer tissues of  the
body.   But it is a great  mistake  to  assert, as it was  formerly common
to do, that  it penetrates the harder organs, such  as bones,  teeth,  carti-
lage, &c.  Its purpose obviously is, to alloAv a certain degree  of move-
ment of the parts which  it  unites ; and hence Ave find it  entering  much
more largely into the  composition of the  Mammary gland (Avhich, from
its attachment to the great pectoral muscle, must have its parts capable
of being shifted upon one another), than into that of the Liver, Kid-
neys, &c.  It also serves as the bed, in Avhich blood-vessels, nerves, and
lymphatics may  be carried into the substance of the different organs;
and it  often undergoes a degree of condensation, in order to form a
sheath for the larger trunks, which gives it  almost the characters of a
Fibrous Membrane.
  196.  The quantity of fluid in the interstices of  Areolar tissue is sub-
ject to considerable variations; but these depend rather upon  the state
of fulness  or emptiness  of the vessels Avhich  traverse it, and upon  the
condition of the walls of  those vessels, than upon any change in  the
tissue  itself.  It has been shown that, Avhen an albuminous fluid is in
contact with an  animal membrane, the Avatery part of the fluid will pass 
                          AREOLAR TISSUES.                      123

through  by transudation;  but that the albuminous  matter will  be for
the most part kept back, so that only a very small proportion of it is to
be found in the transuded liquid.   This  appears to be a  sufficient ex-
planation of the presence of a Aveak serous fluid in the cavities of areolar
tissue;  and there is not any  necessity, therefore, to  imagine the exist-
ence of a secreting power, either in  the  areolar tissue itself, or  in the
Avails of the capillaries Avhich  traverse it.  When there is  a  want of
firmness or tone in the walls of the vessels, producing (as we  shall here-
after see, § 609) an increased pressure of the contained  fluid  on their
Avails, and diminished resistance, the Avatery part of the blood will have
an unusual tendency to transudation:  and Ave accordingly find that it
then distends the areolae, and produces dropsy.  The physical  arrange-
ment of the parts of the  tissue is so much altered, that  its elasticity is
impaired ; and it consequently pits on  pressure,—that is, Avhen  pressure
has made an indentation in the  surface, this is not immediately  filled up
Avhen the pressure is AvithdraAvn, but a pit remains for some  seconds or
even minutes.   The free  communication  which exists  among the inter-
stices, is shoAvn by the influence of gravity upon the seat of  the dropsi-
cal effusion; this always having the greatest tendency to manifest itself
in the most depending parts,—a result,  however, which is also  due to
the increased delay that  takes  place  in  the circulation  in such  parts,
when the vessels are deficient in tone.   The freedom of  communication
is still more  shown, however, by the fact, that either air or  water  may
be made to pass by a moderate continued pressure,  into almost every
part of the body containing Areolar tissue ; although introduced at  only
a single point.  In  this manner it is  the  habit of  butchers to  inflate
veal: and impostors have  thus  blown  up the  scalps and  faces of their
children,  in order to  excite  commiseration.   The  whole body  has been
thus  distended with air  by  emphysema  in  the lung;  the  air  having
escaped from the air-cells into the surrounding areolar tissue, and thence,
by continuity of this tissue with that  of the  body  in
general at the  root  or apex of the  lungs, into the     Fis- 8-
entire fabric.
  197.  The  structure  of the  Serous and Synovial
Membranes is essentially the same Avith that of Areolar
tissue.   It is  the  peculiar character  of these  mem-
branes  to form  closed bags or sacs,  having  a very
smooth and  glistening inner surface,  and containing
a fluid  more or  less  allied  in composition  to  the
serum of the blood.   The disposition of  the Syno-
vial membranes may be understood  by studying one
of the simpler  forms of the joints, such as is repre-
sented  in  the  accompanvin£ diagram; but although T,  ,  ..    ,  , .
                     r  "  p    °    '          , o   Ideal section of a Joint;
oriqinalhi continuous over the surfaces of the  Articu- —<*. a, the extremities of
.  ' .^,  .•',      ,i   ci     • l     r.     i      ■      the two articulated bones;
lar Cartilages,  the  Synovial  membrane does not con- b, b, the  layers of oarti-
tinue to be distinctly traceable after the joint has come J*f "hJJ%£™ai  ^™:
into plav,  and  its vessels retreat from  the portion over Cranes covering tnearti-
  . . F   "i           n               ,     £ .  .        culated  surfaces,   and
which  the two  surtaces are exposed to  friction, but passing from one to the
from a circle round its margin,  from which the Carti- °
lage is nourished (§ 278).—The  arrangement  of the Serous membranes
 
124      STRUCTURE AND ENDOWMENTS  OF ANIMAL TISSUES.

is usually much more complicated.  These  line the three great cavities
of the body,—the head, chest, and abdomen,—together with their sub-
divisions ; enveloping the viscera which these contain, so as to  afford
them an external coating over  every part save that by Avhich they are
suspended ; and being then reflected over the interior of the cavity, so as
to form a shut sac intervening between its outer Avails and its contents.
The chief purpose of this appears to be,  to  facilitate the movements of
the  contained organs,  by forming smooth surfaces which shall  freely
glide over each other;  this is evidently of great importance, where such
constantly moving organs as the heart and  lungs are concerned.
  198. The free or unattached surface of these membranes  is covered
Avith a layer of cells;  but these constitute a distinct tissue, the Epithe-
lium, of  which an account will  be given hereafter.   The epithelium lies
upon a continuous sheet of membrane, of extreme delicacy, in Avhich no
definite structure can be  discovered; the nature of this, Avhich is called
the  basement or primary  membrane, will be  presently considered (§ 206).
Beneath  this is a layer of condensed  Areolar tissue, which  constitutes
the  chief thickness of the serous membrane, and confers  upon it its
strength and elasticity;  this gradually passes into  that  laxer variety,
by Avhich the membrane  is attached to the parts it lines, and which is
commonly knoAvn as the  sub-serous tissue.   The yellow fibrous element
enters largely into the composition of the membrane itself;  and its fila-
ments interlace in a beautiful network, which confers upon it equal elas-
ticity in every direction.   The membrane is traversed by blood-vessels,
nerves, and lymphatics, in  varying proportions; some of the  synovial
membranes, especially that of  the knee-joint, are furnished  with little
fringe-like  projections, which  are extremely vascular, and  which seem
especially concerned in the secretion of the synovial fluid.  The fluid of
the  serous cavities is so nearly  the same  as the serum of the blood, that
the  simple act of transudation is sufficient to account for its presence in
their sacs ; on the other  hand,  that of the Synovial capsules, and of the
Bursae Mucosae Avhich resemble them, may  be considered as serum with
from 6 to 8 per cent, of additional albumen.
  199.  The elements of Areolar tissue enter largely also into two other
textures, which perform a most important share in both the Organic and
the  Animal functions ;—namely, the Mucous Membranes and the Skin.
These textures are continuous with each other; and may, in fact, be con-
sidered as one and the same, modified in its different parts according to
the  function it is destined to perform.   Thus it is everywhere extremely
vascular; but the supply of blood in the  Skin is chiefly destined for the
nervous system,  and is necessary to the act of sensation; whilst that of
the  internal skin or Mucous Membrane is rather subsenrient  to the pro-
cesses  of absorption and secretion.   This  tissue  is continued inAvards
from the external surface of the body, by the several orifices and outlets
of its cavities; and it is further  continued most extensively from  its
primary internal prolongations, into the inmost recesses of the glandular
structures.
  200.  Thus the Gastro-intestinal mucous membrane commences at the
mouth, and lines the whole alimentary canal from the mouth  to the anus,
where it again becomes  continuous with the  skin; and it sends off as 
SKIN, AND  MUCOUS MEMBRANES.
125
branches, the membranous linings of the ducts of the salivary glands,
pancreas, and liver; these membranes  proceed into all the subdivisions
of the ducts, and line the ultimate follicles or caeca  in which  they ter-
minate.   Again, the Bronchio-pulmonary mucous membrane commences
at the nose, and passes along the air-passages, down the trachea, through
the bronchi and their subdivisions, to line the ultimate air-cells of the
lungs; communicating in its course with the gastro-intestinal.  Another
mucous membrane of small extent commences at the puncta lachrymalia,
lines the lachrymal sac and the nasal duct, and becomes continuous with
the preceding.   Another, Avhich may be considered a kind of offset from
either of the first two, passes up from the pharynx along the Eustachian
tube, and lines the  cavity of the  tympanum.
   201. Near  the opposite termination of  the alimentary canal, more-
over, Ave have the Genito-urinary mucous membranes; these commence
in the male by a single external  orifice, that of the  urethra;—passing
backwards along the urethra, the genital division is given  off, to line the
seminal  ducts, the vesiculse seminales, the vasa deferentia,  and the secre-
ting tubili of the testis;  another  division proceeds along the ducts of
the prostrate gland, to line its ultimate follicles, and  another along the
ducts of Cowper's glands; whilst the urinary division lines the bladder,
passes up along the ureters to the kidney, and then becomes continuous
Avith the membrane  of  the tubuli uriniferi.  In the female, the urinary
division commences  at once from  the vulva;  whilst  the genital passes
along the vagina into the uterus, and thence along  the Fallopian tubes
to their  fimbriated  extremities, where  it becomes  continuous  with the
serous lining of the abdominal cavity, the peritoneum.
   202. Besides the  glandular prolongations here enumerated, there are
many others,  both  from the internal and  external  surface.  Thus we
have the Mammary mucous membrane, commencing from  the orifices of
the lactiferous ducts,  passing inwards  to* line their subdivisions, and
forming  the walls of the  ultimate follicles.   In  the  same manner the
Lachrymal mucous  membrane is prolonged from the conjunctival mucous
membrane, Avhich covers the eye and lines the eyelids, and which is con-
tinuous with the skin at their edges.   There are several minute glands,
again, in the substance of the skin, and in the walls  of the alimentary
canal,  which  need   not  be here  enumerated;  but  which contribute
immensely to the extension of the surface of  the  mucous  membrane, a
prolongation of this being the essential  constituent  in every  one.  In
their simplest  form, these glandulae  are  nothing more than little pits
or depressions of the surface; these are found both in the Skin  and
Mucous membrane,  and are particularly destined for  the production  of
their protective secretions, hereafter  to be described.
   203. We have seen, then, that  the essential character of the Mucous
membranes, as regards their arrangement,  is altogether different from
that of the serous and synovial membranes.   For whilst the latter form
shut sacs, the contents of which are  destined to undergo  little change,
the former constitute the walls of tubes or cavities, in Avhich constant
change is taking place, and which have free  outward  communications.
Thus in the gastro-intestinal mucous membrane, we have an inlet for the
reception of the food, and a cavity for its  solution, the walls  of which 
126      STRUCTURE AND ENDOWMENTS  OF ANIMAL TISSUES.

are endoAved in a remarkable degree with absorbing  powrer, whilst they
are also furnished with numerous glandulae, wdiich pour the solvent fluid
into the cavity.   On the other hand, it has an outlet, through which the
indigestible residuum is cast forth, together with the excretions from the
various glands that pour their products into the alimentary tube.   In the
bronchio-pulmonary apparatus, the same outlet serves for the introduc-
tion and for the expulsion of the air ; and here, too, is continual change.
In other cases, there is but a single outlet; and the change is of a simpler
character, consisting merely in the expulsion of  the  matters  eliminated
from the blood by the agency of the glands.   Now it is,  as we shall see
hereafter, in the digestion and absorption of food,  on the one hand, and
in the rejection of effete matters on the other, that the  commencement
and termination of the nutrient processes consist;  and these  operations
are performed by the system of  Mucous membranes, including in  that
general term the Skin, which is an important organ of excretion, besides
serving as the medium through which sensory impressions of a general
character are received by the Nervous system.
   204. The Mucous Membrane  may be said, like the Serous, to consist
of three chief parts;—the epithelium or epidermis covering its free sur-
face;—the subjacent basement-membrane;—and the areolar tissue, with
its vessels, nerves, &c, which forms the thickness of the membrane, and
connects it Avith the subjacent parts.  The Epidermis and Epithelium
alike consist of cells;  but the function of the former (which consists of
several layers, of which the outer are dry and horny) is  simply protec-
tion to the delicate organs beneath;  whilst that of the latter is  essen-
tially connected with the process of Secretion, as will be shown hereafter.
The basement-membrane resembles that of the serous  membranes; but
its separate existence  is unusually evident in some parts  where it exists
alone, as in the tubuli uriniferi of the kidney; whilst  it  can with diffi-
culty  be  demonstrated  in others, as the skin.  The Areolar tissue of
Mucous membranes usually makes up the greatest part  of their thick-
ness ; and it is so distinct from that  of the layers  beneath, constituting
the sub-mucous tissue, as to be readily separable from them.   It  differs
not in any important particular, hoAvever, from the same tissue else-
where ; and  the white  and fibrous elements  may  be detected in it in
varying  proportions,  in  different  parts,—the  latter being especially
abundant in the skin and lungs,  which owe to it their peculiar elasticity.
Hence the Mucous  membranes yield Gelatine in  abundance, on  being
boiled.  The skin also  appears to  contain  some  of the non-striated
Muscular fibre (§ 337), in varying proportions in its  different parts.
   205. The relative amount of Blood-vessels, Nerves, and Lymphatics,
as already mentioned, is subject to  great variation, according  to the
part of the system examined.   The first, however, are most  constantly
abundant, being required in the  Skin for sensation (Fig.  9), and  in the
Mucous membranes for absorption and secretion (Figs. 10, 11,12).  In
fact we might say of many of the mucous membranes, especially those
of the glands, that their whole purpose is to give support to the secreting
cells,  and to convey blood-vessels into their  immediate  neighborhood,
whence these cells may obtain materials for their development.   The
Skin is the only part of the whole system which is largely supplied with 
BASEMENT OR  PRIMARY MEMBRANE.
127
Nerves (Fig.  13), except  the Conjunctival  membrane and  the Mucous
membrane of the mouth and nose;  hence the sensibility of  the internal
Fie. 9.
Fig. 10.
Distribution of Capillaries at the surface
   of the skin of the finger.
Distribution of Capillaries in the Villi of
         the Intestine.
mucous membrane is usually low, although its importance in the organic
functions is so great.   The Skin is copiously supplied with Lymphatics ;
Ficr. 11.
Distribution of Capillaries around follicles
     of Mucous Membrane.
Distribution of Capillaries around the follicles
         of Parotid Gland.
and  the first part  of  the alimentary canal Avith Lacteals; some of the
glandular  organs  are  also  largely  supplied  with  Lymphatics.—The
FiK. 13.
          Distribution of the tactile nerves at the extremity of the human thumb, as
                   seen in a thin perpendicular section of the skin.

Areolar tissue, whether existing  separately,  or  forming  a part of the
Serous and  Mucous Membranes, is  capable of being  very quickly and
completely regenerated; indeed, we often find that losses of substance
in other tissues are replaced by means of it.


               3.  Of the Basement  or Primary Membrane.

  206.^ In many  parts of  the Animal  body, we meet with membranous
expansions of  extreme delicacy and  transparency, in Avhich no definite 
128      STRUCTURE AND ENDOAVMENTS  OF ANIMAL TISSUES.
structure  can be  discovered ;  and  these seem,  like the simple fibres
already described, to have  been formed, rather directly from the nutri-
tive fluid, than  indirectly by any previous  process  of transformation.
Hence we may regard such  membranes and fibres, as constituting the
most simple or elementary forms of  Animal  tissue.   The characters of
membranes  of this  kind  were first pointed out by Mr. BoAvman  and
Prof. Goodsir; by the former  of whom it was termed basement-mem-
brane, as being the foundation  or resting-place for the epithelium-cells
which cover its free  surface (§ 231); whilst  by the latter it Avas termed
the primary membrane, as furnishing the germs of those cells.  These
terms appear equally appropriate, and  may be  used indifferently.—In
its  very simplest form, the basement-membrane is  a pellicle of such
extreme delicacy, that its thickness scarcely admits of being  measured;
it is to  all appearance perfectly homogeneous, and presents not  the
slightest trace of structure under the highest powers of the microscope,
appearing like a thin  film of coagulated gelatine.   Examples of  this
kind may be easily  procured,  by acting upon the  inner layer of  any
bivalve  shell with dilute acid; this dissolves away the calcareous matter,
                and leaves the  basement-membrane.  In other cases,
                however, the  membrane is  not so homogeneous; a
                number  of minute granules  being  scattered,  with
                more or  less uniformity,  through  the transparent
                substance.   And we  not unfrequently find,  in place
                of  these uniformly distributed  granules, a series of
                distinct spots, arranged at  equal or variable distances,
                and in  different  directions,  as  shown in  Figure 14
                Moreover, the membrane thus constituted  is disposed
                to break up into portions of equal size, each of which
                contains one of  these spots;  whilst in the more homo-
  Portion of the primary        «             ,    ,    .,  ,     „  .       .
membrane of the human genous  iorms previously  described, we  nnd  no  such
Sawlthd"tsr geyrm?nai tendency, no appearance of any definite arrangement
d?ffu%dover\"vecentrest)einS perceptible when they are torn.—Hence it would
                seem as if the first and simplest form  were produced
by the simple consolidation of a thin layer  of  homogeneous  fluid;  the
second,  by a layer of such  fluid, including  nuclear  granules; and the
third, by the coalescence of flattened cells, whose  further development
had been checked, but whose nuclei continue to perform their peculiar
functions (§ 212).  We find  the primary membrane, under one or other
of these forms, on all the free surfaces of the body, beneath the epithe-
lial or epidermic cells.  Thus, as already mentioned, it constitutes the
outer layer of the true Skin; it lines all the cavities formed by Mucous
membranes,  and is prolonged into all  the ducts and  ultimate follicles
and tubuli of the Glands which  are connected with them (§199): indeed
it may be  said in many instances to be  the sole constituent of the walls
of these follicles and tubuli, the subjacent tissue not being continued to
their finest ramifications.  Again,  it forms  the innermost layer of the
Serous and Synovial membranes; and it also lines the blood-vessels  and
lymphatics, forming  the sole constituent of  the walls of their minutest
divisions.
  207.  In every one of these cases, we find the free aspect of the Pri-
 
BASEMENT OR PRIMARY MEMBRANE.
129
mary Membrane in contact with cells, which form a more or less conti-
nuous layer upon its surface.   These cells can only receive their nutri-
ment by the imbibition of fluid, through the  primary membrane, from
the blood brought to its attached surface, by the capillary A'essels of the
tissue  Avith Avhich  it is  in  relation.   Thus in the Skin and  Mucous
membranes, a very copious supply of blood is brought  to the attached
surface of  the primary  membrane, by the minutely-distributed  capilla-
ries which  form a large part of the subjacent tissue ; and it is from these
that the epidermis and epithelium draw their  nourishment, through the
primary membrane.  In like  manner, the ultimate follicles and tubuli
of  the Glands are surrounded by a copious network of  capillaries (Fig.
12); and it is from these, through the primary membrane, that the cells
of  these follicles draw  their  nourishment.  Hence this membrane, in
every instance, forms a complete septum, on the one hand between the
stream of blood in the vessels and the surrounding tissues, since it forms
the lining  even of  the  minutest capillaries; and on the other between
the fluids in the interstices of the substance of the true skin, the  mucous
membranes, &c, and the cells covering their free  surfaces.   It is evi-
dent, therefore, that whilst bounding these tissues  and restraining the
too-free passage of fluids from their surfaces,  it alloAvs the transudation
of  a sufficient amount for the nutrition of the cells Avhich lie upon it;
and, as Ave shall presently see, these cells frequently pass  through  all
their stages of growth so rapidly, that a very free supply of nutriment
must be required by them.   Hence, notwithstanding its apparent homo-
geneousness, the primary membrane must have a structure which  readily
admits the passage of fluid.   In this  respect it  corresponds with the
membrane, Avhich forms the Avail of the  cells of both Animal and Vege-
table tissues ;  for this also appears completely homogeneous and struc-
tureless, when  seen under its simplest  aspect, and yet alloAvs the free
passage of fluids from one cell to another.
  208.  But it is probable that this membrane performs a  much more
important office  than that of simply limiting the fluids, whilst allowing
the requisite transudation.   We can scarcely  account for the new pro-
duction of cells, Avhich (as will presently appear) is continually taking
place on its surface, without  referring to it  as the originator of these
cells,—that is, as the source of their germs.  The neAV generations  of cells
cannot here be developed by the  reproductive powers of the old ones
(§ 212); since the latter are often completely cast off entire, before they
liberate the reproductive granules;  or they undergo changes  Avhich
evidently unfit them for such a  purpose.  Thus  in the Epidermis we
shall find that they become flattened into dry scales, forming an  almost
horny layer on the  surface of the body ;  whilst the new cells are origina-
ting beneath, from the surface of the basement-membrane (§ 224 and Fig.
20).  Hence Ave cannot find any other origin for these cells, than in the
basement-membrane itself; and there seems every  probability that the
granules, which have been mentioned as being frequently diffused through
it, are  in reality the germs  of  cells to  be developed from its surface  ;
Avhilst the distinct spots are collections of similar granules, each of which
may give origin  to a large number of such cells, Avhich spring from them
as from a centre.  We shall presently see that these  " germinal centres" 
130      STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES.
closely resemble the nuclei of cells in general, from which it is unques-
                 tionable that the new crops of cells may arise (§ 212).
                 The  only difference is,  that in the latter  case,  the
                 groups of new cells are for a time contained within the
                 parent-cell  (Fig.  18); whilst  in the former they are
                 developed on the free surface of the basement-mem-
                 brane.   In Fig. 15 is shoAvn  a portion  of  the same
   component cells of  membrane as that represented in Fig. 14; but having:
  primary membrane,  ,        .    ,       *•     ,         °.,  .     .     °
  with adherent epithe-  been rendered transparent by acetic acid, its real nature
                 as a layer of flattened nucleated cells is more obvious ;
the nucleus or germinal spot  of the central cell  has given  origin to a
cluster of oval epithelial cells, of which  five still adhere to it.
  209.  Hence we are probably to regard this primary or basement-mem-
brane as a transitional, rather than as a permanent structure ;  and to
look upon it as furnishing the germs of all the cells, which are  developed
upon its surface ;  as well as serving for the medium, through which they
are supplied with nutriment.   It must be continually undergoing disin-
tegration, therefore, on  its free  surface: and  must be  as continually
renewed, at the side in relation with the blood-vessels.

      4.  Of Simple Isolated Cells, employed in the  Organic Functions.

  210.  The active functions of the Animal body are performed, to a
much greater  extent than was until lately believed,  by  the  agency of
simple isolated cells; of which every one grows and lives quite inde-
pendently of the  rest, just  as if it were one of the  simplest Cellular
Plants (§ 30); but of which all are dependent upon the general nutritive
fluid for the materials of their  development, imbibing it from the cur-
rents that circulate in their neighborhood.   It may be said, indeed,
that all the Vegetative  functions of the  body,—all  the processes of
Nutrition and Reproduction,—all those  operations,  in short,  which
are  common to  Plants  and  Animals,—are performed in the  Animal
and  Vegetable structures by  the  very same means, the agency of cells;
and this is true, not only of the  healthy actions, but of various morbid
operations, in which the unusual development of cells, possessing peculiar
endowments, performs a most conspicuous part.   Hence it will be neces-
sary to enter somewhat at large into the history of  cell-development in
the Animal body : and the various modifications under  which this process
may take place.  In fact, a knowledge of the Physiology of  Cells may
be regarded as the foundation of all  accurate  acquaintance Avith that
department of the  Science, which relates to the Nutritive and  Repro-
ductive processes;  and it has a  considerable bearing, as we shall see
hereafter, upon the history of the purely Animal functions.
 ^  211. The history of the Animal cell, in its simplest form, is essen-
tially that of the Vegetable cell  of the lowest kind: excepting in so far
as it is dependent for its nutriment upon organic compounds  previously
elaborated,  instead  of generating these for itself.  It  lives  for itself
and by itself; and is dependent upon nothing but a due supply of nutri-
ment, and of the appropriate stimuli, for  the continuance of  its  growth
and for the due performance of all its functions, until  its term of life be
expired.  In whatever method it originates (and we shall presently see
 
SIMPLE  ISOLATED CELLS.
131
that the life of an independent cell may commence in various modes), it
attracts to itself, assimilates, and organizes, the particles of the nutrient
fluid in its neighborhood ; converts some of them into the substance of
its cell-wall, whilst it draAvs others into its cavity; in this manner  the
cell  gradually increases in size;  and whilst it is  itself approaching  the
term of its life, it may make preparation for its  renewal, by the  deve-
lopment of reproductive particles in its interior,  which may become the
germs of neAV cells, when set free from the cavity of the parent.   In the
interior of most Animal cells, usually attached to some part of their wall,
is seen a collection of granular matter, which is called the nucleus (Fig.
16 a).  This appears to be the  centre of the vital  forces of the cell;
being  the part through  which it specially exerts its agency on  the sub-
stances brought under its influence ; and being also the chief instrument
in the reproductive  operation.  There is reason to believe, indeed, that
nuclear bodies may  exert their vital power, and may  effect the  transfor-
                                Fig. 16.
                  Cells from chorda dorsalis of Lamprey, a, a, nuclei.
mation or new arrangement of organic compounds, without  the forma-
tion of a cell-membrane;  the purpose of the latter, indeed, being appa-
rently to bound or limit the substances drawn together by the nucleus,
and to cut them off from others in their neighborhood.   When the for-
mation of a cell is complete, and it is  not destined to reproduce its kind,
the nucleus frequently disappears ; this is the case,  for example, with
the Red corpuscles of the Blood of Mammalia (§ 215), and also with Fat-
cells (§ 257).—So far as is yet known, however, the  composition of the
cell-wall is everywhere the same ; being that of Proteine.   It is in the
nature of the contents of the cell,  that (as among the cells of Plants) the
greatest diversity exists ;  and we shall find that the purposes of the dif-
ferent groups of  cells, in the Animal  economy, depend upon the nature
of the products they secrete, and upon the mode in which these products
are given  back,  after they have been subjected  to  the action of the
cells.
   212. New cells may originate in one of the two principal modes: either
directly from a  pre-existing cell; or by an  entirely new production in
the midst of an  organizable blastema.   The development  of new cells
from  a pre-existing cell,  again, may  take place in one  of two modes;
either by  the subdivision  of the  parent-cell, or by the  production of a
number of new cells in its interior; the nucleus, in each case, appearing
to perform an important part in the  process.   Of the multiplication of
cells by subdivision, we have  a characteristic example in the growth of
Cartilage, which  repeats in adult age  the process by which the develop-
ment of the " germinal mass"  takes place at the earliest period of embry- 
132     STRUCTURE  AND ENDOWMENTS  OF ANIMAL TISSUES.

onic life (chap, xi.)  The process of subdivision seems to commence in
the nucleus, which tends to separate itself into two equal  parts; and
each  of these draws around it a portion of the contents of the cell, so
that the cell-wall,  which is at first  merely inflected inwards by  a sort of
hour-glass  contraction, at last forms a  complete  partition between  the
two halves  of the  original cavity (Fig. 17,  a-d).   The process  of subdi-
vision may  be again repeated, either in the same or in a contrary direc-
tion,  so  as  to produce four  cells,  either linearly arranged (f,  g, h), or
clustered together (e); and this duplication may take place until a large
mass has been produced by the subdivision of a single original cell.—In
 Multiplication of Cartilage-cells by duplication :—A, original cell; b, the same beginning to divide; C, the
same showing complete division of the nucleus; d, the same with the halves of the nucleus separated, and
the cavity of the cell subdivided; E, continuation of the same process, with cleavage in contrary direction, to
form a cluster of four cells; F, o, h, production of a longitudinal series of cells, by continuation of cleavage
in the same direction.
other cases, however, the nucleus appears to break up at once into several
fragments, each of which may draw  around it  a portion of the contents
of the parent-cell, which becomes invested by a cell-wall of its OAvn ; and
thus the cavity of the parent-cell may at once become filled with a whole
brood of young  cells, without  any successive subdivision.   Of  this pro-
cess we frequently have examples in  the case of morbid growths, in which
the multiplication of cells often takes place with great rapidity (Fig. 18).
Fig. 18.
      Parent-cells, a, a, of cancerous structure, containing secondary cells, b, b, each having one,
                            two, or three, nuclei, c, c.
Generally speaking, the former method seems to prevail in structures
which have a comparatively permanent  destination ; whilst the latter is
adopted in cases in Avhich the life of the cells thus generated is but transi- 
SIMPLE  ISOLATED  CELLS.
133
tory, or in Avhich they are not destined to reproduce themselves.   Thus
the follicles  of  Glands (§ 238) are but parent-cells, in whose wall an
opening has been formed  for the liberation of the cells of the neAV gene-
ration (Avhich are the real instruments of  the secreting  process) as fast
as they are formed ; and  from the nuclei or " germinal spots" of  these
parent-cells, which  occupy the blind extremity of the follicles, successive
crops of young cells are generated, at the  expense of the fresh materials
which the nuclei are continually drawing from the blood.   So the nuclear
particles scattered through the " basement-membranes" (§ 208), probably
give origin to the  epithelial cells developed upon  their free  surfaces ;
even  though these nuclei have never  been  themselves  included Avithin
cells.
   213. In the production of cells de novo in the midst of an organiza-
ble blastema, or plastic exudation, Ave cannot trace with the same dis-
tinctness the instrumentality of pre-existing cells.   This blastema, when
first effused,  presents the appearance of a  homogeneous, semi-fluid, sub-
stance ;  as it solidifies, hoAvever, it becomes dimly shaded  by minute
dots ;  and as it  is acquiring further consistence, some of these dots seem
to aggregate, so as to form  little round or  oval clusters bearing a strong
resemblance  to  cell-nuclei.   These  bodies appear to be the centres of
the further changes which take place in the blastema; for if it be about
to undergo a development into a fibrous tissue, they seem to be the cen-
tres from Avhich the fibrillation takes place;  whilst if a cellular struc-
ture is to be generated, it is from them that the cells take their origin.
The first  stage of the latter process appears to consist in the accumula-
tion round each nucleus  of the substance which the cell is to  include;
and around this the cell-membrane is  subsequently developed.  Such is
the  mode, then, in which the development  of  new structures, for  the
filling-up of  losses of substance,  is provided  for; and it  appears from
the observations of Mr. Paget, that whilst the immediate fibrillation of
the  blastema takes place in the case of effusions which  are  secluded
from the  air and which undergo organization under the most favourable
circumstances, a production of cells takes place when the blastema is
poured out upon  the surface of  an open  wound, where the  contact  of
air, and other sources of irritation, interfere with  the organizing pro-
cess, and occasion a tendency to degradation in the newly-generating
tissue.—Such a production of cells de novo in the midst of an organiza-
ble blastema, does not constitute, hoAvever, any real exception to  the
general rule, of the dependence of the life of every cell upon that of a
pre-existing  cell.   For it is pretty certain that the blastema is itself the
product of the formative agency  of certain cells expressly provided for
its elaboration; and it does not seem improbable that these cells, in
bursting  and setting  free  the  plastic fluid which they have  prepared,
should diffuse through it their own  nuclear or germinal particles  in  a
state of solution or extremely minute division; and that these, attract-
ing  each other  in the act of solidification, should  act as neAV centres of
cell-groAvth, just as if they  were still contained Avithin the parent-cell.
   214. The  very simplest and  most independent condition of the Ani-
mal Cell is  probably  to be found in the Blood,  the  Chyle, and the
Lymph;  in  all  of which liquids we meet with floating cells, which are 
134      STRUCTURE AND ENDOWMENTS  OF ANIMAL TISSUES.

completely isolated from one another, and which  are consequently just
as independent as the vesicles of the Red Snow or other simple cellular
Plants.  Indeed  in  the  nature of their  habitat,  we may compare them
with the Yeast-Plant;  for as this will only vegetate in a saccharine fluid
containing vegetable albumen, so do we find  that  these floating  cells
will  only grow and  multiply in the albuminous  fluids of animals.  In
their general appearance they very closely correspond Avith the figure
already given as  the type of the simple cell.  Their diameter is pretty
uniform  in the different fluids of the body, and even in different animals;
being for the most  part  about l-3000th of an inch.  They are some-
times nearly spherical, and  sometimes flattened ;  when they present the
latter  shape,  they may be  made to swell out into the  spherical  form
(see Frontispiece, Figs. 4 and 5) by the  action of water,  which they im-
bibe according to the laws of Endosmose,—the thinner fluid, water,
passing towards the  more viscid contents of the cells, and mingling with
them.   By the continuance of this kind of action, the cell will be caused
to burst.  These cells, which are known  as  the corpuscles of the Chyle
and  Lymph,  and as  the  White Corpuscles of the  Blood,  are observed to
contain a number of minute molecules in their interior [Front. Fig. 4);
        Colorless cells, with active molecules, and fibres of fibrine, from Herpes labialis.

and at a certain stage of their  development,—probably that which im-
mediately precedes the maturation  and rupture of the  parent-cell,—
these molecules  may be seen, with a good Microscope, in active move-
ment within the cavity.   The action of a very dilute solution of potash
causes the immediate rupture  of these cells, and the discharge of the
contained molecules, which are probably the germs of new cells of a
similar character.  And when  they rupture spontaneously, which they
are much disposed to  do under the influence of contact with  air, the
fluid which they set free shows  an obvious tendency to assume a fibrous
arrangement.—The cells which  are found in many fibrinous  exudations
resemble the colorless  corpuscles of the  blood in all essential particu-
lars  (Fig. 19).   Hence  it  may  be  concluded that they  belong to the
same class ; being probably developed  from granular germs set  free
from the blood,  along with the matter of the fibrinous exudation itself.
   215. Besides  the  cells  already mentioned, the blood of Vertebrated
animals also  contains others, which are distinguished by their red color
and  flattened form.  These are  equally isolated,  and lead an indepen- 
RED  CORPUSCLES OF BLOOD.
135
dent life;  undergoing all  their changes whilst floating  in  the rapidly-
circulating current.   These  Red  Corpuscles can scarcely be said to
exist in the blood of  Invertebrated animals, and their proportion in the
blood of Vertebrata varies considerably in the several  groups of that
sub-kingdom;  they are altogether wanting in the blood of the Amphi-
oxus or Lancelet, which,  although essentially a Fish,  has many pecu-
liarities that connect it Avith the lower divisions of the Animal  senes.
They present,  in  every instance, the form of a flattened disk, which is
circular  in Man and in  most  Mammalia (Front. Fig. 1), but which is
oval in Birds,  Reptiles, and Fishes, and in a few Mammals (Front. Fig.
6).  This  disk is in both instances  a flattened cell, whose walls are pel-
lucid and  colorless,  but  whose contents  are  colored.  Like  the cor-
puscles already described, they may be caused to swell up and burst,
by the imbibition of water; and the perfect transparency and the homo-
geneous character of their walls then become  evident.   (Front. Fig. 8,
e,)—These Red  Corpuscles are not only  distinguished from the others
by the color of their contents; they are also characterized by the ab-
sence of the separate molecules, which formed so distinctive a feature
in the preceding; and in Oviparous Vertebrata by the presence  of  a
distinct central spot  or nucleus, which appears to  be  composed  of an
aggregation of minute granules, analogous  to  those elsewhere diffused
through the interior of the cell.  The nucleus (where  it exists) may be
easily obtained separate from the cell-Avail and its contents, by treating
the red corpuscles with water.   The first effect of this is to render the
nucleus rather more distinct, as is  seen by contrasting the corpuscle
which has  been thus slightly acted on (Front. Fig. 8, a), with the un-
altered corpuscle (Front. Fig. 6) of the same animal.  After a  short
time, the corpuscle swells  out and  becomes  more circular (Front.  Fig.
8, b); and in a short time longer, the nucleus  is seen, not  in the centre
of the disk, but near  its margin (Front. Fig. 8, c, d).  Finally, the wall
of the cell ruptures;  the  nucleus and its  other contents are set free;
and whilst the  coloring matter is diffused through the surrounding fluid,
the cell-walls  and the nuclei are separably  distinguishable.   (Front.
Fig. 8, e.)—It is remarkable,  however,  that  the Red corpuscles  of the
blood of Mammals should possess  no obvious nucleus; the dark  spot
Avhich is seen in their centre (Front. Fig. 1), being merely an  effect of
refraction,   in  consequence of  the  double-concave  form  of  the  disk.
When the  corpuscles  are treated with water, so that their form becomes
first flat, and then double-convex, the dark spot disappears ; whilst, on
the other hand, it is made  more evident Avhen the concavity is increased
by the partial emptying  of the cell, Avhich  may be  accomplished by
treating  the blood-corpuscles  with  fluids  of greater density than  their
own contents.
   216.  The size of the Red Corpuscles is not altogether uniform in the
same blood; thus it varies in  that of Man from about the  l-4000th to
the l-2800th of an inch.   But we generally find that  there is an  ave-
rage size,  which  is pretty constantly maintained among  the  different
individuals  of  the  same species; that of Man may be stated at  about
l-3400th of an inch.   The round corpuscles  of the Mammalia do not
in general  depart very Avidely from  this standard; except in the case of 
136      STRUCTURE AND ENDOAVMENTS OF  ANIMAL TISSUES.

the Musk-Deer, in which they are less  than 1-12000th of an inch in
diameter.  It is in the  Camel tribe alone  that we find  oval  corpuscles
among Mammals ;  these  have  about the same average length as  the
round corpuscles of Man, but little more  than half the breadth.—In
Birds, the corpuscles are occasionally almost circular;  but  in  general
their diameters are to each other as 1J or 2 to 1.   The  size  of  the cor-
puscles is usually greater according to the size of the Bird ;  thus among
the Ostrich tribe, the long diameter is about  l-1650th of an inch, and
the short diameter l-3000th ; whilst among the small Sparrows, Finches,
&c,  the long diameter  is  about  l-2400th,  and the  short frequently
does not exceed  half that amount.—It is in  Reptiles  that Ave find  the
largest red corpuscles;  and it is in their blood, therefore, that Ave can
best  study the characters of these bodies.  The  blood-disks of the Frog,
from  the facility with Avhich they may be  obtained, are particularly
suitable for the purpose ; their long diameter is about the l-1000th of an
inch, whilst their short or transverse diameter is about l-1800th.  The
curious Proteus, Siren, and other allied species, Avhich retain  their gills
through  their whole lives, are  distinguished  by the enormous size of
their blood-disks.   The long diameter of the  corpuscles of the  Proteus
is about l-337th of an inch;  they are consequently almost distinguisha-
ble with the unaided eye.  The  long diameter of  the corpuscles of the
Siren is about l-435th of an inch, and their short diameter about l-800th;
the long  diameter of the nuclei  of  these corpuscles  is about  l-1000th,
and the short diameter about l-2000th of  an inch,—so that the nuclei
are about three times as long, and  nearly tAvice as broad, as  the entire
human corpuscles.
  217.  The  relation between  the White or Colorless  and the Red
Corpuscles of the Blood can only be determined by attentively watching
their development, and  tracing  them through  all  the  stages of their
growth.  Although our knoAvledge  on this subject is far from complete,
yet there seems  much reason to  believe,  from the observations of Mr.
Wharton Jones  on the different forms of blood-cells  presented in the
several classes of animals, and from those of Mr.  Paget and other phy-
siologists on the several gradations  of structure exhibited in the blood-
cells of  Mammalia, that  the red corpuscles  have their origin  in  the
colorless, and that  the different  forms of blood-cell presented  in diffe-
rent  groups of animals  are, in fact, progressive stages in the same de-
velopmental process, which may be checked at any one of them.—Thus
among the loAver  Invertebrata,  the cells Avhich  are observed  to float in
their circulating fluid, seem  to be little else than  aggregations  of gra-
nules, presenting a tuberculated  surface; no cavity can be distinguished;
and  it is with difficulty  that the  presence of a distinct cell-Avail can be
demonstrated.   This form, Avhich is designated  by Mr. Wharton Jones
as the " coarse granule-cell," presents itself also among the  chyle and
lymph-corpuscles of Vertebrated  animals, and  is occasionally met with
in their blood.—In other Invertebrata, the  blood-cell  undergoes a fur-
ther  development;  for the cell-Avail becomes more distinct, and the gra-
nules are so much more minute as to give to the entire cell a  somewhat
nebulous aspect, its surface  being now smooth instead  of tuberculated.
This form of corpuscle, also, termed  by  Mr. Wharton Jones the " fine 
RED  CORPUSCLES OF BLOOD.
137
granule-cell," is found in the chyle and lymph, and occasionally in  the
blood,  of Vertebrated animals.—The next stage in the history of deve-
lopment, is the aggregation of the granules into a  distinct nucleus, and
the clearing up  of the general  cavity of the cell; and  thus  is formed
the "colorless nucleated cell," which is the highest grade  that  the
blood-cell attains in the Invertebrated series; the number  of such cells
being greater in each class, the closer is its  approximation to  the Ver-
tebrated sub-kingdom.   This phase presents itself  also in the blood of
Vertebrata, as a transition stage betAveen the chyle- and lymph-corpus-
cle, and the  proper  blood-disk or red-corpuscle.—Thus,  then,  we  see
that the cells which are found in the circulating  fluid of Invertebrated
animals, correspond rather with those of  the Chyle and Lymph of Ver-
tebrata, than with those which  are  characteristic  of the  Blood of  the
latter.
   218. The  next stage of development seems to consist in the acquire-
ment of the  peculiar red color; and in the change  of form,  from  the
spherical to the flattened or discoidal.  Thus is produced  the  "colored
nucleated  cell," Avhich is the  characteristic grade  of the  blood-disk of
the Oviparous Vertebrata in general.   This grade may be occasionally
seen in the blood of the adult Mammal, as the transition-stage between
the colorless  nucleated  cell,  and the  non-nucleated cells  which  are
proper to it;  but it is more easily made out in the blood of the  em-
bryo.—The non-nucleated red  corpuscles Avhich are  characteristic of
Mammalia, are regarded by Mr. Wharton Jones  as the escaped nuclei
of the preceding, which  have undergone development into cells; but it
seems  much more probable, that they are the same cells in a yet more
advanced  stage of development, the  nuclei  having  been absorbed, as
often happens in the  case of other cells.
   219. There can  be no doubt that, like  all other cells, each Blood-
corpuscle has its proper term  of life, and that it degenerates  and  dies
when  this  is  expired; if it Avere not, therefore, for the continual pro-
duction of neAV cells,  in the  manner just described, the Blood would
soon lose  its due proportion  of  these components, since there is no
reason to believe that the fully-formed red corpuscle can regenerate its
kind,  although  multiplication by subdivision may take  place in  an
earlier stage  of  its  development.  When much blood has been drawn
from the body, the proportion of red corpuscles in the remaining fluid
is at  first  considerably loAvered;  the  fluid portion of the blood being
replaced almost immediately, Avhilst  the floating  cells require time for
their  regeneration.   Their amount  progressively increases,  hoAvever,
until it has reached its proper standard, provided that  a  due  supply of
the materials be afforded.  We shall presently  see  that  one of these
materials is Iron ; and it is well knoAvn that iron administered internally
is an important aid in recovery from severe hemorrhages, as well as  a
valuable remedy for  certain constitutional states, in  which there is  a
diminished poAver  of producing  red corpuscles.  Thus  in  Chlorosis,
under  the  administration of iron, the amount of red corpuscles in  the
blood has been doubled within a short period.  In  the  healthy state of
the system, the  constant production,  and the constant death and disin-
tegration,  balance one another.  In  some instances  (as  in Chlorosis), 
 138     STRUCTURE AND  ENDOAVMENTS OF ANIMAL TISSUES.

 the production is not sufficient to make up  for the loss by death ; and
 the total amount in  the  blood undergoes an extraordinary diminution,
 sometimes even to less than a quarter of their proper  proportion.   In
 other cases, under the influence of  excessive nutriment (as in the state
 termed Plethora), the proportion of Red Corpuscles is increased beyond
 the normal amount;  and in this condition, the loss of a small quantity
 of blood may be a preservative  from  the evils to which it is  incident,
 from Hemorrhage of various kinds.
   220. The Red Corpuscles  make  their first appearance  in  the blood
 of the Embryo, however, long before  the formation of chyle and lymph
 commences;  and they appear to be  formed by  the  metamorphosis of
 some of the cells which constitute the inner layer of the germinal mem-
 brane (chap, xi.)  These cells are  at first nearly spherical, and are full
 of particles of a yellowish substance like  fatty matter;  in the midst
 of which, though somewhat obscured  by them, a central nucleus may be
 seen.   The development of these embryo-cells into the oval red corpus-
 cles of  the Oviparous Vertebrata, is stated by Mr. Paget to be effected
 by the  gradual clearing-up, as  if by division and liquefaction,  of the
 contained particles, the acquirement of the blood-color and of the ellip-
 tical form, the flattening of the cell, and the more prominent appearance
 of the nucleus.   The first set of blood-disks is nucleated in Mammalia,
 as well as in  Oviparous Vertebrata  ; and they occasionally present indi-
 cations of being  in course  of multiplication  by subdivision.  They
 gradually  disappear from the  blood,  however,   when  the  chyle  and
 lymph-corpuscles first  present  themselves  in the circulating  current;
 and thenceforth the  Red corpuscles seem to be formed at the expense of
 the latter alone.  It is curious that this change should usually coincide,
 in the Tadpole, with the time  at which the external branchiae disappear;
 and, in warm-blooded animals, with the period at which the branchial
 fissures are closed in the  neck, and the course  of the  circulation is
 altered  (chap, vi.)
  221.  The chemical composition of the Red Corpuscles presents  cer-
 tain  peculiarities which  require notice; that of the White,  or  Color-
 less,  however, has  not been specially examined.  When   the  Red
 Corpuscles are separated from the other constituents  of the blood, and
 are treated with water, their contents are speedily diffused through the
 fluid (§  215);  and from this may be abstracted two distinct substances,
 Avhich are designated Globuline  and Hcematine.—The  former does not
 seem to differ  from Albumen in any greater degree, than may be attri-
 buted to the presence of the walls  and nuclei  of the corpuscles, from
 which it cannot be separated;  and it is probably common to the White,
 as well  as the  Red.—It  is in the Red alone, however, that the Hxmatine
 exists.  The composition of this substance is notably different from thaf
 of the proteine-compounds ; the proportion of carbon to the other ingre-
 dients being-very much  greater; and  a definite quantity of iron being
 an essential part of it.  Its formula is 44 Carbon, 22 Hydrogen, 3 Ni-
trogen,  6 Oxygen, and 1 Iron.  When completely separated from Albu-
minous  matter, it is a dark brown substance, incapable of coagulation,
nearly insoluble in water, alcohol, ether, acids, or alkalies, alone; but
readily  soluble in alcohol mixed either with sulphuric acid or ammonia. 
RED  CORPUSCLES OF BLOOD.
139
 The solution, even when diluted, has a dark  color; and possesses all
 the properties of the coloring  matter of venous blood.  The iron  may
 be separated from the haematine by strong reagents which combine with
 the former, and the latter still  possesses its characteristic color.   This
 hue cannot be dependent, therefore, on the presence of iron in the state
 of peroxide; as some have supposed.   On  the other hand, the iron is
 most certainly united firmly with the ingredients  of the  hsematine, as
 contained  in the red corpuscles ;  for this may be digested in  dilute sul-
 phuric  or  muriatic acid for many days, without the least diminution in
 the quantity of iron, the usual amount of which may be afterwards ob-
 tained  by combustion  from the haematine that has been subjected to
 this treatment.   This experiment seems further to prove, that the iron
 cannot be united with  the  haematine in the state of either protoxide or
 peroxide, as maintained by Liebig; since weak  acids would then dissolve
 it out.   Regarding the nature of this compound, and the changes which
 it undergoes in respiration, there  is still much to be learned; and  until
 these  points have  been more  fully elucidated, the  precise uses of the
 Red Corpuscles in the  animal economy cannot be  understood.  There
 is evidence, however, that the production of Haematine is (like the pro-
 duction of the red coloring matter of the Protococcus nivalis,  § 26), a
 result of chemical action taking place in  the  cells themselves; for no
 substance  resembling Haematine can  be found in  the liquid in which
 these cells  float, and scarely  a trace of  iron can be detected in it;
 whilst,  on  the other hand, the  fluid portion of  the chyle  holds  a  large
 quantity of iron in solution, which seems to be drawn into the red cor-
 puscles, and united with the other constituents of haematine, as soon as
 ever it  is delivered into the circulating current.
   222.  It has been usually supposed,  until  recently, that the  difference
 in color between Arterial and Venous blood is due to different states
 of combination of the Haematine they respectively contain, with Oxygen
 and Carbonic  acid.   For in its passage through the capillaries of the
 system, the arterial blood loses its  bright florid hue,  and assumes the
 dark purple tint which distinguishes ordinary venous blood; and the
 converse change takes place in the capillaries of the lungs, the original
 florid hue  being  recovered.  Now it  is certain that the  blood, in its
 change from the arterial  to the  venous condition, loses  oxygen, and
 becomes charged with an increased amount of carbonic acid, although
 its precise mode of combination is  not known;  on the other hand,  in its
 return  from the venous to the arterial  state,  the  blood  gives  off this
 additional  charge of  carbonic  acid, and imbibes oxygen.   The change
 of color, under similar conditions, takes place  out of the body, as well
 as in it.   Thus if venous blood be exposed for a short time to the air,
 its surface becomes florid; and the non-extension of this  change to the
 interior of the mass is evidently due to the impossibility of bringing air
 into relation with every particle of the blood, in the manner  which the
 lungs are so admirably contrived to effect.   If  venous blood be exposed
 to pure oxygen, the change of  color will take place still more speedily;
and it is not prevented  by the interposition of a thick animal membrane,
such as a bladder, between the blood and the gas.    On the other hand,
if arterial blood be exposed to carbonic  acid, it loses  its brilliant hue, 
140      STRUCTURE AND ENDOWMENTS  OF ANIMAL TISSUES.

and is rendered as dark  as  venous blood; or even darker, if exposed
very completely to its influence.   The simple removal of this carbonic
acid is not sufficient to restore the original  color; for this removal may
be effected by hydrogen, Avhich has  the  power of dissolving out (so to
speak) the carbonic acid  diffused through  the blood; but the arterial
hue is not restored unless oxygen be present, or saline matter be added
to the blood.—Recent  observations seem  to render it  probable that
these variations are due, not  so  much  to changes of composition in the
Haematine, as to changes of form in the Corpuscles Avhich contain it.
For when the haematine has been separated and diffused through water,
it  is  neither  darkened  by carbonic acid,  nor brightened by  oxygen,
unless  some corpuscles  be floating in the solution.  And it appears
that the effect of oxygen,  like that of saline solutions, is to contract the
corpuscles and to  thicken their walls, thus, by  altering their mode of
reflecting light, making them appear bright red;  whilst carbonic acid,
like water may be seen  to occasion  a  dilatation  of the  corpuscle, and
a thinning of its walls (Avhich are at least dissolved by it), in a degree
that  is probably sufficient to account for the darkening of the hue of
the mass.
   223. These changes in  the condition of the  Red corpuscles (Avhatever
their  precise  nature may be), taken in  connexion with the  fact, that
these bodies are almost completely restricted to the blood of Vertebrata
(whose respiration is much more energetic than that of any Invertebrated
animals save Insects, which have a special provision of a different cha-
racter), and that their proportion to the  whole mass of the blood corre-
sponds Avith the activity of the respiratory function,—leave little doubt
that they are  actively  (but  not exclusively)  concerned  as carriers of
Oxygen from  the lungs to the tissues, and of Carbonic acid from the
tissues to the lungs; and  that they have little other direct concern in
the functions of Nutrition, than the fulfilment  of this duty.   Their com-
plete absence  in the loAver Invertebrated animals, in  the earliest condi-
tion of the higher, and in newly-forming parts until  these are penetrated
by blood-vessels, seems  tp indicate that  they have no  immediate con-
nexion with  even the  most energetic operations of growth  and develop-
ment ;  whilst, on the other hand, there is  abundant evidence, that the
normal activity of the animal functions is mainly dependent upon their
presence in the blood  in due  proportion.
   224. Next in independence to the cells or corpuscles floating in the
animal fluids,  are those which cover the free membranous surfaces of
the body, and form the  Epidermis and Epithelium.  Between  these
two structures there is  no more  real difference than there  is betAveen
the Skin and the Mucous  membranes.   The one is  continuous with the
other; they are both formed  of  the same  elements; they are cast off
and renewed in the same manner; the history of the life of  the indivi-
dual cells of  each  is nearly identical;  but  there is an  important diffe-
rence in  the purposes which  they respectively serve in the general eco-
nomy.  The Epidermis or Cuticle covers  the exterior surfaces of the
body, as a thin semitransparent pellicle,  which is apparently homoge-
neous  in its texture, is not  traversed by vessels  or nerves,  and Avas
formerly supposed to  be an inorganic exudation from the surface of the 
SIMPLE ISOLATED CELLS.—EPIDERMIS.
141
true  skin, designed  for its  protection.  It is  now  known, however, to
consist of a series of layers of cells, which are continually wearing off
at the external surface, and are  being renewed at the  surface of the
true skin; so that the newest and deepest  layers gradually become the
oldest and most superficial, and are at last thrown off bv slow desqua-
mation.   Occasionally  this desquamation  of the cuticle "is much more
rapid;  as after  Scarlatina  and  other inflammatory  affections  of the
Skin.                                              J
   225. In their progress  from the internal to the  external surface of
the  Epidermis, the  cells  undergo  a  series of well-marked  changes.
When we examine the innermost  layer, we find it  soft and granular;
consisting of nuclei, in various stages  of  development into cells, held
together by^ a tenacious semifluid substance.   This was formerly consi-
dered as a distinct tissue, and was supposed  to be the peculiar seat of the
color of the skin ; it received the designation of rete mucosum.   Passing
outwards, we find the cells more completely formed; at first nearly sphe-
rical in shape ;  but becoming polygonal where they are flattened against
one  another.  As we proceed  further towards the  surface, we perceive
that the cells are  gradually more and more flattened, until they become
mere horny scales, their cavity  being obliterated;  their origin  is  indi-
cated, however, by the  nucleus  in the  centre of each.  This flattening
appears to result from the gradual desiccation or drying up of  the con-
tents of the  cells, which results from their exposure to the air.   Thus
each cell of the Epidermis is developed from the nucleus on the surface
of the basement-membrane—which nucleus  is  probably furnished  by the
membrane itself (§ 208),—and is gradually brought to the surface  by the
development  of new cells beneath, and the removal  of the  superficial
layers; whilst at  the same time  it is  progressively changed in form,
until it is  converted into  a flattened scale.    The accompanying  repre-
Fig. 20.
Fig. 21.

  Oblique section of Epidermis, showing the
progressive development of its component
cells;—a, nuclei, resting upon the surface
of the cutis vera/; these nuclei are seen to
be gradually developed into cells, at 6, c, and
d; and the cells are flattened into lamellte
forming the exterior portion of the epidermis
at«.
  Horny Epidermis, from conjunctiva covering
the cornea; a, single scales; 6. single lamina of
epithelium; below is seen a double layer of the
same.
sentation of an oblique section of the Epidermis, exhibits  the principal
gradations of its component structures. 
142      STRUCTURE AND ENDOWMENTS  OF ANIMAL TISSUES.

  226. The Epidermis covers the whole  exterior surface of  the  body;
not excepting  the  Conjunctiva of the eye, on which, however, it has
more the character of an Epithelium, and the Cornea, on which  it par-
ticipates  in  the horny character of  the Epidermic covering (Fig. 21).
The continuity is well  seen in the cast skin or slough of the Snake; in
which the covering of the front of the eye is found to  be as perfectly
exuviated as that of any part  of the body.  The number of layers varies
greatly in different parts; being usually found to be the greatest where
these is most pressure or friction.  Thus  on the  soles of the feet, par-
ticularly  at the heel and the  ball of  the great  toe, the Epidermis is ex-
tremely thick; and the palms  of  the hands of the laboring man are
distinguished  by the  increased density  of their  horny covering.  It
would seem as if the irritation of the skin  stimulated it to an increased
production of this substance.  The  Epidermic membrane is pierced by
the excretory ducts of the sweat-glands, and  of the  sebaceous follicles,
which  lie in the true  skin  and immediately beneath it; or we should
rather say that it  is continuous with the delicate  epithelial lining of
these.  The  Nails  may be considered as nothing more than an altered
form of Epidermis.   When  exanined near their origin, they are found
to consist of cells which gradually dry into  scales; and these remain
coherent together.  A new production is continually  taking place in the
groove of the skin, in which the root of the nail is imbedded ;  and pro-
bably also from the whole subjacent  surface.
  227. The Epidermis, when analysed, is found to differ from the pro-
teine-compounds in its  conposition;  but  not in  any  very striking de-
gree.   The proportion of its elements is  considered to be 48 Carbon,
39 Hydrogen, 7 Nitrogen, 17  Oxygen :  and this corresponds  exactly
with the  composition of the substance of which Nails, Horn, Hair, and
Wool are constituted.   It seems probable, however, that the cell-walk
are formed, as  elsewhere, of Fibrine; and that the horny  matter is a
secretion in their interior, which  is  drawn from the  elements of blood
during their growth and development.
  228. The Epidermis appears solely destined for the protection of the
true Skin; both from  the mechanical injury and the pain which the
slightest abrasion would produce; and from the irritating effects of ex-
posure to the external air and of changes of temperature.  We perceive
the value of  this protection,  when the Epidermis  has  been accidentally
removed.   It is very speedily replaced, however;  the increased deter-
mination of blood to the Skin, which is the  consequence of the irrita-
tion, being favorable to  the  rapid production of  Epidermic  cells on its
surface.
  229. Mingled with the Epidermic cells we  find others which secrete
coloring  matter instead  of  horn;  these are termed Pigment-cells.
They are not readily distinguishable in  the  epidermis  of  the White
races, except in certain parts, such as the areola around the nipple, and
in freckles, naevi, &c.   But  they are very obvious, on account of their
dark hue, in  the newer layers of the Epidermis of the Negro and other
colored races ;  and, like the true  Epidermic cells, they dry  up and
become flattened scales in their passage towards the surface, thus con-
stantly remaining dispersed through the Epidermis, and giving it a dark 
SIMPLE ISOLATED  CELLS.—EPIDERMIS.
143
tint when it is separated and held up to the light.  In all races of men,
hoAvever, we find the most remarkable  development of Pigment-cells on
the inner surface of the Choroid coat of  the eye, where they form seve-
ral layers, known as the Pigmentum nigrum.   Here they have a very
regular arrangement; which is best seen where they cover the blood-
vessels  of the Choroid coat in a single layer, as shown in Fig.  22.
Fig. 22.
Fi<r. 23.
                                                Corpuscles of Pigment,     ]
                                              magnified 300 diameters;—
                                              a, cell; b, nucleus.
  A portion of the choroid ccat from the eye of the Ox, showing the pig-
ment-cells, where they cover, a, a, a, the veins, in a single layer; 6, 6,
ramifications of the veins near the ciliary ligament, covered with less
regular pigment-cells; c, c, spaces between the vessels, more thickly
covered with pigment-cells.

When examined separately, they are found to have  a polygonal form
(Fig. 23,  a), and to  have  a distinct nucleus (b) in  their interior.   The
black color  is given by the accumulation, within the cell, of a number
of flat rounded or oval granules, measuring about l-20,000th of an inch
in diameter, and a quarter as  much in thickness ;  these, Avhen separately
viewed, are observed to be transparent, not black and opaque ; and they
exhibit an active movement Avhen set free from the  cell, and even whilst
enclosed  within  it.   The  pigment-cells are  not  always of a simple
rounded or polygonal form ; they sometimes present remarkable stellate
prolongations, under which form they are well seen in the skin of the
Frog.—The Chemical nature of the  black  pigment  has not yet been
made evident;  it has been shown, however, to have a  close relation Avith
that  of the  Cuttle-fish ink or  Sepia, which derives its color from  the
pigment-cells lining the ink-bag; and to include a larger proportion of
Carbon than most other organic substances,—every 100 parts contain-
ing 58J of this element.
   230. That the development of the  Pigment-cells, or at least the for-
mation of their peculiar secretion, is in some degree due to the influence
of Light,  seems  evident from the facts  already mentioned  (§ 93).  To
these it may be added, that the neAV-born infants of the Negro and other
dark  races do not exhibit nearly the same depth of  color in their skins,
as that which they present after the lapse  of  a few days;  which seems
to indicate that exposure to light is necessary for the full  development
of the characteristic hue.  An occasional development of dark pigment-
cells  takes place during pregnancy in some females of  the fair races;
thus it is very common to meet with an extremely dark and broad areola 
144      STRUCTURE AND ENDOWMENTS  OF ANIMAL TISSUES.

round the nipple of pregnant women;  and  sometimes large  patches of
the cutaneous surface, on the lower part of the body especially, become
almost as dark as the skin of the Negro.  On the other hand, individuals
are occasionally seen with  an entire deficiency of  pigment-cells, or  at
least of their proper secretion, not merely  in the  skin, but in the eye;
such are termed Albinoes ;  and they are met Avith  as well among the
fair,  as among the dark races.  The  absence of  color  usually shows
itself also in the hair ;  which is almost Avhite.
  231.  The Epithelium may be designated as a delicate cuticle, covering
the free internal surfaces of the body ;  and  apparently designed, in some
instances, simply for their protection;  whilst in other  cases, as Ave shall
presently find, it serves purposes of far greater importance.  It has long
been knoAvn that the Epidermis might be traced continuously from the
lips to the mucous membrane of the mouth, and thence down the oesopha-
gus into the stomach;  and that in  the strong muscular stomach or giz-
zard of the graniArorous birds, it becomes  quite a  firm  horny lining.
But  it has been only ascertained by the use of the  Microscope, that a
continuous layer of cells may be traced, not merely along the Avhole sur-
face  of  the mucous membrane lining the  alimentary canal, but likewise
along the free surfaces of all other Mucous membranes, with their pro-
longations into follicles and glands; as well as on Serous  and Synovial
membranes, and the lining membrane of the  heart, blood-vessels, and
absorbents.   The  Epithelial cells,  being always  in  contact with fluids,
do not dry up into scales like those of the Epidermis; and  they differ
from them also in regard to the nature of the matter which they secrete
in their interior.   In this respect, however, the Epithelial cells of dif-
ferent  parts are unlike one another, fully as much  as  any of them are
unlike the cells of the Epidermis ; for Ave shall find that all the secretions
of the body are  the product of the elaboration of Epithelium  cells;
and  consequently there are as many varieties of endowment, in these
important bodies, as there are varieties in the result of their action.
  232.  The Epithelium covering the Serous and  Synovial membranes,
and  the lining of  the blood-vessels, is  composed of flattened polygonal
cells (resembling those  shown in Fig. 23), lying in apposition with each
other, so as to form a kind of pavement;  hence this form is termed pave-
ment- or tessellated-^^\the\mm.   There is  no reason to  believe that it
possesses any active endoAvments in these situations  ; since it does not
appear to be concerned in the elaboration of any peculiar  secretion.  It
has been already pointed out (§ 196), that the fluid of serous membranes
is separated from the blood  by a simple act of  mechanical transudation
(which often takes place to a great  extent after death); the walls of the
blood-vessels do not appear to be concerned in forming any peculiar
secretion ;  and the only product of this kind, which indicates any special
endowment in the epithelium-cells, is the synovia, Avhich is probably
elaborated by the cells covering the vascular fringes of the  synovial
membrane, formerly mentioned (§ 198).   The cells draw it from the blood,
during the progress of  their growth, form it as a  secretion within them-
selves, and then cast it into the general cavity of the joint (when their
term of individual life is ended), either by the rupture or the liquefac-
tion  of  their walls.  In other cases, it Avould  seem  as if  the  epithelial
cells were not frequently cast off and renewed, but possessed a considera- 
               SIMPLE ISOLATED  CELLS.—EPITHELIUM.            145

ble permanency.   It is to be remembered  that, in  the healthy state of
the serous and synovial membranes, and in that of the lining membrane
of the blood-vessels and absorbents,  they are entirely  secluded  from
sources of irritation ;  and that they lead a sort of passive life, very dif-
ferent from the active  life of the mucous membrane.   In fact, it would
appear to be the  sole  object of the serous membranes, to enclose and
suspend the viscera, in such a manner as to allow of the access of blood-
vessels, nerves, gland-ducts, &c.;  and at the same  time to permit  them
the required freedom of motion, and to provide against the irritation of
opposing parts, by furnishing an  extremely smooth and  moistened sur-
face, Avherever friction takes place.   Hence we find membranes, with all
the characters of serous surfaces,  in the false joints formed by ununited
fractures, and in other similar situations.
   233. The Epithelium of the  Mucous membranes and their  prolonga-
tions,  is found under two principal forms, the tessellated,  and the cylin-
drical.  An example of the Tessellated form is shoAvn in Fig. 24, which
sIioavs the separate epithelium cells of the mucous membrane of the mouth,
as they are frequently met with in saliva.   The cells are not always so
polygonal in form, however; sometimes  retaining their rounded or oval
form,  and being  separated by considerable interstices, so that they can
scarcely be said to form a continuous  layer.   A specimen of this kind
is seen in Fig. 25, which represents a group of epithelium  cells from one
of the smaller bronchial tubes.   This form of tessellated epithelium is
more commonly met with, where the secreting operations are more active,
the life of the  cells consequently shorter, and the renewal of them more
frequent;  so that they have not time, so to speak, to be developed into
a  more continuous layer.  The Cylinder-Epithelium  is very differently
constituted.   Its component cells and cylinders are  arranged side by
side;  one  extremity of each cylinder resting upon the basement-mem-

                                                Fig. 25.
            Separated Epithelium-cells, a,            Pavement-Epithelium of the
           with nuclei, 6, and nucleoli, c.           Mucous Membrane of  the
           from mucous  membrane  of           smaller bronchial  tubes; a,
           mouth.                           nuclei with double nucleoli,

brane, whilst the other forms part of the free  surface.  The perfect
cylindrical form is only shoAvn, when the surface on Avhich the cylinders-
rest is flat or nearly so.   When it is convex, the lower ends or bases of
the cells are of much  smaller diameter than the upper or free extremi-
ties ; and thus each has the form of a  truncated  cone, rather than, of ai,
cylinder.  (Fig. 26.)   This is well seen in the cells, which cover the>w$&
of the intestinal canal.  (Fig. 29.)   On the other hand, Avhere theocjdinr-
der-epithelium  lies upon a concave surface, the  free extremities, df tbie
cells may be smaller than those which are attached..  Sometime* eaeh. 
146      STRUCTURE  AND  ENDOWMENTS OF  ANIMAL TISSUES.

cylinder is formed from more than one cell, as is shown by the nuclei it
contains;  although its  cavity'seems to be continuous from end to end.
And occasionally the cylinders arise by stalk-like  prolongations, from
   Vibratile or ciliated Epithelium;—a, nucleated cells, resting on their smaller extremities; 6, cilia.

a tessellated epithelium beneath.  The two forms of Epithelium pass into
one another at various points;  and various transitional  forms are then
seen,—the tessellated  scales appearing to rise more and more from the
surface,  until they project as long-stalked cells, truncated cones,  or
cylinders.
   234.  Both  these principal  forms of Epithelial  cells  are frequently
observed to be  fringed at their free margins  with  delicate filaments,
which are termed cilia; and these,  although of extreme  minuteness, are
organs of great importance in the  animal economy,  through the extra-
ordinary motor power  with Avhich they  are endowed.  The form of the
ciliary filaments is usually a little flattened, and tapering gradually from
the base to the point.   Their size is extremely variable ;  the largest that
have been observed being about l-500th of an inch in  length, and the
smallest about l-13000th.  When  in motion, each filament appears to
bend from its root to its point, returning again to its original state, like
the stalks of wheat when depressed by the wind; and when a number are
affected in succession  with  this motion, the appearance of progressive
waves following one another is produced, as when a wheatfield is agitated
by frequent gusts.  When the  ciliary motion is taking  place in full
activity, however, nothing whatever can be distinguished, but the whirl
of particles  in  the surrounding fluid;  and it  is only when the rate of
movement slackens, that the shape and size of the cilia,  and the manner
in which their stroke  is made, can be clearly seen.  The  motion of the
cilia is not only quite  independent (in  all the higher animals  at  least)
of the will of the animal, but is also independent  even of  the life of the
rest of the body; being  seen  after the death  of the animal, and pro-
ceeding with  perfect regularity in parts separated from the body.   Thus
isolated epithelium cells have been seen to swim about actively in Avater,
by the agency of their  cilia, for some hours after they have been detached,
from the mucous surface of  the  nose; and the ciliary movement has
 been seen fifteen days after  death in the body of a Tortoise, in which
 putrefaction was far advanced.  In the gills of the River Mussel, which
 are among the best objects for the study of it, the  movement  endures
 with  similar pertinacity.
   235.  The  purpose  of  this  ciliary movement  is obviously to propel
 fluids over the surface on which it takes place; and it  is consequently
 limited in the higher  animals  to the internal surfaces of the body, and
 always  takes place in the direction of  the outlets, towards which it aids 
SIMPLE ISOLATED  CELLS.—EPITHELIUM.
147
in propelling the various products of secretion.   The case is^ different,
hoAvever, among animals of the loAver classes, especially those inhabiting
the water.  Thus the  external surface of the gills of Fishes, Tadpoles,
&c, is furnished with cilia; the continual  movement of which  reneAVS
the water in contact with them, and thus promotes the aeration of the
blood.  In the loAver Mollusca, and in many Zoophytes, which pass their
lives rooted to one spot, the motion of the cilia serves  not merely to
produce currents for respiration, but likeAvise to draAV into the mouth the
minute particles that serve as food.  And in the free-moving Animal-
cules, of  various kinds, the cilia  are the sole instruments  which they
possess, not merely for producing those currents in the water which may
bring them the requisite supply of air and food, but also for propelling
their own bodies through the Avater.  This  is the  case, too, with many
larger animals of the  class Acalephae (Jelly-fish), which  move through
the water sometimes  with great activity, by the combined action of the
vast numbers of cilia  that clothe the margins of their external surfaces.
In these  latter cases it would seem as if the  ciliary moA'ement were more
under the control of the will of the animal, than it  is where it is con-
cerned only in  the organic functions.  In what way the will can influence
it, hoAvever, it does not seem easy to say;  since the ciliated epithelium-
cells appear to be perfectly disconnected from the surface on Avhich they
lie,  and cannot, therefore, receive any direct influence from their nerves.
Of the cause of the movement of the cilia themselves, no account can be
given ; they are usually far  too  small  to  contain  even the minutest
fibrillae of muscle;  and Ave must regard them as being, like those fibrillae,
organs sui generis, having their own peculiar endowment,—which is, in
the higher animals at least, that of continuing in ceaseless vibration,
during the Avhole term of the life of the cells to which they are attached.
The length of  time during which  the ciliary movement continues after
the general death  of  the  body, is much less in the warm-blooded than
in the cold-blooded animals ;  and in this  respect it corresponds with
the degree of persistence of  muscular irritability,  and of other vital
endowments.
   236. The Tessellated-Epithelium, as  already mentioned, covers the
Serous and Synovial  membranes, the lining membranes  of the blood-
vessels and absorbents, and the Mucous membranes with their glandular
prolongations, except  where the cylinder-epithelium exists.  It presents
itself, with some modifications  presently to  be noticed, in  the ultimate
follicles of all glands,  and also in the smaller bronchial tubes.  In this
latter situation it is furnished with cilia; and  these are also found on
the cells  of the tessellated epithelium, which covers the delicate pia mater
lining  the  cerebral  cavities.   The cylinder-Epithelium commences at
the cardiac orifice  of the stomach, and lines the  whole intestinal  tube;
and, generally speaking, it lines the larger gland-ducts, giving place to
the tessellated form in their smaller ramifications.   A similar epithelium,
furnished with cilia, is found lining the air  passages and  their various
offsets,—the nasal cavities, frontal sinuses, maxillary antra, lachrymal
ducts and sac, the  posterior surface of the pendulous  velum of the palate
and  fauces, the Eustachian tubes, the  larynx, trachea, and bronchi,—
becoming continuous, hoAvever, in the finer  divisions of the latter, with 
148      STRUCTURE AND ENDOAVMENTS OF ANIMAL TISSUES.

the ciliated pavement-epithelium.   The upper part  of the  vagina, the
uterus, and the Fallopian tubes, are also furnished with a ciliated Cylin-
der-Epithelium.   The function of the cilia in all these cases  appears to
be  the same; that of  propelling the  viscid secretions,  Avhich would
othenvise accumulate on these membranes, towards the exterior orifices,
Avhence they may be carried off.
   237. The simplest office which the Epithelium-cells of  Mucous mem-
branes perform, appear to be that  of elaborating  a peculiar secretion
termed Mucus; Avhich is destined to protect them  from  the  contact of
air, or from that of the various irritating substances to which they are
exposed, in consequence of their peculiar position and functions.  This
Mucus is a transparent semifluid substance, distinguished by its peculiar
tenacity or viscidity.  It is quite insoluble in water ; but is readily dis-
solved by dilute alkaline solutions, from which it is precipitated again by
the addition  of an acid.   A  substance  resembling Mucus may be pro-
duced from any fibrinous  exudation, or even from  pus,  by treating it
Avith a small quantity of liquor potassae.  The  secretion of Mucus,  like
the formation of Epidermis, appears to  take place with an activity pro-
portioned to  the degree of irritation of the subjacent membrane.   On
many parts of  the mucous surface, a sufficient supply is afforded by the
epithelium-cells which cover it; but  in other situations, especially along
the alimentary canal, the demand is much greater, and it is probably
supplied not merely by the cells of the  surface, but by those lining the
crypts or follicles  which are formed  by involutions of it.
  238. The Epithelium-cells  which are thus being  continually reneAved
on  the Mucous surfaces, commonly seem  to have their  origin in the
granular germs diffused through the basement-membrane; but it is dif-
ferent in regard to the  cells of the follicles, which seem rather to occupy
their  cavity than merely to  line their  walls, and which appear to be in
course of continual production from a germinal spot,  or collection of re-
productive granules, at the blind extremity of  the follicle.   This is the
case in the ultimate follicles of the more complex glands; which may be
regarded as so many repetitions of the simple crypts or follicles in the
substance of the mucous membranes;—the only difference  being, that
Fig. 27.                   Fig. 28.              Fig. 29.
 Two follicles from the liver of Carcinus    Ultimate follicles of Mammary    Secreting cells of
manas (Common Crab), with their con-  gland with their secreting cells,   Human Liver; a, nu-
tained secreting cells.               a, a ;—b, b, the nuclei.          cleus; 6, nucleolus; c,
                                                     oil-particles.

the former pour their secretion into a branch of a duct,  which unites with
the_ other ramifications to form a trunk; and this trunk conveys them to
theirdestination in some cavity lined by a mucous membrane;—whilst
the simple follicles or crypts at once pour forth their secretion upon the 
             SIMPLE ISOLATED CELLS.—SECRETING  CELLS.          149

surface of the membrane.   The preceding figure (27) represents tAvo
follicles of the liver of the Common Crab, which are seen  to be  filled
with secreting cells; it  seems evident, from the comparative  sizes of
these cells in different parts, that they originate at  the blind extremity
of the follicle, Avhere there is a germinal spot;  and  that, as  they recede
from  that spot, they gradually increase in  size, and become filled with
their  characteristic secretion, being at the same time pushed onwards
towards the outlet by the continual new growth of cells at the germinal
spot.   In Fig. 28 are shown the corresponding ultimate follicles of the
Mammary gland; filled, like the preceding,  with secreting cells.
   239. The  whole  of the acts, then, by which t-he separation of the dif-
ferent Secretions from the Circulating  fluid is accomplished, really con-
sist in the growth and nutrition of a certain set  of cells, usually covering
the free surfaces of the body, both internal and external, or  lining  cavi-
ties Avhich have a ready communication with these by means  of ducts or
canals.*   These cells differ  widely from  one another, in regard to the
kind of matter which they appropriate and  assemble in  their cavities;
although  the nature of their walls is probably the same  throughout.
Thus Ave find biliary matter and oil, easily recognisable by their  color
and refracting poAver, in the cells of the liver;  milk in the  cells of the
Mammary gland; sebaceous or fatty matter in the cells of the sebaceous
follicles of the skin ; and so on.  All these substances are derived  from
the blood; being either contained in it previously, or being elaborated
from  its constituents by a simple  process of  transformation,—as, for
example, that Avhich converts the albumen of the blood into  the caseine
of milk.   Hence they may be considered as the peculiar aliments of the
several groups of cells ; whose acts of nutrition are the means of drawing
them  off, or secreting them, from the general circulating fluid.   When
they have attained their full groAvth, and accomplished their term of life,
their  Avails either burst or dissolve aAvay, and  thus the contents of the
cells are delivered into the cavity, or  upon the surface, at  Avhich  they
are required.   Now as all the canals of the glands open either directly
outwards upon the surface, or into  cavities which communicate with the
exterior, it is evident that the  various  products of the  action of these
epithelial cells must be destined to be  cast forth from the body.   This
Ave  shall find to be the case ;  some  of them, as the bile and urine, being
excretions, of Avhich it is necessary to get rid by the most direct channel;
whilst others, like the tears, the saliva, the gastric  fluid, the milk, &c,
are separated from the blood, not  so much for  its purification, but be-
cause they are required to answer certain purposes in the economy.
   210.  Now whilst thus actively concerned  in  the  Nutritive functions
of the economy, and exercising in  the  highest  degree their  poAvers of
selection and transformation,  these Secreting cells appear to have nothing
to do  with the operation of Reproduction.   We have seen that they do
not even regenerate themselves;  all their energies being, as it were, con-
centrated  upon their OAvn growth ;  and the successive production of new
broods of  them  being  provided for by other means.  Throughout the
organized creation, it appears to be necessary that the true act of Gene-
* The Synovial secretion is perhaps the only one which is poured into a closed 
150      STRUCTURE AND ENDOWMENTS  OF ANIMAL TISSUES.

ration should be  performed by the reunion  of the products of two dis-
tinct orders of cells ; the coalescence of which produces a germ, that is
the starting point of a new  organism;—thus  differing from  the act of
Reproduction by gemmation or budding,  which essentially consists in an
extension or out-groAVth from the original organism, by the subdivision
of cells in the manner  already described (§ 212).   Among  the lowest
Cellular Plants, in which every cell is apparently similar to the  rest,
this operation is effected by  the  "conjugation"  of any pair (as it would
seem) of the cells  which have been produced by multiplication from the
original germ.   But in all  the  higher organisms, both Vegetable and
Animal, Ave find that certain cells are set apart for  this purpose;  and
that there is  an  obvious distinction  betAveen the "germ-cells,"  from
within which the germ ultimately makes its appearance, and the " sperm-
cells," which  communicate to them a fertilizing influence.   Still in the
loAver tribes, both of Plants  and Animals,  we find that " sperm-cells"
and  "germ-cells" are deA'eloped in the midst  of the  ordinary tissues of
the body;  and it is  only as Ave  ascend the scale, and find the principle
of division of labor carried out in other Avays,  that we meet, as in Man,
with particular organs set apart for their evolution, and find these organs
appropriate respectively to distinct individuals.
   241. The spermatic cells of Man are  developed within the tubuli of
the Testicle;  where they appear to hold exactly the same relation to
the membranous walls of those  tubuli, as do  the secreting cells to the
tubes and follicles of the proper Glands,  being, in fact, the representa-
tives of their epithelial cells (Fig. 30, a).   Each of these developes in its
interior a variable number of secondary cells, or " vesicles of evolution ;"
and within every  one of these is produced  a single thread-like body,
dilated at  one extremity, and  possessed of a remarkable  self-moving
power, which.is termed a Spermatozoa.  Sometimes the  vesicles of evolu-
tion remain enclosed within the parent-cell, until their spermatozoa have
been completely developed, and have been set free by their rupture (b);
Fig. 30.
 Formation of Spermatozoa within seminal cells; a, the original nucleated cell; 6, the same enlarged, with
the formation of the Spermatozoa in progress; c, the Spermatozoa nearly complete, but still enclosed within
the cell.

and thus, when they have all performed their office, the parent-cell con-
tains nothing but a bundle of spermatozoa (c), whose dispersion takes
place as soon as its cell-wall gives way.  From the very peculiar motion
they possess, the  Spermatozoa Avere long regarded as  distinct and 
           SIMPLE ISOLATED CELLS.—REPRODUCTIVE CELLS.       151

independent animalcules;  it is now generally admitted, hoAvever, that
they have no more claim to a distinct animal character, than have the
ciliated epithelia of mucous membrane, which will  likewise continue in
movement  Avhen separated from  the body.   Similar bodies are  formed
by  all the  higher Cryptogamic Plants;  and it  appears from late re-
searches that  their office, as in Animals, is to fertilize the contents  of
the "germ-cells," with which their self-moving poAver brings them into
contact (chap, xi.)  It is a curious fact that the seminal cells, in which
the Spermatozoa are formed, are ejected from the gland in certain Crus-
tacea, not only before they have burst and set free their Spermatozoa,
but even long before the development of the Spermatozoa  in their inte-
rior is completed;—thus affording a complete demonstration of their
independent vitality.
   242. The " germ cells," in like manner, are very commonly developed
among the lower Animals as the epithelia of the tubes or follicles which
constitute the ovary; but in Man and the higher Animals, the ovary is
a solid organ, and the germ-cells are developed in  its substance, lying
in  the midst  of the dense fibrous  tissue  which forms its  parenchyma.
These germ-cells, Avhich are known as "ovisacs," like the sperm-cells,
develope secondary cells or ova in their interior; each ovisac, however,
producing but a single ovum.   The ovum, again, contains a tertiary cell,
the germinal vesicle, whose  contents appear to mingle with those of the
sperm-cell in the act of  fecundation,  so that the fertilized germ  is the
result; the remaining contents of the ovum being the nutritive materials,
at the expense  of which  this germ undergoes  its first  development
(chap, xi.)
   243. We now proceed to a class of cells, which  are equally indepen-/
dent  of each  other, which begin and end their  lives as cells, without'
undergoing any transformation, but which form part of the substance of
the fabric,  instead of lying upon its free surfaces and being continually
cast off from them.  Still their individual history is much the same  as
that of the cells already noticed;  and they differ chiefly in  regard to the
destination of their products.   The first group of this class deserving a
separate notice,  is that which effects the introduction of aliment into the
body ; of those kinds of aliment, at least, which are  not received in solu-
tion by any more direct means.   Along the greater part of the intestinal
tube,  from the point at which the hepatic  and pancreatic ducts enter  it,
to the rectum, we find the mucous membrane furnished with a vast num-
ber of minute tufts or folds, by  which its free surface is vastly extended;
these are termed villi.   They may be compared to the ultimate root-
fibres of trees, both in structure and function;  for each of them gives
origin to a minute lacteal or chyle-absorbing vessel, which occupies its
centre ; whilst it also  contains  a copious network of blood-vessels (Fig.
10, p. 127), which appears likewise to participate in the act of absorp-
tion, by taking up substances that are in complete solution.  Now at the
end of every  villus, there may be seen, whilst the process of digestion
and absorption  is going on, a cluster of minute opalescent globules,  in
the midst of which the origin  of the lacteal is lost.  These  globules,
whose size varies from l-1000th to  l-2000th of an  inch, are composed
of a milky fluid, which is evidently  the same with that which is found  in 
152      STRUCTURE AND ENDOWMENTS OF  ANIMAL TISSUES.

the lacteals ;  and it is  maintained by Prof.  Goodsir, Avho first brought
them  into  notice, that  these globules are really cells, and  that it is by

                                Fig.  31.
                          CO
-f
 Diagram of mucous membrane of Jejunum, when absorption is not going on; a, epithelium of a villus; b}
secreting epithelium of a follicle ; c, c, c, primary membrane, with its germinal spots of nuclei, d, d; e, genua
of absorbent vesicles; /, vessels and lacteals of villus.

their growth and nutrition that the milky fluid, or chyle, is selected from
the  contents of the  digestive cavity.   Their function, therefore, would
be precisely the converse of that of the secreting cells already described ;
whilst the history of  their  individual lives is  the same.   These absorbent
cells draw their materials  from the  fluid in  the digestive  cavity, instead
of from the blood; and Avhen they  burst  or liquefy, they set free their
contents where they  may be taken up by a lacteal and  conveyed  into
the circulating current, instead of  pouring  them into a cavity through
which they will be shortly expelled.   In  the intervals of  the digestive
process,  however,  the extremities of the villi are comparatively flaccid;
and  instead of cells,  they  shoAV merely a collection of granular  particles
(Fig. 31, e), which are considered  by Prof. Goodsir to  be  cell-germs.
There is considerable doubt, however,  whether these supposed  cells are
anything else  than  oil-globules; and  Avhether  the real agents  in  the
selection of chyle  are  not the epithelium-cells  covering the villus (a),
within which chylous-looking globules have been occasionally seen, when
digestion was going on.
   244. Although the Mucous  membrane of the  intestinal tube is  the
only channel through Avhich insoluble  nutriment can be absorbed in the
completely formed Mammal, and the only situation, therefore,  in which
we meet with these absorbent cells, there are other situations in Avhich
similar cells perform analogous duties in  the embryo.  Thus the Chick
derives its nutriment, whilst in the  egg, from the substance of  the yolk,
by absorption through  the blood-vessels spread out in the vascular layer
of the germinal membrane surrounding the yolk ; Avhich vessels  answer
to the blood-vessels and lacteals of the permanent digestive cavity,  and
are raised into folds or villi as the contents of the yolk-bag are diminished.
Now the ends of the  vessels  are separated from  the fluid  contents of the
yolk-bag, by a layer of cells ; Avhich seems to have for its  object to select
and  prepare the materials supplied  by the yolk, for being received into
the  absorbent  vessels.
   245. In like manner, the  embryo of the  Mammal is nourished, up to 
ABSORBENT CELLS.
153
the time of its birth, through the medium of its umbilical vessels ; the
ramifications of Avhich  form  tufts, that dip down, as it  were,  into the
maternal  blood, and receive from it the materials  destined to the nutri-
tion of the foetus, besides effecting the aeration of the blood of the latter,
by exposing it to the more oxygenated blood of the mother.   Now around
the capillary loop of the foetal tuft, there is a layer of cells, closely re-
sembling  the absorbent cells of the villi; and these are enclosed in a cap
of basement-membrane, Avhich completes the foetal portion  of  the tuft,
and renders it comparable in all essential respects to the intestinal villus.
It is again surrounded, however, by  another layer of membrane and of
cells, belonging to the  maternal system;—the  derivation  and  arrange-
ment of which will be  explained hereafter.   The maternal cells (b, Fig.
32), may be regarded as the first selectors of nutriment from the circu-
                                Fig.  32.
  Extremity of a placental villus:—a, external membrane of the villus, continuous with the lining mem-
brane of the vascular system of the mother; 6, external cells of the villus, belonging to the placental decidua;
c, c, germinal centres of the external cells; d, the space between the maternal and foetal portions of the
villus; e, the internal membrane of the villus, continuous with the external membrane of the chorion; /,
the internal cells of the villus, belonging to the chorion ; g, the loop of umbilical vessels.

lating fluid of the parent: the materials, partially prepared by them,  are
poured into the cavity (d)  surrounding the extremity of the tuft;  and
from  this they are taken up by the foetal  cells (/), Avhich further elabo-
rate them, and  impart  them to the capillary loop (g) of  the umbilical
vessels.
   24G. Thus we see that the several functions of Selection, Absorption,
Assimilation, Respiration,  Secretion, and Reproduction, are  performed
by the agency of cells in the Animal as in the Vegetable kingdom,—in
the complex Human organism, as in the  humblest  Cryptogamic Plant:
the only difference being, that  in the latter there is a greater division of
labor, different groups of cells  being appropriated to different functions,
in the general economy, whilst the history  of  their own  processes of
nutrition and decay is everywhere essentially the same.   Thus we have
seen that the Absorbent cells, at the extremities  of the intestinal or pla-
cental villi, select  and  draw into  themselves, as  the materials  of their
OAvn groAvth,  certain  substances  in  their neighborhood;  which are still
as much external to the tissues of the body, as are the fluids surrounding
the roots of plants.   Having come  to  their  full term of life, they give
up their contents to  the absorbent  vessels, which carry  them into  the
general current of the circulation, where they are mingled with the fluid
previously assimilated,—the blood.  Whilst passing through the vessels,
they are subjected to the action of the various cells (all of Avhich we have
seen to be  successive  phases of the  same type) which float  in the circu-
lating current; and by these they seem to be gradually assimilated, or 
154      STRUCTURE AND ENDOAVMENTS OF ANIMAL  TISSUES.

converted into a  substance  of a  more directly organizable character.
The special function of the red corpuscles peculiar to Vertebrated animals,
though not yet accurately knoAvn, seems intimately connected with the
process of Respiration.   Next we have various groups of cells, external
to the vessels, on the free surfaces of the body ; whose office it is to draAV
from the blood certain  materials,  which are destined for  Secretion or
separation from it; either for the  sake of preserving  that fluid in its re-
quisite purity, or for   answering  some  other  purpose in  the system.
These cells grow at the expense of the substances which they draAY  into
themselves from the blood ; and on their dissolution, they cast forth their
contents  on the free surfaces communicating Avith the exterior of the
body, to which they are in  time  conveyed.   And,  lastly, we  have a
special set of  Generative cells, destined in the  one sex to prepare the
germs of neAV beings;   and in the  other to elaborate  a product essential
to their fertilization.
   247. The cells which are thus the active instruments of the Organic
functions, are usually produced and succeed one another with a rapidity
proportional to the  energy of those functions, though the causes Avhich
influence their growth  and  decay are not always evident.  Thus  it is
certain that, cceteris paribus, the  rate  of  production of the  Secreting
cells depends upon the abundance of the materials supplied  by the circu-
lating current, which they are destined to eliminate  from  it.  But this
is by no  means the sole condition  of their development; for, as Ave shall
see hereafter,  these materials  may accumulate  unduly in the blood,
through the insufficient activity of the cells Avhich are destined to sepa-
rate them; whilst, on the other hand, the presence of certain substances
in the blood appears to accelerate their production.   Of these  stimuli,
Mercury is one of the  most powerful; and Ave  have continual opportu-
nities of witnessing its effects, in  giving an  increased  activity to the
secreting actions.   There is  probably not a gland in the body, which is
not in some degree influenced by its presence in the blood ; but the liver,
the kidneys,  the  salivary glands, and the glandulse of  the  intestinal
canal, appear to be those most affected by its stimulating poAvers.  The
action of the  glands, in other words the development of the  secreting
cells, appears to be influenced by mental emotions ; being sometimes accele-
rated, and sometimes retarded, through their agency.   This is  especially
the case in regard to the secretion of Milk, Tears, Saliva, and Gastric juice.
It seems probable that  the  influence  thus  manifested is partly  exerted
through the capillary circulation, Avhich is knoAvn to be powerfully affected
by mental  emotions, as  in the acts of blushing and erection ; and  that the
increased production of the secretion is immediately due to the increased
flow of blood to the gland.   But there are other phenomena Avhich show
that the development and actions of the secreting cells are  more  directly
influenced by the nervous system ; these will be hereafter  considered
(chap. IX.)

    5.  Of Cells connected together as permanent constituents of the Tissues.

   248.  We now pass on to consider those Cells, which enter  as compo-
nent elements into the  solid and permanent fabric of the body,  and which 
           CELLS CONNECTED TOGETHER IN  SOLID TISSUES.       155

do not take so active a part in its vital operations.   These we shall find
to be usually more or less closely connected together, either by a general
enveloping membrane, or by an intercellular substance, which is  inter-
posed  between their walls,  and  holds them together by  its adhesive
properties.
  219. The presence of a general enveloping membrane (where it is not
a secondary formation) appears to depend upon the persistence of the
original cell-walls ;  which, instead of liquefying or thinning away, when
distended by the multiplication of cells in their interior, are thickened or
strengthened by additional nutrition.   Such is perhaps the case with the
sacculi in which the cells of Adipose tissue (§ 257) are often found clustered
together;  but  this  condition is usually much  more  obvious in  many
tumors, whose development depends upon an abnormal process of growth.
  250. Where such  enveloping  membranes are wanting, we frequently
find the component cells of the permanent  tissues of Animals (like those
of the  higher plants) held together by an intercellular substance; which
generally presents no distinct traces of organization; and which usually
consists of Gelatine, or of a substance allied to it  in composition.   The
proportion of this substance to the cells may vary in different cases; and
very different characters may thus be presented by a tissue made up of
the same elements.   Thus the subjoined figure (33) represents a portion
of one  of the animal layers included betAveen the calcareous laminae of a
bivalve shell:  in Avhich we see  on the one  side  a  number of nuclei  or
incipient cells, scattered through a bed of homogeneous intercellular sub-
stance, and bearing but a very small proportion to it; whilst the opposite
end exhibits a set of polygonal cells,  in close contact with  each other,
the intercellular substance being only represented by the thick dark
lines, Avhich mark the boundaries of  the  cells, and which are rather
thicker at the angles of the latter.   BetAveen these tAvo extremes, we
observe every stage  of transition.
  251. The presence of a very large  amount of intercellular substance,
through which minute cells are scattered at considerable interA'als (Fig.
33, a), is  characteristic of various forms  of Cartilage ;  and more par-
ticularly of that soft semi-cartilaginous structure, of which the Jelly-fish
are for the most part composed.   In  other forms of cartilage, Ave find
the cells more developed, and in closer proximity  to each other,  the
proportion of the intercellular  substance being at the same time  dimi-
nished (as  seen at b and  c, Fig.  33); but  it is  not often,  save in  the
embryonic structures, that Ave find the cells in such close proximity, and
the intercellular substance so nearly  Avanting, as at d.   Such examples
do occasionally present themselves, hoAvever, even in the soft  tissues.
Thus  the  chorda dorsalis, Avhich replaces  the vertebral  column in  the
lowest Fishes, and of which the analogue is found in the  embryos of the
higher Vertebrata, is made  up of a structure of this  kind (Fig. 16).
The  true  Skin in  the Short  Sun-fish, is replaced by a similar layer of
cellular tissue, Avhich extends over the whole body, varying in thickness
from one-fourth of an inch to six inches.  And in the Lancelet (a  little
fish which is  destitute of so  many of the  characters of a Vertebrated
animal, that its right to  a place in  that division has been doubted), a
considerable portion of the fabric is made up of a similar parenchyma. 
156*-      STRUCTURE  AND ENDOAVMENTS OF ANIMAL TISSUES.
  252.  Now Ave shall find that one method, by which the requisite firm-
ness and solidity are given to the animal  fabric, consists in  the depo-

                                 Fig. 33.
 Portion of shell-membrane, showing the origin of cells in the midst of horny intercellular substance; a,
nuclei; b, incipient cells; c, the same further advanced, but separated by intercellular substance; d, the
cells become polygonal by mutual pressure.

sition of earthy substances in the  interior  of such cells, by  a peculiar
secreting action of their own.  Thus in Shell, we find them  completely
filled up with carbonate of lime ;  and in the enamel of Teeth with phos-
phate of lime.   When this is the case, there is a tendency to an apparent
coalescence of the cells, by the obliteration of their partitions  ; or rather,
perhaps, by the removal of the whole intercellular substance from between
them, the  actual  cell-walls being so very thin, that they are  not distin-
guishable.   The incipient stages of this coalescence, as seen in another
portion of the same membrane as that represented in the last figure, are
shown in Fig.  34.  At  a, the nucleated cells are very distinct;  and are
separated  by a  large quantity of intercellular  substance.   At b, they
approach each other more closely, the amount of intercellular substance
being less ;  the Avidest intervals are seen at the angles  of the cells.  At
c,  the approximation  is much closer;  and the cell-Avails are scarcely dis-
tinguishable at the points Avhere they come into  immediate  contact.
Proceeding further, we observe that  the partitions are much less com-
plete ; so  that the originally distinct cellular character  of the membrane
 Portion of shell-membrane, showing the gradual coalescence of distinct cells; at a, the cells separated by
      intercellular substance; at b, the partitions are thinner; and at c, they almost disappear.

is  chiefly indicated by the bright nuclei, which  are regularly dispersed
through it,  and by the triangular dark spots, which show the remains of 
FUSIFORM CELLS.
157
the intercellular substance  at  the  angles Avhere three cells join each
other.   The coalescence may be traced further than  it is shown to do
in the figure;  so that if it  were  not for the evidence afforded by the
transition-stages here represented, it would be difficult to  prove that the
membranous layer had its origin in cells.
   253.  These facts, respecting the gradual coalescence of cells, explain
not merely certain appearances  presented in  Tooth, Shell, &c (here-
after to be described); but also those which are exhibited  by the Base-
ment-membrane, as already  detailed (§ 206).
   254. There  is no evidence, in the preceding case, that the cavities of  .
the cells coalesce ; and there is no reason Avhy they should do so.   But
we often find such a union,  Avhere the production of  a continuous tube
is required.   The  long straight open ducts, through wThich the sap of
Plants rises in the stem, are unquestionably formed by a coalescence of
the cavities of cells of a cylindrical form,  placed regularly end to end;
and it seems probable that the netAvork of anastomosing vessels, through
which the elaborated sap finds its way to the various  parts of the vege-
table  fabric,  is  formed,  in  like  manner,  by the coalescence  of cells,
arranged obliquely and transversely in regard to one  another.   In  like
manner, the capillary Blood-vessels of Animals  are usually believed to
originate in toavs of  cells, the  cavities of which have run together by
the obliteration of the transverse  partitions ; as the persistent nuclei of
such cells may be occasionally brought into vieAV in the walls of  the  ca-
pillaries.  And the same appears  to be the origin of the tubular fibres of
Muscular and  Nervous tissue, which contain the elements  characteristic
of those tissues; these elements,—the fibrillse of muscle and the granular
pith of the nerve-tube,—being evidently the secondary products of parent-
cells, which seem  to  remain as their investing tubuli, in the  Avails of
which the original nuclei are often to be seen (§ 338 and  388).
   255.  Besides  these changes, the original cells may  often  undergo
marked alterations of form;  and this quite independently of any pressure
to which they may be subject.   Thus the pigment-cells, as already men-
tioned (§ 229), frequently exhibit a curious stellate  form; arising from
the development of radiating prolongations, which  are  put forth from
the original spheroid.  A form which is frequently assumed by the cells
that are developed in fibrinous or plastic exudations, and which is  also
met Avith in  the cells of tumors, both  malignant  (or  Cancerous) and
non-malignant, is that which has received the designation  of fusiform or
spindle-like, from  its prolonged shape  and  pointed  extremities.   The
various stages of transition, which may be observed betAveen the simple
rounded cell and the fusiform cell, have been shoAvn in Fig. 7; and it is
there seen that,  Avhen the transformation has gone to its utmost extent,
the nucleus of the cell is no longer visible, so that  it bears a close re-
semblance to  a simple fibre.  Such cells are  found  amongst the  simple
fibrous tissues, and, in the opinion  of many, they give origin to them.
The appearance of tissue composed of fusiform cells, is shown in Fig. 35;
this is seldom  met  with as a permanent part of the normal fabric; but it
is a frequent  product of  morbid action.
   256.  We now proceed with the description of the various tissues in
the Human body,  which are composed  of cells  united or  transformed 
158       STRUCTURE  AND  ENDOWMENTS OF  ANIMAL TISSUES.

in the  foregoing manner;  and  we  shall  commence Avith  Adipose  or
Fatty tissue, which may be considered as a sort of link, connecting the
permanent tissues with those Avhich are more actively concerned in the
processes of Nutrition, Secretion, &c.
                 Fig. 35.
 Fusiform tissue of plastic
exudations; a, fusiform bo-
dies without nuclei; b, nu-
cleated fusiform cells; c,
granular intercellular sub-
stance.
   257. The Adipose tissue is composed of isolated cells, which have the
power of appropriating fatty matter from the  blood, precisely in the
same manner as the secreting cells appropriate the elements of bile, milk,
&c.  These cells are sometimes dispersed in the interspaces of the Areolar
tissue; whilst in other cases they are aggregated in distinct  masses,—
constituting the  proper Adipose  tissue.   In the former case they are
held in their places by fibres, that traverse the areolae in different direc-
tions; whilst in the latter, each small cluster of  fat-cells is included in
a common envelope, on the exterior of which the blood-vessels  ramify;
and these sacculi  are held  together by  areolar tissue.  We are thus
probably to regard each fatty mass in the light of a gland, or assemblage
of secreting cells, penetrated  by blood-vessels,  and bound  together by
fibrous tissue; but having its follicles closed  instead  of open (which
appears to be the early condition of the follicles of all glands, § 238):
and consequently retaining its secretion within itself, instead of pouring
it forth into a channel for excretion.
                                Fig. 37.
Capillary network around Fat-cells.
  258.  The individual fat-cells always present a nearly spherical or sphc<
roidal form ; sometimes, however, when they are closely pressed together,
 Areolar and Adipose tissue ; a, a,
fat-cells;  6, b, fibres of areolar
tissues. 
ADIPOSE TISSUE.
159
they become somewhat polyhedral, from the  flattening  of  their walls
against each other. Their intervals are traversed by a minute network
of blood-vessels (Fig. 37), from which  they derive their secretion; and
it is probably by the constant moistening of their walls with a watery
fluid, that their contents are  retained  without  the  least transudation,
although they are quite fluid at the temperature of the living body.   If
the watery fluid of the cell-walls of a mass of Fat be allowed to dry up,
and it be kept  at a temperature of 100°, the escape of the contained oily
matter is soon  perceptible.—By this provision, the fatty matter is alto-
gether prevented  from escaping from  the  cells of the living tissues, by
gravitation  or pressure; and as it is not itself  liable to undergo change
Avhen  secluded from the air, it may remain stored up, apparently unal-
tered, for almost an unlimited period.
   259. The consistency, as well  as the Chemical  constitution,  of the
fatty matter contained in the Adipose cells, varies in  different animals,
according  to the relative proportions  of  three component substances
Avhich may  be distinguished  in it,—Stearine,  Margarine, and Oleine.
The two former are solid when isolated, and  the latter is fluid ;  but at
the ordinary temperature of the warm-blooded animal, they are dissolved
in it.  Of  these, Stearine is the most  solid; and it is the most largely
present, therefore, in the hardest  fatty matter, such as mutton-suet.  It
is crystalline like spermaceti; it is not at all greasy betAveen the fingers,
and it melts at 143°.  It is insoluble in Avater, and in cold  alcohol and
ether: but  it  dissolves in boiling alcohol or ether, crystallizing as it
cools.   The substance termed Margarine exists along with  stearine in
most fats, but  it  is the  principal  solid constituent of Human fat, and
also of Olive oil.  It corresponds with Stearine in  many of  its proper-
ties, and is nearly allied to it  in Chemical composition ;  but it is much
more soluble in alcohol and ether,  and  it melts at 118°.  On the other
hand,  Oleine, when pure, remains fluid at the zero of Fahrenheit's ther-
mometer ;  and it is  soluble in cold ether, from which  it  can only be
separated by the evaporation  of the latter.  It exists in  small quantity
in the various solid fats ;  but it constitutes the great mass of the liquid
fixed oils.  The tendency of these  to solidification by cold, depends upon
the proportion of stearine or  margarine they may contain.
   260. All these substances are neutral compounds formed by the union
of Stearic, Margaric, and Oleic acids,  respectively, with  a base  termed
Glycerine; this base may be  obtained  from any fatty matter, by treat-
ing it with an alkali, which unites with the  acid and  forms a soap,
setting free the Glycerine.  They contain no Nitrogen ;  and their pro-
portion of Oxygen is extremely small in comparison with their amount
of  Carbon and Hydrogen: thus Stearine has 142 Carbon and 141 Hy-
drogen to 17 Oxygen: and in the other substances  the proportions are
similar.  The fatty bodies appear to be mutually convertible; thus mar-
garic  acid  may be procured from  stearic acid,  by subjecting  it to dry
distillation ; and there is ample evidence that animals supplied with one
of  them may produce the others from it.
   261. Since these Fatty matters are abundantly supplied by the Vege-
table kingdom, and are found to exist  largely  in substances  which were
not previously supposed  to contain them, it is not requisite to suppose, 
160      STRUCTURE  AND  ENDOAVMENTS OF ANIMAL TISSUES.

that Animals usually  elaborate them  by any transforming process from
the elements of their ordinary food.   The mode in Avhich they are taken
into  the blood, and the uses to Avhich  they are subservient, will be here-
after investigated: but it may be here remarked, that the portion sepa-
rated from the circulating fluid to form the Adipose tissue, is only that
Avhich can be spared from the other purposes, to which the fatty matters
have to be applied.  Hence the production of this tissue depends in part
upon the amount of Fatty matter taken in as food; but  this is not en-
tirely the case,  as some have maintained; for there is sufficient evidence
that animals may produce fatty matter  by a process of chemical trans-
formation, from the starch or sugar of their food, Avhen  there is an un-
usual deficiency of it in their aliment.
   262.  The development of Adipose  tissue in the body appears to an-
swer several distinct purposes.   It fills up interstices, and forms a kind
of pad or cushion for the support of movable  parts;  and so  necessary
does it  seem for this  purpose, that even in cases of great emaciation,
some fat is ahvays found to remain, especially at the base of the heart
around  the origin  of  the great vessels,  and in the orbit of the eye.  It
also  assists in the retention of  the animal temperature  by its non-con-
ducting  power; and  Ave accordingly find a thick  layer  of it, in those
warm-blooded mammals that inhabit  the  seas,—either immediately  be-
neath their skin, or incorporated with its substance.  Its most important
use,  however, is to serve as a reservoir of combustible  matter, at  the
expense of which  the respiration may be maintained Avhen  other mate-
rials are  deficient; thus Ave find  that  the  respiration of  hibernating
animals  is kept up,  during the period when they cease  taking food
(§ 121), by the  consumption of the store  of fat which was  laid up in
their bodies, previously to their passing into that state; and  it is also
to be noticed that  herbivorous animals,  whose food is scanty during the
winter,  usually  exhibit a  strong  tendency to  such an  accumulation,
during the latter part of the summer, when their  food is most rich and
abundant, in order to supply the  increased demand created by the low
external temperature  of the Avinter season.  Other circumstances being
the same, it appears that the length of time during which a warm-blooded
animal  can live without food, depends upon the quantity of fat in its
body; for the rapid lowering of its temperature, which is the immediate
cause of its death (§ 117), takes place as soon as the whole of this store
has been exhausted.  Of the means by which the fatty secretion is taken
back again into the current of the circulation, Avhen it  is required  for
use in the system,  we know nothing whatever.
   263.  In order that it may be applied to the maintenance of the  ani-
mal  heat, the fatty matter must be received back  into the  blood;  and
although Ave have no  certain knowledge of the  mode in which this is
accomplished, yet it may be surmised to be as folloAvs.  The Blood nor-
mally contains a certain  amount  of fatty matter, held  in  solution by
combination with its  alkali; and should  this be  exhausted by the  com-
bustive process,  the circulating current will draw into itself a fresh sup-
ply from the interior  of  the  fat-cells;—it  having  been shown by Mat-
teucci that oleaginous particles will pass through animal membranes by 
CARTILAGE.
161
endosmose, to diffuse themselves through  an aqueous liquid, provided
the latter be alkaline.
   264. In the simpler forms of Cartilage, we have an example of a tissue
of remarkable permanence, composed entirely of cells scattered through
an intercellular  substance.   This  substance has  received  the  distin-
guishing appellation of Chondrine, which  marks it as the  solidifying
ingredient of Cartilage (§177).  All the Cartilages of the foetus,—those
which are to be  converted into  bone, as  well as  those which  are to
remain unossified,—are composed of it; and yet, as soon as  the process
of Ossification commences, the chondrine is replaced by Gelatine, which
is the sole organic constituent of  the animal  basis  of  bones.   The per-
manent cartilages, however, still  contain only Chondrine; but if acci-
dental bony  deposits should take place  in them (as frequently happens
in old persons, especially in the  cartilages  of the  ribs), the Chondrine
gives  place to Gelatine.  This change of composition  is coincident, as
we shall hereafter see, with a complete change in texture;  the basis of
■bony tissue not being Cartilage (as commonly imagined), but consisting
of a substance much more nearly allied to  the white fibrous  tissue.—It
is only in  the pure cellular cartilages, in which the intercellular sub-
stance presents no trace of organization, that  Chondrine occurs.  Those
of the/fo-o-cartilages (§ 269), in which  the  intercellular substance  has
the characters of the White fibrous tissue, yield gelatine  on  boiling, in
the manner of the ligaments  and tendons; whilst  those which contain
much of the Yellow or elastic  tissue, undergo very little change by boil-
ing, and only yield,  after several  days, a small quantity of  an extract
which  does not form a jelly, but which has the other chemical properties
of Chondrine.
   265. Besides the  organic compounds already described, most Carti-
lages  contain  a certain  amount of mineral  matter, which forms an  ash
when  they are calcined.   This ash contains a large proportion of car-
bonate and sulphate of soda, together with  carbonate of lime,  and a
small  quantity of  phosphate of lime; as  age advances, the  proportion
of the soluble compounds diminishes, and the phosphate of  lime predo-
minates.  This is especially the  case in  the costal cartilages, which
almost invariably become converted into a semi-ossified substance, in
old persons ;  and it is remarkable that, even before they have  themselves
become thus  condensed, they are united  by ossific  matter, when they
have undergone fracture.
                              Fig. 38.
Cartilage of Mouse's ear.
  266.  When a  pure Cellular  Cartilage is examined microscopically
its cells are seen to lie, sometimes singly, and sometimes in clusters  of 
162      STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES.

two, three, or four, in cavities excavated in the intercellular  substance.
These occur at very variable distances; for in some instances they are
packed together as closely as the cells of a vegetable parenchyma (Fig.
38);  whilst in others the principal mass is  composed of intercellular
substance, through which  the cells are interspersed at Avide intervals.
From the various appearances Avhich may be observed  in  the same car-
tilage at different stages of  its  groAvth, it would appear that the compo-
nent  cells multiply by the doubling process  already described (§ 212);
that they then separate from one another, each of them drawing toAvards
itself (as it were) an envelope of intercellular substance;  and that, by
the repetition of the  same process, the  number of cells in the cartilage
may be indefinitely multiplied.
   267.  Various  stages  of this history are shown in the accompanying
figure (Fig. 39), which is taken  from a  section of the cartilaginous bran-
chial ray of the larva or tadpole of the Rana esculenta, or Edible Frog.
In the centre  of the figure are shown  three  separate  cellst which have
evidently been at one time in closer proximity with each other.   In one
of these cells, the nucleus is seen to be developing two new cells in its
interior; and  a continuation of this process would give rise to the appear-
ance shown at b, Avhere  two  cells are shown in close contact,  being evi-
dently the offspring of  the same parent.  Now if each of these cells in
like manner developes tAvo others within itself, a cluster of four will be
developed, as shown at  a; and after a time, intercellular substance being

                                Fig. 39.
Section of the Branchial cartilage of Tadpole;  a, group of four cells, separating from each other; 6, pair
        of cells in apposition; c, c, nuclei of cartilage cells; d, cavity containing three cells.

accumulated around each, their walls will separate, and they will acquire
the character of distinct cells.  It would seem as if, in other cases, one
of the first pair of cells developes another pair in its interior, whilst the
other (from some  unknown cause), does  not at once  proceed to do so;
and thus only three cartilage-cells instead of four are clustered together
in the cavity, as seen at d.
   268.  The primitive cellular organization now described is retained in
some Cartilages through the whole duration of their existence.   This is
the case, for example, in  most  of  the articular cartilages of joints; in
the cartilaginous portion of the septum narium, in the cartilages of the
alae and point of the nose, in  the semilunar cartilages  of the eyelids; in
the cartilages of  the larynx (with the exception of the  epiglottis), the 
CARTILAGE.
163
cartilages of the trachea and bronchial tubes, the cartilages of the ribs,
and the ensiform cartilage of the sternum.  When partial ossific depo-
sits take place, it is usually in the substance  of  cellular, rather than in
that of fibrous cartilage.
   269. When the intercellular substance, instead of being homogeneous,
has a fibrous  character, the tissue  called Fibro-Cartilage is produced;
and this may be either elastic or non-elastic, according as the yellow or
the white form of fibrous structure prevails.   In  some  instances, the
fibrous structure  is so predominant over  the cellular, that the tissue
has rather the character  of  a ligament than of  a cartilage.  The white
fibrous structure is seen in all those cartilages, Avhich unite the bones by
synchondrosis, and which are destined not merely to sustain pressure,
but also to resist tension.  This is the case especially in the substances
which intervene betAveen the vertebrae, and which connect the bones of
the pelvis ; these in adult Man are destitute of  cartilage-corpuscles, ex-
cept in and pear their centres;  but in the lower Vertebrata, and in the
early condition of the higher, the fibrous structure  is confined  to the
exterior, and the Avhole interior is occupied  by the ordinary cartilage-
corpuscles.   The yellow-fibrous or reticulated structure is best  seen in
the epiglottis, and in the concha  of the  ear;  in  the former  of these,
scarcely any trace of cartilage corpuscles remains ;  and  in  the latter,
the cellular structure  is only to be met with towards the tip.
   270. We have seen that the elements  of the  cellular tissues hitherto
described, do not come into  direct contact with  the blood-vessels.  The
Epidermic and Epithelial cells are  separated from  them by the conti-
nuous  layer of basement-membrane,  which forms the surface of the true
skin,  of the  mucous  membranes, of the glandular  follicles produced
from them, &c.  In like  manner, the cells of Adipose tissue are formed
within  membranous   bags;   around which the blood-vessels  form  a
minute network.   The  cells  of Cartilage are not  nourished  in  any
more direct manner; and are sometimes at a  considerable distance from
                                Fig. 40.
  Vessels situated between the attached synovial membrane, and the articular cartilage, at the point where
the ligamentum teres is inserted in the head of the os femoris of the human subject, between the third and
fourth months of foetal life;—a. the surface of the articular cartilage; b, the vessels between the articular
cartilage and the synovial membrane ; c, the surface to which the ligamentum teres was attached ; d, the
vein; e, the artery.

the nearest vessels.  It is  certain that the substance of the permanent
cellular Cartilage is not permeated, in a state of health, eAren by the
minutest  nutrient vessels;  none such being brought into view under the 
164      STRUCTURE AND ENDOAVMENTS OF ANIMAL  TISSUES.

highest magnifying power.   They are, however, surrounded by vessels
(Fig. 40), which  form large ampullae or  varicose dilatations at their
edges, or  spread over their surfaces; and it  is  by  the  fluid which is
drawn from them by the Cartilage-cells, that  the latter  are nourished.
The nutrition  of a mass of Cartilage thus seems to bear a strong resem-
blance to  that of the  thick  fleshy Sea-weeds, which  are in like manner
composed entirely of cells, with intercellular substance disposed between
them in greater or smaller amount.  The cells in nearest proximity to
the  nutrient fluid, draw from it the requisite  materials, and transmit
these to the cells in the interior of the mass, receiving a fresh supply in
their turn from the source in their OAvn neighborhood.   When the Ar-
ticular or other  cellular  Cartilages are inflamed, however, we find ves-
sels  passing into their  substance; but these vessels are formed in  an
entirely neAV tissue, Avhich is the product  of the  inflammatory process,
and cannot be said to belong to the Cartilage itself.
   271.  The temporary Cartilages, which  have  a like cellular structure,
but which are destined to undergo metamorphosis into Bone, are equally
destitute of vessels when their mass is small; but if  their thickness ex-
ceed an eighth of an inch, they are permeated  by canals for the trans-
mission of vessels.  Still these vessels do not ramify with any minute-
ness in the tissue; and they leave large islets, in which the  nutritive
process must take place on the plan just described.
   272.  The Fibro-Cartilages, formed as it were by the intermingling of
two  distinct elementary structures, have a degree of vascularity propor-
tioned to the amount of the fibrous tissue which they  contain ; but these
vessels do not penetrate  the  cellular  portions,  where such  are distinct
from the mixed structure.
   273.  The  Cartilaginous  tissue  appears to  be more  removed than
almost any other in the body from the general tide of nutritive action.
Its properties are simply of a physical character ; and they are not im-
paired for a long time after the death of the tissue,  its tendency to de-
composition being very slight, so long as it is exposed  to ordinary tem-
peratures.  It is  protected  by its toughness and elasticity from those
mechanical injuries to Avhich softer or more brittle  tissues  are liable;
and consequently it has  little need of any active power of reparation.
When loss of substance occurs as a result of disease or accident, this seems
never to be repaired by  real cartilaginous substance;  but  the space is
filled up by a fibrous tissue developed from the reparative blastema (§ 213).
It is in this tissue that the new vessels are found, which have been erro-
neously- supposed to penetrate  the cartilage when it  becomes inflamed;
the  fact being, that the vessels  are restricted to the  "false membrane"
formed in the inflammatory process, which takes the place of the carti-
laginous tissue that has disappeared in consequence of imperfect  nutri-
tion or degeneration.
   274. The  Cornea of  the Eye bears  a  superficial resemblance to
 Cartilage; but it corresponds  rather with Fibrous  Membranes  in  its
 elementary structure.   Besides its anterior or conjunctival layer, which
 consists of epithelium,  and its  posterior layer of cells constituting  the
 epithelium of the aqueous  humor, the Cornea  has  been shown by Mr.
 Bowman to consist of three layers, of which the anterior and the poste-
 rior (which are very thin) have some of  the characters of the yellow 
CELLS  CONNECTED TOGETHER.—CORNEA.
165
elastic tissue, whilst the middle  one, which forms its  principal thick-
ness,  is composed of white fibres  interlaced together in such a manner
Fie. 41.
   Nutrient Vessels of the Cornea;—a, Superficial vessels belonging to the Conjunctival membrane and
  continued over the margin of the Cornea; b, Vessels of the Sclerotic, returning at the mIrgi™onhe Cornea

  as to  form  numerous lamellae, their interspaces or areolae having the
  form  of tubes regularly  arranged and constructed at intervals,  so as
  not to be unlike rows of cartilage-cells, for Avhich in fact they have been
  mistaken.—Two sets of vessels, a superficial and a deep-seated, surround
  the margin of the cornea.   The  former  (Fig. 41  a), belong rather to
  the Conjunctival  membrane, which forms the outer layer of the cornea •
 and they are prolonged to the distance of l-8th  or half a line from its
 margin, then returning as  veins.  The latter (b) do not pass  into  the
 true Cornea,  but  terminate in dilatations  from which  veins arise, just
 where it becomes continuous with the sclerotic.  In diseased  conditions
 of the Cornea, hoAvever, both sets of vessels extend themselves through
 it.  Notwithstanding the absence of vessels in the healthy aondition of
 the corneal tissue, incised wounds of its substance  commonly heal very
 readily, as is  well seen after the operation for  Cataract; but there is a
 danger in  carrying the incision around a large proportion of its margin,
 lest the tissue should be too much cut off from the supply of nutriment
 afforded by the ampullae of  the vessels that surround it.
   275. The  Crystalline Lens of the Eye  approaches  Cartilage, in its
 structure and mode  of nutrition, more nearly than any other tissue.
 It may  be separated into numerous  laminae; which  are composed of
 fibres that lock  into  one another, by their delicately-toothed margins.
 Each of these fibres  appears to be made up of a series of cells, linearly
 arranged,  which  coalesce at  an  early  period.   The  lens is  not per-
 meated by blood-vessels ; at least after it has been completely  formed •
 these being confined to the capsule.   During the early part of foetal
life, and in inflammatory  conditions of the  Capsular  membrane,  both
its anterior and its posterior portions are  distinctly vascular; but  at a
later period, only  the posterior half  of the Capsule has vessels distri-
buted upon its surface.   It has been  shown  by optical • experiments 
166      STRUCTURE AND ENDOWMENTS  OF ANIMAL TISSUES.

devised for the purpose, that  a moderate  vascularity of the posterior
capsule does not  interfere with distinct vision;  whilst if the  anterior
capsule were  traversed  by vessels, the picture on the retina Avould be
no  longer clear.—The  substance  of which the lens is  composed, ap-
pears to  be soluble Albumen,  or  perhaps more  closely resembles the
Globulin of the blood.
  276.  The  Vitreous body, which  fills the  greater part of the globe of
the eye, also seems to possess a cellular structure; the cells  containing
a fluid, which is  little else than water  holding  in  solution  a small
quantity of albumen and  saline matter; and the membrane which forms
their walls being  so pellucid as to be scarcely distinguishable.   Indeed
the cellular character of this substance is chiefly inferred from the fact,
that when its  capsule  or enveloping membrane  is punctured, even in
several places, the contained fluid  does not speedily drain  away,—as
it would do if it were merely contained in  the interstices of  an areolar
tissue.  The blood-vessels  Avhich traverse the Vitreous  body  do not
send branches into its substance ; and it must derive its nutriment from
those  which are  distributed minutely upon its general  envelope,  and
probably  also from the large  plexiform  vessels of the ciliary processes
of the Choroid  coat.
  277.  Before proceeding to describe the  structure of Bone, to which
it seems natural to pass on from Cartilage, it Avill  be useful to advert to
the modes in which the tissues  of Invertebrated animals are consolidated
by  deposits of  solid matter, in order that they may afford the requisite
support and  protection, without that interstitial  growth  which is pecu-
liar to the skeletons of the Vertebrated classes.—Commencing with the
Polypifera, or Coral-forming animals, we observe  that their strong axes
or sheaths are destined only to give support to their softer  structures,
and that the parts  once consolidated undergo no subsequent change.  It
was formerly imagined, that the stony Corals were " built up" by the
animals which form them, somewhat in the same  manner as  a Bee con-
structs  its cell.  But it is now fully demonstrated, that  the  calcareous
matter (which here consists solely of the Carbonate of Lime) is combined
with the living tissue ; and that the most solid mass of Coral thus has
an  organized basis.   The proportion of earthy to animal matter, how-
ever, is so great  in these structures, that very  little, if any, nutrient
changes can take 'place in their tissues, when once it has become con-
solidated.  Such  changes are  not, however, required.   The substance
thus developed by the  attractive power  of the soft gelatinous  tissues,
which draw into themselves the small quantity of  calcareous  matter dis-
solved in  the surrounding water, is so little disposed to undergo change,
that it will maintain its solidity for  centuries^  and even when acted on
by  water or by heat, it does not undergo disintegration, for its calcareous
particles  arrange  themselves in a  neAV method, and become converted
into a solid crystalline rock.   Such rocks, the product of the metamor-
phosis of ancient  coral-formations,  make up a large proportion of the
external  crust  of the earth.   The  solid  stem  or  sheath,  once con-
solidated, appears to undergo no further change in  the living  Coral-
structure ; for its  increase takes place, not by  interstitial but by super-
ficial deposit,—that is, not by  the  diffusion of new matter through its 
SHELLS  OF MOLLUSCA.
167
whole  substance,  separating  from each other the parts formerly de-
posited but by the mere addition of particles to its surface and  extre-
mities.   In this manner the growth of  a solid Coral-structure may
go  to  an enormous extent;  the surface  at which  the  consolidating
action is going  on,  being the only part  alive, that  is exhibiting any
vital change;  and all  the rest  of the mass being henceforth  perfectly
inert.
  278. In the class of Echinodermata, which includes  the  Star-fish,
Sea-Urchin,  &c,  we find the  calcareous structure presenting  a very
elaborate organization ; as an  example of this, we shall select the shell
of the Echinus, commonly known as the Sea-Egg.  This  shell is made
up  of  a number of plates, more or less regularly hexagonal, and fitted
together so as completely to enclose  the animal, except  at two  points
one of which is left open for the mouth, the  other for the anus.  On
the surface of these plates are little tubercles, for the articulation of the
spines, which  serve as instruments  of defence and of locomotion.  The
substance of the shell and of the spines is exactly alike ; being a sort of
areolar tissue, consolidated by  the deposition of calcareous matter, and
having an innumerable number  of interspaces or minute cancelli, freely
communicating with each  other.  The arrangement of this calcareous
netAvork in the spines is  most  varied and elaborate; and causes thin
sections of  them to  be  among the most beautiful of all microscopic ob-
jects.   The external and internal  surface of each plate, in the shell of
the living Echinus, is covered with a membrane, from Avhich its nutrition
                              Fig. 42.
     Portion of the shell of the Echinus, showing at a the constituent plates, and at 6 the calcified
                     areolar tissue, of which they are composed.

is derived; this membrane  dips  down into the spaces between the ad-
jacent plates ;  but it  does  not penetrate the substance of the plates
themselves, nor  does it  transmit vessels  to  their interior.   A simi-
lar  membrane covers  and encircles the spines;  and  it also  connects
these with the shell, being continuous with the membrane that envelopes
the latter.  Thus each plate and  spine is itself completely extra-vascu-
lar ;  but it is enclosed in a soft membrane, which furnishes  (whether
by  vessels or otherwise, has not  yet been  ascertained), the elements of
its nutrition.
  279. But  we  do  not  here find any evidence of interstitial growth;
nor is there  any reason why such should  be required.  For the tissue 
168      STRUCTURE AND ENDOWMENTS  OF ANIMAL TISSUES.

of which it is composed, although of such extreme delicacy, is of great
permanence,  and does not exhibit the slightest tendency to decay, how-
ever long it is  preserved; so that, when once consolidated, it appears
to undergo no further change in the living animal.   The growth of the
animal, however, requires a corresponding enlargement of its enveloping
shell; and this is provided for by the simple process of  superficial de-
posit, through the subdivision of the whole shell into component plates.
For by the addition of  new matter at the edge of each plate, by the
consolidation of a portion of the soft membrane that  intervenes between
the adjacent plates,  the whole  shell is enlarged,  without  losing its
globular  form.   At the  same time it is strengthened in a corresponding
degree, by the  consolidation of the soft  tissue at  the surface  of each
plate.  And, in like manner, the spines  are enlarged and  lengthened
by the progressive formation of new layers, each on the exterior of the
preceding; so that a transverse section exhibits a number of concentric
rings, like those  of an Exogenous tree.—Thus even in  the  growth of
this  complex and elaborate  structure, we  recognise the principle  of
superficial deposit, which we shall find to be universal amongst the hard
parts of the Invertebrata :  notwithstanding that, at  first sight it would
have  appeared impossible to provide on this plan for the gradual en-
largement of a  globular  shell,  completely enclosing the animal, and
therefore required to keep pace with the latter in its rate of increase.
   280. Among  the Mollusca, we find the  body sometimes  altogether
destitute of solid organs of support, protection, or locomotion,—as is the
case, for example, in the Slug;  and the movements are  feeble and the
habits inert, the muscles having no fixed points for their attachment,
and acting Avithout any of the advantages of leverage.   In other cases,
we find the body more or less completely protected by the Shell; which
is  sufficiently large in some instances to  cover the body completely,
whilst in others it affords only a partial investment.   The plan on which
this shell is formed, however, is very different from that which has just
been described; being much  less complex.   The   Univalve  shells, or
those formed in one piece are always of a conical form ;  the cone being
sometimes simple, as in the Limpet;  in other cases being  spirally coiled,
as in the Snail.   Now the base of this cone is open; and through this,
the animal can project  its movable parts.  When  its  increasing size
requires  additional accommodation, it is obvious that an  addition to the
large end of the cone  will increase its diameter and its length  at the
same time; so as to afford the required space,  Avithout any alteration in
the form or dimensions  of the older  and smaller portions of the cone.
This  last, indeed, is  frequently  quitted  by the animal, and remains
empty ;  being sometimes  separated from the latter portions, by one or
more partitions thrown across by the  animal,—as is seen especially in
the Nautilus and other chambered  shells.   Besides the new  matter
added to the  mouth of the shell, a thin layer is usually formed over its
whole interior surface;  so that the lining of the neAV part is continuous
with that of the old.—In the Bivalve shells, Ave  trace this mode of in-
crease Avithout any difficulty; especially in such shells  as  that of the
Oyster, in which the successive laminae  remain  distinct.  Each lamina
is  interior to the preceding, being formed on  the living surface of the 
SHELLS OF MOLLUSCA.
169
animal;  but it also projects beyond it, so as to enlarge the  capacity of
the shell;  and as the separation of the valves affords free exit to those
parts of  the animal, which  are capable of being projected  beyond the
she  , there is obviously no need of any other provision to maintain the
shell in its natural form.—Thus in the shells of the Mollusca, increase
takes place at the surfaces and edges only.
  281. The proportion of organic and calcareous matter in Shell differs
considerably in the various tribes.  The former is sometimes present in
such small amount, that it can scarcely be detected; and the condition
ol the calcareous matter then obviously approaches that of a crystalline
deposit.   But  in  other  instances, the animal  basis  is very obvious-
remaining as a thick  consistent membrane, after all the calcareous mat-
ter  has been dissolved away by an acid.  This membrane is formed of
an aggregation of cells arranged with great regularity (Fig. 43, a);  the
cavities of which are  filled with carbonate of lime in a crystalline state.

                              Fig. 43.
ra#ia		
elfHi		
ill»		
ISPlpf mm		
       Prismatic cellular structure of shell of Pinna :-a, surface of lamina; b, vertical section.

 i?de;rr!rr ?f ^.cfs aPF0AaAches the hexagonal; their diameter varies
 m different shells from l-100th to l-2800th of an inch ; their thickness
 also is extremely variable,  even in different  parts  of the same shell.

 irr^l^nnT^68 .meft.WltM lamina 0f such tenuitJ> ™ ^ot to mea-
 sure l-100th of an inch in thickness, whilst in other instances, a single
 WIT7-  AVV thldTSS °f half an  inch> or  even (^  certain lar°ge
 hp1ni?vC1fl8)*0f T !rC,h 0r, m0re'   In this case> the cells> instead of
 being  thin flat scales like the tessellated-epithelium (§ 233), are long
 prisms somewhat like the cells of the cylinder-epithelium  (Fig. 26),
 with their wals flattened against each other.  The appearance Avhich is
 then presented by a vertical section of them, is represented in Fig. 43, b.
 The long prismatic cells are there  seen to be marked by delicate trans-
 verse striae, and these, taken in connexion  with other indications, appear
 to show, that every such prism is in  reality formed by the coalescence
 of a pile of flat cells  resembling those which are seen in the very thin
 lamina* just described; so that the thickness of the layer depends upon
 the number of the cellular laminae which  have  coalesced to form its
 component prisms.  This  character is of interest,  as representing on a
magnified scale a corresponding  appearance in the Enamel  of  human 
170      STRUCTUKE  AND  ENDOWMENTS OF ANIMAL TISSUES.

Teeth, which we shall presently find to be formed upon  the very same
plan (§ 318).
  282.  We are to regard this kind  of  shell-substance,  therefore, as
formed by  the secreting action of the epithelial cells covering the mantle
of the animal,—which membrane, though it  answers in position to the
skin,  has the soft, spongy,  glandular character of a mucous membrane.
These draw calcareous matter into their cavities, as a part of their own
process of  growth: this matter being  supplied  from the fluids of the
vascular surface beneath.  Now when  these calcigerous cells are sepa-
rated by intercellular substance, they remain distinct through the whole
of their lives, and they form by  their cohesion a tenacious membrane,
that retains its consistency after the removal of the  calcareous matter.
-But this is only the case in certain groups of shells, chiefly belonging to
the bivalve division.  When the intercellular substance is wanting,  and
the cells come into close contact, their partitions become indistinct on
account of their extreme tenuity ; and not unfrequently a fusion of the
whole substance appears to take place, by the dissolution of the original
cell-walls,  so that it becomes  more or  less homogenous,—traces of the
original cellular structure  being here and there distinguishable (§ 252).
  283.  Sometimes where this fusion has taken place, so as to obliterate
the original cell-structure,  Ave find the almost homogeneous  substance
traversed by a series  of  tubuli, not arranged, hoAvever, in  any very defi-
nite direction, but forming an irregular netAvork (Fig. 44).   These tubes
vary  in size from l-2000th to l-20,000th of  an inch;  but their general
diameter, in the shells in which they most abound, is l-4500th of an inch.
In the larger tubuli, something of a bed-like structure may occasionally
be seen : as if their interior were occupied by rounded granules arranged
in a linear direction.   Although it might be supposed that this structure
is destined to convey nutrient fluid into the substance of the shell, yet
there is no evidence that such is the fact; and, on the contrary, there is
ample evidence that, even in shells most copiously traversed by these
tubuli, no processes of interstitial growth or  renewal take place.   The
permanent character  of the substance of all Shells, when  once it is fully
formed, is  as remarkable as that of Coral; and as the adaptation of their
Fig. 44.
Tuhular shell-structure, from Anomia.
size, to that of the animals to which they belong, is entirely effected by
additions to their surfaces and edges, no interstitial deposit  can have a
share in producing it. 
SHELLS  OF CRUSTACEA.
171
  284.  Among the Articulated classes, we still find that the skeleton is
altogether external, and belongs therefore to the cutaneous system ; but
it is formed upon a very different plan from the shells of the  Mollusca,
being closely fitted to the body, and enveloping every part of it;  conse-
quently it must increase in capacity, with the advancing growth of the
contained structures.   Moreover it is destined not merely to afford sup-
port and protection to these, but  to serve for the attachment of the mus-
cles by which the body and  limbs are moved; and the hard  envelopes
of the latter serve, like the bones of the Vertebrata, as levers by which
the  motor  poAvers of the muscles  are more advantageously employed.
Again, the hard envelopes of the body and limbs are not formed  of dis-
tinct plates, like those of the  Echinus-shell; but are only divided  by
sutures at the joints, for the  purpose of permitting the requisite freedom
of motion.    It might have been thought that here, if anywhere, a process
of  interstitial growth  would have existed, to  adapt the capacity of the
envelopes to the dimensions of the contained parts, as the  latter increase
with the growth of the animal;  but, true to the general  principle, that
epidermic structures are not  only extra-vascular, but that they undergo
no change when they are once fully formed, we find that the hard enve-
lopes of Articulated animals  are thrown off, or exuviated, when the con-
tained parts require an  increase of room; and that a new covering is
formed from their surface, adapted to their enlarged dimensions.
   285. This is well  knoAvn to occur at certain intervals in Crabs, Lob-
sters, and other Crustacea ;  which thus exuviate not merely  the outer
shell, with the continuation of  the  epidermis over the eyes, but also its
internal reflection, Avhich forms the lining of the oesophagus and stomach,
and the tendinous plates by which the muscles are attached to the lining
of  the shell.  A similar moulting may be  observed to occur in some of
the minute Entomostracous Crustacea of our  pools, every two or three
jdays, even after the animals  seem to be full grown.  During the early
groAvth of Insects, Spiders, Centipedes, &c, a similar moult is frequently
repeated at  short intervals ;  but after these animals have  attained their
full groAvth, which is the case with Insects at  their  last change, no fur-
ther moulting takes place, the  necessity  for it  having  ceased.  This
moulting is  precisely analogous to the exfoliation and new formation of
the  Epidermis, in Man and  most other Vertebrata; differing from it
only in this,  that the latter is constantly taking place to  a small extent,
whilst the  former is completely effected at certain intervals, andvthen
ceases.  We have examples of a periodical complete moult in Vertebrata,
howeA'er, among Serpents and Frogs.
   286. The  structure of the  hard envelopes of Articulated animals cor-
responds with that of the Epidermis and its appendages in Man.  The
firm casings  of Beetles, for example, are formed of layers of  epidermic
cells, united together,  and having their cavities filled  by a horny secre-
tion.  The  densest structure is found in  the calcareous shells  of the
Crustacea;  Avhich consists of a  substance precisely analogous  to the
Dentine of Teeth  (§  311);  covered on the  exterior with a layer  of
pigment-cells.  The calcareous  matter  consists  chiefly of carbonate
of lime ; but traces of the phosphate are also found.   The animal basis
has a firm consistent structure, resembling that of teeth.  A thin ver- 
172      STRUCTURE  AND  ENDOWMENTS OF ANIMAL  TISSUES.

tical section shows the tubuli running nearly parallel, but with occasional
undulations, from the internal  surface  towards the external ; but  no
traces of the original calcigerous cells can be detected in the fully-formed
shell, the process of fusion having gone so far as to  obliterate  them.
The manner in which these tubuli are formed, will be  presently con-
sidered, under the head of Dental substance.
  287.  Noav the condition of the osseous skeleton of vertebrated animals
is altogether different.  Its purpose is still only mechanical;  its sole use
being, to afford support and protection to the softer  textures, and to
form  inflexible levers by the action of  the muscles, upon Avhich motion
may be given to the different  parts of  the fabric.  But it forms a part
of the internal substance  of their bodies;  and as these grow in every
part, and not merely by addition to this  or  that- portion, so  must the
Bones also, in order to keep pace with the rest of the structure.  Hence
we find them so formed, that the process of interstitial deposition may
be continually going on  in their fabric, as in that of  the softer tissues;
and the changes in their substance do not cease, even Avhen they have
acquired their  full size.   The subsequent continuance of these changes
appears destined, not so  much to repair any waste occasioned by decom-
position,—for this must  be very trifling in  a tissue of such solidity,—as
to keep the fabric in a condition, in which it may repair the injuries in
its  substance occasioned by accident or disease.  The degree of this
reparative power is proportional, as we shall presently see, to  the activity
of the normal changes,  which are continually taking  place in the bone ;
and is thus much greater in youth than in middle life, and greater in
the vigor of manhood than in old age.
  288.  The structure of Bones is Avell  adapted to demonstrate the dis-
tinction  between the tissues themselves, and  those subsidiary  parts, by
which  they are connected with the rest of the fabric.  We have seen
that Cartilage  is essentially non-vascular ; that is, even when it exists,
in a considerable mass, it is not  traversed by vessels, but is nourished
by absorption from the fluids contained  in the vessels distributed on its
exterior.  Now every mass of Bone is penetrated by vessels ; nevertheless
these do not penetrate its  ultimate substance, and may be easily sepa-
rated from it, leaving the bone itself as it was.   In fact, as  Prof. Goodsir
observes, " a well-macerated bone is  one of the most easily made, and,
at the same time one of the most curious anatomical  preparations.  It
is a perfect example of a  texture completely isolated ;  the vessels, nerves,
membranes, and fat, are all separated,  and nothing is left but the non-
vascular osseous substance."   Precisely the  same may be  said of the
substance of a  Tooth, from wrhich the vascular lining of the  pulp-cavity
has been removed; for  it  then possesses  neither vessels,  nerves, nor
lymphatics ; and yet, as Ave shall  presently see, it has a highly-organized
structure, peculiar to itself.
  289.  The general characters of Osseous texture vary according  to the
shape of the Bone, and the part of it examined.  Thus in the long bones
we  find the shaft  pierced  by a central  canal, which runs continuously
from  one extremity to  the other; and  the  hollow cylinder which  sur-
rounds this is very compact in its structure.   On the other hand, the
dilated  ends of the bone are not penetrated by the large central canal; 
STRUCTURE  OF BONE.
173
Fig. 45.
Extremity of Os femo-
nor are they composed of solid osseous substance.   They are made up
of cancellated structure, as it is termed ;  that is, of osseous lamellae and
fibres  interwoven  together (like those  of  areolar
tissue, on a  larger scale) so as  to form a multitude
of minute chambers or cancelli, freely communicating
with  each  other, and with the  cavity of  the  shaft;
whilst the whole is  capped  Avith a thin layer of solid
bone.   Again, in the thin  flat bones, as the scapula,
we find the  two surfaces composed  of solid  osseous
texture, with more  or less  of cancellated structure
interposed  betAveen  the layers.   And in the thicker
flat bones, as  the parietal,  frontal,  &c, this cancel-
lated structure becomes very  distinct, and forms  the
diploe; this, however, is  sometimes  deficient, leaving
a  cavity analogous  to the canal of the long bones:
whilst the plates which form the surfaces of the bone
(the external  and  internal  tables of the  skull),  re-  ^a^^l^^\^t
semble in  their thickness and solidity, as  well  as in  of bone, in contact with
the intimate structure presently to  be described, the  &, canceiii!
shaft or hollow cylinder of  those  bones.  Finally, we
frequently meet (especially in  the Ethmoid and Sphenoid  bones)  with
thin lamellae of osseous  substance,  resembling  those  which elsewhere
form the boundaries  of the cancelli;  these consist of but  one  layer of
bony matter, and shoAV none of the varieties previously adverted to ;  they
are not penetrated by vessels,  but are nourished only by their surfaces ;
and they consequently exhibit to us the elements  of the osseous struc-
ture in their simplest form.   It will be desirable, therefore, to commence
with the description of these.
   290. When a thin natural lamella of this kind is examined, it is found
to be chiefly  made up of a  substance  which is  nearly homogeneous,
sometimes exhibiting indistinct traces of a fibrous  arrangement;  this,
however, may  be generally resolved by prolonged boiling, into an assem-
blage of minute granules, varying in size from l-6000th  to l-14,000th
                                Fig. 46.
    Lacunae of Osseous substance, magnified 500 diameters:—a, central cavity; 6, its ramifications.
of an inch, Avhich are more or less angular in  shape, and seem to cohere
by the medium of some  second  substance, which  is  dissolved by the
boiling.  They are composed of Calcareous salts, apparently in chemical
union Avith the Gelatine that  forms the basis of the osseous  substance.
In the midst of this granular substance, a number of dark spots are to
be observed, the form  of  Avhich is very peculiar.  In their general out-
line, they are  usually somewhat  oval;  but they send forth numerous
radiating prolongations of extreme minuteness, which may be  frequently 
174      STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES.

traced to a considerable distance.   These  spots, known as the osseous
corpuscles (sometimes termed the Purkinjean corpuscles, after the name
of their discoverer), are highly characteristic of the true bony structure,
being never deficient in  the minutest parts of  the bones of the higher
animals,  although those of Fishes  are  frequently  destitute  of them.
These corpuscles  were formerly  supposed,  from their dark appearance,
to be opaque, and to consist of aggregations of calcareous matter which
Avould not transmit the  light: but it is now quite certain, that they are
lacunce  or open spaces ; and that the radiating prolongations from them,
which are far smaller than the minutest capillary  vessel, are canaliculi,
or delicate tubes.   Of these canaliculi, some  may be seen to interlace
freely with each other, whilst others proceed  toAvards the surface of the
bony lamella; and thus a system of passages,  not by any means wide
enough to admit  the blood-corpuscles, but  capable of  transmitting the
fluid elements of the blood, or matters selected from them, is established
through the whole substance of the lamella.
  291.  The lacunae of Human bone have an average length of l-1800th
of an inch;  and  they are usually  about half as wide,  and one-third as
thick.   The diameter of the canaliculi is from  l-12,000th  to  l-20,000th
of an inch.   Their size and form  differ greatly, however, in the different
classes of Vertebrata; so that it is  usually possible to refer a mere frag-
ment of bone to  its proper group,  by an examination of its  minute
structure.  The succeeding figure  represents the arrangement of these
lacunae  and canaliculi in the bony scale of  a  fish (the Lepidosteus);
which is almost the only existing representative of a large class of bony-
scaled  Fishes, that formerly tenanted  the seas.  Its  lacunae  will be
seen to differ greatly in form from those of  human bone ; and the
canaliculi which proceed from them are much feAver in  number.—The
purpose of this penetration of the osseous texture by such a complicated
apparatus of tubuli, can scarcely be anything  else than the maintenance
                               Fig. 47.
  Section of the bony scale of Lepidosteus;—a, showing the regular distribution of the lacunas and of
             the connecting canaliculi; 6, small portion more highly magnified.
of its vitality by the continual percolation of nutrient fluid, drawn  into
the system of lacunae and canaliculi from the neighboring blood-vessels.
Thus the nutrition of the ultimate osseous  texture,  though  carried on
upon the same  general  plan  Avith  that of Cartilage, differs in this :—
that there is a  provision in Bone for the ready transmission of nutrient
matter through its texture, by  means of minute channels, which does
not exist in Cartilage; a difference obviously required  by the greater
solidity of the substance of the former, which  does not allow of the
diffused imbibition, that is permitted by the softer and moister nature
of the latter.  We shall presently  find  that these  channels are only 
STRUCTURE  OF BONE.
175
  formed at a late stage of the development of bone, where the remaining
  tissue has acquired its completest consolidation.
  m  292. Now, as already remarked, the  simple structure just  described
  is found,  not  merely in the delicate plates which form the thinnest part
  of certain bones in Man, but also in those lamella?, which form the walls
  of the cancelli  of the larger and thicker  bones.  Every one  of these
  lamellae repeats, in fact, the same history.   The  cancelli are lined  by a
  membrane derived from  that of  the cavity of  the shaft,  over which
  blood-vessels are minutely distributed; between these blood-vessels and
  the osseous texture is a layer of cells; and  from the materials selected
  and  communicated  by these, each  lamella is  nourished,  through its
  system of radiating  canaliculi  and nutritive centres.  The cancelli,  at
  the time  of their formation  in  the  foetal bone, are entirely filled  with
  such  cells; which appear (as will be presently explained) to  be the
  descendants of  the  cells  of the  original cartilage;  but in the adult
  bone, a large  proportion of them are filled with fatty matter, which
  they secrete into their  cavities.—The Vessels  of  the  cancellated struc-
  ture at the extremities of the long bones are derived from those of the
  medullary cavity, which is penetrated by large trunks from the exterior;
  and ,n the flat bones, they form  a  system of their own, connected with
  the vessels of  the exterior by several  smaller trunks.
    293. The solid osseous texture which forms  the cylindrical shafts of
  the long bones,  and  the thick external plates  of  the denser flat bones,
  is not cut  off from  nutritive action in  the degree in which it might seem
  to be; tor it is penetrated by a series of large canals, termed  the
  Haversian (after Clopton Havers,  their discoverer), which form  a net-
  work  in  its interior, and  which serve for  the transmission of  blood-
  vessels through its  substance (Fig. 48).  These canals, in the long bones,
  run for the most part in  a direction parallel to
  the central cavity ; and they communicate with
  this, with  the external surface, with the cancelli,
  and Avith  each  other, by frequent transverse
  branches;  so that  the whole system  forms an
  irregular  network,  pervading every part  of the
  solid texture, and adapted for the establishment
  of vascular communications throughout.  The
  diameter of the  Haversian canals  varies from
  l-2500th to l-200th of an inch,  or more; their
^ average diameter may be stated at about l-500th
 of  an  inch.  They are  lined by a membrane
 which  is continuous with that of the external
 surface, and which  carries this  inwards, so to
 speak,  to form  the lining membrane of the cen-
 tral cavity, and of the cancelli;—and the cavity
 of the  tube encloses  a single twig of an artery
 or  vein.    Thus  we  may  consider  the whole
 Osseous texture  as enclosed in a membranous
bag; on which blood-vessels  are minutely dis-
tributed, and which is so carried into  the bone
by involutions and prolongations, that no  part
of  the latter is  ever far removed from a vascular surface.
 Haversian Canal, seen on a lon-
gitudinal section of the compact
tissue of the shaft of one of the
long bones; 1, arterial canal;
2, venous canal; 3, dilatation of
another venous canal. 
176       STRUCTURE  AND ENDOWMENTS OF  ANIMAL TISSUES.

  294. In the  adult  bone, the cells which fill  the remaining cavity of
these canals secrete fatty matter.   This is particularly evident in  the
case of the central cavity, which may be  considered as an immensely
dilated Haversian canal,  where  they constitute the medulla or marrow.
It does not appear that  these take any active part in the nutrition of
the bone; indeed, in  the  bones of Birds  the shaft is entirely  hollow,
and  air is admitted into it from  the lungs, so that its lining membrane
is rendered subservient to the  aeration of the blood.
  295. The arrangement of the elementary parts of the  osseous sub-
stance  around  the Haversian  canals is  very interesting  and beautiful.
When a transverse section of  a long bone is made, the open orifices of
the longitudinal canals present themselves at intervals, sometimes con-
nected by a transverse canal where the section happens to traverse this.
Around these orifices, we see  the  osseous matter arranged in*the form
of cylinders, which appear to  be marked by concentric circles (Fig.  49,
1, 2).   Now when one of  these circles is minutely examined, it is found
to be  made up  of a  series of lacunae, analogous to those already  de-
scribed;  these, however,  are  seldom  or never so continuous as to form
a complete  circle.  The long sides of  the  lacunae  are  directed, the one

                                 Fig. 49.
 Minute structure of bone, drawn with the microscope from nature.  Magnified 300 diameters.  1. One of
the Haversian canals surrounded by its concentric lamellae. The lacunae are seen between the lamellse;
but the radiating tubuli are omitted.  2. An Haversian canal with its concentric laminse, lacunas, and.
radiating tubuli.  3. The area of one of the canals. 4, 4. Direction of the lamellae of the great medullary
canal. Between the lamellae at the upper part of the figure, several very long lacuna; with their tubuli are
seen.  In the lower part of the figure the outlines of three  other canals are given, in order to show their
form and mode of arrangement in the entire bone.

towards the Haversian canal (3) in the centre, the other  toAvards the
circular row next beyond it.   .And  when the course of the  canaliculi is
traced, it is  found that these converge  on the inner  side  towards the
central canal,  inosculating with those  of the series next within, whilst
those of the outer side pass outwards in a radiating or diverging direc-
tion, to inosculate with those of the series next external.   Thus a com-
plete communication is  formed, by means of this  system  of  radiating
canaliculi  and intervening lacunae,  between  the central  canal and the 
COMPOSITION OF  BONE.
177
 outermost cylindrical lamella of bony matter ;  and each of these lamellae
 derives its nourishment from the vessels of the central  canal, through
 the lamellae which intervene  between itself and the vascular  membrane
 lining that tube.
   296. Thus every  one of the Haversian canals is the centre  of a
 cylindrical ossicle, which  is complete in itself,  as  far  as  its elementary
 structure is concerned, and Avhich has  no dependence  on, or connexion
 with, other similar ossicles.   These  are arranged, hoAvever, side by side,
 like sticks in a fagot; they are bound together  by a thin cylinder  of
 bone,  on  the exterior  of all, Avhich derives its nourishment from the
 periosteum, or enveloping membrane ; in like manner, the hollow bundle
 is  lined by  a  similar cylinder, which surrounds  the great  medullary
 cavity, and  is nourished by its vascular membranes; and  the spaces
 that  here and there intervene between  the ossicles  are filled up with
 laminae, which are parallel to those of the external and internal cylinders,
 and which seem  to derive their nutriment from them (Fig. 49, 4).   In
 this manner, the whole  structure acquires great density  and solidity.—
 The structure of the outer and inner tables of the skull, and of other
 thick solid layers of bone, is precisely similar; except that the  Haver-
 sian canals have no such definite directions,  and  form  an irregular net-
 work.
   297. Thus we see  that each of the lamellae of bone surrounding  an
 Haversian canal,  or bounding the cancelli, may be  regarded as a repe-
 tition of the simple bony plate, which draAvs  its nourishment direct from
 the vascular  membrane  covering  its surface, by  means of its system of
 lacunae and  canaliculi.   The  membrane lining  the Haversian  canals,
 cancelli,  and central  medullary cavity, is an  internal prolongation of
 that which clothes the exterior ;—just  as the  mucous  membranes, with
 their extensions  into glandular structures, are internal prolongations of
 the true  skin.   Every Haversian canal  and every cancellus are repe-
 titions of each other in all essential particulars, their form alone being
 different.  The central medullary canal  is  but an enlarged  Haversian
 canal or cancellus.  And the Avhole cylindrical shaft is  a collection of
 ossicles, each of which is a miniature representation  of  itself, being a
 hollow cylinder, Avith a central vascular cavity.
   298.  The principal features of the Chemical constitution of Bone are
 easily made evident.   After all the accessory parts have  been removed,
 and nothing remains but the real osseous texture, this may be separated,
 by simple  processes, into its tAvo grand constituents,—the animal basis,
 and the  calcareous matter.   The latter  may  be  entirely removed by
 maceration of the bone in dilute Muriatic or Nitric acid ; and a substance
 of cartilaginous appearance  is then left, which, Avhen  submitted to the
 action of boiling  water  for a short time, is almost entirely dissolved
 aAvay, and the solution forms  a dense jelly on cooling.  The same  sub-
 stance,  Gelatine, may be  obtained by long boiling under pressure, from
 previously unaltered bone; and the  calcareous  matter  is then left  in a
friable condition.   By submitting a bone  to  a heat sufficient  to decom-
pose the animal matter,  Avithout dissipating any of the earthy particles,
we may obtain  the whole  calcareous matter in situ;  but the slightest
violence  is sufficient  to  disintegrate it.  The bones  of persons long 
178      STRUCTURE  AND ENDOWMENTS OF ANIMAL TISSUES.

buried are often found in this condition; their form and position being
retained until they are exposed to the air, or are a  little shaken, when
they crumble to dust.  The proportion of the earthy matter of Bones
to the animal basis may  be  differently stated, according as we include,
in our estimate of the latter, the contents of the medullary cavity, the
Haversian  canals, and the cancelli, or confine ourselves  to that portion
only of the animal matter Avhich is united with the  calcareous element
in the proper osseous tissue.   According to  the recent experiments of
Dr.  Stark,*  the relative amount  of the  two elements, in the latter
estimate, is subject to very little variation, either in the different classes
of animals, or  in the same species at different ages, the animal matter
composing  about one-third, or 33^ per cent., and the mineral matter
two-thirds, or 66^ per cent.   The degree of hardness of bones does not
altogether depend, therefore, on the amount of earthy matter they may
contain; for the flexible, semi-transparent, easily-divided  bones of Fish
contain as large an amount of animal matter, as the ivory-like leg-bones
of the  Deer or  Sheep.   The usual analyses of Bone, however, have
been made  upon the former kind  of estimate : and they  show that the
proportion of the earthy matter to  the whole of the animal  substance
contained in bone varies  much  in different  animals, in the same animal
in different ages, and even in different parts of the same skeleton.  The
reason of this will be apparent, Avhen the history of the growth of Bone
has been explained; since there is a gradual filling up of all the cavities
at first occupied by fat-cells,  vessels,  &c,  which  does not cease with
adult age, but which continues  during the whole of life.   In this manner
the bones of old persons acquire a high degree of solidity, but they
become brittle in proportion to their hardness.  From the same cause,
the more solid bones contain  a  larger proportion of bone-earth than
those of a spongy or cancellated texture ;  the temporal bone, for exam-
ple, containing  63J per cent., whilst the scapula possesses only 54 per
cent.   In the former of these  bones, the proportion  is nearly th©\ same
as that which exists in pure  osseous tissue, the amount of the remaining
tissues which it includes  being very small,  on account  of  the solidity of
the bone;  but  the latter contains in its cancelli a large  quantity of
blood-vessels, fat-cells, &c,  which  swell the proportion of the animal
matter from 33J to 46 per cent.
   299. The Lime of bones is for the most part in a state of Phosphate,
especially among  the  higher  animals ; the remainder is  a carbonate.
In Human bones, the proportion of the latter seems to be  about one-
sixth or one-seventh  of the whole amount of bone-earth.   In the bones
of the loAver animals, however,  the  proportion of Carbonate  is greater;
and it is curious that in  callus, exostosis,  and other  irregular osseous
formations in  the  higher animals, the proportion of  the  Carbonate
should be much greater than in the sound  bone.   In caries, however,
the proportion  of the Carbonate  is  less than usual.   The  composition
of the Phosphate of Lime in  Bones, is somewhat  peculiar;  eight pro-
portions of the  base being united with three of the acid.   According to
Professor Graham, it is to be regarded  as  a compound  of two tribasic

            * Edinburgh Medical  and Surgical Journal, April, 184o. 
COMPOSITION OF BONE.
179
phosphates; one atom of the neutral phosphate (in which one propor-
tional of the acid  is united Avith tAvo of lime and one of water), being
united Avith two proportionals of the alkaline phosphate (in which one
part of  acid is united with  three of the base), together with an atom of
water, Avhich is driven off  by  calcination.   Besides these components,
some Chemists assert that  a small  quantity of Fluoride of calcium is
present in Bone; but this  is rather  doubtful,  since it has been shoAvn
by Dr.  G.  0. Rees that the solvent action  upon glass, which  has been
supposed to be characteristic of fluoric acid, may be imitated by  phos-
phoric acid in combination with water, Avhich, if heated upon glass of
inferior quality until  it volatilizes,  will act upon  it with considerable
energy.—Other  saline matters, such as phosphate of magnesia, oxides
of iron and manganese, and chloride of sodium, are found in bones in
small amount.
   300.  The first development of Bone  is usually preceded by the for-
mation of a Cartilaginous structure, which occupies the place  after-
Avards to be taken by the bone; and it is commonly considered that the
bone is formed  by the calcification  of the cartilage-substance.   This,
however, does not appear  to be the case, as will be  presently  shoAvn;
and it Avould probably be more correct to say that the cartilage is super-
seded by bone.  Moreover, Bone is frequently developed in the sub-
stance of Fibrous membranes ; and  the structure produced by this intra-
membranous ossification cannot be distinguished from that which is
generated by the intra-cartilaginous.—We shall commence the  history
of the  development of Bone, with the period in  Avhich its  condition
resembles  that of  the permanent  Cartilages.  As  already mentioned,
there is no essential difference between  the temporary  and permanent
Cartilages, in regard  to their ultimate structure; the former, however,
are more commonly traversed by vessels, especially when their mass is
considerable.  These vessels, however, do not  pass  at once  from the
exterior of the cartilage into its substance;  but they are  conveyed in-
wards along canals, which are lined by an extension of the perichondrium
or investing membrane, and which  may thus be  regarded'as so  many
involutions of the outer surface of the cartilage.   These canals are espe-
cially developed  at certain points, which are to be the centres of the
ossifying process ; of these puncta  ossificationis, we usually find one in
the centre  of the shaft of a long bone, and one in each of its epiphyses;
in the flat bones there is one in the middle of the surface, and one in
each of the principal  processes.  Up to a late stage of the ossifying
process, the parts which contain distinct centres are  not connected by
bony union, so thatthey fall apart  by maceration ;  and even when they
should normally unite, they sometimes remain separate,—as in the case
of the Frontal, bone,  in  which we frequently meet Avith a continuation
of the sagittal-suture  down the middle, dividing it into two equal halves,
which have originated in two distinct centres of ossification.   It is inte-
resting  to remark that, in the two lowest classes of Vertebrata,—Fishes
and Reptiles,—we find the several parts of the osseous system present-
ing in a permanent form, many of the conditions which are transitory
in the higher; thus the  different portions of each vertebra, the body,
lateral arches, spinous and transverse processes, &c, which have their 
180      STRUCTURE AND ENDOWMENTS OF  ANIMAL TISSUES.

distinct centres  of ossification, but Avhich early  unite in Man, remain
permanently distinct in  the loAver  Fishes ;  the division of the frontal
bone, just adverted to, is  constant amongst Fishes and Reptiles ; and in
these classes we meet with  a permanent separation of the parts of  the
occipital and temporal bones, which, being formed from distinct centres
of ossification, are at first distinct in the higher animals.
   301. During the  formation  of the punctum ossificationis,  and  the
spread of  the vessels into the cartilaginous matrix, certain changes  are
taking place in the substance of the latter, preparatory to its conversion
into bone.   Instead of single  isolated  cells,  or groups of tAvo, three, or
four, such as we have .seen  to be  characteristic  of ordinary Cartilage
(§ 267), we  find, as Ave approach the ossifying centre, clusters  made up
of a larger  number, Avhich appear to be formed by a continuance of  the
same multiplying  process as that  already  described (Fig.  50).   And

                                   Fig. 50.
 Section of Cartilage near the seat of ossification;  each single cell having given birth to four, five, or six
cells, which form clusters. These clusters become larger towards the right of.the figure, and their  cells
more numerous and larger; their long diameter being l-1500th of an inch.

when we pass still nearer, we see that these clusters are composed of a
yet  greater number of  cells, which are arranged  in long rows, Avhose
direction corresponds with the longitudinal axis of the  bone ; these clus-
ters are still separated by intercellular  substance, and it is in  this, that
the ossific .matter is first deposited (Fig. 51).   Thus if we separate  the
                                  Fig. 51.
  The same cartilage at the seat of ossification: tbe clusters of cells are arranged in columns ; tho intercel-
lular spaces between the columns being l-3250th of an inch in breadth.  To the right of the figure, osseous
fibres are seen occupying the intercellular spaces, at first bounding the clusters laterally, then splitting
them longitudinally and encircling each separate cell.  Tbe greater opacity of the right hand border is due
to a threefold cause, the increase of osseous fibres, the opacity of the contents of the cells, and the multipli-
cation of oil-globules.

cartilaginous  and the osseous substance  at this period,  we find  that the
ends of the rows of cartilage-cells  are received into deep narrow cups of 
CONVERSION OF CARTILAGE INTO BONE.
181
bone, formed by the  transformation of the intercellular substance be-
tween them.   Immediately upon the ossifying surface, the nuclei, which
were before closely compressed, separate considerably from one another,
by the increase of material within the cells ;  and the nuclei themselves
become larger and more transparent.  These changes constitute the first
stage of the process of ossification, Avhich extends only  to the calcifica-
tion of  the intercellular substance ; in this stage there are no  blood-
vessels directly concerned.   The  bony lamellae thus formed, mark out
the boundaries of the cancelli  and  Haversian  canals, which are after-
wards to occupy  a part of the  space that is hitherto filled by the rows
of cartilage corpuscles.
   302.  Up to this  point, there is no essential difference  in the accounts
of those Avho have most carefully studied the process of ossification ; but
in regard to the  history of its  subsequent stages, there  is much discre-
pancy; and this especially with respect to the origin of the bone-lacunae,
which some regard  as metamorphosed cartilage-cells, others as the spaces
originally occupied by their nuclei,  whilst others do not regard them as
 in  any way derived from the cartilage-cells, but consider them as a new
formation.  Much  may doubtless be urged in favor of  each view; the
author's OAvn observations incline him to the latter, and lead  him to
regard  the lacuna?  as cells, Avhich like  the  pigment-cells of Batrachia,
ifec, have sent out the stellate  prolongations that constitute the canali-
culi.   All stages of gradation  may be  traced, between  simple  rounded
cavities,—whose correspondence in size Avith the cells that are scattered
in the midst of the consolidating  blastema leaves scarcely any doubt of
their identity with these,—and  the lenticular lacuna with numbers of
canaliculi proceeding from it.   These gradations  are particularly well
seen during the process of ossification ;  so that  it seems probable that
the radiating extension of the  cells takes place during the consolidation
of  the  surrounding  tissue.—It is an additional argument against the
idea that the bone-lacunae in any Avay originate from the cartilage-cells,
that  they are found  to present  exactly the  same characters  in bone
Avhich is developed  in the  substance  of fibrous  membrane (after the
manner to be presently described), and in the formation  of which, there-
fore, cartilage has had no participation.
   303. Although, in a large proportion of the skeleton, the formation
of Bone is thus preceded by that  of cartilage, yet such  is by no  means
invariably or necessarily the case ; for the flat bones, such as the scapula,
and those forming  the roof  of the skull, haA'e usually  only a centre of
cartilage,  beyond  Avhich the  ossifying process extends  in  membrane
only.   This membrane  is chiefly composed of fibrous  fasciculi, corre-
sponding with those  of the Avhite fibrous tissues; but amongst these are
seen numerous cells,  some about  the size of blood-discs, but others two
or three times larger, containing granular matter;  and a soft amorphous
or faintly granular matter is also found interposed amidst the fibres and
cells.   The process  of ossification here seems  essentially to consist in
the  consolidation  of the  fibres by earthy matter; for the first bony
deposit is seen as an irregular reticulation, very loose and open towards
its  edges, and there  frequently presenting itself in the  form of distinct
spicula, Avhich are continuous with fasciculi of fibres in  the surrounding 
182      STRUCTURE  AND  ENDOWMENTS OF ANIMAL  TISSUES.

membrane.  The limits of  the calcifying deposit may be traced by the
opaque and granular character of the parts affected by it; and it gradu-
ally extends itself, involving more and more of the surrounding mem-
brane, until the foundation is laid for the entire bone.  Everywhere the
part most recently formed consists of a very open reticulation of fibro-
calcareous spicula, whilst the older part is  rendered harder and more
compact by the increase in the number of these spicula.  As the process
advances, and the plate of bone thickens, a  series of grooves or furrows,
radiating from the ossifying centre, are found upon its surface; and
these, by a further increase  in thickness, occasioned by a deposit  of
ossific matter all around them, are gradually converted into close canals
(the Haversian), which contain blood-vessels, and are lined by processes
of the investing membrane.  The lacunae and canaliculi seem  to take
their origin in the cells which are interspersed among the fibres, their
prolongations  extending themselves, and insinuating themselves through
the spaces left between the interlacing fibres, whilst the process  of cal-
cification is going on.
   304.  The first osseous tissue  which is formed by either of these pro-
cesses, has an  irregular cancellated structure, analogous to that which
is found at the extremities of the long  bones in adults.   This is gradu-
ally modified  by changes Avhich essentially consist in  absorption and
new deposition; for the  absorptive process first unites minute  areolae
into larger ones, by removing their partitions ; and it is upon their in-
terior walls that neAV osseous lamellae are noAV deposited, from materials
supplied by the blastema they contain.   It is by a process of this kind,
that the central medullary cavity is first formed in  the bones of young
animals.  At  an  early period,  no such cavity exists, and  its place is
occupied by small cancelli;  this is the permanent condition  of the bones
in most Reptiles.   The cancelli gradually enlarge, hoAvever ; and those
within the shaft  coalesce Avith one another until  a continuous  tube is
formed, around  which the  cancelli are  large, open, and irregular.  At
the same time, the diameter of  the surrounding shaft is increasing by
the process of interstitial groAvth just described ; so that the size of the
medullary cavity at last becomes greater than that of  the  whole shaft
Avhen its formation commenced.   The aggregation of the osseous matter
in a holloAV cylinder, instead of a solid one, is the form most favorable
to strength, as may be easily proved upon mechanical  principles.  The
same arrangement is adopted in the  arts, Avherever it is desired to obtain
the greatest strength with a limited amount of material.
   305.  The growth of Bones takes  place by the addition of new tissue
to the part already formed; but this addition may take place  in three
modes,—namely, by the development of neAV bone in the cartilage yet
remaining between the different centres of ossification;  by the develop-
ment of new bone in  the membrane covering  the  surface;  and by the
interstitial formation of neAV layers Avithin the Haversian  canals and
cancelli  of the  part  already formed, by which the  requisite solidity is
given to it.  Of the first process Ave have  the  most characteristic ex-
ample in  the  increase in length of a long bone, by the ossification of
the  cartilage which intervenes  betAveen the shaft  and the epiphyses,
and  Avhich continues to groAV, up to  the time of the final union of these 
REGENERATION OF  BONE.
183
parts.   Thus it Avas long since proved by the experiments of Hales and
Hunter, that the growth of a long bone takes place chiefly  towards the
extremities ; for they found that, when metallic substances were inserted
in the shaft of a growing bone of  a young animal, the distance between
them was but little  altered after  a  long interval,  whilst the space be-
tAveen the extremities of the bone  had greatly increased.  And it seems
that, at a later  period, when the epiphyses have  become completely
united to the  shaft, an elongation continues to take place, by the slow
ossification of the articular cartilage.—Again,  the bone is progressively
increased in thickness, by  the gradual production of neAV osseous matter
upon its surface; this production being effected by the conversion of
the inner layer of the periosteum, the fibres of which are found to be
continuous Avith those of the animal  matrix of the surface of the bone.—
And it is by the successive formation  of neAV  layers of osseous tissue,
one Avithin another,  giving the appearance of concentric rings Avhen the
 Haversian canals are cut  across (Fig. 49), that  the proportion of hard
 to soft parts in bone is gradually increased; the calibre of the Haver-
 sian canals   being  correspondingly diminished.  Of  this  we  have a
 curious  exemplification in the antlers of the  deer, in  Avhich the cavity
 of the canals is gradually  choked up  by the formation of  osseous
tissue, until the vascular supply is cut off, and the death of the bone is
the  result.
   306. The difference in the  relations of the  Osseous  substance to the
Arascular netAvork, at different ages,—accounting for the variations in the
 rapidity of its nutrition and reparation,—is well displayed in the effects
of Madder.   This substance  has a peculiar  affinity for Phosphate of
Lime;  so that Avhen the  latter  is  formed  by precipitation in a fluid
tinged with madder, it attracts color to it  in its  descent, and  falls to
the  bottom richly tinted.  Now  Avhen animals  are fed  with this sub-
stance, it is  found that their  bones become tinged with it, the period
required  being in the inverse proportion to  their age.   Thus in very
young'animals a single day suffices  to  color the entire skeleton, for in
them there is no osseous  matter  far from the vascular surfaces; Avhen
sections are  made, hoAvever, of the  bones thus  tinged, it is found that
the  color is  confined to the immediate neighborhood  of the Haversian
 canals, each of Avhich is encircled  by a crimson ring.  In  full-groAvn
 animals, the bones are very slowly tinged; because the osseous  texture
 is much  more consolidated and less  permeablje to fluid than in earlier
 life; and because the vascular membrane lining the Haversian canals
 is removed further from  the outer and older layers  of osseous tissue
 which surround  them, by  the  interposition  of newer concentric layers,
 which diminish the diameter of the  canals.  In the bones of half-groAvn
 animals, a part  of the bone is nearly in the perfect condition, Avhile  a
 part is new  and easily colored;  so that the  action of this substance
 enables us to distinguish the new from the old.
   307.  The Regeneration of Bone, after loss of its substance by disease
 or injury, is extremely complete; in fact there  is  no other structure of
 so complex a nature,  Avhich is capable of being so thoroughly repaired.
 Although the regenerative poAver appears to  be so much less in Verte-
 brated animals, than it is in the  loAver Invertebrata, yet it is probably 
184      STRUCTURE  AND  ENDOAVMENTS OF ANIMAL  TISSUES.

not at all lower in reality,—the neAV structures  actually formed  being
as complex in the one case as in the  other.  It is  noAvhere, perhaps,
more remarkably manifested, than in the re-formation  of nearly an  entire
bone, Avhen the original one had been  lost by disease ; all the attach-
ments of muscles and ligaments, as Avell as the external form and inter-
nal  structure, being ultimately found as complete in the neAV bone, as
they originally Avere in that which it  has replaced.   Much discussion
has taken place in regard to the degree, in which the different membra-
nous structures, that surround bone and penetrate  its substance, par-
ticipate in its regeneration; some having supposed  the  periosteum to
have the power of itself forming neAV bone, others attributing the same
power to  the membrane lining  the medullary cavities.   It appears cer-
tain, hoAvever, that new osseous tissue may be formed in a great variety
of modes.  It has been ascertained by Mr. Paget* that it may be pro-
duced through the intermediation of perfect fibrous  tissue, either when
this  previously existed as such  (as  the  periosteum or interosseous mem-
brane), or Avhen it  has been neAvly formed by the fibrillation of the plastic
fluid effused as the material for reparation.  The agency of the perios-
teum is  seen in many cases of necrosis, in Avhich that membrane  has
been completely detached  from the dead shaft, and  ne>v bone has been
generated from its interior.  The ossification of a neAvly-produced fibrous
membrane is believed by Mr. Paget to  be the ordinary mode of repara-
tion  of fractures of the  skull; and it takes place in a  manner essentially
the same as that of the  original intra-membranous development of bone.
But  new bone may also be formed, according to that  most excellent ob-
server, by ossification of the fibrous tissue in its rudimental  state (§ 193).
In abnormal  bone-growths, it  sometimes  appears as if  the tissue had
been formed by the ossification of  cells; but  more commonly the calci-
fication takes place in an  earlier stage of tissue-production,  that of  the
"nucleated blastema," in which a granular osseous deposit is seen, Avhich
gradually increases so as to form the lamellae of a fine  cancellous texture,
at the same time inclosing the nuclei, which seem to occupy the places
afterwards to remain as the lacuna).  It is seldom that the reparation
of bone takes place through the intermediation of cartilage ; though this
is occasionally formed, rather, perhaps,  in the loAver animals than in the
human subject.
   308. The reparation  of  Bone, after disease or  injury, takes place ex-
actly upon the same plan as its first formation.  A plastic or  organiza-
ble exudation is first poured out from the  neighboring blood-vessels,
and  this forms a sort of bed or matrix, in Avhich the subsequent processes
take place.  At first all new bone possesses a minutely cancellous  struc-
ture, much like that of  the foetal  bones in their  first construction;  but
this  gradually assimilates  itself to the  structure  of  the bones which it
repairs, its outer portions acquiring a  more  compact laminated  struc-
ture, while its interior substance acquires Avider cancellous spaces,  and a
perfect medulla.  When the shaft of a long bone of an animal has been
fractured  through,  and the extremities have been brought evenly to-
gether, it is found  that the new matter first ossified is that which oc-
cupies the central  portion  of the  deposit, and which  thus connects the
* Lectures on Reproduction and Repair, Medical Gazette, 1849. 
FORMATION  OF TEETH.
185
medullary  cavities  of the  broken ends, forming a  kind  of plug that
enters each.   This was termed by Dupuytren, by whom it Avas first dis-
tinctly described, the provisional callus.  This is usually formed in the
course of five or six weeks, or less in young subjects; but  at that period
the contiguous surfaces  of the bone itself are  not  cemented by bony
union ; and the formation of the permanent callus occupies some months,
during which the provisional callus is  gradually absorbed, and the con-
tinuity of the medullary canal restored, in the same manner as  it was
at first established.   The permanent callus has all the characters of true
bone.  It  seems to have been established by the observations of Mr.
Paget, however, that these  statements do not usually apply to the case
of Man ; in Avhom, Avhen the limb is kept at rest, the union between the
fractured ends is accomplished by ossification of the substance connect-
ing them, without the intermediation of a provisional callus; this being
only formed Avhen the portions of the bone are kept in continual move-
ment.
   309. The most extensive reparation is seen, when the shaft of a long
bone is destroyed by disease.  If violent inflammation occur in its tissue,
the death  of the fabric  is frequently the  consequence,—apparently
through  the blocking-up of the canals with the products of the inflam-
matory action, and the consequent cessation of the supply of nutriment.
It is  not often that  the Avhole  thickness of the bone becomes necrosed
at once;  more commonly this result is confined to its outer  or its inner
layers.   When this is the case, the neAV formation takes place from the
part that remains sound; the external layers, Avhich receive their vas-
cular supply from the periosteum and from  the  Haversian  canals con-
tinued inAvards from it, throAving out neAV matter on their interior, Avhich
is gradually  converted  into bone; whilst the  internal layers,  if they
should be the  parts remaining uninjured, do the same on their exterior,
deriving  their materials  from the medullary membrane and its prolonga-
tions  into  their Haversian  canals.  But it sometimes  happens that
the Avhole shaft  suffers  necrosis; and  as  the medullary membrane and
the entire system of Haversian canals have lost their vitality, reparation
can only take  place from  the periosteum, and from  the living bone at
the tAvo  extremities.  This is  consequently a very sIoav process;  more
especially as  the epiphyses, having been  originally  formed  as distinct
parts from  the shaft, do not seem  able to contribute much to  the regene-
ration of the latter.
   310. We next proceed to the Teeth, Avhich are organs of  mechanical
attrition, developed in  the first part of the alimentary canal, for the
purpose of  comminuting the food conveyed into it.   Their place of ori-
gin is altogether different from that of bone,  as they commence in little
papillary elevations of the mucous membrane covering the jaw ; but the
substance from which they are formed is the same primitive cellular tissue,
as that in which Cartilage itself  originates.  We may best  understand
the structure  and development of the Teeth  in Man, by first  inquiring
into the characters presented by those of some  of  the lower animals,
and the history of their eArolution.   In the foetal Shark, the first appear-
ance of the tooth is in the form of a minute papilla on the mucous mem-
brane covering the  jaAvs;  the substance of this papilla is composed of
ispherical cells, which are imbedded in a kind of gelatinous substance 
186      STRUCTURE  AND  ENDOAVMENTS OF ANIMAL  TISSUES.

resembling that of incipient cartilage; whilst its exterior  is composed
of a dense, structureless, pellucid membrane.   The cellular masses not
at first permeated by vessels ; but a small  arterial branch is distributed
to each papilla, and? spreads out into a tuft  of capillaries at  its base
(Fig. 52).  The papilla gradually enlarges, by the formation of neAV cells
at the part immediately adjacent to the blood-vessels, which supply the
material requisite for their development;  and Avhen it  has acquired its
full size, the process of calcification takes place, by which it is converted
into Dentine, the substance most characteristic of teeth.
   311.  This Dentine, Avhich in the Elephant's tusk is  knoAvn as Ivory,
is a firm substance, in which mineral matter predominates to a greater
extent than in bone; but wdiich still  has a definite animal basis, that
retains its form Avhen the  calcareous matter has been removed by mace-
ration in acid.   In every  100 parts, the animal matter forms about 28;
and of  the mineral portion, phosphate of lime constitutes about  M\
parts, carbonate of lime 5-£ parts, and phosphate of magnesia and soda,
Avith chloride of sodium, about 21 parts.  When it is fractured, it seems
to possess a fibrous appearance;  the fibres radiating from the centre of
the tooth toAvards its circumference.   But Avhen a thin section of it is
submitted to the microscope, it is  seen that  this fibrous  appearance is
due to a peculiar structure in the dentine, which the unaided eye cannot
discover.   The dentinal substance is  itself very transparent; but  it is
traversed by minute tubuli, which appear as dark lines, generally in very
close  approximation, running  from the internal  portion  of the tooth
towards the surface,  and  exhibiting numerous minute  undulations, and
sometimes  more decided  curvatures, in their course (Fig.  53).  They 
STRUCTURE  AND  DEVELOPMENT OF DENTINE.          187
Fig. 53.
occasionally divide into two branches, Avhich continue to run at a little
distance from one  another in  the same direction ; and they also fre-
quently  give off small lateral branches, Avhich again send  off smaller
ones.   In some  animals the tubuli may be traced at their  extremities
into minute cells, or cavities, analogous to the lacunae of bone;  and the
lateral  branchlets  occasionally terminate in such cavities, which  are
called the intertubular cells.  The diameter of the tubuli at their largest
part, averages l-10,000th of an inch ; their smallest  branches are im-
measurably fine.   It is impossible  that  CAren the largest of them can
receive  blood, as their diameter is far less than that of the blood-discs;
but it is probable that, like the canaliculi of bone, they may absorb nu-
trient matter from the vascular surface, Avith which their internal  ex-
tremities are in  relation.
   312.  In the Teeth of Man and of most Mammalia, we find the central
portion IioIIoav;  and lined, in the living tooth, by a vascular membrane.
This cavity, with its vascular wall, is analogous  to a large cancellus or
Haversian canal of Bone; and, as Ave shall  presently see, it is formed
in a similar manner.   Upon the walls of the
cavity, all the tubuli open;  and they radiate
from this towards  the  surface of the upper
part of the tooth,  as shown  in  the accom-
panying figure.   The  central  cavity is con-
tinued as a canal through each fang  or root;
and the dental tubes in like manner  radiate
from this, toAvards the surface of the fang.—In
the teeth of many of the lower animals, how-
ever, Ave find a netAvork of  canals extending
through the substance of the tooth, instead of
a  single cavity; and these canals  are  fre-
quently continuous  Avith the Haversian canals
of the subjacent bone, so that the analogy
betAveen the tAvo is complete.   From each
canal the dentinal tubuli radiate, just in the
manner of the  canaliculi  of bone (§ 295); and thus we may regard a
tooth of this kind as repeating, in each of the parts surrounding one of
these canals, the structure of  the  human tooth.
   313.  The process by which the cellular mass, or pulp,  of the dental
papilla  becomes converted into the Dentine of the  perfect tooth, has not
been so clearly made out, as to be beyond all question.  The folloAving,
hoAvever, is the  account given of it by Mr. Tomes,* Avho has very care-
fully examined it.  The dentinal pulp is at first composed of a mass of
nucleated  cells,  held  together by a meshwork of delicate  fibres and
bands constituting  an imperfect form of areolar tissue, the interspaces
betAveen them being occupied by a homogeneous plasma.  In the part
which is nearest to the coronal surface of the tooth Avhen calcification is
about to commence, the areolar tissue has usually disappeared, and its
place is  occupied  by a finely-granular gelatinous substance.   The cells
are at  first disposed Avithout  any regularity in  the midst of this;  but
 Oblique section of Dentine of hu-
man tooth, highly magnified, show-
ing the parallel tubuli.
* Lectures on Dental Physiology and Surgery. 
188      STRUCTURE  AND  ENDOAVMENTS OF ANIMAL  TISSUES.

immediately preceding the conversion of the pulp into dentine, the cells
are seen to enlarge, and to become arranged in lines, nearly parallel to
each other, and perpendicular to the coronal surface of the tooth ;  they
then elongate in such a manner, that their extremities come into appo-
sition ;  and they finally coalesce,  so  that each toav of  cells forms  a
continuous  tube.   Whilst this  change is in  progress, the gelatinous
intercellular substance is becoming consolidated by calcareous deposit;
Avhich also hardens tbe thick Avails  of the  tubes, so  that only their inte-
rior, formed by the coalescence of the original cell cavities, remains
uncondensed, thus constituting the dentinal tubuli.    This process takes
place first on the surface of the pulp, and gradually extends inwards.
As the  more external and larger cells become hardened,  the inner ones
increase in size, assume the linear arrangement, and in  their turn be-
come converted, writh the intercellular substance, into tubular dentine;
until at last the great bulk of the pulp  is  thus transformed, leaving only
a comparatively small portion, Avhich, with its nerves and blood-vessels,
occupies the central cavity of the tooth.
   314.  Thus the substance of the  outer portion of the pulp is actually
converted into dentine, and does not form it by a process of excretion,
as Avas  formerly supposed.   Sometimes   it happens that  the normal
changes are interrupted, and that some of  the original meshAvork remains
persistent; and it is probably to this that the appearance of large cells,
not unfrequently seen in Human teeth (Fig. 53) is due.—Although in
the most characteristic form of Dentine, no blood-vessels exist, yet there
are certain species, both among Mammals, Reptiles, and Fishes, in which
the Dentine is traversed by cylindrical prolongations of the central cavity,
conveying  blood-vessels into  its substance ; and the presence of these
medullary  canals,  giving  to  the  Dentine a  vascular  character,  thus
increases its resemblance to bone.—The central portion of the pulp  is
sometimes converted into a substance still more nearly resembling bone,
having  its  stellate  lacunae as well as its vascular canals.   This change
is normal or regular in certain animals, as in the extinct Iguanodon and
Icthyosaurus, and in  the  Cachalot or Sperm-whale ;  and the ossified
pulp bears  a close resemblance to  the  bones of the  respective animals,
although it is not formed  in  continuity with them.  A similar change
occurs in the Human tooth ;—sometimes, it would  appear,  rapidly, as
the result of disease;  but in general more slowly, increasing gradually
with the advance of age.
   315.  It is not easy to ascertain the amount of nutritive change that
takes place in  the substance  of Dentine, Avhen it is  once fully formed.
When young animals are fed with coloring-matter, it is found to tinge
their teeth, as well  as their bones;  and if the tooth be in  process  of
rapid formation at the time of the experiment, the progressive calcifica-
tion of  the pulp from Avithout imvards, is  marked by a  series of concen-
tric  lines.   Even  in  the adult, some tinge Avill  result from a prolonged
use of this substance; and it has been noticed that the teeth of persons
who have long suffered from Jaundice sometimes acquire  a tinge of bile.
These facts sIioav that, even after the complete  consolidation  of the
dentine, it  is  still pervious to fluids :  and that in this manner it may
draAv into itself, from the vascular lining of the  pulp-caA'ity, a substance 
          SUCCESSION OF TEETH.—STRUCTURE OF ENAMEL.       189

capable of repairing its structure,  is proved by the circumstance that a
neAV layer of hard matter  is occasionally throAvn out upon a surface
Avhich has been laid bare by caries.
   316.  In those simple teeth Avhich consist solely of  Dentine, the mode
of production already described,—that of the consolidation of a papilla
upon the  mucous  membrane of the mouth,—is all which  is requisite.
When  the formation  of the tooth itself is  complete, it may remain
attached only to the mucous membrane, which is the case in the Shark,
or it may  groAV downwards, by the addition of neAV dental  structure at
its base, until it comes in contact Avith the bone of the jaw.  Where it
is only attached to the mucous membrane, as in the Shark, it is very
liable to be torn away; but a new tooth, formed from a distinct papilla,
is  ready  to replace  it; and this  process is  continually repeated, the
development  of neAV papillae being  apparently unlimited.  On the other
hand, Avhere the root of the tooth comes in contact with the jaAV,  it may
completely coalesce with it, which  is the case in many Fishes, the Ha-
versian  canals of the bones being continued  as medullary canals  into
the dentine;  or it may send long spreading roots into the  bone, Avhich
are united to it at their extremities.  In the classes of Fishes and Rep-
tiles (with scarcely any exceptions) the teeth  are  by no means  perma-
nent, as among Mammalia; but new teeth are continually succeeding
the old ones.   The mode in Avhich  these teeth originate, by small buds
from the capsules of the preceding,  will be understood when the capsular
development of all the  higher forms of the dental apparatus has been
described.
  317. It is obvious that there is no provision, in the simple calcifica-
tion of the dental papilla, for any variations of density,  other than those
which may result from the different degrees of hardness in the substance
of the dentine itself.  Now in the teeth of Man and most  other Mam-
mals, and in  those of many Reptiles and some Fishes, Ave find two other
substances, one of them harder, and the other softer, than Dentine; the
former is termed Enamel;  and the latter  Cementum  or Crusta petrosa.
For the development of these, a peculiar modification of the apparatus
is requisite.
  318. The Enamel is composed of long prismatic cells, exactly resem-
bling those of the prismatic shell-substance formerly described (§ 281),
but on a far more  minute scale ; the  diameter of the  cells not being
more, in Man, than  l-5600th of an inch.  The length of the prisms
corresponds with the thickness of the layer of enamel; and  the tAvo sur-
faces of this layer present the ends of the prisms, which are usually more
or  less  regularly  hexagonal.  The quantity  of animal matter in the
enamel  of the adult  is  extremely minute,—not above 2 parts in 100;
and it is only at a very early age that the true character of the animal
structure can be distinctly seen.   Of the 98 parts of mineral matter in
the enamel, 885- consist (according to Berzelius) of phosphate of lime, 8
of carbonate of lime, and 1| of phosphate of magnesia.   The course of
the prismatic cells is more or less  Avavy;  and they are marked by nu-
merous  transverse striae, resembling those of the prismatic  shell-sub-
stance, and probably originating in the same cause,—the coalescence of
a line of shorter cells, to form  the lengthened prism.  The Enamel  is 
190      STRUCTURE AND  ENDOAVMENTS OF  ANIMAL TISSUES.
  Vertical section of human
molar tooth :—1, enamel; 2, ce-
mentum or crusta petrosa; 3,
dentine or ivory; 4, osseous ex-
crescence, arising from hypertro
usually destitute  of  tubuli; but Mr. Tomes  has shoAvn  that it is occa-
sionally penetrated by prolongations of the  tubuli  of the dentine, and
that this peculiarity, Avhich is occasional and abnormal  in Man, is cha-
racteristic of the teeth of many Marsupials.  In density and resisting
power, the Enamel  far surpasses any other organized  tissue, and ap-
                      proaches some  of the hardest  of mineral sub-
                      stances.  In Man, and in Carnivorous  animals,
                      it covers the crown of  the  tooth only, with a
                      simple  cap or  superficial layer of tolerably uni-
                      form thickness (Fig.  54, 1), which  follows the
                      surface  of  the dentine  in  all  its  inequalities;
                      and its component prisms are directed  at  right
                      angles  to that surface,  their inner  extremities
                      resting in slight  but  regular depressions on the
                      exterior of the dentine.   In  the  teeth of  many
                      Herbivorous  animals,   however,  the  Enamel
                      forms (with the cementum)  a series  of vertical
                      plates, which dip doAvn (as it Avere) into the sub-
                      stance of the  dentine, and  present  their  edges,
                      alternately with  it,  at  the  grinding surface of
                      the tooth;  and  there is in such teeth no con-
                      tinuous  layer  of dentine over the crown.   The
                      purpose of this arrangement is evidently to pro-
                      vide, by the unequal wear  of these three sub-
                    „  stances,—of  Avhich the  Enamel is  the hardest
phy of cementum; 5, cavity 5 6,       '                         pi
osseous ceiis at outer part of and the Cementuui the softest,—for the constant
                      maintenance of a rough  surface, adapted  to tri-
turate the tough vegetable substances on which these  animals feed.—
The Enamel is the least constant of the Dental tissues.   It is more fre-
quently absent than present  in the  teeth  of the class of  Fishes; it is
Avanting  in the entire order of Ophidia (Serpents) among existing Rep-
tiles ;  and it  forms no part of the teeth of the Edentata (Sloths, &c.)
and Cetacea (Whales) amongst Mammals.
  319. The Cementum, or Crusta Petrosa,  has the characters of true
Bone; possessing its distinctive stellate  lacunae and radiating canaliculi.
Where it exists in small amount, we do not find it traversed by medullary
canals; but, like Dentine, it  is  occasionally furnished  with them, and
thus resembles Bone in every particular. These medullary canals enter
its  substance from the exterior of the tooth; and consequently pass to-
wards those, which radiate from the central cavity  towards  the surface
of the dentine, where it possesses  a similar vascularity,  as was remarka-
bly the case in the  teeth of the extinct Megatherium.   In the Human
tooth, however, the  Cementum has no such vascularity.  It forms a thin
layer, which envelopes the root of the tooth, commencing near the ter-
mination of the capping of Enamel (Fig. 54, 2). This layer is very sub-
ject to have its  thickness increased, especially at  the extremity of the
fangs, by hypertrophy,  resulting from inflammation;  and sometimes
large exostoses are thus formed (Fig. 54, 4), which  very much increases
the difficulty of extracting the tooth.  When the tooth is first developed,
the Cementum envelopes its crown, as Avell  as its  body and  root; but 
FORMATION  OF DENTAL CAPSULE.
191
the layer is very thin where it covers the Enamel, and being soft, it is
soon Avorn aAvay by use.  In the teeth of many Herbivorous Mammals,
it dips down with the Enamel to form the vertical plates of  the interior
of the tooth ; and in the teeth of the Edentata as Avell .as of many Rep-
tiles and Fishes,  it forms a thick continuous envelope over the whole  of
the surface, until worn  away at the crown.
  320. The.development of these additional structures  is provided  for
by  the enclosure of the primitive  papilla, from Avhich  the  Dentine is
formed, within a  Capsule, which, at one period, completely covers it in:
betAveen the inner surface of the capsule, and  the outer  surface of the
dentinal papilla,  a sort  of epithelium is developed, by the calcification
of which, the Enamel  is formed;  and  the  Cementum is generated  by
the conversion of the capsule itself into a bony  substance.   The pro-
cesses by Avhich this capsular  investment is produced,  and the tooth
completed and evolved, -will now be  briefly described, as they occur in
the Human foetus.
  321. The dental papillae begin  to make  their  appearance, at about
the seventh week  of  embryonic life, upon the mucous  membrane cover-
ing the bottom of a  deep narrow groove (Fig. 55, a,)  that  runs along
the edge of the jaw  (Fig. 55, b);  and during the  tenth week, processes
from the sides  of this  "primitive  dental groove," particularly the ex-
ternal one, begin  to approach one another,#so  as  to  divide  it, by their
meeting, into a series of open follicles, at the bottom of  which  the pa-
pillae may still be seen.   At  the thirteenth  week  all  the follicles being
completed, the papillae,  which were at first round blunt masses of cells,
begin to assume forms more characteristic of the teeth which are to  be
developed from them; and by their rapid growth, they  protrude from
                               Fig. 55.
   Successive stages of the development of the deciduous or temporary teeth, and of the origin of the
                        sacs of the permanent set.

the mouths of the follicles (Fig. 55, c).   At  the  same time, the edges
of the follicles are lengthened into little valve-like processes, or oper-
cula,  which are destined to meet and form  covers to  the follicles (Fig.
55, d).   There are tAvo of these opercula in the Incisive follicles, three
for the Canines, and four or five for the Molars.  And by the fourteenth
Aveek, the tAvo lips of the dental groove meet over the mouths of the
follicles,  so as completely to enclose each papilla  in a distinct capsule
(Fig.  55, e).   At  this period, before the calcification  of  the primitiA'e
pulps  commences, a provision is made for the production  of the  second
or permanent molars; Avhose capsules originate in buds or offsets from
the upper part of the capsules of the temporary or milk-teeth (Fig. 55,/). 
192      STRUCTURE AND ENDOAVMENTS OF  ANIMAL TISSUES.

