MANUAL of V PHYSIOLOGY. t BY WILLIAM SENHOUSE KIRKES, M.D., LirENTIATE OF THE ROYAL COLLEGE OF IMIY.ICIAN'S ; REGISTEAR, AND DEMONSTRATOR OF MORLID ANATOMY, AT ST. BARTHOLOMEW'S HOSPITAL. ASSISTED BY JAMES PAGET, F.R.S., LECTURER ON GENERAL ANATOMY AND PHYSIOLOGY AT ST. BARTHOLOMEW'S HOSPITAL. Sttonb Etruruan, from ij)* .gtconfc 3LonI)on SEiitioit. with ONE HUNDRED AND SIXTY-FIVE ILLUSTRATIONS. PHILADELPHIA: BLANC HARD AND LEA. 1853. y Entered according to Act of Congress in the year 1852, by BLAXCHARD AND LEA, in the Clerk's Office of the District Court of the United States in and for the Eastern District of Pennsylvania. PHILADELPHIA : T. K. AND P. G. COLLINS, PRINTERS. fc AMERICAN PUBLISHERS' NOTICE. This edition has been reprinted from the Second London Edition with but little alteration. The industry of the authors having em- bodied in it such notices of the more recent investigations and discoveries as were compatible with the design of the work, but few additions have been found requisite. The principal change will be observed in the introduction of a large number of new and superior illustrations, which, it is hoped, will render the facts advanced more easy of comprehension by the student. As in the former American Edition, the steel plates of the original have been engraved on wood, and scattered through the text, in their appro- priate places, as more convenient for reference; and the title of "Manual" has been retained, in place of "Handbook," as being better suited to the character of the work. The editorial supervision to which it has been subjected in its passage through the press is a guarantee that the present edition will in no way detract from the reputation which the work has so deservedly attained. Philadelphia, January, 1853. PREFACE TO THE SECOND EDITION. The rapid sale of the First Edition of this Handbook, together with the general expressions of approval which have been kindly bestowed on the work, seemed sufficient to justify me in adhering to the plan and arrangement originally adopted. In preparing the Second Edition, therefore, little else has been done than care- fully revising each portion of the work, and making such additions and alterations as our advancing knowledge in the science appeared to demand. WILLIAM SENHOUSE KIRKES. Saint Bartholomew's Hospital, November, 1851. 1* PREFACE TO THE FIRST EDITION. The publishers of Dr. Baly's edition of " Midler's Elements of Physiology" had long designed to render that admirable work more available for the general use of students. They had proposed the reduction of its principal contents into a volume more nearly proportionate to the share of time which can be devoted to Physi- ology, as only one of many subjects to be studied, in the period of pupillage. The present work was commenced with the intention of fulfilling their design ; it was announced as a " Handbook of Physiology on the Basis of Midler's Elements;" and many of its chapters, namely, those on Motion, Voice and Speech, the Senses, Generation, and Development, are chiefly abstracts of corresponding portions of that work, and of the Supplement by Dr. Baly and myself. But, in the rest of the subjects, it was found that the progress of Physiology during seven years had so increased or modified the facts, and some even of the principles of the science, that " Midler's Elements," and the notes added by Dr. Baly, could only be employed as among the best authorities and examples. The design was, therefore, departed from, so far as it concerned the construction of a Handbook on the basis of Miiller. In writing the present work, the primary object has been to give such an account of the facts and generally admitted principles of Physiology as may be conveniently consulted by any engaged in the study of the Science; and, more especially, such an one as the student may most advantageously use during his attendance upon Lectures, and in preparing for examinations. The brevity essential to this plan required that only so much of Anatomy, Chemistry, and the other sciences allied to Physiology, should be introduced as might serve to remind the reader of knowledge already viii preface to the first edition. acquired, or to be obtained, by the study of works devoted to these subjects. For the same end, it was necessary to omit all discus- sions of unsettled questions and expressions of personal opinion; but ample references are given, not only to works in which these may be read, but to those by which the study of Physiology may be, in its widest extent, pursued. For the convenience of students, the subjects are arranged on a plan corresponding with that in which they«are taught in the courses of Lectures on Physiology, delivered in the principal metropolitan schools of medicine. I cannot sufficiently express my obligations to Mr. Paget, from whom I have received the most liberal aid in every stage of the work, and who has, moreover, afforded me access to his manuscript notes of Lectures. I have also to offer my best thanks to Dr. Baly, for many kind suggestions made by him in the course of the work. WILLIAM SENHOUSE KIRKES. College Of Saint Bartholomew's Hospital, Sept. 29, 1848. CONTENTS. Introduction PAGE 13 CHAPTER I. Chemical composition of the human body . 14 CHAPTER II. Structural composition of the human body 29 CHAPTER III. Vital properties of the organs and tissues of the human body 37 CHAPTER IV. The blood ...... Coagulation of the blood . Conditions affecting coagulation The blood-corpuscles, or blood-cells The serum..... Chemical composition of the blood . Vital properties and actions of the blood 42 43 48 50 53 53 01 CHAPTER V. Circulation of the blood .... Of the action of the heaet Action of the valves of the heart Sounds and impulse of the heart Frequency and force of the heart's action Cause of the rhythmic action of the heart Effects of the heart's action . 70 72 75 79 84 8G. 89 contents. The arteries .... The pulse ..... Force of the blood in the arteries The capillaries .... The size, number, and arrangement of Circulation of the capillaries The veins ..... Peculiarities of the circulation in Cerebral circulation Erectile structures . capillaries different parts page 90 97 100 103 104 107 110 118 118 120 CHAPTER VI. Respiration ........ Structure of the lungs ..... Movements of respiration .... Movement of the blood in the respiratory organs Changes of the air in respiration Changes produced in the blood by respiration . Influence of the nervous system in respiration Effects of the suspension and arrest of respiration 123 123 125 131 132 139 142 143 CHAPTER VII. Animal heat ........ Sources and mode of production of heat in the body 145 148 CHAPTER VIII. Digestion ........ Changes of the food effected in the mouth Passage of food into the stomach . Digestion of food in the stomach Structure of the stomach .... Secretion and properties of the gastric fluid Changes of the food in the stomach Movements of the stomach .... Influence of the nervous system on gastric digestion Changes of the food in the intestines . Structure and secretions of the intestines The pancreas, and its secretion The liver, and its secretion 155 159 163 105 1G5 1G8 173 178 181 184 184 190 192 contents. xi Changes of the food in the large intestine Movements of the intestines . PAGE 203 204 CHAPTER IX. Absorption ..... Absorption by the lacteal vessels Absorption by the lymphatics . Properties of chyle and lymph Office of the lacteal and lymphatic v Absorption by the bloodvessels esscls and glands 206 207 208 210 214 218 Nutrition and growth Nutrition Growth CHAPTER X. 225 226 237 Secretion Secreting membranes Secreting glands Process of secretion CHAPTER XI. 240 241 247 249 CHAPTER XII. Vascular glands; or glands without ducts 254 CHAPTER XIII. The skin and its secretions Structure of the skin Excretion by the skin CHAPTER XIV. The kidneys and their secretion Structure of the kidneys Secretion of urine . The urine : its general properties Chemical composition of the urine 256 257 260 2G5 265 267 2G9 271 Xll contents. CHAPTER XV. physiology of the meso-cephalon The nervous system ..... Elementary structures of the nervous system Functions of nerve-fibres Functions of nervous centres Cerebro-spinal nervous system Spinal cord and its nerves Functions of the spinal cord The medulla oblongata Its structure . Its functions . Structure and varolii ........ Structure and physiology of the cerebellum Structure and physiology of the cerebrum . Physiology of the cerebral and spinal nerves Physiology of the third, fourth, and sixth cerebral nerves Physiology of the fifth or trigeminal nerve Physiology of the facial nerve Physiology of the glosso-pharyngeal nerve Physiology of the pneumogastric nerve Physiology of the accessory nerve . Physiology of the hypoglossal nerve Physiology of the spinal nerves Physiology of the sympathetic nerve CHAPTER XVI Causes and phenomena of motion Ciliary motion . Muscular and the allied motions Muscular tissue Properties of muscular tissue . CHAPTER XVII Of voice and speech Mode of production of the human voice or cranial 391 391 contents. xni Applications of the voice in singing and speaking Speech ........ PAGE 395 400 CHAPTER XVIII. The senses ....... The sense of smell ..... The sense of sight ..... Of the phenomena of vision Of the reciprocal action of different parts of other ....••• Of the simultaneous action of the two eyes Sense of hearing . Anatomy of the organ of hearing . Physiology of hearing .... Functions of the external ear . Functions of the middle ear; the tympanum fenestrse ..... Functions of the labyrinth Sensibility of the auditory nerve . Sense of taste . ^ense of touch . the retina on ossieula each and CHAPTER XIX. Generation and development Generative organs of the female Unimprcgnated ovum Discharge of the ovum . Impregnation of the ovum Male sexual functions "Dfvfi OPMENT • * • ■* " Changes in the ovum previous to the formation of the embryo Changes of the ovum within the uterus - Development of the embryo..... The chorion and placenta . Development of orgajjs .-..-- Development of the vertebral column and cranium . Development of the face and visceral arches . Development of the extremities ... Development of the vascular system XIV contents. Development of the nervous system .... Development of the organs of sense .... Development of the alimentary canal .... Development of the respiratory apparatus The Wolffian bodies, urinary apparatus, and sexual organs Index.......... List of works referred to ..... PAGE 520 521 523 526 526 531 561 At the end of the volume is a numbered List of Authorities to which reference is made; and with the numbers of these the nume- rals inclosed in parentheses throughout the work, correspond. LIST OF ILLUSTRATIONS. FIG. 1. 2. 3. 4. 5. 6. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 2::. 24. 25. 26. 27. 28. 29. Corpuscles of human blood .... Red particles of the blood of the common fowl . Fibres of unstriped muscle .... Muscular fibre of animal life .... Portion of broken muscular fibre of animal life Fasciculi and fibres of cellular tissue Development of the areolar tissue ; after Schwann Fibres of elastic tissue from the ligamentum flavum of the vertebrae ...... Portion of white fibrous tissue .... Uniform coagulation of blood .... Uniform coagulation with contraction Cupped coagulum ..... Fibrous membrane lining the egg-shell . White corpuscles of the blood .... Development of the first set of blood-corpuscles in the Batra chian larva ...... Development of the first set of blood-corpuscles in the mam malian embryo ..... Development of human lymph- and chyle-corpuscles into blood corpuscles ...... Diagram of the circulating apparatus in mammals and birds Diagram of the semilunar valves of the aorta; after Morgagni Fibrous tissue of a semilunar valve beneath the endocardium Sections of aorta, to show the action of the semilunar valves Haemadynamometer of Poiseuille Bloodvessels of an intestinal villus Distribution of capillaries around follicles of mucous membrane Capillary network of nervous centres Capillary network of fungiform papilla of the tongue . Capillaries in the web of the frog's foot, magnified Arterise helicinse of the penis ; after Midler View of a thin section of the lung of a cat xvi LIST OF ILLUSTRATIONS. FIO. SO. The changes of the thoracic and abdominal walls of the male during respiration ...••• 31. The respiratory movement in the female 32. Mucous membrane of the stomach ; after Boyd 33. Section of mucous membrane of the stomach near the pylorus . 34. One of the tubular follicles of the pig's stomach; after Wasmann 35. Section of the mucous membrane of the small intestine in the dog ofi f a. Transverse section of Lieberkuhn's tubes or follicles \ t b. A single Lieberkiihn's tube J 37. Solitary gland of small intestine ; after Boehm. 38. Tart of a patch of the so-culled Peycr's glands ; after Boehm . 39. Side view of a portion of intestinal mucous membrane of a cat; after Bendz ....... 40. Capillary plexus of the villi of the human small intestine, as seen on the surface, after a successful injection 41. One of the intestinal villi, with the commencement of a lacteal 42. Vertical section of the coats of the small intestine of a dog, show- ing only the commencing portions of the portal vein and the capillaries ...... 43. Cells from the liver ..... 44. Lymphatics of the glans penis, and prepuce; after Breschet 45. Lymphatics of the mucous membrane of the stomach; after Breschet . 46. Capillary bloodvessels and lymphatics from the tail of the tadpole 47. Lymphatic heart (9 lines long, 4 lines broad) of a large species of serpent, the Python bivittatus ; after E. Weber . a. One of the inguinal lymphatic glands injected with mer-~] cury ...... . 48. -| b. One of the superficial lymphatic trunks of the thigh c. One of the femoral lymphatic trunks laid open longitudi- nally to display the valves within it 49. Endosmometer ..... 50. Endosmometer of power 51. Intended to represent the changes undergone by a hair towards the close of its period of existence 52. Section of portion of the upper jaw of a child 53. Scales of tessellated epithelium 54. Cylinders of the intestinal epithelium; after Henle 55. A perpendicular section of the skin of the sole of the foot 56. Sebaceous glands of the skin; after Gurlt 57. A section of the kidney surmounted by the suprarenal capsule 58. Distribution of the renal vessels, from kidney of horse LIST OF ILLUSTRATIONS. xvn 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. GO. 70. 71. 72. 73. 74. 75. 76. 77. 78. 79. 80. 81 82. 83, 85. 86 a. Portion of a secreting canal from the cortical substance of the kidney ..... B. The epithelium or gland-cells C. Portion of a canal fr,om the medullary substance of the kidney ...... Primitive nerve-tubules; after Wagner . a. Diagram of tubular fibre of a spinal nerve b. Tubular fibres c. Gelatinous fibres from the solar plexus R,oots of a dorsal spinal nerve, and its union with sympathetic Terminal nerves on the sac of the second molar tooth of the lower jaw in the sheep ; after Valentin Distribution of the tactile nerves at the surface of the lip Extremities of a nerve of the finger with Pacinian corpuscles attached ...... Pacinian corpuscle from the mesentery of a cat Nerve-corpuscles from a ganglion; after Valentin Ganglion globules, with their processes, nuclei, and nucleoli Connection between nerve-fibres and nerve-corpuscles; after Wagner . . . . Transverse section of the spinal cord Diagram to show the decussation of the fibres within the trunk of a nerve; after Valentin .... Front view of the medulla oblongata Posterior view of the medulla oblongata Sensory and motor column in medulla oblongata Dissection showing relation of fornix Cerebral connection of all the cerebral nerves except the first Vibratile or ciliated epithelium . Nucleated ciliary cells Stages of the development of striped muscular fibre Muscular fibrils of the pig; after Sharpey External and sectional views of the larynx Bird's-eye view of larynx from above 84. Vocal cords; from Prof. Willis Outer wall of the nasal fossa, with the three spongy bones and meatus ...... Olfactory filaments of the dog .... Nerves of the septum of the nose Vertical section of the human retina and hyaloid membrane The yellow spot of the retina occupying the axis of the eye after Soemmering ..... Outer surface of the retina ; after Jacob 267 XV111 LIST OF ILLUSTRATIONS. FIG. 91. 92. 93. 94. 95. 96. 97. 98. 99. 100. 101. 102. 103. 104. 105. 106. 107. 108. 109. 110. 111. 112. 113. 114. 115. 116. 117. 118. 119. 120. 121. Choroid and iris, exposed by turning aside the sclerotica ; from Zinn .....•• / a. Vertical section of the human cornea ^ t b. The posterior epithelium > Position of the lens in the vitreous humor, shown by an imagi nary section; after Arnold . . . • Lens, hardened in spirit, and partially divided along the three interior planes, as well as into lamellae ; after Arnold Vertical section of the eye from before backwards Diagram to show the position and action of the ciliary muscle Diagram to show inversion of image on the retina Diagram illustrative of the results of "attention" to visual im pressions ...... A circle showing the various simple and compound colors of light, and those which are complemental of each other Diagram illustrative of simultaneous action of two eyes Section of eye showing the application, in man " " " " in quadrupeds Diagram showing want of simultaneous action in eye of quad ruped ...... Hypothetical division of optic nerve in chiasm; after Midler Union of correspondent fibres of optic nerves in sensorium Union of correspondent fibres in optic nerve Stereoscopic drawing of a cube Interior of the osseous labyrinth ; from Soemmering . General view of the external, middle, and internal ear; from Scarpa ...... Ossicles of the left ear articulated, and seen from the outside and below; from Arnold .... Propagation of sound through ossicles Tongue, seen on its upper surface; from Soemmering . Papillae of the palm, the cuticle being detached Vessels of papillae, from the heel Section of the Graafian vesicle of a mammal; after Von Baer Ovum of the sow ; after Barry Diagram of a Graafian vesicle, containing an ovum Corpora lutea of different periods ; after Dr. Montgomery Development of the spermatozoids of Certhia familiaris; after Wagner Development of the spermatozoids of the rabbit An ovarian ovum from a bitch in heat The same ovum after the removal of most of the club shaped cells 487 LIST OF ILLUSTRATIONS. XIX FIG. 122. Cleavage of the yelk in ovum of bitch; after Bischoff 123. Cleaving of the yelk after fecundation ; after Bagge . 124. Section of the lining membrane of a human uterus at the period of commencing pregnancy ; after Weber 125. Two thin segments of human decidua after recent impregnation; from Dr. Sharpey ...... 126. A vertical section of the mucous membrane, showing uterine glands of the bitch; from Dr. Sharpey 127. Diagram of part of the decidua and ovum separated, to show their mutual relations; from Dr. Sharpey . 128. Portion of the germinal membrane of a bitch's ovum, with the area pellucida and rudiments of the embryo ; after Bischoff 129. Portion of the germinal membrane, with rudiments of the em- bryo from the ovum of a bitch; after Bischoff 130. Diagram showing vascular area in the chick 131. Embryo of the chick at the commencement of the third day after Wagner ..... 132. Formation of arteriae omphalo-mesentericae 133. Embryo from a bitch at the 23d or 24th day ; after Bischoff 134- A longitudinal section of an embryo chick in the second day of incubation .... 135. Formation of amnion, and vitelline duct 136. Further development of same . 137. Aborted ovum; after Sharpey . 138. Mesentery and intestine of the embryo 139. Omphalo-mesenteric vein in foetus 140, 141, 142. Ovum and embryo; after Midler 143. The lower part of the body of a bitch's embryo ; after Bischoff 144. The lower extremity of an older embryo; after Bischoff 145. Diagram of human ovum, at the time of formation of placenta 146. The villi of the foetal portion of a mature human placenta after Weber ...... 147. Extremity of a villus ; after Weber 148. Transverse section of the uterus and placenta ; J. Reid 149. Connection between the maternal and foetal vessels ; J. Reid 150. Extremity of a placental villus ; after Goodsir 151. Development of the parts of the face in the embryo of Triton tseniatus; after Reichert 152. A human embryo of the fourth week . 153. Capillary bloodvessels of the tail of a young larval frog ; after Kolliker .... 154. Heart of the chick at the 45th, 65th, and 85th hours of incuba- tion ; after Thomson 489 491 492 493 493 495 XX LIST OF ILLUSTRATIONS. FIG. PAGE 155. Heart of a human embryo of about the fifth week; after Von Baer ....... 517 156. Plan of the transformation of the system of aortic arches into the permanent arterial trunks in mammiferous animals; after Von Baer ...... 518 157. Early forms of the brain in the embryo ; after Tiedemann . 521 158. Development of the eye; after Huschke . . • 522 159. An embryo dog; after Bischoff .... 524 160. First appearance of parotid gland in the embryo of a sheep . 525 161. Lobules of the parotid, with the salivary ducts, in the embryo of the sheep at a more advanced stage . . . 525 162. Rudiment of the liver on the intestine of a chick at the fifth day of incubation . . . . . . 526 163. Development of the respiratory organs ; after Rathke . 526 164. Urinary and generative organs of human embryo ; after Midler 527 165. Urinary and generative organs of a human embryo measuring 3 J inches in length ; after Muller .... 528 * INTRODUCTION. Human Physiology is the science which treats of the con- ditions, phenomena, and laws of the life of the human body in the state of health. The phenomena of life manifested in the human body, as in that of all animals, may be arranged in two principal classes; the first comprehending those which are observed, in various degrees of perfection and variously modified, in both vegetables and animals; the second, those which are peculiar to the members of the animal kingdom. The first class of the phenomena of life includes: 1st. The pro- cesses of digestion, absorption, secretion, excretion, circulation, and respiration, which, together with the offices of some parts not yet understood, fulfil their purpose in the formation, movement, and purification of the blood, with the materials for the nutrition of all the tissues of the body; 2d. The processes of growth and nutri- tion, or nutritive assimilation, by which the several parts of the body, obtaining materials from the blood, repair the loss and waste to which they are subject in the discharge of their functions, or through their natural impairment and decay; 3d. The generative processes, for the formation, impregnation, and development of the ova. These are named processes, functions, or phenomena of organic or vegetative life. Those of the first two divisions maintain the existence of the individual being; those of the third maintain that of the species. The second class of vital phenomena includes the functions of sensation and voluntary motion, by which the mind of an animal acquires knowledge of things external to itself, and is enabled to act upon them. These are named phenomena of animal or relative life. But the division of the functions or phenomena of life into these, or any similar classes, is artificial, and must not be taken as indicating absolute difference and dissociation. The organic and the animal life arc knit together and mutually dependent; neither can be long maintained without the other. As all the processes of o 14 CHEMICAL COMPOSITION OF organic life are essential to the maintenance of the organs of animal life, so, in an equal degree, the sensation and voluntary motion of animal life are essential to the taking of food, the discharge of excretions, and other processes of organic life, by which the animal and the species are maintained. All the bodies in which the phenomena of life have been observed are formed of diverse mutually adapted parts, or organs; they are, therefore, called organisms, or organized bodies or parts; their composition and structure, being peculiar, are named organic, and constitute their organization. While alive, also, they manifest certain peculiar vital properties and modes of action. A brief account, therefore, of the chemical composition, general anato- mical structure, and vital properties of the several tissues and organs, will be a necessary preface to the consideration of their actions. CHAPTER I. CHEMICAL COMPOSITION OF THE HUMAN BODY. The following Elementary Substances may be obtained, by chemical analysis, from the human body: Oxygen, Hydrogen, Ni- trogen, Carbon, Sulphur, Phosphorus, Silicon, Chlorine, Eluorine, Potassium, Sodium, Calcium, Magnesium, Iron, and probably, or sometimes, Manganesium, Aluminium, and Copper. Thus, of the fifty-five elements of which all known matter is com- posed, nearly one-third exist in the human body. A few others have been detected in the bodies of other animals; but no element has yet been found in any living body which does not also exist in inorganic matter. Of the elements enumerated above, the first four, because they exist in nearly all animal substances and form the largest parts of all, are named essential elements; the rest, being less constant, and occurring often in only very small quantity, are named incidental elements. But the term incidental must not be understood to im- ply that any of these elements (except, perhaps, the last three) are ess necessary to the right composition of the substances in which they exist than the essential elements are. Sulphur, for example is as constant and necessary a constituent of albumen, and iron of lia^matosine, as any of the elements are. The terms must be taken in only a general sense. No organic substance being known which has not at least three of the first four elements, thty may be con- sidered essential to the formation and existence of organic matter THE HUMAN BODY. 15 But one or more of the other elements added to these, in compara- tively small proportions, contribute to determine, as it were inci- dentally, the peculiarities by which one kind of organic matter is distinguished from another. The elements composing organic and inorganic matter being thus the same, we must look to the modes in which they are com- bined for an explanation of the differences between the two classes of substances. We cannot, indeed, draw an absolute rule of chemi- cal distinction between the two classes, for there are substances which present every gradation of composition between those that are quite organic and those that are inorganic. Such substances of intermediate composition are many that are formed when inorganic matters, taken as nutriment by plants, gradually assume the cha- racters of organic matter, under the influence of the vital properties of the plant; and such are those which are formed in both plants and animals, when, out of the well-organized tissues, or out of the sap or blood, materials are being separated, to form either tissues for mechanical service, or stores for nutriment, or purifying excre- tions. In both cases, the substances that are in the state of transi- tion between the organic and the inorganic, or between the more and the less organized states, may proceed through changes so gradual that no natural line of demarcation between the two states can be discerned; and one cannot say when that which has been called inorganic has acquired the characters of an organic body, or when that which has been organic ceases to deserve the name. Al- cohol, ether, acetic acid, urea, uric acid, and the fatty and oily mat- ters, are such substances of organic origin, and intimately related to such as no one would hesitate to call organic, yet in their sim- plicity and mode of composition they are like inorganic matters. But although no decided difference in chemical characters can be discerned in substances that thus stand, as it were, on the boundary between the organic and the inorganic world, yet, all the substances that form the proper component living tissues of animal bodies are as distinguished from inorganic substances as the actions of living bodies are from the passiveness of dead; and, as a general rule, it may be held that the more active the vital processes are that are carried on in any substance, the more widely do the chemical cha- racters of that substance differ from those of inorganic matter. The chief peculiarities in the chemical characters of animal sub- stances appear to be these three :— 1. The simplest of the compounds naturally formed in the body —of those compounds which, from their being supposed to stand, in order of simplicity, nearest to the elements, are called proxi- mate principles—are composed of at least three elements. In the inorganic world, the most abundant substances are either in the elemental_state, as the oxygen and nitrogen, by the mixture of 16 CHEMICAL COMPOSITION OF which the atmosphere is formed; or, are formed by the union of only two elements, as water, of oxygen and hydrogen, the oxides of calcium, aluminium, and others. In the organic world, the most abundant substances are, in plants, compounds of three ele- ments, as starch, gum, sugar, cellulose, and others composed of carbon, hydrogen, and oxygen ; and in animals, of four or five elements, as albumen, fibrine, gelatine, and other compounds of the four essential elements and sulphur. 2. In the more compound inorganic substances, the several ele- ments of which they consist appear to be combined, or, as it were, put together, in pairs—each element seeming to have more affinity for one of the others than for all the rest. The elements are arranged in what is called a binary mode of combination. But, when any number of elements are combined in an organic compound, * they appear all held together as with one bond, as if each of them were united with equal force to all the others. Thus, for example, carbonate of ammonia, which is regarded as an inorganic salt, is formed of the same four elements as compose most animal matters. Its constitution may be thus expressed:— Owd-ph' \ umting, form carbonic acid, ~| , ,, uxygen, j > I an(| these two uniting, form Hydrogen, } uniting, form ammonia, J carbonate of ammonia. And in the analysis of this substance, the first pair of elements may be separated together in the form of carbonic acid, the second pair remaining as ammonia. But, in stating the composition of an organic body, these four elements would be all placed within one bond, or bracket; and in the analysis of such a compound the elements part asunder, and recombine in compounds, which vary according to the circumstances in which the change takes place, and of which compounds there may be no reason to believe that any previously existed in the substance analyzed. Thus in the decomposition of albumen, carbonic acid, water, ammonia' carbu- retted and sulphuretted hydrogen, and other compounds, would be, not merely separated, but formed out of the elements parting asunder, and combining again according to their several affinities and the circumstances of the case. 3. Not only is a large number of elements combined in an organic compound, but a large number of equivalents or atoms of each of the elements are united to form an equivalent or atom of eL^r ° *? °T ° , Carb°nate °f ainmonia> already ammonia 'Z "^f^ of ™ho™ acid is united with one of carbon 5thT f * °r at°m °f C:"'J0Uic !lcid Consists of ™e of carbon with two of oxygen; and that of ammonia of one of nitrogen with three of hydrogen. But in an equivalent or atom of fibrine, or of albumen, there are of the same ele nents, respec ively THE HUMAN BODY. 17 48, 15, 12, and 39 equivalents, according to Dumas, and nearly ten times as many according to Mulder. And, together with this union of large numbers of equivalents in the organic compound, it is further observable, that the several numbers stand in no simple arithmetical relation one with another, as the numbers of equiva- lents combining in an inorganic compound do. With these peculiarities in the chemical composition of organic bodies we may connect two other consequent facts: the first, that of the large number of different compounds that are formed out of comparatively few elements ; the second, that of their great prone- ness to decomposition. For, it is a general rule, that the greater the number of equivalents or atoms of an element that enter into the formation of an atom of a compound, the less is the stability of that compound. Thus, for example, among the various oxides of lead and other metals, the least stable in their composition are those in which each equivalent has the largest number of equiva- lents of oxygen. So, water, composed of one equivalent of oxygen and one of hydrogen, is not changed by any slight force; but per- oxyde of hydrogen, which has two equivalents of oxygen to one of hydrogen, is among the substances most easily decomposed. The instability on this ground belonging to animal organic com- pounds is augmented, 1st, by their containing nitrogen, which, among all the elements, may be called the least decided in its affinities, that which maintains with least tenacity its combinations with other elements; and, 2dly, by the quantity of water which, in their natural mode of existence, is combined with them, and the presence of which furnishes a most favorable condition for the decomposition of nitrogenous compounds. Such, indeed, is the instability of animal compounds, arising from these several peculi- arities in their constitution, that, in dead and moist animal matter, no more is requisite for the occurrence of decomposition than the presence of atmospheric air and a moderate temperature; conditions so commonly present that the decomposition of dead animal bodies appears to be, and is generally called, spontaneous. The modes of such decomposition vary according to the nature of the original compound, the temperature, the access of oxygen, the presence of microscopic organisms, and other circumstances, and constitute the several processes of decay and putrefaction; in the results of which processes the only general rule seems to be, that the several ele- ments of the original compound finally unite to form those sub- stances whose composition is, under the circumstances, most stable.1 1 An interesting account of the nature of the so-called spontaneous de- composition of dead organic matter is given by Dr. Helmholtz (Ixxx. 1843): for an abstract of the paper see xxv. 1843-4, p. 5. The experiments of Helmholtz show, that although the results of spontaneous decomposition are modified by the presence of infusorial organisms, yet these are not, as has 2* IS CHEMICAL COMPOSITION OF It is not known how far the process of decomposition which thus occurs in dead animal matter is imitated in the living body; but the facility of decomposition which it indicates may be considered in the study of those chemical changes which are constantly effected during life, tranquilly, and without the intervention of any such comparatively violent forces as are used in chemical art. lhe instability which organic compounds show when dead makes them amenable to the chemical forces exercised on them during life by the living tissues—forces inimitably gentle, so slight that their operation is not discernible by any effects besides those which they produce in the living body. What has been said respecting the mode in which the elements are combined in the composition of animal matter refers only to the four essential elements. Little or nothing is known of the mode in which the incidental elements, or their compounds, are combined with the compounds formed of the essential elements; only it is probable that they are combined chemically and as neces- sary parts of the substances they contribute to form. Of the natural organic compounds existing in the human body, some occur almost exclusively in particular tissues or fluids; as the coloring matter of the blood and other fluids, the fatty matter of the nervous organs, &c. But many exist in several different parts, and may, therefore, be now described in general terms. They may be arranged in two classes, namely, the azotized or nitrogenous, and the non-azotized or non-nitrogenous principles. The non-azotized principles include the several fatty, oily, or oleaginous substances, of which, in the human body, the most abundant are named margarine, elaine or oleine, stcarine, choles- tearine, and cerebrine. The fatty substances are, nearly all, compounds of carbon, hy- drogen, and oxygen. They burn with a bright flame, the proportion of oxygen being less than would be sufficient to form water with the hydrogen, or carbonic acid with the carbon, that they contain. They are all lighter than water, nearly all are fluid at the natural temperature of the body, all are insoluble in water, soluble in ether and boiling alcohol, and most of them crystallize when deposited from solution. _ They are nearly all of the kind named fixed oils; none of them is what is called a drying oil, i. c, none so combines with oxygen as to form a resin-like varnish on the substance over which it is spread. been supposed, essential to the occurrence of the process: their existence in large quantities in decomposing animal matters is due to the fact that such decomposition furnishes the most favorable conditions to their devel- opment and lrfe Consult, also, on this subject, Liebig, in the last edition of his Animal Chemistry. THE HUMAN BODY. 19 The oily or fatty matter which, inclosed in minute cells, forms the essential part of the adipose or fatty tissue of the human body, and which is mingled in minute particles in many other tissues and fluids, consists of a mixture of margarine and oleine, the propor- tion of the former being the greater the higher the temperature at which the mixture congeals, and the firmer the mass is when congealed. The animal fats, or suets, that are firmer than human fat, contain also a substance named stearine, which remains solid at or near 130° F. Margarine congeals at about 120°, oleine at about 25°. Their mixture in human fat is a clear yellow oil, of which different specimens congeal at from 45° to 35° F. Mar- garine, when deposited from solution in alcohol, forms fine needle- shaped crystals; and microscopic tufts or balls of such crystals are often found in fat-cells after death, especially in the fat of diseased parts, and old people. According to Schultze, oleine, when acted upon by sulphuric acid and sugar, assumes the same violet-red color as ensues in bile when similarly tested, while the firmer fats are not thus affected, neither are the solid vegetable fats, although vegetable oils are colored like animal oleine (lix. 1850, p. 101). Margarine and oleine, like all the fatty matters with which soaps may be made, and which are therefore named saponifiable, appear to consist of fatty acids combined with a base which is soluble in water.1 When one of these fats is long boiled with an alkali, it is decomposed : the fatty acid, which is named margaric or oleic, ac- cording to the substance employed, unites with the alkali, forming a neutral soapy substance, margarate or oleate of soda, or potash, as the case may be ; and the base of the fat, a sweet, syrupy sub- stance named glycerine, remains. The fatty matter of human adipose tissue may therefore be regarded as a mixture of margarate and oleate of glycerine. Glycerine, moreover, is considered to be an hydrated oxide of a substance called Glyceryl; and margaric acid a compound of a substance named margaryl with oxygen. The formula for margarine is C7(!H730j3 ; that for oleine C^H^O^; that for glycerine CaH705 + HO (cxi. vol. i. p. 70). Cholestearine, or Cholesterine, a fatty matter which docs not melt below 278°, and is, therefore, always solid at the natural temper- ature of the body, may be obtained in small quantity from blood, bile, and nervous matter. It occurs abundantly in many biliary calculi; the pure white crystalline specimens of these concretions being formed of it almost exclusively. Minute rhoraboidal scale- like crystals of it are also often found in morbid secretions, as in cysts, the puriform matter of softening and ulcerating tumors, &c. It is soluble in ether and boiling alcohol; but alkalies do not change 1 See on this subject Mulder (lxi.), Berzclius (xxiv), and Redtenbacher (x. Aug. 1843). 20 CHEMICAL COMPOSITION OF it: it is one of those fatty substances which are not saponifiable. Its formula is C37H3aO (lxxxii. vol. i. p. 83). . The azotized or nitrogenous principles in the human body in- clude two chief classes of substances, namely, the gelatinous and the albuminous. The gelatinous substances are contained in several of the tissues, especially those which serve a passive mechanical office in the economy; as the cellular or fibro-cellular tissue in all parts of the body, the tendons, ligaments, and other fibrous tissues, the cartilages and bones, the skin and serous membranes. These when boiled in water, yield a material, the solution of which remains liquid while it is hot, but becomes solid and jelly-like on cooling. Two varieties of these substances are described, gelatine and chondrine: the latter being derived from cartilages, the former from all the other tissues enumerated above, and, in its purest state, from isinglass, which is the swimming-bladder of the sturgeon, and which, with the exception of about 7 per cent, of its weight, is wholly reducible into gelatine. The most characteristic property of gelatine is that already mentioned, of its solution being liquid when warm, and solidifying or setting when it cools. The tem- perature at which it becomes solid, the proportion of gelatine which must be in solution, and the firmness of the jelly when formed, are various, according to the source, the quantity, and the quality of the gelatine; but, as a general rule, one part of dry gelatine dis- solved in 100 of water, will become solid when cooled to 60°. The solidified jelly may be again made liquid by heating it; and the transitions from the solid to the liquid state, by the alternate ab- straction and addition of heat, may be repeated several times; but at length the gelatine is so far altered, and, apparently, oxidized by the process, that it no longer becomes solid on cooling. Gela- tine in solutions too weak to solidify when cold, is distinguished by being precipitable with alcohol, creosote, tannic acid, and bichloride of mercury, and not precipitable with the ferrocyanide of potassium. The most delicate and striking of these tests is the tannic acid, which is conveniently supplied in an infusion of oak-bark or gall- nuts : it will detect one part of gelatine in 5000 of water; and if the solution of gelatine be strong it forms a singularly dense and heavy precipitate, which has been named tanno-gelatine, and is completely insoluble in water. Gelatine is also distinguished from albuminous substances by assuming a yellowish-brown, instead of a red color, when tested by sulphuric acid and sugar, (Schultee, lix. 18o0, p. 102.) When gelatine is boiled with caustic potash, or with sulphuric acid, it is decomposed, and among the products of its change are two substances named leucine and sugar of gelatine, of which the latter is remarkable for its similarity to the sugars produced from THE HUMAN BODY. 21 vegetable substances, and for being susceptible of crystallization. (Simon, lxxxii. vol. i. p. 195, and Prout, xxi. p. 455; see also lix. 1850, p. 96.) Among the varieties of gelatine derived from different tissues, and from the same sources at different ages, much diversity exists as to the firmness and other characters of the solid formed in the cooling of the solutions. The differences between isinglass, size, and glue in these respects are familiarly known, and afford good examples of the varieties called weak and strong, or low and high, gelatines. The differences are ascribed by Dr. Prout to the quan- tities of water combined in each case with the pure or anhydrous gelatine; and part of this water seems to be chemically combined with the gelatine, for no artificial addition of water to glue would give it the character of size, nor would any abstraction of water from isinglass or size convert it into the hard dry substance of glue. But such a change is effected in the gradual process of nutrition of the tissues; for, as a general rule, the tissues of an old animal yield a much firmer or stronger jelly than the corresponding parts of a young animal of the same species. A similar difference is observable in the leathers formed by the tanning of the skins of young and old animals; a fact which, together with the general similarity of the action of tannic acid upon skin and upon gelatine, makes it probable that gelatine is really (though some chemists hold the contrary) contained as such in the tissues, from which it is obtained by boiling. The analysis of dry gelatine yields C 50.05, H. 6.47, N. 18.35, 0. 25.13 parts in 100: its formula is stated as Cl6H18N4014 (lx. p. 509). Chondrine.-—The variety of gelatine obtained from cartilages agrees with gelatine in that its solution in water solidifies on cooling, though less firmly, and is precipitable with alcohol, creosote, tannic acid, and bichloride of mercury. Like gelatine, also, it is distin- guished from the albuminous substances by not being precipitable with ferrocyanide of potassium; but, unlike gelatine, it is precipi- table with acetic and the mineral and other acids, and with the sulphate of alumina and potash, persulphate of iron and acetate of lead. The albuminous substances are more highly or perfectly organic, i. e., are more different from inorganic bodies than are any of the substances yet considered, or, perhaps, any in the body. The chief of them are, albumen, fibrine, and caseine; but the last being found almost exclusively in milk, will be described with that fluid. Prin- ciples essentially similar to them all are found also in vegetables, especially in the sap and fruits. And substances much resembling, though not classed with, the albuminous, are horny matter and extractive matter. In addition to the chemical properties severally manifested by albumen, fibrine, and caseine, albuminous substances 22 CHEMICAL COMPOSITION OF generally are distinguished from the gelatinous by being changed into a violet-red color when treated with sulphuric acid and sugar, as in Pettenkofer's test for bile. These substances, indeed, undergo changes in color exactly similar to those undergone by bile when exposed to this test. (Schultze, lix. 1850, p. 101.) Millon has also found that albuminous substances assume an intense red color when treated with a solution of quicksilver dissolved in an equal weight of sulphuric acid, and four and a half parts of water. Gela- tinous tissues, however, are similarly affected (xviii. vol. 28). Albumen exists in some of the tissues of the body, especially the nervous, in the lymph, chyle, and blood, and in many morbid fluids, as the serous secretions of dropsy, pus, and others. In the human body it is most abundant, and most nearly pure, in the serum of the blood. In all the forms in which it naturally occurs, it is com- bined with about six per cent, of fatty matter, phosphate of lime, chloride of sodium, and other saline substances. Its most character- istic property is, that both in solution, and in the half solid state in which it exists in white-of-egg, it is coagulated by heat, and in thus becoming solid becomes insoluble in water. The temperature required for the coagulation of albumen is the higher the less the proportion of albumen in the solution submitted to heat. Serum and such strong solutions will begin to coagulate at from 150° to 170°, and these, when the heat is maintained, become almost wholly solid and opaque. But weak solutions require a much higher temperature, even that of boiling, for their coagulation, and either only become milky or opaline, or produce flocculi which are precipitated.1 Albumen, in the state in which it naturally occurs, appears to be but little soluble in pure water, but is soluble in water contain- ing a small proportion of alkali.2 In such solutions it is probably combined chemically with the alkali; it is precipitated from them by alcohol, ether, nitric, and other mineral acids (unless when they are. very dilute), by ferrocyanide of potassium (if before or after adding it the alkali combined with the albumen be neutralized), by bichloride of mercury, acetate of lead, and most metallic salts. These precipitates are not merely solidified albumen, but compounds of albumen, with the acid or base added to it. In the former case the albumen takes the part of a base, as in nitrate of albumen; in the latter, it takes the part of an acid, as in albuminate of oxide of mercury, lead, &c. The precipitates with the metallic salts are soluble in an excess of albumen, and in solutions of chloride of ' For explanation of the conditions in which albumen in the urine and other fluids may not be coagulable by heat, see Dr. Bence Jones, lxxi. vol. xxvn. p. 288. 2 On the mode of preparing albumen soluble in water without any addi- tion, see Wurtz ( xii. Oct. 1844). •> THE HUMAN BODY. 23 sodium and other alkaline salts; and it is, probably, by these means that the salts of iron, mercury, and other metals, taken into the blood, remain dissolved in it. Coagulated albumen, i. e., albumen made solid with heat, is soluble in solutions of caustic alkali, and in acetic acid if it be long digested or boiled with it. With the aid of heat, also, strong hydrochloric acid dissolves albumen previously coagulated, and the solution has a beautiful purple or blue color. The percentage composition of albumen of blood, according to the latest experiments of Mulder (lix. 1847, p. 83), is, carbon, 53.4; hydrogen, 7.1; nitrogen, 15.6; oxygen, 22.3; phosphorus, 0.3; sulphur, 1.3: its formula is not yet certainly known. Fibrine exists, most abundantly, in solution in the blood and the more perfect portions of the lymph and chyle; and in the solid state, in some part of the tissue of voluntary muscles, and occa- sionally in minute particles in the blood. (R. D. Thomson, xvii. April, 1846.) The characteristic property of fibrine is, that, in certain conditions (especially when the blood or other fluid containing it is taken from the living body), it separates from its solution, and sponta- neously assumes the solid form, or coagulates.1 It is on this that the coagulation of the blood (a process to be further described hereafter) depends. If a common clot of blood be pressed in fine linen while a stream of water flows upon it, the whole of the blood- color is gradually removed, and strings and various pieces remain of a soft, yet tough, elastic, and opaque-white substance, which consist of fibrine, impure with a mixture of fatty matter, lymph- corpuscles, shreds of the membranes of red blood-corpuscles, and some saline substances. Fibrine somewhat purer than this may be obtained by stirring blood while it coagulates, and collecting the shreds that attach themselves to the instrument, or by retarding the coagulation, and, while the red blood-corpuscles sink, collect- ing the fibrine unmixed with them. But in neither of these cases is the fibrine perfectly pure. The nature of the process of coagulation will be considered in the account of the blood; it does not appear to be attended with any chemical change, but is comparable with the process in which fluids effused from the bloodvessels in nutrition or inflammation assume organic form and structure. • A very small quantity of fibrine may be so dissolved in serous fluid that it will not spontaneously coagulate. The fluid of common hydrocele does not of itself coagulate; but, as Dr. Buchanan (lxxi. 1836, pp. 52 and 90, 1845, p. 617) has shown, if a piece of washed clot of blood, or of muscle, or some other animal tissue be placed in it, a filmy coagulum of fibrine will form and attach itself to the substance introduced. The film has the fila- mentous appearance of proper fibrine-clot, and is not mixed with corpuscles, as that of blood-clot is. 24 CHEMICAL COMPOSITION OF Chemically, fibrine and albumen cannot be distinguished. All the changes, produced by various agents, in coagulated albumen may be repeated with coagulated fibrine, with no greater differences of result than may be reasonably ascribed to the differences m the mechanical properties of the two substances. Of such differences, the principal are that fibrine immersed in acetic acid swells up and becomes transparent like gelatine, while albumen undergoes no such apparent change; and that deutoxide of hydrogen is decom- posed when in contact with coagulated fibrine, but not with albu- men. Proteine.—It is the opinion of Mulder that animal albumen, fibrine, and caseine, and the corresponding substances derived from vegetables, are all compounds of a substance which he has named proteine, and believes to be composed of the four essential elements alone. He assigns for its composition, carbon 55, hydrogen 7.2, nitrogen 14.5, and oxygen 23.3 per cent.; and for its formula, Caell^NgOjo. Proteine may be obtained by dissolving albumen, fibrine, or caseine in a heated solution of caustic potash (the liquor potassse of the pharmacopoeia will suffice), and adding to the solu- tion enough acetic acid to neutralize it. The proteine, being inso- luble in the neutral salts, is thus precipitated, in the form of a light grayish powdery-looking substance, whose reactions are very similar to those of coagulated albumen. Liebig, however, and Fleitmann (x. b. 61), deny the existence of any such substance as proteine, on the ground that what Mul- der so called and considered to be formed of none but the essential elements, always contains a certain quantity of sulphur, as the albumen or other substance from which it was prepared did. This question is yet undetermined; for since Liebig published his opinion, Mulder has repeated his own, and maintained that, though the proteine prepared as above described does contain sul- phur, yet it is not in the form of elemental sulphur, but in that of hypo-sulphurous acid. He believes albumen, fibrine, and other principles of this group to be compounds of proteine with sulphamid and phosphamid, and that, in dissolving them in potash-lye, these compounds are decomposed with water, ammonia being formed and given off, while sulphurous and phosphorous acids combine with the proteine (lix. 1847, p. 82). The question must, as yet, be thus left; but there seems sufficient probability in Mulder's views to justify the received use of the term proteine compounds, in speak- ing of the class, including fibrine, albumen, and others to which the name of albuminous compounds used to be applied.1 Ilm-ny Matter.—-The substance of the horny tissues, including the hair and nails (with whalebone, hoofs, and horns), probably ' For an account of the oxides and other compounds of proteine, see the Lssay of Mulder quoted above, or Canstatt's Jahresbericht for 1847, p. 82. THE HUMAN BODY. 25 consists, according to Mulder, of proteine with larger proportions of sulphamid than albumen and fibrine contain. Hair contains 10 per cent, and nails 6.8 per cent, of sulphamid. The composition of the latter is 50.1 of the former C. 49.9 6.9 H. 6.4 17.3 N. 17.1 22.5 0. 21.6 3.2 S. 5. The horny substances, to which Simon applies the name of kera- tine, are insoluble in water, alcohol, and ether; soluble in caustic alkalies, and sulphuric, nitric, and hydrochloric acids; and not precipitable from the solution in acids by ferrocyanide of potassium. Mucus, in some of its forms, is related to these horny substances, consisting, in great part, of epithelium detached from the surface of mucous membrane, and floating in a peculiar clear and viscid fluid. But, under the name of mucus, several various substances are in- cluded, of which some are morbid albuminous secretions containing mucus and pus-corpuscles, and others consist of the fluid secretion variously altered, concentrated, or diluted. But the true chemical characters of this fluid are as yet incompletely known. It is gene- rally alkaline, and, when the cells and other corpuscles mingled with it have subsided, is a pellucid fluid, containing, according to Berzelius, 5.33 per cent, of proper mucous matter. This is very little soluble in water; more soluble in water slightly alkaline, and from this solution is precipitated by alcohol, acetic, nitric, sulphuric, and hydrochloric acids. An excess of the last three acids redis- solves the precipitates they severally throw down; and, in the acid solution thus formed, ferrocyanide of potassium produces no pre- cipitate. According to Scherer (x. b. 57), pure mucus, cleared of epithelium, and subtracting 4.1 per cent, of saline matter, contains carbon 52.17, hydrogen 7.01, nitrogen 12.64, oxygen 28.18. Extractive Matters.—Under this name are included substances of mixed and uncertain composition, which form the residue of animal matter when, from almost any of the fluids or solids of the body, the albuminous, gelatinous, and fatty principles have been removed. The remaining animal matter is mixed with various salts, such as lactates, chlorides, and phosphates, and is divisible into two principal portions, of which one is soluble in water alone, the other in alcohol. Doubtless there are in these substances many distinct compounds, of which some exist ready-formed in the body, and some are formed in the changes to which the previous chemical examinations have given rise. Some of these substances have received specific names, according to their most striking characters, as osinazome and 3 C. H. N. 0. with \ P- / S. 26 CHEMICAL COMPOSITION OF zomidine, on which the principal odor and taste of cooked made appear to depend; or according to their source, as ptyahne and phymatine, from the saliva and pancreatic fluid; and part of the extractive matter of the blood appears to be a compound of pro- teine. (Ludwig, x. 1845.) But the true composition, origin, and nature of all these substances are unknown. Kreatine and krea- tinine, two principles which used to be included among the ex- tractive matters of muscular tissue, have been lately studied by Liebig (liv.), who has found them also in the urine, and has thus given additional probability to the suggestion of Berzelius, that the extractive matters are generally the products of the chemical changes that take place in the natural waste and degeneration of the tissues, and are the substances that are to be separated from the tissues for excretion. Such are the chief substances of which the human body is com- posed. They are formed mainly of the four essential elements, and exhibit all those characters which have been mentioned as peculiar to organic bodies; but with the exception of the fatty matters, and perhaps proteine, all appear to contain, besides the four elements, other elements, or even compound substances, such as phosphate of lime, chloride of sodium, or other salts. And all the fluids and tissues of the body appear to consist, chemically speaking, of mix- tures of several of these principles, together with saline matters. Thus, for example, a piece of muscular flesh would yield fibrine, albumen, gelatine, fatty matters, salts of soda, potash, lime, mag- nesia, iron, and other substances which appear passing from the organic towards the inorganic states, as kreatine and others. This mixture of substances may be explained in some measure by the existence of many different structures or tissues in the muscles; the gelatine may be referred principally to the cellular tissue between the fibres, the fatty matter to the adipose tissue in the same position, and part of the albumen to the blood and the fluid by which the tissue is kept moist. But, beyond these general statements, little can be said of the mode in which the chemical compounds are united to form an organized structure ; or of how, in any organic body, the several inorganic and incidental substances are combined with those that are organic and essential. It must suffice, therefore, to mention the several parts in which each of the incidental elements and of their principal compounds occur. Sulphur1 is probably, next to the essential ones, the most nearly constant element in organic compounds. It exists in albumen, fibrine, caseine, and gelatine, combined in all these, probably in ' °!jthe .^ntity of sulphur in different animal substances, see Ruling and others in Llebig's Annalen der Chemie und 1'harmacie, Bd. lviii and Canstatt s Jahresbencht for 1846, p. 90. THE HUMAN BODY. 27 the elemental state, with the other elements. In largest propor- tion it is found in taurine, one of the products of the decomposition of biliary matter, and in the morbid product, cystic oxide : of both these it constitutes about 25 per cent. Among the tissues, and independent of the compounds above named as containing it, sul- phur is most abundant in the hair, cuticle, nails, and other horny tissues, and, according to Lassaigne (lv. Aug. 22, 1843), in fibrous and mucous membranes. Of the compounds of sulphur none are known to exist naturally, except the sulphocyanide of potassium in saliva, and the alkaline sulphates in the urine and sweat. The acid of the sulphates found in the ashes of other animal substances are formed during the burning, through the elemental sulphur combining with oxygen. Phosphorus is found together with sulphur, and probably simi- larly combined as an element, in albumen and fibrine, but not in caseine. It exists also in some tissues, especially in the substance of the brain, from which two fatty acids, containing phosphorus and named oleo-phosphoric and cerebric acid, have been obtained; but, most abundantly, it occurs as phosphoric acid in combination with alkaline and earthy bases—as in the tribasic phosphate of soda in the blood and saliva, the super-phosphates of the muscles and urine, the basic phosphate of lime and magnesia in the bones and teeth. Such phosphates are also found in the ashes of nearly all burnt animal substances, even in tissues so simple that one must assume the phosphate to be a necessary constituent of the substance of the primary cell; for it is probable that these phosphates exist in the tissues ready formed, as they do in caseine, and that they are not, like the sulphates, found in the ashes of animal matters, produced in the combustion. Silicon.—A very small quantity of silica exists, according to Berzelius, in the urine, and, according to Henneberg (x. Bd. 41) and E. Millon (xviii. 1848), in the blood. Traces of it have also been found in bones by V. Bibra, in hair by Van Laer, and in some other parts of the body (lxv. p. 65). Chlorine is abundant in combination with sodium, potassium, ammonium, and other bases in all parts, fluids as well as solid, of the body. Chloride of sodium (common salt) is, indeed, probably the most abundant of all the inorganic compounds in organized bodies. It is also not improbable that chlorine may exist in the gastric fluid in the form of" hydrochloric acid, either free or in com- bination with an organic principle. (Schmidt, lix. 1847, p. 102.) Fluorine—After the observations of Berzelius had been much questioned on which the existence of minute quantities of fluoride of calcium in the bones, teeth, and urine was admitted, they have been fully confirmed by Dr. Daubeny and Mr. Middleton (lxiii. vol. ii. pp. 97, 134), and more recently by Aron Bibra (lxiv.). The salt 28 CHEMICAL COMPOSITION OF is found in the ashes of all bones and teeth; and increased in quantity in fossil bones. Potassium and sodium are constituents of the blood and all the fluids, in various quantities and proportions. They exist in the forms of chlorides, sulphates, and phosphates, and probably, also in combination with albumen, or certain organic acids. Liebig, in his work on the Chemistry of Food, has shown that the juice ex- pressed from muscular flesh always contains a much larger propor- tion of potash-salts than of soda-salts ; while in the blood and other fluids, except the milk, the latter salts always preponderate over the former; so that, for example, for every 100 parts of soda-salts in the blood of the chicken, ox, and horse, there are only 40.8, 5.9, and 9.5 parts of potash-salts; but for every 100 parts of soda- salts in their muscles there are 381, 279, and 285 parts of potash- salts. Calcium.—The salts of lime (oxide of calcium) are by far the most abundant of the earthy salts found in the human body. They exist in the lymph, chyle, and blood in combination with phosphoric acid, the phosphate of lime being probably held in solution by the presence of phosphate of soda. Perhaps no tissue is wholly void of phosphate of lime; but its especial seats are the bones and teeth, in which, together with carbonate and fluate of lime, it is deposited in minute granules, in a peculiar compound, named bone-earth, and containing 51.55 parts of lime, and 48.45 of phosphoric acid Phosphate of lime, probably the tribasic phosphate, is also found in the saliva, milk, bile, and most other secretions, and superphos- phate in the urine, and probably in the gastric fluid. (Blondlot, xvi.) Magnesium appears to be always associated with calcium, and probably exists in the same forms as it; but its proportion is always much smaller, except in the juice expressed from muscles, in the ashes of which magnesia preponderates over lime. (Liebig, liv.) Iron.—The especial place of iron is in thehaematosine, the color- ing-matter of the blood, of which a further account will be given with the chemistry of the blood. Peroxide of iron is found, in very small quantities, in the ashes of bones, muscles, and many tissues, and of lymph and chyle, albumen of serum, fibrine, bile, and other fluids; and a salt of iron, probably a phosphate, exists in considerable quantity in the hair, black pigment, and other deeply colored epithelial or horny substances. Manganesium.— Vauquelin believed he found a trace of the peroxide of this metal in the ashes of hair and bones; but in the more accurate analysis of the former substance by V. Laer, and of the latter by V. Bibra, no mention of manganesium is made. It has been detected in gall-stones (lxxxii. vol. i. p. 4). According to M. E. Millon (xviii. 1848), it exists naturally in blood. THE HUMAN BODY. 29 Aluminium also is stated (Henle, xxxvii. p. 4) to exist in the ashes of hair, bones, and enamel; but neither V. Laer nor V. Bibra mention it. Copper.—-After long disputes, the general existence of copper in the human liver may be regarded as proved by the experiments of Orfila, Heller, and others. It exists in especially large quantity in dark biliary calculi, and we may probably assume that it does not enter into the proper permanent substance of the liver, but is con- tained in the bile, within the bile-cells and ducts, and is destined with it to be excreted. It is true, that Harless and V. Bibra have found it constantly present in the blood, as well as in the liver, of many mollusca and fish: and that in their blood it takes the place of some proportion of the iron contained in the blood of other species, and may be regarded as a normal, necessary constituent; yet, it seems most likely that, in the human body, both copper, manganesium, and aluminium should be regarded as accidental elements, which, being taken in minute quantities with the food, and not excreted at once with the feces, are absorbed and deposited in some tissue or organ, of which, however, they form no necessary part. In the same manner arsenic and lead, being absorbed, may be deposited in the liver and other parts. This view is confirmed by the fact observed by Heller, that although copper is frequently present in the bile of adults, yet it is never found in that of infants (ix. vol. ii. p. 321). The researches of Cattanei di Momo also seemed to prove that neither copper nor lead exists in the bodies of new-born children or infants (xxv. 1843-4, p. 3). CHAPTER II. STRUCTURAL COMPOSITION OF THE HUMAN BODY. The component substances of the body are commonly divided into fluids and solids. The fluids are, 1st, formative fluids, from which are derived the materials for the formation of the solid tissues; and, 2d, secreted fluids, which are separated from the tissues and the blood, through, speaking generally, the operation of special organs, such as cells arranged in glands or membranes. So little can be said that would apply to all the members of either of these classes of the fluids, that a general description of them would be useless; they will therefore be considered in their several more appropriate places.—[See chapters on Blood, Lymph, Chyle, the several Secretions, &c] 3* 30 STRUCTURAL COMPOSITION OF Among the solids of the body, some appear, even with the help of the best microscopic apparatus, perfectly uniform and ^P1® : they show no trace of structure, i. e., of being composed of definitely arranged dissimilar parts. These are named simple, structureless, or amorphous solids. Such are the apparently structureless mass composing the albumen of eggs, and the substance called cytoblastema, or formative substance, in which the nuclei and cells are imbedded in many tissues in progress of development. Such also is the simple membrane which forms the walls of most primary cells, of the finest capillary bloodvessels and gland-ducts, and of the sarco- lemma of muscular fibre; and such the membrane enveloping the vitreous humor of the eye. Such also, having a dimly granular appearance, but no really granular structure, is the intercellular substance of the most perfect cartilage. In the solids which present determinate structure, certain primary forms may be distinguished, which, by their various modi- fications and modes of combination, make up the tissues and organs of the body. Such are: 1. Granules, or molecules, the simplest and minutest of the primary forms. They are particles of various size, from immeasurable minuteness to the l-10000th of an inch in diameter; of various and generally uncertain composition, but usually so affecting light transmitted through them, that at differ- ent focal distances their centre, or margin, or whole substance, ap- pears black. From this character, as well as from their low specific gravity (for in microscopic examinations they always appear lighter than water), and from their solubility in ether when they can be favorably tested, it is probable that most granules are formed of fatty or oily matter; or, since they do not coalesce as minute drops of oil would, that they are particles of oil coated over with albumen deposited on them from the fluid in which they float. (See Ascherson, lxxx. 1848.) In any fluid that is not too viscid, they exhibit the phenomenon of molecular motion, shaking and vibrating incessantly, and sometimes moving through the fluid, under the influence of some unknown force. Granules are either free, as in milk, chyle, milky serum, yelk-sub- stance, and most tissues containing cells with granules ; or inclosed, as are the granules in nerve-corpuscles, gland-cells, and epithelium- cells, the pigment granules in the pigmentum nigrum and medullary substance of the hair; or imbedded, as are the granules of phos- phate and carbonate of lime in bones and teeth. 2. Nuclei, or cytoblasts, appear to be the simplest elementary structures, next to granules. They were thus named in accordance with the hypothesis that they are always connected with cells, or tissues formed from cells, and that in the development of cells, each nucleus is the germ or centre around which the cell is formed. The hypothesis is only partially true, but the terms based on it are THE HUMAN BODY. 31 too familiarly accepted to make it advisable to change them till some more exact and comprehensive hypothesis is formed. Of the corpuscles called nuclei, or cytoblasts, the greater part are minute cellules, or vesicles, with walls formed of simple mem- brane, inclosing a colorless pellucid fluid, and often one or more particles, like minute granules, called nvcleus-corpuscles, or nucleoli. Such vesicular nuclei, without nucleoli, are those of the blood-cor- puscles of oviparous vertebrate animals (figs. 1 and 2); and such, Fig. 1. l Fi;r. I. Corpuscles of human blood, magnified about 500 diameters.—(1) Single particles. 1,1. Their flattened face. 2, A particle seen edgewise. (2) Aggregation of particles in a columnar form. Fi<*. 