HAND-BOOK OF PHYSIOLOGY. KIRKES' HAND-BOOK OF PHYSIOLOGY. HAND-BOOK OF PHYS IO L O G Y. BY W. D. HALLIBURTON, M.D., F.R.S. PROFESSOR OF PHYSIOLOGY, KING'S COLLEGE, LONDON. (Edition. WITH UPWARDS OF SIX HUNDRED ILLUSTRATIONS. INCLUDING SOME COLOURED PLATES. PHILADELPTH3£ P. BLAKISTON, SON & CO., 1012, WALNUT STREET. 1896. PREFACE TO THE FOURTEENTH EDITION. The present edition of this Handbook has been re- arranged, and to a great extent re-written. In fact, with the exception of numerous illustrations, and a few pages here and there which treat of anatomical detail or describe instruments, and which have only been subjected to minor alterations, the book is a new one. In re-writing the book, I have endeavoured to bear in mind, that it is intended for the use of medical students, and I have also retained what has always been one of its special features, namely, that it treats of Histology as well as of Physiology proper. The number of new illustrations is numerous. For leave to borrow blocks or to copy figures I am greatly indebted to the Editors of " Quain's Anatomy," Prof. Schafer, Prof. McKendrick, Dr. Gowers, Dr. Mott, Dr. Brodie, Dr. Starling, and the late Sir George Johnson. The new figures have been executed throughout by Messrs. Walker and Boutall, whom I have to thank for the care which they have bestowed on the work. W. D. HALLIBURTON. King's College, London. JwZy, 1896. CONTENTS. CHAPTER I. PAGE Introductory i Definition of the Science of Physiology i Physiological Methods ......... 3 The Organs, Tissues, and Cells of the Body ..... 4 Animal and Vegetable Cells 6 CHAPTER II. The Animal Cell 8 Protoplasm 8 Nucleus 9 Attraction Sphere 12 Protoplasmic Movement . . . . . . . ..12 Cell-division ........... 16 The Ovum 22 CHAPTER III. Epithelium ... 23 Classification of Epithelium ........ 24 Pavement Epithelium .26 Cubical, Spheroidal and Columnar Epithelium . . . .27 Ciliated Epithelium 30 Ciliary Motion . . . . . . . . . . . 31 Transitional Epithelium 33 Stratified Epithelium .......... 34 Nutrition of Epithelium 36 Chemistry of Epithelium 36 VIII CONTENTS. CHAPTER IV. PAGE The Connective Tissues 37 Areolar Tissue 38 Fibrous Tissue ........... 44 Elastic Tissue 46 Adipose Tissue .... 47 Retiform Tissue. . 50 Adenoid or Lymphoid Tissue . 31 Basement Membranes 3 r Jelly-like Connective Tissue . . . . . . . .52 CHAPTER V. The Connective Tissues-continued 53 Cartilage 33 Bone 38 Ossification . 63 Teeth 70 The Blood 82 CHAPTER VI. Muscular Tissue 84 Voluntary Muscle . . . 85 Red Muscles ........... 93 Cardiac Muscle 93 Plain Muscle 94 Development of Muscle 93 CHAPTER VII. Nerve 96 Structure of 96 Termination of IO2 Development of 104 CHAPTER VIII. Irritability and Contractility 105 CHAPTER IX. Contraction of Muscle-Summary in CONTENTS. IX CHAPTER X. PAGE Changes in Form in a Muscle when it Contracts . . . 112 Instruments used . . . . . ■ • • ■ • TI3 Simple Muscle Curve . . . . . . • - . . 122 The Muscle Wave . . . . . . . • • • I25 Effects of two Successive Stimuli . . . . • . . 126 Effects of more than two Stimuli 127 Tetanus 127 Voluntary Tetanus .128 CHAPTER XI. Extensibility, Elasticity, and Work of Muscle . . . 131 CHAPTER XII. The Electrical Phenomena of Muscle 139 CHAPTER XIII. Thermal and Chemical Changes in Muscle . . . .150 CHAPTER XIV. Comparison of Voluntary and Involuntary Muscle . . . 157 CHAPTER XV. Physiology of Nerve 160 Classification of Nerves 160 Investigation of Nerve-Functions 163 Degeneration of Nerve . . . . . . . . . 163 Roots of the Spinal Nerves 165 Nerve Impulses ........... 168 Chemistry of Nerve 170 CHAPTER XVI. Electrotonus 17! X CONTENTS. CHAPTER XVII. PAGE Nerve-Centres .183 Nerve-Cells . jgy CHAPTER XVIII. Structure of the Spinal Cord 194 CHAPTER XIX. The Brain 2O8 CHAPTER XX. Structure of the Bulb, Pons, and Mid-Brain . . . .212 CHAPTER XXI. Structure of the Cerebellum 228 CHAPTER XXII. Structure of the Cerebrum 231 Histology of the Cortex ......... 237 The Convolutions 242 CHAPTER XXIII. Functions of the Spinal Cord 247 The Cord as an Organ of Conduction ...... 247 Reflex Action of the Cord ......... 249 Reflex Action in Man 251 CHAPTER XXIV. Functions of the Cerebrum 255 Removal of the Cerebrum ........ 256 Localisation of Cerebral Functions 256 CONTENTS. XI CHAPTER XXV. PAGE Functions of the Cerebellum 269 CHAPTER XXVI. Sensation 277 CHAPTER XXVII. Touch 282 Tactile End Organs 282 Sense of Locality 287 Sense of Pressure .......... 289 Sense of Temperature 290 Muscular Sense ........... 290 CHAPTER XXVIII. Taste and Smell 291 Taste . . . . . . . . . ■ • . . 291 Smell 297 CHAPTER XXIX. Hearing 300 Anatomy of the Ear 300 Physiology of Hearing 309 CHAPTER XXX. Voice and Speech 314 Anatomy of the Larynx 314 Movements of the Vocal Cords . 321 The Voice . . 323 Speech 325 Defects of Speech 326 XII CONTENTS. CHAPTER XXXI. The Eye and Vision '327 The Eyeball j28 The Eye as an Optical Instrument 342 The Ophthalmoscope 337 The Perimeter ........... 339 Fovea Centralis .......... 360 Colour Sensations 360 Pigments of the Retina . 365 Various Positions of the Eyeballs 368 Nervous Paths in the Optic Nerves 370 Visual Judgments .......... 372 CHAPTER XXXII. Trophic Nerves 375 CHAPTER XXXIII. The Circulatory System 377 The Heart 378 Course of the Circulation 386 Arteries 387 Veins 390 Capillaries 394 Lymphatic Vessels 397 CHAPTER XXXIV. The Circulation of the Blood 400 CHAPTER XXXV. Physiology of the Heart 406 The Cardiac Cycle 406 Action of the Valves of the Heart ....... 408 Sounds of the Heart . 4IO Coronary Arteries 412 Cardiographs 4!3 Endocardiac Pressure 416 Frequency and Force of the Heart's Action 421 Innervation of the Heart 422 Instruments for studying the excised Frog's Heart . . 433 CONTENTS. XIII CHAPTER XXXVI. PAGE The Circulation in the Blood-vessels 436 Velocity of the Blood-Flow 436 Use of the Elasticity of the Vessels 440 The Pulse 442 Capillary Flow 447 Venous Flow 450 Local Peculiarities of the Circulation . . . . . • 451 Blood-pressure 454 Vaso-motor Nervous System 467 CHAPTER XXXVII. Lymph and Lymphatic Glands 481 Composition of Lymph 481 Lymphatic Glands ....... ... 482 Lymph Flow . 485 Relation of Lymph and Blood ....... 486 Formation of Lymph .......... 486 CHAPTER XXXVIII. The Ductless Glands 488 Spleen 490 Thymus 495 Thyroid 498 Supra-renal Capsules 300 Pituitary Body 303 Pineal Gland ........... 303 Coccygeal and Carotid Glands 304 CHAPTER XXXIX. Respiration Respiratory Apparatus 303 Respiratory Mechanism - Nervous Mechanism of Respiration 324 Special Respiratory Acts 328 Eifect of Respiration on the Circulation 329 Asphyxia 534 Effects of Breathing Gases other than the Atmosphere . . . 537 XIV CONTENTS. CHAPTER XL. PAGE The Chemical Composition of the Body 548 Carbohydrates 549 Fats 555 Proteids . 557 The Polarimeter . ......... 563 Albuminoids ........... Ferments 565 CHAPTER XLI. The Blood 568 Coagulation of the Blood . . . . . . . . . 570 Plasma and Serum . -573 Blood-corpuscles 576 Blood Platelets 581 Development of the Blood-corpuscles .... . . 583 Chemistry of the Blood-corpuscles -587 Compounds of Haemoglobin 391 CHAPTER XLII. The Alimentary Canal 598 CHAPTER XLIII. Food 615 Milk '616 Eggs . . . . . . . . . . • .621 Meat . . . . 621 Flour ... - 622 Bread 623 Cooking of Food 624 Accessories to Food 625 CHAPTER XL1V. Secreting Glands 626 CONTENTS. XV CHAPTER XLV. PAGE Saliva 630 The Salivary Glands ......... 630 Secretory Nerves of Salivary Glands 633 The Saliva ........... 636 CHAPTER XLVI. The Gastric Juice 638 Composition 640 Innervation of the Gastric Glands 641 Action of Gastric Juice . 642 CHAPTER XL VII. Digestion in the Intestines 643 The Pancreas 643 Composition and Action of Pancreatic Juice 646 Intestinal Digestion 648 Leucine and Tyrosine 650 Secretory Nerves of the Pancreas 631 Extirpation of the Pancreas 632 CHAPTER XL VIII. The Liver 653 Functions ............ 659 Bile 659 Glycogenic Function of the Liver ....... 666 CHAPTER XLIX. The Absorption of Food 670 CHAPTER L. The Mechanical Processes of Digestion 674 Defecation ........... 684 XVI CONTENTS. CHAPTER LI. PAGB The Urinary Apparatus 685 Nerves of the Kidney 694 Activity of the Renal Epithelium ....... 697 Work done by the Kidney 698 Extirpation of the Kidneys .... .... 700 Passage of Urine into the Bladder ... , . 700 Micturition .... 701 CHAPTER LIL The Urine ' . . 702 Urea ........... . 704 Uric Acid 710 Hippuric Acid 712 Creatinine 713 Inorganic Constituents of Urine 714 Urinary Deposits . . 717 CHAPTER LIII. The Skin 724 CHAPTER LIV. General Metabolism 734 Discharge of Carbon 737 Discharge of Nitrogen 738 Balance of Income and Discharge in Health 739 Inanition or Starvation 741 Exchange of Material in Diseases 745 Luxus Consumption ......... 746 CHAPTER LV. Animal Heat 749 Regulation of the Temperature of Warm-blooded Animals . . 754 CHAPTER LVI. Tny Reproductive Organs 756 Male Organs -756 Female Organs . . 762 CONTENTS. XVII CHAPTER LV11. Development 768 The Ovum 768 Changes in the Ovum previous to Fecundation .... 769 Impregnation 770 Segmentation 771 Foetal Membranes 780 Development of the Decidua . 782 Development of the Foetal Membranes 783 Development of the Framework of the Body .... 789 Formation of the Head . 791 Development of the Vascular System 795 Development of the Nervous System . 806 Development of the Alimentary Canal .816 Development of the Respiratory Apparatus . . . . . 819 Development of the Genito-urinary Apparatus .... 820 INDEX ........... . 829 FAHRENHEIT and. CENTIGRADE SCALES. MEASUREMENTS. FRENCH INTO ENGLISH. F. C. 500° 260° 401 205 392 200 383 195 374 190 356 180 347 175 338 170 329 165 320 160 311 155 302 150 284 140 275 135 266 130 248 120 239 115 230 no 212 100 203 95 194 90 176 80 167 75 140 60 122 50 113 45 105 40*54 104 40 100 37'8 1 metre 10 decimetres 100 centimetres 1,000 millimetres LENGTH. A grain equals about 1'16 gram., a Troy oz. about 31 gram., a lb. Avoirdupois about4 Kilogrm., and 1 cwt. about 50 Kilogrms. _ = 3977 English inches. (or 1 yard and 34 in.) CAPACITY. 1 decimetre 10 centimetres 100 millimetres = 3'937 inches (or nearly 4 inches) 1,000 cubic decimetres 1,000,000 cubic centimetres = 1 cubic metre. = '3937 or about (nearly f inch.) 1 cubic decimetre or 1,000 cubic centimetres = 1 litre. 1 centimetre 10 millimetres 1 millimetre = nearly inch. Or One Litre = 1 pt. 15 oz. 1 dr. 40. (For simplicity, Litre is used to signify 1 cubic decimetre, a little less than 1 English quart.) Decilitre (100 c.c.) = 3J oz. Centilitre (10 c.c.) = 2| dr. Millilitre (1 c.c.) = 17 m. Decalitre = 2| gal. Hectolitre = 22 gals. Kilolitre (cubic metre) = 274 bushels. A cubic inch = 16'38 c.c.; a cubic foot = 28'315 cubic dec., and a gallon = 4'54 litres. Or, One MEtre = 39'37079 inches. (It is the ten-millionth part of a quarter of the meridian of the earth.) 1 Decimetre = 4 in. 1 Centimetre = in. 1 Millimetre = in. Decametre = 32'80 feet. Hectometre = 109'36 yds. Kilometre = 0'62 miles. One inch = 2'539 Centimetres. One foot = 3'047 Decimetres. One yard = 0'91 of a Metre. One mile = 1 '6o Kilometre. The cubic centimetre (15'432 grains-1 gramme) is a standard at 4° C., the grain at i6°'66 C. CONVERSION SCALE. To convert Grammes to Ounces avoir- dupois, multiply by 20 and divide by 567. To convert Kilogrammes to Pounds, multiply by 1,000 and divide by 454. To convert Litres to Gallons, mul- tiply by 22 and divide by 100. To convert Litres to Pints, multiply by 88 and divide by 50. To convert Millimetres to Inches, multiply by 10 and divide by 254. To convert Metres to Yards, multi- ply by 7° and divide by 64. 98'5 | 36'9 95 35 86 30 77 25 68 20 50 10 41 5 32 o 23 "5 14 -10 + 5 -15 - 4 -20 -13 -25 - 22 "3° - 40 - 40 - 76 -60 WEIGHT. (One gramme is the weight of a cubic centimetre of water at 4° C. at Paris). 1 gramme 10 decigrammes 100 centigrammes 1,000 milligrammes = i5'432349 grs- (or nearly 154.) 1 decigramme ) 10 centigrammes 100 milligrammes J = rather more than i| grain. SURFACE MEASURE. 1 square metre = about 1550 sq. inches. Or 10,000 sq. centimetres, or 1075 sq. ft. t sq. inch = about 6'4 sq. centimetres. 1 sq. foot = ,, 930 ,, ,, 1 centigramme 10 decigrammes = rather more than grain. 1 deg. F. = '54'C. i'8 „ = i°C. 3'6 „ = 20 C. 4'5 >, = 2'5°C. 5'4 „ = 3°C- 1 milligramme = rather more than grain. Or Decigramme = 2 dr. 34 gr. Hectogrm. = 31 oz. (Avoir.) Kilogrm. = 21b. 3 oz. 2 dr. (Avoir.) ENERGY MEASURE. 1 kilogrammetre=about 7'24 ft. pounds. 1 foot pound = ,, '1381 kgm. 1 foot ton = ,, 310 kgm. To convert de- grees F. into de- grees C., subtract 32, and multiply by HEAT EQUIVALENT. 1 kilocalorie = 424 kilogrammetres. ENGLISH MEASURES. To convert de- grees 0. into de- crees F., multiply by f, and add 32°. Apothecaries Weight. 7000 grains = 1 lb. Or 437'5 grains = 1 oz. Avoirdupois Weight. 16 drams = 1 oz. 16 oz. = 1 lb. 28 lbs. = 1 quarter. 4 quarters = 1 cwt. 20 cwt. = 1 ton. Measure of i decimetre, or io centimetres, or 100 millimetres. Highest point of Crest of the Hium. Anterior Su-< perior Spine of the Ilium Symphysis Pubis. DIAGRAM OF THORACIC AND ABDOMINAL REGIONS. A. Aortic Valve. Jf. Mitral Valve. P. Pulmonary Valve. T. Tricuspid Valve. Cranium. 7 Cervical Vertebrae. Clavicle. Scapula. 12 Dorsal Vertebrae. Humerus. 5 Lumbar Vertebrre. Ilium. Ulna. Radius. Pelvis. Bones of the Carpus. Bones of the Meta- carpus. Phalanges of Fingers. Femur. Patella. Tibia. Fibula. Bones of the Tarsus. Bones of the Meta- tarsus. Phalanges of Toes. Ischium Pubes THE SKELETON (after Holden). HANDBOOK OF PHYSIOLOGY. CHAPTER I. INTRODUCTORY. Biology is the science that treats of living things, and it is divided into two main branches, which are called respectively Morphology and Physiology. Morphology is the part of the science that deals with the form or structure of living things, and with the problems of their origin and distribution. Physiology, on the other hand, treats of their functions, that is, the manner in which their individual parts carry out the processes of life. To take an instance : the eye and the liver are two familiar examples of what are called organs; the anatomist studies the structure of these organs, their shape, their size, the tissues of which they are composed, their position in the body, and the variations in their structure met with in different parts of the animal kingdom. The physiologist studies their uses, and seeks to explain how the eye fulfils the function of vision, and how the liver forms bile, and ministers to the needs of the body in other ways. Each of these two great branches of biological science can be further subdivided according as to whether they deal with the animal or the vegetable kingdom ; thus we get vegetable physi- ology and animal physiology. Human Physiology is a large and important branch of animal physiology, and to the student of medicine is obviously the portion of the science that should interest him most. In order to understand morbid or pathological processes it is necessary that the normal or physiological functions should be learnt first. Physiology is not a study which can be put aside and forgotten when a certain examination has been passed ; it has a most direct and intimate bearing in its application to the scientific and successful investigation of disease. It will be my endeavour throughout the subsequent pages of this book to point 2 INTRODUCTORY. [ch. i. out from time to time the practical relationships between physi- ology and pathology. Human physiology will be our chief theme, but it is not a por- tion of the great science that can be studied independently of its other portions. Thus, many of the experiments upon which our knowledge of human physiology rests have been performed principally on certain of the lower animals. In order to obtain a wide view of vital processes it will be occasionally necessary to go still further afield, and call the science of vegetable physiology to our assistance. In another sense, human physiology is in no isolated position. Its study must go hand in hand with the study of anatomy. It is impossible to understand how the body or any part of the body acts unless we know accurately the structure of the organs under consideration. This is especially true for that portion of anatomy which is called Microscopic Anatomy or Histology. Indeed, so close is the relationship between minute structure and function that in this country it is usual for the teacher of physiology to be also the teacher of histology. Another branch of anatomy, namely, Embryology, or the process of growth of the adult from the ovum, falls also within the province of the physiologist. But physiology is not only intimately related in this way to its sister science anatomy, but the sciences of chemistry and physics must also be considered. Indeed, physiology has been sometimes defined as the application of the laws of chemistry and physics to life. That is to say, the same laws that regulate the behaviour of the mineral or inorganic world are also to be found operating in the region of organic beings. If we wish for an example of this we may again go to the eye ; the branch of physics called optics teaches us, among other things, the manner in which images of objects are produced by lenses ; these same laws regulate the formation of the images of external objects upon the sensitive layer of the back of the eye by the series of lenses in the front of that organ. An example of the application of chemical laws to living processes is seen in digestion ; the food contains certain chemical substances which are acted on in a chemical way by the various digestive juices in order to render them of service to the organism. The question arises, however, is there anything else ? Are there any other laws than those of physics and chemistry to be reckoned with ? Is there, for instance, such a thing as " vital force " ? It may be frankly admitted that physiologists at present are not able to explain all vital phenomena by the laws of the physical world ; but as knowledge increases it is more and more abundantly CH. 1.] INTRODUCTORY. 3 shown that the supposition of any special or vital force is un- necessary ; and it should be distinctly recognised that when, in future pages, it is necessary to allude to vital action, it is not because we believe in any specific vital energy, but merely because the phrase is a convenient one for expressing something that we do not fully understand, something that cannot at present be brought into line with the physical and chemical forces that operate in the inorganic world. It will be in connection with the nervous system that we shall principally have recourse to this convenient expression, for it is there that we find the greatest difficulty in reconciling the phenomena of life with those of the non-living. Physiology proper may be conveniently divided into three main branches:- 1. Chemical physiology; or the application of chemistry to living processes. 2. Physical physiology ; or the application of physics to living processes. 3. The physiology of the nervous system where the application of such laws is at present extremely difficult. But just as there is no hard and fast line between physiology and its allies pathology, anatomy, physics, and chemistry, so also there is no absolute separation between its three great divisions; physical, chemical, and so called vital processes have to be con- sidered together. Physiology is a comparatively young science. Though Harvey more than three hundred years ago laid the foundation of our science by his discovery of the circulation of the blood, it is only during the last half century that active growth has occurred. The reasons for this recent progress come under two headings ; those relating to observation and those relating to experiment. The method of observation consists in accurately noting things as they occur in nature ; in other words, the knowledge of anatomy must be accurate before correct deductions as to function are possible. The instrument by which such correct observations can be made is, par excellence, from the physiologist's standpoint, the microscope, and it is the extended use of the microscope, and the knowledge of minute anatomy resulting from that use, that has formed one of the greatest stimuli to the successful progress of physiology during the last fifty years. But important as observation is, it is not the most important method; the method of experiment is still more essential. This consists not in being content with mere reasonings from structure or occurrences seen in nature, but in producing artificially changed 4 INTRODUCTORY. [ch. 1. relationships between the structures, and thus causing new com- binations that if one had waited for Nature herself to produce might have been waited for indefinitely. Anatomy is important, but mere anatomy has often lead people astray when they have tried to reason how an organ works from its structure only. Experiment is much more important; that is, one tests one's theories by seeing whether the occurrences actually take place as one supposes ; and thus the deductions are confirmed or corrected. It is the universal use of this method that has made physiology what it is. Instead of sitting down and trying to reason out how the living machine works, physiologists have actually tried the experiment, and so learnt much more than could possibly have been gained by mere cogitation. Many such experiments involve the use of living animals, but the discovery of anaesthetics, which renders such experiments painless, has got rid of any objection to experiments on the score of pain. We must next proceed to an examination of the general struc- ture of the body, and an explanation of some of the technical terms which will frequently be used hereafter. The adult body consists of a great number of different parts ; and each part has its own special work to do. Such parts of the body are called organs. Each organ does not only its owm special work but acts in harmony with other organs. This rela- tionship between the organs enables us to group them together into what are termed systems. Thus, we have the circulatory system, that is, the group of organs (heart, arteries, veins, &c.) concerned in the circulation of the blood ; the respiratory system, that is, the group of organs (air passages, lungs, &c.) concerned in the act of breathing ; the digestive system, which deals with the digestion of food; the excretory system, with the getting rid of waste products; the muscular system, -with movement; and the skeletal system, with the support of the softer parts of the body. Over and above all these is the nervous system (brain, spinal cord, nerves), the great master system of the body which presides over, controls, and regulates the functions of the other systems. If we proceed still further on our anatomical analysis, and take any organ, we see that it consists of various textures, or, as they are called, elementary tissues. Just as one's garments are made up of textures (cloth, lining, buttons, &c.), so each organ is composed of corresponding tissues. The elementary tissues come under the following four headings :- 1. Epithelial tissues. 2. Connective tissues. 3. Muscular tissues. 4. Nervous tissues. Each of these is again divisible into sub-groups. CH. I.] INTRODUCTORY. 5 Suppose we continue our anatomical analysis still further, we find that the individual tissues are built up of structures which require the microscope for their accurate study. Just as the textures of a garment are made up of threads of various kinds, so also in many of the animal tissues we find threads or fibres, at they are called. But more important than the threads are little masses of living material. Just as the wall of a house is made up of bricks united by cement, so the body walls are built of extremely minute living bricks, united together by different amounts of cementing material. Each one of these living units is called a cell. Some of the tissues already enumerated consist of cells with only very little cement material binding them together; this, for instance, is seen in the epithelial tissues ; but in other tissues, particularly the connec- tive tissues which are not so eminently living as the rest, the amount of cement or intercellular material is much greater, and in this it is that the fibres are developed that confer the neces- sary strength for these binding tissues. If, instead of going to the adult animal, we look at the animal in its earliest stage of development, the ovum, we find that it consists of a single little mass of living material, a single cell. As development pro- gresses it becomes an adherent mass of cells. In the later stages of develop- ment various tissues become differen- tiated from each other by the cells becoming grouped in different ways, by alterations in the shape of the cells, by deposition of intercellular matter between the cells, and by chemical changes in the living matter of the cells themselves. Thus in some situations the cells are grouped into the various epithelial linings; in others the cells become elongated and form muscular fibres; and in others, as in the connective tissues, there is a preponderating amount of intercellular mate- rial which may become permeated with fibres, or be the seat of the deposition of calcareous salts, as in bone. Instances of chemical changes in the cells themselves are seen on the surface of the body where the superficial layers of the epidermis become horny ; in the mucous glands, where they become -filled Space con- • taining liquid. Protoplasm. . Nucleus. Cell-wall. Fig. i.-Vegetable cells. 6 INTRODUCTORY. [ch. 1. with mucin, and in adipose tissue, where they become charged with fat. The term cell was first used by botanists; in the popular sense of the word a cell is a space surrounded by a wall, as the cell of a prison, or the cell of a honey-comb. In the vegetable cell, there is a wall made of the starch-like material called cellulose, within this is the living matter, and a number of large spaces or vacuoles filled with a watery fluid. The use of the term cell by botanists was therefore completely justified. But the animal cell is different; as a rule it has no cell-wall, and no vacuoles. It is just a little naked lump of living material. This living material is jelly-like in consistency, possessing the power of movement, and the name protoplasm has been be- stowed on it. Somewhere in the protoplasm of all cells, generally near the middle in animal cells, is a roundish structure of more solid consistency than the rest of the proto- plasm, called the nucleus. An animal cell may therefore be defined as a mass of protoplasm containing a nucleus. The simplest animals, like the amcebse, consist of one cell only : Fig. 2.-Animal cell consisting of protoplasm containing a nucleus. Fig. 3.-Amoebae : unicellular animals. Fig. 4.-Cells of the yeast plant in pro- cess of budding. the simplest plants, like bacteria, torulse, etc., consist of one cell only. Such organisms are called unicellular. In the progress of their life history the cell divides into two; and the two new cells separate and become independent organisms, to repeat the process later on. CH. 1.] INTRODUCTORY. 7 In the case of the higher animals and plants, they are always unicellular to start with, but on dividing and sub-dividing the resulting cells stick together and subsequently become differ- entiated and altered in the manner already indicated. In spite of these changes, the variety of which produces the great com- plexity of the adult organism, there are certain cells which still Fig'. 5.-Human colourless blood-corpuscle, showing its successive changes of outline within ten minutes when kept moist on a warm stage. (Schofield.) retain their primitive structure; notable among these are the white corpuscles of the blood. All living cells, all living things, whether unicellular or multi- cellular, are characterised by the following : 1. Power of movement; this is seen in amoeboid movement, ciliary movement, muscular movement. 2. Power of assimilation, that is, ability to take in nutrient material or food and convert it into protoplasm. 3. Power of growth; this is a natural consequence of the power of assimilation. 4. Power of reproduction ; this is a variety of growth. 5. Power to excrete; to give out waste materials, the products of other activities. Physiology after all is only a detailed study of these five characteristics of vitality, but before we can proceed to that study, it will be first necessary to devote a few preliminary chapters to a more minute consideration of the animal cell and the elementary tissues or textures of the body. 8 THE ANIMAL CELL. [CH. II CHAPTER IL THE ANIMAL CELL. An animal cell is usually of microscopic dimensions, in the human body varying from 7s4o' to tj-qVo an in diameter. It consists of- 1. Protoplasm. This makes up the main substance of the cell. 2. Nucleus : a vesicular body within the protoplasm, generally situated near the centre of the cell. 3. Attraction sphere : this is another minute particle contained ■within the protoplasm, near the nucleus. These three portions demand separate study. Protoplasm. Until recent years, protoplasm was supposed to be a homo- geneous material entirely destitute of structure, though generally containing minute granules of solid consistency, or globules (vacuoles) containing a watery fluid. It has, however, now been shown with high powers of the microscope that in many cells, the protoplasm consists of two Fig. 6.- (a.) A colourless blood-corpuscle showing the intra-cellular network, and two nuclei with intra-nuclear network. (b.) Coloured blood-corpuscle of newt showing the intra-cellular network of fibrils. Also oval nucleus composed of limiting membrane and fine intra- nuclear network of fibrils, x 800. (Klein and Noble Smith.) parts, a fine network of fibrillte in which the more fluid and apparently structureless portion of the protoplasm is contained. (See figs. 2 and 6.) The network or spongework is called the reticulum or CH. 11.] PROTOPLASM. 9 spongioplasm, and the more fluid portion in its meshes the enchylema or hyaloplasm. The granules in protoplasm are partly thickened portions of the spongioplasm, but in addition to this there appear to be free granules, some fatty in nature (staining black with osmic acid), some composed of the substance called glycogen or animal starch (staining reddish brown with iodine), and sometimes in a few unicellular animals they consist of inorganic (calcareous) matter. But by far the most constant and abundant of the granules are like the main substance of the protoplasm, proteid or albuminous in composition. The chemical structure of protoplasm can only be investigated after the protoplasm has been killed. The substances it yields are (i) Water; protoplasm is semifluid, and at least three- quarters of its weight, often more, are due to water. (2) Proteids. These are the most constant and abundant of the solids. A proteid or albuminous substance consists of carbon, hydrogen, nitrogen, oxygen, with sulphur and phosphorus in small quantities only. In nuclein, a proteid-like substance formed in the nuclei of cells, phosphorus is more abundant. The proteid obtained in greatest abundance in the cell proto- plasm is called a micleo-proteid, that is to say, it is a compound containing varying amounts of this material nuclein with proteid. White of egg is a familiar instance of an albuminous substance or proteid, and the fact (which is also familiar) that this sets on boiling into a solid will serve as a reminder that the greater number of the proteids found in nature have a similar tendency to coagulate under the influence of heat and other agencies. (3) Various other substances occur in smaller proportions, the most constant of which are lecithin, a phosphorised fat; cholesteric, a monatomic alcohol; and inorganic salts, especially phosphates and chlorides of calcium, sodium, and potassium. The large quantity of water present should be particularly noted ; the student when first shown diagrams of the reticulum in protoplasm is apt to imagine that it consists of a firm solid, like a system of wires pervading a jelly. The reticulum is only slightly more solid than the hyaloplasm. The Nucleus. In form the nucleus is generally round or oval, but it may have in some cases an irregular shape, and in other cases there may be more than one nucleus in a cell. 10 THE ANIMAL CELL. [ch. 11. The nucleus exercises a controlling influence over the nutrition and subdivision of the cell; any portion of a cell cut oft' from the nucleus undergoes degenerative changes. A nucleus consists of four parts- 1. The nuclear membrane, which encloses it. 2. A network of fibres in appearance like the spongioplasm of the protoplasm but on a larger scale; that is to say, the threads of which it is composed are much coarser and much more readily seen. The name chromoplasm has been given to this network. 3. The nuclear sap or matrix, a more fluid, and homogeneous substance which occupies the interstices of the spongework of chromoplasm. 4. Nucleoli; these are of two principal varieties; some are knots or thickened portions of the network and others, the true nucleoli, float freely in the nuclear sap. These four parts of the nucleus are represented in the next diagram. Node of network" Node of network- - Nuclear membrane. ..Nucleolus. ..Nuclear matrix. -Nuclear network. Fig. 7.-The resting nucleus-diagrammatic. (Waldeyer.) The next figure gives a view of the nucleus, according to the researches of Rabi. He considers that the fibres of the network may be divided into thick fibres which he terms primary, and thinner connecting branches which he terms secondary (shown only on the right-hand side of the figure). This observer also supposes that the primary fibres have the looped arrangement depicted in the diagram. In the investigation of microscopic objects, a histologist is nearly always obliged to use staining agents; the extremely thin objects he examines are so transparent that, without such stains, much of the structure would be invisible. If such dyes as haematoxylin or safranin are employed, it is the nucleus which becomes most deeply stained, and thus stands out on the lighter background of the protoplasm. ch. 11.] THE NUCLEUS. 11 But the whole nucleus does not stain equally deeply ; it is the chromoplasmic filaments and the nucleoli which have most affinity for the stain, while the nuclear membrane and the nuclear sap are comparatively unaffected. Hence the terms chromatin and achromatin originally introduced by Fleming. The network and the nucleoli are com- posed of chromatic sub- stance or chromatin ; it is so called not because it has any colour in the natural state, but because it has the affinity for colours artificially added to it. For a corresponding reason, achromatin or achromatic substance is the name given to the substances which make up the nuclear membrane and nuclear sap. To these general facts one or two details may be added. Balbiani first showed that the chromoplasmic filaments are apparently transversely marked into alternate dark and light bands ; this is due to the existence of minute highly refracting particles imbedded in regular series in a clear homogeneous and unstainable matrix (see fig. 9). The term chromatin should properly be restricted to these particles. Coming next to the chemical composition of the nucleus, it is found to consist principally of proteid and proteid-like substances. The nuclei of cells may be obtained by subjecting the cells to the action of artificial gastric juice; the protoplasm is nearly entirely dissolved, but the nuclei resist the solvent action of the juice. No doubt the nuclei contain several chemical compounds, but the only one of which we have any accurate knowledge has been termed nuclein, and this is apparently identical with the substance called chromatin by histologists. It is soluble in alkalis, but precipitated by acids ; it is different from a proteid, as it contains in addition to carbon, nitrogen, oxygen, hydrogen and sulphur, an enormous quantity (7 to 8 per cent, or even more) of phosphorus in its molecule. In many cases nucleins contain iron also. Fig. 8.-Diagram of nucleus showing the arrange- ment of chief chromatic filaments. Viewed from the side, the polar end being uppermost. p.c f., primary chromatic filaments ; n., nu- cleolus; n.o.m., node of meshwork. (Rabi.) Kg. 9.-Part of a chromo- plasmic fila- ment, greatly magnified. (Carnoy.) 12 THE ANIMAL CELL. [ch. ii. The Attraction Sphere. Recent research has shown that, in addition to the nucleus and protoplasm, most if not all living cells contain another structure ; it is an extremely minute particle which has been called the "attraction sphere," because it has an apparently attractive influence on the protoplasmic granules which radiate around it. Fig. ii.-Ovum of the worm Asearis, showing a twin attrac- tion sphere. The nucleus with its contorted filament of chro- moplasm is represented, but the protoplasm of the cell is not filled in. (V. Beneden.) Fig. io.-A cell white blood-cor- puscle showing its attraction sphere. In this as in most cases, the attraction sphere lies near the nucleus. (Schafer.) It is most prominent in embryonic cells, and in cells which are dividing or about to divide. Here it becomes double, and it is probable that the attraction sphere takes the lead in cell division, the nucleus and the cell-protoplasm then following suit. Protoplasmic Movement. A cell possesses the power of breathing, that is, taking in oxygen; of nutrition, of building itself up from food materials; and of excretion, or the getting rid of waste material. But the most obvious physiological characteristic of a cell is its power of movement. When an amoeba is observed with a high power of the microscope, it is found to consist of an irregular mass of proto- plasm containing one or more nuclei, the protoplasm itself being more or less granular and vacuolated. If watched for a minute or two, an irregular projection is seen to be gradually thrust out from the main body and retracted ; a second mass is then pro- truded in another direction, and gradually the whole protoplasmic ch. 11.] PROTOPLASMIC MOVEMENT. 13 substance is, as it were, drawn into it. The Axnceba thus comes to occupy a new position, and when this is repeated several times we have locomotion in a definite direction, together with a continual change of form. These movements, when observed in other cells, such as the colour- less blood-corpuscles of higher animals (fig. 13), in the branched cornea cells of the frog and else- where, are hence termed amoe- boid. The projections which are alternately protruded and re- tracted are called A streaming movement is not infrequently seen in certain of the protozoa, in which the mass of protoplasm extends long and fine processes, themselves very Fig. 12.-Amoebee. Fig. 13.-Human colourless blood-corpuscle, showing its successive changes of outline within ten minutes when kept moist on a warm stage. (Schofield.) little moveable, but upon the surface of which freely-moving or streaming granules are seen. A gliding movement, has also been Fig. 14.-(a.) Young vegetable cells, showing cell-cavity entirely filled with granular protoplasm enclosing a large oval nucleus, with one or more nucleoli. (b.) Older cells from same plant, showing distinct cellulose-wall and vacuola- tion of protoplasm. noticed in certain animal cells ; the motile part of the cell being composed of protoplasm bounding a central and more compact mass. By means of the free movement of this layer, the cell may be observed to move along. In vegetable cells the protoplasmic movement can be well seen 14 THE ANIMAL CELL. [ch. 11. in the hairs of the stinging-nettle and Tradescantia and the cells of Vallisneria and Chara; it is marked by the movement of the granules nearly always imbedded in it. For example, if part of a hair of Tradescantia (fig. 15) be viewed under a high magnifying power, streams of protoplasm containing crowds- of granules hurrying along, like the foot-passengers in a busy street, are seen flowing steadily in definite directions, some coursing round the film which lines the interior of the cell-wall, and others flowing towards or away from the irregular mass in the centre of the cell-cavity. Many of these streams of protoplasm run together into larger ones and are lost in the central mass, and thus ceaseless variations of form are produced. The movement of the proto- plasmic granules to or from the periphery is sometimes called vegetable circulation, whereas the movement of the protoplasm round the interior of the cell is called rotation. The first account of the movement of protoplasm was given by Rosel in 1755, as occurring in a small Proteus, probably a large freshwater amoeba. His description was followed twenty years later by Corti's demonstration of the rota- tion of the cell sap in characese, and in the earlier part of the century by Meyer in Vallisneria, 1827; Robert Brown, 1831, in '1 Staminal Hairs of Tradescantia." Then came Dujardin's description of the granular streaming in the pseudopodia of Rhizopods and move- ment in other cells of animal protoplasm (Planarian eggs, v. Siebold, 1841; colourless blood-corpuscles, Wharton Jones, 1846). There is no doubt that the protoplasmic movement is essentially the same thing in both animal and vegetable cells. But in vegetable cells, the cell-wall obliges the movement to occur in the interior, while in the naked animal cells the movement results in an external change of form. Although the movements of amoeboid cells may be loosely described as spontaneous, yet they are produced and increased under the action of external agencies which excite them, and are therefore called stimuli, and if the movement has ceased for the time, as is the case if the temperature is lowered beyond a certain Fig. 15.-Cell of Tradescantia drawn at successive intervals of two minutes.-The cell-contents consist of a central mass connected by many irregular processes to a peripheral film, the whole forming a vacuolated mass of proto- plasm, which is continually changing its shape. (Schofield.) CH. 11.] IRRITABILITY OF PROTOPLASM. 15 point, movement may be set up by raising the temperature. Again, contact with foreign bodies, gentle pressure, certain salts, and electricity, produce or increase the movement in the amoeba. The protoplasm is, therefore, sensitive or irritable to stimuli, and shows its irritability by movement or contraction of its mass. The effects of some of these stimuli may be thus further detailed :- a. Changes of temperature.-Moderate heat acts as a stimulant: the movement stops below o° C. (3 20 F.), and above 40° C. (104° F.); between these two points the movements increase in activity ; the optimum temperature is about 370 to 38° C. Though exposure to a temperature even below o° C. stops the movement of protoplasm, it does not prevent its reappearance if the temperature is raised ; on the other hand, prolonged exposure to a temperature of over 40° C. altogether kills the protoplasm and causes it to enter into a condition of coagula- tion or heat rigor. We have already seen that proteids, the most abun- dant constituents of protoplasm, are coagulated by heat. b. Chemical stimuli. - Distilled water first stimulates then stops amoeboid movement, for by imbibi- tion it causes great swelling and finally bursting of the cells. In some cases, however (myxomycetes), protoplasm can be almost entirely dried up, but remains capable of renewing its movement when again moistened. Dilute salt-solution and very dilute alkalis stimulate the movements temporarily. Acids or strong alkalies permanently stop the movements: ether, chloroform, veratrine and quinine also stop it for a time. Movement is suspended in an atmosphere of hydrogen or carbonic acid and resumed on the admission of air or oxygen ; complete withdrawal of oxygen will aftei' a time kill protoplasm. c. Electrical.-Weak currents stimulate the movement, while strong currents cause the cells to assume a spherical form and to become motionless. Fig. 16.-Cells from the staminal hairs of Tradeseantia. A, fresh in water; B, the same cell after slight electri- cal stimulation; a, b, region stimu- lation ; c, d, clumps and nohs of contracted protoplasm. (Kiihne.) 16 THE ANIMAL CELL. [ch. 11. The amoeboid, movements of the colourless corpuscles of the blood may be readily seen when a drop of blood, from the finger is mixed with salt solution, and examined on a warm stage with the microscope. If the pseudopodium of such a corpuscle is observed under a high power, it will be seen to consist of hyalo- plasm, which has flowed out of its spongy home, the reticulum. Later, however, a portion of the reticular part of the protoplasm may enter the pseudopodium. The cells may be fixed by a jet of steam allowed to play for a moment on the surface of the cover glass. The next figure illustrates one fixed in this way. The essential act in the protrusion of a pseudopodium is the flowing of the hyalo- plasm out of the spongioplasm ; the retrac- tion of the pseudopodium is a return of the hyaloplasm to the spongioplasm. The spongioplasm has an irregular arrangement with openings in all directions, so that the contractility of un- differentiated cells may exhibit itself towards any point of the compass. Pig'. 17.-An Amoeboid cor- puscle of the newt killed by instantaneous appli- cation of steam, showing the appearance of the pseudopodia. (After D. Gunn, Quain's Ana- tomy.) Cell Division. A cell multiplies by dividing into two ; each remains awhile in the resting or non-dividing condition, but later it grows and sub- divides, and the process may be repeated indefinitely. The supreme importance of the cell, the growth of the body from cells, and the fact that cells are the living units of the organism, were first established in the vegetable world by Schleiden, and extended to the animal kingdom by Theodor Schwann. The ideas of physiologists depending on this idea are grouped together as cellular physiology, which under the guidance of Virchow was extended to pathology also : Virchow expressed the doctrine now so familiar as to be almost a truism in the terse phrase omnis cellula a cellula (every cell from a cell). The division of a cell is preceded by division of its nucleus. Nuclear division may be either (i) simple or cZzrecf, which consists in the simple exact division of the nucleus into two equal parts by constriction in the centre, which may have been preceded by division of the nucleoli ; or (2) indirect, which consists ihVi series of changes "which goes on in the arrangement of the nuclear reticulum, resulting in the exact division of the chromatic fibres CH. 11.] CELL DIVISION. 17 into two parts, which form the chromoplasm of the daughter nuclei. The changes in the nucleus during indirect division constitute karyokinesis (jtapvov, a kernel), or mitosis (pdros, a thread), and direct division is called amitotic or akinetic (jilvryris, movement). It is now believed that the mitotic nuclear division is all but, though not quite, universal. Somewhat different accounts of the stages of the nuclear division have been given by different authorities, according to the kind of cell in which the nuclear changes have been studied. The following will summarise the stages of karyo- kinesis as observed by Klein :- The nucleus in a resting condition, i.e., before any changes pre- Fig. 18.-Karyokinesis. a, ordinary nucleus of a columnar epithelial cell; b, c, the same nucleus in the stage of convolution ; d, the wreath or rosette form; e, the aster, or single star; f, a nuclear spindle from the Descemet's endothelium of the frog's cornea; o, h, i, diaster; k, two daughter nuclei. (Klein.) ceding division occur, consists of a very close meshwork of fibrils, which stain deeply with carmine, embedded in a matrix, whicli does not possess this property, the whole nucleus being contained in an envelope. The first change consists of a slight enlargement, the disappearance of the envelope and the increased definition and thickness of the nuclear fibrils, which are also more separated than they were, and stain better. This is the stage of convolution (fig. 18, b, c). The next step in the process is the arrangement of the fibrils into some definite figure by an alternate looping in and oi|t around a central space, by which means the rosette or wreath*stage (fig. 18, d) is reached. The loops of the rosette next become divided at the periphery and their central points become more angular, so that the fibrils divided into portions of about equal length, are doubled at an acute angle, and radiate 18 THE ANIMAL CELL. [ch. 11. in a V-shaped manner from the centre, forming a star (aster) (fig. 18, e), and later from two centres, in which case a double star (diaster) results (fig. 18, G, h, and 1). After remaining almost unchanged for some time, the V-shaped fibres being first re-arranged in the centre, side by side (angle outwards), split longitudinally and separate into two bundles which gradually assume position at either pole. From these groups of fibrils the two nuclei of the new cells are formed (daughter nuclei) (fig. 18, k), and the changes they pass through before reaching the resting condition are exactly those through which the original nucleus (mother nucleus) has gone, but in a reverse order, viz., the star, the rosette, and the convolution. During or shortly after the formation of the daughter nuclei the cell itself becomes constricted and then divides in a line about midway between them. The changes as described are those which are most obvious ; but they take little account of the formation of the spindle seen in fig. 18, F, nor of the part played by the attraction sphere (see p. 12). The work of Waldeyer, Rabi, and others has shown that a more exact description is the following. 1 The process may be divided into the following stages :- 1. The non-dividing nucleus (fig. 19). Node of network Node of network Nuclear membrane. Nucleolus. ■Nuclear matrix. Nuclear' network. Fig. 19.-The resting nucleus. (Waldeyer.) 2. The spirem or skein stage : the nucleoli dissolve, the secondary fibres disappear, and the primary loops running from polar to anti-polar regions remain (figs. 8, 20). 3. Each loop becomes less convoluted and splits longitudinally into two sister threads, and the achromatic spindle appears (fig. 21, a and b). 4. The equatorial stage ; monaster. The nucleus has now two poles, those of the spindle; and at each pole there is a polar corpuscle. Much speculation has taken place in regard to the CH. II.] KARYOKINESIS. 19 origin of the polar corpuscles; there can, however, now be but little doubt that they do not originate from the nucleus, but from the attraction sphere. The division of the attrac- tion sphere into two precedes the commencement of changes in the nucleus ; the two attraction spheres become prominent in cell division, and the connecting achromatic spindle is probably also formed from them. Fig. 20.-Early condition of the skein stage viewed at the polar end. I. c. looped chromatic filament, i. irregular filament. (Rabi.) Achromatic spindle Fig. 21.-Later condition of the skein stage in karyokinesis. a. The thicker primary fibres become less convoluted and the achromatic spindle appears, b. The thick fibres split into two and the achromatic spindle becomes longitudinal. (Waldeyer.) Pole of spindle. Outer granular zone. Split fibres. Inner clear zone. .Polar corpuscle. Fig. 22.-Monaster stage of karyokinesis. (Waldeyer.) At this stage the nuclear membrane is lost, and thus cell protoplasm and nuclear sap become continuous; the protoplasm immediately around the nucleus is clear ; outside this is a granu- 20 THE ANIMAL CELL. [ch. 11. lar zone, and here the granules are arranged radially from the polar corpuscles. The star-like arrangement of these granules is Polar circle Medullary zone X circle Cortical - zone Equatorial zone Equatorial plate of the Chromatic figure Spindle - Pole-body Attraction - sphere zone Fig. 23.-Ovum of the worm Asearis in process of division. The attraction spheres are at opposite ends of the ovum; at the equator of the spindle which unites them, four chromosomes are seen. The protoplasm of the ovum, except in the equatorial zone of the cell, is arranged in lines radiating from the centre (central particle) of the attraction sphere. (Waldeyer.) much better marked in embryonic cells, indeed the lines present very much the appearance of fibrils (see fig. 23). The V-shaped chromoplasmic fibres or chromosomes sink to the equator of the spindle, and arrange themselves so as to stick out horizontally from it. Fine uniting - filaments. Fig. 24.-Metakinesis. a. Early stage. b. Later stage, c. Latest stage-formation of diaster, b. and c. show how the sister threads disentangle themselves from one another. (Waldeyer.) 5. The stage of metakinesis. The sister threads separate, one set going towards one pole, and the other to the other pole of the spindle (fig. 24) : these form the two daughter nuclei. 6. Each daughter nucleus goes backwards through the same series of changes ; the diaster or double star is followed by the dispirem or double skein, until at last two resting nuclei are obtained (fig. 25). A new membrane forms around each daughter nucleus, the spindle atrophies, and the attraction sphere becomes less CH. 11.] KARYOKINESIS. 21 prominent. The division of the cell protoplasm into two parts around the two nuclei begins in the diaster stage, and is complete in the stage represented in fig. 25. The karyokinetic process has been watched in all its stages by more than one observer. The time occupied varies from half an hour to three hours ; the details, however, must be studied in hardened and appropriately stained specimens. They are most readily seen in cells with large nuclei, such as occur in the epi- dermis of amphibians. The process varies a good deal in different animal and vegetable cells; such as in the number of chromosomes, and the relative Remains of spindle. Lighter substance of the nucleus. Cell protoplasm. Hilus. Line of separation of the two cells. Antipole of daughter nucleus. Fig. 25.-Final stages of karyokinesis. In the lower daughter nucleus the changes are still more advanced than in the upper. (Waldeyer.) importance of the different stages. All attempted here has been to give an account of a typical case. The phases may be sum- marised in a tabular way as follows (from Quain's Anatomy):- Network or Reticulum ... 1. Resting condition of mother nucleus (fig. 19). 2. Close skein of fine convoluted fila- I ments (fig. 20). | 3. Open skein of thicker filaments. Spindle appears (fig. 21 a). Skein or Spirem Cleavage4. Movement of V-shaped chromo- somes to middle of nucleus, and each splits into two sister threads (fig. 21 B). Star or Monaster5. Stellate arrangement of V fila- ments at equator of spindle (fig. 22). Divergence or Metakinesis . 6. Separation of cleft filaments and movement along fibres of spindle (fig. 24 A and b). Double Star or Diaster ... 7. Conveyance of V filaments towards poles of spindle (fig. 24 o'). I -- - *T J' 1 8. Open skein in daughter nuclei. 9. Close skein in daughter nuclei (fig. 25)- Double Skein or Dispirem . Network or Reticulum . . .10. Resting condition of daughter nuclei (fig. 25). 22 THE ANIMAL CELL. [ch. 11. The Ovum. The ovary is an organ which produces ova. An ovum is a simple animal cell ; its parts are seen in the next diagram. It is enclosed in a membrane called the zona pellucida or vitelline membrane. The body of the cell is composed of protoplasm loaded Nucleus or germinal vesicle. ■Nucleolus or germinal spot. ■Space left by retraction of protoplasm. •Protoplasm containing yolk spherules. ■Vitelline membrane. Fig. 26.-Representation of a human ovum. (Cadiat.) with granules of food material, and often called the yolk or vitellus. The nucleus and nucleolus are sometimes still called by their old names, germinal vesicle and germinal spot respectively. The formation of ova will form the subject of a chapter later on, but it is convenient here at the outset to state briefly one or two facts, and introduce to the student a few terms which we shall have to employ frequently in the intervening chapters. The ovum first discharges from its interior a portion of its nucleus, which forms two little globules upon it called the polar globules. Fertilisation then occurs; that is to say, the head or nucleus of a male cell called a spermatozoon penetrates into the ovum, and becomes fused with the remains of the female nucleus. Cell division or segmentation then begins, and the early stages are represented in the next figure. Fluid discharged from the cells accumulates within the interior of the mulberry mass seen in fig. 27, d, and later, if a section is cut through it, the cells will be found arranged in three layers. The outermost layer is called the epiblast. The middle layer is called the mesoblast. The innermost layer is called the hypoblast. From these three layers the growth of the rest of the body occurs, nutritive material being derived from the mother in mammals by means of an organ called the placenta. CH. 11.] EPITHELIUM. 23 The epiblast, the outermost layer of the embryo, forms the epidermis, the outermost layer of the adult. It also forms the nervous system. The hypoblast, the innermost layer of the embryo, forms the lining epithelium of the alimentary and respiratory tracts, that is, the innermost layer of the adult. It also forms the cellular elements in the large digestive glands, such as the liver and Fig. 27.-Diagram of an ovum (a) undergoing segmentation. In (6) it has divided into two, in (c) into four; and in (d) the process has resulted in the production of the so-called "mulberry-mass." (Frey.) pancreas, which are originally, like the lungs, outgrowths from the primitive digestive tube. The mesoblast forms the remainder, that is, the great bulk of the body, including the muscular, osseous and other connective tissues; the circulatory and urino-genital systems. CHAPTER III. EPITHELIUM. In the introductory chapter will be found a list of the elementary tissues of which the organs of the body are built up. These may be arranged into the four groups, epithelial, connec- tive, muscular, and nervous. The first of these, the epithelial tissues, follows naturally on a study of the animal cell, as an epithelium may be defined as a tissue composed entirely of cells united by a minimal amount of cementing material. As a rule, an epithelium is spread out as a membrane covering a surface or lining the cavity of a hollow organ. These epithelia may be grouped into two great classes, each of which may be again subdivided according to the shape and arrangement of the cells of which it is composed. The following table gives the principal varieties :- 24 EPITHELIUM. [ch. hi. Class 1.-Simple epithelium ; that is, an epithelium consisting of one layei' of cells only. Its subgroups are as follows :- ci. Pavement epithelium. Fig. 28.-From a section of the lung of a cat, stained with silver nitrate. N. Alveoli or air-cells, lined with large flat, nucleated cells, with some smaller polyhedral nucleated cells. (Klein and Noble Smith.) b. Cubical and columnar epithelium. c. Ciliated epithelium. Class 2.-Compound eprithelium ; that is, an epithelium con- Fig. 29.-Abdominal surface of central tendon of the diaphragm of rabbit, showing the general polygonal shape of the endothelial cells; each cell is nucleated, x 300. (Klein.) sisting of more than one layer of cells. Its subgroups are as follows :- a. Transitional epithelium. b. Stratified epithelium. ch. in.] EPITHELIUM. 25 This classification does not include the more specialised forms of epithelium found in secreting glands, or in the sense organs, Fig- 3°--Peritoneal surface of a portion of the septum of the great lymph-sacs of frog. The stomata, some of which are open, some collapsed, are well shown, x 160. (Klein.) nor structures like hair, and enamel of tooth, which are epithelial in origin. These will be considered in their proper place later on. Fig. 31.-A portion of the great omentum of dog, which shows, amongst the flat endo- thelium of the surface, small and large groups of germinating endothelium, between which are many stomata, x 300. (Klein.) We shall for the present be content to study those already enumerated, and take them one by one. 26 EPITHELIUM. [ch. in. Pavement Epithelium. This consists of a layer of flat cells, arranged like flat pavement- stones accurately fitting together and united by a small amount of cementing material. The structure of the cells and their outlines may be best demonstrated by a process of double staining ; one stain, silver nitrate, to show the cementing material, and another, like logwood, to show up the nuclei of the cells. A portion of the fresh tissue is taken and immersed for a few minutes in a i per cent, solution of nitrate of silver; it is taken out, washed with distilled water, and exposed in water or spirit to sunlight. The silver forms a com- pound with the cement, which in the light is decomposed or re- duced, leading to a fine deposit of silver, showing as black or brown lines between the cells, and accurately defining their outlines. The preparation may then be immersed in some stain like log- wood to bring out the nuclei, and finally mounted in the usual way. The details of histological work can only be properly learnt in a practical class. Fig. 28 shows the appearance presented in a preparation of lung. In the alveoli or air-cells of the lung pavement epithelium of a typical kind is found form- ing a lining membrane. Endothelium.-Epithelium of similar appearance is found lining the interior of the whole of the vascular system, heart, arteries, capillaries, veins, and lymphatics, and in the adjuncts of the circulatory system called the serous membranes (pericardium, peritoneum, etc.). This epithelium is formed from the middle layer of the embryo, the mesoblast ; most other epithelium is derived either from Fig. 32.-Surface view of an artery from the mesentery of a frog, ensheathed in a peri-vascular lymphatic vessel. a. The artery, with its circular mus- cular coat (media) indicated by broad transverse markings, with an indica- tion of the adventitia outside. Z, Lymphatic vessel; its wall is a simple endothelial membrane. (Klein and Noble Smith.) CH. III.J EPITHELIUM. 27 epiblast or hypoblast. Hence it has received a distinct name, viz. : endothelium. The general appearance presented by endothelium in serous membranes is shown in figs. 29, 30, and 31; in blood-vessels in fig- 32- The stomata seen in some of the drawings are minute openings surrounded by more darkly staining cells, which lead from serous cavities into lymphatic vessels. Cubical, Spheroidal, and Columnar Epithelium. In these forms of epithelium, the cells are not flat, but are thick; if they approximate cubes or spheres in shape, the epi- thelium is called cubical or spheroidal respectively. Spheroidal epithelium is found in the alveoli of secreting glands, such as the salivary glands, liver, and pancreas (see figs. 33 and 34), and will Fig'- 33--Glandular epithelium. Small lobule of a mucous gland of the tongue, shoving nucleated glandular cells, x 200. (V. D. Harris.) Fig. 34.-A small piece of the liver of the horse. (Cadiat.) be discussed at length in connection with those organs. Cubical epithelium is found in the alveoli of the thyroid (see fig. 35), in the tubules of the testis, and in the ducts of some glands. In columnar epithelium the cells are tall, and form a kind of palisade or rows of columns. It is found lining the interior of the stomach and intestines, and the ducts of the majority of secreting glands ; it forms also the layer on the outer surface of the ovary. In the intestinal epithelium each cell has a distinct brightly refracting and striated border. Fig. 36 shows two isolated cells of this kind. 28 EPITHELIUM. [ch. hi. The nucleus with its usual network and the vacuolated con- ditions of the protoplasm are very well seen. The attached border is narrower than the free edge. Amoeboid lymph cells are found Fig. 35.-Section of human thyroid ; the few vesicles shown are lined by cubical epithe lium, and contain a colloid material. in the spaces that must necessarily be left when cells of such a shape cover a surface. Fig. 37 shows a row of columnar cells from the rabbit's intestine. The next figure (fig. 38) shows the arrangement of these cells Fig. 36.-Columnar epithelium cells of the rabbit's intestines. The cells have been isolated after maceration in very weak chromic acid. The cells are much vacuolated, and one of them has a fat globule near its attached end. The striated border (str.) is well seen, and the bright disc separating it from the cell protoplasm, n, nucleus with intranuclear network, a, a thinned out winglike projection of the cell which probably fitted between two adjacent cells. (Schafer.) on the surface of a villus, one of the numerous little projections found in the small intestine. ch. in.] EPITHELIUM. 29 The gaps seen there are due to the formation of what are called goblet cells. In some of the columnar cells, a formation Fig. 37.-A row of columnar cells from the rabbit's intestine. Smaller cells are seen between the epithelium cells ; these are lymph-corpuscles. (Schafer.) of granules occurs which consist of a substance called mucigen ; these run together, and are discharged from the cell as a brightly Fig. 38.-Vertical section of an intestinal villus of a cat. a, the striated basilar border of the epithelium; Z>, columnar epithelium; c, goblet cells; d, central lymph-vessel; e, unstriped muscular fibres; f, adenoid stroma of the villus in which are contained lymph-corpuscles. (Klein.) refracting globule of mucin, leaving the cell with open mouth like a goblet, the nucleus being surrounded by the remains of the protoplasm in its narrow stem (see fig. 39). Fig. 39.-Goblet cells. (Klein.) 30 EPITHELIUM. [ch. hi. This transformation is a normal process continually going on throughout life, the discharged mucin contributing to form mucus. The cells themselves may recover their original shape after dis- charge, and repeat the process later on. Ciliated Epithelium. The cells of ciliated epithelium are generally of columnar shape (fig. 40), but they may occasionally be spheroidal (fig. 41). Fig. 40.-Ciliated epithelium from the human trachea. Large fully formed cell. 7>, shorter cell, c, developing cells with more than one nucleus. (Cadiat.) Fig. 41.-Spheroidal ciliated cells from the mouth of the frog, x 300 diame- ters. (Sharpey.) Each cell is surmounted by a bunch of fine tapering filaments. They were originally called cilia because of their resemblance in j-jo. ,2 -Ciliated epithelium of the human trachea. «, layer of longitudinally arranged 6 elastic fibres; 6, basement membrane; c, deepest cells, circular in form; rf, inter- mediate elongated cells ; e, outermost layer of cells fully developed and bearing cilia. X 350. (Kollikcr.) shape to eyelashes. They differ from eyelashes in being extremely small, and in not being stiff; they are in fact composed CH. III.] CILIARY MOTION. 31 of protoplasm. During life these move to and fro, and so produce a current of fluid over the surface they cover. Like columnar cells, they may form goblet cells and discharge mucus. In the larger ciliated cells, it will be seen that the border on which the cilia are set is bright, and composed of little knobs, to each of which a cilium is attached; in some cases the knobs are prolonged into the cell pro- toplasm as filaments or rootlets (fig. 43). The bunch of cilia is homologous with the striated border of columnar cells. Ciliated epithelium is found in the human body, (1) lining the air passages, but not in the alveoli of the lungs; these are lined by pave- ment epithelium; (2) in the Fallopian tubes and upper part of the uterus; (3) in the ducts of the testis known as the vasa efferentia and coni vasculosi; here the cilia are the longest found in the body ; (4) in the ventricles of the brain and central canal of the spinal cord; (5) in the convoluted tubules of the kidney in some animals; (6) the tail of a spermatozoon may also be regarded as a long cilium. In other animals cilia are found in other parts ; for instance, in the frog the mouth and gullet are lined by ciliated cells ; in the tad- pole, the whole surface of the body and especially the gills are covered with cilia. Among the intervertebrates one finds many protozoa com- pletely covered with cilia; in many embryos the cilia are arranged in definite bands round the body ; in the rotifers or wheel animalcules, a ring of cilia round the mouth gives the name to this particular group. The gills of many animals are covered with cilia. Fig. 43. - Ciliated cell from the intes- tine of a mollusc. (Engelmann.) Ciliary Motion. Ciliary motion reminds one of amoeboid movement, bnt it is much more rapid, and more orderly. It consists of a rhythmical movement of the cilia, a bending over, followed by a lessening of the curvature repeated with great, frequency. When living ciliated epithelium, e.y., from the gill of a mussel, or from the mouth of the frog, is examined under the microscope 32 EPITHELIUM. [ch. hi. in a drop of o-6 per cent, solution of common salt (normal saline solution), the cilia are seen to be in constant rapid motion, each cilium being fixed at one end, and swinging or lashing to and fro. The general impression given to the eye of the observer is very similar to that produced by waves in a field of corn, or swiftly running and rippling water, and the result of their move- ment is to produce a continuous current in a definite direction, and this direction is the same on the same surface, being usually in the case of a cavity towards the external orifice. There is not only rhythmicality in the movement of a single cilium, but each acts in harmony with its fellows in the same cell, and on neighbouring cells. The uses of cilia can from the above be almost guessed ; in the respiratory passages they create a current of mucus with entangled dust towards the throat ; in the Fallopian tube or oviduct they assist the ovum on its way to the uterus ; in the gullet of the frog they act downwards and assist swallowing ; in the ciliated protozoa they are locomotive organs. Over the gills of marine animals they keep up a fresh supply of water, and in the case of the rotifers, which are fixed animals, the current of water brings food to the mouth. Ciliary motion is independent of the will, of the direct influence of the nervous system, and of muscular contraction. It may continue for several hours after death or removal from the body, provided the portion of tissue under examination be kept moist. Its independence of the nervous system is shown also in its occurrence in the lowest invertebrate animals which are un- provided with anything analogous to a nervous system, in its persistence in animals killed by prussic acid, by narcotic or other poisons, and after the direct application of narcotics, such as morphia, opium, and belladonna, to the ciliary surface, or of electricity through it. The vapour of chloroform arrests the motion, but it is renewed on the discontinuance of the applica- tion. The movement ceases when the cilia are deprived of oxygen, although it may continue foi' a time in the absence of free oxygen, but is revived on the admission of this gas. Car- bonic acid stops the movement. The contact of various sub- stances, e.g,, bile, strong acids, and alkalis, will stop the motion altogether; but this seems to depend chiefly on destruction of the delicate substance of which the cilia are composed. Tem- peratures above 450 C. and below o° C. stop the movement, whereas moderate heat and dilute alkalis are favourable to the action and revive the movement after temporary cessation. The exact explanation of ciliary movement is not known; whatever CH. in.] TRANSITIONAL EPITHELIUM. 33 may be the exact cause, however, at any rate the movement must depend upon some changes going on in the cell to which the cilia are attached, as when the latter are cut off from the cell the movement ceases, and when severed so that a portion of the cilia are left attached to the cell, the attached and not the severed portions continue the movement. The most probable cause of the movement is that it is part of the inherent power which protoplasm possesses, and that the cilia are but prolongations of the spongioplasm of the cell. It has been suggested by Engelmann that if this be the case, the contractile part of the protoplasm is only on the concave side of a curved cilium, and that when this contracts that the cilium is brought downwards ; where relaxation occurs, the cilium rebounds by the elastic recoil of the convex border. Schafer has suggested that the flow of hyaloplasm backwards and forwards will explain ciliary as it will amoeboid movement. In an amoeboid cell, the spongioplasm is irregular in arrange- ment, hence an outflow of hyaloplasm from it can occur in any direction. But in the curved projection called a cilium, the hyaloplasm can obviously flow in only one direction into the cilium and back again. The flow of more hyaloplasm into the spongioplasm of the cilium will cause it to straighten, the flow of the hyaloplasm back into the body of the cell will cause the cilium to curve. The action of dilute alkalis and acids on cilia is interesting. Dilute acids stop ciliary motion ; and cilia, if allowed to act in salt solution for a time, get more and more languid and finally cease acting ; in popular language they become fatigued. Now we shall find in muscle that fatigue is largely due to the accumu- lation of the acid products of muscular activity; remove the sarko-lactic acid, and fatigue passes off. It is probable that the same occurs in other contractile tissues ; the cilia gradually stop, due to acid products of their activity collecting around them ; neutralise these with dilute alkali, and they resume activity. Transitional Epithelium. This term has been applied to cells which are neither arranged in a single layer, as is the case with simple epithelium, nor yet in many superimposed strata, as in stratified epithelium ; in other words, it is employed when epithelial cells are found in two, three, or four superimposed layers. The upper layer may be either columnar, ciliated, or squa- mous. When the upper layer is columnar or ciliated the second 34 EPITHELIUM. [ch. hi. layer consists of smaller cells fitted into the inequalities of the cells above them, as in the trachea (fig. 40). The epithelium which is met with lining the urinary bladder and ureters is, however, the transitional par excellence. In this variety there are two or three layers of cells, the upper being- more or less flattened according to the full or collapsed condition of the organ, their under surface being marked with one or more depressions, into which the heads of the next layer of club-shaped cells fit. Between the lower and narrower parts of the second row of cells are fixed the irregular cells which constitute the third row, and in like manner sometimes a fourth row (fig. 44). It can be easily understood, therefore, that if a scraping of the mucous membrane of the bladder be teased, and examined under Fig. 44.-Epithelium of the bladder, a, one of the cells of the first row; b, a cell of the second row; c, cells in situ, of first, second, and deepest layers. (Obersteiner.) Kg. 45.-Transitional epithelial cells from a scraping of the mucous membrane of the bladder of the rabbit. (V. D. Harris.) the microscope, cells of a great variety of forms may be made out (fig. 45). Each cell contains a large nucleus, and the larger and superficial cells often possess two. Stratified Epithelium. The term stratified epithelium is employed when the cells forming the epithelium are arranged in a considerable number of superimposed layers. The shape and size of the cells of the different layers, as well as the number of the layers, vary in different situations. Thus the superficial cells are, as a rule, of the squamous, or scaly variety, and the deepest of the columnar form. The cells of the intermediate layers are of different shapes, but those of the middle layers are more or less rounded. The super- ficial cells are broad and overlap by their edges (fig. 46). Their chemical composition is different from that of the underlying cells, as they contain keratin, and are therefore horny in character. CH. III.] STRATIFIED EPITHELIUM. 35 The nucleus is often not apparent. The really cellular nature of even the dry and shrivelled scales cast off from the surface of the epidermis can be proved by the application of caustic potash, which causes them rapidly to swell and assume their original form. The squamous cells exist in the greatest number of layers in the epidermis or superficial part of the skin; the most superficial of these are being continually removed by friction, and new cells from below supply the place of those cast off. Fig. 46.--Epithelium scales from the inside of the mouth. X 260. (Henle.) Fig. 47.-Vertical section of the stratified epithelium of the rabbit's cornea, a, Anterior epithelium, showing the different shapes of the cells at various depths from the free surface; b, a portion of the substance of cornea. (Klein.) The intermediate cells approach more to the flat variety the nearer they are to the surface, and to the columnar as they approach the lowest layer. There may be considerable intercellular intervals; and in many of the deeper layers of epithelium in the mouth and skin the outline of the cells is very irregular, in consequence of processes passing from cell to cell across these in- tervals. Such cells (fig. 48) are termed "ridge and furrow," "cogged" or " prickle " cells. These " prickles " are prolongations of the intracellular network which run across from cell to cell, thus joining them together, the inter- stices being filled by lymph and transparent intercellular cement- Fig. 48.-Jagged cells from the middle layers of stratified epithelium, from a vertical section of the gum of a new-born infant. (Klein.) 36 EPITHELIUM. fCH. in. substance. When this increases in quantity in inflammation the cells are pushed further apart, and the connecting fibrils or " prickles " elongated and therefore more clearly visible. The columnar cells of the deepest layer are distinctly nucleated ; they multiply rapidly by division; and as new cells are. formed beneath, they press the older cells forwards to be in turn pressed forwards themselves towards the surface, gradually altering in shape and chemical composition until they are cast off from the surface. Stratified epithelium is found in the following situations :- (1) Forming the epidermis, covering the whole of the external surface of the body; (2) Covering the mucous membrane of the nose, tongue, mouth, pharynx, and oesophagus; (3) As the con- junctival epithelium, covering the cornea; (4) Lining the vagina and the vaginal part of the cervix uteri. Nutrition of Epithelium. Epithelium has no blood-vessels ; it is nourished by lymph. When the blood is circulating through the thin-walled small blood- vessels in the tissues beneath the epithelium, some of its fluid constituents escapes. This fluid is called lymph ; it penetrates to all parts of the cellular elements of tissues and nourishes them. In the thicker varieties of epithelium, the presence of the irregular minute channels between the prickle cells (fig. 48) enables the lymph to soak more readily between the cells than it would otherwise be able to do. Epithelium is also destitute of nerves as a rule. But in stratified epithelium, particularly that covering the cornea at the front of the eye and in the deeper layers of the epidermis, a plexus of nerve fibrils is found. Chemistry of Epithelium. There is not much to add to what has been already stated concerning cells ; protoplasm and nucleus have the same chemical composition as has been already described in Chapter II. Two new substances have, however, been mentioned in the foregoing- chapter-namely, mucin and keratin. Mucin.-This is a widely distributed substance occurring in epithelial cells or shed out by them (see goblet cells, fig. 39). It also forms the chief constituent of the cementing substance between epithelial cells. We shall again meet with it in the intercellular substance of the connective tissues. The mucin obtained from different sources varies in composition and reaction. CH. IV.] THE CONNECTIVE TISSUES. 37 There are probably several mucins, but they all agree in the following points:- (a) Physical character : viscid and tenacious. (b) Precipitability from solutions by acetic acid. They all dissolve in dilute alkalis, like lime-water. (c) They are all compounds of proteid, with a carbohydrate called animal gum, which by treatment with dilute mineral acid can be hydrated into a reducing but non-fermentable sugar. The substance mucin, when it is formed within cells (goblet cells, cells of mucous glands), is preceded in the cells by granules of a substance which is not mucin, but is readily changed into mucin. This precursor, or mother-substance of mucin, is called mucigen or mucinogen. Keratin, or horny material, is the substance found in the sur- face layers of the epidermis, in hairs, nails, hoofs, and horns. It is very insoluble, and chiefly differs from proteids in its high per- centage of sulphur. These two substances, mucin and keratin, are not proteids, though similar to them. They are members of a heterogeneous group of proteid-like sub- stances which are called albuminoids, and several more members of this group we shall have to consider in our next chapter on the connective tissues. CHAPTER IV. THE CONNECTIVE TISSUES. The connective tissues are the following :- 1. Areolar tissue. 2. Fibrous tissue. 3. Elastic tissue. 4. Adipose tissue. 5. Retiform and lymphoid tissues. 6. Jelly-like tissue. 7. Cartilage. 8. Bone and dentine. 9. Blood. At first sight these numerous tissues appear to form a very heterogeneous group, including the most solid tissues of the body (bone, dentine) and the most fluid (blood). 38 THE CONNECTIVE TISSUES. [ch. iv. But on examining a little more deeply, one finds that the grouping of these apparently different tissues together depends on a number of valid reasons, which may be briefly stated as follows:- 1. They all resemble each other in origin. All are formed from the mesoblast, the middle layer of the embryo. 2. They resemble each other structurally; that is to say, the cellular element is at a minimum, and the intercellular material at a maximum. 3. They resemble each other functionally; they form the skeleton, and act as binding, supporting, or connecting tissues to the softer and more vital tissues. An apology is sometimes made for calling the blood a tissue, because one's preconceived idea of a tissue or texture is that it must be something of a solid nature. But all the tissues contain water. Muscular tissue contains, for instance, at least three- quarters of its weight as water. Blood, after all, is not much more liquid than muscle. Blood, moreover, contains cellular elements analogous to the cells of other tissues, but separated by vast quantities of a fluid intercellular material called blood- plasma. Blood is also mesoblastic, and thus the two first characteristics of a connective tissue are present. It does not fulfil the third condition by contributing to the support of the body as part of the skeleton, but it does so in another sense, and serves to sup- port the body by conveying nutriment to all parts. We may now proceed to a consideration of this long list of tissues, one by one, in the order named. Areolar Tissue. It is convenient to take this first, as it is a very typical connective tissue. It has a wide distribution, and constitutes the subcutaneous, subserous, and submucous tissues. It forms sheaths (fascire) for muscles, nerves, blood-vessels, gdands, and internal organs, binding them in position and penetrating into their interior, supports and connects their individual parts. If one takes a little of the subcutaneous tissue from an animal, and stretches it out on a glass slide, it appears to the naked eye like a soft, fleecy network of fine white fibres, with here and there wider fibres joining it. It is, moreover, elastic. But in order to make out its structure accurately it is necessary to examine the thinnest portions of the film with the microscope, and ch. iv. ] AREOLAR TISSUE. 39 the action of staining and other reagents may then be also studied. By such means it is seen that this typical connective tissue con- Fig. 49.-Bundles of the white fibres of areolar tissue partly unravelled. (After Sharpey.) sists of four different kinds of material, or, as they may be termed, histological elements. They are :-- (a) Cells, or connective-tissue cor- puscles. (b) A homogeneous matrix, ground substance, or intercellular mate- rial. (c) White fibres ) T} (d) Yellow or elastic fibres J are deposited in the matrix. In considering these four histological elements we may first take the fibres, because they are the most obvious and abundant of the structures observable. The white fibres. These are ex- quisitely fine fibres collected into bundles which have a wavy outline. The bundles run in different directions, forming an irregular network, the meshes between which are called areolce ; hence the name areolar. The individual fibres never branch or join other fibres, but they may pass from one bundle to another. On treatment with dilute acetic acid they become swollen and Fig. 50.-Elastic fibres of areolar tissue. (After Schafer.) 40 THE CONNECTIVE TISSUES. [ch. iv indistinct, leaving the other structures mixed with them more apparent. They are composed of the chemical substance called collagen. On boiling they yield gelatin ; some chemists regard collagen as the anhydride of gelatin; but whether this is so or not, the gelatin is undoubtedly derived from the collagen. Gelatin is a proteid-like substance though not a proteid. It belongs to the class of albuminoids. Its most characteristic property is its power of jellying or gelatinising; that is, it is soluble in hot water, but on cooling the solution it sets into a jelly. The yelloiv or elastic fibres. These are seen readily after the white fibres are rendered almost invisible by treatment with dilute acetic acid, or after staining with magenta, for which they have a great affinity. They are bigger than the white fibres, have a distinct outline, and a straight course ; they run singly, branch, and join neigh- bouring fibres. The material of which the elastic fibres are com- posed is called elastin; this is another albuminoid. It is unaltered, as we have seen, by dilute acid. It also resists the action of very strong acid, and is not affected by boiling water. The bundles of white fibres which have been swollen out by dilute acetic acid sometimes ex- hibit constrictions as in fig. 5 1. These are due to elastic fibres or cell processes encircling them and preventing, the swelling at those points. Connective-tissue corpuscles. These are the cells of connective tissue: several varieties may be made out, especially after a preparation has been stained. 1. Flattened cells, branched, and often united by their processes, as in the cornea. 2. Flattened cells, unbranched, and joined edge to edge like the cells of an epithelium; these are well seen in the sheath of a tendon. 3. Plasma cells of Waldeyer, varying greatly in size and form, but not flattened. The protoplasm is much vacuolated. 4. Granule cells : like plasma cells, but containing albuminous granules (stainable by eosin) instead of vacuoles. 5. Wander cells: white blood-corpuscles which have emigrated from the neighbouring blood-vessels. 6. Pigment cells: these are seen in the subcutaneous tissues F1 by iatetiC atid CH. IV.] AREOLAR TISSUE. 41 of many animals, e.g., the frog, and in the choroid coat of the eyeball. Fig. 55 shows a highly magnified view of a small piece of subcutaneous tissue, and illustrates the irregular way in which the fibres and cells are intermixed. Fig. 53.-Ramified pigment- cells, from the tissue of the choroid coat of the eye. x 350. a, cell with pigment ; &, colourless fusiform cells. (Kolli- ker.) Fig. 52.-Horizontal preparation of the cornea of frog, stained in gold chloride; showing the network of branched cornea corpuscles. The ground substance is completely colourless. X 400. (Klein.) But we have still to consider the undifferentiated intercellular material which is called Fig. 54.-Flat, pigmented, branched connective-tissue cells from the sheath of a large blood-vessel of the frog's mesentery ; the pigment is not distributed uniformly through- out the substance of the larger cell, consequently some parts of it look blacker than others (uncontraeted state). In the two smaller cells most of the pigment is withdrawn into the cell-body, so that they appear smaller, blacker, and less branched, x 350. (Klein and Noble Smith.) The grounchsubstance. This may be represented in figure 55 by the white background of the paper. It may, however, be more readily seen in a silver nitrate pre- paration. For the intercellular material has the same property of reducing silver salts in the sunlight that the cement-material of 42 THE CONNECTIVE TISSUES. [ch. iv. epithelium has. It becomes in consequence dark brown, with the exception of the spaces occupied by the corpuscles. Fig. 55.-Areolar tissue. The white fibres are seen in wavy bundles; the elastic fibres form an open network, p, p, plasma cells, g, granule cell, e, e, lamellar cells. /, fibrillated cell. (After Schafer.) The spaces intercommunicate like the cells, and being con- siderably larger than the cells form a ramifying network of irregular channels, which were first termed by v. Recklinghausen Fig. 56.-Ground substance of connective tissue, stained by silver nitrate. The cell spaces are left white. the Saft Kanalchen, or little juice canals. Areolar tissue is certainly provided with blood-vessels, but the tissue elements are, as in all tissues, provided with nutriment by the exudation from the blood CH. IV.] AREOLAR TISSUE. 43 called lymph. The Saft Kantilchen enable the lymph to penetrate to every part of the areolar tissue. Development of Areolar Tissue The mesoblastic cells in those parts where the tissue is to be formed become branched and fusiform. Fig. 57.-Portion of submucous tissue of gravid uterus of sow. a, branched cells, more or less spindle-shaped ; b, bundles of connective tissue. (Klein.) These ultimately become the connective-tissue corpuscles, and they get more and more widely separated by intercellular Fig. 58.-Jelly of Wharton, r, ramified cells intercommunicating by their branches. Z, a row of lymph-cells. /, fibres developing in the ground substance. (Ranvier.) material, partly shed out by the cells themselves, partly shed out from the neighbouring blood-vessels. This becomes the ground substance. The fibres form subsequently in this as crystals may be deposited by a liquid. At one time it was believed that the cells themselves became elongated and converted into fibres. No 44 THE CONNECTIVE TISSUES. [ch. iv. doubt the cells do exercise a controlling influence on fibre-formation in their neighbourhood, but it is extremely doubtful whether they ever become fibres. The formation of fibres is now believed to be intercellular. Some of the fibres formed are of the white, others Fig. 59.-Development of elastic tissue by deposition of fine granules. <7, fibres being formed by rows of elastic granules, p, platelike expansion of elastic substance formed by the fusion of elastic granules. (Ranvier.) of the yellow variety. In the case of the clastic fibres, rows of granules of elastin are first deposited ; these joining together in single or multiple rows form the long fibres : traces of this are seen in transverse markings occasionally noticeable in the larger elastic fibres. Fibrous Tissue. This is a kind of connective tissue in which the white fibres predominate; it is found in tendons and ligaments, in the peri- Fig. 60.-Mature white fibrous tissue of tendon, consisting mainly of fibres with a few scattered fusiform cells. (Stricker.) CH. iv.] FIBROUS TISSUE. 45 osteum, dura mater, true skin, the sclerotic coat of the eye, and in the thicker fasciae and aponeuroses of muscle. Fig'. 61.-Caudal tendon of young rat, showing the arrangement, form, and structure of the tendon cells, x 300. (Klein.) The tissue is one of great strength ; this is conferred upon it by the arrangement of the fibres, the bundles of which run parallel, union here, as elsewhere, giving strength. The fibres of the same bundle now and then intersect each other. The cells in tendons (fig. 61) are forced to take up a similar orderly arrangement, and are arranged in long chains in the ground substance separating the bundles of fibres, and are more or less regularly quadrilateral with large round nuclei containing nu- cleoli, which are generally placed so as to be contiguous in two cells. Each of these cells consists of a thick body, from which processes pass in various directions into, and partially fill up the spaces between, the bundles of fibres. The rows of cells are separated from one another by lines ot cement substance. The cells are generally marked by one or more lines or stripes when viewed longitudinally. This appearance is really produced by the wing-like processes of the Fig. 62.-Transverse section of tendon from a cross section of the tail of a rabbit, showing sheath, fibrous septa, and branched connective- tissue corpuscles. The spaces left white in the drawing repre- sent the tendinous fibres in trans- verse section. X 250. (Klein.) 46 THE CONNECTIVE TISSUES. [ch. iv. cell, which project away from the chief part of the cell in different directions. These processes not being in the same plane as the body of the cell are out of focus, and give rise to these bright stripes when the cells are looked at from above and are in focus. Fig. 63.-Cell spaces of tendon, brought into view by treatment with silver nitrate, The branched character of the cells is seen in transverse section in fig. 62. The cell spaces in which the cells lie are in arrangement like the cells ; they can be brought into relief by staining with silver nitrate (see fig. 63). Elastic Tissue This is a form of connective tissue in which the yellow or elastic fibres predominate. The yellow fibres are larger than are found in areolar tissue (see fig. 64), and are bound into bundles by areolar tissue. It is found in the ligamentum nucha) of the ox, horse, and many other animals; in the ligamenta subflava of man ; in the arteries, constituting the fenestrated coat of Henle; in veins; in the lungs and trachea; in the stylo- hyoid, thyro-hyoid, and crico-thy- roid ligaments; in the true vocal cords. Structure.-Elastic tissue occurs in various forms, from a structure- less, elastic membrane to a tissue whose chief constituents are bundles of fibres crossing each other at different angles ; when seen in bundles elastic fibres are yellowish in colour, but individual fibres are not so distinctly coloured. The Fig. 64.-Elastic fibres from ths liga menta subflava. x 200. (Sharpey.) CH. IV.] ELASTIC AND ADIPOSE TISSUES. 47 larger elastic fibres are often transversely marked, indicating their mode of origin (see p. 44), and on transverse section are seen to be angular. Elastic tissue, being extensible and elastic (i.e., recoiling after it has been stretched), has a most important use in assisting mus- cular tissue in a mechanical way, and so lessening the wear and tear of such an important tissue as muscle. Thus, in the ligamenta subflava of the human vertebral column it assists in the maintenance of the erect posture ; in the ligamentum nuchse in the neck of quadrupeds it assists in the raising of the head and in keeping it in that position. In the arterial walls, and in the air tubes and lungs, it has a similar important action, as we shall see when discussing the subjects of the circulation and respiration. We now come to those forms of con- nective tissue in which the cells rather than the fibres are most prominent. Fig. 65.-Transverse section of a portion of lig. nuchee, show- ing the outline of the fibres, (After stiihr.) Adipose Tissue. Distribution.-In almost all regions of the human body a larger or smaller quantity of adipose or fatty tissue is present; the chief exceptions being the subcu- taneous tissue of the eyelids, penis and scrotum, the nymph se, and the cavity of the cranium. Adipose tissue is also absent from the substance of many organs, as the lungs and liver. Adipose tissue is almost always found seated in areolar tissue, and forms in its meshes little masses of unequal size and irregular shape, to which the term lobules is commonly applied. Structure. - Under the microscope adipose tissue is found to consist essentially of little vesicles or cells which present dark, sharply- defined edges when viewed with transmitted light : they are about or -gfa of an inch in diameter, each consisting of a structureless and colourless membrane or bag formed of the Fig. 66.-Ordinary fat-cells of a fat tract in the omentum of a rat. (Klein.) 48 THE CONNECTIVE TISSUES. ten. IV. remains of the original protoplasm of the cell, filled with fatty matter, which is liquid during life, but in part solidified (or some- Fig. 67.-Group of fat-cells (f c) with capillary vessels (c). (Noble Smith.) times crystallised) after death. A nucleus is always present in some part or other of the cell-protoplasm, but in the ordinary condition of the cell it is not easily or always visible (fig. 67). Fig. 68.-Blood-vessels of adipose tissue, a. Minute flattened fat-lobule, in which the vessels only are represented, a, the terminal artery ; v, the primitive vein ; Z>, the fat- vesicles of one border of the lobule separately represented. X 100. b. Plan of the arrangement of the capillaries (c) on the exterior of the vesicles ; more highly magni- fied. (Todd and Bowman.) This membrane and the nucleus can generally be brought into view by staining the tissue : it can be still more satisfactorily CH. IV.] ADIPOSE TISSUE. 49 demonstrated by extracting the contents of the fat-cells with ether, when the shrunken, shrivelled membranes remain behind. By mutual pressure, fat-cells come to assume a polyhedral figure (fig. 67). When stained with osmic acid fat-cells appear black. The cells are surrounded by capillary blood-vessels (fig. 68); the little clusters thus formed are grouped into small masses, and held so by areolar tissue. The oily matter contained in the cells is composed of the compounds of fatty acids with glycerin, which are named olein, stearin, and palmitin. Development of Adipose Tissue. - Fat-cells are de- veloped from connective-tissue corpuscles ; connective - tissue cells may be found exhibiting every intermediate gradation between an ordinary branched connective-tissue corpuscle and a mature fat-cell. The process of development is as follows : a few small drops of oil make their appearance in the proto- plasm and by their confluence a larger drop is produced (fig. 70) : this gradually increases in size at the expense of the original protoplasm of the cell, which becomes correspondingly diminished in quantity till in the mature cell it only forms a thin film, with a flattened nucleus imbedded in its substance (fig. 66). Vessels and Nerves.-A large number of blood-vessels are found in adipose tissue, which subdivide until each lobule of fat contains a fine meshwork of capillaries ensheathing each individual fat-cell (fig. 68). Although nerve fibres pass through the tissue, no nerves have been demonstrated to terminate in it. The Uses of Adipose Tissue.-Among the uses of adipose tissue these are the chief:- a. It serves as a store of combustible matter which may be reabsorbed into the blood when occasion requires, and, being used up in the metabolism of the tissues, helps to preserve the heat of the body. b. Part of the fat which is situate beneath the skin must, by Fig. 69.-A lobule of developing adipose tissue from an eight months' foetus, a, Spheri- cal or, from pressure, polyhedral cells with large central nucleus, surrounded by a finely reticulated substance staining uniformly with htematoxylin. b, Similar cells with spaces from which the fat has been removed by oil of cloves, c, Similar cells showing how the nucleus with en- closing protoplasm is being pressed to- wards periphery, d, Nucleus of endothe- lium of investing capillaries. (McCarthy.) Drawn by Treves. 50 THE CONNECTIVE TISSUES. [ch. iv. its want of conducting power, assist in preventing undue waste of the heat of the body by escape from the surface. c. As a packing material, fat serves very admirably to fill up spaces, to form a soft and yielding yet elastic material wherewith to wrap tender and delicate structures, or form a bed with like Fig. 70.-Branched connective-tissue corpuscles, developing into fat-cells. (Klein.) qualities on which such structures may lie, not endangered by pressure. As examples of situations in which fat serves such purposes may be mentioned the palms of the hands and soles of the feet and the orbits. d. In the long bones fatty tissue, in the form known as yellow marrow, fills the medullary canal, and supports the small blood- vessels which are distributed from it to the inner part of the substance of the bone. Retiform or reticular tissue is a kind of connective-tissue in which the ground substance is of more fluid consistency than Retiform Tissue Fig. 71.-Retiform tissue from a lymphatic gland, from a section which has been treated with dilute potash. (Schafer.) CH. IV.] LYMPHOID TISSUE. 51 elsewhere. There are few or no elastic fibres in it, but the white fibres run in very fine bundles forming a close network. The bundles are covered and concealed by flattened connective-tissue corpuscles. When these are dissolved by dilute potash, the fibres are plainly seen. Adenoid or Lymphoid Tissue. This is retiform tissue in which the meshes of the network are largely occupied by lymph corpuscles. These are in certain foci actively multiplying; they get into the lymph stream, which washes them into the blood, where they become the colourless Fig. 72.-Part of a section of a lymphatic gland, from which the corpuscles have been for the most part removed, showing the supporting and retiform tissue. (Klein and Noble Smith.) corpuscles. It is found in the lymphatic glands, the thymus, the tonsils, in the follicular glands of the tongue, in Peyer's patches, and in the solitary glands of the intestines, in the Malpighian corpuscles of the spleen, and under the epithelium of many mucous membranes. Basement Membranes. These are homogeneous in appearance, and are found between the epithelium of a mucous membrane and the subjacent con- nective tissue. They are generally formed of flattened connective- tissue corpuscles joined together by their edges, but sometimes THE CONNECTIVE TISSUES. [CH. IV. 52 they are made of condensed ground substance, not of cells, and in other cases again (as in the cornea) they are of elastic nature. Jelly-like Connective Tissue. We have now considered connective tissues in which fibres of one or the other kind predominate, and some in which the cells are in preponderance. We now come lastly to a form of connective tissue in which the ground substance is in excess of the other histological elements. This is called jelly-like connec- tive tissue. The cells and fibres scattered through it are few and far between. It is found largely in the embryo, notably in the Whartonian jelly, which surrounds and protects the blood-vessels of the umbi- lical cord.' In the adult it is found in the vitreous humour of the eye. Various points in the structure of the tissue are illustrated in figs. 58 (p. 43) and 73. The occurrence of large quantities of ground substances in such tissues has enabled physiologists to examine its chemical nature. Its chief constituents are water, and one or more varieties of mucin, with traces of mineral salts. Fig. 73.-Tissue of the jelly of Wharton from umbilical cord, a, connective-tissue corpus- cles ; 6, fasciculi of connective-tissue; c, spherical formative cells. (Frey.) The foregoing tissues are sometimes called the connective tissues proper. The remaining members of the connective tissue group we shall reserve for the next chapter. ch. v.] CARTILAGE. 53 CHAPTER V. THE CONNECTIVE TISSUES (continued}. Cartilage, Bone, Teeth, Blood. Cartilage is popularly termed gristle. It may be divided into two chief kinds : Hyaline cartilage; here the matrix or ground substance is clear and free from fibres : Fibro-cartilage ; here the Cartilage. Fig. 73A.-Section of articular cartilage, a, group of two cells ; b, group of four cells ; d, protoplasm of cell with e, fatty granules ; c, nucleus. (After Schafer.) matrix is pervaded with connective-tissue fibres; when these are of the white variety, the tissue is white fibro-cartilage ; when they are of the yellow or elastic variety, the tissue is yellow or elastic fibro-cartilage. Hyaline Cartilage is found in the following places :- 1. Covering the articular ends of bones; here it is called articular cartilage. 2. Forming the rib-cartilages; here it is called costal cartilage. 54 THE CONNECTIVE TISSUES. [ch. v. 3. The cartilages of the nose, of the windpipe, of the external auditory meatus, and the greater number of the laryngeal cartilages. 4. Temporary cartilage; rods of cartilage which prefigure the majority of the bones in process of development. Articular cartilage : here the cells are rounded and scattered in groups of two and four through the matrix, which is non- fibrillated, and much firmer than the ground-substance of the Fig. 74.-Section of transitional cartilage, a, ordinary cartilage cells; b b, those with processes. (After Schafer.) connective tissues proper; but it is affected in the same way with silver nitrate. In the neighbourhood of the synovial membrane, the connective- tissue fibres of which extend into the matrix, the cells are branched (transitional cartilage). The next figure shows the general arrangement of the cell- groups in a vertical section of articular cartilage. Cartilage is free from blood-vessels, and also from nerves. It is nourished by lymph, but canals connecting the cell-spaces are not evident. Costal cartilage: here the matrix is not quite so clear, and the cells are larger, more angular, and collected into larger groups than in articular cartilage. Under the perichondrium, a CH. V.] FIBRO-CARTILAGE. 55 fibrous membrane which surrounds the rod of cartilage, the cells are flattened and lie parallel to the surface; in the deeper parts they are irregularly ar- ranged ; they frequently contain fat (see fig. 76). The hyaline cartilages of the nose, larynx and trachea (fig. 77) resemble costal cartilage. Hyaline cartilage in many situations (costal, laryngeal, tracheal) shows a tendency to become calcified late in life. On boiling, the ground-substance of cartilage yields a material called chondrin. This resembles gelatin very closely, and the differences in its reactions are due to the fact that chondrin is Fig'- 75--Vertical section of articular cartilage, a, cell-groups arranged parallel to surface, b, cell-groups irregularly arranged, c, cell-groups arranged perpendicularly to surface. Fig. 77.-Ordinary hyaline cartilage from trachea of a child. The cartilage cells are enclosed singly or in pairs in a capsule of hyaline substance. X 150 diams. (Klein and Noble Smith.) Fig. 76.-Costal cartilage from an adult dog, showing the fat-globules in the cartilage-cells. (Cadiat.) not a chemical individual, but a mixture of gelatin with varying amounts of mucin-like substances. White Fibro-Cartilage occurs- 1. As inter-articular fibro-cartilage-e.g., the semilunar carti- lages of the knee-joint. 2. As circumferential or marginal cartilage, as on the edges of the acetabulum and glenoid cavity. 56 THE CONNECTIVE TISSUES. [ch. v. 3. As connectinc/ cartilage-e.g., the inter-vertebral discs. 4. In the sheaths of tendons and sometimes in their substance. In the latter situation the nodule of fibro-cartilage is called a sesamoid fibro-car- tilage, of which a specimen may be found in the tendon of the tibialis posticus in the sole of the foot, and usually in the neighbouring tendon of the peroneus longus. White fibro-cartilage (fig. 78) is composed of cells and a matrix ; the latter, however, being made up almost entirely of fibres of the white variety. In this kind of fibro- cartilage it is not unusual to find a great part of its mass composed almost exclu- sively of fibres, and deriving the name of cartilage only from the fact that in another portion, continuous with it, cartilage-cells may be pretty freely distributed. Yellow or Elastic Fibro- Cartilage is found in the pinna of the external ear, in the epiglottis and cornicula laryngis, and in the Eusta- chian tube. The cells in this variety of cartilage are rounded or oval, with well-marked nuclei and nucleoli (fig. 79). The matrix in which they are seated is pervaded in all directions by fine elastic fibres, which form an intricate interlace- ment about the cells : a small and variable quantity of non-fibrillated hyaline in- tercellular substance is also present around the cells. Development of Cartilage.-Like other connective tissues, ■ Cells of car- tilage. - Fibrous matrix. Fig. 78.-White fibro-cartilage. (Cadiat.) Fig. 79.-Yellow elastic cartilage. (Cadiat.) ch. v.] DEVELOPMENT OF CARTILAGE 57 cartilage originates from mesoblast; the cells are unbran died, and the disposition of the cells in fully formed cartilage in groups of two, four, &c., is due to the fact that each group has originated from the division of a single cell, first into two, each of these again into two, and so on. This process of cell division is accompanied with the usual karyokinetic changes. Each cell deposits on its exterior a sheath or capsule; on Fig. 80.-Plan of multiplication of cells in cartilage, a, cell in its capsule, b, divided into two, each with a capsule, c, primary capsule disappeared, secondary capsules coherent with matrix, d, tertiary division, e, secondary capsules disappeared, tertiary coherent with matrix. (After Sharpey.) division the daughter cells each deposit a new capsule within this, and the process may be repeated (see fig. 80). Thus the cells get more and more separated. The fused capsules form a very large part of the matrix, and indications of their previous existence may sometimes be seen in fully formed cartilage by the presence of faint concentric lines around the cells (see fig. 76). In a variety of cartilage found in the ears of rats and mice called cellular cartilage, the cells never multiply to any great extent, and they are only separated by their thickened capsules. But in most cartilages the cell-capsules will not explain the 58 THE CONNECTIVE TISSUES. [CH. v. origin of the whole matrix, but intercellular material accumulates outside the capsules and still further separates the cells. By certain complicated methods of double staining this twofold manner of formation may be shown very markedly. We have seen that chondrin obtained by boiling cartilage is really a mixture of two substances; one is a mucinoid material, and conies from the capsules ; the other is gelatin, which comes from the rest of the ground-substance which is collagenous. In hyaline cartilage, however, the collagen does not become precipitated to form fibres, but in white fibro-cartilage it does. In yellow fibro- ■cartilage the matrix is pervaded by a deposit of elastin, which results in the formation of a network of elastic fibres. Bone. Chemical composition.-Bone is composed of earthy and animal matter in the proportion of abont 67 per cent, of the former to 33 per cent, of the latter. The earthy matter is composed chiefly of calcium phosphate, but besides this, there is a small quantity (about 11 of the 67 per cent.) of calcium carbonate, calcium fluoride, and magnesium phosphate. The animal matter is chiefly collagen, which is converted into gelatin by boiling. The animal and earthy constituents of bone are so intimately blended and incorporated the one with the other that it is only by severe measures, as for instance by a white heat in one case and by the action of concentrated acids in another, that they can be separated. Their close union too is further shown by the fact that when by acids the earthy matter is dissolved out, or on the other hand when the animal part is burnt out, the shape of the bone is alike preserved. The proportion between these two constituents of bone varies slightly in different bones in the same individual and in the same bone at different ages. Structure.-To the naked eye there appear two kinds of struc- ture in different bones, and in different parts of the same bone, namely, the dense or compact, and the spongy or cancellous tissue. Thus, in making a longitudinal section of a long bone, as the humerus or femur, the articular extremities are found capped on their surface by a thin shell of compact bone, while their interior is made up of the spongy or cancellous tissue. The shaft, on the other hand, is formed almost entirely of a thick layer of the com- pact bone, and this surrounds a central canal, the medullary cavity-so called from its containing the medulla or marrow. CH. V.] MARROW. 59 In the flat bones, as the parietal bone or the scapula, one layer of the cancellous structure lies between two layers of the compact tissue, and in the short and irregular bones, as those of the carpus and tarsus, the cancellous tissue fills the interior, while a thin shell of compact bone forms the outside. Marrow.-There are two distinct varieties of marrow-the red and yellow. Red marrow is that variety which occupies the spaces in the cancellous tissue; it is highly vascular, and thus maintains the nutrition of the spongy bone, the interstices of which it fills. It contains a few fat-cells and a large number of marrow-cells, many of which are undistinguishable from lymphoid corpuscles, and has for a basis a small amount of fibrous tissue. Among the cells are Fig. 81.-Cells of the red marrow of the guinea-pig, highly magnified, a, a large cell, the nucleus of which appears to be partly divided into three by constrictions ; b, a cell, the nucleus of which shows an appearance of being constricted into a number of smaller nuclei; c, a so-called giant cell, or myeloplaxe, with many nuclei; d, a smaller myelo» plaxe, with three nuclei; e-i, proper cells of the marrow. (E. A. Schafer.) some nucleated cells of the same tint as coloured blood-corpuscles There are also a few large cells with many nuclei, termed gian cells or myeloplaxes (fig. 81). Yellow marrow fills the medullary cavity of long bones, and ■consists chiefly of fat-cells with numerous blood-vessels; many of its cells also are in every respect similar to lymphoid corpuscles. From these marrow-cells, especially those of the red marrow, are derived, as we shall presently see, large quantities of red blood-corpuscles. Periosteum and Nutrient Blood-vessels.-The surfaces of bones, except the part covered with articular cartilage, are ■clothed by a tough, fibrous membrane, the periosteum ; and it is from the blood-vessels which are distributed in this membrane, that tlie bones, especially their more compact tissue, are in great 60 THE CONNECTIVE TISSUES. [ch. v. part supplied with nourishment,-minute branches from the periosteal vessels entering the little foramina on the surface of the bone, and finding their way to the Haversian canals, to be immediately described. The long bones are supplied also by a proper nutrient artery which, entering at some part of the shaft so as to reach the medullary canal, breaks up into branches for the supply of the marrow, from which again small vessels are distributed to the interior of the bone. Other small blood- Fig. 82.-Transverse section of compact bony tissue (of humerus). Three of the Haversian canals are seen, with their concentric rings; also the lacunae, with the canaliculi extending from them across the direction of the lamellae. The Haversian apertures were filled with air and debris in grinding down the section, and therefore appear black in the figure, which represents the object as viewed with transmitted light. The Haversian systems are so closely packed in this section, that scarcely any infersfi'iiaZ lamellae are visible. X 150. (Sharpey.) vessels pierce the articular extremities for the supply of the can- cellous tissue. Microscopic Structure of Bone.-Notwithstanding the differences, of arrangement just mentioned, the structure of all bone is found under the microscope to be essentially the same. Examined with a rather high power its substance is found to contain a multitude of small irregular spaces, approximately fusi- form in shape, called lacunae, with very minute canals or canali- culi leading from them, and anastomosing with similar little prolongations from other lacunae (fig. 82). In very thin layers of bone, no other canals than these may be visible ; but on making a transverse section of the compact tissue as of a long CH. V.] BONE. 61 hone, e.g., the humerus or ulna, the arrangement shown in fig. 82 san be seen. The bone seems mapped out into small circular districts, at or ibout the centre of each of which is a hole, around which is an appearance as of concentric layers-the lacunce and canaliculi fol- lowing the same concentric plan of distribution around the small hole in the centre, with which indeed they com- municate. On making a longitu- dinal section, the central holes are found to be simply the cut extremi- ties of small canals which run lengthwise through the bone, anastomosing with each other by late- ral branches (fig. 83), and are called Haversian canals, after the name of the physician, Clopton Havers, who first accu- rately described them. The Haversian canals, the average diameter of which is of an inch, contain blood-vessels, and by means of them blood is conveyed to all, even the densest parts of the bone; the minute canaliculi and lacunae absorbing lymph from the Haversian blood- vessels and conveying it still more intimately to the substance of the bone which they traverse. The blood-vessels enter the Haversian canals both from with- out, by traversing the small holes which exist on the surface of all bones beneath the periosteum, and from within by means of small channels which extend from the medullary cavity, or from the cancellous tissue. The arteries and veins usually occupy separate canals, and the veins, which are the larger, often present, at irregular intervals, small pouch-like dilatations. The lacunae are occupied by branched cells, which are called bone-cells, or bone-corpuscles (fig. 84), which very closely resemble the ordinary branched connective-tissue corpuscles ; each of these little masses of protoplasm ministers to the nutrition of the Fig. 83.-Longitudinal section from the human ulna, showing Haversian canal, lacunae, and canaliculi. (Rollett.) 62 THE CONNECTIVE TISSUES. [ch. v. bone immediately surrounding it, and one lacunar corpuscle com- municates with another, and with its surrounding district, and with the blood-vessels of the Haversian canals, by means of the minute streams of fluent nutrient matter or lymph which occupy the canaliculi. It will be seen from the above description that bone is essentially connective tissue, the ground-sub- stance of which is impreg- nated with lime salts. The bone-corpuscles with their processes, occupying the lacume and canaliculi, correspond exactly to the connective-tissue corpuscles lying in branched spaces. Lamellae of Compact Bone.-In the shaft of a long bone three distinct sets of lamellae can be clearly recognised. 1. Circumferential lamellae; which are most easily traceable just beneath the periosteum, and around the medullary cavity, forming around the latter a series of concentric rings. At a little distance from the medullary and periosteal surfaces (in the deeper portions of the bone) they are more or less interrupted by 2. Haversian lamellae, which are concentrically arranged around the Haversian canals to the number of six to eighteen around each. 3. Interstitial lamellae, which con- nect the systems of Haversian lamellae, filling the spaces between them, and consequently attaining their greatest development where the Haversian systems are few, and vice versd. The ultimate structure of the lamellae is fibrous. If a thin film be peeled off* the surface of a bone, from which the earthy Fig. 84.-Bone-corpuscles with their processes as seen in a thin section of human bone. (RoHett.) Fig. 85.-Thin layer peeled off from a softened bone. This figure, which is intended to represent the reticular struc- ture of alamella, gives abetter idea of the object when held rather farther off than usual from the eye. X 400. (Sharpey.) CH. V.] DEVELOPMENT OF BONE. 63 matter has been removed by acid, and examined with a high power of the microscope, it will be found composed of very slender fibres decussating obliquely, but coalescing at the points of intersection, as if here the fibres were fused rather than woven together (fig. 85). These are called the intercrossing fibres of Sharpey ; they correspond to the white fibres of connective tissuo and form the source of the gelatin obtained by boiling bone. In many cases, as in the parietal bone, the lamellae are per- forated by tapering fibres called the perforating fibres of Sharpey,. resembling in character the ordinary white or rarely the elastic fibrous tissue, which bolt the neighbouring lamellae together, and may be drawn out when the latter are torn asunder (fig. 86). These per- forating fibres originate from ingrowing processes of the periosteum, and in the adult still retain their connection with it. Development of Bone.-From the point of view of their develop- ment, all bones may be subdivided into two classes:- (a.) Those which are ossified directly or from the first in a fibrous membrane afterwards called the periosteum-e.g., the bones forming the vault of the skull, parietal, frontal, and a certain portion of the occipital bones. (6.) Those whose form, previous to ossification, is laid down in hyaline cartilage-e.g., humerus, femur. The process of development, pure and simple, may be best studied in bones which are not preceded by cartilage-i.e., mem- brane-formed (e.g., parietal); and without a knowledge of this process (ossification in membrane), it is impossible to understand the much more complex series of changes through which such a structure as the cartilaginous femur of the foetus passes in its Fig. 86.-Lamellae tom off from a decalcified human parietal bone at some depth from the surface. a, a, lamellae, showing reticular fibres; 6, b, darker part, where several lamellae are superposed; c, perforating fibres. Apertures through which perforating fibres had passed, are seen especially in the lower part, a, a, of the figure. (Allen Thomson.) 64 THE CONNECTIVE TISSUES. [CH. V. transformation into the bony femur of the adult (ossification in cartilage}. Ossification in Membrane.-The membrane, afterwards forming the periosteum, from which such a bone as the parietal is developed, consists of two layers-an external fibrous, and an internal cellular or osteo-genetic. The external layer is made up of ordinary fibrous tissue. The internal layer consists of a network of fine fibrils with a large number of nucleated cells (osteoblasts), some of which are oval, others drawn out into long branched processes : it is more richly supplied with capillaries than the outer layer. It is this portion •of the periosteum which is immediately concerned in the forma- tion of bone. In such a bone as the parietal, ossification is preceded by an increase in the vascularity of this membrane, and then spicules, starting from a centre of ossification near the centre of the future bone, shoot out in all directions towards the periphery. These primary bone spiculee consist of fibres which are termed osteo- genetic fibres; they are composed of a soft transparent substance called osteogen, around and between which calcareous granules are deposited. The fibres in their precalcified state are likened to bundles of white fibrous tissue, to which they are similar in chemical composition, but from which they differ in being stiffer and less wavy. The deposited granules after a time become so numerous as to imprison the fibres, and bony spiculfe result. By the junction of the osteo-genetic fibres and their resulting bony spicuke a meshwork of bone is formed. The osteo-genetic fibres, which become indistinct as calcification proceeds, persist in the lamellee of adult bone as the intercrossing fibres of Sharpey. The osteoblasts, being in part retained within the bony layers thus produced, form bone-corpuscles. On the bony trabeculae first formed, layers of osteoblastic cells from the osteo-genetic layer of the periosteum repeat the process just described ; and as this occurs in several thicknesses, and also at the edges of the spicules previously formed, the bone increases, both in thickness length and breadth. The process is not completed by the time the child is born, hence the fontanelles or still soft places on the heads of infants. Fig. 87 represents a small piece of the growing edge of a parietal bone. The bulk of the primitive spongy bone is in time converted into compact bony tissue, with Haversian systems. Those portions in the interior not converted into bone become filled with the red marrow of the cancellous tissue. Ossification in Cartilage.-Under this heading, taking the ch. v.] DEVELOPMENT OF BONE. 65 femur or any other long bone as an example, we have to consider the process by which the solid cartilaginous rod which represents the bone in the foetus is converted into the hollow cylinder of compact bone with expanded ends formed of cancellous tissue of which the adult bone is made up. We must bear in mind the fact that this foetal cartilaginous femur is many times smaller than even the medullary cavity of the shaft of the mature bone, and, therefore, that not a trace of the original cartilage can be present Fig'. 87.-Part of the growing edge of the developing parietal bone of a fcetal eat. sp, bony spicules with some of the osteoblasts imbedded in them, producing the lacunee; of, osteogenic fibres prolonging the spicules with osteoblasts (osi) between them and applied to them. (G. Lawrence.) in the femur of the adult. Its purpose is indeed purely tem- porary ; and, after its calcification, it is gradually and entirely absorbed. The cartilaginous rod which forms the precursor of a foetal long bone is sheathed in a membrane termed the perichondrium, which so far resembles the periosteum described above, as to consist of two layers, in the deeper one of which spheroidal and branched cells predominate and blood-vessels abound, while the outer layer consists mainly of fibres. Between the cartilaginous prefigurement of which the foetal 66 THE CONNECTIVE TISSUES. [ch. v. long bone consists and the adnlt bone there are several intermediate stages. The process may, how- ever, be most conveniently described as occurring in three principal stages. The, first stage consists of two sets of changes, one in the cartilage, the other under the perichondrium. These take place side by side. In the cartilage the cells in the middle * be- come enlarged and separated from one another. The car- tilage-cells on each side get arranged in rows in the direction of the extremities of the cartilaginous rod. If at this stage one cuts the little embryonic bone with a knife, the knife en- counters resistance, and there is a sensation of grittiness. This is due to the fact that calcareous particles are deposited in the matrix ; and in conse- quence of this the matrix stains differently with his- tological reagents than the unaltered matrix. Simul- taneously with this, the periosteal tissue is forming layer after layer of true bone; this is formed exactly in the same way as in Fig. 88.-Section of two foetal phalanges ; the car- tilage-cells in the centre of B are enlarged and separated from one another by calcified matrix. im, layer of bone deposited under the perios- teum. o, layer of osteoblasts by which this layer was formed. The rows of cartilage-cells are seen on each side of the centre of calcifica- tion. In A, the terminal phalanx, the changes begin at the top. * This is the case in nearly all the long bones, but in the terminal phalanges the change occurs first, not in the middle but at their distal extremities. CH. V.] DEVELOPMENT OF BONE. 67 such a bone as the parietal; by the agency of the osteoblasts, osteogenic fibres, and then spicules of bone, are formed by deposit of calcareous matter. As the layers are formed, some of the osteoblasts get walled in between the layers, and become bone cells. In the later part of this stage the calcareous deposit between the cartilage-cells cuts them off from nutrition, and they in con- Tig. 89.