These offsets are in  the condition of open follicles, communicating Avith
the cavity of the primitive tooth;  but they are gradually closed in, and
detached altogether from the capsules of the milk-teeth (Fig. 55, </, h, i).
   322. Soon after the closure of the follicles of the Milk-teeth, the con-
version of the cells  of the original papilla into Dentine commences, ac-
cording to the method already described (§ 313).   Whilst this  is going
on, the follicles  increase in size, so that  a considerable space  is left be-
tween their inner walls  and the surface of the dental papillae; which
space is filled up with a  gelatinous granular  matter, the Enamel  pulp.
The  portion of this Avhich is  converted into enamel, hoAvever, is very
small; being only a thin layer, which is left on the inner surface of the
capsule after the remainder has disappeared.   The interior of the dental
sac, at the time Avhen the conversion-process, has reached the base of the
dentinal pulp, is in the villous and vascular condition of a Mucous mem-
brane,—which indeed it really is, having been, as Ave have  seen, once
continuous with the lining of  the  mouth;  and the  layer of prismatic
cells  which covers its free surface,  and by the calcification of Avhich the
enamel is produced, may be regarded as an epithelium.   The completion
of the Enamel,  and the  ossification  of  the  capsule  so as to form the
Cementum, take place at a subsequent period.
   323. We have thus seen that the history of the first  development of
the Human teeth may be divided  into three stages, the  papillary, the
follicular, and the saccular.   The  papillary corresponds precisely with
the complete mode  of dental development in the Shark and other Fish,
—as  already mentioned.   The follicular, Avhich commences with the. en-
closure of the papillae in open follicles, and terminates when the papillae
are completely hidden by the closure of the  mouths  of  those  follicles,
has also its permanent representation in the  development of the teeth
of many Reptiles and Fishes; the primitive papillae of Avhich, though
enclosed in follicles, are never covered in at the summit, and  thus free
themselves  from their  envelopes by simply groAving upAvards through
their open mouths.    But in Man, and in all other  animals which agree
with  him in going on to  the saccular stage, there must also be an erup-
tive stage, Avhich consists in  the bursting forth of the tooth from the
enclosing capsule ;  the summit of the tooth being carried against the lid
of the sac, by the growth of its roots (Fig. 55, h).   By the continuance
of the same growth, the teeth  are caused to penetrate the gum, and are
gradually raised above its surface (Fig. 55, i).
   324. All the permanent  teeth,  Avhich  are destined to replace the
temporary set, originate, as already stated, in buds or  offsets  from the
capsules of the latter.   But behind  the last temporary molars, which
are replaced by  the  permanent bicuspids, three  permanent molars are
to be developed, on each side of either jaw.  The first of these is formed
on precisely the same plan Avith the milk-teeth; but is  not  completed
until  a later period.   The capsule  of  the  second is  formed at a  later
period from that of  the first, by a process  of  budding exactly analogous
to that, by Avhich the other  permanent  capsules  are formed  from the
corresponding temporary; and at a still later period, the capsule of the
third permanent molar is formed as a bud from  that of the second.  The
evolution of this molar  does  not usually  take  place, until the system 
DEVELOPMENT AND RENEWAL  OF TEETH.           193
has acquired its  full development;  and the process of budding  then
ceases in  Man,—being limited  to a single  act of reproduction in the
case of the ordinary Milk-teeth, and to a double one in that of  the first
permanent Molar.  In many animals of the lower classes, however, the
process goes on through the whole of life without any limit; the newly-
formed teeth, however, usually taking the places of those of the previous
set, and not being developed at their sides like  the second and third
permanent molars of Man.  By a process  of this  kind, the continual
renewal of the Teeth takes place in those  Reptiles and Fishes, whose
dentition goes on to the saccular stage ; in those at which it stops at the
papillary, the  successive teeth are formed  from new and  independent
papillae.   The analogy between  the continued succession of teeth in the
lower Vertebrata, by the gemmiparous reproduction of their capsules,
and the  development of the capsules of the permanent teeth  of  Man
from those of the temporary set, is  made further evident by the fact,
that a third set  occasionally makes its appearance in persons advanced
in life ; the development of which would not be intelligible, if we could
not refer it to the continuance of the same process in the other capsules,
as that which regularly takes place to a limited extent in the permanent
molars of Man, and which goes on Avithout limit through the Avhole lives
of the lower Vertebrata.
   325. The chief exception to the rule, that no Reptiles  or Fishes  have
permanent teeth, is found in the curious Dicynodon ; an extinct Reptile
which had twTo large tusks  growing from persistent  pulps, like  those of
the Elephant, the front teeth of the Rodentia, and the grinders of the
Edentata.  In such teeth,  the base  of the  pulp  remains unconverted,
and a new development of cells is continually taking place in that situa-
tion ; these new cells are in their turn converted into dentine, in con-
tinuity with that previously formed;  and  thus the tooth or tusk is con-
tinually lengthening at its  base, in a degree which  compensates for its
usual wear at its summit.  If  anything should  prevent that  wear,—
as when the opposite tooth has  been broken off,—there is an absolute
increase in the length of the tooth, from  the continued growth at its
base; which may become a source of great inconvenience to the animal.
There is nothing, in the Human subject, at all analogous to this mode
of ^development from persistent pulps ;  the process being checked by the
closure of  the root around the  base of the pulp, which obstructs the
supply of  blood it receives.
   326. The following table shows the usual periods at which the diffe-
rent teeth of the  tAvo sets first shoAV themselves above  the gum.   It must
be borne in mind, howeArer, that  these  periods are subject to very great
variation; and that the average  alone can therefore  be expressed.
TEMPORARY OR DECIDUOUS TEETH.             PERMANENT TEETH.
	Months:		Tears.
Central Incisors,	7	First Molar, .	. &ito 7
Lateral Incisors,	. 8 — 10	Central Incisors, .	. . 7 — 8
Anterior Molars,	. 12 — 13	Lateral Incisors, .	. 8—9
Canines, .	. 14 — 20	First Bicuspid,	. 9—10
Posterior Molars, .	. 18 — 36	Second Bicuspid, .	. 10—11
		Canines,	. 12 —12}
		Second Molars,	. 12£—14
		Third Molars,	. 16"— 30
 
194     STRUCTURE  AND  ENDOWMENTS OF ANIMAL  TISSUES.

   327. We have  seen that the teeth are formed, in the first instance,
upon the surface  of  the Mucous membrane of the mouth;  and conse-
quently they really form a part of the external or dermo-skeleton, and
not of the  internal or osseous skeleton.  They correspond, therefore,
with the external  skeletons of the Invertebrata;  and thus the  analogy
which has been pointed out, betAveen the enamel of teeth and the pris-
matic  cellular substance of the shells of Mollusca, and between the den-
tine and the shells of the  higher  Crustacea, holds good  also in regard
to the situation of these structures.   Although the teeth are the only
ossified portions of the dermo-skeleton in Man, we find the body par-
tially or completely enclosed  in  an  armor of  bony scales or plates, in
certain Mammalia, Reptiles, and Fishes; and in some  species of the
last-named  class,  which have now ceased  to  exist, the  scales  seem  to
have had the texture  of Enamel.
   328. In connexion with the Teeth, the structure and  development of
the Hair may be described;  this substance being generated very much
in the same manner as dentine,—by the conversion of a pulp enclosed
in  a follicle ; though the  product  of the transformation is different.
The Hair-follicle  is formed by the inversion of the Skin, as the Tooth-
follicle is by an inversion of the Mucous membrane ; and it  is lined by
a continuation  of the epidermis.  From the  bottom of the follicle, a
sort of papilla  rises up, formed of cells; the exterior of this, which is
the densest part,  is known as the  bulb; whilst  the softer  interior  is
termed the  pulp.   The follicle  itself is extremely vascular  ; and even
the bulb is reddened by a minute injection ; though  no  distinct vessels
can  be traced into it.—It  has been imagined until  recently, that the
Hair, like the other extra-vascular tissues, is a mere product of secre-
tion ; its material, which is chiefly horny matter of the same composition
with that of the epidermis and its other appendages  (§ 227), being ela-
borated from the  surface  of the pulp.  This,  however,  proves  to be a
very erroneous account of  it; as is shown by the results of microscopic
inquiries into its structure.  Although the Hairs of different  animals
vary considerably in the appearances they present, we  may generally
distinguish in  them two elementary parts: a cortical or investing sub-
stance, of a fibrous, horny  texture;  and a medullary or pith-like sub-
stance occupying  the interior.   The fullest development of both sib-
stances is to be found in the  spiny hairs  of the Hedge-hog, and in the
quills of  the Porcupine, which are but hairs on a magnified scale.  The
cortical substance forms a dense horny tube, to which  the firmness  of
the structure seems chiefly due ; whilst the medullary substance is com-
posed of an aggregation of very large cells, which seem  not to possess
any fluid contents in  the  part of the hair that  is completely  formed.
The structure of the  feather of Birds is precisely analogous : the cor-
tical horny  tube existing alone in the quill, but being filled with a cel-
lular medulla in the  stem of the feather itself.   The smaller hairs  of
the  Sable (Fig. 56, b) show the cortical and medullary  substances in a
very characteristic form; the former being here plainly seen  to be made
up of flattened imbricated cells resembling those of the epidermis ; whilst
the cells of which the latter is composed are nearly globular.   In the
hair of the  Musk-deer (Fig. 56, a),  we find the medullary substance to 
                STRUCTURE  AND  DEVELOPMENT OF HAIR.           195

'be composed of an assemblage of cells whose walls are flattened against
 each  other, as in  a Vegetable pith ;  whilst  the  cortical envelope is
 scarcely distinguishable.  In  the hair  of the  Mouse and other  small

                                  Fig. 56.
 Structure of Hair:—A, hair of Musk-deer, consisting almost entirely of polygonal cells; B, hair of Sable.
      showing large rounded cells in its interior, covered by imbricated scales or flattened cells.

 Rodents, Ave see the hairy tube crossed at intervals by partitions, which
 are sometimes complete, sometimes  only partial;  these are the Avails of
 the single or double  line of cells, of  which  the medullary substance is
 made up.
   329. In the Human hair, the  representation of the cortical envelope
 of the hair of other animals  is  found in a thin transparent horny film,
 which  is composed of flattened cells or scales, arranged in an imbricated
manner, their edges forming  delicate lines  upon the surface of the  hair,
which  are sometimes  transverse,  sometimes oblique,  and sometimes
apparently spiral (Fig. 57,  a).   Within this  we find  a  cylinder  of
fibrous texture, which forms  the principal  part of the  shaft of the  hair.

                                Fig. 57.
  Structure of the Human Hair:—A, external surface of the shaft, showing the transverse strisc and jagged
boundary caused by the imbrications of the scaly cortex; B,  longitudinal section of the shaft, showing the
fibrous character of the medullary substance, and the arrangement of the pigmentary matter; C, transverse
section, showing the distinction between the cortical and medullary substances and the central collection of
pigmentary matter; D, similar transverse section without the dark centre.

and it is in the centre alone, which is  frequently more distinctly cellular,
that we find any close resemblance to the ordinary condition of the me-
dullary substance.   The constituent fibres of the shaft  are marked out
by  delicate longitudinal strise^which  may be traced in vertical sections
of the hair (b) : but they may be still more  completely demonstrated by 
196      STRUCTURE  AND ENDOWMENTS OF ANIMAL  TISSUES.

crushing the hair, after it has  been macerated for some time in dilute*
acid.   In dark hairs, the pigmentary granules are frequently scattered
between the fibres: but they are frequently found in  greater abundance
in the central cells, where they form a dark spot  in  the  middle of the
transverse section (c).   Sometimes, hoAvever, no such  collection is seen ;
and whatever  pigmentary matter exists in the hair is equally diffused
through the whole of it, or, is even accumulated rather towards its exte-
rior  (d).  This  coloring  matter seems related  to  Haematine;  it is
bleached by chlorine; and when it gives a dark hue to the hair, it usually
contains a good deal of iron.
   330.  The fibres of which the chief part of the shaft is  made up are
probably  cells, which  have become elongated  by the  process  already
noticed (§ 193), and which have at the same time secreted horny matter
into their interior.  This change is  continually going on in the pulp of
the hair, at  the base of the part previously completed; and by the pro-
gressive formation of new cells in the  bulb  a constant growth of the
shaft  is  provided for.   The  central  medullary  substance  is rather
derived from  the cells of the  pulp, in which a continuous growth goes
on, at the same  rate with that of the  bulb.   The imbricated layer of
cells,  which forms the true cortical substance, may be said to be a pro-
longation of the ordinary Epidermis over the surface of the hair, being
developed from the external layer of  the bulb, Avhere  it is  continuous
with the epidermic lining  of the follicle.  Thus the  Hair is constantly
undergoing  elongation by the  addition of new substance at its  base,
precisely in  the same manner as the  teeth of certain  Mammals  grow
from  persistent  pulps.  The part once formed usually  undergoes no
subsequent alteration; but there is evidence  that  it may be affected by
changes at its base,  the  effect  of which is propagated along its whole
extent.   Thus, it is well known that cases are not unfrequent, in which,
under the influence of strong mental emotion, the whole of the hair has
been turned to gray, or even to a silvery white, in the course of single
night; a change which can scarcely be accounted for in any other Avay
than  by supposing  that  a fluid capable  of chemically affecting  the
color, is secreted at the base of the hair, and transmitted by imbibition
through the medullary substance to the opposite extremity.   The know-
ledge  of the organized structure of hair enables us better to understand
some  of the  effects of disease, and especially  of that peculiar  affection
termed Plica Polonica.   The hair of individuals suffering from it is dis-
posed to split  into fibres,  often at  a  considerable  distance  from  the
roots, and to exude a glutinous substance; and these tAvo causes unite
in occasioning that peculiar matting of the hair, which has given origin
to the name of the disease.   In the hair thus affected, there is evidently
a power of transmitting fluid absorbed at  the roots:  and it is said that
even blood exudes from the stumps, when the hairs are  cut off  close to
the skin.

          6. Of Cells coalesced into Tubes with Secondary Deposit.

   331.  Most of the tissues which have been hitherto described, differ in
no essential particulars from those of Plants ;  the chief departure from 
MUSCULAR FIBRE.                      197
the forms presented by the latter, being in the Fibrous tissues, which,
as already observed,  are introduced for the  sake  of facilitating  the
movements of the several parts of the structure, one upon the other.
The various cellular tissues find their exact representatives in those of
the Vegetable  fabric;  and  the denser parts  of the  Animal,  such as
Bone, Cartilage, &c,  are represented by  the  solid substances formed
by the  Plant in the heart-wood of the stem, the stone  of fruits, &c,
these substances acquiring their density in  precisely the  same  manner
with the  Osseous tissues, by  the secreting action  of their own cells,
which draAv a solidifying material from the general  circulating fluid.
But we now come to two tissues of the highest importance in the Animal
fabric, the presence of which is, indeed, its  distinguishing  characteristic.
These are the Muscular and the Nervous tissues.  The  former is  the
one by  which all the  sensible movements of the body  are effected;  and
the latter furnishes the instrument by which  sensations are received,
and by which the will excites  the  muscles to action, besides serving as
the medium for other operations, in Avhich  motion is produced  without
the intervention of either sensation  or will.   These  tissues, with  the
apparatus of bones and joints on which the muscles act,  constitute  the
purely animal portion of the fabric ; and if a being could be constructed
in which they should  be capable of continued activity without any other
assistance, it would be  in all essential particulars an Animal.   But, as
we shall presently see, the plans on  Avhich these  tissues  are formed, in
fact, the very conditions of their existence and activity,  are such, that
they require constant nutrition and  reformation;  so that the Animal
cannot exist without an apparatus for preparing, circulating, and main-
taining in constant purity, a fluid by  which nutrient operations may be
effected, and which shall also be the means of carrying off the products
of the  waste consequent upon  the action  of those tissues.  This appa-
ratus constitutes the  Vegetative portion  of the frame, the elementary
parts concerned in which have been already noticed.
   332.  When we examine an  ordinary Muscle with the  naked eye, we
observe that it is made up of a number of fasciculi or bundles of fibres;
which are arranged side by side with great regularity, in the direction
in which the muscle  is to act, and which are united by  areolar tissue.
These fasciculi may be separated  into smaller parts, Avhich appear  like
simple  fibres; but  when  these are examined  by the microscope, they
are found to be themselves fasciculi, composed of minuter fibres bound
together  by delicate  filaments  of  areolar tissue.   By carefully sepa-
rating these, we may obtain the ultimate  Muscular Fibre.   This fibre
exists under two forms, the striated and the non-striated; the former
makes up the whole substance of those muscles over which  the will  has
control, or Avhich are  usually called into operation through  the  nerves;
whilst the latter exists  in the  muscles which the will  cannot  influence,
and which are excited to contraction  by stimuli that act  directly upon
them.   The muscles of the former class minister especially to the animal
functions; those of the latter to the  functions of organic or  vegetative
life.  The appearance presented  by the striated  fibres  of ordinary
muscles  is shoAvn  in Fig. 58;  that  of the non-striated  fibres  of  the
muscles of organic  life, in Fig. 59. 
198      STRUCTURE AND ENDOWMENTS  OF ANIMAL  TISSUES.
  333.  When the striated fibre, which must be considered as the highest
form of Muscular tissue, is more closely examined,  it is seen to  consist
of a delicate tubular sheath,  quite distinct on  the one  hand from the
areolar texture which binds the fibres into fasciculi, and equally distinct
Fig. 58.
Fig. 59.
 Fasciculus of striated Muscular
Fibre, showing at a, the trans-
verse striae, and at b, the longi-
tudinal strke.
 Non-striated Muscular  Fibre;
at 4, in its natural state; at a,
showing the nuclei after the action
of acetic acid.
from the internal substance of the fibre.  This cannot always be brought
into view, on  account of its  transparency; it becomes most evident,
when, as occasionally  happens, the contents of the fibre are  separated
transversely by the drawing apart of its extremities without the rupture
of the sheath; but  it  may also be sometimes seen rising up in wrinkles
upon the surface of the fibre, when the latter is in  a state of contraction.
This membranous  tube,  which has  been termed  the  Myolemma, has
nothing to do with the production of the stride, these being  due, as will
be presently shown, to the peculiar arrangement of its contents.   It  is
not perforated either  by nerves or capillary vessels, and forms, in fact,
a complete barrier between the real elements of Muscular structure and
the surrounding parts.   That it has no share in the contraction of the
fibre is evident from the fact  just mentioned, in regard to  its wrinkled
aspect when the fibre  is  shortened.
  334.  Although  Muscular Fibres  are commonly described  as  cylin-
drical  in  form, yet they are in  reality rather polygonal,  their  sides
being flattened against those  of the adjoining fibres (Fig. 62).  In some
instances, the angles are sharp and decided; in  others they are rounded
off, so as to leave  spaces between the contiguous fibres, for the passage
of vessels.   In Insects, the fibres often present the form  of flattened
bands, on Avhich the transverse strise are very beautifully marked.   The
size of the fibres is subject to great variation, not merely in different
classes of animals, but in different species, in different sexes of the same
species, and  even  in  different parts  of the same muscle.  Thus  Mr. 
STRIATED MUSCULAR FIBRE.
199
Bowman estimates the  average diameter of the fibres  in  the  Human
male  at  l-352d of an inch;  the largest being l-192d, and  the smallest
l-507th.  In the female, he found the average to be l-454th of an inch;
Avhilst the largest was l-384th, and the smallest l-615th.   The average
size of the Muscular fibre is  greater among Reptiles and Fishes, than
in other Vertebrata;  but, on the other  hand,  the  extremes  are  much
wider.  Thus the dimensions  vary in the Frog from l-100th to 1-lOOOth
of an inch;  and in the Skate from l-65th to l-300th.
   335.  When the striated Muscular Fibre is examined still more closely,
it is found  to contain an assemblage  of very minute elements, Avhich
appear to be flattened disk-like cells, of very uniform size.   These pri-
mitive particles are adherent to each other  both by their flat surfaces,
and by  their edges.  The former adhesion is usually the most powerful,
and causes the substance of the fibre, when  it is broken up, to present
itself in the form of delicate fibrillse, each of which is  composed of  a
single row of the  primitive particles (Fig. 60).  On the  other hand, the
lateral adhesion is sometimes  the stronger, and causes the fibre to break
across into disks,  each of which is composed  of  a layer of the primitive

                                Fig. 60.
             Striated muscular fibre separating into fibrillae (from a Terebratula).

particles (Fig. 61).   That the fibre is a solid collection of these elemen-
tary parts, and  not hollow  in  the centre, as  some have supposed, is
shown by making a  thin transverse section of a fasciculus (Fig. 62); by
which also the polygonal form of the fibre is made apparent.
Fig. 61.
Fig. 62.
 An ultimate fibre, in which the
transverso splitting into disks, in
the direction of the striation of the
ultimate fibrils, is seen.
  Transverse section of ultimate fibres of
the biceps. In this figure the polygonal
forms of the fibres is seen, and their com-
position of the ultimate fibrils.
  336.  When the fibrillae are separately examined, under high magni-
fying power, they arc seen  to present  a cylindrical  or slightly-beaded 
200      STRUCTURE  AND  ENDOWMENTS OF  ANIMAL TISSUES.
Fig. 63.
I
form, and to be made up of a linear aggregation of distinct cells.  We
observe the  same alternation  of light and dark  spaces, as when the
fibrillge are united into fibres or into small bundles;  but it may be
distinctly seen, that each light space is  divided by a  transverse line;
and that there is a  pellucid border at the sides of the dark spaces, as
well  as between their contiguous extremities (Fig. 63).   This pellucid
border seems to  be the cell-Avail; the dark space enclosed by it (which
is  usually bright in  the  centre) being the cavity of the  cell, which is
filled with a highly-refracting substance.   When the fibril is in a state
of relaxation, as seen  at a, the diameter of the cells is greatest in the
longitudinal direction:  but when it is contracted, the fibril increases in
diameter as it diminishes in length; so that the transverse diameter of
each cell  becomes equal to the longitudinal diameter, as seen at b; or
even exceeds it.  Thus the act of Muscular contraction seems  to con-
             sist in a change of form in the cells of the ultimate fibril-
             lae, consequent  upon  an  attraction between the walls of
             their two extremities; and it is interesting to observe, how
             very closely it  thus corresponds  with  the contraction of
             certain Vegetable tissues,  of Avhich  the  component cells
             (§ 345) appear  to  produce a movement, when  they are
             irritated,  by means of  a similar change  of form.  The
             essential difference,  therefore, between the muscular tissue
             of Animals, and the contractile  tissues of  Plants, consists
             in the subjection-of the former to nervous influence (§ 353).
             The diameter of the ultimate fibrillae will of course be sub-
            ject  to variations, in accordance with  their contracted or
             relaxed condition;  but  seems  to be  otherwise  tolerably
             uniform in different animals, being for the most part about
             l-10,000th of an inch.   It has  been observed,  however,
             as high as l-5000th of an inch,  and as low as l-20,000th,
             even when not  put upon  the stretch.  The average dis-
             tance of the striae,  too, is nearly uniform in different ani-
             mals ;  though considerable  variations present themselves
uitSeUrfibriite  iQ every  individual, and  in  different parts  of the  same
             muscle.  Thus the  maximum distance  varies in different
of  striated mus-

nbrii in a state of animals from 1-15,000th to 1-20,000th of an inch;  the mi
ordinary relaxa-   •     n
tion; 6, a fibril in nimum from
a state of partial jy,™,-,  /Jnpq
contraction.      UCdll  UUtfS
                         l-7500th to l-4500th of an inch; while the
                         not  depart  widely  in  any  instance  from
             l-10,000th.
   337. The Muscular tissue of Organic life is very different from that
which has been  now described.   It  exists under two forms;  that  of
fibres  and that of cells.   The  fibres  are  distinguished from  the pre-
ceding by the  absence of transverse markings, but appear to  be tubu-
lar, their contents having  a  granular existence,  without  any  definite
arrangement of the particles into disks or fibrillae.   Their size is usually
much  less than  that of the striated  muscular fibre; but OAving to the
extreme variation in the degree of flattening which they undergo, it is
difficult to make  even an  average estimate of their dimensions.   Those
of the alimentary canal  of Man  are stated by Dr. Baly  to  measure
from about  the l-2500th to the l-5600th part  of  an  inch  in diameter. 
NON-STRIATED MUSCULAR FIBRE.
201
They generally present nodosities or enlargements at frequent intervals
(Fig. 64);  the character of which will be presently apparent.   These
fibres are,  like  those  of  the  other muscles,  arranged in a parallel
manner into bands or  fasciculi;  but these fasciculi are generally in-
terwoven  into  a network, without  having any fixed  points of attach-
ment.   It is of this kind of  structure, that the proper muscular coat of
the oesophagus, of  the stomach and intestinal canal, and of the bladder,
is chiefly composed.   It has been  recently  shown by Prof.  Kolliker,
that contractile tissue exists, even in the adult state  of  the highest
animal, under its very simplest form; that, namely, of cells, which  are
usually more  or less  elongated.  These are  composed  of a soft, light
yellow substance, Avhich swells in water  and  acetic acid,  becoming pale
in the latter, and which is nearly homogeneous, so that it is difficult to
distinguish  the cell-wall clearly from the  cell-contents;  but they  are
especially characterized by the possession of  long staff-like nuclei (Fig.
Fig. 64.
Fig. 65.
  A. A muscular fibre of organic life
from the urinary bladder, magni-
fied COO diameters. Two of the nu-
clei are seen.
  b. A muscular fibre of organic
life, from the stomach, magnified
600 diameters.  The diameter of
this and of the  preceding  fibre,
midway between the nuclei, was
14750th of an inch.
 Fusiform contractile cells :—a,
trabecula of spleen,  with the
cells in situ ; b, a siDgle cell iso-
lated; c, a  similar cell treated
with acetic acid;—a, a, cells; b,b,
nuclei.
65, b, b), which are sometimes only rendered perceptible by acetic acid.
These cells are sometimes so little elongated, especially in the walls of
the blood-vessels, that they might be taken for epithelium-cells;  on the
other hand, they frequently pass into the form of the non-striated fibres
already described.  They are very commonly fusiform (Fig. 65, b), and
are  then arranged in the manner shown  in Fig. 65, a.   This form of
muscular structure is  often found  without any admixture of other tissue,
as in the smaller  arteries, veins, and  lymphatics.   But it is most com-
monly intermixed  with the various forms  of the simple fibrous tissues;
and  in this  state  it is found in the circular  coat of the larger arteries
and veins, in the  erectile tissues  generally,  in the skin, and especially
the dartos, to which  it gives a contractility that  is manifested under
the influence of cold or of mental emotions, and  thus produces  that
general  roughness and rigidity of the  surface which is known as cutis
anserina, whilst it throAVs the scrotum into wrinkles.
  338.  From the study of the early deA-elopment of Muscular Fibre, it 
202      STRUCTURE AND ENDOWMENTS  OF ANIMAL TISSUES.
appears that the Myolemma, or external transparent tube, is  the part
first formed: this being distinctly  visible  long before any traces of
fibrillae can  be observed in it.   This tube takes  its  origin,  like  the
straight ducts of Plants, in cells laid end to end;  the cavities  of  which
coalesce, by the disappearance of the partitions, at a subsequent period.
The nuclei of these original cells may be distinctly seen, for some time
after the appearance of the transverse striae, which indicate the forma-
tion of the fibrillae in their  interior;  and they project considerably from
the sides of the fibres.   In the fully-formed muscle of animal  life, how-
ever, they are  not perceptible, except when the  fibre is treated with
Aveak acid; the effect  of which  is to render  the nuclei more opaque,
whilst  the  surrounding  structure becomes more transparent (Fig. 66).

                                Fig.  66.
ifii
  Mass of ultimate fibres from the pectoralis major of the human foetus, at nine months. These fibres
have been immersed in a solution of tartaric acid, and their " numerous corpuscles, turned in various direc-
 ions, some presenting nucleoli," are shown.

They are usually numerous in proportion to the size of the fibre.  There
is every probability that these nuclei continue to act, like the "germinal
spots" of the glandular follicles,  as centres of nutrition; from which the
minute cells that  compose the fibrillae are developed as they are required.
The diameter of  the Muscular Fibre of the foetus is not above one-third
of that which it possesses in the adult; and-as the size of  the ultimate
particles is the same in both cases, their number must be greatly multi-
plied during the growth of  the structure.   But we shall find reason to
believe, that the decay of  these particles is constantly taking place,
with a rapidity proportional to  the functional activity of  the Muscle;
and their generation, Avhich occurs  as continually, when the nutrient
operations proceed  in their  regular  course, is probably accomplished by
a development from  these  centres,  at the expense of the blood  with
which  the Muscle is copiously  supplied.
  339. From the preceding history it appears, that  there is no diffe-
rence,  at an early  stage of  development,  betAveen the striated and the
non-striated forms of muscular fibre.  Both are simple tubes, containing
a granular matter in which no definite arrangement can be traced,  and
presenting enlargements occasioned by the presence of the  nuclei.   But
whilst  the striated fibre goes on  in its development, until the fibrillse,
with their alternation of light and dark spaces, are fully produced, the
non-striated fibre retains  throughout life its original embryonic condi-
tion.—And it may further be remarked,  that the contractile  cells, of
which  many of the non-striated  muscular structures are composed, are
the permanent types of those  which are found in the  earliest condition 
VESSELS  AND  NERVES OF  MUSCLES.              203
of the heart;  Avhose walls are composed of a  similar tissue, for  some
time after its rhythmical movements have become established.
  340.  We have seen that the  Muscular tissue, properly so called, is
as extra-vascular  as  cartilage or dentine; for  its  fibres are not pene-
trated by vessels; and the nutriment required for the growth of its con-
tained matter is drawn by absorption through the myolemma.  But the
substance of Muscle is extremely vascular; the capillary vessels being
distributed in nearly  parallel lines, in  the minute interspaces betAveen
the fibres (Fig. 67); so that it is probable that there is no fibre, Avhich

                               Fig. 67.
Capillary network of Muscle.
is not in close relation with a capillary.  Hence  there  is  every provi-
sion for the active nutrition .of this tissue ; the arterial circulation bring-
ing the materials for its groAvth and renovation ;  whilst the venous con-
veys away the products of the Avaste or disintegration, Avhich is consequent
upon its active exercise.—The supply of blood  is not merely requisite
for the nutrition of the muscular tissue; but it also affords  a condition
Avhich is requisite for its action.  This condition  is oxygen.   It is not
enough  that blood should  circulate through the muscles; for that blood,
to exercise any beneficial influence, must be arterialized.    Consequently
the muscles of Avarm-blooded animals soon lose theirtcontractile poAver,
after the supply of arterial blood has been suspended, either by the ces-
sation of the circulation,  or by the want of aeration of the blood ; but
those of cold-blooded animals preserve their properties for a much longer
period, in accordance  with the general principle formerly stated,—that
the loAver  the usual amount of vital energy, the longer is its persistence,
after the withdraAval of the conditions on Avhich it is dependent.
  341.  The Muscles of Animal Life are, of all  the tissues Hcept the
Skin, the  most copiously supplied with Nerves.   These, like the blood-
vessels, lie on  the  outside of the Myolemma of each fibre ; and  their
influence must consequently be exerted through it.   The arrangement
of these nerves is shoAvn in the succeeding figure.   Their ultimate fibres
or tubes cannot be said to terminate anywhere in the muscular substance;
for after  issuing from the trunks,  they form a  series of loops, Avhich
either return to the same trunk, or join an  adjacent one (Fig. 68).   The
occasional appearance of  a termination  to  a nervous fibril is  usually
caused by  its dipping doAvn betA\-een the muscular fibres, to pass towards
another stratum ; but it appears from recent inquires to be sometimes
due to a subdivision of the central axis into a brush-like group of minute
fibrillae, Avhich  form a yet  minuter plexus around the muscular fibres.—
The non-striated muscles, hoAvever, are very sparingly  supplied with 
204      STRUCTURE  AND  ENDOWMENTS OF  ANIMAL TISSUES.

nerves; and these are derived (for the most part, if not entirely) from
the Sympathetic system, rather  than from  the Cerebro-spinal.

                              Fig. 68.
         Portion of Muscle, showing the arrangement of the motor nerves supplying it.

  342.  Every Muscular Fibre, of the striated kind at least, is attached
at its extremities to fibrous tissue; through the medium of  which it
exerts its contractile  power on the bone or other  substance, which it is
destined to move.   The muscular fibre  usually ends  abruptly by a per-
fect disk ; and the myolemma seems to terminate there.   The tendinous
fibres are attached to the whole surface of the disk; and seem to spread
themselves from it over the whole myolemma.   Thus the whole muscle
is penetrated by minute fasciculi of tendinous fibres; and these collect
at its extremities  into  a tendon.   Sometimes the muscular fibres  are
attached obliquely to the tendon, which forms a broad band that does
not subdivide ; this is seen in the legs of Insects  and Crustacea, in which
the muscular fibres have  what is called penniform arrangement, being
inserted into the tendon, on either  side,  like the laminre of a feather
into its stem.  The forms which different muscles present, have reference
purely to  the mechanical purposes, which they have respectively to
accomplish.  The elements are the same in  all, both as regards structure
and properties.
  343.  notwithstanding the energy of growth in Muscular tissue, it is
doubtful if it is  ever  regenerated, Avhen there  has been actual loss of
substance.   Wounds  of Muscles  are united by Areolar Tissue, which
gradually becomes condensed ; but its fibres never acquire any degree of
contractility.
  344.  It is probable that the pure Muscular Fibre is  identical in ulti-
mate composition, or nearly so, with the Fibrine of the blood.  It differs,
however, in this: that the fibrine of muscle is soluble in dilute muriatic
acid, whilst that of blood swells up without dissolving.  The fibrine of
veal bears a closer resemblance  to that of blood, than  to that of adult
muscle.   In ordinary muscle, the  solid  matter forms about 23  parts in
100:  the  remainder consisting of water.  The solid  matter contains
about 7i per cent, of fixed salts.
  345.  We now come to investigate the remarkable  property, which is 
CONTRACTILITY OF  ORGANIZED TISSUES.
205
the distinguishing characteristic of Muscular tissue ;—-that of contract-
ing on the application of a stimulus.   Some approaches to this property
are manifested by certain Vegetable structures.   Thus, if the small
enlargement  at  the base  of the footstalk of the leaf of the Sensitive
Plant be touched ever so slightly, the leaf will  be  immediately drawn
down by the  contraction of the tissue of the part irritated.  If the leaf
itself be touched, the same effect results, but apparently through a diffe-
rent channel; the tissue of the leaf contracts where it is touched, and
forces some of its fluid along the vessels of the footstalk into the upper
side of the little excrescence at its base,  by the distension of which the
leaf is  forced down.  In the Dionoea muscipula, or Venus's Fly-trap,
there is a similar transmission of the effect of the stimulus from one
part to another;  for the two lobes of the leaf, which form the trap, are
made to close together, when an insect settles  upon either one of three
spines  which project from the surface of each lobe, or when the points
of these spines are touched with any hard body.   Many other instances
of Vegetable movement might be brought together.   Some of them are
obviously produced by an enlargement or contraction of the  cells, occa-
sioned  by variations in the  amount of  fluid  they contain ; and these
variations depend upon the hygrometric state of the  atmosphere.  With
these we have nothing to do.   But there are many, in  which (as  in
the case of the Sensitive-Plant first mentioned) a stimulus applied to a
part occasions the  immediate contraction of its cells, and a  consequent
motion  in the same part.  And there are also several, in which the con-
traction produces motion in a distant part, as in the Dionoea;  but this
propagation  appears to be of  a simply mechanical character ; being
accomplished through the medium of fluid,  which is forced from one
part by its own contraction, and caused to distend another.
   346. From these examples, however,  it is evident that the property
of contractility is  not entirely restricted to the  Animal kingdom; and
we shall find that the simplest form under  which it  manifests itself  in
the Animal body, bears a close relationship with  that which is displayed
in Plants.  The non-striated fibre of the  alimentary canal, Avhich is sub-
servient to the functions of Vegetative life  alone, is called  into action
much more readily by a stimulus directly applied to itself, than it is in
any other mode.   Such is not the case, hoAvever,  with the striated fibre,
of Avhich the Muscles of Animal life are composed; this being much
more readily called into action  by a peculiar stimulus conveyed  through
the nerves supplying those muscles, than by any other  more  directly
applied to them.
   347. The  Contractility of Muscular  Fibre shoAvs  itself  under two
forms.   Its  most obvious and striking  manifestations  are  those that
occur in the  voluntary muscles and in the heart;  which, when in action,
exhibit powerful contractions alternating with relaxations.  The property
which is  concerned in these is distinguished  as Irritability.   On the
other hand, Ave find that  these same  muscles exhibit a tendency  to  a
moderate and permanent contraction, which is not shown by  them when
they are dead, and which cannot therefore be the result of elasticity  or
of any simple physical property;  this endowment, Avhich seems to exist 
 206      STRUCTURE AND ENDOAVMENTS OF ANIMAL TISSUES.

 in the greatest amount in certain forms of the non-striated fibre, is called
 Tonicity.
   348. That the Irritability of Muscles is a property inherent in them,
 and is in this respect analogous to the peculiar vital endowments of any
 other form of tissue, cannot be any  longer a  matter  of doubt; though
 many  physiologists have sought to show, that it is in some way  derived
 from the nerves.  Not only may an  entire Muscle be made to contract,
 by the application of  a proper stimulus, long after the division of the
 nervous trunks supplying it; but even a single fibre, completely  isolated
 from all its  nervous connexions, may  be seen to contract under the
 Microscope.   Moreover, in the non-striated muscular  fibre, it  is often
 difficult to excite contractions through the nerves at all,  when a stimulus
 directly applied to itself will immediately produce sensible and vigorous
 movements.   The energy of the contractile power depends in great part
 upon the state of  nutrition  of  the muscle; and this again is influenced
 by the degree in which it is exercised.    Now as the Muscles of Animal
 Life are all  excited to action, in the usual state of things, through the
 medium of their nerves,  it folloAVS that if the  nerves be paralysed, the
 muscles will  be seldom or never called  into use.   When  disused, they
 will receive very little nourishment;  the disintegrating  changes will not
 be  counterbalanced by reparative  processes; and  in consequence, the
 muscular structure will be gradually  so far impaired, as to lose its pecu-
 liar properties,—and will even, in time, almost totally  disappear.   Yet
 even after the  almost complete  departure of  muscular  contractility,
 through the  metamorphosis of the structure consequent upon disuse, it
 may be again recovered, if  the muscles  be called into exercise;  but the
 recovery of  the poAver is very sIoav, and proceeds pari passu with the
 improvement in the nutrition of the part, being more tedious in propor-
 tion to the length of the previous disuse.
   349. That the Irritability  of Muscular fibre belongs to itself, and is
 not  derived in any  way from the  nen7es, is further shown in  the fol-
 lowing manner.    If a set of muscles (as those of  the leg  of a Eabbit
 or Frog)  be   repeatedly  thrown  into  action by galvanism, until the
 stimulus will no longer  occasion  their  contraction, their irritability is
 then said  to  be  exhausted;  by rest,  however, it is  recovered,—the nu-
 tritive processes making good the loss previously suffered.  Now it has
 been shown by Dr. J. Reid, that  this  recovery  may take place, even
 after the  division of all  the nerves supplying the limb; provided  that
 the nutrition  of  the part be not interfered  with.  It has  been  further
 shoAvn  by the same excellent  Physiologist, that, if the nerves of a limb
 be divided, the loss or retention of the contractility of  the  muscles en-
 tirely depends upon the degree of exercise to which they are subjected,
 and consequently upon the nutrition  they receive.  The muscles of the
 hind-leg of a Rabbit,  whose sciatic nerve had been divided, Avere found
 to lose their contractility  almost  completely  in  the course of seven
 weeks.   They were much  smaller,  paler  and softer,  than the corre-
 sponding muscles of the opposite leg ; and they scarcely weighed more
than half as much as the latter.  Now when the nerves of both hind-
legs of a Frog  were cut, and the  muscles  of one of the limbs  thus
paralysed were daily exercised by a weak galvanic  battery, while those 
           INHERENT CONTRACTILITY OF  MUSCULAR FIBRE.        207

 of the  other  were alloAved to remain at rest, it was found  after the
 lapse of two  months that the muscles  of the exercised limb retained
 their original size and firmness, and contracted vigorously, whilst those
 of the  other  had  shrunk to one-half their former size.   Though the
 latter still retained their contractility, there could be no doubt that they
 would soon lose it, in consequence of the change already far  advanced
 in their physical structure; this change not being as rapid in cold-blooded
 animals, as in Birds  and Mammals.
   350. By these and other  facts,  then,  it may be regarded, as com-
 pletely proved, that  the Irritability of Muscles  is a vital endowment,
 belonging to  them in virtue of their peculiar  structure ;—that, so long
 as this structure is maintained in its normal condition  by the nutritive
 processes, so long is the property capable of being manifested;—but
•that any cause which interferes with the nutrition of a muscle, impairs
 .or destroys its irritability.   No  cause is  so effectual in doing this,  as
 complete disuse;  and  no means is so sure to produce complete  disuse
 of a muscle,  as the division of its nerve,  since its being called  into
 exercise  in any other  way is very improbable; hence the section  of
 the  nerve is almost certain to produce, in time, the loss of the contrac-
 tility of the muscle.   But if a means be devised, by which the muscle
 may still be  called  into action in  any other way,—as in Dr.  Reid's
 experiment just quoted,—its irritability is retained, because its regular
 nutrition is continued.
   351. We have noAV  to inquire, then,  into  the circumstances  under
 which  this peculiar  endowment  acts; or the means by which it may
 be called into operation, the mode in which the contraction takes place,
 and the conditions Avhich are necessary for its performance.   All Mus-
 cular Fibre, which has  not  lost its  contractility, may be made to  con-
 tract by a stimulus applied directly to itself; and this stimulus may  be
 of different kinds.  The  simplest  is the contact of a solid substance;
 thus Ave may  excite muscular contractions by  simply touching the fibre,
 just as we cause contraction in the  tissue of the Dionoea or  Sensitive
 Plant.   Most substances of  strong  chemical  action, such as  acids and
 alkalies, will  call forth the contractility of muscular fibre, when applied
 to it;  and the same  result is produced by heat, cold, and electricity,—
 the  last named agent being the most powerful of all.   The effect of the
 application of any of these stimuli varies  considerably, according to the
 kind of  Muscle on which it is  exerted.   If  we irritate a portion of a
 muscle composed of striated fibre (any one of  the voluntary muscles, for
 example), the fasciculus  of  fibres Avhich is  touched  will immediately
 contract, and that one only; and  the contracted fasciculus will soon
 relax,  without communicating its movement to any other.
   352. If we irritate a portion  of non-striated fibre,  however, as  that
 of the  Alimentary canal, the fasciculus which is stimulated will contract
 less suddenly, but ultimately to a greater amount; its relaxation will
 be less speedy;  and before it takes place, other fasciculi  in the neigh-
 borhood  begin  to contract; their   contraction   propagates   itself  to
 others; and  so  on.   In this manner, successive contraction and relax-
 ations  may be produced through a considerable  part of the canal, by a
 single  prick with a scalpel;  a sort of wave of contraction being trans- 
208     STRUCTURE AND ENDOWMENTS OF  ANIMAL TISSUES.

mitted in the direction of its length, and being followed by relaxation.
Again, in the Muscular  structure of the Bladder and Uterus, powerful
contractions are excited by irritation, and these produce a great degree
of shortening : but they do not alternate in the healthy state with any
rapid and decided elongation; whilst, on the other hand, an  irritation
applied to one spot causes more extensive contractions, than are seen to
occur as its immediate  consequence in the  preceding cases.  In the
Heart, the Muscular structure of a large part  of the  organ is thrown
into rapid and energetic contraction, by a stimulus applied at  any one
point; and  this contraction is speedily followed by relaxation.   And
in the fibrous tissue of the middle coat of the Arteries, the contraction
takes place  rather after the manner of that of  the  bladder and uterus,
and a prolonged application of the stimulus is often necessary to produce
the effect; but when the  contraction commences, it produces a consi-
derable degree of  shortening, which takes place in other fasciculi than
those directly irritated, and does not speedily give way to relaxation.
   353.  On  the  other hand, when the stimuli  which  excite  muscular
contraction  are applied to the Nerve, which supplies a voluntary muscle
composed of striated fibre, they produce a simultaneous contraction in
the  whole muscle;  the  effect  of the stimulus  being  at  once exerted
upon every part of it.   In the ordinary action of such muscles, the
nervous system  is  always the channel  through which they are  called
into play, whether to carry into effect the determinations of the  mind
(§ 391), or to perform some office necessary to  the continuance of life,
such as the movements  concerned in Respiration (§ 394).   The nerves
of the striated fibre are all derived  at once  from  the  brain or  spinal
cord.  The  ordinary actions of the  non-striated fibre,  on the contrary,
are executed in  respondence  to stimuli  applied directly to  themselves.
It is so difficult to excite contractions in it  through the medium of its
nerves,  that many Physiologists have denied the possibility of doing so;
and the nerves lose their power of conveying the influence  of stimuli
very soon after  death, although the contractility of the  muscles may
remain for a considerable time.  The nerves of the non-striated fibre
are chiefly those belonging to the Sympathetic  system; but, as will be
shown hereafter (chap, xii.), those  which excite motion  are probably
derived in reality from the Cerebro-spinal  system, through the commu-
nicating branches which unite the two.
   354.  When a Muscle is thrown into  contraction, its bulk  does not
appear  to be at all affected.   Its  extremities  approach, so that it is
shortened in the direction of its  fibres; but its diameter  enlarges in
the  same proportion.  It  was formerly supposed that  the ultimate
fibres, in the act of contraction,  threw themselves into zigzag  folds;
but  this is  now well-ascertained  not to be  the  case.  The fibre, like
the entire muscle,  preserves its straight  direction  in  shortening, and
increases in  diameter.   The  fibrillae  themselves, as already mentioned
(§ 336), exhibit an evident change, in regard to the distances of their
successive light and dark portions;  and the fibre, which is made up of
these, exhibits, in  its  contracted  state,  a very  close approximation of
the transverse striae; to  such  an extent that they become two,  three,
or even four times as numerous  in  a given length, as they are in a 
ACT OP  MUSCULAR  CONTRACTION.
209
similar length of a non-contracted fibre.   According to Mr. Bowman's
observations, the contraction usually commences at the extremities  of
a fibre;  but it  may occur  also  at one  or  more  intermediate points.
The first appearance of contraction is a  dark spot, caused by the ap-
proximation  of  the  striae, and this gradually extends itself, so as  to
involve a greater or less proportion of the  length  of the  fibre.   The
approximation  of the solid  portions  forces  out the  fluid, which  was
previously contained amongst the fibrillae;  and this is seen  to lie in
bullae or blebs beneath the myolemma, which is drawn up into Avrinkles.
   355.   The successive  stages of the act of contraction can  only  be
thus observed when  it takes place very" slowly, as in  the rigor mortis,
or sIoav  contraction  after  death, the  phenomena of Avhich will be pre-
sently noticed (§ 367).   But the  resulting change  in muscular fibres,
which have been made  to contract by galvanism or any other stimulus
is essentially the same.   This may be best seen in transparent Entozoa,
Crustacea,  and  others  among the lower Articulated  Animals, Avhilst
alive.   Again, in persons who  have died from  Tetanus, a considerable
number  of the fibres are found to have been ruptured by violent spas-
modic action; the contractile force, called into action by the powerful
stimulation of the nerves, having  overcome  the tendency of the fibre:
and in  such cases,  the same  approximation  of the  transverse striae
and proportional  increase  in the diameter  of the  fibre,  are to  be
observed.
  356. It  appears that, even when considerable force of contraction is
being exerted, the  whole fibre is  seldom or  never in  contraction  at
once; but that  a continual interchange is  taking place amongst  its
different  parts,—some  of  them  passing  from the contracted to the
relaxed state as shown  by  the separation of the  transverse  striae,—
whilst others are taking up the duty, and  passing  from the relaxed to
the contracted condition, as shoAvn by the approximation of the striae.
But it is not only among  the different parts of the  individual fibres,
that this interchange seems  to take place.    There is good reason to
believe, that, when a muscle  is kept in a contracted state, by  an effort
of the will, for any length of time, only a part of its  fibres are in  con-
traction  at any one time; but that a constant interchange of condition
takes place amongst  them, some  contracting while  others are relaxing
so that the entire muscle remains  contracted,  whilst the state of every
individual fibre may have undergone a succession of alterations.   When
the ear is applied to  a muscle in vigorous action, an exceedingly rapid,
faint, silvery vibration is heard, which seems to be attributable to this
constant moArement in its substance.
  357. Thus it  appears that the prolongation of the  contraction of a
muscle through any length of time, is  not opposed to the fact that,  in
the individual fibres, relaxation  speedily follows  contraction; but  is
only a peculiar manifestation of it.   The ordinary movements of the
Heart exhibit a different manifestation; its fibres contracting  simulta-
neously,  and relaxing together, instead of alternating amongst them-
selves like those of  a voluntary  muscle.   The occasional zigzag ar-
rangement of the fibres, which has been supposed to be their contracted
state, is really dependent upon the approximation of  their extremities, 
210      STRUCTURE AND  ENDOAVMENTS  OF ANIMAL TISSUES.

in consequence  of the contraction of some neighboring  fibres, whilst
their own condition  is that of relaxation.   It  may be  artificially  pro-
duced by bringing together the two extremities .of a  fasciculus, after
the irritability of  the  fibre has ceased;  so that the flexure at deter-
minate  points must  be  owing  simply to the physical  arrangement  of
the parts,—perhaps to the passage of nerves or vessels in a transverse
direction.
   358.  We have now  to consider the conditions which are requisite for
the manifestation of Muscular Irritability.   It has been already pointed
out, how close is the dependence of the property upon the due nutrition
of the tissue; but the property'cannot be  long  exercised  except under
another condition, which is consequently of almost equal importance,—
the circulation of  arterial  blood through the substance of the muscle.
The length of time during which  the contractility remains,  after the
circulation has ceased, has been shown by Dr. M. Hall to vary inversely
to the activity of the respiration of the animal.  In cold-h\ooded animals,
the standard of whose respiration is  low, the  contractility remains  for
many hours after death, even in the voluntary muscles; and the muscles
of organic life retain it Avith great tenacity. Thus the heart of a Frog
will go  on pulsating for  many hours  after its removal from the body;
and the heart of a Sturgeon, which had been inflated with  air and hung
up to dry has been seen to continue beating, until  the auricle had be-
come absolutely so dry as  to rustle during  its movements.   An exceed-
ingly feeble Galvanic  current is sufficient to excite the muscles of these
animals  to contraction ; so that Matteucci, in his experiments upon
Animal Electricity, has been accustomed to use the prepared hind-leg
of a Frog as  the best indicator of the passage of an  electric current.
Among warm-blooded animals whose  respiration is  vastly more active,
the duration of the irritability is  proportionally abbreviated;  and the
muscles  of Birds whose  respiration  is  peculiarly energetic,  lose this
property at an earlier period after the,cessation of the circulation, than
do those of Mammals.  From experiments on  the  bodies of  executed
criminals who were previously in good health, Nysten ascertained that,
in the Human subject, the contractility of the  several muscular struc-
tures, as tested  by Galvanism, departs in the following time and order:
—the left  ventricle of the  heart first; the intestinal  canal  at the end of
45 or 55  minutes ; the urinary bladder  nearly  at the  same time ; the
right ventricle after the lapse of an hour ; the oesophagus at  the expira-
tion of an hour and a half;  the iris a quarter of an hour later ; and lastly,
the ventricles of the  heart, especially the   right, which in one instance
contracted 16| hours after death.
   359.  That  the circulation of  arterial or oxygenated blood  through
the muscles, is the essential condition of the continuance  of their irri-
tability  appears from  this,—that after the  general death of the system,
and even after the removal of the brain and  spinal  cord, the  muscles
will preserve their  irritability, and  the action of the heart itself will
continue for  a  long  time, provided  that  the  circulation be  kept up
through the  lungs  by artificial respiration on  the principles hereafter
to be explained (§ 688).  But if, Avhilst the general  circulation  con-
tinues, the  circulation through  a  particular  muscular  part be inter- 
       DEPENDENCE OF MUSCULAR CONTRACTION  UPON  OXYGEN.    211

rupted, that organ will lose  its contractility earlier  than usual.   Thus
it has been shown  by Mr. Erichsen, that,'if the coronary arteries (sup-
plying the substance of the heart) be tied in  a dog  or rabbit, after the
animal has been pithed, and the circulation  is  being maintained by arti-
ficial respiration, the pulsation of the heart  will only go on for about 23
minutes  after the  ligature has been applied,  or about 33 minutes after
the death of the animal;  instead of continuing for 90 minutes, Avhich  it
Avill  do  under  other  circumstances.   Further if blood charged  with
carbonic  acid instead  of with  oxygen, circulate  through the muscles,
their irritability is  speedily impaired, and is  even destroyed.   This  is
best seen, when animals are killed by being  caused to breathe an atmo-
sphere highly  charged  with carbonic  acid;   the irritability of their
muscles  departing  as soon as they are dead.   In fact, the  destruction
of the irritability of the heart, by the circulation of venous blood through
its substance, is one of the immediate causes of death.   A similar effect
is produced by the respiration of other gases, which are either poisonous
in themselves, or which prevent the interchange of  carbonic acid and
oxygen, Avhich ought to take place in  the  lungs.  On the  other hand,
Avhen animals have been  made  to  respire oxygen, and their blood has
been consequently highly arterialized, the contractility of their muscles
is retained  for a longer time than usual.
   360. Hence we may conclude the presence of oxygen in the blood to
be one of the conditions  of  muscular contraction ; although it is much
less essential in the case of cold-blooded, than  in that of warm-blooded
animals.   It is interesting to remark, that the muscles of hybernating
warm-blooded Mammals are reduced for a time to  the level  of those of
cold-blooded animals; their contractility being retained almost as long
as that of  the latter;—thus confirming  the general principle already
stated, as to the relation  betAveen  the  amount of respiration, and the
duration of the irritability.
   361.  The Muscles, as Ave have seen, are largely supplied with blood;
and the flow of blood into them increases Avith  the use that  is made of
them.   The demand for nutrition is obviously  augmented, in proportion
to the activity of the exercise of the Muscular system ; for the slightest
observation suffices to sIioav, that a much smaller amount of nourishment
is sufficient to sustain the  body in  its normal condition, when the Mus-
cular system is not actively exercised, than  when it  is in energetic
operation.   The quantity Avhich is ample for  an  individual leading an
inactive life, is far too little for  the same person in the  full exercise of
his muscular power.—Again, there is evidence  derived from  observation
of the  relative  amount of the  solid matters  excreted  from the  body
under different circumstances (§ 731), that a waste or disintegration of
the' muscular tissue takes  place, whenever it is actively employed;  and
this in a degree strictly proportional to the  amount of force  Avhich it is
called upon to exercise.   In  fact, it would appear  that  this waste  is  a
necessary consequence of  the exercise of  the muscle; every  act of con-
traction involving the death and decomposition of a  certain  amount of
tissue.  And as the presence of oxygen is ahvays  necessary for the
decomposition of organic substances, so we do  find that the  penetration '
of the muscular tissue by oxygenated blood is  essential to the manifes- 
  212      STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES.

  tation of its contractile power.—Every act of contraction, then, may be
  said to involve the death of a  certain amount  of muscular tissue;  and
  on the principles formerly laid down  (chap. I., Sect. 3), we may look
  upon the development of contractile  power as an expenditure of the
  vital force  which that  tissue  previously  possessed, and which ceases
  to  exist  as  such, when  the  elements of  the tissue enter  into neAV
  combinations.
    362. On the  other  hand, the muscular substance is repaired by an
  act  of  nutrition, at the expense  of the fibrine supplied  to  it by the
  circulating  fluid.   There  are  certain  muscles, as  the  heart, and  the
  muscles of respiration, whose action is necessarily constant;  and  their
  reparation must take place as unceasingly as their Avastc.  In these
  muscles no sense of fatigue is ever experienced.  But in the muscles
  which are usually put in action by the will, this is not the case.   Any
  prolonged exertion of them induces fatigue; and this fatigue is an evi-
  dence of their impaired condition, and of the necessity of rest to impart
  to them a renewal of vigor.   The rest of  muscles is  essential to the
  recovery of their  powers; and this recovery is  due  to the nutritive
  operations,  which  then take place unchecked,  and  which  repair  the
  losses previously sustained.   The permanently-increased Hoav of blood
  to a muscle, Avhich takes place when it is  continually being  called into
  vigorous action, is  thus  on the one hand occasioned by the demand for
  oxygenated blood created by its use, whilst  on the other hand it tends
  to increase the power of the muscle by an augmentation of its nutrition.
  Hence it is, that,  the  more a muscle is  exercised, the more vigorous
  and more bulky  does it become.  This  is equally  the case, whether
  the  exercise of the muscle be voluntary or  not.  We  see examples of
  it in the arms of the smith and  in the legs of the opera-dancer;  and
  we have  a  still more  striking manifestation  of  it in  those cases, in
  which an obstruction to the exit of urine through  the urethra, has called
  for  increased efforts on the  part of  the  bladder, the  continuance of
  which gives rise  to an  extraordinary augmentation in  the thickness of
  its muscular coat.
    363. Thus we see that the property of Irritability is a vital endow-
  ment peculiar to muscular tissue,  and dependent for its existence upon
  due  nutrition of that tissue; that it may be called into exercise by cer-
  tain stimuli, applied either to the  muscle itself, or to the nerve supply-
  ing  it, provided that the muscle be also permeated Avith  oxygen; that
  it may be exhausted by repeated stimulation,  but is then recovered by
  rest, provided that there be no obstacle to the  nutrition of the muscle;
  that the nutrition of the  muscle  is impaired by continued repose,  and
  that its irritability diminishes in  the same proportion; that the nutri-
  tion is  increased by frequent use,  and that the poAver of the muscle then
  augments in like degree; and finally, that  the departure of muscular
  power, which ensues  upon the general death  of the system, is depen-
  dent in part upon  the cessation of the supply  of oxygen, and in part
  upon  changes  in the composition of the  muscle itself,  which  are  no
  longer compensated by the functions that keep it in its normal condition
*  during life.—The rapidity of  these changes is  the greatest in warm-
  blooded animals, in which also the muscular irritability is most depen- 
                TONICITY OF  MUSCLES.—RIGOR MORTIS.           213

 dent upon  the  presence of oxygen  in the muscular substance ;  conse-
 quently the irritability departs after death much more speedily in these
 than in cold-blooded animals.
   364. We have now to consider the other form of Contractility ; which
 produces a constant  tendency to contraction in  the  Muscular fibre, but
 which  is so far different from simple Elasticity, that it abates after
 death,  before decomposition has taken place.  This Tonicity manifests
 itself in the retraction Avhich takes place in the ends of a living muscle,
 when it is  divided ;  the retraction being permanent, and greater  than
 that of a dead  muscle.   It also shows  itself in the permanent flexure
 of joints, when, by paralysis  of the  extensors, the tonic contraction  of
 the flexors  is  not  antagonized.   In the  healthy state, it would seem  as
 if the tonicity of the several groups of muscles was so adjusted, as to  be
 in mutual counterpoise;  but  the balance is  destroyed, when, in  conse-
 quence of paralysis, or of impaired nutrition from other causes, the
 tonicity of one  set is Aveakened.  This is the case, for  example,  in the
 lead palsy; in  which the extensors of the forearm and hand lose their
 power, so that the tonic contraction of the flexors keeps the fingers con-
 stantly bent upon the palm.  It Avould seem, hoAvever, that the tonicity
 of the flexors  is usually greater  than that of the extensors ; as the
 former predominate, when  all are equally withdrawn from the control of
 the nervous system,  in profound sleep.
   365. The Tonicity is much greater, relatively to  the amount of irri-
 tability, in the  non-striated, than in the striated fibre; and it is parti-
 cularly remarkable  in  the fibrous coat of the arteries, in which it is
 difficult to  procure any  decided indication of irritability by the applica-
 tion  of stimuli.   It is by this tonicity of the Avails of the arteries, that
 they are kept in a  state of constant moderate  contraction  upon their
 contents ;  and that, when they are emptied, they contract until the
 tube is nearly obliterated.   If its amount be too great  (as sometimes
 happens) the  artery approaches the condition of a rigid tube ; which,  as
 will be shoAvn hereafter (§ 583), is unfavorable to the regularity of the
 flow of blood through it, though the rate is increased.  On the other
 hand, if it be unduly diminished,  the  circulation is retarded, by the
 tendency of the arterial walls to yield too much to the  pulse-Avave.
   366. This  property is  very greatly  affected  by temperature ;  being
 diminished by warmth  and increased by cold.   Thus  when an artery
 is exposed  to the  air for  some time,  the lowering of its  temperature
 occasions  its contraction to such an extent, that its tube may be almost
 obliterated.  And in the operation of crimping fish, immersion of the
 body in cold  water,  after the muscles have  been divided, increases the
 tonic contraction of the muscles, and thus improves the firmness of their
 substance,  which it is the object of this  operation to produce.
1   367. The Rigor  Mortis, or death-stiffening of the  muscles, is pro-
! bably to  be regarded  as  a manifestation of this property, occurring
 after all  the  irritability of the muscles  has departed, but  before any
 putrefactive change  has commenced.  This  phenomenon is rarely ab-
 sent ; although  it may be so  slight, and may last for so short a time,  as
 to escape observation.  The  period which elapses before its commence-
 ment is as variable as its duration ; and both seem to be dependent 
214      STRUCTURE  AND  ENDOAVMENTS OF  ANIMAL TISSUES.

upon the vital condition of the system at the time of death.  When it
has been weakened or depressed by previous  disease, the irritability of
the muscles speedily  departs;  and  the  stiffening comes on early, and
lasts but a short time.  Thus, after death, from Typhus, the limbs have
been sometimes known to stiffen within 15 or 20 minutes.   On the other
hand, when the general vigor of the system  has not  been previously
impaired, and death has resulted from some sudden cause, the irritability
of the muscles is of longer duration, and their stiffening is consequently
deferred.   The commencement of the rigidity usually takes place within
seven hours after  death;  but twenty or even thirty hours  may elapse
before it  shoAvs itself.   Its general duration  is from  tAventy-four to
thirty-six hours;  but it may pass off much more rapidly, or it may be
prolonged through several days.   It affects all the muscles composed of
the striated fibre with nearly the same intensity ; except that the flexors
usually contract  more strongly than  the extensors (as  in  sleep), the
fingers being  closed upon the palm, the hand bending  on the fore-arm,
and the lower jaw being drawn  firmly against the upper.   And it even
manifests itself in  muscles that have been thrown out of use by paralysis,
provided that their nutrition has not been seriously impaired.
   368.  This tonic contraction,  however, is most remarkably manifested
in the  non-striated fibre ;  and especially in the heart and blood-vessels.
As soon as"the muscular walls  of the several cavities lose their irrita-
bility, they begin  to contract forcibly upon their contents, and thus be-
come  stiff and firm,  although  they were previously flaccid.   In  this
manner, the ventricles of the  heart, which  are the first parts  to lose
their irritability,  become  rigid  and contracted w7ithin an  hour or two
after death ; and usually remain in  that state for ten  or  twelve hours,
sometimes for tAventy-four or thirty-six, then again becoming relaxed
and flaccid.  This rigid contracted state  of the heart, in which the Avails
are thickened and  the cavities  diminished, was  formerly supposed to be
a result of disease, and was termed concentric hypertrophy; but it is
noAV known  to be the  natural condition of the organ, at the period when
the rigor  mortis occurs in it.  The contraction of the arterial  tubes is
so great, as to produce for the time a great diminution  in their calibre;
and this doubtless  contributes to the passage of the blood from the arte-
rial into the venous system, Avhich almost invariably takes place within
a feAv hours after death.   The  arteries then  enlarge again, and become
quite flaccid, their tubes being emptied of the previous  contents ; and it
Avas from  this circumstance, that the ancient  physiologists were led to
imagine that the arteries are not destined to carry blood, but air.
   369. As soon as the  Rigor  Mortis  departs,  the muscles  pass into a
state of decomposition; in  fact, it is by the  commencement of decom-
position,  that the  cessation of this vital  property is  occasioned.  Thus
we may regard the Rigor Mortis as the last act of the Muscular  Con-
tractility : and in this respect it corresponds with the coagulation of the
blood, which also is the closing act of its life, when it is drawn from the
living body, or has ceased to circulate (§ 184).  There are, indeed, many
remarkable points  of correspondence between the two phenomena ; which
have induced some physiologists to believe, that rigor mortis is in fact
nothing else than the  coagulation of the blood  in the muscles.   It has 
CONTRACTILE FORCE OF  MUSCLES.
215
been shown by Mr. BoAvman, however, that the stiffening of the muscles
after death is due to the permanent contraction of their component fibres,
and  that  the  coagulation of the blood can  have nothing to do with it.
Nevertheless,  this contraction may be considered as being, for the mus-
cular fibre, a phenomenon  of  very  much  the same kind as the coagula-
tion  of the fluid fibrine of the blood,—especially resembling the subse-
quent contraction of the clot, which takes place gradually, within a few
hours after its separation.  The causes which prevent the coagulation
of the blood after death (§ 187), usually prevent also this last manifes-
tation of the tonicity of the muscles ; their vitality being completely de-
stroyed, like that of the blood, by sudden  and powerful shocks operating
on the nervous  system, or by the complete exhaustion  consequent  upon
violent and long-continued exertion, as Avhen animals are run to death.
And again, the  tonicity of muscles  survives the  freezing process; mani-
festing  itself  by contraction  and  rigidity, in a  muscle  that has  been
frozen immediately after death, and is subsequently thaAved ; just as the
peculiar properties of the fibrine of the blood cause its  coagulation upon
being  thawed, if it have  been frozen immediately  upon  being draAvn
from  the vessels.
   370.  The power by Avhich the elements of Muscular fibre are caused
to approach one another  in the  exercise  of their Contractility, differs
from any  other  with which we are acquainted.   Its complete dependence
upon  the life of the tissue is remarkably shown by the fact (ascertained
by Valentin), that, after  the  cessation of the  irritability, the  muscles
tear with a far less Aveight, than they were previously able to draw, when
excited by galvanism ; so that their contractile force is much greater than
that,  which the simple cohesiveness of the  tissue can sustain.  More-
over,  it has been shown by the experiments  of  Schwann, that the con-
tractile force is  greatest, Avhen the muscle is most extended; so  that,
with the same stimulus, it can overcome a greater resistance by its con-
traction, when it has been previously stretched  to its full length, than it
can when  it has been already  in part shortened  by  the exercise of its
contractile force.   The power diminishes  progressively with the  further
shortening of the muscle; until at last  no further contraction can be
produced  by  any stimulus,  the extreme  limit having  been reached.
Hence it seems  as if the contractile force  of Muscles differs completely
from  other forms of Attraction, as those of Gravitation, Electricity, &c.;
since it is the universal law of their operation, that the force increases,
in proportion to the decrease betAveen the squares of the distances be-
tween the attracting bodies; whilst, in the case of muscle, the force de-
creases, in proportion as the distance  betAveen  the attracting particles
decreases.  But it is to be remembered that the  law of  attraction just
quoted supposes the particles  to be quite free to approach one another;
and this they obviously are not in the contraction of a  muscle, since the
approach cannot take place without a change of place betAveen the solid
and fluid  elements (§ 354).   Hence it is  difficult, if not impossible, to
discover the law, Avhich shall  truly express the nature of the attraction
betAveen the ultimate particles of Muscle at different distances; but the
law discovered  by Schwann expresses the force  actually developed, at
the different states of muscular contraction. 
216      STRUCTURE AND  ENDOWxMENTS OF ANIMAL  TISSUES.

   371.  It has been ascertained by the researches of MM. Becquerel and
Breschet, that the temperature of a muscle rises, when it is thrown into
energetic contraction.  The increase is ordinarily but about 1° Fahr.;
but it may amount to twice as much,  if the muscle be kept in action for
some time, as in the exercise of*saAving.  Tavo causes maybe assigned
for  this increase.   It may  depend upon  the chemical changes which
take place in the  Muscle, as a necessary  condition  of the production
of its  force (§ 361); or  it may  be the result of the  friction  taking
place betAveen different parts, during  the constant interchange of their
actions (§ 356).   Perhaps  both these causes  concur  in producing  the
effect.
   372.  The Nervous System, taken as  a whole, is the instrument of all
those operations, which peculiarly distinguish the Animal from the Plant;
and  it serves many additional  purposes, connected with the Organic or
Vegetative functions, which the  peculiar  arrangements of the Animal
body involve.   Wherever  a distinct Nervous  System  can be made  out
(which has not yet been found possible in the lowest Animals), it con-
sists of two very different  forms of structure; the presence of both of
which, therefore, is essential to our idea of it  as a Avhole.  We observe,
in the first place, that it is formed of  trunks, which are distributed to
the different parts of the body, especially to the muscles and to the sen-
sory surfaces;  and  of the ganglia, which  sometimes appear merely as
knots or enlargements on these  trunks, but Avhich in  other cases, have
rather the character of central masses, from  which the trunks proceed.
Now it is easily established by  experiment,  that the active powers of
the nervous system reside  in the  ganglia; and that the trunks serve
merely as conductors of the influence,  which is to be propagated towards
or from them.   For if a trunk  be divided in any part  of its course, all
the parts to which the portion  thus cut off from  the ganglion is*distri-
buted, are completely paralysed; that is no impression made upon them
is felt as a sensation, and no motion can be excited in  them by any  act
of the mind.   Or if the substance of  the ganglion be destroyed, all  the
parts,  which are  exclusively supplied  by nervous  trunks proceeding
from it, are in like manner paralysed.  But if, Avhen a trunk is divided,
the portion still connected Avith the ganglion be pinched, or othenvise irri-  *
tated, sensations are felt, Avhich are referred to the  points supplied by
the separated portion of the trunk ; Avhich shows that the part remaining
in connexion with  the ganglion is still capable  of conveying impressions,
and  that the ganglion  itself  receives  these impressions and makes them
felt as sensations.   On the other hand, if  the separated portion  of  the
trunk be irritated, motions are  excited in the  muscles which it supplies;
showing that it is still capable of conveying the motor influence, though
cut off from the  usual source of that influence.
   373.  When Ave  minutely examine the trunks of the nerves, we find
that they are composed, in the first  place, of a Neurilemma or nerve-
sheath, consisting of areolar tissue; the office  of Avhich is evidently that
of protecting the nerve-tubes, and of  isolating them  from the surround-
ing structures, at the same time that it  allows blood-vessels to pass into
the interior of the  trunk.  From the interior of  the  neurilemma, thin
layers of areolar tissue pass into the midst of the enclosed bundle of 
STRUCTURE OF  NERVOUS FIBRES  AND TRUNKS.        217
nervous fibres; separating it into numerous smaller fasciculi, which are
thus bound together and supplied with blood-vessels.   The capillaries
are distributed very much on the same plan as those of Muscular tissue
(Fig. 66); the network being  composed of straight vessels, which  run
along  the  course of the nerve, betAveen the nerve-tubes, and which  are
connected  at intervals by transverse vessels. When the neurilemma has
been removed, and the trunk has been separated into its component
fasciculi,  we  may still further  subdivide  the  fasciculi themselves by
careful dissection, until Ave arrive at  the ultimate Nervous Fibre, Avhich
is the  essential element of the  structure.  Two forms of this fibre exist
in the nerves of higher animals, bearing a considerable analogy to  the
tAvo forms of  the muscular fibre; one being  known as the  tubular;
Avhilst the  other, which seems to  be in a state of less complete develop-
ment,  is  distinguished  as  the gelatinous.  These require a separate
description.
   374. The Nervous fibre, in its most  complete form, is distinctly tubular.
It  is composed externally  of  a  very  delicate  transparent membrane,
Avhich is apparently quite homogeneous: this is obviously analogous to
the myolemma of the Muscular fibre, and  serves, like it, to isolate the
contained substance most completely  from surrounding structures. This
membranous tube is not penetrated by blood-vessels, nor does it branch
or anastomose with others;  and there is reason to  believe it to be con-
tinuous from the origin to the termination of the nervous trunk. Within
the tube is a holloAV cylinder, of a material knoAvn as  the White sub-
stance of Schwann, which differs in composition and refracting power
from the matter that occupies the centre of the  tube, and of  Avhich the
outer and inner boundaries  are marked out by tAvo distinct lines.  And
the centre  or axis of the tube  is  occupied  by a transparent substance,
Avhich  is termed the axis cylinder.  There is reason to believe that this last
is the  essential component  of  the nervous fibre; and that  the  hollow
cylinder which surrounds it, serves, like the external investment,  chiefly
for its complete isolation.   The Avhole  of the matter contained  in  the
tubular sheath is extremely soft;  yielding to very slight pressure.  The
tubular sheath itself varies in density in different parts; being stronger
in the  nervous  trunks,  than in the substance of the brain  and spinal
cord.  In the former, it is not  difficult  to  show, that  the regular form
of the nerve-tube is a perfect cylinder; though a little disturbance will
cause  an alteration in this,—a  small  excess of pressure in one  part
forcing the contents of the  tube  towards another, Avhere they are more
free to distend  it, and thus producing a SAvelling.  The greater delicacy
of the tubular sheath in the latter, causes this result to take place Avith
yet more readiness; so that  a very little manipulation, exercised upon
the fibres of the brain  and spinal cord, or on  those of special sense
occasions them to assume a  varicose or  beaded  appearance, which, when
first observed by Ehrenberg, Avas thought to be characteristic of them.
When  the fibres of these parts, hoAvever, are examined without any such
preparation, they are  found to be as cylindrical as the others.—The
diameter of the tubuli is  usually  between l-2000th and l-4000th of an
inch.   Sometimes, hoAvever,  it is as much as l-1500th ; and occasionally
as little as  l-14,000th.  They are larger in  the nerve-trunks than in the 
218      STRUCTURE  AND  ENDOAVMENTS OF ANIMAL  TISSUES.

brain ; and  they diminish  in the latter  as  they approach the cortical
substance.   The fibres of the nerves of special  sense are smaller than
the average, in every part  of their course.
   375.  The gelatinous fibres cannot  be shown  to consist of the same
variety of parts as the preceding; no tubular  envelope can  be  distin-
guished ; and  the white substance of Schwann  seems Avanting.   They
are flattened, soft and homogeneous in their appearance, bearing a con-
siderable resemblance to the unstriped Muscular fibres; and, like them,
they contain numerous  shell-nuclei, which  are arranged with tolerable
regularity.  These nuclei  are  brought into vieAV by acetic acid^ which
dissolves the rest of the fibre, leaving  them unchanged.   The gelatinous
fibres are usually of smaller size than the  tubular, their  diameter ave-
raging between the l-6000th and the.l-4000th of an  inch;  and they
sometimes show a disposition to split  into very delicate fibrillae.   Being
of a yellowish-gray color,  they have  been sometimes distinguished as
the gray fibres.   These two classes of fibres have been supposed to  be
essentially distinct  in character and office;  the "tubular" having been
regarded as  ministering to the Animal functions  of  sensation  and
motion;  and the "gelatinous" as connected with the Organic or nutri-
tive operations.  The facts Avhich   will be  presently stated  (§ 388)
regarding their origin, however, as well as their joint existence in  almost
every nerve, are decidedly adverse to this view ;  and Ave shall find rea-
son to consider them as  differing chiefly in grade of development.  In
fact it appears  that the very same fibre may  be "tubular" in one part
of its course, and "gelatinous" in another.
   376.  The Nerve-fibres appear to run continuously, from one extremity
of a nervous cord to the other, without anything like union  or anasto-
mosis ; each ultimate fibre probably  having its distinct office, which it
cannot share with another.  The fasciculi, or bundles of fibres, however,
occasionally intermix and exchange  fibres with each  other;  and this
interchange may take place among either the fasciculi of the same trunk,
or among those of different trunks.   Its object is evidently  to  diffuse
among the  different  branches  the endowments of a particular set  of
fibres.   Thus we shall hereafter see that, in  all the Spinal Nerves  of
Vertebrata, one  set  of roots ministers to sensation, and  another  to
motion;  the sensory fibres are principally distributed  to the skin, and
the motor fibres to the muscles ; but every branch contains both sensory
and motor  fibres, which are brought  together by the interlacement  of
those connected with both sets of roots.   In the head, Ave have some
nervous trunks which have sensory roots alone; and others which have
motor  roots only ;  these in like manner  acquire each other's functions
in some  degree by an interchange  of  filaments,—the  sensory trunk
receiving motor fibres,  and the  motor trunk  receiving sensory fibres.
An interchange of this kind, upon a very extensive  scale, takes place
between  the Cerebro-spinal system,  Avhose ganglionic centres  are  the
brain and spinal cord, and the Sympathetic system,  Avhose centres con-
sist of a number of scattered ganglia.   The former sends a large number
of fibres into the latter, by the twigs of communication near the  origins
of the Spinal n'erves, as Avell  as by their  connecting branches; whilst 
            COMMUNICATIONS BETAVEEN  NERVOUS TRUNKS.        219

 the latter sends a smaller number of fibres into the former, these being
 chiefly of the gelatinous kind.
   377.  Sometimes we find the fasciculi of several distinct trunks united
 into an  extensive plexus ; the sole  object  of which appears to be,  to
 give a more advantageous distribution to fibres, which all possess corre-
 sponding endowments.  Thus  the brachial plexus  mixes  together the
 fibres arising by five pairs of roots, on either side, from the spinal cord;
 and sends off five principal trunks to supply the arm.   Noav, if each of
 these trunks had arisen by itself, from a distinct segment of  the spinal
 cord, so that the  parts on which it is distributed had only a single  con-
 nexion with  the  nervous centres,  they would have been much more
 liable to paralysis than they are.  By means  of the plexus, every  part
 is supplied  with  fibres arising from each of the  five segments  of the
 spinal cord ; and the  functions of  the  whole must, therefore, be  sus-
 pended, before complete paralysis of any part could occur from a cause
 which operates above the plexus.   This may be experimentally shown
 on the Frog, whose crural plexus is formed by the interlacement of the
 component fasciculi of three trunks on each side ; for section  of the
 roots of  one of these produces little effect on  the general movements of
 the limb; and  even when two are divided, there is no  paralysis of any
 of its actions, all being weakened in nearly an equal degree.—It is  pro-
 bable, hoAvever, that (as  suggested by Dr. Gull) the chief use of this
 arrangement is to bring groups of muscles into relation with the different
 segments of the cord, in such a mode that their actions may be combined
 and harmonized.   We shall hereafter (chap, xh.) find reason to believe
 that the "will does not at once act through the nerves upon the muscles;
 but  that it  plays (so to speak) upon the  spinal cord, each segment  of
 which has its OAvn particular endowments, and ministers to a particular
 set of movements.  And thus, the  greater the variety of movements
 which any part is destined to perform, the more complicated will be.the
 nervous plexus by which its muscles are connected  with the centres of
 motion.
   378. The second primary element  of the  Nervous System,  without
 Avhich the fibrous portion would seem to be totally inoperative,  is com-
 posed of nucleated cells, containing a finely granular  substance, and
 lying somewhat loosely in the midst  of a minute plexus of blood-vessels
 (Fig. 69, a).   Their normal  form may be regarded as globular (hence
 they have been termed nerve- or ganglion-globules); but this  is liable  to
 alteration from the compression they suffer, so that they may become
 oval or polygonal.  The most  remarkable  change of form, however,
 which they undergo, is by an extension into one  or more long processes,
 giving them a caudate  or a stellate aspect (b,  b).  These  processes are
 composed of a finely-granular substance, resembling that of the  interior
 of the vesicle, with Avhich they seem to be distinctly continuous; and if
 traced to  a  distance, they are frequently found to become  continuous
 with the axis-cylinders of the nerve-tubes.  As a general rule, according
 to Professor  Kolliker,  only one  nerve-tube  is connected  with each
ganglion-cell;   but this rule is not  without its  exceptions,  especially
amonc the loAver animals.  The other processes  seem to inosculate with
those of other cells, so as to establish a direct communication between 
220      STRUCTURE AND  ENDOWMENTS OF ANIMAL TISSUES.

them.   In some instances,  again, the ganglionic cell comes into relation
with the  fibre, not by sending out  a prolongation which is continuous
with it, but by being received into  the cavity of the fibre (so to speak),
by an extension of the tubular wall over it.   In other cases, moreover,
the fibres merely pass around and amongst the  ganglionic cells, without
coming into direct connexion with them;  and when this is the case, the
ganglionic cells  retain their simply globular  character.—Besides the
finely granular substance just mentioned,  these cells  usually contain a
collection of pigment granules, which give them a reddish or yellowish-
brown color:  this, however, is frequently absent, especially among the
lower animals.—The size of the vesicles is  liable to great variation ; the
globular ones  are usually  betAveen l-300th and l-1250th of an inch in
diameter.
 Primitive fibres arid ganglionic globules of human brain, after Purkinje.  A, ganglionic globules ljing
amongst varicose nerve-tubes, and blood-vessels, in substance of optic thalamus; a, globule more enlarged;
b, small vascular trunk,  b. b, globules with variously-formed peduncles, from dark portion of crus cerebri.'
350 Diam.

   379.  The vesicles just described are aggregated together in masses of
variable size;  and are  in  some  degree held together  by the  plexus of
blood-vessels (Fig. 70), in the midst of which they lie.  They are sometimes
imbedded in a soft granular substance, which adheres closely to  their
exterior and to their processes;  this is the case in the outer part of the
cortical substance of the human brain.   In  other instances, each cell is
enclosed in a distinct envelope, composed of  smaller  cells, closely adhe-
rent to each other and to the contained cell;  such  an arrangement is
common in the smaller ganglia, and in the inner portion of the cortical
substance of the brain.   The substance which is made up of these pecu-
liar cells,  of the  plexus of the blood-vessels in which they lie, and of the
granular matter that is disposed amongst them, is altogether commonly
known as the  cineritious or gray substance; being distinguished by its
color in Man and the higher animals at least, from the white substance
(composed of nerve-tubes) of which  the trunks of the nerves, as Avell as
a large part of the brain and spinal cord, are made up.  But this dis-
tinction is by no  means constant;  for the  gray color, which is partly 
STRUCTURE OF NERVOUS GANGLIA.               221
due to the pigment-granules of the cells, and  partly to  the redness of
the blood in the vessels, is wanting in the Invertebrata generally, and is
not characteristically seen in the classes of Fishes and Reptiles.  More-
over, when  the ganglionic  substance  exists in small amount, even in
Fig. 70.
Man, its color is not sufficiently intense to serve to distinguish it; and,
as we have already seen, there  are nerve-fibres which possess a  grayish
hue.  The real  distinction evidently lies in the form of the  ultimate
structure, which is fibrous  in the one case, and  cellular or vesicular in
the other; and these terms will be henceforth used to characterize the
two kinds of nervous tissue, which have been now described.
  380.  A ganglion, then, essentially consists of a collection  of nerve-
vesicles or ganglion-globules, interspersed among the nerve-fibres ; and
it is in the presence of  the former that it differs from a plexus, which
it frequently resembles  in  the arrangement  of the latter.   When a
nerve enters  a ganglion,  its component fibres separate and pass through
the  ganglion in different  directions, so  as to  be variously distributed
among the branches which pass  out of  it (Fig. 71); coming, in  their

                               Fig. 71.
1	\v\\\\			^f0:>
				
			WSM	
				ii^Ss^;
iP	==\i	^^^^^i	•,! WM	
		~J-\i_7\(.j^-—	e.	
  Dorsal ganglion of Sympathetic nerve of Mouse;—a, b, cords of connexion with adjacent sympathetic
ganglia; c, c, c, c, branches to the viscera and spinal nerve; d, ganglionic globules or ceUs; e, nervous fibres
crossing the ganglion.

course into relation with the vascular  matter, which occupies  the  inte-
rior of the  ganglion, in one  or more of the  modes  already specified
(§ 378).   Some of the fibres  may terminate in the cells of the  ganglion, 
222      STRUCTURE AND ENDOWMENTS OF ANIMAL  TISSUES.

and new ones may originate  in them; so that there is no constant cor-
respondence between the number of fibres which enter a ganglion, and
of those who pass out of it.—The  only exception  to the  general  fact,
that the vesicular matter occupies  the centre  of the ganglia, occurs  in
the brain of Vertebrata, in which  it is  chiefly disposed on the exterior,
forming the cortical envelope.  The reason for this variation is probably
to be found in the very large amount of this substance, which the brain
of  the  higher Vertebrata  contains; and  in  the  necessity of  the free
access of  blood-vessels  to it,  which is provided for by a great extension
of  its surface beneath the investing vascular membrane (pia mater),
more readily than it could be in any other mode.
   381.  But the vesicular matter is not found in the central masses only
of the Nervous  system ; for it presents itself  also at those parts of the
surface or periphery which are peculiarly destined  to  receive  the im-
pressions  that are to be conveyed  to the central organs.   Thus on the
expansion of  the optic-nerve  which forms the retina, there is a distinct
layer of ganglionic  corpuscles, or  nerve-cells, with a minute plexus of
vessels, possessing all the essential characters  of the vesicular substance
of the brain ; and something of the same  kind has  been  seen in con-
nexion with the corresponding  expansion of the olfactive and auditory
nerves.   Moreover, the study of the history  of the development  of these
organs has shown that the vesicular matter  of the retina is  an  offshoot
(so to speak) from that of  the optic ganglion, that of  the  labyrinth of
the ear being in like manner an offshoot from  that of the auditory gan-
glion.   Thus it is  obvious  that the fibres of the  connecting nerve are
interposed betAveen the cells  of the peripheral and those  of the central
organs, for the  sake of preserving  that connexion between them which
would otherwise have been  interrupted;  and that the vesicular matter
is the active agent  in the origination of those  changes which take place
as a consequence of sensory  impressions, whilst for  the  conduction  of
such changes, the fibrous structure is alone required.
  382.  The  ultimate distribution  of the nerve-fibres  in  the skin and
tongue, however, has not been so clearly made out, nor can their  rela-
tion to cells be distinctly traced.   These fibres are distributed, for the
most part, to  the papillse, in which they can be frequently seen to  form
              Fig. 72.                              Fig. 73.
 Distribution of the tactile nerves at the surface of the
lip; as seen in a thin perpendicular section of the skin.
loops (Fig. 72), accompanied by similar loops of blood-vessels (Fig. 73):
but no such loops can be seen in the fungiform papilke of the tonguej 
            CHEMICAL COMPOSITION OF NERVOUS MATTER.        223

the continuity of whose nerve-fibres cannot be distinguished near their
apices.   Here, however, the history of the development of the nervous
plexuses in the skin seems to show, that the fibres may be considered as
commencing in a peripheral ganglionic expansion ; for it has been shown
by the observations of Prof. Kolliker upon the tail of the tadpole, that
the nervous plexuses  are formed in  the same manner  as the capillary
network; namely, by the inosculation of  the  prolongations of radiating
cells, whose centres are at a  considerable distance from each other.
Hence it is probable that cell-nuclei, or some equivalent centres of force,
remain  in connexion  with  the  peripheral  nerve-fibres of the skin and
tongue, as with those  of other sensory surfaces.   Sometimes the ultimate
plexuses seem to be formed in the Skin, as in  Muscle, by the subdivision
of the primitive fibres (§ 341).
   383.  We have now  to  speak briefly of  the Chemical  Composition of
the Nervous matter;—a consideration which will be presently shoAvn to
be of much importance.  As formerly remarked  (§ 7), the vital activity
of  a  tissue is usually greater, as the proportion  of its  solid to its fluid
contents is less; and this rule  holds good most  strikingly in regard to
the Nervous  substance, the vital activity of which is far  greater  than
that of any other tissue, and  the solid matter of which usually consti-
tutes no more than a  fourth, and occasionally  does not exceed an eighth
of its entire weight.   The proportion of water is greatest in infancy and
least  in middle life; and  it has been observed to be under the average
in idiots.  Of the solid matter of  the brain, about a  third consists of
fibrine or albumen; Avhich is probably the material of the membrane of
the tubuli, as Avell as of the tissue that connects them.—It is chiefly Avith
the Fatty matter, Avhich constitutes  about a third of the  solid substance,
that the attention of  Chemists  has been occupied.  This is stated by M.
Fremy  (one of the most recent analysts) to contain, besides the ordinary
fatty matters, and Cholesterine  or  biliary fat, tAvo peculiar fatty acids,
termed the Cerebric and the Oleo phosphoric.   Cerebric  acid, when  puri-
fied, is  white, and presents itself in crystalline grains.   It contains a
small proportion  of  Phosphorus; and differs from the  ordinary  fatty
matters in containing Nitrogen, as also in containing twice their propor-
tion of  oxygen.   Oleo phosphoric acid is  separated from the former by
its solubility  in ether; it is  of a viscid consistence;  but when boiled
for a long  time in Avater or alcohol, it gradually loses its viscidity, and
resolves itself into a pure oil,  Avhich  is  elaine, while phosphoric acid
remains in the liquor.   The proportion  of phosphorus  in the brain  is
considerable ; being from 8 to 18 parts in 1000 of the whole mass, or from
l-20th  to l-30th of the whole solid matter.   It  seems to be unusually
deficient in  the brain  of  idiots.  The remaining; third and  sometimes
more, is composed of a substance termed Osmazome (Avhich seems  to be
a proteine-compound in a state of  decomposition), together Avith saline
matter. No  satisfactory examination  has yet been made into the com-
parative composition of the vesicular and fibrous substances ; but accord-
ing to Lassaigne, the former contains much more Avater than the latter,
and little colorless fat, but nearly 4 per cent, of red fat, which does not
exist in the other.
   384. Various circumstances lead  to the belief,  that the Nervous tissue, 
 224     STRUCTURE AND ENDOAVMENTS OF  ANIMAL TISSUES.

 during the whole period of active life, is continually undergoing changes
 in its substance, by decay and renewal.  We know that, after death, it
 is one of the first of all the animal tissues to exhibit signs of decompo-
 sition ; and there is no reason to suppose, that  this tendency is absent
 during life.  Hence for the simple maintenance of its normal character,
 a considerable amount of nutritive change must be required.  But many
 circumstances further lead to the conclusion, that, like all other tissues
 actively concerned  in  the  vital operations, Nervous  matter is  subject
 to  a waste or disintegration, which  bears  an  exact  proportion to the
 activity  of its operations;—or, in other  words, that every act of the
 Nervous system  involves the death and decay of a certain amount of
 Nervous matter, the replacement of which will be  requisite in order to
 maintain the system in a state fit for  action.  We shall hereafter see,
 that there  are certain parts of the Nervous system, particularly such as
 put in action the respiratory muscles, which are in a state of unceasing,
 though moderate, activity; and in these, the constant  nutrition  is suffi-
 cient to repair the effects of the constant decay.   But those parts, which
 operate in a more powerful and energetic manner,  and which therefore
 waste  more rapidly  when in action, need a season of rest for their repa-
 ration.  Thus a sense of fatigue is experienced, when the mind has been
 long acting through its instrument—the brain ;  indicating the necessity
 for rest and reparation.  But when  sleep, or cessation of the cerebral
 functions, comes  on, the process of nutrition takes place with unchecked
 energy, counterbalances the results of the previous  waste, and prepares
 the  organ  for a  reneAval of its activity.  In the healthy state of the
 body, when the exertion of the nervous system by day does not exceed
 that, which the repose of the night may compensate, it is maintained in
 a condition which fits it for constant  moderate exercise; but  unusual
 demands upon its poAvers,—whether by the  long-continued and severe
 exercise of  the intellect, by excitement of the emotions, or by the com-
 bination  of both in that state  of anxiety which  the  circumstances  of
 man's  condition  too frequently induce,—occasion  an  unusual  waste,
 which requires, for the complete restoration of its poAvers, a  prolonged
repose.
  385.  There can be no doubt that (from causes which are not known),
 the amount of sleep required by different persons,  for the maintenance
of a healthy condition of the nervous system, varies  considerably; some
being able to dispense Avith  it, to a degree which would be exceedingly
injurious to others of no greater mental activity.   Where a prolonged
exertion of the mind has been made,  and the natural tendency to sleep
has been habitually  resisted, by a strong  effort of the  Avill, injurious
results are  sure to follow.   The bodily health breaks down, and too
frequently  the mind itself is permanently enfeebled.  It is obvious that
the nutrition of the Nervous system becomes completely deranged; and
that the tissue is no longer formed, in the manner requisite for the dis-
charge of its healthy functions.
  386. As the amount of Muscular tissue  that has undergone disinte-
gration, is  represented  (other things being  equal), by the quantity  of
urea in the urine, so do we find that an unusual waste of the nervous
matter is indicated by an increase in the amount of phosphatic deposits. 
           AVASTE  AND  RENEAVAL  OF NERVOUS SUBSTANCE.        225

No others of the soft tissues contain any large proportion  of phospho-
rus ;  and the marked increase in these deposits, Avhich has been con-
tinually observed  to  accompany long-continued wear of mind,  whether
by intellectual exertion, or by anxiety, can scarcely be set  down to any
other  cause.  The most  satisfactory proof is  to  be  found in cases, in
Avhich there is a periodical demand upon the mental powers; as, for
example, among clergymen, in  the preparation  for, and discharge of,
their Sunday duties.   This is found to be almost invariably followed
by the appearance  of a large quantity of the phosphates in  the urine.
And  in  cases, in Avhich constant and  severe  intellectual exertion has
impaired the  nutrition  of the  brain, and has consequently weakened
the mental poAver, it is found that any premature attempt to renew the
activity  of its exercise, causes the reappearance of the excessive phos-
phatic discharge, which indicates an undue waste of nervous matter.
   387. As  the  disintegration of the Nervous* System is thus  propor-
 tional to its exercise, so must its reparation make  a  corresponding de-
 mand upon  the  nutritive processes.  And accordingly we  find, that it
 is very copiously supplied with blood-vessels;  and that the amount of
food, appropriated  to  its  maintenance in an  active  condition, is  very
considerable. This Ave know from the fact, that persons of active minds,
but sedentary bodily habits, commonly require nearly as  much  food as
those, in whom the waste of the Muscular system  is greater, and that
of the Nervous  system less, in virtue  of their bodily activity and the
less energetic operation of their minds.
   388. The nerve-fibres  appear to originate, according to  the observa-
tions  of  Prof. Kolliker, in cells which become fusiform by elongation
(§ 193),  and Avhich  then  coalesce at  their extremities;  and these seem
to  increase,  after  the  first  formation  of the  trunks, by  the  longitu-
dinal subdivision of fusiform cells Avhich had not  previously undergone
complete metamorphosis into fibres,  as well as by the development of
cells de novo.   The nuclei of the original cells may be frequently seen
in the nerve-tubes at a later period, lying between their membranous
walls and the substance deposited in their interior.  The earliest condi-
tion of both forms of nerve-fibre appears to be precisely the same;  but
the gelatinous  remains in a  state nearly resembling this; whilst  the
tubular is developed into a higher form.  Various gradations, indeed,
may be traced between the two.  The vesicular matter appears to be in
a state of continual change, as is the case with all cells whose functions
are active.  The appearances observed by Henle  in  the cortical sub-
stance of the brain lead  to the belief, that there is as continual a suc-
cession of nerve-cells, as there is of epidermic  cells;  their development
commencing at the  surface, where they are most copiously supplied with
blood-vessels from  the investing membrane, and proceeding as they are
carried toAvards the inner layers, Avhere they come into more immediate
relation with the fibrous  portion  of the  nerve-structure.  This  change
of place is probably due to the continual death  and decay of the mature
cells, where  they are connected Avith the fibres;  and the constant pro-
duction of neAV generations at the external surface,—thus  carrying the
previously-formed cells inwards, in precisely the same manner that  the
epidermic cells are  progressh'ely carried outwards.
r                               15 
226      STRUCTURE AND ENDOWxMENTS OF ANIMAL TISSUES.

  389. The regeneration of Nervous  tissue that has been  destroyed,
takes place very readily in  continuity Avith that Avhich is left  sound.
This may be more easily proved by the return of the sensory and motor
endowments of  the  part, whose nerves have been separated, than by
microscopic examination  of the  reunited  trunks themselvef, which is
not  ahvays satisfactory.   All our knoAvledge of the functions  of the
nervous system leads to the belief, that perfect continuity  of the nerve-
tubes is  requisite  for  the conduction of  an  impression  of any kind,
whether  this be destined to produce motion or sensation; and various
facts, well knoAvn to Surgeons, prove that such restoration may be com-
plete.  In the various operations which are practised for the restoration
of lost parts, a portion of tissue removed from one spot is  grafted,  as it
were, upon another; its original attachments are more or less completely
severed,—frequently altogether destroyed,—and new ones are formed.
Now in such  a part, so,long as its original  connexions exist, and the
new ones are  not  completely formed, the  sensation is referred  to  the
spot from which it was taken ; thus when a new nose is made, by partly
detaching  and bringing  down a piece  of  skin  from the  forehead, the
patient at first feels, when anything touches the tip of his nose, as if the
contact Avere  really Avith his forehead.  After time has been  given,
however, for the establishment of new connexions Avith the parts into
whose neighborhood it has been brought, the old connexions  of  the
grafted portion are completely severed; and an interval then  ensues,
during which  it frequently loses all sensibility; but after a time its
power of feeling is restored, and the sensations received through it are
referred  to the right spot.  A more familiar case is the  regeneration of
Skin, containing sensory  nerves, which takes place in the  well-managed
healing  of wounds involving loss  of substance.   Here there must ob-
viously be, not  merely a  prolongation of the  nerve-tubes  from the sub-
jacent and surrounding  trunks, but also  a formation  of new sensory
papillae.   A still more striking example of the regeneration of Nervous
tissue, however, is to be found in those  cases (of which there are now
several on record), in which portions of  the extremities, that  have been
completely severed by accident, have been made to adhere to f.he  stump;
and have, in time, completely recovered their  connexion with the Ner-
vous as  with  the  other  systems,  as is  indicated by the restoration of
their sensory and motor endowments.—Of the degree in which the vesi-
cular substance  of the Nervous system may be regenerated, Ave have no
certain knowledge; but  there can be little doubt, from the activity of
its usual nutritive changes, that a complete reproduction may be effected
in cases  of loss of substance, where it can commence from a neighboring
mass of  the same tissue.
  390. We have now to inquire into the conditions, under  which the
peculiar properties of  the Nervous System are manifested in an active
form; and it will first be desirable to explain, somewhat more in detail,
the  nature of the different operations to which it is subservient.  These
operations present themselves in their most complex  form, in Man  and
the  higher animals; but  they may often be most  satisfactorily  studied
in the lower.   In the first place, when  an  impression is  made upon any
part of  the surface of the body by mechanical contact,  by heat, elec- 
PROPERTIES OF NERVOUS  SYSTEM.
227
tricity, or any other similar agent,—or upon the organs of special sense
(the eye and ear, the nose and  tongue), by light or sound, by odorous
or sapid bodies,—these impressions, in the healthy and wakeful state of
the Nervous system, are felt as sensations ; that is, the  mind is rendered
conscious of thern.   Now there can be no doubt that the mind is imme-
diately influenced, not by the impression in the remote organ, but by a
certain change  in the condition of the brain, excited or aroused by that
Avhich  has originated  elsewhere.  For  if the communication with the
brain be cut off, no impression on the distant parts of the nervous system
is felt, notwithstanding that  the mind  remains perfectly capable of re-
ceiving it.  The mind, then, is only rendered conscious of external objects,
by the influence which they exert upon the brain, or upon  a certain part
of it, Avhich, being the  peculiar seat of sensation, is called the sensorium.
Hence we recognise, in the process by which the mind is  rendered con-
scious  of external objects, three distinct stages;  first, the reception of
the impression  at the extremities  of the  sensory  nerve  ; second, the
conduction  of  the  impression,  along  the trunk of the  nerve, to the
sensorium; third,  the change  excited  by it  in the  sensorium itself,
through  Avhich  sensation is produced.   Here,  then,  the change in the
condition of the nervous  system commences at the circumference, and
is transmitted to the centre;  and the fibres which are concerned in this
transmission are termed sensory.
  391.  On the  other hand, when an emotion, an instinctive impulse, or
an act  of the will,  operates  through  the central  organs  to  produce a
muscular contraction, the first change is in the condition of the vesicular
substance of those organs.  The influence of this change is transmitted
by the motor nerves to the muscles, among which they are distributed  ;
and the desired movement is the  result.   Here,  too, we  have  at  least
three stages; first,  the origination of the change by an impression act-
ing on the central organs ; second, the conduction of that change along
the motor nerves; and third, the stimulation of the muscles to contrac-
tion.   But the  operation here commences at the centre ; and the effects
of the change in the brain are  transmitted to  the  circumference,  by a
set of nervous fibres which are  termed motor.  The complete distinct-
ness of these tAvo classes  of fibres Avas first established by Sir C. Bell.
It is best seen in the nerves of the head, of which some are purely sen-
sory, and others purely motor; but it  may also be clearly proved to
exist at the roots of the  spinal  nerves  (although their trunks possess
mixed  endowments), the posterior being sensory, whilst the anterior are
motor.
  392.  But although sensations can only be felt through the  brain, and
voluntary motions can only be produced by an action of the  mind through
the same organ, yet there are  many changes  in  the  animal body, in
which the nervous system  is concerned, Avhich yet do  not involve the
operation of the brain, being produced without our  consciousness being
necessarily excited, and  Avithout any act of the will, or even  in opposi-
tion to its efforts.   Of these actions, the  spinal cord of Vertebrata, and
its prolongation within the cranium, are the chief instruments; in the
Invertebrate animals, they are performed by various ganglia, which are
usually disposed in the  neighborhood  of the organs to which  they 
 228      STRUCTURE AND ENDOWMENTS  OF ANIMAL TISSUES.

 minister.   If the  spinal cord of a Frog  be divided in its back, above
 the crural plexus, so as entirely to cut off  the  nerves  of the  lower ex-
 tremities  from connexion Avith the brain,  the animal loses all voluntary
 control over these limbs,  and no sign of pain is produced by any injury
 done to  them.  But they are  not  thereby rendered motionless; for
 various stimuli applied to the limbs themselves  will cause movements in
 them.  Thus if the skin of the foot be pinched, or if a flame  be  applied
 to it, the  leg will be violently retracted.  Or, if the cloaca be irritated
 by a probe, the feet will endeavor to  push aAvay the instrument.   We
 have no reason hence to believe, that  the animal feels  the irritation, or
 intends to execute  these movements in  order  to  escape from  it; for
 motions of a similar kind  are exhibited by men, who have suffered injury
 of the lower part of the spinal  cord, and who  are  utterly unconscious,
 either of the irritation which their limbs receive, or  of the actions which
 they perform.
   393. We  are not to  suppose, however, that  the  stimulus  acts  at
 once upon the muscles, without the nervous system being concerned at
 all;  throwing them into  contraction by  its direct influence.   For it is
 quite certain, that unless the nervous trunks  remain  continuous  with
 the spinal cord, and unless the part of the spinal cord with which they
 are  connected remains sound (although  cut off  from connexion  with
 the  parts above, and with the brain), no  action will result.  If the
 trunks be divided, or either of the roots  by which they are  connected
 with  the  spinal cord  be severed, or the lower portion of the spinal
 cord  itself be injured, no  stimulation  will cause  the  muscular move-
 ments just described.   A very good  example of this  necessity for the
 completeness  of the nervous  trunks, which convey impressions  to and
from the  central organ, is found  in the movements of the iris,  for the
 contraction and dilatation of the pupil.  Here the stimulus of light upon
 the retina gives rise to  a  change in the condition of the optic  nerve;
 which, being  transmitted to a certain portion  of the  encephalon with
 which that nerve is connected,  excites  there a motor impulse; and
 this impulse  is conveyed through a distinct  nerve (a branch  of the
 third pair) to the iris, occasioning contraction of the pupil.   Every one
 knows that this adjustment of the size of  the  pupil to the  amount of
 light, is effected without any  exertion of the will on his OAvn part, and
 even without any consciousness that it is taking place.  It is performed,
 too, during profound sleep; when the influence  of light upon  the retina
 excites  no consciousness of its presence,—when no sensation,  therefore,
 is produced by it.
   394.  The class of actions thus performed, is termed reflex ;  and we
 see that every such action involves  the folloAving series  of changes.
 In  the first place,  an  impression is made upon the extremity  of a
 nerve, by some external agent; just  as  when  sensation is to be  pro-
 duced.  Secondly, this impression is  transmitted  by a nervous trunk
 to the spinal  cord  in  Vertebrata, or to  some  ganglionic mass which
 answers to it in the Invertebrata.   But instead of being communicated
 by its means to the mind, and  becoming a sensation, it immediately
 and necessarily executes a motor impulse;  Avhich  is reflected  back  as it
were  to certain muscles,  and,  by their  contraction,  gives  rise to a 
CONDITIONS  OF NERVOUS ACTIONS.              229
movement.  We shall hereafter see, that nearly all those movements in
the animal body,  which are immediately connected  with  the  mainte-
nance of the organic functions,—such  as those of respiration,  degluti-
tion (or swallowing), the expulsion of the faeces, urine, and foetus, &c,
—are performed in this manner.
  395.   Noav there is strong reason to believe  that the changes which
take place in the nervous trunks are of the same nature, whatever  may
be the source from which  they proceed,—whether,  for example, the
movement is simply reflex, whether it proceed  from a mental emotion,
or whether it be executed  in  obedience  to an act  of  the will.  It was
formely supposed that all the afferent  or centripetal  fibres pass up to
the Brain, and that all the efferent or centrifugal fibres pass down  from
the same organ; the Spinal Cord being looked upon as little else  than
a bundle of nerves.  It is now known,  however, that by far the greater
part of the fibres of any trunk terminates in the central organ, to Avhich
that trunk at first proceeds; and that  the Spinal Cord  may be consi-
dered as a series of such ganglionic centres, each receiving the afferent
fibres,  and  giving  origin  to the efferent  of  its own segment.   So,
again,  the special  sensory nerves, the  olfactive,  optic,  auditory, and
gustative, terminate in  their  own ganglionic centres, which lie at the
base of  the  brain, in  immediate  connexion with the summit  of the
spinal  cord, and Avhich are quite  independent  of the cerebrum.  The
apparatus for receiving impressions, and for originating motions, is thus
complete in itself; and the addition of the cerebrum  does not make
any essential difference in its operations, save that this sensori-motor
apparatus (as it may be termed) is made to act through its means as
the agent of  the mind, in addition to its  functions  as  the instrument of
the automatic movements.   We  shall  hereafter see  (chap. XII.),  that
the difference betAveem Instinct and Intelligence  is  closely connected
with the development of the cerebrum ; but that this organ,  even in
that highest grade  of development which it possesses in  Man, has no
other connexion with the sensory organs than that  which it acquires
through  its relation with the sensory ganglia, and has no more power of
exciting  muscular  movement, than by playing  (so  to  speak), upon the
spinal  cord, whose efferent fibres  respond to its mandates, just  as  they
Avould  do to the stimulus, of an impression primarily acting  through that
organ.
   396. Of the mode by which the effects of changes  in one part of the
Nervous  system,  are thus  instantaneously transmitted  to another,
nothing Avhatever is known.  There is evidently a strong analogy betAveen
this phenomenon,  and  the instantaneous transmission of  the  Electric
poAver  along good conductors ; but the relation is  much  more intimate
than  this, for Electricity is  capable  of exciting  Nerve-force  whilst,
conA'ersely,  Nerve-force  can  excite Electricity.   Thus  a  very feeble
galvanic current transmitted along a motor  nerve, serves to excite con-
tractions in the muscles supplied by it;  and in like manner, a galvanic
current  transmitted along any of  the  sensory nerves, give rise to a
sensation of the  kind  to  Avhich  the  nerve ministers.  Moreover  we
shall  hereafter see, that  certain  animals  are capable of generating
Electric power in  a very remarkable manner (chap,  x.) ; and that the 
230     STRUCTURE AND  ENDOAVMENTS OF  ANIMAL TISSUES.

nervous  force is essentially concerned in  this operation.   But, on the
other hand, it is quite certain that the influence  transmitted along the
nerves of the living body  is not ordinary electricity ; for all  attempts
to procure  manifestations  of electric changes in  the state of nerves,
that are acting most energetically on muscles, have completely failed;
and a nerve remains capable of conveying the influence of electricity,
when it has been  rendered unable  to  transmit the influence of the
brain, as by tying a  ligature round it, or  by tightly  compressing it
between the forceps,  Avhich gives no interruption  to the one  agency,
while it completely checks the other.—Notwithstanding, then, the strong
analogy  which exists between these two powers,  we are not Avarranted
in regarding them as identical; but they have towards each other that
relation  of reciprocity, which exists between Electricity and Heat,  or
between Electricity and Magnetism,  each being  convertible  into the
other in a certain definite ratio (§ 53).
   397.  It is more  desirable,  however, that  we should understand the
conditions   under which the  phenomena  of the  Nervous  System take
place, than  that we should spend much time  in discussing the  identity
of  its peculiar powers  with  any others in  Nature.  The  conducting
power of the nervous  fibres appears to remain with little decrease for
some  time after death, especially in cold-blooded animals; for  we can,
by pinching, pricking,  or  otherwise stimulating the motor-trunk, give
rise to contractions  in the muscles supplied by them, exactly as during
life.  This poAver is much lessened by the influence of narcotics; so that
if a nen^ous trunk be soaked in a solution of  opium, belladonna,  or other
powerful narcotic, it ceases to be able to convey the effects of stimuli to
the muscles, some time before the muscles  themselves lose their con-
tractile power.  On the other hand,  it seems to be exalted  by various
irritating influences; so that, when the nervous trunk has  been treated
with strychnia, or when it has been subjected to undue excitement in other
ways, a very slight change is magnified (as it were) during its transmis-
sion,  and produces  effects of unusual intensity.
   398.  Now although the  conducting power of the fibrous structure
will continue for a time, after the circulation through it has ceased, the
peculiar  endowments of the vesicular substance, by which it originates
the changes which the  former transmits, are only manifested,  when
blood is moving through its capillaries.   Thus if the circulation  through
the brain cease but for a moment, total insensibility, and loss of the power
of voluntary motion, immediately surpervene.   The brain is  supplied
with  blood  through four arteries,—the  two internal carotids, and the
two  vertebrals ; and by the  communication of these with  each  other
through  the circle of Willis, the circulation will still be kept up, if only
one of them should  convey blood into the cavity of the cranium.  Hence
it is necessary that the flow of blood should be checked  through  all of
them, in order that the functions of the brain should be suspended; and
the suspension is then  complete and instantaneous.  The best  method
of effecting  this was devised by Sir Astley Cooper.  He tied both the
carotid  arteries in a dog :  which, for the  reasons just mentioned, did
not produce any decided influence on the functions of  the  brain, the
circulation being kept up through  the vertebrals.   But upon compress- 
          EFFECTS OF STIMULANTS  UPON  NERVOUS POWER.       231

ing the latter, so as to suspend the flow of blood through them, imme-
diate insensibility, and loss of voluntary power, were the result.  When
the compression Avas  taken off, the animal immediately returned to its
usual state ;  and again become suddenly insensible, Avhen  the pressure
was reneAved.  Although the functions of the brain were thus suspended,
those of  the  spinal cord were not; as was shown by the occurrence of
convulsive movements.  But  in  the  state called Syncope, or fainting,
the suspension of the circulation, by  a failure in the  heart's action,
causes  an entire loss  of poAver in both these centres ; and a complete
cessation of  muscular movement is  the result.  This  condition  may
come on  instantaneously, under the influence of powerful  mental emo-
tion, or of some  other cause, which  acts primarily in suspending  the
heart's action,  and consequently  in checking the circulation ;  the insen-
sibility,  and  loss of muscular power, are secondary results, depending
upon the suspension of  the powers of  the nervous centres, consequent
upon the cessation of the Aoav of blood  through  them.
   399. The  due activity of the vesicular nervous  matter is  not  only
dependent upon  a sufficient supply of  blood, but it requires  that this
blood should be in a state  of  extreme purity; for  there is no tissue in
the body, whose functions are so readily  deranged, by  any  departure
from the regular standard in the "circulating fluid,—Avhether  this  con-
sist in  the alteration of the proportions of its normal ingredients, or in
the introduction of other substances which have no proper place in  it.
One of  the  most fertile sources of disturbance in the  action of the
brain,  consists  in the  retention of substances within the  blood, which
ought  to be excreted from  it.  We shall hereafter see,  that three of
the largest and most important  organs in  the body,—the lungs, the
liver,  and the kidneys,—have  it for  their special  office, to separate
from the circulating  fluid  the products of the  decomposition, which is
continually  taking place  in  the body; and thereby to  maintain  its
purity and its  fitnesss for its  important functions.  Now if these, from
any cause, even partially fail  in  their office, speedy disturbance of the
functions of the nervous centres is  the result.  Thi^s if the lungs do
not purify the venous blood  of  its impregnation  of carbonic acid, or
restore to it  the proper proportion of oxygen, the functions of the brain^
are seriously affected.   The sensations  become indistinct, the Avill loses
its control  over the  muscles, giddiness and faintness come on, and at
last complete insensibility supervenes.   Corresponding symptoms occur,
though to a  less serious degree,  when the excretion of carbonic acid is
but slightly impeded.  Thus Avhen a number of persons are shut up in
an ill-ventilated  apartment, for a sufficient length of time to raise the
proportion of carbonic acid in the air to 1 or 2  per cent., the continued
purification of their blood by respiration is but insufficiently performed,
for reasons  which will be stated hereafter (chap, viii.) ;  and the car-
bonic acid accumulates in  their  blood  in a sufficient degree, to produce
headache and  obtuseness  of  the mental powers.—Similar results take
place,  as will be shown hereafter, from  the retention of the substances
which  ought to be drawn off by the liver and kidneys: tl ese,  when they
accumulate in even a trifling degree, produce torpor of the functions of
the brain ; and when their proportion  increases, complete cessation of 
232      STRUCTURE  AND ENDOWMENTS OF ANIMAL  TISSUES.

its powers is the  result,  their action being precisely that of narcotic
poisons.   Various substances introduced into the blood may exert similar
influences;  depressing the  activity of the vesicular substance of the
nervous centres, and consequently producing  torpidity, not merely in
regard to the reception of impressions, and the performance of volun-
tary motions, but also in the mental operations generally.
  400.  On the other hand, various conditions  of the blood, especially
those depending on the presence of certain external  agents produce an
undue energy in the functions of the nervous centres; which  energy,
however, is almost invariably accompanied by irregularity, or  want of
balance among the different  actions.  Of this we have a familiar example
in the operation of  alcohol.  Its first effect, when taken in moderate
quantity,  is usually to produce a simple increase in  the activity of the
cerebral functions.  A further dose,  however,  occasions not merely an
increase, but an irregularity ; destroying that power of self-control, which
is so  important a  means  of balancing the different tendencies in the
healthy condition  of  the  mind.   And a still larger dose has the effect
of a narcotic poison; producing diminution or  suspension of activity in
all the functions  of  the  brain.   In some persons, this is the mode in
which the alcohol acts from the first, its stimulating effects being altogether
wanting.—A similar activity is usually produced  by the respiration of
the Nitrous Oxide, which  seems to increase all the powers of the mind,
save that  of self-control, which it diminishes ; the individual, while under
its influence, being the slave of  his impulses, which act on his muscular
system with astonishing energy.   Very analogous to this, is the incipient
stage of mania; which is simply an undue energy of the cerebral func-
tions at first  in  some  degree under the control of  the will, but after-
wards, increasing to an extent  that  renders the  individual completely
powerless over  himself; and showing  itself in the intensity of the sensa-
tions produced by external objects, in the vividness of the trains of
thought (which, being entirely  uncontrolled, succeed each  other  with
apparent irregularity, though probably according to the laAvs of associa-
tion and suggestion), and in the  violence of the muscular actions.  Such
a state may continue for some time, without the intervention of sleep;
but the subsequent exhaustion of nervous power is proportioned to the
duration  of the  excitement; and frequent attacks of  mania  almost
invariably subside at last  into imbecility.
  401.  In these cases of undue excitement, there is obviously an increase
in the supply of blood to the head; as indicated by the suffusion of the
face, the injection of the conjunctiva, the throbbing at  the temples, the
pulsation  of the  carotids;  and we find  that  measures which diminish
the activity of  the circulation through the brain, are those most effec-
tual in  subduing the excitement.  But it  does not  at  all follow, that
this  undue action of the brain should be connected with an excess in the
whole amount of nutritive material,  and  should require general deple-
tion for its treatment.  In fact, a very similar class of  symptoms  may
present itself  under two conditions of  an entirely opposite  kind,—
inflammation, accompanied  with an increase in the proportion of fibrine
in the blood, and requiring treatment of a lowering kind,—and irritation,
depending on  a state of  blood  in which there is a deficiency  of  solid 
       DEPENDENCE  OF NERVOUS  POWER ON SUPPLY OF  BLOOD.   233

materials, and requiring a strengthening and even a stimulating regimen.
The skill of  the  practitioner is  often  put  to the  test, in the due
discrimination between these states.
  402. The  preceding examples mark the influence of various causes
upon the actions of  the vesicular matter of the brain; others might be
adduced to show,  that the vesicular substance of the spinal cord is also
liable to have its poAvers  depressed  or excited; but these  will be best
adverted to  hereafter,  when  the  distinct functions of that  organ are
under  consideration  (chap, xii.)  We may simply notice, that the stimu-
lating  effect  of Strychnia is peculiarly  and  most remarkably exerted
upon the vesicular substance of the  spinal cord; and that a correspond-
ing state, in  which violent convulsive  actions are excited  by the most
trifling causes, sometimes presents itself as a peculiar form of disease,
named Tetanus, which may  be either idiopathic,  depending probably
upon a disordered condition of the blood, or traumatic, consequent upon
the irritation of a wound.
  403. But,  as formerly remarked,  it is not  in the Nervous centres
only, that changes originate.  Whenever an impression is made upon
the surface of the body,  or upon the  organs  of  special sense,  Avhich,
being conducted to the nervous centres, either excites a sensation in the
brain (§ 390), or a reflex action through the  spinal cord (§ 392), the
reception  and propagation of such  impression at the extremities of the
sensory nerves requires a set of conditions of the same kind with those,
Avhich  we  have seen  to exist in the nervous centres.   In fact, if Ave re-
gard the course of  the  motor nerves  as commencing in  the  nervous
centres and  terminating in the muscles, we may with equal justice con-
sider that of the sensory nerves as originating in their peripheral extremi-
ties, and terminating in the sensorium.  And, as already stated (§ 381),
precisely the same kind of vesicular structure exists in  some (probably
in all)  of the peripheral expansions  of  the sensory nerves, as makes up
the gray substance of the brain and spinal cord.  Noav it  is easily shown,
that the circulation of the blood through these parts is just as necessary
for the original  reception of the impressions, as is the circulation through
the brain  to their reception  as  sensations,  and  to  the  origination  of
motor  impulses  by an act of the will.   We find that anything  Avhich
retards the  circulation  through  a  part supplied by  sensory nerves,
diminishes its sensibility;  and that if the flow of blood be completely
stagnated, entire insensibility is the result.  A familiar  example of this
is seen in the  effects of prolonged cold; which, by diminishing, and
then entirely checking, the Aoav  of blood through  the  skin, produces
first numbness, and then complete insensibility of the part.  This result,
however, may be partly due to the direct influence of the cold upon the
nerve-vesicles themselves; depressing  their peculiar vital powers (§ 97).
The same  effect is  produced, however, when  the supply of blood  is
checked in any other way;  as, for example, by pressure on the artery,
or by obstruction in  its interior.   Thus when the main artery of a limb
is tied, numbness of the extremities is immediately perceived; and this
continues, until the circulation is re-established by the collateral branches,
when the usual  amount  of sensibility is restored.  Again, in the gan-
grene  which  depends upon  obstruction  of the  arterial  trunks by  a 
234     . STRUCTURE  AND  ENDOAVMENTS OF ANIMAL TISSUES.

fibrinous clot in their interior, diminution of sensibility, consequent upon
the insufficient circulation, is one of the first symptoms.
  404.  On  the other hand, increased circulation of blood through a
part produces  exaltation of its  sensibility;  that  is, the ordinary im-
pressions  produce changes of unusual energy in  its sensory nerves,
This is particularly evident in the  increased  sensibility  of the genital
organs  of animals during  the period of heat; and in  those of Man,
Avhen in a state of venereal excitement.   Moderate  warmth, friction,
exercise, and other causes which increase the circulation through a part,
also augment its sensibility; and this augmentation is one of the most
constant indications  of that  state of determination of blood, or active
congestion, which usually precedes inflammation, and which exists in the
parts surrounding the centre  of inflammatory action.   But it must be
borne in mind, here as elseAvhere (§ 401),  that such exaltation of func-
tion in a limited part, is quite  consistent with general debility; and in
fact we may often observe, that  the  tendency of such local affections
is particularly great,  when  the blood is in a very poor condition.   (See
chap, v.)
  405.  To  sum up,  then,  we may  compare  the vesicular  substance,
wherever it  exists, to a galvanic combination:  the former being capable
of generating  nervous  influence, and transmitting it along the  fibrous
structure, to the part on  which  it  is to operate;  in  the same manner
as the latter generates  electric power, and transmits it  along the con-
ducting wires, to  the point at which it is to effect a decomposition or
any other change.  In  one of the  most perfect forms of the galvanic
battery (that  invented  by Mr. Smee), although  the metals remain  in-
serted in the  acid solution,  and  are consequently  always ready  for
action, no electricity  is generated until the circuit is complete;  and the
ivaste of the  zinc, produced  by its solution  in the  acid, is  therefore
exactly proportional  to the electric effects to which it gives rise.  The
condition of the nervous system,  in the healthy and waking state, bears
a close  analogy to this;  for it is  in a state constantly ready for action,
but waits to be excited; and its waste is proportional to the activity of
its  function.—The vesicular  matter,  diffused over the  surface of  the
body, is inactive, until an impression is made  upon  it by some external
agent;  but  a change then takes place in its condition  (of which  we
know no more, than  that  the  presence of arterial  blood and a certain
amount of  warmth  are necessary  for it), which  is transmitted to the
central  organs by the sensory  trunks.  It  Avould appear that the excite-
ment of this  change has a tendency to increase the afflux of blood to
the part; thus when  a lozenge or some similar substance is allowed to
lie for a time in contact with the tongue,  or with the side of the mouth,
a roughness is  produced, which  is due to the erection of the sensory
papillae, by  the distension of  their blood-vessels.—On the other hand,
the  change  in the vesicular  matter of the  central  organs, by  which
motion  is produced in the distant muscles, may be excited either by  the
stimulus conA'eyed by the afferent nerves (as  in reflex action, § 392), or
by an act of Mind.   This  act  may be voluntary, originating in the will;
or it may be instinctive  or  emotional, resulting from certain  states of 
CONNEXION BETAVEEN THE  MIND  AND  BRAIN.        235
mind excited  by sensations, and  altogether independent of the will.
Of the mode m Avhich the mind thus acts upon  the  nervous system, we
know nothing whatever, and  probably never shall  be informed in our
present state of being   But it is sufficient for us to be aware  of the
physiological fact of the peculiar connexion  between the mind and the
brain ; a connexion so intimate, as to enable the mind to receive through
tlm body a knowledge of the condition of the Universe around it, and
to impress on the body the results of its own  determinations; and of
such a nature, that the regularity of the  working of the mind itself is
dependent upon the complete organization of the brain in the first in-
stance, upon the constant supply of pure and well elaborated blood, and
upon  all  those^ influences  which favor the due performance  of the
nutritive operations in general. 
BOOK   II.
SPECIAL  PHYSIOLOGY.
                          CHAPTER  IV.

          OF  FOOD, AND THE  DIGESTIVE  PROCESS.

                 1. Sources of the Demand for Aliment.

  406. The dependence of all  Organized beings upon food or aliment,
must be evident from the facts  stated in the preceding portion of this
Treatise.  In  the first place, the germ requires  a  large and constant
supply of materials, with which it may develope itself into the perfect
being,  by the  properties with  which it is endowed.   In all but the
lowest tribes of Plants, we  find the materials required for the  earliest
stages of the process prepared  and  set  apart by the parent.   Thus in
the seed, the germ itself forms but a  small proportion  of  the Avhole
substance, the principal mass being composed of starchy matter which
is laid up there for its nutrition ; and the act of germination consists in
the appropriation of that  nutriment by the germ, and the consequent
development  of the latter,  up  to the point at Avhich it becomes inde-
pendent  of such  assistance, and is able  for  itself to procure, from the
soil and  atmosphere that surround it, the materials for its  continued
growth.   So  in the egg of  the  Animal, the principal mass is composed
of Albumen and oily matter ; the germ itself being, at the time the egg
is first deposited,  a mere point  invisible to the naked eye ;  these mate-
rials serve as  the food or  aliment of the germ, which gradually draws
them  to  itself, and converts them into the materials of its own struc-
ture ; and at the  end of a certain period, the young animal comes forth
from the egg,  ready to obtain for itself the food which is necessary for
its continued increase in size.
   407. In many  instances  among the lower animals, the form in which
the young animal  emerges from the egg  is very  different  from  that
which it  is subsequently to assume ;  and the latter "is only attained by
a process of metamorphosis.  This change has been longest known, and
most fully studied, in the case  of  Insects and Frogs;  which were for-
merly thought to constitute an exception  to all general  rules in this
respect,—the  Insect coming forth from  the egg in the state of a Worm, 
SOURCES  OF DEMAND  FOR FOOD.               237
and the Frog in the condition of a Fish.  But  it is  now known that
changes of form, as complete as these, occur  in  a  large proportion of
the lower tribes of Animals; so that the absence  of them  is the excep-
tion.   The fact seems to be, that the supply of nutriment laid up within
the egg, among the lower classes, is by no means sufficient to carry on
the embryo to the form it is subsequently to attain ; and its development
is  so arranged, that  it may come  into  the world in a  condition which
adapts it to obtain its own nutriment, and thus to acquire for itself the
materials for  its further development.   Thus the Insect, in its larva or
Caterpillar state, is essentially a foetus in regard to its grade of develop-
ment ;  but it  is a foetus capable of acquiring its own food.   In this con-
dition it attains its full growth as regards size, though its form remains
the same ; but it then, in passing into the Chrysalis state, reassumes (as
it were) the condition of an embryo within the  egg,—the development
of various new parts takes place, at the expense of the nutriment stored
up in its tissues,—and it comes forth  as the  perfect Insect.  In  many
of the loAver  tribes, the  animal quits the egg at a still earlier period in
comparison ;  thus it  has been lately shoAvn by M. Milne Edwards, that
some of the long marine worms consist  only of a single segment, form-
ing a kind of head, when  they leave the  egg; and that the other seg-
ments, to the number (it may  be) of  several hundred, are  gradually
developed from this, by a  process that resembles the budding of Plants.
  408." Up to the period, then, when  the full dimensions of the body
have been attained, and the complete evolution of all its organs has taken
place, a due  supply  of  food  is necessary for  these purposes.  In  the
Plant,  nearly the whole  of the alimentary  materials  taken  into  the
system, are thus appropriated;  the extension of its structure going on
almost indefinitely, and the waste occasioned by  decay being compara-
tively small.   Thus the carbon, which  is given out by the respiratory
process in  the form of carbonic acid, bears but  a small proportion to
that Avhich is introduced by the decomposition of that  same gas,  under
the influence of light (§ 81).  And the fall of the  leaves, which takes
place once  a  year or more frequently, and Avhich  gives back  a large
quantity  of the matter that has undergone the organizing process, does
not occur, until by their  means a considerable addition has been made.
to the  solid and permanent substance of the tree.
  409. This  is not the case, however, with the animal.  Its period of
increase  is limited.  The full size of the body is usually attained, and
all the organs acquire their complete evolution, at a comparatively early
period.  The continued supply of  food  is not then requisite for the ex-
tension of the structure, but simply for its maintenance ; and the source
of the demand lies in the  constant waste, to which,  during its period of
activity,  it  is  subjected.  We  have  seen  that  every action of  the
Nervous  and  Muscular systems involves the death  and decay of  a cer-
tain  amount  of the living tissue,—as  is indicated by the appearance of
the products  of that decay in the Excretions, and  a large part of the
demand for  food will be  consequently  occasioned by the necessity for
making good the loss thus sustained.   Hence we find that the demand
for food  bears  a close relation to  the activity of the animal functions;
so that a diet, Avhich would be superfluous and injurious to an individual 
238
OF FOOD AND THE  DIGESTIVE PROCESS.
 of  inert habits, is suitable and beneficial to one who is leading a life of
 continual exertion ; and this difference  manifests itself in the require-
 ments of the same individual who makes a change in his habits—the
 indolent man acquiring an appetite by vigorous exertion, and the active
 man  losing his disposition to hearty feeding by any cause that keeps
 him from his accustomed exercise.   We see precisely the same contrast
 between Animals of different tribes, whose natural instincts lead them
 to  different modes  of life.   The Birds of .most  active flight, and the
 Mammals which are required to put forth  the  greatest efforts to obtain
 their food, need the largest and most constant  supplies of nutriment;
 but even the least active of these classes  stand in  remarkable contrast
 with the inert Reptiles, whose slow and feeble movements are attended
 with  so little waste,  that  they can  sustain  life for weeks  and even
 months, with  little or no diminution of their  usual activity, without a
 fresh supply of food.
   410. The waste  and decay just adverted to, however, do not affect
 the muscular and nervous tissues alone ; for all the operations of nutri-
 tion involve it to a certain extent.   It has been already shown that the
 acts of absorption,  assimilation, respiration,  secretion,  and reproduc-
 tion ;—all those, in. fact, by which the material for the nutrition of the
 nervous and muscular tissues is first prepared, and subsequently main-
 tained in the requisite purity,—are effected in the Animal, as in the
 Plant, by the  agency of cells, which are continually dying and requiring
 renewal.  In  most  Vegetables,  the death of the  parts  concerned in
 these functions takes place simultaneously, as soon as they have per-
 formed them; the whole crop of leaves  ceasing at once to perform its
 proper actions, and  dropping off;—to be replaced by another, at an in-
 terval that solely depends upon the temperature  under which the tree
 is living (§ 99).  In the  evergreen, however, the process bears a close
 resemblance to that which we observe in the Animal; for the leaves die
 one by one,  and not simultaneously ; and are  constantly undergoing
 replacement, so that the vigor of the  system and the  actiA'ity of its
 nutritive processes never suffer a complete suspension.
  411.  In the  Animal  body, the different classes of  cells,  to which
 allusion has been made, are in like  manner constantly undergoing death
 and renewal; and  this with a rapidity  proportioned to  the  energy of
 their functions.   Hence  a supply of food is as requisite, to furnish the
 materials of their growth, as it is in Plants to furnish the materials of
 the growth of the leaves.   A large part of these  materials are subse-
 quently used for other purposes in the economy; thus, as the leaves
 prepare the sap Avhich is to nourish the woody stem, and to form new
 shoots, so do the absorbing and  assimilating cells prepare and  furnish
 the fluid elements of the blood, which are  to repair the waste of nerve
 and muscle, bone and cartilage, &c.   But still  a considerable amount is
 expended in the simple  nutrition  of these organs themselves, whose
 duration is transient, and whose solid parts are cast off as of no  further
use.  Thus the skin and  all the mucous surfaces are continually form-
ing and throwing off epidermic and epithelial cells,  whose formation re-
quires a regular supply of nutriment;  and only a part of this nutriment
(that which occupies the cavity of  the cells) consists of matter,  that is 
SOURCES  OF DEMAND  FOR FOOD.
239
destined to serve some other purpose in'the system, or that has already
ansAvered it; the remainder (that of which the solid walls are composed)
being furnished by  the nutritive materials of the  blood, and  being
henceforth altogether lost to it.—Thus every act of Nutrition involves
a Avaste or decay of Organized tissue.
  412.  We may observe a marked difference, however, between  the
amount of aliment required, and the amount of waste occasioned by the
simple exercise of the  nutritive  or vegetative functions in  the building-
up and  maintenance of the animal body, and  that which results from
the exercise of the animal functions.   The former are carried on, with
scarcely any intermixture of the latter, during foetal life.  The aliment,
in a state of preparation, is introduced into the foetal vessels;  and is
conveyed  by them into  the various parts of the structure, which  are
developed at its expense.   The amount of waste is then very trifling, as
we may judge by the small amount of excretory matter, the product of
the action  of the liver and kidneys, which has accumulated at the time
of birth ;  although these organs have attained  a sufficient development
to act with energy when called  upon to do  so.  But as soon  as  the
movements of the body begin to take place  with activity,  the waste in-
creases greatly; and Ave even observe this immediately after birth, when
a large part of the time is still passed in sleep, but when the actions of
respiration involve a constant employment of muscular power.—In  the
state of profound sleep, at subsequent periods of life, the vegetative
functions are performed, with  no other exercise of the animal powers,
than is requisite to sustain  them ;  and we observe that the waste, and
the demand for food, are then diminished to a very low point.  This is
well seen in many animals, which lead a life of great activity during
the warmer parts of the year, but which pass  the winter in a state of
profound sleep, Avithout, hoAvever, any considerable reduction of tempe-
rature ; the demand for food, instead  of  being  frequent, is only felt at
long intervals,  and the excretions are much reduced in amount.  And
those animals which become completely inert, either by the influence of
cold, or by the drying-up of their tissues, do not suffer from  the pro-
longed deprivation of food; because not only are their animal functions
suspended, but their  nutritive  operations also  are in  complete  abey-
ance ;  and the continual decomposition of their  tissues, Avhich Avould
otherwise be  taking place, is checked by the cold or desiccation; so that
the Avhole series of changes which goes on  in  their active condition, is
completely at a stand.
   413.  But there is another most  important cause of demand for food,
amongst the higher Animals, which does not exist either amongst  the
loAver Animals, or in the Vegetable kingdom.  We have seen (chap, ii.)
that Mammals and  Birds, and  to a  certain  extent  Insects also,  are
able to  sustain the heat of their  bodies at  a fixed standard, and thus
to become  independent of  variations in external temperature.  This
they are  enabled to do, as will be explained  hereafter, by a process
strictly analogous to ordinary  combustion; the carbon and hydrogen
which are directly supplied by their food, or which have been employed
for a time in the composition of  the living tissues and are then set free,
being made to unite with oxygen introduced by the respiratory process, 
240            OF FOOD AND THE  DIGESTIVE PROCESS.
   and thus giving off as much heat as if the same materials were burned
   in a furnace.   And it has been further shown, that the immediate cause
   of death in a warm-blooded animal, from Avhich food has  been entirely
   withheld, is the inability any longer to sustain the temperature, which
   is requisite for the performance of its vital operations (§ 117).  Hence
   Ave see the necessity for a constant supply of aliment, in the case of
   warm-blooded animals,  for  this purpose alone :  and the demand  will
   be chiefly  regulated  by the external temperature.   When the heat is
   rapidly carried off from the surface, by the chilling influence of the
   surrounding air, a much greater amount  of carbon  and hydrogen must
   be consumed  within  the  body,  to maintain its proper heat, than when
   the  air is nearly as warm as the body  itself;  so that  a  diet which is
   appropriate  to  the former circumstances, is superfluous  and injurious
   in the latter; and the food which is amply sufficient in a warm climate,
   is utterly destitute of power to enable the body to resist  the influence
   of severe cold.   This is a fact continually  experienced; both in the
   ordinary  recurrence of changes of  temperature in our  own climate;
   and, still more remarkably, when the same  individual is subjected to
   the  extremes of heat and cold, in successively visiting the tropical  and
   frigid zones.
     414. Thus we  find that in the Animal body, aliment is  ordinarily
 ,'  required for four  different purposes.  First, for the original  construc-
■   tion or building-up of the organism.  Second, to supply  the loss occa-
 !  sioned by its continual decay, even when in a state of repose.   Third,
 i  to compensate  for the waste occasioned by  the active exercise of the
 I  nervous and  muscular systems.  And  Fourth, to supply the materials
   for  the heat-producing  process, by which the temperature of the body
   is kept up.—The  amount  required for these several purposes will vary
   according to  the  conditions of the body, as regards exercise or repose,
 I  and external heat or cold.  It is also  subject to great  variation with
 ,'  difference of age.   During the period of growth, it might  be anticipated
 J  that a larger supply  of food would be required,  than when the  full
   stature has been attained; but a very small daily addition Avould suffice
   in the case of a child or youth, to produce the entire increase of aAvhole
\  year.  Yet  every one knoAvs that the  child requires much  more food
   than the adult, in  proportion to his comparative bulk.   This results from
   the much more rapid change in the constituents of his fabric; Avhich is
   evident from the  large proportional amount of his excretions, from the
\  quickness with  which the effects of illness or  of deficiency of food mani-
1  fest themselves in the diminution of the bulk and firmness of the body,
   from the short duration of life when food is altogether  Avithheld, and
\   from  the readiness with which  losses of substance by disease or injury
   !are repaired, when the nutritive processes are  restored to  their full
   activity.   The  converse of all this holds good in the aged person.   The
   excretions diminish in amount,  the want of food may be sustained for a
   longer period, losses of substance are but slowly repaired, and everything
   indicates that  the interstitial changes  are performed with comparative
\  sloAvness; and,  accordingly, the  demand for  food is far less in propor-
|  tion to the bulk of the body than  it is  in the adult, and  may be even
 V^ absolutely less  than  in the child of a fourth of its weight.
 
          INFLUENCE OF  VARIATIONS  IN SUPPLY  OF FOOD.       241

  415. The demand for food is  increased by any  cause, which creates
an unusual drain or waste in  the  system.   Thus  an extensive  suppu-
rating action can be sustained only by a large supply of highly-nutri-
tious food.  The mother,  who has to furnish  the  daily supply of milk
which constitutes the sole support of her offspring, needs an unusual
sustenance for this purpose.  And  there  are states of the system, in
which the solid tissues seem to possess an unusual tendency to decom-
position,  and in Avhich an  increased supply of aliment is therefore re-
quired.   This  is  the  case, for example, in diabetes; one of the first
symptoms of which disease is the craving appetite,  that seems as if it
would never be satisfied.  And there can be no doubt that, putting
aside  all  the  other  circumstances that have been alluded to, there is
much difference amongst  individuals, in regard to the rapidity of the
changes which their organism undergoes, and the amount of food re-
quired for its maintenance.
  416. The influence of the supply of  food upon the size of the indivi-
dual, is very evident in the Vegetable kingdom ; and it is most strikingly
manifested, Avhen a plant naturally growing in  a poor dry soil is trans-
ferred to a damp rich one, or Avhen we  contrast two or more individuals
of the same species, groAving in localities of opposite  characters.   Thus,
says Mr. Ward, " I have gathered, on  the chalky borders of a wood in
Kent, perfect specimens in full flower of Erythrcea  Centaurium (Com-
mon Centaury), not  more than half an  inch in height; consisting of one
or tAvo pairs of most minute leaves, with one solitary flower: these were
growing on the bare chalk.  By tracing the plant toAvards and in the
wood, I found  it gradually increasing in size, until its full development
was attained in the  open  parts of the wood, where it became a glorious
plant, four or five feet in elevation, and covered with hundreds of  flow-
ers."   On the other hand, by starvation, naturally or artificially induced,
Plants may be dwarfed, or reduced in stature :  thus the Dahlia has been
diminished from six feet to two; the  Spruce Fur from a lofty tree to  a
pigmy bush; and many of the trees of plains become more and  more
dwarfish  as they ascend mountains, till at length they exist as  mere
underwood.  Part of this  effect, however, is doubtless to be  attributed
to diminished  temperature; which, as already remarked, concurs with
deficiency of food in producing inferiority of size.
  417. Variations in the supply of food would not appear to be effec-
tual in producing a corresponding variety of size in the Animal king-
dom :  this is  not,  however,  because animals  are in any degree less
dependent than Plants upon a due supply of food; but because such  a
limitation of the supply, as would  dwarf a  Plant  to any  considerable
extent, would be fatal  to  the life of  an Animal.   On the  other  hand,
an  excess  of food,  which (under favorable circumstances), would pro-
duce great increase in the size of the Plant, would have no correspond-
ing influence on the Animal; for its size appears to be restrained within
much narrower limits,—its period of groAvth being restricted to the early
part of its life, and the dimensions proper to  the  species being rarely
exceeded in any great degree.  Even in the case of giant individuals,
it does not appear that the excess of  size is produced by an over-supply
of food ;  but that the larger supply of food taken in is called for by the 
242
OF FOOD AND  THE DIGESTIVE PROCESS.
unusual wants of the system,—those wants being the result of an extra-
ordinary activity in the processes of growth, and being traceable rather
to the properties inherent in the system, than to any external agencies.
Thus we not unfrequently hear of  children, who have attained an extra-
ordinary size at the age of a few years ; and this excess of size is usually
accompanied with other marks of precocious  development.   We shall
hereafter see, that a provision exists in the Digestive apparatus, which
absolutely prevents the reduction and  preparation  of  the food, in any
amount greatly  surpassing that which the wants of  the system demand
(§ 474); and it is probably to this cause, in part, that we  are to  attri-
bute the small degree of influence exerted by an excess of food, in pro-
ducing an increased development of the Animal frame.
   418. The  influence of a diminished supply of food, in producing a
marked inferiority in  the  size of Animals,  is most effectually  exerted
during those early periods of growth, in which the  condition of the
system is most  purely Vegetative.  Thus it is well known to Entomo-
logists, that, whilst it is rare to  find Insects  departing widely from the
average  size on  the  side of excess, dwarf-individuals, possessing only
half the usual dimensions, or even less, are  not uncommon ; and there
can be little  doubt that these have suffered  from a diminished supply
of nutriment during their larva state.   This variation is  most apt to
present itself in  the  very large species of Beetles, which pass several
years  in the larva state; and  such dwarf  specimens have even been
ranked as sub-species.  Abstinence has been  observed to produce the
effect,  upon some Caterpillars, of diminishing the number of moults and
accelerating  the  transformation ;  in  such cases, the Chrysalis is  more
delicate, and the  size  of the perfect Insect much below the average.
   419. One of the most remarkable examples known, of the effect of
food in modifying  the development  of Animals, is to be found in the
economy of  the Hive-Bee.   In every community, the majority of indi-
viduals consists of neuters; which may be regarded as females, having
the organs of the female sex undeveloped; and Avhich,  whilst incapable
of reproduction, perform all the labors of the hive.  The office  of  con-
tinuing the  race is restricted to  the queen; who  is  the only perfect
female in the community.   If by any accident the  queen be destroyed,
or if she be purposely removed for the sake  of experiment, the bees
choose tAvo or three  from amongst the  neuter larvce, wdiich are being
nurtured in their appropriate cells; and these they cause to be developed
into perfect queens.   The first operation is to change the cells in wThich
they lie into royal cells; these differ considerably from the ordinary ones
in form,  and are  of much larger dimensions.   This is  accomplished by
breaking down the walls of the surrounding cells,  removing the eggs
or grubs they may contain, and  rebuilding the central  cell upon an en-
larged scale, and upon the  same plan as the  royal cells in which the
queens are  ordinarily reared.  Whilst this  operation  is going on, the
maggot is supplied with food of a very different nature from the farina
or bee-bread (composed of  a  mixture of pollen and honey),  which has
been stored  up for the nourishment of the workers; this food being of a
jelly-like  consistence  and  pungent  stimulating  character.  After the
usual transformations, the grub becomes  a perfect queen ; differing from 
EFFECTS  OF EXCESS OF  FOOD.
243
the neuter bee, into which it would have otherwise been changed, no?
only in the development of the reproductive system, but in the general
form of the body, the proportionate  shortness of the wings, the shape
of the  tongue, jaws, and sting, the absence of the hollows on the thighs in
Avhich  the pollen is carried, and the loss of  the power of secreting Avax.
   420. That insufficiency of wholesome food, continued through succes-
sive generations, may produce a marked effect, not merely  upon the
stature, but  upon the form  and condition of  the  body,  even in the
Human race, appears from  many cases, in  which such influence has
operated on an extensive scale.   Thus there are parts of Ireland inha-
bited by a population descended from  those who were  treated by the
English as rebels two centuries since, and who were driven  into  moun-
tainous tracts, bordering on the sea, where they have been since exposed
to the  two great brutalizers  of the human race, hunger and ignorance.
The present  race  is distinguished physically  from the kindred race  of
Meath and other neighboring districts, Avhere the same causes have not
been in  operation, by  their Ioav stature (not exceeding five feet two
inches),  their pot-bellies  and  bow-legs; whilst their open projecting
mouths, with prominent teeth and exposed gums, their advancing cheek-
bones and depressed noses, bear barbarism in their  very front.  " These
spectres of a people that once were well-grown, able-bodied,  and comely,
stalk abroad  into the daylight of civilization, the annual apparitions  of
Irish  ugliness and Irish  want."—The  aboriginal population of New
Holland,  as  a whole, presents a similar aspect; and apparently fromj
the operation of the same causes.
   421. When a larger quantity of azotized food (§429) is habitually
consumed than the Avants  of  the system require, it is not converted into
solid flesh; but it is got rid of by the various  processes of excretion.
The increased production of Muscular fibre  depends, as we have already
seen (§ 362),  upon nothing so much as the  exercise of the  muscle.   It
cannot take place unless the  blood supply it with the materials; but no
degree of the richness of the blood can alone produce it. Consequently, the
accumulation of nutritive matter in the blood is so far from being a con-
dition of health, that it powerfully tends to  produce disease,—either  of
an inflammatory character, if the fibrine predominate,—or  of the he-
morrhagic  character, if the red corpuscles predominate.   This state  is
most apt to present itself in those who live well and take little exercise;
and the remedy for it is either to diminish  the  diet, or to increase the
amount of exercise, so as  to  bring the tAvo into harmony.
   422. The continued over-supply of food has several injurious effects:
it disorders  the digestive processes, by  stimulating  them  to  undue
activity, and  lays the foundation for a complete derangement of them;
it giAres a predisposition to the various diseases of repletion, as already
noticed; and it  throAvs upon  the excreting organs much more than their
proper amount  of labor, besides tending to produce a depraved condi-
tion of the matters to be  drawn off by them,  which renders the proper
act of excretion  still  more difficult.  When this is the case various disor-
ders arise, caused by the retention, within  the  circulating  current,  of
substances Avhich  are very noxious  to  the  general  system, and  which
become most  fertile sources of disease.   What are  commonly regarded 
244            OF FOOD AND THE  DIGESTIVE PROCESS.

as diseases of the biliary and urinary organs, are really, in a large pro-
portion of  cases, nothing else than  disordered actions of those  organs,
occasioned by the irregular mode in which the products of decomposition
are formed within the blood, and dependent upon some error  in diet,
either  as regards quantity or quality.   Thus the "lithic acid diathesis,"
in which there is an undue  proportion of that substance in  the urine,
and of which Gout is a particular manifestation, is due, not to disorder
of the kidney, but to an undue production of lithic acid in the blood;
so long as  the excreting action of the kidney is sufficient to prevent its
accumulation in the blood, so long the general health is but little affect-
ed; but whenever that action  receives a check, various constitutional
symptoms  indicate that the system is disturbed  by the presence of this
product  of decomposition.   And though our remedies  may be rightly
directed, in part, to facilitating its escape through the kidneys, yet the
radical cure is to be sought only in  the regulation of the diet, and in the
prevention of the first production of the substance in question.—Similar
remarks might  probably be applied to the disorders of the Liver; but
we are,  from various  causes, far less perfectly acquainted  with their
character than we are with those of the Kidney.
   423. There is only one tissue, the increase of which is directly pro-
duced  by an over-supply of food.   This is the Adipose or fatty.  It is
formed almost entirely at the expense of the non-azotized constituents
of the food (§ 430);  the walls of the cells, into which  the fatty matter
is secreted, being the only part of this  tissue  that is derived from the
proteine-compounds of the blood.  The production of the adipose tissue
is most  directly favored  by the presence of a large amount  of fatty
matter in  the food ;  but it may also be effected, as will be  presently
shown, by the conversion of starchy and saccharine substances into fatty
compounds.   It cannot occur, unless there be in the food a larger pro-
portion  of substances  that can be thus appropriated, than is sufficient
to maintain the  heat of the system by the respiratory process.  Conse-
quently, whatever increases the demand for heat is unfavorable to the
deposition  of fat; and vice  versd.  The fattening of animals is now
brought to a regular  system; and experience has  showrn that  rest and
a warm temperature, with food containing a large amount of oily mat-
ter, are most conducive to the accumulation.  Rest acts by keeping the
respiration at a low standard;  for it will  hereafter  be shown (chap.
viii.), that a much larger proportion  of carbonic acid is throAvn  off when
the body is in active  movement than when it  is  in  repose.  External
warmth has  the same effect;  the  demand upon the  calorifying power
being  diminished,  and more of the combustible material being left, to be
stored up  as fat.
«   424. The  deposition of fat affords a supply of combustible matter,
against  the time when it may be needed; and it is consequently found,
that the duration of life in warm-blooded animals, when they are com-
pletely deprived of food, is in a great degree proportional to the amount
of fat they have previously accumulated.  There is no sufficient reason
to believe  that fatty matter can be converted, within the animal  body,
into a proteine-compound, which can  serve for the nutrition of  the mus-
cular  and other tissues.   But  the greatest and most  constant waste, 
             VALUE OF DIFFERENT MATERIALS  OF FOOD.         245

when an animal is undergoing starvation, is that which is occasioned by
the heat-producing process;  this, so long as the  supply lasts, is kept up
by the store of fat, which is gradually consumed;  and when it is com-
pletely exhausted, the temperature falls, hour  by hour, until life can no
longer be sustained (§ 117).   The use of this store of fat, in supplying
any temporary deficiency  in the food,  becomes evident from such expe-
riments ; for when it has been completely exhausted, the Avithholding of
a single meal  proves fatal, from the want of power to sustain the calori-
fying process.  We find that animals, Avhich are likely to  suffer from
deficiency of food in  the  Avinter, or which spend that period in a state
of quiescence, have a tendency to accumulate  a store of  fat in the
autumn; which tendency  seems principally to depend upon the nature
of their food.  We observe it chiefly in those Birds and Mammals which
live upon seeds and grains;  and these,  when ripe, contain a large quan-
tity of oily matter, which  thus becomes a valuable store against  the time
of need.  There are many birds such as the beccafico, so much esteemed
in Italy, which are described, if killed at this season, as being  " lumps
of fat."
   425. It  is well known  to breeders  of cattle  that  some  varieties or
breeds have a much greater  tendency to the production  of Adipose tis-
sue than others placed under the same  circumstances ;  and the former
are therefore selected to undergo the fattening process.   Corresponding
differences may be met with among different individuals of  the Human
race; some persons having a remarkable tendency to  the production of
fat, under circumstances Avhich do not seem by  any means favorable to
it, whilst others appear as much indisposed to this deposit.   The latter
condition we notice particularly in that temperament which is commonly
termed the "bilious;"  and  it is important to  bear in mind that, Avhere
such an indisposition  exists, any superfluity of  fatty matter in the food
taken into the system must  be excreted again  through the liver, instead
of being retained  and stored  up  in  the body.   It  is  very desirable,
therefore, that such persons should abstain from any excess of this kind ;
since an habitual call  upon the liver, to relieve the system of a super-
fluity of fatty matter, is certain to produce a disordered state of that
organ; and in order to prevent it, the diet should be  altered, so as to
include less of fatty matter, or the  amount  of exercise should  be in-
creased so that it may be burned off by the additional respiration which
then takes place.                                                ^X~
   426.  We see, then, that  the amount of  food  which can be  properly  „
appropriated  by  the system varies  considerably in different individuals,
and in the same individual under different circumstances.  Consequently
it is impossible to give any  general rule, which shall apply to every one
alike.   The average quantity required by  adult men, leading an active
life, and exposed to the ordinary vicissitudes of temperature in their own
climate, seems to be  from  30 to 36  ounces  of dry aliment.    But  a
healthy condition may be kept up  on scarcely  more than half this allow-
ance, if the muscular poAA*ers are but little  exerted, and the surrounding
temperature be high ; provided that it consists of substances of a nutri-
tious kind, united in proper proportions.
   427.  The value of different substances as aliment depends in the first 
246
OF FOOD AND THE  DIGESTIVE PROCESS.
  place upon the quantity of solid matter  they contain ; being of course
  the greater as the solids form the larger proportion of the entire weight.
:  Many esculent vegetables contain so large a quantity of water, that the
;  nutriment they afford is very slight in proportion  to their bulk.—Next
  it depends upon the proportion of digestible matter which the solid parts
  include;  for it is not every substance containing the  requisite ingre-
  dients that is capable of being reduced to a state which enables it to be
  absorbed.   Thus woody  fibre is  composed of the  same elements as
  starch-gum; but it passes out of  the intestinal  canal unchanged and
  therefore affords no nutriment.  In the same manner, the horny tissues
  of animals  though nearly allied to proteine in their composition, are
  completely destitute of nutritive properties to man and the higher ani-
  mals, because  not capable of being  reduced by their digestive process ;
  though certain insects appear capable of living exclusively upon them.
    428.  But when the watery and indigestible parts of  the food are put
  out of consideration, and our attention is directed only to the soluble
  solids, we find a most important difference in the  chemical composition
  of different substances, which renders them more  or less appropriate to
v the different purposes Avhich have to be answered in the nutrition of the
  body.  It has  been already pointed  out, that Vegetables possess the
-, power of combining the  elements furnished by the inorganic world into
  two classes of compounds,—the ternary, consisting of oxygen, hydrogen,
  and carbon,—and the quarternary,  which consist of these elements, with
<- the addition of azote or  nitrogen.    These two classes  are hence termed
  the non-azotized, and the azotized.
    429. Now the azotized compounds are required  for the reparation of
  the waste  of  the muscular tissue,  and for the general nutrition of the
  body; consequently,  unless the food contain a sufficient proportion of
  these substances,  the body  must  be insufficiently nourished, and the
  strength  must diminish, even though other  elements of the food be in
  superabundance.   The azotized substances formed by Plants are essen-
  tially the same, as already shown  (§  174), with  those Avhich are fur-
  nished by the Albuminous solids and fluids of animals; but the quantity
  of them is usually small in proportion to the non-azotized, being consi-
 i derable only in the Corn-grains, in  the seeds of Leguminous plants, and
  in some  other  products, which the universal experience of  ages has
  demonstrated to be the most  nutritious of Vegetable  substances.   The
  other azotized compounds existing in the  animal body may be elaborated
  by the transformation of these proteine-compounds ;  so that when they
  are duly  supplied, the system cannot become enfeebled for want of sup-
  port.—But  there is  another  azotized compound, Gelatine, that is fur-
  nished by Animals, to which  nothing  analogous exists in  Plants; and
  this, although it cannot sustain life  by itself, is a valuable adjunct to the
  proteine-compounds.  For as the gelatinous tissues suffer waste in com-
  mon with the others, it is evident that if the gelatine be  supplied already
  prepared, it may be at once applied to their nutrition ;  and thus the pro-
  portion of proteine, which they would otherwise require, is not demanded,
  and the labor  of transformation is also  saved.    Further, there is this
  great advantage in combining a proportion of gelatine Avith the food,—
  especially when the digestive powers are feeble,—that being already in a 
VALUE OF DIFFERENT  MATERIALS OF FOOD.         247
state of perfect solution, it is taken up at once by the simple act of physi-
cal absorption or  endosmose, instead of requiring any preliminary pre-
paration .or exertion of vital activity for its absorption.  But there is no
evidence that Gelatine can ever be transformed into a proteine-compound,
and can thus be applied to the nutrition of the muscular and other fibrous
tissues ; and the presumption, derived from the result of various experi-
ments, is very strong the other wray.
  430. The Non-azotized compounds, which are presented to us in great
abundance in the Vegetable kingdom, exist under  various forms;  of  0->-T^
which the principal are starch, sugar, and oil.  The  two former may be  •' <
regarded as belonging to  one class, the Saccharine ;  because we know
that starch and the substances allied to it may be converted into sugar*
by simple chemical processes, and that this  transformation  takes place T
readily both in the Vegetable and Animal economy.   On the other hand,
the Oily matters  contained in  vegetable and animal food,  are usually  "
ranked as a distinct  group of alimentary substances; and  it has been  _
maintained that,  under no circumstances, has the Animal the power of
elaborating  fatty matter from starchy or  saccharine compounds.  But
this is now knoAvn to be an unfounded limitation; since the  transforma-
tion of a saccharine  into  a fatty compound takes place in  the case of „
bees, which  form  wax when fed upon pure sugar, and  it has been recently
shown that it may take place in the laboratory of the Chemist, butyric
acid (the fatty acid of butter) being one of the products of the fermenta-
 tion of sugar, taking  place under peculiar circumstances.   It  appears, /
 indeed, to be one office of the Liver to effect this transformation,  as will
 be explained hereafter.
   431. The great use of these substances in the Animal economy, is to
 support the respiratory process,  and thus maintain  the temperature of
 the body.   We have seen that, in the compounds of the Saccharine group
 (in Avhich Starch is included), the amount  of  oxygen is  no more than
 sufficient to form water with the  hydrogen of the substance  (§ 12), so
 that the carbon is free to combine with the oxygen taken in by the lungs,
 and thus becomes a  source of calorifying power.   Again,  in the oily
 matters taken in  as food, the proportion of oxygen is far smaller, so that
 they contain a large quantity of surplus hydrogen, as  well as of carbon,
 ready to be burned  off in the system, and  thus to  supply  the heat re-
 quired.   This is obviously the ordinary destination of the alimentary
 matters belonging to  these classes; and the  greatest economy in  the
 choice of diet is  therefore exercised, when it is composed of azotized sub-
 stances in sufficient amount  to repair  the waste of the system,  and of
 non-azotized compounds which include free carbon and hydrogen  in suf-
 ficient quantity to develope (with the aid of other processes)  the requisite
 amount of heat  by combination with oxygen. But if there  be a deficiency
 in either of these kinds of aliment, the body  must  suffer.   Should  the
 supply of duly-prepared azotized matter be  less than is required to  re-
 pair the waste of the albuminous and gelatinous tissues, then these di-
 minish in bulk and in vital poAver, though the heat  of the body may be
 kept up to  its proper standard.   But if the non-azotized matter should
 be supplied in sufficient amount, or in a form in  which it cannot be
 appropriated, the heat of the body cannot be sustained in any other • way,
 than by drawing upon the store  of fat previously laid up. 
248            OF  FOOD AND THE DIGESTIVE  PROCESS.
  432. Various circumstances lead to the  belief, that  the  saccharine
compounds are thus carried off by the respiratory process, within a short
time after they have been introduced into the  system.   They have not
been detected in the chyle drawn from the lacteal absorbents; but there
seems reason to believe that, in consequence of their ready solubility,
they are  directly taken up by the blood  (§ 493), and that they are so
rapidly burned off there,  as  to escape notice in  that fluid.  But it has
been  lately shown  by Dr. Buchanan, that,  if the blood  be examined
within a  short time after  a meal  consisting in  part of farinaceous and
saccharine substances, a very appreciable quantity of saccharine matter
is found  in it.  This soon  disappears, however, being eliminated or sepa-
rated from the blood by the action of the lungs.  In fact it is very pro-
bable, that a large proportion of the matter  thus taken  in never enters
the general circulation at  all; as the blood of the mesenteric veins pro-
ceeds to  the lungs,  after passing through the  liver, before it  is trans-
mitted to the systemic arteries, and may there lose its saccharine matter,
as fast as this is taken in from the stomach.   After a meal containing the
ordinary admixture of saccharine, oily, and albuminous compounds, it is
probable that the saccharine are first received into the blood, and are
the first  to be eliminated from it; and that, by the time  they have been
all consumed, the oily matter, introduced through the  more  circuitous
channel of the lacteal system, is ready to answer the same purpose.   If
these are exhausted before a fresh  supply of food  is taken in, cold as
well as hunger is experienced ;  and the body is in  this  condition  pecu-
liarly liable to suffer from any depressing causes, such as a low external
temperature, poisonous miasmata, &c.;  hence the prudence of avoiding
exposure to such influences upon an empty stomach.
  433. We can thus in part account for the fact, Avhich  universal  expe-
rience has established, that  in Avarm-blooded animals, a  mixture of azo-
tized and non-azotized substances is the diet most conducive to the welfare
of the body ; and that, in all but the purely  carnivorous tribes, the diet
provided by Nature consists not only of albuminous, gelatinous, and oily
substances, such as are furnished by the flesh and fat  of animals, but
also of saccharine or farinaceous matter.  This is the diet to  which Man
is evidently best adapted ;  and it is remarkable hoAV completely accordant
is his use of the ordinary  materials of food, with the principles now es-
tablished by chemical and physiological research, in regard to the wants
of his bodily system, and the best mode of supplying them.   Thus, good
AA'heaten  bread contains, more nearly than any other substance in ordi-
nary use, that proportion  of azotized  and  non-azotized  matter, Avhich
is adapted  to repair the Avaste of the system, and to supply the neces-
sary amount of combustible  material, under  the ordinary conditions of
civilized  life in temperate climates;  and we find that  the  health and
strength  can be more perfectly sustained upon that substance, than upon
any other taken alone.  The addition of  a moderate quantity of butter
increases its  heat-producing powers:  and this  is especially useful Avhen
the temperature is  Ioav, under  which condition  there  is usually an in-
creased disposition  to  the  employment of fatty matters as articles of
food.   On the other hand, if the body be subject to  violent  exertion,
advantage is gained by increasing the proportion of the proteine-com-
pounds, by the addition of animal flesh;  and, under any circumstances, 
VALUE OF  DIFFERENT MATERIALS OF FOOD.          249
there is an economy in the use of gelatine, in the form of soup, which
diminishes the demand for other azotized matter.  The use of animal flesh,
hoAvever, as a principal article of diet, except when the individual is lead-
ing the incessantly-active life of a carnivorous animal, is very far from
being economical, and is positively injurious  to the welfare of the body.
   434.  On the other hand, in rice, potatoes, cassava-meal, and similar
substances, the farinaceous or saccharine components form so very large
a proportion of the Avhole mass, and the proteine-compounds are present in
so very small an amount, that they are insufficient to support the bodily
vigor when taken alone, unless a larger quantity be ingested, so as to
supply  the requisite  proportion of azotized  matter.  But Avhen these
substances form  part  of  a mixed diet, the other ingredients  of Avhich
consists of animal flesh, a much smaller quantity of them suffices; and
the same kind  of combination  is then formed, as exists  in the single
article of bread.  Those in Avhose diet the farinaceous elements  predo-
minate largely, and the azotized compounds exist in the smallest amount
compatible with the maintenance of  the bodily vigor, are  exempt from
many diseases incident to those Avho  live more highly; thus among the
potato-eating  Irish, and the oatmeal-feeding Scotch,  gout is a disease
never heard of;  whilst among the richer classes  of  the same countries,
there is no peculiar exemption from it.
   435.  The oily constituents of  food are most abundant in the  diet of
the inhabitants of frigid zones,  who feed upon Avhales, seals, and other
animals loaded with fat, and Avho devour this fat with avidity, as if in-
stinctively guided to  its use.   It is  by the  enormous quantity of this
substance taken in  by them, that they are enabled to pass a large  part
of the year in a temperature beloAV that of our coldest winter, spending
a great  portion of  their time in the open air ; as well as to sustain  the
extreme of cold, to Avhich they are occasionally subjected.   And in  con-
sequence of its being more sloAvly introduced  into the system than most
other substances, a larger quantity may be taken in at one time, Avith-
out palling the appetite ; Avhilst its  bland  and non-irritating  character
favors its  being retained  until it is all  absorbed.   In this manner, the
Esquimaux and Greenlanders  are enabled to  take  in 20 or 30 pounds of
blubber at a meal; and, Avhen thus supplied, to pass several days without
food.  On the other hand, among the inhabitants of warm climates there is
comparatively little disposition to the use of oily matter as food ; and the
quantity of it contained in most articles of their diet is comparatively small.
   436.  In the Milk, Avhich is the sole nutriment of the young Mammalia,
during the period immediately succeeding their birth, Ave find an admix-
ture of albuminous, saccharine,  and oleaginous  substances; AYhich in-
dicates the intention of the Creator, that all  these should  be employed
as components of the ordinary diet.   The Caseine or cheesy matter is a
proteine-compound ; the Butyrine of butter is but a slight modification
of its ordinary fats ; and its Sugar differs from that in common use, only
by its larger proportion of Avater.  The relative amount of these ingre-
dients in the milk of different animals is subject, as Ave shall hereafter
see, to considerable variation;  but they constantly exist, at least in the
milk of  the Herbiv ^rous Mammalia, and of those  Avhich, like Man,  sub-
sist upon a mixed diet.   But it has been recently asserted, that the milk 
250             OF FOOD AND THE DIGESTIVE PROCESS.
of the purely Carnivorous animals is destitute of Sugar, consisting, like
their food, of proteine-compounds and  fatty matter only.
   437.  No fact in Dietetics is better established, than the impossibility
of long  sustaining health,  or even life, upon any single alimentary prin-
ciple.   Neither pure albumen or fibrine, gelatine or gum, sugar or starch,
oil or fat,  taken alone for any length  of time, can serve for the due nu-
trition of the body.  This is partly due, so far  as the non-azotized com-
pounds  are concerned, to their incapability of supplying the waste of the
albuminous tissues.   This reason does not apply, hoAvever, to the pro-
teine-compounds ;  which can  not  only serve for  the reparation of the
body, but can also afford the carbon and hydrogen requisite for the sus-
tenance of its  temperature.   The real cause is to be found in the fact,
that the continued use of single  alimentary substances excites such a
feeling  of disgust, that the animals experimented on seem  at last to
prefer starvation, rather than  the  ingestion of them.  Consequently it
is quite impossible to ascertain, by such experiments, the nutritive poAver
of the different alimentary principles; no animal  being capable of sus-
taining  life upon less than tAvo of them at least.   The same disgust is
experienced by Man, when too long confined to  any article of diet, which
is very  simple  in its composition ;  and a craving for change is then ex-
perienced, which  the strongest will is scarcely  able to resist.   Thus, in
the treatment of Diabetes, a disease in which there is an undue tendency
to the production of sugar in the system, it is very important to abstain
completely from the  introduction of  saccharine or farinaceous matters
in the food; but the craving for  vegetable  food, which is experienced
when the diet  has long  consisted of meat alone, is such as to make per-
severance  in the latter very  difficult; and  a  means has  been latterly
devised of supplying this want without injury, by the use of bread from
which the starchy portion  has been removed, the gluten or azotized
matter  alone being eaten.*
   438.  The organic  compounds, which have been  enumerated as sup-
plying the various wants of the system, would be totally useless without
the admixture  of  certain inorganic substances, which also form  a con-
stituent part of the bodily frame, and  which are constantly being voided
by the excretions, especially  in  the Urine.    These  substances have
various uses in  the  system.    Thus common  Salt,  or the Chloride  of
Sodium, appears to afford, by its decomposition, the muriatic  acid which
is concerned in the digestive process, and the soda which is an important
constituent of  the bile.  Its presence in the serum  of the blood, also,
and in  the various animal fluids which are  derived from this, probably

  * As an illustration of the advantage of this treatment, even in unpromising cases,
the author may cite an instance which has  come under his own observation.   The
patient was  a man of 72 years of age; the disease had lasted at least a year, and was
decidedly on the  increase; considerable  loss  of flesh and of muscular vigor had taken
place ; and the quantity of sugar in the urine was such as to make it quite sweet to the
taste.  By the careful restriction of his diet to animal flesh and gluten-bread, this in-
dividual  kept the disease in complete check for more than 'five years; he  gained flesh,
and improved in strength ; and his urine lost its sweetness.  Having two or three times
ventured upon a return to his ordinary diet, his old symptoms  immediately manifested
themselves, warning him of the necessity of  perseverance in  the strict regimen pre-
scribed for him.  He died at last, at the age of 77 years,  of old age, rather than of any
specific disease. 
               NECESSARY MATERIALS  OF ANIMAL FOOD.           251

 aids in preventing the decomposition of the organic constituents of these
 fluids.— 1 hosphorus  has been supposed, until  recently, to be chiefly re-
 quisite as one of tne materials of the nervous tissue (§  383);  and also,
 when acidified by oxygen, to unite with lime in forming the bone-earth
 by which bone is consolidated.   But there is reason to believe, from the
 results of late inquiries, that the acid and alkaline phosphate of lime and
 soda are  very important constituents of the various fluid secretions, and
 have a large share in their respective actions.—Sulphur exists in small
 quantities in several animal tissues ; but  its  part  appears to be by no
 means so important as that performed by phosphorus.—Lime is required
 for the consolidation  of  the bones, and for the production of the shells
 and other hard parts that form the skeletons  of the  Invertebrata ;  and
 also as the base of the  acid phosphate, which has  been just referred to
 as an important constituent  of  the animal  fluids.—Lastly, Lron is  an
 essential  constituent of Haematosine; and is  consequently required for
 the production of the red corpuscles of the blood in  Vertebrated animals.
   439. These  substances  are contained, more or less abundantly, in
 most of these articles generally used as food;  and where they are defi-
 cient, the animal suffers in consequence, if they be not supplied in any
 other Avay.   Thus common Salt exists, in no inconsiderable amount, in the
 flesh and fluids of animals, in the milk,  and in the substance of the egg;
 it is not  so abundant, however, in Plants; and the deficiency is usually
 supplied to  herbivorous animals in some other way.   Thus/salt is pur-
 posely  mingled with the food of domesticated animals; and  in most
 parts  of  the world inhabited  by wild cattle, there are spots  where it
 exists in the soil, and to which they resort to obtain it.  Such are  the
 "buffalo-licks" of  North America.—Phosphorus exists also, in combi-
 nation with proteine-compounds, in all  animal  substances composed of
 these ;  and  in the state of phosphate, combined with lime, magnesia, and
 soda, it exists largely in many vegetable  substances  ordinarily used  as
 food.   The phosphate of lime is  particularly abundant in the seeds of
 the grasses; and it also exists largely,  in combination with  caseine, in
 Milk.—Sulphur is derived alike from vegetable and animal substances.
 It  exists, in union with proteine-compounds,  in flesh, eggs, and milk;
 also in  several  vegetable substances; and, in  the  form of sulphate  of
 lime, in most of the river and spring water that we drink.
   440. Lime is  one of the  most universally diffused  of all  mineral
 bodies; for there are very few Animal or Vegetable substances in which
 it does not exist.   The principal  forms  in which it is  an element of Ani-
 mal nutrition, are the carbonate and phosphate.  Both these are found
 in the ashes of the grasses, and of other plants  used  as food; the phos-
 phate of lime being particularly abundant (as already mentioned) in the
 corn-grains.  The production of these cannot take place, to their fullest
 extent, unless the soil previously contain phosphate of lime in a state in
 which  the  plant can  receive it;  and it is now understood,  that the
 diminished fertility of many lands is due, in great  part, to the  exhaus-
 tion of the soil as regards this ingredient.   The restoration of the alka-
 line and  earthy phosphates to the soil, in the form  of  manure, is the
 obvious means of preserving its fertility; but so long as a very large
proportion of the excrements  of animals  (the  materials of which are
originally derived from the earth, through the vegetables it supplies) is 
252
OF FOOD AND THE  DIGESTIVE PROCESS.
allowed to run to waste, so long will it  be necessary that  the  requisite
amount of phosphate of lime should be draAvn from foreign sources.
  441.  The phosphate of lime, as already mentioned, seems to perform
important offices of a chemical nature in the animal economy, besides
being the chief solidifying ingredient of bones and teeth;  but the car-
bonate Avould seem  principally destined  to mechanical uses  only ; and
Ave find it predominating,  or existing as  the  sole mineral ingredient, in
those non-vascular tissues of the Invertebrated animals, Avhich give sup-
port and protection to their soft parts (§ 277).  The degree of develop-
ment of these tissues depends in great part upon the supply of carbonate
of lime which the animals receive.  Thus the Mollusca  which inhabit
the sea, find in its waters the proportion of  that substance which they
require; but those dAvelling  in  streams and fresh-water lakes,  which
contain but a small quantity of lime, form very thin shells;  Avhilst  the
very same  species inhabiting lakes, which, from peculiar local causes,
contain  a large impregnation of calcareous matter, form shells  of re-
markable thickness.  The  Crustacea which  periodically throw off their
calcareous  envelope (§ 285), are  enabled to  renew it Avith rapidity by a
very curious provision.   There is laid up in  the Avails of their stomachs
a considerable  supply of  calcareous matter  in the form of little concre-
tions, which are commonly known as "crabs' eyes."  When the shell is
throAvn off, this matter is taken  up  by the circulating current,  and is
thrown out from the surface,  mingled with the animal matter  of  which
the  shell is composed.    This hardens  in a day or tAvo,  and the neAV
covering is complete.   The concretions  in the stomach are then  found
to haA'e disappeared; but  they are gradually  replaced before the supply
of lime they contain is again drawn  upon.   The large amount of carbo-
nate of lime which is required by the laying  Hen, is derived from chalk,
mortar, or other substances containing it, which she is compelled by her
instinct to eat;  and if the supply of these be Avithheld, the eggs  which
she deposits  are soft on the exterior,—not  being  destitute of shell, as
commonly supposed,—but having the fibrous  element of the shell (§ 181)
unconsolidated by the intervening deposit of chalky particles.