2. Red particles of the blood of the common fowl, a, Ordinary appearance when the flat surface is turned towards the eye; 6, appearance which is sometimes presented by the particle when in the same position, and which suggests the idea of a furrow surrounding the central nucleus; c, d, different appearances of the particles when seen edgewise. with nucleoli, are those of epithelium-cells and pigment-cells. But some nuclei appear to be formed of an aggregate of granules im- bedded in a pellucid substance, as, for example, the nuclei of the lymph- and chyle-corpuscles. The composition of the nucleus is uncertain. One of its most general characters, and the most useful in microscopic examinations is, that it is neither dissolved nor made transparent by acetic acid, but acquires, when that fluid is in contact with it, a darker and more distinct outline. On this account, Kblliker (lxxxiv. pp. 144-5) supposes the wall of the nucleus to be composed of a proteine- compound named pyine ; its contents he also considers to be albu- men, and the nucleoli fatty matter; but there is not sufficient evidence for these views. _ Nuclei may be either free or attached. Free nuclei are such as either float in fluid, like those in the gastric juice, which appear to be derived from the secreting cells of the gastric glands, or lie loosely imbedded in solid substance, as in the gray matter of the brain and spinal cord, and most abundantly in some quickly-grow- m?ittached nuclei are either closely imbedded in homogeneous pel- lucid substance, as in rudimental cellular tissue ; or are fixed on the surface of fibres, as on those of organic muscle and organic nerve- 32 STRUCTURAL COMPOSITION OF fibres (fig. 3); or are inclosed in cells, or in tissues formed by the extension or junction of cells. JNu- clei inclosed in cells appear to be attached to the inner surface of the cell wall, projecting into the cav- ity. Their position in relation to the centre or axis of the cell is un- certain ; often, when the cell lies on a flat or broad surface, they ap- pear central, as in blood-corpus- cles, epithelium-cells, whether tes- selated or cylindrical; but, per- haps, more often their position has no regular relation to the centre of the cell. In most instances, each cell contains only a single nucleus; but in cartilage, espe- cially when it is growing or ossify- ing, two or more nuclei in each cell are common ; and the development of new cells is often effected by a division or multiplication of nuclei in the cavity of a parent cell; as in blood-cells, the germinal vesicle, and others. When cells extend and coalesce, so that their walls form tubes or sheaths, the nuclei commonly re- main attached to the inner surface of the wall. Thus they are seen imbedded in the walls of the minutest capillary bloodvessels of, for example, the retina and brain; in the sarcolemma of transversely striated muscular fibres; and in minute gland-tubes. In such cases their arrangement may be irregular, as in the capillaries; or regular, as in the single or alternating double rows of nuclei in different examples of the muscular fibre. Nuclei are most commonly oval or round, and do not generally conform themselves to the diverse shapes that the cells assume; they are, altogether, less variable elements, even in regard to size, than the cells are; of which fact one may see a good example in the uniformity of the nuclei in cells so multiform as those of epi- thelium. But sometimes they appear to be developed into fila- ments, elongating themselves and becoming solid, and uniting end to end for greater length, or by lateral branches to form a net- work. So, according to Henle (xxxvii. p. 194), are formed the filaments of the striated and fenestrated coats of arteries, and the yellow or elastic filaments of cellular tissue, and with organic muscu- Fibres of unstriped muscle: c. In their natural state, a. Treated with ace- tic acid, showing the corpuscles, b. Cor- puscles, or nuclei, detached, showing their various appearances. THE HUMAN BODY. 33 lar fibre, especially in the walls of arteries. The filaments of the cortical substance of hair, and the seminal filaments, or spermato- zoids, appear to be also elongated and divided nuclei. 3. Cells, Primary cells, or Elementary cells, are vesicles or scales of larger average size than nuclei, but, like them, composed, in the normal state, of membranous cell-walls, with, usually, liquid con- tents, and generally round or oval. The cell-wall never presents any appearance of structure : it appears sometimes to be a proteine-substance, as in blood-cells; sometimes a horny matter, as in thick and dried cuticle. In almost all cases (the dry cells of horny tissue, perhaps, alone excepted) the cell-wall is made transparent by acetic acid, which also pene- trates through it and distends it, so that it can hardly be discerned. But in such cases the cell-wall is usually not dissolved ; it may be brought into view again by nearly neutralizing the acid with soda or potash. In some instances, the most developed state of a cell is that in which it has no nucleus, as in the mammalian blood-corpuscles, in which, as will be described, the substance of the nucleus of the lymph- or chyle-corpuscle is gradually all appropriated and changed to the contents of the blood-corpuscle. But, in other instances, especially in old cells, as in those of the nails, the outer layers of epidermis, and the adipose tissue, the nucleus may disappear, wast- ing away; and this is, probably, always a sign of degeneration of the tissue, for a similar wasting of nuclei is commonly observed in all tissues in the state of fatty degeneration. With the exceptions just mentioned, all the cells of the human body appear to contain nuclei. Sometimes the nucleus nearly fills the cavity of the cell, as in lymph- and chyle-corpuscles, in which the cell-wall lies so close round the nucleus, that it can hardly be seen till it is raised up by water or acetic acid insinuat- ing itself between it and the nucleus; and such is the proportion between the nucleus and cell in young epidermis-cells; but more often the nucleus has a diameter from one-fourth to one-tenth less than that of the cell. The simplest shape of cells, and that which is probably the nor- mal shape of the primary cell, is oval or spheroidal, as in cartilage- cells and lymph-corpuscles; but in many instances they are flat- tened and discoid, as in the blood-corpuscles, or scale-like, as in epidermis and tesselated epithelium. By mutual pressure they may become many-sided, as the pigment-cells of the choroidal pigmen- tum nigrum and in close-textured adipose tissue; they may assume a conical or cylindriform or prismatic shape, as in the varieties of cylinder-epithelium and the enamel-tubes; or be caudate, as m certain bodies in the spleen; they may send out exceedingly fine processes in the form of vibratile cilia, or larger processes, with 34 STRUCTURAL COMPOSITION OF which they become stellate, or variously caudate, as in the large nerve- or ganglion-corpuscles, and the epithelium of the choroid plexuses. The contents of cells, including under this term all but their nu- clei, are almost infinitely various, according to the position, office, and age of the cell. In adipose tissue, they are the oily matter of the fat, the mixture of margarine and oleine; in gland-cells, the contents are the proper substance of the secretion, bile, semen, &c, as the case may be; in pigment-cells, they are the pigment-granules that give the color; and in the numerous instances in which the cell-contents can be neither seen because they are pellucid, nor test- ed because of their minute quantity, they are yet, probably, pecu- liar in each tissue, and constitute the greater part of the proper substance of each. Commonly, when the contents are pellucid, they contain granules which float in them; and when water is added and the contents are diluted, the granules display an active molecular movement within the cavity of the cell. Such a move- ment may be seen by adding water to mucus- or pus-corpuscles, or to those of lymph. In a few cases the whole cavity of the cell is filled with granules: it is so in yelk-cells and milk-corpuscles, in the large diseased corpuscles often found among the products of in- flammation, and in some cells when they are the seat of extreme fatty degeneration. The peculiar contents of cells may be often observed to accumulate first around or directly over the nuclei, as in the cells of black pigment, in those of melanotic tumors, and in those of the liver during retention of bile. Intercellular substance is the material in which, in certain tissues, the cells are imbedded. Its quantity is very variable. In the finer epithelia, especially the columnar epithelium on the mucous mem- brane of the intestines, it can be just seen filling the interstices of the close-set cells; here it has no appearance of structure. In car- tilage and bone it forms a large portion of the whole substance of the tissue, and is either homogeneous and finely granular, or osse- ous, or, as in fibro-cartilage, resembles tough tendinous tissue. In Borne cases, the cells are very loosely connected with the intercell- ular substance, and may be nearly separated from it, as in fibro- cartilage; but in some their walls seem amalgamated with it. The foregoing may be regarded as the simplest, and the nearest to the primary, forms assumed in the organization of animal mat- ter ; as the states into which it passes in becoming a solid tissue living or capable of life. By the further development of tissue thus far organized, according to rules which will be hereafter de- scribed, higher or secondary forms are produced, which it will be sufficient in this place merely to enumerate. Such are, 4, Fila- ments, or fibrils.—Threads of exceeding fineness, from l-20000th of an inch upwards. Such filaments are either cylindriform, as are TISSUES OF TnE HUMAN BODY. 35 those of the striated muscular (figs. 4 and 5), and the fibro-cellular or areolar tissue (figs. 6 and 7); or flattened, as are those of the organic muscles (fig. 3), the common elastic tissues (figs. 8 and 9), and the finer variety of the same tissue, which is commonly asso- ciated with the proper white filaments of the fibro-cellular tissue. Filaments usually lie in parallel fasciculi, as in muscular and tend- inous tissues; but in some instances are matted or reticular, with branches and intercommunications, as are the filaments of the mid- dle coat, and of the longitudinally-fibrous coat of arteries; and in other instances, are spirally wound, or very tortuous, as in the common fibro-cellular tissue. Fig. 4. Fig. 5. Fig. i. Muscular fibre of animal life (magnified 5 diameters). A. Small portion, natural Bize. b. Same, magnified 5 diameters, of larger and smaller fasciculi, seen in transverse section. Fig. 5. Portion of broken muscular fibre of animal life (magnified about 700 diameters.) 5. Fibres, in the instances to which the name is commonly applied, are larger than filaments or fibrils, but are by no essential general character distinguished from them. The flattened band- like fibres of the coarser varieties of organic muscles and elastic tissue are the simplest examples of this form; the toothed fibres of the crystalline lens are more complex; and more compound, so as hardly to permit of being classed as elementary forms, are the striated muscular fibres, which consist of bundles of filaments in- closed in separate membranous sheaths, and the cerebro-spinal nerve-fibres in which similar sheaths inclose apparently two varie- ties of nerve-substance. 6 Tubules are formed of simple membrane, such as the minute capillary lymph- and bloodvessels, the investing sheaths of striated 36 TISSUES OF THE HUMAN BODY. muscular and cerebro-spinal nerve-fibres, and the basement-: brane or proper wall of the fine ducts of secreting glands. Fig. 6. FiS- 7- Fig. 6. Fasciculi and fibres of cellular tissue.—The two elements of Areolar tissue, in their natural relations to one another: 1, the white fibrous element, with cell-nuclei, 9, sparingly visible in it; 2, the yellow fibrous element, showing the branching or anastomos- ing character of its fibrilhje; 3, fibrillas of the yellow element, far finer than the rest, but having a similar curly character, 8, nucleolated cell-nuclei, often seen apparently loose.—■ From the areolar tissue under the pectoral muscle, magnified 320 diameters. Fig. 7. Development of the Areolar tissue (white fibrous element); 4, nucleated cells, of a rounded form; 5, 6, 7, the same, elongated in different degrees, and branching. At 7, the elongated extremities have joined others, and are already assuming a distinctly fibrous tis- sue character. (After Schwann.) Most of the tissues which are composed of these primary struc- tures will be briefly described in future chapters, and in connection with the physiology of the organs that they help to form. The insertion of a system of general anatomy would not further the purpose of this work; and would be superfluous while the student has access to such admirable workg devoted to the subject as the Introduction to Quain's Anatomy by Dr. Sharpey, the Physiologi- cal Anatomy of Dr. Todd and Mr. Bowman, the Microscopic Ana- tomy of the Human Body by Mr. Hassall, and the various articles TISSUES OF THE HUMAN BODY. 37 on the tissues published in the Cyclopaedia of Anatomy and Phy- siology. Fig. 8. Fig. 9. Fig. 8. Fibres of elastic tissue from the ligamentum flavum of the vertebrae. (Magnified 320 diameters.) Fig. 9. Portion of white fibrous tissue, magnified 320 diameters. 1, 2. Straight appearance of the tissue when stretched; 3, i, 5. Various wavy appearances which the tissue exhibits when not stretched. CHAPTER III. VITAL PROPERTIES OF THE ORGANS AND TISSUES OF THE HUMAN BODY. Some of the actions observed in living bodies indicate the opera- tion of other properties and forces besides those which can be referred to the chemical and mechanical constitution of organized substances. These properties being the sources of phenomena which are peculiar to living beings, are named vital properties ; the forces issuing from them, vital forces; the acts in which they are ex- pressed, such as those enumerated at pp. 13, 14, are vital acts or vital processes ; and the state in which these processes are displayed is life. . . 1. The most general, perhaps an universal, property of living bodies, is that which is manifested in the ability to form them- 4 38 VITAL PROPERTIES OF THE ORGAN selves out of materials dissimilar from them; as when, for ex- ample, the ovule develops itself from the nutriment of the fluids ot the parent-or when a plant, or any part of one, grows by appro- priating the elements of water, carbonic acid, and ammonia—or when an animal subsists on vegetables, and its blood and various organs are formed from the materials of its food. The force which is manifested in these acts is termed formative force (assimilative, or plastic force); and the processes effected by it are named assimi- lative, nutritive, or formative processes. This power of self-formation from dissimilar materials, which appears to be wholly peculiar to living bodies, and without which, probably, none exists, manifests itself in three modes, which, though they bear different names, yet probably are only three ex- pressions of one force operating in different conditions : they are development, growth, and assimilation, or maintenance. Development is the process by which each tissue or organ of a living body is first formed; or by which one, being already incom- pletely formed, is so changed in shape and composition as to be fitted for a function of a higher kind; or, finally, is advanced to the state in which it exists in the most perfect condition of the species. Growth, which commonly concurs with development and con- tinues after it, is, properly, mere increase of a part by the insertion or superaddition of materials similar to those of which it already consists. In growth, properly so called, no change of form or composition occurs: parts only increase in weight, and, usually, in size; and if they acquire more power, it is only more power of the same kind as that which they before enjoyed. In assimilation, or self-maintenance, living bodies preserve their condition notwithstanding the changes to which they are liable through the influence of external forces and their own natural de- cay; and the stability of composition which they thus display is effected by the continual formation of new particles in the place of those that are impaired and removed. The modes in which these three manifestations of formative power are accomplished will be considered hereafter, especially in the chapters on Nutrition, Secretion, and Development. From these it will appear that the most general, and one of the simplest, modes is in the formation and further development of nucleated cells. The nucleated cell is the form towards which organizing matter most commonly tends, in which it often rests, and through which it usually proceeds in its further developments. In nucleated cells, also, are the best examples of inherent formative power, which, not being consumed in the formation of the cells, remains operative in them, changing them and their contents, and influencing the sur- rounding or intercellular substance in which they are deposited. Thus, whether it be for the preparation of materials from food which TISSUES OF THE HUMAN BODY. 39 may serve to the maintenance of the body, or for the construction of the several tissues, or for the formation or temporary storing-up of the materials that are_io be removed from the body as refuse, in all these, and in nearly all instances of them, the end is attained by or with the help of the formation, continued energy, or dissolu- tion of nucleated cells. The property to which is referred the formative power of living beings, or living parts, is, however, no simple property, such as the " attraction" of mechanical science, or the "affinity" of chemistry. These manifest themselves in acts so simple and almost uniform, that the hypothesis which assumes the existence of such properties supplies at once the language in which their laws of action may be enunciated. But in the simplest exercise of living formative power, even in that which accomplishes the formation of a cell, there is evidence of the operation of many forces. Mechanical force is shown in the determination of the position, shape, and rela- tions of the cell; chemical force, in the determination of the com- position of its walls and contents: and with these, or as if directing them, that vital force, different from them and from all other known physical forces, is in operation, by virtue of which the cell acquires all the properties that characterize the species or organ to which it is attached, and becomes capable of taking part in vital processes —even in such processes as those in which itself originated. Thus the vital formative force seems not to oppose or exclude, but to include and direct the physical forces that issue from the mere matter of the organic body. It may, therefore, be believed that every vital act is accompanied with physical changes in the active matter; but there is no sufficient evidence that such changes ever wholly constitute or make up any of those that are commonly called vital acts. In all those acts or processes some force is ex- ercised peculiar to the state of life, and as different from all recog- nized physical forces as they are different from one another. We cannot tell how much in each act of the living body is physical, and how much depends on the peculiar vital force. The advancing knowledge of the physical sciences does, indeed, prove eyery year that effects, which used to be ascribed to vital forces are due to the operation of the forces of chemistry and mechanics; and it may be observed, generally, that the substances in which the processes of organic life are most actively carried on are those whose chemical com- position is most remote from that of inorganic matter. Still, many things yet remain, observed only in the living body, so completely dependent on the maintenance of the whole state of life, and so different from what physical forces ever accomplish in dead matter, that we cannot refer them to the operation of those forces. Any hypothesis which would abolish the idea of vital formative force, 40 VITAL PROPERTIES OF THE ORGANS AN would be much less reasonable and useful than that which admits it; indeed, unless we admit the existence of such a force in the processes of organic life, and adopt the language which the hypo- thesis suggests, it is hardly possible to express the ordinary tacts of physiology. T 2. Contractility may be reckoned a second vital property. It consists in the power which certain tissues have, during life, of contracting or shortening themselves in a peculiar manner. Such contractility is usually and best observed in fibrous tissues, as in muscles and some kinds of fibro-cellular tissue; but it maybe seen, also, in cells and collections of them, as in the muscles of embryos, while they yet consist of cells, and before any fibres are developed in them. The peculiar contraction of muscular and other organic tissues differs from the contraction of which dead and inorganic matter is capable, both in its modes and in the conditions that give rise to it. The modes of contraction will be described hereafter: the conditions are not only such previous elongation as would be followed by contraction in any elastic body, but various slight changes, such as those produced by the contact of foreign matters, varia- tions of temperature, electricity, &c. These, and whatever will give rise to the peculiar contractions in the organic tissues, are called stimuli. That which most characterizes the contractility of animal tissues is, that the contraction may be excited by the application of a stimulus to the nerves that ramify in them; and, indeed, it is generally through the nerves that the stimulus which produces a contraction is conveyed.,.-: In the chapter on the Muscles, it will be shown that the property of contractility is inherent in the tissues that contract, and is essentially independent of their nerves; so that contraction may take place without the co-operation of the nerves. Therefore, the whole property of irritability, including both the capacity of receiving a direct stimulus and the power of contracting in consequence thereof, may be ascribed to the muscles and other contractile tissues. But, in the ordinary conditions of life, the nerves may be said to be necessary to contractions, because, in these conditions, it is only through the medium of nerves that a stimulus is applied to the contractile tissues, and when the nerves are destroyed contractions do not naturally ensue. The modes in which stimuli are applied to, and their effects conveyed along, nerves to the contractile parts, will be described in the chapter on the Nervous System. One mode is essentially characteristic of animals; that, namely, wherein the contractile tissues are made to act by a Mind, an Anima; which, having knowledge of the existence of tho body, consciousness of power, and TISSUES OF THE HUMAN BODY. 41 will to exercise it, acts on the nervous system,1 and through it on the contractile tissues : thus voluntary motion is produced. 3. The power of conducting or transmitting stimuli or impressions which, in the foregoing paragraphs, has been ascribed to the nerves, constitutes another peculiar vital property. It belongs to the nervous system alone, and may be said to consist in this—that the state, or change, which is produced in the fibre of a nerve by the application of a stimulus of any kind, may be propagated through the whole length of the fibre, so that every part thereof shall, with immeasur- able rapidity, be brought into the same state as the part first stimulated. Thus the stimulus, or rather the change or impression produced by it, is said to be conducted by the nerve; in the same way as it is said electricity is conducted along a wire, although at the instant of contact with the source of electricity the whole wire becomes at once electric. A peculiar force is generated by the change thus produced in nerves, the effect of which force, when the nerve conveys it to a muscle, is shown by the muscle contracting with a force which, other things being equal, is directly proportionate to the intensity of the stimulus. The same force, generated by stimulating nerves, may also be shown by changes in organic processes; as when secretions are augmented, diminished, or altered, in states of nervous excitement. The force thus developed in the nerves has been named vis nervosa, but there is an objection to the use of the term, since some appear to think that force is exercised only when the conduction takes place towards muscles, and that in that case the force itself is in some way transferred from the nerves to the muscle. But a safer hypothesis appears to be that which holds that a peculiar force is generated whenever a nerve is stimulated; and which ascribes to all nerves alike the power of conducting im- pressions; i. e., of propagating the changes produced by stimuli, and the force issuing therefrom. Adopting this hypothesis, we may believe that the different consequences of such conducted force depend on the various connections of the conducting nerve- fibres with the nervous centres and contractile parts, on the nature and strength of the stimulus, and on other circumstances external to the nerves. When an impression is conveyed from any part of the body, along a nerve, to the brain, the mind may take cognizance of it: what"the mind thus becomes conscious of is called a sensation; and the act of the mind noticing it, perception; and all parts through i It does so, at least, in all animals in which a nervous system can be demonstrated.' In those in which none has been yet seen, it must be doubt- ful wbetherthe mind can directly influence the contractile tissues, or whether some nervous material exists which we cannot discover, but through which the mind of the animal can act. Ask 42 THE BLOOD. the nerves of which such sensations may be denyed are «dled sensible or sensitive parts. But in the use of the latter terms it must be remembered that they mean only that certain parts are capable of giving rise to sensations. The mind alone is sentient and percipient; neither tissues, nerves, nor brain could of them- selves, or from any property or change of which they are capable, become in any sense conscious of their condition. _ Such appear to be the peculiar properties of living animal matter. It is all capable of self-formation, and the various modes of forma- tion constitute the principal functions of the organic life of the animal. Some of the living tissues are, also, capable of peculiarly contracting; and the nervous tissue is capable of peculiarly con- ducting the changes or impressions made on it by certain stimuli. These two properties, in their various modifications, are exercised especially in the animal life, and it is by means of these that the mind becomes cognizant of the condition of the body, and through it, of some of the properties of things that act on it, as well as able to will the movements of its several parts. CHAPTER IV. THE BLOOD. The processes enumerated in the first division of the phenomena of Organic Life have their end or purpose in the formation, move- ment, and purification of the blood. The physiology of the blood must, therefore, precede the consideration of these subservient processes. Wherever blood can be seen as it flows in the vessels of a living part, it appears a colorless fluid, containing minute particles, the greater part of which are red, and give the blood its color. The fluid is named liquor sanguinis; the particles are the blood- and lymph-corpuscles, or cells. When blood flows from the living body, it is a thickish heavy fluid, of bright scarlet color when it comes from an artery; deep purple, Modena, or nearly black, when it flows from a vein. Its specific gravity at 60° F. is, on an average, 1055, that of water being reckoned as 1000; and the extremes consistent with health being, according to Nasse, 1050 and 1059 (xv. vol. i. p. 82). Its temperature is, generally, 100° F.; it has a slight alkaline reaction; and emits an odor similar to that which issues from the skin or breath of the animal from which it flows, but fainter. The above-mentioned differences of color in arterial and venous THE BLOOD. 43 blood are sometimes not to be observed. If blood runs very slowly from an artery, as from the bottom of a deep and devious wound, it is generally as dark as venous blood. In climates of high tem- perature also, the arterial blood is dark and hardly to be distin- guished from venous blood. (John Davy, lxxxv. vol. ii. p. 140.) In persons nearly asphyxiated, also, and sometimes under the influence of chloroform or ether, the arterial blood becomes like the venous. But in all these cases the dark blood becomes bright on exposure to the air. The specific gravity of men's blood is, on an average, rather greater than that of women's, because of the larger proportion of red-corpucles in the former; that of robust persons, for the same reason, is greater than that of the feeble. It is always diminished by bleeding; and so quickly that, in a single venesection, the spe- cific gravity of a portion of the blood last drawn is often less than that of the blood that flows first, (J. Davy, lxxxv. vol. ii. p. 28: Polli, xci.) The specific gravity of blood is increased in diseases attended with great watery discharges, as cholera and diabetes; but with these exceptions, a specific gravity above the natural stand- ard would generally indicate a disease attended with plethora, or excess of the animal constituents of blood, and a low specific gravity, a disease of debility or exhaustion. The alkaline reaction is said to be a constant character of blood in all animals and under all circumstances. An exception has been supposed to exist in the case of menstrual blood; but the acid re- action which this sometimes presents is due to the mixture of an acid mucus from the uterus and vagina. Pure menstrual blood, such as may be obtained with a speculum, or from the uteri of women who die during menstruation, is always alkaline, and re- sembles ordinary blood. (Whitehead, lxxxvi.) The odor of blood is easily perceived in the watery vapor, or halitus, as it is called, which rises from blood just drawn: it may also be set free, long afterwards, by adding to the blood a mixture of equal parts of sulphuric acid and water. It is not difficult to tell, by the likeness of the odor to that of the body, the species of domestic animal from which any specimen of blood has been taken : the strong odor of the pig or cat, and the peculiar milky smell of the cow, are especially easy to be thus discerned in their blood. (Barruel, lxxxvii. No. 1.) Coagulation of the Blood. When blood is drawn from the body, and left at rest, certain changes ensue which constitute a kind of rough analysis of it, and are instructive respecting the nature of some of its constituents. After about ten minutes^takiog a general average of many observa- tions, it gradually clots or coagulates, becoming solid, like a soft 44 THE BLOOD. jelly. The clot thus formed has at first the same volume and ap- pearance as the fluid blood had, and, like it, looks quite uniform : the only change seems to be that the blood which was fluid is now solid. But presently, drops of transparent yellowish fluid begin to ooze from the surface of the solid clot; and these gradually collecting, first on its upper surface, and then all around it, the clot, diminished in size, but firmer than it was before, floats in a quantity of yellowish fluid which is named serum, and the quantity of which may continually increase for from twenty-four to forty- eight hours after the clotting of the blood. (Figs. 10 and 11.) Fig. 10. Fig. 11. The changes just described may be thus explained. The liquor sanguinis, or liquid part of the blood, consists of serum, holding fibrine in solution. The peculiar property of fibrine, as already said (p. 15), is its tendency to become solid when at rest, and in" some other conditions. When, then, a quantity of blood is drawn from the vessels, the fibrine being (in the normal state) equally diffused through the whole mass, coagulates, and serum and blood-corpuscles are held, or, as it were, soaked and entangled in the solid substance which it forms. But after healthy fibrine has thus coagulated, it always contracts; and what is generally described as one process of coagulation should rather be regarded as consisting of two parts or stages; namely, first, the simple act of clotting, coagulating, or becoming solid; and, secondly, the contraction or condensing of the solid clot thus formed By this second act, much of the serum which was soaked m the clot is gradually pressed out; and this collects in the vessel round the contracted clot. Thus, by the observation of blood within the vessels, and of the changes which commonly ensue when it is drawn from them, we may distinguish it in three principal constituents, namely, 1st, the hbrme, or coagulating substance, which has been also called the ymph, or coagulating lymph of the blood; 2, the serum, in which the hbrme, before its coagulation, was dissolved : 3, the blood- and lymph-corpuscles. ; inKe, fibrine, is the °nly spontaneously coagulable material m the blood, may be proved in many ways; and, most simply, by THE BLOOD. 45 any means by which a portion of the liquor sanguinis, i. e., the serum and fibrine, can be separated from the red corpuscles before coagulation. This separation is always effected when coagulation is retarded beyond the usual time, or when the red corpuscles, which have a higher specific gravity than the other constituents of the blood, sink more rapidly than usual. Coagulation may be retarded by cold, or by drawing the blood into a vessel containing oil, so that, as the oil floats over it, it may be excluded from the air. The red corpuscles, also, sink quickly in blood drawn from patients with inflammatory diseases, and in horses' blood. In any of these cases the red corpuscles may be observed, while the blood is yet fluid, to sink below its surface ; and the layer beneath which they have sunk, and which has usually an opaline or grayish white tint, will coagulate without them, and form a white clot consisting of fibrine alone, or of fibrine with entangled white corpuscles; for the white corpuscles being very light, tend upwards towards the sur- face of the fluid. The layer of white clot which is thus formed rests on the top of a colored clot of ordinary characters, i. e., of one in which the coagulating fibrine has entangled the red corpus- cles while they were sinking; and, thus placed, it constitutes what has been called a buffy coat. It is also by the action of the fibrine alone that the contraction of the clot is FlS-12- effected, and the contraction is greatest when it is least hindered by particles in- closed in the solid fibrine. Hence, when a buffy coat is formed in the manner just described, it commonly contracts more than the rest of the clot does,, and, draw- ing in at its sides, produces that cupped appearance on the top of the clot which is often characteristic of the existence of inflammation. (Fig. 12.) It is thus evident that the coagulation of the blood is due to its fibrine. We are not able to explain certainly why fibrine coagu- lates ; but the most reasonable hypothesis is, that its coagulation is a process of organization—a spontaneous assumption of organic form and structures which, in favorable conditions, would be the first step towards the formation of a more highly organized tissue. The principal evidence for this view is as follows :— 1. When the coagulation of fibrine is observed with the micro- scope (and this may be easily done in a minute portion of liquor sanguinis, taken when the red corpuscles are subsiding), it appears, when first solidified, soft, and quite homogeneous; but gradually, when it becomes tougher, it acquires the appearance of a closely matted or felt-like mass of fine white pliant fibrils, in which scat- tered granules and white corpuscles are imbedded. Such a fila- 46 TIIE BLOOD. mentous structure, with filaments separable by minute dissection, is observable in all well-formed fibrine clots, and is characteristic of organization ; at least, such structures are not seen in inorganic matter, or in organic matter artificially coagulated, as albumen; but they are found in the more lowly organized tissues, such as that of the membrane of egg-shell (Car- Fig. 13. penter, xciii. 1844, page 1), organizing lymph, some of the least developed tu- mors, &c. (Fig. 13.) 2. The coagulation of fibrine is hard- ly, if at all, distinguishable from that of lymph effused in inflammation. Both alike spontaneously coagulate in the same mode, and acquire the same sim- ple structure ; both alike contract after coagulation ; and generally, when lymph effused in any case of inflammation has little tendency to organization, the fibrine of the patient's blood coagulates very Fibrous membrane lining the weakly and imperfectly. It is on the egg-sheii, and forming the animal wnole probable that the lymph of in- flammatory effusion is the fibrine of the blood only somewhat modified by the disease. But the lymph, after coagulating, goes on to more perfect organization, and becomes a vascular tissue; the fibrine shows no such further change, at least in blood drawn from the body. Yet, seeing that the coagulation of the lymph is the first step in its organization, it appears reasonable to regard the similar change in the fibrine as also organization, and to explain the sub- sequent difference between them by the difference of their exter- nal conditions. The lymph continues to be organized, being placed among living organizing parts that may supply it with the neces- sary materials for its support; the fibrine does not so continue, having none of these, perhaps essential, advantages. 3. The coagulation of blood out of the body is not different from its coagulation, under certain conditions, within the body, as e. g., when the circulation through a vessel is obstructed by a liga- ture, or when blood is effused from its vessels into an adjacent cavity or tissue. And there is sufficient evidence that, in many cases, when the blood thus coagulates in contact with healthy liv- ing tissues, its coagulation is the first step towards a more complete organization, and the formation of vascular tissue; just as the coagulation of the lymph effused in inflammation is the beginning of its complete organization. The instances in which this organiza- tion of blood, or rather of the fibrine of blood, has been best ob- served, are those in which the blood coagulates above a ligature on THE BLOOD. 47 an artery, and forms there what is called the conical clot. Such a clot gradually loses its color by the disintegration of the red corpuscles; by the absorption of its serum it becomes firmer and drier ; then it acquires the structure of fine filamentous tissue, and a supply of bloodvessels; it is more and more intimately united with the walls of the vessel, and at length forms part of the fibrous cord by which, finally, the obliterated portion of the artery is replaced. In this process, which has been minutely examined by Zwicky (lxxxix.), the changes through which the fibrine of the blood passes are, in all essential respects, similar to those which constitute the organization of lymph into false membrane or adhe- sions. Doubtless the process is similar by which effused blood is, in many other cases, organized; as, for example, in apoplexy of the cerebral membranes; in which a thin layer of blood having coagu- lated may be traced through stages of organization till it is nearly all formed into a tough membrane lining the dura mater; or in which, when the effusion of blood is more abundant, a layer on the surface of the clot may form a sac or cyst of finer membrane, vascular and transparent like the arachnoid, and inclosing the rest of the blood (see especially Dr. Burrows, lxxi. 1835; and Mr. P. Hewett, xli. 1845). Such organization of clots of blood is also seen in certain tumors attached to the interior of the heart, and more rarely of the arteries, as well as in the formation of phlebo- lithes, or vein-stones, and of the cords by which veins after slight phlebitis, as in phlegmasia dolens, are often obstructed or oblite- rated.1 Such is the principal evidence for ascribing the coagulation of blood to the organization of the fibrine. It is true that blood, when it coagulates in the living body, does not always proceed to further organization; but this may be ascribed to the failure or absence of certain conditions favorable or perhaps essential to the process. Of such conditions the principal are, 1st, that the blood should be free from the changes induced by certain diseases, as typhus, bronchitis, and other affections attended with dyspnoea; 2d, that it should be little, if at all, exposed to the air; 3d, that, when it coagulates, it should be in contact with healthy, or nearly healthy parts; and 4th, that the clot should be not so large but that it may all be near the living parts. It is probably for want of the third condition that the clots in aneurismal sacs and in cerebral apoplectic effusions are not further organized; the parts with which they lie in contact being hardly able to maintain their own nutrition, or to repair the damages they have themselves sustained. And, in very large effu- 1 Most museums contain such specimens. In the Museum of the Royal College of Surgeons, Nos. 12, 1528, 1732, 1733, 1742, may be referred to: in that of St. Bartholomew's Hospital, Nos. 1 and 3 in Series vi., 107 in Series xiii., 45 and 49 in Series xiv. 48 THE BLOOD. sions, the distance of the central part of the clot from the living tissues seems too great to admit of its further organization; just as, in morbid growths, the central parts are apt to degenerate through the insufficiency of the supply of materials for their nutri- tion. It may be added, that arterial blood is, on the whole, more likely to be completely organized than venous blood is. Effused venous blood often remains for many days or weeks without coagulating; arterial blood very rarely does so; (Grarengeot, xcii. tome ii.;) and clots of venous blood, when partially organized, are very liable to deposits of calcareous matter; a form of degeneration which is sel- dom or never observed in organized clots of arterial blood. The evidence that coagulation of blood is due to the organization of its fibrine, justifies Mr. Hunter's expression, that " coagulation is an operation of life," (i. vol. iii. n. 34;) for here he used the term " life" in the same sense as that in which one speaks of a live egg, or a live seed, implying not an active state of life, but the capacity of developing and acquiring higher organization in conditions favorable to life. And the fact that the coagulation of blood may be suspended for any length of time by freezing it, or by adding to it large quantities of alkaline salts, is no more a disproof of Mr. Hunter's doctrine than the fact that eggs are not developed except at a certain temperature is a proof that they are incapable of de- velopment. Nothing more is proved by such a fact, than that certain external conditions are necessary for the perfect performance of these as of all other " operations of life." Conditions affecting Coagidatipn. Although the coagulation of fibrine is, like other processes of organization, spontaneous, yet it is liable like them to be modified by the conditions in which it is placed, such as temperature, motion, the access of air, the substances with which it is in contact, the mode of death, &c. All these conditions need to be considered in the study of the coagulation of the blood.1 Cold retards the coagulation of blood, and it is said that, so long as blood is kept at a temperature below 40° F., it will not coagulate at all. Freezing the blood, of course, prevents its coagulation; yet it will coagulate, though not firmly, if thawed after being frozen; and it will do so even after it has been frozen for several months. Coagulation is accelerated, but the subsequent contraction of the clot is hindered, by a temperature between 100° and 120°: a higher temperature retards coagulation, or, by coagu- 1 The fullest accounts of them all are given by Nasse (xv. vol. i. Art. Blut.), and Gulliver, in his edition of Hewson's Works for the Sydenham Society. THE BLOOD. 49 latin g the albumen of the serum, prevents the coagulation of the fibrine. Rest is favorable to the coagulation of blood. Blood of which the whole mass is kept in uniform motion, as wh&n a closed vessel completely filled with it is constantly moved, coagulates very slowly and imperfectly. But rest is not essential to coagulation ; for the coagulated fibrine may be quickly obtained from blood by stirring it with a bundle of small twigs; and whenever any rough points of earthy matter or foreign bodies are introduced into the blood- vessels, the blood soon coagulates upon them. Neither is rest suf- ficient for the coagulation of the blood when other conditions are unfavorable ; for coagulation is very slow in the body after death, although the blood is motionless. The free access of air is, perhaps, of all external conditions that which is most favorable to coagulation. It is supposed to be so, by favoring the exhalation of carbonic acid from the blood; for many experiments by Dr. Polli (xc. April, 1843), seem to show that the more freely carbonic acid can be evolved, the quicker is the coagulation of the blood; and that the larger the quantity of the same gas contained in the blood, the slower is its coagulation. The same conclusion is made probable by the blood coagulating very slowly when placed in carbonic acid, but as quickly as usual in hydrogen or nitrogen, into which carbonic acid may be evolved as freely as into atmospheric air. The conclusion, however, is not yet sure ; for Dr. John Davy says that carbonic acid is not evolved from coagulating blood (lxxxv. vol. ii. p. 86). Whatever be the explanation, many facts show that coagulation is accelerated by the free access of air, and retarded by the opposite condition. Thus, it is quick when blood is drawn slowly, and slow when it is drawn quickly ; because in the former case the blood is more fully exposed to the atmosphere than in the latter. To this also we may refer it that coagulation is quicker in shallow than in tall and narrow vessels; because in the former a large surface of blood is exposed to the air ; and to this (at least principally) that the blood in the dead body often remains fluid for from twelve to twenty-four hours after death, and then coagulates within a few minutes after it is let out of the vessels and exposed to the air. Yet, favorable as the access of air may be, we cannot regard the change that it produces as in any sense the cause of the coagulation of the blood ; for coagulation will take place in a vacuum, and in the most remote parts of the body, into which we cannot suppose that free atmospheric air can make its way either during life or after death, as, for example, in the brain. Lastly, the multiplication of points of contact seems a favorable condition for coagulation. Thus, when all other conditions are unfavorable, the blood will coagulate upon rough bodies projecting 5 50 THE BLOOD. into the vessels; as, for example, upon threads passed through arteries or aneurismal sacs, on the heart's valves roughened by in- flammatory deposits, or calcareous accumulations. And, perhaps, this may explain the quicker coagulation of blood after death in the heart with walls made irregular by the fleshy columns, than in the simple smooth-walled arteries and veins. It has been believed, and chiefly on the authority of Mr. Hunter, that, after certain modes of death, the blood does not coagulate : he enumerates the death by lightning, over-exertion (as in animals hunted to death), blows on the stomach, fits of anger. He says : " I have seen instances of them all." Doubtless, he had done so; but the results of such events are not constant. The blood has been often observed coagulated in the bodies of animals killed by lightning or an electric shock; and lately, Mr. Gulliver (lxxi. vol. xli. p. 1087) has published instances in which he found clots in the hearts of hares and stags hunted to death, and of cocks killed in fighting. The Blood- Corpuscles, or Blood- Cells. It has been already said that the clot of blood contains, with the fibrine and the portion of the serum that is soaked in it, the blood- corpuscles or blood-cells. Of these there are two principal forms, the red and the white corpuscles, *f which the latter are in process of being developed into the former. When coagulation has taken place quickly, both kinds of corpuscles may be uniformly diffused through the clot; but, when it has been slow, the red corpuscles, being the heaviest constituent of the blood, tend by gravitation to accumulate at the bottom of the clot; and the white corpuscles, being among the lightest constituents, collect in the upper part, and contribute to the formation of the buffy coat. The human red blood-corpuscles (Fig. 1) are circular flattened cells of different sizes, the majority varying in diameter from l-3000th to 14000th of an inch, and about l-10,000th of an inch in thickness.1 Their borders are rounded ; their surfaces, in their most perfect and usual state, slightly concave; but they readily acquire flat or convex surfaces when the liquor sanguinis being di- luted they are swollen by absorbing more fluid into their cavity. They are composed of a delicate, and probably colorless, membra- nous cell-wall, which incloses a peculiar substance impregnated with the red coloring matter, or haematine. Their cell-walls are tough and elastic ; so that, as they circulate, they admit of elonga- tion and various changes of form in adaptation to the vessels, yet recover their natural shape as soon as they escape from compression. 1 Mr. Gulliver has given, in his edition of Hewson's AVorks, p. 237, a table of the sizes of the red corpuscles in numerous species of vertebrate animals. THE BLOOD. 51 They have no nuclei, and their contents are probably homogeneous; at least they appear so, when their surfaces are flat or slightly con- vex ; it is only when they are concave that the unequal refraction of transmitted light gives the appearance of a central spot, which is brighter or darker than the border, according as it is viewed in or out of focus. In examining a number of red corpuscles with the microscope, it is easy to observe certain natural diversities among them, though they be all taken from the same part. The great majority, indeed, are very uniform; but some are larger than these, and the larger ones generally appear paler and less exactly circular than the rest; their surfaces also are, usually, flat or slightly convex; they often contain a minute shining particle like a nucleolus, and they are lighter than the rest, floating higher in the fluid in which they are placed. These differences are connected with the development of the blood-corpuscles, and will be explained in the account of that process. Other deviations from the general characters assigned to the corpuscles depend on changes that occur after they are taken from the body. Very commonly they assume a granulated form, in consequence, apparently, of a peculiar corrugation of their cell- walls. The larger cells are much less liable to this change than the smaller ones are, and the natural shape may be restored by di- luting the fluid in which the corpuscles float; by such dilution the corpuscles, as already said, may be made to swell up by absorbing the fluid; and, if much water be added, they will become spherical and pellucid, their coloring matter being dissolved, and, as it were, washed out of them. Some of them may thus be burst; the others are made obscure; but many of these may be brought into view again by evaporating, or adding saline matter to, the fluid, so as to restore it to its previous density. The changes thus produced by water are more quickly effected by weak acetic acid, which imme- diately makes the corpuscles pellucid, but dissolves few or none of them, for the addition of an alkali so as to neutralize the acid will restore their form, though not their color.1 A peculiar condition of the red corpuscles in inflammatory bl00(j—a condition which appear! to exist naturally in the blood of }10rses—is the principal cause of the formation of the buffy coat. It gives them a great tendency to adhere together in rolls or columns, like piles of coins, and then, very quickly, these rolls fasten together by their ends, and cluster; so that, when the blood is spread out thinly on a glass, they form a kind of irregular network, with crowds of corpuscles at the several points corresponding with the 1 On the effects of various reagents on the blood see Gulliver (xxviii.), Nasse (xv. art. Blut.)\ and on those produced by gases see Earless (ciii.). 52 THE BLOOD. knots of the net.1 Hence, the clot formed in such a thin layer of blood looks mottled with blotches of pink upon a white ground : in a larger quantity of such blood, as soon as the corpuscles have clus- tered^ and collected in rolls (that is, generally in two or three minutes after the blood is drawn), they begin to sink very quickly; for in the aggregate they present less surface to the resistance of the liquor sanguinis than they would if sinking separately. Thus quickly sinking, they leave above them a layer of liquor sanguinis, and this coagulating forms a buffy coat, the volume of which is augmented by the white corpuscles, which have no tendency to adhere to the red ones, and by their lightness float up clear of them. The white corpuscles are much less numerous than the red ones. On an average, in health, there may be one white to fifty red cor- puscles ; but, in disease, the proportion is often as high as one to ten, and sometimes, even much greater (clxxxix. Jan. 1851, p. 17, and lxxi. 1851, p. 147). They present greater diversities of form than the red ones do; but the gradations between the extreme forms are so regular that no sufficient reason can be found for supposing there is, in healthy blood, more than one species of white corpuscles. In their most general appearance they are cir- cular and nearly spherical, about l-2500th of an inch in diameter, tuberculated on their surfaces (Fig. 14), 1. They have a gray- ish, pearly look, appearing variously shaded or Fig. 14. nebulous, the shading being much darker in , ^gx some than in others. They seem to be formed ^ of some white substance, variously refracting * 3 the light, and containing granules which are in (©) ^) some specimens few and very distinct, in others (though rarely) so numerous that the whole white corpuscles of corpuscle looks like a mass of granules. the blood. i. a corpus- In a few instances, a distinct cell-membrane cie in its natural con- can be at once traced round a corpuscle thus rUSL^ C01TS!?; ^ ^ more commonly, none duced by the action of c:an b.c demonstrated, till water or diluted ace- diiuted acetic acid. ^G acid being added penetrates the corpuscle, a"d lifts up and distends a cell-wall, to the interior of which the material, that before appeared to form the whole corpuscle, remains attached as the nucleus of the cell. Thus these corpuscles are demonstrated to be nucleated cells, the nuclei of which are soft, granular, or tuberculated masses, and fill the cavities of the cells. The diversities presented by the nuclei, 'n! Jhtai?efanCG is r°Pr2j^*£ THE BLOOD. 55 These results of the ultimate analysis of ox's blood afford a re- markable illustration of its general purpose as supplying the mate- rials for the renovation of all the tissues. For the same analysts (Llayfair and Boeckmann) have found that the flesh of the ox yields the same elements in so nearly the same proportions, that the ele- mentary composition of the organic constituents of the blood and flesh may be considered identical, and may be represented for both by the formula C^ILgNgO^. After what has been said of many of the constituents of the blood, in the chapter on the Chemical Composition of the Body, a few words on each of the principal of them may here suffice. The water of the blood is subject to hourly variations in its quantity, according to the period since the taking of food, the amount of bodily exercise, the state of the atmosphere, and all the other events that may affect either the ingestion or the excretion of fluids. According to these conditions, it may vary from 700 to 790 parts in the thousand. Yet uniformity is on the whole maintained ; because nearly all those things which tend to lower the proportion of water in the blood, such as active exercise, or the addition of saline and other solid matter, excite thirst; while, on the other hand, the addition of an excess of water to the blood is quickly fol- lowed by its more copious excretion in sweat and urine. And these means for adjusting the proportion of the water _find_ their purpose in maintaining certain important physical conditions in the blood; such as its proper viscidity, and the degree of its adhesion to the vessels, through which it ought to flow with the least pos- sible resistance from friction. On this also depends, in great mea- sure, the activity of absorption by the bloodvessels, into which no fluids will quickly penetrate, but such as are of less density than the blood. Again, the quantity of water in the blood determines, chiefly, its volume, and thereby the fulness and tension of the^ves- sels, and the quantity of fluid that will exude from them to keep the'tissues moist. Finally, the water is the general solvent of all the other materials of the liquor sanguinis, and its abundance greatly favors organic chemical action: for generally, and within limits of health, the amount of action in the organic life of the several parts of the body is in direct proportion to the quantity of water they severally contain. Compare, e. g., in these respects, the bones or tendons with the muscles; or any tissue with the blood; or any of the tissues of young animals with the same_ in old ones: in all such cases, abundant water appears connected with activity of organic life. It is remarkable that the proportion of water in the blood may b" sometimes increased even during its abstraction from an artery or vein. Thus Dr. Zimmerman (ix. vol. iv. p. 385), in bleeding dogs, found the last drawn portion of blood contain 12 or 13 parts 5G THE BLOOD. more of water in 1000 than the blood first drawn ; and I olli (xci.) notices a corresponding diminution in the specific gravity ot human blood during venesection, and has suggested the only probable ex- planation of the fact, namely, that during bleeding, the bloodvessels absorb very quickly a part of the serous fluid with which all the tis- sues are moistened. The albumen may vary, consistently with health, from 60 to 70 parts in the 1000 of blood. The form in which it exists in the blood is not yet certain. It may be that of simple solution as pure albumen : but it is, more probably, in combination with soda, as an albuminate of soda; for, if serum be much diluted with water, and then neutralized with acetic acid, pure albumen is deposited. Another view, entertained by Enderlin (x. March and April, 1844), is that the albumen is dissolved in the solution of the tribasic phosphate of soda, to which he considers the alkaline reaction of the blood to be due, and solutions of which can dissolve large quantities of albumen and phosphate of lime. Fibrine.—The proportion of fibrine in healthy blood may vary between 2 and 3 parts in 1000. In some diseases, such as typhus, and others of low type, it may be as little as 1.034; in other dis- eases, it is said, it may be increased to as much as 7.528 parts in 1000. But, in estimating the quantity of fibrine, chemists have not taken account of the white corpuscles of the blood. These can- not, by any mode of analysis yet invented, be separated from the fibrine of mammalian blood : their composition is unknown, but their weight is always included in the estimate.of the fibrine. In health, they may, perhaps, add too little to its weight to merit con- sideration : but in many diseases, especially in inflammatory and other blood-diseases in which the fibrine is said to be increased, these corpuscles become so numerous that a large proportion of the supposed increase of the fibrine must be due to their being weighed with it. On this account all the statements respecting the increase of fibrine in certain diseases need revision. The quantity of fibrine appears to be, generally rather greater in arterial than in venous blood; and to be less in the blood of the splenic and portal veins than in ordinary venous blood. According to Denis, the fibrine of venous blood differs from that of arterial in that, when it is fresh and has not been much exposed to the air, it may be dissolved in a slightly heated solution of nitrate of potash. Mulder considers that the apparent peculiarity of the fibrine of arte- rial blood is due to the mixture of oxides of proteine, formed as often as the blood traverses the lungs: and that these, also, consti- tute part of the buffy coat and apparent increase of fibrine in in- flammatory diseases. Globuline, and Hacmatine, or Ilsematosine, mixed in a compound, which has been named Hsemato-globulin, or Cruor, constitute the THE BLOOD. 57 substance contained in the red blood-corpuscles. The cell-walls of these minute bodies cannot, indeed, be completely separated from their contents, or chemically distinguished from the globuline: and hence, perhaps, the diversity in the statements respecting the pro- portions of globuline and haematine. Berzelius states the propor- tion of haematine at 5.8 per cent, of the compound; Schmidt (x. No. lxi. p. 165) at 12.41 per cent, having, perhaps, more com- pletely separated the walls from the contents of the blood-cor- puscles. Globuline appears to be a proteine-compound. According to Si- mon (Ixxxii. vol. i. p. 81) it bears most resemblance to caseine, on which account he named it caseine of blood ; but Liebig and others regard it as more similar to albumen. It is soluble in water, and its solution, when heated, forms a granular coagulum. Its com- position, according to Mulder, is, Carbon.........54.11 Hydrogen . . . . . . . . . 7.17 Nitrogen ......... 15.7 Oxygen.........20.52 Sulphur.........2.5 But the chemical nature of globuline cannot be exactly deter- mined, because it cannot be obtained quite pure from either haematine or the membranous walls of the blood-cells, the mixture of which, as Henle suggests (xxxvii. p. 55), is probably sufficient to explain its apparent differences from common albumen. Hsematosine, or Haematine, is distinguished from all other animal matter by its peculiar blood-color, and by the changes which this color presents when, being incorporated in the blood-corpuscles, it is exposed to oxygen, carbonic acid, and other gases. It is so- luble in water, by which, as already said, it may, with the globuline, be washed out of the blood-corpuscles : and from this solution it is precipitated by most metallic salts, and by concentrated acids. In the living, or recent, state of the blood-corpuscles, the haematine is confined within their cell-walls, and appears to be insoluble in the serum; but when the blood begins to decompose, and the cell- walls, losing their texture, permit the outward passage of their contents, both the haematine and the globuline are dissolved in the serum, which thus becomes blood-colored, and may impart its tinge to the surrounding parts. With the globuline, also, haematine ap- pears to be coagulated by heat; but, according to Mulder, it is only inclosed in the coagulum of the globuline, without being itself eoagulable. In the purest state in which it can be obtained,1 it is so far changed as to be insoluble in water, of a deep brackish- 1 On the several modes of obtaining it, see Simon (Ixxxii.), Fownes (lx.), Griffiths (cii.), and Garrod (xxx. 1848, vol. i. p. 054). 58 THE BLOOD. brown color, and not liable to change of color on exposure to gases. Boiling alcohol will dissolve small quantities of it, and it is freely soluble in alcohol acidulated with sulphuric, hydrochloric, or nitric acid, and in weak solutions of potash, soda, or ammonia. According to Mulder, pure haematine consists of Carbon........65.84 per cent. Hydrogen . . . • • • • .0.37 Nitrogen .....••• 10.4 Oxygen . . . • • • " • H.7o Iron ........6.64 and he assigns for its formula C44H22N306Fe. The presence of so large a proportion of iron constitutes a pe- culiar feature in haematine. The mode in which the metal exists in it has been much discussed. By some it is supposed to be in the form of an oxide, or a salt, or in the form of peroxide in arterial blood, and carbonate of the protoxide of iron in venous blood. (Liebig, xi.) But the greater probability is, that the iron is com- bined as an element with the four essential elements, in the same manner as, it is held, sulphur is combined with them in albumen, fibrine, cystic oxide, &c. The principal evidence for this view, which is especially supported by Scherer and Mulder, is, 1, that when chlorine, which would not decompose an oxide of iron, is passed through a solution of haematine, chloride of iron is formed, and the iron thus removed from the other elements of the haema- tine, is replaced by chlorous acid; 2, that all the iron may be re- moved from haematine by sulphuric acid, without abstracting from it any of its oxygen, which would not be possible if the iron were more intimately united with the oxygen than with the other ele- ments of the haematine; 3, that pure haematine may be exposed for several days to the action of dilute hydrochloric or sulphuric acid, without any loss of its iron; though these acids would dis- solve an oxide of iron or decompose a carbonate. The peculiar color of haematine depends less on the iron than on its other constituents, for, as Scherer and Mulder have shown, haematine may retain its color after all the iron is extracted from it. Therefore the changes of color produced by respiration, and the contact of gases with the blood, cannot be referred to any change in the state of the iron in the haematine. It is, indeed, very doubtful whether the rapid change of color which is effected in respiration, and on the contact of various gases, can be referred to any chemical changes whatever in the haematine ; much more probably, it is due to changes in the form of the blood-corpuscles and their consequently different modes of reflecting and transmitting light. For, 1, the changes of color produced by carbonic acid and oxygen, mixed with a solution of the coloring matter of the blood, are very THE BLOOD. 59 slight; they are generally scarcely perceptible, and when they are seen they are slowly produced, or are not more than may be ex- plained by the action of the gases on some corpuscles still suspend- ed in the solution ; 2, the same changes of color as are produced by carbonic acid and oxygen acting on the corpuscles, may be pro- duced by distilled water, and strong solutions of alkaline salts. A black clot of blood becomes at once scarlet by washing it with salt, and is not blackened again by carbonic acid; a scarlet one is made black by washing it with distilled water, and is only very slowly reddened again by the contact of oxygen. Now the changes thus produced by salt and by water acting on the corpuscles, are not produced by the addition of the same substances to a solution of haematine, and are not connected with any chemical change in that substance or in the corpuscles ; but they are connected with alterations in the shape of the red corpuscles, for saline solutions, if denser than the liquor sanguinis, contract and shrivel up the corpuscles, making them deeply bi-concave; and distilled water has the contrary effect, swelling out the corpuscles and making them thickly bi-convex or spherical. Changes corresponding with these are produced by the contact of oxygen and of carbonic acid with the corpuscles : the former contracting them, and making their cell- membranes thick and granular; the latter dilating them, and thin- ning, and finally dissolving their cell-walls; and effecting these changes in a degree which, however slight it may appear in a single corpuscle, is enough to account for the change of color in a mass of blood. Herein, then, is a sufficient explanation of the changes that the corpuscles undergo, without supposing any im- mediate chemical alteration in the haematine ; an alteration which should take place as well in a solution of haematine as in the cor- puscles. The opinion that the instantaneous change of color which takes place in blood exposed to the action of oxygen or carbonic acid is due to a physical rather than a chemical alteration in the corpus- cles, is quite consistent with the probability that the corpuscles are chemically changed by the longer action of those gases dissolved in the blood. It appears that entire blood will absorb much more oxygen than either serum or liquor sanguinis alone will; as if it were chiefly with the corpuscles that the absorbed oxygen combines. If this be true, we may conclude, from the whole, that the oxygen, by first contracting the corpuscles and thickening their walls, makes them so reflect light as to appear, in mass, bright-red, and then chemically combines with them; and that carbonic acid, by dilat- ing them and thinning their walls, makes them reflect less light, and appear, in mass, nearly black; but we have no means of de- termining how large a portion of the oxygen inspired combines 60 THE BLOOD. with the corpuscles, nor whether that portion combines with the haematine or the globuline, or equally with both.1 The enumeration of the fatty matters of the blood makes it probable that most of those which are found in the tissues or se- cretions exist also ready-formed in the blood; for it contains the cholestearine of the bile, the cerebrine and phosphorized fat of the brain, and the margaric and oleic acids of common fat. The fat named seroline appears to be peculiar to the blood. The volatile fatty acid is that on which the odor of the blood mainly depends; and it is supposed, that when the sulphuric acid is added (see p. 43,) it evolves the odor by combining with the base with which, naturally, this fat is neutralized. The fatty matters of the blood are subject to much variation in quantity, being commonly increased after every meal in which fat, or starch, or saccharine substances have been taken. At such times, the fatty particles of the chyle, added quickly to the blood, are only gradually assimilated; and their quantity may be sufficient to make the serum of the blood opaque, or even milk-like.3 Of the inorganic constituents of the blood—the substances which remain as ashes after its complete burning—one may observe in general their small quantity in proportion to that of animal matter contained in it. Those among them of peculiar interest are the phosphate and carbonate of soda, and the phosphate of lime. It appears most probable that the blood owes its alkaline reaction to both these salts of soda. The existence of the tribasic phos- phate, a salt consisting of one equivalent of phosphoric acid with two of soda and one of basic water (P05-|-2Na 0 +HO), was proved by Enderlin (x. 1844). His experiments showed that the alkaline reaction of the blood, and of solutions of its ashes, could not depend, as some had supposed, on free soda; and seemed to prove, also, that the blood could not contain an alkaline carbonate. He, therefore, concluded that the alkaline reaction of the blood is wholly due to the tribasic phosphate of soda, and Liebig has sup- ported this view by showing that it is impossible to evolve any car- bonic acid by adding hydrochloric acid to a very concentrated solu- tion of the alkaline salts of the blood. But Marchand (cxxiv. vol. xxxvii. p. 321), Lehmann (lix. 1847, p. 88), and others, explain this by the fact that the carbonic acid being evolved in a solution so ca- pable of absorbing that gas as one of the tribasic phosphate of soda would be at once dissolved therein, and would not appear as gas 1 The treatises on this subject are discussed by Scherer in his several re- ports in Canstatt's Jahresberichte since 1844. The chief original works are those of Scherer, Bruch, and Reuter, in Ilenle and Pfeufer's Zeitschrift from 184ri to 1847 ; Donders, Harless, Marchand, and Mulder. 2 On the subject of milky serum, see Dr. R. D. Thomson (xxx. May 1845), and Dr. Buchanan (lxxi. Oct. 1844). THE BLOOD. 61 escaping. A recent very careful series of analyses, by Lehmann, seem to have proved the existence of both the carbonate and the tribasic phosphate in the blood, and that they are, jointly, the source of its alkaline reaction. The quantities of the alkaline salts set down in the table at page 54 are adopted from his analysis. In illustration of the characters which the blood may derive from the phosphate of soda, Liebig points out the large capacity which solutions of that salt have of absorbing carbonic acid gas, and then very readily giving it off again when agitated in atmo- spheric air, or when the atmospheric pressure is diminished. It is probably also by means of this salt that the phosphate of lime is held in solution in the blood in a form in which it is not soluble in water, or in a solution of albumen.1 Of the remaining constituents of the blood—the oxide and phos- phate of iron referred to exist in the liquor sanguinis, independent of the iron in the corpuscles. What concerns the urea present in healthy blood, will be stated in speaking of the secretion of urine : the existence of biliary coloring matter in it will be referred to in the section on bile ; and the gases it contains in the chapter on respiration. Vital Properties and Actions of the Blood.2 The life of the blood is manifested, as already said, in its coagu- lation, and the subsequent more perfect organization which it may attain when it coagulates among healthy living tissues. But, in a higher degree, its life is shown in its development and self-mainte- nance, in its liability to idiopathic disease and death, and in the purpose and relation which connect it with the other living parts. In the development of the blood, little more can be traced than the processes by which the corpuscles and fibrine are formed. _ In all the Vertebrata, two sets of red corpuscles are developed at different periods of life : a first set, which exist alone in the blood, till lymph and chyle begin to be formed; and a second set, which are formed from the lymph- and chyle-corpuscles, and gradually supersede the first set. The corpuscles of the first set are, in the first instance, part of the embryo-cells which form the mucous or vegetative layer of the embryos in Mammalia and birds, and the whole inner sur- ' The student will do well to refer to the interesting observations of Lic- hW in his Chemistry of Food, respecting the mode in which the phosphate of°soda is formed for the blood of herbivorous animals who take, in then- food, phosphate of potash and chloride of sodium: and respecting the mutual action of the alkaline phosphate in the blood and the acid phosphate in the juice of the muscles. 2 The following portion of this chapter contains an abstract of part of the Lectures on the Life of the Blood, delivered by Mr. Paget, at the Col- lege of Surgeons, in May, 1848. 