-Ossification in cartilage showing stage of irruption. The shrunken cartilage-cells are seen in the primary areolte. At ir an irruption of the subperiosteal tissue has penetrated, the subperiosteal bony crust. sequence waste, leaving spaces that are called the primary areolce. The calcareous deposit creeps up between the rows of cartilage- cells, enclosing them in calcified boxes containing one, two, or more cells each. The wasting of the cells leads here also to the formation of primary areolse. We may roughly compare the two sets of cells engaged in the process to two races of settlers in a new country. The cartilage- cells constitute one race, and so successfully build for themselves ■calcareous homes as to be completely boxed up ; so they waste 68 THE CONNECTIVE TISSUES. [ch. v. and disappear, leaving only the walls of their home enclosing the spaces called primary areolse. The osteoblasts, the other race of cells under the perichondrium, are forming layers of true bone in that situation. Some, it is true, get walled in in the process, and become bone-corpuscles, but the system of intercommunicating lacunae and canaliculi maintains their nutrition. These twTo races are working side by side, and at first do not interfere with each other. But soon comes a declaration of war, and we enter upon the second stage of ossification, which is very appropriately called the stage of irruption (fig. 89). Breaches occur in the bony wall which the osteoblasts have built like a girdle round the calcifying cartilage, and through these the peri- chondrial tissue pours an invading army into the calcified cartilage. This consists of osteoblasts, the bone formers; osteoclasts, or the bone destroyers; the latter are large cells, similar to the myelo- plaxes found in marrow (fig. 81). There are also a few fibres, and a store of nutrient supply in the shape of blood-vessels. Having got inside, the osteoclasts set to work to demolish the homes of the cartilage-cells, the primary areolse, and thus large spaces are formed, which are called the secondary areolae, or the medullary sgjaces. On the ruins of the calcified cartilage the osteoblasts proceed to deposit true bone in layers, just as they were wont to do in their own country, under the periosteum. The third stage of ossification is a repetition of these two stages towards the extremities of the cartilage. The cartilage-cells get flattened and arranged in rows ; calcareous deposit occurs around these, and primary areolae result; then follows the advance of the subperiosteal tissue, the demolition of the primary areolae, the formation of secondary areolae, and the deposit of true bone. At the same time, layer upon layer is still being deposited beneath the periosteum, and these, from being at first a mere girdle round the waist of the bone, now extend towards its extremities. The next figure is a magnified view of the line of advance. The bone which is first formed is less regularly lamellar than that of the adult. The lamellae are not deposited till after birth, and their formation is preceded by a considerable amount of absorption. To carry our simile further, the osteoblasts are not satisfied with the rough constructions that they were first able to- make, but having exterminated the cartilage, they destroy (again through the agency of the regiment of giant osteoclasts) their first work, and build regular lamellte, leaving lacunse for the accommodation of those who desire to retire from active warfare. About this time, too, the marrow cavity is formed by the absorption of the bony tissue that originally occupied the centre ch. v.] DEVELOPMENT OF BONE. 69 of the shaft. Here the osteoclasts have again to do the work, and, with this final act of destruction, all remains of any calcified cartilage of the foetal bone entirely disappear. The formation of a so-called cartilage bone is thus, after all, a formation of bone by subperiosteal tissue, just as it is in the so-called membrane bone. After a time the cartilage at the ends of the shaft begins to ossify independently, and the epiphyses are formed. They are not joined on to the shaft till late in life, so that growth of the bone can con- tinue till union takes place. Bone also grows in width by the deposition of layers under the perios- teum, like successive rings formed under the bark of a growing tree. This was shown long before the histological details which we have described were made out by Sharpey. Silver rings were placed by Duha- mel around the bones of young pigeons. When killed later, the rings were completely covered in by bone; and in the animals killed last, were even found in the central cavity. Another series of experi- ments with pigs were made by the celebrated John Hunter. The young animals were fed alternately on ordinary food and food dyed by the red pigment marrow. The new bony tissue acts like what dyers called a " mordant : " it fixes the dye, and the rings of bone deposited during the madder periods were distinctly red in colour. The importance of the periosteum in bone formation is now recognized by surgeons. When removing a piece of bone they are careful, if possible, to leave the periosteum behind : this leads to regeneration of the lost 'bone. If it is absolutely necessary to remove the periosteum, successful cases have occurred in which the living periosteum from an animal has effectively been transplanted. Fig. 90.-Longitudinal section of ossi- fying cartilage. Calcified trabeculse are seen extending between the columns of cartilage-cells, c, car- tilage-cells ; a, b, secondary areolae, x 140. (Sharpey.) 70 THE CONNECTIVE TISSUES. [CH. V. During the course of his life, man, in common with most other mammals, is provided with two sets of teeth; the first set, called the temporary or milk-teeth, makes its appearance in infancy, and is in the course of a few years shed and replaced by the second or set. The temporary oi' milk-teeth have only a very limited term of existence. They are ten in num- ber iu each jaw, namely, on either side from the middle line two mczsors, one canine, and two de- ciduous molars, and are replaced by ten perma- nent teeth. The num- ber of permanent teeth in each jaw is, however, increased to sixteen by the development of three molars on each side of the jaw, which are called the permanent or true molars. The following formula shows, at a glance, the comparative arrangement and number of the temporary and permanent teeth:- The Teeth. Fig. 91.-Normal well-formed jaws, from which the alveolar plate has been in great part removed, so as to expose the developing permanent teeth in their crypts in the jaws. (Tomes.) Temporary Teeth. Middle Line of Jaw. MOLARS. CANINE. INCISORS. 2 12 1 INCISORS. CANINE. MOLARS. 2 I 2=10 2 12 2 I 2=10 Permanent Teeth. BICUSPIDS TRUE OR PRE- CANINE. INCISORS. MOEARS. M0LARS> 3 2 I 2 BICUSPIDS INCISORS. CANINE. OR PRE- 1RUE MOLARS. M0LARS- 2 12 3 3212 2 12 3 Middle Line of Jaw. ch. v.] THE TEETH. 71 From this formula it will be seen that the two bicuspid or pre- molar teeth in the adult are the successors of the two deciduous molars in the child. They differ from them, however, in some respects, the temporary molars having a stronger likeness to the 2)ermanent than to their immediate descendants the so-called bicuspids, besides occupying more space in the jaws. The temporary incisors and canines differ from their successors but little except in their smaller size and the abrupt manner in which their enamel terminates at the necks of the teeth, forming a ridge or thick edge. Their colour is more of a bluish white than of a yellowish shade. The following tables show the average times of eruption of the Temporary and Permanent teeth. In both cases the eruption of any given tooth of the lower precedes, as a rule, that of the corresponding tooth of the upper jaw. Temporary or Milk Teeth. The figures indicate in months the age at which each tooth appears. INCISORS. DECIDUOUS FIRST MOLARS. CANINES. DECIDUOUS SECOND MOLARS. 6 12 18 24 Permanent Teeth. FIRST MOLARS. INCISORS. BICUSPIDS OR PRE- MOLARS. CANINES. SECOND MOLARS. THIRD MOLARS OR WISDOMS. CENTRALS. LATERALS. FIRST. SECOND. 6 7 8 9 IO 11 12 17 to 25 The age at which each tooth is cut is indicated in this table in years. The times of eruption given in the above tables are only approximate : the limits of normal variation being tolerably wide. Certain diseases affecting the bony skeleton, e.y., Rickets, retard the eruptive period considerably. It is important to notice that it is a molar which is the first tooth to be cut in the permanent dentition, not an incisor as in the case of the temporary set, and also that it appears behind the last deciduous molar on each side. The third molars, often called Wisdoms, are sometimes unerupted through life from want of sufficient jaw space and the presence of 72 THE CONNECTIVE TISSUES. [ch. v. the other teeth ; cases of whole families in which their absence is a characteristic feature are occasionally met with. When the teeth are fully erupted it will be observed that the upper incisors and canines project obliquely over the lower front teeth and the external cusps of the upper bicuspids and molars lie outside those of the corresponding teeth in the lower jaw. This arrangement allows to some extent of a scissor-like action in dividing and biting food in the case of incisors ; and a grinding motion in that of the bicuspids and molars when the side to side movements of the lower jaw bring the external cusps of the lower teeth into direct articulation with those of the upper, and then cause them to glide down the inclined surfaces of the external and up the internal cusps of these same upper teeth during the act of mastication. The work of the canine teeth in man is similar to that of his incisors. Besides being a firmly implanted tooth and one of stronger substance than the others, the canine tooth is important in preserving the shape of the angle of the mouth, and by its shape, whether pointed or blunt, long or short, it becomes a character tooth of the dentition as a whole in both males and females. Another feature in the fully developed and properly articulated set of teeth is that no two teeth oppose each other only, but that each tooth antagonises with two, except the upper Wisdom, usually a small tooth. This is the result of the greater width of the upper incisors, which so arranges the " bite " of the other teeth that the lower canine closes in front of the upper one. Should a tooth be lost, therefore, it does not follow that its former opponent remaining in the mouth is rendered useless and thereby liable to be removed from the jaw by a gradual process of extrusion commonly seen in teeth that have no work to perform by reason of absence of antagonists. Structure of a Tooth. A tooth is generally described as possessing a crown, neck, and root or roots. The crown is the portion which projects beyond the level of the gum. The neck is that constricted portion just below the crown which is embraced by the free edges of the gum, and the root includes all below this. On making longitudinal and transverse sections through its centre (figs. 92, 93), a tooth is found to be composed of a hard ch. v.] STRUCTURE OF TEETH. 73 material, dentine or ivory, which is moulded around a central cavity which resembles in general shape the outline of the tooth; the Fig. 92.-a. Longitudinal section of a human molar tooth ; c, cement; d, dentine ; e, enamel; v. pulp-cavity (Owen). b. Transverse section. The letters indicate the same as in a. Enamel.- Dentine. Periosteum of alveolus. Cement. Cement." Lower jaw- bone. Fig. 93.-Premolar tooth of cat in situ. 74 THE CONNECTIVE TISSUES. [ch. v. cavity is called the pulp cavity from its containing the very vascular and sensitive pulp. The tooth-pulp is composed of loose connective tissue, blood- vessels, nerves, and large numbers of cells of varying shapes, e.g., fusiform, stellate, and on the surface in close connection with the dentine a specialised layer of cells called odontoblasts, which are elongated columnar-looking cells with a large nucleus at the tapering ends farthest from the dentine. The blood-vessels and nerves enter the pulp through a small opening at the apical extremity of each root. The exact termina- tions of the nerves are not definitely known. They have never Fig. 94.-Section of a portion of the dentine and cement from the middle of the root of an incisor tooth, a, dental tubules ramifying and terminating, some of them in the inter- globular spaces b and c; d, inner layer of the cement with numerous closely set canaliculi. e, outer layer of cement; /, lacunee ; g, canaliculi. x 350. (Kulliker.) been observed to enter the dentinal tubes. No lymphatics have been seen in the pulp. A layer of very hard calcareous matter, the enamel, caps that part of the dentine which projects beyond the level of the gum ; while sheathing the portion of dentine which is. beneath the level of the gum, is a layer of true bone, called the cement or crusta petrosa. At the neck of the tooth, where the enamel and cement come into contact, each is reduced to an exceedingly thin layer; the cement overlapping the enamel and being prolonged over it. On the surface of the crown of the tooth, when it first comes through the jawr, is a thin membrane called Nasmyth's membrane, or the cuticle of the tooth. The covering of enamel becomes thicker towards the crown, and the cement towards the lower end or apex of the root. Chemical composition.-Dentine closely resembles bone in chemical composition. It contains, however, rather less animal Dentine or Ivory. ch. v.] STRUCTURE OF TEETH. 75 matter; the proportion in a hundred parts being about twenty- eight animal to seventy-two of earthy. The former, like the animal matter of bone, may be converted into gelatin by boiling. It also contains a trace of fat. The earthy matter is made up chiefly of calcium phosphate, with a small portion of the carbonate, and traces of calcium fluoride and magnesium phosphate. Structure.-Under the microscope dentine is seen to be finely channelled by a multitude of delicate tubes, which, by their inner ends communicate with the pulp-cavity, and by their outer ex- tremities come into contact with the under part of the enamel and cement, and sometimes even penetrate them for a greater or less distance (figs. 95, 96). The matrix in which these tubes lie is composed of " a reticulum of fine fibres of connective tissue modi- Fig. 95.-Enamel prisms. A, fragments and single prisms of the transversely-striated enamel, isolated by the action of hydrochloric acid. B, surface of a small fragment of enamel, showing the hexagonal ends of the fibres with darker centres, x 350. (Kolliker.) fled by calcification, and where that process is complete, entirely hidden by the densely deposited lime salts " (Mummery). In their course from the pulp-cavity to the surface, the minute tubes form gentle and nearly parallel curves and divide and sub- divide dichotomously, but without much lessening of their calibre until they are approaching the peripheral termination. From their sides proceed other exceedingly minute secondary canals, which extend into the dentine between the tubules and anastomose with each other. The tubules of the dentine, the average diameter of which at their inner and larger extremity is /q o of an inch, contain fine prolongations from the tooth-pulp, 76 THE CONNECTIVE TISSUES. [ch. v. which give the dentine a certain faint sensitiveness under ordi- nary circumstances and, without doubt, have to do also with its nutrition. These prolongations from the tooth-pulp are processes of the dentine-cells or odontoblasts which are columnar cells lining the pulp-cavity; the relation of these processes to the tubules in which they lie being precisely similar to that of the processes of the bone- corpuscles to the canaliculi of bone. The outer portion of the dentine, underlying the cement, and the enamel to a much lesser degree, forms a more or less distinct layer termed the granular or interglobular layer. It is characterised by the presence of a number of irregular minute cell-like cavities. The explanation of these will be seen when we study the development of a tooth. Enamel. Chemical composition. - The enamel, which is by far the hardest tissue in the body, is composed of the same inorganic compounds that enter into the composition of dentine and bone. Its animal matter, however, amounts only to about 2 or 3 per cent., and does not yield gelatin on boiling. According to Tomes it contains no animal matter at all. Gelatin is a characteristic product of connective tissue, and enamel is not a connective tissue, but is epithelial in origin. Examined under the microscope, enamel is found composed of six-sided prisms (figs. 95, 96) yoVo of an inch in diameter, which are set on end on the surface of the dentine, and fit into corresponding depres- sions in the same. They radiate in such a manner from the dentine that at the top of the tooth they are more or less vertical, while towards the sides they tend to the horizontal direction. Like the dentine tubules, they are not straight, but disposed in wavy and parallel curves. The prisms are marked by transverse lines, and are solid. The enamel-prisms are connected together by a very minute Fig. 96.-Thin section of the enamel, and a part of the dentine. a, cuticular pellicle of the enamel (Nasmyth's membrane); b, enamel columns with fissures between them and cross striae; c, larger cavities in the enamel, communicating with the extremities of some of the dentinal tubules (d). X 350. (Kolliker.) ch. v.] DEVELOPMENT OF TEETH. 77 quantity of hyaline cement substance. In the deeper part of the enamel, between the prisms, are often small lacunae, which have the processes or fibrils of the dentinal tubes in con- nection with them (fig. 96, c). Crusta Petrosa. The crusta petrosa, or cement (fig. 94, e, d), is composed of true bone, and in it are lacunse (/) and canaliculi (y), which sometimes communicate with the outer finely branched ends of the dentinal tubules, and generally with the inter- globular spaces. Its laminse are bolted to- gether by perforating fibres like those of ordinary bone (Shar- pey's fibres). Cement differs from ordinary bone in possessing no Haversian canals, or, if at all, only in the thickest part. Such canals are more often met with in teeth with the cement hypertro- phied than in the normal tooth. Kg. 97.-Section of the upper jaw of a foetal sheep. A.-1, common enamel germ dipping down into the mucous membrane; 2, palatine process of jaw ; 3, Rete Malpighi. B.-Section similar to A, but passing through one of the special enamel germs here becoming flask-shaped; c, c'. epithe- lium of mouth; /, neck; body of special enamel germ. C.-A later stage; c, outline of epithelium of gum; /, neck of enamel germ; enamel organ ; p, papilla; s, dental sac form- ing ; f p, the enamel germ of permanent tooth; m, bone of jaw ; v, vessels cut across. (Waldeyer and Kblliker.) Development of the Teeth. The first step in the development of the teeth consists in a downward growth (fig. 97, a, i) from the deeper layer of stratified epithelium of the mucous membrane of the mouth, which first 78 THE CONNECTIVE TISSUES. [ch. v. "becomes thickened in the neighbourhood of the maxillae or jaws now in the course of formation. This process passes downward into a recess of the imperfectly developed tissue of the embryonic jaw. The downward epithelial growth forms the common enamel or dental germ, and its position is indicated by a slight groove in the mucous membrane of the jaw. The next step in the process consists in the elongation downward of the enamel groove and of the enamel germ and the inclination outward of the deeper part (fig. 97, B, f'\ which is now inclined at an angle with the upper portion or neck (/), and has become bulbous. After this there is an increased development at certain points corresponding to the situations of the future milk-teeth. The common enamel germ becomes extended by further growth into a number of special enamel germs corre- sponding to each of the above-mentioned milk teeth, and connected to the com- mon germ by a narrow neck. Each tooth is thus placed in its own special recess in the embryonic jaw (fig- 97? As these changes pro- ceed, there grows up from the underlying connective tissue into each enamel germ (fig. 97, c, p), a dis- tinct vascular papilla (den- tal paprilld), and upon it the enamel germ becomes moulded, and presents the appearance of a cap of two layers of epithelium separated by an interval (fig. 97, c, /'). Whilst part of the sub-epithelial tissue is elevated to form the dental papilla, the part which bounds the embryonic teeth forms the dental sac (fig. 97, c, s); and the rudiment of the jaw sends up processes forming partitions between the teeth. In this way small chambers are produced in which the dental sacs are contained, and thus the sockets of the teeth are formed. The papilla is composed of nucleated cells arranged in a mesh- work, the outer or peripheral part being covered with a layer of columnar nucleated cells called odontoblasts. The odontoblasts form the dentine, while the remainder of the papilla forms the tooth-pulp. Fig. 98.-Part of section of developing tooth of a young rat, showing the mode of deposition of the dentine. M ighly magnified. a, outer layer of fully formed dentine; &, uncalcified matrix with one or two nodules of calcareous matter near the calcified parts ; c, odontoblasts send- ing processes into the dentine ; d, pulp ; e, fusiform orwedge-shape cells found between odontoblasts; /, stellate cells of pulp in fibrous connective tissue. The section is stained with carmine, which colours the uncalcified matrix but not the calcified part. (E. A. Schiifer.) ch. v.] DEVELOPMENT OF TEETH. 79 The method of the formation of the dentine by the odontoblasts is given in Quain's Anatomy as follows :- These cells, either by secretion, or as some think by direct transformation of the onter part of each, form a layer of dentinal matrix on the apex of the papilla, or if the tooth has more than one cusp then at the apex of each cusp. This layer is first uncalcified (odontogen), but globules of calcareous matter soon appear' in it. These, becoming more numerous, blend into the first cap of dentine. In the mean- while the odontoblasts have formed a second layer of odontogen within this (fig. 98), and this in turn be- comes calcified ; thus layer after layer is formed, each extending laterally further than its predecessor; the layers blend except in some places; here portions of odontogen remain, which in a tooth macerated for histological purposes get destroyed, and appear as the interglolndar spaces (fig. 94), so called because bounded by the deposit of calcareous salts, which occurs, as we have already seen, in the form of globules. As the odontoblasts re- tire towards the centre, depositing layer after layer of dentine, they leave behind them long filaments of their protoplasm around which the calcareous deposit is moulded ; thus the dentinal tubules occupied by the processes of the odontoblasts are formed. The other cells of the dental papilla form the cells of the pulp. Formation of the enamel.-The portion of the enamel or dental germ that covers the dental papilla is at this stage called the enamel organ. This consists of four parts (see figs. 99 and 100) :- Fig. 99.-Vertical transverse section of the dental sac, pulp, &c. of a kitten, a, dental papilla or pulp; &, the cap of dentine formed upon the summit; c, its covering of enamel; d, inner layer of epithelium of the enamel organ; gelatinous tissue; /, outer epithelial layer of the enamel organ; g, inner layer, and A, outer layer of dental sac. x 14. (Thiersch.) 80 THE CONNECTIVE TISSUES. [ch. v. 1. A layer of columnar epithelium cells in contact with the dentine. These are called the enamel cells, or adamanto- blasts. 2. Two or three layers of smaller polyhedral nucleated cells, the stratum intermedium of Hannover. Fig. 100.-Highly magnified view of a piece of the enamel organ in a kitten's canine. d, superficial layer of dentine, e, newly formed enamel stained black by osmic acid. T, Tomes' processes from the adamantoblasts, ad.; str. int., stratum intermedium of the enamel organ, p, branched cells of the enamel pulp. (Rose.) 3. A matrix of non-vascular jelly-like tissue containing stellate cells. 4. An outer membrane of several layers of flattened epithelium cells. The first three layers on an enlarged scale are seen in Fig. TOO. The enamel prisms are formed by the agency of the ends of the adamantoblasts which abut on the dental papilla. Each forms a fine deposit of globules staining with osmic acid and resembling keratin in its resistance to mineral acid. At one time it was believed that each adamantoblast was itself calcified and converted into an enamel prism, but this view has been disproved by recent research. The layer of keratin-like material is outside the bodies of the cells, although a process of each adamantoblast extends into it as a tapering fibre (process of Tomes), and it is usually produced simultaneously with the first layer of uncalcified dentine ; when it undergoes calcification, the first layer of enamel is complete. The adamantoblasts then ch. v.] DEVELOPMENT OF TEETH. 81 repeat the process, first causing a deposition of keratin-like materia], and this in turn is calcified, and so on. During the formation of layer after layer of enamel, the adamantoblasts retire. By the time the enamel is approaching completion the other layers of the enamel organ have almost disappeared, and they entirely disappear when the tooth emerges through the gum. But for some little time there is a somewhat more persistent mem- brane covering the crown ; this is Nasmyth's membrane, or the enamel cuticle ; this is the last formed keratinous layer of enamel which has remained uncalcified. As with the dentine, the formation of enamel appears first on the apex of each cusp. The cement or crust a petrosa is formed from the internal tissue of the tooth sac, the structure and function of which are identical with those of the osteogenetic layer of the periosteum ; or, in other words, ossification in membrane occurs in it. The outer layer or portion of the membrane of the tooth sac forms the dental periosteum. This periosteum, when the tooth is fully formed, is not only a means of attachment of the tooth to its socket, but also in con- junction with the p>ulp a source of nourishment to it. Additional laminae of cement are added to the root from time to time during the life of the tooth (as is especially well seen in the abnormal con- dition called an exostosfs), by the process of ossification taking place in the periosteum. On the other hand, absorption of the root (such as occurs when the milk-teeth are shed) is due to the action of the osteoclasts of the same membrane. In this manner the first set of teeth, or the milk-teeth, are formed; and each tooth, as it grows, presses at length on the wall of the sac enclosing it, and, causing its absorption, is cut, to use a familiar phrase. The temporary or milk-teeth are later replaced by the growth of the permanent teeth, which push their way up from beneath them. Each temporary tooth is replaced by a tooth of the permanent set which is developed from a small sac which was originally an offshoot from the sac of the temporary tooth which precedes it, and called the cavity of reserve (fig. 98, c, /jo). Thus the tem- porary incisors and canines are succeeded by the corresponding permanent ones, the temporary first molar by the first bicuspid ; the temporary second molar developes two offshoots, one for the second bicuspid, the other for the permanent first molar. The permanent second molar is budded off from the first permanent molar, and the wisdom from the permanent second molar. 82 THE CONNECTIVE TISSUES. [ch. v. The development of the temporary teeth commences about the sixth week of intra-uterine life, after the laying down of the bony structure of the jaws. Their permanent successors begin to form about the sixteenth week of intra-uterine life. The second permanent molars originate about the third month after birth, and the wisdom teeth about the third year. The Blood. A fidl consideration of the blood will come later, when we know more about the chemical aspects of physiology, but it will be impossible to discuss all the other phenomena we shall have to study in the meanwhile without some elementary knowledge of the principal properties of this fluid. For that reason, and also to complete our list of the connective tissues, we may here rapidly and briefly enumerate its principal characters. The blood is a fluid which holds in suspension large numbers of solid particles which are called the corpuscles. The fluid itself is called the plasma or liquor sanguinis. It is a richly albuminous fluid ; and one of the proteids in it is called fibrinogen. After blood is shed it rapidly becomes viscous, and then sets into a jelly. The jelly contracts and squeezes out of the clot a straw-coloured fluid called serum, in which the shrunken clot then floats. The formation of threads of a solid proteid called fibrin from the soluble proteid we have called fibrinogen is the essential act of coagulation; this, with the corpuscles it entangles, is called the clot. Serum is plasma minus fibrin. The following scheme shows the relationships of the constituents of the blood at a glance :- Plasma Corpuscles Scrum Fibrin ... . Blood Clot. The corpuscles are of two chief kinds, the red and the white. The white corpuscles are typical animal cells, and we have already made their acquaintance when speaking about amoeboid movements. The red corpuscles are much more numerous than the white, averaging in man 5,000,000 per cubic millimetre, or 400 to 500 red to each white corpuscle. It is these red corpuscles that give the red colour to the blood. They vary in size and structure in CH. v.] THE BLOOD. 83 different groups of the vertebrates. In mammals they are bi- concave (except in the camel tribe, where they are biconvex) non-nucleated discs, in man hich diameter ; during foetal life nucleated red corpuscles are, however, found. In birds, reptiles, amphibians and fishes they are biconvex oval discs with a nucleus : they are largest in the amphibia. The most important and abundant of the constituents of the red corpuscles is the pigment which is called hemoglobin. This is a proteid-like substance, but is remarkable as containing a small amount of iron (less than o'5 per cent.). The blood during life is in constant movement. It leaves the heart by the vessels called arteries, and returns to the heart by the vessels called veins ; the terminations of the arteries and the commencements of the veins are, in the tissues, connected by the thin-walled microscopic vessels called capillaries. In the capil- laries, leakage of the blood-plasma occurs; this exuded fluid carries nutriment from the blood to the tissue-elements, and removes from them the waste products of their activity. The lymph is collected by lymphatic vessels, which converge to the main lymphatic, called the thoracic duct. This opens into the large veins near to their entrance into the heart ; and thus the lymph is returned to the blood. But blood is also a carrier of oxygen, and it is the pigment haemoglobin which is the oxygen carrier; in the lungs the haemoglobin combines with the oxygen of the air, and forms a loose compound of a bright scarlet colour called oxyhemoglobin. This arterial or oxygenated blood is taken to the heart'and thence propelled by the arteries all over the body, where the tissues take the respiratory oxygen from the haemoglobin, and this removal of oxygen changes the colour of blood to the bluish-red tint it has in the veins. The veins take the blood minus a large quantity of oxygen and plus a large quantity of carbonic acid received in exchange from the tissues to the heart, which sends it to the lungs to get rid of its surplus carbonic acid, and replenish its store of oxygen; then the same round begins over again. 84 MUSCULAR TISSUE. [CH. VI. CHAPTER VI. MUSCULAR TISSUE. Muscle is popularly known as flesh. It possesses the power of contraction, and is, in the higher animals, the tissue by which their movements are executed. The muscles may be divided from a physiological standpoint into two great classes, the volun- tary muscles, those which are under the control of the will, and the involuntary muscles, those which are not. The contraction of the involuntary muscles is, however, controlled by the nervous system, only by a different part of the nervous system from that which controls the activity of the voluntary muscles. When muscular tissue is examined with the microscope, it is seen to be made up of small elongated thread-like structures, which are called muscular fibres ; these are bound into bundles by connective tissue, and in the involuntary muscles there is in addition a certain amount of cement substance, stainable by nitrate of silver, between the fibres. The muscular fibres are not all alike; those of the voluntary muscles are seen by the microscope to be marked by alternate dark and light stripings or striations; these are called trans- versely striated muscular fibres. The involuntary fibres have not got these markings as a rule. There is one important exception to this rule, namely, in the case of the heart, the muscular fibres of which are involuntary, but transversely seriated. There are, however, histological differences between cardiac muscle and the ordinary voluntary striated muscles. The unstriated involuntary muscular fibres found in the walls of the stomach, intestine, bladder, blood-vessels, uterus, and other contractile organs are generally spoken of as plain muscular fibres. From the histological standpoint there are, therefore, three varieties of muscular fibres found in the body of the higher animals : transversely striated, cardiac, and plain muscular fibres. The relationship of this histological classification to the physio- logical classification into voluntary and involuntary is shown in the following table i. Transversely striated muscular fibres : a. In skeletal muscle .... Voluntary. b. In cardiac muscle .... 2. Plain muscular fibres : In blood-vessels, intestine, uterus, bladder, etc Involuntary. CH. VI.] VOLUNTARY MUSCLE. 85 All kinds of muscular tissue are therefore composed of fibres, but the fibres are essentially different from those we have hitherto studied in the connective tissues. There, it will be remembered, the fibres are developed between the cells; here, in muscle, the fibres are developed from the cells; that is, the cells themselves become elongated to form the muscular fibres. Voluntary Muscle. The voluntary muscles are those which are sometimes called skeletal, constituting the whole of the muscular apparatus attached to the bones.* Each muscle is enclosed in a sheath of areolar tissue, called the Epimysium ; this sends in partitions, or septa, dividing off the fibres into fasciculi, or bundles; the sheath of each bundle may be called the Perimysium. Between the individual fibres is a small amount of loose areolar tissue, called the Endomysium. The blood-vessels and nerves for the muscle are distributed in this areolar tissue. The fibres vary in thickness and length a good deal, but they Fig', ioi.-A. branched muscular fibre from the frog's tongue. (Kulliker.) average inch in diameter, and about x inch in length. Each fibre is 'cylindrical in shape, with rounded ends; many become pro- longed into tendon bundles (fig. no), by which the muscle is at- tached to bone. As a ride they are unbranched, but the muscles of the face and tongue divide into numerous branches before being inserted to the under surface of the skin, or mucous membrane. The fibres in these situations are also finer than in the majority of the voluntary muscles. Each fibre consists of a sheath, called * The muscular fibres of the pharynx, part of the oesophagus, and of the muscles of the internal ear, though not under the control of the will, have the same structure as the voluntary muscular fibres. 86 MUSCULAR TISSUE. [ch. vi. the sarcolemma, enclosing a soft material called the contractile substance. The sarcolemma is homogeneous, elastic in nature, and especially tough in fish and amphibia. It may readily be demonstrated in a microscopic preparation of fresh muscular fibres by aPPtying gentle pressure to the cover slip ; the contractile sub- stance is thereby ruptured, leaving the sarcolemma bridging the space (fig. 102). To the sarcolemma are seen adhering some nuclei. The contractile substance within the sheath is made up of alternate discs of dark and light substance. Muscular fibres also contain oval nuclei. In mammalian muscle these are situated just beneath the sarcolemma ; but in frog's muscle they occur also in the thickness of the muscular fibre. The chromoplasm of the nucleus has generally a spiral Fig. 102.-Muscular fibre torn across, the sarcolemma still connecting the two parts of the fibre. (Todd and Bow- man.) Fig. 103.-Muscular fibre of a mammal highly mag- nified. The surface of the fibre is accurately focussed. (Schafer.) arrangement, and often there is a little granular protoplasm (well seen in the muscular fibres of the diaphragm) around each pole of the nucleus. The foregoing facts can be made out with a low power of the microscope ; on examining muscular fibres with a high power other details can be seen. Treatment with different reagents brings out still further points of structure. These are differently described and differently interpreted by different histologists; and perhaps no subject in the whole of microscopic anatomy has been more keenly debated than the structure of a muscular fibre, and the meaning of the changes that occur when it contracts. A good deal of the difficulty has doubtless arisen from the fact that a muscular fibre is cylindrical, and if one focusses the surface one gets different optical effects from those obtained by focussing deep in the substance of the fibre. I shall, in the following account of the intimate structure of striated muscle, adhere very closely to the writings of Professor Schafer. ch. vi.] VOLUNTARY MUSCLE. 87 If the surface is carefully focussed rows of apparent granules are seen lying at the boundaries of the light streaks, and fine longitudinal lines passing through the dark streaks may be detected uniting the apparent granules (fig. 103). In specimens treated with dilute acids or gold chloride, the granules are seen to be connected side by side, or transversely also. This reticulum, with its longitudinal and transverse meshes, was at one time considered to be the essential contractile portion of the muscular fibre; it was thought that on contraction the transverse networks, with their enlargements, the granules, became increased by the longitudinal strands diminishing in Fig. 105.-Transverse section through muscular fibres of human tongue. The nuclei are deeply stained, situated at the inside of the sar- colemma. Each muscle fibre shows " Cohnheim's areas." X 450. (Klein and Noble Smith.) Fig. X04.-Portion of muscle-fibre of water-beetle, showing network very plainly. One of the transverse networks is split off, and some of the longitudinal bars are shown broken off. (After Melland.) length and running into them. Most histologists have rejected this idea, and regard the network as mere interstitial substance lying between the essentially contractile portions of the muscle. A muscular fibre is thus made up of what are variously called fibrils, muscle-columns or sarcostyles; and the longitudinal inter- stitial substance with cross networks comprising the reticulum just referred to is called sarcoplasm. By the use of certain reagents, such as osmic acid or alcohol, the muscle-columns or sarcostyles may be completely separated from one another. A transverse section of a muscular fibre (fig. 105) shows £he sections of these sarcostyles ; the interstitial sarcoplasm is repre- sented as white in the drawing. The angular fields separated by sarcoplasm may still be called by their old name, areas of Cohnheim. 88 MUSCULAR TISSUE. [ch. vi. If, instead of focussing the surface of a fibre, it is observed in its depth, a fine dotted line is seen bisecting each light stripe ; this has been variously termed D able's line, or Krause's membrane. At one time this was believed to be an actual membrane con- tinuous with the sarcolemma. It is probably very largely an optical effect, caused by light being transmitted between discs of different refrangibility. If cross membranes do exist they are not very resistant; this was well shown by an accidental observation first made by Kuhne, Fig. 106.-A. Portion of a medium-sized human muscular fibre. X 800. B. Separated bundles of fibrils equally magnified ; a, a, larger, and Z>, 6, smaller collections ; c, still smaller; d, d, the smallest which could be detached, possibly representing a single series of sareous elements. (Sharpey.) and subsequently seen by others. A minute thread-worm, called the Myorectes, was observed crawling up the interior of the con- tractile substance of a muscular fibre; it crawled without any opposition from membranes, and the track it left, closed up slowly behind it without interfering with the normal cross-striations of the contractile substance. This observation strikingly illustrates the fact that the contractile substance in a muscular fibre is fluid, but only semi-fluid, as the closing of its track occurs slowly as a hole always closes in a viscous material. Another appearance which is sometimes seen is a fine clear line CH. VI.] VOLUNTARY MUSCLE. 89 running across the fibre in the middle of each dark band. It is called Hensen's line. A muscular fibre may not only be broken up into fibrils or muscle-columns, but under the influence of some reagents like dilute hydrochloric acid, it can be broken up into discs, the cleavage occurring in the centre of each light stripe. Bowman, the earliest to study muscular fibres with profitable results, con- cluded that the subdivision of a fibre into fibrils was a phenomenon of the same kind as the cross cleavage into discs. He considered that both were artificially produced by a separation in one or the other direction of particles of the fibre he called " sarcous ele- ments." The cleavage into discs is however much rarer than the separation into fibrils ; indeed, indications of the fibrils are seen in perfectly fresh muscle before any reagent has been added, and this is markedly evident in the wing muscles of many insects. It is now believed that a muscular fibre is built up of contiguous fibrils or sarcostyles, while cleavage into discs is a purely artificial phenomenon. Haycraft, who has recently investigated the question of muscu- lar structure, has arrived at the conclusion that the cross striation is entirely due to optical phenomena. The sarcostyles are vari- cose, and where they are enlarged different refractive effects will be produced from those caused by the intermediate narrow por- tions. This view he has very ingeniously supported by taking negative casts of muscular fibres by pressing them on to the sur- face of collodion films. The collodion cast shows alternate dark and light bands like the muscular fibres. Schafer is unable to accept this view; he regards the substance of the sarcostyle in its dark stripes as being of different composi tion, and not merely of different diameter, from the sarcostyle in the region of the light stripes; it certainly stains very differently with many reagents, especially chloride of gold. His views regard- ing the intimate structure of a sarcostyle have been worked out chiefly in the wing muscles of insects, where the sarcostyles are separated by a considerable quantity of interstitial sarcoplasm, and a brief summary of his conclusions is as follows :- Each sarcostyle is subdivided in the middle of each light stripe by transverse lines (membranes of Krause) into successive por- tions, which may be termed sarcomeres. Each sarcomere is occu- pied by a portion of the dark stripe of the whole fibre; this portion of the dark stripe may be called a sarcous element* The * Notice that this expression has a different meaning from what it originally had when used by Bowman. 90 MUSCULAR TISSUE. [ch. vi. sarcous element is really double, and in the stretched fibre (fig. 10713) separates into two at the line of Hensen. At either end of the sarcous element is a clear interval separating it from Krause's membrane ; this clear interval is more evident in the extended sarcomere (fig. 10 713), but diminishes on contraction (fig. 107A). The cause of this is to be found in the structure of the sarcous element. It is pervaded with longitudinal canals or pores open towards Krause's membrane, but closed at Hensen's Fig. 107.-Sarcostyles from the wing-muscles of a wasp. a. a'. Sarcostyles showing degrees of con- traction. b. A sarcostyle extended with the sarcous elements separated into two parts. c. Sarcostyles moderately extended (semidia- grammatic). (E. A. Schafer.) Fig. 108.-Diagram of a sarcomere in a moderately extended condition, a, and in a con- tracted condition, b. k, k, Krause's membranes; n, plane of Hensen; s.e., poriferous sarcous ele- ment. (E. A. Schafer.) line. In the contracted muscle the clear part of the muscle sub- stance passes into these pores, disappears from view to a great extent, swells up the sarcous element, widens it and shortens the sarcomere. In the extended muscle, on the other hand, the clear substance passes out from the pores of the sarcous element, and lies between it and the membrane of Krause ; this lengthens and narrows the sarcomere. This is shown in the diagrams. It may be added that the sarcous element does hot lie free in the middle of the sarcomere, but is attached at the sides to a fine enclosing envelope, and at either end to Krause's membrane by fine lines running through the clear substance (fig. 108). This view is interesting, because it brings into harmony amoe- boid, ciliary, and muscular movement. In all three instances we have protoplasm composed of two materials, spongioplasm and hyaloplasm. In amoeboid movement the irregular arrangement of the spongioplasm allows the hyaloplasm to flow in and out of it in any direction. In ciliary movement the flow is limited by the CH. VI.] VOLUNTARY MUSCLE. 91 arrangement of the spongioplasm to one direction ; hence the limitation of the movement in one direction (see p. 33). In muscle, also, the definite arrangement of the spongioplasm (repre- sented by the sarcous element) in a longitudinal direction directs the movement of the hyaloplasm (represented by the clear sub- Fig'. 109.-Wave of contraction passing over a muscular fibre of water-beetle, r, r, por- tions of the fibre at rest; c, contracted part; 1, 1, intermediate condition. (Schafer.) stance of the light stripe), so that it must flow either in or out in that particular direction. The muscular fibre is made up of sarcostyles and the sarcostyle of sarcomeres. The contraction of the whole muscle is only the sum total of the contraction of all the constituent sarcomeres. In an ordinary muscular fibre it is stated that when it con- tracts, not only docs it become thicker and shorter, but the light 92 MUSCULAR TISSUE'. [ch. vi. stripes become dark and the dark stripes light. This again is only an optical illusion, and is produced by the alterations in the shape of the sarcostyles, affecting the sarcoplasm that lies between them. When the sarcous elements swell during contraction, the sarcoplasm accumulates Fig. in. - Three muscular fibres running longitudinally, and two bundles of fibres in transverse section, M. from the tongue. The capillaries, C, are injected, x 150. (Klein and Noble Smith.) opposite the membranes of Krause, and diminishes in amount opposite the sar- cous elements ; the accumulation of sar- coplasm in the previously light stripes makes them appear darker by contrast than the dark stripes proper. This is very well shown in fig. 109. There is no true reversal of the stripings in the sarcostyles themselves. Fig. no. - Termination of a muscular fibre in a tendon- bundle. m, sarcolemma ; s, the same passing over the endofbundle; p,extremity of muscular substance c, retracted from the end of sarcolemma tube; t, ten- don bundle fixed to sarco- lemma. (Ranvier.) That this is the case can be seen very well when a muscular fibre is examined with polarised light. A polarising microscope contains a Nicol's prism beneath the stage of the microscope which polarises the light passing through the object placed on the stage. The eye-piece contains another Nicol's prism, which detects this fact. If the two Nicols are parallel, the light passing through the first passes also through the second ; but if the second is at right angles to the first, the light cannot traverse it and the field appears dark. If an object on the microscope stage is doubly refracting it will appear bright in this dark field ; if it remains dark it is singly refract- ing. The sarcoplasm is singly refracting or isotropous ; it remains dark in the dark field of the polarising microscope. The muscle columns or sarco- styles are in great measure doubly refracting or anisotropous. and appear bright in the dark field of the polarising microscope. The sarcostyle, how- CH. VI.] RED AND CARDIAC MUSCLE. 93 ever, is not wholly doubly refracting ; the sarcous elements are doubly refracting, and the clear intervals are singly refracting. On contraction there is no reversal of these appearances, though of course the relative thickness of the singly refracting intervals varies inversely with that of the doubly refracting sarcous elements. Ending of Muscle in Tendon.-A tendon-bundle passes to each muscular fibre, and becomes firmly united to the sarcolemma. The areolar tissue between the tendon-bundles becomes also con- tinuous with that between the muscular fibres. Blood-vessels of Muscle.-The arteries break up into capillaries, which run longitudinally in the endomysium, transverse branches connecting them. No blood-vessels ever penetrate the sarco- lemma. The muscular fibres are thus, like other tissues, nourished by the exudation from the blood called lymph. The lymph is removed by lymphatic vessels found in the perimysium. The nerves of voluntary muscle pierce the sarcolemma, and ter- minate in expansions called end-plates, to be described later on. Red Muscles. Ill many animals, such as the rabbit, and some fishes, most of the muscles are pale, but some few (like the diaphragm, crureus, soleus, semi-membranosus in the rabbit) are red. These muscles contract more slowly than the pale muscles, and their red tint is due to haemoglobin contained within their contractile substance. In addition to these physiological distinctions, there are histo- logical differences between them and ordinary striped muscle. These histological differences are the following :- 1. Their muscular fibres are thinner. 2. They have more sarcoplasm. 3. Longitudinal striation is more distinct. 4. Transverse striation is more irregular than usual. 5. Their nuclei are situated not only under the sarcolemma, but also in the thickness of the fibre. 6. The transverse loops of the capillary network are dilated into little reservoirs, far beyond the size of ordinary capillaries. Cardiac Muscle. The muscular fibres of the heart, unlike those of most of the involuntary muscles, are striated ; but although, in this respect, they resemble the skeletal muscles, they have distinguishing characteristics of their own. The fibres which lie side by side are united at frequent intervals by short branches (fig. 112). The 94 MUSCULAR TISSUE. [ch. vi. fibres are smaller than those of the ordinary striated muscles, and their transverse striation is less marked. No sarcolemma can be discerned. One nucleus is situated in the middle of the substance of Fig. 112. -Muscular fibre cells from the heart. (E. A. Schafer.) each fibre. At the junctions of the fibres there is a certain amount of cementing material, stainable by silver nitrate. Plain Muscle. Plain muscle forms the proper muscular coats (i.) of the digestive canal from the middle of the oesophagus to the internal sphincter ani; (2.) of the ureters and urinary bladder; (3.) of the trachea and bronchi; (4.) of the ducts of glands; (5.) of the gall-bladder; (6.) of the vesicuhe seminales ; (7.) of the uterus and Fallopian tubes ; (8.) of blood-vessels and lymphatics ; (9.) of the iris, and ciliary muscle of the eye. This form of tissue also enters largely into the composition (10.) of the tunica dartos, the contraction of which is the principal cause of the wrinkling and contraction of the scrotum on exposure to cold. It occurs also in the skin generally, being found .both in the secreting part of the sweat glands and in small bundles attached to the hair follicles; it also occurs in the areola of the nipple. It is composed of long, fusi- form cells (fig. 113); which vary in length, but are not as a rule more than inch long. Each cell has an oval or rod-shaped nucleus. The cell substance is longitudinally but not transversely striated. Each cell or fibre, as it may also be termed, has a deli- cate sheath. The fibres are collected into fasciculi, and united CH. VI.] DEVELOPMENT OF MUSCLE. 95 by cementing material, which can be stained by silver nitrate. This intercellular substance is bridged across by fine filaments passing from cell to cell. The nerves in involuntary muscle (both cardiac and plain) do Fig. 113.-Muscular fibre-cells from the muscular coat of intestine-highly magnified. Note the longitudinal striation, and in the broken fibre the sheath is visible. not terminate in end-plates, but by plexuses or networks, which ramify between and around the muscular fibres. Development of Muscular Fibres. All muscular fibres (except those of the sweat glands which are epiblastic) originate from the mesoblast. The yj/om fibres are simply elongated cells in which the nucleus becomes rod- shaped. In cardiac muscle, the likeness to the original cells from which the fibres are formed is not altogether lost, and in certain situations (immediately beneath the lining membrane of the ventricles) there are found peculiar fibres called after their &\scoNerer Purkinje's fibres ; these are large clear quadrangular cells with granular protoplasm containing several nuclei in the centre, and striated at the margin. It appears that the differen- tiation of these cells is arrested at an early stage, though they continue to grow in size. Voluntary muscular fibres are developed from cells which be- 96 NERVE. [ch. vii. come elongated, and the nuclei of which multiply. In most striated muscle fibres the nuclei ultimately take up a position beneath the cell-wall or sarcolemma which is formed on the surface. Striations appear first along one side, and extend round the fibre (fig. 114), then they extend into the centre. During life new fibres appear to be formed in part by a longi- tudinal splitting of pre-existing fibres ; this is preceded by a multiplication of nuclei ; and in part by the lengthening and differentiation of em- bryonic cells (sarcoplasts) found between the fully formed fibres. Bundles of fine muscular fibres enclosed within a thick sheath of connective tissue are found at the point of entrance of the nerve into the muscle. The whole structure is called a muscle spindle. There has been considerable discussion as to the meaning of the muscle spindle ; some consider the fibres in it are formed from sarcoplasts, and are, in fact, young muscular fibres ; others think they are fibres undergoing degeneration, and others still, looking at their relatively large nerve supply, believe them to be sensory end organs. In plain muscle, growth occurs in a similar way : this is well illustrated in the enlargement of the uterus during pregnancy; this is due in part to the growth of the pre-existing fibres, and in part to the formation of new fibres from small granular cells lying between them. After parturition the fibres shrink to their original size, but many undergo fatty degeneration and are removed by absorption. Fig. 114.-Deve- iar'flbrefrom Rau- vier.) CHAPTER VII. NERVE. Nervous tissue is the material of which the nervous system is composed. The nervous system is composed of two parts, the central nervous system, and the peripheral nervous system. The central nervous system consists of the brain and spinal cord ; the CH. VII.] NERVE. 97 peripheral nervous system consists of the nerves, which conduct the impulses to and from the central nervous system, and thus bring the nerve centres into relationship with other parts of the body. Some of the nerves conduct impulses from the nerve-centres and are called efferent; those which conduct impulses in the opposite direction are called afferent. When one wishes to Fig. 115.-Two nerve-fibres of sciatic nerve, a. Node of Ranvier, b. Axis-cylinder, c. Sheath of Schwann, with nuclei. Medullary sheath is not stained. X 300. (Klein and Noble Smith.) Fig. 116.-Axis cylinder, high- ly magnified, showing its component fibres. (M. Schultze.) move the hand, the nervous impulse starts in the brain and passes down the efferent or motor nerves to the muscles of the hand, 'which contract; when one feels pain in the hand, afferent or sensory nerves convey an impulse to the brain which is there interpreted as a sensation. If all the nerves going to the hand are cut through, all communication with the nerve-centres is destroyed, and the hand loses the power of moving under the influence of the will, and the brain receives no impulses from the hand, or as we say the hand has lost sensi- bility. This distinction between efferent and afferent nerves is a physiological one, which we shall work out more thoroughly later on. No histological distinction can be made out between motor and sensory nerves, and it is histological structure which we 98 NERVE. Tch. vii. wish to dwell upon in this chapter. Under the microscope nervous tissue is found to consist essentially of two elements, nerve-cells and nerve-fibres. The nerve-cells are contained in the brain and spinal cord, and in smaller collections of cells on the course of the nerves called ganglia. The part of the nerve-centres containing cells is called grey matter. The nerve-fibres are contained in the nerves, and in the white matter of brain and spinal cord. The nerve-fibres are long branches from the nerve- cells, which become sheathed in a manner to be immediately described. Nerve-cells differ in size, shape, and arrange- ment, and we shall discuss these fully when we get to the nerve-centres. For the present it will be convenient to confine ourselves to the nerve-fibres as they are found in a nerve. Nerve-fibres are of two his- tological kinds, medullated and •non-medullated. Medullated . nerve-fibres are found in the* white matter of the nerve- centres and in the nerves origi- nating from the brain and spinal cord. Non-medullated nerve-fibres are those which occur in the sympathetic nerves. The medullated. or white fibres are characterised by a sheath of white colour, fatty in nature, and stained black by osmic acid ; it is called the medullary sheath or white substance of Schwann; this sheathes the essential part of the fibre which is a process from a nerve-cell, and is called the axis cylinder. Outside the medullary sheath is a thin homogeneous membrane of elastic nature called the jyrimitive sheath or neurilemma. Fig. 118.-Anodeof Ran- vier in a medullated nerve - fibre, viewed from above. The me- dullary sheath is inter- rupted, and the primi- tive sheath thickened. Copied from Axel Key and Retzius. X 750. (Klein&Noble Smith.) Fig.117.-Nerve- fibre stained with osmic acid. (Key and Retzius.) CH. VII.] NERVE. 99 The axis cylinder is a soft transparent thread in the middle of the fibre; it is made up of exceedingly fine fibrils which stain readily with gold chloride. The medullary sheath gives a characteristic double contour and tubular appearance to the fibre. It is interrupted at regular intervals known as the nodes of Ranvier. The stretch of nerve between two nodes is called an inter-node, and in the middle of each inter-node is a nucleus which belongs to the primitive sheath. Besides these interruptions, a variable number of oblique clefts are also seen, Fig. 119.-Small branch of a muscular nerve of the frog, near its termination, showing divisions of the fibres, a, into two ; b, into three. X 350. (Kolliker.) dividing the sheath into medullary segments (fig. 117) ; but most if not all of these ai'e produced artificially in the preparation of the specimen. The medullary sheath also contains a horny substance called neurokeratin : the arrangement of this substance is in the form of a network or reticulum holding the fatty matter of the sheath in its meshes. The occurrence of horny matter in the epidermis, in the development of the enamel of teeth and in nerve is an interesting chemical reminder that all these tissues originate from the same embryonic layer, the epiblast. The fatty matter consists largely of lecithin, a phosphorised fat, and cholesterin, a monatomic alcohol. We shall make a more intimate acquaint- ance with these chemical materials at a later stage in our. studies. Near their terminations the nerve-fibres branch : the branching occurs at a node (fig. 119). 100 NERVE. [ch. vii Staining with silver nitrate produces a peculiar appearance at the nodes, forming what is known as the crosses of Banvier. One limb of the cross is produced by the dark staining of Fig. i2o.-Several fibres of a bundle of medullated nerve-fibres acted upon by silver nitrate to show peculiar behaviour of nodes of Ranvier, N, towards this reagent. The silver has penetrated at the nodes, and has stained the axis-cylinder, M, for a short distance. S, the white substance. (Klein and Noble Smith.) cement substance which occurs between the segments of the neurilemma; the other limb of the cross is clue to the staining of •Fig. 121.-Transverse section of the sciatic nerve of a cat about x roo.-It consists of bundles {funiculi) of nerve-fibres ensheathed in a fibrous supporting capsule, ejoi- neurium, A ; each bundle has a special sheath (not sufiiciently marked out from the epineurium in the figure) or perineurium B ; the nerve-fibres N /are separated from one another by endoneurium; L, lymph spaces ; Ar, artery; V, vein; F, fat. Somewhat diagrammatic. (V. D. Harris.) ch. vn.] NERVE. 101 the axis cylinder in a number of minute transverse bands (Fromann's lines), which is here not invested by any medullary sheath. The arrangement of the nerve-fibres in a nerve is best seen in a transverse section. The nerve is composed of a number of bundles or funiculi of Fig. 122.- Section across the second thoracic anterior root of the dog, stained with osmic acid. (Gaskell.) nerve fibres by connective tissue. The sheath of the whole nerve is called the epineurium ; that of the funiculi the perineurium ; that which passes between the fibres in a funiculus, the endo- neurium (fig. 121). The size of the nerve fibres varies ; the largest fibres are found in the spinal nerves, where they are 14'4 to 19/z in diameter.* Others mixed with these measure i'8 to 3'6p. These small nerve-fibres are the visceral nerves ; they pass to sympathetic ganglia, whence * p. = micro-millimetre = T(J-0 millimetre. 102 NERVE. [ch. vii. they emerge as non-medullated fibres, and are distributed to in- voluntary muscle. They are well seen in sections stained by osmic acid, the black rings being the stained medullary sheaths (fig. 122). The non-medullated fibres or fibres of Remak have no medullary sheath and are therefore devoid of the double contour of the medullated fibres, and are unaffected in appearance by Fig. 123.-Grey, or non-medullated nerve-fibres. A. From a branch of the olfactory nerve of the sheep ; two dark-bordered or white fibres from the fifth pair are asso- ciated with the pale olfactory fibres. B. From the sympathetic nerve, x 450. (Max Schultze.) osmic acid. They consist of an axis cylinder covered by a nucleated fibrillated sheath. They branch frequently. Though principally found in the .sympathetic nerves, a few are found in the spinal nerves mixed with the medullated fibres. Termination of Motor Nerves. In the voluntary muscles the nerve-fibres have special end organs called end-plates. The fibre branches two or three times (figs. 119, 124), and each branch goes to a muscular fibre. Here the neurilemma becomes continuous with the sarcolemma, the medul- lary sheath stops short, and the axis cylinder branches repeatedly. This ramification is embedded in a layer of granular protoplasm containing numerous nuclei. Considerable variation in shape of the end plates occurs in different parts of the animal kingdom. Somewhat similar nerve-endings are seen in tendon; these however are doubtless sensory (figs. 125, 126). In the involuntary muscles, the fibres which are for the most part non-medullated form complicated plexuses near their termination. CH. VII.] END-PLATES. 103 The plexus of Auerbach between the muscular coats of the intestine is a typical case. Groups of nerve-cells will be noticed at the junc- tions of the fine nervous cords. From these plexuses fine branches Fig. 124.-From a preparation of the nerve-termination in the muscular fibres of a snake, a, End-plate seen only broad surfaced, b, End- plate seen as narrow surface. (Lingard and Klein.) pass off and bifurcate at frequent intervals, until at last ultimate fibrillae are reached. These subdivisions of the axis cylinders do not anastomose with one another, but they come into close Fig. 125.-Termination of medullated nerve-fibres in tendon near the mus- cular insertion. (Golgi.) Fig. 126.-One of the reticulated end-plates of fig. 114, more highly magnified, a, medullated nerve-fibre; ft, reticulated end-plates. (Golgi.) relationship with the involuntary muscular fibres; though some histologists have stated that they end in the nuclei of the muscular fibres, it is now believed that they do not pass into their interior. The terminations of sensory nerves are in some cases plexuses, 104 NERVE [ch. VII, in others special end organs. We shall deal with these in our study of sensation. Fig. 127.-Plexus of Auerbach, between the two layers of the muscular coat of the intestine. (Cadiat.) Development of Nerve-fibres. A nerve-fibre is primarily an outgrowth from a nerve-cell, as is shown in the accompanying diagram. As a rule a nerve-cell only Fig. 128.-Multipolar nerve-cell from anterior bone of spinal cord ; a, axis cylinder process. (Max Schultze.) CH. VII.] IRRITABILITY. 105 gives off one process which becomes the axis cylinder of a nerve- fibre. It acquires a medullary sheath when it passes into the white matter of the brain or spinal cord, and a primitive sheath when it leaves the nerve-centre and gets into the nerve. But at first the axis cylinder is not sheathed at all. The formation of the sheaths is still a matter of doubt, but the generally accepted opinion is that the primitive sheath is formed by cells which become flattened out and wrapped round the fibre end to end. These are separated at the nodes by inter- cellular or cement substance stainable by silver nitrate (fig. 120). These cells are probably mesoblastic. The medullary sheath is formed according to some by a fatty change occurring in the parts of these same cells which are nearest to the axis cylinder, but it is more probable that it is formed from the peripheral layer of the axis cylinder ; the presence of neurokeratin in it distinctly points to an epiblastic origin. CHAPTER VIII. IRRITABILITY AND CONTRACTILITY. Irritability or Excitability is the power that certain tissues possess of responding by some change to the action of an external agent. This external agent is called a stimulus. Undifferentiated cells like white blood corpuscles are irritable; when stimuli are applied to them they execute the movements we have learnt to call amoeboid. Ciliated epithelium cells and muscular fibres are irritable ; they also execute movements under the influence of stimuli. Nerves are irritable; when they are stimulated, a change is produced in them ; this change is propagated along the nerve and is called a nervous impulse; there is no change of form in the nerve visible to the highest powers of the microscope ; much more delicate and sensitive instruments than a microscope must be employed to obtain evidence of a change in the nerve; it is of a molecular nature. But the irritability of nerve is readily manifested by the results the nervous impulse produces in the organ to which it goes ; thus the stimulation of a motor nerve produces a nervous impulse in that nerve which, when it reaches 106 IRRITABILITY AND CONTRACTILITY. [ch. vin. a muscle causes the muscle to contract: stimulation of a sensory nerve produces a nervous impulse in that nerve which when it reaches the brain causes a sensation. Secreting glands are irritable ; when irritated or stimulated they secrete. The electrical organs found in many fishes like the electric eel, and torpedo ray, are irritable ; when they are stimulated they give rise to an electrical discharge. Contractility is the power that certain tissues possess of responding to a stimulus by change of form. Contractility and irritability do not necessarily go together; thus both muscle and nerve are irritable, but of the two only muscle is contractile. Some movements visible to the microscope are not due to contractility; thus granules in protoplasm or in a vacuole may Fig-. 130. - Pigment-cells from the retina, a, cells still cohering, seen on their surface; a, nu- cleus indistinctly seen. In the other cells the nucleus is concealed by the pigment granules. b, two cells seen in profile; a, the outer or posterior part containing scarcely any pig- ment. X 370. (Henle.) Fig. 129.-Frog's pigment cells. often be seen to exhibit irregular, shaking movements due simply to vibrations transmitted to them from the outside. Such movement is known as Brownian movement. Instances of contractility are seen in the following cases :- 1. The movements of protoplasm seen in simple animal and vegetable cells; in the former we have already considered streaming, gliding, and amoeboid movement (see p. 13); in the latter case we have noted the rotatory movements of the protoplasm within the cell wall in certain plants (see p. 14). 2. The movements of pigment cells. These are well seen under the skin of such an animal as the frog; under the influence of electricity and of other stimuli, especially of light, the pigment granules are massed together in the body of the cell, leaving the processes quite transparent (fig. 129). If the stimulus is removed the granules gradually extend into the processes again. Thus the skin of the frog is sometimes uniformly dusky, and sometimes quite light coloured. The chamafleon is an animal CH. VIII.] RHYTHMICALITY. 107 which has become almost proverbial as it possesses the same power to a marked degree. This function is a protective one; the animal approximates in colour that of its surroundings, and so escapes detection. In the retina we shall find a layer of pigment cells (fig. 130), the granules in which are capable of moving in the protoplasm in a somewhat similar way; the normal stimulus here also is light. 3. Ciliary movement; here we have a much more orderly movement which has already been described (see p. 31). 4. In Vorticellse, a spiral thread of protoplasm in their stalk enables them by contracting it to lower the bell at the end of the stalk. 5. In certain of the higher plants, such as the sensitive and carnivorous plants, movements of the stalks and sensitive hairs of the leaves occur under the influence of stimuli. 6. Muscular movement. This for the student of human physiology is the most important of the series; it is by their muscles that the higher animals (man included) execute the greater number of their movements. If we contrast together amoeboid, ciliary and muscular move- ment, we find that they differ from each other very considerably. Amoeboid movement can occur in any part of an amoeboid cell, and in any direction. Ciliary and muscular movement are limited to one direction ; but they are all essentially similar, consisting of the movement of hyaloplasm in and out of spongio- plasm ; it is the arrangement of the spongioplasm that limits and controls the movement of the hyaloplasm (see also p. go). Rhythmicality.-In some forms of movement there is not only order in direction, but order in time also. This is seen in ciliary movement, and in many involuntary forms of muscular tissue, such as the heart. Here periods of contraction alternate with periods of rest, and this occurs at regular intervals. Under the influence of certain saline solutions, voluntary muscles may be made artificially to exhibit rhythmic contractions. A familiar instance of rhythmic movement in the inorganic world is seen in a water-tap nearly turned off but dripping ; water accumulates at the mouth of the tap till the drop is big- enough to fall; it falls, and the process is repeated. If, instead of water, gum or treacle, or some other viscous substance is watched under similar circumstances, the drops fall much more slowly ; each drop has to get bigger before it possesses enough energy to fall. Thus we may get different degrees or rates of rhythmic movement. So in the body, during the period of rest, the cilium or the heart is accumulating potential energy, till, as it 108 IRRITABILITY AND CONTRACTILITY. [ch. viii. were, it becomes so charged that it discharges; potential energy is converted into kinetic energy or movement. When contraction travels as a wave along muscular fibres, or from one muscular fibre to another, the term peristalsis is applied. These waves are well seen in such a muscular tube as the intestine, and are instrumental in hurrying its contents along. The heart's contraction is a similar but more complicated peristalsis occurring in a rhythmic manner. The physiology of muscle and nerve furnish us with the best means of studying irritability and contractility. We shall have to consider these two tissues together to a large extent, but confine our attention at the outset to the voluntary muscles. The question may be first asked, what evidence there is of irritability in muscle ? May not the irritability be a property of the nerve-fibres which are distributed throughout the muscle and terminate in its fibres ? The doctrine of independent muscular irritability was enunciated by Haller more than a century ago, and was afterwards keenly debated. It was finally settled by an experiment of Claude Bernard which can be easily repeated by every student. If a frog is taken and its brain destroyed by pithing, it loses consciousness but the circulation goes on, and the tissues of its body retain their vitality for a considerable time. If now a few drops of a solution of curare, the Indian arrow poison, are injected with a small syringe under the skin of its back, it loses in a few minutes all power of movement. If next the sciatic or any other nerve going to muscle is dissected out and stimulated, no movement occurs in the muscles to which it is distributed. Curare paralyses the motor end-plates, so that for all practical purposes the muscles are nerveless ; or rather nervous impulses cannot get past the end-plates and cause any effect on the muscles. But if the muscles are stimulated themselves they contract. Another proof that muscle possesses inherent irritability was adduced by Kuhne. In part of some of the frog's muscles (e.y. the sartorius) there are no nerves at all • yet they are irritable and contract when stimulated. The evidence of the statement just made that the poisonous effect of curare is on the end-plates is the following :-The experiment described proves it is not the muscles that are paralysed. It must therefore be either the nerves, or the links between the nerve-fibres and the muscular fibres. By a process of exclusion we arrive at the conclusion that it is these end-plates, for the following experiment shows it is not the nerves. The CH. VIII.] STIMULI. 109 frog is pithed as before, and then one of its legs is tightly ligatured so as to include everything except the sciatic nerve of that leg. Curare is injected and soon spreads by the circulating blood all over the body except to the leg protected by the ligature. It can get to the sciatic nerve of that leg because that was not tied in with the rest. The sciatic nerve of the other leg is now dissected out; when the muscles supplied by it cease to contract when the nerve is stimulated, the frog may be considered to be fully under the influence of the drug. But on stimulating the sciatic nerve of the protected limb, the muscles respond normally ; this shows that the nerve which has been exposed to the action of the poison has not been affected by it. Varieties of Stimuli. The normal stimulus that leads to muscular contraction is a nervous impulse; this is converted into a muscular impulse (visible as a contraction) at the end-plates. This nervous impulse starts at the nerve- centre, brain or spinal cord, and travels down the nerve to the muscle. In a reflex action the nervous impulse in the nerve-centre is started by a sensory impulse from the periphery ; thus when one puts one hand on some- thing unpleasantly hot, the hand is removed ; the hot substance causes a nervous impulse to travel to the brain, and the brain reflects down to the muscles of the hand another impulse by the motor-nerves which causes the muscles to contract in such a manner as to move the hand out of the way. But the details of muscular contraction can be more readily studied in muscles removed from the body of such an animal as the frog, and made to contract by artificial stimuli. When we have considered these, we can return to the lessons they teach us about the normal contractions in our own bodies. The first thing to do is to make from a pithed frog a muscle- nerve preparation ; the muscle usually selected is the gastroc- nemius, the large muscle of the calf of the leg, with the sciatic nerve attached. For some experiments the sartorius or gracilis may be used; but nearly all can be demonstrated on the gastrocnemius. Fig. 131.-Muscle-nerve preparation, f, femur ; x, nerve ; t, tendo Achillis. (M'Kendrick.) 110 IRRITABILITY AND CONTRACTILITY. [ch. viii. The tendon of the gastrocnemius may be tied to a lever with a flag at the end of it, and thus its contractions rendered more evident; the bone at the other end being fixed in a clamp. Stimuli may be applied either to the nerve or to the muscle. If the stimulus is applied to the nerve, it is called indirect stimulation ; the stimulus starts a nervous impulse which travels to the muscle; the muscle is thus stimulated as it is in voluntary contraction by a nervous impulse. Stimulation of the muscle itself is called direct stimulation. These stimuli may be : 1. Mechanical ; for instance a pinch or blow. 2. Chemical ; for instance salt or acid sprinkled on the nerve or muscle. 3. Thermal; for instance touching the nerve or muscle with a hot wire. 4. Electrical; the constant or the induced current may be used. In all cases the result of the stimulation is a muscular contraction. Of all methods of artificial stimulation, the electrical is the one most generally employed, because it is more under control and the strength and duration of the stimuli (shocks) can be regulated easily. We shall therefore have to study some electrical apparatus. Chemical stimuli are peculiar, for some which affect muscle do not affect nerve, aild vice versa; thus glycerine stimulates nerve, but not muscle; ammonia stimulates muscle, but not motor nerves. We may regard stimuli as liberators of energy; muscle and nerve and other irritable structures undergo disturbances in consequence of a stimulus. The disturbance is some form of movement, visible movement in the caseJof muscle, molecular movement in the case of nerve. A stimulus may be regarded as added motion. Dr. Gowers compares it to the blow that causes dynamite to explode, or the match applied to a train of gun- powder. A very slight blow will explode a large quantity of dynamite ; a very small spark will fire a long train of gunpowder. So in muscle or nerve the effect is often out of all proportion to the strength of the stimulus ; a light touch on the surface of the body may elicit very forcible nervous and muscular disturb- ances ; and moreover, the effect of the stimulus is propagated along the nerve or muscle without loss. CH. IX. 1 CONTRACTION OF MUSCLE. 111 CHAPTER IX. CONTRACTION OF MUSCLE. Muscle undergoes many changes when it contracts; they may be enumerated under the following five heads:- 1. Changes in form. 2. Changes in extensibility and elasticity. 3. Changes in temperature. 4. Changes in electrical condition. 5. Chemical changes. In brief, each of these changes is as follows :- 1. Changes in form.-The muscle becomes shorter, and at the same time thicker. The amount of shortening varies from 65 to 85 per cent, of the total length of the muscle. Up to a certain point, increase of the strength of the stimulus increases the amount of contraction. Fatigue diminishes and up to about 330 C. the application of heat increases the amount of contraction. Beyond this temperature the muscular substance begins to be permanently contracted, and a condition called heat rigor, due to coagulation of the muscle proteids, sets in a little over 40° C. What the muscle loses in length it gains in width ; there is no appreciable change of volume. Among the changes in form must also be mentioned those changes in the individual muscular fibres which require a microscope for their investigation; these have been already considered (see p. 90). 2. Changes in elasticity and extensibility.-The contracted muscle is more stretched by a weight in proportion to its length than an uncontracted muscle with the same weight applied to it; the extensibility of contracted muscle is increased ; its elasticity is diminished. 3. Changes in temperature.--When muscle is at work 'or contracting, more energetic chemical changes are occurring than when it is at rest; more heat is produced and its temperature rises. 4. Changes in electrical condition.-A contracted muscle is electrically negative to an uncontracted muscle. 5. Chemical changes.---These consist in an increased consump- tion of oxygen, an increased output of waste materials such as 112 MUSCULAR CONTRACTION. [ch. x. carbonic acid, and sarco-lactic acid. After prolonged contraction the muscle consequently acquires an acid reaction. These five sets of changes will form the subjects of the following five chapters. CHAPTER X. CHANGE IN FORM IN A MUSCLE WHEN IT CONTRACTS. Though it has been known since the time of Erasistratus (b.c. 304) that a muscle becomes thicker and shorter when it contracts, it was not until the invention of the graphic method by Ludwig and Helmholtz, about fifty years ago, that we pos- sessed any accurate knowledge of this change. The main fact just stated may be seen by simply looking at a contracting- muscle, such as the biceps of one's own arm; but more elaborate apparatus is necessary for studying the various phases in contrac- tion and the different kinds of contraction that may occur. These may be readily demonstrated on the ordinary muscle- nerve preparation (gastrocnemius and sciatic nerve) from a frog. By the graphic method, one means that the movement is re- corded by a writing. We shall find that the same method is applied to the heart's movements, respiratory movements, blood pressure, and many other important problems in physiology. The special branch of the graphic method we have now to study is called myography; the instrument for writing is called a myo- graph ; the writing itself is called a myogram. Put briefly, a myograph consists of a writing point at the end of a lever attached to the muscle, and a writing surface which travels at a uniform rate, on which the writing point inscribes its movement. The first thing, however, that is wanted is something to stimu- late the muscle and make it contract; the stimulus is usually applied to the nerve, and the form of stimulus most frequently employed is electrical. The galvanic battery in most common use is the Daniell cell. It consists of a well-amalgamated zinc rod immersed in a cylinder of porous earthenware containing 10 per cent, sulphuric acid; this is contained within a copper vessel (represented as trans- parent for diagrammatic purposes in fig. 132) filled with saturated ch. x.] BATTERIES AND KEYS. 113 solution of copper sulphate. Each metal has a binding screw attached to it, to which wires can be fastened. The zinc rod is called the positive element, the copper the negative element. The distal ends of the wires attached to these are called poles or electrodes, and the pair of electrodes may be conveniently held in a special form of holder. The electrode attached to the positive element (zinc) is called the negative pole or kathode; that attached to the negative ele- ment (copper) is called the positive pole or anode. If now the two elec- trodes are connected together, an electrical, galvanic or constant cur- rent flows from the copper to the zinc outside the battery, and from the zinc to the copper through the fluids of the battery; if the elec- trodes are not connected the circle is broken, and no current can flow at all. If now a nerve or muscle is laid across the two h SO- Cm An- Fig. 132.-Diagram of a Daniell's battery.. B. Mercury Key. electrodes the circuit is completed, and it will be noticed at the moment of completion of the circuit the muscle enters into contraction; if the muscle is lifted off the electrodes, another contraction occurs at the moment the circuit is broken. The same thing is done more conveniently by means of a key: fig. 133 Fig. 133.-A. Du Bois Reymond's Key. 114 INSTRUMENTS. [ch. x. represents two common forms of key. A key is a piece of apparatus by which the current can be allowed to pass or not through the nerve or muscle laid on the electrodes. When the key is open the current is broken, as in the next figure (fig. 134) when it is closed the current is allowed to pass. The opening of the key is called break; the closing of the key is called make. A contraction occurs only at make and break, not while the current is quietly traversing the nerve or muscle. Fig. 134- But it will be seen in the Du Bois Reymond key (fig. 133) that there are four binding screws. This key is used as a bridge or short Fig. 135. circuiting key, and for many reasons this is the best way to use it. The above diagram (fig. 135) represents this diagrammatically. The two wires from the battery go one to each side of the key; the electrodes come off one from each side of the key. When the key is open no current can get across it, and therefore all the current has to go to the electrodes with the nerve resting on them ; but when the key is closed, the current is cut off from the nerve, as then practically all of it goes by the metal bridge, or short cut, back to the battery. Theoretically a small amount of current goes through the nerve ; but the resistance of animal tissues to electrical currents is enormous as compared to that of metal, and the amount of electricity that flows through a conductor is inversely proportional to the resistance ; the resistance in the metal bridge is so small that for all practical purposes, all the current passes through it. Another form of electrical stimulus is the induced current, pro- duced in an induction coil. If one has a battery of which the metals are connected by a wire, we have seen the current in the wire travels from the CH. x.] THE INDUCTION COIL. 115 copper to the zinc ; if we have a key on the course of this wire the current can be made or broken at will. If in the neighbour- hood of this wire we have a second wire forming a complete circle, nothing whatever occurs in it while the current is flowing through the first wire, but at the instant of making or breaking the current in the first or primary wire, a momentary electrical current occurs in the secondary wire, which is called an induced current; and if the secondary wire is not a complete circle, but its two ends are connected by a nerve, this induction shock tra- Fig. 136.-Du Bois Reymond's induction coil. verses the nerve and stimulates it; this causes a nervous impulse to travel to the muscle, which in consequence contracts. If the first and second wires are coiled, the effect is increased, because each turn of the primary coil acts inductively on each turn of the secondary coil. The direction of the current induced in the secondary coil is the same as that of the current in the primary coil at the break; in the opposite direction at the make. The nearer the secondary coil is to the primary the stronger are the currents induced in the former. Fig. 136 represents the Du Bois Reymond coil, the one gene- rally employed in physiological experiments, c is the primary coil, and cl and d' its two ends, which are attached to the battery, a key being interposed for making and breaking ; g is the secon- dary coil, the two terminals of which are at its far end ; to these the electrodes to the nerve are attached ; the distance between 116 INSTRUMENTS. [ch. x. the two coils, and so the strength of the induction currents can be varied at will. It is only when the primary current is made or broken, or its intensity increased or diminished, that induction shocks occur in the secondary circuit which stimulate the nerve. When one wishes to make or break the current rapidly the automatic interrupter or Wagner's hammer seen at the right-hand end of the diagram is included in the circuit. The next thing to be noticed is that the break effects are stronger than the make effects; this is easily felt by placing the electrodes on the tongue. This is due to what is called Faraday's extra current. This is a current produced in the primary coil by the inductive influence of contiguous turns of that wire on each other; its direction is against that of the battery current at make, and so the make shock is lessened. At the break there can be no extra current, because the circuit being broken there can be no current at all. The same difference of strength occurs alternately in the repeated shocks produced by Wagner's hammer. Helmholtz, to obviate this, introduced a modification now known after him. It consists in bridging the current by a side wire, so that the current entirely ceases in the primary coil, but is alter- nately strengthened and weakened by the rise and fall of the hammer; the strengthening corresponds to the ordinary make, and is weakened by the make extra current, which occurs in the opposite direction to the battery current; the break is also incomplete,- and so it is weakened by the break extra current, which being in the same direction as the battery current impedes its disappearance. The two next diagrams show the way the interrupter acts. We are supposed to be looking at the end of the primary coil; the battery wires are attached to the binding screws A and E (fig. 137). The current now passes to the primary coil by the pillar on the left and the spring or handle of the hammer as far as the screw (C) in the middle of the top of the diagram; after going round the primary coil, one turn only of which is seen, it twists round a pillar of soft iron on the right-hand side, and then to the screw E and back to the battery ; the result of a current going around a bar of soft iron is to make it a magnet, so it attracts the hammer, and draws the spring away from the top screw, and thus breaks the current; the current ceases, the soft iron is no longer a magnet, so it releases the hammer and con- tact is restored by the spring; then the same thing starts over again, and so a succession of break and make shocks occurs alternately and automatically. CH. x.] THE INDUCTION COIL. 117 In Helmholtz' modification (fig. 138) the battery wires are connected as before. The interrupter is bridged by a wire from B to C (also shown in fig. 136). C is raised out of reach, and the lower screw F is brought within reach of the spring. Owing Fig- 137- to the wire BC, the vibration of the hammer never entirely breaks the current. Instead of Wagner's hammer a long vibrating reed constructed on the same principle is often used. This has the advantage Kg. 138. that the rate of vibration can be varied at will by means of a sliding clamp which fixes the reed so that different lengths of it can be made to vibrate. If a long piece of reed vibrates, it does so slowly, and thus successive induction shocks at long intervals can be sent into the nerve. But if one wishes to 118 INSTRUMENTS. [ch. x. stimulate a nerve more rapidly, the length of reed allowed to vibrate can be shortened. In Ewald's modification of the coil there is another simple method of modifying the rate of the interrupter. But an hour spent in the laboratory with an induction coil and cell will teach the Fig. 139.-Myograph of von Helmholtz, shown in an incomplete form, a, forceps for holding frog's femur ; ft, gastrocnemius; c, sciatic nerve; ci, scale pan; e, marker recording on cylinder; /, counterpoise. (M'Kendrick.) student much more easily all these facts than apy amount of reading and description. We can pass now to the myograph. There are many different forms of this instrument. Fig. 139 shows Helmholtz' instru- ment. The bony origin of the gastrocnemius is held firmly by forceps, the tendo Achillis tied to a weighted lever ; the end of the lever is provided with a writing-point such as a piece of pointed parchment; when the muscle contracts it pulls the lever up, and this move- ment is magnified at the end of the lever. The writing-point scratches on a piece of glazed paper covered with a layer of soot; the paper is wrapped round a cylinder. When the lever goes up the writing-point will mark an up-stroke; when it falls it ch. x.] MYOGRAPHS. 119 will mark a down-stroke, and if the cylinder is travelling, the down-stroke will be written on a different part of the paper than the up-stroke; thus a muscle curve or myogram is ob- tained. The paper may then be removed, varnished, and preserved. Fig. 140 shows a somewhat different arrangement. The muscle is fixed horizontally on a piece of cork B, one end being fixed by a pin thrust through the knee-joint into the cork; the tendo Achillis is tied to a weighted lever: the lever Fig. 140.-Arrangement of the apparatus necessary for recording muscle contractions with a revolving cylinder carrying smoked paper. A, revolving cylinder ; B, the muscle arranged upon a cork-covered board which is capable of being raised or lowered on the upright, which also can be moved along a solid triangular bar of metal attached to the base of the recording apparatus-the tendon of the gastrocnemius is attached to the writing lever, properly weighted, by a ligature. The electrodes from the secondary coil pass to the nerve-being, for the sake of convenience, first of all brought to a short- circuiting key, D (Du Bois Reymond's) ; C, the induction coil; F, the battery (in this flg. a bichromate one); E, the key (Morse's) in the primary circuit. is so arranged that it rests on a screw till the muscle begins to contract; the muscle therefore does not feel the weight till it begins to contract, and gives a better contraction than if it had been previously strained by the weight. This arrangement is called after-loading. 120 INSTRUMENTS. [ch. x. The writing surface is again a travelling cylinder tightly covered with smoked glazed paper. The rest of the apparatus shows how cell, coil, keys, and electrodes are applied with the object of stimulating the nerve. The key E makes and breaks the primary circuit, but the effect is only felt by the muscle-nerve preparation when the short-circuiting key D in the secondary circuit is opened. Instead of the key A it is better to have what is called a " kick-over " key which the cylinder by means of a bar projecting from it knocks over and so breaks the primary circuit during the course of a revolution. The exact position of the writing-point at the moment of break, that is the moment of excitation, can then be marked on the blackened paper. Besides the travelling cylinder there are other forms of writing surface. Thus fig. 141 represents the sp>ring myograph of Du Bois Reymond. Here a blackened glass plate is shot along by the recoil of a spring ; as it travels it kicks over a key, and the result of this, the muscular contraction, is written on the plate. The pendulum myograph (fig. 142) is another form. Here the movement of the pendulum along a certain arc is substituted for the clockwork of the cylinder, or the spring of Du Bois Reymond. The pendulum carries a smoked glass plate upon which the writing-point of the muscle lever is made to mark. The break Fig. 141-: ■Du Bois Reymond's spring myograph. (M'Kendrick.) CH. X.] . MYOGRAPHS. 121 shock is sent into the muscle-nerve preparation by the pendulum Fig. 142.-Simple form of pendulum myograph and accessory parts. A, pivot upon which pendulum swings ; B, catch on lower end of myograph opening the key, C, in its swing; D, a spring-catch which retains myograph, as indicated by dotted lines, and on pressing down the handle of which the pendulum swings along the arc to D on the left of figure, and is caught by its spring. in its swing opening a key in the primary circuit. This is shown in an enlarged scale in BC (fig. 142). Fig. 143.-Moist Chamber. 122 INSTRUMENTS. [,CH. X. To keep the preparation fresh during an experiment, it should be covered with a glass shade, the air of which is kept moist by means of wet blotting paper. A somewhat elaborate form of moist chamber is shown in fig. 143. The last piece of apparatus necessary is a time-marker, so that the events recorded in the myogram can be timed. The simplest time-marker is a tuning-fork vibrating 100 times a second. This is struck, and by means of a writing-point fixed on to one of the prongs of the fork, these vibrations may be written beneath the myogram. More elaborate forms of electrical time-markers or chronographs are frequently employed. The Simple Muscle Curve. We can nowT pass on to results, and study first the result of a single induction shock upon a muscle-nerve preparation. Fig. 144.-Simple muscle-curve. (M. Foster.) A single momentary stimulation causes a single or single mus- cular contraction, or as it is often called a twitch. The graphic record of such a contraction is called the simple muscle curve. One of these is shown in the preceding figure (fig. 144). The upper line (m) represents the curve traced by the end of the lever in connection with a muscle after stimulation of the muscle by a single induction-shock : the middle-line (Z) is that described by a lever, which indicates by a sudden drop the exact instant at which the induction-shock is given. The lower wavy line (f) is traced by a tuning-fork vibrating 100 times a second,, and serves to measure precisely the time occupied in each part of the contraction. ch. x.J THE SIMPLE MUSCLE CURVE. 123 It will be observed that after the stimulus has been applied as indicated by the vertical line s, there is an interval before the contraction commences, as indicated by the line c. This interval, termed (a) the latent period, when measured by the number of vibrations of the tuning-fork between the lines s and c, is found to be about During the latent period there is no apparent change in the muscle. The second part is the (6) stage of contraction proper. The lever is raised by the contraction of the muscle. The contraction is at first very rapid, but then progresses more slowly to its maximum, indicated by the line mx, drawn through its highest point. It occupies in the figure T£ySec. (c) The next stage, stage of elongation. After reaching its highest point, the lever begins to descend, in consequence of the elongation of the muscle. At first the fall is rapid, but then becomes more gradual until the lever reaches the abscissa or base line, and the muscle attains its pre-contraction length, indicated in the figure by the line c. The stage occupies Very often after the main con- traction the lever rises once or twice to a slight degree, producing small curves (as in fig. 146). These contractions, due to the elasticity of the muscle, are called (cf) Stage of elastic after- vibration, or contraction remainder. The whole contraction occupies about of a second. With regard to the latent period, it should be pointed out that if the muscle is stimulated indirectly, i.e., through its nerve, some of apparent lost time is occupied in the propagation of the nervous impulse along the nerve. To obtain the true latent period, this must be deducted. Then there is generally latency in the apparatus, friction of the lever, &c., to be taken into account. This can be got rid of by photographing the contracting muscle, on a sensitive photographic plate travelling at an accurately- timed rate. By such means it is found that the true latent period is much shorter than was formerly supposed. It is only yyy of a second. In red muscles it is longer. We now come to the action of various factors in modifying the character of the simple muscle nerve. 1. Influence of strength of stimulus.-A minimal stimulus is that which is just strong enough to give a contraction. If the strength of stimulus is increased the amount of contraction as measured by the height of the curve is increased, until a certain point is reached (maximal stimulus), beyond which increase in the stimulus produces no increase in the amount of con- traction. The latent period is shorter with a strong than with a weak stimulus. 2. Influence of load.-Up to a certain point increase of load increases the amount of contraction, beyond which it diminishes, until at last a weight 124 MUSCULAR CONTRACTION. [ch. x. is reached which the muscle is unable to lift. The latent period is some- what longer with a heavy load than with a light one. 3. Influence of fatigue.-This can be very well illustrated by letting the muscle write a curve with every revolution of the cylinder until it ceases to contract altogether. The next diagram shows the early stages of fatigue. Fig. 145.-Fatigue. At first the contractions improve, each being a little higher than the pre- ceding ; this is known as the beneficial effect of contraction, and the graphic record is called a staircase. Then the contractions get less and less. But what is most noticeable is that the contraction is much more prolonged ; the latent period gets longer ; the period of contraction gets longer ; and the period of relaxation gets very much longer ; there is a condition known as contracture, so that the original base line is not reached by the time the next stimulus arrives. In the last stages of fatigue, contracture passes off. Fig. 146.-Effect of temperature on a single muscular contraction; N, normal; H, warm ; Ci, cooling; C2, very cold; P, point of stimulation. The above tracing is a con- siderably reduced fac-simile of a tracing taken with the pendulum myograph. 4. Effect of temperature.-Cold at first increases the height of contrac- tion, then diminishes it ; otherwise the effect is very like that of fatigue increasing the duration of all stages of the curve. Moderate warmth increases the height and diminishes the duration of all stages of the curve, latent period included. This may be Fig. 147.-Veratrine curve, taken on a very slowly-travelling cylinder; the time tracing indicates seconds. ch. x.] THE MUSCLE-WAVE. 125 readily shown by dropping some warm salt solution* on to the muscle before taking its curve. Too great heat (above 40° c.) induces heat rigor due to the coagulation of the muscle-proteids. 5. Effect of veratrine.-If this is injected into the frog before the muscle- nerve preparation is made, the very remarkable result seen in the preceding diagram is produced on stimulation ; there is an enormous prolongation of the period of relaxation ; marked by a secondary rise, and sometimes by tremors. After repeated stimulation this effect passes off, but returns after a period of rest. The Muscle-Wave. The first part of a muscle which contracts is the part where the nerve fibres enter; but nerve impulses are so rapidly carried to all the fibres that for practical purposes they all contract together. But in a nerveless muscle, that is one rendered physio- Fig-. 148.-Arrangement for tracing the muscle-wave. (McKendrick.) logically nerveless by curare, if one end of the muscle is stimu- lated, the contraction travels as a wave of thickening to the other end of the muscle, and the rate of propagation of this wave can be recorded graphically. The above figure (fig. 148) represents one of the numerous methods that have been devised for this purpose. A muscle with long parallel fibres, like the sartorius, is taken ; it * Physiological saline solution used for bathing living tissue is a 0.65 per cent, solution of sodium chloride. 126 MUSCULAR CONTRACTION. [ch. x. is represented diagrammatically in the figure. It is stimulated at the end, where the two wires, + and -, are placed ; it is grasped in two places by pincers, which are opened by the wave of thickening ; the opening of the first pair of pincers (1) presses on a drum or tambour connected to a second tambour with a re- cording lever (1'), and this lever goes up first; the lever (2') of the tambour connected with the second pair of pincers (2) goes up later. If the length of muscle between the pairs of pincers is measured, and by a time-tracing the delay in the raising of the second lever is ascertained, we have the arithmetical data for calculating the rate of propagation of the muscle wave. It is about 3 metres per second in frog's muscle, but is hastened by warmth and delayed by cold and fatigue. The Effect of Two successive Stimuli. If a second stimulus follows the first stimulus, so that the muscle receives the second stimulus before it has finished con- Fig. 149. -Tracing of a double muscle-curve. To be read from left to right. While the muscle was engaged in the first contraction (whose complete course, had nothing inter- vened, is indicated by the dotted line), a second induction-shock was thrown in, at such a time that the second contraction began just as the first was beginning to decline. The second curve is seen to start from the first, as does the first from the base line. (M. Foster.) tracting under the influence of the first, a second curve will be wadded to the first, as shown in the next diagram. The third little •curve is only due to elastic after-vibration. This is called super- position, or summation of effects. If the two stimuli are in such close succession that the second occurs during the latent period of the first, the result will differ according as the stimuli are maximal or submaximal. If they •are maximal, the second stimulus is without effect; but if sub- CH. X.] TETANUS. 127 maximal, the two stimuli are added together, and though pro- ducing a simple muscle curve, produce one which is bigger than either would have produced separately. This is summation of stimuli. Effect of More than Two Stimuli. Just as a second stimulus adds its curve to that written as the result of the first, so a third stimulus superposes its effect on the Fig. 150.-Curve of tetanus, obtained from the gastrocnemius of a frog, where the shocks were sent in from an induction coil, about sixteen times a second, by the interruption of the primary current by means of a vibrating spring, which dipped into a cup of mercury, and broke the primary current at each vibration. (Tracing to be read right to left.) second ; a fourth on the third, and so on. Each successive incre- ment is, however, smaller than the preceding, and at last the muscle remains at a maximum contraction, till it begins to relax from fatigue. Fig'. 151.-Curve of tetanus, from a series of very rapid shocks from a magnetic interrupter. (Tracing to be read right to left.) A succession of stimuli may be sent into the nerve of a nerve- muscle preparation by means of the Wagner's hammer of a coil, or the vibrating reed previously described (p. 117). This method of stimulation is called faradisation. Figs. 150 and 151 show 128 MUSCULAR CONTRACTION. [CH. X; the kind of tracings one obtains. The number of contractions corresponds to the number of stimulations; the condition of pro- longed contraction so produced, the muscle never relaxing com- pletely between the individual contractions of which it is made up, is called tetanus : incomplete tetanus, or clonus, when the individual contractions are discernible (fig. 150); complete tetanus, as in fig. 151, when the contractions are completely fused to form a continuous line without waves. The rate of faradisation necessary to cause complete tetanus varies a good deal; for frog's muscle it averages 15 per second ; for the pale-muscles of the rabbit, 20 per second ; for the more slowly contracting red muscles of the same animal, 1 o per second ; and for the extremely slowly contracting muscles of the tortoise 2 per second is enough. With fatigue, the rate necessary to pro- duce complete tetanus is diminished. Voluntary Tetanus. We have seen that voluntary muscles under the influence of artificial stimuli may be made to contract in two ways; a single excitation causes a single contraction ; a rapid series of excitations causes a series of contractions which fuse to form tetanus. We now come to the important question, in which of these two ways does voluntary muscle ordinarily contract in the body 1 The answer to this is, that voluntary contraction is always a tetanus, never a twitch. The nerve-cells from which the motor fibres originate do not possess the power of sending isolated impulses to the muscles ; they send a series of impulses which result in a muscular tetanus*, or voluntary tetanus, as it may conveniently be termed. If a stethoscope is placed over any muscle of the human body, such as the biceps, a low sound is heard. The tone of this sound, which was investigated by Wollaston, and later by Helmholtz, corresponds to thirty-six vibrations per second; this was regarded as the first overture of a note of eighteen vibrations per second, and for a long time 18 per second was believed to be the rate of voluntary tetanus. The so-called "muscle sound" is, however, no indication of the rate of muscular vibration. Any irregular sound of low intensity will produce the same note ; it is, in fact, the natural resonance- * The use of the word tetanus in physiology must not be confounded with the disease known by the same name, in which the most marked symptom is an intense condition of muscular tetanus or cramp. CH. X.] VOLUNTARY TETANUS. 129 tone of the membrana tympani of the ear, and, therefore, selected by the organ of hearing when we listen to any irregular mixture of low tones and noises. A much more certain indication of the rate of voluntary tetanus is obtained by the graphic method. The myographs hitherto described are obviously inapplicable to the investigation of such a problem in man. The instrument employed may be termed a transmission myograph. The next figure shows the recording part of the apparatus. It is called a Marcy's Tambour. It consists of a drum, on the membrane of which is a metallic disc fastened to one end of a lever, the far extremity of which carries a writing point. The interior of the drum is connected by an india-rubber tube (seen at the right hand end of the drawing) to a second tambour called Screw to regulate elevation of lever. Writing lever. Tambour. Tube to receiving tambour. Fig. 152.-Marey's Tambour, to which the movement of the column of air in the first tambour is conducted by a tube, and from which it is communicated by the ' lever to a revolving cylinder, so that the tracing of the movement of the impulse beat is obtained. the receiving tambour, in which the writing lever is absent. Now if the receiving tambour is held in the hand, and the thumb presses on the metallic disc on the surface of its membrane, the air within it is set into vibrations of the same rate as those occurring in the thumb muscles ; and these are propagated to the recording tambour and are written in a magnified form by the end of the lever on a recording travelling surface. The tracing obtained is very like that in fig. 150; it is an incomplete tetanus, which by a time marker can be seen to be made up of 1 o to 1 2 vibrations a second. In some diseases these tremors are much increased, as in the clonic convulsions of epilepsy, or those produced by strychnine poisoning, but the rate is the same. Similar tracings can be obtained in animals by strapping the receiving tambour on the surface of a muscle, and causing it to 130 MUSCULAR CONTRACTION. [ch. x. contract by stimulating the brain or spinal cord. The rate of stimulation makes no difference • however slow or fast the stimuli occur, the nerve-cells of the central nervous system give out impulses at their own normal rate. The same is seen in a reflex action. If a tracing is taken from a frog's gastrocnemius, the muscle being left in connection with the rest of the body, its tendon only being severed and tied to a lever, and if the sciatic nerve of the other leg is cut through, and the end attached to the spinal cord is stimulated, an impulse passes up to the cells of the cord, and is then reflected down to the gastrocnemius, under observation. The impulse has thus to traverse nerve-cells ; the rate of stimulation then makes no difference : the reflex contraction occurs at the same rate, i o or 12 per second. But now a difficulty arises; if a twitch only occupies of a second, there would be time for ten complete twitches in a second ; they would not fuse to form even an incomplete tetanus. There must be some means by which each individual contraction can be lengthened till it fuses with the next contraction ; or, in other words, our results of electrical stimulation of excised muscles, must not be applied without reserve to the contraction of the intact muscles in the living body in response to the will. Lever Systems.-The arrangement of the muscles, tendons, and bones present examples of the three systems of levers which will be known to anyone who has studied mechanics; the student of anatomy will have no difficulty in finding examples of all three systems in the body. What is most striking is that the majority of cases are levers of the third kind, in which there is a loss of the mechanical power of a lever, though a gain in the rapidity and extent of the movement. Most muscular acts involve the action of several muscles, often of many muscles. The acts of walking and running are examples of very complicated muscular actions in which it is necessary not only that many muscles should take part, but also must do so in their proper order and in due relation to the action of auxiliary and antagonistic muscles. This harmony in a complicated mus- cular action is called co-ordination. By the device of taking instantaneous photographs at rapidly repeated intervals during a muscular act, the details of different modes of locomotion in man and other animals have been very thoroughly worked out. With this branch of research the name of Prof. Marey is intimately associated. CH. XI.] EXTENSIBILITY OF MUSCLE. 131 CHAPTER XL EXTENSIBILITY, ELASTICITY, AND WORK' OF MUSCLE. Muscle is both extensible and elastic. It is stretched by a weight, that is, it possesses extensibility ; when the weight is taken off, it returns to its original length, that is, it possesses elasticity. The two properties do not necessarily go together; thus a piece of putty is very extensible, but it is not elastic ; a piece of steel or a ball of ivory are only slightly extensible, but after the stretching force has been removed they return to their original size and shape very perfectly. A substance is said to be strongly elastic, when it offers a great resistance to external forces : steel and ivory are strongly elastic. Elastic Band Muscle Fig. 153.-(After Waller.) A substance is said to be perfectly elastic, when its return to its original shape is absolute; again steel and ivory may be quoted as examples. Muscle is very extensible, i.e., it is easily stretched ; it is feebly elastic, i.e., it opposes no great resistance to external force ; it is, however, very perfectly elastic ; that is, it returns to its original shape very exactly after stretching. This is true in the case of living muscle within the body, but after very great stretching 132 EXTENSIBILITY, AND WORK OF MUSCLE, [ch. xi. even in the body, and still more so after removal from the body when it begins to undergo degenerative changes culminating in death, its elasticity is less perfect. The cohesion of muscular tissue is less than that of tendon. E. Weber stated that a frog's muscle one centimetre square in transverse section will support a weight of a kilogramme (over 2 lbs.) without rupture, but this diminishes as the muscle gradually dies. The extensibility of any material may be studied and recorded by measuring the increase of length which occurs when that material is loaded with different weights. In figure 139 showing Helmholtz' myograph, different weights may be placed in the scale pan beneath the muscle, and the increase of length recorded on a stationary blackened cylinder by the downward movement of the writing point; the cylinder may then be moved on a short distance, more weight added, and the additional increase of length similarly recorded, and so on foi' a succession of weights. If this experiment is done with some non-living substance like a steel spring or a piece of india-rubber instead of a living muscle, it is found that the amount of stretching is proportional to the weight; a weight - 2 produces an extension twice as great as that produced by a weight = 1 ; in this way one obtains a tracing like that seen on the left hand of figure 153, and the dotted line drawn through the lowest points of the extensions is a straight one. With muscle, however, this is different; each successive ad- dition of the same weight produces smaller and smaller incre- ments of extension, and the dotted line obtained is a curve. A continuous curve of extensibility may be obtained by placing a gradually and steadily increasing force beneath the muscle instead of a succession of weights added at intervals. The most convenient way of doing this is to use a steel spring which is gradually and steadily extended ; and the writing point inscribes its excursion on a slowly moving cylinder. If, then, after the muscle has been stretched, the steel spring is gradually and steadily relaxed, the muscle relaxes and again writes a curve now in the reverse direction, until it regains its original length.* But in muscles removed from the body, unless they arc very slightly loaded, the return to the original length is never complete ; the muscle is permanently longer to a slight extent, which varies with the amount of the previous loading. * A mathematical examination of these curves shows that they are not rectangular hyperboles, as'they were once considered. They are very vari- able in form and cannot be identified with any known mathematical curve.' CH. XI.] EXTENSIBILITY OF MUSCLE. 133 If the muscle is slowly loaded and slowly unloaded the curva- ture of its tracing is much more marked than if the experiment be done rapidly. The following three tracings are reproduced from some obtained by Dr. Brodie. In the method used, the curves are not com- plicated by the curve of a lever, but the movement was simply magnified by a beam of lighu falling on a mirror attached to the end of the muscle, and reflected on to a travelling photographic plate. Each tracing is to be read from right to left; the first one (A) shows the result of stretching a steel spring by a steadily increasing force ; the end of the spring gets lower and lower, and de- scribes a straight line ; at the apex of the tracing- unloading began and went g on steadily till the spring once more regained its initial length. The up- stroke, like the downstroke, is a straight line. In B and C muscles were used ; it will be noticed that the q muscle does not regain its original length after unload- ing, and that after unload- ing the upward tendency of the tracing represents after-relaxation. In B, the extension was applied rapidly, the tracing is almost a straight line ; in C, the extension was brought about more slowly, and the tracing is a curve; in both cases the tracing of the period of unloading shows more curvature. This introduces us to what is called after extension and after relaxation. That is to say, after a muscle is weighted there is an immediate elongation, followed by a gradual elongation which continues for some time; or if a muscle has been weighted and is then unloaded there is an immediate slackening, followed by a gradual after relaxation. This may be shown by looking at the graphic records shown in the next diagram. It will be noticed that the extension is Fig. 154.-Curves of extensibility. 134 EXTENSIBILITY, AND WORK OF MUSCLE. [CH. XI. greatest when the muscle is in a contracted condition, and smallest when it is dead (in rigor). In fatigue the after extension is very marked, and the return after unloading very imperfect. We may now give the results of an actual experiment; a frog's gastrocnemius was loaded with successive weights of 50, 100, 150, etc., grammes, and its length carefully measured in centi- metres. Load .... 50 100 150 200 250 300 Total extension . . 3-26 8 9'5 10 10'3 Increment of extension - 2'8 2 15 0'5 0'3 Figure 155 shows that the contracted muscle is more exten- sible than the uncontracted muscle. This may be still further illustrated by an example given on the opposite page in the form of a diagram. The thick lines represent the contracted muscle, the thin ones the uncontracted. It is repre- sented as being stretched by dif- ferent weights indicated along the top line ; and the lengths under the influence of these weights are separated by equal distances. Thus A C represents the length of the uncontracted muscle, A B of the contracted muscle when unloaded. A' C' and A7 B' the same under the in- fluence of a weight of 50 grammes, and so on. The curve connecting the ends of the lengths of the contracted muscle falls faster than that obtained from the uncontracted one, until at the point P under the influence of a weight of 250 grammes, the two curves meet; that is to say, 250 grammes is the weight which the muscle is just unable to lift. Suppose a muscle has to lift the weight of 200 grammes, it begins with a length A" C7/, but when it con- tracts it has a length A" B7Z, that is, it has contracted a distance In rigor r In tetanus ■ Normal ■ Fatigued - Fig. 155.-Extensibility of muscle in different states; tested by 50 grammes applied for short periods. Tracings to be read from left to right. (After Waller.) CH. XI.] EXTENSIBILITY OF MUSCLE. 135 of B" Cv, which is very small; when it has to lift a less weight it contracts more, when a greater weight it contracts less ; till when it contracts least it lifts the greatest weight. This experiment illustrates the general truth that when a muscle is contracted it is more extensible. At the point P the energy tending to shorten the muscle (its contractile power) is exactly equal to the energy tending to lengthen it against its elastic force. Thus we have the apparent paradox at this point that a muscle when contracted has exactly the same length as when uncontracted; but this is a matter of everyday experience ; if one tries to lift a weight beyond one's strength, one fails to raise it, but nevertheless one's muscles have been contracting in the effort; they have not contracted in the narrow sense of 0 50 100 150 200 250 Contracted Uncontracted- Fig. 156. becoming shorter, but that is not the only change a muscle undergoes when it contracts ; the other changes, electrical, ther- mal, chemical, etc., have taken place, as evidenced in one's own person by the fact that the individual has got warm in his efforts, or may even feel fatigue afterwards. But the paradox does not end here, for if diagram 156 is again looked at, it will be seen that beyond the point P the two curves cross ; in other words, the muscle may even elongate due to increase of extensibility when it contracts. This is known after its discoverer as Weber's paradox. Influence of Temperature on Extensibility.-If a piece of iced india-rubber is taken and stretched by a weight, its retractility when the weight is removed is very small. If, now, when the weight is on it, it is warmed at one point as by placing the hand on it, its retractility is increased and it contracts, raising the 136 EXTENSIBILITY, AND WORK OF MUSCLE. [CH. XI. weight. Some physiologists have considered that muscular con- traction can be explained in this way ; they have supposed that the heat formed in muscular contraction acts like warmth as applied to india-rubber. This view is, however, very far-fetched. It is much more probable that there is no causal relationship between the temperature-change and the extensibility-change which occur when muscle contracts; both are simultaneously produced by a common cause, called a stimulus. Moreover, the influence of heat on muscle is by no means the same as that on india-rubber. This influence is not invariable, and at certain temperatures near the freezing-point, and under the influence of certain weights, actual elongation may occur when the temperature is raised. Muscular Tonus. In the living animal muscles are more or less stretched, but never taut between their two attachments. They are in a state of tonicity or tonus, and when divided they contract and the two parts separate. Thus a muscle, even at rest, is in a favourable condition to contract without losing time or energy in taking in slack. Muscular tonus is under the control of the nervous system; the muscles lengthen when their nerves are divided, or when they are rendered physiologically nerveless by curare. Besides the nervous system, the state of muscular nutrition dependent on a due supply of healthy blood must also be reckoned as important in maintaining muscular tonus. The question of muscular work is intimately associated with that of elasticity. In a technical sense, work (W) is the product of the load (/) and the height (A) to which it is raised. W = I X h. Thus in fig. 156, when the muscle is unloaded the work done is m'Z: W = BC x 0 = 0. When the load is 250, again the work done is nil, because then h = 0. With the load 50, W = 50 X B'C'. If the height is measured in feet and the load in pounds, work is expressed in terms of foot-pounds. If the height is measured in millimetres or metres, and the load in grammes, the work is expressed in gramme-millimetres or gramme-metres respectively. This may be shown diagrammatically by marking on a hori- zontal base line or abscissa distances proportionate to different weights, and vertical lines (ordinates) drawn through these would represent the height to which they are lifted (see fig. 157). Work of Muscle. CH. XI.] MUSCULAR WORK. 137 In the diagram the figures along the base line represent grammes, and the figures along the vertical line represent milli- metres. The work done as indicated by the first line is 10X5 = 50 gramme-millimetres, the next 20x6 = 120 gramme-millimetres, Fig. 157.-Diagram to show the mode of measuring muscle work. (McKendriek.) and so on, while the last on the right, 100X3 = 300 gramme- millimetres. It is thus seen that the height of a muscle curve is no measure of the work done by the muscle unless the weight lifted is taken into account as well. The following figures are taken from an actual experiment done with the frog's gastrocnemius (Weber):- Weight lifted. Height. Work done. q grammes 27'6 millimetres 138 gramme-millimetres 15 25-1 376 >• 25 ii'45 „ 286 30 7'3 219 „ The work increases with the weight up to a certain maximum, after which a diminution occurs, more or less rapidly, according as the muscle is fatigued. Similar experiments have been made in human beings, weights being lifted by the calf muscles, or elbow muscles, leverage being allowed for. In the higher animals the energy so obtained com- pared to those of the frog is about twice as great for the same volume of muscular tissue. Fig. 158 represents a common form of dynamometer for clinical use, employed in testing the muscles of the arms and Fig'. 158.-Dynamometer. 138 EXTENSIBILITY, AND WORK OF MUSCLE. [CH. xi hands. It is squeezed by the hand, and an index represents kilo- grammes of pressure. The muscle, regarded as a machine, is sometimes compared to artificial machines like a steam-engine. A steam-engine is sup- plied with fuel, the latent energy of which is transformed into work and heat. The carbon of the coal unites with oxygen to form carbonic acid, and it is in this process of combustion or oxidation that heat and work are liberated. Similar, though more complicated, combustions occur in muscle. In a steam- engine a good deal of fuel is consumed, but there is great economy in the consumption of the living muscular material. Take the work done by a gramme (about 15 grains) of muscle in raising a weight of 4 grammes to the height of 4 metres (about 13 feet); in doing this work probably less than a thousandth part of the muscle has been consumed in the process. Next let us consider the relationship between the work and the heat produced. An ordinary locomotive wastes about 96 per cent, of its available energy as heat, only 4 per cent, being represented as work. In the best triple-expansion steam-engine the work done rises to 12-5 per cent, of the total energy. In muscle, various experimenters give different numbers. Thus, Fick calculated that 33 per cent, of the mechanical energy is available as work; later he found this estimate too high, and stated the number as 25 ; Chauveau gives 12 to 15 ; McKendrick 17. Thus muscle is a little more economical than the best steam-engines; but the muscle has this great advantage over any engine, for the heat it produces is not wasted, but is used for keeping up the body temperature, which, if it fell below a certain point, would lead to death not only of the muscles but of the body generally. CH. XII.] ELECTRICAL PHENOMENA OF MUSCLE. 