         2. Of the Digestive Apparatus, and its Actions is general.

  442. It has been already pointed out, that the nature of the food of
Animals is so far different from that of Plants, as to require the  prepa-
ratory process  of Digestion, before its nutritious part can be taken up
by the absorbent vessels  and received  into  the  system.   This process
may be said to  have three different purposes in vieAv: the reduction of
the alimentary  matter to a fluid form, so that it may become capable of
absorption ; the separation of that portion of it which is fit to be assi-
milated  or converted into organized texture, from  that  Avhich  cannot
serve this  purpose, and which is at  once rejected; and the alteration,
to a certain extent when  required, of the chemical  constitution of the
former, Avhich prepares it for the important  changes it is  subsequently
to undergo.  The simplest conditions requisite  for the accomplishment
of these purposes are the folloAving:  a fluid  capable  of performing  the
solution, and of effecting the required chemical changes; a fluid capable
of separating  the  excrementitious matter, by a process  analogous to 
SIMPLEST  FORMS  OF DIGESTIVE  APPARATUS.          253
chemical precipitation;  and a  cavity or sac in which these operations
may be performed.
  443.  In the loAvest Animals, Ave find this cavity formed upon a very
simple plan ; the digestive sac  being a mere excavation in the solid tissue
of the body, lined with a membrane which is an inverted continuation
of the external integument, and communicating with the exterior by one
orifice only, through Avhich food is drawn in, and excrementitious matter
rejected.   In the  little  Hydra,  or  fresh-Avater Polype, the  external
covering of the body and the lining of the stomach correspond so closely
in their structure, their actions differing only with their  situation,  as  to
be mutually  convertible;  for the animal  maybe turned  completely in-
side-out, without  its functions being  deranged.  The fluid necessary to
dissolve the food, knoAvn  by the  name of "gastric  fluid" or  "gastric
juice" is secreted in the Avails of the  stomach; and, from the trans-
parency of the tissues, the Avhole process may be watched.  The prey
is frequently, and indeed generally, introduced alive, by the contractile
poAver of  the arms, which coil round it, and  gradually draw it  into the
mouth or  entrance to the stomach;  and its movements may often be
observed to continue for some time after  it has been swallowed.  In a
little time, however, its outline appears less  distinct, and a turbid film
partly conceals it;  the soft parts are soon dissolved and reduced to a
fluid state; and any firm indigestible portions which the body may con-
tain, are rejected through the aperture by which it entered.  The nutri-
tive matter is absorbed by the Avails of the stomach,  every part of which
appears to be endoAved Avith equal power in this respect:  and it is  con-
veyed to the remoter parts of  the arms by the simple imbibition of one
part from  another, without any proper circulation through vessels.
  444.  In Polypes of  a higher conformation, however, the  digestive
cavity is  provided  with  a second orifice: from  the dilated  cavity or
stomach, an  intestinal tube proceeds; and this has a termination dis-
tinct from the mouth, though often in its neighborhood.   The  food, be-
fore  entering the stomach, is submitted to a  poAverful  triturating appa-
ratus, resembling the gizzard of birds, by Avhich it is broken doAvn; and
in the digestive cavity it is  submitted, not merely to the action of the
gastric  fluid, but also to that  of the bile, which is secreted  in  little
follicles in the Avails of the stomach, and  Avhich is poured into its cavity
during the process of digestion,—being  easily recognised by its bright
yelloAv color.  The excrementitious matter  is  rejected in the  form  of
little pellets, through the intestinal tube.
  445.  As Ave ascend the Animal scale, we find the  digestive apparatus
gradually increased in complexity; but its  essential characters remain
the same.  Near the entrance  to the  stomach,  we usually find an appa-
ratus for  effecting the mechanical reduction of the food, by Avhich its
subsequent solution may be rendered  more easy.  This  may consist  of
a set of teeth ; either fixed in the mouth, as  in Mammalia and Reptiles ;
or more particularly besetting  the pharynx,  as in Fishes;  or  attached
to the Avails Of the stomach, as in Crustacea.   Or it may be  formed by
the tongue, conArerted into a sort of rasp ; as  in the common Limpet,
which thus reduces the sea-Aveeds that constitute its  chief food.  Or the
same purpose may be answered by a gizzard,  or first  stomach, with dense 
254
OF  FOOD AND THE  DIGESTIVE  PROCESS.
 muscular and tendinous walls ; such as we find in the grain-eating Birds,
 and many Insects, and in certain  Molluscs  and Polypes.   But where
 the food is already composed of very minute particles, or is received in
 a liquid state (as in the case of those animals which live upon the juices
 of  others), or is easily acted on by the gastric juice, no such preparation
 is requisite.
    446.  Before  the food reaches the true digestive stomach, it is  some-
 times delayed in  a previous cavity, in order  that it may be macerated
 in fluid, and may be thoroughly saturated with it.   This is the purpose
 of  the crop  of  Birds, and of the  first stomach  of Ruminant  animals.
 When this incorporation with fluid is  not effected before the food is sub-
 jected to the triturating process, it usually takes  place concurrently with
 it;  and in those  animals which reduce their food  in the mouth by the
 process of mastication, there is a  special secretion  of fluid  into  that
 cavity, for this purpose; this  fluid is  termed Saliva, and  the act  by
 Avhich it is  incorporated with  the food  is  termed insalivation.   ]The
 mechanical reduction  of the  aliment, and its  incorporation with fluid,
 constitute, as we shall hereafter see, a very  important  preparation for
 the true digestive process./
    447.  This process, among the higher animals,  takes place exclusively,
I or  nearly so, in the stomach ; the form of which varies Avith the charac-
 ter  of the  food.   When  this is of a  nature to be easily acted on by
 the gastric fluid,  the stomach is a simple enlargement of the alimentary
 canal, almost in the direct line  betAveen the oesophagus and the intestinal
                                    Fig. 74.
  A vertical and longitudinal section of the Human stomach and duodenum, made in such a direction as to
include the two orifices of the stomach. 1. The oesophagus ; upon its internal surface the plicated arrange-
ment of the cuticular epithelium is shown.  2. The cardiac orifice of the stomach, around which the fringed
border of the cuticular epithelium is seen. 3. The great end of the stomach.  4. Its lesser or pyloric end.
5. The lesser curve. 6. The greater curve. 7. The dilatation at the lesser end of the stomach, which has
received from AVillis the name of antrum of the pylorus. This may be regarded as the rudiment of a second
stomach.  8. The rugae of the stomach formed by the mucous membrane : their longitudinal  direction is
shown. 9.  The pylorus.  10. The oblique portion of the duodenum.  11. The descending portion. 12.  The
pancreatic duct and the ductus communis eholedochus, close to their termination. 13. The papilla upon
which the ducts open.  14. The transverse portion of the duodenum. 15. The commencement  of the jeju-
num. In the interior of the duodenum and jejunum, the valvulse conniventes are seen.


tube ; so that there  is  little  provision  for the delay of the food  in  its
cavity.   But when the aliment is such as to be less  easily reduced, and 
DIGESTIVE  APPARATUS OF  MAN.
255
requires to be  submitted to the action of the gastric fluid for a longer
period,  the stomach forms a more considerable enlargement, and is placed
more out of the direct line betAveen the oesophagus and the commence-
ment of the intestine.  The former condition obtains in the Carnivora,
and particularly in those which  live more upon blood than  upon flesh,—
such as Weasels, Stoats, &c, in which this  part of the alimentary tube
is almost straight; the latter condition is found among the Herbivora,
and the provision for the delay of the aliment attains its greatest com-
plexity  in  the  Ruminant animals.   The  form  of  the human stomach
(Fig. 74) is intermediate between that of purely carnivorous and purely
herbivorous animals.   As  in the former, there is a direct  passage  from
the cardiac orifice, or entrance of the oesophagus, to the pyloric orifice,
or commencement of  the  intestine ; but there is also  a  considerable
dilatation or cul de sac, which is out of that line; and it  appears  that,
during  the digestive process, there  is a constriction across  the stomach,
Avhich separates the  cardiac portion from  the  pyloric, and causes the
retention of the food in the dilated part or large  extremity.   The gas-
tric fluid is still secreted in the walls of this organ, by scattered follicles
Avhich pour their products  into its cavity through  separate orifices; but
the bile is  elaborated by a distinct organ, altogether removed  from it,
Avhich transmits its secretion by a single duct, that opens into the intes-
tinal tube at a  short distance from its commencement; and at the same
point is delivered the pancreatic  secretion, which, as we shall hereafter
see (§ 480), takes an important share in the preparation of the alimentary
products.
   448.  The action of the Stomach  is restricted, in the higher animals,
to the reduction of the food by the solvent  poAvers of the gastric juice],
and to  the absorption (by the  vessels in its walls) of those parts of it*
which are in a  state of the most perfect  solution.   The changes whichi
are produced by the admixture of the biliary and pancreatic fluids  take)
place in the intestine; and the principal  part of the nutritive elements!
of the food are taken up  by the absorbent vessels of the walls of the \
intestine, after that process has been accomplished.   It would seem as!
if the preparation  of  the  food  for absorption Avere  not by any means I
completed, in this  first portion  of  the alimentary canal;  for it is still(
destined to pass through a long and convoluted tube, which is sometimes j
extended to an  extraordinary degree; and in this passage it is gradually
exhausted of its nutritious matter.   The  length of the intestinal canal/
bears a close relation to the character of the food.  In the Carnivorous!
animals, Avhose aliment is  easily dissolved and prepared for conversion t
into blood, the  intestine is comparatively short;  thus in the Lion and 1
other Felines it is no more than three times the length of the body; /
and in some of the bloodsucking Bats, it is almost straight  and simple, j
On the  other hand, in Herbivorous  animals  it  is  of  enormous  length;]
thus in  the Sheep  it is about twenty-eight times as  long as  the body.
In  animals Avhose  diet is mixed,  its length is intermediate between
these extremes ; thus in Man, the whole length of the intestinal tube is \
about thirty feet, or between five and six  times that of the body.   The /
intestine is of much smaller diameter along  its first portion, than it is :
nearer  its  termination; and it  is  consequently distinguished into  the / 
256
OF FOOD AND THE  DIGESTIVE PROCESS.
small and the large.  In  the small intestine, which constitutes in Man
about five-sixths of the Avhole, the  surface of the mucous membrane is
greatly extended by the valvuloe conniventes, which are folds or dupli-
catures,  often  several lines  in  breadth,  not entirely surrounding the
intestine, but extending for about one-half, or three-fourths of its cir-
cumference.   These are wanting at the lower part of the ileum.  The
whole  surface  of the  mucous membrane  of the  small intestine, below
the entrance of the biliary ducts, is thickly covered with villi, or little
root-like projections, in Avhich the proper  absorbent vessels originate.
No proper valvulse  conniventes  exist in the large intestine; the only
extensions of the mucous membrane being  crescentic folds at the edges
of the sacculi  or  pouch-like  dilatations  in its walls;  and the villi are
comparatively few in number, gradually disappearing towards the termi-
nation of the intestine.
  449.  The  mucous membrane  of the  alimentary canal, through its
whole course, is studded with the orifices of numerous scattered glands
which lie in its thickness,  or  immediately beneath it.   The simplest of
these are the follicles of Lieberkuhn, which are small  pouches,  formed
by an inflexion of  the mucous surface, analogous to the follicles of other
mucous membranes and apparently destined for  the elaboration of the
     p.   _r         protective secretion (§ 237,  see Fig. 31,  b).   These
                   follicles in the  small  intestine, are very simple in
                   their  character,  and not  very  deep;   and  their
                   apertures, which are small,  are  situated  for the
                   most  part around the bases of the villi.   In the
                   large intestine  they  are more prolonged, especially
                   towards  the  extremity of the rectum, where they
 vnious coat of smaii in- form  a  distinct layer, the  component tubes  of
the"foiircies'oTueberkFhn wmcn  are visible to  the naked eye; they probably
sued with tenacious white form  the peculiarly thick and tenacious mucus of
                   that part.  These mucous  follicles  become  parti-
cularly evident when the membrane is inflamed; for they then  secrete
an  opaque whitish matter, which  is absent in  the healthy  state, and
which  distinguishes their  orifices (Fig. 75).  A modified  kind of  these
follicles, rather  more complex in structure, is found abundantly in the
stomach; where it is concerned  in  the  secretion of the gastric fluid
(§ 469).
  450.  The  coats of the intestine contain other glandulae, of which some
appear  destined, like  the preceding, to elaborate fluids of use in the
system; Avhilst others serve  rather to draw off from  the blood  certain
products of  decomposition,  which  are  to be excreted from it.  The
former are known as the glands of Brunner, and the latter as those of
Peyer, after  the names  of  their  respective discoverers.   The  glands of
Brunner are situated  in  the duodenum, and lie, not in the mucous but
in  the sub-mucous tissue.  Though  their  size  is only about that of a
hemp-seed, they are of very complex structure, consisting  of  several
hundred follicles, clustered round the ramifications of an  excretory duct,
so  as to resemble the salivary glands (Fig. 79); and  each pours  its
secretion through  a single  orifice into the intestinal tube.  Athough
nothing is certainly knoAvn of the properties of the fluid secreted by these
 
              ALIMENTARY  CANAL  AND  ITS MOVEMENTS.          257

glandulae, yet there is strong reason to belieA'e, from their position and
character, that it assists the pancreatic and  biliary secretions  in  pre-
paring the alimentary materials for absorption (§ 480).—The glands of
Peyer are either solitary or  agminated; the  latter
form large patches, which are made of aggregations       FiS- 76.
of  the former.   Each solitary  gland in its  closed        r.-^7p\
state  consists of a spheroidal vesicle  (Fig. 76, a), a      -^y''^;'M
Avhich is  half imbedded  in  the  mucous  membrane,        i':-}^}
but which  also forms an elevated  projection  above         ■^m^r
jt; and  this  projection is surrounded by a ring or
zone  of openings which lead into an annular cluster
of Lieberkuhnian  follicles.   On rupturing one of
these vesicles, its cavity  is found to contain a gray-
ish white matter, interspersed with cells in various Pe^n^^u^fShe
stages of development; and these products appear to large intestine:—a, as seen
      b. o    l.i       ,            •    n i      • t   from above, when closed; b,
  e set tree by the spontaneous opening oi the vesicle, as seen in section, after ha-
which takes place  when it  has  become mature, by desSofSer^uhnoneeit0her
the thinning away of its wall at  its most projecting side-
part (Fig. 76, b).  In any one  of the agminated  glands,  some  of the
vesicles are usually found to be open, and others closed.   The closed
condition is not,  as was  once supposed, peculiar to the Peyerian glan-
dulae ; since, as  will be  shown  hereafter, it  is the general rule  for
other glandular follicles  in  an early stage of their development to be
equally closed (§ 718).   Of the  nature of the  secretions  of the Pey-
erian glandulae,  nothing  has been  positively  ascertained; but some
probable inferences from well-known facts  will  be  stated  hereafter
(§ 749).

                 3. Movements of the Alimentary Caned.

  451. The food which is conveyed to the mouth, is grasped with the
lips, by a muscular effort, which  is voluntary in the adult under ordinary
circumstances,  but  Avhich may be  performed  automatically when  the
influence of the will is  withdrawn; in the infant, as among the lower
animals,  the action seems purely automatic,- the nipple of  the  mother
being firmly grasped by the lips when introduced  between them, even
after  the brain has been removed.—By the  act of mastication, Avhich
then  succeeds, the food  is  triturated and mingled with  the  salivary
secretion ; and is thus prepared  for  the  further process of solution, to
which it  is to be  subjected in the stomach.   The degree of this prepa-
ration, and the form of the  instruments by which it is effected, vary in
different animals, according to the nature of the food.   In  those Carni-
vora whose aliment  consists  exclusively of flesh, very little mastication
is nece'ssary, because this substance  is  very  readily acted on  by  the
gastric fluid; and Ave accordingly find the molar teeth raised into sharp
cutting edges, and Avorking against each other  with  a  scissors-like
action (the only  one  permitted by. the  articulation  of  the jaw), so as
simply to divide the food.   On  the  other hand,  in those Herbivora
Avhose food consists of tough vegetable substances, such as the leaves of
grasses, or the stems and roots  of other plants, we find the molar or
                                 17
w 
258
OF FOOD AND THE DIGESTIVE PROCESS.
       grinding teeth peculiarly adapted  to  its reduction;  their surface being
       extended horizontally, and being  kept continually rough, by the alter-
       nation of vertical plates of different degrees of hardness; and the lower
       jaw being so connected with the skull, that great freedom of motion is
       permitted.  In  Man we find an intermediate conformation, as regards
       both  the teeth  and the  articulation  of the  jaw;  for the molar teeth
       possess broad surfaces which are covered with a  continuous  coat of
       enamel, but which are raised  into rounded tubercles ; and the articula-
       tion of the jaw allows it a degree of freedom, which is much greater
       than that possessed  by the Carnivora;  although inferior to that which,
       exists in many Herbivora.  The whole apparatus of Mastication is so
       formed in  Man, as to  lead to the  conclusion that he is destined to  live
       on a mixed diet, composed in part of animal flesh, and in part of vege-
       table substances that  are  sufficiently soft  to be reduced  by the simple
       act of crushing,  or  by the slight  trituration for which the molar teeth
       are adapted.
         452.  The mechanical reduction  of the food by Mastication, and the
       incorporation  of the Salivary secretion with its substance, constitute a
       very important step in the Digestive process.  We shall hereafter see
       that the operations, to which the alimentary matter is subjected in  the
       stomach,  are  of a purely  Chemical  nature ;  and  this  preparation is
       exactly of  the  same  character as that,  which the  Chemist  finds it
       advantageous to make, Avhen he is operating on a substance of difficult
       solution.   For nothing is so favorable to  the action of the solvent, as
       the previous reduction of  the matter to be dissolved, and its thorough
       incorporation with (he fluid that is to act upon it.   We shall hereafter
       see, that  the relative properties of the  Saliva and  of the gastric fluid
       are such, that, by  the minute admixture of the  food with the former,
       the latter finds access to every particle of it.  Hence the  practice of
       eating so rapidly, that Mastication and Insalivation are insufficiently
       performed, is extremely  injurious ; since it throws more work upon the
       Stomach than it ought to perform, by rendering  its solvent action more
       difficult.  There can be no doubt that, by the prolonged continuance of
   ,    it, a foundation  is  laid for the distressing complaint termed Dyspepsia,
       or difficulty of digestion; »and where any form of this complaint exists,
       too much attention cannot be paid to the efficient reduction of the food
       in the mouth.
          453. When the  aliment has  been sufficiently triturated, it  is con-
       veyed into the Pharynx  by  the act  of  Deglutition  or swalloAving.
 ,  , •'i This  act involves  a great many  distinct movements, into a  minute
'" y   description of which we shall not here enter; but it is desirable that
       its general nature should be well understood.  It is one  of those most
     .purely reflex in its character (§  394),  and  is not capable of being per-
 ^y   formed or even  controlled by a voluntary effort.  This stateme'nt may
       seem inconsistent with the fact, that we SAvallow w-hen we will; but it
       is not so in reality.   The  muscular movements which are concerned in
»W    deglutition, are  called forth by nerves that proceed from the spinal cord,
       not  from the brain ; these  motor nerves are excited  to  action, by the
       contact of solid or fluid matters Avith  the mucous surface of the fauces,
       and in  no other* way.   The impression  produced by the contact is 
MOVEMENTS OF C3SOPHAGUS.                 259
conveyed to the Medulla  Oblongata, or that portion of the spinal cord
which lies  Avithin  the  cranium, by afferent nerves that  terminate  in
it; and, in immediate respondence to that impression, a motor impulse
is transmitted from it, which calls the muscle into the combined action
necessary to produce the  movement.   Now this contact also  produces
a sensation, provided the  brain be sound and awake, because nervous
fibres proceed from the mucous surface to the brain as well  as to the
spinal cord ;  but this sensation is not a necessary link in the  chain of
actions, by Avhich the movement is produced; for the act of Deglutition
takes place during profound sleep,  when all  sensation is suspended,
and  it may  be excited even after the brain has been removed.   It
seems to be voluntary, under ordinary circumstances, simply because it
is by an  act of the will, that the matter  to be swallowed is carried
backAvards into contact  with  the fauces; but that it is not so in reality,
is shoAvn by the fact, that when this  impression has once been made
with sufficient force, we  cannot by any effort  of the will, prevent the
action.   We have a good example of. this in the following circumstance,
of no very unfrequent occurrence.  The tickling of the upper part of the
fauces with a feather is often practised to induce vomiting ; but if the
end of the  feather be carried too low down,  it excites the act of degluti-
tion  instead;  the feather  is grasped by the pharnyx and draAvn down-
Avards; and if it be not  held tenaciously between the fingers, it is drawn
from them and carried doAvmvards into the stomach.
  454. The carrying-back of the  alimentary matter, so  that it reaches
the fauces  or upper part of the pharynx, is  principally accomplished by
the tongue ; when it has passed the anterior palatine arcnes, these con-
tract and close over the tongue, so as to prevent the return of the food /
into the mouth;  and at  the same time  the posterior palatine arches
and  the uvula are so drawn  together, as to prevent its passage into the >
posterior  nares.    The larnyx is drawn  forward beneath the root of
the tongue, and the epiglottis is pressed down over  the  rima  glottidis,
so that nothing can  enter  the latter, unless draAvn towards  it by an
act of inspiration.   When fairly Avithin  the pharynx,  the alimentary
matter  is  seized by the  constrictors which enclose  that  part of the
alimentary  tube, and is drawn downwards by them into  the  oesopha-
gus, which is the cylindrical  continuation of it.   The  continued action
of the constrictors serve to propel the food  along the oesophagus ;  their
movement  being of a  reflex  nature,  excited by  the  contact of the
substance  contained  in the tube, with  its lining membrane,—which
produces an impression that  is transmitted to the medulla oblongata,
and is reflected back as a motor  impulse to the muscles.  We  have sv
here a  distinct case of reflex action without  sensation;  for we have . \.
no consciousness of the ordinary passage of food down  the oesophagus,
unless it occasion pressure on the surrounding parts through its bulk,
or unduly irritate the lining membrane by  its  high  or low temperature
or its acrid qualities; and yet it may be shoAvn by experiment, that the >■
completeness of the nervous  circle is requisite for the excitement of the
movement which Avill not take place when it is  interrupted either by
division of  the  nerves, or by  destruction or paralysis of the medulla
oblongata. 
260            OF FOOD AND THE  DIGESTIVE  PROCESS.
               455. The progress of the  food along the  (Esophagus  is  aided by
I*  j r  ,      the action of the muscular  coat peculiar  to  it.  This is composed of
fi'f'3'''*^ ,}  nfHhe non-striated fibre ;  and,  like that of the intestinal canal further on,
ji!"    J.*^*   it is usually  stimulated  to  contraction by the  direct contact of the
l'^^*"^1' "stimulus, and not either by the will,  or by the reflex action of the spinal
  '-'    ' '     cord.   The movement produced by it is of the peristaltic or wave-like
         >••*   kind;  the  contractions being limited  to  one portion of  the tube, and
  r    ':      being propagated along it from  above downwards.   This action  con-
     ", ;//*   tinues  after  the division of all  the  nerves supplying the oesophagus;
  T',--'"'"    and it cannot, therefore, be  dependent upon  the brain or spinal  cord.
             It may be  observed to take place in  a  rhythmical manner (that is, at
             short and tolerably regular intervals), whilst a meal is being SAvallowed;
             but  as the stomach becomes full,  the intervals are longer and   the
         ■ • .   wave-like  contractions less  frequent.  The degree  in which the action
             of the oesophagus  alone, without that of the surrounding  muscles, is
             capable of propelling the food into the stomach, seems to  vary in diffe-
             rent animals.  When the latter are paralysed in the Dog,  by section of
             the nerves that supply^them, the food that has entered the oesophagus is
             still propelled into the  stomach;  but this  is  not the  case in the Rab-
             bit, the action of its oesophageal fibres not being sufficient to carry the
             food onwards  to the stomach, though it will  expel it from the  divided
             extremity of  the tube  when it is cut across.   The usual  peristaltic
             movements  of the oesophagus are  reversed in Vomiting : and this rever-
           ; sion has been observed even  after the separation of the  stomach from
             the oesophagus, as  a consequence of the injection of tartar emetic into
             the veins.
          k    456. At  the point where the  oesophagus  enters the Stomach,—the
           ;  cardiac  orifice of the latter,—there is a sort of sphincter, or circular
             muscle, which is  usually closed.   This opens when there is a sufficient
             pressure on it, made by the  accumulated  food propelled  by the move-
             ments of the oesophagus above; and  it then closes again, so as to*retain
             the food in the stomach.   The closure is  due to reflex action ; for when
             the nerves supplying it are divided, the sphincter no  longer  contracts,
             and the food regurgitates into the oesophagus.  The opening of the car-
             diac orifice is one of the first acts which takes place in vomiting.
               457. In ruminating  animals, there  is a very remarkable  conforma-

                                           Fig. 77.
     Compound Stomach of Sheep :—a, oesophagus ; 6, paunch; c, second, or honeycomb stomach;
            d, third stomach, or many-plies; e, fourth stomach or reed;/, pylorus.

tion at the lower end of the oesophagus, which is  destined to regulate
the passage of food  into the different compartments  of  the  stomach, 
DIGESTIVE APPARATUS OF  RUMINANTS.
261
according as it has been submitted to the  second mastication, or  not.
The oesophagus (Fig. 77, a) does  not terminate  at its  opening into the
first stomach  or paunch  (b),  but it is  continued onwards as  a deep
groove with two lips (Fig. 78): by the closure of these lips it is made
to_ form a  tube, which  serves  to convey the food  onwards  into  the
third stomach  ; but Avhen they separate, the food  is  allowed  to  pass
either into the first  or the second  stomach.  When  the  food is  first
swallowed,  it undergoes  but very little  mastication;  it is consequently
firm in its consistence, and is brought  down to the termination  of the
oesophagus in dry bulky masses.  These separate the lips of the groove
or demi-canal (Fig. 78, e, e), and  pass into the first and  second stomachs.
After they have been macerated in the fluids of these cavities, they are
returned to the mouth by a reverse peristaltic action of the oesophagus;
this return takes place in a very regular manner, the food being shaped
into globular pellets by compression  within a sort of mould formed by

                                Fig. 78.
  Section of part of the Stomach of the Sheep, to show the demi-canal of the oesophagus; the mucous mem-
 brane is for the most part removed, to show the arrangement of the muscular fibres. At a is seen the
 termination of the oesophageal tube, the cut edge of whose mucous membrane is shown at 6. The lining of
 the first stomach is shown at c, c; and the mucous membrane of the second  stomach is seen to be raised
 from the subjacent fibres at d.  At e, e, the lips of the demi-canal are seen bounding the groove, at the
 lower end of which is the entrance to the third stomach or many-plies.

the demi-canal, and these pellets being conveyed to the mouth at regular
intervals,  apparently by a rhythmical movement of the oesophagus.   It
is  then subjected to a  prolonged  mastication  within  the mouth  (the
"cheAving of the cud"), by which it is thoroughly  triturated and im-
pregnated Avith saliva ;  after  Avhich  it is  again  swallowed in  a pulpy
semi-fluid state.   It noAV  passes  along the groove  which forms the con-
tinuation  of the  oesophagus, Avithout opening its lips ; and is thus  con-
veyed into the third stomach (Fig. 77,  d),  whence it passes to the fourth 
262            OF  FOOD AND THE  DIGESTIVE PROCESS.

(e), in which alone the true digestive process takes place.   Noav, that the
condition of the food as to bulk and solidity is the circumstance which
determines the opening or  closure of the  lips of the groove, and which
consequently occasions its  passage  into the first and second stomachs,
or into the third and fourth, appears  from the experiment of Flourens;
who found that when the food, the first time of being SAvallowed, was
artificially reduced to a soft and pulpy condition, it passed for the most part
along the demi-canal into the third  stomach, as if it had been ruminated,
—only a small portion finding its way into the first and second stomachs.
How far the actions of this curious apparatus are dependent upon ner-
vous influence,—or how far they are  due to the exercise of the contrac-
tility of the muscular fibre, directly excited by the  contact of the sub-
stances with the  lining membrane  of the  tubes and cavities,—has not
yet been clearly ascertained.
  458. The food, when introduced into the stomach, and submitted to
the solvent action of its secretions,  is also subjected to a peculiar move-
ment, which is effected  by the muscular walls  of that organ.  The pur-
pose of this motion  is obviously to keep the contents of the stomach in
that state  of  constant agitation,  which  is  most  favorable to  their
chemical solution ;  and particularly to bring  every portion of  the ali-
mentary matter into contact  Avith the walls of the stomach, so as to be
subjected to the action  of  the  fluid, which is poured forth from  them
during the digestive process.   The  movement is produced by the  alter-
nate shortening and relaxation of  the various fasciculi, which  are dis-
posed in almost every direction throughout the muscular wall of the
stomach;  and it seems to produce  a  kind of revolution of the contents
of the stomach, sometimes in the direction of  its length, and sometimes
transversely.  Its result is well shown in the hair-balls, which are occa-
sionally found in the  stomachs of animals, that  have swallowed hair
from time to time through licking their skins ;  the component hairs not
being pressed  together confusedly, but being worked together in  regular
directions,  and so  interwoven  that they  cannot be readily separated.
As digestion proceeds, the dissolved fluid escapes, little by  little, through
the pyloric orifice, which closes itself firmly against the passage of
solid bodies; and this motion continues until the stomach is completely
emptied;  when it ceases, until  food is again  introduced.   The bulk of
the alimentary mass diminishes rapidly, as the solvent process  is near
its completion ; and the  separation  of the fluid  product or chyme is
aided by a peculiar action of the transverse  fasciculi, which surround
the stomach at about four inches from its pyloric extremity.   These
shorten in  such a manner, as to produce a sort of hour-glass separation
between the portions of the stomach  on either side of it; and the fluid
solution, being received by the  pyloric or smaller portion, is pumped
away through the pylorus; whilst the solid matter yet undissolved is
retained in the larger division.
  459. The degree in which  these  movements are dependent upon the
nervous system, or  are  under its control or direction, has not yet been
clearly ascertained.   Distinct movements may be excited in the stomach
of a  Rabbit, if it be distended  with food, by irritating the par vagum
soon after  the death of the animal; these movements seem to commence 
MOVEMENTS OF STOMACH  AND INTESTINE.           263
from the cardiac orifice, and  then to spread themselves peristaltically
along the Avails of the stomach; but no such movements can be excited
if  the stomach be empty.   On the other hand, there is distinct proof
that all the movements necessary to  digestion may take place after the
section of that nerve; although the first effect of the operation appears
to  be to suspend them completely.  It is probable that the movements
of the stomach are more regular and  energetic in Herbivorous animals,
Avhose food is difficult of  digestion,  than they  are  in  the  Carnivora,
whose aliment is dissolved with comparative facility.
  460. From the time that the  ingested matter enters the Intestinal
tube, it is propelled onAvards by the peristaltic contractions  of  its mus-
cular coat; which are excited, independently of all nervous influence,
by the  contact of the aliment, or  by that of the secretions mingled Avith
it in its passage along the canal.   These last appear to  have an impor-
tant effect; for we find that, when the bile-duct is tied, so as to prevent
the bile from entering the intestine, constipation  always occurs; whilst
an  increase of  the  biliary and otheV  secretions, consequent upon the
action  of  mercury or  upon any  other cause, produces an  increased
peristaltic movement, and a more rapid  discharge of the excrementitious
matter.   During the passage of the alimentary matter along the small
intestine,  as we  shall see hereafter,  a  large proportion  of its fluid  is
removed, by the absorbent  poAver of  the villi;  and the residue is again
brought, therefore, to a more solid consistence.   This residue  consists
in  part of those portions of the aliment, which are not capable  of being
dissolved or finely divided, so as to be received by the absorbents; and
in  part of  the matters  poured into the  alimentary canal, by the various
glands that discharge their contents  into it, for  the purpose  of being
carried  out of the body.   The faeces Avhich  are  thus  formed, are  pro-
pelled through the large intestine, by the continued peristaltic  action of
its walls, until they arrive at the rectum.
   461.  That  the ordinary peristaltic action of the intestinal canal is
independent of nervous influence, is sufficiently indicated  by the fact,
that it  will continue when the tube is completely separated  from  all
connexion with the nervous centres;  as well  as by the difficulty, already
adverted  to (§ 353), of exciting  contractions in  the muscular  coat  by
any stimulation of its nerves.  All the nerves of the intestine, from  its
commencement at the pyloric orifice  of the  stomach to  its termination
at the anus, are derived from the ganglia of the sympathetic system;
but  there  is evidence  that those Avhich influence  its  movements  are
really derived from the spinal cord (see chap, xii.)   Although the will
has no influence whatever on the peristaltic movement, yet the emotions
seem to affect it; and it is probably  to  convey their influence, that the
intestinal canal is supplied Avith motor nerves.   It is also furnished with
sensory  nerves,  which form part of the trunks  of the  sympathetic
system, but which really pass onwards  to the brain ;  these do not, how-
ever, make us conscious of the passage of the  alimentary matter along
the canal, so long as it is  in a state  of health; but in various diseased
conditions, they give rise to sensations  of the most painful nature.
   462. For the occasional discharge of the faeces from the rectum, and
for the  retention of them at other  times, we find the outlet or  anal 
264
OF FOOD AND  THE DIGESTIVE PROCESS.
orifice  provided with an additional muscular apparatus, Avhich is con-
nected with the spinal system of nerves.  The act of Defecation is due
to the pressure upon the contents of the rectum, which is occasioned by
the combined contraction of the diaphragm and the abdominal muscles ;
whilst, on the other hand, the retention of the faeces is due to the con-
tractile power of the sphincter muscle which surrounds the  anus.   The
action  of the sphincter  ani, like that of the sphincter of the cardia, is a
reflex one;  dependent upon the connexion of the muscle, by excitor and
motor nerves, with the  spinal cord.   If the  lower portion of the cord
be destroyed, or  if the nerves be divided, the sphincter  loses  its con-
tractile power, and becomes flaccid.   When in proper action, however,
its power is sufficient to prevent  the escape of the contents of  the rec-
tum ; until  the expulsive  force  becomes very strong,  in consequence
either of the quantity of faeces which have accumulated, or the acridity
of  their character.   In  either  case, the  impression  made  upon the
mucous membrane of the rectum is conveyed to the spinal cord; and,
by a reflex motor impulse,  the mMscles of  defecation are thrown  into
combined action, the  resistance of the sphincter is overcome, and the
faeces are expelled.  An unduly irritable state of the mucous membrane,
or a disordered state of the excrementitious matter (resulting from the
irritating character of the substances  swalloAved, from the acrid charac-
ter of  the secretions poured into  the canal, or from an  unusual change
in  the  aliment  during the digestive  process),  may occasion unduly
frequent calls  upon  the muscles of defecation, which the sphincter  is
unable to resist.  On the other hand, if  the progress  of the faeces be
delayed in  the large intestine, by deficient peristaltic  movement, they
accumulate higher up,  and the act of defecation is not excited.
   463. Although the sphincter ani on the one hand,  and the  muscles of
defecation  on the other, are called into action by the reflex poAver of the
spinal  cord, and  are so  far  involuntary in their operation, yet they are
also in  some degree subject (in Man  at  least) to the influence of the
will.   The resistance of the sphincter may be increased by a voluntary
effort,  when it is desired to retain the faeces  in opposition  to  the power
of the expulsors; and  it is only when the latter operate with excessive
force, that  they  can overcome it.   On the  other hand, the expulsors
may be called  into action, or may be aided, by the will, when  the stimu-
lus to their movement  received through the spinal cord Avould not other-
wise be strong enough; and  the fasces  may thus  be evacuated hy a
voluntary effort,  at a time when they Avould not othenvise be discharged.

   4. Of the Secretions poured into the Alimentary Canal, and of Changes
                      which they effect in its contents.

   464. The whole Mucous  Membrane of the Alimentary canal, from
the mouth  to the anus,  is covered during health with  that peculiar viscid
secretion,  termed mucus,  of  which the  characters have  been already
described (§ 237).  This is formed, partly, on the free surface of the
membrane itself, but chiefly in the numerous follicles or depressions by
which that  surface is increased;  and it appears destined for the protec-
tion of the  delicate highly vascular membrane from undue irritation,  by
i 
              SALIVARY GLANDS AND THEIR SECRETIONS.          265

 the contact of the substances which are passing through the alimentary ,A  ■ ;
 tube.   When these are unusually acrid, the secretion of mucus is  aug--
 mented in quantity, and is increased in viscidit}% so as to form an effec-
 tive  sheath  to the membrane, which  Avould otherwise suffer  severely.'
 When  this secretion is deficient, the membrane is irritated by the con-
 tact  of any but  the  blandest  substances;  and  the class of  remedies
 termed demulcents are useful in coating and protecting it.
   465. JDuring the mastication of the food in the mouth, the Salivary f
 secretion is poured in, for the purpose of being mingled with it, and of      v^ri
 rendering the act of mastication  more easy.   This secretion is formed gJtt-w.-^W
 by three  pairs of glands,—the  Parotid, the  Sub-lingual, and  the  Sub-.*
 maxillary ; these arc composed of minute follicles,                     "^
 Avhose diameter is about l-1000th of an inch, con-        FiS- 79-
 nected together by branches of their duct, upon
 Avhich they are set like grapes upon their stalk,    JjjjM   jj|k         -vX'^' *"*
 surrounded by a plexus of blood-vessels, and bound   iHp^ffifBKfoi1       -« ^JP^1,
 together  by areolar  tissue.  Within the follicles  |||JH^^^HS^^^
 are  the true secreting cells (§  238);  by whose
 groAvth and  development, the material of the se-
 cretion is  separated from  the blood.  These sali-
 vary cells  are often to be recognised in the saliva;
 they must not, however, be confounded with the
 epithelium-cells of the mucous membrane of the
 mouth, Avhich are much larger.  The fluid ob-           ""'""'  '     iL^ir1
 tained  from  the  mouth is not  pure saliva; for   Lobule of Parotid  Gland of ;,?,
 ,,           ,, ,.       ,i   •,  ir» •    •   i  i   •  i  new-born  infant, filled with
 the  mUCUS OI  the mOUth  itself IS mingled  With mercury;  magnified 50 dia-
 the  secretion  from the salivary glands.   If the meters-                ^
 proportion of the former  be considerable, it  gives to the fluid  of the    ; {,  u-,o
 mouth  an acid reaction; whilst, if the latter be predominant (Avhich it
 is directly before, and during, the act of eating),  the fluid of the mouth
 has an alkaline reaction.   It  may be  sometimes observed,  that the
 saliva of  the  mouth  will  strike a blue color  with reddened  litmus-
 paper,  whilst it turns  blue litmus-paper red;  thus shoAving the presence
 both of an acid and an alkali in a state of imperfect neutralization.
^  466. The  solid matter  of the Salivary secretion  is about 1 per  cent.^C,
 of the  whole; and this consists in  part  of animal principles,  and in        „,
 part* of saline substances.   The animal matter  consists  of ozmazome,,/p^ Atvvvi
 mucus, and a  peculiar substance termed ptyaline  or  salivary matter ;j     ,/
 Avhich is soluble in water and insoluble in alcohol, and which is yet dif-
 ferent  from  both albumen  and gelatine.   This substance  appears  to
 have a decided effect in producing the metamorphosis of certain alimen-
 tary substances,  on Avhich it   acts like a ferment.   Starch may  be^
 comrerted into sugar,  and sugar into lactic acid, by its agency ;  and, if\
 acidified,  it  has a certain solvent  poA\Ter for caseine, animal  flesh, and/      '   .
 other proteine-compounds.  Its chemical  nature has not yet been pre-  *    t •
 cisely determined.  The  saline  constituents  of the Saliva are  nearly \ *vv-v/l
 identical  with  those of the  blood; the chlorides of sodium and potassium' *'-
 form considerably more  than half;  and the remainder consists  chieflyv>  ftp
 of the  tribasic phosphate of soda,  to Avhich the alkaline reaction of- the }
 fluid is due, Avith the  phosphates of lime, magnesia, and iron.   It  is  of' 
266
OF FOOD AND THE DIGESTIVE PROCESS.
          the earthy phosphates, that the tartar which collects about the teeth is
 ;T~r  "\.  chiefly composed, the particles of these being held together by  about
  cj''4^  20 per cent, of animal  matter;  and the composition of the concretions,
          Avhich occasionally obstruct the salivary ducts, is nearly the same.
             467.  The quantity of Saliva formed during the twenty-four hours,
          has been estimated at from 15 to 20 ounces;  but on this* point  it is im-
          possible to speak with  certainty.  The secretion  is by no means con-
          stantly flowing;  indeed,  it  is  almost entirely suspended,  when  the
          masticator muscles and tongue are at perfect rest, unless it be excited
          by any mental cause;  and hence  it is,  that  the  mouth  becomes  dry
          during sleep, if it be not kept closed.  The Aoav of Saliva takes place
        s  just Avhen it is most wanted;  that is, \rhen food has been taken into the
          mouth, and when the operation of mastication is going on.  But it will
          also take place, especially in a hungry  person, at  the  sight, or even at
          the idea, of savory food;  as is  implied by the common expression of
          the "mouth watering"  for such an object.   The influence thus exercised
          over it by the emotional state of the mind, is probably conveyed to the
          salivary glands by the Fifth  pair;  which contains  many of the gray or
          organic filaments, and which seems to take the place, in the Head, of a
          distinct sympathetic system.
             468. Having been conveyed  into the stomach, the food is  submitted
 «.', •      to the action of the gastric fluid, Avhich is secreted in the walls of that
          organ.  This  fluid is  not present  in the empty stomach; its secretion
          being excited by the presence of food, or by the irritation of the walls
 V^V.      of the organ  by some solid body.   In  the intervals betAveen  the diges-
          tive process,  the  mucous membrane   is  of a  light  pink hue; but it
         - becomes more turgid  with blood, when the presence of food calls for
;.,        the activity  of its secreting  processes.  It is of a soft and  velvet-like
          appearance; and it is constantly covered with a very thin transparent vis-
          cid mucus, which  has neither acid nor  alkaline reaction.   By applying
 V        aliment or other stimulants to the internal coat of  the  stomach, and by
          observing the effect through a magnifying glass, numerous minute pa-
£'         pillae can be seen to erect themselves upon the mucous membrane, so as
       f   to rise through the coating of mucus ;  and from these is poured forth a
  %A^  .   pure, limpid, colorless, slightly viscid fluid,  having  a distinctly acid
fy "P\"   •  reaction, which is the Gastric juice.  This fluid is secreted by follicles,
          which are lodged in the walls of the stomach, and which closely resemble
 ^';       those that elsewhere  secrete  mucus;  but  they are  usually of more
          complex structure, and are more numerous.
             469. If the Mucous membrane of the stomach be divided by a section
          perpendicular to its walls, it is seen to  be made up, as it were, of tubu-
          lar follicles closely applied to each other ; their blind extremities resting
          upon the submucous tissue, and their open ends being directed towards
          the cavity of  the stomach.   In some situations  these tubuli are short
          and straight; in other parts they are longer, and present an appearance
          of irregular dilatation or partial convolution (Fig. 80, 1).   This is their
          usual character, especially near the cardiac orifice of the  stomach; but
    :;   . "  ryear  the pyloric orifice they have a much more complex structure (Fig.
          '80, 2).   These tubular follicles are arranged  in  bundles or groups, and
        . are surrounded and  bound  together by a fine areolar membrane;  and 
GASTRIC  FLUID.
267
this  also  serves to convey vessels  from  the  submucous tissue, which
ramify among the follicles, and supply the materials for their secretion.
Ffc. 80.
Fig. 81.
i:;y


<h

 Olandulw from the coats of the Stomach,
magnified 45 diam.;—1, from the middle of
the stomach; 2, from the neighborhood of the
pylorus.
-r,- r-'-^ "<g.'^T'7\
 Porlion of the Mu-
cous membrane of the
Stomach, showing the
entrance to its secreting
tubes, in pits upon its
surface.
The number of tubuli in each group is by no means constant.  The fol- / '
licles do not,  in  general, open directly upon the surface ;  but into the
bottom  of  small  depressions or pits, which may be seen to  cover the
membrane (Fig. 81).  These pits are more or less circular in form;  and V v    ;^
are separated  from one another by membranous partitions, which vary in /
depth, and sometimes by pointed processes, which are capable of erect-.
ing themselves in the manner just described.  The diameter  of these pits
varies from about l-100th to  l-250th of an inch; it is always greatest "l
near the pylorus.  The number of the gastric follicles opening into each, /
is usually from three to five ;  but there are sometimes more.                   •
  470.  The chemical composition of the Gastric fluid has been a subject if / ^/v*/"^
of much discussion, and can scarcely yet be regarded as precisely deter-   r
mined; possibly  it may vary  in its nature, according to the state of the
system, and the kind of animal from which it is obtained.   In all cases,
hoAvever, this  fluid appears  to contain a free acid, together Avith a pecu-  A-n/n"'
liar organic compound,  Pepsine, which seems like albumen in a state of
change.  It is in regard to the nature of the free  acids that Chemist^
are most at issue. Muriatic, phosphoric,  acetic, lactic, and butyric acids
have each been detected in the gastric fluid ; but there  are great diffi-        h.
culties in the Avay of determining  which  of these acids are free,  and
which are in combination.  Thus, although it is very easy to obtain free *^v
\s\.iw i "
%*A*m\
muriatic acid by distillation of the gastric fluid, yet this is by no means <k
an adequate proof  of the previous presence of the acid in a free state;
for it  has been found that free lactic acid will decompose chloride of
sodium at an elevated temperature, forming (with water) lactate of soda
and muriatic acid; so that the lactic may be the free acid of the gastric
fluid, the muriatic having been formed  during the distillation, at the
expense of  the chloride  of sodium, Avhich is a  constituent of the gastric
fluid.   It has been further determined, that muriatic and  lactic acids
both possess a remarkable solvent  power for albuminous matters, when
assisted by  pepsine;  so  that  it  is probable that they may replace one
another.—The properties of the  organic compound, named  Pepsine,
which  is  peculiar to the gastric fluid, have  been  principally studied  in

hpi 
268
OF FOOD AND  THE DIGESTIVE PROCESS.
    that form of it obtained from the mucous membrane of the stomach of
    the Pig, Avhich bears a close resemblance  to that of Man.  When this
    membrane is digested in  a large quantity of warm water, it is purified
    from the various soluble substances it may contain ; but the pepsine is
    not taken up, as it is not soluble in Avarm Avater.  By continuing the
  , .digestion in cold  water,  the  pepsine  is  then extracted nearly pure.
  ,  When this  solution is evaporated to dryness there remains  a  brown,
    grayish, viscid  mass, having  the  appearance of an  extract, and the
    odor of  glue.  A similar  substance  may be obtained  by adding strong
    alcohol to a fresh solution of pepsine  ; for the latter is then precipitated
    in white flocks, which may be collected on a filter, and which produce a
    gray compact mass Avhen dried.  Pepsine  enters into chemical combina-
    tion  with many  acids ;  forming compounds Avhich  still redden  litmus-
    paper ;  and this appears  to be its condition in the gastric juice.
      471.  The muriate and acetate of pepsine possess a very remarkable
    solvent poAver for albuminous substances.   A liquid which contains only
    17 ten-thousandths of acetate of pepsine, and 6 drops of muriatic acid
    per  ounce,  possesses solvent poAver  enough  to  dissolve a thin  slice of
 I-  coagulated  albumen, in the course of six or eight hours' digestion.  With
    12 drops of muriatic acid per ounce, the same quantity of Avhite of egg
    is dissolved in two  hours.   A liquid which contains  only half a grain of
    acetate of pepsine,  and to which the muriatic acid and Avhite of egg are
    alternately added,  so long as the latter is  dissolved, is capable of taking
    up 210 grains of coagulated white of egg, at a temperature betAveen 95°
    and 104°.   The same acid with pepsine dissolves blood, fibrine, meat,
  ")  and cheese ; whilst the acid without the pepsine requires a very long
    time to do  so  at ordinary temperatures.  Very dilute muriatic  acid,
    however, at the  boiling point,  dissolves  these  albuminous substances;
    and the solution has the same characters, as that Avhich is made by the
  !  agency  of  pepsine.  The horny  tissues,  such as the epidermis, horn,
    hair, &cv  and the yellow fibrous  tissue, are  not affected by the acid
    solution of  pepsine.—It appears from these experiments, that the acid
    is the real solvent; and that the action of the pepsine is limited to dis-
    posing the  albuminous matter  for solution, producing in it  a  change
    analogous to that  which  may be effected  by heat.   Hence  it may be
    considered, like  ptyaline, as a sort of ferment;  its office being to pro-
    duce a tendency to change, on the substances on Avhich  it acts, without
    itself entering into new combinations with any of their elements.
      472.  These experiments appear  to afford an explanation of the pro-
    perties of the gastric fluid, as ascertained by direct  experiment upon it.
    When drawn direct from  the human stomach, it  is found to possess the
    poAver of dissolving various kinds of alimentary substances, whilst these
 ,.  are submitted to its action at a constant temperature  of 100° (which is
    about that of the stomach), and are  frequently agitated.   The solution
iAy' appears to  be in all  respects as perfect  as that, which naturally takes
 ,■■'•> place in the stomach ; but a longer time is required  to make it.  This
i. •  is easily accounted for by the difference of the conditions ; for no ordi-
    nary agitation can  produce the  same effect Avith  the curious movements
    of the stomach (§ 458); fresh gastric fluid is poured  out, as it is wanted,
    during the natural process of  digestion; and the continual removal of 
PROPERTIES OF PEPSINE AND OF GASTRIC FLUID.       269
the matter Avhich has been already dissolved, by its  exit through  the
pylorus, is of course  favorable to the action of the solvent upon  the
remainder.   The quantity of food, which a given amount of gastric fluid
can dissolve, is limited;  precisely as in the case of  the  acidulous solu- ^X-JSth
tion of  pepsine.  The marked influence of temperature upon its action/
is shoAvn by the fact,  that fresh gastric fluid has scarcely any influence^
on the matter submitted  to it, when the bottle  is exposed  to cold  air,)
instead  of being kept at  a temperature  of 100°.  Hence the use of a
large  quantity of cold water at meal-times, or  of ice  afterwards, must
retard the digestive process.
  473.  The  pulpy substance,  which is the product  of the reducing
action of the gastric juice, is termed Chyme.   Its consistence will of 7  'SJIawvj
course vary, in some  degree, with  the  relative quantity of solids and (  ""  /'
liquids ingested.   In  general it is grayish, semifluid, and homogeneous;
and possesses a slightly acid taste, but is otherwise insipid.   When  the
food has been of  a rich oily character, the Chyme  possesses a  creamy
aspect;  but when it  has contained  a large proportion  of farinaceous
matter,  it has rather the  appearance of gruel.   The state in which  the
various  alimentary principles  exist in it, has not yet been accurately
determined ;  the  following, however, may be near the truth.—The Pro-1    ,
teine-compounds,  Avhether derived from  Animal or Vegetable food,  are 'A^
all reduced  to a  state of solution, if the gastric digestion have been
properly performed;  and in this  state, they have all the properties of
Albumen.—Gelatine will be dissolved or  not, according to its previous^,
condition;  if it exist  in  a tissue from which it cannot  readily be  ex- ,
tracted, it Avill pass forth almost unchanged;  but Avhen ingested in a  ^ t  /  ,
state  of  solution,  it remains so ; and if it have been preAaously prepared '^^^w
for solution by boiling, its solution is completed in  the  stomach.   Its
condition, hoAvever, is altered in the process; for it loses the power of
gelatinizing,  and  cannot  be precipitated by chlorine; so  that it cannot
be detected as such either in  the blood  or  the  chyle  into  which it is '
received.—The Gummy matters of Vegetables are dissolved, Avhen they's  n
exist in a soluble  form;  as in  the case of pure gum, pectine, and dex- r    'V\*J*V
trine  or starch-gum.   It  does not  appear, however, that  any further \^^JJ:'%\^\j
conversion of  Starch  is effected by the gastric fluid; for  if no saliva be '/1^9'^
admitted into the stomach, no sugar is  generated there by the meta-
morphosis of the  starch Avhich it may contain.   But the continued intro-
duction of the saliva ordinarily occasions the continuance of the process, /,, J  -
although the presence of the free acid of the gastric fluid in some degree W a,u^ ■'■
interferes Avith it; and it is not until this has been neutralized by  thel^^U**-1-
admixture of  the biliary  and pancreatic  secretions, that the metamor->'<■ -vVk<•■'•'
phosis of the starch  is  actively  renewed.  Any sugar that may have'*? *vfu
been taken in as such, or  that may have been produced from starch by the !
converting power of the  saliva, is reduced to the state of complete solu-
tion.—Oily matters do not appear to be  in any way acted upon, other-.
wise than by being set free by the solution of the envelopes which may*)^t.;-.  ; •- ^tf
have contained them  (e.  g. fat cells), and  by being dispersed through J 7!^tX
the mass ; their state of  division, however, does not seem to  be yet fine^ '°~    ,,/^j
enough  to uIIoav of their absorption.—Most other substances, as resins,
Avoody fibre, horny matter, yellow fibrous tissue, &c, pass  unchanged^ 'w- 
270
OF FOOD AND THE  DIGESTIVE PROCESS.
            from the stomach, and undergo no  subsequent alteration in the  intes-
            tinal canal; so that they are discharged among the faeces as completely
            useless.
         ~l-   474.'We have now to  notice the  conditions, under which the  Gas-
         /  trie fluid is secreted ;  the  knowledge of which is of great practical im-
            portance.  We have seen  that it is not poured forth,  except Avhen food
            is introduced into the stomach, or when its Avails are irritated in some
            other mode; and there is reason to believe, that it  is not preAdously
            secreted and  stored up  in the follicles, but that the  act  of secretion
            itself is  due to the stimulus applied to  the mucous membrane.  The
            quantity of the  fluid  then poured  into  the stomach, hoAvever, is not
            regulated by the amount of food  ingested, so much as by the Avants of
            the system ; and as only a definite quantity  of food can be acted  on by
            a given amount of  gastric juice, any superfluity remains undissolved for
            some time,—either continuing in the stomach until a fresh supply of the
            solvent is secreted, or passing into the intestinal canal in a crude  state,
            and becoming a source  of irritation, pain, and disease.  The use of a
            small quantity of salt, pepper, mustard, or other stimulating substances,
            appears to produce a  gently stimulating effect upon  the mucous mem-
            brane, and by  causing an increased afflux of blood,  to  augment the
        •'■•'■' quantity of the gastric fluid poured forth.   Any excess of these or other
            irritants, however,  produces a disordered condition of the mucous mem-
            brane, Avhich is very unfavorable  to  the  digestive process.   It becomes
            red  and dry, with an insufficient secretion of mucus; the epithelial
            lining is abraded, so  that  the mucous coat is left entirely bare ; and
          '■"'irregular circumscribed patches of  a deeper hue, sometimes with small
            aphthous crusts,  present themselves here and there on  the  walls of the
            stomach.   Similar results follow excess in eating.  When these changes
            are inconsiderable, the appetite is not much impaired,  the  tongue does
            not indicate disorder, and the digestive process may be performed;  but
  , , ., >;"    if  they proceed further, dryness of the mouth, thirst, accelerated pulse,
       •   ,; foulness of the tongue, and other symptoms of febrile  irritation, mani-
            fest themselves ; and no  gastric secretion  can then be excited by the
            stimulus of food.  Similar results  may  follow the  excitement  of the
          ,. emotions ;  and those of a depressing nature seem especially to produce
            a  pale  flaccid condition  of the  mucous membrane, which is  equally
            unfavorable to the  due secretion of gastric fluid.
              475.  That the amount of the secretion is ordinarily proportioned to
r,.•■■  ■      ithe Avants of the system,—that the  introduction of  any superfluous
?.        '   aliment into the stomach is not only useless but injurious, as giving  rise
/           to irritation,—that incipient disorder of  the  stomach may occur, render-
            ing it less fit than usual for the discharge of its important duties, without
            manifesting itself by the condition of the tongue,—that when the tongue
            does indicate disorder of the stomach, such  disorder is usually consider-
            able,—and that every particle of food ingested, in such states as prevent
            the secretion of gastric fluid, is a source of fresh irritation,—are truths
            which cannot be too constantly kept in mind.  There can  be no  doubt
            that the habit of taking more food than the system  requires, is  a very
            prevalent one ;  and that it is persevered in, because no evil result seems
            to folloAV.  But when it is borne in mind that this habit must keep the 
SECRETION OF GASTRIC FLUID.
271
stomach in a state of continual irritation, however slight, it can scarcely
be doubted that the foundation is thus laid for future disorder of a more
serious kind.   Two circumstances especially tend to maintain this prac-
tice  in adults, independently  of the mere disposition to gratify  the
palate.   One is the habit of eating the same amount of food, as during
the period of growth, when more Avas  required  by the system.  The
other is the custom of eating too fast;  and this is injurious,—both by
preventing sufficient mastication, and thus throwing  on  the  stomach
more than its proper duty,—and also by causing an over-supply of food
to be ingested, before  there is time for  the feeling of satisfaction to
replace that of hunger (§ 486).
  476. Of the Albuminous, Saccharine, and other matters dissolved in >,. .
the Chyme, there is reason  to  believe that  part are absorbed through
the blood-vessels so  copiously  distributed  on the walls of the stomach
(§ 492).   The  remainder, with the undissolved matters, pass into the
duodenum, Avhere the chyme is mingled  with the biliary and pancreatic
secretions.—The secretion of Bile is evidently  a process of the highest  /
importance in the economy; as we may  judge alike from the size of the' -•
Liver and the supply of blood it receives, and from the  rapidly  fatal
effects of  its suspension.  That a part  of it is  purely  excrementitious,
and is poured into the intestinal  tube for the purpose  of being carried,
out of the body, cannot  be questioned; but there is  strong evidence,
that a part of it is destined  to be  absorbed again, after  performing some
action of  importance upon  the contents of the alimentary canal.—Iri :>
all but the very lowest animals, we find  traces  of a bile-secreting appa^ f- ;
ratus ; and this  is  almost constantly situated in the immediate  neigh-;
borhood of the stomach.  In many cases, the secretion is poured directly,
into the cavity  of that organ; but in most, it is conveyed (as in Man)
into the intestinal tube near its commencement.   Hence it seems clear
from the  disposition of the  biliary apparatus, that it has  a purpose to
serve in connexion with the digestive function, and is not destined solely
for the  elaboration  of a  product which  is  to be cast out  of the body;
since, if the latter were  the case, that product  would be carried out
immediately, like the urinary  excretion, and would  not be discharged
into  the  alimentary canal  high  up.—This  conclusion  is  confirmed  by
experiment; for it has been shown that, if the bile-duct be divided, and
be made to discharge its contents  externally through a fistulous orifice
in the walls of the abdomen, instead of  into the intestinal canal, those  '\
animals  Avhich  survive  the  immediate effects of the operation, exhibit
indications of the imperfect performance of the digestive operation.   At
first they  eat much, but  their food does not seem to impart to them an
adequate amount of nutrition; afterwards they lose their  appetite, be-
come thin, and usually die  after an interval of some months  passed in
this state.   If, however, they be allowed  to lick the  orifice, so as to
receive the  fluid discharged from it into their stomachs, these injurious
results do not folloAV.—The observation  of disease in the human subject
leads to similar conclusions ; for, when the biliary  secretion is deficient,
or its floAV into the intestine is obstructed, the digestive  processes are
evidently  disordered;  the  peristaltic action of the bowels is not  duly
performed;  the  faeces  are  white  and clayey;  and there  is an obvious 
272
OF FOOD AND THE  DIGESTIVE  PROCESS.
   insufficiency  in the  supply of nutriment prepared for  the absorbent
   vessels.
jjt   477. On the other hand, that one  great  object of the secretion is to
   withdraAV  from the Blood certain  products  of the decomposition of the
   tissues, which would otherAvise accumulate in it, and Avould be deleterious
   to its character, is shown by evidence yet more decisive.   We find that
   the action of the liver is constant, and not  occasional, like that of the
,.,»' Salivary  and Gastric glands;  and that, if  anything interfere with the
   secreting  process,  and thereby  cause the accumulation of the elements
   of the bile in the  blood, the effects of their presence are  immediately
   manifested in the disorder of other functions, especially those of the
   nervous system (§ 399); and the  continued suspension  of the function
   leads to a fatal result, unless the  elements  of the bile are drawn off (as
   sometimes happens) by the urinary organs.   When the secreting action
   of the liver has once been performed, an  obstruction to the discharge of
   the bile into the intestine does not seem to be so immediately injurious.
   The fluid accumulates, and distends  the bile-ducts and the gall-bladder;
   and Avhen  they^ are  completely filled, part of it is reabsorbed into the
   blood, apparently in a changed condition, since it does not then produce
   the same injurious effects, as result from  the accumulation  of  the same
   materials, previously to the action of the Liver upon  them.  The color-
   ing matter seems to be very readily taken back into the circulating
   system; and is deposited by it in almost  every tissue of the body.
     478. Although the secreting action of the Liver  is constant, yet the
   discharge of bile into the intestine is certainly favored by  the presence
   of chyme in the latter.   The purpose of the gall-bladder is obviously to
   permit the accumulation of bile, when it is  not wanted in the intestine;
   and we find it most constantly  present in those tribes of animals, which
   live upon  animal food,  and Avhich therefore take their  aliment at inter-
   vals ;  Avhilst it  is more frequently  absent in those herbivorous animals,
   in  which  the digestive  process is almost constantly  going on.   The
   middle coat of the bile-ducts is clearly  muscular, and has a peristaltic
   action  like  that of the intestinal  canal;  this action may be excited by
   galvanism, or by irritation  of the branches of the Sympathetic nerve,
   by which it is supplied.   The mucous coat  of the ductus choledochus is
   disposed in valvular  folds, in such a manner as to prevent  the reflux of
   the bile or of the contents of the  intestine; and a still further security
   is afforded by the valvular  covering to the  orifice of the duct, Avhich is
   furnished  by the mucous covering of the intestine itself.  The flow of
   bile into the intestine, when its presence is needed  there, is  commonly
   imputed to the pressure of the distended Duodenum against the gall-
   bladder ;  but it is probable that the contractility of the  muscular coat
   of the duct itself, which may be excited either through the  sympathetic
   nerve, or by  irritation at the orifice of the  duct (as in  the case of the
   Salivary glands), is the  real cause of the discharge of the fluid.   It is
   an interesting fact, Avhich proves how much the passage of the Bile into
   the  Intestine is dependent upon  the presence of aliment in the latter,
   that the gall-bladder is almost invariably found turgid  in  persons Avho
   have died of starvation ; the secretion haA'ing accumulated, through the
   want of demand for it, although there was no obstacle to  its exit. 
SECRETION OF BILE.
273
  479.  The composition  of the  Bile, and  the  structure of the organ
which  elaborates  it,  will be more appropriately considered hereafter,
Avhen the Secreting apparatus generally is being  described (chap, ix.)
At present we have to inquire what is the precise effect of its admixture
with the products of digestion, and what is the  purpose  which this  ad-
mixture serves.   In the first place, it may be stated that biliary matter
is essentially a soap, formed  by the union of a fatty acid with  a  soda-
base ; and that it serves the purpose of neutralizing the acidity of  the
chyme,  which is derived from the gastric juice; the biliary acids falling,  v •
doAvn as an  insoluble precipitate, Avhen thus deprived of their  soda.
Further, the  bile shares with the pancreatic  fluid in  that  emulsifying
poAver, by which the fatty matters of the food are reduced to a  state of
such fine division, as  to  be rendered  capable  of being  absorbed; and
thus it happens that the introduction  of these matters into the system,
through the  medium of  the  lacteal absorbents (§ 494), does not take
place until after the  chyme has been mingled Avith the biliary and pan-
creatic  secretions.  Again, it has  been asserted (but the fact  has  not
been fully substantiated),  that the admixture of biliary matter produces
a conversion of  saccharine in fatty  compounds.  When  fresh bile is
mingled with neAvly-formed chyme, in a glass vessel, the mixture  sepa-  ja^
rates into three distinct parts; a reddish-brown  sediment at the bottom,  ^"
a wdiey-colored fluid  in the  centre, and a  creamy pellicle  at the top.*,
The central stratum probably contains the  albuminous, gelatinous, sac- "■
charine, and  other matters in a state  of solution;  the superficial pellicle
may be looked upon as consisting chiefly of oleaginous matter destined
for absorption ; whilst the sediment, partly consisting of the unreducible
portion of the food, and partly of the biliary matter itself, is evidently
excrementitious.
   480. Tho  Pancreatic secretion has a chemical constitution very ana-  \
logous  to that of Saliva;  but the peculiar  organic  compound which it
contains, has been found  by  M. Bernard to possess a power of emulsify-
ing fatty matter, when mingled  with it; and there is strong reason to
believe that the chief purpose of this secretion is to effect such a change
in the condition of the oleaginous constituents  of the chyme,  as may
prepare them for absorption.   But further, the neutralization of the acid
of the gastric fluid now allows the metamorphosis of starch to be recom-
menced ; and as there is eAddence that the production of sugar continues
to take place during the passage of the chymous mass along the small
intestines, in animals  Avhose  food is partly  or completely vegetable, the
pancreatic fluid, which has been experimentally ascertained  to possess  -
this power, is probably the chief agent by Avhich the conversion is  effected.
It may be surmised, further, that the glandulae  of Brunner (§ 450) par-
ticipate in the functions of the Pancreas;  being, perhaps, the chief
agents  in the elimination of that "succus  entericus," which has been
experimentally found to concur with the biliary and pancreatic fluids in
the emulsification of fatty matters.
   481. During the passage  of the contents of  the Intestine, now aug-
mented by the biliary secretion, along the  canal, the nutritious portion '" '
is gradually withdrawn  by the absorbent vessels on its Avails;  and the
excrementitious matter alone remains, increased in amount by the pro-
                                  18 
  274           OF FOOD AND THE DIGESTIVE PROCESS.

  ducts of the secretion of the Peyerian and other glandulae, with Avhich
  the mucous lining of the lower intestines is studded.   Many of the loAver
  animals are furnished, at the part where the small  intestine enters  the
  large, with a caecum, resembling that which in Man is termed the vermi-
  form appendage of the caecum, but greatly exceeding it in  size.   Some-
  times Ave find two caeca instead of one ;  and these are much prolonged,
  so  as  to form tubes  of  considerable length.  It has been ascertained
  that, in herbivorous animals, a distinctly acid secretion is formed by the
  cascum, during the digestive process; and there is reason to  believe,
  that the food there undergoes a second process, analogous to that to which
  it has been submitted in the stomach, and  fitted to  extract from it any
  undissolved alimentary matter which it may still contain.   There is no
  reason to believe, however, that any such  process takes place in Man,
  whose real caecum is rudimentary,—the part of  the intestine which  has
  received the  name, being merely the dilated commencement of the colon.
  The act of Defecation, by Avhich the excrementitious matter is discharged,
  has been already noticed (§ 462); the Absorption  of nutritive matter
  will be treated of in the succeeding Chapter.

                     5.  Of Hunger,  Satiety, and Thirst.

     482.  The want of  solid aliment is  indicated  by the sensation of
  Hunger; and the deficiency of fluid by  that of Thirst.   On the other
  hand, the presence of a sufficiency of food or liquid in  the  stomach is
  indicated by the sense of  Satiety.   These sensations are intended as
  our guides, in regard to  the amount of aliment we take  in.   What is
  the real seat of these sensations, and on  what conditions do they  de-
  pend?
     483. The  sense of Hunger is referred to the stomach, and seems  im-
  mediately to  depend upon  a certain condition of that  organ; but what
  that condition is, has  not yet been precisely ascertained.   It is not pro-
educed by mere emptiness of the stomach, as some have  supposed;  for,
  if the previous meal  have  been sufficient, the food  passes entirely from
  the cavity  of the stomach,  before a renewal of the sensation is felt.  It
  cannot be due to the  action of the  gastric fluid upon the coats of  the
  stomach themselves ;  because this  fluid is not poured into  the stomach,
  except when  the production of it is stimulated  by  the irritation of  the
  secreting follicles.  It has  been attributed to distension of the  gastric
  follicles by the secreted fluid ;  but there is no evidence that the  fluid is
  secreted before it is wanted; and moreover, it is well known that mental
  emotion can  dissipate in a  moment the keenest  appetite, and it is diffi-
  cult to imagine  how this  can occasion the emptying of the follicles.
  Perhaps the most satisfactory view is that, which attributes the  sense of
  hunger to  a determination  of blood to the stomach, preparing it for  the
  secretion of gastric fluid; since this is quite adequate to account for  the
  impression made upon the  nerves;  and it accords  with what has just
  been stated of the influence  of mental emotions,  since we know that
  these have a  powerful effect upon the circulation of blood in the minute
  vessels (§ 603).
     484. Although the sense of Hunger is  immediately dependent, in 
SENSE  OF nUNGER AND SATIETY.
275
great part at least, upon the condition of the stomach, yet it is also in-
dicative of the condition of the general system ; being extremely strong,
when the  body has undergone an unusual waste Avithout a due supply of
food, even though the stomach  be in a state of distension ; whilst it is
not  experienced, if, through the general inactivity of the  system, the
last  supply has not been exhausted,  even though the stomach has been
long empty.   It is well known that,  when food is deficient,  the attempt
to allay the pangs of hunger by filling the stomach with  non-nutritious
substances, is  only temporarily  successful;  the feeling soon  returning
with  increased violence,  though  it  has  received a temporary  check.
The  reason for this is obviously, that the general system has received
no satisfaction, although the stomach has been caused to secrete gastric
fluid by the contact of solid matter with its Avails:  so that although the
state on which hunger immediately depends, has been for a time relieved,
this  state is soon renewed, unless the solid  matter  introduced into the
stomach be of an alimentary character, and  be  dissolved and  carried
into the system.
   485. When the food is nutritious in  its character, but  of small  bulk
experience has shoAvn the  advantage  of mixing it with  non-nutritious
substances, in order to give  it bulk and solidity ; for if this be  not done,
it does not exert its  due stimulating influence upon the  stomach; the
gastric juice is not poured forth in proper quantity ; and the result is,
that  neither is the sense of hunger  relieved, nor are  the wants  of the
body satisfied.   Thus the Kamschatdales are in  the habit of mixing
earth or sawdust with the train-oil, on which alone they are frequently
reduced to live.  The Veddahs  or wild hunters of Ceylon, on the same
principle, mingle the pounded fibres of soft  and decayed wood with the
honey on which they feed when  meat is not to be  had;  and  on one  of
them being asked the reason of  the  practice, he replied, " I cannot tell
you, but I know that the belly must be filled."  It has been found  that
soups and fluid  diet are not more  readily converted into  chyme than
solid aliment, and are not alone fit for the support of the body in health;
and  it is often to be  observed, in disordered states of the stomach,  that
it can retain a small quantity of easily  digested solid food, when a thin
broth would be rejected.
  486. The sense of Satiety is  the  opposite  of Hunger; and  like  it,
depends on  tAvo sets of conditions,—the state of the stomach, and  that
of the general system.  It is  produced in the  first  instance  by the
ingestion  of solid matter into the stomach, which gives rise to the  feel-
ing of fulness ; but  this is only a part of the sensation which ought  to
be experienced ;  and it is only Avhen the act of digestion  is being duly
performed, and nutritive matter is being absorbed into the vessels,  that
the  peculiar feeling  of satisfaction is excited, which indicates that the
wants of  the system at large are being supplied.—It  has been very
justly remarked  by  Dr. Beaumont, that  the  cessation of the demand
set up by the system, rather than the positive feeling of  satiety,  should
be the guide in regulating  the quantity of food taken into the stomach.
The  sense of satiety is beyond the point of healthful indulgence ; and is
Nature's earliest  indication of an abuse  and overburden  of her powers 
276            OF FOOD  AND  THE DIGESTIVE PROCESS.
to replenish the system.   The proper intimation is the  pleasurable sen-
sation which is experienced, when the cravings of the appetite are first
allayed ; since, if  the stomach  be sufficiently distended with wholesome
food for this to be the case, it is  next to  certain that the digestion  of
that food will supply Avhat is required for the nutrition of the body.   It
is  only when the  substance with which  the stomach  is distended, is not
of a digestible character, that the feelings excited by the state of  that
organ are anything but a correct index of the wants  of the system.
  487.  The Par Vagum is evidently the  nerve, which conveys to the
sensorium the  impression  of the state of the stomach, and  which  is
therefore the immediate  excitor of the sensation of hunger  or of the
feeling of satiety.  But  it is evident from experiments upon animals,
that it  is not the only source, through which  they  are incited to  take
food, and are  informed  Avhen  they  have  ingested  enough ;  and it  is
probable that  the  Sympathetic  nerve is the channel, through Avhich the
wants of the system are made known, and through which, in particular,
the feeling of general exhaustion is excited, that is experienced when
there has been an unusual waste, or Avhen the proper  supply has been
too long withheld.
   488.  The conditions of the sense of Thirst are very analogous  to
those of hunger ;  that is, it indicates the deficiency  of fluid in the body
at large; but  the immediate seat of the feeling is a  part of the ali-
mentary canal,—not the  stomach, however, but  the fauces.  It is re-
lieved  by the introduction of fluid into the circulating  system,  through
any channel;  whilst the mere contact of fluids with  the surface  to
which  the sensation is referred, produces only a temporary effect, un-
less absorption takes  place.  If  liquids be introduced into  the stomach
by an  cesophagus-tube, they are  just as  effectual in allaying thirst,  as
if they Avere swallowed in the ordinary manner; and  the  same result
follows  the injection of fluid into the veins (as was most  remarkably
the case when  this method of  treatment Avas  practised in the Asiatic
Cholera), or the absorption of fluid through  the  skin or the lower part
of  the  alimentary canal.   The  deficiency of fluid in the body  may
arise,—and Thirst may consequently  be induced,—either by an un-
usually small  supply of  fluid, or by excessive  loss  of  the fluids of the
body, as by perspiration, diarrhoea, &c.   But it may also be occasioned
by the impression made  by particular kinds of food or drink  upon the
alimentary canal; thus salted or highly-spiced meat, fermented liquors
when too little diluted, and other similar irritating agents, excite thirst;
the purpose of Avhich  sensation is evidently to cause the ingestion  of
fluid, by which these substances may be  diluted, and  their irritating
action prevented. 
ABSORPTION AND SANGUIFICATION.
277
                          CHAPTER V.

                 ABSORPTION AND SANGUIFICATION.

                1. Absorption from the Digestive Cavity.

  489.  So long as the Alimentary matter is contained in the digestive
cavity, it is as far from being conducive to the nutrition of the system,
as if it Avere  in  contact  with the  external surface.  It is only when
absorbed into the vessels, and carried  by the  circulating current into
the remote portions of the body, that it really becomes  useful in main-
taining the vigor of  the  system, by replacing that which  has decayed,
and by affording the materials for  the various  organic processes Avhich
are continually going on.  Among the Invertebrated animals,  Ave find
the reception of alimentary  matter into the circulating  system to be
entirely accomplished through the  medium of the veins, Avhich are dis-
tributed upon the walls of the digestive cavity.  We  not unfrequently
observe, that the intestinal tube is completely enclosed within  a large
venous sinus, so that its  whole external surface is bathed  with blood ;
and into this sinus, the alimentary  materials Avould appear to transude,
through the walls of the intestinal  canal, to become  mingled with the
blood, and to be conveyed with its  current into the  remote portions of
the body.   Among the Vertebrata, we find an additional set of vessels,
interposed betAveen the walls of  the intestine and the sanguiferous sys-
tem, for the purpose, as it would seem,  of taking up that portion of the
nutritive matter Avhich is not in a state of perfect solution, and of pre-
paring it for being  introduced into the current  of the blood.   These
vessels  are the lacteals  or absorbents.  They are very copiously distri-
buted upon the walls  of the smaller  intestine,  commencing near the
entrance  of the biliary and  pancreatic ducts; the  walls of the large
intestine are less abundantly supplied with them, and they do not show
themselves in the villi  Avhich are found on some parts of the lining mem-
brane of the stomach, although the walls of that viscus are supplied with
lymphatic absorbents.
   490. Nevertheless it is quite certain, that substances may pass into
the current of the circulation, which have been prevented from  passing
further than the stomach; thus, if a solution  of Epsom-salts be  intro-
duced into the  stomach of an animal, and its passage into the intestine
be prevented by a ligature  around the  pylorus, its purgative  action
will be exerted nearly as soon, as  if the  communication  between the
stomach and intestines  had been left quite  free;  or  if a solution of
prussiate   of potash be  introduced  into the stomach  under  similar
circumstances,  the presence  of that salt  in the blood may be speedily
demonstrated by chemical tests.   It appears  from  the  experiments  of
MM.  Tiedemann and Gmelin, that when various  substances were min-
gled with  the food, Avhich, by their color, odor, or  chemical properties
might be easily detected,—such as  gamboge, madder, rhubarb, camphor, 
278
ABSORPTION AND SANGUIFICATION.
musk,  asafoetida, and  saline compounds,—they Avere seldom  found in
the chyle, though many of them were detected in the blood and in the
urine.   The coloring matter appeared to  be  seldom absorbed at all;
the odorous substances were generally detected in the venous blood and
in the  urine, but not in the chyle ;  whilst, of the  saline substances,
many Avere found in the blood and in the urine, and  only a very few in
the chyle.
  491.  This passage  of substances in a state of perfect solution, from
the stomach into  the blood-vessels, is probably due to the operation of
that peculiar modification  of Capillary Attraction, which is called En-
dosmose.   When tAvo  fluids differing  in  density are  separated by a
thin animal or  vegetable membrane,  there is  a tendency to mutual
admixture  through  the pores of the  membrane ;  but the less dense
fluid will transude Avith  much  greater  facility than the more dense;
and consequently there will be a considerable  increase on  the  side of
the denser' fluid; Avhilst very little of this, in comparison, will have
passed towards the less dense.   When one of the fluids is contained in
a sac or cavity, the flow of the  other  toAvards it is termed Endosmose,
or floAV-inAvards; whilst the contrary  current is termed Exosmose or
floAv-outwards.   Thus if the caecum of a  fowl, filled  Avith  syrup  or
gum-Avater, be tied to the  end  of  a tube, and be  immersed in pure
water, the latter Avill penetrate the  caecum  by Endosmose, and will so
increase the volume  of its  contents, as  to  cause the fluid to  rise  to a
considerable height in  the attached tube.    On the other hand, a small
proportion of the gum  or syrup will find its way into  the  surrounding
fluid by Exosmose.   But  if the caecum  were  filled with water,  and
Avere immersed in a solution of gum or sugar, it would soon be nearly
emptied, the  Exosmose  being  much stronger  than  the  Endosmose.
It is in this manner that Ave may cause the flattened corpuscles of the
blood to be distended  into  spheres, by treating them with water; or
may empty them  almost  completely,  by  immersing them  in syrup
(§ 215); since their  contents  are  more  dense than  the  surrounding
fluid in the first  case, so that they will be  augmented by Endosmose;
whilst  they are  less dense  in the second,  so  as to be  diminished by
Exosmose.
  492.  Now it seems to  be in  this manner, that substances contained
in the  cavity of the stomach, and perfectly dissolved by its fluids,  are
received into the blood-vessels; for as  the  blood  is the fluid of greater
density,  it will  have a tendency  to  draw  towards  it, by Endosmose,
the saline and other matters, which are in a state  of perfect solution
in the  stomach.   The  Mucous membrane,  which forms the inner  wall
of that organ, is most copiously supplied  with  blood-vessels;  partly,
indeed, that they may afford the materials of the gastric secretion; but
partly, also, that they may  take up the substances, which are capable
of entering the circulating current  by this direct channel.  The move-
ment of blood  through the vessels, tends to accelerate  the  permeation
of liquids through their  walls, in a very  remarkable  degree: as  may
be shown by the following simple experiment.   If a membranous tube,
such as a piece of the -small intestine or of a large  vein of an animal,
be  fixed by one extremity to  an opening at the bottom of  a vessel 
         ABSORPTION OF SOLUBLE MATTERS  INTO THE VEINS.     279

filled  with Avater, and  have a  stopcock attached at the other extre-
mity,  and be then  immersed in Avater  acidulated with  sulphuric  or
hydrochloric acid, it will be some time before the acid will penetrate to
the interior  of  the  tube, Avhich is  distended with water;  but if  the
stopcock be  opened, and  the water be  allowed to discharge itself, the
presence  of  the  acid will be  immediately  discovered (by  tincture of
litmus) in the liquid  which flows out.   Thus  the continuance of the Cir-
culation  is  obviously one  of  the  most  important of  the  conditions of
Absorption.   It is  whilst passing through  the system of  capillaries,
which forms  a minute plexus immediately beneath the free  surface of
the mucous membrane, that the blood thus receives an admixture of the
soluble matters contained within the  digestive cavity; and  hence it is
that  these substances are  detectible in the blood of the gastric and
mesenteric veins, sooner than in any part of the arterial system.  They
are very rapidly diffused, however, through the general circulation ; and
may even show themselves in the excretions within so short a  period,
that it is obvious that they must  have  been absorbed immediately on
their  introduction into the stomach.   Thus Mr. Erichsen found that he
Avas able to  detect the presence of ferrocyanide of  potassium in  the
urine, within one minute after it had been swallowed in solution.  This,
however, was only Avhen it Avas taken after a long  fast; more commonly
the absorption is less rapid; and if the substance be introduced within an
hour or two after a full meal, it may be as much as half an hour before
its presence in the urine gives evidence of  its having been received into
the circulating current.   Although it is difficult to speak with certainty
on the point, yet there  appears a strong probability, that, both in the
stomach  and intestinal tube, the absorption of nutritive matters in a
state  of perfect  solution  (such as gum, sugar, pectine,  gelatine, and
soluble albumen) is thus accomplished through the medium of the blood-
vessels ;  Avhich also take up the chief supply of water that is required by
the system.  It is difficult else to  see the purpose of  the extraordinary
vascularity of the mucous membrane, and in particular of those filaments
                 Distribution of Capillaiies in the Villi of the Intestine.

or narrow folds, termed villi, which so thickly cover its surface.   Each
of these villi is  furnished  Avith a plexus of minute  blood-vessels, of
which the larger  branches may even be seen  with the naked eye, when
they are distended with  blood or with a colored injection.  By these
villi,  the vascular  surface of  the mucous  membrane is enormously
extended.  In  Man, they are commonly cylindrical or nearly so, and
are from about a  quarter  of a line, to a line  and a half in length;  but 
280
ABSORPTION AND SANGUIFICATION.
in many of the loAver animals, they are spread out into broader laminae
at the base,  and are connected together so as to form ridges or folds.
   493.  The nutritive materials taken up by the blood-vessels of the ali-
mentary canal, are not conveyed directly into the general circulation;
for they are first submitted to the  agency of the Liver.   All  the veins
which return the blood from the gastro-intestinal capillaries,  converge
into the portal trunk, which distributes it to the various portions of that
secreting apparatus; and there  is strong  reason to  believe,  that  not
merely  is the fluid there depurated of some matters Avhose  presence
Avould be injurious, but that the Liver exercises a  poAverful  assimilating
action upon the proper nutritive substances,  rendering them  fitter to
become components of the Blood.  For the blood of the portal vein,
when examined during digestion, is found to contain a large proportion
of albumen and comparatively little  fibrine ; whilst  in  that Avhich has
passed through the liver, the amount of fibrine  has  undergone a large
increase.  Again, it appears that fatty matters  are  elaborated  in  the
liver, either from saccharine substances, or from albuminous compounds;
for even when no fat can be detected in the  blood of the vena portae,
that of the hepatic vein contains it in considerable amount.  So, again,
it appears that the liver elaborates  from some other constituents  of  the
blood a saccharine compound (diabetic  sugar), which is destined for im-
mediate  elimination by the lungs, and which, being much more readily
carried  off by the respiratory process than either grape-sugar or cane-
sugar, may be regarded as its most appropriate  pabulum.   Further, if
white of egg mixed Avith Avater be injected into any of the systemic veins,
distinct evidence of the presence  of albumen is speedily traceable in the
urine ; showing that this substance has not  been properly assimilated.
But if the same fluid be injected into the  portal system, no trace of its
presence in the urine is found.   So, again, when a solution of sugar is
injected into the general venous system, this substance soon sIioavs itself
in the urinary excretion;  but if  the  same  injection be  made into the
vena portas, so that the sugar is obliged  to  pass through the liver, no
such elimination takes place, it being  then  assimilated with the blood.
The liver, however, is  not required to effect a corresponding change in
the fatty matters taken up from the  food;  for these are  received into
the blood through the absorbents, rather than through the sanguiferous
vessels ;  and it is found that if fatty matters be injected into the general
circulation, no effect is produced on the urine.*
   494.  Every one of the intestinal  Villi,  however, also contains  the
commencement of a proper lacteal vessel; the portion of the absorbent
system specially  adapted for the reception of alimentary matters from
without, being thus distinguished, on  account of the milky aspect of the
fluid Avhich is  found Avithin it.  The folloAving figure (83) represents the
appearance offered by the incipient lacteals, in a  villus of the jejunum
of a young man, who had  been  hung  soon after taking a full meal of
farinaceous  food.   The trunk that issues from the villus is formed by
the confluence of several smaller  branches, whose origin it is difficult to
trace ;  but it is probable that they  form loops by  anastomosis with each
  * See the recent Lectures and Memoirs of M. CI.  Bernard, in "L'Union Me"dicale,"
and the  " Gazette Me"dicale," for 1850. 
        ABSORPTION OF ALCOHOL, ETC.—MESENTERIC GLANDS.     281

other, so that there is no proper free extremity in any case.   It is quite
certain  that the lacteals never open by free orifices  upon the surface
of the intestine, as Avas formerly  imagined.   And there seems good
reason to believe, that either by the  cells  asserted by  Prof. Goodsir
to be developed Avithin the free  extremities of the villi, or by the epi-
thelial cells  which cover  their extremities (§ 243), a
selection is made of those substances Avhich are proper      FiS- 83-
to be received into the special Absorbent system.
  495.  It is particularly  important to keep  in  view
the difference betAveen  the  tAvo modes, by  which ali-
mentary substances are introduced  into the system,
when we are  treating  those disordered  states, in
which the digestive process is imperfectly performed,
or  is altogether  suspended.   There  can  be  little
doubt that the immediate cause of death, in many
diseases  of exhaustion, is the want of power to main-
tain the heat of the  body;  the  stomach  not being
 r i  .   v   . p   i    i.i       -ii   i            Uneoltae Intestinal Villi,
able to digest iood, and the special  absorbent poAver with the commencement of
of the lacteals being altogether  suspended, so  that alacteal-
the  inanition is as  complete,  as  if  food were  altogether withheld.
Now under such circumstances, it becomes a matter of the greatest im-
portance to present a supply of combustible matter,  in such a form that
it may be introduced into the circulating system by  simple Endosmose ;
and the  value which experience has assigned to broths  and to thin fari-
naceous solutions, and still more,  to diluted  alcoholic drinks, frequently
repeated, under such circumstances,  seems  to depend in great part upon
the facility with which they may be thus absorbed.   The good effects
of alcohol, cautiously administered,  are no doubt owing in  part  to its
specific  influence upon the nervous  system;  but that they are also due
to its heat-producing power,  appears from the results of the administra-
tion of  frequently-repeated  doses,  in  states of utter exhaustion,—the
temperature of the body being kept up so long as they are continued,
and falling when they  are  intermitted  (§ 118).  As the alcohol is thus
burned off nearly as  fast as it  is introduced, it never accumulates  in
sufficient quantity, to produce its usual violently-stimulating effects upon
the nervous system.

2. Passage of the Chyle along the Lacteals, and its admixture with the Lymph
                   collected from the general System.

   496.  The  Lacteal vessels, which commence on the surface of the in-
testines, run together on  their walls,  and form  larger  trunks,  which
converge and unite with  each other in the  mesentery;  and the main
trunks  thus formed then  enter  certain bodies, which are commonly
knoAvn as the " mesenteric glands."  Their structure, hoAvever, does not
seem to  correspond with that of the proper glands ; as they are simply
composed of  lacteal  trunks, convoluted into knots, and dilated  into
larger cavities,  amongst which bloodvessels are  minutely distributed.
These blood-vessels have no direct  communication with the interior of
the lacteals; but are separated from them by the membranous walls of
 
282
ABSORPTION AND SANGUIFICATION.
both sets of tubes.   The epithelium, which lines the  absorbent vessel,
undergoes a marked  change Avhere the vessel enters  the gland, and be-
comes more like that of the proper glandular follicles in its character.
Instead of being flat and scale-like, and forming a single layer in close
apposition with the basement-membrane,  as it does  in the  lacteal tubes
before they enter the gland  and after they have  emerged from it, we
find it composed, within the gland, of numerous layers of spherical  nu-
cleated cells (Figs. 84 and 85);  of which the superficial ones are easily
detached, and appear to be identical with the cells that are found float-
ing in the  chyle.  The purpose of the cells  will be presently inquired
into.
              Fig. 84.                          Fig. 85.
  Diagram of an Absorbent Gland, showing the     Portion of intra-glandular Absorbent, showing
 intra-glandular network, and the transition from   along the lower edge the thickness of the germi-
 the scale-like epithelia of the exlra-glandular ab-   nal membrane, and  upon it, the thick layer of
 sorbents, to the nucleated cells of the intra-glan-   glandular epithelial cells.
 dular.
   497. After emerging from  the mesenteric glands, the  lacteal trunks
 converge, with occasional union, until they discharge their contents into
 the receptaculum chyli, which is situated at the front  of the body of the
 second lumbar vertebra.   Into the same cavity are  poured the contents
 of a part of the other divisions of  the Absorbent system; which is dis-
 tributed through the body in general, and which, from the transparency
 of the fluid or lymph it contains, is termed the lymphatic system. From
 the receptaculum chyli, arises the thoracic duct; which passes  upwards
 in front of  the spine, receiving other lymphatic trunks in its course, to
 terminate at the junction of the left subclavian and jugular veins ; where
 it delivers its contents into the sanguiferous system (Fig. 86).  A smaller
 duct receives some of the lymphatics of the right side, and there termi-
 nates  at a  corresponding part of  the  venous system; but  it  does not
 receive any of the contents of the lacteals.
  498. The Lymphatic system is  evidently  allied  very  closely to the
 lacteal, in its general purposes; and makes its first  appearance in the
 same  class  of animals, namely, in  fishes.  The vessels  of  which it is
 composed are distributed through most of the softer tissues of the body,
 and are particularly abundant in the skin.   They have never been'found
 to commence by closed or open extremities ; but seem to form a net-
 work, from  Avhich the trunks  arise.   In their course they pass through
glandulae, disposed  in different parts of the body, which  exactly re-
semble in structure  those which  are  found  upon  the lacteals in the
mesentery.  And they at last terminate, as already shown, in the same
general receptacle with the lacteals.   Hence it cannot  be  reasonably
doubted that the fluid which they absorb from the various tissues of the
body, is destined to become again subservient to nutrition ; being poured
back into the current of  the blood,  along with  the new materials, which 
LACTEAL  AND LYMPHATIC SYSTEMS.
283
Fig. 86.
are now for the  first time  being introduced into it.   That the special
Absorbent apparatus of Vertebrated animals has for part of its functions,
to effect a change  in the materials
absorbed, and thus to aid in fitting
them for introduction into the blood,
seems  apparent  from  the  facts of
Comparative Anatomy ; which show
that, the more distinct the blood is
from the chyle and lymph, the more
marked is the provision for delaying
the latter in the  absorbent  system,
and for subjecting it to preliminary
change.
   499. The course of the Absorbent
vessels in Fishes is short and simple;
they are not furnished with glands ;
and they  pour their contents into
the blood-vessels at several different
parts of the body.  In this class the
blood contains fewer red corpuscles,
and its coagulating power is feebler,
than in any other Vertebrata. And
in the  lowest tribes, in which  the
Vertebrated character is almost en-
tirely  wanting,  and in Avhich  the
blood is almost pale, no special ap-
sorbent system  has  yet been  dis-
covered.—In Reptiles, the length of
the Absorbent vessels is remarkably
increased by  their doublings  and
convolutions; so  that the system ap-
pears to be more highly developed,
than in either of the warm-blooded
classes.  But this superiority is not
real; for there is yet no trace of the
glands, Avhich concentrate, as it were,
the assimilation power, of a long se-
ries of vessels. Moreover, we often
find the lymphatics of this class fur-
nished with pulsating dilatations, or
lymphatic  hearts ; which have  for
their office to propel the lymph into
the  venous  system.   In the Frog
there ar e t wo pair s of these; one situ-
ated just beneath the skin (through
Avhich its pulsations are readily seen
in the  living animal),  immediately
behind the  hip-joint;  the other pair being more deeply  seated at the
upper part of the chest.   The former receive  the lymph of the posterior
part of the body, and pour  it into the  veins  proceeding from the same
  The course and termination of the Thoracic
Duct. 1. The arch*of the aorta. 2. The thoracic
aorta.  3. The  abdominal aorta; showing its
principal branches divided near their origin. 4.
The arteria innominata, dividing into the right
carotid and right subclavian arteries.  5. The
left carotid. 6. The left subclavian.  7. The su-
perior cava formed by the union, of, 8, the two
venae innominatae; and these by the junction, 9,
of the internal jugular and subclavian vein at
each side. 10. The greater vena azygos. 11. The
termination of the lesser in the greater vena
azygos.  12. The receptaculum chyli; several
lymphatic trunks are seen opening into it. 13.
The thoracic duct dividing opposite the middle of
the dorsal vertebras into two branches, which soon
reunite ; the course of the duct behind the arch of
the aorta and left subclavian artery is shown by a
dotted line. 14. The duct making its turn at the
root of the neck, and receiving several lymphatic
trunks previously to terminating in the posterior
aspect of the junction of the internal jugular and
subclavian vein. 15. The termination of the trunk
of the ductus lymphaticus dexter. 
284
ABSORPTION AND SANGUIFICATION.
part; the latter collect that which is transmitted from the anterior part
of the body and  head, and  empty their contents into the jugular vein.
Their pulsations are totally independent of the action of the heart, and
of the respiratory movements ;  since they continue after the removal of
the former, and for an hour of  two subsequently to the death and com-
plete dismemberment of  the animal.   They usually take place at the
rate of about  sixty in a minute; but they are by no  means  regular,
and are not synchronous on the two sides.
   500. In  Birds, we find the Absorbent system existing in  a more per-
fect form; its diffused plexuses and convolutions being replaced by glands;
in which the contained fluid is brought into closer proximity  with the
blood, and in which it is subjected to the influence of assimilating cells.
These, however, are not very numerous; being principally found on the
lymphatics of the  upper extremities.   The  absorbents,  in this class,
terminate  principally by two  thoracic ducts, one  on  each side, which
enter the  jugular veins by several orifices.    There  are, however, two
other entrances, as in Reptiles, into the veins  of  the lower extremity;
and these  are connected by two large dilatations of  the  lymphatics,
which are  evidently analogous to the lymphatic hearts of Reptiles, but
which have little or no power of spontaneous contraction.—In Mam-
malia, the  Absorbent system presents itself in its most developed and
concentrated  state.   The vessels  possess firmer walls,  and  are more
copiously provided  with valves, than  in the classes beneath; and the
glands are much  more numerous, particularly upon  the  vessels  that
receive or  imbibe substances from without,—as  those  of the  digestive
cavity, the skin, and the lungs.  The  terminations of the absorbents in
the veins are  usually restricted,  as in Man,  to the single  point  of  en-
trance of the thoracic duct on either side ;  but  they are sometimes more
numerous ; and certain variations in  the arrangement  of the  thoracic
ducts, which occasionally present themselves as  irregularities  in  Man,
are the ordinary condition  of  these parts  in  some of  the  lower Mam-
malia.
   501.  With  regard to the source  of the matters absorbed by  the
Lymphatics, it is difficult to speak with certainty. We shall presently
see that their contents bear a close resemblance to the  fluid element of
the  blood, or " liquor sanguinis," in  a  state of dilution; it is  very
probable that they partly consist  of the  residual fluid, Avhich, having
escaped  from  the blood-vessels into the  tissues, has furnished the latter
with the materials  of their nutrition, and is now to be returned to the
former.  But they may include, also, those particles of the  solid frame-
work which have lost their vital powers,  and  which are, therefore, not
fit to be  retained as components of the living  system, but Avhich have not
undergone a degree of decay which  prevents  them from  serving, like
matter derived from the dead bodies of other animals, as a material for re-
construction, when it has been  again subjected  to the organizing process.
   502.  It was formerly supposed (and the doctrine Avas particularly in-
culcated by the celebrated  John Hunter) that the office of the Lymphatic
system is to take up and remove all the effete  matter, that  is to be cast
out of the  body, being no longer fit  for its  nutrition.   But for such a
supposition there  is no  adequate foundation.   It seems absurd to ima- 
ABSORPTION BY  THE LYMPHATICS.
285
gine, that this effete matter Avould be mingled with the newly-ingested
aliment, and Avould be poured  back with it into the general current of
the circulation, instead of being at once carried out of the system.  And
the idea is directly negatived, as we shall  presently see,  by the actual
composition of the lymph draAvn from these vessels; the solid matter of
which consists, in  great part at  least, of substances of a  nutritive cha-
racter.   It is  true  that other substances are  occasionally found in the
lymphatics; thus, when  the gall-bladder  and bile-ducts are over-dis-
tended with bile,  in  consequence of some obstruction to its exit, the
lymphatics  of the  liver  are  found to contain  a  biliary  fluid.   In like
manner,  the lymphatics  in  the neighborhood of a large abscess have
been found to contain pus.   When  the  limb  of an animal, round the
upper part  of Avhich  a bandage is tied, is kept for some hours in  tepid
milk, the lymphatics  of  the skin are found  distended with  that  fluid.
And when saline  solutions  are applied to  the  skin, they are usually
detected more readily in the lymphatics, than  in the veins.   But  these
facts only prove, that the lymphatics very readily imbibe soluble sub-
stances with which they are in proximity; and this imbibition seems to
take place on  the same physical principles, as the imbibition of soluble
substances by the  veins of the intestinal canal.
   503.  The more  ready absorption of such substances by the lympha-
tics, than  by  the  veins,  of the  cutaneous  surfaces,—contrary to Avhat
obtains in the alimentary canal,—is easily accounted for, by the very
abundant distribution of the lymphatics  in  the skin, and the ready
access which  fluids can  obtain  to  their walls.   In other  tissues it  is
different:  thus  it  appears that  saline  matters injected  into the lungs
are  detected  much sooner in the serum of the blood, than they are in
the lymph ; and make their appearance earlier in the  left cavities of
the  heart, to  which they Avould be conveyed by the pulmonary vein,
than in the right, which they would  reach  through the thoracic duct
and descending cava.  This is obviously due to the minute distribution
of the blood-vessels upon the walls of the air-cells; which makes them
far  more  ready channels for the imbibition of fluid,  than the lym-
phatics could  be.—In regard to the occasional absorption of pus from
the cavity of an abscess or of an open ulcer,  by the lymphatics, it is to
be remarked that  the absorbent vessels must themselves probably be
laid open  by ulceration ; since in no other way can we understand the
entrance of the globules, so  large as those of  pus, into their interior.
   504. In regard to  the cause of the movement of the. chyle and lymph
along the absorbent vessels, from their  commencement to their termina-
tion in  the central  receptacle, no very definite account can be given.
The middle coat  of  these vessels has a fibrous texture;  and the fibres
bear some resemblance  to  that of  the non-striated muscle.  In the
thoracic duct, this fibrous structure is  more evident; and distinct con-
tractions have been excited in it, by irritating the sympathetic trunks
from which it receives its nenres, and the roots of the spinal nerves with
which those trunks are connected.   Hence it seems probable, that there
is a sort  of  peristaltic  contraction of the  walls  of  the  absorbents,
analogous to  that which takes  place in the  intestinal tube,  serving to
driA'e their contents  slowly  onwards; their reflux being prevented by 
286
ABSORPTION AND SANGUIFICATION.
 the valves, with which  they are copiously furnished.  Moreover, it is
 probable that the general movements of  the  body may concur with  the
 contractile power of the absorbent vessels themselves, to urge their con-
 tents  onwards;  for  almost every  change in  position must  occasion
 increased pressure on some portion of them, which will  propel the fluid
 contents in the  sole direction permitted by the valves, and thus give
 them an additional  impulse towards the  trunks, in which they are col-
 lected for delivery into  the blood-vessels.

 3.  Of the Spleen,  and other Glandular Appendages to the Lymphatic System.

   505.  The structure and functions of the Spleen, and of certain other
 organs  allied  to it in  character, have  been among the  most obscure
 subjects in Anatomy and Physiology;  and they are far  from having
 been yet fully  elucidated.   There  seems sufficient evidence, however,
 for regarding  them  in  the  light  of  appendages  to the  Absorbent
 system, and as concerned, like it, in the process of Sanguification, or
 the preparation  of Blood.   Hence  this  appears to  be the most  appro-
 priate place for  such a brief notice of them as the  present state  of  our
 knowledge admits.
   506.  The Spleen is certainly to be regarded as an organ of compound
 structure, having at least two sets of functions to fulfil.   It is essen-
 tially composed of a fibrous  membrane, which constitutes its exterior
 envelope, and which sends prolongations in  all directions across its in-
 terior, so as to divide it  into a number  of minute cavities of  irregular
 form, freely communicating with each  other.  In many  animals, this
 fibrous  envelope, and  the  prolongations or trabecular which it sends
 through the substance of the organ, are distinctly muscular ; containing
 a large proportion of  the  peculiar fusiform contractile cells formerly
 described (§ 337).  These, however, do  not  present  themselves  in  the
 Human spleen;  and its  trabeculae do not appear to have any contractile
 property.  The  areolae  formed  by  the trabecular  tissue,  commonly
 known as the splenic follicles, are differently occupied  in different ani-
 mals.   In the  Ruminants  they are lined  by a continuation of  the
 splenic  vein, which dilates into a cavernous  structure, capable  of re-
 ceiving  a very large quantity of blood.   In Man, however, they have
 no  communication  with the splenic vein, and  are chiefly occupied by
 the Malpighian  corpuscles and the  parenchymatous tissue, which, in  the
 Ruminants, are  limited  to the partitions  between the venous  cells.  The
 Malpighian corpuscles of the spleen are  whitish spherical bodies, which
 are always connected with the smaller arteries, like currants with their
 stalks;  being sometimes in immediate  contact with them,  but  more
 commonly being connected by a  peduncle.  Their  size,  when fully
 formed, varies from l-3d to l-6th  of a  line.  Each of them  contains,
 as its constant and  essential elements, nucleated cells from l-4000th to
 l-2500th of an  inch in diameter, pale  and faintly  granular, together
 with free  nuclei, as  well as larger cells of  l-2000th of  an inch in
 diameter, Avhich sometimes contain  AA'hat appear  to  be red blood-cor-
t puscles.   These are enclosed in a  capsule, which  has no orifice,  and 
              STRUCTURE AND FUNCTIONS OF THE SPLEEN.        287

 Avhich appears  to  be comparable to the elementary  vesicles  of other
 glands, before they  have acquired an  outlet by  the  rupture  of their
 Avails (§ 718).   Whatever may be the function of these bodies, it is pro-
 bable that it must  be completed by the  rupture  of the capsules  and the
 discharge of their  contents;  and that new corpuscles are continually in
 course of development.—The true splenic parenchyma consists in great
 part of  cells which  correspond in  appearance with those of the Malpi-
 ghian corpuscles; but in addition  to these, there are cells which bear
 a strong  likeness to the colorless  ' granule-cells' of the blood  (§ 217),
 and others which resemble young red-corpuscles.  These elements, also,
 present  indications  of being in a  state of continual  development and
 degeneration; and form small irregular groups of various sizes, which
 are  clustered especially  on  the sheaths of the vessels,  the  trabecular
 partitions, and the exterior of the Malpighian capsules.—A considerable
 part of  the contents of the  splenic areolae has  been found by Prof.
 Kolliker to consist of blood-corpuscles in various stages of degenerative
 metamorphosis.  These are aggregated in small masses,  each of which
 acquires  an investing membrane, that may contain from one to twenty
 corpuscles; and the blood-cells gradually diminish in size, and undergo
 a transition into pigment-granules ;  so  that  the containing cells are
 converted into  pigmentary granule-cells, which  at  last,  by a gradual
 loss of color of the granules, become quite  pale.  Such  cells are found
 in the blood drawn from the  splenic vein, the vena portae, and the in-
 ferior  cava; and occasionally in that  of  other veins.  These  curious
 collections of degenerating blood-corpuscles, however,  are  not  peculiar
 to the spleen, for they have been found by Prof. Kolliker in other parts,
 such as the substance of  the muscles.
   507. In regard to the functions of the Spleen, great uncertainty still
 exists.   It appears from  the foregoing account of its  structure, that it
 may be  regarded as an  organ of  duplex  character,  and probably of
 double  function.   In the Ruminants, the cavernous dilatations  of the
 veins enable it to hold, upon  occasion, a large quantity  of blood; and
 their walls are so elastic, that their cavities may be greatly distended
 Avith a very moderate force;  the Spleen of the sheep,  which  weighs
 about 4 oz., being easily made to  contain about  30 oz. of water.  This
 peculiar  distensibility evidently points  to  the  Spleen,  as a  kind  of
 reservoir, connected Avith the Portal  circulation, for the purpose  of
 relieving  the portal vessels from undue pressure or distension, under a
 great variety of circumstances.   The portal system is well known  to
 be destitute of valves, so  that the splenic vein communicates freely with
 the Avhole of it;  and thus, if any obstruction exists to the flow of blood
 through  the liver,  or any peculiar  pressure elsewhere  prevents the
 mesenteric veins from dilating to their  full extent, the general circula-
 tion  is not disturbed, the Spleen affording a kind  of safety-valve.   That
 any  cause of  congestion  of  the  Portal system peculiarly affects the
 Spleen, has been proved by experiment; for, after the portal vein has
been tied, the spleen of an  animal that  previously Aveighed only 2 oz.,
has been  found to increase to 20  oz.—Again,  the Spleen appears to
serve as a reservoir, into Avhich superfluous blood may be carried, during
the  digestive  process.  When  the  alimentary canal is distended with 
288
ABSORPTION AND SANGUIFICATION.
food,  and a  great afflux of arterial blood  takes  place  to  the  mucous
membrane, the veins of the portal  system will be liable  to increased
pressure from Avithout, whilst their  contents will be  augmented by the
quantity of fluid neAvly absorbed from the  alimentary canal.  In  this,
as in  the preceding  cases, the distensibility of the spleen  makes  it a
kind of safety-valve, by which undue distension of the portal system is
relieved.  It has been ascertained that its  maximum  volume is attained
about five hours after a meal, Avhen the process of chymification  is at an
end, and that of absorption is taking place with  activity; and the in-
crease is proportional rather to the amount of the fluids ingested, than
to that of the solids.—Although the Human Spleen has no true  cavern-
ous structure, yet its veins are obviously very distensible, so that a great
accumulation of blood may  take place in it.  Thus, in Asphyxia, Avhen
the circulation of blood is checked in the Lungs, and Avhen the  stagna-
tion extends itself backAvards to  the right side of the heart, the vena
cava,  and thence to the portal  system,  the Spleen is often found after
death  to be  enormously distended with blood.   And in the cold stage
of intermittent fever, in which a great quantity of blood is  driven from
the surface  towards the  internal  organs, the Spleen receives  a large
portion of it, so that its increased  size becomes  quite perceptible;  and
in cases of confirmed Ague, the  Spleen becomes permanently enlarged,
forming what is popularly known as  the  "ague-cake."
   508. But besides this safety-valve function, there can be little ques-
tion that the  Spleen performs some other, which is related more closely
to the nutritive operations,  and  which in some degree correspond with
that performed by the Absorbent glandulae.   The  multitude  of  glandu-
lar cells in  immediate  relation  with blood-vessels, and the appearances
of rapid development  and degeneration which these present, taken in
connexion with the fact that there is no other outlet for the products of
their  action than  that  which is afforded by the  veins, clearly indicate
that whatever this product may be, it is destined to form  part of the
blood; and that the  Spleen is,  therefore, an organ of sanguification.
This view is confirmed by the remarkable  fact, ascertained by recent
experiments,  that  after the  spleen  has been extirpated, the lymphatic
glands of the neighborhood increase in size, and cluster  together as
they enlarge, so as to form an organ which  at least equals  the original
spleen in volume.  This circumstance explains the reason for the almost
invariable negative result of  the extirpation of the spleen;  for although
the operation has been frequently  practised, with  the  view of deter-
mining the functions of the  organ  by the  symptoms presented by the
animals after its removal, no decided change in the  ordinary course of
their vital phenomena has ever been  observed, and the  health, if at all
disturbed for  a time, is afterwards  completely regained.   Now if the
principal function  of the Spleen  be the same with that of the lymphatic
glands in general, it is easy to understand hoAV its loss  may be  at once
compensated  by an increased action on  their  part, and  how it  may be
permanently  replaced by an increased development of certain of those
bodies.—It is worthy of remark, that a Spleen is found in all Verte-
brated animals which have  a distinct Absorbent system; but that no
orgaji  exactly corresponding with  it  exists  in the Invertebrata, which 
            STRUCTURE  AND  FUNCTIONS  OF THE  SPLEEN.         289

are destitute of that system,—although the distensible cellated cavities,
apparently destined to perform its safety-valve  function, exist in some
of the higher among them.   This is an additional reason for regarding
its parenchymatous portion as essentially a part of the assimilating ap-
paratus of the Absorbent system.
  509.  It would further seem as if the Spleen were specially concerned
in the development of  the red corpusdes of the blood;  since its paren-
chyma contains cells which resemble these in various  stages of develop-
ment.   But this  organ also appears to promote  the disintegration of
those red corpuscles Avhich have become effete;  and this so powerfully,
that the blood of the  splenic vein contains a far less proportion of red
corpuscles, and a far  greater  proportion of albumen, than that of any
other vessels in the body.   It is supposed by  Prof.  Kolliker, that the
dissolved blood-corpuscles are subservient to the formation of bile;  the
coloring matter of which is nearly allied to that of the blood.
   510.  The Supra-Renal Capsules seem  to correspond with the Spleen
in  their essential structure, whilst in  the arrangement  of their compo-
nent parts, they bear more resemblance to the Kidney.   Their exterior
or cortical portion, in  Man and  the Mammalia generally, is formed of
straight arteries, which  divide into a minute capillary network; and
from this arise venous branches,  which form  a  minute  plexus  through
the internal or medullary substance, pouring its contents into a large
central cavity, Avhich  is the  dilated commencement  of  the supra-renal
vein.  In the lower Vertebrata, hoAvever, there  is  no  distinction of cor-
tical and medullary substance; the distribution  of vessels being nearly
the same throughout.  As in the Spleen, we find in the interspaces of
the vascular plexus a parenchymatous structure; partly composed of
free cells and nuclei in various stages  of  development;  and  partly con-
sisting  of closed granular vesicles, within which similar  cells  and nuclei
are included.  There  is ground for asserting  that  these  vesicles are
themselves  at first in the condition of simple  cells, and that the cells
Avhich they include are a secondary product.   In Man  and the Mam-
malia, these vessels are confined  to the cortical  substance, and have the
form of elongated tubes, lying betAveen its parallel blood-vessels. Thus,
then, the supra-renal capsules possess  the essential structure of a gland,
in every respect save the Avant of an excretory duct; and Avhatever may
bo the product of their cell action, this must be received back into the
blood again.  The fluid contents of these bodies  are rich in proteine-
compounds and in fat; and it can be  scarcely doubted that these mate-
rials here undergo an elaboration, Avhich renders them  better fitted for
the nutrition of the system.  It does not seem unlikely that these bodies-,
like the Spleen, have a double function;  and that, besides participating
in  the general actions of the Absorbent glandulae, they^may serveas-a
divftrticnjnm forthe Renal circulation, when  from  any cause the se-
curing  funHTorTbf  imeKidneys is retarded or  checked, and  the move-
ment of blood through  them is  stagnated.—The Supra-Renal  capsules
of Man attain a very  large size early  in foetal life, surpassing the true
Kidneys in dimension up to the tenth or  twelfth week;  but  they affcer-
Avards  diminish relatively to  the latter,  and are  evidently subordinate
organs during the Avhole remainder of life.  In most of the lower ani- 
290
ABSORPTION AND SANGUIFICATION.
   mals, hoAvever, these bodies  retain through life the same relative deve-
   lopment.
     511.  The Thymus Gland is  another body, which seems referable to
   the same group;  having all the essential characters  of  a true gland
   (§ 714), save an excretory duct;  and its function  being evidently con-
   nected,  during the early period of life at least, Avith the elaboration of
   nutritive matter, which is to be .reintroduced  into the circulating cur-
   rent.   Its  elementary  structure  may be best understood from  the
   simple form it presents Avhen it is first capable of being  distinguished
   in  the embryo.   It then consists of a single tube,  closed  at both ends,
   and  filled with  granular matter; and its subsequent development con-
   sists in the lateral growth  of branching off-shoots from this  central
   tubular  axis.  In  its mature state, therefore, it consists  of an  assem-
   blage of glandular follicles, Avhich are surrounded by a plexus of blood-
   vessels ;  and these  follicles all communicate with the central reservoir,
   from which, hoAvever, there is no outlet.   The cavities of the follicles
   contain  a fluid, in which a number  of corpuscles are found, giving it a
   granular appearance.   These corpuscles are for the most part  in the
   condition of nuclei; but fully developed cells are found among them, at
   the  period  when the  function  of this  body seems most  active.  The
   chemical nature of  the contents at this period, closely resembles that of
   the  ordinary proteine-compounds.—It has been  commonly stated, that
   the Thymus attains its greatest development, in relation to the  rest of
   the body, during the latter part of foetal life; and it has been considered
   as an organ peculiarly connected with the embryonic  condition.  But
   this is a mistake; for the greatest activity in the growth of this organ
   manifests itself,  in  the Human infant, soon after birth ; and it is then,
   too, that its functional energy seems the greatest.   This rapid state of
   growth,  however, soon subsides into one of less activity, which merely
   serves to keep up its proportion to the rest of the  body; and its increase
   usually ceases  altogether  at  the  age of about two years.  From  that
   time, during a variable number of years, it remains stationary in point
   of size;  but,  if  the individual be  adequately nourished,  it gradually
   assumes the character of a mass of fat, by the development of the cor-
   puscles of its interior  into fat-cells, which secrete adipose matter from
   the blood.   This change in  its  function is most remarkable in hiber-
   nating Mammals;  in  which  the  development of the  organ continues,
   even in an increasing ratio, until  the animal reaches adult  age, when it
   includes a large quantity of  fatty matter.   The same is the case, gene-
   rally speaking, among Reptiles.   It is an important fact in the history
   of this  organ, that it is  not to be detected in  Fishes; and does not
   appear to exist, either in the tadpole state of the Batrachian  reptiles,
   or in the Perennibranchiate group; so that we may regard it as essen-
   tially connected with pulmonic respiration.*
 j   512. Various facts  lead to the conclusion,  that  the  function  of the
|  Thymus, at the period of its highest development, is that of elaborating
?  and storing-up nutritive materials, to supply the demand Avhich is pecu-
$   liarly active  during the early period of extra-uterine life.   The  elabo-

           * See Mr. Simon's admirable Prize Essay on the Thymus Gland. 
THYMUS GLAND.
291
rating action probably corresponds with that, which is  exerted by the
glands of the Absorbent system ; and the  product, as in the preceding
cases, seems to be taken back into the circulation.   The provision of a
store  of  nutritive matter seems a most valuable one, under the circum-
stances in which it is met Avith; the waste being more rapid and variable
than in adults,  and the  supply not constant.   Thus it has been noticed
that,  in  over-driven lambs, the  thymus  soon shrinks  remarkably ; but
that it becomes as quickly distended  again  during rest and plentiful
nourishment.   As the demand becomes less energetic, and as the sup-
plies furnished  by other organs become more adequate to meet  it, the
Thymus  diminishes in size, and no longer performs the  same function.
It then obviously serves to provide a store of material, not for the nutri-
tion of the body, but for the respiratory process, when  this has to be
carried on for long periods (as  in hibernating Mammals, and in Rep-
tiles), Avithout a fresh supply of food.—It is  possible  that the Thymus
gland may further stand in the same relation to the Lungs, as the Spleen
to the Liver, and the Supra-Renal capsules to the Kidneys ;  that is, as
a diverticulum for the blood transmitted through the bronchial arteries
(which are the  nutritive vessels of the Lungs), before the Lungs acquire
their  full development  in comparison with other organs, or when any
cause subsequently obstructs the circulation through their capillaries.
   513. The  Thyroid Gland bears  a general analogy to the Thymus;
but its vesicles are distinct from  each other, and do  not communicate
with any common reservoir.   They are surrounded, like  the  vesicles of
the true  glands, with a minute capillary plexus ;  and  in the fluid they
contain,  numerous corpuscles are found  suspended, which appear  to be
cell-nuclei, in a state of more or less advanced development.   This body
is supplied with arteries of considerable size; and with peculiarly large
lymphatics.  Though proportionably larger  in the  foetus than  in the
adult, it remains of considerable size during the  whole of life.—It ap-
pears, from the recent inquiries of Mr. Simon,* that  a Thyroid  gland,
or some  organ  representing it in place and office,  exists in  all Verte-
brated animals.  It presents its  simplest form in the class  of Fishes ;
in some  of Avhich it appears to consist merely of a plexus of capillary
vessels, connected with the origin of  the cerebral vessels, and capable,
by its distensibility, of  relieving the  latter, in case of any obstruction
to the proper movement of blood through them.  In the  higher forms
of this organ,  the glandular structure,—consisting of the closed vesicles
over  Avhich the capillary plexus is distributed, and of their cellular con^
tents,—is superadded :  and the organ then appears, likejjii£ Spifiga, to
be destined for two different uses ; namely, to^sgjxe as  a diverticulum
to>-tke_Cerebral circulation; and to aid  in theidaboratiofPof nutritive
matter/which  Is  "probably taken up by the Absorbent system,  to be
again poured by it into the general current of  the circulation.    *——
   514.  Thus the Spleen, the Supra-Renal Capsules, the  Thymus  Gland,
and the  Thyroid Gland, all seem to share in the  preparation of the nu-
tritive materials of the blood; in fact, Ave may regard them all as toge-
ther  constituting an  elaborating apparatus, which is precisely analogous
Philosophical Transactions, 1844. 
292
ABSORPTION AND SANGUIFICATION.
to that of the ordinary glands, but of Avhich the  elementary parts are
scattered through the body instead of being collected into one compact
structure, and of Avhich the product is received back into the blood, in-
stead of being discharged through  an  efferent duct, upon the surface,
or into an open cavity, of the body.  And it is a  remarkable confirma-
tion of this vieAv, that Prof. Goodsir should have  ascertained that the
last three of these organs are in continuity Avith each other,  during the
early part of foetal life; and  that  they are  in  reality portions  of the
blastoderma or germinal membrane (chap, xi.), which retain their ori-
ginal simplicity of  structure, whilst other parts undergo changes of form
and texture, and which continue to perform their original function, save
that the materials of their elaborating  action are now supplied  by the
blood instead of by the  yolk.   The probable uses  of  these bodies, as
diverticula to the circulation through  other organs,  render them liable
to occasional distension with blood; and it seems  determined that this
blood shall not lie useless, but shall be subservient to the action in ques-
tion ; the gland-cells that line  the cavities of the organ withdraAving cer-
tain constituents of the blood, to restore them to the circulating current,
in a state of more  complete preparation for the operations of Nutrition.
Their function is very probably vicarious ; that is, the determination of
blood is greatest (through the  state of  the other organs), at  one  time to
one of these bodies, and at another time to another.   Hence the effects
of the loss of any  one of  them are not serious;  as the others are ena-
bled in a great degree to discharge its  duty.

         4. Composition and Properties of the Chyle and Lymph.

   515. The chief chemical difference between the Chyle and the Lymph,
consists in a much smaller proportion of solid matter in the  latter, and
in the  almost entire absence of fat, which is an important constituent of
the former.   This is well shown in the folloAving  comparative analyses,
performed by Dr.  G. 0. Rees, of  the  fluids  obtained from  the  lacteal
and lymphatic vessels of a donkey, previously to their entrance into the
thoracic duct;  the animal having had a full meal  seven hours before its
death.

    Water,     -      -      -
    Albuminous matter (coagulable by heat),
    Fibrinous matter (spontaneously coagulable),
    Animal extractive matter, soluble in water and alcohol,
    Animal extractive matter, soluble in water only,
    Fatty matter,   ------
    Salts;—Alkaline chloride, sulphate and carbonate, with
     traces of alkaline phosphate, oxide of iron,

                                                 100000    100000
   The Lymph  obtained from the  neck of a horse has  been recently
analyzed by Nasse, with nearly the same result.   He found it to  contain
95 per cent, of  Avater; and the 5 per cent,  of solid matter  was  chiefly
composed of albumen and fibrine, with Avatery extractive,—scarcely a
trace of fat being  discoverable.   The  proportions of saline matter were
found  to be remarkably coincident Avith those Avhich exist in the serum
Chyle.	Lymph.
90-237	95-536
3-516	1-200
0-370	0-120
0-332	0-240
1-233	1-319
3-601	a trace
0-711	0-585
 
CHARACTERS  OF THE  CHYLE.
293
of the blood ;  as might be expected, from the fact, that the fluid portion
of the lymph must have its origin in that which  has transuded through
the blood-vessels:  the absolute  quantity, however, is rather less.   A
similar  analysis of the Chyle of a cat by Nasse, has given  results very
closely  correspondent Avith  that  of Dr. Rees;  for the  proportion  of
water Avas  90-5 per  cent.; and  of the  9-5 parts  of  solid matter, the
albumen, fibrine,  and extractive amounted  to more than  5, and the fat
to more than 3 parts.   Dr. Rees  has  also analyzed  the fluid of the
Thoracic duct of  Man,  Avhich consists  of  chyle with  an admixture  of
lymph ; and he found this to contain about 90-5 per  cent, of water, 7
parts  of albumen and fibrine, 1 part of aqueous alcoholic extractive, and
not quite one part of fatty matter, with  about J per cent, of  salines.
The composition of this fluid more resembles that of the lymph than that
of the chyle;  the  proportion of  the fatty to  the albuminous matter
being small.   This was probably due to  the circumstance, that the sub-
ject from which it Avas obtained (an executed criminal)  had eaten but
little  for some hours before his death.
   516.  The  characters of the Chyle are not the same in every part of
the Lacteal system ;  for the  fluid undergoes a very important series  of
changes  in its characters, in its transit from the Avails of the intestines
to the receptaculum chyli.   The fluid draAvn from the  lacteals that tra-
verse  the intestinal walls, has no power of spontaneous coagulation;
whence we may infer that it contains little  or no Fibrine.   It contains
Albumen in a state of complete solution, as we may ascertain by the
influence of heat or acid in producing coagulation.  And it includes a
quantity of fatty  matter, which is not dissolved, but suspended in the
form  of globules of variable size.   The quantity of this evidently varies
with the character of the food ; it is more abundant, for instance, in the
chyle of Man and the Carnivora, than in that of the Herbivora.  It is
generally supposed that  the  milky color of the chyle is  owing to the
oil-globules ; but Mr. Gulliver has pointed out that it is really due  to
an  immense  multitude of far more minute  particles,  AA'hich  he has
described under the name of the molecular base of the  chyle.   These
molecules are most  abundant in rich, milky, opaque  chyle; whilst  in
poorer chyle,  Avhich is semi-transparent, the particles float separately,
and often exhibit the vivid motions common to the  most  minute mole-
cules  of various substances.   Such is their minuteness, that, even with
the best instruments, it is  impossible to determine either their form  or
their  dimensions with exactness; they seem, hoAvever, to be generally
spherical;  and their diameter may be estimated at  between l-36,000th
and l-24,000th of an inch.   Their chemical nature is as yet uncertain;
they are remarkable for their  unchangeableness, when  submitted to the
action of numerous reagents, Avhich quickly  affect the  proper  Chyle-
corpuscles ; whilst  their  ready solubility in Ether would  seem to indi-
cate that they are of an  oily or fatty nature.        ^
  517.  The milky aspect which the serum of blood  sometimes exhibits,
is due to an admixture of this molecular base.  It  may be particularly
noticed, Avhen  blood is drawn a feAV hours after a full meal, that has been
preceded by a long fast.  By recent experiments it has been found, that
the serum  begins  to shoAV this turbidity about  half an  hour after the 
294
ABSORPTION AND SANGUIFICATION.
meal has been taken;  and that the turbidity increases for some hours
subsequently, after Avhich it disappears.  The period at which the dis-
coloration is greatest, and the length of time during Avhich it continues,
vary according to  the digestibility  of the  food.   When the serum  is
allowed to remain at rest, the opaque  matter  rises to the surface, pre-
senting very much the  appearance of  cream ;  and when  separately
examined, it  has  been found to contain a proteine-compound, mingled
with oily matter,—the relative amount of the tAvo  appearing to depend
in part upon the characters of the food ingested.   Hence it would seem
probable that the molecular base of the chyle is partly derived from
albuminous matter of the food;  more especially as it is known that oily
particles, when introduced into  an albuminous fluid, become surrounded
with a pseudo-membranous pellicle.  The gradual disappearance of the
turbidity of the serum, indicates that the substance which occasioned it
no longer exists as such  in the circulating current; being either drawn
off by the nutritive or secretory operations, or being converted  by the
assimilating process into  the ordinary constituents of the blood.
   518. During the passage of the Chyle along the lacteals, towards the
Mesenteric glands,  it undergoes  two important changes; the presence
of Fibrine begins to manifest itself by the spontaneous coagulability  of
the fluid ; and  the oil-globules  diminish  in  proportional amount.  The
fibrine appears to be formed at the expense  of  the albumen ;  as this
latter  ingredient undergoes a  slight  diminution.  It is in the chyle
drawn from the  neighborhood of the  mesenteric glands, that we first
meet  with the peculiar  floating cells,  or chyle-corpuscles, formerly
adverted to (§ 214), in any number.  The average diameter of  these is
about  l-4600th of an inch ; but  they  vary from about l-7000th to
l-2600th,—that is, from  a diameter about half that of the human blood-
corpuscles, to  a size about one-third larger.   This variation probably
depends in great part upon the period of their growth.  They are usually
minutely granulated on the surface, seldom exhibiting any regular nuclei,
even when treated Avith  acetic  acid ;  but  three  or four  central parti-
cles may sometimes be distinguished in the larger ones.   These corpus-
cles are particularly abundant in the chyle obtained by puncturing the
mesenteric glands themselves;  and there can be  little doubt, that they
are identical with  the altered epithelium-cells, which  line the  lacteal
tubes in their course through those bodies (§ 496).
   519. The glandular character of these cells,  and their continued pre-
sence  in  the  circulating  fluid, seem  to  indicate that  they have  an
important concern  in  the  process of assimilation,—that  is, in the con-
version of the crude elements derived from the food, into an organizable
matter adapted to  the nutrition of the body; in other  words, in the
conversion of Albumen into Fibrine; which change would seem to take
place to a considerable extent in the Mesenteric glands.   For it is only
in the  Chyle which is drawn from the  lacteals intervening between the
mesenteric glands and the receptaculum chyli,  that  the spontaneous
coagulability  of the fluid  is so complete, as to produce a perfect separa-
tion  into  clot  and serum.  The  former is a  consistent mass,  which,
when examined with the microscope, is found  to  include many of the
chyle-corpuscles,  each of them being surrounded  Avith a delicate  film  of 
PROGRESSIVE ELABORATION  OF BLOOD.
295
oil;  the latter bears a close resemblance to the serum of the blood, but
has some of the chyle-corpuscles  suspended in it.   Considerable diffe-
rences present themselves, however, both in the perfection of the coagu-
lation, and in its duration.  Sometimes the chyle sets  into a jelly-like
mass ; which, without any separation into coagulum and serum, liquefies
again at the end of half an hour, and remains in this  state.   The coagu-
lation is usually most complete in the fluid drawn from the receptaculum
chyli and thoracic duct; and here the resemblance between the floating
cells, and the white  or colorless corpuscles of the  blood, becomes  very
striking.
  520.  The Lymph, or fluid of the Lymphatics, differs from the Chyle,
as already remarked, in its  comparative transparency:  its want of the
opacity or opalescence, which is characteristic of the latter, being due
to the absence, not merely of oil-globules, but also of the "molecular
base."  It contains  floating cells, which bear a close  resemblance to
those of the Chyle on  the one hand, and to the colorless corpuscles of
the Blood on the other; and these, as in the preceding case, are most
numerous in  the fluid  which is drawn from  the lymphatics that have
passed through the glands, and in that obtained from the glands them-
selves.  Lymph coagulates  like chyle ; a colorless  clot being formed,
which encloses the greater part of the corpuscles.  The lacteals may be
regarded as  the  Lymphatics  of  the intestinal  Avails and mesentery ;
performing the function of interstitial absorption, as well as effecting the
introduction of alimentary substances from without.  During the inter-
vals  of digestion, they  contain a fluid, Avhich is in all respects conform-
able  to the lymph of the lymphatic trunks.
  521.  Thus by  the admixture of the aliment newly introduced from
without, with the matter Avhich has been taken up in the various parts
of the system,  and by the  preparation  which these undergo in their
course  toAvards  the  thoracic duct, a  fluid  is  prepared, which bears a
strong resemblance to  blood in every particular, save the presence of
red corpuscles.   Even these may sometimes  be found in the contents
of the thoracic duct, in sufficient amount to  communicate  to  them a
perceptible red tinge;  but it is doubtful whether they have not found
their way thither accidentally,—some of the lymphatic or lacteal trunks,
which have been divided in the dissection necessary to expose the duct,
having taken up blood  by their open mouths, and rapidly transmitted it
into  the general  receptacle.  The fluid of the  thoracic duct may be
compared to the blood of Invertebrated animals;  from  which  the red
corpuscles are almost  or altogether  absent;  but which  contains white
or colorless  corpuscles, and  which possesses  but a  slight coagulating
power,  in consequence of its small  proportion  of  fibrine.  And  we
hence see, why these animals  should  require no special  absorbent sys-
tem ; since the blood-vessels convey a fluid, which is  itself so analogous
to the chyle and lymph to be  absorbed,  that  the latter may be at once
introduced into it, Avithout injuring its qualities. 
296
ABSORPTION AND SANGUIFICATION.
        5. Absorption from the External and Pulmonary Surface.

  522. Although the  Mucous Membrane of the Alimentary Canal is
the special channel for the introduction of nutritive or other substances
into the system, it is by no means the only one.   The  Skin coA'ering
the body, and  the Mucous Membrane  prolonged into the Lungs, are
also capable of absorbing  liquids and vapors, and of introducing them
into the Circulation ;  although they serve  this  purpose less  in  Man
and the higher animals, than in some  of the  loAver.  Their  utility in
this respect  is  best shown, when,  from peculiar  circumstances, the
function  of the digestive  cavity cannot  be properly performed;  and
when, therefore, the  system has been more  than  usually drained of its
fluids, and stands in  need of a fresh supply.  Thus shipwrecked sailors,
and others, who are  suffering from thirst, owing to the Avant  of fresh
water,  find it  greatly alleviated, or altogether  relieved, by dipping
their clothes  into the sea,  and putting them on Avhilst still wet, or by
frequently immersing  their own bodies.  In  a case of dysphagia, in
which neither solid nor  fluid  nutriment could be introduced into the
stomach, the patient was kept  alive  for  a  considerable time, and his
sufferings  greatly alleviated, by the administration of nutritive clysters,
and by the immersion  of his body in a bath of tepid milk and Avater,
night and morning.  Under this system, the weight  of the body, which
had previously  been rapidly diminishing, remained stationary, although
the amount of the excretions was  increased : and the use of the bath
had a special influence in assuaging  the  thirst,  which was previously
distressing.  It appeared that the  water of the  urinary  excretion,
amounting to from 24 oz. to 36 oz. per day, must  have been entirely
supplied from this latter  source.  Again, a man who had lost nearly
31bs. by perspiration, during an hour and a quarter's labor in a very
hot atmosphere, regained 8 oz. by immersion in a warm bath at 95° for
half an hour.  In these cases it appears probable, from the experiments
already noticed (§ 502),  that the Lymphatics,  rather  than the blood-
vessels, are the chief agents in the absorbing process; not,  however,
from any powers peculiar to them, but  merely on account of the thin-
ness of their  walls, and their very copious distribution  in the skin.
  523. Absorption may also take place from an atmosphere  saturated
with watery vapor.    Of  this we  have  a  very  curious proof in the
Frog;  whose urinary bladder (which serves as a sort of reservoir for
water)  has been observed to be  refilled, after  having been emptied, by
placing the  animal  in  an atmosphere  loaded   with watery vapor.
Numerous instances  are on record which prove that such absorption
may take  place in Man, to a  very  considerable  extent; though the
proportion introduced  through the Skin, and through the Lungs, can-
not be exactly ascertained.  The ready introduction of volatile matter
into the system, through  the  latter  channel, is  a  matter of familiar
experience;  thus if we breathe  an atmosphere  through which the
vapor  of   turpentine  is  diffused,  it soon produces the  characteristic
odor of violets in the  urinary secretion.  And  it is  probably in this
manner, that a large number  of those putrescent miasmata  and other
zymotic poisons are introduced, which are such  fertile causes of disease. 
COMPOSITION OF THE BLOOD.
297
            6.  Of the Composition and Properties of the Blood.

  524.  Having traced the steps by which the Blood is elaborated,  and
prepared for circulation through the  body, and having  (in the former
part of the volume)  inquired  into  the characters of  its chief consti-
tuents, Ave  have now to consider the fluid as a whole, to study the usual
proportions of  these constituents, and the properties which they impart
to it.
  525.  The Blood, whilst circulating  in  the  living  vessels,  may be
seen to consist^of a, tlranRpairpnt».,nfiady.Jin1orl£a&JLiud, termed Liquor
Sanguinis  ; in which  the  Corpuscles, to which the blood owes  its  red
hue, as well as the white  or  colorless  corpuscles, are  freely suspended
and carried along by the current.   On the other  hand, when the blood
has been  drawn from  the  body, and is allowed to remain at rest, a
spontaneous coagulation takes  place, separating it into  Clot and Serum.
The Clot is composed of a  network of Fibrine, in the  meshes of which
the Corpuscles, both red  and  colorless, are involved ;  and  the  Serum
is the same with the  liquor sanguinis deprived of its  Fibrine.  When
the Serum  is heated, it coagulates, showing the  presence of Albumen.
And if it be exposed to a high  temperature, sufficient to  decompose the
animal matter, a  considerable amount  of earthy and  alkaline Salts
remains.   Thus we  have four  principal  components in  the Blood:—
namely,  Fibrine, Albumen, Corpuscles, and  Saline  matter.  In  the
circulating Blood they are thus combined:—
           Fibrine         A
           Albumen        v In solution, forming Liquor Sanguinis.
           Salts          J
           lied  Corpuscles,—Suspended in Liquor Sanguinis.

But in coagulated blood they are thus  combined:—

           D , n     ,    >  Crassamentum or Clot.
           Red Corpuscles  $
           Albumen        )  t>    ...   , ,.   „       ^
           a ,.            >  Remaining in solution, forming Serum.

A certain amount of Serum, hoAvever,  is involved in the Crassamentum ;
and  can only be separated by cutting  the clot  into  thin  slices,  and
carefully washing it.
  526.  The components  of the Blood  may  be separated, and their
amount estimated,  in  various  Avays.   Thus, if fresh-drawn blood  be
continually stirred  with  a stick,  or be  " Avhipped" with a bunch of
twigs, the  fibrine  coagulates in  the form  of  strings,  which adhere to
the Avood, and may thus be AvithdraAvn ;  whilst the red corpuscles then
remain  suspended in the  serum,  gradually sinking to  tbe bottom in
virtue of their greater specific  graATity.   On the  other hand,  the Red
Corpuscles may be separated, in those animals in Avhich  they are large
enough, by passing the blood through a filter ; haA'ing previously mingled
with it some substance which retards, but does  not prevent its coagula-
tion*  (§ 185).  The liquor  sanguinis is thus separated from the blood-

  * This experiment cannot be performed -with Human blood, because the corpuscles
are small enough to pass through the pores of any filter that allows the liquor sanguinis
to permeate it; but it answers very well with Frog's blood. 
298               ABSORPTION AND SANGUIFICATION.
discs;  and the former coagulates,  whilst the blood-discs are retained
upon the filter.   The experiment  convincingly proves, that the act of
coagulation is not due to  the  red  corpuscles, as Avas at  one  time ima-
gined.—The  ordinary  act of  coagulation, by withdrawing the Fibrine
and Corpuscles, makes it  easy to estimate the proportion  of  Albumen
and of Saline matter in  the  Blood, Avhen due alloAvance  is made for
the quantity of Serum  retained in the Clot;  and the  relative propor-
tions of these may  be determined, by evaporating the fluid, so as to
obtain  the Avhole amount  of solid  matter it contains, and by then cal-
cining  the residuum, so as  to  ascertain how much of this is  a mineral
ash,—the remainder being chiefly Albumen.—The solid  matter of the
blood also contains  various Fatty  substances, which may be removed
from it by ether.  Some  of these appear  to  correspond Avith the con-
stituents of  ordinary Fat  (§ 261); whilst another contains phosphorus,
and seems allied to the peculiar fatty acids  of  Neurine (§ 383); and
another has some of the properties of Cholesterine, the fatty matter of
the Bile (§ 724).—Besides these, there are  certain substances known
under the name of Extractive ; one group of which is soluble in water,
and another in Alcohol.   Of the precise nature of these,  little is knoAvn.
They have been aptly termed  " ill-defined" animal principles; and it is
probable that they may include various substances in a state of change
or disintegration, which are being  eliminated from the Blood by the
processes of Excretion.
  527.  The general  result of numerous  recent  analyses of the Blood
may be thus stated.   The whole  amount of solid matter is rather
greater in the Male than in the Female; being, on the  average, about
210 parts in  1000 in  the former, and 200 in the latter.  This diffe-
rence,  hoAvever, chiefly depends on the larger  proportion of red corpus-
cles contained in  the blood of the male.   The proportion of  Albumen
seems  more  constant than that of  the  other constitutents of Blood;
seldom varying beyond 5 or 6  parts, in either sex, above or below 70
in 1000.  The quantity of Corpuscles  appears  liable  to considerably
greater variation ; the superiority  on the side  of the Male, however,
being very strongly marked in the maximum and minimum, as well as
in the average.   We may  regard its average in the Male  as about 132in
1000 parts of blood; but it may fall to 110-5 parts, without the health
being seriously affected; whilst, on the other  hand, it may arise to 186
without any manifestation of disease.  In the Female, its average may
be about 120 parts in 1000 ; but it may fall to as little as 71'4, and may
rise to  167, consistently with ordinary health.  The range of variation
is thus much  greater in the Female than in the Male ; the  minimum
being considerably less, in the former, than half the maximum:  whilst
in the  latter it is much more.   This is probably due in part to the fact,
that the loss by the Catamenial discharge may produce a great tempo-
rary depression  in  the proportion  of the  Corpuscles.   The aArerage
proportion of Fibrine seems to be  no more than  2*5 in  the Male ;  and
though it may rise to as much  as 3-5 or even 4, without disordering the
system, it does not seem to fall below 2, in the state of ordinary health.
The average in the Female is  probably about 2-3; the proportion may
rise to  3, or fall to 1*8; but the variation seems less considerable  in the 
COMPOSITION OF THE BLOOD.
299
Female than in the Male.—Much is probably yet to be learned, regard-
ing the influence of different kinds of food recently taken, on the pro-
portion of these constituents of the blood ;  and it does not seem  un-
likely, from what has been already  stated (§  517),  that the quantity of
fatty matter is especially liable to  variation, in  accordance  Avith  the
amount contained in  the food, and the time which has elapsed since the
last meal.
   528. The Saline constituents  of  the  blood, obtained by drying and
incinerating  the whole mass, usually amount to betAveen 6 and 7 parts
in 1000.   More than half of their  total quantity is  composed of the
Chlorides of Sodium and Potassium ; and the remainder is made up of
the tribasic Phosphate of Soda, the  Phosphates of  Lime and Magnesia,
Sulphate  of  Soda, and a  little Phosphate  and  Oxide of Iron.  Of
these the chief  part  are dissolved in the Serum ; but  the Earthy Phos-
phates, which are insoluble by themselves, are probably combined Avith
the  proteine-compound (§ 175); and the Iron  is  contained,  chiefly  or
entirely, in the red corpuscles.   It  is difficult to speak with  certainty,
from the examination of the ashes of the blood, as to the  state of the
saline constituents of the circulating fluid.  Thus the Serum has an
alkaline reaction ;  and this  has been supposed to be due to the pre-
sence of alkaline Carbonates.  Moreover, the presence of the Lactates
of potash and soda  seems probable; for it is certain that lactic acid is
normally introduced  into the blood, and is  also  eliminated from  it;
and  the rapidity Avith which the  lactates are removed as such,  or are
converted into  carbonates, seems to  afford  a sufficient explanation  of
the difficulty in demonstrating the presence of this  acid in the circula-
ting fluid.  Although the ashes of the entire mass of blood do not effer-
vesce on the addition of an acid,  effervescence takes place, Avhen  acid is
added to the ashes of the Serum; showing the existence in it, either of
alkaline Carbonates, or  of Lactates which have been reduced  to the
state of Carbonates by incineration.  When the entire mass of blood is
incinerated, hoAvever, enough phosphoric  acid is produced from the phos-
phorized fats, to neutralize the alkaline carbonates, and thus to prevent
their presence  from being recognised.   The  alkaline  reaction of the
blood, however,  is certainly dependent in part upon the presence of the
tribasic phosphate of soda, which appears to confer upon the serum a
special poAver of absorbing carbonic acid.
   529. The following appear, from  the considerations stated in the pre-
ceding part of the Volume, to be the chief uses of the  principal consti-
tuents of the Blood, in the general economy.  The Fibrine is the material,
Avhich is most completely prepared for organization, and Avhich supplies
what is requisite for the nutrition of the larger proportion of the solid
tissues of the body.   It is, therefore, being continually AvithdraAvn from
the blood by the  nutritive operations ; and the demand  appears  to be
supplied, in part by the influx of Fibrine that has been prepared in the
Absorbent  system, and in part by the continued transformation of Albu-
men, which takes place during the circulation of the Blood.  If a proper
amount of  Fibrine be not present in  the Blood, its physical properties
are so far  altered, by the diminution of its viscidity,  that it Avill not
circulate through the capillaries as  readily as before ;  a certain degree 
300
ABSORPTION AND SANGUIFICATION.
  of viscidity having been  experimentally  found to be favorable to the
  movement of  fluid  through glass or metallic tubes of small bore.—The
  Albumen of the blood is the raw material, at the expense of A\Thich not
  only the Fibrine, but many other substances are generated during the
  nutritive  process.  All the Albuminous compounds  of the  Secretions,
  the Horny matter of the Epidermic tissues, the Gelatine of the simple
  fibrous tissues, the solid materials of the Red Corpuscles,  and other sub-
  stances, may  be regarded as almost certainly produced by the transfor-
  mation of the Albumen of the Blood : and a continual  supply of this
  from the food is therefore requisite, to preserve the due proportion in
  the circulating fluid.—The Red Corpuscles, Avhich (it will be remembered)
  are  almost exclusively confined to Vertebrated animals, appear to be
  more  connected  Avith the function of Respiration,  than Avith that of
  Nutrition ; and the stimulating action of Arterial blood, especially upon
  the Muscular  and Nervous tissues, appear chiefly to depend upon their
  presence.   It has  been observed  in  particular, that their presence is
  more effectual in stimulating the heart's  action, than is  that  of either
  of the other  constituents  of the blood.   In addition to what has been
  already stated (§ 219), in reference to their continual disintegration and
  reneAval, it may be mentioned, that when the blood  of one  animal Avas
  injected by Magendie  into the  veins of another having discs of very
  different  size  and form, the original Red Corpuscles soon disappeared,
  and were replaced by those characteristic of the species,  in whose veins
  the fluid was  circulating.
    530. The use of the  Saline matter is evidently in  part to  prevent de-
  composition in the  circulating Blood; but  also to supply the mineral
  materials, requisite for  the generation of the tissues, and entering into
  the composition of  the  secretions.   It is by the saline and  albuminous
  matters in conjunction, that the specific gravity of the Liquor Sanguinis
  is kept up to  the point, at which it is  equivalent to that of the contents
  of the Red Corpuscles;  and it is only in this condition, that the forma-
  tion of the latter can duly take place.   The Fatty matters of the blood
  are evidently  derived from the food, either directly, or by a transforma-
  tion of its farinaceous  ingredients (§ 430);  and they are chiefly  appro-
  priated to the maintenance of the combustive  process.   But there is
  reason to believe, that Oleaginous  matter performs  a most important
  part in the incipient stages of Animal nutrition ; and that its presence
  is not less essential to the formation of "cells, than is  that of the albumi-
  nous matter which forms their chief component, all nuclei being observed
  to include fatty particles.   That which may be superfluous is either de-
  posited in the cells of Adipose Tissue, or it is eliminated by the Liver,
  the Sebaceous follicles  of the Skin, and, in the female Avhen nursing, by
  theJVlammary glands.
    531. The proportion of these components  of the Blood  is liable to
,  undergo changes in disease, which extend far beyond the widest limits
\ which have  been  mentioned as  consistent  with  health.   Thus, the
; quantity of Fibrine exhibits a remarkable increase in Inflammation; the
 \ amount  then  found in  the blood being from 5 or 6 parts in 1000 to 9,
 • 10,  or even 10£, according to the extent and  intensity  of the disease.
 ;On  the  other hand, it presents a  remarkable diminution  in  Typhoid 
COMPOSITION OF BLOOD IN DISEASE.
301
fevers;  the quantity being  sometimes as little as 0-9.   If any decided
Inflammation should develope itself, however, in the course of the Fever,
the proportion of Fibrine rises accordingly.  A deficiency of Fibrine in
the blood  predisposes  to Hemorrhages,  Congestions,  &c, either into
the substance of the tissues, or on the surface  of membranes ; and these
conditions are well knoAvn to be of frequent occurrence as complications
of febrile  disorders.   An  excess  of Fibrine  is not much  affected by
copious  bleeding,  even if this be frequently repeated; but there is
reason to think, that the administration  of  Mercury has a tendency to
restrain its production.
   532.  It is difficult to say what amount of Red Corpuscles should  be
regarded as excessive; since, as we have seen, they may augment to a
great degree, without  disturbing the health.   When they are present
in an amount  much above the average, they seem concerned in pro-
ducing  the condition  termed Plethora;  which marks  a "high condi-
tion" of the system, and which  borders  upon  various diseases, espe-
cially those of Congestion, and  Hemorrhages.  To  these a peculiar
liability then exists; because, although the proportion of Fibrine in the
blood is not absolutely low, it is  low in reference to that of the Red
Corpuscles.  Plethoric  persons do not seem more  liable to Inflamma-
tion, than are those of Aveakly constitution.   The quantity of the Red
Corpuscles is rapidly diminished by frequent bleeding ;  and hence it is
loAvered .by repeated Hemorrhages.   On  the  other hand, it is speedily
restored to its usual standard under the influence of nutritious diet, if
the digestive powers have not been too much weakened to make use of
this.—The  proportion of  Red  Corpuscles undergoes a  marked diminu-
tion in various forms of Aneemia; and particularly in Chlorosis.   In
severe cases of this  latter disease, it has been found  as low as 27 in
1000; and it not unfrequently sinks to 40 or 50.  The  marked influ-
ence  of the administration of Iron, in  favoring the reproduction  of
Red Corpuscles, has been already noticed (§ 219).
   533.  The proportion of  Albumen in the blood  seems less liable to
change, except in the condition termed Albuminuria, in which a large
quantity of Albumen  appears in the Urine.   When this condition is
permanently established, it is indicative  of  the  existence of  serious
organic disease of the kidney; but it may occur for a short time under
the influence  of simple  congestion  of  that  organ,  which causes  an
escape  of the  Albuminous part  of the blood, together with the water
which  is filtered  off  (as it were) in this gland  (§ 728).  Now Avhen
Albuminuria is fully established, there is a marked diminution in the
quantity of Albumen  in  the  serum of the  blood; and this diminution
is  constantly proportional  to the amount of  Albumen present in  the
Urine.—The proportion of Saline matter appears to undergo  less alte-
ration in disease, than that of the other constituents of the blood;  and
has not been found to have a regular correspondence, either in  the way
of excess or diminution, with any particular morbid state.
   534.  The condition  of  the  Blood may be affected,  not merely  by
alteration  in the proportions of its normal ingredients,  but  by  the pre-
sence of other substances;—either such as are generated in it, and are
constantly being eliminated from it in health, but have accumulated to 
302
ABSORPTION AND SANGUIFICATION.
an abnormal  degree;—or such as  have found their way into it from
without.  Thus,  Carbonic Acid, Urea and  Lithic Acid,  Cholesterine
and other elements of Bile, and other matters which it is the office of
the Excreting organs to remove,  may  accumulate  in the  blood, and
may become  fertile sources of disease,  by  their injurious  influences.
The introduction  of various  Mineral substances,  by absorption from
Avithout, changes the composition of the  normal elements  of  the Blood,
and thus affects  their vital  properties; thus  strong saline solutions
diminish  or destroy the coagulating power  of the  Fibrine.  But the
most  remarkable cases of depravation of the Blood, by  the introduc-
tion of matters from without, are those which result from the action of
ferments,—exciting such  Chemical changes  in the  constitution  of the
fluid,  that its Avhole character is  speedily changed,  and  its vital pro-
perties are altogether destroyed.   Of  such an occurrence  we have a
marked  example  in the  various forms  of malignant fevers; in which
the introduction of a very minute quantity of  noxious matter into the
blood, either through the  lungs or through the  skin, produces a speedy
alteration in  the characters of the whole mass of  the blood, the func-
tion of  every organ in the body  is disordered, and decomposition of
the solids and fluids takes place to a considerable extent, even  before
the circulation ceases, and whilst consciousness  yet remains.  The
train  of symptoms produced by the  bite of venomous Serpents, and of
rabid  animals, appears referable to the same cause,—the alteration in
the condition of  the whole  current  of blood, by the introduction of a
minute quantity of a substance that acts as a ferment.
   535. The  Coagulation  of the blood,  as already explained, depends
upon  the passage of its Fibrine from the fluid state  to the solid (§ 184);
consequently, if the Fibrine be separated from the other elements, no
coagulation takes  place.   On the other hand, if the amount  of Fibrine
be larger than ordinary, the coagulum possesses an unusual degree of
firmness.  The length of time  which elapses  before  coagulation,  and
the degree in which the Clot solidifies,  vary considerably ;  in general
they are in  the  inverse  proportion to  each  other.   Thus, if a large
quantity of blood be withdrawn from the vessels of  an  animal  at the
same  time, or within short  intervals, the portions that last flow  coagu-
late much more rapidly, but much less firmly, than those  first obtained,
in consequence of the  diminished proportion of fibrine.   On  the other
hand,  when the fibrine is  in excess, its coagulation is  unusually de-
layed.   From this delay an important change results, in the mode in
which the coagulation takes place; for the  red corpuscles,  instead of
being  uniformly diffused through the coagulum, have time to  sink to the
bottom, in virtue of their greater specific gravity;  and the upper part
of the clot is consequently made  up  of Fibrine, almost exclusively,
whilst the lower is chiefly formed by the aggregation of the red cor-
puscles.   Hence  the upper layer  is almost destitute  of  color (whence
it has  received the name of buffy coat),  and  is remarkably tenacious in
its character; whilst the lower is very deep in hue, and very friable in
consistence.   When the  fibrillated network  forming the  buffy coat
undergoes the slow contraction,  which is characteristic of highly-elabo- 
COAGULATION  OF BLOOD—BUFFY COAT.
303
rated fibrine subsequently to its coagulation, it draAvs in the edges  of
the upper surface of the clot, giving it a cupped appearance.
   536.  The  Buffy Coat may present itself under  a great variety  of
conditions; and it can no longer, therefore, be regarded, as it formerly
was,  a  sign  of the Inflammatory state.   It is most  fully developed
when acute  Inflammation  exists; because in  that  condition  all the
circumstances  which favor  it are  present.  That it may be produced
by any cause,  which occasions delay in the coagulation of the blood,
is  evident from the fact, that healthy blood may be made to exhibit  it,
by adding a solution  of  a neutral  salt, which retards, but does not
prevent its coagulation.   But the blood may coagulate with its ordinary
rapidity, or even  more  speedily than  usual; and may yet exhibit the
Buffy Coat.   And,  moreover, the separation of  the Fibrine  and the
Red  Corpuscles may take place in  films  of blood  so  thin, as not  to
admit of a stratum of one being laid over the other ;  the tAvo elements
separating from each other laterally, and the films acquiring a speckled
or mottled appearance, equally characteristic of the  Inflammatory con-
dition with the Buffy Coat  itself.  Hence  the separation must  be due
in such cases,  to  other  causes than  gravity: and recent observations
have  accounted for it,  by shoAving that the Red Corpuscles have an
unusual attraction for one another in the  inflammatory state, causing
their  coalscence in  piles and masses; whilst the particles  of Fibrine
have also a peculiar strong  attraction for  each other.   Thus  there  is
a poAverful tendency, that draws together the components of each kind,
and consequently tends  to  separate them from  the  others; and  when
this separation takes place, the difference in the specific gravity of the

                               Fig. 87.
 The microscopic appearance of a drop of blood in the inflammatory condition. The red corpuscles lose
their circular form, and adhere together ; the white corpuscles remain apart, and are more  abundant than
usual.

two  elements decides their respective  situations.—The  peculiar  ten-
dency of the Red corpuscles to unite,  in the Inflammatory state, serves
to  distinguish this condition eA7en in a single drop of blood;  and it  is
then  that the White  corpuscles  may be most easily distinguished, as
they are seen apart from the rest of the  mass,  having no tendency to 
304
CIRCULATION OF NUTRITIVE FLUID.
unite with it.   In fact, the white corpuscles are not found in company
with the red, in the ordinary coagulum, but rather with the fibrinous
portion; and when they are peculiarly abundant, as they  usually are
in Inflammatory blood, they may form a considerable proportion of the
buffy coat.
   537.  The Buffy Coat  may present itself, without the least increase
in the normal  quantity  of  Fibrine, and without any  approach  to the
Inflammatory  state;  simply because the Fibrine is present in exces-
sive amount, in relation to the amount of Red corpuscles, the latter
being much below  their usual proportion.   Thus in severe Chlorosis,
the buffy coat is almost  as strongly marked, as in the severest  Inflam-
mation ; but the two conditions are at  once  distinguished by the rela-
tive proportions of solid matter in  the blood, as  indicated by the size
of the Coagulum.   For  in  Chlorosis, the coagulum is very small, [in
consequence of the reduced proportion  of  Corpuscles, and  is almost
invariably found  floating in the serum ; whilst in the ordinary Inflam-
matory condition, it is of full  size, frequently adhering to the side  of
the vessel.
                          CHAPTER VI.

                 OF THE CIRCULATION OF THE BLOOD.

        1.  Nature and Objects of the Circulation of Nutrient Fluid.

  538.  The nutritive fluid,—the elements of which are thus partly taken
up and  prepared by the Absorbent system, but are in great part  also
imbibed through the  Blood-vessels distributed upon  the walls  of the
digestive cavity, and assimilated by the liver (§  493),—is carried  into
the various parts of the system, by the act of Circulation.  This move-
ment answers various purposes.  It furnishes all the tissues, which are
to deriAre nutriment from the Blood, with a constantly-reneAved  supply
of the materials which they severally require; and in this manner  it  is
subservient to the growth, not only of those  tissues which form part  of
the solid structure of the body, but also of those various cells, covering
its free  surfaces, which are being continually cast off and renewed, and
which, in the course  of their development, separate  from the blood the
products that are to  perform ulterior purposes  in the economy, or are
to be removed as altogether effete.   Thus the Circulation is subsement
to the functions of Nutrition and Secretion.   In  the  exercise of these
functions, different materials are draAvn from the blood by the several
tissues it supplies.  Thus the nutrition of the muscle requires fibrine;
that of  the nerve requires fatty-matter; that of the bone draws off gela-
tine and earthy salts; that of the hepatic cells removes the fatty  matter
and other  elements  of bile; that  of the milk-cells  (during lactation)
separates albuminous, fatty, and  saccharine  substances;—and  so on.
Thus various  portions of the blood, when returning from the  several 
CIRCULATION OF NUTRITIVE FLUID.
305
 organs through which they have been transmitted, have undergone very
 different changes by the nutritive  and secreting processes, according to
 the function of the organs which they have supplied ;  and if  the  same
 portion of  the circulating  fluid were constantly  being  transmitted to
 each organ, and returning from it, its composition would speedily undergo
 a change that Avould render  it no  longer  fit for its purposes.  By the
 union  of the different local circulations, however, into one general circu-
 lation, this  change is prevented,  and the whole  mass of the blood is
 maintained  in its normal or regular condition ; for as its  composition is
 such, as  to supply all parts of the body, in the state of health, with the
 proportions of nutritive material which they respectively need; and as
 the returning currents  are all  mingled together  in the vessels, before
 being  again distributed to the system, each part supplies what the other
 has been deprived of, and  thus the normal proportion of ingredients in
 the whole mass of the blood is constantly kept up, whilst in each of its
 separate streams it is undergoing  an  alteration of a different kind.
   539. But these processes alone  might be  carried on  by the aid of a
 much  less  rapid Circulation, than that which exists  in Man and the
 higher animals.   We do, in fact, occasionally meet with examples in
 Avhich  they  continue for some time, under  an almost total stagnation of
 the current.   There are  others,  however, which require a much more
 rapid and uninterrupted movement of the circulating  fluid.   We have
 already seen that, for the action of the Nervous  and Muscular tissues,
 oxygen is necessary; and the amount of that gas contained in the blood
 circulating through these tissues Avould be very speedily exhausted, if it
 were not continually renewed ; whilst the carbonic  acid, which is formed
 at the expense of that oxygen, would speedily accumulate to an injurious
 degree, if it were not carried off  as fast as it is  produced.   Hence we
 find that, in all Animals, the maintenance of the Respiration, by carry-
 ing Oxygen from the respiratory surface  into the  different parts of the
 system, and by conveying  back Carbonic acid to  be thrown  off at the
 Respiratory surface, is  one of the  great purposes  of the  Circulation of
 the blood;  and its extreme importance  is  shoAvn by the very speedy
 check, which the  interruption of this function produces in the  movement
 of the  blood, in warm-blooded animals.   Thus in a Bird or Mammal,
 completely cut off from  Oxygen, the  circulation in the lungs  will  come
 to a stop, which stoppage  will  necessarily  extend itself over  the whole
 body, in little more than three minutes.   We find, then, that the rate
 of the  circulation in different animals bears  a relation to  the  energy of
 their Respiration; and this energy is  closely connected with the general
 activity of their functions, but particularly with that of the Nervous and
 Muscular systems, which are most dependent for  the exercise of  their
 powers upon a continually fresh supply of oxygen, and  upon the un-
 ceasing removal of the  carbonic acid which is generated in  their sub-
stance.

             2. Different forms of the Circulating Apparatus.

  540. It is desirable that  the Circulating apparatus  should be first
 studied in its very simplest form,—that which it possesses in Plants and
                                 20 
306
CIRCULATION OF NUTRITIVE  FLUID.
in the lowest  tribes of Animals; as in this way alone can the forces
which are concerned  in  the  movement of the  fluid,  be rightly appre-
ciated.  In all the higher Plants, the ascending or crude sap is  to be
distinguished from  the elaborated or descending sap.   The  former of
these fluids should be compared rather with the chyle than with [the
blood of Animals ; for it is not yet fully prepared to  take part in the
nutrition and extension of the  structure.   But there are some  circum-
stances attending  its movement, Avhich throw  light  upon other  more
complicated phenomena.   The ascending sap  consists principally of
water; which is imbibed, together  Avith  various  substances  which it
holds in solution, by the  delicate  tissue at the soft  extremities of the
root-fibres, or spongioles.   The poAver of forcing upwards. a column of
sap, which exists in these bodies, and  Avhich seems due to Endosmose
(§ 491), is shoAvn by very simple experiments.   If  the stem of  a Vine,
or of any tree in which the sap rises rapidly, be cut across when in full
leaf, the sap continues to flow  from the lower extremity ; and this with
such force as to  distend wTith violence, or  even to burst, a bladder tied
firmly over the  cut surface.   If, instead of a bladder, a bent  tube be
attached to this, and mercury be poured into  it so  as to indicate the
pressure exerted, it is found that the rise  of the sap  takes place with a
force equal to the pressure of from one to three atmospheres (from 15
to 45 lbs. upon the square inch)—or  even more.   Thus the ascent of
the sap is partly due  to a powerful vis a tergo,  or  impelling  influence,
derived from the point where the absorption takes place.
  541. But,  on  the other hand, if the upper extremity be placed with
the cut surface  of the  stem in water, a  continued  absorption of that
fluid will take place, as  is evidenced by the withdrawal of the water
from the vessel; the fluid  Avhich  is thus  taken up,  however, is not
retained within the stem  and  branches, but is  carried into the leaves,
and is thence dissipated  by exhalation.   It  is obvious, then, that the
vis a tergo is not the sole cause of the ascent of the sap; but that a
vis a froute also exists, by which the fluid is drawn toAvards the parts in
which it is to be employed.  This is  further made apparent  by a few
simple experiments.  If  a branch, when thus actively absorbing  fluid,
be carried into a dark room, the absorption and ascent of  fluid imme-
diately cease almost completely ; and are renewed again, so soon as the
leaves are  again exposed to light.   Noav we  know, from  other experi-
ments, that light stimulates the exhaling process (§ 87), whilst darkness
checks it;  and the cessation of the demand in the leaves thus produces
a cessation in the absorption at the lower extremity of  the stem.   And
this is the  case, also, in the natural  condition of the plant; as is  easily
shoAvn by immersing  the  roots in water, and observing the respective
quantities which are removed by absorption during sunshine, shade, and
darkness.  On the other  hand the movement of the sap may be  excited,
when  it would not  otherwise take place, by the production of a  demand
at the extremities of the branches ; thus if a branch of a vine growing
in  the open  air, be  introduced into a hot-house, and be subjected to
artificial heat during the winter, its buds will  be  developed,  its leaves
will expand, and these will draAv fluid to themselves  through the roots
and stems, which are still inactive as regards the remainder of the tree. 
CIRCULATION OF ELABORATED SAP.
307
  And the natural commencement of the movement of the ascending sap,
  which takes place with the returning warmth of spring,  has been  ex-
  perimentally shown to occur, in the first instance, not in the neighbor-
  hood of the roots, but nearest the extremities  of the branches;  the
  exhalation  of fluid from the expanding buds being  the first  process,
  and a demand for fluid being thus created, which  is supplied by the flow
  that is thus excited in  the lower part of the stem,—this, again, being
  supplied from  the roots, which are thus caused  to recommence  their
  absorbent function.
    542.  Thus we see  that, in the  ascending sap, the movement  is  en-
  tirely regulated by the  demand for fluid occasioned by the actions of the
  leaves ; even though  it is in great part  dependent on  the via a  tergo
  which has its seat in  the spongioles.   Not even this force, however,—
  poAvcrful as it has been shoAvn to be,—can produce the continuance of
  the  upward flow, when the exhalation  from the leaves is checked by
  darkness, and when the demand occasioned by the action of these organs
  is consequently  suspended.
    543  The movement of the descending  sap offers numerous points
  which deserve to be carefully considered.   This fluid is strictly compa-
  rable to the blood of animals;  having undergone  a preparation or
  elaboration in the leaves, which adapts it to the nutrition and extension
  ot the structure, and  to the  formation  of the various  secretions of the
  plant.  A great part  of the  fluid of the ascending sap has  been lost  by
  exhalation:  and the  remainder,  thus  concentrated,  receives  a large
  additional supply of solid matter through the agency of the green cells
  of the leafy parts, which  take in carbon  from the atmosphere (§ 83) •
  so that it now includes a considerable amount of gummy matter in the
  state prepared for being converted into solid tissue, as well as numerous
 other compounds.  Now this elaborated  sap seems to be conveyed into
 the various parts of the system, partly by transmission from one cell to
 another, but partly through  the agency of a network  of vessels, which
 takes its origin  in the leaves, and extends along the  branches to the
 stem and roots, chiefly in the bark of those parts.  These  vessels are
 strictly analogous to the capillaries or  small blood-vessels of  Animals •
 but they differ from them in  this,—that the  capillary network of Ani-
 mals communicates on either  side with  larger trunks,  being formed,  in
 fact, by the interlacement or  anastomosis of  their  minutest branches' —
 whilst the network of nutritive  vessels in Plants is  everywhere con-
 tinuous with itself, not having any communication  Avith large vessels, so
 that the  fluid prepared in the leaves  commences a circulation there
 which is continued on  the same  plan, until it has found its way to its'
 remote destination in the roots.
   544. The  natural movement  of the  elaborated sap  through these
 vessels may be studied, under favorable circumstances, with the assis-
 tance of the microscope ;  the requisite conditions being, that the part
 should be  sufficiently transparent for  the vessels to be  distinctly seen
 that the  sap shall  contain globules  in sufficient  number to  allow its'
movement to be distinguished by their means, and that the  circulation
should be observed without the separation of the organ examined from
the rest of the Plant, Avhich would produce irregular movements  by the 
308
CIRCULATION OF NUTRITIVE FLUID.
escape of the sap  from the wounded part.   These  conditions may be
attained in many Plants;—most  conveniently, perhaps, in the stipules
of the Ficus  elastica, one of the trees which affords the largest supplies
of Caoutchouc; and it is then found that the movement takes place in
the following manner.   Distinct  currents are  seen, passing along the
straightest and  most  continuous vessels, and crossing by the lateral
connecting branches of the network.  These  currents follow no  deter-
minate direction ;  some proceeding up, and  others down; some to the
left, and others  to the right;  not unfrequently a complete  stoppage is
seen in  one or more of the channels, without  any obvious  obstruction;
and the movement then recommences, perhaps in  the opposite direction.
The influence of  a force, developed by the act of circulation, which
determines the direction of the movement, appears from this : that if a
tube be cut off,  so as  to  give its contents  an equally free exit at both
ends, the  sap  only flows  out at one  extremity.   The  movement is
retarded by lowering  the  temperature of the surrounding air, and it is
completely checked by extreme  cold; it is   capable of being  renewed
by moderate warmth; and  a further  addition  of heat increases its
rapidity.  By a strong electric  shock, the force  by  which the liquid is
propelled seems to be  altogether destroyed; for the movement  then
ceases entirely.
   545.  Noav it  is quite certain  that this circulation cannot be due to
any vis a tergo ; both because it is not constant  in its direction in par-
ticular vessels ;  and because there is no organ in Avhich any propelling
force, that could extend itself through such a complex system of vessels,
may be  developed.  Nor can it be in any way due to the force of gravity;
for although  this may assist the descent of the fluids through the stem,
it is totally opposed to its ascent  from the ends of its branches toAvards
their origin,  when, as  often happens, the latter are  at the higher level.
Moreover,  it  may be  noticed that this  circulation takes place most
readily, in parts that are undergoing a rapid development; and that its
energy corresponds with the vitality of the  part.  Further, it may be
observed to continue for some time  in parts that have been completely
detached  from the rest; and  on  which  neither vis  a tergo, nor  vis a
fronte, can have any influence.   It  is evident, then, that the force,—
whatever be its nature,—by which this continued moA'ement is  kept up,
must be developed by the processes  to which that movement is subser-
vient ;  in other words,  that the changes involved in the acts of nutrition
and secretion are the real source  of  the  motor power.  The manner in
which they become so, is the  next  object of our inquiry;  and on this
subject, some new views have  recently been  put forth by  Prof. Draper,
which seem to account well for the phenomena.
   546.  It is capable of being shown, by experiments on organic bodies,
that, if two  liquids communicate with each other  through a capillary
tube, for the walls of which they both have an affinity, but this affinity
is stronger in the one liquid than in  the  other, a movement will ensue;
the liquid which has the greatest affinity being absorbed most energeti-
cally into the tube, and driving  the other before it.  The same result
occurs when  the fluid  is drawn, not  into a  single tube, but into a net-
work of tubes, permeating a  solid  structure; for if this porous struc- 
            CIRCULATION IN PLANTS AND LOWER ANIMALS.       309

ture be previously saturated  with the fluid, for which it has the less
degree of  attraction, this will be  driven out and replaced by that for
which  it has the greater affinity, when  it is  permitted to  absorb this.
Now if, in its passage through the porous solid, the liquid undergo such
a change, that its affinity is diminished,  it is obvious  that, according to
the principle just explained, it must be  driven out by a fresh supply of
the original  liquid,  and that thus a continual  movement in the same
direction would be produced.
  547. Now this is precisely  that  which seems  to  take place  in the
organized  tissue, permeated  by nutritious fluid.   The particles  of this
fluid, and the solid matter through which it is distributed, have a certain
affinity for each  other;  which is exercised in the nutritive changes, to
which the fluid becomes  subservient during the course of  its circulation.
Certain matters  are drawn from it, in  one  part, for the support and
increase of the woody tissue ; in another  part, the secreting cells demand
the materials Avhich are requisite  for their growth,—as starch, oil, resin,
&c.; and thus in every  part that is traversed by the vessels, there are
certain affinities between the solids and the fluids, Avhich are continually
being developed  afresh  by acts of growth, as fast as those which pre-
viously existed are satisfied  or neutralized  by  the  changes that have
already occurred.  Thus in the circulation of the elaborated sap, there
is a constant attraction of its particles towards the walls  of the vessels,
and a continual series of changes produced in the fluid, as the result of
that attraction.   The fluid, which has  given up  to a certain tissue some
of its materials, no longer has  the same  attraction for that tissue; and
it is consequently driven from it by the superior attraction  then pos-
sessed  by the tissue for  another  portion of the fluid, which is ready to
undergo the same changes, to be  in its turn rejected for a fresh supply.
Thus in a groAving part,  there is  a constantly-reneAved attraction for the
nutritive fluid, Avhich has not yet  traversed it; whilst, on the other hand,
there is a diminished attraction for the fluid, which has yielded  up the
nutritive materials required by the particular tissues  of the part; and
thus the former is continually driving  the latter before it.
  548. But the  fluid which is  thus repelled from  one part,  may still be
attracted  toAvards another; because that portion of its contents  which
the latter  requires, may not yet  have  been remoAred  from it.   And in
this manner, it would seem, the flow of sap is maintained, through the
whole capillary netAvork, until it  is altogether exhausted of  its nutritive
matter.   The source of the movement is thus entirely to be looked for
in the changes Avhich take place in the act of growth; and the influence
of heat, cold, and other agents, upon the movement is exercised through
their power of accelerating or retarding those changes.—The fluid which
thus descends through the stem and roots, seems to be at last almost en
tirely exhausted ; a portion of it appears to find  its way into the interior
of the stem,  and to be mingled with the  ascending current; but  all  the
rest seems to  have been entirely appropriated  by the different tissues,
through which it has circulated.   Thus there is no need of any general
receptacle, into which it may be  collected, and from  which  it may take
a fresh departure;—such as is afforded by the heart  of Animals.  And
as the purpose of this  circulation is only to supply the nutritive mate- 
310
CIRCULATION OF NUTRITIVE FLUID.
rials, and not to convey oxygen,—this element being but little required
in the vegetative  processes, and being supplied  by other means,—the
same energy and rapidity are not required in it, as need to be provided
for in the higher Animals.
   549. A condition of the Circulating system very similar  to this, exists
in several of the  lower animals, as well as in the embryo-state of the
higher.  In the very lowest no blood-vessels are required, for the same
reason that no sap-vessels exist  in the loAvest Plants;—namely, because
every part absorbs and  assimilates nutritious  fluid for itself, so that it
does not require a supply from vessels.   As, in the sea-Aveeds, the Avhole
substance is nourished  by direct absorption from the fluid  in contact
with the external surface, every part  of which seems  endowed with the
same absorbent poAver, so  in Zoophytes do we find; that the whole sub-
stance is nourished  by direct  absorption  from  the internal surface,
which forms the lining of the  digestive cavity.  In the same manner,
the Aeration  of the  animal fluids,—or the  exposure of them to  the air
contained in  water, by  Avhich they may part with  carbonic acid and
imbibe oxygen,—is provided for, not by any special respiratory organs,
but by the contact of Avater with  every part of the soft  external and
internal surfaces.   Further, as the substance of their body is nearly of
the same kind in  every part, they do not require the continual inter-
change of the fluid distributed  to its several portions.   Thus no circu-
lation is necessary, in these simple animals, either for  the nutrition of
their tissues,  or for  the aeration of the fluids.   The same is the case
with others of the  loAver  tribes ; as well as with the embryo of the higher
Animals,  at the earliest  period of their development.   Thus the lowest
Entozoa, or parasitic worms, have a digestive cavity channelled  out, as
it Avere, in their soft gelatinous tissues;  and from the walls of this, the
nourishment is drawn  by the several component parts of  those tissues,
without the  mediation of vessels.  And  the embryo even of Man, in its
early condition, consists of an aggregation of cells, each of which absorbs
for itself from the nutritious fluid with  Avhich it is surrounded, and goes
through all its functions independently of the rest.
  550. Proceeding a little higher, we find the first appearance of proper
vessels in the  higher Entozoa, and in the Echinodermata.  These vessels
take  up  the nutritive fluid  from  the Avails  of the digestive  cavity, on
which they are spread out,  just  as the roots of Plants do from the soil.
They then unite into  trunks, by  which the fluid is  conveyed to the more
distant parts of the structure, in the same manner as the ascending sap
is conveyed  to the leaves by the vessels of the stem and branches; and
these trunks  again subdivide, and  form a netAvork of capillary vessels,
which are dispersed through the several parts of the fabric ;  some of them
being very abundantly distributed upon  a portion  of  the surface, which
is particularly destined to  perform the respiratory function.   Through
these capillary vessels, the fluid seems  to move in very much the same
manner, as through the system of anastomosing vessels in Plants ;—that
is, its motion is due, rather to  forces which are developed  during its
circulation, than to any vis  a tergo  derived  from the contractile  power
of a propelling organ.   But there is this difference:  that, after  having
traversed  the  minute vessels,  and yielded  up to the tissues  a part 
CIRCULATION IN  ARTICULATED ANIMALS.
311
of the solid  matter which it  contains, the fluid  is collected^ again by
other trunks, Avhich convey it  back to the point from which it started;
there it  is mingled with the fluid that has been newly  absorbed, and
with that which has undergone aeration; and  it  is then distributed, as
before, through the general capillary network  of the body.
  551.  Now this is very  much the condition of the Human embryo, at
the time Avhen  vessels  are first developed in its substance.   These ves-
sels are formed by the coalescence  of cells ;  and from the contents of
these cells, Avhich have been imbibed from the yolk, the first blood seems
to be derived.   The first formation of  blood-vessels  takes  place, not in
that part of the embryonic structure which is to be developed into the
perfect animal, but in  a membranous expansion from it, which surrounds
the yolk, and which answers the  purpose of a temporary stomach.  A
capillary network  is  formed  in  a limited portion of this membrane,
termed the vascular area (Fig. 88);  and this is not by the branching of

                               Fig.  88.
  Vascular area of Fowl's eggs, at the beginning of the third day of incubation;—a, a, yolk; 6, b, b, b, venous
sinus bounding the area; c, aorta; d, punctum saliens, or incipient heart; e,e,area pellucida; /,/, arteries
of the vascular area; g, g, veins; h, eye.
larger trunks, these trunks being subsequently formed by  the reunion
of the capillaries.   The first movement of the blood is towards the cen-
tral spot, in Avhich  the organs  of  the permanent structure are  being
evolved; and it takes place before the incipient heart has acquired any
muscularity, so that it must be quite independent of any contractile force
exerted by that organ.   Here too, then, we perceive that the circulation
is essentially capillary ; and that it is sustained by forces very different
from  those, of Avhich the  action is most evident  to us in the higher
animals.
   552. As Ave  ascend the animal scale, however, we find that provision
is made for a more regular and vigorous Circulation of the  Blood, than
that which exists in the lowest  classes.  Even in the class of Echino-
dermata (including the Star-fish and Sea-urchin), a portion  of the prin-
cipal  vessel is peculiarly endowed with contractile power; and this may
be  seen in constant  pulsation, like the  heart of the higher animals,
alternately contracting, to  propel the fluid it contains, through the ves- 
312
CIRCULATION OF  NUTRITIVE  FLUID.
sels that issue from it, and then dilating, to receive a fresh supply from
the vessels that pour their contents  into it.   It  seems  quite  certain,
however, from the extent of the vascular system of these animals, that
the influence of such  a pulsatile cavity must be quite insufficient to keep
up the movement of blood through it.  A similar provision is observable
in the lower tribes of Worms, in which this contractile vessel lies along
the back; propelling the blood forwards, by a sort of peristaltic move-
ment, through  trunks which pass out at its anterior termination; and
receiving it again after it has circulated  through the system, by vessels
which enter at its posterior extremity.  In the higher orders of Worms,
in the Myriapoda or  Centipede tribe, and in Lnsects, we find this dorsal
vessel divided by transverse partitions containing  valves, into separate
cavities which answer to the different segments of the body.  Each  of
these is, to a certain  extent, the heart of its own segment, receiving and
propelling blood by trunks which open  into it; but they all participate
in the more  general circulation just described, a large portion of the
blood being poured into the hindermost segment, transmitted forwards
from  cavity to  cavity through the valves which  separate them, and  at
last propelled through trunks that issue from the most anterior segment.
In some instances we find that two or three of these trunks, on either
side, pass round the  oesophagus, and reunite beloAv  it, so as to enclose
it in a sort of collar; and  they form a main trunk by this union, which
runs backwards along the under surface of  the body, and which distri-
butes the blood  to its  different organs  by lateral branches.  These
subdivide into a capillary network, and the  returning vessels, Avhich
originate in this network, pour  the blood which has circulated through
it into  the  posterior  cavity  of  the dorsal  vessel.—Still it is  very
evident  from the  observation  of  the circulation in  those transparent
species in Avhich the Avhole process can be distinctly  watched under the
Microscope, that the contractile power of the dorsal vessel  is far from
sufficient of itself to sustain the Circulation; and that the movement of
the blood through the  capillary network  is in  part due to forces deve-
loped during its progress, being often retarded or accelerated in parti-
cular spots, without any visible change in the propelling force of the
eentral organ.  Moreover, the  blood, during some  part of its course,
almost always escapes from the proper vessels into lacunce channelled
among the tissues;  and over its flow through these, no central impelling
organ can have much influence.
   553. In most of these animals, there are distinct organs  of Respira-
tion,  confined to some one part of the body ; and we often find that the
vessels which  convey blood to them, are furnished with distinct con-
tractile portions, like so many supplementary hearts, for the purpose of
propelling the blood  through  them more energetically.   In  proportion
as we ascend the series of Articulated animals, do we find for the most
part, a more vigorous and regular circulation, both for the nutrition of
the system,  and for the transmission  of the blood through the respira-
tory  organs; but  there is an  exception in the case of insects, which
deserves special notice.  In this class, the circulation is much less vigo-
rous than it is in  other Articulated animals  of similar complexity of
structure; though it might have been anticipated, that the extraordinary 
CIRCULATION IN  MOLLUSCS.
313
activity of their movements would necessitate a  corresponding rapidity
in the circulating  current,  especially for the purpose  of conveying an
extraordinary supply of oxygen to the  nervous  and muscular systems.
But this  is  provided for in another way; the air being  conveyed to
these tissues not through the blood, but by direct  transmission through
the minute  ramifications of the air-tubes or tracheae,  which penetrate
the very smallest organs of the body (§ 659).
   554. The condition of the Circulating apparatus in the Embryo of
higher animals, at a period a little advanced beyond that just alluded to,
presents a striking analogy with that last described ; for the  heart, at
the time of its first formation, seems like a mere  dilatation of the princi-
pal vascular trunk, having  thickened walls, in which, after  a time, mus-
cular fibre begins to be developed,  and the  contractile  power manifests
itself.   The pulsation of this heart,  hoAvever, does not seem to extend
its influence immediately through the vascular area ; the capillary circu-
lation in which, remains for some time in great degree independent of it.
There is no resemblance inform, hoAvever, between the dorsal vessel of
Insects, and the incipient heart of the higher  animals; since the latter
is never  much prolonged, and  speedily becomes doubled  (as  it were)
upon itself;  and its first division into  distinct cavities is merely for the
purpose of separating its receiving portion, or auricle, from its propelling
portion,  or ventricle.    But the general condition of the  Circulating
system is  much the same in the two  cases;  and it is further alike in
this,—that it is not always easy to show that the vessels haA^e distinct
walls,  as  they frequently seem like  mere  channels excavated in the
tissues.
   555. We may next  turn  our  attention briefly  to the condition of the
Circulating apparatus  in  the Molluscous classes, Avhich has lately been
found to  present  some  very peculiar  characters.  In  these  it would
seem as if the moving power were more  concentrated in the heart, than
in the preceding;  for  this organ seems no longer like a mere dilatation
of the vascular trunk,  but is a distinct sac with muscular walls, usually
having at least tAvo  cavities, an  auricle and a ventricle.   The usual
course of  the Circulation is the  following.  The blood, expelled from
the ventricle of the heart,  passes  along the main systemic artery, or
aorta; which distributes it  to the body at large.  It is then  collected
again, and transmitted to the respiratory organs ; in Avhich it is exposed,
either to the air contained in the surrounding Avater, or (in the terres-
trial Molluscs) more directly to  the atmosphere; and from these  it is
returned to the heart, to be again transmitted  to the system.—Thus we
see that the heart of these  animals receives and impels aerated blood;
and that its office is, to send that blood  to the capillaries of the general
system.   Hence it may be  called a systemic heart.
   556. The  blood, in  the first  part of its  course, passes  through  dis-
tinct vessels : it has been lately shoAvn, however, that in  the Molluscs
in general, the blood which has passed through the systemic capillaries,
and  is on its Avay to the respiratory organs, is no longer thus  confined,
but that itjneanders through passages  or lacunar, which are channelled
out in the tissues, and which even communicate freely with the abdomi-
nal  cavity in Avhich the viscera lie ; so that their whole exterior is 
314
CIRCULATION OF NUTRITIVE  FLUID.
bathed by the circulating fluid.  It is perhaps in this part of its course,
that it most readily takes up the fresh nutrient materials, which have
been prepared by the digestive  process, and which Avould, under such
circumstances,  find their way with  comparative facility from the inner
surface of their walls, to the outer.—After being thus  diffused, in its
venous or carbonized state, through the substance of the tissues and
through the visceral  cavity, it is again  collected into  distinct  trunks;
and these convey it to the respiratory organs.—Now although it cannot
be doubted, that the impelling power of the heart is the chief cause of
the movement of the blood through the systemic vessels, yet it would
seem impossible  to  suppose, that  this power  can be exerted over the
unrestrained currents, in which it  is  diffused  through the body, after
passing through the systemic capillaries ; and it can scarcely be doubted,
that its passage through the capillaries of the respiratory organs is due
to the power which  is  developed in themselves,  under the  conditions
already alluded to.
  557. There is a very curious phenomenon to be observed  in the  cir-
culation of some of the  loAvest Molluscs ; namely, the continual  reversal
of the course of the  current.  The  heart, in these animals, is much less
perfectly formed, than  in the higher tribes; and seems more like  the
mere contractile  dilatation  of the   principal trunk, which is the sole
representative  of that  organ  in the Echinodermata.  The circulating
fluid is sometimes transmitted first  to the system;. and, after being dis-
tributed to its different  parts by jhe  ramifications of the main artery,
it meanders through the channels  excavated  in its tissues;  and then
flows toAvards  the  respiratory surface, after passing over which, it re-
turns to the heart.  But after  a certain  duration of its  flow in this
direction, the current  stops,  and  then  recommences  in the contrary
direction,—proceeding  first to the  respiratory  organs, and then to the
system in general.   It would seem  as if  in this, one of the lowest forms
of animals possessing a distinct  Circulation, the central poAver were not
yet  sufficiently strong,  to determine  the  course  which  the  fluid is to
take :  so that it undergoes continual vacillations.   In  a group  of Com-
pound Polypes, to which  this  class of Molluscs  has  many  points  of
affinity, there is a movement of fluid  through  the stem  and branches,
which in  like manner  continually  changes its direction.  This move-
ment, hoAvever, can scarcely be regarded in the light  of a proper Cir-
culation ; since the tubes in  which it occurs are in direct communication
with the digestive cavities of the Polypes. But the flow seems  altogether
independent of any mechanical propulsions  ; and takes  place most ener-
getically and regularly towards parts in Avhich new  growth is going on.
  558. We have noAv to consider the chief forms in which  the Circu-
lating apparatus presents itself in the Vertebrated classes ; and first in
that of Fishes.  We have here, as  in Molluscs, a heart with two cavi-
ties, an auricle and a ventricle ; this heart, however, is not placed at the
commencement of the  systemic  circulation, but at the origin of the
respiratory vessels.   The blood which it receives and propels, is venous
or carbonized; this is transmitted  along a main  trunk, which  speedily
subdivides into lateral  branches or arches ; and these distribute  it to
the fringes of gills, that hang on the sides  of the neck.  By  the action 
                        CIRCULATION IN FISHES.                    315

 of the water on the gills, the blood is aerated in its passage through them ;
 and it is  then collected  by a  series of converging vessels,  which re-
 unite  to form the  great systemic artery, or aorta.
 By the ramifications of this artery, the blood, now        FiS- 89-
 aerated,  is  distributed  through the system,  and
 affords the  requisite nourishment  and  stimulation
 to its  tissues.   Returning from the systemic capil-
 laries  in a venous state, the  blood  of the head and
 anterior  portion of the body finds its Avay at once
 into  the  great systemic  vein,  or vena  cava, by
 Avhich it is  conveyed  back  to the auricle  of the
 heart; but that which has traversed the capillaries
 of the posterior part  of the body, and of the ab-
 dominal viscera, is conveyed by a distinct  system
 of veins  to  the liver  and the kidneys.  In these
 organs,  the  veins  again subdivide into a network
 of capillaries,  which  is  distributed  through  the
 secreting  structure, and which  serves to afford to
 the secreting cells the  materials of their  develop-
 ment.   This is  termed the portal system of vessels.  ~.
 -171     ,1      -n  •    /• .1   v       -ii-i       .1     Diagram of the Circulating
 .rJrom  the capillaries of the  liver and kidneys, the Apparatus of Fishes:— a, the
 blood  is finally collected by  the  hepatic and renal trunkVuppiylngniebranrfiia'i
 veins,  which convey it into  the vena cava;  where ^ll^X^^^n-
 it is mingled with  the blood that  has  not passed veycd *y «> «> the  branchial
 r     i   ,               i-i            11   veins, to /, the aorta, which
 through those organs,  and is thus conveyed to the distributes it to the system;
 Vionrl-                                             thence it is collected, and re-
 neart.                                            turned to the auricle, by the
   559. The  heart  of Fishes, then,  belongs to the ^^ich unite in the vena
 respiratory  circulation.   It propels venous  blood
 to  the capillaries  of  the  gills,  in Avhich  it is aerated;   returning
 from these,  the aerated  blood  is transmitted  through  a second  set
 of capillaries, those of the system,  in which it  again becomes venous :
 whilst a portion of  this blood  is made to traverse a third set of  capil-
 laries, those  of the  liver  and  kidneys,  before  it is  again  subjected
 to the propelling poAver of the heart.   Noav  as  the heart,  instead of
 being  stronger  than it is in animals with the complete  double  circu-
 lation  presently to  be described,—in which the greater part of  the
 blood  propelled by it only traverses one  set  of  capillaries, and  never
 more than tAvo,—is much Aveaker in proportion, it is evident that here,
 too,  a supplementary  poAver must exist,  by which  the flow of  blood
 through  the  capillaries is aided, and on Avhich, indeed, the portal   circu-
 lation  must greatly depend.
   560. An extremely interesting aspect of the circulating apparatus is
 presented by the Amphioxus or  Lancelot;  an animal which presents the
 general form of a Fish, and which can scarcely be  referred to  any other
group ; but  in Avhich  the characters of the Vertebrated  series are  de-
graded (as it were) to the level of the lower Molluscous and Vermiform
classes.  The blood, which is white, moves through distinct vessels,  but
there is no proper heart;  and the vascular trunks present several dila-
tations, in different  parts, which have muscular  walls, and show con-
tractile power.   Thus  the circulation is carried  on, not  through  the 
316
CIRCULATION OF  NUTRITIVE FLUID.
agency of a central impelling organ, as in other Fishes ; but by a power
Avhich  is scattered  or diffused  through various  parts  of the  system of
blood-vessels,  as in the lower Invertebrata.—The respiratory apparatus,
also, is formed upon a type much lower than that of Fishes ; for it con-
sists simply of a dilatation of the first part  of the alimentary canal, or
pharynx, upon the walls  of which the  blood  is distributed  in divided
streams, its cavity being  filled with water,  which  serves to  aerate  the
blood.   This  is precisely  the type, on which the respiration is effected,
in those lowest Molluscs,  of which mention has just been  made, as ex-
hibiting alternations  in the direction of the  circulating current (§ 557).
In other  respects, however,  the arrangement of the vascular system
in this extraordinary animal corresponds  with that  which  obtains in
Fishes.
   561. It  is  requisite that, in  the class of Fishes, the  whole of  the
A'enous blood  returned from the system should pass through the respi-
ratory organs before being again transmitted to the  body ;  since  the
aerating action of the small quantity  of air diffused through the Avater,
would  otherAvise be insufficient for its  renovation.  But in Reptiles, all
                  of which breathe  air during their  adult  condition,
                  the case is very different; for if the whole current of
                  their blood were exposed to the  atmosphere, before
                  being again sent to the body, the  quantity of oxygen
                  conveyed into the tissues would be too  great, and
                  would have an over-stimulating effect.   The plan of
                  the  Circulation is, therefore, differently arranged in
                  Reptiles.  We find the  heart  to  consist  of  three
                  cavities ; tAvo auricles and one ventricle.  From  the
                  ventricle  issues a  single trunk, which  speedily sub-
                  divides ; some of its branches proceeding to the lungs,
                  and others to the  body.   The  blood which is trans-
                  mitted  through this  trunk, is of  a  mixed character,
                  as we shall presently see ; being neither fully aerated,
                  nor  yet highly  carbonized.   It  contains  sufficient
                  oxygen, to stimulate the  nervous  and muscular sys-
                  tems of these comparatively inert animals ; whilst it
tio^ln'RTptnes^-a.'sin^e a^so contains enough of carbonic  acid,  to require
ventricle,  receiving the being exposed to the atmosphere through the medium
aerated blood from  6, the  «  ,°  . *      m,   , ,   i  i • 1  1         ii     1
pulmonary auricle, and ve-ot the lungs.  Ihe blood which has passed through
temic^u^cfe^nd^o^l- tne systemic  capillaries, and which  has  been thereby
tonthPeartu0imon\?iXceadfira^ rendered completely venous, is returned  to  one of
ries a, and part to the sys- the auricles—the  systemic—by the vena cava.   On
                  the other hand, the blood which has passed through
the capillaries of the lungs, and which has been thereby rendered com-
pletely arterial, is returned through  the pulmonary vein  to  the  other
auricle,—the pulmonary.   Thus one of the  auricles exclusively receives
aerated, and the other carbonated  blood ; and  as  both pour  their con-
tents into the common ventricle, the  blood which that cavity contains
and propels is of a mixed character.
   562. Various  modifications of  this form of Circulating  apparatus
exist in the different  groups of reptiles.  In the lowest  among them,
 
CIRCULATION IN REPTILES.
317
which  breathe permanently by gills like Fishes, besides possessing
imperfectly-developed lungs, the apparatus exhibits a  blending of both
plans; for a small  portion  of  the blood,  which  is propelled by each
contraction of the  ventricle, passes directly to the lungs;  the principal
part of it being at once distributed  to  the gills, as in Fishes.  After
passing through these, it is transmitted to the general  system; and on
returning thence, in a completely venous state, it  is mingled with the
blood which has been arterialized in the lungs.   This latter, hoAvever,
bears so small  a proportion to the  rest, that, if the aeration were not
partly  effected by the gills,  it would be insufficient for  the wants of the
animal.  The tadpoles of the  common Frog and Water Newt, as well
as of other species  which, like  them,  begin  life in the general condition
of Fish, present a similar condition at one period of their change.   At
first, the whole aeration  is  effected by means of gills, the lungs being
in  a  rudimentary  or undeveloped state; and the entire circulation  is
carried on as in Fishes, the pulmonary vessels being scarcely traceable.
As the lungs begin to be developed, however, a  portion of the blood is
sent to them ; and at  the  same time,  communicating passages which
previously existed,  betAveen  the vessels that convey blood  to the  gills,
and those that return it  from  them,  are increased  in  size;  so that a
certain proportion of the blood is transmitted to  the  system, Avithout
having passed through the gills at all.   By a further  increase in the
diameter of  these, the whole current of blood takes this direction, the
gills being no longer serviceable; and as,  at the  same time, the lungs
are attaining  their full  development, the aeration which they effect  in
the blood transmitted to  them becomes sufficient, and the whole circula-
tion is thus permanently established on the Reptilian type.
   563.  On the  other hand, among the  higher Reptiles,  we find the
circulating apparatus presenting  approaches to  the form it possesses
in Birds and Mammals.   For the  ventricle is divided, more or less
completely, into tAvo cavities,  one of which  propels aerated  blood  to
the system, whilst the other  transmits venous blood to the  lungs.   A
certain amount, of mixture  of  arterial  and venous Wood  always takes
place, however,  either in the  heart  itself or in  the  vessels; so  that
the blood which the body receives, is never purely arterial.  But this
mixture is sometimes effected  in  such  a  manner, that pure arterial
blood is  sent  to  the head  and anterior extremities;  though the  re-
mainder of the body receives a half-aerated fluid.   This is accomplished
in the Crocodile, by a provision very similar to  that  which exists  in
the foetus of warm-blooded  animals, (chap, xi.)   The portal circulation
in Reptiles is carried on  nearly upon the same  plan as in Fishes.   It
receives  the blood  from  the posterior extremities  and from the  tail,
as well as from the abdominal viscera; and this blood  is distributed by
the portal capillaries, not only through  the liver, but also through the
kidneys, although  the latter also  receive  arterial  branches from the
aorta.   The fact that the kidneys are supplied from the general portal
circulation in  Fishes and Reptiles, has  an important  bearing on the
difference in the arrangement  of their OAvn vessels, which will be here-
after shown to exist, betAveen the kidneys of these animals and those  of
Birds and Mammals (§ 728). 
318
CIRCULATION OF NUTRITIVE FLUID.
  564.  In the warm-blooded division of the  Vertebrated series, which
includes the classes of Birds and Mammals, we  find  the whole circu-
lation possessed of a greatly-increased energy;  but the  distinguishing
peculiarity of the  apparatus in these animals, is that conformation of
the heart and vessels  which  secures  a complete  double circulation of
Fig. 91.
                     the blood; that is, which provides for the aeration
                     of every particle of the venous blood which has re-
                     turned from the system, before it is  again sent into
                     the tissues.   The  heart may be regarded  as con-
                     sisting of tAvo distinct parts,—a systemic heart, like
                     that of the  Molluscs, forming its left side,—and a
                     respiratory heart, like that of Fishes, constituting
                     its  right.    Each  of these parts has  a receiving
                     cavity or auricle,  and  an  impelling cavity or ven-
                     tricle.  The  cavities  of the two sides are completely
                     separated from one  another, in the adult  state at
                     least; though their Avails are united, for economy of
                     material.  It is obvious that much  is saved in this
                     manner ; since, as the contractions of the auricles
                     and of the ventricles on the two sides occur  simul-
                     taneously, the pressure of blood in the one is partly
  Diagram of the Circulating    ,     •  i  i   ,i       xi     ^i     l        -.   ?
Apparatus in Mammals and  antagonized by that on the  other, wherever it acts
KiT^tes-MeMcwai  on  the wall that is common to both.  This antago-
th^IffiMridetfeerighi  ™sm *s  not complete, however ; since the systemic
ventricle, propelling venous  ventricle contracts with far  greater force than the
blood through e, the pulmo-    .             i,i      ni,      , i         ii
nary artery,  to /, the capii-  pulmonary ; and  the wall  between them must  be
inriciefrl«ilto| tof ^d  capable  of resisting the difference  of  pressure on
wood fro^ tte pulmonary  jts tw0 gjjgg thus occasioned.   The blood which is
vein, and, delivering it to the             '
left  ventricle, h, which pro-  returned from the system, in a venous state, through
pels it, through the aorta, i,  ,              ,   ,i   • i ,    • i       i   i • i •
to the systemic capillaries,  the vena cava to  the  right -auricle, and Avhich is
4ew vdns, and carried %&  poured by it into the right ventricle, is impelled by
ctavtahe6heartthr°ughthe7ena  tne  latter through  the capillaries of  the  lungs,
                 v   where it undergoes aeration.   Returning thence in
an  arterialized state, it is conveyed into  the left auricle,  and thence
flows into the left  ventricle; by  which it is propelled through the great
systemic artery or  aorta, and through its  ramifications  to the general
system.                              ♦*
   565.  The greater  part of  the  blood Avhich has been rendered venous
by passing through the systemic capillaries, is collected  by the systemic
veins, and is returned directly to the heart through the vena cava.  But
a portion is still employed  for the distinct circulation,  which is destined
to  supply the materials for  the secreting  action of  the liver.   The
blood that has  traversed the capillaries  of the walls of the alimentary
canal, and of the other viscera concerned in digestion,  is collected again
by  the  converging veins into a large venous trunk, the  vena portse, by
which it is distributed through  the liver.  This vessel, although formed
by the  convergence  of  veins, and conveying venous  blood,  has really
the character of an artery in an equal  degree;  for it  subdivides  and
ramifies after its entrance into  the liver, so as to form a  netAVork of
capillaries, from  which the blood is  again  collected, and thence trans- 
CIRCULATION  IN MAMMALS.
319
mitted by  the hepatic vein  to  the vena  cava.  Thus  that portion  of
blood  Avhich supplies the liver with the materials of its secreting action,
passes through  two sets of capillaries, between the time  of its leaving

                                  Fig.  92.
  Anatomy of the Human Heart and Lungs.  1. The right ventricle; the vessels to the right of the figure
are the middle coronary artery and veins; and those to its left, the anterior coronary artery and veins.
2. The left ventricle.  3. The right auricle. 4. The left auricle.  5. The pulmonary artery. 6. The right
pulmonary artery. 7. The left pulmonary artery. 8. The remains of the ductus arteriosus. 9. The arch
of the aorta.  10. The superior vena cava. 11. The right arteria innominata, and in front of it the vena
innominata. 12. The right subclavian vein, and behind it its corresponding artery. 13. The right common
carotid artery and vein.  14. The left vena innominata.  15. The left carotid artery and vein. 16. The left
subclavian vein  and artery.  17. The trachea.  18. The right bronchus.  19. The left bronchus. 20, 20.
The pulmonary veins; 18, 20, form the root of the right lung; and 7, 19, 20, the root of the left.  21. The
superior lobe of the right lung,  22. Its middle lobe.  23. Its inferior lobe.  24. The superior lobe of the
left lung.  25. Its inferior lobe.'

the heart and its return  to it.   The  portal  circulation in Birds, as in
Reptiles and Fishes, receives the  blood from the posterior part of the
body, and from the extremities; but the portal blood is only  conveyed
to the liver; the kidneys being supplied by the renal artery.
   566.  This perfect form of the  Circulating apparatus is only attained,
in the  warm-blooded  animal, after  a  series of transformations, which
strongly remind  us of  the  permanent  forms  presented by the vascular
system in Fishes and  Reptiles.   Thus in  the embryo of the Chick at
about the  60th  hour,  and in that of  the Dog at about the  21st day,
the  curved  and  dilated tube, of which the  heart  previously  consisted
(§ 554), is found to  be  distinctly  divided into an  auricle and a ven-
tricle.    From the latter originates  the  main arterial  trunk, which
divides into four  pairs of lateral branches;  and  these pass round the
pharynx, precisely in the position  and direction of  the arterie's of the
gills  of  Fishes.    They do not, however, distribute  the  blood  to gill-
tufts; for none such  are developed in the  embryo of the warm-blooded
animal:  but they  meet again  below  the pharynx,  to  form  a  trunk,
Avhich supplies the general  circulation.   Within a  short  period, how-
ever, the Avhole plan of the circulation undergoes a change.  The auricle 
320
CIRCULATION OF  NUTRITIVE  FLUID.
         and the ventricle are each divided  by a partition, that  is developed in
         the middle of the heart;  and thus the two  auricles and the tAvo ven-
         tricles are formed.   Whilst this is  going  on, a  change takes place also
         in the vessels that arise from the heart; for the  arterial trunk, that was
         previously single, undergoes a division into two distinct tubes ; one of
         which is  connected with  the  left  ventricle,  and  becomes  the  aorta,
         whilst the other originates in the right ventricle, and becomes the pul-
         monary artery.   Of the four pairs of branchial arches, some are sub-
         sequently obliterated;  whilst others undergo changes that end in their
         becoming the arch of the aorta, the right and left pulmonary arteries,
         and the right and left subclavians.
            567.  The muscular power of the heart is much greater in the warm-
         blooded than in  the cold-blooded  Vertebrata, in proportion to the ex-
         tent of  the circulation which it is  concerned in  maintaining; and it is
       r_evidently destined to take a much larger share in the propulsion of  the
         fluid,  than it is in  the  lower tribes.  Marry  Bh-ysiologisLs^indeecLAfe
         of opinion that the jnojjenient  gXJ^hsJolpo^^_entirely'due to the actum,
         ofLtdiehearj;  and~tnis view appears toBe sup"ported by the results of
       [numerous experiments upon the circulation.  But it  is very difficult, if
       """"hot""impossible, to make  experiments  that  shall be  really satisfactory
         upon  this point; and it  appears safer to trust to  the " experiments
         ready prepared for  us by Nature," as Cuvier  termed them,—namely,
         those  lower forms of animated being, in  which various  diversities of
         structure present themsehres, and  in  which we can  study the regular
         and undisturbed effects of these.   Thus we  have seen  that, in Plants
         and the lowest Animals, Avhich have  no central impelling  cavity,  the
         movement of the nutritive fluid is  entirely dependent upon  the power
         that  is  diffused through the network  of vessels in which it circulates.
         As Ave  ascend the  series,  we find an organ of impulsion   developed
         upon  a  certain  part of the vascular system, whose object it is to give
         increased energy and regularity to the movement.  And ascending still
         higher, we find the moving power gradually concentrated, as it were, in
         this organ;  yet it is not  altogether withdrawn from  the capillary net-
         work, as we shall see from several facts to be presently adduced.  The
         particular actions of the  Heart, the Arteries, the Capillaries, and the
         Veins, will now be considered in more detail.