6 62 THE BLOOD. face of the vitelline membrane, in the embryos of fish and reptiles. In the latter class, certain of these cells are laid out in the plan of the future heart and chief bloodvessels, before the walls of those organs are yet formed, and before the blood has begun to move. As described by Vogt (cv.), Kolliker (xxxi. 1846), and Cramer (lxxx. 1848, p. 631), they are large, colorless, vesicular, spherical cells, full of yellowish particles of a substance like fatty matter (Fig. 15, a); many of which particles are quadrangular and flatten- Fig. 15. » E P Development of the first set of blood-corpuscles in the Batrachian larva. A. An embryo- cell filled with fatty-looking particles. B, C, D, and E. Successive stages in the transition of the embryo-cell to a blood-corpuscle, as described in the text. F. A fully-formed blood- corpuscle. ed, and have been called stearine-plates, though they are not proved to consist of that or any other unmixed fatty substance. Among these particles each cell has a central nucleus, which, however, is at first much obscured by them. The development of these embryo- cells into the complete form of the corpuscles is effected by the gra- dual clearing up, as if by division and liquefaction, of the contained particles, the acquirement of blood-color, and of the elliptical form, the flattening of the cell, and the more prominent appearance of the nucleus. The changes are so slowly accomplished in the tadpole of the frog, and in other batrachian embryos, that they may be easily traced in the blood while it flows in their transparent parts; a similar process appears to occur in the development of the blood-corpuscles of fish (Vogt, civ.), and there is little doubt that a similar one obtains in birds, though, since it is completed in the first forty hours of incuba- tion, it is very difficult to trace it in all its successive stages. We have, however, seen in the heart of the chick, after from thirty to forty hours' incubation, some colorless spherical cells, with fatty- looking particles and granules, exactly similar to those of the mucous layer of the germinal area of the ovum. "With these were others, which appeared to be of the same origin, but to have undergone THE BLOOD. 63 changes similar to those above described; a clearing up of the fatty particles, acquirement of color, reduction of size, and more distinct appearance of the nucleus. The assumption of the flattened ellipti- cal form occurs in birds at a later period. In mammalian embryos, also, the earliest blood-corpuscles ap- pear to be a portion of the cells of the vegetative or mucous layer of the germinal area. They are large, spherical or oval, pellucid and colorless, nucleated, and full of minute granules (Fig. 16, a). In these we have observed (as Kblliker (xxxiii. 1846), Fahrner, (cvi.) and others have done), a process of multiplication by bi-par- tition of the nucleus, each half of which, either by appropriating half the cell, or by developing a cell around itself, becomes the central nucleus of a new cell differing from the parent-cell from which it escapes, in little except in being smaller and more gene- rally circular (b, c, and d). The subsequent changes of these cells resemble those already described; they gradually acquire the blood-color, their granules disappear, and their surfaces become smoother and more uniform (e and f). Fig. 16. Development of the first set of blood-corpuscles in the mammalian embryo. A. A dotted, nucleated embryo-cell in process of conversion into a blood-corpuscle: the nucleus provided with a nucleolus, b. A similar cell with a dividing nucleus; at c, the division of the nucleus is complete; at d, the cell also is dividing, e. A blood-corpuscle almost complete, but still containing a few granules. F. Perfect blood-corpuscle. It seems, moreover, that in the mammalian corpuscles, these changes may ensue as well during, as after, the multiplication by partition of the nucleus: for red corpuscles are not unfrequent in mammalian embryos containing two nuclei, and we have seen some with three, and even four, nuclei. The development of the first set of blood-corpuscles appears, thus, to be uniform in all the classes of Vertebrata; namely, in all, from the embryo-cells of the vitelline membrane or germinal area, into nucleated red blood-cells; the principal visible changes being 64 THE BLOOD. the disappearance of granules or fatty-looking substance, the greater prominence of the nucleus, and the acquirement of color. And, in their most perfect state, the corpuscles of the first set are in all the vertebrate classes nucleated cells. Those of the human embryo are circular, thickly disk-shaped, full-colored, and, on an average, about l-2500th of an inch in diameter; their nuclei, which are about l-5000th of an inch in diameter, are central, circular, very little prominent on the surfaces of the cell, and apparently slightly granular or tuberculated. In a few instances, cells are found with two nuclei; and such cells are usually large and elliptical, with one of the nuclei near each end of their long axis. When, in the development of the embryo, the lymph and chyle begin to be formed and added to the blood, their corpuscles are developed so as to supersede those produced in the manner just described. In some species (as in the frog) the first appearance of lymph- and chyle-corpuscles in the blood exactly corresponds with the time at which the external branchiae disappear; in others, as in the chick, rabbit, ferret, and sheep, their appearance coincides with the closure of the branchial fissures.1 After they have once appeared the new blood-corpuscles appear to be derived exclusively through them. For some time, indeed, the two sets of corpuscles appear mingled in the blood. In this case, in Mammalia, the white corpuscles of the first set (if any remain in this stage) are distinguished from those of the second set, by their larger size, distinct cell-walls, small, well-defined central nuclei, and their pel- lucid contents with very minute scattered dots or granules; and the red corpuscles of the first set are always characterized by their larger size and their nuclei, which, if not at once distinct, are ren- dered so by the addition of water. But, gradually, while the corpuscles of the second set are increasing, those of the first dis- appear, and we believe they would not be found in a human embryo of more than two months old, unless in cases of arrested development; in such an one, where the abdominal walls were incomplete, we found the two sets of corpuscles mixed in the blood of a fetus between three and four months old. The origin and first formation of the lymph and chyle, and of their corpuscles, will be described in the chapter on Absorption; the structure of the corpuscles (which are the white or colorless corpuscles of most writers, the granule-cells of Mr. Wharton Jones) is described already (p. 52). In the different vertebrate classes there is much greater similarity in these corpuscles than in the 1 These instances prove a frequent coincidence in the development of the blood by the production of a'new set of corpuscles through lymph and chyle, and of the respiratory apparatus by the suppression of the external branchial organs. But in the Triton punctatus we have found lymph-cor- puscles in the blood while its long-retained external branchiae still exist. THE BLOOD. 65 red blood-corpuscles of the second set. Except for some difference of size, the same general description might apply to all; and some features in the development are alike in all, namely, the gradual clearing up, as if by deliquescence, of their granular contents, and a commensurate acquirement of color. But, while in the corpus- cles of the oviparous Vertebrata the outer part alone of their gra- nular contents thus clears up, and the central part remains as the small nucleus of the complete blood-cell, in man, and all Mamma- lia, the whole of the contents clear up, acquire a uniform color, and become the homogeneous contents of a ceil without a nucleus. The principal steps in the development of the human lymph- or chyle-corpuscle into the red-corpuscle, may be traced in specimens of blood in which these white corpuscles are numerous. The white corpuscle, at first tuberculated, containing many granules, and darkly-shaded (Fig. 17, a), becomes smoother, paler, less granular, and more dimly shaded or nebulous (Fig. 17, b) : changes corre- sponding with those which Mr. Wharton Jones describes as from the coarsely to the finely granular stage of the granule-cell (xliii. 1846). In these stages, the cell-wall may be easily raised from its contents by the contact and penetration of acetic acid, or by the longer action of water (Fig. 17, c); and, according to the stage of Fig. 17. Development of human lymph- and chyle-corpuscles into blood-corpuscles. A. A lymph or white blood-corpuscle, b. The same in process of conversion into a red corpuscle, c. A lymph-corpuscle with the cell-wall raised up round it by the action of water, d. A lymph- corpuscle from which the granules have almost all disappeared, e. A lymph-corpuscle acquiring color; a single granule, like a nucleus, remains, f. A red corpuscle fully developed. development, so, as already stated, are the various appearances which the contents of the cell thus acted on present. In the regu- lar progress of development, it becomes at length impossible to raise the cell-wall from its contents. Then the corpuscles acquire a pale tinge of blood-color; and this always coincides with the 6* 66 TnE BLOOD. softening of the shadows which before made them look nebulous, and with the final vanishing of all the granules, with the exception sometimes of one which remains some time longer like a shining particle in the corpuscle, and has probably been often mistaken for a nucleus (Fig. 17, e). The blood-color now deepens, and at the same rate the corpuscles become smooth and uniform; biconcave, having previously gradually changed the nearly spherical form for a lenticular or flattened one; smaller, apparently by condensation of their substance, for at the same time they become less amenable to the influence of water; more liable to corrugation and to collect in clusters; and heavier, so that the smallest and fullest-colored corpuscles always lie deepest in the field. Thus the most deve- loped state of the mammalian red-corpuscles appears to be that in which it is full-colored, circular, biconcave, small, uniform, and heavy: this also is the state in which they appear to live the longer and most active portion of their lives. This mode of development of new blood-corpuscles from those of the lymph and chyle, continues throughout life. New corpuscles never appear to be produced from the germs of old ones; when a corpuscle is past its perfection, it degenerates, and probably lique- fies. The changes of such degeneration have not been clearly seen in mammalian corpuscles; but they are probably nearly similar to what occur in those of fish and reptiles, in which the old and de- generate corpuscles appear perfectly white and pellucid (not shaded or granular, like the lymph-corpuscles), smaller than they were, and, in some instances, cracked, or as if eroded. The nuclei ap- pear to degenerate with the cells, but, because of their darker and harder outlines, remain longer distinct, and often look like free nuclei, unless the dim cell-wall round them be carefully searched for. But in this process no germ for a new corpuscle issues from the transient cell. Every new corpuscle forms itself in and from the materials of the lymph and chyle, and is perfected in the blood; and the blood is maintained by constant repetitions of this process. Herein, also, is provision for the welfare of the body; for, if the blood-corpuscles were, like many cells, derived from germs formed in their predecessors, then every loss of blood would involve the loss, not only of the corpuscles escaping at the time, but of all those that, in after time, should have descended from the lost ones and their germs. But new blood being made from lymph and chyle, its losses to any amount can be repaired, pro- vided the processes for the formation of those fluids are not dis- turbed. The development of fibrine appears to proceed commensurately with that of the second set of corpuscles. In the earliest state of the chyle, no fibrine exists; but when chyle-corpuscles arc formed, the fluid in which they float is spontaneously coagulable; and the THE BLOOD. 67 fibrine, whose existence is thus proved, appears to increase as the chyle proceeds onwards to the blood, and passes through the lac- teal glands. Yet, in the most perfect chyle and lymph the fibrine is less abundant, and coagulates less firmly than in the blood: we may therefore assume that its development, like that of the corpuscles, is perfected in the blood itself. From what has been said, it will have appeared that when the blood is once formed, its growth and maintenance are effected by the constant repetition of the development of new portions. In the same proportion as the blood yields its materials for the main- tenance and repair of the several solid tissues, and for secretions, so are new materials supplied to it in the lymph and chyle, and, by development, made like it. The part of the process which relates to the formation of new corpuscles and fibrine has been described; but it is probably only a small portion of the whole process; for the assimilation of the new materials to the blood must be perfect, in regard to all those immeasurably minute particulars by which the blood is adapted for the nutrition of every tissue, and the maintenance of every peculiarity of each. How precise the assi- milation must be for such an adaptation, may be conceived from some of the cases in which the blood is altered by disease, and, by assimilation, is maintained in its altered state. For example, by the insertion of vaccine matter, the blood is for a short time mani- festly diseased; however minute the portion of virus, it affects and alters, in some way, the whole of the blood. And the alteration thus produced, inconceivably slight as it must be, is long maintained; for, even very long after a successful vaccination, a second inser- tion of the virus may have no effect, the blood being no longer amenable to its influence, because the new blood, formed after the vaccination, is made like to the blood as altered by the vaccine virus; in other words, the blood exactly assimilates to its altered self the materials derived from the lymph and chyle. So, in all probability, are maintained the morbid states of the blood which exist in syphilis, and many other chronic diseases: the blood, once inoculated, retaining, by the exactness of its assimilation, the taint which it received, though after a time, it may not have in it one of the particles on which the taint first passed. In health, we cannot see the precision of the adjustment of the blood to the tissues; but we may imagine it from the small influences by which, as in vaccination, it is disturbed, and we may be sure that the new blood is as perfectly assimilated to the healthy standard, as, in disease, it is-assimilated to the most minutely altered standard.1 The assimilation of the blood is probably effected, essentially and finally, by the formative power (see p. 38) which the blood 1 Corresponding facts in relation to the maintenance of the tissues by assimilation will be mentioned in the chapter on Nutrition. 68 THE BLOOD. possesses in common with the solid tissues. But it is ministered to and assisted by the actions of other parts; as, 1st, the digestive and absorbent systems, with probably the liver, and most or all of the vascular glands, whose especial office is to prepare materials, not only enough, but exactly fit to form the new blood; and, 2dly the excretory organs, through which the blood separates from itself materials which are refuse, such as the waste substance of the tissues, the urea, carbonic acid, &c, or are unfit to form part of its essential constituents, such as some of the materials taken tor food and drink, and absorbed by the bloodvessels of the digestive canal without being formed into chyle. But, 3dly, the precise constitu- tion of the blood is adjusted by the balance of the nutritive pro- cesses for maintaining the several tissues, so that none of the materials appropriate for the maintenance of any part may remain in excess in the blood. Each part, by taking from the blood the materials it requires for its maintenance is, as Treviranus observed (lxviii. bd. i. p. 401), in the relation of an excretory organ to all the rest. For example, if the muscles did not take materials for their nutrition, there might be an excess of fibrine and their other constituents in the blood; if the bones did not do so, the salts of lime would be in excess, and so on.1 The formative power by which the blood maintains itself is, perhaps, inherent in its whole substance. No sufficient reason appears for considering that it belongs to the corpuscles more than to any of the other highly organic constituents of the blood; neither is there any evidence for determining the particular functions of the corpuscles; only, it is probable that they help in the formation of the fibrine and other appropriate materials for the nutrition of the tissues by acting like gland-cells, that is, by forming or elaborating in their cavities materials which they may discharge when perfect. (See Secretion.) Both white and red corpuscles may do this, but the red ones more perfectly than the white; since, as a general rule, rudimentary parts have the same function as the perfect parts into which they are developed, but discharge that function with less power. The purpose of the blood thus developed and maintained appears, in the perfect state, to be threefold; namely, 1st, to provide mate- rials appropriate for the nutrition and maintenance of all the parts of the body; 2d, to convey to the several parts oxygen, whether for the discharge of their functions, or for combination with their refuse matters; 3d, to bring from the same parts those refuse matters, and convey them to where they may be discharged. The first is the principal and essential purpose of the blood; the second and third are subordinate purposes, which the blood discharges, as 1 See further on this subject, lxxi. vol. xxxix. p. 938, and succeeding Lectures. THE blood. 69 it were, by the way, and which will be considered in future chap- ters. Of the first purpose little more is known than that the blood does provide the materials for the maintenance of the body; and that they are not all in the blood in the same chemical compounds as they form in the tissues. Gelatine, for example, which forms so large a part of the tissues, does not exist in the blood, and must therefore be formed from some of its albuminous or proteine compounds while the tissues in which it is found are being de- veloped. It may be observed that the changes which materials taken from the blood and forming tissues undergo, though always processes of development in regard to structure, are sometimes degenerations in a chemical sense. The case of the gelatinous tissues is an example of this; however highly organized their structure, their chemical composition is lower than that of the blood, gelatine being-, as Dr. Prout has shown (xxi. p. 455), the least remote from inorganic matter of any of the nitrogenous animal compounds. Thus, the providing of materials for the gelatinous tissues may be regarded as the lowest part of this office of the blood; the highest is, probably, the provision for the nervous and animal muscular systems. To these, and especially to the brain, the development of the blood appears to be peculiarly adjusted. Thus, in the In- vertebrata that have blood, the observations of Mr. Wharton Jones (xliii. 1846) show that the blood-corpuscles are not developed beyond the stage which the lymph-corpuscles commonly attain in the oviparous Yertebrata, though, up to that stage, and in it, they are very similar to the lymph-corpuscles. Among the Yertebrata, the Branchiostoma, as a connecting link between the two classes, appears to have the same form of blood-corpuscles as the Inverte- brata; but in other fish we find, coincidently with the great ad- vance in the development of a brain and spinal cord, the introduc- tion of a proportionally larger quantity of blood, and of red cor- puscles formed by a further development of such corpuscles as are the most perfect in the Invertebrata. In the transition from fish to reptiles, the greater development of the brain is associated with a general further increase in the quantity and velocity of the blood; aud in that from reptiles to birds with a yet much larger increase in its quantity and velocity, an augmentation of the proportion of fibrine, and a great multiplication of blood-corpuscles with reduc- tion of their size. Lastly, with the greater development of the brain in Mammalia, we find the development of the blood-corpus- cles into a higher form than they have in any other Vertebrata: for, though the nucleated cell is commonly regarded as a higher development than the cell without a nucleus, yet since, in the blood of the mammalian embryo, the latter supersedes the former, and is 70 circulation. adapted to the general advance of development, we may be sure that, in this instance at least, the cell without a nucleus is the higher form. Thus it appears that in the same proportion as animals occupy a higher position in the scale of beings, so have they both a larger quantity and a higher quality of blood. Their position in the scale is determined by the development of the central nervous sys- tem, in adaptation to which, more or less directly, the other systems relating to the life of the individual are adjusted. The adjust- ment of the characters of the organic life to the central nervous system is effected through the intervention of the blood, to the formation of which all the organs of that life minister, and which we may, therefore, regard as the highest member of the parts con- cerned in the organic life, in the same sense as we regard the brain as the highest of the organs of the animal life. And this emi- nence of the blood is shown, first, by its chemical composition, which, as we have seen, is more highly organic than that of the greater part of the tissues; second, by the time at which it first appears in the embryo, in which, as the brain and spinal cord pre- cede, in their rudiments, the other and subordinate organs of ani- mal life, so the blood appears before any of the persistent organs of the organic life; third, by the complexity and number of the processes through which it is elaborated, including all those the his- tory of which is now to be traced till we come to that of the Nervous system. CHAPTER Y. CIRCULATION OF THE BLOOD. The purposes which have been assigned to the blood, those, namely, of conveying oxygen and nutritive materials to the several parts of the body, and of carrying away from them to excretory organs their refuse matters—require that it should be constantly moving through all the parts, and, at certain periods, should be ex- posed to the atmosphere in order that it may imbibe oxygen, and emit carbonic acid and water, the substances into which the prin- cipal refuse matter is combined. To this end it is provided, in man and all warm-blooded animals, that all the blood which has passed once through the several parts of the body, shall traverse the lungs, and be exposed to the atmosphere before it again takes the same course. This is effected by what is called a double circulation, or, more properly, a single complete circulation in two nearly separate CIRCULATION. 71 parts; the organs for which are, a heart with two separated com- partments, or sides, and arteries and veins so connected with each compartment of the heart, that the arteries proceeding from the one may lead to the veins belonging to the other. The course through which blood moves in such a circulation may be thus briefly described. (Fig. 18.) Commencing, we will suppose, at the left ventricle of the heart, blood is impelled into the aorta and along its successive branches, the systemic arteries, through which all the organs of the body, except the finer textures of the lungs, derive all their blood. Through these arteries it is conveyed into the systemic capillaries, the minute vessels which lie intermediately between the arteries and veins of every part, and in which the blood is brought most nearly into contact with the very substance of the organs. From these it passes into the systemic veins, through the main trunks of which, the venae cavae, it flows into the right auricle, and thence into the right ventricle of the heart. This com- pletes what is called the systemic circulation, or systemic or general part of the circulation. In the right ventricle, the blood enters the pulmonary or lesser circulation, in which it passes from the right ventricle through the pulmonary artery and its branches in the lungs to the capillaries, in which it is brought nearest to the atmosphere. From the pul- monary capillaries the blood enters, in con- verging streams, the pulmonary veins, which carry it to the left auricle, whence, having thus traversed the pulmonary part of the circulation, it passes again into the left ven- tricle, where, in the case here supposed, it started on its course. The blood in the left ventricle is arterial (see p. 42, &c), and charged with oxygen in greater proportion than carbonic acid, as well as with materials for the supply of the organs. So it remains in all the systemic arteries; but in the systematic capillaries it parts with portions of those materials, and its oxygen is, in great measure consumed in uniting with the hydro-carbonous and other substances which enter the bloodvessels from the refuse matter of the tissues. Thus the blood acquires the venous characters; and Diagram of the Circu- lating Apparatus in Mam- mals and Birds: a, the heart containing four cavi- ties ; b, vena cava, deliver- ing venous blood into c, the right auricle ; d, the right ventricle propelling venous blood through e, the pulmo- nary artery, to /, the capil- laries of the lungs; g, the left auricle, receiving the aerated blood from the pul- monary vein, and delivering it to the left ventricle, h, which propels it through the aorta, i, to the systemic ca- pillaries j, whence it is col- lected by the veins, and car- ried back to the heart through the vena cava, 6. 72 CIRCULATION. in this state it traverses the systemic veins, the right side of the heart, and the pulmonary arteries; but in the pulmonary capillary vessels, emitting carbonic acid and water, and imbibing oxygen, it becomes again arterial, and so passes on to the left ventricle. A subordinate kind of circulation is inserted in the liver, and is called the portal circulation. The veins belonging to that part of the systemic vessels which are appropriated to the organsof diges- tion, form a common trunk called vena portae ; and this, instead of joining at once with the other main trunks of the systemic veins, enters the substance of the liver. There the vena portae, branch- ing like an artery, carries its share of the blood into capillaries, through which it passes into the hepatic veins, then goes through their largest branches into the vena cava inferior, one. of the two main trunks of the systemic venous system, where the portal circu- lation terminates by mingling its blood with that which in the vena cava inferior has nearly reached the end of the systemic circula- tion. The principal force provided for constantly moving the blood through this course is that of the muscular substance of the heart; other assistant forces are those of the elastic walls of the arteries, the pressure of the muscles among which some of the veins run, and the movements of the walls of the chest in respiration. The right direction of the blood's course is determined and maintained by valves placed between each auricle and ventricle of the heart, at the orifices of communication between the ventricles and the main arterial trunks, and in most of the veins; which valves open to permit the movement of the blood in the course described, but close when any force tends to move it in the contrary direction. Wc shall consider separately each member of the system of organs for the circulation : and first— THE ACTION OF THE HEART. The heart's action in propelling the blood consists in the succes- sive alternate contractions and dilatations of the muscular walls of its two auricles and two ventricles. The auricles contract simulta- neously ; so do the ventricles; their dilatations, also, are severally simultaneous; and the contractions of the one pair of cavities are synchronous with the dilatations of the other. The description of the action of the heart may best be com- menced at that period in each action which immediately precedes the beat of the heart against the side of the chest, and by a very small interval more precedes the pulse at the wrist. For at this time, which corresponds with the pause between the two sounds of the heart, the whole heart is in a passive state; the walls.of both auricles and ventricles are relaxed, and their cavities are being CIRCULATION. 73 dilated. The auricles are gradually filling with blood flowing into them from the veins; and a portion of this blood passes at once through them into the ventricles, the opening between the cavity of each auricle and that of its corresponding ventricle being, dur- ing all the pause, free and patent. The auricles, however, receiv- ing more blood than at once passes through them to the ventricles, become, near the end of the pause, fully distended; then, in the end of the pause, they contract and empty their contents into the ventricles. The contraction of the auricles is sudden, and very quick; it commences at the entrance of the great veins into them, and is thence propagated towards the auriculo-ventricular opening; but the last part which contracts is the auricular appendix. The effect of this contraction of the auricles is to propel nearly the whole of their blood into the ventricles. The reflux of blood into the great veins is hindered by the simultaneous contraction of the muscular coats with which they are provided for some distance before their entrance into the auricles; a contraction which, how- ever, is not so complete but that a small quantity of blood. does regurgitate, i. e., flow backwards into the veins, at each auricular contraction. The effect of this regurgitation from the right auricle is limited by the valves at the junction of the subclavian and in- ternal jugular veins, beyond which the blood cannot move back- wards; and the coronary vein is preserved fxom it by a valve at its mouth. The blood which is thus driven, by the contraction of the auricles, into the corresponding ventricles, being added to that which had already flowed into them during the heart's pause, is sufficient to complete the distension of the ventricles. Thus distended, they im- mediately contract; so immediately, indeed, that their contraction looks as if it were continuous with that of the auricles. They con- tract much more slowly than the auricles, and simultaneously in every part, the whole wall of each ventricle being drawn up uni- formly towards the origin of the artery at its base, diminishing the cavity in every diameter, but especially in length, so that the heart assumes a shorter and more globular form than it had in the relaxed and distended state of the ventricles. In this complete and uniform contraction, the ventricles probably always thoroughly empty themselves, differing in this respect from the auricles, in which, even after their completest contraction, a small quantity of blood remains. The form and position of the fleshy columns on the internal walls of the ventricle appear, indeed, especially adapted to produce this obliteration of their cavities during their contraction; and the completeness of the closure may often be observed on mak- ing a transverse section of a heart shortly after death, in any case in which the contraction of the rigor mortis is very marked. In 74 CIRCULATION. such a case only a central fissure may be discernible to the eye in the place of the cavity of each ventricle. At the same time that the walls of the ventricles contract, the fleshy columns contract also, and draw away the aunculo-yentricular valves from the internal surface of the ventricles against which they had lain while the blood was flowing into the ventricles. Ihe blood thus passes beneath or behind the valves and, being pressed by the contracting walls of the ventricles, pushes the valves up- wards and inwards, and brings their margins into apposition, so that they close the auriculo-ventricular openings, and prevent the backward passage of the blood into the auricles. The whole force of the ventricular contraction is thus directed to the propulsion of the blood through their arterial orifices. During the time which elapses.between the end of one contraction of the ventricles and the commencement of another, the communication between them and the great arteries—the aorta on the left side, the pulmonary artery on the right—is closed by the three semilunar valves situated at the orifice of each vessel. But the force with which the current of blood is propelled by the contraction of the ventricle separates these valves from their contact with each other and presses them back against the sides of the artery, making a free passage for the stream of blood. Then, as soon as the ventricular contraction ceases, the elastic walls of the distended artery recoil, and by pressing the blood behind the valves force them down towards the centre of the vessel, and spread them out so as to close the orifice and prevent any of the blood flowing back into the ventricles. As soon as the contraction of the auricles is completed, they begin again to dilate, and to be filled again with blood which flows into them in a steady stream through the great venous trunks. They are thus filling during all the time in which the ventricles are contracting; and the contraction of the ventricles being ended, they also again dilate and receive again the blood that flows into them from the auricles. By the time that the ventricles are thus from one-third to two-thirds full, the auricles are distended; these, then suddenly contracting, fill up the ventricles as already described. Thus the action of the auricles consists in a succession of quick contractions and slow dilatations; that of the ventricles in a suc- cession of contractions and dilatations of nearly equal length. Of the period occupied by a complete action of the heart, the auricles are engaged for about one-eighth in contraction, and seven-eighths in dilating and receiving blood; while the ventricles are occupied for one-half in contracting, and the other in dilating. The following table will explain the order of the actions already described, and their coincidence, with the sounds and impulse of the heart, of which we shall next speak. It supposes the period occu- CIRCULATION. 75 pied by a complete set of the actions of the heart to be divided into eight parts, and if the case be taken of a person with a pulse beating sixty times in a minute, these parts may represent eighths of a second. Eighths of a second. Last part of the pause . . 1 Aur. contracting : ventr. distended. 1st sound and impulse . . 4 Ventr. contracting: aur. dilating. 2d sound .... 2 Ventr. dilating: aur. dilating. Pause . . . . .1 Ventr. dilating: aur. distended. Action of the Valves of the Heart. The periods in which the several valves of the heart are in action may be connected with the foregoing table; for the auriculo-ven- tricular valves are closed, and the arterial valves are open during the whole time of the ventricular contraction; while, during the dilatation and distension of the ventricles the latter valves are shut, the former open. Thus the valves are all alternately open and shut for nearly equal periods of time; and each half or side of the heart, through the action of its valves, may be compared with a kind of forcing pump, like the common enema-syringe with two valves, of which one admits the fluid on raising the piston, but is closed again when the piston is forced down; while the other opens for the escape of the fluid, but closes when the piston is raised, so as to prevent the regurgitation of the fluid already forced through it. The ventricular dilatation is here represented by the raising up of the piston; the valve thus admitting fluid represents the auri- culo-ventricular valve, which is closed again when the piston is forced down, i. e., when the ventricle contracts, and the other, i. e., the arterial, valve opens. The arterial, semilunar, or sigmoid valves (Fig. 19) are, as Fig 19. Diagram of the semilunar valves of the aorta (after Morgagni). a. Corpus Arantii on the free border. 6. Attached border, c. Orifices of the coronary arteries. already said, brought into action by the pressure of the arterial blood forced back towards the ventricles, when the elastic walls of 76 CIRCULATION. the arteries recoil after being dilated by the blood propelled into them in the previous contraction of the ventricle. The dilatation of the arteries is, in a peculiar manner, adapted to bring the valves into action. The lower borders of the semilunar valves are attached to the inner surface of a tendinous ring, which is, as it were, inlaid, at the orifice of the artery, between the muscular fibres of the ven- tricle, and the elastic fibres of the walls of the artery (Fig. 20). The tissue of this ring is tough, Fig. 20. and does not admit of ex- Fibrous tissue of a semilunar valve beneath the °^ tOUgn> close-textured, endocardium. fibrous tissue, with strong interwoven cords, and co- vered with epithelium. Hence, when the ventricle propels blood through the orifice and into the canal of the artery, the lateral pres- sure which it exercises is sufficient to dilate the walls of the artery, but not enough to stretch in an equal degree, if at all, the unyield- ing valves and the ring to which their lower borders are attached. The effect, therefore, of each such propulsion of blood from the ventricle is, that the wall of the first portion of the artery is dilated into three pouches behind the valves, while the free margins of the valves, which had previously lain in contact with the inner surface of the artery (as at A, Fig. 21) are drawn inwards towards its centre Fig. 21. Sections of aorta, to show the action of the semilunar valves, a is intended to show the valves, represented by the dotted lines, in contact with the arterial walls, represented by the continuous outer line. B (after Hunter) shows the arterial wall distended into three pouches (a); and drawn away from the valves which are straightened into the form of an equilateral triangle, as represented by the dotted lines, c (after Retzius, cxii.) shows the margins of the valves when in action; a, the pouches between the valves and the arterial wall; b, the apposed edges; c, the apposed surfaces of the valves; d, mouths of coronary arteries; e, cut edge of aorta. CIRCULATION. 77 (Fig. 21, b). Their positions may be explained by the following diagrams, in which the continuous lines represent a transverse sec- tion of the arterial walls, the dotted ones the edges of the valves, first, when the valves are in contact with the walls (a), and, secondly, when the walls being dilated the valves are drawn away from them (b). This position of the valves and arterial walls is retained so long as the ventricle continues in contraction; but, as soon as it relaxes, and the dilated arterial walls can recoil by their elasticity, they press the blood as well towards the ventricles as onwards in the course of the circulation. Part of the blood thus pressed back lies in the pouches (a, Fig. 21, b) between the valves and the arte- rial walls; and the valves are by it pressed together till their mar- gins meet in three lines radiating from the centre to the circum- ference of the artery, as in c, Fig. 21. The contact of the valves in this position, and the complete closure of the arterial orifice, are secured by a peculiar construc- tion of their borders. Among the cords which are interwoven in the substance of the valves, are two of greater strength and promi- nence than the rest; of which one extends along the free border of each valve, and the other forms a double curve or festoon just below the free border. Each of these cords is attached at its ends to the outer angles of its valve, and in the middle to the corpus Arantii, a small mass of fibrous tissue at the centre of the border of each valve; they thus inclose a space from a line to a line and a half in width, in which space the substance of the valve is much thinner and more pliant than elsewhere. When the valves are pressed down, all these parts or spaces of their surfaces come into contact, and the closure of the arterial orifice is thus secured by the apposi- tion not of the mere margins and thin edges of the valves, but of all those parts of the surfaces of each which lie between the free edges and the cords next below them. (See Fig. 21, C, c.) These parts are firmly pressed together, and the greater the pressure that falls on them, the closer and more secure is their apposition. The corpora Arantii meet at the centre of the arterial orifice when the valves are down, and they probably assist in the closure; but they are not essential to it, for, not unfrequently, the valves of the pul- monary artery have none, but are extended in larger, thin, flapping margins. In this form of valves, also, the inlaid cords are less dis- tinct than in those with corpora Arantii; yet the closure by contact of their surfaces is not less secure. The auriculo-veutricular valves, like those already described, se- cure the closure of the orifices at which they are placed by the con- tact of parts of their surfaces; but in most other respects their mode of action is peculiar. The valve between the right auricle and ven- tricle is named tricuspid, because it presents three principal cusps 78 CIRCULATION. or pointed portions, and that between the left auricle and ventricle bicuspid, or mitral, because it has two such portions. But in both valves there is between each two principal portions a smaller one; so that more properly, the tricuspid may be described as consisting of six, and the mitral of four, portions. Each portion^ is of tri- angular form, its apex and sides lying free in the cavity of the ventricle, and its base being fixed to a tendinous ring, which en- circles the orifice between the auricle and ventricle, and receives the insertions of the muscular fibres of both. In each principal portion may be distinguished a middle-piece, extending from its base to its apex, and including about half its width; this piece is thicker, and much tougher and tighter than the border-pieces which are attached loose and flapping at its sides. While the bases of the several portions of the valves are fixed to the tendinous rings, their ventricular surfaces and borders are fastened by slender tendinous cords to the walls of the ventricles or to muscular columns or processes (carnese columnse) projecting from the walls into the cavities of the ventricles. In each ventri- cle there are as many of these columns as there are principal por- tions in the corresponding valve. Of the tendinous cords, besides some which pass from the walls of the ventricle and the fleshy columns to the tendinous ring, there are some, of principal strength, which pass from the same parts to the edges of the middle pieces of the several chief portions of the vaj^e. The ends of these cords are spread out in the substance of the valve, giving its middle-piece its peculiar strength and toughness; and from their sides numerous other more slender and branching cords are given off, which are attached all over the ventricular surface of the adjacent border- pieces of the principal portions of the valves, as well as to those smaller portions, which have been mentioned as lying between each two principal ones. Moreover, the muscular columns are so placed, that, from the summit of each, tendinous cords may proceed to the adjacent halves of two of the principal divisions, and to one inter- mediate or smaller division of the valve. These valves are brought into action at the instant in which the contraction of the ventricles begins. When not in action, all their portions probably lie close to the ventricular walls; but the con- traction of the fleshy columns, being simultaneous with that of the walls of the ventricles, draws out the several portions of the valves as soon as ever the ventricle begins to contract. The arrangement just mentioned, in which each column is connected with two of the principal and one of the intermediate divisions of the valve, further secures that the contraction of the columns shall approximate all the several portions of the valve, bringing them towards the centre of the orifice they are to close. Then, as the contracting walls of the ventricles press on the blood, the valves are pressed up by it CIRCULATION. 79 towards the auriculo-ventricular orifices, till their free edges and parts of their borders come into contact. In this position they are held secure, even though "the form and size of the orifice and the ventricle may change during the continued contraction; for the border-pieces are held by their mutual apposition and • the equal pressure of the blood on their ventricular surfaces; and the middle pieces are secure by their great strength, and by the attachment of the tendinous cords along their margins, these cords being always held tight by the contraction of the muscular columns. A peculiar advantage, derived from the projection of these columns into the cavity of the ventricle, seems to be that they prevent the valve from being everted into the auricle; for, when the ventricle contracts, and the parts of its walls to which, through the medium of the columns, the tendinous cords are fixed, approach the base of the heart and the auriculo-ventricular orifices, there would be a tend- ency to slackness of the cords, and the valves might be everted, if it were not that while the wall of the ventricle is drawn towards the orifice, the end of the simultaneously-contracting fleshy column is drawn away from it, and the cords are held tight. What has been said applies equally to the auriculo-ventricular valves on both sides of the heart, and of both alike the closure is generally complete every time the ventricles contract. But in some circumstances the closure of the tricuspid valve is not com- plete, and a certain quantity of blood is forced back into the auricle; and, since this may be advantageous, by preventing the over-filling of the vessels of the lungs, it has been called the safety-valve action of this valve (Hunter, Wilkinson King). The circumstances in which it usually happens are those in which the vessels of the lung are already full enough when the right ventri- cle contracts, as, e. g., in certain pulmonary diseases, in very active exertion, and in great efforts. In these cases, perhaps be- cause the right ventricle cannot contract quickly or completely enough, the tricuspid valve does not completely close, and the regurgitation of blood may be indicated by a pulsation in the jugu- lar veins synchronous with that in the carotid arteries. Sounds and Impulse of the Heart. When the ear is placed over the region of the heart, two sounds may be heard at every beat of the heart, which follow in quick succession, and are succeeded by a pause or period of silence. The first sound is dull and prolonged; its commencement coincides with the impulse of the heart, and just precedes the pulse at the wrist. The second is a shorter and sharper sound, with a some- what flapping character, and follows close after the arterial pulse. If the period of time occupied by the two sounds and by the sub- sequent pause—which together constitute the rhythm of the 80 CIRCULATION. heart—be divided into four equal parts, the first sound, and the very short interval between it and the second, will be found to occupy the first two parts, or half the period of the rhythm; the second sound rather less than another part, and the pause rather more than the fourth part. The events which correspond, in point of time, with the first sound, and which may therefore contribute to its production, or to morbid changes in its characters, are (as expressed in the table at p. 75) the contraction of the ventricles, the first part of the dilata- tion of the auricles, the closure of the auriculo-ventricular valves, the openness of the semilunar valves, and the propulsion of blood into the arteries. The sound is succeeded, in about one-thirtieth of a second, by the pulsation of the facial artery, and in about one- sixth of a second, by the pulsation of the arteries at the wrist. The second sound, in point of time, immediately follows the cessa- tion of the ventricular contraction, and corresponds (as the same table shows) with the closure of the semilunar valves, the con- tinued dilatation of the auricles, the commencing dilatation of the ventricles, and the opening of the auriculo-ventricular valves. The pause immediately follows the second sound, and corresponds in its first half with the completed distension of the auricles, and in its second with their contraction, and the distension of the ventricles, the auriculo-ventricular valves being all the time open, and the arterial valves closed. The results of numerous investigations into the cause of the sounds of the heart have shown that the first sound is chiefly due to the contraction of the muscular fibres of the ventricles, which, like the contraction of muscle in other parts, is accompanied with the production of a certain amount of sound. In the case of the ventricular contraction, the sound is rendered peculiarly loud and distinct by a large mass of fibres being simultaneously in action, and by the force with which their contraction takes place. The sound emitted by them is rendered clearer and more distinct by the tense state in which the auriculo-ventricular valves are held during the continuance of the ventricular contraction; for, in this condition, they probably vibrate, like sounding-boards, with the vibrations communicated to them through the tense tendinous cords that attach them to the vibrating and sounding muscles. Thus the valves may increase the sound produced by the muscles; and, besides, it is not improbable that the suddenness with which they are put on the stretch at the commencement of the ventricular contraction may be productive of a certain amount of sound origi- nating in themselves. This supposition is supported by the fact observed by Valentin (iv. Bd. i. p. 427), that if a portion of a horse s intestine, tied at one end, be moderately filled with water without any admixture of air, and have a syringe containing water CIRCULATION. 81 fitted to the other end, the first sound of the heart is exactly imi- tated by forcing in more water, and thus suddenly rendering the walls of the intestine more tense. Some share in the production or modification of the first sound of the heart is probably due, also, to the impulse of the apex against the walls of the chest; for, when this impulse is prevent- ed, as in cases of congenital or artificial exposure of the heart, the intensity of the first sound is said to be diminished. The sudden pressing back of the semilunar valves, and the rush of blood through the orifices of the aorta and the pulmonary artery may also, in some measure, affect the first sound. These circumstances, however, must be regarded as only secondary conditions in the production of a sound which has its source in the muscular con- traction of the ventricles; and they would have scarcely deserved notice, but that, in cases of disease, alterations in them may change the character of the first sound. The fact that the sound emitted by the contraction of a morbidly enlarged and thickened ventricle is less clear and loud than that produced by a healthy one, may seem opposed to the above expla- nation ; for the inference is natural that increase in the bulk of a muscle will be accompanied with increase in the sound given out by it during its contraction. But it may be that, in the contrac- tion of an hypertrophied ventricle, the sound emitted by the super- ficial fibres alone is conveyed to the ear; that produced by the contraction of the deeper ones being obscured by the thickness of the tissue through which it has to pass in its transit to the sur- face. It is probable, also, that the contraction of the fibres of an hypertrophied ventricle is not quite simultaneous; and this may account for the first sound in such cases being unusually pro- longed, and less loud and distinct. When, however, the ventricu- lar walls are unusually thin, the first sound is peculiarly loud and clear, because the sound emitted by the contraction of every fibre is transmitted to the ear, and because all the fibres contract simul- taneously. The share, also, which the tension of the auriculo- ventricular valves and the passage of the blood through the orifices of the great arteries contribute towards the first sound, is more perceptible in the latter than in the former case, probably be- cause it more readily admits of being transmitted through thin, than through the hypertrophied, walls of a ventricle. The cause of the second sound appears to be more simple than that of the first. It is probably due almost entirely to the sudden tightening of the semilunar valves when they are pressed down across the orifice of the aorta and pulmonary artery; for, of the other events which take place during the second sound, none is calculated to produce sound. The influence of the valves in pro- ducing the sound is illustrated by the experiment already quoted 82 CIRCULATION. from Valentin, and by others performed on large animals such as calves, in which the results could be fully appreciated. In these two delicate curved needles were inserted, one into the aorta and another into the pulmonary artery, below the line of attachment of the semilunar valves; and, after being carried upwards about half an inch, were brought out again through the coats of the respec- tive vessels, so that in each vessel one valve was included between the arterial walls and the wire. Upon applying the stethoscope to the vessels after such an operation, the second sound has ceased to be audible. Disease of these valves, when so extensive as to inter- fere with their efficient action, also often demonstrates tha same fact by modifying or destroying the distinctness of the second sound. The second sound does not continue all the time the semilunar valves are closed, probably because it is only produced by the tightening and soon-ending vibration of the valves. The contraction of the auricles which takes place in the end of the pause is inaudible outside the chest, but may be heard when the heart is exposed and the stethoscope placed on it, as a slight sound preceding and continued into the louder sound of the ven- tricular contraction. The Impulse of the Heart.—At the commencement of each ventricular contraction, the heart may be felt to beat with a shock or impulse against the walls of the chest. This impulse is most evident in the space between the fifth and sixth ribs, betweeen one and two inches to the left of the sternum. The force of the im- pulse, and the extent to which it may be perceived beyond this point, vary considerably in different individuals, and in the same individuals under different circumstances. It is felt more distinct- ly, and over a larger extent of surface, in emaciated than in fat and robust persons, and more during a forced expiration than in a deep inspiration; for, in the one case, the intervention of a thick layer of fat or muscle between the heart and the surface of the chest, and in the other the inflation of the portion of lung which overlaps the heart, prevents the impulse from being fully transmitted to the surface. An excited action of the heart, and especially an hyper- trophied condition of the ventricles, will increase the impulse, while a depressed condition, or an atrophied state of the ventricular walls, will diminish it. The impulse of the heart is probably the result of several cir- cumstances which, acting in combination, have a tendency to rotate the whole organ from left to right, and to tilt its apex forwards and upwards, so that it is made to strike against the walls of the chest. Apparently the most important of these circumstances is the con- traction of the spiral muscular fibres of the ventricles, and especially of certain of these fibres which, according to Dr. Reid (lxxiii. vol. ii. p. 606), arise from the base of the ventricular septum, pass CIRCULATION. 83 downwards and forwards, forming part of the septum, then emerge and curve spirally around the apex and adjacent portion of the heart. The general direction of these fibres is from right to left, and the great mass of them pass in at the apex of the left ventricle, and assist in forming the muscular columns within it. The fibres on the front of the heart and on the right side of the left ventricle being much longer than those on the back and left side of it, will contract most, as they all draw up toward the tendinous rings; in their contraction they tend to draw the apex forwards and upwards; and probably, also, by their spiral turning round the apex, they make it rotate slightly from the left towards the right side of the chest. The whole extent of movement thus produced is but slight; the apex of the heart is carried through a curved line of about half an inch in length, at the end of which it touches the intercostal space, and is felt externally. The movement affects the apex of the heart much more than its body and base ; their relation to the walls of the chest undergoes little change in the several actions of the heart, and the change is the less perceptible because the tissues that intervene between them and the wall of the chest shift and adapt themselves to their several movements. The condition which, next to the action of these fibres, contri- butes most to the occurrence and character of the impulse of the heart is its change of shape ; for, during the contraction of the ventricles, it becomes more globular, and bulges so much that, according to Dr. Mitchell (cxvi. Nov. 1844), and M. Kiwisch (lix. 1846), this change alone is sufficient to produce the impression of an impulse when the finger is placed over the bulging portion of the heart, either at the front of the chest or under the diaphragm. The pro- duction of the impulse is, perhaps, further assisted by the tendency of the aorta to straighten itself and diminish its curvature when distended with the blood impelled by the ventricle; and, by the elastic recoil of all the parts about the base of the heart, which, ac- cording to the experiments of Kiirschner (xv. Art. Herzthatigheit), are stretched downwards and backwards by the blood flowing into the auricles and ventricles during the dilatation of the latter, but recover themselves when, at the beginning of the contraction of the ventricles, the flow through the auriculo-ventricular orifices is stop- ped. But these can be only accessory conditions in the perfect state of things : for the same tilting movement of the heart ensues when its apex is cut off, and no tension or change of form can be produced by the blood. The cause of the impulse must therefore be in the walls of the heart itself; and when the apex of the heart is cut off, and the continuity of most of those fibres, to whose action we have ascribed the impulse in the perfect state of the organ, is destroyed, then we may believe that the fibres remaining in the body and base of the heart, and having the same general spiral di- 84 CIRCULATION. 130 to 115 115 to 100 100 to 90 90 to 85 85 to 80 75 to 70 65 to 50 rection from right to left and from above downwards, are sufficient to produce a similar tilting movement. Frequency and Force of the Heart's Action. The frequency with which the heart performs the actions we have described may be counted by the pulses at the wrist, or in any other artery; for these correspond with the contractions of the ventricles. The heart of a healthy adult man in the middle period of life acts from seventy to seventy-five times in a minute. The frequency of the heart's action gradually diminishes from the commencement to the end of life, thus :— In the embryo, the average number of pulses in a minute is 150 Just after birth......from 140 to 130 During the first year . During the second year During the third year About the seventh year About the fourteenth year In the middle period of life In old age In persons of sanguine temperament the heart acts somewhat more frequently than in those of the phlegmatic; and in the female sex more frequently than in the male. • After a meal its action is accelerated, and still more so during bodily exertion or mental excitement; it is slower during sleep. The effect of disease in producing temporary increase or diminution of the frequency of the heart's action is well known. From the observations of several experimenters, it appears that in the state of health, the pulse is most frequent in the morning, and becomes gradually slower as the day advances; and that this diminution of frequency is both more regular and more rapid in the evening than in the morning. It is found, also, that, as a general rule, the pulse, especially in the adult male, is more frequent in the standing than in the sitting posture, and in this than in the recumbent position; the difference being greatest between the standing and the sitting posture. This effect of change of posture is greater as the frequency of the pulse is greater, and accordingly, is more marked in the morning than in the evening. Dr. Guy, by supporting the body in different postures, without the aid of muscular effort of the indi- vidual, has proved that the increased frequency of the pulse in the sitting and standing positions is dependent on the muscular exer- tion engaged in maintaining them ; the usual effect of these postures on the pulse being almost entirely prevented when the usually at- tendant muscular exertion was rendered unnecessary (lviii. Nos. 6 and 7). The effect of food, like that of change of posture, is CIRCULATION. 85 greater in the morning than in the evening. According to Parrot the frequency of the pulse increases in a corresponding ratio with the elevation above the sea; though it must be stated that other observers have found no such difference from change of elevation. (See especially Mr. R. H. Hunter, lxxi. Aug. 9, 1850.) In health, there is observed a nearly uniform relation between the frequency of the pulse and of the respirations; the proportion being, on an average, one of the latter to three or four of the for- mer. The same relation is generally maintained in the cases in which the pulse is naturally accelerated, as after food or exercise: but in disease this relation usually ceases to exist. In many affec- tions accompanied with increased frequency of the pulse, the respi- ration is, indeed, also accelerated, yet the degree of its acceleration bears no definite proportion to the increased number of the heart's actions : and in many other cases the pulse becomes more frequent without any accompanying increase in the number of respirations; or, the respiration alone may be accelerated, the number of pulsa- tions remaining stationary, or even falling below the ordinary stand- ard. (On the whole of this subject, the article Pulse, by Dr. Guy, in the Cyclopaedia of Anatomy and Physiology, may be advantage- ously consulted.) The force with which the left ventricle of the heart contracts is about double that exerted by the contraction of the right : being equal (according to Valentin) to about l-50th of the weight of the whole body, that of the right being equal only to l-100th of the same (iv. Bd. 1, p. 415, &c). This difference in the amount of force exerted by the contraction of the two ventricles, results from the walls of the left ventricle being about twice as thick as those of the right. And the difference is adapted to the greater degree of resistance which the left ventricle has to overcome, compared with that to be overcome by the right: the former having to propel blood through every part of the body, the latter only through the lungs. The capacity of the two ventricles is probably exactly the same. It is difficult to determine with certainty how much this may be- but, taking the mean of various estimates, it may be inferred that each ventricle is able to contain, on the average, about three ounces of blood, the whole of which is impelled into their respective arte- ries at each contraction. The capacity of the auricles is rather less than that of the ventricles : the thickness of their walls is consider- ably less. The latter condition is adapted to the small amount of force which the auricles require, in order to empty themselves into their adjoining ventricles; the former, to the ventricles being partly filled with blood before the auricles contract. The force exercised by the auricles in their contraction has not been calculated. Neither is it known with what amount of 8 86 CIRCULATION. force either the auricles or the ventricles dilate : but there is no evidence for the opinion that in their dilatation they can materially assist the circulation by any such action as that of a sucking-pump, or a caoutchouc bag, in drawing blood into their cavities. That the force the ventricles exercise in dilatation is very slight was proved by Oesterreicher (c. p. 33). He removed the heart of a frog from the body, and laid upon it a substance sufficiently heavy to press it flat, and yet so small as not to conceal the heart from view; he then observed that during the contraction of the heart the weight was raised, but that during its dilatation the heart remained flat. And the same was shown by Dr. Clendinning, who, applying the points of a pair of spring callipers on the heart of a live ass, found that their points were separated as often as the heart swelled up in the contraction of the ventricles, but approached each other by the force of the spring when the ventricles dilated. Seeing how slight the force exerted in the dilatation of the ventricles is, it has been sup- posed that they are only dilated by the pressure of the blood im- pelled from the auricles; but that both ventricles and auricles di- late spontaneously is proved by their continuing their successive contractions and dilatations when the heart is removed, or even when they are separated from one another, and when therefore no such force as the pressure of blood can be exercised to dilate them. By such spontaneous dilatation they at least offer no resist- ance to the influx of blood, and save the force which would other- wise be required to dilate them. Cause of the Rhythmic Action of the Heart. It has been attempted in various ways to account for the exist- ence and continuance of those peculiar rhythmic movements by which the action of the heart is distinguished from that of all the other muscles. By some it has been supposed that the contact of arterial blood with the lining membrane of the left cavities of the heart, and of venous blood with that of the right cavities, furnishes a stimulus, in answer to which the walls of these cavities contract. And they explain the rhythmic order in which these contractions ensue, by supposing that the same act—the systole, which expels the stimulating fluid from the ventricles, causes the auricles to be filled from the veins; and that the contraction of the auricles thereupon induced gives rise, in its turn, to the filling and conse- quent contraction of the ventricles. But this, and all hypotheses concerning the action of the heart which suppose the necessity of the contact of blood, or any such stimulus, are disproved by the fact that the heart, especially in Amphibia and fishes, will con- tinue to contract and dilate regularly and in rhythmic order after it is removed from the body, completely emptied of blood, and CIRCULATION. 87 even placed in a vacuum where it cannot receive the stimulus of the atmospheric air. The influence of the mind, and of some affections of the brain and spinal cord upon the action of the heart, proves that it is not altogether, or at all times, independent of the cerebro-spinal nerv- ous system. Yet the numerous experiments instituted for the purpose of determining the exact relation in which the heart stands towards this system, have failed to prove that the action is directly governed by the power of any portion of the brain or spinal cord. The results of the experiments are, in many instances, contradictory; but a general conclusion from them may be, that no uniform and decided alteration in the movements of the heart is produced by irritation of any part of either of those nervous centres. Sudden destruction of either the brain or spinal cord alone, or of both together, produces, immediately, a temporary interruption or ces- sation of the heart's action; but this appears to be only an effect of the shock of so severe an injury; for, in some such cases, the movements of the heart are subsequently resumed, and if artificial respiration be kept up, may continue for a considerable time; and may then again be arrested by a violent shock applied through an injury of the stomach. While, therefore, we must admit an indi- rect or occasional influence exercised by, or through, the brain and spinal cord upon the movements of the heart, and may believe this influence to be the greater the more highly the several organs are developed, yet it is clear that we cannot ascribe the regular deter- mination and direction of the movements to them, in the same way as we may ascribe to the medulla oblongata the power of deter- mining and regulating the involuntary and, in some degree, rhythmical, movements of respiration. The persistence of the movements of the heart in their regular rhythmic order, after its removal from the body, and their capa- bility of being then re-excited by an ordinary stimulus after they have ceased, prove that the cause of these movements must be resident within the heart itself. And it seems probable, from the experiments and observations of Remak (cxxx. No. ii. 1840), Volk- mann (lxxx. 1844, p. 424), Dr. Robert Lee (cxxiii. 1847, and lxxi. vol. xlv. p. 224), and others (see xxv. 1844-5, p. 13), that it may be connected with the existence of numerous minute ganglia of the sympathetic nervous system, which, with connecting nerve- fibres, are distributed through the substance of the heart. These ganglia appear to act as so many centres or organs for the produc- tion of motor impulse; while the connecting nerve-fibres unite them into one system, and enable them to act in concert and direct their impulses so as to excite in regular series the successive con- tractions of the several muscles of the heart. The mode in which ganglia thus act as centres and co-ordinators of nervous power will 88 CIRCULATION. be described in the chapter on the Nervous System; audit will appear probable that the chief peculiarity of the heart, in this regard, is due to the number of its ganglia, and the apparently equal power which they all exercise ; so that there is no one part of the heart whose action, more than another's, determines the actions of the rest. Thus, if the heart of a reptile be bisected, the rhythmic successive actions of auricle and ventricle will go on in both halves: we therefore cannot say that the action of the right side determines or regulates that of the left, or vice versa; and we must suppose that when they act together in the perfect heart, it is because they are both, as it were, set to the same time. Neither can we say that the auricles determine the action of the ventricles; for, if they are separated, they will both contract and dilate in regular, though not necessarily similar, succession. A fact pointed out by Mr. Maiden, shows how the several portions of each cavity are similarly adjusted to act alike, yet independently of each other. If a point of the surface of the ventricle of a turtle's or frog's heart be irritated, it will immediately contract, and very quickly afterwards all the rest of the ventricle will contract: but, in the close of this general contraction, the part that was irritated and contracted first, is slightly distended or pouched out, showing that it was adjusted to contract in and for only a certain time, and that therefore as it began to contract first, so it first began to dilate.1 The connection of the action of the heart with the other organs, and the influences to which it is subject through them, are explica- ble by the connection of its nervous system with the other ganglia of the sympathetic, and with the brain and spinal cord through, chiefly, the pneumogastric nerves. But this influence is proved in a much more striking manner by the phenomena of disease than by any experimental or other physiological observations. The influence of a shock in arresting or modifying the action of the heart—its very slow action after compression of the brain, or in- jury to the cervical portion of the spinal cord—its irregularities and palpitations in dyspepsia and hysteria—are better evidence for the connection of the heart with the other organs through the nervous system, than any results obtained by experiments. The best of such results are recorded by E. H. Weber (xv. Art. Mus- kelbeicegvng), who found that the electro-magnetic stimulus ap- plied in the frog to the bulbus arteriosus, around which the prin- cipal fibres of the sympathetic nerves supplying the heart are placed, accelerated and strengthened the heart's action; but, ap- 1 The experiment also proves the uniform and simultaneous action of the whole wall of the ventricle, and the advantage thereof; since if one part of the wall ceased to contract before the rest it would be pouched' out by the communicated pressure of the blood still compressed by the con- tinuing contraction of the rest of the wall. CIRCULATION. 