139 CHAPTER XII. THE ELECTRICAL PHENOMENA OF MUSCLE. We have seen that the chemical processes occurring in muscular contraction lead to a transformation of energy into work and heat. These changes are accompanied with electrical disturbances also. The history* of animal electricity forms one of the most fascinating of chapters in physiological discovery. It dates from 1786, when Galvani made his first observations. Galvani was Professor of Anatomy and Physiology at the University of Bologna, and his wife was one day preparing some frogs' legs for dinner, when she noticed that the apparently dead legs became convulsed when sparks were emitted from a frictional electrical machine which stood by. Galvani then wished to try the effect of lightning and atmospheric electricity on animal tissues. So he hung up some frogs' legs to the iron trellis-work round the roof of his house by means of copper hooks, and saw that they con- tracted whenever the wind blew them against the iron. He imagined this to be due to electricity secreted by the animal tissues, and this new principle was called Galvanism. But all his friends did not agree with this idea, and most prominent among his opponents was Volta, Professor of Physics at another Italian university, Pavia. He considered that the muscular con- tractions were not due to animal electricity, but to artificial electricity produced by contact with different metals. The controversy was a keen and lengthy one, and was ter- minated by the death of Galvani in 1798. Before he died, how- ever, he gave to the world the experiment known as " contraction without metals," which we shall study presently, and which con- clusively proved the existence of animal electricity. Volta, how- ever, never believed in it. 1 n his hand electricity took a physical turn, and the year after Galvani's death he invented the Voltaic pile, the progenitor of our modern batteries. Volta was right in maintaining that galvanism can be produced independently of animals, but wrong in denying that electrical currents could be * For a full and interesting account of this subject the reader is referred to Professor McKendrick's " Text-book of Physiology," Vol. I., chap, xviii. The account in the text is mainly a brief summary of this chapter. 140 THE ELECTRICAL PHENOMENA OF MUSCLE. [CH. XII. obtained from animal tissues. Galvani was right in maintain- ing the existence of animal electricity, but wrong in supposing that the contact of dissimilar metals with tissues proved his point. This conclusion has been arrived at by certain new methods of investigation. In 1820 Oersted discovered electro-magnetism: that is, when a galvanic current passes along a wire near a magnetic needle, the needle is deflected one way or the other, according to the direction of the current. This led to the inven- tion of the astatic needle and the galvanometer, an instrument by which very weak electrical currents can be detected. For a long time the subject of animal electricity, however, fell largely into disrepute, because of the quackery that grew up around it. It is not entirely free from this evil nowadays; but the scientific in- vestigation of the subject has led to a considerable increase of knowledge, and among the names of modern physiologists associa- ted with it must be particularly mentioned those of Du Bois Reymond and Hermann. Before we can study these it is, however, necessary that we should understand the instruments employed. The Galvanometer.-The essential part of a galvanometer is a magnetic needle suspended by a delicate thread ; a wire coils Tig- i.'9- Fig. 160. round it; and if a current flows through the wire, the needle is deflected. Suppose a man to be swimming with the current with his face to the needle, the north pole is turned to the left hand. But such a simple instrument as that shown in fig. 159 would not detect the feeble currents obtained from animal tissues. It is necessary to increase the delicacy of the apparatus, and this is done in several ways. In the first place, the needle must be rendered astatic, that is, independent of the earth's magnetism. The simplest way of doing this is to fix two needles together (as CH. XII. THE GALVANOMETER. 141 shown in fig. 160), the north pole of one pointing the same way as the south pole of the other. The current is led over one needle and then over the other; the effect is to produce a deflection in each in the same direction, and so the sensitiveness of the instrument is doubled. If now the wire is coiled not only once, but twice or more in the same position, each coil has its effect on the needles ; the effect of multiplying a weak current in this way is accomplished' in actual galvanometers by many hundreds of turns of fine wire. Kg. 161. - .Reflecting galvanometer. (Thomson.) A. The galvanometer, which consists of two sys- tems of small astatic needles suspended by a fine hair from a support, so that each set of needles is within a coil of fine insulated copper wire, that forming the lower coil is wound in an opposite direction to the upper. Attached to the upper- set of needles is a small mirror about i inch in diameter; the light from the lamp at B is thrown upon this little mirror, and is reflected upon the scale on the other side of B, notshown in figure. The coils u I are arranged upon brass uprights, and their ends are carried to the binding screws. The whole apparatus is placed upon a vulcanite plate capable of being levelled by the screw supports, and is covered by a brass-bound glass shade, the cover of which is also of brass, and supports a brass rod b, on which moves a weak curved magnet m. C is the shunt by means of which the amount of the current sent into the galvanometer may be regulated. When in use the scale is placed about three feet from the galvano- meter, which is arranged east and west, the lamp is lighted, the mirror is made to swing, and the light from the lamp is adjusted to fall upon it, and it is then regulated until the reflected spot of light from it falls upon the zero of the scale. The wires from the non-polarisable electrodes touching the muscle are attached to the outer binding screws of the galvanometer, a key intervening for short circuiting, or if a portion only of the current is to pass into the galvanometer, the shunt should intervene as well with the appropriate plug in. When a current passes into the galvanometer the needles and, with them, the mirror, are turned to the right or left according to the direction of the current. The amount of the deflection of the needle is marked on the scale by the spot of light travelling along it. 142 THE ELECTRICAL PHENOMENA OF MUSCLE. [CH. XII. Fig. 161 illustrates the best galvanometer: that of Sir William Thomson (now Lord Kelvin). It is called a reflecting galvanometer, because the observer does not actually watch the moving needle, but a spot of light reflected from a little mirror, which is attached to and moves with the needle. A very small movement of the needle is rendered evident, because the move- ment of the spot of light being, as it were, at the end of a long lever-namely, the beam of light, magnifies it. Non-polarisable Electrodes.-If a galvanometer is connected with a muscle by wires which touch the muscle, electrical currents would be obtained in the circuit which are set up by the contact of metal with muscle. The currents so obtained would be no evidence of electro-motive force in the muscle itself. It is there- fore necessary that the wires from the galvanometer should have interposed between them and the muscle some form of electrodes which are non-polarisable. Fig. 162 shows one of the earliest non-polarisable electrodes of Du Bois Reymond. It consists of a zinc trough on a vulcanite base. The inner surface of the trough is amalgamated and nearly filled with a saturated solution of zinc sulphate. Into the trough is placed a cushion of blotting-paper, which projects over the edge of the trough ; on it there is a pad of china clay or kaolin, moistened with physiological salt solution (o-6per cent. NaCl); on this pad one end of the muscle rests. The binding screw (/•) connects the instrument to the galvanometer; the other end, or the middle of the same muscle, is connected by another non-polarisable electrode in the same way to the other side of the galvanometer. If there is any electrical dif- ference of potential (that is, difference in amount of positive or negative electricity) between the two parts of the muscle thus led off, there will be a swing of the galvanometer needle ; the galvano- meter detects the existence and direction of any current that occurs. Fig. 163 shows a more convenient form of non-polarisable electrodes. Fig. 162.-Non-polarisable elec- trode of Du Bois Keymond. (McKcndrick.) In order to measure the strength (electromotive force) of such currents, the mere amount of swing of the needle is only a very rough indication, and in accurate work the following arrangement must be used (fig. 164). The electromotive force is usually measured in terms of a standard Daniell cell. The two surfaces of the muscle (M) are connected to a galvanometer (B) ; the needle swings, and then a fraction of a Daniell cell is introduced in the reverse direction so as to neutralise the muscle current, and bring CH. XII.] NON-POLARISABLE ELECTRODES. 143 back the needle to rest. From the Daniel K, wires pass to the ends a, b of a long platinum wire of high resistance, called the compensator ; c is a slider on this wire ; a and c are connected to the galvanometer, the com- Fig. 163.-Diagram of Du Bois Reymond's non-polarisable electrodes, a, glass tube filled with a saturated solution of zinc sulphate, in the end, c, of which is china clay drawn out to a point; in the solution a well amalgamated zinc rod is immersed and con- nected, by means of the wire a, with the galvanometer. The remainder of the apparatus is simply for convenience'of application. The muscle and the end of the second electrode are to the right of the figure. mutator C enabling the observer to ensure that the current from the Daniell passes in the opposite direction to that produced by the muscle. If the slider c is placed at the end b of the compensator, the whole strength of the Daniell will be sent through the galvanometer and will more than Fig. 164.-Arrangement for measuring the electromotive force of muscle. (McKendrick.) neutralise the muscle current ; if c is half way between a and J, half the Daniell's strength will be sent in ; but this is also too much ; ac will be found to be only quite a small fraction of ab ; and this fraction will correspond to a proportional fraction of the electromotive force of the Daniel cell. Lippmann'3 Capillary Electrometer.-This instrument is often used 144 THE ELECTRICAL PHENOMENA OF MUSCLE. [CH. XII. instead of the galvanometer, It consists of a glass tube drawn out at one end to a fine capillary and filled with mercury. It is connected to an apparatus by which the pressure on this mercury can be lowered or increased. Fig. 163.-Lippmann's Capillary Electrometer. 1. Pressure apparatus and microscope on stand of which the capillary tube is fixed. 2. Capillary tube, fixed in outer tube containing 10 per cent, sulphuric acid : the platinum wires are also shown. 3. Capillary and column of mercury as seen in the field of the microscope. Fig. 166.-Frog's heart. Diphasic variation. Simultaneous photograph of a single beat (upper black line), and the accompanying electrical change indicated by the level of the black area, which shows the varying level of mercury in a capillary electrometer. (Waller.) C'U. Xll.j THE RHEOTOME. 145 The capillary is enclosed within another tube filled with 10 per cent, sul- phuric acid. Two platinum wires fused through the glass, pass respectively into the mercury and the acid, the other ends of these wires are connected by electrodes to two portions of the surface of a muscle. The capillary Fig. 167.-Human heart. Diphasic variation, ee, and simultaneous cardiogram, co. Time tt is marked in second. The lead offs to the capillary electrometer were from the mouth to the sulphuric acid, and from the left foot to the mercury. (Waller.) tube is observed by a microscope ; the surface of the mercury is in a state of tension which is easily increased or diminished by variations of electrical potential, and the mercury moves in the direction of the negative pole. If the shadow of the mercurial column is thrown upon a travelling sensi- tive photographic plate, photographs are obtained which show the electrical Fig. 168.-Scheme of a Rheotome. (Waller.) variations in a living tissue in a graphic manner. The instrument is exceed- ingly sensitive and its indications are practically instantaneous. Figs. 166 and 167 indicate the kind of result one obtains with the heart, which will be more fully discussed when we are considering that organ. The Rheotome.-This is an instrument by means of which the time of 146 THE ELECTRICAL PHENOMENA OF MUSCLE. [CH. XII. the occurrence of electrical disturbances in relation to the contraction of a muscle can be determined. This is in principle effected by a revolving bar carrying two contacts, one in the primary or exciting circuit (1. 1, 1, 1), one in the galvanometer circuit (2, 2, 2, 2). The bar revolves, and by making or breaking the primary circuit, sends an induction shock into the nerve at the same instant. The muscle is connected by non-polarisable electrodes to the galvano- meter ; this circuit includes the brass blocks 2, 2, on the disc over which the bar revolves, and a compensator not shown in the figure to neutralise any current set up by the muscle in a state of rest. If an electrical change occurs in the muscle, it is only noticed by the galvanometer if at the same time the bar on its revolution connects the two brass blocks on the disc, and so completes the circuit. The apparatus can be set so that the bar makes the primary contact (1, 1) simultaneously with the galvanometer contacts, or that the galvanometer contact is made, 1, 2, 3, &c. hundredths of a second later than the primary contact. If the two are closed simultaneously the electrical condition of the muscle is tapped off at the moment of excitation ; if the galvanometer contact is closed jU, &c. second after excitation, the electrical condition of the muscle at that particular instant is ascertained. By a number of experiments with different intervals between the making of the two contacts, one ascertains how long after the excitation the change in the electrical condition of the muscle takes place. We can now pass on to a consideration of results. In muscles that are removed from the body, it is found that on leading off two parts of their surface to a galvanometer, that the Fig. 169.-Diagram of the currents in a muscle prism. (Du Bois Reymond.) galvanometer needle generally swings. The most marked result is obtained with a piece of muscle in which the fibres run parallel to one another, and the longitudinal surface is connected with one of the cut ends by a wire (2 in fig. 169). On the course of the wire a galvanometer indicates that a current flows from the centre to the cut end outside the muscle, and from the cut end to the centre inside the muscle. If, now, the muscle is made to contract, the needle returns more or less completely to the position of rest. Du Bois Reymond, who first described these facts, called the CH. XII.] CURRENTS OF REST AND ACTION. 147 first current the current of rest, and the second current, which occurs at the instant of commencing contraction, the current of action ; the change in direction being indicated by the expression negative variation ; this means that the current of action is in the opposite direction to the current of rest, and therefore lessens or neutralises it. Du Bois Reymond explained this by supposing that a muscular fibre is built up of molecules, each of which is positive in the centre and negative at both ends. So when a muscle is cut across, a number of the negative ends of these molecules is exposed. On contraction the difference between the centre and ends of each molecule is lessened, and the resultant effect on the whole muscle (made up of such molecules) is similar. There is no doubt about the facts as described by Du Bois Reymond. We now adopt, however, an entirely different view of their meaning : in causing this revolution of ideas the principal part has been played by Herrmann. The new idea is that the so-called current of rest does not exist; it is really a current produced by injury, and is now generally called a demarcation current : the more the ends of the muscle are injured the more negative they become; and when they are connected to the uninjured centre, a current naturally is set up as described by Du Bois Reymond. If a muscle is absolutely uninjured it is iso- electric ; that is, it gives no current at all when two parts of it are connected together by a wire. We may put the main conclusions concerning this subject in the form of a number of propositions:- 1. Uninjured muscle at rest is iso-electric. 2. Dead muscle is iso-electric. 3. But dead muscle is negative to uninjured living muscle. 4. Dying muscle, i.e., injured muscle, is also negative to un- injured muscle ; thus we get a demarcation current on connecting the uninjured to the injured part of a muscle. 5. Not only are dying and dead portions of muscles negative to uninjured resting muscle, but the same is true for con- tracting muscle at the moment it begins to contract. 6. When a muscle removed from the body contracts, the previously positive part in the centre contracts most because it is least injured ; hence, the electrical condition of the centre approaches that of the injured ends; hence, the demarcation current is diminished ; thus, Du Bois Reymond's negative variation is accounted for. .7. In a curarised muscle, the wave of contraction (see p. 125) is accompanied by a wave of increased negativity travelling 148 THE ELECTRICAL PHENOMENA OF MUSCLE, [ch. Ail. at the same rate. This is followed very rapidly by a return to the original condition; hence the change is often spoken of as a diphasic one. 8. The electrical change in a contracting muscle accompanies the commencement of the other changes. This has been ascertained by the use of the capillary electrometer, which confirms the earlier experiments made with the rheotome. The electrical change lasts only a few thousandths of a second, and is over long before the other changes in form, Ac. are completed. Prof. Burdon Sanderson gives the following numbers from experiments with the frog's gastrocnemius. When the muscle is excited through its nerve, the electrical response begins TqVo the change of form second after the stimulation ; the second phase, that is, the return to the previous condition, begins twIjw second aftei' excita- tion. When the muscle is directly excited, the latent period is much shorter, the change in form beginning y-yVo and the electrical change in less than a second after excitation. Muscle is not the only tissue which exhibits electrical phenomena. A nerve which is uninjured is iso-electric; injury causes a demar- cation current; activity is accompanied with a similar diphasic wave travelling along the nerve simultaneously with the nervous impulse. The activity of secreting glands, and also of the retina, is accompanied with electrical changes of the same kind. But the most prominent exhibition of animal electricity is seen in the electric organs of electric fishes. In some of these fishes the electric organ is modified muscle, in which a series, as it were, of hypertrophied end plates correspond to the plates in a voltaic pile. In other fishes the electric organ is composed of modified skin glands. But in each case the electric discharge is the principal phenomenon that accompanies activity. The Rheoscopic Frog. The electrical changes in muscle can be detected by a much simpler instrument than the galvanometer or electrometer. This is known as the physiological rheoscope, and consists of an ordinary muscle nerve preparation from a fresh and vigorous frog. The nerve is stimulated by the electrical changes occurring in muscles, and the nervous impulse so generated causes a contrac- tion of the muscles of the rheoscopic preparation. The following are the principal experiments that can be shown in this way :- CH. XII.] THE RHEOSCOPIC FROG. 149 i. Contraction without metals. If the nerve of a nerve-muscle preparation A is dropped upon another muscle B (or upon its own muscle) it will be stimulated by the injury current of the muscle on which it is dropped, and lead to a contraction of the muscle which it supplies. The experiment succeeds best if the nerve is Fig. 170.-Galvani's experiment without metals. dropped across a longitudinal surface and a freshly made trans- verse section. 2. Secondary contraction. This is caused by the current of action. If, while the nerve of A is resting on the muscle B, the latter is made to contract by the stimulation of its nerve; the nerve of A is stimulated by the electrical variation which accom- panies the contraction of the muscle B, and so a contraction Fig. 171.-Secondary contraction, of muscle A is produced. This is called secondary contraction. It may be either a secondary twitch or secondary tetanus, accord- ing as to whether the muscle B is made to contract singly or tetanically. 3. Secondary contraction from the heart. If an excised but still beating frog's heart is used instead of muscle B, and the nerve of A laid across it, each heart's beat, accompanied as it is by an electrical variation, will stimulate the nerve and cause a twitch in the rheoscopic muscle A., 150 CHANGES IN MUSCLE. [CH. XIII, CHAPTER XIII. In muscular contraction there is a transformation of the poten- tial energy of chemical affinity into other forms of energy, espe- cially work and heat. Heat is a form of motion, in which there is movement of molecules ; work is a form of motion in which there is movement of masses. The fact that when a blacksmith hammers a piece of iron it becomes hot is a familiar illustration of the transformation of one mode of movement into the other. Heat is measured in heat-units or calories. One calorie is the energy required to raise the temperature of i gramme of water from o° to i°C.; and this in terms of work is equal to 425'5 gramme-metres; that is, the energy required to raise the weight of 425'5 grammes to the height of 1 metre. A muscle when uncontracted is nevertheless not at absolute rest, We have already seen that it possesses tonus or tone; it also possesses what we may call chemical tone; that is, chemical changes are occurring in it, and consequently heat is being pro- duced. But when it contracts, the liberation of energy is increased; work is done, and more heat is produced; the heat produced represents more of the energy than the work done. The more resistance that is offered to a muscular contraction, the more is the work done relatively increased and the heat diminished. The amount of heat produced is increased by increasing the tension of the muscle. It diminishes as fatigue comes on. On increasing the strength of the stimulus the amount of heat increases faster, proportionately, than the work performed. If work is done by a few large contractions, more heat is pro- duced than if the same work is done by a larger number of smaller contractions ; that is, more chemical decomposition occurs, and fatigue ensues more rapidly in the first case. This fact is within the personal experience of everyone. If one ascends a tower, the work done is the raising of the weight of one's body to the top of the tower. If the staircase in the tower has a gentle slope, each stair being low, far less fatigue is experienced than if one ascended to the same height by a smaller number of steeper steps. On a cold day one keeps oneself warm by muscular exercise; this common fact is confirmed by more accurate experiments on THERMAL AND CHEMICAL CHANGES IN MUSCLE. CH. XIII.] THERMAL CHANGES. 151 isolated muscles, the heat produced being sufficient to raise tempo- rarily the temperature of the muscle. This can be shown in larger animals by inserting a thermometer between the thigh muscles and stimulating the spinal cord. The rise of temperature may amount to several degrees. In the case of frog's muscles, Helmholtz found that, after tetanising them for two or three minutes, the temperature rises 0'14° to o'i8° 0.; and for each single twitch Heidenhain gives a rise of temperature of from o'ooi0 to 0'005° For the detection of such small rises in temperature a thermo- pile, and not a thermometer, is employed. A thermopile consists of a junction of two different metals; the metals are connected by wires to a galvanometer. If the junction is heated an electrical current passes round the circuit, and is detected by the galvanometer. The metals usually employed are b4>A a«-b b<-a a«-«-<-b b-»->~»a 7 Couple. 2 Couples. 3 Couples. Fig. 172.-Scheme of thermo-electric couples. (After Waller.) iron and German silver, or antimony and bismuth. If the number of couples in the circuit is increased, each is affected in the same way, and thus the electrical current is increased through the galvanometer. The arrangement is shown in the fig. 172, which also indicates the direction of the currents produced, the metals employed being antimony and bismuth. By using 16 couples of this kind Helmholtz was able to detect a change of Tinnr a degree Centigrade. Within certain limits, the strength of the current is directly proportional to the rise of temperature at the junction. If two couples are in circuit, as shown in the second diagram, and they are heated equally, no current will pass through the galvanometer, the current through one couple being opposed by the current through the other. But if the two couples are heated unequally, the direction of swing of the galvanometer needle indicates which is the warmer. To apply this to the frog's gastro- cnemius, plunge several needle-shaped couples (diagram 3) into a frog's gastrocnemius of one side and the same number of couples into the gastrocnemius of the other side, and then excite first one then the other sciatic nerve, a deflection of the galvanometer will 152 CHANGES IN MUSCLE. fCH. XIII. be observed first in one, then in the other direction, indicating the production of heat first on one side, then on the other. Chemical Changes in Muscles. The chemical changes which are normally occurring in a resting muscle are much increased when it contracts. Waste products of oxidation are discharged, and the most abundant of these is carbonic acid. Sarco-lactic acid is also produced, and the alkaline reaction of a normal muscle is replaced by an acid one. The muscles of animals hunted to death are acid ; the acid reaction to litmus paper of a frog's gastrocnemius can be readily shown after it has been tetanised for 10 to 15 minutes. The quantity of oxygen consumed is increased, but the con- sumption of oxygen will not account for the much greater increase in the discharge of carbonic acid. This is illustrated by the following table :- Venous Blood. 0, less than Arterial Blood. CO 2 more than Arterial Blood. Of resting muscle... 9 per cent. 671 per cent. Of active muscle ... 12'26 per cent. 1079 per cent. Indeed, a muscle can be made to contract and give off oxidation products like carbonic acid in an atmosphere containing no oxygen at all. The oxygen used is thus stored up in the muscle pre- viously. Herrmann has supposed that the oxygen enters into the formation of a complex hypothetical compound he calls inogen. On contraction he considers this is broken up into carbonic acid, lactic acid, and a proteid residue of myosin. There are other chemical changes in the muscle when it contracts-namely, a change of glycogen into sugar, and an increase of nitrogenous waste. The question whether urea is in- creased during muscular activity is, however, a much debated one, and we shall return to it when we are studying the urine. What is certain is that the increased consumption of carbon (possibly in large measure derived from the carbohydrate stored in the muscle) is a much more marked and immediate feature than an increase in the consumption of nitrogen, CH. XIII.] FATIGUE. 153 Fatigue. If the nerve of a nerve-muscle preparation is continually stimulated, the muscular contractions become more prolonged (see p. 124), smaller in extent, and finally cease altogether. The muscle is said to be fatigued : this is due to the consump- tion of the substances available for the supply of energy in the muscle, but more particularly to the accumulation of waste products of contraction; of these, sarco-lactic acid is probably the principal one. Fatigue may be artificially induced in a muscle by feeding it on a weak solution of lactic acid, and then removed by washing out the muscle with salt solution containing a minute trace of an alkali. If the muscle is left to itself in the body, the blood stream washes away the accumulation of acid products, and fatigue passes off. The question next presents itself, where is the seat of fatigue ? Is it in the nerve, the muscle, or the end-plates ? If, after fatigue has ensued and excitation of the nerve of the preparation produces no more contractions, the muscle is itself stimulated, it contracts ; this shows it is still irritable, and, therefore, not the seat of fatigue. If an animal is poisoned with curare, and it is kept alive by artificial respiration, excitation of a motor nerve produces no con- traction of the muscles it supplies. If one goes on stimulating the nerve for many hours, until the effect of the curare has disappeared, the block at the end-plates is removed and the muscles contract: the seat of exhaustion is therefore not in the nerves. By a process of exclusion it has thus been localised in the nerve-endings. When the muscle is fatigued in the intact body, there is, how- ever, another factor to be considered beyond the mere local poisoning of the end-plates. This is the effect of the products of contraction passing into the circulation and poisoning the central nervous system. It is a matter of common experience that one's mental state influences markedly the onset of fatigue and the amount of muscular work one can do. This aspect of the question has been specially studied by Mosso; he invented an instrument called the ergograph. The arm, hand, and all the fingers but one are fixed in a suitable holder; the free finger repeatedly lifts a weight over a pulley, and the height to which it is raised is registered by a marker on a blackened surface. By the use of this instrument he has arrived at the conclusion that the state of the brain and central nervous system generally js a iiiost important factor in fatigue, and that the fatigue products 154 CHANGES IN MUSCLE. [ch. xiii. produced in the muscles during work cause some of their injurious effects by acting on the central nervous system and diminishing its power of sending out impulses. Rigor Mortis. After the muscles of the dead body have lost their irritability or capability of being excited to contract by the application of a stimulus, they spontaneously pass into a contracted condition. It affects all the muscles of the body; and, when external circum- stances do not prevent it, commonly fixes the limbs in that which is their natural posture of equilibrium or rest. Hence, and from the simultaneous contraction of all the muscles of the trunk, is produced a general stiffening of the body, constituting rigor mortis or post-mortem rigidity. When this condition has set in, the muscle (a) becomes acid in reaction (due to development of sarco-lactic acid), (6) gives of carbonic acid in great excess, and (c) becomes shortened and opaque. Rigor comes on much more rapidly after muscular activity, and is hastened by warmth. The immediate cause of rigor is a chemical one-namely, the coagulation of the muscle plasma. We may distinguish three main stages-i. Gradual coagulation. 2. Contraction of coagulated muscle-clot (myosin), and squeezing out of muscle- serum. 3. Putrefaction. It has been noticed that the relaxation in muscles after rigor sometimes occurs too quickly to be caused by putrefaction, and the suggestion that in such cases at any rate such relaxation is due to a ferment-action is very plausible. It is known that pepsin is present in muscle, and that this ferment will act in an acid medium. The conditions for the solution of the coagulated myosin are therefore present as the reaction of rigored muscle is acid. Order of Occurrence.-The muscles are not affected simul- taneously by rigor mortis. It affects the neck and lower jaw first; next, the upper extremities, extending from above down- wards ; and lastly, reaches the lower limbs; in some rare in- stances only, it affects the lower extremities before, or simul- taneously with, the upper extremities. It usually ceases in the order in which it begins : first at the head, then in the upper extremities, and lastly in the lower extremities. It never com- mences earlier than ten minutes, and never later than seven hours after death ; and its duration is greater in proportion to CH. XIII.] CHEMISTRY OF MUSCLE. 155 the lateness of its accession. Heat is developed during the pas- sage of a muscular fibre into the condition of rigor mortis. Since rigidity does not ensue until muscles have lost the capacity of being excited by external stimuli, it follows that all circumstances which cause a speedy exhaustion of muscular irri- tability, induce an early occurrence of the rigidity, while con- ditions by which the disappearance of the irritability is delayed, are succeeded by a tardy onset of this rigidity. Hence its speedy occurrence, and equally speedy departure in the bodies of persons exhausted by chronic diseases; and its tardy onset and long continuance after sudden death from acute diseases. In some cases of sudden death from lightning, violent injuries, or paroxysms of passion, rigor mortis has been said not to occur at all; but this is not always the case. It may, indeed, be doubted whether there is really a complete absence of the post-mortem rigidity in any such cases; for the experiments of Brown-Sequard make it. probable that the rigidity may supervene immediately after death, and then pass away with such rapidity as to be scarcely observable. The occurrence of rigor mortis is not prevented by the previous existence of paralysis in a part, provided the paralysis has not been attended with very imperfect nutrition of the muscular tissue. Chemical Composition of Muscle. The phenomena of igor mortis will be more intelligible if we consider the chemical composition of muscle. The connective tissue of muscle resembles connective tissue elsewhere : the gelatin and fat obtained in analyses of muscle are derived from this tissue. The sarcolemma is composed of a sub- stance which resembles elastin in its solubilities. The contractile substance within the muscular fibres is, during life, of semi-liquid consistency, and contains a large percentage of protcids and smaller quantities of extractives and inorganic salts. By the use of a press this substance can be squeezed out of per- fectly fresh muscles, and it is then called the muscle-plasma. After death, muscle-plasma, like blood-plasma, coagulates (thus causing the stiffening known as rigor mortis). The solid clot corresponding to the fibrin from blood-plasma is called myosin, and the liquid residue is called the muscle-serum. Pursuing the analogy further, it is found that the coagulation of both muscle-plasma and blood-plasma can be prevented by cold, by strong solutions of neutral salts, and by potassium oxalate, 156 CHANGES IN MUSCLE. [CH. XIII. which precipitates, as the insoluble oxalate of calcium, the lime salts essential for the coagulation process. In both cases the clotting is produced by the action of a ferment developed after death. In both cases the precursoi' of the solid clot is a proteid of the globulin class which previously existed in solution. Fibrin in the blood-clot is formed from the previously soluble fibrinogen of the blood-plasma. Myosin in the muscle-clot is formed from the previously soluble myosinogen * of the muscle- plasma. When the blood-clot contracts it squeezes out blood- serum ; when the muscle-clot contracts it squeezes out muscle- serum. The muscle-serum contains small quantities of albuminous material, together with the extractives and salts of the muscle. The origin of the sarco-lactic acid is a controversial question ; some consider it originates from the carbohydrate (glycogen and sugar); others think it comes from the proteid molecules in the muscle. The general composition of muscular tissue is the following :- Water . . . . .75 per cent. Solids . . . 25 ,, Proteids . . . . 18 „ Gelatin . . . . Fat .... 2 to 5 Extractives . . . . 0'5 „ Inorganic salts . . . 1 to 2 ,, The proteids, as already stated, chiefly pass into the clot: very little is found in the muscle-serum. The extractives comprise a large number of organic substances, all present in small quantities, some of which are nitrogenous, like creatine, creatinine, xanthine, and hypoxanthine; the rest are non-nitrogenous-namely, fats, glycogen, sugar, inosite, and the variety of lactic acid known as sarco-lactic acid. The inorganic salts are chiefly salts of potassium, especially potassium phosphate. The condition of dead muscle reminds one something of con- tracted muscle. Indeed, the similarity is so striking that Herr- mann has propounded the idea that contracted muscle is muscle on the road to death, the differences between the two being of degree only. He considers that, on contraction, inogen (see p. 152) is broken up into carbonic acid, sarco-lactic acid, and myosin ; on death the same change occurs, only to a much more marked extent. * The myosin precursors are really two in number, paramyosinogen, which is coagulated by heat at 470-50°, and myosinogen, which is coagulated by heat at 56° C, Off. xiii.j INVOLUNTARY MUSCLE. 157 This idea is a far-fetched one, but it is a useful reminder of the similarities of the two cases. In chemical condition, contracted and dead muscle are alike, so far as the formation of acid products is concerned; there is, however, no evidence of any formation of a muscle-clot (myosin) during the contraction of living muscle, as there is in dead muscle. Then heat is produced in both cases, and in both cases also the muscle is negatively electrical to un- contracted muscle. Here, however, the analogy must end : for living contracted muscle is irritable, dead muscle is not. Living contracted muscle is more extensible than uncontracted muscle; muscle in rigor mortis is not so (see fig. 153, p. 134). CHAPTER XIV. COMPARISON OF VOLUNTARY AND INVOLUNTARY MUSCLE. The main difference between voluntary and involuntary muscle is the difference expressed in their names. Voluntary muscle is under the control of that portion of the central nervous system the activity of which is accompanied by conscious volition. Involuntary muscle on the other hand, is, as a rule, also under the control of the central nervous system, but of a portion of the central nervous system the activity of which is independent of volition. There appear, however, to be exceptions to this rule, and the involuntary muscle executes its contractions independently of nervous control; that is to say, it is sometimes in the truest sense of the term really involuntary. This is very markedly seen in the developing heart of the embryo, which begins to beat before any nerve fibres have grown into it from the central nervous system. Another characteristic of involuntary muscle is a tendency to regular alternate periods of rest and activity, or rhythmicality. This is best exemplified in the heart, but it is also seen in the lymphatic vessels, especially the lymph hearts of the frog, and the mesenteric lymphatic vessels (lacteals) of many animals. It is seen in the veins of the bat's wing, and in the muscular tissue of the spleen. A third characteristic of involuntary muscle is peristalsis. If 158 VOLUNTARY AND INVOLUNTARY MUSCLE. [CH. XIV. any point of a tube of smooth muscle such as the small intestine is stimulated, a ring-like constriction is produced at this point. After lasting some time at this spot it slowly passes along the tube in both directions at the rate of 20 to 30 millimetres per second. This advancing peristaltic wave normally takes place in only one direction, and so serves to drive on the contents of the tube. Involuntary muscle nearly always contains numerous plexuses of non-medullated nerve-fibres with ganglion cells; so that much discussion has taken place on the question whether the phenomena of rhythmicality and peristalsis are properties of the muscular tissue itself or of the nerves mixed with it. The evidence available (namely, that portions of muscular tissue entirely free from nerves act in the same way as those that possess nerves), indicates that it is the muscular rather than the nervous tissues that possess these properties ; though it can hardly be doubted that under usual circumstances the contraction of involuntary muscle is influenced and controlled by nervous agency. The artificial stimuli employed for smooth muscle are the same as those used for striated muscle; single induction shocks are often ineffectual to produce contraction, but the make, and to a less extent the break of a constant current will act as a stimulus. The faradic current is a good stimulus, but it never throws involuntary muscle into tetanus; in the heart, strong stimulation will sometimes effect a partial fusion of the beats, but never complete tetanus. The rate of stimulation makes no difference; in fact, very often a rapid rate of stimulation calls forth less rapidly occurring contractions than a slow rate. A stimulus strong enough to produce a contraction at all usually elicits a maximum contraction, but the phenomenon known as the staircase (see p. 124) is generally better marked in the case of the heart than in that of voluntary muscle. The contraction of smooth muscle is so sluggish that the various stages of latent period, shortening and relaxation can be followed with the eye ; the latent period often exceeds half a second in duration. The normal contraction of voluntary muscle is a tetanus (voluntary tetanus); the normal contraction of plain muscle is a much prolonged single contraction. A very valuable piece of evidence in this direction is seen in the experiment on the heart with the physiological rhcoscope (see p. 149). Each time the heart contracts the rheoscopic preparation executes a single twitch, not a tetanus. This is an indication that the electrical CH. XIV.] INVOLUNTARY MUSCLE. 159 change is a single one, and not a succession of changes such as would occur in tetanus. When, however, this single electrical change is examined with the electrometer, it is seen that it really is a diphasic one. It is, however, only different in degree from the change which produces the current of action in a voluntary muscle. If a voluntary muscle is stimulated at one end, a wave of contraction travels along it to the other. Suppose two points of the muscle (a) and (6) are connected by non-polarisable electrodes to a galvanometer; as soon as the wave of contraction reaches (a), this point becomes negative to (6), and therefore a current flows from (&) to (a). A moment later the two points are equipotential, and no current flows ; a thousandth of a second later this balance is upset, and now (5) is negative to (c) and the galvanometer needle moves in the opposite direction. The varia- tion is here also diphasic ; but in a slowly contracting tissue like the heart the two phases are separated by a prolonged period of equipotentiality, and thus they are rendered more distinct. The illustrations already given (figs. 166 and 167) show this fact graphically. But though involuntary muscle cannot be thrown into tetanus, it has the property of entering into a condition of sustained contraction called tonus. We shall have to consider this ques- tion again in connection with the plain muscular tissue of the arterioles. Involuntary muscle when it contracts undergoes thermal and chemical changes similar to those we have dealt with in the case of the voluntary muscles. The nerve-endings in involuntary muscle require a much larger dose of curare to affect them than the end-plates in voluntary muscle. The phenomena of rigor mortis in involuntary muscle have never been fully studied. What has been found is that the chemical composition of involuntary muscle differs in no note- worthy manner from that of voluntary muscle, and on death the muscle becomes acid; such products as carbonic acid and sarco- lactic acid are formed. In the heart, stomach and uterus rigidity has been noted, but in the case of the other involuntary muscles it has never been satisfactorily observed. 160 PHYSIOLOGY OF NERVE. [cfi. Yv. CHAPTER XV. PHYSIOLOGY OF NERVE. Many points relating to the physiology of nerve have been already studied in connection with muscle. But there still remain further questions upon which we have hardly touched as yet. Classification of Nerves. The nerve fibres which form the conducting portions of the nervous system may be classified into three main groups, according to the direction in which they normally conduct nerve impulses. These three classes are 1. Efferent nerve fibres. 2. Afferent nerve fibres. 3. Inter-central nerve fibres. 1. Efferent or centrifugal nerves are those which conduct impulses from the central nervous system (brain and spinal cord) to other parts of the body. When for instance there is a wish to move the hand, the impulse starts in the brain, and travels a certain distance down the spinal cord ; it leaves the spinal cord by one of the spinal nerves, and so reaches the muscles of the hand which are thrown into contraction. This is called a motor nerve, but all efferent nerves are not motor, some cause secretion instead of movement, and others may cause a stoppage of movement, etc. A list of the efferent nerves is as follows :- a. Motor. b. Accelerator. c. Inhibitory. d. Secretory. e. Electrical. f. Trophic. a. Motor nerves. Some of these go to voluntary muscles; others to involuntary muscles, such as the vaso-motor nerves which supply the muscular tissue in the walls of arteries. b. Accelerator nerves are those which produce an increase in the rate of rhythmical action. An instance of these is seen in the sympathetic nerves that supply the heart. ch. xv.] CLASSIFICATION OF NERVES. 161 c. Inhibitory nerves are those which cause a slowing in the rate of rhythmical action, or it may be its complete cessation. Inhibitory nerves are found supplying many kinds of in- voluntary muscle j a very typical instance is found in the inhibitory fibres of the heart which are contained within the trunk of the vagus nerve.* d. Secretory nerves are found supplying many secreting glands, such as the salivary glands, pancreas, gastric glands, and sweat glands. The impulse which travels down a secretory nerve causes a formation of the secretion in the gland it supplies. e. Electrical nerves are found in the few fishes which possess electrical organs. The impulse which travels down these nerves causes the electrical organ to be thrown into activity. /. Trophic nerves are those which control the nutrition of the part they supply. 2. Afferent or centripetal nerves are those which conduct impulses in the reverse direction, namely from all parts of the body to the central nervous system. When one feels pain in the finger, the nerves of the finger are stimulated, an impulse travels up the nerves to the spinal cord, and then to the brain. The mental process set up in the brain is called a sensation; the sensation, however, is referred to the end of the nerve where the impulse started, and the sensation of pain does not appear to occur in the brain, but in the finger. This is an instance of a sensory nerve ; and the terms afferent and sensory mean very nearly the same thing. The nerves of sensation may be grouped as follows :- a. The nerves of special sense, sight, hearing, taste, smell and touch. b. The nerves of general sensibility, that is of a vague kind of sensation not referable to any of the five special senses just enumerated ; as instances we may take the vague feelings of comfort or discomfort in the interior of the body. c. Nerves of pain. These do not appear to be anatomically distinct from the others, but any excessive stimulation of a sensory nerve whether of the special or general kind will cause pain. The words " sensory " and " afferent," however, are not quite * The question has been much debated whether voluntary muscle is pro- vided with inhibitory nerves ; they do, however, appear to be present in certain nerves supplying the muscles of the claws of lobsters and similar crustaceans. 162 PHYSIOLOGY OF NERVE. [ch. xv. synonymous. Just as we may have efferent impulses leaving the brain for the heart or blood-vessels of which we have no conscious knowledge, so also afferent impulses may travel to the central nervous system which excite no conscious feelings. The afferent nerves to the cerebellum form a very good instance of these. Then, too, the excitation of many afferent nerves will excite what are called reflex actions. We are very often conscious of the sensations that form the cause of a reflex action, but we do not necessarily have such sensations. Many reflex actions, for instance, occur during sleep; many may be executed by the spinal cord even after it has been severed from the brain, and so the brain cannot be aware of what is occurring. A reflex action is an action which is the result of an afferent impulse. Thus a speck of dust falls into the eye, and causes movements of the eyelids to get rid of the offending object. The dust excites the sensory nerve-endings of the conjunctiva, an impulse travels to the centre of this nerve in the brain, and from the brain a reflected impulse travels to the muscles of the eyelid. As an instance of a reflex action in which secretion is concerned, take the watering of the mouth which occurs when food is seen or smelt. The nerves of sight or smell convey an afferent impulse to the brain, which reflects, down the secretory nerves, an impulse which excites the salivary glands to activity. < These, however, are instances of reflex action which are accompanied with conscious sensation, but like all pure reflex actions are not under the control of the will. An instance of a reflex action not accompanied with conscious- ness is seen in a man with his spinal cord cut across or crushed, so that any communication between his brain and his legs is impossible. He cannot move his legs voluntarily and is un- conscious of any feelings in them. Yet when the soles of his feet are tickled he draws his legs up, the centre of reflex action being in the grey matter of the lower region of the spinal cord. For a reflex action, three things are necessary : (i) an afferent nerve, (2) a nerve-centre consisting of nerve-cells to receive the afferent impulse and send out an efferent impulse, and (3) an efferent nerve along which the efferent impulse may travel. If the reflex action is a movement, the afferent nerve is called excito-motor ; if it is a secretion, the afferent nerve is called excito- secretory, and similarly afferent nerves may also be excito-accelera- tor, excito-inhibitory, etc. CH. XV.] SECTION AND STIMULATION OF NERVE. 163 3. Intercentral nerves are those which connect nerve-centres together; they connect different parts of brain, and of the cord to one another, and we shall find in our study of the nerve-centres that they are complex in their arrangement. Investigation of the Functions of a Nerve. There are always two main experiments by which the function of a nerve may be ascertained. The first is section, the second is stimulation. Section consists in cutting the nerve and observing the loss of function that ensues. Thus, if a motor nerve is cut, motion of the muscles it supplies can no longer be produced by activity of the nerve-centre; the muscle is paralysed. If a sensory nerve is cut, the result is loss of sensation in the part it comes from. Stimulation of the cut nerve is the opposite experiment. When a nerve is cut across, one piece of it is still connected with the brain or spinal cord; this is called the central end; the other piece, called the peripheral end, is still connected with some peripheral part of the body. Both the central and the peripheral end should be stimulated ; this is usually done by means of induction shocks. In the case of a motor nerve, stimulation of the central end produces no result; stimulation of the peripheral end produces a nervous impulse which excites the muscles to con- tract. In the case of a sensory nerve, stimulation of the peripheral end has no result, but stimulation of the central end causes a sensation, usually a painful one, and reflex actions, which are the result of the sensation. When a nerve is cut across, there are other results than the loss of function just mentioned; and even though the nerve is still left within the body with a normal supply of blood, it becomes less and less irritable, till at last it ceases to respond to stimuli altogether. This diminution of excitability starts from the point of section and travels to the periphery, but is tem- porarily preceded by a wave of increased excitability travelling in the same direction (Ritter-Valli law). This loss of excitability of nerve is accompanied with degen- erative changes which are of so great importance as to demand a separate section. Degeneration of Nerve. Suppose a nerve is cut right across, the piece of the nerve left in connection with the brain or spinal cord remains healthy both 164 PHYSIOLOGY OF NERVE. [ch. xv. in structure and functions ; but the peripheral piece of the nerve loses its functions and undergoes what is generally called after the discoverer of the process, Wallerian degeneration. A nerve is made up of nerve-fibres, and each nerve-fibre is essentially a branch of a nerve-cell; when the nerve is cut the axis cylinders in the peripheral portion are separated from the cells of which they are Fig. 173.-Degeneration and Regeneration of nerve-fibres, a, nerve-fibre, fifty hours after operation, m y, medullary sheath breakingup into myelin drops, p, granular proto- plasm replacing myelin, n, nucleus, g, primitive sheath, b, nerve-fibre after four days, cy, axis cylinder partly broken up and enclosed in portions of myelin, c, a moi e advanced stage in which the medullary sheath has almost disappeared. Numerous nuclei, n" are seen, n, commencing regeneration ; several fibres (z', Z") have sprouted from the somewhat bulbous cut end (i) of the nerve, a, an axis cylinder which has not yet acquired its medullary sheath, s, s' primitive sheath of the original fibre. (Ranvier.) branches, and from which they have grown. These separated portions of the axis cylinders die, and the mednllary sheath of each undergoes a gradual process of disintegration into droplets of myelin, which are ultimately absorbed and removed by the lymphatics. At the same time there is a multiplication of the nuclei of the primitive sheath. This degenerative process begins CH. XV.] ROOTS OF THE SPINAL NERVES. 165 two or three days after the section has been made. In the case of the non-medullated fibres, there is no medullary sheath to exhibit the disintegration changes just alluded to ; and the nuclei of the sheath do not multiply ; there is simply death of the axis cylinder. The degeneration occurs simultaneously throughout the whole extent of the nerve; it does not start from the section and travel to the periphery. A great amount of attention has been directed to this process of degeneration, because it has formed a valuable method of research in tracing nervous tracts, and ascertaining the nerve-cells from which they originate. It must not, however, be regarded as an isolated phenomenon in physiology; it is only an illustration of the universal truth that any portion of a cell (in this case the axis cylinder process) cut off from the nucleus of the cell degen- erates and dies. If a nerve is simply cut, and allowed to heal, regeneration of function in time occurs. This is hastened by the surgeon suturing the cut ends of the nerve together. It must not, how- ever, be supposed that this is due to a restoration of the structure of the fibres in the peripheral portions of the cut nerve. It is due to new nerve-fibres sprouting out from the central end of the cut nerve, and growing distalwards in the old sheaths. This is illustrated in D, fig. 173. Functions of the Roots of the Spinal Nerves. The general truths enunciated in the two preceding sections are well illustrated by the experiments made to determine the functions of the roots of the spinal nerves. Each spinal nerve originates from the spinal cord by two roots. One of these is called the anterior or ventral root : it consists of nerve-fibres which originate from the large multipolar cells in that portion of the grey matter in the interior of the spinal cord which we shall presently learn to call the anterior horn. These nerve-fibres are all medullated ; the large ones join up with the posterior root to form the spinal nerve; the small nerve-fibres leave the root an pass to the sympathetic chain, whence they are distributed as non-medullated fibres to the involuntary muscular fibres of the blood-vessels, etc.* * Recent researches indicate that in some animals like frogs, some of these small fibres for the supply of involuntary muscles leave the cord by the posterior roots. 166 PHYSIOLOGY OF NERVE. [ch. xv. The other root, the posterior or dorsal root, has upon it a col- lection of nerve-cells forming the spinal ganglion. Each nerve- cell is enclosed within a nucleated sheath of connective tissue origin, and it is from these nerve-cells that the fibres of the posterior roots grow. In the embryo, each nerve-cell has two processes (fig. 174, a), one of which grows to the spinal cord, where it terminates by branching around the multipolar cells of the grey matter; the other process grows outwards to the peri- phery. In the adult the two processes coalesce in the first part of their course, forming a T-shaped junction. The first experiments on the functions of the spinal nerve-roots were performed in this country by Sir Charles Bell (1811), and in France by Magendie (1822). These observers found that on section of the anterior roots there resulted paralysis of the muscles supplied by the nerves; on section of the posterior roots there was loss of sensation. These experiments clearly pointed to the conclusion that the an- terior roots contain the efferent (motor) fibres; and the posterior roots the afferent (sensory) fibres. This conclusion was confirmed by the experiment of stimula- tion. Stimulation of the peripheral end of the cut anterior root caused muscular movement; of the central end, no effect. Stimulation of the central end of the cut posterior root caused pain and reflex movements; of the peripheral end, no effect. Recurrent sensibility.-One of the statements just made requires a slight modification; namely, excitation of the peripheral end of a divided anterior root will evoke pain and reflex movements, as well as direct movements ; that is to say the anterior root though composed mainly of motor fibres contains a few sensory fibres coming probably from the membranes of the spinal cord, and Fig. 174.-a, Bipolar cell from spinal ganglion of a 4| weeks embryo (after His). n, nucleus; the arrows indicate the direction in which the nerve processes grow, one to the spinal cord, the other to the periphery, b, a cell from a spinal ganglion of the adult; the two processes have coalesced to form a T-shaped junction. CH. XV.] WALLERIAN DEGENERATION. 167 then running into the posterior root with the rest of the sensory fibres. They often, however, run down the mixed nerve a con- siderable distance before returning to the posterioi' roots. Degeneration of roots.-The facts in connection with this sub- ject were made out by Waller (1850), and may be best understood by referring to the next diagram. A represents a section of the mixed nerve beyond the union of the roots ; the whole nerve beyond the section degenerates, and is shaded black. B represents the result of section of the anterior root; only the anterior root-fibres degenerate; the sensory fibres of the posterior root remain intact. The small mcdullated nerve-fibres (not shown in the diagram) also degenerate as far as the ganglion cells of the sympathetic system with which they communicate. The recur- rent sensory fibres in this root do not degenerate with the others, but are found degenerated in the part of the anterior root still attached to the spinal cord. Section of the posterior root always produces the same physiological effect (loss of sensation) wherever the section is made, but the degeneration effect is different according as the section is made on the proximal or distal side of the ganglion. This is illustrated by diagrams C and D. If the section is made beyond the ganglion the degeneration occurs as shown in C beyond the section in the peripheral portion of the posterior root-fibres; the anterior root remains intact except for the recurrent sensory fibres which it contains. If the section is made as in D, between the ganglion and the cord, the only piece that degenerates is the piece severed from the ganglion and running into the cord; these fibres may be traced up in the posterior columns of the spinal cord until they terminate in grey matter which they do at different levels. The whole of the sensory fibres including the recurrent ones which are still attached to the ganglion remain histologically healthy. Fig. 175.-Diagram to illustrate Wallerian de- generation of nerve-roots. 168 PHYSIOLOGY OF NERVE. [ch. xv. The next figure is one of the original illustrations made by Dr. Waller, and not published until the publication of his son's text- book quite recently. I am indebted to the present Dr. Waller for permission to reproduce it. These facts of degeneration teach us, what we also learn from the study of embryology, that the nerve-fibres of the anterior root are connected to the nerve-cells within the spinal cord, while the posterior root-fibres are connected to the cells of the spinal ganglia; or to put it another way, the trophic centres which control the nutrition of the nerve-fibres is situated within the cord for the anterior roots, and within the spinal ganglia for the posterior roots. Changes in a Nerve during Activity. When a nerve is stimulated, the change produced in it is called a nervous impulse ; this change travels along the nerve, and the propagation of some change is evident from the effects which follow : sensation, movement, secretion, &c.; but in the nerve itself very little change can be detected. There is no change in form ; the most delicate thermo-piles have failed to detect any production of heat, and we are also ignorant of any chemi- cal changes. The only alteration which can be detected as evidence of this molecular change in a nerve is the electrical one. Healthy nerve is iso-electric, but during the passage of nervous impulse along it there is a wave of momentarily increased negativity, which travels at the same rate as the nervous impulse. This can be detected by the instruments already described in connection with muscle. Velocity of a Nerve Impulse. A nervous impulse is not electricity ; compared to electricity its rate of propagation is extremely slow. It has been measured in motor-nerves as follows : a muscle-nerve preparation is made with as long a nerve as possible ; the nerve is stimulated first as near to the muscle, and then as far from the muscle as possible. Fig. 176.- Groups of fibres from the anterior and posterior roots several days after sec- tion of both roots close to the cord; the anterior fibres are degenerated ; the posterior, being still in connection with the nerve-cells from which they grew, are normal. CH. XV.] NERVE IMPULSES. 169 The moment of stimulation and the moment of commencing con- traction is measured by taking muscle tracings on a rapidly moving surface in the usual way, with a time-tracing beneath. The con- traction ensues later, when the nerve is stimulated at a distance from the muscle, than in the other case, and the difference in the two cases gives the time occupied in the passage of the impulse along the piece of nerve, the length of which can be easily measured. A similar experiment can be performed on man by means of the transmission myograph (see p. 129). If a tracing of the contrac- tion of the thumb muscles is taken, the two stimuli may be successively applied through the moistened skin, first at the brachial plexus below the clavicle, and secondly, at the median nerve at the bend of the elbow. Another method, largely employed by Bernstein, is to take the electrical change as the indication of the impulse. The rheotome is the instrument used. If fig. 168 is referred to, and a dong nerve substituted for the muscle-nerve preparation, the stimulus is applied at one end, and the change in the electrical condition of the nerve is recorded by the galvanometer, which is connected to the other end of the nerve. The time measurement is effected by the adjustment of the rheotome, which must be such as to tap off the electrical change at the moment it occurs. The rate of the transmission of nervous impulses discovered by these methods is, in a frog's motor-nerve, 28 to 30 metres a second; in human motor-nerves, 3 3 metres a second ; in sensory nerves, 30 to 33 metres a second Direction of a Nerve Impulse. Nerve impulses are conducted normally in only one direction : in efferent nerves to, in afferent nerves from the nerve-centres. But there are some experiments which point to the conduction occurring under certain circumstances in both directions. Thus, in the rheotome experiment just described, if the nerve is stimulated in the middle instead of at one end, the electrical change (the evidence of an impulse) is found to be conducted towards both ends of the nerve. Kuhne's gracilis experiment proves the same point. The gracilis muscle of the frog (fig. 177) is in two portions, with a tendinous intersection, and supplied by a nerve that divides into two bundles; excitation strictly limited to one of these bundles causes both portions of the muscle to contract. 170 PHYSIOLOGY OF NERVE. [ch. xv. Older experiments, designed to prove the same point, were per- formed by Paul Bert. He grafted the tip of a rat's tail either to the back of the same rat, or to the nose of another. When union had been effected, the tail was amputated near its base. After a time, irritation of the end of the trunk-like appendage on the back or nose of the rat gave rise to sensation. The impulse thus passed from base to tip, instead of from tip to base, as formerly. This experiment does not, however, prove the point at all; for all the original nerve- fibres in the tail must have degenerated, and the restoration of sensation was due to new fibres, which had grown into the tail. Exactly the same objection holds to another series of experiments, in which the motor and sensory nerves of the tongue were divided and united crosswise. Restoration of both movement and sensation does occur, but is owing to new nerve-fibres growing out from the central stumps of the cut nerves. Fig. 177.-Gracilis of frog. Chemistry of Nervous Tissues. The nervous tissues contain a large amount of water • it is present in larger amount (85 to 90 per cent.) in grey matter than in white matter (about 70 per cent.); in early than in adult life; in the brain than in the spinal cord ; in the spinal cord than in nerves. The solids contain :- a. Proteids : these comprise about half the solid matter in grey matter, and about one-third of the solid matter in white matter and nerve. In other words, proteid is most abundant where the cells are situated, which is what one would expect. The proteids found are nucleo-proteid and globulin. b. Albuminoids : (1) neuro-keratin contained within the white substance of Schwann, and forming the chemical basis of neuroglia, the supporting tissue of the nerve-centres; (2) nuclein from the nuclei of the cells. c. Fatty materials : the most important of these is lecithin, a complex fat containing phosphorus and nitrogen, in addition to carbon, hydrogen and oxygen. d. Cerebrins : nitrogenous substances of unknown constitution which yield a reducing sugar (galactose) on hydration. CH. XV. ] ELECTROTONUS. 171 e. Cholesterin : a crystalline alcohol which we shall study more fully in connection with bile, where it is also found. f. Extractives, similar to those found in muscle, but in very minute quantity. g. Gelatin and fat from the adherent connective tissue. h. Inorganic salts. The following table gives some of the quantitative analyses that have been made of the solids in percentages :- Portion of Nervous System. Pro- teids. Lecithin. Choleste- rin and Fat. Cere- brin. Neuro- keratin. Other organic matters. Salts. - - - - Grey matter 1 17 °'5 i'5 of Brain ... J 55 19 0 7 White matter 1 9'5 o*6 of Brain ...J 25 IO 52 3 3 < - - < Spinal Cord 23 75* I'l Human Sciatic 1 / - -- - X Nerve J 36 32 12 II 3 4 Nothing is known of the changes these undergo during the activity of nerve. When nervous tissues die, they, like muscles and all organs of the body, become acid from the development of lactic acid. In Wallerian degeneration, the staining reactions indicate that the lecithin, the principal constituent of the medullary sheath, is replaced by ordinary fats. But this has not been proved by actual chemical analysis. CHAPTER XVI. ELECTROTONUS. When a constant current is thrown into a nerve, there is an excitation which leads to a nervous impulse producing muscular contraction of the muscle at the end of the nerve. Similarly, there is another contraction when the current is taken out. While the current is flowing through the nerve, the muscle is quiescent. But while the current is flowing there are changes in the nerve, both as regards its electrical condition and its 172 ELECTROTONUS. [CH. XVI excitability. These changes are summed up in the expression electrotonus. In the investigation of this subject, the instruments employed are the same as those already described with the addition of two others, that it will be convenient to describe before passing on to the study of electrotonus itself. These are the reverser or commutator, and the rheochord. PohVs commutator is the form of reverser generally employed. It consists of a block of ebonite provided with six pools of Fig. 178.-Pohl's commutator, with cross wires, mercury, each of which is provided with a binding screw. The corner pools are connected by diagonal cross wires, and by a cradle consisting of an insulating handle fixed to two arcs of copper wire which can be tilted so that the two middle pools can be brought into communication with either of the two lateral pairs of pools. Fig. 178 shows how, by altering the position of the cradle, the direction of the current from one electrode to the other is reversed. The numbers 1, 2, 3, &c. indicate the path of the current in the two cases. Sometimes the reverser is used without the cross wires for a different purpose. The battery wires are connected as before with the middle mercury pools. Each lateral pair of pools is connected by wires to a pair of electrodes. The two pairs of electrodes may be applied to two portions of a nerve, or to two different nerves, and by tilting the cradle to right or left the current can be sent through one or the other pair of electrodes. The Rheochord is an instrument by means of which the strength of a constant current passed through a nerve may be varied. It consists of a long wire (?•, r, »•) stretched on a board. This is placed as a bridge oil the course of the battery current. (See fig. 179.) The current is thus divided into two parts: one part CH. xvi.] RHEOCHORD. 173 through the bridge, the other through the nerve which is laid across the two terminal wires. The resistance through the bridge is varied by the position of the slider (ss). The farther the slider is from the battery end of the instrument, the longer is the bridge, and the higher its resistance, so that less current goes that way, and more to the nerve. Fig. 179.-Simple rheochord. The next figure shows the more complicated form of rheochord invented by Poggendorf. The number of turns of wire is greater, so that the resistance can be varied to a much greater extent than in the simpler form of the instrument. The term " electro tonus " includes two sets of changes in the Fig. 180.-Poggendorf's rheochord. (McKendrick.) nerve; first an electrical change, and secondly a change in excitability. We will take the electrical change first. Electrotonie currents.-The constant current is passed through the nerve from a battery, non-polarisable electrodes being used; it is called the polarising current. If portions of the nerve beyond the electrodes are connected ("led offJ') as in the figure by non-polarisable electrodes to galvanometers, a current will in each case be indicated by the swing of the galvano- meter needles. The electrotonic current in the neighbourhood of 174 ELECTROTONUS. [ch. xvi. the negative pole or kathode is called the kathelectrotonic current; and that in the neighbourhood of the anode is called the anelectrotonic current. In both cases the electrotonic current has the same direction as the polarising current. These currents are dependent on the physical integrity of medullated nerve ; they are not found in muscle, tendon, or non-medullated nerve ; they are absent or diminished in dead or degenerated nerve. They can, howrever, be very successfully imitated in a model made of zinc wire encased in cotton soaked with salt solution. The electrotonic currents must be carefully distinguished from the normal current of action, which is a momentary change rapidly propagated with a nervous impulse which may be produced by any method of stimulation. The electrotonic currents are pro- duced only by an electrical (polarising) current; they vary in Anclectrotonic Current Katelectrotonic Current Polarising Current Fig. 181.-Electrotonic currents. intensity with the polarising current, and last as long as the polarising current passes through the nerve. After the polarising current is removed, after-electrotonic currents occur in different directions in the three regions tested. (a) In the intrapolar region, the after-current is opposite in direction to the original polarising current; unless the polarising current is strong and of short duration, when it is in the same direction. (Z») In the kathelectrotonic region, the after-current has the same direction as the kathelectrotonic current, (c) In the anelectrotonic region, the after-current has at first the same, then the opposite direction as the anelectrotonic current. The experiment known as the paradoxical contraction depends upon electrotonic currents. The sciatic nerve of the frog divides in the lower part of the thigh into two parts. If one division is cut across, and its central end stimulated electrically (the spinal cord having been previously destroyed), the muscles supplied by the other branch contract; the nerve fibres in this branch having ch. xvi.] ELECTROTONUS. 175 been stimulated by the electrotonic variation in the divided branch.* Electrotonic alterations of excitability.-When a constant current is passed through a nerve, the excitability of the nerve is increased in the region of the kathode, and diminished in the region of the anode. When the current is taken out the excitability is temporarily increased in the neighbourhood of the anode, and diminished in that of the kathode. This may be shown in the case of a motor nerve by the fol- lowing experiment. The next diagram represents the apparatus used :- Reuerser Key Coil EXCITING CIRCUIT Muscle Fig. 182.-Diagram of apparatus used in testing electrotonic alterations of excitability. An exciting circuit for single induction shocks is arranged in the usual way, the exciting electrodes being placed on the nerve near the muscle. A polarising circuit is also arranged, and in- cludes a battery, key, and reverser ; the current is passed into the nerve by means of non-polarisable electrodes. When the polarising current is thrown into the nerve, or taken out, a con- traction of the muscle occurs, but these contractions may be dis- regarded for the present. The exciting circuit is arranged with the secondary coil so far from the primary that the muscle responds to break only, and the tracing may be recorded on a stationary blackened cylinder. * The term " paradoxical contraction " used in this sense must be carefully distinguished from the same term as employed by Westphal and adopted by many physicians. He uses it for a slow tonic contraction occurring in a muscle when its attachments are suddenly brought nearer together. This is best seen in the tibialis anticus, and the contraction may last several minutes. This condition is much more marked in certain diseases, but its explanation is not known. 176 ELECTROTONUS. [ch. xvi. Move the cylinder on a short distance, and repeat this. The height of the lines drawn may be taken as a measure of the excitability of the nerve. Now throw in the polarising current in a descending direction (i.e., towards the muscle); the kathode is then the non-polarisable electrode near to the exciting elec- trodes. While the polarising current is flowing, take some more tracings by breaking the exciting current. The increase in the excitability of the nerve is shown by the much larger contrac- tions of the muscle; probably a contraction will be obtained now at both make and break of the exciting current. After removing the polarising current, the contractions obtained by exciting the nerve will be for a short time smaller than the normal, but soon return to their original size. Exactly the reverse occurs when the polarising current is ascending, i.e., from the muscle towards the spinal cord. The non-polarisable electrode near the exciting electrodes is now the anode. While the polarising current is passing, the excitability of the nerve is diminished so that induction shocks which pre- viously produced contractions of a certain size, now produce smaller contractions, or none at all. On removing the polarising current, the after-effect is increase of excitability. The following figure is a reproduction of a tracing from an Fig. 183.-Electrotonus. M, make'; B, break. actual experiment. The after-effects are not shown. N repre- sents a series of contractions obtained when the nerve is normal K when it is kathelectrotonic, A when it is anelectrotonic. CH. XVI.] LAW OF CONTRACTION. 177 Exactly similar results are obtained if one uses mechanical stimuli, such as hammering the nerve instead of induction shocks. The same is true for chemical stimuli. If the exciting electrodes are removed, and salt sprinkled on the nerve near the muscle, the latter soon begins to quiver; its contractions are increased by throwing in a descending and diminished by an ascending polarising current. The increase in irritability is called katelectrotonus, and the decrease is called anelectrotonus. As there is between the electrodes both an increase and a decrease of irritability on the passage of a polarising current, it must be evident that the in- crease must shade off into the decrease, and that there must be Fig. 184.-Diagram illustrating the effects of various intensities of the polarising currents. n, n', nerve; a, anode; k, kathode; the curves above indicate increase, and those below decrease of irritability, and when the current is small the increase and decrease are both small, with the neutral point near a, and as the current is increased in strength, the changes in irritability are greater, and the neutral point approaches fc. a neutral point where there is neither increase nor decrease of irritability. The position of this neutral point is found to vary with the intensity of the polarising current-when the current is weak the point is nearer the anode, when strong nearer the kathode (fig. 184). Pfliiger's Law of Contraction. The constant current sometimes causes a contraction both at make and break, sometimes at make only, sometimes at break only. The difference depends on the strength and direction of the current; and follows from the electrotonic conditions of ex- citability we have been studying. Increase of excitability acts as a stimulus ; so that at the make the kathode is the stimu- lating electrode, and at the break the anode is the stimulating- electrode. 178 ELECTROTONUS. [ch. xvi- The facts may be demonstrated in the following way : from a battery lead the wires to the middle screws of a reverser (with cross wires), interposing a key; from one pair of end screws of the reverser lead wires to the binding screws of the rheochord ; from these same screws of the rheochord the non-polarisable electrodes lead to the nerve of a nerve-muscle preparation. The strength of the current is varied by the slider S. The nearer S is to the binding Big. 185.- Arrangement of apparatus for demonstrating Pfliiger's law. screws the less is the resistance in the rheochord circuit, and the less the current through the nerve. With a weak current, a contraction occurs at make only, but more readily, i.e. with a weaker current when its direction is descending, i.e. towards the muscle. With a stronger current (ascending or descending) con- traction occurs both at make and break. With a very strong- current (six Groves), the contraction occurs only at make with a descending current; and only at break with an ascending current. The contractions produced in the muscle of a nerve-muscle preparation by a constant current have been arranged in a table which is known as Pfliiger's Law of Contraction. It is really only a statement as to when a contraction may be expected: Strength of Current used. 1 Descending Current. Ascending Current. Make. Break. Make. Break. Very Weak Yes. No. No. No. Weak Yes. No. Yes. No. Moderate Yes. Yes. Yes. Yes. Strong Yes. No. No. Yes. The increase of irritability at the kathode when the current is made is more potent to produce a contraction than the rise of CH. XVI.] LAW OF CONTRACTION. 179 irritability at the anode when the current is broken; and so with weak currents the only effect is a contraction at the make of both currents. The descending current is more potent than the ascending (and with still weaker currents is the only one which produces any effect), since the kathode is near the muscle. In the case of the ascending current the impulse produced by the stimulus has to pass through a district of diminished irritability, which with a very strong current acts as a block, being of con- siderable amount and extent (see fig. 184), but with a weak current being less considerable both in intensity and extent, only slightly affects the contraction. As the current is stronger how- ever, recovery from anelectrotonus is able to produce a contraction as well as katelectrotonus, and a contraction occurs both at the make and the break of the current. The absence of contraction with a very strong current at the break of the ascending current is due to the fall in irritability at the kathode blocking the stimulus due to the rise in irritability at the anode. Thus we have seen that two circumstances influence the effect of the constant current upon a nerve, viz., the strength and direction of the current. It is also necessary that the stimulus should be applied suddenly and not gradually, and that the irri- tability of the nerve should be normal ; not increased or diminished. Sometimes (when the preparation is specially irritable) instead of a simple contraction a tetanus occurs at the make or break of the constant current. This is liable to occur at the break of a strong ascending current which has been passing for some time into the preparation, or at the make of a strong descending current; both being conditions which increase the excitability of the piece of nerve nearest to the muscle; this is called Ritter's tetanus, and may be increased by passing a current in the opposite direction or stopped by passing a current in the same direction. The main fact that the kathode is the stimulating electrode at the make, and the anode at the break, may be shown in nerve by using a current strong enough to give a contraction both at make and break, and then ligaturing the nerve between the two electrodes so tightly that a nervous impulse cannot traverse the block produced. On making a descending current a contraction occurs, the kathode being nearest to the muscle, and the kathode is the stimulating electrode ; at the break no contraction occurs, because the impulse starts at the anode which is cut off from the muscle by the ligature. With an ascending current, one gets just the reverse, namely, a contraction at the break only. 180 ELECTROTONUS. [ch. xvi. The same is true for muscle; if a curarised, that is, a physio- logically nerveless muscle, is arranged as in the experiment for demonstrating the muscle wave (see fig. 148, p. 125), and a non- polarisable electrode placed at each end, the muscle wave at the make of a constant current starts at the negative electrode (kathode) and at the break at the positive electrode (anode). An induced current in the secondary circuit of an inductorium may be regarded as a current of such short duration that the opening and closing are fused in their effects. This is true for all induction currents, whether produced by the make or break of the primary circuit. The kathode will always be the more effective in causing contraction. Response of Human Muscles and Nerves to Electrical Stimulation. Perhaps the most important outcome of this study of the response of muscle and nerve to electrical stimulation is its application to the muscles and nerves of the human body, because here it forms a most valuable method of diagnosis in cases of disease. The following account of this is chiefly an abstract from Dr. Gowers' Manual of Diseases of the Nervous System. In the normal state, nerves can be stimulated either by induction shocks, or by the make and break of a constant current. In the case of the motor-nerves this is shown by the contraction of the muscles they supply; and in the case of the sensory nerves by the sensations that are produced. In the case of the sensory nerves, the sensation produced by the constant current is most intense at the instant of make and break, or when the strength of the current is changed in the direction either of diminution or increase; but there is a slight sensation due doubtless to the electrotonic alterations in excitability which we have been study- ing, during the whole time that the current is passing. When the nutrition of the nerves is impaired, much stronger currents of both the induced and constant kinds are necessary to evoke muscular contractions than in the normal state. M hen the nerves are completely degenerated (as for instance when they are cut off from the spinal cord, or when the cells in the cord from which they originate are themselves degenerated as in infantile paralysis) no muscular contraction can be obtained on stimulating the nerves even with the strongest currents. The changes in the excitability of the muscles is less simple, because in them there are two excitable structures, the termina- CH. XVI.] REACTION OF DEGENERATION. 181 tions of the nerves, and the muscular fibres themselves. Of these, the nerve-fibres are the more sensitive to induction currents, and the faradic stimulation of a muscle under normal circumstances is by means of these motor nerve-endings. Thus we find that its excitability corresponds in degree to that of the motor-nerve supplying it. The muscular fibres are, even in the normal state, less sensitive to faradism (that is, a succession of induction shocks) than the nerve, because they are incapable of ready response to stimuli so very short in duration as are the shocks of which a faradic current consists. (The proof of this consists in the fact that under the influence of curare, which renders the muscle practically nerveless, the muscle requires a much stronger faradic current to stimulate it than in the normal state.) But when the nerve is degenerated, the slowly interrupted constant current stimulates the muscle as readily as in the normal state; a contrac- tion occurs when the circuit is completed or broken-distinctly slower than that which occurs when the nerve-fibres are intact, and due to the stimulation of the muscular fibres themselves. The fact that, under normal circumstances, the contraction which is caused by the constant current is as quick as that produced by an induction shock, is ground for believing that in health the constant, like the induced current, causes the muscle to contract chiefly by exciting the motor-nerves within it. When the motor-nerve is degenerated, and will not respond to any form of electrical stimulation, the muscle also loses all its power of response to induction shocks. The nerve-degeneration is accompanied by changes in the nutrition of the muscular fibres as is evidenced by their rapid wasting, and any power of response to faradism it possessed in the normal state is lost. But the response to the voltaic current remains, and is indeed more ready than in health, doubtless in consequence of nutritive changes which develop what the older pathologists called, truly enough, " irritable weakness." There is, moreover, a qualitative as well as a quantitative change. In health the first contraction to occur on gradually increasing the strength of the current is at the negative pole, when the circuit is closed (see Pfluger's law), and a stronger current is required before closure-contraction occurs at the positive pole. But in the morbid state we are discussing, closure-contraction may occur at the positive pole as readily as or even more readily than at the negative pole. This condition, the reasons for which we do not know, is called the " Reaction of Degeneration." Suppose a patient comes before one with muscular paralysis. 182 ELECTROTONUS. [ch. xvi. This may be due to disease of the nerves, of the cells of the spinal cord, or of the brain. If the paralysis is due to brain disease, the muscles will be slightly wasted owing to disuse, but the electrical irritability of the muscles and nerves will be normal, as they are still in connection with the nerve-cells of the spinal cord that control their nutrition. But if the paralysis is due to disease either of the spinal cord or of the nerves, this nutritive influence can no longer be exercised over the nerves or muscles. The nerves will degenerate; the muscles waste rapidly; the irritability of the nerves to both forms of electrical stimulation will be lost; the muscles will not respond to the faradic current, but in relation to the constant current they will exhibit what we have called the " reaction of degeneration." This illustrates the value of the electrical method as a means of diagnosis, that is, of finding out what is the matter with a patient. It is also a valuable means of treatment; by making the muscles contract artificially, their nutrition is kept up until restoration of the nerves or nerve-centres is brought about. Another illustration will indicate that the facts regarding electrotonic variation of excitability are true for sensory as well as for motor nerves; in a case of neuralgia, relief will often be obtained by passing a constant current through the nerve; but the pole applied to the nerve must be the anode which produces diminution of excitability, not the kathode which produces the reverse. Waller has pointed out that Pfliiger's law of contraction, as formulated for frogs' muscles and nerves, is true for human muscles and nerves in the main, but there are certain discrepancies. These arise from the method Fig. 186.-Electrodes applied to the skin over a nerve-trunk. In a the polar area is anelectrotonic, and the peripolar kathelectrotonic. The former condition, therefore, preponderates, since the current is more concentrated. In b the conditions are reversed, the polar zone corresponding here to the kathode. (After Waller.) necessarily employed in man being different from those used with a muscle- nerve preparation. In a muscle-nerve preparation the nerve is dissected out, the two electrodes placed on it. and the current has of necessity to traverse the piece of nerve between the two electrodes. In man, the current is applied by means of electrodes or rheophores which consist of metal discs CH. XVII.] NERVE-CENTRES. 183 covered with wash leather, and soaked in brine. One of these is placed on the moistened skin over the nerve, and the other to some indifferent point such as the back. The current finds its way from one electrode to the other, not necessarily through the nerves to any great extent (though it will be concentrated at the nerve as it leaves the anode or reaches the kathode), but diffuses widely through the body, seeking the paths of least resistance. Thus it is impossible to get pure anodic or kathodic effects. If the anode is applied over the nerve, the current enters by a series of points (polar zone), and leaves by a second series of points (peripolar zone). The second series of points is very close to the first, as the current leaves the nerve as soon as possible, seeking less resistant paths. The polar zone will be in the con- dition of anelectrotonus, the peripolar in that of kathelectrotonus, so that although the former effect will predominate, the points being more con- centrated, the latter effect may prevent a pure anelectrotonic effect being observed. Excitability and Conductivity.-Excitability of a nerve and its conduc- tivity generally vary one with the other. The following experiment, however, indicates that these two properties of nerve do not necessarily go together. The nerve of a frog's leg is led through a glass tube, the ends of which are sealed with clay, but the nerve must not be compressed. The tube is supplied with an inlet and outlet, so that gases may be passed through it. Two pairs of electrodes are arranged, so that the nerve can be stimulated either within or outside the tube. If carbonic acid is passed through the tube, and the nerve stimulated by an induction shock within the tube, the muscle does not respond ; but on stimulating outside the tube, the muscle contracts. The nerve is, therefore, not excitable, though it will conduct impulses. If alcohol vapour is used, conductivity vanishes before excitability. Cold acts like carbonic acid ; localised cold applied to nerve, however, increases its excitability to the constant current, and also to mechanical and thermal stimuli (Gotch). Waller tests the excitability of nerve by the amount of current of action it gives rise to ; not by the amount of contraction in the muscle to -which it leads. He finds that the effect of carbonic acid is to depress the activity of nerve ; but after the gas is removed, there is greatly increased activity. Ether acts similarly ; but with chloroform there is no recovery. CHAPTER XVII. NERVE-CENTRES. The nerve-centres consist of the brain and spinal cord; they are characterised by containing nerve-cells, from which the nerve- fibres of the nerves originate. Small collections of nerve-cells are found also in portions of the peripheral nervous system, where they are called ganglia. The spinal ganglia on the posterior roots of the spinal nerves, and the sympathetic ganglia are instances of these. 184 NERVE-CENTRES. [ch. xvii. The general arrangement of the cerebro-spinal axis is given in the next diagram, and some anatomical details concerning the membranes of the brain and cord may here be conveniently added. Membranes of the Brain and. Spinal Cord.-The Brain and Spinal Cord are enve- loped in three mem- branes- (i) the Dura Mater, (2) the Arach- noid, (3) the Pia Mater. (1) The Dura Mater, or external covering, is a tough membrane com- posed of bundles of connective tissue which cross at various angles, and in whose interstices branched connective- tissue corpuscles lie : it is lined by a thin elastic membrane, and on the inner surface and where it is not adherent to the bone, on the outer sur- face also is a layer of endothelial cells very similar to those found in serous membranes. (2) The Arachnoid is a much more delicate membrane, very similar in structure to the dura mater, and lined on its outer or free surface by an endothelial mem- brane. (3) The Pia Mater consists of two chief layers, between which numerous blood-vessels Fig. 187.-View of the cere- bro-spinal axis of the nervous system. The right half of the cranium and trunk of the body has been removed by a vertical section; the membranes of the brain and spinal cord have also been removed, and the roots and first part of the fifth and ninth cranial, and of all the spinal nerves of the right side, have been dissected out and laid separately on the wall of the skull and on the several vertebrae opposite to the place of their natural exit from the cranio-spinal cavity. (After Bourgery.) CH. XVII.] GREY AND WHITE MATTER. 185 ramify. Between the arachnoid and pia mater is a network of fibrous- tissue trabeculae sheathed with endothelial cells : these sub-arachnoid trabeculae divide up the sub-arachnoid space into a number of irregular sinuses. There are some similar trabeculae, but much fewer in number, travers- ing the sub-dural space, i.e., the space between the dura mater and arachnoid. Pacchionian bodies are growths from the sub-arachnoid network of connective-tissue trabeculae which project through small holes in the inner layers of the dura mater into the venous sinuses of that membrane. The venous sinuses of the dura mater have been injected from the sub-arach- noidal space through the intermediation of these villous outgrowths. The more intimate structure of the brain and spinal cord we shall consider at length in subsequent chapters. For the present we shall deal with some of the general aspects of the nerve- centres, both as regards structure and function. Both brain and spinal cord consist of two kinds of tissue, easily distinguishable by the naked eye. They are called respectively white matter and grey matter. White matter is composed of medullated nerve-fibres, which differ in structure from the medullated fibres of nerve by having no primitive sheath. Grey matter is the true central material so far as regards func- Fig. 188.-a. Branched neuroglia-cells. (After Stohr.) tion; that is to say, it is the part which receives and sends out nervous impulses; it is characterised by containing nerve-cells and their branches. In the brain the grey matter is chiefly situated on the surface, forming what is called the cortex ; the white matter and certain subsidiary masses of grey matter are in the interior. 186 NERVE-CENTRES. [ch. xvn. In the spinal cord, the grey matter is in the interior, the white matter outside. In both grey and white matter the nerve-cells and nerve-fibres are supported by a peculiar tissue which is called neuroglia. It is composed of cells and fibres, the latter being prolonged from the cells. Some of the fibres are radially arranged. They start from the outer ends of the ciliated epithelium cells that line the central canal of the spinal cord and the ventricles of the brain, and diverge constantly branching towards the surface of the organ, where they end by slight enlargements attached to the pia mater. The other fibres of the tissue are cell processes of the neuroglia or glia cells proper, or spider cells as they are some- times termed (see fig. 188). Fig. 189.-Different forms of ganglion cells. A, a, round ball-shaped unipolar cell from the human Gasserian ganglion. A X 240. a x 80. Two cells only show the process /; b, spindle-shaped ; c, multipolar ganglion cell from the spinal cord of the ox. X 80. d, D, Purkinje ganglion cells from human cerebellum; ax, axis-cylinder process ; P, protoplasmic process; h, h, two cells surrounded with a nucleated sheath. (Stohr.) Neuroglia is thus a connective tissue in function, but it is not one in origin. Like the rest of the nervous system, it originates from the outermost layer of the embryo, the epiblast. All true connective tissues are mesoblastic. Chemically it is very different from connective tissues. It consists of an insoluble material called neuro-keratin, or nerve- horn, similar to the horny substance keratin which is found in the surface layers of the epidermis. CH. XVII.] NERVE-CELLS. 187 Varieties of Nerve-Cells. Nerve-cells differ a good deal both in shape and size. The nucleus is generally large and spherical, with a distinct nucleolus. The principal varieties in shape of nerve-cells are shown in fig. 189. They may be roughly divided into three groups, according to the number of their processes, into unipolar, bipolar, and multipolar cells. Unipolar cells, or cells with one process, are found in the spinal ganglia (fig. 189, a). They are spherical in shape, are enclosed in a nucleated sheath, and the single process after a short course joins one of the nerve-fibres traversing the ganglion by a T's^aPe(i junction. It has, however, already been ex- plained (p. 166, fig. 174) that these unipolar cells are in reality bipolar, the two processes having become amalgamated for a short distance. Bipolar cells are cells with two branches. The embryonic condition of the cells of a spinal ganglion form one example of these. In many lower animals the two pro- cesses come off from the opposite ends of the cells (fig. 189, b) ; the cell appears as a nucleated enlarge- ment on the course of a nerve-fibre. In some cases, however, where there appear to be two fibres connected with a cell, one of them is really derived from another cell elsewhere, and is passing to end in a ramification which envelopes the ganglion cell; it may, as in the sympathetic ganglia of the frog, be coiled spirally around the issuing nerve process (see fig- 19°)- Multipolar nerve-cells : here the cell becomes angular or stel- late. It has been thought in some instances, as in the sympa- thetic ganglia (fig. 191), that all the processes become nerve- Fig'. 190.-Sympathetic ganglion cell of a frog, highly magnified. (Beale.) 188 NERVE-CENTRES. [ch. xvii. fibres, but this is extremely unlikely, and in most instances, as in the large cells of the grey matter of the spinal cord (fig. 189 c, and fig. 192), only one process becomes the axis cylinder of a nerve-fibre, the others dividing and subdividing in a ramified manner until they end in an arborescence of fine twigs. Many of these cells, especially those in the spinal cord, have a Fig. 191.-An isolated sympathetic ganglion cell of man, showing sheath with nucleated- cell lining, B. A. Ganglion cell, with nucleus and nucleolus. C. Branched process. D. L'nbranched process. (Key and Retzius.) x 750. finely fibrillar structure, and the fibrils can be traced into the axis cylinder process and the other branches of the cell. Other- wise the cells have a finely granular appearance,* and often possess a deposit of yellowish pigment at one side of the nucleus. In preparations made by Golgi's nitrate of silver method, the cells and their processes are stained black by a deposit of silver, so that the processes can be traced to their remotest ramifica- tions. By this method it is found that the axis cylinder process is not unbranched (as represented in fig. 192), but invariably * These granules are of some significance ; in fatigue of the cell, as after an epileptic fit, they disappear, being apparently used up during the dis- charge of energy (Nissl). CH. XVII.J NERVE-CELLS. 189 gives off fine side branches which are called collaterals, which ramify in the adjacent nerve substance. The axis cylinder acquires the sheaths, and is converted into a nerve-fibre. This nerve-fibre sometimes, as in the nerve-centres, after a more or less extended course breaks up into a terminal arborescence envelop- ing other nerve-cells. Even the long type of axis cylinder which passes into the nerves ultimately ends in a similar manner; we have already seen an instance of this in the end-plates of volun- tary muscle. The grey matter of the cerebellum contains a large number of very small nerve-cells, and one layer of large cells which are shown in fig. 189, n. They are flask-shaped, and are called the cells of Purkinje. The neck of the flask breaks up into branches, Fig. 192.-Multipolar nerve-cell from anterior bone of spinal cord; a, axis cylinder process. (Max Schultze.) and the axis cylinder process (a x) comes off from the base of the flask. In the grey matter of the cerebrum the nerve-cells are various in shape and size, but the most characteristic cells are large, and pyramidal in shape. They are especially large and numerous in what are called the motor areas of the brain. The apex of the cell is directed to the surface ; from the angles branching pro- cesses originate ; the axis cylinder process comes off from the base of the pyramid. The next figure shows a section of cere- bral cortex prepared by Golgi's method. The study of the nervous system by the valuable method in- troduced by Golgi has led to some new conceptions as to its structure. The whole nervous system consists of nerve-cells and their branches, supported by neuroglia in the central nervous system, and by connective tissue in the nerves Some of the 190 NERVE-CENTRES. [ch. xvii. branches of a nerve-cell break up almost immediately into smaller branches ending in arborescences of fine twigs ; these branches have been called dendrons or dendrites ; one branch becomes the long axis cylinder of a nerve-fibre, but it also ultimately ter- minates in an arborisation. It may be called the axis cylinder process, or the axon, or the neuron. The term neuron, however, is applied by some writers to the complete nerve-unit, that is, the Fig. 193.-Cerebral cortex of mammal, prepared by Golgi's method, a, b, c, d, nerve' cells ; e, neuroglia-cells. (R. J. Cajal.) body of the cell, and all its branches. Some observers have sup- posed that the axis cylinder process is the only one that conducts nerve impulses, the dendrons being rootlets which suck up nutri- ment for the nerve-cell. This exclusive view has not, however, been generally accepted; the dendrons may be nutritive, but it is believed that they also, like the rest of the nerve-unit, are con- cerned in the conduction of nerve impulses. The next idea which it is necessary to grasp is, that each nerve-unit (cell plus branches of both kinds) is anatomically inde- pendent of every other nerve-unit. There is no anastomosis of the branches from one nerve-cell with those of another; the CH, XVII.] REFLEX ACTION. 191 arborisations interlace and intermingle, and nerve impulses are transmitted from one nerve-unit to another, but not by continuous structures. The impulses are transmitted through contiguous, but not through continuous structures. This idea is a little difficult to grasp at first. The old notion of a reflex action was the following : a sensory nerve-fibre is stimulated at S (fig. 194), the impulse is carried up to a sensory nerve-cell (S 0), transmitted to a motor nerve-cell (M C) by branching pro- cesses connecting the two (I), and then reflected down the motor nerve-fibre to the muscle (M) which executes the action. Fig. 194.-Reflex action : old idea. The figure on the next page is a diagram of our new notion of a reflex action. Excitation occurs at S, as before, and the impulse is transmitted by the sensory nerve-fibre to the nerve-centre, where it ends not in a nerve-cell, but by arborising around a nerve-cell and its dendrons. The only nerve-cell in actual continuity with the sensory nerve-fibre is the one in the spinal ganglion (G) from which it grew. The terminal arborisation of the sensory nerve-fibre merely in- terlaces with the dendrons of the motor nerve-cell; yet simply by this contiguity, or touching, the motor nerve-cell (M C) is affected and sends an impulse by its axis cylinder process to the muscle (M). A very rough illustration which may help one in realising this 192 NERVE-CENTRES. [ch. xvii. may be taken as follows : Suppose two trees standing side by side ; their stems will represent the axis cylinders ; their branches the dendrons. If the trees are close together the branches of one will intermingle with those of the other: there is no actual branch from the one which becomes continuous with any branch of the other; but yet if the stem of one is vigorously shaken, the close intermixture of the branches will affect the other so that it also moves. Another very important general idea which we must next get hold of, is that a nervous impulse does not necessarily travel along the same nerve-fibre all the way, but there is what we may term a system of relays. The nervous system is very often compared to a telegraphic system throughout a country. The Fig'. 195.-Reflex action : modern idea. telegraph offices represent the nerve-centres, the afferent nerve- fibres correspond to the wires that carry the messages to the central offices, and the efferent nerve-fibres are represented by the wires that convey messages from the central offices to more or less distant parts of the country. This illustration will serve us very well for our present purpose, provided that it is always remembered that a nervous impulse is not electricity. Suppose, now, one wishes to send a message from the metropolis, which will represent the brain, to a distant house, say in the Highlands of Scotland. There is no wire straight from London to that house, but the message ultimately reaches the house; one wire CH. XVII.] RELAYS AND CELL-STATIONS. 193 takes the message to Edinburgh; another wire carries it on to the telegraph station in the town nearest to the house in question ; and the last part of the journey is accomplished by a messenger on foot or horseback. There arc at least two relays on the journey. It is just the same with the nervous system. Suppose one wishes to move the arm; the impulse starts in the nerve-cells of the brain, but there are no fibres that go straight from the brain to the muscles of the arm. The impulse travels down the spinal cord, by what are called pyra- midal fibres, to the nerve-cells of the spinal cord, and from these fresh nerve- fibres pass to the arm-muscles, and con- tinue the impulse. Here again the con- nection between the nerve-units is by con- tiguity, not by continuity. This is shown in the accompanying diagram. The cell of the cerebral grey matter is represented by C C, the pyramidal nerve-fibre ar- borises around the cell of the spinal cord (S C) from which the motor nerve-fibre arises, and which carries on the impulse. The spinal cord cells are thus surrounded by arborisations derived not only from the sensory nerves (S), but by fibres from the upper part of the nervous system. We now see how it is possible that reflex ac- tions in the cord may be controlled by impulses from the brain. The sheaths of the nerve-fibres are not shown in the diagram. The system of relays is still more com- plicated in the case of sensory impulses, as we shall see later on ; the same is true for the motor path to involuntary muscle, accessory cell-stations being situa- ted in the sympathetic ganglia. In concluding this chapter we may return for a moment to the subject of de- generation. If the nerve-fibre is cut off from its connection with the spinal nerve-cell, the peripheral end degenerates as far as the muscle. Fig. 196.-Diagram of an element of the motor path. U.S., upper seg- ment; L.S., lower seg- ment ; C.C., cell of cere- bral cortex; S.C., cell of spinal cord, in anterior cornu ; M, the muscle ; S, path from sensory nerve roots. (After Gowers.) 194 STRUCTURE OF THE SPINAL CORD. [ch. xviii. Suppose, now, the pyramidal fibre were cut across, the piece still attached to the brain-cell would remain healthy, but the peri- pheral end would degenerate as far as its arborisation round the spinal cell (S C), but not beyond. We can thus use the degeneration method to trace out tracts of nerve-fibres in the white matter of the central nervous system. The histological changes in the fibres is here the same as that already described in the nerves, except that, as there is no primitive sheath, there can be no multiplication of its nuclei; there is instead an over- growth of neuroglia. Degenerated tracts consequently stain differently from healthy white matter, and can be by this means easily traced. Another method of research which leads to the same results as the degeneration method is called the embryological method. The nerve-fibres which grow from different groups of nerve-cells do so at different dates, and so by examining brains and cords of embryos of different ages, one is able to make out individual tracts before they have blended in the general mass of white matter. CHAPTER XV1I1. STRUCTURE OF THE SPINAL CORD. The spinal cord is a cylindriform column of nerve-substance connected above with the brain through the medium of the bulb, and terminating below, about the lower border of the first lumbar vertebra, in a slender filament of grey substance, the filum terminale, which lies in the midst of the roots of many nerves forming the cauda equina. It is composed of white and grey nervous substance, of which the former is situated externally, and constitutes its chief portion, while the latter occupies its central or axial portion, and is so arranged, that on the surface of a transverse section of the cord it appears like two crescentic masses (the horns of each of which are called respectively the anterior and posterior cornua) connected together by a narrower portion or isthmus (fig. 197). Passing through the centre of this isthmus in a longitudinal direction is a minute canal (central canal), which is continued through the whole length of the cord, and opens above into the space at the CH. XVIII.] THE SPINAL CORD. 195 back of the medulla oblongata and pons Varolii, called the fourth ventricle. It is lined by a layer of columnar ciliated epithelium, and contains a fluid called cerebro-spinal fluid. The spinal cord consists of two symmetrical halves, separated anteriorly and posteriorly by vertical fissures (the posterior Fig. 197.-Different views of a portion of the spinal cord from the cervical region, with the roots of the nerves (slightly enlarged);! In a, the anterior surface of the specimen is shown ; the anterior nerve-root of its right side being divided ; in b, a view of the right side is given ; in c, the upper surface is shown; in d, the nerve-roots and ganglion are shown from below, i, the anterior median fissure; 2, posterior median fissure ; 3, anterior lateral depression, over which the anterior nerve-roots are seen to spread; 4, posterior lateral groove, into which the posterior roots are seen to sink ; 5, anterior roots passing the ganglion; 5', in a, the anterior root divided; 6, the posterior roots, the fibres of which pass into the ganglion 6'; 7, the united or com pound nerve; 7', the posterior primary branch, seen in a and d to be derived in pal* from the anterior and in part from the posterior root. (Allen Thomson.) fissure being deeper, but less wide and distinct than the anterior), and united in the middle by nervous matter which is usually described as forming two commissures-an anterior com- missure in front of the central canal, consisting of medullated nerve-fibres, and a posterior commissure behind the central canal consisting also of medullated nerve-fibres, but with more neuroglia, which gives the grey aspect to this commissure (fig. 197, b). Each half of the spinal cord is marked on the sides (obscurely at the lower part, but distinctly above) by two longitudinal furrows, which divide it into three portions, columns, or tracts, an anterior, lateral, and posterior. From the groove between the anterior and 196 STRUCTURE OF THE SPINAL CORD. [ch. xviii. lateral columns spring the anterior roots of the spinal nerves (b and c, 5) ; and just in front of the groove between the lateral and posterior column arise the posterior roots of the same (b, 6): a pair of roots on each side corresponds to each vertebra (fig. 197). White matter.-The white matter of the cord is made up of medullated nerve-fibres, of different sizes, and arranged longitudi- nally, and of a supporting material of two kinds, viz. :-(a) ordi- nary fibrous connective-tissue with elastic fibres, which is con- nected with septa from the pia mater which pass into the cord to carry the blood-vessels. (6) Neuroglia. The processes of the neuroglia-cells are arranged so as to support the nerve-fibres which arc without the usual external nerve sheaths. The general rule respecting the size of different parts of the cord is, that each part is in direct proportion to the size and number of nerve - roots given off from it, and has but little relation to the size or number of those given off be- low it. Thus the cord is very large in the middle and lower part of its cervical portion, whence arise the large nerve-roots for the formation of the brachial plexuses and the supply of the up- per extremities, and again en- larges at the lowest part of its dorsal portion and the upper part of its lumbar, at the origins of the large nerves which after forming the lumbar and sacral plexuses, are distributed to the lower extremities. The chief cause of the greater size at these parts of the spinal cord is increase in the quantity of grey matter; the white part of the cord (especially the lateral columns) becomes gradually and progressively smaller from above down- wards, a certain number of fibres coming down from the brain passing into the spinal grey matter at different levels. Grey matter.-The arev matter of the cord consists of nerve- Fig. 198.-Section of grey matter of anterior cornu of a calf's spinal cord; d, nerve-fibres of white matter in transverse section, showing axis-cylinder in centre of each; a, large stellate nerve-cells with their nuclei and prolongations. (Cadiat.) CH. XVIII.] CELLS IN THE GREY MATTER. 197 fibres, most of which are very fine and delicate, of nerve-cells with branching processes, and of an extremely delicate network of the primitive fibrillae of axis-cylinders. This fine plexus is called Gerlactis network, and is mingled with the meshes of neuroglia. The neuroglia of the grey matter resembles that of the white, but instead of everywhere forming a close net- work to support the nerve-fibres, here and there it is in the form of a more open sponge-work to support the nerve-cells. It is especially developed around the central canal, which is lined with columnar ciliated epithelium, the cells of which -at their outer end terminate in fine processes, which join the neuroglia network surrounding the canal, and form the substantia gelatinosa centralis. It is also developed at the tip of the posterior cornu of grey matter, forming what is known as the substantia gelatinosa lateralis of Rolando, which is much enlarged in the upper cervical region. Groups of cells in grey matter.-The multipolar cells are either scattered singly or arranged in groups, of which the following are to be distinguished on either side-certain of the groups being- more or less marked in all of the regions of the cord, viz., those (a) in the anterior cornu, and (6) those in the posterior cornu. (a) The cells in the anterior cornu are large and branching, and each gives rise to an axis-cylinder process which passes out in the anterior nerve-root. These cells are everywhere con- spicuous, but are particularly numerous in the cervical and lumbar enlargements. In these districts they may be divided into several groups-(i.) a group of large cells close to the tip of the inner part of the anterior cornu-all the cells of the anterior cornu in the dorsal or thoracic region are said to belong to this group ; (ii.) several lateral groups (2, a, b, and c, fig. 199) on the outer side of the grey matter, and (iii.) a certain number of cells at the base of the inner part of the anterior cornu particularly well marked in the thoracic region, (b) Cells of the posterior cornu-these are not numerous; they are small and branched, and each has an axis-cylinder process passing off; but these processes do not pass into the posterior nerve-roots. The groups are two at least in number, viz., (i.) in connection with the edge of the grey matter externally, where it is considerably broken up by the passage of bundles of fibres through it, and called the lateral reticular formation ; and (ii.) in connection with a similar reticular formation, more at the tip of the grey matter of the posterior cornu; this is known as the posterior reticular formation. STRUCTURE OF THE SPINAL CORD. [ch. xviii. The other groups of cells (not represented in fig. 199) are confined to the thoracic region of the cord, and are two in num- ber, viz. : one situated at the base of the posterior cornu, formed of large fusiform cells, constitutes the posterior vesicular column of Lockhart Clarke (fig. 203, c c), and the other situated on the outer portion of the grey matter, about midway between the Fig. 199.-Section of spinal cOrd, one half of which (left) shows the tracts of the white matter, and the other half (right) shows the position of the nerve-cells in the grey matter. 7, 10, 9, and 3 are tracts of descending degeneration; 1, 4, 6, and 8, of ascending degeneration. Semidiagrammatic. (After Sherrington.) anterior and posterior cornua, constitutes the cells of the inter- medio-lateral tract (fig. 203, 1 t). These cells arc small and spindle-shaped, and are found in the upper lumbar as well as in the thoracic region. Columns and tracts in the white matter of the spinal cord.-In addition to the columns of the white matter which are marked out by the points from which the nerve-roots issue, and which are the anterior, the lateral and posterior, the posterior is further divided by a septum of the pia mater into two almost equal parts, consti- tuting the postero-external column, or column of Burdach (fig. 199, 2), and the postero-median, or column of Goll (fig. 199, 1). In addition to these columns, however, it has been shown that the white matter can be still further subdivided. This sub- division lias been accomplished by evidence of several kinds, that ch. xvni.j TRACTS IN THE WHITE MATTER. 199 the parts, or as they arc called, tracts in the white matter, per- form different functions in the conduction of impulses. The methods of observation are the following :- (а) The embryological method. It has been found that if the development of the spinal cord be carefully observed at different stages that certain groups of the nerve-fibres put on their myelin sheath at earlier periods than others, and that the different groups of fibres can therefore be traced in various direc- tions. This is also known as the method of Flechsig. (б) Wallerian or degeneration method.-This method depends upon the fact that if a nerve-fibre is separated from its nerve- cell, it wastes or degenerates. It consists in tracing the course of tracts of degenerated fibres, which result from an injury to any part of the central nervous system. When fibres degenerate below a lesion the tract is said to be of descending degeneration, and when the fibres degenerate in the opposite direction, the tract is one of ascending degeneration. By modern methods of staining of the central nervous system it has proved comparatively easy to distinguish degenerated parts in sections of the cord and of other portions of the central nervous system. Degenerated fibres have a different staining reaction when the sections are stained by what are called Weigert's and Pal's methods, which consist of subjecting them to a special solution of haematoxylin, and then to special differentiating solutions. The degenerated fibres appear light yellow, whereas the healthy fibres are a deep blue. Marchi's method is even better. By Marchi's solution (a mixture of Midler's fluid and osmic acid) degenerated fibres are stained black, the rest of the tissue being unstained. Accidents to the central nervous system in man have given us much information upon this subject, but this has of late years been supplemented and largely extended by experiments on animals, particularly upon monkeys; and considerable light has been by these means shed upon the conduction of impulses to and from the nervous system by the study of the results of section of different parts of the central nervous system, and of the spinal nerve-roots. By these methods the tracts in the white matter have now been mapped out, and the principal ones are shown in the left half of fig. 199. But as they are there all put together, it will be a better way of studying the subject to enumerate the ascending and descending tracts with separate diagrams. It will be convenient to begin by considering the result of cutting through the roots of the spinal nerves. Cutting the anterior roots produces no degeneration in the 200 STRUCTURE OF THE SPINAL CORD. [ch. xviii. cord ; the fibres of the anterior roots come off from the large cells of the anterior horn, and degeneration is found only on the distal side of the point of section, that is in the motor nerve- fibres of the nerves. Cutting the posterior roots between the spinal ganglia and the cord leaves the peripheral part of the nerve healthy, and degeneration occurs in the portion of the root which runs into the cord, the fibres being cut off* from the cells of the spinal ganglion from which they grow. These degenerated nerve-fibres may be traced up the cord for a considerable distance. Each posterior root-fibre when it enters the cord bifurcates, the main branch passing upwards, and the shorter branch downwards, so that the degeneration is seen in a small tract called the comma tract (fig. 199, 3) immediately below the point of entrance of the cut posterior root. The upgoing fibre is contained in the posterior column of white matter, and it terminates in one or other collec- tions of grey matter either in the cord itself, or in the medulla oblongata. Fig. 200 represents in a schematic way the manner in which the fibres of the two roots of a spinal nerve are connected to the grey matter in the cord. 1, 2, 3, 4 represent four cells of the anterior horn. Each gives rise to an axis-cylinder process A, one of which is shown terminating in its final ramification in the end plate of a muscular fibre M. Each of these four cells.is further surrounded by an arborisation derived from the fibres of the pyramidal tract P, which comes down from the brain. A fibre of the posterior root is also shown; this originates from the cell G of the spinal ganglion ; the process of this cell bifurcates, one branch (B) passing to the periphery where it ends in an arborescence in the skin (S); the arrow by the side of this branch represents the direction of conduction of the sensory impulses from the skin. An arrow in the opposite direction would indicate the direction of its growth. The other branch C passes into the spinal cord, where it again bifurcates ; the branch E, a short one, passes downwards and ends in an arborisation around one of the cells P of the posterior cornu; from which a new axis-cylinder arises, and terminates around one of the multipolar cells (4) of the anterior horn. The main division D travels up in the posterior column of the cord, and ends in grey matter at various levels. Some collaterals (5) terminate by arborising directly around the anterior cornual cells ; others (6) do so with an intermediate cell station in a pos- terior cornual cell ; others (7) arborise around the cells of Clarke's CH. XVIII.] COURSE OF FIBRES IN CORD. 201 column (C) in the thoracic region of the cord, and from these cells fresh axis-cylinders carry up the impulse to the cerebellum in what are called the cerebellar tracts, while the main fibre (8) may terminate in any of these ways at a higher level in the cord, or above the cord in the medulla oblongata. When we become Fig. 200.-Course of nerve-fibres in spinal cord (after Schafer) acquainted with the structure of the medulla oblongata, we shall be able to trace these fibres further. In general terms the anterior root-fibres pass out of the grey matter of the anterior horns, and after a short course leave the spinal cord in the anterior spinal nerve-roots. The posterior roots on the other hand do not pass to any great extent into the grey matter immediately, but into the white matter on the inner side of the posterior horn ; in other words they go into the column of Burdach (fig. 199, 2); they pass up in this column but gradually approach the middle line, and are continued upwards to the medulla in the column of Goll; but as they go up they become less numerous, as some terminate in the grey matter of the cord on the way in the manner described. A few fibres of the posterior root, 202 STRUCTURE OF THE SPINAL CORD. [CH. XVIII. however, travel for a short distance in a small tract on the outer side of the posterior horn ; this is called the tract of Lissauer (4 in fig. 199) ; the comma tract (3) has been already explained. Suppose now one cuts through several posterior roots between the spinal ganglia and the cord, so that the course of degenera- tion may be more readily traced. Immediately below the points of entrance of these nerve-roots, the comma tract will be found degenerated; immediately above, the degenerated fibres will be found in the column of Bnrdach ; higher up in the cord they will be less numerous, and have approached the middle line; the fibres which enter the cord lowest get ultimately nearest the middle line, so that the greater part of the column of Goll is made up of sensory fibres from the legs; the fibres which enter the cord last, for instance those from the upper limbs and neck, pursue their course in the inner part of the column of Burdach. The next figure shows the degeneration in a section of the Fig. 201.-Degeneration in column of Goll after section of posterior nerve-roots. spinal cord, after the division of a number of nerve-roots on one side. The section is taken high up, so that all the degenerated fibre have passed into the column of Goll on the same side ; the inner set (i) are shaded differently from the outer set (2), indicat- ing that those nearest the middle line come from the lowest nerve-roots. We may pass from this to consider the tracts of degeneration that occur when the spinal cord is cut right across in the thoracic region. Some tracts will be found degenerated in the piece of cord below the lesion; these consist of nerve-fibres that are connected with the nerve-cells in the brain ; they are called the CH. XVIII.] DESCENDING DEGENERATION. 203 pyramidal. tracts. Other tracts are found degenerated in the piece of cord above the lesion ; these consist of nerve-fibres that are connected with the nerve-cells of the spinal ganglia, or with the cells of the spinal cord itself below the lesion and are passing upwards. The tracts which degenerate downwards are the motor tracts ; the tracts that degenerate upwards are the sensory tracts. If the animal is killed a few weeks after the operation, its cord removed, and microscopic sections of it made and stained in an appropriate manner, the ascending tracts will be found degene- rated in the piece of cord above the lesion ; the descending tracts degenerated in the piece of cord below the lesion. The two next figures represent these. Tracts of descending degeneration (fig. 202). Pyramidal Antero-lateral descending Direct Pyramidal Fig. 202.