•                                3. Action of the Heart.

            568.  The Heart  is a hollow muscle, endowed in an eminent  degree
         with the property of irritability; by which is meant, the capability of
         being easily excited to movements of contraction alternating with relaxa-
         tion (§ 347).  At first sight, its actions  seem different from  that of the
         muscles which are called into action by the  impulse of the will;  for in
         these  there is apparently no such alternation, the  state of  contraction
         being kept up as long as the  will  operates.  But it has been already
         explained that, even in these, the  individual fibres  are  probably in a
         state  of continual alternation of contraction and relaxation, during their
         active condition,—one set taking up the action, whilst another is return-
         ing to the state of relaxation.   Hence the chief peculiarity in the Heart's 
MOVING POAVERS  OF THE CIRCULATION.
321
 action consists  in this,—the  whole  mass  of  fibres of each  division
 of the organ contract and relax together.  The contraction  of the  tAvo
 ventricles is perfectly synchronous, as is  that of the two auricles;  but
 the contraction  of the auricles is synchronous Avith the dilatation of the
 ventricles,  and vice versa.   The regularity of this alternation, however,
 is somewhat  disturbed,  when the irritability of the  heart is becoming
 exhausted ; and both sets of movements will  continue, when  the auricle
 and ventricle have been separated from one  another.  The regular suc-
 cession, in  the natural state, is doubtless in part due to the fact, that
 the transmission  of blood  from the auricle  into the ventricle,  by the
 contraction of the former, is the stimulus Avhich most effectually excites
 the latter to contraction; Avhilst the ventricle is contracting, the auricle,
 now free to dilate, is distended by the flow of blood from the veins that
 open into it; and this flow stimulates it to renewed contraction, just at the
 time when  the contraction of the ventricle has been  completed, and its
 state of relaxation enables it to receive the blood poured in through the
 orifice leading from the auricles.
   569. In  the living animal, the auricular and ventricular movements
 succeed one another with great regularity;  and, when  the  circulation
 is proceeding with vigor, scarcely any  appreciable  pause can be  dis-
 covered betAveen the different acts.   The  contraction or  systole of the
 Auricles takes place precisely at the same moment with the  dilatation or
 diastole of the Ventricles; and, as soon as the latter are full, and  the
 former are  empty, the diastole  of the Auricles and the  systole of the
 Ventricles,  immediately  succeed.  The systole  of  the Ventricles occa-
 sion the propulsion of blood into the  arterial system;  and  this action
 produces the pulse, as will be explained hereafter.   And it  also corre-
 sponds Avith the impulse or  stroke of the heart  against  the parietes of
 the chest.  This impulse is  not  produced, as some  have supposed, by
 the swinging of the entire heart forwards; but by the peculiar mode in
 Avhich the Ventricular  systole takes  place.  In the  contraction of its
 walls,  every dimension  is lessened;  but shortening is the most percep-
 tible change, the vertical diameter of the  Ventricle  being the greatest.
 Owing to the peculiar spiral disposition  of the fibres of the heart, its
 apex is not  simply draAvn upwards by their contraction,  but  it is made
 to describe  a spiral movement, from  right to  left, and from behind for-
 wards  ; and it is in this manner, that it is  caused to strike against  the
 side of the chest.
   570. The systole of the Ventricles is immediately followed by their
 diastole;  but the commencement of this has been observed to occur at
 a small interval previous to the contraction of the Auricles;  and some-
 times a brief interval of repose may be noticed, separating the first stage
 of the  Ventricular  diastole, which may be  partly due to the simple elas-
 ticity of the walls of the Ventricles, from the  second, which is accompa-
nied by the systole of the Auricles, and in which the blood of the latter
is forcibly propelled into them.   When the circulation is being  carried
on regularly, the blood is propelled into  the  Ventricles  with sufficient
force to dilate them strongly;  so that the  hand  closed upon  the heart
is opened with violence.   Even the auricles dilate with more  force than
it seems easy to account for  by the vis a tergo of  the blood in the venous
                                 21 
322
CIRCULATION OF NUTRITIVE  FLUID.
system; which is small compared with that which the fluid possesses in
the arteries.
  571.  The natural movements of the Heart  are  accompanied  by cer-
tain sounds, which are heard when the ear is  applied over the  cardiac
region; and an acquaintance with these  sounds and with their causes
is of much importance, since the alterations which they undergo in dis-
ease, afford us some of our most accurate  information in regard to the
nature of the morbid  affection.  Concurrently with the impulse of the
heart against  the chest, a  dull and  prolonged sound  is heard;  this,
which is termed the first  sound,  marks the ventricular systole, and is
synchronous Avith the pulsation  in the  arteries.   The second sound,
which  is short  and sharp, folloAVS immediately upon the conclusion of
the first; and it must therefore be produced  during the first stage of
the Ventricular diastole, before the  systole of the Auricles has  com-
menced.  It  is followed by a brief  interval of repose, which occurs
during  the remainder of  the  Ventricular  diastole and the Auricular
systole; and this is succeeded  by a recurrence of  the  first  sound.  If
the whole period between two successive pulsations be divided into four
parts, it is estimated that the first sound usually occupies two of these;
and the second sound, and the interval, one part each.
   572.  Now in order to  understand  the  causes of these sounds, it is
necessary to sfudy the course  of the blood through the heart  a  little
more  in detail.   When the Ventricles, distended  with blood, are con-
tracting upon their contents, they eject them forcibly through the narrow
orifice of the aorta and pulmonary artery ; and the semilunar valves,
which  guard these orifices, are thrown back against the  walls of the
arteries.  The regurgitation of the blood into the auricles is prevented
by the action of the mitral and tricuspid  valves ; but the flaps of  these
do not suddenly fall against each other, when the blood first begins to
press them together; being restrained by the chordae  tendinece.   The
connexion of these Avith the carnear columnar, which form part of the
ventricular walls, and contract simultaneously with them,  appears to
have this use,—that the flaps of the valves, which are completely thrown
back during the  preceding  rush of blood from the auricles to the ven-
tricles,  may be drawn into a favorable position  for the blood to get
behind them  and  bring  them  together, so as completely to close the
orifice.   As soon as the ventricular diastole begins to take place  (even
before the contraction of the auricles  has commenced), there will be a
tendency of the blood, that has just been propelled into the aorta and
pulmonary artery, to  flow back to the heart;  but this  regurgitation is
completely prevented by the semilunar valves of these orifices, which
are immediately filled-out by the backAvard tendency of the  blood, and
which meet in such a  manner  as  completely to close the orifices.   This
closure is much more  sudden than that of the mitral and tricuspid valves,
being altogether unrestrained.
   573. The first  sound is certainly in part due  to the impulse of the
heart against the thoracic parietes; as is  prOved by the fact, that when
the impulse is prevented, the  sound  is much diminished in intensity:
also by the circumstance, that, when  the ventricles  contract  with  vigor,
the greatest intensity of the sound is  over the point of percussion.   But 
SOUNDS OF THE HEART.
323
 that it is not entirely due to this cause, is also  sufficiently evident from
 tAvo circumstances;—its prolonged  character, which could scarcely be
 given by  a momentary impulse;—and its  continuance, though with
 diminished intensity, Avhen the parietes of the chest are wanting, and
 even after the complete  removal of the heart from the body.   Moreover,
 the duration of the first sound is  much increased by any morbid state
 of the orifices of the ventricles, which  obstructs  the exit of the blood.
 Much discussion has taken place as to the cause of that part of it, Avhich
 is not due to the  impulse;  some having attributed it  to the muscular
 contraction  of  the walls of the ventricles, others to the flow of blood
 over the irregular surfaces of  their interior, and  others  to the rush of
 the fluid through the narrow orifices leading to the aorta  and pulmonary
 artery.   There can be little doubt, that the first and last  of these causes
 are both concerned in producing the  sound.   For as a  sound may be
 distinctly heard by means  of  the  stethoscope, when the heart  is con-
 tracting vigorously out of the body, and Avhen no blood is  propelled by it,
 nothing else than muscular contraction can be then regarded as its source;
 and there is other evidence, that sound may  be produced by this cause,
 since the vigorous contraction  of any other large muscle gives rise to a
 continued tingling, which  may be  heard through  the stethoscope.  But
 when the heart is contracting  in its natural  position, and is propelling
 the blood with its ordinary vigor, the sound is heard in  its greatest in-
 tensity at  the base of the heart, i. e., at the origin of the  great arteries;
 and since any obstruction to the exit of the blood through  them increases
 the intensity as well as the length  of the  sound, it can  scarcely be
 doubted that it  is partly due to the rush of the blood through the con-
 tracted entrances of  these vessels.   A very similar sound, known as the
 "bruit de soufllet" or bellows-sound, may be heard through  the  stetho-
 scope, over any large artery, when it is compressed, so as to permit the
 passage of blood less readily than usual.   Thus the ordinary first sound
 maybe regarded as composite in its nature; being made up of the sound
 produced by the impulse of the heart against the  parietes of the  chest,
 of the muscular sound occasioned by the forcible contraction of the thick
 walls of the ventricles, and of the sound generated by the friction of the
 particles of blood against each other, and against the boundaries of the
 narrowing orifices which lead into the vessels.
   574. The  cause of the  second sound is simpler, and more easily un-
 derstood.  It is due to the sudden filling-out of the semilunar valves
 Avith blood, at the moment  when the ventricular systole has ceased, and
 Avhen the commencing diastole  produces a tendency to the regurgitation
 of blood from the aorta  and pulmonary artery.  The sudden passage of
 the valves, from a state of  complete relaxation to one of complete ten-
 sion, occasions a sort of  click ;  A\Thich is the second sound of the heart.
 That this is the real  cause, has noAv been  fully demonstrated.  If one
 of the valves be hooked  back  against  the side of  the artery, by the in-
 troduction  of  a curved needle, so that a reflux of blood is  permitted, the
 sound is entirely suppressed.   And if the complete closure of the valves
be preArented by disease, so that their tension  is diminished,  and a cer-
tain amount of regurgitation takes  place, the second sound is no longer
heard in its proper intensity: Avhilst, on the other hand, a sound  analo- 
324              CIRCULATION OF NUTRITIVE FLUID.
gous to the first, and sometimes prolonged over the whole interval of
repose, indicates the reflux of the blood into the ventricles.  When the
semilunar valves are thickened by a morbid deposit, their surface rough-
ened, and their opening narrowed, the first sound becomes harsher and
sharper; and the second sound acquires the same character,—the back-
ward as  well as  the forward flow  of the blood being  affected by this
cause.
   575. The  natural movements of  the  mitral  and tricuspid valves,
appear to be accomplished with perfect  freedom from  sound; for the
size of the orifices which they guard  prevents any  considerable friction
of the blood, in its flow from one cavity to the other ; and their closure
when  the ventricular systole begins, does not  take  place Avith the
rapidity and suddenness of that of  the  semilunar valves.  But when
their  structure is changed  by disease, their  action  is not so  noiseless;
and they give rise to various morbid  sounds, Avhich are heard in  addi-
tion  to the ordinary sounds, and wThich may even obscure them  alto-
gether.  In the same manner, the ordinary movements of the  heart do
not produce any audible friction-sound, between  the two surfaces of
the pericardium, that which covers the heart, and that Avhich  lines the
pericardial sac.  These surfaces are kept moist, in health, by the serous
fluid constantly exhaling from  them; and they are extremely smooth,
so that they glide  over  one another  noiselessly.   But  if  they become
dry,  as in  the  first  stage  of inflammation, a  slight creaking is heard,
accompanying both the ordinary sounds  of the  heart, and  somewhat
resembling the rustling  of paper.   And  if they are roughened by the
deposit of inflammatory exudations, this "to and fro" sound becomes of
a harsher character.
  576. The walls of the  left ventricle are  considerably thicker  than
those of the right; and the  contractile power is greater.   This difference
is obviously required, by the difference in length between  the  systemic
and the pulmonary vessels ; the amount of force* necessary to drive the
blood through  the latter, being  far  inferior to that which is  requisite
to propel  it  through the former.  The average thickness of the  walls
of the left Ventricle  is about 4J  lines ; being  somewhat  greater  than
this  at the middle  of  the heart, and less at  its  apex.   The average
thickness of the  walls of  the right ventricle is not more than 1J  line  ;
being a little greater than this at the base, and less at the apex of the
heart.  The left  auricle is  somewhat thicker than the right.   The
capacities of all the four cavities  are nearly equal; each of  them, in
the full-sized heart, holding about tAvo ounces, of fluid.  The Ventricles
are, perhaps, a little larger than their respective Auricles ; but there is
no very  positive  difference in  capacity, between  the Ventricles and
Auricles of the two sides.
   577. The  quantity of blood which is propelled  at  each Ventricular
systole, cannot, therefore, exceed two ounces ;  and it is probably some-
Avhat less, as the ventricles do not seem to empty themselves  completely
at each contraction.   Now the whole quantity  of the blood seems  to be
about one-fifth  of  the entire weight of the body; so that it will amount
to about 28 lbs. in an individual of 140 lbs. weight.  Allowing 75 pul-
sations to a minute, 150 oz. (or 9 lbs. 6 oz.) of blood would pass through 
CIRCUMSTANCES  AFFECTING  RATE OF  PULSE.          325
  each ventricle of the heart in that time; consequently  nearly  three
  minutes would  be required for the passage of the entire mass of the
  blood through the whole circle of its movement, if  its rate be entirely
  determined by the impulses it receives from this central organ.   But it
  appears, from various experiments, that the rate of circulation is much
  more rapid than this.   For if a solution of any salt, easily detectible in
  the blood be injected into one of the large veins near  the heart, it may
  be traced  in  the arterial circulation in from  15 to  20 seconds  after-
  wards ; during  which interval it must  have  traversed the whole pul-
  monary system  of vessels,  and passed through both sides of the heart.
  And  if the salt be one, which acts poAverfully  on the heart itself,—as is
  the case with Nitrate of Baryta or Nitrate  of Potass,—this action is
  manifested almost at the same moment with the appearance of the salt
  in the arteries of other parts; thus showing that it  has been  conveyed
  by the coronary arteries into  the capillaries of the  heart itself.  The
  period  required  for the transmission of a saline substance  from the
  veins of the upper part of the body to  those of the  lower,—which can
  scarcely be accomplished through  any  more  direct channel  than the
  current that  returns to the heart, then passes through the lungs back
  to the  heart  again, and  then  flows through  the systemic arteries and
  capillaries to the veins,—is accomplished in little more than 20 seconds,
 even  in  an animal  so  large as a Horse.   It appears, then, that even
 the vigorous  and constant  action of the Heart is  not alone sufficient
 to maintain  the circulation at  its ordinary  rate; and  we  are not
 justified, therefore,  in  excluding those  sources  of  movement in the
 higher animals, which evidently exert so important an  influence in the
 loAver.
   578.  The force with which the heart propels  the  blood  is such, that
 if a vertical pipe  be inserted into the Carotid artery of  a horse the
 blood> will  sometimes rise  in  it  to  a height  of 10  feet.   From  com-
 parative  experiments upon  other animals, it  has  been estimated that
 the vigorous  action of the  heart in Man  would sustain  a column of
 blood  in his  aorta  about 7£ feet high; or, in  other  words,  that the
 force Avith which the heart  ordinarily  propels the blood  through the
 aorta, is equal to that  which would be  generated by  the  weight  of a
 column  of blood  of the same  size,  and  7* feet high; which weight
 would be about 4J lbs.   But the  force which  must  be exerted by the
 heart  to sustain such a column, may be shown,  upon physical principles,
 to be  as much  greater than  this,  as  the area  of a plane passing
 through the base and apex of the left ventricle is  greater than  that
 of the transverse section of the  aorta ;  and as  the proportion  of  these
 areae is about 3 : 1, the real  force of the  heart may  be stated  at about
 13 lbs.
 _ 579. The number  of contractions of the heart, in a given  time,  is
 liable  to  great variations within the limits of  health,  from several
 causes ;  the chief of which  are diversities  of Age  and Sex, amount
of Muscular exertion,  the  condition  of  the Mind,  the  state of the
Digestive system,  and the period of  the  Day.  The  following  are the
points  of greatest importance, in regard  to the action  of these several
influences. 
326
CIRCULATION OF NUTRITIVE FLUID.
  Age.—The pulse of the newly-born infant averages from 130 to 140
per minute ; and this rate gradually diminishes, until, in adult age, the
pulse averages from 70 to 80; and  in the decline of life from 50 to 65.
  Sex.—The pulse of the adult female exceeds that  of the adult male
in frequency, by about 10 or 12 beats in  a minute; and it is also more
liable to  disturbance from other causes.
  Muscular Exertion.—The effect  of this  in accelerating the pulse  is
well known; but as  the amount of  change depends upon the degree  of
exertion, no general statement can  be made on the subject.   The con-
tinued influence of a moderate degree of muscular exertion, is shown by
the effect of posture upon the pulse.   Thus the pulse is on the average
from 7 to 10 beats faster (per minute) in the standing than in the sitting
posture;  and 4 or  5 beats faster in the sitting than  in the  recumbent
posture.  This  amount  of variation is  temporarily increased by the
muscular effort required for the change of posture; but this  soon sub-
sides into the continued rate, which the permanent maintenance  of the
new posture involves.  There are certain states of the system, in which
the heart's  action is increased  to  a most  violent degree, by a simple
change of posture;  and in which,  therefore, it  is necessary that even
this slight movement should be made with gentleness and caution.
  Mental Condition.—The action of the  heart is peculiarly influenced,
as every  one is aware, by  the excitement of the emotions.   This is  a
fact to Avhich, however familiar, the  medical  practitioner should con-
stantly direct his attention.  The trifling agitation  occasioned by the
entrance  of  the medical man will produce,  in many patients, such an
acceleration of the pulse, as would be very alarming, if its true cause
were not  known.   And the real rate of the pulse cannot be ascertained,
until time  has  been  permitted  for the agitation to subside; which is
favored,  also, by the influence of a gentle manner  and tranquillizing
conversation.  The operation of the intellectual powers does  not seem
to affect  the rate of  the heart's  movement in any other way, than by
inducing  a general state of feverishness, if it be too long or too ener-
getically  kept up.
  State of the Digestive System.—The pulse is quickened during the
digestion of a meal; but no exact numeral  statement can be made on
this subject.
  Period of the Day.—The frequency of the pulse appears to be some-
what greater in  the  morning than  it is in the evening; and the tem-
porary action of any of the preceding causes, more quickly subsides  in
the evening than in the morning.
  580. The movements  of the  heart have rjeen supposed to depend
upon a constant  supply of nervous influence, generated by the cerebro-
spinal system, and transmitted  through the  sympathetic nerve, the
branches of which  are copiously  distributed  to it.  And  this idea
seemed to  derive  support from  the fact,  that, when the brain and
spinal cord are removed, or when large portions of  them are suddenly
destroyed, by crushing or by the breaking-up of their substance in any
other mode,  the  movements of the  heart  are  arrested.  But  it has
been shown that the brain and  spinal cord may be gradually removed,
without any such consequence; and  the occasional production of foetuses 
              EQUALIZATION  OF FLOAV IN THE ARTERIES.          327

destitute of those centres, but  possessing a regularly-pulsating heart,
is another proof that .the movements of this organ do not depend upon
a  supply  of  nervous influence  derived  from  them.  Still  they are
capable of  being influenced  by impressions transmitted through the
nerves.  It has been ascertained by Valentin, that, after the heart has
ceased to beat, its  contractions may be  re-excited by stimulating the
roots of the  Spinal Accessory nerve, or  of the  first  four Cervical
nerves; the influence of that stimulation being  conveyed to the heart
by the Sympathetic system, the cardiac portion of which communicates
with these nerves.  Irritation of the Par Vagum, also, has a tendency
to accelerate  the  heart's  action, or to re-excite  it when it has ceased;
but the complete severance  of both its trunks produces little disturb-
ance  in the regularity of  the movement.  The action of the heart may
be also affected  more  directly through the Sympathetic system; thus
it is excited by irritation  of the cervical ganglia,  especially the  first;
whilst continued  pressure  upon  the cardiac nerve, by an enlarged
bronchial gland, has appeared to be the cause of its occasional suspen-
sion.   It is without doubt through its nervous connexions, and probably
through the sympathetic system, that the heart receives the influence of
mental emotions.
   581.  The movements of the  heart may be suspended, or altogether
checked, by sudden  and violent  impressions on the  nervous centres,
even though these do not occasion any perceptible breach of substance.
Thus  in concussion of the brain, there  is not merely insensibility, but
also a complete suspension of the circulation, occasioned by a failure  of
the heart's  power.  This suspension may be permanent, so that ani-
mation  cannot be restored;  or it may be  temporary, as in ordinary
fainting.  The well-knoAvn influence  of blows upon the epigastrium,  in
producing sudden death, is probably to be attributed to a similar cause,
—namely, the shock thus  communicated to the extensive plexus  of gan-
glionic nerves, radiating from the semilunar ganglia, and proceeding  to
the abdominal vriscera.  Violent impressions upon other nervous expan-
sions  may  produce  a dangerous weakening of  the heart's  contractile
power;  this is the case, for example, with extensive burns, which may
produce faintness, and even death, especially in children, by the depres-
sion Avhich  they induce.  Many other causes of sudden  suspension  of
the heart's  action might  be  enumerated; but they may be  generally
traced to a strong impression upon the nervous system;  though of the
mode in which this operates Ave know nothing.

                4. Movement  of the Blood in the Arteries.

   582.  The Blood, thus propelled from the Heart into the Arteries by
a series of interrupted jets, Avould continue to flow in the same manner,
if it were not for  the equalization of its movement, effected by the pro-
perties of the  arterial walls.   This influence is exerted by the middle or
fibrous coat, which consists in part of yellow  elastic tissue (§ 189), and
in part of non-striated muscular fibre (§ 337).   The proportion of  these
two components  varies in arteries of different calibre;  the muscular 
328
CIRCULATION OF NUTRITIVE FLUID.
tissue being thicker in the smaller branches, and the elastic tissue being
found in larger amount in the main trunks.
   583.  It is chiefly to the simple physical property of Elasticity, thus
possessed by the Arterial  tubes, that Ave owe the equalization  of  the
flow of blood; and we may hence understand the reason Avhy the trunks
that; are  in nearest connexion  with the heart,  should be those most
endowed with it.   If a forcing-pump were to inject water, by successive
strokes, into a system of tubes with perfectly unyielding  walls, the flow
of fluid at the further  extremities  of these  tubes would be  as much
interrupted as its entrance into them.   But  if the pump be connected
with an air-vessel (as in the common fire engine), so that a part of the
force of each stroke is expended in compressing the air, the expansion
of this, during the  interval betAveen the  successive strokes, produces a
continuous flow of water along the tubes.   Or if the  tubes  themselves
were  endowed with a certain degree of  elasticity, which should allow
them to  dilate  near their  commencement,  so  as to  receive the new
charge  of fluid, and Avhich should occasion a continued  pressure upon
the fluid  during the interval of the stroke,  the same equalizing effect
Avould be  produced.  This  is precisely the case with the arterial system;
the intermittent jets, by which the blood is propelled from the heart,
are speedily converted into a continued stream ; so that, at even a mode-
rate distance from the heart, the only indication of its interrupted action
is presented  by the greater or  less rapidity of the flow;  and this gives
rise, when an artery is divided, to an alternate rise and fall of the jet
of blood, and, in the ordinary circulation, to the phenomenon called the
pulse.   This is due to an increase in the dimension of the arterial tube,
both in  length and breadth, with each additional ingress of  blood;  the
increase in length is the more considerable of the two effects, and causes
the artery to be somewhat  lifted from its  seat.  During the intervals, a
quantity  of  blood  corresponding  to that which  had  entered, escapes
by the further extremity of the tube; and thus the artery  is enabled
to contract to  its previous dimensions, and  to return to its  bed.   We
may compare the pulse, therefore, to a wave, which commences  in  the
heart, and travels onwards through the  arterial system.
   584.  In the  large arteries near the heart, the pulsation is  always
precisely  synchronous with the ventricular systole;  but it takes place
somewhat later in the arteries at a distance from the  heart;  the  time
required for the transmission  of  the wave being  proportioned  to  the
degree in which the walls of the arteries yield to it.   If they were quite
rigid, the egress at one extremity must take place at the precise moment,
that the fluid is forced into the other.   On  the other hand, if the walls
be too easily distensible, they yield to  the propelling force in  such a
degree that it is entirely  expended upon them; and  the fluid  is  not
moved onward  at all or but very sloAvly.  In the healthy state of the
arterial walls, they  should  contract  upon their contents  Avith sufficient
force to equalize the flow of blood,  and to prevent the pulse-Avave from
occupying more than one-sixth  or one-seventh of a second, in its propa-
gation to  the remotest arteries  of the system; and the pulse should be
full, producing a prolonged but gentle elevation beneath the finger, and
capable of resisting moderate  pressure.   This condition  is dependent 
               EFFECTS  OF MUSCULARITY OF ARTERIES.            329

in great part upon the due tonicity of the muscular coat of the arteries
(§ 365).  When this tonicity is in excess, the  walls of the arteries are
too rigid ; the pulse at the wrist is  felt to occur exactly at the same
time  Avith the ventricular systole; and its character is that of strength,
incom possibility, and sustained poAver, though it may be even sloAver
than  usual.  This is the case  in Avhat is commonly termed " high corfdi-
tion" of the system ; which predisposes to inflammatory disorders, but
which renders it less susceptible than usual to the influence of mala-
ria, contagious miasmata, or other causes of a  depressing  character.
On the other hand, when the  tonicity of the arteries  is less than it
should be, their walls  yield too much to the pulse-wave ;  so that  the
pulse at the wrist is often felt even after the second sound is heard ;  and
the pulse itself is jerking,  unsteady, and too easily compressible.   This
loose relaxed state  of  the  vessels is  the  most unfavorable that can be
to regularity  and vigor  of  the circulation ;  and it manifests its  ill
effects in the general condition of the  system,  which is then peculiarly
prone to suffer from the  agency of malaria,  infectious miasmata, or
any other depressing causes.
   585. Although many  Physiologists have denied that the Arteries
possess real Muscular contractility in any degree,  yet there can  be  no
longer any  doubt on the subject; since numerous experimenters have
succeeded in producing distinct contraction in  their walls, by the appli-
cation of those stimuli, Avhich act upon muscular fibre in general.   More-
over it has  been ascertained, that when an artery is dilated by the blood
injected into  it  from the heart, it  reacts  with a force superior to the
impulse to Avhich it yielded ; and that,  if a portion of an artery from an
animal recently dead, in which  the vital properties are still  preserved,
and a similar  portion from an animal that has been dead some days, in
Avhich nothing but the elasticity remains, be distended Avith equal force,
the former  contracts to a much greater degree than the latter after the
distending force is  withdraAvn.—One use of this contractile poAver may
very  probably be, to assist the Heart in maintaining the flow of blood;
for if the arterial walls yield rapidly to the ingress of blood, and then,
contract upon  their contents with a force greater than that which  dis-
tended them,  the  current must necessarily be  propelled  onwards with
greater force.  This supplementary propelling force, on the part of the
arteries, may  serve  as a compensation to that diminution of the heart's
power which must result from the increased friction of the blood against
the Avails of the vessels  occasioned by their subdivision ; and we thus
observe even in the highest animals, some traces of that diffused agency
on Avhich the Circulation is so much more dependent in the loAver tribes..
   586. It seems  probable, however,  that  one chief use of the Muscu-_
larity oT The Arterial walls,  consists in its regulation of lhe_diameter
ofTnVfubes, In liceoToTance with the quantity ofMoj>pMo_l^ conducted
through^henTTcT any party the proper ainouTiFT^emg determined by
c^uTnltancesattlTeTrrire:—Such local changes may form a part  of the
regular series  of actions of the  human  body, as Avhen the  Uterine and
Mammary  arteries  undergo  enlargement, at the periods  of pregnancy
and parturition ; and they  occur still more frequently in diseases,  which
are attended by increased  action of  particular organs.  In such  cases, 
330              CIRCULATION OF NUTRITIVE FLUID.

it cannot be vis a tergo of the Heart, that occasions the enlargement of
certain arterial trunks, and of no others; since any increase in its pro-
pulsive power would affect  all  alike.  It must  be, therefore, through
a power inherent in themselves, that the dilatation takes place ; and
there seems much reason for attributing to  the  Sympathetic system of
nerves a control 0Arer this poAver, and consequently the  office  of regu-
lating the  local distribution of blood, in accordance Avith the Avants of
the different parts.   It is well known that the nerves of this system are
copiously distributed upon the arterial Avails ;  and it  has been experi-
mentally shown, that they have the  power of producing contractions in
the larger  arteries.   Moreover, there is  every  reason to believe that
the diameter of the  Capillary blood-vessels, and  the rate of the  move-
ment of the blood through them  is  much  influenced  by these nerves
(§ 603); and it seems highly probable, therefore, that they should have
a corresponding influence upon the size  of the trunks, from Avhich these
capillaries  are derived.
  587. The arterial system  possesses nearly the  same relative  capacity
in every part;  that  is, if a section could be made  through  all the sys-
temic arteries  at a  certain distance from  the  heart, the united areas
would be found equal to that of the  aorta ;  and  those of the branches
of the pulmonary arteries Avould equal those of their trunk.  This re-
sults  from  the fact, that, at every subdivision, the united areas of the
branches are almost precisely equal to  that of the trunks from which
they proceed ;  although the united diameters of the former far exceed
that of the latter.   According  to a well-known mathematical laAv, the
areas of circles are  as the  squares of the  diameters ;  consequently, in
making such comparisons, it is necessary to square the diameters of the
trunk and those of the branches, and to contrast the  former with the
sum of the latter.  Thus a trunk Avhose  diameter is 7, may subdivide
into two branches, each having a diameter  of nearly 5 ; for the square
of 7 is 49, and twice the square of 5 is 50.   Or a trunk whose diameter
is 17 may subdivide  into  three  branches whose  diameters are 10, 10,
and 9 J (making 29J as the sum of the diameters); for the square of the
diameter of the trunk is 289, wdiile  the sum of the squares ef those of
the branches is 290J.  It appears,  hoAvever, from Mr.  Paget's recent
admeasurements,  that there is seldom  an exact equality betAveen the
area of the trunk and that of  its branches;  the area sometimes increas-
ing, and sometimes diminishing.  The  former seems  the general rule
in the upper extremities; the latter in the loAver.   Thus the area of the
trunk of the external carotid is to that of its branches, as 100 to 119;
Avhilst the  area of the abdominal aorta, just before its  final  division,
is to that of its branches as 100  to 89.
  588. In  almost every part of their course,  the ramifications of the
arteries communicate freely with each other, by  anastomosis ;  and this
communication is most important, as affording the  means by which the
circulation is sustained, Avhen the current through the main trunk is ob-
structed.   There is  scarcely an artery in  the body except the aorta,
which may not be tied, with the certainty that the blood will still be
conducted  to  its destination,  by the collateral circulation.   At first,
the quantity which thus passes is very insignificant, and is by no means 
ARRANGEMENT  OF CAPILLARY VESSELS.
331
sufficient to supply what is needed ; thus, when the femoral artery has
been tied for popliteal aneurism, the limb becomes cold, and the sensi-
bility of the surface and its muscular power are alike diminished.   In a
few hours,  hoAvever, its warmth returns, and its sensibility and muscular
poAver are restored ; indicating that its circulation  has been already re-
established through the collateral branches.   And Avhere an opportunity
presents itself at a subsequent period for examining  the state  of the
vessels in such a limb, it is found that  an  extraordinary  enlargement
has taken  place in arteries that were previously of  insignificant size,
which form a communication betAveen the branches that issue above and
below the interruption.  Moreover,  it is commonly found that the main
trunk has  become completely impervious above the  part where  it Avas
obliterated by the ligature, up to the point at which the nearest  lateral
branch  is given off.—Even the abdominal aorta has  been tied in dogs,
without fatal results ; the circulation in the posterior part of the body,
and in  the hinder extremities, being then maintained chiefly by the
inosculation of the external mammary artery with the epigastric, upon
the parietes of the abdomen.

                 5. Movement of Blood in the Capillaries.

  589.  The  ultimate  ramifications  of  the arteries pass so  insensibly
into those  of the Veins, that  no  definite line of demarcation between
them can be draAvn;  and although we are in the  habit of speaking of
the " Capillaries" as  a distinct systeni of vessels, yet it ought  to be
strictly borne  in  mind, that they differ  only in size  from the vessels
from which they receive their blood on  the one  side, and into  which
they pour it on the other.   It was at one  time supposed, that they Avere
merely channels or passages, excavated in the tissues, having no definite
walls of their OAvn.   This is probably true of them in  the  lower  tribes
of  Animals; and  it  may also be the  case  at an early stage of their
development in the  higher.  But Avhen their formation is complete,
they undoubtedly  possess walls of a fibrous texture, as distinct as those
of the arteries  and veins, though of extreme thinness.  From the occa-
sional appearance of  bodies resembling cell-nuclei, in the substance  of
the Avails of the capillaries, it  has been  thought  that their  tubes  are
formed, in  the first instance, by the coalescence of cells arranged in a
linear direction ; and this idea receives confirmation from the fact, that
the ducts of Plants are undoubtedly formed in  this manner, and not by
the mere retirement of the tissues on either side, leaving an intervening
channel.   The closely-reticulated  structure usually formed by the capil-
laries, has  been commonly regarded  as distinguishing them  both from
the arteries and the veins; and it is not unusual to  speak of the arteries
as delivering the blood into the " capillary netAvork," and  of the veins
as receiving the fluid that has traversed this.  Such expressions  are
not incorrect as implying the simple fact, that betAveen the  arteries and
the veins is a netAvork of minute  vessels, through Avhich  the  blood has
to travel Avhen proceeding from  one to  the other; but these vessels
must  not be regarded as belonging to a distinct  class,  being nothing 
332
CIRCULATION OF  NUTRITIVE FLUID.
else  than the minutest  subdivisions of the  veins  and  arteries, which
commonly inosculate freely with each other.
  590.  The degree of this inosculation, and the consequent form of the
capillary netAvork, are subject, however, to very great variations ; and
these may be generally shown to have a  relation to the form of the
ultimate elements of the tissues, Avhich are traversed by  the capillaries.
Thus we have seen in the capillaries of Muscle, that the  major part run
parallel to the course of the fibres, lying in the minute interspaces be-
tAveen  them  (Fig. 67); a few transverse branches  serving to connect
them with each other.   A  similar distribution prevails in the capillaries
of the Nervous trunks ; but those of the Nervous centres are arranged
in the  form  of a minute network, so  as  completely  to  traverse every
part of the structure (Fig. 93).  Again, we observe that the capillaries
of Glands form a minute network around the secreting  follicle  (Fig.
94); and a similar  arrangement prevails  in  the capillaries of the air-
cells of the lungs, which are set so closely together, that it would  seem
Fig. 93.                            Fig. 94.
as if the purpose were to cover the surface with blood as completely as
possible, consistently with  its being  retained Avithin vessels, and not
spread out into a continuous film (Fig. 106).   A network of very much
the same  character  is found in the villi of the mucous membrane (Fig.
Fig. 95.                            Fig. 96.
,   Capillary Network in simple mucous mem-
      brane of palpebral conjunctiva.
82), on the ordinary surface of simple mucous membrane (Fig. 95), and
on that of the choroid coat of the eye (Fig. 96).  Where the surface of
the mucous membrane is depressed into follicles, the arrangement of the 
DISTRIBUTION OF THE CAPILLARIES.
333
capillaries has  an evident reference  to  these (Fig. 97) ;  whilst, on the
other hand,  Avhere  the  surface  of the skin is raised up  into sensory
papillse,  the capillary network  sends  looped prolongations into them
which are found accompanying their  nerves (Figs.  98 and 99).
  591.  It cannot be supposed  that the arrangement  of the vessels has
any further influence upon the function of the  part they traverse, than
Fig. 97.                            Fig. 98.
that which it derives from the regulation of the supply of blood afforded
to each indiAridual  portion of the structure.  The form of the capillary

                               Fig. 99.
                 Capillary Network of fungiform papilla of the tongue.

network is evidently determined by that of the elements of the  tissues
permeated by it; these are the real operative instruments in every part;
and  the  distribution  of the blood-vessels  is  so arranged, as to afford
them precisely the amount of nourishment they respectively require.
Thus Ave have seen, that there are many living parts, possessing most
important functions, in the human body, which  are not in any direct
relation Avith blood-vessels, and which yet derive their whole nutriment,
and  the materials of their functional  operations, from the blood.  This
is  the case, for example, Avith the  whole of the epithelial and epidermic
cells; and also with  the articular cartilages, and the substance of the
teeth. Even in bone, the islets  between the  Haversian canals, Avhich
are completely unpenetrated by vessels, are of considerable size.   Such
islets must everywhere exist, between the meshes of  the capillary net-
Avork ; so that the question of the vascularity or non-vascularity of a
tissue is  one of degree only;—the ultimate fibre of  muscle or nerve,
and the cells and fibres of other tissues, being as completely non-vascular,
as the entire substance of a tooth  or of an articular cartilage ; the latter
being nourished, like  the former, by  imbibition from the  surrounding
vessels.
   592. The  term " Capillary" may be employed  in an  extended or a 
334
CIRCULATION OF NUTRITIVE  FLUID.
restricted sense;  in the former it includes all the minute vessels which
pass betAveen the arteries and the veins;  in the latter it is applied only
to those which admit no more than a single file of blood-discs at once,
and  excludes  those Avhich  admit two, three, or  even four rows, even
although  they establish a  direct communication from one side of the
network to the other.   The former application of the term is the most
convenient, although perhaps not the most strictly accurate;  and it will
be therefore here employed in its extended sense.  And this is rendered
more correct by the fact, that the  size of the individual capillaries  is
by no means permanent; an enlargement often taking place in one, and
a contraction in another, at the same time:  so that vessels, Avhich were
previously true capillaries, no longer remain such; and passages, which
were previously  of far  greater  calibre,  are  reduced to the average
diameter.
  593.  The opinion was long entertained, that there are vessels adapted
to the supply of the Avhite  or  colorless tissues ; carrying from  the
arteries only the fluid  portion of  the blood, or  liquor sanguinis, and
leaving the rest  behind.   No other  such vessels have been really ob-
served, however,  than the capillaries  in a state of unusual contraction,
as just now mentioned.   And it may be safely affirmed, that  the suppo-
sition of  their existence is not required.  For  any  one who observes
the smallest capillary vessels under  the microscope,  may perceive,  that
the current of blood Avhich passes  through them is almost free from
color,—as the red corpuscles themselves appear to be, when  spread
out in a single layer.  Tissues which are  rather  scantily  permeated by
such  vessels, therefore, may still be white; and it  is only  where the
netAvork is very close, and the quantity of blood Avhich  passes through
it is consequently great, that a perceptible color will  be  communicated
by the red corpuscles.  And we  have seen, that  the idea that Nutrition
can only be carried on by direct  communication with vessels, is entirely
unfounded; the tissues  into Avhich no bloodvessels can be traced, being
adapted to nourish  themselves, like cellular Plants, by the imbibition of
fluid at their surfaces, on Avhich vessels are (for the most part) copiously
distributed.
  594. That the  blood can only minister  to the operations of Nutrition,
Secretion, &c, whilst it  is circulating through the Capillaries, is eAddent
from several considerations.   The thickness  of the walls  of the  larger
vessels interposes an  effectual barrier to  its  transudation ; and so com-
pletely  is  the blood cut  off even from penetrating these, that they do
not derive their own  nourishment from the blood which Aoavs through
their tubes, but from  a capillary netAvork  in  their own substance, which
is supplied by vessels from collateral branches,—these being termed the
vasa vasorum.  MoreoA'er it is by  the inosculation of  the  capillaries
alone, that the minute  network  is formed, Avhich serves to bring the
blood into proximity with the minute parts of the tissues to be nourished;
thus let it be supposed that the  minute arteries of Muscle were to ter-
minate in  veins, without undergoing  further subdivision, the islets left
betAveen their anastomosing branches would be far too large, and the
nutritive materials  Avould consequently not  be supplied  with sufficient
readiness,  eA'en supposing  that  it  could  freely permeate the Avails of 
VARYING SIZE OF THE  CAPILLARIES.
335
these vessels.—The capillaries, then, must not be regarded as altogether
distinct in their  endowments, from the vessels with which they are con-
nected on  either side;  but merely  as  intended, by their  minute sub-
division and inosculation, to bring the blood into sufficiently close rela-
tion with the tissues they are to nourish, and to allow a greater degree
of transudation  of its elements by  the comparative thinness of their
Avails.
  595.  When the Aoav  of blood through the Capillaries of a  trans-
parent part,  such as the web of a Frog's  foot, is  observed  with  the
microscope, it appears at first to take place with great  evenness  and
regularity.   The influence of the contractions of the heart may be seen
to  extend  itself into the smaller arteries ; the blood moving onwards
in  them with  a somewhat jerking motion.  But  this influence alto-
gether disappears in  the capillary network; the flow of blood through
this being  even and  continuous, except when the action of the heart is
becoming Aveak  and  irregular,  or   when its influence is impeded   by
obstruction in the vessels leading to  the part,—the blood being then
 impelled by a succession  of jerks,  with intervals of complete repose.
But on watching the movement for some time, various changes may
be  observed, which cannot be  attributed to the heart's  influence,  and
Avhich  shoAV  that  a certain regulating or distributive power  exists in
the Avails of the capillaries, or in the tissues which they traverse.   Not
only do  Ave occasionally perceive some of the tubes  enlarging, so as to
admit  several files of blood-discs instead  of one,  whilst others  that
previously  received  several now only  admit one;—but we also  see
vessels  coming  into  vieAV, which were not previously  noticed,  whilst
other vessels seem to become obliterated.   This apparently new  forma-
tion and obliteration  of vessels, hoAvever,  does not really take  place;
for a "more close examination shows, that  the former of  these appear-
ances  is due to the entrance  of red  corpuscles  into passages  which
existed before, but which were in such  a state of contraction as enabled
them only to admit  the  fluid portion  of the blood; whilst, by  a  con-
verse change in certain capillaries,  from  the dilated to the contracted
state, the appearance of obliteration  is produced,  the  red  corpuscles
being  excluded,  and the transparent fluid of the blood being alone
transmitted by them.
   596. But  these are by no  means  all the irregularities which  may
be  detected by  a close  scrutiny  of the Capillary circulation.  The
velocity of the  current is liable to great and sudden variations, which
 cannot be  accounted  for by any change in the heart's action, or in the
 supply of blood afforded by the arteries; and this  change may manifest
itself,  either in the  Avhole capillary network of a part, or in a portion
 of  it;  the  circulation taking  place  with  diminished rapidity  in one
part, and  with  increased  energy in another, though both are supplied
by the same trunk.  These variations  are sometimes manifested by the
 complete change in the  direction  of  the  movement, in  certain of the
transverse or communicating branches ; this movement taking place, of
 course, from the stronger toAvards the weaker current.  Not unfrequently
 an   entire  stagnation,  of longer  or shorter duration,  precedes the
reversal of the  direction.  Irregularities of this kind are most frequent 
336
CIRCULATION OF NUTRITIVE  FLUID.
when the heart's  action is enfeebled or partially interrupted ; and it
Avould thus appear, that the local influences by Avhich they are produced
are overcome by the propelling  power of the central organ, when this
is acting Avith its full vigor.  When the Avhole current has nearly  stag-
nated,  and a fresh impulse from  the  heart renews it, the movement is
seldom uniform through the entire plexus supplied by one trunk ; but is
much greater in some of the tubes than in others,—the  variation being
in no degree connected with their size, and being very different  in its
amount at short intervals.
  597.  All these  circumstances  indicate that the movement of  blood
through the capillaries is very much influenced by local forces ; although
these forces are  not  sufficiently poAverful, in the  higher animals,  to
maintain it alone.  And  from other facts  it appears, that the condi-
tions necessary  for  the energetic Aoav  of blood  through these vessels,
are nothing else than the active performance of  the nutritive and other
operations, to Avhich they are subservient.  The examination of a single
one of these processes, Avill afford  us the requisite proof.  The  blood
when circulating through the systemic capillaries, yields a portion of its
oxygen to  the  tissues it permeates,  and receives from  them carbonic
acid.   On  the other hand, when passing through the pulmonary capil-
laries, it gives  up its  carbonic acid  to  the  atmosphere, and imbibes a
fresh supply of oxygen.   Noav if either of these changes be prevented
from taking  place, a retardation  and even a complete stagnation of the
blood  will  take  place,—the  flow  through  the  capillaries being noAV
resisted, instead of accelerated, by the relation which the blood bears to
the tissues.  Thus it  has  been shoAvn, that  if an animal be  partially
deprived of oxygen, so that the arterial blood is not duly aerated (rather
resembling  the ordinary   venous blood), and cannot exert its proper
action  on  the  tissues, the pressure upon the walls  of the  systemic
arteries  is increased,  although the  supply of blood propelled by the
heart, and  the propulsive poAver of the heart itself are diminished; and
this plainly indicates a retardation in the systemic capillaries, producing
an undue accumulation in  the  arteries.   On the other hand, the sus-
pension of the supply of oxygen to the lungs, either by an obstruction
in the air passages, or by the substitution of some other gas, brings the
pulmonary circulation  to a stand in  a very  short  time, the blood not
being able  to undergo its usual  changes in the capillaries of those organs;
and  by this stagnation, the whole movement of blood is speedily checked.
The readmission of oxygen, if the suspension of the circulation  have
not been too long  continued, occasions the renewal of the movement in
the capillaries, and thence  in the Avhole circle of A'essels; and this  even
after the heart has ceased to propel blood towards the lungs.
  598.  The principles already noticed  (§ 547), as put forth  by Prof.
Draper, seem fully adequate to explain these phenomena.   The arterial
blood,—containing oxygen Avith which it is  ready to part, and being
prepared to receive  in exchange  the  carbonic acid which the tissues set
free,—must obviously have a greater affinity for the tissues, than venous
blood, in Avhich  both these  changes have been already effected.   Conse-
quently, upon mere physical principles, the arterial blood, which enters
the systemic capillaries on  one side, must drive  before  it, and expel  on 
MOVEMENTS OF BLOOD IN  CAPILLARIES.
337
the other Bide of the network,  the blood  which has  become venous
whilst traversing it.   But if the blood which enters the capillaries have
no such affinity, no such motor power can be developed.  On the  other
hand, in the capillaries of the lungs, the opposite affinities prevail.  The
venous blood and the air in the pulmonary cells have a mutual attrac-
tion, which is satisfied  by the exchange  of oxygen and carbonic acid
that takes place through  the walls of the  capillaries;  and when the
blood has become arterialized, it no longer has  any attraction for  the
air.   Upon the very same  principle, therefore,  the  venous blood  will
drive the arterial before it in the pulmonary capillaries, whilst respira-
tion is properly going on;  but if the supply of  oxygen be interrupted,
so that the blood is no longer aerated, no change in the affinities takes
place whilst it traverses the  capillary network; the  blood, continuing
venous, still  retains its need of a change, and its attraction for the walls
of  the  capillaries;  and its egress into the  pulmonary veins is thus
resisted, rather than aided, by the force generated in the lungs.
   599.  The  change in the  condition of the  blood, in regard  to the
relative proportions of its oxygen and  carbonic acid, is the only one to
which the Pulmonary circulation is subservient; but  in the  Systemic
circulation,  the changes are  of  a  much more complex nature,—every
distinct  organ  attracting  to  itself the peculiar  substances which  it
requires as the  materials of its own nutrition, and the nature of the
affinities thus generated being consequently different in each case.   But
the same law holds  good in all instances.  Thus the blood conveyed to
the  liver by the portal vein, contains  the materials at the expense of
which the bile-secreting cells are developed; consequently the tissue of
the liver, which is principally made  up of these cells,  possesses a certain
degree of affinity or attraction for blood containing these materials ; and
this is diminished, so soon as they have been drawn from it into the cells
around.  Consequently the blood of the portal vein will drive before it,
into the hepatic vein, the  blood which has traversed the capillaries of
the portal system, and which has given up, in doing so, the elements of
bile to the solid  tissues of the liver.  The same principle holds good in
every other case.
   600. We are  noAV prepared, therefore, to understand  the general
principle, that the rapidity of the  circulation of a part will  depend in
great measure upon the activity of  the functional changes taking place
in  it,—the  heart's  action, and the state of the general  circulation,
remaining the same.  When, by the heightened vitality, or the unusual
exercise, of  a part,  the changes which the blood naturally undergoes in
it are increased  in  amount, the affinities which draw the arterial blood
into the capillaries  are stronger, and more  speedily satisfied, and the
venous blood  is therefore  driven out with increased energy.  Thus  a
larger quantity of blood will pass through the capillaries of the part in
a given time, without any enlargement of their calibre, and even though
it be somewhat diminished; but the size of the arteries by which it  is
supplied soon undergoes  an increase,  which adapts it to supply the
increased demand.   Any circumstance, then, which increases the func-
tional energy of a part,  or stimulates it to increased nutrition,  will
occasion an  increase in the  supply of blood, altogether irrespectively
                                  22 
338
CIRCULATION OF NUTRITIVE  FLUID.
of any change  in the  heart's action.   This principle  has long  been
known, and has been  expressed in the concise  adage,  " Ubi  stimulus,
ibi fluxus;" which  those Physiologists  Avho maintain that  the Circu-
lation  is maintained and governed by  the  heart  alone, cast into un-
merited neglect.
  601.  An undue acceleration of the local circulation, arising from  an
excess  of functional activity in the part, and unaccompanied by any
other change, constitutes the state knoAvn as active congestion, or deter-
mination  of blood.   This may be  artificially produced  by the applica-
tion  of gentle stimulants; and it is usually the first change that occurs,
when their action proves  sufficiently violent to produce Inflammation.
From that state, however, it is distinguished by this important charac-
ter,—that there is merely an exaltation of the natural function, but no
change.  Moreover we  shall presently see that, in Inflammation, there
is a stagnation of  blood,  not an  acceleration.   We  frequently  meet
with  cases, in  which this  active  congestion  becomes  very manifest:
especially in persons of active minds, who  exert their mental  poAvers
too violently, and who thereby induce  an  habitually-increased  flow of
blood  towards  the head, manifested in  the  increased pulsation of the
carotids, the suffusion of the face and eyes, and the heat of the surface.
The balance of the circulation being  thus  disturbed,  there  is almost
invariably a diminished  energy of the movement  of  blood in other
organs, especially the extremities;  as indicated  by their habitual  cold-
ness and  lividity.   In the  treatment   of such a state  (which is  often
the precursor of serious disease), it should  be our object to restore the
circulation in the extremities, by friction, exercise, &c.;  and to abate
the flow of blood toAvards the head, by restraining the functional activity
of the brain, by the application of cold to the surface, by keeping the
head high during sleep, and other means of  similar tendency.
   602. There  is another  condition of the capillary  circulation, also
known under the name of Congestion, which  is  precisely the opposite
of the preceding.  In  this state, there  is deficient functional  energy in
the part, and the circulation through  it is consequently retarded,—as
in the lungs where there is a partial obstruction  to the aeration of the
blood.  The same cause produces a deficient tonicity  of the Arteries,
and allows their walls  to be unduly distended by the vis a tergo of the
blood;  and consequently there  is a great accumulation of blood in the
part, with a retarded movement.   This condition, like  the preceding,
predisposes to  Inflammation, although in a different mode, as will be
explained  hereafter  (§ 631).  It is relieved by causes which promote
the  action of the part; thus congestion of the lungs, occasioned by  the
effusion of fluid into the air-cells, Avhich creates an obstacle to the aera-
tion  of the blood,  disappears when that  effusion is  absorbed.  And
congestion  of  the liver,  the result  of  deficient secreting poAver in  the
organ, is relieved by mercurial and other medicines, which promote  the
flow of bile by stimulating the growth of the hepatic cells.
   603. The Capillaries, like the Arteries,  possess a power  of contrac-
tion and dilatation,  which seems to be very  much under the influence of
the  Nervous System, and particularly of that part of it which  conveys
the  influence of the Emotions.  We have a visible  example of this in- 
MOVEMENT OF BLOOD IN THE VEINS.
339
 fluence in the act of blushing, Avhich consists in a sudden enlargement
 of the capillaries and small vessels of the surface;  whilst the converse
 state  of  pallor, Avhich  often alternates with it  under the influence of
 strong emotion, is evidently due to an unusual contraction of the same
 vessels.  But  the effects  of this influence are no less sensible in  other
 cases ; and  particularly  in the regulation  of the  quantity of certain
 secretions, in accordance with the mental state,  or the condition of the
 system generally.   To the mode in which this regulation is effected, the
 act of  blushing  seems to  afford  us the key; for it indicates that the
 supply of blood afforded to the glands, may be entirely governed by the
 influence of the  nervous  system upon the calibre of the arteries.   Thus
 the nursing  mother, at the sight, or even  at the thought, of  her child,
 Avhen  the usual time for  suckling approaches, feels a rush of blood to
 the breast, exactly  resembling  that which  takes place to the cheeks in
 blushing, and popularly termed "the draught;"  this rush occasions an
 almost immediate increase in the secretion.  In like manner we may
 explain the influence of the mental state upon the amount of  the secre-
 tions of the lachrymal, the salivary, and many other glands ; its influence
 upon  their quality, must probably be  effected through changes in the
 condition of the  blood itself.
   604. The supply  of Nervous agency from the Cerebro-spinal system,
 has been  clearly  proved to exert no direct influence  in maintaining the
 capillary  circulation ; since  the latter continues  as usual, after all the
 nerves of a part  have been divided.  This  is obviously due to the fact,
 that the operations  of nutrition, secretion, &c, are  essentially indepen-
 dent of this  agency.  But as they are in  some degree influenced by it,
 so  will the capillary circulation be affected, through  its  connexion with
 them.   In this manner Ave are  to explain the effect of violent impres-
 sions  upon the nervous centres in bringing to a stand, not merely the
 action  of the Heart (§ 581), but the Capillary circulation  all over the
 body.   The  general vitality of the system appears to be at once de-
 stroyed ;  so that  the capillary circulation, which may usually be seen to
 continue in the web  of a frog's foot for some time after the interruption
 of  the  heart's  action, is  immediately suspended by crushing  the brain
 Avith a hammer.

               6. Of the Movement of Blood in the Veins.

   605. The Arenous  system is formed by the reunion of the small trunks,
 Avhich  originate in  the capillary network ; and  it carries back to the
 heart the  blood Avhich has been transmitted through this.   This blood is
 dark  or carbonated in the systemic veins; whilst it is bright or oxy-
genated in the pulmonary veins.  The  structure of the veins is essen-
tially the  same vvith that of the arteries; but the fibrous tissue of their
middle coat less decidedly exhibits the  characters, either of the yellow
elastic tissue, or of non-striated muscle.   Still it  possesses  no inconside-
rable amount of Elasticity;  and a certain degree of  Muscular contrac-
tility also.  The  Avhole capacity of the Venous system is at least twice
and perhaps  more nearly three times, that of the  Arterial;  and the rate
of motion of the  blood in it must be proportionably sloAver. 
340              CIRCULATION OF  NUTRITIVE  FLUID.
  606.  The movement of the Blood through the Veins is, without doubt,
chiefly effected by the vis a tergo, or propulsive force, which results from
the contractile power of the heart  and arteries, aided by the power
generated  in  the capillary vessels.   The  intermittent flow,  which is
caused by the interrupted action of the former, is usually so far equalized
during the passage of the blood through the capillary network, that no
pulsation can be shoAvn to exist in  the veins; but instances occasionally
present themselves, in which  a venous pulse may be clearly perceived.
The Venous Circulation  is affected,  however, by certain other  causes,
which exert little influence on the movement  of blood in the  Arteries.
One of these is the frequently recurring action of Muscles,  to which the
Veins  are peculiarly subject, on  account of their position.   In every
instance in which Muscular  movement takes  place, a portion  of  the
Veins of the part will undergo  compression;  and as the  blood is pre-
vented by the  valves in the veins, from being driven back into the small
vessels, it  is necessarily forced  onwards  towards the heart.  As each
set  of muscles is relaxed, the veins  that were  compressed by it fill out
again, to be again compressed on  the  renewal of the force.   Thus  we
see how the general Muscular movements of the body have an important
influence in maintaining the Venous Circulation,—how continued exer-
cise, involving the alternate contraction and relaxation of several groups
of muscles, must send the blood  more rapidly towards the heart,  and
thus increase the rapidity of its pulsations,—and how the sudden  and
simultaneous action of a large number of muscles after repose (as when
we  rise  up from the sitting or lying, to the standing posture), may drive
the blood to the heart with  so violent an impetus, as to produce even
fatal results, if,  by any diseased condition of  that organ,  it should be
rendered unable  to  dispose Avith  sufficient rapidity of  the quantity of
blood thus transmitted to it.
  607.  The Respiratory movements  exert a slight  influence  upon the
flow of blood through the large veins near the  heart; for as the  chest is
a closed cavity, in Avhich a partial  vacuum is produced by the act of In-
spiration, whilst its contents  are  compressed by the  act of Expiration,
the former state will favor the movement of blood from the large veins
on  the exterior of the cavity, towards the heart, whilst the latter condi-
tion will retard it.   This produces the phenomenon termed the respira-
tory pulse ; which may be seen in the veins of the neck and shoulders in
thin persons, and especially in those who are suffering from pulmonary
diseases.   The influence  of the Respiratory movements is made  evident
by  introducing a tube into  the Jugular vein, the lower end  of which
dips into water;  for an alternate elevation and depression of  the water
into the tube is then witnessed, showing the suction poAver of the Inspi-
ratory movement, and the expellent force of  the Expiratory  act.   On
the other  hand,_ the Expiratory movement, while it directly  tends to
cause accumulation in the veins, will assist the heart in propelling  the
blood in the Arteries; and by the combined action of these two causes
is produced, among  other effects,  the  rising and  sinking of the Brain,
synchronously with expiration and inspiration, which are observed when
a portion of the cranium is removed.
  608.  A pulsatory movement may be  occasioned in the  great veins 
RESPIRATORY PULSE—VENOUS PULSE.
341
near the heart, by another cause entirely distinct from the preceding;
namely, the regurgitation of blood from the ventricle into the auricle, and
thence into the venae cavae, during the ventricular systole ;  and the pul-
sation thus occasioned is synchronous, therefore, with that in the arteries
(proceeding backwards, however, from the heart), instead of correspond-
ing with the respiratory movement.   This regurgitation may take place,
not from any disease in the valves on the right  side of the heart, but
simply from  over-distension of its cavities, resulting fr.om  any obstruc-
tion to the circulation of blood through the lungs; for Avhen this occurs,
the tricuspid valve does  not completely  close, and allows  a portion of
the blood to  escape from  the ventricle backwards into the  auricle and
venae cavae.  This want of complete closure, constituting what has been
termed the "safety-valve function" of the tricuspid valve, has been par-
ticularly noticed in diving animals, in which the circulation through the
lungs  is liable to be temporarily suspended.   The  venous pulsation
which is thus produced, may be  noticed in almost every case of long-
standing dyspnoea; especially Avhen  this is accompanied (as it usually
is) by hypertrophy and dilatation of the  right ventricle of the heart.
  609.  The Venous circulation is much more liable than the Arterial,
to be influenced by the force of Gravity; and this influence  is particu-
larly noticeable, when the tonicity of the vessels is deficient.   The fol-
lowing experiments performed by Dr.  Williams, to elucidate the influence
of deficient firmness in the walls of the vessels,  and of gravitation, over
the movement of  fluids  through tubes, throw great light on the causes
of Venous Congestion.  A tube with  two equal arms having been fitted
to a syringe, a brass tube two feet long, having several right angles in
its course, Avas adapted to one of them,  whilst to the  other was  tied a
portion of  a rabbit's intestine four feet long, and of calibre double that
of the brass  tube, this being  arranged in curves and coils, but without
angles and  crossings.   When the two  ends were raised to the  same
height,  the small metal tube discharged from  two  to five times the
quantity of water discharged in a given time by the  larger but mem-
branous  tube; the difference being  greatest, Avhen the  strokes of the
piston were most forcible  and sudden, by which the intestine was much
dilated at its syringe end, but conveyed  very little more water.   When
the discharging ends were raised  a feAV inches  higher,  the difference in-
creased considerably, the amount of fluid discharged  by the gut being
much diminished ; and when the ends were raised to the height of eight
or ten inches, the gut ceased to discharge, each  stroke only moving the
column of Avater in it, and  this subsiding  again, without rising high
enough to overflow.   When the force of the stroke was increased, the
part of the intestine nearest the syringe  burst.
  610.  From these experiments it is easy to understand, how any defi-
ciency of tone in the Venous system will tend to prevent the ascent of
the blood from the depending parts of the body, and will consequently
occasion an increased pressure on the walls of  the vessels, and an aug-
mentation in the quantity of blood they  contain.  All these conditions
are peculiarly favorable to the escape of the watery part  of the blood
from the small vessels ; and  this  may either infiltrate into the  areolar
tissue, or it  may be poured into  some  neighboring serous cavity, pro- 
342
CIRCULATION OF NUTRITIVE FLUID.
ducing  dropsy.   Thus  it  happens, that such effusions may often be
traced to that state of deficient vigor of the system, which peculiarly
manifests itself in want of tone of the blood-vessels ;  and that  it is re-
lieved by remedies which tend to restore this.  In many young  females
of leuco-phlegmatic  temperament, for example, there  is a  tendency to
swelling of the feet, by oedematous effusion  into the  areolar tissue, in
consequence of the depending position of the limbs; the oedema disap-
pears during the night, but returns during the day, and is at its maxi-
mum  in the evening.   And the congestion  which  frequently manifests
itself in the posterior parts of the  body, towards the close of exhausting
diseases, in Avhich the patient has lain much upon his back, is attributable
to a similar cause ;  of such congestion,  effusions into the various serous
cavities are frequent results ; and such effusions,  taking place during
the last hours of life, are often erroneously regarded as the cause of
death.  To the same cause we are to attribute the  varicose  state of the
veins  of the leg, which  is so common amongst persons of relaxed fibre,
and especially in those whose habits require them to be much in the erect
posture; and thi3 distension occasionally proceeds  to complete rupture,
the causes of which are fully elucidated by the experiments just cited.
   611.  It has been thought that the  circulation  Avithin the Cranium
takes place under different conditions from  that of other parts of the
body.   For as the cranium is a closed cavity,—a certain part of which
is occupied by the cerebral substance and its membranes, the remainder
being filled up with blood,—it has  been argued  that the amount of
blood in the vessels of the  brain must be ahvays the same ; and that
any disturbance  of its circulation must be due to a difference  in the
relative  quantity of blood in the arteries and the  veins.   This idea
appeared to  derive  support from the  results  of  experiments, which
showed  that the blood is retained  in  the vessels within  the cranium of
animals bled  to death, unless an  opening be made in the  skull, so as
to allow the air to exert the same pressure upon these vessels,  as  upon
those of other parts.   But such experiments do not at all  sanction the
assertion, that the  quantity of blood within the cranium is constant;
on the contrary, we have reason to believe that it  undergoes as  much
change  as in  other parts.  For  although the cerebral substance be
incompressible, yet  its bulk is subject to constant  variation, according
to the quantity of fluid it contains; and the presence of the cerebro-
spinal fluid in the sub-arachnoid  cavity of the brain and  spinal cord,
appears  to be peculiarly destined to favor this continual change,—the
proportions of it contained in the spinal and  the  cerebral cavities, re-
spectively, being governed by the bulk of  the other contents  of the
cranium.   Thus if the vessels of the cerebrum be in their ordinary  state
of fulness, a certain amount of fluid is  present in  the sub-arachnoid
cavity of the brain ; this will be pressed out  into the spinal portion of
the cavity, if  the cerebral vessels be unusually distended  with  blood;
Avhilst it will be increased from  the latter  source, so as to fill up the
vacant  space within  the cranium, if the cerebral vessels be unusually
empty. 
OF NUTRITION.
343
                         CHAPTER VII.

                           OF  NUTRITION.

               1.  Selecting Power of the Individual Parts.

  612.  The  Blood  which  is carried into the different parts  of  the
system, by the Circulating apparatus, is the source  from Avhich all  the
organs and tissues of the body derive the materials of their growth and
development; and, as we have seen, it is distributed by the  Capillaries
of the several tissues, with a degree of minuteness, which varies accord-
ing to the activity of the nutrient  operations taking place in the indi-
vidual  parts.    Thus, in Nerve and  Muscle,  Mucous Membrane and
Skin, a constant decay of the old, and a development of new tissue,  are
taking place, when these organs are in a state of functional activity;
and a  copious supply of blood  is  carried through  every part of their
substance; whilst  in  Cartilage and Bone, Tendon  and  Ligament,  the
amount of interchange  is very small, and is effected  by a much less
minute reticulation of capillary blood-vessels.
  613.   The materials of the nutritive process  being prepared in  the
blood, the process  of nutrition is the act of each individual part; which
grows and developes itself, in virtue of its own inherent poAvers, as long
as the requisite conditions are supplied.  The mode in which this takes
place, in each individual tissue,  has been already explained  in the for-
mer part of this Treatise.   We  have  seen that,  in  the  great majority
of cases, the act of Nutrition  is, in fact, a process of cell-growth;  and
that it takes place under the same  conditions as the production  of  the
simple isolated cell, which constitutes  the whole of the humblest forms
of Cryptogamic Vegetation,—namely, that it grows from a germ, Avhich
draws to itself the materials of its nutrition, and  gives to some of them
a neAV arrangement, whereby they form  the cell-Avail, whilst  others  are
introduced into the cell-cavity,—and then when it has passed through
its  regular series of changes, it dies,  and sets free its  contents.   We
have seen that,  in some  cases, the  germs  are  prepared by previously-
existing cells of the same kind;  whilst in others they are furnished by
certain "nutritive  centres," which seem to be constantly engaged in the
preparation of  them, deriving their materials from the  blood.  Fre-
quently it  would seem as if the original or parent-cell is able to con-
tinue the production of secondary cells to an unlimited extent, even
though  it may  have  itself undergone a considerable change  of form.
Thus the ultimate  follicles of Glands seem to  be at first closed cells,
which  subsequently open at  the part nearest to  the duct, and establish
a connexion with it; and having thus  changed  their condition, they go
on developing new generations of secreting cells in  their interior, from
their own nuclei or germinal centres, to an unlimited extent.   In like
manner,  the parent-cells of Muscular Fibre, which have coalesced to
form the tubular Myolemma, seem  to  continue to develope new fibrillae
from their nuclei,  notwithstanding their  change of form. 
344
OF NUTRITION.
  614.  The selecting power,  which is possessed by the germs  of each
kind of tissue,  and which  enables  them to  draAv from the blood the
materials which they severally require for their development, manifests
itself also in the mode in which substances, that are abnormally present
in the blood, affect the condition and development of the solid tissues.
Thus we find that  the presence of  a  certain  quantity of Arsenic in  the
blood, will  produce a state of irritation in all the Mucous membranes of
the body.   The continued introduction  of Lead  into  the  circulating
system, occasions a modification in the nutrition of the extensor muscles
of the  fore-arm,  producing the form  of partial paralysis commonly
termed  "wrist-drop" and  the  existence of this  modification  is shown
by the  presence of lead  in  the  palsied muscles.   Here we  have to
remark the symmetrical  nature of  the affection, consequent upon  the
occurrence of the same  disorder in the corresponding parts of the  two
sides of the body;  for these muscles appear to have the same kind of
tendency to attract lead from the  circulating current, in a  degree that
is equal on the two sides, as  they  have to  draw from  the blood  the
materials  of their regular growth,  and to develope themselves  in an
exactly similar manner.  In like manner, the cutaneous eruptions, which
are  occasionally produced  by the internal  exhibition of iodide of po-
tassium, are found to be almost precisely symmetrical; the  presence of
the medicine in the blood being the  occasion  of  a disordered  nutrition
of certain parts of the skin; and the selecting power of particular spots
being evinced,  by  the exact correspondence of the parts affected on the
tAvo sides.
  615.  The same appears to  be  the case  with  regard to  substances,
whose presence in the blood is rather the result  of a disordered condi-
tion of the digestive and assimilating processes, than of their  direct
introduction from without.   Thus  in  Lepra and  Psoriasis,—chronic
diseases of the Skin, which seem  to have their  origin in a disordered
state of the blood, rather than in  the solid tissues affected,—we find a
remarkable tendency to the repetition of the patches  on the two sides
of the body, or on the  corresponding  parts  of the  limbs;  and this we
must attribute to the peculiar attraction,  existing  between  the  solid
tissues  of those parts, and the  morbid matter circulating through  them.
So in those chronic forms of  Gout and Rheumatism, which modify the
nutrition of the joints,  producing a deposit of "chalk-stones,"  or  per-
manent distortion and  stiffening,  we almost  invariably find the  corre-
sponding joints of the two sides affected.  The chief exceptions to the
general principle,  that the presence  of morbid or extraneous matters in
the blood  affects  corresponding parts alike, are found to exist  where
there  is much febrile  disturbance,  or where local causes  produce a
peculiar tendency  to disorder of a single part.   The nearer the character
of the morbid process is to that of the ordinary nutritive operations, the
more  nearly  does it  approach these, in the symmetry with which it
developes itself.*

  * See Dr. W. Budd's valuable Paper on the "Symmetry of Disease," in vol. xxv. of
the Medico-Chirurgical Transactions. 
VARYING ACTIVITY OF THE NUTRITIVE PROCESSES.       345
               2. Varying Activity of the Nutritive Processes.

    616. The  nutritive operations go  on with  very great  variations  in
 their_ relative activity, under  different circumstances.   As  a general
 rule it may be stated that, the greater the demand for the  functional
 activity of the organ or tissue, the more energetic is its nutrition; and
 vice versd.  Now this is readily understood, when it is considered, that
 the  active state of many structures  essentially  consists  in  an act of
 nutrition ; thus the energy  of the  secreting processes  is  really de-
 pendent, as we have seen, upon the growth of  the secreting cells,  which
 make up the  essential part of the gland, and the  energy of the absorb-
 ing  and assimilating process  is  dependent upon  the  development of
 the cells, which select and elaborate the nutrient matter.  This growth
 is regulated  mainly by the supply of blood; being  increased  by the
 afflux of blood towards the part, in consequence of the influence of the
 nerves upon  the vessels, or through any other  change in the  current of
 the circulation.   Thus the secretions are increased in  amount, by emo-
 tions of the mind, that act (probably through the sympathetic nerve)  in
 regulating the calibre of the arteries supplying their respective glands:
 or the interruption of the function   of one  gland shall occasion an
 increased  nutrition, and consequently an augumented secretion, in its
 fellow,—as when one of the kidneys is hypertrophied, through  a dis-
 ease in the other, that renders it  incapable of  performing  its  office.
 Still it would appear, that there may be variations in the activity of
 these organs,  resulting from causes inherent in themselves (of the nature
 of which we know little or nothing); and that here, as elsewhere,  active
 nutritive operations will promote the circulation of blood through the
 parts, whilst a languid state of the function  will retard it.-
   617. In  certain other tissues, however, the functional  activity would
 seem  to  consist rather in a  waste or  decay of  structures previously
 developed  ; this is the case especially in Nerve and Muscle, which are
 found to undergo disintegration, in exact proportion to  the degree in
 which they are  exercised; whilst  the  degree  in which  this waste  is
 repaired, depends upon the supply of nutritive material, the  quiescent
 state of the  part,  and other circumstances.   But  even here we find
 that  functional  activity occasions  increased  nutrition;  in the  same
 manner as burning a lamp with a high  flame increases the amount of
 fluid draAvn up by the wick.  For neither the  nerves nor  the muscles
 can act with energy, without a large supply of  arterial blood;  and this
 is drawn to them on the  principles already  mentioned  (§ 60) as in-
 creasing the  energy  of  the local  circulation.   The determination of
 blood to the parts,  thus established, favors  their  increased nutrition;
 and thus we find that, under favorable  circumstances, any  set of Mus-
 cles,  which is habitually exercised,  undergoes  a  great increase of de-
 velopment  ; whilst,  in like manner  the  Nervous centres, if too great a
 demand be made upon their activity, are liable to become hypertrophied
 (especially in  young persons),  and  may thus  become subject to disor-
 ders,  which temporarily or permanently destroy their powers.   In these
 cases, then, the  functional activity  determines  the increased supply of
blood, and  occasions the augmented growth;  and increased nutrition will 
346
OF NUTRITION.
rarely take place in  these  tissues without an  especial stimulus of this
kind.  Thus we find that, when a larger supply of nutritive matter is
introduced into the circulation, than is required to  repair the Avaste  of
these tissues, they do not undergo  an increased  development in conse-
quence ;  but an augmented nutrition  is produced, either in the adipose
tissue, or in the glandular structures by Avhich the superfluous matter is
eliminated from the system.
  618. Augmented nutrition, or Hypertrophy, then, may result in cer-
tain organs, from an excessive supply of their nutrient materials; as in
the case  of the kidney, just mentioned; or as in the enlargement which
Ave  not unfrequently  meet with in the livers  of  those, who have resided
long in warm  climates, and Avho have  not  sufficiently restricted their
supply of non-azotized food to the small amount required for respiration
at an elevated temperature, thereby sending an over-supply of that par-
ticular class of bodies, to  be separated from  the  blood  by the  liver.
Or, in other cases, the increase of functional activity may be the imme-
diate cause of the increased nutrition ;  and this we see, not  only in the
nervous centres and  voluntary  muscles, which are put in  action by the
will, but in parts over Avhich  the mind has no control.  Thus the heart
becomes  hypertrophied, when an obstruction exists  in the pulmonary or
systemic circulation, to overcome which, increased energy of  contraction
is required; and in the same manner  the muscular  coats of the urinary
and gall-bladder acquire  an extraordinary increase of thickness, when
long-continued obstruction, by calculi or stricture in the  canals issuing
from them, impedes  the free exit of  their contents.   Sometimes, how-
ever, a local hypertrophy takes place, which cannot be accounted  for in
either of these modes; as when a single  finger  is  enlarged out  of all
proportion to. the rest, or the  whole  of one hand  increases to a much
greater size than the other, by the  existence (as it  would seem)  in  the
individual part of that tendency to unusual development, which, when it
affects the whole body uniformly, produces a gigantic stature.
  619.  Now a  precisely reversed  series of conditions diminishes the
activity  of the nutrient  processes, and  induces  a  state of Atrophy.
If there  be a deficiency in the  general amount  of nutriment introduced
into the  system by absorption, a general atrophy results ; and the waste
being more rapid than the supply, there is  a diminution in  the volume
of all the tissues excepting the nervous, Avhich seems  to have its  nutri-
tion kept up even  to the  last, at the expense of all  the  rest.  Such a
condition results not merely from  the  Avant of food, but also from the
Avant of  power  to assimilate it; and thus emaciation may take place
to an excessive degree, when  food  of the  most nutritive character is
copiously supplied, and when the appetite for it is  vehement;  in conse-
quence of disorder in the mesenteric  glands, or  in some other  part of
the apparatus particularly concerned  in the elaboration of fibrine.  A
partial atrophy may result, in like  manner, from a deficiency of the
materials required for the formation  of  an  individual tissue or  organ;
thus the adipose tissue throughout  the body, may be atrophied, in con-
sequence of an absence of those materials in the food, which  are capable
of being converted into fatty  matter.   Or  a  particular organ may be
atrophied, by a diminution of  the circulating  current that  should flow 
          VARYING  ACTIVITY OF  THE NUTRITIVE PROCESSES.       347

 to it, either in consequence of obstruction in the trunk or by the par-
 tial diversion of thestream of blood in another direction; and thus the liver,
 which is much more developed in  the foetus, relatively to the rest of the
 body, than it is in the  adult, undergoes a  partial atrophy immediately
 after birth, in consequence of the change in the whole course of the cir-
 culation Avhich takes place as soon as the lungs are expanded.
   620. But partial atrophy may  also take piace from  causes inherent
 in a particular organ.   Thus we occasionally meet Avith  limbs, which
 are " blighted," never attaining their due size relatively to the remainder
 of the body, yet not exhibiting any defect of organization.   Here there
 would seem to be, from some   unknown  cause, a deficient power of
 growth; analogous  to that which, when acting on the body in general,
 confines it Avithin  dwarfish dimensions.—One of the most frequent causes
 of partial atrophy, hoAvever, is Avant of functional activity in the organ ;
 and this is  particularly the case in regard to the Muscular and Nervous
 systems.   Thus, as already remarked (§  348),  any set of Muscles that
 is long disused, becomes partially atrophied;  which is probably due to
 the feebleness and  languor  of  the circulation, consequent upon  the
 absence of  the demand for arterial blood.  As  soon as the parts  are
 called into use again, their nutrition improves.   So, also,  in regard to
 the Nerves  ; the nutrition of both the fibrous  and vesicular structures
 appears to be entirely dependent  upon the activity of  their function ;
 and as the former are inert without the latter, we may say  that the due
 nutrition of the nervous system entirely depends upon the functional
 activity of  the vesicular matter.   Of this we have a well-marked illus-
 tration in the fact, that when the cornea has been rendered so  opaque
 by disease or accident, as to prevent the penetration of any light to the
 interior of the eye, the retina  and  the optic  nerve lose after a time their
 characteristic structure ; so that scarcely a trace of the peculiar globules
 of the former or of the nerve-tubes of the latter can be found in  them.
   621. In the healthy  condition  of the organism, however, the nutri-
 tion in every part of the body goes on in  a degree  sufficient to  keep it
 constantly ready  for the  performance of  its appropriate function;  a
 regular supply of the requisite materials being furnished in  the aliment,
 and being prepared by the assimilating processes; and  the products of
 the Avaste or decay of the tissues, together  with such alimentary mate-
 rials  as may be superfluous, being carried off  by the excreting opera-
 tions.   When the  nutrition and the waste are equal, the weight of the
 body remains  the  same ; and this is commonly the case in adult age.
 But during the earlier periods of life, the powers of growth are greater;
 the demand for food is very large  in proportion to the bulk of the body;
 and  though the Avaste is rapid, and the  excreting process very active
 (as evinced  by the large amount of urea and of carbonic acid set free),
 the growth  predominates over the decay, and the  development  of the
 whole structure  proceeds at a gradually-decreasing  rate, until the full
 stature and  bulk are attained.  The energy of the nutritive process is
 particularly manifested, in  the rapidity and  completeness with which
severe injuries,  occasioned by disease  or  accident, are repaired.  In
advanced  life, on  the contrary,  although the  Avaste is comparatively
small, the renewing  processes are enfeebled in a still greater degree; 
348
OF NUTRITION.
and there is a gradual  diminution in the stature and bulk of the body,
and in its physical powers.   All the functions are performed with de-
creased energy ; and the comparative inertness of the nutritive processes
is seen in the difficulty with Avhich the effects  of severe injuries are
repaired, in the length of time requisite for the purpose, and frequently
in  the imperfection of the result.
  622. During the successive periods of life, there are many remarkable
changes in the relative nutrition of different organs ; Avhich we can  attri-
bute to nothing else, than to inherent differences in their own powers of
development.   Thus, during the early stages of foetal existence, the
greatest  energy of growth is seen  in certain parts which  are to answer
but a temporary purpose, and which are afterwards completely atrophied.
This is the case, for example, with the Corpora Wolffiana, which seems
to  ansAver  the  purpose of  temporary kidneys,  and  in connexion with
which the permanent kidneys and the genital organs are developed ; and
of these bodies, though of large size in the early embryo,  and evidently
of great  importance, no trace whatever is afterwards to be discovered.
So in regard to the Supra-Renal capsules, the  Thymus and  Thyroid
glands, and other  organs, we find their proportional size  the greatest,
and their function evidently the most active, during foetal  existence and
in  early infancy; after which their bulk diminishes in proportion to the
rest of the body, and their  functional activity  seems  almost at an end.
  623. Even in  the relative development of the organs  which  form
essential  parts of the permanent structure, Ave find  considerable varia-
tions at different periods of life.   Thus the eArolution of the generate
system  does not  usually  take  place,  until the  rest of  the  body is
approaching its maturity;  but cases sometimes  occur, in  which this
apparatus attains its full development, both in the male and the female,
at  a  very early period of childhood, and seems capable of performing
its functions.  In  the Human species, these organs, when once  evolved,
remain always in a  state of preparation for  the performance  of  their
function, unless  thay are atrophied through complete  disuse,  or  have
lost  their vigor by age, or through  excessive demands upon their acti-
vity ; but in most of the lower animals, the development of these organs
is periodical through the whole  of life, taking place at a certain season
of the year, and being greatly influenced,  it would  appear,  by the
external  temperature, and by the supply of food.  Thus in the Sparrow
the testes are no larger  than mustard-seeds, during the greater part of the
year ; but  in the spring, they acquire  the size of large peas,  and it is
then only that they possess any procreative power.
   624. We are not always to judge of the degree  of development of
organs, hoAvever, by their size alone ; for the completeness of their struc-
ture, and their  aptitude for the performance of  their functions, must
also be taken into the account.    Thus in the new-born infant, the organs
of Digestion and Assimilation, though of small size, are so completely
formed as to be able at once to take on the duty of receiving and pre-
paring the  nutritive materials, provided these are supplied in a form
adapted to their powers; the Circulating apparatus is fully adequate to
transmit  the products of the action of those organs  to  the  body in
general,  and to  bring  back the results of its  continual decay; and the 
VARIATIONS OF NUTRITION  WITH AGE.
349
Respiratory organs, together with other parts of the Excretory appa-
ratus, are so completely  evolved, as to be able to ^separate the effete
matter, and to  cast it out of the system with an energy  equivalent to
that of the organs by which new matter is introduced and  appropriated.
On the other hand, the Brain, although of larger comparative size  at
birth,  than at any subsequent  period of life, is but  very imperfectly
developed;  for its structure is not yet so far completed, as to prepare
it for a state of high functional  activity.  In fact it would seem as if
the use of the organ, as called forth by the new circumstances in  which
the infant is placed as  soon as it comes into  the world, is essential to
its complete development;  and the  same may be said of  the Muscular
system.
   625.  During the whole period of infancy and childhood, the current
of nutrition seems peculiarly directed towards the  brain ; for though
its size  does not  continue to increase, in  proportion with that of the
remainder of the body, its  structure is evidently being rendered more
perfect, and its functional activity is excited with remarkable  facility.
Hence it  is  peculiarly liable to  be acted on by various  causes  which
may produce disease ; and  the  operation  of remedies, which  specially
affect that organ, is far more poAverful than at any other period of life.
Thus, whilst, a child will bear a fourth, or even a third of the dose of a
purgative adequate for an adult,  it is strongly affected by an eighth, or
even a twelfth  of the dose of a narcotic or a stimulant that would be
required to produce a corresponding effect in middle life.   This peculiar
impressibility of  the nervous system, resulting from the activity  of the
nutrient processes which  are taking place  in  it, manifests itself also in
other ways ; thus children  are peculiarly liable to have its poAvers de-
pressed by any sudden  shock, such as a  dIoav, or an extensive burn or
laceration; whilst, on the  other hand, if the depression be not fatal,
they recover from its effects much more  speedily than an adult  would
do from a similar condition.
   626. During the periods of youth and adolescence, the chief energy
of  development  (except  in regard to the generative system, already
noticed), appears to be directed towards the Muscular apparatus ;  Avhich
then increases in vigor, in a degree which surpasses its increase of
size ; and the circulating and respiratory organs, upon whose energetic
action there is then a corresponding demand, are  peculiarly liable to
disturbance of function, inducing disease in themselves or in other parts.
The maladies of this period are for the most part of a sthenic or inflam-
matory character; resulting, as we shall  presently see, from  the  exces-
sive activity of the assimilating processes, which are disposed to produce
more fibrine than the wants of the body require.   Or if, on the other
hand, there be an imperfect elaboration  of  the nutrient materials, as
happens in the tubercular diathesis, its  effects are  peculiarly liable to
manifest themselves at this period, when the demand for nutritive matter
is greatly augmented by the activity of the muscular system.
   G27. In adult age, there should be such a balance of all  the functions,
arising from the due de\7elopment and proper use of  each organ, as may
preserve the body in the state of health and  vigor, without any marked
change in the  relative dimensions  of its different parts, through  a long 
350
OF NUTRITION.
series of years.   The  digestive, assimilating, and excreting organs, as
they were the first to come to  maturity, are commonly the first to fail
in their activity ; but  this is very generally the result of over-exertion
of their powers, the amount of food introduced into the stomach being
rarely (among the higher and  middle classes  of society  at  least) kept
down to the real wants of the system.   The muscular apparatus usually
experiences the effects of this diminished  nutrition,  sooner than  the
nervous system ; the vigor of the  latter being often sustained  in a
remarkable degree (as shown by the  energy of the mental operations)
through a protracted life, when it has not been overtasked at an earlier
period.   The  very slight impairment of  the nutrition  of the  nervous
system, during the general emaciation which results from a wasting dis-
ease, or during that  more gradual decline of the bodily vigor which is
consequent upon advancing age, is a phenomenon which strongly marks
it out as the part  of the body, to the maintenance of  whose integrity
everything else is subservient;  and this is still more remarkably shown
in the phenomena of starvation, in which state, notAvithstanding the dis-
appearance of the Avhole of the fat, and the reduction of the weight of
the body in general by about 40 per cent., the nervous system  appears
to lose little or none of its substance (§ 117).

                 3.  Of Death, or Cessation of Nutrition.

  628.  The general cessation of the NutritiAre operations, in Death,
usually depends, as formerly explained (§ 65), upon the cessation of the
supply of Nutriment, in consequence of the stagnation of the Circulating
current; and  this stagnation  may result from the direct operation  of
three causes ;  namely,—failure in the propulsive power of the Heart,
or Syncope,—obstruction  to the flow of blood through the  pulmonary
capillaries, consequent upon a  deficient  supply of air, or Asphyxia,—
and a  disordered state of the  blood itself  (§ 534), which at the same
time weakens the power of the heart, and prevents the performance of
those changes in the  systemic capillaries, Avhich  afford  a poAverful
auxiliary to the  circulation; a mode of death, for which  the term Ne-
crarmia has been proposed.  Each of these conditions may be dependent
upon a variety of remote  causes, which cannot be here particularized.
But it is evident that, when either one of them has been established, the
nutritive processes must speedily cease,  although they  may continue for
a short time at the expense of  the  blood  in the capillaries of the part.
The  cooling of the body is  another cause of their cessation ; and this
is one reason Avhy molecular death (or the death of the individual organs
and tissues) follows so  much more closely on somatic death (or  the ces-
sation of the  circulating  and respiratory functions),  in  warm-blooded
than in cold-blooded animals.   In either case, however, the solid tissues
may preserve for a  time  their  independent vitality; and changes may
take place  in  them, which  indicate the  continuance of their nutritive
actions to a certain extent, even when they have been disconnected from
the body.  There are undoubtedly cases, however, in  which the loss of
vital power is as complete and immediate in  the solids  as in the fluids;
the want of ability to avail themselves of  nutriment being as decided in 
VARIOUS CAUSES  OF DEATH.
351
the former, as  the  deficiency of supply is in the latter.  This is seen,
for example, Avhen death results from a sudden and violent shock, which
destroys the vitality of the Avhole system alike (§ 604);  molecular death
being here consentaneous with somatic.
   629.  But as each component part of the Animal fabric has an indi-
vidual life of its own, so must it have a limited duration of its own ;  the
period of termination of its vital activity, or its death, being quite inde-
pendent of  that of the body at large, excepting in so far as the opera-
tions of the latter are requisite to afford  it a constant supply of  appro-
priate nutriment, and  to maintain its temperature at the proper eleva-
tion.   It is perfectly compatible, on the other hand, with the Life of
the entire organism, that certain  parts of it should  be continually in
course  of decay and reneAval; and, in fact, we find that the most  im-
portant parts in the vital functions are performed by tissues whose indi-
vidual duration is comparatively brief, but which are renewed as  fast as
they  degenerate.   We have a Avell-marked example of this in the case
of the  leaves of trees, which are the chief agents in the  preparation of
the nutritious fluid, at whose expense the permanent tissues of the trunk
and branches are generated ;  and although there is nothing in the Ani-
mal body at all comparable to the complete exuviation which commonly
takes place in the Plant at the close of the season of vegetative activity,
yet there is a continual death and separation of parts  that  have per-
formed  their function, which in the end makes up a much larger aggregate.
Thus there is scarcely a less complete renewal of the epidermis in Man,
in the course of twelve months, than there is in  Serpents, Frogs, &c,
which throw it off periodically ;  the only difference being, that  in  the
one case the whole is exuviated and renewed at once, Avhilst in the other
there is a continual  interchange.   In the exuviation  of the antlers of
the Deer, and of the milk-teeth of all Mammalia,  we have very marked
examples of this limitation of the life of individual parts, even  in  the
highest Animals ;  and  as a general proposition it may be stated, that
every part must degenerate, when it  has gone through the whole  series
of changes in which its Life consists, and that it must then either die
and decay, or must  be so altered  in its constitution, as to be  able to
remain  inactive without further change.
   630.  Hence we see  that the duration of vital activity must be  carteris
paribus, in the inverse ratio of its energy ; that is, the life of any part,
or of the entire organism, must  be shortened by any excess of func-
tional activity ; whilst it may be prolonged by such a degree of  repose,
as does not involve an impairment of its nutrition.  We see this most
remarkably exemplified in  the case of cold-blooded animals; the dura-
tion of whose lives, after they have sustained some fatal injury (such as
the removal of the  heart or of the lungs), or are placed in any other
circumstances  incompatible with its continuance, is in the inverse pro-
portion to the  elevation of the temperature to which they are exposed,
and therefore to the degree of their vital activity (§ 128).  Now although
this variation is comparatively little observable in the rate of life of that
portion of the fabric  of Avarm-blooded animals which is concerned in
their organic functions (the  temperature  to which it is  subjected being
nearly  constant),  it is  clearly seen  in those organs Avhose  functional 
352
OF NUTRITION.
activity is more under the control of the individual, and is therefore less
constant.   Thus, in Man, we continually notice that the duration of the
powers of the Brain  and the Generative system is  the longest, when
these organs have been moderately exercised; and that it is much cur-
tailed by the  excessive use of either.   The  duration of their activity,
however, is not increased by partial or entire disuse of the organs ; for
this  induces a state of atrophy, on the principles  already mentioned.
Now we have  every reason  to  believe, that what is  true of  individual
parts and  organs, is, true also  of the whole  structure; and that the
existence of the entire bodily fabric may thus come  to an end, without
any  special disease, in  consequence of the limit originally set  to its
powers of self-renovation.   It is but rarely, however, that  this occurs;
the various accidents of life, the neglect of ordinary  precautions for the
preservation of  health, and hereditary tendencies to various kinds of   ,
morbid action, being too frequently the means  of cutting off the term of
Human existence, long before its natural expiration.

          4. Disordered  Conditions of the Nutritive Processes.

   631.  Having thus passed in review the general  conditions,  under
which the  ordinary Nutritive processes take  place,  it  may  be well to
add  a few words in relation to two of their  abnormal states; one or
other of which is concerned in a very large  proportion of  the diseases
that afflict the human race.  In one of these, there is a  tendency  to the
excessive production of  fibrine in the blood; whilst  in  the other,  there
is a  want of the  proper  nutritive  power  in  the  tissues, which is  appa-
rently due to an imperfect elaboration of that important material.  The
one  of these conditions is termed Inflammation ; whilst the other, Avhich
is less active, but more insidious, is known as the Tubercular Diathesis.
   632.  The extraordinary tendency to the production of Fibrine  in the
blood, which has been already noticed (§ 531) as one of the most im-
portant characters of Inflammation, seems to be always conjoined with
a depressed vitality of the tissues of some part of the body, which indis-
poses them to the performance of their regular nutritive operations ; and
this part may undergo a variety of changes, according to the degree in
which it is affected.   The depressed condition  of its nutritive operations
involves, on the principles explained in the preceding chapter, a languor
in the movement of blood through it, together with a distensible state
of the capillaries, which causes them to contain a far greater amount of
that fluid than under ordinary circumstances.  On the other hand, there
is a tendency to the production  of an increased amount  of plastic mate-
rial  in the blood, as if for  the  reparation of  the part whose vitality is
lowered.  What is the immediate cause of this production is still doubt-
ful ; but we see the consequences of the deficiency of it in those asthenic
or unhealthy Inflammations, which so  frequently involve the destruction
of a large amount of tissue ; the degeneration of the part first affected
soon extending itself to others, if there be no limit set up by the repara-
tive powers of the blood.—In ordinary or sthenic Inflammation, of which
the  increase of fibrine in the blood, and a diminution in the power of
appropriating it  on the  part of the tissues, are  the  most  characteristic 
suppuration;  ulceration; gangrene.            353
 phenomena, the  simplest result  is the effusion of fibrinous matter, or
 organizable lymph, into the substance of the part inflamed, or upon  the
 nearest free surface; and thus is produced a condensation of the tissue,
 or a new growth upon the  membrane.  But when  the depression of
 vitality is more complete, the tissue at that spot gradually dies and dis-
 integrates ; and Avhilst itself undergoing such changes, it gives origin to
 similar changes in the effused fibrine, which it converts  from a plastic
 or organizable deposit, into an aplastic or  unorganizable one, namely,
 pus, the cells of Avhich degenerate without  passing into  any higher or
 more permanent form of tissue,  whilst the liquid through which they
 are  dispersed  has  lost its  coagulating  power.  Thus is produced  the
 Suppurating process; which may either take place in a  cavity thus  ex-
 cavated in the substance of a tissue or organ; or on a free surface.  In
 either case, the  surrounding tissues,  which are less inflamed,  and in
 which the vitality is impaired but not destroyed, become consolidated
 by a deposition of organizable fibrine, which prevents the infiltration of
 pus through their substance.  If this  should not occur, through a want
 of power  to  generate well-elaborated fibrine, the  suppurating  process
 extends itself rapidly, with the most calamitous results;  the properties
 of pus being such, as to produce a  tendency to decomposition,  both in
 the blood, and in the solid tissues into the substance of which it  may be
 carried.
   633.  Another consequence of Inflammation is Ulceration, which is
 a breach of surface  caused  by the same process  as  that Avhich forms
 the  cavity of an abscess,—namely, the degeneration of the inflamed
 tissue, and the removal of its particles, either  by absorption,  or  by
 solution and  ejection in pus.   Many ulcers commence as abscesses near
 the surface, which at last come  to  open upon it; and  others are pre-
 ceded by inflammation of  the  superficial tissues, which die and  are
 thrown off, leaving a vacuity, which may be  subsequently increased by
 the  extension  of the  degeneration of the  deeper parts.   These may
 either die and be thrown  off en masse, constituting what is known  as
 the "sloughing ulcer," or they may disintegrate more slowly, and may
 be dissolved  in the discharge from the ulcerated surface.  This dis-
 charge, when  proceeding  from a spreading ulcer, is  usually of a thin
 ichorous quality, and has  the  power of exciting unhealthy action even
 in healthy parts  to Avhich it may be applied; and it is its change to
 what has been designated as  "laudable pus," that indicates the cessa-
 tion of the destructive, and the  commencement  of the reparative pro-
 cess (§ 636).
   634.  The  state of Gangrene, which  consists  in the  entire  loss of
 vitality of the part, with a complete cessation of the circulation through
 it, is commonly regarded as a result of Inflammation,  when this process
 occurs in its most intense  form; but it may be more rightly considered
 as the ultimate consequence of the  causes which produce Inflammation.
 For it is an essential part, as Ave have seen, of  the condition of Inflam-
mation,  that the vitality of the affected tissues should be lowered; and
thus  there is in them always  a tendency to  death, which is most com-
pletely developed in Gangrene.   We  have  a Avell-marked  example  of
this complete destruction of the life of a part,  by the intense operation 
354
OF NUTRITION.
of causes, which, when less potent, occasion Inflammation, in the case
of frost-bites  produced by Cold; for this agent  at  the same time pro-
duces contraction of the  blood-vessels, and depression  of  the  vital
powers  of the  solid tissues, proceeding to the complete destruction of
them;  whilst in the parts adjoining those which are  actually killed, the
inflammatory state is developed, an  effusion  of fibrine being produced,
which serves to plug up the mouths of the vessels, and thus  to prevent
hemorrhage, when the mortified part drops off.   Here we see, that the
violent action  of  cold completely destroys the vitality of the part most
exposed to it; and  this by its direct  influence on the properties of the
organized structure.  No inflammation can take place in the part thus
killed,  because the vital  processes are altogether brought  to an end.
But inflammation takes place  in  the adjoining  parts, which are less
seriously affected; for the depression of their vital powers  occasions
the result already adverted to,—namely, the production of an increased
amount of fibrine in the  blood,  and an infiltration of  this  substance
into their tissues.   The same is the  case, with regard to the operation
of other powerful agents;  such as those which (like Caustic Potass, or
Sulphuric Acid) destroy the vitality of the parts to  which they are ap-
plied, by the chemical decomposition of their tissues.  The Inflammatory
process is set up, not in the parts which are killed by the application,
but in the surrounding tissues, whose vitality has been simply depressed;
and thus, when the slough, or dead part, is  cast off, there is a prepara-
tion  for  the development  of new tissue to  supply  its  place, from the
superabundant plastic materials of the surrounding parts.
  635,  If, then, we limit the term Inflammation, as  there seems reason
to do, to that state, in which there is a tendency to stagnated  circula-
tion, with increased  production of Fibrine, in the vessels of the  part,
we see that  the Gangrene cannot be a  result of  that process, which is
one rather of reparation than of  destruction.  But Gangrene proceeds,
where we can distinctly trace its  causes, from the violent operation of
the same agents,  as those which, in a less degree, produce Inflammation.
And where  this last process is not set up at the line  of demarcation
between the living and the dead parts, Gangrene, like Suppuration, has
a tendency to  spread;  the influence of  the decay, which is taking  place
in one part, having a tendency to propagate itself, to  the  adjoining
tissue ;  and a  constantly-extending destruction being thus produced.
  636.  We  have now to speak  of those reparative processes, by which
the effects of disease or  injuries are more  or less  perfectly recovered
from.—The healing of a  simple  wound may take  place by the direct
adhesion of its walls,  when they  can be drawn  closely  together; but
more  frequently it  is accomplished by  the intermediation  of a thin
layer of "coagulable lymph," which may be thrown  out for the purpose
of reparation, without the existence of inflammatory action.   But the
reparation of  wounds, in which there has been so great  a loss  of sub-
stance   that neither direct  nor  indirect adhesion  can  take place, is
accomplished  by the  gradual  development  of new  tissue from the
"nucleated  blastema" with  which  the cavity is   first  filled.    This,
however, may occur in two very different modes;  and from  the inqui-
ries of Mr.  Paget it appears that the determination  of one or the other
# 
REPARATIVE  PROCESS—GRANULATION.
355
 of them is chiefly dependent on the condition of the wound, as to seclu-
 sion from  air, or exposure  to it.  When  the  reparative effusion is
 poured  out into a  subcutaneous wound,  the  "nucleated blastema"
 appears to  be  gradually developed into fibrous tissue without any loss,
^and  usually with freedom from local  inflammation (beyond what may
 be the immediate result of  the  injury), as well  as from constitutional
 irritation.  This process seems to take place naturally in cold-blooded
 animals, even  in superficial wounds; the contact of  air not producing
 that disturbance in it,  which it occasions  in warm-blooded animals.
 And Nature  frequently endeavors (so  to speak)  to  bring  it  about in
 the superficial wounds of warm-blooded animals, by the formation of a
 large scab, which protects the exposed  surface; but this happens much
 less  frequently in the Human subject, than  it does  among the lower
 animals; the  unnatural  conditions in  which a large proportion of the
 more civilized  races habitually live (especially deficient purity of the
 air,  continual excess in diet, and the  frequent  abuse of  stimulants),
 being  obviously unfavorable  to it.   The application of  steam to
 wounded surfaces  has been  found to favor the reparation by the most
 healthy process; and the formation  of an  artificial scab by  means of
 resinous unguents has  also  been practised Avith advantage.   It  is the
 duty of the Surgeon to endeavor to promote it by every means in his
 power; since it is the method of healing, which is not merely  the most
 desirable as regards its  economy of nutritive material and  freedom
 from constitutional  irritation, but which most completely supplies the
 loss of substance, so that the  cicatrix  does  not contract.   The newly-
 formed fibrous  tissue becomes  vascular, by the extension  of loops or
 arches from the adjacent capillaries, and  of other loops from these;
 and  subsequently  other structures—such  as bone,  lymphatics, and
 nerves,—may be developed  in  it. True Cartilaginous tissue, and the
 higher form of Muscular fibre, however, seem never to be thus generated
 de novo in the  new tissues of a repaired part;  so that wounds of Carti-
 lages and Muscles are united by simple fibrous texture.
   637. In an open wound, on  the other hand, which is healing by the
 process termed  Granulation, the "nucleated blastema" is rapidly deve-
 loped into cells, amongst which vessels speedily extend  themselves;  but
 the vitality of  this tissue is very low, and  that part  of it  which is ex-
 posed to  the  air passes into the condition of pus, its cells being either
 imperfectly developed from the first,  or  speedily undergoing degenera-
 tion.  Thus there is a constant waste of plastic material, the amount of
 which, in the case of an extensive suppurating sore,  must be  a serious
 drain upon  the  system;  whilst at  the same time, the local inflammation
 is greater, and  gives rise to more or less of  constitutional disturbance;
 and the formation of neAV tissue  is so  much  less  complete, that by its
 subsequent  degeneration, and removal by absorption,  a contracted cica-
 trix is produced, Avhich is different from  the original texture.   The new
 tissue is here produced by a metamorphosis of cells into fibres; and this
 change is taking place in the deeper part of the granulation-structure,
 whilst the more superficial is degenerating  into  pus.—The difference
 between the two modes of reparation  now described, is  often one of life
 and  death, especially in the case of large burns of the trunk in children; 
356
OF NUTRITION.
for it frequently happens that the patient sinks under the great consti-
tutional disturbance occasioned by a large suppurating surface, although
he may have survived the immediate shock of the injury.
   638.  If the Fibrine of the Blood, however, be not well elaborated, it
does not possess its due organizability; and thus instead of being con-
verted,  either when effused as an Inflammatory product, or in the ordi-
nary Nutritive process into solid tissue, proper to the part in  Avhich it
is  deposited,  it is liberated from  the vessels in a state, which  prevents
any but  a very  imperfect  structure from  being developed by it, and
which tends  to  very speedy degeneration.  This is the condition of the
Tubercular  substance, which  is so often found to replace  the proper
tissue,  especially in  the lungs; being slowly deposited there, by a sort
of degradation of  the regular nutritive operations; and being effused in
larger  quantity, when the inflammatory process  is  set up.   There is
every degree of  gradation between the plastic  or organizable deposit  of
well-elaborated Fibrine, the caco-plastic or imperfectly-organizable mat-
ter of Tubercle,  and the aplastic or non-organizable matter of Pus.  The
microscopic examination  of tubercular  deposits shows, that they  some-
times contain fully-developed cells and fibres, analogous to  those  of
fibrinous exudations; but that more frequently, the cells and fibres are
imperfectly formed, and are accompanied by a large quantity of a gra-
nular substance, which strongly resembles coagulated Albumen; and
that in  many cases, there is  scarcely any trace of organization  in the
mass.   The greatest degree of organization is  found in the semi-trans-
parent, miliary,  gray, and tough yellow forms of Tubercle ;  the least in
the opaque, crude, or yellow  Tubercle.—It is the opinion of some emi-
nent Pathologists, that Tubercular matter is  always deposited  in  the
first instance in the cellular form; but that it  tends to undergo a rapid
and complete degeneration.
   639.  The  constitutional state,  which predisposes  to this perversion
of the ordinary nutritive operations, and which is knoAvn as the  Tuber-
cular Diathesis,  may be  the result of the continued  operation of any
causes, that tend to depress the vital powers;  such as insufficient nutri-
tion, habitual exposure to cold and damp, protracted  mental depression,
&c.; or it may be derived  from the operation  of the same  or other
causes on the ancestors of the individual, being one  of those  disorders
which  has a peculiar tendency to  become hereditary.  The treatment
must be directed to the invigoration of the system by good food,  active
exercise, pure air, warm clothing, and cheerful occupations; and by the
due  employment  of those means, at a sufficiently early period, many
valuable lives may be saved, which would otherwise fall  a sacrifice to
Tubercular disease in the  lungs, or other important organs.—Much
reason  has lately presented  itself for  the belief,  that a deficiency of
appropriate  oleaginous constituents in the food exerts a marked influ-
ence in the production of the Tubercular diathesis.   This would appear
to be indicated  by the very marked benefit which  has been derived in
the treatment of Pulmonary Consumption and other tubercular diseases,
from the use  of Cod-liver oil, or of other easily assimilated fish-oils.
And the same  view is confirmed  by the remarkable  exemption of the
Icelanders (whose diet is extremely oleaginous)  from Tubercular  dis- 
        NATURE AND CONDITIONS  OF  THE RESPIRATORY PROCESS.  357

 eases, notwithstanding that the general habits of the people would seem
 peculiarly favorable to their production.
  _ 640. There  is another remarkable class of diseases, resulting from a
 disordered  condition of the nutritive processes ;—those,  namely of a
 malignant nature.   We not unfrequently meet with abnormal growths
 of  a  fatty, cartilaginous, fibrous,  or  bony structure;  which appear  to
 originate in some  perverted action  of the part itself, and which have
 little tendency to  reappear  in the  same part, when they have been
 removed, still  less, to  reappear  in  distant parts.  But the various
 forms of Malignant or  Cancerous disease are peculiar in this,—that
 they  are composed  of cells, sometimes of a globular form (see Fig. 18),
 sometimes elongated or  spindle-shaped, having a power of rapid°multi-
 plication, and not capable of changing into any kind of normal  tissue.
 When a truly cancerous growth has once established itself in  any part
 of  the body,  it may increase  to an  unlimited  extent,  obtaining its
 nourishment from  the blood-vessels  in  its neighborhood,  and destroy-
 ing the surrounding parts  by its pressure, as  well as by drawing-off
 their supply of aliment.   When it  has developed itself to  a  consi-
 derable degree in  one  part,  it is very liable to make its appearance
 in others, especially when the original growth  has been removed ;  and
 hence the judicious surgeon  is  disinclined  to remove   a  Cancerous
 growth of any  but the most limited kind; knowing that  the  disease is
 almost certain  to reappear.   There is a strong  analogy between such
 Cancerous growths,  and  the low forms of Fungoid Vegetation, which
 develope themselves in the  interior of the higher  Plants, and even  in
 Animal bodies; and in both cases, the  disease may be propagated by
 inoculation  from one individual to another.  But still it appears pro-
 bable that Cancerous disease, like tubercular, is of constitutional origin ;
 and the peculiar tissue which characterizes it, is perhaps to be regarded
 simply as_tlm manifestation  of the presence of a morbid matter in the
 blood, which is  thus removed from  the circulating current;  just as fatty
 matter is removed by an increased formation  of Adipose  tissue, or  as
the elements of the  excretions are eliminated by an increased growth of
the gland-cells of which they are the appropriate pabulum.
                       CHAPTER VIII.

                          OF RESPIRATION.

       1. Essential nature and Conditions of the Respiratory Process.

  641.  The function  of  Respiration  essentially  consists in an inter-
change of oxygen and  carbonic acid, between  the blood of the Animal
and  the  surrounding medium ; carbonic acid being given  out by the
blood, and  oxygen entering in  its stead.  It has  been already noticed
(§ 84) that this  function  is performed likewise  by Plants;  although,
in  consequence  of their  deriving a large part of their food  from the 
358
OF RESPIRATION.
 atmosphere by a converse process—the absorption of carbon and  the
 liberation of  oxygen,—their  true respiration is  commonly overlooked.
 It may,  therefore,  be regarded as common to  all Organized beings.
 Every one is  conscious, in his own person, of the  imperative demand
 for the due performance of this operation.   If the breath be purposely
 held  for  a few  seconds,  a  feeling of  distress  is  experienced, Avhich
 increases every moment, and  at  last prompts irresistibly to the respi-
 ratory movement.   And if the admission of air to the lungs be  in any
 way  prevented, the respiratory movements are at first increased in
 energy, violent efforts are made to obtain the needed  supply; these
 are succeeded by irregular convulsive actions,  and at the same time
 insensibility comes on; and within  a  short time all movement ceases,
 the circulation of the blood is suspended, and a  stop is put to all  the
 vital  operations of the body.   This state, which is termed Asphyxia,
 usually comes on, in a warm-blooded animal,   within ten minutes of
 the time when the respiration is completely checked ; thus affording  the
 most convincing proof of the importance of that function in the Animal
 economy.   In many cold-blooded tribes, hoAvever, a much longer sus-
 pension may be borne with impunity ;  as also by  warm-blooded animals,
 when the general activity  of their functions is lowered in the state of
 hibernation (§ 121).  We shall  now  inquire into  the sources  of  the
 necessity for  this interchange of oxygen and  carbonic acid; and  the
 mode in which the suspension  of it acts upon the  system at large.
   642.  All Organized bodies, as already explained, are  liable to con-
 tinual decay, even whilst they are most actively engaged  in performing
 the actions of Life;  and one of the chief products of that decay  is car-
 bonic acid.  A large quantity of this gas is set free, during the decom-
 position of almost every kind  of  organized  matter; the carbon  of  the
 substance being united with  oxygen  supplied  by the air.   Hence  we
 find, that the  formation and liberation of carbonic acid  goes on with
 great rapidity after  death, both in the Plant and in the Animal; and
 that it takes place also, to  a very great extent, in the period that often
 precedes the death  of the  body,  during  which a  general decomposition
 of the tissues  is going on.   Thus in Plants, as soon  as they become
unhealthy, the extrication of carbon  in the form  of  carbonic acid takes
 place in greater amount, than  its  fixation from the carbonic acid of the
 atmosphere ; and  the same change normally takes place during the
period that immediately precedes  the  annual fall of the leaves, their
tissue being no longer able to perform  its proper functions, and giving
 rise by its incipient decay, to  a large increase  in the quantity of car-
 bonic acid set  free.  The  same thing probably happens in  the Animal
 body, during the progress of many diseases  which are attended with  an
 extraordinary  tendency to decomposition in  the solids and fluids; for in
 such cases the blood usually exhibits an unusually dark hue, indicating
that it has not been properly freed from  the usual amount of carbonic
acid which it has received  from the tissues.  It has not yet been accu-
rately determined, however, Avhether there is an increase in the amount
of carbonic acid actually thrown off in such cases.
  643. Hence, the  first object of the Respiratory process,  which is
common to all forms of Organized being,  is  to extricate from  the body 
SOURCES OF EXCRETION  OF CARBONIC  ACID.         359
the carbonic acid, which is one of the products of the continual decom-
position of its tissues.   The softness of many of the tissues of Animals,
and the large quantity of fluid contained in their bodies, render them
more prone than Plants to this  kind of  decomposition ;  and, in warm-
blooded animals, the high temperature at which the fabric  is usually
maintained, adds considerably to  the degree of this tendency ; so that
the waste of their tissues, from  this cause  alone, is  as  much  greater
than  that of cold-blooded animals, as the latter is than that of Plants.
But when the temperature of the Reptile is raised by external heat to
the level of that of the Mammal, its need for  respiration increases, owing
to the augmented Avaste of the tissues.   When, on the other hand, the
Avarm-blooded Mammal is reduced, in  the state of hibernation, to  the
level of the cold-blooded Reptile, the waste  of its tissues diminishes to
such  an extent, as to require  but  a very small exertion of the respiratory
process to get rid of the carbonic acid, which is one of its chief products.
And  in those animals Avhich are capable of retaining their vitality when
frozen (§  136), or Avhen  their tissues are completely  dried up  (§ 159),
the decomposition for the time is entirely suspended, and consequently
there is no  carbonic acid to be set free.
   644.  But another source  of Carbonic acid to be  set free  by  the
Respiratory process, and one wdrich is  peculiar to Animals, consists in
the  rapid changes  which take  place  in the Muscular  and  Nervous
tissues,  during the period of  their activity.   It has been already shown
(§ 361), that there is strong  reason to  believe the Avaste  or decomposi-
tion of the  Muscular tissue to  be  in exact proportion to  the  degree in
which it is exerted; every development of muscular force being accom-
panied by a change in the condition of  a  certain amount  of tissue.  In
order that  this  change  may  take place, the presence  of  Oxygen is
essential; and one  of the products of the  union of oxygen with  the
elements of muscular fibre, is carbonic  acid.  The same  may probably
be  said of  the Nervous tissue (§ 384).   Hence  it may be stated as a
general principle, that the peculiar waste of  the Muscular and Nervous
substances,  which is a condition of their  functional activity, and which
is altogether distinct from the general slow decay that  is  common to
these tissues with others, is another  source  of the carbonic acid Avhich
is set free from the animal body:  and that the amount thus generated
will consequently depend  upon the degree,  in which  these tissues  are
exercised.   In animals  which are chiefly made  up  of  the  organs of
vegetative life, in whose bodies the nervous  and  muscular tissues form
but  a very small  part, and  in Avhose  tranquil  plant-like  existence
there is but very little  demand upon  the exercise of these structures,
the quantity of  carbonic  acid thus liberated will be extremely small.
On the other hand, in  animals, whose bodies are chiefly composed of
muscle, and whose life is an almost ceaseless  round of exertion,  the
quantity of  carbonic acid thus liberated is very considerable.
  645.  We  are  enabled to  trace the  connexion betAveen  the  amount
of muscular exertion, and the quantity of carbonic acid set free in  the
act of respiration, in  the class of Insects,  better than  in any other.
They have no fixed temperature to maintain ;  and they are consequently
not in the condition  of Avarm-blooded animals, in which the quantity of 
360
OF RESPIRATION.
carbonic acid set free is kept  up to a more regular standard  by the
provision  to  be presently noticed.   On the  other  hand, they are  pre-
eminent among all Animals, in regard to the energy of their muscular
power in relation to the bulk of their bodies ; and  the waste  of muscu-
lar tissue during their state of activity must  therefore be very great.
Thus a Humble  Bee  has  been found to produce  one-third of a cubic
inch of carbonic  acid  in the course of a single hour, during Avhich its
whole body Avas in a state  of constant movement, from  the excitement
consequent upon its capture ;  and yet during the whole twenty-four hours
of the succeeding day, which it passed in a state of comparative rest,
the quantity  of carbonic  acid generated by it was absolutely less.
   646.  Besides these  sources of Carbonic acid, which are common to
all animals, there is another, Avhich  appears to be peculiar to the two
highest classes, Birds and  Mammals.  These are capable of  maintain-
ing a constantly-elevated  temperature, so  long as they  are supplied
with a proper amount of  appropriate food  ; and  their power of doing so
appears to depend  upon the direct  combination of certain elements of
the food, with the oxygen  of the air, by a process analogous to com-
bustion ; these elements  having been introduced into the blood for that
purpose, but  not having formed a part of  any of the solid tissues of the
body, unless  they have been deposited in the form of fat.  The nature
of  these  substances  has been  already noticed (§ 430).   It is  quite
clear that they cannot be applied in their original form, to  the nutri-
tion of the tissues that originate in proteine-compounds ; and it is tole-
rably certain that, in the ordinary condition of  the body, they undergo
no such conversion, as Avould adapt them  to that purpose.   The Liver
seems to afford a channel, by which some of the fatty matters are drawn
off from the blood;  but  even  these  seem, in part  at least, to be reab-
sorbed (§ 725), and  to be thrown off by the respiratory process.
   647.  The  quantity  of  Carbonic  acid, that  is  generated directly
from  the  elements of the food, seems to  vary considerably in different
animals, and in different states of the same individual.  In  the  Carni-
vorous tribes, which spend the greater part of  their time in  a state of
activity, it is probable that the quantity Avhich is generated  by the
waste or metamorphosis  of the tissues is sufficient for  the  maintenance
of the required temperature,—and that little or none of  the carbonic
acid set free  in respiration  is derived  from the direct combustion of the
materials of the food.  But  in  Herbivorous animals of comparatively
inert  habits,  the  amount of metamorphosis  of the tissues' is far from
being sufficient; and a large part of the  food, consisting as  it  does of
substances that cannot  be applied  to the nutrition of the  tissues, is
made to enter into  direct combination with the  oxygen of the air, and
thus  to compensate  for  the  deficiency.   In Man and  other animals,
which can sustain considerable variations of climate,  and  can  adapt
themselves to a great diversity of habits, the quantity of carbonic acid
formed by the direct combination of  the  elements  of the food Avith the
oxygen  of the air, will differ extremely under  different  circumstances.
It Avill serve as the complement of that Avhich is formed in other  ways;
so that  it Avill  diminish with the  increase, and will  increase with the
diminution, of muscular  activity.   On the other  hand, it will vary in 
          SOURCES OF CARBONIC ACID  IN THE  ANIMAL  BODY.     361

 accordance with the external temperature; increasing with its diminu-
 tion,  as  more heat must then be generated; and  diminishing with its
 increase.—In all cases, if a sufficient  supply of food be not furnished,
 the store of  fat is  drawn upon ; and  if this be exhausted, the animal
 dies of cold (§ 117).
    648. To recapitulate, then; the sources of Carbonic acid  in the Ani-
 mal body are threefold.—1.  The continual decay of the tissues ;  which
 is  common to all organized  bodies ; which is  diminished  by cold and
 dryness,  and increased by  warmth and moisture;  which  takes  place
 with increased  rapidity at  the approach  of death,  whether this affect
 the body at large, or only an  individual part;  and which  goes on un-
 checked,  when the actions of nutrition  have ceased altogether.—2. The
 metamorphosis,  which is peculiar to the Nervous and Muscular  tissues;
 which is the very  condition  of their  activity; and which, therefore,
 bears a,  direct relation  to the degree  in which they are exerted.—3.
 The direct conversion of the carbon  of the food  into  carbonic  acid;    *
 which is peculiar to  warm-blooded  animals; and which seems  to vary
 in  quantity, in accordance with the amount of heat to be generated.
    649. Now  the function of Respiration has for its object, not merely
 to  extricate ^the  Carbonic acid  which  is generated in the  system, but
 likewise to introduce the Oxygen Avhich is  required  to form that car-
 bonic acid; the proportion of oxygen in the tissues, and in  the combus-
 tible materials   of the  blood, not  being  sufficient for  this purpose.
 Hence it is not enough, that  the carbonic acid should be removed; for
 this may be accomplished by causing an animal to  breathe an atmo-
 sphere which  contains no oxygen.  Any cold-blooded animal, such as a
 Frog or a Snail,  may be kept in hydrogen or nitrogen for several hours
 or  even days; and will give  out, during  that time, an amount of car-
 bonic acid nearly as great, as if  it had  been respiring atmospheric air.
 But the continued production of carbonic acid must have a  limit,  occa-
 sioned by the want of  oxygen, and death will then supervene.—On
 the other hand,  a supply of oxygen may be freely afforded; and yet
 the presence of even a small  amount of carbonic acid in the surround-
 ing atmosphere (in addition  to  that  which is normally present in  it,
 § 81) will impede the extrication of that substance from the blood; and
 if the excess be considerable, the carbonic acid  will  not be set free at
 all; so that the same injurious results follow, as if  respiration Avere al-
 together prevented from  taking place.
  650.  These two actions are accomplished by the very same act; ad-
 vantage being taken  of  the  property of "mutual  diffusion," Avhich is
 common to all gaseous substances that do not unite  chemically with one
 another.  In virtue of this property; Hydrogen, the  lightest of gases,
 and Carbonic  acid, one of the heaviest,  when introduced into the same
 vessel, will be found in a short time to have uniformly mixed, notwith-
 standing the difference of their specific  gravities, which are  as 1 to 22.
 Now this  intermixture will take place, when the two gases are sepa-
 rated by a porous septum ; each  gas  passing towards the other, by an
 action  resembling  the   Endosmose  and Exosmose  of liquids (§ 491).
And it may also take place, when one of the gases is diffused through
a liquid;  provided that the other  gas is likewise  capable of being 
362
OF RESPIRATION.
absorbed by the  liquid.   In this manner,  as already  mentioned, the
surface of venous blood,  enclosed in a bladder, may be made to exhibit
the arterial  hue, by suspending the  bladder in  an  atmosphere  of
oxygen;  for the carbonic  acid of  the blood, and the surrounding
oxygen, will overcome by their  mutual attraction the  obstacle inter-
posed by the bladder ; and the former will be lifted out, so to  speak, and
will be replaced by the latter.  It has  been found by experiment, that
the free carbonic  acid diffused through  blood, may be more  completely
extricated from the liquid, by exposing  it to hydrogen, than by placing
it  under  the  vacuum of an air-pump ; for in the  latter case there is
nothing to replace it, and the attraction between the  gas and the liquid
tends to  resist the  exhausting  influence of the vacuum;  whilst in the
former, the blood receives one gas  in  exchange for the other, so that
the whole force of the tendency to mutual diffusion is exercised in lift-
ing out the carbonic acid.
   651. The immediate  purpose  of the organs of Respiration, then,—
whatever may  be the variety in their form,—is  this: to  expose  the
blood to the air, in a state of such minute division as to present a very
extended surface,  a thin membrane  only being  interposed between
them.  For this purpose we find  a certain organ, or set of organs, spe-
cially set apart  in  all  the higher animals;  and this is  formed by a
prolongation of the general surface,  either  externally or  internally,
according to the mode in  which the respiration is accomplished.  Thus
in Fishes and aquatic Molluscs,  the blood is  aerated by  exposure, not
directly to  the atmosphere, but  to  the air which  is  dissolved in  the
water they inhabit; and  the respiratory apparatus is  formed in  them
of an  extension of the external  surface,  at a particular part, into  innu-
merable delicate fringe-like processes, the gills (Fig. 100);  every  divi-
sion of which contains a  network of blood-vessels (Fig. 104); so that

                              Fig. 100.
        Doris Johnstoni, a Nudibranchiate Gasteropod, showing the luft of external gills.

the amount  of  blood which is exposed to  the surrounding medium  at
any one time, is collectively very great, although the quantity contained
in each gill-filament is very minute.   On the other hand, in all the air-
breathing Vertebrata, the blood is exposed  to the atmosphere, through
the medium of an  internal membranous prolongation, which is conti-
nuous Avith the  mucous membrane lining the mouth and nostrils; this
forms a pair of sacs, termed lungs, communicating with the back of the
mouth by means of a tube  called  the  trachea or windpipe, through
which  air  is freely admitted  to  the cavities thus formed (Fig.  105). 
RESPIRATION IN MOLLUSCS.
363
 The blood  is minutely distributed on the walls of these  sacs by a close
 network of Capillary vessels (Fig. 106); and not only on the external
 walls, but  also on  numerous partitions  by which the cavities  are  sub-
 divided with more  or less minuteness, so as greatly to extend the vas-
 cular surface.
    652.  Such is the essential nature of  the Respiratory apparatus ; but
 in order that it  may be carried into that vigorous operation, which  is
 required in the higher classes of animals, various supplementary arrange-
 ments are made,  for the purpose of  promoting the due influence  of the
 air upon the blood.  In the first  place, the capillary vessels of the respi-
 ratory surface are connected with arterial trunks, which issue immedi-
 ately from  the heart, and which thus convey a constant stream of blood
 from that  organ; whilst they  give  origin to venous  trunks, which ter-
 minate  directly in the heart, and which are  ready to convey back  to it
 the blood that has undergone  aeration.  Thus  by the energetic  action
 of  the heart, and by the force  generated in the capillaries of  the lungs
 (§ 598), a constant renewal is effected  in the blood, which is exposed to
 the air  through  the medium of these organs.  On the other hand, the
 renewal of the blood Avould be useless, unless a fresh supply  of air were
 continually being introduced, and that Avhich had been vitiated, by the
 loss of its oxygen and the admixture of carbonic acid, were removed;
 and this is effected by a series of muscular movements, which are adapted
 for the  alternate expulsion  of  the vitiated air from  the  lungs, and for
 the introduction  of  a  fresh supply  of  pure  air from the atmosphere.
 These movements are kept up by a certain part of the nervous system;
 but they are not dependent upon any exertion of the will, for they con-
 tinue during profound sleep, and in other states in which even conscious-
 ness is altogether suspended.

     2. Different forms of the Respiratory Apparatus in the lower Animals.

   653.  Before proceeding to consider, in more detail, the structure  and
 actions  of  the respiratory apparatus in Man, Ave may advantageously
 glance at the mode, in  which this function is  effected  in the lower  ani-
 mals.—In the lowest and simplest, Avhich are inhabitants of  the water,
 Ave do not find any special apparatus  for the aeration of the fluids  of the
 body ; this being accomplished by the exposure of them to the surround-
 ing medium, through the thin integument; and the interchange of the
 layer of water (holding air in solution) in contact with the aerating  sur-
 face, is effected either by the general movements of the body, or by the
 action of the cilia (§ 234) which produce the currents necessary for  this
 purpose.  Not unfrequently, the internal surfaces—such as the walls of
 the  stomach and  of  other cavities,—seem  as much  concerned in  this
 function as  the external, or even more so ; these cavities being distended
 with Avater taken in  through  the mouth, and this water being frequently
 renewed  by the ejection of that which has been vitiated, and by the in-
 troduction  of a fresh supply.   This  is  the case in the Sea Anemone,
 for example, and  in many other polypes ;  and there are certain higher
forms  of the  same class, in Avhich there is  a great  dilatation of  the
pharynx, Avhich seems peculiarly destined for  the aeration of the fluids, 
364
OF RESPIRATION.
—being filled with water, and then suddenly emptied at tolerably regu-
lar intervals.
  654.  In the various classes of the Molluscous sub-kingdom, we find
the respiration  provided for, by the adaptation  of distinct organs for the
purpose.  As most of the animals of this group are inhabitants of the
water, the  respiration is  usually carried on by means  of gills, rather
than by any organ resembling a lung.  The latter is found, however, in
a feAV species; such as the Snail, Slug, and other terrestrial air-breathing
Molluscs, and usually consists of a simple cavity, situated in the back,
communicating directly with the air through an aperture in the  skin,
and  having  its Avails covered Avith a  minute network  of blood-vessels.
The form and position of the gills differ extremely in the several classes
of Molluscous animals.  In the lowest the respiratory surface is formed,
as in the higher Polypes, by a dilatation of the Pharynx;  but  sometimes,
instead  of surrounding a large cavity, it forms a special riband-like fold
of membrane, passing from  one end of it to the  other, on which the
blood is minutely  distributed.   In  this  group  of animals,  there  is a
regular system of canals for the conveyance of the blood; but these, in
many parts of the system, and especially on the respiratory  membrane,
do  not  seem to  be furnished with distinct walls, and  are rather  mere
channels excavated  in the tissues.  And the circulation is  liable  to a
continual change in its direction, the blood being sometimes transmitted
to the respiratory surface before it proceeds  to the body, and sometimes
after it  has traversed the other tissues (§ 557).   The Avater in contact
with  the respiratory surface  is continually renewed by the action of the
cilia, with  which it is thickly covered.
   655.  In  certain of the Molluscs,  inhabiting bivalve shells, we find
that the internal surface of the mantle or skin that lines  the valves, is
the special organ of respiration ;  the external water having free access
to these by the separation of the skin along the edges of the valve, so
that  it enters  the cavity in which the vriscera  are lodged,  and bathes
their exterior.   But  in most bivalve molluscs, the  internal surface of
the mantle is doubled (as it were) into four  riband-like folds, which are
delicately  fringed  at their  edges, and  which have, in fact the   same
essential structure as  the gills of higher animals (§ 663).  To  these the
blood is transmitted,  Avhen it  has  been rendered venous by traversing
the vessels of the body generally :  and in these it is  exposed, through
a surface Avhich is greatly extended by the minute division of  the fringes,
to the action of water introduced from without, and constantly renewed
by ciliary  action.  In many of these animals, as in the common Oyster,
the two lobes of the mantle  are so completely separated, that the Avater
can still enter freely between the valves,  but  in general, they are  more
or less  united,  so that the cavity in which the gills lie is partially closed.
There is always  a provision, however, for the free access of water from
without by means of  two apertures, one Tor its  entrance  and the  other
for its ejection, and  in certain species which burrow deeply in sand or
mud, these apertures are furnished with  long tubes, or  siphons,  which
convey the  water from near the entrance of the  burroAV, and carry it
 thither again.   In these also, a continual flow of water over the  respi- 
RESPIRATION IN WORMS AND  CRUSTACEA.           365
 ratory surface  is maintained by the vibration of the  cilia, with which
 they are clothed.
   656.  The position of the gills, in the Mollusca of higher organization,
 is extremely variable.   Sometimes they are disposed upon the external
 surface of the body, and form delicate leaf-like  or
 arborescent appendages (Fig. 101); whilst in other        FiS- 101-
 cases they  are enclosed in a special cavity or gill-
 chamber, to which water is  freely admitted  from
 Avithout; a continual interchange  being provided
 for, either  by ciliary action,  or  by  muscular move-
 ments specially  adapted  for the  purpose.   The
 blood is conveyed to  them, after  having become
 venous in traversing the capillaries of the general
 system, by means of  large  channels  and sinuses
 excavated in the several parts of the body (§ 556);
 and after being aerated in the  gills, it  returns to  ,
 the heart to be again conveyed to the system.   In   One of the art^'went pro-
 the  Cuttle-fish  tribe,  there  are  supplementary  S JllTtL^ltS
 hearts at the  origin of  the  branchial arteries,  or  and enlarsed-
 vessels that distribute blood to the gills;  and  these  have evidently for
 their  purpose, to render  the  respiratory circulation more energetic, and
 thus  to increase the aeration  of the  blood, in the degree required for the
 vigorous habits of these  animals, Avhich present  a  remarkable contrast
 to the sluggish, inert character of the  Mollusca in general.—In these
 classes,  taken  as a whole, the respiration is  low in its  amount.   The
 blood contains no red corpuscles, excepting perhaps in the highest class;
 and the change in its composition, which  is effected by the air, is con-
 fined, therefore, to the fluid plasma, or liquor sanguinis.   And as  it is
 not exposed directly to the air, except in a few  species, but  to the air
 contained in the water  inhabited by the animals, this change cannot be
 very energetically performed.  But as the life of these animals is chiefly
 vegetative,—as their movements, except in the highest classes, are  few
 and feeble,—and as they maintain no independent  heat,—there is but
 little  need of that interchange, which it is the  object of the respiratory
 process to effect; and these animals can sustain the complete suspension
 of it for a long time.
 _ 657. Among many of the Articulated tribes,  the  respiration is  car-
 ried on upon a similar plan.   In some of the lowest, such as  the Tape-
 worm of the intestinal canal, there is no special provision for  the aera-
 tion of the fluids; the soft integument permitting the extrication of car-
 bonic acid,  and imbibition of  oxygen, in  the required degree.  This is
 but very small,  however; the life of these animals being  almost purely
 vegetative.   In the Marine Worms, which constitute a numerous and
 interesting  group,  endowed with considerable locomotive  powers, and
 leading a life of almost constant activity, there is, on the  other hand,  a
 special provision for this function ; the blood  being  transmitted, in the
 course of its circulation, to a series of gill-tufts, which are composed of
a delicate membrane prolonged from the external surface of the body,
and which sometimes have the form of branching trees, and sometimes
of delicate brushes  made up of a bundle of distinct filaments.   In either 
366
OF RESPIRATION.
case, the  filaments are traversed by blood-vessels, and are  adapted to
bring the blood into close relation with the surrounding water;  and the
continual interchange of the latter is provided for by the restless move-
ments  of the body.  The tufts are sometimes arranged along every seg-
ment of the body ; and their multiplication  prevents them from indivi-
dually attaining any considerable size.   In other cases, they are disposed
at intervals; and they are then larger, being less numerous.   Their most
beautiful development is where they are present  on the  head only, the
rest of the body being enclosed in a shelly or sandy tube, as in the Ser-
pulse and Terebellee.   The gill-tufts then frequently present the appear-
ance of a flower,  endowed, when alive, Avith the most brilliant and deli-
cate hues.  In many animals of this group,  there is a small supplemen-
tary heart at the base of every one of the vessels that distribute the
blood to the gills ; and this is obviously designed to  aid in the respiratory
circulation, for which the feeble action of the dorsal vessel  would not
furnish sufficient power (§ 552).
   658. The higher Articulated classes are,  for the most part, adapted
to atmospheric respiration, on the plan to be presently explained; but
there is one class, that of Crustacea, whose respiration  is still  carried
on through the medium of water.   In the lowest  forms of this group,
there is no special respiratory apparatus; the general surface being soft
enough to admit of the required aeration of  the fluids through its own
substance, and the  animal functions  being performed with so  little
activity, that a very small amount of interchange  is required.   In the
higher orders, however, Avhose bodies are encased Avithin a hard envelope,
we find external  gills, like  those  of  many Molluscs; and  these are
attached to the most movable parts of  the body,—one or more pairs of
legs being in some instances kept in constant agitation, for the purpose
of producing currents in the surrounding fluid, that may serve  for the
aeration of the blood.  In the Crab-tribe, Avhich constitutes the  highest
family of this class, the gills are  themselves enclosed within a  cavity,
formed by a sort of doubling of the hard integument  of the  under side
of the  body ; and  a constant stream of Avater is maintained through this,
by means of a  peculiar valve, situated in the exit pipe;  the continual
movement of the valve causing a regular stream  of water to  issue from
the gill-chamber, and thus occasioning the entrance of a constantly-fresh
supply.   In these, also, we find a dilatation, the walls of which seem to
have contractile powers, at the commencement of each  artery that dis-
tributes the blood to the gills; and this connects the venous  blood from
the various channels, in Avhich it has meandered  through the body.  It
is by the enclosure of the gills within a cavity, and by the consequent
protection of them from the drying  influence of the air (which would
prevent their function from being duly performed), that Crabs and other
allied  species are  enabled to live for a considerable time out of water;
and the Land-Crabs, as they are termed, are adapted to spend the  greater
part of their lives at a distance from  the  sea, by means of a special
glandular apparatus within the gill-cavity,  which  secretes a fluid that
preserves the surface of the gills in the moist condition requisite for the
aeration of the blood through its membrane.  Thus the Land-Crabs are 
               RESPIRATION  IN INSECTS AND SPIDERS.            367

air-breathing  animals (except  at  certain seasons, when they frequent
the sea-shores), although they breathe by gills.
   659. In Insects and other proper air-breathing Articulata, however,
the character of  the respiratory apparatus is very different.   The tran-
sition from one form to the other is effected through such animals as the
Leech and the Earthworm, which  seem able to live almost equally well
in air or water, and whose  respiration appears to  be carried on chiefly,
if not entirely, through the medium of the external surface alone. These
animals are furnished with a series of small sacs, disposed  at regular
intervals  along each side of  the body, and opening by a row of pores,
which  are termed spiracles or stigmata; but these sacculi do not seem
to participate  in  the  respiratory  function, their office being  rather to
secrete a  protective mucus.  But  in the  Myriapods, these sacculi  are
respiratory organs, and communicate more or less freely with each other.
And  in Insects,  the spiracles,  instead  of forming  the  entrances to so
many distinct  sacs, open  into a pair of large tubes, one of which tra-
verses the body  on either side, along  its  whole length.   These tubes,
termed tracheoe, have  many communications  with each other  across the
body ; and they branch out into innumerable prolongations, the ultimate
ramifications of which are  distributed  to  every portion  of  the system.
They occasionally present dilatations of considerable size (Fig. 102, a);
especially in the  thoracic region of the body, in those insects  which are
endoAved with great powers of flight. These  dilatations or air-sacs appear
destined to serve as reservoirs of air, during the time  that the insect is
upon the  wing, its spiracles being then  partially  closed ;  and they may
also be useful in diminishing the specific gravity  of the body.   The air-
tubes are  prevented from having  their cavity obliterated  through the
pressure of  the surrounding parts, by means  of an elastic spiral fibre;
Avhich  winds round them,  between their  outer and inner  membrane,
from one  extremity to the  other (Fig. 102, b) ;  and which answers the
purpose of  the  cartilaginous rings and  plates,  in  the trachea  and
bronchi of air-breathing Vertebrata.

                               Fig. 102.
                        A
 Respiratory apparatus of Insects : a, air vesicles and part of tracheal system of Scolia horlorum. b, por-
     tion of one of the great longitudinal trachese of Carabus auratus, with one of its spiracles.

   660. In this  manner, the air that is introduced through the spiracles
is  carried into every  part  of the body, and  is brought  into immediate
relation with the  tissues to be aerated;  so that the carbonic acid which
 
368
OF RESPIRATION.
they set free  is communicated at once to the atmosphere, instead  of
being taken up by the blood; and the  oxygen they require is imbibed
in the same manner.   And thus we see how the respiration of this inte-
resting  class, which is unequalled  for its energy when thebody is in a
state  of activity, is provided for without an active circulation of blood,
and Avithout the presence of red corpuscles,—Avhich elsewhere  seem to
be essential conditions of the interchange of  oxygen and carbonic acid
between the air and the  tissues, wherever this takes place to  any great
extent.
   661.  In the Spider tribe,  we return to a more  concentrated form of
the respiratory apparatus; but, notAvithstanding that it is limited within
much narrower dimensions externally, it  exposes a very large  amount
of surface on  its interior.   It consists of a series of sacs, much less
numerous  than in  the lower Articulata,  and not communicating  with
each  other.  Their  lining membrane, however, is doubled into a series
of folds, which lie in proximity with  each other,  like the leaves of a
book, and which  thus present a  very extensive  surface within a very
small space.  Over this surface  the blood is distributed in  a  minute
capillary  network;  and thus it comes into immediate relation with the
air, which is received into the cavity through its aperture or spiracle.
The alternate admission and expulsion of air seem to be provided for,
as in Insects,  by movements of the body, which first  empty the cavities
or air-tubes by compression, and then allow them to be refilled  by their
own elasticity, the pressure being relaxed.  The respiratory cavities in the
Spider-tribe have received the name of pulmonary branchiar,  from their
analogy, on the one hand, with the lungs  of higher animals, and, on the
other, with the branchial sac or gill-cavity of the higher Crustacea, the
gills  in which are formed by prolongations of the lining membrane, like
the leaf-like folds in the air-cavities  of the Spider-tribe.
   662. The accompanying diagram  will giAre an idea of the relations of
these different forms of the respiratory apparatus,  amongst themselves,
and to  that of Vertebrata.  Let  the  line A B  represent the   general

                               Fig.  103.
  Diagram illustrating different forms of the Respiratory apparatus: a, simple leaf-like gill; b, simple
            respiratory sac; c, divided gill; d, divided sac; e, pulmonary hranchia.

surface of the animal;  the continuations of that line on its  upper side
being its external prolongations;  and those below, its internal prolon-
gations or  reflexions.   Now at a is seen the  character of the simple
foliaceous or leaf-like gill, such as is found in the lower aquatic animals;
presenting merely a flat expanded  surface  in  contact with  the water,
over which the blood may be distributed.   At  b is shown a correspond- 
RESPIRATION IN  FISHES.
369
ingly simple inversion;  such as that which forms the respiratory sac of
the Leech, having the  blood-vessels  distributed upon its  walls.   A
higher form of the gill, such as  is found  in Fishes and in the higher
aquatic Invertebrata, is seen at c, the surface  being greatly extended,
by  subdivision into minute  filaments.   A more complex form of the
pulmonary  apparatus, such  as  is found in the higher Vertebrata, is
shown at d, the blood being distributed, not merely to its outer Avails,
but to the minute partitions  which subdivide its cavity into cells.   And
at e  is represented  the respiratory organ of  the  Spider-tribe, which
bears an obvious  resemblance to the lung of the Vertebrated animal,
shown at d; whilst it is evidently  as nearly allied to  the  gill shown
at c, provided this be imagined to be sunk  within a cavity formed by a
depression of the external  surface, instead of projecting beyond it.—
Thus we  see how very  close is the real resemblance  between  all the
forms of the respiratory  apparatus, however unlike each other they may
at first sight appear to be.
   663. Thegills  of Fishes correspond with those of the higher  Mollusca
in all essential particulars; but they are more largely developed in pro-
portion to the size of the body; and they are placed in  a situation, that
enables them to receive a more regular and constantly-changed supply,
Fig. 104.
  Capillary network of a pair of [leaflets of the gills of the Eel:—a, a, branches of the branchial artery con-
 veying venous blood; b, b, branches of the branchial vein, returning aerated blood.  The disappearance of
 the dark shading of the network, as it traverses the gill, is designed to indicate the change in the character
 of the blood, as it passes from one side to the other.

 both of blood and water.  The gills are suspended to bony ,or cartilagi-
 nous arches, of which three, four, or more, are fixed on  either  side  of
 the neck ; and the fringes hang loosely within the cavity, which  commu-
 nicates on the one hand with the mouth, and on the other with the ex-
 terior  of  the  body.  The mechanism of respiration is very complex in
 these animals;  and is  evidently adapted  to produce the most effectual
 aeration possible.  The mouth is first distended with  water;  and its
 muscles are then thrown into contraction, in such a manner as to expel
 the water, through the aperture on either side of the pharynx, into the
 gill-cavity.  At the same time,  the bony arches are lifted and separated
 from each  other, by the  action of muscles especially adapted  to  this
 purpose;  so that the gill-fringes may hang freely, and may present no
 obstacle to  the flow of  the Avater betAveen them.  When  they have been
 thus bathed with the aerating liquid, and  their blood has  undergone the
necessary change, the  water is expelled through the outward aperture
on each  side  of  the  back of the neck ; which is furnished with  a large
flap  or valvular cover,  termed  the operculum.   In  some of the  cartila- 
370
OF RESPIRATION.
ginous Fishes, each branchial arch is enclosed in  a separate cavity;
which communicates on the inner  side with the pharynx by an orifice
peculiar  to itself, and  by  another  orifice with  the external surface.
Thus there is a series of external  openings, instead  of a single  one, on
each side of the neck ;  and these sometimes amount to six or seven, as
in the Lamprey, reminding us of the spiracles of Articulated animals;
whilst there is a corresponding series of internal  openings in  the pha-
rynx  on  either side, or into a tube that communicates with it.
   664. It is well known, that most Fishes speedily die when  removed
from the water; and it  can be easily shown, that the deficient aeration
of the blood is the immediate cause of their death.   But as it might
have  been expected, that the  atmosphere Avould exert a  much more
energetic influence upon the blood  contained in the  gills, than that
which is  exercised by  the  air  contained in  the Avater, the  question
naturally arises, how this  deficient aeration comes to  pass.   It is chiefly
due  to the two following causes:—the drying-up of the membrane of
the gills  themselves, where it is exposed to the air, so that the aeration
of the blood is impeded ;—and the flapping together of the filaments of
the gills, which no longer  hang  loosely and  apart, but adhere in such
a manner  as to prevent  the exposure of the greater portion of their
surface to the air.  Those fishes can live longest out of water, in Avhich
the external gill-openings are very small  so that the gill-cavity may
be kept full of fluid;  and there are certain species which are provided,
like  the  Land-crab, with  a particular apparatus for keeping the gills
moist, and which  perform long migrations over land in search of food
even (it is said) ascending trees.   These are exceptions  to  the  general
rule.
  665. The  respiration of Fishes is much more energetic than that of
any of the lower aquatic animals ; and this is partly due to the great
extension of the  surface  of the gills, partly to the  provision just ex-
plained for maintaining a constant Aoav of fresh water over their sur-
face, and partly to  the  position of the heart at the base of the main
trunk that conveys the  blood to the gills (§ 588), by which the regular
propulsion of that fluid  through these  organs is secured.   Their blood
too, is furnished with red corpuscles ; which  give important aid in con-
veying oxygen from the gills to  the remote tissues of the body, and in
returning the carbonic  acid to be excreted.   The proportion of these
varies^ considerably, in  the different species of the class, being very
small in those that approach most nearly to the Invertebrata ; and there
is  even an entire absence of them in  one  remarkable fish, the Amphi-
oxus or Lancelot; whilst they are present in large numbers in the blood
of certain Fishes, which have great vascular activity, and can maintain
a high independent temperature.
  Q6Q. It would seem, however,  that not even this high  amount of
respiration is ahvays sufficient for  Fishes which live  in small collections
of water, where their temperature is liable to be greatly augmented by
the heat  of summer ; under which condition, there is an increased prone-
ness to disintegration in their tissues, and a corresponding necessity for
the extrication of carbonic acid and for the absorption of oxygen.  Many
fresh-water fishes, under such circumstances, may be seen to come to the 
RESPIRATION IN REPTILES.
371
 surface and to swallow air;  and it would seem as if the interior of the
 intestinal canal then served  the purpose of a respiratory surface, the
 air being expelled from the anus, deprived of a large part of its oxygen,
 and highly charged with carbonic acid.
   667. In  addition to  their  apparatus for  aquatic respiration, many
 Fishes are provided, in their air-bladder, with the rudiments of  the air-
 breathing apparatus of higher animals; although  it is  only in certain
 species,  which  approach Reptiles in  their  general organization,  that
 this  really  affords any aid in  the  aeration of the  blood.  The air-
 bladder  in  its simplest condition is  entirely closed;  and it  is  then
 obviously incapable  of taking any share  in the respiratory function,
 although it seems to be an organ of some importance  to the animal, in
 regulating its specific gravity, and altering its  position in the water.
 In other cases, it communicates with the  intestinal tube by a short,
 Avide canal, termed the  ductus  pneumaticus; and this  may serve to
 admit air, Avhich is  taken  into the alimentary tube by the process of
 swalloAving just mentioned.    In the Reptilian Fishes,  just adverted to,
 the air-bladder forms a double sac, which is  evidently the representative
 of the double lung of the  air-breathing Vertebrata;  and it communi-
 cates with the back of the  mouth by a regular trachea or windpipe,
 which has a muscular valve  at its commencement, serving to open or
 to close its orifice.  Some of these fishes are able to live for a consider-
 able  time out of wrater,  their respiration  being maintained by these
 rudimentary lungs; and  they  can also make a hissing sound,  by the
 expulsion of the contents of  the air-sacs through the narrow glottis, or
 entrance  to the trachea.
   668. The condition of the  Respiratory apparatus, and the mode in
 which the function is performed in the  class of Reptiles, are peculiarly
 interesting; as  it is in this class, that we first meet with the complete
 adaptation of the Vertebrated  structure to the aeration of the blood by
 the direct influence  of the  atmosphere.  Their  general  habits of life
 require but a very feeble amount of aeration, especially at moderate
 temperatures ;  their muscular and nervous  systems being usually exer-
 cised in a very low degree;  their movements being sluggish, and their
 perceptions obtuse.   In fact, they may be considered,  on the whole, as
 the  most vegetative of all Vertebrated animals.   In  accordance with
 this character, the lungs  are so constructed as not to  expose any .very
 large amount of blood to the air at  any one time;  and,  as we have
 already seen (§ 563), only a  portion of the  stream  of the circulation is
 diverted to the lungs ; the main current being sent to  the system, with
 only that amount of aeration, which it  has  derived  from the admixture
 of the portion of blood  that has been  aerated in  the lungs, with  the
 A'enous current that has last been returned from the system.
   669.  The lungs of Reptiles  are, for the  most part, capacious sacs,
 occupying a considerable part of the cavity of the trunk; but they are
very slightly subdivided, so that the amount of surface  they can  expose
is  really small.  Where any subdivision exists, it is usually at the upper
extremity of the lung, near the point of entrance of the bronchial tube;
and where there is no actual subdivision of the cavity, we usually find
that its surface is extended in  this situation, by  the formation of a
number of little depressions or pouches  in its walls,  upon which  the 
372
OF RESPIRATION.
Fig. 105.
Section of the Lung of the Turtle.
blood-vessels are minutely distributed.   The greatest amount of subdi-
vision is seen in the  lungs of the Turtle  tribe; but  even in these, the
partitions scarcely form a complete division at any part  of  the lungs;
and  the ultimate air-cells are of very large  size (Fig. 105).  The  air-
                       sacs of  Reptiles are  not  filled,  like  those of
                       Mammalia, by an act of inspiration, but hy a
                       process of swallowing, which is  comparatively
                       tedious ; and, from the small amount of aerating
                       surface, in proportion to the  amount of air thus
                       received into the cavity, one inflation of the air-
                       sacs lasts for  a considerable time.  When the
                       replacement of  oxygen by carbonic  acid has
                       proceeded to an extent that  renders the air no
                       longer fit to remain  in the lungs, these cavities
                       are  emptied by pressure exercised  upon them
                       by the muscles of the  trunk ; and the slow exit
                       of the air through the narrow glottis is accom-
                       panied by a prolonged hissing sound, which is
                       the only sort of  voice that is possessed by the
                       greater  part  of the Reptile class.  The lungs
                       are again filled by the swallowing-process; and
                       all goes  on as before.
                          670. Now in the Frog tribe, which forms the
                       lowest order of Reptiles (and which is sometimes
ranked as a distinct class, under the title of  Amphibia),  the respiration
during  the  early or Tadpole state, is aquatic; being  carried on by
means of  gills, and conducted exactly upon the plan of that of Fishes.
The lungs are not developed, until a period long subsequent to the ani-
mal's emersion from the egg; and as  soon as they are  ready to  come
into play, an  alteration begins to take place in  the circulating system,
by which the current of blood is  diverted towards them, and away from
the gills (§ 562).  This change takes place to its full extent in the Frog,
Toad, Newt, and their allies; which henceforth have a respiration and
a circulation exactly analogous to that  of Reptiles in general; but it is
checked in the Proteus, Siren, and other species, Avhich form the peren-
nibranchiate group,—so called from the persistent  character of  their
gills, which still remain in action, the lungs never being  sufficiently  de-
veloped to maintain the  respiration by themselves.  The curious  influ-
ence which  Light possesses  on this metamorphosis, has been already
referred to  (§ 95).
   671. This order Batrachia  is  further distinguished from other Rep-
tiles, even when the metamorphosis is complete, by the softness and
nakedness of the skin, which is destitute of the scales and horny plates
that cover it in the Lizards,  Serpents,  and Tortoises.  The  skin of the
Frog tribe  is a very important  organ  of respiration, being  richly sup-
plied with blood-vessels, and exposing their contents to the influence of
the air, under  circumstances  nearly as  favorable as those afforded by
the imperfectly-developed lungs of these animals.   Thus a Frog, from
which the lungs have been removed, will live a  considerable time  at  a
moderate temperature, if its skin be freely exposed to a  moist air; for,
in consequence of the peculiar mode in Avhich the circulation is carried 
                RESPIRATION IN REPTILES  AND  BIRDS.            373

 on in these  animals  (§ 561), the  interruption to the  flow of  blood
 through the lungs does not, as in the higher classes, produce a stagna-
 tion  of  the general current through the body;  and the blood receives,
 in its course through  the  skin, a sufficient  amount  of aeration for the
 support  of life.   Indeed,  at a low temperature,  the influence of  water
 on the skin is sufficient (by means  of  the air included  in the liquid) to
 remove the small amount of carbonic acid then ready for excretion, and
 to supply the requisite amount of oxygen;  and Frogs may thus live
 beneath the water for any length of time, without coming to the surface
 to breathe.^  But with the rise of the temperature of their bodies, their
 blood requires a  higher degree of aeration;  and they then come to the
 surface to take in  air by  the mouth. Avhich aerates the blood through
 the lungs.  It appears that, during the heat of summer, the pulmonary
 respiration, and  the influence of the water on the skin, are not sufficient;
 as it is  found that  Frogs  die, if they are  confined to the water under
 such circumstances,—their natural habit being to quit the water at
 such times, so that the air may exert  its  full influence on their skin as
 Avell  as on  their  lungs.  They do not, however, quit the neighborhood
 of water, and soon die if exposed to a dry atmosphere, for, if the skin
 become dry, its aerating function  can be no  longer performed.   The
 same result happens if the passage of gases through the skin be impeded
 by smearing it over with any unctuous substance.  We shall presently
 find reason to believe that  this cutaneous respiration is a very important
 part  of the function, even in Man and Mammalia.
   672. The class of Birds presents a most striking contrast to that of
 Reptiles, in regard to the energy of the respiratory function, and the
 extent of  the apparatus destined to its performance.   The air-cells are
 considerably diminished in size, so that the extent of surface over which
 they expose the  blood to  the air is greatly increased; and there even
 seems reason to believe that the air comes into direct contact  Avith the
 vessels of the very close capillary plexus which intervenes between the
 air-cells.   But the subdivision of the lungs is not carried to the  same
 degree of minuteness as it  is in Mammalia;  and  the required  extent of
 surface would not be afforded by  the lungs alone.  In addition to these
 organs, we find large  air-sacs, communicating with them, disposed in
 different  parts of the body,—such as the abdominal cavity, the inter-
 spaces among  the  muscles,  the  spaces  betAveen  the  muscles and the
 skin,  &c.   These very greatly increase  the respiratory  surface,  their
 lining membrane being extremely vascular, and  adapted to expose the
 blood to  the influence of the air.   In most Birds, the bones themselves
 are hollow, and the lining membrane of their cavities serves as an addi-
 tional aerating surface, the air being introduced into the interior of the
 bones, by canals that communicate directly with the lungs.   So free is
 this  communication, that the respiration  has  been known to  be main-
 tained through the fractured humerus of an Albatross, when an attempt
 was made  to destroy the  bird  by compressing its  trachea.  Thus the
respiratory surface is  extended into the  remoter parts of the system,
very  much as in  Insects;  and the hollowness of the  bones, together
with the  presence of numerous air-sacs in different parts  of the body,
contribute to diminish  its  specific gravity.  The large quantity of air 
374
OF RESPIRATION.
thus included in different  portions of the frame, also serves, like that
contained in the air-sacs of Insects, as a reservoir for the supply of the
principal  aerating  organs  during active flight, when the  respiratory
movements are less free.
   673.  The mechanism of  Respiration in Birds is very  different from
that which  produces the respiratory movements  in  Mammalia.  The
cavities of the chest and thorax are not  yet separated by a diaphragm ;
except in  a very small number of species, that approach most nearly to
the next class.  But, on the other hand, the whole cavity of the trunk
is more completely  enclosed in a bony casing, the ribs being connected
with the sternum by osseous prolongations from  the latter, instead of
by cartilages, and the sternum itself being so largely developed, as to
cover almost the entire front of the body.  Now the natural condition
of this bony framework is such, that, when no pressure is made upon it,
the  cavity it encloses is in a state  of distension;  and the  state of
emptiness can only  be produced by a forcible compression of the frame-
work, through  an exertion  of muscular  power.   The lungs, instead of
being freely suspended in the cavity of the chest, as in  Mammalia, are
attached to the ribs; and their OAvn tissue is endoAved with  a degree of
elasticity, which causes them  to  dilate when  they are permitted to do
so.  In the state of distension, therefore which is natural to  the cavity
of the trunk, the lungs are expanded,  and  fill  themselves with air,
which  they  draw in through the  trachea;  and   this  condition they
retain,  until,  by the action of the  external muscles upon the bony
framework, the cavity of the trunk is diminished, and the air is expelled
from the lungs and  air-sacs, which are again filled as soon as the pres-
sure is taken off.  As the air-sacs chiefly communicate with the part of
the lungs that is  most distant from the trachea,  the air  has  to  traverse
the whole extent of those last organs, both when it is being drawn into
the air-sacs,  and when it  is being  expelled  from them;  so that it is
made to serve for the aeration of the blood in the most effectual manner.
   674.  Thus, although  the respiratory  apparatus of Birds does  not
possess the highly concentrated development which we  shall find it to
present in Mammals, it serves, by the extension of the aerating surface
through the  body,  to bring the  air and the blood into most intimate
relation;  and the energy of the function is further provided for, by the
mode in which the pulmonary circulation is carried on (a distinct heart,
as it Avere, being provided for it, § 564), as well  as by the arrangement
of the blood-vessels, which  transmit to the respiratory organs the whole
of the blood that has been returned in a carbonated state by the great
veins of the system.  The very large proportion of red corpuscles con-
tained  in the  blood  gives  additional  effect to  these provisions.  The
very high amount of respiration which is natural  to  Birds,  and  which
cannot be suspended  even for a short time without fatal consequences,
has a direct relation (as already explained) with  their extraordinary
muscular  activity, as well  as  with the high bodily temperature which
they are fitted to maintain, and which cannot be lowered in any great
degree without  the suspension  of their other  functions.   Birds  are
peculiarly susceptible of impurities in the atmosphere;  and it has been
shown  by experiment, that if  a  Bird,  a  Mammal, and a  Reptile, be 
RESPIRATION IN BIRDS AND MAMMALS.
375
placed together in a limited  quantity of air, which gradually becomes
vitiated by their respiration, the Bird will die  first, the Mammal  next,
and  the  Reptile  last.  Or  if the Bird  be placed alone in  a limited
quantity of  air, and be left until the atmosphere is so  vitiated as to be
no longer capable  of supporting its life, a Mammal will still live for a
time in  that atmosphere; and  when it  is no longer fit to sustain the
life of the Mammal, the Reptile may still breathe it without injury for
a considerable  period.  There is strong  reason to believe, indeed, that,
in former epochs  of the Earth's  history, when the  Reptile  class was
predominant, supplying the place of Mammals  on land,  and of Birds in
the air, the  atmosphere  was so highly charged writh carbonic acid, as
not to be capable of sustaining the life of the higher air-breathing Ver-
         3.  Mechanism of Respiration in Mammalia and in Man.

   675. It is in the  class of Mammalia that we find  the Respiratory
 apparatus presenting  its  highest degree  of concentration;  and  the
 arrangements for its action the most complete.   The Lungs are divided
 into cavities of extreme minuteness, so that the extent of surface Avhich
 they expose is  enormously increased.   These cavities, or  air-cells,  are
 all connected  with  the trachea by means of  the bronchial  tubes and
 their  minute subdivisions;  but, on account of the minuteness of  these
 passages,  a  considerable force would be required to inflate the air-cells
 with air, if  their distension Avere to be accomplished by the propulsion
 of air through  the  trachea, as we have seen to be the normal mode of
 inspiration in Reptiles.  Moreover,  if the  air were introduced in this
 manner, the air-cells would be the last portions of the pulmonary struc-
 ture that would be distended by it, as well  as the first to be emptied
 Avhen  the air is forced  out again by external pressure.  The mechanism
 of Respiration  in  Mammalia, however, is  so arranged, that the  air is
 most  effectually drawn  into the lungs, instead  of  being forced  into
 them; and  the distension of the air-cells is far more complete than it
 could be rendered in the latter method, besides being accomplished in a
 much shorter time.
   676. The general principle of the operation is this.   The lungs are
 suspended in a cavity that is completely closed,  being bounded above
 and  around by the bony framework of the thorax, the interspaces  of
 which are filled up by muscles and  membranes,  and being entirely cut
 off from  the abdomen below by the diaphragm.  Under  ordinary cir-
 cumstances, the lungs completely fill the cavity;  their external surface,
 covered by  the pleura, being everywhere in contact with the pleural
 lining of the thorax.  But the  capacity of the thoracic cavity is sus-
 ceptible of being greatly altered by the movements of the ribs, and  by
the actions of the diaphragm and  abdominal muscles, as will presently
be explained in more  detail.  When  it is diminished, the lungs are
compressed, and a  portion of the air contained in  them is expelled
through the trachea.   On the other  hand, when it is increased, the
elasticity of the air Avithin  the lungs causes them immediately to dilate,
so as  to fill the vacuum that would  otherwise  exist in  the  thoracic 
376
OF RESPIRATION.
cavity;  and a rush of air takes place down  the air-tubes, and into the
remotest air-cells, to equalize the density of the air they include (which
has been rarefied by the dilatation of the containing cavities) Avith that,
of the surrounding atmosphere.
  677.  The  diameter of  the  ultimate  air-cells  of  the Human  lung
varies from  about the l-200th to the l-70th  of  an inch.   Their shape
is  irregular, and their walls are, for  the most part, flattened against
each  other.  Each of the ultimate ramifications of the bronchial tubes
communicates with  a cluster of these air-cells grouped around it; those
which are in immediate proximity with the  tube open into  it oy well-
defined  circular apertures,  and  the others communicate with  it by open-
ing into these and into each other.   Each air-cell is  lined by an exten-
sion of  the mucous membrane from the bronchial  tubes;  but this  does
not seem to be furnished with an  epithelial  covering.  Between the
adjacent air-cells, is a network  of fibrous tissue, that  forms the connect-
ing medium by which they are  held  together; this tissue appears to  be
of the elastic  kind.   The  pulmonary arteries subdivide into branches,
whose ultimate ramifications form an extremely minute capillary plexus;
and this is disposed betAveen the walls of the adjacent air-cells, so that
each portion of this plexus comes into relation with the air (through the
lining membrane of the contiguous air-cells) on both sides,—an arrange-
ment  which is obviously the most favorable  that can be  to the aeration
of the contained blood.   It has been calculated  by  M. Rochoux,  that
the number of  air-cells grouped around each terminal bronchus is little
less than 18,000; and that the total  number in  the lungs  amounts to
six hundred millions.   If this estimate be even a remote approximation
to the truth, it is evident that  the amount  of surface  exposed  by the
walls  of these  minute cavities,  must  be  very many times greater  than
that of  the whole exterior  of the body.
Fig. 106.
            Arrangement of the Capillaries of the air-cells of the Human Lung.