89 plied to the pulsating part of the vena cava inferior, where are the principal filaments it derives from the pneumogastric nerves, re- tarded the action. He is disposed, therefore, to think that, in general, stimuli conveyed through the sympathetic nerve would accelerate, and through the pneumogastric would retard, the heart's action. The latter conclusion is corroborated by the fact, also stated by him, that the heart's action is retarded by stimulus applied to any part between the corpora quadrigemina and poste- rior part of the fourth ventricle of the brain, or to both trunks of the pneumogastric nerves at once, or by division of both pneumo- gastric nerves in the necks of Mammalia. Effects of the Heart's Action. That the contractions of the heart supply alone a sufficient force for the circulation of the blood appears to be established by these following facts. 1. When the heart is removed, or the aorta tied, the circulation stops abruptly and completely : and the circulation in any part may be so arrested by tying its main arteries. 2. Magendie found, by dividing transversely all the parts of a limb except its main artery and vein, that the current through the latter was completely and at once controlled by pressure on the former : although here, as in the former cases, the vessels beyond the ob- struction might have maintained the stream of blood if they had possessed any propulsive power. 3. Dr. Sharpey (xciv. vol. lxiii. p. 20) injected bullock's blood into the thoracic aorta of a dog re- cently killed, after tying the abdominal aorta above the renal arteries, and found that, with a force just equal to that by which the ventricle commonly impels the blood in the dog, the blood that he injected into the aorta passed in a free stream out of the trunk of the vena cava inferior. It thus traversed both the sys- temic and hepatic capillaries; and when the aorta was not tied above the renals, blood injected under the same pressure flowed freely through the vessels of the lower extremities. A pressure equal to that of one and a half or two inches of mercury was, in the same way, found sufficient to propel blood through the vessels of the lungs. How this force of the ventricles is applied, directed, and assisted in the several orders of vessels will appear in the following sections. The best special treatises to which the student can refer for details and discussions respecting the action of the heart are, the articles Heart and Circulation, in the Cyclopaedia of Anatomy and Physi- ology, the Reports of the Medical Section of the British Associa- tion, in the Medical Gazette, vols. xix. and xxi., and the works of Mr. Hunter (i. vol. iii. p. 173), of Dr. Hope (cxvii.), and of Dr. C. J. B. Williams (cxviii). 8* 90 CIRCULATION. THE ARTERIES. For the purpose of explaining the influence of the arteries in the circulation it will be sufficient to consider the walls of an artery as containing, at the most, five distinct layers, or coats, namely, an external, an elastic, a muscular, an internal coat, and an epithelial lining.1 The external coat is constructed of ordinary fibro-cellular tissue, the fibres of which are arranged, for the most part, in a longitudinal direction. It forms a strong, tough investment, which, though capable of extension, appears principally designed to strengthen the walls of the artery, and to guard against their excessive distension from the force of the heart's action. It serves another purpose also in affording a suitable tissue for the ramifica- tions of the vasa arteriarum, or nutritive vessels for the supply of the arterial walls. The internal arterial coat (the striated or fenes- trated coat of Ilenle) consists of a very thin and brittle membrane. It possesses little elasticity, and is thrown into folds or wrinkles, when an artery contracts. Its internal surface is lined with a delicate layer of epithelium, which makes it smooth and polished, and furnishes a nearly impermeable surface, along which the blood may flow with the smallest possible amount of resistance from fric- tion. The elastic and muscular coats are the seats of those properties by which arteries chiefly influence the circulation. Previous to the time of John Hunter, the distinction between these two coats, which constitute the chief thickness of the arterial wall appears to have been overlooked, and it was usual to describe them together as a single tissue under the denomination of the middle, fibrous, or elastic coat. But in the admirable account which Hunter gave of the properties of arteries, proof was afforded of the dissimilarity in structure and function between the inner and outer portions of this supposed single coat. And recent observations have shown the accuracy of his account, and furnished additional facts in confirma- tion of it. The outer of these two coats is made up almost entirely of fibres of yellow elastic tissues, and constitutes, as Hunter named it, the elastic coat. The inner consists of circularly-arranged, pale, flat fibres, which differ in no essential respect from the fibres of organic muscle, such as compose the muscular coat of the stomach and intestines; but are mingled with more filaments of fine elastic tissue. Its chemical characters are equally similar to those of organic muscle. The older analyses, in which the similarity was not detected, were probably made of the walls of the largest arteries in which clastic tissue alone exists. By later analysis, Dr. Retzius 1 For the best account of these structures see Henle (xxxvii. p. 494), or an abstract of his observations (xxv. 1812, p. 39); also Kolliker (lix. 1847). CIRCULATION. 91 (cvii. vol. i. p. 171) has found in this coat a proteine-compound, which neither cellular nor elastic tissue contain; and Dr. Donders (cviii. 1846, p. 67) has proved the same more perfectly. When, he says, strong nitric acid is applied to any compound of proteine, it forms with it what is termed xantho-proteinic acid, which, with ammonia, produces a yellow xantho-proteinate of ammonia. On applying this test, with the requisite cautions, to the coats of blood- vessels, he found that the muscular arterial coat alone assumed the characteristic yellow color. The other coats, as well as all the coats of veins, remained unchanged in color. He found also that potash acts on the coat of arteries, as on organic muscle, separating its fibres, making them granular, and finally dissolving them. For this coat, therefore, the name of muscular, applied by Hunter, may be retained. These two coats exist in different relative degrees of thickness in different arteries; and, in general, are in an inverse ratio to each other, for the arteries which possess most elastic tissue have the least muscular tissue, while those whose walls are most muscular are in general the least elastic. In the large arteries, such as the aorta and its main branches, scarcely a trace of the muscular coat can be found, nearly the whole thickness of their walls consisting of elastic tissue. But in the arteries farther removed from the heart, and of smaller size, the proportionate thickness of the elastic coat gradually diminishes, while, as a general rule, that of the mus- cular coat progressively increases. Moreover, in the arteries of certain organs, probably of those in which the supply of blood is subject to greater than usual variations in adaptation to fluctuations in the amount of function they discharge, there is a proportionately greater development of the muscular coat. Of the properties which the arteries possess in these two coats, the muscularity has its seat exclusively in the muscular coat, and no artery without this coat would present any contraction similar to that of muscles. But elasticity is a property not exclusively, though especially, seated in the elastic coat; rather, all the coats, except perhaps the internal, are in some measure elastic, and will recoil after being distended; and the effect their elasticity produces is yet further assisted by the elasticity of the tissues around them. The purposes of the elasticity of arteries are chiefly twofold: 1st. It guards them from the suddenly exerted pressure to which they are subjected at each contraction of the ventricles. In every such contraction, the contents of the ventricles are forced into the arteries more quickly than they can be discharged into and through the capillaries. The blood, therefore, being for an instant resisted in its onward course, a part of the force with which it was impelled is directed against the sides of the arteries; under this force, which 92 CIRCULATION. might burst a brittle tube, their elastic walls dilate, stretching enough to receive the blood, and as they stretch becoming more tense and more resisting. Thus, by yielding, they as it were break the shock of the force impelling the blood, and exhaust it before they are in danger of bursting, through being overstretched. Elasticity is thus advantageous in all arteries, but chiefly so in the aorta and its large branches, which are provided, as already said, with a large quantity of elastic tissue, in adaptation to the great force of the left ventricle, which falls first on them, and to the increased pressure of the arterial blood in violent expiratory efforts. On the subsidence of the pressure, when the ventricles cease con- tracting, the arteries are able, by the same elasticity, to resume their former caliber; and in thus doing, they manifest the second chief purpose of their elasticity, that, namely, of equalizing the current of the blood by maintaining pressure on the blood in the arteries during the periods at which the ventricles are at rest or dilating. If some such method as this had not been adopted, if, for example, the arteries had been rigid tubes, the blood, instead of flowing as it does, in a constant stream, would have been pro- pelled through the arterial system in a series of jerks corresponding to the ventricular contractions, with intervals of almost complete rest during the inaction of the ventricles. But, in the actual con- dition of the arteries, the force of the successive contractions of the ventricles is expended partly in the direct propulsion of the blood, and partly in dilating the elastic arteries; and in the intervals between the contractions of the ventricles, the force of the recoil- ing and contracting arteries is employed in continuing the same direct propulsion. Of course, the pressure exercised by the recoil- ing arteries is equally diffused in every direction through the blood, and the blood would tend to move backwards as well as onwards; but all movement backwards is prevented by the closure of the arterial valves, which takes place at the very commencement of the recoil of the arterial walls. By this exercise of the elasticity of the arteries, all the force of the ventricles is made advantageous to the circulation; for that part of their force which is expended in dilating the arteries is restored in full, according to the law of action of elastic bodies, by which they return to the state of rest with a force equal to that by which they were disturbed therefrom. There is thus no loss of force; but neither is there any gain, for the elastic walls of the artery cannot originate any force for the propulsion of the blood— they only restore that which they received from the ventricles; they would not contract, had they not been first dilated, any more than a spiral spring would shorten itself unless it were first elongated. The advantage of elasticity in this regard is, therefore, not that it CIRCULATION. 93 increases, but that it equalizes or diffuses the forces derived from the periodic contractions of the ventricles. The force with which the arteries are dilated every time the ventricles contract might be said to be received by them in store, to be all given out again in the next succeeding period of dilatation of the ventricles. It is by this equalizing influence of the successive branches of every artery that, at length, the intermittent accelerations produced in the arte- rial current by the action of the heart cease to be observable, and the jetting stream is converted into the continuous and equable movement of the blood which we see in the capillaries and veins. Two other purposes served by the elasticity of arteries must not be overlooked. One is the capacity which the arteries have for receiving more than the average quantity of blood, both every time the ventricles contract, and when the supply of blood to the whole body or any part of it is, for a time, unusually large. In all such cases the enlargement of the arteries is effected by increase of both their diameter and their length; and the elongation appears to be, generally, more considerable than the dilatation. The other pur- pose served by the elasticity is, that, by means of it, the arteries are enabled to adapt themselves to the different movements of the several parts of the body. The evidence for the muscularity of arteries needs probably to be given at length, since, even recently, some physiologists have denied that the arterial walls possess any property analogous to muscular contractility. We have already referred to Mr. Hunter's account of the muscular structure of the inner layer of the middle coat of all but the largest arteries, and to the fact, first observed by Ilenle, that this layer is composed of fibres, in all respects similar to those of organic muscles, though mingled with fine elastic filaments. The observation of the action of arteries will show, 1st, The opera- tion of a contractile power in arteries, essentially distinct from their elasticity; and 2dly, the identity of this power with muscular contractility. When a small artery in the living subject is exposed to the air or cold, it gradually but manifestly contracts. Hunter (i. vol. iii. p. 157) observed that the posterior tibial artery of a dog when laid bare became in a short time so much contracted as almost to pre- vent the transmission of blood; and the observation has been often and variously confirmed. Simple elasticity could not effect this; for after death, when the vital muscular power has ceased, and the* mechanical elastic one alone operates, the contracted artery dilates again. When an artery is cut across, its divided ends contract, and the orifices may be completely closed. The rapidity and completeness of this contraction are different in different animals; they are gene- 94 CIRCULATION. rally greater in young than in ol-d animals; and less, apparently, in man than in animals. The contraction is generally increased by the application of cold, or of any simple stimulating substances, or by mechanically irritating the cut ends of the artery, as by pricking or twisting them. Such irritation would not be followed by these effects if the arteries had no other power of contracting than that depending upon elasticity. The contractile property of arteries continues many hours after death, and thus affords an opportunity of distinguishing it from elasticity. When a portion of an artery, for example, the splenic artery, of a recently killed animal, is exposed, it gradually contracts, and its canal may be thus completely closed: in this contracted state it remains for a time, varying from a few hours to two days; then it dilates again, and permanently retains the same size. If while contracted, the artery be forcibly distended, its contractility is destroyed, and it holds a middle or natural size. This persistence of the contractile property after death was well shown in an observation of Hunter, which may be mentioned as proving also the greater degree of contractility possessed by the smaller than the larger arteries. Having injected the uterus of a cow, which had been removed from the animal upwards of twenty- four hours, he found, after the lapse of another day, that the larger vessels had become much more turgid than when he injected them, and that the smaller arteries had contracted so as to force the in- jection back into the larger ones. The results of an experiment which Hunter made with the vessels of an umbilical cord, prove still more strikingly the long continu- ance of the contractile power of arteries after death. In a woman delivered on a Thursday afternoon, the umbilical cord was separated from the foetus, having been first tied in two places, and then cut between, so that the blood contained in the cord and placenta was confined in them. On the following morning, Hunter tied a string round the cord, about an inch below the other ligature, that the blood might still be confined in the placenta and remaining cord. Having cut off this piece, and allowed all the blood to escape from its vessels, he attentively observed to what size the ends of the cut arteries were brought by the elasticity of their coats, and then laid aside the piece of cord to see the influence of the contractile power of its vessels. On Saturday morning, the day after, the mouths of the arteries were completely closed up. He repeated the experi- ment the same day with another portion of the same cord, and on the following morning found the results to be precisely similar. On the Sunday, he performed the experiment the third time, but the artery seemed then to have lost its contractility, for, on the Monday morning, the mouths of the cut arteries were found open. In each of these experiments there was but little alteration per- ceived in the orifices of the veins (i. vol. iii. p. 158). CIRCULATION. 95 The influence of cold in increasing the contraction of a divided artery has been referred to: it has been shown, also, by Schwann, in an experiment on the mesentery of a living toad. Having ex- tended the mesentery under the microscope, he placed upon it a few drops of water, the temperature of which was some degrees lower than that of the atmosphere. The contraction of the vessels soon commenced, and gradually increased until, at the expiration of ten or fifteen minutes, the diameter of the canal of an artery, which at first was 0.0724 of an English line, was reduced to 0.0276. The arteries then dilated again, and at the expiration of half an hour had acquired nearly the original size. By renewing the application of the water, the contraction was reproduced : in this way the ex- periment could be performed several times on the same artery. The veins did not contract. It is thus proved that cold will excite contraction in the walls of very small, as well as of comparatively large arteries : it could not produce such contraction in |a merely elastic substance; but it is a stimulus to the organic muscular fibres in many other parts, as well as in the arterial coats; as, e. g., in the walls of the bronchi, and the dartos. Lastly, satisfactory evidence of the muscularity of the arterial coats is furnished by the experiments of Ed. and E. H. Weber (lxxx. 1847, p. 232), and of Professor Kblliker (exc. July, 1850, p. 241), in which they applied the stimulus of electro-magnetism to small arteries. One principal circumstance which induced Midler to deny the muscularity of arteries, was the seeming impossibility of producing contractions in arteries by galvanic and electric stimuli, which excite all true muscular tissues to manifest contraction. An explanation of the failure may be found in the circumstance that, in nearly all the experiments, the arteries examined were of large size, such as the aorta and the carotid, in which there is little or no muscular tissue. The experiments of the Webers were performed on the small mesenteric arteries of frogs; and the most striking results were obtained, when the diameter of the vessels examined did not exceed from l-7th to l-17th of a Paris line. When a vessel of this size was exposed to the electric current, its diameter, in from five to ten seconds, became one-third less, and the area of its section about one-half. On continuing the stimulus, the narrowing gradu- ally increased, until the caliber of the tube became from three to six times smaller than it was at first, so that only a single row of blood-corpuscles could pass along it at once; and eventually the vessel was closed and the current of blood arrested. Thus, with the exception of the largest trunks, the arteries ap- pear to offer every necessary evidence of muscularity. One of their coats has the structure and chemical composition of organic muscle; and, like such muscle, they contract on exposure, on division and mechanical irritation, after death, with a kind of rigor mortis, on the application of cold, and under the stimulus of electricity : in all 96 CIRCULATION. these contractions they are reduced to a size less than that which their mere elasticity would give them, and from all they are com- monly, after a time, again by their elasticity restored to a larger size. With regard to the purpose served by the muscular coat of the arteries, there appears no sufficient reason for supposing that it, in any way, assists in propelling the onward current of blood. It could not do so unless it possessed the property of alternately con- tracting and relaxing coincidently with the relaxation and contrac- tion of the ventricles; or unless it had a kind of peristaltic or vermicular movement commencing at the heart, and thence propa- gating itself rapidly along the several arteries; but there is no evidence to show that the arteries ever contract in either of these modes. The most probable office of the muscular coat is that of regulating the quantity of blood to be received by each part, and of adjusting it to the requirements of each, according to various circumstances ; but, chiefly and most naturally, according to the ac- tivity with which the functions of each part are at different times performed. The amount of function discharged by each organ of the body varies at different times, and the variations often quickly succeed each other, so that, like the brain, for example, during sleep and waking, within the same hour, a part may be now very active and then inactive. In all its active exercise of function, such a part requires a larger supply of blood than is sufficient for it during the times when it is comparatively inactive. It is evident that the heart cannot regulate the supply to each part at different periods, neither could it be regulated by any general and uniform contraction of the arteries; but it may be regulated by the power which the arte- ries of each part have, in their muscular coat, of contracting so as to diminish, and of passively dilating or yielding so as to permit an increase of the supply of blood, according as the requirements of the part may demand. And thus, while the ventricles of the heart determine the total quantity of blood to be sent onwards at each contraction, and the force of its propulsion, and while the large and merely elastic arteries distribute it and equalize its stream, the smaller arteries with muscular coats add to these two purposes, that of regulating and determining the proportion of the whole quantity of blood which shall be distributed to each part. The contraction of an artery may also be regarded as fulfilling a natural purpose when, the artery being cut, it first limits and then arrests the escape of blood. It is only because of such contraction that we are free from danger through even very slight wounds; for it is only when the artery is closed that the processes for the more permanent and secure prevention of bleeding are established. There are occasions in which the whole of the arteries appear to be contracted; such are those of diseases attended with a small hard pulse, but they probably occur only in morbid conditions, and physiology appears incapable of explaining them. CIRCULATION. 97 The normal contraction of arteries is probably excited through the instrumentality of the nerve-fibres of the sympathetic system dis- tributed in their walls, and connected through the medium of ganglia with the fibres supplying the organ to which such arteries convey blood. From what has been said in the preceding pages, it appears that the office of the arteries in the circulation is, 1st, the conveyance and distribution of blood to the several parts; 2d, the equalization of the current, and the conversion of the pulsatile, jetting movement given to the blood by the ventricles, into the uniform flow ; 3d, the regulation of the supply of blood to each part. In explanation of the. mode in which by the combination of the elastic and muscular coats of arteries this threefold office is accomplished, we may use, as a summary of what has been already said, the words of Mr. Hunter, who observes that, " there are three states in which an artery is found, viz. 1st, the natural pervious state; 2d, the stretched; and 3d, the contracted state, which may or may not be pervious. The natural pervious state is that to which the elastic power naturally brings a vessel which has been stretched beyond or contracted within the extent which it held in a state of rest. The stretched is that state produced by the impulse of the blood in consequence of the contraction of the heart; from which it is again brought back to the natural state by the elastic power, per- haps assisted by the muscular. The contracted state of an artery arises from the action of the muscular power, and is again restored to the natural state by the elastic," (i. vol. iii. p. 159.) The Pulse. The jetting movement of the blood, which, as just stated, it is one of the offices of the arteries to change and put an end to, is the cause of the pulse, and therefore needs a separate consideration. We have already said that as the blood is not able to pass through the arteries so quickly as it is forced into them by the ventricle, on account of the resistance it experiences in the capillaries, a part of the force with which the heart impels the blood is exercised upon the walls of the vessels which it distends. The distension of each artery increases both its length and its diameter; but the elonga- tion is the most considerable. In their elongation the arteries change their form, the straight ones becoming curved, and those already curved becoming more so ;l but they recover their previous 1 There is, perhaps, an exception to this in the case of the aorta, of which the curve is by some .supposed to be diminished when it is elongated; but if this be so, it is because only one end of the arch is immovable; the other end, with the heart, may move forward slightly when the ventricles con- tract. 9 98 CIRCULATION. form as well as their diameter when the ventricular contraction ceases, and their elastic walls recoil. The increase of their curves which accompanies the distension of arteries, and the succeeding recoil may be well seen in the prominent temporal artery of an old person. The elongation of the artery is in such a case quite mani- fest. The dilatation or increase of the diameter of the artery is less evident. In several reptiles it may be seen without aid in the immediate vicinity of the heart (i. vol. iii. p. 216, note), and it may be watched with a simple magnifying glass, in the aorta of the tadpole. Its slight amount in smaller arteries, the difficulty of observing them in opaque parts, and the rapidity with which it takes place, are sufficient to account for its being, in Mammalia, imperceptible to the eye. But in these also experiment has made its occurrence probable. Poiseuille (lxii. t. ix. p. 44) laid bare the common carotid of a living horse for the space of about twelve inches, and passed beneath it a tube of metal open at one side, which he afterwards closed by means of a narrower portion, so as to complete the tube; he then stopped the ends of the tube, and filled the interior around the artery with water, by means of a glass tube which was connected with the metallic tube. At every pulsation the water rose 70 millimetres1 in the glass tube, the diameter of which was 3 millimetres, and fell again the same dis- tance during each interval. The included portion of artery mea- sured in length 180 millimetres, and its capacity equalled 11,440 cubic millimetres; and since at every beat of the heart it under- went an increase of capacity equal to a column of water of 3 mil- limetres in diameter and 70 millimetres in height, or about 494 cubic millimetres, it follows that it was enlarged about l-23d of its capacity. It is probable that part of this enlargement was owing to dilatation : and Flourens, in evidence of such dilatation, says he encircled a large artery with a thin elastic metallic ring cleft at one point, and that at the moment of pulsation the cleft part be- came perceptibly widened. This dilatation of an artery, and its elongation producing curva- ture, or increasing its natural curves, are sensible to the finger placed over it, and produce the pulse. The mind cannot distin- guish the sensation produced by the dilatation, from that produced by the elongation and curving; that which it perceives most plainly is the curving, or, as it may be called, the locomotion of the artery; the portion that is under the finger slightly shifting its place as it lengthens in pulsation. Such, it is generally agreed, is the cause of the pulse felt in any artery; it is produced by the elongation and dilatation of the part under the finger, when it receives its portion of the fresh quantity 1 A millimetre equals 0.0393708 of an English inch. CIRCULATION. 99 of blood just discharged from the left ventricle into the aorta and its branches. But some doubt still exists in reference to the man- ner in which the pulse is propagated from one part of the arterial system to another. According to the theory of the pulse advanced by E. H. Weber (Ixvi.), and adopted by Midler, the impulse given to the blood by the heart distends first merely the arteries nearest to the heart. These, by their elasticity, again contract, and thus cause the distension of the next portion of the arterial system, which, also, in its turn, by contracting, forces the blood into the next portions, and so on; so that a certain interval of time, although a very short one, elapses before this undulation, resulting from the successive compressions of the blood and the dilatation and contraction of the arteries, reaches the most distant parts of the system. In this view, the arterial pulse is regarded as the effect of an oscillation or undulation, produced first by the pressure on the blood in the aorta by the contracting left ventricle, and thence pro- pagated along the walls of the arteries, and along the blood itself. But against this theory a very forcible objection presents itself in the fact recently pointed out by Mr. F. H. Colt (lxi. vol. xxxvi. p. 456), viz., that the pulse is perceived in every part of the arte- rial system previous to the occurrence of the second sound of the heart, that is, previous to the closure of the aortic valves. Now, if the pulse were the effect of a wave propagated by the alternate dilatation and contraction of successive portions of the arterial tube, it ought in all the arteries except those " nearest to the heart" to follow, or coincide with, but could never precede, the second sound of the heart; for the first effect of the elastic recoil of the arteries first dilated (which recoil, on Weber's theory, causes the dilatation and pulse in the arteries following those nearest to the heart), is the closure of the aortic valves, and their closure produces the second sound. It appears impossible to reconcile Weber's theory with this fact; the contraction of the arteries nearest to the heart, which he supposes to produce the pulse in those further on, cer- tainly produces, by closing the valves, the second sound of the heart; yet, this sound always, or nearly always, follows the pulse even in the most distant arteries in which it can be felt. It seems certain, therefore, that the contraction of the arteries nearest the heart does not take place till after the pulse in the more distant ones. The theory proposed by Mr. Colt, which seems to reconcile all the facts of the case, and especially those two which appear most opposed, namely, that the pulse always precedes the second sound of the heart, and yet is later in the arteries far from the heart than in those near it, may be thus stated : It supposes that the blood which is impelled onwards by the left ventricle does not so impart its pressure to what the arteries already contain as to dilate the whole arterial system at once; but that, as it enters the arteries, 100 CIRCULATION. it displaces and propels what they before contained, and flows on with what may be called a head-wave, like that which Is formed when a rapid stream of water overtakes another moving more slowly. The slower stream offers resistance to the more rapid one, till their velocities are equalized: and, because of such resistance, some of the force of the more rapid stream of blood just expelled from the ventricle, is diverted laterally, and with the rising of the wave the arteries nearest the heart are dilated and elongated. They do not at once recoil, but continue to be distended so long as blood is entering them from the ventricle. The wave at the head of the more rapid stream of blood runs on, propelled and maintained in its velocity by the continuous contraction of the ventricle : and it thus dilates in succession every portion of the arterial system, and produces the pulse in all. The rate of its movement, which repre- sents also the velocity of the blood in the arteries during the ven- tricular contraction, may be estimated by the interval between the pulses near and far from the heart. At length, the whole arterial system (wherein a pulse can be felt) is dilated; and at this time, when the wave we have supposed has reached all the smaller arteries, the entire system may be said to be simultaneously dilated; then it begins to contract, and the contractions of its several parts ensue in the same succession as the dilatations commencing at the heart. The contraction of the first portion produces the closure of the valves and the second sound of the heart; and both it and the progressive contractions of all the more distant parts maintain, as already said, that pressure on the blood during the inaction of the ventricle by which the stream of the arterial blood is sustained between the jets, and is finely equalized by the time it reaches the capillaries. It may seem an objection to this theory that it would probably require a larger quantity of blood to dilate all the arteries than can be discharged by the ventricle at each contraction. But the quantity necessary for such a purpose is less than might be sup- posed. Injections of the arteries prove that, including all down to those of about one-eighth of a line in diameter, they do not con- tain, on an average, more than one and a half pints of fluid even when distended. There can be no doubt, therefore, that the three ounces which the ventricle is supposed to discharge at each contrac- tion, being added to that which already fills the arteries, would be sufficient to distend them all. Force of the Blood in the Arteries. The force with which the ventricles act in their contraction, and the reasons for believing it sufficient for the circulation of the blood, have been already mentioned. Both calculation and experiment have proved that very little of this force is consumed in the arteries. CIRCULATION. 101 Fie. 22. Dr. Thomas Young (xliii. vol. xcix.) calculated that the loss of force in overcoming friction and other hindrances in the arteries would be so slight, that if one tube were introduced into the aorta, and another into any other artery, even into one as fine as a hair, the blood would rise in the tube from the small vessel to within two inches of the height to which it would rise from the large vessel. The correctness of the calculation is established by the experiments of Poiseuille (lxii. t. viii. p. 272), who invented an instrument named a haemadynamometer, for estimating the statical pressure exercised by the blood upon the walls of the arteries. It consists of a long glass tube bent so as to have a short horizontal por- tion (Fig. 22, a), a branch b descend- ing at right angles from it, and a long ascending branch d, e, c. Mercury poured into the ascending and de- scending portions will necessarily have the same level in both branches, and in a perpendicular position the height of its column must be the same in both. If, now, the blood is made to flow from an artery, through the horizontal portion of the tube (which should contain a solution of carbon- ate of potash to prevent coagulation) into the descending branch, it will exert on the mercury a pressure equal to the force by which it is moved in the arteries, and the mercury will, in consequence, descend in this branch, and ascend in the other. The depth to which it sinks in the one branch, added to the height to which it rises in the other^ will give the whole height of the column of mercury which balances the pressure exerted by the blood; the weight of the blood which takes the place of the mercury in the descending branch, and which is more than ten times less than the same quantity of quick- silver, being subtracted. Poiseuille thus calculated the force with which the blood moves in an artery, aceord- 9* Haemadynamometer of Poiseuille. A bent glass tube, filled with mercury in the lower part, a d e. The horizon- tal part, b, is provided with a brass head, which fits in the artery. A small quantity of a solution of the carbonate of soda is interposed be- tween the mercury and the blood, to prevent its coagulation. When the blood presses on the fluid in the hori- zontal limb, the rise of the mercury towards e, measured from the. level to which it has fallen towards a, gives the pressure under which the blood moves. 102 CIRCULATION. ing to the laws of hydrostatics, from the diameter of the artery, and the height of the column of quicksilver; that is to say, from the weight of a column of mercury whose base is a circle of the same diameter as the artery, and whose height is equal to the difference in the levels of the mercury in the two branches of the instrument. He found the blood's pressure equal in all the arteries examined; difference in size, and distance from the heart, being unattended by any corresponding difference of force in the circulation. The height of the column of mercury displaced by the blood was the same in all the arteries of the same animal. From the mean result of several observations on horses and dogs, he calculated that the force with which the blood is moved in any large artery is capable of supporting a column of mercury six inches and one and a half lines in height, or a column of water seven feet one line in height. With these results the more recent observa- tions of Valentin (iv. p. 441), Spengler (lxxx. 1844), and Lud- wig (lxxx. 1847, p. 242), closely accord. Poiseuille's experiments having thus shown to him that the force of the blood's motion is the same in the most different arteries, he concluded that to mea- sure the amount of the blood's pressure in any artery of which the caliber is known, it is necessary merely to multiply the area of a transverse section of the vessel by the height of the column of mercury which is already known to be supported by the force of the blood in any part of the arterial system. The weight of a column of mercury of the dimensions thus found will represent the pressure exerted by the column of blood. And assuming that the mean of the greatest and least height of the column of mercury found by experiments on different animals to be supported by the force of the blood in them, is equivalent to the height of the column which the force of the blood in the human aorta would support, he calculated that about 4 lbs. 4 oz. avoirdupois will in- dicate the static force with which the blood is impelled into the human aorta. By the same calculation, he estimates the force of the circulation in the aorta of the mare to be about 11 lbs. 9 oz. avoirdupois: and that in the radial artery at the human wrist only 4 drs. We have already seen that the muscular force of the right ventricle is equal to only half that of the left, consequently, if Poiseuille's estimate of the latter is correct, the force with which the blood is propelled into the lungs will only be equal to 2 lbs. 2 oz. avoirdupois. The amounts above stated indicate the pressure exerted by the blood at the several parts of the arterial system at the time of the ventricular contraction. During the dilatation this pressure is somewhat diminished. Hales observed that the column of blood in the tube inserted into an artery falls an inch or rather more after each pulse; Ludwig (lxxx. 1847, p. 242) has observed the same, CIRCULATION. 103 and recorded it more minutely. The pressure is also influenced by the various circumstances which affect the action of the heart; the diminution or increase of the pressure being proportioned to the weaker or stronger action of this organ. Valentin observed that, by increasing the amount of blood by the injection of a fresh quantity into it, the pressure in the vessels was also increased, while a contrary effect ensued on diminishing the quantity of blood. Poiseuille, Ludwig, and others have confirmed what Haller and Magendie observed, namely, that the strength of the blood's im- pulse in the arteries is increased during expiration ; in which act the chest is contracted, and the large vessels in consequence com- pressed. The column of mercury in the haemadynanionieter rises somewhat at each expiration, and falls during inspiration. The extent of the rise and fall of the mercury observed by Poiseuille was the same in arteries the distance of which from the heart was different; and in ordinary tranquil respiration amounted to from four to ten lines. The decrease of the arterial blood's impulse in inspiration is in some persons so great, that the pulse at the radial artery becomes imperceptible when inspiration is long continued and the breath held ; its increase in expiration is well shown in the increased pain and throbbing of an inflamed part during coughing, and in the frequency of rupture in diseased arteries during violent expiratory efforts. These things will be again referred to in speak- ing of the movement of the venous blood. THE CAPILLARIES. In all organic textures the transmission of the blood from the miuute branches of the arteries to the minute veins is effected through a network of microscopic vessels, in the meshes of which the proper substance of the tissue lies. (Fig. 23.) This may be seen in all minutely injected preparations; and, during life, by the aid of the microscope, in any transparent parts, such as the web of the frog's foot, the lungs, tongue, and urinary bladder of the frog, the tail or external branchiae of the tadpole, the incubated egg, young fishes, the wings of the but, and the mesentery of all Vertc- brata, and even in some opaque textures of the larva of the sala- mander by means of a simple microscope. The ramifications of the minute arteries form repeated anastomoses with each other, and give off the capillaries, which, by their anastomoses, compose a con- tinuous and uniform network, from which the venous radicles, on the other hand, take their rise. The reticulated vessels, connecting the arteries and veins, are called capillary, on account of their minute size; and intermediate vessels, on account of their position. The point at which the arteries terminate and the minute veins 104 CIRCULATION. commence cannot be exactly defined, for the transition is gradual; but the intermediate network has, nevertheless, this peculiarity, that the small vessels which compose it maintain the same diameter throughout; they do not di- minish in diameter in one direction, like arteries and veins; and the meshes of the network that they compose are more uniform in shape and size than are those formed by the anastomoses of the minute arteries and veins. The diameter of the capillary vessels varies somewhat in the different tex- tures of the body, the most common size being about l-3000th of an inch. Among the smallest may be mentioned those of the brain, and of the mucous membrane of the intestines; among the largest, those of the skin, and especially of the medulla of bones. The form of the capillary network presents considerable variety in the dif- ferent textures of the body; the varie- ties consisting principally of modifica- tions of two chief kinds of mesh, the rounded and the elongated. That kind in which the meshes or interspaces have a roundish form is the most common, and prevails in those parts in which the capillary network is most dense, such as the lungs, most glands, and mucous membranes, and the cutis. (Fig. 24.) The Fig- 24. Fig. 25. Distribution of capillaries around fol- Capillary network of nervous centres. licles of mucous membrane. meshes of this kind of network are not quite circular, but more or less angular, sometimes presenting a nearly regular quadrangular Fig. 23. Bloodvessels of an intestinal vil- lus, representing the arrangement of capillaries between the ultimate venous and arterial branches; 1,1. the arteries; 2, the vein. CIRCULATION. 105 or polygonal form, but being more frequently irregular. _ The capil- lary network with elongated meshes is observed in parts in which the vessels are arranged among bundles of fine tubes or fibres, as in muscles and nerves. (Fig. 25.) In such parts the meshes usually have the form of a parallelogram, the short sides of which may be from three to eight or ten times less than the long ones; the long sides always corresponding to the axis of the fibre or tube, by which it is placed. The appearance both of the rounded and elongated mesh is much varied according as the vessels com- posing it have a straight or tortuous form. Sometimes the capillaries have a looped arrangement, a single ca- pillary projecting from the common network into some prominent organ, and returning after forming one or more loops, as in the papillae of the tongue and skin. (Fig. 26.) What- ever be the form of the capillary net- work in any tissue or organ, it is, as a rule, found to prevail in the COr- Capnlary nctworkof fungiform pa- ' f ., . . pilla of the tongue. responding parts of all animals. The number of the capillaries and the size of the meshes in differ- ent parts determine in general the degree of vascularity^ of those parts. The parts in which the network of capillaries is closest, that is, in which the meshes or interspaces are the smallest, are the lungs and the choroid membrane of the eye. In the iris and ciliary body, the interspaces are somewhat wider, yet very small. In the human liver, the interspaces are of the same size, or even smaller than the capillary vessels themselves. In the human lung they are smaller than the vessels. In the human kidney, and in the kidney of the dog, the diameter of the injected capillaries, compared with that of the interspaces, is in the proportion of one to four, or of one to three. The brain receives a very large quan- tity of blood; but the capillaries in which the blood is distributed through its substance are very minute, and less numerous than in some other parts; their diameter, according to E. H. Weber, com- pared with the long diameter of the meshes, being in the propor- tion of one to eight or ten ; compared with the transverse diameter, in the proportion of one to four or six. In the mucous mem- branes—for example, in the conjunctiva—and in the cutis vera, the capillary vessels are much larger than in the brain, and the interspaces narrower—namely, not more than three or four times wider than the vessels. In the periosteum, the meshes are much larger. In the cellular coat of arteries, the width of the meshes is ten times that of the vessels (Henle). It may be held as a general rule that the more active the func- 106 CIRCULATION. tions of an organ are the more vascular it is, that is, the closer is its capillary network and the larger its supply of blood. Hence the narrowness of the interspaces in all glandular organs, in mu- cous membranes, and in growing parts; their much greater width in bones, ligaments, and other very tough and comparatively inac- tive tissues; and the complete absence of vessels in cartilages, the dense tendons of adults, and such parts, in which, probably, very little organic change occurs after they are once formed. But the general rule must be modified by the consideration that some organs, such as the brain, though they have small and not closely arranged capillaries, may receive large supplies of blood by reason of its more rapid movement. When an organ has large arterial trunks and a comparatively small supply of capillaries, the move- ment of the blood through it will be so quick, that it may in a given time receive as much fresh blood as a more vascular part with smaller trunks, though at any given instant the less vascular part will have in it a smaller quantity of blood. Microscopic observations and minute injections have shown that the capillary vessels are merely the fine tubes which form the me- dium of transition from arteries to veins, and that no other kind of vessel arises from them; that the minute arteries have no other mode of termination than the communication with the veins by means of the capillaries; in a word, that there are no exhalant or other vessels terminating by open extremities. The hypothesis of the existence of such vessels is alike unnecessary to the explanation of secretion, growth, nutrition, and all the other functions of a part. All that these functions require of the vessels is, that, for each tissue, the blood should be brought so near to the active ele- ments of the tissue, that some of its fluid part may be absorbed or imbibed by them. And the distance through which such imbibition is effected is not always small; in the brain, for example, those portions of its substance which lie in the middle of the wide meshes of the capillary network must be nourished by imbibing fluid through the spaces between them and the nearest vessels; so also in bone, where the vessels are yet wider apart. But the instances in which such imbibition is effected through the greatest distance are in what are called non-vascular tissues, which receive no bloodvessels into their own substance, and are nourished by the fluid absorbed from the vessels of the adjacent vascular parts; as the cornea from the vessels of the conjunctiva, the lens from those of the posterior layer of its capsule, and the articular cartilage from the vessels of the subjacent bone. The difference of nutrition in vascular and non-vascular tissues is only one of degree; in all parts alike the elementary structures are outside the vessels, and obtain new materials from the blood by imbibition; but the imbi- CIRCULATION. 107 bition has to be accomplished through a greater distance in the less than in the more vascular parts. The structure of the capillaries offers little hindrance to such imbibition. Their walls are composed of exceedingly fine, trans- parent, and apparently homogeneous membrane, in which are im- bedded, here and there, minute oval corpuscles, probably the per- sistent nuclei of the cells from which the vessels were originally formed. Only in the largest capillaries are there traces of an epi- thelial lining like that of the arteries, or of filaments, like rudi- ments of a circularly fibrous coat. In the Circulation in the Capillaries, as seen in any transparent part of a living adult animal by means of the microscope (Fig. 27), the blood flows with a constant equable motion. In very young animals the motion, though continuous, is accelerated at intervals, corresponding to the pulse in the larger arteries, and a similar mo- tion of the blood is also seen in the capillaries of adult animals when they are feeble : if their exhaustion is so great that the power of the heart is still more diminished, the red corpuscles are observed to have merely the periodic motion, and to remain stationary in the intervals ; while, if the debility of the animal is extreme, they even recede somewhat after each impulse, apparently because of the elasticity of the capillaries and the tissues round them. These observations may be added to those already (p. 89) advanced, to prove that, even in the state of great debility, the action of the heart is sufficient to impel the blood through the capillary ves- sels. Moreover, Dr. Marshall Hall (xciv. 1S43) having placed the pectoral fin of an eel in the field of the microscope, and com- pressed it by the weight of a heavy probe, observed that the movement of the blood in the capillaries be- came obviously pulsatory, the pul- sations being synchronous with the contractions of the ventricle. This power of the heart to propel the blood through a second set of capil- laries, affords an explanation of the foetal circulation, and of the more difficult problem, the circulation through the acardiac foetus. The pulsatory motion of the blood in the capillaries cannot be attributed to an action in these vessels; Fig. 2< Capillaries in the web of the frog'a foot magnified. 108 CIRCULATION. for, when the animal is tranquil, they present not the slightest change in their diameter. It is in the capillaries that the chief resistance is offered to the progress of the blood; for in them the friction of the blood, is greatly increased by the enormous multiplication of the surfaces with which it is brought in contact. The velocity of the blood is, also, in them reduced to its minimum, because of the widening of the stream. If, as Professor Muller says, the sectional area of all the branches of a vessel united were always the same as that of the vessel from which they arise, and if the aggregate sectional area of the capillary vessels were equal to that of the aorta, the mean rapidity of the blood's motion in the capillaries would be the same as in the aorta and largest arteries; and, if a similar corre- spondence of capacity existed in the veins and arteries, there would be an equal correspondence in the rapidity of the circulation in them. It is quite true that the force with which the blood is pro- pelled in the arteries, as shown by the quantity of blood which es- capes from them in a certain space of time, is much greater than that with which it moves in the veins; but this force has to over- come all the resistance offered in the arterial and capillary system— the heart itself, indeed, must overcome this resistance; so that the excess of the force of the blood's motion in the arteries is expended in overcoming this resistance, and the rapidity of the circulation in the arteries, even from the commencement of the aorta, would be the same as in the veins and capillaries, if the aggregate capacity of each of the three systems of vessels were the same. But since the aggregate sectional area of the branches is greater than that of the trunk from which they arise, the rapidity of the blood's motion will necessarily be greater in the trunk, and will diminish in proportion as the aggregate capacity of the vessels in- creases during their ramification : in the same manner as other things being equal, the velocity of a stream diminishes' as it widens. The observations of Hales (lxvii. vol. ii.), E. II. Weber (lxxx. 1838, p. 450) and Valentin (iv. vol. i. p. 468) agree very closely as to the rate of the blood in the capillaries of the frog : and the mean of their estimates gives the velocity of the systemic capillary circulation at about one inch per minute. Through the pulmonic capillaries, the rate of motion according to Hales, is about five times that through the systemic ones. The velocity in the ca- pillaries of warm-blooded animals is greater, but has not yet been accurately estimated. If it be assumed to be three times as great as in the frog, still the estimate may seem too low, and inconsistent with the facts which show that the whole circulation is accom- plished in about a minute. But the whole length of capillary ves- sels, through which any given portion of blood has to pass pro- CIRCULATION. 109 bably does not exceed l-30th of an inch; and, therefore, the time required for each quantity of blood to traverse its own appointed portion of the general capillary system will not be more than two seconds: while in the pulmonic capillary system the length of time required will be even much less than this. The estimates given above are drawn from observations of the movements of the red blood-corpuscles, which move in the centre of the stream. At the circumference of the stream, in contact with the walls of the vessels, and adhering to them, there is a layer of liquor sanguinis, which appears to be motionless. The existence of this still layer, as it is termed, is inferred both from the general fact that such an one exists in all fine tubes traversed by fluid, and from what can be seen in watching the movements of the blood-corpuscles. The red corpuscles occupy the middle of the stream, and move with comparative rapidity; the colorless lymph-corpuscles run much more slowly by the walls of the vessels; while next to the wall there is often a transparent space in which the fluid appears to be at rest; for if any of the corpuscles happen to be forced within it, they move more slowly than before, rolling lazily along the side of the vessel, and often adhering to its wall. Part of this slow movement of the pale corpuscles, and their occa- sional stoppage may be due, as E. H. Weber has suggested, to their having a natural tendency to adhere to the walls of the ves- sels. Sometimes, indeed, when the motion of the blood is not strong, many of the white corpuscles collect in a capillary vessel, and for a time entirely prevent the passage of the red corpuscles. But there is no doubt such a still layer of liquor sanguinis exists next the walls of the vessels, and it is between it and the tissues around the vessels that those interchanges of particles take place which ensue in nutrition, secretion, and absorption by the blood- vessels ; interchanges which are probably facilitated by the tran- quillity of the fluids between which they are effected. There is no reason for supposing that either the pale or red cor- puscles ever remain permanently fixed to the wall of the vessels, and become united with them, or that they pass through the walls to enter into the structure of the tissues. Their office appears to be fulfilled exclusively within the vessels. The reasons have been already stated for holding that the power of the heart is sufficient to propel the blood through the capillary system (see p. 89). There is, therefore, no need of an hypothe- sis of any action of the capillaries for the regular propulsion of the blood through them; nor is it probable that they have such an office. There is some evidence for supposing that, under certain circumstances, the capillaries can exercise an influence on their contents; but, as it will appear, the phenomena on which this sun- position is founded, are as well explained by referring them to the 110 CIRCULATION. action of the small arteries, or to the relation which exists between the tissues outside, and the blood within, the capillaries. Thus, the capillaries contract on the application of cold : but this may be due not to any contraction similar to that of muscular tissue, but to the elasticity of the capillaries, and the surrounding tissues which close in, when, by the contraction of the small arteries (which as already stated, can be made to contract by cold), the flow of blood into the capillaries is diminished. The apparent contraction of the capillaries on the application of certain irritating substances, and during fear, and their dilatation in blushing, may be similarly referred to either their own action, or that of the small arteries. When the access of oxygen to the lungs is prevented, the circu- lation through the pulmonic capillaries is gradually retarded, the blood-corpuscles cluster together, and their movement is eventually almost arrested, even while the action of the heart continues. In inflammation, also, the capillaries of an inflamed part are enlarged and distended with blood, which either moves very slowly, or is completely at rest. In both these cases the change may be due to some influence exercised by the capillaries on the blood; but, in both also, it may be that an alteration in the blood increases the adhesion of its particles to one another, and to the walls of the capillaries, to an amount which the propelling action of the heart is not able to overcome. The temporary increase in the size of the capillaries, and in the quantity of blood moving through them in any part, during an un- usually active discharge of its functions, has been cited as evidence of their exercising some power to determine the amount of blood that shall traverse them. Instances of such enlargement are seen in the turgescence of an actively secreting or quickly growing part. But the control here displayed may be exercised, not by the capil- laries, but by that relation, whatever be its nature, which exists between every tissue and the blood, and by which the condition of the tissue determines the quantity of blood to be supplied to it; as in the rudimcntal state, the condition of each organ or tissue de- termines the first formation and supply of blood to it. It thus appears that the capillaries are capable of contraction and dilatation, but we cannot say whether these changes of size are effected by their proper and primary agency, or are the results of their being merely extensile and elastic, and thereby capable of adapting themselves to changes in the quantity of blood impelled into them. We seem to have at present no means of determining this question. The Veins. In structure, the coats of veins bear a general resemblance to those of arteries. They possess, however, no complete elastic coat; what CIRCULATION. Ill elastic tissue they have is interwoven in their fibro-cellular tissue, which, being itself also extensile and elastic, enables them to re- cover from the temporary extensions to which they are liable. That part of their walls, also, which corresponds with the muscular coat of the arteries is composed of fibres resembling those of fibro- cellular tissue, rather than of organic muscle. It is probable, however, that this fibro-cellular tissue is contractile like that of the skin, and that through it the veins possess some power of independ- ently contracting on their contents. (See cxc. July, 1850, p. 241.) To the great trunks of the veins, where they are near the auricles, more of this power is given by a circular layer of striated muscular fibres like those of the auricles, which takes the place of the con- tractile fibro-cellular tissue, and the action of which has been already referred to (p. 73). But, in the rest of the veins, the contractile power is probably weak and of slow action; sufficient, however, to enable them to adapt themselves to the size required when they receive less than their average supply of blood through the arteries; while to the usual, or more than the usual, supply they are adapted by the extensile and elastic property of all their coats. The chief influence that the veins have in the circulation is effected with the help of the valves, which are placed in all veins that are subject to local pressure from the muscles between or near which they run. The general construction of these valves is similar to that of the semilunar valves of the aorta and pulmonary artery, already described (p. 75); but their free margins are turned in the opposite direction, i. e., towards the heart, so as to stop any move- ment of blood backwards in the veins. They are commonly placed in pairs, at various distances in different veins, but almost uniformly in each. In the smaller veins, single valves are often met with : and three or four are sometimes placed together, or near one an- other, in the largest veins, such as the subclavian, and at their junction with the jugular veins. The valves are semilunar : the unattached edge being in some examples concave, in others straight. They are composed of inextensile fibrous tissue, and are covered with epithelium like that lining the veins. During the period of their inaction, when the venous blood is flowing in its proper direc- tion, they lie by the sides of the veins; but when in action, they close together like the valves of the arterieSj and offer a complete barrier to any backward movement of the blood. The principal obstacle to the circulation is already overcome when the blood has traversed the capillaries; and the force of the heart which is not yet consumed, is sufficient to complete its pas- sage through the veins, in which the obstructions to its movement are very slight. For the formidable obstacle supposed to be pre- sented by the gravitation of the blood has no real existence, since the pressure exercised by the column of blood in the arteries will 112 CIRCULATION. be always sufficient to support a column of venous blood of the same height as itself: the two columns mutually balancing each other. Indeed, so long as both arteries and veins contain continu- ous columns of blood, the force of gravitation, whatever be the po- sition of the body, can have no power to move or resist the motion of any part of the blood in any direction: as if one had a circular tube full of fluid at every part, the fluid might be made to circu- late with equal facility in either direction, or in any position of the tube. The lowest bloodvessels have, of course, to bear the greatest amount of pressure; the pressure on each part being directly pro- portionate to the height of the column of blood above it: hence their liability to distension. But this pressure bears equally on both arteries and veins, and cannot either move, or resist the mo- tion of, the fluid they contain, so long as the columns of fluid are in both of equal height, and continuous. Their condition may in this respect be compared with that of a double bent tube, full of fluid, held vertically; whatever be the height and gravitation of the columns of fluid, neither of them can move of its own weight, each being supported by the other; yet the least pressure on the top of either column will lift up the other; so, when the body is erect, the least pressure on the column of arterial blood may lift up the venous blood, and, were it not for the valves, the least pressure on the venous might lift up the arterial column. In experiments to determine what proportion of the force of the left ventricle remains to propel the blood in the veins, Valentin found that the pressure of the blood in the jugular vein of a dog, as estimated by the haemadynamometer, did not amount to more than 1-llth or l-12th of that in the carotid artery of the same animal; and this estimate is confirmed, in the instances of several other arteries and their corresponding veins, by Mogk (xxxiii. 1845, p. 33). In the upper part of the inferior vena cava, Valentin could scarcely detect the existence of any pressure, nearly the whole force received from the heart having been, apparently, consumed during the passage of the blood through the capillaries (iv. p. 477). But, slight as this remanent force might be (and the experiment in which it was estimated would reduce the force of the heart below its natural standard), it would be enough to complete the circula- tion of the blood; for, as already stated (p. 86), the spontaneous dilatation of the auricles and ventricles, though it may not be forci- ble enough to assist the movement of blood into them, is adapted to offer to that movement no obstacle. Some assistance is given to the venous circulation by the respi- ratory movements of the chest; and some occasionally, but very effectually and timely, by the actions of the muscles, capable of pressing on such veins as have valves. The effect of muscular pressure on such veins may be thus ex- CIRCULATION. 113 plained. When pressure is applied to any part of a vein, and the current of blood in it is obstructed, the portion behind the seat of pressure becomes swollen, and distended as far back as to the next pair of valves. These, acting like the arterial valves, and being like them inextensile, both in themselves and at their margins of attachment, do not follow the vein in its distension, but are drawn out towards the axis of the canal. Then, if the pressure continues on the vein, the compressed blood, tending to move equally in all directions, presses the valves down into contact at their free edges, and they close the vein and prevent regurgitation of the blood. Thus, whatever force is exercised by the pressure of the muscles on the veins is distributed partly in pressing the blood onwards in the proper course of the circulation, and partly in pressing it back- wards and closing the valves behind. The circulation might lose as much as it gains by such compres- sion of the veins, if it were not for the numerous anastomoses by which they communicate with one another; for through these, the closing up of the venous channel by the backward pressure is pre- vented from being any serious hindrance to the circulation, since the blood, of which the onward course is arrested by the closed valves, can at once pass through some anastomosing channel, and proceed on its way by another vein. Thus, therefore, the effect of muscular pressure upon veins that have valves is turned almost en- tirely to the advantage of the circulation; the pressure of the blood onwards is all advantageous, and the pressure of the blood backwards is prevented from being a hindrance by the closure of the valves and the anastomoses of the veins. The effects of such muscular pressure are well shown by the ac- celeration of the stream of blood, when, in venesection, the muscles of the forearm are put in action, and by the general acceleration of the circulation during active exercise; and the numerous move- ments which are continually taking place in the body while awake, though their single effects may be less striking, must be an import- ant auxiliary to the venous circulation. Yet they are not essential; for the venous circulation continues unimpaired in parts at rest, in paralyzed limbs, and in parts in which the veins are not subject to any muscular pressure. The respiratory movements of the chest also assist the circulation of the blood in the systemic veins : at least, the more forcible respiratory movements do; the ordinary ones are too weak to pro- duce any considerable effect. The effect of expiration in increasing the pressure of the blood in the arteries, has been already men- tioned (page 103), and is minutely illustrated by the experiments of Luclwig (lxxx. 1847, p. 242). It acts as the pressure of con- tracting muscles does upon the veins, and is advantageous to the movement of arterial blood, while the aortic valves are closed, be- 114 CIRCULATION. cause, during this time, the backward pressure cannot wholly neu- tralize the benefit of the pressure forwards. The increased pres- sure on the blood in the arteries, during expiration, is also propa- gated through the capillaries to some of the veins; for Magendie has shown that the stream of venous blood from the lower end of a divided vein becomes stronger during each expiration. But on the whole, little advantage is derived to the circulation from the movements of expiration, since the same pressure which drives on the arterial blood with increased force must, in some de- gree, retard the blood in the veins, and obstruct its passage to the heart. The effect of such retardations is shown in the swelling up of the veins of the head and neck, and the lividity of the face, during coughing, straining, and similar violent expiratory efforts. The effects shown in these instances are due, both to some regurgi- tation of blood in the great veins, and to the accumulation of blood in the veins of the head and face, which are constantly more and more filled by the influx from the arteries, and are not able to empty themselves into the vena cava superior. The regurgitation, however, is stopped or much diminished by the valves at the junc- tion of the jugular and subclavian veins, by which, also, the disad- vantageous effects of the forced expirations are limited. The act of inspiration is favorable to the venous circulation, and its effect is not quite counterbalanced by its tending to draw the arterial as well as the venous blood towards the cavity of the chest. When the chest is enlarged in inspiration, the additional space within it is filled, chiefly by the fresh quantity of air which passes through the trachea and bronchial passages to the vesicular structure of the lungs. But the blood, being like the air, subject to the at- mospheric pressure, some of it, also, is at the same time pressed towards the expanding cavity of the chest, and therein towards the heart. The effect of this in the arterial current is hindered by the aortic valves, while they are closed; and is less than it is on the venous current, in the same proportion as the orifice of the aorta is less than the united orifices of the two venae cavse. Sir David Barry was the first who showed plainly this effect of inspiration on the venous circulation, and mentions the following experiment in proof of it. He introduced one end of a bent glass tube into the jugular vein of an animal, the vein being tied above the point where the tube was inserted; the inferior end of the tube was immersed in some colored fluid. He then observed that at the time of eadh inspiration the fluid ascended in the tube, while, during expiration, it either remained stationary, or even sank. Poiseuille confirmed the truth of this observation in a more accu- rate manner, by means of his hgemadynainometer. And a like confirmation has been more recently furnished by Valentin (iv. p. 478), and in minute details by Ludwig (lxxx. 1847,- p. 242). CIRCULATION. 