-Descending tracts of degeneration. (i.) The crossed pyramidal tract.-This tract is situated in the lateral column on the outer side of the posterior cornu of grey matter. At the lower part of the spinal cord it extends to the margin, but higher up it becomes displaced from this position by the interpolation of another tract of fibres, to be presently described, viz., the direct cerebellar tract. The crossed pyramidal tract is large, and may touch the tip of grey matter of the posterior cornu, but is separated from it elsewhere. In shape on cross section it is somewhat like a lens, but varies in different regions of the cord, and diminishes in size from the cervical region downwards, its fibres passing off as they descend, to arborise around the nerve-cells and their branchings in the grey matter of the cord. The fibres of which this tract is composed are moderately large, but arc mixed with some that are smaller. 204 STRUCTURE OF THE SPINAL CORD. [ch. xviii. (ii.) The direct or uncrossed pyramidal tract.-This tract is situated in the anterior column by the side of the anterior fissure. It is smaller than (i.), and is not present in all animals, though conspicuous in the human cord. It can be traced upwards to the brain, and downwards as far as the mid or lower thoracic region, where it ends. The two pyramidal tracts come down from the brain; in the medulla oblongata, the greater number of the pyramidal fibres cross over to the other side of the cord which they descend; hence the term crossed pyramidal tract; a smaller collection of the pyramidal fibres goes straight on, on the same side of the cord, and crosses at different levels in the anterior commissure of the cord lower down; hence the disappearance of the direct pyramidal tract in the lower part of the cord. The fact that the crossed pyramidal tract of one side is the fellow of the direct pyramidal tract of the other side, is indicated in the diagram by the direc- tion of shading. (iii.) Antero-lateral descending tract. ■- An extensive tract, elongated but narrow, and reaching from the crossed to the direct pyramidal tract. It is a mixed tract, since not all of its fibres degenerate below the lesion. (iv.) Comma tract is a small tract of fibres which degenerate below section or injury of the cord. It is only found for a few millimetres below the actual lesion; though it degene- rates downwards it is in reality a sensory tract, being com- posed, as we have already seen, of the branches of the enter- ing posterior root-fibres which pass downwards on entering the cord. Tracts of ascending degeneration (fig. 203). (i.) Postero-median column.-This tract degenerates upwards on injury or on section of the cord, as well as on section of the posterior nerve-roots. It exists throughout the whole of the cord from below up, and can be traced into the bulb. It consists of fine fibres. The figure represents a section prepared from a piece of cord some distance above the injury, so that the degenerated fibres which begin in the column of Burdach have passed into the column of Goll. (ii.) Dorsal or direct cerebellar tract.-This tract is situated on the outer part of the cord between the crossed pyramidal tract and the margin. It is found in the cervical, thoracic, and upper lumbar regions of the cord, and increases in size from below upwards. It degenerates on injury or section of the cord itself, but not on section of the posterior nerve-roots. As its name implies, it passes up into the cerebellum. Its fibres are large, CH. XVIII.] ASCENDING DEGENERATION. 205 and originate from the cells of Clarke's column of the same side of the cord. (iii.) Ventral cerebellar tract, called also the antero-lateral ascending tract, or tract of Gowers.-This tract is situated at the marginof the cordoutsideof the corresponding descending tract. Its fibres are of various sizes, and originate from cells situated in the base of the anterior horn of the opposite side of the spinal cord, in the lower thoracic and lumbar region ; the fibres pass through the grey commissure and anterior horn of the opposite side, and travel up the tract of Gowers to terminate above principally in the cerebellum, but partly in the corpora quadrigemina. It is thus chiefly a crossed cerebellar tract. Kg-. 203.-Ascending tracts of degeneration. The diagram also indicates the position of Clarke's column (C.C.) and the intermedio-lateral tract (I.T.) in the lateral horn. (iv.) Tract of Lissauer, or posterior marginal zone.-A small tract of ascending fibres situated at the outer side of the tip of the posterior cornu. It is made up of fibres of the posterior nerve-roots. Hemisection.-If the operation performed is not a complete cutting of the spinal cord across transversely, but a cutting of half the cord across, it is termed hemisection. Complete transverse section of the spinal cord leads to :- 1. Loss of motion of the parts supplied by the nerves below the section on both sides of the body. 2. Loss of sensation in the same regions. 3. Degeneration, ascending and descending, on both sides of the cord. 206 STRUCTURE OF THE SPINAL CORD. [ch. xviii. Ilemisection leads to :- 1. Loss of motion of the parts supplied by the nerves below the section on the same side of the body as the injury. 2. Loss of sensation in the same region. 3. Degeneration, ascending and descending, nearly entirely confined to the same side of the cord as the injury. These are shown in the next figure. Fig. 204.-The above diagrams are reproductions of photo-micrographs from the spinal cord of a monkey in which the operation of right hemiseetion had been performed some weeks previously (Mott). The sections were stained by Weigert's method, by which the grey matter is bleached, while the healthy white matter remains dark blue. The degenerated tracts are also bleached. A is a section of the cord in the thoracic region below the lesion; the crossed pyramidal tract is degenerated. B is a section lower down in the lumbar enlargement; the degenerated pyramidal tract is now smaller. C is a section in the thoracic region some little distance above the lesion. The degenerated tracts seen are in the outer pari of Goll's column and in the direct cerebellar tract. D is a section higher up in the cervical region; the degeneration in GoH'scolumnnow occupies a median position ; the degenerations in the direct cerebellar tract, and in the tract of Gowers are also well shown. Notice that in all cases, the degenerated tracts are on the same side as the injury. Differences in different regions of the spinal cord.-The outline of the grey matter and. the relative proportion of the white matter varies CH. XVIII.] REGIONS OF THE SPINAL CORD. 207 in different regions of the spinal cord, and it is, therefore, possible to tell approximately from what region any given transverse section of the spinal cord has been taken. The white matter increases in amount from below upwards. The amount of grey matter, varies ; it is greatest in the cervical and lumbar enlargements, viz., at and about the 5th lumbar-and 6th cervical nerve, and least in the thoracic region. The greatest development of grey matter corresponds with greatest number of nerve-fibres passing from the cord. In the cervical enlargement the grey matter occupies a large proportion of the section, the grey commissure is short and thick, the anterior horn is blunt, whilst the posterior is somewhat tapering. The anterior and posterior roots run some distance through the white matter before they reach the periphery. At the extreme upper part of the cervical region, the end of the posterior horn is swollen out by excess of neuroglia into a rounded mass called the substantia gelatinosa of Rolando., The cervical cord is wider from side to side than from before back ; this is owing to the great width of the lateral columns. In the dorsal region the grey matter bears only a small relation to the white, and the posterior roots in particular run a long course through the white matter before they leave the cord ; the grey commissure is thinner and narrower than in the cervical region. The intermedio-lateral tract is here most marked, and forms a prominence often called the lateral horn. This is shown in fig. 203 (1 T). Clarke's column is also confined to this region of the cord ; the position of the cells which make up this column is shown in the same figure (c c). The cord in this region is circular in transverse section (see also fig. 204 C.). In the lumbar enlargement the grey matter again bears a very large pro- portion to the whole size of the transverse section, but its posterior cornua are shorter and blunter than they are in the cervical region. The grey commissure is short and extremely narrow. The cord is circular on trans- verse section. yl£ the upper part of the conus medullaris, which is the portion of the cord immediately below the lumbar enlargement, the grey substance occupies nearly the whole of the transverse section, as it is only invested by a thin layer of white substance. This thin layer is wanting in the neighbourhood of the posterior nerve-roots. The grey commissure is extremely thick. At the level of the fifth sacral vertebra the grey matter is again in excess, and the central canal is enlarged, appearing T-shaped in section ; whilst in the upper portion of the filum terminate the grey matter is uniform in shape without any central canal. 208 THE BRAIN. fCH. XIX, CHAPTER XIX. THE BRAIN. A student's first glance at a brain, or at such a drawing of it as is given in fig. 205, will be sufficient to convince him of its complicated structure. It certainly is extremely complex, but by studying it systematically we shall find that a knowledge of the Fig. 205.-Base of the brain. 1, superior longitudinal fissure; 2, 2', 2", anterior cerebral lobe ; 3, fissure of Sylvius, between anterior and 4, 4', 4", middle cerebral lobe ; 5, 5', posterior lobe; 6, medulla oblongata; the figure is in the right anterior pyramid; 7, 8, 9, 10, the cerebellum; +, the inferior vermiform process. The figures from I. to IX. are placed against the corresponding cerebral nerves; III. is placed on the right crus cerebri. VI. and VII. on the pons Varolii; X. the first cervical or suboccipital nerve. (Allen Thomson.) J. essential facts in its anatomy will be attainable with comparative ease. An acquaintance with the structure of the brain is, more- over, essential for understanding its functions. So we shall devote this and a few succeeding chapters to anatomical conside- rations, before passing on to the study of its physiology. CH. XIX.] THE BRAIN. 209 Aii outline diagram of its parts, such as is presented in the next figure, will indicate the various parts of the brain which we shall have to take into consideration. At the lowest part of the brain, continuing the spinal cord upwards, is the medulla oblongata or bulb (D). Next comes the pons Varolii (C), very appropriately called the bridge, because in it are the connections between the bulb and the upper regions of the brain, and between the cerebellum or small brain (B) and the rest of the nervous system. The mid-brain comes next (a, 6), and this leads into the • Fig. 206.-Plan in outline of the brain, as seen from the right side. The parts are represented as separated from one another somewhat more than natural, so as to show their connections. A, cerebrum ; f, g, h, its anterior, middle, and posterior lobes ; e, fissure of Sylvius ; B, cerebellum; C, pons Varolii ; D, medulla oblongata; a, peduncles of the cerebrum; b, c, d, superior, middle, and inferior peduncles of the cerebellum. (From Quain.) peduncles or crura of the cerebrum (A), the largest section of the brain. Through the brain runs a cavity filled with cerebro-spinal fluid, and lined by ciliated epithelium; this is continuous with the central canal of the spinal cord. In the brain, however, it does not remain a simple canal, but is enlarged at intervals into what are called the ventricles. There is one ventricle in each half or hemisphere of the cerebrum, these are called the lateral ventricles, they open into the third ventricle, which is in the middle line ; and then a narrow canal (aqueduct of Sylvius') leads from this to the/ourtfA ventricle, which is placed on the back of the bulb and 210 THE BRAIN. [ch. xix. pons, which form its floor ; its roof is formed partly by the over- hanging cerebellum (fig. 206), partly by pia mater. This piece of pia mater is pierced by a hole (Foramen of Magendie), and so the cerebro-spinal fluid in the interior of the cerebro-spinal cavity is con- tinuous with that which bathes the external surface of brain and cord in the sub-arachnoid space. The fourth ventricle leads into the central canal of the spinal cord. The fifth ventricle in the central structures of the brain does not communicate with the others. The cerebro-spinal fluid is a thin watery fluid, containing a small quantity of salts and proteids in solution, and a- substance which gives Trommel's test for sugar ; it is, however, not sugar, but a substance of the aromatic group,, called pyrocatechin. Before passing on to describe these portions of the brain one by one, it will be convenient to state first a few general facts. (i.) Inthebulb, at the lower part the distribution of grey matter follows that which prevails in the cord. Higher up the chief part is found towards the posterior or dorsal aspect, surrounding the central canal. When the central canal opens out into the fourth ven- tricle, the grey matter comes to that surface chiefly, and is found to consist more particularly, on either side, of the nuclei of origin of the cranial nerves, viz., the 12th, nth, 10th, 9th, and 8th, and more externally of the nucleus gracilis and nucleus cuneatus. In ad- dition to these masses of grey matter there are the olivary bodies towards the ventral surface with the accessory olives and the external arcuate nuclei, placed at the tip of the anterior fissure on either side on the ventral surface of the anterior pyramids. (ii.) In the pons Varolii.-In addition to the origins of nerves Fig. 207.-Diagrammatic hori- zontal section of a vertebrate brain. The figures serve both forthis and thenext diagram. Mb, mid-brain : what lies in front of this is the fore-, and what lies behind, the hind- brain ; Lt, lamina teimina- lis; Olf, olfactory lobes ; Bmp, hemispheres ; Th. E, thalamencephalon; Pn,pineal gland ; Py, pituitary body ; F.M., foramen of Munro ; cs, corpus striatum ; Th, optic thalamus ; CC, crura cerebri: the mass lying above the canal represents the corpora quadrigemina ; Cb, cerebel- lum ; I-IX, the nine pairs of cranial nerves; i, olfactory ventricle; 2, lateral ventricle; 3, third ventricle ; 4, fourth ventricle ; +, iter a tertio ad quartum ventriculum. (Huxley.) CH. XIX.] THE BRAIN. 211 in the floor of the fourth ventricle on the dorsal aspect of the pons, viz., of the 7th, 6th, and 5th nerves, there are several masses of grey matter, viz., in the back part, the superior olive and in the front part the locus coeruleus, as well as small amounts of the same material mixed with fibres in the more ventral surface. (iii.) In the mid-brain, the grey matter preponderates in the corpora quadrigemina, and corpora geniculata. It is also found surrounding the aqueduct of Sylvius, and in other parts of the crura, notably such masses as the red nucleus and locus niger. (iv.) In the cerebral hemispheres, the cerebral cortex is made up Fig. 208.-Longitudinal and vertical diagrammatic section of a vertebrate brain. Letters as before. Lamina terminals is represented by the strong black line joining Pn and Py. (Huxley.) of grey matter which encloses white matter, and the corpora striata and optic thalami are made up chiefly of grey matter. (v.) In the cerebellum, the grey matter forms the encasing material of the white matter. In the interior too there are masses of grey matter forming the corpora dentata. Speaking generally, there are two main collections of grey matter-that on the surface, and that in the interior bordering on the cerebro-spinal cavity, and subdivided into various masses (corpora striata, optic thalami, &c.), whose names have been men- tioned, but whose closer acquaintance we shall make presently. The cerebral or cranial nerves, some of which have also been mentioned, are those which originate from the brain ; there are twelve pairs of these altogether, and the majority originate from nerve-cells in the grey matter of the floor of the fourth ventricle or its immediate neighbourhood. In the foetus the central nervous system is formed by an in- folding of a portion of the surface epiblast. Tins becomes a tube of nervous matter, which loses all connection with the surface of the body, though later in life this is in a sense re-established by 212 STRUCTURE OF THE BULB, PONS, & MID-BRAIN, [ch. xx. the nerves that grow from the brain and cord to the surface. The anterior end of this tube becomes greatly thickened, to form the brain, its cavity becoming the cerebral ventricles ; the rest of the tube becomes the spinal cord. The primitive brain is at first subdivided into three parts, the primary cerebral vesicles; the first and third of these again subdivide, so that there are ultimately five divisions, which have received the following names :- 1. Pros-encephalon, or fore brain. This is developed into the cerebrum with the corpora striata. It encloses the lateral ventricles. 2. Thalam-encephalon, or tivixt brain. This is developed into the parts including the optic thalami, which enclose the third ventricle. 3. Mes-encephalon, or mid brain, consists of the parts which enclose the aqueduct of Sylvius-namely, the corpora quadri- gemina, which form its dorsal, and the crura cerebri, which form its ventral aspect. 4. Met-encephalon, or hind brain, which forms the cere- bellum and pons. 5. Ep-encephalon, or after brain, which forms the bulb or medulla oblongata. Figs. 207 and 208 represent a diagrammatic view of a verte- brate brain; the attachment of the pineal gland, pituitary body, and olfactory (I) and optic (II) nerves is also shown. CHAPTER XX. STRUCTURE OF THE BULB, PONS, AND MID-BRAIN. We may study the bulb and pons by examining first the anterior or ventral, then the posterior or dorsal aspect, and last of all the interior. Anterior Aspect. The bulb is seen to be roughly shaped, like an inverted trun- cated cone, larger than the spinal cord, and enlarging as it goes up until it terminates in the still larger pons (/>). In the middle line is a groove, which is a continuation upwards of the CH. XX.] THE SURFACE OF THE BULB. 213 anterior median fissure of the spinal cord ; the columns of the bulb are, speaking roughly, continuations upwards of those of the cord, but there is a considerable rearrangement of the fibres in each. Thus the prominent columns in the middle line, called the pyramids (a a), are composed of the pyramidal fibres, which Fig'. 210.-Dorsal or posterior surface of the pons Varolii, corpora quad- rigemina, and medulla oblongata. The peduncles of the cerebellum are cut short at the side, a, a, the upper pair of corpora quadri- gemina; b, b, the lower ; /. /, supe- rior peduncles of the cerebellum; c, eminence connected with the nucleus of the hypoglossal nerve; e, that of the glosso-pharyngeal nerve ; i, that of the vagus nerve ; d, d, restiform bodies; p, p, poste- rior columns ; v, v, groove in the middle of the fourth ventricle, ending below in the calamus scrip- torius ; 7, 7, roots of the auditory nerves. Fig. 209.-Ventral or anterior surface of the pons Varolii, and medulla oblon- gata. a, a, anterior pyramids; b, their decussation; c, c, olivary bodies; d, d, restiform bodies; e, arciform fibres ; /, fibres passing from the an- terior column of the cord to the cere- bellum ; g, anterior column of the spinal cord; h, lateral column; p, pons Varolii; i, its upper fibres; 5, 5, roots of the fifth pair of nerves. in the spinal cord are situated principally in the lateral columns of the opposite side (crossed pyramidal tracts). The decussation or crossing of the pyramids (6) occurs at their lower part ; a small collection of the pyramidal fibres is, however, continued down the cord in the anterior column of the same side of the cord (direct pyramidal tract) : these cross at different levels in the cord. On the outer side of each pyramid is an oval prominence (c c), which is not represented in the spinal cord at all. These are 214 STRUCTURE OF THE BULB, PONS, & MID-BRAIN. [CH. XX. called the olivary bodies or olives ; they consist of white matter outside, with grey and white matter in their interior. The restiform bodies at the sides (d cZ) are the continuation upwards of those fibres from cord and bulb which enter the cerebellum, and the upper part of each restiform body is called the inferior peduncle of the cerebellum.* Posterior Aspect. Fig. 210 shows a surface view of the back of the bulb, pons, and mid-brain. Again we recognise some of the parts of the spinal cord continued upwards, though generally with new names, and again we see certain new structures. The posterior median fissure is continued upwards, and on each side of it is the prolongation upwards of the posterior columns of the cord. The column of Goll is now called the Funiculus gracilis, and the column of Burdach the Funiculus cuneatus. The two funiculi graciles lie at first side by side, but soon diverge and form the two lower boundaries of a diamond-shaped space called the floor of the fourth ventricle ; this is made of grey matter ; the central canal of the cord gets nearer and nearer to the dorsal surface of the bulb, till at last it opens out on the back of the bulb, and its surrounding grey matter is spread out to form the floor of the fourth ventricle. The two upper boun- daries of the diamond-shaped space are made by the superior peduncles of the cerebellum, which contain fibres coming down through the mid-brain from the cerebrum. The middle peduncles of the cerebellum are principally made up of fibres running from one cerebellar hemisphere to the other through the pons. Running down the centre of the floor of the fourth ventricle is a shallow groove ; on each side of this is a rounded longitudinal eminence called the funiculus teres ; running across the middle of the floor are a number of fibres (the strice acousticce'), which join the auditory nerve. In the upper part of the diagram the mid-brain, with the corpora quadrigemina (a a, b b), is shown. Here there is once more a canal which penetrates the substance of the mid-brain, and is called the aqueduct of Sylvius, or the iter a tertio ad quartum ventriculum,; it leads, as its second name indicates, from the third to the fourth ventricle. * Each half of cerebellum has three peduncles : inferior, middle, and superior. CH. XX.J THE CRANIAL NERVES. 215 Origin of the Cranial Nerves. Each cranial nerve is said to have two origins : a deep origin, i.e., the region of grey matter 'where its fibres actually arise from nerve-cells; and a superficial origin, the region of the brain's sur- Fig. 211.-Dorsal or posterior view of the medulla, fourth ventricle, and mesencephalon (natural size). p.n., line of the posterior roots of the spinal nerves ; p.m.f., posterior median fissure; f.g., funiculus gracilis ; ch, its continuation, called the clava ; f.c., funiculus cuneatus ; f.R., funiculus of Rolando ; r.lj., restiform body; c.s., cala- mus scriptorius; I, section of ligula or taenia ; part of choroid plexus is seen beneath it; l.r., lateral recess of the ventricle; str., striae acustieae; i.f., inferior fossa; s./., posterior fossa ; between it and the median sulcus is the fasciculus teres ; cbl., cut surface of the cerebellar hemisphere; md., central or grey matter; s.m.v., superior medullary velum ; Ing., ligula ; s.c.p., superior cerebellar peduncle cut longitudinally ; cr., combined section of the three cerebellar peduncles ; c.q.s., c.q.i., corpora quadri- gemina (superior and inferior) ; /r., fraenulum; /., fibres of the fillet seen on the surface of the tegmentum ; c.. erusti; Z.7., lateral groove; c. g.i., corpus genieulum intemus; t.h., posterior part of thalamus; p., pineal body. The Roman numbers indicate the corresponding cranial nerves. (E. A. Schafer.) face where the nerve, after coursing through the brain substance, actually leaves it for its destination. The deep origins of the cranial nerves will especially interest us as students of physiology. There are twelve pairs of cranial 216 STRUCTURE OF THE BULB, RONS, & MID-BRAIN. [ch. xx. nerves altogether, and of these, ten originate from the floor of the fourth ventricle or the neighbouring grey matter. This will, moreover, be a convenient place to describe not only the origin, but also in brief the functions of the cranial nerves. Fig. 211 shows a surface view of this part of the brain with the nerves attached, and the following one (fig. 212) shows in a diagrammatic way their deep and superficial origins. It will be noticed that many of the cranial nerves are not mixed like the spi- nal nerves, but are either wholly sensory or wholly motor in function, 1. Olfactory nerve.- This is the nerve of smell. 2. Optic nerve.-This is the nerve of sight. These two pairs of sen- sory nerves come under a different category from the rest, and will be de- scribed later. It is the remaining ten pairs that originate from the tract of grey matter we are discussing just now. 3. Motor oculi. -This is wholly motor. It sup- plies the following ex- trinsic eye-muscles : su- perior rectus, inferior rec- tus, internal rectus, and inferior oblique ; and the following intrinsic eye-muscles (?>., within the eyeball itself) : ciliary muscle, and the sphincter fibres of the iris. Its deep origin is in the grey matter on the side of the Sylvian aqueduct underneath the corpora quadrigemina. 4. Trochlear.-This is wholly motor too. It supplies the superior oblique muscle of the eyeball. It takes origin from the grey matter immediately below the centre of the third (not shown in figure) ; its centre is connected with that of the third and sixth nerves. 6. Aljehicens.-It is convenient to take this next. It also is. Fig. 212.-Fourth ventricle, with the medulla ob- longata and the corpora quadrigemina. The Roman numbers indicate superficial origins of the cranial nerves, while the other numbers indicate their deep oiigins, or the position of their central nuclei. 8, 8', 8", 8"'. auditory nuclei nerves ; t, funiculus teres ; A, B, corpora quad- rigemina ; c, <7, corpus geniculatum ; p, c, pe- dunculus cerebri; m, c, p, middle cerebellar peduncle; s, c, p, superior cerebellar peduncle; i, c, p, inferior cerebellar peduncle ; I, c, locus cceruleus; e. t, eminentia teres ; a, c, ala cinerea ; o, n, accessory nucleus ; 0, obex ; c, clava ; f, c, funiculus cuneatis; f, g, funiculus gracilis. CH. XX.] THE CRANIAL NERVES. 217 wholly motor, supplying the external rectus of the eyeball. Its centre is in the upper part of the floor of the fourth ventricle, near the middle line. 5. Trigeminal.-This is a mixed nerve. Its smaller motor division supplies the muscles of mastication; its larger sensory division, the Gasserian ganglion on which corresponds to the spinal ganglion on a spinal nerve, is the great sensory nerve of the face and head. Its deep origin is also double. The motor-centre is internal to the sensory, and from it reach a number of fibres stretching upwards as far as the anterior corpus quadrigeminum; this is termed its descending root; it is also connected with the locus cceruleus. The sensory centre or nucleus outside the motor has connected with it a tract of fibres from the cord as low as the second cervical nerve (ascending root) (fig. 212, 5). 7. Facial.-This is the great motor nerve of the face muscles. When it is paralysed, the muscles of the face being all powerless, the countenance acquires on the paralysed side a characteristic, vacant look, from the absence of all expression : the angle of the mouth is lower, and the paralysed half of the mouth looks longer than that on the other side ; the eye has an unmeaning stare, owing to the paralysis of the orbicularis palpebrarum. All these peculiarities increase, the longer the paralysis lasts : and their appearance is exaggerated 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 and cheeks 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 feeding, the lips and cheek are powerless, and on account of paralysis of the buccinator muscle food lodges between the cheek and gums. The deep origin of this nerve is shown in the diagram below that of the fifth, and to the outer side of that of the sixth nerve. The chorda tympani nerve, one of the branches of the seventh, we shall study in connection with secretion and vaso-dilatation. 8. Auditory.-This arises from two nuclei, median and lateral, in the floor of the fourth ventricle, in the anterior part of the bulb in front and to the side of the twelfth nerve ; it extends from the middle line to the outside margin of the ventricle. There is also an accessory nucleus situated on the ventral surface of the restiform body. The nerve leaves the sur- face of the brain from the ventral surface of the fore-part of the restiform body at the hind margin of the pons in two roots. One winds round the restiform body dorsal to it and the other passes 218 STRUCTURE OF THE BULB, RONS, & MID-BRAIN, [ch. xx. median to it. The former is called the dorsal root. The latter is called the ventral root. Most of the fibres of the dorsal root (cochlear) end in cells of the accessory nucleus, but fibres emerging from this nucleus pass inwards to the bulb, superficially, forming the strice acousticce in the floor of the fourth ventricle and end in the median nucleus. Most of the fibres of the ventral root (vestibular) end in cells of the lateral nucleus. The cells of the median nucleus are small, those of the lateral nucleus, large. The cochlear branch is the auditory nerve proper, and the vestibular is distributed to the semicircular canals, the utricle and saccule, parts of the internal ear not directly concerned with hearing. 9, io, ii.-These three nerves are called respectively the glosso-pharyngeal, vagus or pneumogastric, and spinal accessory. They arise from an area of grey matter, reaching from about the middle of the floor of the fourth ventricle down into the spinal cord, as low as the origin of the sixth or seventh cervical nerves. The nuclei of the three nerves are closely connected with each other. In addition to this combined nucleus there are certain lateral contributions, namely:-i. the nucleus ambiguus, which lies on the lateral side of the reticular formation and is an accessory origin of the vagus; ii. the fasciculus solitarius, situated in the bulb, ventral and a little lateral to the combined nucleus, is also called the ascending root of the glosso-pharyngeal nerve or the respiratory bundle; and iii. the spinal portion which takes origin from a group of cells lying in the extreme lateral margin of the anterior cornu. This is the origin of the spinal accessory, it corresponds to the antero-lateral nucleus of the bulb, and the lateral part of the grey matter of the spinal cord. The fibres of the spinal origin of the nerve pass from these cells through the lateral column to the surface of the cord. The fibres from the combined mccleus, chiefly from the median part, pass in a ventral and lateral direction through the reticular formation, then ventral to or through the gelatinous substance and the strand of fibres connected with the fifth nerve, to the surface of bulb. The bundles of fibres of the fasciculus solitarius start in the lateral grey matter of the cervical cord and higher in the reticular formation of the bulb, run longitudinally forwards to pass into the roots of the ninth nerve. The glosso-pharyngeal nerve gives filaments through its tympanic branch (Jacobson's nerve), to the fenestra ovalis and fenestra rotunda, and the Eustachian tube, parts of the middle ear; also, CH. XX.] THE CRANIAL NERVES. 219 to the carotid plexus, and through the petrosal nerve, to the •spheno-palatine ganglion. After communicating, either within or without the cranium, with the vagus, it leaves the cranium, with the sympathetic, digastric branch of the facial, and the accessory nerve, and 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 csecum in the middle line, and to near the tip at the sides and inferior part. Functions.-The glosso-pharyngeal nerve contains some motor fibres to some of the pharyngeal muscles, together with those of ■common sensation and the sense of taste. The vagus or pneumogastric nerve has most varied functions, giving branches to the pharynx, larynx, oesophagus, stomach, lungs, heart, intestines, liver, and spleen. Some fibres are afferent and some efferent. It will, however, be best to postpone ■our study of the functions of this nerve until we know more about the organs it supplies. It should be particularly noted that the principal origin of this nerve is at the lower end of the ventricular floor, or as it is generally called, the calamus scriptorius. The spinal accessory nerve arises by two distinct origins-one from a centre in the floor of the fourth ventricle, and connected with the glosso-pharyngeal-vagus-nucleus ; the other, from the ■outer side of the anterior cornu of the spinal cord as low down as the fifth or sixth cervical nerve. The fibres from the two origins ■come together at the j ugular foramen, but separate again into twQ. branches, the inner of which, arising from the medulla, joins the vagus, to which it supplies its motor and cardio-inhibitory fibres, consisting of small medullated nerve-fibres, whilst the outer, consisting of large medullated fibres, supplies the trapezius .and sterno-mastoid muscles. The external branch, which is the larger of fhe two, is composed almost exclusively of motor fibres. The internal branch of the accessory nerve supplies chiefly viscero- motor filaments to the vagus. The muscles of the larynx, all of which are supplied, apparently, by branches of the vagus, derive their motor nerves from the accessory • and (which is a very significant fact) Vrolik states that in the chimpanzee the internal branch of the accessory does not join the vagus at all, but goes direct to the larynx. 12. The hypoglossal nerve arises from a large celled and long nucleus in the bulb, close to the middle line, inside the combined nucleus of tire ninth, tenth, and eleventh nerves. Fibres from 220 STRUCTURE OF THE BULB, PONS, & MIO-BRAIN, [ch. xx. this nucleus run from the ventral surface through the reticular formation in a series of bundles, and it emerges from a groove between the anterior pyramid and olivary body. It is connected with the vagus, the superior cervical ganglion of the sympathetic and with the upper cervical nerves. Distribution.-This nerve is the motor nerve to the muscles, connected with the hyoid bone, including those of the tongue. CEREBELLAR 4- HEMISPHERE Fig. 213.-Diagrammatic representation of dorsal aspect of medulla, pons, and mid-brain. It supplies through its descending branch (descendens noni), the sterno-hyoid, sterno-thyroid, and omo-hyoid; through a special branch, the thyro-hyoid, and through its lingual branches, the- genio-hyoid, stylo-glossus, hyo-glossus, and genio-hyo-glossus and linguales. As a motor nerve, its influence on all the muscles enumerated above is shown by their movements when it is irritated, and by their loss of power when it is paralysed. A mere enumeration of the destination of the nerves arising in the bulb shows how supremely important this small area of the- ■ch. xx.] INTERIOR OF THE BULB. 221 brain is for carrying on the organic functions of life. It contains centres which regulate deglutition, vomiting, the secretion of saliva, sweat, &c., respiration, the heart's movements, and the vaso-motor nerves. These and others we shall have to make a very complete study of when discussing the various subjects referred to. The Internal structure of the Bulb, Pons, and Mid-brain. The structure of the interior of these parts is best studied in a series of transverse sections. We will limit ourselves to seven, the level of which is indicated in fig. 213. The cerebellum has been bisected into two halves and turned outwards, its upper Fig. 214.-Section through the bulb at level of the decussation of the pyramids, g, funi- culus gracilis, continuation of column of Goll; c, funiculus cuneatus, continuation of column of Burdach; k, substantia gelatinosa of Rolando, continuation of posterior horn of spinal cord ; l, continuation of lateral column of cord; a, remains of part of the anterior horn, separated from the rest of the grey matter of the pyramidal fibres p, which are crossing from the pyramid of the medulla to the posterior part of the lateral column of the opposite side. (After L. Clarke.) peduncles cut through to render the parts more evident. The position of our seven sections is indicated by the transverse lines numbered i to 7. First section.-This is taken at the lowest level of the bulb, through the region of the decussation of the pyramids. The similarity to the cervical cord will be at once recognised; the passage of the pyramidal fibres (P) from the anterior part of the bulb to the crossed pyramidal tract of the opposite side of the cord cuts off the tip of the anterior horn (A), which in sections higher up appears as an isolated mass of grey matter, called the lateral nucleus (fig. 215, nl). The V formed by the two posterior horns is opened out, and thus the grey matter with the central 222 STRUCTURE OF THE BULB, RONS, & MID-BRAIN, [ch. xx. canal is brought nearer to the dorsal aspect of the bulb ; the tip of the cornu swells out to form the substantia gelatinosa of Rolando (R), which causes a prominence on the surface called the tubercle of Rolando ; G and C are the funiculi gracilis and cuneatus respectively, the continuations upwards of the columns of Goll and Burdach. Second section.-This is taken through the upper part of the pyramidal decussation. Beginning in the middle line at the Fig. 215.-Anterior or dorsal section of the medulla oblongata in the region of the superior pyramidal decussation, a.m.f., anterior median fissure; f.a., superficial arciform fibres emerging from the fissure; py., pyramid; a.a.r., nuclei of arciform fibres ; /.a1., deep arciform becoming superficial ; 0, lower end of olivary nucleus; n. I, nucleus lateralis; /.»•., formatio reticularis ; f.a'2., arciformfibres proceeding from the formatio- reticularis ; g, substantia gelatinosa of Rolando ; a.V., ascending root of fifth nerve ; n.c., nucleus cuneatus; n.c.', external cuneate nucleus; n.g., nucleus gracilis; f.g., funiculus gracilis; p.m.J., posterior median fissure; c.c., central canal surrounded by grey matter, in which are n.XI., nucleus of the spinal accessory, and n.XII., nucleus of the hypoglossal; s.d., superior pyramidal decussation (decussation of fillet). (Modified from Schwalbe.) top of the diagram, we see first the posterior median fissure below which is the grey matter enclosing the central canal (c.c.), and containing the nuclei of the eleventh and twelfth nerves; the funi- culus gracilis (f.gi) comes next, and then the funiculus cuneatus (/.c.) ; these two funiculi have now grey matter in their interior : these masses of grey matter are called respectively nucleus gracilis (n.gd) and nucleus cuneatus (n.c.'); the fibres which have ascended the posterior columns of the cord terminate by arborising around the CH. XX.] INTERIOR OF THE BULB. 223 cells of this grey matter; the fibres from the lower part of the body ending in the nucleus gracilis, and those from the upper part of the body in the nucleus cuneatus. These nuclei form a most important position of relay in the course of the afferent fibres, from cord to brain. The new fibres arising from the cells of these nuclei pass in a number of different directions, and break up the rest of the grey matter into what is called the formatter reticularis. The fibres may be divided into three main groups ; they are termed arcuate or arciform fibres. 1. The external arcuate fibres {fa, f a1) course round the ven- tral surface of the bulb. 2. Some of these turn round sharply (/a2) to the restiform body of the same side. 3. The internal arcuate fibres are those which pass into the formatio reticularis and cross with their fellows at the median raphe, forming what is sometimes called the superior p>yramidal decussation, but which should be more properly called the decus- sation of the fillet. . The fillet fibres, after having crossed to the other side, become a longitudinal bundle, which lies just dorsal to the pyramid {p>y\ and continues upwards to various parts of the cerebrum, passing, however, through one or more cell stations (positions of relay) before ultimately arriving at the cortex. We now see that the brain has a crossed relationship to the body, the left half of the brain governing the, right half of the body, and vice versd, both as regards motion and sensation ; the motor- fibres mostly cross at the decussation of the pyramids, some few (those in the direct pyramidal tract) crossing at lower levels in the cord; the sensory fibres mostly cross at the decussation of the fillet, though some few cross at lower levels in the cord, soon- after their entrance into the cord by the posterior nerve-roots. Other points to be noticed in the section are the substantia gelatinosa of Rolando (y) (remains of posterior cornu), now sepa- rated from the surface by the ascending root of the fifth nerve (a V) ; the lateral nucleus (n I) ; the lower part of the grey matter of the olivary body (o o1), and most anteriorly the pyramid {py\ Third section.-This is taken at about the middle of the olivary body, and passes'also through the lower part of the floor of the fourth ventricle. The central canal has now opened out into the fourth ventricle, and the grey matter on its floor contains the nuclei of the twelfth and tenth nerves ; bundles of the fibres of these nerves course through the substance of the bulb, leaving it at the places indicated in the diagram. The nucleus gracilis, nucleus cuneatus, and tubercle of Rolando 224 STRUCTURE OF THE BULB, PONS, & MID-BRAIN, [ch. xx. are pushed into a more lateral position; the restiform body (Cr) now forms a well-marked prominence, and the olivary body is well seen with its dentate nucleus ; from the open mouth of this corrugated layer of grey matter a large number of fibres issue, and passing through the raphe, course as internal arcuate fibres to the opposite restiform body, and thus to the cerebellum; some Tig. 216.-Section of the medulla oblongata at about the middle of the olivary body. t.l.a., anterior median fissure; n.ar., nucleus arciformis; p, pyramid ; XII., bundle of hypoglossal nerve emerging from the surface ; at b, it is seen coursing between the pyramid and the olivary nucleus, o ; f.a.e., external arciform fibres ; n.l., nucleus lateralis ; a., arciform fibres passing towards restiform body, partly through the sub- stantia gelatinosa, g., partly superficial to the ascending root of the fifth nerve, a. V. ; X., bundle of vagus root emerging; f.r., formatio reticularis ; c.r., corpus restiform, beginning to be formed, chiefly by arciform fibres ; superficial and deep ; n.c., nucleus cuneatus ; n.g., nucleus gracilis ; t, attachment of the ligula ; f.s., funiculus solitarius ; n.X., n.X.', two parts of the vagus nucleus ; n.XII., hypoglossal nucleus ; n.i., nucleus of the funiculus teres; n.am., nucleus ambiguus ; r., raphe ; A., continuation of the anterior column of cord ; o', o", accessory olivary nucleus ; p.o., pedunculus olivee. (Modified from Schwalbe.) ■pass to the restiform body of the same side; the continuation of the direct cerebellar tract of the cord also passes into the resti- form body. Its fibres terminate by arborisations round Purkinje's •cells in the vermis. The funiculus solitarius and nucleus ambiguus, also seen in this section, have been already alluded to in our •account of the origin of the cranial nerves (p. 218). Fourth, section.-This is taken through the middle of the pons, and shows much the same kind of arrangement as in the upper part of the bulb. The general appearance of the section CH. xx.] INTERIOR OF THE BULB. 225 is, however, modified by a number of transversely coursing bundles of fibres, most of which are passing from the cerebellar hemispheres to the raphe, and form the middle cerebellar peduncles. Intermingled with these is a considerable amount of grey matter (nuclei pontis). The continuation upwards of the pyramids (py) is embedded between these transverse bundles, and separated by them from the reticular formation; the deeper transverse fibres, which form the trapezium (t), connect the supe- rior olivary nucleus of one side to the accessory auditory nucleus Fig. 217.-Section across the pons, about the middle of the fourth ventricle, py., pyramidal bundles; po., transverse fibres passing po1, behind, andjt>o2, in front of py ; r., raphe; o.s., superior olive; a. V., bundles of ascending root of V. nerve enclosed in a pro- longation of the substance of Rolando ; VI., the sixth nerve, n.VI., its nucleus ; VII., facial nerve; FZZ.a., intermediate portion, n.VII., its nucleus ; VIII., auditory nerve, n.VIII., lateral nucleus of the auditory. (After Quain.) of the other side. The large olivary nucleus is no longer seen, but one or two small collections of grey matter (os) represent it, and constitute the superior olivary nucleus. The nerves taking origin in this region of the floor of the fourth ventricle are the sixth, seventh, and eighth. The fifth nerve originates higher up, where the floor of the fourth ventricle is narrowing, till at last, in the region of the mid-brain, we once more get a canal (Sylvian aqueduct) corresponding to the central canal of the spinal cord. The reticular formation between the grey floor of the ventricle and the trapezium is a continuation upwards of the reticular 226 STRUCTURE OF THE BULB, PONS, & MID-BRAIN. [CH. xx. formation seen in previous sections. It consists of white fibres passing transversely in different directions, between which there are fibres running longitudinally, and a considerable amount of grey matter. In such a complex system of intercrossing fibres, it is extremely difficult to separate any definite tracts, but there are at least two longitudinal tracts of fibres in it which a little higher up in the mid-brain are separated off from the rest; one of these is the fillet, the origin of which in the nuclei gracilis and cuneatus of the opposite side we have already seen; the other is the posterior longitudinal bundle, which is stated by some to be a continuation upwards of some of the fibres of the anterior column of the cord ; it certainly contains fibres connecting the nuclei of the third and sixth nerves. These are shown in the Fifth and. Sixth sections, which are taken through the Fig. 218.-Outline of two sections across the mid-brain: A, through the middle of the inferior; B, through the middle of the superior corpora quadrigemina C.Q. Cr., crusta ; S.N., substantia nigra-shown only on one side; T, tegmentum; S, Sylvian aqueduct, with its surrounding grey matter; L. G., lateral groove; p.l., posterior longitudinal bundle; d. V, descending root of the fifth nerve; S.C.P., superior cerebellar peduncle; F, fillet; III., third nerve. The dotted circle in B represents the situation of the tegmental nucleus. (After Schafer.) In B the three divisions of the crusta are indicated on one side. The pyramidal fibres (By) are in the middle, and the fronto-cerebellar (F.C.) and temporo-occipital cerebellar (T.O.C.) at the sides. mid-brain, and are drawn on a smaller scale than the others we have been examining ; they represent the actual size of the sec- tions obtained from the human subject. Near the middle is the Sylvian aqueduct, with its lining of ciliated epithelium. In the grey matter which surrounds it are large nerve-cells, from which the fourth nerve, and higher up the third nerve, originate; the fibres of the third nerve are seen issuing from these in fig. 218, B III. The reticular formation of the pons is continued up into the mid-brain, and is called the tegmentum. Its transverse fibres include the decussating fibres of the superior peduncles of the cerebellum. The fibres of the fillet partly pass in an oblique manner to the side of the mid-brain, and ter- ch. xx. ] THE CRUS. 227 minate in the grey matter of the corpora quadrigemina (C Q); this is called the lateral fillet; the rest of the fillet (mesial fillet') goes on through the crus, and has been traced into the optic, thalamus ; from here fresh nerve-fibres, forming a new relay, con- tinue the afferent impulses to the cortex of the cerebrum. The pyramidal bundles of the pons are continued upwards, and form the middle third of the crusta (cr) or pes. The crusta and tegmentum are separated by a layer of grey matter called the substantia nigra (S N). There is also grey matter in the teg- mentum itself, which is called the tegmental or red nucleus. The corpora quadrigemina are formed mainly of grey matter; from each a bundle of white fibres passes upwards and forwards to the geniculate bodies, eventually joining the optic tract of the same side. The white layer on the surface of the grey matter of the C. quadrigemina is derived from the optic tract; these fibres come from the retina, and terminate by arborising around the cells of the grey matter of the C. quadrigemina. The further relationships of these parts of the brain we shall study in con- nection with vision. Seventh, section.-This is through the crus. It is made up of crusta (which contains the motor fibres), tegmentum (which contains the sensory fibres, especially in the bundle called the mesial fillet), and the substantia nigra, the grey matter which separates them. The destination of one of the spinal cord tracts we have not yet mentioned ; this is the tract of Gowers. This is continued up through the ventral part of the pons lateral to the pyramidal bundles ; when it reaches the superior cerebellar peduncles the main part of the tract takes a sharp backward turn and enters the middle lobe or vermis of the cerebellum by the superior peduncle and superior medullary velum. Some of the fibres of the tract are continued, however, into the corpora quadrigemina. Fig. 219.-Section through crus of cerebrum. Cr, crusta; S.N.,substantia nigra; T, tegmentum 228 STRUCTURE OF THE CEREBELLUM. [CH. XXI. CHAPTER XXL The Cerebellum is composed of an elongated central portion or lobe, called the vermis or vermiform process, and two hemispheres. Each hemisphere is connected with its fellow, not only by means of the vermiform process, but also by a bundle of fibres called the middle peduncle (the latter forming the greater part of the STRUCTURE OF THE CEREBELLUM. Fig'. 220.-Cerebellum in section and fourth ventricle, with the neighbouring parts. 1, median groove of fourth ventricle, ending below in the calamus scriptorius, with the longitudinal eminences formed by the fasciculi teretes, one on each side ; 2, the same groove, at the place where the white streaks of the auditory nerve emerge from it to cross the floor of the ventricle ; 3, inferior crus or peduncle of the cerebellum, formed by the restiform body; 4, funiculus gracilis ; above this is the calamus scriptorius ; 5, superior crus of cerebellum ; 6, 6, fillet to the side of the crura cerebri; 7, 7, lateral grooves of the crura cerebri; 8, corpora quadri gemma. (From Sappey after Hirschfeld and LeveillC.) transverse fibres of the pons Varolii), while the superior peduncles, which decussate in the mid-brain, connect it with the cerebrum (5, fig. 220), and the inferior crura (restiform bodies) connect it with the medulla oblongata (3, fig. 220). The cerebellum is composed of white and grey matter, the latter being external, like that of the cerebrum, and like it, infolded, so that a larger area may be contained in a given space. The convolutions of the grey matter, however, are arranged after a different pattern, as shown in fig. 220. The tree-like CH. XXI.] THE CEREBELLUM. 229 arrangement of the white matter has given rise to the name arbor vitas. Besides the grey substance on the surface, there are, in the centre of the white substance of each hemisphere, small masses of grey matter, the largest of which, called the corpus den- tatum (fig. 221, cd), resembles very closely the corpus dentatum of the olivary body of the medulla oblongata in appearance. If a section is taken through the: cortical portion of the Fig. 221.-Outline sketch of a section of the cerebellum, showing the corpus dentatum. The section has been carried through the left lateral part of the pons, so as to divide the superior peduncle and pass nearly through the middle of the left cerebellar hemi- sphere. The olivary body has also been divided longitudinally so as to expose in section its corpus dentatum. cr, crus cerebri; /, fillet; q, corpora quadrigemina; s p, superior peduncle of the cerebellum divided ; m p, middle peduncle or lateral part of the pons Varolii, with fibres passing from it into the white stem; a v, continuation of the white stem radiating towards the arbor vitae of the folia; c d, corpus den- tatum ; o, olivary body with its corpus dentatum ; p, pyramid. (Allen Thomson.) J. cerebellum, the following distinct layers can be seen (fig. 222) by microscopic examination. Underneath the pia mater is the external layer of grey matter ; it is formed chiefly of fine nerve-fibres with small nerve-cells scattered through it. Into its outer part, processes of pia mater pass vertically ; these convey blood-vessels. There are also here numerous long tapering neuroglia-cells. The internal or granular layer of grey matter is made up of a large number of small nerve- cells mixed with a few larger ones, and some neuroglia-cells. Between the two layers is an incomplete stratum of large flask- shaped cells, called the cells of Purkinje. Each of these gives off from its base a fine process which becomes the axis-cylinder of one of the medullated fibres of the white matter; the neck of the flask passing in the opposite direction breaks up into dendrons which pass into the external layer of grey matter. By Golgi's method (fig. 223) these dendrons have been shown to spread out in planes transverse to the direction of the lamellae of the organ. Each cell of Purkinje is further invested by arborisations of two sets of nerve-fibres. One of these (originating from the fibres 230 STRUCTURE OF THE CEREBELLUM. fCH. XXI. Fig. 222.-Vertical section of dog's cerebellum; p m, pia mater ; p, corpuscles of Purkinje, which are branched nerve-cells lying in a single layer and sending single processes downwards and more numerous ones upwards, which branch continuously and extend through the deep " molecular layer " towards the free surface; g, dense layer of ganglionic corpuscles; /, layer of nerve-fibres, with a few scattered ganglionic cor- puscles. This last layer (//) constitutes part of the white matter of the cerebellum, while the layers between it and the free surface are grey matter. (Klein and Noble Smith.) CH. XXI.] CEREBELLUM AND CEREBRUM. 231 of the white matter which are not continuous as axis-cylinders from the cells of Purkinje) forms a basket-work round the Fig. 223.-Section of cerebellar cortex, stained by Golgi's method; 1. taken across the lamina; n. in the direction of the lamina; A, outer or molecular layer; b, inner or granule layer ; c, white matter, a, Cell of Purkinje ; b, small cells of inner layer ; c, dendrons of these cells ; d, axis-cylinder process of one of these cells becoming longitu- dinal in the outer layer ; e, bifurcation of one of these ; g, a similar cell lying in the white matter. (Ramon y Cajal.) dendrons ; the other (originating as axis-cylinder processes from the nerve-cells of the external layer) forms a felt-work of fibrils round the body of the cell. CHAPTER XXII. STRUCTURE OF THE CEREBRUM. The large size and complexity of the cerebrum distinguishes the brain of man from that of the lower animals ; the amount of convolution of its surface corresponds roughly with the degree of intelligence. The cerebrum consists of two halves called cerebral hemispheres, separated by a deep longitudinal fissure and connected by a large band of transverse commissural fibres known as the corpus callosum. 232 STRUCTURE OF THE CEREBRUM. [CH. XXII. The interior of each hemisphere contains a cavity of complicated shape called the lateral ventricle ; the lateral ventricles open into Fig. 224.-View of the Corpus Callosum from above. |.-The upper surface of the corpus callosum has been fully exposed by separating the cerebral hemispheres and throwing them to the side ; the gyrus foroicatus has been detached, and the transverse fibres of the corpus callosum traced for some distance into the cerebral medullary substance. 1, the upper surface of the corpus callosum ; 2, median furrow or raphe ; 3, longitudinal striae bounding the furrow ; 4, swelling formed by the transverse bands as they pass into the cerebrum ; 5, anterior extremity or knee of the corpus callosum ; 6, posterior extremity ; 7, anterior, and 8, posterior part of the mass of fibres proceeding from the corpus callosum; 9, margin of the swelling; 10, anterior part of the convolution of the corpus callosum; 11, hem or band of union of this convolution; 12, internal con- volutions of the parietal lobe; 13, upper surface of the cerebellum. (Sappey, after Foville.) the third ventricle. These and the corpus callosum are shown in fig. 224. The next figure represents a dissected brain in which the corpus callosum has been removed; the ventricles are seen better. Each hemisphere is covered with grey matter, which passes down into the fissures that abound on its exterior. This surface grey matter is called the cerebral cortex. The amount of this grey matter varies directly with the amount of convolution of the surface. Under it white matter is situated ; and at the base there are masses of grey matter; part of these basal ganglia are CH. XXII.] INTERIOR OF CEREBRUM. 233 seen forming part of the wall of the ventricles. The anterior basal ganglion is called the corpus striatum; it is divided into two parts called the lenticular or extraventricular nucleus, and the caudate or intraventricular nucleus. It has received the Fig. 225.-Dissection of brain, from above, exposing the lateral fourth and fifth ventricles with the surrounding parts. J.-a, anterior part, or genu of corpus callosum ; &, corpus striatum.; V, the corpus striatum of left side, dissected so as to expose its grey sub- stance ; c, points by a line to the tenia semicircularis ; d, optic thalamus ; e, anterior pillars of fornix divided ; below they are seen descending in front of the third ventricle, and between them is seen part of the anterior commissure; in front of the letter e is seen the slit-like fifth ventricle, between the two laminae of the septum lucidum ; f, soft or middle commissure ; g is placed in the posterior part of the third ventricle ; immediately behind the latter are the posterior commissure (just visible) and the pineal gland, the two crura of which extend forwards along the inner and upper margins of the optic thalami; h and i, the corpora quadrigemina ; k, superior crus of cerebellum; close to k is the valve of Vieussens, which has been divided so as to expose the fourth ventricle; I, hippocampus major and corpus fimbriatum, or tenia hippocampi ; m, hippocampus minor; n, eminentia collateralis ; 0, fourth ventricle ; p, posterior surface of medulla oblongata ; r, section of cerebellum; s, upper part of left hemisphere of cerebellum exposed by the removal of part of the posterior cerebral lobe. (Hirschfeld and Leveilld.) latter name because it is seen in the interior of the ventricle. The posterior basal ganglion is called the optic thalamus. Passing up between the basal ganglia are the white fibres which enter the cerebral hemisphere from the crus ; these consti- 234 STRUCTURE OF THE CEREBRUM. [ch. xxii tute the internal capsule. This passes in front between the two subdivisions of the corpus striatum, and behind between the optic thalamus and the lenticular nucleus of the corpus striatum. The relationship of these parts is best seen in a vertical section ; such as is represented in the next diagram. One hemisphere is seen, with portions of the other. The sur- face darkly shaded indicates the grey matter of the cortex, which passes down into the fissures; one very extensive set of convolu- Fig. 226.-Vertical section through the cerebrum and basic ganglia to show the relations of the latter, co, cerebral convolutions; cc., corpus callosum; v.l., lateral ventricle ; /, fornix ; vIII., third ventricle ; k.c., caudate nucleus ; th, optic thalamus ; n.h, lenti- cular nucleus ; c.i., internal capsule ; cl., claustrum; c.e., external capsule ; m, corpus mammillare ; t.o., optic tract; s.t.t., stria terminalis ; n.a., nucleus amygdales ; cm, soft commissure. (Schwalbe.) tions (co i) passes deeply into the substance of the hemisphere, this is called the Island of Red ; the lowest stratum of grey matter is separated from this to form a narrow isolated strip of grey matter called the claustrum (cl). In the middle line from above down are seen the great longitudinal fissure extending as far as (cc) the corpus callosum, the band of white matter that forms the great commissure between the two hemispheres; be- neath this are the lateral ventricles which communicate by the foramen of Monro with the third ventricle : the fornix is indicated by the letter/. Contributing to the floor of the lateral ven- tricle, one next sees the optic thalamus (th), and the tail end of the nucleus caudatus (n c) ; the section being taken somewhat posteriorly. The nucleus lenticularis is marked nl; and the CH. XXII.] THE INTERNAL CAPSULE. 235 band of white fibres passing up between it and the thalamus is called the internal capsule (ci); the narrow piece of white matter between the claustrum and the lenticular nucleus is called the external capsule. For the student of medicine the internal capsule is one of the most important areas of the brain. Into it are continued up the fibres which we have previously traced as far as the crus cerebri; the motor-fibres of the crusta are continued into the anterior two- thirds of its posterior limb (i.e. behind the genu * in fig. 227); the sensory fibres of the tegmentum into the posterior third of this limb. When these fibres get beyond the narrow pass between the basal ganglia, they spread out in a fan-like manner and are distributed to the grey cortex; the motor-fibres going to the motor convolutions around the fissure of Rolando; the sensory fibres to the same convolutions and also to others behind these which are asso- ciated with special sensations. The name corona radiata is applied to the fan-like spreading of the fibres; the fibres as they pass through the handle of the fan, or internal capsule, communicate with the nerve-cells of the grey matter of the basal ganglia; the pyramidal fibres on their way down to the medulla and cord from the motor areas of the brain send off collaterals or side branches which arborise around the cells of the corpus striatum, and to a lesser degree around those of the optic thalamus ; the axis- cylinder processes of these cells pass out to join the pyramidal tract on its downward course. The sensory fibres on their way up may pass straight on to the cortex, but the majority, especially those in the fillet, terminate by arborising found the cells of the optic thalamus, and in the subthalamic area. This, in fact, is another cell-station or position of relay : the fibres passing out from the cells of the thalamus continue the impulse on to the cortex. The importance of the internal capsule is rendered evident when one considers the blood supply of these parts; at the an- terior and posterior perforated spots, numerous small blood-vessels enter for the supply of the basal ganglia, and these are liable to become diseased, and if they rupture, a condition called apoplexy is the result; if the haemorrhage is excessive, death may result almost immediately ; but if the patient recovers, a condition of paralysis more or less permanent remains behind; and a very large amount of paralysis results from a comparatively limited lesion, because so many fibres are congregated together in this narrow isthmus of white matter. If the haemorrhage is in the anterior part of one internal capsule, motor paralysis of the opposite side of the body (hemiplegia) will be the most marked 236 STRUCTURE OF THE CEREBRUM. [ch. xxii. symptom. If the haemorrhage occurs in the posterior part, sen- sory paralysis of the opposite side of the body will be the most marked symptom. If the motor-fibres are affected, degeneration will occur in the pyramidal tract and can be traced through the pes of the crus and mid-brain to the pyramid of the pons and bulb, and then in the crossed pyramidal tract of the opposite side and the direct pyramidal tract of the same side of the cord. Figure 227 represents a horizontal view through the hemi- Fig. 227.-Diagram to show the connection of the Frontal Occipital Lohes with the Cere- bellum, &c. The dotted lines passing in the crusta (t.oc), outside the motor fibres, indicate the connection between the temporo-occipital lobe and the cerebellum, f.c., the fronto-cerebellar fibres, which pass internally to the motor tract in the crusta; i.f., fibres from the caudate nucleus to the pons, fb., frontal lobe ; Oc., occipital lobe; af., ascending frontal; ap., ascending parietal convolutions ; pcf., precentral fissure in front of the ascending frontal convolution ; fr., fissure of Rolando ; iff., interparietal fissure. A section of crus is lettered on the left side. S.N., substantia nigra; py., pyramidal motor fibres, which on the right are shown as continuous lines converging to pass through the posterior limb of i.c., internal capsule (the knee or elbow of which is shown thus *) upwards into the hemisphere and downwards through the pons to cross at the medulla in the anterior pyramids. (Gowers.) sphere. The internal capsule (c) at the point * makes a bend called the genu or knee, behind which the motor-fibres, and more posteriorly still the sensory-fibres, pass. The connection between cerebrum and cerebellum is also indicated; one cerebral hemisphere being connected with the cerebellar hemisphere of the opposite side by the superior cerebellar peduncle which decussates with its fellow in the mid-brain. CH. XXII.] THE CEREBRAL CORTEX. 237 Histological Structure of the Cerebral Cortex. The cortex is generally described as consisting of the following- five layers (Meynert):- i. Superficial layer with abun- dance of neuroglia and a few small multipolar ganglion-cells. 2. A thin layer of a large number of closely packed small ganglion-cells of pyramidal shape. 3. The most important layer, and the thickest of all: it contains many large pyramidal ganglion-cells, each with a process running off from the apex vertically towards the free surface, and lateral processes at the base which are always branched. Also a median process from the base of each cell which becomes con- tinuous with the axis-cylinder of a nerve-fibre. The bundles of fibres spread out in this layer. 4. Numerous ganglion-cells, some large and others small, form- ing the granular formation of Meynert. 5. Spindle-shaped and branched ganglion-cells of mode- rate size arranged chiefly parallel to the free surface (fig. 228). This layer is remarkable in being broken up by fibres arranged in groups passing to the outei' layers. It is a noticeable fact that the different layers do not bear the same relation to one another in thickness in different regions. In the area about the fissure of Ro- lando, wrhich we shall presently see is called the motor area, the large pyramidal cells of the third layer are conspicuous in size and number, and numerous large cells are found in the fourth layer.. These latter attain their greatest development in the pre-central Fig. 228.-The layers of the cortical grey matter of the cerebrum. (Meynert.) 238 STRUCTURE OF THE CEREBRUM. [ch. xxii. and post-central convolutions. The granular layer is very marked in the occipital region, forming a distinct and broad division of the fourth layer. The large cells are scarce. In the frontal region, the pyramidal and fourth layers are well marked but the cells are A, medium-sized pyramidal cell of the second layer. B, large pyramidal cell of third layer. C, polymorphous cell of fourth layer. D, cell of which the axis-cylinder process is ascending. E, neuroglia cell. F, cell of the first, or molecular, layer, forming an intermediate cell-station between sensory fibres and motor cells. G, sensory fibre from the white matter. H, white matter. I, collateral of the white matter. (Ramon y Cajal.) Fig'. 229.-Principal types of cells in the cerebral cortex. less numerous ; the nuclear layer is very distinct. The pyramidal cells are those from which the motor or efferent fibres originate. The separation of the fifth layer from the rest to form the claustrum in the region of the Island of Reil has been already alluded to (p. 234). By Golgi's method the arrangement of these cells has been re- cently made out much better. The above diagram (fig. 229) is taken from Ramon y Cajal's Croonian Lecture, and the following two (figs. 230 and 231) are from photo-micrographs kindly lent CH. XXII.] CELLS OF THE CORTEX. 239 me by Dr. Mott. Fig. 230 represents a section through the motor cortex of the human brain, and shows very beautifully the large Fig. 230.-Human cerebral cortex: Golgi's method. (Mott.) Fig. 231.-Human cerebral cortex: Golgi's method. (Mott.) pyramidal cells with dendrons passing off from their corners, and the axis-cylinder process passing from the base of each towards 240 STRUCTURE OF THE CEREBRUM. [ch. xxii. the white matter, giving off collaterals on the way. Neuroglia- cells are also seen. Fig 231 is a high-power view of the same. An interest- ing additional point is also brought out ; in the lower part of the diagram some of the neuroglia-cells are seen in the sheath of a small blood-vessel. Cajal considers that such cells may by their movements assist the dilatation of the vessels. The cells of the cortex thus give rise to the motor or efferent fibres ; these pass into the white matter of the interior of the brain. These go either directly or by collaterals, (1) to the cortex of more or less distant convolutions. These are called Association ■fibres. (2) Others pass to the corpus callosum, and so reach the cortex of the opposite hemisphere. These are called Commissural fibres. In each case they terminate by arborisations around the cells of the grey matter of the cortex; while others again, especially those of the largest pyramidal cells, extend downward through the corona radiata and interna] capsule and become, (3) fibres of the pyramidal tract. These are called Projection fibres. As they pass down they give off collaterals to the adjacent grey matter, to the opposite hemisphere vid the corpus callosum, to the corpus striatum and the optic thalamus, where they terminate by arbori- sations : the main fibres terminate in arborisations round the multipolar cells of the anterior horn of the spinal cord. The cells of the cortex are, in addition to all this, surrounded by the arborising terminations of the sensory nerve-fibres, which, after relays at various cell-stations, ultimately reach the cortex. We are now in a position to complete diagram 200, and obtain an idea of the relations of the principal cells and fibres of the cerebro-spinal nervous system to one another. Pyr. is a cell of the Rolandic area of the cerebral cortex ; AX is its axis- cylinder process which passes down in the pyramidal tract, and crosses the middle line AB at the pyramidal decussation. It gives off collaterals, one of which (caZZ) is shown passing in the corpus callosum to terminate in an arborisation in the cortex of the opposite hemisphere ; another (str) passes into the corpus striatum. In the cord collaterals pass off and end in arbori- sations round cells of the anterior horn of the spinal cord (see also fig. 200) ; the main fibre has a similar termination. The motor nerve-fibre passes from the anterior cornual cell to muscular fibres where it ends in the terminal arborisation of the end-plate. Coming now to the sensory fibres, a cell of one of the spinal ganglia is shown. Its axis-cylinder process bifurcates, and one branch passes to the periphery ending in arborisations in skin and tendon. The other (central) branch bifurcates on entering the cord, and its divisions pass upwards and downwards, the latter for a short distance only; the terminations of this descending branch and of collaterals of the ascending branch round the cells of the spinal cord are more fully shown in figure 200. The main ascending branch arborises around a cell of the nucleus gracilis (n.g.) or nucleus cuneatus in the posterior columns of the bulb ; the axis-cylinder process of CH. XXII.] PROJECTION SYSTEMS. 241 this cell passes over to the other side as an internal arcuate fibre (I.A.), and becomes longitudinal as one of the fibres of the mesial fillet (F), which terminates round a cell of the optic thalamus (O.T.), from which a new axis- cylinder process passes to form an arborisation around the dendrons of one of Fig. 232.-Scheme of relationship of cells and fibres of brain and cord. (In the preparation of this diagram I have received considerable assistance from Dr. Mott.) the cerebral cells (Cajal's nerve-unit of association A.C.N.) in the surface layer of the cortical grey matter (shown on a larger scale in fig. 229 f) ; the axis- cylinder process of A.C.N. arborises round the dendrons of the pyramidal cell from which we started. In this way one gets a complete physiological circle of nerve-units ; the segments of the circle are, however, anatomically distinct, and the impulses travel through contiguous, not through continuous, structures. The simple arrows indicate the direction of the impulses in the efferent projection system ; the feathered arrows in the afferent projection system. 242 STRUCTURE OF THE CEREBRUM. [ch. xxii. Next we come to the connections of the cerebellum. One of the colla- terals of the sensory nerve-fibres arborises round a cell of Clarke's column, from which a fibre of the direct cerebellar tract passes to end in an arbori- sation around a cell in the vermis of the cerebellum. P is one of the cells of Purkinje, the axis-cylinder process of which P.ax passes to the cerebro- spinal axis ; it is depicted as passing down to envelop one of the cells of the anterior horn ; but this has never been satisfactorily demonstrated ; so a dotted line has been used to indicate this uncertainty. The origin and destination of the tract of Gowers, which are also matters of doubt, are not shown in the diagram ; the fibres of communication from the cerebral to the opposite cerebellar hemisphere, which pass through the superior cerebellar peduncle, are also omitted. The sympathetic system, with its numerous cell stations in the sympathetic ganglia, we shall study in connection with the blood-vessels and viscera to which the sympathetic fibres are distributed. G.M is the grey matter which is continuous from spinal cord to the optic thalamus, and through this certain afferent impulses, such as those of pain, travel upwards. The Convolutions of the Cerebrum. The surface of the brain is marked by a great number of de- pressions which are called fissures or sulci, and it is this folding of the surface that enables a very large amount of the precious material called the grey matter of the cortex to be packed within the narrow compass of the cranium. In the lowest vertebrates the surface of the brain is smooth, but going higher in the animal scale the fissures make their appearance, reaching their greatest degree of complexity in the higher apes and in man. In a certain embryonic stage of the human foetus the brain is also smooth, but as development progresses the sulci appear, until the climax is reached in the brain of the adult. The sulci, which make their appearance first, both in the animal scale and in the development of the human foetus, are the same. They remain in the adult as the deepest and best marked sulci; they are called the primary fissures or sulci, and they divide the brain into lobes ; the remaining sulci, called the secon- dary fissures or sulci, further subdivide each lobe into convolutions or gyri. A first glance at an adult human brain reveals what appears to be a hopeless puzzle; this, however, is reduced to order when one studies the brain in different stages of development, or compares the brain of man with that of the lower animals. The monkey's brain in particular has given the key to the puzzle, because there the primary fissures are not obscured by the complexity and contorted arrangement of secondary fissures. The next figure, comparing the brain of one of the lower monkeys CH. XXII.] CEREBRAL CONVOLUTIONS. 243 with that of the child at birth, shows the close family likeness in the two cases. Fig. 233. A. Brain of adult Macacque monkey. B. Brain of child shortly before birth. The two brains are very much alike, but the growth forwards of the frontal lobes even at this early stage of development of the human brain is quite well seen. S, fissure of Sylvius ; R, fissure of Rolando. Fig. 234 gives a representation of the brain of one of the higher monkeys, the orang-outang, where there is an inter- Fig. 234.-Brain of the Orang, j natural size, showing the arrangement of the con- volutions. Sy, fissure of Sylvius; Ji, fissure of Rolando ; EP, external perpendicular fissure; Olf, olfactory lobe; Ob, cerebellum; PV, pons Varolii; MO, medulla oblongata. As contrasted with the human brain, the frontal lobe is short and small relatively, the fissure of Sylvius is oblique, the temporo-sphenoidal lobe very promi- nent, and the external perpendicular fissure very well marked. (Gratiolet.) mediate condition of complexity by which we are led lastly to the human brain. Let us take first the outer surface of the human hemisphere ; the primary fissures are- i. The fissure of Sylvius ; this divides into two limbs, the pos- 244 STRUCTURE OF THE CEREBRUM. [ch. xxii. terior of which is the larger, and runs backwards and upwards, and the anterior limb, which passing into the substance of the hemisphere, forms the Island of Reil. 2. The fissure of Rolando, running from about the middle of the top of the diagram downwards and forwards. 3. The external parieto-occipital fissure (Par. oc. f) parallel to the fissure of Rolando but more posterior and much shorter ; in some monkeys it is longer. Fig. 235.-Right cerebral hemisphere, outer surface. These three fissures divide the brain into five lobes :- 1. The frontal lobe ; in front of the fissure of Rolando. 2. The parietal lobe; between the fissure of Rolando and the external parieto-occipital fissure. 3. The occipital lobe; behind the external parieto-occipital fissure. 4. The temporal or temporo-sphenoidal lobe ; below the fissure of Sylvius. 5. The Island of Reil. It will be noticed that the names of the lobes correspond to those of the bones of the cranial vault which cover them. There is no exact correspondence between the bones and the lobes, but the exact position of the various convolutions in relation to the surface of the skull is a matter of anatomy, which, in these days of brain-surgery, is of overwhelming importance to the surgeon. The position of a localised disease in the brain can be determined very accurately, as we shall see later, by the symptoms exhibited by the patient, and it would be obviously inconvenient to the patient if the surgeon was unable to trephine over the exact spot under which the diseased convolution lies, but had to make a number of exploratory holes to find out where he was. Each lobe is divided into convolutions by secondary fissures. 1. The frontal lobe is divided by the central frontal sulcus CH. XXII.] LOBES OF THE BRAIN. 245 which runs upwards parallel to the fissure of Rolando, and two transverse frontal sulci, upper and lower, into four convolutions; namely, the ascending frontal convolution in front of the fissure of Rolando, and three transverse frontal convolutions, upper, middle, and lower, which run outwards and forwards from it. 2. The parietal lobe has one important secondary sulcus, at first running parallel to the fissure of Rolando and then turning back parallel to the margin of the brain. It is called the intra- parietal sulcus. The lobe is thus divided into the ascending parietal convolution behind the fissure of Rolando, the supra- marginal convocation between the intra-parietal sulcus, and the fissure of Sylvius; the angular convolution which turns round the end of the Sylvian fissure, and the superior parietal convolution, or parietal lobule, in front of the external parieto-occipital fissure. 3. The occipital lobe is divided into upper, middle, and lower occipital convolutions by two secondary fissures running across it. 4. The temporal lobe is similarly divided into upper, middle, and loiver temporal convolutions by two fissures running parallel Fig. 236.-Right cerebral hemisphere, mesia surface. to the fissure of Sylvius; the upper of these fissures is called the parallel fissure. 5. The Island of Beil is divisible into convolutions by the breaking up of the anterior limb of the Sylvian fissure. Coming now to the mesial surface of the hemisphere (fig. 236), its subdivisions are made evident by cutting through the corpus cal- losum which unites the hemisphere to its fellow. The subdivision into lobes is not so apparent here as on the external surface of the hemisphere, so we may pass at once to the convolutions into which it is broken up by fissures. 246 STRUCTURE OF THE CEREBRUM. [CH. XXII. In the middle the corpus callosum is seen cut across; above it and parallel to its upper border is a fissure called the calloso- marginal fissure which turns up and ends on the surface near the upper end of the fissure of Rolando. The convolution above this is called the marginal convolution, and the one below it the callosal convolution or gyrus fornicatus. The deep fissure below the corpus callosum running from its posterior end forwards and downwards is called the dentate fissure; this forms a projection seen in the interior of the lateral ventricle and called there the hippocampus major; it is some- times called the hippocampal convolution which, togethei' with the gyrus fornicatus above the corpus callosum, constitutes the limbic lobe. Below the dentate fissure is another called the collateral fissure, above which is the uncinate convolution, and be- low which is the inferior tem- poral convolution which we have previously seen on the external surface of the hemisphere (see fig. 235). In the occipital region the internal parieto-occipital fis- sure, which is a continuation of the external parieto-occipital fissure, passes downwards and forwards till it meets the cal- carine fissure ; these two enclose between them a wedge-shaped piece of brain called the cuneus or cuneate lobule; the square piece above it being called the precuneus or quadrilateral lobule. The only convolutions now left are those which are placed on the surface of the frontal lobe that rests on the orbital plate of the frontal bone; they are shown in fig. 205, 2 2' 2", and may be seen diagrammatically in fig. 237, the end of the temporal lobe being cut off to expose the convolu- tions of the central lobe or Island of Reil. Along the edge is the continuation of the marginal convolution (m) ; next comes the olfactory sulcus (o) in which the olfactory tract and bulb lie ; then the triradiate orbital sulcus which divides the rest of this surface into three convolutions. Fig. 237.-Orbital surface of frontal lobe. M, marginal convolution. O, olfactory sulcus. O.S, orbital sulcus. I, island of Reil. 8. a, anterior limb of Sylvian fissure. S.p. posterior limb of Sylvian fissure. A.P.S., anterior perforated spot. CH. xxin.] FUNCTIONS OF THE SPINAL CORD. 247 CHAPTER XXIII. FUNCTIONS OF THE SPINAL CORD. The functions of the spinal cord fall into two categories: functions of the grey matter, which consist in the reflection of afferent impulses, and their conversion into efferent impulses (reflex action) ; and functions of the white matter, which are those of conduction. The Cord, as an Organ of Conduction. We have studied at some length the various paths in the white matter, and so we have the materials at hand for recapitulating the main facts in connection with the physiological aspect of the problem. Complete section of the spinal cord in animals, and diseases or injuries of the cord or spinal canal in man, which practically cut the cord in two, lead to certain histological changes of a degene- rative nature, which we have already studied, and to physiological results, which are briefly-(i) paralysis, both motor and sensory, of the parts of the body supplied by spinal nerves which originate below the point of injury; and (2) increased reflex irritability of the same parts, the reason for which we shall study immediately. Hemisection of the cord leads to degenerative changes on the same side of the cord, and loss of motion and sensation on the same side of the body below the lesion. The motor path in the cord from the brain is the pyramidal tract; the anatomy of this tract is fully described in Chapters XVIII. to XXL, and we need do no more here than remind the reader that it originates from the pyramidal cells of the cortex of the opposite cerebral hemisphere, and that the principal decussa- tion occurs at the lower part of the bulb. The sensory tracts are more complex, on account of the nume- rous cell-stations on their course. The path for tactile and mus- cular sense impressions is up the posterior columns to the nucleus gracilis and nucleus cuneatus; thence by the internal arcuate fibres and fillet to the optic thalamus, and thence by the poste- rior part of the internal capsule to the Rolandic area of the oppo- site cerebral hemisphere ; the decussation of the fillet occurs in the bulb. Brown Sequard and Schiff, two of the earliest to work at the 248 FUNCTIONS OF THE SPINAL CORD. [cn. xxiii. subject of conducting paths in the cord, arrived at the conclusion that painful impressions travelled to the brain by the grey matter of the cord. This conclusion was regarded as paradoxical, for white matter is conducting, grey matter is central or reflecting. But the conclusion is not so paradoxical as it appears at first sight, for we now know the grey matter is made up of nerve- units, communicating physiologically by their interlacement of dendrons; and it is quite easy to understand that impulses may travel up grey matter through a vast series of cell stations or positions of relay. The more exact methods of modern research have gone far to justify the conclusions of Schiff and Brown- Sequard, and it is now generally held that the impulses due to painful impressions, and also those produced by heat and cold, travel up to the optic thalamus by the loopings of fibres from cell to cell through the tract of grey matter, which is continuous from cord to optic thalamus (fig. 232); from the optic thalamus the fibres of the corona radiata carry on the impulse to the cortex. These conclusions are confirmed by the phenomena seen in certain diseases. One of the most instructive of diseases from the physio- logical standpoint is called locomotor ataxy. This disease is a widespread affection of the sensory apparatus, both central and peripheral; it sometimes leads to almost complete loss of sensa- tion of all kinds in various parts of the body, particularly the lower limbs, but it is sometimes selective, muscular and tactile sense being abolished, when painful and thermal sensations are felt. This at any rate shows that different channels conduct these impressions to the brain. Some sensory or rather afferent impulses reach the cerebellum via the cells of Clarke's column and the direct or dorsal cerebellar tract to the restiform body and inferior peduncle of the cere- bellum. It terminates in the vermis or middle lobe of the cere- bellum ; the fibres of the tract of Gowers originate in cells at the base of the anterior horn of the opposite side of the cord, and those of its fibres which enter the cerebellum do so by its supe- rior peduncle, and these also end in the vermis. This leaves us still one more set of fibres to consider; these are the fibres that leave the cerebellum and travel up to the brain and down the cord. They, like most of the other tracts, have been investigated by the degeneration method. Their exact course is, however, uncertain, though probably they ultimately terminate by arborising round the multipolar cells of the cerebrum and of the anterior horn of the cord (see dotted line in fig. 232). CH. XXIII.] REFLEX ACTIONS. 249 Reflex Action of the Spinal Cord. There are two theories of a speculative nature regarding the relationship of reflex and voluntary actions : one is, that all actions are in essence reflex, and that the so-called voluntary actions are modified reflexes, in which the afferent impulse to act, though often obscure, is nevertheless by seeking always to be found. Put in popular language, this theory implies that we have really no such thing as a will of our own, but our actions are simply the result of external circumstances. The other theory is the exact opposite-namely, that all actions are in the beginning voluntary, and become reflex by practice in the lifetime of the individual, or the lifetime of his ancestors, who transmit this acquired character to their descen- dants. This is not the place to discuss a philosophical question of this kind, and still less the debated question whether acquired characters are transmissible by inheritance. The distinction be- tween voluntary and reflex actions is a useful practical one, and certainly it cannot be doubted that many practised actions become reflex in the lifetime of every one of us. Take walking as an example : at first the act of locomotion is one in which the brain is concerned ; it is an action demanding the concentration of the attention ; but later on the action is largely carried out by the spinal cord, the afferent impulses to the cord from the feet directing the efferent impulses to the muscles concerned. The reflex actions of the spinal cord may first be studied in a brainless frog, as in this animal the spinal cord possesses a great power of controlling very complex reflex actions. Reflexes in a Brainless Frog. After destruction of the brain the shock of the operation ren- ders the animal for a short time motionless and irresponsive to stimuli, but in a few minutes it gradually assumes a position which differs but little from that of a living conscious frog. If thrown into water it will swim; if placed on a slanting board it will crawl up it (Goltz); if stroked on the flanks it will croak (Goltz); if it is laid on its back, and a small piece of blotting- paper moistened with acid be placed on the skin, it will generally succeed in kicking it off; if a foot is pinched it will draw the foot away; if left perfectly quiet it remains motionless. 250 FUNCTIONS OF THE SPINAL CORD. [ch. xxiii The muscular response that follows an excitation of the sur- face is purposive and constant, the path along which the impulse is propagated being definite. Under certain abnormal conditions, however, the propagation of the impulse in the cord is widespread, the normal paths being, as it were, broken down. This is seen in the convulsions that occur on slight excitation in animals or men who have suffered from profuse haemorrhage, or in the disease called lockjaw or tetanus. Such a condition is easily demonstrable in a frog under the influence of strychnine : after the injection of a few drops of a i per cent, solution under the skin, cutaneous excitation no longer produces co-ordinated muscular responses, but paroxysms of convulsions, in which the frog assumes a characteristic attitude, with arms flexed and legs extended. Spreading of reflexes.-If one lower limb is excited, it is that limb which responds: if the excitation is a strong one it will spread to the limb of the opposite side, and if stronger still to the upper limbs also. Cumulation of reflexes.-This is well illustrated by Turck's method. If a number of beakers of water are prepared, acidulated with i, 2, 4, &c. parts of sulphuric acid per 1,000, and the tips of the frog's toes are immersed in the weakest, the frog at first takes no notice of the fact, but in time the cumulation or sum- mation of the sensory impulses causes the animal to withdraw its feet. If this is repeated with the stronger liquids in succession, the time that intervenes before the muscles respond becomes less and less. This method also serves to test reflex irritability when the frog is under the influence of various drugs. Inhibition of reflexes.-If instead of destroying the whole brain, the cerebrum only is destroyed, and the optic lobes left intact, response to excitation is much slower, the influence of the remain- ing part of the brain inhibiting the reflex action of the cord. Or if in doing the experiment with acid just described the toes of the other foot are being simultaneously pinched, the response to the acid is delayed. Inhibition, or delay of reflex time is thus pro- duced by other sensations, which, as it were, take up the attention of the cord. This influence of the brain on the cord is also illustrated in man, by the fact that a strong effort of the will can control many reflex actions. It is, for instance, possible to subdue the tendency to sneeze ; if one puts accidentally one's hand in a flame, the natural reflex is to withdraw it : yet it is well known that Cranmer, when being burnt at the stake, held his hand in the flames till it was consumed. CH. XXIII.] REFLEX ACTION IN MAN. 251 After the spinal cord has been divided by injury or disease in the thoracic region, the brain can no longer exert this controlling action ; hence the cord having it, as it were, all its own way, has its reflex irritability increased. This increase of reflex irritability is well seen in the disease called lateral sclerosis ; here the lateral columns, including the pyramidal tract, become degenerated, and so the path from the brain to the cells of the cord is in great measure destroyed. In these patients the increase of reflex irr - tability may become a very distressing symptom, slight excita- tions, like a movement of the bed-clothes, exciting powerful con vulsive spasms of the legs. Reflex time.-In the frog, deducting the time taken in the transmission of impulses along nerves, the time consumed in the cord (reflex time) varies from o-oo8 to 0'015 second; if the reflex crosses to the other side it is one-third longer. It is lessened by heat, and under the influence of a strong stimulus. Reflex Action in Man. The reflexes obtainable in man form a most important factor in diagnosis of diseases of the nervous system; each action is effected through an afferent sensory nerve, a system of nerve-cells in the cord termed the reflex centre, and an efferent motor nerve ; the whole constitutes what is called the reflex arc. The absence of certain reflexes may determine the position in the spinal cord, which is the seat of disease. Two forms of reflex action must be distinguished :- 1. Superficial reflexes. These are true reflex actions, and are excited by stimulation of the skin. 2. Deep reflexes or tendon reflexes. This is a most undesirable name, as they are not true reflex actions. Superficial Reflexes.-These are obtained by a gentle stimu- lation, such as a touch on the skin ; the muscles beneath are usually affected, but muscles at a distance may be affected also. Thus a prick near the knee will cause a reflex flexion of the hip. The most important of these reflexes are :- a. Plantar reflex: withdrawal of the feet when the soles are tickled. b. Gluteal reflex: a contraction in the gluteus when the skin over it is stimulated. c. Cremasteric reflex: a retraction of the testicle when the skin on the inner side of the thigh is stimulated. d. Abdominal reflex in the muscles of the abdominal wall when the skin over the side of the abdomen is stroked ; the upper part 252 FUNCTIONS OF THE SPINAL CORD. [CH. XXIII. of this reflex is a very definite contraction at the epigastrium, and has been termed the epigastric reflex. e. A series of similar reflex actions may be obtained in the muscles of the back, the highest being in the muscles of the scapula. f. In the region of the cranial nerves the most important reflexes are those of the eye-(i) the conjunctival reflex, the movement of the eyelids when the front of the eyeball is touched ; and (ii) the contraction of the iris on exposure of the eye to light, and its dilatation on stimulation of the skin of the neck. Tendon Reflexes.--When the muscles are in a state of slight tension, a tap on their tendons will cause them to contract. The Fig. 238.-The Knee-jerk. (Gowers.) two so-called tendon reflexes which arc generally examined are the patella tendon reflex or knee-jerk, and the foot phenomenon or ankle-clonus. The knee-jerk.-The quadriceps muscle is slightly stretched by putting one knee over the other; a slight blow on the patellar tendon causes a movement of the foot forwards, as indicated in the dotted line of fig. 238. This phenomenon is present in health. Ankle-clonus.-This is elicited as depicted in the next figure : the hand is pressed against the sole of the foot, the calf muscles are thus put on the stretch and they contract, and if the pressure is kept up a quick succession or clonic series of contractions is obtained. This, however, is not readily obtained in health. CH. XXIII.] TENDON REFLEXES. 253 These phenomena arc not true reflexes ; the time that inter- venes between the tap and the response is so short that they must be due to direct stimulation of the muscles or of their tendons. Nevertheless, the idea that they are reflex is supported by the following facts :- 1. There are nerves in tendon. 2. The phenomena depend for their occurrence on the integrity of the reflex arc. Disease or injury to the afferent nerve, efferent nerve, or spinal grey matter, abolishes them. Thus they cannot be obtained in locomotor ataxy (damage to the posterior nerve- Fig. 239.-Ankle-clonus. (Gowers.) roots), or in infantile paralysis (damage to the anterior horns of grey matter). 3. They are excessive in those conditions that increase the true reflex irritability, such as severance of brain from cord, and in lateral sclerosis. How, then, is it possible to reconcile these two sets of facts ? The explanation advanced by Dr. Gowers does so best; it is briefly as follows :- (i) The tendon reflexes are not reflexes, but are due to direct stimulation of the muscle itself. (ii) In order that the muscle may respond it is necessary that it be in an irritable condition; this is accomplished by putting it slightly on the stretch, and so calling forth the condition called tonus (see p. 136), a readiness to contract on the slighcst provo- cation. (iii) Muscular tonus depends on the integrity of the reflex arc. 254 FUNCTIONS OF THE SPINAL CORD. [ch. xxiii. (iv) Hence injury to any part of the reflex arc, by abolishing the healthy tone of a muscle, deprives it of that intensely irritable condition necessary for the production of these so-called reflex actions. Reaction Time in Man.-The term reaction time is applied to the time occupied in that complex response to a pre-arranged stimulus in which the brain as well as the cord comes into play. It is sometimes called the Fig. 240.-Reaction time. personal equation. It may be most readily measured by the electrical method, and the following diagram will illustrate one of the numerous arrangements which have been proposed for the purpose. In the primary circuit two keys A and B are included, and a chrono- Fig. 241.-The Dilemma. .graph (i), arranged, to write on a revolving cylinder (fast rate). Another chronograph (2), marking i-iooths of a second, is placed below this. The experiment is performed by two persons Cand D. The key A, under the control of C is opened. The key B, under the control of D is closed. The electrodes E are applied to some part of D's body. C closes A. The primary circuit is made, and the chronograph moves. As soon as D feels the shock he opens B, the current is thus broken, and the chronograph lever returns to rest. Measure the time between the two movements of the chronograph (1), by means of the time tracing written by chronograph (2). It usually varies from 0'15 to o-2 second, but is increased in :- The Dilemma.-The primary circuit is arranged as before. Lead the CH. XXIV.] FUNCTIONS OF THE CEREBRUM. 255 wires from the secondary coil to the middle screws of a reverser without cross wires. To each pair of end screws, attach a pair of electrodes E and E', applied to different parts of B's body (Fig. 241). Arrange previously that D is to open B, when one part is stimulated, but not the other, C adjusting the reverser unknown to B. Under these cir- cumstances the reaction time is longer. The reaction time in response to various kinds of stimuli, sound, light, pain, etc., varies a good deal; the condition of the subject of the experiment is also an important factor. This, however, is really a practical branch of psychology, and has recently been much worked at by students of that science. Special Centres in the Cord.-In addition to the general function of reflex action, the grey matter also exercises control of certain special actions. A full consideration of each will be given in the proper place, but the principal centres of this kind may be enumerated as follows :- Ciliospinal centre ; this centre controlling the dilatation of the pupil is situated in the lower cervical region, reaching as far down as the origin of the first to the third thoracic nerve. Centres for defoecation, micturition, erection, andparturition, are situated in the lumbar enlargement of the cord. Subsidiary vaso-motor centres are scattered through the grey matter, the principal vaso-motor centre being situated in the bulb. CHAPTER XXIV. FUNCTIONS OF THE CEREBRUM. The brain is the seat of those psychical or mental processes which are called volition and consciousness; volition is the starting point in motor activity ; consciousness is the final phase of sensory impressions. In the days of the ancients very curious ideas prevailed as to the use of the brain. It is true that Alkmaon as early as 580 B.c. placed the seat of consciousness in the brain, but this view was of the nature of a guess, and did not meet with general accept- ance ; and two hundred years later Aristotle considered that the principal use of the brain was to cool the hot vapours rising from the heart. At this time the seat of mental processes, especially those of an emotional kind, was supposed to be in the heart, an idea now confined to poets, or in the bowels, as those acquainted with such ancient writings as the Bible will know. As time went on truer notions regarding the brain came to the fore; thus Herophilus (300 b.c.) was aware of the danger attend- ing injury to the medulla; Aretseus and Cassius (97 a.d.) knew that injury to one side of the brain produced paralysis of the 256 FUNCTIONS OF THE CEREBRUM. [ch. xxiv. opposite side of the body, and Galen (131-203 a.d.) was acquainted with the main motor and sensory tracts in brain and cord. Be- tween that time and this, most of the celebrated anatomists have contributed something to our knowledge, and one may particularly mention Vesalius, Sylvius, Rolando, Gall, Cams, Willis, and Burdach; many of these names are familiar because certain structures in the brain to which they called attention have been christened after them. The erroneous notion that the brain was not excitable by stimuli lasted even to the days of Flourens and Magendie. In modern times, new methods of research in the microscopic and experimental direction have produced results which perhaps in no other branch of physiology have been of such immediate benefit to the human race as those in connection with the brain. Effects of Removal of the Cerebrum. When the cerebral hemispheres are removed in a frog, it is deprived of volition and of consciousness; it remains perfectly quiescent unless stimulated; it is entirely devoid of initiatory power, but as we have already seen, it will execute reflex actions many of which are of a complex nature (see p. 249). A pigeon treated in the same way remains perfectly motionless and unconscious unless it is disturbed. When disturbed in any way it will move, for instance, when thrown into the air it will fly; but these movements are, as in the frog, purely reflex in character. In mammals the operation of extirpation of the brain is attended with such severe haemorrhage that the animal dies very rapidly, but in some few cases where the animal has been kept alive, the phenomena they exhibit are precisely similar to those shown by a frog or pigeon. In the case of the dog portions of the cortex have been removed piecemeal by Goltz of Strasburg, until at last nearly the whole of the cortex has been extirpated. Such animals, strangely enough, carry out co-ordinated movements very well, and even manifest some degree of intelligence. It appears there- fore, that in the dog, there is not so much localisation of function as in animals higher in the scale, and that the portions of the brain that arc left are able to take on to some extent the functions of those which have been removed. Localisation of Cerebral Functions. When the main function of the cerebrum was understood, CH. xxiv.] CEREBRAL LOCALISATION. 257 physiologists were divided into two schools ; those who thought that the brain acted as a whole, and those who thought that different parts of the brain had different functions to perform. One of the most prominent of the first school was Flourens, and Goltz, whose work has been done chiefly on dogs, is about the only prominent living survivor of this set of physiologists. Gradually as better methods have come in, and especially since monkeys have been use for experiment, those who believe in the localisation of function have multiplied; and now, localisation of cerebral function is more than a theory, it is an accepted fact. Perhaps the best practical evidence of this is the fact that experi- ments on monkeys have been taken as the basis for surgical operations on the human brain, and with perfect success. The earliest to work in the direction of localisation were Hitzig and Fritsch. The subject was then taken up by Ferrier and Yeo, and later by Schafer, Horsley, etc., in this country, and by Munk and many others in Germany. In addition to those who have studied the matter from the experimental standpoint, must also be reckoned the work of pathologists, who in the post-mortem room have examined the brains of patients dying from cerebral disease, and carefully compared the position of the disease with the symptoms exhibited by the patients during life. In this way two series of independent investigations have led to the same results; both methods are essential, as many minor details dis- covered by the one method correct the erroneous conclusions which are apt to be sometimes drawn by those who devote their entire attention to the other. The main point which these researches have brought out is the overwhelming importance of the cortex; it contains the highest cerebral centres. Before Hitzig began his work, the corpus striatum was regarded as the great motor centre, and the optic thalamus as the chief centre of sensation ; very little note was taken of the cortex, it appears to have been almost regarded as a kind of ornamental finish to the brain. The idea that the basal ganglia were so important arose from the examination of the brains of people who had died from, or at least suffered from, cerebral haemorrhage. The most common situation for cerebral haemorrhage, is either in the region of the corpus striatum or optic thalamus ; it was noticed that motor paralysis was the most marked symptom if the corpus striatum was injured, and sensory paralysis if the optic thalamus was injured. The paralysis, however, is due, not to injury of the basal ganglia, but of the neighbouring internal capsule. The internal capsule consists in front of the motor-fibres passing 258 FUNCTIONS OF THE CEREBRUM. [CH. XXIV. down from the cortex to the cord, and behind of the sensory fibres passing up from cord to the cortex in front. Hence, if these fibres are ploughed up by the escaping blood, paralysis naturally is the result. If a haemorrhage or injury is so limited as to affect the basal ganglia only, and not the fibres that pass between them, the resulting paralysis is slight or absent. The question will next be asked : What, then, is the function of the basal ganglia ? They are what we may term subsidiary centres; the corpus striatum, principally in connection with move- ment, and the optic thalamus, in connection with sensation, and especially with the sense of vision as its name indicates. A subsidiary centre may be compared to a subordinate official in an army. The principal centre may be compared to the commander-in-chief. This highest officer gives a general order for the movement of a body of troops in a certain direction; we may compare this to the principal motor-centre of the cortex sending out an impulse for the movements of the muscles of a limb. But the general does not give the order himself to each individual soldier, any more than the cerebral cortex does to each individual muscle; but the order is first given to subordinate officers, who arrange exactly how the movement shall be executed, and their orders are in the end distributed to the individual men, who must move in harmony with their fellows with regard to both time and space. So the subsidiary nerve-centres or positions of relay enable the impulse to be widely distributed by collaterals to numerous muscles which contract in a similar orderly, har- monious, and co-ordinate manner. The subject of muscular co-ordination we shall consider at greater length in the next chapter on the functions of the cerebellum. There is just the same sort of thing in the reverse direction in the matter of sensory impulses. Just as a private in the army, when he wishes to communicate with the general, does so through one or several subordinate officers, so the sensory impulse passes through many cell-stations or subsidiary centres on the way to the highest centre where the mental process called sensation, that is, the appreciation of the impulse, takes place. There are two great experimental methods in determining the function of any part of the cerebrum. The first is stimulation; the second is extirpation. These words almost explain them- selves ; in stimulation a weak interrupted induction current is applied by means of electrodes to the convolution under investi- gation, and the resulting movement of the muscles of the body, if any occurs, is noticed. In extirpation the piece of brain is removed, and the resulting paralysis if any, is observed. on. xxiv.] MOTOR AND SENSORY AREAS. 259 By such means the cortex has been mapped out into what we may provisionally term motor areas, and sensory areas. Motor areas.-Stimulation of these produces movement of some part of the opposite side of the body ; excitation of the same spot is always followed by the same movement in the same animal. In different animals excitation of anatomically corre- sponding spots produces similar or corresponding results. It is this which has enabled one to apply the results of stimulating- areas of the monkey's brain to the elucidation of the function of the similar brain of man. Extirpation, or removal, of these areas produces paralysis of the same muscles which are thrown into action by stimulation. Sensory areas.-Stimulation of these produces no direct movements, but doubtless sets up a sensation called a subjective sensation; that is, one produced in the animal's own brain, and this indirectly leads to movements which are reflex; thus on stimulating the auditory area there is a pricking up of the ears ; on stimulating the visual area there is a turning of the head and eyes in the direction of the supposed visual impulse. That such movements are reflex and not direct is shown by the long- period of delay intervening between the stimulation and the movement. Extirpation of a sensory area leads to loss of the sense in question. The rougher experiments performed by nature in the shape of diseases of the brain produce corresponding results. Some diseases are of the nature of extirpation. An instance of this is cerebral haemorrhage. If the haemorrhage is in the region of the internal capsule, it cuts through fibres to the muscles of the whole of the opposite side of the body, as they are all collected together in a narrow compass, and the condition obtained is called hemiplegia. The varieties of hemiplegia are numerous, according as motor or sensory fibres are most affected, and in one variety of hemiplegia, called crossed hemiplegia, the face is paralysed on one side of the body, the limbs on the other ; this is due to injury of the nerve-tracts in the bulb subsequent to the crossing of the fibres to the nucleus of the seventh nerve, but above the crossing of the pyramids (just below the asterisk in fig- 251). If now the haemorrhage occurs on the surface of the brain, a much more limited paralysis, called monoplegia, is the result; if the arm area is affected, there will be paralysis of the opposite arm; if the leg area, of the opposite leg; if a sensory area, there will be loss of the corresponding sense. 260 FUNCTIONS OF THE CEREBRUM. [CH. xxiv. Some diseases, on the other hand, act as the induction currents do in artificial stimulation; they irritate the surface of the brain ; such a disease is a tumour growing in the membranes of the Fig-. 243. Figs. 242 and 243.-Brain of dog, viewed from above and in profile. F, frontal fissure sometimes termed crucial sulcus, corresponding to the fissure of Rolando in man. (S', fissure of Sylvius, around which the four longitudinal convolutions are concentrically arranged ; 1, flexion of head on the neck, in the median line ; 2, flexion of head on the neck, with rotation towards the side of the stimulus; 3, 4, flexion and extension of anterior limb ; 5, 6, flexion and extension of posterior limb ; 7, 8, 9. contraction of orbicularis oculi, and the facial muscles in general. The unshaded part is that exposed by opening the skull. (Dalton.) CH. XXIV.] JACKSONIAN EPILEPSY. 261 brain ; if the tumour irritates a piece of tlie motor area, there will be involuntary movements in the corresponding region of the body • these move- ments may culmi- nate in the pro- duction of epilep- tiform convulsions commencing in the arm, leg, or other part of the body which corresponds to the brain area irritated. It is these cases of " Jacksonian Epi- lepsy " which have given the best re- sults in surgery ; the movement produced is an indication of the area of the brain which is being irritated, and the surgeon after trephining is able to remove the source of the mischief. If the area of the brain which is irritated is a sensory area, the result produced is a subjective sensation, similar to what we imagine is produced in animals with an electric current. We may now proceed from these general con- siderations to particular points, and give maps of the brain to show the areas we have been speak- ing of. Fig. 242 is a view of the dog's brain. It is convenient to take this first because it was the starting-point of the experimental work on the subject in the hands of Hitzig and Fritsch. If the text beneath the figure is consulted, it will be seen that the motor areas, mapped out by the Fig. 244. Fig- 245. Figs. 244 and. 245.-Diagrams of monkey's brain to show the effects of electric stimulation of certain spots. (According to Ferrier.) 262 FUNCTIONS OF THE CEREBRUM. [on. xxiv. method of stimulation, are situated in the neighbourhood of the crucial sulcus, which corresponds to the fissure of Rolando in man. Coming next to the brain of the monkey, figures 244, 245 are reproductions from Ferrier. He marked out the surface into a number of circles, stimulation of each of which produced move- ments of various sets of muscles, face, arm, and leg from below upwards ; extirpation of these same areas produced the corre- sponding paralysis. It will be further noticed that these areas are all grouped around the fissure of Rolando, particularly in the ascending frontal and ascending parietal convolutions; hence the term Rolandic area which is often applied to this region of the brain. These facts, however, are of principal interest because of their application to the human brain, to which we now pass. The following maps of the human brain were prepared from data partly derived from the examination of the monkey's brain, and partly from the post-mortem examination of human brains in cases of brain disease. Fig. 246 shows the outer surface of the right cerebral hemisphere with the names of the principal convolutions and fissures inserted. Fig. 247 gives the corresponding surface of the left hemisphere with the principal motor and sensory areas marked. Fig. 248 shows the mesial surface of the right hemi- sphere with the names of the convolutions and fissures. Fig. 249 gives the corresponding surface of the left hemisphere with the functional areas marked. Motor areas of the Human Brain.-Roughly, these occupy the convolutions around the fissure of Rolando, and turn over the edge of the hemisphere into the marginal convolution of the mesial surface ; from below, up and backwards, we have the areas for the head, arm, and leg in the order named. More accurately the areas are as follows :- Fig. 246.-Right cerebral hemisphere, outer surface. CH. XXIV.] MOTOR AREAS. 263 Head, neck, and face : lower two-thirds of the ascending frontal, bases of the lower and middle transverse frontal, a small piece reaching the front of the motor region in the marginal of the mesial surface. Fig. 247.-Left cerebral hemisphere, outer surface. Fig. 248.-Right cerebral hemisphere, mesial surface. Fig. 249.-Left cerebral hemisphere, mesial surface. 264 FUNCTIONS OF THE CEREBRUM. [ch. xxiv- Upper limb : upper third of the ascending frontal, the base of the upper transverse frontal, the ascending parietal (where the centres of hand and wrist are situated), and the piece of the marginal behind the head-centre. Lower limb : the parietal lobule, and the posterior part of the marginal. Trunk : the marginal between the leg and arm areas. The next diagram (fig. 250) shows the relative position of the several motor-tracts in their course from cortex to crus, according to Dr. Gowers. Fig. 250.-Diagram to show the relative positions of the several motor tracts in their course from the cortex to the crus. The section through the convolutions is vertical; that through the internal capsule, I, C, horizontal; that through the crus again vertical. C, N, caudate nucleus; O, TH, optic thalamus; L2 and L3, middle and outer part of lenticular nucleus ; /, a, I, face, arm, and leg fibres. The words in italics indicate corresponding cortical centres. (Gowers.) The following diagram shows a vertical section through the brain, and enables one to trace the motor-tracts to the nucleus of the seventh or ■ facial nerve of the opposite side, and to the pyramidal tracts of the spinal cord. In this and in the preceding figure the letterpress underneath them should be carefully con- sulted. The marginal convolution was first investigated by Schafer and Horsley, and to them belongs the credit of discovering the centre for the trunk muscles. If one marginal convolution is removed in an animal, there is much more marked paralysis of the opposite limbs than of the trunk ; if the two marginal convolutions are removed, there is very complete paralysis of the trunk as well as of the limbs. In cases of hemiplegia in man, there is usually CH. XXIV.] SPEECH CENTRE. 265 very little paralysis of the trunk muscles. It is the muscles which act normally unilaterally that are most paralysed. The muscles of the trunk always normally move bilaterally; thus we use both sides of our chest in breathing; both sets of back muscles in maintaining an erect position, and so on. The spinal centres of the muscles of the two sides are, no doubt, connected Fig. 251.-Fibres are seen passing from the cortex of the Rolandic area through the corona radiata to the internal capsule (I.C.); a few collaterals to the corpus callosum (C.C.) are also put in. I.R., island of Reil; Ch, claustrum; O.T., optic thalamus; C.N., caudate nucleus; L.N., lenticular nucleus. The asterisk indicates the place of decussation of the face fibres to VII., the nucleus of the seventh nerve. C.P.T., crossed pyramidal tract; D.P.T., direct pyramidal tract. by commissural fibres, and therefore can be affected from both sides of the brain. The Speech centre.-This is surrounded by a dotted circle in fig. 247. There are other centres concerned in speech besides this, but this is the centre for the muscular actions concerned in speech. The discovery of this centre was the earliest feat in the direction of cerebral localisation. It was discovered by a French physician named Broca; he noticed that patients who died after haemorrhage in the brain, but who previous to death ex- hibited a curious disorder of speech called aphasia, were found, after death, to have the seat of the haemorrhage in this convolu- tion. The convolution is generally called Broca's convolution. Experiments on animals are obviously useless in discovering the centre for speech. The most curious fact about the speech-centre is that it is unilateral; it is situated only on the left side of the brain, except 266 FUNCTIONS OF THE CEREBRUM. [ch. xxiv. in left-handed people, where it is on the right. We are thus left-brained so far as the finer movements of the hand-muscles are concerned, and we are also left-brained in regard to speech, an action which is apparently bilateral. The Sensory areas of the Human Brain.--These are much less accurately mapped out than the motor areas. Roughly speaking, they are situated behind the motor areas. The. visual area is situated in the occipital lobe, and the an- gular gyrus. Dr. Gowers, on clinical grounds, regards the angular gyrus as the higher psychical centre for vision, and corresponds to the opposite eye. But experimentally we know much more about the relationship of the occipital lobes to vision. Extirpa- tion of one occipital lobe in an animal, or disease of that lobe in man, produces blindness of the same side of each retina; this condition is called hemianopsia. If, for instance, the right occipital lobe is removed, the result is blindness of the temporal half of the right retina, and the nasal half of the left retina, leading to an inability to see things in the left half of the field of vision; the animal turns its head and eyes to the same side as the lesion, or in technical language there is conjugate deviation of head and eyes to the right. Stimulation of the visual area (and this is true for both the occipital lobe and the angular gyrus) leads, no doubt, to a sub- jective visual sensation of the corresponding halves of the two retinte. Suppose the right visual area is stimulated, the subjec tive sensation will be on the right halves of the retinae; the animal therefore imagines light is falling on its eyes from the left and so there is conjugate deviation of the head and eyes to the left ; that is, the opposite side to that stimulated. The auditory area was localised by Ferrier in the superior temporo-sphenoidal convolution. But there is considerable doubt whether this is correct; it is so much more difficult to tell when an animal is deaf than when it is blind. Similar uncertainty exists as to the situations for taste and smell. No doubt they are closely connected, and they have been placed provisionally in the uncinate convolution, and tip of the temporo-sphenoidal lobe. The large size of these parts in animals with a keen sense of smell lends support to this idea. Tactile sensibility was localised by Schafer in the limbic lobe, but there is so much doubt about this, that a query is placed after the words " tactile sense," in the gyrus fornicatus in fig. 249. Munk's view, supported in this country by Bastian, Mott, and numerous others, is that the sensory fibres from the skin and CH. XXIV.] SENSORI-MOTOR AREAS. 267 muscles terminate in the Rolandic area; and the histological researches of Golgi and Ramon y Cajal (see figs. 229 and 232) point to the same conclusion. This is, in fact, what one would expect, volition and consciousness are associated together so closely physiologically, that anatomically we should expect to find the commencement of the volitional fibres contiguous to the termina- tions of the sensory fibres. That this is really the case has been shown by a careful examination of the sensation in animals in which the Rolandic area has been removed, and in cases of hemi- plegia in man. The most delicate test is to place a clip on the fingers or toes, taking care the animal does not see the clip put on. If there is loss of tactile sensibility the monkey either takes no notice at all of the clip or removes it after a long delay. Whereas if sensation is perfect the monkey at once seizes the clip and flings it away. It is found that the intensity of both the motor and sensory paralysis are directly proportional to each other. Hence the term motor area which we have been provi- sionally employing for the Rolandic area, should be replaced by the more correct term sensori-motor area. The question will then be asked, what is the function of the gyrus fornicatus ; on removal of this convolution there certainly is loss of sensation; this has been explained by the fact that on removing this area of grey matter it is almost impossible to avoid injury to the white matter beneath it, and thus there will be loss of function due to division of the fibres on the way to the mar- ginal convolution, which is like the Rolandic area, sensori-motor in function. It is, however, quite possible that the gyrus fornicatus has something to do with sensation. The term tactile sense is a very general one ; it really includes the true sense of touch from the skin, the muscular sense, general sensibility, the sensation of pain, and the sensations of heat and cold. It is the first two (true tactile sense, and the muscular sense) which are so important in the regulation of the resulting muscular movements. The others probably go by a different channel to the brain (see p. 248), and probably a different destination, it may be the limbic lobe, in the brain. On referring once more to the maps of the brain, it will be seen that there is a large blank in the anterior part of the frontal region. This is left blank because its function is absolutely un- known. Extirpation or stimulation of this part of the brain in animals produces no appreciable result. It has also been removed accidentally in man, as in the celebrated American crowbar acci- dent ; owing to the premature explosion of a charge of dynamite 268 FUNCTIONS OF THE CEREBRUM. [ch. xxiv in one of the American mines, a crowbar was sent through the frontal region of the foreman's head, removing the anterior part of his brain. He, however, recovered, and no noteworthy symp- toms were observed in him during the rest of his life. He, indeed, returned to his work as overseer of the mine. The large size of this portion of the brain is very distinctive of the human brain, and it has therefore been supposed that here is the seat of the intellectual faculties. This may be so, but experimental physiology lends no support to this view, as the sensory centres (and sensations are the materials for intellect) are situated behind or within, and not in front, of the Rolandic area. Degeneration Tracts after Injury of the Rolandic Area. After our long study of the motor tracts in the central nervous system, it might seem unnecessary to recapitulate the cause of the degenerated tracts which are produced by injury to the CORTEX PONS INTERNAL CAPSULE CORD MID. BRAIN Fig. 252.- Degeneration after destruction of the Rolandic area of the right hemisphere. (After Gowers.) Rolandic area. Repetition in connection with this important subject, however, calls for no apology, and the above diagram will put the matter in rather a fresh light. The shaded area in each case represents the injured or degenerated material; a in the cortex, B in the anterior part of the internal capsule, c in the middle of the crusta of crus and mid-brain, D in the pyramidal bundles of the pons, e in the pyramid of the bulb, and f in the crossed and direct pyramidal tract of the cord. OH. xxv.] FUNCTIONS OF THE CEREBELLUM. 