  678. The larger bronchial tubes are more or less cartilaginous; but the
smaller branches  do not possess any such deposit in their walls, though
still  retaining  their  circular  form.  We find in  the  latter a fibrous
structure, which seems to possess the properties of  non-striated muscle;
and  by this, the diameter of these  tubes appears to be  governed.   The 
STRUCTURE OF  THE  LUNGS IN  MAN.
377
contractility of the walls of the smaller  bronchi may be excited by
chemical, electrical, or mechanical stimuli applied to themselves ; though
it is not so readily caused to manifest itself by stimulating the nerves.
By the continued  influence  of galvanism,  bronchial  tubes of  a line in
diameter have been made to contract, until  their cavity Avas nearly oblite-
rated ;  and it has  been found by Volkmann that a similar effect may be
produced by  galvanising the  Par Vagum.   Supposing the  muscular
fibres of the bronchial tubes to contract during expiration, the effect of
such contraction would be to diminish both the length and the  diameter
of the tubes, and thus to force out their contained air.   Whether such
contraction, alternating with relaxation, takes  place automatically, as a
part of the ordinary rhythmical  movements of respiration, has not yet
been clearly made out;  but in its tonic form, it manifests itself strongly
in certain diseased conditions, especially  in spasmodic  Asthma, which
appears essentially to  consist  in a  contracted  state  of  the smaller
bronchial passages, occasioning an interruption to  the  passage of air
through them.  It is interesting  to observe, that the contractility of the
muscular walls of these tubes has  been  experimentally found  to be
greatly diminished by the application of vegetable narcotics, especially
stramonium and belladonna,—substances which are well-known to have
a powerful remedial influence in spasmodic  Asthma.
   679. The Lungs themselves appear to  be,  almost  entirely, passive
instruments of the Respiratory function.  Their contraction when over-
distended, and their dilatation after extreme pressure, may be partly due
to the elasticity of their structure ; which seems to produce, Avhen acting
by itself, a moderately-distended state of the air-cavities.   This, too, is
the condition that seems most natural to the cavity of  the chest;  the
fullest dilatation, or the most complete contraction, of which it is  capa-
ble, being only accomplished by a forcible  effort.
   680.  The  dilatation  of  the cavity of the chest, which constitutes
Inspiration, is accomplished by two sets of movements;—the elevation
of the ribs, and the depression of the diaphragm.  From the  peculiar
mode in which the ribs are articulated with  the spinal  column at one
extremity,  and from the angle which they make with the cartilages that
connect them to the sternum at  the other,  the  act of elevation  tends to
bring the ribs and the cartilages more into a straight line, and  to carry
the former to a greater distance from the  median plane of the body,
whilst the sternum is also throAvn fonvards.   Consequently the eleva-
tion of the ribs increases the capacity of the thorax, upwards, fonvards,
and laterally.  The moArement is chiefly accomplished  by  the Scaleni
muscles, Avhich draw up  the  first  rib; and by the Intercostals, which
draAv the other ribs into  nearer  proximity  with each other, so  that the
total amount of movement in  each rib increases as we pass from above
downwards,—every one being  drawn  up  by  its   connexion with the
one above it, and being  drawn nearer to  it by the action of  its  own
intercostals.   The elevation of the ribs  is  further assisted  by  the Ser-
ratus magnus, and  by other muscles connected Avith the spine and the
scapula; and when the respiratory movement is very forcibly performed,
the scapula is itself drawn  upwards, by the muscles that descend to it
from the neck, thus producing an increased elevation of the ribs, and 
378
OF RESPIRATION.
an unusual enlargement of the upper part of the thoracic cavity.—When
the Expiratory  action is to be  performed, the descent of  the rib is
occasioned by the  muscles of the spine and  abdomen, which proceed
upwards from the loAver part of the trunk ; and this action is aided by
the elasticity of the costal cartilages.
  681.  In the ordinary act of inspiration,  however, the Diaphragm
performs  the most important part.   The  contraction  of this  muscle
changes its upper surface,  from  the  high arch  that  it  forms Avhen
relaxed and pushed upwards by the viscera below,  to a much  more  level
state;  though it never approaches very closely to a plane;  being some-
what convex, even when the fullest inspiration has been  taken.  When
thus drawn  down, it  presses upon the abdominal viscera, and causes
them to project forwards, which  they are allowed  to do, by the relaxa-
tion of the  abdominal muscles.  In  tranquil breathing, this action is
alone nearly sufficient to produce  the  requisite  enlargement of the tho-
racic cavity ;  the position of the ribs being very little altered.  In the
expiratory movement, the diaphragm is altogether passive;  for, being
in a state  of relaxation, it is forced upwards by the abdominal viscera,
which are pressed inwards by the contraction of the abdominal muscles.
These last, therefore,  are the main instruments  of  the expiratory move-
ment;  diminishing the  cavity of the  chest by elevating  its floor, at
the same time that they draw its bony framework into a narrower com-
pass.
  682.  In this manner, by the regularly-alternating dilatation and con-
traction of the thoracic cavity, the air within the lungs is alternately in-
creased and diminished in  amount;  and  thus a  regular  exchange is
secured.   This exchange, however, can only affect at any one time a
certain proportion of the air in the lungs ;  thus it  is probable, that the
quantity remaining in these  organs  after  ordinary expiration is above
100 cubic  inches, whilst the amount usually expired is not  above  20
cubic inches.  Indeed if it were not for the tendency of gases to mutual
diffusion, the air in the remote air-cells might never be renewed.—If
any aperture exist, by which air could obtain direct access to the pleural
cavity,  the  lungs  would not be  dilated by its  enlargement; for the
vacuum would be supplied much more readily, by  the direct  ingress of
the air (provided the aperture be large enough), than by the  distension
of the  lung.  Thus a large  penetrating wound of the  thoracic cavity
may completely throw out of use the lung  of that side ; and the same
result will follow, when an  aperture forms by ulceration  in the  sub-
stance  of the lung itself, establishing a free communication betAveen the
pleural cavity and one of the bronchial tubes;  so that, of the air which
rushes in by the trachea, to compensate for the enlargement of the tho-
racic cavity, a great  part  goes  at once into  that cavity, without  con-
tributing  to the  distension of the lungs, and  therefore without semng
for  the aeration  of the blood.
  683.  The number of the respiratory movements (that is, of the acts
of inspiration and expiration taken together) may be probably estimated
at from 14 to  18 per minute, in a state of health, and of repose of body
and mind.    Of these,  the greater  part are  moderate in amount, involv-
ing little movement except in the diaphragm; but a  greater exertion, 
,                MECnANISM  OF RESPIRATION  IN MAN.            379

attended with a decided  elevation of the ribs,  is usually made at every
fifth recurrence.   The frequency of the respiratory movements, hoAv-
ever, is liable to be greatly increased by various causes, such as violent
muscular exertion, mental  emotion, or quickened circulation ; whilst it
may be diminished by the torpidity of the nervous centres, on whose
agency the movement depends,—as we see  in apoplexy, narcotic poison-
ing, &c.  An acceleration seems very constantly to take place in dis-
eases, which unfit a part of the lung for the performance of  its func-
tion ;  and the rate bears a proportion to the amount thus thrown out of
use.  Thus,  the usual proportion between the respiratory movements and
the  pulse being  as 1 to 4-£  or  5, it may become  in Pneumonia as 1
to 3, or even in  severe  cases 1  to 2 ;  the increase in the number of
respiratory  movements being much greater in  proportion, than the aug-
mentation of the rate of the pulse.  But it must be remembered by the
practitioner, that a simply hysterical state  may  produce, in young
females, an  extraordinary acceleration of  the respiration; the number
of movements being sometimes no less than  100  per minute.  There
will be a great increase, also, in the number of inspirations, when  the
regular movements are prevented from being fully performed, by any
cause  that  affects their mechanism, even  whilst the lungs  themselves
are quite sound.   Thus in  inflammation  of the  pleura, or pericardium,
or in rheumatic  affections of the intercostal muscles, the full  action of
the ribs is prevented by the pain which the movements produce ; and
the same is  the case in regard to the diaphragm, when  the peritoneum
or the abdominal viscera are  affected with inflammation.   Under such
circumstances, there is an involuntary tendency  to make up for the de-
ficiency in the amount of the respiratory movements, by an  increase in
their number.
   684. The combined actions of the  respiratory muscles, which have
been noAV explained, belong to the group termed  reflex ; being the result
of the operation of a certain part of the nervous centres, which does
not im'olve  the will, or even  sensation, and which may continue when
all the other parts of the nervous centres have been removed.  In the
Invertebrated Animals, we commonly find  a distinct ganglionic centre
set apart for the  performance of the  respiratory movements ; and the
division of  the nervous  centres in Vertebrated animals,  which is the
seat of the same function, may be clearly marked out, although it is not
so isolated from the rest.  It is, in fact, that segment of the Medulla
Oblongata and upper part of the Spinal Cord, wrhich is connected with
the 5th, 7th, and 8th pairs of cephalic nerves, and  with  the  phrenic.
The entire brain  may be removed from aboAre (by  successive slicing),
and  the whole spinal cord may be destroyed beloAV;  and yet the respi-
ratory movements of the diaphragm  will still continue,—those  of the
intercostal and other muscles being of course suspended, by the destruc-
tion  of that  portion of  the cord from Avhich their nerves arise.  But if
the spinal cord be divided, betAveen the point  at which it receives the
5th and 8th  pairs of nerves, and that  at which it gives origin to the
phrenic, the movements of the diaphragm immediately cease;  and this
is the reason Avhy death is so instantaneous, in  cases of luxation or frac-
ture of the higher cervical vertebrae, causing  pressure upon the spinal 
380                      OF  RESPIRATION.
cord just beloAV its exit from the  cranium; whilst  if  the  injury  take
place below the origin of the phrenic nerve, life  may be prolonged for
some time.
  685. The Respiratory movements, like other reflex  actions (§ 394),
depend upon a stimulus of some kind, originating at the extremities of
the nerves, propagated  towards  the  centre by the afferent trunks, and
giving rise  to a motor  impulse, which is  transmitted along the efferent
or motor nerves to the  muscles, and  which occasions their contraction.
Now the importance of the respiratory function  to the maintenance of
life which has already been sufficiently  pointed out,  necessitates an
ample provision for its due performance ; and thus  we find that the
stimulus  for  the  excitement  of  the movements may be transmitted
through several channels.   Its chief source, no doubt, is  in the lungs;
and arises from the presence of venous blood  in  the capillaries, and of
carbonic acid in the air-cells.  Under ordinary circumstances,—that is,
when the blood is being duly aerated, and the air being properly renewed,
—the impression thus made upon the nerves of the lungs is so faint, that
we  cannot perceive it, even when we  specially direct our attention  to it.
But if we suspend  the movements for a moment or two, we immediately
experience a sensible  uneasiness.   The Par Vagum is obviously the
channel,  through which this impression is conveyed to  the nervous cen-
tres ; and it is found that, if the trunk of this  nerve be divided on both
sides, the respiratory movements are greatly diminished in frequency.
Hence it is undoubtedly one of the principal excitors of the respiratory
movements.
  686. But the sensory nerves of  the general surface, and more parti-
cularly the sensory portion of  the Fifth pair, which  supplies the  face,
are most important auxiliaries, as excitor nerves ;  the inspiratory move-
ment being peculiarly  and forcibly excited  by impressions  made  upon
them, especially by the contact of  cold  air or  water with the  face.
The poAver of the impression made by the air upon  the general surface,
and particularly upon  the face, in exciting  the inspiratory movement,
is well  seen in the  case of  the first inspiration of the  new-born infant,
which appears to  be  excited  solely in this  manner.   An inspiratory
effort is often made,  as soon as the face has  emerged from the Vagina
of  the mother; whilst, on  the  other  hand, if the face be  prevented
from coming  into  contact  Avith cool air, the inspiratory effort may be
wanting.   When it does not duly take  place, it  may often be  excited
by  a slap with the  flap of  the hand upon the nates  or abdomen; a fact
which  shows  the special  influence  of  impressions upon  the general
surface, in  rousing the motor impulse  in the Medulla  Oblongata, and
in causing its transmission to the muscles.  The deep inspirations which
follow a  dash of cold water upon the  face, or the descent of  the cold
douche or of  the divided streams of  the shower-bath upon the body, or
the shock  of immersion  in the cold  plunge-bath, all testify to the
powerful influence  of such impressions  in the  adult;  and the efficacy
of  other kinds of irritation of the skin, such  as  beating with holly-
twigs, in maintaining the respiratory movements in  cases of  narcotic
poisoning,  shows  that  the  required impressions are  not  restricted to
the contact of cold air  or water.   It seems probable, from various facts, 
        REFLEX CHARACTER OF THE RESPIRATORY MOVEMENT.    381

that the  presence  of  venous blood in the arterial  capillaries of the
system, and  the  consequent  stagnation in  the current through them
(§ 597), may exert an influence through the Sympathetic nerves: which
may be transmitted, by the copious inosculations of that system with
the Par Vagum, to  the Medulla Oblongata; and which may there serve
as a valuable auxiliary in exciting the respiratory movements.
   687.  Of the  mode in which the  impressions,  thus transmitted to the
Medulla Oblongata, act in exciting  the motor impulses which  issue
from  it, nothing is known; but these impulses, directed  along the
phrenic, intercostal, and  other nerves, produce the requisite movements.
When  the stimulus is unusually  strong, various nerves and muscles
are put in action, which do not co-operate in the ordinary movements
of inspiration ; and it; may sometimes be observed, that movements are
thus excited in parts, which will not act in obedience to the will, being
to  all appearance  completely paralysed.  This  fact shows  how  com-
pletely the class of  actions in question is independent of the influence of
the mind; but  we must  not  lose  sight of the  control which the mind,
especially in the  higher  classes of animals, possesses over them.  Va-
rious actions of the respiratory muscles, particularly those of weeping
and laughing, are the most direct means of expressing the passions and
emotions  of the mind;  and are involuntarily excited by these.  And,
again,  the respiratory actions are  placed in a certain  degree under the
control of the Will; in order that they may be subservient to the pro-
duction of vocal sounds, and to the actions of speech, singing, &c.  The
Avill cannot long  suspend the respiratory movements; for the stimulus
to their involuntary performance  soon becomes too powerful to be any
longer resisted.  And it  is well that it should be so ;  for  if the perfor-
mance of this  most important  function were left to our own choice, a
few moments of forgetfulness would be productive of fatal results.  But
it is to the power which the will possesses, of directing and  controlling
the respiratory movements, that Ave owe the faculty of producing  arti-
culate  sounds,  and  thus  of  holding the most direct and intimate con-
verse with each other.
   688.  It is essential for the due performance of the respiratory move-
ments, that  the portion  of the nervous centres, on Avhich they depend,
should be  in a state of  activity.   This  is the case, under ordinary cir-
cumstances, throughout life.  The state of perfect quiescence, to which
the Brain is liable, never affects the Medulla Oblongata;  and the respi-
ratory movements are  consequently kept up with as much  regularity
and energy (in proportion to the requirements of the system), during
our sleeping, as during  our Avaking hours.   But if any cause induce
torpidity of the medulla oblongata, the respiratory movements are then
retarded,  or  even suspended altogether;  and all the consequences of
the cessation of the aeration of the blood speedily develope themselves
(§ 706).   This  is seen in apoplexy ; when the pressure, or other cause
of suspended activity, which at first affected the brain alone, gradually
propagates its influence downwards.   The same is the case in narcotic
poisoning  ; in which also the brain is the first  to be affected,  and  may
suffer alone;  but if the noxious influence be propagated  to the medulla
oblongata, it manifests  itself in the retardation of the respiratory move- 
382
OF RESPIRATION.
ments, and, when  sufficiently poAverful, in their complete  suspension.
Under such circumstances, it is requisite to resort to all possible means
of keeping  up  the respiratory movements;  and when  these fail, arti-
ficial respiration  may be  successfully employed.    For  if, by  such
means, the  circulation  can be prevented from  failing  for  a sufficient
length of time, the ordinary processes of nutrition go on,  the poisonous
matter is gradually decomposed, or eliminated by the secreting organs;
and the nervous centres resume their usual functions.   A torpid condi-
tion  of the medulla oblongata, inducing a retardation of the respiratory
movements, seems to be one of the morbid  conditions  attendant upon
typhoid fever;  and probably depends in the first instance upon a disor-
dered state of the blood, which does not exert its usual vivifying influ-
ence.  In  such cases, the proportion of the respiratory movements to
the pulse sinks as low as 1 to  6, or even 1 to 8; and thus the due aera-
tion  of the blood is not performed, and its  stimulating properties  are
still further diminished.

                4.  Chemical Phenomena of Respiration.

  689. Having  now fully considered the means, by Avhich  the Atmo-
sphere and the Blood are brought into relation in the lungs, we have
to examine into the results  of their  mutual action.   It will be remem-
bered that the Atmosphere contains about 21 per  cent, of Oxygen to
79 of Nitrogen, by measure; or 23 parts of Oxygen to 77 of Nitrogen,
by weight.  The  changes which  it undergoes in  Respiration  may be
considered under four heads:—1. The disappearance of Oxygen, which
is absorbed.  2. The presence of Carbonic Acid, which has been exhaled.
3. The absorption of Nitrogen.   4. The  exhalation  of Nitrogen.   Of
these, the first two are by  far the most  important.—It Avas formerly
supposed  that  the Oxygen which  disappears, is the precise  equivalent
of the Carbonic Acid Avhich is set free (the  latter gas containing  its
own  bulk of the  former);  and that the union of the absorbed oxygen
with the  carbon to be eliminated, takes place in the lungs.   It is now
known, hoAvever, that the carbonic acid is given out ready formed, its
production having  taken place  at the  expense of oxygen  preA'iously
contained in the blood; and that a much larger proportion of oxygen is
usually absorbed, than is contained in  the  carbonic acid  exhaled,  the
difference sometimes  exceeding the third part  of the carbonic acid
formed, [whilst it  is sometimes  so small  that it may  be  disregarded.
This diversity seems to  depend,  partly upon  the  constitution  of  the
species experimented on, and partly upon the degree of development of
the individual, but in great part upon the nature of  the  food; it having
been established  by the  recent  experiments of MM. Regnault  and
Reiset, that the quantity of oxygen absorbed into the system is much
greater on an animal diet, than on a farinaceous.   It is certain that, of
this absorbed oxygen, a part must  enter into  combination with the  sul-
phur and phosphorus of the original components of the body, converting
these into sulphuric and phosphoric acids ; and the remainder must enter
into  other chemical combinations, very probably uniting  with the hydro- 
AMOUNT  OF CARBONIC  ACID EXHALED.
383
gen of the fatty matter, to form part of the Avater which is exhaled from
the lungs.
  690.  This  interchange would  seem  to depend  upon the tendency
which all gases have  to mutual  admixture, when  they are  separated
by a porous septum.   According  to the law discovered by Prof. Gra-
ham,  the  relative  volumes of the gases which will thus replace each
other, are inversely as the square-roots of their specific gravities; thus,
the specific gravity of oxygen being to that of hydrogen as 16 to 1, the
replacing  volume  of oxygen is to  that of hydrogen as 1 to 4.   The
same  holds good,  when one  of the gases  is absorbed by a liquid;  pro-
vided the replacing gas be also capable of being absorbed  to the same
extent.   On  this  principle, the replacing volume of oxygen  is to  that
of carbonic acid  as 1174 to 1000; but as the actual  amounts inter-
changed do not constantly follow this ratio, it is obvious that they are
liable to be modified by other conditions, these being chiefly (it seems
probable) the relative quantities of the two gases already present in the
blood, and the relative facility with which they are absorbed into it or
extricated from it.
  691.  It is  difficult  to  form an  exact estimate of the  actual quantity
of Carbon, thrown off from  the  lungs in  the  form of Carbonic Acid
during any lengthened period:   since the amount disengaged during
experiments carried on for a limited time,  cannot, for many reasons, be
taken as affording a  fair average.   Thus the quantity will vary with
the external temperature, with the  state of previous rest  or activity,
Avith the length of time that has elapsed since a meal, and particularly
with the general development of  the body.  The  amount  of carbonic
acid exhaled is greatly increased by external cold;  as  is shown in the
results of such experiments  as the  following.—Small Birds and Mam-
mals having been enclosed in a limited quantity of air, for  the space of
an  hour, at ordinary temperatures,  the quantity of carbonic  acid they
produced was noted.   The experiment Avas then repeated at a tempera-
ture nearly approaching  that of the body;  and was performed a third
time at a temperature of about 32°.  The following are the comparative
amounts.
	Temp. 59°—6S°	Temp. 86°—106°	Temp, ahout
	Grammes.	Grammes.	Grammes,
A Canary,	0-250	0129	0-325
A Turtle-dove,	0-684	0-366	0-974
Two Mice,	0-498	0-268	0-531
A Guinea-pig,	2-080	1-453	3-006
Thus it Avould appear that the quantity of carbonic acid exhaled between
86° and 106° is not much more than half of that which is exhaled be-
tAveen 59° and 68°; and is only about tioo-fifths of that which is given
off at 32°.
  692.  The quantity of Carbonic Acid exhaled during exercise, and for
a certain  time after it, and  also  after a full meal, is considerably in-
creased ; Avhilst on the other hand, it is greatly diminished during sleep.
Thus  a person who was excreting 145 grains of carbon per hour, whilst
fasting and at rest, excreted 165 after  dinner, and 190 after breakfast
and a Avalk: whilst he only excreted 100 during  sleep.   The variation 
384
OF RESPIRATION.
with the general development of the body, and also with the sex and
age, is considerable.   Thus, the exhalation is almost always greater in
males, than in females of the same age, at every period of life except
childhood.   In males,  the quantity  increases regularly  from eight to
thirty years of age, remaining nearly stationary  until  forty ;—thus it
averages 77*5 grains of carbon per hour at  eight years; 135 grains at
fifteen; 176*7 grains at twenty;  and 189 grains from thirty to forty.
Between forty and fifty, there is a well-marked diminution, the  average
being then 156 grains; and the diminution continues up to extreme old
age, when the amount exhaled scarcely exceeds that which is extricated
at ten years of  age;  thus, .between  sixty  and  eighty, it was 142*5
grains; and  in a man of  a  hundred and two,  it was only 91*5 grains.
These average results, however, are widely departed from in individual
cases; an extraordinary development of the  muscular system being al-
ways  accompanied by a  high rate of  extrication of carbon;  and  vice
versd.   Thus a  man  of  remarkable  muscular vigor,  whose age  was
twenty-six years, exhaled 217 grains of carbon  in an hour;  a robust
man of sixty exhaled 209*4  grains; and an old man of ninety-tAvo, who
in his younger days had possessed uncommon muscular power, and who
preserved a remarkable degree of energy, still gave forth at the rate of
151 grains per  hour.  On  the other hand,  a  man of slight muscular
development, at the age of forty-five, only exhaled 132 grains;  and an-
other at  the age of seventy-six, only 92-4 grains.
   693. In females, nearly the same proportional increase goes on, up
to the time of puberty; when the quantity abruptly ceases to increase,
and remains stationary so long as menstruation continues regular.   The
average quantity of carbonic acid  exhaled by girls nearly approaching
puberty, is about 100 grains per hour ;  and  it  remains at this standard
until  nearly the close of menstrual life.  At the period of the cessation
of the catamienia, it  undergoes a perceptible increase; the average be-
tAA'een forty and fifty years  of age, being about 130 grains per  hour;
and the  quantity exhaled in a woman of great muscular development,
and of forty-four years of age, rising to 152-4 grains in an hour.  After
the age of fifty,  or thereabouts, the quantity decreases, as in men.  It is
remarkable that, during  pregnancy, there  is the  same increase in the
exhalation of carbon, as  there  is after the  final cessation of the cata-
menia; and  the same takes place, if the menstrual discharge  be tem-
porarily  suspended, through any other cause.
   694. It is obviously difficult, then, to obtain  exact estimates, from
any experiments conducted for a short time only, of the total amount
of Carbon thrown off during a lengthened period; since the condition
of the individual varies so greatly at different times; and the variation
amongst different individuals is so great.  Moreover, of the total amount
of carbon excreted in a gaseous form, a certain part is undoubtedly set
free from the skin;  but the  proportion of this does not seem to be con-
siderable.  As a means  of  measuring the whole quantity of carbonic
acid set free, without causing the respiratory movements to be  per-
formed in any unnatural manner, Prof. Scharling constructed  an  air-
tight  chamber, of dimensions sufficient to allow an individual to remain
in it  for  some  time without inconvenience; and so arranged, that he 
AMOUNT  OF CARBONIC ACID  EXHALED.
385
 could eat and drink, read, or sleep Avithin it.  This was connected with
 an apparatus, by Avhich the air was continually renewed; and  the  air
 drawn off was carefully analysed, in order to determine the  quantity of
 carbonic acid contained in it.   The average per hour, in different states
 having been ascertained, it  was calculated that, allowing seven hours
 for  sleep in adults,  and nine hours for children, the total amount of
 carbon consumed in the tAventy-four was as follows:

      No.                                Weighing.      Grains.   Oz. Troy.
      1. A male, aged thirty-five,.....'131  lbs.      3387 or  7-0
      2. A male, aged sixteen,......115^ lbs.      3453 or  7-2
      3. A soldier, aged twenty-eight,  .  .  .  164  lbs.     ' 3699 or  7-7
      4. A girl, aged nineteen,......Ill  lbs.      2540 or  5-3
      5. A boy, aged nearly ten,.....44  lbs.      2054 or  4-3
      6. A girl, aged ten,.......46  lbs.      1930 or  40

   695. This  estimate is perhaps rather too low, as it  does not take
 sufficient account of the great increase which is  produced by exercise.
 Another  method has been adopted by Prof.  Liebig, who  endeavored to
 ascertain the total amount  of carbon  excreted from the body in  the
 form of carbonic acid, by comparing the  amount of carbon taken in as
 food, with  that contained in the faeces and urine ; the difference being
 set  down to the  account of respiration.  His estimate amounts to  the
 very large  sum  of 13*9 oz. of solid carbon per day, which he considers
 to be thus set free by the lungs and skin; but this is almost certainly
 above the truth.  The observations were made upon a body of  soldiers,
 who Avere subjected  to  severe  daily exertion ;  and they were  far from
 being exactly conducted, many of the items being set doAvn by guess
 only, whilst of others no account whatever was taken.   We may per-
 haps consider 10 or 11 oz. as  more  nearly representing  the amount of
 carbon consumed by adult men exposed to severe exertion ;  whilst from
 Prof. Scharling's experiments  it may be inferred, that from 7  to 8 oz.
 of carbon are thrown off during the twenty-four hours, by the lungs and
 skin of adult men not  using  much  active exertion ; to  Avhich  another
 ounce or  tAvo may be added, as the increased quantity excreted during
 moderate exercise.   In a subsequent series of experiments, Prof. Schar-
 ling ascertained  the proportion  of carbonic acid given off from  the
 general surface of the human  body,  to be from about  l-30th to  l-50th
 of the whole amount; so that, adopting  l-40th as the average, out of
 eight ounces of carbon exhaled, only one-fifth of an ounce is set free in
 the form of carbonic acid from the Skin.
   696. If  we assume 10 oz.  or  4800  grains  of solid Carbon, as  the
 total amount excreted from the lungs  and skin of a male adult, using
 active  exercise, in the  course  of twenty-four  hours, Ave  find that  the
•^volume of  carbonic acid thus generated must  be nearly 37,000 cubic
 inches, or  more than 21  cubic feet.   Of this, about 16  cubic  feet are
 probably extricated from the lungs.  But it  is probable, that about 10
 cubic  feet  per  day is nearer  the  ordinary average.  Now it has been
 ascertained, that the whole quantity of air  Avhich passes through  the
 chest  during that time under  similar circumstances, is about 266 cubic
 feet; so  that the proportion of carbonic acid contained in the  expired
 air  seems to average about 4 per  cent.  It is  certain, however,  that
                                  25 
386
OF RESPIRATION.
this proportion may rise  much  higher;  particularly when  the  respira-
tory movements are slowly and laboriously performed.  Now in order
that the blood should  be properly aerated, it  is requisite that the air
should contain no previous impregnation of carbonic acid; since the
diffusion  of even a moderate  percentage  of that  gas through the
inspired  air, seriously impedes the exhalation of more.   Thus  it was
found by Messrs. Allen  and Pepys, that when 300 cubic inches of air
were respired  for three  minutes, only 28J inches  of  carbonic acid (or
somewhat more than 9 per cent.) were present in it;  though the rate
of its production  in a parallel experiment, in which fresh air was taken
in at each inspiration, Avas  32 cubic inches  per minute, or 96  cubic
inches in three minutes.  It appears from the experiments of Dr. Snow,
that the presence of carbonic acid in the atmosphere  acts more delete-
riously on the  system,  in proportion as the normal  quantity of oxygen
has been reduced; and hence, that  the substitution  of  carbonic acid for
oxygen by the respiratory  process,  vitiates the air far more  effectually
than the  introduction of a surplus of carbonic acid, the normal quantity
of oxygen  being still  present.  He concludes from  his  experiments
upon the lower animals, that 5 or  6 per cent, of carbonic acid cannot
exist in an atmosphere respired  by Man, without danger  to life ; and
that less  than  half this amount would soon  be fatal, when it is formed
at the expense of the oxygen of the air.  A still smaller proportion  is
capable  of  producing  very  injurious  results.  Thus the discomforts
occasioned by  the presence of a crowded audience in  a church, lecture-
room, or theatre, which is  not provided with  sufficient ventilation, are
due in great part to the  continued respiration of air, which  becomes
loaded in the course of an  hour or two with carbonic acid gas,  to the
amount of from  one-half  or  two per cent.,—as has been ascertained
both by direct experiment, and by calculation.   And there  can be  no
reasonable  doubt, that the habitual  respiration of such  air, in the nar-
row and noisome dwellings of the poor, or in croAvded factories and
workshops, has a tendency to  produce, both  directly and  indirectly,
much loss  of  physical and mental  vigor, and also to blunt the acute-
ness of the moral  feelings ; its influence being specially noticed in in-
creasing the predisposition to Epidemic  diseases, and in augmenting the
fatality of  their attacks.
   697.  The effects of  a  simple deficiency of Oxygen in the respired air,
are experienced by those who breathe  a rarefied  atmosphere, such  as
that which exists on the summits of high mountains.   All persons who
have made such ascents, have  experienced  the  insufficiency  of rarefied
air  to sustain the degree of  respiration required for active  exertion.
As  long as the body  remains at rest,  no inconvenience is  perceived;
but as soon as the muscular system is  put into action, the insufficiency
of the  supply of oxygen is manifested by the  feeling of distress and
languor; which becomes so severe, that  the  individual,  if unused  to
sueh ascents, is  obliged to  stop and  take breath at every few steps.
The necessity for doing so will be easily understood,  when it is remem-
bered that Avhen the pressure  of the atmosphere is reduced to half  its
usual amount, the bulk of  a given weight of air will be  twice  as great
as at the  surface of the earth, or the same measure  will  Aveigh only 
CHANGES EFFECTED IN  THE BLOOD.
387
half as much.  Consequently, when the chest is  completely filled with
air, the  real  quantity  of  oxygen  included*-in it, is only half of that
which is drawn in by a corresponding inspiration at the earth's surface.
   698. With regard to the absorption and  exhalation of Nitrogen, it
seems probable that both these  processes are constantly  going on;  but
that their relative activity  varies under different conditions.  Thus it
has   been  ascertained  by  MM. Regnault and  Reiset,  that  although
Avarm-blooded animals,  when subjected to  their ordinary regimen, usu-
ally increase the amount of nitrogen in the atmosphere, yet that, when
food is Avithheld, or the animals are fed upon a diet to which they are
unaccustomed, an absorption of nitrogen takes place ; this being  parti-
cularly remarkable  in  the hibernating Mammalia, in which  the gain in
weight by  the absorption of oxygen and nitrogen  even exceeds the loss
occasioned by the exhalation of carbon.
   699.  Having thus considered the changes produced  by the Respira-
tory function, in the air submitted to it, Ave  have next to inquire into
converse series of changes effected  by it in  the  blood.   The  nature of
these cannot be well stated with  precision, as  they have  not yet been
fully determined.   It was  formerly supposed,  that the venous  blood
arrives at the lungs charged with carbon, and that this carbon is united
with the oxygen of the air in the  lungs themselves.   Numerous facts,
however, go to prove, that the  blood comes  to  the lungs charged with
carbonic acid; and  that  it gives out this  ready formed, and receives
oxygen in its stead.  Thus it has been already shown, that there is a
positive  disappearance of oxygen, more of that element being withdrawn
from the atmosphere, than is restored to it in the condition of carbonic
acid ;  so that Ave know that the surplus must be received into the blood.
Further, cold-blooded  animals  may be made to breathe  nitrogen  or
hydrogen for a sufficient length  of time, to cause a large  quantity of
carbonic acid to be disengaged; and this must have been brought to the
lungs ready formed, since no oxygen was present there to generate it.
Lastly, it can be shown by experiment, that oxygen,  carbonic acid, and
nitrogen exist in a free state in  blood, arterial as well as venous;  but
that the proportion  of oxygen is  greater  in arterial than in venous
blood, whilst that of carbonic acid is  less.   The following table expres-
ses the percentage of each kind of gas in the two sorts  of blood respec-
tively, as deduced from the experiments of Magnus.

                                       Arterial Blood.  Venous Blood.
           Carbonic acid,.....62-3        71-6
           Oxygen.......23-2        15-3
           Nitrogen,......14-5        131

Thus it appears that the quantity  of nitrogen is very  nearly the same
in both,  as would  be anticipated from Avhat has been already stated in
regard to its  non-participation in the respiratory process ;  whilst  about
one-third of the free oxygen of arterial blood  disappears  during its cir-
culation  in the systemic  capillaries,  to be replaced by an  equivalent
amount of carbonic acid;  and a converse change takes place in the pul-
monary capillaries, this additional portion of  free carbonic acid  being
set free,  and replaced by oxygen. 
388
OF RESPIRATION.
  700. Thus it  is evident, that a part of the change effected in the
Blood consists in  an alteration in the proportion  of  the  gases which
always exist in it, either  entirely free,  or in a state of such loose com-
bination  that they can be  removed by the air-pump.  But it may be
suspected, that a portion of the effect consists in the oxidation of the
proteine  of the fibrinous constituent; since  the fibrine  of arterial blood
possesses properties that distinguish it from that of venous.  It has
been usually supposed that the hematosine  of the red corpuscles under-
goes a change under the  influence of  oxygen in the lungs, and a con-
verse change in  the systemic capillaries, Avhere it is  subjected to the
influence of carbonic acid;  this  change being indicated by the altera-
tion in the color of  the red  corpuscles.   The alteration  in  question,
however, seems  due rather to a physical  than to a  chemical  change
(§ 222);  and we have no direct evidence, though much that it is indirect,
of the special influence of the aeration of  the blood upon the contents
of the red  corpuscles.  It  appears tolerably certain that a part of the
oxygen imbibed  in the lungs, is  appropriated to the  oxidation of the
matters  set free by the  decomposition of the  solid  tissues;  whilst
another  part enters  into  combination with fatty, saccharine,  farina-
ceous, and  other matters, existing in the blood itself, and destined to be
carried off  in the form of carbonic acid and water, without ever enter-
ing into  the composition  of  the  solid fabric.  The relative amounts
of carbonic acid  formed in  these two modes, vary in  different animals
and in different states of the same individual:  for a  man in a warm
atmosphere, taking a moderate amount of exercise, may thus set free, by
the waste of his  muscular and other tissues, a sufficient quantity of car-
bon for the maintenance of his animal heat by its union Avith  oxygen ; but
this is far from being sufficient, when a larger amount  of heat must be
evolved,  to sustain the temperature of  the body in  a colder climate.
  701. The blood parts in  the lungs with a very large amount of mois-
ture ; for the inspired air is  always saturated with fluid, as soon as it
reaches the air-cells;  and,  as it is heated at the same time to about 98°,
it thus  receiA'es  a considerable  addition,  even if it  were previously
charged  with as  much as it could contain at a lower temperature.  The
total  quantity of  fluid thus  disengaged will  vary, therefore, with the
amount previously contained in the atmosphere, being greater as this
was less, and vice versd ; but the quantity  that usually passes off seems
to be from 16 to 20  ounces  in  the twenty-four hours.  It cannot be
doubted,  that a great part  of this water is  a simple exhalation of that
which has  been absorbed; but, on the other hand, it seems probable
that a portion of it may  be  actually formed in  the system, by the
union of a portion  of  the oxygen absorbed  in the lungs,  with the
hydrogen of the combustible matters of  the blood.  In the  various
forms of  saccharine and farinaceous aliments, the proportion of hydrogen
and oxygen are such as Avould of themselves form  water,  when the
carbon is withdrawn; but in  oily and  fatty  matters, the proportion of
oxygen is far too small thus to neutralize the hydrogen ;  and it seems
likely that by their oxidation  in the blood, as by their combustion else-
where, Avater is actually generated by  the union of atmospheric  oxygen 
ASPHYXIA—ITS CAUSES.                    389
with their hydrogen, at the same time that carbonic acid is produced by
its union with their carbon.
  702.  Along  Avith the Avater thus extricated from the lungs, a certain
amount of organic matter is set free.  If the fluid be collected  in  a
closed vessel, and be  exposed to warmth,  a very evident putrid  odor
is exhaled from it;  and if  the  expired air  be made to pass through
sulphuric  acid, that liquid  is colored red.  Every one knows that the
breath itself possesses, occasionally in some persons, and  constantly in
others, a foetid taint;  when  this does not proceed from carious teeth,
ulceration in the air-passages or lungs, or other similar causes, it must
result from  the excretion of the odorous matter, in combination  with
watery  vapor,  from  the  pulmonary surface.   That this is the  true
account  of  it  seems   evident, from the  analogous phenomenon  of the
exhalation  of  turpentine,  camphor,  alcohol,  and other  odorous  sub-
stances, Avhich  have been introduced  into the venous system, either by
natural absorption, or by direct  injection ;  and also from the  sudden-
ness Avith Avhich the odor  manifests itself, when the digestive apparatus
is slightly disordered.
                                                             -^
      5. Effects of Insufficiency, or Suspension, of the Aerating Process.

  703.  The change which the Blood  undergoes, by being  brought into
relation with atmospheric air in the respiratory organs, is  so important
to life, that  the entire  suspension of it inevitably  produces a fatal ter-
mination,  at no remote period; and if it be  insufficiently  performed,
various  disorders in the system are nearly sure to manifest themselves.
The state Avhich  is induced by the  entire  suspension of  the  aerating
process, is termed Asphyxia; a word which literally means the absence
of pulse, and Avould be applicable  therefore  to the stoppage  of the cir-
culation from any cause; though  it  is  usually employed  to designate
the particular condition resulting from suspended respiration.  Asphyxia
may be produced in aquatic animals, as well as in those which breathe air,
by cutting them oft" from the influence of the atmosphere ; for if a fish
be placed  in water from Avhich the air has been expelled by boiling, it is
precisely in  the condition  of an air-breathing animal placed in a vacuum,
since it  has  no power of obtaining oxygen by decomposing the  Avater it
inhabits, and is entirely dependent for  the aeration of its blood, upon
the air that is  absorbed by the liquid.  Again, if a fish  be placed  in
water impregnated Avith  carbonic acid, its death is nearly  as instan-
taneous as that of an air-breathing animal immersed in an atmosphere
of that gas.
  704.  Asphyxia may result from a great variety of causes.   Thus there
may be a mechanical obstruction  to the entrance of air through the
trachea; as  in  hanging, strangulation, or drowning; or as in occlusion
of the aperture of the glottis, by oedema of its lips, or by  the  presence
of a foreign  body in the larynx.   Or, again, the passage may be perfectly
free, and yet no air may enter, in consequence of some obstacle to the
performance of the respiratory movements.   This obstacle may be me-
chanical ; as when a quantity of earth has fallen round the body, in such a
manner as completely to prevent the distension of the chest and abdomen.
Or it may result (and this is a most frequent occurrence) from torpidity 
390
OF RESPIRATION.
or complete inactivity of the ganglionic centre,  which is concerned in
the respiratory actions ; or from interruption to  the  transmission of its
influence along the nervous trunks.  Further, Avhen there is no obstacle
to the free ingress or egress of air, Asphyxia may be produced by the want
of oxygen in the atmosphere that is respired, or  by the presence of car-
bonic acid in too large an amount.  And  the presence  of other  gases,
which exert a  directly poisonous influence on the blood,—such as sulphu-
retted hydrogen,—produces a state, which may be included under the
same general description.
   705.  Now when, from any of these causes, the free exchange of car-
bonic acid for  oxygen in the pulmonary capillaries is checked, the  first
effect of the interruption appears to be, the stagnation of the blood in
the pulmonary capillaries.   This stagnation is evidently due, not to any
deficiency of power in the heart; for that  organ  is not yet affected ; but
to the insufficiency of the heart's power acting alone, to drive the blood
through the pulmonary capillaries : the force  which should be generated
by chemical changes in them (§ 598), being  deficient.   The stagnation
is not, hoAvever, complete at first; since the quantity of oxygen contained
in the, lungs is sufficient  to produce an imperfect arterialization of the
blood;  and the blood thus partially changed  is  transmitted to the left
side  of the heart, and is thence propelled  to the system.   Owing  to its
half-venous' condition, it cannot exert its usual stimulating influence on
the tissues, especially the muscular and nervous  ; and their powers are
consequently weakened.   For the same reason,  it does not receive its
usual auxiliary force in  the  systemic  capillaries  (§ 599) ;  since the
changes, which it ought to undergo in them,  can only -be partially per-
formed.
   706.  As the air included in the lungs loses more and more of its oxy-
gen, and becomes more and more charged with carbonic acid, the aeration
of the blood in the  pulmonary capillaries  becomes more  and  more im-
perfect  ; the quantity of blood which is allowed to return to the heart is
gradually diminished, and its condition become  more and more venous;
and  at last, the pulmonary circulation  is  altogether suspended.  From
the relation which the respiratory circulation bears  to the systemic, in
all the higher  classes of animals, save Reptiles,  it folloAvs that the sys-
temic circulation must in  like manner be brought to a stand.  The venous
blood accumulates in the pulmonary artery, in  consequence of the ob-
struction of its capillaries; it distends the right caAnties of the heart;
and  the  accumulation extends to the  venous  system  of the body in
general, especially affecting those organs which naturally receive a large
quantity of venous blood, such as the liver and spleen.   The arterial
system, on the other hand, is emptied in a corresponding degree; nearly
all its blood  having  passed  through the systemic capillaries; and no
fresh supplies being  received  from the heart.   From  this  deficiency,
and  from the venous character of the blood which the systemic arteries
do contain, it  results that the nervous  and muscular systems lose their
power; insensibility comes on, at first accompanied  with  irregular con-
vulsive  movements; but in a  short time there is a total cessation  of all
movement except that of the heart; and the pulsations of that organ be-
come feebler and feebler, until they cease altogether.  The immediate
cause of the cessation of the heart's action appears to be different on the 
            ASPHYXIA—ITS PHENOMENA AND TREATMENT.         391
                             t
 two sides.  Both are equally affected by the want of arterial blood in the
 capillaries of their own substance ; but the right side suffers from over
 distension,  which produces  a sort of paralysis  of its muscular tissue;
 Avhilst the left side retains  its contractility, but is  not excited  to con-
 traction,  for want of the stimulus of arterial blood in  its cavities.
   707. In  those warm-blooded  animals which  are  not endowed Avith
 any special provision for enabling them to sustain life during the pro-
 longed suspension of the  respiratory process, insensibility and  loss of
 Aroluntary power almost invariably supervene within a minute and a
 half after the admission of air  to the lungs  has been  entirely pre-
 vented ; though  the  respiratory  efforts and convulsive actions, which
 are dependent upon  the medulla oblongata and spinal cord, may con-
 tinue for  a minute or two longer.  The circulation generally comes to
 a complete stand Avithin ten minutes at farthest.—The chief exceptions
 are in the case of diving animals, which are provided with large arterial
 and venous reservoirs, that  serve to maintain  the  circulation  during
 a prolonged  suspension of  the  respiratory process ; for  the  arterial
 plexuses being ordinarily filled, they afford a supply of aerated blood
 to the systemic capillaries, Avhen other blood is wanting;  and the reser-
 voirs connected with  the venous system, which were previously  empty,
 receive this blood, and  prevent it from exercising undue  pressure  on
 the  heart.  To such an extent is this provision carried in some ani-
 mals, that the Whale has been knoAvn to  remain under water  for  an
 hour.   Another exception exists in  the  case of hibernating Mammals,
 which are reduced for a time to the condition of cold-blooded animals,
 and which  can, like  the latter, sustain a  prolonged  suspension  of the
 aerating process.   And  there is reason to believe that, in the state of
 Syncope or fainting,—in which the circulation is already reduced to a
 very low amount,  in consequence of a partial failure in the heart's
 poAver, all the functions of the body being nearly suspended, and  the
 demand for aeration being  consequently  very small,—the respiration
 may be suspended for a long period, even  in the Human subject, with-
 out fatal results.   Thus more than one case has been credibly recorded,
 in which recovery has taken place after complete submersion for more
 than three-quarters of  an hour;  and  it is probable that, in these in-
 stances, a state of  Syncope came on at  the moment of the immersion,
 through the influence of mental emotion, or of concussion of the brain.
   708.  In the restoration of an  animal from the state of Asphyxia, it
 is above all things  of importance to renew the air in  the lungs; for in
 this way  the  blood in  the  pulmonary capillaries will be aerated, the
 capillary circulation will be re-established, the right  side of  the heart
 will be relieved of its excessive load of venous blood, and the left side
 Avill receive the stimulus of  a fresh supply  of arterial blood ; so that, if
 its movements have not  ceased altogether, it may be  speedily restored
 to due activity.  At the same time, the temperature of the body  should
be kept up by artificial warmth ; and the circulation in the skin should
be excited by  friction.  Where no other means  are at hand for intro-
ducing pure air into the lungs (of which means  the application of gal-
vanism along the course of the phrenic nerve, so as to produce contraction
of the diaphragm, Avill probably be the most effectual), the  object may 
392
OF RESPIRATION.
be attained by forciby compressing the trunk on all sides, so as to empty
the lungs as much as possible, and then allowing the chest to dilate
again, by the elasticity of its walls.  In this manner, a large  proportion
of the carbonic acid may be expelled, and a considerable proportion of
fresh air introduced, in  the course of a few minutes.   If  air be blown
into the lungs by the bellows, great care must be taken to prevent  the
employment  of too much force, which  is likely to produce  rupture of
the air-cells.
  709. Now when, from the more prolonged action  of various  causes
that impede the due performance of the respiratory function, the aera-
tion of the blood in the lungs is insufficient for health, though not such
as  to produce  a  complete  stagnation  of the  movement,  a  variety of
results may follow; of Avhich some, or others, will manifest themselves,
according to the condition of the general system, and the peculiarities
of the individual.  Thus deficient respiration seems  to  favor  the  re-
tention of fatty matter in the system ; and this  not merely in the con-
dition of Adipose tissue, which (unless  it accumulate to excess)  may be
regarded as  a  healthy product;  but also in  the place of the  normal
components of  other tissues, as the muscular and glandular,  giving rise
to the condition  Avhich is termed  "fatty degeneration."—Again,  the
due elaboration of the fibrine of the blood is undoubtedly prevented by
an  habitually-deficient respiration; and various  diseases,  Avhich result
from the imperfect performance of  this  elaboration, consequently mani-
fest themselves.   The Scrofulous diathesis is thus frequently connected
with an  unusually small capacity of the chest.—Further, an habitual
deficiency of respiration may impede, though it does not check,  the cir-
culation in the lungs ; and thus a tendency arises, in various pulmonary
diseases,  to  an overloading of the pulmonary arteries, to a dilatation
of the right  cavities of the heart, and to a congestion of the venous
system in general, as marked by lividity of the surface, by venous pul-
sation, &c.   This  state may result, not merely from  obstruction in  the
lungs themselves, but from deficiency of the respiratory movements,
consequent upon  torpidity  of the  medulla  oblongata (as in apoplexy
and narcotic poisoning)  or upon partial interruption of the nervous
circle requisite for all reflex movements.  Thus Avhen  the  par vagum is
divided,  the  number of respiratory movements is  greatly diminished,
and a partial stagnation of  the blood in the lungs is  the  result.  The
same happens in certain forms of  typhoid fever, in Avhich the respira-
tory movements are preternaturally slow, in  consequence of torpidity
of the medulla oblongata.   Noav in this state, an effusion of the watery
part of the  blood into the air-cells of the lungs (as  in other  cases of
obstructed circulation) is very liable to occur;  and when the lungs  are
thus loaded with fluid, the  respiratory process is  still more impeded,
and the disorder has thus a tendency to increase itself. 
NATURE OF  THE SECRETING PROCESS.
393
                          CHAPTER IX.

                           OF  SECRETION.

    1. Of the secreting process in general; and of the instruments by which
                             it is effected.

   710. We have seen that, in the process of Nutrition, the circulating
 current not  only  deposits the  materials,  Avhich  are  required  for the
 renovation  of the solid  tissues;  but  also takes  back the substances,
 Avhich are produced by the decay of these, and which are destined  to
 be throAvn oft' from the body.   We have also  seen, that it supplies the
 materials of certain fluids, which are separated from it to effect various
 purposes in the economy ; such as  the Salivary and Gastric fluids, which
 have for their office to assist in  the  reduction of the food.  Now the
 process by which  the  fluids of  the latter  kind are separated from the
 Blood, is  precisely the same in character as that by which the products
 of decay are eliminated from it;  and the structure of the organs con-
 cerned in the two  is essentially the  same.  Hence both processes are
 commonly included under the  general term  Secretion,  which simply
 denotes separation.  Considered in its  most general point of  vieAv, this
 designation may be applied to  every act,  by  which substances of any
 kind  are  separated from  the  blood.   Thus the  function of  the  cells,
 which are concerned in the elaboration of  the organizable plasma, may
 be termed one of Secretion, because they draw from the blood a supply
 of Albumen, upon which  their converting action is exercised; but as
 the product  of their operation  is returned to the blood again, and is
 employed for  higher purposes in  the  economy, the process  is usually
 termed Assimilation.   In the same manner,  the  elaborating action  of
 the Lymphatic Glands, with the Spleen, Thymus Gland, &c, is not
 usually termed Secretion ; since,  although it  is exercised  upon matter
 draAvn from  the blood, the product appears to-be delivered  back into
 the circulating current, through the medium of the Lymphatic System.
 (chap, v.)   With much more  justice, however, the process of Respira-
 tion may be regarded  as one  of Secretion; for it consists, as we have
 seen, in the constant elimination of a substance from  the blood, Avhich
 cannot be retained in it without the most injurious consequences.
   711. There is an important difference in the characters of the  prin-
 cipal  products of  the  Secreting  process, which is connected  with the
 purposes that are to be answered  by their separation.  Some of  these
 products are altogether different from the  ordinary constituents of the
 animal fabric, and from the materials which the  blood supplies for the
nutrition of these.  So different are they, that their  presence  in the
circulating current has an injurious effect; and the great object of  their
separation is  the maintenance of the  purity of the blood.  These poi-
sons, for such they may be considered, are generated  in the system by
the decay  and decomposition to  which its several parts are liable; and
they are just as noxious to it,  as if they Avere absorbed from without. 
394
OF SECRETION.
We have seen that the retention of Carbonic acid in the blood for even
a feAV minutes is  fatal, both  by putting a stop to the  circulation,  and
by acting unfavorably upon the substance of some of the most  impor-
tant organs in the body.   The same fatal result attends the retention of
Urea and of Biliary matter, Avhich are among the other products of the
decomposition of the tissues; but,  although as  certain,  it is  not so
speedy, because  the  general  circulation is  not affected by the  loss of
secreting power on  the part of the Kidneys and the Liver, and because
the accumulation of the noxious matter is slower.—On  the other hand,
the ingredients that  are  met  Avith  in those  secreted  fluids, which are
destined to answer  some purpose in  the economy,  almost invariably
bear a close correspondence with the ordinary materials  of the blood.
Thus in  the Salivary, Gastric, Pancreatic,  and Lachrymal fluids, the
principal part of the solid matter consists of the saline and of the albu-
minous constituents of the blood, the latter in a  more  or less  altered
condition.   In Milk, again, we trace  the ordinary constituents of the
blood, with very  little change.  Thus it  appears,  that the separation of
these fluids is not required so  much to maintain the Blood in a state of
purity, as  to supply Avhat is needed for some subsequent operation in
the economy ; and hence, if the secreting process be interrupted, in the
case of any one of  them, the suspension  has usually no further effect,
than that of disturbing the process to  Avhich the fluid is usually  subser-
vient.  If the secretion of Gastric fluid be checked, for example, under
the influence of strong  mental emotion, the  Digestive operation is pre-
vented from taking place.
  712. The essential character of the true  Secreting  operation seems
to consist,—not in the nature  of the action  itself, for  this is  identical
with those of Assimilation and Nutrition, being, as we have seen (§ 239),
a process of  cell-groAvth,—but in the position in which  the  cells are
developed,  and the mode in  Avhich the  products of   their action are
afterwards disposed of.   Thus the cells at the extremities of the intes-
tinal Villi (§ 243), the cells  of which the  Adipose tissue is  made up
(§ 257), and the cells of which the greater part of the substance of the
Liver is formed  (§ 239), all have  an attraction for fatty matter;  and
draw it from the neighboring fluids, at the expense of which they are
developed, to  store it up in their own cavities.   But  the cells   of the
first kind, when  they have come to maturity, set free  their  contents,
which are delivered to the absorbent vessels, to be introduced into the
circulating  current;—those of the second kind seem  more permanent
in their character, and retain their contents, so  as  to  form part of the
ordinary tissues of  the body,  until  they are required  to  give them up
for other purposes, when the matters which they have  temporarily
separated from the  circulating current, are restored to it  again without
change;—and the cells of the third  class, when they liberate their con-
tents (which  they  are  continually doing),  cast them forth  into the
hepatic  ducts, by  which they are  carried  into  the   intestinal  canal,
whence a portion of them at least is  directly conveyed out of the body.
  713. It is, then,  in the position of the Secreting cells,—which causes
the product of their action to be delivered upon  a free  surface, commu-
nicating,  more or less directly, with an external  outlet,—that their 
             SIMPLEST FORM  OF GLANDULAR STRUCTURE.         395

distinctive character depends.  All the proper Secretions are thus either
poured  out upon  the  exterior  of the body, or into cavities provided
Avith orifices that  lead to it.   Thus we  shall see that  a considerable
quantity of solid matter,  and a very large quantity of fluid, of which
it is desirable that the system should be  freed, are carried off from the
Cutaneous surface.   Another most  important  secretion, containing a
large quantity of solid matter,  and serving  also to  regulate the quan-
tity of  fluid in  the  body,—namely, the Urinary,—is  set free. by a
channel expressly adapted to convey it  directly out of  the body.   The
same may be said of the Mammary  Secretion, which is  separated from
the blood, not to  preserve its purity nor to  answer  any purpose in the
economy of the individual, but to afford  nutriment to  another being.
And of the matters secreted  by the  very numerous  glandulae  situated
in the walls of the Intestinal canal, a  great  part  are obviously poured
into it for no other purpose, than that they may be  carried out of the
body by the readiest channel.
   714.  The cells covering the simple  membranes  that form the free
surfaces of the body, whether external or  internal,  are  all entitled to
be  regarded as  secreting cells, since they  separate various products
from the  blood, which are  not  again to  be returned  to it.  But the
secreting action of some of these seems to have for its  object the pro-
tection of the surface; thus the Epidermic cells secrete  a horny matter,
by  which density and firmness are imparted with the cuticle ; Avhilst by
the epithelial  cells of the Mucous membrane of the alimentary canal,
and of  other  parts, their protective  Mucus  seems  to be elaborated.
But in  general we find that special  organs, termed Glands,  are set
apart for the production  of the chief  secretions; and we  have now to
consider the essential  structure  of  these organs, and the mode of their
operation.—A true Gland may be  said to consist of a closely-packed
collection  of follicles,  all of  which  open  into a  common  channel, by
Avhich the product of the glandular action  is collected and delivered.
The follicles contain the secreting  cells  in their  cavities ;  whilst  their
exterior is in  contact with a network of blood-vessels, from which the
cells draw the materials of their  groAvth and  development (Fig. 94).
In  any one  of the  higher  animals, we  may trace out a  series  of pro-
gressive stages of complexity,  in  the  various glands included within
their fabric ;  and in following any one of  the  glands that attain the
highest degree of  development (such as  the Liver or Kidney), through
the ascending scale ©f the Animal series, we should trace a very similar
gradation from the simplest to the most complex form.
  715.  That  there  is nothing in the form or disposition of the compo-
nents of  the glandular  structure, which can have  any influence upon
the character  of  the secretion  it elaborates, is  evident  from  the fact,
that the  very  same product,—e. g.,  the  Bile, or the Urine,—is found
to  issue from  nearly  every variety of secreting  structure, as Ave trace
it through the  different croups of the Animal kingdom.   The  peculiar
power  by Avhich  one orJPm separates from the blood  the elements of
the Bile, and another the elements of the Urine, whilst a third merely
seems to draw off a certain amount of  its albuminous  and saline con-
stituents, is obviously the  attribute  of the ultimate  secreting  cells, 
396
OF SECRETION.
which are the real agents in the secreting process (§ 239).   Why one
set of cells should secrete Bile, another Urea, and  so on, we do not
know; but we are  equally ignorant of  the  reason,  for which one  set
of cells converts  itself  into Bone,  another into  Muscle, and so on.
This  variety  in  the endowments  of the  cells, by  whose aggregation
and  conversion the  fabric of the  higher Animals is  made up,  is a fact
which we cannot explain, and which must be regarded (for the present,
at least), as one of the "ultimate facts"  of Physiological Science.
   716.  Passing by the  extended membranous  surfaces, and the protec-
tive  cells with Avhich they are covered, we find that  the  simplest form
of a  secreting organ is composed of an inversion of that  surface into a
series of follicles, which discharge their contents  upon it by separate
orifices.   Of  this we have  an example  in the gastric follicles, even in
the higher animals;  the apparatus for the secretion of the Gastric fluid
never attaining any higher  condition, than that of  a series of distinct
follicles, lodged in the walls of the stomach, and pouring their products
into  its  cavity by separate  apertures.   In Fig. 107 is  represented a
a portion of the Ventriculus succenturiatus of a Falcon ; in which the
simplest form of such  follicles is seen.  A  somewhat more  complex
condition is seen in some of the Gastric follicles of the Human stomach
(Fig. 80); the surface of each  follicle being  further  extended by a sort
of doubling upon  itself,  so as to form  the commencement  of  secondary
follicles, which open out of the  cavity of the primary one.—Now a con-
dition of this kind is common to all glands, in an  early stage of  their
evolution; and in  this stage, we meet Avith them,  either  by  examining
them in the lowest animals in Avhich they  present  themselves, or by
looking  to  an  early period of their embryonic  development in the
highest.   Thus, for example, the Liver consists, in certain Polypes and
in the lowest Mollusca, of a series of isolated  follicles,  lodged  in the
walls of the stomach, and pouring  their product into  its cavity by sepa-
rate  orifices;  these follicles being recognised  as constituting a biliary
apparatus, by the color of  their secretion.   And in the Chick,  at an
Fig. 107.                           Fig. 108.
          Glandular follicles in ventriculus      Origin of the Liver from the intestinal wall, in
*•           succenturiatus of Falcon.          the embryo of the Fowl, on the fourth day of in-
                                     cubation :—a, heart; b, intestine; c, everted por-
                                     tion giving origin to liver ; d, liver ; e, portion of
                                     yolk-bag.      jjL


     early period of incubation,  the condition of the Liver is essentially the
     same with the preceding; for it consists of a cluster  of isolated follicles
     not lodged in the Avails of the intestine, but clustered  around a sort  of 
DIFFERENT  FORMS OF GLANDULAR  STRUCTURE.        397
bud  or diverticulum of the intestinal tube, which is the first condition
of the hepatic  duct, and into which they discharge  themselves (Fig.
108).   So, again, the Pancreas first  presents  itself in  the condition of

             Fig. 109.                           Fig. 110.
    Rudimentary Pancreas from Cod;—a,          Mammary Gland of Ornithorhyncus.
   pyloric extremity of stomach; 6, intes-
   tine.
a group of prolonged follicles, or cseca, clustered  round the commence-
ment  of the intestinal tube ; and this is its permanent condition in many
Fishes (Fig. 109).  And  the Mammary  Gland  possesses an  equally
simple structure in the  lowest of all the Mammalia (to which group it is
restricted); namely, in the Ornithorhyncus (Fig.  110).
   717.  The  next grade  of  complexity is seen, Avhere  a cluster of the
ultimate follicles  open into  one  common  duct, which  discharges their
product by a single outlet;  a single gland often containing a number  of
such clusters, and  having, therefore, several excretory  ducts.   A good
example of such a condition,  in  which the  clusters  remain isolated
from  one  another,  is  seen  in the Meibomian glands of the  eyelid
(Fig.  Ill); each of which consists of a double  ro>v of follicles, set upon
a long straight duct,  that receives the  products of their secreting action

            Fig. 111.                            Fig. 112.
    Meibomian glands of upper lid of          Portion of Cowper's gland from Hedgehog; the
         new-born infant.                     follicles distended with air.

and  pours  them  out  upon  the edge of the eyelid.   And of the more
complex form, in  Avhich a number of such clusters are  bound  together
 
398
OF SECRETION.
in one glandular mass, we have an illustration in  the accessory  glands
of  the genital  apparatus, in several animals, which discharge  their
secretion  into the  urethra by numerous  outlets (Fig. 112);  or in the
Mammary glands  of Mammalia  in  general, the  ultimate follicles  of
which are clustered upon  ducts  that  coalesce to a considerable extent,
though continuing to form  several distinct  trunks even to their termi-
nation.  Such glands may be subdivided, therefore,  into  glandulse  or

                               Fig. 113.
                   Lobule of Lachrymal Gland; from foetal sheep.

lobules, that  remain entirely distinct from each other (Fig. 113).—In
the highest form of Gland,  however, all the ducts unite; so as to form a
single  canal, which conveys away the products  of the secreting action
of the entire mass.  This  is the condition in which we find the Liver
to exist, in most of the higher animals ;  also the Pancreas, the Parotid
Gland, and many others.   In  some of these cases, Ave may  still sepa-
rate the  gland into numerous distinct  lobules, which  are clustered upon
the excretory duct  and its branches, like grapes upon a stalk; in others,
however, the branches of the excretory duct do not  confine themselves
to ramifying, but  inosculate, so as to  form a  network,  which passes
through  the whole substance of the gland, and which connects together
its different parts, so as to render the  division into lobules less distinct.
This seems to be the case in regard to the Liver  of the higher Verte-
brata (§  723).
  718. Whatever degree of complexity, however, prevails in the gene-
ral  arrangement of the elements of  the Glands in higher animals, these
Fig. 114.                             Fig. 115.
Two follicles from the liver of Carcinus mcenas (Com-    Ultimate follicles of Mammary gland with their
  mon Crab), with their contained secreting cells.         secreting cells, a, a ;—b, b, the nuclei.

elements are themselves  everywhere  the same; consisting of follicles
that enclose  the real  secreting cells  (Figs. 114 and 115).  Noav from 
DIFFERENT FORMS OF  GLANDULAR STRUCTURE.       399
 the history of the development of Glands in general, it appears that the
follicles may be- considered as parent-cells and that  the  secreting cells
 in the interior  constitute a second generation,  developed from the
 nuclei germinal spots on the walls of the first.  Of such parent-cells, we
 have characteristic examples in the Peyerian glandulae of the intestinal
 canal (§ 450), and also in the Thyroid gland (§ 513) and Supra-Renal
 capsules (§ 510);  and the most elaborate  glands, in their earliest stage
 of development, present a similar condition.   These closed cells become
 follicles, by opening at one extremity, either  upon a neighboring free
 surface,  or into  a  canal Avhich is prolonged from it.   Thus the first rudi-
 ment of the Liver is formed by a thickening of the cellular  mass in the
 walls of the alimentary canal,  at the spot in which the hepatic duct
 is subsequently to  discharge itself.   This thickening increases, so as
 to form a projection upon  the exterior of the canal; and soon after-
 wards the lining membrane dips down into it, so  that a kind of caecum
 is formed, surrounded by a  mass  of cells, as shown in Fig. 108.  The
 increase of the  mass  seems  to take place by a continual new budding-
 forth of cells from its peripheral portion, which takes place to a consi-
 derable extent before the caecum in the interior begins to ramify.   Gra-
 dually, however, the cells of the exterior become  metamorphosed into
 fibrous  tissue for  the investment  of the  organ; those of the interior
 break down into ducts which form  continuations of the principal canal;
 whilst those which  occupy the  intervening  space, and Avhich form the
 bulk of the gland, seem to be developed into follicles, and to give  origin
 to the proper secreting cells.   As this is  going on, the hepatic mass is
 gradually removed to a distance from the wall of the alimentary canal;
 and  the caecum is  narrowed and lengthened, so as to become a mere con-
 necting pedicle, forming, in fact, the main trunk of the hepatic duct.—
 The development  of the Pancreas, Salivary glands, &c, seems to  follow
 the same plan.
   719.  It has been pointed  out by Prof. Goodsir, that the continued
 development and decay of the glandular structure,—in other Avords, the
 elaboration of its secretion,—may take place in two different modes.  In
 one class of Glands, the parent-cell, having begun to develope new cells
 in its interior, gives way at  one point, and bursts into  the excretory
 duct, so as to become an open follicle, instead of a closed cell:  its con-
 tained or  secondary cells, in the  progress  of their own groAvth, draAv
 into themselves the matter to be eliminated from the blood, and, having
 attained their full  term of life, burst or liquefy, so as to discharge their
 contents into the  cavity of  the follicle, whence they pass by its  open
 orifice into the excretory duct: and a continual new production of secon-
 dary cells  takes place from the  germinal spot, or nucleus, at the  extre-
mity of the follicle, which  is  here a permanent structure.  In this form
of gland, we may  frequently observe the secreting cells existing in vari-
ous stages  of development within a single follicle;  their  size increasing,
and the character  of their contents becoming more distinct in propor-
tion  to  their distance  from the germinal spot (which  is at the blind
termination of the follicle), and  their consequent proximity to the outlet
(Fig.  114).  In some varieties of  such  glands,  however,  as in  the
greatly-prolonged  follicles or  tubuli uriniferi of the kidney,  the produc- 
400
OF SECRETION.
tion of new cells does not take place from a single  germinal spot at the
extremity of the follicle, but from a number of points scattered through
its entire length.—In  the  second type  of Glandular  structures, the
parent-cell does not remain as a permanent follicle;  but, having  come
to maturity and formed  a  connexion with the excretory duct, it dis-
charges its entire  contents into the latter, and  then shrivels up and
disappears, to be replaced by newly-developed follicles.  In each parent-
cell of a gland formed upon this type, we shall find all its secondary or
secreting cells  at nearly the same  grade of development; but the  diffe-
rent parent-cells, of which the  parenchyma of the gland is composed, are
in very different stages of growth at any one period: some having dis-
charged their  contents  and being in progress of disappearance, whilst
others are just arriving at maturity and connecting themselves with the
excretory duct; others exhibiting an earlier degree of development of
the secondary cells ; others presenting  the latter in their incipient con-
dition ; whilst others are  themselves just starting into existence, and as
yet exhibit no traces of a secondary generation.—The former seems to
be the  usual  type of  the ordinary glands; the latter is chiefly, if not
entirely, to be  met with in the Spermatic glands.

                     2.  Of the Liver and the Bile.

   720.  The LiArer is more  rarely absent than any other Gland;  being
discoverable, under some  form or other, in all but the very lowest mem-
bers of the Animal kingdom.   Its simple  condition  in  the higher Po-
lypes has been already noticed (§ 716);  and it  is met with,  under an
almost  equally simple form, in the Starfish.   As  we ascend the scale,
hoAvever, we find it assuming a much greater importance, and presenting
a great increase in size.   This is particularly the case  in the Mollus-
cous classes; and also  in  the Crustacea, a class which, in mode of respi-
ration and in general habits, bears a great resemblance to the Mollusca.
In nearly all such animals the Liver makes up  a large proportion of
the mass of the body.  It usually consists  of  a series of large follicles,

               Fig. 116.                       Fig. 117.
    Lobule of Liver of Squilla Mantis; exterior.   Lobule of Liver of Squilla Mantis, cut open.

which branch  out  into smaller ones (Figs. 116 and 117), and of  which
several open into one excretory duct;  but these ducts remain  separate,
and  discharge their contents into  the  intestine by several distinct ori-
 
COMPARATIVE  STRUCTURE OF THE LIVER.          401
 fices.—In Insects  and  other air-breathing  Articulata, however,  the
 Liver  is  much less developed; and  its  type remains  much  simpler.
 We usually find  it consisting of a small number of caecal tubuli, which
 open separately into the intestinal canal, just below the stomach.  These
 tubuli are often so long, as to pass several times from one extremity of
 the visceral cavity to  the other, being  doubled upon  themselves;  in
 other instances, we find that  the  principal tube or canal is beset with
 rows of short follicles,  somewhat in the manner of Fig. 111.  But they
 never cluster together so as to form a solid glandular mass.   The low
 development of  the liver  in  these animals, bears an evident relation
 with the  high development of their respiratory apparatus; whilst,  the
 respiration being  comparatively  feeble  in the aquatic Mollusca and
 Crustacea, the development of the liver in those classes  is enormous.
   721. There  is much  difficulty in ascertaining the mode in which  the
 elementary constituents of the Liver are arranged, in the  fully-developed
 condition of that organ in  the higher Vertebrata.  At  an early period of
 its development, as already remarked,  it may be easily shown  to con-
 sist, in the Fowl, of a series of distinct caeca, clustered  round a projec-
 tion from the intestinal canal, and opening separately into it (Fig. 108);
 and it is a peculiarly interesting fact, that this very condition should
 exist as the permanent form of the Liver, in that curious little fish,  the
 Amphioxus or  Lancelot, which retains the embryonic type  in so many
 parts of its conformation.  In the Tadpole,  again, the  distinct caeca  are
 very evident (Fig. 118); but here we see that the projection of  the
 intestinal  canal,  instead of being a simple wide caecum,  has  become
 extended in length and contracted in  diameter, at the  same  time  di-
 viding and subdividing,  so as to  form  an  arborescent  excretory duct,
 whose ramifications extend through the  entire  glandular  mass.   In  this
 manner, then, is  formed the complex system of hepatic ducts, which we
 find in the liver of the higher Vertebrata, branching out from the main
 trunk.  But the  mode in which the ultimate ramifications of these are
 arranged, and their relations with  the secreting cells which make up the
parenchyma of the gland, have not yet  been fully elucidated.   The fol-
lowing are the principal facts, that have been ascertained  on  the sub-
ject.
   722.  The entire Liver is made up of a vast number of minute lobules,
 of irregular form, but about the average size of a millet-seed.  Each of
these lobules contains  the component  elements  of which the entire
organ  is  made up; namely, branches of the  hepatic  artery and vein,
branches of the portal vein, branches of the hepatic ducts, and secreting
cells.  The lobules  are connected together in part by areolar tissue, but
in great part by the anastomosis of the  blood-vessels and hepatic ducts,
which  supply  the  adjoining lobules;  indeed  there is  frequently  no
definite division of  the glandular substance into lobules, other than that
which is marked out by the arrangement of these canals  (Figs. 119 and
121).  The branches of the Hepatic Artery are principally distributed
upon the  Avails of the hepatic ducts, and upon the trunks and branches
of the portal and  hepatic veins, supplying them with their  vasa vasorum ;
also upon  Glisson's capsule  and its prolongations into  the substance  of
the liver, -which prolongations  form the greatest part of the connecting 
402
OF SECRETION.
structure  that  holds together the several elements.   There is strong
reason to believe,  that  the blood which  the  liver  receives from the
Fig. 118.
        Liver of Tadpole; showing distinct and free caecal terminations of the biliary ducts.

hepatic artery is  not destined to supply the materials for the biliary
secretion, until it  has become venous by travelling through the network,
in which it is subservient to the nutrition of the tissues it permeates, as
it is in other parts  of the systemic capillary system.—The supply of
blood, from  which the  materials  of  the  biliary secretion are chiefly
drawn, is  afforded by the  Vena Portar, which collects it as a Vein from
the chylopoietic viscera, and which then subdivides as an Artery to dis-
tribute it  to the different parts of the  Liver.   Its branches proceed to

                               Fig. 119.
the capsules of the lobules, covering the whole  external surface of the
latter  with their ramifications, and  sending capillary twigs inwards,
which  converge towards the centre of each lobule (Fig. 119, 2, 2).  As 
STRUCTURE OF THE LIVER.
403
the principal branches of these veins ramify in  the  spaces  between the
lobules, they are termed mfer-lobular veins.—On the other hand, the
branches of the Hepatic  Vein pass from the trunk to the centre of each
lobule, from  which they send out  diverging capillary tAvigs (1, 1), to-
wards  the  circumference;  and  these last, coming  into connexion with
the converging capillaries of  the portal vein, establish a free capillary
communication between the interior and exterior of  each lobule.   Thus
the portal blood is first  distributed to  its exterior,  then  penetrates its
substance,  and then, after  permeating  the  parenchymatous substance
in numerous minutely-divided streams, is  collected  and carried off by
the hepatic vein, of which a twig originates in the centre of each lobule.
OAving to  the peculiar position  of the  branches of  the hepatic vein in
the centre of each lobule, the lobules are appended  to its  main trunks
almost in the manner of leaves upon a stem (Fig.  120).   The precise
relation of the capillaries of  the hepatic artery with those of the portal

                               Fig. 120.
and venous systems has not yet been well ascertained; but there seems
reason to believe, with Mr.  Kiernan,  that  the  arterial capillaries dis-
charge themselves  into the ultimate ramifications of the portal vein;
and that thus the blood of the former,  having become venous by trans-
mission through  the nutritive  capillaries of the  liver, mingles with the
other venous blood collected by the venae portae,  to  supply the  mate-
rials of the secretory function,  Avhich are eliminated from it during its
passage into the hepatic vein.
   723.  The Hepatic Ducts also seem to form a plexus which surrounds
the lobules, connecting them together, and sending  branches towards
the interior of each.  The mode in which they terminate, hoAvever, and
the precise relation in Avhich they stand to the hepatic cells, which form
nearly the  entire  parenchyma of the Gland, has not  yet  been com-
pletely elucidated.   There seems reason to believe,  hoAvever, that the
tubular plexus extends  throughout the substance  of  the lobule, filling
up the entire space not occupied by the blood-vessels (its membranous
Avail, however, being with  difficulty traceable,  owing to its extreme
tenuity);  and that the  hepatic cells are contained Avithin it, as  within
the follicles or tubes of  ordinary glands.  These  cells (which are easily
Obtained in a separate condition by scraping a piece of fresh  Liver)
are of a flattened spheroidal or polygonal form ; and their diameter is
usually from l-800th to l-1600th of an inch.  Each cell presents a dis- 
404
OF SECRETION.
tinct nucleus;  and it is usually around this, that the yellowish hue of
the cell is the  deepest.   The cavity of the cell  is chiefly occupied by
biliary matter, much of which is in  the condition of fine granular parti-
cles too minute to be measured.   In the midst of these, there are usually
  Horizontal section of two superficial lobules, showing the interlobular plexus of biliary ducts:—1,1, in-
tralobular veins; 2, 2, trunks of biliary ducts, proceeding from the plexus which traverses the lobules; 3,
nterlobular tissue; 4, parenchyma of the lobules.

one or two large adipose globules, or five or six small ones (Fig. 122);
but the amount of this  fatty matter is liable to great variations (§ 754).
                           The biliary matter which these cells contain,
             I22-          marks them out as the real agents in the se-
                           creting process ;  this process consisting, it is
                           evident, in the growth of the hepatic cells,
                           Avhich, in the  course of their  development,
                           eliminate from the blood the biliary matter,
                           for which they have a special affinity.  The
Glandular ceiis of Liver -.-a, nucleus;  mode in  which the particles  thus eliminated
  b, nucleolus; c, adipose particles.        -i.  i      i  •     i  i     •
                           are discharged  into the hepatic ducts, to be
by them conveyed to the intestine, cannot be understood, until the rela-
tion between the  secreting cells and the  ultimate  ramifications of the
ducts shall have been more precisely determined.
  724. The Bile which has  been secreted by the hepatic  cells,  and
Avhich has found  its  way into  the ramifications  of the hepatic  ducts,
may be directly  conveyed by the  trunk of the latter into the intestine,
or it may regurgitate along the cystic ducts into the gall-bladder.  It is
probable  that  the secreting  process is constantly going on; although,
as in  other  cases, it  may  vary  in  its  degree of  activity at  different
times.  When the process of digestion is taking place, and the small
intestine is filled with  chyme, there  is probably an uninterrupted  flow
of bile into its cavity;  but when the intestine  is empty, the bile seems
not to be admitted into it, but rather to flow back into the gall-bladder,
in which it is stored up, as in a  reservoir, until the time when it may
be needed.   In this reservoir it undergoes a certain degree of concen-
tration, by  the absorption of  its watery part;  and it also  becomes
mixed Avith a large proportion of  mucus, which is secreted by the walls
of the gall-bladder.—As the  analyses  of Bile have been chiefly made
upon the fluid obtained from this receptacle,  they probably  over-esti-
mate the proportion of solid matter  contained in this secretion; Avhich 
COMPOSITION OF THE BILE.
405
 is usually stated at from 8 to 9| per cent.  Of this solid matter, about a
 tenth consists of alkaline and earthy salts, corresponding with those of
 the blood; whilst  the  remainder is made up  of organic constituents.
 These are very  readily decomposed, and enter  into  neAV combinations
 with the  substances  employed to separate them;  so  that  different
 Chemists, by employing different  means of analysis,  have obtained re-
 sults which  seem far from conformable.   All are agreed,  however, that
 the chief part of the solid ingredients of bile are allied to fat in compo-
 sition ; consisting of a  very  large proportion of carbon and  hydrogen,
 and of a  comparatively small amount of oxygen and azote.  According
 to  Prof. Liebig,  the organic portion of ox-bile may be represented by
 the formula  76  Carbon, 6Q Hydrogen, 22 Oxygen,  and 2  Nitrogen,
 with a considerable proportion of Sulphur.  This substance, essentially
 corresponding with the  bilic acid, choleic acid, bilin, picromel, &c, of
 different  Chemists, seems to  be  a fatty acid (§ 261), united with soda, so
 as  to constitute a soap.  In healthy bile, the proportion of Cholesterine
 appears to be very small, and it is  held in solution  by the  preceding
 ingredient;  but  in many disordered states, and especially  in  disease of
 the Gall-Bladder, this element is  present in  much larger amount; and
 it usually forms  the principal, if  not the sole ingredient in biliary con-
 cretions.   It is a white crystallizable  fatty matter,  somewhat resem-
 bling  spermaceti,  free  from taste  and odor, and  composed  almost
 entirely of carbon and hydrogen ;  its formula is 36  Carbon, 32 Hydro-
 gen,  and  1 Oxygen.—The  coloring  matter  of Bile is a  substance
 distinct from  the preceding;  that of the Ox  and  other graminiA'orous
 animals appears  to be identical,  or  nearly so, with the  chlorophyll of
 the leaAres on which they feed ; but that of Human bile seems to possess
 different properties, and to be derived from the  proper constituents of
 the blood.
   725. Regarding the  destination and purposes of this secretion in  the
 Animal economy, the following  may be  considered as a tolerably com-
 plete summary; though it is  impossible to speak with precision on some
 points,  since  the  organic  constituents  of the Bile are liable to be so
 easily altered by various reagents,  that they are with difficulty recog-
 nised.—A portion of the  Bile unquestionably passes  off,  in  Man and
 most other animals, with  the faeces ;  this portion, which includes the
 coloring matter,  is probably that which would  be most  injurious, if
 retained in the blood,  and is most purely excrementitious.  In bilious
 diarrhoea,  and under the influence of purgatives, especially mercurials,
 a large quantity of bile is  discharged per anum, apparently almost
 unchanged.   But in the healthy state, a portion, at least,  of the soapy
 compound seems  destined for reabsorption.   Just as ox-gall  is com-
 monly used to remove  grease-spots, by its solvent power for  fatty mat-
 ter, so does the bile seem to act  in the living body, by  rendering soluble
 the fatty  matters of the fcod, and thus enabling them to  be absorbed
by the lacteals (§ 479).   The fatty matter of the bile  when reabsorbed
 with that  of the neAvly-ingested  food, is probably, like it, carried off by
the respiratory process :  but it  is easily shoAvn, that the biliary matter
cannot  supply more than one-sixth or one-eighth of the amount of
carbon eliminated from  the  lungs in the form of  carbonic acid ; and 
406
OF SECRETION.
that it cannot be (as supposed by Liebig) the chief fuel of the process
of combustion, which is kept up through the agency of those organs.—
The secreting action of the Liver, by Avhich a certain product is entirely
separated from the blood, constitutes, however, only a part of the action
of that organ; since, as already shown (§ 493), the  changes which it
effects in  the  alimentary  materials newly  introduced  into the current
of the circulation,  are at least equally important.  A large part  of the
bulk of the Liver, in many of the lower animals, is made up of oleagi-
nous matter;  which appears to accumulate  in the hepatic cells,  giving
them almost the character of  fat-cells, in  proportion  as the respiratory
function is inactive.  Thus, the liver is very large and fatty in Mollusca
and Crustacea;  whilst, on the other hand, in Insects it is comparatively
undeveloped.  In  Fishes, again, it is rich in oily matter, but in  Mam-
malia it is much less fatty in the state of health; whilst in the liver of
Birds, scarcely any traces of fat are  to be found.
   726.  The elements  of the  bile may be  altogether supplied by the
disintegration of the tissues; and this must certainly  be the case, Avhen
the amount of food taken is no  more than  enough to supply the waste
of the system.   We may  regard  it, then, as one office of the Liver, to
remove from  the blood such products  of that disintegration, as are rich
in carbon and hydrogen.  It may be pretty certainly affirmed, however,
that biliary matter does not pre-exist as such in the blood; but that its
elements must be originally present  there, under some more  pernicious
form.  For it is found  that the total suspension  of the  secreting  action
of the Liver, whereby  the excrementitious matter is left to accumulate
in the blood,  has a much more prejudicial  effect upon the system, than
the reabsorption of Bile after it has been  secreted, in consequence of
an obstruction to its discharge through the ductus choledochus; so that
it  may be inferred that the noxious products of the disintegration of
the tissues are  transformed into  comparatively innocent components
of Bile, in the very act  of secretion.—But  there can be little  doubt,
that the Liver has also  for  its office, to draw off from the  blood  any
superfluity which may exist in the non-azotized compounds derived from
the  food, beyond  the  amount that  is  requisite for  the supply  of the
respiratory process, or  that can be  deposited as fat.   For  we  conti-
nually witness the results of habitual  excess in the amount of such sub-
stances, in producing that state of the system commonly termed bilious;
of which all the symptoms are referable to the accumulation of the ele-
ments of the  bile in the blood, and the consequent deterioration  in the
purity of the circulating fluid.  Where  a tendency to such a state
exists, proper means should be taken  to stimulate the liver to increased
activity; but the chief reliance should be  placed on the  avoidance of
those articles of diet, which contain a large proportion of non-azotized
matter, and  on  abstinence from superfluous  nutriment of any  de-
scription.

                   3.  Of the Kidneys and the Urine.

   727. The Kidneys are  perhaps the most purely excreting organs in
the body ;  their function being to separate from the blood certain matters 
STRUCTURE OF THE KIDNEY.
407
that would be injurious to it if retained, and these matters being des-
tined to immediate  and complete removal from the system.  We have
seen that, in the Lungs, the excretion of Carbonic acid is made subser-
vient to the absorption of Oxygen ;  and the separation of a fatty acid
from the blood, which is effected by  the Liver, is a means of introducing
a new supply of fatty matter  into  the system.   There is no ulterior
purpose of this kind in the secreting action of the Kidney ; the product
of which is invariably conveyed directly to an outlet, by which it may be
discharged from the body.   Some traces of Urinary organs may be de-
tected in most of the higher Invertebrata; but it is in Fishes, that they
first present a considerable development; and in ascending through the
Vertebrated series, we find them  rapidly increasing in the complexity
of their organization, and in their functional importance, although their
size and extent  are not  so great.  In  Fishes, the Kidneys very com-
monly extend the  whole  length of  the  abdomen; and they consist of
tufts of uniform-sized tubules, Avhich shoot out transversely at intervals
from  the  long ureter, and which are connected together by a loose web

         Fig. 123.
 Embryo of Green Lizard;—a, heart;
b, duplex aorta; c, vena cava; ^intes-
tine ; e, liver;/, rudiment of AVolffian
body;#, g, rudiments of extremities.
 of areolar tissue, that supports the network of vessels distributed upon
 their walls.  This condition of the Urinary organs is very analogous to
 that of the Corpus Wolffianum  or temporary kidney of the embryo of
 higher animals (Fig. 123,/).  A similar condition is found in the true
 Kidney of higher animals at an early grade of development (as shown
 in Fig. 124);  the tubuli uriniferi being short  and  straight.   In their
 more advanced condition, however, they become long  and convoluted;
 and the ramifications of the capillary vessels come into very close rela-
 tion with them (Fig. 125).  It is in the higher Reptiles, that we first
 meet  with the distinction between the cortical and medullary substance ;
 the former being the part in which the blood-vessels are most copiously
 distributed, and in which the tubuli  have the most convoluted arrange-
 ment ; and  the  latter consisting  chiefly of straight tubuli, converging
 towards the points at Avhich  they discharge themselves into the ureter
(Fig.  126).  The bundles of tubuli and their vascular plexuses remain
distinct, however, in Birds and in the lower Mammalia, so as to give to 
408                           OF  SECRETION.
the whole gland a lobulated character;  but in the Human Kidney they
come  into  closer contact;  and  the  vascular  connexion  between the

                    Fig. 125.
ias5«
  Portion of Kidney from Coluber:—a, a, vascular
trunk; b, 6, ureter j  c, c, converging fasciculi of tubuli
uriniferi.
                                                     Pyramidal fasciculi of tubuli
                                                    uriniferi of Bird, terminating in
                                                    one of the branches of the ureter.

plexuses of the different bundles is such, as  to prevent  any separation
into distinct lobules.
   728.  The act of secretion appears to be effected,  as in other Glands,
by the epithelial cells lining the tubuli uriniferi; these cells drawing the
materials of their  development  from  the  vascular  plexus  upon  the
exterior of these tubuli;  and delivering them  up, when  they have com-
pleted their own term  of existence, to be carried off through  the open
Fig. 127.
Fig. 128.

2  \M■
  A section of the Kidney, surmounted by the supra-re-
nal capsule; the swellings upon the surface mark the
original constitution of the organ, as made up of dis-
tinct lobes.—1. The supra renal capsule.  2. The vas-
cular portion of the kidney. 3,3. Its tubular portion,
consisting of cones.  4,4. Two of the papilla: project-
ing into their corresponding calices.  5, 5, 5. The
three infundibuli; the middle 5 is  situated in the
mouth of a calyx. 6. The pelvis. 7. The ureter.
  Portion of the  Kidney of a new-
born infant:—a, natural size; 1, 1,
Corpora Malpighiana, as dispersed
points in the cortical substance; 2,
papilla; n, a smaller part magnified;
1, 1. Corpora Malpighiana; 2, 2. Tu-
buli uriniferi.
orifices of the tubuli.   But  the  Kidney contains another apparatus,  of
a very peculiar  description;  which  appears  specially destined for the 
STRUCTURE  OF THE KIDNEY.
409
separation of the superfluous fluid of the system.   When a section of the
Kidney is slightly magnified (Fig. 128, b), the cut surface is seen to be
studded by a number of little dark points ; each one of which, when exa-
mined under a higher  magnifying power, is found to consist of a knot of
minute blood-vessels, formed by the convolutions of thin-walled capilla-
ries (Fig. 12!), m).  It has been  shown by Mr. Bowman, that each one
of these knots is  included in a flask-like capsular dilatation, connected
with one of the tubuli uriniferi; several such capsules, it appears, being
usually developed from  the  sides of each  tubulus, like currants upon a
stalk.  Each of these vascular tufts  (called Malpighian bodies,  after
their  discoverer) is  directly supplied by a branch of the renal artery
(Fig.  129, af);  which, upon piercing  the capsule,  subdivides into a
group of  capillaries ;  and these, after forming the convoluted  tuft, coa-
lesce  into a  single efferent trunk (ef), which may be considered as re-
presenting (in  a small way) the vena  portse.  For  the  efferent trunks
of the Malpighian bodies discharge their blood into the capillary plexus,
^Fig. 129.
  Distribution of the Renal vessels, from Kidney of Horse:—a, branch of Renal artery; af, afferent vessel;
 m, m, Malpighian tufts; ef, ef, efferent vessels; p, vascular plexus surrounding the tubes; st, straight tube;
 ct, convoluted tube.  Magnified about 30 diameters.                        /

 which surrounds the tubuli uriniferi,  and from which the solid  matter
 of the urinary secretion is elaborated  ; just as the vena  portce supplies
 the  capillary plexus, from  which the  biliary  secretion is elaborated in
 the  liver.  In Reptiles (in  Avhich, as in Fishes,  the kidney is partly
 supplied by the hepatic portal system), the efferent vessels of  the Mal-
 pighian bodies unite with branches of  the portal vein to  form the secre-
 ting plexus around the tubuli uriniferi;  and even in Birds this arrange-
 ment still  seems  to prevail to a  certain  extent.   Thus all the blood
 which the  secreting plexus  receives,  has already passed, in each case,
 through  a set of capillaries within or Avithout  the organ ;  those, namely,
 of the Malpighian bodies, or those of the parts supplying the hepatic
 portal system.—The special purpose  of  the Malpighian bodies appears
 to be, to alloAv of the transudation of the water of the blood, which is
 filtered off  (so to speak) through the thin walls of their capillaries, and
thus passes into the tubuli uriniferi.   It is well known that  the fluid
and solid constituents of the urinary secretions bear no constant relation
to each other; the  amount of fluid depending mainly upon the degree 
410
OF SECRETION.
of fulness of the blood-vessels; whilst the amount of solid matter is
governed, as we shall presently see, by the previous waste of the tissues.
The quantity of fluid in the blood-vessels is governed  by the relative
amount that has been absorbed, and that Avhich has been exhaled from
the skin ; so that the  quantity to be drawn off by the  Kidneys is^ in-
creased, either  by augmented absorption, or by diminished exhalation.
The Malpighian bodies seem to act the part of a  system of regulating
valves ; permitting the transudation of only enough fluid to dissolve the
solid matter, when there is no superfluity of  water in the vessels ; but
allowing the escape of an almost unlimited amount of it,  when increased
imbibition has rendered the vessels unusually turgid.
  729. The average amount of Urine excreted  in twenty-four hours,
by adults Avho do not drink more than  the Avants of nature require, is
probably from  30 to 40 oz.; and its  average  specific gravity may be
about  1020.   The  quantity of fluid is  usually less, and  the  specific
gravity of the  secretion consequently greater,  in summer  than in Avin-
ter, on account of the larger  proportion of fluid exhaled by  the skin
during the former season.   The quantity of solid matter  has been found
to vary, within the limits of ordinary health, from 3*6 to 6*7 per cent.;
and the extent  of variation in disease  is doubtless much greater.  About
one-third of the solid matter is made up of alkaline and earthy salts; and
the remainder is made up of organic compounds.   The  salts are partly
those of the blood, which will not be  separated during the transudation
of the serum through the membranous walls of the Malpighian capilla-
ries,  although the albuminous matter  is  kept back (§ 196).  But there
is a  much larger proportion of the alkaline and earthy phosphates in
the urine, than is present in the blood;  and this  is  liable to a further
increase under  circumstances to be presently alluded to.—The  urine is
normally acid,  but  the degree of its acidity has been  shoAvn  by Dr.
Bence Jones to be continually changing, and to be considerably affected
by food; being augmented by vegetable, and decreased by animal food.
What  the acid  may be  to which  the acidity is due, is  yet uncertain;
possibly it is not always the same.
  730. The organic compounds present in  the  Urinary secretion (in its
healthy state at least) are undoubtedly the  result of the  waste  or disin-
tegration of the animal fabric; as Avell as (in certain cases) of the de-
composition of  constituents of the blood, which have never undergone
conversion into organized tissue.   Their unfitness  to be  retained within
the system, is  proved by the fatal results which speedily ensue when
their  elimination by the secreting process  receives a check;  and also
by the crystalline form, in which  the  most  characteristic of them pre-
sent  themselves,—such a form being altogether incompatible  Avith the
possession  of plastic or organizable  properties.   Various  well-defined
compounds present themselves  in  the Urine  of different classes of ani-
mals ;  and they are  nearly all peculiarly rich in Nitrogen and  deficient
in Carbon, as compared with the Albuminous compounds.  Thus, whilst
the proportion  of Nitrogen in Albumen is (by weight) as 1: 6-24 of the
whole, it is in  Urea as 1: 2*14, in Allantoin as 1: 2*21, in Kreatinine
as 1: 2-69, in Uric acid as 1 : 3-00, and in Kreatine as 1 : 3-12.   The
only  exception  is in the case of Hippuric  acid,  which is discharged 
COMPOSITION OF THE URINE.
411
largely by herbivorous  animals, and in which the proportion of Nitro-
gen is as  1:  12-14.   On  the other hand, whilst the proportion of  Car-
bon in Albumen is as 1 :  1-80, it is in Kreatinine as 1 : 2-35,  in Krea-
tine as 1  : 2-73, in Uric Acid as 1 : 9-80, in Allantoin 1 : 3*87, and in
Urea  as 1 : 5*00.  Here, again, Hippuric acid is  exceptional;  for its
Carbon is as 1 : 1*57 of the whole, or in larger proportion than in Al-
bumen.—We may say, then, that the characteristic components of the
Urinary secretion are such products of the waste of the azotized tissues,
as, from containing nitrogen in large  proportion, are not adapted for
elimination, either by the respiratory  process, or by the biliary excre-
tion.   The only exception is in the case of Hippuric acid; and  the large
proportion of carbon and the small proportion of nitrogen contained in
this substance, appear due to the  great excess of non-azotized compounds
in the food of  the animals voiding it.
   731. Of  the compounds just enumerated,  the  most  important,  in
Man, is that Avhich  is named Urea.   It  exists in Urine in a state of
perfect solution;  and may  be readily separated from it in the form of
transparent colorless crystals, which have a  faint and peculiar but  not
urinous odor.  In its ultimate composition it is identical Avith Cyanate
of  Ammonia, being made up of 2 Carbon, 4  Hydrogen, 2 Nitrogen, and
2 Oxygen,—a  formula much more simple  than that of almost any other
organic substance.   The amount of Urea in the Urine is liable to very
great variation, in accordance with the degree in which the disintegrating
process has been taking place in the  solid  fabric;  and also in confor-
mity with the amount of azotized matter, which has been  taken in as
food.  Supposing that the latter Avere so precisely adjusted to the wants
of  the system, as  to supply  only that Avhich is  required for its mainte-
nance, Ave might then  measure the amount of previous waste, by the
quantity of Urea present  in the Urine.   There  can be no doubt as to
the fact, that,  other things being equal, the  amount of Urea is greatly
increased by any unusual  exertion of  the Muscular system; but such
an  increase cannot be  invariably, or even  usually, attributed to  this
cause;  since it is equally certain, that any superfluity in the amount of
azotized matter received into the blood, must be drawn off by the uri-
nary excretion, and thus that an increase in  the quantity of urea may
be occasioned by an excessive use of proteine-compounds  as articles of
food.   The  average proportion of  Urea, under ordinary circumstances
as to diet and  exercise, seems to be from 20 to 35 parts in 1000; but
it may be raised  to 45  parts by violent exercise, and to 53 parts by an
exclusively animal diet; whilst  it  may fall  as Ioav as from 12 to 15
parts,  Avhen the diet is  deficient  in azotized matter.  The total  daily
excretion  of Urea in adult males  seems  to  average about 430 grains,
and that  of  females nearly  300 grains, but  these  averages may be
Avidely  departed  from,  on the  side  either  of  excess or  diminution,
according to the  circumstances  already noticed.   It  is interesting to
observe, that children of eight years old excrete, on the average, half
as much Urea as adults; -whilst, in very old persons, the quantity sinks
to one-third, or even less.   In proportion to their relative bulks, there-
fore, children  excrete at least tAvo or three times the  quantity of urea
that is set free  by adults, and four or  five times that which is  excreted 
412
OF SECRETION.
by old persons,—a fact which corresponds with other indications of the
far greater rapidity  of  interstitial change in the earlier periods of life,
than in adult or advanced age.
  732.  There is an organic compound, nearly allied to Urea in compo-
sition, but differing from it in its distinctly acid  properties, and also in
its comparative insolubility.   This substance, termed Uric or Lithic
Acid, forms but a small proportion of  the solid matter of Human Urine
in the state  of  health;  but it is  the chief  element in the Urine of the
lower Vertebrata; and its presence in  too large a proportion  is a fre-
quent  source of disease in Man.   Its ultimate  composition is 10 Car-
bon,  4 Hydrogen, 4  Nitrogen, 6 Oxygen; it  crystallizes in fine  scales
of a brilliant Avhite color and  silky lustre; and it is so sparingly solu-
ble  in water, that at least 10,000 times its own Aveight  of fluid  is re-
quired to dissolve it.  In healthy Human urine, it is in a state of perfect
solution, but it is precipitated by the addition of a small quantity of any
acid,  even the Carbonic:  it is evident, therefore, that it is  held in solu-
tion  by union with some base,  and it seems probable  that  this base is
ammonia.   According to Dr. Bence Jones,  the first precipitate thrown
down by the addition of hydrochloric acid to ordinary urine, is urate of
ammonia, which is less soluble in acid  than  in neutral or alkaline urine;
and it is only after prolonged  contact Avith the hydrochloric acid, that
this  salt is  decomposed, and uric acid  left behind.   The  solubility of
urate of ammonia is much greater in warm than in cold urine ; and  hence
it frequently happens, that urine which is clear when voided, gives  a pre-
cipitate of urate of  ammonia on  cooling.
  733. The proportion of Uric acid in healthy urine seldom rises above
1 part in 1000, and  the quantity excreted daily is usually  from 6 to 10
grains.  The  circumstances  under Avhich it  varies, however, have not
been  clearly determined.   The absolute quantity in the urine bears no
proportion to its acidity, nor is it indicated by the amount of  deposit;
for the acidity  of the urine depends upon the presence of  other  acids;
and  a deposit  of urate of ammonia may be due to an excess  of acid,
diminishing its solubility, rather than  to  an excess of the substance
itself.  Thus it may happen  that a  precipitate  of urate  of  ammonia
may be formed, when it is not present in any undue proportion, in con-
sequence of the acid state of the  urine ;  Avhilst, on the other hand, there
may be a large excess of urate  of ammonia in the urine without any
precipitate,  if  the urine should be alkaline.   In disordered states of the
system, there is often a great increase in the amount of uric acid in the
urine;  and  there can be no doubt that this increase is partly controlla-
ble by the reduction of the proportion  of  azotized matter in the food.
In some of these cases, free uric acid  is deposited, in consequence of
the decomposition of the urate of ammonia by a large excess of acid in
the urine.   In attacks of gout, urate  of soda is separated  from the cir-
culating blood, and is deposited in the tissues around the affected joints,
forming the concretions termed "chalk'stones ;" and in this state of the
system, uric acid may be detected in the blood.
   734. There  seems reason  to believe that we are to  regard Hippuric
acid as a normal  element of the Urine of Man;  although it has been
usually supposed to  be restricted to the Herbivorous quadrupeds, Avhere 
COMPOSITION OF THE URINE.
413
it replaces Uric Acid.  Its composition and  properties are very diffe-
rent from those of that substance.  When pure, it forms long, transpa-
rent, four-sided prisms;  it is soluble in 400  parts  of  cold Avater, and
dissolves readily at a boiling heat; and it has a strong  acid  reaction,
with a bitterish taste.  It is composed of 18 Carbon, 8  Hydrogen, 1
Nitrogen, and 5 Oxygen, with 1  equiv. of Water.   When exposed to a
high temperature, or subjected to the  putrefactive  process, it is partly
converted into Benzoic acid ; and it is on the presence of the latter in
putrefied  Human  Urine, that the belief in the existence of  Hippuric
acid in the same fluid  when fresh, is chiefly grounded.   It is  a curious
fact, that the administration of Benzoic acid  causes the  appearance of
a large additional quantity of Hippuric acid  in the  Urine, so that  its
presence  is  then  sufficiently evident;  and this seems  to be due to the
union of the benzoic acid Avithin the system, either with glycocoll (§ 176),
or Avith the elements which would have formed it;  for one equivalent of
benzoic acid, added  to one of glycocoll, giA^es the precise equivalent of
hippuric acid.  It does not appear that, as once asserted, the adminis-
tration of  benzoic acid diminishes the quantity of uric acid  normally
present in the urine; but it seems to bring down the excess, where such
exists, to about the normal quantity; and it may thus be employed with
advantage in cases of uric acid gravel.
   735.  Much discussion  has taken place, as to  the normal presence of
Lactic acid in the urine ; and the question cannot even now be  regarded
as completely determined.  It  is certain that the  peculiar  crystalline
compound, procurable by treating the urine with zinc in solution, is not
as Avas formerly maintained, a lactate of zinc; but that its composition
is altogether different, as will be  presently  explained.   But, on  the
other hand, it appears from  the researches  of Lehmann  and others,
that lactic acid is  usually present in  small quantity in healthy urine ;
and that its quantity may be increased under such conditions,  as either
tend to augment  the quantity of  lactic acid in the blood,  or to obstruct
its elimination  by the respiratory process.  Thus  an excess of farina-
ceous food, which furnishes sugar and  lactic  acid  faster than  they can
be throAvn  off as carbonic acid  and Avater; or an excess of exertion  of
the muscles, of whose disintegration lactic acid appears  to  be one  of
the results ;  or pulmonary diseases, Avhich interfere with  the normal
aeration of the blood; all favor  the appearance of lactic acid  in the
urine.   It seems to be constant in herbivorous animals, and in patients
suffering under chronic bronchitis, pulmonary emphysema, and similar
disorders.—The urine of Man, more uniformly contains, however, tAvo
substances  termed Kreatine  and Kreatinine; which  seem to  be the
result of the degeneration of the  muscles, as they may be obtained from
the juice of raw flesh.   The former of these,  which is  found in largest
amount, is a neutral substance,  crystallizing  in long prisms, sparingly
soluble  in cold water, but dissolving readily  in warm.   By the action
of strong  acids,  kreatine may  be  readily converted  into kreatinine,
which only differs from it in composition by containing two proportion-
als less of the elements of water, but is a  substance of very  different
chemical relations, having a  strong  alkaline reaction, and serving as a
powerful organic base  to acids.   When long boiled with caustic baryta, 
414
OF SECRETION.
kreatinine is gradually resolved into urea ;  and thus it would seem as if
they hold an intermediate position between the components of the orga-
nized tissues and the urea which may be considered as the ultimate pro-
duct of their metamorphosis within the body.  The peculiar crystalline
compound just referred to, as having been  formerly supposed to be lac-
tate of  zinc, has been shown to be really  formed by a  combination of
zinc with a compound of kreatine and kreatinine.
  736.  Of the substances ranked under the head of Extractive Matters,
little is definitely known;  it appears, however, from  recent researches,
that they are peculiarly rich in carbon, and that  they are liable to be
greatly augmented, either by an excess of non-azotized matter in the
food, or by any impediment to the  action of the liver or  lungs.   A
yellow extractive has  been separated  by Scherer,  which he regards as
proceeding from the final metamorphosis of the haematine of the blood ;
and this seems nearly related to the purpurine, which sometimes gives
a deep color to the sediment of urate of ammonia, and which  is pecu-
liarly liable to appear when the functional  activity  of the liver is below
par.  The ordinary coloring-matter of  bile often presents itself in the
urine in cases  of jaundice:  and, as Dr. Golding Bird has  pointed out,
there is a close  similarity in composition  between these three colored
compounds, indicating a derivation from the same source.  A sulphur
extractive has also been obtained from  the urine:  in Avhich (as in the
bile) there is a considerable proportion of free sulphur.
  737.  The Urine also contains a considerable amount of Saline matter,
of  which the acids as well as the bases are  derived from  the  mineral
kingdom; and the excretion of them, after they have served their pur-
pose in the  economy, appears to be one of the chief functions of the
Kidney.  Of these a part may find  their  way directly into the urine
from the serum of the blood, when its Avater is being filtered off (so to
speak) through the walls  of  the Malpighian  capillaries; for although,
from the peculiar  properties of  animal  membranes (§ 196),  the  albumi-
nous constituents of the serum are  held back, the saline matter, which is
in a state of perfect solution, must pass with the  water.  This  is pro-
bably the chief source of the large quantity of the muriates  of soda and
ammonia contained in the urine.  But the Urinary secretion seems to be
specially destined  to eliminate the saline compounds, which are formed
by  the acidification of the Sulphur and Phosphorus, taken in with the
proteine-compounds as food.  These substances are united with Oxygen
in the system, and are thus converted  into Sulphuric and  Phosphoric
acids; which acids unite with  alkaline bases,  that were  ingested in
combination with  Citric,  Tartaric,  Oxalic,  and other  organic  acids;
the  latter  undergoing decomposition within  the  system,  and  leaving
the  bases  ready to unite  with others.  Such AYeakly-combined bases
abound in the food of  Herbivorous animals ; and  their  urine is almost
invariably alkaline, the quantity of the Sulphuric and Phosphoric acids
generated in the system not being sufficient to neutralize  it.   On the
other hand, they are  nearly absent in the  food of the Carnivora;  and
their urine is therefore almost invariably acid, from the want of neutra-
lization of the Sulphuric and Phosphoric acids.
  738.  The Alkaline Sulphates, whether taken in  as such, or formed in 
COMPOSITION OF THE URINE.
415
 the manner now described, are soluble enough to be always passed off in
 a fluid form;  but this is not uniformly the  case with  the Phosphates,
 which are frequently deposited as sediments of a dead-Avhite aspect,
 sometimes crystalline, and sometimes wholly or partly amorphous.  The
 crystalline  sediment consists of the triple phosphate,  or phosphate of
 ammonia and magnesia ;  the  amorphous  contains an admixture of the
 phosphate of  lime.   The urine, Avhen these are deposited,  is usually
 alkaline,  sometimes very decidedly so ; and there is reason to think that,
 in many  cases, this alkaline character, and  the deposit of  phosphatic
 sediments, are due  to an alkaline secretion from the walls of the bladder
 and urinary passages, Avhich results from an irritable state of their mem-
 brane,—the urine,  as secreted by the kidney, having its  usual properties.
 That an alkaline condition of the urine, resulting from  the presence of
 an unusual amount of bases, is capable of producing a phosphatic deposit,
 is shown  by the simple experiment of adding ammonia to healthy urine,
 which will occasion a precipitation of the triple phosphate.
   739. But there can be little doubt, that a frequent  cause of the de-
 posit is excessive production  of phosphate salts, arising from the in-
 creased Avaste or disintegration of Nervous  matter, which takes place
 when it is in  a  state of unusual activity, either from  intense  thought,
 from prolonged exertion, or from continued anxiety.  The general prin-
 ciples already set forth, in regard to the  dependence of the functional
 activity  of the  Nervous  Centres upon a  supply of arterialized blood
 (§ 384), shoAV  the probability that every act of theirs involves  the oxy-
 genation  of a  certain quantity of nervous matter.  In this oxygenation,
 phosphoric acid  will be produced, from the large amount of phosphorus
 contained in the nervous  matter; and this will unite in part with am-
 monia, Avhich  is perhaps set free by the same metamorphosis,  or is de-
 rived from other sources;  and in part with bases derived from the food.
 The experience of every studious man must have shown  him (if he make
 any observations on the matter at all) the frequent coincidence between
 the presence of phosphatic deposits in his urine, and  an excess of men-
 tal labor ; and there are many instances  on  record, in  which  the peri-
 odical recurrence of the latter has been  so  invariably folloAved by the
 recurrence of  the former,  that no reasonable  doubt can  exist  as to their
 mutual connexion.
   740.  It is very important for the successful  treatment of those  Uri-
 nary deposits, Avhich consist of the normal  elements  of the Urine,—
 namely, Lithic Acid, and the  Phosphates,—that the  leading facts al-
 ready stated should be borne continually in mind.   In the  first place,
 these sediments may depend upon  the general  condition of the fluid,
 and not  upon any  excess in the  constituents  of which they are com-
 posed ; thus a lithic deposit may result  from the presence of  an excess
 of some other  acid  in the urine ; and a  phosphatic  sediment  may be
 produced  by the excess of bases.   In  such  cases, then, our  treatment
 should be directed,  not to diminish the quantity of the  peculiar consti-
 tuents of  the  deposits, but to rectify the state of the Urine on which
 their  precipitation  depends.  But, in  the second place, the sediments
may be  present in such great amount, as to indicate that their consti-
tuents are present in the urine to an excessive  degree; and our treat- 
416
OF SECRETION.
 ment must then be directed towards the diminution of the quantity pro-
 duced.   Thus the  tendency to  lithic  acid deposit may be frequently
 cured, by simply  diminishing the  quantity  of azotized matter  in the
 food; and the undue formation of the phosphates may be often kept in
 check by that mental repose, which is peculiarly required after long-
 continued and severe exercise of the intellectual  faculties, or strong ex-
 citement of the  feelings.
   741.  There is no doubt whatever, that the total  suspension  of the
 Urinary secretion  is productive of  rapidly-fatal  results, from the accu-
 mulation of the elements of the secretion in  the blood;  and it would
 appear, that the tissue on Avhich their presence  in the circulating fluid
 exerts the most  injurious effects, is the Nervous. It  is probable that
 Urea is the substance  which is most directly concerned in producing
 the noxious influence; and we see an effort  made by the system (so to
 speak) to get rid of it, in those cases  in which a discharge of urinous
 fluid takes place by unusual channels, such as from  the mucous mem-
 brane of the stomach, the mamma, the umbilicus, the  nose, &c, when
 the usual secreting action of the Kidney has been suspended. Although
 the accounts of such  cases  have been treated  with ridicule  by some
 Physiologists, yet  there  seems no valid reason to discredit them, when
 it is borne in mind that, in persons who have died from the complete
 suspension of the secretion,  effusions containing  urea have  been found
 in the serous cavities of the trunk, and in the ventricles of the brain.
 The poisonous influence  of an accumulation  of urea in  the  blood,when
 strongly exerted, produces in the first instance irregular or convulsive
 movements, which are dependent upon irritation of  the Spinal  system
 of nerves ; then  loss of consciousness, depending  upon the suspension of
 the powers of the Brain  ; and lastly, complete suspension of the  powers
 of the spinal system, so  that the ordinary reflex  actions cease, and life
 becomes extinct from the stoppage of the respiratory movements (§ 688).
 There is reason  to  believe, that many convulsive  motions,  for which no
 obvious cause can be assigned, have their origin in a disordered condi-
 tion of the blood, resulting from imperfect elimination of Urea;  thus it
 has been ascertained that, in  several cases of  puerperal  convulsions,
 urea was present  in the blood; the functional power of the  kidney
 being diminished by chronic disease.   It  is especially to  be noticed,
 that most of the  cases  in  which the urinary secretion is  discharged
 through  some irregular channel, occur in  persons who have been  sub-
ject to those convulsive  affections,  which  are commonly designated as
 hysterical;  and that the discharge  of a large quantity  of urine through
 the natural channel, is often the termination  of an hysterical paroxysm.
 It is desirable, therefore, that in all such obscure cases, the state of the
 urinary secretion should be  carefully looked  to.

              4. Of the Cutaneous and Intestinal Glandulae.

   742.  The Glandulae Avhich are disposed in the  substance of the Skin,
 and in the walls of the  Intestinal canal, although individually minute,
 make up by their aggregation an excreting  apparatus of no mean im-
 portance. . The  Skin is the seat of two processes in  particular ;  one of 
CUTANEOUS GLANDULE.
417
which is  destined to free the  blood of a large quantity of fluid;  and
the other to draAv off a considerable amount of solid matter.   To effect
these processes, we  meet with  two distinct classes of  glandulae in its
substance;  the  Sudoriparous or sweat-glands;  and the Sebaceous or oil-
glands.   They  are both formed, however, upon the same  simple plan;
and can frequently be distinguished only by the nature of their secreted
product.
   743. The Sudoriparous or perspiratory glandulae form small  oval or
globular masses,  situated just beneath the cutis, in almost every  part of
the surface of the body.   Each is formed by the convolution of a single
tube; which thence runs towards  the  surface  as the  efferent duct,
making  numerous spiral  turns in its  passage through  the  skin,  and
penetrating the epidermis rather obliquely, so that its orifice  is covered
by a sort of little  valve  of scarf-skin, which  is lifted  up as  the  fluid
issues from it  (Fig. 130).  The convoluted knot, of which the gland

                                 Fig. 130.
  The anatomy of the Skin :—1. The Epidermis, showing the oblique laminae of which it is composed, and
 the imbricated disposition of the ridges upon its surface. 2. The Rete Mucosum, or deep layer of the epi-
 dermis.  3. Two of the quadrilateral papillary clumps, such as are seen in the palm of the hand or sole of
 the foot; they are composed of minute conical papillae.  4. The deep layer of the cutis, the Corium. 5. Adi-
 pose cells.  6. A Sudoriparous gland with its spiral duct, such as is seen in the palm of the hand or sole of
 the foot.  7. Another Sudoriparous gland with a straighter duct, such as is seen in the scalp. 8. Two
 hairs from the scalp, enclosed in their follicles; their relative depth in the skin is preserved.  9. A pair of
 Sebaceous glands, opening by short ducts into the follicle of the hair.

 consists, is copiously supplied with blood-vessels.  On the palm of  the
 hand,  the  sole of the foot, and the extremities of the fingers, the  aper-
 tures of the perspiratory ducts  are  visible to the naked eye, being situ-
 ated at regular distances along the little ridges of sensory papillae, and
giving to the latter the appearance of being crossed by transverse  lines.
According to Mr. Erasmus Wilson,  as many as 3528 of these glandulae
exist in  a square  inch  of surface on the palm of the hand; and as every
tube, when straightened  out, is  about a quarter of an inch in length, it
folloAvs that in a square inch  of skin from the palm of the hand,  there
exists a length of tube equal  to 882  inches, or 73J feet. • The number 
418
OF SECRETION.
of glandulae in other parts of the skin is sometimes greater, but gene-
rally less than this; and, according to Mr. Wilson, about 2800 may be
taken as the average number of pores in each square inch throughout
the body.  Now  the number  of square inches of surface, in a man of
ordinary stature,  is about 2500; the number of  pores, therefore, is
seven millions ; and the number of inches of perspiratory tubing would
thus be 1,750,000, or  145,833 feet,  or  48,611 yards,  or nearly  28
miles.
  744.  From this extensive system of glandulae,  a  secretion of watery
fluid is  continually taking place;  and a considerable amount of  solid
matter also is draAvn off by the epithelium-cells that line the tubuli.
Under ordinary circumstances,  the  fluid is carried off in the state of
vapor, forming the insensible  perspiration;  and it  is only  when  its
amount  is considerably increased, or when the surrounding air is already
so loaded .with moisture as to be incapable of receiving more, that the
fluid remains in the form of  sensible perspiration upon the surface of
the skin.   It is difficult to estimate  the proportion  of solid matter con-
tained in this secretion; partly  on account of the great  variations in
the amount of fluid  eliminated  by the Sudoriparous glands, which are
governed by the temperature of the skin ;  and partly because the secre-
tion can scarcely  be collected for analysis free from the sebaceous and
other matters which accumulate  on the surface of the  skin.  According
to Anselmino it varies  from f- to If per cent.; and consists in part of
lactic acid, to which the acid reaction  and sour  smell of  the  secretion
are due; in part of a proteine-compound, which is probably furnished by
the epithelium-cells that line the tubes; and in part of saline matters,
directly proceeding from the serum of the blood.  Urea  has  been re-
cently detected in the perspiration of the inhabitants  of warm climates.
  745.  The amount of  fluid excreted  from  the skin is almost entirely
dependent  upon the temperature of the surrounding medium ;  being in-
creased  with its rise, and  diminished with  its fall.   The  object of this
variation is very  evident; being the regulation  of the temperature of
the body.   When the  surface is exposed to a high degree of external
heat,  the increased amount of fluid set free from the perspiratory glands
becomes the means of keeping down its own temperature;  for this fluid
is then carried  off in a state of vapor, as fast  as it is set free; and
in its change of form, it withdraAvs a large quantity of caloric from the
surface.  But if  the hot atmosphere be already loaded with vapor, this
cooling poAver cannot be exerted; the temperature of the body  is raised,
and death  supervenes, if the experiment be long continued.  The cause
of the increased secretion is probably to be  looked  for in the increased
determination of blood to  the skin, which takes place under the stimulus
of heat.—The entire loss by Exhalation from the lungs and skin, during
the twenty-four hours,  seems to average a little above 2 lbs.  In a warm
dry atmosphere, however, it has been found to rise to as much as 5 lbs.
whilst in a cold damp one, it may be lowered to If lb.  Of this  quantity,
the pulmonary exhalation is usually somewhat less  than one-third, and
the cutaneous somewhat more than  two-thirds;  but when the quantity
of fluid  lost is unusually great, the increase must  be chiefly in the Cuta-
neous exhalation; since, as already pointed  out (§ 701), the amount of 
                      CUTANEOUS EXHALATION.                   419

exhalation  from the lungs is not  influenced by the external tempera-
ture, but only by the degree in which the surrounding air is previously
saturated with moisture.
  746.  The variations  in  the  amount  of  fluid set free  by Cutaneous
and  Pulmonary  Exhalation,  are counterbalanced by the  regulating
action of the Kidney; which  allows a larger proportion of  water to be
strained off in a liquid state from the blood-vessels, as the Exhalation
is less,—and vice versd.   The Cutaneous  and Urinary excretions seem
to be vicarious, not merely in regard to the amount of fluid which they
carry off from the blood, but also in respect  to the solid matter which
they eliminate from it.   It appears  that  at least 100 grains of effete
azotized matter  are daily thrown off  from the  skin;  and any cause
which checks this  excretion, must increase the labor of the Kidneys,
or produce an accumulation  of noxious matter  in the  blood.   Hence
attention to the functions of  the skin, at all times a matter of great
importance, is peculiarly required in the treatment of Urinary diseases ;
and  it will be often found that no means is so  useful  in  removing the
lithic acid deposit, as  copious ablution and friction of  the skin, com-
bined with exercise.  When  the Exhalent  action of the skin is com-
pletely  checked by the application of an' impermeable varnish, the effect
is not (as might be anticipated) an elevation of the temperature of the
body; on the contrary it is lowered, in consequence, as it  would appear,
of the interruption to the aeration of the blood through the skin, Avhich
is a function of such importance in  the lower animals (§ 671), and  of
no trifling account in Man; and in a short time,  a fatal  result ensues.
A partial suppression by the same means gives  rise to febrile symptoms,
and  to  Albuminuria, or escape of the  albuminous part  of  the  liquor
sanguinis into the  urinary tubes,  in  consequence  (it would appear) of
the increased  determination which then takes  place towards  the Kid-
neys.   These facts are interesting, as throwing  light upon the febrile
disturbance which accompanies those cutaneous diseases that  affect the
whole surface of  the skin at once, and interfere Avith its functions; and
as partly accounting also  for  the Albuminuria which  frequently mani-
fests itself during their progress, especially in  Scarlatina.
  747.  The Skin is likewise furnished with numerous Sebaceous glands,
which are distributed more or  less closely  over  the whole  surface of the
body; being least abundant Avhere the Perspiratory glandulae are most
numerous; and vice versd.  They are altogether absent on the palms of
the hands and the soles of the feet; and are particularly frequent in the
skin of  the face and in the scalp.  They differ  greatly in  size and in de-
gree of  complexity; sometimes consisting of short straight follicles; some-
times closely resembling the Sudoriparous glandulae, the tubes, however,
being usually straighter and wider;  and  being sometimes  much more
complex in structure, consisting of a number of distinct sacculi clustered
around  the extremity of a common duct, into which they open, and form-
ing little arborescent masses about the size of millet-seeds.  In some situa-
tions they acquire still greater complexity.  Thus the Meibomian glan-
dulae, which are found at the edges of the eyelids, and which secrete an
unctuous matter for their lubrication, are long  sacculi branching  out  at
the sides (Fig. Ill); and the glandulae of the ear passage, which secrete 
420                        OF SECRETION.

its cerumen or Avaxy matter, and which belong to the general Sebaceous
system, are formed of long tubes, highly contorted, and copiously sup-
plied with blood-vessels.   In  the hairy parts  of  the  skin, we  usually
find  a pair of  Sebaceous follicles opening  into  the  passage through
which every hair ascends (Fig. 130, 9).  The purpose  of the sebaceous
secretion is evidently to prevent the skin from being dried and cracked
by the influence  of the sun and air.  It is much more abundant in those
races of mankind which are formed to exist  in warm climates, than in
the races that naturally  inhabit  cold  countries; and the former are
accustomed to aid its preservative power, by lubricating their skin with
vegetable  oils of various kinds; which process they find to  be of use, in
protecting it from the scorching influence of the solar rays.—The Seba-
ceous follicles are  frequently the residence  of a  curious  parasite, the
Demodex  folliculorum, which  is  stated  by Mr. Erasmus Wilson to be
present in great numbers  in the skin of almost all inhabitants of large
towns; the  activity of their cutaneous glandular  system  being much
checked by the want of free exposure  to pure air, and by inert habits of
life.
  748.  To what extent  the  Sebaceous secretion can  be  regarded as
destined to free the Blood from deleterious matters, it may not, perhaps,
be very easy to say; but with  regard  to the functions of the Skin taken
altogether, as a channel for the elimination of morbific matters from the
blood, it is probable that  they have  been much underrated;  and that
much more use  might be made of it in  the treatment of diseases,—
especially of such as depend upon the presence of  some morbific matter
in the  circulating current,—than is commonly thought  advisable.   We
see that Nature  frequently uses it for this purpose; a copious perspira-
tion being often  the turning-point or crisis of febrile diseases, removing
the cause  of the malady from the blood, and  allowing the restorative
powers free play.  Again, certain forms  of Rheumatism are charac-
terized by copious  acid perspirations; and  instead of endeavoring to
check these, we should rather encourage them, as the best means of
freeing the blood from its undue accumulation  of  lactic acid.  And it
is recorded  that in the "sweating sickness," which spread throughout
Europe in  the  16th  century,, no  remedies seemed  of any  avail but
diaphoretics; which, aiding the powers of nature,  concurred with them
to purify the blood  of its  morbific matter.  The hot-air bath, in some
cases, and the Avet  sheet (which, as used by  the Hydropathists, is  one
of the most powerful of  all diaphoretics), will be probably employed
more extensively as theurapeutic agents, in proportion as the importance
of acting  on the Skin, as  an extensive collection of glandulae, comes to
be better understood.  The absurdity of the  "Hydropathic" treatment
consists in its indiscriminate application  to a great variety of diseases;
no person who has watched its operation, can deny that it is a remedy of
a most powerful  kind;  and if  its agency be fairly tested, there is strong
reason to belieA^e, that it will  be found to be  the most valuable curative
means we possess for various specific  diseases, Avhich  depend  upon the
presence of" a definite "materies morbi" in the blood,  especially Gout
and  chronic Rheumatism; as  well  as for  that depressed  state of the 
CUTANEOUS  EXHALATION.
421
general system, Avhich results from the " wear and  tear" of the bodily
and mental powers.
  749.  The Mucous surface of the Alimentary Canal is furnished, like
the skin, with a vast  number of glandulae, varying in complexity, from
the simple follicle, to a mass  consisting of numerous lobules opening
into a  common excretory duct.  The functions  of these, as already
pointed out, are equally various.   The simple follicles appear destined,
for the most  part, to secrete  the protective mucus,  which intervenes
between the membranous wall and  the  substances  contained  in  the
canal, and which serves to protect the former from the irritating action
of the latter.   The  more  complex  follicles  of the  Stomach  elaborate
the  Gastric fluid, which  is the prime  agent in  the  digestive process
(§ 496).  The still more elaborate glandulae of Brunner, situated in the
walls of the  duodenum, also seem  to  furnish a product which is  con-
cerned  in the digestive operation  (§ 480).   But there is  strong  reason
to believe, that the function of the Peyerian glandulae, which beset the
walls of the lower part of the intestinal canal, is purely excretory; and
that they are  destined to eliminate putrescent matters from the blood,
and to  convey them, by  the readiest channel, completely out  of the
body.   That the  putrescent elements of the faeces are not immediately
derived from the  food taken in, so much  as from the secreting action of
the intestinal glandulae, appears from this consideration ;—that faecal
matter  is still discharged, even  in considerable  quantities, long after
the intestinal  tube has been completely emptied of  its alimentary  con-
tents.   We see this  in the course of many diseases,  when food is not
taken for  many  days, during which  time the  bowels  are completely
emptied of their  previous contents by  repeated evacuations ; and what-
ever then passes,  must be derived from the intestinal  Avails themselves.
Sometimes a copious flux of putrescent matter continues to take place
spontaneously; Avhilst it is often produced by the agency of purgative
medicine.  The " colliquative  diarrhoea," which  frequently comes on
at the close of exhausting diseases, and Avhich  usually  precedes death
by starvation, appears to depend, not  so much upon a disordered state
of the intestinal  glandulae, as  upon the general  disintegration  of the
solids of the body, which calls them into extraordinary activity, for the
purpose of separating the decomposing matter.
  750.  Thus we  perceive, that we have  here, also, to  watch for  the
indications of  Nature; and that  this extensive  system  of intestinal
glandulae, being the principal channel  for the elimination of putrescent
matters  from  the blood,  should be especially attended  to, when there
is reason to think-that such matters are present in too  large an amount.
Hence,  when diarrhoea is  already existing, we may often  do more good
by alloAving it to  take its course, or even by increasing it by the agency
of  purgative  medicines,  than by attempting to check  it,  and  thus
causing the retention of the morbid matter  in the  circulating current.
But, on  the other hand, it is necessary to bear  in mind the  extreme
irritability of  the intestinal mucous membrane; and carefully to avoid
exciting it, when  it is already in  excess, or when there is danger  that
it will supervene,—as in that form of Fever in Avhich there is a peculiar 
422
OF SECRETION.
liability to inflammation and  ulceration of the  walls of the alimentary
canal, and of their contained glandulae.

             5. General Summary of the Excreting Processes.

   751.  We have now  passed  in review the various processes, by Avhich
the products of the  disintegration of the animal tissues are  carried
off;  and  we have seen that  the necessity for their removal is much
more urgent than for  their replacement.  A cold-blooded animal may
subsist for some weeks, or even months,  without a fresh supply of food,
the waste of its tissues being so small, if it remain in a state of rest, as
to be quite  compatible with the continuance of its  life ;  and a warm-
blooded animal may live for many days or even weeks, provided that it
has in its  body a store of fat  sufficient to  keep up its heat by the com-
bustive process.  But  in either case, if  the exhalation of  carbonic acid
by the lungs, the elimination of biliary matter by the liver, the separa-
tion of urea or uric acid by the kidneys, or the withdraAval of putrescent
matter by the intestinal glandulae, be completely checked, a fatal result
speedily ensues ;—more speedily in warm-blooded animals, than in those
which  cannot sustain  a high  independent temperature, on account of
the greater proneness  to decomposition in  the bodies of the former, than
in those  of the latter;—and  more speedily  in the  latter, when  their
bodies  are kept at an  elevated temperature by the warmth of the sur-
rounding  medium, that when  the degree of heat is so low, that there is
little proneness to spontaneous change in the substance of  their bodies.
   752.  It may be taken as a general principle, in  regard to the Ex-
creting  processes (including Respiration), that they have a  threefold
purpose ;—in the first  place, to carry off the normal results of the waste
or disintegration of  the solid tissues, and  of the  decomposition of the
fluids;—in  the second place, to draAv  off the  superfluous alimentary
matter, which though  received into the circulating current, is not con-
verted into solid tissue, in  consequence of the want of demand for it;
—and  in  the third  place,  to carry off the abnormal  products, which
occasionally result  from  irregular or  morbid changes  in the system.
Thus by the Lungs  are excreted a large  amount of carbon, and  some
hydrogen, resulting from  the disintegration of the tissues,  especially
the nervous and muscular; the same elements, in animals that take in a
large proportion of farinaceous or oleaginous aliment, may be derived im-
mediately from the food, without any previous conversion into solid tissue;
and there  can be little doubt that the respiratory function is also an impor-
tant means of purifying the  blood from  various deleterious matters,
either  introduced from without (such as narcotic poisons), or generated
within the body (such  as the poison of fever).*  And  it  is important
to bear this last circumstance  in mind ; since it enables us to understand
how, if  time be given, the  system frees itself  from such  noxious sub-
stances ; and points out the duty of the medical attendant to be rather that

  * There is strong reason to believe that, in many instances, a small amount of poi-
sonous matter introduced from without, in the  form of a contagion or miasm, may lead,
by a process resembling fermentation, to the production of a large quantity of similar
noxious substances in the animal fluids. 
GENERAL RELATIONS OF EXCRETING PROCESSES.        423
of supporting the powers of the body by judiciously-devised means, and
of aiding the elimination of the morbid matter  through the lungs and
skin by a copious supply of pure air, than  of interfering more actively
to promote that which Nature is already effecting in  the most advanta-
geous manner.
   753.  In like manner, the Liver is charged  with the separation  of
hydrocarbon, in a fluid form; for which  a supply of oxygen  is not
requisite.  This product is  partly derived from  the waste of the sys-
tem ; but the arrangement of the biliary vessels leads to the belief, that
part of it may be  at  once derived from crude matter,  taken  up  by the
mesenteric veins, and eliminated from them by the hepatic cells, with-
out ever passing into the  general  circulation.   And various facts seem
to indicate, that the  Liver is also destined to remove from  the blood
extraneous  substances, which are  noxious to it.   Thus, in cases Avhere
death has resulted  from the prolonged introduction of the salts of Cop-
per into the  system, a considerable amount of that  metal has been ob-
tained from  the substance  of the gland.
   754.  It has been already pointed out (§ 726), that in those tribes  of
animals Avhose respiration  is feeble, a considerable part of the mass  of
the liver is composed of fatty  matter; and this condition may  be in-
duced, as a  state of disease, in Avarm-blooded, energetically-respiring
Birds and Mammals, by impediments to  the due performance  of the
respiratory process.  This is remarkably shown in the treatment of the
geese which  are to furnish  the celebrated Strasburg pates.  The unfor-
tunate bird  is closely confined  at  a high  temperature;
so that the respiration is reduced to its minimum amount,
by the combined effects of warmth and muscular inac-
tion ; and it  is then crammed with maize, which contains
a large  amount of oily matter.   The consequence is, that
its liver soon enlarges, and becomes unusually fatty;  its
cells being gorged with oil-globules, instead of each con-    Hepatic cells
...     b b b a1         & ,      '  j .,  .  ,,       i     gorged with fat:
taming  no more than one or  two :  and it  is then ready   — a, atrophied
for the epicureans Avho set so high a value on the pate   pose^giobui'es341"
de foie gras.   A similar diseased  condition of the liver
frequently presents  itself in  Man, in connexion  with  chronic dis-
orders of the respiratory organs, Avhich diminish  the amount  of hydro-
carbon eliminated through  their  agency ;  this " fatty liver" is peculiarly
common in  the advanced  stages  of Phthisis.   It may arise, however,
from  a local disorder  of  nutrition, such  as that which produces the
fatty degeneration of other organs.  Such a fatty degeneration  may
occur in the  Kidney,  for example, as a consequence of  inflammation  of
its tissues.
   755.  With regard  to the Kidneys, it  has been already pointed out
that they are  the  special  emunctories of the azotized  products  of the
decomposition of the tissues; and  that they serve also to convey away
the overplus of such earthy and  alkaline salts,  as are readily soluble.
Moreover, it has been shown  that  the  surplus proteine-compounds,
which  are not  required for  the  nutrition of the  system,  must be
excreted by  their agency,  after  having  been metamorphosed  into urea.
And we have now to  notice, that other matters of an injurious charac-
 
424
OF SECRETION.
  ter, Avhether introduced from without, or generated within the system,
  are drawn off by the same channel.  Thus the saline compounds, taken
  up  by the absorbent process (§ 493), are  for  the most  part  set free
  through these organs; especially when their properties are such, as to
  excite the action  of the kidneys in a peculiar degree.  Thus, Prussiate
  of Potash has  been detected in the urine,  within one minute after it
  has been introduced into the stomach.   It  has been sometimes noticed
  that  Iodide of Potassium, when administered as a medicine, is  retained
  within the body for some  days,  producing extensive cutaneous erup-
  tions, or some  other unusual consequence ; and  that it  then suddenly
  begins to pass off by the kidneys,  and  is excreted in  very large  quan-
  tities.   Further,  it  has  been shown  by Dr. Letheby,  that poisonous
  substances (such as arsenious  acid),  introduced  into the system  in
  small but frequently-repeated doses, may  be carried  out of the body
  with  such  rapidity  as  to be prevented from exerting their injurious
  effects, provided that diuretics be administered at  the same time.  The
  effect of the inhalation of the  vapor  of  turpentine, even  in a very
  diluted state, in speedily imparting to the urine the odor  of violets, is
  an evidence that not merely the actual substances imbibed, but  new and
  peculiar compounds to which they  give rise, are thus eliminated by the
  Kidneys.
    756.  The most singular  variations  in the excretory function of the
  Kidneys are seen, however,  when  the Urine is charged with substances
  Avhich are  not only foreign to it, but  are altogether foreign to the
r~healthy body.   The most remarkable  instance  of this  is seen in the
  disease termed Diabetes, in which  a large quantity  of Sugar is formed,
  either directly from the food, or by the  disintegration  of the solid
/ tissues; and in which  this  compound is eliminated by  the Kidneys,
  imparting  to  the  urine a saccharine  taste.  And another example of
  the same general  fact is  seen in the " oxalic diathesis," in which an
  unusual arrangement  of the elements that usually form  urea or uric
  acid,  gives rise to a new and peculiar  compound, oxalate  of  ammonia;
  this being drawn off by the kidneys, and  being  decomposed by the
  calcareous matter present in the urine, gives rise to a deposit of oxalate
  of lime.  In the treatment of such diseases, our attention must be given,
  not so much to the  secreting organ, as  to the condition of the system at
  large, of which the character of the secreted product is  the  indication
  or exponent.
    757.  To  what has  already  been stated in  regard to the exhalant
  functions of the Lungs and Skin,  it may be added  that many  states of
  disease are marked  by  an  unusual odor emitted from the body; and
  there can be little doubt that the peculiar odorous matter is pre-formed
  in the blood,—as Ave know that   the ordinary  scent of any species
  (whether Man, Dog, Horse, Goat, &c.) may  be set free from the blood
  of that species, by  the  addition   of sulphuric  acid.  The existence of
  such odors, therefore, is  not  to be attributed to disordered function in
  the excreting organs;  but to the formation  of morbid products in the
  interior of the  body, which these organs do their best to remove.  The
  foetid breath, which  frequently accompanies an attack of  indigestion, is
  another instance of the power of the  lungs to eliminate, not merely 
HEAT OF ANIMALS AND PLANTS.
425
Carbonic  acid, but other products of the changes in composition which
the food undergoes when introduced into the system.
   758. The  same remarks apply, and with yet greater force, to the
Intestinal  glandulae; whose function  it is, not  merely  to  remove the
putrescent matter ordinarily formed by the disintegration of the tis-
sues, or by the decomposition of  unassimilated food, but also  to  draw
off the still more offensive products of such changes as take  place in
disease.  Thus there are  conditions of the system, in which, Avithout
any well-marked  disorder, the faeces emit a  peculiarly foetid  odor ;
and with  these is almost  always  associated a depressed state of mind.
Noav it can  scarcely be doubted,  that the  real fault  is here rather in
the  early part of the nutritive operations,  than in the excretory func-
tion ;  and that the foetor of the contents of the intestine depends upon
the undue formation of putrescent matter in the system, which, by taint-
ing  the blood, causes its action upon the brain to become unhealthy.
The object of the physician will be here to eliminate  the morbid pro-
duct, by the  moderate use of purgatives;  and  so to regulate  the diet
and regimen, as to correct the tendency to its formation.—An excessive
foetor  in the  evacuations, as well as in the exhalations from the skin
and lungs, is peculiarly  characteristic of those very severe forms of
typhus (noAV, happily,  of comparatively rare  occurrence), which are
termed putrid fevers.^.  Here the whole of  the  solids and fluids of the
body appear  to have an  unusual  tendency to decomposition, in conse-
quence of  the introduction of some morbid  agent, which acts as a fer-
ment ; and the system attempts to free itself from the products of that
decomposition, by  the various organs of excretion, particularly the Skin
and Intestinal surface.
   759. It  is of great importance that the Student should form  clear
conceptions on this subject;  and that he should not (as too  often hap-
pens),  by directing his remedies to the mere symptoms or results of a
disease, act in precise opposition to the natural tendency of the system
to free itself from some unusual noxious matter,  through those channels
Avhich  are  ordinarily destined to carry off only the regular  products of
its disintegration.
                          CHAPTER  X.

  OF THE  DEVELOPMENT OF HEAT,  LIGHT,  AND  ELECTRICITY, IN THE
                           ANIMAL BODY.

  760.  It has been shown, in an earlier part of this volume (chap,  ii.),
that all Vital  actions require a certain amount of Heat for their  per-
formance ; and  that  there is  a great variety amongst  the different
classes of Animals, both in regard to the degree of Heat which  is most
favorable  to  the several processes  of their economy, and in regard to
their  own  power of sustaining  it, independently of oscillations in  the 
426
DEVELOPMENT OF HEAT,  ETC.
temperature of the surrounding medium.  As  a general rule, the In-
vertebrated  animals  are cold-blooded; that is, they have  little or no
power  of sustaining an  independent temperature.   The degree  of
energy  of their  vital actions  entirely depends,  therefore,  upon  the
warmth they receive from the  air or water they inhabit; they have no
power of resisting the depressing influence of cold ; and they are gene-
rally so organized, as to pass into a state of complete inaction or torpi-
dity, when the temperature sinks below a, certain point,—after gradu-
ally becoming more and more inert with every diminution in  the  heat
of their bodies.   The same is true, also, of most Fishes and Reptiles :
but the animals of the former class, from the more equable temperature
of the medium they inhabit, are not so liable to be reduced to inaction
as the latter ; being usually so organized, as to retain their activity so
long as the water around them continues liquid;  and being actually
imbedded in a frozen state, when the water around them  is converted
into ice, without the loss of their vitality.  There are certain Fishes,
hoAvever,—such  as the  Thunny, Sword-fish, and  other large species of
the Mackerel tribe,—which are  able  to  sustain a temperature consi-
derably above that of  the sea they inhabit; thus in the Bonito, the
heat of the body has been found to be  99°, Avhen the temperature of
the surrounding sea was but 80J°.   It is not probable, however, that
the temperature  of the body would be kept up to  the same standard,
if that of the sea should be considerably lowered ; but it Avould proba-
bly remain at from 18°  to  20° above the latter.   And in like manner,
it has been noticed that many of the more active Reptiles possess the
power of sustaining the  temperature of their bodies at 10° or 15° above
that of the surrounding air.
   761.  The classes  of  animals which  are  especially endowed with the
power of producing  and  maintaining heat,  are Insects,  Birds,  and
Mammalia.   The remarkable variations  which present themselves in
the temperature of the first of these classes, and the connexion of these
variations with the condition  of  the  animals  in  regard to activity or
repose, have already been sufficiently noticed  (§ 123).—The  tempera-
ture of Birds is higher than that of any other class of animals;  varying
from 100° to 111° or 112°.  The loAvest degree is found in some of the
aquatic species, as the Gull, and  in those which principally live on the
ground, as the Fowl tribe ;  and the highest in  the birds of most active
flight,  as  the  SAvalloAV.   The  temperature of the Mammalia  seems to
range from  about 96° to 104° ; that of Man has been observed  as low
as 96J°, and as  high as 102°.   The variations  are  dependent in part
upon the temperature of the external air;  but are influenced also by
the general condition of the body as to repose or activity, the period of
the day, the time that has elapsed since  a  meal, &c   A someAvhat
larger amount of caloric is generated during the day, than in the night;
and the body is usually warmer, by a degree  or  tAvo, at noon, than at
midnight.   There is also  a slight increase during the digestion of a
meal; and exercise is a powerful means of raising the temperature.—
The range of temperature is much greater in disease ;  thus the thermo-
meter  has been seen to rise to 106° in  Scarlatina and Typhus, and to 
HEAT OF ANIMALS AND PLANTS.
427
 110?° in Tetanus;  whilst it has fallen to 82° in Spasmodic Asthma, and
 to 77° in Cyanosis and Asiatic Cholera.
   762. In searching for the conditions on which this production of heat
 within  the  Animal body is dependent, it is very important to bear in
 mind, that a similar generation of Caloric may be observed in the Vege-
 table  kingdom.  It appears from the most  recent and exact experi-
 ments, that all  living Plants are somewhat warmer than similar  dead
 plants exposed to the same atmosphere;  and that the elevation is the
 greatest in  the leaves  and young stems, in which the  most active vital
 changes are taking place.  But the most decided production of heat
 occurs  in the flowering  of certain Plants, such  as the Arum, which
 have large fleshy receptacles, on which a great number of blossoms are
 crowded ; thus  a  thermometer placed in the centre of five spadixes of
 the Arum cordifolium has been seen to rise to 111°, and one placed in
 the midst of twelve spadixes has risen to 121°, whilst  the temperature
 of the surrounding air was only 66°.  In the germination of seeds, also,
 a great elevation of temperature occurs, which is rendered most evident
 by bringing together a number of seeds, as in the process of malting, so
 that  the caloric is  not dissipated as fast as  it is generated;  the ther-
 mometer, placed in the midst of a mass of seeds  in active germination,
 has been seen to rise to 110°.
   763. Thus it is  evident that the chemical changes which are involved
 in the operations of Nutrition, are capable of setting free a large amount
 of heat; which, although ordinarily dissipated from the vegetating sur-
 face too speedily to manifest itself, becomes sensible enough, when this
 rapid loss is checked.   If we further examine into the  nature of the
 chemical changes  which appear most concerned in this elevation  of
 temperature, we find that they uniformly consist  in the  combination of
 the carbon  of the plant with the oxygen  of the atmosphere;  so that a
 large quantity of carbonic acid is formed and set free,  precisely in the
 manner of the Respiration  of Animals.   This process  is so  slowly per-
 formed, in  the ordinary growth of Plants, that  it is  concealed (as it
 were) by the converse change,—the fixation of carbon from the carbonic
 acid of the atmosphere, under the influence of light (§ 83).  But it takes
 place with extraordinary energy  during flowering and germination;  a
 large quantity of carbon being set free, by union with the oxygen of the
 air;  and the starchy matter of the receptacle, or of the seed, being con-
 verted into sugar.   Now it has been ascertained by careful experiments,
 that  the amount of heat generated is  in close relation  with the amount
 of carbonic acid set free ;  and that, if the formation of the latter be pre-
 vented, by placing the flower or the seed  in  nitrogen or  hydrogen, no
 elevation of temperature takes place ; whilst, if the process be stimu-
 lated  by pure oxygen, so  that  a larger proportion of  carbonic acid is
 evolved, the elevation of temperature is  more rapid and considerable
 than usual.
   764.  Upon examining into the conditions under which Caloric is gene-
 rated in the Animal body, Ave find them essentially the same.  Wherever
the temperature of the  body is maintained at  a regular standard, so as
to be independent  of variations  in  the Avarmth of the surrounding me-
dium, we find a provision for exposing the blood most  freely to the in- 
428
ANIMAL HEAT.
fluence of oxygen, and for extricating its carbonic acid ;  thus  in  Birds
and Mammals, the blood is  distributed, in  a minute capillary  netAvork,
on the walls of the pulmonary air-cells, the gaseous contents of Avhich
are continually renewed; and in  Insects, the air is carried into every
part of the body by the ramifying tracheae.   We constantly find a pro-
portion between the amount  of heat evolved, and that of carbonic acid
generated; this is peculiarly evident in Insects, whose respiration and
calorification vary so  remarkably (§ 123); but  it  is also  proved by
comparing the amount of carbonic acid generated by warm-blooded ani-
mals, Avhen the external temperature is low, and when more heat must
be evolved to keep the temperature of their bodies up to its proper stan-
dard, with that generated by the same animals in a warmer atmosphere,
when the proper animal heat is diminished in amount (§ 691).
  765. The sources of the Carbonic Acid thrown off by the lungs, have
been  already pointed  out (chap,  viii.) : it is partly deriAred  from the
metamorphosis  of the tissues ; but partly, in all but purely carnivorous
animals, more directly from  the non-azotized portion of the food.  The
precise mode in which the carbon thus supplied is united with the oxy-
gen derived from the atmosphere, is not yet known ; but it is certain that,
in whatever  manner the combination may take place, a certain measure
of caloric  must be generated.  It appears, however, from various ex-
periments, that the whole quantity of caloric generated by an animal in
a given time, is greater than  that which would be evolved by  the com-
bustion of the carbon, included in the  carbonic acid  evolved during the
same time.  Hence it is evident that other chemical  processes occurring
within the body are concerned in the maintenance of the temperature;
and it is not difficult to point to some of these.  It is probable, in the first
place, that some of the Hydrogen of the food may be " burned off" by
union with the  oxygen of the atmosphere, so as to form  part  of the
water which is exhaled from  the lungs.  Again,  the sulphur and phos-
phorus of the food are converted, by  oxygenation,  into sulphuric and
phosphoric acids; in  which  process, heat must  be  generated.   In the
composition of urea, moreover, oxygen is present in much larger propor-
tion, than it  is in the proteine-compounds by the metamorphosis of which
it is  formed; so that in its production  too, caloric  will be generated.  In
fact it may be stated as a general truth, that the whole excess of the oxy-
gen absorbed, over that Avhich is contained in the carbonic acid exhaled
(§ 689), must be applied to purposes in the laboratory of the system, in
which caloric will be disengaged.   Still, the amount of Carbonic Acid
exhaled must always be the measure of the chemical  processes, by which
heat  is generated in  the body;  because it  is itself the result  of the
chief of these processes (the union of carbon and oxygen), and because
the surplus amount of oxygen which is absorbed, and  which is applied
to other purposes, is closely  related to  it.
  766. The power of maintaining a high independent temperature  is
usually much less in young warm-blooded  animals, than in adults.
There are considerable variations  in  this  respect, however,  amongst
different species; for where  the young animal is born in such an ad-
vanced condition, as to be thenceforth almost independent of  parental
assistance, it is capable of maintaining its own temperature; but where 
REGULATION OF  HEAT IN MAN.
429
 it is born in such a state, as to require to be supplied with food  by the
 parent for some time, it is also more or less dependent upon the warmth
 imparted to it from the parental body.  This is peculiarly the case with
 the young of the  Human  species,  which is longer  dependent upon
 parental aid,  than  that of any other animal.  In the case of children
 born very prematurely, the  careful sustenance of their heat is  one of
 the points most to be attended to in rearing them; and even the most
 vigorous infants, born at the full time, are far from being able to keep
 up their proper standard without assistance, if exposed to a cool atmo-
 sphere.  It has been ascertained that, during the first month of infant
 life, the mortality in winter is nearly double that  of  summer,—being
 1-39 in January to  0*78 in July; and this striking difference cannot be
 attributed to any other cause, than the  injurious  influence of external
 cold, which the calorifying  powers of the infant do not enable it to
 resist.   As age advances, the power of generating heat increases,  and
 the body becomes much more  independent of external vicissitudes; so
 that, in adult life, the  winter  mortality is  to  that of summer, only as
 1'05 to 0-91, or less than one-sixth more.  In advanced age, the  calori-
 fying power again diminishes; and this we should anticipate from the
 general torpor of the nutritive operations in old persons.  Between 50
 and 65 years  of  age,  the relative winter and summer  mortality  are
 nearly  as in the first month of infancy; and  at  90 years, the average
 mortality of winter  is much more than twice that  of  summer, being as
 1-58 to 0-64.
   767. It appears that  there is  a difference in calorifying power,  not
 merely at different  ages, but at  different seasons: the amount of heat
 generated in summer not being sufficient, in many animals, to prevent
 the  body from  being cooled down  by prolonged exposure  to a tempera-
 ture, which is  natural  to them in winter.  To what extent this  is the
 case with Man, it  is difficult  to say.  His constitution  is distinguished
 by its power of adapting itself to circumstances; and he can live under
 extremes of temperature more wide than those, which most other  ani-
 mals can endure (§ 113).  Whether  in the torrid  zone, or in the arctic
 regions, he can maintain his  healthy condition under favorable circum-
 stances; in each  case his  natural appetite leading him to the use of
 that kind and  amount of food, which  is best suited to the wants  of  his
 system.   But the  longer he has  been habituated  to  a very warm or a
 very cold climate,  the more difficult  he at first finds it to live comforta-
 bly in one of an opposite character; as his constitution, having become
 adapted to one particular set of circumstances, requires time to accom-
 modate itself to an opposite one.
   768.  The means  by which the  heat of the  body is prevented from
 rising above its normal standard,  even  in the midst of a very high tem-
 perature in the surrounding air, are  of the most simple character.  The
 excreting action of the  skin is directly stimulated  by the application of
 warmth to the  surface;   and the  fluid  which is poured forth, being  im-
 mediately vaporized, converts a large  quantity of  sensible caloric into
 latent, and thus keeps down the temperature of the skin.   By this pro-
vision, the body may be exposed  with impunity to dry air of 600° or
more, so long as the supply of fluid  is  maintained.  But it cannot long 
430
ANIMAL LUMINOSITY.
 sustain exposure to air saturated with vapor, even though it may not be
 many degrees hotter than the body;  because the  cooling act of evapo-
 ration from the skin cannot then be carried on.
   769. The evolution of Light is a very interesting phenomenon, chiefly
 witnessed among the lower animals, and usually supposed not to  occur
 in  any class above  Fishes.   It  is particularly remarkable among the
 Radiata and inferior Mollusca.  A large proportion of the Acalephce, or
 Jelly-fish tribe, possess the property of luminousness in a greater or less
 degree; and it is to small animals of this class, which sometimes multi-
 ply to an amazing extent,  that the beautiful phenomenon of phosphores-
 cence of the sea is chiefly due.  In the midst of the soft diffused  light
 thus occasioned, brilliant stars, ribands,  and globes of fire are frequently
 seen;  these  appearances being  due to  the luminosity of the larger
 species of the same  tribe, or to that of other marine animals.—Some of
 the most remarkable examples of luminosity, in regard to the brilliancy
 of  the light  emitted, occur in the class of Insects.   Here the emission
 is confined to one portion of the  body,  or to two or  more isolated spots,
 instead of being diffused over a larger surface; and it is proportionally
 increased in intensity.   The phenomenon  of Animal Luminousness
 appears usually attributable to the formation of a peculiar secretion;
 which in many instances, continues to shine after removal from the
 animal, so long as  it is  exposed to  the influence of  oxygen: and it
 seems not unreasonable to believe that  it depends upon a slow process
 of  combustion, analogous to  that which takes place  when phosphorus is
 exposed to the  air.   There is a special  provision in Insects, for convey-
 ing a large supply of air through the peculiar substance, which is depo-
 sited beneath the luminous spots;  and  the power which Glow-worms, Fire-
 flies, &c, possess, of suddenly extinguishing their light and  as suddenly
 reneAving it, seems to depend upon their control over the air-aperture or
 spiracle by  which air is  admitted, the stoppage  of the supply of air
 causing the  immediate cessation of the luminousness, and its readmission
 occasioning  a renewal of the process on which it depends.—It is proba-
 ble, however, that in certain cases the luminosity is rather of  an electrical
 character.  There are several of the smaller Annelida or marine Worms,
 which are brilliantly luminous when irritated;  the luminosity having the
 character, however, of a succession of sparks, rather  than  of  a steady
 glow.  It appears from the experiments of M. Quatrefages, that this pecu-
 liar luminosity is the especial attribute of the muscular system; and that
 it is produced with every act of muscular contraction  in these  animals.
  770.  Although no such luminosity is  commonly  manifested in any of
 the higher Vertebrata, or in Man,  yet  there are well-authenticated
 cases, in which the phenomenon has presented itself in the living Human
subject,*—luminous  emanations from dead animal matter  being of no
unfrequent occurrence.  In  most of these  cases, however,  the indivi-
duals  exhibiting the luminosity had suffered from consumption, or some
other  wasting disease, and were near  the close of their lives at the
time; so that it is probable that a  decomposition  of the  tissues  was

  *  See an account of several cases of the Evolution of Light in the Livin°- Human
Subject, by Sir  Henry Marsh,  M.D., M.R.I.A., &c. 
ANIMAL ELECTRICITY.—ELECTRIC FISHES.
431
 actually in progress, analogous to that which, when it occurs after death,
 imparts luminosity to the decaying body.  One instance is recorded, in
 which a  large cancerous sore of the breast emitted light enough to en-
 able  the hands on a watch-dial to be distinctly seen Avhen it was  held
 within a few inches of the ulcer; "here, too, decomposition Avas obviously
 going on, and the phosphorescent matter produced by it was exposed to
 the oxygenating action  of the atmosphere.
   771. Slight manifestations of free Electricity, or, in other words, dis-
 turbances of Electric equilibrium, are very frequent in living animals;
 and they are readily accounted for, when we bear in  mind  that  nearly
 all chemical changes are attended with  some  alteration  in  the  electric
 state of the bodies concerned;  and when we consider the number and
 variety of such changes in the living animal body.  When slight, how-
 ever, they can only be detected  by refined means of observation ;  and it
 is only when they are considerable, that they attract notice.   The most
 remarkable examples of the evolution of free Electricity in Animals, are
 to be found in certain species  of the class of Fishes; the best knoAvn of
 which are the  Torpedo  or Electric Ray,  and  the Gymnotus or Electric
 Eel.   These possess organs, in which Electricity may be generated and
 accumulated in large quantities, and from which it may be discharged
 at will.  The shock of a large  and  vigorous Gymnotus is sufficiently
 powerful to kill small animals, and  to paralyse large ones,  such  as  men
 and horses ;  that of the Torpedo is less severe, but it is sufficient to be-
 numb the hand that touches it.
   772. The electric organs of the Torpedo (which, from being found on
 European shores, has been  the  most studied) are of flattened  shape,
 and occupy the front and sides of the body;  forming two large masses,
 which extend  backwards and  outAvards from each side of the head.
 They are composed of  tAvo  layers of membrane separated by a consi-
 derable  space  ; and  this space  is  divided by vertical partitions  into
 hexagonal cells like those  of a honeycomb, the  ends of  which are
 directed towards the two surfaces of  the body.  These cells, which are
 filled with a whitish soft pulp, somewhat resembling the  substance of
 the brain, but containing more water, are again subdivided horizontally
 by membranous partitions; and  all these partitions  are profusely  sup-
 plied with blood-vessels  and nerves.—The electrical organs of the Gym-
 notus are essentially the same in structure ; but they differ in shape, in
 accordance  with the conformation  of the  animal.—In these, and the
 other Electrical fishes,  the electric organs are supplied Avith nerves of
 very great size, larger than any  others in the same animals, and larger
 than any nerves in other animals of similar bulk.  These  nerves arise
 from  a peculiar ganglionic  enlargement of the Medulla  Oblongata,
 termed the  electric lobe, and seem chiefly analogous to the pneumogas-
 trics of other animals.
  773. The following conditions  appear to be essential to  the manifes-
 tation of the Electric powers of  these animals.  Two parts of the body
 must be touched at the  same time; and these two must  be  in different
 electrical  states.  The  most energetic discharge is procured from the
Torpedo, by touching its back and belly simultaneously; the electricity
of the back being positive, and that of  the belly negative.  When tAvo 
432        ELECTRIC FISHES.—ELECTRICITY OF  MUSCLES.

parts of the same surface, at an equal distance from the electric organ,
are touched, no effect is produced, as they are equally charged with the
same electricity ;  but if one point be further from it than the other, a
discharge occurs, the intensity of which is proportioned to the difference
in the distance of the points from  the electric organ.   However much
a Torpedo is  irritated, no discharge can take place through a single
point;  but the fish makes  an effort to bring  the border of the other
surface in contact with the offending body, through which a shock is
then  transmitted.   This, indeed, is probably  the usual way in which
the discharge  is effected.—The identity of animal with common Elec-
tricity is  proved,  not merely by the similarity of the effects upon  the
feelings produced  by the  shock of both;  but also by the fact that  a
spark may  be obtained, and chemical decompositions effected, by  the
former, precisely as by the latter.
   774. The power of the animal over the actions  of its Electric organs,
is dependent  upon their  connexion with the  nervous  centres.  If all
the nerve-trunks  supplying the organ on one side be divided, the ani-
mal's control  over  that organ will be destroyed ;  but the  power of  the
other  may  remain uninjured.  If the nerves be partially divided on
either or both sides, the power is retained by the portions  of the organs
which are still connected with the centres  by the trunks  that  remain.
Even slices  of the organ, entirely separated from the body except by a
nervous fibre, may exhibit  electrical properties.  Discharges  may be
produced, by  irritating the part of  the  nervous centres from which  the
trunks proceed, so long as the latter are  entire ; or by irritating the
portions of the divided trunks which remain in connexion with  the
electric organs; or even  by irritating portions of the  electric  organs
themselves,  when  separated  from  the nervous centres.—In all these
respects, there is a strong analogy between the action of the nerves on
the Electric organs, and their action on  the Muscles.   The connexion
of the organs specially appropriated to each of these actions with  the
Nervous system, the dependence of their functions  upon  the integrity
of this connexion  and upon the state of activity of the  central organs,
the influence  of stimulation  applied to  the nervous centres or trunks,
the results  of ligature or section of the  nerve, and  the effects of poi-
sonous  agents, are all so remarkably analogous  in  the  two cases, that
it seems scarcely possible to doubt that the Nervous force is the agent
Avhich is instrumental in producing both sets of phenomena.  Still, how-
ever,  no proof whatever can be derived from this source, of the  identity
of nervous influence with any form of Electricity ; since all that can be
legitimately inferred from it  is, that Nerve-force acting through a parti-
cular organic structure developes Electricity, in virtue of the correlation
formerly explained (§ 396).
  775. It is another interesting point of  analogy between the action
of Muscles,  and that of the Electrical organs, that the former (as is
now fully proved by the elaborate and exact  researches of Matteucci)
is attended with electric disturbance.  In any  fresh vigorous muscle, in
a state of passive or tonic  contraction,  there is a continual  electric
current from the interior to the exterior,  sufficient to excite the leg of
a frog to energetic contraction, when its  nerve is  so  applied to  the 
MANIFESTATIONS OF  ELECTRICITY.
433
 muscle, as t.o receive the influence of this current.   And a much more
 powerful current is  produced, when the muscle  is thrown, by a stimu-
 lus applied to its own nerve, into a state of energetic contraction.  The
 explanation of the  constant direction of the current, from the interior
 toAvards the exterior of the muscle, seems to be, that the changes con-
 nected with the nutrition and  disintegration of the  muscular  tissue go
 on more energetically in its interior, than they do  nearer its surface,
 where the  proper  muscular fibres  are mingled with  a  large proportion
 of areolar  and tendinous  substance.
   776. It  was  observed by  Galvani that there exists  in the Frog,
 during its  whole life, a continual  current  of Electricity passing from
 its extremities towards its head ; and as no such current has been detect-
 ed in any  other animal, it has been termed the  courant propre, or pe-
 culiar current, of the Frog.   It bears this curious analogy to the elec-
 tric discharges of Fishes,—that it is not manifested if the connexion be
 made  between corresponding points of the opposite sides, but that it
 shows itself when the communication is made between  points higher or
 lower  in the body, whether on the same  or on opposite  sides.—There
 noAv seems reason to  believe, however, from the observations  of Mat-
 teucci, that this "proper current of the frog" is but a special case of the
 ordinary muscular current, depending upon the peculiar arrangement of
 the muscular and tendinous elements in this animal.   Both currents are
 alike influenced by agents which affect the  vitality of the muscle; and
 it is curious that poisoning with sulphuretted hydrogen  should almost
 immediately put an  end  to each,  although ordinary narcotic poisons
 have very  little influence.
   777. Manifestations of Electricity may be produced, in most animals
 having a soft fur, by rubbing the surface, especially in  dry weather;
 this is a fact sufficiently well known in regard to  the domestic Cat.
 Some  individuals of the Human race exhibit spontaneous manifestations
 of electricity, which are occasionally of very remarkable power.  There
 are persons, for instance, who  scarcely ever pull off articles  of dress
 that have  been worn next their skin, without  sparks and a crackling
 noise being produced, especially in dry weather.  This  is partly due,
 however, to the  friction of these materials  with  the surface,  and with
 each other.  But the case of a lady has been recently put on record,
 who was for many months in an electric  state so different from that of
 surrounding bodies,  that, whenever she was but slightly insulated by a
 carpet or  other feebly-conducting  medium,  sparks passed between her
 person and any object which she  approached.   When she was most
 favorably circumstanced,  four sparks  per minute would  pass between
 her finger and the brass ball of a stove, at a distance of 1J inch.   Va-
 rious experiments were tried, with the vieAv  of ascertaining if  the Elec-
 tricity  Avas produced by the friction of articles of dress ; but no change
in these seemed to modify its intensity.   From  the  pain  which accom-
panied the  passage of  the sparks,  this condition  was  a source  of much
discomfort  to the subject of it.
28 
434
OF GENERATION AND DEVELOPMENT.
                           CHAPTER XI.

                .OF GENERATION AND DEVELOPMENT.

               1.  General View of the Nature of the Process.

   778. There is  no one of the functions  of living beings, that distin-
guishes them in  a more striking  and evident manner  from  the  inert
bodies which  surround  them, than the process of Generation.  By this
function, each race of Plants and  Animals is perpetuated; whilst the
individuals composing it successively disappear from the surface of the
earth, by that death and decay which are the common lot of all.   There
are certain tribes, in which the death of the parent is necessary for the
liberation of the germs from which a new race  is  to spring up.   This
is the case, for example, in some  of the  simplest  Cellular Plants;  in
which every cell  lives for itself alone, and  performs its  whole series of
vital operations independently of  the rest.  But as, in  more  complex
organisms, we find  certain cells  set apart for  Absorption, others for
Secretion, &c, so do we find a particular  group of cells set  apart  for
Reproduction; and  these  go through a series of  changes peculiar to
themselves, without interfering with the general life of the structure.—
It is in the  Vegetable kingdom, that the essential character of the Ge-
nerative process can be best studied ; and we shall, in the first instance,
therefore, inquire into  the  nature and import of the principal pheno-
mena which it presents.
   779. If we take as our starting-point the  simple  cell in which the in-
dividuality of the lowest Algae seems to reside, Ave find that this cell,

                               Fig. 132.
 Various stages of development of Bcematococcus binalis:—a, a, simple rounded cells; b, elongated cells,
the endochrome preparing to divide; c, c, cells in which the division has taken place; d, cluster of four cells
formed by a repetition of the same process.

under the  influence of light  and  warmth, and supplied  with aliment,
multiplies itself to an extent that almost  seems unlimited; and this by
a process of duplication exactly  analogous to that which has been al-
ready described (§ 212) as taking place in Cartilage;—the chief  diffe- 
SIMPLEST FORMS  OF GENERATIVE PROCESS.           435
rencc being, that in the latter the fission commences in the nucleus, each
half of which seems to draw  around it a portion of the contents of  the
cell, whilst in the former the fission shows  itself at once in the entire
endochrome, there being no distinct nucleus (Fig. 132).  Now although
the effect of this operation is to produce a  great number of new cells,
yet it cannot be truly  considered as  an act of  Generation;  for it is ob-
viously analogous to that multiplication of the component  cells, which
takes place as a part of every process of development in the most com-
plex organisms; the only difference  being, that  the new  cells are here
in great^ degree independent one of another, so as  to be able  to maintain
their existence  when isolated ;  whilst among the higher tribes,  there is
so close a  relation of mutual  dependence between the component cells,
that they  cannot continue to live if  separated from one another.  And
we shall hereafter  see, that  the  early  development of the embryonic
Fig. 133.
  Successive stages of development of simpler Algse:—a, individual cells of Protococcus viridis; b, c, clusters
formed by their multiplication; r, filament of Schizognium murale ; e, a similar filament, subdividing late-
rally, which constitutes the early form of the Utvacece ; p, o, portions of the expanded thallus of Viva furfu-
racea, formed by the continuance of the same process of transverse subdivision.

mass, even in the highest  Animals presents phenomena in all respects
comparable  to  this multiplication of the simplest of Cellular  Plants by
successional subdivision (§ 805);  all the descendants of the original cell,
however, here remaining  in mutual apposition, and concurring to make
up  what is commonly designated as a single individual, whilst in the 
436
OF GENERATION AND  DEVELOPMENT.
simplest Plants, these cells, as their number is successively augmented
by fission, arejnore and more widely separated from  each other, and
may disperse  themselves over an extensive surface.—If, however, they
should remain in  connexion with each other, they may  form clusters,
or fronds (expanded leafy surfaces), according to the direction in which
the subdivision takes place (Fig. 133);  and this  without the slightest
departure from the original cellular type  which is preserved throughout
the structure,  every part being exactly similar to every other.   In these
composite  organisms, we usually find a provision, not merely for  the
extension of the original structure, but for the multiplication of indivi-
duals, Avhich being still referable to the general type of cell-subdivision
must be considered as a process of development, rather than of genera-
tion.  This consists in the emission, from the interior of  certain of the
cells, of broods of young cells formed in their interior;  and these, in
the loAver aquatic  plants, are very commonly furnished with cilia, by
the agency of which they  are  dispersed  through the water, beginning
to develope themselves into the likeness  of the organisms from which
they sprang,  as soon as their  movement has  ceased.    These " zoo-
spores," as they are termed, must be regarded as the representatives of
the gemmar or buds  of higher Plants.   The latter are usually developed
in continuity  with the stock from  which they originate;  but there are
many instances in which they  are spontaneously  detached; and there
are few cases in  which  they will not continue their  existence under
favorable  circumstances, when  artificially separated  from it, as is
practised in the operations of grafting, budding, &c.
   780.  The true  Generative  process, on the other hand,  seems to con-
sist, throughout the Vegetable kingdom, in the reunion of the contents
of tAvo  cells  which  have been separated in the process of development
and multiplication, and in the production  of a germ as the result of this
reunion, which is  usually very different in its characters and properties
from  either of the  cells  whose contents  have contributed to form it.
This process has been observed  to take place in the Vegetable kingdom,
under  three  principal forms, which  seem to  be characteristic of  the
lowest  Cryptogamia, of  the  higher Cryptogamia,  and  of the Phanero-
gamia,  respectively.—The first of these  presents  itself in those simple
Cellular Plants, in which, whether the cells remain in connexion or not,
their endowments are all of the same nature.  At a certain time of the
year (it would seem) in each species, the cells  approach one another in
pairs,  and their  endochromes (or colored contents) are  intermingled
(Fig. 134, a), either by the rupture of both cells (1), or by the formation
of a direct communication from the interior of one to that of the other,
in  Avhich  case the union of the two endochromes may  take place either
in  the  connecting channel (2), or in one of the pairs of cells (3).   Of
this process, Avhich is known as Conjugation, the result is the formation
 of a body known  as a Sporangium, which may be considered as the first
 product of the true generative process; and from this sporangium (which
 is a single cell, or a pair or cluster of cells) a " neAV generation" is de-
 veloped by the subsequent process of fission and multiplication,  There
 is here no definite distinction of the sexes, the conjugating  cells being
 apparently alike  in their endowments;  such a distinction is shadowed 
SIMPLEST  FORMS OF  GENERATIVE PROCESS.           437
forth, however, where the sporangium is developed within  one of them.
—The  second form of the true generative process  is  seen even  in the
higher  Algae; and, although the  extent of its prevalence has not yet
been clearly  determined, it  is probably  common  to  the  Liverworts,
Mosses, and Ferns, it being in the last  of  these groups that it has  been
most satisfactorily  made  out.   In  conformity  with the  separation or
specialization of organs which is characteristic of these Plants, we find
that the Generative power is now limited to certain small parts of them,
and that these produce two orders of cells,  very distinct in their  endow-
ments,  which  may be called  respectively  "sperm-cells,"  and "germ-
cells."   It is  from the latter  that the new plant originates ; but  this it
can only do, when the fertilizing influence of the former has been  con-
veyed  to  it;  and the provision for this  purpose  is  very remarkable.
The  sperm-cells,  developed within bodies termed  antheridia, form in
their interior, as  their  characteristic products,  minute  spirally-coiled
filaments, usually furnished with cilia at one  extremity, and bearing  a
very close resemblance to the spermatozoa of animals  (§ 785).   These,
when liberated from  the cells  within which they were formed,  possess  a
very active power of movement, in virtue of which they make  their way
to the germ-cells;  and when they have  impinged against these, there is
reason  to believe that they dissolve  away, and that  the product of  their
diffluence is  absorbed into the germ-cells and  mingles with the contents
of the latter, the formation of a germ being the result of this intermix-
ture (Fig. 134, b).   Here, then, we have  the  distinction of sexes  well

                                 Fig.  134.
  Diagram representing the three principal forms of the Generative process in Plants:—A, conjugation of
 inferior Cryptogamia; formation of the sporangium, 6, by admixture of the discharged endochromes of the
 parent-cells, a, a ; 2, production of the sporangium, 6, within a dilatation formed by the union of the two
 parent-cells; 3, production of the sporangium, b, by the passage of the endochrome of cell a, into that of cell
 a*, marking out a sexual difference.—b, fertilization of germ in higher Cryptogamia; a, sperm-cell discharg-
 ing its spiral filament, a*, germ-cell, against which one of these filaments is impinging, b, germ produced
 by their contact.—c, fertilization of germ in Phanerogamia; a, germ-cell, or pollen-grain, sending its  pro-
 longed tube dawn the style, until it reaches a*, the germ-cell, enclosed in the ovule, the section of whose
 coats is shown at c; from the contact of the two, is produced the germ b.


marked ;  but both sperm-cells and germ-cells  are usually developed  in
the same  organism, and are alike the product of a single original germ.
Throughout the Cryptogamic  series,  the fertilized germ  appears to  be 
438
OF GENERATION AND DEVELOPMENT.
thrown at once upon the world, and is dependent for its supply of food
upon its own absorbing and assimilating powers; these enable it to mul-
tiply itself by fission, sometimes to a vast extent; and thus an elaborate
and  complex organism (such as a Tree-fern) may be produced.—In the
third form of the generative process, which is peculiar to Phanerogamia
(or Flowering Plants), there is the  same distinction between  " sperm-
cells" and " germ-cells;" but  the mode in which the action of the former
upon the latter is brought about, is very different.   The  " sperm-cell,"
which is known as the pollen-grain,  and is developed in  the  anthers of
the flower, does not here evolve self-moAring filaments, but, when it falls
upon the apex of the style,  puts forth long tubes, which insinuate  them-
selves down between its  loosely-connected tissue,  until they reach the
ovary at its base.   Here they meet with the ovules, which are in reality
" germ-cells" imbedded in a mass  of nutriment stored up by the parent;
and  the pollen-tube, entering  the  micropyle  or foramen of the  ovule,
penetrates into  such close  approximation to  the  germ-cell  contained
within it, that its contents find a ready passage by  endosmose into the
latter (Fig.  134, c).   Here, again, therefore, we have the same essential
phenomenon,—the intermixture of the contents of the  sperm-cell and
of the germ-cell, as the condition for the development of the  true germ.
But  this germ, still making its first appearance as a single  cell within
the ovule, is supplied with nutriment by its parent; and this not merely
whilst  the  ovule  remains  in  connexion with  the organism  which
evolved it, but for some time subsequently, the store laid up around it
in the seed being the material at the expense  of which  its early  deve-
lopment takes place.   It is  not, in fact, until its true leaves  have  been
evolved and its root-fibres have penetrated the  soil,  which  takes  place
in the  act  of germination,  that it  becomes capable of absorbing and
assimilating  nutriment  for itself.  As soon, however,  as this  takes
place, the young plant becomes independent of further assistance; and
all its  subsequent growth is provided for by its own powers.   In pro-
cess  of time, its generative apparatus is evolved; and here, too, Ave find,
that the two sets of sexual  organs are usually developed in the  same
organism, it being only a small proportion of Phanerogamia  that are
dioecious,  i. e., that have the male  or  staminiferous flowers, and the
female or pistilline, restricted  to different individuals.*
   781.  The history of embryonic Development in Flowering Plants,
presents some interesting points  of correspondence with that of the
higher  Animals.—The germ that  is  developed  within the germ-cell
(here designated the "embryonic vesicle" of the ovule)  as the product
of the admixture of its contents with those of the sperm-cell  (or pollen-
grain), is itself a single cell; and the early history of its development
closely resembles that  which may be observed in all the inferior Plants.
In the first place it subdivides into tAvo, each of these into two others,
and  so on ;  its first nisus or tendency being  to the production, not of
the parts which are to  be evolved into the stem, roots, leaves,"&c,  of the
perfect plant, but of a leaf-like expansion, which may be likened to the

  * For a more particular account of the recent discoveries, on which the above account
of the Generation of Plants is based, see the Author's " Principles of General and Com-
parative Physiology," chap, xviii., sect. 2. 
SIMPLEST  FORMS OF  GENERATIVE PROCESS.          439
 frond of the Cryptogamia, and of which the function is only temporary.
 It is by this organ, the single or double cotyledon, that the nourishment
 provided in the ovule is absorbed and prepared for the development of
 the young Plant;  the  permanent fabric of which,  even at the time of
 the  maturity of the seed, forms but a small  proportion of the entire
 embryonic  structure.   In the act of germination, however, the perma-
 nent portions are developed at the expense of the  temporary, the plu-
 mula and radicle absorbing the nourishment which  has been elaborated
 by the cotyledons ; and having fulfilled  its transient purpose, and com-
 pleted its term of life, the first leaf-like expansion withers and dies.
 The tissues of the young Plant are  at first of the simplest possible
 character;  but as the organs characteristic of its  adult condition  are
 one after another  put forth  (always originating in peculiar groups of
 cells), so do Ave find  that the spiral vessels,  woody fibre,  &c, charac-
 teristic of the higher organisms, gradually make their appearance.—Thus
 we see  that  even the  highest Plants  have to pass through conditions
 closely conformable to  those which are permanently shown in the lower;
 and  that the parts which are first formed are  destined for only a tem-
 porary purpose, that of preparing nourishment for the evolution of more
 permanent structures.   We shall find,  in tracing the history of the de-
 velopment of the higher Animals, that exactly  the same general fact
 may be observed, in even a more striking manner;  the number of diffe-
 rent  stages  being greater, and a yet larger proportion  of the parts first
 formed having a merely temporary purpose, and being destined to an
 early decay, as  soon as  the more permanent  parts of  the fabric shall
 have been evolved.
   782. Among many of the lower Animals, a  multiplication  of indivi-
 duals takes place  by a process that closely resembles the budding of
 Plants;  this also must  be regarded, not as a proper act  of  Generation,
 but as a modification of the ordinary Nutritive  process.  The same may
 be said of the powers of reparation, which every animal body possesses
 in  a greater or less degree, but which are by far the  most remarkable
 among the  lower tribes ; for when an entire member is renewed (as in
 the Star-fish), or even the whole body is regenerated from a small frag-
 ment (which is  the case in many Polypes), it is by a process exactly
 analogous to that Avhich is concerned in the reparation  of the simplest
 wound in our own  bodies, and  which, as already explained (§ 636), is
 but a modification of the process that is constantly renewing, more or
 less rapidly, every  portion of their fabric.  Although the  buds  thus
 produced and separated are usually developed into  the likeness of the
 parent stock, yet this is sometimes not the case, the stock possessing one
 form, and the bud another, which may be quite different; as when certain
 fixed composite  Zoophytes bud  off free-moving solitary  Medusae, these
 last depositing ova, from  which  the  Zoophytic type is regenerated.
 When, however, this  phenomena, to which the name  of " alterations
 of generations" has been given (erroneously, in the Author's opinion),*
is carefully examined, it is found that the  bud thus detached is really

  *For a discussion of  this subject, see the Author's " Principles of General and Com-
parative Physiology," chap, xviii., sect. 1. 
440             OF  GENERATION AND DEVELOPMENT.
the generative apparatus of the parent stock,, furnished (it may be) with
nutrient and locomotive organs of its OAvn;  and that neither can be re-
garded as a complete organism without the  other.  Thus, the Medusa
contains the proper  generative  apparatus of  the Zoophyte,  which
developes no other; and  the " aggregate" Salpae that are  budded-
forth from a kind of stalk in the interior  of the  "solitary" form, must
be regarded as  altogether constituting its  true  generative apparatus,
since it never produces any other.   In all  instances it  will be found,
that whatever may be the variations which  present themselves in  the
entire history of any species, the immediate  product of  the true Gene-
rative act is always the same.
  783. This act,  in Animals as in  Plants,  requires the concurrent
action of two sets of organs, evolving "sperm-cells" and "germ-cells"
respectively;  and  it  is curious that these  should present  the closest
approach to those  of  the higher  Cryptogamia, rather  than to those of
Plants above or  below them in the scale.   The two sets  of organs may
be united in the same individual, as they  are in most Plants ;  and  the
ova  may be fertilized from the seminal cells of the same  being;—as
happens in  many  Zoophytes and in some of the  lowest  tribes of Mol-
lusca.   Or,  the two sets of organs being present  in  each individual, it
may not be capable of self-impregnation; but, in the congress of  two
individuals,  each impregnates, and is  impregnated  by, the other;—as
may be observed in the Snail, and many  of the  higher  Molluscs.  Or
the sexes may be altogether distinct; one individual  possessing only the
male or spermatic organs; and the other the female,  or germ-nourishing
apparatus ;—this is observed in the higher classes of the Radiated, Mollus-
cous, and articulated sub-kingdoms and it  is the case in all Vertebrata.
  784.  The earliest part of the history of Embryonic Development is
nearly the same in all Animals ; for it consists in the  multiplication of
the single cell of Avhich the original germ is composed,  until a cluster is
formed, all the cells of which appear to be in all respects similar to  one
another.  Each  of these cells  either takes into itself,  or draAvs around
it, a portion of  the vitellus or yolk,  which is the  nutrient substance of
the ovum;  and thus  either the whole of this vitellus, or a portion of it,
is subdivided into a number of minute spherules, altogether constituting
what is known as the "mulberry mass" (Fig. 135).  The former seems
to be the case, when the grade of development of  the organism which is
to be formed at  the expense of the yolk is very low; whilst  the  latter
plan is followed, when the  yolk is destined to afford a  prolonged suste-
nance to the embryo, which attains a high degree  of development Avhilst
supported upon it alone.   Thus among the  Invertebrata generally, we
find that the embryo  comes forth from the egg  in a  very simple  condi-
tion, a large part  of its structure having undergone but little change
from the state of the "mulberry mass;" and in  these, the Avhole yolk
undergoes subdivision.  The same is the  case, too,  in the Batrachian
Reptiles, which  issue from the egg in a form  very  different from that
into which they are  to be subsequently  developed; and it is the case
even with Mammalia, but for a very different  reason,  their embryonic
structure, formed at  the expense of the yolk, being  destined to acquire
additional material for its full development from a source altogether 
SIMPLEST FORMS OF  GENERATIVE  PROCESS.          441
different.   In the highest Mollusca, however, as also in Fishes, Reptiles,
and  Birds, the portion of  the yolk which undergoes subdivision is com-
paratively small; and  the great mass of the vitellus is  destined  to be
                               Fig. 135.
                   ABC          D


               09
                  E          F         G           H
  Successive stages of segmentation in the vitellus of the Ovum of Ascaris acuminata :—a, ovum recently
 impregnated, the yolk-bag slightly separated from the enveloping membrane ;  b, first fission into two
 halves; c, second fission, forming four segments; d, yolk, now divided into numerous segments; e, formation
 of "mulberry mass" by further segmentation; r, the mass of cells now beginning  to show the form of the
 future worm; o, further progress of its evolution: h, the worm, formed by the conversion of the yolk-cells
 now nearly mature.

 subsequently absorbed into the  substance of  the germ, by a  process
 analogous  to that by Avhich the  food of the adult is  imbibed.   Hence
 the portion of their yolk which undergoes subdivision, and helps to con-
 stitute  the "mulberry mass," may be termed  the "germ-yolk," whilst
 the remainder may be designated as the "food-yolk."
   785. When the whole of the yolk is taken  into the  mulberry mass,
 the  formation of the embryo is usually  the result of  the  progressive
 metamorphosis of its parts; the cells of the surface being converted into
 the  integument, and  those  of  the inner part into the internal  organs.
 This is the case, for example, in the Intestinal Worm, some of the stages
 in whose development are  shown in  Fig.  135.  The  embryonic  con-
 dition  of many of the organs  is frequently  retained, at the time when
 the young animal comes forth  from  the egg ;  those parts  only being
 completed  which are necessary to enable it  to  obtain its nutriment.—
 Other  organs are  subsequently  evolved, at the  expense of the food thus
 introduced;  and thus a  complete change or metamorphosis may  take
 place, in regard alike to external form and to internal structure, betAveen
 the  larval   and adult  states.   Of this  phenomenon  Ave  have  charac-
 teristic examples in the groups  of Insects and Batrachia ; and although
 it was  formerly considered  exceptional, it is now knoAvn to be  the or-
 dinary  occurrence among  the  lower  tribes of animals  ; it  being com-
 paratively  rare for any of them to come forth from the  egg  under their
 adult forms.  This  change  is sometimes obviously gradual, as in  the
 progressive advance of the  Tadpole into the condition of the Frog ;  but
it is  sometimes apparently sudden, as when the Chrysalis skin is thrown
off, and  the perfect  Insect comes forth.  In the latter case, however,
the change is really just as gradual as  in the former;  since the develop-
ment of the organs characteristic of the perfect Insect is taking place
during  the Avhole  of the Chrysalis period, to be displayed and brought
 
442
OF GENERATION  AND DEVELOPMENT.
into use at its termination.  Thus the whole life of the Insect, up to its
last change, may be regarded as one of prolonged  embryonic develop-
ment ;  and the same  may be said of that of the Frog, up  to the time
when its permanent organs are fully evolved.—No  such ostensible me-
tamorphosis takes place, however, in any of the animals which are pro-
vided with a "food-yolk;" for this  supplies that material for the con-
tinued  development of the embryo  within  the egg, which is elsewhere
to be obtained out of  it; and thus the embryo is supported, until it has
nearly attained its adult  condition, although far from having acquired
its adult size.   Noav in all these  cases, it is very interesting  to remark
that the first nisus is  towards an  extension of  the embryonic mass as a
membranous  expansion (evidently  analogous  to the cotyledon of the
Flowering Plants, § 781) over the  "food-yolk;" in this "germinal mem-
brane," which forms a sort of temporary stomach, blood-vessels are de-
veloped, which absorb the prepared nutriment  and convey it to the per-
manent portion of the embryonic  structure;  and when its function is
completed, the store of aliment being exhausted, and the proper nutrient
apparatus of the embryo  being ready for  action, we lose sight of it al-
together. We shall find that a similar germinal membrane is formed in
the Human ovum, although there is no " food-yolk ;" its formation being
apparently requisite for ulterior  purposes,  and the portion of the mul-
berry mass which gives origin to  the  permanent part of the  embryonic
structure being comparatively small.