115 The effect of inspiration on the veins is observable only in the large ones near the thorax. Poiseuille could not detect it by means of his instrument in veins more distant from the heart—for exam- ple, in the veins of the extremities. And its beneficial effect would be neutralized were it not for the valves ; for he found, when he re- peated Sir D. Barry's experiment, and passed the tube so far along the veins that it went beyond the valves nearest to the heart, as much fluid was forced back into the tube in every expiration as was drawn in through it in every inspiration. On the whole, therefore, the respiratory movements of the chest are advantageous to the systemic circulation; on the pulmonary circulation they appear to produce no effect. The additional force which expiration gives to the arterial current is not counterbalanced by the retardation of the venous current, because the valves of the veins closing, limit the regurgitation from the chest; and though the blood behind or above these valves is retarded while they are close, yet it goes on with accumulated force as soon as they are open again. On the other hand, the retardation of the arterial blood by the acts of inspiration is less than the acceleration of the venous blood, because the orifice of the aorta is less than that of the venae cavae. The disturbance which hurried respiration and struggling pro- duce in the movement of the venous blood, makes it exceedingly difficult to determine the average force or velocity of the venous stream. The velocity of the blood is greater in the veins than in the capillaries, but less than in the arteries; and to this are adapted the relative capacities of the arterial and venous systems; for since the veins must return to the heart all the blood that they receive from it, in a given time, through the arteries, their larger size and proportionally greater number must compensate for the slower movement of the blood through them. If an accurate esti- mate of the proportionate areas of arteries and the veins corre- sponding to them could be made, we might, from the velocity of the arterial current, calculate that of the venous. Perhaps a fair approximation to such an estimate is, that the capacity of the veins is about three times as great as that of the arterial system; and that the velocity of the blood's motion is about one-third less in the former than in the latter. And this is not a slow move- ment ; for, if we stop the circulation at the beginning of any super- ficial vein, and empty the upper part of the vein, immediately upon removing the finger the blood will move along the vein faster than the eye can follow it. The rate at which the blood moves in the veins gradually increases the nearer it approaches the heart, for the sectional area of the venous trunks, compared with that of the branches opening into them, becomes gradually less as the trunks advance towards the heart. 116 CIRCULATION. Having now considered the share which each of the circulatory organs has in the propulsion and direction of the blood, we may speak of their combined effects, especially in regard to the velocity with which the movement of the blood through the whole round of the circulation is accomplished. As Midler says, the rate of the blood's motion in the vessels must not be judged of by the rapidity with which it flows from a vessel when divided. In the latter case, the rate of motion is the result of the entire pressure to which the whole mass of blood is subjected in the vascular system, and which at the point of the incision in the vessel meets with no re- sistance. In the closed vessels, on the contrary, no portion of blood can be moved forwards but by impelling on the whole mass, and by overcoming the resistance arising from friction in the smaller vessels. From the rate at which the blood escapes from opened vessels, we could only judge, in general, that its velocity is, as already said, greater in arteries than in veins, and in both than in the capillaries. More satisfactory data for the estimates are afforded by the results of experiments to ascertain the rapidity with which poisons introduced into the blood are transmitted from one part of the vascular system to another. From eighteen such experiments on horses, Hering deduced that the time required for the passage of a solution of ferrocyanide of potassium, mixed with the blood, from one jugular vein (through the right side of the heart, the pul- monary circulation, the left cavities of the heart, and the general circulation) to the jugular vein of the opposite side, varies from twenty to thirty seconds. The same substance was transmitted from the jugular vein to the great saphena in twenty seconds; from the jugular yein to the masseteric artery in between fifteen and thirty seconds, to the facial artery in one experiment in be- tween ten and fifteen seconds, in another experiment in between twenty and twenty-five seconds: in its transit from the jugular vein to the metatarsal artery it occupied between twenty and thirty seconds, and in one instance more than forty seconds. The re- sult was nearly the same whatever was the rate of the heart's action. Poiseuille's observations (xxxi. 1843) accord completely with the above; and show, moreover, that when the ferrocyanide is in- jected into the blood with other substances, such as acetate of ammonia, or nitrate of potash (solutions of which, as other experi- ments have shown, pass quickly through capillary tubes), the pas- sage from one jugular vein to the other is effected in from eighteen to twenty-four seconds; while, if instead of these, alcohol is added, the passage is not completed until from forty to forty-five seconds after injection. Still greater rapidity of transit has been observed by Mr. J. Blake (xciv. Oct. 1841), who found that nitrate of ba- CIRCULATION. 117 ryta injected into the jugular vein of a horse could be detected in blood drawn from the carotid artery of the opposite side in from fifteen to twenty seconds after the injection. In sixteen seconds a solution of nitrate of potash, injected into the jugular vein of a horse, caused complete arrest of the heart's action, by entering and diffusing itself through the coronary arteries. In a dog, the poi- sonous effects of strychnia on the nervous system were manifested in twelve seconds after injection into the.jugular vein; in a fowl, in six and a half seconds, and in a rabbit, in four and a half seconds. In all these experiments, it is assumed that the substance in- jected moves with the blood, and at the same rate as it, and does not move from one part of the organs of circulation to another by diffusing itself through the blood or tissues more quickly than the blood moves. The assumption is so probable that it may be con- sidered nearly certain that the times above mentioned, as occupied in the passage of the injected substances, are those in which the portion of blood into which each was injected was carried from one part to another of the vascular system. It would, therefore, ap- pear that a portion of blood cfn traverse the entire course of the circulation, in the horse, in half a minute; of course, it would re- quire longer to traverse the vessels of the most distant part of the extremities than to go through those of the neck; but taking an average length of vessels to be traversed, and assuming, as we may, that the movement of blood in the human subject is not slower than in the horse, it may be concluded that one minute, which is the estimate usually adopted of the average time in which the blood completes its entire circuit in man, is rather above than below the actual rate. Another mode of estimating the general velocity of the circulat- ing blood is by calculating it from the quantity of blood supposed to be contained in the body, and from the quantity which can pass through the heart in each of its actions. But the conclusions arrived at by this method are less satisfactory. For the estimates both of the total quantity of blood, and of the capacity of the cavi- ties of the heart, are as yet only approximated to the truth. Still, the most recent and careful of the estimates thus made accord with those already mentioned; for Valentin has, from these data, calcu- lated that the blood may all pass through the heart in from 43 !J- to 62 § seconds. The estimate from the speed at which the blood may be seen moving in transparent parts is not opposed to this. For, as already stated (p. 108), though the movement through the capil- laries may be very slow, yet the length of capillary vessel through which any portion of blood has to pass is very small. If we esti- mate that length at the tenth of an inch, and suppose the velocity of the blood therein to be only one inch per minute, then each por- 118 CIRCULATION. tion of blood may traverse its own distance of the capillary system in about six seconds. There would, thus, be plenty of time left for the blood to travel through its circuit in the larger vessels, in which the greatest length of tube that it can have to traverse in the human subject does not exceed ten feet. All the estimates here given are averages; but of course the time in which a given portion of blood passes from one side of the heart to the other varies much, according to the organ it has to traverse. The blood which circulates from the left ventricle, through the coronary vessels, to the right side of the heart, re- quires a far shorter time for the completion of its course than the blood which flows from the left side of the heart to the feet, and back again to the right side of the heart; for the circulation from the left to the right cavities of the heart may be represented as forming a number of arches, varying in size, and requiring pro- portionately various times for the blood to traverse them; the smallest of these arches being formed by the circulation through the coronary vessels of the heart itself. The course of the blood from the right side of the heart, through the lungs, to the left, is shorter than most of the arches described by the systemic circula- tion, and in it the blood flows, caeteris paribus, much quicker than in most of the vessels which belong to the aortic circulation. For although the quantity of blood contained, at any instant, in the greater circulation of the body is far greater than the quantity within the lesser circulation, yet, in any given space of time, as much blood must pass through the lungs as passes in the same time through the systemic circulation. If the systemic vessels contain five times as much blood as the pulmonary, the blood in them must move five times as slow as in these; else, the right side of the heart would be either overfilled or not filled enough. Peculiarities of the Circulation in different Parts. The most remarkable peculiarities attending the circulation of blood through different organs are observed in the cases of the lungs, the liver, the brain, and the erectile organs. The pulmonary and portal circulations have been already alluded to (p. 72), and will be again noticed, when considering the functions of the lungs and liver. The chief circumstances requiring notice in relation to the cere- bral circulation, are observed in the arrangement and distribution of the vessels of the brain, and in the conditions attending the amount of blood usually contained within the cranium. The functions of the brain seem to require that it should receive a large supply of blood. This is accomplished through the number and size of its arteries, the two internal carotids, and the two verte- brals. But it appears to be further necessary that the force with CIRCULATION. 119 which this blood is sent to the brain should be less, or, at least, subject to less variation from external circumstances, than it is in other parts. This object is effected by several provisions; such as the tortuosity of the large arteries, and their wide anastomoses in the formation of the circle of Willis, which will insure that the supply of blood to the brain may be uniform, though it may by an accident be diminished, or in some way changed, through one or more of the principal arteries. The transit of the large arteries through bone, especially the carotid canal of the temporal bone, may prevent any undue distension; and uniformity of supply is, further, insured by the arrangement of the vessels in the pia mater, in which, previous to their distribution to the substance of the brain, the large arteries branch and divide into innumerable minute ra- muscles and capillaries, which, after frequent communications with one another, enter the brain, and carry into nearly every part of it uniform and equable streams of blood. The arrangement of the veins within the cranium is also peculiar. The large venous trunks or sinuses are formed so as to be scarcely capable of change of size; and composed as they are of the tough tissue of the dura mater, and, in some instances, bounded on one side by the bony cranium, they are not compressible by any force which the fulness of the arteries might exercise through the sub- stance of the brain; nor do they admit of distension when the flow of venous blood from the brain is obstructed. The general uniformity in the supply of blood to the brain, which is thus secured, is well adapted, not only to its functions, but also to its condition as a mass of nearly incompressible substance placed in a cavity with unyielding walls. These conditions of the brain and skull have appeared, indeed, to some, enough to justify the opinion that the quantity of blood in the brain must be at all times the same ; and that the quantity of blood received within any given time through the arteries must be always, and at the same time, exactly equal to that removed by the veins. In accordance with this supposition, the symptoms commonly referred to either excess or deficiency of blood in the brain, were ascribed to a disturbance in the balance between the quantity of arterial and that of venous blood. Some experiments performed by Dr. Kellie appeared to establish the correctness of this view. He believed that in animals bled to death, while all the other organs of the body were nearly emptied of blood, the vessels of the brain contained almost their ordi- nary quantity; but that if, previous to bleeding an animal, he made a hole in its cranium, and thus exposed the brain, equally with the other organs, to the influence of atmospheric pressure, its vessels, like those of other parts of the body, were emptied as the animal bled to death. But Dr. Burrows (lxxi. May, 1843, and xcv.) having repeated these experiments, and performed additional ones, 120 CIRCULATION. has obtained different results. He found that in animals bled to death, without any aperture being made in the cranium, the brain became pale and anaemic like other parts. And, in proof that, dur- ing life, the cerebral circulation is influenced by the same general circumstances that influence the circulation elsewhere, he found congestion of the cerebral vessels in rabbits killed by strangling or drowning; while in others, killed by prussic acid, he observed that the quantity of blood in the cavity of the cranium was determined by the position in which the animal was placed after death, the cerebral vessels being congested when the animal was suspended with its head downwards, and comparatively empty when the ani- mal was kept suspended by the ears. He concluded, therefore, that although the total volume of the contents of the cranium is probably nearly always the same, yet the quantity of blood in it is liable to variation, its increase or diminution being accompanied by a simul- taneous diminution or increase in the quantity of the cerebro-spinal fluid, which, by readily admitting of being removed from one part of the brain and spinal cord to another, and of being rapidly ab- sorbed, and as readily effused, would serve as a kind of supplemental fluid to the other contents of the cranium, to keep it uniformly filled incase of variations in their quantity. (See also Eekr. cxxix.) But such variations occur only in a normal condition ; in ordinary states, and in health, it is probable that the arrangements of the vessels, to which we have referred, insure to the brain a supply of blood which is both uniform and guarded from the accidental disturbances to which the supply for all other organs is liable even in circum- stances consistent with health. Erectile structure.—The instances of greatest variation in the quantity of blood contained, at different times, in the same organs, are found in certain structures which, under ordinary circumstances, are soft and flaccid, but, at certain times, receive an unusually large quantity of blood, become distended and swollen by it, and pus3 into the state which has been termed erection. Such structures arc the corpora cavernosa and corpus spongiosum of the penis in the male, and the clitoris in the female; and, in a less degree, the nipple of the mammary gland in both sexes. The corpus cavernosum penis, which is the best example of an erectile structure, has an external fibrous membrane, or sheath, from the inner surface of which numerous fine lamellae pass into the interior of the body, di- viding its cavity into small compartments, which look like cells when they are inflated. Within these is situated the plexus ot veins upon which the peculiar erectile property of the organ mainly depends. It consists of short veins, which very closely interlace and anastomose with each other in all directions, and admit of great variation of size, collapsing in the passive state of the organ, but, for erection, capable of an amount of dilatation which exceeds CIRCULATION. 121 beyond comparison that of the arteries and veins which convey the blood to and from them. The strong fibrous tissue lying in the intervals of the venous plexus, and the external fibrous membrane or sheath with which it is connected, limit the distension of the vessels, and, during the state of erection, give to the penis its condition of tension and firmness. The same general condition of vessels exists in the corpus spongiosum urethrae, but around the urethra the fibrous tissue is much weaker than around the body of the penis, and around the glans there is none. The venous blood is returned from the plexuses by comparatively small veins; those from the glans and the fore part of the urethra empty themselves into the dorsal vein of the penis; those from the corpus cavernosum pass into deeper veins, which issue from the corpora cavernosa at the crura penis; and those from the rest of the urethra and bulb pass more directly into the plexus of the veins about the prostate. For all these veins one condition is the same ; namely, that they are liable to the pressure of muscles when they leave the penis. The vena dorsalis penis may be compressed by the uniting tendons of the ischio-cavernosi; the crura penis, and the veins issuing from them, are under the same muscles; and the veins of the bulb are subject to the compression of the bulbo-cavernosi. (See Krause, lxxx. 1837 ; Kobelt, cxxvii. and xxv. 1843-4, p. 5S.) Midler described a peculiar arrangement of the arteries of the corpora cavernosa; one set of them he described as the ultimate ramuscles, which terminate in the minute radicles of the veins, and are destined for the nutrition of the part: the others as coming off from the side of the arteries, and consisting of short tendril-like branches, terminating abruptly Fig. 28. by a rounded, apparently closed extremity, turned back somewhat on itself. He named these arteriae helicinae, and thought they pro- jected into the venous cells, as represented in the annexed figure. His account has been con- firmed by Erdl (lxxx. 1841), but is denied by Valentin (lxxx. 1838) and Berres (cxxviii. 60, xxxi.), who think that what Miiller called heli- cine arteries were small arteries running in the septa and bands of fibrous tissues which intercept the corpus cavernosum, and which, when cut or torn across, assume a contorted somewhat spiral figure. Erection results from the distension of the venous plexuses with blood. . The principal exciting cause, in the erection of the penis, is nervous irritation, originating in the part itself or derived from the brain and spinal cord. The nervous influence is communicated to the penis by the pudic nerves which ramify in its vascular tissue; and Guenther (xcvi. 1828, p. 364) has observed that, after their division in the horse, the penis is no longer capable of erection. It 11 122 CIRCULATION. affords a good example of the subjection of the circulation in an in- dividual organ to the influence of the nerves; but the mode in which they excite a greater influx of blood is not with certainty known. The most probable explanation is that recently offered by Pro- fessor Kblliker, who ascribes the distension of the venous plexuses to the influence of organic muscular fibres, which he finds in abund- ance in the corpora cavernosa of the penis, from the bulb to the glans, also in the clitoris and other parts capable of erection. While erectile organs are flaccid and at rest, these contractile fibres exercise an amount of pressure on the plexuses of vessels distri- buted amongst them sufficient to prevent their distension with blood. But when, through the influence of their nerves, these parts are stimulated to erection, the action of these fibres is suspended, and the plexuses thus liberated from pressure yield to the distending force of the blood, which probably at the same time arrives in greater quantity owing to a simultaneous dilatation of the arteries of the parts, and thus the plexuses become filled, and remain so until the stimulus to erection subsides, when the organic muscular fibres again contract, and so gradually expel the excess of blood from the previously distended vessels. The influence of cold in producing extreme contraction and shrinking of erectile organs, and the opposite effect of warmth in inducing fulness and distension of these parts, are among the arguments used by Kblliker in support of this opinion (cxci.). The accurate dissections and experiments of Kobelt (cxxvii.), extending and confirming those of Le Gros Clark (lxxi. vol. xviii. p. 437) and Krause (lxxx. 1837), have shown that this influx of the blood, however explained, is the first condition necessary for erection, and that through it alone, much enlargement and tumes- cence of the penis may ensue. But the erection is probably not complete, nor maintained for any time, except when, together with this influx, the muscles already mentioned contract, and by com- pressing the veins, stop the efflux of blood, or prevent it from being as great as the influx.1 It appears, however, to be only the most perfect kind of erection that needs the help of muscles to compress the veins; for none such can assist the erection of the nipples, or that amount of tum- escence just falling short of erection of which the spleen and many other parts are capable. For such turgescence nothing more seems • Kolliker, however, seems to doubt the existence of any such additional influence in the production of erection, and believes that the mere accumu- lation of blood in the venous plexuses, in consequence of the relaxation of the muscular fibres by which they are surrounded, is sufficient to produce the distension and firmness characteristic of the erectile state. RESPIRATION. 123 necessary than a large plexiform arrangement of the veins and such arteries as may admit, upon local occasions, augmented quantities of blood. CHAPTER VI. RESPIRATION. As the blood circulates through the various parts of the body, and fulfils its office by nourishing the several tissues and supplying to secreting organs the materials necessary for their different se- cretions, it is deprived of part of its nutritive constituents, and be- comes charged with impurities resulting from the deterioration of the tissues. It is, therefore, necessary that fresh supplies of nutri- ment should be continually added to the blood, and that provision should be made for the removal of the impurities. The first of these objects is accomplished by the processes of digestion and absorp- tion. The second is principally effected by the agency of the various excretory organs through which are removed the several impurities with which the blood is charged, whether these impuri- ties are derived altogether from the degeneration of tissues, or in part, also, from the elements of unassimilated food. One of the most important and abundant of the impurities is carbonic acid, the removal of which, and the introduction of fresh quantities of oxygen, constitute the chief purpose of respiration—a process which, because of its intimate relation to the circulation, may be considered here rather than with the other excretory functions. Structure of the Isungs.1 The respiratory process in man, and all Mammalia, is chiefly carried on in the minute cavities within the lungs, called air-ret Is, or pulmonary vesicles. Each lobule, or small subdivision of the lung, consists of a collection of such air-cells, clustered upon and opening into minute branches of the bronchial tubes, and having their walls overlaid with capillaries derived from the terminal branches of the pulmonary artery. The bronchial tube belonging to each lung passes into its sub- stance, dividing and subdividing, but without anastomosis, and sending branches to every part of the organ. All the larger branches have walls formed of tough membrane with organic-muscular circu- 1 The best recent essays on the Structure of the Lungs are those by Rainey (xli. vol. xxviiL); Addison (xliii. 1812); Bourgery (xix. 1812); Moleschott (cxxi.); and Adriani (cxcvii.). 124 RESPIRATION. lar fibres, giving them some power of spontaneous contraction, por- tions of cartilaginous rings by which they are held open, and longi- tudinal bundles of elastic tissue for greater power of recoil after ex- tension : they are lined with mucous membrane, the surface of which is covered with vibratile ciliary epithelium. But when the bronchi, by successive branchings, are reduced to about 1,100th of an inch in diameter, they lose these structures, and their walls are formed of only a tough, elastic membrane, with traces of fibrous, probably muscular,1 structure, over which the capillaries are spread in a very dense network, and on various parts of which air-cells irregularly open. Tubes of this kind are named by Mr. Rainey intercellular passages. The air-cells opening into them may be placed singly on their walls, like recesses from them (Fig. 29); but Fig. 29. View of a thin section of the lung of a cat, which had been injected by the pulmonary artery with gelatine, so as to fill bloodvessels and air-cells, and had been sliced when cold. a a a. Air-cells and lobular passage in section. 6 6. Their wall in section, c. Their wall in face. d. Extremely faint nucleus in the same, e e. Capillaries, h. Nucleus in wall of capil- lary, n. Small pulmonary artery or vein with simple wall. Magnified 250 diameters. more often are arranged in rows like minuter sacculated tubes; so that a succession or series of cells, all opening into one another, open by a common orifice into the tube. The cells are of various 1 Moleschott (exxi.), and Kolliker (excii. 1849). RESPIRATION. 125 forms, according to the mutual pressure to which they are subject; their walls are nearly in contact, and they vary from l-120th to l-1200th of an inch in diameter. (Moleschott, cxxi.) Their walls are formed of fine membrane, similar to that of the intercellular pas- sages, and continuous with it, which is folded on itself so as to form a sharp-edged border at each circular orifice of communication be- tween contiguous air-cells, or between the cells and the bronchial passages. The cells have no epithelial lining; but on the exterior of the membrane of which they are constructed a network of pul- monary capillaries is spread out so densely that the interspaces or meshes are even narrower than the vessels, which are, on an aver- age, 1-3000th of an inch in diameter. Between the atmospheric air in the cells and the blood in these vessels nothing intervenes but the thin membranes of the cells and capillaries; and the expos- ure of the blood to the air is the more complete because the folds of membrane between contiguous cells, and, often, the spaces be- tween the walls of the same, contain only a single layer of capillaries, both sides of which are thus at once exposed to the air. The cells situated nearest to the centre of the lung are smaller, and their networks of capillaries are closer than those nearer to the circumference, in adaptation to the more ready supply of fresh air to the central than the peripheral portion of the lungs. The cells of adjacent lobules do not communicate; and those of the same lobule, or proceeding from the same intercellular passage, do so as a general rule only near angles of bifurcation; so that, when any bronchial tube is closed or obstructed, the supply of air is lost for all the cells opening into it or its branches. Movements of Respiration. The movements for taking into the lungs fresh air, and for ex- pelling from them the air that has been changed by the respiratory process, are those of inspiration and expiration. The former is performed by the contraction of muscles bounding or attached to the exterior of the chest; the latter, chiefly and usually, by the elastic contraction or recoil of the lungs and the walls of the chest, after they have been dilated in the act of inspiration. The chest is a cavity closed on every side from the entrance of air; its immediate boundary is formed by its lining membrane, the pleura; its walls, external to the pleura, are partly osseous and un- yielding, though movable, partly muscular, partly elastic. It is filled by the lungs and heart and their larger vessels, and these fill it equally in all its alterations of size; when it enlarges, they re- ceive more air and blood; when it contracts, air and blood pass out of them; and the lungs and part of the heart are always in contact with every part of its internal surface. The changes produced by the respiratory movements on the circulation of blood have been 11* 126 RESPIRATION. noticed already (p. 112); the much greater changes of air will now be alone considered. Air, as already said, fills all the air-tubes and cells of the lungs, and through their medium the pressure of the atmosphere is com- municated, through the open glottis, to the whole of the interior of the cavity of the chest, and balances the pressure of the atmo- sphere on the exterior of the chest. The force, therefore, which is required for the expansion of the chest in inspiration is not more than is necessary for moving the weight of its walls and those of the abdomen, and overcoming their elasticity and that of the lungs. The expressions which imply the necessity of forming a vacuum in the chest to draw in air, are inaccurate or exaggerated; the pres- sure of the atmosphere on both the inside and outside of the chest being equal, its walls are free to move so long as the glottis is open. When they are raised, so as to expand the chest, the pressure on the exterior of the lungs is made somewhat less than that of the atmospheric air on their interior: the excess of pressure, therefore, impels more air into them through the trachea. When, on the other hand, the walls of the chest contract, or allow the elastic tissue of the lungs and pulmonary pleura to contract, the pressure is greater on the exterior than on the interior of the lungs, and air is forced out of them through the trachea. In its enlargement in inspiration the capacity of the chest is commonly increased in all directions; and chiefly in its vertical diameter and at its posterior part, so as to insure the expansion of the great masses of lung which lie in the hollows at the back of the chest, by the sides of the spine. But its mode of increase presents some peculiarities in different persons and circumstances. In young children, the inspiration is effected almost entirely by the diaphragm, which being highly arched in expiration, becomes flatter as it contracts, and descending presses on the abdominal viscera, and pushes forward the front walls of the abdomen. The move- ment of the abdominal walls being here more manifest than that of any other part, it is usual to call this the abdominal mode or type of respiration. In adult men, together with the descent of the diaphragm, and the pushing forward of the front wall of the abdomen, the lower part of the chest and the sternum are subject to a wide movement in inspiration. In women, the movement ap- pears less extensive in the lower, and more so in the upper part of the chest; a mode of breathing to which a greater mobility of the first rib is adapted, and which may have for its object the provision of sufficient space for respiration when the lower part of the chest is encroached upon by the pregnant uterus. 31M. Beau and Maissiat (exxii. 1842-3), call the former the inferior costal, and the latter the superior costal, type of respiration ; but the annexed diagrams from Mr. Hutchinson's paper (xli. vol. xxix.) will explain RESPIRATION. 127 the difference better than the names, which imply a greater diversity than naturally exists in the modes of inspiration. Fig. 30. Fig. 31. Fig. 30. The changes of the thoracic and abdominal walls of the male during respira- tion. The back is supposed to be fixed in order to throw forward the respiratory movement as much as possible. The outer black continuous line in front represents the ordinary breathing movement: the anterior margin of it being the boundary of inspiration, the posterior margin the limit of expiration. The line is thicker over the abdomen, since the ordinary respiratory movement is chiefly abdominal: thin over the chest, for there is less movement over that region. The dotted line indicates the movement on deep inspiration, during which the sternum advances while the abdomen recedes. Fi". 31. The respiratory movement in the female. The lines indicate the same changes as in the last figure. The thickness of the continuous line over the sternum shows the larger extent of the ordinary breathing movement over that region in the female than in the male. From the enlargement produced in inspiration, the chest and lungs return, with expiration, by their elasticity.1 The costal car- ' For an account of the particular muscles engaged in inspiration, see the article Thorax, by Mr. Hutchinson, in the Cyclopaedia of Anatomy ; a paper on the Movements of Respiration, by Dr. Sibson, in the Medico-Chi- rurgical Transactions for 1848 ; and as well as these the student may refer to Mr. Hutchinson's paper in the twenty-ninth volume of the Medico-Chi- rur«ical Transactions: Mr. Sibson's papers in the rhilosophical Transac- tions for 1846, and the forty-first volume of the Medical Gazette; Dr. J. Ileid's article Respiration in the Cyclopaedia of Anatomy; and the papers of MM. Beau and Maissiat (exxii. 18-12 and 1843). 128 RESPIRATION. tilages are the chief seats of elastic power in the walls of the chest, and add to the force of an ordinary expiration, i. e., of such an one as is made in tranquil breathing. But the walls of the chest hinder deeper expirations, such as are made in straining, coughing, and the like; for these, muscular efforts are required, and when the chest is contracted or compressed by those efforts, it recovers its average capacity by its elasticity. Independently of its elasticity, the walls of the chest follow, in expiration, the elastic recoil or contraction of the lungs, which have elastic tissue in the bronchial tubes and air-cells and in their investing pleura, and are always kept on the stretch, ready to contract as soon as the muscular effort for expanding the chest ceases, contracting with the more force the more they have been expanded, and never in health contracting so much as they might, or as they do when the chest is opened and atmospheric pressure is directly admitted to their external surface. The quantity of air that is changed in the lungs in each act of ordinary tranquil breathing is variable, and is very difficult to esti- mate, because it is hardly possible to breathe naturally while, as in an experiment, one is attending to the process. The best esti- mate, perhaps, is that by Mr. Coathupe (xciii. 1839) who states the quantity at from twenty to twenty-five cubic inches; and this is probably as near as possible to the truth in the case of healthy young and middle-aged men; but Bourgery (exxii. 1S43) is most likely right in saying that old people, even in health, habitually breathe more deeply, and change in each respiration a larger quan- tity of air than younger persons do. This quantity, being that habitually and almost uniformly changed in breathing, is called by Mr. Hutchinson (xli. vol. xxix.) breathing air. The quantity over and above this which a man can draw into the lungs in the deepest inspiration, he names com- plemental air : its amount is various, as will be presently shown. After ordinary expiration, such as that which expels the breathing air, a certain quantity of air remains in the lungs, which may be expelled by a forcible and deeper expiration: this he terms reserve air. But, even after the most violent expiratory effort, the lungs are not completely emptied; a certain quantity always remains in them, over which there is no voluntary control, and which may be called residual air. Its amount depends in great measure on the absolute size of the chest, and has been variously estimated at from forty to two hundred and sixty cubic inches. The greatest respiratory capacity of the chest is indicated by the quantity of air which a person can expel from his lungs by a forci- ble expiration after the deepest inspiration that he can make. Mr. Hutchinson names this the vital capacity; it expresses the power which a person has of breathing in the emergencies of active RESPIRATION. 129 exercise, violence, and disease; and in healthy men it varies accord- ing to stature, weight, and age. It is found by Mr. Hutchinson, from whom nearly all our infor- mation on this subject is derived, that at a temperature of 60° F. 225 cubic inches is the average vital capacity of a healthy person, five feet seven inches in height. For every inch of height above this standard the capacity is increased, on an average, by eight cubic inches; and for every inch below, it is diminished by the same amount. This relation of capacity to height is quite inde- pendent of the absolute capacity of the cavity of the chest; for the cubic contents of the chest do not always, or even generally, in- crease with the stature of the body; and a person of small absolute capacity of chest may have a large capacity of respiration, and vice versa. The capacity of respiration is determined only by the mobility of the walls of the chest; but, why this mobility should increase in a definite ratio with the height of the body is yet unex- plained ; and must be difficult of solution, seeing that the height of the body is chiefly determined by that of the legs, and not by that of the trunk or the depth of the chest. But the vast number of observations made by Mr. Hutchinson leave no doubt of the fact, as stated above. The influence of weight on the capacity of respiration is less manifest and considerable than that of height; and it is difficult to arrive at any definite conclusions on this point, because the natu- ral average weight of a healthy man in relation to stature has not yet been determined. As a general statement, however, it may be said that the capacity of respiration is not affected by weights under 161 pounds, or W\ stones; but that, above this point, it is diminished at the rate of one cubic inch for every additional pound up to 196 pounds, or 14 stones; so that, for example, while a man of five feet six inches, and weighing less than 11 \ stones, should be able to expire 217 cubic inches, one of the same height, weighing 12 J stones, might expire only 203 cubic inches. By age, the capacity appears to be increased from about the fif- teenth to the thirty-fifth year, at the rate of five cubic inches per year; from thirty-five to sixty-five it diminishes at the rate of about one and a half cubic inch per year; so that the capacity of respiration of a man sixty years old would be about 30 cubic inches less than that of a man forty years old, of the same height and weight. Mr. Hutchinson's observations were made almost exclusively on men; and his conclusions are, perhaps, true of them alone; for women, according to Bourgery, have only half the capacity of breathing that men of the same age have. The number of respirations in a healthy adult person usually ranges from fourteen to eighteen per minute. According to Mr. \ 130 RESPIRATION. Hutchinson, the force with which the inspiratory muscles are capa- ble of acting is greatest in individuals of the height of from five feet seven inches to five feet eight inches, and will elevate a column of three inches of mercury. Above this height, the force decreases as the stature increases ; so that the average of men of six feet can elevate only about two and a half inches of mercury. The force manifested in the strongest expiratory acts is, on the average, one- third greater than that exercised in inspiration. But this differ- ence is in great measure due to the power exerted by the elastic reaction of the walls of the chest; and it is also much influenced by the disproportionate strength which the expiratory muscles attain, through being called into use for other purposes than that of simple expiration. The force of the inspiratory act is, there- fore, better adapted than that of the expiratory for testing the muscular strength of the body. Much of the force exerted in inspiration is employed in overcom- ing the resistance offered by the elasticity of the walls of the chest and of the lungs. Mr. Hutchinson estimated the amount of this elastic resistance by observing the elevation of a column of mercury raised by the return of air forced, after death, into the lungs, in quantity equal to the known capacity of respiration during life; and he calculated that in a man capable of breathing 200 cubic inches of air, the muscular power expended upon the elasticity of the walls of the chest, in making the deepest inspira- tion, would be equal to the raising of at least 301 pounds avoirdu- pois. In tranquil respiration, supposing the amount of breathing air to be twenty cubic inches, the resistance of the walls of the chest would be equal to lifting more than 200 pounds. The elastic force exerted in ordinary expiration must therefore be much greater than enough to lift this weight; because in it the elastic force of the lungs is also in action—a force which is not included in these estimates, because the lungs were in both cases burst by the air forced into them. It is probable that in the ordinary quiet respiration, which is performed without consciousness or effort of the will, the only forces engaged are those of the inspiratory muscles, and the elas- ticity of the walls of the chest and the lungs. And it is not known under what circumstances the contractile power which the bron- chial tubes, and perhaps the air-cells, possess, by means of their organic muscular fibres, is brought into action. It is possible, as Dr. R. Hall (C. C.) has lately maintained, that it may assist in expiration; but it is more likely that its chief purpose is to regu- late and adapt, in some measure, the quantity of air admitted to the lungs, and to each part of them, according to the supply of blood. Another purpose probably sometimes served by the mus- cular fibres of the bronchial tubes is that of contracting upon and RESPIRATION. 131 gradually expelling collections of mucus, which may have accumu- lated within the tubes, and cannot be ejected by forced expiratory efforts, owing to collapse or other morbid conditions of the portion of lung proceeding from the obstructed tubes. (Dr. W. T. Gairdner, clxxxix. May, 1851.) The muscular action in the lungs, morbidly excited, is probably the chief cause of the phenomena of spasmodic asthma. It may be demonstrated by galvanizing the lungs shortly after taking them from the body : under such a stimulus they contract so as to lift up water placed in a tube introduced into the trachea (C. J. B. Williams, cxxxi. p. 588); and Yolkmann (xv. art. Nerveiqdiysio- logie, p. 586), has shown that they may be made to contract by stimulating their nerves. He tied a glass tube, drawn fine at one end, into the trachea of a beheaded animal, and when the small end was turned to the flame of a candle, he galvanized the pneu- mogastric trunk; each time he did so, the flame was blown, and once it was blown out. The changes of the air in the lungs effected by these respiratory movements are assisted by the various conditions of the air itself. According to the law observed in the diffusion of gases, the car- bonic acid evolved in the air-cells will, independently of any respi- ratory movement, tend to leave the lungs, by diffusing itself into the external air where it exists in less proportion; and, according to the same law, the oxygen of the atmospheric air will tend of itself towards the air-cells in which its proportion is less than in the air in the bronchial tubes or external to the body. But for this tendency of the oxygen and carbonic acid to mix uniformly, within and without the lungs, the reserve and residual air would, probably, be very injuriously charged with carbonic acid; for the respiratory movements alone are not enough to empty the air-cells, and perhaps expel only the air which lies in the larger bronchial tubes. Probably also the change is assisted by the different tem- perature of the air within and without the lungs; and by the action of the cilia on the mucous membrane of the bronchial tubes, the continual vibrations of which may serve to prevent the adhe- sion of the air to the moist surface of the membrane. Movement of the Blood in the Respiratory Organs. To meet the air thus alternately moved into and out of the air- cells and minute bronchial tubes, the blood is propelled from the right ventricle through the pulmonary capillaries in steady streams, and slowly enough to permit every minute portion of it to be for a few seconds exposed to the air, with only the thin walls of the capillary vessels and air-cells intervening. The pulmonary circula- tion is of the simplest kind; for the pulmonary artery branches regularly; its successive branches run in straight lines, and do not 132 RESPIRATION. anastomose; the capillary plexus is uniformly spread over the air- cells and intercellular passages; and the veins derived from it pro- ceed in a course as simple and uniform as that of the arteries, their branches converging, but not anastomosing. The veins have no valves, or only small imperfect ones prolonged from their angles of junction, and incapable of closing the orifice of either of the veins between which they are placed. The pulmonary circulation also is unaffected by changes of atmospheric pressure, and is not exposed to the influence of the pressure of muscles: the force by which it is accomplished, and the course of the blood, are alike simple. The blood carried through the- pulmonary artery, being venous till it comes to the capillaries, is unfit for the nutrition of any parts of the lungs, except those in which it flows through the capillaries: to these it probably supplies nutritive materials as soon as it is itself arterialized. For the nutrition of the rest of the lungs, including the pleura, interlobular tissue, bronchial tubes and glands, and the walls of the larger bloodvessels, a special supply of arterial blood is furnished through one or two bronchial arteries, the branches of which ramify in all these parts. The blood of the bronchial artery, when, having served for the nutrition of these parts, it has become venous, is carried, partly, into the branches of a bronchial vein, distributed in the parts about the root of the lung, and partly into the small branches of the pulmonary artery, or, more directly, into the pulmonary capillaries, whence, being with the rest of their blood arterialized, it is carried to the pulmonary veins and left side of the heart. Changes of the Air in Respiration. By their contact in the lungs the composition of both air and blood is changed. The alterations of the former being manifest, simpler than those of the latter, and in some degree illustrative of them, may be considered first. The atmosphere we breathe has, in every situation in which it has been examined in its natural state, an uniform composition. It appears to be a mixture of oxygen, nitrogen, carbonic acid, and watery vapor. Of every 100 volumes of pure atmospheric air, 79 volumes (on an average) consist of nitrogen, the remaining 21 of oxygen : but the proportions of these gases are subject to variations of 2 or 3 parts in 1000, in situations where the oxygen is much exposed to absorption, as over the sea when there is no wind. (Lewy, xviii. 1842.) The proportion of carbonic acid is extremely small; 10,000 volumes of atmospheric air contain, according to M. do Saussure, only 4.15 of carbonic acicl. In the open country, he found the maximum proportion of this gas to be 5.74, the mini- mum 3.15 in 10,000 parts. In the town of Geneva, the air con- tained 0.31 more carbonic acid than in the country. M. Boussin- RESPIRATION. 133 gault, however, has lately found (xii. 1844), from numerous analy- ses, which are confirmed by those of M. Lewy, that the quantity of carbonic acid in the air of Paris (which may be taken as an exam- ple of a large town), is not above the average of the quantity con- tained in the air of the country. This average he finds to be 3.97 volumes in 10,000; and his estimate may be considered gene- rally true, except for localities, such as mines, crowded rooms, vol- canic districts, and others in which large quantities of carbonic acid are constantly exhaling. The quantity of watery vapor varies greatly, according to the temperature, and other circumstances, but some is never absent from the atmosphere. Besides these, its constant constituents, the atmosphere usually contains minute fractional quantities of ammo- nia, and other accidental substances; but, as far as is at present known, none of these have any particular relation to the respira- tory process, and the consideration of them may therefore be omitted. The changes produced by respiration on the atmospheric air are, that 1, it is warmed; 2, its carbonic acid is increased; 3, its oxy- gen is diminished; 4, its watery vapor is increased. 1. The expired air, heated by its contact with the interior of the lungs, is (at least in most climates) hotter than the inspired air. Its temperature varies between 97° and 99?°, the lower tempera- ture being observed when the air has remained but a short time in the lungs rather than when it was inhaled at a very low tempera- ture ; for whatever the temperature when inhaled may be, the air nearly acquires that of the blood before it is expelled from the chest. 2. The carbonic acid in respired air is always increased; but the quantity exhaled in a given time is subject to change from various circumstances. It may be stated, as a general average, deduced from the results of experiments of Valentin and Brunner (iv. vol. i. 547), that, under ordinary circumstances, the quantity of this gas exhaled into the air breathed by a healthy adult man amounts to 1345.3 cubic inches, or about 636 grains per hour. According to this estimate, which corresponds very closely with the one furnished by Sir II. Davy, and does not widely differ from those obtained by Allen and Pepys, and by Lavoisier, the weight of carbon excreted from the lungs is about 173 grains per hour, or 8 ounces in the course of twenty-four hours. Andral and Gavar- ret (cix.) calculate the average quantity of carbon excreted from the lungs of a healthy adult man at 9 ounces per day. Mr. Coa- thupe (xciii. 1839) makes it scarcely 5 ounces; while Liebig (xi. 3d edit. p. 14) estimates the total quantity excreted from the lungs 12 134 RESPIRATION. and skin together at 13.9 ounces. Some of these discrepancies may be due to the variations to which the exhalation of carbonic acid is liable in different circumstances; for, even in health, the quantity varies according to age, sex, diversities in the respiratory movements, external temperature, the degree of purity of the re- spired air, and other circumstances. Each of these circumstances deserves a brief notice, because they afford evidence concerning either the sources of the carbonic acid exhaled, or the mode in which it is separated from the blood. a. Influence of Age and Sex.—According to Andral and Ga- varret (cix.), the quantity of carbonic acid exhaled into the air breathed by males, regularly increases from eight to thirty years of age; from thirty to forty it is stationary or diminishes a little; from forty to fifty the diminution is greater; and from fifty to ex- treme age it goes on diminishing till it scarcely exceeds the quan- tity exhaled at ten years old. In females (in whom the quantity exhaled is always less than in males of the same age), the same re- gular increase in quantity goes on from the eighth year to the age of puberty, when the quantity abruptly ceases to increase, and re- mains stationary so long as they continue to menstruate. When, however, menstruation has ceased, either in advancing years, or in pregnancy, or morbid amenorrhcea, the exhalation of carbonic acid again augments; but when menstruation ceases naturally, it soon decreases again at the same rate as it does in old men. b. Influence of Respiratory Movements.—According to Dr. Vier- ordt (ex.), the more quickly the movements of respiration are per- formed the smaller is the proportionate quantity of carbonic acid contained in each volume of the expired air. Thus he found, that, with six respirations per minute, the quantity of expired carbonic acid was 5.528 per cent.; with twelve respirations, 4.262 per cent.; with twenty-four, 3.355; with forty-eight, 2.984; and with ninety- six, 2.662. Although, however, the proportionate quantity of car- bonic acid is thus diminished during frequent respiration, yet the absolute amount exhaled into the air within a given time is in- creased thereby, owing to the larger quantity of air which is breathed in the time. This is the case, whether the respiration be voluntarily accelerated or is naturally increased in frequency, as it is after feeding, active exercise, &c. By diminishing the frequency and increasing the depth of respiration, the percentage proportion of carbonic acid in the expired air is diminished; being in the deepest respiration as much as 1.97 per cent, less than in ordinary breathing. But for this proportionate diminution, also, there is a full compensation in the greater total volume of air which is thus breathed. Finally, the last half of a volume of expired air con- tains more carbonic acid than the half first expired; a circumstance which is explained by the one portion of air coming from the remote RESPIRATION. 135 parts of the lungs, where it has been in more immediate and pro- longed contact with the blood than the other has, which comes chiefly from the larger bronchial tubes. c. Influence of external Temperature.—The observations made by Vierordt, at various temperatures between 38° F. and 75° F., show that within this range every rise equal to 10° F. causes a diminution of about two cubic inches in the quantity of carbonic acid exhaled per minute. Letellier (xii. 1845), from experiments performed on animals at much higher and lower temperatures than the above, also finds that the higher the temperature of the respired air (as far as 104° F.), the less is the amount of carbonic acid ex- haled into it, whilst the nearer it approaches zero the more does the carbonic acid increase. The greatest quantity exhaled at the lower temperatures he found to be about twice as much as the smallest exhaled at the higher temperatures. d. Purity of the respired Air.—The average quantity of carbonic acid given out by the lungs constitutes about 4.48 per cent, of the expired air, but if the air which is breathed be previously impreg- nated with carbonic acid (as is the case when the same air is fre- quently respired), then the quantity of carbonic acid exhaled be- comes much less. This is shown by the results of two experiments performed by Allen and Pepys (xliii. 1808-9). In one, in which fresh air was taken in at each inspiration, thirty-two cubic inches of carbonic acid were exhaled in a minute; whilst in the other, in which the same air was respired repeatedly, the quantity of carbonic acid emitted in the same time was only 9.5 cubic inches. They found also that however often the same air may be respired, even if until it will no longer sustain life, it does not become charged with more than 10 per cent, of carbonic acid. The necessity of a constant supply of fresh air, by means of ventilation, through rooms in which many persons are breathing together, or in which, from any other source, much carbonic acid is evolved, is thus rendered obvious; for even when the air is not completely irrespirable, yet in the same proportion as it is already charged with carbonic acid, does the further extrication of that gas from the lungs suffer hin- drance. The period of day seems to exercise a slight influence on the amount of carbonic acid exhaled in a given time, though beyond the fact that the quantity exhaled is much less by night than by day, we are scarcely yet in a position to state that variations in the amount exhaled occur at uniform periods of the day, independent of the influence of other circumstances. By the use of food the quantity is increased, whilst by fasting it is diminished; and, ac- cording to Regnault and Ileiset, it is greater when animals are fed on farinaceous food than when fed on meat. Spirituous drinks, especially when taken on an empty stomach, produce an immediate 136 RESPIRATION. and marked diminution in the quantity of this gas exhaled. Bodily exercise, in moderation, increases the quantity to about one-third more than it is during rest; and for about an hour after exercise the volume of the air expired in the minute is increased about 118 cubic inches: and the quantity of carbonic acid about 7.8 cubic inches per minute. During sleep, on the other hand, there is a considerable diminution in the quantity of this gas evolved; a result probably in great measure dependent on the tranquillity of the breathing: directly after waking, there is a great, though quickly transitory, increase in the amount exhaled. A larger quantity is exhaled when the barometer is low than when it is high. 3. The Oxygen in respired air is always less than in the same air before respiration, and its diminution is generally proportionate to the increase of the carbonic acid. The experiments of Valentin and Brunner (iv. Bd. i.) seem to show that for every volume of carbonic acid exhaled into the air, 1.17421 volumes of oxygen are absorbed from it; and that when the average quantity of carbonic acid, i. e., 1345.3 cubic inches, or 635.85 grains, is exhaled in the hour, the quantity of oxygen absorbed in the same time is 1583.6 cubic inches, or 541.5 grains. According to this estimate, there is more oxygen absorbed than is exhaled with carbon in the car- bonic acid, for oxygen combines with carbon to form carbonic acid without change of volume; and to this general conclusion, namely, that the volume of air expired in a given time is less than that of the air inspired (allowance being made for the expansion in being heated), and that the loss is due to a portion of oxygen absorbed and not returned in the exhaled carbonic acid, all observers agree, though as to the actual quantity of oxygen so absorbed they differ even widely. The quantity of oxygen that does not combine with the carbon given off in carbonic acid from the lungs, is probably disposed of in forming some of the carbonic acid and water given off from the skin, and in combining with sulphur and phosphorus to form part of the acids of the sulphates and phosphates excreted in the urine, and probably, also, from the experiments of Dr. Bence Jones (vi. April, 1851, and lxxxviii. August 30, 1851), with the nitrogen of the decomposing nitrogenous tissues. The quantity of oxygen consumed seems to vary much not only in different individuals, but in the same individual at different periods : thus it is considerably influenced by food, being greater in dogs when fed on farinaceous than on animal food, and much diminished during fasting, while it varies at different stages of digestion. Ani- mals of small size consume a relatively much greater amount of oxy- gen than larger ones. The quantity of oxygen in the atmosphere surrounding animals, appears to have very little influence on the RESPIRATION. 137 amount of this gas absorbed by them, for the quantity consumed is not greater even though an excess of oxygen be added to the atmosphere experimented with. (Regnault and Reiset, cxc. Julv, 1850, p. 252.) J Valentin and Brunner's estimates of the interchange of the oxygen and carbonic acid in respiration, led them to believe that the exchanged quantities of the two gases are always in the propor- tion of their diffusion-volumes; a conclusion which, if it were established, would justify us in explaining the process of respira- tion as one due only to the properties of the gases, and their tendency to diffuse or mingle with one another in certain fixed proportions by volume. But the grounds are not yet strong enough for so weighty a conclusion; and against it appear the many instances of deviation from the law, which seem proved by good experiments.1 If the exchange of gases were according to the law of diffusion, their proportions ought never to vary; however much the circum- stances of the general economy might change the cjuantity of one (as of the carbonic acid in the instances already quoted), the quantity of the other should be equally and at the same time changed. Whereas, in many experiments, and even in some of Valentin's, the proportion of oxygen absorbed has been less or more than, according to the law of diffusion, it should have been. Especially, the experiments of Dulong and Despretz seemed to show that, in Carnivora, the oxygen absorbed always bears a larger proportion to the carbonic acid evolved than it does in Herbivora; and the recent careful experiments of Regnault and Reiset (xviii. 1848) confirm this, while they in no case show such a proportion between the gases exchanged as, according to the law of diffusion, there should be. They show that in the dog for every 100 parts of carbonic acid formed in twenty-four hours, 134.3 parts of oxygen were, on the average, absorbed; and in the rabbit and hen for every 100 parts of carbonic acid, 109.34 parts of oxygen; while, according to the diffusion-law, the proportion should be always 117.42 parts of oxygen to 100 of carbonic acid. It may be added, that the conditions of the gases engaged in respiration are not those in which the law of diffusion would exactly hold. The law requires that both gases should be free and under equal pressure; while, in the actual case, the gas in the blood is dissolved, under pressure, and separated by a membrane from that into which it is to diffuse. It is possible that these peculiarities of the conditions may account for the deviations from the law while it is really in operation; but many more facts than are yet ascer- tained will be necessary to prove this. 1 The objections to Valentin's theory, and his answers to them, are all in his Annual Reports on Physiology, in Canstatt's Jahresberichte, since 1843. 12* 138 RESPIRATION. ' The Nitrogen of the atmosphere, in relation to the respiratory process, is supposed to serve only mechanically, by diluting the oxygen, and moderating its action upon the system. This purpose, or the mode of expressing it, has been lately denied by Liebig (xi. 3d edit. p. 184), on the ground that, if we suppose the nitrogen removed, the amount of oxygen in a given space would not be altered. But although it be true that if all the nitrogen of the at- mosphere were removed, and not replaced by any other gas, the oxygen might still extend over the whole space at present occupied by the mixture of which the atmosphere is composed; yet since, under ordinary circumstances, oxygen and nitrogen, when mixed together in the ratio of one volume to four, produce a mixture which occupies precisely five volumes, with all the properties of atmo- spheric air, it must result that a given volume of atmosphere drawn into the lungs contains four-fifths less weight of oxygen than an equal volume composed entirely of oxygen. The greater rapidity and brilliancy with which combustion goes on in an atmosphere of oxy- gen than in one of common air, and the increased rapidity with which the ordinary effects of respiration are produced when oxygen instead of atmospheric air is breathed, seem to leave no doubt that the nitrogen with which the oxygen of the atmosphere is mixed has the effect of diluting this gas, in the same sense and degree as one part of alcohol is diluted when mixed with four parts of water. It has been often discussed whether nitrogen be ever absorbed or exhaled from the atmosphere in respiration. That it may, in some conditions, be either absorbed or exhaled is proved by experi- ments of Allen and Pepys, who found that when guinea-pigs were made to breathe in a mixture of oxygen and hydrogen, nitrogen was exhaled, and in a quantity exceeding the volume of the whole body of the animal. The lower animals also, especially insects, are said to exhale nitrogen (Treviranus, xxxii. p. 330, Am. Ed.), and fishes to absorb it from the water in which they breathe, though they do not absorb hydrogen. (Humboldt, xxxii. p. 330, Am. Ed.) But we cannot, from these facts, safely conclude what is the case in the ordinary conditions of life. In the earlier experiments, it seemed as if small quantities of nitrogen were sometimes absorbed, and sometimes exhaled, in respiration in atmospheric air. The later experiments of M. Boussingault (xviii. 1846), on turtle- doves, would prove that, besides the nitrogen excreted from the digestive canal and kidneys, nearly 2? grains are daily discharged from the skin and liings; and those of MM. Regnault and Reiset (xviii. 1848, and liii. 1849) on dogs, rabbits, and fowls, prove con- stantly a certain exhalation of nitrogen, the proportion seeming to vary according to the nature of the food, while they also find that during prolonged fasting, nitrogen instead of being exhaled is absorbed. RESPIRATION. 139 4. Watery Vapor is, under ordinary circumstances, always exhaled from the lungs in breathing. The quantity emitted is, as a general rule, sufficient to saturate the expired air (Valentin, iv. Bd. i. p. 547), or very nearly so (Moleschott, cxxi.). Its absolute amount is, therefore, influenced by the following circumstances. First, by the volume of air expired; for the greater this is, the greater also will be the quantity of moisture exhaled. Secondly, by the quantity of watery vapor contained in the air previous to its being inspired; because the greater this is, the less will be the amount required to complete the saturation of the air. Thirdly, by the temperature of the expired air: for the higher this is the greater will be the quantity of watery vapor required to saturate the air. Fourthly, by the length of time which each volume of in- spired air is allowed to remain in the lungs; for it seems probable that, although during ordinary respiration the expired air is always saturated with watery vapor, yet when respiration is performed very rapidly the air has scarcely time to be raised to the highest temperature, or be fully charged with moisture ere it is expelled. For ordinary cases, however, it may be held that the expired air is saturated with watery vapor, and hence is derivable a means of estimating the quantity exhaled in any given time; namely, by subtracting the quantity contained in the air inspired from the quantity which (at the same barometric pressure) would saturate the same air at the temperature of expiration, which is ordinarily about 99°. And, on the other hand, if the quantity of watery vapor in the expired air be estimated, the quantity of air itself may from it be determined, being as much as that quantity of watery vapor would saturate at the ascertained temperature and barometric pressure. The quantity of water exhaled from the lungs in twenty-four hours ranges (according to the various modifying circumstances already mentioned) from about 3000 to 13,000 grains (6 to 27 ounces). Some of this is probably formed by the combination of the excess of oxygen absorbed in the lungs with the hydrogen of the blood; but the far larger proportion of it must be the mere exhalation of the water of the blood, taking place from the surface of the air-passages and cells, as it does from the free surfaces of all moist animal membranes, particularly at the high temperature of warm-blooded animals. It is exhaled from the lungs whatever be the gas respired, continuing to be expelled even in hydrogen gas. Changes produced in the Blood, by Respiration. The most obvious change which the blood undergoes in its pas- sage through the lungs is that of color, the dark crimson of venous blood being exchanged for the bright scarlet of arterial blood. The circumstances which have been supposed to give rise 140 RESPIRATION. to this change, the conditions capable of effecting it independent of respiration, and some other differences between arterial and ve- nous blood, were discussed in the chapter on Blood (page 59). The change in color is, indeed, the most striking, and may ap- pear the most important, which the blood undergoes in its passage through the lungs; yet, perhaps, its importance is very little, except so far as it is an indication of other and essential alterations effected in the composition of the blood. Of these alterations the principal are, 1st, that the blood after passing through the lungs is 1° or 2° warmer than it was before; 2d, that it coagulates sooner and more firmly, and contains, apparently, more fibrine; 3d, that it contains more oxygen, less carbonic acid, and less nitrogen. The difference last named is, probably, the most important. It might be assumed from what has ibeen said of the changes in the inspired air, and it is proved, at least in regard to the first two gases, by examination of the blood itself. The existence of carbonic acid in both arterial and venous blood has been proved by several experimenters, who have obtained ap- preciable quantities of it by exposing the blood to the vacuum of the air-pump, or, more certainly, by agitating it with atmospheric air, oxygen, or other gases, such as hydrogen or nitrogen. By the latter process carbonic acid may always be extracted from venous blood. Some, indeed, have failed to procure any gas from blood by means of the air-pump; but this may be explained by the fact observed by Magnus, that carbonic acid is not given out until the air in which the blood is placed is so rarefied that it supports only one inch of mercury. Heat also, commonly fails to evolve carbonic acid from blood: probably because, as also observed by Magnus, a temperature high enough to set free this gas coagulates the albu- men of the blood, and if albumen, impregnated with carbonic acid, is once coagulated, the gas cannot be separated from it again by means of heat. The uncertainty of former experiments is corrected by the re- cent researches of Magnus (xvii. 1845), from which it appears sure that carbonic acid, oxygen, and nitrogen exist, both in arterial and venous blood. Their relative proportions differ in the two kinds of blood. The quantity of oxygen contained in arterial blood is twice as great as that in venous blood; being equal to from 10 to 10 £ per cent, of the volume of the former, and only about 5 per cent, of the volume of the latter. The quantity of carbonic acid, on the other hand, is less in arterial than in venous blood, amount- ing to about 20 volumes per cent, in the former, and 25 per. cent. in the latter. The quantity of nitrogen contained in the blood varies from about 1.7 to 3.3 per cent. : its relative proportion in arterial and in venous blood does not appear to differ much; but RESPIRATION. 