269 CHAPTER XXV. FUNCTIONS OF THE CEREBELLUM. In past times there have been several views held as to the functions of the cerebellum. One of the oldest of these was the idea that the cerebellum was associated with the function of generation; another view, first promulgated by Willis, was that the cerebellum contained the centres which regulate the functions of organic life; this arose from the circumstance that diseases of the cerebellum are often associated with nausea and vomiting; it is a familiar fact that in displacements of equilibrium such as occur on board ship in a rough sea, or in the disease called Memere's disease, sickness is a frequent result j it appears from this that the cerebellum does receive from or send to the viscera cer- tain impulses. The third and last of these older theories was that the cerebellum was the centre for sensation. This arose from the fact that certain of the afferent channels of the spinal cord were traced into the cerebellum. The impulses that travel along these, however, though afferent, are not truly sensory, and their reception in the cerebellum is not associated with consciousness. The true function of the cerebellum was first pointed out by Flourens, and our knowledge about it has not advanced much from the condition in which Flourens left it. He showed that the cerebellum is the great centre for the co-ordination of muscu- lar movement, and especially for that variety of co-ordination which is called equilibration-that is, the harmonious adjustment of the working of the muscles which maintain the body in a position of equilibrium. It must not be supposed from this that the cerebellum is the sole centre for co-ordination. We have already seen that all the machinery necessary for carrying out very complicated locomotive movement is present in the spinal cord. The higher centres set this machinery going, and the work of arranging what muscles are to act, and in what order, is carried out by the whole of the grey matter from the corpora striata to the end of the spinal cord, including such outgrowths as the corpora quadrigemina and cerebellum. An instance of a complex co-ordinated movement is seen in what we learnt to call in the last chapter conjugate deviation of head and eyes. The higher cortical centre gives the general word of command to turn the head and eyes to the right: 270 FUNCTIONS OF THE CEREBELLUM. [ch. xxv. the subsidiary centres or subordinate officials arrange that this is to be accomplished by the external rectus of the right eye supplied by the right sixth nerve, the internal rectus of the left eye supplied by the left third nerve, and numerous muscles of neck and back of both sides supplied by numerous nerves. We thus see how the complicated intercrossing of fibres and connec- tions of the centres of the various nerves are brought into play. The functions of the cerebellum are investigated by the same two methods of experiment (stimulation and extirpation) that are employed in similar researches on the cerebrum. The anatomical connections of the cerebellum with other parts of the cerebro- spinal axis have been chiefly elucidated by the degeneration method. Each side of the cerebellum has three peduncles ; the superior peduncle connecting it to the opposite hemisphere of the cerebrum, the inferior peduncle connecting it to the same side of the spinal cord, and the middle peduncle contains fibres which link the two halves of the cerebellum together in a physiological though not in an anatomical sense. The inferior peduncle termi- nates in the vermis ; in some of the lower animals the vermis is practically the only part of the cerebellum which is present, and it is this part of the cerebellum which is principally con- cerned in the co-ordination of the bodily movements. The cere- bellar hemispheres are especially connected with the opposite cerebral hemispheres ; and possibly just as the different regions of the body have corresponding areas in the cerebrum, so also they are similarly represented in the cerebellum; but localisation of function in the cerebellum has not gone sufficiently far yet to make this a certainty. After hemi-extirpation, degeneration occurs in the peduncles of the same side ; there are, therefore, no commissural fibres that actually pass from one hemisphere to the other. In the superior peduncle, the degenerated fibres pass chiefly to the tegmental nucleus of the opposite side, but partly to that of the same side ; in other words, the decussation of these peduncles in the mid-brain is not complete. Some fibres are traceable to the optic thalamus. The middle peduncle is completely degenerated as far as the raphe, where they intermingle with the fibres from the opposite side. The inferior peduncle was stated by Luciani and Marchi to be also degenerated, and these observers traced the fibres down into the cord. But their state- ments have not been confirmed by the most recent and careful work of Ferrier and Risien Russell. In other words, we are entirely ignorant of any fibres passing out from the cerebellum to the cord (hence the use of a dotted line in figure 232) ; if they do exist their course has yet to be discovered. Some degenerated fibres in the inferior peduncle have been traced to the opposite lower olivary body which completely atrophies. If the cerebellum is removed in an animal, or if it is the seat of disease in man, the result is a condition of slight CH. XXV.] EQUILIBRATION. 271 muscular weakness; but the principal symptom observed is inco-ordination, chiefly evidenced by a staggering gait similar to that observable in a drunken man. It is called cerebellar ataxy. In order that the cerebellum may duly execute its function of equilibration it is necessary that it should send out impulses ; this it does by fibres that leave its cells and pass out through its peduncles; they pass out to the opposite cerebral hemisphere, and so influence the discharge of the impulses from the cortex of the cerebrum. It is quite pos- sible that impulses pass out to the cord (see dotted line in fig. 232), but we are entirely igno- rant of their course if they do exist. The degeneration method has not led to their discovery. The only way of which we have any certain knowledge by means of which the cerebellum influ- ences the motor discharge is as an elaborate cell station on the course of sensory impulses to the cerebrum. The cerebellum thus acts upon the muscles of the same side of the body in conjunction with the cerebral hemispheres of the opposite side. The close inter-relation of one cerebral with the opposite cerebellar hemisphere is shown in cases of brain disease, in which atrophy of one cerebellar hemisphere follows that of the opposite cerebral hemisphere (see fig. 253). In order that the cerebellum may send out impulses in this way, it is necessary that it receive impulses which guide it by keeping it informed of the position of the body in space. These afferent impulses are of four kinds, namely :- Fig. 253.-This is a reproduction of a photograph of a lunatic's brain lent me by Dr. Fricke. One cerebral and the opposite cerebellar hemisphere are atrophied. 1. Tactile. 2. Muscular. 3. Visual. 4. Labyrinthine. We will take these one by one :- i. Tactile impressions.-The importance of impulses from the skin is shown in those diseases of the sensory tracts (especially locomotor ataxy) where there is anaesthesia of the soles of the feet. In such cases the patient cannot stand with his eyes shut. The same effect may be produced experimentally by freezing the soles of the feet. Again, if the skin is stripped from the hind limbs of a brain- 272 FUNCTIONS OF THE CEREBELLUM. [ch. xxv. less frog, it is unable to execute such reflex actions as climbing an inclined plane, which it can do quite well when the skin is uninjured. 2. Muscular impressions.-Quite as important as the tactile sense from the skin is the muscular sense, the sense which enables us to know what we are doing with our muscles. We have hitherto chiefly spoken of the muscular nerves as being motor ; they also contain sensory fibres; these pass from the muscles, and their tendons to the posterior roots of the spinal nerves, and the impulses ascend the sensory tracts through cord and brain to reach the cerebellum and the Rolandic area. In some cases of locomotor ataxy there is little or no loss of tactile sensibility, and the condition of inco-ordination is then due to the loss of the muscular sense. 3. Visual impressions. - The use of visual impressions in guiding the nervous centres for the maintenance of equilibrium is seen in those cases of locomotor ataxy where there is loss of equi- librium when the patient closes his eyes. Destruction of the eyes in animals often causes them to spin round and lose their balance. The giddiness experienced by many people on looking at moving water, or after the onset of a squint, or when objects are viewed under unusual circum- stances, as in the ascent of a mountain railway, is due to the same thing. The importance of keeping one's eyes open is brought home to one very forcibly when one is walking in a perilous position, as along the edge of a precipice, where an upset of the equilibrium would be attended with serious consequences. 4. Labyrinthine impressions.-These are the most important of all; they are the impressions that reach the central nervous system from that part of the internal ear called the labyrinth. Here, however, we must pause to consider first some anatomical facts in connection with the semicircular canals that make up the Fig. 254.-Right bony labyrinth, viewed from the outer side. The specimen here represented is prepared by sepa- rating piecemeal the looser substance of the petrous bone from the dense walls which immediately enclose the labyrinth. 1, the vestibule ; 2, fen- estra ovalis; 3, superior semicircular canal; 4, horizontal or external canal; 5, posterior canal; *, ampullae of the semicircular canals; 6, first turn of the cochlea; 7, second turn; 8, apex ; 9, fenestra rotunda. The smaller figure in outline below shows the natural size. (Sommering.) 1 ch. xxv.] SEMICIRCULAR CANALS. 273 labyrinth. Fig. 254 is an external view of the internal ear ; it is enclosed within the petrous portion of the temporal bone; and consists of three parts-the vestibule (1), the three semicircular canals (3, 4, 5) which open into the vestibule, and the tube, coiled like a shell, called the cochlea (6, 7, 8). The cochlea is the part of the apparatus which is concerned in the reception of auditory impressions ; it is supplied by the cochlear division of the eighth or auditory nerve. The remainder of the internal ear is concerned not in hearing, but in the reception of the impressions we are now studying. Within the vestibule are two chambers made of membrane, called the utricle and the saccule : these communicate with one another and with the canal Fig. 255.-Section of human semicircular canal. 1, bone ; 2, periosteum ; 3, 3, fibrous bands connecting the periosteum to 4, the outer fibrous coat of the membranous canal; 5, tunica propria ; 6, epithelium. 1 of the cochlea. Within each bony semicircular canal is a mem- branous semicircular canal of similar shape. Each canal is filled with a watery fluid called endolymph, and separated from the bony canal by another fluid called perilymph. Each canal has a swelling at one end called the ampulla. The membranous canals open into the utricle; the horizontal canal by each of its ends ; the superior and posterior vertical canals by three openings, these two canals being connected at their non-ampullary ends. Eig. 255 shows in transverse section the way in which the membranous is contained within the bony canal; the membranous canal consists of three layers, the outer of which is fibrous and continuous with the periosteum that lines the bony canal ; then comes the tunica propria, composed of homogeneous material, and 274 FUNCTIONS OF THE CEREBELLUM. [ch. xxv. thrown into papillae except just where the attachment of the membranous to the bony canal is closest; and the innermost layer is a somewhat flattened epithelium. At the ampulla there is a different appearance; the tunica propria is raised into a hillock called the crista acustica (see fig. 256) ; the cells of the epithelium become columnar in shape, and to some of them fibres of the auditory nerve pass, arborising round them ; these cells are provided with stiff hairs, which pro- ject into what is called the cupula, a mass of mucus-like material containing oboliths or crystals of calcium carbonate. Between Fig. 256.-Section through the wall of the ampulla of a semicircular canal, passing through the crista aeustica. i, epithelium ; 2, tunica propria ; 3, fibrous layer of canal; N, bundles of nerve fibres ; C, cupula, into which the hairs of the hair-cells project. the hair-cells are fibre-cells which act as supports (fig. 257). When the endolymph in the interior of the canals is thrown into vibra- tion, the hairs of the hair-cells are affected, and a nervous impulse set up in the contiguous nerve, which carry it to the central nervous system. The walls of the saccule and utricle are similar in composition, and each has a similar hillock, called a macula, to the hair-cells on which nerve-fibres are distributed. The macula of the utricle and the cristse of the superior and horizontal canals are supplied by the vestibular division of the eighth or auditory nerve. The macula of the saccule and the crista of the posterior canal are supplied by a branch of the cochlear division of the same nerve. When these canals are diseased in man, as in Meniere's disease, there are disturbances of equilibrium; a feeling of giddiness, which may lead to the patient's falling down; this is associated with nausea and vomiting. In animals similar results are pro- CH. XXV.] SEMICIRCULAR CANALS. 275 duced, and the subject has been chiefly worked out on birds by h lourens, where the canals are large and readily exposed, and more recently in fishes by Lee. 1 bus if the horizontal canal is divided in a pigeon, the head is tin own into a series of oscillations in a horizontal plane, which aie increased by section of the corresponding canal of the opposite side. After section of the vertical canals, the forced move- ments are in a vertical plane, and the animal tends to turn •somersaults. "When the whole of the canals are destroyed on both sides the disturbances of equi- librium are of the most pro- nounced character. Goltz de- scribes a pigeon so treated which always kept its head with the occiput touching the breast, the vertex directed downwards, with the right eye looking to the left and the left looking to the right, the head being incessantly swung in a pen- dulum-like manner. Cyon says it is almost impossible to give an idea of the perpetual move- ments to which the animal is subject. It can neither stand, nor lie still, nor fly, nor maintain any fixed attitude. It executes violent somersaults, now forwards, now backwards, rolls round and round, or springs in the air and falls back to recommence anew. It is necessary to envelope the animals in some soft covering to prevent them dashing themselves to pieces by the violence of their movements, and even then not .always with success. The extreme agitation is manifest only during the first few days following the operation, and the animal may then be set free without danger; but it is still unable to stand or walk, and tumultuous movements come on from the .slightest disturbance. But after the lapse of a fortnight it is .able to maintain its upright position with some support. At this stage it resembles an animal painfully learning to stand and walk. In this it relies mainly on its vision, and it is only necessary to cover the eyes with a hood to dispel all the fruits of this new •education, and cause the reappearance of all the motor disorders." (F errier.) Fig. 257.-1, hair-cell; 3,hair-cell, show- ing the hair broken, and the base of the hair, split into its constituent fibrils; 2, fibre-cell; N, bundle of nerve fibres which have lost their medullary sheath, and terminate by arborising round the base of the hair- cells ; A.B., surface of tunica propria. 276 FUNCTIONS OF THE CEREBELLUM. [ch. xxv. It is these canals which enable all of us to know in which direction we are being moved, even though our eyes are bandaged, and the feet are not allowed to touch the ground. On being whirled round, such a person knows in which direction he is- being moved, and when the whirling stops he seems, especially if he opens his eyes, to be whirling in the opposite direction, due- to the rebound of the fluid in the canals. The forced movements just described in animals are due either to the absence of the normal sensations from the canals, or to delusive sensations- arising from their irritation, and the animal makes efforts to correct the movement which it imagines it is being subjected to. It will be noticed that the canals of each side are in three- Fig. 258.-Diagram of semicircular canals, to show their positions in three planes at right angles to each other. It will be seen that the two horizontal canals (H) lie in the- same plane: and that the superior vertical of one side (S) lies in a plane parallel to that or the posterior vertical (P) of the other. (After Ewald.) planes at right angles to each other, and we learn the movements of our body with regard to the three dimensions of space by means of impressions from the ampullary endings of the auditory nerve ; these impressions are set up by the varying pressure of the endolymph in the ampullae. Thus a sudden turning of the head from right to left will cause- movement of the endolymph towards, and therefore increased pressure on, the ampullary nerve-endings of the right horizontal canal, and diminished pressure on the corresponding nerve-endings- of the left side. " One canal can be affected by, and transmit the sensation of rotation about one axis in one direction only ; and for complete perception of rotation in any direction about any axis, six canals are required in three pairs, each pair being in the same or parallel planes, and their ampullae turned opposite ways. Each pair would thus be sensitive to any rotation about a line at right angles to its plane or planes, the one canal being influenced by ■CH. XXVI.] SENSATION. 277 rotation in one direction, the other by rotation in the opposite •direction." (Crum-Brown.) The two horizontal canals are in the same plane; the posterior vertical of one side is in a plane parallel to that of the superior vertical of the other side (see fig. 258). These four sets of impressions (tactile, muscular, visual, and labyrinthine) reach the cerebellum by its peduncles ; from the eyes through the superior peduncle, from the semicircular canals through the middle and inferior peduncles, and from the body generally through the restiform body or inferior peduncle. Section and stimulation of the peduncles cause inco-ordination, chiefly evidenced by rotatory and circus movements similar to those that occur when the nerve-endings in the semicircular canals are •destroyed or stimulated. Stimulation of the cerebellum itself- and this has been done through the skull in man-causes giddi- ness, and consequent muscular efforts to correct it. The results of stimulation, indeed, are precisely analogous to those of extir- pation, only in the reverse direction. CHAPTER XXVI. SENSATION. Before passing to the study of the various special senses, there are a number of general considerations in connection with the subject of sensation that demand our attention. The psychologist divides the mental phenomena, which the physiologist localises in the brain, into three main categories : - 1. Intellectual : perceiving, remembering, reasoning, &c. 2. Emotional : joy, love, hate, anger, &c. 3. Volitional : purposing, deliberating, doing. These are all closely connected together, and are all present in each healthy brain; but according as one or other may predomi- nate, we speak of intellectual, emotional, or strong-willed indi- viduals. The connection is especially close between intellect and will, which represent as it were the two sides of what we may call a conscious reflex action; the intellect giving the reason or stimulus for the exercise of the volitional power. The emotions are more complex, and we shall not discuss them; they are elaborate mental processes, in which sensations predominate. 278 SENSATION. [ch. xxvi. The intellectual faculties are derived from the senses; sensa- tions form the materials for intellect; in other words, we know and learn from what we see, feel, and hear. People born blind or deaf thus labour under the great disadvantage of having one or the other channel of knowledge closed; they, however, make up for this in some measure by an education, and consequent increased sensibility of the channels that remain open. The simplest mental operation is a sensation-that is, the conscious reception on the mind of an impression from the exter- nal world. For this the following things are necessary :- 1. A stimulus. 2. A nerve-ending to receive it. 3. A path to the brain. 4. A part of the brain to receive the impulse. During the whole operation, moreover, there must be atten- tion ; it is quite possible, for instance, in a dreamy person, that he may look at a thing without seeing it, or be present at a lecture without hearing it. The brain refers the sensation to the nerve-endings that received the stimulus ; thus pain in the finger is referred to the finger, the sight of an object to the eyes, Ac. If the ulnar nerve is stimulated by a knock on the elbow, the sensation is referred to the fingers where the nerve is distributed; if the stump of a recently amputated leg be stimulated, the brain not having got used to the new condition of things, refers the sensation to the toes, which still seem to be present. Perception is a more complicated mental process; it consists in the grouping of sensations, and the imagining of the object from which they arise, and which is called the percept. The smell, the taste, the colour, &c. of an orange are all sensations; the grouping of these together constitutes the perception of an orange. Each mental process leaves an impress on the mind ; these impressions build up memory, or representative imagination; this may be reproductive, as in recalling a friend's face ; or con- structive, as in picturing the face of an historical person. The more complex intellectual operations consist in the forma- tion of concepts, and reasoning the grouping and discrimination of conceptions. Just as perception is built up of sensations, so conception is built up of perceptions. Thus the orange of our previous example is learnt to be one of similar substances called fruits; fruits to be products of the vegetable, as distinguished from the animal world, and so on. This is seen in the education of a child; at first scattered sen- sations only are perceived, and education consists in learning CH. XXVI.] NERVE-ENDINGS. 279 what these sensations correspond to in the external world, and how they may be classified. The other mental facnlties are in the same way built of simpler material; the volitional operations are at first simple responses to external conditions ; later on they become more complex and representative, culminating in speech, the most complicated movement of all. The emotions, too, are at first simple, and merely exaggerated sensations; the higher ones are complex and representative. The nerve-endings that receive the impression from the exter- nal world are of various kinds. They may be simply ramifying and interlacing plexuses of nerve-fibrils, as in the cornea, parts of the skin, and in the interior of the body; this kind of nerve- ending is chiefly associated with general sensibility, that vague kind of sensation which cannot be put under any of the special headings-taste, sight, hearing, touch, and smell. The nerve-end- ings of the nerves of special sense are usually end-organs of a spe- cialised kind. The most frequent kind of sensory end-organ is made of what is called nerve-epithelium ; certain epithelial cells of the surface of the body become peculiarly modified, and grouped in special ways to receive the impressions from the outer world; these send a nervous impulse into the arborisations of the termi- nation of the axis-cylinders of the nerves which envelope the cells. One of these varieties of nerve-epithelium we have already made the acquaintance of, in the hair-cells of the semicircular canals; we shall find other kinds in the hair-cells of the cochlea, in the rods and cones of the retina, &c. Pain is due to an excessive stimulation of the other sensory nerves, but there is some evidence that it may be a distinct sen- sation. Thus in some cases of diseases of sensory channels, tactile sensation may be intact, but sensitiveness to pain absent, and vice versd; see also pp. 248 and 267. The other essential anatomical necessities for a sensation, the channels to the brain with their numerous cell-stations on the road, and the parts of the brain to which these tracts pass, we have already dwelt upon. Some of these points we shall, how- ever, be obliged to return to, especially in connection with vision. But here it is sufficient to insist on the necessity of the presence not only of the end-organ, but also of the nervous tracts and centres. Blindness, for instance, may not only be due to disease of the eye, but also to disease of the optic nerve, or of the parts of the brain which are connected with vision. A small stimulus, or a small increase or decrease in a big stimulus, will have no effect; a light touch, a feeble light, a gentle sound, may be so slight as to produce no effect on the 280 SENSATION. [ch. xxvi. brain. The smallest stimulus that produces an effect is called the lower limit of excitation or the liminal (from limen, a threshold) intensity of the sensation. The height of sensibility or maximum of excitation is such a strong stimulus, beyond which the brain is incapable of recognising any increase ; a bright light, for instance, may be so intense that any increase in its brightness is not perceptible. Between/ these two extremes we have what is called the range of sensibility. Most of our ordinary sensations fall somewhere about the middle of the range, and Weber's or Fechner's law is a law that regulates the proportion between the stimulus and the sensation, and which is operative for this region of the range of sensibility. In general terms it may be stated that sensations increase as the logarithm of the stimuli; or, in order that the intensity of a sensation may increase in arith- metical progression, the stimulus must increase in a geometrical progression. A definite example will help us to understand these mathe- matical terms a little better. We will select our example from the sense of vision, because the intensity of the cause of visual sensations, light, is easily measurable. Suppose a room lighted by 100 candles, and one candle more is brought in, the increase of light produced by the extra candle is quite perceptible to the eye ; or if a candle were removed, the decrease in light would be perfectly appreciable. Next suppose the room lighted by 1,000 candles, and one extra was brought in, no difference would be seen in the amount of illumination; in order to notice increase or decrease in the light it would be necessary to bring in ten extra candles, or take away ten of the candles, as the case might be. In each case an increment or decrease of one-hundredth of the original light is necessary to cause a corresponding increase or diminution in the sensation. This is after all a perfectly familiar fact; a farthing.rushlight will increase the illumination in a dimly-lighted cellar, but it makes no apparent difference in the bright sunshine. The magnitude of the fraction representing the increment of stimulus necessary to produce an increase of sensation determines what is called the discriminative sensibility. This fraction differs considerably for different sense-organs; thus :- For light it is For sound it is For weight it is T\-. For temperature it is For tactile pressure to -j- in different parts of the body. Another general consideration in connection with sensation is CH. xxvi.J SPECIFIC NERVE ENERGY. 281 that the sensation lasts longer than the stimulus; a familiar instance of this is the sting after a blow. The after-sensations, as they are called, have been specially studied in connection with the eye (see After-images). Subjective sensations are those which are not produced by stimuli in the external world, but arise in one's own inner con- sciousness ; they are illustrated by the sensations experienced during sleep (dreams), and in the illusions to which mad and ■delirious people are subject. Homologous stimuli.-Each kind of peripheral end-organ is specially suited to respond to a certain kind of stimulus. The homologous stimuli of the organs of special sense may be divided into :- 1. Vibrations set up at a distance without actual contact with the object; for instance, light and radiant heat. 2. Changes produced by actual contact with the object; for instance, in the production of sensations of taste, touch, weight, .and alteration of temperature by conduction ; in the case of the •olfactory end-organs, the sensation is also excited by material particles given off by the odoriferous body, and borne by the air to the nostrils. In sound also, though there is no actual contact of the ear with the vibrating body which emits the sound, the ■organ of hearing is excited by waves of material substance, first of air, then of bones, then of endolymph, and these excite the nerve-endings of the internal ear. When the eye is excited by any other kind of stimulus than by light, which is its adequate or homologous stimulus, the sensation ■experienced is light all the same ; for instance, one sees sparks when the eyeball is struck ; singing in the ears, the result of an accumulation of wax against the membrana tympani, is a similar ■example. This brings us to the conclusion of this chapter by leading to the question, Is there such a thing as specific nerve energy 2 It is an old question, but the answer has still to be found. Sight is a different thing from hearing, and both are different from taste and smell. What is the difference really due to ? Can it be explained by supposing that the nervous impulse along the optic nerve is a different kind of molecular change from that which accompanies gustatory or auditory impulses ? Or can it be explained by supposing that the main difference is in the end- organ, or in the psychical process which interprets the impulse from the end-organ? Until we know more about the nature of the molecular change which constitutes a nervous impulse, it is merely a matter of speculation whether specific nerve-energy exists. 282 TOUCH. [CH. XXVII. CHAPTER XXVII. TOUCH. Under the general heading Touch we shall include the various kinds of sensory impressions that start from the skin and muscles. Tactile End Organs. First, however, it is necessary to study the varieties of end organs concerned in the reception of the impressions. They are of numerous kinds, but the following are the principal ones :- Pacinian Corpuscles.-These are named after their discoverer, Pacini. They are little oval bodies, situated on some of the cerebro-spinal and sympathetic nerves, especially the cuta- neous nerves of the hands and feet, where they lie deeply placed in the true skin. They often occur on the nerves of the mesentery of some animals like the cat. They have been observed also in the pancreas, lymphatic glands and thyroid glands, as well as in the penis. They are about inch long. Each corpuscle is attached by a narrow pedicle to the nerve on which it is situated, and is formed of several concentric layers of membrane, con- sisting of a hyaline ground-membrane with connective-tissue fibres each layer being lined by endothelium (figs. 260, 261); through its pedicle passes a single nerve-fibre, which, after traversing the several concentric layers and their immediate spaces, loses its medullary sheath and enters a central core, at or near the distal end of which it terminates in an arborisation. Some of these layers are continuous with those of the perineurium, but some are superadded. In some cases two nerves have been seen enter- ing one Pacinian body, and in others a nerve after passing un- altered through one has been observed to terminate in a second Pacinian corpuscle. Fig. 259.-Extremities of a nerve of the finger with Pacinian corpuscles attached, about the natural size. (Adapted from Henle and Kolliker.) CH. XXVII.] TACTILE END ORGANS. 283 The corpuscles of Herbst (fig. 262) are closely allied to Pacinian corpuscles, except that they are smaller and longer witli a row of nuclei around the central termination of the nerve in the core. They have been found chiefly in the tongues and bills of Fig. 260.-Pacinian corpuscle of the cat's mesentery. The stalk consists of a nerve-fibre (N) with its thick outer sheath. The peripheral capsules of the Pacinian corpuscle are continuous with the outer sheath of the stalk. The intermediary part becomes much narrower near the entrance of the axis-cylinder into the clear central core. A hook-shaped termination (T) is seen in the upper part. A blood-vessel (V) enters the Pacinian corpuscle, and approaches the end ; it possesses a sheath which is the continuation of the peripheral capsules of the Pacinian corpuscle. X 100. (Klein and Noble Smith.) ducks. The capsules are nearer together, and towards the centre the endothelial sheath appears to be absent. End.-bu.lbs are found in the conjunctiva (where in man they are but in most animals oblong), in the glans penis and clitoris, in the skin, in the lips, in the epineurium of nerve- 284 TOUCH. [ch. xxvii. trunks, and in tendon; each is about inch in diameter, oval or spheroidal, and is composed of a medullated nerve-fibre, which Fig. 261.-Summit of a Pacinian corpuscle of the human finger, showing the endothelial membranes lining the capsules. X 220. (Klein and Noble Smith.) terminates in corpuscles of various shapes, with a capsule con- taining a transparent or striated core, in the centre of which Fig. 262.-A corpuscle of Herbst, from the tongue of a duck, e, medullated nerve cut away. (Klein.) Fig. 263.-End-bulb of Krause, a, me- dullated nerve-fibre; b, capsule of corpuscle. terminates the axis-cylinder of the nerve-fibre, the ending of which is somewhat clnbbed (fig. 263). CH. XXVII.] TACTILE CORPUSCLES. 285 Touch-corpuscles (figs. 264, 266) are found in the papillae of the skin of the fingers and toes. They are small oblong masses, about -g-g-jj- inch long, and inch broad, composed of connective-tissue, surrounded by elastic fibres and a capsule of more or less numerous nucleated cells. They do not occur in all Fig. 264.-Papilla.' from the skin of the hand, freed from the cuticle and exhibiting tactile corpuscles, a. Simple papilla with four nerve-fibres ; a, tactile corpuscle ; b, nerves with winding fibres c and e. b. Papilla treated with acetic acid ; a, cortical layer with cells and fine elastic filaments ; b, tactile corpuscle with transverse nuclei; c, entering nerve; d and e, nerve-fibres winding round the corpuscle. X 350 (Kiilliker.) the papillae of the parts where they are found, and, as a rule, in the papillae in which they are present there are no blood- vessels. Fig. 265.-A corpuscle of Grandry, from the tongue of a duck. Fig. 266.-A touch-corpuscle from the skin of the human hand, stained with gold chloride. The peculiar way in which the medullated nerve winds round and round the corpuscle before it enters it is shown in fig. 264. It loses its sheath before it enters into the interior, and then 286 TOUCH. [ch. xxvii. its axis-cylinder branches, and the branches after either a straight or convoluted course terminate within the organ. The corpuscles of Grandry (fig. 265) form another variety Fig. 267.-Termination of medullated nerve-fibres in tendon near the mus- cular insertion. (Golgi.) Fig. 268.-One of the reticulated end-plates of fig. 114, more highly magnified. «, medullated nerve-fibre; &, reticulated end-plates. (Golgi.) and have been noticed in the beaks and tongues of birds. They consist of corpuscles oval or spherical, contained within a delicate nucleated sheath and containing several cells, two or more compressed vertically. The cells are granular and transparent, with a nucleus. The nerve enters on one side and, laying aside its medullary sheath, terminates between the cells in flattened expansions. Nerve terminations, sensory in function, are found in inter-muscular tissue (figs. 267, 268), and also in tendon. The former are reticulated end-plates, and the latter are something like small Pacinian corpuscles (fig. 269). The muscle-spindle (see p. 96) is supposed by some to be sensory. In addition to the special end organs, sensory fibres may terminate in plexuses, as in the sub-epithelial and the intra- epithelial plexus of the cornea (fig. 270). We may now proceed to the consideration of the sense of touch itself; it may be divided into three kinds :- 1. The sense of locality. 2. The sense of pressure. Fig. 269.-A termination of a medullated nerve-fibre in tendon, lower half with convoluted medul- lated nerve-fibre. (Golgi.) CH. XXVII.] SENSE OF LOCALITY. 287 3. The sense of temperature. When any object rests on the skin, it is possible by these three varieties of tactile sense to ascertain its shape and the part of the skin which it touches (sense of locality); to estimate its weight even if it is not lifted (sense of pressure); if it is lifted the muscular sense is called into play; and, thirdly, by the temperature sense we determine whether it is hot or cold. The end organs in the skin are numerous, and it is quite possible that these sensations are received by different kinds of end organs, though we are not acquainted with which corresponds to which. It is also not possible to draw a hard- and-fast line between touch proper on the one hand and general sensibility and pain on the other. The facts of disease, especially in that disease of the sensory tracts called loco- motor ataxy, point to the conclusion that these varieties of sensation are transmitted to the central nervous system by different tracts, and re- ceived and interpreted there by dif- ferent areas (see pp. 248, 267, and 279)- Fig. 270.-Vertical section of rab- bit's cornea, stained, with gold chloride. The nerves n, termin- ate in a plexus under and with- in the epithelial layer, e. The Sense of Locality. The acuteness of the sense of locality on different parts of the surface is proportioned to the power which such parts possess of distinguishing and isolating the sensations produced by two points placed close together. This power depends in part on the number of nerve-fibres distributed to the part; for the fewer the fibres which an organ receives, the more likely is it that several impressions on different contiguous points will act on only one nerve-fibre, and hence produce but one sensation. Experiments have been made to determine the tactile properties of different parts of the skin, as measured by this power of distinguishing distances. These consist in touching the skin, while the eyes are 288 TOUCH. [ch. xxvii closed, with the points of a pair of compasses, and in ascertaining how close the points of the compasses may be brought to each other, and still be felt as two points. Table of variations in the tactile sensibility of different parts.- The measurement indicates the least distance at which the two points of a pair of compasses could be separately distinguished. (E. H. Weber.) Tip of tongue ........ inch i mm. Palmar surface of third phalanx of forefinger . . „ 2 „ Palmar surface of second phalanges of fingers . . 1 „ 4 „ Red surface of under-lip 0 » 4 » Tip of the nose . I » 6 „ Middle of dorsum of tongue 3 » 8 » Palm of hand „ 10 ,, Centre of hard palate i » 12 „ Dorsal surface of first phalanges of fingers . • £> „ 14 >r Back of hand ...... . . ii „ 25 „ Dorsum of foot near toes . . . . . . i| „ 37 ,, Gluteal region . . . . . . . . ij „ 37 „ Sacral region ii >, 37 >, Upper and lower parts of forearm . . . . . ij „ 37 ,, Back of neck near occiput ...... 2 ,, 50 „ Upper dorsal and mid-lumbar regions . . . . 2 „ 50 ,, Middle part of forearm 2) „ 62 „ Middle of thigh . ,, 62 ,, Mid-cervical region 21 „ 62 „ Mid-dorsal region 2| „ 62 „ Moreover, in the case of the limbs, it was found that before they were recognised as two, the points of the compasses had to- be further separated when the line joining them was in the long axis of the limb, than when in the transverse direction. According to Weber the mind estimates the distance between two points by the number of unexcited nerve-endings which in- tervene between the two points touched. It would appear that a certain number of intervening unexcited nerve-endings are necessary before the points touched can be recognised as separate, and the greater this number the more clearly are the points of contact distinguished as separate. But the number of nerve- endings is not the only factor in the case, for by practice the delicacy of a sense of touch may be very much increased. A familiar illustration occurs in the case of the blind, who, by con- stant practice, can acquire the power of reading raised letters the forms of which are almost if not quite undistinguishable by the sense of touch to an ordinary person. The power of correctly localizing sensations of touch is gradu- ally derived from experience. Thus infants when in pain simply cry but make no effort to remove the cause of irritation, as an CH. XXVII.] SENSE OF PRESSURE. 289 older child or adult would, doubtless on account of their imperfect knowledge of its exact situation. As education proceeds the brain gets to know more and more accurately the surface of the body, and the map of the surface in the brain is most accurately known where there is most practice of the sense of touch. The great delicacy of the tongue as a touch organ can be explained by the fact that this organ has to rely upon the sense of touch alone. Usually, in ascertaining the sbApe of an object or the part of the skin it touches, we use our eyes as well. In the case of the interior of the mouth this is impossible. The different degrees of sensitiveness possessed by different parts may give rise to errors of judgment in estimating the distance between two points where the skin is touched. Thus, if the blunted points of a pair of compasses (maintained at a constant distance apart) are slowly drawn over the skin of the cheek towards the lips, it is almost impossible to resist the con- clusion that the distance between the points is gradually increasing. When they reach the lips they seem to be considerably further apart than on the cheek. Thus, too, our estimate of the size of a cavity in a tooth is usually exaggerated when based upon sensa- tion derived from the tongue alone. Another curious illusion may here be mentioned. If we close the eyes, and place a marble between the crossed fore and middle fingers, we seem to be touching two marbles. This illusion is due to an error of judgment. The marble is touched by two surfaces which, under ordinary circumstances, could only be touched by two separate marbles, hence, the mind, taking no cognizance of the fact that the fingers are crossed, forms the conclusion that the two sensations are due to two marbles. The Sense of Pressure. The sense of pressure may be estimated by the ability of the skin to distinguish different weights placed upon it. There must be no lifting of the weight, or the muscular sense is brought into play also. That this is a different sense from that of locality is seen by the fact that its acuteness is differently distributed ; the forearm, for instance, is as sensitive in this direction as the skin of the palm. The tip of the tongue is the most discriminative portion of the body for locality, but it is not so for pressure ; one cannot, for instance feel one's radial pulse with the tongue. The fraction which by Weber's law represents the discriminative sensibility varies from 1 at the finger tip to at the shoulder blade. 290 TOUCH. [CH. XXVII. The Sense of Temperature. Here again the distribution of acuteness is different; the tip of the finger is not nearly so sensitive as the forearm or the cheek, to which a washerwoman generally holds her iron when forming a judgment of its temperature. The fraction which represents the discriminative sensibility is approximately It has been further shown that there are two kinds of nerve- endings for temperature in the skin which are respectively ex- cited by heat and cold. Thus, if a small metallic pencil kept warm by a stream of water inside it, is moved over the surface, there are some points where the sensation is merely tactile, and at others the pencil will feel un- comfortably hot; these spots are called heat spots. Cold spots may be similarly mapped out by the use of a cold pencil. The accompanying figure (fig. 271) indicates a small piece of the skin of the thigh with the heat spots horizontally, and the cold spots vertically shaded. Fig. 271.-Heat and cold spots (after Goldscheider). The Muscular Sense. The muscular sense has been much discussed; some have even denied its existence, and supposed that it is merely a variety of the tactile sense; when the muscles contract they press upon the skin over them and the joints. No doubt the tactile sense of pressure helps us to know what we are doing with our muscles, but there are two sets of facts which show that the muscular sense proper is different from the tactile sense. One of these is that the muscular sense estimated by the lifting of weights, or by the amount of convergence of the axes of the eyes in looking at objects at different distances is much more sensitive than the tactile sense of pressure; the fraction representing the discriminative sensibility being only instead of or i which is the fraction for the pressure sense. The other set of facts are obtained from the study of disease; locomotor ataxy is a selective disease; it may pick out certain sensory tracts and leave others for a time CH. XXVIII.] TASTE. 291 intact; in this way the muscular sense may be destroyed without the tactile sense being much affected. Those who believe in the muscular sense are again divided into two sets ; some believe that the muscular sense is an accom- paniment of the efferent impulse and it is variously spoken of as an estimation of will force, or a sense of expended energy ; others, and among these the majority of physiologists must be included, look upon the sense as due to afferent impulses from the muscles to the brain. The " estimation of will force " doctrine is put out of court by the fact that people know when their muscles are contracting, and whether they are contracting much or little, when there is no expenditure of will force at all, as when the muscles are made to contract artificially under the influence of electrical stimulation. There are now many anatomical facts which point to the correctness of the view that the muscular sense is a true sense. Many years ago it was shown that out of the nerve-fibres which go to the frog's Sartorius some few degen- erate after section of the posterior nerve-roots. This has been more recently demonstrated to be also the case in the muscles of mammals by Sherrington. The discovery of sensory nerve- endings in muscle and tendon points in the same direction. CHAPTER XXVIII. TASTE AND SMELL. These two senses are very closely allied to one another, and it will therefore be convenient to consider them in one chapter. Taste. Certain anatomical facts must be studied first in connection with the tongue, the upper surface of which is concerned in the reception of taste stimuli. The tongue is a muscular organ covered by mucous membrane. The muscles, which form the greater part of the substance of the tongue {intrinsic muscles) are termed linguales; and by these, which are attached to the mucous membrane, its smaller and more delicate movements are performed. 292 TASTE AND SMELL. [ch. xxviii. By other muscles (extrinsic muscles), as the genio-hyoglossus, the styloglossus, &c., the tongue is fixed to surrounding parts : and by these its larger movements are performed. The mucous membrane of the tongue resembles other mucous Fig-. 272.-Fapillar surface of the tongue, with the fauces and tonsils. 1, 1, circumvallate papillfe, in front of 2, the foramen crecum ; 3, fungiform papilla?; 4, filiform and conical papillfe; 5, transverse and oblique rugee ; 6, mucous glands at the base of the tongue and in the fauces; 7, tonsils; 8, part of the epiglottis; 9, median glosso- epiglottidean fold (frfenum epiglottidis). (From Sappey.) membranes in essential points of structure, but contains papillue, peculiar to itself. The tongue is beset with numerous mucous follicles and glands. CH. XXVIH.l LINGUAL PAPILLJE. 293 The larger papillae, of the tongue are thickly set over the anterior two-thirds of its upper surface, or dorsum (fig. 272), and give to it its characteristic roughness. In carnivorous animals, especially those of the cat tribe, the papillae attain a large size, and are developed into sharp re- curved horny spines. Such papillae cannot be regarded as sensitive, but they enable the tongue to play the part of a rasp, as in scraping bones, or of a comb in cleaning fur. The papillae of the tongue present several diversities of form; three principal varieties may be distinguished, namely, the (1) circumvallate, the (2) fungiform, and the (3) conical and filiform papillae. They are all formed by a projection of the corium of the mucous membrane, covered by stratified epithelium, and contain special branches of blood-vessels and nerves. The corium in each kind is studded by minute conical processes or microscopic papillae. (1.) Circumvallate.-These pa- pillae (fig. 274), eight or ten in number, are situate in a V-shaped line at the base of the tongue (1, 1, fig. 272). They are circular eleva- tions, from to jo-th of an inch wide (1 to 2 mm), each with a slight central depression, and sur- rounded by a circular moat, at the outside of which again is a slightly elevated ring or rampart; their walls contain taste-buds. Into the moat that surrounds the central tower, a few little serous glands open. (2.) Fungiform.-The fungiform papillae (3, fig. 272) are scattered chiefly over the sides and tip, and sparingly over the middle of the dorsum, of the tongue ; their name is derived from their being shaped like a puff-ball fungus. (See fig. 275B.) (3.) Conical and Filiform.-These, which are the most abund- ant papillae, are scattered over the whole upper surface of the Fig. 273.-Section of a mucous gland from the tongue. A, opening of the duct on the free surface ; C, basement membrane with nu- clei ; B, flattened epithelial cells lining duct. The duct divides into several branches, which are convoluted and end blindly, being lined throughout by columnar epithelium. D, lumen of one of the tubuli of the gland, x 90. (Klein and Noble Smith.) 294 TASTE AND SMELL. [ch. xxviii. tongue, but especially over the middle of the dorsum. They vary in shape, some being conical (simple or compound) and others filiform ; they are covered by a thick layer of epithelium, which is arranged over them, either in an imbricated manner, or is pro- longed from their surface in the form of fine stiff projections Fig. 274.-Vertical section of a circumvallate papilla of the calf. 1 and 3, epithelial layers covering it; 2, taste buds; 4 and 4', duct of serous gland opening out into the pit in which papilla is situated; 5 and 6, nerves ramifying within the papilla. (Engelmann.) (fig. 276). From their structure, it is likely that these papillae have a mechanical and tactile function, rather than that of taste ; the latter sense being probably seated especially in the other two varieties of papillae, the circumvallate and the fungiform. Fig. 275.-Surface and section of the fungiform papillae. A, the surface of a fungiform papilla, partially denuded of its epithelium; p, secondary papillae ; e, epithelium. B, section of a fungiform papilla with the blood-vessels injected; a, artery; v, vein ; c, capillary loops of similar papillae in the neighbouring structure of the tongue; <7, capillary loops of the secondary papillae ; e, epithelium. (From Kolliker, after Todd and Bowman.) In the circumvallate papillae of the tongue of man peculiar structures known as taste-buds have been discovered. They are of an oval shape, and consist of a number of closely packed, very narrow and fusiform, cells (gustatory cells}. This central CH. XXVIII. J NERVES OF TASTE. 295 core of gustatory cells is enclosed in a single layer of broader fusiform cells (encasing cells'). The gustatory cells terminate in fine stiff spikes which project on the free surface (fig. 277, a). These bodies also occur side by side in considerable numbers in the epithelium of the papilla foliata, which is situated near the root of the tongue in the rabbit, and is composed of a number of closely packed papillae very similar in structure to the circum- vallate papillae of man. Similar taste-buds have been observed scattered over the posterior third of the tongue and the pharynx, as low as the posterior (laryngeal) sur- face of the epiglottis. The gustatory cells in the interior of the taste- buds are surrounded by arborisations of the ter- minations of the glosso- pharyngeal nerve. The middle of the dorsum of the tongue is not endowed to any great degree with the sense of taste ; the tip and mar- gins, and especially the posterior third of the dorsum (be., in the region of the taste-buds), pos- sess this faculty. The anterior part of the tongue is supplied by the lingual branch of the fifth nerve and the chorda tympani, and the posterior third by the glosso-pharyngeal nerve. Considerable discussion has arisen whether there is more than one nerve of taste. The view generally held by physiologists is that the glosso-pharyngeal nerve is the nerve of taste, and the lingual the nerve of tactile sensation. Nevertheless, the lingual and the chorda tympani do contain taste-fibres, which may be, Fig. 276.-Two filiform papillee, one with epithelium, the other without. V •-P, the substance of the papillae dividing at their upper extremities into secondary papillae; a, artery, and v, vein, dividing into capillary loops ; e, epithelial covering, lami- nated between the papillee, but extended into hair-like processes, /, from the extremities of the secondary papillae. (From Kdlliker, after Todd and Bowman.) 296 TASTE AND SMELL. [ch. xxviii. however, ultimately derived from the glosso-pharyngeal by its communication with the fifth and chorda tympani in the tym- panic plexus. Dr. Gowers, on the other hand, holds that the Fig. 277.-Taste-goblet from dog's epiglottis (laryngeal surface near the base), precisely similar in structure to those found in the tongue, a, depression in epithelium over goblet; below the letter are seen the fine hair-like processes in which the cells termi- nate ; c, two nuclei of the axial (gustatory) cells. The more superficial nuclei belong to the superficial (encasing) cells ; the converging lines indicate the fusiform shape of the encasing cells, x 400. (Schofield.) true nerve of taste is the fifth, and that the taste-fibres in the glosso-pharyngeal come ultimately from the fifth. Tastes may be classified into- i. Sweet. 2. Bitter. 3. Acid. 4. Saline. Sweet is neutralised by acid as well as by bitter tastes. Acids and salines apparently affect nerves of tactile sense as well as those of taste proper. Sweet tastes are best appreciated by the tip, acid by the side, and bitter tastes by the back of the tongue. Flavours are really odours. The substance to be tasted must be dissolved ; here there is a striking contrast to the sense of smell. In testing the sense of taste in a patient, the tongue should be protruded, and drops of the substance to be tasted applied with a camel's hair brush to the different parts; the subject of the experiment must signify by signs his sensations, for if he withdraws the tongue to speak, the material gets widely spread. The more concentrated the solution, and the larger the surface acted on, the more intense is the taste; some tastes are perceived more rapidly than others, saline tastes the most rapidly of all. The best temperature of the substance to be tasted is from io° to 350 C. Very high or very low temperatures deaden the sense. It is possible by chewing the leaves of an Indian plant (Gym- CH. XXVIII.] SMELL. 297 nema sylvestre) to do away with the power of tasting bitters and sweets, while the taste for acids and salts remains. The delicacy of the sense of taste is sufficient to discern i part of sulphuric acid in 1,000 of water; the sense may be improved by practice, as in professional tea-tasters. Here again we will take anatomical considerations before studying the physiology of the sense of smell. The nasal cavities are divided into three districts called re- spectively, (a) regio vestibu- laris, which is the entrance to the cavity. It is lined with a mucous membrane closely resembling the skin, and contains hairs {vibrissae) with sebaceous glands; {b)regio respiratoria, which includes the lower meatus of the nose, and all the rest of the nasal passages except (c); its mucous membrane is covered by cilia- ted epithelium. The corium is thick and consists of fibrous connective tissue, it contains a certain number of tubular mucous and serous glands ; (c) regio olfactoria, includes the anterior two-thirds of the superior meatus, the middle meatus, and the upper half of the septum nasi. It is considerably larger in animals like the dog, with a keener sense of smell than we possess. It consists of a thicker mucous membrane than in (6), made up of loose areolar connective tissue covered by epithelium of a special variety, resting upon a basement membrane. The cells of the epithelium are of several kinds, first, columnar cells not ciliated (fig. 278), with the broad end at the surface, and below tapering into an irregular branched process or processes, the terminations of which pass into the next layer. The second kind consists of a small cell body with large spherical nucleus, situated between the ends of the first kind of cell, and sending upwards a process to the surface between the cells of the first kind, and from the other pole of the nucleus a process towards the corium. The latter process is very delicate and may be varicose. The upper process Smell. Fig. 278.-Cells from the olfactor5r region of the rabbit, st, supporting cells; r, r', olfactorial cells; f, ciliated cell; s, cilia-like process ; 6, cells from Bow- man's gland. (Stohr.) 298 TASTE AND SMELL. [ch. xxviii. is prolonged beyond the surface, where it becomes stiff, and in some animals, like the frog, provided with hairs. These cells, which are called olfactorial cells, are numerous, and the nuclei of Fig. 279.-Nerves of the septum nasi, seen from the right side, j.-I, the olfactory bulb ; 1, the olfactory nerves passing through the foramina of the cribriform plate, and de- scending to be distributed on the septum; 2, the internal or septal twig of the nasal branch of the ophthalmic nerve; 3, naso-palatine nerves. (From Sappey, after Hirsch- feld and Leveille.) the cells not being on the same level, a comparatively thick nuclear layer is the result. In the corium are a number of serous glands called Bowman's glands. They open upon the surface by fine ducts passing- up between the epithelium cells. The distribution of the olfactory nerves which penetrate the cribri- form plate of the ethmoid bone and pass to this region of the nasal mu- cous membrane is shown in fig. 279. The nerve-fibres come into contact with the cells we have termed olfac- torial; the columnar cells between these act as supports to them. The olfactory tract is an outgrowth of the brain which was originally hol- low, and remains so in many animals ; in man the cavity is obliterated, and the centre is occupied by neuroglia: outside this the white fibres lie, and a thin superficial layer of neuroglia covers these. Fig. 280.-Semi-diagrammatic sec- tion through the olfactory mu- cous membrane of the new-born child, a, non-nuclear; and ft, nucleated portions of the epi- thelium ; c, nerves ; dd, Bow- man's glands. (M. Schultze.) CH. xxviii.J OLFACTORY BULB. 299 The olfactory bulb has a more complicated structure; above there is first a continuation of the tract (white fibres enclosing neuroglia); below this four layers are distinguishable; they are shown in the accompanying diagram from Ramon y Cajal's work, the histological method used being Golgi's. (1) A layer of white fibres containing numerous small cells, or " granules " (d). (2) A layer of large nerve-cells called " mitral cells " (c), with smaller cells (a) mixed with them. The axis-cylinder processes of these cells pass up into the layer above and eventually become Fig. 281.-Nervous mechanism of the olfactory apparatus, a, bipolar cells of the olfactory apparatus (Max Schultze's olfactorial cells) ; b, olfactory glomeruli; c, mitral cells ; n, granule of white layer ; e, external root of the olfactory tract ; f, grey matter of the sphenoidal region of the cortex ; a, small cell of the mitral layer; b, basket of a glomerulus ; c, spiny basket of a granule ; e, collateral of the axis-cylinder process of a mitral cell; /, collaterals terminating in the molecular layer of the frontal and sphenoidal convolutions ; <7, superficial triangular cells of the cortex ; h, supporting epithelium cells of the olfactory mucous membrane. (Ramon y Cajal.) fibres of the olfactory tract e, which passes to the grey matter of the base of the brain f. They give off numerous collaterals on the way (e,/). (3) The layer of olfactory glomeruli (b). Each glomerulus is a basket-work of fibrils derived on the one hand from the termi- nal arborisations of the mitral cells, and on the other from similar arborisations of the non-medullated fibres which form the next layer. (4) The layer of olfactory nerve-fibres.-These are non- medullated ; they continue upwards the bipolar olfactory cells, or as we have already termed them, the olfactorial cells of the mucous membrane. An exceptional point in connection with the olfactory nervous 300 HEARING. [ch. XXIX. apparatus is that there is no decussation of the olfactory tracts. In testing a patient's sense of smell, substances like musk or assafoetida should be employed ; pungent substances like ammonia affect the nerves of tactile sense (5th nerve) more than the olfactory nerves. The sense of smell is excited either by solid or gaseous particles; these affect the terminations of the olfactorial or bipolar cells and the path to the brain of the nervous impulse so set up we have already indicated. Liquids, unless they are volatile (that is, give off vapours), do not, as a rule, excite the sense ; thus Weber could not smell the slightest odour when his nostrils were com- pletely filled with water containing eau-de-Cologne. It is matter of common experience that odours and flavours (which are really odours) cannot be perceived readily when the amount of moisture in the nose is increased, as when one has a bad cold. On the other hand, the mucous membrane must not be too dry; this also impairs the delicacy of the sense. The delicacy of the sense is most remarkable ; thus, Valentin calculates that Too 0 0 0 0 0 0 a grain of musk can be distinctly smelt; and even this can be improved by practice, as in certain tribes of Indians. W e cannot at present give a scientific classification of odours ; the only possible classification into pleasant and unpleasant is a matter of individual education and taste to a great extent. CHAPTER XXIX. HEARING. Anatomy of the Ear. The Organ of Hearing is divided into three parts, (i) the external, (2) the middle, and (3) the internal ear. The two first are only accessory to the third or internal ear, which contains the essential parts of the organ of hearing. The accompanying figure shows the relation of these divisions, one to the others (fig. 282). External Ear.-The external ear consists of the pinna and the external auditory meatus. CH. XXIX.] THE ORGAN OF HEARING. 301 The principal parts of the pinna are two prominent rims enclosed one within the other {helix and antihelix), and en- closing a central hollow named the concha; in front of the concha, a prominence directed backwards, the tragus, and opposite to this one directed forwards, the antitragus. From the concha, Fig. 282.-Diagrammatic view from before of the parts composing the organ of hearing of the left side. The temporal bone of the left side, with the accompanying soft parts, has been detached from the head, and a section has been earned through it trans- versely, so as to remove the front of the meatus externus, half the tympanic mem- brane, the upper and anterior wall of the tympanum and Eustachian tube. The meatus intemus has also been opened, and the bony labyrinth exposed by the removal of the surrounding parts of the petrous bone. 1, the pinna and lobe; 2, 2', meatus externus ; 2', membrana tympani; 3, cavity of the tympanum ; 3', its opening back- wards into the mastoid cells ; between 3 and 3', the chain of small bones ; 4, Eusta- chian tube ; 5, meatus internus, containing the facial (uppermost) and the auditory nerves ; 6, placed on the vestibule of the labyrinth above the fenestra ovalis ; a, apex of the petrous bone; &, internal carotid artery ; c, styloid process ; d, facial 'nerve issuing from the stylo-mastoid foramen ; e, mastoid process ; /, squamous part of the bone covered by integument, &c. (Arnold.) the auditory canal, with a slight arch directed upwards, passes inwards and a little forwards to the membrana tympani, to which it thus serves to convey the vibrating air. Its outer part consists of fibro-cartilage continued from the concha; its inner part of bone. Both are lined by skin continuous with that of the pinna, and extending over the outer part of the membrana tympani. Towards the outer part of the canal are fine hairs and sebaceous glands, while deeper in the canal are small glands, resembling the sweat-glands in structure, which secrete the cerumen. 302 HEARING. [ch. xxix. Middle Ear or Tympanum.-The middle ear, or tympanum or drum (3, fig. 282), is separated by the membrana tympani from the external auditory meatus. It is a cavity in the temporal bone, opening through its anterior and inner wall into the Eustachian tube, a cylindriform flattened canal, dilated at both ends, com- posed partly of bone and partly of elastic cartilage, and lined with mucous membrane, which forms a communication between the tympanum and the pharynx. It opens into the cavity of the pharynx just behind the posterior aperture of the nostrils. The cavity of the tympanum communicates posteriorly with air- cavities, the mastoid cells in the mastoid process of the temporal bone ; but its only opening to the external air is through the / Fi g. 2 83.-The hammer- bone or malleus, seen from the front. 1, the head; 2, neck; 3, short process; 4, handle. (Schwalbe.) Fig. 285.-The stapes, or stirrup-bone. 1, base; 2 and 3, arch; 4, head of bone, which articu- lates with orbicular process of the incus; 5, constricted part of neck; 6, one of the crura. (Schwalbe.) Fig. 284.-The incus, or anvil-bone. 1, body ; 2, ridged articulation for the malleus; 4, processus brevis, with 5, rough articular surface for ligament of incus; 6, processus magnus, with articu- lating surface for stapes ; 7, nu- trient foramen. (Schwalbe.) Eustachian tube (4, fig. 282). The walls of the tympanum are osseous, except where apertures in them are closed with mem- brane, as at the fenestra rotunda, and fenestra ovalis, and at the outer part where the bone is replaced by the membrana tympani. The cavity of the tympanum is lined with mucous membrane, the epithelium of which is ciliated and continuous with that of the pharynx. It contains a chain of small bones which extends from the membrana tympani to the fenestra ovalis. The membrana tympani is placed in a slanting direction at the bottom of the external auditory canal, its plane being at an angle of about 450 with the lower wall of the canal. It is formed of tough and tense fibres, some running radially, some circu- larly ; its edges are set in a bony groove ; its outer surface is covered with a continuation of the cutaneous lining of the CH. XXIX.] THE EAR OSSICLES. 303 auditory canal, its inner surface with the mucous membrane of the tympanum. The ossicles are three in number; named malleus, incus, and stapes. The malleus, or hammer-bone, is attached by a long slightly-curved process, called its handle, which is inserted between the layers of the membrana tympani; the line of attachment being vertical, including the whole length of the handle, and extending from the upper border to the centre of the membrane. The head of the malleus is irregularly rounded ; its neck, or the line of boundary between it and the handle, supports two pro- cesses : a short conical one, which receives the insertion of the tensor tympani, and a slender one, processus gracilis, which extends forwards, and is attached to the Avail of the cavity at the Glaserian fissure. The incus, or anvil-bone, shaped like a bicuspid mo- lar tooth, is articulated by its broader part, corres- ponding with the surface of the crown of the tooth, to the malleus. Of its two fang-like processes, one, directed backwards, has a free end lodged in a de- pression in the mastoid bone; the other, curved downwards, longer and more pointed, articulates by means of a roundish tubercle, formerly called os orbiculare, with the stapes, a little bone shaped exactly like a stirrup, of which the base or bar fits into the membrane of the fenestra ovalis. To the neck of the stapes, a short process, corresponding with the loop of the stirrup, is attached the stapedius muscle. The bones of the ear are covered with mucous membrane reflected over them from the wall of the tympanum; and are moveable both altogether and slightly one upon the other. The malleus moves and vibrates with every movement and vibration of the membrana tympani, and its movements are communicated through the incus to the stapes, and through it to the membrane closing the fenestra ovalis. The Internal Ear.-The proper organ of hearing is formed by the distribution of the auditory nerve within the internal Fig. 286.-Interior view of the tympanum, with membrana tympani and bones in natural position. 1, Membrana tympani; 2, Eusta- chian tube ; 3, tensor tympani muscle ; 4, lig. mallei super.; 6, chorda-tympani nerve; a, b, and c, sinuses about ossicula. (Schwalbe.) 304 HEARING. [ch. xxix. ear, or labyrinth, a set of cavities within the petrous portion of the temporal bone. The bone which forms the walls of these cavities is denser than that around it, and forms the osseous labyrinth; the membrane within the cavities forms the mem- branous labyrinth. The membranous labyrinth contains a fluid called endolymph; while outside it, between it and the osseous Fig. 287.-Right bony labyrinth, viewed from the outer side. The specimen here represented is prepared by sepa- rating piecemeal the looser substance of the petrous bone from the dense walls which immediately enclose the labyrinth. 1, the vestibule; 2, fen- estra ovalis; 3, superior semicircular canal; 4,horizontal or external canal; 5, posterior canal; *, ampullae of the semicircular canals ; 6, first turn of the cochlea ; 7, second turn; 8, apex ; 9, fenestra rotunda. The smaller figure in outline below shows the 2i natural size. - (Summering.) Fig. 288.-View of the interior of the left labyrinth. The bony wall of the laby- rinth is removed superiorly and exter- nally. 1, fovea hemielliptica; 2, fovea hemispherica ; 3, common opening of the superior and posterior semicircular canals ; 4, opening of the aqueduct of the vestibule ; 5, the superior ; 6, the posterior, and 7, the external semicir- cular canals; 8, spiral tube of the cochlea (seala tympani) ; 9, opening of the aqueduct of the cochlea ; 10, placed on the lamina spiralis in the scala ves- tibuli. ?i (Simmering.) 1 labyrinth, is a fluid called perilymph. This fluid is not pure lymph ; as it contains mucin. The osseous labyrinth consists of three principal parts, namely the vestibule, the cochlea, and the semicircular canals. The vestibule is the middle cavity of the labyrinth, and the central organ of the whole auditory apparatus. It presents, in its inner wall, several openings for the entrance of the divisions of the auditory nerve; in its outer wall, the fenestra ovalis (2, fig. 287), an opening filled by membrane in which is inserted the base of the stapes ; in its posterior and superior walls, five openings by which the semicircular canals com- municate with it : in its anterior wall, an opening leading into CH. XXIX. J THE MEMBRANOUS LABYRINTH. 305 the cochlea. The structure of the semicircular canals is described in Chapter XXV. The cochlea (6, 7, 8, figs. 287 and 288), a small organ, shaped like a snail-shell, is situated in front of the vestibule, its base resting on the bottom of the internal meatus, where some apertures transmit to it the cochlear filaments of the auditory nerve. In its axis, the cochlea is traversed by a conical column, the modiolus, around which a spiral canal winds with two turns and a half from the base to the apex. At the apex of the cochlea the canal is closed ; at the base it presents three openings, of which one, already mentioned, communicates with the vestibule; another, called fenestra rotunda, is separated by a membrane from the cavity of the tympanum ; the third is the orifice of the aquce- ductus cochleae., a canal leading to the jugular fossa of the petrous bone. The spiral canal is divided into two passages, or seal® (staircases), by a partition formed partly of bone, the lamina spiralis, connected with the modiolus, and partly of a membrane called the basilar membrane. The Membranous Labyrinth.-The membranous labyrinth Fig. 289.-Diagram of the right membranous labyrinth. TJ, utricle, into which the three semicircular canals open ; S, saccule, communicating with the cochlea (C) by C.R., the canalis reunions, and with the utricle by a canal having on it an enlargement, the saccus endolymphaticus (S.E.). The dark shading represents the places of termination of the auditory nerve, namely, in the maculse of the utricle and saccule ; the cristse in the ampullary ends of the three semicircular canals ; and in the whole length of the canal of the cochlea. corresponds generally with the form of the osseous labyrinth, so far as regards the vestibule and semicircular canals, but is separated from the walls of these parts by perilymph, except where the nerves enter into connection within it. The labyrinth is a closed membrane containing endolymph, which is of much the same composition as perilymph, but contains less solid matter. It is somewhat viscid, as is the perilymph, and it is secreted by the epithelium lining its cavity ; all the sonorous vibrations impressing the auditory nerves 306 HEARING. [ch. xxix. in these parts of the internal ear, are conducted through fluid to a membrane suspended in and containing fluid. In the cochlea, the membranous labyrinth completes the septum between the two scalce, and encloses a spiral canal, previously mentioned, called the canalis cochleae (fig. 290). The fluid in the scalce of the cochlea is continuous with the perilymph in the vestibule and semicircular canals. The vestibular portion of the membranous labyrinth comprises two communicating cavities, of which the larger and upper is named the utricle; the lower, the saccule. They are lodged in depressions in the bony labyrinth, termed respectively fovea hemielliptica and fovea hemispherica. Into the former open the orifices of the membranous semicircular canals ; into the latter by the canalis reuniens the canal of the cochlea. The accom- panying diagram (fig. 289) gives the relationship of all these parts to one another. Auditory Nerve.-All the organs now described are provided for the appropriate exposure of the filaments of the auditory nerve to vibrations. It enters the bony canal (the meatus auditorius internus), with the facial nerve and the nervus inter- medins, and, traversing the bone, enters the labyrinth at the angle between the base of the cochlea and the vestibule, in two divisions ; one for the vestibule and semicircular canals, and the other for the cochlea. There are two branches for the vestibule, one, superior, dis- tributed to the utricle and to the superior and horizontal semi- circular canals, and the other, inferior, which arises from the cochlear nerve, ends in the saccule and posterior semicircular canal. Where the nerve comes in connection with the utricle and saccule, the structure of the membrane is modified and the places are called maculae acusticce. At the ampullae of the semicir- cular canals, too, the structure is altered, becoming elevated into a ridge, which projects into the interior of the cavity, forming the crista acustica. The distribution of the rest of the cochlear nerve occurs along the whole length of the canal of the cochlea. The structure of the membranous canals has been given in Chapter XXV., so we can pass at once to the cochlea. This is best seen in vertical section ; the cavity is divided into two scales, partly by bone (the spiral lamina), partly by mem- brane (the basilar membrane) ; the other end of the basilar membrane is attached to the bone by a ligament (the spiral ligament), formerly supposed to be a muscle (Bowman's muscle); the two spiral staircases or scalee are named scala vestibuli and scala tympani (fig. 291). At the apex of the cochlea, the spiral lamina ends in a small hamulus, the inner and concave part of CH. XXIX.] THE COCHLEA. 307 which, being detached from the summit of the modiolus, leaves a small aperture named the helicotrema, by which the two scalse, separated in all the rest of their length, communicate. Besides the scala vestibuli and scala tympani, there is a third space between them, called scala media or canal of the cochlea (CC, fig. 291). In section it is triangular, its external wall being- formed by the wall of the co- chlea, its upper wall (separating it from the scala vestibuli) by the membrane of Reissner, and its lower wall (separating it from the scala tympani) by the basilar membrane, these two meeting at the outer edge of the bony lamina spiralis. Following the turns of the cochlea to its apex, the scala media there terminates blindly ; while towards the base of the cochlea it is also closed with the exception of a very narrow passage (canalis reunions) uniting it with the saccule. The scala media (like the rest of the mem- branous labyrinth) contains endolymph. Organ of Corti.- U pon the basilar mem- brane are arranged cells of various shapes. About midway be- tween the outer edge of the lamina spiralis and the outer wall of the cochlea are situa- ted the rods of Corti. Viewed sideways, they are seen to consist of an external and in- ternal pillar, each ris- ing from an expanded foot or base attached to the basilar membrane (0, n, fig. 292). They slant inwards towards each other, and each ends in a swelling termed the head ; the head of the inner pillar overlying that of the outer (fig. 292). Fig. 290.-View of the osseous cochlea divided through the middle, r, cen- tral canal of the modiolus; 2, lamina spiralis ossea; 3, scala tympani; 4, scala vestibuli; 5, porous substance of the modiolus near one of the sec- tions of the canalis spiralis modioli, f. (Arnold.) Fig. 291.-Section through one of the coils of the cochlea (diagrammatic). ST, scala tympani ; SV, scala ves- tibuli ; CO, canalis cochleae or canalis membranaceus ; R, membrane of Reissner ; Iso, lamina spiralis ossea ; Ils, limbus laminae spiralis; ss, sulcus spiralis ; nc, cochlear nerve ; gs, ganglion spirale ; t, membrana tectoria ; (below the membrana tectoria is the lamina reticularis) ; b, membranabasilaris ; Co,rods of Corti; Isp, ligamentum spirale. (Quain.) 308 HEARING. [ch. xxix. Each pair of pillars forms, as it were, a pointed roof arching over a space, and by a succession of them, a tunnel is formed. There are about 3,000 of these pairs of pillars, in proceeding, from the base of the cochlea towards its apex. They are found progressively to increase in length, and become more oblique; in other words the tunnel becomes wider, but diminishes in height as we approach the apex of the cochlea. Leaning against these- external and internal pillars are certain other cells, called hair- cells, which terminate in small hair-like processes. There are several rows of these on the outer and one row on the inner side. Between them are certain supporting cells called cells of Deiters. Fig-. 292.-Vertical section of the organ of Corti from the dog. 1 to 2, homogeneous layer of the membrana basilaris ; u, vestibular layer; v, tympanal layer, with nuclei and protoplasm ; a, prolongation of tympanal periosteum of lamina spiralis ossea ; c, thickened commencement of the membrana basilaris near the point of per- foration of the nerves h ; d, blood-vessel (vas spirale); e, blood-vessel; /, nerves ; g, the epithelium of the sulcus spiralis internus ; i, internal hair-cell, with basil process k, surrounded with nuclei and protoplasm (of the granular layer), into which the nerve-fibres radiate; I, hairs of the internal hair-cell; n, base or foot of inner pillar of organ of Corti; m, head of the same uniting with the corresponding part of an external pillar, whose under half is missing, while the next pillar beyond, ot presents both middle portion and base ; r s d, three external hair-cells ; t, bases of two neighbouring hair or tufted cells; x, supporting cell of Deiters; w, nerve-fibre arborising round the first of the external hair-cells ; 11 to I, lamina reticularis. X 800. (Waldeyer.) Most of the above details are shown in the accompanying figure (fig. 292). This structure rests upon the basilar membrane; it is roofed in by a fenestrated membrane or lamina reticularis into- the fenestrse of which the tops of the various rods and cells are received. When viewed from above, the organ of Corti shows a remarkable semblance to the key-board of a piano. The top of the organ is roofed by the membrana tectoria (fig. 291, t) that extends from the end of the limbus (Us, fig. 291), a connective- tissue structure on the spiral lamina. In close relation with the- hair-cells which form the auditory nerve epithelium, are filaments. CH. XXIX.] PHYSIOLOGY OF HEARING. 309 of the auditory nerve. These are derived from the cochlear division already mentioned. This passes up the axis of the cochlea, and in its course gives off fibres to the lamina spiralis. These fibres are thick at their origin, but thin out peripherally, and containing bipolar ganglion cells form the ganglion spirals. Beyond the ganglion at the edge of the lamina the fibres pass up and become connected with the organ of Corti, arborising around the hair-cells. Physiology of Hearing. Sounds are caused by vibrations ; when a bell or a piano-string is struck, it is thrown into a series of rapid regular vibrations; the more rapidly the vibrations occur the higher is the pitch of the musical note, that is, it is shriller. The vibrations are trans- mitted as waves through the air, and ultimately affect the hair-cells at the extremities of the auditory nerve in the cochlea. The semicircular canals are not concerned in the sense of hearing; their function in connection with equilibration is described in Chapter XXV. The external and middle ears are conducting ; the internal ear is conducting and receptive. In the external ear the vibrations travel through air; in the middle ear through solid structures-membranes and bones : and in the internal ear through fluid, first through the perilymph on the far side of the fenestra ovalis; and then the vibrations pass through the basilar membrane, and membrane of Reissner, and set the endolymph of the canal of the cochlea in motion. This is the normal way in which the vibrations pass, but the endolymph may be affected in other ways, for instance through the other bones of the head ; one can, for example, hear the ticking of one's watch when it is placed between the teeth. From this fact is derived a valuable practical method of distinguishing in a deaf person, what part of the organ of hearing is at fault. The patient may not be able to hear a watch or a tuning-fork when it is held close to the ear; but if he can hear it when it is placed between his teeth or on his forehead, the malady is localised in either the external or middle ear; if he can hear it in neither situation it is a much more serious case, for then the internal ear or the nervous mechanism of hearing is at fault. In connection with the external ear there is not much more to be said; the pinna in many animals is large and acts as a kind of natural ear-trumpet to collect the vibrations of the air ; in man this function is to a very great extent lost, and though there are muscles present to move it into appropriate postures, they are not 310 HEARING. [ch. XXIX. under the control of the will in the majority of people, and are functionless, ancestral vestiges. In the middle ear, however, there are several points to be considered, namely, the action of the membrana tympani, of the ossicles of the tympanic muscles, and of the Eustachian tube. The Membrana Tympani.-This membrane, unlike that of ordinary drums, can take up and vibrate in response to, not only its own fundamental tone, but to an immense range of tones differing from each other by as much as seven octaves. This would clearly be impossible if it were an evenly stretched membrane. It is not evenly nor very tightly stretched, but owing to its attachment to the chain of ossicles it is slightly funnel- shaped ; the ossicles also damp the continuance of the vibrations. When the membrane gets too tightly stretched, by increase or decrease of the pressure of the air in the tympanum, then the sense of hearing is dulled. The pressure in the tympanic cavity is kept the same as that of the atmosphere by the Eustachian tube, which leads from the cavity to the pharynx and so to the external air. The Eustachian tube is not, however, always open; it is opened by the action of the tensor palati during swallowing. Suppose it were closed owing to swelling of its mucous membrane- this often happens in inflammation of the throat-the result would be what is called Eustachian or throat deafness, and this is relieved by passing a catheter so as to open the tube. When the tube is closed, the blood in the vessels of the tympanic wall takes up oxygen from the imprisoned air, and gives off carbonic acid in exchange; but the amount of carbonic acid given out is less than the amount of oxygen removed, so that the total quantity of gases within the tympanum is reduced, and its pressure consequently becomes less than that of the atmosphere, so the membrane is cupped inwards; it is this increased tightening of the membrane that produces deafness. There is also an accumulation of mucus. When one makes a violent expiration, as in sneezing, some air is often forced through the Eustachian tube into the tympanum. The ears feel as though they were bulged out, as indeed the membrana tympani is, and there is again partial deafness, which sensations are at once relieved by swallowing so as to open the Eustachian tube and so re-establish equality of pressure once more. The ossicles communicate the vibrations of the membrana tympani (to which the handle of the malleus is fixed) to the membrane which closes the fenestra ovalis (to which the foot of the stapes is attached). Thus the vibrations are communicated to the fluid of the internal ear which is situated on the other side of the oval window. CH. xxix.J ACTION OF THE EAR OSSICLES. 311 The following diagram will assist us in understanding how this is brought about. The bones all vibrate as if they were one, the slight movements between the individual bones being inappreci- able. The utility of there being several bones is seen when the vibrations are excessive; the small amount of "give" at the articulations is really protective and tends to prevent fractures. The handle of the malleus is inserted between the layers of the tympanic membrane; the processus gracilis (79 g) has its end A attached to the tympanic wall on the inner aspect of the Glaserian fissure ; the end (B) of the short process (s 79) of the incus is fastened by a ligament to the opposite wall of the tympanic cavity; the end D of the long pro- cess of the incus articulates with the stirrup, the base of which is turned towards the reader. The handle vibrates with the mem- brana tympani; and the vibrations of the whole chain take place round the axis of rotation AB. Every time C comes forwards D comes forwards, but by drawing per- pendiculars from 0 and D to the axis of rotation, it is found that D is about of the distance from the axis that C is. So in the transmission of the vibrations from membrane to membrane across the bony chain, the amplitude of the vibration is decreased by about J, and the force is correspondingly increased. The final movement of the stapes is, however, always very small; it varies from to less than 10q00 of a millimetre. The action of the tensor tympani by pulling in the handle of the malleus increases the tension of the membrana tympani. It is supplied by the fifth nerve. It is doubtful if the laxator tympani is a true muscle. The stapedius attached to the neck of the stapes tilts it backwards. It is supplied by the seventh nerve. We have still to consider the use of the fenestra rotunda ; this is closed by a membrane, and its action is to act as a vent for the vibrations of the perilymph. The next very simple diagram wil explain how this happens. The cochlea is supposed to be uncoiled ; the scala vestibuli leads from the fenestra ovalis, to the other side of which the Foot of Stapes Fig. 293.-Diagrammatic view of ear ossicles. 312 HEARING. [ch. xxix. stapes is attached; the scala tympani leads to the fenestra rotunda; the two scalse communicate at the helicotrema, and are separated from the canal of the cochlea by the basilar membrane, and the membrane of Reissner. CR is the canalis reunions leading to the saccule. The two scalte contain perilymph ; the canal of the cochlea contains endolymph which is set in vibration by the perilymph through the membranes. Every time the membrane of the oval window is bulged in by the stirrup, the membrane of the round window is bulged out, and vice versa. If there were no vent in this way the propagation of vibrations through the fluid would be impossible. The theories in connection with the cochlea are two in number : F. Oval is Stapes Scala Vestibuli (Perilymph) C.R. Scala Tympani (Perilymph) Helicotrema F. Rotunda Fig. 294.-Diagram to illustrate the use of the fenestra rotunda. one is Helmholtz' piano theory; the other is the telephone theory of Rutherford and Waller. The Piano Theory.-If one sings a note in front of a piano, the string of the piano that emits that note will take up the vibra- tion and answer; another note will elicit an answer from another string. It was supposed by Helmholtz that there is an analogous arrangement in the cochlea. Different parts of the organ of Corti will respond to different notes as do the strings of a piano. At first he thought it was the rods of Corti which acted in this way, but when it was shown that in birds there are no rods, he referred it to the different fibres of the basilar membrane. This is supported by the fact that this membrane increases in breadth from below upwards ; low notes will set in sympathetic vibration the long fibres of the upper part, and high notes the short fibres of the lower part of the organ. These responsive vibrations extend to the hair-cells resting on these particular portions of the membrane, and give rise to excitations which, conducted along the nerve-fibres to the brain, produce different auditory sensations. This theory therefore localises the analysis of sounds in the cochlea. The niembrana tectoria acts as a damping mechanism. The Telephone Theory.-Just as in a telephone one membrane CH. XXIX.] THE RANGE OF HEARING. 313 vibrates in response to a sound but at different rates for different sounds, so in this theory it is supposed that the basilar membrane vibrates as a whole, the hair-cells on it are affected, the nerve impulse travels to the brain, and the analysis of the sound occurs there. In other words, the basilar membrane acts very much like the membranatympani. "It is the internal drum-head, repeating the complex vibrations of the membrana tympani, and vibrating in its entire area to all sounds-although more in some parts than in others-giving what we may designate as acoustic pressure patterns between the membrana tectoria and the subjacent field of hair-cells. In place of an analysis by sympathetic vibration of particular radial fibres, it may be imagined that varying com- binations of sound give varying pressure patterns, comparable to the varying retinal images of external objects." (Waller.) The Range of hearing is more extensive than that of voice. Sounds can be heard that are produced by 30 vibrations per second, up to those caused by 30,000 to 40,000 vibrations per second; and in this range as many as 6,000 variations of pitch can be perceived, or about twice as many as the pairs of arches of Corti. Two sounds can be recognised as distinct if the interval between them is less than 0'002 second (Exner), a fact that shows us the perfection of the damping as well as of the vibrating mechanism. The distinction between musical notes is not equally obvious to all observers. People differ a good deal in the musical element in their nature. But in all there is a limit to the perception of high-pitched notes. In Gallon's whistle, one has an instrument by which the rate of vibration of the air which produces the sound can be increased ; it gets shriller and shriller, and at last when the vibration frequency exceeds 30,000 or 40,000, the sound becomes inaudible. Probably many animals, however, are able to hear much higher notes than we can detect. An estimate, by the sense of hearing alone of the direction in which a sound comes, is always most imperfect. 314 VOICE AND SPEECH. [CH. XXX. CHAPTER XXX. VOICE AND SPEECH. The fundamental tones of the voice are produced by the cur- rent of expired air causing the vibration of the vocal cords, two elastic bands contained in a cartilaginous box placed at the top of the wind-pipe or trachea. This box is called the larynx. The sounds produced here are modified by other parts like the tongue, teeth, and lips, as will be explained later on. Anatomy of the Larynx. The cartilag-es of the larynx are the thyroid, the cricoid, the two ary- tenoids. These are the most important for voice production ; they are made of hyaline cartilage. Then there is the epiglottis, two cornicular, and two cuneiform cartilages. These are made of yellow fibro-cartilage. Cornu min. Cornu maj. Cornu sup. ,m. Sterno-hyoideus. tn. Sterno-hyoideus. Dig. crico-thyr. med. Cart, cricoidea Dig. crico-tracheee Cart, traeheale m. Sterno-hyoideus. m. Crico-thyroideus. Fig. 295.-The larynx, as seen from the front, showing the cartilages and ligaments. The muscles, with the exception of one crico-thyroid, are cut off short. (Stoerk.) The thyroid cartilage (fig, 296. 1 to 4) does not form a complete ring around the larynx, but only covers the front portion. It forms the pro- minence in front of the throat known as Adam's apple, (o') The cricoid cartilage (fig. 296, 5, 6), on the other hand, is a complete ring; the back part of the ring being much broader than the front. On the top of this CH. xxx.] THE LARYNX. 315 broad portion of the cricoid are (J) the arytenoid cartilages (fig. 296, 7), the connection between the cricoid below and arytenoid cartilages above being a joint with synovial membrane and ligaments, the latter permitting tolerably free motion between them. But although the arytenoid cartilages can move on the cricoid, they accompany the latter in all its move- ments. Fig. 296.-Cartilages of the larynx seen from the front. 1 to 4, thyroid cartilage; 1, vertical ridge or pomum Adami; 2, right ala; 3, superior, and 4, inferior cornu of the right side; 5, 6, cricoid cartilage; 5, inside of the posterior part; 6, anterior narrow part of the ring ; 7, arytenoid cartilages, x f. The cornicular cartilages, or cartilages of Santorini, are perched on the top of the arytenoids ; the cuneiform cartilages, or cartilages of Wrisberg, are in a fold of mucous membrane ; the epiglottis looks like a lid to the whole (fig. 297). Lig. ary-epiglott. Cart. Wrishergii. Cart. Santorini. Cart, ary ten. Proc, muscul. Lig. cnco-aryten. Lig. cerato-crico. post.sup. Cornu infer. Lig. carat-crico. post. inf. Cart, tracheae. .< Fars membran. Fig. 297.-The larynx as seen from behind after removal of the muscles. The cartilages and ligaments only remain. (Stoerk.) The thyroid cartilage is also connected with the cricoid, by the crico- thyroid membrane, and also by joints with synovial membranes ; the lower cornua of the thyroid clasping the cricoid between them, but not so tightly but that the thyroid can revolve, within a certain range, 316 VOICE AND SPEECH. [ch. xxx. around an axis passing transversely through the two joints at which the cricoid is clasped. The vocal cords are attached (behind) to the front portion of the base of the arytenoid cartilages, and (in front) to the re-entering angle at the back of the thyroid ; it is evident, therefore, that all movements of either of these cartilages must produce an effect on them of some kind or other. Inasmuch, too. as the arytenoid cartilages rept on the top of the back portion of the cricoid cartilage, and are connected with it by capsular and other ligaments, all movements of the cricoid cartilage must move the arytenoid cartilages, and also produce an effect on the vocal cords. Muscles.-The muscles of the larynx are divided into intrinsic and ex- trinsic. The intrinsic are named from their attachments to the various cartilages ; the extrinsic are those which connect the larynx to other parts like the hyoid bone. The attachments and the action of the intrinsic muscles are given in the following table. All the muscles are in pairs except the arytenoideus. Table of the several Groups of the Intrinsic Muscles of the Larynx and their Attachments. Group. Muscle. Attachments. Action. I. Abductoi'S. Crico-aryte- noidei pos- tici. This pair of muscles arises, on either side, from the posterior surface of the corresponding half of the cricoid cartilage. From this depression their fibres converge on either side upwards and outwards to be inserted into the outer angle of the. base of the arytenoid cartilages behind the crico- arytenoid laterales. Draw inwards and backwards the outer angle of arytenoid cartilages, and so rotate out- wards the pro- cessus vocalis and widen the glottis. II. and III. Adductors and Sphincters. Thyro- ary - epiglottici. A pair of muscles. Flat and narrow, which arise on either side from the processus mus- cularis of the arytenoid carti- lage, then passing upwards and inwards cross one another in the middle line to be in- serted into the upper half of the lateral border of the oppo- site arytenoid cartilage and the posterior border of the cartilage of Santorini. The lower fibres run forwards and downwards to be inserted into the thyroid cartilage near the commissure. The fibres at- tached to the cartilage of San- torini are continued forwards and upwards into the ary-epi- glottic fold. Help to narrow or close the rima glottidis. ch. xxx.] MUSCLES OF LARYNX. 317 Group. Muscle. Attachments.' Action. II. and. III. Arytenoide- A single muscle. It is attached Draws together Adductors us. to the borders of the arytenoid the arytenoid and cartilages, its fibres running cartilages and Sphincters horizontally between the two. also depresses -continued. them. When the ■ muscle is paralyzed, the inter -cartilagi- nous part of the glottis cannot come together. Tliyro-aryte- A pair of muscles. Each noidei. may be further divided into two layers, internal and ex- ternal. The external fibres arise side by side from the lower half of the internal sur- face of the thyroid cartilage, close to the angle, and from the fibrous expansion of the crico- thyroid membrane, and are inserted into the lateral border of the arytenoid cartilage. The inner fibres run horizontally, to be attached to the lower half of this border, and the outer fibres pass obliquely out- wards to be inserted into the upper half, whilst some pass to the cartilage of Wrisberg and the ary-epiglottic fold. The internal fibres arise inter- Render the vocal nally to those just described, cords tense and and running parallel to and in rotate the aryte- the substance of the vocal cord noid cartilages are attached posteriorly to the and approxi- processus vocalis along its mate their an- whole length and to the adj a- terior angles or cent part of the outer surface of the arytenoid cartilage. vocal processes. Crico-arytc- A pair of muscles. They arise Approximate the noidei late- on either side from the middle vocal cords by rales. third of the upper border of drawing the the cricoid cartilage and are processus mus- inserted into the whole ante- cularis of the rior margin of the base of the arytenoid earth arytenoid cartilage. Some of lages forwards their fibres join the thyroid- and downwards ary-epiglottici. and so rotate the processus vocalis inwards. 318 VOICE AND SPEECH. [ch. xxx. Group. Muscle. Attachments. Action. IV. Tensors. Crico-thyroi- dei. A pair of fan-shaped muscles attached on either side to the cricoid cartilage below ; from the mesial line in front for nearly one-half of its lateral circumference backwards the fibres pass upwards and out- wards to be attached to the lower border of the thyroid cartilage and to the front border of its lower cornea. The thyroid carti- lage being fixed by its extrinsic muscles, the front of the cri- coid cartilage is drawn upwards, and its back, with the aryte- noids attached, is drawn down. Hence the vocal cords are elon- gated antero- posteriorly and put upon the stretch. Pa- ralysis of these muscles causes an inability to produce high notes. Nerve Supply. - The larynx is supplied by two branches of the vagus; the superior laryngeal is the sensory nerve ; by its external branch, how- ever, it supplies one mus- cle, namely the crico- thyroid. The rest of the muscles are supplied by the inferior laryngeal nerve, the fibres of which, however, come from the spinal accessory, not the vagus proper. Mucous membrane.- The larynx is lined with a mucous membrane con- tinuous with that of the trachea ; this is covered with ciliated epithe- lium except over the vocal cords and epiglot- tis, where it is strati- fied. The vocal cords are thickened bands Lig. ary-epiglott. Cart. Wrisbergii Cart. Santorini mm. Aryten. obliqu. Crico-arytenoid. post. Cornu inferior Lag. cerato-cric. Fars post. inf. membrani Pars eartilag. Fig. 298.-The larynx as seen from behind. To show the intrinsic muscles posteriorly. (Stoerk.) CH. XXX.] THE LARYNGOSCOPE. 319 of elastic tissue in this mucous membrane which run from before back as already described. The chink between them is called the rima glottidis (see fig. 299). Two ridges of mucous membrane above and parallel to these are called the false vocal cords; between the true and false vocal cord on each side is a recess called the ventricle; and it is in the mucous membrane here that the cuneiform cartilages are imbedded. The laryngoscope is an instrument employed in investigating during life the condition of the pharynx, larynx, and trachea. It consists of a large concave mirror with perforated centre, and of a smaller mirror fixed in a Fig. 299.--Vertical section through the larynx, passing from side to side. H, hyoid hone ; T, thyroid cartilage ; T.C.M., thyro-cricoid membrane ; C, cricoid cartilage ; Tr, first ring of trachea ; T.A., thyro-arytenoid muscle ; R.G., rima glottidis; V.C, vocal cord ; V, ventricle ; F.V.C., false vocal cord. long handle. It is thus used : the patient is placed in a chair, a good light (argand burner, or lamp) is arranged on one side of, and a little above his head. The operator fixes the large mirror round his head in such a manner, that he looks through the central aperture with one eye. He then seats himself opposite the patient, and so alters the position of the mirror, which is for this purpose provided with a ball-and-socket joint, that a beam of light is reflected on the lips of the patient. The patient is now directed to throw his head slightly backwards, and to open his mouth ; the reflection from the mirror lights up the cavity of the mouth, and by a little alteration of the distance between the operator and the patient the point at which the greatest amount of light is reflected by the mirror-in other words, its focal length-is readily discovered. The small mirror fixed in the handle is then warmed, either by holding it over the lamp, or by putting it into a vessel of warm water ; this is necessary to prevent the condensation of breath upon its surface. The degree of heat is 320 VOICE AND SPEECH. [CH. XXX. regulated by applying the back of the mirror to the hand or cheek, when it should feel warm without being painful. After these preliminaries the patient is directed to put out his tongue, which is held by the left hand gently but firmly against the lower teeth by means of a handkerchief. The warm mirror is passed to the back of the mouth, until it rests upon and slightly raises the base of the uvula, and at the same time the light is directed upon it : an inverted image of the larynx and trachea will be seen in the mirror. If the dorsum of the tongue is alone seen, the handle of the mirror must be slightly lowered until the larynx comes into view ; care should be taken, however, not to move the Fig. 300.-The parts of the Laryngoscope. mirror upon the uvula, as it excites retching. The observation should not be prolonged, but should rather be repeated at short intervals. The structures seen will vary somewhat according to the condition of the parts as to inspiration, expiration, phonation, &c.; they are (fig. 302) first, and apparently at the posterior part, the base of the tongue, immediately below which is the arcuate outline of the epiglottis, with its cushion or tubercle. Then are seen in the central line the true vocal cords, white and shining in their normal condition. The cords approximate (in the inverted image) posteriorly ; between them is left a chink, narrow whilst a high note is being sung, wide during a deep inspiration. On each side of the true vocal cords, and on a higher level, are the pink false vocal cords. Still more externally than the false vocal cords is the aryteno-epiglottidean fold, in which are situated upon each side three small elevations ; of these the most external is the cartilage of Wrisberg, the intermediate is the cartilage of Santorini, whilst the summit of the arytenoid cartilage is in front, and. CH. XXX.] MOVEMENTS OF THE VOCAL CORDS. 321 somewhat below the preceding, being only seen during deep inspiration. The rings of the trachea, and even the bifurcation of the trachea itself, if the patient be directed to draw a deep breath, may be seen in the interval between the true vocal cords. Fig. 301.-To show the position of the operator and patient when using the Laryngoscope Movements of the Vocal Cords. In Respiration.-The position of the vocal cords in ordinary tranquil breathing is so adapted by the muscles, that the opening of the glottis is wide and triangular (fig. 302, b), becoming a little wider at each inspiration, and a little narrower at each expiration. On making a rapid and deep inspiration the opening of the glottis is widely dilated (fig. 302, c), and somewhat lozenge-shaped. In Vocalisation.-At the moment of the emission of a note, the chink is narrowed, the margins of the arytenoid cartilages being brought into contact and the edges of the vocal cords approximated and made parallel; at the same time their tension is much in- creased. The higher the note produced, the tenser do the cords become (fig. 302, a); and the range of a voice depends, in the main, on the extent to which the degree of tension of the vocal cords can be thus altered. In the production of a high note the vocal cords are brought well within sight, so as to be plainly visible with the help of the laryngoscope. In the utterance of low-pitched tones, on the other hand, the epiglottis is depressed and brought over them, and the arytenoid cartilages look as if they were trying to hide themselves under it (fig. 303). The epiglottis, by being somewhat pressed down so as to cover the superior cavity of the larynx, serves to render the notes deeper in tone and at the same time somewhat duller. The degree of approximation of the vocal cords also usually 322 VOICE AND SPEECH. Fch. xxx. corresponds with the height of the note produced; but the width of the aperture has no essential influence on the height of the note, as long as the vocal cords have the same tension : only Fig. 302.-Three laryngoscopic views of the superior aperture of the larynx and surround- ing parts. A, the glottis during the emission of a high note in singing; B, in easy and quiet inhalation of air; C, in the state of widest possible dilatation, as in inhaling a very deep breath. The diagrams A', B', and C', show in horizontal sections of the glottis the position of the vocal ligaments and arytenoid cartilages in the three several states represented in the other figures. In all the figures so far as marked, the letters indicate the parts as follows, viz.: I, the base of the tongue ; e, the upper free part of the epiglottis ; e', the tubercle or cushion of the epiglottis; ph, part of the anterior wall of the pharynx behind the larynx ; in the margin of the aryteno-epiglottidean fold, iv, the swelling of the membrane caused by the cartilages of Wrisberg ; ,s-, that of the cartilages of Santorini; a, the tip or summit of the arytenoid cartilages ; c v, the true vocal cords or lips of the rima glottidis ; c v s, the superior or false vocal cords ; between them the ventricle of the larynx; in C, tr is placed on the anterior wall of the receding, trachea, and b indicates the commencement of the two bronchi beyond the bifurcation which may be brought into view in this state of extreme dilatation. (Quain, after Czermak.) with a wide aperture the tone is more difficult to produce and is less perfect, the rushing of the air through the aperture being heard at the same time. No true vocal sound is produced at the posterior part of the aperture of the glottis, that, viz., which is formed by the space between the arytenoid cartilages. CH. XXX.] THE VOICE. 323 The Voice. The human musical instrument is often compared to a reed organ-pipe : certainly the notes produced by such pipes in the vox humana stop of organs is very like the human voice. Here there is not only the vibration of a column of air, but also of a reed, which corresponds to the vocal cords in the air-chamber composed of the trachea and the bronchial system beneath it. The pharynx, mouth, and nasal ca- vities above the glottis are re- sonating cavities, which by al- terations in their shape and size, are able to pick out and em- phasize certain component parts of the fundamental tones pro- duced in the larynx. The na- tural voice is often called the chest voice. The falsetto voice is differently explained by different observers ; on laryngoscopic ex- amination, the glottis is found to be widely open, so that there is an absence of chest resonance; some have supposed that the attachment of the thyro-arytenoid muscle to the vocal cord renders it capable of acting like the finger on a violin string, part of the cord being allowed to vibrate while the rest is held still. Such a shortening of a vibrating string would produce a higher pitched note than is natural. Musical sounds differ from one another in three ways :- 1. In pitch. This depends on the rate of vibration ; and in the case of a string, the pitch increases with the tension, and diminishes with the length of the string. The vocal cords of a woman are shorter than those of a man, hence the higher pitched voice of women. The average length of the female cord is ii'5 millimetres; this can be stretched to 14; the male cord averages 15*5 and can be stretched to 19'5 millimetres. 2. In loudness.-This depends on the amplitude of the vibra- tions, and is increased by the force of the expiratory blast which sets the cords in motion. 3. In " timbrel-This is the difference of character which distin- guishes one voice, or one musical instrument, from another. It is due to admixture of the primary vibrations with secondary vibrations or overtones. If one takes a tracing of a tuning-fork on a revolving Fig. 303.-View of the upper part of the larynx as seen by means of the laryn- goscope during the utterance of a bass note, e, epiglottis; «, tubercles of the cartilages of Santorini; a, arytenoid cartilages ; z, base of the tongue ; ph, the posterior wall of the pharynx. (Czermak.) 324 VOICE AND SPEECH. [ch. xxx. cylinder, it writes a simple series of up and down waves corresponding in rate to the note of the fork. Other musical instruments do not lend themselves to this form of graphic record, but their vibra- tions can be rendered visible by allowing them to act on a small sensitive gas-flame ; this bobs up and down, and if the reflection of this flame is allowed to fall on a series of mirrors, the top of the continuous image formed is seen to present waves. The mirrors are usually arranged on the four lateral sides of a cube which is rapidly rotated. If one sings a note on to the mem- brane in the side of the gas-chamber with which the flame is in Fig. 304.-Konig's apparatus for obtaining flame pictures of musical notes. connection, the waves seen are not simple np and down ones, but the primary large waves are complicated by smaller ones on their surface, at twice, thrice, &c., the rate of the primary vibra- tion. The richer a voice, the richer the sound of a musical instrument, the more numerous are these overtones or har- monics. The range of the voice is seldom, except in celebrated singers, more than two-and-a-half octaves, and for different voices this is in different parts of the musical scale. Although the voice is usually produced by the expiratory blast, by practice one can employ the inspiratory blast; this con- stitutes the form of speech known as ventriloquism. The voice does not appear to come out of the speaker's mouth ; and as •CH. XXX.] SPEECH. 325 we never readily distinguish the direction in which the sounds reach our ear, the ventriloquist, by directing the attention of the audience to various parts of the room, is able to make them imagine the voice is proceeding from those parts. Speech. This is due to the modification produced in the fundamental laryngeal notes, by the resonating cavities above the vocal cords. By modifying the size and shape of the pharynx, mouth, and nose, certain overtones or harmonics are picked out and exaggerated ; this gives us the vowel sound; the consonants are produced by interruptions, more or less complete, of the outflowing air in •different situations. The soft palate is raised at each word. When the larynx is passive, and the resonating cavities alone ■come into play, then we get whispering. The pitch of the Vowels has been estimated musically ; u has the lowest pitch, then 0, a (as in father), a (as in cane), i, and e. We may give a few ex- amples of the shape of the resonating cavities in pronouncing vowel sounds, and producing their characteristic timbre : when sounding a (in father) the mouth has the shape of a funnel wide in front ; the tongue lies on the floor of the mouth ; the lips are wide open ; the soft palate is moderately and the larynx slightly raised. In pronouncing u (co), the cavity of the mouth is shaped like a capacious flask with a short narrow neck. The whole resonating cavity is then longest, the lips being protruded as far as possible ; the larynx is depressed and the root of the tongue approaches the fauces. In pronouncing 0, the neck of the flask is shorter and wider, the lips being nearer the teeth ; the larynx is slightly higher than in sound- ing oo. In pronouncing e, the flask is a small one with a long narrow neck. The resonating chamber is then shortest as the larynx is raised as much as possible, and the mouth is bounded by the teeth, the lips being retracted : the approach of the tongue near the hard palate makes the long neck of the flask. The Consonants are produced by a more or less complete closure of cer- tain doors on the course of the outgoing blast. If the closure is complete, and the blast suddenly opens the door, the result is an explosive; if the door is partly closed, and the air rushes with a hiss through it, the result is an aspirate ; if the door is nearly closed and its margins are thrown into vibration, the result is a vibrative; if the mouth is closed, and the sound has to find its way out through the nose, the result is a resonant. These doors are four in number; Briicke called them the articulation positions. They are- 1. Between the lips. 2. Between the tongue and hard palate. 3. Between the tongue and soft palate. 4. Between the vocal cords. The following table classifies the principal consonants according to this plan :- 326 VOICE AND SPEECH. [ch. xxx Articulation position. Explosives. Aspirates. Vibratives. Resonants i B. P. .. . F, V, W. . - M. 2 T. D. .. . S.Z.L, Sch, Th. . R. N. 3 K. G. .. J, Ch. Palatal R. .. Palatal N. 4 - H. .. R of lower Saxon.. - Defects of Speech. Speech is an action confined to man ; experiments on animals are there- fore here inapplicable ; hence our knowledge of the nervous mechanism of speech defects depends on deductions drawn from the clinical and patho- logical study of disease. Speech may be absent in certain forms of lunacy, and temporarily in that defect of will called hysteria. It may be absent owing to congenital defects. Children born deaf are dumb also. This is because we think with remembered sounds, and in a person bom deaf the auditory centres are never set into activity. By edu- cating the child by the visual inlet, it can be taught to think with the remembered shapes of the mouth and expressions of the face produced in the act of speaking, and so can itself speak in time. If a child becomes deaf before it is six or seven years old, there is a liability it will forget the speech it has learnt, and so become dumb. In congenital hemiplegia there may be speechlessness, especially if the injury is due to meningeal haemorrhage affecting the grey cortex of the left hemisphere. These children generally talk late, the right side of the brain taking on the function of the left. Disorders of speech and voice occur from affections of the larynx, and of the nerves which supply the larynx. Stammering is a want of co- ordination between the various muscles employed in the act of speaking. Perhaps the most interesting of the disorders of speech, however, are- those due to brain disease. These fall into three principal categories:- 1. Aphemia.-A difficulty or inability to utter or articulate words. It is often associated with difficulty of swallowing, and occurs in lesions of the base of the brain, especially of pons and bulb. The blurring of speech noticed in most cases of apoplexy may also be included under this head. 2. Aphasia.-This is a complex condition in which the will to speak exists, and also the ability to speak, but the connection between the two is- broken down. When the patient speaks, the words which he utters are well pronounced, but are not those he wishes to utter. This is often asso- ciated with Agraphia, a similar condition in respect to writing. It is the form of disordered speech associated with disorganisation of Broca's con- volution. 3. Amnesia.-This term includes a large class of cases in which the main symptom is loss of memory for words, or a defect of the association of ideas- of things with ideas of words, not as in aphasia with ideas of verbal action. Amnesia is associated with lesions of the intellectual, i.e., the sensory centres of the cortex behind the Rolandic area. We have seen that in this region of the brain there are two important centres, the visual and the auditory, and the parts of these which are associated with words may be called the visual word-centre and the auditory word-centre. They have not, however, been anatomically localised. In amnesia, either these centres- themselves, or the tracts that connect them, are diseased or broken down. With regard to the auditory word-centre, impressions for the sounds of words are revived in one of these ways :- a. Spontaneous or volitional; owing to accumulated traces which con- stitute memory, a man when he wants to express his thoughts in words remembers the sounds it is necessary to use ; impulses pass to the motor- CH. XXXI.] THE EYE AND VISION. 327 centre (Broca's convolution), thence to the nerve-centres, nerves, and muscles of the larynx, mouth, chest, &c., and the man speaks. b. In slight disease of the auditory word-centre, he is unable to do this, but if his mind is set into a certain groove he will speak ; thus if the alphabet or a well-known piece of poetry be started for him he will finish it by himself. c. Mimetic. In more severe cases, a more powerful stimulus still is needed ; he will repeat any words after another person, but forget them immediately afterwards. With regard to the visual word-centre as tested by writing, there are also three ways of reviving impressions for written words or letters. (a.) Spontaneous or normal. (&.) A train of thought must first be set going ; as, for instance, converting printed words into written characters. (c.) Mimetic ; he can only write from a copy. Two operations require the combined activity of both centres ; the first of these is reading aloud, the second is writing from dictation. In reading aloud, the impression of the words enters by the eyes, reaches the visual word-centre, travels across to the auditory word-centre, where the sounds of the words are revived and the person pronounces them. Writing from dictation is just the opposite ; there the impressions of the words enter by the ears, reach the auditory word-centre, travel across to the visual word-centre, where the shapes of the words are revived and the person writes them. In the investigation of any case of this kind there are always the follows ing six things to be inquired into :- 1. Can the patient understand spoken words ? (The patient, of course, not being deaf.) If he cannot, the auditory word-centre is deranged. 2. Can he repeat words when requested ? This tests the emission fibres from the auditory word-centre which pass through the motor-centres for speech in Broca's convolution. If he cannot do this, the patient has aphasia. 3. Can he write from dictation ? If he cannot, either the auditory or visual word-centre, or the fibres passing from the one to the other, are injured. 4. Does he understand printed matter, and can he point out printed letters and words ? Can he read to himself 1 (The patient, of course, not being blind.) This tests the visual word-centre. 5. Can he copy written words ? This tests the channels from the visual word-centre to the motor-centres for movements of the hand in writing. 6. Can he read aloud, or, what is the same thing, name objects he sees ? This is the opposite to writing from dictation, and tests the healthiness of the word-centres or the fibres which connect the visual to the auditory word-centre. CHAPTER XXXI. THE EYE AND VISION. The eyeball is contained in the cavity of the skull called the orbit; here also are vessels and nerves for the supply of the eye- ball, muscles to move it, and a quantity of adipose tissue. In the front of the eyeball are the lids and lacrimal apparatus. 328 THE EYE AND VISION. [ch. xxxi. The eyelids consist of two moveable folds of skin, each of which is kept in shape by a thin plate of fibrous tissue called the tarsus. Along their free edges are inserted a number of curved hairs (eyelashes'), which, when the lids are half closed, serve to protect the eye from dust and other foreign bodies : their tactile sensibility is also very delicate. On the inner surface of the tarsus are disposed a number of small racemose glands (Meibomian), the ducts of which open near the free edge of the lid. The orbital surface of each lid is lined by a delicate, highly sensitive mucous membrane (conjunctiva), which is continuous with the skin at the free edge of each lid, and after lining the inner surface of the eyelid is reflected on to the eyeball, being somewhat loosely adherent to the sclerotic coat. Its epithelium, which is columnar, is continued over the cornea as its anterior epithelium, where it becomes stratified. At the inner edge of the eye the conjunctiva becomes continuous with the mucous lining of the lacrimal sac and duct, which again is continuous with the mucous membrane of the inferior meatus of the nose. The lacrimal gland, composed of several lobules made up of acini resembling the serous salivary glands, is lodged in the upper and outer angle of the orbit. Its secretion, which issues from several ducts on the inner surface of the upper lid, under ordinary circumstances just suffices to keep the conjunctiva moist. It passes out through two small openings (puncta lacrimalia) near the inner angle of the eye, one in each lid, into the lacrimal sac, and thence along the nasal duct into the inferior meatus of the nose. The excessive secretion poured out under the influence of any irritating vapour or painful emotion overflows the lower lid in the form of tears. The eyelids are closed by the contraction of a sphincter muscle (orbicularis), supplied by the facial nerve; the upper lid is raised by the levator palpebroe superioris, which is supplied by the third nerve. The Eyeball. The eyeball or the organ of vision (fig. 305) consists of a variety of structures which may be thus enumerated :- The sclerotic, or outermost coat, envelops about five-sixths of the eyeball: continuous with it, in front, and occupying the remaining sixth, is the cornea. Immediately within the sclerotic is the choroid coat, and within the choroid is the retina. The interior of the eyeball is filled by the aqueous and vitreous humours CH. XXXI.] THE EYEBALL. 329 and the crystalline lens; but, also, there is suspended in the interior a contractile and perforated curtain,-the iris, for regu- lating the admission of light, and behind at the junction of the sclerotic and cornea is the ciliary muscle, the function of which is to adapt the eye for seeing objects at various distances. Structure of the Sclerotic Coat.--The sclerotic coat is com- -Sclerotic coat -Choroid coat -Retina -Vitreous humour Ciliary muscle - Ciliary process- Canal of Petit- Comea- Anterior chamber- Lens- Iris Ciliary process- Ciliary muscle- Fig. 305.-Section of the anterior four-fifths of the eyeball posed of white fibrous tissue, with some elastic fibres near the inner surface, arranged in variously disposed and interlacing Fig. 306.-Vertical section of rabbit's cornea, a, Anterior epithelium, showing the different shapes of the cells at various depths from the free surface ; b, portion of the substance of cornea. (Klein.) layers/ . Many of the bundles of fibres cross the others almost at right angles. It is strong, tough, and opaque, and not very 330 THE EYE AND VISION. [CH. XXXI. elastic. It is separated from the choroid by a lymphatic space- (perichoroidal), and this is in connection with smaller spaces lined with endothelium in the sclerotic coat itself. There is a lym- Fig. 307.-Horizontal preparation of cornea of frog; showing the network of branched; cornea-corpuscles. The ground substance is completely colourless, x 400. (Klein.) phatic space also outside the sclerotic separating it from a loose investment of connective tissue, containing some smooth muscular fibres, called the capsule of Tenon. The innermost layer is made Fig. 308.-Surface view of part of lamella of kitten's cornea, prepared first with caustic- potash and then with nitrate of silver. (By this method the branched cornea-corpuscles with their granular protoplasm and large oval nuclei are brought out.) x 450. (Klein and Noble Smith.) up of loose connective tissue and pigment-cells, and is called tho lamina fusca. Structure of the Cornea.-The cornea is a transparent membrane- which forms a segment of a smaller sphere than the rest of the- eyeball, and is let in, as it were, into the sclerotic, with which it is. CH. xxxi.J THE CORNEA. 331 continuous all round. It is covered by stratified epithelium (a, fig. 306), consisting of seven or eight layers of cells, of which the superficial ones are flattened and scaly, and the deeper ones, more or less columnar. Immediately beneath this is the anterior homogeneous lamina of Bowman, which differs, only in being more condensed tissue, from the general structure of the cornea. This latter tissue, as well as its epithelium, is, in the adult, completely destitute of blood-vessels ; it consists of an intercellular ground-substance of rather obscurely fibrillated flattened bundles of connective tissue, arranged parallel to the free surface, and forming the boundaries of branched anastomos- ing spaces in which the corneal cor- puscles lie. These corneal corpuscles have been seen to execute amoeboid movement. At its posterior surface the cornea is limited by the posterior homogeneous lamina, or membrane of Descemet, which is elastic in nature, and lastly a single stratum of cubical epithelial cells (fig. 309, d). Nerves. - The nerves of the cornea are both large and numerous: they are derived from the ciliary nerves. They traverse the substance of the cornea, in which some of them near the anterior surface break up into axis cylinders, and their primitive fibrillae. The latter form a plexus immediately beneath the epithelium, from which delicate fibrils pass up between the cells anasto- mosing with horizontal branches, and forming an intra-epithelial plexus. Most of the primitive fibrillae have a beaded or varicose appearance. The cornea has no blood-vessels penetrating its structure, nor yet lymphatic vessels proper. It is nourished by the circulation of lymph in the spaces in which the cornea corpuscles lie. These communicate freely and form a lymph- canalicular system. Fig. 309.-Vertical section of rab- bit's cornea, stained with gold chloride, e, Stratified anterior epithelium. Immediately be- neath this is the anterior homo- geneous lamina of Bowman, n, Nerves forming a delicate sub- epithelial plexus, and sending up fine twigs between the epi- thelial cells to end in a second plexus on the free surface ; d, Descemet's membrane, consist- ing of a fine elastic layer, and a single layer of epithelial cells; the substance of the cornea, /, is seen to be fibrillated, and con- tains many layers of branched corpuscles, arranged parallel to the free surface, and here seen edgewise. (Schofield.) 