                        2. Action of the Male.

  786.  The share in  the  Reproductive Function, which belongs to the
Male  Sex, essentially consists in the formation and liberation of the
fertilizing bodies  termed Spermatozoa.   These are  prepared within
peculiar cells, as already described  (§ 241)  ; and the " sperm cells" are
either scattered  through the soft parenchyma of the body, as happens
among some of the lowest animals ;  or they are confined to certain parts
of it, as in those a little more elevated in the scale;  or they are formed
within  follicles or tubes, clustered together into ian organ of a glandular
character, known as the Testis.   Such an organ is  found in all Insects
and Mollusca ;  as well as in Vertebrated Animals.   In the first of these
classes, it is formed on the general plan of their proper glands  (§ 720);
being usually composed of tubes, more  or less elongated,  and  some-
times  terminating in  enlarged follicles.  In the Molluscs,  on the other
hand, it is almost invariably composed of clusters of follicles.   In either
case, the  seminal  cells are  developed within  the tubes or follicles, a9
are the  ordinary  secreting cells of the Liver or  Kidney within the
tubes or follicles of those glands; and their contents are discharged by
an excretory duct, which terminates in an organ that conveys them  out
of the  body, either emitting them into the  surrounding Avater (as  hap-
pens with many Mollusca), or depositing them within the body of the
female.  It  is curious that, in some  of  the loAvest Fishes,  we should
return  to one  of the simplest conditions   of  this  organ,—a  mass of
vesicles,  without  any excretory  duct.   In these  cases the secretion
formed within the vesicles escapes, by their rupture, into the abdominal 
ACTION  OF THE MALE.
443
Fig. 136.
Anatomy of the Testis :—
cavity;  whence it passes out  by openings that lead directly to the ex-
terior.—The Testis in Man is formed, in every essential particular, upon
the plan of the ordinary Glands.  It consists of several distinct lobules,
separated by processes of the  fibrous  envelope, or
tunica albuginea, which pass down between them;
and each lobule consists of a mass of convoluted
tubuli seminiferi, through which blood-vessels are
minutely distributed.   The diameter of these tubuli
is  tolerably  uniform;  being,  when they are not
over-distended, from l-195th to l-170th of an inch.
They form frequent anastomoses with  each other;
and on  this account it  is difficult to trace out their
free or  caecal extremities.  The tubuli of each testis
discharge  their contents into  an efferent  duct, the
Vas deferens;  and  by  this the product is conveyed
into the Vesiculi seminalis on  each  side, which, like
the gall-bladder and urinary bladder, serves to store
up the  secretion until  the proper  time arrives for
discharging  it.  The product of the action of the
Testis consists  of a fluid, through  which the Sper-
matozoa are  diffused ; these last bodies being usually
set free by the rupture of the  seminal  cells, before
they leave the  tubuli of the testis.   It  is difficult to
determine the precise characters of the fluid portion \ \ ^l-^^t\^iZt\i
of the secretion; as this is  mingled with other se- s! 3. The lobuii testis. 4, 4.
         ,    ,   ',     r  ,1   t\  .   .   i   i    i/> Tne vasa recta. 5. The rete
cretions (such as that or the Prostate gland, and ot testis. 6.The vasaefferentia,
the  mucous  lining  of  the Vesiculae seminales and gInTehdlc^nS1uiLDldiaa»7amepr7'
spermatic  ducts), before it is  emitted.   And an ^rh^Tobtmfcftt
exact analysis  is  not of much consequence;  since epididymus. s. The body of
,,         r      i   i ,  .i  ,  ,i       ■!•            „ the epididymus. 9. The globus
there can be no doubt  that  the peculiar powers of minor of the epididymus. 10.
the  fluid depend  upon the  Spermatozoa.   It  may TasecuTSmdaberrrnans. n" The
be stated, however, that the  Spermatic fluid has
an  alkaline  reaction,  and  that it  contains albumen, together  with  a
peculiar animal principle termed Spermatine; and that it also includes
saline matter,  consisting chiefly of  the  muriates and phosphates, espe-
cially the latter, which form crystals when the fluid has stood for some
little time.
   787.  The  Spermatozoa, or minute filamentous bodies set free  by the
rupture of  the spermatic cells, are distinguished  by their power of
spontaneous movement, which  occasioned  them to be long regarded as
proper  Animalcules. It is now clear, hoAvever, from the history of their
development, as Avell as from other considerations, that they cannot be
justly regarded in this  light;  and that they are analogous to the repro-
ductive particles of Plants, which, in many  cases,  exhibit  a spontaneous
motion  of extraordinary activity, after they have been set free from the
parent  structure.   The Human Spermatozoa  consists  of a little  oval
flattened "body," from the l-600th to the l-800th of a line in length;
from Avhich proceeds a  filiform  "tail,"  gradually tapering to a very fine
point, of l-50th  or at most l-40th of a line in  length.   The whole is
perfectly transparent;  and nothing that can be called structure  can be 
444
OF GENERATION AND  DEVELOPMENT.
satisfactorily distinguished within it.  The movements  are  principally
excited  by the  undulations of the tail, which give a propulsive action
to the body.  They may continue for many hours after the emission of
the fluid;  and they are not checked  by its admixture with other  secre-
tions, such  as  the urine, and the prostatic fluid.  When the seminal
fluid remains in contact with a living surface (as when deposited in the
generative organs of  the female),  the Spermatozoa may retain  their
vitality  for some days; and an  instance has  already been referred to
(§ 240), in Avhich the later stages of the development  of the Sperma-
tozoa actually take  place in  this situation,—the seminal fluid emitted
by the male (among many Crustacea) not containing any Spermatozoa
completely formed, but  numerous spermatic cells, which undergo the
remainder of their development, and  then  rupture and set  free  their
contents,  within the oviducts of  the female.
   788.  The power of  procreation  does not exist in  the Human  Male
(except in rare cases)  until  the  age of from 14 to  16 years;  at  which
epoch, the sexual  organs undergo a much-increased development; and
the instinctive desire, which leads to the use of them, is awakened in
the mind.   From that time to an advanced age, the procreative power
remains, in the healthy state of the system; unless it be exhausted by
excessive  use of it, or by too energetic a direction  of the  mental or
corporeal  powers to some other object.   The formation  of Seminal fluid
being, like the  proper  acts of  Secretion, very much influenced by con-
ditions of the Nervous System,  is increased  by the continual direction
of the mind toAvards objects which  arouse the sexual propensity; and
thus, if sexual  intercourse be very frequent, a much  larger quantity of
the  fluid  will be produced than  if it is more rarely emitted, although
the  amount  discharged  on each occasion will  be less.   The formation
of this product is evidently a great tax upon the corporeal powers; and
it is a well-knoAvn fact, that the highest  degree of bodily and  mental
vigor is inconsistent with more than a very moderate indulgence in
sexual intercourse; whilst nothing is more certain to reduce the powers,
both of body and mind, than excess  in this respect.
   789.  It may be  stated as  a  general  laAv,  prevailing equally in the
Vegetable and Animal kingdoms,—that the development of the indi-
vidual,  and the reproduction  of the  species, stand  in an inverse  ratio
to each other.   We have seen  that,  in many organized beings, the
death of the parent is  necessary to the production of a new generation;
and even  in numerous  species of Insects it folloAvs very speedily upon
the sexual intercourse.  It is a curious  fact, that Insects which usually
die, the male almost immediately after the act  of  copulation,  and the
female very soon after the  deposition of the  eggs, may be  kept alive
for many  Aveeks or even  months, by simply preventing the copulation.
And there can be no doubt that, in the Human race, early death  is by
no means an unfrequent  result of  the  excessive or premature employ-
ment of  the genital  organs; and  Avhere  this  does not produce  an
immediately-fatal result, it lays the  foundation  of  future debility, that
contributes to  produce any forms of disease to which there may be a
constitutional predisposition,  especially those of a Scrofulous nature.
   790.  The emission of the Spermatic fluid is an, act of a purely  reflex 
ACTION  OF THE  FEMALE.
445
nature; the Will having  no poAver either to effect or to restrain  it.
The stimulus is given by the friction of the surface of  the Glans  Penis
against  the rugous walls  of the Vagina;  the sensibility of the  organ
being at the same  time much  increased, by the determination of blood
to it.   The impression is  at last sufficiently strong to produce, through
the medium of the lower part  of the Spinal cord  (which is  the gangli-
onic centre of the circle of afferent and efferent nerves connected with
this organ), a reflex contraction of the muscles surrounding the Vesi-
culae  seminales.   These  receptacles  discharge their  contents  (which
consist partly of the Spermatic fluid, and partly of a secretion  of their
OAvn)  into  the Urethra;  and from this they are expelled,  Avith  some
degree of force, and with a kind of spasmodic action,  by its own Com-
pressor muscles.  Although the sensations  concerned in  this act are
ordinarily  most  acutely pleasurable, yet there appears to  be sufficient
evidence that  they are by no means essential to its performance;  and
that the impression  conveyed to  the  Spinal cord may excite the con-
traction of the Ejaculator muscles, like other reflex operations, without
producing  sensation (§ 394).

                       3. Action of the Female.

   791.  The share of the  Female in the Generative act is greater than
that of the Male;  for she not only furnishes,  in the "germ-cell," a
product which is as essential  as that supplied by  the "sperm-cell" for
the first formation of the  germ; but she also supplies it with the mate-
rials Avhich it requires for its development, up to the condition in which
it can support its own life.   The  mode  in which  this  is accomplished,
is essentially  the same Avith  that  in which the process is  effected in
Plants.  In certain  parts of  the female structure are  developed pecu-
liar bodies termed ova ; Avhich contain, not merely the germ-cells, but in
addition a store of nutriment adapted to supply the wants of the germ.
The fertilizing influence finds  its  way into these; and the germs thus
produced begin to grow at the expense of  the material with which they
are surrounded.  This, as already pointed out, may enable the embryo
to develope itself,  without any further assistance  (save a warm tempe-
rature) into the form it is permanently to assume ; as in the case of
Birds and  Reptiles, Avhich do not  come forth from the investments of
the egg, until they have attained the  form characteristic of the  group
to Avhich they belong.  Or it  may only serve for  the  early part of the
process ; and one  of two  methods may then  be employed to complete
it;—either a  new  connexion is formed between the  parent  and the
embryo, by which the former continues to supply the  latter with nutri-
ment, more directly from  its blood,—as is the case with Mammalia,-^or
the embryo issues from the egg, in a  condition very unlike  that which
it is permanently to attain, but in a form Avhich enables it to acquire its
own nourishment,  and to  pass through the latter stages of its evolution
quite independently of any assistance from its parent: this is the case
with a large proportion of the Invertebrata.
   792.  The Ova,  like the seminal cells, are  scattered through the soft
parenchyma of the body, in animals of the  lowest class;  but they are 
446              OF GENERATION  AND  DEVELOPMENT.
more commonly developed  in  certain distinct  portions of  the fabric ;
being sometimes formed in the  midst of solid masses of areolar or cel-
lular texture;  whilst in other instances they are  developed,  like the
spermatic cells, in the interior of tubes  and vesicles  resembling  those
of glands, and furnished with  an excretory duct.   The latter condition
obtains in the greater proportion of  the higher Invertebrated animals,
and in some Fishes; but in the Vertebrated classes we  return to the
type which characterizes  the egg-producing organs in many Zoophytes,
—namely, the  development of the egg in  the midst of a mass of solid
parenchyma, from which it gradually makes it way,  to escape into the
visceral cavity.  The Ovarium of the Mammal, Bird, or Reptile, as well
as that of most Fishes, differs entirely, therefore, from that of the In-
vertebrata ;  for  the latter have all the  essential characters of true
glands;  whilst the former are nothing  else than masses of parenchyma,
copiously supplied with blood-vessels,  and  having  dispersed through
their substance certain peculiar cells, termed ovisacs, within which the
ova are developed.  In order  that  the latter may be  set  free, not only
must the ovisac itself burst (like parent-cells in general), but the pecu-
liar tissue and  the envelopes  of the ovarium must likewise give  way.
When the ova thus escape into the  abdominal cavity, they may lie there
for some time, at last  to be discharged through  simple openings in its
walls, as happens in those  Fishes which  have this form of ovarium : or
they may be at once received  into  the trumpet-shaped  expansion of
tubes, that shall convey them  to these orifices.   These tubes are termed
oviducts, in  common with  the excretory ducts of the glandular ovaria
of Invertebrated animals; for their function is the same,—that of con-
veying the  ova to the outlet by which they are extruded from the body.

                                Fig. 137.
 The Uterus with its appendages viewed on their anterior aspect. 1. The body of the uterus. 2. Its fundus.
3. Its cervix. 4. The os uteri. 5. The vagina; the number is placed on the posterior raphe or columna,
from which the transverse rugae are seen passing off at each side.  6, 6. The broad ligament of the uterus.
7. A convexity of the broad ligament formed by the ovary.  8, 8. The round ligaments of the uterus. 9, 9.
The Fallopian tubes.  10,10. The fimbriated extremities of the Fallopian tubes ; on the left side the mouth
of the tube is turned forwards in order to show its ostium abdominale. 11. The ovary. 12. The utero-ova-
rian ligament.  13. The Fallopio-ovarian ligament, upon which some small fimbriae are continued for a short
distance.  14. The peritoneum of the anterior surface of the uterus.  This membrane is removed on the left
Bide, but on the right is continuous with the anterior layer of the broad ligament.


They are represented in Mammalia by the Fallopian tubes, which  are
true oviducts, although they terminate  in  the uterus instead of proceed-
ing directly to  the  outlet.  And  it is  by the  fimbriated extremities of
the Fallopian tubes  (Fig. 137,  10, 10), which  apply themselves closely 
            STRUCTURE AND  DEVELOPMENT OF THE  OVUM.         447

to the surface  of the  ovaries at the time  of  the discharge of  the ova,
that these are received  and conveyed to  the uterus, instead of being
allowed (as  in  some of the lower animals) to fall into the  abdominal
cavity.
   793. There  are  many cases among the lower  classes, in which the
ovum is retained within the oviducts, so that the young comes into the
world alive; and there are few in Avhich, during this delay, it receives
a direct supply of additional nourishment from  the fluids  of  its parent.
It is in the  Mammalia, however, that we find  the  most remarkable and
complete provision for this purpose.   Still, the lowest division of  this
group approximates closely, in the type of its generative apparatus to
the Oviparous Vertebrata; for the oviducts of the Monotremata remain
distinct from each other, and terminate separately, in the uro-genital
canal, each of them  having first undergone  dilatation into a uterine
cavity, so that these animals have  two completely distinct uteri.  In
the Marsupialia, there is a closer approximation of the two lateral sets.
of  organs  on the median  line; for the oviducts converge toAvards one
another, and meet on the median  line, but without coalescing; so that
these animals have a true "double uterus," opening by two orifices into
the vaginal  canal,—a condition which  is sometimes met with as a mal-
formation in the Human female.  The vaginal canal, however, is also
double; which  is less frequently observed in the Human species.   The
two preceding orders constitute the sub-class of Lmplacental Mammals;
the development of their ova within the uteri being cut short at a period
anterior to  the formation  of the  placenta  (§ 818).—As  we ascend
through the series of Placental Mammals, we, find the lateral coales-
cence of the uterine dilatations of the Fallopian tubes becoming more
and more  complete.  It first  shows itself in the vagina, which is every-
where single, although a trace of separation into  two  lateral halves is
seen in the Mare, Ass, Cow, Pig, and Sloth, in which animals it is tra-
versed, in the virgin state, by a narrow, vertical  partition.  In  many
of the Rodentia, the  uterus  still  remains  completely divided into two
lateral hahres,  opening into the vagina by separate  orifices; whilst in
others, these coalesce at  their lower  portion, forming a rudiment of the
true "body" of the uterus  of the Human female.  This part increases
in the more  elevated Herbivora and Carnivora, at the expense of  the
lateral ununited portions, Avhich are now termed the "cornua;" but even
in the lower Quadrumana,  the uterus is somewhat cleft at its summit,
and the "angles," into  which the oviducts enter, form a considerable
part of the  whole  organ.  As we ascend  through the Quadrumanous
series towards Man, we find the "body" of the uterus increasing, and
the "angles"  diminishing in proportion,  until the  original division is
completely lost sight of,  except in the slight dilatation of  the cavity at
the points at which the Fallopian tubes enter it.
  794. Having thus briefly  noticed the most important  characters of
the organs provided for the original production  and for the subsequent
reception of the ova, we have now to inquire  into the history of their
development.—The essential  structure of the ovule, or unfertilized egg,
appears to  be  the same in  all animals.  It  consists externally of a
membranous sac, so termed, from the nature of its contents, the vitelline 
448
OF GENERATION  AND DEVELOPMENT.
 membrane, or yolk-bag.  The vitellus, or yolk, consists chiefly of albu-
 men and oil-globules; and floating in this fluid is seen a cell of peculiar
 aspect, termed  the  germinal vesicle, upon the walls of which is a very
 distinct nucleus, termed the germinal spot.—The layer of albumen sur-
 rounding the yolk, and termed the white of the Bird's egg, together with
 the membrane Avhich envelopes this and  forms the basis of the shell, are
 not added until after  the ovum has  left the ovarium.   They are  not
 present in the eggs of many of the loAver Invertebrata;  these  con-
 sisting merely of the parts which are formed within the ovarium.
   795.  The structure  of the  ovule in  Mammals differs in no essential
 particular from that just  described; but the yolk is   much less  in
 amount than in the ovules  of  Invertebrated  animals;  since only the
 very earliest stages of the development of the embryo are to  take place
 at its  expense. The  vitelline  membrane is of peculiar thickness and
 transparency;  and  as, when  the ovum is compressed under  the micro-
• scope, it is seen as a broad transparent  belt, it is  commonly known as
 the zona pellucida.  We shall  find that the  ovule, after leaving  the
 ovarium and receiving the fertilizing influence, becomes enclosed, whilst
 passing through the Fallopian tube, with a layer of albuminous matter,
 which represents the white of the Bird's egg; and with an  additional
 fibrous envelope, which corresponds with the membrane enveloping  the
 latter.   This fibrous membrane, termed the  Chorion, afterwards  be-
 comes subservient,  however, to various important changes ;  by means of
 which the ovum is  again brought into  connexion with the  parent, to
 derive from the blood of the  latter  the  materials requisite for the con-
 tinued  development of the embryo.   These  changes will be described
 hereafter (§§ 811, 818).
   796.  The Ovisac of Mammalia forms the inner layer of what is
 termed the Graafian follicle, after the name of its  discoverer;  and in-
 stead of closely enveloping the ovulum,  as it does in oviparous animals,
 it contains, in  addition to it, a quantity of granular matter,  consisting
 of cells arranged in membranous layers, together  with  fluid.  In  the
 immature ovisac, these cells occupy nearly the whole space betAveen  the
 ovisac  and the ovum, but they  gradually dissolve  aAvay, especially on
 the side nearest the surface of the ovary; and  at the same time an al-
 buminous  fluid is  effused from  the deeper part  of the  ovisac, which
 pushes before   it  the residual layer of cells that immediately sur-
 rounds the ovum (forming the discus proligerus),  and thus  carries it
 against  the opposite wall.  The outer layer of the Graffian follicle is
 formed by a thickening and condensation  of the  surrounding paren-
 chyma of the ovarium ; and it is quite distinct from the ovisac Avhich it
 envelopes.   It is   extremely  vascular,  and  is evidently destined  to
 afford to the contained structures the materials for their development,
 Avhich they receive and appropriate by their own  powers of absorption
 and assimilation.
   797. The Mammalian Ovarium may be seen, even in  the  foetal ani-
 mal, to contain immature  ova, enclosed  within their ovisacs  ; and  the
 several parts of the former may  be clearly distinguished in those which
 are in  the  more advanced stages  of development.   It  appears that,
 during the period of childhood, there is  a continual rupture of the ovi- 
MENSTRUAL DISCHARGE.                   449
sacs (or parent-cells), and discharge  of ova, at the surface of  the ova-
rium ; but these ova never attain so high a degree of development, as to
render them fit for impregnation.   Their evolution  takes place more
completely, as well as  more rapidly, at the period of puberty, when
there is a greatly-increased  determination  of blood  to the  genital
organs, and a correspondingly-augmented energy in their nutritive opera-
tions.  . At this epoch, th_ parenchyma  of the ovarium is crowded with
ovisacs ; which are still so minute,  that in the Ox, according  to  Dr.
Barry's computation, a  cubic inch would contain 200 millions of them.
Some of those nearest  the surface, however, are  continually  attaining
increased development;  and a rupture of some of the Graafian follicles,
and a discharge of ova prepared for impregnation, from the exterior of
the ovarium,  thenceforth take place,  with more or less tendency to
periodicity, during the whole time that the female is in  a state of apti-
tude for procreation.
   798. In the Human female,  the period of Puberty usually occurs be-
tween  the 13th and 16th years.  The differences in the time of its advent
partly depend upon individual constitution, and partly upon various ex-
ternal  circumstances, such as  temperature, habits of life,  &c.  As  a
general  rule, habitual  exposure to a warm atmosphere,  an inert life,
sensual indulgence, and circumstances that  excite the sexual feelings,
favor the approach of Puberty;  whilst  a cold  climate and hardy life
retard  it.  The appearance of the Catamenial  discharge  usually takes
place Avhilst the evolution of the genital organs is in progress ; and it is a
decided indication, when it occurs, that the aptitude for procreation has
been attained. It is not unfrequently delayed much  longer, however; and
its absence is by no means to be regarded as a proof of inability to con-
ceive.   The Catamenial fluid, as it proceeds from  the lining membrane
of the Uterus, seems to be nothing else than Blood; but  in its passage
through the vagina, this is deprived of its coagulating power by admix-
ture Avith the vaginal mucus.  The appearance  of clots in the discharge
may usually be regarded as an indication than an excess of blood is
escaping from the uterine surface.   In some cases  of difficult Menstrua-
tion, Avhich  seem  to depend upon a state of low  inflammation in the
Uterus, the fibrine has such a tendency to become organized, as to form
shreds, or layers of false membrane, which  sometimes plug  up the os
uteri.   The healthy Menstrual secretion is remarkable for its  very acid
character.—It has been recently maintained  that this periodical dis-
charge  of blood  from  the lining membrane of  the uterus is dependent
upon the ovarian  oestrum ; but there seems adequate  reason for the
belief,  that, although the two phenomena are  usually consentaneous,
yet that they are essentially independent;  since,  each  occasionally
recurs  without being accompanied by the other.  The  catamenial  dis-
charge usually makes its appearance pretty regularly (save during preg-
nancy and lactation) at intervals of 28 days ; but there are many females
in Avhom its recurrence takes place with no less regularity at shorter or
at longer intervals.  The duration of the flow, too, is subject to great
variations; for in some  individuals it does not last above a  day or two,
whilst in others it continues a week or more.
   799.  This flux  of blood from the  lining  membrane of  the  Uterus is
                                 29 
450             OF GENERATION  AND DEVELOPMENT.
not  confined to the Human female, as was  formerly supposed; but
occurs in some of the lower Mammalia in  the state of heat, or periodi-
cal aptitude for procreation, at Avhich time  the  ovarium contains ova
ready for impregnation.  The chief peculiarity attending its appearance
in the Human female, is its regular  monthly return.  In  the natural
condition  of many of the lower Mammalia, as in Oviparous  animals,
the period of heat recurs at some one time of the  year,—usually in
the spring; or,  in  the  smaller and more prolific species, from two to
six times.   And in those which have undergone a change by domesti-
cation,  the recurrence  is  usually  irregular,  depending upon various
circumstances of regimen,  temperature, &c.   The  general  analogy
between the Menstruation of the Human  female and the Heat of the
lower Mammalia,—consisting in the peculiar aptitude for impregnation
which then exists in consequence of the maturation of ova in the ovarium,
—cannot now be questioned; but it appears that in the Human female
ova may be matured and impregnated at  any part of the period which
elapses between  the occurrences of the Catamenial discharge;  though
it is certain that the aptitude for  conception is  much greater, during
the few days which  precede and follow the menstrual period, than at
any intervening time.  The  duration of the  period of aptitude for pro-
creation, which is marked by the  continued appearance of the  Cata-
menia, is more limited in Women than in Men;  usually terminating at
about the  45th year.  It  is  sometimes prolonged, however, for ten or
even fifteen years longer;  but cases are rare, in which women above 50
years of age haAre borne children.   There  is usually no menstrual flow
during  pregnancy and  lactation;  in  fact the cessation  of the  Cata-
menia is usually  one of  the  first signs indicating that conception has
taken place.   It is by no  means  uncommon, however, for them to
appear once or twice subsequently to Conception; and their appearance
during Lactation, especially if it be much prolonged, is still more fre-
quent ;  hence it  might be inferred, that the continuance of Lactation
may not prevent a  fresh  conception,—which is  found to be true in
practice.
  800.  We shall now take  a brief survey of the changes which  occur
in the Ovulum, when it is being prepared for  fecundation ;  and of the
principal features  of its subsequent  development.—Up  to  the period
when the Ovule  is  nearly brought  to  maturity, it remains  suspended
in the centre of the  cavity of the Ovisac; but it then  begins to  move
towards that side of  the Graafian  follicle,  which  is nearest  the surface
of the ovarium.  An important change is at the  same time occurring
in the Graafian follicle itself; for whilst the part with which the  ovule
comes in contact gradually  thins  away, the  outer or  vascular layer of
the remainder, especially on that side most  deeply  imbedded in the
ovary,  becomes  much increased  in thickness;  and a great  increase
takes place at  that  part, in the cellular layer that lines  the ovisac,
which presents a reddish glutinous  aspect.  This subsequently under-
goes a still greater augmentation, and becomes more fleshy; projecting
like a  mass of  granulations from  the  interior  of the ovisac, and
receiving blood-vessels  which pass into it from the vascular  membrane
that surrounds it.   At the same  time, the wall of the Graafian follicle 
FERTILIZATION  OF THE  OVUM.                451
 is thrown into wrinkles, which are directed towards the interior, so as
 to occasion the  contraction of the cavity; and  thus  it comes to  be
 entirely filled with  the  new growth, the centre  of which is marked  by
 a sort  of  stelliform  cicatrix.  This substance speedily becomes of a
 paler hue  than at first,  and is  known from its  color as  the  corpus
 luteum.—The  escape of the  ovule from the  ovarium involves  pro-
 cesses which are essentially the same, whether it be  impregnated or
 not;  but  the subsequent changes differ in the two cases, so that the
 corpus  luteum which accompanies  the pregnant state  is  usually a
 much  larger  and  more  highly-organized body,  than  that  which  is
 found in  the  ovary of  the  unimpregnated  female.   This difference
 may be due in part  to the  absence, in the latter case, of that special
 determination of blood to the genital organs, which  takes  place in the
 former.—It is obvious, then, that the presence of a small and imper-
 fect corpus luteum in  the  ovary, merely indicates  that an ovum  has
 been matured  and discharged, and affords  no  evidence  of impreg-
 nation  or sexual  intercourse.  The  presence of a large and character-
 istic  corpus luteum,  on the  other hand, may be regarded as affording
 undoubted  evidence  that impregnation has taken place.  When fully
 formed, the corpus luteum  may be rather more than  half an inch in
 one of its  diameters,  and rather  less in the other; it  usually forms a
 projection on the surface  of the ovary, and occupies from one-fourth to
 one-half of  the  whole area of  its  section.   It is frequently, how-
 ever, much smaller  than this; and on the whole it  may be said that the
 presence  of a corpus luteum  as  large as a full-sized pea, is toler-
 ably certain evidence of impregnation.  After delivery, the size of the
 corpus luteum rapidly diminishes,  and  in a few months  it ceases to be
 recognisable as such;  the  cicatrix  by which the ovum has escaped,
 however, remains visible for some  time longer.
   801. The increase of size which is  observable  in the ovule  that  is
 being prepared for  fecundation, is  chiefly due to an augmentation in the
 substance  of the Yolk ; and this  also  becomes more firm and granular
 than before.  But  the most curious change is that which takes place in
 the Germinal Vesicle; for this, although previously in the centre of the
 yolk, now moves  up towards the side of it which is nearest the surface
 of the ovary, and  becomes flattened against the yolk-bag.  At the same
 time, it ceases to  present its ordinary pellucidity and becomes obscure ;
 and this alteration  appears to be due to the development of a brood of
 young cells in its interior.  From the recent observations of Mr. New-
 port and others, it would  seem that it then bursts and  sets these free,
 so that  they become diffused through the yolk ;  and as this change may
 happen before fecundation, it must be regarded as  being preparatory to
 it, or at any rate  as being independent of it.
  802.  The ova  thus matured and prepared  for  fecundation, are dis-
 charged from the  surface of the ovary by the process already described,
 Avhether sexual intercourse take place or not; and being received into
 the Fallopian tubes, they are by them  conducted  towards the uterus.
 Their transmission  may be effected by a kind of peristaltic movement
Avhich is observed to  take place  in these tubes during the periods of
heat; or, it may be, by the action of the cilia which line them; the di- 
452
OF GENERATION AND  DEVELOPMENT.
rection of both movements being  the  same,—namely, from the ovaries
towards the uterus.  If in their course they should not receive the fer-
tilizing influence, they appear soon to die and to disintegrate ; but if they
should be impregnated by contact with the spermatic fluid, they almost
immediately begin to undergo the first of those changes, which tend to
the production of a new organism.
  803. Much discussion has taken place, with regard to the exact point
at which the fertilization of the  ovulum takes place ; but this does not
seem to be a matter of much consequence, as we find the order of the diffe-
rent steps to vary  considerably in  different classes of animals.  Thus in
many aquatic Mollusca, and even in a large  proportion of the class of
Fishes, there  is no act of copulation whatever ; but the spermatic fluid
when emitted by the male, is diffused through the water, and fertilizes the
ova which have been deposited by the female in his neighborhood.  In the
Frog, again, and in  other Reptiles, the spermatic fluid is emitted upon
the ova, at  the  time that they  are  being extruded  by the female.   In
many Insects and  Crustacea, in which a single congress often serves to
fertilize many thousand eggs, the deposition of which occupies  a period
of several weeks or months, the  spermatic fluid is received and  stored
up in a saccular dilatation of the oviduct of the female, which is termed
the spermo-theca ;  and in this manner it serves to impregnate the ova, as
they are  successively developed, and are conveyed to  the outlet of the
oviduct.   In Birds, we find that  ova are often set free from the ovarium
in a state of full maturity, but without fertilization ; and that  they re-
ceive their  additional layer of  albumen and their shelly envelope, in
passing down the oviduct, so as,  at the time of their  deposition, to differ
in no obvious  particular from fertile eggs.  It is doubtful, in regard to
Mammalia,  whether the act of fertilization takes place before the ovum
has been completely extricated  from the ovisac, or  subsequently to its
finally  quitting the ovarium and being received into  the Fallopian tube.
It is quite certain  that the spermatozoa frequently, if not invariably,
find their way to the surface of the ovary,  being  carried thither by
their own spontaneous movements ; and it seems on the whole most  pro-
bable, that  the fertilization of the ova usually takes place before they
have entirely escaped from the  ovisac, or whilst they are still in the
commencement of the Fallopian tube.   It is not unlikely that the place of
the act of fecundation varies, according to the point at which the ovule
and the seminal fluid first come into  contact,—which may depend upon
the degree of maturity of the ova at  the period of copulation.
  804. Everything indicates that the contact of the  Spermatozoon with
the Ovulum is the one thing needful in the act of fecundation; and there
is strong reason to believe, from  Mr. Newport's recent observations,  that
when this contact occurs, the spermatozoa undergo solution ; and that it
is in the absorption  of the product of  that solution  into the interior of
the ovum (thus blending, as in Plants, the contents of the " sperm-cell"
with those of the  " germ-cell"), that the act of fecundation  essentially
consists.   Availing himself of the agency of caustic potass, which has
been found to be a powerful solvent of the spermatozoa, Mr.  N. applied
this  agent to the  ova at  determinate  periods  after the  application of
these bodies, which he had separated from the liquor seminis by filtra- 
FERTILIZATION  OF THE  OVUM.
453
Fig. 138.
tion ; and  he found that when the interval of time between  the one
application and  the other was  only one  or  two seconds, only the early
stages of the  process took place, and  no embryo was  produced; when
an interval of five seconds was allowed, very few embryos were produced
from  a large number of ova ;  but when the  interval was fifteen seconds
or more, the proportion of embryos produced was much greater.  Thus
it seems obvious that time  is  an important element in the fertilizing
process ; and that  fertilization may be incompletely effected, for want
of a sufficient penetration of the product of the diffluence of the Sperma-
tozoa.  How this product acts upon the contents of the ovum, however,
and whether one or many of the cells set free by the rupture or  solution
of the germinal vesicle are fertilized by it, have not yet been ascertained.
   805. The first change which can be observed to be consequent upon
fecundation in the Mammalian ovum is the " segmentation" of the yolk;
the entire  mass of which, though  previously
compact and uniform,  resolves itself,  first
into two, then into four, then into  eight seg-
ments (Fig. 138);  each segment containing  a
transparent vesicle, which may  be surmised
to be a descendant of the original  germ-cell.
By a continuance  of the same  process,  the
whole cavity of the vitelline sac,  or zona  pel-
lucida, becomes  occupied by spherical parti-
cles of yolk (each containing a pellucid par-
ticle), the  aggregation of which gives  it  a
mulberry-like appearance;  and  by  its fur-
ther continuance, the component  dells becom-
ing more and more minute, the mass comes
to present a  uniform finely-granular aspect.
At this stage it does not appear  that the se-
veral segments of  the  yolk hare  a  distinct
enveloping  membrane;  but  an  envelope is
now formed around each of them, converting
it into a cell, of which the included  particle
forms the nucleus.   This  happens first to the
peripheral  portions of the mass; and as its
cells  are fully developed, they arrange them-
selves at the  surface of the yolk into a kind
of membrane; at the same time assuming  a
pentagonal or hexagonal  shape from mutual
pressure, so as to resemble  pavement epithe-
lium (Fig. 139).   As  the globular  masses of
the interior are gradually converted into cells,
,i_    i        ,.i      p     j       I.    Progressive stages in the segmenta-
tney also pass to the  surface and accumulate tion oftheviteiius of the Mammalian
there; thus increasing  the  thickness  of  the SSZrtJSwKffi'rfSfiS
envelope alreadv formed by the  more super- into.two; c, further subdivision, pro-
      r        J     ...  ',         .    r   ducing numerous segments.
ficial  layer of cells, while the central part of
the yolk  remains, filled  only with a clear fluid, which  seems to be  the
product of  the liquefaction of some of the interior spherules.  By this
process, the external part of the yolk is converted into a kind of secon-
 
454
OF GENERATION AND  DEVELOPMENT.
dary envelope, constituting the germinal membrane; and as this forms
a complete sac enveloping the liquefied yolk of the interior, and as the

                               Fig. 139.
                      A                      B
  Latter stage in the segmentation of the yolk of the Mammalian Ovum;—at A is shown the " mulberry
ma,ss" formed by the minute subdivision of the vitelline spheres; at B, a further increase has brought its
surface into contact with the vitelline membrane, against which the spherules are flattened.

whole structure of the future embryo originates in its substance, it has
been termed by Bischoff the blastodermic vesicle.
   806.  The Blastodermic  Vesicle very  soon after  its formation, pre-
sents at one point an opaque roundish  spot, which  is produced by an
accumulation of cells and nuclei of less transparency than elsewhere;
and it is within this, which is termed the area germinativa, that  all the
structures of the permanent organism  originate.  When seen in section,
this mass of  cells presents the aspect shown at Fig. 140, c.  The Ger-
minal Membrane increases in extent and thickness by  the production of
new cells; and it subdivides into  two layers, which,  although both at
first composed  of cells,  soon  presents distinctive characters, and are
concerned in very different ulterior operations.  The outer one of these
is commonly known  as the  serous layer, and the inner as the mucous;
the division  is at first most evident in  the  neighborhood of  the area
germinativa; but it soon extends from this point, and  implicates nearly
the whole of the germinal membrane.
   807.  The area  germinativa soon loses the rounded  form which it at
first possessed, and becomes first oval, and then pear-shaped.   Whilst

                               Fig. 140.
    Plan of early Uterine Ovum.  Within the external ring, or zona pellucida, are the blastodermic
                  vesicle, a; the yolk, b; and the incipient embryo, c.

this change is taking place in  it, there gradually appears in its centre
a clear space, termed the area pellucida ; and this is bounded externally 
GERMINAL MEMBRANE.
455
by a more opaque circle (whose opacity is due to the greater accumula-
tion  of cells and nuclei in that part), which subsequently becomes the
area vasculosa.  In the formation of these two spaces, both the serous
and mucous layers of  the germinal membrane seem to take their share;
but the foundation of  the vertebral column and nervous centres appears
to be laid chiefly if not entirely in the serous layer;  whilst the mucous
is afterwards concerned more especially in the formation of the nutritive
apparatus.  Between  these  a  third  layer subsequently makes  its ap-
pearance ; which,  as  the  first  vessels of the embryonic  structure are
formed in it, is termed the vascular layer.
  808.  Thus the first development  of the Mammalian  embryo  is into
a sac, enclosing the store of nutriment that has been prepared for it,—
in fact, a stomach; and we shall presently see, that it is by the agency
of the walls of this sac, that  the nutrient materials which it encloses
are prepared for being appropriated  to the development of the more
permanent part of the fabric,  which is to be evolved from the centre of
the mulberry mass.  But  we may here stop to  notice the interesting
fact, that the development of the ovum in  the lowest classes of animals
may  almost be said  to cease  at this point; the external layer of the
germinal membrane remaining  as  the integument;  the internal layer
becoming the  lining  of the stomach; and the space occupied by the
yolk forming the  digestive  cavity, into which an entrance or mouth is
formed, by  the thinning away of the germinal membrane at a certain
point, round which tentaeula or prolonged lips  are usually developed.
This is the essential part of  the history of development in the simpler
Polypes; and we see how remarkably it corresponds with the history of
development of the lower Cryptogamic plants, in which the first-formed
membranous expansion, or primary  frond, remains as  the permanent
leaf.—In the Mammalia, on the  other hand, the greater part of the
germinal  membrane, and  of the cavity which it forms, have a merely
temporary purpose; being  cast off, when they  have performed their
function, like the cotyledons of Flowering Plants.
  809.  During the time which is occupied by these important changes,
the Ovum passes through  the Fallopian tubes, and makes its way into
the Uterus.   During its transit through the Fallopian tubes, the Mam-
malian ovum,—like the ovum  of Birds in  its passage through the ovi-
duct,—receives an  additional layer of albuminous matter secreted from
the walls  of the tube; and  this is surrounded by a fibrous membrane,
whose structure and mode  of formation have been described on a former
occasion (§§ 181,182). The outer layer  of this envelope, in the egg
of the Bird, is further consolidated by the deposition of particles of car-
bonate of lime in  its areolae; but it undergoes no higher organization.
In the Mammal, however,  this  new envelope (termed the  Chorion) is a
formation of great importance; being the medium  through which the
whole subsequent nutrition  of the embryo  is derived.   This is at first
taken in by means of a number of villous processes, proceeding from
the entire  surface of  the Chorion, and giving  it a spongy or shaggy
appearance;  these processes (which are  composed of  nucleated cells)
serve as absorbing  radicles, which draw in the  fluids afforded by the
parent; and they thus make up for the early exhaustion of the small 
456
OF GENERATION AND  DEVELOPMENT.
supply of nutritious matter stored  up in the ovum itself.   The con-
tained embryo appropriates the fluid which is thus  imbibed, by simple
absorption through its surface; and  thus  it is nourished until a more
special provision for its development comes into action.  The structure
of this  organ, termed  the Placenta, cannot  be understood, until the
concurrent changes in the lining  membrane of the Uterus have  been
considered.
  810.  This membrane, in its natural condition, presents  on its free
surface the orifices of numerous cylindrical follicles ; which are arranged
parallel to each other, and at right angles to the surface.  In the spaces
between these follicles, the blood-vessels form a dense capillary network.
When impregnation  takes  place, this mucous  membrane swells and be-
comes lax;  its capillaries  increase in  size; the  follicles are developed
into glandular cavities, and become turgid with a white epithelium ; and
the interfollicular spaces are crowded with nucleated cells, Avhich fill up
the meshes  of the capillary network.   In this  peculiar condition, the
uterine  mucous membrane is termed the  Decidua.  At a later period,
the decidua may be found to consist of two distinct layers; the decidua
ver/z, lining  the uterus ;  and the decidua reflexa, covering the exterior
of the ovum.  Much discussion  has taken place with  regard to the
mode in which the  decidua reflexa originates, and the question cannot
even now be considered as  determined.  The view recently put forth by
Coste is, perhaps, as probable as any.   He considers that the ovum on
its entrance into the uterine cavity, is partly imbedded in its thick vas-
cular lining  membrane,  and that this swells up around it, like the
granulations around the pea in an issue;  so that at last  the OArum be-
comes completely invested  by the special  envelope thus formed, which
closes in around  it, constituting the decidua reflexa,  and which is  at
first not in contact with the decidua vera at any part save Avhere it has
sprung  from it.  As the  ovum increases in  size, the decidua reflexa
grows with  it, and is  thus gradually brought  into contact with the
decidua vera  which  lines  the uterus, the cavity between them  being
obliterated; and at a later  period,  the two coalesce, so that they are no
longer distinguishable from each other.
  811.  When the ovum has arrived in the Uterus, therefore, and the
villous tufts  of its chorion are developed, these  come into contactj in
the first instance, with  the epithelial layer which intervenes between
them and  the vascular decidua.  Through  this cellular  membrane,
therefore,  the ovum must derive  its nutriment from the  vascular sur-
face ; and it cannot be deemed improbable, that its office  is to draw
from the subjacent  vessels the materials which are  to serve for the
nutrition of the  ovum, and  to  present it to the villous  tufts of the
chorion.  Each  of  these,  as already mentioned,  is  composed of an
assemblage  of nucleated cells, which  are found in various stages of
development; and the villus seems to  elongate  by  the development of
new cells from the germinal spot  at its free extremity, whilst, like the
spongiole of the  plant, it draws in nutriment  from the soil  in which it
is  imbedded.  On the  other  hand,  the  Decidua at this  early period,
appears to   be  actively  employed  in preparing  nutriment  for  the
embryo; for its cellular layer is  so abundant,  as to  form abed into 
          FORMATION OF DECIDUA, AND VILLI OF  CHORION.       457

which the tufts of the chorion  are  received;  whilst  its follicles are
enlarged into glandulae of sufficient size,  to allow  these villi (in  some
Mammals at least) to extend  themselves into  their interior.—In its
earliest grade of development,  as already remarked, the  chorion and
its  villi  contain no vessels;  and the fluid drawn in  by the tufts is
communicated to the embryo, by the absorbing powers  of  its germinal
membrane.   But when the  tufts  are  penetrated  by blood-vessels, and
their communication with the embryo becomes much more direct, the
means by which they communicate with the parent are  found to be
essentially the  same;—namely,  a double  layer of cells,  one  layer
belonging to the foetal tuft, the other  to the vascular maternal surface.
(See § 819.)
  812. We now return  to  the  Embryo itself; the general  history of
whose development has been already traced, up to the period  at which
the cluster of cells in the Area Germinativa is about to give  origin to
the permanent structures of the foetus.  The parts first formed in the
embryo  of Vertebrated animals,  are such as most characteristically
distinguish them from all others ;—namely, the Vertebral  Column,  and
Spinal Cord.   The  first indication of  these consists in the  formation
of what is termed the primitive trace,  which is a shallow groove, lying
between two  oval ridges  (Fig. 141, v),  known  as the  laminar dorsales.
The form of  these  is changed with that of the area pellucida; at first
they are oval, then pear-shaped, and  at last become of a violin-shape.
At the same  time  they  rise more and more from  the  surface of the
Area  pellucida, so as to form ridges of higher elevation,  with  a  deeper
groove  between  them;  and  the  summits  of these ridges  tend  to
approach one another, and gradually unite, so as  to convert the  groove
into a tube.   It is  within this,  that  the Cerebrospinal Axis is after-
Ayards formed; the brain being developed in the anterior dilated por-
tion, and the spinal cord in the posterior  more  contracted part.  The
Fig. 141.
  The germ and surrounding parts, from a more advanced Uterine Ovum:—6, blastoderma, or germinal
 membrane; a g, area germinativa; c, cephalic extremity of the germ ; v, first indications of vertebrae; q, cau-
 dal extremity.

former  remains  unclosed  much longer  than the latter.   The  tube
within which the neural  axis is  thus  enclosed, and  which is entirely
composed of  nucleated cells (Fig. 16),  is termed  the  Chorda  dorsalis ;
and it retains its embryonic type, in many of the lower Fishes,  which
neArer possess a true vertebral column.  The  elements of the vertebrae 
458              OF GENERATION AND  DEVELOPMENT.
are developed in a fibrous membrane,  with which  the  chorda dorsalis
subsequently becomes invested ; the neural arches being  the  parts  first
formed.
  813.  During the progress of this change, another very important one
is  taking place,  which has  reference to the nutrition  of the embryo
during its further development.  This is the formation of vessels  in the
substance of the germinal membrane;  which vessels  serve to take up
the nourishment supplied by the yolk, as well as that  derived from the
chorion externally, and to convey it through the tissues  of the embryo.
These Vessels are first seen in that part of the vascular lamina  of the
germinal  membrane, which  immediately  surrounds  the embryo;  and
they form a network, bounded by  a circular channel, which is  known
under the name  of  the  Vascular Area  (Fig.  142).  This  gradually
extends itself, until the vessels spread over the  whole of the germinal
membrane.   The vessels are probably formed by the coalescence  of the
original cells of the layer; and the first blood-discs  which they contain
seem  to originate in the nuclei of these cells.  This network  of vessels
serves to receive the  nutritious matter  contained in the yolk-bag,  and
to convey it to the embryo ;  but the act of absorption seems  to be  per-
formed here as elsewhere,  by cells, a layer of Avhich  always  intervenes
between the vascular layer  and the yolk  itself.   These cells probably
correspond in function with those of the villi of  the intestinal canal in
the adult (§ 243); as the vessels of the yolk-bag,  or temporary diges-
Fig. 142.
 Vascular area of Fowl's egg, at the beginning of the third day of incubation; a, a, yolk; 6, 6, b, b, venous
sinus bounding tbe area, c, aorta; d, punctum saliens, or incipient heart; e, e, area pellucida; /, /, arteries
of the vascular area; g, g, veins; h, eye.

tive cavity,  represent those of the alimentary canal, to be  afterwards
developed from a portion of it.  The vessels of the yolk-bag terminate
in two large trunks, which enter  the embryo at  the  point that after-
wards becomes the umbilicus, and which are  known  as the Omphalo-
Mesenteric,  Meseraic, or  Vitelline vessels (Fig. 146,  q, r).   The first
movement of fluid takes place towards the embryo ; and this may be
witnessed before any distinct heart is evolved. 
FORMATION OF VESSELS AND DIGESTIVE CAVITY.        459
  814.  The formation of the Heart  takes place in a thickened portion
of the Vascular layer; by the liquefaction of  the interior of a mass  of
cells, of which the  exterior  constitute the  first  walls  of  the cavity.
These gradually acquire firmness and consistency, and are endowed
Avith a contractile power that  enables  them to  execute regular pulsa-
tions.   In this  early condition,  the  heart is  known  as the punctum
saliens  (d, Fig. 142).  The first appearance of the heart in the Chick
is at about the 27th hour;  the time of its formation in  Mammalia has
not been distinctly ascertained.
  815.  Concurrently with the formation of the Vascular system, the
production of the permanent Digestive cavity commences.   This origi-
nates in the separation of  a small portion of the yolk-bag, lying imme-
diately  beneath the  embryo, from the general cavity, in the following
manner.—At about the 25th hour of incubation, in the Fowl's egg, the
parts of the germinal membrane  which lie beyond the extremities, and
Fig. 143.
Fig. 144.
 Diagram of Mammalian Ovum at later stage;
the digestive cavity beginning to be  separated
from the jrolk-sac, and the amnion beginning to
be formed:'— a, chorion; 6, yolk sac; c, embryo ;d
and e, folds of the serous layer rising up to form
the Amnion.
SftffiKBS5^
 The Amnion in process of formation, by the
arching-over of the serous lamina:—a, the cho-
rion ; 6, the yolk-bag, surrounded by serous and
vascular laminae; c, the embryo: d, e, and/, ex-
ternal and internal folds of the serous layer,
forming the amnion; g, incipient allantois.
which spread out from the sides of the embryo, are doubled in, so as to
make a depression upon the  yolk; and their  folded edges gradually
approach one another under the abdominal surface of the embryo, so as
at last to meet and enclose a cavity, which is at first simple in its form,
but which is subsequently rendered more complex  by the  prolongation
and involution  of its walls  in various parts, so as to form the stomach
and intestinal  tube (Figs. 143, 144, 145).   This digestive cavity com-
municates for some time with the yolk-bag (from which it has been thus
pinched off, as it were), by a wide opening, that is left by the imperfect
meeting of the folds of the germinal membrane that constitute its walls.
In the Mammalia, this orifice is gradually narroAved, and is at last com-
pletely closed; and the yolk-bag, thus  separated, is afterwards  thrown
off.  It may be detected, however, upon the  umbilical cord, up to a late
period of pregnancy, and is known as the Umbilical vesicle (Fig. 146,  t).
In Birds,  and other  oviparous  animals,  the  whole of the yolk-bag is 
460
OF GENERATION AND DEVELOPMENT.
ultimately drawn into the abdomen of the embryo; the  former gradu-
ally shrinking, as its contents are exhausted;  and the latter enlarging,
so  as  to  receive it  as  a little pouch  or appendage.  In Fishes, the
hatching  of the egg very commonly takes place before the  process has
been completed; so  that the little Fish swims  about with the yolk-bag
hanging from its body.
  816. Whilst these processes are  going on in the Vascular and Mu-
cous layers  of the  germinal  membrane,  a remarkable change is taking
place in that portion of the Serous  lamina, which surrounds the Area
pellucida.   This rises up  on either side in two  folds (Fig.  143); and
these  gradually approach one  another,  at  last  meeting in the  space
between the general  envelope and  the embryo, so as to form an  addi-
tional investment to the latter.   As  each fold contains two layers of
membrane,  a double envelope is thus formed;  of which the outer layer
(Fig. 144, d, e, and Fig. 145, h) aftenvards adheres to  the  inner sur-
face of the  chorion,  the original yolk-bag, or Zona pellucida, being now
lost sight of; whilst the inner one (Fig. 144, /, /,  and Fig. 145,/)
remains as a distinct sac, to which the name of Amnion is given.   This
takes place during the third day in  the Chick; but the period at which
it occurs in the Human Ovum has not yet been clearly ascertained.
  817. The embryo, like the adult, has. need of Respiration; in  order
that the  carbonic acid set free in  the Nutritive operations may be re-
moved from its fluids.  In Fishes, the surrounding water acts with suffi-
cient power upon  the vessels of the yolk-bag, to produce the required
aeration,  up to the  time when the  gills  of the young animal are  ready
to come into play.   But in the higher oviparous animals, Avhose  deve-
lopment proceeds further before they leave the egg, a more special pro-
vision is made for the purpose.  A bag, termed the allantois, sprouts
(as  it were) from the lower  end of the body (Fig. 144, g); and gradu-
ally enlarges, passing round the embryo (Fig. 145, g),  so as in  Birds
Fig. 145.
 Diagram representing a Human Ovum in second month:—a, 1, smooth portion of chorion; a, 2, villous
portion of chorion; k, k, elongated villi, beginning to collect into Placenta; b, yolk sac or umbilical vesicle;
c, embryo; /, amnion (inner layer); g, allantois; h, outer layer of amnion, coalescing with chorion.

almost completely to enclose it, intervening between the germinal mem-
brane and the shell, and receiving the direct  influence of the air that
penetrates the latter.  It is thus the temporary lung of the air-breathing 
                RESPIRATION OF EMBRYO;—ALLANTOIS.            461

oviparous animal;  and it serves for the  aeration of its fluids, up to the
time when it  quits  the egg.   In the ovum  of the Mammal, the chief
office of the Allantois is to convey the vessels of the embryo to the cho-
rion ;  and its  extent bears a pretty close correspondence with the extent
of surface through which  the chorion comes into vascular connexion with
the decidua,—this extent varying considerably in the different orders of
Mammalia.   Thus in the Carnivora, whose placenta extends like a band
around the whole ovum, the allantois  also lines the whole inner surface
of the chorion, except where the umbilical vesicle  comes  into  contact
with it.   On the other hand, in Man  and the Quadrumana, whose pla-
centa is restricted to one  spot,  the allantois conveys the foetal vessels to
one portion only of the chorion (Fig. 146); although, according to Coste,

                                 Fig. 146.
  Diagram of the Circulation in the Human Ovum, at the commencement of the formation of the Placenta;
—a, venous sinus, receiving all the systemic veins; 6, right auricle; b', left auricle; c, right ventricle; d,
bulbus aorticus, subdividing into e, e', e", branchial arteries; /, arterial trunk formed by their confluence ;
g, vena azygos superior; h, confluence of the superior and inferior azygos; j, vena cava inferior; A;, vena
azygos inferior; m, descending aorta; n, n, umbilical arteries proceeding from it; o, umbilical vein; q,
omphalo-mesenterlc vein ; r, omphalo-mesenteric artery, distributed on the walls of the vitelline vesicle, t ;
v, ductus venosus;  y, vitelline duct; z, chorion.

it completely surrounds the embryo.   When these vessels have reached
the Chorion,  they ramify in  its  substance, and  send  filaments into  its
villi;  and  in proportion  as these villi form that connexion with the
uterine structure which has  been  already described (§ 811),  do the ves-
sels  increase  in size.   They then pass directly  from the foetus to the
chorion;  and the  allantois, which  is  no longer  of any  use, ceases to
present itself as a  distinct sac.   The lower part of it, however, pinched
off (as it were)  from the rest, remains  as the  Urinary  bladder; and the 
462
OF GENERATION AND  DEVELOPMENT.
Urachus or suspensory ligament of the latter  represents the duct by
which the Allantois was originally connected with the abdominal cavity.
   818.  The connexion which is thus formed  between the Vascular sys-
tem of the foetus and that of the parent, is the only one that exists in
the lower Mammalia; which are  thus  properly  designated as "nonpla-
cental."  Each villus of the Chorion  contains a capillary  loop; this
is enclosed in a layer of cells; and this again in a lamina of basement-
membrane ;—the whole forming the foetal tuft.   This comes into contact
with  the cellular  decidua, which lies upon the basement-membrane
covering the vascular layer of the decidua.  Now the Placenta is com-
posed of these very elements, arranged in a  more complex manner.   It
is formed by an extension of the vascular tufts of the Chorion at certain
parts; and a corresponding adaptation,  on the part of the Uterine struc-
ture,  to afford to  these an increased supply of nutritious fluid.  These
specially-prolonged portions are scattered, in the Buminants and some
other Mammalia, over the whole surface  of the Chorion, forming what
are termed the Cotyledons; but in the  higher orders, and in Man, they
are concentrated  in one spot, forming  the Placenta.   In some  of the
lower tribes, the maternal  and foetal portions of the placenta may be
very easily separated; the former consisting of the  thickened decidua;
and the latter being composed  of the prolonged and ramifying  vascu-
lar tufts of the Chorion, which dip down into it.  But  in the Human
placenta, the two  elements  are mingled together through its whole
substance.
   819.  The foetal portion of the Placenta consists of  the branches  of
the umbilical vessels ; which subdivide at the point at which they enter
the mass, and form, by their minute  ramifications, a large part of its
substance.   Each villus  contains a  capillary  vessel,  which  forms a
series of loops, communicating with an  artery on the one side, and with
a  vein on  the other; but the same  capillary may  enter several villi,
before re-entering a larger vessel.   The vessels of the villi (Fig. 147, g)
are covered, as in the chorion, by a layer of cells (/), enclosed in base-
ment-membrane (e); but the  foetal tuft thus formed is enclosed in a
second series of envelopes (a, b, c), derived from the maternal portion
of the placenta;  a space (d) being left between the two, however,  at
the extremity of  the tuft.   The vascular  tufts not unfrequently extend
beyond the uterine surface of the placenta, and  dip  down into the ute-
rine sinuses, where they are bathed in the  maternal blood.—The ma-
ternal portion of  the  Placenta  may be  regarded as a large sac, formed
by a  prolongation  of the  internal coat  of  the great  uterine vessels.
Against the foetal surface of this sac, the  tufts just  described may be
said to   push themselves, so as to dip down into it,  carrying  before
them a portion of its thin wall, which constitutes a sheath to each tuft.
In this manner, the whole interior of the placental cavity, is intersected
by numerous tufts of foetal vessels, disposed in fringes, and bound down
by the membrane that forms its proper wall; just as the intestines are
covered and held  in their places by the  peritoneum.  Now as this dila-
tation of the  uterine blood-vessels  carries the  decidua before it, every
one of the vascular tufts that dips down into it  will be  covered with a 
STRUCTURE  OF  PLACENTA.
463
layer of the cellular structure of the latter (Fig. 148, and Fig.  149, e);
and this will also  form  a part  of  all  the  bands  that  connect and tie
Fig. 147.
Fig.  148.
  Extremity of a Placental villus;—a, external mem-
brane of the villus, continuous with the lining mem-
brane of the vascular system of the mother; 6, external
cells of the villus, belonging to the placental decidua;
c, c, germinal centres of the external cells ; d, the space
between the maternal and foetal portions of the villus;
e, the internal membrane of the villus, continuous with
the external membrane of the chorion; /, the internal
cells of the villus, belonging to the chorion; g, the loop
of umbilical vessels.
  Portion of the external mem-
brane, with the external cells,
of a  Placental villus;—a, cells
seen through the membrane; 6,
cells  seen from within the vil-
lus ; e, cells seen in profile along
the edge of the villus.
down the tufts (Fig.  149, g).   The blood is conveyed into the cavity of
the placenta by the  "curling arteries," so named from  their peculiar
course (Fig. 149, c), which proceed from the arteries of the uterus ; and

                                    Fig. 149.
  Diagram illustrating the arrangement of the Placental Decidua:—a, decidua in contact with the interior
of the uterus; 6, venous sinus passing obliquely through it by a valvular opening; c, a curling artery
passing in tbe same direction;  d, tbe lining membrane of the maternal vascular system, passing in from
the artery and vein, lining the bag of the placenta, and covering e, e, the foetal tufts passing on to them
from their stems from the foetal side of the cavity, also by the terminal decidual bars f,f, from the uterine
sido, and from one tuft to the other by the lateral bar, g; h, h, separated foetal tufts, showing the internal
membrane and cells,  which, with the loops of umbilical vessels, constitute the true foetal portion of the
tufts.
it is returned by large,  short,  straight  trunks, which pass obliquely
through the decidua (Fig. 149, b), and discharge their contents into the
great uterine  sinuses.
   820.  There is no more  direct communication between the Mother^
and Foetus than this;  all the observations  which  have been  supposed
to prove  a direct vascular  continuity, being certainly fallacious.   The
function of the Placenta  is manifestly double.   The foetal tufts draw,
from the maternal  blood,  the   materials  which  are required for  the
nutrition of  the  embryo,—these  materials having  been  first  selected 
464
OF GENERATION AND  DEVELOPMENT.
and partly  elaborated by the two  sets of  intervening cells;  and  in
this character, the foetal tufts resemble the villi of the intestinal surface,
which dip down into the fluids of the alimentary canal, and absorb the
nutritive material which they furnish.   But the  Placenta also serves
as a respiratory organ; aerating the blood of the  foetus, by exposing it
to the influence of the oxygenated blood of the  Mother;  and in this
respect the foetal tufts bear a  close correspondence with  the  gills  of
aquatic animals,  bringing  the blood into relation with a surrounding
fluid medium containing  oxygen, which  is  imbibed  by  the  blood  in
exchange for the carbonic acid given off.    And it is probable, too,
that the Placenta is to be regarded as an excreting organ; serving for
the  removal, through the maternal blood,  of excrementitious  matter,
whose continued circulation in  the blood of the foetus  would  be preju-
dicial to it.
   821. The formation of the  Human Placenta commences in the latter
part of  the second  month of utero-gestation;  during the  third,  it
acquires its  proper  character;  and it subsequently goes on increasing,
in accordance with the growth of the ovum.   The vessels of the Uterus
undergo great enlargement throughout;  but especially at the  part  to
which the  Placenta  is  attached;  and the  blood, in  moving through
them, produces a peculiar murmur, which  is  usually audible with dis-
tinctness at an early period of  pregnancy, and which may be regarded
(Avhen due care is taken  to avoid  sources of fallacy) as one of its most
unequivocal  physical signs.   The sound  is  most commonly heard near
the situation of the Fallopian  tube of the right side; and it corresponds
with the pulse  of the mother.  *
   822. It would be  inconsistent with the character and objects of this
Treatise, to follow in any detail, the history of the  development of the
Foetus, during its intra-uterine  life ; and a general account of the evo-
lution of most of the chief organs, has been  given in  connexion with
that of their structure.   The condition  of the circulating apparatus,
however, at  the  period of birth, deserves especial  notice.  A  general
account of the  development of the simple pulsating trunk, which consti-
tutes its first form, into the four-cavitied heart of the higher Vertebrata,
and of the conversion of  the  single trunk  proceeding from it, with  its
four pairs of branchial  arches,  into the aorta and  pulmonary arteries,
with their  chief  subdivisions, has been  already given (§ 566).   Up to
the  time of birth, however, the partition betAveen the Auricles is incom-
plete ; a large aperture, the foramen ovale,  existing in  it.  There is also
a direct communication between the pulmonary artery and the aorta,  by
the  ductus arteriosus; and another direct  channel between  the umbi-
lical vein and the vena cava, by the ductus venosus.
   823. The following is the course of the  Circulation  of  the Blood  in
the  Foetus.  The fluid brought  from the  Placenta by the umbilical vein
.(Fig. 150,  3), is partly conveyed at once to  the vena cava ascendens,  by
means of the ductus venosus (5), and partly flows  through two trunks
(4, 4), that unite with the portal vein (7) returning the blood from the
intestines, into the substance  of the liver, thence  to be returned to the
vena cava by the hepatic vein. Having thus been transmitted through the
two great depurating organs,  the placenta and the liver, the blood that 
FCETAL CIRCULATION.
465
 enters the vena cava is purely arterial
 in its character;  but being mixed in
 the vessels Avith the venous blood that
 is returned from the trunk and lower
 extremities, it  loses  this character in
 some  degree,  by  the  time that  it
 reaches  the   heart.    In  the  right
 auricle, Avhich it then enters, it would
 also be  mixed with the venous blood
 which is brought down  from the head
 and upper extremities  by  the  de-
 scending cava;  were  it  not that  a
 very  curious provision exists to  im-
 pede  (if it does not entirely  prevent)
 any further  admixture.    This  con-
 sists in the arrangement of the Eusta-
 chian valve, which directs the arterial
 current  (that flows  upward  through
 the ascending cava)  into the left side
 of  the  heart,  through the  foramen
 ovale, whilst it  directs  the  venous
 current (that is being returned by the
 descending cava) into the  right ven-
 tricle.  When the ventricles contract,
 the arterial blood contained in  the
 left is propelled  into  the  ascending
 Aorta, and supplies the branches that
 proceed  to the head and  upper  ex-
 tremities,  before  it undergoes  any
 further admixture ; Avhilst the venous
 blood contained in the right ventricle,
 is forced  into the Pulmonary artery,
 and thence through the ductus  arte-
 riosus (17), which is  like  a continua-
 tion of its  trunk, into the descending
 aorta, mingling with  the arterial cur-
 rent Avhich that vessel previously con-
 veyed, and thus supplying  the trunk
 and lower  extremities with a mixed
 fluid.   A portion  of this is conveyed,
 by the umbilical arteries,  to the Pla-
 centa ;  in  which  it  undergoes   the
 renovating  influence  of the maternal
 blood, and from which it  is returned
 in a state of purity.
   824. Hence the head and superior
extremities,  whose   development   is
required  to be  in advance  of  that
of the loAver,  are supplied with blood
nearly as pure as that Avhich returns
from the  placenta;  whilst  the  rest
Fig. 150.
  The Foetal Circulation:—1. The umbilical cord,
consisting of the umbilical vein and two um-
bilical  arteries ; proceeding from the placenta
(2).  3. The umbilical vein dividing into three
branches; two  (4, 4) to be distributed  to the'
liver; and one (5), the ductus venosus, which
enters the inferior vena cava (6).  7. The portal
vein, returning the blood from the intestines, and
uniting with the right hepatic branch.  8. The
right auricle ; the course of the bloood is denoted
by the  arrow,  proceeding from 8, to 9. the left
auricle. 10. The left ventricle ; the  blood fol-
lowing the arrow to the arch of the aorta (11),
to be distributed to the branches given off by the
arch to the head and upper extremities. The ar-
rows 12 and 13, represent the return of the blood
from the head  and upper extremities through
the jugular and subclavian veins, to the supe-
rior vena cava (14), to the right auricle (8), and
in the  course of the arrow through the right
ventricle (15), to the pulmonary artery (16). 17.
The ductus arteriosus, which appears to be a
proper  continuation of the pulmonary artery;
the offsets at each side are the right and left
pulmonary artery cut off; these are of extremely
small size as compared with the ductus arterio-
sus. The ductus arteriosus joins the descending
aorta (18, 18), which divides into the common
iliacs, and these into the internal iliacs, which
become the hypogastric arteries (19), and return
the blood along the umbilical cord to the pla-
centa ;  while the other divisions, the external
iliacs (20), are continued into the lower extremi-
ties. The arrows at the terminations of these
vessels mark the return of the venous blood by
the veins to the Inferior cava. 
466
OF GENERATION AND DEVELOPMENT.
of the body receives  a mixture of this, with what has previously cir-
culated through  the  system.   The  Pulmonary arteries convey  little
or no  blood through the lungs ;  the current of blood, propelled from the
right  ventricle, passing directly omvards through the ductus arteriosus,
into the aorta.—At birth, hoAvever, the course of the circulation under-
goes  great changes, that it may be adapted  to the new mode, in which
the infant is henceforth to  obtain its nutrition and to carry on its respi-
ration.  As soon as the lungs  are distended by the first  inspiration, a
portion of the blood of the pulmonary artery is diverted into them, and
there  undergoes aeration ;  and, as this proportion increases, with the
full activity of the lungs, the ductus arteriosus gradually shrinks, and its
cavity finally becomes obliterated.   At the same time, the foramen ovale
is closed by a valvular fold  ; and thus the direct communication between
the two auricles is cut off.  When  these changes  have been  accom-
plished, the circulation, which  was before carried on upon the  plan of
that of the higher Reptiles (§ 563), becomes that of the complete warm-
blooded animal; all the blood which has been returned in a venous state
to the right side of the heart, being transmitted through the lungs, be-
fore it can reach the left side, or be propelled from its arterial trunks.—
It is by no means unfrequent, however, for some arrest of devolopment
to prevent the completion of these  changes ;  and various malformations,
involving  an imperfect  discharge of the  circulating  and  respiratory
functions, may  hence  result.
   825. The average length of  time, which elapses between  Conception
and Parturition, in the Human female, appears to be 280 days, or 40
weeks.  There  can be little doubt, however,  that Gestation may be
occasionally prolonged for  one, two, or even three weeks, beyond that
period ; such prolongation not being at all unfrequent amongst the lower
animals ;  and numerous well-authenticated instances of it, in the Human
female, being upon record.  Upon what circumstances this departure
from  the usual  rule is dependent,  has not yet been ascertained; but it
is a remarkable circumstance, ascertained by the observations of cattle-
breeders, that the male has  an influence upon the length of gestation,—
a  large proportion of cows  in calf by certain bulls exceeding the  usual
period, and a small proportion  falling  short of it.  In such  cases, we
must  attribute the  prolongation of the period to some peculiarity in the
embryo, derived from its male parent.
   826. The shortest  period at Avhich Gestation may terminate, consis-
tently with the life of the child, has  not yet been precisely ascertained;
the difficulty  of determining the precise date of  conception being usually
such,  in this  case as  in the preceding, as to prevent  the exact length
of the Gestation from being known.   Thus, the commencement of preg-
nancy being fixed by the time  of the cessation  of  the Catamenia,  when
there  is no more definite guide, it  is obvious  that the act of Conception
may have taken place during any  part of the interval  that has  elapsed
since  the  last monthly period;  and thus a doubt  may exist as to the
length of the  Gestation, to the extent of from one to three weeks.   There
are very satisfactory  cases  on  record, in which, from the  degree of de-
velopment of the infant at  birth,  as well as  from  other circumstances,
it might be certainly known not  to haAre  attained  26 or 27  weeks, or 
LENGTH OF  GESTATION.
467
little more than six months;  and in which, by careful treatment, the
infant was reared  in  a condition  of health  and vigor.   And there  is
reason to  believe, that infants have lived for some time, and might pro-
bably have been reared under better management, that  were born as
early as the 24th or 25th week.
  827.  The act of Parturition, by which the foetus is expelled from the
Uterus, is accomplished in part by the  contractile poAver  of the Uterus
itself; and  in  part by the  combined  operation  of the  various  mus-
cles, Avhich press upon the abdominal cavity, and which effect the expul-
sion of the faeces and urine.   No definite  account can be given of the
reasons, Avhy this change should take place at the period which has been
mentioned as its usual date ; but we are as much in the dark in regard
to other  periodic  phenomena of Animal life; and  we must probably
look for its source in the maturation of the  placental structure, which
prepares it for  detachment (like the dropping-off of a ripe fruit), and in
the complete evolution of the contractile  tissue of  the uterus, the con-
tractions  of which may be considered to commence spontaneously  when
it has attained  a certain epoch in its growth, just as do those of the
heart in the embryo  (§ 814).   For  some  days previously to  the  com-
mencement of labor,  there is  usually a slow contraction  of the fibres
of the fundus and body of the uterus, and  a  yielding of those of the
cervix; so that the child lies  lower, and the size of the abdomen  dimi-
nishes.   This slow contraction is probably not dependent upon any act
of the nervous  system; but upon the direct  excitement of the contrac-
tility of the muscular substance of the uterus.  When labor properly
commences, however, the  Spinal system of nerves comes into  play, and
the uterine  contractions are of a reflex nature.  As  before, hoAvever,
the act of contraction is confined to the fundus and body  of the uterus ;
the fibres of the cervix uteri, and of the vagina, being in a state of re-
laxation,  Avhich alloAvs them to yield to the pressure  of the child's head.
In the first stage of labor, the Uterine  contractions appear to be  alone
concerned;  and it is  not until the head cf the child is passing through
the os uteri, and is entering the vagina,  that  the assistance of the ab-
dominal  muscles is called  in.  These  act, in  the first instance,  as in
ordinary expiration ; but their power is much increased by the  voluntary
retention  of the breath, so that the whole of their contractile force may
be applied to the  expulsion of the foetus.   In  a later stage  of labor,
this retention of the breath becomes involuntary, during the accession of
the " pains ;" and the expulsion of the foetus is commonly effected Avith
considerable force,  especially  if the previous resistance has been con-
siderable.
  828.  The same  action  Avhich expels  the foetus, usually detaches the
placenta; and if the  uterus contract with  sufficient force, after this has
been thrown off, the orifices of the vessels Avhich communicated with it
are so effectually closed, that  little or no hemorrhage from that source
takes place.  When efficient  contractions do not occur,  they may fre-
quently be excited by pressure upon the uterus itself; by the applica-
tion of cold to the abdominal surface, to the extremities,  and  (in severe
hemorrhage) to the entire body; or  by the application of the child to
the nipple, Avhich  will frequently at  once succeed in producing the de- 
468
OF GENERATION AND DEVELOPMENT.
sired effect.  The efficacy of these means,—the latter in particular,—
obviously depends upon the influence of the spinal cord  and its nerves
upon the muscular fibres of the uterus;  the  application of cold to the
surface, or  the  irritation of the nipple, occasioning a reflex action in
the uterus.  But it is probable that this organ has also considerable poAver
of contracting,  independently of the nervous system; thus there are
well-authenticated cases on record, in which the foetus has been  expelled
after the somatic death (§ 65) of the parent; Avhich must have been in
consequence of  the persistence of the independent contractility of the
Uterus, and the relaxed state of  all the parts through  which the child
had  to make its exit.
   829.  The cause of the occasional occurrence of  the parturient efforts
at an unusually early period, is as little understood as that of their ordi-
nary action.  There are some individuals, in whom this regularly  happens
at a certain month; so that it seems to be an action natural to  them.
In many  cases,  however, it may be traced to some undue exertion of
body, or mental excitement; and not unfrequently  to a general constitu-
tional irritability, Avhich renders  the  system liable  to be  derange d by
very trifling causes.   Premature  labor is ahvays to be prevented, if
possible, being injurious alike to both mother and child; and for  this pre-
vention we have chiefly to rely upon rest and tranquillity of mind  and
body, and upon the careful avoidance of all those exciting causes, which
are liable to produce uterine contractions by their operation upon the
nervous system ; whilst, at  the  same time, any measures which will in-
vigorate the body, without stimulating it, should not be overlooked.
  830.  A peculiar preparation is made, in the females of the class Mam-
malia, for the sustenance of the infant during a long period after birth.
This consists in  the secretion of a fluid, from  the  glands termed Mam-
mary, which contains all the elements that are required for the develop-
ment of the body of the infant, during  the  first  year.   These glands
present themselves in an almost rudimentary  state, in some of  the non-
                      placental animals of the class ; consisting only of
            151-        a feAV large follicles, which open separately upon
                      the surface (Fig. 110).   In the higher Mammalia,
                      hoAvever, we find it composed of vast numbers of
                      minute follicles, clustered  together upon excre-
                      tory  ducts.   The general arrangement of these,
                      in the human subject, is seen in Fig. 151; and in
                      Fig.  115, the  character of the  follicles  them-
                      selves, and  of the  secreting epithelial cells they
 Termination of portion of        .             ,         i  1 •  i        • p •
miik-duct in foiiicics; from a   contain, as seen under a much higher magnifying
c^r1aeniS^          power, has been already shown.  Each Mammary
                      gland consists of a number of, glandulae, which
are held together by areolar and fibrous  tissue ; this  arrangement may
probably have reference to  the  mobility, which it is requisite  that the
different parts of the mass should possess, one upon the other, in con-
sequence of its situation upon the pectoralis  muscle.  The  ducts con-
verge and unite together ; so as at last to form ten or twelve principal
trunks, Avhich terminate in the nipple.  At the base of the nipple, these
tubes dilate into reservoirs, Avhich extend beneath the areola, and to
 
MAMMARY GLAND.
469
 some distance into the gland, when the breast is in a state of lactation.
 These, which are much larger in many of the loAver Mammalia than
 they are in the Human female, seem to have for their office to contain
 a store  of milk, sufficient  to supply the immediate wants of  the child
 when it is first applied to the breast;  so that it shall not be disappointed,
 but shall be induced to proceed with sucking, until  the  draught be oc-
 casioned (§ 836).
   831.  The Mammary gland may be  detected at an early period of foetal
 existence, and  it  then  presents no difference in the male and female;
 and it continues  to grow, in  each sex, in  proportion  to the body  at
 large, up to the period of  puberty.  At that epoch, however, the gland
 begins  to undergo a great enlargement in the  female;  and by the age
 of twenty, it attains its full size previous to lactation.  Even then, how-
 ever, the milk-follicles cannot be injected from the tubes.  During preg-
 nancy,  the mammary glands receive  a greatly-increased quantity  of
 blood.   This determination often  commences very early: and  produces
 a feeling of tenderness and  distension,  which is a valuable sign (where
 it occurs in conjunction with others) of conception  having taken place.
 The vascularity of the gland continues to increase  during pregnancy;
 and, at the  time of parturition, its lobulated character can be distinctly
 felt.   The follicles cannot  be readily injected, however,  until the gland
 is in a  state of  complete functional activity; i. e., during lactation.—
 The Mammary gland of the Male does not undergo this increase of deve-
 lopment, except under certain peculiar  circumstances to be presently
 noticed (§ 836); and it remains a sort of miniature picture of that of the
 female, varying in diameter from that of a large pea to an inch or even
 two inches.
   832.  The Milk, secreted by the Mammary glands, consists of Water,
 holding  in solution the peculiar Albuminous substance termed  Caseine,
 and various  Saline ingredients, together with (in most cases) a certain
 form of Sugar; and having Oleaginous globules  suspended in it.  These
 globules appear to be surrounded  by  a thin pellicle, which keeps them
 asunder, so  long as the milk remains at rest.—The existence  of these
 elements in  ordinary Milk, as that of the Coav, is made apparent by the
 processes to which it is subjected in domestic economy.  If it be allowed
 to stand for some  time, exposed to the air,  a large part of  the oleagi-
 nous globules come to the surface,  in  consequence of their inferior spe-
 cific gravity; and thus is formed the  cream, which  includes  also a con-
 siderable amount of caseine, with the sugar and salts of the milk.  These
 may be  partly  separated by the continued agitation  of the cream, as in
 the process of churning ;  this, by rupturing the emrelopes of the oil-glo-
 bules, separates it into butter, formed by their aggregation, and butter-
 milk, containing the  caseine, sugar, &c.  A considerable quantity of
 caseine, however, is still entangled with the oleaginous matter;  and this
 has a tendency to decompose, so  as  to render  the  butter rancid.  It
may be separated by keeping the butter melted at a temperature  of 180°,
when the caseine  will fall  to the  bottom, leaving the butter pure and
 much less liable to change;  an operation which is commonly knoAvn as
 the clarifying of butter.—The Milk, after the  cream has  been removed,
still contains the greater  part of its caseine and sugar.  If it  be kept 
470
OF GENERATION AND  DEVELOPMENT.
long enough, a spontaneous change takes place in  its composition;  an
incipient change in the caseine being the cause of the conversion of the
sugar into lactic acid; and this coagulating the caseine, by precipitating
it in small flakes.   The same precipitation may be accomplished at any
time by the agency of  various acids, especially the acetic, which does
not act upon Albumen;  but Caseine cannot  be  coagulated, like  albu-
men, by the influence of heat alone.  The most complete  coagulation
of Caseine is  effected by the agency of the dried stomach of  the calf,
knoAvn as rennet; Avhich exerts so powerful an influence as to coagulate
the caseine of  1,800 times  its weight of milk.  It is thus that, as in
the making  of cheese, the curd is separated from the whey ;  the former
consisting chiefly of the caseine ; whilst the latter  contains a large
proportion of the saline and  saccharine matter,  which entered into the
original composition of the milk.  These may be readily separated  by
evaporation.
  833.  The principal characters of Caseine have been already stated
(§ 172).—The Oleaginous matter  consists, like the  fats in  general, of
the two substances, elaine and stearine; but  it  also  contains another
substance peculiar to it, which is termed butyrine.   This last (to which
the characteristic smell  and  taste of butter  are due) is  converted  by
saponification into three volatile acids, of strong animal odor,  to which
the names of butyric, capric, and caproic acids have been given.  This
change may be effected, at any  period, by treating the butyrine with
alkalies ; but it may also take place by spontaneous decomposition, which
is favored by  time and moderate warmth.—The Sugar of Milk is pecu-
liar as containing nearly 12 per cent, of water: so that  it may be con-
sidered as really a hydrate of sugar.   It is nearly identical  in its com-
position with starch ; and may, like it, be converted  into true sugar by
the agency  of sulphuric acid.   But it is chiefly remarkable for  its
proneness to conversion  into lactic acid, under  the influence of a fer-
ment or decomposing azotized substance.—The Saline  matter contained
in Milk appears to be nearly  identical with that of the blood;  with a
larger  proportion, however, of the phosphates of lime and magnesia,
which amount to  2 or 2J parts in  1000.   These are  held  in  solution
chiefly by  the Caseine,  which has a remarkable power  of combining
Avith them.
  834.  Thus ordinary Milk contains the three classes of organic prin-
ciples,  which form the chief part of the food  of  animals,—namely, the
albuminous,  the saccharine,  and the oleaginous;  together with the mi-
neral elements, which are required for the development and consolida-
tion  of the  fabric of the infant.   It would appear, however, that the
combination of all these is not necessary;  but rather  has  reference to
the composition of the food on which the animal is destined to  be after-
wards supported.   Thus it has been lately shown  that,  in the Carnivora,
the milk contains no sugar; which principle  is  altogether wanting in
the food of the adult.  Amongst the different species of Herbivorous
animals, the proportion  of the several ingredients varies considerably;
and  it is also  liable to  considerable variation in accordance with the
nature of the food, the amount of exercise taken by  the animal that
affords it, and other circumstances.   Thus in the milk of the Coav, Goat, 
COMPOSITION OF MILK.
471
 and Sheep, the average proportions of Caseine, Butter, and  Sugar are
 nearly the same one with another,  each amounting to  from 3 to 5 per
 cent.   In the milk of the  Ass and Mare, on the other hand, the pro-
 portion of Caseine is under 2 per cent., the oleaginous constituents are
 scarcely traceable, whilst the sugar and allied substances rise to nearly
 9 per cent.   In the  human female, the  saccharine  and oleaginous ele-
 ments  are  both present in large amount;  whilst the  Caseine  forms a
 moderate  proportion.—The  proportion of  saccharine and  oleaginous
 elements appears to  be  considerably affected  by  the amount in  which
 these are present in the food; and by the degree in which the quantity
 ingested is  consumed by the respiratory process.  Thus, a low external
 temperature,  and out-door exercise, by increasing the  production of
 carbonic acid from the lungs,  occasion the consumption  of the oleagi-
 nous and saccharine  matters,  which might  otherwise  pass into the
 milk, and thus diminish the amount of cream.   On the other hand, exer-
 cise favors the secretion of caseine; which would  seem  to shoAV, that
 this ingredient is derived from the disintegration of  the azotized tissues.
 Thus in Switzerland, the cattle which pasture  in exposed situations, and
 which are obliged to use a great deal of muscular exertion,  yield a very
 small quantity of butter, but  an unusually large proportion of  cheese;
 yet the same  cattle, Avhen stall-fed,  give a large quantity  of butter, and
 very little cheese.
   835.  The Milk first secreted after parturition,  knoAvn  as the Colos-
 trum, is very  different  from ordinary milk,  and possesses  a strongly-
 purgative action, which is useful in  clearing  the bowels of the infant,
 from the various secretions Avhich have accumulated in them at birth,
 constituting the meconium.   The Colostrum,  when examined Avith the
 Microscope, is found to contain a multitude of large yellow granulated
 corpuscles ; each of which seems composed of a number of small  grains
 aggregated  together.  The Colostric character is sometimes retained
 for some time after birth, and severely affects the health of the infant.
 This may happen without any peculiarity in  the  ordinary characters
 of the secretion, Avhich has all the appearance of healthy milk ; but the
 Microscope at once detects the difference, by the presence of the colos-
 tric corpuscles.
   836.  The formation of this  Secretion is influenced by the Nervous
 system,  to a  greater degree,  perhaps,  than that  of any other.   The
 process may go  on continuously, to a slight  degree,  during the Avhole
 period of lactation; but it is  only  in animals that have  special reser-
 voirs for the purpose, that any accumulation  of   the fluid  can take
 place.   In the Human female,  as Ave have seen, these are so minute as
 to hold but a  trifling quantity  of milk ; and the  greater part of the
 secretion is  actually  formed whilst the child  is at the  breast.   The
 irritation of the nipple produced  by the act of  suction,  and the mental
 emotion  connected with it, concur to produce an  increased flow of
 blood into the  gland,  Avhich is  known to  Nurses as  the draught;  and
 thus the  secretion is for the  time  greatly  augmented.  The draught
 may be produced simply by the  emotional  state of mind, as by the
 thought of the child when absent; and the irritation  of  the nipple may
alone occasion it; but the tAvo influences  usually act  simultaneously. 
472             OF  GENERATION AND DEVELOPMENT.

The most remarkable examples of the influence of such stimuli on the
Mammary secretion, are those in which milk has  been produced by
girls and  old women, and even by men, in quantity sufficient  for the
support of an infant.   The  application of the child to the  nipple in
order to tranquillize it, the irritation produced by its efforts at suction,
and  the strong  desire to furnish milk, seem in the first instance to
occasion an augmented nutrition of the gland, so that it becomes fit for
the performance of its function ; and then  to produce in it that state of
functional activity, the result of which is the production of Milk.
  837.  It is  not only in this way,  that  the Mammary  secretion is
influenced by the condition of the mind;  for it is peculiarly liable to
be affected as  to quality, by the habitual state of the feelings, or even
by their temporary excitement.  Thus a fretful temper  not only lessens
the quantity of  milk,  but makes it  thin  and serous, and gives it an
irritating quality ; and the same effect will be produced for a  time by a
fit of anger.  Under the influence of  grief or anxiety,  the secretion is
either checked altogether, or it is diminished in amount, and deteriorated
in quality.  The secretion is  usually checked altogether by terror ;  and
under the  influence  of violent passion,  it may  be  so  changed  in its
characters, as to produce the most injurious and even fatal consequences
to the infant.   So many instances are noAV on record, in Avhich children,
that have been suckled within a few  minutes after  the mothers have
been in a state of violent rage or terror, have died suddenly in convul-
sive attacks, that the occurrence can scarcely  be set doAvn as  a mere
coincidence ; and certain as  we are  of the deleterious effects of less
severe  emotions  upon the properties  of the milk,  it does not  seem un-
likely that, in these cases, the bland nutritious fluid should be  converted
into  a poison of  rapid and deadly operation.
  838.  Of the quantity of Milk ordinarily secreted by a  good Nurse,
it is impossible to form any definite  idea ;  as the amount which can be
artificially drawn, affords no  criterion of that which is ordinarily secreted
at the time of the draught.   The quantity Avhich can be squeezed from
either breast at  any  one time, and,  which, therefore, must have been
contained in its  tubes and reservoirs, is about two ounces.  The amount
secreted Avill depend upon several circumstances; such as the nature
and amount of the ingesta; the state of bodily health; and the condi-
tion  of the mind.  An adequate but not excessive supply of  nutritious
food, in which the farinaceous, oleaginous, and  albuminous  principles
are duly  blended;  a vigorous but  not plethoric constitution, regular
habits,  and moderate  exercise, together with  a  cheerful and tranquil
temper,  altogether  produce   the  most beneficial  influence   upon the
secretion.   It is seldom that stimulating liquors, which are so commonly
indulged in, are anything but prejudicial;  but the unmeasured  con-
demnation of  them,  in which some writers have  indulged, is  certainly
injudicious ; as experience amply demonstrates the improvement  in the
condition both of mother and infant, Avhich  occasionally  results from
the moderate employment of  them.—In the administration of  medicines
to the  mother, it is very desirable that the tendency of soluble saline
substances to pass into the milk, and thus  to affect the  child,  should be
borne in mind.  The  vegetable substances used in medicine seem to 
              GENERAL FUNCTIONS OF  NERVOUS SYSTEM.          473

have much less  disposition to pass off by this secretion; and they are
consequently  to be preferred during lactation.
  839.   From the close  correspondence which exists between the ele-
ments of the Milk and those of the Blood, it is evident that we cannot
expect to trace the  existence of the former, as such, in the circulating
current.   It is interesting, however,   to remark,  that  a preparation
appears to be  taking place in the laboratory  of the system, for the pro-
duction of this  secretion, long before the period of parturition.   The
Urine of pregnant Avomen  almost  invariably contains  a peculiar  sub-
stance termed kiestine, which  is nearly related to  caseine, and which
disappears from the urine as soon as  lactation  has fully  commenced.
It would seem, therefore, that a  compound  of this  nature is in course
of preparation  during pregnancy;  and that  it is  eliminated  by the
kidney, until  the  Mammary Gland is prepared for  the active perform-
ance  of  its  functions.—That the  kidney may relieve  the system from
the  accumulation  of other constituents  of  the mammary secretion,
appears  from  a case recently  put on record; in which the urine of a
parturient female, who did not suckle her infant, was  found to contain
a considerable quantity  of butyric acid, during several days.   There
can be no doubt that, in ordinary states of the system, this secretion can-
not be required for the depuration of the blood, since it does not occur
in the male at all,  and is present in the female at particular times only.
But these facts afford ground to believe  that, when the  process is going
on, certain products are generated in the system, Avhich are not found
there at other times.   And it is quite certain that the  sudden checking
of the secretion, or the reabsorption of the fluid already  poured out,
occasioning  an accumulation of these substances in the circulating cur-
rent,  may give rise to very  injurious consequences.   Some very curious
instances are on record, in which a transference of the secreting power
to some other  surface has taken place under such circumstances; so as
to relieve the system from the accumulation in question.
                          CHAPTER XII.

             OF THE NERVOUS  SYSTEM AND ITS ACTIONS.

    1.  General View of the operations, of which the  Nervous System is the
                             instrument.

   840.  We have now considered  the  entire series of those  operations,
Avhich make up the vegetative or organic life of the Animal; including those
functions by Avhich the germ is prepared, by which it is nourished until
it can be left to its OAvn powers, by which its continued development is
effected until the  fabric characteristic of the  adult  has  been built up,
and by Avhich the normal constitution is maintained through a length-
ened period,—so long as the  necessary materials  are supplied, and no 
474          OF THE NERVOUS SYSTEM AND ITS ACTIONS.

 check or hindrance is interposed, by external influences, to that regular
 sequence of changes, on which  the continuance of its poAvers depends.
 In  this survey it will have  been perceived, that the essential parts of
 these operations are, in Animals as in  Plants, completely independent
 of the influence of  that which  constitutes  the peculiar endowment of
 Animals; namely, the Nervous System.
  a.  The Reduction of the  food in the Stomach, by the solvent power
 of the gastric fluid, is a purely chemical operation, with which the Ner-
 vous  System has  nothing whatever to do, excepting  that  it perhaps
 accelerates the process, by stimulating the Muscular coat of the stomach
 to that  peculiar series of contractions,  which keeps the contents of  the
 cavity in continual  movement,  and  favors the action  of  the solvent
 upon it.
  b.  In the process of Absorption, by which the nutritive materials,
 with other substances, are introduced into the vessels, the Nervous Sys-
 tem has no participation ; this being a purely vegetative operation, partly
 dependent upon  the simple  physical conditions which produce Endos-
 mose, and partly on a process of cell-groAvth.
  c.  The Assimilation of the neAV material, effected, as we have seen
 reason  to  believe, by another set  of independent cells, can  receive  but
 little influence  from the Nervous  System, and  is obA'iously  capable of
 taking place without its aid.
  d.  The  Circulation of the Blood, again, though  dependent in part
 upon the impulsive poAver of a Muscular organ, the heart, is not on that
 account brought into closer dependence upon the Nervous System; for
 we  have seen that  the contractions of the heart result from  its own
 inherent poAvers,  so  as to  continue after it has been completely detached
 from the body ;  and that the capillary poiver, which is the chief agent
 in the movement of  the blood in the loAver animals, and Avhich exerts an
 important subsidiary action in the higher, is the result of the exercise
 of certain affinities betAveen  the  blood  and the surrounding tissues, in
 which the Nervous System can have no immediate concern.
   e.  The act of Nutrition, in which every  tissue draAvs from the circu-
 lating blood the  materials for its  own continued growth and develop-
 ment, and by which it incorporates these with its own substance, is but
 a continuance of the same kind of operation  as that which  takes place
 in  the  early  development  of the embryo, long  anteriorly  to the first
 appearance of the nervous  system,—namely, a process of cell-develop-
 ment and metamorphosis, Avhich must be, from its very nature, indepen-
 dent of Nervous agency.
  /.  The same may be said of the Secreting operation  in general;   for
 this  essentially consists in the separation of certain products from the
 blood, by  cells situated upon free surfaces;  Avhich  thus remove those
 products from the interior of the fabric.
   g. And the interchange of oxygen  and carbonic acid, which takes
 place between the atmosphere and the  venous blood, Avhen brought  into
 mutual relation in the lungs, and which is the essential  part of the func-
 tion of Respiration, is an operation of a merely physical character, with
 Avhich the Nervous system can have no direct  concern.
   h. Finally, the  development of the " sperm-germ-cells" in the  one 
GENERAL FUNCTIONS OF NERVOUS  SYSTEM.          475
 sex, and of the ova containing germ-cells in the other, the subsequent
 fertilization of the latter by the former, and the changes consequent
 upon that act, together making up the  function of Generation, may be
 all regarded as modifications  of the ordinary Nutritive processes; and
 are effected, like these, by the  inherent powers of the parts concerned
 in them, at the expense of the materials  supplied by the blood, without
 any direct dependence upon the Nervous system.
  ^ 841.  Still, although the various processes, which make up the essen-
 tial part  of the nutritive operations, in Animals as in Plants, are no
 more dependent on any peculiar influence derived from a Nervous sys-
 tem, in  the former, than they are in the latter, it must  be evident, from
 the details already given, that there must be in Animals various acces-
 sory changes, which are requisite for the continuance  of the former, and
 which can only be effected by  the peculiar powers with which  Animals
 are endoAved.—Thus, to commence with  Digestion:  this  preliminary
 process, Avhich the nature of  the food of the plant renders unnecessary
 for its maintenance, can only  be accomplished by the introduction of the
 food into a cavity or  sac, in which  it may be submitted to the action of
 the solvent fluid.   The operation  of grasping and swallowing the food,
 wherever it is performed, is  accomplished through the agency of the
 Nervous system ; and if it be checked by the loss of Nervous power, the
 Digestive process must cease for want of material.—So, again, although
 interchange  of gaseous  ingredients betAveen  the  atmosphere  and the
 circulating fluid may  take place with sufficient energy in Plants and the
 lower Animals, through the mere exposure of the general surface to the
 atmosphere, yet we find that, in all the higher Animals, certain move-
 ments are requisite, for the continual reneAval of the air or water Avhich
 are in contact with one side of the respiratory surface, and of the blood
 which is in relation with the other: for the direction of which move-
 ments a  Nervous system is requisite.—In the excretory  processes, more-
 over, the removal  of the effete matters  from the body can  only be
 accomplished, in  the  higher Animals, by certain combined movements;
 the object of which is, to take up the  products that  are separated by
 the action of the proper secreting cells, and to carry them to the exte-
 rior of the body,  there to be set free;  and these combined  movements
 can only be effected by the agency of the Nervous system.—Lastly, in
 the act of Reproduction, the arrangement of the sexual organs in Animals
 requires  that  a certain  set of movements should be  adapted to bring
 together the contents of the "sperm-cells" of the male, and of the "germ-
 cells" of the female;  and also for the expulsion of the  ovum from the
 body of  the latter,  in a state of more  or less advanced development.
 For these movements  a special arrangement is made, in the construction
 of the Nervous system, and in the application of its peculiar poAvers.
  842. Thus we see that, although the Organic functions of the Animal
 are essentially independent of the Nervous System, this system affords
 the conditions  which  are  requisite for their continued maintenance;
 being the instrument whereby the muscles are called into action for the
performance of the  various combined actions, that constitute  the mecha-
nism (so  to speak) by which the Vegetative part of the fabric is com-
bined with the Animal portion of the organism.  We are not to suppose, 
476          OF THE NERVOUS SYSTEM  AND ITS ACTIONS.
hoAvever, that every movement which takes place in the Animal body is
dependent upon the Nervous System ; for we have seen that the Muscu-
lar tissue may be employed to perform  contractions excited by stimuli
applied to itself, and that it may thus  execute a set of movements in
which the nervous system has no direct participation. And it is desirable
that the Student should observe, that these are, in all instances, those
most directly connected with the Vegetative functions, and, at the same
time, those of the simplest and most straightforward character.—Thus,
the peristaltic movement, by which the  alimentary and foecal matters
are propelled along the Intestinal tube, results from the direct excite-
ment of the contractility of its muscular wralls, and is entirely indepen-
dent of Nervous agency;  and this movement is  accomplished by the
successive  contraction of the different fasciculi surrounding the tube,
which take up  (as it were)  each others'  action (§ 352).   So  again, the
successive  contractions  and  dilatations of  the  cavities  of the Heart,
which perform so important a part in the Circulation of the blood, are
the result of  the properties inherent in that organ; the muscular fibres
of which are excited to  a  peculiar rhythmical and consentaneous con-
traction, by the flow of  blood into the cavities  when dilating.  More-
over, in the Excretory ducts of various glands, we find a Muscular coat,
by which the fluids  secreted in the  glands are  propelled towards their
outlet on the exterior of the body, or on one of its free internal surfaces.
  843.  In these instances, then, we observe that the simple Contractility
of Muscular  structure, excited by direct  stimulation, is applied to effect
the movements most closely connected with the Organic functions. With
the processes, therefore, which take place in the penetralia of the system,
the Nervous  System has no direct concern.  Its office is to guard the
portals for entrance and exit; and to fill those chambers, which admit
the new materials from  the external world; or to empty the receptacles,
which collect from the interior of  the system the effete matters that are
to be cast out from  it.   And we find that, for these offices, the Nervous
system is  employed in its very simplest mode of operation ;—that which
does not involve  Sensation, Intelligence, Will, or  even  Instinct (in the
proper sense  of that term), but which may take place independently of
all consciousness,—by the  simple reflexion of an  impression, conveyed
to a ganglionic centre by one set of fibres proceeding towards  it  from
the circumference, along another set which  passes  from it to the  mus-
cles, and calls them into operation (§  394).   This reflex function, there-
fore, is the simplest application  of the Nervous System in the Animal
body.   We shall presently see  reason to believe, that a very large pro-
portion of the movements of many of the lower animals are of this reflex
character; and that they are not necessarily accompanied by sensation,
although this may usually be aroused by the same  cause which produces
them.   As we  rise, however, in the scale of Animal existence,  we  find
the reflex movements forming a smaller and smaller proportion of the
whole;  until, in Man, they constitute so limited  a part of the entire
series of movements of which the Nervous system is the agent, that their
very existence has been overlooked.
  844.  But the main purpose of  the Nervous System is to serve as the 
         DEPENDENCE OF MENTAL ACTIONS UPON SENSATIONS.     477

instrument  of the Psychical* powers, which are  the distinguishing
attribute of the Animal.   It  has been already pointed out, that the pos-
session of Consciousness (or  of the capability of  receiving sensations),
and the power of executing Spontaneous Movements (that is, movements
which are not immediately dependent upon external stimuli),  constitute
the essential features in which the Animal differs from the Plant.  All
the other differences in  structure, that respectively characterize these
two classes of living beings,  are subordinate to this one  leading distinc-
tion,—the presence of a Nervous system,  and  of  its peculiar attributes
in the one,—and  its  absence in  the  other.  Noav when Ave attempt  to
analyze  those peculiar attributes, we may resolve  them, like the pro-
perties of the material body,  into different groups.   We find that the
first excitement of all mental changes, whether these  involve  the action
of the feelings  or of the reason, depends upon sensations;  which are
produced by impressions made upon  the nerves of certain parts of the
body, and  are  conveyed  by these to a particular  ganglionic centre,
which is termed the  sensorium,—being the part in which Sensation,  or
the capability of feeling external impressions, especially resides.
   845. Noav there are numerous actions,  especially  among  the  lower
Animals, which  seem to  be  as far removed from the influence of the
Will,  and as little directed  by Intelligence, as the Reflex movements
themselves;  but  which,  nevertheless, depend upon  sensation for their
excitement.   The sensation may immediately direct the  movement, and
may call the muscular apparatus into  action in  such a  manner, as, with-
out any calculation of consequences, any intentional adaptation of means
to ends, any exertion of the reason,  or  any employment of a discrimi-
nating Will,  to  produce an  action, or train of actions,  as directly and
obviously adapted to  the Avell-being  of the individual, as we  have seen
those  of the reflex character to be.  Of this we  have an  excellent
example in  the  act  of Sneezing; the purpose of Avhich is obviously  to
expel  from  the  nasal passages those irritating matters,  the sense  of
whose presence  excites the complicated  assemblage of muscular move-
ments, concerned in the operation.  This class of actions may be  appro-
priately termed  the Consensual; and under it Ave may  include most  of
those  purely instinctive  actions  of  the loAver animals, Avhich,  being
prompted by sensations, cannot be assigned to the reflex group.   These
seem  to make up, with  the reflex,  nearly the whole of the Animal
functions in  many tribes;  but  they are found  to  be  gradually brought
under the domination of  the Intelligence and Will, as we  rise towards
Man,  in whom  these faculties are most strongly developed, so  as  to
keep the Consensual as well as the Reflex actions quite in subordination
to the more  elevated purposes of his existence.—Closely allied, however,
to these, are the purely Emotional movements; in which the sensation
excites a mental  feeling  or  impulse, that  reacts  upon the  muscular
system  without  giving  rise  to  any distinct  idea,  and consequently
without having  called the intellect  and will  into  exercise.   In fact,
these  emotional movements  are  often performed  in opposition  to the

  * This term, derived from the Greek -ji/;^, is used to designate the sensorial and
mental endowments of Animals, in the most comprehensive acceptation of those terms. 
478         OF THE NERVOUS SYSTEM  AND ITS ACTIONS.
strongest  efforts  of the will  to  restrain them;  as when laughter is
provoked  by some ludicrous sight or sound, or by the remembrance of
such, at an unseasonable time.  It is probable, from the  strong  mani-
festations of emotion  exhibited  by  many of the lower animals, that
some of  the actions which  Ave assemble under the general designation
of instinctive, are to be referred to this group.
  846. There are many sensations, however,  which do not thus imme-
diately give rise  to muscular movements;  their operation being rather
that of stimulating to action the Intellectual powers.  There can be
little doubt that all Mental processes are dependent, in the first instance,
upon Sensations,  which serve to  the Mind the same  kind of purpose
that food and air fulfil  in  the  economy of the body.   If  we could
imagine  a being  to come into the Avorld with its mental faculties fully
prepared for action, but destitute of  any power of receiving sensations,
these faculties would never be aroused from the condition in Avhich they
are  in profound  sleep; and  the  being must remain in a state of com-
plete unconsciousness,  because there  is nothing of which it can be made
conscious, no kind of idea which  can be aroused within it.  But after
the mind has once been in active  operation, the destruction of all future
power of  receiving sensations would  not reduce it again to the inactive
condition.   For sensations are so stored up in the mind by the power
of Memory, that  they may give rise to  ideas at any future  time;  and
thus the  mind may feed, as  it were,  upon  the past.   Now  the ideas
which are excited by sensations, and which are colored by the state of
Feeling which accompanies them, become the  subjects  of  Reasoning
processes more or less complex,  sometimes of the utmost brevity and
simplicity, sometimes of the  most refined  and intricate nature.   These
reasoning processes, when they result in a determination to execute a
particular movement,  execute that movement  by an  act  of Volition;
the peculiar character  of which is that it is the expression of a definite
purpose, of a designed adaptation of means to ends, on the part of the
individual performing it, instead of being the result of the mere  blind,
indiscriminating  impulse, which  seems  to be  the mainspring  of the
instinctive operations.  It is in  Man  that we find the highest develop-
ment of  the  reasoning faculties; but it is quite absurd to limit them to
him, as some have done, since  no  impartial observer can doubt that
many of the lower animals can execute reasoning processes, as complete
in their way as those of Man, though much more limited in their scope.
   847.  Thus, then, wTe have  to consider the Nervous system  under four
heads;—first, as  the instrument of the Reflex actions;—second, as the
instrument of the Consensual actions;—third,  as the instrument of the
Emotional actions;—and fourth, as the instrument of the Intellectual
processes and of Voluntary movements.  There is reason  to believe
that the Nervous Centre from which the muscles derive their impulse to
contract is the same, whether the movement be prompted by  an impres-
sion which does  not  excite  the  consciousness, by a  sensation, by an
emotion, or by a  volition; and that this instrument may be played upon,
.so to speak, by other centres, which minister to these functions respec-
tively.   In order  that the  relations of the component  parts of the
Nervous  apparatus may be  better  understood,  it will be desirable to 
NERVOUS SYSTEM OF  RADIATA.
479
  take a brief survey of its comparative structure in the principal groups
  ot Animals ; and to inquire what actions may be justly attributed to its
  several divisions in each  instance,—commencing with those in which
  the  structure is  the simplest, and  the variety of  actions  the smallest;
  and passing on gradually to those in which the structure is increased in
  complexity by the addition of new  and distinct parts, and in which the
  actions present a corresponding variety.

         2.  Comparative Structure and Action of the Nervous System.

    848.  From what has been  already said (§ 373-9) of the  characters
  ot the two elementary forms of the Nervous tissue, it is evident that no
  JNervous system  can exist, in which both these  forms should  not be
  present.   We look, therefore, for  ganglia composed  of  the vesicular
  nervous substance, and serving as  the centres of nervous power; and
  lor cords or trunks, composed of the tubular substance, and  serving to
  communicate between the ganglia and the parts with which they are to
  be functionally connected.   Now it is quite certain that, at present, no
  such Nervous apparatus can be detected in many of the lowest Animals;
  and  some Physiologists have had recourse to the supposition of their
  possessing  a "diffused" nervous system; that is, of  their  possessing
  nervous particles, in  a  separate  form, incorporated, as it were, with
  their tissues.  But we have seen that each tissue possesses its own pro-
  perties,  and can perform its own  actions independently of the rest;__
  that  even the contractility of Muscular fibre is by  no means dependent
  upon the Nervous system, though usually called into play through its
  means;—and that the simplest office of a Nervous  System  is  to pro-
  duce  a  muscular movement in respondence  to a  certain impression;
 which action requires that it  should  have an internuncial or commu-
 nicating poAver, only to be exercised (so far as we at present  know) by
 continuous  fibres.  _ The  apparent  absence of a  Nervous  system is
 doubtless to be attributed, in many instances,  to the general softness of
 the tissues of the  body, which  prevents it from being clearly  made out
 among them.  And it is  to  be  remembered, that,  on the  principles
 already  stated,  we  should expect to find it  bearing a much smaller
 proportion  to the  entire structure,  in  the  lowest  Animals,  whose
 functions are chiefly Vegetative,—than  in the highest, in which the
 vegetative  functions  seem  destined merely  for  the development  of
 the Nervous and  Muscular systems, and  for  the  sustenance of  their
 powers.
   849. Among the Radiated  classes, the parts of Avhose bodies are
 arranged in a circular manner around the  mouth, and repeat each other
 more  or  less  precisely, the Nenrous system presents a  corresponding
 form.  In  the Star-fish, for  example, Avhich is  one of the highest  of
 these  animals, it forms  a ring, which surrounds the mouth ;  this  ring
 consists of  nervous  cords, which form  communications  between the
 several ganglia, one  of which is placed at the base of each ray.   The
number of these  ganglia corresponds with that of  the  rays  or  arms •
being five in the common  Star-fish ; and from nine to fifteen, in the"
species possessing those several numbers of  members.   The ganglia 
480          OF THE  NERVOUS SYSTEM  AND  ITS  ACTIONS.
 appear to be all similar to one another in function, as they are in the
 distribution of their branches ;  every one of them sending a large trunk
 along its OAvn ray, and tAvo small filaments to the organs  in the central
 disk.   The rays being all so similar in structure, as to be exact repeti-
 tions of each other, it would appear that none of the  ganglia can  have
 any controlling power over the rest.  All the rays (in certain species)
 have at their extremities what  seem  to be very imperfect eyes ;  and so
 far  as  these  can  aid  in directing the movements of the animal,  it ia
 obvious that they will do so towards all sides  alike.   Hence there is no
 one part, which corresponds to the head  of  higher animals ; and the
 ganglia of the nervous system,  like the parts they supply, are but repe-
 titions  of one another, and are capable of acting  quite independently.
 Each would perform its own individual functions if separated from the
 rest; but, in the entire animal, their actions are all connected with  each
 other by the circular cord,  which  passes from every one of the five  gan-
 glia to  those on either side of it.  We shall find that, in Articulated
 and Vertebrated animals, there is a similar repetition of  corresponding
 ganglia on  the  two sides  of the  median plane of the body;  and that
 these are  connected by transverse bands, analogous  in function to the
 circular cord of the Star-fish.   Moreover,  we shall see a like repetition
 of ganglia, almost or precisely similar in function, in passing from one
 extremity of these animals to  the other;  and these  ganglia  are  con-
 nected  by longitudinal cords, whose function is in like manner commis-
 sural.—From the  best judgment wre can  form of  the actions  of  the
 Star-fish,  by comparing them with the corresponding  actions of higher
 animals, we may fairly regard the  greater number of them as simply re-
flex ; being  performed in  direct  respondence to external  stimuli, the
 impression made by which is propagated to one or more of the ganglia,
 and excites in them a motor impulse.  How far the movements of these
 animals are  indicative of  sensation, we have  not  the power of deter-
 mining ; but it may be safely affirmed, that they afford no indication of
 the exercise of reasoning faculties or of voluntary power.
   850.  Perhaps the  simplest form  of a Nervous  system is  that  pre-
 sented by  certain of the loAver Mollusca;  for the body not here  pos-
 sessing any repetition of similar parts, the  nervous system is destitute
 of that multiplication of ganglia which we see in the Star-fish; whilst
 the limited nature of the animal  powers involves a corresponding  sim-
 plicity  in  the  integral  parts  of  their  instrument.   The  animals, to
 which reference is  here made,  form the class Tunicata, Avhich is inter-
 mediate, in many respects, between the ordinary Molluscs and the Zoo-
 phytes.  They consist essentially of an external membranous bag or
 tunic,  within Avhich is a  muscular envelope,  and  again  Avithin  this a
 respiratory sac,  which may  be considered as the -dilated pharynx of the
 animal.  At the bottom of this last, is the entrance to the  stomach;
 which,  with the other viscera,  lies at the lower end of  the  muscular
 sac.  The external  envelopes  have  two orifices ;  a  mouth,  to admit
 water into the pharyngeal sac  ; and an  anal  orifice, for the  expulsion
 of the water which has served for respiration, and of that which has
 passed  through  the alimentary canal, together with  the faecal  matter,
 the ova, &c.   A current of water is  continually being drawn into  the 
NERVOUS SYSTEM OF  MOLLUSCA.                481
  pharyngeal sac, by the  action of the cilia that line it;  and of this, a
  part is driven  into the stomach, conveying  to it the necessary supply
  of  aliment in  a  very finely-divided state;  Avhilst a part  is destined
  merely for the aeration of the  circulating fluid,  and is  transmitted
  more  directly  to  the anal orifice,  after  having  served that purpose.
  These animals  are for the most part fixed to one spot, during all but
  the earliest period of their existence; and they give  but  little external
  manifestation of life, beyond  the continual entrance  and  exit of the
  currents already adverted to, which  being effected by ciliary action, is
  altogether independent  of the nervous system  (§  234).   When any
  substance is^ drawn in by the current, however, the entrance of which
  would be injurious, it excites  a general contraction of  the mantle or
  muscular  envelope;  and this causes a jet of water to issue  from one or
  both  orifices, which carries  the  offending body to a distance.   And,
 in the  same  manner,  if the  exterior  of the body be touched,  the
 mantle suddenly and violently contracts, and expels the contents of the
 sac.
   851.  These are  the only actions, so far  as we know, which the Ner-
 vous system of these animals is destined to perform.  They do  not ex-
 hibit the least trace of eyes,  or of other organs  of  special  sense; and
 the only parts that appear peculiarly  sensitive, are the small tentacula,
 or feelers, that  guard the oral orifice.   BetAAreen  the tAvo apertures in
 the  mantle, Ave  find a solitary ganglion, which receives  branches from
 both orifices, and sends  others over the muscular sac.  This, so  far  as
 we knoAv at present, constitutes the whole nervous system of the animal;
 and it is fully sufficient to account for  the  movements which have been
 described.   For  the impression produced  by the  contact of any hard
 substance with the  tentacula, or with the general surface of  the mantle,
 being conveyed  by the afferent fibres of this ganglion, will  excite in it
 a reflex motor impulse ; which, being transmitted to the  muscular fibres
 of the contractile sac, as well as to those circular  bands that surround
 the orifices and  act as sphincters, will produce the movements in ques-
 tion.
   852. In the Conchifera, or Molluscs inhabiting bivalve shells, there
 are invariably two  ganglia, having different functions.   The larger  of
 these (Plate II., Fig. 1, e), corresponding to the single ganglion of the
 Tunicata, is situated towards  the posterior  end of the body (that is the
 end most distant from the mouth), in  the neighborhood of the posterior
 muscle that draws the valves together; and its branches are  distributed
 to that muscle, to the mantle, to the gills (d, d), and to the  siphons  (e,
 e)> by which  the water  is introduced and carried  off.   But we find
 another  ganglion, or rather pair of ganglia (a, a),  situated  near the
 front of  the .body,  either upon the oesophagus, or at its  sides;  these
 ganglia are connected with the very sensitive tentacula  which guard
 the mouth; and  they may  be regarded  as presenting the first approach,
 both in position  and functions, to  the  brain of higher animals.  In  the
 Oyster, and others of the lower Conchifera, which have no  foot, these
are the only principal ganglia: but in those having a foot,—which is a
muscular tongue-like organ,—we  find an additional  ganglion (b) con-
nected with it.   This is the case in the Solen, or animal of the Razor-
                                 31 
482          OF THE  NERVOUS SYSTEM AND ITS  ACTIONS.
shell; whose foot is a very powerful boring instrument, enabling it to
penetrate deeply into the sand.—Here,  then, we have three distinct
kinds of ganglionic centres; every one  of which may be  doubled, or
repeated on the tAvo sides of the body.  First, the cephalic ganglia, a, a,
which are probably the sole instruments of sensation and of the consen-
sual movements; these are almost invariably double, being connected
together by a  transverse band, which arches over the oesophagus.  Se-
cond, the pedal ganglion, b, which is usually single, in conformity with
the single character  of the  organ  it supplies; but in one  very rare
Bivalve Mollusc,  the foot is double, and  the  pedal ganglion  is  double
also.   Third,  the respiratory ganglion, c, which  frequently presents a
form that indicates a partial  dhrision  into .two halves, corresponding
with  the  repetition of the organs it supplies  on the two  sides of the
body.   Besides these principal centres, Ave meet  with numerous smaller
ones upon the nervous cords (/, /, and g, g,) which proceed from them
to the different parts of the general muscular envelope or mantle.
   853.  Now it will be observed, that the  two cephalic ganglia a, a, are
connected with the pedal ganglion, b, by  means of a pair of trunks pro-
ceeding from the  former to the latter; and that  they are, in like man-
ner, separately connected with the respiratory or branchial  ganglion, c.
There is good  reason to believe, that the pedal  and branchial ganglia
minister to the purely reflex action of the organs they respectively sup-
ply ; and that they would serve this purpose as well, if altogether  cut off
from connexion with the cephalic ganglia: whilst the  cephalic, being the
instruments of the actions which are called forth by sensation, exert a
general control and direction over the movements of the animal, through
the medium of the trunks by Avhich they communicate with the ganglia
in immediate  connexion with  the muscular apparatus.   It is difficult,
however, to make satisfactory experiments upon this subject in these
animals, their movements being for  the most part slow and feeble,  and
their nervous  system not readily accessible;  and our idea of the  re-
spective functions of their ganglia is chiefly founded upon  the  distribu-
tion of  their nerves, and upon the analogous  operations of the ganglia
that correspond to them in other animals.
   854.  In ascending through the series of the Mollusca, we find  the
Nervous system increasing in  complexity, in accordance with the gene-
ral organization  of the  body:  the  addition  of new organs  of  special
sensation,  and of new  parts  to  be  moved by  muscles, involving  the
.addition of new ganglionic  centres, Avhose functions  are  respectively
adapted to these purposes.   But we  find no other multiplication of
similar centres, than a doubling on the two sides of the body; excepting
in a few cases, where the organs they supply are correspondingly mul-
tiplied.   We have a very characteristic  example of this in the arms of
;the €uttle-fish, which are  furnished  with great numbers of contractile
suckers, every one possessing a ganglion  of its own.  Here we can trace
very clearly the  distinction between the reflex actions of each individual
sucker, depending upon the powers of its own ganglion; and the move-
ment prompted by sensation which results from its connexion with the
cephalic ganglia.  The nervous trunk, which proceeds to each arm, may
be distinctly  divided into two tracts ; in  one  of which are contained 
NERVOUS SYSTEM OF  MOLLUSCA.
483
 the ganglia which peculiarly appertain to the  suckers, and which are
 connected with them by distinct filaments; whilst in the other there is
 nothing but fibrous  structure, forming a direct  communication between
 these and the cephalic ganglia; so that each sucker has a separate rela-
 tion with a ganglion of its own, whilst all are alike connected with the
 cephalic ganglia, and are placed under their control.   We see the results
 of this arrangement,  in the modes in which the  contractile power of the
 suckers may be called into operation. When the animal embraces any sub-
 stance with its arm (being directed to this action by its sight or other sensa-
 tion) it can bring all the suckers simultaneously to bear upon it; evidently
 by a voluntary or instinctive impulse transmitted along the  connecting
 cords that proceed from the cephalic ganglia to the ganglia of the suckers.
 On the other  hand, any individual  sucker may be made to contract and
 attach  itself  by  placing a substance in contact with it alone;  and this
 action will take place equally well when  the arm is separated from the
 body, or  even in a small piece of  the arm when recently severed from
 the rest,—thus proving that, when it  is directly excited by an impres-
 sion made upon itself, it is a reflex  act, quite independent of the cephalic
 ganglia, not involving sensation, and taking  place through the medium
 of its OAvn ganglion alone.
   855.  In the  Molluscous classes, generally speaking, the  Nervous
 system bears  but a  small  proportion to  the whole mass of the body;
 and  the part of  it which  ministers to the general movements of the
 fabric, is  often small  in  proportion to those which serve  some special
 purpose, such  as the actions  of respiration.  This is what  we should
 expect from the  general inertness of their character, and from the small
 amount of muscular structure which they possess.   On the other hand,
 in the Articulated classes, in which the  locomotive apparatus is highly
 developed, and its actions of the most energetic kind, we find the Ner-
 vous system almost entirely subservient  to this function.   In its usual
 form, it consists  of a chain of ganglia,  connected by a double cord;
 commencing  in  the head, and  passing  backwards  through the  body
 (Plate II., Fig.  2).   The ganglia, though they usually appear single,
 are really double; being composed of two equal halves, sometimes closely
 united on the  median line, but occasionally remaining separate, like the
 cephalic  ganglia  of the Solen (Fig. 1, a,  a),  and being united together
 by a transA'erse  commissural trunk.  In like manner, the longitudinal
 cord, though really double (as seen in the upper part  of Fig. 2), often
 appears to be  single,  in consequence of  the  close approximation of its
 lateral halves  (as in  the lower  part  of Fig. 2).  In general we find a
 ganglion in each segment; giving off nerves to  the muscles of the legs,
 as in Insects,  Centipedes, &c;  or to the muscles that move the rings of
 the body, where no extremities are  developed, as in the leech, worm, &c.
 In the  lower Vermiform (or worm-like) tribes, especially in the marine
 species, the number of segments  is  frequently very great, amounting
 even to several hundreds ; and the  number of ganglia follows the same
 proportion.  Whatever be their  degree of multiplication, they seem but
repetitions of  one another; the functions  of each  segment being the
 same Avith those of the rest.  The cephalic ganglia, however, are ahvays
larger  and more important;  they are connected with  the organs of 
484         OF THE NERVOUS SYSTEM AND ITS ACTIONS.
special sense ; and they evidently possess a power of directing and con-
trolling  the  movements of the  entire body; whilst the  power of each
ganglion of the trunk is confined to its OAvn segment.—The longitudinal
ganglionic cord  of Articulata occupies a position which seems at first
sight altogether different from that of the nervous system of Vertebrated
animals;  being found in the neighborhood of the ventral or inferior
surface  of their bodies; instead of lying just beneath  their dorsal or
upper surface.   There  is  reason, however,  for regarding the whole of
the body of these  animals as having an inverted  position ; so that they
may be  considered as really crawling upon  their backs.   On this view,
their longitudinal  nervous tract corresponds with the  spinal cord of
Vertebrata in position, as we shall find that it does in function.
   856.  We shall  draw our chief illustrations of the  structure of the
nervous system in  the Articulated series, from the class  of Insects;  in
which it has been particularly examined.  In these animals the number
of segments never  exceeds twelve (exclusive of the head), either in their
larva, pupa, or imago states; and the  total number of pairs of ganglia,
therefore, never exceeds thirteen, including the cephalic  ganglia.   These,
in the larva,  are nearly equal in size, one to another (Plate II., Fig. 2,
a, and 1-12); the functions of the different segments of the body being
almost uniform ; and the development of the organs of special sense not
being such as to involve any considerable predominance in  the size of
the cephalic  ganglia.   We observe,  at the anterior extremity, the pair
of cephalic  ganglia (a); from which proceeds, on  each  side, a cord of
communication to the first ganglion (1) of the trunk.  This double cord,
with the ganglia above and below, thus forms a ring which embraces the
oesophagus; the cephalic ganglia being situated on the upper side of it,
whilst the ganglionic column of the trunk lies beneath the alimentary
canal along its whole length.   In the Sphinx  ligustri,  or Privet Hawk-
moth, the nervous  system of whose  larva is here represented, the last
two segments of the body are drawn together  as it were, into one; and
instead of  distinct 11th and  12th ganglia, we find but  a single mass
nearly double the size of the  rest, and  obviously formed of the elements
that would have otherwise gone to form the  two.
   857.  When the  structure of the chain of ganglia is more particularly
inquired into, it is  found to consist of tAvo distinct tracts ; one of which
is composed  of nervous fibres  only,  and passes backwards from the
cephalic  ganglia, over the surface of  all the ganglia  of the trunk;
whilst the other  includes the  ganglia themselves.   Hence every part of
the body has tAvo  sets of nervous connexions ; a  direct one with the
ganglion of its own segment,  and an indirect with the  cephalic ganglia.
Impressions made upon  the afferent fibres, which  proceed from any part
of the body to the cephalic ganglia, become  sensations when conveyed
to the latter: Avhilst in respondence  to these, the influence of  sensa-
tions received by  the  cephalic  ganglia, and  operating  through  them,
harmonizes and  directs  the general  movements of the body, by means
of the communicating cords proceeding from them.   For  the  reflex
operations on the other  hand, the  ganglia of the ventral cord are suffi-
cient; each one  ministering to the actions of its own  segment, and, to
a certain extent also, to those of  other segments.  It has been  ascer- 
NERVOUS  SYSTEM  OF ARTICULATA.
485
tained by the careful dissections  of Mr. Newport, that of the fibres con-
stituting the roots, by which the nerves  are implanted in  the ganglia,
some  pass into the vesicular matter of the ganglion, and, after coming
into relation with  its  vesicular substance, pass out again on the same
side (Fig. 152,/,  k);  whilst a second set, after traversing the vesicular
Fig. 152.
  Portion of the ganglionic tract of Polydesmus maculatus:—6, inter-ganglionic cord; c, anterior nerves; d,
 posterior nerves; /. lc, fibres of reflex action; g, h, commissural fibres; i, longitudinal fibres, softened and en-
 larged, as they pass through ganglionic matter.

matter, passes  out by the trunks proceeding from the opposite side of
the same ganglion ; and a third set runs along the portion  of the cord
which  connects the ganglia of different segments, and enters  the ner-
vous trunks that issue from them, at a distance of one or more ganglia
above or below.   Thus it  appears, that an impression conveyed  by an
afferent fibre to any ganglion,  may excite  a motion in the  muscles of
the same side of its OAvn segment; or in those  of  the  opposite side; or
in those  of segments  at a  greater or  less  distance,  according to  the
point at  Avhich  the efferent fibres leave the  cord.
   858.  The  general conformation of Articulated  animals, and  the
 arrangement of the parts of their nervous system, render  them  pecu-
liarly favorable subjects for the  study of  the  reflex actions ;  some of
the principal phenomena of  which will now be described.   If the head
of a Centipede  be cut off, whilst it is in motion, the body will  continue
to move  onAvards by the action of its legs ;  and the same will take place
in the  separate parts, if the body be divided  into several distinct por-
tions.  After these actions have come to  an end, they may be excited
again, by irritating  any part of  the  nervous centres,  or the cut  extre-
mity of the nervous cord.   The body is moved fonvards by  the regular
and successive  action of the legs, as in the natural state; but its  move-
ments are always  fonvards, never backAvards, and are only directed to
one  side Avhen  the  fonvard movement is  checked by an interposed ob-
stacle.   Hence,  although they might  seem to indicate consciousness
and a guiding  will,  they do not so in reality;  for they are  carried on,
as it were, mechanically ;  and shoAv no direction of object, no avoidance
of danger.   If  the body be  opposed in its progress by an  obstacle of
not more than half of its own height, it mounts over it, and moves  directly
onAvards  as in  its natural state  ; but if the obstacle be equal to its own
height its progress  is  arrested, and the cut extremity of the body re- 
486          OF THE  NERVOUS SYSTEM  AND ITS ACTIONS.
mains forced up against the opposing substance, the legs still continuing
to move.—If, again, the nervous cord of a Centipede be divided in the
middle of the trunk, so that  the hinder legs are cut off from connexion
with the  cephalic ganglia, they will continue to move, but not  in har-
mony with those  of the fore  part of the body; being  completely para-
lysed, as far as the animal's controlling  power is concerned;  though
still capable of performing  reflex movements, by the influence of their
own ganglia, which may thus continue to propel the body, in  opposition
to the determination of the animal itself.—The case  is still  more re-
markable, when the nervous cord is not merely divided, but  a  portion
of it is entirely removed from the middle of the trunk ; for the anterior
legs still remain obedient to the animal's control;  the legs of the seg-
ments from which the nervous cord has been  removed, are  altogether
motionless; whilst those  of the posterior segments  continue  to act,
through  the reflex powers of their own  ganglia,  in  a  manner which
shows that the animal has no power of checking or directing them.
  859. The stimulus  to the reflex movements  of the  legs, in the fore-
going cases, appears  to be given by the contact of the extremities with
the solid surface  on which they rest.   In other cases, the appropriate
impression can only be made by the contact of liquid; thus a Dytiscus
(a kind of  water-beetle) having  had its cephalic ganglia removed, re-
mained motionless, so  long as it rested upon a dry surface;  but when
cast into water, it executed the usual swimming  motions with great
energy and rapidity,  striking all its comrades to one side by  its vio-
lence, and persisting in these  for  more  than half  an hour.  Other
movements,  again, may be  excited  through  the  respiratory surface.
Thus, if  the head of  a Centipede be cut  off,  and, while it  remains  at
rest, some irritating vapor  (such as that of ammonia  or muriatic acid)
be caused to enter the air-tubes on  one  side of the  trunk, the body will
be immediately bent in the opposite direction, so  as to  withdraw itself
as much  as  possible from the influence  of the  vapor; if the same irri-
tation be then applied  on  the  other side, the reverse  movement will
take place; and the body may be caused  to  bend  in two or three diffe-
rent curves, by  bringing  the irritating  vapor into  the neighborhood
of different parts  of either  side.  This  movement  is evidently  a reflex
one, and serves to withdraw  the  entrances of the air-tubes  from the
source of irritation; in the same manner as the acts of coughing and
sneezing in the higher animals  cause the  expulsion, from their  air-pas-
sages, of solid, liquid, or gaseous  irritating  matters,  which may  have
found their way into them.
  860. From  these and similar facts  it appears, that the ordinary
movements of the legs and wings of Articulated animals are of  a reflex
nature,  and may  be  effected solely through the  ganglia with which
these organs are  severally  connected;  whilst in the perfect being, they
are harmonized,  controlled, and directed by the  instinctive  guidance,
which depends upon sensations acting through the cephalic ganglia and
the fibres proceeding  from  them.   There is  strong reason to  believe,
that the  operations to which  these ganglia are  subservient, are almost
entirely  of a consensual nature; being immediately prompted by sensa-
tions, chiefly  those of sight, and seldom  involving any processes of  a 
           REFLEX AND CONSENSUAL ACTIONS  OF INSECTS.        487

truly rational character.   When we attentively consider the habits of
these animals, we find  that  their actions, though evidently directed  to
the attainment  of certain ends, are very far from being of the same
spontaneous nature, or from possessing  the  same designed adaptation
of means to ends, as those performed by ourselves, or  by the more in-
telligent Vertebrata, under like circumstances.  We judge of this by
their unvarying  character, the different individuals of  the same species
executing precisely  the same movements  when  the circumstances are
the same; and by the very elaborate nature of the mental operations
which would  be required, in many instances, to arrive at  the same
results by an effort of  reason.  Of such we cannot have a more remark-
able  example, than  is to be found in the operations of Bees, Wasps,
and other social Insects; which construct habitations for themselves,
upon a plan which the most enlightened human intelligence, working
according to the most  refined geometrical principles, could not surpass;
but which yet do so without education communicated by their parents,
or  progressive  attempts of their own, and with  no trace of hesitation,
confusion, or interruption,—the  different individuals of a  community all
laboring effectively for one common purpose, because  their instinctive
or consensual  impulses are the same.
   861.  It is interesting to remark that, in the change from  the Larva
to the  perfect or Imago state  of  the  Insect,  the Cephalic  ganglia
undergo a great increase in size.  (Plate II.,  Fig.  3, a,  a.)   This
evidently has reference to the increased development  of the organs of
special  sense in  the  latter;  the eyes  being much more  perfectly
formed; antennae and  other appendages used for  feeling being evolved;
and rudimentary  organs of hearing  and  smell being  added.   In
respondence  to  the  new  sensations, which  the  animal must  thus
acquire, a  great number of new instinctive actions  are  manifested;
indeed  it may  be  said, that the  instincts of the perfect  Insect  have
frequently nothing  in  common  with those of  the Larva.   The  latter
have  reference to  the  acquirement  of  food ;  the former chiefly relate
to the acts of reproduction, and to the provisions^requisite for the de-
posit and protection  of the eggs and the early  nutrition  of the young.
—We find another  important  change in the nervous  system of the
adult or perfect Insect;  namely, the  concentration of the  ganglionic
matter of the  ventral cord in the thoracic region (e,f);  with the three
segments of which, the three pairs of legs and the tAvo  pairs of wings
are  connected.  The  nine  segments of the abdomen, in the perfect
Insect,  give attachment to no organs of motions, and are seldom  them-
selves very movable;  and  we find that  the ganglia which correspond
with  them have undergone  no  increase in size, but have rather dimi-
.nished,  and have  sometimes almost  completely disappeared.   Where
the last segment, however,  is furnished with  a  particularly  movable
appendage, such as  a  sting,  or an  ovipositor, Ave always find a large
ganglion in  connexion  with it.
   862.  These  ganglia of the  ventral  cord evidently  correspond  in
function with the  pedal ganglion of the Mollusca;  being  so  many
repetitions of it;  in accordance with the number of members.   We
have  noAV to  speak of a system of respiratory ganglia,  which also are 
488         OF THE NERVOUS SYSTEM  AND ITS ACTIONS.
repeated in  like manner, in accordance  Avith the  condition  of the
respiratory apparatus ;  this being diffused  through the Avhole body, in
most of the Articulata,  instead of being restricted  to one spot as in the
Mollusca.  The system of respiratory nerves consists of  a chain of
minute ganglia, lying upon the larger cord, and sending off its delicate
nerves betAveen those  that proceed  from the ganglia of the latter, as
seen in Fig. 2.  These respiratory ganglia and  their  nerves  are best
seen in the thoracic portion of the cord, where the cords of communica-
tion between the pedal ganglia diverge or separate from one  another.
And this is  particularly the case in  the  Pupa state, when the whole
cord is being shortened, and their  divergence is increased.  The tho-
racic portion of the cord, in the  Pupa of the Sphinx ligustri, is shown
in Plate II.,  Fig. 4; where a, b, and  c, represent the 2d, 3d,  and 4th
double ganglia of  the  ventral  cord; d,  d, the  cords  of connexion
between  them, here Avidely diverging  laterally;  and e,  e,  the  small
respiratory ganglia, which  are  connected  with each other by delicate
filaments that pass over the ganglia of the  ventral cord, and Avhich send
off lateral branches, that are distributed to the air-tubes and other parts
of the respiratory apparatus, and communicate with those of the other
system.
  863. Besides the respiratory system of  ganglia and  nerves, there is
in Insects, as in some Molluscs,  a set of minute ganglia, which is espe-
cially connected with the acts of mastication and SAvalloAving, its fila-
ments being distributed to the muscles of the mouth and  pharynx, and
some of its ganglia  being even found on the stomach, Avhere that organ
is remarkable for its muscular powers.  The number and arrangement
of these  ganglia vary considerably in different animals, even in those
of the same group:  but some traces of this distinct  system,  which is
designated as the stomato-gastric, may always be found.   One of the
minute ganglia appertaining to it, and  forming its  anterior termination,
is seen to lie  on the median line,  in front of the great cephalic  ganglia,
in Plate II.,  Fig. 3,  c.  From this a trunk passes backwards along the
oesophagus; which may be  likened to the oesophageal branches of the
Par vagum in Vertebrata.   Two other small ganglia communicating
with this, are seen at d, d.
  864. We are not without  traces,  moreover, among  Invertebrated
animals, of the Sympathetic system of the  higher  classes;  though it is
quite a mistake to compare the entire system of nerves and ganglia in
the former, with the Sympathetic system of the latter, as was formerly
done.  The  chief distribution of the  branches of the Sympathetic of
Vertebrata is upon the walls of the blood-vessels, and upon the muscular
substance of  the heart and alimentary canal; and it  is by the passage
of some  of the filaments,  from the system of  minute  ganglia just,
pointed out, to the dorsal vessel, that  we recognise it as combining the
functions of the Sympathetic  Avith those of the gastric and cardiac por-
tions of the Par vagum.  It will be remembered that there is a frequent
inosculation between these  two nerves, even in the highest animals.
  865. Thus we have seen that, in Invertebrated animals, the Nervous
System  consists of  a series of isolated ganglia, connected  together by
fibrous trunks.  The number of these ganglia, and the variety  of  their 
NERVOUS SYSTEM OF  INVERTEBRATA.
489
 function, entirely depend upon the number and variety of the organs to
 be supplied.  In the lowest Mollusca, the regulation of  the ingress and
 egress of water seems almost the only function  to be performed; and
 here we have but a single ganglion.   In the Star-fish, we have  five  or
 more ganglia; but they are all repetitions one of another, and are  ob-
 viously the centres of action to the several segments to which they
 respectively belong, neither having a  predominance over the rest. And
 in the higher Mollusca, and in Articulata, we have a ganglion, or more
 commonly a pair of ganglia, situated  at the anterior extremity of the
 body, connected with  the organs of special sensation,  and evidently ex-
 erting a dominant influence over the  rest.  In  the lower Mollusca, we
 have but a single ganglion for general locomotion; but  this is doubled
 laterally, and repeated longitudinally in the Articulata, in accordance
 with the multiplication of their  locomotive  organs, so as  to form the
 ventral cord.   In like manner, the Mollusca possess a single ganglionic
 centre for  the  respiratory movements; and  this is repeated  in every
 segment of the Articulata, forming a chain of respiratory ganglia, which
 regulates the actions of the extensively-diffused respiratory apparatus of
 these animals.  The acts of  mastication and  deglutition, again, in both
 sub-kingdoms, are immediately dependent upon a distinct set of gan-
 glionic centres; which are connected,  however, like the preceding, with
 the cephalic ganglia.  And we have further seen, that, wherever special
 organs are developed,  whose operations depend upon muscular contrac-
 tion, ganglionic centres are developed  in immediate relation with  them ;
 so as to  enable them  to  act  by their simple reflex poAver,  as well  as
 under the direction of  the cephalic ganglia; as in the case of the suckers
 of  the  Cuttle-fish.—Now when  we inquire into  the  relation of the
 cephalic ganglia of Invertebrata with the Brain  of the higher  Verte-
 brated animals, we find that these organs cannot be compared in their
 totality; for that the former are the representatives of a certain portion
 only, and that usually but a small one,  of the latter.   The  cephalic
 ganglia of the Centipede, for example,  receive nerve-trunks from the
 eyes, the antennae, and other sensory organs, and give off motor  nerves
 to  the different movable parts  of the head; and the history of their
 development, which has been studied by Mr. Newport, shows that they
 may be considered as the coalesced ganglia of the four segments  of
 which the anterior part of the head  is  composed ; whilst the first sub-
 oesophageal  ganglia are formed by the coalescence of the four segments
 entering into the composition of  the  posterior part of the head.  The
 increased  bulk of the cephalic  ganglia in the higher Articulata,  and
 especially in the perfect Insect, is obviously for the most  part dependent
 upon the increased development  of the  visual apparatus, for we  find it
 everyAvhere  proportional to it; and  thus we may look upon  them as
 mainly optic ganglia, serving to direct  the  actions of the  animal through
 the sense of sight.—There is no part of  these organs which can be con-
 sidered as superadded to the ganglionic masses which are the immediate
 centres  of the cephalic  nerves;  consequently there is  nothing  which
 can be likened either to the Cerebrum or  to the Cerebellum of Verte-
 brata. And the representative of these cephalic ganglia in  the Verte-
brated Encephalon, is  that series  of ganglionic centres  at  the base of 
490          OF THE  NERVOUS SYSTEM AND ITS ACTIONS.
the brain, which constitute (as we shall  presently find) its fundamental
portion, and with  which all  the cephalic nerves  are  immediately con-
nected.
  866.  When we  direct  our attention  to  the  nervous system of the
Vertebrated series, we  perceive that  it differs  from that of the Inver-
tebrated classes we have been considering, in two remarkable features.
In these last it has seemed but as  a mere appendage to the rest of the
system, designed to bring its several parts into more advantageous rela-
tion.   On  the other hand, in the Vertebrata,  the Avhole structure ap-
pears  subservient to it, and designed but  to  carry its purposes into
operation.—Again,  in  the  Invertebrata, we do  not find  any special
adaptation of  the  organs of support for the protection  of  the Nervous
System ; for it is  either enclosed, with the other soft parts  of  the body,
in one general hard tegument, as in the Star-fish  and other Echinoder-
mata, and  in Insects, Crustacea, and other  Articulata; or it receives  a
still more imperfect protection, or  even none at all, as in the Mollusca.
Noav in the Vertebrata, we find a special and complex bony apparatus,
adapted in the most perfect  manner for the protection of  the Nervous
system ; and it is, in fact, the  possession of a  jointed  spinal column,
and of  its  cranial expansion, which best characterizes the group.
   867. The Nervous System of Vertebrata  is  not merely remarkable
for its high development,  relatively to the remainder of the structure:
it is also  distinguished by the possession  of parts, to  which  we have
nothing analogous in the lower tribes; and by the mode in which these
are concentrated  and combined, so  as to  form  one continuous mass,
instead of  consisting of a series of scattered ganglia.—The chief parts
which  are newly introduced (so to  speak) in  this sub-kingdom, are the
Cerebral Hemispheres  and Cerebellum ; of Avhich there are  no  traces
whatever in the loAver Articulata and Mollusca, and but very uncertain
representations in the highest.  These are superimposed, as it  were,
upon the  cephalic ganglia connected with the  organs of special sense,
and upon  the cords that connect them with the first ganglion  of the
trunk.—Again, we find that the locomotive ganglia, which formed the
long knotted cord of the Articulata, are united with the centres  of the
respiratory system, and with  those of the  stomato-gastric system, to
form one continuous tract, which  commences anteriorly from the gan-
glia of special sense, and runs backwards*  without  interruption,  in the
canal of the Vertebral column, forming the spinal cord.  This is a con-
tinuous instead of an  interrupted ganglionic  mass; it is composed  of
two lateral halves, precisely similar  to each other;  and each of these
consists of two parts, as distinct  from each other as the  two tracts  in
the ventral cord of the Articulata,—namely, a  fibrous structure, which
connects every part of  it with the  Encephalon (or collection of nervous
masses within the cranium),  and Avhich also serves  to connect together
the different  parts of the cord itself,—and a  vesicular portion, which
forms  the proper centre of  the greater part,  if not the whole, of the
fibres entering into the roots of those nerves.   The  anterior  portion  of
the Spinal cord, which is prolonged into the cranium,  and comes into
  * When we speak of the Vertebrata generally, their bodies are of course supposed to
be in a horizontal position,—not vertical, as in Man. 
NERVOUS SYSTEM OF  VERTEBRATA.
491
 immediate relation with the Encephalon, is termed the Medulla Oblon-
 gata.   It is in this, that  the  centres of the respiratory and stomato-
 gastric nerves are found;  the situation of these important ganglia within
 the cranium,  being obviously  destined to protect  them from those inju-
■ ries, to Avhich the Spinal Cord itself is liable.
   868. Thus, then, Ave are led  to recognise in the Nervous  system of
 .Vertebrata the following fundamental  parts.—1. A system  of ganglia
 subservient to the reflex  actions of the organs of locomotion, and cor-
 responding with the chain of pedal or  locomotive ganglia that makes
 up the chief part of the ventral  cord of the Articulata; in this system,
 the gray  or vesicular matter forms one continuous tract, which occupies
 the interior of the Spinal Cord.—2. A ganglionic centre for the  move-
 ments of respiration, and another  for those of mastication and degluti-
 tion ;  these, with part  of  the preceding, make up the proper substance
 of the Medulla Oblongata.—3. A series of ganglia, in immediate con-
 nexion with the organs of Special Sense;  these are situated within  the
 cranium,  at the  anterior  extremity of the  Medulla Oblongata; and, in
 the lowest Vertebrata,  they constitute by far the largest portion of the
 entire Encephalon.—4. The  Cerebellum,  Avhich is  a sort of off-shoot
 from the upper  extremity of the Medulla Oblongata, lying behind the
 preceding.—5. The Cerebral Hemispheres,  a pair of ganglionic masses,
 which  lie  upon the ganglia of special sense, capping them over more
 or less completely, according to their relative development.—These last
 two organs  exist  in the lowest Vertebrata,  as in Invertebrated animals
 generally, in  quite a rudimentary state; but their development, rela-
 tively  to other parts  of the Encephalon, and to  the entire  bulk of  the
 animal, increases as Ave ascend the scale; so that in Man and the higher
 Mammalia  they  constitute by far  the largest portion  of the Nervous
 centres, and are essential  to the greater part of  the operations of  the
 Nervous system.   The  development of the  Cerebral Hemispheres  holds
 a  close relation with  the increase of the Lntelligence, and with the pre-
 dominance  of the Will  over the  involuntary impulses.   The  increased
 size of the Cerebellum, on the other hand, seems connected with  the
 necessity  which exists, for the adjustment and combination  of the loco-
 motive powers, when the  variety in  the movements  performed by  the
 animal is  great, and a  more perfect harmony is required among them.
 —A sketch  of the mode in which these different parts  are  combined
 and arranged  in the several classes of Vertebrata, and of their relative
development in each, will aid us in the subsequent more detailed exami-
nation  of  their functions.
   869. In the class of Fishes, taken as a whole, the Encephalon  bears
a much smaller proportion to the Spinal  Cord, than in  the  higher Ver-
tebrata.   In the  curious Amphioxus, or Lancelot, there is  no discover-
able nenous mass anterior to the Medulla Oblongata;  and wre have here,
therefore,  an animal regularly formed upon  the plan Avhich occasionally
presents  itself as a monstrosity  in  Man,—namely, having the Spinal
Cord and  Medulla Oblongata for the Avhole  of the nervous centres, and
being anencephalous, or destitute of any proper encephalon.  In  some
of the  lowest  Vermiform (Avorm-like) Fishes, such as  the Lamprey, the
cephalic masses are very  little more developed  in proportion to the 
492          OF THE  NERVOUS SYSTEM AND ITS ACTIONS.
Spinal Cord, than are  the  cephalic ganglia of Insects in reference to
their chain of ventral ganglia.   But as the organs  of special sense ac-
quire a more  complete evolution, we find  the  ganglia connected with
them  presenting a greatly-increased  size.  On opening  the  cranial
caA'ity  of a Fish,  we usually observe four nervous masses (three of them
in pairs) lying, one in  front of  the other, nearly in the same line with
the Spinal cord.   The first or most anterior of  these are the Olfactoryt
ganglia (Plate II., Figs. 5, 6,  7, a), or the ganglia  of  the nerves of
smell;  the nature of which is knoAvn, from their being situated at the
origin  of  the Olfactory nerves.   In the shark  and some other Fishes,
these  are  separated from  the  rest by peduncles or foot-stalks;  a fact
of much interest, as explaining the arrangement which we find in  Man.
What is commonly termed  the trunk of his Olfactive nerve is really the
commissure connecting the OlfactiA'e ganglion  (known as  the bulbous
enlargement that lies upon the cribriform  plate of the Ethmoid  bone)
with the  other portions of his Encephalon:  the  proper fibres of the
nerve being those which come off from  this ganglion, in  the numerous
branches that proceed  from it into the nasal cavity.—Behind the Olfac-
tive ganglia is a pair of  masses, b, b, of which the relative  size varies
greatly in  different Fishes.  Thus, in the  Perch, whose Encephalon  is
here figured, their size is  intermediate between that of the first and
third pairs; being as much inferior to that  of the third, as it is superior
to that of the first.   On the other hand, in the Shark and several  other
Fishes, they' are considerably larger than the succeeding pair.  These
second  ganglia are commonly considered as the rudiments of the  Cere-
bral Hemispheres;  but there seems reason  for regarding  them  as  being
chiefly the representatives of the Corpora Striata;  the existence of a
cerebrum  being only indicated by  a  thin layer of  vesicular matter,
which  overlies the ventricle that is found  in these bodies in the brains
of Cartilaginous  Fishes  alone.—Behind them,  and forming the  third
pair  of ganglionic masses, c, c,  are two large  bodies, from Avhich the
optic nerves arise; these evidently represent the Optic ganglia, which
constitute the principal mass of the cephalic ganglia in Insects and the
higher Mollusca, and with  Avhich the Corpora Quadrigemina of higher
Vertebrata  partly  correspond  ; but they  probably  represent  also the
Thalami Optici of the  brain  of Man and  the higher animals.—At the
back of these, overlying the top of the spinal cord, is a single  mass, d,
the Cerebellum.  This, also, varies  greatly in its relative  dimensions,
being much more highly developed in the active and rapacious Sharks,
than it  is  in Fishes of inferior muscular  energy and variety of move-
ment.—The Spinal Cord e, is divided at the top by a fissure, which  is
most wide and deep beneath the cerebellum, where there is a complete
separation between its two halves.  This  opening corresponds to that,
through which  the oesophagus  passes in the Invertebrata; but as the
entire nervous mass of Vertebrated animals lies above the  alimentary
canal (or nearer the dorsal surface), it  does not serve the same purpose
in them;  and in the higher classes, the fissure is almost entirely closed
by the union of the tAvo halves on the  median  plane,  the fourth ven-
tricle, hoAvever, being a remnant  of it.  This  cavity is partly seen in
Fig. 7,  Avhich is a vertical section of the brain  whose upper and under 
NERVOUS CENTRES  OF REPTILES.
493
   surfaces  are shown in Figs. 5 and 6.—In the  lateral strands of the
   Medulla  Oblongata, close to the fourth ventricle, there is  a pair of gan-
   glionic centres  (characterized by the  presence of vesicular matter),  in
   which the auditory nerve terminates ; and these are sometimes deA'eloped
   as distinct ganglionic enlargements.  Other separate ganglia, sometimes
   of considerable size, are very commonly found at the origin of the Par
   Vagum.—It is  curious to notice the very large comparative size of the
   Pineal gland (/), and of the Pituitary body (h), in this class;  the  func-
   tions of these organs are entirely unknown.
    ^ 870. The analogy of the Optic lobes of Fishes to  the Corpora Quad-
   rigemina and  Thalami  Optici of the  fully-formed brain of the higher
   Vertebrata, is not so complete as it is to certain parts which occupy their
   place  at an earlier period.  In  the Human Embryo, at  about the 6th
,   week, the Encephalon consists of a series of vesicles arranged in a line

                                  Fig. 153.
  Human embryo of sixth week, enlarged about three times:— a, vesicle of corpora quadrigemina; 6, vesi-
 cle of cerebral hemispheres; c, vesicle of thalami optici and third ventricle; d, vesicle for cerebellum and
 medulla oblongata; e, auditory vesicle; f, olfactory fossa; /(, liver; ** caudal extremity.

 with each other ;  of which those that represent the cerebrum (b, Fig. 153)
 are the smallest, whilst  that which represents the cerebellum (d) is the
 largest.   BetAveen the cerebral and the cerebellic vesicles are two others
 (c, and a), of which the posterior one is the Optic ganglion, and answers
 to the Tubercula Quadrigemina; whilst the anterior contains the Third
 Ventricle, and corresponds in some degree to the Thalami Optici.  This
 condition is precisely represented in the Lamprey;  but in most Fishes,
 the optic ganglia, and the  parts surrounding  the third ventricle, form
 but one lobe ; so that  the third ventricle seems hollowed out of the cptic
 ganglia, as shown in Fig. 7, c, (Plate II.)
  871. The  Encephalon of Reptiles does  not show any considerable
 advance in its general structure, above that of the higher Fishes.  The
 Cerebral  Hemispheres (Plate II.  Figs. 8, 9, 10, b), are always much
larger than the Olfactive and Optic  ganglia; and they generally cover-
in the  latter (c, c) in part,  by their posterior extremities.   The Cere-
bellum is almost invariably of small  proportionate dimensions;  and this
is especially the case in the Frog, in which it does not even cover-in the
fourth ventricle.   This low development of  the Cerebellum in Reptiles,
is what might be anticipated from the general inertness of those ani-
mals, and the Avant of variety in their movements.   The Spinal Cord is 
494      *  OF  THE NERVOUS  SYSTEM AND ITS ACTIONS.

still very large, in proportion  to the nervous masses contained in the
skull;  and, as we shall hereafter see, its poAver of keeping up the move-
ments of the body, after it has been cut off from all connexion, with the
brain, is very considerable.—We find that, in Reptiles, as in Fishes, the
Spinal Cord may have a nearly uniform size from one extremity to the
other, like the ventral  cord of  the lower Articulata; or it may present
considerable enlargements  at particular spots,  like the ganglionic cord
in the thoracic  regions  of  Insects.  This difference depends upon the
degree of development of the  special locomotive organs.   Thus in the
Eel and Serpent, whose movements are accomplished by the undulations
of the entire trunk, and which  are destitute of members,  we find a uni-
form development of ganglionic matter in the spinal cord.   On the other
hand, in the Flying-fish, in which the pectoral fins or anterior extremi-
ties effect the greater part of the propulsion of the body, we find a great
ganglionic enlargement of  the  Spinal  cord, at the part in which the
nerves of those members are connected: in the  Frog, whose movements
are chiefly effected by the posterior extremities, Ave find a similar enlarge-
ment at the roots of the crural nerves : and  in the  Turtles and Lizards,
the two pairs of whose  members are nearly equal in function, and serve
to effect the principal movements of  the body, we find  an anterior and
posterior enlargement  of the  Spinal Cord,  corresponding to the parts
with which the nenes of these  members are  connected.
   872.  We find in Birds a considerable advance in the character of the
Encephalon, towards that which it presents in Mammalia.  The Cere-
bral  Hemispheres (Plate II., Figs. 11, 12, 13, b) are greatly increased
in size; and they  cover-in, not  merely the olfactory ganglia, but in
great part also, the optic  ganglia.  The  former are of  comparatively
small size; the organ of smell in Birds not being much developed. The
latter are very large, in conformity with the acuteness of  sight which is
highly  characteristic of the class.  The  cerebellum is of large size, as
we should expect  from the number and complexity of the muscular
movements performed by animals of  this class;  but it is  still undivided
into hemispheres.  The Spinal  Cord is still of considerable size in com-
parison Avith  the Encephalon;  and  it  is  much enlarged at the points
whence the legs and wings originate.   In  the  species which have the
most energetic flight,  such as the  Swallow, the enlargement is the
greatest  where the nerves  of the wings come off;  but in those which,
like the Ostrich, move principally by running on the ground, the poste-
rior enlargement, from  which the legs are  supplied with nerves,  is much
the more considerable.
   873.  In the Mammalia we find the size and general development of
the Encephalon presenting  a gradual increase,  as we ascend the series,
from the non-placental  Monotremes and Marsupials, towards Man.   In
the former, the  Hemispheres  exhibit no  convolutions;  and the great
transverse commissure, or connecting band of fibrous structure, termed
the corpus callosum, is deficient.   As we rise through the true viviparous
division of the class, we notice  a gradually-increasing prolongation of
the Cerebral  Hemispheres  backwards; so that first the  optic ganglia,
and then the  cerebellum, are  covered-in  by them.   The latter partly
shows itself, however, in  all but Man  and  the  Quadrumana, when we 
           NERVOUS CENTRES OF MAMMALIA—SPINAL CORD.       495

 look at the brain from above downwards: as we  see in the Encephalon
 of  the  Sheep (Plate II., Figs. 14,  15,  d).  The Cerebral hemispheres
 increase,  not only in size ;  but also in complexity of structure, both
 external and internal.  Their exterior,  instead of remaining smooth, is
 marked by convolutions ; which serve to extend very greatly the amount
 of  surface over which blood-vessels can pass into the gray substance.
 Their internal structure becomes more complex,  in the same proportion
 as their size and the depth of their convolutions increase; and in Man
 all  these conditions present themselves in a far  higher degree than in
 any other animal. ^ The number of commissural bands, connecting the
 two hemispheres with each other transversely, and uniting their anterior
 and posterior portions, is very greatly increased ;  and in fact, a large
 proportion of their mass is composed, in Man, and the higher Mammalia,
 of fibres of this character.—In proportion  to  the increase of  the  Cere-
 bral hemispheres,  there is a relative diminution in the size of the ganglia
 of special sense ; but  their dimensions, as compared with the entire bulk
 of  the animal, are by no means reduced, but are even increased.  The
 Olfactive ganglia (Fig. 14, a) are  always readily discoverable;  being
 separated from the remainder of the encephalic  masses by a  peduncle
 on  each side.  The Optic ganglia (Fig. 15,  c), on the other  hand, are
 so completely covered-in by the Hemispheres,  that it is only  when the
 latter are turned aside that we can discern them.   They  differ in exter-
 nal aspect from the optic  ganglia of Birds  and the lower Vertebrata;
 being divided by a transverse furrow into anterior and  posterior emi-
 nences, whence they are known  as the Corpora Quadrigemina.   The
 Auditory ganglia are  lodged in the  substance of the Medulla Oblongata,
 forming  the  ''gray nuclei" of the strands termed the "posterior pyra-,
 mids;"  and similar nuclei in the "restiform bodies"  are the ganglionic
 centres  of the Glosso-pharyngeal nerves, and perhaps minister to the
 sense of Taste.  Besides these, however, are the tAvo large bodies termed
 the Corpora Striata and Thalami Optici,  which have been commonly
 considered as appendages of the Cerebrum, but which must undoubtedly
 be regarded as independent of it, and as themselves  constituting gan-
 glionic centres, whose development  bears no constant proportion to that
 of the Cerebrum.   From the peculiar relation presently to be described
 (§ 901), Avhich these bodies bear on  the  one  hand to  the Spinal  Cord,
 and on the other  to the rest of the Encephalon,  there seems strong
 reason  to believe that they together constitute the ganglionic centre of
 the sense of Touch, and of the motions which are automatically prompted
 by it.—The Cerebellum is chiefly remarkable for the development of its
 lateral parts  or hemispheres,  and for the intricate arrangement of the
 gray and Avhite matter in them (Fig. 15, d); the central portion,  some-
 times called the vermiform process, is relatively less developed than in
 the  lower Vertebrata,  where it forms the entire  organ.—The  Spinal
 Cord is  much reduced in size, when  compared with other parts of the
nervous centres;  the  motions of the animals  of this  class being more
dependent upon their  will  or guided by their sensations ; and the simply
reflex actions bearing a  much  smaller proportion to the rest.   The
development  of ganglionic enlargements, in accordance with the presence
or absence of high locomotive  powers in  the extremities, follows  the
same rule as in the preceding classes. 
496          OF THE  NERVOUS SYSTEM  AND ITS ACTIONS.
             3.  Functions of the Spinal Cord and its Nerves.

  874.  In commencing our more  detailed examination into the func-
tions of the different parts of  the Nervous system in Vertebrated ani-
mals,  it seems best to commence with the Spinal Cord ; this being the
portion whose presence is most essential to the continuance of life.  As
already mentioned,  Infants are sometimes born Avithout any Cerebrum
or Cerebellum ; and such have existed for several hours or  even days,
breathing, crying, sucking,  and performing various other  movements.
The Cerebrum and Cerebellum have been experimentally removed  from
Birds and young Mammalia, thus  reducing these beings  to a  similar
condition ; and  all  their  vital operations  have,  nevertheless,  been so
regularly  performed, as  to enable  them  to  live for weeks,  or  even
months.   In the  Amphioxus, as already remarked, we have an example
of a completely-formed adult animal, in which no rudiment of  a Cere-
brum  or Cerebellum can be detected.  And in ordinary profound sleep,
or in apoplexy, the functions of these organs are  so completely sus-
pended, that the  animal is, in  all essential particulars,  in the same con-'
dition for a time as  if destitute of them.   It is possible, indeed, to re-
duce a vertebrated animal to the condition (so far as its nervous system is
concerned) of an Ascidian Mollusc (§ 850) ; for it may continue to exist
for some time, when not  merely the Cerebrum  and Cerebellum  haA'e
been removed from  above, but when nearly the whole  Spinal Cord has
been removed from below,—that part only of the  latter being left, Avhich
is the centre of  the respiratory actions,  and which  corresponds to the
single ganglion of the Tunicata.   On the other  hand, no  animal can
exist by its Encephalon alone, the Spinal Cord being  destroyed or re-
moved ; for  the reflex actions of the latter are so essential  to the con-
tinuance  of its respiration,  and consequently of  its  circulation, that if
they be suspended (by the destruction of the portion of the cord Avhich
is concerned in them), all  the organic functions must soon cease.
  875.  Although the Spinal Cord was formerly regarded as little  else
than a bundle of nerves proceeding from the Brain, yet its  true rank,
as  a distinct centre  of nervous  poAver, is noAV  universally admitted.
That the  actions prompted by it, when these  do not originate in one
of the higher centres, are of a purely reflex nature,—consisting in the
excitement of muscular movements in respondence  to external  impres-
sions, Avithout the necessary intervention of sensation,—appears to be
a necessary inference from the facts that have been brought to  light by
experiment and  observation.   Experiments on the nature  of this  func-
tion are best made upon cold-blooded animals ; as their general functions
are less disturbed by the  effects of  severe injuries of  the nervous sys-
tem, than are those of  Birds and Mammals.   When the Cerebrum has
been removed, or its functions have been  suspended by a  severe blow
upon the head, a variety of motions may be excited by their appropriate
stimuli.   Thus,  if the edge of the eyelid be touched with a  straw, the
lid  immediately  closes.  If  a  candle be brought near the eye, the pupil
contracts.   If liquid be poured into the  mouth, or a solid substance be
pushed  within the grasp of the muscles of deglutition, it  is  swallowed. 
REFLEX FUNCTION OF THE  SPINAL CORD.         497
If the foot be pinched, or burned with a lighted taper, it is withdrawn;
and (if the animal experimented on be a Frog) the animal will leap away,
as if  to escape from the source of irritation.  If the cloaca be irritated
with a probe,  the hind-legs will endeavor to push it  away.
   876.  Now the performance of these, as well as of other movements,
many of them most remarkably adapted to an evident  purpose, might
be  supposed to indicate,  that sensations are called up by the impres-
sions ; and that the animal can not only feel, but can voluntarily direct
its  movements, so as to get rid of the irritation which annoys it.  But
such  an inference Avould  be inconsistent with other facts.—In the first
place, the  motions  performed by an animal under  such circumstances
are never  spontaneous, but are  ahvays  excited by  a stimulus of some
kind.   Thus,  a decapitated  Frog, after  the  first  violent convulsive
movements occasioned by the operation  have passed away, remains at
rest until  it is  touched : and then  the  leg, or its  whole body may be
thrown  into sudden action, Avhich immediately subsides again.   In the
same  manner, the act of  swallowing is  not performed, except when it
is excited  by the contact of food or liquid ; and  even the respiratory
movements, spontaneous as they seem to be, would not continue,  unless
they were continually re-excited  by the presence of  venous blood in the
vessels.   These  movements are  necessarily linked  with the stimulus
that excites them ; that is, the same stimulus will always  produce the
same  movement, when the condition of the body is  the  same.  Hence
it is evident,  that the judgment and will are not concerned in pro-
ducing them ;  and that the adaptiveness of  the movements is  no proof
of the existence of consciousness and discrimination in  the being that
executes them,—the  adaptation being made for the  being,  by  the pecu-
liar structure  of its  nervous apparatus', which causes a certain  move-
ment  to be executed in respondence to a given impression,—not by it.
An animal thus circumstanced may be not  unaptly  compared to  an au-
tomaton ; in which  particular movements, adapted to produce a given
effect, are produced by touching  certain springs.   Here  the adaptation
was in the  mind of the maker or designer of the automaton ;  and so it
evidently is, in regard to the reflex or consensual movements of animals,
as well as with respect to the various operations of  their nutritive sys-
tem, over Avhich they have no control, yet Avhich concur most admira-
bly to a common end.
  877.  Again, we find that such  movements  may  be performed,  not
only Avhen the Brain has been removed, the Spinal  cord remaining en-
tire, but also when the Spinal cord  has been itself cut across, so as to
be divided into two or more portions, each of them  completely isolated
from each other, and from other parts of the  nervous centres.   Thus,
if the head of a Frog be cut off, and its spinal cord be divided in the
middle of the back, so that its fore-legs remain connected  with  the
upper part, and its hind-legs with the loAver, each pair of members may
be excited  to  movement by a stimulus  applied to  itself;  but the two
pairs will not exhibit  any consentaneous motions,  as they will do when
the spinal  cord is undivided.   Or, if the  Spinal cord  be cut  across,
without the removal of the Brain, the lower limbs may be excited to move-
ment,  by an appropriate stimulus, though they are completely paralysed 
498          OF THE NERVOUS SYSTEM  AND  ITS ACTIONS.
to the will ;  whilst the upper remains under the control of the animal,
as completely as before.   Now it  is not conceivable that, in this  last
case,  sensation and volition should exist in that portion of the  spinal
cord,  which remains connected with the nerves of the posterior extremi-
ties, but which is cut off from the  brain.  For if it were so, there must
be two distinct centres in the same animals, the attributes of the brain
not being affected ; and, by dividing the spinal cord into  two or more
segments, Ave might thus create in the  body of one  animal two or more
distinct centres of sensation, independent of that which still holds its
proper  place  in  the Encephalon.   To say that tAvo or more distinct
centres  of sensation are present in such a case, would really be in effect
the same as saying, that there are two or more distinct minds  in  one
body,—which is manifestly absurd.
   878.  But the best proofs of the limitation of  the  endowments  of the
Spinal Cord, are  derived from the phenomena presented by the Human
subject, in cases  where that organ has suffered injury, by  disease or
accident, in the middle of the back.  We find that, when this injury has
been  severe enough  to produce  the effect of a complete division  of the
Cord, there is not only a total want of voluntary control over the lower
extremities, but a complete absence of sensation also,—the individual
not being  in the least conscious of any impression made upon them.
When the lower segment  of the Cord remains sound, and its nervous
connexions with  the limbs are  unimpaired, distinct reflex movements
may be  excited in the limbs,  by stimuli directly applied to  them,  as, for
instance, by pinching the skin, tickling the sole of the foot, or applying
a hot  plate to its  surface ;  and this without the  least sensation, on the
part of  the patient, either of  the cause of the moA7ement, or of the move-
ment  itself.  This fact, taken in connexion with the preceding experi-
ments, both upon Vertebrated and Articulated animals, distinctly proves
that Sensation  is  not  a necessary link in  the chain of  reflex actions;
but that all which is required is an afferent fibre, capable of receiving
the impression made upon the surface, and of conveying it to the centre;
a ganglionic centre, composed of vesicular nervous substance, into which
the afferent fibre passes; and an efferent fibre,  capable  of transmitting
the motor impulse, from the ganglionic centre, to the muscle which  is to
be thrown into contraction.
   879.  These  conditions are realized  in  the  Spinal Cord.  We  may
have  reflex actions excited through any one isolated segment of it, as
through a single ganglion of the  ventral cord of Articulata ; but  they
are then confined to the part supplied  by the nerves of that segment.
Thus, if the  spinal cord of a Frog be divided just above the origin of
the crural nerves, the hind-legs  may  be throAvn into reflex contraction
by various stimuli applied to themselves ;  but the fore-legs will exhibit
no movement of this kind.   But Avhen  the brain has been removed, and
the Spinal Cord  is left  entire, movements may be excited in  distant
parts, as, for example, in the fore-legs, by any powerful irritation of the
posterior extremities, and vice versd.   This is particularly well seen in
the convulsive  movements, Avhich take place in certain disordered states
of the nervous  system; a slight local irritation being sufficient to throAV
almost any muscle of the body into a state of energetic action (§ 885). 
REFLEX FUNCTION OF  THE SPINAL CORD.
499
And a similar state may be artificially induced, by applying Strychnine
(in solution) to the Spinal  Cord of a decapitated Frog.
 ^ 880.  The minute anatomy of the Spinal Cord is a subject of great
difficulty; and our notions of the course of the fibres within it are rather
founded  upon  physiological  phenomena,  and upon  the more  evident
structure of the ventral  column in Articulata, than upon what  can be
clearly demonstrated in Vertebrated animals.  The roots of the Spinal
nerves are all distinctly separable into  an anterior and a posterior fasci-
culus ; and it is certain that  these fasciculi have entirely opposite func-
tions.   If they be  laid bare, and the  anterior fasciculus of any spinal
nerve be touched, violent contractions  are immediately seen in the mus-
cles supplied by that nerve; these contractions are as strongly manifested
if the  anterior  roots be divided, and  their separated end be irritated;
whilst no such result follows, whatever amount of irritation be applied to
the ends still in connexion with the cord.  Notwithstanding these violent
movements, the animal shows  little or no sign of pain.   On the other
hand, if the posterior roots be irritated, the animal gives signs of acute
pain, and no vigorous muscular contractions are produced.  The move-
ments Avhich are Avitnessed are evidently of a reflex nature, being called
forth through the anterior roots; as is proved by their cessation when
these are divided.  Further,  if the posterior  roots be divided, and  the
separated ends  be  irritated,  no effect  Avhatever is produced; no move-
ment is excited;  and no sensation is occasioned;  but  if the ends still in
connexion with the cord be irritated, the animal shows  signs of  pain as
before.—Hence it  is evident,  that the posterior roots are made up of
afferent  fibres, that is, of  the  fibres Avhich convey impression  towards
the nervous centres; which  impressions, if confined  to  the cord itself,
excite reflex actions; whilst,  if conveyed to the brain, they produce sen-
sations.  On the other hand  it is equally evident, that the anterior roots
are composed of efferent or  motor fibres, Avhich serve to comrey to the
muscles the motor impulses  originating in the nervous centres; these
impulses may be occasioned by the  reflex action of the  Spinal cord ; or
they may descend from the Brain,  wThere they have been generated by
a Consensual or Emotional impulse, or by  an  act of the Will.
  881.  The Spinal  Cord is a completely double tract;  being composed
of two distinct halves, united together on  the median  plane by nume-
rous commissural fibres.   This  union  is much closer in Man  and  the
Mammalia, than it  is in the  lower Vertebrata; but the  division is still
marked externally, by a deep fissure on the anterior surface of the cord,
and by a shalloAver one on its posterior aspect.  Its surface is traversed,
moreover, by two furrows on  each side; so  that each half is divided into
three columns,  the anterior, lateral, and posterior.  The anterior roots
of the spinal  nerves join the Cord for the most  part along the line of
the anterior furrow;  and the posterior along the line of  the posterior
furrow; so that  the middle  or lateral column lies between them, the
anterior  column  being altogether in  front of them,  and  the posterior
column behind  them.  When a transverse  section of  the Cord is made,
it is seen to contain, on each side, a crescentic patch  of  gray or vesicu-
lar  substance; the  points of  each crescent  are directed towards the
anterior and posterior furrows of its own side respectively; whilst the
convexities of the two crescents approach one another near the  median 
500          OF THE NERVOUS SYSTEM  AND ITS ACTION?.
plane, and are connected by a transverse tract of gray matter.  The
remainder  of  the  cord  is made up  of white or tubular  substance, the
course of whose fibres is for the most part longitudinal.  The posterior
peak of the crescentic patch of gray matter approaches  very closely to
the bottom of the  posterior furrow;  Avhilst the anterior  peak does not
come into nearly the same degree of proximity with the bottom of the
anterior furroAv.   Hence, it is considered by some, that  the lateral or
middle columns of  the cord, being much less completely isolated from the
anterior columns than they are from the posterior, should be associated
with the former, under the name of antero-lateral columns.
  882. Upon tracing the roots of the nerves into  the  substance of the
Cord, the connexion of  a part of their fibres with  its gray or vesicular
substance is easily made  evident.   Of these fibres, therefore, it serves
as the proper  ganglionic centre.   There is reason  to believe, both from
anatomical investigation,  and from  physiological  phenomena,  that, as
in the Articulata  (§ 857), a part of  the afferent or excitor fibres,  after
traversing the gray substance, pass out on the same side  as the efferent
or motor; whilst another portion crosses to the opposite  side, and forms
part of its efferent trunks.—It is pretty certain that other fibres of the
roots become continuous with the longitudinal fibres that  form the white
strands  of the Spinal Cord;  but it is by no  means  certain, on that
account, that they  pass  on to the Brain ;  and, in fact, there is adequate
evidence  that if any of  the  fibres thus establish a direct  communication
between the Encephalic centres and the spinal nerve-trunks, their pro-
portion must be very small,  the chief part of the longitudinal strands of
the Cord  being apparently  made up of  commissural fibres which esta-
blish an intimate connexion betAveen its different segments, as in Insects.
This will  appear from the facts to be next stated.   The thickness of the
Spinal  Cord differs  considerably at its different  parts.   Thus in the
cervical region, there is an  enlargement  corresponding with the origins
of the nerves  that form the brachial plexus;  this enlargement is partly
caused by an increase in the amount  of gray matter; but the amount of
fibrous structure also, is much greater than  at  the  upper part of the
cervical region.    On the other  hand, there is a still  greater  enlarge-
ment of the cord in the lumbar region, at the part Avhence the nerves
of the loAver extremities arise ; and  this   enlargement  is  caused by the
great  increase in the amount both of the  gray matter  and of the white
at that point.  It may  be  easily shoAvn  by direct  measurement, that
the fibrous strands of the upper cervical region would not by any means
serve to  carry omvard to the brain  those  of the lumbar region alone,
much less with the addition of other fibres proceeding from all the inter-
mediate nerves.   Further, if  the  fibrous  strands Avere  for the  most
part (as formerly supposed) directly continuous  betAveen the brain and
the roots of the spinal nerves, the white portion of the Spinal  Cord, in
such animals as Serpents, in which  it  has no ganglionic enlargements,
should progressively diminish in diameter  with every  pair of nerves
into which it  sends  fibres, from  its  cephalic to its  caudal  extremity;
this, however, is by  no means the  case, the Spinal cord of Serpents
being remarkable  for its uniform  diameter throughout.—It is  obvious,
then, that if any of  the longitudinal fibres of the cord should thus pass 
REFLEX FUNCTION OF THE SPINAL  CORD.
501
direct from the Encephalon to the nerve-roots, their proportion must be
very small.  And Ave shall see that all the phenomena which have been
supposed to indicate an  immediate communication between the brain
and the nerves, admit of another and more satisfactory explanation.
   883. It was supposed by Sir C. Bell (who was the first to determine
the relative functions of  the  two roots of the spinal nerves in Verte-
brated animals),  that the anterior  columns of the spinal cord have a
function corresponding  to that of the anterior roots of the spinal nerves;
and the posterior columns with the posterior roots.   But  from  the diffi-
culty of tracing the  connexion  between the longitudinal fibres of the
cord  and any portion of the  roots, it is at present  impossible to  say
hoAv far there is any anatomical reason for the assumption of this cor-
respondence; and it is  quite certain that the physiological  facts at
present known, from  observation of the effects of disease  or injury upon
different tracts of the spinal cord, do not bear out the supposition.  As
to what the precise functions of the several columns are,  however, it is
not easy to form  any other  conjecture, that shall be consistent  with all
the phenomena at present knoAvn.
   884. Of the particular  Reflex actions to which the Spinal Cord (using
that term in its limited sense, as excluding the Medulla  Oblongata) is
subservient,  those most  connected  with  the  organic functions have
already been noticed.  They are chiefly of an expulsive kind; being
destined to force  out the contents of various cavities of the body.  Thus
the ordinary acts of defecation and urination, the ejaculatio seminis and
parturition, are all reflex  actions, over Avhich the will has a greater or
less degree of control; being able to keep the two former ones in check,
so long as the stimulus  is not very violent, and being also capable of
effecting them by itself; but having no control  over the  two latter,
either by way of  acceleration  or prevention, when once the stimulus by
which they are excited  has come into full action.—The movements of
the posterior extremities are among the most remarkable of those, Avhich
seem  due to the action of the  proper Spinal Cord.   It has been already
noticed,  that these  may be  excited, eATen in  Man, when  the spinal
cord has been severed in the middle Avithout injury to its lower segment;
and it is remarkable, that gentle stimuli, applied to the skin of the  sole
Qf the foot, appear the most capable of producing them.   We haAre seen
Iioav completely, in the lower  animals, the  acts of progression  may  be
sustained, by the  repeated stimulus of the contact of the  ground, or of
fluid,  without any influence from the cephalic ganglia;  the power of
these being limited, it would seem, to the control and  direction of them.
And there is strong reason to  believe that so  far as  the  ordinary acts
of locomotion are concerned, the movements of the inferior extremities
in Man may  be performed on the  same plan,'being  continued  by  the
reflex influence of the successive  impulses of 'the feet upqn the ground,
when  once set in action by the will, whilst Ave are  Avalking steadily  on-
Avards,—the  mind b^ing at  the  same  time  occupied  by  some  train of
thought,  which engrosses  its whole  attention.   Theirs are< fcAv  persons,
to Avhom it has not occasionally happened that, on awakingt (as it were)
from their revery, they  have found themselves  in a place  very different
from that to  which they had intended going;  and  even  Avhen tYe Ton- 
           502         OF  THE NERVOUS  SYSTEM AND ITS ACTIONS.

           sciousness is sufficiently on the alert to allow sensations to guide, direct,
           and control the motions  of the limbs, their actions  appear to be per-
           formed without the agency of the will, which  may be  entirely concen-
           trated  upon  some  interior mental operation.   It  is  certain  that, in
           Birds, the movements of flight may be performed  after the removal of
           the Cerebrum.
             885. There are many irregular or abnormal  reflex  actions, performed
■'it          through the instrumentality of the Spinal Cord, the study of which is of
           the highest importance to the Medical  Man.  It is probable that all Con-
           vulsive movements are produced  through its agency and  that of the
           Medulla  Oblongata; for it has  been  found, by repeated experiments,
           that these movements  are never produced  by injuries  of the  Cerebral
           hemispheres.—Convulsive movements  may be of three kinds.  1. They
           may be simply reflex ; being the natural result  of some extraordinary
           irritation.  2. They may be simply centric ; depending upon a peculiar
           condition of the ganglionic centre of  the Spinal Cord, which  occasions
           muscular movements without any stimulation.   This may be dependent
           upon an  abnormal  state of the  Blood.  We know that it  may be pro-
           duced by the introduction of certain poisons (as  strychnia)  into the cir-
           culation; and it is  probable that morbid  matters generated within the
           body may have the same effect.   3.  They may  depend upon  the  com-
           bined action of  both principles;  the  nervous  centres  being in a very
           irritable  state, Avhich causes very slight irritations (such as  would other-
           wise be inoperative) to excite violent reflex or  convulsive  movements.
           This last is by far  the most  common cause of the  convulsive actions,
           that occur in  various diseased conditions of the system.  Thus,  convul-
           sions are not  unfrequent in  children, during the period  of  teething;
           being produced by the  irritation, which results from the pressure of the
           tooth, as it rises against the unyielding gum.  In this case, the stimulus
           would scarcely be  sufficient to  produce  the violent  result,  were it not
           for a peculiarly excitable state of the Spinal Cord, brought  about by
           various causes.  In like manner, when  such an excitable  state exists,
           convulsions may be occasioned  by the presence of intestinal worms, of
           irritating substances, or even simply of undigested matters, in the ali-
           mentary  canal;  and will cease  as soon as they are cleared  out, in the
           same manner  as  the  convulsions of teething  may often  be at once
           checked, by the free lancing of the gums.
              886. The  influence of the  condition of the Spinal Cord itself,  is
           peculiarly seen in  the  convulsive  diseases  termed Hydrophobia, Teta-
           nus, Epilepsy, and Hysteria.—In  the first of these, not only the Spinal
           Cord, but the Medulla Oblongata, and the ganglia of Special Sense,
           are involved;  their peculiar condition being the  result, it would appear,
           of the introduction of a poison  into the blood.   It is most remarkable
           that the  Cerebrum should so completely escape its influence.  When
           the state of intense excitability  in these  centres,  is once  established,
           the slighest stimulus is sufficient  to bring about convulsive movements
           of the utmost violence.   It is characteristic of this complaint, that the
           stimuli most effectual in exciting the  movements,  are those Avhich act
           through  the nerves of special  sense; thus the sight  or the  sound  of
           water AV'ill bring on the paroxysm; and any attempt to taste it increases 
           HYDROPHOBIA ; TETANUS ;  EPILEPSY ;  HYSTERIA.       503

the severity of the convulsions.—In tetanus there appears to be a simi-
larly excitable state  of the  Spinal Cord  and Medulla Oblongata, not
involving the ganglia  of  special sense.   This may  be the result  of
causes altogether  internal, as  in the idiopathic form of the disease;  in
Avhich the  condition  exactly resembles that, which may be artificially
induced by the administration of Strychnine, or  by its application  to
the cord.   Or it  may be  first occasioned by  some local irritation,  as
that of  a lacerated wound;  the  irritation of  the  injured nerve being
propagated to the nervous centres, and establishing the  excitable state
in them.  When the  complaint has once  established itself,  the removal
of the original cause  of irritation (as by the amputation of the injured'
limb) is seldom of any avail;  since the slightest  impressions upon al-
most any part of  the body, are sufficient to excite the tetanic spasm.—
In  like  manner,  Epilepsy, which  consists in convulsive actions  with
temporary suspension of the functions of the Encephalon,  may result
from the irritation of local causes, like the convulsions of teething; and
may, like them, cease when-the sources of irritation are removed.  But
when it becomes confirmed, it seems to involve a disorder of the nervous
centres, which no  local treatment can influence.
  887.  These and other forms of Convulsive disorder, when productive
of a fatal result, usually act by suspending the respiratory movements;
the muscles which effect  these being  fixed by the  spasms, so that the
air  cannot pass either in or out, and suffocation takes place as completely
as if  the entrance to  the air-passages were closed.   It is remarkable
that every one of  them may be imitated  by Hysteria ;  a state  of the
nervous system, in which there is a peculiar excitability, but in Avhich
there is no such fixed  tendency  to irregular action, as Avould indicate
any positive disease,—one form of  convulsion  often taking  the place of
another,  at short  intervals, with the  most wonderful  variety.   It Avill
often be found, that the  convulsions may  be immediately  traced  to
some local irritation ; thus they are particularly liable to occur  at the
catamenial periods, especially if the menstrual flux be deficient;  but it
does not seem improbable, that here too the presence of morbid matters
in the blood  has  much to do with the development  of that peculiar
excitability, Avhich gives  to  slight  local irritations such  a powerful
agency.
  888.  The statement  that the Spinal Nerves arise  by double roots, is
not without exception  as  regards  some,  Avhich arise from its  cranial
prolongation, and  which are distributed to  the  parts of the head and
neck.  The first spinal nerve,  or sub-occipital  (the  10th pair of  Willis)
not unfrequently arises by a single set of roots, from the anterior por-
tion of the cord ;  and it is then  purely motor, except in virtue  of its
inosculation with  other nerves.  The  Hypoglossal (9th pair  of  Willis)
appears to be  also a  purely motor nerve; arising by one set of roots,
and beino- distributed  entirely to the muscles of the  tongue, which
organ derives its  sensibility from other nerves.  The Glossopharyn-
geal usually  arises from  a  single set of roots, and these  correspond
with the posterior roots of the spinal nerves ; in some animals, howrever,
and occasionally  in  man, there is  a distinct anterior root, and  the
nerve acquires direct  motor  functions.   It may  in  some  respects  be 
504         OF THE NERVOUS SYSTEM AND ITS ACTIONS.

considered as making up, with the preceding, an ordinary spinal nerve.
The Spinal Accessory, again, appears to be chiefly or entirely a motor
nerve  at its origin;  and in like manner  the Pneumogastric, or Par
Vagum,  seems at its roots to correspond with the posterior roots of the
ordinary spinal  nerves, and  to execute functions analogous to theirs ;
but these two nerves  exchange fibres, so that each acquires in part the
endoAvments of the other.  The Facial nerve (or portio dura of the  7th),
which  is the nerve  that  supplies the muscles of the head  in general,
arises  by a single root, and is exclusively  motor  in its properties,—
except in branches which have received sensory filaments by inoscula-
tion with other  nerves.   The same is the case, also, with  the Motor
Nerves of the Orbit? (the 6th,  4th, and 3d, of Willis), which  arise by
single  roots, and which have no  sensory  endowments but those which
they obtain by inosculation with the Fifth pair.—On the other hand,
the Fifth pair arises by a double root; that which corresponds  to the
anterior  or  motor root of the spinal  nerves is very small, however, and
only enters the third division of the nerve, which  supplies the muscles
concerned in mastication; the other root, corresponding with the pos-
terior roots of the spinal nerves, is of large  size, and its branches are
distributed to the face and head, endoAving them with sensibility.   Thus
the sensory division  of  the fifth pair, being distributed,  not merely to
the same parts with its  motor division, but also to the parts which de-
rive their  motor endoAvments from  the  Facial nerve,  and from the
nerves  of the orbit, may be regarded as making up, together Avith all of
them, one ordinary Spinal nerve.