141 from its being commonly exhaled in small quantity from the lungs, it may be believed to be greater in the venous blood. These facts are supported by those already mentioned, concern- ing the exhalation of nitrogen by animals breathing in oxygen and hydrogen, and of carbonic acid by frogs breathing in nitrogen. The gases could not be so exhaled did they not exist in solution in the blood. And there can therefore be little doubt which of the proposed theories of respiration should be chosen for the explanation of the process. Till the existence of the gases in the blood was clearly proved, the theory most favored was, that the oxygen of the atmospheric air permeates the membranous walls of the air- cells, enters the blood, and there at once combines with carbon derived from the disintegrated tissues, to form carbonic acid, which escapes, together with the greater part of the nitrogen previously absorbed from the atmosphere. It could be well objected, even when the existence of gases in the blood was doubtful, that if this theory were true, the lungs ought to be much warmer than other parts of the body, through the quantity of heat given out in the quick union of the carbon with the oxygen of the atmosphere; and that such was not found to be the case: the temperature of the blood in the left side of the heart being never more than one or two degrees higher than that in the right. Lagrange and Hassenfratz (xxxii. p. 350), impressed with this and other objections, proposed the theory which, with some modi- fications, has been more recently advocated by Magnus and others, and has been shown by them to be sufficient for the explanation of most of the phenomena yet observed in this part of the respiratory process. According to this theory, the oxygen absorbed into the blood from the atmospheric air in the lungs, is in part dissolved, and probably, also, in part loosely combined chemically with one or other of its ingredients. In this condition the oxygen is carried in the arterial blood to the various parts of the body, and with it, is, in the capillary system of vessels, brought into near relation or contact with the elementary parts of the tissues. Herein, co-ope- rating probably in the process of nutrition, or the removal of dis- integrated parts of the tissues, "about one-half of the oxygen which the arterial blood contains disappears, and a jiroportionate quantity of carbonic acid and water is formed. The venous blood, contain- ing the new-formed carbonic acid, returns to the lungs, where a portion of the carbonic acid is exhaled, and a fresh supply of oxy- gen is again taken in. Whether part, or the whole, of the oxygen absorbed during re- spiration is at once united chemically with any of the constituents of the blood has not been determined. By some it is supposed to combine with the red corpuscles, by others with the fibrine. It appears most probable that the greater part of the gas is held in 142 RESPIRATION. solution by the fluid part of the blood: if combined, it must be very loosely so, till it reaches the capillaries. The same may be said with respect to the carbonic acid. How the exchange of the gases is effected has been already considered; if the diffusion theory be not received, we must suppose the emission and imbibi- tion to be effected after the plan of the secretion and absorption of fluids by other organs; a supposition which is favored by the close analogy in structure between the lungs and the secreting glands. Influence of the Nervous System in Respiration. The respiratory functions are in two respects subject to the in- fluence of the nervous system : namely, 1st, in the movements for the introduction and exit of air; and, 2dly, in the interchange of the gases. These will be more particularly considered in the sec- tions on the Medulla Oblongata and Pneumogastric Nerves. It may suffice to state here, that the respiratory movements and their regular rhythm, so far as they are involuntary and inde- pendent of consciousness (as in all ordinary occasions they are), are under the absolute governance of the medulla oblongata, which, as a nervous centre, receives the impression of the "necessity of breathing," and reflects it to the phrenic and such other motor nerves as will bring into co-ordinate and adapted action the mus- cles necessary to inspiration. But the respiratory movements may be voluntarily performed or variously directed, and the mind may be conscious of the necessi- ty of breathing, either when it attends the sensations to which that necessity gives rise, or when those sensations are more than commonly intense. In these cases, we may believe that the brain, as well as the medulla oblongata, is engaged in the process; for we have no evidence of the mind exercising either perception or will through any other organ than the brain. But even when the brain is thus in action, it appears to be the medulla oblongata which combines the several respiratory muscles to act together. In such acts, for example, as those of coughing and sneezing, the mind must first perceive the irritation at the larynx or nose, and exercise a certain degree of will in determining the actions, as, e. g., in the taking of the deep inspiration that always precedes them. But the mode in which the acts are performed, and the combination of muscles to effect them, are determined by the medulla oblongata, independent of the will, and have the peculiar character of reflex involuntary movements, in being always, and without practice or experience, precisely adapted to the end or purpose. In these and in all the other extraordinary respiratory actions, such as are seen in dyspnoea, or in straining, yawning, hiccough, and others, the medulla oblongata brings into adapted combination RESPIRATION. 143 of action many other muscles besides those commonly exerted in respiration. Almost all the muscles of the body, in violent efforts of dyspnoea, coughing, and the like, may be brought into action at once, or in quick succession; but, more particularly, the muscles of the larynx, face, scapula, spine, and abdomen, co-operate in these efforts with the muscles of the chest. These, therefore, are often classed as secondary muscles of respiration; and the nerves supplying them, including, especially, the facial, pneumogastric, spinal accessory, and external respiratory nerves, were classed by Sir Charles Bell with the phrenic, as the respiratory system of nerves. There appears, however, no propriety in making a separate system of these nerves, since their mode of action is not peculiar, and many beside them co-operate in the respiratory acts. That which is peculiar in the nervous influence directing the extraordinary move- ments of respiration is, that so many nerves are combined towards one purpose by the power of a distinct nervous centre, the medulla oblongata. In other than respiratory movements, these nerves may act singly or together, without the medulla oblongata; but, after it is destroyed, no movement adapted to respiration can be per- formed by any of the muscles, even though the part of the spinal cord from which they arise be perfect. The phrenic nerves, for example, are unable to excite respiratory movements of the dia- phragm when their connection with the medulla oblongata is cut off, though their connection with the spinal cord may be uninjured. The influence exercised through the pneumogastric nerves upon the functions of the lungs cannot be considered separately from their relation to the muscles of the larynx, and must therefore be deferred to the section particularly treating of the nerves. Effects of the Suspension and Arrest of Respiration. These deserve some consideration, because of the illustration which they afford of the nature of the normal processes of respira- tion and circulation. When the process of respiration is stopped, either by arresting the respiratory movements, or permitting them to continue in an atmosphere deprived of uncombined oxygen, the circulation of blood through the lungs is retarded, and, at length, stopped. The immediate effect of such retarded circulation is an obstruction to the exit of blood from the right ventricle : this is followed by delay in the return of venous blood to the heart; and to this succeeds venous congestion of the nervous centres, and all the other organs of the body. In such retardation, also, an unu- sually small supply of blood is transmitted through the lungs to the left side of the heart; and this small quantity is venous. The condition, then, in which a suffocated animal dies is, com- monly, that the left side of the heart is nearly empty, while the lungs, right side of the heart, and other organs are gorged with 144 RESPIRATION. venous blood. To this condition many things contribute. 1. The obstructed passage of blood through the lungs, which appears to be the first of the events leading to suffocation, seems to depend on the cessation of the interchange of gases, as if blood charged with carbonic acid could not pass freely through the pulmonary cap*il- laries. That such may be the case, is shown by Mr. Wharton Jones's observation that the circulation in the web of the frog's foot, may be retarded or arrested by directing on the web a stream of carbonic acid, under the influence of which, the blood-corpuscles appear to cluster and stagnate in the vessels. But the stagnation of blood in the pulmonary capillaries would not be enough to stop the circulation, unless the actions of the heart were also weakened ; for Mr. Erichsen (xciv. vol. lxiii. p. 22), having pithed dogs, and tied the right bronchus, and maintained artificial respiration in the left lung, found that, so long as the heart's action continued, nearly as much black blood flowed through a right pulmonary vein, as red blood through a left one. Therefore, 2dly, the fatal result is due, in some measure, to the weakened action of the right side of the heart, in consequence, probably, of its over-distension by blood continually flowing into it; this flow being, probably, much increased by the powerful but fruitless efforts continually made at inspiration. (Eccles., lxxi. vol. xliv. p. 657.) Thirdly, because of the obstruction at the right side of the heart, there must be venous congestion in the medulla oblongata and nervous centres; and this evil is augmented by the left ventricle receiving and propelling none but venous blood. Hence, slowness and disorder of the respiratory movements, and the movements of the heart, may be added. But this alone does not explain asphyxia, for Mr. Erichsen found that a dog was asphyxiated in the ordinary time, although arterial blood was made to circulate through the nervous centres during the whole time. However, under all these conditions combined, the heart at length ceases to aot. The time at which the complete cessation ensues is uncertain. The domestic Mammalia usually perish, after submersion in water, in about three minutes : there are exceptional cases, in which animals and human beings have been revived, after being under water for a longer period. According to Mr. Erichsen (1. c. p. 30), in dogs suffocated by drowning, the voluntary movements cease in 1| mi- nutes; the involuntary in 2i after submersion; the ventricular contractions continue for a period, ranging from 6£ to 14 minutes, the average time being 9=} minutes; and the blood in the arteries becomes as black as that in the veins, in about 1£ minutes. In the human subject, he .thinks that the ventricular contractions always cease at or before the expiration of five minutes after complete sub- mersion ; for persons are rarely, if ever saved, if they have been ANIMAL HEAT. 145 under water more than four minutes. The instances in which re- covery has taken place after a longer immersion are probably to be explained by the occurrence of fainting at the moment of the acci- dent ; for, with the circulation enfeebled, the deprivation of air may be endured much longer than it can while the blood still cir- culates quickly, and accumulates carbonic acid. It is to the accumulation of carbonic acid in the blood, and its conveyance into the organs that we must, in the first place, ascribe the phenomena of asphyxia. For when this does not happen, all the other conditions may exist without injury; as they do, for example, in hybernating, warm-blooded animals. In these, life is supported for many months in atmospheres in which the same animals, in their full activity, would be speedily suffocated. During the periods of complete torpor, their respiration entirely ceases; the heart acts very slowly and feebly; the processes of organic life are all but suspended, and the animal may with impunity be com- pletely deprived of atmospheric air. Spallanzani kept a marmot, in this torpid state, immersed for four hours in carbonic acid gas, without its suffering any apparent inconvenience. Dr. Marshall Hall kept a lethargic bat under water for sixteen minutes, and a lethargic hedgehog for 22 \ minutes: and neither of the animals appeared injured by the experiment (lxxiii. vol ii. p. 771). CHAPTER VII. ANIMAL HEAT. Intimately associated with the process of respiration are the production of animal heat, and the maintenance of a uniform temperature of the body; conditions as essential to the continuance of life, in warm-blooded animals, as the extrication of carbonic acid and the absorption of oxygen are. The average temperature of the human body, in those internal parts which are most easily accessible, such as the mouth and rectum, may be estimated at from 98° to 100° F. In children, the temperature is commonly as high as 102° F. In old persons, it is about the same as in adults (Davy, xliii. 1844). Of the external parts of the body the temperature becomes lower the further they are removed from the centre of the body; thus, in the human sub- ject, a thermometer placed in the axilla was found by Dr. John Davy to stand at 98° F., at the loins it indicated a temperature of 96J°, on the thigh 94°, on the leg 93° or 91°, on the sole of the 13 146 ANIMAL HEAT. foot 90° (xliii. 1844). In disease, the temperature of the body may deviate several degrees above and below the average of health. In some diseases, as scarlatina and typhus, it rises as high as 106° or 107° F.; and in children, M. Roger has observed the temperature of the skin to be raised to 108.5° Fah. (exxii. 1844). In the morbus cceruleus, in which there is defective arterialization of the blood from malformation of the heart, the temperature of the body is often as low as 79° or 77£°; in the Asiatic cholera, a thermo- meter placed in the mouth sometimes rises only to 77° or 79°. M. Roger observed the temperature of the body in children to be sometimes reduced in disease to 74.3. The temperature of the body, in health, is about 1J° F. lower during sleep than while awake. According to Dr. Davy (exxiii. June 1845), it is highest in the morning after rising from sleep, continues high but fluctuating till evening, and is lowest about midnight. Sustained mental exertion elevates it slightly; continued bodily exercise does so to a considerable extent; after feeding also it is somewhat raised. All these facts are important, both as showing variations in the temperature of the body correspondent with those in the production of carbonic acid in the same circum- stances, and as proving that the influence which slight changes in the organic economy of warm-blooded animals have is as great or greater than that exercised by even extreme variation in the external temperature to which they are exposed. For in warm climates, Dr. Davy found the temperature of the interior of the body only from 2.7° to 3.6° F. higher than in temperate climates; and during the voyage of the " Bonite," the French naturalists, who had an opportunity of observing the influence of various climates on the same persons, found that the temperature of the human body rises and falls in only a slight degree, even in extremes of external temperatures; that it falls slowly in passing from hot to cold climates, and rises more rapidly in returning towards the torrid zone: but that these changes in the temperature of the body are more considerable in some individuals than in others (xviii. 1838, p. 456). The temperature maintained by Mammalia, in an active state of life, according to the tables of Tiedemann and Rudolphi, averages 101°. The extremes recorded by them were 96° and 106°, the former in the narwhal, the latter in a bat (Vespertilio Pipistrella). In birds, the average is as high as 107° ; the highest temperature, 111.25°, being in the small species, the linnets, &c. (exxv. p. 234). Among reptiles, Dr. John Davy found that while the medium they were in was 75°, their average temperature was 82.5°. As a gene- ral rule, their temperature, though it falls with that of the sur- rounding medium, is, in temperate media, two or more degrees higher; and though it rises also with that of the medium, yet at ANIMAL HEAT. 147 very high degrees ceases to do so, and remains even lower than that of the medium. Fish, insects, and other Invertebrata present, as a general rule, the same temperature as the medium in which they live, whether that be high or low; only, among fish, the tun- ny-tribe, with strong hearts, and red meat-like muscles, and more blood than the average of fish have, are generally 7° warmer than the water around them. The difference, therefore, between what are commonly called the warm- and the cold-blooded animals, is not one of absolutely higher or lower temperature; for the animals, which to us, in a temperate climate, feel cold (being, like the air or water, colder than the sur- faces of our bodies), would, in an external temperature of 100° or 200°/ have nearly the same temperature, and feel hot to us. The real difference is, as Mr. Hunter expressed it (i. vol. iii. p. 16, and vol. iv. p. 131, et seq.), that what we call warm-blooded animals (birds and Mammalia), have a certain " permanent heat in all atmospheres," while the temperature of the others, which we call cold-blooded, is " variable with every atmosphere." The power of maintaining an uniform temperature, which Mam- malia and birds possess, is combined with the want of power to endure such changes of temperature as are harmless to the other classes; and when their power of resisting change of temperature ceases, they suffer serious disturbances or die. M. Magendie has shown that birds and rabbits die, when, being exposed to great ex- ternal heat, their temperature is raised as much as 9° above the natural standard ; but they bear a reduction of the temperature of the interior of the body to a much greater amount before very dangerous or fatal consequences ensue (exciii. 1850). In all the ordinary circumstances of life, the maintenance of uni- form temperature is effected by the production of heat sufficient to compensate for that which is constantly lost in radiation into the medium in which we live, or in combination with the fluids evapo- rating from the exposed surfaces of the body. The losses thus sustained are extremely various in different cir- cumstances ; and the degrees of power which animals possess of adapting themselves to such differences are equally various. Some live best in cold regions, where they produce abundant heat for radiation, and cannot endure the heat of warm climates, where the heat that they habitually produce would, probably, be excessive, and by its continual, though perhaps small, excess, would generate disease ; others, naturally inhabiting warm climates, die if removed to cold ones, as if because their power of producing heat were not quite sufficient to compensate for the constantly larger abstractions 1 Humboldt and Bonpland saw fish thrown up from volcanoes alive, and apparently in health, along with water and vapor which raised the thermo- meter to 210°. 148 ANIMAL HEAT. of it by radiation. Man, with the aid of intellect for the provision of artificial clothing, and with command over food, is, in these re- spects, superior to all other creatures ; possessing the greatest power of adaptation to external temperature, and being capable of endur- ing extreme degrees of heat as well as of cold without injury to health. His power of adaptation is sufficient for the maintenance of an uniform temperature in a range of upwards of 200° Fah- renheit ; a power which is only shared by some of the domestic animals who are his companions in his various abodes. Sources and Mode of Production of Heat in the Body. To explain the production of heat in the body, several theories have been advanced; but it now appears almost certain that the correct one is that which refers the generation of heat, primarily and in general, to certain chemical processes going on in the system; but admits, at the same time, that as these chemical changes are carried on in parts whose functions are, to a certain extent, under the influence of the nervous system, therefore the production of heat is liable to be modified, either locally or in every part, by the operation of that system. In explaining the chemical changes effected in the process of respiration (p. 139), it. was stated that the oxygen of the atmo- sphere taken into the blood is, most probably, combined in the systemic capillary vessels with the carbon and the hydrogen of disin- tegrated and absorbed tissues, and certain elements of food which have not been converted into tissues. That such a combination, between the oxygen of the atmosphere and the carbon and hydro- gen in the blood, is continually taking place, is made nearly certain by the fact that a larger amount of carbon and hydrogen is con- stantly being added to the blood from the food than is required for the ordinary purposes of nutrition, and that a quantity of oxygen is also constantly being absorbed from the air in the lungs, of the disposal of which no account can be given except by regarding it as combining, for the most part, with the excess of carbon and hydrogen, and being evaporated in the form of carbonic acid and water. In other words, the blood of warm-blooded animals appears to be always receiving from the digestive canal and the lungs more carbon, hydrogen and oxygen, than are consumed in the repair of the tissues; and to be always emitting carbonic acid and water, for which no other source can be ascribed than the combination of these elements. In the processes of such combination, heat must be continually produced in the animal body. The same amount of heat will be evolved in the union of any given quantities of carbon and oxygen, and of hydrogen and oxygen, whether the combination be rapid and evident, as in ordinary combustion; or slow and im- perceptible, as in the changes which are believed to occur in ANIMAL HEAT. 149 the living body. And since the heat thus arising will be gene- rated wherever the blood is carried, every part of the body will be heated equally; or so nearly equally that the rapid circulation of the blood will quickly remove any diversities of temperature in different parts. To establish this theory, it needs to be shown that the quantity of carbon and hydrogen which, in a given time, unites in the body with oxygen, is sufficient to account for the amount of heat gene- rated in the animal within the same time: an amount capable of maintaining the temperature of the body at from 98° to 100°, notwithstanding a large loss by radiation and evaporation.1 An attempt to determine this point was made by Dulong and Despretz. Dulong introduced different mammiferous animals, carni- vorous as well as herbivorous, into a receiver, in which the changes produced in the air by respiration, and the volume of the different products, could be determined at the same time that the amount of heat lost by the animal could be ascertained. His experiments led him to conclude, among other points, that supposing all the oxygen, absorbed into the blood from the air in the lungs, were combined with carbon and hydrogen in the system, and that as much heat were thus generated as would be developed during the quick com- bustion of equal quantities of oxygen and«arbon, and of oxygen and hydrogen, still, the whole quantity of heat produced would amount to only from 3-4ths to 4-5ths of that which is developed during the same space of time by carnivorous as well as herbivorous animals. Despretz placed animals in a vessel surrounded with water; an uninterrupted current of air to and from the vessel was maintained, and the volume and composition of the air employed were ascer- tained both before and after the experiment (which was continued 1J or two hours), as well as the increase in the temperature of the surrounding water during it: by this means it was found that the heat which should have been generated, according to the chemi- cal theory of respiration, would account for from 0.76 to 0.91 only of that which the animals really gave out during the same time. The failure of these experiments to account for all the heat pro- duced, threw doubts on the chemical theory of animal heat (as the proposed explanation has been called), till Liebig lately showed that Dulong and Despretz were in error in their conclusions, from having formed too low an estimate of the heat produced in the combustion of carbon and hydrogen. On repeating their experi- ments, and using the more accurate numbers to represent these com- 1 Some heat will also be generated in the combination of sulphur and phosphorus with oxygen, to which reference has been made (p. 130); but the amount thus produced has not been estimated, and need not be con- sidered in the exposition of a theory which can, at present, be stated in only the most general terms. 13* 150 ANIMAL HEAT. bustion-heats, Liebig finds reason to believe that the quantity of heat which would be generated, by the union of the oxygen absorbed into the blood from the atmosphere with the carbon and hydrogen taken into the system as food, is sufficient to account for the whole of the caloric formed in the animal body.1 Many things observed in the economy and habits of animals are explicable by this theory, and are, therefore, evidence for its truth. Thus, as a general rule, in the various classes of animals, as well as in individual examples of each class, the quantity of heat gene- rated in the body is in direct proportion to the activity of the respi- ratory process. The highest animal temperature, for example, is found in birds, in-whom the function of respiration is most actively performed. In Mammalia, the process of respiration is less active, and the average temperature of the body less, than in birds. In reptiles, both the respiration and the heat are at a much lower standard; whilst in animals below them, in which the function of respiration is at the lowest point, a power of producing heat is, in ordinary circumstances, hardly discernible. Among these lower animals, however, the observations of Mr. Newport (xliii. 1837) supply confirmatory evidence. He shows that the larva, in which the respiratory organs are smaller in comparison with the size of the body, has a lower temperature than the perfect insect. Volant insects have the highest temperature, and they have always the largest respiratory organs and breathe the greatest quantity of air: while among terrestrial insects, those also produce the most heat which have the largest respiratory organs and breathe the most air. During sleep, hybernation, and other states of inaction, respiration is slower or suspended, and the temperature is proportionally diminished; while on the other hand, when the insect is most active and respiring most voluminously, its amount of temperature is at its maximum, and corresponds with the quantity of respiration. Neither the rapidity of the circulation nor the size of the nervous system, according to Mr. Newport, presents such a constant rela- tion to the evolution of heat. Similar evidence in favor of this theory of animal heat is fur- nished by the fact that heat is sometimes evolved by plants, in a quantity which appears to be in direct proportion to the amount of oxygen they at the same time absorb and convert into carbonic acid. For example, their evolution of heat is most evident during flower- ing and the germination of seeds, the times at which the largest amount of carbonic acid is exhaled. The quantity and quality of food consumed by man and animals in the different climates and seasons, also appear to be adapted to 1 Liebig's estimates and calculations maybe referred to in the Lancet (Feb. 1845). ANIMAL HEAT. 151 the production of various amounts of heat by the combination of carbon and hydrogen with oxygen. In northern regions, for ex- ample, and in the colder seasons of more southern climes, the quantity of food consumed is (speaking very generally) greater than is consumed by the same men or animals in opposite conditions of climate and seasons. And the food which appears naturally adapted to the inhabitants of the coldest climates, such as the se- veral fatty and oily substances, abounds in carbon and hydrogen, and is fitted to combine with the large quantities of oxygen which, breathing cold dense air, they absorb from their lungs. The influence of the nervous system in modifying the production of heat has been already referred to. The experiments and observ- ations which best illustrate it are those showing, first, that when the supply of nervous influence to a part is cut off, the temperature of that part falls below its ordinary degree; and, secondly, that when death is caused by severe injury to or removal of the nervous centres, the temperature of the body rapidly falls, even though artificial respiration be performed, the circulation. maintained, and to all appearance the ordinary chemical changes of the body be com- pletely effected. It has been repeatedly noticed that, after division of the nerves of a limb, its temperature falls r and this diminution of heat has been remarked still more plainly in limbs deprived of nervous influence by paralysis. For example, Mr. Earle (xli. vol. vii. p. 173) found the temperature of^the hand of a paralyzed arm to be 70°, while that of the sound side had a temperature of 92° F. On electrifying the paralyzed limb, the temperature rose to 77°. In •. another case, the temperature of the paralyzed finger was 56° F., while that of the unaffected hand was 62°. Sir B. C. Brodie (xliii. 1811 and 1812) found, that if artificial respiration was kept up in animals killed by decapitation, division of the medulla oblongata, destruction of the brain, or poisoning with Woorara poison, the action of the heart continued, and the blood underwent the usual changes in the lungs, as shown by the analysis of the air respired, but that the heat of the body was not maintained : on the contrary, being cooled by the air forced into the lungs, it became cold more rapidly than the body of an animal in which artificial respiration was not kept up. With equal certainty, though less definitely, the influence of the nervous system on the production of heat is shown in the rapid and momentary increase of temperature, sometimes general, at other times quite local, which is observed in states of nervous excitement; in the general increase of warmth of the body, sometimes amounting to perspiration, which is excited by passions of the mind ; in the sudden rush of heat to the face, which is not a mere sensation; and in the equally rapid diminution of temperature in the depressing passions. But none of these instances suffices to prove that heat is 152 ANIMAL HEAT. generated by mere nervous action, independent of any chemical change; all are as well explicable on the supposition that the influ- ence of the nervous system alters in some way, the chemical pro- cesses from which the heat is commonly generated. There are ample proofs that the nervous system, especially in the most highly organized animals, does so modify all the functions of organic life; and it appears more reasonable to suppose that it thus influences the production of heat, than to ascribe it to any more direct agency. [According to the recent experiments of M. CI. Bernard,1 it ap- pears that an elevation of temperature constantly takes place on one side of the face, when the trunk which unites the Sympathetic ganglia of the neck on that side is cut through; this increase being not only perceptible to the touch, but showing itself by a thermo- meter introduced into the nostrils or ears, even to the extent of from 7° to 11° Fahr. When the superior cervical ganglion is re- moved, the same effect is produced, but with yet greater intensity. This difference is maintained for many months, and is not con- nected with the occurrence of inflammation, congestion, oedema, or any other pathological change in the part; moreover, it is not pre- vented from manifesting itself by the division of any of the cerebro- spinal nerves of the face. It is remarkable that the sensibility of the parts thus affected should be no less augmented than their temperature. Dr. Brown-Sequard has observed the same remarkable pheno- mena as those detailed by M. CI. Bernard, but he regards them as mere results of the paralysis, and of the consequent dilatation of the bloodvessels. In consequence of this dilatation, the blood reaches the part supplied by the nerve in larger quantities; the nutrition is therefore more active. The increased sensibility is a result of the augmented vital properties of the nerves when their nutrition is increased. Dr. Brown-Sequard has likewise noticed the increase of temperature of the ear over that of the rectum, to the amount of one or two degrees Fahr.; but it must be remem- bered that the temperature of the rectum is a little lower than that of the blood, and as the ear is gorged with that fluid, it is easy to understand why it should possess its temperature. Many facts prove that the degree of temperature and sensibility in a part are in direct ratio with the amount of blood circulating in it. If galvanism be applied to the superior portion of the sympathetic nerve after it has been cut in the neck, the vessels of the face and ear, after a short time, begin to contract, and subsequently resume their normal condition, if they do not even diminish. Cuineidently with this diminution, there is a decrease of the temperature and sensibility of the face and ear, until the palsied and sound side are alike in this respect. 1 Gazette Medicale, Fevr. 21, 1852. ANIMAL HEAT. 153 When the galvanic current ceases to act, the vessels again dilate, and all the phenomena discovered by M. Bernard reappear. It hence appears that the only direct effect of section of the cervical portion of the sympathetic is the paralysis and consequent dilatation of the bloodvessels. Another deduction from these experiments is, that the sympathetic sends motor fibres to many of the bloodvessels of the head.1] In the foregoing pages, the illustrations of the power of main- taining an uniform temperature have had reference to the ordinary case of man living in a medium colder than his body, and therefore losing heat both by radiation and evaporation. The losses in these two ways will bear, in general, an inverse proportion to one another; the small loss of heat in evaporation in cold climates may go far to compensate for the greater loss by radiation; as, on the other hand, the great amount of fluid evaporated in hot air may remove nearly as much heat as is commonly lost by both radiation and evaporation in ordinary temperatures. Thus, it is possible that the quantities of heat required for the maintenance of an uniform proper temperature in various climates and seasons are not so dif- ferent as they may, at first thought, seem : but on these points no accurate information has been yet obtained.2 Neither, as to the maintenance of the temperature of the body in hot air is more known than that great heat can for a time be borne with little change in the proper temperature of the body, provided the air be dry. Sir Charles Blagden and others supported a temperature varying between 198° and 211° F. in dry air for several minutes; and in a subsequent experiment he remained eight minutes in a temperature of 260°. Delaroche and Berger (exxxii.) observed that the temperature of rabbits was raised only a few de- grees when they were exposed to heat varying from 122° to 194°. But such heats are not tolerable when the air is moist as well as hot, so as to prevent evaporation from the body. M. C. James (xix. April, 1844) states that, in the vapor baths of Nero, he was almost suffocated in a temperature of 112°, while in the caves of Testaccio, in which the air is dry, he was but little discomfited by a temperature of 176°. In the former, evaporation from the skin was impossible; in the latter, it was probably abundant, and the layer of vapor which would rise from all the surface of the body 1 Vide Phil. Med. Exam., N. S., vol. viii. No. viii. August, 1852. 2 Vierordt has made estimates of the heat given out, per minute, from the lungs in warming the inspired air, and in combination with the evapo- rated water; it would be enough to heat (at the most) 90.84 grains of water from 32° to 2123 (ex. p. 23G). At this rate, the loss by evaporation from the skin and lungs together may be roughly estimated at enough to heat nearly 4000 grains of water from 32° to 212°. 154 ANIMAL HEAT. would, by its very slowly conducting power, defend it for a time from the full action of the external heat. It remains to notice certain conditions by which the production of heat is modified. The effects of age are noticeable. M. Edwards found the power of generating heat to be less in old people ; and the same was ob- served by Dr. Davy (xliii. 1844); who, in eight people, between eighty-seven and ninety-five years old, found that, although the average temperature of the body was not lower than that of younger persons, yet the power of resisting cold was less in them—exposure to a low temperature causing a greater reduction of heat than in young persons. The same rapid diminution of temperature was -observed by M. Edwards in the new-born young of most carnivorous and rodent animals when they were removed from the parent, the temperature of the atmosphere being between 50° and 53 J° F.; whereas, while lying close to the body of the mother, their temperature was only 2 or 3 degrees lower than hers. The same law applies to the young of birds. Young sparrows, a week after they were hatched, had a temperature of 95° to 97°, while in the nest; but when taken from it, their temperature fell in one hour to 66 J°, the temperature of the atmosphere being at the time 62 i°. It appears from his investiga- tions that, in respect of the power of generating heat, some Mamma- lia are born in a less developed condition than others ; and that the young of dogs, cats, and rabbits, for example, are inferior to the young of those animals which are not born blind. The need of external warmth to keep up the temperature of new-born children is well known; the researches of M. Edwards show that the want of it is, as Hunter suggested, a much more frequent cause of death in new-born children than is generally supposed, and furnish a strong argument against the idea that children, by early exposure to cold, can soon be hardened into resisting its injurious influence. Active exercise, as already stated, raises the temperature of the body. This may be partly ascribed to the fact that every muscu- lar contraction is attended by the development of one or two de- grees of heat in the acting muscle ; and that the heat is increased according to the number and rapidity of these contractions, and may be quickly diffused by the blood circulating from the heated muscles. Possibly, also, some heat may be generated in the various movements, stretchings, and recoilings of the other tissues, as the arteries, whose elastic walls, alternately dilated and contracted, may give out some heat, just as caoutchouc, alternately stretched and recoiling, becomes hot. But the heat thus developed cannot be so much as some have supposed. (Winn, xvii. Ser. 3, vol. xiv. p. 174; Winter, xxx. 1843, p. 794.) DIGESTION. 155 The influence of external coverings for the body must not be un- noticed. In warm-blooded animals they are always adapted, among many purposes, to the maintenance of uniform temperature; and man adapts for himself such as are, for the same purpose, fitted to the various climates to which he is exposed. By their means, and by his command over food and fire, per- haps as much as by his capacity of developing appropriate amounts of heat, he maintains his temperature on all accessible parts of the surface of the earth. CHAPTER VIII. DIGESTION. Digestion is the process by which those parts of our food which may be employed in the formation and repair of the tissues, or in the production of heat, are made fit to be absorbed and added to the blood. Food may be considered in its relation to the two purposes above mentioned; and the various articles of food may be artificially classified according as they are chiefly subservient to one or the other of these purposes. All articles of food that are to be em- ployed in the production of heat, must contain a larger proportion of carbon and hydrogen than is sufficient to form water with the oxygen that they contain ; and none are appropriate for the main- tenance of any tissues (except the adipose) unless they contain ni- trogen, and are capable of conversion into the nitrogenous principles of the blood. The name of nutritive or plastic is given to those principles of food which admit of conversion into the albumen or fibrine of the blood, and of being subsequently assimilated, through the medium of the blood, by the tissues. And those principles, comprising the greater part of the non-nitrogenous materials of food, in the form of fat, starch, sugar, gum, and other similar substances, which are believed to be employed in the production of heat, are named ca- lorifacient, or sometimes respiratory food. An easier division of foods than this, according to their destina- tion, is derived from their origins; for all consist of either animal or vegetable substances. No substance can afford nutriment, even though it contain all the elements of organic bodies, unless it have all the natural peculiarities of organic composition, and contain, incorporated with its other elements, some of those derived from the mineral kingdom, which, as incidental elements (p. 14), are 156 digestion. found in the organized tissues; such as sulphur, iron, lime, mag- nesia, &c. Man is supported as well by food constituted wholly of animal substances, as by that which is formed entirely of vegetable mat- ters ; and the structure of his teeth, as well as experience, seems to point out that he is destined for a mixed kind of aliment. In the case of carnivorous animals, the food upon which they exist, con- sisting as it does of the flesh and blood of other animals, not only contains all the elements of which their own blood and tissues are composed, but contains them combined, probably, in the same forms. Therefore, little more may seem requisite, in the prepara- tion of this kind of food for the nutrition of the body, than that it should be dissolved and conveyed into the blood in a condition capable of being reorganized. But in the case of the herbivorous animals, which feed exclusively upon vegetable substances, it might seem as if there would be greater difficulty in procuring food capa- ble of assimilation into their blood and tissues. But the chief or- dinary articles of vegetable food contain substances identical, in composition, with the albumen, fibrine, and caseine, which consti- tute the principal nutritive materials in animal food. Albumen is abundant in the juices and seeds of nearly all vegetables ; the gluten which exists, especially in corn, and other seeds of grasses, as well as in their juices, is identical in composition with fibrine, and is now commonly named vegetable fibrine; and the substance named legumin, which is obtained especially from peas, beans, and other seeds of leguminous plants, and from the potato, is identi- cal with the caseine of milk. All these vegetable substances are, equally with the corresponding animal principles, and in the same manner, capable of conversion into blood and tissues ; and, like the blood and tissues in both classes of animals, the nitrogenous food of both may be regarded as in all essential respects similar. An apparently more considerable difference between animal and vegetable food consists in the different kind, and proportionately larger quantity, of the non-nitrogenous principles contained in the latter. The only non-nitrogenous organic substances in animal food are furnished by the fat, and, in some instances, by the vegetable matters that may chance to be in the digestive canals of such ani- mals as are eaten whole. The amount of these is far less than that of the non-nitrogenous substances consumed by herbivorous animals, in their quantities of starch, sugar, gum, oil, and other ternary com- pounds. Yet, that the final destination of the ternary principles is the same in both classes, is almost proved by the ability of man and many other animals to subsist, and, apparently, to maintain an identical composition and an uniform temperature, with food of either kind. Again, the several alimentary substances, from both animal and DIGESTION. 157 vegetable substances, may be arranged, according to the system of Dr. Prout, in three classes, under the names of albuminous, sac- charine, and oleaginous principles. In the albuminous group, are included all the nitrogenous principles, whether derived from the animal or from the vegetable kingdom. These comprise albumen, fibrine, caseine, gelatine, and chondrine; the two latter substances being classed under this head on account of their bearing a closer resemblance to the albuminous than to any other principles of food. The saccharine group comprises substances derived exclusively from the vegetable kingdom, viz., sugar itself, and the various principles capable of being converted into it, as starch, gum, pectine, and lig- nine, or woody fibre: all of which are composed of carbon, hydrogen, and oxygen, with the two latter in the proportion in which they form water. The oleaginous group includes the various kinds of fatty and oily principles, which occur abundantly in both the animal and vegetable kingdoms. All are composed principally of carbon and hydrogen: the quantity of the former element usually exceeding that of the latter; and both being more than sufficient to form water with the oxygen they contain. Besides these three principal divi- sions, Dr. Prout makes a fourth division for the aqueous part of food. For, besides that water constitutes nearly four-fifths of the total weight of the animal body, and must, therefore, enter largely into the composition of food, it is highly probable that it plays an im- portant part in the various transformations undergone in the system, and thus contributes materially to the nutrition of the different textures. It has been already said that animals cannot subsist on any but organic substances, and that these must contain the incidental ele- ments and compounds which are naturally combined with them; in other words, not even organic compounds are nutritive unless they are supplied in their natural state. The most singular instance of this fact is, perhaps, that of the production of scurvy by the want of vegetable food, and its cure by giving vegetables; which, how- ever, must be either raw, or simply preserved, or so cooked that their saline constituents may not be removed from them. Pure fibrine, pure gelatine, and other principles purified from the sub- stances naturally mingled with them, are incapable of supporting life for more than a brief time. Moreover, health cannot be maintained by any number of sub- stances derived exclusively from one of the three groups of ali- mentary principles. A mixture of nitrogenous and non-nitrogenous substances, together with the inorgauic principles which are sever- ally contained in them, is essential to the well-being, and, generally, even to the existence of an animal. The truth of this is demon- strated by experiments performed for the purpose, and is illustrated by the composition of the food prepared by nature as the exclusive 14 158 DIGESTION. source of nourishment to the young of Mammalia, namely, milk. In milk, the albuminous group of aliments is represented by the caseine, the oleaginous by the butter, the aqueous by the water, the saccharine by the sugar of milk.1 Milk, likewise, contains phosphate of lime, alkaline and other salts, and a trace of iron; so that it may be briefly said to include all the substances which the tissues of the growing animal need for their nutrition, and which are required for the production of animal heat. The yelk and albumen of eggs are in the same relation, as food for the embryos of oviparous animals, as milk is to the young of Mammalia, and afford another example of mixed food being provided as the most perfect for nutrition. The experiments illustrating the same principle have been chiefly performed by Magendie (cxxxiii.). Dogs were fed exclusively on sugar and distilled water. During the first seven or eight days they were brisk and active, and took their food and drink as usual; but in the course of the second week they began to get thin, although their appetite continued good, and they took daily between six and eight ounces of sugar. The emaciation increased during the third week, and they became feeble, and lost their activity and appetite. At the same time an ulcer formed on each cornea, followed by an escape of the humors of the eye; this took place in repeated ex- periments. The animals still continued to eat three or four ounces of sugar daily; but became at length so feeble as to be incapable of motion, and died on a day varying from the 31st to the 34th. On dissection, their bodies presented all the appearances produced by death from starvation ; indeed, dogs will live almost the same length of time without any food at all. When dogs were fed exclusively on gum, results almost similar to the above ensued. When they were kept on olive-oil and water, all the phenomena produced were the same, except that no ulcera- tion of the cornea took place : the effects were also the same with butter. Tiedemann and Gmelin obtained very similar results. They fed different geese, one with sugar and water, another with gum and water, and a third with starch and water. All gradually lost weight. The one fed with gum died on the sixteenth day; that fed with sugar on the twenty-second; the third, which was fed with starch, on the twenty-fourth, and another on the twenty- seventh day; having lost, during these periods, from one-sixth to one-half of their weight. The experiments of Chossat (xix. Oct. 1843) and Letellier (xii. 1844) prove the same; and in men, the 1 At least it is so in the milk of herbivorous animals; but, according to Dumas (xix. Oct. 1845), sugar does not exist in the milk of Carnivora, except when some saccharine or farinaceous principle is mixed with their food; its place in their natural milk is filled, as it is in their food, by the fatty matter. DIGESTION. 159 same is shown by the various diseases to whicL they who consume but little nitrogenous food are liable, and especially, as Dr. Budd has shown, by the affection of the cornea which is observed in Hindoos feeding almost exclusively on rice. But it is not only the non-nitrogenous substances, which, taken alone, are insufficient for the maintenance of health. The experiments of the Academies of France and Amsterdam were equally conclusive that gelatine alone soon ceases to be nutritive (xxv. 1843-4, p. 35). These facts prove the necessity of a mixture of elementary principles in the food; and, beyond this, Magendie's further ex- periments appear to prove that animals cannot live long if fed exclusively on any single article of food (except milk), even al- though it contains principles belonging to each of the three groups of alimentary substances. For example (to mention only some of his results), a dog fed on white bread, wheat, and water, did not live more than fifty days; rabbits and guinea-pigs fed on any one of the following substances—wheat, oats, barley, cabbage, or car- rots—died with all the signs of inanition in fifteen days; while, if the same substances were given simultaneously, or in succession, the animals suffered no ill effect. Changes of the Food effected in the Mouth. The first of the series of changes to which the food is subjected in the digestive canal takes place in the cavity of the mouth; the solid articles of food are here submitted to the action of the teeth, whereby they are divided and crushed, and by being at the same time mixed with the fluids of the mouth, are reduced to a soft pulp capable of being easily swallowed. The fluids with which the food is mixed in the mouth consist of the secretion of the salivary glands and the mucus secreted by the lining membrane of the whole buc- cal cavity. The glands concerned in the production of saliva are very exten- sive, and, in man and Mammalia generally, are presented in the form of four pairs of large glands, the parotid, submaxillary, sub- lingual, and intralingual, and numerous smaller bodies, of similar structure and with separate ducts, which are scattered thickly be- neath the mucous membrane of the lips, cheeks, soft palate, and root of the tongue. These all have the structure common to what are termed conglomerate glands, which will be spoken of in the chapter on Secretion. Saliva, as it commonly flows from the mouth, is mixed with the secretion of the mucous membrane, and often with air-bubbles, which, being retained by its viscidity, make it frothy. When obtained from the parotid ducts, and free from mucus, saliva is a transparent watery fluid, the specific gravity of which varies from 1.006 to 1.009, and in which, when examined with 160 DIGESTION. the microscope, are found floating a number of minute particles, derived from the secreting ducts and vesicles of the glands. In the impure or mixed saliva are found, besides these particles, numerous epithelial scales separated from the surface of the mu- cous membrane of the mouth and tongue, and mucous corpuscles, discharged for the most part from the tonsils, which, when the saliva is collected in a deep vessel, and left at rest, subside in the form of a white opaque matter, leaving the supernatant salivary fluid transparent and colorless, or with a pale bluish-gray tint. In reaction, the saliva, when first secreted, appears to be always alkaline; and that from the parotid gland is said to be more strong- ly alkaline than that from the other salivary glands. This alkaline condition is most evident when digestion is going on, and, accord- ing to Dr. Wright (xxx. 1842-3), the degree of alkalinity of the saliva bears a direct proportion to the acidity of the gastric fluid secreted at the same time. During fasting, the saliva, although secreted alkaline, shortly becomes acid; and it does so especially when secreted slowly, and allowed to mix with the acid mucus of the mouth, by which its alkaline reaction is destroyed. According to Dr. Wright (xxx. March, 1842), whose analysis does not materially differ from the more recent analyses of Fre- richs (lix. 1850, p. 136), and others, the composition of saliva is— Water .... 988.1 Mucus ... 2.G Ptyaline . . .1.8 Ashes . . . .4.1 Fatty matter . . .5 Loss . . . .1.2 Albumen (v:ith soda) . 1.7 1000.0 Ptyaline is the name given to a supposed peculiar animal matter, which is insoluble in alcohol. By Mialhe, it is stated to be closely analogous to the vegetable substance, termed diastase; but it is more commonly regarded as belonging to the ill-defined class of extractive matters. The ashes of saliva have been analyzed by Enderlin (x. 1844), who found that they consist of substances very similar to those in the ashes of blood, and believes that the alkalinity of the saliva, like that of the blood, is due to the tribasic phosphate of soda. The other salts which he found in it were chlorides of sodium and potassium, sulphate of soda, and phosphates of lime, magnesia, and of iron. Saliva is also said to contain a small quantity of sul- pho-cyanogen: the presence of which is indicated by a deep red color when saliva is mixed with a neutral solution of a salt of the peroxide of iron. See, on the question, Pettenkofer (lix. 1846, p. 115) and Strahl (lix. 1847, p. 100). The tartar which collects on the human teeth consists almost entirely of the earthy phosphates, combined with about 19 per cent, of animal matter, and containing shells of infusoria, and other accidental mixtures. The rate at which saliva is secreted is subject to considerable DIGESTION. 161 variation. When the tongue and muscles concerned in mastication are at rest, and the nerves of the mouth are subject to no unusual stimulus, the quantity secreted is not more than sufficient, with the mucus, to keep the mouth moist. But the flow is much accelerated when the movements of mastication take place, and especially when they are combined with the presence of food in the mouth. It may be excited, also, even when the mouth is at rest, by the mental im- pressions produced by the sight or thought of food. Under these varying circumstances, the quantity of saliva secreted in twenty- four hours varies also; its average amount probably ranges from fifteen to twenty ounces. In a man who had a fistulous opening of the parotid duct, Mitscherlich found that the quantity of saliva discharged from it during twenty-four hours, was from two to three ounces; and the saliva collected from the mouth during the same period, and derived from the other salivary glands, amounted to six times more than that from the one parotid. The purposes served by saliva are of several kinds. In the first place, acting mechanically, it keeps the mouth in a due condition of moisture, facilitating the movements of the tongue in speaking, and the mastication of food. Thus also it serves in dissolving sapid substances, and rendering them capable of exciting the nerves of taste. But the principal mechanical purpose of the saliva is that, by mixing with the food during mastication, it makes it a soft pulpy mass, such as may be easily swallowed. To this purpose the saliva is adapted both by quantity and quality. For, speaking generally, the quantity secreted during feeding is in direct proportion to the dryness and hardness of the food : as M. Lassaigne has shown, by a table of the quantity produced in the mastication of a hundred parts of each of several kinds of food : thirty parts suffice for a hun- dred partsof crumb of bread, butnot less than 120 for the crusts; 42.5 parts of saliva are produced for the hundred of roast meat; 3.7 for as much of apples; and so on, according to the general rule above stated. The quality of saliva is equally adapted to this end. It is easy to see how much more readily it mixes with most kinds of food than water alone does ; and M. Bernard believes, from his experiments, that the saliva from the parotid, labial, and other small glands, being more aqueous than the rest, is that which is chiefly braided and mixed with the food in mastication ; while the more viscid mucoid secretion of the submaxillary, palatine, and tonsillitic glands, is spread over the surface of the softened mass to enable it to slide more easily through the fauces and oesophagus. Beyond these, its mechanical purposes, there are reasons for supposing that saliva performs some chemical part in the digestion of the food. The chief of these reasons are, the nuinber and siz.o of the glxnds engaged in the secretion ; the variety of substances 14* 162 DIGESTION. which enter into its composition, and which can scarcely be sup- posed to be of use so far as its mechanical properties are concerned; the quantity which is secreted, not only during mastication, but after the food has passed into the stomach, especially in old persons, who, from their loss of teeth, frequently swallow their food in an imperfectly masticated state; the fact that the saliva secreted during digestion is more alkaline than at other times; and, lastly, the re- sults of certain experiments. Among the experiments are those of Spallanzani and Reaumur, who found that food inclosed in perforated tubes, and introduced into the stomach of an animal, was more quickly digested when it had been previously impregnated with saliva than when it was moistened with water. Dr. Wright, also, found that if the oeso- phagus of a dog is tied, and food mixed with water alone is placed in the stomach, the food will remain undigested though the stomach may secrete abundant acid fluid; but if the same food were mixed with saliva, and the rest of the experiment similarly performed, the food was readily digested. But although it may hence appear that the saliva has more than a mechanical influence in promoting digestion, yet the nature of the chemical part it takes is uncertain. Its composition, as traced by chemical analysis, offers no guide. Its alkalinity, though, as already stated, it appears to increase in the same proportion as the acidity of the secretion of the stomach both in health and disease, is never sufficient to neutralize the gastric fluid ; the contents of the stomach, including as they do the saliva swallowed, are always acid. The very short time during which the saliva remains in contact with the food before it is neutralized by the acid of the stomach, precludes the notion that the alkali is the principal constituent by which it assists in digestion. Its organic principles may, however, have more power ; for numerous experiments, easily repeated, show that when saliva or a portion of salivary gland is added to starch- paste, the starch is quickly transformed into dextrine, and grape- sugar ; and when common raw starch is masticated and mingled with saliva, and kept with it at a temperature of 90° or 100°, the starch-grains are cracked or eroded, and their contents are trans- formed in the same manner as the starch-paste.1 Changes similar to these are effected on the starch of farinaceous food (especially after cooking) in the stomach ; and it is reasonable to refer them to the action of the saliva, because the acid of the gastric fluid tends to retard or prevent, rather than favor, the transformation of 1 See on these points Leuchs (xxxii. p. 577), Mialhe (xii. 1845), Wright (xxx. 1812-8), Lehmann (xiv. 1843,) a report of the Academy of Sciences, translated in the Medical Gazettp, vol. xxxvii. p. 788, and Valentin's sum- mary in Canstatt's Jahresberichte for 1850, p. 131, DIGESTION. 163 the starch. It may therefore be held that a purpose served by the saliva in the digestive process is that of assisting in the transform- ation of the starch, which enters so largely into the composition of most articles of vegetable food, and which (being naturally in- soluble) is converted into the soluble dextrine or grape-sugar, and made fit for absorption. It appears from the experiments of Magendie (xviii. July, 1S46) and Bernard (lix. 1847, p. 117) that many substances besides saliva may excite this transformation of starch, such as pieces of the mucous membrane of the mouth, bladder, rectum, and other parts, various animal and vegetable tissues, and even morbid pro- ducts ; but the gastric fluid will not produce the same effect. The property, therefore, cannot be assigned to any peculiar organic principle in the saliva. The part of the saliva which appears most active is that secreted by the small glands and the mucous mem- brane of the mouth. (Bernard, 1. c. Magendie, cxc. Oct. 1839, p. 546; see also Frerichs, lix. 1850, p. 134.) If the influence of saliva in assisting the digestion of farinaceous food be admitted, and the tendency of all the recent observations on the subject is in favor of this view, we have yet to seek for the corresponding pur- pose served by the saliva of Carnivora who consume no such food ; for on this point there is at present but little information. It is, indeed, unknown whether saliva exercises any kind of digestive power over animal substances; no such power has been clearly shown by any of the numerous experiments intended to display it.1 PASSAGE OP FOOD INTO THE STOMACH. When properly masticated, the food is transmitted in successive portions to the stomach by the act of deglutition or swallowing. This act, for the purpose of description, may be divided into three parts. In the first, particles of food collected to a morsel glide between the surface of the tongue and the palatine arch, till they have passed the anterior arch of the fauces; in the second, the morsel is carried through the pharynx; and in the third, it reaches the stomach through the oesophagus. These three acts follow each other rapidly. The first is performed voluntarily by the muscles of the tongue and cheeks. The second also is effected with the aid of muscles which are in part endued with voluntary motion, such 1 On the action of Saliva, as well as on other points connected with the process of digestion, the student will find much valuable information in the instructive lectures by Dr. Bence Jones, now publishing in the Medical Times. [See also Amer. Journ. of Med. Sciences, Oct. 1851, for a resume" of M. CI. Bernard's experiments. Also Carpenter's Human Physiology, pp. 027, 8, et seq.~\ 164 DIGESTION. as the muscles of the soft palate and pharynx ; but it is, nevertheless, an involuntary act, and takes place without our being able to prevent it, as soon as a morsel of food, drink, or saliva is carried backwards to a certain point of the tongue's surface. When we appear to swallow voluntarily, we only convey, through the first act of deglutition, a portion of food or saliva beyond the anterior arch of the palate; then, the substance acts as a stimulus, which, in accord- ance with the laws of reflex movements hereafter to be described, is carried by the sensitive nerves to the medulla oblongata, where it is reflected to the motor nerves, and an involuntary adapted action of the muscles of the palate and pharynx ensues. The third act of deglutition takes place in the oesophagus, the muscular fibres of which are entirely beyond the influence of the will. The second act of deglutition is the most complicated because the food must pass by the posterior orifice of the nose and the rima glottidis of the larynx, without touching them. When it has been brought, by the first act, behind the anterior arches of the palate, it is moved onwards by the tongue being carried backwards, and by the muscles of the anterior arches contracting behind it. The root of the tongue being retracted, and the larynx being raised with the pharynx and carried forwards under the tongue, the epiglottis is pressed over the rima glottidis, and the morsel glides past it; the closure of the glottis being additionally secured by the simultaneous contraction of its own muscles, so that even when the epiglottis is destroyed there is little danger of food or drink passing into the larynx, so long as its muscles can act freely. At the same time, the approximation of the sides of the posterior palatine arch, which move quickly inwards like side-curtains, closes the passage into the upper part of the pharynx and the posterior nares, and forms an inclined plane, along the under surface of which the morsel descends; when the pharynx, raised up to receive it, in its turn contracts, and forces it onwards into the oesophagus. In the third act, in which the food passes through the oesophagus, every part of that tube, as it receives the morsel, and is dilated by it, is stimulated to contract; hence an undulatory contraction of the oesophagus, which is easily observable in horses while drinking, proceeds rapidly along the tube. It is only when the morsels swallowed are large, or taken too quickly in succession, that the progressive contraction of the oesophagus is slow, and attended with pain. Besides the actions ensuing in the oesophagus during the passage of food, certain rhythmic contractions have been observed at its lower part, independently of deglutition. They are produced by the fibres near the cardiac orifice of the stomach, which fibres are usually in a state of contraction, especially when the stomach is DIGESTION. 165 full, and appear to act as a kind of sphincter to prevent the regurgitation of food. During vomiting they are relaxed; and at the same time, the whole muscular tissue of the tube is said to perform an anti-peristaltic motion, the reverse of that which it executes during deglutition. When vomiting has been produced by the injection of tartar emetic into the veins, these anti-peristaltic motions of the oesophagus are said to be continued, even though the tube is separated from the stomach. DIGESTION OF FOOD IN THE STOMACH. Structure of the Stomach. It appears to be an universal character of animals, that they have an internal cavity for the production of a chemical change in the aliment—a cavity for digestion : and when this cavity is com- pound, the part in which the food undergoes its principal and most important changes in the stomach. In man, and those Mammalia which are provided with a single stomach, its walls consist of three distinct layers, or coats, viz. : an external peritoneal, an internal mucous, and an intermediate mus- cular coat, with bloodvessels, lymphatics, and nerves distributed in and between them. In relation to the physiology of the stomach in digestion, only the muscular and mucous coats need be con- sidered. The muscular coat of the stomach consists of three separate layers, or sets of fibres, which, according to their several directions, are named the longitudinal, circular, and oblique. The longitu- dinal set are the most superficial; they are continuous with the longitudinal fibres of the oesophagus, and spread out in a diverging man- ner over the great end and sides of the stomach. They extend as far as the pylorus, being espe- cially distinct at the lesser or upper curvature of the stomach, along which they pass in seve- ral strong bands. The next set are the circu- lar or transverse fibres, which more or less completely encircle all parts of the stomach; they are most abundant at the middle, and in the pyloric portion of the organ, and some form the chief part of the thick projecting ring thc stomachf fat: but, in the more superficial parts they are exceedingly small or entirely obliterated. The papillae are minute conical or cylindriform elevations, which are more prominent and more densely set at some parts, as the palmar surface of the hands and fingers, than at others (Fig. 55). On these parts, and on the plantar surface of the feet- and toes, they are disposed in double rows, in parallel curved lines separated from each other by depressions. Thus they may be seen easily on the palm; whereon each raised line is composed of a double row of papillae, and is intersected by short transverse lines or furrows corresponding with the interspaces between the successive pairs of the papillae. In the middle of each of these transverse furrows, and irregu- larly scattered between the bases of the papillae in other parts of the surface of the body, are the orifices of ducts of the sudoripa- rous glands, by which it is probable that a large portion of the aqueous and gaseous materials excreted by the skin are separated. Each of these glands consists of a small lobular mass, which appears formed of a coil of tubular gland-duct, surrounded by blood- vessels and imbedded in the subcutaneous adipose tissue (Fig. 55). From this mass the duct ascends, for a short distance, in a spiral manner through the deeper part of the cutis; then passing straight, and then sometimes again becoming spiral, it passes through the 22* 258 THE SKIN. cuticle and opens by an oblique valve-like aperture. In the parts where the epidermis is thin, the ducts themselves are thinner and more nearly straight in their course. The canal of theduct, which maintains nearly the same diameter throughout, is lined with a Fig. 55. A perpendicular section of the skin of the sole of the foot, showing : 1. Tlie salient lines of the external surface of the skin cut perpendicularly. 2. The furrows or wrinkles of the same. 0. The epidermis or cuticle, as formed by its superimposed layers. 4. The rete mucosum. 5. The cutis vera, with its cellular fibres pressed into fasciculi and each directed towards the papillae. 6. The papillae, each of which answers to the prominences on the ex- ternal surface of the skin. 7. The small furrows between the papillae. 8. The deeper furrows which are between each couple of the papillae. 9. Colls filled with fat, and seen between the bands of fibres. 10. The adipose layer with numerous fat vesicles. 11. Cellu- lar fibres of the adipose tissue, continuous with the sub-cutaneous cellular tissue, and with that of the cutis vera. 12. The sudoriferous follicles. 13. The spiral or sudoriferous canals. 1-1. The infundibular shaped pores or orifices of the&e canals. layer of epithelium continuous with the epidermis; and its walls are formed of pellucid membrane continuous with the surface of the cutis. The sudoriparous glands are abundantly distributed over the whole surface of the body, but are especially numerous, as well as very large, in the skin of the palm of the hand, where, according to Krause, they amount to 2736 in each superficial square inch (xv. article Haul), and according to Mr. Erasmus Wilson to as many as 3528 (xxxv.). They are almost equally abundant and large in the skin of the sole. The glands by which the peculiar odorous THE SKIN. 259 am- matter of the axillre is secreted form a nearly complete layer under the cutis, and are like the ordinary sudoripa- rous glands, except in being larger and having very short ducts (Robin, xix. Sept. 1845; Horner, xxxvi. Jan. 1846): In the neck and back, where they are least numerous, the glands amount to 417 on the square inch (Krause). Their total number Krause esti- mates at 2,381,248 ; and, supposing the ori- fice of each gland to present a surface of l-56th of a line in diameter, he reckons that the whole of the glands would present an evaporating surface of about eight square inches. Besides the perspiration, the skin secretes a peculiar fatty matter, and for this purpose is provided with another set of special organs, termed sebaceous glands (Fig. 56), which, like the sudoriparous glands, are abundantly distributed over most parts of the body. They are most numerous in parts largely sup- plied with hair, as the scalp and face, and are thickly distributed about the entrance of the various passages into the body, as the anus, nose, lips, and external ear. They are en- tirely absent from the palmar surfaces of the hands and the plantar surfaces of the feet. They are minutely lobulated glands, which appear composed of an aggregate of small vesicles or sacculi filled with opaque white substance, like soft ointment.1 Minute capillary vessels overspread them; and their ducts, which have a beaded ap- pearance, as if formed of rows of cells, open either on the surface of the skin close to a hair, or, which is more usual, directly into the follicle of the hair. In the latter case, there are generally two glands to each hair (Fig. 56). The ducts of these glands are very commonly tenanted by one or more entozoa, of a species named Acarus folliculorum. (Erasmus Wilson, xliii. 1844, p. 305; Gruby, xviii. March 1845.) The hair-follicles, into which the sebaceous glands open, may also be reckoned among the secreting organs of the skin, since it is only at their lowest part that the material produced from their walls is appropriated to the growth of hair. The follicles are * The peculiar bitter yellow substance secreted by the skin of the external auditory passage is named cerumen, and the glands themselves cermnuious glands ; but they do not diifer in structure from the other'sebaccous glands. Sebaceous glands of the skin, after Gurlt :a,a, se- baceous glands, opening into the follicle of the hair by efferent ducts; b, a hair on its follicle. 