332 THE EYE AND VISION. [ch. xxxi. Structure of the Choroid Coat (funica vascvdosa).-This coat is attached, to the inner layer of the sclerotic in front at the corneo-scleral junction and behind at the entrance of the optic nerve, elsewhere it is con- nected to it only by loose connective tissue. Its external coat is formed chiefly of elastic fibres and large pig- ment corpuscles loosely arranged and containing lymphatic spaces lined with endothelium. This is the supra- choroidea. More internally is a layer of arteries and veins arranged in a system of venous whorls, together with elastic fibres and pigment cells. The lymphatics, too, are well developed around the blood-vessels, and there are besides distinct lymph spaces lined with endothelium. Internally to this is a layer of fine capillaries, very dense and derived from the arteries of the outer coat and ending in veins in that coat. It contains corpuscles without pigment, and lymph spaces which surround Fig. 310.-Section through the choroid coat of the human eye. 1, elastic membrane, structureless or finely flbril- lated ; 2, chorio-capiilaris or tunica Ruyschiana ; 3, Pro- per substance of the choroid with large vessels cut through ; 4, suprachoroidea; 5, sclerotic. (Schwalbe.) Fig. 31X.-Section through the eye carried through the ciliary processes. 1, cornea; 2, membrane of Descemet; 3, sclerotic; 3', cpmeo-scleral junction; 4, canal of Schlemm; 5, vein; .6, nucleated network on inner wall of canal of Sehlemm; 7, lig. pectinatum iridis, abc ; 8, iris stroma; 9, pigment of iris ; 10, ciliary processes ; 11, ciliary muscle; 12, choroid tissue; 13, meridional and 14, radiating fibres of ciliary muscle ; 15, ring-muscle of Muller; 16, circular or angular bundles of ciliary muscle. (Schwalbe.) the blood-vessels (memhrana chorio-capillaris). It is separated from the retina by a fine elastic membrane (membrane of Bruch'), which is either structureless or finely fibrillated. The choroid coat ends in front in what are called the ciliary CH. XXXI.] THE IRIS. 333 processes (figs. 311, 312). These consist of from 70 to 80 meridion- ally arranged radiating plaits, which consist of blood-vessels, fibrous connective tissue, and pigment corpuscles. They are lined by a continuation of the membrane of Bruch. The ciliary processes ter- minate abruptly at the margin of the lens. The ciliary muscle (13, 14 and 15, fig. 311), takes origin at the corneo - scleral junction. It is a ring of muscle, 3 mm. broad and 8 mm. thick, made up of fibres running in three directions, (a) Meridional fibres near the sclerotic and passing to the choroid ; (6) radial fibres passing to be inserted into the choroid behind the ciliary pro- cesses ; and (c) circular fibres (muscle of Muller), more inter- nal ; they constitute a sphincter. The Iris.-The iris is a con- tinuation of the choroid inwards beyond the ciliary processes. It is a fibro-muscular membrane perforated by a central aperture, the pupil. It is made up chiefly of blood-vessels and connective tissue, with pigment and unstriated muscle. Posteriorly is a layer of pigment cells (uvea}, which is a con- tinuation forwards of the pigment layer of the retina. The structure of the iris proper is made of connective tissue in front with corpuscles which may or may not be pigmented, and behind of similar tissue supporting blood-vessels enclosed in connective tissue. The pigment cells are usually well developed here, as are also many nerve-fibres radiating towards the pupil. Surrounding the pupil is a layer of circular unstriped muscle, the sphincter pupillce. In some animals there are also muscle-fibres which radiate from the sphincter in the substance of the iris forming the dilatator pupillce. The iris is covered anteriorly by a layer of epithelium continued upon it from the posterior surfaceof the cornea. The Lens.-The lens is situated behind the iris, being enclosed in a distinct capsule, the posterior surface of which is less thick than the anterior. It is supported in place by the suspensory ligament, fused to the anterior surface of the capsule. The suspensory ligament is derived from the hyaloid membrane, which encloses the vitreous humour. It is made up of a series of concentric laminae (fig. 313), which when it has been hardened, can be peeled off like the coats of Fig'. 312.-Ciliary processes, as seen from behind, i, posterior surface of the iris, with the sphincter muscle of the pupil; 2, anterioi' part of the choroid coat; 3, one of the ciliary processes, of which about seventy are represented. J. 334 THE EYE AND VISION. [ch. xxxi. an onion. The laminae consist of long ribbon-shaped fibres, which in the course of development have originated from cells. The fibres near the margin have nuclei and are smooth, those near the centre are without nuclei and have serrated edges. They are hexagonal in transverse section. The fibres are united together by a scanty amount of cement substance. The central portion (nucleus) of the lens is the hardest. The epithelium of the lens con- sists of a layer of cubical cells an- teriorly, which merges at the equator into the lens fibres. The develop- ment of the lens explains this trans- ition. The lens at first consists of a closed sac composed of a single layer of epithelium. The cells of the posterior part soon elongate for- wards and obliterate the cavity ; the anterior cells do not grow, but at the edge they become continuous with the posterior cells, which are gradually developed into fibres (fig. 314). The principal chemical constituent of the lens is a proteid of the globulin class called crystallin. Corneoscleral junction.-At this junction the relation of parts (fig. 311) is so important as to need a short description. In the neighbourhood, the iris and ciliary processes join with the cornea. The proper substance of the cornea and the posterior elastic lamina Fig. 313.-Laminated structure of the crystalline lens. The lamince are split up after hardening in alcohol. 1, the denser central part or nucleus ; 2, the succes- sive external layers, y. (Arnold.) Fig. 314.-Meridional section through the lens of a rabbit. 1, Lens capsule; 2, epithelium of lens ; 3, transition of the epithelium into the fibres ; 4, lens fibres. (Bubuchin.) become continuous with the iris, at the angle of the iris, and the iris sends forwards processes towards the posterior elastic lamina, forming the ligamentuni pectinatum iridis, and these join with fibres of the elastic lamina. The epithelial covering of the posterior surface of the cornea is, as we have seen, continuous over the front of the iris. At the iridic angle, the compact inner substance of the cornea is looser, and between the bundles are lymph spaces filled with fluid, called the spaces of Fontana. They are little developed in the human cornea. CH. xxxi. j THE RETINA. 335 The spaces which are present in the broken up bundles of corneal tissue at the angle of the iris, are continuous with the larger lymphatic space of the anterior chamber. Above the angle at the corneo-scleral junction is a canal, which is called the canal of Schlemm. It is a lymphatic channel. Structure of the Retina.-The retina (fig. 315) is a delicate membrane, concave with the concavity directed forwards and apparently ending in front, near the outer part of the ciliary processes, in a finely notched edge,-the ora serrata, but really represented by the uvea to the .very margin of the pupil. It results from the expansion of the optic nerve, of whose terminal fibres, de- prived of their external white substance, together with nerve-cells, it is es- sentially composed. The presence of nerve-cells in the retina which come into contact with the rods and cones (visual nerve- epithelium) reminds us that the optic, like the olfactory nerve, is not a mere nerve, but an out- growth of the brain. Exactly in the centre of the retina is a round yellowish elevated spot, about -AT of an inch (1 mm.) in diameter, hav- ing a depression in the centre, called after its discoverer the macula lutea, or yellow spot of Soemmering. The depres- sion in its centre is called the fovea centralis. About T\j- of an inch (2'5 mm.) to the inner side of the yellow spot, is the point (optic disc or white spot) at which the optic nerve enters the eyeball, and begins to spread out its fibres into the retina. Fig. 315.-A Section of the retina, choroid, and part of the sclerotic, moderately magnified; a, mem- brana limitans interna; b, nerve-fibre layer traversed by Muller's sustentacular fibres; c, ganglion-cell layer; d, internal molecular layer; e, internal nuclear layer; /, external molecular layer; g, external nuclear layer; ft, membrana limitans externa, running along the lower part of i, the layer of rods and cones ; ft, pigment cell layer; Im, internal and external vascular portions of the choroid, the first containing capillaries, the second larger blood- vessels, cut in transverse section ; n, sclerotic. (W. Pye.) 336 THE EYE AND VISION. [ch. xxxr. The optic nerve passes forwards from the ventral surface of the cerebrum towards the orbit enclosed in prolongations of the membranes, which cover the brain. The external sheath at the entrance of the nerve into the eyeball becomes continuous with the sclerotic, which at this part is perforated by holes tO' allow of the passage of the optic nerve fibres, the perforated part being the lamina cribrosa. The pia mater here becomes incomplete, and the subarachnoid and the superarachnoid spaces become continuous. The pia mater sends in processes into the nerve to support the fibres. The fibres of the nerve themselves are exceedingly fine, and are surrounded by the myelin sheath, but do not possess the ordinary external nerve sheath. As they pass into the retina they lose their myelin sheaths and proceed as axis cylinders. Neuroglia supports the nerve fibres in the optic nerve trunk. In the centre of the nerve is a small artery, the arteria centralis retinae. The number of fibres in the optic nerve is said to be upwards of 500,000. The axis cylinders pass on to the retina, turning over the edges of theporus opticus, to be distributed on the inner surface of the retina, as far as the ora serrata, as the layer of optic nerve-fibres. The retina consists of certain elements arranged in ten layers, from within outwards (figs. 315,. 3i6> 3*7)- 1. Membrana limitans interna : This so-called membrane in con- tact with the vitreous humour is formed by the junction laterally of the bases of the sustentacular or supporting fibres of Muller, which bear the same relation to the retina as the neuroglia does to the brain. The supporting character of these fibres may be seen in fig. 316. 2. Optic nerve fibres.-This layer is of very varying thickness in different parts of the retina: it consists of non-medullated fibres which interlace, and some of which are continuous with processes of the large nerve- Inter-nuclear layer. Spongio- blasts. Inner mole- cular layer. Fig. 316.-Diagram showing the susten- tacular fibres of the retina ; /, fibre- basket above the external limiting membrane ; m, nucleus of the fibre ; r, base of the fibre. (From MacKen- drick, after Stbhr.) CH. XXXI.] THE RETINA. 337 cells forming the next layer. The fibres are supported by the sustentacular fibres. They become less and less numerous ante- riorly and end at the ora serrata. 3. Layer of ganglion cells, consisting of large multipolar nerve- cells with large and round nuclei, forming either a single layer, or as in some parts of the retina, especially near the macula lutea, where this layer is very thick, it consists of several strata of nerve- cells. They are arranged with their single axis cylinder-processes inwards. These pass into and are continuous with the layer of optic nerve-fibres. Ex- ternally the cells send off several branched processes which pass into the next layer. 4. Inner molecular layer. -This presents a finely granulated appearance. It consists of neuroglia tra- versed by numberless very fine fibrillar processes of the nerve-cells and the minute branchings of the processes of the bipolar cells of the next layer. 5. Inner nuclear layer. -This consists chiefly of numerous small round cells, with a very small quantity of protoplasm surrounding a large ovoid nucleus ; they are generally bipolar, giving off one process outwards and another inwards. One process passes inwards to intermingle with the arbo- risation of the ganglion cells, the other outwards to similarly arborise with the branchings of the rod and cone fibres. Some cells, called spongioblasts, however, only send off one process, which passes inwards. The large oval nuclei (fig. 316) belonging to the Mullerian fibres occur in this layer. Inner limb of rod. Rod fibre. Bipolar cell. Fig. 317.-Diagram showing the nervous ele- ments of retina, i, nerve fibre to ganglion cell; 2, processes of ganglion cell going out- wards : 3, nerve fibre passing direct to bipolar cell in inner nuclear layer ; 4, process of gan- glion cell towards bipolar cell; 5, fibre from cone-granule breaking up into fibrils which arborise round the branches of bipolar cells. (From MacKendrick, after Stdhr.) 338 THE EYE AND VISION. [ch. xxxi. 6. Order molecular layer.-This layer closely resembles the inner molecular layer, but is much thinner. It contains the branchings of the rod and cone fibres on the one hand and of the bipolar cells on the other. 7. Externalnuclear layer. -This layer consists of small cells resembling at first sight those of the in- ternal nuclear layer ; they are classed as rod and cone fibres, according as they are connected with the rodsand cones respectively, and will be described with them. They are lodged in the meshes of a framework, which is due to the break- ing up of the Mullerian fibres. 8. Membrana limitans externa.-A delicate well- defined membrane, clearly marking the internal limit of the rod and cone layer, and made up of the junction of the bases of the sustentacular fibres externally. Fig. 318.-The posterior half of the retina of the left eye, viewed from before ; s, the cut edge of the sclerotic coat; ch, the choroid; r, the retina; in the interior at the middle the macula lutea with the depression of the fovea centralis is represented by a slight oval shade ; towards the left side the light spot indicates the colliculus or eminence at the entrance of the optic nerve, from the centre of which the arteria centralis is seen spreading its branches into the retina, leaving the part occupied by the macula comparatively free. (After Henle.) Fig. 319.-Section through half of the macula lutea and fovea centralis of human retina. a, fovea, b, descent of the macula towards fovea. The numbers indicate the layers of the retina. (Kuhnt.) Small hairlike processes project outwards between the rods and cones to support them. 9. Layer of rods and cones.-This layer is the nerve-epithelium CH. xxxi.] THE RODS AND CONES. 339 of the retina. It consists of two kinds of cells, rods and cones, which are arranged at right angles to the external limitans mem- brane, and supported by hairlike process (basket) proceeding from the latter for a short distance. The rods.-Each rod (fig. 317) is made up of two parts, very different in structure, called the outer and inner limbs. The outer limb of the rods is about 3 op long and 2p broad, is transparent, and doubly refractive. It is said to be made up of fine superimposed discs. It stains with osmic acid but not with haematoxylin, and resembles in some ways the myelin sheath of a medullated nerve. It swells up on exposure to light, and is the part of the layer in which the pigment called visual purple is found. The inner limb is about as long but slightly broader than the outer, is longitudi- nally striated at its outer and granular at its inner part. It stains with haematoxylin but not with osmic acid. Each rod so constructed is connected internally with a rod fibre, very fine, but here and there varicose; in the middle of the fibre is a rod granule, really the nucleus of the rod, striped broadly transversely, and situated about the middle of the external nuclear layer; the internal end of the rod fibre terminates in minute branchings in the outer molecular layer. The cones. - Each cone (fig. 317), like the rods, is made up of two limbs, outer and inner. The outer limb is tapering and not cylindrical like the corresponding part of the rod, and about one- third only of its length. There is, moreover, no visual purple found in the cone. The inner limb of the cone is broader in the centre. It is protoplasmic, and under the influence of light has been seen to execute movements. In birds there is often a coloured oil globule present here. Each cone is in connection by its internal end with a cone fibre, which has much the same structure as the rod fibre, but is much stouter and has its nucleus quite near to the external limiting membrane. Its inner end terminates by branchings in the external molecular layer. In the rod and cone layer of birds, the cones usually predomi- nate largely in number, whereas in man the rods are by far the more numerous, except in the fovea centralis, where cones only are Fig. 320.-Pigment-cells from the retina, a, cells still cohering, seen on their surface; a, nucleus indistinctly seen. In the other cells the nucleus is concealed by the pig- ment granules, b, two cells seen in profile ; a, the outer or posterior part containing scarcely any pigment. X 370. (Henle.) 340 THE EYE AND VISION. [ch. xxxi. present, as is the case at the anterior part of the retina near the ora serrata. The number of cones has been estimated at 3,000,000. 10. Pigment-cell layer consists of a single layer of polygonal cells, mostly six-sided, which send down a beard-like fringe to surround the outer ends of the rods. It is this layer which is continuous with the uvea, where, however, the cells become rounded, and arranged two or three deep. The next figure represents the structure of the retina as made out by Golgi's method. Differences in Structure of different parts.-Towards the centre of the macula lutea all the layers of the retina become greatly thinned out and almost disappear, except the rod and cone layer, which considerably increases in thickness, but at the fovea centralis Fig. 321.-Scheme of the retinal elements. A, cones of the fovea centralis ; B, granules (nuclei) of these cones; C, articulation between the cones and bipolar ceils in external molecular layer; D, articulation between the bipolar and ganglion cells in the internal molecular layer; a and ft, rods and cones in other regions of the retina ; c, bipolar cell destined for the cones; d, bipolar cell destined for the rods; e, ganglion cells; f, spongioblast; g, efferent fibre (? trophic), originating from the cell in geniculate body; h, optic nerve; », terminal arborisations of optic nerve fibres in geniculate body ; j, fibres from the cells of geniculate body on the way to cerebral cortex. (R. y Cajal.) comes to consist almost entirely of long slender cones and cone- fibres, which curve towards the periphery. They are supported by neuroglia, which is also found internally as a thin layer. There are capillaries here, but none of the larger branches of the retinal arteries. Towards the edge of the macula lutea, not only are all the layers present, but the ganglionic layer consists of several strata of cells, and with this increase there is also an increase in the thickness of the inner nuclear layer. Towards the centre the layers diminish in this order : optic nerve fibres, ganglionic layer, inner molecular layer, and inner nuclear layer. The rods grow scanty and then are absent. At the ora serrata the layers are not perfect and disappear in ch. xxxi. ] THE RETINA. 341 this order: nerve-fibres and ganglion cells, then the rods, leaving only the inner limbs of the cones, next these cease, then the outer molecular layer, the inner and outer nuclear layers coalescing, and finally the inner molecular layer also is unrepresented. At the pars-ciliaris retime, the retina is represented by a layer of columnar cells, derived from the fusion of the nuclear layers. The cells externally are in contact with the pigment layer of the retina, which is continued over the ciliary processes and back of the iris. At the entrance of the optic nerve the only structures pre- sent are nerve-fibres. The chambers of the eye.-The anterior chamber is the space behind the cornea and in front of the lens. It is filled with aqueous humour, which is diluted lymph. The posterior chamber, or that behind the lens, contains the vitreous humour, which is a jelly-like connective tissue (see p. 52). It is enclosed in a membrane called membrana hyaloidea. In front it is continuous with the capsule of the lens; round the edge of the lens the canal left is called the Canal of Petit, the membrane itself being the Zonule of Zinn. The hyaloid mem- brane separates the vitreous from the retina. Blood-vessels of the Eyeball.-The eye is very richly supplied with blood-vessels. In addition to the conjunctival vessels which are derived from the palpebral and lacrimal arteries, there are at least two other distinct sets of vessels supplying the tunics of the eyeball. (1) The vessels of the sclerotic, choroid, and iris, and (2) the vessels of the retina. (1.) These are the short and long posterior ciliary arteries which pierce the sclerotic in the posterior half of the eyeball, and the anterior ciliary which enter near the insertions of the recti. These vessels anastomose and form a very rich choroidal plexus ; they also supply the iris and ciliary processes, forming a very highly vascular circle round the outer margin of the iris and adjoining portion of the sclerotic. The distinctness of these vessels from those of the conjunctiva is well seen in the difference between the bright red of blood-shot eyes (conjunctival congestion), and the pink zone surrounding the cornea which indicates deep-seated ciliary congestion. (2.) The retinal vessels (fig. 318) are derived from the arteria centralis retinae., which enters the eyeball along the centre of the optic nerve. They ramify all over the retina, in its inner layers. They can be seen by ophthalmoscopic examination. 342 THE EYE AND VISION. [CH. XXXI. The Eye as an Optical Instrument. The eye may be compared to a photographic camera, and the transparent media correspond to the photographic lens. In such a camera images of external objects are thrown upon a ground- glass screen at the back of a box, the interior of which is painted black. In the eye, the camera is represented by the eye-ball with its pigment, the screen by the layer of rods and cones of the retina, and the lens by the refracting media. In the case of the camera, the screen is enabled to receive clear images of objects at different distances, by an apparatus for focussing. The corresponding contrivance in the eye is called accommo- dation. The iris, which is capable of allowing more or less light to pass into the eye, corresponds with the diaphragms used in the photo- graphic apparatus. The refractive media are the cornea, aqueous humor, crystal- line lens, and vitreous humor. The most refraction or bending of the rays of light occurs where they pass from the air into the cornea; they are again bent slightly in passing through the crystalline lens. Alterations in the anterior curvature of the crystalline lens lead to what we have already termed accommo- dation-that is, the power the eye has for adjusting itself to objects at different distances. In its simplest form, we may consider the refraction through a simple transparent spherical surface, separating two media of different density. The rays of light which fall upon the surface exactly perpen- dicularly do not suffer refraction, but pass through, cutting the optic axis (0 A, fig. 322), a line which passes exactly through the centre of the surface, at a certain point, the nodal point (fig. 322, N), or centre of curvature. Any rays which do not so strike the curved surface are refracted towards the optical axis. Rays which impinge upon the spherical surface parallel to the optical axis, will meet at a point behind, upon the said axis which is called the chief posterior focus (fig. 322, I\); and again there is a point in the optical axis in front of the surface, rays of light from which so strike the surface that they are refracted in a line parallel with the axis d f"-, this point (fig. 322, F2) is called the chief anterior focus. The optic axis cuts the surface at what is called the principal point. It is quite obvious that the eye, even in the simplified form above indicated, is a much more complicated optical apparatus CH. XXXI.] REFRACTION. 343 than the one described in the figure. It is, however, possible to reduce the refractive surfaces and media to a simpler form when the refractive indices of the different media and the Fig. 322.-Diagram of a simple optical system (after M. Poster). The curved surface, &, d, is.supposed to separate a less refractive medium towards the left from a more refractive medium towards the right. curvature of each surface are known. These data are as follows :- Index of refraction of cornea = i'37 „ „ aqueous and vitreous = i-34 to 1'36 , _ J 1-4 in outer to 1-45 | in inner part. Radius of curvature of cornea = 7'8 mm. „ „ anterior surface of lens = 10 ,, ,, „ posterior „ = 6 „ Distance from anterior surface of cornea and anterior surface of lens = 3'6 „ Distance from posterior surface of cornea and posterior surface of lens = 7'2 Distance from posterior surface of lens to ' retina = 15'0 „ With these data, it has been found comparatively easy to reduce by calculation the different surfaces of different curvature, into one mean curved surface of known curvature, and the differently refracting media, into one mean medium the refractive power of which is known. The simplest so-called schematic eye formed upon this prin- ciple, suggested by Listing as the reduced eye, has the following dimensions :- From anterior surface of cornea to the principal point . . . = 2'3448 mm. From the nodal point to the posterior sur- face of lens . . . . . . = '4764 mm. Posterior chief focus lies behind cornea . = 22'8237 mm. Anterior chief focus in front of cornea . . = 12'8326 mm. Radius of curvature of ideal surface . . = 5'1248111111. 344 THE EYE AND VISION. [ch. XXXI. The term index of refraction means the ratio of the sine of the angle of incidence to that of the angle of refraction; this is shown in the next figure. Fig. 323.-If P P' is a line which separates two media, the lower one being the denser, and O A is a ray of light falling on it, it is bent at O towards the normal or perpendicular line N N'. A O is called the incident ray, and O B the refracted ray ; A 0 N is called the angle of incidence (i), and N'OP the angle of refraction (r). If any distance O X is measured off along O A. and an equal distance O X' along O B and perpen- O Y diculars drawn to N N , then = index of refraction. In this reduced or simplified eye, the principal posterior focus, about 23 mm. behind the spherical surface, would correspond to the Fig. 324.-Diagram of the optical angle. position of the retina behind the anterior surface of cornea. The refracting surface would be situated about midway between the posterior surface of the cornea and the anterior surface of the lens. ch. xxxi. ] REFRACTION. 345 The optical axis of the eye is a line drawn through the centres of curvature of the cornea and lens, prolonged back- ward to touch the retina between the porus opticus and fovea centralis, and this differs from the visual axis which passes through the nodal point of the reduced eye to the fovea centralis; this forms an angle of 5°with the optical axis. But for practical purposes the optical axis and the visual axis may be considered to be identical. The visual or optical angle (fig. 324) is included between the lines drawn from the borders of any object to the nodal point; if the lines be prolonged backwards they include an equal angle. It has been shown by Helmholtz that the smallest angular distance between two points which can be appre- Fig. 325.-Diagram of the course of the rays of light, to show how an image is formed upon retina. The surface C C should he supposed to represent the ideal curvature. ciated =50 seconds, the size of the retinal image being this practically corresponds to the diameter of the cones at the fovea centralis which - 3/z, the distance between the centres of two adjacent cones being = 4/a. Any object-take, for instance, the arrow A B (fig. 325)- may be considered as a series of points from each of which a pencil of light diverges to the eye. Take, for instance, the rays diverging from the tip of the arrow A ; C C represents the cur- vature of the schematic or reduced eye ; the ray which passes through the centre of the circle of which C C is part is not refracted ; this point is represented as an asterisk in fig. 325 ; it is near the posterior surface of the crystalline lens; the ray A C, which is parallel to the optic axis 0 O', is refracted through the principal posterior focus P, and cuts the first ray at the point A' on the retina. All the other rays from A meet at the same point. Similarly the other end of the arrow B is focussed at B', and all other points have corresponding focusses. It will thus be seen that an inverted image is formed on the 346 THE EYE AND VISION. [ch. xxxi. retina of external objects. The retina is a curved screen, but the images fall only on a small area of the retina under normal circumstances; hence for practical purposes this small area may be regarded as flat. The question then arises, Why is it that objects do not appear to us to be upside down ? This is easily understood when we remember that the sensation of sight occurs not in the eye, but in the brain. By education the brain learns that the tops of objects excite certain portions of the retina, and the lower parts of objects other portions of the retina. That these portions of the retina are reversed in position to the parts of the object does not matter at all, any more than it matters when one's photograph arrives home from the photographers that it was wrong way up in the photographer's camera -one puts it right way up in the photograph album. Accommodation. The power of accommodation, or the adaptation of the eye to vision at different distances, is primarily due to a varying shape of the lens, its front surface becoming more or less convex, according as the distance of the object looked at is near or far. The nearer the object, the more convex, up to a certain limit, the front surface of the lens becomes, and vice versa ; the back surface taking no share in the production of the effect required. And this surface, which during rest is more convex than the anterior, becomes the less convex of the two during accommodation. The following simple experiment illustrates this point: If a lighted candle be held a little to one side of a person's eye an observer looking at the eye from the other side sees three images of the flame (fig. 326). The first and brightest is (1) a small erect image formed by the anterior convex surface of the cornea ; the second (2) is also erect, but larger and less distinct than the preceding, and is formed at the anterior convex surface of the lens ; the third (3) is smaller, inverted, and indistinct; it is formed at the posterior surface of the lens, which is concave forwards, and therefore, like all concave mirrors, gives an inverted image. If now the eye under observation be made to look at a near object the second image becomes smaller, Fig. 326.-Diagram showing three reflections of a candle, i, From the anterior surface of cornea ; 2, from the anterior surface of lens; 3, from the posterior surface of lens. For further explanation, see text. The experiment is hest performed by employing an instrument invented by Helmholtz, termed a Phakoscope (see flg. 328). ch. xxxi. ] SANSON'S IMAGES. 347 clearer, and approaches the first. If the eye be now adjusted for a far point, the second image enlarges again, becomes less distinct, and recedes from the first. In both cases alike the first and third images remain unaltered in size, distinctness, and relative position. This proves that during accommodation for near objects the curvature of the cornea, and of the posterior surface of the lens, remains unaltered, while the anterior surface of the lens becomes more convex and approaches the cornea. The experiment (fig. 327) is more striking when two candles are used, and the images of the two candles from the front surface of the lens during accommodation not only approach those from Fig. 327.-Diagram of Sanson's images. A, when the eyes are not, and B, when they are focussed for near objects. The fig. to the right in A and B is the inverted image from the posterior surface of the lens. the cornea, but also approach one another, and become some- what smaller. (Sanson's Images.) Helmholtz' Phakoscope (fig. 328) is a triangular box with arrangements for demonstrating this experiment. Mechanism of accommodation.-The lens having no inherent power of contraction, its changes of outline must be produced by some power from without; this power is supplied by the ciliary muscle. Its action is to draw forwards the choroid, and by so doing to slacken the tension of the suspensory ligament of the lens which arises from it. The anterior surface of the lens is kept flattened by the action of this ligament. The ciliary muscle during accommodation, by diminishing its tension, dimi- nishes to a proportional degree the flattening of which it is the cause. On diminution or cessation of the action of the ciliary muscle, the lens returns to its former shape, by virtue of the elasticity of the suspensory ligament (fig. 329). From this it will appear that the eye is usually focussed for distant objects. In viewing near objects the ciliary muscle contracts, the opposite effect taking place on withdrawal of the attention from near objects, and fixing it on those distant. 348 THE EYE AND VISION. CH. XXXI. Range of Distinct Vision. Near-point.-In every eye there is a limit to the power of accommodation. If a book be brought Fig. 328.-Phakoscope of Helmholtz. At B B' are two prisms, by which the light of a candle is concentrated on the eye of the person experimented with, which is looking through a hole in the third angle of the box opposite to the window C. A is the aperture for the eye of the observer. The observer notices three double images, as in fig. 327, reflected from the eye under examination when the eye is fixed upon a distant object; the position of the images having been noticed, the eye is then made to focus a near object, such as a reed pushed up at C; the images from the anterior surface of the lens will be observed to move towards each other, in consequence of the lens becoming more convex. nearer and nearer to the eye, the type at last becomes indistinct, and cannot be brought into focus by any effort of accommodation, Fig. 329.-Diagram representing by dotted lines the alteration in the shape of the lens on accommodation for near objects. (E. Landolt.) CH. XXXI. ] SCHEINER'S EXPERIMENT. 349 however strong. This which is termed the near-p>oint, can be determined by the following experiment (JScheiner). Two small holes are pricked in a card with a pin not more than a twelfth of an inch (2 mm.) apart, at any rate their distance from each other must not exceed the diameter of the pupil. The card is held close in front of the eye, and a small needle viewed through the pin-holes. At a moderate distance it can be clearly focussed, but when brought nearer, beyond a certain point, the image appears double or at any rate blurred. This point where the needle ceases to appear single is the near-point. Its distance from the eye can of course be readily measured. It is usually about 5 or 6 inches (13 cm.). In the accompanying figure (fig. 330) the lens b repre- sents the eye ; ef the two pin-holes in the card, nn the retina; a represents the position of the needle. When the needle is at a moderate distance, the two pencils of light coming from e and f, are focussed at a single point on the retina nn. If the needle be Pig. 330.-Diagram of experiment to ascertain the minimum distance of distinct vision. brought nearer than the near-point, the strongest effort of accom- modation is not sufficient to focus the two pencils, they meet at a point behind the retina. The effect is the same as if the retina were shifted forward to mm. Two images h.g. are formed, one from each hole. It is interesting to note that when two images are produced, the lower one g really appears in the position Q, while the upper one appears in the position p. This may be readily verified by covering the holes in succession. During accommodation two other changes take place in the eyes: (1) The eyes converge by the action of the ocular muscles. (2) The pupils contract. The contraction of all of the muscles which have to do with accommodation, viz. of the ciliary muscle, of the recti muscles, and of the sphincter pupillfe is under the control of the third nerve. 350 THE EYE AND VISION. [ch. xxxi. Defects in the Optical Apparatus. Under this head we may consider the defects known as (i) Myopia, (2) Hypermetropia, (3) Astigmatism, (4) Spherical Aber- ration, (5) Chromatic Aberration. The normal (emmetropic) eye is so adjusted that parallel rays Fig. 331.-Diagrams showing-i, normal (emmetropic) eye bringing parallel rays exactly to a focus on the retina ; 2, normal eye adapted to a near point; without accommo- dation the rays would he focussed behind the retina, but by increasing the curvature of the anterior surface of the lens (shown by a dotted line) the rays are focussed on the retina (as indicated by the meeting of the two dotted lines); 3, hypermetropic eye ; in this case the axis of the eye is shorter than normal; parallel rays are focussed behind the retina; 4, myopic eye; in this case the axis of the eye is abnormally long; parallel rays are focussed in front of the retina. are brought exactly to a focus on the retina without any effort of accommodation (i, fig. 331). Hence all objects except near ones (practically all objects more than twenty feet off) are seen without CH. XXXI.] ERRORS OF REFRACTION. 351 any effort of accommodation; in other words, the far-point of the normal eye is at an infinite distance. In viewing near objects we are conscious of the effort (the contraction of the ciliary muscle) by which the anterior surface of the lens is rendered more convex, and rays which would otherwise be focussed behind the retina are converged upon the retina (see dotted lines 2, fig. 331). 1. Myopia, (short-sight) (4, fig. 331).-This defect is due to an abnormal elongation of the eyeball. The retina is too far from the lens and consequently parallel rays are focussed in front of the retina, and, crossing, form little circles on the retina ; thus the images of distant objects are blurred and indistinct. The eye is, as it were, permanently adjusted for a near-point. Rays from a point near the eye are exactly focussed in the retina. But those which issue from any object beyond a certain distance (/ar- point} cannot be distinctly focussed. This defect is corrected by concave glasses which cause the rays entering the eye to diverge : hence they do not come to a focus so soon. Such glasses of course are only needed to give a clear vision of distant objects. For near objects, except in extreme cases, they are not required. 2. Hypermetropia (3, fig. 331).-This is the reverse defect. The eyeball is too short. Parallel rays are focussed behind the retina: an effort of accommodation is required to focus even parallel rays on the retina; and when they are divergent, as in viewing a near object, the accommodation is insufficient to focus them. Thus in well-marked cases distant objects require an effort of accommodation and near- ones a very powerful effort, and the ciliary muscle is, therefore, constantly acting. This defect is obviated by the use of convex glasses, which render the pencils of light more convergent. Such glasses are of course especially needed for near objects, as in reading, etc. They rest the eye by relieving the ciliary muscle from excessive work. 3. Astigmatism.-This defect, which was first discovered by Airy, is due to a greater curvature of the eye in one meridian than in others. The eye may be even myopic in one plane and hypermetropic in others. Thus vertical and horizontal lines crossing each other cannot both be focussed at once ; one set stand out clearly and the others are blurred and indistinct. This defect, which is present in a slight degree in all eyes, is generally seated in the cornea, but occasionally in the lens as well; it may be corrected by the use of cylindrical glasses (i.e., curved only in one direction). 4. Spherical Aberration.-The rays of a cone of light from an object situated at the side of the field of vision do not meet all in the same point, owing to their unequal refraction; for the 352 THE EYE AND VISION. [ch. xxxi. refraction of the rays which pass through the circumference of a lens is greater than that of those traversing its central portion. This defect is known as spherical aberration, and in the camera, telescope, microscope, and other optical instruments, it is remedied by the interposition of a screen with a circular aperture in the path of the rays of light, cutting off all the marginal rays and only allowing the passage of those near the centre. Such correc- tion is effected in the eye by the iris, which prevents the rays from passing through any part of the refractive apparatus but its centre. The posterior surface of the iris is coated with pigment, to prevent the passage of rays of light through its substance. The image of an object will be most defined and distinct when the pupil is narrow, the object at the proper distance for vision, and the light abundant; so that, while a sufficient number of rays are admitted, the narrowness of the pupil may prevent the production of indistinctness of the image by spherical aberration. Distinctness of vision is further secured by the pigment of the outer surface of the retina, the posterior surface of the iris and the ciliary processes, which absorbs the greater part of light that may be reflected within the eye, and prevents their being thrown again upon the retina so as to interfere with the images there formed. 5. Chromatic Aberration.-In the passage of light through an ordinary convex lens, decomposition of each ray into its ele- mentary colours, commonly ensues, and a coloured margin appears around the image, owing to the unequal refraction which the elementary colours undergo. In optical instruments this, which is termed chromatic aberration, is corrected by the use of two or more lenses, differing in shape and density, the second of which continues or increases the refraction of the rays produced by the first, but by recombining the individual parts of each ray into its original white light, corrects any chromatic aberration which may have resulted from the first. It is probable that the unequal refractive power of the transparent media in front of the retina may be the means by which the eye is enabled to guard against the effect of chromatic aberration. The human eye is achromatic, however, only so long as the image is received at its focal distance upon the retina, or so long as the eye adapts itself to the different distances of sight. If either of these conditions be interfered with, a more or less distinct appearance of colours is produced. From the insufficient adjustment of the image of a small white object, it appears surrounded by a sort of halo or fringe. This phenomenon is termed Irradiation. It is from this reason that a CH. xxxi.] THE IRIS. 353 white square on a black ground appears larger than a black square of the same size on a white ground. Defective Accommodation-Presbyopia.-This condition is due to the gradual loss of the power of accommodation which is part of the general decay of old age. In consequence the patient is obliged in reading to hold the book further and further away in order to focus the letters, till at last the letters are held too far for distinct vision. The defect is remedied by weak convex glasses. It is due chiefly to the gradual increase in density of the lens, which is unable to swell out and become convex when near objects are looked at, and also to a weakening of the ciliary muscle, and a general loss of elasticity in the parts concerned in the mechanism. Functions of the Iris. The iris has three uses :- 1. To act as a diaphragm in order to lessen spherical aberra- tion in the manner just described. 2. To regulate the amount of light entering the eye. In a bright light the pupil contracts; in a dim light it enlarges. This may be perfectly well seen in one's own iris by looking at jt in a mirror while one alternately turns a gas light up and down. 3. By its contraction during accommodation it supports the action of the ciliary muscle. The muscular fibres (unstriped in mammals, striped in birds) of the iris are arranged circularly around the margin of the pupil, and radiatingly from its margin. The radiating fibres are best seen in the eyes of birds and otters; some look upon them as elastic in nature, but there is little doubt that they are con- tractile. Those who believe they are not contractile explain dilatation of the pupil as due to inhibition of the circular fibres. But if the iris is stimulated near its outer margin at three different points simultaneously the pupil assumes a triangular shape, the angles of the triangle corresponding to the points stimulated; this must be due to contraction of three strands of the radiating muscle ; inhibition of the circular fibres would occur equally all round. The iris is supplied by three sets of nerve-fibres contained in the ciliary nerves. (a) The third nerve supplies the circular fibres. (J) The cervical sympathetic supplies the radiating fibres. The cilio-spinal centre which governs them is in the cervical region of the cord (see p. 255). (c) Fibres of the fifth nerve which are probably sensory. The experiments on these nerves are those of section and 354 THE EYE AND VISION. [ch. xxxi. stimulation of the peripheral ends; the usual experiments by which the functions of a nerve are discovered. Nerve. Experiment. Effect on pupil. Third .... Third . . Sympathetic Sympathetic . . . Both nerves together . Section . Stimulation . . Section . Stimulation Stimulation . j Dilatation. Contraction. Contraction. Dilatation. Contraction overcomes the dilatation. Certain drugs dilate the pupil. These are called mydriatics; atropine is a well-known example. Others cause the pupil to contract. These are called myotics; physostigmine and opium (taken internally) are instances. Different myotics and mydri- atics act in different ways, some exerting their activity on the muscular, and others on the nervous structures of the iris. Reflex actions of the iris.-When the iris contracts under the influence of light, the sensory nerve is the optic, and the motor the third nerve. The central connection of the two nerves in the region of the mid-brain we shall see later on (fig. 344). The iris also contracts on accommodation; and the reflex path con- cerned in this action is a different one from that concerned in the light reflex, as this reflex often remains in cases of locomotor ataxy, after there is an entire loss of the reflex to light (Argyll- Robertson pupil). On painful stimulation of any part of the body, there is reflex dilatation of the pupil. This is accompanied by starting of the eyeballs, due to contraction of the plain muscle in the capsule of Tenon, which, like the dilatator fibres of the iris, is supplied by the cervical sympathetic nerve. We may sum up the principal conditions under which the pupil contracts and dilates in the following table :- Causes of- Contraction of the Pupil. 1. Stimulation of third nerve. 2. Paralysis of cervical sympathetic. 3. When the eye is exposed to light. 4. When accommodation occurs. 5. Under the local influence of physostigmine. 6. Under the influence of opium. 7. During sleep. Dilatation of the Pupil. 1. Paralysis of the third nerve. 2. Stimulation of the cervical sympa- thetic. 3. In the dark. 4. When the accommodation is relaxed. 5. Under the local influence of atro- pine. This drug also paralyses the ciliary muscle. 6. In the last stage of asphyxia. 7. In deep chloroform narcosis. 8. Under the influence of certain emotions, such as fear. 9. During pain. CH. XXXI. J THE BLIND SPOT. 355 There is a close connection of the centres that govern the activity of the two irises. If one eye is shaded by the hand, its pupil will of course dilate, but the pupil of the other eye will also dilate. The two pupils always contract or dilate together unless the cause is the local injury to the nerves of one side or the local action of drugs. Functions of the Retina. The Retina is the nervous coat of the eye ; it contains the layei' of nerve-epithelium (rods and cones) which is capable of receiving the stimulus of light, and transforming it into a nervous impulse which passes to the brain by the optic nerve. The bacillary layer, or layer of rods and cones, is at the back of all the other retinal layers, which the light has to pene- trate before it can affect this layer. The proofs of the statement that it is the layer of the retina which is capable of stimulation by light are the following :- (i) The point of entrance of the optic nerve into the retina, where the rods and cones are absent, is insensitive to light, and is called the blind spot. The phenomenon itself is very readily demonstrated. If we direct one eye, the other being closed, upon a point at such a distance to the side of any distance, that the image of the latter must fall upon the retina at the point of entrance of the optic nerve, this image is lost. If, for example, we close the left eye, and direct the axis of the right eye steadily towards the circular spot here represented, while the page is held at a distance of about six inches from the eye, both dot and cross are visible. On gradually increasing the distance between the eye and the object, by removing the book farther and farther from the face, and still keeping the right eye steadily on the dot, it will be found that suddenly the cross disappears from view, while on removing the book still farther, it suddenly comes in sight again. The cause of this phenonemon is simply that the portion of retina which is occupied by the entrance of the optic nerve is quite blind; and therefore when the images of objects fall on it they cease to be visible. By a psychical process the blind spot is not normally perceived. (2) In the fovea centralis and macula lutea which contain rods and cones but no optic nerve-fibres, and the other layers of the retina are thinned down to a minimum, light produces the greatest effect. In the latter, cones occur in large numbers, and in the former cones 356 THE EYE AND VISION. [CH. XXXI. without rods are found, whereas in the rest of the retina which is not so sensitive to light, there are fewer cones than rods. We may conclude, therefore, that cones are even more important to vision than rods. (3) If a small lighted candle be moved to and fro at the side of and close to one eye in a dark room while the eyes look steadily forward into the darkness, a remarkable branch- ing figure (Purkinje's figures) is seen floating before the eye, con- sisting of dark lines on a reddish ground. As the candle moves, the figure moves in the opposite direction, and from its whole appear- ance there can be no doubt that it is a reversed picture of the retinal vessels projected before the eye. This remarkable appear- ance is due to shadows of the retinal vessels cast by the candle. Under ordinary circumstances, the brain has learnt to disregard these shadows, and it is only when they are thrown upon the retina in an unusual slanting direction that they are perceived. The branches of these vessels are distributed in the nerve-fibre and ganglionic layers ; and since the light of the candle falls on the retinal vessels from in front, the shadow is cast behind them, and hence those elements of the retina which perceive the shadows must also lie behind the vessels. Here, then, we have a clear proof that the light-perceiving elements of the retina are not the inner layers of the retina, but the external layer of the retina, rods and cones, which indeed are the special terminations of the optic nerve-fibres. Duration of Visual Sensations.-The duration of the sensation produced by a luminous impression on the retina is always greater than that of the impression which produces it. However brief the luminous impression, the effect on the retina always lasts for about one-eighth of a second. Thus, supposing an object in motion, say a horse, to be revealed on a dark night by a flash of lightning. The object would be seen apparently for an eighth of a second, but it would not appear in motion ; because, although the image remained on the retina for this time, it was really revealed for such an extremely short period (a flash of lightning being almost instantaneous) that no appreciable movement on the part of the object could have taken place in the period during which it was revealed to the retina of the observer. And the same fact is proved in a reverse way. The spokes of a rapidly revolving wheel are not seen as distinct objects, because at every point of the field of vision over which the revolving spokes pass, a given impression has not faded before another comes to replace it. Thus every part of the interior of the wheel appears occupied. These after-sensations are called after-images. They are of CH. XXXI.] THE OPHTHALMOSCOPE. 357 two kinds, positive and negative. Positive after-images are those which resemble the original image in distribution of light and shade, and colour. Negative after-images which occur after strong, and especially after prolonged, excitation of the retina, are those in which the light parts appear dark, the dark parts light, and the coloured parts of the opposite or contrast colour. Hence the image of a bright object, as of the panes of a window through which the light is shining, may be perceived in the retina for a considerable period, if we have previously kept our eyes fixed for some time on it. But the image in this case is negative. If, however, after shutting the eyes for some time, we open them and look at an object for an instant, and again close them, the after-image is positive. The Ophthalmoscope. Every one is perfectly familiar with the fact, that it is quite impossible to see the fundus or back of another person's eye by simply looking into it. The interior of the eye forms a perfectly black background.* The same remark applies to an ordinary photographic camera, and may be illustrated by the difficulty we experience in seeing into a room from the street through the window unless the room be lighted within. In the case of the eye this fact is partly due to the feebleness of the light reflected from the retina, most of it being absorbed by the retinal pigment; but far more to the fact that every such ray is reflected straight to the source of light (e.g., candle), and cannot, therefore, be seen by the unaided eye without intercepting the incident light from the candle, as well as the reflected rays from the retina. This difficulty is surmounted by the use of the ophthalmoscope. The ophthalmoscope was invented by Helmholtz ; as a mirror for reflecting the light into the eye, he employed a bundle of thin glass plates ; this mirror was transparent, and so he was able to look through it in the same direction as that of the rays of the light it reflected. It is almost impossible to over-estimate the boon this instrument has been to mankind ; previous to this * In some animals (e.g., the cat), the pigment is absent from a portion of the retinal epithelium ; this forms the Tapetum lueidum. The use of this is supposed to be to increase the sensitiveness of the retina, the light being reflected back through the layer of rods and cones. It is certainly the case that these animals are able to see clearly with less light than we can, hence the popular idea that a cat can see in the dark. In fishes a tapetum lueidum is often present; here the brightness is increased by crystals of guanine. 358 THE EYE AND VISION. [ch. xxxi. in the examination of cases of eye disease, the principal evidence on which the surgeon had to rely was that derived from the patient's sensations; now he can look for himself. The instrument, however, has been greatly modified since Helmholtz' time; the principal modification being the substitu- tion of a concave mirror of silvered glass for the bundle of glass plates; this is mounted on a handle, and is perforated in the centre by a small hole through which the observer can look. The methods of examining the eye with this instrument are-the direct and the indirect: both methods of investigation should be employed. A drop of a solution of atropine (two grains to the ounce) or of homatropine hydrobromate, should be instilled about twenty minutes before the examination is commenced ; the ciliary muscle is thereby paralysed, the Fig. 332.-Diagram to illustrate the action of the Ophthalmoscope, when a plane concave glass is used, c, observer's eye. The light reflected from any point, d, on retina of a, would naturally be focussed at e; if the lens b is used it would be focussed at i, in other words, at back of c. The image would be enlarged, as though of g, and would be inverted. The perforated concave mirror through which c is looking is not shown. (After McGregor Robertson.) power of accommodation is abolished, and the pupil is dilated. This will materially facilitate the examination ; but it is quite possible to observe all the details to be presently described without the use of this drug. The room being now darkened, the observer seats himself in front of the person whose eye he is about to examine, placing himself upon a somewhat higher level. A brilliant and steady light is placed close to the left ear of the patient. The atropine having been put into the right eye only of the patient this eye is examined. Taking the mirror in his right hand, and looking through the central hole, the operator directs a beam of light into the eye Fig. 333.-Diagram to illustrate action of ophthalmoscope when a bi-convex glass is used. The fig. d on retina of a is under ordinary conditions focussed at f and inverted. If the lens b be placed between the eyes, the image h is seen by the eye c as an enlarged image. The perforated concave minor through which c is ..looking is not shown. (After McGregor Robertson.) CH. XXXI.] THE OPHTHALMOSCOPE. 359 of the patient. A red glare, known as the reflex, is seen ; it is due to the illumination of the retina. The patient is then told to look at the little finger of the observer's right hand as he holds the mirror ; to effect this the eye is rotated somewhat inwards, and at the same time the reflex changes from red to a lighter colour, owing to the reflection from the optic disc. The observer now approximates the mirror, and with his eye to the eye of the patient, taking care to keep the light fixed upon the pupil, so as not to lose the reflex. At a certain point, which varies with different eyes, but is usually when there is an interval of about two or three inches between the observed and the observing eye, the vessels of the retina will become visible, Examine carefully the fundus of the eye, i.e., the red surface-until the optic disc is seen ; trace its circular outline, and observe the small central white spot, the porus opticus, or physiological pit: near the centre is the central artery of the retina breaking up upon the disc into branches ; veins also are present, and correspond roughly to the course of the arteries. Trace the vessels over the disc on to the retina. Somewhat to the outer side, and only visible after some practice, is the yelloio spot, with the smaller lighter- coloured fovea centralis in its centre. This constitutes the direct method of examination (fig. 332) ; by it the various details of the fundus are seen as they really exist, and it is this method which should be adopted for ordinary use. If the observer is myopic or hyperme- tropic, he will be unable to employ the direct method of examination until he has remedied his defective vision by the use of proper glasses. In the indirect method (fig. 333) the patient is placed as before, and the operator holds the mirror in his right hand at a dis- tance of twelve to eighteen inches from the patient's right eye. At the same time he rests his left little finger lightly upon the right temple, and holding a convex lens be- tween his thumb and forefinger,two or three inches in front of the patient's eye, directs the light through the lens into the eye. The red reflex, and subsequently the white one, having been gained, the operator slowly moves his mirror, and with it his eye, towards or away from the face of the patient, until the outline of one of the retinal vessels becomes visible, when very slight movements on the part of the ope- rator will suffice to bring into view the details of the fundus above described, but the image will be much smaller and in- erte d. The appearances seen are de- picted in fig. 318. The lens should be kept fixed at a distance of two or three inches, the mirror being alone moved until the disc becomes visible : should the image of the mirror, however, obscure the disc, the lens may be slightly tilted. Kg. 334.-The ophthalmoscope. The small upper mirror is for direct the larger for indirect illumina- tion. The Perimeter This is an instrument for mapping out the field of vision. It 360 THE EYE AND VISION. [ch. xxxi. consists of a graduated arc, which can be moved into any position, and thus when rotated it traces out a hollow hemisphere. In the centre of this the eye under examination is placed, the other eye being closed. The examiner then determines on the surface of the hemisphere those points at which the patient just ceases or just begins to See a small object moved along the arc of the circle. These points are plotted out on a chart graduated in degrees, and by connecting them the outline of the field of vision is obtained. The next figure shows one of the forms of perimeter very generally employed, and fig. 336 represents one of the charts provided with the instrument. The blind spot is shown, and the dotted line represents the normal average field of vision for the right eye. It will be seen that the field of vision is most extensive on the outer side; it is less on the inner side because of the presence of the nose. By the use of the same instrument, it is found that the colour of a coloured object is not distinguishable at the margin, but only towards the centre of the field of vision, but there are differences for different colours ; thus a blue object is seen over a wider field than a red, and a red over a wider field than a green object. In disease of the optic nerve, contraction of the field of vision for white and coloured objects is found. This is often seen before any change in the optic nerve is discoverable by the ophthalmoscope. The Fovea Centralis. This is the region of most acute vision ; when we want to see an object distinctly we look straight at it. It is also the region where the colours of objects are best distinguishable. It is, however, stated to be less sensitive from one point of view than the zone immediately surrounding it; that is to say, the minimum intensity of white light which will cause an impression is some what greater. But with this exception, the sensibility of the retina diminishes steadily from centre to circumference. The yellow spot of one's own eye can be rendered evident by what is called Clerk-Maxwell's experiment :-on looking through a solution of chrome-alum in a bottle with parallel sides, an oval purplish spot is seen in the green colour of the alum. This is due to the pigment of the yellow spot. Colour Sensations. If a ray of sunlight be allowed to pass through a prism, ch. xxxi. ] THE PERIMETER. 361 Fig. 335.-Priestley Smith's Perimeter. Fig. 336.-Perimeter chart. 362 THE EYE AND VISION. [CH. xxxi. it is decomposed by its passage into rays of different colours, which are called the colours of the spectrum j they are red, orange, yellow, green, blue, indigo, and violet. The red rays are the least turned out of their course by the prism, and the violet the most, whilst the other colours occupy in order places between these two extremes. The differences in the colour of the rays depend upon the number of vibrations producing each, the red rays being the least rapid and the violet the most. In addition to the coloured rays of the spectrum, there are others which are invisible but which have definite properties, those to the left of the red, and less refrangible, being the calorific rays which act upon the thermometer, and those to the right of the violet, which are called the actinic or chemical rays, have a powerful chemical action. White light may be built from its constituents either physically, as by a second prism reversing the dispersion produced by the first, or physiologically by causing the colours of the spectrum to fall on the retina in rapid succession. The best way to study the effects of mixing colour sensations is by means of a rapidly revolving disc to which two or more coloured sectors are fixed. Each colour is viewed in rapid succession, and owing to the persistence of retinal impressions, the two or more constituent colour impressions blend and give a single compound colour (Maxwell). It is found that white light can be produced by the mixture of three primary colours, or even of two colours in certain propor- tions. These pairs of colours, which are roughly red and green, orange and blue, and violet and yellow, are called complementary. The colours are not of equal stimulation energy, otherwise thev might be arranged around a circle; they are more properly arranged in a triangle, with red, green, and violet at the angles. The red, green, and violet are selected on the theory of Helmholtz that they constitute the three primary colour sensations ; other colours being mixtures of these. Thus, the orange and yellow between the red and green are mixtures of the red and green sensations; the blue a mixture of Fig. 337.-Colour triangle. CH. XXXI. ] COLOUR VISION. 363 green and violet; and the purples (which are not represented in the spectrum) of red and violet. Join the three angles red, green, and violet, and one gets white light; or join the blue and orange, which comes to the same thing, and one also gets white. Blue and orange on Maxwell's disc give white ; but it is well known that a mixture of blue and orange paint gives green ; how can one explain this 1 Suppose the paint is laid on white paper ; the white light from the paper on its way to the eye passes through transparent particles of blue and orange pigment; the blue particles only let the green and violet sensations reach the eye, and cut off the red ; the yellow particles only let the red and green through, and cut off the violet. The red and violet being thus cut off, the green sensation is the only one which reaches the eye. The experiments which led Helmholtz and others to the selec- tion of green, red, and violet as the three fundamental colour sensations were performed in this way : the eye under- goes exhaustion to a colour when exposed to it for some time; suppose, for instance, the eye is fatigued for red, and is then exposed to a pure yellow light, such as that given off by the sodium flame, the yellow then appears greenish; or fatigue the eye for green and then expose it to blue, the blue will have a violet tint. By the repetition of numerous experiments of this kind, it was found that the fatigue experienced manifested itself in three colours, red, green, and violet, which were therefore selected as the three fundamental colour sensations. The theory of colour vision constructed on these data was originated by Young, and independently discovered and elaborated by Helmholtz. It is consequently known as the Young-Helmholtz theory. This theory teaches that there are in the retina rods or cones which answer to each of these primary colours, whereas the innumerable intermediate shades of colour are produced by stimulation of the three primary colour terminals in different degrees, the sensation of white being produced when the three elements are equally excited. Thus if the retina is stimu- lated by rays of certain wave length, at the red end of the Fig. 338.-Diagram of the three primary colour sensations. (Young-Helmholtz theory.) 1, is the red ; 2, green, and 3, violet, primary colour sensations. The lettering indicates the colours of the spectrum. The diagram indicates by the height of the curve to what extent the several primary sensations of colour are ex- cited by vibrations of different wave lengths. 364 THE EYE AND VISION. [ch. xxxi. spectrum, the terminals of the other colours, green and violet, are hardly stimulated at all, but the red terminals are strongly stimulated, the resulting sensation being red. The orange rays excite the red terminals considerably, the green rather more, and the violet slightly, the resulting sensation being that of orange, and so on (fig. 338). Another theory of colour (Hering's) supposes that there are six primary colour sensations, viz.:-three pairs of antagonistic or complemental colours, black and white, red and green, and yellow and blue; and that these are produced by the changes either of disintegration or of assimilation taking place in certain substances, somewhat it may be supposed of the nature of the visual purple, which (the theory supposes to) exist in the retina. Each of the substances corresponding to a pair of colours, is capable of undergoing two changes, one of construction and the other of disintegration, with the result of producing one or other colour. For instance, in the white-black substance, when disintegration is in excess of construction or assimilation, the sensation is white, and when assimilation is in excess of disintegration the reverse is the case; and similarly with the red-green substance, and with the yellow-blue substance. When the repair and disintegration are equal with the first substance, the visual sensation is grey; but in the other pairs when this is the case, no sensation occurs. The rays of the spectrum to the left produce changes in the red- green substance only, with a resulting sensation of red, whilst the (orange) rays further to the right affect both the red-green and the yellow-blue substances; blue rays cause constructive changes in the yellow-blue substances, but none in the red-green and so on. These changes produced in the visual substances in the retina are perceived by the brain as sensations of colour. Neither theory satisfactorily accounts for all the numerous complicated problems presented in the physiology of colour vision. One of these problems is colour blindness or Daltonism, a by no means uncommon visual defect. One of the commonest forms is the inability to distinguish between red and green. Helmholtz's explanation of such a condition is, that the elements of the retina which receive the impression of red, etc., are absent, or very imperfectly developed, and Hering's would be that the red-green substance is absent from the retina. Other varieties of colour- blindness in which the other colour-perceiving elements are absent have been shown to exist occasionally. Hering's theory appears to meet the difficulty best, for if the red element of Helmholtz were absent, the patient ought not to be able to perceive white sensations, of which red is a constituent CH. XXXI.] RETINAL PIGMENTS. 365 part: whereas, according to Hering's theory, the white-black visual substance remains intact. Pigments of the Retina. The method by which a ray of light is able to stimulate the endings of the optic nerve in the retina in such a manner that a visual sensation is perceived by the cerebrum is not yet understood. It is supposed that the change effected by the agency of the light which falls upon the retina is in fact a chemical alteration in the protoplasm, and that this change stimulates the optic nerve-endings. The discovery of a certain temporary reddish-purple pigmentation of the outer limbs of the retinal rods in certain animals (e.g., frogs) which had been killed in the dark, forming the so-called rhodopsin or visual purple, appeared likely to offer some explanation of the matter, especially as it was also found that the pigmentation disappeared when the retina was exposed to light, and reappeared when the light was removed, and also that it underwent distinct changes of colour when other than white light was used. It was also found that if the operation were performed quickly enough, the image of an object (optogram might be fixed in the pigment on the retina by soaking the retina of an animal which has been killed in the dark, in alum solution. The visual purple cannot however be absolutely essential to the due production of visual sensations, as it is absent from the retinal cones, and from the macula lutea and fovea centralis of the human retina, and does not appear to exist at all in the retinee of many animals, e.g., bat, dove, and hen, which are, nevertheless, possessed of good vision. However the fact remains that light falling upon the retina (а) bleaches the visual purple, and this must be considered as one of its effects. If it produces chemical changes in other substances, these must be colourless and so extremely difficult to discover. The rhodopsin is derived in some way from the black pigment (melanin or fuscin) of the polygonal epithelium of the retina, since the colour is not renewed after bleaching if the retina be detached from its pigment layer. Certain pigments, not sensitive to light, are contained in the inner segments of the cones. These coloured bodies are oil globules of various colours, red, green, and yellow, called chromophanes, and are found in the retinas of marsupials (but not other mammals), birds, reptiles, and fishes. Practically nothing is known about the yellow pigment of the yellow spot. (б) The second change produced by the action of the light upon the retina is the movement of the pigment cells. On being 366 THE EYE AND VISION. [CH. XXXI. stimulated by light the granules of pigment in the cells which overlie the outer part of the rod and cone layer of the retina become diffused in the parts of the cells between the rods and cones, the melanin or fuscin granules (which are generally rod- shaped, looking almost like crystals), passing down into the processes of the cells, (c) A movement of the cones and possibly of the rods occurs, as has been already incidentally mentioned ; in the light the cones shorten and in the dark they lengthen (Engelmann). According to the careful researches of Dewar and McKendrick, and of Holmgren, it appears that the stimulus of light is able to produce (tZ) a variation of the natural electrical currents of the retina. The current is at first increased and then diminished ; this is the electrical expression of those chemical changes in the retina of which we have already spoken. Protrusion of the eyeballs occurs (i) when the blood-vessels of the orbit are congested; (2) when contraction of the plain muscular fibres of the capsule of Tenon takes place; these are innervated by the cervical sympathetic nerve; and (3) in the disease called exophthalmic goitre. Retraction occurs (1) when the lids are closed forcibly; (2) when the blood-vessels, of the orbit are comparatively empty ; (3) when the fat in the orbit is reduced in quantity as during starvation; and (4) on section or paralysis of the cervical sympathetic nerves. The most important movements, however, are those produced by the six ocular muscles. Movements of the Eyeball. The eyeball possesses movement around three axes indicated in fig. 339 viz. an antero-posterior, a vertical, and a transverse, passing through a centre of rotation a little behind the centre of the optic axis. The move- ments are accomplished by pairs of muscles. Direction of movement. By what muscles accomplished. InwardsInternal rectus. OutwardsExternal rectus. Superior rectus. Inferior oblique,. Upwards Inferior rectus. Superior oblique. Downwards Inwards and upwards Internal and superior rectus. Inferior oblique. Inwards and downwards . . . Internal and inferior rectus. Superior oblique. Outwards and upwards . External and superior rectus. Inferior oblique. Outwards and downwards . . . External and inferior rectus, Superior oblique. CH. XXXI.] MUSCLES OF THE EYEBALL. 367 These muscles are all supplied by the third nerve except the superior oblique which is supplied by the fourth and the external rectus by the sixth nerve. The muscles of the two eyes act simultaneously, so that images of the objects looked at may fall on corresponding points of the two retinae. The inner side of one retina corresponds to the outer side of the other, so that any movement of one eye inwards must be accompanied by a movement of the other eye outwards. Fig- 339-- Diagram of the axes of rotation to the eye. The thin lines indicate axes of rotation, the thick the position of muscular attachment. If one eyeball is forcibly fixed by pressing the finger against it so that it cannot follow the movement of the other, the result is double vision {diplopia), because the image of the objects looked at will fall on points of the two retina) which do not correspond. The same is experienced in a squint, until the brain learns to disregard the image from one eye. If the external rectus is paralysed, the eye will squint inwards; if this occurs in the right eye the false image will lie on the right side of the yellow spot, and appear in the field of vision to the left of the true image. If the third nerve is paralysed, the case is a more complicated one : owing to the paralysis of the levator palpebrae superioris, the patient will be unable to raise his upper lid (ptosis), and so in order to see will walk with his chin in the 368 THE EYE AND VISION. [.CH. XXXI. air. If the paralysis is on the right side, the eyeball will squint downwards and to the right; the false image will be formed below and to the right of the yellow spot, and the apparent image in the field of vision will consequently appear above and to the left of the true image, and owing to the squint being an oblique one, the false image will slant in a corresponding direction. Various Positions of the Eyeballs. All the movements of the eyeball take place around the point of rotation, which is situated i "7 7 mm. behind the centre of the visual axis, or 10'9 mm. behind the point of the cornea. The three axes around which the movements occur are :- 1. The visual or antero-posterior axis. 2. The transverse axis, which connects the points of rotation of the two eyes. 3. The vertical axis, which passes at right angles through the point of intersection of the other two axes. The line which connects the fixed point in the outer world at which the eye is looking to the point of rotation is called the visual line. The plane which passes through the two visual lines is called the visual plane. The various positions of the eyeballs are designated primary secondary, and tertiary. The primary position occurs when both eyes are parallel, the visual lines being horizontal. Secondary positions are of two kinds :- (1) The visual lines are parallel but directed either upwards or downwards from the horizontal. (2) The visual lines are horizontal, but converge towards one another. Tertiary positions are those in which the visual lines are not horizontal, and converge towards one another. Both eyes are moved simultaneously, even if one of them happens to be blind. They are moved so that the object in the outer world is focussed on the two yellow spots, or other corre- sponding points of the two retina). The images which do not fall on corresponding points are seen double, but these are disregarded by the brain, which only pays attention to those images which fall on corresponding points. The following diagrams will assist us in understanding more fully what is meant by corresponding or identical points of the two retina). If R and L (fig. 340) represent the right and left retina) ch. xxxi.] THE HOROPTER. 369 respectively, 0 and O' the two yellow spots are identical; so are A and A', both being the same distance above 0 and O'. But the corresponding point to B on the inner side of 0 in the right retina, is B', a point to the same distance on the outer side of 0 in the left retina; similarly C and C' are identical. The two blind spots X and X' are not identical. Fig. 341 shows the same thing in rather a different way ; Fig. 340.-Identical points of the retinee, A and B represent a horizontal section through the two retinae ; the points a a', b b', and c c', being identical. In the lower part of the diagram is shown the way in which the brain combines the images in the two retinae, one as it were overlapping so as to coincide with the other. The Horopter is the name given to the surface in the outer world which contains all the points which fall on the identical points of the retinae. The shape of the horopter will vary with the position of the eye- balls. In the primary position, and in the first variety of the secondary position, the visual lines are parallel ; hence the horopter will be a plane at infinity, or at a great distance. In the other variety of the secondary position, and in tertiary positions in which the visual lines converge as when looking at a near object, the horopter is a circle which passes through the nodal points of the two eyes, and through the fixed point (I) in the outer world at which the eye is looking, and which will con- sequently fall on the two yellow spots (O,and Oz). All other points in this circle (II, III) will fall on identical points of the two retinae. The image of II will fall on A and A'; of III on B, and Bz; it is Fig. 341.-Diagram to show the correspond- ing parts of both retina. 370 THE EYE AND VISION. [CH. XXXI. a very simple mathematical problem to prove that OA = O'A', and OB = O'B'. This, however, applies to man only, or to animals with both eyes in front of the head ; in the horse, where the eyes are more lateral in position, and the visual lines diverge, the horopter will be very different. Nervous Paths in the Optic Nerves. The correspondence of the two retinae and of the movements of the eyeballs is produced by a close connection of the centres controlling these phenomena, and by the arrangement of the nerve-fibres in the optic nerves. The crossing of the nerve- fibres at the optic chiasma is incomplete, and the following dia- gram (fig. 343) gives a simple idea of the way the fibres go. It will be seen that it is only the fibres from the inner portions Left Retina Right Retina Hemisphere Hemisphere Fig. 342.-The Horopter, when the eyes are convergent. Fig. 343.-Course of fibres at optic chiasma. of the retinae that cross; and that those represented by con- tinuous lines from the right side of the two retinae ultimately reach the right hemisphere, and those represented by interrupted lines from the left side of the two retinae ultimately reach the left hemisphere. The two halves of the retinae are not, however, separated by a hard-and-fast line from one another; this is repre- sented by the two halves being depicted as slightly overlapping, and this comes to the same thing as saying that the central region of each retina is represented in each hemisphere. The finely dotted lines representing fibres connecting the two retinae and the two hemispheres are problematical. CH. xxxi.] THE OPTIC NERVE-FIBRES. 371 The part of the hemisphere concerned in vision is the occipital lobe, and the reader should turn back to our previous considera- tion of this subject in connection with cerebral localisation, the phenomena of hemianopsia (p. 266), and the conjugate devia- tion of head and eyes (pp. 266, 269). The following illustration, though only diagrammatic, will assist the reader in more fully Fig-. 344.-Relations of nerve cells and fibres of visual apparatus. (After Schtifer.) comprehending the paths of visual impulses, and the central con- nections of the nerves and nerve-centres concerned in the pro- cess. The fibres to the lateral geniculate body end there by arborising around its cells, and a fresh relay of fibres from these cells passes to the cortex of the occipital lobe. Those to the ante- rior corpus quadrigeminum are continued on by a fresh relay to the nucleus of the third nerve, the cells of which are also sur- rounded by arborisations of the axis cylinder processes of the cortical cells. 372 THE EYE AND VISION. [ch. XXXI. Visual Judgments. The psychical or mental processes which constitute the visual sensation proper have been studied to a far greater degree than is possible in connection with other forms of sensation. We have already seen that in spite of the reversion of the image in the retina, the mind sees objects in their proper posi- tion, the sense of sight being here educated in great measure by that of touch. We are also not conscious of the blind spot. This is partly due to the fact that those images which fall on the blind spot of one eye are not focussed there in the other eye. But even when one looks at objects with one eye, there is no blank, the area corresponding to the blind spot being closed up by a mental process. Our estimate of the size of various objects is based partly on the visual angle under which they are seen, but much more on the estimate we form of their distance. Thus a lofty mountain many miles off may be seen under the same visual angle as a small hill near at hand, but we infer that the former is much the larger object because we know it is much further off than the hill. Our estimate of distance is often erro- neous, and consequently the estimate of size also. Thus persons seen walking on the top of a small hill against a clear twilight sky appear unusually large, because we over-estimate their dis- tance, and for similar reasons most objects in a fog appear immensely magnified. The same mental process gives rise to the idea of depth in the field of vision ; this idea being fixed in our mind principally by the circumstance that, as we ourselves move forwards, different images in succession become depicted on our retina, so that we seem to pass between these images, which to the mind is the same thing as passing between the objects themselves. The action of the sense of vision in relation to external objects is, therefore, quite different from that of the sense of touch. The objects of the latter sense are immediately present to it; and our own body, with which they come in contact, is the measure of their size. The part of a table touched by the hand appears as large as the part of the hand receiving an impression from it, for the part of our body in which a sensation is excited, is here the measure by which we judge of the magnitude of the object. In the sense of vision, on the contrary, the images of objects are mere fractions of the objects themselves, realised upon the retina, the extent of which remains constantly the same. But the CH. XXXI.] VISUAL JUDGMENTS. 373 imagination, which analyses the sensations of vision, invests the images of objects, together with the whole field of vision in the retina, with very varying dimensions; the relative size of the image in proportion to the whole field of vision, or of the affected parts of the retina to the whole retina, alone remaining unaltered. The estimation of the form of bodies by sight is the result partly of the mere sensation, and partly of the association of ideas. Since the form of the images perceived by the retina depends wholly on the outline of the part of the retina affected, the sensation alone is adequate to the distinction of only super- ficial forms from each other, as of a square from a circle. But the idea of a solid body like a sphere, or a body of three dimensions, like a cube, can only be attained by the action of the mind constructing it from the different superficial images seen in different positions of the eye with regard to the object, and, as shown by Wheatstone and illustrated in the stereoscope, Fig. 345.-Diagrams to illustrate how a judgment of a figure of three dimensions is obtained from two different perspective projections of the body being presented simultaneously to the mind by the two eyes. Hence, when, in adult age, sight is suddenly restored to persons blind from infancy, all objects in the field of vision appear at first as if painted flat on one surface; and no idea of solidity is formed until after long exercise of the sense of vision combined with that of touch. Thus, if a cube, for example, be held at a moderate distance before the eyes, and viewed with each eye successively while the head is kept perfectly steady, a (fig. 345) will be the picture presented to the right eye, and b that seen by the left eye. Wheatstone has shown that on this circumstance depends in a great measure our conviction of the solidity of an object, or of its projection in relief. If different perspective drawings of a solid body, one representing the image seen by the right eye, the other that seen by the left (for example, the drawing of a cube, a, b, fig. 345) be presented to corresponding parts of the two retina), as may be readily done by means of the stereoscope, the 374 THE EYE AND VISION. [CH. XXXI. mind will perceive not merely a single representation of the object, but a body projecting in relief, the exact counterpart of that from which the drawings were made. By transposing two stereoscopic pictures a reverse effect is produced ; the elevated parts appear to be depressed, and vice versd. An instrument contrived with this purpose is termed a pseudoscope. Viewed with this instrument a bust appears as a hollow mask, and as may readily be imagined the effect is most bewildering. The clearness with which an object is perceived irrespective of accommodation, would appear to depend largely on the number of rods and cones which its retinal image covers. Hence the nearer an object is to the eye (within moderate limits) the more Fig. 346.-Diagram to illustrate visual illusions. clearly are all its details seen. Moreover, if we want carefully to examine any object, we always direct the eyes straight to it, so that its image shall fall .on the yellow spot where an image of a given area will cover a larger number of cones than anywhere else in the retina. It has been found that the images of two points Fig. 347.-Parallel puzzle. must be at least 3 p apart on the yellow spot in order to be distinguished separately; if the images arc nearer together, the points appear as one. The diameter of each cone in this part of the retina is about 3 p. CH. XXXII.] TROPHIC NERVES. 375 Visual judgments are not always correct; there are a large number of puzzles and toys which depend on visual illusions. Two of the best known are represented in the preceding diagrams. In fig. 346, a, b, and c are of the same size; but a looks taller than b, while c appears to cover a less area than either. In fig. 347, the horizontal lines are parallel, though they do not appear so, owing to the mind being distracted by the inter- crossing lines. CHAPTER XXXII. TROPHIC NERVES. Nerves exercise a trophic or nutritive influence over the tissues and organs they supply. The chemical changes that occur during the nutrition of a living cell may be summed up in the word metabolism; and this includes two kinds of changes; anabolic phenomena, that is the process of building up protoplasm from food material; and katabolic phenomena, those in which there is a breaking down of protoplasm, and a consequent formation of simpler waste products. Some nerves increase the building-up stage of metabolism; these are termed anabolic. Such a nerve is the vagus in reference to the heart; when it is stimulated the heart beats more slowly or may stop, and is thus enabled to rest and repair its waste. The opposite kind of nerves (katabolic), are those which lead to increase of work and so increased wear and'tear and formation of waste products. Such a nerve in reference to the heart is the sympathetic. There has been considerable diversity of opinion as to whether trophic nerve-fibres are a distinct anatomical set of nerve-fibres, or whether all nerves in addition to their other functions exercise a trophic influence. When a nerve going to an organ is cut, the wasting of the nerve itself beyond the cut constitutes what we have learnt to call Wallerian degeneration, but the wasting process continues beyond the nerve, the muscles it supplies waste also, and waste much more rapidly than can be explained by simple disuse. The same is seen in the testicle after section of the spermatic cord ; and in the disease of joints called Charcot's disease, the trophic 376 TROPHIC NERVES. [ch. xxxii changes are to be explained by disease of the nerves supplying them. From these, and numerous other instances that might be given, there is no question that nerves do exert a trophic influence; the question, however, whether this is due to special nerve-fibres has been chiefly worked out in connection with the fifth cranial nerve. After the division of this nerve there is loss of sensation in the corresponding side of the face : the cornea in two or three days begins to get opaque, and this is followed by a slow inflammatory process which may lead to a destruction not only of the cornea, but of the whole eyeball. The same is seen in man ; when the fifth nerve is diseased or pressed upon by a tumour beyond the Gasserian ganglion the result is loss of sensation in the face and conjunctiva, an eruption (herpes) appears on the face, and ulceration of the cornea leading in time to disintegration of the eyeball may occur too. In disease of the spinal ganglia there is a similar herpetic eruption on the skin (shingles). In the case of the fifth nerve the evidence that there are special nerve fibres to which these trophic changes are due, is an experiment by Meissner and Buttner, who found that division of the most internal fibres is most potent in producing them. Those, however, who do not believe in special trophic nerves, attribute the changes in the eyeball to its loss of sensation. Dust, etc., is not felt by the cornea, it is therefore allowed to accumulate and set up inflammation. This is supported by the fact that if the eyeball is protected by sewing the eyelids together the trophic results do not ensue. On the other hand, in paralysis of the seventh nerve, the eyeball is much more exposed, and yet no trophic results follow. Others have attributed the change to increased vascularity due to disordered vaso-motor changes : but this is negatived by the fact that in disease of the cervical sympathetic, the disordered vaso-motor phenomena which ensue do not lead to the disorders of nutrition we have described. There can, therefore, be but little doubt that we have to deal with the trophic influence of nerves ; * but the dust, etc., which falls on the cornea must be regarded as the exciting cause of the ulceration. The division or disease of the nerve acts as the predisposing cause. The eyeball is more than usually prone to undergo inflammatory changes, with very small provocation. * The proof, however, that there are distinct nerve-fibres anatomically is not very conclusive. CH. XXXIII.'] THE CIRCULATORY SYSTEM. 377 Exactly the same explanation holds in the case of the influence of the vagi on the lungs. If both these nerves are divided in an animal, it usually dies within a week or a fortnight from a form of pneumonia called vagus pneumonia, in which gangrene of the lung substance is a marked characteristic. Here the predisposing cause is the division of the trophic fibres in the vagi nerves; the exciting cause is the entrance of particles of food into the air passages, which on account of the loss of sensation in the larynx and neighbouring parts are not coughed up. Another trophic result that follows division of the vagi is fatty degeneration of the heart. We will conclude by giving one more instance of trophic dis- turbance due to nervous disease, and this is the case of bed-sores. Many bed-sores are due to prolonged confinement in bed with bad nursing. These sores are of slow onset. But there is one class of bed-sores seen especially in cases of paralysis of spinal origin which are acute; they come on in three to five days after the onset of the paralysis in spite of the most careful attention; they cannot be explained by vaso-motor disturbance nor by loss of sensation ; and, in fact, there is no doubt that they are of trophic origin. The nutrition of the skin is so greatly impaired that the mere contact of it with the bed for a few days is sufficient to act as the exciting cause of the sore. CHAPTER XXXIII. THE CIRCULATORY SYSTEM. The circulatory system is the collection of organs which have for their function, the circulation of the blood. It consists of the heart, the vessels (arteries) that carry the blood from the heart to other parts of the body, the veins or vessels that carry the blood back to the heart again, and the capillaries, a network of minute tubes which connect the terminations of the smallest arteries to the commencements of the smallest veins. We shall also have to consider in connection with the circulatory system, (i) the lymphatics, which are vessels that convey back the lymph (the fluid which exudes through the thin walls of the blood-capillaries) to the large veins near to their entrance into the heart, and (2) the large lymph spaces contained in the serous membranes. 378 THE CIRCULATORY SYSTEM. [CH. XXXIII. The Heart. This is the great central pump of the circulatory system. It is contained in the chest or thorax. It lies between the right and left lungs (fig. 348), and is enclosed in a covering called the pericardium. The pericardium is an instance of a serous membrane. Like all serous membranes it consists of two layers; each consists of fibrous tissue containing a large amount of elastic fibres ; one layer envelopes the heart and forms its outer covering Larynx. Trachea.. Aorta. . Right Lung. Pulmonary Artery. I Left Lung. Diaphragm. Fig'. 348.-View of heart and lungs in situ. The front portion of the chest-wall and the outer or parietal layers of the pleurae and pericardium have been removed. The lungs are partly collapsed. or epicardium ; this is the visceral layer of the pericardium ; the other layer of the pericardium, called its parietal layer, is situated at some little distance from the heart, being attached below to the diaphragm, the partition between the thorax and the abdomen. The visceral and parietal layers are continuous for a short distance along the great vessels at the base of the heart, and so form a closed sac. This sac is lined by endothelium; in health it contains just enough lymph to lubricate the two surfaces and enable them to glide over each other smoothly during the movements of the heart. The presence of elastic fibres in the membrane enables the epicardium to follow without hindrance the changing shape of the heart itself. CH. XXXIII. J THE HEART. 379 The pericardium is a comparatively simple serous membrane, because the organ it encloses is a single one of simple external form. All serous membranes are of similar structure : thus the pleura which encloses the lung, and the peritoneum which encloses the abdominal viscera differ from it only in anatomical arrangement. The great complexity of the peritoneum is due to its enclosing so many organs. Each consists of a visceral layer applied to the organ or organs it encloses ; and a parietal layer continuous with this in contiguity with the parietes or body-walls. The Chambers of the Heart.-The interior of the heart is divided by a longitudinal partition in such a manner as to form two chief chambers or cavities-right and left. Each of these chambers is again subdivided transversely into an upper and a lower portion, called respectively, auricle and ventricle, which freely communicate one with the other ; the aperture of communi- cation, however, is guarded by valves, so disposed as to allow blood to pass freely from the auricle into the ventricle, but not in the opposite direction. There are thus four cavities in the heart -the auricle and ventricle of one side being quite separate from those of the other (figs. 349, 350). Hight Auricle.-The right auricle is situated at the right part of the base of the heart as viewed from the front. It is a thin walled cavity of more or less quadrilateral shape, prolonged at one corner into a tongue-shaped portion, the right auricular appendix, which slightly overlaps the exit of the great artery, the aorta, from the heart. The interior is smooth, being lined with the general lining of the heart, the endocardium, and into it open the superior and inferior venae cavse, or great veins, which convey the blood from all parts of the body to the heart. The opening of the inferior cava is protected and partly covered by a membrane called the Eustachian valve. In the posterior wall of the auricle is a slight depression called the fossa ovalis, which corresponds to an open- ing between the right and left auricles which exists in foetal life. The coronary sinus, or the dilated portion of the right coronary vein, also opens into this chamber. Right Ventricle.-The right ventricle occupies the chief part of the anterior surface of the heart, as well as a small part of the posterior surface ; it forms the right margin of the heart. It takes no part in the formation of the apex. On section its cavity, in consequence of the encroachment upon it of the septum ventri- culorum, is semilunar or crescentic (fig. 351); into it are two openings, the auriculo-ventricular at the base and the opening of the pulmonary artery also at the base, but more to the left; both orifices are guarded by valves, the former called tricuspid and the latter semilunar. In this ventricle are also the pro- 380 THE CIRCULATORY SYSTEM. [ch. xxxiii. j ections of the muscular tissue called columnce carnece (described at length p. 384). Left Auricle.-The left auricle is situated at the left and posterior Fig1. 349.-The right auricle and ventricle opened, and a part of their right and anterior walls removed, so as to show their interior. J.-1, superior vena cava ; 2, inferior vena cava : 2', hepatic veins cut short; 3, right auricle ; 3', placed in the fossa ovalis, below which is the Eustachian valve ; 3", is placed close to the aperture of the coronary vein; + +, placed in the auriculo-ventricular groove, where a narrow portion of the adjacent walls of the auricle and ventricle has been preserved; 4, 4, cavity of the right ventricle, the upper figure is immediately below the semilunar valves ; 4', large columna carnea or musculus papillaris; 5, 5', 5", tricuspid valve; 6, placed in the interior of the pulmonary artery, a part of the anterior wall of that vessel having been removed, and a narrow portion of it preserved at its commencement, where the semilunar valves are attached; 7, concavity of the aortic arch close to the cord of the ductus arteriosus; 8, ascending part or sinus of the arch covered at its commencement by the auricular appendix and pulmonary artery; 9, placed between the innominate and left carotid arteries; 10, appendix of the left auricle; n, 11, the outside of the left ventricle, the lower figure near the apex. (Allen Thomson.) part of the base of the heart, and is best seen from behind. It is quadrilateral, and receives on either side two pulmonary veins. The auricular appendix is the only part of the auricle seen from CH. XXXIII.] THE HEART. 381 the front, and corresponds with that on the right side, but is thicker, and the interior is smoother. The left auricle is only slightly thicker than the right. The left auriculo-ventricular Fig. 350.-The left auricle and ventricle opened and a part of their anterior and left walls removed, |.-The pulmonary artery has been divided at its commencement; the opening into the left ventricle is carried a short distance into the aorta between two of the segments of the semilunar valves ; and the left part of the auricle with its appendix has been removed. The right auricle is out of view. 1, the two right pul- monary veins cut short; their openings are seen within the auricle; 1', placed within the cavity of the auricle on the left side of the septum and on the part which forms the remains of the valve of the foramen ovale, of which the crescentic fold is seen towards the left hand of r'; 2, a narrow portion of the wall of the auricle and ventricle preserved round the auriculo-ventricular orifice ; 3, 3', the cut surface of the walls of the ventricle, seen to become very much thinner towards 3", at the apex; 4, a small part of the anterior wall of the left ventricle which has been preserved with the principal anterior columna camea or musculus papillaris attached to it; 5,5, musculi papillares ; 5', the left side of the septum, between the two ventricles, within the cavity of the left ventricle; 6, 6', the mitral valve; 7, placed in the interior of the aorta, near its com- mencement and above the three segments of its semilunar valve which are hanging loosely together; 7', the exterior of the great aortic sinus ; 8, the root of the pulmonary artery and its semilunar valves; 8', the separated portion of the pulmonary artery remaining attached to the aorta by 9, the cord of the ductus arteriosus ; 10, the arteries rising from the summit of the aortic arch. (Allen Thomson.) 382 THE CIRCULATORY SYSTEM. [ch. xxxiii. orifice is oval, and a little smaller than that on the right side of the heart. There is a slight vestige of the foramen between the auricles, which exists in foetal life, on the septum between them. Left Ventricle.-Though taking part to a comparatively slight extent in the anterior surface, the left ventricle occupies the chief part of the posterior surface. In it are two openings very close together, viz. the auriculo-ventricular and the aortic, guarded by the valves corresponding to those of the right side of the heart, viz. the bicuspid or mitral and the semilunar. The first opening Cavity of right ventricle Cavity of left ventricle. Fig. 351.-Transverse section of bullock's heart in a state of cadaveric rigidity. (Dalton. is at the left and back part of the base of the ventricle, and the aortic in front and towards the right. In this ventricle, as in the right, are the column® carne®, which are smaller but more closely reticulated. They are chiefly found near the apex and along the posterior wall. They will be again referred to with the description of the valves. The walls of the left ventricle, which are nearly half an inch in thickness, are, with the exception of the apex, twice or three times as thick as those of the right. Capacity of the Chambers.--During life each ventricle is capable of containing about four to six ounces (about 180 grms.) of blood. The capacity of the auricles after death is rather less than that of the ventricles : the thickness of their walls is con- siderably 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 circumstance of the ventricles being partly filled with blood before the auricles contract. Size and. Weight of the Heart.-The heart is about 5 inches long (about 12'6 cm.), inches (8 cm.) greatest width, and inches (6'3 cm.) in its extreme thickness. The average weight of the heart in the adult is from 9 to 10 ounces (about 300 grms.); its weight gradually increases throughout life till middle age ; it diminishes in old age. CH. XXXIII.] THE HEART. 383 Structure.-The walls of the heart are constructed almost entirely of layers of muscular fibres ; but a ring of connective tissue, to which some of the muscular fibres are attached, is inserted between each auricle and ventricle, and forms the Fig. 352.-Network of muscular fibres from the heart of a pig. The nuclei are well shown. X 450. (Klein and Noble Smith.) boundary of the auriculo-ventricular opening. Fibrous tissue also exists at the origins of the pulmonary artery and aorta. The muscular fibres of each auricle are in part continuous with those of the other, and partly separate; and the same remark holds true for the ventricles. The minute structure of the striated muscular fibres of the heart has been already described (p. 93). Endocardium.-As the heart is clothed on the outside by the epicardium, so its cavities are lined by a smooth and shining membrane, or endocardium, which is directly continuous with the internal lining of the arteries and veins. The endocardium is composed of connective tissue with a large admixture of elastic fibres ; and on its inner surface is laid down a single tesselated layer of flattened endothelial cells. Here and there unstriped muscular fibres are sometimes found in the tissue of the endo- cardium. Valves.-The arrangement of the heart's valves is such that the blood can pass only in one direction (fig. 353). The tricuspid valve (5, fig. 349) presents three principal cusps or subdivisions, and the mitral or bicuspid valve has two such portions (6, fig. 350). 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 triangular form. Its base is continuous with the bases of the neighbouring portions, so as to form an annular membrane around the auriculo-ventricular 384 THE CIRCULATORY SYSTEM. [ch. xxxiii. opening, and is fixed to a tendinous ring which encircles the orifice between the auricle and ventricle and receives the inser- tions of the muscular fibres of both. In each principal cusp may- be distinguished a central part, extending from base to apex, and including about half its width. It is thicker and much tougher than the border pieces or edges. While the bases of the cusps of the valves are fixed to the ten- dinous rings, their ventricular surface and borders are fastened by slender tendinous fibres, the chordae, tendinece, to the internal surface Fig. 353--Diagram of the circulation through the heart. (Dalton.) of the walls of the ventricles, the muscular fibres of which project into the ventricular cavity in the form of bundles or columns- the columnce carnece. These columns are not all alike, for while some are attached along their whole length on one side, and by their extremities, others are attached only by their extremities; and a third set, to which the name musculi papillares has been given, are attached to the wall of the ventricle by one extremity only, the other projecting, papilla-like, into the cavity of the ventricle (4, fig. 350), and having attached to it chordae tendineae. Of the tendinous cords, besides those which pass from the walls of the ventricle and the musculi papillares to the margins of the valves, there are some of especial strength, which pass from the same parts to the edges of the middle and thicker portions of the CH. XXXIII.] VALVES OF THE HEART. 385 cusps before referred to. 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 the 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 one between each two principal ones. Moreover, the musculi papillares are so placed that, from the summit of each, tendinous cords proceed to the adjacent halves of two of the principal divisions, and to one inter- mediate or smaller division, of the valve The preceding description applies equally to the mitral and tricuspid valve ; but it should be added that, the mitral is con- siderably thicker and stronger than the tricuspid, in accordance with the greater force which it is called upon to resist. The semilunar valves guard the orifices of the pulmonary artery and of the aorta. They are nearly alike on both sides of the heart; but the aortic valves are altogether thicker and more strongly constructed than the pulmonary valves, in accordance with the greater pressure which they have to withstand. Each valve consists of three parts which are of semilunar shape, the convex margin of each being attached to a fibrous ring at the place of junction of the artery to the ventricle, and the concave or nearly straight border being free, so as to form a little pouch like a watch-pocket (7, fig. 350). In the centre of the free edge of the pouch, which contains a fine cord of fibrous tissue, is a small fibrous nodule, the corpus Arantii, and from this and from the attached border fine fibres extend into every part of the mid substance of the valve, except a small lunated space just within the free edge, on each side of the corpus Arantii. Here the valve is thinnest, and composed of little more than the endocardium. Thus constructed and attached, the three semilunar pouches are placed side by side around the arterial orifice of each ventricle, which can be separated by the blood passing out of the ventricle, but which immediately afterwards are pressed together so as to prevent any return (6, fig. 349, and 7, fig. 350). This will be again referred to. Opposite each of the semilunar cusps, both in the aorta and pulmonary artery, there is a bulging outwards of the wall of the vessel : these bulgings are called the sinuses of Valsalva. Structure.-The valves of the heart are formed of a layer of closely woven connective and elastic tissue, over which, on every part, is reflected the endocardium. 386 THE CIRCULATORY SYSTEM. [-CH. XXXIII. Course of the Circulation. The blood is conveyed away from the left ventricle (as in the diagram, fig. 354) by the aorta to the arteries, and returned to Pulmonary capillaries. i , Pulmonary veins Aorta. Arteries to head and neck. Left auricle. Left ventricle. • Gastric and in- testinal vessels. First renal cir- culation. •Systemic capillaries. Pulmonary' |artery. Superior cava or- vein from head and neck. Right auricle.' Inferior vena?" cava. Right ventricle. Portal circula- tion. Second renal- circulation. the right auricle by the veins, the arteries and veins being con- tinuous with each other at the far end by means of the capillaries. From the right auricle the blood passes to the right ventricle, and then by the pulmonary artery, which divides, into two, one for each lung, then through the pulmonary capillaries, and through the pulmonary veins (two from each lung) to the left auricle. From here it passes into the left ventricle, which brings us back to where we started from. The complete circulation is thus made up of two circuits, the one, a shorter circuit from the right side of the heart to the lungs and back again to the left side of the heart; the other and larger circuit, from the left side of the heart to all parts of the body and back again to the right side. The circulations through the lungs and through the system generally are respectively Fig. 354.-Diagram of the circulation. CH. XXXIII. 1 .f'\_ THE ARTERIES. 387 named the Pulmonary and Systemic, or lesser and greater circulations. It will be noticed also in the same figure that a portion of the stream of blood having been diverted once into the capillaries of the intestinal canal, and some other organs, and gathered up again into a single stream, is a second time divided in its passage through the liver, before it finally reaches the heart and completes a revolution. This subordinate stream through the liver is called the Portal circulation. A somewhat similar accessory circulation is that through the kidneys, called the Renal circulation. The difference of colours in fig. 354 indicates roughly the difference between arterial and venous blood. The blood is oxygenated in the lungs, and the formation of oxyhaemoglobin gives to the blood a bright red colour. This oxygenated or arterial blood (contained in the pulmonary veins, the left side of the heart, and systemic arteries) is in part reduced in the tissues, and the deoxy- genated haemoglobin is darker in tint than the ■oxyhaemoglobin ; this venous blood passes by the systemic veins to the right side of the heart ;and pulmonary artery to the lungs, where it once more receives a fresh supply of oxygen. The Arteries. Distribution.-The arterial system begins at the left ventricle in a single large trunk, the aorta, which almost immediately after its ■origin gives off in the thorax three large branches for the supply of the head, neck, .and upper extremities ; it then traverses the thorax and abdomen, giving off branches, ;some large and some small, for the supply of the various organs and tissues it passes on its way. In the abdomen it divides into two ■chief branches, for the supply of the lower ■extremities. The arterial branches wherever given off divide and subdivide, until the calibre of each subdivision becomes very minute, and these minute vessels pass into capillaries. Arteries ;are, as a rule, placed in situations protected from pressure and •other dangers, and are, with few exceptions, straight in their ■course, and frequently communicate (anastomose or inosculate) "with other arteries. The branches are usually given off at an ;acutc angle, and the area of the branches of an artery generally texcecds that of the parent trunk ; and as the distance from the Fig-. 355.-Minute artery viewed in longitudinal section, e. Nucleated endothelial membrane, with faint nuclei in lumen, looked at from above, i. Elastic mem- brane. m. Muscular coat or tunica media. a. Tunica adventitia. (Klein and Noble Smith.) x 250. 388 THE CIRCULATORY SYSTEM. [CH. xxxm. origin is increased, the area of the combined branches is increased also. After death, arteries are usually found dilated (not collapsed Fig. 356.-Transverse section through a large branch of the inferior mesenteric artery of a pig. e, endothelial membrane; i, tunica elastica interna, no subendothelial layer is seen; m, muscular tunica media, containing only a few wavy elastic fibres; e, e, tunica elastica externa, dividing the media from the connective-tissue adventitia, a. (Klein and Noble Smith.) x 350. as the veins are) and empty, and it was to this fact that their name (apTi]pia, the windpipe) was given them, as the ancients believed that they conveyed air to the various parts of the body. As regards the arterial system of the lungs, the pulmonary artery is distributed much as the arteries belonging to the general systemic circulation. Structure.-The walls of the arte- ries are composed of three coats, termed (a) the external or tunica adventitia, (6) the middle or tunica media, and (c) the internal or tunica intima. (a) The external coat or tunica adventitia (figs. 355 and 356, a), the strongest and toughest part of the wall of the artery, is formed of areolar tissue, with which is mingled throughout a network of elastic fibres. At the inner part of this outer coat the elastic network forms, in some arteries, so distinct ■fin- 357--Muscular fibre-cells from human arteries, magnified. 350 diameters. (Kolliker.) a. Nu- cleus. b. A fibre-cell treated with acetic acid. CH. XXXIII.] THE ARTERIES. 389 a layer as to be sometimes called the external elastic coat (fig. 356, e). . (6) The middle coat (fig. 356, m) is composed of both muscular and elastic fibres, with a certain proportion of areolar tissue. In the larger arteries (fig. 358) its thickness is comparatively as well as absolutely much greater than in the small, constituting, as it Endothelium. Sub-endothelial layer. Elastic intima. Middle coat. Fig. 358.-Transverse section of aorta through internal and about half the middle coat. does, the greater part of the arterial wall. The muscular fibres are unstriped (fig. 357), and are arranged for the most part trans- versely to the long axis of the artery; while the elastic element, taking also a transverse direction, is disposed in the form of closely interwoven and branching fibres, which intersect in all parts the layers of muscular fibre. In arteries of various sizes there is a difference in the proportion of the muscular and elastic element, elastic tissue preponderating in the largest arteries, and unstriped muscle in those of medium and small size. (c) The internal coat is formed by a layer of elastic tissue, 390 THE CIRCULATORY SYSTEM. [ch. xxxiii. called the fenestrated membrane of Henle. Its inner surface is lined with a delicate layer of elongated endothelial cells (fig. 356, e), which make it smooth, so that the blood may flow with the smallest possible amount of resist- ance from friction. Immediately external to the endothelial lining of the artery is fine connective tissue, sub-endothelial layer, with branched corpuscles. Thus the internal coat con- sists of three parts, (a) an endothelial lining, (6) the sub-endothelial layer, and (c) elastic layer. Vasa Vasorum. - The walls of the arteries are, like other parts of the body, supplied with little arteries, ending in capil- laries and veins, which, branching throughout the external coat, extend for some distance into the middle, but do not reach the internal coat. These nutrient vessels are called rasa vasorum. Nerves.-Most of the arteries are surrounded by a plexus of sympathetic nerves, which twine around the vessel very much like ivy round a tree : and ganglia are found at frequent in- tervals. They terminate in a plexus between the muscular fibres (fig. 359). Fig. 359.-Ramification of nerves and terminaticn in the muscular coat of a small artery of the frog. (Arnold.) Distribution.-The venous system begins in small vessels which are slightly larger than the capillaries from which they spring. These vessels are gathered up into larger and larger trunks until they terminate (as regards the systemic circulation) in the two venae cavie and the coronary veins, which enter the right auricle, and (as regards the pulmonary circulation) in four pulmonary veins, which enter the left auricle. The total capacity of the veins diminishes as they approach the heart; but, as a rule, their capacity exceeds by twice or three times that of their correspond- ing arteries. The pulmonary veins, however, are an exception to this rule, as they do not exceed in capacity the pulmonary The Veins. ch. xxxiii.] ' THE VEINS. 391 arteries. The veins are found after death more or less collapsed, owing to their want of elasticity. They are usually distributed in a superficial and a deep set which communicate frequently in their course. Structure.-In structure the coats of veins bear a general re- semblance to those of arteries (fig. 360). Thus, they possess outer, middle, and internal coats. The outer coat is con- structed of areolar tissue like that of the arteries, but is thicker. In some veins it contains muscular fibre- cells, which arc arranged longitudinally. The middle coat is con- siderably thinner than that of the arteries; it contains circular unstriped muscular fibres, mingled with a few elastic fibres and a large pro- portion of white fibrous tissue. In the large veins, near the heart, namely, the venae cavce and pulmonary veins, the middle coat is replaced, for some distance from the heart, by circularly arranged striped muscular fibres, continuous with those of the auricles. The internal coat of veins has a very thin fenestrated membrane, which may be absent in the smaller veins. The endothelium is made up of cells elongated in the direction of the vessel, but wider than in the arteries. Valves.-The chief influ- ence which the veins have in the circulation, is effected with the help of the valves, con- tained in all veins subject to local pressure from the muscles between or near which they run. The general construction Fig. 360.-Transverse section through a small artery and vein of the mucous membrane of a child's epiglottis; the artery is thick- walled and the vein thin-walled, a. Artery, the letter is placed in the lumen of the vessel, e. Endothelial cells with nuclei clearly visible; these cells appear very thick from the contracted state of the vessel. Outside it a double wavy line marks the elastic layer of the tunica intima, m. Tunica media, consisting of unstriped muscular fibres cir- cularly arranged ; their nuclei are well seen. a. Part of the tunica adventitia showing bundles of connective-tissue fibre in section, with the circular nuclei of the connective- tissue corpuscles. This coat gradually merges into the surrounding connective- tissue. v. In the lumen of the vein. The other letters indicate the same as in the artery. The muscular coat of the vein (m) is seen to be much thinner than that of the artery. X 350. (Klein and Noble Smith.) 392 THE CIRCULATORY SYSTEM. [ch. xxxiii. of these valves is similar to that of the semilunar valves of the aorta and pulmonary artery, already described ; but their free margins are turned in the opposite direction, i.e., towards the heart, Fig. 361.-Diagram showing valves of veins, a, part of a vein laid open and spread out, with two pairs of valves, b, longitudinal section of a vein, showing the apposition of the edges of the valves in their closed state, c portion of a distended vein, exhibiting a swelling in the situation of a pair of valves. so as to prevent any movement of blood backward. They are commonly placed in pairs, at various distances in different veins, but almost uniformly in each (fig. 361). In the smaller veins Fig. 362.-a, vein with valves open, b, with valves closed; stream of blood passing off by lateral channel. (Dalton.) single valves are often met with; and three or four are sometimes placed together, or near one another, in the largest veins, such as the subclavian, at their junction with the jugular veins. CH. XXXIII.] THE VALVES OF VEINS. 393 The valves are semilunar; the unattached edge being in some examples concave, in others straight. They are composed of an outgrowth of the subendothelial tissue covered with endothe- lium. Their situation in the superficial veins of the forearm is readily discovered by pressing along their surface, in the direction opposite to the venous current, i.e., from the elbow towards the wrist; when little swellings (fig. 361, c) appear in the position of each pair of valves. These swellings at once disappear when the pressure is removed. pig. -Surface view of an artery from the mesentery of a frog, ensheathed in a peri-vascular lymphatic vessel, a, the artery, with its circular mus- cular coat (-media) indicated by broad transverse markings, with an indica- tion of the adventitia outside. Z, lymphatic vessel; its wall is a simple endothelial membrane. (Klein and Noble Smith.) Valves are not equally numerous in all veins, and in many they are absent altogether. They are most numerous in the veins of the extremities, and more so in those of the leg than the arm. They are commonly absent in veins of less than a line in dia- meter, and, as a general rule, there are few or none in those which are not subject to muscular pressure. Among those veins which have no valves may be mentioned the superior and infe- rior vena cava, the pulmonary veins, the veins in the interior of 394 THE CIRCULATORY SYSTEM. [ch. xxxiii. the cranium and vertebral column, the veins of bone, and the umbilical vein. Lymphatics of Arteries and Veins.-Lymphatic spaces are present in the coats of both arteries and veins. In the external coat of large vessels they form a plexus of more or less tubular vessels. In smaller vessels they appear as spaces lined by endo- thelium. Sometimes, as in the arteries of the omentum, mesentery, and membranes of the brain, in the pulmonary, hepatic, and splenic arteries, the spaces arc continuous with vessels which dis- tinctly ensheatn them-perivascular lymphatics (fig. 363). The Capillaries. In all vascular textures except some parts of the corpora caver- nosa of the penis, and of the uterine placenta, and of the spleen, the transmission of the blood from the minute branches of the arteries to the minute veins is effected through a network of capillaries. Their walls are composed of endothelium-a single layer of elongated flattened and nucleated cells, so joined and dovetailed together as to form a continuous transparent membrane (fig. 364). Fig. 36).-Capillary bicod-vessels from the omentum of rabbit, showing the nucleated endothelial membrane of which they are composed. (Klein and Noble Smith.) Here and there the endothelial cells do not fit quite accurately; the space is filled up with cement material ; these spots are called i)seudo-stomata. The diameter of the capillary vessels varies somewhat in the different tissues of the body, the most common size being about of an inch, 12 /z. Among the smallest may be mentioned CH. XXXIII.] THE CAPILLARIES. 395 those of the brain, and of the follicles of the mucous membrane of the intestines; among the largest, those of the skin, lungs, and especially those of the medulla of bones. The size of capillaries varies necessarily in different animals in relation to the size of their blood-corpuscles: thus, in the Proteus, the capillary circulation can just be discerned with the naked eye. The/orm. of the capillary network presents considerable variety in the different tissues of the body : the varieties consisting principally of modifications of two chief kinds of mesh, the rounded and the elongated. That kind in which the meshes or Fig. 365.-Network of capillary ves- sels of the air-cells of the horse's lung magnified, a, a, capillaries proceeding from Z>, b, terminal branches of the pulmonary ar- tery. (Frey.) Fig. 366.-Injected capil- lary vessels of muscle seen with a low mag- nifying power. (Sharpey.) 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 (fig. 365), most glands and mucous membranes, and the cutis. The meshes of this kind of network are not quite circular but more or less angular, sometimes pre- senting a nearly regular quadrangular or polygonal form, but being more frequently irregular. The capillary 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. In such parts, the meshes form parallelograms (fig. 366), the short sides of which may be from three to eight or ten times less than the long ones; the long sides being more or less parallel to the long axis of the fibres. 