                 4.  Functions of the Medulla Oblongata.

  889.  This portion of the nervous centres, as already stated, does not
differ in  any essential particular from the  Spinal Cord, of which it may
be considered as a cranial  prolongation.   But the arrangement of its
constituent parts is peculiar; for whilst it is the medium by which the
various strands of the Spinal Cord are   connected  with the different
portions  of the Encephalon, it  is also remarkable  as  being the gan-
glionic centre, concerned  in the  maintenance of the  action  of respi-
ration, and in the ingestion of food.  Four principal  strands of nervous
matter may be distinguished anatomically, in each of its lateral halves ;
these  are,  anteriorly,  the  Anterior Pyramids; next,  the  Olivary
bodies  ; next the Restiform bodies; and lastly, the Posterior Pyramids.
It Avill be presently  seen, hoAvever,  that the  physiological  relations of
these strands, as indicated by their connexions with  the  Encephalon
above,  with the Spinal Cord below, and with the nerves that have  their
contres in them, are very different from what their mere relative  posi-
tions Avould indicate.—The gray  or vesicular substance in  this part no
longer holds the same  relation to the white, that  it  possesses in the
Spinal Cord; but is principally aggregated in three  pairs of gangli-
onic centres, of Avhich the anterior  forms the nucleus  of  the Olivary
body, the lateral of the Restiform, and the posterior of the Posterior
Pyramidal.
   890. The Anterior Pyramids consist  entirely of fibrous structure, 
STRUCTURE OF THE MEDULLA OBLONGATA.
505
   and establish a communication betAveen  the  motor tract at the base of
   the Encephalon (which is chiefly derived from the Corpora Striata) and
   the  anterior and  antero-lateral  columns of the  Spinal  Cord.   They
   have also a connexion with the Cerebellum.  A large part  of  its fibres
   decussate, those that proceed from  the  right  hemisphere passing into
   the left side of the cord, and those from  the left hemisphere  into the
   right side of the cord; an arrangement  which fully explains  the fact,
   that in Hemiplegia, the paralytic affection  of  the body is  on the side
   opposite to that of the lesion of the brain.   A small proportion of the
   fibres  of  the  anterior  pyramids does not  decussate;  and this passes
   doAvn,  with fibres  from the olivary columns, into  the anterior columns
   of the cord; whilst  the decussating fibres dip  more deeply away  from
   the anterior surface  of the cord, and connect themselves rather with its
   lateral or middle columns.
      891. The fibrous portion of the Olivary body is connected above with
   the Motor tract, with the Corpora Quadrigemina, and  with the Cere-
/  bellum, and below with the anterior columns of the Spinal Cord.—The
   vesicular  nucleus  of the Olivary body, on  the other  hand, which  is
   knoAvn  as the corpus dentatum.,  seems to  be especially connected with
   the origins of the nerves concerned in the regulation of the movements
   of the tongue;  thus we find that anteriorly a portion of  the roots  of
   the hypoglossal, which is the motor nerve  of the  tongue, issue  from
   it; whilst posteriorly, a portion of the roots of the glossopharyngeal,
   which  is  one of the sensory nerves of the tongue, seem to terminate
   in it.
      892. The fibres of the Restiform columns are continuous above with
   those  of  the  hemispheres of the  Cerebellum  ; and beloAV they  pass,
   Avithout  decussation, chiefly into the posterior  columns of the Spinal
   Cord, a band  of arciform fibres, however,  crossing over to the anterior
   and lateral columns  on  each side.   The ganglia  imbedded  in  these
   columns,  hoAvever, seem  to possess a completely  independent function ;
   being the centres of the Pneumogastric nerves, which are the chief ex-
   citors  of  the  Respiratory movements, as  Avell  as of a portion of the
   Glosso-pharyngeal nerves.
      893. The Posterior Pyramids are two small strands  of fibrous struc-
   ture, lying between the t\vo restiform bodies, and occupying the portion
   of the  Medulla Oblongata on either side of the posterior median furrow.
   They may be traced upAvards  into the Thalami Optici,  and downwards
   into the posterior columns and the posterior  part of the lateral.   They
   undergo a decussation in their upward course ; but it  is  not certain
   Avhether this decussation involves all  their fibres.—The gray nuclei  of
   the  Posterior Pyramids,  which  are situated immediately  beneath the
   fourth ventricle, are the ganglionic centres of  the Auditory nerves, or
   the proper Auditory ganglia.
      894.  When Ave consider these various  lines of communication simply
   in their Physiological relations, as establishing  connexions  between the
   Encephalon above and the Spinal  Cord beloAV, it  will be convenient
   first to notice  and -put aside the Cerebellar.  Of these  there are tAvo
   sets ; the principal  forming the Restiform  bodies, which connect the
   Cerebellum with the posterior columns of the Spinal cord; whilst there 
506          OF THE NERVOUS SYSTEM AND ITS ACTIONS.

is  another division, Avhich comes  into  connexion  through  the Olivary
and Pyramidal bodies,  with the anterior and antero-lateral columns.—
The remaining fibres, which constitute what are improperly called the
Crura Cerebri, may be considered as forming two principal tracts, the
sensory and the motor; these being distinguished as such  by the cha-
racter of the nerves which  arise in their  course.   The sensory  tract
passes upwards from the posterior columns of  the Spinal Cord, and the
posterior part of the lateral, to the Thalami Optici;  it is obviously con-
tinuous below with the  tract in which the posterior roots of the Spinal
nerves terminate, and  in its upward course it  receives the large or sen-
sory root of the  Fifth  pair;  whilst passing through the  Pons Varolii, it
undergoes a partial decussation.  On  the other hand,  the motor  tract
may be regarded  as descending from  the  Corpora Striata and Tuber-
cula Quadrigemina into the anterior and antero-lateral  columns of the
Spinal Cord ; in its course it gives off the roots of all  the motor nerves
usually considered as cranial: and the greater part of its fibres undergo
decussation  beloAV the  Pons Varolii.—The functions of  the Medulla
Oblongata are, therefore, of a double character;—to bring the higher
parts of the Encephalon into connexion with the Spinal Cord  and the
Nerves that issue from it; and  to serve as a centre for  the reflex
movements, performed through the nerves that issue from it.   In  both
respects it corresponds  precisely with any segment  of the  Spinal  Cord
itself; and there  is no  reason to believe,  that  it possesses any other or
more special endowments.   The importance, however, of the reflex acts
of Respiration and  Deglutition, over Avhich it presides,  causes this por-
tion  of the Medulla to be the one, whose integrity is  most essential to
the preservation  of life; and therefore it seems to  possess a character
more distinctive than it really has.
                               Fig. 154.
• Dissection of the Medulla Oblongata, to show the connexions of its several strands:—A, corpus striatum;
b, thalamus opticus; c, d, corpora quadrigemina; e, commissure connecting them with the cerebellum; P,
corpora restiformia; P, r, pons varolii; it, st, sensory tract; mt, mt, motor tract; g, olivary tract; p, pyra-
midal tract; og, olivary ganglion ; op, optic nerve; 3i», root of the third pair (motor); 5s, sensory root of the
fifth pair.

   895.  The chief excitor nerve of the Respiratory movements, as already
stated (§§ 685-687) is the afferent portions of the Par Vagum : but  the 
      STRUCTURE AND FUNCTIONS OF THE MEDULLA OBLONGATA.   507

afferent portion of the Fifth  pair is also a powerful excitor;  and the
afferent portions of all the spinal nerves, conveying impressions  from
the general surface of the body, are also capable of contributing to the
excitement necessary for the production  of the movement.—The  chief
motor  nerves  are  the phrenic and intercostals; which, though issuing
from the Cord at a considerable  space  lower doAvn, probably originate
in the Medulla Oblongata.   The  motor portions of several other spinal
nerves are also partly concerned;  as are also the Facial  nerve, the
motor portion of the Par Vagum,  and  the  Spinal Accessory.   The
ordinary movements of Respiration  involve little action  of  any motor
nerves but the Phrenic and Intercostal;  and it is only when an excess
of the stimulus (produced, for  example, by too long a suspension of the
aerating process) excites extraordinary movements, that the nerves last
enumerated are called into action.
   896. The acts  of Prehension  of food Avith lips, and of Mastication,
though usually effected by voluntary  power in the  adult, seem to be
capable of taking place as  a part of the reflex operation of the  Medulla
Oblongata, in  the Infant, as in the loAver  animals.  This is particularly
evident in the  prehension of the nipple by the  lips of the infant,  and
the act of suction which the contact of that body (or of any resembling
it) seems to excite.   The experiments provided for us by nature, in the
production of anencephalous monstrosities, fully prove that the integrity
of the nervous connexion  of  the lips  and respiratory organs Avith the
Medulla Oblongata, is alone sufficient for the performance of this action;
and experiments upon young  animals, from which the  brain has  been
removed, establish the same fact.   Thus Mr. Grainger found  that,  upon
introducing his finger, moistened with milk, or with sugar and water,
between  the lips of  a puppy thus  mutilated,  the act of suction  was
excited;  and not merely the act of suction itself, but other movements
having a relation to it; for as the puppy lay on its side, sucking the
finger, it pushed out its feet, in the same  manner as  young  pigs exert
theirs in  compressing the soav's dugs.   This action seems akin  to many
of those, by Avhich  the lower  animals take in their food; and  Ave may
thus recognise in  the Medulla Oblongata a distinct centre of reflex
action for  the reception and  deglutition  of aliment, analogous to the
stomato-gastric ganglia of Invertebrated animals.
   897. In the movements  of Deglutition, which, as formerly explained
(§ 453), are purely reflex, the  chief  excitor is undoubtedly the afferent
portion of the Glosso-pharyngeal nerve.  It is found that, if the trunk of
this nerve, or  its pharyngeal (but not its lingual) branches, be pinched,
pricked, or otherwise irritated, whilst still  in connexion with the Medulla
Oblongata, the movements concerned in the  act of  swallowing are
excited.   The  same occurs if, when the  trunk  of the  Glosso-pharyn-
geal has been  divided, the cut  extremity in connexion with the  Medulla
Oblongata is irritated; but little or no muscular  contraction is produced
by irritation of the  separated extremity; whence it is  apparent, that
the Glosso-pharyngeal has little  or no direct motor  poAver, but acts as
an  excitor.  In this  it appears  to be assisted by the branches of the
Fifth  pair distributed upon the  fauces; and  probably, also,  by the
branches of the superior laryngeal distributed upon the Pharynx.  The 
508         OF  THE NERVOUS SYSTEM  AND  ITS  ACTIONS.

motor influence,  which is generated in respondence to the stimulus thus
conveyed, appears to act chiefly through the branches of the Par Vagum,
Avhich are distributed to most of the muscles concerned in SAvallowing;
but the Facial,  the Hypoglossal, the motor  portion  of the Fifth,  and
perhaps also the motor portions of some of the Cervical nerves, are also
concerned in the movement, and  may effect it, though with  difficulty,
after the pharyngeal branches of the Par Vagum have been divided.
  898.  In the  propulsion of the  food doAvn the Oesophagus, to which
the glosso-pharyngeal  nerve does not extend, the muscular contraction,
so far as it is of a reflex nature (§ 455), must depend upon the oesopha-
geal branches of the  Par Vagum alone; their afferent portion being
the excitor, and  their motor portion  giving the requisite stimulus to the
muscles.  The same  must be the case in regard to the muscular contrac-
tions of the cardiac  and pyloric sphincters, and of the Avails of the sto-
mach, so far as regards their  dependence upon the nervous system at
all; but the degree of this is doubtful.
   899.  There are other  reflex actions  of the Medulla Oblongata, con-
nected with the  regulation of the  aperture of the Glottis;  these, which
are effected through  the superior and inferior laryngeal branches of the
Par Vagum, will be better noticed, Avhen the  actions of the  Larynx are
under consideration (§  976).—In like manner  the reflex action concerned
in the regulation of the aperture of the Pupil, will be more conveniently
noticed in the sketch to be hereafter given of the Physiology of  Vision
(§ 969).

                 5. Functions of the Sensory  Ganglia.

   900.  All the  nerves of Sensation, both general and special, may be
traced into a series of ganglionic masses lying at the base of the brain;
Avhich seem to constitute  their OAvn  particular centres.  Thus Ave have
seen in Fishes, the Olfactive,  Optic, and Auditory ganglia, marked out
as such, by the  termination of the nerves proceeding from the organs of
smell,  sight, and hearing, in these masses respectively.  These ganglia
bear an evident  correspondence with the cephalic ganglia of the Inverte-
brata; which must chiefly, hoAvever, be regarded as optic ganglia, since
the development of the  eyes  far  surpasses that of the other organs of
special  sense.   On  the other  hand, they find their representatives in
certain organs at the base of the brain, in Man and the higher Mamma-
lia ; which, though small in proportion to the whole Encephalon,  are
capable  of being clearly marked  out  as  the ganglionic centres of the
several nerves of sense.—Thus, anteriorly,  Ave have the Olfactive gan-
glia, in what are commonly termed the bulbous expansions of the Olfac-
tive nerve; Avhich, however, are real  ganglia, containing gray or vesicular
substance; and  their separation from the general mass of the Encepha-
lon, by the peduncles  or footstalks commonly termed the trunks of the
olfactory nerves, finds its analogy in many species of Fish (§869).  The
ganglionic nature of these masses is more evident in many of the lower
Mammalia, in Avhich the organ of smell is highly developed, than it is
in Man, whose olfactive poAvers are comparatively moderate.—At some
distance behind  these, we have the representatives of the Optic Ganglia, 
THALAMI  OPTICI, AND CORPORA  STRIATA.           509
m the  Tubercula  Quadrigemina, to which  the principal  part  of the
roots of the Optic nerve may be traced.  Although these bodies are so
small in Man, as to be apparently insignificant, yet they are relatively
larger,  and form a more  evidently-important part of the encephalon, in
many of the lower Mammalia;  though still presenting the same general
aspect.—The Auditory ganglia seldom  form distinct lobes or projec-
tions ; but are usually lodged  in  the substance  of the Medulla Oblon-
gata.   lheir_ real  character  is most evident in  certain Fishes, as the
Carp; in which we find  the Auditory Nerve having as distinct a gan-
glionic centre as the Optic.  In higher animals, however, we are able to
trace the Auditory nerve into  a small mass  of gray matter, which lies
on each  side of  the Fourth Ventricle;  and  although this is lodged in
the midst of parts whose function is altogether different, yet there seems
no reason for doubting that it has  a character of its own, and that it is
really the ganglion of the auditory nerve.—We are not able to fix upon
any such mass of  gray matter,  as  the distinct Gustatory ganglion;  nor

                               Fig. 155.
  Diagram of the relation of the Sensori-motor tract at the Vase of the Brain, to the Cerebrum, as seen in
horizontal section:—olf, olfactive ganglia; opt, optic ganglia; aud, auditory ganglia; cs, corpora striata;
thai, thalami optici; a, a, olfactive nerves; b, b, optic nerves; c, c, auditory nerves.

is it necessary to attempt to do so ; for, as we shall see hereafter, there
is strong reason to regard the sense of Taste as only a refined kind of
Touch, combined with the sense  of Smell.
  901. At the  base  of  the Cerebral  Hemispheres, Ave find tAvo gan-
glionic masses  on either side; through which all the fibres pass that
connect the Hemispheres with the  Medulla Oblongata.   These are the
Corpora  Striata,  and Thalami  Optici.   Upon  tracing  fonvards  the
tract of motor fibres that ascend from the Anterior Pyramids, we find
it passing chiefly into the Corpora Striata; whilst if we follow the 
510          OF THE  NERVOUS SYSTEM  AND  ITS ACTIONS.
Sensory Column  that  ascends  from the Posterior Pyramids, Ave shall
find  it to enter the Thalami Optici.   These bodies have been usually
considered as mere appendages to the Cerebrum ; but the fact that they
are independent centres of action is fully established by the presence of
a large  quantity  of vesicular matter  in their substance;  and  there is
now a sufficiently large amount of evidence, both anatomical and physio-
logical,  to render it probable that the fibres Avhich seem to  pass through
them from the Crura Cerebri, and then  to radiate towards the periphery
of the Cerebral Hemispheres, do  not do so in reality, but that these
ganglionic masses receive, on the one  hand, the  fibres that ascend to
them from the Medulla Oblongata, and, on the other, are the  point of
departure of a new set, passing to the  proper  Cerebrum.   Looking to
the connexion of the Thalami  Optici with the sensory tract, it may be
regarded as not improbable that Ave may consider them as the ganglionic
centres  of common sensation; standing in the  same relation to the sen-
sory nerves, that converge from various parts  of  the  body towards the
Encephalon, as do the Optic and other  ganglia to their nerves of special
sensation.   And as these last  give origin to  motor  fibres, so may Ave
regard the ganglionic matter of the Corpora Striata (Avhich are in close
connexion with the Thalami) as probably sharing in the same function;
giving origin to  the motor fibres, which produce the  respondent  con-
sensual movements; just as the anterior peak of gray matter  in the
Spinal Cord gives  exit to the  motor filaments, which effect the reflex
movements  excited through the afferent fibres forming part of the pos-
terior roots.
   902.  The functions of this series  of ganglia  may be more certainly
determined by the aid of Comparative  Anatomy,  than by experimental
mutilations.   Reverting to the class  of  Fishes, we  find that it there
constitutes, with the Cerebellum, nearly the entire Encephalon ;  scarcely
a rudiment of  the true Cerebellum being  discoverable in that group.*
And when Ave descend to the  Im^ertebrata, Ave find  the cephalic masses
entirely to consist of the ganglionic centres  of the nerves of sense and
motion.   There can scarcely be a reasonable doubt, that these  Cephalic
ganglia are the seat of consciousness and the sources of those movements
which are directed  by sensation, in  such animals as present  this low
type of nervous organization;  and there is no adequate reason  for the
belief that the  superaddition of the Cerebral Hemispheres in the Verte-
brated series alters the endoAvments  of the  Sensory Ganglia  on which
they are superimposed;  on the contrary,  we everywhere  see  that the
addition of neAV ganglionic  centres, as instruments  of new functions,
leaves those which were  previously existing in  the  discharge  of their
original duties.  Hence Ave should be led to  regard them as the  centres
of consciousness, even in Man, each pair of ganglionic centres  minister-
ing to that peculiar kind of sensation for Avhich its nerves and the organs
they supply are set apart; thus we should consider the Optic ganglia to
be the seat  of Visual sensations, the Auditory to be the seat of the sense
of hearing, and so on.   And we should also consider them as the instru-

  * The ganglionic masses, commonly designated as the Cerebral lobes or hemispheres,
must be really likened in great part (as already stated \ 869) to the Corpora Striata. 
FUNCTIONS OF THE SENSORY GANGLIA.
511
 ments whereby sensations, of whatever kind, either originate or direct
 Automatic movements.
   903. So far as the results  of  experiments  can be relied on, they
 afford a confirmation of these views, by showing that sensory impressions
 can be felt, and that automatic movements  of  a higher kind than  the
 simply reflex can be called into play after the removal of the Cerebral
 Hemispheres, provided that these ganglia  be  left intact.   Thus, if a
 Bird  be  thus  mutilated, it  maintains its  equilibrium, and recovers it
 when it has been disturbed; if pushed, it walks ; if throAvn into  the air,
 it flies.  A pigeon deprived of its cerebrum  has been  observed  to seek
 out the light parts of a partially-illuminated room in  which it was con-
 fined, and to  avoid  objects that lay in its  Avay;  and at night,  when
 sleeping Avith  closed eyes and its head under its Aving, it raised its head
 and opened its eyes upon  the slightest noise.  So, again, the removal
 or destruction of one  pair  of  these Sensory centres appears to involve
 the loss of the particular  sense to which it  ministers; and frequently,
 also,  to occasion such a disturbance in the ordinary movements of  the
 animal, as to show the importance of these centres in  regulating them.
 Such experiments have been  chiefly made upon the  Optic ganglia, or
 Corpora Quadrigemina, the partial loss of which on one  side produces
 temporary blindness in the eye of the opposite side, and  partial loss of
 muscular poAver on the opposite side of the body;  and the removal of a
 larger portion, or the complete  extirpation of it, occasions permanent
 blindness  and immobility of the pupil, and  temporary muscular weak-
 ness,  on  the  opposite  side.   This temporary disorder of the  muscular
 system sometimes manifests itself in a tendency to move on the  axis, as
 if the animal Avere giddy;  and sometimes in  irregular  convulsive move-
 ments.  Here, then, we have proof of the necessity of the integrity of
 this ganglionic centre for the possession of the sense of vision;  and Ave
 have further proof that the ganglion  is connected with  the  muscular
 apparatus, by  motor nerves issuing from it.   The reason why the eye
 of the opposite side is  affected is to be found in the decussation of the
 optic  nerves, a point to be immediately adverted  to (§ 907).   The in-
 fluence of the  operation on the muscles of the opposite side of the body
 is at  once understood from the fact of the decussation of the motor
 fibres in the anterior pyramids (§ 890).   Similar disturbances of move-
 ment  have been produced by injuries to the organs  of sense themselves,
 or to  the nerves  connecting  them Avith the sensorial centres.   Thus, if
 one of the eyes of a pigeon be blindfolded, or its humors be evacuated,
 vertiginous motions ensue ;  and section of one of the semicircular canals
 of the ear in pigeons  and rabbits has been found  to occasion constant
 efforts to  move in the plane of that canal,  thus  confirming  the belief
 that the function of these canals is to indicate  the direction of sounds
 (§ 952).
   904. Notwithstanding that, in Man, the high development of Lntel-
 ligcnce and the exercise of the Will, supersede  in great degree the
 operations of Instinct, Ave still find that  there  are in  ourselves  certain
movements which can be distinguished as neither voluntary nor simply
reflex, and which are examples  of the method of operation that seems to
be the chief source  of the  actions of the lowTer Vertebrata, as of the 
512         OF  THE NERVOUS SYSTEM  AND ITS ACTIONS.

Invertebrated  classes in general.  These  movements are as automatic
and involuntary  as are the ordinary reflex actions, but differ from them
in requiring that the impressions which originate them should he felt as
sensations ;  and hence they  are  conveniently designated as  consensual.
As examples of  this group, we may advert to the start upon a loud  and
unexpected  sound; the sudden closure of the eyes to a dazzling light,
or on the approach of bodies  that  might injure  them, Avhich has been
observed to  take place even  in cases in which the  eyelids could not be
voluntarily closed; the act of sneezing excited by an irritation of the
nostril, and sometimes  also by a dazzling light; the semi-convulsive
movements and  the laughter called forth by tickling;  and the vomiting
occasioned by the sight or the smell of a loathsome object.   So, again,
the act of yawning is ordinarily called forth by certain uneasy sensa-
tions within ourselves, but also  by the  sight or hearing of the act as
performed  by another.   Various  phenomena of  disease  exhibit  the
powerful influence of  sensations in producing  automatic motions.  As
instances of this kind, we may refer to the effects of the sight or the
sound  of liquids, or of the slightest  currents of air, in exciting  the
Hydrophobic paroxysm; whilst in  many Hysteric subjects  the sight of
a paroxysm in another individual is the most certain means of its induc-
tion in themselves.   The most remarkable examples, hoAvever,  of auto-
matic movements depending upon sensations,  are  those which we come
to perform  habitually, and, as  we commonly say, mechanically, Avhen
the  attention and  the voluntary effort are  directed in quite a  different
channel.  Thus the man who  is walking through  the streets in a com-
plete revery, unravelling some knotty  subject,  or working out a mathe-
matical problem, not only performs the movements of progression, which
may be simply reflex, with great regularity, but also directs these in a
manner which plainly indicates  the guidance of sensations.  Thus, he
will  avoid obstacles in the line of his path, and he Avill folloAV the course
Avhich he has been accustomed to take, although he  may have  intended
to pass along some very different route; and it is not until his attention
is recalled to his situation, that his train  of  thought suffers the  least
intermission, or  that his will is brought to bear upon his motions.
   905. We may trace the agency  of the Sensory Ganglia,  however, in
the Human subject, not merely in  their direct and independent opera-
tion upon the muscular system, but also  in the manner in Avhich they
participate  in all Voluntary actions.   The existence of a Sensation of
some kind,  in connexion with  a Muscular  exertion, seems  essential to
the continuance of the latter.    Our ordinary movements are guided by
what is termed the Muscular  Sense ; that is, by a feeling of the  con-
dition of the muscle, that comes to us through its OAvn sensory nerves.
How necessary this is to  the  exercise  of  muscular power  may be best
judged of from  cases in Avhich it has been  lost.   Thus, a  woman  Avho
had suffered complete loss of sensation in one arm, but who retained its
motor power, found that she could not  support  her infant  upon it,
without constantly looking at  the child; and that, if she Avere to remove
her eyes for a moment, the child would fall in spite of her knowledge
that her infant wras resting upon her arm, and of her desire to sustain
it.  Here,  the  muscular sense being  entirely deficient, the  sense of 
DECUSSATION OF THE OPTIC NERVE.
513
vision supplied Avhat Avas required, so long as it was exercised upon the
object; but as soon as this guiding influence wras withdrawn, the strong-
est  will could not sustain the muscular contraction.  Again, in the
production  of vocal sounds, the nice adjustment of the muscles of the
larynx, which is requisite  to  produce determinate  tones,  can only be
effected in obedience to a mental conception of the tone to be  uttered;
and this conception cannot be formed, unless the sense of hearing has
previously brought similar tones to the mind.   Hence it is that persons
who are born deaf are also dumb.   They may have no malformation of
the organs of speech; but they are incapable of  uttering distinct vocal
sounds or musical tones, because they have not the guiding  conception,
or recalled sensation, of the nature of these.   By long training, and by
efforts directed by the muscular sense of the larynx itself, some persons
thus circumstanced have acquired the power of speech ; but the want of
sufficiently definite control over the vocal muscles is always very evident
in their use of the organ.
   906. Quitting  noAV  the functions  of the Sensory Ganglia, we  have
briefly to notice certain peculiarities in the characters of the Nerves to
which they serve as the centres.  And of these peculiarities,  there is
one of a very remarkable  nature, which  is common to the three nerves
of special sense,—namely, the Olfactive, Optic,  and  Auditory ;—that
they  are not in the least degree endowed Avith common sensibility; so
that they may be  cut, stretched, pinched, &c, without producing the
least pain.    Consequently, the ordinary sensibility of the surfaces  they
supply is entirely due  to the branches of the Fifth pair, which are  dis-
tributed upon them ;  and  Ave may have  a loss of either the general or
special sensibility of any of the organs of sense, without the other being
affected, save indirectly.  Again, we do not find that irritation of these
nerves produces any other purely  reflex  movements, than  such as are
connected with the operations of  the organs of  sense,  in  which  they
respectively originate.  Thus the Olfactory nerve cannot, by any irrita-
tion, be made to excite a reflex movement; the only reflex  action that
can  be excited by irritating the Optic  nerve, is contraction  of the
Pupil; and the regulation of the tension  of the Membrana Tympani (if,
as is probable, this is effected  by the motor poAver  of the Facial nerve,
excited by impressions made upon the organ  of sense),  appears to be
the only reflex action  to which the Auditory nerve can minister.
   907.  There is a further peculiarity, of a very  marked kind,  attend-
ing the course of  the  Optic nerves;  this is the crossing or  decussation
Avhich they undergo, more or less completely,  whilst  proceeding  from
their ganglia to the eyes.   In some  of the lower animals, in which the
two eyes (from their lateral position) have  entirely different spheres of
vision, the decussation is complete ;  the  whole of  the fibres  from the
right Optic ganglion passing into the left eye, and vice versd.  This is
the case for  example,  with most  of the Osseous  Fishes (as the cod,
halibut, &c); and also, in great part at least,  with Birds.  In  the
Human subject, however, and in animals which, like him, have the two
eyes looking in the same direction, the decussation seems less complete ;
but there is a very remarkable arrangement of  the fibres, which seems
destined  to  bring the two eyes into peculiarly  consentaneous action. 
514          OF THE  NERVOUS SYSTEM AND  ITS  ACTIONS.

The posterior border of the Optic Chiasma is formed exclusively of co?n-
missural fibres, which pass from one optic ganglion to the other,  with-
out entering  the real  optic nerve.   Again, the anterior border of the
chiasma is composed of fibres, which seem, in like manner, to act as a
commissure between the two  retinar; passing from  one to  the other,
without any  connexion with the optic ganglia.   The tract  which lies
between the two borders,  and occupies the  middle  of the chiasma, is
the true optic nerve;  and in this it Avould appear that a portion of the
fibres decussates, whilst another portion passes directly  from each Op-
tic ganglion into the corresponding eye.  The fibres which proceed from
the ganglia to the retinae, and constitute the proper  optic nerves, may
be distinguished into an  internal  and  external tract.   Of these, the
external, on each side, passes directly onwards to the eye of that  side;
whilst the internal crosses over to the eye of the opposite side. The dis-
tribution of these two sets of fibres in the retina of each side respectively,
is such that, according to Mr. Mayo, the fibres from  either  optic gan-
glion will be distributed to its own side of both eyes;—the  right  optic
ganglion being thus  exclusively connected  with the outer part of the
retina of the right eye, and with the inner part of the retina  of the left
eye;  and the left optic ganglion being, in like  manner, connected  ex-
clusively with the  outer side of the left retina, and with the  inner side
of the right.   Now as either side  of the  eye receives the images  of
objects, which are on  the other side of its axis, it follows, if this account
of their distribution be correct, that in Man, as  in  the lower animals,
each ganglion receives the sensations of objects situated on the opposite
sides  of the body.  The purposes of this decussation may be, to  bring
the visual impressions, which are so  important in directing the move-
ments of the body, into proper  harmony with the motor apparatus ;  so
that, the decussation of the motor fibres in the pyramids being accom-
panied by a  decussation of the optic nerves, the  same effect  is produced
as if neither decussated,—which  last is the case  with Invertebrated
animals in general.

                     6. Functions of the Cerebellum.

  908.  Much discussion has taken place, of late years, respecting  the
uses of the  Cerebellum ; and many experiments have  been made  to
determine them.   That it is in some way connected  with the powers of
motion, might be  inferred  from its  connexion with the antero-lateral
columns of the Spinal Cord, as well as with the posterior ; and the com-
parative size of the organ,  in different orders of Vertebrated animals,
gives us some indication of what the nature  of its functions may be. For
Ave find its degree of development corresponding  pretty closely with  the
variety and energy of the muscular movements which are habitually exe-
cuted by the species ; the organ being the largest in those animals  Avhich
require the combined effort of a great variety of muscles to maintain their
usual position, or to execute their ordinary movements;  whilst it is  the
smallest in those which require no muscular  exertion for the one pur-
pose, and little combination of different actions for the other.  Thus
in animals that habitually rest and  move upon four legs, there  is com- 
FUNCTIONS OF THE  CEREBELLUM.
515
paratively little  occasion for any organ  to  combine and harmonize
the actions of their several  muscles ;  and  in these, the Cerebellum  is
usually small.  But  among the more active predaceous Fishes (as the
Shark), Birds of  the most powerful and varied flight (as the  SAvallow),
and such Mammals as can maintain the erect position, and can use their
extremities for other  purposes than support and motion,—we find the
Cerebellum  of much  greater size, relatively to  the remainder of the
Encephalon.   There is a marked advance in this respect, as we ascend
through the series of Quadrumanous animals ;  from the Baboons, which
usually walk on all-fours, to the semi-erect Apes, which often  stand and
move on their hind-legs only.    The greatest development of the Cere-
bellum is found in Man; who surpasses all other animals in the number
and variety  of the combinations of muscular  movement which  his
ordinary actions  involve, as well  as of those  which he is  capable, by
practice, of learning to execute.
   909.  From experiments  upon  all classes  of Vertebrated animals,
it has been found that, when the Cerebellum is  removed, the power of
walking, springing, flying, standing, or maintaining the equilibrium  of
the body, is destroyed.   It does  not seem that the animal has in any
degree lost the voluntary power over its individual muscles; but it  can-
not combine their actions for any general movements of the  body.   The
reflex  movements, such  as  those  of respiration,  remain  unimpaired.
When an animal thus mutilated is laid on its back, it cannot recover its
former  posture;  but it moves its  limbs, or flutters its wings, and evi-
dently is not in a state of stupor.   When  placed in the erect position,
it staggers and falls like a drunken man; not, hoAvever, without making
efforts to maintain its balance.   Phrenologists, who attribute a different
function to the Cerebellum  have attempted to put aside  these results,
on  the ground that the severity of the operation is alone  sufficient to
produce them ; but, as we shall presently see,  many animals may be
subjected to a much more severe operation, the removal of the Cerebral
hemispheres,  without the loss of the power of combining and  harmoniz-
ing the muscular actions, provided the Cerebellum be  left uninjured.—
Thus, then, the idea of the functions of the Cerebellum, which we derive
from Comparative Anatomy, seems fully borne out by  the results of ex-
periment ; and it is also consistent Avith the indications which may be
drawn from the observations of  Pathological  phenomena.    When the
Cerebellum is affected with chronic disease, the motor function is seldom
destroyed  ; but the same kind of want of combining power jsIioavs itself,
as when the organ has been purposely mutilated.   Some kind of lesion
of the  motor function is invariably to be  obsen'ed ; whilst the mental
powers may or may not  be  affected,—probably according to  the influ-
ence of the disease in  the Cerebellum, upon other  parts.  ^ The same
absence of any direct connexion  with the Psychical poAvers,  is shoAvn in
the fact, that inflammation of the membranes covering it, if confined to
the Cerebellum, does not produce delirium.  Sudden effusions of blood
into its substance may  produce apoplexy or paralysis ; but  this may
occur as a consequence  of effusions into  any  part of the Encephalon,
and does not indicate that the Cerebellum  has anything to  do with the
mental functions, or with the power of the Will over the muscles. 
516         OF  THE NERVOUS  SYSTEM AND ITS ACTIONS.

  910. There  is  another  doctrine, however, in regard to the functions
of the Cerebellum, first propounded by  Gall; which ought not  to  be
altogether passed by.   According  to the system of Phrenologists, the
Cerebellum is the organ of the sexual instinct; and its connexion with
the motor function is limited to the  performance of the movements, to
which that instinct leads.  This doctrine derives no support,  however,
from the fact* supplied by Comparative  Anatomy; for there is a com-
plete  want of correspondence  between the size  of the  Cerebellum in
different  animals, and the power of  their  sexual  instinct.—Again,
although Pathology has been  appealed to, as showing a decided con-
nexion between  disease of the  Cerebellum and affection of the Genital
organs (manifesting itself  in priapism, turgescence of the testes, seminal
emissions, &c), yet it  appears,  on a careful  examination of evidence,
that such a sympathy is  comparatively ^rare, not  being displayed  in
more than one out of every seventeen cases of Cerebellic disease.   And
where it is manifested, it is explicable quite readily by the known fact,
that this kind of excitement of the genital organs may be produced  by
excitement of the spinal cord  and  medulla  oblongata.—Little  or  no
light has been thrown on  this question by experiment.  It was asserted
by  Gall, that  the Cerebellum is very small in castrated animals; but
this assertion has been met by  the most positive counter-statements on
the part of Leuret, who has shown that the average weight of the Cere-
bellum (both absolutely and in  proportion to the weight of the  entire
encephalon) is even greater in  Geldings, than in Stallions or Mares.—It
is  asserted, however, that the results of observation in Man lead to a
positive conclusion, that  the size  of the Cerebellum is a measure of
the intensity of  the sexual instinct  in the individual.   This  assertion
has been met by the counter-statement of others, that no such relation
exists.  There are, of course, very  great difficulties in regard to the
collection of accurate  information on  this subject;  and the question
must be at present regarded as sub judice.
  911. It may be added,  that  the idea of a special connexion between
the sexual instinct and the Cerebellum, is not inconsistent with the view
of its function  previously stated ; and  it would seem to  derive  some
confirmation from the fact, that an unusual amount of muscular  exer-
tion appears to have a peculiar tendency to depress the sexual  passion,
even whilst it increases the general vigor of the system.  If the  Cere-
bellum be really connected with both  kinds of functions, it  does not
seem  unlikely that the excessive  employment of it upon one, should
diminish  its energy in regard  to the  other.   Further, it seems not
improbable,  that the Lobes of the Cerebellum are the  parts specially
concerned in the regulation of the muscular movements ; whilst the
central portion (constituting the Vermiform  process in Man, but form-
ing the entire cerebellum  of many  of the lower Vertebrata, such  as the
Frog) may contain the centre of the sexual sensations, and may thus be
the instrument of the consensual actions  to Avhich they give rise.

                    7. Functions of the Cerebrum.
  912. The view which has been taken of the Comparative structure of
the Nervous system, in different animals, leads  to  the conclusion, that 
RELATIVE DEVELOPMENT OF THE CEREBELLUM.        517
the Cerebral Hemispheres are far from  being  the essential parts of the
apparatus they were formerly imagined to be ;  and that they are, on
the contrary, superadded organs, of which we find no distinct represen-
tatives in the Invertebrata, and of which the first  appearance (in the
class of Fishes) exhibits  them in the light  of appendages, destined to
perform some  special function  peculiar to Vertebrated animals.   The
results of the removal of the Cerebral Hemispheres,  in animals to which
the shock of the operation does  not prove immediately fatal,  fully con-
firms  this view;  and must  appear  extraordinary to those,  who have
been  accustomed to regard these organs as the centre  of all energy.
Not only Reptiles, but Birds and Mammalia, if their physical wants be
supplied, may survive the removal of the whole Cerebrum for weeks, or
even months.  If the entire mass be taken away at once,  the operation
is usually fatal; but if it be  removed by successive slices, the shock is
less severe, and the depression it produces in the  organic functions is
soon  recovered  from.   It is  difficult  to substantiate the existence of
actual sensation in  animals  thus circumstanced; but their movements
appear to be of a higher kind, as already remarked (§ 903), than those
resulting from mere  reflex action.  Thus they will  eat food,  when it is
put into  their mouths ; although they do not go to seek it.  If violently
aroused,  the animal has all the manner of one waking from sleep; and
it manifests about the  same  degree of consciousness as a  sleeping
Man, whose torpor is not too profound to prevent his suffering from an
uneasy position, and who moves himself to  amend  it.  In both cases,
the movements are consensual only, and do not  indicate  any  voluntary
power ; and we  may well believe that, in the former case  as  in  the
latter, though  felt they are not remembered; an  active state  of the
Cerebrum being essential to memory, though not to sensations, which
simply excite certain  actions.—When  the  Cerebral Hemispheres are
being removed, slice by slice, it  is noticed that injuries of these organs
neither occasion any  signs of pain, nor give  rise  to convulsive move-
ments.   Even  the  Thalami  and Corpora Striata  may   be   wounded,
without the  excitement of convulsions;  Avhilst, if the  incisions involve
the Tubercula Quadrigemina, convulsions uniformly occur.  It has been
often observed in Man, that, when it has been  necessary to separate
protruded portions of the brain from the healthy part, no  sensation was
produced, even though the mind Avas perfectly clear at the time.  Hence
it would  appear that neither is  the Cerebrum itself  the centre of sensa-
tion, nor is it so connected with that centre, as  to  be able to convey to
it sensory impressions of  an  ordinary kind.   This  is analogous to the
condition of the nerves of special sense, as already remarked.   That no
irritation of the  cerebral substance should excite convulsive movements,
is a very remarkable circumstance;  and it  seems  to indicate, that the
changes  which mental operations produce in the cerebral fibres, cannot
be imitated, as changes  in other motor fibres may be, by physical im-
pressions.                                                   .
   913. As already stated, the relative amount  of  Lntelhgence in diffe-
rent animals bears so close a correspondence with the relative size and
development of the Cerebral Hemispheres, that it can scarcely be ques-
tioned that these constitute  the organ  of the Reasoning faculties, and 
518         OF  THE NERVOUS  SYSTEM AND ITS ACTIONS.

issue the mandates by which the Will calls the muscles into action.  It
must be borne in mind, however, that size is not by any means the only
indication of their comparative development.  As we advance from the
lower to the higher Vertebrata, we observe a marked advance in the
complexity of the structure of the  Cerebrum.  Its surface  becomes
marked  by convolutions, that greatly  increase the  area  over  which
blood-vessels can enter it from the surrounding membranes; and in pro-
portion to the  increase in the number and depth of these, do we find an
increase in the thickness of the layer of gray matter, which is the source
of all the powers of the organ.  The arrangement of the white or fibrous
tissue, which  forms the  interior  of the mass,  also  increases in  com-
plexity ;  and as  we ascend even from the lower Mammalia  up to  Man,
we trace a 'marked increase in the number of the fibres,  which  esta-
blish communication between different parts  of the organ. It is, in fact,
not merely from the different parts  of the gray matter which forms the
surface of the  hemispheres, that these commissural fibres arise ; but also
from those isolated portions of vesicular substance, which are found in
different parts of their  interior;  and an extremely complex system is
thus formed, which is  still but very  imperfectly understood.
   914. The most important group of commissural fibres, is that which
connects the Sensory with  the Hemispheric Ganglia;  that is,  which
radiates from  the Thalami Optici and Corpora Striata, to the stratum of
gray  matter which forms  the convoluted  surface  of the Cerebrum.
These fibres constitute,  in  fact, the principal part of the white sub-
stance of the brain ; the remainder  being made up by the commissures
to be presently described, and by commissural fibres which (it is pro-
bable) connect the different parts  of the  Cerebral  surface with each
other.   It was  formerly  supposed  (and is  still maintained by many
Anatomists), that the radiating fibres which may be traced  to the Cor-
pora Striata and Thalami Optici, pass through these bodies, and are
continuous with  the Crura Cerebri,  and consequently with  the sensory
and motor tracts of the Medulla Oblongata.  But  Avhen the small size
of the Crura Cerebri is  compared with the relatively enormous bulk of
the radiating  fibres, it is obvious that the former can only  contain but
a very small proportion of the latter; and  as  no absolute continuity
has been traced, it appears more conformable to Anatomical and Phy-
siological probability,  to  believe that the fibres of the Crura Cerebri
pass  no  further upwards than  the Sensory Ganglia, and that the
radiating fibres  take  a fresh departure from these  bodies, to pass to-
wards the surface of the Cerebrum.—Thus, then, we  should be  led  to
regard the Spinal Cord, Medulla  Oblongata, and  chain  of Sensory
Ganglia, as precisely representing the entire Nervous System of Insects,
the character  of whose action is essentially automatic ;  and to consider
the Cerebrum as an organ superadded at  its summit, receiving  all its
incitement to action  from impressions  transmitted  to it  through the
Sensory Ganglia, and carrying into effect  its volitional determinations
and  emotional impulses, not (as formerly supposed) by immediately ex-
citing muscular movements  through nervous  communications passing
direct from the  convoluted  surface  of  the  Cerebrum,  but  by playing
downwards upon the  Automatic apparatus by Avhich its mandates are 
            COMMISSURES OF  THE  CEREBRAL  HEMISPHERES.        519

carried  into  effect (Figs. 155,  156).   Of this view  we  shall presently
find that there is strong physiological evidence.

                                Fig. 156.
  Diagram of the mutual relations of the principal Encephalic centres, as shown in a vertical section :—a,
 Cerebrum; b, Cerebellum; c, Sensori-motor tract, including the Olfactive ganglion olf the Optic opt, and
 the Auditory aud, with the Thalami Optici thai, and the Corpora Striata cs; r>, Medulla Oblongata; e, Spinal
 Cord:—a, olfactive nerve; 6,optic; c, auditory; d, pneumogastric; e, hypoglossal; /, spinal: fibres of the
 medullary substance of the cerebrum are shown, connecting its ganglionic surface with the sensori-motor
 tract.

   915.  The two Hemispheres are united  on the median line by several
 transverse  commissures; of which the Corpus  Callosum is  the most  im-
 portant.   This consists of a mass of  fibres  very  closely interlaced  to-
 gether ; which may be traced into the substance of the  hemispheres on
 each side,  particularly at their  lower part, Avhere  they are connected
 with the thalami optici and corpora striata.   It is difficult, if not impos-
 sible, to trace its fibres any  further;  but  there can be  little doubt that
 they radiate, with the fibres proceeding from the bodies just named, to
 the different parts of the surface  of the hemispheres.   This commissure
 is altogether  absent in Fish, Reptiles, and Birds; and it is partially or
 completely wanting in the  Mammalia with least  perfect  brain, as  the
 Rodents  and  Marsupials.—The  other transverse  commissures  rather
 belong  to the Sensory Ganglia than to the Cerebral hemispheres. Thus
 the anterior commissure  particularly  unites the Corpora Striata of  the
 two sides; but many of its fibres pass through those organs, and radiate
 towards the convolutions  of the hemispheres, especially  those  of  the
 middle  lobe.   This  commissure is  particularly large in those  Marsu-
 pials, in  which the corpus  callosum  is deficient.—The posterior com-
 missure is a band of fibres which connects the Optic Thalami; crossing
over from the posterior extremity of one to that of the other.—Besides
 these, there are other groups of fibres, which seem to have similar com-
missural functions, but which are intermingled with vesicular substance. 
520         OF  THE NERVOUS  SYSTEM AND ITS ACTIONS.

Such are the soft commissure, which also extends between the Thalami;
the Pons Tarini, Avhich extends between the two crura or peduncles of
the Cerebrum;  and the Tuber  Cinereum, which  seems to unite  the
optic tracts with the thalami, the corpus callosum, the fornix, &c, and
to be a common  point of meeting for several distinct groups of fibres.
   916.  The anterior and posterior parts of the hemispheres, moreover,
are connected by longitudinal Commissures, of which some lie above,
and some below, the corpus callosum ; and of these, also, a part belong
to the  Sensory  Ganglia.  Above the transverse  fibres  of the corpus
callosum, there is  a longitudinal tract on  each side of the median line,
which serves to  connect the convolutions of the anterior  and  posterior
lobes of  the brain.—And above this, again, is the superior longitudinal
commissure, which is formed by the fibrous matter of the great convolu-
tion nearest the median plane  on the upper surface  of the brain, and
which connects the convolutions of the anterior and  middle lobe with
those of the posterior.—Beneath the great transverse commissure, we
find the most extensive of all the longitudinal commissures, namely, the
Fornix.  This is connected in front with the optic thalami, the mammil-
lary bodies,  the  tuber cinereum, &c;  and behind, it  spreads  its fibres
over the hippocampi (major and minor), which  are  nothing else than
peculiar convolutions that  project into the posterior and  descending
cornua of the lateral ventricles.  The fourth longitudinal cotnmissure is
the tarnia semicircularis, which forms  part of the same system of fibres
with the fornix; connecting the corpus mammilare and thalamus opticus
with the middle  lobe of the cerebral hemisphere.  If, as Dr. Todd  has
remarked, we could  take away the corpus callosum, the gray matter of
the internal convolution, and the  ventricular  prominence  of the optic
thalami, then  all these commissures  would fall together, and become
united as one and  the same series  of  longitudinal  fibres.—It is curious
that there should  be no direct  communication betAveen the Cerebral
hemispheres and the Cerebellum ; the only commissural  band  between
them  being the processus a cerebello  ad testes, which passes  onwards,
through  the Tubercula Quadrigemina, to the Thalamus Opticus on each
side.   This would seem  to confirm the idea of  the complete distinctness
of their functions.
   917.  The Cerebrum appears to be the instrument of all those psychi-
cal operations, which are  superadded, in Man and  the higher Verte-
brata,  to mere sensations.  The impressions which are merely felt in
the sensorium, give rise, Avhen they pass  upAvards  into the Cerebrum,
to Ldeas, which  then become the material  (so to speak) of all the higher
mental  processes.  These  processes  may be  ranked under two  dis-
tinct heads, namely, the Emotional and the Intelligential; the former
being most intimately connected with the sensations Avhich prompt them,
whilst the latter  are commonly of  a much  more abstract  character.
The Emotions may, in fact, be considered  as feelings of pleasure or pain
associated with  particular  classes of ideas;  and it  is  this association
which gives them the character of the moving or active powers of the
mind, and which makes them, either  directly or indirectly, the springs
of  the greater  part of  our actions.   When strongly excited,  the Emo-
tions may produce  movements which  the  Will may  not be able to re- 
FUNCTIONS OF THE CEREBRUM.
521
 strain ; as when Ave burst into laughter at some ludicrous image presented
 to the mind, either by a present sensation or by an act of the  Memory
 or Imagination, notwithstanding the strongest inducements presented by
 "time, place,  and circumstance" to a  preservation  of  our  gravity.
 The  distinctness of the character of Emotional and Volitional move-
 ments is  further evident  from this, that  cases of paralysis  not unfre-
 quently occur (especially in the facial nerves, through which most of the
 muscles of "expression" are excited to action), in which the muscles are
 obedient to one class of impulses,  Avhile the other exerts no power over
 them.  Thus, in one instance, the muscles of  one side of  the face were
 palsied in such  a  manner, that the patient could not voluntarily close
 his eye, nor draw his mouth towards that side ; yet when any ludicrous
 circumstance caused him  to laugh, their usual play  was  manifested in
 the expression of his countenance.  And in another case, the  muscles
 were obedient  to the will; but when the  individual laughed or cried
 under the influence of an emotion, it was only on one side of his face.
 To these  may be added another case, in which the right arm was com-
 pletely palsied, so that the individual had not the least  voluntary power
 over  it; yet it was violently agitated, whenever he met a friend whom he
 desired to  greet.—The influence  of an undue tendency to Emotional
 excitement, is remarkably seen in  what are ordinarily termed Hysterical
 states of  the system; in which violent  convulsive paroxysms  are fre-
 quently brought on by the most trivial  causes, if these should  call the
 passions or  affections of the mind into undue activity.   There can be no
 doubt that  many of the peculiar  actions performed  by the subject of
 what is termed Mesmeric influence, are the result of a condition of this
 nature.   There appears to be, in such persons, a proneness to  activity
 of the  consensual  and emotional parts of the nervous centres, Avhich
 manifests itself most strongly when the control of the Avill is withdrawn;
 and  thus very slight  impressions produce very powerful involuntary
 movements,—especially when this response is favored by  the  strong
 desire, on the part of the patient, to exhibit any particular manifestation
 that is known to be expected by the bystanders.
   918. It has been supposed by some, that the Emotional movements
 of Man and the  higher animals may be ranked  in the same category
 with  Lnstinctive actions   of the lower;  and that the Desires of the
 former  are  comparable to the instinctive Propensities of  the  latter.
 But this comparison is erroneous; for what Ave term propensities (among
 the lower animals)  are nothing else than tendencies to  perform particular
 movements  in respondence to particular sensations, without any idea of
 the purpose of the movement or of the object AA'hich has excited it; whilst an
 Emotion involves an idea of the object which has called it up, and a Desire
■ involves a conception of the object  to be obtained.—The imitative actions
 afford a good  example of the difference betAveen a  propensity and a
 desire.  The former is manifested in such imitative movements  as are
 purely consensual; the sensation,  Avhich is the mainspring of the action
 in each case, exciting  a  respondent automatic movement, as when we
 yawn involuntarily from  seeing  or hearing  the action  performed by
 another or as when children learn undesignedly to  perform many of
 the movements Avhich they Avitness in adults.  This propensity to  in- 
522          OF THE NERVOUS SYSTEM AND ITS ACTIONS.

voluntary imitation is much stronger in some individuals than in others.
On the other hand, imitative actions may be voluntarily performed,  as
the result of a desire to execute them, which involves a distinct idea  of
the object;  and the moving force  of this desire is derived from the
pleasure which the individual derives from  the performance, and Avhich
he finds  either in  the act itself, or in the enjoyment which it affords to
others, or in its prospective benefits  (pecuniary or otherwise) to himself.
Thus we see that the Mind (properly so called) is concerned in all Emo-
tional actions; whilst there is no evidence of the participation of any higher  N
attribute than sensation in the purely Instinctive acts; and even this is
not a requisite link in the chain, by which many of  the  movements are
excited, that are usually grouped together under that designation.
   919.  Again, the Emotions may be excited by operations of the Mind
itself, as well as by  sensations  immediately received  from  without.
Thus, involuntary laughter may result from a ludicrous idea, called up
by some train of association, and having no obvious  connexion with the
sensation which first set this process in operation ; and the various move-
ments of the face and person by which Actors endeavor to express strong
emotions, are most effectual in conveying their meaning, when they result
from  the actual working of the emotions  in the mind of the performer,
who has by an effort of the will, identified himself (so  to speak) with
the character he personates.   A still more remarkable  case is that  in
which paroxysms  of Hysterical convulsion, in  themselves beyond the
power of the Will to excite or control, are brought  on  by a  voluntary
effort; this being  exerted, not in the attempts to  perform the move-
ments, but  in  "getting up," so to  speak, the state of feeling, from
which, AAThen it is once excited, the movements spontaneously flow.  In
all these instances, and others of  like nature, it would  seem as if the
agency of the Cerebrum produced the same condition in the Automatic
centres, as  that which  is more directly excited  by  sensations received
through their own afferent nerves.—But  on the other  hand, the Emo-
tions, by their influence on the Reasoning processes, are largely  con-
cerned in many actions which are strictly voluntary; in fact it may be
questioned Avhether there are any of  our actions, the  power necessary for
whose performance is not derived, directly or indirectly, from emotional
states of mind; all our motives to any  kind of exertion being found, if care-
fully  analysed, to have reference to pleasure to be derived, or pain to be
avoided, either in  the very performance of the action, or in the conse-
quences Avhich our reasoning processes connect with  it.   And it will  be
found that  the difference betAveen those persons who are said to act from
feeling, and those who are said to be guided by reason, is  not precisely
what  these terms imply ;  for the actions of both are equally determined
by the motives supplied by emotional states ; and the difference rather •
lies in this, that one class act on their first impulses  without considering
the  consequences, whilst the  other  calculate  the remoter results, and
weigh the future pain against the present pleasure, the ultimate enjoy-
ment against the immediate  distress.—The Emotional  states are pecu-
liarly liable to be influenced by the condition  of the corporeal system ;
thus a very slight depravation of the blood may produce an irresistible
tendency to take a gloomy view of everything  to which the mind  may 
FUNCTIONS OF THE CEREBRUM.
523
 be directed, and especially of all that relates to the individual;  whilst
 that condition  of  perfect health which is derived from wholesome  re-
 creation, fresh  air, active exercise, &c, is almost always accompanied
 Avith a degree of cheerfulness and elasticity, which occasions even real
 evils to be but comparatively little felt.
   920.  When  we  turn our  attention to the Intelligential actions of
 which  the Cerebrum appears (in our present state of being) to  be the
 exclusive instrument, we  perceive that the attribute by which they are
 distinguished both from the Instinctive and Emotional, is  their inten-
 tional or purposive performance, in accordance with the mental concep-
 tion of the  object to be attained, and the intellectual belief as  to the
 most  advantageous  means  of  accomplishing  it.  The decision thus
 formed by the Reasoning processes, is put into operation by the Will;
 and thus it is the characteristic  of a Voluntary act, that it is designed
 by  the  individual to  answer a certain  purpose which is distinctly
 present to the mind.—Now when we come to  analyze the faculties
 concerned in this  class of operations,  we  find that the one most closely
 related  to the simple Sensorial  powers already treated of,  and  at the
 same  time  most  essential  to all the "higher  operations, is Memory.
 This faculty is  one of  those first awakened in the opening mind  of the
 Infant; and we find  traces of it in animals that seem  to be otherwise
 guided by pure Instinct.   It obviously affords the first steps  towards
 the exercise of the  reasoning poAvers ;  since no experience can be gained
 without it, and  the foundation of all intelligent adaptation of means to
 ends lies  in the application of the knoAvledge which has been acquired
 and stored up in the  mind.  There  is strong reason to believe that this
 attribute  belongs  to the  Cerebrum  exclusively;  no  impression made
 upon the Sensorial centres  being ever  remembered,  unless they are
 registered (as it were)  in  this organ.  And further, there  is evidence
 that no impression of this kind  once made upon  the Cerebrum is ever
 entirely lost, in the  normal state;  although disease  or accident will
 sometimes occasion a  complete  destruction  of  the  memory,  or wrill
 obliterate the remembrance of a particular class of objects or of ideas.
 All memory, hoAvever,  seems to  depend upon the  principle  of Sugges-
 tion ;  one idea being linked with another, or with a  particular  sensa-
 tion, in such  a manner as  to be called  up by its recurrence ; and  a
 period of many years frequently intervening, without  that combination
 of circumstances presenting itself Avhich is requisite to arouse the dor-
 mant  impression of  some early event.   Sometimes  this  combination
 occurs in Dreaming,  Delirium, or Insanity, three states which agree in
 this, that the Will has no control  over the  current  of thought; and
 ideas are thus recalled, of which the mind in a state of healthy activity
■has no remembrance.
   921. It is upon the ideas  aroused in the mind by Sensorial changes,
 or  recalled  by Conception, or evolved  by  the process of Reflection (in
 Avhich the  mind  perceives  its  OAvn  operations,  and  traces relations
 amongst  its objects  of thought), or  generated  by  the Lmagination
 (which  really acts,  however, rather by combining into new forms, than
 by creating altogether  de novo), that  all  acts of  Reasoning are based.
 These consist, for the most part, in the aggregation and collocation  of 
524         OF THE NERVOUS SYSTEM AND ITS ACTIONS.

ideas, the decomposition of complex ideas  into more simple ones, and
the combination of simple ideas into general expressions; in which are
exercised the faculty of Comparison, by which  the  relations and con-
nexions  of ideas are perceived, that  of Abstraction, by which we  fix
our attention on any particular qualities  of the  object of our thought,
and isolate it from  the  rest, and  that  of  Generalization, by which  we
grasp  in our minds some definite notions in regard  to  the general
relations of those objects.  These are the processes chiefly concerned
in  the simple  acquirement of KnoAvledge, Avith which  class of opera-
tions the Emotional part  of  our nature has  very little participation,
save  as  furnishing  the desire which may be   the  necessary incitement
to the exertion of  the intellect.   A  certain measure of  intellectual
activity seems  natural  to  Man, provided  that the development of the
mind has taken place  under  favorable circumstances;  and our highest
pleasures are connected with the healthful and almost spontaneous exer-
cise of its faculties.
   922. But the Will possesses a determining power over  the  mental
as Avell as  over the bodily operations; and it  is, in fact, this determin-
ing power which is the source of the self-control that characterizes the
well-regulated  mind of  Man, and  distinguishes  him  alike from  the
madman and the brute.  The regulation of our  conduct consists in the
application  of  our reasoning  poAvers to  the circumstances of our con-
dition, and in  the due regulation of those emotional tendencies, which
(as already pointed out, § 919) are the moving springs  of our actions.
However powerful  these  tendencies  may  be, there can be no doubt,
that we  possess within  ourselves the  means of checking them,  by
withdrawing our minds by a  voluntary effort  from the  thoughts which
they suggest, as well as by calling forth  opposing influences within us,
so  that  the decision which  is  finally arrived at is  something  very
different  from  that  which the first "balance  of motives" Avould  have
produced.   It is  the  deficiency or entire loss  of this power of  self-
control that usually constitutes  the first step in the  development of
Insanity; for this state generally consists, not so much in  a perversion
of  the reasoning processes,  as  in a  disorder of the emotional state,
which  causes the  patient  to  dwell upon particular  trains of thought,
until his  feelings in  relation to them become exaggerated or perverted;
and at last intellectual delusions arise, from the habit of vieAving every-
thing that comes before  the mind  through  a distorted medium,  and
from the substitution of the patient's morbid imaginings for real occur-
rences.   In what is  now termed impulsive Insanity, there is intellectual
perversion ; but a desire of some kind is so powerfully excited, that the
Will cannot control  it.   And every phase may be witnessed between a
state of this kind, which renders the individual an irresponsible agent,
and that  mental condition in which the individual, though originally  fully
able to control  himself, habitually gives Avay to his passions, and thus,
by their  continual indulgence, at  last alloAvs them to become the domi-
nant powers of his mind.
   923. Although the Will has been usually regarded as directly deter-
mining those muscular  movements Avhich  are  usually distinguished as
Voluntary, through  the intermediation of fibres originating in the cere- 
FUNCTIONS OF THE CEREBRUM.
525
 bral convolutions and proceeding to the muscles, yet a careful analysis
 of the process fully bears out  the idea already put forwards, that the
 Will really operates through  the Automatic apparatus, exciting parti-
 cular groups of muscular actions, just as they would be called forth by
 sensations directly excited by external objects.   For it has been shown
 that the Craniospinal  Axis (consisting of the Sensory Ganglia, Me-
 dulla  Oblongata, and Spinal Cord) receives all the sensory nerves, and
 gives origin to all the motor;  and that the fibres which pass between
 the Cerebral convolutions  and  the Sensory Ganglia, probably serve to
 bring these centres into mutual relation, and are  not continuous with
 those of any nerves, either sensory or motor.  And we might expect,
 therefore that  the addition of a cerebrum  to  this  automatic apparatus
 would have the effect of supplying a new stimulus to movement, which,
 whilst proceeding from mental  operations,  should still act  through the
 same mechanism as that already provided for the reflex and consensual
 movements.   Now, when we attentively consider the nature of what we
 are accustomed to call voluntary action, we perceive that the agency of
 the Will is limited to the  determination of the result; and that it has
 nothing to  do with  the selection and co-ordination of the individual
 movements, by which that result is brought about.   If it were other-
 wise, we should be dependent upon our anatomical knowledge, for our
 power of performing even the simplest movements of the body.  Again,
 there  are very few cases in which we  can single  out any indivfdual
 muscle, and put it into action independently of others;  and  the cases in
 which we can do  so, are  those in which a single muscle is concerned in
 producing the  result, as in the elevation  of the eyelid; and  we then
 really single out the muscle by "willing" the result.  Thus, then, how-
 ever startling the position may at first appear, Ave have a right to affirm
 that the Will  cannot exert  any direct  or  immediate  power  over the
 muscles; but that its determinations are carried into effect through an
 intermediate mechanism, which,  without any further guidance on our own
 part, selects and  combines the particular muscles whose contractions are
 requisite to produce the  desired movement.  We have seen  that the
 Sensorial centres  play (so  to speak) upon the Cerebrum, sending to it
 impressions of a kind fitted to call forth its peculiar activity as an instru-
 ment of purely mental operations ; and in return, the Cerebrum appears
 to play downwards upon  the motor portion of the automatic apparatus,
 sending to it volitional impulses  Avhich  excite its motorial activity.  And
 thus Ave see  that the very same action may be excited through an impres-
 sion conveyed to the centres of  the whole system through some one or
 more nerves of the external senses,  or  through the  fibres converging to
 them from the cerebral  convolutions,  Avhich  have been not  unaptly
 called " the  nerves of the internal senses ;" and may hence be automatic
 in the first case, and voluntary in the second.   For example, in the act
 of Coughing, we  have the very same  combination  and succession  of
diverse but mutually related actions, whether the operation be excited
by the presence of an irritating  particle  in  the air-passages, or be per-
formed as the consequence of a voluntary effort.   And a little attention
to his own  consciousness will satisfy the reader, that as regards the
selection and co-ordination of the movements which are concerned, the 
526         OF  THE NERVOUS  SYSTEM AND ITS ACTIONS.

Intelligence and Will are no more concerned in the one case than in the
other.—And hence it  follows,  that all the  movements which are per-
formed by the instrumentality of the  Cerebro-spinal system of ganglia
and nerves,  are in their essential nature automatic; and that their cha-
racter  as Reflex, Instinctive,  Emotional,  or Voluntary, is  entirely
dependent on the nature and  seat  of the  impulses which respectively
originate them.
   924.  There are  various  conditions,  some  of  them natural, others
morbid, in which the distinctness of the functions of the Cerebral Hemi-
spheres is  well marked.    Thus  in  profound sleep  they seem  to be
entirely dormant; the Spinal  Cord and  Medulla  Oblongata, by which
the necessary reflex actions are carried on, being alone  in  a state of
activity.  In this condition, the Sensory ganglia  also appear to be in a
torpid state; but in less  profound sleep, actions are  often performed,
which may be referred  to the consensual group,—being such as the sen-
sation would immediately  prompt, without any reflection, and not being
remembered in the waking state.  Thus we turn in our beds, under the
influence of an uneasy sensation ;  or  we  give some sign of recognition
when our names are called.  The first of these appears to be a purely
consensual movement,  being as automatic as  if it Avere a reflex action;
the other seems to have become as  automatic by the influence of habit,
and to belong to that  class of secondarily automatic actions, in which
the movement, though at first directed  by the will, has  become, after
very frequent performance, so  closely associated with  the  guiding sug-
gestion, as to be called forth by it  alone (§ 904).—In the Coma of Apo-
plexy, Narcotic  Poisoning, &c, we witness the same  gradations as in
ordinary sleep.  When it is least profound,  it seems to affect the Cere-
bral hemispheres alone  ; the Sensory Ganglia being still, in some degree,
open to the  reception of impressions.  When complete, hoAvever, none
but reflex actions can be excited;  and if it  advance to a fatal termina-
tion, it does so by the  supervention of the same state of torpidity in the
Medulla Oblongata, whereby the respiratory movements are  brought to
a close.  These movements do  not cease  until the power of deglutition
has been lost, and until the eye ceases to close,  when the edge of the
lid is irritated;  but when this  is the case,  a fatal  termination may be
apprehended, as  it is  thus shown that  the torpor is extending to the
Spinal system of Nerves.—In  the  condition of Dreaming,  it would seem
as if the Cerebrum were partially active; a train of thought  being sug-
gested, frequently by sensations from without; which is carried on with-
out any controlling or directing power on the part of the Mind;  and
which is not corrected, or is only modified in a limited degree, by the
knowledge acquired by experience.  This condition is still more remark-
able in Somnambulism, or (as it has been better termed) Sleep-walking;
in which the dreams are not only acted, but may be often  acted on with
the utmost facility,—a suggestion  conveyed through any of  the senses
excepting sight (which is  usually in abeyance) being apprehended and
followed-up Avith the utmost readiness, and, in like manner, with little
or no correction from experience.   Between this  condition, and that of
ordinary dreaming, on the one hand,  and that of complete insensibility
on the  other, there is  every shade of variety;  which is presented 
FUNCTIONS OF THE SYMPATHETIC  SYSTEM.          527
 by different  individuals, or by the  same individual at different times.
 The Cerebellum, in the Sleep-walking state, seems to be frequently in a
 condition of peculiar  activity;  a remarkable  power of balancing and
 combining the movements of the body, being often exhibited.
   925. On the other hand, there may be an undue exaltation  or a per-
 version of mental activity, without  any affection of the sensorial appa-
 ratus.  This is well seen, for example, in  the first stage of Alcoholic
 excitement,  and in that of Mania, Phrenitis, and  other  disorders in
 which the Cerebral Hemispheres are especially affected.   Frequently,
 as in the case of Alcohol, Opium, Haschish, &c, we may directly attri-
 bute  the morbid action of the Cerebrum to the presence of a  poison in
 the blood which permeates it; and there is  strong reason to believe that
 many other forms of delirium are partly due to a perverted state of that
 fluid.   On the other hand,  there  can be  no doubt that an  extreme
 depression of intellectual power, as well as of the emotional  state, is
 often to be attributed  to a depravation of the blood;  a slight accumula-
 tion of bile being very prone to occasion this state in some individuals,
 and an entire change being effected  by a mild dose of  mercurial prepa-
 rations, which, by eliminating the bile, restores the circulating fluid to
 its proper purity.  And it may be fairly suspected, that the foul atmo-
 sphere in the midst  of which a large class of our  population habitually
 lives, has the effect, by keeping  their blood charged Avith  noxious mat-
 ters, of so perverting the actions of the brain, that neither the intellec-
 tual powers nor the moral sense can be duly exercised; and thus it may
 be anticipated that Sanitary Reform will largely benefit not merely the
 corporeal but the mental  and moral  health of those, Avhose position is at
 present one of fearful  degradation from the want of it.

                 8.  Functions of the Sympathetic System.

   926. The Cerebro-Spinal  apparatus, of which the several parts have
 now been described, is not the only system of ganglia and nerve-trunks,
 that is contained within  the body of a Vertebrated animal.  There is
 another system, having its own set of centres, and its own distribution
 of branches,  characterized also  by  a peculiarity  in  the  nature of  the
 nervous fibres of which its trunks are composed, and communicating at
 numerous points with  the preceding.  It will be remembered that, in
 front of the vertebral column, there  is a series of ganglia on each side ;
 communicating, on  the one hand, Avith the  spinal nerves, as they issue
 from the vertebral canal; and also connecting themselves with the two
 large Semilunar ganglia, which lie amidst the abdominal viscera; as well
 as Avith a series of ganglia, that is found near the base of the heart.   In
 the head, also, there are  numerous  scattered ganglia, which evidently
 belong to the same system ; having several communications  with the
 cephalic nerves ;  and being also connected Avith the chain of ganglia in
 the neck.   The branches  proceeding from this series of ganglia are dis-
tributed, not to the  skin and muscles (like  those  of the cerebro-spinal
system), but to the organs of digestion and secretion, to the heart and
lungs,  and particularly to the Avails of the blood-vessels, on Avhich they
form a plexus whose branches probably accompany their minutest rami- 
528          OF THE  NERVOUS SYSTEM  AND  ITS ACTIONS.

fications.   The peculiar connexion of this system  of nerves with the
organs of vegetative life, has caused it to receive the designation of the
Nervous System of Organic  Life; the Cerebro-Spinal  system being
termed the Nervous System of Animal Life.   It is also not unfrequently
termed the ganglionic system; on account of the separation of its centres
into  scattered ganglia, which forms a striking contrast to the concentra-
tion  that is so evident in the  Cerebro-spinal system.   But this term is
objectionable, as leading to a supposed analogy between this system and
the general nervous system of Invertebrata, whose  centres are equally
scattered;—an analogy which is completely erroneous, since as we have
seen, this last is chiefly the representative of the Cerebro-Spinal system
of Vertebrated animals.  The term Sympathetic is perhaps  the best;
although it must not be supposed  that this system of nerves  is the
instrument of by any means all the sympathies, which manifest them-
selves between different organs.
   927.  The sympathetic system contains both classes of nervous fibres;
—the ordinary white  tubular fibres,  all of  which are probably derived
from the Cerebro-Spinal system ; and the gray or gelatinous fibres, part
of which seem to belong to itself (§ 375).   Thus we may consider each
system as intermingling itself with the other ;—'the  Cerebro-Spinal sys-
tem  transmitting some  of its  fibres, both motor and sensory, into the
Sympathetic;—whilst the Sympathetic is represented in the Cerebro-
Spinal system, by certain fibres and collections  of  vesicular matter of
its own.  The trunks  that proceed from  the Semilunar ganglia, are
almost entirely composed of gray or  organic fibres;  whence it is evident
that  these ganglia are to be regarded as the true centres of the Sympa-
thetic system.   On the other hand, the trunks which issue  from the
chain of spinal ganglia, contain a large admixture  of white or tubular
fibres.
   928.  The Sympathetic nerves possess a certain  degree of power of
exciting Muscular contractions, in the various parts to which they are
distributed.  Thus by irritating them, immediately after the death of
an animal,  contractions may be excited in any  part of the alimentary
canal, from the pharynx to  the rectum, according  to the trunks which
are irritated,—in the  heart,  after its ordinary movements have ceased,
—in the aorta, vena cava, and thoracic duct,—in the  ductus choledochus,
uterus, fallopian tubes, vas deferens, and vesiculse seminales.  But the
very same contractions  may be excited, by irritating  the roots of those
Spinal nerves, from which the Sympathetic  trunks  receive  their white
fibres;  and there is,  consequently,  strong  reason  to believe that the
motor power of the latter is  entirely  dependent upon the Cerebro-spinal
system.  Whatever sensory endowments the  Sympathetic trunks pos-
sess, are probably to be referred to the same connexion.  In  the ordi-
nary condition of the body, these are not manifested.   The  parts ex-
clusively supplied by  Sympathetic trunks do not appear to be in the
least degree sensible; and no  sign of pain is given when the Sympa-
thetic trunks themselves are irritated.  But in certain diseased conditions
of those organs, violent pains are  felt in  them; and these pains can
only be produced through the  medium of fibres communicating with the
sensorium through the spinal nerves. 
                 GENERAL AND SPECIAL SENSATIONS.             529

  929.  It is difficult to speak with  any precision, as to the functions
of the  Sympathetic system.   There is  much  reason  to believe,  how-
ever,^ that it constitutes the channel through  which the  passions and
emotions of the mind affect the Organic functions ;  and this especially
through its power of regulating the calibre  of  the arteries.  We  have
examples of the influence of these states upon the Circulation, in the
palpitation of the heart which  is produced by an agitated state of feel-
ing ; in the Syncope, or suspension of the heart's action, which  some-
times comes  on from a sudden shock; in  the  acts  of blushing and
turning pale, which consists in the dilatation or contraction of the small
arteries ; in the sudden increase of the salivary,  lachrymal,  and  mam-
mary secretions, under the influence of particular states of mind,  which
increase is probably due to the  temporary dilatation of the arteries that
supply the glands, as in the act of blushing; and in many other phe-
nomena.  It is  probable  that  the  Sympathetic  system not only thus
brings the Organic functions into relation with  the Animal,  but that it
also  tends to harmonize the former with each other,  so as to bring the
various acts of  secretion, nutrition, &c, into mutual conformity.  For
whilst the quantity of a secreted product, or the amount of tissue gene-
rated in a part, may be affected  by an increase or diminution in the
calibre of the vessels supplying it, the quality of the secretion, or the
character  of the tissue,  may  be  likewise affected  (there  seems  valid
reason to believe) by that Nervous force, whose  relations to the Physical
and Chemical, as well as  to  all  other  Vital  forces, are so intimate
(§ 396).
                         CHAPTER XIII.

                OF SENSATION, GENERAL AND SPECIAL.

                      1.  Of Sensation in general.

  930. All  beings  of  a  truly  Animal  Nature possess (there is  good
reason to believe), a consciousness of their own existence, first derived
from a feeling  of some of the corporeal changes taking place within
themselves; and also a greater or less amount of sensibility to the con-
dition of external things.   This consciousness of  what is taking place
within and around the individual, is  all derived from impressions made
upon its afferent nervous fibres ; which, being conveyed  by them to the
central sensorium, are there felt (§  390).  Of the mode  in  which the
impression, hitherto a change  of a physical character, is there made to
act  upon  the mind,  we are  absolutely  ignorant; we  only know the
fact.  Although we commonly refer  our various sensations to the parts
at which  the impressions are made,—as, for instance,  when we say
that we have a pain in the hand, or an ache in the leg,—we  really use
incorrect language;  for, though we  may  refer our  sensations to the 
530
OF SENSATION IN GENERAL.
parts where the impression is first made on the nerves,  they are really
felt in the brain.   This is evident from two facts;—first, that if the
nervous communication between the part and the brain be  interrupted,
no impressions, however violent, can make themselves felt; and second,
that if the trunk of the nerve be irritated or pinched,  anywhere in its
course, the pain which  is felt is referred, not to the  point  injured,  but
to the surface to Avhich these nerves are distributed.   Hence the well-
known fact  that, for some  time  after  the  amputation of a limb,  the
patient feels  pains, which he refers to the fingers or toes that have been
removed; this continues until the irritation of the cut extremities of the
nervous trunks has subsided.
   931. It would seem probable that,  among the lower  tribes  of Ani-
mals, there exists no other kind of sensibility, than that termed general
or common ;  which pervades, in a greater  or less degree, nearly every
part of the bodies of the higher.   It is by this, that we  feel those  im-
pressions, made upon  our bodies by the objects around us, which  pro-
duce  the various modifications  of pain, the sense of contact or resistance,
the sense of  variations of temperature, and others of  a similar character.
From what was formerly stated (§ 403) of  the dependence of the  im-
pressibility of  the sensory nerves, upon  the activity of the circulation
in the neighborhood of their extremities, it is  obvious  that  no parts
destitute of blood-vessels can receive such impressions, or  (in common
language) can possess sensibility.   Accordingly  Ave find  that  the hair,
nails, teeth, cartilages,  and other parts that  are altogether  extra-vascu-
lar, are themselves destitute  of sensibility; although  certain parts con-
nected with them, such as the bulb of the hair, or the vascular membrane
lining the pulp-cavity of the tooth, may be  acutely  sensitive.   Again,
in tendons, ligaments,  fibro-cartilages, bones, &c, whose  substance con-
tains  very few vessels, there is but  a very  Ioav amount of sensibility.
On the  other  hand, the skin and  other parts, which  are peculiarly
adapted to receive such impressions, are extremely vascular;  and  it is
interesting to observe,  that some of the tissues just mentioned become
acutely  sensible,  when new vessels  form  in them  in consequence of
diseased  action.  It does not  necessarily folloAV,  hoAvever, that parts
should be sensible in a degree proportional to the amount of blood they
may contain;  for this blood may be sent to them for other purposes,
and they may contain  but a small number  of  sensory nerves.   Thus,
although it is a condition necessary to the action of  Muscles, that  they
should be copiously supplied Avith blood (§ 359), they are by no means
acutely sensible;  and, in like  manner,  Glands, which receive a large
amount  of blood for their peculiar purposes, are far from possessing a
high  degree  of sensibility.
   932. But  besides the general or  common sensibility, which is dif-
fused over the greater part  of  the body, in most animals, there are cer-
tain  parts, which are endowed with  the property of receiving impres-
sions of a peculiar or special kind, such as sounds or  odors,  that would
have no influence on the rest; and the sensations  which  these excite,
being of  a kind very  different from  those  already  mentioned, arouse
ideas in our minds, which we should never  have gained without them.
 Thus, although we can acquire a knowledge of  the shape  and  position 
GENERAL AND SPECIAL  SENSATIONS.             531
of objects by the touch, we could form no  notion  of their color with-
out  sight, of their sounds without  hearing, or of their odors without
Bmell.   The  nerves Avhich convey these special impressions, as already
mentioned, are not able  to receive  those  of a common kind; thus the
eye, however well fitted  for  seeing, would not feel the  touch of the
finger, if  it were not supplied by branches from the Fifth pair, as well
as by the Optic.  Nor can the different  nerves of special  sensation  be
affected by impressions that are adapted  to  operate on others; thus the
ear cannot distinguish the slightest difference between a luminous and
a dark object;  nor could the  eye distinguish a sounding  body from a
silent one,  except when  the  vibrations  can be seen.   But  Electricity
possesses  the remarkable power,  when  transmitted along the  several
nerves of special sense, of exciting the sensations peculiar  to each; and
thus, by proper management,  this single agent may be made to produce
flashes  of light,  distinct  sounds,  a  phosphoric odor, a peculiar taste,
and  a pricking feeling, in the  same individual,  at one time.   Each kind
of sensation may also be excited, however, by mechanical irritation of
the nerve which is subservient to  it.—The  feeling  of pain may be  in-
duced by impressions made upon the nerves of special sense, as Avell as
upon those of feeling, if  these impressions be too  violent  or excessive.
Thus the dazzling of the eye by a strong light,  and still more, the
action  of a moderate  light  in an irritable state of the  retina,—sudden
loud sounds,  or even sounds of moderate intensity but of peculiar harsh-
ness,—powerful odors, or  even such  as are agreeable in moderation,—
produce feelings of uneasiness, which may be properly called painful,
even though they are different from  those excited through  the nerves of
common sensation.
  933.  As a general rule, it may  be  stated, that the violent excite-
ment of any sensation is  disagreeable; even when  the same sensation,
experienced in a moderate degree, may be a source  of extreme pleasure.
But  the question of degree is relative rather than  absolute: that is, a
sensation  may  be felt as extremely violent by  one individual, whilst
another, who is more  accustomed to  sensations of the same kind, is not
disagreeably affected  by it.   Thus our sensations of heat  and cold are
entirely governed by the previous  condition of the  parts affected; as is
shown by  the well-known  experiment of  putting one hand  in hot water,
the other in  cold,  and then transferring them  both to tepid water,—
which will seem cool  to the one hand and warm  to the  other.  The
same is  the case in regard to light and sound,  smell and taste.   A per-
son going out of a totally dark room, into one moderately  bright, is for
the time painfully impressed by the  light, but soon  becomes habituated
to it; whilst  another, who enters it from a room brilliantly illuminated,
will consider it dark and gloomy.                    .
  934. The intensity with Avhich sensations are felt, therefore, depends
upon the degree of change which  they produce in the sensorium.  The
more frequent  the recurrence of any  particular  sensation, the more
does  the system become adapted to  it, and  the less change does it pro-
duce.  It is,  therefore, perceived in  a less and less  degree, and at last
it ceases to  excite attention.  The stoppage of a  constantly-recurring
sensation, however, will produce  a  change, which  makes  as strong  an 
532
OF SENSATION IN GENERAL.
impression on the system as its first commencement; thus there are
persons, who have become so habituated  to the sound of a waterfall or
even of a forge-hammer, that they cannot sleep anywhere but in its
vicinity; and it is well known that, when a person has gone  to  sleep
under the influence of some continuous  or frequently-recurring sound
(such as the voice of a reader, the dropping  of  water, the  tread of a
sentinel, &c), the cessation of the sound will cause his awaking.
  935.  The acuteness  of  particular sensations  is influenced  in  a re-
markable degree by the attention they receive from the mind.  If the
mind be entirely inactive, as in profound  sleep,  no sensation whatever
is produced  by very feeble impressions; on  the  other hand, when the
mind is from any cause strongly directed upon them, impressions very
feeble in themselves  produce  sensations  of even painful acuteness.  It
is in this manner, that the habit of attending to  sensations  of  any par-
ticular class increases  their  vividness;  so that  they are  at once per-
ceived by an individual on the watch for them, when they do not ex-
cite the observation of others.  We may even, by a strong effort, direct
the mind  into one particular channel, so as to receive only those sensa-
tions which  have reference to it, and to be unconscious quoad all others.
Thus, the application of the  mind to some particular train of thought
may prevent our being conscious of anything that is going on around or
within us,—the conversation of friends,—the striking of the clock,—
the calls of hunger, &c.  This abstraction may be altogether voluntary;
and  the possession of the power of thus withdrawing  the mind at will
from the influence of external disturbing causes, and of fixing  it upon
any particular train of ideas, is an extremely valuable one.   But it may
also be involuntary, and may be a source  of inconvenience  from its
tendency  to recur at  improper  times,—producing  the habitual state
which is known as absence of mind or reverie.
  936.  It is desirable that we should make a distinction, between the
sensations themselves,  and the ideas which are the  immediate results of
those sensations, when they are perceived by the  mind.  These ideas
relate to the cause of the sensation, or the object by which the impres-
sion is made.  Thus, the formation  of the picture of an object, upon
the retina, produces  a certain impression upon the optic nerve;  which,
being conveyed to  the sensorium,  excites a corresponding sensation,
with which, in all ordinary cases, we immediately connect  an idea of
the nature  of the object.  So closely, indeed, is this idea  usually re-
lated to the sensation,  that we are not in the habit of making a distinc-
tion between them.   Thus,  I may say at this moment, " I see a book
on the table before me;"  the fact being, that I am conscious of a certain
picture, which conveys to my mind  the ideas of  a book and of a table,
and of their relative positions; these ideas being (in  Man) the result of
experience  and  associations,—in  fact,  originating  in the immediate
application  of  the knowledge we have previously acquired, that  a cer-
tain object,  whose picture we  see, is a book, another object  a table, and
so on.  We are liable  to be  deceived on this assumption;  as when, by
a clever imitation, a picture on a plane surface is made to represent an
object in relief, so perfectly as at once to excite the idea of  the latter, 
INTUITIVE AND ACQUIRED PERCEPTIONS.
533
—which may not be corrected, until we have ascertained by the touch,
the flatness of the real object.
  937.  This production of ideas, by the agency of sensations, is a pro-
cess altogether  mental, and  dependent upon the laws of Mind.   We
find that some of these perceptions or elementary notions are intuitive:
that is,  they are prior to all experience, and are as necessarily connected
with the sensation  which  produces them, as reflex movements are with
the impression  that excites  them.   This seems to be the  case, for
example, with regard to erect vision.   There is  no reason whatever to
think, that either infants or any of the lower animals see objects in an
inverted position, until they have corrected their notion by the touch;
for there is no reason why the  inverted  picture on the retina  should
give rise to the idea of the inversion  of  the object.   The picture is so
received by  the mind, as to convey to us  an idea  of the  position of
external objects, which harmonizes with the ideas we derive through the
touch;  and whilst we are in such complete ignorance of the manner in
which the  mind becomes  conscious of the sensation  at all, we need not
feel any difficulty about the  mode  in  which this conformity is effected.
But in Man, as already  stated, the attaching definite ideas to certain
groups  of lines, colors, &c, with respect to  the objects they represent,
is a subsequent process, in Avhich experience  and memory are essentially
concerned; as Ave see particularly well, in cases presently to be referred
to, in which the sense  of sight has been acquired comparatively late in
life, and in which the mode of using it, and of  connecting the  sensa-
tions  received  through it with  those  received  through the touch, has
had to  be learned, by a long-continued  training.   The  elementary
notions thus formed,—which may, by  long habit, present themselves as
immediately and unquestionably, as if they were intuitive,—are termed
acquired perceptions.
   938.  It  is probable that, among the lower animals, the proportion  of
intuitive perceptions is much greater than in Man; whilst, on the other
hand, his power of acquiring perceptions is  much greater than theirs.
So that, whilst the young of the lower animals very soon becomes pos-
sessed of all the knowledge which is necessary  for the acquirement  of
its food, the construction  of its habitation, &c, its range is very limited,
and it is incapable of  attaching any ideas to a great variety of objects,
of  which the Human  mind  takes cognizance.  This correspondence
between the acquired perceptions of Man, and the intuitive perceptions
of  many of the loAver animals,  is  strikingly evident  in regard to the
power  of measuring distance.   This is acquired very gradually by the
Human infant,  or  by  a person  who has  first obtained the faculty  of
sight later in life ;  but it  is obviously  possessed  by  many of the lower
animals, to Avhose maintenance  it is  essential, immediately upon their
"entrance into the  world.  Thus, a Fly-catcher, immediately after its
exit from the egg, has been known to peck at and capture an insect,—
an action which requires a very exact  appreciation of distance, as well
as a power of precisely regulating the muscular movements in accordance
with it. 
534
OF SENSATION.
                       2.  Of the Sense of Touch.

   939. By the sense of Touch is usually understood that modification
of the common sensibility of the body, of which the surface of the skin
is the especial seat, but which exists also in some of its internal reflexions.
In some animals, as in Man, nearly the whole exterior of the  body is
endowed with it, in  no inconsiderable degree; whilst in others, as the
greater number of Mammalia, most Birds, Reptiles, and Fishes, and a
large proportion of the Invertebrata, the greater part of the body is so
covered with hairs, scales, bony or horny plates, shells  of various kinds,
complete horny envelopes, &c, as to  be  nearly insensible; and the
faculty is restricted to particular portions of the surface, or to organs
projecting from it, which often  possess a peculiarly high degree of this
endowment.   Even in Man, the  acuteness  of the sensibility  of the
cutaneous surface  varies greatly in different parts, being greatest at the
extremities of the  fingers and in the lips, and least  in  the skin of the
trunk, arm, and thigh.   Thus the  two  points of a  pair of compasses
(rendered blunt by bits of cork) can be separately distinguished by the
point of the middle finger, when approximated so closely as l-3d of a
line;  whilst they require to be opened so widely as 30  lines from  each
other, to be separately distinguished, when  pressed  upon the skin over
the spine, or upon that of the middle of the arm or thigh.
   940. The impressions that  produce the sense  of  touch are received
through the sensory papillar, with which the surface of  the true Skin is
beset,—more or less closely, according to the part of it that is examined.
These papillee are  minute  elevations, which  enclose  loops of capillary
vessels (Fig. 157), and branches of the sensory nerves.  With regard

                               Fig. 157.
Capillary network at margin of lips.
to the precise course of the latter, there is some uncertainty; but  it is
probable,  from analogy,  that  the  representation  given  of  them by
Gerber  (Fig.  158) is  in the main  correct;  and that each loop of the
Sensory nerve is  connected with one or more vesicular foci, on some
change in which the formation of the sensory impression is immediately
dependent (§ 382).   It is peculiar to the sense of Touch, and to that of
Taste (which is closely related to it) that the impression must be made
by the contact of the  object itself  with  the sensory  surface, and  not
through any  intermediate agency.   The only  exception to this is in
regard  to the sense   of  Temperature,  which  seems to be  in many
respects different from ordinary touch; here  the proximity of the warm 
SENSE  OF TOUCH.
535
or cold body is sufficient, the impressions  being  made after the manner
of those of odors, sounds, &c.   It is worth remarking, with reference
to the question of the special nature of the sensory fibres, which are the

                              Fig. 158.
Distribution of the tactile nerves at the extremity of the Human Thumb, as seen in a thin perpendicular
                            section of the skin.

channel of these impressions, that no mechanical irritation of the nerves
of common  sensation ever seems to  excite sensations of heat or cold;
these being apparently almost as distinct from the sense of contact as
they are from that of light or sound.
  941.  The only idea communicated to our minds, when this sense is
exercised in  its simplest form, is  that  of resistance; and we  cannot
acquire a notion of the size or  shape of  an object, or of the nature of
its surface, through this sense alone, unless we move the object over our
own sensory organ, or pass the latter over  the former.  By the various
degrees of resistance which we  then encounter, we form our estimate of
the hardness or softness of the body.  By the  impressions made upon
our sensory papillae, when they are passed  over its surface, we form our
idea of its  smoothness  or roughness.  But it  is through the muscular
sense which renders us  cognizant of the  relative position of the fingers,
the amount  of movement  the  hand  has performed in passing over the
object, and of other impressions  of like  nature, that we acquire our
notions of the size and figure of the object; and hence we perceive that
the sense of  touch, without  the power  of giving motion to the tactile
organ, would haA'e been of comparatively little use.  It is chiefly in the
variety of movements of which  the hand of Man is capable,—thus con-
ducive  as they  are, not merely to his  prehensile powers, but to the
exercise of his sensory  endowments,—that it is  superior to that of any
other animal; and it cannot be doubted  that  this  affords us  a  very
important means of  acquiring information in  regard to the external
world, and especially of correcting many  vague and fallacious notions
which Ave should derive from the sense of Sight, if used alone.  On the
other hand, it must be  evident that our  knowledge would have but a
very limited  range, if this sense were the only medium through which
we'could acquire ideas.  Of this Ave have the clearest evidence in the
very imperfect development of the mental powers in those unfortunate
persons who have suffered  under the  deprivation of sight and hearing
from their birth, and who have been consequently cut off from the most
direct means  of profiting  by the  knoAvledge possessed by their fellow-
beings, through want of poAver to use the organs of speech.  It is only 
536
OF SENSATION.
where such individuals have fallen under the care of judicious and perse-
vering instructors, that their mental powers have been called into their
due activity, or that any ideas have been awakened, beyond those imme-
diately connected with  the  gratification of the  animal wants, or  with
painful or pleasurable sensations.   Thus a mind quite capable of being
aroused to activity and enjoyment, may remain  in a condition nearly
allied to that of idiocy, simply for  want of the sensations requisite to
produce ideas of a  higher and more abstract character than those de-
rived through the sense of Touch, Taste, and Smell.
  942. For the exercise of the sense of Temperature,  the integrity of
the sensory apparatus  contained in the Skin  appears  to be requisite;
for  it has been  ascertained  by the recent experiments of Prof. Weber,
that if the  integuments be  removed, the application  of hot  or cold
bodies only causes pain, their elevation  or depression  of temperature
not being perceived ; and the same is the  case when hot or cold bodies
are applied  to  the nerve-trunks.   It is worthy of note that there are
many cases on  record in which the  sense of Temperature has been lost,
while the ordinary tactile sense remained; and the former is sometimes
preserved when there is a  complete  loss of every other kind of  sensi-
bility.  So again we find that the subjective sensations of temperature
(that is, sensations which originate from changes in the body itself, not
from external  impressions)  are frequently excited quite independently
of the tactual sensations; a person being  sensible of heat or of chilli-
ness in some part of his body, without any real alteration of its tempera-
ture, and without any corresponding affection of  the tactual sensations.
It is curious that the intensity of the sensation  of temperature should
depend, not merely upon the relative  degree of heat to which the part
is exposed (§ 933), but also upon the extent of the surface over which it
is applied ; a weaker impression made on a larger surface seeming more
powerful than a stronger impression made on a  small  surface.   Thus,
if the forefinger of  one  hand be immersed in water  at 104°, and the
Avhole of the other hand be plunged in water at 102°, the cooler water
will be thought the warmer;  whence  the well-known fact that water in
which a finger can be  held without  discomfort, will produce a scalding
sensation when the entire hand is immersed in it.

                       3. Of the Sense of Taste.

  943. The sense of Taste, like that of Touch, is excited by the direct
contact of particular substances with certain parts of the body;  but it
is of a much  more refined nature than touch, inasmuch as it communi-
cates to us a knowledge of properties which that sense would not reveal
to us.  All substances, however, do  not  make  an impression on the
organ of Taste.  Some have  a strong savor, others  a  slight one, and
others are  altogether  insipid.   The  cause of these differences is not
altogether understood;  but it may be remarked, that, in general, bodies
which cannot be dissolved in water, alcohol, &c,  and which thus cannot
be presented to the gustative papillae in a state  of solution, have no
taste.   This sense has for  its chief purpose to direct animals in their
choice of food; hence its organ is always placed at the entrance to the 
SENSE  OF TASTE.
537
digestive canal.  In higher animals, the Tongue is the principal seat of
it;  but other  parts  of the mouth  are  also capable  of  receiving the
impression of certain savors.  The mucous membrane which covers the
tongue is  Copiously supplied with papillae, of various  forms and sizes.
Those of simplest structure closely resemble the cutaneous papilla ; but
there are  others, which resemble clusters of such papillae,  each being
composed  of a fasciculus of looped capillaries (Fig. 159), with a bundle

                               Fig. 159.
                 Capillary network of fungiform papillae of the Tongue.

of nerve-fibres, whose precise mode of  termination it  has not yet  been
found  possible  to  ascertain.  These fungiform papillae,   which  are
covered with a very thin epithelium, are probably the special  instru-
ments of the sense of taste ;  for the exercise of which it seems probable
that the sapid substances must penetrate, in solution,  to the interior of
the papilla.   When these papillae are called into action by the contact
of substances  having a  strong savor,  they  not unfrequently become
very turgid, by a distension  of their vessels  analogoas to  that which
occurs in erection ;  and  they rise  up from the  surface of the mucous
membrane, so as to produce a decided roughness of  its surface.  The
conical papillae, on the other hand, are furnished with thick epithelial
investments,  which are  sometimes prolonged into  filamentous  appen-
dages ; and, looking to their  higher development among other animals,
and  the offices to Avhich  they are there  subservient, it  seems probable
that their functions are purely mechanical, and that they serve especially
to cleanse the teeth from adhering particles.   The  nerve-fibres  can  be
seen to form distinct loops in their interior, at some distance from the
apex.
   944. There  has been  much discrepancy  of opinion as to the nerve
which is specially concerned  in the sense of Taste.   The tongue of Man
is  supplied by two sensory nerves : the lingual branch of the Fifth pair;
and the Glosso-pharyngeal.  The former chiefly supplies the upper sur-
face of  the front of  the tongue,  and is copiously distributed  to the
papillae near the tip.  The latter is mostly distributed upon  the mucous
surface of the Fauces, and upon the back of the tongue ; but it sends a
branch fonvards, beneath the lateral  margin on eaeh side, which supplies
the edges and inferior surface of the  tip of the tongue,  and inosculates
with the preceding.  There is  reason to believe, from experiment, that
the gustative sensibility  of the  tongue is not destroyed by section of
either of these nerves; though the operation produces a total or  partial
loss of sensibility over certain parts of  the surface.   There  seems  good
reason to conclude, that  the lingual  branch  of the  Fifth  pair is the 
538
OF SENSATION.
nerve, through which the sense of Taste, as  well  as that  of Touch, is
exercised, in the parts of the tongue to which it is specially distributed,
—which are those that  possess both senses in the  most acute degree;
and that the Glosso-pharyngeal is subservient to the  same functions in
the parts supplied by it, being probably the exclusive channel, also,
through wdiich the impressions made by  disagreeable substances taken
into the mouth, are propagated to the  Medulla Oblongata, so as to pro-
duce nausea and excite  efforts to  vomit.  The latter nerve  is also, as
we have seen, the  principal channel of the impressions that give rise to
the reflex act of swallowing ; with which the  fifth pair is concerned in a
much inferior degree (§  897).
  945.  A considerable  part of the impression produced by  many  sub-
stances taken into the mouth, is  received through the  sense of Smell,
rather than through that of Taste.  Of this, any one may easily satisfy
himself, by closing the  nostrils, and breathing through  the mouth only,
whilst holding in his mouth, or even rubbing between his tongue and his
palate, some aromatic substance ;  its taste is then scarcely  recognised,
although it is immediately perceived  when  the nasal passages are re-
opened, and  its effluvia are drawn into them.   There are  many sub-
stances,  however, which  have no aromatic  or volatile  character;  and
whose taste,  though not in the least dependent  upon the  action of the
nose, is nevertheless of a poAverful character.  Some of these produce,
by irritating the mucous membrane, a  sense of pungency,  allied to that
which the same substances (mustard,  for instance)  will produce, when
applied to the skin for a sufficient length of time, especially  if the Epi-
dermis have  been  removed.  Such sensations, therefore,  are evidently
of the same kind Avith those of Touch, differing from them only in the
degree of sensibility of the organ through which they are received.   But
there are others which  produce  sensations entirely different from  any
that can be received through the  skin, and which  are  properly distin-
guished, therefore, as gustative ;  such are common Salt, which  may be
considered as a type of the saline  taste, Sugar, the type of the saccharine,
Quinine of  the bitter, and Tannin of  the astringent, and  Citric acid of
the sour.   All such substances, therefore, are said to possess sapid pro-
perties, exciting distinctive tastes, quite irrespectively  of  any aromatic
or odoriferous properties which they may also possess, as well as of their
stimulating action on the skin.

                       4.  Of the  Sense of Smell.

   946.  Certain bodies  possess the property of exciting sensations  of a
peculiar nature, which cannot be  perceived  by the  organs  of  taste or
touch,  but which seem to depend upon the diffusion  of the  particles of
the substance through* the surrounding air, in a state of extreme minute-
ness.   As the solubility of a substance in liquid seems  a necessary  con-
dition of its exciting the sense of  Taste, so does its volatility, or tendency
to a vaporous state, appear requisite for its having  Odorous properties.
Most volatile substances are more or less  odorous : whilst those which
do not readily transform themselves  into vapor,  usually  possess  little
or no fragrance in the liquid or solid state, but acquire strong odorous 
SENSE  OF SMELL.
539
properties, as soon as they are converted  into vapor,—by the  aid  of
heat for example.   There are some solid substances, which possess very
strong odorous  properties,  without losing weight in any  appreciable
degree by the diffusion of their particles through the air.   This is the
case, for example, with Musk; a grain of which has been  kept  freely
exposed to the air of  a room, whose doors and windows were constantly
open, for a period of ten years ; during  which time the air, thus con-
tinually changed, was completely impregnated with the odor of  musk;
and yet,  at the end of that time, the particle was not  found to have per-
ceptibly diminished in weight.  We can only attribute this result to the
extreme minuteness of the division of the odorous particles of this sub-
stance.   There are other odorous solids, such as Camphor, which rapidly
lose  weight by  the loss  of  particles  from their surface,  when  freely
exposed to the air.
  947.  The conditions of the sense of Smell are very simple.  The Ol-
factory nerve is minutely distributed over the Schneiderian membrane,
which is  itself highly  vascular.  The arrangement of the ultimate fibres
of this nerve has  not been  ascertained.   The Schneiderian membrane
is kept constantly but moderately moist, by a mucous secretion from its
surface ; and this  condition is essential to the  acute perception of odors.
If the mucous surface be too dry,  as happens Avhen the  fifth pair  is

                              Fig. 160.
septum nasi.

paralysed, the sensation is blunted, or even destroyed; and the same
effect is produced by the presence  of too copious a secretion, as when 
540
OF SENSATION.
we are suffering under an ordinary cold.—The highest part of the nasal
fossae appears to be that, in which there is the most acute sensibility to
odors; and  hence it is, that, when we snuff the air,  so  as  to direct it
into this portion of the cavity, we perceive delicate odors, which would
otherwise have escaped us.  The acuteness of the sense of Smell depends
in no small degree, upon the extent of surface exposed by the membrane
lining  the nasal cavity; and  in this respect Man is far surpassed by
many  of the lower  Mammalia, especially the  Ruminants,  which are
warned by its means of the proximity of their enemies.  The habit of
attention to sensory  impressions  of this  class,  however, very much
heightens their acuteness: hence in those who suffer under blindness and
deafness conjointly, it is  usually the principal means by which indivi-
duals are distinguished, and the presence of strangers recognised; and
there are cases, in which  individuals  in a state of somnambulism have
exhibited a degree of acuteness of smell, quite comparable to that which
is characteristic of Deer, Antelopes, &c.
  948. Besides ministering to  the sense of Smell, by stimulating the
secreting powers of its surface, the Fifth pair has another very impor-
tant function,—that of endowing the interior of the nose with common
sensibility, and thus receiving the impression produced  by acrid or pun-
gent substances, which act upon it in the  same way as they do upon the
tongue.   Such substances are felt, by the  irritation they produce, rather
than smell; and the sensation they occasion gives  rise to the consensual
act of sneezing, by which a violent blast of air is directed  through the
nasal passages, in such a manner as to clear them of the irritating mat-
ter, whether solid (as snuff), fluid or gaseous.  Hence this  action  may
be excited by the contact  of an irritant with the  Schneiderian mem-
brane, after the olfactory nerve has been divided,  if the branches of the
fifth pair be entire; whilst  it does not take place  when the fifth pair is
paralysed, even though the sense of smell is retained.

                       5. Of the Sense of Hearing.

   949.  By this sense we become acquainted with the sounds produced
by bodies in a certain state of vibration ; the vibrations being propagated
through  the surrounding medium,  by the corresponding waves or undu-
lations Avhich  they  produce in it.   Although air is the usual  medium
through which sound is propagated, yet liquids or solids may  answer the
same purpose.  On the other hand, no sound can be propagated through
a perfect vacuum.—It is a fact of much importance, in regard to the
action  of the Organ of Hearing,  that sonorous  vibrations which have
been excited, and are being transmitted, in a medium  of one kind, are
 not imparted with the same readiness to others.   The  following conclu-
 sions have been drawn from experimental inquiries on  this subject.
   I. Vibrations excited  in solid bodies, may be transmitted to water
 without much loss of their intensity;  although not with the same readi-
 ness that they would be communicated to another solid.
   II. On the other hand, vibrations  excited in water lose  something  of
 their intensity in being propagated to solids;  but they are returned,  as
 it were, by these solids to the liquid, so that the  sound is more  loudly 
SENSE  OF HEARING.
541
 heard in the neighborhood of these bodies, than  it  would otherwise
 have been.
   III.  The sonorous vibrations are much more  weakened in the trans-
 mission of solids to air; and those of air make but  little impression on
 solids.
   IV. Sonorous vibrations in  water are transmitted but feebly to air;
 and those which are taking place in air are with difficulty communicated
 to water ; but the communication is rendered more easy by the inter-
 vention of a membrane extended between them.
   The application of these conclusions, in the Physiology of Hearing,
 will be presently apparent.
   950.  It is  on the Auditory nerve  (commonly  termed  the Portio
 Mollis of the  7th pair), that the sonorous undulations make their  im-
 pression ; but we invariably find, that this impression is made through
 the  medium  of a liquid, contained  in  a cavity,  on the walls of which
 the ultimate branches of this nerve are distributed.   The simplest form
 of the organ of Hearing, such as we find in Cephalopods and in certain
 Fishes, consists merely of  a cavity  excavated in the solid framework
 of the head; which cavity is filled with liquid, and lined by a membrane
 on  Avhich the auditory nerve is distributed.   These animals  are inhabi-
 tants of the water ; and the sonorous vibrations  excited in this medium
 being communicated  to the solid parts of the head, will be by  them
 again transmitted to  the  contained fluid, without  much diminution of
 their intensity ; according to principles I. and II.—In certain Crustacea,
 however, whose organ of  hearing is  contained in the base of the anten-
 nae,  as well as in most Fishes, we find the auditory cavity or vestibule
 no longer entirely closed; but having an  aperture on its external side,
 which is covered in by a membrane.   Here the vibrations of the liquid
 within the cavity will be more directly excited by those  of the  sur-
 rounding medium, for if this be water,  it will propagate its undulations
 into the cavity, with little  interruption  from the membrane stretched
 across its mouth ; whilst, if it be air, the interposition of this very mem-
 brane will greatly assist in the transmission of the  vibrations to  the
 water of the auditory cavity, according  to principle IV.  In most of the
 animals  which have the organ of hearing constructed  upon  this simple
 plan, the force of the vibrations of the fluid within the cavity is increased
 by  several minute  stony concretions (termed otolithes), which are sus-
 pended in it.   These act  according to principle II.   Some traces of
 them are found  in the  higher animals ; in which they  are for the most
 part  superseded, however, by an apparatus  better adapted to augment
 the intensity of the sonorous vibrations.
  951. This  apparatus consists, in  all Vertebrated animals  above  the
 inferior  Reptiles,  of the  tympanum or drum, with its membrane and
 chain of  bones;  together Avith, in the mammalia, the external ear;
 which is adapted  to  direct  itself, more or less completely, towards  the
 point from which the sonorous  vibrations proceed, and to give  them a
 degree  of preliminary concentration.    The  tympanic  apparatus is
 interposed betAveen the external ear  and the membrane covering  the
foramen ovale, which  is the entrance to the  real auditory cavity; and
its purpose is evidently, to receive the sonorous vibrations from the  air, 
542
OF SENSATION.
              and  to  transmit  them to that membrane,  in  such a manner  that the
              vibrations thus excited in the latter may be  much more powerful, than
              they would be if the air acted immediately upon  it, as in the loAver
              Vertebrata.—The usual  condition of the Membrana Tympani appears
              to be rather lax; and, when in this condition, it vibrates in accordance
              with grave or deep tones.  By  the action of  the tensor tympani it may
i; j|             be tightened, so  as to  vibrate in accordance  with sharper or higher
 |             tones ;  but it will then be less able to receive the impressions of deeper
 ;             sounds.   This state we may  easily induce artificially, by holding the
              breath,  and forcing air from  the  throat into the Eustachian tube, so as
              to make the membrane bulge out by pressure from  within ;  or by ex-
              hausting the cavity by an effort  at  inspiration, with  the mouth and
              nostrils  closed, which will cause  the membrane to  be pressed inwards
              by the external air.   In either case, the hearing is immediately found
              to be imperfect;  but  the  deficiency relates only to grave sounds, acute
              ones being heard even more plainly  than before.   There is a different
              limit to the acuteness  of the  sounds, of which the ear can naturally
              take cognizance, in different persons.   If the sound be so high in pitch,
              that the  membrana  tympani  cannot vibrate  in unison  with  it,  the
              individual will  not  hear it, although it may be loud;  and it  ha3 been
              noticed, that certain  individuals  cannot hear the very shrill tones pro-
              duced by particular Insects, or  even  Birds, which are distinctly audible
              to others.
                952.  Not only do we  find the  tympanic  apparatus superadded,  in
              the higher forms of the organ of Hearing, but  also  the Semicircular
              Canals,  and the Cochlea.—The former exist in  all Vertebrata,  save the
              loAvest Fishes ; and in nearly  every  case, they  are three in  number,
              and  lie  in three  different planes.    Hence it  has been supposed, with
              some probability, that they assist in producing the idea of the direction
              of sounds.   The Cochlea does not exist at all in Fishes; and in  Reptiles
              its condition is quite rudimentary.  In Birds, this cavity is more com-
              pletely  formed, though the passage is nearly straight instead of spiral;
              of its real character, however, there can  be no doubt, from its being
              divided,  like the cochlea of  Man,  by  a membranous  partition,  on
              which the ramifications of the  auditory nerve  are  spread out.   This
              appendage has been supposed to be the organ that enables us  to judge
              of the pitch of sounds;  an idea which derives  some  confirmation from
              the correspondence between the development of the cochlea in different
              animals, and the  variety in the  pitch (or length of  the scale) of the
              sounds which it is  important  that they should hear  distinctly, espe-
              cially the voices  of their own kind.—That the Vestibule with  the
              passages proceeding from it, constitutes the true organ of hearing, even
              in Man, is  evident from the  fact,  that  when (as  not unfrequently
              happens)  the  tympanic  apparatus  has been  entirely  destroyed  by
              disease,  so as  to  reduce the organ to the condition of that in Avhich no
              such apparatus exists, the faculty of Hearing is by no means abolished,
              although it is deadened.
                953.  The faculty of Hearing, like  other  senses, may be very much
              increased  in acuteness by cultivation; but  this improvement  depends
              rather upon the  habit of attention to the  faintest impressions made 
                           SENSE OF HEARING.                       543

upon the organ,  than upon any change in the organ itself.   This habit
may be cultivated in regard to sounds of some one particular class;  all
others, being heard as  by an  ordinary person.   Thus, the  watchful
North American Indian recognises  footsteps, and can  even distinguish

                                 Fig. 161.
  A diagram of the Ear:—p. The pina.  t. The tympanum. I. The labyrinth. 1. The upper part of the
helix.  2. The antihelix. 3. The tragus. 4. The antitragus. 5. The lobulus.  6. The concha. 7. The upper
part of the fossa scaphoidea.  8. The meatus.  9. The membrana tympani, divided by the section.  10. The
three little bones, crossing the area of the tympanum, malleus, incus, and stapes: the foot of the stapes
blocks up the fenestra ovalis upon the inner wall of the tympanum.  11. The promontory. 12. The fenestra
rotunda; the dark opening above the ossicula leads into the mastoid cells. 13. The Eustachian tube; the
little canal upon this tube contains the tensor tympani muscle  in its passage to the  tympanum.  1-1. The
vestibule. 15. The three  semicircular canals,  horizontal, perpendicular, and oblique. 16. The ampullae
upon tbe perpendicular and horizontal canals.  17. The cochlea. 18. A depression between  the convexities
of the two tubuli which communicate with the tympanum and vestibule: the one  is the scala tympani,
terminating at 12; the other is the scala vestibuli.

between  the tread  of  friends  and foes ;  whilst  his white companion,
who  has  lived among the  busy hum  of  cities,  is  unconscious of the
slightest  sound.  Yet  the  latter  may  be  a musician,  capable  of dis-
tinguishing the  tones of all the different instruments in  a  large  orches-
tra, of following any one of them through  the part  which  it  performs,
and of detecting the least discord in the blended  effects  of the Avhole,—
effects which would be  to the unsophisticated  Indian but  an  indistinct
mass of sound.   In the same manner, a person who  has lived much  in
the country, is able  to distinguish  the note  of  every species of  bird
that lends its voice  to the general chorus of nature; whilst the  inhabi-
tant  of  a  town  hears  only  a confused assemblage of  shrill sounds,
which may impart to him a disagreeable rather than a pleasurable sen-
sation.
   954. In all continued sounds  or tones, there are  several  points  to be
attended  to.   In the first  place, we take cognizance  of  their pitch;
which depends  upon the number of vibrations  in  a given time,—the
high  notes  being produced by the most rapid  vibrations, and the low
notes by the sloAvest.   The ear can appreciate tones produced  by 24,000 
544
OF SENSATION.
impulses per second, the pitch of which is about four octaves above the
highest F of the piano-forte.   On the other hand, no sequence of vibra-
tions fewer  than  7 or 8 in a second, can produce a continuous tone,
because the impression left by each impulse has passed away, before the
next succeeds ; and there is consequently nothing more than a succession
of distinct beats.  The strength or loudness of musical tones  depends
(other  things being equal) on the force and extent of the vibrations, com-
municated by the sounding body to the medium which propagates them.
This will diminish, however, with distance, which softens loud tones by
lowering the intensity of the undulations, as a consequence of their more
extensive diffusion.   The  causes  of  the difference in the  timbre,  or
quality of musical tones,—such,  for  instance, as those  which exist
between the tones of a flute, a violin,  a  trumpet,  and a human  voice,
all sounding a note  of the same  pitch, are unknown: but they pro-
bably depend upon differences of form in the undulations.   Our ideas
of the  direction and distance of sounds, are for  the most part formed by
habit.   Of the former we probably judge in great degree, by the rela-
tive intensity of the impressions received by the two ears; though we
may form some notion of it by a single  ear, if the idea just stated  as
to the  use of the semicircular canals (§ 952), be correct.—Of  the dis-
tance of the sounding  body, we judge by the  intensity of the sound,
comparing it with that which we know the same body to produce when
nearer to us.  The Ear may be  deceived in this respect as well as  the
eye; thus the  effect of a full band at  a  distance may be given by  the
subdued tones of a concealed orchestra close by us; and the Ventrilo-
quist produces  his deception,  by  imitating as  closely as possible,  not
the sounds themselves, but the manner in which they would strike  our
ears.

                       6. Of the Sense of Sight.

   955. By the faculty of  Sight we are made acquainted in  the first
place,  with the existence of Light; and  by the medium of that  agent
we take cognizance of the form,  size,  color,  position,  &c, of  bodies
that transmit or reflect it.   As to  the mode in  which luminous impres-
sions are propagated  through  space, philosophers are at present  unde-
termined ; and the question is  of no physiological importance, since all
are agreed  as to the  laws which  regulate their transmission.   These
laws, which will be found at large  in any Treatise on Natural  Philoso-
phy,* may be briefly  stated as follows.
   I. Light  travels in straight  lines,  so long  as the medium  through
Avhich  it passes is of uniform density.
   II. When the rays  of light pass from  a  rarer medium into a  denser
one, they are refracted towards a  line drawn perpendicular to  the sur-
face they  are entering.
   III.  When the rays of light pass from a denser medium into  a rarer
one, they are refracted from the perpendicular.
   IV.  When rays proceeding  from the several points of a luminous
object, at a distance, fall upon a double convex lens, they  are  brought
* See Dr. Golding Bird's Manual, Chap. XXII. 
SENSE  OF SIGHT.
545
to a focus upon the other side of it; in such a manner that an inverted
picture of the object is formed upon a screen, placed in the proper posi-
tion to receive it.   Thus in Fig. 162, a b is the object,  and e f the

                              Fig. 162.
lens ; the rays issuing from the two extremities and  the centre of the
object, are brought  to a corresponding focus at a less distance on the
other side of it, so as to form a  distinct  picture; but as the rays from
A are brought to a focus at d, and those  from B at  c, the picture will
be inverted.
  V. The further the object is  removed from  the  lens, the nearer will
the picture be brought to it, and the smaller will it be.
  VI. If the screen be not held precisely in  the focus of the lens, but
a little nearer, or further off, the  picture will be indistinct;  for the
rays which form it will  either not  have met, or they will have crossed
each other.
  956. The Eye, in  its most perfect form—such as it possesses in Man
and  the  higher animals,—is an  optical instrument of wonderful com-
pleteness ;  designed  to form an  exact picture of  surrounding objects
upon the Retina or expanded  surface  of the Optic nerve, by which the
impression is conveyed to the brain.  The rays of light, which diverge
from the several  points of any object, and  fall upon the front of the
cornea, are refracted by its convex surface, whilst passing through it
into the eye, and are made to converge  slightly.   They are brought
more  closely together  by the crystalline lens, which they reach after
passing through the  pupil; and  its refracting influence, together with
that produced by the vitreous  humor, is such as to cause the rays, that
issued  from each point,  to meet in a focus  on  the retina.   In this
manner, a complete  inverted image is formed, as shown in Fig.  163;
which represents a vertical section  of the  eye, and  the general course

                              Fig.  163.
of the rays in its interior.   As in the preceding figure, the rays which
issue from the point A are brought to a focus at D; whilst those diverg-
ing from b are made to converge  upon  the retina  at c.—The Retina,
which is itself  so thin as to  be  nearly  transparent, is spread  over the 
546
OF SENSATION.
layer of black  pigment, which lines  the choroid coat.   The purpose of
this is evidently to absorb the rays of light that form the picture, imme-
diately after they have passed through the retina ; in this manner, they
are prevented from being reflected from one part of the interior of the
globe to another; which would cause great confusion and indistinctness
in the picture.   Hence it is that, in those albino individuals (both of
the Human race, and among the  loAver animals), in  whose eyes this
pigment  is  deficient, vision  is extremely  imperfect, except in a  very
feeble light; for  the  vascularity of  the  choroid and  iris is such as to
give to these membranes a bright red hue, which enables them power-
fully  to reflect the light that reaches  the interior of  the eye, when they
are not prevented from doing so  by the interposition of the pigmentary
layer.
   957. The Eye is so constructed, as  to avoid certain errors and de-
fects, to which all  ordinary optical instruments  are liable.  One  of
these imperfections, termed spherical aberration, results from the fact,
that the  rays of light, passing through a convex lens  whose curvature
is circular, are not all brought to their  proper foci, those which  have
passed through the exterior of the lens being  made to converge sooner
than those Avhich have traversed  its central portion.  The result of this
imperfection is, that the image  is  deficient in  clearness,  unless only
the central part  of the lens be employed.—The other  source of imper-
fection is what is termed chromatic aberration ; and it results from the
unequal  degree in  Avhich the differently-colored rays  are refracted, so
that they are  brought to a focus at different points.   The violet rays,
being the most refrangible, are soonest brought to a focus; and the red
being the least refrangible, have their  focus  at the greatest  distance
from  the lens.   Hence it is  impossible to obtain an  image  by an ordi-
nary  lens,  in  which  the  colors of  the  object are accurately  repre-
sented ; for the foci of its differently-colored portions will be different;
and its white rays will be decomposed, so that  the outlines will be sur-
rounded by colored  fringes.—The  Optician is  enabled to  correct the
effects of these  aberrations, by combining lenses of different densities
and curvatures;  so arranged as  to correct each other's errors, without
neutralizing the refractive power.  Thi3  is precisely the plan adopted
in the construction of the Eye;  which, when  perfectly formed, and in
a healthy state, forms an accurate picture of the object upon the retina,
free from either spherical or chromatic aberration.  This is effected  by
the  combination of humors  of different  densities,  having curvatures
precisely adapted to the required purpose.
   958.  There are certain variations, hoAvever, in  the  conformation of
the eye which diminish the perfection of its result.  Thus  the Cornea
may be too convex,  and the  whole  refractive  powrer  too great; so that
the image of an object at a moderate distance is formed in front  of the
retina, instead of upon it.   When this is the case, a distinct image can
only  be formed, by bringing the  object  nearer to the eye; the effect of
which will be, to throw the  picture further back.   Such an eye is said
to be myopic, or short-sighted; and  its  imperfection may be corrected
by placing a concave lens in front of the cornea, of a curvature adapted
to neutralize Avhat is  superfluous in  the  convexity  of the latter.—On 
OF THE EVE  AS AN  OPTICAL INSTRUMENT.
547
the other hand, if the  cornea be too flat, and  the  refractive  power of
the humors be  too  low, the  com'ergent rays proceeding from an object
at a moderate  distance will not  meet upon the retina, but behind it (if
they were allowed to pass on);  consequently the picture is indistinct;
and it can  only be made clear, either by withdrawing the object to a
greater distance, which will bring the focus of the eye nearer to its front,
or by interposing a convex lens to increase the refractive power of the
eye.   Such a condition is termed presbyopic (from its being common in
aged persons),  or long-sighted.   It may proceed to such an extent, that
not even the removal of the object to any distance can permit the forma-
tion of a distinct picture;  so that the assistance of a convex lens must
be obtained even to see remote objects clearly; though a less degree of
convexity will  be required, than  for the  clear vision of nearer objects.
This state is particularly well marked after the operation for cataract;
for the removal of the crystalline lens so greatly diminishes the refrac-
tive poAver of the eye, as to  render necessary the assistance of convex
lenses of high  curvature.
   959. The power, by which a healthy, well-formed eye can accommo-
date itself to the distinct vision of objects at varying distances, is a very
remarkable one ; and its rationale is not yet properly understood.   Ac-
cording to the  laws already stated (§ 955, V. and VI.) the picture of a
near object can only be distinct  when formed more remotely from the
lens than the picture of a  distant object.  Consequently when the eye,
that  has been looking at a distant  object, and has seen it clearly, is
turned to a near object, a distinct picture of the latter cannot be formed
without some alteration, either in the  distance between  the  refractive
surfaces and the retina, or in the  curvature of the former.   It seems
most probable  that, in the Human eye, this adjustment is  chiefly effected
by the automatic contraction of  the  ciliary muscle; which, by drawing
the lens nearer to the iris, will thus increase its distance from the retina.
But this may not be the sole  change.
   960. The various  humors  and containing  membranes  of  the Eye,
thus answer the purpose of  a most delicate and self-adjusting Optical
Fig. 164.
                 Distribution of Capillaries in vascular layer of Retina.

instrument; the  sole part, which  is immediately concerned in the act
of sensation, being the Retina, or net-like expansion of the Optic nerve, 
518
OF SENSATION.
which lies  between the black  pigment and the vitreous humor.   It is
in this structure, that the presence of cells at the peripheral as Avell as
the central extremities of the  afferent  nerves  (§  381), may be most
clearly demonstrated.   They can  scarcely be distinguished,  in many
animals,  from the cells of the vesicular matter of the brain ;  and, like
the latter,  they lie  in the  midst of a plexus of capillary blood-vessels
(Fig. 164), which supplies  the  materials requisite for their groAvth and
activity.   For the  maintenance of the due nutrition of this organ it is
requisite that it should be occasionally called into use.   If its functional
power be destroyed by opacity of the anterior portion of the eye, the
nutrition of the retina and optic nerve suffers to such a degree that these
parts cease, after a time, to exhibit their characteristic structure ;  thus
showing  that the general rules  already stated (chap, vii.) in regard to
the connexion between the functional  activity and  the  due nutrition of
tissues and organs, hold  good with respect to the Nervous structure.—
The fibres  of the Optic nerve when they diverge  to form the Retina, lose
their tubular structure; their central  axis  only being continued, in the
form of  gray fibres (§ 375), and some of these becoming directly conti-
nuous with the caudate vesicles (§ 378).
   961. The  picture of  external objects,  which  is  formed  upon  the
Retina, closely resembles that which we  see in  a Camera Obscura.   It
represents  the outlines,  colors, lights and shades, and relative posi-
tions, of  the objects before us ;  but  these  do not necessarily convey to
the mind the knowledge of their real forms, characters, or distances.
The perception of the latter, as already remarked (§ 936), is a mental
process;  and it  may be intuitive or acquired,—the  latter, it would
seem, being the general condition of the function in Man, the former in
the lower  animals.  The  Infant is  educating  his perceptive powers,
long before any indications present themselves, of the exercise of higher
mental faculties.  By the  combination, especially, of the sensations of
sight and touch,  he is learning to judge of  the surfaces of objects as
they feel, by the appearance they present,—to form an  idea of  their
distance, by the  mode in which his eyes are directed towards them,—
and to estimate  their  size, by combining the notions obtained through
the picture on the  retina, with  those  he acquires by the movement of
his hands over their different parts.  A  simple illustration will show,
how closely the  ideas excited by the two sets of sensations, are blended
in our minds.  The idea of smoothness is one which has reference to the
touch; and yet  it constantly occurs to us,  on looking at a surface which
reflects light in  a particular manner.  On  the  other hand, the  idea of
polish is  essentially visual, having reference to the reflection of light from
the surface of the object;  and yet it would occur to us from the sensa-
tions conveyed through the touch, even in the dark.
   962. That this sort of combination is not intuitive in Man, but is the
result  of experience, is evident from  the numerous observations made
upon those who had acquired the sense of Sight for the first time, after
long familiarity with the characters of objects as perceived through the
Touch.  Thus a boy of four years old, upon whom the operation for con-
genital  cataract had  been very successfully performed, continued to
find his way about his father's  house, rather by feeling with his hands, 
SENSE  OF VISION.
549
as he had been formerly accustomed to do, than by  his newly-acquired
sense of Sight; being evidently perplexed, rather than assisted  by the
sensations  which he  derived through it.   But  when learning a new
locality, he employed his sight and evidently perceiA'ed  the increase
of facility which  he derived from  it.   Among the many interesting
particulars recorded  of the  youth on whom  Cheselden  operated with
equal success, it is  mentioned  that, although perfectly familiar with a
dog and a cat by feeling them, and quite able to distinguish between them
by his sight,  it was long before he associated his  visual with his tactual
sensations, so as to be able to name  either animal by sight alone.—The
question was put by the celebrated Locke, whether a person born blind,
who was able by  his  touch to distinguish a cube  from  a  sphere, would,
on suddenly  obtaining his sight, be able  to recognise each by the latter
sense;  the reply was given in the negative;  and  the experience of the
cases just referred  to, as well as of many others, fully justifies such an
answer.
   963. Still  there are,  even  in Man, certain  intuitive  perceptions,
which afford great assistance in the formation  of  ideas regarding ex-
ternal objects through the visual sense.   And  the first of these is the
power by  which we recognise their  erect  position, notwithstanding the
inversion of  the image  upon  the retina.   This is certainly  not a matter
of experience;  nor is it capable of explanation (as  some have thought)
by a reference to the direction in which the  rays fall  upon the retina.
It is  the mind which rectifies the inversion ;  and as already remarked,
it is just as difficult to understand how the inverted image on the retina
should be taken cognizance of by the mind at all, as  it  is to comprehend
hoAV it should  be  thus rectified.  In fact, there is  no real connexion
whatever, between  the inversion of the image upon the retina, and that
wrong perception  of  external objects, which some have  thought to be
its  necessary consequence.   Any distortion  of the picture, giving a
wrong vieAv of the relative positions of the objects represented, Avould be
attended with a different result.—The same may be  said of the cause of
the singleness  of  the sensation perceived by the mind,  although  an
image is formed upon  the retina of each eye, of  those objects at  least,
which lie in the field of vision that is common to both.  This blending
of the  pictures, formed upon the two retinae, into a single perception,
appears to be, in part at least, the effect of habit.  For Avhen the images
do not fall upon those parts of the two retinae Avhich are accustomed to act
together, double vision is the result.  Thus if, when looking steadily at
an object, we press one of the eyeballs sideways with the finger, we see
two representations of the object; and the same thing frequently occurs,
as a result of an affection of  the nerves or muscles of  one  or both eyes
(as in ordinary strabismus or squinting),  or from some  change in the
nervous centres, as in various disorders of the Encephalon, and in  in-
toxication.  If this condition should be permanent, howeA'er, we usually
find  that the individual becomes accustomed to  the double  images, or
rather ceases to perceive that  they  are double;  probably because the
mind becomes habituated to receive the impressions from the tAvo  parts
of the retinse Avhich now act together.  And  if,  after  the double vision
has passed away, the conformity of the tAvo eyes be  restored (as by the 
550
OF SENSATION.
operation for the cure of squinting), there is double vision for some little
time, although the two parts of the retinas, which originally acted toge-
ther, are now brought into their pristine position.
   964. But the images thus combined are far from being identical; and
one of the most remarkable of all our intuitive perceptions, is that by
which they are reconciled and combined, and are caused to give rise to
an idea that differs essentially from either image.  No near object can
be seen by the two eyes in  the same manner; of this  the  reader  may
easily convince himself, by  holding up a  thin book, in such a position
that its back shall be in a line with  the nose, and at a moderate distance
from it; and by looking at the book, first with one eye, and then  with
the other.   He will find that he gains a different view of the object with
each eye, when used separately;  so that  if he were to represent it, as
he actually sees  it under these circumstances, he would  have two per-
spective delineations differing from one another, because  drawn from
different points.   But on looking  at the object  Avith the two eyes con-
jointly, there is no confusion between these pictures; nor does the mind
dwell upon either of them singly; but the union of the two intuitively
gives us the idea of a solid projecting body,—such an  idea as we could  .
only have otherwise acquired by the exercise  of the  sense of touch.
That this is really the case, has been proved by experiments with a very
ingenious instrument, the Stereoscope, invented  by Prof. Wheatstone;
which is so  contrived, as to bring  to the two eyes, by reflection from
mirrors, two different  pictures, such as would be accurate representa-
tions of a solid object, as seen by the tAvo eyes respectively.   When the
arrangement is such, as  to  bring  the images of  these pictures to those
parts of the retinas, which would have been occupied  by the images of
the solid (supposing that to have  been before the eyes), the mind will
perceive, not one or other of the  single  representations of the object,
nor a confused union of the two, but a  body projecting in relief, the
exact counterpart of that from which the drawings were made.—Thus
the combination  of the two pictures, and the perception of an object
different from either of them, is effected  by a mental process of an in-
stinctive kind, of the nature of which we know nothing further.
   965. When  two pictures, representing dissimilar objects,  are pro-
jected upon the retinae of the two eyes by means  of the  Stereoscope, the
result is a curious one.  The mind perceives only one of them,  the
other being completely excluded for a time; but it commonly happens
that, after one has been seen for  a short period, the  other begins  to
attract attention and takes its place, the first entirely disappearing;  so
that  there is no confusion or intermingling of  images, except at the
moment of change.   The  Will may determine, to  a  certain   extent,
which object shall be seen ; but not  entirely; for if one picture be more
illuminated than the other,  it will be seen during a larger proportion of
the time.—An interesting variation of  this experiment may be made,
without  the aid of the Stereoscope, by  holding a piece of blue glass
before one eye, and a piece of yellow glass before the other.   The re-
sult will be, not that everything will be seen of  a green color, but that
the surrounding  objects will be seen  alternately blue  and yellow; or
sometimes the field of vision will  be blue spotted with yellow, alterna- 
ESTIMATE OF DISTANCE BY VISION.
551
ting with yellow spotted with blue.   Thus, when we have two dissimilar
objects before the eyes, our attention cannot be kept upon either, to the
exclusion  of  the other, but is alternately and involuntarily directed,
either in part or completely, to one and the other.
  966.  Our  ideas of the distance of near objects is evidently acquired
from experience; and is  suggested by the muscular sensations which
are produced by the contraction of the  adductor muscles of the eyes.
When we direct our eyes towards a  near object, a certain degree of con-
vergence  takes place between  their axes; the degree increasing as the
distance between the object and  the eyes diminishes;  and vice versd.
We instinctively interpret the  sensations thus  produced,  in  such  a
manner as to be able  to compare, with great accuracy, the relative dis-
tances of two objects that are not remote from the eyes.  This intuition,
however,  is  evidently  one of the acquired kind;  as may be  seen by
watching the actions of an infant,  or of a person who has recently be-
come possessed of Vision.   When an object is held before the eyes, and
an attempt is made to grasp it, the manner, in which the attempt is made
clearly shows, that  there  is• no  power  of forming a preciseidea of its
situation, such as that which exists  in many of the lower animals from
their first entrance into the world (§ 938).  The impressions made upon
the eyes have to be corrected by those  received through the touch, be-
fore the  power of judging of distance is acquired.  How much this
power depends upon  the  conjoint use of both eyes, is  evident from the
 difficulty with which any actions, that require an exact appreciation of
 distance, are performed by those Avho have lost the sight of  one eye,
 until they have acquired neAV modes of judging of it.
    967. In regard to remote objects, we have not the same guide; since
 the convergence of the eyes, in viewing them, is so slight that  the axes
 are virtually parallel.  Our judgment of their distance is chiefly founded
 upon their apparent size, if their actual size  be known to us; and also
 upon the extent of ground, Avhich we see to intervene between ourselves
 and the  object.  But if we do not know their  actual size, and are so
 situated  that we cannot  estimate  the intervening space, we  form our
 iudgment chiefly from the greater or less distinctness of their color and
 outline.   Hence our idea of it will be  very much affected by varying
 states of the atmosphere;  a slight  haziness increasing the apparent dis-
 tance; whilst a peculiarly clear  state of the  air will seem to  cause  re-
 mote  objects to approach much more closely.   This want of conver-
 gence between the axes of the two  eyes, has the  further effect of causing
 the pictures upon the two retinae to  be nearly identical; and conse-
 quently the idea of projection is  not so strongly excited ; nor are we able
 to distinguish with the same certainty between a well-painted picture,
 in which the lights and shades are  preserved, and the objects themselves

 m968e Our notion of the size  of an  object is closely connected with
 that of its distance.   It is founded upon the dimensions of the picture
 nroiected on the retina; and  the dimensions of this picture will vary,
 according  to  the laws  of  optics (§ 955) inversely as the distance,—
 beincr  fo? example, twice as great when the object  is  vieAved at the
  distance  of  one foot as when it is carried to the distance of two feet.
  When  we  know  the relative  distances of  two  objects,  the estima- 
552
OF SENSATION.
tion  of their real comparative sizes from their apparent sizes is easily
effected by a simple process of mind; but this is not the case, when we
only guess at their distances ;  and our estimate of the  size of objects,
even moderately remote, is as much affected by states of the atmosphere
as that of their distance,—the  one being, in  fact, proportional to the
other.   Thus a slight mist, which gives the idea of increased  distance,
will also augment the  apparent size; because in order that an object
two miles off, should  produce  a picture upon the retina  of the same
extent with that  made by an object one mile off, it must  have double
the dimensions.   It is  evident that our perception of the size of objects
must be acquired by experience, in the same  manner as that  of their
distance has been shown to be.
   969. We  have now to  consider briefly some other  phenomena of
Vision, in which the acts of mind, that have been just  alluded to, do
not participate.—The  contraction of the Pupil,  under  the stimulus of
light, seems to be effected  by a sphincter  muscle, which surrounds the
orifice, and which is put in action by a branch of the Third pair of
nerves.  This is  an action with which the will has nothing to do ; and
it takes place entirely  without our consciousness.  Although  it is  due
to the  stimulus of light, yet there is reason to believe that the conscious-
ness of the presence of light is  not requisite;  and that it  is, therefore,
a  purely reflex action.  The Optic nerve  seems  to  be  the  channel
through  which the  impression  is conveyed  to  the nervous centres;
whilst  the Third pair is that through which  the motor impulse  is con-
veyed  to the iris; but  there is some ground for the idea that the Fifth
pair may in some degree convey the  requisite  stimulus, when the optic
nerve has been divided.  That the dilatation of the pupil is a muscular
action, appears probable from the fact that  the radiating  fibres of the
iris are of  the same character with the circular;  both sets  constituting,
in Man,  a peculiar variety of the non-striated form of muscular tissue.
Through what nervous channel,  however, the stimulus to this  action is
conveyed, has not yet been clearly made out.   The contraction of the
pupil is evidently designed to exclude from the interior of the eye, such
an amount of light as would be injurious to it; whilst its  dilatation in
opposite circumstances admits  the  greatest possible number  of rays.
There  is a contraction  of the pupils, however, Avhich takes place without
any change in the amount of light.   This occurs  when the  two eyes are
made to converge strongly upon any object brought very near them;
and its purpose appears to be,  to prevent  rays  from entering the eye
at such a wide angle, as would render it impossible  for them  to be all
brought  to their proper  foci,  and would  thus produce an indistinct
image.
   970. In the use of the Eye, like that of the Ear, there is a tendency
to blend into one continuous image a succession of luminous impressions
made at short intervals;  upon which fact  depend a number of* curious
optical illusions.   The length of the greatest interval that can elapse
without  an interruption of the  presence of the image (in other words
the duration  of the visual  impression), may be measured by causing a
luminous object to whirl round, and by ascertaining the longest period
that may be allowed for each revolution, consistently with the complete- 
OF THE VOICE AND SPEECH.
553
ness of the circle of light thus formed.   By experiments of this kind,
the time has been found to vary, in different individuals, or  in different
states of the same individual, from about l-4th to l-10th of a second :
that is, the impression must be repeated from four to ten times in each
second to insure the continuousness of the image.
  971.  The impressions  of variety of color, are produced by the diffe-
rently-colored rays, which objects reflect or transmit to the eye.   It
is curious that some persons, whose  sight  is perfectly good for forms,
distances, &c, are unable to discriminate colors.  This curious affection
has received the name of Daltonism; from the  circumstance that the
celebrated Dalton Avas an example of it.   There are numerous modifi-
cations of it; the want of power  to discriminate color being total  in
some;  whilst in others it extends only to  certain shades of  color, or to
the complementary colors.
  972.  When the  retina  has  been  exposed for some  time  to a strong
impression of some particular  kind, it seems less susceptible of feebler
impressions of the same kind ;  thus if we look at any brightly-luminous
object, and then turn our eyes upon  a sheet of paper, we shall perceive
a dark spot upon it:  the  portion of the retina, which  had received the
brighter image not being affected by the fainter one.  Again,  when
the eyes have received a  strong impression from a colored  object, the
spot Avhich is seen Avhen the eyes  are directed  upon a white  surface ex-
hibits  the complementary color;  for the retina  has been so strongly
affected in the part that originally received the image,  by its vivid hue,
that it does not perceive the fainter hue of the same kind in the object
to which it is then  turned, and it  is  impressed only by the remaining
rays forming  the  complementary colors.   This explanation applies to
the phenomena of the colored shadoAvs which are often seen at sunset,
and of those which may be seen in  a room whose light enters through
colored glass or drapery.  For if the prevailing light be of one  color,
—orange or red for instance,—the eye will not take cognizance of that
color in the faint light of the shadoAvs ; and Avill see  only  its comple-
ment, blue or green.   If the shadoAv be viewed through a tube, in such
a manner that the general colored  ground is excluded, it presents the
ordinary tint.
                         CHAPTER XIV.

                    OF THE VOICE  AND SPEECH.

  973.  There is one particular application of Muscular power in Man,
which deserves special consideration, as being that by which he effects
his most complete and intimate communication with his felloAvs ;—that,
namely, by which his organ of Voice is put  into action.  In  all air-
breathing Vertebrata, the production of sound depends  upon the pas-
sage of air through a certain portion of the respiratory tubes, which is 
554
OF THE VOICE AND SPEECH.
so constructed as to set it in vibration,  as  it passes  forth from  the
lungs.—In Reptiles, the vibrating apparatus is situated at the point,
where the trachea opens into the front of the pharynx;  it is of very
simple construction,  however, being only composed of a slit bounded by
two  contractile  lips;  and  few of  the animals of this  class  can  pro-
duce  any other sound than a hiss, which, owing to the  great  capacity
of their lungs, is often very much  prolonged.—In Birds,  the  situation
of the vocal organ is very different.   The  trachea opens into  the front
of the  pharynx as in Reptiles, by a mere slit;  the borders  of which
have  no other movement than that  of approaching one another, so as
to close the  aperture when  necessary.   This appears to be the instru-
ment for  regulating  the ingress and egress of air, in conformity Avith
the wants of the respiratory function.  The vocal larynx of Birds is
situated at the lower extremity of  the trachea, just Avhere  it subdivides
into the bronchial tubes; and  it  is of very complex  construction, espe-
cially in  the  singing  birds.—In Mammalia,  on the  other hand,  the
vocal organ  and the  regulator of  the respiration are united in  one
larynx, which is situated at the top  of the trachea.  There are few, if
any,  of this  class, Avhich  have not some  vocal sound;  but the variety
and expressiveness which can  be  given to it, .differ considerably in the
several orders;  being by far the greatest in Man, who alone, there is
reason  to believe, has the power of  producing articulate  sounds, or pro-
per language.
   974. The Larynx is built up as  it were, upon the Cricoid  cartilage
(Fig. 165, x w r u),  Avhich  surmounts the trachea, and which  might be

                               Fig. 165.
                                          i
 Bird's-eye view of Larynx from above, after Willis:— g e h, the thyroid cartilage, embracing the ring of
the cricoid r u x w, and turning upon the axis x z, which passes through the lower horns ; n f, n f, the ary-
tenoid cartilages, connected by the arytenoideus transversus; t v, t v, the vocal ligaments; n x, the right
cricoarytenoideus lateralis (the left being removed); v k f, the left thyro-arytenoideus (the right being re-
moved) ; n I, n I, the crico-ary tenoidei postici; b b, the crico-arytenoid ligaments.


considered as  its highest  ring modified in form, its depth from  above
downwards being  much  greater posteriorly than anteriorly.   This  is
embraced, as it  were, by the thyroid cartilage (g eh);  which is arti- 
          REGULATION OF THE APERTURE  OF THE  GLOTTIS.       555

culated to the sides of  the Cricoid by its lower horns, round the extre-
mities of  which it may be considered to rotate, as on a pivot.   In this
manner, the front of  the Thyroid  cartilage may be lifted up,  or  de-
pressed, by the muscles which  act  upon it; whilst  the position of its
posterior part is  but  little changed.   Upon the  upper surface of  the
back of the Cricoid cartilage, are seated the two small Arytenoid carti-
lages (n f) ; these are so tied to the cricoid by a bundle of strong liga-
ments (b b), as to have a sort of  rotation upon an articulating  surface,
which  enables them to be approximated or  separated from each other,
—their inner edges being nearly parallel in the first case,  but slanting
away from each other in the second.   To the summit of these cartilages
are attached the Chordse vocales, or vocal ligaments (t u), composed of
yellow fibrous or elastic tissue.  These stretch across to the front of
the Thyroid cartilage; and it is  upon their condition and relative situ-
ation,  that the absence or the production of vocal tones, and  all their
modifications of pitch, depend.   They are rendered tense by the  de-
pression of the front of the  Thyroid cartilage, and  relaxed by its ele-
vation ; by which action the pitch of the  tones is regulated.   But  for
the production of any vocal tones Avhatever, they must be  brought into
a nearly parallel condition, by  the mutual  approximation of the points
of the arytenoid cartilages to which  they  are  attached ;  whilst  in  the
intervals of vocalization, these are separated, and the rima glottidis,  or
fissure between the chordae vocales, assumes the form of a narrow V,
with its point directed backwards.
  975. Thus there are two sets of movements concerned  in  the act  of
vocalization;—the  regulation  of the relative  position  of  the Vocal
Cords, Avhich is effected by the  movements  of the Arytenoid cartilages;
—and the regulation  of  their  tension,  which  is  determined by  the
movements of  the  Thyroid  cartilage.  The  Arytenoid  cartilages  are
made to diverge from one another  by means of  the  Crico-arytenoidei
postici of  the  two sides (n I, N I), which proceed from their outer cor-
ners and turn somewhat round  the  edge of the Cricoid,  to be  attached
to the lower part of its back ; their action is to draw the  outer corners
of the  Arytenoid cartilages outwards and downwards,  so  that  the
points to which the vocal ligaments are  attached are separated from
one another, and the rima glottidis  is thrown  open.   The action  of
these muscles is antagonized  by that of the Arytenoideus transversus,
which  draAvs  together the  Arytenoid cartilages ; and by that of  the
Crico-arytenoidei laterales of the two sides (n x), which run  forwards
and doAvnwards from  the outer  corners of the Arytenoid cartilages, and
tend by their  contraction to bring  together their  anterior points,  to
which the Vocal ligaments are  attached.—The depression of the front
of the  Thyroid cartilage, and the consequent tension of the Vocal liga-
ments, is occasioned by the  conjoint action  of the Crico-thyroidei of
the  two sides, Avhich  occasions the  Thyroid and  Cricoid  cartilages  to
rotate, the one upon the other,  at the  articulation formed by the infe-
rior  cornua  of the   former;  and  this action will  be  assisted by  the
Sterno-thyroidei, which tend  to depress the  front of  the  Thyroid carti-
lage  by pulling from  a fixed  point  below.  On the other hand,  the
elevation of the front of the Thyroid cartilage,  and the relaxation  of 
556
OF THE VOICE AND SPEECH.
the Vocal ligaments are affected by the  contraction of the Thyro-ary-
tenoidei of the two sides (v k f),  whose attachments  are the same as
those of the Vocal ligaments themselves ; and this is aided by the Thyro-
hyoidei, which will tend to draw up the front of the Thyroid  cartilage,
acting from a fixed point above.
   976.  The muscles which govern the aperture of the glottis,—those
namely,  which separate and bring together the arytenoid cartilages,
and thus widen or contract the space between the posterior extremities
of the vocal ligaments,—have important  functions in connexion with
the Respiratory actions in general; standing as guards, so to  speak, at
the entrance of the lungs.   We can entirely close the glottis through
their means, by an effort of the Will, either  during inspiration or expi-
ration ; and it is a spasmodic movement of this sort, which  is concerned
in the acts of Coughing and  Sneezing, the purpose of  Avhich is to expel
by a sudden and powerful blast of  air,  any irritating substances,
whether solid, liquid, or gaseous,  Avhich have found their way into the
air-passages.  These muscles appear to be under the  sole direction of
the inferior or recurrent laryngeal nerve ; which seems to possess ex-
clusively motor endowments.  When this nerve is divided, on  each side,
or when the par vagum is divided above its  origin, the  muscles  of the
larynx (with the exception of the crico-thyroid) are paralysed; and the
aperture of the glottis may remain open, or may be entirely closed, ac-
cording to the manner in which its lips  are affected by the currents of
air in ingress or egress.  It is found that,  under such circumstances,
tranquil respiration may be carried on; but that any violent ingress or
egress of air will tend to drive the lips of the glottis (these being in a
state of complete relaxation) into apposition with each other, so as com-
pletely to close the aperture.   The character of the superior laryngeal
nerve appears to be almost exclusively afferent; no muscle, except the
crico-thyroid, being thrown into contraction  when it is irritated;  whilst,
on the other hand, if it be divided, neither the act of coughing, nor any
reflex respiratory  movement whatever, can be excited, by irritating the
lining membrane of the larynx.
   977.  During  the ordinary acts of inspiration  and  expiration, the
Chordae vocales appear to be widely separated from each other,  and to
be in a state of  the freest possible  relaxation.   In order  to  produce a
vocal sound, they must be made  to approach  one another,  and their
inner faces must be brought  into  parallelism;  both of which ends are
accomplished by the rotation of the Arytenoid cartilages;  whilst, at the
same time, they must be put into  a certain  degree of tension,  by the
depression of the Thyroid  cartilage.   Both of these movements take
place consentaneously,  and  are mutually  adapted to each other; the
vocal ligaments being approximated, and the rima glottidis consequently
narroAved, at the same time that their tension is increased.  There is a
certain aperture,  which is favorable to the production of each tone,
although the pitch itself is governed by the tension of  the Vocal  Cords;
and it is, perhaps, to a  want of consent betAveen the tAvo, that the pecu-
liarly discordant nature of some voices, which  appear  incapable of pro-
ducing a distinct musical tone, is due.
   978.  It has been fully proved, by  the researches of  Willis, Miiller, 
           REGULATION OF THE PITCH OF  VOCAL SOUNDS.       557

and others, that the action of  the Vocal ligaments, in the production of
sound, bears no resemblance to that of vibrating strings; and that it is
not comparable to that  of  the mouth-piece of the flute-yipes of the Or-
gan;  but  that  it is, in  all essential particulars, the same  with  that of
the reeds of the Hautboy or Clarionet, or the tongues of the  Accordion
or Concertina.  All the phenomena attending the production of Musical
tones are fully  explicable on this hypothesis; except the production of
falsetto notes,  which has  not  yet  been clearly accounted  for.—The
power Avhich the Will possesses, of determining, with the most  perfect
precision,  the exact degree of tension which these  ligaments shall re-
ceive, is extremely remarkable.   Their  average length in the  Male,
in the state of  repose, is estimated by Miiller at  about 73-100ths of an
inch ) whilst, in the state of greatest tension, it is about 93-100ths ; the
whole difference, therefore, is not above  20-100ths, or one-fifth of an
inch.  In the  female glottis, their average dimensions  are  about 51-
lOOths, and 63-100ths,  respectively;  so that the difference is here only
12-100ths, or less than  one-eighth of an  inch.   Now the  natural com-
pass of the voice, in most  persons who have cultivated the vocal organ,
may be stated at about two octaves, or 24 semitones.   Within each
semitone,  a singer of ordinary capability could produce at least ten dis-
tinct intervals; so that, for the total number of intervals, 240 is a very
moderate  estimate.  There must, therefore, be at least 240 different
states of tension  of the vocal  cords, every one of which can  be  at once
determined by the will, when a distinct conception exists of the tone to
be produced (§  905); and, as  the whole variation in their  length is not
more than one-fifth of an  inch,  even in Man, the variation required, to
pass from one interval  to  another, will not  be more than l-1200th of
an  inch.—And yet  this estimate is much  below that, which might be
truly made from the  performance of a  practised vocalist.   The cele-
brated Madame  Mara is said  to have been able to sound 50 different
intervals betAveen each semitone; the compass of her voice was at least
40  semitones, so that the total number of  intervals was  2000.  The
extreme variation in the  length  of  the vocal cords, even taking the
larger scale of  the Male larynx, not being  more than  one-fifth of an
inch, it may be said that she was able to determine the contractions of
her vocal muscles to the ten-thousandth of an inch.
  979.  It is on account of the  greater length of the Vocal cords, that
the pitch of the voice is much lower in Man  than in Woman : but this
difference does  not arise until  the end of the period of  childhood,—the
size of the larynx being about the same in the Boy and Girl, up to the
age  of 14 or  15 years, but  then undergoing a rapid increase in the
former, whilst it  remains nearly stationary in the  latter.   Hence it is
that Boys, as well as Girls and Women, sing treble ;  whilst  Men sing
tenor, which is  about an octave lower than the treble;  or bass, which is
several notes lower still.  The cause  of  the variation  in the  timbre or
quality in different voices  is  not certainly known;  but it appears to be
due, in part, to differences in  the degree of flexibility and smoothness
in the cartilages of the larynx.   In Women and children, these carti-
lages are  usually soft and flexible, and the voice is clear and smooth  ;
Avhilst in men, and in Avomen  whose voices have a masculine roughness, 
558
OF THE VOICE AND SPEECH.
the cartilages are harder, and are sometimes almost completely ossified.
The loudness of the voice depends  in  part  upon the force  with which
the air is expelled from the lungs ; but  the  variations in  this respect
which exist among  different individuals, seem partly due to the degree
in which its resonance is increased by the vibration of  the other parts
of  the larynx, and  of the  neighboring  cavities.   In  the  HoAvling
Monkeys of America,  there  are  several  pouches  opening  from the
larynx, which seem destined to increase the volume of tone that issues
from it;—one of these is excavated in  the substance of the  hyoid bone
itself.   Although  these Monkeys are   of inconsiderable size, yet their
voices  are louder than the roaring of lions, and are distinctly audible at
the distance of two miles; and when a  number of them are congregated
together, the effect is terrific.                                 %
   980. The vocal  sounds produced by the action  of the larynx are of
very different characters; and may be distinguished  into  the cry, the
song, and the ordinary or acquired  voice.   The cry is generally a sharp
sound, having little modulation or accuracy of pitch, and being usually
disagreeable in its timbre or  quality.  It is that  by which animals
express their unpleasing emotions, especially  pain  or terror; and the
Human  infant, like  many  of the lower animals, can  utter no  other
sound.—In song, by the regulation of  the vocal cords,  definite and sus-
tained musical  tones are produced, Avhich can be changed or modulated
at  the  will of the individual.  Different species of Birds  have their
respective  songs ;  which are partly instinctive, and  partly acquired by
education.   In  Man, the poAver of song is entirely acquired ; but some
individuals possess a much greater  facility in acquiring  it than others,
—this superiority  appearing to depend upon their more precise con-
ception of the tones to be sounded, as well as their more ready imitation,
—besides  differences in the construction  of the  larynx  itself.   The
larynx of an accomplished vocalist, obedient  to  the expression  of the
emotions, as well as to  the dictates of  the will, may be  said  to be the
most perfect musical instrument ever constructed.—The voice is a sound
more resembling  the  cry, in regard  to the absence  of any sustained
musical  tone;  but it differs from  the cry, both  in  the quality of its
tone, and  in the  modulation of which it  is capable  by the Avill.  In
ordinary conversation,  the voice passes  through a great  variety of
musical tones, in the course of a single sentence, or even a single word,
sliding imperceptibly from one to another;  and it is  when we attempt
to fix it definitely  to a certain pitch, that we change it  from the speak-
ing to the singing tone.
   981. The poAver of producing articulate sounds, from the combination
of which Speech results, is altogether independent of the Larynx ; being
due to the action of the muscles of the  mouth, tongue  and palate.
Distinctly-articulate sounds  may  be  produced  without any  vocal or
laryngeal tone, as when we whisper;  and  it  has been  experimentally
shoAvn, that the only condition necessary for this mode of speech is the
propulsion of a current of air  through the mouth,  from back to front.
On the other hand, Ave may have the most perfect laryngeal tone without
any articulation;  as in the production  of musical sounds, not connected
with words.  But  in ordinary speech,  the laryngeal tone is modified by 
          ARTICULATE  SPEECH—VOAVELS  AND CONSONANTS.       559

the various  organs which intervene between  the  larynx and  the  os
externum.   The simplest of these  modifications  is that by which the
 Vowel  sounds are  produced,—these  sounds being continuous tones,
modified by  the form of the aperture through  which they pass out.
Thus, let the reader open his mouth to  the  widest  dimensions, depress
the tongue, and raise the velum palati, so  as to make the exit of air  as
free as possible; on then making a vocal  sound,  he will find that this
has the character of the vowel a in  ah.  On the other hand, if he draw
together the lips, still keeping the  tongue  depressed, he will pass to the
sound  represented  in the English  language by oo, in the Continental
languages by u.  By attention to the production  of other vowel sounds,
it Avill  be found that they are capable of being formed  by similar modi-
fications in  the form of the buccal cavity  and the size of the buccal
orifice; and that they are capable of being sustained for any length  of
time.   There is an exception, however, in  regard  to the sound of the
English i, as in fine, which is, in reality, a diphthongal sound, produced
in the act of transition from a peculiar indefinite murmur to the sound
of the long e, which takes  its place AA'hen  Ave attempt to continue  it.
The short voAvel sounds, moreover, such as a in fat, e  in met, o  in pot,
&c, are not  capable of being perfectly prolonged,  as  they require, for
their true enunciation, to be immediately followed by a consonant.—A
tolerably good artificial imitation of VoAvel sounds has been effected  by
means of a reed-pipe representing  the larynx, surmounted by an India-
rubber  ball, Avith an orifice, representing  the cavity and  orifice of the
mouth.  By modifying the form of the ball, the different  vowels could
be sounded during the action of the reed.
   982. In the production of the sounds termed Consonants, the breath
suffers a more or less complete interruption, in its passage through the
parts anterior to the larynx.  The most natural  primary division  of
these sounds is into those which require a total stoppage of the breath
at the  moment previous to their  being pronounced, and  which,  there-
fore, cannot be prolonged; and those in pronouncing  which the inter-
ruption is partial, and Avhich can, like the vowel sounds, be prolonged
ad libitum.   The former have received the designation of explosive con-
sonants ; the latter are termed  continuous.  In pronouncing any conso-
nants of the explosive class, the posterior  nares are completely closed;
and the whole current of air is directed through the mouth.  This may
be checked by the approximation of the lips, as  in pronouncing b and  p;
 by the approximation of the point of the tongue to the front of the palate,
 as in pronouncing d and t; or by the approximation of the middle  of
the tongue to the arch of the palate, as in pronouncing the hard g or k.
 The difference betAveen b, d, and#, on the one hand, andp, t, and k, on the
 other,  depends simply upon the greater extent  of the  meeting  surfaces
in the former case than  in the latter.  In  sounding  some of the con-
tinuous consonants, the  air is not allowed to pass through the nose; but
the interruption in the mouth is incomplete; this is the case with v and
/,  s and z.   In others, the posterior nares are  not  closed, and  the  air
has a nearly free passage, either through the nose alone, as in m and n,
 or through the nose and mouth conjointly, as in I and  r.  The sound of
 h is a mere  aspiration, caused by an increased force of breath; and that 
560
OF THE VOICE  AND SPEECH.
of the guttural ch, as it exists  in Welsh, Gaelic, and  most Continental
languages, is an aspiration  modified  by the elevation  of the tongue,
which causes a slight obstruction to the air, and an increased resonance
in the back of the mouth.
   983.  The study of the mode in  Avhich the different  Consonants are
produced, is of particular importance  to those who labor under  defec-
tive speech, especially that  difficulty  which is known as Stammering.
This  very  annoying impediment is  occasioned  by a want  of proper
control  over the muscles concerned in Articulation;  which,  instead of
obeying the Will, are sometimes affected with an involuntary  or spas-
modic action,  that interrupts the pronunciation of particular  words,—
just as, in Chorea, the  muscles of the limbs are interrupted by spas-
modic twitchings, in the performance  of any voluntary movement.  In
fact, persons affected with general  Chorea frequently stammer;  show-
ing that ordinary Stammering may be considered as  a kind of local
Chorea.  The analogy between the two states is further indicated by
the corresponding influence  of  excited Emotions in aggravating both.
   It is  in the pronunciation of the consonants  of the explosive class,
that the stammerer usually experiences the greatest  difficulty; for the
total  interruption to the breath, which they occasion,  is frequently con-
tinued involuntarily ;* so that either the expiration is entirely checked,
the whole frame being frequently thrown into the most distressing semi-
convulsive  movements, or  the sound comes  out  in jerks.  Sometimes,
however, the spasmodic action occurs in  the pronunciation of vowels and
continuous consonants ; the stammerer prolonging his expiration, with-
out being able to check it.
   984.  The best method of curing  this defect (where there is  no mal-
formation of the organs  of speech, but merely a want  of poAver to  use
them  aright), is to study the  particular difficulty under which the indi-
vidual labors; and then to  cause  him  to  practise  systematically  the
various  movements concerned in the production of the sounds in ques-
tion, at first separately, and  afterwards in combination,—until he feels'
that his voluntary control  over  the  muscles  is complete.   The patient
would at first  do well to  practise sentences,  from which the  explosive
consonants are omitted; his  chief difficulty, arising from the  spasmodic
suspension of the expiratory movement, being thus  avoided.  Having
mastered these, he may pass on to  others, in Avhich the difficult letters
are sparingly introduced;  and may finally accustom himself to the  use
of ordinary language.  One of the chief points  to be  aimed  at, is to
make the patient feel that  he has command over his muscles of articula-
tion ;  and this is best done, by gradually leading him from that  which
he can do,  to that which he fears to attempt.

  * The interruption of the expiratory movement in Stammering, is usually stated to
take place in the glottis ; but the Author is satisfied that, in  all ordinary cases at least,
it is in that condition of the mouth, which is preparatory to the pronunciation of one of
the explosive consonants. 
INDEX.
N.B. The numbers refer to the Paragraphs.
Aberration corrected in eye, 957.
Absorbent Cells, 243-245.
           Vessels, 489.
Absorption, from alimentary canal, 489, 490;
               by  lacteals,  243,  244;  by
               blood-vessels, 491-493.
            from  general  and pulmonary
               surfaces, 522, 523.
            interstitial, by lymphatics, 502,
               503; by blood-vessels, 502,
               503.
Adipose tissue, 257-263, 423-425.
Age, influence of, on pulse, 579; on nutrition,
  622-627.
Air-cells of lungs, 676-679.
Albumen, 167-171.
          conversion of, into fibrine, 519.
Albuminous compounds of Plants, 174.
Albuminuria, 533, 746.
Alga?,  development and generation of, 779,
  780.                                 •
Aliment, sources of demand for, 406-415.
         effect of variations in supply of, 416-
           426.
         relative value  of different kinds of,
           427-441.
         necessity for mixture in, 437.
Allantois, formation of,  817.
Amnion, formation of, 816.
Amphioxus, see Lancelot.
Anterior Pyramids, 890.
Apoplexy, 688, 924.
Area germinativa, 806.
     pellucida, 807.
     vasculosa, 551, 813.
Areolar tissue,  194-196, 205.
Arteries, movement of Blood in, 582-588.
         elasticity of, 583; tonicity  of, 584;
           contractility of, 585, 586;  pulsa-
          tion of, 583, 584; uniform capacity
           of, 587; anastomosis of, 588.
Articulata, circulation in, 552,553; respi-
  ration  in, 657-661;   nervous system  in,
  856-863.
Articulate speech, 981, 982.
Asphyxia, 628, 703-709.
Assimilating cells, 514, 519.
Assimilation, 519; by the liver, 493.
Asthma, 678.
Atrophy, 619, 620.
Attention, effects of, 935.
Auditory ganglia, 900.
         nerve, 950.
Azotized Compounds in Plants, 174,428, 429.
                    in  Animals,  167-179,
                      428, 429.
                    destination of, in food,
                       429, 433.

Basement membrane, 206-209.
Batrachia, respiration of, 670, 671.
Bile, composition and properties of, 724-726.
     uses of, in digestion, 476-479.
Birds,  circulation in, 564, 565 ; respiration in,
  672-674; lymphatic system in, 500; nerv-
  ous centres of, 872; heat of, 761.
Blastodermic vesicle, 805, 806.
Blastema, organizable, 213.
Blood,  composition of, 525-528; uses of seve-
         ral constituents of, 529,530; changes
         of, in disease, 531-534.
       corpuscles of, white, 214; red, 215-223.
       coagulation of, 535.
       buffy coat of, 536, 537.
       rate of  movement of, 577.
       influence of respiration on, 699-702.
Blushing, 603.
Bone, structure and composition of, 287-299 j
  development of, 300-306; regeneration of,
  307-309.
Brunner's glands, 450, 480.
Buffy coat of blood, 536, 537.
Butyric acid, 430, 833.

Ccecum, secondary digestion in, 481.
Canaliculi of Bone, 290.
Cancelli of Bone, 289.
Cancer-cells, 212, 255.
Capillaries, movement of Blood in, 589-604 ;
              variations in its rate, 597.
            variations  in size of, 595, 603;
              influence  of nerves upon, 603,
              604.
           independent force generated in,
              598-600.
Carbotiic acid,  decomposition of,  by Plants,
                 79-87; necessity for excre-
                 tion of, 641; sources of, in
                 Animal bodies, 642-648.
               mode of its extrication, 649-
                 652; amount set free, 691-
                 698.
Cartilage, 264-273 ; multiplication of cells of,
  212;  "ossification of, 300-303.
Caseine, 172, 832.
Catamenia, 798, 799.
36 
562                                   INDEX.
Cells, vegetable, general history of, 26-42.
      animal,  general history of,  210,  211;
        production of, 212, 213.
      isolated, various forms of, 214-216.
      the immediate agents in Organic func-
        tions, 246, 247.
      union of, 248-254.
      coalescence of, 252-254.
      changes of form in,  255.
Cementum, 319.
Centipede, experiments on, 858, 859.
Cerebellum, 867-869.
            functions of, 908-911.
Cerebric acid, 383.
Cerebrum, 867-873.
           functions of, 912-925.
Chlorosis, state of blood in, 219, 532, 537.
Cholesterine, 724.
Chondrine, 177.
Chorda dorsalis, 211, 251, 812.
Chord Be vocales, 974-979.
Chorea, 983.
Chorion, 795, 809,  817, 818.
Chyle, composition and properties of, 515-519.
      corpuscles of, 214, 518, 519.
Chyme, 473, 476.
Cilia, 234, 235.
Cineritious substance, 379.
Circulation, 538,539; in Plants, 540-548 ;
        in lowest Animals, 549, 550; in Echi-
        nodermata, 552;  in Articulata,  552,
        553; in Mollusca, 555-557; in Fishes,
        558-560;  in  Reptiles, 561-563;  in
        Birds and Mammals, 564, 565.
      in  early  embrvo, 551,  554, 566;  in
        foetus at birth, 823, 824.
Coagulation, of Albumen,  168, 169; of Blood,
  535; of Caseine, 172;  of  Chyle, 518;  of
  Fibrine, 180-187.
Cochlea, 952.
Cold,degree of, sustainable by Plants, 110,111.
     degree of, sustainable by Animals,  136.
Colostrum, 835.
Colors, perception of, 971.
Commissures of brain, 913-916.
Complementary  colors, 972.
Conchifera, nervous system of, 852, 853.
Concussion, 581.
Congestion, arterial,  601, 602.
            venous, 609, 610.
Consensual actions, 903-905.
Consonants, 982.
Contractility of Muscle, 347.
            of Vegetable tissues,  345, 346.
Convulsive actions, 885.
Coral, 277.
Cornea, 274.
Corpora  Malpighiana of the Spleen, 506;  of
  the*Kidney, 728.
Corpora Quadrigemina, 873, 900, 902.
        Striata, 901.
        Wolffiana, 727.
Corpus Callosum, 915.
       Luteum, 800.
Corpuscles of blood, red, 215-223; white, 214.
           of Chyle and Lymph, 214.
Correlation of Forces, 53-61.
Cortical substance  of brain, 380.
Cranium, circulation in, 611.
Crura cerebri, 894.
Crustacea,  geographical distribution of,  133 ;
  shells of, 286; respiration of, 658.
Crusta petrosa, 319.
 Cryptogamia, generation of, 780.
 Crystalline lens, 275.
 Cuttle-fish, nervous cords  in arms of, 854.

 Daltonism, 971.
 Death, somatic, 65, 68, 69, 628, 629.
        molecular, 66, 67.
 Decidua, 810,811.
 Decussation, of Anterior Pyramids, 890; of
   Posterior Pyramids, 893 ; of Optic Nerves,
   907.
 Defecation, 462, 463.
 Deglutition, 453, 454, 897.
 Dentine, 311-316.
 Determination of blood, 601.
 Development,  early history of, in Plants, 781;
   in Animals,  784, 785; see Embryo.
 Developmental process, influence of heat on,
   124-127.
 Diffusion,  mutual, of gases, 650.
 Digestion, organs of, 442-450.
            nature of the process of, 471, 472.
 Disintegration of tissues,  617; of  Muscular
   tissue, 361;  of Nervous  tissue, 381.
 Distances, estimate of,  966, 967.
 Doris, gills of, 651, 656.
 Dormant Vitality, 43-46.
 Double vision, 963.
 Draper, Prof.,  his views on the capillary cir-
   culation, 545-548, 598, 599.
 Dreaming, 924.
 Duration of pregnancy, 825, 826.
         of impressions on Ear, 956.
         of impressions on Eye, 970.
 Dytiscus, experiments on, 859.

 Ear, structure  of, 950-952.
 Echinodermata, shells of, 278, 279; circula-
   tion in, 552.
 Electricity, development of, in Animals, 771-
              777; in Torpedo and Gymno-
              tus, 771-774 ; in Muscles, 775;
              in the Frog, 776 ; in higher ani-
              mals,  777.
            influence of, on organized bodies,
              142; on  Vegetation,  143, 144,
              effects of shocks of, 145 ; influ-
              ence of, on  Animals, 146-148;
              on  Muscles,  351;  on nerves,
              396, 932.
 Embryo, early development of, 805, 808;  for-
   mation of vertebral column in, 812; forma-
   tion of vessels in, 813;  formation of heart
   in,  814;  formation  of digestive cavity in,
   815 ; circulation in, 551, 554, 556.
 Emotional movements, 917, 919.
 Emotions, influence of, on hunger, 483; on
   salivary secretion, 467;  on heart's  action,
   580; on capillary circulation, 603; on mam-
   mary secretion, 836,  837.
 Enamel, 318.
 Endosmose, 491, 492.
 Entozoa, circulation in, 549, 550.
 Epidermis, 224-228.
 Epilepsy,  886.
 Epithelium, 231-239.
 Erect vision, 963.
 Exhalation of water,  from lungs, 701;  from
              cutaneous surface, 743-746.
            of organic matter, 702, 746.
 Excreting processes, general review of, 751-
   759.
1 Eye, structure of, 956-960. 
INDEX.
563
Facial nerve, 888.
Fat, 257-263, 423-425.
Fatty liver, 754.
Fecundation of ovum, 803, 804.
Ferments, action of, on blood, 534.
Fertilization of ovum, 803, 804.
Fibre, white, 189, 191.
       yellow, 190, 192.
Fibres, formation of, from cells, 193.
Fibrillation, 183, 213.
Fibrine, coagulation of, 180-187.
        composition of, 178, 179.
        production of, 519.
Fibrous membranes, 188.
Fibrous tissues, simple, 188-193.
Fibro-Cartilage, 188, 269, 272.
Fifth Pair, 686, 888.
Fishes, lymphatic system in, 499 ;  circulation
  in, 558-560 ;  respiration in, 663-667; heat
  of,  760; electricity of, 771-774;  nervous
  centres  in, 869, 870.
Foetus, circulation in, 822-824.
Follicles of glands, 238, 714-719.
Follicles of Lieberkiihn, 449.
Food, see  Aliment.
Force, abstract nature of, 18, 71.
Forces, Physical, see Physical Forces.
        Vital, see Vital Forces;  their relation
          to the Physical, 61.
 Gall-bladder, 478.
 Ganglion, 380.
 Gangrene, 634.
 Gases, mutual diffusion of, 650.
 Gastric fluid, properties and actions of, 468-
                472.
              conditions of its secretion, 474,
                475.
 Gastric follicles, 469.
 Gelatinous nerve-fibres, 375.
 Gelatine, 175, 176, 264, 298; uses of, as food,
   429.
 Gemmation, in Plants, 779;  in Animals, 782.
 Generation, essential character of the pro-
   cess, 780-783; action of the male  in, 786-
   790; action of the female in, 791-804.
 Geographical distribution of Animals, 133.
              distribution of  Plants, 102-106.
 Germ-cells, 240, 242, 780, 783.
 Germinal membrane, 805.
          spot, 794.
          vesicle, 794.
 Gestation, duration of, 825, 826.
 Gills, structure of, 651, 655, 656, 663.
 Glands, essential parts of, 238, 714-719.
 Globuline, 221.
 Glosso-pharyngeal nerve, 888, 897.
 Glottis, regulation of aperture of, 976.
 Glycerine, 260.
 Glycocoll, 176, 734.
 Gout, 422,615.
 Graafian Vesicle, 796.
 Granulation,  637.
 Gravity, influence of, on venous circulation,
  609,  610.
 Gray nerve-fibres, 375.
 Gymnotus, 771-774.

 Haematine, 221,222.
Hair, 328-330.
Haversian canals, 293-297.
Hearing, sense of, 949-954.
 Heart, action of, 568-570;  sounds of, 571-
          575; propulsive power of, 576-578 ;
          frequency of contractions of, 579.
        power of,  independent  of nervous
          agency, 580;  influenced by mental
          emotions, 580 ; by state of nervous
          system, 581.
        first development of, in embryo, 554,
          566,814.
 Heat, amount of, developed  in Insects, 123 ;
         in  Fishes,  760;  in Birds, 761; in
         Mammals, 761; in Plants, 762.
       development of,  chiefly  dependent on
         production of carbonic acid, 763, 764 ;
         but  partly on  other oxidizing pro-
         cesses,  765; inferior in young ani-
         mals, 766.
       of body,  kept down by  perspiration,
         745,768.
       its influence upon vital activity in gene-
         ral, 97,98; upon Vegetation, 99-111;
         upon Animal life, 112-141.
       degree of, sustainable by Animals, 138-
         141; by Plants, 108.
 Hemispheres, Cerebral, 912-916.
 Hippuric acid, 730, 734.
 Hunger, sense of, 483-485.
 Hybernation, 120, 121.
 Hydra, stomach of, 443.
 Hydrophobia, 886, 908.
 Hypertrophy, 617, 618.
 Hypo-glossal nerve, 888.
 Hysteria, 741,887, 908.

 Inanition, Chossat's experiments on, 117.
 Incubation, heat  supplied in, 125-127.
 Inflammation, nature of the process, 631,632 ;
  state of the blood in, 531, 536.
 Inorganic substances in food, 438-441.
 Insalivation,  446, 451, 452.
 Insects,  circulation  in, 552; respiration  in,
  659, 660 ; nervous system of, 856-864;  re-
  flex actions of,  858-860;  instinctive actions
  of, 860, 861; heat of,  123.
 Instinctive actions of Man, 904.
 Intelligence,  913, 920.
 Intercellular  substance, 248-253.
 Intestinal canal, structure of, 447-450; move-
  ments of, 460, 461.
 Iris, movements of, 969.
 Irritability of Muscles, 348-363.

 Kidneys,  structure of,  727,  728; action  of.
  729-741,755.
 Kreatine, 735.
 Kreatinine, 735.

 Lacteals, 494, 496, 499.
 Lactic acid, in gastric fluid, 470; in urine, 735.
 Lacunae of Bone,  290.
 Laminae dorsales, 812.                    •>
 Lancelot, 215, 251, 560, 869.
Laryngeal nerves, 976.
Larynx, structure and actions of, 974, 979.
Lead-palsy, 614.
Leucin, 170, 176.
Light, laws of transmission and refraction  of,
         955.
       influence of, on vegetation,  79-92;  on
         growth and development of animals,
         93-96.
       emission of, by animals, 769; by man.
         770. 
564
INDEX.
Life, idea of, 49 ; conditions of, 70; duration
  of, 25.
Lime, in Animal body, 438, 440, 441.
Lithic acid, 732, 733.
            diathesis, 422, 733.
Liver,  structure of, 720-723;  assimilating
  action of, 493; secreting action of, 476-479,
  724-726.
Luminousness, animal, 769, 770.
Lungs, structure of,  676-679.
Lymph,composition and properties of, 515,520.
Lymphatics, 498-503.

Male, action of, in reproduction, 786-790.
Malignant diseases, 640.
Malpighian bodies, of Kidney, 728; of Spleen,
  506.
Mammalta, lymphatic system in,  500; cir-
  culation in, 564, 565; respiration in, 674-
  676;  heat evolved in, 761; nervous centres
  of, 873; ovisac of, 795.
Mammary glands, 830, 831.
Margarine, 259.
Mastication, act of, 451, 452, 896.
Medulla  Oblongata, structure of,  889-893;
  functions of, 894-899.
Memory, 924.
Menstrual secretion, 798,799.
Mesenteric glands, 496.
Metamorphosis of animals, 407, 784, 785 ; in-
  fluence of heat upon, 127.
Milk,  436; composition and  properties of,
         832-834.
       circumstances influencing secretion of,
         836-839.
Mineral ingredients of food, 438-441.
Moisture, proportion of, in different parts of
  the body, 149-152 ; necessity of, for growth
  of Plants and Animals, 153-157;  effects of
  withdrawal of, 158-161.
Mollusca,  circulating system of, 555-557;
  respiratory organs  of,  654-656; nervous
  system of, 850-854.
Mucous membrane, 199-204.
Mucus, 237, 464.
Mulberry mass, 784, 805.
Muscles, contractility of, 345-371 ; irritability
           of, 348-363 ;  tonicity of, 364-366 ;
           rigor  mortis of,  367-369;  pecu-
           liar force of, 370; heat evolved
           by, 371; electricity evolved by,
           775.
         energy of,  dependent on supply of
           blood, 358-360.
         disintegration of, 361.
Muscular  fibre,  striated,  332-336 ;   non-
  striated, 337; development of, 338,  339;
  vessels and nerves of, 340, 341.
Muscular sense, 904.
Myolemma, 333.
Myopia, 958.

Nail, 226.
Nervous System, general view of actions of,
  840-848.
Nervous System, in  Radiata,  849;  in  Tuni-
  cata,  850, 851 ; in Bivalve  Mollusca, 852,
  853;  in higher Mollusca, 854; in Articu-
  lata, 855-863 ; in Vertebrata, 867, 868; in
  Fishes, 869, 870; in Reptiles,  871 ;  in
  Birds, 872; in Mammals, 873.
Nervous Tissue,  372-405 ;  fibrous, 373-377;
  vesicular, 378, 379 ; arrangement of  ele-
  ments of, in  nervous centres, 380;  in peri-
  phery, 381, 382; chemical composition of,
  383; disintegration and  renewal of, 384-
  389 ; conditions of activity of, 390-404.
Neurilemma, 373.
Nucleus, 211.
Nutrition, 612-615.
           activity  of,  dependent on func-
             tional activity of parts, 616-627.

Oesophagus, passage of  food along, 455, 898.
Oily compounds in  food, 430, 432, 435; im-
  portance of,  530.
Oleine, 259.
Oleo-phosphoric acid, 383.
Olfactive lobes, 8G9-873, 900.
Olfactory nerve, 906, 946, 947.
Olivary bodies, 891.
Omphalo-mesenteric vessels, 813.
Optic lobes, 869-873, 900.
Optic nerves,  906, 907, 960.
Orbit, motor nerves  of the, 888.
Organized structures, general  characters of,
  2-15.
Osseous tissue, see Bone.
Ossification, 300-303.
Otolithes, 950.
Ova of animals, 791-795.
Ovarium, 793-797.
Ovisac, 792,796.
Oxygen, necessity for, in animal  body, 649 ;
  mode of introduction of, 650-652.

Pancreatic secretion, 480.
Papilla?, sensory, of skin, 382,940 ; of tongue,
  943.
Parturition, 827-829.
Par Vagum, 459, 487, 580,  685, 686, 888,895,
  897-899.
Pedal ganglia  in Mollusca,  852, 853.
              in Articulata, 857, 862.
Pepsine, 470, 471.
Perception, nature of, 936,  937.
Perceptions, tactual, 941; visual, 961-968.
Peristaltic movement, 352,  460.
Perspiration, 743-746.
Peyer's glands, 450, 749.
Phosphate of lime, in food,  438-441 ;  in bone,
  298.
Phosphatic deposits, 386, 738-740.
Phosphorus, in animal body, 438, 439.
Phanerogamia, generation  and development
  of, 780, 781.
Physical Forces,  19-23, 52;  correlation of,
  53-55;  relations  of, to Vital, 61-63; their
  influence on vital action, 73-78.
Pigment-cells, 229, 230.
Placenta, structure of, 818-820.
Placental tufts, 245, 818.
Plants, heat of, 762; circulation in, 540-548 ;
  respiration  in, 84, 641, 642; reproduction
  in, 778-781.
Pneumogastric nerve, see Par  Vagum.
Polypes, digestive process in, 443, 444.
Posterior Pyramids, 893.
Pregnancy, duration of,  825, 826.
Prehension of food, 896.
Presbyopia, 958.
Primary membrane, 206-209.
Primitive trace, 812.
Proteine,  171.
Puberty in male, 788 ; in female, 798.
Pulp of hair, 328, 330. 
INDEX.
565
Pulp of teeth, 310, 313, 314.
Pulsations of heart, 579.
Pulse, in arteries, 583, 584; respiratory, in
  veins, 607.
Pupil, changes in diameter of, 969.
Purpurine, 736.
Pus, 632-637.
Pyramids, anterior, 890.
          posterior, 893.

Radiata, Nervous system of, 849.
Receptaculum Chyli, 497.
Red corpuscles of blood,  characters of, 215,
  216; development of, 217-220.
Reduction of food, provisions for, 445.
Reflex actions, nature of, 392-396.
              of Articulata,  858-860;  of
                Mollusca, 850, 851, 854 ; of
                Vertebrata, 875-879, 884.
Regeneration of parts, influence of heat upon,
  129; of nerve, 389.
Reproductive cells, 240-242.
Reptiles, circulation in, 562, 563; lymphatic
  system  in, 499;  respiration in, 668-671;
  nervous centres in, 871.
Respiration,  nature  of  the   process, 641;
                sources  of demand for it,
                642-648; mode  of its per-
                formance, 649-652.
              organs *of,  in lowest  animals,
                653 ; in Mollusca, 654-656 ;
                in Annelida, 657; in  Crus-
                tacea, 658; in Insects, 659,
                660 ;  in Spiders,  661 ;  in
   v            Fishes,  663-667;  in Rep-
                tiles,  668-671 ;  in Birds,
                672-674; in Mammalia and
                Man, 675-678.
              movements of, 679-688.
              chemical phenomena of, 689-
                702.
              insufficient, effects  of,  703-
                709.
Respiratory nerves, in Insects, 862; in  Mol-
  luscs,  850-853;  in Vertebrata,  684-688,
  895.
Respiratory pulse, 607.
Restiform bodies, 892.
Retina, general structure  of, 960.
Rigor Mortis,  367-369.
Ruminating stomach, 457.
Saccharine compounds in food, 430-434, 493.
Salivary glands, 465.
        secretion, 446, 466, 467.
Satiety, sense of, 486,
Sebaceous follicles, 747, 748.
Secreting  cells, 238, 239, 712-714.
Secretion, nature of the process, 710-713.
           effects of suppression of, 711.
Segmentation of vitellus, 805.
Selecting  power of individual parts, 612-615.
Semicircular canals, 952.
Sensation, 389, 390, 930,  931;  nerves of, 389,
  390, 900, 901; general and special, 932.
Sensations, regulation ot muscular movement
  by, 902-904.
Sensorium, 390.                .
Sensory Ganglia, 900,  901;  functions of, 902
  -905.
Sensory nerves, 906, 907.
Serous membranes, 197, 198.
Shell, of Echinodermata, 278, 279 ; of Mol-
  lusca, 280-285 ; of Articulata, 284-286.
Sight, sense of, 955-972.
Single vision with two eyes, 963.
Size of objects, estimate of, 968.
Skin, 199, 205, 742-748, 940.  '
Sleep, 924.
Smell, sense of, 946-948.
Sneezing, 948.
Solen, nervous system of, 852, 853.
Somnambulism, 924.
Sounds, propagation of, 949; qualities of, 954.
Sounds of heart, 571-575.
Speech, 981, 982.
Sperm-cells, 240, 241, 780, 783.
Spermatic fluid, 786, 787; emission of, 790.
Spermatozoa, 240, 786, 787; use of, in fecun-
  dation, 803, 804.
Sphinx ligustri, nervous system of, 856, 857.
Spiders, respiratory organs of, 661.
Spinal Cord, 867,  868; its independence of
  the Brain, 874-879 ; structure of, 880-883 ;
  reflex actions of, 884 ; disordered states of.
  885-887.
Spinal accessory nerve, 580, 888.
Spinal nerves, origin of, 880, 882.
              peculiar, 888.
Spiracles of Insects, 659.
Spleen, structure of, 506 ; uses of, 507-509.
Stammering, 983, 984.
Starchy compounds in food, 430-434.
Star-fish, nervous system of, 849.
Starvation, Chossat's experiments on, 117.
Stearine, 259.
Stereoscope, 964, 965.
Stomach, 447, 448;  movements of, 458, 459.
         in Ruminants, 457.
Stomato-gastric nerves of Invertebrata, 863.
                      of Vertebrata, 896.
Strabismus, 963.
Suction, act of, 896.
Sudoriparous glandulae, 743, 744.
Supra-renal capsules,  510.
Sympathetic  System, in Man,  functions of,
               926-929.
             traces of,  among Invertebrata,
               864.
Syncope, 581,628.
Synovial membranes,  197, 198.

Tadpole, respiration of, 670; metamorphosis
  of, 670.
Ttiste, nerves  of, 944.
      sense of, 945.
Teeth,  structure and  development  of,  310-
  327.
Temperature, sense of, 933, 942.
Testis, structure of, 786.
Tetanus, 886.
Thalami Optici, 901.
Thirst, 488.
Thoracic duct, 497.
Thymus Gland, 511,512.
Thyroid Gland, 513.
Tickling, 907.
Tongue, papilla? of, 943.
Tonicity of arteries, 365, 584; of muscle, 364
  -366.
Torpedo, electricity of, 771-774.
Torpidity, induced  by cold, 97, 98, 136; by
  want of moisture, 158-161.
Touch, sense of, 939-942.
Tubercula quadrigemina, 869, 870-873,900. 
566
INDEX.
Tubercular diathesis, 626, 638, 639.
Tunicata, nervous system of, 850, 851.
Tympanum, 951.
Tyrosin, 170.

Ulceration, 633.
Umbilical vessels, 818.
Umbilical vesicle, 815.
Urea, 73Q, 731.
Uric acid, 732, 733.
Urine,  composition and  properties  of,  729-
  740; effects of suppression of, 741.
Uterus, comparative structure of, 793 ; partu-
  rient action of, 827.

Vascular area, 551, 813, 814.
Vegetable kingdom, office of, 15.
Vegetation, influence  of  Light upon,  79-92;
  influence of Heat upon, 98-107;  influence
  of Electricity upon,  143-145.
Veins, movement of blood in, 605-610; pul-
  sation in, 607, 608.
Venous congestion, 609, 610.
Villi of mucous membrane, 243, 492 ; of yolk-
  bag, 244; of placenta. 245.
Vital Actions, 16-18, 47-51.
Vital Forces, 17, 24, 52; their relations to
  each other, 58-60; to the  Physical  forces,
  61-63.
Vital Stimuli, 61.
Vitellus, 784, 794; segmentation of, 805.
Vitreous body of eye, 276.
Vocal ligaments, 974-979.
Voice, production of, 973-979.
Voluntary movements, nature of, 923.
Vowel sounds, production of, 981.