260 THE SKIN. tubular depressions from the surface of the skin, descending into the subcutaneous fat, generally to a greater depth than the sudori- parous glands, and at their deepest part enlarging in a bulbous form, and often curving from their previous rectilinear course. They are lined throughout with cells of epithelium, continuous with those of the epidermis, and their walls are formed of pellucid membrane, which, commonly, in the follicles of the largest hairs, has the structure of vascular fibro-cellular tissue. The cells lining the deepest part of the follicles, contribute to the growth of the hair; those in the rest of their extent, though like epithelial-cells in their arrangement, are doubtless gland-cells in function, and secrete a part of the material by which the hairs and the surface of the skin are anointed.1 Such are the glands of the skin : but it is with it, as with the glands in general; together with the formed secretions, fluids pass through it by mere oozing from the bloodvessels, and gases and watery vapor are exhaled from its free surface. This evaporation, however, is much limited by the epidermis, which is composed of layers of tessellated or pavement-epithelium cells. Its cells are flattened, oval, or polygonal, and average about l-1900th of an inch in diameter; each contains a nucleus, which again contains one or more distinct, and several paler granules. The thickness of the epidermis on different portions of the skin, is directly pro- portioned to the friction, pressure, and other sources of injury to which it is exposed; and the more it is subjected to such injury, with- in certain limits, the more does it grow, and the thicker and more horny docs it become ; for it serves as well to protect the sensitive and vascular cutis from injury from without, as to limit the evapo- ration of fluid from the bloodvessels. The adaptation of the epi- dermis to the latter purpose, may be well shown by exposing to the air two dead hands or feet, of which one has its epidermis perfect, and the other is deprived of it; in a day, the skin of the latter will become brown, dry, and horn-like, while that of the former will almost retain its natural moisture. Excretion by the Skin. The skin, as already stated, is the seat of a twofold excretion ; of that formed by the sebaceous glands and hair-follicles, and of the more watery fluid, the sweat or perspiration, eliminated by the sudoriparous glands. The secretion of the sebaceous glands and hair-follicles (for their products cannot be separated) consist of cast-off epithelium ceils, with nuclei and granules, together with an oily matter, extractive 1 On the Structure of the Skin and its Glands, see Breschet (xlv.); Henle (xxxvii.); E. Wilson (xliv.) ; Todd and Bowman (xxxix.), and other works on General Anatomy. THE SKIN. 261 matter, and stearine ; in certain parts, also, it is mixed with a pe- culiar odorous principle. It is, perhaps, nearly similar in composi- tion to the unctuous coating, or vernix caseosa, which is formed on the body of the foetus while in the uterus, and which contains large quantities, both of oleine and margarine (J7Dav}r, xii. vol. xxvii. p. 187). Its purpose seems to be that of keeping the skin moist and supple, and, by its oily nature, of both hindering the evaporation from the surface, and guarding the skin from the effects of the long-continued action of moisture. But while it thus serves local purposes, its removal from the body entitles it to be reckoned among the excretions of the skin ; though the share it has in the purifying of the blood, cannot be discerned. The fluid secreted by the sudoriparous glands is usually formed so gradually that the watery portions of it escape by evaporation as fast as it reaches the surface. But, during strong exercise, expo- sure to great external warmth, in some diseases, and when evapo- ration is prevented by the application of oiled silk or plaster, the secretion becomes more sensible and collects on the skin in the form of drops of fluid. A good analysis of the secretion of these glands, unmixed with other fluids secreted from the skin, can scarcely be made; for the quantity that can be collected pure is very small. Krause (iv.) in a few drops from the palm of the hand found an acid reaction, oily matter, and margarine, with water, and no other examination has been made. The perspiration of the skin, as the term is commonly employed in physiology, includes all that portion of the secretions and exu- dations from the skin that is capable of evaporation; the sweat includes all that may be collected in drops of fluid on the surface of the skin. The former is also often called insensible perspiration: the latter, sensible perspiration. The fluids are the same, except that the sweat is commonly mingled with various substances lying on the surface of the skin. The contents of the sweat are, in part, matters capable of assuming the form of vapor, such as carbonic acid and water, and in part, other matters which are deposited on the skin, and mixed with the sebaceous secretion. Thenard col- lected the perspiration in a flannel shirt which had been washed in distilled water, and found in it chloride of sodium, acetic acid, some phosphate of soda, traces of phosphate of lime, and oxide of iron, together with an animal substance. In sweat which had run from the_ forehead in drops, Berzelius found lactic acid, chloride of sodium, and muriate of ammonia (xxiv). Anselmino placed his arm in a glass cylinder, and closed the opening around it with oiled silk, taking care that the arm touched the glass at no point. The cutaneous exhalation collected on the interior of the glass, and ran down as a fluid: on analyzing this, he found water, acetate of ammonia, and carbonic acid; and in the ashes of the dried residue 262 THE SKIN. of sweat he found carbonate, sulphate, and phosphate of soda, and some potash, with chloride of sodium, phosphate and carbonate of lime, and traces of oxide of iron. But of these several substances none need particular consideration except the carbonic acid and water. The quantity of watery vapor excreted from the skin was esti- mated very carefully by Lavoisier and Seguin. The latter chemist inclosed his body in an air-tight bag, with a mouth-piece. The bag being closed by a strong band above, and the mouth-piece adjusted and gummed to the skin around the mouth, he was weighed, and then remained quiet for several hours, after which time he was again weighed. The difference in the two weights indicated the amount of loss by pulmonary exhalation. Having taken off the air-tight dress, he was immediately weighed again, and a fourth time, after a certain interval. The difference between the two weights last ascertained gave the amount of the cutaneous and pulmonary exhalation together; by subtracting from this the loss by pulmonary exhalation alone while he was in the air-tight dress, he ascertained the amount of cutaneous transpiration. The repetition of these experiments during a long period, showed that during a state of rest the average loss by cutaneous and pulmonary exhala- tion in a minute is from seventeen to eighteen grains—the minimum eleven grains, the maximum thirty-two grains; and that of the eighteen grains, eleven pass off by the skin, and seven by the lungs. The maximum loss by exhalation, cutaneous and pulmo- nary, in twenty-four hours, is 5 lbs.; the minimum, 1 lb. 11 oz. 4 dr. (xlii. 1790). Valentin found the whole quantity lost by exhalation from the cutaneous and respiratory surfaces of a healthy man, who consumed daily 40,000 grains of food and drink, to be 19,000 grains, or 3? lbs. Subtracting from this, for the pulmonary exhalation, 5000 grains, and for the excess of the weight of the exhaled carbonic acid over that of the equal volume of the inspired oxygen, 2256 grains, the remainder, 11,744 grains, or about 2zlbs., may represent an average amount of cutaneous exhalation in the day. The large quantity of watery vapor thus exhaled from the skin, will prove that the amount excreted by simple transudation through the cuticle must be very large, if we may take Krause's estimate of about eight square inches for the total evaporating surface of the sudoriparous glands; for not more than about 3365 grains could be evaporated from such a surface in twenty-four hours, under the ordinary circumstances in which the surface of the skin is placed (xxv. 1843-4, p. 40). This estimate is not an improbable one, for it agrees very closely with that of Milne Edwards, who calculated that when the temperature of the atmosphere is not above GS° F., the glandular secretion of the skin contributes only l-6th to the total sum of cutaneous exhalations (xlvi). THE SKIN. 263 The quantity of watery vapor lost by transpiration, is of course influenced by all external circumstances which effect the exhalation from other evaporating surfaces, such as the temperature, the hy- grometric state, and the stillness, of the atmosphere. But, of the variations to which it is subject under the influence of these condi- tions, no calculation has been yet made. Neither, until recently, has there been any estimate of the quan- tity of carbonic acid exhaled by the skin on an average, or in vari- ous circumstances. Regnault and Reiset have attempted to supply this defect, and conclude, from some careful experiments, that the quantity of carbonic acid generated by the body of a warm-blooded animal is about l-50th of that furnished by the pulmonary respira- tion (liii. 1849). The cutaneous exhalation is most abundant in the lower classes of animals, more particularly the naked Amphibia, as frogs and toads, whose skin is thin and moist, and readily permits an interchange of gases between the blood circulating in it and the surrounding atmosphere. Bischoff found that, after the lungs of frogs had been tied and cut out, about a quarter of a cubic inch of carbonic acid gas continues to be exhaled by the skin. And this quantity is very large, when it is remembered that a full-sized frog will generate only about half a cubic inch of carbonic acid by his lungs and skin together in six hours (Milne Edwards and Muller, xxxii. p. 297, Am. Ed.). That the respiratory function of the skin is per- haps even more considerable in the higher animals than appears to be the case from the experiments of Regnault and Reiset just al- luded to, is made probable by the fact observed by Fourcault (xviii. March, 1844), Magendie (xix. Dec. 1843), and others, that if the skin is covered with an impermeable varnish, or the body inclosed, all but the head, in a caoutchouc dress, animals soon die, as if as- phyxiated; their heart and lungs being gorged with blood, and their temperatures during life gradually falling many degrees, and sometimes as much as 36° F. below the ordinary standard (Magen- die). Results so serious as these could not be consequent on the retention of water alone, for that might be discharged through the kidneys and lungs, or some other internal surface. Absorption by the skin has been already mentioned, as an instance in which that process is most actively accomplished. Metallic preparations rubbed into the skin have the same action as when given internally, only in a less degree. Mercury applied in this manner exerts its specific influence upon syphilis, and excites sali- vation ; potassio-tartrate of antimony may excite vomiting, or an eruption extending over the whole body; and arsenic may produce poisonous effects. Vegetable matters also, if soluble, or already in solution, give rise to their peculiar effects, as cathartics, narcotics, and the like, when rubbed into the skin. The effect of rubbing is 264 THE SKIN. probably to convey the particles of the matter into the orifices of the glands, whence they are more readily absorbed than they would be through the epidermis. When simply left in contact with the skin, substances, unless in a fluid state, are seldom absorbed. It has long been a contested question whether the skin covered with its epidermis has the power of absorbing water, and it is a point the more difficult to determine, because the skin loses water by evaporation. But from the result of many experiments it may now be regarded as a well-ascertained fact that such absorption really occurs. M. Edwards has proved that the absorption of water by the surface of the body may take place in the lower animals very rapidly. Not only frogs, which have a thin skin, but lizards, in which the cuticle is thicker than in man, after having lost weight by being Sept for some time in a dry atmosphere, were found to recover both their weight and plumpness very rapidly when im- mersed in water. When merely the tail, posterior extremities, and posterior part of the body of the lizard were immersed, the water absorbed was distributed throughout the system (xlvi.). And a like absorption through the skin, though to a less extent, may take place also in man. Br. Madden, having ascertained the loss of weight by cutaneous and pulmonary transpiration that occurred during half an hour in the air, entered the bath, and remained immersed during the same period of time, breathing through a tube which commuuicated with the air exterior to the room. He was then carefully dried and again weighed. Twelve experiments were performed in this manner; and in ten there was a gain of weight, varying from 2 scruples to 5 drachms and 1 scruple, or a mean .gain of 1 drachm, 2 scruples, and 13 grains. The loss in the air during the same length of time (half an hour), varied in ten experiments from 2} drachms to 1 ounce 2} scruples, or, in the mean, was about 6i drachms. So that, admitting the supposition that the cutaneous transpiration was entirely suspended, and estimating the loss by pulmonary exhalation at 3 drachms, there was in these ten ex- periments of Dr. Madden an average absorption of 4 drachms 1 scruple and 3 grains, by the surface of the body, during half an hour (xlvii). In four experiments performed by M. Berthold, the gain in weight was greater than in those of Dr. Madden (lxxx. 1838, p. 177). In severe cases of dysphagia, when not even fluids can be taken into the stomach, immersion in a bath of warm water or of milk and water may assuage the thirst: and it has been found in such cases that the weight of the body is increased by the immersion. Sailors, also, when destitute of fresh water, find their urgent thirst allayed by soaking their clothes in salt Avater and wearing them iu THE KIDNEYS AND THEIR SECRETION. 265 that state; but these effects may be in part due to the hindrance to the evaporation of water from the skin. The absorption, also, of different kinds of gas by the skin is proved by the experiments of Abernethy, Cruikshank, Beddoes, and others. In these cases, of course, the absorbed gases combine with the fluids, and lose the gaseous form. Several physiologists have observed an absorption of nitrogen by the skin : Beddoes says that he saw the arm of a negro become pale for a short time when immersed in chlorine; and Abernethy observed that when he held his hands in oxygen, nitrogen, carbonic acid, and other gases contained in jars over mercury, the volume of the gases became considerably diminished. The share which the evaporation from the skin has in the main- tenance of the uniform temperature of the body, and as one of the conditions to which the production of heat needs to be adapted, is already mentioned (p. 153). CHAPTER XIV. THE KIDNEYS AND THEIR SECRETION. Structure of the Kidneys. The kidneys, provided especially for the excretion of the refuse nitrogen, phosphorus and sulphur, lime and magnesia, have the general structures of glands arranged in a manner distinguishing them from all other excretory organs. In each kidney numerous secreting tubes (tubuli uriniferi) are collected in bundles, in from ten to twenty separated conical or pyramidal portions (pyramids or cones of Maipighi), which together constitute the tubular portion of the kidney. The apices of the cones converge, and project into calyces, which are branches of a large cavity called the pelvis of the kidney, that leads to the ureter, its excretory duct (Fig. 57). The trunks of the urine-tubes open at the extremities or papillae of the pyramids, and their branches running in straight and somewhat divergent course towards the surface of the kidney, as they ap- proach it, become tortuous, and, winding in various directions, ter- minate in, or bear on small pedicles proceeding from their walls, dilated, flask-shaped sacculi, named capsules of Maipighi. Those that bear capsules at their sides, probably unite with one another in loops, or terminate in simply closed ends. 23 266 THE KIDNEYS. FlS- 57. The small branches of the renal arte- ries ramify very abundantly in the parts of the kidney near its surface, and be- tween the several pyramids; and pre- dominating over the tubules, have ob- tained for these cortical parts of the kidney the name of vascular portion. Before dividing into capillaries, they form vascular tufts or little balls, called Malpighian corpuscles or glomerules. In the formation of these, each minute artery divides into four or more small tortuous branches, which run on the surface of the corpuscles, and give off many branches that fill up the spaces between and within them, and lead to a small vein which usually emerges from the corpuscles at the same part as the artery enters it. Thus, each Malpighian corpuscle appears as if suspended by a small short pedicle, formed of its artery and vein. Each lies within a Malpighian capsule, or attached to its exterior (Hyrtl, lxxxviii. April, 1846; Bidder, lxxx. 1845), and from the vein of each proceed capillaries, which ramify in close net- works over the urine-tubes (Fig. 58). Thus, therefore, the circulation of the kidney is peculiar in that the capillaries, from which the blood is chiefly derived to form the urine, are like the divisions of a vein rather than of an artery: for the branchings of the arte- ries in the Malpighian tufts or corpuscles, and the collection of their branches again into the small efferent vessels, give that vessel the character of a vein, and make the capillary circulation over the urine-tubes analogous to the portal circulation through the liver, an analogy which is the closer, because in fish and Amphibia the kidney receives not only a renal artery, whose branches form the Malpighian bodies, but also a large renal (or renal-portal) vein, bringing, for the secretion of urine, the venous blood of the hinder parts of the body, and giving off the capillaries which ramify upon the urine-tubes (Bowman, xliii. 1842). The urine-tubes are minute canals of about 1-700th of an inch in diameter, formed of pellucid, simple, or basement-membrane, and lined throughout with nucleated gland-cells, arranged like an epithelium, of spheroidal form, and darkly dotted or granulated A section of the Kidney, sur- mounted by the suprarenal cap- sule ; the swellings upon the sur- face mark the original constitu- tion of the organ, as made up of distinct lobules. — 1. The supra- renal capsule. 2. The vascular portion of the kidney. 3, 3. Its tubular portion, consisting of cones, i, 4. Two of the papillae projecting into their correspond- ing calyces. 5, 5, 5. The three infundibula; the middle 5 is situ- ated in the mouth of a-calyx. C. The pelvis. 7. The ureter. THE KIDNEYS. 267 (see Fig. 59). Not unfrequently, portions of tubes, especially of those that are convoluted or tortuous, appear nearly filled with such cells, or thin separated nuclei, as if the urine were filtered Fig. 58. 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. Fig. 09. A, Portion of a secreting canal from the cortical substance of the kidney, b, The epithelium or gland-cells, more highly magnified (TOO times), c, Portion of a canal from the medullary substance of the kidney. At one part the basement-membrane has no epithelium lining it. through them in its way to the pelvis. The same kind of epithe- lium is continued into the Malpighian capsules, and lines their whole internal surface, and, if they contain Malpighian tufts, is reflected over them like a serous membrane.1 Secretion of Urine. The separation of urine from the blood is probably effected, like other secretions, by the agency of the gland-cells, and equally in all parts of the urine-tubes. The urea and uric acid, and perhaps some of the other constituents existing ready formed in the blood, 1 In the frog, triton, and probahly most or all other naked Amphibia, the epithelium at and just -within the neck or commencing dilatation of the Malpighian capsule is ciliated. This fact (first observed by Mr. Bowman) is, perhaps, connected with the peculiar arrangement of the seminal tubes or branches of the vasa deferentia, which open into one end of the Malpi- ghian capsules, while the urine-tubes open into the others. The cilia work towards the seminal tubes, and would prevent the seminal fluid from min- gling with the urine (see Bidder, cliv., and Ludwig, lix. 1817). 268 THE KIDNEYS. may need only separation, that is, they may pass from the blood to the urine without further elaboration ; but this is not the case with some of the other principles of the urine, such as the acid phosphates and the sulphates, for these salts do not exist in the blood, and must be formed by the chemical agency of the colls. The large size of the renal arteries and veins permits so rapid a transit of the blood through the kidneys, that the whole of the blood is purified by them. The secretion of urine is rapid in com- parison with other secretions, and as each portion is secreted it propels those already in the tubes onwards into the pelvis. Thence through the ureter the urine passes into the bladder, into which its rate and mode of entrance has been watched in cases of ectopia vesicae, i.e., of such fissures in the anterior and lower part of the walls of the abdomen, and of the front wall of the bladder, that its hinder wall with the orifices of the ureters is exposed to view. The best observations on such a case were made by Mr. Erichsen (lxxi. 1845). The urine does not enter the bladder at any regular rate, nor is there a synchronism in its movement through the two ureters. During fasting, two or three drops enter the bladder every minute, each drop as it enters first raising up the little papilla on which, in these cases, the ureter opens, and then passing slowly through its orifice, which at once again closes like a sphincter. In the recumbent posture, the urine collects for a little time in the ureters, then flows gently, and if the body is raised, runs from them in a stream till they are empty. Its flow is increased in deep inspiration, or straining, and in active exercise, and in fifteen or twenty minutes after a meal. The same observations, also, showed how fast some substances pass from the stomach through the circulation, and through the vessels of the kidneys. Ferrocyanate of potash so passed on one occasion in a minute : vegetable substances, such as rhubarb, oc- cupied from sixteen to thirty-five minutes; neutral alkaline salts with vegetable bases, which were generally decomposed in transitu, made the urine alkaline in from twenty-eight to forty-seven min- utes. But the times of passage varied much ; and the transit was always slow when the substances were taken during digestion. The urine collecting in the urinary bladder is prevented from regurgitation into the ureters by the mode in which they pass through the walls of the bladder, namely, by their lying for be- tween half and three-quarters of an inch between the muscular and mucous coats, and then turning rather abruptly forwards, and opening through the latter. It collects till the distension of the bladder is felt either by direct sensation, or, in ordiuary cases, by a transferred sensation at and near the orifice of the urethra. Then, the effort of the will being directed primarily to th,e muscles of the abdomen, and through them (by reason of its tendency to act with THE KIDNEYS. 269 them, to the urinary bladder), the latter, though its muscular walls are really composed of involuntary muscle, contracts, and expels the urine. The muscular fibres behind the ureters, where they lie between the muscular and mucous coat of the bladder, compress these canals as they contract for the expulsion of the urine; and the vesical orifice of the urethra, which appears to be closed only by the elasticity of the surrounding parts, is forced open by the press- ure of the urine while the bladder is contracting, and again closes by the same elasticity when the bladder ceases to contract. The Urine : its general Properties. Healthy urine is a clear limpid fluid, of a pale yellow or amber color, with a peculiar faint aromatic odor, which becomes pungent and ammoniacal when decomposition takes place. The urine, though usually clear and transparent at first, often, as it cools, becomes opaque and turbid from the deposition of part of its con- stituents previously held in solution; and this may be consistent with health, though it is only in disease that, in the temperature of 98° or 100°, at which it is voided, the urine is turbid even when first expelled. Although ordinarily of a pale amber color, yet, consistently with health, the urine may be nearly colorless, or of a brownish or deep orange tint; and between these extremes it may present every shade of color. When secreted, and, most commonly, when first voided, the urine has a distinctly acid reaction in man and all carnivorous animals, and it thus remains till it is neutralized or made alkaline by the ammonia developed in it by decomposition. In most herbivorous aninials, on the contrary, the urine is alkaline and turbid. The difference depends, not on any peculiarity in the mode of secretion, but on the differences in the food on which the two classes subsist; for when carnivorous animals, such as dogs, are restricted to a vegetable diet, their urine becomes pale, turbid, and alkaline, like that of an herbivorous animal, but resumes its former acidity on the return to an animal diet; while the urine voided by herbivorous animals, e. g., rabbits fed for some time exclusively upon animal sub- stances, presents the acid reaction and other qualities of the urine of Carnivora, its ordinary alkalinity being restored only on the substitution of a vegetable for the animal diet (Bernard, xviii. 1. Lumbar bulbs. E. An inch lower. F. Very near the lower end. a. Anterior sur- face, p. Posterior surface. The points of emergence of the anterior and posterior roots of the nerves are also seen. to the conduction of found that their weights, in this order, were 219, 293, 163, and 281 grains. On mea- surement, he found that the areas of the transverse sections of the gray matter in them were (in the same order) 13, 28, 11, and 25 square lines; and those of the white matter 109, 142, 89, and 121 square lines. It thus appeared that the quantity of white or fibrous substance of the cord is absolutely less at the cervical than at the lowest part of the lumbar portion ; which it could not be, if the cord in its progress from below upwards retained any quantity of the fibres successively received from the roots of the spinal nerves. (Fig. 70.) On the other hand, the enlargement and in- creased weight of the cord at parts exactly corresponding to the origin of the larger and most numerous nerves, and its diminution immediately above and below such parts, make it most probable that the fibres com- posing the roots of those nerves arise directly from the largest parts of the cord, and not from any parts higher up. Moreover, the appearances of transverse sections of the cord indicate that many fibres pass inwards towards the centre of the cord, either directly or more or less obliquely. Many of the fibres thus passing may be com- missural, i. e., fibres connecting different parts of the cord, and having no direct connection with the nerve-roots: and the observations of Stilling and Wallach (clvii.) may be fallacious because of the low magnifying powers they employed; but the appearance of fibres pass- ing from the nerve-roots transversely into the substance of the cord, is too clear to be de- ceptive. They appear to pass into the gray substance of the cord; and the probability that they do so is increased by the finding in all parts of the gray substance such nerve- vesicles as give origin to nerve-fibres (Kbl- liker, cxiv.; Hannover, cxix.; and others). It may be added that there is no sufficient evidence for the supposition that an uninter- rupted continuity of nerve-fibres is essential impressions on the spinal nerves to and from THE SPINAL CORD. 305 the brain : such impressions may be as well transmitted through the nerve-vesicles of the cord and the filaments connecting them. On these grounds it will be assumed, in what follows, that the fibres of the roots of the spinal nerves terminate in, or are con- nected with, nerve-corpuscles not far from the parts of the cord at which they severally penetrate it; and that, therefore, the connec- tion of function between the brain and spinal nerves is maintained almost exclusively by mea/is of gray nervous substance, or of fibres which are placed as commissures or connectors between the brain and medulla oblongata, and the several parts of the spinal cord.1 The nerves of the spinal cord consist of thirty-one pairs, issuing from the sides of the whole length of the cord; their numbers corresponding with the intervertebral foramina, through which they pass. Each nerve arises by two roots, an anterior and posterior, the latter being the largest. The roots emerge through separate apertures of the sheath of dura mater surrounding the cord; and directly after their emergence, while the roots lie in the interver- tebral foramen, a ganglion is formed on the posterior root. The anterior root lies in contact with the anterior surface of the gang- lion, but none of its fibres intermingle with those in the ganglion. But immediately beyond the ganglion, the two roots coalesce, and, by the mingling of their fibres, form a compound or mixed spinal nerve, which, after issuing from the intervertebral canal, divides into an anterior and posterior branch, each containing fibres from both the roots. (Fig. 71.) The anterior root of each spinal nerve arises by numerous separate and converging fasciculi from the anterior column of the cord; the pos- terior root by more numerous parallel fasciculi, from the posterior column, or, rather, from the posterior part of the lateral column; for if a fissure be directed inwards from the groove be- tween the middle and posterior columns, the posterior roots will remain attached to the former. The anterior roots of each spinal nerve consist exclusively of motor fibres; the posterior as ex- clusively of sensitive fibres. For the knowledge of this important fact, and much of the conse- quent progress of the physiology of the nervous system, science is indebted to Sir Charles Bell. It is proved in 1 On the anatomy of the spinal cord, consult any of the principal sys- tematic treatises; or Grainger (clii.); Todd (lxxiii. art. Nervous Cinlres); Longet (exxxvi.) ; Stilling and Wallach (clvii.). 26* Diagram to show the decussation of the fibres within the trunk of a nerve.— (After Valentin.) 306 FUNCTIONS OF THE SPINAL CORD. various ways. Division of the anterior roots of one or more nerves is followed by complete loss of motion in the parts supplied by the fibres of such roots ; but the sensation of the same parts remains perfect. Division of the posterior roots destroys the sensibility of the parts supplied by their fibres, while the power of motion continues unimpaired. Moreover, irritation of the ends of the distal portions of the divided anterior roots of a nerve excites muscular movements ; irritation of the ends of the proximal por- tions, which are still in connection with the cord, are followed by no effect. Irritation of the distal portions of the divided pos- terior roots, on the other hand, produces no muscular movements, and no manifestation of pain ; for, as already stated, sensitive nerves convey impressions only towards the nervous centres : but irritation of the proximal portions of these roots elicits signs of intense suf- fering. Occasionally, also, under this last irritation, muscular movements ensue ; but these are either voluntary, or the result of the irritation being reflected from the sensitive to the motor fibres. As an example of the experiments, of which the preceding para- graph gives a summary account, this may be mentioned : If, in a frog, the three posterior roots of the nerves going to the hinder extremity, be divided on the left side, and the three anterior roots of the corresponding nerves on the right side, the left extremity will be deprived of sensation, the right of motion. If the foot of the right leg, which is still endowed with sensation, but not with the power of motion, be cut off, the frog will give evidence of feeling pain, by movements of all parts of the body except the right leg itself, in which he feels the pain. If, on the contrary, the foot of the left leg, which has the power of motion but is deprived of sensation, is cut off, the frog does not feel it, and no movement follows, except the twitching of the muscles irritated by cutting them or their tendons. Functions of the Spinal Cord. The spinal cord manifests all the properties already assigned to such centres (see p. 298). 1. It is capable of conducting impressions, or states of nervous excitement. Through it, all the impressions made upon the peri- pheral extremities or other parts of the spinal sensitive nerves are conducted to the brain, where alone they can be perceived by the mind. Through it, also, the stimulus of the will, applied to the brain, is capable of exciting the action of the muscles supplied from it with motor nerves. And for all these conductions of im- pressions to and fro between the brain and the spinal nerves, the perfect state of the cord is necessary; for when any part of it is destroyed, and its communication with the brain is interrupted, impressions on the sensitive nerves given off from it below the FUNCTIONS OF THE SPINAL CORD. 307 seat of injury, cease to be propagated to the brain; and the mind loses the power of voluntarily exciting the motor nerves proceed- ing from the portion of cord isolated from the brain. Illustrations of this are furnished by various examples of para- lysis, but by none better than by the common paraplegia, or loss of sensation and voluntary motion in the lower part of the body, in consequence of destructive disease or injury of a portion, including the whole thickness, of the spinal cord. Such lesions destroy the communication between the brain and all parts of the spinal cord below the seat of injury, and consequently cut off from their con- nection with the mind, the various organs supplied with nerves issuing from those parts of the cord. But, if this lower portion of the cord preserves its integrity, the various parts of the body supplied with nerves from it, though cut off from the brain, will nevertheless be subject to the influence of the cord, and, as pre- sently to be shown, will indicate its other powers as a nervous centre. From what has been already said, it will appear probable that the conduction of impressions along the cord is effected (at least for a part of the distance) through the gray substance, i. e., through the.nerve-corpuscles and filaments connecting them. But there is reason to believe that all parts of the cord are not alike able to con- duct all impressions; and that, rather, as there are separate nerve- fibres for motor and for sensitive impressions, so, in the cord, sepa- rate and determinate parts serve to conduct the same impressions. The consideration of this point involves the question of the func- tions of the columns of the cord. The question is, whether the anterior and posterior columns correspond to the anterior and pos- terior roots respectively : whether the anterior columns contain only motor, the posterior only sensitive fibres. It was difficult to decide this, even when it was supposed that the fibres of the roots of the nerves were continued uninterruptedly to the brain : and the difficulty is much increased if we believe that they are not so continued, and that conduction may take place as well through the gray substance of the cord as through its fibres. Experiments, especially those of Longet (cxxxvi.) and Van Deen (clviii.), have shown that irritations of the anterior columns of the spinal cord are followed by convulsive movements of all the parts supplied with motor nerves from and below the irritated part, but give rise to no manifestations of pain: while irritation of the poste- rior columns appears to cause excruciating pain, without producing any muscular movement besides such as may be the result of voli- tion, or the reflection of the stimulus from the irritated cord to the roots of motor nerves. Again, when the spinal cord is completely divided, irritation of the posterior columns of the lower part which is cut off from the brain, produces no effect.: irritation of the aute- 308 FUNCTIONS OF THE SPINAL CORD. rior columns of the same part excites violent movements. And, in the same experiment, irritation of the divided anterior columns of the portion of the cord still connected with the brain, produces no effect: but irritation of the divided posterior columns of the same portion produces acute pain and reflex movements (Longet). Again, when both the anterior columns of the cord are divided, the power of voluntary movement over the parts supplied with nerves below the point of division is completely lost: the sensibility of the same parts being unimpaired. When both posterior columns are divided, sensation in the parts supplied by nerves from below the injured point is lost, while the power of movement over such parts remains perfect (Van Deen). [It has been shown by Dr. Brown- Sequard, that when the posterior column on one side is cut, there is a loss of sensibility in the opposite side of the body, thus proving a crossing of the fibres of the sensory, as well as of the motor tract.] The results of these experiments would seem to prove that the effects of the division of the anterior or posterior columns of the cord are exactly the same as those of division of the anterior or posterior roots of the spinal nerves, and that therefore one might be justified in calling the anterior the motor, and the posterior the sensitive, columns of the cord. Yet there are reasons for hesita- tion. For the posterior roots of the spinal nerves are connected (as already stated), not with the posterior columns, but with the posterior part of the lateral columns; and neither the injuries in experiments, nor the results of disease, can be so precisely limited as to discern the difference of the effects of injury of the posterior columns, from those of the immediately-adjacent portions of the lateral columns. Neither is it likely that the fibres of the columns are the sole, or even the principal, conductors of impressions: at the most, therefore, we should not be justified in assuming more than that the posterior half of the cord corresponds with the sensi- tive roots, the anterior with the motor. And even this statement, though there may be little doubt of its general truth, should be held as likely to require modifications; for the results of diseases and injuries of different parts of the human cord are not always in accordance with it. Though many cases have seemed confirmatory of it,1 yet some have been observed directly contrary to it; cases, 1 See especially a case by Begin, quoted, with others, by Longet (cxxxvi. vol. i. p. 331). A man Avas stabbed at the back of the neck, and the point of the knife passed obliquely forwards between the sixth and seventh cervi- cal vertebra}, dividing the corresponding antero-lateral and anterior columns of the cord on the right side. During the six days in which he survived the injury, there existed a complete paralysis of motion in the right lower ex- tremity, and incomplete paralysis of motion in the right upper extremity, but sensibility was perfect. FUNCTIONS OF THE SPINAL CORD. 309 for example, in which complete loss of motion, without any im- pairment of sensation, was an accompaniment of lesion of the pos- terior columns of the cord, the anterior being apparently entire (Stanley, xii. vol. xxiii.; Webster, xii. vol. xxvi.).. The foregoing observations have related only to the conduction of impressions along the Cord; they may also be conducted in some directions across it. Thus, if the brain and medulla oblongata be removed, irritation of either posterior column of the upper end of the cord will cause general movements of muscles, the impression being conveyed across to the anterior ^columns and roots; for the movements do not happen if the anterior roots are divided. If one half of the cord be divided at a certain part, and the other half at a certain distance from that part, impressions (at least sensitive ones) may be conducted through the intermediate portion of the cord from one side to the other (Van Deen); and this may be ef- fected though only a portion of the gray substance be left to con- nect the portions of cord above and below. But impressions do not seem to be conveyed from the anterior columns to the posterior, nor from one anterior column to the other; so that, as in the case already cited from Begin, after the division of one anterior column, including the anterior part of the gray matter in it, the will has no power over the muscles deriving nerves from or below the in- jured part of the column.1 2. In the second place, the spinal cord as a nervous centre, or rather as an aggregate of many nervous centres, has the power of com- municating impressions from fibre to fibre in the several ways al- ready mentioned (p. 299). Examples of the transference and radiation of impressions in the cord have been given; and that the transference at least takes place in the cord, and not in the brain, is nearly proved by the cases of pain felt in the knee, and not in the hip, in diseases of the hip; of pain felt in the urethra or glans penis, and not in the bladder, in calculus; for if both the primary and the secondary or trans- ferred impressions were in the brain, both should be always felt. Of radiation of impressions there are, perhaps, no means of deciding whether they take place in the spinal cord or in the brain : but the analogy of the cases of transference makes it probable that the communication is, in this, also, effected in the cord. 1 For a complete discussion of this subject, and for the arguments in fa- vor of the posterior columns of the cord being composed of fibres forming commissural connections between its several parts, see Todd (lxxiii. art AVrvous Centres ; and clix). The best evidence for the sensitive and motor functions being appropriate to the posterior and anterior columns is iii Lon- get (cxxxvi). Many interesting facts are in Sir Charles Bell's works (cxlii.); Muller (xxxii.) ; and Grainger (clii.). 310 FUNCTIONS OF THE SPINAL CORD. The power, as a nervous centre, of communicating impressions from sensitive to motor, or, more strictly, from centripetal to cen- trifugal nerve-fibres, is what is usually discussed as the reflex func- tion of the spinal cord. Its general mode of action, its general, though incomplete, independence of consciousness, the will and the brain, and the conditions necessary for its perfection have been al- ready stated (p. 300). These points, and the extent in which the power operates in the production of the natural reflex movements of the body, have now to be further illustrated. They will be de- scribed in terms adapted to the general rules of reflection of im- pressions in nervous centres, avoiding all such terms as might seem to imply that the power of the spinal cord in reflecting is different in kind from that of all other nervous centres. The occurrence of movements under the influence of the spinal cord, and independent of the will, is well exemplified in the acts of swallowing, in which a portion of food carried by voluntary efforts into the fauces, is conveyed by successive involuntary contractions of the constrictors of the pharynx and muscular walls of the oeso- phagus into the stomach. These contractions are excited by the stimulus of the food on the centripetal nerves of the pharynx and oesophagus being first conducted to the spinal cord and medulla ob- longata, and thence reflected through the motor nerves of these parts.1 All these movements of the pharynx and oesophagus are involun- tary ; the will cannot arrest them or modify them ; and though the mind has a certain consciousness of the food passing, which becomes less as the food passes further, yet that this is not necessary to the act of deglutition, is shown by its occurring when the influence of the mind is completely removed; as when food is introduced into the fauces or pharynx during a state of complete coma, or in a brainless animal (flrainger, clii.). So, also, for example, under the influence of the spinal cord, the involuntary and unfelt muscular contraction of the sphincter ani is maintained when the mind is completely inactive, as in deep sleep, but ceases when the lower part of the cord is destroyed, and cannot be maintained by the will. The independence of the mind manifested by the reflecting power of the cord, is further shown in the more perfect occurrence of the reflex movements when the spinal cord and the brain are discon- 1 It is customary to call the nerves thus conducting impressions to be re- flected excito-motory, and the nerves by which the impressions are reflected reflecto-motory ; and corresponding terms are applied in explanation of the reflex acts of the cord. They are here avoided, both for the reason given in the preceding paragraph, and because they are apt to lead the student to believe that the nerves contain one set of fibres for the conduction of im- pressions to and from the brain, and another for the conduction of them to and from the spinal cord; the improbability of which will appear from what is said of the structure of the cord in p. 303. FUNCTIONS OF THE SPINAL CORD. 311 nected, as in decapitated animals, and in cases of injuries or dis- eases so affecting the sjiinal cord as to divide or disorganize its whole thickness at any part whose perfection is not essential to life. Thus, when the head of a lizard is cut off, the trunk remains stand- ing on the feet, and the body writhes when the skin is irritated. If the animal is cut in two, the lower portion can be excited to motion as well as the upper portion; the tail may be divided into several segments, and each segment, in which any portion of spinal cord is contained, contracts on the slightest touch; even the ex- tremity of the tail moves as before, as soon it is touched. All the portions of the animal in which these movements can be excited, contain some part of the spinal cord; and it is evidently the cause of the motions excited by touching the surface; for they cannot be excited in parts of the animal, however large, if no cord is con- tained in them. Mechanical irritation of the skin excites not the slightest motion in the leg when it is separated from the body; yet the extremity of the tail moves as soon as it is touched. With the same power of the spinal cord in reflecting impressions, an eel, or a frog, or any other cold-blooded animal, will move long after it is deprived of its head, and when, however much the movement may indicate purpose, it is not probable that consciousness or will has any share in them. And so, in the human subject, or any warm-blooded animal, when the cord is completely divided across, or so diseased at some part that the influence of the mind cannot be conveyed to the parts below it, the irritation of any part of the surface, supplied by nerves given off from the cord below the seat of injury, is commonly followed by spasmodic and irregular reflex movement, even though in the healthy state of the cord such invo- luntary movements could not be excited when the attention of the mind was directed to the irritating cause. In the fact last mentioned is an illustration of an important dif- ference between the warm-blooded and the lower animals in regard to the reflecting power of the spinal cord (or its homologue in the In vertebrata), and the share which it and the brain have respect- ively, in determining the several natural movements of the body. When, for example, a frog's head is cut off, the limbs remain in or assume a natural position; resume it when disturbed; and when the abdomen or back is irritated, the feet are moved with the manifest purpose of pushing away the irritation. It is as if the mind of the animal were still engaged in the acts.1 But, in divi- 1 The evident adaptation and purpose in the movements of the cold- blooded animals have led some to think that they must be conscious and capable of will without their brains. But purposive movements are no proof of consciousness or will in the creature manifesting them. The movements of the limbs of headless frogs are not more purposive than the movements of our own respiratory museles are; in which we know that neither will nor consciousness is at all times concerned. * 312 FUNCTIONS OF THE SPINAL CORD. sion of the human spinal cord, the lower extremities fall into any position that their weight and the resistance of surrounding objects combine to give them: if the body is irritated, they do not move towards the irritation; and if themselves are touched, the con- sequent movements are disorderly and purposeless. Now, if we are justified by analogy in assuming that the will of the frog can- not act more than the will of man through the spinal cord sepa- rated from the brain, then it must be admitted that many more of the natural and purposive movements of the body can be performed under the sole influence of the cord in the frog than in man; and what is true in the instances of these two species is generally true also of the whole class of cold-blooded as distinguished from warm-blooded-animals. It may not, indeed, be. assumed that the acts of standing, leaping, and other movements which decapitated cold-blooded animals can perform, are also always, in the entire and healthy state, performed involuntarily and under the sole influence of the cord; but it is probable that such acts may be, and com- monly are, so performed, the mind of the animal having only the same kind of influence in modifying and directing them, as the mind of man has in modifying and directing the movements of the respiratory muscles. The fact that such movements as are produced by irritating the skin of the lower extremities in the human subject, after division or disorganization of a part of the spinal cord, do not follow the same irritation when the mind is active and connected with the cord through the brain, is, probably, due to the mind ordinarily perceiving the irritation and instantly controlling the muscles of the irritated and other parts; for, even when the cord is perfect, such involuntary movements will often follow irritation if it be applied when the mind is wholly occupied. When, for example, one is anxiously thinking, even slight stimuli will produce involun- tary and reflex movements. So, also, during sleep such reflex movements may be observed when the skin is touched or tickled; for example, when one touches with a finger the palm of the hand of a sleeping child, the finger is grasped—the impression on the skin of the palm producing a reflex movement of the muscles which close the hand. But when the child is awake, no such effect is produced by a similar touch. On the whole, it may, from these and like facts, be concluded that the proper reflex acts, performed under the influence of the reflecting power of the spinal cord, are essentially independent of the brain, and may be performed perfectly when the brain is sepa- rated from the cord: that these include a mUch larger number of the natural and purposive movements of the lower animals than of the warm-blooded animals and man; and that over nearly all of them the mind may exercise, through the brain, some control; FUNCTIONS OF THE SPINAL CORD. 313 determining, directing, hindering, or modifying them, either by direct action or by its power over associated muscles. In this fact, that the reflex movements from the cord may be perfectly performed without the intervention of consciousness or will, yet are amenable to the control of the will, we may see their admirable adaptation to the well-being of the body. Thus, for example, the respiratory movements maybe performed while the mind is, in other things, fully occupied, or in sleep powerless; yet, in an emergency, the mind can direct and strengthen them; and it can adapt them to the several acts of speech, effort, &c. Being, for ordinary purposes, independent of the will and consciousness, they are performed perfectly, without experience or education of the mind; yet they may be employed to other and extraordinary uses when the mind wills, and so far as it acquires power over them. Being commonly independent of the brain, their constant continuance does not produce weariness; for it is only in the brain that it ftr any other sensation can be perceived. The subjection of the muscles to both the spinal cord and the brain makes it difficult to determine in man what movements or what share in any of them can be assigned to the reflecting power of the cord. The fact that, after division or disorganization of a part of the cord, movem^ats, and even forcible though purposeless ones, are produced in the lower limbs when the skin is irritated, proves that the spinal cord can supply nervous force for the action of the muscles that are, naturally, most under the control of the will: and it is, therefore, not improbable that, for even the voluntary action of those muscles, when the cord is perfect, it may supply the force, and the will the direction. For instances in which it supplies both force and direction, that is, both excites and determines the combination of muscles, may be mentioned the acts of the abdominal muscles in vomiting and voiding the contents of the bladder and rectum: in both of which, though, after the period of infancy, the mind may have power of postponing or modifying the act, there are all the evidences of reflex action; namely, the necessary prece- dence of a stimulus, the independence of the will, and, sometimes, of consciousness, the combination of many muscles, the perfection of the act without the help of education or experience, and its failure or imperfection in disease of the lower part of the cord. The emission of semen is equally a reflex act governed by the spinal cord: the irritation of the glans penis, conducted to the spinal cord and thence reflected, excites the successive and co-ordinate contrac- tions of the muscular fibres of the vasa deferentia and vesicuke scminales, and of the bulbo-cavernosi and other muscles of the urethra; and a forcible expulsion of semen takes place, over which the mind has little or no control, and which, in cases of paraplegia, may be unfelt. The erection of the penis also, as already explained 314 FUNCTIONS OF THE SPINAL CORD. (page 121), appears to be in part the result of a reflex contraction of the muscles by which the veins returning the blood from the penis are compressed. Irritation of the vagina in sexual intercourse appears also to be propagated to the spinal cord, and thence reflected to the motor nerves supplying the Fallopian tubes. The involun- tary action of the uterus in expelling its contents during parturition, is also of a purely reflex kind, dependent in part upon the spinal cord, though in part also upon the sympathetic system: its inde- pendence of the brain and the mind was proved by cases of delivery in paraplegic women, and is now more abundantly shown in the use of chloroform. Besides these acts, regularly performed under the influence of the reflecting power of the spinal cord, others are manifested in accidents; such as the movements of the limbs and other parts, to guard the body against the effects of sudden danger. When, for ■ efample, a limb is pricked or struck, it is instantly and involuntarily withdrawn from the instrument of injury; a threatened blow on the face causes involuntary closure of the eye. And the same pre- servative tendency of the reflex power of the cord is shown in the outstretched arms when falling forwards, and their reversed position when falling backwards. To these instances of spinal reflex action some add yet many more, including nearly all the acts which seem to be performed unconsciously, such as those of standing, walking, and the like. But these are not involuntary acts; they are not accomplished with- out the active co-operation of the brain, for they are impossible in coma, sleep, paraplegia, and complete mental abstraction; they all require education for their perfection; their force is not proportioned to any%xternal stimulus exciting them; they produce weariness; in short, they appear to be only examples of how small an amount of attention and will are necessary for the performance of habitual acts. The phenomena of spinal reflex actions in man are much more striking and unmixed in cases of disease. In some of these, the effect of a morbid irritation, or a morbid irritability of the cord, is very simple; as when the local irritation of sensitive fibres, being propagated to the spinal cord, excites merely local spasms—spasms, namely, of those muscles, the motor fibres of which arise from the same part of the spinal cord as the sensitive fibres that are irritated. Of such a case, we have instances in the spasms and tremors of limbs on which a severe burn is inflicted, &c. In other instances, in which we must assume that the cord is morbidly more irritable, i. c, apt to issue more nervous force than is proportionate to the stimulus applied to it, a slight impression on a sensitive nerve produces extensive reflex movements. This ap- FUNCTIONS OF THE SPINAL CORD. 315 pears to be the condition in tetanus, in which a slight touch on the skin may throw the whole body into convulsions. A similar state is induced by the introduction of strychnia, and, in frogs, of opium, into the blood; and numerous experiments on frogs thus made tetanic have shown that the tetanus is wholly unconnected with the brain, and depends on the state induced in the spinal cord. It may have seemed to be implied that the spinal cord, as a single nervous centre, reflects alike from all parts all the impressions con- ducted to it. But it is more probable that it should be regarded as a collection of nervous centres united in a continuous column. This is made probable by the fact that segments of the cord may act as distinct nervous centres, and excite motions in the parts supplied with nerves given off from them; as well as by the anal- ogy of certain cases in which the muscular movements of single organs are under the control of certain circumscribed portions of the cord. Thus Volkmann (lxxx. 1844) has shown that the rhythmical movements of the anterior pair of lymphatic hearts in the frog depend upon nervous influence derived from the portion of spinal cord corresponding to the third vertebra, and those of the posterior pair on influence supplied by the portion of cord opposite the eighth vertebra. The movements of the hearts continue, though the whole of the cord, except the above portions, be destroyed; but on the instant of destroying either of these portions, though all the rest of the cord is untouched, the movements of the corresponding hearts cease. What appears to be thus proved in regard to two portions of the cord, may be inferred to prevail in other portions also; and the inference is reconcilable with most of the facts known concerning the physiology of the cord. It might be supposed that each portion of the cord is as the nervous centre of a certain region, receiving and issuing impressions from and to the several nerve-fibres immediately connected with it. But some experiments by Engelhardt and Harless have made it probable (if the case of frogs may be taken as an example of a general truth), that different portions of the length of the cord are assigned for the government of different kinds of movements. The results of Harless's experiments may be thus expressed, in a scheme in which each number represents that of the vertebra opposite to which the irritation was applied to the spinal cord:— Irritation at the . . 1st vertebra. No movement. Flexion of upper ex- j . tremities decreasing L, ] F1exion of loAvcr extrem- as the irritation is I 3d I ^ies decreasing as the applied higher. -,., irritation is applied ri ° [4th " J lower. 316 FUNCTIONS OF THE SPINAL CORD. f5th " Least or no effect. Extension of upper ex- I Gth « . Extension of lower ex- tremities decreasing tremities deceasing as the irritation is - ., ■.. ,.- „ :, ,.,... as the irritation is applied lusher. ,. , , *l ° q,, u J applied lower. Other of Ilarless's experiments appeared to show that the only portion of the frog's cord capable of reflecting impressions to the motor nerves of the extremities, is that between the third and fifth vertebra. For, by cutting away the cord from below upwards, the power of reflecting so as to produce movements in the lower ex- tremities is lost, when the section comes to the sixth vertebra, and that of reflecting to the upper extremities, when the section reaches the fourth vertebra. The influence of the spinal cord on the sphincter ani has been already mentioned. It maintains this muscle in permanent con- traction, so that, except in the act of defecation, the orifice of the anus is always closed. This influence of the cord resembles its common reflex action in being involuntary, although the will can act on the muscle to make it contract more, or to permit its dilata- tion, and in that the constant action of the muscle is not felt, nor diminished in sleep, nor productive of fatigue. But the act is different from ordinary reflex acts in being nearly constant. In this respect it resembles that condition of muscles which has been called Tone,1 or passive contraction; a state in which they always appear to be when not active in health, and in which, though called inactive, they appear to be in slight contraction, and cer- tainly are not relaxed, as they are long after death, or when the spinal cord is destroyed. This tone of all the muscles of the trunk and limbs seems to depend on the spinal cord, as the con- traction of the sphincter ani docs. If an animal is killed by injury or removal of the brain, the tone of the muscles may be left, and the limbs feel firm, as during sleep; but if the spinal cord be destroyed, the sphincter ani relaxes, and all the muscles feel loose, and flabby, and atonic, and remain so till the rigor mortis com- mences. For the further study of the functions of the spinal cord, it need scarcely be said that the works of Sir Charles Bell and Dr. Marshall Hall, are the most important. The other principal writings are those of Prochaska (cliii.); Magendie (civ.); Muller (xxxii.) ; 1 This kind of tone must be distinguished from that mere firmness and tension which it is customary to ascribe with the name of tone to all tissues that feel robust and not flabby, as well as to muscles. The tone peculiar to mu.scles, and perhaps to other contractile parts, has in it a degree of vital contraction; that of other tissues is only due to their being well nourished, and therefore compact and tense. THE MEDULLA OBLONGATA. 317 Grainger (clii.); Newport (xliii. 1844); Volkmann (lxxx. 1838); Dr. W. Budd (xii. vol. xxii.); Carpenter (cxxxi.); Todd (lxxiii. art. Nervous Centres); Barlow (lxxi. vol. xii.). THE MEDULLA OBLONGATA. Its Structure. The medulla oblongata is a mass of gray and white nervous sub- stance contained within the cavity of the cranium, forming part of the cephalic prolongation of the spinal cord, and connecting it with the brain. The gray substance which it contains is situated in the interior, variously divided into masses and laminae by the white or fibrous substance, which is arranged partly in external columns, and partly in fasciculi traversing the central gray matter. The medulla oblongata is larger than any part of the spinal cord. Its columns are pyriform, enlarging as they proceed towards the brain, continuous with those of the spinal cord, more pro- minent than they are, and separated from one another by deeper grooves. In front, are two corresponding with the anterior columns of the cord, and named anterior pyramids, or corpora pyramidal la (Fig. 72); they are separated from each other by a deep anterior median fissure, at the bot- tom of which fibres appear decussating, ?'. e., crossing one another and changing sides. In this maimer, nearly all the fibres of each pyramid pass over, and turning back- wards become continuous with the op- posite lateral columns of the cord; those which do not decussate are directly con- tinuous with the anterior column of the cord. Traced upwards, the fibres of the anterior pyramids pass through the inferi- or part of the pons Varolii, and then, form- ing the lower part of the crura cerebri, pro- ceed through the optic thalami and cor- pora striata to be distributed in the substance of the cerebral hemispheres.1 (Fig. 74, p.) 1 The expression " continuous fibres," and the like, appear to be usually understood as meaning that certain primitive nerve-fibres pass Avithout in- terruption from one part to the other of those named. But such continuity of primitive fibres through long distances in the nervous centres is very far from proved. The apparent continuity of fasciculi (which is all that dissec- tion can yet trace) is explicable on the supposition that many comparatively Fig. 72. Front view of the medulla ob- longata : p, p. Pyramidal bodies, decussating at d. o, o. Olivary- bodies, r, r. Eestiform bodies. a, a. Arciform fibres, v. Lower fibres of the Pons Variolii. 318 THE MEDULLA OBLONGATA. Fig. 73. External to each anterior pyramid is a prominent oval body (the olivary body), the fibres in and around which are continuous below with those of the corresponding anterior tracts of the cord, while above they pass into the deeper longitudinal fibres of the medulla oblongata, along which they may be traced through the crura cerebri into the lower parts of the optic thalami and corpora striata. The corpora olivaria are formed of portions of gray substance imbedded in fibres, and elevating them. Immediately behind the corpora olivaria, on each side, is a small depressed tract of fibrous matter, distinguished from the olivary tract, because its fibres, instead of passing onwards longitudinally to the cerebrum, go outwards transversely through the pons into the cerebellum. (Fig. 74.) These tracts are named the lateral tracts, and are interesting in that the facial nerve emerges through them, and probably derives from them its connection with the mo- tor portion of the medulla oblon- gata and cord. Behind the lateral tract' on each side, is the corpus rest!forme, a large column of nerve-fibres, which, with its continued fibres below, forms the rcstiform tract. (Fig. 73.) It is continuous be- low with the posterior columns of the cord, while above, its fibres may be traced transversely through the pons into the cere- bellum. Those of each body, form a large portion of the cor- responding cms ccrebelli, and are distributed to the correspond- ing hemisphere of the cerebellum, whence it is probable that con- tinuations from them pass into the cerebrum. The rcstiform bodies are sopa- cd of c, c. posterior columns, and df d, lateral rated from each other, postei'iur- part of the antero-lateral columns of the cord. ]y} by two narrow columns, the a. a. Olivary columns-, as seen on the floor of , , , , • i „ „ , ' , „ A . ' . . . x, iiosii ran- )a/ramu/s, or posterior the fourth ventricle, separated by s, the me- ' . , ' y ' x . of pyranuaid tracts, one on each side of the posterior fissure; and Posterior view of the medulla oblongata: p,p. Posterior pyramids, separated by the pos- terior fissure, r, r. Rcstiform bodies, compos- dian fissure, and crossed ly some fibres origin of n, n, the seventh pair of nerves. short fibres lie parallel, with the ends of each inlaid among many others. In such a case there would be an apparent continuity of fibres : jUst as there is, for example, when one untwists and picks out a long cord of silk or wool, in which each fibre is short, and yet each fasciculus appears to be continued through the whole cord. THE MEDULLA OBLONGATA. 319 by the Lower angle of the fourth ventricle. (Fig. 73.) The fibres of these tracts are continuous below, with a narrow column, which, about the middle of the cervical portion of the cord begins to be, as it were, set off from the posterior columns, by a narrow groove. They seem to pass upwards longitudinally through the pons, and thence in connection with the processes that unite the cerebrum with the cerebellum, under the corpora quadrigemina, and into the crus cerebri of the opposite side. (Fig. 74, w.) This drawing is from a dissection made on a piece of brain, which had been hardened in spirits. It exhibits the course of the sensory columns from the medulla oblongata to the thalamus. C. Anterior optic tubercle. ». Posterior ditto. I & C. Inter-cerebral commissure, or processus e cerebello ad testes, h. Spinal cord. k. Thalamus optici.' m. Corpus stria- tum, u. Crus cerebri, w. Corpus restiforme. x, x. Pons Varolii, b. Optic nerve, c. Third pair, b c. Locus niger. p t. Pyramidal, or motor tract. st,st,st. Sensory tracts—the pos- terior third of the antero-lateral column, s c. Sensory root of the fifth pair of nerves. Deeper than the posterior pyramidal tracts, and forming slight elevations on each side of the middle line of the fourth ventricle, are other two, named the round tracts. They appear to be com- posed of the middle or axial portions of the anterior and lateral columns, which, as they pass upwards, are, as it were, exposed from behind, by the divergence of the rcstiform and posterior pyra- midal tracts. The round tracts pass longitudinally through the pons, and thence proceed, decussating, under the corpora quadrige- mina, to the fibres of the crura cerebri. The continuation of the gray matter of the cord, into the me- dulla oblongata, forms the gray matter covering the floor of the fourth ventricle, and diffused beneath its surface. The separation of the posterior internal, and rcstiform tracts leaves open, in the 320 THE MEDULLA OBLONGATA. fourth ventricle, the upper portion of the canal, which, in the early fcetal state, extends -through the whole length of the gray matter of the spinal cord, and is continuous above with the cerebral ventricles. It is unfortunate that even a much deeper study than is here sketched of the anatomy of the medulla oblongata, affords very little insight into its physiology. The interest connected with the tracing of the continuities of its several columns with those of the spinal cord lies, chiefly, in the fact that nerves of similar function arise from both. Thus, from the anterior pyramids, and their con- tinuation in the crura cerebri, arise the motor third, and sixth pairs of cerebral nerves. From the groove between the anterior pyra- mids and the olivary tracts (a groove continuous with that in which all the motor roots of the spinal nerves emerge), arises the motor hypoglossal nerve. From the lateral and the round tracts, formed of fibres continuous with the anterior and lateral columns of the cord, arise the motor facial, and fourth or trochlear nerves ; while from the front of the restiform tracts, in a line continuous with the groove between the posterior and lateral columns of the cord, spring the roots of the sensitive glosso-pharyngeal, and pneumo- gastric nerves. There is, thus, the closest analogy in structure, and, probably also in the general endowments of their several parts, between the medulla oblongata and the spinal cord. The difference in size and form appears due, chiefly, first, to the divergence, enlargement, and decussation of the several columns, as they pass to be connected with the cerebellum or the cerebrum; and, secondly, to the inser- tion of new quantities of gray matter, in the olivary bodies and other parts, in adaptation to the higher office, and wider range of influence, which the medulla oblongata as a nervous centre exer- cises. Functions of the Medulla Oblongata. In its functions, the medulla oblongata differs from the spinal cord chiefly in the importance and extent of the actions that it governs. Like the cord, it may be regarded first, as conducting impressions, in which office it has a wider extent of function than any other part of the nervous system, since it is obvious that all impressions passing to and fro between the brain and the spinal cord, and all nerves arising below the pons, must be transmitted through it. The modes of conduction through the medulla oblon- gata are probably similar to those through the cord. In the same degree as it is probable that the spinal cord transmits motor im- pressions in its anterior columns, and sensitive impressions chiefly along its posterior columns, so is it that the medulla oblongata con- ducts motor impressions along its anterior pyramidal and olivary THE MEDULLA OBLONGATA. 