396 THE CIRCULATORY SYSTEM. [ch. xxxiii. The number of the capillaries and the size of the meshes in different parts determine in general the degree of vascularity of those parts. The capillary network is closest in the lungs and in the choroid coat of the eye. It may be held as a general rule, that the more active the Lymphatics of head and neck, right. Lymphatics of head, and neck, left. Thoracic duct. Left subclavian vein. Thoracic duct. Lacteal?. .Right internal jugular vein. Right subclavian vein. Lymphatics of right arm. Receptaculum chyli, Lymphatics of lower extremities. Lymphatics of lower extremities. Fig. 367.-Diagram of the principal groups of Lymphatic vessels. (From Quain.) functions of an organ are, the more vascular it is. Hence the narrowness of the interspaces in all glandular organs, in mucous membranes, and in growing parts; their much greater width in bones, ligaments, and other tough and comparatively inactive tissues. CH. XXXIII.] LYMPHATICS. 397 Lymphatic Vessels. The blood leaves the heart by the arteries; it returns to the heart by the veins; but this last statement requires modification, for in the capillaries some of the blood-plasma escapes into the cel] spaces of the tissues and nourishes the tissue-elements. This fluid, called lymph, is gathered up and carried back again into the blood by a system of vessels called lymphatics. Fig. 368.-Superficial lymphatics of right groin and upper part of thigh. - 1. Upper inguinal glands. 2, 2'. Lower or inguinal or femoral glands. 3, 3'. Plexus of lymphatics in the course of the long saphenous vein. (Mascagni.) • The principal vessels of the lymphatic system are, in structure, like small thin-walled veins, provided with numerous valves. The beaded appearance of the lymphatic vessels shown in figs. 368 and 369 is due to the presence of these valves. They com- mence in fine microscopic lymph-capillaries, in the organs and tissues of the body, and they end in two trunks which open into the large veins near the heart (fig. 367). The fluid which they contain, unlike the blood, passes only in one direction, 398 THE CIRCULATORY SYSTEM. [CH. XXXIII. namely, from the fine branches to the trunk, and so to the large veins, on entering which they are mingled with the stream of blood and form part of its constituents. In fig. 367 the greater part of the contents of the lymphatic system of vessels will be seen to pass through a comparatively large trunk called the thoracic duct, which finally empties its contents into the blood- Fig. 369.-Lymphatic vessels of the head and neck and the upper part of the trank (Mascagni). L-The chest and pericardium have been opened on the left side, and the left mamma detached and thrown outwards over the left arm, so as to expose a great part of its deep surface. The principal lymphatic vessels and glands are shown on the side of the head and face, and in the neck, axilla, and mediastinum. Between the left internal jugular vein and the common carotid artery, the upper ascending part of the thoracic duct marked 1, and above this, and descending to 2, the arch and last part of the duct. The termination of the upper lymphatics of the diaphragm in the mediastinal glands, as well as the cardiac and the deep mammary lymphatics, is also shown. stream, at the junction of the internal jugular and subclavian veins of the left side. There is a smaller duct on the right side. The lymphatic vessels of the intestinal canal are called lacteals, because during digestion the fluid contained in them resembles milk in appearance; and the lymph in the lacteals during the CH. XXXIII. : LYMPHATICS. 399 per iod of digestion is called chyle. There is< no distinction of structure, however, between lacteals and lymphatics. In' some parts of its course the lymph-stream must pass through lymphatic glands, to be described later on. Origin of Lymph Capillaries.- The lymphatic capillaries commence most commonly either (a) in closely- meshed networks, or (6) in irregular lacunar spaces between the various structures of which the different organs are composed. In serous membranes such as the omentum and mesentery they occur as a con- nected system of very irregular branched spaces partly occupied by connective-tissue corpuscles, and both in these and in many other tissues are found to communicate freely with regular lymphatic vessels. In many cases, though they are formed mostly by the chinks and crannies between the blood-vessels, secreting ducts, and other parts which may happen to form the framework of the organ in which they exist, they are lined by a distinct layer of endothelium. The lacteals offer an illustration of another mode of origin, namely (c) in blind dilated extremities in the villi of the small intestine. Structure of Lymph Capillaries.- The structure of lymphatic capil- laries is very similar to that of blood- capillaries their Avails consist of a single layer of elongated endothelial cells with sinuous outline, which cohere along their edges to form a delicate membrane. They differ from blood - capillaries mainly in their larger and very variable calibre, and in their numerous communications w'ith the spaces of the lymph-canalicular system. Fig. 370.- Superficial lymphatics of the forearm and palm of the hand. |.-5. Two small glands at the bend of the arm. 6. Radial lymphatic vessels. 7. Ulnar lymphatic vessels. 8, 8'. Palmar arch of lymphatics. 9, 9'; Outer and inner sets of ves- sels. b. Cephalic vein. d. Radial vein. e. Median vein. f. Ulnar vein. The lymphatics are repre- sented as lying on the deep fascia. (Mascagni.) 400 THE CIRCULATION OF THE BLOOD. [ch. xxxiv. In certain parts of the body, stomata exist, by which lymphatic capillaries directly communicate with parts formerly supposed to be closed cavities. They have been found in the pleura, and in other serous membranes ; a serous cavity thus forms a large lymph-sinus or widening out of the lymph-capillary system with which it directly communicates. A very typical plexus of lymphatic capillaries is seen in the central tendon of the diaphragm. Fig. 371 represents the appear- ance presented after staining with silver nitrate. Fig. 371.-Lymphatics of central tendon of rabbit's diaphragm, stained with silver nitrate. The shaded background is composed of bundles of white fibres, between which the lymphatics lie. I, Lymphatics lined by long narrow endothelial cells, and showing v valves at frequent intervals. (Schofield.) CHAPTER XXXIV. THE CIRCULATION OE THE BLOOD In the preceding chapter, we have seen what the course of the circulation is, and we have devoted considerable space to a study of the structure of the heart and vessels. We have now to CH. XXXIV.] HARVEY'S DISCOVERIES. 401 approach the more strictly physiological side of the subject, and study the means by which the blood is kept in movement, so that it may convey nutriment to all parts, and remove from those parts the waste products of their activity. Previous to the time of Harvey, the vaguest notions prevailed regarding the use and movements of the blood. The arteries were supposed by some to contain air, by others to contain a more subtle essence called animal spirits ; the animal spirits were supposed to start from the ventricles of the brain, and they were controlled by the soul which was situated in the pineal gland. How the animal spirits got into the arteries was an anatomical detail which was bridged across by the imagination. There was an idea that the blood moved, but this was con- sidered to be a haphazard to and fro movement, and confined to the veins. The proofs that the movement is a movement in a circle were discovered by William Harvey, and to this eminent discoverer also belongs the credit of pointing out the methods by which every physiological problem must be studied. In the first place there must be correct anatomical knowledge, and in the second there must be experiment, by which deductions from structure can be tested; moreover, this second method is by far the more important of the two. Harvey's proofs of the circulation came under these two heads. The structural or anatomical facts upon which he relied were the following: 1. The existence of two distinct sets of tubes in connection with the heart, namely the. arteries and the veins. 2. The existence in one of these, the veins, of valves which would only allow the passage of the blood in one direction. His experimental facts were the following : 3. That the blood spurts with great force and in a jerky manner from an artery opened during life, each jerk corresponding with a beat of the heart. 4. That if the large veins near the heart are tied, the heart becomes pale, flaccid, and bloodless, and on removal of the ligature the blood again flows into the heart. 5. If the aorta is tied, the heart becomes distended with blood, and cannot empty itself until the ligature is removed. 6. The preceding experiments were performed on animals, but by the following experiment he showed that the circulation is a fact in man also ; if a ligature is drawn tightly round a limb, no blood can enter it, and it becomes pale and cold. If the ligature is somewhat relaxed so that blood can enter but cannot leave the limb, it becomes swollen. If the ligature is removed, the limb soon regains its normal appearance. 402 THE CIRCULATION OF THE BLOOD. [ch. xxxiv. 7. Harvey also drew attention to the fact that there is general constitutional disturbance resulting from the introduction of a poison at a single point, and that this can only be explained by a movement of the circulating fluid all over the body. Since Harvey's time many other proofs have accumulated; for instance :- 8. If an artery is wounded, haemorrhage may be stopped by pressure applied between the heart and the wound; but in the case of a wound in a vein, the pressure must be applied beyond the seat of injury. 9. If a substance which like ferrocyanide of potassium can be readily detected, is injected at a certain point into a blood vessel, it will after the lapse of a short interval have entirely traversed the circulation and be found in the blood collected from the same point. 10. Our increased knowledge of the structure of the heart and its valves has shown that its structure is such as to permit the blood to pass in one direction only. 11. Perhaps the most satisfactory proof of the circulation is one within the reach of every student, though beyond that of Harvey. It consists in actually seeing the passage of the blood from small arteries through capillaries into veins in the trans- parent parts of animals, such as the tail of a tadpole or the web of a frog's foot. Harvey could not follow this part of the circulation, for he had no lenses sufficiently powerful to enable him to see it. Harvey's idea of the circulation here was that the arteries carried the blood to the tissues, which he considered to be of the nature of a sponge, and the veins collected the blood again, much in the same way as drainage pipes would collect the water of a swamp. The discovery that the ends of the arteries are connected to the commencements of veins by a definite system of small tubes we now call capillaries, was made by Malpighi, in the year 1661. He first observed them in the tail of the tadpole, and Leeuwenhoek, seven years later, saw the circulation in the lung of the frog. We can now proceed to study some of the principles on which the circulation depends :- The simplest possible way in which we could represent the circulatory system is shown in fig. 372 A. Here there is a closed ring containing fluid, and upon one point of the tube is an enlargement (H) which will correspond to the heart. It is obvious that if such a ring made of an ordinary Higginson s syringe and a tube were placed upon the table, there would be no movement of the fluid in it; in order to make the fluid move CH. XXXIV.] SCHEMA OF CIRCULATION. 403 there must be a difference of pressure between different parts of the fluid, and this difference of pressure is caused in the fluid by the pressure on it of the heart walls. If, for instance, one takes the syringe in one's hand and squeezes it, one imitates a con- traction of the heart : if the syringe has no valves, the fluid would pass out of each end of it in the direction of the two arrows placed outside the ring. When the pressure on the syringe is relaxed (this would correspond to the interval between the heart beats), the fluid would return into the heart again in Fig. 372.-Simple schema of the circulation. the direction of the two arrows placed inside the ring. This, however, would be merely a to and fro movement, not a circula- tion. Fig. 372 B shows how this to and fro movement could, by the presence of valves, be converted into a circulation; when the heart contracts the fluid could pass only in the direction of the outer arrow; when the heart relaxes it could pass only in the direction of the inner arrow ; the direction of both arrows is the same, and so if the contraction and relaxation of the heart are repeated often enough the fluid will move round and round within the tubular ring. The main factor in the circulation is difference of pressure. Fluid always flows in the direction of pressure ; it could no more flow from a place where the pressure is low to where it is high than it could flow uphill. This difference of pressure is produced in the first instance by the contraction of the heart, but we shall find in our study of the vessels that some of this pressure is stored up in the elastic arterial walls, and keeps up the circulation particularly during the periods of rest of the heart. Before passing on to consider the physiology of heart and vessels at greater length, let us take a few types of the circulatory system from different parts of the animal kingdom. 404 THE CIRCULATION OF THE BLOOD. [ch. xxxiv. In worms, and in the lowest vertebrate Amphioxus, the cir- culatory system is almost as simple as in the scheme just described ; the heart is a long contractile tube provided with valves, which contracts and presses the blood forwards into the aorta at its anterior end; this divides into arteries for the supply of the body ; the blood passes through these to capillaries, and 'is collected by veins which converge to one or two main trunks that enter the heart at its posterior end. In fishes, the heart is a little more complicated ; it is divided into a number of chambers placed in single file, one in front of each other; the most posterior which receives the veins is called Fig. 373..-The heart of a frog (Rana esculenta) from the front. V, ventricle ; Ad, right auricle; .Ls, left auricle; 7?, bulbus arteriosus, dividing into right and left aorta. (Ecker.) the sinus venosus; this contracts and forces the blood into the next chamber called the auricle ; this forces the blood into the next cavity, that of the ventricle, and last of all is the aortic bulb. From the bulb, branches pass to the gills where they break up into capillaries, and the blood is aerated : it then once more enters larger vessels which unite to form the dorsal aorta, whence the blood is distributed by arteries to all parts of the body ; here it enters the capillaries, then the veins which enter the sinus (whence we started) by a few large trunks. Taking the frog as an instance of an amphibian, we find the heart more complex still, and the simple peristaltic action of the CH. xxxiv.] THE FROG'S HEART. 405 heart muscle as we have described it in the hearts of worm and fish, becomes correspondingly modified. There is only one ventricle, but there are two auricles, right and left. The ventricle contains mixed blood, since it receives arterial blood from the left auricle (which is the smaller of the two), and venous blood from the right auricle ; the right auricle receives the venous blood from the sinus, which in turn receives it from the systemic veins. The left auricle, as in man, receives the blood from the pulmonary veins. When the ventricle contracts, it forces the blood onward into Fig. 374.-The heart of a frog (Rana esculenta) from the back, s.v., sinus venosus opened ; c.s.s., left vena cava superior; c.s.d., right vena cava superior; c.i., vena cava inferior; v.p., vena pulmonales ; A.cl., right auricle ; A.s., left auricle ; A.p., opening of com- munication between the right auricle and the sinus venosus. x 2]-3. (Ecker.) the aortic bulb which divides into branches on each side for the supply of the head (fig. 373, 1), lungs and skin (fig. 373, 3), and the third branch (fig. 373, 2) unites with its fellow of the opposite side to form the dorsal aorta for the supply of the rest of the body. Passing from the amphibians to the reptiles, we find the division of the ventricle into two beginning, but it is not complete till we reach the birds. The heart reaches its fullest development in mammals, and we have already described the human as an example of the mammalian heart. The sinus venosus is not present as a distinct chamber in the mammalian heart, but is represented by that portion of the right auricle at which the large veins enter. 406 PHYSIOLOGY OF THE HEART. [CH. XXXV. CHAPTER XXXV. PHYSIOLOGY OF THE HEART. The Cardiac Cycle. The series of changes that occur in the heart constitute the cardiac cycle. This must be distinguished from the course of the circulation. The term cycle indicates that if one observes the heart at any particular moment, the heart from that moment onwards undergoes certain changes until it once more assumes the same condition that it had at the moment when the observa- tion commenced, when the cycle is again repeated, and so on. This series of changes consists of alternate contraction and relaxation. Contraction is known as systole, and relaxation as diastole. The contraction of the two auricles takes place simultaneously, and constitutes the auricular systole; this is followed by the simultaneous contraction of the two ventricles, ventricular systole, and that by a period during which the whole of the heart is in a state of diastole; then the cycle again commences with the auricular systole. Taking 7 2 as the average number of heart beats per minute, each cycle will occupy of a minute or a little more than o-8 of a second. This may be approximately distributed in the following way : Auricular systole about o'i + Auricular diastole 0-7 = o-8 Ventricular systole about 0'3 + Ventricular diastole 0'5 = o-8 Total systole about 0-4 + Joint auricular and ventricular diastole 0'4 = o-8 If the speed of the heart is quickened, the time occupied by each cycle is diminished, but the diminution affects chiefly the diastole. These different parts of the cycle must next be studied in detail. The Auricular Diastole.-During this time, the blood from the large veins is flowing into the auricles, the pressure in the veins though very low being greater than that in the empty auricles. This expands the auricles, and during the last part of the auricular diastole the blood passes on into the ventricles. The dilatation of the auricles is assisted by the elastic traction of the lungs. The lungs being in a closed cavity, the thorax, and being distended with air, are in virtue of their elasticity always tending ch. xxxv.J THE CARDIAC CYCLE. 407 to recoil and squeeze the air out of their interior; in so doing they drag upon any other organ with which their surface is in contact : this elastic traction will be greatest when the lungs are most distended, that is during inspiration, and will be more felt by the thin-walled auricles than by the thick-walled ventricles of the heart. The Auricular Systole is sudden and very rapid; by contracting, the auricles empty themselves into the ventricles. It commences at the entrance of the great veins and is thence propagated towards the auriculo-ventricular opening. The reason why the blood does not pass backwards into the veins, but onward into the ventricles is again a question of pressure ; the pressure in the relaxed ventricles, which is so small as to exert a suction action on the auricular blood, is less than in the veins. Moreover, the auriculo-ventricular orifice is large and widely dilated, whereas the mouths of the veins are constricted by the contraction of their muscular coats. Though there is no regurgitation of the blood backwards into the veins, there is a stagnation of the flow of blood onwards to the auricles. The veins have no valves at their entrance into the auricles, except the coronary vein which does possess a valve; there are valves, however, at the j unction of the subclavian and internal jugular veins. Ventricular Diastole; during the last part of the auricular diastole, and the whole of the auricular systole, the ventricles have been relaxed and then filled with blood. The dilatation of the ven- tricles is chiefly brought about in virtue of their elasticity ; this is chiefly evident in the left ventricle with its thick muscular coat. It is equal to 23 mm. of mercury, and is quite independent of the elastic traction of the lungs, which, however, in the case of the thinner-walled right ventricle comes into play. The Ventricular Systole ; this is the contraction of the ventricles, and it occupies more time than the auricular systole ; when it occurs the auriculo-ventricular valves are closed to prevent regurgitation into the auricles, and when the force of the systole is greatest, and the pressure within the ventricles exceeds that in the large arteries which originate from them, the semilunar valves are opened, and the ventricles empty themselves, the left into the aorta, the right into the pulmonary artery. Each ventricle ejects about 4 ozs. of blood with each contraction ; the left in virtue of its thicker walls acting about thrice as forcibly as the right. The greater force of the left ventricle is necessary as it has to overcome the resistance of the small vessels all over the body; whereas the right ventricle has only to overcome peri- pheral resistance in the pulmonary district. 408 PHYSIOLOGY OF THE HEART. [ch. xxxv. The shape of both ventricles during systole is generally described as undergoing an alteration, the diameter in the plane of the base being diminished, and the length of the ventricles slightly lessened. The whole heart, moreover, moves towards the right and forwards, twisting on its long axis and exposing more of the left ventricle anteriorly than when it is at rest. These movements, which were first described by Harvey together with the hardening that occurs when the ventricles contract, have been since Harvey's time believed to be the cause of the cardiac impulse or apex beat which is to be felt in the fifth intercostal space about three inches from the middle line. It has, however, been recently shown by Haycraft that these changes only occur when the chest walls are open. When the heart con- tracts in a closed thorax it undergoes no change in shape, as the contraction is concentric, that is equal in all directions. The diminution of the heart's volume which occurs in systole cannot be the cause of the apex beat; it would rather tend to draw the chest wall inwards than push it outwards. Doubtless the apex beat is produced by the increased pressure in the aorta being transmitted backwards to the heart, and causing it to press more closely than it does in diastole against the chest walls. Action of the Valves of the Heart. (i) The Auricula-Ventricular.-The distension of the ventricles with blood continues throughout the whole period of their diastole. The auriculo-ventricular valves are gradually brought into place by some of the blood getting behind the cusps and floating them up; and by the time that the diastole is complete, the valves are in apposition, and they are firmly closed by the reflex current caused by the systole of the ventricles. The diminution in the size of the heart during ventricular systole is well marked in the neighbourhood of the auriculo-ventricular rings, and this aids in rendering the auriculo-ventricular valves competent to close the openings, by greatly diminishing their diameter. The margins of the cusps of the valves are still more secured in apposition with another, by the simultaneous contraction of the musculi papillares, whose chordee tendinese have a special mode of attachment for this object. The cusps of the auriculo-ventricular valves meet not by their edges only, but by the opposed surfaces of their thin outer borders. The musculi papillares prevent the auriculo-ventricular valves from being everted into the auricle. For the chordae tendineae might allow the valves to be pressed back into the auricle, were it not that when the wall of the ventricle is brought by its con- traction nearer the auriculo-ventricular orifice, the musculi papillares more than compensate for this by their own contraction -holding the chords tight, and, by pulling down the valves, adding slightly to the force with which the blood is expelled. These statements apply equally to the auriculo-ventricular valves on both sides of the heart; the closure of both is generally ch. xxxv.] VALVES OF THE HEART. 409 complete every time the ventricles contract. But in some circum- stances the tricuspid valve does not completely close, and a certain quantity of hlood is forced back into the auricle. This has been called the safety-valve action. The circumstances in which it usually happens are those in which the vessels of the lung are already completely full when the right ventricle contracts, as, e.g., in certain pulmonary diseases, and in very active muscular exertions. In these cases, the tricuspid valve does not completely close, and the regurgitation of the blood may be indicated by a pulsation in the jugular veins synchronous with that in the carotid arteries. 2. The Semilunar Valves.-It has been found that the com- mencement of the ventricular systole precedes the opening of the semilunar valves by a fraction of a second. This shows that the intraventricular pressure does not exceed the arterial pressure until the systole has actually begun, for the opening of the valves takes place at once when there is a distinct difference in favour of the intraventricular over the arterial pressure, and they continue open only as long as this difference continues. When the arterial begins to exceed the intraventricular pressure, there is, as it were, a reflux of blood towards the heart, and the valves close. The dilatation of the arteries is, in a peculiar manner, adapted to bring this about. The lower borders of the semilunar valves are attached to the inner surface of the ten- dinous ring, which is 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. The tissue of this ring is tough, and does not admit of extension under such pressure as it is commonly exposed to; the valves are equally inextensile, being- formed of fibrous tissue. Hence, when the ventricle propels blood through the orifice and into the canal of the artery, the lateral pressure 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 unyielding valves and the ring to which their lower borders are attached. The effect, therefore, of each such propul- sion of blood from the ventricle is, that the wall of the first por- Fig. 375.-Sections of aorta, to show the action of the semilunar valves, a is intended to show the valves, represented by the dotted lines, lying near 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. 410 PHYSIOLOGY OF THE HEART. [ch. xxxv. tion of the artery is dilated into three pouches behind the valves, while the free margins of the valves are drawn inward towards its centre (fig. 375, b). Their positions may be explained by the diagrams, in which the continuous lines represent a transverse section of the arterial w'alls, the dotted ones the edges of the valves, first, when the valves are nearest to the walls (a), as in the dead heart, 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, the blood is forced backwards towards the ventricles and onwards in the course of the circulation. Part of the blood thus forced back lies in the pouches (sinuses of Valsalva) (a, fig. 375, b) between the valves and the arterial walls; and the valves are by it pressed together till their thin lunated margins meet in three lines radiating from the centre to the circumference of the artery (7 and 8, fig. 350). The Sounds 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 joawse or period of silence. The first or systolic sound is dull and prolonged ; its commence- ment coincides with the impulse of the heart against the chest wall, and just precedes the pulse at the wrist. The second or diastolic sound is shorter and sharper, with a somewhat flapping character, and follows close after the arterial pulse. The periods of time occupied respectively by the two sounds taken together and by the pause, are nearly equal. Thus, according to Walshe, if the cardiac cycle be divided into tenths, the first sound occu- pies ytj; the second sound Tqy; the first pause (almost imper- ceptible) y-g-; and the second pause -f'y. The sounds may be expressed by the words lilbb-ditp. The events which correspond, in point of time, with the first sound, are (i) the contraction of the ventricles, (2) the first part of the dilatation of the auricles, (3) the tension of the auriculo- ventricular valves, (4) the opening of the semilunar valves, and (5) the propulsion of blood into the arteries. The sound is suc- ceeded, in about one-thirtieth of a second, by the pulsation of the facial arteries, and in about one-sixth of a second, by the pulsa- tion of the arteries at the wrist. The second' sound, in point of time, immediately follows the cessation of the ventricular con- CH. XXXV.] SOUNDS OF THE HEART. 411 traction, and corresponds with (a) the tension of the semilunar valves, (6) the continued dilatation of the auricles, (c) the com- mencing dilation of the ventricles, and (cl) the opening of the auriculo-ventricular valves. The pause immediately follows the second sound, and corresponds in its first part with the completed distension of the auricles, and in its second with their contraction, and the completed distension of the ventricles; the auriculo- Fig. 376.-Scheme of cardiac cycle. The inner circle shows the events which occur within the heart; the outer the relation of the sounds and pauses to these events. (Sharpey and Gairdner.) ventricular valves being all the time of the pause open, and the arterial valves closed. Causes.-The exact cause of the first sound of the heart is a matter of discussion. Two factors probably enter into it, viz., first, the vibration of the auricido-ventricidar valves and the chorda tendinece. This vibration is produced by the increased intraven- tricular pressure set up when the ventricular systole commences, which puts the valves on the stretch. The question whether this stretched condition of the valve continues throughout the whole of the ventricular systole cannot be definitely settled, but if it does not, the valvular element may possibly take part in the production of the first part of the first sound only. It is not unlikely too that the vibration of the ventricular walls them- selves, and of the aorta and pulmonary artery, all of which parts are suddenly put into a state of tension at the moment' of ven- tricular contraction, may have some part in producing the first sound. Secondly, the muscular sound produced by contraction of the mass of muscular fibres which form the ventricle Looking 412 PHYSIOLOGY OF THE HEART. [ch. xxxv. upon the contraction of the heart as a single contraction and not as a series of contractions or tetanus, it is at first sight difficult to see why there should be any muscular sound at all when the heart contracts, as a single muscular contraction does not produce sound. It has been suggested, however, that it arises from the repeated unequal tension produced when the wave of muscular contractions passes along the very intricately arranged fibres of the ventricular walls. There can be no doubt, however, that the valvular element is the more important of the two factors. The cause of the second sound is more simple than that of the first. It is entirely due to the vibration consequent on the sudden closure of the semilunar valves when they are pressed down across the orifices of the aorta and pulmonary artery. The influence of these valves in producing the sound was first demon- strated by Hope, who experimented with the hearts of calves. In these experiments 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 respective 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 ceased to be audible. Disease of these valves, when sufficient to interfere with their efficient action, also demonstrates the same fact by modifying the second sound or destroying its distinctness. The contraction of the auricles which takes place in the end of the pause is inaudible outside the chest. The first sound is heard most distinctly at the apex beat in the fifth interspace; the second sound is best heard over the second right costal cartilage-that is, the place where the aorta lies nearest to the surface. The pulmonary and aortic valves generally close simultaneously. In some cases, however, the aortic may close slightly before the pulmonary valves, giving rise to a 1 reduplicated second sound.' The pulmonary contribution to this sound is best heard over the second left cartilage. The Coronary Arteries. The coronary arteries are the first branches of the aorta; they originate from the sinuses of Valsalva, and are destined for the supply of the heart itself; the entrance of the corresponding vein, the coronary vein, into the right auricle, we have already seen. CH. XXXV. ] CARDIOGRAPHS. 413 Ligature of the coronary arteries causes almost immediate death; the heart, deprived of its normal blood supply, beats irregularly, twitches, and then ceases to contract altogether. In fatty degeneration of the heart in man, sudden death is by no means infrequent. This is in many cases due to a growth in thickness of the walls of the coronary arteries called atheroma, which progresses until the lumen of these arteries is obliterated, and the man dies almost as if they had been ligatured. Self-steering Action of the Heart.-This expression, which is now only of historical interest, was originated by Briicke. He supposed that the semi- lunar valves closed the orifices of the coronary arteries during the systole of the heart. Unlike all the other arteries of the body, the coronary arteries would therefore fill only during diastole, and this increased fulness of the vessels in the heart walls during diastole would assist the ventricle to dilate. This, however, is incorrect ; the valves do not cover the mouths of the arteries ; and when the coronary arteries are cut they spurt, like all other arteries, most forcibly during the systole of the ventricle. Cardiographs, A cardiograph is an instrument for obtaining a graphic record of the heart's movements. In animals the heart may be exposed, and levers placed in connection with its various parts may be employed to write on a revolving blackened surface. A simple instrument applicable to the frog's heart is the fol- Fig. 377.-Simple cardiograph for frog's heart. The sternum of the frog having been removed, the pericardium opened, and the fraenum (a small band from the back of the heart to the pericardium) divided, the heart is pulled through the opening, a minute hook placed in its apex, and this is fixed by a silk thread to a lever pivoted at f as in the figure. The cardiac wave of contraction starts at the sinus, this is followed by the auricular systole, and that by the ventricular systole and 414 PHYSIOLOGY OF THE HEART. [ch. xxxv. pause. This is recorded as in the next figure by movements of the writing point at the end of the long arm of the lever. Such apparatus is, however, not applicable to the human heart, and all the various forms of cardiograph devised are modifications of Fig. 378.-Cardiogram of frog's heart, c, showing auricular, followed by ventricular beat; t, time in half seconds. Marey's tambours. One of those most frequently used is depicted in the next two diagrams. It (fig. 379) consists of a cup-shaped metal box over the open front of which is stretched an elastic india-rubber membrane, upon which is fixed a small knob of hard wood or ivory. This knob, however, may be attached, Tube to communicate with tambour. Tympanum. Ivory Tape to attach the instrument knoh. to the chest. Fig. 379.- Cardiograph. (Sanderson's.) as in the figure, to the side of the box by means of a spring, and may be made to act upon a metal disc attached to the elastic membrane. CH. XXXV.] CARDIOGRAPHS. 415 The knob is for application to the chest-wall over the apex beat. The box or tympanum communicates by means of an air-tight tube with the interior of a second tympanum, in connection with which is a long and light lever. The shock of the heart's impulse being communicated to the ivory knob and through it to the first tympanum, the effect is at once transmitted by the column of air in the elastic tube to the interior of the second tympanum, also closed, and through the elastic and movable lid. Screw to regulate elevation of lever. Fig. 380.-Marcy's Tambour, to which the movement of the column of air in the first tympanum is conducted by a tube, and from which it is communicated by the lever to a revolving cylinder, so that the tracing of the movement of the impulse beat is obtained. Writing lever. Tambour. Tube of cardiograph. of the latter to the lever, which is placed in connection with a registering apparatus, which consists of a cylinder covered with smoked paper, revolving with a definite velocity. The point of the lever writes upon the paper, and a tracing of the heart's impulse or cardiogram is thus obtained. Fig. 380 represents a typical tracing obtained in this way. The first small rise of the lever is caused by the auricular, the second larger rise by the ventricular systole; the downstroke represents the pause, the tremors at the commencement of which are partly instrumental and partly caused by the closure of the semilunar valves. Fig. 381.-Cardiogram from human heart. The variations in the individual heats are due to the influence of the respiratory movements on the heart. To be read from left to right. Another method of obtaining a tracing from one's own heart consists in dispensing with the first tambour, and placing the 416 PHYSIOLOGY OF THE HEART. [ch. xxxv. tube of the recording tambour in one's mouth, and holding the breath though keeping the glottis open. The chest then acts as the first tambour, and the movements of the lever (cardio- pneumatogram) may be written in the usual way. Endocardiac Pressure. The tracings of the cardiograph are, however, very variable, and their interpretation is a matter of discussion. A much better method of obtaining a graphic record of the events of the cardiac cycle consists in connecting the interior of an animal's heart with recording apparatus. There are several methods by which the endocardiac pressure may be recorded. By placing three small india-rubber air-bags or cardiac sounds down the jugular vein into the interior respectively of the right auricle and the right ventricle, and in an intercostal space in front of the heart of a living animal (horse), and placing these Fig. 382.-Apparatus of MM. Chauveau and Marey for estimating the variations of endo- cardial pressure, and production of impulse of the heart. bags, by means of long narrow tubes, in communication with three tambours with levers, arranged one over the others in con- nection with a registering apparatus (fig. 382), Chauveau and Marey were able to record and measure the variations of the endocardial pressure and the comparative duration of the contrac- tions of the auricles and ventricles. By means of the same appa- ratus, the synchronism of the impulse with the contraction of the ventricles is also shown. In the tracing (fig. 383), the intervals between the vertical CH. XXXV.] ENDOCARDIAC PRESSURE. 417 lines represent periods of a tenth of a second. The parts on which any given vertical line falls represent simultaneous events. It will be seen that the contraction of the auricle, indicated by the marked curve at a in first tracing, causes a slight increase of pressure in the ventricle, which is shown at a' in the second tracing, and produces also a slight impulse, which is indicated by A,z in the third tracing. The closure of the semilunar valves causes a momentarily increased pressure in the ventricle at d', affects the pressure in the auricle D, and is also shown in the tracing of the impulse also, d'z. The large curve of the ventricular and the impulse tracings, Fig. 383.-Tracings of (1), Intra-auricular, and (2), Intra-ventricular p essures, and (3), of the impulse of the heart, to be read from left to right, obtained by Chauveau and Marey's apparatus. between a' and d', and a" and d", are caused by the ventricular contraction, while the smaller undulations, between b and c, b' and c', b" and c", are caused by the vibrations consequent on the tightening and closure of the auriculo-ventricular valves. Much objection has, however, been taken to this method of investigation. First, because it does not admit of both positive and negative pressure being recorded. Secondly, because the method is only applicable to large animals, such as the horse. Thirdly, because the intraventricular changes of pressure are communicated to the recording tambour by a long elastic column of air ; and fourthly, because the tambour arrangement has a tendency to record inertia vibrations. Kolleston has re-investi- gated the subject with a more suitable apparatus. His method is as follows : a window is made in the chest of an anaesthetized and curarized animal, and an appropriately curved glass cannula introduced through an opening in the auricular appendix. The 418 PHYSIOLOGY OF THE HEART. [ch. xxxv. cannula is then passed through the auriculo-ventricular orifice without causing any appreciable regurgitation, into the ventricle, or it may be introduced into the cavity of the right or left ven- tricle by an opening made in the apex of the heart. In some experiments the trochar is pushed through the chest-wall into the ventricular cavity. The apparatus is filled with a solution of leech extract in '75 per cent, saline solution, or with a solution of sodium bicarbonate of specific gravity 1083. The animals employed were chiefly dogs. The movement of the column of blood is communicated to the writing lever by means of a vulca- nite piston which moves with little friction in a brass tube Fig. 384.-Apparatus for recording the e.idocardial pressure. (Rolleston.) connected with the glass cannula by means of a short connecting tube. When the lower part of the tube (a) is placed in communica- tion with one of the cavities of the heart, the movements of the piston are recorded by means of the lever (c). Attached to the lever is a section of a pulley (h), the axis of which coincides with that of the steel ribbon (e) ; while, firmly fixed to the piston, is ch. xxxv.J ENDOCARDIAC PRESSURE. 419 the curved steel piston rod (i), from the top of which a strong silk thread (j) passes downwards into the groove on the pulley. This thread (j), after being twisted several times round a small pin at the side of the lever, enters the groove in the pulley from above downwards, and then passes to be fixed to the lower part of the curve on the piston-rod as shown in the smaller figure. The rise and fall of the lever (c) is controlled by the resistance to torsion of the steel ribbon (e), to the midle of which one end of the lever is securely fixed by a light screw clamp (f). At some distance from this clamp-the distance varying with the degree of resistance which it is desired to give to the movements of the lever-are two holders (g g) which securely clamp the steel ribbon. As the torsion of a steel wire' or strip follows Hooke's law, the torsion being proportional to the twisting force-the movements Fig. 385.-Endocardial pressure-curve from the left ventricle. The thorax was opened and a cannula introduced through the apex of the ventricle ; abscissa is line of atmo- spheric pressure, o to d represents ventricular contraction; from n to the next rise at g represents the ventricular diastole. of the lever point are proportional to the force employed to twist the steel strip or ribbon-in other words, to the pressures which act on the piston (b). To make it possible to record satisfactorily the very varying ventricular and auricular pressures, the resistance to torsion of a steel ribbon adapts itself very conveniently. This resistance can be varied in two ways, first by using one or more pieces of steel ribbon or by using strips of different thick- nesses ; or secondly, by varying the distance between the holders (g g) and the central part of the steel ribbon to which the lever is attached. Rolleston's conclusions are as follows :- i. That there is no distinct and separate auricular contraction marked in the curves obtained from either right or left ventricles, 420 PHYSIOLOGY OF THE HEART. [ch. xxxv. the auricular and ventricular rises of pressure being merged into one continuous rise. 2. That the auriculo-ventricular valves arc closed before any great rise of pressure within the ventricle above that which results from the auricular systole (a, fig. 386). The closure of Fig. 386.-Curve from left ventricle ; abscissa shows atmospheric pressure tlie valve occurs probably in the lower third of the rise a b (fig. 386), and does not produce any notch or wave. 3. That the semilunar valves open at the in the ventri- cular systole, situated (at g) about or a little above the junction of the middle or upper third of the ascending line (a b), and the closure about or a little before the shoulder (d). 4. That the minimum pressure in the ventricle may fall below that of the atmosphere, but that the amount varies considerably. Another method of overcoming the imperfections of Marey's tambour is by the use of Hurthle's manometer. In this the tam- bour is very small, the membrane is made of thick rubber, and the whole, including the tube that connects it to the heart, is filled with a strong saline solution (saturated solution of sodium sulphate). Fig. 387.-Hurthle's manometer. The tracing obtained by this instrument, when connected with the interior of the ventricle, is represented in the next figure. The auricular systole causes a small rise of pressure a b ; it lasts about '05 second. It is immediately followed by the ven- tricular contraction, which lasts from b to d. From b to c the ventricle is getting up pressure, so that at c it equals the aortic CH. XXXV.] FREQUENCY AND FORCE OF THE HEART. 421 pressure. This takes '02 to '04 second. Beyond c the aortic valves open, and blood is driven into the aorta ; the outflow lasts from c to d (-2 second). At d the ventricle relaxes. The flat part of the curve is spoken of as the systolic plateau, and accord- ing to the state of the heart and the peripheral resistance may present a gradual ascent or descent; it occupies about • 18 second. Almost immediately after the relaxation begins, the intraven- tricular pressure falls below the aortic, so that the aortic valves close near the upper part of the descent at e. The amount of pressure in the heart is measured by a mano- Fig. 388.- Curve of intra-ventrieuiar pressure. After Hurthle.) meter, which is connected to the heart by a tube containing a valve. This was first used by Goltz and Gaule. If the valve permits fluid to go only from the heart, the manometer will indicate the maximum pressure ever attained during the cycle. If it is turned the other way, it will indicate the minimum pressure. The following are some of the measurements taken from the dog's heart in terms of millimetres of mercury :- Maximum pressure. Minimum pressure. Left ventricle . 140 mm. - 30 to 40 mm. Hight ventricle . . 60 mm. 15 mm. Right auricle . 20 mm. 7 to 8 mm. By a negative ( - ) pressure one means that the mercury is sucked up in the limb of the manometer towards the heart. Frequency and Force of the Heart's Action. The heart of a healthy adult man contracts about 72 times in a minute; but many circumstances cause this rate, which of course corresponds with that of the arterial pulse, to vary even in health. The chief are age, temperament, sex, food and drink, exercise, time of day, posture, atmospheric pressure, temperature. Some figures in reference to the influence of age are appended The frequency of the heart's action gradually diminishes from 422 PHYSIOLOGY" OF THE HEART [ch. xxxv. the commencement to near the end of life, but is said to rise again somewhat in extreme old age, thus :- Before birth the average number of pulsations per minute is 150 Just after birth . from 140 to 130 During the first year 130 to 115 During the second year . . . 11510100 During the third year 100 to 90 About the seventh year . . . from 90 to 85 About the fourteenth year . . . 85 to 80 In adult age . . 80 to 70 In old age . . . 70 to 60 In decrepitude . . 75 to 65 In health there is observed a nearly uniform relation between the frequency of the beats of the heart and of the respirations ; the proportion being, on an average, 1 respiration to 3 or 4 beats. The same relation is generally maintained in the cases in which the action of the heart is naturally accelerated, as after food or exercise ; but in disease this relation may cease. In estimating the work done by any machine it is usual to express it in terms of the unit of work. In England, the unit of work is the foot-pound, and is defined to be the energy expended in raising a unit of weight (1 lb.) through a unit of height (1 ft.) : in France, the gram-metre. The work done by the heart at each contraction can be readily found by multiplying the weight of blood expelled by the ventricles by the height to which the blood rises in a tube tied into an artery. This height is probably about 2 metres (7 ft.) in man. Taking the weight of blood expelled from the left ventricle at each systole as 125 grammes (4 oz.) and the average pressure in the aorta as 150 mm. mercury (2 metres blood), the work done at each contraction will be 250 gram-metres. To this must be added 80 gram-metres for the work done by the right ventricle. If the heart beats 7 2 times a minute, it will do 30,000 kilogramme-metres of work in the 24 hours, or about a quarter of the work performed by a labourer working under supervision for eight hours. (Waller.) Innervation of the Heart. The nerves of the heart, which under normal circumstances control its movements, are the following :- 1. Cardiac branches of the vagus. 2. The cardiac branches of the sympathetic. 3. The intrinsic nerves of the heart. These were formerly regarded as more or less independent of the other two sets of fibres; we now know, however, that they are merely the termi- CH. XXXV.] THE CARDIAC VAGUS. 423 nations of the other nerves in the heart-wall. For convenience of description, however, we will keep the old name. The Vagus.-This arises from the grey matter in the floor of the fourth ventricle, at the point of the calamus scriptorius. It leaves the bulb by some 10-15 bundles behind the ninth nerve, and leaves the skull by the jugular foramen, having upon it a ganglion called the jugular ganglion. It gives off branches to the vessels of the meninges and to the ear, and then receives certain connecting branches : (a) from the glosso-pharyngeal, the physiological meaning of which is not known; and (6) it receives the whole inner division of the spinal accessory nerve. This nerve arises from a centre in the bulb close to and below the vagal nucleus; the outer half of the same nerve arises from spinal roots, and supplies the sterno-mastoid and trapezius. It is the fibres of the spinal accessory that join the vagus that give to the vagus its cardio-inhibitory function; and the spinal acces- sory nucleus, not that of the vagus proper, is the cardio-inhibitory centre. The vagus then gives off branches to the pharynx, larynx, Fig. 389.-Tracing showing the actions of the vagus on the heart. Aur., auricular; Vent., ventricular tracing. The part between perpendicular lines indicates period of vagus stimulation. (7.8 indicates that the secondary coil was 8 c.wi. from the primary. The part of tracing to the left shows the regular contractions of moderate height before stimulation. During stimulation, and for some time after, the beats of auricle and ventricle are arrested. After they commence again they are single at first, but soon acquire a much greater amplitude than before the application of the stimulus. (From Brunton, after Gaskell.) heart, lungs, oesophagus, and stomach, the remainder joining the coeliac plexus, and contributing to the nerve supply of various abdominal organs. We have, however, in this place only to deal with the cardiac fibres. It has been known since the experiments of the Bros. Weber in 1845, that stimulation of one or both vagi produces slowing of the beats of the heart. It has since been shown in all of the vertebrate animals experimented with, that this is the normal 424 PHYSIOLOGY OF THE HEART. [ch. xxxv. ,ction of vagus stimulation. Moreover, section of one nerve, or at any rate of both vagi, produces acceleration of the pulse, and stimulation of the distal or peripheral end of. the divided nerve produces normally slowing or stopping of the heart's beats. It appears that any kind of stimulus produces the same effect, either chemical, mechanical, electrical, or thermal, but that of these the most potent is a rapidly interrupted induction current. A certain amount of confusion has arisen as to the effect of vagus stimulation in consequence of the fact that within the trunk of the nerve is contained, in some animals, fibres of the sympathetic, and it depends to some extent upon the exact position of the application of the stimulus, as to the exact effect produced. Speaking generally, however, excitation of any part of the trunk of the vagus produces inhibition, the stimulus being particularly potent if applied to the termination of the vagi in the heart itself, where they enter the substance of the organ at the situation of the sinus ganglia. The stimulus may be applied to either vagus with effect, although it is frequently more potent if applied to Fig'. 390.-Tracing showing diminished amplitude and slowing of the pulsations of the auricle and ventricle without complete stoppage during irritation of the vagus. (From Brunton, after Gaskell.) the nerve on the right side. The effect of the stimulus is not imme- diately seen ; one or more beats may occur before stoppage of the heart takes place, and slight stimulation may produce only slowing and not complete stoppage of the heart. The stoppage may be due either to prolongation of the diastole, as is usually the case, or to diminution of the systole. Vagus stimulation inhibits the spon- taneous beats of the heart only, it does not do away with the irritability of the heart-muscle, since mechanical stimulation may bring out a beat during the still-stand caused by vagus stimula- tion. The inhibition of the beats varies in duration, but if the stimulation be a prolonged one, the beats may reappear before the current is shut off When the beats reappear, the first few are usually feeble, and may be auricular only ; after a time the CH. XXXV.] THE CARDIAC SYMPATHETIC. 425 contractions become stronger, and very soon exceed both in ampli- tude and frequency those which occurred before the application of the stimulus (figs. 389, 390). One branch of the vagus is called the depressor ; it is only a separate nerve in a few animals. Unlike the inhibitory branches, it is afferent, not efferent; it carries impulses to the vaso-motor centre in the bulb from the heart. We shall study its use in connection with blood pressure. The Sympathetic.-The influence of the sympathetic is the reverse of that of the vagus. Stimulation of the sympathetic, even of one side, produces acceleration of the heart-beats, and according to certain observers, section of the same nerve produces lowing. The acceleration produced by stimulation of the sym- pathetic fibres is accompanied by increased force, and so the action of the nerve is more properly termed augmentor. The action of the sympathetic differs from that of the vagus in -several particulars besides the augmentation which is produced : first, the stimulus required to produce any effect must be more powerful than is the case with the vagus stimulation ; secondly, a longer time elapses before the effect is manifest; and thirdly, the augmentation is followed by exhaustion, the beats being after a time feeble and less frequent. The fibres of the sympathetic system, which influence the heart-beat in the frog, leave the spinal cord by the anterior root of the third spinal nerve, and pass thence by the ramus commu- nicans to the third spinal ganglion, thence to the second spinal ganglion, and thence by the annulus of Vieussens (round the subclavian artery) to the first spinal ganglion, and thence in the main trunk of the sympathetic, to near the exit of the vagus from the cranium, where it joins that nerve and runs down to the heart within its sheath, forming the joint vago-sympathetic trunk. These fibres are indicated by the dark line in the figure on the next page. The fibres of the sympathetic seen running up into the skull are for the supply of blood-vessels there. It should be noted that the frog has no spinal accessory nerve, so that in this animal the cardio-inhibitory fibres are true vagus fibres. From the fact that the augmentor fibres are joined to the vagus trunk, it may be understood that the effect of the stimula- tion of the vagus in the frog is not in all cases purely inhibitory, but may be augmentor, according to the position where the stimulus is applied, the intensity of the stimulus, and the con- dition of the heart; if it is beating strongly a slight vagus stimulation will produce immediate inhibition. 426 PHYSIOLOGY OF THE HEART. [ch. xxxv. In the dog, the augmentor fibres leave the cord by the second and third dorsal nerves, and possibly by anterior roots of two or more lower nerves, passing by the rami communicantes to the ganglion stellatum, or first thoracic ganglion, thence by the annulus of Vieussens to the inferior cervical ganglion of the sympathetic; fibres from the annulus, or from the inferior cervical ganglion proceed to the heart. In man, the cardiac branches of the sympathetic probably originate in the same way; they pass to the heart from the Fig'. 391.-Heart nerves of frog. annulus of Vieussens, and cervical sympathetic in superior, middle and lower bundles of fibres. These pass to the cardiac plexus and reach the heart surrounding the coronary vessels. They probably contain vaso-motor fibres for these vessels, as well as the more important fibres for the heart itself. The proof of the statement previously made that in mammals the fibres in the vagus, by means of which inhibitory influences are conveyed to the heart, are derived from the spinal accessory CH. xxxv,] INHIBITION. 427 nerve is the following :-When that nerve is divided certain fibres of the vagus trunk degenerate, and afterwards stimulation of the vagus trunk no longer produces inhibition. The spinal accessory fibres are fine medullated fibres, 2 /z to 3 p. in diameter, and may be traced to the heart, and lose their medulla in the ganglia of that organ. Some, however, arc non-medullated when they enter. They pass out of Remak's ganglion more generally as medullated nerves, forming the nerve-fibres of the septum, but after these septal fibres issue from Bidder's ganglia, they enter the ventricle as non-medullated nerves, in the frog at any rate, but in mammals a few fibres are medullated. The position of these ganglia we shall study immediately. The sympathetic fibres, on the other hand, reach the heart as non-medullated fibres, having lost their medulla in the sympathetic ganglia. The course of the augmentor fibres in the spinal cord is not known, but it is thought that in all probability they are connected with an augmentor centre in the medulla. The circulation of venous blood appears to stimulate the inhibitory centre, and of highly oxygenated the augmentor centre. Influence of Drugs.-The question of the action of drugs on the heart forms a large branch of pharmacology. We shall be content here with mentioning two only, as they are largely used for experimental purposes by physiologists. Atropine produces considerable augmentation of the heart-beats by paralysing the inhibitory mechanism. Muscarine (obtained from poisonous fungi) produces marked slowing, and in larger doses stoppage of the heart. It produces a similar effect to that of prolonged vagus stimulation, and, as in that case, the effect can be removed by the action of atropine. The action of atropine cannot, however, be antagonised by muscarine. That these drugs act on the nerves, and not the muscular substance of the heart, is shown by the fact that in the hearts of early embryos, so early that no nerves have yet grown to the heart, these drugs have little or no effect. (Pickering.) Reflex Inhibition.- Thus there is no doubt that the vagi nerves arc simply the media of an inhibitory or restraining influence over the action of the heart which is conveyed through them from the centre in the medulla oblongata which is always in operation. The restraining influence of the centre in the medulla may be reflexly increased by stimulation of almost any afferent nerve, particularly of the abdominal sympathetic, so as to produce slowing or stoppage of the heart, through impulses passing down the vagi. As an example of this reflex stimu- lation, the well-known effect on the heart of a violent blow 428 PHYSIOLOGY OF THE HEART. [ch. xxxv. on the epigastrium may be referred to. The stoppage of the heart's action in this case is due to the conveyance of the stimulus by fibres of the sympathetic (afferent) to the medulla oblongata, and its subsequent reflection through the vagi (efferent) to the muscular substance of the heart. Chloroform vapour and tobacco smoke in some people and animals, by acting on the terminations of the vagi or their branches in the respiratory system, may also produce reflex inhibition of the heart. Some very remarkable facts concerning the readiness by which reflex inhibition of the fish's heart may be produced were made out by Prof. McWilliam; any slight irritation of the tail, gills, mucous membrane of mouth and pharynx, or of the parietal peritoneum, causing the heart to stop beating. In connection with the subject of reflex inhibition, it may be mentioned in conclusion that though we have no volun- tary control over the heart's movements, yet cerebral excitement will produce an effect on the rate of the heart, as in certain emotional conditions. Intracardiac Nerves.-The heart beats after its removal from the body ; in the case of the frog and other cold-blooded animals, this will go on for hours, and under favourable circum- stances for days. In the case of the mammal, it is more a ques- tion of minutes. At one time this was supposed to be due to its intrinsic nervous system; the heart was regarded almost as a com- plete organism, possessing not only parts capable of movement, but also a nervous system to initiate and regulate those move- ments. We now, however, look upon the muscular tissue of the heart, rather than its nerves, as the tissue which possesses the power of rhythmical movement, because muscular tissue which has no nerves at all possesses this property. For instance, the ventricle apex of the frog's or tortoise's heart possesses no nerves, but if it is cut off and fed with a suitable nutritive fluid at considerable pressure, it will beat rhythmically (Gaskell) The apparatus by which this may be accomplished we shall study at the end of this chapter. The middle third of the ureter is another instance of muscular tissue free from nerves, but which nevertheless executes peristaltic movements. Perhaps, however, the most striking instance is that of the foetal heart, which begins to beat directly it is formed, long before any nerves have grown into it. The power of rhythmical peristalsis therefore resides in the muscular tissue itself, though normally during life it is controlled and regulated by the nerves that supply it. The intracardiac nerves have been chiefly studied in the frog ; the two vago-sympathetic nerves terminate in various groups of ch. xxxV.J REMAK'S GANGLION. 429 ganglion cells ; of these the most important are Remak's ganglion, situated at the junction of the sinus with the right auricle ; and Bidder's, at the junction of the auricles and ventricle. From the Fig. 392. Course of the nerves in the auricular partition wall of the heart of a frog, rf, dorsal branch ; v, ventral branch. (Ecker.) ganglion cells fibres spread out over the walls of sinus, auricles, and the upper part of the ventricle. Remak's ganglion used to be called the local inhibitory centre of the heart; it is really the Fig. 393.-Isolated nerve-cells from the frog's heart. I. Usual form. II. Twin cell. C, capsule ; N, nucleus ; N', nucleolus ; P, process. (From Ecker.) termination of the inhibitory fibres, and stimulation of the heart at the sino-auricular junction is the most certain way of obtain- ing stoppage of the heart. Bidder's ganglion was called the local accelerator centre for a corresponding reason. The two preceding figures show the vagal terminations in Remak's ganglion (fig. 392), some isolated nerve-cells from the frog's heart (fig. 393) ; and fig. 394 is a rough diagram to indi- cate the positions of the principal ganglia. 430 PHYSIOLOGY OF THE HEART. ed!. xxxv. In connection with the rhythmic contraction of muscle, it is necessary to allude to what is known as Stannius' experiment. This experiment consists in applying a tight ligature to the heart between the sinus and the right auricle, the effect of which is to stop the beat of the heart below' the ligature, whilst the sinus continues to beat. If a second ligature be applied at the junc- tion of the auricles and ventricle, the ventricle begins to beat, whilst the auricles continue quiescent. In both cases the quies- cent parts of the heart may be made to give single contractions in response to mechanical or electrical stimulation. A consider- Fig. 394.-Diagram of ganglia in frog's heart. R, Remak's, B, Bidder's ganglion S, sinus ; A, auricle; V, ventricle. able amount of discussion has arisen as to the explanation of these phenomena. It was suggested that the action of the liga ture is to stimulate some inhibitory nervous mechanism in the sinus, whereby the auricles and ventricle can no longer continue to contract, but this suggestion must certainly be given up if the present theory as to the functions of the nerve ganglia be correct. The effect of Stannius' ligature is simply an example of what has been called by Gaskell blocking. The explanation of this term is as follows :-It appears that under normal conditions the wave of contraction in the heart starts at the sinus and travels downwards over the auricles to the ventricle, the irrita- bility of the muscle and the power of rhythmic contractility being greatest in the sinus, less in the auricles, and still less in the ventricle, whilst under ordinary conditions the apical portion of the ventricle exhibits very slight irritability and still less power of spontaneous contraction. Thus it may be supposed that the wave of contraction beginning at the sinus is more or less blocked by a ring of muscle at lower irritability at its junction with the auricles, and again the wave in the auricles is similarly delayed in its passage over to the ventricle by a ring of lesser irritability, and thus the wave of contraction starting at the sinus is broken as it were both at the auricles and at the ventricle. By an arrangement of ligatures, or better, of a system of clamps, one part of the heart may be isolated from the other portion, and the contraction when stimulated by an induction shock may be CH. XXXV.] THE STANXJUS HEART. 431 made to stop in the portion of the heart-muscle in which it begins. It is not unlikely that the contraction of one portion of the heart acts as a stimulus to the next portion, and that the sinus contraction generally begins first, since the sinus is the most irritable to stimuli, and possesses the power of rhythmic contractility to the most highly developed degree. It must not be thought, however, that the wave of contraction is incapable of passing over the heart in any other direction than from the sinus downwards ; it has been shown that by application of appropriate stimuli at appropriate instants, the natural sequence of beats may be reversed, and the contraction starting at the arterial part of the ventricle may pass upwards to the auricles and then to the sinus in order. An exceedingly interesting fact with regard to the passage of the wave in any direction has been made out by partial division of the muscular fibres at any point, whereby one part of the wall of the heart is left connected with the other parts by a small portion of undivided muscular tissue, and the wave of contraction then being only able to pass to the next portion of the wall every second or third beat. Thus division of the muscle has much the same effect as partial clamping it in the same position, or of a ligature similarly applied, but not tied tightly. The first Stannius ligature acts as a partial or complete block, and pre- vents the stimulus of the sinus-beat from passing further down the heart, but parts below the ligature may be made to contract by stimuli applied to them directly. The second Stannius liga- ture acts as a stimulus to the ventricle. Instead of applying the second ligature, the experiment may be varied by cutting off the heart beyond the first ligature; the stimulation caused by cutting produces waves of contraction that travel over auricles and ventricle. The importance of the sinus as the starting-point of the peri- stalsis can be very well shown by warming it. If the whole heart is warmed by bathing it in salt solution at about the body temperature, it beats faster; this is due to the sinus starting a larger number of peristaltic waves; that this is the case may be demonstrated by warming localised portions of the heart by a small galvano cautery; if the sinus is warmed the heart beats faster, but if the auricles or ventricle are warmed there is no alteration in the heart's rate. The sinus in the frog's heart, and that portion of the right auricle of the mammal's heart which corresponds to the sinus, is always the last portion of the heart to cease beating on death, or after removal from the body (uZtwna moriens). This is an addi 432 PHYSIOLOGY OF THE HEART. fen. xxxv. tional proof of the superior rhythmical power possessed by this part of the heart. The fact that the Stannius heart is quiescent has enabled physiologists to study the effects of stimuli upon heart muscle. A single stimulus produces a single contraction, which has a long latent period, is slow, and propagated as a wave over the heart at the rate of to 4 inch, or io-15 cm. a second. A second stimulus causes a rather larger contraction, a third one larger still, and so on for some four or five beats, when the size of the contraction becomes constant. This staircase phenomenon, as it is called, is also seen in voluntary muscle, but it is more marked in the heart. The following tracing shows the result of an actual experiment:- Fig. 395.-Staircase from frog's heart. This was obtained from a Stannius preparation ; an induction shock being sent into it with every revolution of the cylinder (rapid rate). The contractions became larger with every beat. To be read from right to left. There are, however, more marked differences than this between voluntary and heart muscle. The first of these is, that the amount of contraction does not vary with the strength of the stimulation. A stimulus strong enough to produce a contraction at all brings out as big a beat as the strongest. The second is, that the heart muscle has a long refractory period ; that is to say, after the application of a stimulus, a second stimulus will not cause a second contraction until after the lapse of a certain interval called the refractory period. The third difference depends on the second, and consists in the fact that the heart muscle can never be thrown into complete tetanus by a rapid series of stimu- lations ; with a strong current there is a partial fusion of the beats, but this is entirely independent of the rate of faradisation. Indeed, as a rule, the heart responds by fewer beats to a rapid than to a slow rate of stimulation. In spite of these differences there are many and important resemblances between heart muscle and voluntary muscle. The thermal and chemical changes are similar; there is a using-up of oxygen and a production of carbonic acid and sarco- CH. XXXV.] PERFUSION CANNULA. 433 lactic acid. The using-up of oxygen was well illustrated by an experiment of Prof. Yeo's. He passed a weak solution of oxy- haemoglobin through an excised beating frog's heart, and found that after it had passed through the heart, the solution became less oxygenated and venous in colour. The electrical changes are also similar, and have already been dwelt upon in Chapters XII. and XIV. We may, however, add to the facts there described another experiment which was performed by Dr. Gaskell. He argued that if the vagus is an anabolic nerve, and the sympathetic a katabolic nerve (for explanation of these terms see p. 375), they ought to produce different electrical as well as different nutritional effects. So he took a strip of the auricle of the tortoise which was still in connection with the two nerves ; on stimulating the vagus it became electrically more negative, and on stimulating the sympathetic it became more positive, a result which justified his argument. Instruments for Studying the Excised Frog's Heart. If a frog's heart is simply excised and allowed to remain with- out being fed, it ceases to beat after a time varying from a few minutes to an hour or so, but if it is fed with a nutritive fluid, it will continue to beat for many hours. A very good nutritive fluid is defibrinated blood di- luted with twice its volume of physio- logical saline solution. Dr. Ringer has, however, shown that nearly as good results are obtained with physiological saline solution to which minute quanti- ties of calcium and potassium salts have been added; in other words, the inorganic salts of the blood will main- tain cardiac activity for a time without the addition of any organic material. The fluid is passed through the heart by means of a perfusion cannula (fig. 396). The heart is tied on to the end of the cannula; the fluid enters by one and leaves by the other tube. There have been innumerable instruments devised for obtaining graphic records of the heart's movements under these circum- stances, but we will be content with describing three only. They Fig. 3g6.--Kroneeker's Perfusion Cannula, for supplying Fluids to the interior of the Frog's Heart. It consists of a double tube, one outside the other; the end view is shown in the engraving. The inner tube branches out to the left; thus, when the ventricle is tied to the outer tube of the cannula, a current of liquid can be made to pass into the heart by one tube and out through the other. 434 PHYSIOLOGY OF THE HEART. [cn. xxxv. have been much used in the investigation of drugs on the heart, and the results obtained have been of much service to physicians. (1) The heart having been securely tied on to the perfusion cannula, the circulating fluid is passed through it. One stem of the cannula is then attached by the small side branch on the left in fig. 396 by a tube containing salt solution to a small mercurial manometer, provided with a float, on the top of which is a writing style. The apparatus is arranged so that the movements of the mercury can be recorded by the float and the writing style on a slowly revolving drum. The movements of the mercury are due to change in the endocardiac pressure ; the other two instruments we shall describe are constructed on the opposite principle : the heart is enclosed in a chamber filled with oil, and the movements of this oil outside the heart are registered. (2) Roy's Tonometer (fig. 397) : A small bell-jar, open above, but provided with a firmly fitting cork, in which is fixed a double cannula, is adjustable by a smoothly ground base upon a circular brass plate, about 2 to 3 inches in diameter. The junction is made complete by greasing the base with lard. In the plate, which is, fixed to a stand adjustable on an upright, are two Fig. 397.-Roy's Tonometer. holes, one in the centre, a large one about one-third of an inch in diameter, to which is fixed below a brass grooved collar, about half an inch deep ; the other hole is the opening into a pipe provided with a stopcock. The opening provided with the collar is closed at the lower part with a membrane, which is closely tied by means of a ligature around the groove at the lower edge of the collar. To this membrane a piece of cork is fastened by sealing-wax, from which passes a wire, which is attached to a lever (cut short in the diagram) fixed on a stage below the apparatus. When using the apparatus, the bell-jar is filled with olive oil. The heart CH. XXXV.] HEART PLETHYSMOGRAPH. 435 of a large frog is prepared and the cannula fixed in the cork is firmly tied into it; the tubes of the cannula communicate with the reservoir of circu- lating fluid on the one hand, and with a vessel to receive it after it has run through on the other. The cannula with heart attached is passed into the oil, and the cork firmly secured. Every time the heart enlarges, the mem- brane is pressed down ; every time the heart contracts the membrane is pulled up ; the lever follows and magnifies these movements. The lever is adjusted to a convenient elevation and allowed to write on a moving drum. After a short time the heart may stop beating ; but two wires are arranged, the one in the cannula, the other projecting from the plate in such a way that the heart can be moved against them by shifting the position of the bell-jar a little. The wires act as electrodes, and can be made to communi- cate with an induction apparatus, so that induction shocks can be sent into Fig. 398.-Schafer's heart plethysmograph. the heart to produce contractions. After a greater or less' time the heart ceases to beat altogether ; before doing so it becomes irregular. A frequent form of irregularity seen consists of groups of contractions each showing a staircase, separated by long intervals of quiescence. (Luciani's Groups.) (3) Schafer's Heart-pletkysmograph.-The principle of this apparatus is the same as Roy's. A diagrammatic sketch of it is given in fig. 398. The heart, tied on to a double cannula, is inserted into an air-tight vessel con- taining oil. On one side of the vessel is a tube, in which a lightly-moving piston is fitted ; to this a writing point is attached. The piston is moved backwards and forwards by the changes of volume in the heart causing the oil to alternately recede from and pass into this side tube. The correspond- ing tube on the other side can be opened and the tube with the piston closed when one wishes to cease recording the movements. 436 THE CIRCULATION IN THE BLOOD-VESSELS. [CH. XXXVI. CHAPTER XXXVI. THE CIRCULATION IN THE BLOOD-VESSELS. The circulation through the vessels is accomplished by the heart as the primary propelling force ; the pressure in the heart is greater than that in the arteries ; the arterial pressure (which is kept high not only by the heart's force, but by the existence of resistance at the periphery) is greater than that in the capil- laries, and the pressure is lowest in the veins, especially at their entrance into the heart; and fluid always flows in the direction of pressure. Before, however, passing on to the all-important question of blood-pressure, we must first consider various other phenomena in connection with the flow in the vessels, such as the velocity of the stream, and the character of the flow in different parts of the vascular circuit. The Velocity of the Blood-Flow The velocity of the blood-current at any given point in the various divisions of the circulatory system is inversely propor- tional to their sectional area at that point. If 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 if a similar correspondence of capacity existed in the veins and arteries, there would be an equal correspondence in the rapidity of the circulation in them. But when an artery divides, the sectional area of its branches is greater than that of the artery from which they originate. The only exception to this rule is seen in the division of the aorta into the two common iliac arteries. It is the same with the veins, the sectional area of a vein formed by the union of smaller veins is less than the total sectional area of its tributaries. From the aorta onwards to the capillaries there is a gradual increase of the area of the stream with a corresponding diminution of its velocity; from the capillaries onwards to the heart there is a gradual de- crease of the bed of the stream and a corresponding increase in its velocity. In other words the arterial and venous systems may be repre- sented by two truncated cones with their apices directed towards the heart; the area of their united base (the sectional area CH. XXXVI. ] LUDWIG'S STROMUHR. 437 of the capillaries) is about 700 times as great as that of the truncated apex representing the aorta. Thus the velocity of blood in the capillaries is not more than of that in the aorta. The veins are larger than the corresponding arteries, and so the rate there is proportionally slower. In the Arteries.-The velocity of the stream of blood is greater in the arteries than in any other part of the circulatory system, and in them it is greatest in the neighbourhood of the heart, and during the ventricular systole. The rate of movement diminishes during the diastole of the ventricles, and in the parts of the arterial system most distant from the heart. A few of the results obtained by different observers may be here given. In the carotid of the dog, the velocity is 205-350 mm. per second. „ „ „ horse „ 306 „ „ metatarsal ,, „ 56 „ In very round numbers we may state the average speed in the large arteries as a foot per second. Estimation of the Velocity.-Various instru- ments have been devised for measuring the velocity of the blood-stream in the arteries. Ludwig's Stromuhr (fig. 399) consists of a U-shaped glass tube dilated at a and a', the ends of which h and i, are of known calibre. The bulbs can be filled by a common opening at k. The instrument is so con- trived that at b and b', the glass part is firmly fixed into metal cylinders, attached to a circular hori- zontal table c o', capable of horizontal movement on a similar table d d' about the vertical axis marked in figure by a dotted line. The opening in c o', when the instrument is in position, as in fig. 399, corresponds exactly with those in d d'; but if c c' be turned at right angles to its present position, there is no communication between h and a, and i and a', but lb communicates directly with i ; and if turned through two right angles d communicates with d, and c with d', and there is no direct commu- nication between lb and i. The experiment is per- formed in the following way :-'The artery to be experimented upon is divided and connected with two cannulre and tubes which fit it accurately with h and i-lb the central end, and i the peripheral ; the bulb a is filled with olive oil up to a point rather lower than lb, and a' and the remainder of a is filled with defibrinated blood ; the tube on It is then carefully clamped : the tubes d and d' are also filled with defibrinated blood. When everything is ready, the blood is allowed to flow into a through h, and it pushes before it the oil, and that the defibrinated blood into the artery through i, and replaces it in a'; when the blood reaches the former level of the oil in a, the disc c c' is turned rapidly through two right angles, and the blood Fig. 399.-Ludwig's Stromuhr. 438 THE CIRCULATION IN THE BLOOD-VESSELS. [CH. xxxvi. flowing through d into a' again displaces the oil which is driven into a. This is repeated several times, and the duration of the experiment noted. The capacity of a and a! is known ; the diameter of the artery is also known by its corresponding with the cannulae of known diameter, and as the number of times a has been filled in a given time is known, the velocity of the current can be calculated. Chauveau's instrument (fig. 400) consists of a thin brass tube, a, in one side of which is a small perforation closed by thin vulcanised india-rubber. Passing through the rubber is a fine lever, one end of which, slightly flat- tened, extends into the lumen of the tube, while the other moves over the face of a dial. The tube is inserted into the interior of an artery, and ligatures applied to fix it, so that the movement of the blood may, in flowing Fig. 400.-Diagram of Chauveau's Dromograph. a, Brass tube for introduction into the lumen of the artery, and containing an index-needle, which passes through the elastic membrane in its side, and moves by the impulse of the blood current, c, Graduated scale, for measuring the extent of the oscillations of the needle. through the tube, be indicated by the movement of the outer extremity of the lever on the face of the dial. The Heematochometer of Vierordt resembles in principle that of Chauveau. The Heemadromometer of Volkmann, one of earliest instruments devised for this purpose, is simply a long glass U-tube of the same calibre as the artery under investigation. It is provided with a stop-cock, so that at a given moment the blood can be admitted, and the time that the blood takes to reach its other end is observed. In the Capillaries.-The microscopic observations of E. H. Weber and Valentin agree very closely as to the rate of the blood-current in the capillaries of the frog; and the mean of their estimates gives the velocity of the systemic capillary circulation at about one inch (25 mm.) per minute. The velocity in the capil- laries of warm-blooded animals is greater. In the dog to -pit- inch (-5 to '75 mm.) a second. This may seem inconsistent with the facts, which show that the whole circulation is accomplished in less than half a minute. But the whole length of capillary vessels through which any given portion of blood has to pass, probably does not exceed from to tilth of an inch (-5 mm.); CH. xxxvi.] THE RATE OF BLOOD FLOW. 439 and therefore the time required for each quantity of blood to traverse its own appointed portion of the general capillary system will scarcely amount to a second. In the Veins.-The velocity of the blood is greater in the veins than in the capillaries, but less than in the arteries: this fact depends upon the relative capacities of the arterial and venous systems. If an accurate estimate of the proportionate areas of arteries and the veins corresponding to them could be made, we might, from the velocity of the arterial current, calculate that of the venous. A usual estimate is, that the capacity of the veins is about twice or three times as great as that of the arteries, and that the velocity of the blood's motion is, therefore, about twice or three times as great in the arteries as in the veins, 8 inches (200 mm.) a second. 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. Of the Circulation as a whole.-Among the earliest investigators of the question how long an entire circulation takes was Hering. He injected a solution of potassium ferrocyanide into the central end of a divided jugular vein and collected the blood either from the other end of the same vein, or from the corresponding vein of the other side. The substance injected is one that can be readily detected by a chemical test (the prussian blue reaction). Vierordt improved this method by collecting the blood as it flowed out, in a rotating disc divided into a number of compartments. The blood was tested in each compartment, and the ferrocyanide discovered in one which in the case of the horse received the blood about half a minute after the injection had been made. The experiment was performed in a large number of animals, and the following were a few of the results obtained : In the horse . . . 3 r seconds. „ dog . . . . 16 „ „ cat .... 6.5 „ „ fowl . . . . 5 At first sight these numbers show no agreement, but in each case it was found that the time occupied was 27 heart beats. The dog's heart, for instance, beats twice as fast as the horse's, and so the time of the entire circulation only occupies half as much time. If, now, this is applied to man, 27 heart beats, the heart beat- ing 72 times a minute, will occupy 23'2 seconds; and this was 440 THE CIRCULATION IN THE BLOOD-VESSELS, [ch. xxxvi. until recently taken as the time of the complete circulation, and from it the following calculation was made; 2 7 heart beats will propel all the blood round the body, so that if we multiply 27 by the capacity of the left ventricle, we obtain the total amount of blood in the body ; thus : 27 X 187'5 = 5,062 grammes, which is equal to about one- thirteenth of the body-weight of a man weighing 65 kilograms. The question, however, has recently been re-investigated by Prof. Stewart by new and improved methods, which have shown that the time of the circulation is about 15 seconds, that is con- siderably less than was found by the researches of Hering and Vierordt. The great objection to the older method is the fact that haemorrhage is occurring throughout the experiment, and this would materially weaken the heart and slow down the cir- culation. The substance Stewart injects is methylene blue; this can be readily detected through the vessel walls, and the time of its reappearance noted ; this method gives the same results as a more complicated method involving the use of a galvanometer previously adopted by the same investigator. Stewart has applied his method also for determining the time occupied by the passage of blood through various districts of the circulation; among the organs, for instance, he finds that the circulation through the kidneys is slowest. The Use of the Elasticity of the Vessels. If a pump is connected with a rigid tube, such as a glass tube, every time that a certain amount is forced into one end of the tube an exactly equal quantity will be forced out at the other end. During the intervals between the pumpings, the flow will cease. If the far end of the tube is partially closed, the flow will still be intermittent, only the quantity injected and the quantity ejected, though still of equal volume, will be diminished. If we employ an elastic tube instead of a rigid tube, and the end is left freely open, the flow will still be intermittent as in the case of the rigid tube; but if the end of the elastic tube is narrowed by a clamp the intermittent flow will be converted into a more or less perfectly constant flow. Each stroke of the pump forces a certain amount of fluid into the tube, but owing to the peripheral resistance, it cannot all escape at once, and part of the force of the pump is spent in distending the walls of the tube. This distended elastic tube, however, tends to empty itself, and forces out the fluid which distends it before the next stroke of the pump takes place. One part of the fluid is therefore forced ch. xxxvi. ] ELASTICITY OF THE VESSELS. 441 out by the immediate effect of the pump, and another part by the elastic recoil of the tube between the strokes. If the rate of the pump and the distension of the tube which it produces is sufficiently great, the fluid forced out between the strokes will be equal to that entering at each stroke and thus the outflow becomes continuous. Let us now apply this to the body. At each beat the left ventricle forces about four ounces of blood into the already full arterial system. The arteries are elastic tubes, and the amount of elastic tissue is greatest in the large arteries. The first effect of the extra four ounces is to distend the aorta still further; the elastic recoil of the walls drives on another portion of blood which distends the next section of the arterial wall, and this wave of distension is transmitted along the arteries with gradually diminishing force as the total arterial stream becomes larger. This wave constitutes the pulse. Between the strokes of the pump, or, in other words, during the periods of diastole, the arteries tend to return to their original size, and so drive the blood on. The flow, therefore, does not cease during the heart's inactivity, but although the force is an intermittent one, the flow through the capillaries and the veins beyond is a constant one, all trace of the pulse having disap- peared. The peripheral resistance which keeps up the blood- pressure in the arteries, and like the clamp on our india-rubber tube, assists in the conversion of the intermittent into a continu- ous stream is to be found in the arterioles or small arteries, just before the blood passes into what we may term the vast capillary lake. These small arteries with their relative excess of muscular tissue, which in health is always in a tonic state of moderate con- traction, play the part of a multitudinous system of stop-cocks. The large arteries contain a considerable amount of muscular as well as elastic tissue. This co-operates with the elastic tissue in adapting the calibre of the vessels to the quantity of blood they contain. For the amount of blood in the vessels is never constant, and were the elastic tissue only present, the pressure exercised by the walls of the containing vessels on the contained blood would be sometimes very small, sometimes too great. The presence of a contractile clement, however, provides for a certain uniformity in the amount of pressure exercised. There appears no reason to suppose that the muscular coat assists in propelling the onward current of blood, except in virtue of the fact that muscular tissue is elastic, and therefore co-operates in the large arteries with the elastic tissue in keeping up the constant flow in the way already described. 442 THE CIRCULATION IN THE BLOOD-VESSELS. [CH. XXXVI. The contractility of the arterial walls fulfils a useful purpose in checking haemorrhage should a small vessel be cut as it assists in the closure of the cut end, and this in conjunction with the coagulation of the blood arrests the escape of blood. The Pulse. This is the most characteristic feature of the arterial flow. It is a wave of expansion which travels along the arteries due to the propulsion of the contents of the left ventricle into the already full arterial system. The more distant the artery from the heart, the longer is the interval that elapses between the ventricular beat and the arrival of the pulse. Thus it is felt in the carotid earlier than in the radial artery, and it is still later in the dorsal artery of the foot. The difference of time is, however, very slight; it is only a minute fraction of a second; for a distinction must be drawn between the propagation of the pulse and the rate of blood flow in the arteries ; the wave travels at the rate of from 5 to io metres a second, that is twenty or thirty times the rate of the blood current. The pulse may be compared to a wave produced by the wind travelling rapidly down a sluggishly-flowing river. A physician usually feels the pulse in the radial artery, since this is near the surface, and supported by bone. It is a most valuable indication of the condition of the patient's heart and vessels. It is necessary in feeling a pulse to note the following points :- 1. Its frequency ; that is the number of pulse beats per minute. This gives the rate of the heart beats. 2. Its length ; that is how long a time each pulse-beat occupies. 3. Its strength; whether it is a strong, bounding pulse, or a feeble beat; this indicates the force with which the heart is beating. 4. Its regularity or irregularity; irregularity may occur owing to irregular cardiac action either in force or in rhythm. 5. Its tension; that is the force necessary to obliterate it. This gives an indication of the state of the arterial walls and the peripheral resistance. In disease there are certain variations in the pulse, of which wo will mention only two; namely, the pulse, due to the heart missing a beat every now and then; and the water hammer pulse, due either to aortic regurgitation or to a loss of elasticity of the arterial walls, cither of these circumstances diminishing the onward flow of blood during the heart's diastole, and thus CH. xxxvi.] SPHYGMOGRAPHS. 443 the suddenness of the impact of the blood on the arterial wall during systole is increased. When this condition is due to arterial disease, such as atheroma or calcification, this sudden Fig. 401.-Marey's Sphygmograph, modified by Mahomed. pulse combined with the increased brittleness of the arteries may- lead to rupture of the walls, and this is especially serious if it occurs in the arteries of the brain (apoplexy). In order to study the pulse more fully, it is necessary to obtain Fig. 402.-Diagram of the lever of the Sphygmograph. a graphic record of the pulse-beat, and this is accomplished by the use of an instrument called the sphygmograph. This instrument consists of a series of levers, one end of which is a button placed over the artery, and the other end is provided with a writing point to inscribe the magnified record of the arterial movement on a travelling surface. 444 THE CIRCULATION TN THE BLOOD-VESSELS, [ch. XXXVI. The instruments most frequently used are those of Marey, one of the numerous modifications of which is represented in figures 401, 402, and 403, and of Dudgeon (fig. 404). Fig. 403.-The Sphygmograph applied to the arm. Each instrument is provided with an arrangement by which the pressure can be adjusted so as to obtain the best record. The measurement of the pressure is, however, rough, and both instru- Fig. 404.-Dudgeon's Sphygmograph. The dotted outline represents the piece of blackened paper on which the sphygmogram is written. ments have the disadvantage of giving oscillations of their own to the sphygmogram; this is specially noticeable in Dudgeon's sphygmograph. But these defects may be overcome by the use of more complicated apparatus, such as the sphygmometer of Boy and Adami and the more recently invented sphygmomanometer of Mosso, both of which, however, are unsuitable for clinical use. CH. XXXVI. ] SPHYGMOGRAPHS. 445 Fig. 405 represents a typical sphygmographic tracing obtained from the radial artery. It consists of an upstroke due to the expansion of the artery, and a downstroke due to its retrac- tion. The descent is more gradual than the upstroke, because the elastic recoil acts more constantly and steadily than the heart-beat. On the descent are several secondary (katacrotic) elevations. A is the primary, or percussion wave; C is the pre-dicrotic, or tidal wave ; D is the dicrotic wave, and E the post-dicrotic wave, and of these there may be several. In some cases there is a secondary wave on the upstroke, which is called an anacrotic wave. These various secondary waves have received different inter- pretations, but the best way of explaining them is derived from information obtained by tak- ing simultaneous tracings of the pulse, aortic pressure, apex beat, and interventricular pressure, as in the researches of Hurthle. By this means it is found that the primary and pre-dicrotic waves occur during the systole of the heart, and the remainder of the waves during the diastole. The closure of the aortic valves occurs just before the dicrotic wave. The secondary waves, other than the dicrotic wave, are due to the elastic tension of the arteries, and are increased in number when the tension of the arteries is greatest; the tauter an elastic substance is, the more does it tend to vibrate under the influence of any fresh force suddenly applied to it. Some of the post-dicrotic waves are also doubtless instrumental in origin. The dicrotic wave is of different origin. It was at one time thought that this wave was reflected from the periphery, but this view is at once excluded by the fact that wherever we take the pulse- tracing, whether from the aorta, carotid, radial, dorsalis pedis, etc., this secondary elevation is always situated at the same dis- tance from the beginning of the primary elevation, showing that it is centrifugal, travelling in the same direction as the primary wave, and having its origin in the commencement of the arteria system. Moreover, a single reflected wave from the periphery would be impossible, as the waves reflected from one part would be interfered with by those from other parts; and a reflected .Fig. 405.-Diagram of pulse-tracing. Amp- stroke ; b, down-stroke ; c, pre-dicrotic wave; d, dicrotic; e, post-dicrotic wave. 446 THE CIRCULATION IN THE BLOOD-VESSELS. [CH. XXXVI. wave would be increased by high peripheral resistance, and not diminished as the dicrotic wave is. The primary cause of the dicrotic wave is the closure of the semilunar valves; the inflow of the blood into the aorta sud- denly ceases, and the blood is driven up against the closed aortic doors by the elastic recoil of the aorta; the wave rebounds from here and is propagated through the arterial system as the dicrotic elevation. The systolic secondary waves, namely, the pre-dicrotic and the Fig. 406.-Anacrotic pulse. anacrotic when it is present, are due to elastic vibrations of the aortic wall and perhaps of the heart wall itself; they arc in- creased by an increase in the peripheral resistance. In our study of endo-cardiac pressure, we saw that the systolic plateau sometimes has an ascending, sometimes a descending, slope (see p. 421); we now come to the explanation of this fact. If after the first sudden rise of pressure in the aorta the peripheral resistance is low and the blood can escape more rapidly than it is thrown in, the plateau will sink. If, on the other hand, the peripheral resistance is high, the aortic pressure will rise as long as the blood is flowing in, and we get an ascending systolic plateau and an anacrotic pulse. Thus an anacrotic pulse is seen in Bright's disease, where the peripheral resistance is very high. The production of the dicrotic wave is favoured by relaxation of the arterioles when the heart is beating forcibly as in fever, and to a certain extent after taking alcohol. Such a pulse is called a dicrotic pulse, and the second beat can be easily felt by the finger on the radial artery. The main waves of a pulse tracing can be demonstrated with- out the use of any instruments at all by allowing the blood to Fig. 407.-Dicrotic pulse. ch. xxxvi.J THE CAPILLARY FLOW. 447 spurt from a cut artery on to the surface of a piece of white paper travelling past it. We thus obtain what is very appro- priately called a hcem-autograph (fig. 408). If a long pulse-tracing is taken, the effect of the respiration can be seen causing an in- crease of pressure, and a slight acceleration of the heart's beats during inspiration. This we shall study at greater length in connection with blood-pressure. The Rate of Propagation of the Pulse-Wave.-The method of ascertaining this may be illustrated by the use of a long elastic tube into which fluid is forced by the sudden stroke of a pump. If a series of levers are placed along the tube at measured distances those nearest the pump will rise first, those farthest from it last. If these are arranged to write on a revolving cylinder under one another, this will be shown graphi- cally, and the time interval between their movements can be measured by a time tracing. The same principle is applied to the arteries of the body; a series of Marey's tambours are applied to the heart and to various arteries at known distances from the heart; then levers are arranged to write immediately under one another, as in fig. 382. The difference in the time of their up-strokes is measured by a time tracing in the usual way. Fig. 408.-Haim-auto- graph, to be read from left to right. The Capillary Flow. When the capillary circulation is examined in any transparent part of a living animal by means of the microscope (fig. 409), the blood is seen to flow with a constant equable motion; the red blood - corpuscles moving along, mostly in single file, and bending in various ways to accommodate themselves to the tortuous course of the capillary, but instantly re- covering their normal outline on reaching a wider vessel. At the circumference of the stream in the larger capillaries, but especially well marked in the small arteries and veins, in contact with the walls of the vessel, and adher- ing to them, there is a layer of liquor sanguinis which moves more slowly than the blood in the centre. The existence of this still layer, as it is termed, is Fig. 409.-Capillaries (C) in the web of the frog's foot connecting a small artery (A) with a small vein (V). (After Allen Thomson.) 448 THE CIRCULATION IN THE BLOOD-VESSELS, [ch. xxxvi inferred both from the general fact that such an one exists in all tubes traversed by fluid, and from what can be seen in watch- ing the movements of the blood-corpuscles. Anyone who has rowed on a river will know that the swiftest current is in the middle of the stream. The red corpuscles occupy the middle of the stream and move with comparative rapidity; the colourless corpuscles run much more slowly by the walls of the vessel; while next to the wall there is a transparent space in which the fluid is at comparative 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 colourless corpuscles and their occa- sional stoppage may be due to their having a natural tendency to adhere to the walls of the vessels. 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. When the peripheral resistance is greatly diminished by the dilatation of the small arteries and capillaries, so much blood passes on from the arteries into the capillaries at each stroke of the heart, that there is not sufficient remaining in the arteries to distend them. Thus, the intermittent current of the ventricular systole is not converted into a continuous stream by the elasticity of the arteries before the capillaries are reached; and so intermit- tency of the flow occurs both in capillaries and veins and a pulse is produced. The same phenomenon may occur when the arteries become rigid from disease, and when the beat of the heart is so slow or so feeble that the blood at each cardiac systole has time to pass on to the capillaries before the next stroke occurs ; the amount of blood sent at each stroke being insufficient to properly distend the elastic arteries. It was formerly supposed that the occurrence of any transuda- tion from the interior of the capillaries into the midst of the surrounding tissues was confined, in the absence of injury, strictly to the fluid part of the blood; in other words, that the corpuscles could not escape from the circulating stream, unless the wall of the containing blood-vessel was ruptured. Augustus Waller, affirmed, in 1846, that he had seen blood corpuscles, both red and white, pass bodily through the wall of the capillary vessel in which they were contained (thus confirming what had been stated a short time previously by Addison); and that, as no opening could be seen before their escape, so none could be observed after- wards-so rapidly was the part healed. But these observations did not attract much notice until the phenomena of escape of the CH. XXXVI.] DIAPEDESIS. 449 blood-corpuscles from the capillaries and minute veins, apart from mechanical injury, were rediscovered by Cohnheim in 1867. Cohnheim's experiment demonstrating the passage of the cor- puscles through the wall of the blood-vessel, is performed in the following manner : A frog is curarized, and the abdomen having been opened a portion of small intestine is drawn out, and its transparent mesentery spread out under a microscope. After a variable time, occupied by dilatation, following contraction of the minute vessels and accompanying quickening of the blood-stream, there ensues a retarda- tion of the current, and blood-corpuscles, both red and white, begin to make their way through the capillaries and small veins. The process of diapedesis of the red cor- puscles, which occurs under circumstances of impeded venous circulation, and consequently increased blood-pressure, resembles closely the migration of the leucocytes, with the exception that they are squeezed through the wall of the vessel, and do not, like them, work their way through by amoeboid move- ment. Various explanations of these phenomena have been suggested. Some believe that pseudo-stomata between contiguous endothe- lial cells provide the means of escape for the blood-corpuscles. But the chief share in the process is to be found in the vital en- dowments with respect to mobility and contraction of the parts concerned-both of the corpuscles and of the capillary wall itself. Diapedesis or emigration of the white cor- puscles occurs to a small extent in health. But it is much in- creased in inflammation, and may go on so as to form a large collection of leucocytes outside the vessels. Such a collection is called an abscess, and the corpuscles are called pus corpuscles ; they are, however, dead leucocytes, and show a considerable amount of fatty degeneration in their protoplasm. The emigration of red corpuscles is only seen in inflammation and is a passive process; it occurs when the holes made by the emigrating leucocytes do not close up immediately and so the red corpuscles escape too. The real meaning of the process of inflammation is a subject Fig. 410.-A large capil- lary from the frog's mesentery eight hours after irritation had been set up, showing emigration of leuco- cytes. a, Cells in the act of traversing the capillary wall; b, some already escaped. (Frey.) 450 THE CIRCULATION IN THE BLOOD-VESSELS. [CH. XXXVI. which is being much discussed now, but it may be interesting to state briefly the views of Metschnikoff, who has in recent years been one of the most prominent investigators of the subject. Even if these views do not represent the whole truth, it can hardly be doubted that the phenomena described play a very important part in the process. Metschnikoff teaches that the vascular phenomena of inflammation have for their object an increase in the emigration of leucocytes, which have the power of devouring the irritant substance, and removing the tissues killed by the lesion. They are therefore called phagocytes. It may be that the microbic influence, or the influence of the chemical poisons they produce, is too powerful for the leucocytes; then they are destroyed and the dead leucocytes become pus corpuscles ; but if the leucocytes are successful in destroying the foreign body, micro-organisms, and disintegrated tissues, they disappear, wan- dering back to the blood-vessels, and the lost tissue is replaced by a regeneration of the surrounding tissues. The circulation through the capillaries must, of necessity, be largely influenced by that which occurs in the vessels on either side of them in the arteries or the veins ; their intermediate position causing them to feel at once any alteration in the size, rate, or pressure of the arterial or venous blood-stream. Thus, the apparent contraction of the capillaries, on the application of certain irritating substances, and during fear, and their dilatation in blushing, may be referred primarily to the action of the small arteries. The Venous Flow. The blood-current in the veins is maintained (a) primarily by the vis a tergo of the contraction of the left ventricle; but very effectual assistance to the flow is afforded (b) by the action of the muscles capable of pressing on the veins with valves, as well as (c) by the suction action of the heart, and the aspiratory action of the thorax (vis a froute). The effect of muscular pressure upon the circulation may be thus explained. 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 the next pair of valves, which are in consequence closed. 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 backwards and closing the valves behind. The circulation might lose as much as it gains by such an CH. xxxvi.] THE VENOUS FLOW. 451 action, if it were not for the numerous communications which the veins make with one another ; through these, the closing up of the venous channel by the backward pressure is prevented from being any serious hindrance to the circulation, since the blood, on 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, the effect of muscular pressure upon veins which have valves, is turned almost entirely to the advantage of the circulation; the pressure of the blood onwards is all advantageous, and the pressure of the blood back- wards is prevented from being a hindrance by the closure of the valves and the anastomoses of the veins. In the web of the bat's wing, the veins are furnished with valves, and possess the remarkable property of rhythmical con- traction and dilatation, whereby the current of blood within them is distinctly accelerated. The contraction occurs, on an average, about ten times in a minute; the existence of valves preventing regurgitation, the entire effect of the contractions is auxiliary to the onward current of blood. Analogous phenomena have been observed in other animals. A venous pulse is observed under the conditions previously described when the arterioles are dilated so that the arterial pulse passes through the capillaries to the veins. A venous pulse is also seen in the superior or inferior vena cava near to their entrance into the heart; this corresponds to variations of the pressure in the right auricle. When the ventricle is contracting there is a slow rise due to the fact that the blood cannot get into the ventricle and so distends the auricle ; a second short sharp elevation of pressure is produced by the auricular systole. Alterations of venous pressure are also produced in the great veins by the respiratory movements, the pressure sinking during inspiration, and rising during expiration. Local Peculiarities of the Circulation. The most remarkable peculiarities attending the circulation of blood through different organs are observed in the cases of the brain, erectile organs, lungs, liver, spleen, and kidneys. In the Brain.-For the due performance of its functions the brain requires a large supply of blood. This object is effected through the number and size of its arteries, the two internal carotids, and the two vertebrals. It is further necessary that the force with which this blood is sent to the brain should be less, or at least should be subject to less variation from external circum- 452 THE CIRCULATION IN THE BLOOD-VESSELS, [ch. xxxvi. stances than it is in other parts, and so the large arteries are very tortuous and anastomose freely in the circle of Willis, which thus insures that the supply of blood to the brain is 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 ensured by the arrangement of the vessels in the pia mater, in which, previous to their distribution to the sub- stance of the brain, the large arteries break up and divide into innumerable minute branches ending in capillaries, which, after frequent communication with one another, enter the brain, and carry into nearly every part of it uniform and equable streams of blood. The arteries are also enveloped in a special lymphatic sheath. 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 compres- sible by any force which the fulness of the arteries might exercise through the substance of the brain; nor do they admit of disten- sion 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 sub- stance placed in a cavity with unyielding walls. A very small amount of artificial compression of the brain is fatal. These conditions of the brain and skull formerly appeared, indeed, enough to justify the opinion that the quantity of blood in the brain must be at all times the same. But it was found that in animals bled to death, without any aperture being made in the cranium, the brain became pale and aneemic like other parts. And in death from strangling or drowning, there was congestion of the cerebral vessels ; while in death by prussic acid, 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 animal was kept suspended by the ears. Thus, it was concluded, 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 simultaneous diminution or increase in the quantity of the cerebro- spinal fluid, which, by readily admitting of being removed from CH. XXXVI. ] CIRCULATION IN THE BRAIN. 453 one part of the brain and spinal cord to another, and of being rapidly absorbed, and as readily effused, would serve as a kind of supplemental fluid to the other contents of the cranium, to keep it uniformly filled in case of variations in their quantity. And there can be no doubt that, although the arrangements of the blood-vessels, to which reference has been made, ensure to the brain an amount of blood which is tolerably uniform, yet, inas- much as with every beat of the heart and every act of respiration, and under many other circumstances, the quantity of blood in the cavity of the cranium is constantly varying, it is plain that, were there not provision made for the possible displacement of some of the contents of the unyielding bony case in which the brain is contained, there would be often alternations of excessive pressure with insufficient supply of blood. In Erectile Structures.-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 cir- cumstances, are soft and flaccid, but, at certain times, receive an ' unusually large quantity of blood, become distended and swollen by it, and pass into the state which has been termed erection. Such structures are the corpora cavernosa and corpus spongiosum of the penis in the male, and the clitoris in the female; and, to 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; and from the inner surface of the latter are prolonged numerous fine lamellae which divide its cavity into small compartments. Within these is situated the plexus of veins upon which the peculiar erectile property of the organ mainly depends. It con- sists of short veins which very closely interlace and anastomose with each other in all directions, and admit of great variations of size, collapsing in the passive state of the organ, but capable of an amount of dilatation which exceeds 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 plexuses, 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. For all these veins one condition is the same ; namely, that they are liable to 454 THE CIRCULATION IN THE BLOOD-VESSELS, [ch. xxxvi. the pressure of muscles when they leave the penis. The muscles chiefly concerned in this action are the erector penis and accelerator urinae. 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 after their division the penis is no longer capable of erection. Erection is not complete, nor maintained for any time except when, together with the influx of blood, the muscles mentioned contract, and by compressing the veins, stop the efflux of blood, or prevent it from being as great as the influx. The circulation in the Lungs, Liver, Spleen and Kidneys will be described under their respective heads. The circulation of the blood depends on the existence of different pressure in different parts of the circulatory system; Blood-pressure. Fig. 411.-Height of blood-pressure (bp) in lv, left ventricle, a, arteries ;c, capillaries ; v, veins; ra, right auricle; 00, line of no pressure. (After Starling.) there is a diminution of pressure from the heart onwards through arteries, capillaries, and veins, back to the heart again. Fig. 411 represents roughly the fall of pressure along the vascular system. It falls slowly in the great arteries; at the end of the arterial system it falls suddenly and extensively just beyond the resistance of the arterioles ; it again falls gradually through the capillaries and veins till in the large veins near the heart it is negative. A curve of blood-pressure is thus very different from one of velocity ; the velocity like the pressure falls from the arteries to the capillaries, but unlike it, rises again in the veins. We must now study the methods by which blood-pressure is measured and recorded, and the main causes that produce variations in its amount. CH. XXXVI.] BLOOD-PRESSURE. 455 In order to do this in the simplest way, it will be first neces- sary to examine how we may measure pressure in an artificial schema of the circulation. Take the simplest possible case of a fluid flowing from a reservoir, R (fig. 412), along a tube, which we will imagine is open at the other end. In the course of the tube we will suppose three upright glass tubes (A, B, and D) are inserted at equal distances. Between B and D there is a bladder, which may be divided into a number of channels by packing it with tow to represent the capillaries, Fig. 412.-Schema to illustrate blood-pressure. and between B and C a clip which can be tightened or loosened at will, and which will roughly represent the peripheral resistance pro- duced by the constricted arterioles. The far end of the tube is pro- vided with a stop-cock. If this stop-cock is closed there will naturally be no flow of fluid, and the fluid will rise to an equal height indi- cated by the dotted line in all the upright tubes. This shows that the pressure in all parts of the tube is the same. The upright tubes which measure the lateral pressure exerted by the fluid on the wall of the main tube are called manometers or pressure measurers. The lateral pressure of a fluid is equal to the forward pressure. If now the stop-cock is opened, the fluid flows on account of the difference of pressure brought by gravitation; the height of the fluid in the manometers indicates that the pressure is greatest in R, less in A, less still in B, and least of all in D. On account of the peripheral resistance of the arterioles and 456 THE CIRCULATION IN THE BLOOD-VESSELS. [CH. XXXVI. capillaries, the pressure is very small in the veins as indicated by the height of the fluid in the manometer D. The difference between D and B is much more marked than the difference between B and A. If the fluid which flows out of the end of the tube is collected in a jug and poured back into R we complete the circulation. But the schema is an extremely rough one ; and is especially faulty in that the pressure which starts at R is nearly constant and not intermittent. This may be remedied by taking R in the hand, and raising and lowering it alternately. The pressure in the manometers bobs up and down with every rise and fall of R : this is least marked in D. The greater and the faster the movement of R, the greater is the rise of arterial pressure. This is a rough illustration of the fact that increase in the force and frequency of the heart's beat causes a rise of arterial pressure. Again, if more fluid is poured into R, there is a corresponding rise in fluid in the manometers. This illustrates the rise of pressure produced by an increase in the contents of the vascular system. And this schema, rough though it is, also serves to illustrate the third important factor in the maintenance of the blood- pressure, namely, the peripheral resistance. This is done by means of the clip E; if the clip is tightened, one imitates increased constriction of the arterioles; if it is loosened, one imitates dilata- tion of the arterioles. If it is closed entirely, the fluid in A and B rises to the same level as that in R; the pressure of R is not felt at all by C and D, which empty themselves, and the flow ceases. If the clip E is only tightened so as not to be quite closed, the arterial pressure (in A and B) rises, and the venous pressure falls ; if the clip is freely opened, the arterial pressure falls, and the venous pressure rises. These same facts can be demonstrated by a more perfect cir- culation schema such as is represented in the next diagram. The heart (H) is represented by a Higginson's syringe, which is worked with the hand; the tube from it represents the arterial system, the clip E the resistance of the arterioles; C is the capillary lake, from which the vein (larger than the artery) leads back to the reservoir from which H pumps, the fluid. A and B are two manometers which respectively indicate arterial and venous pressures. Only in place of using straight tubes, mercurial manometers are used instead. Each of these is a (J-tube about half filled with mercury, and united to the artery or vein by a tube containing fluid. If the mercury in the two limbs of the (J is at the same level, the pressure of the fluid in connection with one limb is exactly equal to the atmospheric pressure. The mercury, however, is pushed up in the far limb of the manometer CH. xxxvi.] BLOOD-PRESSURE. 457 connected to the artery, the pressure there being greater than that of the atmosphere; this is therefore called positive pressure, and the total amount of pressure, usually measured in millimetres, is the difference between the levels a and a. The manometer B attached to the vein, however, indicates a negative }>ressure (b 6'), that is, a pressure less than that of the atmosphere, so that the mercury in the limb nearest the vein is sucked up. Anderson Stuart's sphygmoscope (fig. 414) is a much more complete schema. It consists of a long leaden tube filled with fluid, the two ends of which are connected by an india-rubber tube on which is a valved syringe to represent the heart. On the course of the tube are a large number of open-mouthed upright manometers which indicate the pressure when the syringe is worked, and confer on the tube the necessary elasticity which causes the disappearance of the pulse in the middle region which represents the capillaries. The long leaden tube is twisted round a cylinder so that the manometers are placed closely side by side. We can now pass on to the methods adopted in the investiga- tions of blood-pressure in animals. The fact that the blood exerts considerable pressure on the arterial walls may be readily shown by puncturing any artery; the blood is propelled with great force through the opening, and the jet rises to a considerable height; in the case of a small artery, where the pres- sure is lower, the jet is not so high as in a large artery: the jerky character of the outflow due to the intermittent action of the heart is also seen. If a vein is similarly injured, the blood is expelled with much less force and the flow is continuous, not intermittent. Fig. 413.-Schema of the circulation. (After Starling.) 458 THE CIRCULATION IN THE BLOOD-VESSELS, [ch. xxxvi. The first to make an advance on this very rough method of demonstrating blood-pressure was the Rev. Stephen Hales, Vicar of Teddington (1727). He inserted, using a goose-quill as a cannula, a glass tube at right angles to the femoral artery of a horse, and noted the height to which the blood rose in it. This is a method like that we used in the first schema we described. The blood rose to the height of about 8 feet, and having reached its highest point, it oscillated with the heart-beats, and also with the respiration ; each inspiration causing a rise, each expiration a Fig. 414.-Anderson Stuart's Sphygmoscope. fall of pressure ; each cardiac systole causing a smaller rise, each diastole a smaller fall. The method taught Hales these primary truths in connection with arterial pressure, but it possesses many disadvantages; in the first place the blood in the glass tube very soon clots, and in the second place, a column of liquid eight feet high is an inconvenient one to work with. The first of these disadvantages was overcome to a great extent by Vierordt, who attached a tube filled with saturated solution of sodium carbonate to the artery, and the blood-pressure was measured by the height of the column of this saline solution which the blood would support. ch. xxxvi.] THE KYMOGRAPH. 459 The second disadvantage was overcome by Poiseuille, who introduced the heavy liquid mercury as the substance on which the blood excited its pressure; and the U-shaped mercurial manometer was connected to the artery by a tube filled with sodium carbonate solution to delay clotting. The study of blood-pressure cannot, however, be considered to have been in a satisfactory condition until the introduction by Carl Ludwig of the Kymograph ; that is to say, Poiseuille's hcemo- was combined with apparatus for obtaining a graphic B.P.tracing Abscissa Fig'. 415.-Diagram of mercurial kymograph. record of the oscillations of the mercury. The name kymograph ■or wave-writer we shall see immediately is a very suitable one. A skeleton sketch of the apparatus is given in fig. 415. The artery is exposed and clipped, so that no haemorrhage occurs; it is then opened and a glass cannula inserted and firmly tied in. The form of cannula usually employed (Francois Franck's) is shown on a larger scale at A ; the narrow part with the neck in it, is tied into the artery towards the heart; the cross piece of the T is united to the manometer, the third limb is closed by a short piece of india-rubber tubing which is kept closed by a clip; and only opened on emergencies, such as to clear out a clot with a feather should one form in the cannula during the progress of an •experiment. 460 THE CIRCULATION IN THE BLOOD-VESSELS, [ch. xxxvi. The tube by means of which the cannula is united to the manometer is not an elastic one, but is made of flexible metal, so that none of the arterial force may be wasted in expanding it. The tube, cannula and proximal limb of the manometer are all filled with a saturated solution of sodium carbonate, sodium sulphate, or other salt which will mix with blood and delay its clotting. Before the clip is removed from the artery, the pressure is first got up by a syringe (or pressure bottle containing the same saline solution suspended at a good height above the Fig. 416.