White fibrous tissue, 189, 191.
Worm tribes, circulation  in, 552;  respiration
  in, 657.
                                       >
Yellow  fibrous tissue,«190, 192.
Yolk, see  Vitellus.

Zona pellucida, 795.
Zoospores of Alga?, 779.
/ 
CARPENTER'S  HUMAN  PHYSIOLOGY.
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positive elegance."—Med. Examiner. 
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accomplished.  The chapters follow each other in the order in which dissections are usually con-
ducled in this country, and as each region is taken up, every detail regarding it is fully described
and illustrated, so that the student is not interrupted in his labors, by the necessity of referring from
one portion of the volume to another.
  However  valuable  may be  the  " Dissector's
Guides" which we, of late, have had occasion to
notice, we feel confident that the work of Dr. Allen
is superior to any of them.  We  believe with the
author, that none is so fully illustrated as this, and
the nrrangement of the work  is such as to facilitate
the labors of  the student in  acquiring a thorough
practical knowledge of Anatomy.   We most cordi-
Blly recommend it to their attention.—Western Lan-
cet, Dec. 1856.
  We believe it to be one  of the most useful works
upon the subject ever written.  It is handsomely
illustrated, well printed, and will be found of con-
venient size for use in the dissecting-room.—Med.
Examiner, Dec. 1856.
    From Prof. J. S. Davis, University of Va.

  I am not acquainted with any 'work that attains so
fully the object which it proposes.

From C. P. Fanner, M. D., Demonstrator, Uni-
             versity of Michigan.

  I  have examined the work briefly, but even this
examination has convinced me that it is an excellent
guide for the Dissector.  Its illustrations are beau-
tiful, and more than I have seen in a work of this
kind.  I shall take great pleasure in recommending
it to my classes as the text-book of the dissecting-
room.
                         ANALYTICAL  COMPENDIUM
OF  MEDICAL  SCIENCE,  containing Anatomy, Physiology, Surgery, Midwifery,
  Chemistry, Materia Medica, Therapeutics, and Practice of Medicine.  By John Neill, M. D.,
  and F. G-. Smith, M. D.  New and enlarged edition, one thick volume royal 12mo. of over
  1000 pages, with 374 illustrations.  $3 00.  OF See Neill, p. 24.
               ABEL (F.  A.),  F. C.S. AND  C.  L.  BLOXAM.

HANDBOOK  OF  CHEMISTRY,  Theoretical,  Practical, and Technical; with a
  Recommendatory Preface by Dr. Hofmann. In one large octavo volume, extra cloth, of 662
        with illustrations.  $3 25.
                         ASHWELL (SAMUEL), M.D.,
                  Obstetric Physician and Lecturer to Guy's Hospital, London.

A PRACTICAL TREATISE ON THE DISEASES PECULIAR TO WOMEN
  Illustrated by Cases derived from Hospital and Private Practice.  Third American, from the Third
  and revised London edition.  In one octavo volume, extra cloth, of 528 pages.  $3  00.
 The most useful practical work on the subject in
the  English language. — Boston Med. and  Surg.
Journal.
  The most able,  and certainly the most standard
and practical, work on female diseases that we have
yet seen.—Medico-Chirurgical Review.
                            ARNOTT  (NEILL),  M.  D.
ELEMENTS   OF  PHYSICS;  or  Natural Philosophy, General  and Medical.
  Written for universal use, in plain or non-technical language.  A new edition, by Isaac Hays,
  M. D.  Complete in one octavo volume, leather, of 484 pages, with about two hundred illustra-
  tions.  $2 50.                         _____________

                        BUDD (GEORGE),  M. D., F. R. S.,
                      Professor of Medicine in King's College, London.

ON DISEASES OF THE  LIVER.    Third   American,  from  the  third and
  enlarged London edition.  In one very handsome octavo volume, extra cloth, with four beauti-
  fully colored plates, and numerous wood-cuts.  pp. 500.  $3 00.  (Just Issued.)
  Has fairly established for itself a place among the
classical medical literature of England.—British
and Foreign Medico-Chir. Review, July, 1857.
  Dr. Budd's Treatise on Diseases of the Liver is
now a standard work in Medical literature, and dur-
ing the intervals which have elapsed between the
successive editions, the author has incorporated into
the text the most striking novelties which have cha-
racterized the recent progress of hepatic physiology
and pathology; so that although the size of the book
is not perceptibly changed, the history of liver dis-
eases is made more complete, and is kept upon a level
with the progress of modern science.  It is the  best
work on Diseases of the Liver in any language.__
London Med. Times and Gazette, June 27, 1857.
 This work, now the standard book of reference on
the diseases of which it treats, has been carefully
revised, and many new illustrations of the views of
the learned author  added in the present edition.__
Dublin Quarterly Journal, Aug. 1S57.
BY THE SAME AUTHOR.
ON THE  ORGANIC  DISEASES  AND FUNCTIONAL DISORDERS OF
  THE STOMACH.  In one neat octavo volume, extra cloth.  $1 50.
  From the high position occupied by Dr. Budd as
a teacher, a writer, and a practitioner, it is  almost
needless to state that the present book may be con-
Rultedwith great advantage. It is written in an easy
style, the subjects are well arranged, and the practi-
cal precepts, both of diagnosis and treatment, denote
the character of a thoughtful and experienced phy-
sician.—London Med. Times and Gazette. 
4
BLANCHARD  &  LEA'S MEDICAL
                           BUCKNILL (J. C), M. D.,
               Medical Superintendent of the Devon County Lunatic Asylum; and
                           DANIEL H.  TUKE,  M. D.,
                        Visiting Medical Officer to the York Retreat.

A MANUAL OF  PSYCHOLOGICAL  MEDICINE;  containing the  History,
  Nosology, Description, Statistics, Diagnosis, Pathology, and Treatment of INSANITY.  With
  a Plate.  In one handsome octavo volume, of 536 pages.  $3 00.  (Now Ready, July, 1858.)
  The increase of mental disease in its various forms, and the difficult questions to which it is
constantly giving rise, render the subject one of daily enhanced  interest, requiring on the part of
the physician a constantly greater familiarity with this, the most perplexing branch of his profes-
sion.  At the same time there  has been for some years no work accessible in this country, present-
ing the results of recent investigations in the Diagnosis and Prognosis  of Insanity, and the greatly
improved methods of treatment which have done so much  in alleviating the condition or restoring
the health of the insane.  To  fill this vacancy the publishers present this  volume, assured that
the distinguished reputation and experience of the authors will entitle  it at once to the confidence
of both student and practitioner.   Its scope may be gathered from the declaration of the authors
that "their aim has been to supply a text book which may serve as a guide in the  acquisition of
*ueh knowledge, sufficiently elementary to be adapted to the wants of  the student, and sufficiently
modern in its views and explicit in its teaching to suffice for the demands of the practitioner."
               BENNETT (J.  HUGHES), M.D.,  F. R. S. E.,
               Professor of Clinical Medicine in the University of Edinburgh, &c.

THE  PATHOLOGY AND  TREATMENT OF PULMONARY TUBERCU-
  LOSIS, and on the Local Medication of Pharyngeal and Laryngeal Diseases frequently mistaken
  for or associated with, Phthisis.  One vol. 8vo.,extra cloth, with wood-cuts.  pp. 130.  $1 25.
                          BENNETT  (HENRY), M. D.
A PRACTICAL TREATISE  ON INFLAMMATION OF THE UTERUS,
  ITS CERVIX AND APPENDAGES, and on its connection with Uterine Disease.  New and
  much enlarged edition, preparing by the author for publication in 1859.
  We are firmly of opinion that in proportion as a
Knowledge of uterine, diseases becomes more appre-
ciated, this work will be proportionably established
as a text-book in the profession.—The Lancet.
  When, a few years back, the first edition of the
present work was published, the subject was one al-
most entirely unknown to the obstetrical celebrities
of the day ; and even now we have reason to know
that the bulk of the profession are not fully alive to
the importance and frequency of the disease of which
it takes cognizance.  The present edition is so much
enlarged, altered, and improved, that it can scarcely
be considered the same work.—Dr. Ranking's Ab-
stract.
                                BY THE SAME AUTHOR.

A REVIEW OF  THE  PRESENT  STATE  OF UTERINE  PATHOLOGY.
  8vo., 75 pages, flexible cloth, 50 cents.
                     BIRD (GOLDING), A. M.,  M. D.,  Sec.
URINARY   DEPOSITS:  THEIR  DIAGNOSIS,   PATHOLOGY,   AND
  THERAPEUTICAL INDICATIONS.  A new and enlarged American, from a late improved
  London edition. With over sixty illustrations.  In one royal 12mo. vol, extra cloth, pp.372. $130.
  It can scarcely be necessary for us to say anything
uf the merits of this well-known Treatise, which so
admirably brings into practical application the re-
sults of those microscopical and chemical researches
regarding the physiology and pathology of the uri-
aary secretion, which have contributed  so much to
the  increase of our diagnostic powers, and to the
extension and satisfactoryemploymentof our thera-
peutic resources.  In the preparation of this new
edition of his work, it is obvious that Dr. Golding
Bird has spared no pains to render it a faithful repre-
sentation of the present state of scientific knowledge
on thesubject it embraces.— The British and Foreign
Medico-Chirurgical Review.
                         BOWMAN (JOHN  EJ,  M.D.
PRACTICAL HANDBOOK  OF  MEDICAL  CHEMISTRY.   Second Ame-
  rican, from the third and revised English Edition.  In one neat volume, royal 12mo., extra cloth,
  with numerous illustrations,  pp. 288.  $1 25.
                                 BY THE SAME AUTHOR.

INTRODUCTION  TO  PRACTICAL   CHEMISTRY,   INCLUDING  ANA-
  LYSIS.  Second American, from the second and revised London edition.  With numerous illus-
  trations. In one neat vol., royal 12mo., extra cloth, pp. 350.  $1 25.
BEALE ON THE LAWS OF HEALTH IN RE-
  LATION TO MIND AND BODY. A Series of
  Letters from an old Practitioner to a Patient.  In
  one volume, royal  12mo., extra cloth,  pp. 296.
  80 cents.

tfUSHNAN'S PHYSIOLOGY OF ANIMAL AND
  VEGETABLE LIFE ; a Popular Treatise on the
  Functions and Phenomena of Organic Life.  In
  one handsome royal ISmo. volume, extra cloth,
  with over 100 illustrations,  pp.234. 80 cents.
BUCKLER ON THE ETIOLOGY, PATHOLOGY,
  AND TREATMENT OF  FIBRO-BRONCHI-
  TIS AND RHEUMATIC  PNEUMONIA.  In
  one 8vo. volume, extra cloth,  pp.150.  SI 25.
BLOOD AND  URINE (MANUALS ON).  BY
  JOHN  WILLIAM  GRIFFITH,  G. OWEN
  REESE, AND ALFRED MARKWICK.   One
  thick volume, royal 12mo.,  extra cloth,  with
  plates, pp. 460.  SI 25.
BRODIE'S  CLINICAL  LECTURES ON SUR-
  GERY. 1 vol. 8vo. cloth.  350 pp. SI 25. 
AND SCIENTIFIC PUBLICATIONS.
5
                              BARCLAY (A.  W.)  M. D.,
                        Assistant Physician to St. George's Hospital, &c.

A  MANUAL  OF  MEDICAL DIAGNOSIS;  being an Analysis  of the Signs
  and Symptoms of Disease.  In one neat octavo volume, extra cloth, of 424 pages.  $2 00.  (A new
  work, now ready.)
  Of works exclusively devoted to  this important deficiency, is the object of Dr. Barclay's Manual.
branch, our profession has at command, compara- j The task of composing such a work is neither an
tively, but few, and, therefore, in the publication of easy nor a light one; but Dr. Barclay has performed
the present work, Messrs. Blanchard  k Lea have ! it in a manner which meets our most  unqualified
conferred a great favor upon us.  Dr. Barclay, from approbation.  He is no mere theorist; he knows his
having occupied, for ajonj* period,  the position of work thoroughly, and in attempting to perform it,
                                               has not exceeded his powers.—British Med. Journal,
Medical Registrar at St.  George's Hospital, pos
sessed advantages for correct observation and reli-
able conclusions, as to the significance of symptoms,
which have fallen to  the  lot of but few, either in
his own or any other country.  He  has carefully
systematized the results of his observation of over
twelve thousand  patients, and by his  diligence and
judicious  classification, the profession  has been
presented  with the most  convenient  and reliable
work on the subject of Diagnosis that it has been
our good fortune ever to  examine; we can, there-
fore, say of Dr. Barclay's work, that, from his sys-
tematic manner of arrangement, his work is one of
the best works " for reference" in the daily emer-
gencies of the  practitioner, with which we are ac-
quainted ;  but, at the same time, we would recom-
mend our  readers,  especially  the younger ones, to
rend thoroughly and study  diligently the whole work,
and the ''emergencies" will not  occur  so often.—
Southern Med. and Surg.  Journ., March, 1858.
   To give this information, to supply this admitted
Dec. 5, 1857.
  We venture to predict that the work will be de-
servedly popular, and soon become, like Watson^s
Practice, an indispensable necessity to the practi-
tioner.—iV. A. Med. Journal, April, 1S58.
  An inestimable work of reference for the young
practitioner and student.—Nashville Med. Journal,
May, 1858.
  We hope the volume will have an extensive cir-
culation, not among students of medicine only, but
practitioners also.  They will never regret a faith-
ful study of its pages.—Cincinnati Lancet, Mar. '58.
   This Manual of Medical  Diagnosis is one of the
most scientific, useful, and instructive works of its
kind that we have ever read, and Dr.  Barclay has
done good service to medical science in collecting,
arranging, and analyzing the signs and symptoms
of so many diseases. — N.  J. Med.  and Surg. Re-
porter, March, 1858.
                          BARLOW  (GEORGE  H.),  M. D.
                           Physician to Guy's Hospital, London, &c.
A MANUAL OF THE  PRACTICE OF MEDICINE.  With Additions by D.
  F. Condie, M. D., author of "A Practical Treatise on Diseases of Children," &c.  In one hand-
  some octavo volume, leather, of over 600 pages.  $2 75.
  We recommend Dr. Barlow's Manual in the warm-
est manner as a most valuable vade-mecum.  We
have had frequent  occasion to consult it, and have
found it clear, concise, practical, and sound. It is
eminently  a practical  work,  containing all that is
essential, and avoiding useless theoretical discus-
sion.  The work supplies what  has been for some
time wanting, a manual of practice based upon mo-
dern discoveries in pathology and rational views of
treatment of disease.  It is especially intended for
the use of students and junior practitioners, but it
will be found hardly less useful to the experienced
physician.  The American editor has added to the
work three chapters—on Cholera Infantum, Yellow
Fever, and Cerebro-spinal Meningitis.  These addi-
tions, the two first of which are indispensable to a
work on practice destined for the profession in this
country, are executed with great judgment and fi-
delity, by Dr.  Condie, who has  also succeeded hap-
pily in imitating the conciseness and clearness of
style which are such agreeable characteristics of
the original book.—Boston Med. and Surg. Journal.
                            BARTLETT (ELISHA),  M. D.
THE  HISTORY,  DIAGNOSIS,  AND  TREATMENT  OF THE  FEVERS
  OF THE UNITED STATES.  A new and revised edition.   By Alonzo Clark, M. D., Prof.
  of Pathology and Practical Medicine in the N. Y. College of Physicians and  Surgeons, &c.  In
  one octavo volume, of six hundred pages, extra cloth.  Price $3 00.
  It is the best work on fevers which has emanated i logy.  His annotations add much to the interest of
from the American press, and the present editor has  the work, and have brought it well up to the condi-
carefully availed himself of all information exist-
ing upon the subject in the Old and New World, so
that the doctrines advanced are brought down to the
latest date in  the progress of  this department of
Medical Science.—London Med. Times and Gazette,
May 2, 1857.
  This excellent monograph on febrile disease, has
stood deservedly high since its first publication.  It
will be seen that it has now reached its fourth edi-
tion under the supervision of Prof. A. Clark, a gen-
tleman who, from the nature of his studies and pur-
suits, is well calculated  to appreciate and discuss
the many intricate and difficult questions in patho-
tion of the science as it exists at the present day
in regard to this class of diseases.—Southern Med.
and Surg.  Journal, Mar. 1857.
  It is a work of great practical value and interest,
containing much that is new relative to the several
diseases of which it treats, and, with the additions
of the editor, is fully up to the times.  The distinct-
ive features of the different forms of fever are plainly
and forcibly portrayed, and the lines of demarcation
carefully and accurately drawn, and to the Ameri-
can practitioner is a more valuable and safe guide
than any work  on  fever extant.—Ohio Med. and
Surg. Journal, May, 1857.
                             BROWN  (ISAAC  BAKER),
                         Surgeon-Accoucheur to St. Mary's Hospital, &c.

ON SOME DISEASES OF WOMEN ADMITTING OF SURGICAL TREAT-
  MENT.  With handsome illustrations.  One vol. 8vo., extra cloth, pp. 276.  $ I 60.
  Mr Brown has earned for himself a high reputa-
tion in the operative treatment of sundry diseases
and injuries to which females are peculiarly subject.
and merit the careful attention of every  surgeon-
accoucheur.—Association Journal.
                                                 We have no hesitation in recommending this book
We can truly say of his workthat it is an important, ^ the carefu, attention of all ,nr     °who make
addition to obstetrical lit«»t"f-.  "'£7 | female complaints a part of their study and practice.
suggestions and contrivances which Mr Brown de-  _DubHn q\iarterly JoumaL       *    p
Bcribes, exhibit much practical sagacity and skill, |          "* 
i)                      BLANCHARD & LEA'S  MEDICAL

             CARPENTER (WILLIAM B.),  M. D., F. R. S., &c,
           Examiner in Physiology and Comparative Anatomy in the University of London.

PRINCIPLES OF HUMAN  PHYSIOLOGY; with  their chief applications to
  Psychology, Pathology, Therapeutics, Hygiene, and Forensic Medicine. A new American, from
  the last and revised London edition.  With nearly three hundred illustrations.  Edited, with addi-
  tions, by Francis Gurney Smith, M. D., Professor of the Institutes of Medicine in the Pennsyl-
  vania Medical College, &c. In one very large and beautiful octavo volume, of about nine hundred
  large pages, handsomely printed and strongly bound in leather, with raised bands.   $4 25.

  In the preparation of this new edition, the author has spared no labor to render it, as heretofore,
a complete and lucid  exposition of the most advanced condition  of its important  subject.   The
amount of the additions required to effect this object thoroughly, joined to the former large size ol
the volume, presenting objections arising from the unwieldy bulk of the work, he has omitted all
those portions  not  bearing  directly upon Human Physiology, designing  to incorporate them in
his forthcoming Treatise on General Physiology.  As a full and accurate text-book on the Phy-
i-iology of Man, the work in its present  condition therefore presents even greater claims upon
the student and physician than  those  which have heretofore won  for it the very wide and distin-
guished favor which it has so long enjoyed.  The additions of Prof. Smith will be found to supply
whatever may  have been wanting to the American  student, while the  introduction of many new
illustrations, and  the most careful  mechanical execution, render the volume one of the most at-
tractive as yet issued.
  For upwards of thirteen years Dr. Carpenter's
work has been considered by the profession gene-
rally, both in this country and England, as the most
valuable compendium on the subject of physiology
in our language. This distinction it owes to the high
attainments and unwearied industry of its accom-
plished author.  The present edition (which, like the
last American one, was prepared by the author him-
self), is the result of such extensive revision, that it
may almost be considered  a new work.  We need
hardly say, in concluding this brief notice, that while
the work is indispensable to every student of medi-
cine in this country, it will amply repay the practi-
tioner for its perusal by  the interest and value of its
contents.—Boston Med.  and Surg. Journal.

  This is a standard work—the text-book used by all
medical students who read the English  language.
It has passed  through several  editions in order to
keep pace with the rapidly growing science of Phy-
siology.  Nothing need  be said in its praise, for its
merits are universally known ;  we have nothing to
say of its defects, for they only  appear where  the
science of which it treats is incomplete.—Western
Lancet.
  The most complete exposition of physiology which
any language  can at present give.—Brit, and For.
Med.-Chirurg. Review.

  The greatest, the  most reliable, and the best book
on the subject which we know  of in the English
language.—Stethoscope.
  This book should not only be read but thoroughly
Btudied by every member of the profession.  None
are too wise or old, to be benefited thereby.  But
especially to the younger class would we cordially
commend it as best fitted of any work in the English
language to qualify them for the reception and com-
prehension of those truths which are daily being de-
veloped in physiology.—Medical Counsellor.
  Without pretending to it, it is  an encyclopedia of
the subject, accurate and complete in all  respects—
a truthful reflection of the advanced state at which
the science has now arrived.—Dublin Quarterly
Journal of Medical Science.
  A truly magnificent work—in itself a perfect phy-
siological study.—Ranking's Abstract.
  This work stands without its  fellow.   It is one
few men in Europe could have undertaken;  it is one
  To eulogize this great work would be superfluous
We should observe, however, that in this edition
the author has remodelled a large portion  of the
former, and the editor has added much matter of in-
terest, especially in the form of illustrations.  We
may confidently recommend it as the most complete
work on Human Physiology  in  our  language.—
Southern Med. and Surg. Journal, December, 1855,
  The most  complete work on the science  in our
language.—Am. Med. Journal.
  The most complete work now extant in our lan-
guage.—N. O. Med. Register.
  The best text-book in  the language on this ex-
tensive subject.—London  Med.  Times.
  A complete cyclopaedia  of this branch of science.
—N. Y. Med. Times.
  The profession of this country, and  perhaps  also
of Europe, have anxiously and for some time awaited
the announcement of this  new edition of Carpenter's
Human Physiology.  His former editions have for
many years been almost the only text-book on Phy-
siology in all our medical schools, and its circula-
tion among the profession has  been unsurpassed by
any work in any department of medical science.
  It is quite unnecessary for us to speak of thii
work as its  merits would justify.  The mere an-
nouncement of its appearance will afford the highest
pleasure to every student of Physiology, while its
perusal  will be of infinite service in advancing
physiological science.—Ohio Med. and Surg. Journ,
no man, we believe, could have brought to so suc-
cessful an issue as Dr. Carpenter.  It required for
its production a physiologist at once deeply read in
the labors of others, capable of taking a general,
critical, and unprejudiced view of those labors, and
of combining the varied, heterogeneous materials at
his disposal, so as to form an harmonious whole.
We feel that this abstract can give the reader a very
imperfect idea of the fulness of  this work, and no
idea of its unity, of the admirable manner in which
material has been brought, from the most various
sources, to conduce to its completeness, of the lucid-
ity of the reasoning it contains, or of the clearness
of language in which the whole is clothed.  Not the
profession only, but the  scientific world at large,
must feel deeply indebted to Dr. Carpenter  for this
 freat work.  It must, indeed, add largely even  to
 is high reputation.—Medical Times.
                          BY the same author.  (Lately Issued.)

PRINCIPLES OF COMPARATIVE  PHYSIOLOGY.   New American,  from
  the Fourth and Revised London edition.  In one large and handsome octavo volume, with ovet
  three hundred beautiful illustrations,  pp. 752.  Extra cloth, $4 80; leather, raised bands, f 5 25.

  The delay which has existed in the appearance of this work has been caused by the very thorough
revision and remodelling which it has undergone at the  hands of the author, and the large number
of new illustrations which have been prepared  for it.  It will, therefore,  be found  almost a new
work, and fully up to the day in every department of the  subject, rendering it a reliable text-book
for all students engaged in this branch of science.  Every effort  has been made to render its typo-
graphical finish and mechanical execution worthy of its exalted reputation, and creditable to the
mechanical arts of this country. 
AND  SCIENTIFIC  PUBLICATIONS.
7
                CARPENTER (WILLIAM  B.)f  M. D., F. R. S.,
          Examiner in Physiology and Comparative Anatomy in the University of London.

THE  MICROSCOPE AND ITS  REVELATIONS.    With  an Appendix con-
  taining the Applications of the Microscope to Clinical Medicine, &c.  By F. G. Smith, M. D.
  Illustrated by four hundred and thirty-four beautiful engravings on wood.  In one large and very
  handsome octavo volume, of 724 pages, extra cloth, $4 00;  leather, $4  50.

  Dr. Carpenter's position as a microscopist and physiologist, and his great experience as a teacher
eminently qualify him to produce what has long been wanted—a good text-book on the practical
use of the microscope.  In the present volume his object has  been, as stated  in his Preface, " to
combine, within a moderate compass, that information with regard to the use of his ' tools,' which
is most essential to the working microscopist, with such an account of the objects best fitted for
Uis study, as might qualify him to comprehend what  he observes,  and might thus prepare him to
benefit science, whilst expanding and refreshing his own mind."  That he has succeeded in accom-
plishing this, no one acquainted with his previous labors can doubt.
  The great importance of the microscope as a means of diagnosis, and the number of microsco-
pists who are also physicians, have induced the American publishers, with the author's approval, to
add an Appendix, carefully prepared by Professor Smith, on the applications of the instrument to
clinical  medicine, together with an  account  of American Microscopes, their modifications  and
accessories.  This portion of the work is illustrated  with nearly one hundred wood-cuts, and, it is
hoped, will adapt the volume more particularly to the use of the American student.
  Every care has been taken in the mechanical  execution of  the work, which is confidently pre-
sented as in no respect inferior to the choicest productions of  the London press.
  The mode in which the author has executed his intentions may be gathered from the following
condensed synopsis of the
                                      CONTENTS.

Introduction—History of the Microscope.   Chap.  I. Optical Principles of  the  Microscope.
  Chap. II.  Construction of the Microscope.  Chap.  III.  Accessory Apparatus.  Chap. IV.
  Management of the  Microscope  Chap. V.  Preparation, Mounting, and Collection of Objects.
  Chap. VI.  Microscopic Forms of Vegetable Life—Protophytes.  Chap. VII. Higher Cryptoga-
  mia.  Chap. VIII. Phanerogamic Plants.  Chap. IX. Microscopic Forms of Animal Life—Pro-
  tozoa—Animalcules.  Chap. X. Foraminifera, Polycystina, and Sponges.  Chap. XI. Zoophytes.
   Chap. XII. Echinodermata.  Chap. XIII.  Polyzoa and  Compound  Tunicata.  Chap. XIV.
   Molluscous Animals Generally.  Chap. XV. Annulosa.  Chap. XVI. Crustacea.  Chap. XVII.
   Insects and Arachnida.  Chap.  XVIII. Vertebrated  Animals.   Chap.  XIX. Applications of the
   Microscope to Geology.  Chap. XX. Inorganic or Mineral Kingdom—Polarization.  Appendix.
   Microscope as a means of Diagnosis—Injections—Microscopes of American Manufacture.
  Those who are acquainted with Dr. Carpenter's
previous writings on Animal and Vegetable Physio-
fogy, will fully understand how vast a store of know-
ledge he is able to bring to bear upon so comprehen-
sive a subject as the revelations of the microscope ;
and even those who have no previous acquaintance
with the construction or uses of this instrument,
will find abundance of information conveyed in clear
and simple  language.—Med.  Times and Gazette.
  Although  originally not  intended as  a strictly
medical work, the additions by Prof. Smith give it
a positive claim upon the profession, for which we
doubt not he will receive their sincere thanks.   In-
deed, we know not where the student of medicine
will find such a complete and satisfactory collection
of microscopic facts bearing  upon physiology and
practical medicine as is contained in Prof. Smith's
appendix; and this of itself, it seems to us, is fully
worth the cost of the volume.—Louisville Medical
Review, Nov. 1856.
                                 BY THE SAME AUTHOR.

ELEMENTS  (OR MANUAL) OF PHYSIOLOGY, INCLUDING  PHYSIO-
  LOGICAL ANATOMY.  Second American, from a new  and revised London edition.  With
  one hundred and ninety illustrations.  In one very handsome octavo volume, leather,   pp. 566.
  $3 00.
  In publishing the first edition of this work, its title was altered from that of the London  volume,
by the substitution of the word " Elements" for that of " Manual," and with the author's  sanction
the title of " Elements" is still retained as being more  expressive of the scope of the treatise.
  To say that it is the best manual of Physiology   Those who have occasion for an elementary trea-
now before the public, would not do sufficient justice tise on Physiology, cannot do better than to possess
to the author.—Buffalo Medical Journal.         themselves ofthe manual of Dr. Carpenter .—Medical
  In his former works it would seem that he had Examiner.
exhausted the subjectof Physiology. In the  present,   The best and most complete expose of modern
heeives theessence, as it were, ofthe whole.—N. Y. Physiology, in one volume extant in the English
Journal of Medicine.                           language.—St. Louis Medical Journal.

                           BY the  same author.  (Preparing.)

PRINCIPLES  OF  GENERAL  PHYSIOLOGY,  INCLUDING  ORGANIC
  CHEMISTRY  AND HISTOLOGY".  With a  General Sketch of the Vegetable and Animal
  Kingdom.   In one large and very handsome octavo volume, with several hundred illustrations.
  The subject of general physiology having been omitted in the last editions ot the author's " Com-
parative Physiology"  and "Human Physiology," he  has undertaken to prepare a volume which
shall present it more thoroughly and fully than has yet been attempted, and which may be regarded
tis an introduction to his other works.
                                 BY  THE SAME AUTHOR.

A PRIZE ESSAY ON THE  USE OF ALCOHOLIC LIQUORS IN  HEALTH
  AND DISEASE.  New edition, with a Preface by D. F. Condie, M. D., and explanations of
  scientific words.  In one neat  12mo. volume, extra cloth,  pp. 178.  50 cents. 
s
BLANCHARD & LEA'S MEDICAL
                           CONDIE (D. F.),  M. D., &.C.

A PRACTICAL  TREATISE ON THE DISEASES OF CHILDREN.   Fifth
  edition, revised and augmented.  In one large volume, 8vo., leather, of over 750 pages. $3 25.
  (Now Ready, December, 1858.)
  In presenting a new and revised edition cf this favorite work, the publishers have only to state
that the author ha* endeavored to render it in every respect "a complete and faithful exposition of
the pathology and therapeutics of the maladies incident to the earlier stages  of existence—a full
and exact account ofthe diseases of infancy and childhood."  To accomplish this he has subjected
the whole work to a careful and thorough revision, rewriting a considerable portion, and adding
several new chapters.  In this manner it is hoped that any deficiencies which may have previously
existed have been supplied, that the recent labors of practitioners and observers have been tho-
roughly incorporated, and that in every point the work will be found to maintain the high reputation
it has enjoyed as a complete and thoroughly practical book of reierence in infantile affections.
  A few notices of previous editions are subjoined.
                                              We pronounced the first edition to be the best
                                            work on the diseases of children in the English
                                            language, and, notwithstanding all that has been
                                            published, we still regard it in that light.—Medical
                                             Examiner.
  Dr. Condie's scholarship, acumen, industry, and
practical sense are manifested in this, as in all his
numerous contributions to science.—Dr. Holmes's
Report to the American Medical Association.
  Taken as a whole, in our judgment. Dr. Condie's
Treatise is  the one from the perusal of which the
practitioner in this country will rise with the great-
est satisfaction.—Western Journal of Medicine and
Surgery.
  One of the best works upon the Diseases of Chil-
aren  in the English language.—Western Lancet.
  We feel assured  from actual experience that no
physician's library can be complete without a copy
of this work.—N. Y. Journal of Medicine.
  A veritable pediatric encyclopaedia, and an honoi
to American medical literature.—Ohio Medical and
Surgical Journal.
  We feel persuaded that the American medical pro-
fession will soon regard  it not only as a very good,
but as the vert best "Practical Treatise on the
Diseases of Children."—American Medical Journal.
  In  the department of infantile therapeutics, the
work of Dr. Condie is considered one  of the best
which has been published in  the  English language.
—The Stethoscope.
  The value of works by native authors on the dis-
 eases which the physician is called upon to combat,
 will be appreciated by all; and the work of Dr. Con-
 die has gained for itself the character of a safe guide
 for students, and a useful work for consultation by
 those engaged in practice.—N. Y. Med. Times.
  This is the fourth edition of this deservedly popu-
 lar treatise.  During the interval since the last edi-
 tion, it has been subjected to a thorough revision
 by  the author;  and all new observations  in  the
 pathology and therapeutics of children have been
 includea in the present volume.  As we said  btfore,
 we do not know of a better book on diseases of chil-
 dren, and to a large part of its recommendations we
 yield an unhesitating concurrence.—Buffalo Med.
 Journal.
  Perhaps the most full and complete work now be-
 fore the profession ofthe United States; indeed, we
 may say in the English language. It is vastly supe-
 rior to most of its predecessors.—Transylvania Med.
Journal,
            CHRISTISON (ROBERT),  M.  D., V. P.  R. S. E.,  &c.
A DISPENSATORY;  or, Commentary  on the  Pharmacopoeias of Great Britain
  and the United States;  comprising the Natural History, Description, Chemistry, Pharmacy, Ac-
  tions, Uses, and Doses  of the Articles of the Materia Medica.   Second edition, revised and im-
  proved, with a Supplement containing the most  important New Remedies.   With copious Addi-
  tions, and two hundred and thirteen large wood-engravings.  By R. Eglesfeld Griffith, M. D.
  In one very large and handsome octavo volume, leather, raised bands, of over 1000 pages.  $3 50.

                        COOPER  (BRANSBY B )   F  R. S.
LECTURES ON  THE  PRINCIPLES  AND ' PRACTICE  OF  SURGERY-
  In one very large octavo volume, extra cloth, of 750 pages.  $3  00.
COOPER  ON  DISLOCATIONS  AND FRAC-
  TURES OF THE JOINTS —Edited by Bransby
  B. Cooper, F. R. S., &c.  With  additional Ob-
  servations by Prof. J. C. Warren. A new Ame-
  rican edition.  In one handsome octavo volume,
  extra cloth, of about 500 pages, with numerous
  illustrations on wood. $3 25.
COOPER ON THE ANATOMY AND DISEASES
  OF THE BREAST, with twenty-five Miscellane-
  ous and Surgical Papers.  One large volume, im-
  perial 8vo., extra cloth, with 252 figures, on 36
  plates.  $2 50.
COOPER  ON  THE  STRUCTURE AND DIS-
  EASES  OF  THE  TESTIS, AND ON  THE
  THYMUS  GLAND.   One vol. imperial 8vo., ex-
  tra cloth, with 177 figures on 29 plates. $2 00.
COPLAND ON THE CAUSES, NATURE, AND
  TREATMENT OF PALSY AND APOPLEXY.
  In one volume, royal 12mo., extra cloth, pp. 326.
  80 cents.

CLYMER ON FEVERS;  THEIR DIAGNOSIS,
  PATHOLOGY,  AND  TREATMENT  In one
  octavo volume, leather, of 600 pages.  $1 50.

COLOMBAT DE L'ISERE ON THE DISEASES
  OF FEMALES,  and on  the special  Hygiene of
  their Sex.  Translated, with many Notes and Ad-
  ditions, by C. D. Meigs, M.D.  Second edition,
  revised and improved.  In one large volume, oc-
  tavo, leather, with numerous wood-cuts.  pp. 720.
  $3 50.
                           CARSON (JOSEPH), M.D.,
           Professor of Materia Medica and Pharmacy in the University of Pennsylvania.
SYNOPSIS OF  THE  COURSE  OF LECTURES ON  MATERIA  MEDICA
  AND PHARMACY, delivered in the  University of Pennsylvania.  Second  and  revised edi-
  tion.  In one very neat octavo volume, extra cloth, of 208 pages.   $1  50.
                          CURLING  (T.  B.),  F. R.S.,
             Surgeon to the London Hospital, President of the Hunterian Society, &c.
A PRACTICAL  TREATISE ON DISEASES OF THE  TESTIS,  SPERMA-
  TIC CORD, AND SCROTUM.  Second American, from the second and enlarged English edi-
  tion.  In one handsome octavo volume, extra cloth, with numerous illustrations, pp. 420. $2 00. 
AND  SCIENTIFIC  PUBLICATIONS.
               CHURCHILL  (FLEETWOOD),  M. D.,  M. R. I. A.
ON  THE  THEORY  AND  PRACTICE  OF  MIDWIFERY.   Edited,  with
  Notes and Additions, by D.  Francis Condie, M. D., author  of  a  "Practical  Treatise on the
  Diseases  of Children," &c.  With  139  illustrations.  In one very handsome octavo volume,
  leather,   pp.510.   $3 00.
  To bestow praise on a book that has received such
marked approbation would be superfluous. We need
only say, therefore, that if the first edition was
thought worthy of a  favorable reception  by the
medical public, we can confidently affirm that this
will be found much  more  so.  The lecturer, the
practitioner, and the student, may all have recourse
to its pages, and derive from their perusal much in-
terest and instruction in everything relating to theo-
retical and practical midwifery.—Dublin Quarterly
Journal of Medical Science.

  A work of very great merit, and such as we can
confidently recommend to the study of every obste-
tric practitioner.—London Medical Gazette.

  This is certainly the most perfect system extant.
It is the best  adapted for  the purposes of  a text-
book, and that which he whose  necessities confine
him to one book, should select in preference to all
others.—Southern Medical and Surgical Journal.

  The most popular -work on midwifery ever issued
from the American press.—Charleston Med. Journal.

  Were we reduced to the necessity of having but
one  work on midwifery, and permitted to choose,
we  would unhesitatingly take Churchill.—Western
Med. and Surg. Journal.
  It is  impossible to  conceive a more useful and
elegant  manual  than  Dr.  Churchill's  Practice of
Midwifery.—Provincial Medical Journal.

  Certainly, in our opinion, the  very best work on
the subject which exists.—N. Y.  Annalist.
                            BY  THE  SAME AUTHOR.   (Just Issued.)
ON THE  DISEASES OF INFANTS  AND  CHILDREN.   Second American
  Edition, revised and enlarged by the author.  Edited, with Notes, by W. V. Keating, M. D.  In
  one large and handsome volume, extra cloth, of over 700 pages.  $3 00, or in leather, $3 25.

  In preparing this work  a second time for the  American profession, the author has  spared no
labor in giving it a very thorough revision, introducing several new chapters, and rewriting others,
while every portion of the volume has been subjected  to a severe scrutiny.  The efforts of the
American editor have been directed to supplying such information relative to matters peculiar
to this  country as might  have escaped  the attention  of the author,  and the whole may, there-
fore, be safely pronounced one  of the most complete works on the  subject accessible to the Ame-
rican Profession.  By an alteration in the size of the  page, these very extensive additions have
been accommodated without unduly increasing the size of the work.
                                                contribution for the illustration of its topics.  The
                                                material thus derived has been used with consummate
                                                skill, and the result has been a work creditablealike
                                                                           -N. A. Medico-Chir.
  No work holds a higher position, or is more de-
serving of being placed  in the hands of the tyro,
the advanced student, or the practitioner.—Medical
Examiner.

  Previous editions, under the editorial supervision
of Prof R.  M. Huston,  have been  received with
marked favor,  and they deserved it; but this, re-
printed from a very late Dublin edition, carefully
revised and brought up by the author  to the present
time, does present an unusually accurate and able
exposition of every important particular embraced
in the department of midwifery. * * The clearness,
directness, and  precision of its teachings, together
with the great amount of statistical research which
its text exhibits, have served to place it already in
the foremost rank of works in  this department of re-
medial science.—N. O. Med. and Surg. Journal.

  In our opinion, it forms one of the best if not the
very best text-book and epitome of obstetric science
which we at present possess in the English lan-
guage.—Monthly Journal of Medical Science.

  The clearness and precision of style in which it is
written, and the great amount of statistical research
which it contains, have served to place it in the first
rank of works in this departmentof medical science.
— N. Y. Journal of Medicine.

  Few treatises will  be found better adapted as a
text-book for the  student, or as a manual for the
frequent consultation of the young practitioner.—
American Medical Journal.
  This work contains a vast amount of interesting
matter, which is so well  arianged and  so curtly
worded that the book may be regarded as an ency-
clopaedia of information upon the subject  of which
it treats.  It is certainly also a monument of Dr.
Churchill's untiring industry, inasmuch as there is
not a single work upon the diseases of children with
which we are acquainted that is not fully referred
to and quoted from in its pages, and scarcely a con-
tribution of the least importance to any British or
Foreign Medical Journal, for some years past, which
is not duly noticed.—London Lancet, Feb. 20, lt>58.
  Availing himself of every fresh source of informa-
tion, Dr. Churchill endeavored, with his accustomed
industry and perseverance, to bring his work up to
the present state of medical knowledge in all the
subjects of which it treats; and in  this endeavor he
has, we feel bound to say, been eminently success-
ful.  Besides the addition of more than one hundred
and thirty pages of matter, we  observe that some
entirely new and important chapters are introduced,
viz : on paralysis, syphilis, phthisis, sclerema, &c.
&c.  As the work now stands, it is, we believe, the
most comprehensive in  the English language  upon
the diseases incident to early life.—Dublin Quarterly
Journal, Feb. 1858.
  It brings before the reader an amount of informa-
tion not comprised in any similar production in the
language. The amount of labor consumed upon its
production can only be conceived by those who have
been similarly occupied, every work of note pub-
lished witnin the last twenty-five years in the dit-
ferent languages of Europe having been laid under I and Surgical Journal
                                   BY THE SAME  AUTHOR.

ESSAYS  ON  THE  PUERPERAL FEVER, AND OTHER DISEASES PE-
  CULIAR TO WOMEN.   Selected from the writings of British Authors previous to the close of
  the Eighteenth Century.  In one neat octavo volume, extra cloth, of about 450 pages.   $2 50.
to the author and his country.
Review, May, 18^8.
  After this meagre, and we know,  very imperfect
notice of Dr. Churchill's work, we  shall  conclude
by saying, that it is one that cannot fail from its co-
piousness, extensive research, and general accuracy,
to exalt still higher the reputation of the author in
this country.  The American reader will beoarticu-
larly pleased to find that Dr. Churchill has done full
justice throughout his work to the various American
authors  on this subject.  The names of  Dewees,
Eberle, Condie, and Stewart, occur on nearly every
page, and these authors are constantly referred toby
the author in terms ofthe highest praise,  and with
the most liberal courtesy.—The Medical Examiner.

  We recommend the work of Dr. Churchill most
cordially,  both to students and practitioners, as a
valuable and;reliable guide in the treatment of the dis-
eases of children.—Am. Journ.of the  Med.  Sciences.
  We know of no work on this department of Prac-
tical Medicine which presents so candid and unpre-
judiced a  statement or posting  up  of our  actual
knowledge as this.— N. Y. Journal of Medicine.
  Its claims to merit both as a scientific and practi-
cal  work, are  of  the  highest order.  Whilst we
would not elevate it above every other treatise on
the same subject, we certainly believe that very few
are equal to it, and none  superior.—Southern Med. 
10
BLANCHARD  & LEA'S  MEDICAL
           CHURCHILL  (FLEETWOOD), M. D.,  M. R.  I. A.,  &c.

ON THE  DISEASES  OF WOMEN;  including those of Pregnancy and  Child-
  bed.  A new American edition, revised by the Author.   With Notes and Additions, bv D  Fran-
  cis Condie, M. D., author of "A Practical Treatise on the Diseases of Children."  With nume-
  rous illustrations.  In one large  and handsome octavo volume, leather, of 76S pages. (Just Issued,
  1857.)  $3 00.
  This edition of Dr. Churchill's very popular treatise  may almost be termed a new work,  so
thoroughly has he revised it in every portion.   It will be found greatly enlarged, and thoroughly
brought up to the most recent condition of the subject, while the very handsome series of  illustra-
tions introduced, representing such pathological conditions as can be accurately portrayed, present
a novel feature, and afFord valuable  assistance  to the young practitioner.  Such additions as ap-
peared desirable for the American  student have  been made by the editor, Dr. Condie,  while a
marked improvement in the  mechanical execution keeps pace with the advance in all other respects
which the volume has undergone, while the price has been kept at the former very moderate rate.
                                                extent that Dr. Churchill  does.  His, indeed, is the
                                                only thorough treatise we know of on the  subject;
                                                and it may be commended to practitioners  and stu-
  It comprises, unquestionably, one of the most ex-
act and comprehensive expositions of the present
state of medical knowledge in respect to the diseases
of women that has yet been published.—Am. Journ.
Med. Sciences, July, 1857.
  We hail with much pleasure the volume  before
us, thoroughly revised, corrected, and brought up
to the latest date, by Dr. Churchill  himself, and
rendered still more valuable by notes, from the ex-
perienced and able pen of Dr. D. F. Condie, of Phil-
adelphia.— Southern Med. and Surg. Journal, Oct.
1657.
  This work is the most reliable -which we possess
on this subject;  and is deservedly popular with the
profession.—Charleston Med. Journal, July, 1857.
  Dr. Churchill's treatise on the Diseases of Women
is, perhaps,  the most popular of his works with the
profession in this country.   It has been very gene-
rally received both as a text-book and manual of
practice.  The  present edition has undergone the
most elaborate revision, and additions of an import-
ant character have been made, to render it a com-
plete exponent of the present state of our knowledge
of these diseases.—N.  Y. Journ. of Med., Sept. 1857.
   We now regretfully take leave of Dr. Churchill's
book.  Had  our typographical limits permitted, we
should gladly have borrowed more from its richly
stored pages.  In  conclusion, we heartily recom-
mend it to  the profession, and would at the same
time express our firm conviction that itwill not only
add to the reputation of its author, but will pTove a
work  of great  and extensive utility to  obstetric
practitioners.—Dublin Medical Press.
   We know of no author who deserves that appro-
bation, on "the diseases of females," to the same
dents as a masterpiece in its particular department.
The former editions of this work have been com-
mended strongly in this journal, and they have won
their way to an extended, and a well-deseTved popu-
larity.  This fifth edition, before us, is well  calcu-
lated to maintain Dr.  Churchill's high reputation.
It was revised and enlarged by the author, for his
American publishers, and it seems to us that there is
scarcely any species of desirable information on itB
subjects that may not  be found in this work.—Tht
Western Journal  of Medicine and Surgery.

  We are gratified to announce a new and revised
edition of Dr. Churchill's valuable work on the dis-
eases of females   We have ever regarded it as one
of the very best works on the subjects embraced
within its scope, in the English  language; and the
present edition, enlarged and revised by the author,
renders it still more entitled to the confidence of the
profession.  The valuable notes  of Prof.  Huston
have been retained, and contribute, in no small de-
gree,  to  enhance the  value of the work.  It is a
source of congratulation that the publishers have
permitted the author  to be, in  this  instance, his
own editor, thus securing all the revision  which
an author alone is capable of making.—The Western
Lancet.
  As  a comprehensive manual for students, or  a
work of  reference for practitioners, we only speak
with common justice when we say that it surpasses
any other  that  has ever issued on the same sub-
ject from the British press.—The Dublin Quarterly
Journal.
                              DICKSON  (S.  H.),  M. D.,
           Professor of Practice of Medicine in the Jefferson Medical College, Philadelphia.
ELEMENTS  OF  MEDICINE;  a Compendious View  of Pathology and Thera-
  peutics, or the History and Treatment of Diseases.  In one large and handsome octavo volume,
  of 750 pages, leather.  $3 75.
  As an American text-book on the Practice of Medicine for the student, and as a condensed work
of reference for the practitioner, this volume will have strong claims on the attention ofthe profession.
Few physicians have had wider opportunities than the author for observation and experience, and
few perhaps have used them better.  As the result of a life of study and practice, therefore, the
present volume will doubtless be received with the welcome it deserves.
  This book is eminently what it professes to be; a
distinguished merit in these days.  Designed for
" Teachers and Students of Medicine," and admira-
bly suited to their wantB, we think it will be received,
on its own merits, with a hearty welcome.—Boston
Med. and Surg. Journal.

  Indited by one of the most accomplished writers
of our country, as well as by one who has long held
a high position among teachers and practitioners of
medicine, this work is entitled to patronage and
careful study.  The learned author has endeavored
to condense in  this volume most of the practical
matter contained in his former productions, so as to
adapt it to  the use of those who have not time to
devote to moreextensiveworks.—SouthernMed. and
Surg. Journal.

  Prof. Dickson's work supplies, to a great extent,
a desideratum long felt in American medicine.—N.
O. Med. and Surg. Journal,

  Estimating this work according to the purpose for
which it is designed,  we must think highly of its
merits, and we have no hesitation in predicting for
it a favorable reception by both students and teachers.
  Not professing to be a complete and comprehensive
treatise, it will not be found full in detail, nor filled
with discussions of theories  and opinions, but em-
bracing all that is essential in theory and practice,
it is admirably adapted to the wants of the American
student.  Avoiding all that is uncertain, it presents
more clearly to the mind of the reader that which is
established and verified by experience.   The varied
and extensive reading of the author is conspicuously
apparent, and all the recent improvements and dis-
coveries in therapeutics and pathology are chroni-
cled in its pages.—Charleston Med. Journal.
  In the first part of the work the subject of gene-
ral pathology is presented in outline, giving a btau-
tiful picture of  its  distinguishing features, and
throughout the succeeding chapters  we find that  he
has kept scrupulously within the bounds of sound
reasoning  and  legitimate deduction.   Upon the
whole, we do not hesitate to pronounce it a superior
work  in its  class, and that  Dr. Dickson  merits a
place in the first rank of American writers.—Western
Lancet. 
AND SCIENTIFIC  PUBLICATIONS.
11
                      DRUITT (ROBERT),  M.R. C.8.,  &.C.
THE PRINCIPLES AND  PRACTICE  OF  MODERN SURGERY.   Edited
  by F. W. Sargent, M. D., author of " Minor Surgery," &c.  Illustrated with one hundred and
  ninety-three wood-engravings.  In one very handsomely printed octavo volume, leather, of 576
  large pages.  $3 00.
  Dr. Druitt's researches into the literature of his ; ments of theoretical and practical Surgery—that it
subject have been not only extensive, but well di-  is found to contain reliable and authentic informa-
rected ; the most discordant authors are fairly and ! tion on the nature and treatment of nearly all surgi-
impartially quoted, and, while due credit is given | cal affections—is a sufficient reason for the liberal
to each, their respective merits  are weighed with  patronage it has obtained.  The editor, Dr. F. W.
an unprejudiced hand.  The grain of wheat is pre- i Sargent, has contributed much to enhance the value
                                               of the work, by such American improvements as are
                                               calculated more perfectly  to adapt it to our own
                                               views  and  practice  in this  country.  It  abounds
                                               everywhere with spirited and life-like illustrations,
                                               which  to the young  surgeon, especially, are of no
                                               minor consideration.  Every medical man frequently
                                               needs just such a work as this, for immediate refer-
                                               ence in moments of sudden emergency, when he has
                                               not time to  consult more elaborate treatises.—The
                                               Ohio Medical and Surgical Journal.
                                                 No work, in our opinion, equals it in presenting
                                               so much valuable surgical matter in so  small  a
                                               sompass.—St. Louis Med. and Surgical Journal.
served, and the chaff is unmercifully stripped off.
The arrangement is simple and philosophical, and
the style, though clear and interesting, is so precise,
that the book contains more information condensed
into a few words than any other surgical work with
which we are acquainted.—London Medical Times
mnd Gazette.
  Druitt's Surgery is too well known to the Ameri-
can medical profession to require its announcement
anywhere.  Probably no work of the  kind has ever
been more cordially received and extensively circu-
lated than this.  The fact  that it comprehends in a
comparatively small compass, all the essential ele-
                           DALTON,  JR. (J.  C),  M.  D.
                 Professor of Physiology in the College of Physicians, New York.

A TREATISE  ON HUMAN PHYSIOLOGY, designed for the  use of Students
  and Practitioners of Medicine.  With two hundred and fifty-four illustrations on wood.   In one
  very beautiful octavo volume, of over 600 pages, extra cloth, $1 00; leather, raised bands, $4 25.
  (Now ready, Jan. 1859.)
  The object of the author has  been to present a condensed view of the present  condition of his
subject, divested of mere theoretical views and hypothetical reasonings, but  comprehending all
important details which may be received as  definitely settled.  His long experience as  an  investi-
gator and as a teacher has given him peculiar advantages in this, and he has endeavored wherever
practicable to show the means by which results have been reached, so as to afford the student the
means of pursuing original research, as well as a complete text-book of  the science in its most ad-
vanced condition.  Of the numerous illustrations, all are original with the exception of eleven, so
that  the whole possesses a completeness  and authority not otherwise attainable, and  in the me-
chanical execution every care  has been taken to present one of the handsomest volumes as yet
produced by the American press.
  To our mind, fulfils  in a most admirable manner I pressed into a reasonable compass, embracing like-
the objects contemplated  by the author.  The broad | wise the results of recent laborers in this department
field  of physiology has been traversed with discri-  of our science.—Med. and Surg.  Reporter, Jan. 29,
mination,  and its most valuable  acquisitions com- | 1859.

           DUNGLISON,  FORBES,  TWEEDIE, AND  CONOLLY.
THE CYCLOPiEDIA  OF  PRACTICAL MEDICINE: comprising Treatises on
  the Nature and Treatment of Diseases, Materia Medica, and Therapeutics, Diseases  of Women
  and Children, Medical Jurisprudence,  &c. &c.  In four large  super-royal octavo volumes, of
  3254 double-columned pages, strongly and handsomely bound, with raised bands.  $12 00.
  *  * This work contains no less than four hundred and eighteen distinct treatises, contributed by
sixty-eight distinguished physicians, rendering it  a  complete library of reference for the  country
practitioner
  The most complete work on Practical Medicine
extant;  or,  at  least,  in  our  language.—Buffalo
Medical and Surgical Journal.
  For reference, it is above all price to every prac-
titioner.—Western Lancet.
  One of the most valuable medical publications of
the day__as a work of reference  it is invaluable-
Western Journal of Medicine and Surgery.
  It has been to us, both as learner and teacher, a
work for ready and frequent reference, one in which
modern  English medicine is exhibited in the most
advantageous light.—Medical Examiner.

  We rejoice that this work is to be placed within
the reach of the profession in this country, it being
unquestionably one of very great value to the prac-
titioner.  This estimate of it has not been formed
from a hasty examination, but after an intimate ac-
quaintance derived from frequent consultation of it
during  the past nine or ten years.   The editors are
practitioners of established reputation, and the list
of contributors embraces many of the most eminen
professors and teachers of London, Edinburgh, Dub-
lin, and Glasgow.  It is, indeed, the great merit of
this work that the principal articles have been fur-
nished  by practitioners who have not only devoted
especial attention to the diseases  about which they
have written, but have also enjoyed opportunities
for an extensive practical acquaintance with them,
and whose reputation carries the assurance of their
competency justly  to appreciate the  opinions of
others, while  it stamps their own doctrines with
high and just  authority.—American Medical Journ.
DEWEES'S COMPREHENSIVE SYSTEM OF
  MIDWIFERY.  Illustrated by occasional cases
  and many ensravings.  Twelfth edition, with the
  author's last improvements and corrections  In
  one octavo volume, extra cloth, of 600 pages. S3 20.
DEWEES'S TREATISE  ON  THE PHYSICAL
  AND MEDICAL  TREATMENT  OF CHILD-
  REN.  The last edition.  In one volume, octavo,
  extra cloth, 548 pages.  $2 80.
DEWEES'S TREATISE  ON THE DISEASES
  OF FEMALES.  Tenth edition. In one volume,
  octavo   xtra cloth, 532 pages, with plates. S3 00.
DANA ON ZOOPHYTES AND CORALS.  In one
  volume, imperial quarto, extra cloth, with wood-
  cuts. $15 00.  Also, AN ATLAS, in one volume,
  imperial folio, with sixty-one magnificent colored
  plates.  Bound in half morocco.  830 00.
DE LA BECHE'S  GEOLOGICAL  OHSERVER.
  In one very large and handsome octavo volume, ex-
  tra cloth, of 700 pages, with 300 wood-cuts. $4 00.
FRICK ON  RENAL AFFECTIONS; their Diag-
  nosis and Pathology.   With illustrations. One
  volume, royal 12mo., extra cloth. 75 cents. 
12
BLANCHARD  <&  LEA'S  MEDICAL
                         DUNGLISON   (ROBLEY),  M.D.,
          Professor of Institutes of Medicine in the Jefferson Medical College, Philadelphia.

            NEW  AND ENLARGED  EDITION, Now Ready, 1858.

MEDICAL  LEXICON;   a Dictionary of  Medical  Science,  containing a concise
  Explanation ofthe various Subjects and Terms of Anatomy, Physiology, Pathology, Hygiene,
  Therapeutics. Pharmacology, Pharmacy, Surgery, Obstetrics, Medical Jurisprudence, Dentistry,
  &c.   Notices of Climate and of Mineral Waters; Formulae for Officinal, Empirical, and Dietetic
  Preparations, &c.   With French and other Synonymes.  Fifteenth edition, revised and very
  greatly enlarged.  In one very large and handsome octavo volume, of 992 double-columned pages,
  in small type; strongly bound in leather, with raised bands.   Price $4  00.

  Especial care has  been devoted in the preparation of this  edition to render it in every respect
worthy a continuance of the very remarkable favor which it has hitherto enjoyed.  The rapid
sale of Fifteen  large editions,  and  the constantly increasing  demand, show that it is regarded by
the profession as the standard authority.  Stimulated by this fact, the author has endeavored in the
present revision to introduce whatever might be necessary " to make it a satisfactory and desira-
ble—if not indispensable—lexicon,  in which the student may search  without disappointment for
every term that has been legitimated  in the nomenclature of the science."  To accomplish this,
large additions have been found requisite, and  the extent of the author's  labors may be  estimated
from the fact that about Six Thousand subjects and  terms  have been introduced  throughout, ren-
dering the whole number of definitions about  Sixty Thousand, to accommodate which, the num-
ber of pages has been increased by nearly a hundred, notwithstanding an enlargement in the size
ofthe pase.  The medical press, both in this country and in England, has pronounced the work in-
dispensable to all medical students and practitioners, and the present improved edition will not lose
that enviable reputation.
  The publishers have endeavored to render the mechanical execution worthy of a volume of such
universal use in daily reference. The greatest care  hns been exercised to obtain the typographical
accuracy so necessary in  a work of the kind.   By the small but exceedingly clear type employed,
an  immense amount of matter is condensed in its thousand  ample pages, while the binding will be
found strong and durable.  With all these improvements and enlargements, the price has been kept
at the  former very moderate rate, placing it within the reach of all.
  This work, the appearance of the fifteenth edition
of which, it has become our duty and pleasure to
announce, is perhaps the most stupendous monument
of labor and erudition in medical literature.  One
would hardly suppose after constant use of the pre-
ceding editions, where we have never failed to find
a sufficiently full explanation of ever) medical term,
that in this edition  " about six  thousand  subjects
and terms have been added,''' with a careful revision
and correction of the entire work. It is only neces-
sary to announce the advent of this edition to make
it occupy the place ofthe preceding one on the table
of every medical man, as it is without doubt the best
and most comprehensive work of the kind which has
ever appeared.—Buffalo Med. Journ., Jan. 1858.

  The work is a monument  of patient research,
skilful judgment, and vast physical labor, that will
perpetuate the name of the author more effectually
than any  possible device of stone  or metal.  Dr,
Dunglison deserves the thanks not only of the Ame-
rican profession, but of the whole medical world.—
North Am. Medico-Chir. Review, Jan. 1858.
  A Medical Dictionary better adapted for the wants
of the profession than any other with which we are
acquainted, and of a character which places it far
above comparison and  competition.—Am. Journ.
Med. Sciences, Jan. 1858.
  We need only say, that the addition of 6,000 new
terms, with their accompanying definitions, may be
said to constitute a new work, by itself.  We  have
examined  the Dictionary attentively, and are  most
happy to pronounce it unrivalled of its  kind.  The
erudition displayed, and the extraordinary industry
which must have been demanded, in its preparation
and perfection, redound to  the lasting credit  of its
author, and have furnished us with a volume indis-
pensable at the  present day, to all who would find
themselves au niveau with the highest standards of
medical information.—Boston Medical and Surgical
Journal, Dec. 31, 1857.
  Good lexicons and encyclopedic works generally,
are the most labor-saving  contrivances which lite-
rary men enjoy; and the labor which is  required to
produce them in the perfect manner of this example
is something appalling to contemplate.  The author
tells us in his preface that he has added about six
thousand terms and subjects to this edition, which,
before, was considered universally as the best work
of the kind in any language.—Silliman's Journal,
March, 1858.
  He has razed his gigantic structure to the founda-
tions, and remodelled and reconstructed the entire
pile.  No less than six thousand additional subjects
and terms are illustrated and analyzed in this new
edition,  swelling  the grand  aggregate  to beyond
sjxty thousand !  Thus is placed before the profes-
sion a complete and  thorough exponent of medical
terminology, without rival or possibility of rivalry.
—Nashville Journ. of Med. and Surg., Jan. 1858.
  It is universally acknowledged, we believe, that
this work is incomparably  the best and most com-
plete  Medical  Lexicon in the  English language.
The amount of labor which the distinguished author
has bestowed upon it is truly wonderful, and the
learning  and research displayed in its preparation
are equally remarkable.  Comment and commenda-
tion are unnecessary, as no one at the present day
thinks of purchasing any other Medical Dictionary
than this.—St. Louis Med. and Surg. Journ., Jan.
1858.
  It is the foundation stone of a  good medical libra-
ry, and should always be included in the first list of
books purchased by the medical student.—Am. Med.
Monthly, Jan. 1858.
  A very perfect work of the kind, undoubtedly the
most perfect in the  English language.—Med. and
Surg. Reporter, Jan. 1858.
  It is now emphatically the Medical Dictionary of
the English language, and for it there is no substi-
tute.—N. H. Med. Journ., Jan.  1858.
  It is scarcely necessary to remark that any medi-
cal library wanting a copy of Dunglison's Lexicon
must be imperfect.—Cin. Lancet, Jan. 1858.
  We have ever considered it the best authority pub-
lished, and the present edition we may safely say has
no equal in the world.—Peninsular Med. Journal,
Jan.1858.
  The most complete authority on the subject to be
found in any language.— Va.Med. Journal, Feb. '58.
BY THE SAME AUTHOR.
THE PKACTICE  OP  MEDICINE.   A Treatise on Special Pathology and The-
  rapeutics.  Third Edition.   In two large octavo volumes, leather, of 1,500 pages.  $6 25. 
AND  SCIENTIFIC  PUBLICATIONS.
13
                         DUNGLISON  (ROBLEY),  M.D.,
          Professor of Institutes of Medicine in the Jefferson Medical College, Philadelphia.

HUMAN   PHYSIOLOGY.   Eighth  edition.    Thoroughly revised  and  exten-
  sively modified and enlarged, with five hundred and thirty-two illustrations.  In two large and
  handsomely printed octavo volumes, leather, of about 1500 pages.   $7 00.

  In revising this work for its eighth appearance, the author has spared no labor to render it worthy
a continuance ofthe very great favor which has been extended to it by the profession.  The whole
contents have been rearranged, and to a great extent  remodelled ; the investigations which of late
years have been so numerous and so important, have been carefully examined and incorporated,
and the work in every respect has been brought up to a level with the present state of the subject.
The object of the author has been to render it a concise but comprehensive treatise, containing the
whole body of physiological  science, to which the student and man of science can at all times refer
with the certainty of finding whatever they are in search of, fully presented in  all its aspects; and
on no former edition has the author bestowed  more labor to secure this result.
  We believe that it can truly  be said, no more com-
plete  repertory of tacts upon the  subject treated,
can anywhere be found.  The  author lias, moreover,
that enviable tact at description and that facility
and ease of expression which  render him peculiarly
acceptable to the casual, or  the studious reader
This faculty, so requisite in  setting forth many
erraver and less attractive subjects, lends additional
charms to one always fascinating.—Boston  Med.
and Surg. Journal, Sept. 1856.
                                                The best work of the kind  in the English lan-
                                              guage.—Silliman's Journal.
                                                The present edition the author has made a perfect
                                              mirror of the science as it is at the present  hour.
                                              As a work upon physiology proper, the science of
                                              the functions performed by the body, the student will
                                              find it all  he wishes.—Nashville Journ. of Med.
                                              Sept. 1856.
                                                That he has succeeded, most admirably succeeded
                                              in his purpose, is apparent from the appearance of
  The most  complete and  satisfactory system of j an eighth edition.  It is  now the great encyclopaedia
Physiology in the English  language.—Amer. Med  on the subject, and worthy of a place in every phy-
Journal.                                       [ sician's library.— Western Lancet, Sept. 1856.

                           BY the same author.  (Now Ready.)

GENERAL   THERAPEUTICS   AND  MATERIA MEDICA;  adapted  for a
  Medical Text-book.   With Indexes of Remedies and of Diseases and their Remedies.  Sixth
  Edition, revised and improved.  With one hundred and ninety-three illustrations.  In two large
  and handsomely printed octavo vols., leather,  of about 1100 pages. $6 00.

                                 From the Author's Preface.

  " Another edition of this work being called for, the author has subjected  it to a thorough and careful
revision.  It has been gratifying to him that it has been found so extensively useful by those for whom
it was especially intended, as to require  that a.sizth edition should be issued in so short a time afier
the publication of a fifth.  Grateful for the favorable reception of the work by the profession, he has
bestowed on the preparation of the present edition all those cares which were demanded by the
former editions,  and has spared no pains to render it a faithful epitome  of General Therapeutics
and Materia Medica.  The copious Indexes of Remedies and of Diseases and their Remedies can-
not fail, the author conceives, to add materially to the value  of the work."
  This work is too widely and too favorably known  to require more than the  assurance that the
author has revised it with his customary industry,  introducing whatever has been found necessary
to bring it on a level with  the most advanced condition ofthe subject.  The  number of illustration's
has been somewhat enlarged, and the mechanical execution  of the volumes will be  found to have
undergone a decided improvement.
  The work will, we have little doubt, be bought
and read by the majority of medical students; its
size, arrangement, and reliability recommend it to
all;  no  one, we  venture to preilict, will  study it
without profit, and there are few to whom it. will
not be in some measure useful as a work of refer-
ence.  The young practitioner, more especially, will
find the copious indexes appendid to this edition of
great assistance in the selection and pieparation of
suitable formulas.— Charleston Med. Journ. and Re-
view, Jan. 1858.
  In announcing a new edition of Dr. Dunglison's
General Ttierapeutics and Materia Medica, we have
no words of commendation to bestow upon a work
whose merits have been heretofore so often and so
justly extolled.  It must not be supposed, however,
that the present is a mere reprint of the previous
edition: the character of  the author for laborious
research, judicious analysis, and  clearness of ex-
pression, is fully sustained by  the numerous addi-
tions he has made to the work, and the careful re-
Vision to which he has subjected the  whole.—JV. A.
Medico-Chir. Review, Jan. 1858.

                         by the  same  author.  (.4 new Edition.)

NEW REMEDIES, WITH  FORMULAE FOR THEIR  PREPARATION AND
  ADMINISTRATION. Seventh edition, with extensive Additions.  In one very large octavo
  volume, leather, of 770 pages.   $3 75.
  Another edition of the  " New Remedies"  having been called for, the author has endeavored  to
add everything of moment that has appeared since the publication of the last edition.
  The articles treated of in the former editions will be found  to have undergone considerable ex-
pansion in this, in order that the author might  be enabled to introduce, as far as practicable, the
results of the subsequent experience of others, as well as of his  own observation and reflection ;
and to make the work still more deserving of the extended circulation with which  the preceding
editions have been favored by the profession.   By an enlargement of the page, the numerous addi-
tions have been incorporated  without greatly increasing the bulk of the volume.—Preface.
                                                 The great learning of the author, and his remark-
                                               able industry in  pushing his researches into every
                                               source whence information is derivable,have enabled
                                               him to throw together  an extensive mass of facts
                                               and statements, accompanied  by full reference to
                                               authorities; which  last feature renders the work
                                               practically valuable to investigators who desire tp
                                               examine the original papers.—The American Journal
                                               of Pharmacy.
  One of the most useful of the author's works.—
Southern Medical and Surgical Journal.
  This elaborate  and useful volume  should be
found in every medical library, for as a book of re-
ference, for physicians, it is  unsurpassed by any
other work in existence, and the double index for
diseases and for  remedies, will be found greatly to
enhance its value.—New York Med. Gazette. 
14
BLANCHARD  & LEA'S MEDICAL
                                ERICHSEN  (JOHN),
                    Professor of Surgery in University College, London, &c.

THE  SCIENCE  AND ART  OF SURGERY;  being a Treatise on Surgical
  Injuries, Diseases, and Operations. New and improved American, from the second enlarged
  and carefully revised London edition.  Illustrated with over four hundred engravings on wood.
  In one large and  handsome octavo  volume, cf one thousand closely printed pages,  leather,
  raised bands.  $4 50.  (Now Ready, January, 1859.)
  The very distinguished favor with which this work has been received on both sides ofthe Atlan-
tic has stimulated the author to render it even more worthy of the position which it has so rapidly
attained as a standard authority.  Every portion has been carefully  revised,  numerous additions
have been made, and the most watchful care has been exercised  to render it a complete exponent
ofthe most advanced condition of surgical science.  In this manner the work has been enlarged by
about a  hundred pages, while the series of engravings has  been increased  by more than a hundred,
rendering it one of the most thoroughly illustrated volumes before the profession.  The additions of
the author having rendered unnecessary most of the notes of the former American editor, but little
has been added in this country; some few notes and occasional illustrations have, however, been
introduced to elucidate American modes of practice.
  It is, in our humble judgment, decidedly the best
Dook of the kind in the English language. Strange
that just such books are notoftener produced by pub-
lic teachers of surgery in this  country and Great
Britain.  Indeed, it is a matter of great astonishment,
hut no less true than astonishing, that ofthe many
works on surgery republished in this country within
the last  fifteen or twenty years as text-books for
medical students, this is the only one that even ap-
proximates to the fulfilment of the peculiar wants of
youngmen just entenngupon the study of this branch
of the profession.— Western Jour .of Med. and Surgery.
  Its value is greatly enhanced by a very copious
well-arranged index. We regard this as one of the
most valuable contributions to modern surgery.  To
one entering his novitiate of practice, we regard it
the most serviceable guide which he can consult. He
will find a fulness of detail leading him through every
step ofthe operation, and not deserting him until the
final issue ofthe case is decided.  For the same rea-
son we recommend it to those whose routine of prac-
tice lies in such parts of the country  that they must
rarely encounter cases requiring surgical manage-
ment.—Stethoscope.
  Embracing, as will be perceived, the whole surgi-
cal domain, and each division of itself almost com-
plete and perfect, each chapterfull and explicit, each
subject faithfully exhibited, we can only express oui
estimate of it in the aggregate. We consider it an
excellent contribution to surgery, as probably the
best single volume  now extant on the subject, and
with great pleasure we add  it to onr text-books.—
Nashville Journal of Medicine and Surgery.
  Prof. Erichsen's work, for its size, has not been
surpassed; his nine hundred and eight pages,  pro-
fusely illustrated, are rich in physiological, patholo-
gical, and operative suggestions, doctrines, details,
and processes;  and will prove a reliable resource
for information, both to physician and surgeon, in the
hour of peril.—JV. 0. Med. and Surg. Journal.

  We are acquainted with no other  work wherein
so much good sense, sound principle, and practical
inferences, stamp every page.—American Lancet.
                            ELLIS  (BENJAMIN), M.D.
THE MEDICAL  FORMULARY:  being a Collection  of Prescriptions,  derived
  from the writings and practice of many of the most eminent physicians of America and Europe.
  Together with the usual Dietetic Preparations and Antidotes for Poisons.  To which  is added
  an Appendix, on the Endermic use of Medicines, and on the use of Ether and Chloroform.   The
  whole  accompanied with a few brief Pharmaceutic and Medical Observations.   Tenth edition,
  revised and much extended by Robert P. Thomas, M.  D., Professor of Materia Medica in the
  Philadelphia College of Pharmacy.  In one neat octavo volume, extra cloth, of 296 pages. $1 75.

                        FOWNES  (GEORGE), PH. D., &c.
ELEMENTARY   CHEMISTRY;  Theoretical and Practical.   With numerous
  illustrations.   Edited, with Additions, by Robert Bridges,  M. D.  In  one large royal 12mo.
                                                In leather, $1  50; extra cloth, $ 1 35.
                                                The work of Dr. Fownes has long been before
                                              the public, and its merits have been fully appreci-
                                              ated  as  the best text-book on chemistry now in
                                              existence.  We do not, of course, place it in a rank
                                              superior  to the works of Brande, Graham, Turner,
                                              Gregory, or Gmelin, but we say that, as a work
                                              for students, it is preferable to any of them.—Lon-
                                              don Journal of Medicine.
                                                A work well adapted to the wants of the student
                                              It is an  excellent exposition of the chief doctrines
                                              and facts of modern chemistry. The size of the work,
                                              and still  more the condensed yet perspicuous style
                                              in which it is  written, absolve it from the charges
                                              very properly  urged against most manuals termed
                                              popular.—Edinburgh Journal of Medical Science.
  volume, of over 550 pages, with 181 wood-cuts
  We know of no better text-book, especially in the
difficult  department of  organic chemistry,  upon
which it is particularly full and satisfactory. We
would recommend it  to preceptors as  a capital
" office book" for their students who are beginners
in Chemistry.  It is copiously illustrated with ex-
cellent wood-cuts, and altogether admirably  "got
up."—N. J. Medical Reporter.
  A standard manual, which  has long enjoyed the
reputation of embodying much knowledge in a small
space.  The author has achieved the difficult task of
condensation with masterly tact.  His book is con-
cise without being dry, and brief without being too
dogmatical or general.—Virginia Med. and Surgical
Journal.
                          FISKE FUND  PRIZE ESSAYS.
THE EFFECTS OF CLIMATE ON TUBERCULOUS DISEASE.   By Edwin
  Lee, M. R. C. S., London, and THE INFLUENCE OF PREGNANCY ON THE DEVELOP-
  MENT OF TUBERCLES.  By Edward Warren, M. D.,  of Edenton, N. C.   Together in
  one neat octavo volume, extra cloth.  $1 00.  (Just Ready.)

                       FERGUSSON  (WILLIAM), F. R. S.,
                     Professor of Surgery in King's College, London, &c.

A  SYSTEM  OF PRACTICAL SURGERY.   Fourth American,  from the third
  and enlarged London edition.  In one large and beautifully printed octavo volume, of about 700
  pages, with 393 handsome illustrations, leather.  $3 00.
  No work was ever written which more  nearly I  The addition of many new pages makes this work
comprehended the necessities of  the  student and | more than ever indispensable to the student and prac-
practitioner, and was more carefully arranged to titioner.—Ranking's Abstract.
that single purpose than this.—N. Y. Med. Journal. | 
AND SCIENTIFIC  PUBLICATIONS.
15
                             FLINT (AUSTIN), M.  D.,
        Professor of the Theory and Practice of Medicine in the University of Louisville, &c.
                                (An Important New Work.)
PHYSICAL EXPLORATION AND DIAGNOSIS OF DISEASES  AFFECT-
  ING THE RESPIRATORY ORGANS.  In  one large and  handsome octavo volume, extra
  cloth, 636 pages.  $3 00.
 We can only state our general impression of the
high  value of this work, and cordially recommend
it to all. We regard it, in point both of arrangement
and of the marked ability of its treatment of the sub-
jects, as destined  to take the first rank in works of
this class.  So far asour information extends, it has
at present no equal.  To the practitioner, as well as
the student, it will be invaluable in clearing up the
diagnosis of doubtful  cases, and in shedding light
upon difficult phenomena.—Buffalo Med. Journal.
 This is the most elaborate work devoted exclu-
sively to the physical exploration of diseases of the
lungs, with which we are acquainted in the English
language.—Boston Med. and Surg. Journal.
  A work of original observation of the highest merit.
We recommend the treatise to every one who wishes
to become a correct auscultator.  Based to  a very
large extent  upon  eases numerically examined, it
carries the evidence of careful study and discrimina-
tion upon every pasje.  It does credit to the autnor,
and, through him, to the profession in this country.
It is, what we cannot call every book upon auscul-
tation, a readable book.—Am. Jour. Med. Sciences.
                                  NOW COMPLETE,

                          GRAHAM (THOMAS), F.  R. S.,
THE ELEMENTS  OF  INORGANIC  CHEMISTRY, including  the Applica-
  tions ofthe Science in the Arts. New and much enlarged edition, by Henry Watts and Robert
  Bridges, M. D.   Complete in one large and handsome  octavo volume, of over 800 very large
  pages, with two hundred and thirty-two wood-cuts, extra cloth.  $4 00.
  **,£ Part II., completing the work from p. 431 to end, with Index, Title Matter, &c, may be
had separate, cloth backs and paper sides.  Price $2 50.
  The long delay which has intervened since the appearance of the first portion of this work, has
rendered necessary an Appendix, embodying the  numerous and important investigations and dis-
coveries of the last few years in the subjects  contained in Part I.  This occupies a large portion
of Part II., and will be found to present a complete abstract of the most recent researches in the
general principles of the science, as well as all details necessary to bring the whole work thoroughly
up to the present time in all departments of Inorganic Chemistry.
  Gentlemen desirous of completing their copies of the work are requested to apply for Part II.
without delay.   It will be sent by mail, prepaid, on receipt of the amount, $2 50.
                                                From Prof.  Wolcott Gibbs, N. Y. Free Academy.
                                                               May 25, 1658.
                                                The work is an admirable one in all respects,and
                                              its republication here cannot fail to exert a positive
                                              influence upon the progress of science in this country.

                                              From Prof. E. N. Horsford. Harvard College, April
                                                                 27, 185S.

                                                 It has, in its earlier and less perfect editions, been
                                              familiar to me, and  the excellence of its plan and
                                              the clearness and completeness of its discussions,
                                              have long been my admiration.
  It is a very acceptable addition to the library of
standard boohs of every chemical student. No reader
of English works on this science can afford to be
without this edition of Prof. Graham's Elements.—
Silliman's Journal, March, 1858.

From Prof.  J. L. Crawcour,  New  Orleans School
            of Medicine, May 9, 1858.
  It is beyond all questicn the best systematic work
on Chemistry in the  English  language, and I am
gratified to find that an American edition at a mo-
derate price has been issued, so as to place  it within
the means of students.  It will be the only text-book
1 shall now recommend to my class.
                      GRIFFITH  (ROBERT E.),  M. D.,  &.c.
A UNIVERSAL FORMULARY, containing the methods of Preparing and Ad-
  ministering Officinal and other Medicines.  The whole adapted to  Physicians and Pharmaceu-
  tists.  Second Edition, thoroughly revised, with numerous additions, by Robert P. Thomas,
  M. D. Professor of Materia Medica in the Philadelphia College of Pharmacy.   In one large and
  handsome octavo volume, extra cloth, of 650 pages, double columns.  "
  $3 25.
00; or bound in sheep,
  It was a work requiring much perseverance, and
when published was looked upon as by far the  best
work of its kind that had issued from the American
press.  Prof Thomas has certainly "improved," as
well as added to this Formulary, and has rendered it
additionally deserving of the confidence of pharma-
ceutists and physicians.—Am. Journal of Pharmacy.
  We are happy to announce a new and improved
edition of this, one ofthe most valuable and useful
works that have emanated from an American  pen.
It would do credit to any country, and will be found
of daily usefulness to practitioners of medicine; it is
better adapted to their purposes than the dispensato-
ries.— Southern Med. and Surg. Journal.
  It is one ofthe most useful books a country practi-
nistering medicines that can be desired by the physi-
cian and pharmaceutist.— Western Lancet.
  The amountof useful, every-day matter.for a prac
ticing physician, is really immense.—Boston Med.
and Surg. Journal.
  We predict a great sale for this work, and we espe-
cially recommend it to all medical teachers.—Rich-
mond Stethoscope.
  This edition of Dr. Griffith's work has been grea'ly
improved by the revision and ample additions of Dr.
Thomas, and is now,  we believe, one of the most
complete works of its kind in any language.  The
additions amount to about seventy  pages, and no
effort has been spared to include in them all the re-
cent  improvements which have been published in
tionercan possibly have in his possession.—Medical  medical journals, and systematic treatises.  A work
Chronicle.                                      of this kind appears to us indispensable 1o the physi-
  This is a work of six hundred and fifty-one pages,  cian, and there is none we can more cordially recom-
embracin«- all on the subject of preparing and admi-  mend.—N. Y. Journal of Medicine.
                           GLUGE (GOTTLIEB),  M. D.,
         Professor of Physiology and Pathological Anatomy in the University of Brussels, &c.

AN  ATLAS  OF  PATHOLOGICAL  HISTOLOGY.   Translated, with  Notes
  and Additions, by Joseph Leidy, M. D., Professor of Anatomy in the University of Pennsylva-
  nia.   In one volume, very large imperial quarto, extra cloth, with 320 figures, plain and colored,
  on twelve copperplates.  $5 00. 
16
BLANCHARD  &  LEA'S  MEDICAL
                          GROSS (SAMUEL D.),  M.  D.,
             Professor of Surgery in the Jefferson Medical College of Philadelphia, tec.
                            New Edition (Now Ready.)
ELEMENTS  OF PATHOLOGICAL ANATOMY.   Third edition, thoroughly
  revised and greatly improved.   In one large and very handsome octavo volume, with about three
  hundred and fifty beautiful illustrations, of which a large  number are from original drawings.
  Price in extra cloth, $4 75; leather, raised bands, $5 25.
  The very rapid advances in the Science of Pathological Anatomy during the last few years have
rendered essential a thorough modification  of this work, with a view of making it a correct expo-
nent of the present state of the subject.   The  very careful manner in which this task has been
executed, and the amount of alteration which it has  undergone, have enabled the author to say that
'' with  the many changes and improvements now introduced, the work may be regarded almost as
a new treatise," while the  efforts of the author have been seconded as regards  the  mechanical
execution of the volume, rendering it one of the handsomest productions of the American press.
A very large number of new and beautiful original illustrations have been introduced, and the work,
it is hoped, will fully maintain the reputation hitherto enjoyed by it of a complete and practical ex-
position of its difficult and important subject.
  We most sincerely congratulate the author on the    We have been favorably impressed with the gene-
successful manner in which he has accomplished his  ral manner in which Dr. Gross has executed his task
proposed object.  His book is most admirably cal-  of affording a comprehensive digest of  the present
culated to fill up a blank which has long been felt to  state ofthe literature of Pathological Anatomy, and
exist in this department of medical literature, and
as such must become very widely circulated amongst
all-classes of  the profession. — Dublin Quarterly
Journ. of Med. Science, Nov. 1857.
have much pleasure in recommending his work to
our readers, as we believe one well deserving ot
diligent perusal and careful study.—Montreal Med.
Chron., Sept. 1857.
                                 BY THE SAME AUTHOR.
A  PRACTICAL   TREATISE  ON  THE   DISEASES,   INJURIES,  AND
  MALFORMATIONS OF THE URINARY BLADDER, THE PROSTATE GLAND, AND
  THE  URETHRA.  Second Edition, revised and much enlarged, with one hundred and eighty-
  four illustrations.  In one large and very handsome octavo volume, of over nine hundred pages.
  m leather, raised bands, $5 25; extra cloth, $4 75.
  A volume replete with truths and principles ofthe
utmost value in the investigation of these diseases.—
American Medical Journal.
  On the appearance of the first edition of this work,
the leading English medical review predicted that it
would have a " permanent place in the literature of
surgery worthy to rank with the best works of the
present age."  This prediction has been amply ful-
filled. Dr. Gross's treatise has been found to sup-
ply completely the want which has been felt ever
since the elevation of surgery to the rank of a science,
of a good practical treatise on the diseases of the
bladder and its accessory organs. Philosophical in
its design, methodical in its arrangement, ample and
                                            sound in its practical details, it may in truth be said
                                            to leave scarcely anything to be desired on so im-
                                            portant a subject, and with the additions and modi-
                                            fications resulting  from future discoveries and im-
                                            provements, it will probably remain one of the most
                                            valuable works on this subject so long as the science
                                            of medicine shall  exist.—Boston Med. and Surg.
                                            Journal.
                                              Whoever will peruse the vast amount of valuable
                                            practical information it contains, and which we
                                            have been unable  even to notice, will, we think,
                                            agree with us, that there is no work in the English
                                            language which can make any just pretensions to
                                            be its equal.—iV. Y. Journal of Medicine.
                                  BY THE SAME AUTHOR.
A PRACTICAL TREATISE ON FOREIGN BODIES IN THE AIR-PAS-
  SAGES.  In one handsome octavo volume, extra cloth, with illustrations,  pp. 468.   $2 75,

                            BY THE SAME AUTHOR.  (In press.)
A SYSTEM  OF SURGERY j Diagnostic, Pathological, Therapeutic, and Opera-
  tive.  With very numerous engravings on wood.  In two large octavo volumes.
                          GIBSON  (WILLIAM),  M. D.,
                Late Professor of Surgery in the University of Pennsylvania, &c

INSTITUTES  AND  PRACTICE  OF SURGERY;  being  Outlines of a Course
  of Lectures.   Eighth edition, improved and altered.  With thirty-four pla'es. In two handsome
  octavo volumes, containing about 1000 pages, leather, raised bands.  $6 50.
                            GRAY  (HENRY),  F. R. S.,
                   Lecturer on Anatomy at St. George's Hospital, London, &c.

ANATOMY,  DESCRIPTIVE  AND  SURGICAL.   The  Drawings by H. V.
  Carter, M. D., late Demonstrator on Anatomy at St. George's Hospital; the Dissections jointly
  by the Author and  Dr. Carter. In  one magnificent imperial octavo volume, with 363  large
  and elaborate engravings on wood.  (At press.)
  The author  has endeavored  in this work to cover a more extended  range of subjects than  is
customary in the ordinary text-books, by giving not only the details necessary for the student, but
also ihe application of those details in the practice of medicine and surgery, thus rendering it not
only a guide for the learner, but an admirable work of reference for the active practitioner.  The
engravings form a special feature in the work, many of them being the  size of nature, nearly all
original, and having the names of the various parts printed on the body of the cut, in place of figures
of reference with descriptions at the foot.  They thus form a complete and splendid series, which
will greatlv assist the student ia obtaining a clear idea of Anatomy, and will also serve to refresh
the memory of those who may find in the exigencies of practice the necessity of recalling the de-
tails of tne dissecting room.
GARDNER'S MEDICAL  CHEMISTRY, for the
  use of Students and the Profession.  In one royal
  l'2mo. vol., ex. cloth, pp. 396, with illustrations.
  81 00.
HUGHES'  CLINICAL INTRODUCTION  TO
THE PRACTICE OF AUSCULTATION AND
OTHER MODES OF PHYSICAL DIAGNOSIS,
IN DISEASES OF THE LUNGS AND HEART.
Second American, from the second London edition.
1 vol. royal l'imo., ex. cloth, pp. 304.  $1 00. 
AND  SCIENTIFIC  PUBLICATIONS.
17
                        HOBLYN (RICHARD D.),  M. D.
A DICTIONARY  OF  THE  TERMS  USED  IN  MEDICINE  AND  THE
  COLLATERAL SCIENCES.  A new American edition.  Revised, with numerous Additions,
  by Isaac Hays, M. D., editor of the " American Journal ofthe Medical Sciences."  In one large
  royal 12mo. volume, leather, of over 500 double columned pages.   $1 50.
  If the frequency with which we have referred to
this volume since its reception from the publisher,
two or three weeks ago, be any criterion for the
future, the binding will goon have to be renewed, even
with cureful handling.  We find that Dr. Hays has
done the profession great service by his careful and
industrious labors.  The Dictionary has thus become
eminently suited to our  medical brethren in  this
country.  The additions by Dr. Hays are in brackets,
and we believe there is not a single page bnt bears
these insignia; in every instance which we have thus
far noticed, the additions are really needed and ex-
ceedingly valuable. We heartily commend  the work
to all who wish to be au courant in medical termi-
nology.— Boston Med. and Surg. Journal.

  To both practitioner and student, we recommend
this dictionary as being convenient in size,  accurate
in definition, and sufficiently full and complete for
ordinary consultation.—Charleston Med. Journ. and
Review.
  Hoblyn has always been a favorite dictionary, and
in its present enlarged and improved form will  give
greater satisfaction than ever. The American editor,
Dr. Hays, has made many very valuable additions.
—N. J. Med. Reporter.
  To supply the want of the medical reader arising
from this  cause, we  know of no dictionary better
arranged and adapted than the one bearing the above
title.  It is not encumbered with the obsolete terms
of a bygone age, but it contains all that are now in
use ; embracing every department of medical science
down  to the very latest date.  The volume is  of a
convenient size to be used  by the medical  student,
and yet large enough to make a respectable appear-
ance in the library of a physician.—Western Lancet.
  Hoblyn's Dictionary has long been a favorite with
us. It is the best book of definitions we have, and
ought always  to be upon the student's  table.—
Southern Med. and Surg. Journal.
                     HUNTER  (JOHN).
(See "Ricord," page 26, for Ricord's edition of Hunter on Venereal.)
             HOLLAND (SIR  HENRY),  BART.,  M.D..F. R. S.,
                      Physician in Ordinary to the Queen of England, &c.

MEDICAL NOTES AND REFLECTIONS.  From the third London edition.
  In one handsome octavo volume, extra cloth,  $3 00.
                          HABERSHON  (S.  O.),  M. D.,
   Assistant Physician to and Lecturer on Materia Medica and Therapeutics at Guy's Hospital, &c.

PATHOLOGICAL AND  PRACTICAL  OBSERVATIONS ON DISEASES
  OF THE  ALIMENTARY CANAL, CESOPHAGUS, STOMACH, O/ECUM, AND INTES-
  TINES.  With illustrations on wood.  In one handsome  octavo volume.  (Publishing in the
  Medical News and Library for 1858 and 1859.)
                        HORNER (WILLIAM E.), M. D.,
                    Professor of Anatomy in the University of Pennsylvania.
SPECIAL  ANATOMY  AND   HISTOLOGY.   Eighth  edition.   Extensively
  revised and modified.   In  two large octavo volumes, extra cloth, of more than one thousand
  pages, handsomely printed, with over three hundred illustrations.  $6 00.
ANATOMICAL  ATLAS.
  page 28.
 BY THE SAME AUTHOR.

By Professors  Horner and  Smith.
See  Smith,
                       HAMILTON (FRANK H.),  M.  D.,
                     Professor of Surgery, in Buffalo Medical College, &c.
A COMPLETE TREATISE  ON  FRACTURES AND  DISLOCATIONS.
  one handsome octavo volume, with several hundred illustrations.   (Preparing.)
In
                       JONES (T.  WHARTON),  F. R. S.,
         Professor of Ophthalmic Medicine and Surgery in University College, London, &c.

THE PRINCIPLES  AND PRACTICE OF  OPHTHALMIC   MEDICINE
  AND SURGERY.  With one hundred and ten illustrations.   Second American from the second
  and revised London edition, with additions by Edward Hartshorne, M. D., Surgeon to Wills'
  Hospital, &c.   In one large, handsome royal 12mo. volume, extra cloth, of  500 pages.  $1 50.
  We are confident that the reader will find, on
perusal, that the execution ofthe work amply fulfils
the promise of the pre/ace, and sustains, in every
point, the already high reputation of the author as
an ophthalmic  surgeon as well as a physiologist
and pathologist. The  book is evidently the result
of much labor and research, iind has been written
with the greatest care and attention; it possesses
that best quality which a general work, like a sys-
tem or manual can show, viz : the quality of having
all the materials whencesoever derived, so thorough-
ly wrought up, and digested in the author's mind,
as to come forth with the freshness and impressive-
ness of an original production.  We entertain little
doubt that this book will become what its author
hoped it might become, a manual for daily reference
and consultation by the student and the general prac-
titioner.  The work is marked by that correctness,
clearness, and precision of style which distinguish
all the productions of the learned author.—British
and For. Med. Review. 
18
BLANCHARD  &  LEA'S  MEDICAL
JONES (C. HANDFIELD), F. R. S., &. EDWARD  H.  SIEVEKING, M.D.,
                Assistant Physicians and Lecturers in St. Mary's Hospital, London.

A  MANUAL OF  PATHOLOGICAL  ANATOMY.  First American Edition,
  Revised.  With three hundred and ninety-seven handsome wood engravings.   In one large and
  beautiful octavo volume of nearly 750 pages, leather.   $3 75.
  As a concise text-book, containing, in a condensed
form, a complete outline of what is known in the
domain of Pathological  Anatomy, it is perhaps the
best work in the English language.  Its great merit
consists in its completeness and brevity, and in this
respect it supplies a great desideratum  in our lite-
rature.  Heretofore  the student of pathology was
obliged to glean from a great number of monographs,
and the field was so extensive that but few cultivated
it with  any  degree of success.  As a simple work
of reference, therefore,  it is of great value to the
student of pathological  anatomy, and should be in
every physician's library.—Western Lancet.
  In  offering the above titled work to the public, the
authors have not attempted to intrude new views on
their professional brethren, but simply to lay before
them, what has long been wanted, an outline of the
present condition of pathological anatomy.  In this
they have been completely successful.  The work is
one of the best  compilations which we have ever
perused.—Charleston Medical Journal and Review.
  We urge upon our readers and the profession gene-
rally the importance of informing themselves in re-
gard to modern views of pathology, and recommend
to them to procure the work before us as the best
means of obtaining this information.—Stethoscope.
  From  the casual examination we have given we
are inclined to  regard it as a text-book, plain, ra-
tional, and intelligible, such a book as the practical
man needs for daily reference.  For this reason it
will be likely tp be largely useful, as it suits itself
to those busy men who have little time for minute
investigation, and prefer a summary to an elaborate
treatise.—Buffalo Medical Journal.
                    KIRKES (WILLIAM  SENHOUSE),  M. D.,
               Demonstrator of Morbid Anatomy at St. Bartholomew's Hospital, &c.

A  MANUAL  OF  PHYSIOLOGY.    A  new  American, from  the  third  and
  improved London edition.  With two hundred illustrations.   In one large and handsome royal
  12mo. volume,  leather,   pp. 586.  $2 00.  (Just Issued, 1857.)
  In again passing this work through his hands, the author has endeavored to render it a correct
exposition of the  present condition of the science, making such  alterations  and additions as have
been dictated by further experience, or  as the progress of investigation has rendered desirable.   In
every point of mechanical execution the publishers have sought to make it superior to former edi-
tions, and at the very low price at which it is offered, it will be found one of the handsomest and
cheapest volumes before the profession.
  In making these improvements, care has been exercised not unduly to  increase its  size, thus
maintaining its distinctive characteristic of presenting within a moderate compass a clear and con-
nected view of its subjects, sufficient for the wants of the student.
                                                One of the very best handbooks of Physiology we
                                               possess—presenting just such an outline of the sci-
  This is a new and very much improved edition of
Dr. Kirkes' well-known Handbook of Physiology.
Originally constructed on the basis of the admirable
treatise of Muller, it has in successive editions de-
veloped itself into an almost original work, though
no change has been made in the plan or arrangement.
It combines conciseness with  completeness, and is,
therefore, admirably adapted for consultation by the
busy practitioner.—Dublin Quarterly Journal, Feb.
1857.
  Its excellence is in its compactness, its clearness,
and its carefully cited  authorities.  It is the most
convenient of text-books. These gentlemen, Messrs.
Kirkes and Paget, have really an immense talent for
silence, which is not so common or so cheap as prat-
ing people fancy.  They have the gift of telling us
what we want to know, without thinking it  neces-
sary to tell us all  they know.—Boston Med. and
Surg. Journal, May 14, 1857.
ence, comprising an account of its leading facts and
generally admitted principles, as the student requires
during his attendance upon a course of lectures, or
for reference whilst preparing for examination.—
Am. Medical Journal.
  We need only say, that, without entering into dis-
cussions of unsettled questions, it contains all  the
recent improvements in this department of medical
science.  For the student beginning this study, and
the practitioner who has but leisure to refresh  his
memory, this book is invaluable, as  it contains all
that it is important to know, without special details,
which are  read with interest only  by those who
would make a specialty, or desire to  possess a criti-
cal knowledge of the subject.—Charleston Medical
Journal.
KNAPP'S TECHNOLOGY ; or, Chemistry applied
  to the Arts and to Manufactures.  Edited, with
  numerous Notes and Additions, by Dr. Edmund
  Ronalds and Dr. Thomas Richardson.  First
American edition, with Notes and Additions, by
Prof. Walter R. Johnson.   In two handsome
octavo volumes, extra cloth, with about 500 wood-
engravings.  $6 00.
                               LUDLOW (J.  L.), M.  D.

A  MANUAL  OF  EXAMINATIONS  upon  Anatomy,  Physiology,  Surgery,
  Practice of Medicine, Obstetrics, Materia Medica, Chemistry, Pharmacy, and Therapeutics.  To
  which is added a Medical Formulary.   Designed for Students of Medicine throughout the United
  States.   Third edition, thoroughly revised and greatly extended  and enlarged.   With three
  hundred and  seventy illustrations.  In one large and  handsome royal 12mo. volume,  leather, of
  over 800 closely printed pages.  (Just Issued.)  $2 50.
  The great popularity of this volume, and the numerous demands for it during the two years in which
it has been out of print, have induced the author in its  revision to spare no pains to render  it a
correct and accurate digest ofthe most recent condition of all the branches of medical science.  In
many respects  it may, therefore, be regarded rather as a  new book than a new edition, an entire
section on Physiology having been  added, as also  one on Organic  Chemistry,  and many portions
having been rewritten.  A  very complete series of illustrations has been introduced,  and every
care ha.s been taken in the mechanical execution to render it a convenient and satisfactory book for
study or reference.
  The arrangement of the volume in the form of question and answer renders  it  especially suited
for the office examination of students and for those preparing for graduation.
  We know of no better companion for  the student  crammed into his head by the various professors to
during the hours spent in the lecture room, or to re-  whom he is  compelled to listen.—Western Lancet,
fresh, at a glance, his memory of the various topics  May, 1857. 
AND  SCIENTIFIC  PUBLICATIONS.
19
                               LEHMANN  (C.  G.)
PHYSIOLOGICAL  CHEMISTRY.   Translated from the second  edition by
  George E. Day, M. D., F. R. S., &c, edited by R. E. Rogers, M. D., Professor of Chemistry
  in  the Medical Department of the  University of Pennsylvania, with illustrations selected from
  Funke's Atlas of Physiological Chemistry, and an Appendix of plates.  Complete in two large
  and handsome octavo volumes, extra cloth, containing 1200 pages, with nearly two hundred illus-
  trations.  $6 00.
  This great work, universally acknowledged as the most complete and authoritative exposition of
the principles and details of Zoochemistry, in its  passage through the press, has  received from
Professor Rogers such care as was necessary to present it in a correct and reliable form.  To such
a work additions were deemed superfluous, but several years  having elapsed between the appear-
ance in Germany of the first and last volume, the latter contained a supplement, embodying nume-
rous corrections and additions resulting from  the advance ofthe science.  These have all been incor-
porated in the text in their appropriate places, while the subjects have been still further elucidated by
the insertion of illustrations from the Atlas of Dr. OttoFunke. With the view of supplying the student
with the means of  convenient comparison, a large number of wood-cuts, from works  on kindred
subjects, have also been added in the form of an Appendix of Plates.  The work is, therefore, pre-
sented as in every  way worthy the attention  of all who desire to be familiar with the modern facts
and doctrines of Physiological Science.
  The most important contribution as yet made to
it treats.—Edinburgh Monthly Journal of Medical
Science.
  Already well known and appreciated by the scien-
tific world, Professor Lehmann's great work re-
quires no laudatory sentences, as, under a new garb,
it is now presented to us.  The little space at our
command would ill suffice to set forth even a small
portion of its excellences.—Boston Med. and Surg.
Journal, Dec. 1855.
Physiological Chemistry.—Am. Journal Med. Sci
tnces, Jan. 1856.
  The present volumes belong to the small class of
medical literature which comprises elaborate works
of the highest orderof merit.—Montreal Med. Chron-
icle, Jan. 1856.
  The work of Lehmann stands unrivalled  as the
most comprehensive book of reference and informa-
tion extant on every branch of the subject on which

                           BY THE SAME AUTHOR.   (Just Issued.)

MANUAL OF CHEMICAL PHYSIOLOGY.   Translated  from  the German,
  with Notes and Additions,  by J. Cheston Morris, M. D., with an  Introductory Essay on Vital
  Force, by Samuel Jackson, M. D., Professor of the Institutes of Medicine in the University of
  Pennsylvania.  With illustrations on wood.   In one very handsome octavo volume, extra cloth,
  of 336 pages.  $2 25.
                         From Prof. Jackson's Introductory Essay.
  In adopting the  handbook of Dr. Lehmann as a manual of Organic  Chemistry for the use of the
students of the University, and in recommending his original work of Physiological Chemistry
for their more mature studies, the high value of his researches, and the great weight of his autho-
rity in that important department of medical science are fully recognized.

                         LAWRENCE (W.),  F. R. S., &.C.
A  TREATISE ON  DISEASES  OF  THE  EYE.  A   new edition,  edited,
  with numerous additions, and  243 illustrations, by Isaac Hays, M. D., Surgeon to Will's Hospi-
  tal, &c.  In one very large and handsome octavo volume, of 950 pages, strongly bound  in leather
  ....'«U ..n.'tt^.l UnJa   <HF\ Oft
                                                                                . leather
  with raised bands.  '$5 00.
  This admirable treatise—the safest guide and most
comprehensive work of reference, which is within
the reach ofthe profession.—Stethoscope.
likely that thiB great work will cease to merit the
confidence and preference of students or practition-
ers.  Its ample extent:—nearly one thousand large
octavo pages—has enabled both author and editor to
  This  standard text-book on the department of  do justice to all the details of this subject, and con-
whieh it treats, has not been superseded, by any or  dense in this single volume the present state of our
all of the  numerous publications on the subject  knowledge of the whole science in this department,
heretofore issued.  Nor with the multiplied improve- | whereby its practical value cannot be excelled.—N.
ments of Dr. Hays, the American editor, is it at all i Y. Med. Gaz.

                   LAYCOCK (THOMAS), M.  D., F. R. S. E.,
          Professor of Practical and Clinical Medicine in the University of Edinburgh, &c.
LECTURES  ON THE  PRINCIPLES  AND  METHODS   OF  MEDICAL
  OBSERVATION AND RESEARCH.  For the Use of Advanced Students and Junior  Prac-
  titioners. In one very neat royal 12mo. volume, extra cloth. Price $1 00.

                         LALLEMAND  AND WILSON.
A   PRACTICAL  TREATISE   ON   THE  CAUSES,  SYMPTOMS,  AND
  TREATMENT OF SPERMATORRHOEA.  By M.  Lallemand   Translated and edited ^y
  Henry  J  McDougall.  Third American edition.   To which is added---—ON DISEASES
  OF'THE  VESICULJE SEMINALES; and their associated organs.  With special  refer-
  ence to  the Morbid Secretions of the Prostatic and Urethral Mucous Membrane.  By Marris
  Wilson M. D.  In one neat octavo volume, of about 400 pp., extra cloth. $2 00. (Now Ready.)
  Although the views of M. Lallemand on Spermatorrhoea have  unquestionably exercised a very
grea influence, and the treatment advocated by him has been very genera ly adopted, still, a num-
ber of years  having elapsed since his work was given to the world the publishers have thought that
the vakie  of the present edition would be enhanced by the addition of-he little treatise of Dr.
Morris Wilson.  In it the causes ofthe different varieties of Spermatorrhoea are investigated with
 he aid of modern pathology, from which, combined with  the most recent experience of  the pro-
 essfon.Tte attempt is made  to deduce a rational system  of curative treatment  Whatever defi-
ciencies may have been caused in the work of M. Lallemand by the progress of medical science, will,
it is hoped, be in this manner supplied. 
20
BLANCHARD  &  LEA'S  MEDICAL
                            LA ROCHE  (R.),  M. D.,  &.c.
YELLOW  FEVER,  considered  in its  Historical, Pathological, Etiological,  and
  Therapeutical Relations.  Including a Sketch of the Disease as it has occurred in  Philadelphia
  from 1699 to 1854, with an examination of the connections between it and the fevers known under
  the same  name  in other parts of temperate as well as in tropical regions.   In  two large and
  handsome octavo volumes of nearly 1500 pages, extra cloth.   $7 00.
 From Professor S. H. Dickson, Charleston, S. C.,
              September 18, 1855.
  A monument of intelligent and well applied re-
search, almost without example.  It is, indeed, in
itself, a large library, and is destined to constitute
the special resort as a book of reference, in the
Bubject of which it treats, to all future time.
  We have not time at present, engaged as we are,
by day and by night, in the work of combating this
very disease, now prevailing in our city, to do more
than give this cursory notice of what we consider
as undoubtedly the  most able and  erudite medical
publication our country has yet produced.  But in
view of the startling fact, that this, the most malig-
nant and unmanageable  disease of modern times,
has for several years been prevailing in our country
to a greater  extent than  ever before;  that it  is no
longer confined to either large  or small cities, but
penetrates country villages, plantations, and farm-
houses; that it is treated with  scarcely better  suc-
cess now than thirty or forty years ago; that  there
is vast mischief done by ignorant pretenders to know-
ledge in regard to the disease, and in view of the pro-
bability that a majority of southern physicians will
be called  upon to treat the disease, we trust that this
able and comprehensive treatise will be very gene-
rally read in the south.—Memphis Med. Recorder.
  This is decidedly the great American medical work
of the day—a full, complete, and systematic treatise,
unequalled by any other upon the all-important sub-
ject of Yellow Fever.  The laborious, indefatigable,
and learned author has devoted to it many years of
arduous research and careful study, and the result
is such as will reflect the highest honor upon the
author and our country.—Southern Med. and Surg.
Journal.
  The genius and scholarship of this great physician
could not have been better  employed than in the
erection of this towering monument to his own fame,
and to the glory of the medical literature of his own
country. It is destined to remain the great autho-
rity upon the subject of Yellow Fever.  The student
and physician will  find in these volumes a risuml
of the sum total of the knowledge of the world upon
the awful scourge which they so elaborately discuss.
The style is so soft and so pure as to refresh and in-
vigorate the mind while  absorbing the thoughts of
the gifted author,  while the publishers have suc-
ceeded in bringing the externals into a most felicitous
harmony with the  inspiration that dwells within.
Take it all in  all, it is a book we have often dreamed
of, but dreamed not that it  would ever meet our
waking eye as a tangible reality.—Nashville Journal
of Medicine.
  We deem it fortunate  that the splendid work of
Dr. La Roche should have been issued from the press
at this particular time.  The want of a reliable di-
gest of all that is known in relation to this frightful
malady has long been felt—a want very satisfactorily
met in the work before us.   We deem it but faint
praise to say that Dr. La Roche has succeeded in
presenting the profession with an able and  complete
monograph, one which will  find its way into every
well ordered library.—Va. Stethoscope.
                                     BY THE SAME AUTHOR.
PNEUMONIA; its Supposed Connection, Pathological  and  Etiological, with Au-
  tumnal Fevers, including an Inquiry into the Existence and Morbid Agency of Malaria.  In one
  handsome octavo volume, extra cloth, of 500 pages.  $3 00.

                              MILLER (HENRY), M. D.,
     Professor of Obstetrics and Diseases of Women and Children in the University of Louisville.
PRINCIPLES  AND  PRACTICE OF OBSTETRICS, &c.; including the Treat-
  ment of Chronic Inflammation of the  Cervix and Body of the Uterus considered  as a frequent
  cause of Abortion.  With about one hundred illustrations on wood.  In one very handsome oc-
  tavo volume, of over 600 pages.  (Now Ready.)  $3 75.
  The reputation of Dr. Miller as an obstetrician is too widely spread to require the attention ol
the profession to be specially called to a  volume containing the experience of his long and extensive
practice.  The very favorable reception accorded to his " Treatise on Human Parturition," issued
some years since, is an earnest that the  present work will fulfil the author's intention of providing
within a moderate compass a complete and trustworthy text-book for the student, and book of re-
ference for the practitioner.  Based to a certain extent upon the former work, but enlarged to more
than double its size, and almost wholly  rewritten, it presents, besides the matured experience of
the author, the most recent views and investigations of modern obstetric writers, such as Dubois,
Cazeaux,  Simpson, Tyler Smith, &c, thus embodying the results not  only of the American,
but also of the Paris, the London, and the Edinburgh obstetric schools.  The author's position for so
many years as a teacher of his favorite  branch, has given him a familiarity with the wants of stu-
dents and a facility of conveying  instruction, which cannot fail  to render the volume eminently
adapted to its purposes.
                                                  In fact, this volume must take its place among the
                                                standard systematic treatises on obstetrics ; a posi-
                                                tion to which its merits justly entitle it.  The style
                                                is such that the descriptions are clear, and each sub-
                                                ject is discussed and elucidated with due regard to
                                                its practical bearings, which cannot fail to make it
                                                acceptable and valuable to both students and prac-
                                                titioners.  We cannot, however, close  this brief
                                                notice without  congratulating the author and the
                                                profession on the production  of such an excellent
                                                treatise.  The author is a western man of whom we
                                                feel proud, and  we cannot but think that his book
                                                will find many readers and warm admirers wherever
                                                obstetrics is taught and studied as a science and an
                                                art.—The Cincinnati Lancet and Observer, Feb. 1858.
                                                  A most respectable and  valuable addition to our
                                                home medical  literature,  and one reflecting credit
                                                alike on the author and the institution to which he
                                                is attached.  The student will find in this work a
                                                most useful guide to his studies; the country prac-
                                                titioner, rusty in his reading, can obtain from its
                                                pages a fair resume of the modern literature of the
                                                science; and we hope to see this American produc-
                                                tion generally consulted  by  the profession.— Va.
                                                Med. Journal, Feb. 1858.
  We congratulate the author that the task is done.
We congratulate him that he has given to the medi-
cal public, a work which will secure for him a high
and permanent position among the standard autho-
rities on  the principles and practice of obstetrics.
Congratulations are not less due to the medical pro-
fession of this country, on the acquisition of a trea-
tise embodying the results of the studies, reflections,
and experience of Prof. Miller.  Few men, if any,
in this country, are more competent than he to write
on this department of medicine.  Engaged for thirty-
five years in an extended practice of obstetrics, for
many years a teacher of this branch of instruction
in one of the largest of our institutions, a diligent
student as well as a careful observer, an original and
independent  thinker, wedded  to  no hobbies, ever
ready to consider without prejudice new views, and
to adopt innovations if they are really improvements,
and withal a  clear,  agreeable writer, a practical
treatise from his pen could not fail to possess great
value. Returning to Prof.  Miller's work we have
only to add that we hope most sincerely it will he in
the hands of every reading and thinking practitioner
of this country.—Buffalo Med Journal, Mar. 1858. 
AND SCIENTIFIC  PUBLICATIONS.                   21
                          MEIGS (CHARLES D.),  M. D.,
             Professor of Obstetrics, Ice. in the Jefferson Medical College, Philadelphia.

OBSTETRICS:  THE  SCIENCE  AND  THE  ART.  Third  edition, revised
  and improved.  With one hundred and twenty-nine illustrations.  In one beautifully printed octavo
  volume, leather, of seven hundred and fifty-two large pages.  $3 75.
  The rapid demand for  another edition of this work is a  sufficient expression of the favorable
verdict of the profession.   In thus preparing it a third time for the press, the author has endeavored
to render it in every respect worthy of the favor which  it has received.   To accomplish this he
has thoroughly revised it in every part.  Some portions have been rewritten, others added, new
illustrations have been in many instances substituted for such as were not  deemed  satisfactory,
while, by an alteration in the typographical arrangement, the size ofthe work has not been increased,
and the price remains unaltered.   In its present improved form, it is,  therefore, hoped that the work
will continue to meet the wants of the American profession  as a sound, practical, and extended
System of Midwifery.
  Though the work has received only five pages of
enlargement, its chapters throughout wear the im-
press of careful revision. Expunging and rewriting,
remodelling its sentences, with occasional new ma-
terial, all evince a lively desire that it shall deserve
to be regarded as improved in manner as well as
matter.  In the matter, every stroke of the pen has
increased the value of the book, both in expungings
and additions —Western Lancet, Jan. 1857.
  The best American work on Midwifery that is
accessible  to the student and practitioner—N. W.
Med. and Surg. Journal, Jan. 1857.
  This is a standard work by a great American Ob-
stetrician.  It is the third and last edition, and, in
the language of the preface, the author has "brought
the subject up to the latest dates  of real improve-
ment in our art and Science."—Nashville Journ. of
Med. and Surg., May, 1857.
                          BY THE same author.  (Lately Issued.)

WOMAN:  HER DISEASES  AND  THEIR  REMEDIES.   A Series of  Lec-
  tures to his Class.  Third and  Improved edition.  In one large and beautifully printed octavo
  volume, leather.    pp. 672.  $3 60.
  The gratifying appreciation of his labors, as evinced by the exhaustion of two large impressions
of this work within a few years, has not been  lost upon the author, who has endeavored in every
way to  render it worthy of the favor with which it has been received.   The opportunity thus
afforded for a second revision has been improved, and the work is now presented as in every way
superior to its predecessors, additions  and  alterations having been made whenever the advance of
science has rendered them desirable.  The typographical execution of  the work will also be found
to have undergone a similar improvement, and the work is now confidently presented as in every
way worthy the position it has acquired as the  standard American text-book on the Diseases of
Females.
  It contains a vast amount of practical knowledge.
by one who has accurately observed and retained
the experience of many years, and who tells the re-
sult in a free, familiar, and pleasant manner.—Dub-
lin Quarterly Journal.
  There is an ofT-hand fervor, a glow, and a warm-
heartedness infecting the eff jrt of Dr. Meigs, which
is  entirely captivating, and which absolutely hur-
ries the reader through from beginning to end. Be-
sides, the book  teems with solid instruction, and
it  shows the very highest evidence of ability, viz.,
the clearness with which the information  is pre-
sented. We know of no better test of one's under-
standing a subject than the evidence of the power
of lucidly explaining it.  The most elementary, as
well  as the obscurest subjects, under the pencil of
Prof. Meigs, are isolated and made to stand out in

                                   BY THE SAME AUTHOR.

ON    THE   NATURE,   SIGNS,  AND   TREATMENT   OF  CHILDBED
  FEVER.  In a Series of Letters  addressed to the Students of his  Class.  In one handsome
  octavo volume, extra cloth, of  365 pages.  $2 50.
such bold relief, as to produce distinct impressions
upon the mind and  memory  ofthe reader. — Th*
Charleston Med. Journal.
  Professor Meigs has enlarged and amended this
great work, for such it  unquestionably is, having
passed the ordeal of criticism at home and abroad,
but been improved thereby ;  for in this new edition
the author has introduced real improvements, and
increased the value  and utility of  the book im-
measurably.  It presents so many  novel, bright,
and sparkling thoughts;  such an exuberance of new
ideas on almost every page, that we confess our-
selves to have become  enamored  with the book
and its author; and cannot withhold our congratu-
lations from our Philadelphia confreres, that such a
teacher is in their service.—N. Y. Med. Gazette.
  The instructive and interesting author of this
work, whose previous labors in the department of
medicine which he so sedulously cultivates, have
placed his countrymen under deep and abiding obli-
gations, again challenges their admiration  in the
fresh and vigorous, attractive and racy pages before
us.  It. is a delectable book. * *  * This treatise
upon child-bed fevers will have an extensive sale,
being destined, as it deserves, to find a place in the
library of every practitioner who scorns to lag in the
rear.—Nashville Journal of Medicine and Surgery.

                       BY THE SAME AUTHOR ',  WITH COLORED PLATES.

A TREATISE  ON ACUTE AND CHRONIC DISEASES  OF  THE  NECK
  OF THE UTERUS.  With numerous plates, drawn and colored from nature in the highest
  style of art.  In one handsome octavo volume, extra cloth.  $4 50.
  This book will add more to his fame than either
of those which bear his name.  Indeed we doubt
whether any material improvement will be made on
the teachings of this volume for a century to come,
since it is so eminently practical, and based on  pro-
found knowledge of the science  and  consummate
skill in the art of healing, and ratified by an ample
and extensive experience, such as few men have the
industry or good  fortune to acquire.—N. Y. Med.
Gazette.
MAYNE'S DISPENSATORY  AND   THERA-
  PEUTICAL REMEMBRANCER.  Comprising
  the  entire lists of Materia Medica, with every
  Practical Formula contained in the three British
  Pharmacopoeias.  Edited, with the addition ofthe
  Formula? of the U. S. Pharmacopoeia, by R. E.
  Griffith,M.D.  112mo. vol. ex.cl.,300pp. 75 c.
MALGAIGNE'S OPERATIVE SURGERY, based
  on Normal and Pathological Anatomy.   Trans-
  lated from the French by Frederick Brittan,
  A. B.,M. D. Withnumerousillustrationsonwood.
  In one handsome octavo volume, extra cloth, of
  nearly six hundred pages.  $2 25. 
22                     BLANCHARD  &  LEA'S  MEDICAL
                        MACLISE  (JOSEPH),  SURGEON.
SURGICAL  ANATOMY.   Forming one  volume,  very large  imperial  quarto.
  With sixty-eight large and splendid Plates, drawn in the best style and beautifully colored.  Con-
  taining one hundred and ninety Figures, many of them the size of life.  Together with copious
  and explanatory letter-press.  Strongly and handsomely bound in  extra cloth, being one of the
  cheapest and best executed Surgical works as yet issued in this country.  $11 00.
  %* The size of this work prevents its transmission through the post-office as a whole, but those
who desire to have  copies forwarded by mail, can receive them  in five parts, done up in  stout
wrappers.  Price $9 00.
                                                 Of great value to the student engaged in dissect-
                                               ing, and to the surgeon at a distance from the means
                                               of keeping up his anatomical knowledge.—Medical
                                               Times.
                                                 A work which has no parallel in point of accu-
                                               racy and cheapness in the English language.—N. Y.
                                               Journal of Medicine.
                                                 To all engaged in the study or practice of theii
                                               profession, such a work is almost indispensable.—
                                               Dublin Quarterly Medical Journal.

                                                 Country practitioners will find these plates of im-
                                               mense value.—N. Y. Medical Gazette.

                                                 We are  extremely gratified  to  announce to the
                                               profession  the completion of this truly magnificent
                                               work, which, as a whole,  certainly stands unri-
                                               valled, both for accuracy of drawing, beauty of
                                               coloring, and all the requisite explanations of the
                                               subject in hand.—The New Orleans Medical and
                                               Surgical Journal.

                                                 This is by far the ablest work on Surgical Ana-
                                               tomy  that has come under our observation.  We
                                               know of no other work that would justify a stu-
                                               dent, in any degree, for neglect of actual dissec-
                                               tion.  In those sudden emergencies that so often
                                               arise, and which require the instantaneous command
                                               of minute anatomical knowledge, a work of this kind
                                               keeps the details ofthe dissecting-room perpetually
                                               fresh in the memory.—The Western Journal of Medi-
                                               cine and Surgery.

        The very low price at which this work is furnished, and the beauty of its execution,
require an extended sale to compensate the publishers for the heavy expenses incurred.

                            MORLAND  (W. W.),  M.  D.
                        Fellow of the Massachusetts Medical Society, &c.
DISEASES OF THE  URINARY ORGANS; a  Compendium  of their Diagnosis,
  Pathology, and Treatment.  With illustrations.   In one large  and handsome octavo volume, of
  about 600 pages, extra cloth.  (Now ready, Oct. 1858.)  Price $3 50.
  This volume, it is hoped, will supply the want of a work  presenting within convenient compass
the whole subject of the diseases to which all the urinary organs are liable, with their treatment,
both medical and surgical.   The aim ofthe author has heen  throughout to condense the results of
the most recent investigations in a clear and succinct manner,  omitting nothing of practical  im-
portance, without,  at the same time, embarrassing  the student with unnecessary speculations.
Various elaborate and  important works have recently appeared on different departments of the
subject, but none, it is believed, which thoroughly covers the whole  ground  in  the manner which
Dr. Morland has attempted.

                             MACKENZIE (W.), M.D.,
                 Surgeon Oculist in Scotland in ordinary to Her Majesty, &c. &c.
A  PRACTICAL TREATISE  ON  DISEASES  AND INJURIES OF  THE
  EYE.  To which is prefixed an Anatomical Introduction explanatory of a Horizontal Section of
  the Human Eyeball, by Thomas  Wharton Jones, F. R. S.  From the Fourth Revised and En-
  larged London Edition. With Notes and Additions by Addinell Hewson,  M. D., Surgeon to
  Wills Hospital, &c. &c. In one very large and handsome octavo volume, leather, raised bands, with
  plates and numerous  wood-cuts.  $5 25.
  One of the greatest artistic triumphs of the age
in Surgical Anatomy.—British American Medical
Journal.
  Too much cannot be said in its praise; indeed,
we have not language to do it justice.—Ohio Medi-
cal and Surgical Journal.
  The most admirable surgical atlas we have seen.
To the practitioner deprived of demonstrative dis-
sections upon the human Bubject, it is an invaluable
companion.—N. J. Medical Reporter.
  The most accurately engraved and beautifully
colored plates  we have ever seen in an American
book—one of the best and cheapest surgical works
ever published.—Buffalo Medical Journal.
  It is very rare that so elegantly printed, so well
illustrated, and so useful  a work, is offered at so
moderate a price.—Charleston Medical Journal.
  Its plates can boast a superiority which places
them almost beyond the reach of competition.—Medi-
cal Examiner.
  Every practitioner, we think, should have a work
of this kind within reach.—Southern Medical and
Surgical Journal.
  No such lithographic illustrations of surgical re-
gions have hitherto, we think, been given.—Boston
Medical and Surgical Journal.
  As a surgical  anatomist, Mr. Maclise has proba-
bly no superior—British and Foreign Medico-Chi-
rurgical Review.
  The treatise of Dr. Mackenzie indisputably holds
the first place, and forms, in respect of learning and
research, an Encyclopaedia unequalled in extent by
any other work ofthe kind, either English or foreign.
—Dixon on Diseases ofthe Eye.

  Few modern books on any department of medicine
or surgery have met with such extended circulation,
or have procured for their authors a like amount of
European celebrity.  The immense research which
it displayed, the thorough  acquaintance with the
subject, practically as well as theoretically, and the
able manner in which the author's stores of learning
and experience were rendered available for general
use, at once procured for the first edition, as well on
the continent as in this country, that high position
as a standard work which each successive edition
has more firmly established, in spite of the attrac-
tions of several rivals of no mean ability.  We con-
sider it the duty of every one who has the love of his
profession and the welfare of his patient at heart, to
make himself familiar with this the most complete
work in the English language upon the diseases of
the eye.—Med. Times and Gazette.
  The fourth edition of this standard work will no
doubt be as fully appreciated as the three former edi-
tions.  It is unnecessary to  say a word in its praise,
for the verdict has  already been passed upon it by
the most competent judges, and " Mackenzie on the
Eye" has justly obtained a reputation which it is
no figure of speech to call world-wide.—British and
Foreign Medico-Chirurgical Review.
  This new  edition of Dr.  Mackenzie's celebrated
treatise on diseases of the eye, is truly a miracle of
industry and learning.  We need scarcely  say  that
he has entirely exhausted the subject of his specialty.
—Dublin Quarterly Journal. 
AND SCIENTIFIC PUBLICATIONS.
23
                           MILLER (JAMES),  F. R. S. E.,
DDTXTrnm        Professor of Surgery in the University of Edinburgh, &c.

  idSSJ h  ?S  0F  SURGE^Y.   Fourth American, from the third and revised
  hundrerUnH r!!!?"'      °"e '^  and veT beautiful volume, leather, of 700 pages, with two
  nuntlred and forty exquisite illustrations on wood.  $3 75.
  The work of Mr. Miller is too well and too favor-
ably known among us, as one of our best text-books,
to render any further notice of it necessary than the
announcement  of a new edition, the fourth in our
country, a proof of its extensive circulation among
us.  As a concise and reliable exposition of the sci-
ence of modern surgery, it stands deservedly hieh—
we know not its superior.—Boston Med. and  Surg.
Journal.
  It presents the most satisfactory exposition ofthe
modern doctrines of the principles of surgery to be
found many volume in any language.—N. Y. Journal
of Medicine.
  The work takes rank with Watson's Practice of
Physic; it certainly does not fall behind that great
work in soundness of principle or depth of reason-
ing and research.  No physician who values his re-

                           by the same author.  (Now Ready.)

THE  PRACTICE  OF  SURGERY.    Fourth  American  from the  last  Edin-
  burgh edition.   Revised  by the American editor.  Illustrated by three hundred and sixty-four
  engravings on wood.  In one large octavo volume, leather, of nearly 700 pages.  $3 75.
putation, or seeks the interests of his clients, can
acquit himself before his God and the world without
making himself familiar with the sound and philo-
sophical views developed in the foregoing book.—
New Orleans Med. and Surg. Journal.
  Without doubt the ablest exposition of the prin-
ciples of that branch of the  healing art in any lan-
guage. This opinion, deliberately formed after a
careful study of the  first edition, we have had no
cause to change on  examining the second.  This
edition has  undergone thorough revision by the au-
thor; many expressions  have been  modified, and a
mass of new matter introduced.  The book is got up
in the finest style, and is an evidence of the progress
of typography in  our country.—Charleston Medical
Journal and Review.
  No encomium of ours could add  to the popularity
of Miller's Surgery.  Its reputation in this country
is unsurpassed by that of any other work, and, when
taken in connection with the author's Principles of
Surgery, constitutes  a whole, without reference to
to which no conscientious surgeon would be willing
practice his art.—Southern Med. and Surg. Journal.
  It is seldom that two volumes have ever made so
profound  an  impression in so short a time as  the
"Principles" and the "Practice" of Surgery by
Mr. Miller—or so richly merited the reputation they
have acquired.   The author is an eminently sensi-
ble, practical, and well-informed man, who knows
exactly what he is talking about and exactly how to
talk it.—Kentucky Medical Recorder.
  By the almost unanimous voice of the profession,
his works, both on the principles and practice of
surgery have been assigned the highest rank.  If we
were limited to but one work on surgery, that one
should be Miller's, as we regard it as superior to all
others.—St. Louis Med. and Surg. Journal.

  The author has in this and his " Principles," pre-
sented to the profession one of the most complete and
reliable systems of Surgery extant.  His style of
writing is original, impressive, and engaging, ener-
getic, concise, and lucid. Few have the faculty of
condensing so much in small space, and at the same
time so persistently holding the attention. Whether
as a text-book for  students  or a book of reference
for practitioners, it cannot be too strongly recom-
mended.—Southern Journal of Med. and Physical
Sciences.
               MONTGOMERY (W. F.),  M. D., M. R. I. A., &c,
        Professor of Midwifery in the King and Queen's College of Physicians in Ireland, &c.

AN  EXPOSITION OF THE SIGNS  AND SYMPTOMS  OF  PREGNANCY.
  With some other Papers on Subjects connected with Midwifery.  From the second and enlarged
  English edition.  With two  exquisite colored  plates, and numerous wood-cuts.   In one  very
  handsome octavo volume, extra cloth, of nearly 600 pages.  (Just Issued, 1857.)  $3 75.
  The present edition of this classical volume is fairly entitled to be regarded as anew work, every
sentence having been carefully  rewritten, and the whole increased to more 1han double the original
size.   The title of the  work scarcely  does justice  to the extent and importance of the topics
brought under consideration, embracing, with  the exception  of the operative procedures of  mid-
wifery, almost everything connected with obstelries, either directly or incidentally; and there are
few physicians who will not find in  its pages much that will  prove of great interest and value in
their daily practice.  The special Essays on the Period of Human Gestation, the Signs of Delivery,
and the Spontaneous Amputation and other Lesions of the Foetus in Utero present topics ofthe
highest interest fully treated and beautifully illustrated.
  In every point of mechanical execution the work will be found one of the handsomest yet issued
from the American press.
  A book unusually rich in practical suggestions.—
Am. Journal Med. Sciences, Jan. 1857.
  These several  subjects so interesting  in them-
selves, and so important, every one of them, to the
most delicate and precious of social relations, con-
trolling often the honor and domestic peace  of a
family, the legitimacy of offspring, or the  life of its
parent, are all treated with an elegance of diction,
fulness of illustrations, acuteness and justice of rea-
soning, unparalleled in obstetrics, and unsurpassed in
medicine.  The reader's  interest can never flag, so
fresh, and vigorous, and classical is  our author's
style;  and one forgets,  in the renewed  charm of
every page, that it, and every line, and every word
has been weighed and reweighed through years of
preparation; that this is of all others the book of
Obstetric Law, on each of its several topics; on all
points connected with pregnancy, to be everywhere
received as a manual of special  jurisprudence, at
once announcing fact, affording argument, establish-
ing precedent, and governing alike the juryman, ad-
vocate, and judge.  It is not merely in its legal re-
lations that we find this work so interesting.  Hardly
a page but that has  its hints or facts important to
the general practitioner; and not a chapter without
especial matter for the  anatomist, physiologist, or
pathologist. — iV. A. Med.-Chir. Review.  March,
1857.
      MOHR (FRANCIS), PH.  D., AND REDWOOD (THEOPH I LUS).
PRACTICAL   PHARMACY.   Comprising the Arrangements,  Apparatus, and
  Manipulations of the Pharmaceutical Shop and Laboratory.  Edited, with extensive Additions,
  by  Prof. William Procter, of the Philadelphia  College  of Pharmacy.  In one handsomely
  printed octavo volume, extra cloth, of 570  pages, with over 500 engravings on wood.   $2 75. 
24
BLANCHARD  &  LEA'S MEDICAL
                               NEILL (JOHN), M. D.,
                         Surgeon to the Pennsylvania Hospital, &c.; and

                      FRANCIS  GURNEY  SMITH, M.D.,
              Professor of Institutes of Medicine in the Pennsylvania Medical College.

AN  ANALYTICAL  COMPENDIUM   OF  THE   VARIOUS   BRANCHES
  OF MEDICAL SCIENCE ; for the Use and Examination of  Students.  A new edition, revised
  and improved.  In one very large and  handsomely printed  royal  12mo. volume, of about one
  thousand pages, with 374 wood-cuts.  Strongly bound in leather, with raised bands.  $3 00.
  The very flattering reception which has been accorded to this work, and the high estimate placed
upon it by the profession, as evinced by the constant and increasing demand which has rapidly ex-
hausted two large editions, have stimulated the authors to render the volume in its present revision
more worthy of the success which has attended it.  It has accordingly been thoroughly examined,
and such errors as had on former occasions escaped observation  have been corrected, and whatever
additions were necessary to  maintain it on a level with the advance of science have been introduced.
The extended series of illustrations has been  still further increased and much improved, while, by
a slight enlargement of the page, these various additions have been incorporated without increasing
the bulk of the volume.
  The work is, therefore, again presented as eminently worthy of the favor with which  it has hitherto
been received.  As a book for daily reference by the student requiring a guide to his more elaborate
text-books, as a manual for preceptors desiring to stimulate their students by frequent and accurate
examination, or as a source from which the practitioners of older date may easily and cheaply acquire
a knowledge ofthe changes and improvement in professional science, its reputation is permanently
established. -
  The best work of the kind with which we are
acquainted.—Med. Examiner.
  Having made free use of this volume in our ex-
aminations of pupils, we  can speak from experi-
ence in recommending it as an admirable compend
for students, and as especially useful to preceptors
who examine their pupils.  It will save the teacher
much labor by enabling him  readily to recall all of
the points  upon which  his  pupils should be ex-
amined.   A work of this sort should be in the hands
of every one who takes pupils into his office with a
view of examining them ; and this is unquestionably
the best of its class.—Transylvania Med. Journal.
In the rapid course of lectures, where work for
the students is heavy, and review necessary for an
examination, a compend is not only valuable, but
it is almost a sine qua non.  The one before us is,
in most of the divisions, the most unexceptionable
of all books of the kind that we know of.   The
newest and soundest doctrines and the latest im-
provements and discoveries are explicitly, though
concisely, laid before the student. There is a class
to whom we very sincerely commend this cheap book
as worth its weight in silver—that class is the gradu-
ates in medicine of more than ten years' standing,
who  have not studied medicine since.  They  will
perhaps find out from it that the science is not exactly
now what it was when they left it off.—The Stetho-
scope.
                              NEILL (JOHN),  M. D.,
                  Professor of Surgery in the Pennsylvania Medical College, &c.
OUTLINES OF THE VEINS AND LYMPHATICS.   With handsome colored
  plates.  1 vol., cloth. $1 25.
OUTLINES OF THE  NERVES.   With handsome plates.   1  vol., cloth. $1 25.
               NELIGAN (J.  MOORE), M. D., M. R.  I.A., &.C.
                             (A splendid work.  Just Issued.)

ATLAS OF CUTANEOUS DISEASES.   In  one beautiful quarto volume, extra
  cloth, with splendid colored plates, presenting nearly one hundred elaborate representations of
  disease.  $4 50.
  This beautiful volume is intended as a complete and accurate representation of all the varieties
of Diseases of the Skin.  While it can be consulted in conjunction with any work on Practice, it has
especial reference to the author's "Treatise on Diseases ofthe Skin," so favorably received by the
profession some years since.  The publishers feel justified in saying that few more beautifully exe-
cuted plates have ever been presented  to the profession of this country.
  A compend which will very much aid the practi-
tioner in this difficult branch of diagnosis.  Taken
with the beautiful plates of the Atlae,  which are
remarkable for their accuracy and beauty of color-
ing, it constitutes  a  very valuable addition to the
library of a practical man.— Buffalo Med. Journal,
Sept. 1856.
  Nothing is often more difficult than the diagnosis
of disease ofthe skin; and hitherto, the only works
containing illustrations have been at rather incon-
venient prices—prices, indeed, that prevented gene-
ral use.   The work before  us will supply a want
Ion? felt, and minister to a more perfect acquaintance
with the  nature and treatment of  a very frequent
and troublesome form of disease.—Ohio Med. and
Surg. Journal, July, 1856.

  Neligan's Atlas of Cutaneous Diseases supplies a
long existent desideratum much felt by the largest
class of our profession. It presents, in quarto Bize,
16 plates, each containing  from 3 to 6 figures, and
forming in all a total of 90 distinct representations
of the different species of  skin affections, grouped
together in genera or families.  The  illustrations
have been taken from nature, and have been copied
with such fidelity that they present a striking picture
of life; in which the reduced scale aptly serves to
give, at a coup d'ail, the  remarkable peculiarities
of each individual variety.   And while thus the dis
ease is rendered more definable, there is yet no loss
of proportion incurred by the necessary concentra-
tion.  Each figure is highly colored, and so truthful
has the artist been that the mostfastid'ous observer
could not justly take exception to the correctness of
the execution of the pictures under his scrutiny.—
Montreal Med. Chronicle.
                                  BY THE SAME AUTHOR.
I  PRACTICAL  TREATISE  ON  DISEASES  OF  THE  SKIN.   Second
 American edition.  In one neat royal 12mo. volume, extra cloth, of 334 pages.  $1 00.
  S®~ The two volumes will be sent by mail on receipt of Five Dollars.
OWEN ON THE DIFFERENT FORMS OF I
  THE SKELETON,  AND OF THE TEETH.
One vol. royal 12mo., extra cloth, with numerous
illustrations.  SI 25. 
AND SCIENTIFIC  PUBLICATIONS.                   25
                              PEASLEE (E.  R.), M. D.,
          Professor of Physiology and General Pathology in the New York Medical College.

HUMAN HISTOLOGY, in its relations to Anatomy, Physiology, and Pathology;
  for the use of Medical Students.   With four hundred and thirty-four illustrations.  In one hand-
  some octavo volume, of over 600 pages.   (Now Ready, 1858.)  $3 75.
  It embraces a library upon the topics discussed i ceive this contributnn to physiological science,
within itself, and is just what the teacher and learner "  Not with vain thanks, but with acceptance boun-
need.  Another advantage, by no means to be over-
looked, everything of real value in the wide range
which it embraces, is with great skill compressed
into an octavo volume of but little more than six
hundred pages.  We have not only the whole  sub-
ject of Histology, interesting in itself, ably and fully
discussed, but what is of infinitely greater  interest
to the student, because of greater practical value,
are its relations to Anatomy, Physiology, and Pa-
thology, which are here fully and satisfactorily set
forth.  These great supporting branches of practical
medicine are thus linked together, and while estab-
lishing and illustrating each other, are interwoven
into a harmonious whole.  We commend the work
to students and physicians generally. — Nashville
Journ. of Med. and Surgery, Dec. 1857.
  It far surpasses our expectation.  We never  con-
ceived the possibility of compressing so much valu-
able information into so compact a form.  We will
not consume space with commendations.  We re-
teous."   We have already  paid it the  practical
compliment of making abundant use of it in the
preparation of our lectures, and also of recommend-
ing its further perusal most cordially to our alumni;
a recommendation which we now  extend  to  our
readers.—Memphis Med. Recorder, Jan. 185S.
  We would recommend it to the medical student
and practitioner, as containing a summary of all that
is known ofthe important subjects which it treats;
of all that is contained in the great works of Simon
and Lehmann, and the organic chemists in general.
Master this one volume, we would say to the mtdical
student and practitioner—master this book and you
know all that is known of the great fundamental
principles of medicine, and we have io hesitation
in saying that it is an honor to the American medi-
cal profession that one of its members should have
produced it__St. Louis Med. and Surg. Journal,
March, 1858.
             PEREIRA (JONATHAN), M.  D., F.  R. S., AND  L. S.

THE   ELEMENTS  OF  MATERIA   MEDICA  AND   THERAPEUTICS.
  Third American edition, enlarged and improved by the author; including Notices of most of the
  Medicinal Substances in use in the civilized world, and forming an Encyclopaedia of Materia
  Medica.  Edited, with Additions, by Joseph Carson, M. D., Professor of Materia Medica and
  Pharmacy in the University of Pennsylvania.  In two very large octavo volumes of 2100 pages,
  on small type, with about  500 illustrations on stone and wood, strongly bound in leather, with
  raised bands.  $9 00.
                               PARRISH  (EDWARD),
  Lecturer on Practical Pharmacy and Materia Medica in the Pennsylvania Academy of Medicine, &c.

AN INTRODUCTION  TO  PRACTICAL  PHARMACY.  Designed as a  Text-
  Book lor  the  Student, and as a Guide for the Physician  and Pharmaceutist.  With many For-
  mulae and Prescriptions.  In one handsome octavo volume, extra cloth, of 550 pages, with 243
  Illustrations.  $2 75.
  A careful examination of this work enables us to
speak of it in the highest terms, as being the best
treatise on practical pharmacy with which we are
acquainted, and an invaluable vade-mecum, not only
to the apothecary and to those practitioners who are
accustomed to prepare their own medicines, but to
every medical man and medical student.  Through-
out the work are interspersed valuable tables, useful
formulae, and practical hints, and the whole is illus-
trated by a large number of excellent wood-engrav-
ings.—Boston Med. and Surg. Journal.
T This is altogether one ofthe most useful books we
have seen.  It is just what we have long felt to be
needed  by apothecaries, students, and practitioners
of medicine, most of whom in this country have to
put up their own prescriptions.  It bears, upon every
page, the impress of practical knowledge, conveyed
in a plain common sense manner, and adapted to the
comprehension  of all who may  read it.  No detail
has been omitted, however trivial it may seem, al-
though really important to the dispenser of medicine.
—Southern Med. and Surg. Journal.
  To both the country practitioner and the city apo-
thecary this  work of Mr. Parrish  is a godsend.  A
eareful study of its contents will give  the young
graduate a familiarity with  the value and mode  of
administering his prescuptions, which will be of as
much use to his patient as to  himself.— Va. Med.
Journal.
  Mr. Parrish has rendered a very acceptable service
to the practitioner and student, by furnishing this
book, which contains the leading facts and principles
of the science of Pharmacy, conveniently arranged
for study, and with special reference to those features
of the subject which possess an especial practical in-
terest to the physician. It furnishes  the student, at
the  commencement  of his studies, with that infor-
mation which is of the greatest importance in ini-
tiating him into the domain of Chemistry and Materia
Medica; it familiarizes him with the compounding
of drugs, and supplies those minutiae  which but few
practitioners can impart.  The  junior  practitioner
will, also, find this volume replete with instruction.
—Charleston Med. Journal and  Review, Mar. 1856.

  There is no useful information in the details of the
apothecary's or country physician's office conducted
according to science  that is omitted.  The young
physician will find it an encyclopedia of indispensa-
ble  medical knowledge, from the purchase of a spa-
tula to the compounding  of  the most learned pre-
scriptions. The work is by the ablest pharmaceutist
in the United States, and must meet with an im-
mense sale.—Nashville Journal of Medicine, April,
1856.

  We are glad to receive this excellent work.  It
will supply a want long felt by  the profession, and
especially by the student ot Pharmacy.  A  large
majority of physicians are obliged to compound
their own medicines, and to them a work of this
kind is indispensable.—N. O. Medical and Surgical
Journal.
                              PARKER (LANGSTON),
                         Surgeon to the Queen's Hospital, Birmingham.

THE MODERN  TREATMENT OF SYPHILITIC  DISEASES, BOTH  PRI-
  MARY AND SECONDARY;  comprising the Treatment of Constitutional and Confirmed Syphi-
  lis  by a safe and successful method.  With numerous Cases, Formulae, and Clinical Observa-
  tions.  From the Third and entirely rewritten  London edition.  In  one neat  octavo volume,
  extra cloth, of 316 pages.   $1 75. 
26
BL.ANCHARD &  LEA'S  MEDICAL
                        PIRRIE (WILLIAM), F.  R. S. E.,
                      Professor of Surgery in the University of Aberdeen.

THE  PRINCIPLES  AND  PRACTICE  OF  SURGERY.   Edited by John
  Neill, M. D., Professor of Surgery in the Penna. Medical College, Surgeon to the Pennsylvania
  Hospital, &c.  In one very handsome octavo volume, leather, of 780 pages, with 316 illustrations,
  $3 75.
  We know of no other surgical work of a reason-
able size, wherein there is so much theory and prac-
tice, or where subjects are more soundly or clearly
taught.—The Stethoscope.
  There  is scarcely a disease of the bones or soft
parts, fracture, or dislocation, that is not illustrated
by accurate wood-engravings.  Then, again, every
instrument employed hy the surgeon is thus repre-
sented.  These engravings are not only correct, but
really beautiful, showing the astonishing degree of
perfection to which the art of wood-engraving has
arrived.  Prof. Pirrie, in the work before us, has
elaborately discussed the principles of surgery, and
a safe and effectual practice predicated upon them.
Perhaps no work upon this subject heretofore issued
is so full upon the science of the art of surgery.—
Nashville Journal of Medicine and Surgery.
                               RICORD (P.), M. D.,
A TREATISE ON THE VENEREAL DISEASE.   By John Hunter, F. R. S.
  With copious Additions, by Ph. Ricord, M. D.  Translated and Edited, with Notes, by Freeman
  J. Bumstead, M. D , Lecturer on Venereal at the College of Physicians and Surgeons, New York.
  Second edition, revised, containing presume of Ricord's  Recent Lectures on Chancre.  In
  one handsome octavo volume, extra cloth, of 550 pages, with eight plates.  $3 25. (Now Ready,
  December, 1858.)
  In revising this work, the editor has endeavored to introduce whatever matter of interest the re-
cent investigations of syphilographers have added to our knowledge of the subject.  The principal
source from which this has been derived is the volume of "Lectures on Chancre," published a few
months since by M. Ricord, which affords a large amount of new and instructive material on many
controverted points.   In the previous edition, M. Ricord's additions amounted to nearly one-third
ofthe whole, and with the matter now introduced, the work may be considered to present his views
and experience more thoroughly and completely than any other.  The value of the original treatise
of Mr. Hunter is too well known to require praise.  Perhaps no medical work in the English lan-
guage has so thoroughly stood the test of time, or has  so completely assumed the position  of a
cla-sie, and a volume like ihe present, containing the united  labors of the highest  authorities on so
difficult and important a subject, becomes indispensable  to all who desire  to keep themselves on a
level with the progress of medical science.
  Every  one will recognize the attractiveness and
value which this work derives from thus presenting
the opinions of these two masters side by side. But,
it must be admitted, what has  made the fortune of
the book, is the fact that it contains the "most com-
plete embodiment of the veritable doctrines of  the
Hopital du Midi," which has ever been made public.
The doctrinal ideas  of M. Ricord, ideas which, if not
universally adopted, are inconteslably dominant, have
heretofore only been interpreted by more or less skilful
                                  BY THE SAME AUTHOR.
RICORD'S  LETTERS ON SYPHILIS.  Translated by  W. P. Lattimore, M. D.
   In one neat octavo volume, of 270 pages, extra cloth.  $2 00.

                         ROKITANSKY  (CARL), M.D.,
    Curator of the Imperial Pathological Museum, and Professor at the University of Vienna,  &c.
A MANUAL OF  PATHOLOGICAL  ANATOMY.   Four  volumes,  octavo,
secretaries, sometimes accredited and sometimes not.
  In the notes to Hunter, the master substitutes him-
self for his interpreters, and gives hisoriginal thoughts
lo the world in a lucid and perfectly intelligible man-
ner.  In conclusion we can say that this is incon-
testably the best treatise on syphilis with which we
are acquainted, and, as we do not often employ the
phrase, we may be excused for expressing the hope
that it may find a place in the library of every phy-
sician.— Virginia Med. and Surg, Journal.
  bound in two, extra cloth, of about 1200 pages.
  kins, C. H. Moore, and G. E. Day.  $5 50
  The profession is too well acquainted with the re-
putation of Rokitansky's work to need our assur-
ance that this is one of the most profound, thorough,
and valuable books  ever issued from the medical
press.  It is sui generis, and has no standard of com-
parison. It is only necessary to announce that it is
issued in a form as cheap as is compatible with its
size and preservation, and its  sale follows as a
matter  of course. No library can be called com-
plete without it.—Buffalo Med. Journal.

  An attempt to give our readers any adequate idea
of the  vast amount  of instruction accumulated in
these volumes, would be feeble and hopeless. The
effort  of the distinguished author to concentrate
in a small space his great fund of knowledge, has
 Translated by W. E. Swaine, Edward Sieve-


bo charged his text with valuable truths, that any
attempt of a reviewer to epitomize is at once para-
lyzed, and must end in a failure.—Western Lancet.
  As this is the highest source of knowledge upon
the  important subject of which it  treats, no real
student can afford to be without it.  The American
publishers have entitled themselves to the thanks of
the profession of their country, for this timeous and
beautiful edition.—Nashville Journal of Medicine.
  As a book of reference, therefore, this work must
prove of inestimable value, and we cannot too highly
recommend it to the profession.—Charleston Med.
Journal and Review.
  This book is a necessity to every practitioner.—
Am. Med. Monthly.
                           RIGBY  (EDWARD),  M.D.,
                    Senior Physician to the General Lying-in Hospital, &c.
A  SYSTEM  OF  MIDWIFERY.  With Notes and  Additional  Illustrations.
  Second American Edition.  One volume octavo, extra cloth, 422 pages.  $2 50.
                        by the same author.  (Now Ready, 1857.)
ON THE  CONSTITUTIONAL  TREATMENT  OF  FEMALE  DISEASES.
  In one neat royal 12mo. volume, extra cloth, of about 250 pages.  $1  00.

ROYLE'S  MATERIA  MEDICA AND  THERAPEUTICS;  including  the
  Preparations of the Pharmacopoeias of London, Edinburgh, Dublin, and of the United States.
  With many new medicines.  Edited by Joseph Carson, M. D.  With ninety-eight illustrations.
  In one large octavo volume, extra cloth, of about 700 pages.  $3  00. 
AND  SCIENTIFIC  PUBLICATIONS
27
                     RAMSBOTHAM  (FRANCIS  H.), M.D.
THE PRINCIPLES  AND PRACTICE  OF  OBSTETRIC  MEDICINE AND
  SURGERY, in reference to the Process of Parturition.  A new and enlarged edition, thoroughly
  revised by the Author.  With Additions by W. V. Keating, M. D.  In one large and handsome
  imperial octavo volume, of 650  pages, strongly bound in leather, with raised bands;  with sixty-
  four beautiful Plates, and numerous Wood-cuts in the  text, containing in all nearly two hundred
  large and beautiful figures. $5 00.
  In calling the attention of the profession to the new edition of this standard work, the publishers
would remark that no efforts have been spared to secure for it a continuance and extension of the
remarkable favor .with which it has been received.   The last London issue, which was considera-
bly enlarged, has received a further revision from the author, especially for this  country.  Its pas-
sage through the press here has  been supervised by Dr. Keating, who has made numerous addi-
tions with a view  of presenting  more fully whatever was necessary to adapt it thoroughly to
American modes of practice. In its mechanical execution, n like superiority over former editions
will be found.
                          From Prof. Hodge, of the University of Pa.
  To the American public, it is most valuable, from its intrinsic undoubted excellence, and as being
the best authorized exponent of British Midwifery. Its circulation will, I trust, be extensive throughout
our country.
  The publishers have shown their appreciation of
the merits of this work and secured its success by
the truly elegant style in which they have brought
it out, excelling themselves in its production, espe-
cially in its plates.  It is dedicated to Prof. Meigs,
and  has the emphatic endorsement of Prof. Hodge,
as the best exponent of British Midwifery.  We
know of no text-book which deserves in all respects
to be more highly recommended to students, and we
could wish to see it in the hands of every practitioner,
for they will find it invaluable for reference.—Med.
Gazette.
  But once in a long time some brilliant genius rears
his head above  the horizon of science, and illumi-
nates and purifies every department that he investi-
gates j and his works become types, by which innu-
merable imitators model their feeble productions.
Such a genius we find in the younger Ramsbotham,
and such a type we find in the work now before us.
The binding, paper, type, the engravings and  wood-
cuts are all so excellent as to make this book one of
the finest specimens of the art of printing that have
given such a world-wide reputation to  its  enter-
prising and liberal publishers. We welcome  Rams-
botham's Principles and Practice of Obstetric Medi-
cine and  Surgery to our library, and confidently
recommend it to our readers, with the assurance
that it will not disappoint their most sanguine ex-
pectations.—Western Lancet.
  It is unnecessary to say anything in regard to the
utility of this work.  It is already appreciated in our
country for the value of the matter, the clearness of
its style, and the fulness of its illustrations. To the
physician's library it is indispensable, while to the
student as a  text-book, from which to extract the
material for laying the foundation of an education on
obstetrical science, it has no superior.—Ohio Med.
and Surg. Journal.

  We will only add that the student will learn from
it all he need to know, and the practitioner will find
it, as a book of reference, surpassed by none other.—
Stethoscope.
  The character and merits of Dr. Ramsbotham's
work are  so well known and thoroughly established,
that comment is unnecessary and praise superfluous.
The illustrations, which are numerous and accurate,
are executed in the highest style of art. We cannot
too highly recommend the work to our readers.—St.
Louis Med. and Surg. Journal.
                      SCHOEDLER (FRIEDRICH), PH.D.,
                        Professor of the Natural Sciences at Worms, &c.

THE BOOK  OF  NATURE; an Elementary Introduction  to the  Sciences  of
  Physics, Astronomy, Chemistry, Mineralogy, Geology, Botany, Zoology, and Physiology.  First
  American  edition, with a Glossary and other Additions and Improvements; from  the second
  English edition.  Translated from  the  sixth German edition, by Henry Medlock, F. C. S., &c.
  In one volume, small octavo, extra  cloth, pp. 692, with 679 illustrations.  $1 80.

                           SMITH   (HENRY  H.), M.D.,
                   Professor of Surgery in the University of Pennsylvania, &c.

MINOR SURGERY;  or, Hints on the Every-day Duties of the Surgeon.   Illus-
  trated by two hundred and forty-seven illustrations.  Third  and enlarged edition.  In one hand-
  some royal 12mo. volume,   pp.  456.  In leather, $2 25; extra cloth, $2 00.
                               BY  THE SAME AUTHOR, AND
                        HORNER (WILLIAM  E.),  M.D.,
                  Late Professor of Anatomy in the University of Pennsylvania.

AN ANATOMICAL ATLAS, illustrative of the Structure of the Human Body.
  In one  volume, large imperial octavo, extra cloth, with about six hundred and fifty beautiful
  figures.  $3  00.
  These figures are well selected,  and  present a
complete and accurate representation of  that won-
derful fabric, the  human body.  The plan of  this
Atlas, which renders it so  peculiarly convenient
for the student, and its superb artistical execution,
have been already pointed out.  We must congratu-
late the student upon the completion of this Atlas,
as it is the most convenient work of the kind that
has yet appeared ; and we must add, the very beau-
tiful manner in which it is "got up" is so creditable
to the country  as to be flattering to our  national
pride.—American Medical Journal.
                           SMITH (W.  TYLER),  M. D.,
                       Physician Accoucheur to St. Mary's Hospital, &c.
ON  PARTURITION,   AND   THE  PRINCIPLES  AND  PRACTICE  OF
  OBSTETRICS.  In one royal 12mo. volume, extra cloth, of 400 pages.  $1 25.
BY THE SAME AUTHOR.
A PRACTICAL TREATISE  ON THE  PATHOLOGY AND TREATMENT
  nv t FUOORRHCEA.  With numerous illustrations.  In one very handsome octavo volume
  extra cloth, of about 250 pages.  $1 50. 
28
BLANCHARD  & LEA'S  MEDICAL
       SHARPEY (WILLIAM),  M.  D., JONES QUAIN,  M. D., AND
                        RICHARD QUAIN, F. R. S., &c.
HUMAN  ANATOMY.   Revised, with Notes and Additions, by Joseph Leidt,
  M. D., Professor of Anatomy in the University of Pennsylvania.   Complete in two large octavo
  volumes, leather, of about thirteen hundred pages.  Beautifully illustrated with over five hundred
  engravings on wood.  $6 00.
  We have no hesitation in recommending this trea- I perhaps, in any language, which brings the state
tise on  anatomy as the most complete on that sub-  of  knowledge  forward to the most recent disco-
ject in  the English language;  and the only one, | veries.—The Edinburgh Med. and Surg. Journal.

                             SARGENT  (F.  W.), M. D.           /
ON BANDAGING AND OTHER  OPERATIONS OF MINOR SURGERY.
  Second edition, enlarged.  One handsome royal 12mo. vol., of nearly 400 ps(ges, with 182 wood-
  cuts.  Extra cloth, $1 40; leather, $1 50.                            / I
  Sargent's Minor Surgery has always been popular,   A work that has been^scjioi
and deservedly so. It furnishes that knowledge of the
most frequently requisite performances of surgical
art which cannot be entirely understood by attend-
ing clinical lectures.  The art of bandaging, which
is regularly taught in Europe, is very frequently
overlooked by teachers in this country; the student
and junior practitioner, therefore, may often require
that knowledge which this little volume so tersely
and happily supplies.—Charleston Med. Journ. and
Review, March, 1856.
  A work that has been^scjiong and favorably known
to the profession as Dry Sargent's Minor Surgery,
needs no commendation from us.  We would remark,
however, in this connection,,that minor surgery sel-
dom gets that attention in ^>ur schools that its im-
portance deserves.   Our larger works are also very
defective in their teaching on these small practical
points. This little book ^ill supply the void which
all must feel who have not studied its pages.—West-
ern Lancet, March, 1856/
SKEY'S OPERATIVE SURGERY.  In one very
  handsome octavo volume, extra cloth, of over 650
  pages, with about one hundred wood-cuts. $3 25.
STANLEY'S  TREATISE  ON  DISEASES  OF
  THE BONES. Inone volume, octavo, extra cloth,
  286 pages. $1 50.
SOLLY ON THE HUMAN BRAIN; its Structure,
  Physiology, and Diseases.  From the Second and
/
  much enlarged London  edition.  In  one octavo
  volume, extra cloth, of 500 pages, with 1-20 wood-
  cuts. $2 00.  /

SIMON'S GENERAL PATHOLOGY, as conduc-
  ive to the Establishment of Rational Principles
  for the prevention and Cure of Disease. In one
  neat octavo volume, extra cloth, of 212 pages.
  $1 25.
                            TANNER (T.  H.),  M. D.,
                          Physician to the Hospital for Women, &c.

A MANUAL OF CLINICAL  MEDICINE AND  PHYSICAL DIAGNOSIS.
  To  which  is added The Code of Ethics of the American Medical Association.   Second
  American Edition.  In one neat volume, small 12mo., extra cloth, 87| cents.
  The work is an honor to its writer, and must ob-ttioners,it has  only to be seen, to win for itself a
tain a  wide circulation by its intrinsic merit alone/1 place upon the shelves  of every medical  library.
Suited alike to the wants of students and practf- |—Boston Med  and Surg. Journal.
                                   Now Complete.
                 TODD (ROBERT BENTLEY), M. D., F. R. S.,
                    Professor of Physiology in King's College, London; and
                         WILLIAM  BOWMAN, F. R. S.,
                     Demonstrator of Anatomy in King's College, London.

THE PHYSIOLOGICAL ANATOMY AND PHYSIOLOGY OF MAN.  With
  about three hundred large and beautiful  illustrations on wood.  Complete  in one large octavo
  volume, of 950 pages, leather. Price $4 50.
  1^° Gentlemen who have received portions of this work, as published in the " Medical News
and Library," can now complete their copies, if immediate application be made.  It will be fur-
nished as follows, free by mail, in paper covers, with cloth backs.
  Parts I., II., III. (pp. 25 to 552), $2 50.
  Part  IV. (pp. 553 to end, with Title, Preface, Contents, &c), $2 00.
  Or, Part IV., Section II. (pp. 725 to end, with Title, Preface,  Contents, &c), $1 25.
                                              One of the very best books ever issued from any
  A magnificent contribution to British medicine,
and the American physician who shall fail to peruse
it, will have failed to read one of the most instruc-
tive books of the nineteenth century.—N. O. Med.
and Surg. Journal, Sept. 1857.
  It is more concise than Carpenter's Principles, and
more modern than the accessible edition of Mailer's
Elements; its details are brief, but sufficient; its
descriptions vivid; its illustrations exact and copi-
ous ;  and its  language terse and perspicuous.—
Charleston Med. Journal, July, 1857.
  We know of no work on the subject of physiology
so well adapted to the wants of the medical student.
Its completion has been thus long delayed, that the
authors might secure accuracy by personal observa-
tion.—St. Louis Med. and Surg. Journal, Sept. '57.
medical press.  We think it indispensable to every
reading medica) man, and it may, with all propriety,
and with the utmost advantage be made a text-book
by any student who would  tnoroughly comprehend
the groundwork of medicine.—N. O. Med. Newst
June, 1857.

  Our notice, though it conveys but a very feeble
and imperfect idea of the magnitude and importance
of the work now under consideration, already tran-
scends our limits ; and, with the indulgtnce of our
readers, and the hope that they will peruse the book
for themselves, as we feel we can with  confidence
recommend it, we leave it in their hands for them
to judge of its merits.—The Northwestern Med. and
Surg. Journal, Oct. 1857.
                      TODD  (R. B.),  M. D.,  F. R. S.,  &c.
CLINICAL LECTURES ON  CERTAIN  DISEASES  OF  THE  URINARY
  ORGANS AND ON  DROPSIES.  In one octavo volume.  (Just Issued, 1857.)  $1 50. 
AND  SCIENTIFIC  PUBLICATIONS.
29
                     TAYLOR (ALFRED S.), M. D., F. R. S.,
                Lecturer on Medical Jurisprudence and Chemistry in Guy's Hospital.

MEDICAL JURISPRUDENCE.   Fourth American  Edition.   With Notes and
  References to American Decisions, by Edward Hartshorne, M. D. In one large octavo volume,
  leather, of over seven hundred pages.   $3 00.
  This standard work has lately received a very thorough revision at the hands ofthe author, who
has introduced whatever was necessary to render it complete and satisfactory in  carrying out  the
objects in view.  The editor has likewise used  every exertion to make it equally thorough with
regard to all matter;- relating to the practice of this country.  In  doing this, he has carefully ex-
amined all that has appeared on the subject since the publication ofthe last edition, and has incorpo-
rated all the new information thus presented.  The work has thus been considerably increased in
size, notwithstanding which, it has been kept at its former very moderate price, and in every respect
it will be found worthy of a continuance of the remarkable favor which has carried it through so
many editions on both sides of the Atlantic.   A few notices of the former editions are appended.
we do not hesitate to affirm that after having once
commenced its perusal, few could be prevailed upon
to desist before completing it.  In the last London
edition, all the newly observed and accurately re-
corded facts have been inserted, including much that
is recent of Chemical, Microscopical, and Patholo-
gical research, besides papers on numerous subjects
never before published .-Charleston Medical Journal
and Review.

  It is not excess of praise to say that  the volume
before us is the very best treatise extant on Medical
Jurisprudence. In saying this, we do not wish to
be understood as detracting from the merits of the
excellent works of Beck, Ryan, Traill, Guy, and
others; but in interest and value we think it must
be conceded that Taylor is superior to anything that
has preceded it.—N. W. Medical and Surg. Journal.
  We know of no work on Medical Jurisprudence
which contains in the same space anything like the
same amount of valuable matter.—N. Y. Journal of
Medicine.
  No work upon the subject can be put into the
hands of students either of law or medicine which
will engage them more closely or profitably; and
none could be offered  to the busy practitioner of
either calling,  for the purpose of casual or hasty
reference, that would be more likely to afford the aid
desired. We therefore recommend it as the best and
safest manual for daily use.—American Journal oj
Medical Sciences.
  This work of Dr. Taylor's is generally acknow-
ledged  to be one of the ablest extant on the subject
of medical jurisprudence.  It is certainly one of the
most attractive books that we have met with; sup-
plying  so  much both to interest  and instruct, that

                         by the same author.  (Nearly Ready.)

ON POISONS, IN  RELATION TO   MEDICAL  JURISPRUDENCE  AND
  MEDICINE.  Second American,  from a second and revised London edition.   In one large
  octavo volume.
  The length of time which has elapsed since  the first appearance of this work, has wrought so
great a change in the subject, as to reauire a very thorough revision to adapt the volume to the
present wants of the profession. ThelraRid advance of Chemistry has introduced into we many
new substances which may become  fajal through accident, carelessness, or design—while at the
same time it has likewise designated  new and more exact modes of counteracting or detecting those
previously treated of.  Mr. Taylor's position as the leading medical jurist of England, has during
this period conferred on him extraordinary advantages in acquiring experience in all that relates to
this department, nearly all cases of moment being  referred to  him for examination, as an expert
whose testimony is generally accepted as final.  The results of his labors, therefore, as gathered
together in this volume, carefully weighed  and sifted, and presented  in the clear and intelligible
style for which he is noted, may be  received as an acknowledged  authoritj-, and as a guide to be
followed with implicit confidence.

                            WILSON  (MARRIS), M. D.

ON  DISEASES OF THE  VESICUL.E  SEMINALES.
   page 19.

                     WILLIAMS  (C. J. B.i,  M. D., F. R. S.,
                  Professor of Clinical Medicine in University College, London, &c.

PRINCIPLES  OF MEDICINE.   An Elementaiy View  of the  Causes, Nature,
   Treatment, Diagnosis, and Prognosis of Disease; with brief remarks on Hygienics, or the pre-
   servation of health.  A new American, from  the third and revised London edition.  In one octavo
   volume, leather, of about 500 pages.  $2 50.  (Just Issued.)
   The very recent and thorough revision which this work has  enjoyed at the hands of the author
 has brought it so completely up to the present state ofthe subject that in reproducing it no additions
 have been found necessary.   The  success which the work has heretofore met shows that its im-
 portance  has been appreciated, and in its present form it will be found eminently worthy a continu-
ance of the same favor, possessing as it does the strongest claims to the attention of the medical
student and practitioner, from the admirable manner in which the various inquiries in the different
branches  of pathology are investigated, combined and generalized by an experienced practical phy-
sician, and directly applied to the investigation and  treatment of disease.
See  "Lallemand,"
  We find that the deeply-interesting matter and
Btyle of this book have so far fascinated us, that we
have unconsciously hung upon its pages, not too
long, indeed, for our own profit, but longer than re-
viewers can be permitted to indulge.  AVe leave the
further analysis to the student and practitioner.  Our
judgment ((f the work has already been sufficiently
exDresscd.  It is a judgment of almost unqualified
nraise   The work is not of a controversial, but of
a didictic character; and  as such we hail  it, and
recommend it for a text-book, guide, and constant
companion to every practitioner and every student
who wishes to extricate himself from the well-worn
ruts of empiricism, and to base his practice of medi-
cine upon principles.—London Lancet.

  A text-book to which no other in our language is
comparable.—Charleston Medical Journal.
  No work has ever achieved or maintained a more
deserved reputation.— Va. Med. and Surg. Journal.
WHITEHEAD ON THE CAUSES AND TREAT- I
  MBNT OF ABORTION AND  STERILITY. |
Second American Edition.  In one volume, octa-
vo, extra cloth, pp. 308.  $1  75. 
30
BLANCHARD &  LEA'S  MEDICAL
            New and much enlarged edition—(Now Ready, Oct. 1858.)
                        WATSON (THOMAS),  M.D.,  &c.
                        Late Physician to the Middlesex Hospital, &c.

LECTURES  ON   THE  PRINCIPLES  AND  PRACTICE  OF  PHYSIC.
  Delivered at King's  College, London.  A new American, from the last revised and enlarged
  English edition,  with Additions, by D. Francis Condie, M. D., author of" A Practical Treatise
  on the Diseases of Children," &c.  With one hundred and eighty.five illustrations on wood.  In
  one very  large and handsome volume, imperial octavo, of over 1200  closely printed pages in
  small type ; the whole strongly bound in leather, with raised bands. Price $4 25.
  The publishers feel that they are rendering a service to the American profession in presenting at
so very moderate a price this vast  body of sound practical  information.   Whether as a guide for
the student entering on a course of instruction, or as a book of reference for daily consultation by
the practitioner,  "  Watson's Practice" has long been regarded as second to none; the soundness
and fulness of its teachings, the breadth and liberality of its views, and the easy and flowing style
in which it is written having won for it the position of a general favorite.  That this high reputa-
tion might be fully maintained, the  author has subjected  it to a thorough revision; every portion
has been  examined with the aid of the most recent researches in pathology, and  the results of
modern investigations in both theoretical and practical subjects have  been carefully weighed and
embodied throughout its pages.   The watchful  scrutiny of the editor  has likewise introduced
whatever possesses immediate importance to the American physician  in relation to diseases inci-
dent to our climate which are little  known in England, as well as those points in which experience
here has led to different modes of practice ; and he has also added largely to the series of  illustra-
tions, believing that in  this manner valuable assistance may be conveyed to the student in elucidat-
ing the text. The work will, therefore, be found thoroughly on a level with the most advanced
state of medical science on  both sides of the Atlantic.
   The additions which the work has received are shown by  the fact  that notwithstanding an  en-
largement in the size of the page, more than two hundred additional pages have been necessary
to accommodate the two large volumes of the London edition (which sells at ten dollars), within
the compass of a single volume, and in its present form it contains  the matter of at  least  three
ordinary octavos.   Believing it to  be a work which should lie on the  table of every physician, and
be in the hands of every student, the publishers have put it at a price within the reach of all, making
it one of the cheapest books as yet presented to the American profession, while at the same time
the beauty of its mechanical execution renders it an exceedingly attractive volume.
   It would appear almost superfluous to adduce commendatory notices of a work which has so
long been established in the  position of a standard authority  as "Watson's Practice."  A few ex-
tracts are, however, subjoined from reviews of the new and improved edition.
  The fourth edition now appears, so carefully re-
vised, as to add considerably to the value of a book
already acknowledged, wherever the English  lan-
guage is read, to be beyond all comparison the  best
  The lecturer's skill, his wisdom,his learning,are
equalled by the ease of his graceful diction, his elo-
quence,  and the far higher qualities of candor, of
courtesy, «f modesty, and of generous appreciation
s> stematic work on the Principles and Practice of | of merit in others.  May he long remain to instruct
Physic in  the whole range of medical literature.
Every lecture contains proof of the extreme anxiety
ofthe author to keep pace with the advancing know-
ledge of the day, and to bring  the results of  the
labors, not only of physicians, but of chemists and
histologists, before his readers, wherever they can
be turned to useful account.  And this is done with
such a cordial appreciation of the merit due to  the
industrious observer, such a generous desire to  en-
courage younger and rising men, and such a candid
acknowledgment of his  own  obligations to them,
that one scarcely knows whether to admire most the
pure, simple, forcible English—the vast amount of
useful  practical  information condensed into  the
Lectures—or the manly, kind-hearted, unassuming
character of the lecturer shining through  his work.
—London Med. Times and Gazette, Oct. 31, 1857.
  Thus these admirable volumes come  before  the
profession in their fourth edition, abounding in those
distinguished  attributes of moderation, judgment,
erudite cultivation, clearness, and eloquence, with
which  they were from  the first invested, but  yet
richer than before in the results of more prolonged
observation, and  in the able appreciation of  the
latest advances in pathology and medicine by  one
of the most profound medical thinkers of the day.—
London Lancet, Nov. 14, 1857.
us, and to enjoy, in the glorious sunset of his de-
clining years, the honors, the confidence and love
gained during his  useful life.—N. A.  Med -Chir.
Review, July, 1858.

  Watson's unrivalled,   perhaps  unapproachable
work  on  Practice—the copious additions made to
which (the fourth edition) have given it all the no-
velty  and much of the interest of a new book.—
Charleston Med. Journal, July, 1858.

  Lecturers, practitioners, and students of medicine
will equally hail the reappearance of the work of
Dr. Watson in the form of anew—a fourth—edition.
We merely do justice to our own feelings, and, we
are sure,  of the whole profession, if we thank him
for having, in the  trouble and  tuTmoil of a large
practice,  made leisure to supply the hiatus caused
by the exhaustion of the publisher's stock of the
third  edition, which has been severely felt for the
last three years. For Dr. Watson has not merely
caused the lectures  to be reprinted, but scattered
through the whole work we find additions or altera-
tions  which prove that the author has in every way
sought to bring up his teaching to the  level of che
most recent acquisitions in science.—Brit, and For.
Medico-Chir. Review, Jan. 1858.
                               WHAT TO  OBSERVE
AT  THE  BEDSIDE  AND   AFTER  DEATH,   IN  MEDICAL  CASES.
  Published under the authority of the London Society for Medical Observation.  A new American,
  from the second and revised London edition.  In one very handsome volume, royal 12mo., extra
  cloth.  $1 00.
  To the observer who prefers accuracy to blunders j   One of the finest aids to a young practitioner w«
and precision to carelessness, this little book is in-  have ever seen.—Peninsular Journal of Medicine.
valuable.—N. H. Journal of Medicine.            I
                                   WILDE (W.  R.),
                  Surgeon to St. Mark's Ophthalmic and Aural Hospital, Dublin.
AURAL SURGERY, AND  THE  NATURE  AND TREATMENT  OF  DIS-
  EASES OF THE EAR.   In one handsome octavo volume, extra cloth, of 476 pages, with
  illustrations.   $2 80. 
AND SCIENTIFIC  PUBLICATIONS.
31
            New and much enlarged edition—(Now Ready, Oct. 1858.)

                          WILSON  (ERASMUS),  F. R. S.,
A  SYSTEM  OF  HUMAN ANATOMY, General  and Special.   A new and  re-
  vised American, from the last and enlarged Engli>h Edition.  Edited by W. H. Gobrecht, M. D.,
  Professor of Anatomy in the Philadelphia Medical College, &c.  Illustrated with three hundred
  and ninety-seven engravings on wood.  In one large  and exquisitely printed octavo volume, of
  over 600 large pages; leather.  $3 25.
  The publishers trust that the well earned reputation so long enjoyed by this work willjbe more
than maintained bythe present edition.   Besides a very thorough revision by the author, it has been
most carefully examined by the editor, and the efforts of both have been  directed to  introducing
everything which increased experience in its use has suggested as desirable to render it a complete
text-book for those seeking to obtain or to renew an acquaintance with Human Anatomy.  The
amount of additions which it has thus received  may be estimated from the fact that  the present
edition contains over one-fourth more matter than the last, rendering a smaller type and an enlarged
page requisite to keep the volume within a convenient size.  The author has  not only thus added
largely to the work, but he has  also made alterations  throughout, wherever there  appeared the
opportunity of improving the arrangement or style, so as to present every fact in its  most appro-
priate  manner,  and to render the whole as  clear and intelligible as  possible.  The editor  has
exercised the utmost caution to  obtain entire  accuracy in the text, and has largely increased the
number of illustrations, of which there are about one hundred  and fifty more in  this edition than
in the last, thus bringing distinctly before tbe eye  ofthe student everything of interest or importance.
  The publishers have felt that neither  care nor expense should be spared to  render the external
finish ofthe volume worthy of the universal favor with which it has been received bythe American
profession, and they have endeavored consequently to produce in its mechanical execution an im-
provement corresponding with that which the text has enjoyed.  It will therefore be found one of
the  handsomest specimens of typography as yet produced in this country, and in all respects suited
to the  office table of the practitioner, notwithstanding the  exceedingly  low price at  which it has
been placed.
  A few notices of former editions are subjoined.
  This is probably the prettiest medical book ever j evident.  Let  students, by all means examine the
published, and we believe that its intrinsic merits
are in keeping with its exterior advantages, having
examined it sufficiently to satisfy us that it may be
recommended to the student as no less distinguished
by its accuracy and clearness of description than by
its typographical elegance.  The wood-cuts are ex-
quisite.—British and Foreign Medical Review.    ,
  An elegant edition of one  of  the most useful and
accurate systems of anatomical science which has
been issued from the press. The  illustrations are
really beautiful.  In its style the work is extremely
concise and intelligible.  No one  can possibly take
up this volume without being struck with the great
 beauty of its mechanical execution, and the clear-
ness of the descriptions which it contains is equally

                          BY THE SAME AUTHOR.  (Just Issued.)

 ON DISEASES  OF THE SKIN.   Fourth and enlarged American, from the last
   and improved London edition.   In one large octavo volume, of 650 pages, extra cloth, $2 75.
   The writings of Wilson, upon diseases of the skin,
claims of this work on their notice, before they pur-
chase a text-book  of the vitally important science
which this volume so  fully and easily unfolds.—
Lancet.

  In every respect,  this work, as an anatomical
guide for the student who seeks to obtain know-
ledge which  he has  not yet acquired, and for the
practitioner who wishes to keep up that which he
find-", gradually fading from his mind, merits our
warmest and most  decided praise.—Med. Gazette.
  We regard it as the  best system now extant for
students.—Western Lancet.
  It therefore receives our highest commendation.—
Southern Med. and Surg. Journal.
are by fir  the most  scientific and  practical  that
have ever been presented to the  medical world on
this subject. The present edition is a great improve-
ment on all its predecessors.  To dwell upon all the
greut merits and high claims of the work before us,
seriatim, would indeed be an agreeable service;  it
would  be a mental homage which we could freely
offer, but we should thus occupy an  undue amount
of space in this Journal.  We will, howiver.  look
at some of the more  salient points  with  which it
abounds, and which make itincompuraoiy superior in
excellence to all other treatises on the subject of der-
matology.  No mere speculative views are allowed
a place in this volume, which, without a doubt, will,
for a very long period, be acknowledged as the chief
standard work on dermatology.  The principles of
an enlightened and rational therapeia are introduced
on every appropriate occasion.  The general prac-
titioner and surgeon who, peradventure, may have
for years regarded cutaneous maladies as scarcely
worthy their attention, because, forsooth, they are
not  fatal in their  tendency; or who, if they have
attempted their cure, have followed the blind guid-
ance of empiricism, will almost assuredly be roused
to a new and becoming interest  in this department
of practice, through the inspiring agency of this
book.—Am. Jour. Med. Science, Oct. 1857.
                                     ALSO, NOW READY,

A  SERIES  OF  PLATES ILLUSTRATING WILSON ON  DISEASES  OF
  THE SKIN ;  consisting of nineteen beautifully executed plates, of which twelve are exquisitely
  colored, presenting the Normal Anatomy and Pathology of the Skin, and containing accurate re-
  presentations of about one hundred varieties of disease, most of them the size of nature.  Price
  in cloth $4 25.
  In beauty of drawing and accuracy and finish  of coloring these  plates will be found equal to
anything of the kind as yet issued in this country.
  One of the best specimens of colored lithographic
illustrations that have ever been published in this
country.  Tlie representations of  diseases of  the
skin, even to the most minute shade of coloring, are
remarkably accurate, giving the student or practi-
tioner a very correct idea of the disease he is study-
ing.  We  know of no work so well adapted to the
wants of the general practitioner as Wilson's, with
the accompanying  plates. — Med. and Surg. Re-
porter, -May, 1*56.
  We have already expressed our high appreciation
of Mr. Wilson's treatise on Diseases of the Skin.
The  plates are  comprised  in a  separate volume,
which we counsel all those who possess the text to
purchase.  It is a beautiful specimen of color print-
ing, and the repiesentations of the various forms of
skin disease are as faithful as is  possible in plates
of the size.—Boston Med. and Surg. Journal, April
8, IboS. 
32         BLANCHARD & LEAS MEDICAL PUBLICATIONS.
                     WILSON (EftASMUSI,  M. D.,  F. R.  S.
                               Lecturer on Anatomy, London.

THE  DISSECTOR'S MANUAL;  or, Practical and Surgical Anatomy.  Third
  American, from  the last revised and enlarged English edition.  Modified and  rearranged, by
  William Hunt, M. D., Demonstrator of Anatomy in the  University of Pennsylvania. In one
  large and handsome royal 12mo. volume, leather, of 582 pages, with 154 illustrations.  $2  00.
  The modifications and additions which this work has received in  passing recently through tbe
author's hands, is sufficiently indicated  by the fact that it is enlarged  by more than one hundred
pages, notwithstanding that it is printed in smaller type, and with  a greatly enlarged page.
                                 BY THE SAME AUTHOR.

ON  CONSTITUTIONAL   AND  HEREDITARY  SYPHILIS,   AND   ON
  SYPHILITIC ERUPTIONS.  In one small octavo volume, extra cloth, beautifully printed, with
  four exquisite colored plates, presenting more than thirty varieties of syphilitic eruptions. $2 25.
                                 BY THE SAME AUTHOR.

HEALTHY SKIN;  A Popular Treatise on  the  Skin  and Hair, their  Preserva-
  tion and Management.   Second American, from the fourth London edition.   One neat volume,
  royal 12mo.; extra cloth, of about 300 pages, with numerous illustrations.  $1 00; paper cover,
  75 cents.

                                  (Now Complete.)

                           WEST  (CHARLES),  M. D.,
Accoucheur to and Lecturer on Midwifery at St. Bartholomew's Hospital, Physician to the  Hospital for
                                    Sick Children, &c.

LECTURES  ON  THE DISEASES OF WOMEN.   Now  complete in  one hand-
  some octavo volume, extra cloth, of about 500 pages ; price $2 50.

Also, for sale  separate, Part II, being pp.  309 to end, with Index, Title matter,
  &c, 8vo., cloth, price $1.
EP Copies done up in paper covers, for mailing, will be sent, free of postage, to any address within
  the United States on receipt of One Dollar in current funds or postage stamps.   Subscribers to the
  "Medical News and Library" who received the first portion of this work as published in 1856
  and 1857, should lose no time in securing the completion.
Part I will no longer be sold separate.
  As the first part of this work formed a complete treatise on the Diseases of the Uterus, so Part
II is complete in itself as a text-book  on the affections of the Uterine  Appendages, the Ovaries,
Vagina, Bladder,  and External Organs.  It will be found fully to maintain the high character ac-
quired by the preceding portion immediately on its appearance,  and  the whole will  constitute a
reliable text-book on this  interesting and difficult branch of practice.

  A few notices of Part I are added.
painstaking, practical physician is apparent on every
page.—N. Y. Journal of Medicine, March, 1858.
  We know of no treatise of the kind so complete
and yet so compact.—Chicago Med. Journal, Janu-
ary, 1858.
  A fairer, more honest, more earnest, and more re-
liable investigator of the many diseases of women
and children  is not  to be found in any country.—
Southern Med. and Surg. Journal, January 1853.

  We gladly recommend his Lectures as in the high-
est degree  instructive to all who are interested in
obstetric practice.—London Lancet.

  We have to say of it, briefly and decidedly, that
it is the best work on the subject in any language;
and that it stamps Dr. West as the facile princeps
of British obstetric authors.—Edinb. Med. Journ.
  As a writer, Dr. West stands, in our opinion, se-
cond only to Watson, the " Macaulay of Medicine;"
he possesses that happy faculty of clothing instruc-
tion in easy  garments; combining  pleasure with
profit, he leads his pupils,  in spite  of the ancient
proverb, along a royal road to learning.  His work
is one which will not satisfy the extreme on either
side, but it is one that will please the great ma-
jority who are seeking truth, and one that will con-
vince the student that he has committed himself to
a candid, safe, and valuable guide.  We anticipate
with pleasure the appearance of the second part of
the work, which, if it equals this part, will com-
plete one of our very best volumes upon diseases of
females.—N. A. Med.-Chirurg. Review, July, 1858.
  We must now conclude this hastily written sketch
with the confident assurance to our readers that the
work will well  repay perusal.  The conscientious,
                          BY THE SAME AUTHOR.  (Just Issued.)

LECTURES  ON  THE  DISEASES  OF INFANCY  AND CHILDHOOD.
  Second American, from the Second and Enlarged London  edition.   In one volume, octavo,
  extra cloth, of nearly five hundred pages.  $2 00.
  We take leave of Dr. West with great respect for ligation by this able, thorough, and finished work
his  attainments, a due appreciation of his acute upon a subject which almost daily taxes to the ut-
powers of observation, and  a deep sense of obliga- most the skill of the general practitioner.  He haa
tion for this valuable  contribution  to our profes- with singular felicity threaded his way through ail
sional literature.  His book is undoubtedly in many the tortuous labyrinths of the difficult subject he has
respects the best we possess on diseases of children, undertaken to elucidate, and has in  many  of the
Dublin Quarterly Journal of Medical Science.      darkest corners left a  light, which will never be
  Dr. West haB placed the profession under deep ob- extinguished.—IVosAirfH* Medical Journal.

                                 BY THE SAME AUTHOR.

AN ENQUIRY INTO THE PATHOLOGICAL IMPORTANCE  OF ULCER-
  ATION OF THE  OS UTERI.  In one neat octavo volume, extra cloth. $1  00.

                           YOUATT  (WILLIAM), V. S.

THE   DOG.  Edited by E. J.  Lewis,  M. D.   With numerous  and beautiful
  illustrations   In one very handsome volume, crown Svo., crimson cloth, gilt. $ 1 25. 
) 
 

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