321 tracts, and sensitive ones along its posterior and rcstiform tracts. This, which might be expected from the continuity of the columns, in the two parts, and the similarity of the nerves arising from them, is further rendered probable by experiments and the results of disease. Magendie divided one of the anterior pyramidal tracts of the medulla oblongata, and observed complete loss of the motor power over one-half of the body, while its sensation seemed to be unimpaired (cxli. t. i. p. 285). In Longet's experiments on dogs and rabbits, irritation of the anterior pyramids appeared to be un- productive of pain, but the slightest touch of the restiform bodies elicited signs of acute suffering (exxxvi. t. i. p. 400). Among the corresponding evidences furnished by disease, Lebert mentions a case in which great disorder of the power of motion with unimpaired sensation, resulted from an affection of the anterior portion of the medulla oblongata; the posterior portion being apparently unharmed (exxxvi. t. i. p. 407). The decussation of part of the fibres of the anterior pyramids of the medulla oblongata, and their crossing into the lateral tracts of the opposite side of the cord, make it probable that the motor impressions proceeding from the brain would, by traversing one pyramid, pass across to the opposite side of the spinal cord. Thus are explained the phenomena of cross-paralysis, as it is termed, i. e., of the loss of motion, in cerebral apoplexy, being always on the side opposite to that on which the effusion of blood has taken place. But, in the present state of the anatomy of the medulla oblongata, it is not possible to explain why the loss of sensation is also on the side opposite the injury or disease of the brain; for there is no evidence of a decussation of posterior fibres nearly equal to that which ensues among the anterior fibres of the medulla oblongata and upper part of the cord. See on the subject, Longet (exxxvi. t. i. p. 400). The functions of the medulla oblongata as a nervous centre are more immediately important to the maintenance of life than those of any other part of the nervous system, since from it alone issues the nervous force necessary for the performance of respiration and deglutition. It has been proved by repeated experiments, espe- cially by those of Legallois (exxxix. t. i. p. 64), Fiourens (cxl.), and Longet (exxxvi.), that the entire brain may be gradually cut away in successive portions, and yet life may continue for a considerable time, and the respiratory movements be uninterrupted. Life may, also, continue when the spinal cord is cut away in successive por- tions from below upwards as high as the point of origin of the phrenic nerve, or in animals without a diaphragm, such as birds or reptiles, even as high as the medulla oblongata. In Amphibia, these two experiments have been combined: the brain being all re- moved from above, and the cord from below; and so long as the 322 THE MEDULLA OBLONGATA. medulla oblongata was intact, respiration and life were maintained. But if, in any animal, the medulla oblongata is wounded, particu- larly if it is wounded in its central part, opposite the origin of the pneumogastric nerves, the respiratory movements cease, and the animal dies as if asphyxiated. And this effect ensues even when all parts of the nervous system, except the medulla oblongata, are left intact.1 Injury and disease in men prove the same as these experiments on animals. Numerous instances are recorded, especially by Sir Charles Boll (cxlii.),in which injury to the human medulla oblon- gata has produced instantaneous death; and, indeed, it is through injury of it, or of the part of the cord connecting it with the origin of the phrenic nerve, that death is commonly produced in fractures and diseases with sudden displacement of the upper cer- vical vertebra. The centre whence "the nervous force for the production of com- bined respiratory movements appears to issue is in the interior of that part of the medulla oblongata from which the pneumogastric nerves arise; for with care the medulla oblongata may be divided to within a few lines of this part, and its exterior may be removed without the stoppage of respiration; but it immediately ceases when this part is invaded. This is not because the integrity of the pneumogastric nerves is essential to the respiratory movements, for both these nerves may be divided without more immediate effect than a retardation of these movements. The conclusion, therefore, may safely be, that this part of the medulla oblongata is the nervous centre wherein the impulses producing the respiratory movements originate, and whence they issue in rhythm and adaptation. The power by which the medulla oblongata governs and com- bines the action of various muscles for the respiratory movements is an Instance of the power of reflection, which it possesses in common with all nervous centres. Its general mode of action, as well as the degree in which the mind may take part in respiration, and the number of nerves and muscles which, under the governance of the medulla oblongata, may be combined in the forcible respira- tory movements, have been already briefly described (see p. 142). That which seems most peculiar in this centre of respiratory action is its wide range of connection, the number of nerves by which the centripetal impression to excite motion may be conducted, and the number and distance of those through which the motor impulse may be directed. The principal centripetal nerves engaged in respiration are the pneumogastric, whose branches supplying the lungs appear to convey the most acute impression of the "necessity 1 Death in such cases may not be immediate, especially if the temperature of the animal be previously reduced. (B. Seure reason, which may be instructed otherwise than through the senses, and exercised independently of the brain. The evidences that the cerebral hemispheres are, in the sense and degree indicated above, the organs of the mind are chiefly these: 1. That any severe injury of them, such as a general concussion, or sudden pressure by apoplexy, may instantly deprive a man of all power of manifesting externally any mental faculty. 2. That in the same general proportion as the higher sensuous mental faculties are developed in the vertebrate animals, and in man at different ages, the more are the size of the cerebral hemi- spheres developed in comparison with the rest of the cerebro-spinal system. 3. That no other part of the nervous system bears a corresponding proportion to the development of the mental faculties. 4. That congenital and other morbid defects of the cerebral hemi- spheres are, in general, accompanied with corresponding deficiency in the range or power of the intellectual faculties and the higher instincts. To explain such facts, no hypothesis (if it must be so called, 1 By understanding, or intellect, is here meant the "faculty of judging ac- cording to sense ;" a faculty, therefore, which has to do with none but sensi- ble things and the ideas derived from them. It is often called "reason," or the reasoning faculty; but the term " reason" is here applied only to the higher faculty which has cognizance of necessary truths, and of things above the senses—that which Scripture designates, or includes in the designation, the " Spirit of man."—In the use and adaptation of the terms here employed, the example of Coleridge is followed. See his " Aids to lteticotion." THE CEREBRAL HEMISPHERES. 337 while we have regard only to the facts of science) is so sufficient as that which supposes an immaterial principle, not necessarily dependent for its existence on the brain, but incapable of external manifestation, or of knowledge of external things, except through the medium of the brain, and the nervous organs connected there- with. Such a principle would remain itself unchanged, in the case of injury or disease of the brain; but its external manifesta- tions, and all its acts performed in connection with the brain, would be hindered or disturbed; as, for example, the work of any artist might be stopped or spoiled through deficiency or badness of his implements of art. And in the operations of such a principle, it might well be supposed that the power with which its several faculties are manifested would bear a direct proportion to the size of the organs through which they are manifested; for whether we suppose or not that the principle itself may, in different individuals, have different degrees of power, yet its power of manifestation or perception through the cerebral hemispheres, may vary as those organs do. But while this may be true respecting those parts of the mind which have to do with the things of sense, it would require much more and different evidence and arguments to make it probable that the cerebral hemispheres, or any other parts of the brain, are in any meaning of the term, the organs of those parts or powers of the mind which are occupied with things above the senses. The reason or spirit of man, which has knowledge of divine truths, and the conscience, with its natural discernment of moral right and wrong, cannot be proved to have any connection with the brain. In the complex life we live, they are, indeed, often exercised in questions in which the intellect or some other lower mental faculty is also concerned; and in all such cases men's actions are deter- mined as good or bad, according to the degree in which they are guided by the higher or by the lower faculties. But the reason and the conscience must be exercised independently of the brain, when they are engaged in the contemplation of things which have not been learned through the senses, or through any intellectual consideration of sensible things. All that a man feels in himself, and can observe in others, of the subjects in which his reason and his conscience are most naturally engaged; of the mode in which they are exercised, and the disturbance to which they are liable by the perception or ideas of sensible things; of the manner and sources of their instruction; of their natural superiority and supremacy over all the other faculties of the mind; and of his consciousness of responsibility for their use; all teaches him that these faculties are wholly different, not in degree only, nor as dif- ferent members of one order, but in kind and very nature from all else of which he is composed; all, if rightly considered, must 29 338 THE CEREBRAL HEMISPHERES. incline him to receive and hold fast the clearer truth which Reve- lation has given of the nature and destinies of the spirit to which these, his highest faculties, belong. Respecting the mode in which the mental principle operates in its connection with the brain there is no evidence whatever. But it appears that, for all but its highest intellectual acts, one of the cerebral hemispheres is sufficient. For numerous cases are recorded in which no mental defect was observed, although one cerebral hemisphere was so disorganized or atrophied, that it could not be supposed capable of discharging its functions. The remaining hemisphere was in these cases adequate to the functions generally discharged by both; but the mind does not seem in any of these cases to have been tested in very high intellectual exercises; so that it is not certain that one hemisphere will suffice for these. In general, the mind combines, as one sensation, the impressions which it derives from one object through both hemispheres, and the ideas to which the two such impressions give rise are single; and in gen- eral, also, the mind acts alike in and through both the hemispheres: its actions being, if one may so speak, symmetrical as the hemi- spheres are. But it would appear that when one hemisphere is disordered, the same object may produce two sensations, and suggest simultaneously different ideas: or, at the same time, two trains of thought may be carried on by the one mind acting, and being acted upon, differently in the two hemispheres. Thus are explicable some of the incoherences of dreaming and delirium; and, especially, those singular cases in which a person in delirium, puzzled by the two different, and seemingly simultaneous, trains of thought in which he is engaged, fancies himself two persons, and, as another, holds conversation with himself.1 In relation to common sensation and the effort of the will, the impressions to and from the hemispheres of the brain are carried across the middle line; so that, in destruction or compression of either hemisphere, whatever effects are produced in loss of sensa- tion or voluntary motion, are observed on the side of the body opposite to that on which the brain is injured. In speaking hitherto of the cerebral hemispheres as the organs of the mind, they have been regarded as if they were single organs, of which all parts are equally appropriate for the exercise of each of the mental faculties. But it is a more probable theory that each faculty has a special portion of the brain appropriated to it as its 1 See Dr. Holland's essay on this subject (clxvii.); and Dr. Wigan's essay, and other Works, on the Duality of the Mind, or, as it would be better called, of the Brain, for every reasonable person is as conscious of his unity as of his identity; indeed, the idea of personal identity involves that of unity. THE CEREBRAL HEMISPHERES. 339 proper organ. For this theory, the principal evidences among those collected by Drs. Gall and Spurzheim are as follows: 1. That it is in accordance with the physiology of the other compound organs or systems in the body, in which each part has its special function; as, for example, of the digestive system, in which the stomach, liver, and other organs perform each their separate share in the general pro- cess of digestion of the food. .2. That, in different individuals, the several mental functions are manifested in very different degrees. Even in early childhood, before education can be imagined to have exercised any influence on the mind, children exhibit various dis- positions, each presents some predominant propensity, or evinces a singular aptness in some study or pursuit; and it is a matter of daily observation that every one has his peculiar talent or propen- sity. But it is difficult to imagine how this could be the case, if the manifestation of each faculty depended on the whole of the brain; different conditions of the whole mass might affect the mind generally, depressing or exalting all its functions in an equal degree, but could not permit one faculty to be strongly and another weakly manifested. 3. The plurality of organs in the brain is supported by the phenomena of some forms of mental derangement. It is not usual for all the mental faculties in an insane person to be equally disordered; it often happens that the strength of some is increased, while that of others is diminished; and in many cases one function only of the mind is deranged, while all the rest are performed in a natural manner. 4. The same opinion is supported by the fact that the several mental faculties are developed to their greatest strength at different periods of life, some being exercised with great energy in,childhood, others only in adult age; and that, as their energy de- creases in old age, there is not a gradual and equal diminution of power in all of them at once, but, on the contrary, a diminution in one or more, while others retain their full strength, or even increase in power. 5. The plurality of cerebral organs appears to be indi- cated by the phenomena of dreams, in which only a part of the mental faculties are at rest or asleep, while the others are awake, and, it is presumed, are exercised through the medium of the parts of the brain appropriated to them. 6. It is stated that the exami- nation of the brains of individuals, each remarkable for some pecu- liar propensity or talent, has always demonstrated a corresponding development of a certain portion of the brain. These facts have been so illustrated and adapted by phrenologists, that the theory of the plurality of organs in the cerebrum, thus made probable, has been commonly regarded as peculiar to phre- nology, and as so essentially connected with it, that, if the system of Grail and Spurzheim be untrue, this theory cannot be maintained. But it is plain that all the system of phrenology built upon the theory may be false, and -the theory itself true: for the school of 340 TDE CEREBRAL HEMISPHERES. Grail and Spurzheim assume, not only this theory, but also that they have determined all the primitive faculties of which the mind consists, i. e., all the faculties to which special organs must be assigned, and the places of all those organs, in the cerebral hemi- spheres, and the cerebellum. Possibly this may be a system of error, founded on a true theory: the cerebrum may have many organs, and the mind as many faculties; but what are the faculties that require separate organs, and where those organs are, may be subjects of which only the first or most general knowledge is yet attained. At any rate, the phrenological physiology of the brain could not be introduced here without more discussion and objection than is consistent with the plan of this work.1 Of the physiology of the other parts of the brain, little or nothing can be said. Of the offices of the corpus callosum, (Fig. 75, K,) or great transverse and oblique commissure of the brain, nothing positive is known. But instances in which it was absent, or very deficient, either without any evident mental defect, or with only such as might be ascribed to coincident affections of other parts, make it probable that the office which is commonly assigned to it, of enabling the two sides of the brain to act in concord, is exercised only in the highest acts of which the mind, acting on the brain, is capable. And this view is confirmed by the very late period of its development, and by its absence in all but the placental Mammalia.3 • To the fornix and other commissures no special functions can be assigned; but it is a reasonable hypothesis that they connect the actions of the parts between which they are severally placed. (Fig. 75.) _ As little is known of the functions of the pineal and pituitary glands. Indeed, Oesterlen raises the question whether either their structure or functions are those of nervous organs, and classes them among the glands without ducts (cli). PHYSIOLOGY OF THE CEREBRAL AND SPINAL NERVES. The cerebral nerves are twelve pairs, and the spinal nerves thirty- one pairs, symmetrically arranged on each side of what, reduced to 1 The phrenological writings of Mr. Combe, and the " Brain and its Phy- siology," by Mr. Noble, are, probably, the best for the medical student who desires to read the arguments in favor of the system. The objections against it may be read in an article in the British and Foreign Medical Re- view, October, 1846; and in the article Phrenology, in the Penny Cyclo- paedia, from which the above is chiefly taken. 2 See cases of congenital deficiency of the corpus callosum, by Mr. Paget and Mr. Henry, in the twenty-ninth and thirty-first volumes of the Medico- Chirurgical Transactions. THE CEREBRAL NERVES. 341 its simplest form, may be regarded as a column or axis of nervous matter, extending from the olfactory bulbs on the ethmoid bone, to Fig. 75. This figure has been introduced with the view of assisting the student in his study of the relations of the inferior longitudinal commissure or fornix, which may be described as commencing in the centre of the thalamus nervi optici (l), proceeding from thence to the base of the brain, where it suddenly bends upwards and forwards, forming by this turn the knuckle (b), which is called corpus albicans or mammillare. This body receives a few fibres (a) from the locus niger (6) in the crus cerebri (5), running forward from thence towards the anterior commissure, receiving fibres from the convolutions at the base of the brain, cross- ing and as it were kneeling upon the anterior commissure (s), and ascending towards the great transverse commissure, forms the anterior pillar of the fornix (c), receiving fibres in its course from the under and front part of the anterior lobes, and thus forming the septum lucidum (d) ; running back from thence, passing in its course backwards over the thalamus nervi optici (l), it spreads laterally, constituting that portion which is called the body of the fornix (e) ; descending again at the back>art of the brain it forms the descending or pos- terior pillar of the fornix tenia hippocampi (v), some of its fibres running back to be connected with the posterior lobes (i); others crossing the projection called hippocampus major (a), to be connected with the middle lobe, and others again passing over the pes hippocampi (h) to be connected with the anterior portion of the middle lobe. Thus does this commissure con- nect different portions of the convoluted surface of the brain together, which are inferior to the great transverse commissure, and on the same side of the mesial line. a. Fibres of the inferior longitudinal commissure, or fornix, from the locus niger. b. Corpus mammillare. o. Anterior pillars of inferior longitudinal commissure, or fornix. D. Septum lucidum. E. I'.'idy of the fornix, or centre of the commissure. F. Taenia hippocampi, or descending fibres of the inferior longitudinal commissure, a. Fibres covering the hippocampus major, n. Fibres covering the pes hippocampi, i. Fibres covering the hippocampus minor. K. Great transverse commissure divided in the mesial line. s. Posterior cerebralganglion, or thalamus. I. Anterior commissure. 5. Section of the crus cerebri. 6. Locus niger. 7. Anterior cere- bral ganglion, or corpus striatum, partially scraped away. the filum terminale of the spinal cord in the lumbar and sacral por- tion of the vertebral canal. The spinal nerves all present certain 29* 342 THE CEREBRAL NERVES. characters in common, such as their double roots; the isolation of the fibres of sensation in the posterior roots, and of those of motion in the anterior roots; the formation of the ganglia on the posterior root; and the subsequent mingling of the fibres in trunks and branches of mixed functions. Similar characters probably belong essentially to the cerebral nerves; but even when one includes the nerves of special sense, it is not possible to discern a conformity of arrange- ment in any besides the fifth or trifacial, which, from its many analogies to the spinal nerves, Sir Charles Bell designated as the spinal nerve of the head. According to their several functions, the cerebral or cranial nerves may be thus arranged :— Nerves of special sense . Olfactory, optic, auditory, part of the glosso- pharyngeal, and the lingual branch of the fifth. " of common sensation The greater portion of the fifth, and part of the glosso-pharyngeal. " of motion . . Third, fourth, lesser division of the fifth, sixth, facial, and hypoglossal. Mixed nerves . . . Pneumogastric, and accessory. The physiology of the several nerves of the special senses will be considered with the organs of those senses. Physiology of the Third, Fourth, and Sixth Cerebral or Cranial Nerves. The physiology of these nerves may be in some degree combined, because of their intimate connection with each other in the actions of the muscles of the eyeball, which they supply. They are pro- bably all formed exclusively of motor fibres : some pain is indicated when the trunk of the third nerve is irritated near its origin, but this may be because of some filaments of the fifth nerve running backwards to the brain in the trunk of the third, or because adja- cent sensitive parts are involved in the irritation. The third nerve, or motor oculi, (Fig. 70, c,) supplies the levator palpebrae superioris muscle, and, of the muscles of the eyeball, all but the obliquus superior or trochlearis, to which the fourth nerve is appropriated, and the rectus externus which receives the sixth nerve. Through the medium of the ophthalmic or lenticular gan- glion, of which it forms what is called the short root, it also sup- plies the motor filaments to the iris. When the third nerve is irritated within the skull, all these muscles to which it is distributed are convulsed. When it is para- lyzed or divided, the following effects ensue: first, the upper eye- lid can be no longer raised by the levator palpebrae, but drops, and remains gently closed over the eye, under the unbalanced influence of the orbicularis palpebrarum, which is supplied by the facial THE CEREBRAL NERVES. 343 Fig. 76. The drawing exhibits the cerebral connection of all the cerebral nerves except the 1st. It is from a sketch of Solly's, taken from two dissections of this part. D. Posterior optic tubercle. The generative bodies of the thalamus are just above it. e. Cerebellum, h. Spinal cord. I. Tuber einereum. k. Optic thalamus divided perpendicularly, w. Corpus restiforme. x. Pons Varolii. 6 6. Optic nerves: this nerve is traced on the left side back beneath the optic thalamus and round the crus cerebri. It divides into four roots; the first (g g) plunges into the substance of the thalamus, the next runs over the external geniculate body and surface of the thalamus, the third goes to the anterior optic tubercle, the fourth runs to D, the testis or posterior optic tubercle. C. Third pair common oculo- muscular, arising by two roots like the spinal roots of the spinal nerves, the upper from the gray neurine of the locus niger, the lower from the continuation of the pyramidal columns in the crus cerebri and^ Pons Varolii, p t. d. Fourth pair, apparently arising from the inter-cerebral commissure (i c), but really plunging down to the olivary tract (o t) as it ascends to the optic tubercles, e m. Motor or non-ganglionie root of the'fifth pair, arising from the posterior edge of the olivary tract, e. Sensory root of the fifth pair running down between the olivary tract and restiform body to the sensory tract. /. Sixth pair, or ab- duecns, arising from the pyramidal tract, g. Seventh pair, facial nerve, or portio dura, arising by an anterior portion from the olivary tract and by a posterior portion from the cerebellic fibres of the anterior columns as they ascend on the corpus restiforme, w. h. Eighth pair, portio mollis, or auditory nerve, with its two roots embracing the restiform body. i. Ninth pair, or glosso-pharyngeal; amiy. Tenth pair, or par vagum, plunging into the restiform ganglion. J J. Fibres of the optio nerve plunging into the thalamus; immedi- ately below these letters is the corpus geniculatum externum. 1c. Eleventh pair, or lingual nerve; the olivary body has been nearly sliced off and turned out of its natural position; gome of the filaments of the lingual nerve are traced into the deeper portion of the gan- glion, which is left in its situation; others wliich are the highest are evidently connected with the pyramidal tract. 341 THE CEREBRAL NERVES. nerve : secondly, the eye is turned outwards by the unbalanced action of the rectus externus, to which the sixth nerve is appro- priated; and hence, from the irregularity of the axes of the eyes, double-sight is often experienced when a single object is within view of both the eyes: thirdly, the eye cannot be moved either up- wards, downwards, or inwards : fourthly, the pupil is dilated. The relation of the third nerve to the iris is of peculiar interest. In ordinary circumstances, the contraction of the iris is a reflex action, which may be explained as produced by the stimulus of light on the retina being conveyed by the optic nerve to the brain (probably to the corpora quadrigemina), and thence reflected through the third nerve to the iris. Hence the iris ceases to act when either the optic or the third nerve is divided or destroyed, or when the corpora quadrigemina are destroyed or much compressed. But when the optic nerve is divided, the contraction of the iris may be excited by irritating that portion of the nerve which is connected with the brain; and when the third nerve is divided, the irritation of its distal portion, will still excite contraction 01 the iris in which its fibres are distributed. The contraction of the iris thus shows all the character of a reflex act, and in ordinary cases requires the concurrent action of the optic nerve, corpora quadrigemina, and third nerve; and probably, also, seeing the peculiarities of its perfect mode of action, the ophthalmic ganglion. But, besides, both irides will contract their pupils under the reflected stimulus of light falling on only one retina or under irritation of one optic nerve. Thus in amau- rosis of one eye, its pupil may contract when the other eye is ex- posed to a stronger light: and generally the contraction of each of the pupils appears to be in direct proportion to the total quantity of light which stimulates either one or both retinae, according as one or both eyes are open. The iris acts also in association with certain other muscles sup- plied by the third nerve: thus, when the eye is directed inwards, or upwards and inwards, by the action of the third nerve distri- buted in the rectus internus and rectus superior, the iris contracts, as if under direct voluntary influence. The will cannot, however, act on the iris alone through the third nerve ; but this aptness to contract in association with the other muscles supplied by the third, may be sufficient to make it act even in total blindness and insen- sibility of the retina, whenever these muscles are contracted. The contraction of the pupils when the eyes are moved inwards, as in looking at a near object, has probably the purpose of excluding those outermost rays of light which would be too far divergent to be refracted to a clear image on the retina; and the dilatation in looking straight forwards, as in looking at a distant object, permits THE CEREBRAL NERVES. 345 the admission of the largest number of rays, of which none are too divergent to be so refracted.1 The fourth nerve, or Nervus trochlcaris or patheticus, (Fig. 76, D,) is exclusively motor, and supplies only the trochlearis or obliquus superior muscle of the eyeball. This muscle acts spasmodically when the nerve is irritated, and is paralyzed when the nerve is divided or otherwise hindered from its function. From this paralysis results a very slight, if any, deviation of the eye from its normal direction ; -the pupil is directed a very little upwards and outwards by the unbalanced action of the obliquus inferior, and a peculiar kind of double vision is produced in which the same object appears' as two, placed one above the other, but again appears single when the head is inclined towards the shoulder of the opposite side to that on which the superior oblique is paralyzed (Szokalski; in Longet, exxxvi. vol. ii. p. 398). These, phenomena are explained by the peculiar actions of the oblique muscles, which, as Hunter2 showed (i. vol. iv. p. 274), rotate the eye round its anteropos- terior axis, or round such an imaginary line as would nearly cor- respond with the prolongation of the optic nerve. Thus, when the head is bent down for a certain distance towards either (say the left) shoulder, the corresponding points of the retinas of both eyes may be held on a level horizontal line by the superior oblique of the right eye rotating the inner part of the eye downwards, and the inferior oblique of the left eye rotating the inner part of its eye upwards. Thus in health, the mind receives a similar and single impression from an object whether the head is erect or turned towards either side, through the action of the inferior oblique of one eye being associated with that of the superior oblique of the other. And thus in disease, when one superior oblique is paralyzed, the inner half of the retina of that eye is rotated upwards, and when the image of any object falls on it, the mind refers that ob- ject to a point lower than that to which it refers the image of the same object on the other retina, though all the inner parts of the latter retina are really lower than the corresponding points of the retina on the paralyzed side. The sixth nerve, Nervus abducens or ocularis cxternus, (Fig. 76, F,) is also, like the fourth, exclusively motor, and supplies only the rec- tus externus muscle.3 The rectus externus is, therefore, convulsed, 1 On the contractions of the iris, and the functions of all its nerves, see Dr. Radclyffe Hall's essays (xciv. 1846). 2 And more lately Hueck (clxvi.); Volkmann (xv. art. Schen.) ; and Dr. G. Johnson (lxxiii. art. Orbit). 3 In several animals, it sends filaments to the iris (Itadclyffe Hall) ; and it has probably done so in man, in some instances in which the iris has not 346 THE CEREBRAL NERVES. and the eye is turned outwards, when the sixth nerve is irritated ; and the muscle paralyzed when the nerve is disorganized, com- pressed, or divided. In all such cases of paralysis, the eye squints inwards and cannot be moved outwards. In its course through the cavernous sinus, the sixth nerve forms larger communications with the sympathetic nerve, than any other nerve within the cavity of the skull does; and, on this ground, used to be considered as giving origin to the sympathetic. But the import of these communications with the sympathetic, and the subsequent distribution of its filaments after joining the sixth nerve, are quite unknown ; and there is no reason to believe that ' the sixth nerve is, in function, more closely connected with the sympathetic than any other cerebral nerve is. The question has often suggested itself, why the six muscles of the eyeball should be supplied by three motor nerves when all of them are within reach of the branches of one nerve; and the true explanation would have more interest than attaches to the move- ments of the eye alone, since it is probable that we have in this in- stance, within a small space, an example of some general rule, according to which, associate or antagonist muscles are supplied with motor nerves. Now, in the several movements of the eyes, we sometimes have to act with symmetrically placed muscles, as when both eyes are turned upwards or downwards, inwards or outwards.1 All the symmetrically placed muscles are supplied with symmetrical nerves, ■i. e., with corresponding branches of the same nerves on the two sides; and the action of these symmetrical muscles is easy and natural, as we have a natural tendency to symmetrical movement of most parts. But, because of this tendency to symmetrical movements of muscles supplied by symmetrical nerves, it would appear as if, when the two eyes are to be moved otherwise than symmetrically, the muscles to effect such a movement must be sup- plied with different nerves. So, when the two eyes are to be turned towards one side, say the right, by the action of the rectus externus of the right eye, and the rectus internus of the left, it appears as if the tendency to action, through the similar branches of corresponding nerves (which would move both eyes inwards or outwards) were corrected, by one of these muscles being supplied by the sixth, and the other by the third nerve. So, with the been paralyzed, while all the other parts supplied by the third nerve were. (See Grant, in Longet, exxxvi. t. ii. p. 088). 1 It is sometimes said that the two external recti cannot be put in action simultaneously: yet they are so, when the eyes, having been both directed inwards, are restored tp the position which they have in looking straight forwards. THE CEREBRAL NERVES. 347 oblique muscles : the simplest and easiest actions would be through branches of the corresponding nerves, acting similarly as symme- trical muscles; but the necessary movements of the two eyes re- quire the contraction of the superior oblique of one side, to be associated with the contraction of the inferior oblique, and the re- laxation of the superior oblique of the opposite side. For this, the fourth nerve of one side is made to act with a branch of the third nerve of the other side; as if, thus, the tendency to simulta- neous action through the similar nerves of the two sides were prevented. At any rate, the rule of distribution of nerves here, seems to be, that when, in frequent and necessary movements, any muscle has to act with the antagonist of its fellow on the opposite side, it and its fellow's antagonist are supplied from different nerves. Physiology of the Fifth or Trigeminal Nerve. The fifth, trigeminal, or trifacial nerve, (Fig. 76, E,) resembles, as already stated, the spinal nerves, in that its branches are derived through two roots; namely, the portio major, the filaments of which expand to receive the corpuscles that form the Casserian ganglion, and the portio minor, which has no ganglion, and passes under the ganglion of the portio major to join the third branch or division which issues from it. The first and second divisions of the nerve, which arise wholly from the ganglion of the portio major, are purely sensitive. The third division, which is formed in part by the portio minor, and in part from the Casserian ganglion, is, in its trunk and many of its branches, both motor and sensitive. Through the branches of the greater or ganglionic portion of the fifth nerve, all the anterior and antero-lateral parts of the face and head, with the exception of the skin of the parotid region (which derives branches from the cervical spinal nerves,) acquire common sensibility; and among these parts may be included the organs of special sense, from which common sensations are conveyed through the fifth nerve, and their peculiar sensations through their several nerves of special sense. All the muscles, also, acquire muscular sensibility through the filaments of the ganglionic portion of the fifth nerve distributed to them with their proper motor nerves. Through branches of the lesser or non-ganglionic portion of the fifth the muscles of mastication, namely, the temporal masseter, two pterygoid, anterior part of the digastric, and mylo-hyoid, derive their motor nerves. The motor function of these branches is proved by the violent contraction of all the muscles of mastication in ex- perimental irritation of the third, or inferior maxillary, division of the nerve; by paralysis of the same muscles when it is divided or disorganized, or from any reason deprived of power; and by the retention of the power of these muscles when all those supplied by 348 THE CEREBRAL NERVES. the facial nerve lose their power through paralysis of that nerve. The last instance proves best that, though the buccinator muscle gives passage to, and receives some filaments from, a buccal branch of the inferior division of the fifth nerve, yet it derives its motor power from the facial, for it is paralyzed together with the other muscles that are supplied by the facial, but retains its power when the other muscles of mastication are paralyzed. It is probable, therefore, that the buccal branch of the fifth contains only sensitive fibres; and that of these some are supplied to the buccinator muscle, as to all the other muscles some sensitive fibres are distributed to confer muscular sensibility. The sensitive function of the branches of the greater division of the fifth nerve is proved by all the usual evidences, such as their distribution in parts that are sensitive and not capable of muscular contraction, the exceeding sensibility of some of these parts, their loss of sensation when the nerve is paralyzed or divided, the pain without convulsions produced by morbid or experimental irritation of the trunk or branches of the nerve, and the analogy of this por- tion of the fifth to the posterior root of a spinal nerve. (See Lon- get and others.) But although formed of sensitive filaments exclusively, the branches of the greater or ganglionic portion of the fifth nerve ex- ercise a manifold influence on the movements of the muscles of the head and face and other parts in which tbey are distributed. They do so, in the first place, by providing the muscles themselves with that sensibility, without which the mind, being unconscious of their position and state, cannot voluntarily exercise them. It is, probably, for conferring this sensibility on the muscles, that the branches of the fifth nerve anastomose so frequently with those of the facial and hypoglossal, and the nerves of the muscles of the eye; and it is because of the loss of this sensibility that when the fifth nerve is divided, animals are always slow and awkward in the movements of the muscles of the face and head, or hold them still, or guide their movements by the sight of the objects towards which they wish to move. Again, the fifth nerve has an indirect influence on the muscular movements by conveying sensations of the state and position of the skin and other parts; which the mind perceiving is enabled to de- termine appropriate acts. Thus, when the fifth nerve, or its infra- orbital branch is divided, the movements of the lips in feeding may cease or be imperfect; a fact which led Sir Charles Bell into one of the very few errors of his physiology of the nerves. He sup- posed that the motion of the upper lip, in grasping food, depended directly on the infra-orbital nerve; for he found that after he had divided that nerve on both sides in an ass, it no longer seized the food with its lips, but merely pressed them against the ground, and TIIE CEREBRAL NERVES. 349 used the tongue for the prehension of the food. 31r. Mayo correct- ed this error. He found, indeed, that, after the infra-orbital nerve had been divided, the animal did not seize its food with the lip, and could not use it well during mastication, but that it could open the lips. He, therefore, justly attributed the phenomena in Sir C. Bell's experiments to the loss of sensation in the lips; the animal not being able to feel the food, and, therefore, although it had the power to seize it, not knowing how or where to use that power. Lastly, the fifth nerve has an intimate connection with muscular movement through the many reflex acts of muscles of which it is the necessary excitant. Hence, when it is divided, and can no longer convey impressions to the nervous centres to be thence re- flected, the irritation of the conjunctiva produces no closure of the eye, the mechauical irritation of the nose excites no sneezing, that of the tongue no flowing of saliva; and although tears and saliva may flow naturally, their efflux is not increased by the mechanical or chemical or other stimuli, to the indirect or reflected influence of which it is liable in the perfect state of this nerve. The fifth nerve, through its ciliary branches and the branch which forms the long root of the ciliary or ophthalmic ganglion, exercises, also, some influence on the movements of the iris. When the trunk or the ophthalmic portion is divided, the pupil becomes, according to Valentin (iv. vol. ii. p. 666), contracted in men and rabbits, and dilated in cats and dogs; but, in all cases, becomes immovable, even under all the varieties of the stimulus of light. How the fifth nerve thus effects the iris is unexplained; according to Longet, the same effects are produced by destruction of the superior cervical ganglion of the sympathetic, so that possibly they are due to the injury of those filaments of the sympathetic which, after joining the trunk of the fifth at and beyond the Casserian ganglion, proceed with the branches of its ophthalmic division to the iris: or, as Dr. R. Hall ingeniously suggests, the influence of the fifth nerve on the movements of the iris may be ascribed to the affection of vision in consequence of the disturbed circulation or nutrition in the retina, when the normal influence of the fifth nerve and ciliary ganglion is disturbed. In such disturbance, increased circulation making the retina more irritable might induce extreme contraction of the iris; or, under moderate stimulus of light, pro- ducing partial blindness, might induce dilatation ; but it does not appear why, if this be the true explanation, the iris should in either case be immovable and unaffected by the various degrees of light. Furthermore, the complete paralysis or division of the fifth nerve, by the morbid effects which it produces in the organs of special sense, makes it probable that, in the normal state, the fifth nerve exercises some direct influence on all these organs or their func- 30 350 THE CEREBRAL NERVES. tions. Thus, after such complete paralysis, within a period varying from twenty-four hours to a week, the cornea begins to be opaque; then it grows completely white; a low destructive inflammatory process ensues in the conjunctiva, sclerotica, and interior parts of the eye; and within one or a few weeks, the whole eye may be quite disorganized, and the cornea may slough or be penetrated by a large ulcer. The sense of smell (and not merely that of mechani- cal irritation in the nose), may be at the same time lost, or gravely impaired; so may the hearing; and commonly, whenever the fifth nerve is paralyzed, the tongue loses the sense of taste in its ante- rior and lateral parts, i. e., in the portion in wliich the lingual or gustatory branch of the inferior maxillary division of the fifth is distributed. The loss of the sense of taste may be due to the lingual branch of the fifth nerve being, really, a nerve of special sense ; or it may be because it supplies, in the anterior and lateral parts of the tongue, a necessary condition for the proper nutrition of that part. But, deferring this question till the glosso-pharyngeal nerve is to be considered, it may be observed that in some brief time after complete paralysis, or division, of the fifth nerve, the power of all the organs of the special senses may be lost; they may lose not merely their sensibility to common impressions, for which they all depend directly on the fifth nerve, but also their sensibility to the several peculiar impressions for the reception and conduction of which they are purposely constructed and supplied with special nerves besides the fifth. The facts observed in these cases1 can, perhaps, be only explained by the influence which the fifth nerve exercises on the nutritive processes in the organs of the special senses. It is not unreasonable to believe that, in paralysis of the fifth nerve, their tissues may be the seats of such changes as are seen in the laxity,. the vascular congestion, oedema, and other affections of the skin of the face and other tegumentary parts which also accompany the paralysis; and that these changes, which may appear unimportant when they affect external parts, are sufficient to destroy that refine- ment of structure by which the organs of the special senses are adapted to their functions. In the chapter on Nutrition (p. 236), the question is mentioned whether of the two, the fifth or the sympathetic nerve, conveys the impression by which the nutrition of the parts is influenced; and it is stated that Magendie and Longet have observed that the destruction of the eye ensues more quickly after division of the trunk of the fifth beyond the Casserian ganglion, or after division of the ophthalmic branch, than after division of the roots of the fifth between the brain and the ganglion. Hence it would appear _' Two of the best cases are published, with analysis of others, by Mr. Dixon, in the Medico-Chirurgical Transactions, vol. xxviii. THE CEREBRAL NERVES. 351 as if the influence on nutrition were conveyed through the filaments of the sympathetic, which join the branches of the fifth nerve at and beyond the Casserian ganglion, rather than through the fila- ments of the fifth itself; and this is confirmed by some experiments in which extirpation of the superior cervical ganglion of the sym- pathetic appeared to produce the same destructive disease of the eye as commonly follows the division of the fifth nerve. And yet, that the filaments of the fifth nerve, as well as those of the sympathetic, may conduct such influence appears certain from the cases, including that by Mr. Stanley, in which the source of the paralysis of the fifth nerve was near the brain, or at its very origin, before it receives any communication from the sympathetic nerve. The problem, therefore, cannot yet be certainly solved. The existence of ganglia of the sympathetic in connection with all the principal divisions of the fifth nerve where it gives off those branches which supply the organs of special sense—for example, the connection of the ophthalmic ganglion with the ophthalmic nerve at the origin of the ciliary nerves; of the sphenopalatine ganglion with the superior maxillary division where it gives its branches to the nose and the palate; of the otic ganglion with the inferior maxillary near the giving off of filaments to the internal ear; and of the submaxillary ganglion with the lingual branch of the fifth—all these connections suggest that a peculiar and probably Conjoint influence of the sympathetic and fifth nerves is exercised in the nutrition of the organs of the special senses; and the results of experiment and disease confirm this by showing that the nutrition of the organs may be impaired in consequence of impairment of the power of either of the nerves. A singular connection between the fifth nerve and the sense of sight is shown in cases of no unfrequent occurrence, in which blows or other injuries implicating the frontal nerve as it passes over the brow are followed by total blindness in the corresponding eye. The blindness appears to be the consequence of defective nutrition of the retina; for although, in some cases, it has ensued immedi- ately, as if from concussion of the retina, yet in some it has coma on gradually, like slowly progressive amaurosis, and in softie with inflammatory disorganization, followed by atrophy of the whole eye.1 And, again, the fifth nerve is shown intimately connected with the third by cases in wliich paralysis of the third has followed neural- gia of the fifth; and not less, by the influence of belladonna applied to the filaments of the fifth, and producing a kind of paralysis of the iris through a reflected narcotizing influence on the branches of the third. 1 Such a case is recorded by Snabilic in the Ncdcrlandsch Lancet, August, 1S46. 352 THE CEREBRAL NERVES. Physiology of the Facial Nerve. The facial, or portia dura of the seventh pair of nerves, (Fig. 76, g,) is the motor nerve of all the muscles of the face, including the platysma, but not including any of the muscles of mastication already enumerated (p. 34S); it supplies, also, through the connection of its trunk with the Vidian nerve, by the petrosal nerves, some of the muscles, most probably the levator palati and azygos uvulae, of the soft palate; by its tympanic branches it supplies the stapedius and laxator tympani, and through the otic ganglion the tensor tympani; through the chorda tympani it supplies the lingualis and some other muscular fibres of the tongue; and by branches given off before it comes upon the face, it supplies the muscles of the external ear, the posterior part of the cligastricus, and the stylo-hyoicleus.1 To all these muscles it is the sole motor nerve, and it is probably exclusively motor in its power; no pain is produced by irritating it near its origin (Valentin), and the indications of pain which are elicited when any of its branches are irritated may be explained by the abundant anastomoses which, in all parts of its course, it forms with sensitive nerves, whose filaments being mingled with its own are the true source of the pain. Such anastomoses are effected with the fifth nerve through the petrosal branches of the Vidian, and probably also through the chorda tympani, and with the pneu- mogastric nerve through its auricular branch, even before the facial leaves the cranium. When the facial nerve is divided, or in any other way paralyzed, the loss of power in the muscles which it supplies, while proving the nature and extent of its functions, displays also the necessity of its perfection for the perfect exercise of all the organs of the special senses. Thus, in paralysis of the facial nerve, the orbicu- laris palpebrarum being powerless, the eye remains open through the unbalanced action of the levator palpebrae, and the conjunctiva, thus continually exposed to the air and the contact of dust, is liable to repeated inflammation, which may end in thickening and opa- city of both its own tissue and that of the cornea. These changes, however, ensue much more slowly than those which follow paralysis of the fifth nerve, and never bear the same destructive character; both because the nutrition of the eye is not directly interfered with, and because the globe can still be moved upwards and inwards, so as to carry the cornea partially under the angle of the upper eyelid in winking and sleeping. In paralysis of the facial nerve, also, tears are apt to flow constantly over the face, apparently because of the paralysis of the tensor tarsi muscle, and the loss of 1 On the minute anatomy of the facial" nerve, see especially Morganti (cxx. 1845, or an abstract in xxv. 1844—'), p. 53); and lieck (clxiv.). THE CEREBRAL NERVES. 353 the proper direction and form of the orifice of the puncta lacryma- lia. By these things the sense of sight is impaired. The sense of hearing, also, is impaired in many cases of paralysis of the facial nerve ; not only in such as are instances of simulta- neous disease in the auditory nerves, but in such as may be ex- plained by the loss of power in the muscles of the internal ear. The sense of smell is commonly, at the same time, impaired through the inability to draw air briskly towards the upper part of the nasal cavities, in which part alone, the olfactory nerve is distributed ; because, to draw the air perfectly in this direction, the action of the dilators and compressors of the nostrils should be perfect. Lastly, the sense of taste is impaired, or may be wholly lost in paralysis of the facial nerve, provided the source of the paralysis be in some part of the nerve, between its origin, and the giving off of the chorda tympani. This result, which has been observed in many instances of disease of the facial nerve in man,1 appears ex- plicable only by the influence which, through the chorda tympani, it exercises on the movements of the lingualis and the adjacent muscular fibres of the tongue; and, according to some, or probably in some animals, on the movements of the stylo-glossus. This result is not due to any gustatory fibres conveyed by the chorda tympani from the Vidian nerve to the tongue; for the loss of taste is observed when the facial nerve is paralyzed by some affection be- hind the junction of the great petrosal branch of the Vidian, when, therefore, whatever filaments of the Vidian there may be in the chorda tympani, are unaffected. We can, therefore, only suppose that the accurate movement of these muscles of the tongue is es- sential to the exercise of taste; a fact, if it be so, which is the more singular, because the sense of taste is not materially impaired in cases of paralysis of all the other muscles of the tongue, through injury of the hypoglossal nerve. Together with these effects of paralysis of the facial nerve, the muscles of the face being all powerless, the countenance acquires on the paralyzed side a characteristic, vacant look, from the ab- sence of all expression : the angle of the mouth is lower, and the paralyzed half of the mouth looks longer than that on the other side ; the eye has an unmeaning stare. All these peculiarities in- crease, the longer the paralysis lasts; and their appearance is exag- gerated, when, at any time, the muscles of the opposite side of the face are made active in any expression, or in any of their ordinary functions. In an attempt to blow or whistle, one side of the mouth 1 See especially C. Bernard (exxii. 1844). See also Gnarini (cxx. 1812), and Verga (xc. 1843; and for evidences against this view, see Morganti (cxx. 1845 and 1846). He maintains that the chorda tympani is formed exclusively of sensitive fibres; but in this, he is most probably wrong. 30* 354 THE CEREBRAL NERVES. and cheek acts properly, but the other side is motionless, or flaps loosely at the impulse of the expired air; so, in trying to suck, one side only of the mouth acts; in feeling, the lip and cheek are powerless, and food lodges between the cheek and gum. The number of movements concerned in respiration, which are performed under the control of the facial nerve, and the great share which it has in the movements most expressive of the states of the mind, led Sir Charles Bell to place the facial in his class of respiratory nerves. But there are no instances in which, when unable to act under ordinary stimuli, or in other functions, thf facial nerve has yet been capable of action in respiratory move- ments ; its paralysis, when complete, is so in respect to every func- tion alike. As a nerve of expression, it must not be considered independent of the fifth nerve, with which it forms so numerous anastomoses; for, although it is through the facial nerve alone that all the muscles of the face are put into their naturally ex- pressive actions, yet the power which the mind has of suppressing, controlling, and imitating, or acting all these expressions, can only be exercised by voluntary and well-educated actions, directed through the facial nerve with the guidance of the knowledge of the state' and position of every muscle; which knowledge is acquired only through the fifth nerve, which confers sensibility on the muscles, and appears, for this purpose, to be more abundantly supplied to the muscles of the face, than any other sensitive nerve is to those of other parts. It has been already said, that the facial nerve perhaps supplies the levator palati and azygos uvulae muscles with motor power; but the same is also ascribed, as probable, to the pneumogastric and accessory nerves. The evidence for the facial is, chiefly, the fact that when it is paralyzed the uvula often deviates to the opposite side, and recovers its median position when the paralysis ceases; a condition which is also said to be sometimes observed when the petrosal nerves, through which alone the facial can supply the palate, are injured in fracture of the base of the skull. The mid- dle posterior palatine nerve, also, passes into the levator palati and azygos uvulae, and may, through the petrosal nerves and spheno- palatine ganglion, receive filaments from the facial nerve. But, on the other hand, irritation of the trunk of the facial nerve produces no contractions of these muscles of the palate (Ilein, lxxx. 1844; Valentin, iii.); and the experiments of Hein seemed to show that such contractions did follow the irritation of the pneumogastric and accessory nerves, from one or both of which filaments pass to the palate through branches of the glosso-pharyngcal.1 1 The several cases relating to this question are given in xxv. 1843—5. THE CEREBRAL NERVES. 355 Physiology of the Glosso-Pharyngcal Nerve. The glosso-pharyngeal nerves, in the enumeration of (Fig. 76, i,) the cerebral nerves by numbers according to the position in which they leave the cranium, are considered as divisions of the eighth pair of nerves, in wliich term are included with them the pneumogastric and accessory nerves. But the union of the nerves under one term is inconvenient, although in some parts the glosso-pharyngeal and pneumogastric are so combined in their distribution that it is im- possible to separate them in either anatomy or physiology. The glosso-pharyngeal nerve appears to give filaments through its tympanic branch (Jacobson's nerve), to the fenestra ovalis, and fenestra rotunda, and the Eustachian tube; also, to the carotid plexus, and, through the lesser petrosal nerve, to the sphenopala- tine ganglion.1 After communicating, either within or without the cranium, with the pneumogastric, and, soon after it leaves the cranium, with the sympathetic, digastric branch of the facial, and the accessory nerve, the glosso-pharyngeal nerve parts into the two principal divisions indicated by its name, and supplies the mucous membrane of the posterior and lateral walls of the upper part of the pharynx, the Eustachian tube, the arches of the palate, the tonsils and their mucous membrane, and the tongue as far forwards as the foramen caicuin in the middle line, and to near the tip at the sides and inferior part. Some experiments make it probable that the glosso-pharyngeal nerve contains, even at its origin, some motor fibres, together with those of common sensation and the sense of taste. For Volkmann (lxxx. 1840), and Hein (lxxx. 1844), when they divided the nerve within the skull, and then irritated its distal portion, saw move- ments of the pharynx and of the palate and its arches, which appeared to be due to contractions of the stylo-pharyngeus, and, perhaps, also, of the palato-glossus muscles. And the recent ex- periments of Biffi and Morganti (lxxx. 1847, p. 360), confirm these, although the former ones (cxx. 1847) did not. Whatever motor influence, therefore, is conveyed directly through branches of the glosso-pharyngeal may be ascribed to the filaments of the pneumo- gastric or accessory that are mingled with it. The experiments of Dr. John Reid (xciv. 1838), confirming those of Panizza and Longet, tend to the same conclusions; and their results probably express nearly all the truth regarding the part of the glosso-pharyngeal which is distributed to the pharynx. These results were that: 1. Pain was produced when the nerve, particularly its pharyngeal branches, were irritated. 2. Irritation of the nerve before the giving off its pharyngeal branches, or of 1 See especially Beck (clxiv.). 356 THE CEREBRAL NERVES. any of these branches, gave rise to extensive muscular motions of the throat and lower part of face: but, when the nerve was divided, these motions were excited by irritating the upper or cranial portion, while irritation of the lower end, or that in connection with the muscles, was followed by no movement; so that these motions must have depended on a reflex influence transmitted to the muscles through other nerves by the intervention of the nervous centres. 3. When the functions of the brain and medulla oblongata were arrested by poisoning the animal with prussic acid, irritation of the glosso-pharyngeal nerve, before it was joined by any branches from the pneumogastric, gave rise to no movements in the muscles of the pharynx or other parts to which it was distributed; while, on irritating the pharyngeal branch of the pneumogastric, or the glosso- pharyngeal nerve, after it had received the communicating branches just alluded to, vigorous movements of all the pharyngeal muscles and of the upper part of the oesophagus followed. The most probable conclusion, therefore, "may be that what motor influence the glosso-pharyngeal nerve may seem to exercise, is due either to the filaments of the pneumogastric or accessory that are mingled with it, or to impressions conveyed through it to the medulla oblongata, and thence reflected to muscles through motor nerves, especially the pneumogastric, accessory, and facial. Thus, j» the glosso-pharyngeal nerve excites through the medium of the medulla oblongata the actions of the muscles of deglutition. It is the chief centripetal nerve engaged in these actions; yet not the only one, for, as Dr. John Reid has shown, the acts are scarcely disturbed or retarded when both the glosso-pharyngeal nerves are divided. But besides being thus a nerve of common sensation in the parts which it supplies, and a centripetal nerve through which impressions are conveyed to be reflected to the adjacent muscles, the glosso- pharyngeal is also a nerve of special sensation; being the gustatory nerve, or nerve of taste in all the parts of the tongue to which it is distributed. After many discussions, the question, which is the nerve of taste?—the lingual branch of the fifth, or the glosso- pharyngeal ?—may be most probably answered by stating that they are both nerves of this special function. For very numerous experiments and cases have shown that when the trunk of the fifth nerve or its lingual branch is paralyzed or divided, the sense of taste is completely lost in the superior surface of the anterior and lateral parts of the tongue. The loss is instantaneous after division of the nerve; and, therefore, cannot be ascribed to the defective nutrition of the part, though to this, perhaps, may be ascribed the more complete and general loss of the sense of taste when the whole of the fifth nerve has been long paralyzed. But, on the other hand, while the loss of taste in the part of the THE CEREBRAL NERVES. 357 tongue to which the lingual branch of the fifth nerve is distributed proves that to be a gustatory nerve, the fact that the sense of taste is at the same time retained in the posterior and postcro-lateral parts of the tongue, and in the soft palate and its anterior arch, to which (and to some parts of which exclusively) the glosso-pharyngeal is distributed, proves that this also must be a gustatory nerve. In a patient lately in St. Bartholomew's Hospital, the left lingual branch of the fifth nerve was divided in removing a portion of the lower jaw: she lost both common sensation and the sensation of taste in the tip and anterior parts of the left half of the tongue, but retained both in all the rest of the tongue. M. Lisfranc and others have noted similar cases; and the phenomena in them are so simple and clear, that there can scarcely be any fallacy in the conclusion that the lingual branches of both the fifth and the glosso-pharyngeal nerves are gustatory nerves in the parts of the tongue which they severally supply. This conclusion is confirmed by some experiments on animals ;l and, perhaps, more satisfactorily as concerns the sense of taste in man, by observation of the parts of the tongue and fauces in which the sense is most acute. According to Valentin's experiments made on thirty students, the parts of the tongue from which the clearest sensations of taste are derived, are the base, as far as the foramen cascum and lines diverging forwards on each side from it; the posterior palatine arches down to the epiglottis ; the tonsils and upper part of the pharynx over the root of the tongue. These are the seats of the distribution of the glosso-pharyngeal nerve. The anterior dorsal surface, and parts of the anterior and inferior parts of the tongue, in which the lingual branch of the fifth is alone dis- tributed, conveyed no sense of taste in the majority of the subjects of Valentin's experiments; but even if this were generally the case, it would not invalidate the conclusion that, in those who have the sense of taste in the anterior and upper part of the tongue, the lingual branch of the fifth is the nerve by which it is exercised. And the same may be said of the soft palate and uvula; in those who have the sense of taste in these parts its nerves must be branches of the fifth; for, unless it be through the minute branch which passes into the Jacobsonian plexus, and might thence pass through the inferior petrosal nerve and spheno-palatine ganglion, the glosso-pharyngeal nerve can send no filaments to the soft palate. 1 Namely, those of Magendie, Mayo, Muller, and Kornfeld (see Miillcr xxxii. p. 770, Am. Ed.) ; and most completely by those of Dr. Alcock (lxxi. 1830), and of Morganti and Biffi (cxx. 1847). (In the coiici'ary are the experiments of I'anizza (recorded by Dr. Burrows, lxxi. vol. xvi.) ; of Valentin (iii. and iv.), and of Wagner (xxxviii. No. 75). Some explanation of the probable source of the contradiction is given by Morganti (/. c.). 358 THE CEREBRAL NERVES. Physiology of the Pneumogastric Nerve. The pneumogastric nerve, nervus vagus, or par vagum, (Fig. 76, if) has, of all the cranial and spinal nerves, the most various distri- bution, and influences the most various functions, either through its own filaments or those which, derived from other nerves, are mingled in its branches. The parts supplied by the branches of the pneumogastric nerve are as follows : by its pharyngeal branches, which enter the pha- ryngeal plexus, a large portion of the mucous membrane, and, probably, all the muscles of the pharynx; by the superior laryn- geal nerve, the mucous membrane of the under surface of the epi- glottis, the glottis, and the greater part of the larynx, and the crico- thyroid muscle; by the inferior laryngeal nerve, the mucous mem- brane and muscular fibres of the trachea, the lower part of the pharynx and larynx, and all the muscles of the larynx except the crico-thyroid; by oesophageal branches, the mucous membrane and muscular coats of the oesophagus. Moreover, the branches of the pneumogastric nerve form a large portion of the supply of nerves to the heart and the great arteries through the cardiac nerves, derived from both the trunk and the recurrent nerve ; to the lungs, through both the anterior and the posterior pulmonary plexuses; and to the stomach by its terminal branches passing over the walls of that organ. From the parts thus enumerated as receiving nerves from the pneumogastric, it might be assumed that it is a nerve of mixed function, both sensitive and motor. Experiments prove that it is so from its origin, for the irritation of its roots, even within the cranial cavity, produces both pain and convulsive movements of the larynx and pharynx, and when it is divided within the skull the same movements follow the irritation of the distal portion, showing that they are not due to reflex action. Similar experiments prove that, through its whole course, it contains both sensitive and motor fibres, but after it has emerged from the skull, and, in some in- stances even sooner, it enters into so many anastomoses that it is hard to say whether the filaments it contains are, from their origin, its own, or whether they are derived from other nerves combining with it. This is particularly the case with the filaments of the sympathetic nerve, which are abundantly added to nearly all the branches of the pneumogastric. The likeness to the sympathetic which it thus acquires, is further increased by its containing many filaments derived, not from the brain, but from its own petrosal ganglia, in which filaments originate, in the same manner as in the ganglia of the sympathetic, so abundantly that the trunk of the nerve is visibly larger below the ganglia than above them (Bidder and Volkmann, xv. article Nervenphysiologic). Next to THE CEREBRAL NERVES. 359 the sympathetic nerve, that which most importantly communicates with the pneumogastric is the accessory nerve, whose internal branch joins its trunk, and is lost in it. . Properly, therefore, the pneumogastric might be regarded as a triple-mixed nerve; having, out of its own sources, motor, sensi- tive, and sympathetic or ganglionic nerve-fibres ; and to this natural complexity it adds that which it derives from the reception of fila- ments from the sympathetic, accessory, and cervical nerves, and, probably, the glosso-pharyngeal and facial. The most probable account of the particular functions which the branches of the pneumogastric nerve discharge in the several parts to wliich they are distributed may be drawn from Dr. John Reid's experiments on dogs (xciv. vols. xlix. and li.). They show that —1. The pharyngeal branch is the principal, if not the sole, motor nerve of the pharynx and soft palate,1 and is most probably wholly motor, a part of its motor fibres being derived from the internal branch of the accessory nerve. 2. The inferior laryngeal nerve is the motor nerve of the larynx, irritation of it producing vigorous movements of the arytenoid cartilages; while irritation of the superior laryngeal nerve gives rise to no action in any of the mus- cles attached to the arytenoid cartilages, but merely to contractions of the crico-thyroid muscle. 3. The superior laryngeal nerve is chiefly sensitive; the inferior, for the most part, motor; for divi- sion of the recurrent nerves puts an end to the motions of the glottis, but without lessening the sensibility of the mucous mem- brane; and division of the superior laryngeal nerves leaves the movements of the glottis unaffected, but deprives it of its sensibility. 4. The motions of the oesophagus are dependent on motor fibres of the pneumogastric, and are probably excited by impressions made upon sensitive fibres of the same; for irritation of its trunk excites motions of the oesophagus, which extend over the cardiac portion of the stomach; and division of the trunk paralyzes the oesophagus, which then becomes distended with the food. 5. The cardiac branches of the pneumogastric nerve are one, but not the sole channel through which the influence of the central organs and of mental emotions is transmitted to the heart. 6. The pulmonary branches form the principal, but not the only channel by which the impressions on the mucous surface of the lungs that excite respiration are transmitted to the medulla oblongata. Dr. Reid was unable to determine whether they contain motor fibres; but reasons for believing that they do so have been already given (p. 131). From these results, and referring to what has been said in former 1 On the probable influence of the facial in the movements of the palate, see p. 354 ; and on the glosso-pharyngeal, see p. 355. 360 THE CEREBRAL NERVES. chapters, the share which the pneumogastric nerve takes in the functions of the several parts to wliich it sends branches may be understood:— 1. In deglutition, the motions of the pharynx are of the reflex kind. The stimulus of the food, or other substance to be swallowed, acting on the filaments of the glosso-pharyngeal, the filaments of the superior laryngeal given to the pharynx, and the cervical nerves, is conducted to the medulla oblongata, where it is reflected, chiefly, through the pneumogastric to the muscles of the pharynx, and, perhaps, also of the soft palate (see further, pp. 164 and 323). 2. In the functions of the larynx, the sensitive filaments of the pneumogastric supply that acute sensibility by which the glottis is guarded against the ingress of foreign bodies, or of irrcspirable gases. The contact of these stimulates the filaments of the supe- rior laryngeal branch of the pneumogastric ; and the impression conveyed to the medulla oblongata, whether it produces sensation or not, is reflected to the filaments of the recurrent or inferior laryngeal branch, and excites contraction of the muscles that close the glottis. Both these branches of the pneumogastric co-operate also in the production and regulation of the voice; the inferior laryngeal determining the contraction of the muscles that vary the tension of the vocal cords, and the superior laryngeal conveying to the mind the sensations of the state of these muscles necessary for their continuous guidance. And both the branches co-operate in the actions of the larynx, in the ordinary slight dilatation and contraction of the glottis in the acts of expiration and inspiration, and more evidently in those of coughing and other forcible respira- tory movements (p. 142). 3. It is partly through their influence on the sensibility and muscular movements in the larynx, that the pneumogastric nerves exercise so great an influence on the respiratory process, and that the division of both the nerves is commonly fatal. To determine how death is in these cases produced has been the object of innu- merable and often contradictory experiments. It is probably pro- duced differently in different cases, and in many is the result of several co-operating causes. Thus, after division of both the nerves, the respiration at once becomes slower, the number of respi- rations in a given time being commonly diminished to one-half (Emmert, xxxii. p. 371; J. Reid, xciv. 1839); probably, because the pneumogastric nerves are the principal conductors of the im- pression of the necessity of breathing to the medulla oblongata. Respiration does not cease, for it is probable that the impression may be conveyed to the medulla oblongata, through the sensitive nerves of all parts m wliich the imperfectly aerated blood Hows (see p. 182); yet the respiration being r