-The Manometer of Ludwig's Kymograph. It is also shown in fig. 417, d, c, e. The mercury which partially fills the tube supports a float in form of a piston, nearly filling the tube; a wire is fixed to the float, and the writing style or pen is guided by passing through the brass cap of the tube fixed to the wire; the pressure is communi- cated to the mercury by means of a flexible metal tube filled with fluid. apparatus and connected to it by a tube), so that the mercury rises in the distal limb to a height greater than that of the anticipated blood-pressure; this prevents blood passing into the cannula when the arterial clip is removed. In the distal limb of the U-tube, floating on the surface of the mercury, is an ivory float, from which a long steel wire extends upwards, and terminates in a writing point. The writing point may be a stiff piece of parchment or a bristle which writes on a moving surface covered with smoked paper, or a small brush kept full of ink which writes on a long roll of white paper also made to travel by clockwork in front of it. When the two limbs of the mercury are at rest, the writing point inscribes a base line or CH. XXXVI.] THE KYMOGRAPH. 461 abscissa on the travelling surface ; when the pressure is got up by the syringe it writes a line at a higher level. When the arterial clip is removed it writes waves as shown in the diagram (fig. 415), the large waves corresponding to respiration (the rise of pressure in most animals accompanying inspiration),* the smaller ones to the individual heart-beats. The blood-pressure is really twice as great as that indicated by the height of the tracing above the abscissa, Fig. 417.-Diagram of mercurial kymograph, a, revolving cylinder, worked by a clock- work arrangement contained in the box (b), the speed being regulated by a fan above the box; cylinder supported by an upright (6), and capable of being raised or lowered by a screw (a), by a handle attached to it; d, c, b, represent mercurial manometer, a somewhat different form of which is shown in next figure. because if the manometer is of equal bore throughout, the mercury falls in one limb the same distance that it rises in the other; the true pressure being the difference of level between a and a (fig. 415). Fig. 416 shows a more complete view of the manometer, and fig. 417 is a diagram of the arrangement by means of which it is made into a kymograph. * The explanation of the respiratory curves on the tracing is postponed till after we have studied Respiration. 462 THE CIRCULATION IN THE BLOOD-VESSELS. [CH. xxxvi. Fig. 418 shows atypical normal arterial blood-pressure tracing on a larger scale. In taking a tracing of venous blood-pressure, the pressure is so low and corresponds to so few millimetres of mercury, that a Fig. 418.-Normal tracing of arterial pressure in the rabbit obtained with the mercurial kymograph. The smaller undulations correspond with the heart-beats the larger curves with the respiratory movements. (Burdon-Sanderson.) saline solution is usually employed instead of mercury. If the vein which is investigated is near the heart, a venous pulse is exhibited on the tracing, with small waves as before corre- Fig. 419.-A form of Fick's Spring Kymograph, a, tube to be connected with artery ; c, hollow spring, the movement of which moves b, the writing lever ; e, screw to regulate height of b ; d, outside protective spring ; g, screw to fix on the upright of the support. spending to heart-beats, and larger waves to respiration, only the respiratory rise in pressure now accompanies expiration (see p. 451). CH. xxxvi.] THE KYMOGRAPH. 463 The capillary pressure is estimated by the amount of pressure necessary to blanch the skin ; this has been done in animals and men (v. Kries, Roy and Brown). Other manometers are often employed instead of the mercurial one. Fick's is one of these. The vessel is connected as before with the manometer, and the pressure got up by the use of a syringe (which is seen in fig. 420), before the clip is removed from the artery. The manometer itself is a hollow C-shaped spring Fig. 420.-Fick's Kymograph, improved by Hering (after McKendrick). a, hollow spring filled with alcohol, bearing lever arrangement b, d, c, to which is attached the marker e; the rod c passes downwards into the tube /, containing castor oil, which offers resistance to the oscillations of c ; g, syringe for filling the leaden tube b with saturated sulphate of sodium solution, and to apply sufficient pressure as to prevent the blood from passing into the tube h at i, the cannula inserted into the vessel; Z, abscissa- marker, which can be applied to the moving surface by turning the screw m ; k, screw for adjusting the whole apparatus to the moving surface; o, screw for elevating or depressing by a rack-and-pinion movement the Kymograph ; n, screw for adjusting the position of the tube/. filled with liquid ; this opens with increase, and closes with decrease of pressure, and the movements of the spring are com- municated to a lever provided with a writing point. Hurthle's manometer (see p. 420) is also very much used. The advantage of these forms of manometer is that the character of 464 THE CIRCULATION IN THE BLOOD-VESSELS. [CH. xxxvi. each individual movement is much better seen ; in the case of a heavy liquid like mercury the inertia is so great that it cannot catch the finer movements which we have seen as secondary vibrations on the pulse wave. If Fick's or Hurthle's manometer is employed, and the surface travels sufficiently fast, these can be ■recorded (see fig. 421). We may now proceed to give some results. The following table gives the probable average height of blood-pressure in various parts of the vascular system in man. They have been very largely inferred from experiments on animals :- Fig. 421.-Normal arterial tracing obtained with Fick's Kymograph in the dog. (Burdon-Sanderson.) Large arteries (e.g. carotid) + 140 mm. (about 6 inches) mercury Medium arteries (e.g. radial) . + no mm. mercury. Capillaries . . + 15 to + 20 „ ,, Small veins of arm . . . + 9 „ ,, Portal vein . . . + 10 ,, ,, Inferior vena cava . . . + 3 ,, ,, Large veins of neck . from o to - 8 „ „ (Starling). These pressures are, however, subject to considerable variations ; the principal factors that cause variation are the following :- Increase of arterial blood-pressure is produced by 1. Increase in the rate and power of the heart. 2. Increase in the quantity of blood (plethora, after a meal, after transfusion). 3. Increase in the contraction of the arterioles. Decrease in the arterial blood-pressure is produced by 1. Decrease in the rate and force of the heart. 2. Decrease in the quantity of blood (e.g. after hsemorrhage). 3. Decrease in the contraction of the arterioles. The above is true for general arterial pressure; but if we are investigating local arterial pressure in any organ, the increase or decrease in the size of the arterioles of other areas may make its effect felt in the special area under investigation. CH. xxxvi.] VENOUS PRESSURE. 465 Venous pressure varies in the opposite way to arterial pressure, in so far as the first and third factors are concerned. Like arterial pressure it is increased by plethora, diminished by anaemia. It is increased by a decrease in the rate and force of the heart, and by a dilatation of the arterioles. It is diminished by the opposites. It is quite easy to understand how this is; when the heart beats with increased force, it naturally raises the pressure in the arteries; but an increase during systole in the force Fig. 422.-Effect of weak stimulation of vagus on arterial blood-pressure, bp, blood- pressure ; a, abscissa or base line; t, time in seconds. Note fall of blood-pressure and slow heart beats. of propulsion into the arteries means an increase also during diastole in the force of suction upon the venous blood, that is, a reduction of the pressure there; it becomes more negative than it usually is. With regard to the arterioles, contraction in the arterioles means a rise in pressure in the arteries, just as narrowing the doors of a theatre during the exit of the audience will increase the pressure behind the doors; but a contraction of the arterioles causes a fall in pressure in the capillaries and veins beyond them, just as the narrowing of the theatre doors will lessen the congestion in the street outside of them. 466 THE CIRCULATION IN THE BLOOD-VESSELS, [ch. xxxvi. Captillary pressure is increased by 1. Dilatation of the arterioles ; the blood-pressure of the large arteries is then more readily propagated into them. 2. The size of the arterioles remaining the same, increase of arterial pressure from any other cause will produce a rise of capillary pressure. 3. By narrowing the veins leading from the capillary area; complete closure of the veins may quadruple the capillary pressure. This leads secondarily to an increased formation of lymph (dropsy); Fig. 423.-Effect of strong stimulation of vagus on arterial blood-pressure. Note stoppage of heart and fall of blood-pressure nearly to zero ; after the recommencement of the heart, the blood-pressure rises, as in fig. 422, above the normal for a short time. it is seen in the pressure of a tumour on the veins coming from the legs. 4. Any circumstance that leads to increased pressure in the veins will act similarly ; this is illustrated by the effects pro- duced by gravity in the circulation as in alterations of posture. Capillary pressure is decreased by the opposite conditions. The pressure in the Pulmonary Circulation is roughly about one- third of what it is in the systemic vessels. The influence of the Cardiac Vagus on blood-pressure. The importance of the heart's action in the maintenance of blood- CH. XXXVI. j VASO-MOTOR NERVES. 467 pressure is well shown by the effect that stimulation of the vagus nerve has on the blood-pressure curve. If the vagus of an animal is exposed and cut through, and the peripheral end stimulated, the result is that the heart is slowed or stopped; the arterial blood-pressure falls simultaneously; the fall being especially sudden and great if the heart is completely stopped. There is a rise in venous pressure. The effect on arterial pressure is shown in the two accompanying tracings; fig. 422 representing the effect of partial, and fig. 423 of complete stoppage of the heart; in both cases the animal used was a rabbit, and the artery the carotid. The effects of stimulating the central end of the vagus and other nerves cannot be understood until we have studied the vaso-motor nervous system, to the consideration of which we now pass. The Vaso-motor Nervous System. The vaso-motor nervous system consists of the vaso-motor centre situated in the bulb, of certain subsidiary vaso-motor centres in the spinal cord, and of vaso-motor nerves, which are of two kinds, -(a) those the stimulation of which causes constriction of the vessels; these are called vaso-constrictor nerves; (b) those the stimulation of which causes dilatation of the vessels; these are called vaso-dilatator nerves. The following names are associated with the history of the subject. The muscular structure of arteries was first described by Henle in 1841 ; in 1852 Brown Sequard made a study of the vaso-constrictor, or, as he termed them, tonic nerves. The vaso- motor centre was discovered by Schiff (1855), and more accurately localised by Ludwig (1871). The dilatator nerves were also discovered by Schiff; at first they were termed paretic nerves. Other names which must be mentioned in connection with the subject are those of Claude Bernard, Heidenhain, and in more recent years, Gaskell, Langley, and Ramon y Cajal. The nerves supply the muscular tissue in the walls of the blood- vessels and regulate their calibre, but exert their most important action in the vessels which contain relatively the greatest amount of muscular tissue, namely, the small arteries or arterioles. Under ordinary circumstances, the arterioles are maintained in a state of moderate or tonic contraction, and this constitutes the peripheral resistance, the use of which is to keep up the arterial pressure which must be high in order to force the blood in a continuous stream through the capillaries and veins back to the heart. 468 THE CIRCULATION IN THE BLOOD-VESSELS. [CH. XXXVI. Another function which is served by this muscular tissue is to regulate the amount of blood which flows through the capillaries of any organ in proportion to its needs. During digestion, for instance, it is necessary that the digestive organs should be supplied with a large quantity of blood; for this purpose the arterioles of the splanchnic area are relaxed, and there is a vast amount of blood in this area, and therefore a correspond- ingly small amount in other areas, such as the skin; this accounts for the sensation of chilliness experienced after a full meal. The skin vessels form another good example; one of the most important uses of the skin is to get rid of the heat of the body in such a way that the body temperature remains constant; when excess of heat is produced there is also an increase in the loss of heat; the skin vessels are then dilated and so more blood is exposed on the surface, and thus increase in the radiation of heat from the surface is brought about. On the other hand, when it is necessary that the heat produced should be kept in the body, the loss of heat is diminished by a constriction of the skin vessels, as in cold weather. The altera- tions of the calibre of the vessels is brought about by the action of the vaso-motor nervous system on the muscular tissue of the arterioles. There are certain organs of the body in which the necessity for alterations in their blood supply does not exist. Such organs are the lungs and the brain. It is in the vessels of these organs that the influence of vaso-motor nerves is at a minimum. The pulmonary vessels are supplied by nerves, which have been discovered by stimulating certain nerve-roots in the upper thoracic region; but the existence of vaso-motor nerves in the case of the brain has been denied altogether; at any rate they have still to be discovered. The vaso-motor centre lies in the grey matter of the floor of the fourth ventricle ; it is a few millimetres in length reaching from the upper part of the floor to within about 4 mm. of the calamus scriptorius. The position of this centre has been discovered by the following means ; when it is destroyed the tone of the small vessels is no longer kept up, and in consequence there is a great and universal fall in arterial blood-pressure ; when it is stimulated there is an increase in the constriction of the arterioles all over the body, and therefore a rise of blood-pressure. Its upper and lower limits have been accurately determined in the following ■way ; a series of animals is taken and the central nervous system divided in a different place in each ; the cerebrum and cerebellum may be cut off without affecting blood-pressure, the vaso-motor CH. xxxvi.] VASO-MOTOR NERVES. 469 centre must therefore be below these; if the section is made just above the medulla, the blood-pressure still remains high, and it is not till the upper limit of the centre is passed that the blood- pressure falls. Similarly in another series of animals, if the cervical cord is cut through, and the animal kept alive by artificial respiration, there is an enormous fall of pressure due to the influence of the centre being removed from the vessels ; in other experiments the section is made higher and higher, and the same result noted, until at last the lower limit of the centre is passed, and the fall of pressure is less and less marked the higher one goes there, until in the animal in which the section is made at the upper boundary of the centre, the blood-pressure is not affected at all, and the centre can be influenced reflexly by the stimulation of afferent nerves, the pressor and depressor nerves, which we shall be considering immediately. After the destruction of the vaso-motor centre in the bulb, there is a fall of pressure. If the animal is kept alive, the vessels after a time recover their tone, and the arterial pressure rises; this is due to the existence of subsidiary vaso-motor centres in the spinal cord ; for on the subsequent destruction of the spinal cord the vessels again lose their tone and the blood- pressure sinks. We owe a good deal of our knowledge concerning the distribu- tion of the vaso-motor nerves to Dr. Gaskell, whose researches may be summarized as follows :- The vaso-motor nerves travel down the lateral column of the spinal cord, and terminate by arborising around the cells of the grey matter of the subsidiary vaso-motor centres, the exact anatomical position of which is uncertain. From these cells fresh axis-cylinder processes originate which pass out as the small medullated nerve-fibres in the anterior roots of the spinal nerves. The vaso-constrictor nerves for the whole body leave the spinal cord by the anterior roots of the spinal nerves from the second thoracic to the second lumbar both inclusive. They leave the roots by the white rami communicantes and pass into the ganglia of the sympathetic chain which lies on each side along the front of the vertebral column. The ganglia on this chain (the lateral ganglia of Gaskell) may also be called the chain of vaso-motor ganglia, because here are situated cell stations on the course of the vaso-constrictor nerves. That is to say, the small medullated nerve-fibres terminate by arborising around the cells of these ganglia, and a fresh relay of axis-cylinder processes from these cells carries on the impulses. 470 THE CIRCULATION IN THE BLOOD-VESSELS, [ch. xxxvi. The following figure represents diagrammatically how this occurs. The sheaths of the fibres are not represented. The cell station of any particular fibre is not necessarily situated in the first ganglion to which it passes : the fibres of the white ramus communicans of the second thoracic do not for instance all have their cell stations in the second thoracic ganglion, Fig. 424.-Transverse section through half the spinal cord, showing the ganglia. A, ante- rior cornual cells ; B, axis cylinder process of one of these going to posterior root; C, anterior (motor) root; D, posterior (sensory) root; E, spinal ganglion or posterior root; F, sympathetic ganglion; G, ramus communicans; H, posterior branch of spinal nerve; I, anterior branch of spinal nerve; a, long collaterals from posterior root fibres reaching to anterior horn; &, short collaterals passing to Clarke's column; c, cell in Clarke's column sending an axis cylinder (rf) process to the direct cerebellar tract; e, fibre of the anterior root; f, axis cylinder from sympathetic ganglion cell, dividing into two branches, one to the periphery, the other towards the cord ; g, fibre of the anterior root terminating by an arborisation in the sympathetic ganglion; h, sympa- thetic fibre passing to periphery. but may pass upwards or downwards in the chain to a more or less distant ganglion before they terminate by arborising around a cell or cells. The vaso-constrictor nerves, however, have all cell stations somewhere in the lateral or sympathetic chain, and the new axis- cylinders that arise from the cells of the ganglia differ from those which terminate there in the circumstance that they never acquire a medullary sheath, but they are pale, grey or non- CH. XXXVI.] VASO-MOTOR NERVES. 471 medullated fibres. Those which are destined for the supply of the vessels of the head and neck pass into the ganglion stellatuni or first thoracic ganglion, from here through the annulus of Vieussens to the inferior cervical ganglion, and thence along the sympathetic trunk to their destination. Those for the body wall and limbs pass back from the sympa- thetic ganglia to the spinal nerves by the grey rami communi- cantes, and are distributed with the other spinal nerve-fibres. Those for the interior of the body pass into the various plexuses of sympathetic nerves in the thorax and abdomen and are distributed to the vessels of the thoracic and abdominal viscera. This set includes the most important vaso-motor nerves of the body, the splanchnics. The vaso-dilatator nerves in part accompany those first described, but they are not limited to the outflow from the second thoracic to the second lumbar. Thus the nervi erigentes originate as white rami communicantes from the second and third sacral nerves, and the chorda tympani, another good example of a vaso-dilatator nerve, is a branch of the seventh cranial nerve. The vaso-dilatator nerves also differ from the vaso-constrictors oy not communicating with cell stations in the sympathetic chain : they pass through these ganglia, retaining their medullary sheath, and have their cell stations in the collateral ganglia (such as the semilunar) or in the terminal ganglia on the walls of the blood- vessels themselves. These vaso-motor nerves, whether they are constrictor or dilatator, differ very markedly from the spinal nerve-fibres which are distributed to voluntary muscles in being ganglionated; that is having cell stations or positions of relay on their course to the muscular fibres they supply. The existence of cell stations between the central nervous system and the muscular fibres is not confined to the nerves of blood-vessels, but is found also in the nerves which supply the heart, and other viscera. Moreover, the nerves which supply the voluntary muscles are motor in function; inhibitory fibres to the voluntary muscles of vertebrates do not exist. But in the case of the involuntary muscles there are usually two sets of nerve-fibres with opposite functions. In the case of the heart, we have an accelerator set, which course through the sympathetic, and an inhibitory set which course through the spinal accessory and vagus. In the case of the vessels, we have an accelerator set, which we 472 THE CIRCULATION IN THE BLOOD-VESSELS, [ch. XXXVI. have hitherto called vaso-constrictors, and an inhibitory set we have been calling vaso-dilatators. In the case of the other contractile viscera, we have also viscero- accelerator and viscero-inhibitory which respectively hasten and lessen their peristaltic movements. Adopting Gaskell's nomenclature we may further term the accelerator groups of nerves, katabolic as they increase the activity of the muscles they supply, bringing about an increase of wear and tear, and an increase in the discharge of waste material. The inhibitory nerves on the other hand are anabolic as they produce a condition of rest in the tissues they supply, and so an opportunity for repair or constructive metabolism. In all cases the medullary sheath of the katabolic nerves is lost in cell stations situated near the origin of the nerves from the central nervous system; the medullary sheath of the anabolic nerves is lost in cell stations near their terminations. Langley has more recently investigated the distribution of sympathetic nerves by another method, and has published his researches in a number of very complete articles. We have only space here to describe his method, the results being largely of an anatomical kind. He finds that Gaskell's histological test (the loss of the medullary sheath) is by no means an absolutely trust- worthy one, as to the situation of the cell station on the course of a sympathetic fibre. But he employs a physiological test. Nicotine in certain doses paralyses nerve cells, but not nerve- fibres •, if this alkaloid is injected into an animal, stimulation of the anterior nerve-roots produces no movements of the involun- tary muscles, as the paralyzed nerve stations on the course of the nerve-fibres act as blocks in the propagation of the impulse. If the nicotine is applied locally by painting it over one or more ganglia, there will only be a block in those fibres which have their cell stations in those particular ganglia. We shall come to a definite instance of the usefulness of this method in connection with the nerves and ganglia of the submaxillary gland. We may now ask what is the object that is served by the existence of ganglia on the course of these nerves. It appears to be a means of distri- buting nerve-fibres to a vast area of muscular tissue by means of a com- paratively small number of nerve-fibres that leave the central nervous system ; for each fibre that leaves the central nervous system arborises around a number of cells, and thus the impulse it carries is transferred to a number of new axis-cylinder processes. In some cases, it is true, a single nerve-fibre will divide into multitudin- ous branches to accomplish the same object (as in the supply of the electric organ of Malaptemrus, the fibres to the millions of its sub-divisions all originating from a single axis-cylinder), but the usual way appears to be a combination of this method with that of subsidiary cell-stations. CH. XXXVI.] VASO-MOTOR CENTRE. 473 At one time a ganglion was supposed to be the seat of reflex action. The submaxillary ganglion was the battle-field in which this question was fought out. In all the researches of Langley and Anderson, who have investigated every ganglion in the body, they have never found that a ganglion is the seat of a reflex action. The only instances where such a thing seemed possible was the following:-When all the nervous connec- tions of the inferior mesenteric ganglion are divided except the hypogastric nerves, stimulation of the central end of one hypogastric causes contraction of the bladder, the efferent path to which is the other hypogastric nerve. In addition they observed an apparent reflex excitation of the nerve sup- plying the erector muscles of the hairs (pilo-motor nerves) through other sympathetic ganglia. In neither case is the action truly reflex, but is caused by the stimulation of the central ends of motor fibres which issue from the spinal cord, and which after passing through the ganglion send branches down each hypogastric nerve. We can now pass from these anatomical considerations to the experiments which have been performed in connection with the vaso-motor nerves and their centre. The vaso-motor centre can be excited directly as by induction currents; the result is an increase of blood-pressure owing to an increase produced in the contraction of the peripheral arterioles. It can also be excited by the action of poisons in the blood which circulates through it; thus strophanthus or digitalis causes a marked rise of general arterial pressure due to the con- striction of the peripheral vessels brought about by impulses from the centre. It is also excited by venous blood as in asphyxia ; the rise of blood-pressure which occurs during the first part of asphyxia is due to constriction of peripheral vessels; the fall during the last stage of asphyxia is largely due to heart failure. We shall study asphyxia more at length in connection with re- spiration. During the period of increased pressure, waves are often observed on the blood-pressure curve which arise from a slow rhythmic action of the vaso-motor centre. The centre alternately sends out stronger and weaker constrictor impulses. They are known as the Traube-Hering waves and are much slower in their rhythm than the waves on the curve which are due to respiration. They are not peculiar to asphyxia, but are frequently seen in tracings from normal animals. Fig. 425 represents tracings obtained from a dog under the influence of morphia and curare. The upper curve taken while artificial respiration was being carried on shows the three sets of waves, first the oscillations due to the heart beats, next in size those due to the respiratory movements, which in their turn are superposed on the prolonged Traube-Hering curves. The lower curve was taken immediately after the cessation of the artificial 474 THE CIRCULATION IN THE BLOOD-VESSELS. [CH. xxxvi. respiration and shows only the heart beats and the Traube-Hering waves. . The Vaso-motor centre may be excited reflexly. The afferent impulses to the vaso-motor centre may be divided into pressor and depressor. All sensory nerves are pressor nerves. The sciatic or the vagus nerves may be taken as instances; when they are divided and their central ends stimulated, the result is a rise of blood-pressure due to the stimulation of the vaso-motor centre, and a consequent Fig. 425.-Arterial blood-pressure tracings showing Traube-Hering curves. (Starling.) constriction of the arterioles, especially in the splanchnic area. Fig. 426 shows the result of such an experiment. It is convenient in performing such an experiment to administer curare as well as an ansesthetic to the animal, in order to obviate reflex muscular struggles. In addition to the general rise of arterial pressure there is vaso- dilatation in the area of distribution of the sensory nerve stimu- lated (Loven); this causes a swelling of the leg if the sciatic is the nerve experimented with. The purpose of such an arrange- ment is that where a limb is injured a good supply of blood is necessary for the process of repair ; this is accomplished by (1) the rise of blood-pressure in the arterial system produced by the CH. XXXVI.J DEPRESSOR NERVE. 475 stimulation of pressor fibres, and (2) the vascular dilatation in the limb itself. Depressor nerve.-In most animals the depressor fibres are bound up in the trunk of the vagus, but in some, like the rabbit, cat and horse, the nerve runs up as a separate branch from the heart and joins the vagus or its superior laryngeal branch and ultimately reaches the vaso-motor centre. When this nerve is stimulated (the vagi having been previously divided to prevent Fig. 426.-Result on arterial blood-pressure curve of stimulating the central end of cut sciatic nerve in rabbit, bp, blood-pressure; a, abscissa or base line; t, time in seconds ; e, signal of period of excitation of the nerve. reflex inhibition of the heart), a marked fall of arterial blood-pres- sure is produced (see fig. 427). Stimulation of this nerve affects the vaso-motor centre in such a way that the normal constrictor impulses that pass down the vaso-constrictor nerves are inhibited. The fall of pressure is very slight after section of the splanchnic nerves, showing that the splanchnic area is the part of the body most affected. The normal function of this nerve is to adapt the heart's action to the peripheral resistance : if the constriction of the arterioles is too high for the heart to overcome, an impulse by this nerve to the vaso-motor centre produces reflexly a lessening of the peripheral resistance. 476 THE CIRCULATION IN THE BLOOD-VESSELS, [ch. xxxvi. N.B.-The term depressor should be carefully distinguished from inhibi- tory ; stimulation of the peripheral end of the vagus produces a fall of blood-pressure due to inhibition (slowing or stoppage) of the heart (see figs. 422 and 423) ; stimulation of the central end of the depressor nerve produces a lowering of blood-pressure for a different reason, namely a reflex relaxation of the splanchnic arterioles. Experiments on Vaso-motor nerves.-The experiments on the vaso-motor nerves are similar to those performed on other Fig. 427.-Tracing showing the effect on blood-pressure of stimulating the central end of the Depressor nerve in the rabbit. To be read from right to left. T, indicates the rate at which the recording-surface was travelling, the intervals correspond to seconds; C, the moment of entrance of current; 0, moment at which it was shut off. The effect is some time in developing, and lasts after the current has been taken off. The larger undulations are the respiratory curves; the pulse oscillations are very small. (Foster.) nerves when one wishes to ascertain their functions. They consist of section and excitation. Section of a vaso-constrictor nerve such as the splanchnic causes a loss of normal arterial tone, and consequently the part supplied by the nerve becomes flushed with blood. Stimulation of the peripheral end causes the vessels to constrict and the part to become comparatively pale and bloodless. This can be very readily demonstrated on the ear of the rabbit. This is a classical experiment associated with the name of Claude Bernard. Division of the cervical sympathetic produces an increased redness of the side of the head, and looking at the ear, the transparency of which enables one to follow the phenomena easily, the central artery with its branches is seen to become larger, and many small branches not previously visible come into view. The ear feels hotter, though this effect soon passes off as the exposure of a large quantity of blood to the air causes a rapid loss of heat. On stimulating the peripheral end of the cut nerve, the ear resumes CH. XXXVI.] VASO-DILATATOR NERVES. 477 its normal condition and then becomes paler than usual owing to excessive constriction of the vessels. The first part of the experiment, the dilatation following section, can be demonstrated in a very simple way, by pressing the thumb nail forcibly on the nerve where it lies by the side of the central artery of the ear. Section of a vaso-dilatator nerve such as the chorda tympani produces no effect on the vessels, but stimulation of its peripheral end causes great enlargement of all the arterioles, so that the sub- maxillary gland and the neighbouring parts supplied by the nerve become red, and gorged with blood, and the pulse is propagated through to the veins; the circulation through the capillaries is so rapid that the blood loses very little of its oxygen, and is there- fore arterial in colour in the veins. Another effect, free secretion of saliva, we shall study in connection with that subject. Other examples of vaso-dilatator nerves are the nervi erigentes to the erectile tissue of the penis, &c., and of the lingual nerve to the vessels of the tongue. It is, however, probable that all the vessels of the body receive both constrictor and dilatator nerves. But the presence of the latter is difficult to determine unless they are present in excess; if they are not, stimulation affects the constrictors most. The ■effect of section is also inconclusive ; for if a mixed nerve is cut the only effect observed is a dilatation due to removal of the tonic •constrictor influence. To solve this difficult problem, two methods are in use. 1. The method of degeneration.-If the sciatic nerve is cut, the vessels of the limb dilate. This passes off in a day or two. If the peripheral end of the nerve is then stimulated, the vessels are dilated, as the constrictor fibres degenerate earliest, and so one gets a result due to the stimulation of the still intact dilatator fibres. 2. The method of slowly interrupted shocks.-If a mixed nerve is stimulated with the usual rapidly interrupted faradic current, the effect is constriction; but if the induction shocks are sent in at long intervals (e.g. at intervals of a second), vaso-dilatator effects are often obtained. This can be readily demonstrated on the kidney vessels by stimulation of the anterior root of the tenth thoracic nerve in the two ways just indicated. The action of vaso-motor nerves can be studied in another way, than by the use of the mercurial or other forms of mano- meter, which is the only method we have considered so far. The second method, which is often used together with the manometer, consists in the use of an instrument which records 478 THE CIRCULATION IN THE BLOOD-VESSELS. [CH. XXXVI. variations in the volume of any limb, or organ of an animal. Such an instrument is called a plethysmograph. One of these instruments applied to the human arm is shown in the next figure. Every time the arm expands with every heart's systole, a little of the fluid in the plethysmograph is expelled and raises the lever. Variations in volume due to respiration are also seen in the tracing. The same instrument in a modified form applied to such organs as the spleen and kidney is generally called an oncometer, and Fig. 428.-Plethysmograph. By means of this apparatus, the alteration in volume of the arm e, which is enclosed in a glass tube, a, filled with fluid, the opening through which it passes being firmly closed by a thick gutta-percha band, y, is communicated to the lever, d, and registered by a recording apparatus. The fluid in a communicates with that in b, the upper limit of which is above that in a. The chief alterations in volume are due to alteration in the blood contained in the arm. When the volume is increased, fluid passes out of the glass cylinder, and the lever, d, also is raised, and when a decrease takes place the fluid returns again from b to a. It will therefore be evident that the apparatus is capable of recording alterations of blood-pressure in the arm. the recording part of the apparatus, the oncograph. These instruments we owe to Prof. Roy, and the next two figures represent respectively sections of the kidney oncometer and oncograph. Each consists of a metal capsule, of shape suitable to enclose the organ : its two halves are jointed together, and fit accurately except at one opening which is left for the vessels of the organ. A delicate membrane is attached to the rim of each half, the space between which and the metal is filled with warm oil. The tube from the oncometer is connected to the oil-containing cavity by a tube also containing oil to the oncograph. An increase in the volume of the organ squeezes the oil out of the oncometer into the oncograph and so produces a rise of the oncograph CH. XXXVI.] THE ONCOMETER. 479 Fig. 429.-Diagram of Roy's Oncometer, a, represents the kidney enclosed in a metal box, which opens by hinge/; b, the renal vessels and duet. Surrounding the kidney are two chambers formed by membranes, the edges of which are firmly fixed by being clamped between the outside metal capsule, and one (not represented in the figure) inside, the two being firmly screwed together by screws at h, and below. The mem- branous chamber below is filled with a varying amount of warm oil, according to the size of the kidney experimented with, through the opening then closed with the plug i. After the kidney has been enclosed in the capsule, the membranous chamber above is filled with warm oil through the tube e, which is then closed by a tap (not represented in the diagram); the tube d communicates with a recording apparatus, and any alteration in the volume of the kidney is communicated by,the oil in the tube to the chamber d of the Oncograph, fig. 430. Fig. 430.-Roy's Oncograph, or apparatus for recording alterations in the volume of the kidney, &c., as shown by the oncometer-a, upright, supporting recording lever I, which is raised or lowered by needle b, which works through /, and which is attached to the piston e, working in the chamber d, with which the tube from the oncometer com- municates. The oil is prevented from being squeezed out as the piston descends by a. membrane, which is clamped between the ring-shaped surfaces of cylinder by the screw i working upwards; the tube h is for filling the instrument. 480 THE CIRCULATION IN THE BLOOD-VESSELS. [CH. XXXVI. piston and lever; a contraction of the organ produces a fall of the lever. Very good results are obtained by using saline solution instead of oil; and Prof. Schafer has recently shown in connection with the spleen that a spleen box of simple shape covered with a glass plate, made air-tight with vaseline, except where it communicates by a tube with a Marey's tambour, gives a far more delicate record of the splenic alterations of volume than the oncometer. If now we are investigating the action of the anterior root of tenth thoracic nerve on the vessels of the kidney, a tracing is taken simultaneously of the arterial blood-pressure in the carotid, and of the volume of the kidney by the oncometer. On stimu- lating the nerve rapidly, there is a slight rise of arterial pressure, but a large fall of the oncograph lever showing that the kidney has diminished in volume. It is evident that there must be an active contraction of the arterioles of the kidney, causing it to diminish in size, for the blood-pressure tracing shows that there is no failure of the heart's activity to account for it. We shall return to the subject of the oncometer in connection with the spleen and kidney. The vaso-motor nervous system is influenced to some extent by conditions of the cerebrum, some emotions, such as fear, causing pallor (vaso-constriction), and others causing blushing (vaso- dilatation). It is almost impossible to over-estimate the importance of the study of vaso-motor phenomena, as a means of explaining certain pathological conditions; our knowledge of the processes concerned in inflammation is a case in point. Disorders of the vessels due to vaso-motor disturbances are generally called angio-neuroses. Of these we may mention the following :- Tache cerebrate is due to abnormal sensitiveness of the vascular nerves; drawing the finger-nail across the skin causes an imme- diate wheal, or at least a red mark that lasts a considerable time. At one time this was considered characteristic of affections of the cerebral meninges like tubercular meningitis, and was conse- quently called the " meningeal streak." It, however, occurs in a variety of pathological conditions of the nervous system both cerebral and spinal. In certain forms of migraine there is a peculiar kind of head- ache confined to half the head, and called hemi-crania. This is associated with an abnormal constriction of the vessels on the affected side. In certain conditions which lead to angina pectoris the pain in CH. xxxvii.] LYMPH AND LYMPHATIC GLANDS. 481 the heart is due to its being unable to overcome an immense peripheral resistance, and the condition is relieved by the adminis- tration of drugs like amyl-nitrite or nitro-glycerine, which relax the vessels and cause universal blushing. Raynauds disease is one in which there is a localised constric- tion of the vessels which is so effectual as to entirely cut off the blood supply to the capillary areas beyond, and if this lasts any considerable time may lead to gangrene of the parts in question. CHAPTER XXXVII. LYMPH AND LYMPHATIC GLANDS. As the blood circulates through the capillary blood-vessels some of its liquid constituents exude through the thin walls of these vessels, carrying nutriment to the tissue elements. This exudation is called lymph ; it receives from the tissues the pro- ducts of their activity, and is collected by the lymph channels, which converge to the thoracic duct--the main lymphatic vessel -and thus the lymph once more re-enters the blood stream near to the entrance of the large systemic veins into the right auricle. Lymph is a fluid, which comes into much more intimate re- lationship with metabolic processes in the tissues than the blood; in fact, there is only one situation-the spleen-where the blood comes into actual contact with the elements,-that is, cells, fibres, Ac.,-of a tissue. Composition of Lymph. Lymph is alkaline; its specific gravity is about 1015, and after it leaves the vessels it clots, forming a colourless coagulum of fibrin. It is like blood-plasma in composition, only diluted so far as its proteid constituents are concerned. This is due to the fact that proteids do not pass readily through membranes. The proteids present are called fibrinogen, serum globulin, and serum albumin ; these we shall study with the blood-plasma. The salts are similar to those of blood-plasma, and are present in the same proportions. The waste products, like carbonic acid and urea, are more abundant in lymph than in blood. The total amount of solids dissolved in lymph is about 6 per cent., more than half of which is proteid in nature. 482 LYMPH AND LYMPHATIC GLANDS. [CH. XXXVII. When examined with the microscope the transparent lymph is found to contain colourless corpuscles, which are called lymphocytes ; these are cells with large nuclei and comparatively little proto- plasm. They pass with the lymph into the blood, where they undergo growth, and are called leucocytes. All the lymphatics pass at some point of their course through lymphatic glands, which are the factories of these corpuscles. Lymphocytes also pass into the lymph stream wherever lymphoid tissue is found, as in the tonsils, thymus, Malpighian bodies of the spleen, Peyer's patches, and the solitary glands of the intestine, &c. The lymph that leaves these tissues is richer in lymph-cells than that which enters them. When lymph is collected from the thoracic duct after a meal containing fat, it is found to be milky. This is due to the presence in the lymph of minutely subdivided fat particles absorbed from the interior of the alimentary canal. The lymph is then called chyle. The fat particles constitute what used to be called the molecular basis of chyle. If the abdomen is opened during the process of fat absorption, the lymphatics are seen as white lines, due to their containing this milky fluid. They are consequently called lacteals. The structure and arrangement of the lymphatic vessels is given in Chapter XXXIIL, and we have now to proceed to the study of the structure of The Lymphatic Glands. Lymphatic glands are round or oval bodies varying in size from a hemp-seed to a bean, interposed in the course of the lymphatic Fig. 431.-Section of a mesenteric gland from the ox, slightly magnified, a, Hilus ; b (in the central part of the figure), medullary substance; c, cortical substance with indistinct alveoli; rf, capsule. (Kiilliker.) vessels, and through which the chief part of the lymph passes in its course to be discharged into the blood-vessels. They are found CH. XXXVII.] LYMPHATIC GLANDS. 483 in great numbers in the mesentery, and along the great vessels of the abdomen, thorax, and neck ; in the axilla and groin ; a few in the popliteal space, but not further down the leg, and in the arm as far as the elbow. A lymphatic gland is covered externally by a capsule of connective-tissue, generally containing some unstriped muscle. At the inner side of the gland, which is somewhat concave (hilus), (fig. 431), the capsule sends inwards processes called trabeculae in which the blood-vessels are contained, and these join with other processes prolonged from the inner surface of the Fig. 432.-Diagrammatic section of lymphatic gland, a.l., afferent; e.l., efferent lympha- tics ; C, cortical substance; l.h., reticulating cords of medullary substance ; l.s., lymph-path; fibrous coat sending in trabeculse tr. into the substance of the gland. (Sharpey.) part of the capsule covering the convex or outer part of the gland; they have a structure similar to that of the capsule, and entering the gland from all sides, and freely communicating, form a fibrous scaffolding. The interior of the gland is seen on sec- tion, even when examined with the naked eye, to be made up of two parts, an outer or cortical, which is light coloured, and an inner of redder appearance, the medullary portion (figs. 431, 432). In the outer part, or cortex, of the gland (fig. 432) the intervals between the trabecula) are large and regular; they are termed 484 LYMPH AND LYMPHATIC GLANDS. [CH. XXXVII. alveoli; whilst in the more central or medullary part is a finer meshwork formed by an irregular anastomosis of the trabecular processes. Within the alveoli of the cortex and in the meshwork formed by the trabeculae in the medulla, is contained lymphoid tissue ; this occupies the central part of each alveolus ; but at the periphery surrounding the central portion and immediately next the capsule and trabeculae, is a more open meshwork of retiform tissue constituting the lymph path, and containing few lymph- corpuscles. At the inner part of the alveolus, the central mass divides into two or more smaller rounded or cord-like masses, which joining with those from the other alveoli, form a much Fig. 433.-A small portion of medullary substance from a mesenteric gland of the ox, d,d, trabeculae ; a, part of a cord of glandular substances from which all but a few of the lymph-corpuscles have been washed out to show its supporting meshwork of retiform tissue and its capillary blood-vessels (which have been injected, and are dark in the figure); b, b, lymph-path, of which the retiform tissue is represented only at c, c. X 300. (Kolliker.) closer arrangement than in the cortex; spaces (fig. 433, J) are left within those anastomosing cords, in which are found portions of the trabecular meshwork and the continuation of the lymph path. The lymph enters the gland by several afferent vessels, which pierce the capsule and open into the lymph-path ; at the same CH. XXXVII.] THE LYMPH FLOW. 485 time they lay aside all their coats except the endothelial lining, which is continuous with the lining of the lymph-path. The efferent vessels begin in the medullary part of the gland, and are continuous with the lymph-path here as the afferent vessels were with the cortical portion ; the endothelium of one is continuous with that of the other. The efferent vessels leave the gland at the hilus, and generally either at once, or very soon after, join together to form a single vessel. Blood-vessels which enter and leave the gland at the hilus are freely distributed to the trabecular tissue and to the lymphoid tissue. The Lymph Flow The flow of the lymph towards the point of its discharge into the veins is brought about by several agencies. With the help of the valvular mechanism all occasional pressure on the exterior of the lymphatic and lacteal vessels propels the lymph onward j thus muscular and other external pressure accelerates the flow of the lymph as it does that of the blood in the veins. The action of the muscular fibres of the small intestine, and probably the layer of unstriped muscle present in each intestinal villus, assist in propelling the chyle; for, in the small intestine of a mouse, the chyle has been seen moving with intermittent propul- sions that appear to correspond with the peristaltic movements of the intestine. But for the general propulsion of the lymph and chyle, it is probable that, together with the vis a tergo resulting from external pressure, some of the force may be derived from the contractility of the vessel's own walls. The respiratory movements, also, favour the current of lymph through the thoracic duct as they do the current of blood in the thoracic veins. Lymph-Hearts.-In reptiles and some birds, an important auxiliary to the movement of the lymph and chyle is supplied in certain muscular sacs, named lymph-hearts, and it has been shown that the caudal heart of the eel is a lymph-heart also. The number and position of these organs vary. In frogs and toads there are usually four, two anterior and two posterior. Into each of these cavities several lymphatics open, the orifices of the vessels being guarded by valves, which prevent the retrograde passage of the lymph. From each heart a single vessel proceeds, and conveys the lymph directly into the venous system. Blood is prevented from passing into the lymphatic heart by a valve at its orifice. The muscular coat of these hearts is of variable thickness ; in some cases it can only be discovered by means of the microscope ; but in every case it is composed of striped fibres. The contractions of the hearts are rhythmical, occurring about sixty times in a minute. The pulsations of the cervical pair are not always synchronous with those of the pair in the ischiatic 486 LYMPH AND LYMPHATIC GLANDS. [CH. xxxvn. region, and even the corresponding sacs of opposite sides are not always synchronous in their action. Unlike the contractions of the blood-heart, those of the lymph-heart appear to be directly dependent upon a certain limited portion of the spinal cord. For Volkmann found that so long as the portion of spinal cord corresponding to the third vertebra of the frog was uninjured, the cervical pair of lymphatic hearts continued pulsating after all the rest of the spinal cord and the brain were destroyed ; while destruction of this portion, even though all other parts of the nervous centres were uninjured, instantly arrested the heart's movements. The posterior, or ischiatic, pair of lymph- hearts were found to be governed, in like manner, by the portion of spinal cord corresponding to the eighth vertebra. Division of the posterior spinal roots did not arrest the movements ; but division of the anterior roots caused them to cease at once. Relation of Lymph and Blood. The volume of blood in the body remains remarkably constant. If the amount is increased by injection of fluids, at first its specific gravity is lessened, but in a short time, often in a few minutes, it returns to the normal. The excess of fluid is got rid of in two ways : (x) by the kidneys, which secrete profusely; and (2) by the tissues, which become more watery in consequence. After the renal arteries are ligatured, and the kidney is conse- quently thrown out of action, the excess of water passes only into the tissues. On the other hand, a deficiency of blood is soon remedied by a transfer of water from the tissues to the blood through the intermediation of the lymph. Carl Ludwig taught that the lymph flow is conditioned by two factors : first, differences in the pressure of the blood in the capillaries and the fluid in the tissue spaces, giving rise to a filtration of fluid through the capillary walls; and secondly, chemical differences between these two fluids, setting up osmotic interchanges through the wall of the blood-vessel. Formation of Lymph Osmosis.-The phenomenon of the passage of fluids through animal membrane, which occurs quite independently of vital conditions, was first demonstrated by Dutrochet. The instrument which he employed in his experiments was named an endosmometer. One form of this, represented in the figure, consists of a graduated tube expanded into an open-mouthed bell at one end, over which a portion of membrane is tied. If the bell is filled with a solution of a salt-say sodium chloride-and is immersed in water, the water will pass into the solution, and part of the salt will pass out into the water ; the water, however, will pass into the solution much more rapidly than the salt will pass out into the water, and the diluted solution will rise in the tube. It is to this passage of fluids through membrane that the term osmosis is applied. CH. XXXVII.] FORMATION OF LYMPH. 487 The nature of the membrane used as a septum, and its affinity for the fluids subjected to experiment, have an important influence, as might be anticipated, on the rapidity and duration of the osmotic current. Thus, if a piece of ordinary bladder be used as a septum between water and alcohol, the current is almost solely from the water to the alcohol, on account of the much greater affinity of water for this kind of membrane ; while, on the other hand, in the case of a membrane of caoutchouc, the alcohol, from its greater affinity for this substance, would pass freely into the water. If the lymph is produced by a simple act of filtration, then the amount of lymph must rise and sink with the value of D-cl; D representing the capillary blood pressure, and d the pressure in the tissue spaces. In support of this mechanical theory, various workers in Ludwig's laboratory showed that in- creased capillary pressure due to obstruction of the venous outflow increases the amount of lymph formed ; and that diminution of the pressure in the lymph spaces, by squeezing out the lymph previously contained in them, leads to an increase in the transudation. On the other hand, there were some facts which could not be well explained by the filtration theory, among which may be mentioned the action of curare in causing an increase of lymph flow. Heidenhain was the first to fully recognise that the laws of filtration and osmosis as applied to dead membranes may be considerably modified when the membranes are composed of living cells; and he considers that the formation of lymph is due to the selective or secretory activity of the endothelial walls of the capillaries. This so-called vital action of the endothelial cells is seen in the fact that after the injection of sugar into the blood, in a short time the percentage of sugar in the lymph becomes higher than that in the blood. There must, therefore, be some activity of the endothelial cells in picking out the sugar from the blood and passing it on to the lymph. Heidenhain is also the inventor of the term lymphagogues (literally lymph drivers). These are substances like curare, which have a specific action in causing an increased lymph flow. Heidenhain considers the majority of these act by stimulating the endothelial cells to activity. This conclusion, however, has been subjected to much criticism. In this country the question has been taken up by Dr. Starling, who has shown that the influence of vital action is not so marked as Heidenhain supposes it to be, Fig. 434- - En- dosmometer. 488 THE DUCTLESS GLANDS. [ch. xxxvtii. but that most of the phenomena in connection with lymph formation can be explained by the simpler mechanical theory. The question, however, is just now much under discussion, and without pronouncing a definite opinion one way or the other, we may conclude by stating briefly the view held by Starling on the subject. The amount of lymph produced in any part depends on two factors :- 1. The pressure at which the blood is flowing through the capillaries. Heidenhain took the arterial pressure in his experi- ments as the measure of the capillary pressure; Starling points out, very justly, that this is incorrect, as there is between the arteries and the capillaries the peripheral resistance in the arterioles. 2. The permeability of the capillary wall. This varies enormously in different regions ; it is greatest in the liver, so that an intracapillary pressure which would cause lymph to flow here is without effect on the production of lymph in the limbs. The flow of lymph may therefore be increased in two ways :- 1. By increasing the intracapillary pressure. This may be done locally by ligaturing the veins of an organ; or generally by injecting a large amount of fluid into the circulation, or by the injection of such substances as sugar and salt (Heidenhain's first class of lymphagogues) into the blood. These attract water from the tissues into the blood, and thus increase the volume of the circulating fluid and raise the intracapillary pressure. 2. By increasing the permeability of the capillary wall by injuring its vitality. This may be done locally by scalding a part; or generally, by injecting certain poisonous substances, such as peptone, leech extract, decoction of mussels, <fcc. (Heidenhain's second class of lymphagogues). These act chiefly on the liver capillaries; curare acts chiefly on the limb capillaries. CHAPTER XXXVIII. THE DUCTLESS GLANDS. The ductless glands form a heterogeneous group of organs, most of which are related in function or development with the circula- tory system. They include the lymphatic glands, the spleen, CH. XXXVIII.] THE DUCTLESS GLANDS. 489 the thymus, the thyroid, the suprarenal capsules, the pineal body, the pituitary body, and the carotid and coccygeal glands. The function of a gland that has a duct is a comparatively simple physiological problem, but the use of ductless glands has long been a puzzle to investigators. Recent research has, however, shown that most of, if not all the ductless glands do form a secretion, and this internal secretion, as it is termed, leaves the gland by the venous blood or lymph, and thus is distributed and ministers to the needs of parts of the body elsewhere. Many of the glands which possess ducts and form an external secretion, form an internal secretion as well. Among these the liver, pancreas, and kidney must be mentioned. In many cases the internal secretion is essential for life, and removal of the gland that forms it, leads to a condition of disease culminating in death. In other cases the internal secretion is not essential, or its place is taken by that formed in similar glands in other parts of the body. The body is a complex machine; each part of the machine has its own work to do, but must work harmoniously with other parts. Just as a watch will stop if any of its numerous wheels gets broken, so the metabolic cycle will become disarranged or cease altogether if any of the links in the chain break down. In unravelling the part which the ductless glands play in this cycle, it is at present impossible in many cases to state precisely what the particular function of each is; all one can say is that when the gland is removed or its function interfered with, that the metabolic round is broken somehow, and that this upsets the whole of the machinery of the body. The difficulty of investi- gating this subject is increased by the fact that it is impossible to get the internal secretion in a state of purity and examine it; it is always mixed with, and masked by the lymph or blood into which it is poured. In spite of this, however, our knowledge in this branch of physiology is increasing, particularly in connection with some of these ductless glands. The methods which have been employed are the following :- 1. Extirpation.-The gland in question is removed, and the effect of the absence of the internal secretion noted. 2. Disease.-In cases where the function of the gland is in abeyance, owing to its being diseased, the symptoms are also closely observed. 3. Injection of Extracts.-The gland is taken in a fresh condi- tion; an extract is made of it, and this is injected into the circulation of healthy animals, and into that of those animals 490 THE DUCTLESS GLANDS. [ch. xxxviii. from which the gland has been previously removed, and the effects watched. 4. Transplantation.-After the gland is removed and the usual effect produced, the same gland from another animal is transplanted into the first animal and restoration of function looked for. The case of the lymphatic glands we have already studied ; they form an internal secretion which consists of lymph-cells, and these furnish the blood with its most important supply of colour- less corpuscles. Removal of lymphatic glands is not fatal, as the other lymphatic glands, and other collections of lymphoid tissue that remain behind, carry on the work of those that are removed. The Spleen. The Spleen is the largest of the ductless glands ; it is situated to the left of the stomach, between it and the diaphragm. It is of a deep red colour and of variable shape. Vessels enter and leave the gland at a depression on the inner side called the hilus. Structure.-The spleen is covered externally almost completely by a serous coat derived from the peritoneum, while within this is the proper fibrous coat or capsule of the organ. The latter is composed of connective-tissue, with a large preponderance of elastic fibres and a certain proportion of unstriated muscular tissue. Prolonged from its inner surface are fibrous processes or trabecula;, containing much unstriated muscle, which enter the interior of the organ and, dividing and anastomosing in all parts, form a supporting framework in the interstices of which the proper substance of the spleen (spleen-pulp) is contained. At the hilus of the spleen, the blood-vessels, nerves, and lym- phatics enter or leave, and the fibrous coat is prolonged into the spleen substance in the form of investing sheaths for the arteries and veins, which sheaths again are continuous with the trabeculae before referred to. The spleen-pulp, which is of a dark red or reddish-brown colour, is composed chiefly of cells, imbedded in a network formed of fibres, and the branchings of large nucleated cells. The network so formed is thus very like a coarse kind of retiform tissue. The spaces of this network are only partially occupied by cells and form a freely communicating system. Of the cells some are granular corpuscles resembling the lymph-corpuscles, both in general appear- ance and in being able to perform amoeboid movements; others are red blood-corpuscles of normal appearance or variously changed; while there are also large cells containing either a pig- CH. XXXVIII.] THE SPLEEN. 491 meat allied to the colouring matter of the blood, or rounded coi- puscles like red corpuscles. The splenic artery, after entering the spleen by its concave surface, divides and subdivides, with but little anastomosis between its branches ; at the same time its branches arc sheathed by the prolongations of the fibrous coat, which they, so to speak, Fig. 435.-Section of.injected dog's spleen; c, capsule; 6-, trabeculae; m, two Malpighian bodies with numerous small arteries and capillaries; a, artery; I, lymphoid tissue, consisting of closely-packed lymphoid cells supported by very delicate retiform tissue ; a light space unoccupied by cells is seen all round the trabeculfe, which corresponds to the " lymph path " in lymphatic glands. (Schofield.) carry into the spleen with them. The arteries soon leave the trabeculae, and their outer coat is then replaced by one of lym- phoid tissue ; they end in an open brushwork of capillaries, the endothelial cells of which become continuous with those of the rete of the spleen-pulp. The veins begin by a similar open set of capillaries from the large blood spaces of the pulp. The veins soon pass into the trabeculae, and ultimately unite to form the 492 THE DUCTLESS GLANDS. [ch. xxxvni. splenic vein. This arrangement readily allows lymphoid and other corpuscles to be swept into the blood-current. On the face of a section of the spleen can be usually seen readily with the naked eye, minute, scattered rounded or oval whitish spots, mostly from to inch (f to -§■ mm.) in diameter. These are the Malpighian cor- puscles of the spleen, and are situated on the sheaths of the minute splenic arteries. They are in fact outgrowths of the outer coat of lymphoid tissue just referred to (see fig. 437). Blood capillaries traverse the Mal- pighian corpuscles and form a plexus in their interior. The structure of a Malpighian corpuscle of the spleen is practically iden- tical with that of a lymphoid nodule. Functions.-These are the following :- (1.) The spleen, like the lymphatic glands, is engaged in the Fig. 436.-Reticulum of the spleen of a cat, shown by injection with gelatine. (Cadiat.) Fig. 437.-Section of spleen of cat. a, a', Malpighian corpuscles, in case of a', in connection with small artery, &; b, b', small arteries; c, section of trabeculae. ch. xxxvni.] THE SPLEEN. 493 formation of colourless blood-corpuscles. For it is quite certain, that the blood of the splenic vein contains an unusually large amount of white corpuscles ; and in the disease termed leucocythvemia, in which the white corpuscles of the blood are remarkably increased in number, there is found a hypertrophied condition of the spleen, especially of the Malpighian corpuscles. The white corpuscles formed in the spleen also doubtless partly leave that organ by lymphatic vessels. By stimulating the spleen to contract in a case of splenic leucocythsemia by means of an electric current applied over it through the skin, the number of leucocytes in the blood is almost immediately increased. Removal of the spleen is not fatal; but after its removal there is an overgrowth of the lymphatic glands to make up for its absence. (2.) It forms coloured corpuscles at any rate in some animals; in these animals, cells are found in the spleen similar to those we have described in red marrow, and called hcematoblasts. In these animals, if the spleen is removed, the red marrow hypertrophies. (3.) There is reason to believe, that in the spleen many of the red corpuscles of the blood, those probably which have discharged their office and are worn out, undergo disintegration; for in the coloured portions of the spleen-pulp an abundance of such cor- puscles, in various stages of degeneration, are found, and in those cases of disease in which the destruction of blood-corpuscles is increased (pernicious anaemia) iron accumulates in the spleen as in the liver. It was formerly supposed that the spleen broke down the corpuscles and liberated haemoglobin, which passing- in the blood of the splenic vein to the liver was discharged by that organ as bile-pigment. But this is not the case ; the disin- tegration does not proceed so far as to actually liberate haemo- globin ; there is no free haemoglobin in the blood of the splenic vein. (4.) From the almost constant presence of uric acid, in larger quantities than in other organs, as well as of the nitrogenous bodies, xanthine and hypoxanthine, in the spleen, some share in nitrogenous metabolism may be fairly inferred to occur in it. (5.) Besides these direct offices, the spleen fulfils some purpose in regard to the portal circulation, with which it is in close con- nection. From the readiness with which it admits of being distended, and from the fact that it is generally small while gastric digestion is going on, and enlarges when that act is con- cluded, it is supposed to act as a kind of vascular reservoir, or diverticulum to the portal system, or more particularly to the vessels 494 THE DUCTLESS GLANDS. [ch. xxxviii. of the stomach. That it may serve such a purpose is also made probable by the enlargement which it undergoes in certain affections of the heart and liver, attended with obstruction to the passage Big. 438.-Roy's Oncometer for spleen ; A, open ; B, closed. of blood through the latter organ, and by its diminution when the congestion of the portal system is relieved by discharges from the CH. XXXVIII.] THE THYMUS. 495 bowels, or by the effusion of blood into the stomach. This mechanical influence on the circulation, however, can hardly be supposed to be more than a very subordinate function. Influence of the Nervous System upon the Spleen.-When the spleen is enlarged after digestion, its enlargement is probably due to two causes : (i) a relaxation of the muscular tissue which forms so large a part of its framework ; (2) a dilatation of the vessels. Both these phenomena are under control of the nervous system. It has been found by experiment that when the splenic nerves are cut the spleen enlarges, and that contraction can be brought about (1) by stimulation of the spinal cord or of the divided nerves; (2) reflexly by stimulation of the central stumps of certain divided nerves, e.c/., vagus and sciatic; (3) by local stimulation by an electric current; (4) the administration of quinine and some other drugs. It has been shown by the oncometer of Roy that the spleen undergoes rhythmical contractions and dilatations, due to the contraction and relaxation of the muscular tissue in its capsule and trabecula). A tracing also shows waves due to the rhythmical alterations of the general blood-pressure. The form of oncometer adapted for the shape of the spleen of most animals is shown in the next figure. In most mammals the spleen is not kidney-shaped as in man, but narrow and ribbon shaped. The general principles of the oncometer have been explained on p. 478, where also it is mentioned that by a simpler plethysmographic method Schafer has obtained tracings which show first, the large waves occurring about once a minute, due to the splenic systole and diastole; secondly, smaller waves on this, due to the effect of respiration on the blood-pressure ; and on these smaller waves still, corresponding with the individual heart-beats. The large waves, due to the splenic contractility still go on after the division of all the splenic nerves. The Thymus. This gland is a temporary organ ; it attains its greatest size early after birth, and after the second year gradually diminishes, until in adult life hardly a vestige remains. At its greatest develop- ment it is a long narrow body, situated in the front of the chest behind the sternum and partly in the lower part of the neck. It is of a reddish or greyish colour, and is distinctly lobulated. Structure.-The gland is surrounded by a fibrous capsule, which sends in processes, forming trabeculae, which divide the gland 496 THE DUCTLESS GLANDS. [ch. xxxviii. into lobes, and carry the blood- and lymph-vessels. The large trabeculse branch into small ones, which divide the lobes into lobules. The lobules are further subdivided into follicles by fine connective-tissue. A follicle (fig. 441) is seen.on section to be more or less polyhedral in shape, and consists of cortical and medullary portions, both of which are composed of adenoid or lymphoid tissue, but in the medullary portion the matrix is coarser, Fig. 439.-Transverse section of a lobule of an injected infantile thymus gland, a, capsule of connective-tissue surrounding the lobule ; ft, membrane of the glandular vesicles ; c, cavity of the lobule, from which the larger blood-vessels are seen to extend towards and ramify in the spheroidal masses of the lobule. X 30. (Kolliker.) and is not so filled up with lymphoid corpuscles as in the cortex. Scattered in the lymphoid tissue of the medulla are the concentric corpuscles of Hassall, which consist of a nucleated granular centre, surrounded by flattened nucleated epithelial cells. These are islands of epithelial cells cut off from the epithelium of the pharynx in process of development. They are not occluded blood-vessels, as was at one time supposed (fig. 442). They remind one somewhat of the epithelial nests seen in some varieties of cancer. The arteries radiate from the centre of the gland. Lymph sinuses may be seen occasionally surrounding a greater or smaller por- tion of the periphery of the follicles (Klein). The nerves are very minute. From the thymus various substances may be extracted, many of CH. XXXVIII.] THE THYMUS. 497 them similar to those obtained from the spleen, e.g., xanthine, hypoxanthine, and leucine. The main constituent of the cells is proteid, and especially Fig. 440.-Thymus of a calf, a, cortex of follicle; b, medulla; c, interfollicular tissue magnified about twelve times. (Watney.) nucleo-proteid. Indeed the thymus is usually employed as the source of nucleo-proteid when one wishes to inject that substance into the blood-vessels of an animal to produce intravascular Fig. 441.-From a horizontal section through superfi- cial part of the thymus of a calf, slightly magni- fied. Showing in the centre a follicle of poly- gonal shape with simi- larly shaped follicles round it. (Klein and Noble Smith.) Fig. 442.-The reticulum of the thymus, a, epithelial ele- ments ; &, corpuscles of Has- sall. (Cadiat.) clotting. It is, however, not characteristic of the thymus, but is found in all protoplasm. The method of preparation will be given later. 498 THE DUCTLESS GLANDS. [ch. XXXVIII Function.-The thymus takes part in producing the colourless corpuscles like other varieties of lymphoid tissue. Respecting the thymus gland in the hybernating animals, in which it exists throughout life, as each successive period of hyber- nating approaches, the thymus greatly enlarges and becomes laden with fat, which accumulates in it and in fat glands con- nected with it, in even larger proportions than it does in the ordinary seats of adipose tissue. Hence it appears to serve for the storing up of materials which, being reabsorbed in inactivity of the hybernating period, may maintain the respiration and the temperature of the body in the reduced state to which they fall during that time. Some observers state that it is also a source of the red blood-corpuscles, at any rate in early life. The Thyroid. The thyroid gland is situated in the neck. It consists of two lobes, one on each side of the trachea, extending upwards to the Fig. 443 .-Part of a section of the human thyroid, a, fibrous capsule; b, thyroid vesicles filled with, e, colloid substance; c, supporting fibrous tissue ; d, short columnar cells lining vesicles; f, arteries; g, veins filled with blood; h, lymphatic vessel filled with colloid substance, x. (S. K. Alcock.) thyroid cartilage, covering its inferior cornu and part of its body; these lobes are connected across the middle line by a middle lobe CH. XXXVIII.] THE THYROID. 499 or isthmus. It is highly vascular, and varies in size in different individuals. Structure.-The gland is encased in a capsule of dense areolar tissue. This sends in strong fibrous trabeculae, ■which enclose the thyroid vesicles-which are rounded or oblong irregular sacs, con- sisting of a wall of thin hyaline membrane lined by a single layer of short cylindrical or cubical cells. These vesicles are filled with transparent colloid nucleo-albuminous material. The colloid substance increases with age, and the cavities appear to coalesce. In the interstitial connective-tissue is a round meshed capillary plexus, and a large number of lymphatics. The nerves adhere closely to the vessels. In the vesicles there are in addition to the yellowish glassy colloid material, epithelium cells, colourless blood-corpuscles, and also coloured corpuscles undergoing disintegration. Function.-It is difficult to state definitely the function of the thyroid body; it is one of those organs of great importance in the metabolic round; and its removal or disease is followed by general disturbances. It no doubt forms an internal secretion; to this the colloid material mentioned contributes, as it is found in the lymphatic vessels of the organ. When the gland is diseased in children and its function obliterated, a species of idiocy is produced called cretinism. The same condition in adults is called myxoedema; the most marked symptoms of this condition are slowness, both of body and mind, usually associated with tremors and twitchings. There is also a peculiar condition of the skin leading to the overgrowth of the subcutaneous tissues, which in time is replaced by fat; the hair falls off, the hands become spade-like; the whole body is unwieldy and clumsy like the mind. A similar condition occurs after the thyroid is completely removed surgically; this is called cachexia strumipriva; this operation, which was performed previous to our knowledge of the importance of the thyroid, is not regarded as justifiable nowadays. Lastly, in many animals removal of the thyroid produces analogous symptoms, in the overgrowth of the connective-tissues especially under the skin, and in the nervous symptoms (twitch- ings, convulsions, etc.). The term Myxoedema was originally given under the erroneous idea that the swelling of the body is due to mucin. In the early stages of the disease there is a slight increase of mucin, because all new connective-tissues contain a relatively large amount of ground substance, the most abundant constituent of which, next 500 THE DUCTLESS GLANDS. [ch. xxxviii. to water, is mucin. But there is nothing characteristic about that. 1 he discovery of the relationships between the thyroid and these morbid conditions is especially interesting, because important practical results in their treatment have followed close on the heels of experimental investigation. The missing internal secretion of the thyroid may be replaced in these animals and patients by grafting the thyroid of another animal into the abdomen; or more simply by injecting thyroid extract subcu- taneously ; or even by feeding on the thyroid of other animals. This treatment, which has to be kept up for the rest of the patient's life, is entirely successful. Chemical physiologists have been searching recently to try and discover what the active material in thyroid extract is, which produces such marvellous results ; some look upon it as nucleo-proteid ; others as a crystal- line substance called thyreo-antitoxin (CGH11N.,O.) by Fraenkel, its discoverer; others again (Baumann, and Roos) are inclined to attribute the efficacy of thyroid extract to a substance they have separated from the gland and which stands almost unique among physiological compounds by containing a large percentage of iodine in its molecule. But whatever the chemical composition of the active principle of the thyroid s internal secretion may be, there can be no doubt that its action under normal circumstances is as a regulator of metabolic processes, especially in the central nervous system. The Supra-renal Capsules. These are two triangular or cocked-hat-shaped bodies each resting by its lower border upon the upper border of the kidney. /S'tfrwctatre.-The gland is surrounded by an outer sheath of connective-tissue, which sends in fine prolongations forming the framework of the gland. The gland tissue proper consists of an outside himer cortical portion and an inside soft dark medullary portion. (i.) Fhe cortical portion is divided into (fig. 444) columnar groups of cells {zona fasciculata). Immediately under the capsule, however, the groups are more rounded {zona glomerulosa), while next to the medulla they have a reticular arrangement {zona reticularis'). The cells themselves are polyhedral, each with a clear round nucleus, and often with oil globules on their proto- plasm. The blood-vessels run in the fibrous septa between the columns, but do not penetrate between the cells. (2.) 1 he medullary substance consists of a coarse rounded or CH. XXXVIII.] SUPRA-RENAL CAPSULES. 501 irregular meshwork of fibrous tissue, in the alveoli of which are masses of multinucleated protoplasm (fig. 445); numerous blood- Fig. 444.-Vertical section through part of the cortical portion of supra-renal of guinea-pig. a, capsule; b, zona glomerulosa; c, zona fasciculata; d, connective-tissue supporting the columns of the cells of the latter, and also indicating the position of the blood- vessels. x. (S. K. Alcock.) Fig. 445.-Section thiough a portion of the medullary pait of the supra-renal of guinea- pig. The vessels are very numerous, and the fibrous stroma more distinct than in the cortex, and is moreover reticulat :d. The cells are irregular and larger, clean, and free from oil globules, x . (S. K. A'cock.) 502 THE DUCTLESS GLANDS. [CH. XXXVIII. vessels; and an abundance of nerve fibres and cells. The cells are very irregular in shape and size, poor in fat, and often branched ; the nerves run through the cortical substance, and anastomose over the medullary portion. The cells of the medulla are characterised by taking a brown stain with chromic acid, provided the organ is quite fresh. The substance to which this is due is only found in the body in the medulla of the suprarenals. Its chemical composition is uncertain, but in many of its characters it is similar to lecithin. This substance is probably the material to which the action of suprarenal extracts is due, and by inference the most important constituent of the internal secretion of the gland. Function.-The immense importance of the suprarenal bodies was first indicated by Addison, who, in 1855, pointed out that the disease now known by his name is associated with pathological alterations of these glands. This was tested experimentally by Brown-Sequard, who found a few years later that removal of the suprarenals in animals is invariably and rapidly fatal. The symptoms are practically the same (although more acute) as those of Addison's disease, namely, great muscular weakness, loss of vascular tone and nervous prostration. The pigmentation of the skin, however, which is a marked symptom in Addison's disease, is not seen in animals. The experiments of Brown- Sequard attracted much attention at the time they were per- formed, but were almost forgotten until quite recently, when they were confirmed by Abelous, Schafer, and others. The effects on the muscular system are the most marked results both after removal of the capsules and 'after injection of an extract of the glands. The effect of injecting such an extract on the voluntary muscles is to increase their tone, so that a tracing obtained from them resembles that produced by a small dose of veratrine, namely, a prolongation of the period of relaxation. The effect on involuntary muscle is equally marked ; there is an enormous rise of arterial blood pressure due chiefly to a contraction of the arterioles. This is produced by the direct action of the extract on the muscular tissue of the arterioles, not an indirect one through the vaso-motor centre. The active chemical substance in the extract that produces the effect is still a matter of uncer- tainty ; it is, however, confined to the medulla of the capsules, and is absent from the capsules in cases of Addison's disease. The capsules, therefore, form something which is distributed to the muscles and is essential for their normal tone; when they are removed the poisonous effects seen are the result of the absence of this internal secretion. CH. xxxvm.] PITUITARY AND PINEAL BODIES. 503 Whether this discovery will lead to the same important practical results as in the case of the thyroid and myxoedema must be left to the future to decide. There is already some evidence to show that injection of suprarenal extract is beneficial in cases of Addison's disease. The Pituitary Body. This body is a small reddish-grey mass, occupying the sella turcica of the sphenoid bone. Structure.-It consists of two lobes-a small posterior one, consisting of nervous tissue; an anterior larger one, resembling the thyroid in structure. A canal lined with flattened or with ciliated epithelium, passes through the anterior lobe ; it is con- nected with the infundibulum. The alveoli are approximately spherical; they are filled with nucleated cells of various sizes and, shapes not unlike ganglion cells, collected together into rounded masses, filling the vesicles, and contained in a semi-fluid granular substance. The vesicles are enclosed by connective- tissue, rich in capillaries. Nothing is known of the function of the pituitary body. The Pineal Gland. This gland, which is a small reddish body, is placed beneath the back part of the corpus callosum, and rests upon the corpora quadrigemina. Structure,.-It contains a central cavity lined with ciliated epithelium. The gland substance proper is divisible into-(i) An outer cortical layer, analogous in structure to the anterior lobe of the pituitary body; and (2) An inner central layer, wholly nervous. The cortical layer consists of a number of closed follicles, containing (a) cells of variable shape, rounded, elongated, or stellate ; (6) fusiform cells. There is also present brain-sand, gritty matter consisting of round particles aggregated into small masses. The central substance consists of white and grey matter. The blood-vessels are small, and form a very delicate capillary plexus. The pineal gland is the atrophied remains of a third eye situated centrally. This eye is formed in a more perfect condition, though covered by skin, in certain lizards, such as Hatteria. 504 RESPIRATION. [ch. xxxix. The Coccygeal and Carotid Glands. These so-called glands are situated, the one in front of the tip of the coccyx, and the other at the point of bifurcation of the common carotid artery on each side. They are made up of a plexus of small arteries, are enclosed and supported by a capsule of fibrous tissue, which contains connective-tissue corpuscles. The blood- vessels are surrounded by one or more layers of cells like secreting- cells, which are said to be modified plasma cells of the connective- tissue. The function of these bodies is unknown. CHAPTER XXXIX. RESPIRATION. The respiratory apparatus consists of the lungs, and of the air- passages which lead to them. In marine animals, the gills fulfil the same functions as the lungs of air-breathing animals. The muscles which move the thorax and the nerves that supply them must also be included under the general heading, Respiratory System; and using this expression in the widest sense, it includes practically all the tissues of the body, since they are all concerned in the using up of oxygen and the production of waste products like carbonic acid. Essentially a lung or gill is constructed of a thin membrane, one surface of which is exposed to the air or water, as the case may be, while, on the other, is a network of blood-vessels,- the only separation between the blood and aerating medium being the thin wall of the blood-vessels, and the fine membrane on one side of which vessels are distributed. The difference between the simplest and the most complicated respiratory membrane is one of degree only. The lungs or gills are only the medium for the exchange, on the part of the blood, of carbonic acid for oxygen. They arc not the seat, in any special manner, of those combustion-processes of which the production of carbonic acid is the final result. These processes occur in all parts of the body in the substance of the tissues. CH. XXXIX.] ORGANS OF RESPIRATION. 505 The Respiratory Apparatus. The object of respiration being the interchange of gases in the lungs, it is necessary that the atmospheric air shall pass into them and that the changed air should be expelled from them. The lungs are contained in the chest or thorax, which is a closed cavity having no communication with the outside except by means of the respiratory passages. The air enters these passages through the nostrils or through the mouth, whence it passes through the larynx into the trachea or windpipe, which about the middle of the chest divides into two tubes, bronchi, one to each (right and left) lung. The Larynx is the upper part of the passage, and has been already described in Chapter XXX. The Trachea and Bronchi.-The trachea extends from the cricoid cartilage, which is on a level with the fifth cervical vertebra, to a point opposite the third dorsal vertebra, where it divides into the two bronchi, one for each lung (fig. 446). It measures, on an average, four or four-and-a-half inches in length, and from three- quarters of an inch to an inch in diameter, and is essentially a tube of fibro-elastic membrane, within the layers of which are enclosed a series of cartilaginous rings, from sixteen to twenty in number. These rings extend only around the front and sides of the trachea (about two-thirds of its circumference), and are deficient behind ; the interval between their posterior extremities being bridged over by a continuation of the fibrous membrane in which they are enclosed (fig. 447). The cartilages of the trachea and bronchial tubes are of the hyaline variety. Immediately within this tube, at the back, is a layer of unstriped muscular fibres, which extends, transversely, between the ends of the cartilaginous rings to which they are attached, and opposite the intervals between them, also; their function being to diminish, when required, the calibre of the trachea by approximating the ends of the cartilages. Outside these are a few longitudinal bundles of muscular tissue, which, like the preceding, are attached both to the fibrous and cartilaginous framework. The mucous membrane consists to a great extent of loose lymphoid tissue, separated from the ciliated epithelium which lines it by a homogeneous basement membrane. The epithelium is formed of several layers, of which the most superficial layer is ciliated ; while between these cells are smaller elongated cells prolonged up towards the surface and down to the basement membrane. Beneath these are one or more layers of more 506 RESPIRATION. [CH. XXXIX. irregularly shaped cells (fig. 451). Many of the superficial cells are of the goblet variety. In the deeper part of the mucosa are Fig. 446.-Outline showing the general form of the larynx, trachea, and bronchi, as seen from before, h, the great cornu of the hyoid bone; e, epiglottis ; t, superior, and inferior cornu of the thyroid carti- lage ; c, middle of the cricoid cartilage; tr, the trachea, showing sixteen cartila- ginous rings; &, the right, and the left bronchus. (Allen Thomson.) x J. Fig. 447.-Outline showing the general form of the larynx, trachea, and bronchi, as seen from behind, h, great cornu of the hyoid bone ; t, superior, and the inferior cornu of the thyroid cartilage; e, epiglottis; a, points.to the back of both the arytenoid cartilages, which are surmounted by the comicula; c, the middle ridge on the back of the cricoid cartilage; tr, the posterior membranous part of the trachea; ft, l>', right and left bronchi. (Allen Thomson.) J. many elastic fibres between which lie connective-tissue corpuscles and capillary blood-vessels. CH. XXXIX.] THE TRACHEA. 507 Numerous mucous glands are situate on the exterior and in the substance of the fibrous framework of the trachea ; their ducts per- forate the various structures which form the wall of the trachea, and open through the mucous membrane into the interior. Fig. 448.-Section of the trachea, a, columnar ciliated epithelium; & and c, proper stimcture of the mucous membrane, containing elastic fibres cut across transversely; d, submucous tissue containing mucous glands, e, separated from the hyaline cartilage, g, by a fine fibrous tissue, h, external investment of fine fibrous tissue. (S. K Alcock.) The two bronchi into which the trachea divides, of which the right is shorter, broader, and more horizontal than the left (fig. 446), resemble the trachea in structure, with the difference that in them there is a distinct layer of unstriped muscle arranged circularly beneath the mucous membrane, forming the muscularis mucosae. On entering the substance of the lungs the cartilaginous rings, although they still form only larger or 508 RESPIRATION. [ch. xxxix. smaller segments of a circle, are no longer confined to the front and sides of the tubes, but arc distributed impartially to all parts of their circumference. The bronchi divide and sub-divide, in the substance of the lungs, into a number of smaller and smaller branches, which penetrate into every part of the organ, until at length they end in the smaller sub-divisions of the lungs, called lobules. All the larger branches have walls formed of tough membrane, containing portions of cartilaginous rings, by which they are held open, and unstriped muscular fibres, as well as longitudinal Fig. 449.-Transverse section of a bronchus, about | inch in diameter, e, Epithelium (ciliated) ; immediately beneath it is the mucous membrane or internal tibrous layer, of varying' thickness ; m, muscular layer ; s.m, submucous tissue ; /, fibrous tissue ; c, cartilage enclosed within the layers of fibrous tissue ; g, mucous gland. (F. E. Schulze.) bundles of elastic tissue. They are lined by mucous membrane, the surface of which, like that of the larynx and trachea, is covered with ciliated epithelium, but the several layers become less and less distinct until the lining consists of a single layer of short columnar cells covered with cilia (fig. 449). The mucous membrane is abundantly provided with mucous glands. As the bronchi become smaller and smaller, and their walls thinner, the cartilaginous rings become scarcer and more irregu- lar, until, in the smaller bronchial tubes, they are represented only by minute and scattered cartilaginous flakes. And when the bronchi, by successive branchings, are reduced to about T\j- of an inch ('6 mm.) in diameter, they lose their cartilaginous element altogether, and their walls are formed only of a fibrous elastic membrane, with circular muscular fibres ; they are still lined, however, by a thin mucous membrane, with ciliated epithelium, the length of the cells bearing the cilia having become CH. XXXIX.] THE PLEURA. 509 so far diminished, that the cells are now cubical. In the smaller bronchi the circular muscular fibres are relatively more abundant than in the larger bronchi, and form a distinct circular coat. The Lungs and Pleurae.-The lungs occupy the greater por- tion of the thorax. They are of a spongy elastic texture, and are composed of numerous minute air-sacs, and on section every here and there, the air-tubes may be seen cut across. A fragment of lung, unless from a child that has never breathed, floats in water; no other tissue does this. Fig'. 450.-Transverse section of the chest. Each lung is enveloped by a serous membrane-the joZeura, one layer of which adheres closely to its surface, and provides it with its smooth and slippery covering, while the other adheres to the inner surface of the chest-wall. The continuity of the two layers, which form a closed sac, as in the case of other serous membranes, will be best understood by reference to fig. 450. The appearance of a space, however, between the pleura which covers the lung (visceral layer), and that which lines the inner surface of the chest (parietal layer), is inserted in the drawing only for the sake of distinctness. It does not really exist. The layers are, in health, everywhere in contact, one with the other; and between them is only just .so much fluid as will ensure the lungs gliding easily, in their expansion and contraction, on the inner surface of the parietal layer, which lines the chest-wall. If, however, an opening be made so as to permit air or fluid to enter the pleural sac, the lung, in virtue of its elasticity, recoils, and a considerable space is left between it and the chest-wall. In other words, the natural elasticity of the lungs would cause 510 RESPIRATION. [ch. xxxix. them at all times to contract away from the ribs, were it not that the contraction is resisted by atmospheric pressure which bears Fig. 451.-Ciliated epithelium of the human trachea, a, layer of longitudinally arranged elastic fibres ; b, basement membrane ; c, deepest cell's, circular in form; d, inter- mediate elongated cells ; e, outermost layer of cells fully developed and bearing cilia, x 350. (Kolliker.) only on the inner surface of the air-tubes and air-cells. On the admission of air into the pleural sac, atmospheric pressure bears alike on the inner and outer sur- faces of the lung, and their elastic recoil is no longer prevented. Each lung is partially sub- divided into separate portions Fig. 452.-Terminal branch of a bronchial tube, with its infundibula and air-cells, from the margin of the lung of a monkey, injected with quicksilver, a, terminal bronchial twig; b b, infundibula and air- cells. x 10. (F. E. Schulze.) Fig. 453- - Two small infundibula or groups of air-cells, a a, with air-cells, b b, and the ultimate bronchial tubes, c c, with which the air-cells commu- nicate. From a new-born child. (Kolliker.) called lobes ; the right lung into three lobes, and the left into two. Each of these lobes, again, is composed of a large number of minute parts, called lobules. Each pulmonary lobule may be considered to be a lung in miniature, consisting as it does of a CH. XXXIX.] THE LUNGS. 511 branch of the bronchial tube, of air-cells, blood-vessels, nerves, and lymphatics, with a sparing amount of areolar tissue. On entering a lobule, the small bronchial tube, the structure of which has been just described (a, fig. 452), divides and sub- divides ; its walls at the same time becoming thinner and thinner, until at length they are formed only of a thin membrane of areolar, muscular, and elastic tissue, lined by a layer of pavement epithelium, not provided with cilia. At the same time, they are altered in shape; each of the minute terminal branches widening out funnel-wise, and its walls being pouched out irregularly into small saccular dilatations, called air-cells (fig. 452, b). Such a funnel-shaped terminal branch of the bronchial tube, with its group of pouches or air-cells, has been called an infundibulum (figs. 452, 453), and the irregular oblong space in its centre, with which the air-cells communicate, an intercellular passage. The air-cells, or air-vesicles, may be placed singly, like recesses from the intercellular passage, but more often they are arranged in groups or even in rows, like minute sacculated tubes ; so that a short series of vesicles, all communicating with one another, open by a common orifice into the tube. The vesicles are of various forms, according to the mutual pressure to which they are subject; their walls are nearly in contact, and they vary from -gip- to y-j- of an inch ('5 to '3 mm.) in diameter. Their walls are formed of fine membrane, like those of the intercel- lular passage; this membrane is folded on itself so as to form a sharp-edged border at each circular orifice of communication between contiguous air-vesicles, or between the vesicles and the bronchial passages. Numerous fibres of elastic tissue are spread out between contiguous air-cells, and many of these; are attached to the outer surface of the fine membrane of which each cell is composed, imparting to it additional strength, and the power of recoil after distension. The vesicles are lined by a layer of pavement epithelium (fig. 454), not provided with cilia. Outside the air vesicles, a network of pulmonary capillaries is spread out so densely (fig. 455), that the interspaces or meshes are even narrower than the vessels, which are, on an average, of an inch (8p) in diameter. Between the air in the cells and the blood in these vessels, nothing intervenes but the thin Avails of the cells and capillaries ; and the exposure of the blood to the air is the more complete, because the folds of membrane between contiguous cells, and often the spaces between 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 arrangement of the capillaries is shown on a larger scale in fig. 365. 512 RESPIRATION. [ch. xxxix. The vesicles of adjacent lobules do not communicate; 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. Fig. 454.-Section of lung stained -with silver nitrate. A. D., alveolar duct or intercellular passage ; S, alveolar septa ; N, alveoli or air-cells, lined with large flat cells, with some smaller polyhedral cells; M, plain muscular fibres surrounding the alveolar duct. (Klein and Noble Smith.) Blood-supply.-The lungs receive blood from two sources, (a) the pulmonary artery, (6) the bronchial arteries. The former conveys venous blood to the lungs to be arterialized, and this blood takes no share in the nutrition of the pulmonary tissues through which it passes. The branches of the bronchial arteries convey arterial blood for the nutrition of the walls of the bronchi, of the largei* pulmonary vessels, of the interlobular con- nective tissue, &c.; the blood of the bronchial vessels is returned chiefly through the bronchial and partly through the pulmonary veins. Lymphatics.-The lymphatics are arranged in three sets:- i. Irregular lacunae in the walls of the alveoli or air-cells. The lymphatic vessels which lead from these accompany the pul- monary vessels towards the root of the lung. 2. Irregular anas- tomosing spaces in the walls of the bronchi. 3. Lymph-spaces in the pulmonary pleura. The lymphatic vessels from all these irregular sinuses pass in towards the root of the lung to reach the bronchial glands. CH. XXXIX.] MECHANISM OF RESPIRATION. 513 Nerves.-The nerves of the lung are to be traced from the anterior and posterior pulmonary plexuses, which are formed by branches of the vagus and sympathetic. The nerves follow the Fig. 455.-Capillary network of the pulmonary blood-vessels in the human lung, x 60. (Kolliker.) course of the vessels and bronchi, and in the walls of the latter many small ganglia are situated. The Respiratory Mechanism Respiration consists of the alternate expansion and contraction of the thorax, by means of which air is drawn into or expelled from the lungs. These acts are called Inspiration and Expiration respectively. For the inspiration of air into the lungs it is evident that all that is necessary is such a movement of the side-walls or floor of the chest, or of both, that the capacity of the interior shall be- enlarged. By such increase of capacity there will be a diminution of the pressure of the air in the lungs, and a fresh quantity will enter through the larynx and trachea to equalise the pressure on the inside and outside of the chest.' For the expiration of air, on the other hand, it is also evident that, by an opposite movement which shall diminish the capacity of the chest, the pressure in the interior will be increased, and air will be expelled, until the pressure within and without the chest are again equal. In both cases the air passes through the trachea and larynx, whether in entering or leaving the lungs, there being no other communication with the exterior of the 514 RESPIRATION. [ch. xxxix. body ; and the lung, for the same reason, remains, under all the circumstances described, closely in contact with the walls and floor of the chest. To speak of expansion of the chest, is to speak also of expansion of the lung. The movements of the lung are therefore passive, not active, and depend on the movements of the closed cavity in which they are contained. A perforation of the chest-wall would mean that the lung in that side would no longer be of use ; a similar inj ury on the other side (double pneumo- thorax) would cause death. If the two layers of the pleura were adherent, those portions of the lung would be expanded most where the movements of the chest are greatest. The existence of the two layers prevents this, and thus the lung is equally expanded throughout. Inspiration.-The enlargement of the chest in inspiration is a muscular act; the effect of the action of the inspiratory muscles being an increase in the size of the chest cavity (a) in the vertical, and (6) in the lateral and antero-posterior diameters. The muscles engaged in ordinary inspiration are the diaphragm ; the external intercostals; parts of the internal intercostals ; the leva- tores costarum; and serratus posticus superior. (<z.) The vertical diameter of the chest is increased by the con- traction and consequent descent of the diaphragm; at rest, the diaphragm is dome-shaped with the convexity upwards; the central tendon forms a slight depression in the middle of this dome. On contraction the muscular fibres shorten and so the convexity of the double dome is lessened. The central tendon, which was formerly regarded as remaining fixed, is drawn down a certain distance, but the chief movement is at the sides. For the effective action of this muscle, its attachment to the lower ribs is kept fixed by the contraction of the quadratus lumborum. The diaphragm is supplied by the nerves. (5.) The increase in the lateral and antero-posterior diameters of the chest is effected by the raising of the ribs, the upper ones being fixed by the scaleni. The greater number of the ribs are attached very obliquely to the spine and sternum. The elevation of the ribs takes place both in front and at the sides-the hinder ends being prevented from performing any upward movement by their attachment to the spine. The move- ment of the front extremities of the ribs is of necessity accom- panied by an upward and forward movement of the sternum to which they are attached, the movement being greater at the lowei' end than at the upper end of the latter bone. The axes of rotation in these movements arc two; one corre- sponding with a line drawn through the two articulations which CH. xxxix.] MECHANISM OF RESPIRATION. 515 the rib forms with the spine (a, b, fig. 456); and the other with a line drawn from one of these (head of rib) to the sternum Fig. 456.-Diagram of axes of movement of ribs. (A B, fig. 456) ; the motion of the rib around the latter axis being somewhat after the fashion of raising the handle of a bucket. Fig. 457.-Diagram of movement of a rib in inspiration. The elevation of the ribs is accompanied by a slight opening out of the angle which the bony part forms with its cartilage (fig. L L 2 516 RESPIRATION. [ch. xxxix. 45 7? A); and thus an additional means is provided for increasing the antero-posterior diameter of the chest. The muscles by which the ribs are raised, in ordinary quiet inspiration, are the external intercostals, and that portion of the internal intercostals which is situate between the costal cartilages; and these are assisted by the levatores costarum, and the serratus posticus superior. The action of the levatores and the serratus is very simple. Their fibres, arising from the spine as a fixed point, pass obliquely downwards and forwards to the ribs, and neces- sarily raise the latter when they contract. The action of the intercostal muscles is not quite so simple, inasmuch as, passing merely from rib to rib, they seem at first sight to have no fixed point towards which they can pull the bones to which they are attached. In tranquil breathing, the expansive movements of the lower part of the chest are greater than those of the upper. In forced inspiration, on the other hand, the greatest extent of movement appears to be in the upper antero-posterior diameter. In extraordinary or forced inspiration, as in violent exercise, or in cases in which there is some interference with the due entrance of air into the chest, and in which, therefore, strong efforts are necessary, other muscles than those just enumerated, are pressed into service. It is impossible to separate by a hard-and-fast line the muscles of ordinary from those of extraordinary inspiration; but there is no doubt that the following are but little used as respiratory agents, except in cases in which unusual efforts are required-the sternomastoid, the serratus magnus, the perforates, and the trapezius. The expansion of the chest in inspiration presents some pecu- liarities in different persons. In young children, it is effected chiefly by the diaphragm, which being highly arched in expira- tion, becomes flatter as it contracts, and, descending, presses on the abdominal viscera, and pushes forward the front walls of the abdomen. The movement of the abdominal walls being here more manifest than that of any other part, it is usual to call this the abdominal type of respiration. In men, together with the descent of the diaphragm, and the pushing forward of the front wall of the abdomen, the chest and the sternum are subject to a wide movement in inspiration (inferior costal type). In women, the movement appears less extensive in the lower, and more so in the upper, part of the chest (superior costal type). There are also differences in different animals. In the frog, for example, the air is forced into the lungs by the raising of the floor of the mouth, the mouth and nostrils being closed. CH. XXXIX.] MECHANISM OF RESPIRATION. 517 Expiration.-From the enlargement produced in inspiration, the chest and lungs return in ordinary tranquil expiration, by their elasticity; the force employed by the inspiratory muscles in distending the chest and overcoming the elastic resistance of the lungs and chest-walls, being returned as an expiratory effort when the muscles are relaxed. This elastic recoil of the chest and lungs is sufficient, in ordinary quiet breathing, to expel air from the lungs in the intervals of inspiration, and no muscular power is required. In all voluntary expiratory efforts, however, as in speaking, singing, blowing, and the like, and in many involuntary actions also, as sneezing, coughing, etc., something more than merely passive elastic power is necessary, and the proper expira- tory muscles are brought into action. By far the chief of these are the abdominal muscles, which, by pressing on the viscera of the abdomen, push up the floor of the chest formed by the dia- phragm, and by thus making pressure, on the lungs, expel air from them through the trachea and larynx. All muscles, however, which depress the ribs, must act also as muscles of expiration, and therefore we must conclude that the abdominal muscles are assisted in their action by the interosseous part of the internal inter-costals, the triangularis sterni, the serratus posticus inferior, and quadratus lumborum. When by the efforts of the expiratory muscles, the chest has been squeezed to less than its average diameter, it again, on relaxation of the muscles, returns to the normal dimensions by virtue of its elasticity. The construction of the chest-walls, therefore, admirably adapts them for recoiling against and resisting as well undue contraction as undue dilatation. In the natural condition of the parts, the lungs can never contract to the utmost, but are always more or less " on the stretch," being kept closely in contact with the inner surface of the walls of the chest by cohesion as well as by atmospheric pressure, and can contract away from these only when, by some means or other, as by making an opening into the pleural cavity, or by the effusion of fluid there, the pressure on the exterior and interior of the lungs becomes equal. Methods of recording1 Respiratory Movements. The movements of respiration may be recorded graphically in several ways. One method is to introduce a tube into the trachea of an animal, and to connect this tube by some gutta-percha tubing with a T-piece intro- duced into the cork of a large-sized bottle, the other end of the T having attached to it a second piece of tubing, which can remain open or can be partially or completely closed by means of a screw clamp. Into the cork is inserted a second piece of glass tubing connected with a Marey's tambour by suitable tubing. This second tube communicates any alteration of the 518 RESPIRATION. [CH. XXXIX. Fig. 458.-Stethograph. h, tambour fixed at right angles to plate of steel /; c and d, arms by which instrument is attached to chest by belt e. When the chest expands, the arms are pulled asunder, which bends the steel plate, and the tambour is affected by the pressure of b, which is attached to it on the one hand, and to the upright in connection with horizontal screw g. (Modified from Marey's instrrunent.) Tambour. Ivory button. Tube to commu- nicate with re- cording tam- bour. Ball to fill appa- ratus with air. Fig. 459.-Stetliograph. (Burdon-Sanderson.) CH. xxxix.] STETHOGRAPHS. 519 pressure in the bottle to the tambour, and this may be made to write on a recording surface. There are various instruments for recording the movements of the chest by application of apparatus to the exterior. Such is the stethograph of Burdon- Sanderson. This consists of a frame formed of two parallel steel bars joined by a third at one end. At the free end of the bars is attached a leather strap, by means of which the apparatus may be suspended from the neck. Attached to the inner end of one bar is a tambour and ivory button, to the end of the other an ivory button. When in use, the apparatus is suspended with the transverse bar posteriorly, the button of the tambour is placed on the part of the chest the movement of which it is desired to record, and the other button is made to press upon the corresponding point on the other side of the chest, so that the chest is, as it were, held between a pair of callipers. The tambour is connected by tubing and a j-piece with a recording tambour Kg. 460.-Tracing of the normal diaphragm respirations of rabbit, a, with quick move- ment of drum, b, with slow movement. J, inspiration; e, expiration. To be read from left to right. (Marckwald.) and with a ball, by means of which air can be squeezed into the cavity of the tympanum. When in work the tube connected with the air ball is shut off by means of a screw clamp. The movement of the chest is thus com- municated to the recording tambour. A simpler form of this apparatus consists of a thick india-rubber bag of elliptical shape about three inches long, to one end of which a rigid gutta- percha tube is attached. This bag may be fixed at any required place on the chest by means of a strap and buckle. By means of the gutta-percha tube the variations of the pressure of air in the bag produced by the movements of the chest are communicated to a recording tambour. This apparatus is a simplified form of Marey's stethograph (fig. 458). The variations of intrapleural pressure may be recorded by the intro- duction of a cannula into the pleural or pericardial cavity, which is con- nected with a mercurial manometer. 520 RESPIRATION. [ch. xxxix. Finally, it has been found possible in various ways to record the dia- phragmatic movements by the insertion of an elastic bag connected with a tambour into the abdomen below it (phrenograph), by the insertion of needles into different parts of its structure, or by recording the contraction of isolated strips of the diaphragm. Such a strip attached in the rabbit to the xiphisternal cartilage may be detached, and attached by a thread to a recording lever. This method was largely used by Head ; the strip of the diaphragm serves as a sample of the diaphragm. Fig. 460 shows a tracing obtained in this way ; but in tracings taken with a stethograph, or any of the numerous arrangement of tambours which are applied to the chest-walls of men and animals, the large up-and-down strokes due to the respiratory movements have upon them smaller waves due to heart-beats. The acts of expansion and contraction of the chest take up under ordinary circumstances a nearly equal time. The act of inspiring air, however, especially in women and children, is a little shorter than that of expelling it, and there is commonly a very slight pause between the end of expiration and the beginning of the next inspiration. The respiratory rhythm may be thus expressed :- Inspiration ...... 6 Expiration 7 or 8 A very slight pause. If the ear be placed in contact with the wall of the chest, or be separated from it only by a good conductor of sound or stetho- scope, a faint respiratory or vesicular murmur is heard during inspiration. This sound varies somewhat in different parts-being loudest or coarsest in the neighbourhood of the trachea and large bronchi (tracheal and bronchial breathing), and fading off into a faint sighing as the ear is placed at a distance from these (vesi- cular breathing). It is best heard in children, and in them a faint murmur is heard in expiration also. The cause of the vesicular murmur has received various explanations; but most observers hold that the sound is produced by the air passing- through the glottis and larger tubes, and that this sound is modified in its conduction through the substance of the lung. The alterations in the normal breath sounds, and the various additions to it that occur in different diseased conditions, can only be properly studied at the bedside. Respiratory movements of the Nostrils and of the Glottis.- During the action of the muscles which directly draw air into the chest, those which guard the opening through which it enters are not passive. In hurried breathing the instinctive dilata- tion of the nostrils is well seen, although under ordinary con- ditions it may not be noticeable. The opening at the upper CH. xxxix.] VITAL CAPACITY. 521 part of the larynx, however, or rima glottidis, is dilated at each inspiration for the more ready passage of air, and becomes smaller at each expiration; its condition, therefore, corresponding during respiration with that of the walls of the chest. There is a further likeness between the two acts in that, under ordinary circumstances, the dilatation of the rima glottidis is a muscular act and its narrowing chiefly an elastic recoil. Terms used to express Quantity of Air breathed.- a. Breathing or tidal air, is the quantity of air which is habitually and almost uniformly changed in each act of breathing. In a healthy adult man it is about 30 cubic inches, or about 500 ccm., or half a litre. It will be seen that this amount of air is not nearly sufficient to fill the lungs; in fact it only passes into the upper respiratory passages; the air finds its way into the alveoli by the much slower process of diffusion, the oxygen diffusing downwards, and the carbonic acid diffusing upwards. b. Complemental air, is the quantity ovei' and above this which can be drawn into the lungs in the deepest inspiration; its amount varies, but it may be reckoned as 100 cubic inches, or about 1,600 ccm. c. Reserve or supplemental air.-After ordinary expiration, such as that which expels the breathing or tidal air, a certain quantity of air, about 100 cubic inches (1,600 ccm.) remains in the lungs, which may be expelled by a forcible and deeper expiration. This is termed reserve or supplemental air. d. Residual air is the quantity which still remains in the lungs after the most violent expiratory effort. Its amount depends in great measure on the absolute size of the chest, but may be estimated at about 100 cubic inches, or about 1,600 ccm. to 2,000 ccm. The total quantity of air which passes into and out of the lungs of an adult, at rest, in 24 hours, is about 686,000 cubic inches. This quantity, however, is largely increased by exertion ; the average amount for a hard-working labourer in the same time being 1,568,390 cubic inches. e. Respiratory or Vital Capacity.-The greatest respiratory capacity of the chest is indicated by the quantity of air which a person can expel from his lungs by a forcible expiration after the deepest inspiration possible. The average capacity of an adult, at 15'4° C. (6o° F.), is about 225 to 250 cubic inches, or 3,500 to 4,000 ccm. It is the sum of the complemental, tidal, and supple- mental air. The respiratory capacity, or as John Hutchinson called it, vital capacity, is usually measured by a modified gasometer "or spirometer, into which the 522 RESPIRATION. [ch. xxxix. experimenter breathes,-making the most prolonged expiration possible after the deepest possible inspiration. The quantity of air which is thus expelled from the lungs is indicated by the height to which the air-chamber of the spirometer rises ; and by means of a scale placed in connection with this, the number of cubic inches is read off. In healthy men, the respiratory capacity varies chiefly with the stature, weight, and age. It was found by Hutchinson, from whom most of our informa- tion on this subject is derived, that at a temperature of 15'4° C. (6o° F.), 225 cubic inches is the average vital or respiratory capacity of a healthy person, five feet seven inches in height. Circumstances affecting the amount of respiratory capacity.-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. 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 natural average weight of a healthy man in relation to stature has not yet been determined. As a general state- ment, however, it may be said that the capacity of respiration is not affected by weights under 161 pounds, or 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. By age, the capacity is increased from about the fifteenth 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 of sixty years old would be about 30 cubic inches less than that of a man forty years old, of the same height and weight. Sex.-The vital capacity of an adult man to that of a woman of the same height is 10 to 7. (John Hutchinson.) The number of respirations in a healthy adult person usually ranges from 14 to 18 per minute. It is greater in infancy and childhood. It varies also much according to different circum- stances, such as exercise or rest, health, or disease, etc. Varia- tions in the number of respirations correspond ordinarily with similar variations in the pulsations of the heart. In health the proportion is about 1 to 4, or 1 to 5, and when the rapidity of the heart's action is increased, that of the chest movement is commonly increased also ; but not in every case in equal propor- tion. It happens occasionally in disease, especially of the lungs or air-passages, that the number of respiratory acts increases in quicker proportion than the beats of the pulse; and, in other affections, much more commonly, that the number of the pulse- beats is greater in proportion than that of the respirations. The Force of Inspiratory and Expiratory Muscles.-The force with which the inspiratory muscles are capable of acting is greatest in individuals of the height of from five feet seven CH. xxxix.] FORCE OF RESPIRATION. 523 inches to five feet eight inches, and will elevate a column of nearly three inches (abont 60 mm.) 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 expira- tory acts is, on the average, one-third greater than that exercised in inspiration. But this difference 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, from their being called into use for other purposes than that of simple expiration. The force of the inspiratory act is, therefore, better adapted than that of the expiratory for testing the muscular strength of the body. (John Hutchinson.) In ordinary quiet breathing, there is a negative pressure of only i mm. during inspiration, and a positive pressure of from 2 to 3 mm. mercury during expiration. The instrument used by Hutchinson to gauge the inspiratory and expira- tory power was a mercurial manometer, to which was attached a tube fitting the nostrils, and through which the inspiratory or expiratory effort was made. 1 The greater part of the force exerted in deep inspiration is employed in overcoming the resistance offered by the elasticity of the lungs. The amount of this elastic resistance was estimated 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 Hutchinson calculated, according to the well-known hydro- static law of equality of pressures (as shown in the Bramah press), that the total force to be overcome by the muscles in the act of inspiring 200 cubic inches of air is more than 450 lbs. By connecting a mercurial manometer with the interior of the pleural cavity, the elastic tension of the lung is found to vary from 6 mm. during expiration to 30 mm. during inspiration when the distension and therefore the tendency to recoil is greatest. It is possible that the contractile power which the bronchial tubes and air-vesicles possess, by means of their muscular fibres, may assist in expiration; but it is more likely that its chief purpose is to regulate 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 : the muscular tissue also contracts upon and gradually expels collections of mucus, which may have accumulated within the tubes, and which cannot be ejected by forced expiratory efforts, owing to collapse or other morbid condi- 524 RESPIRATION. [CH. XXXIX. tions of the portion of lung connected with the obstructed tubes (Gairdner). The Nervous Mechanism of Respiration. In the central nervous system there is a specialised, small district called the Respiratory centre. This gives out impulses which travel down the spinal cord to the branches of the spinal nerves that innervate the muscles of respiration. It also receives various afferent fibres, the most important of which are contained in the trunk of the vagus. The vagus is chiefly an afferent nerve in relation to respiration. It, however, also is in a minor degree efferent, for it supplies the muscular tissue of the lungs and bronchial tubes, and exercises a trophic influence on the lung. The respiratory centre was discovered by Flourens; it is situated at the tip of the calamus scriptorius, and almost exactly coincides in position with the centre of the vagus. The existence of sub- sidiary respiratory centres in the spinal cord has been mooted, but the balance of experimental evidence is against their existence. Flourens found that when the respiratory centre is destroyed, respiration at once ceases, and the animal dies. He therefore called it the " vital knot " (noeud vitale). The centre is affected not only by the afferent impulses which reach it from the vagus, but also by those from the cerebrum ; so that we have a limited amount of voluntary control over the respiratory movement. The sensory nerves of the skin have also an eftect. The action of the cold air on the body of a new-born child is no doubt the principal afferent cause of the first respirations. During fcetal life, the need of the embryo for oxygen is very small, and is amply met by the transference of oxygen from the maternal blocd through the thin wTalls of the fcetal capillaries in the placenta. The application of cold water to the skin always causes a deep inspiration; this is another instance of the reflex effect which follows stimulation of the cutaneous nerves. Stimulation of the central end of the splanchnics causes expiration. Stimulation of the central end of the glosso-pharyngeal causes an inhibition of the respiratory movements for a short period; this accounts for the very necessary cessation of breathing during swallowing. Stimu- lation of the peripheral end of the cut superior laryngeal nerve, or of its terminations in the mucous membrane of the larynx, as ■when a crumb is " swallowed the wrong way;" produces inhibition of inspiratory and increase of expiratory efforts, culminating in coughing. CH. XXXIX.] RESPIRATORY NERVES. 525 These nerves, however, are none of them in constant action as the vagi are, and the influence of the vagus is somewhat compli- cated. Still, respiration continues after the vagi are cut. The character of the respiration becomes altered, especially if both nerves are severed; it is slower and deeper. The animal, how- ever, lives a considerable time; a warm-blooded animal usually dies after about a week or ten days from vagus pneumonia, due to the removal of trophic influences from the lungs. Cold-blooded animals live longer; they exhibit fatty degeneration of the heart- muscle also. The question has been much debated whether the activity of the respiratory centre is automatic or reflex; that is to say, whether the rhythmic discharges proceeding from it depend merely on local changes induced by the condition of its blood supply, or on the repeated stimulations it receives by afferent nerves. There appears every reason to believe that the centre has the power of automatism, but this is never excited under normal circumstances. Normally the respiratory process is a series of reflex actions. The evidence in favour of the automatic activity of the centre is the following : - (i.) If the spinal cord is cut just below the bulb, respiration ceases, except in the case of the facial and laryngeal muscles, which are supplied by nerves that originate above the point of injury. The alee nasi work vigorously. Such respiration is not effective in drawing any air into the chest, and so the animal soon dies; but the forcible efforts of these muscles show that the respiratory centre is in a state of activity sending out impulses to them. If the two vagus nerves are cut, these movements con- tinue ; this shows that afferent impulses from the vagus are not essential. As the blood gets more and more venous, the move- ments become more pronounced. The question has been much debated whether this increased activity of the respiratory centre is due to increase of carbonic acid, or decrease of oxygen in the blood which it receives. The balance of evidence shows that the diminution in the oxygen is the more important factor of the two. (2.) In asphyxia, one always gets great increase of respiratory activity, called dyspnoea ; this is produced by the stimulation of the centre by venous blood. It is not due (or not wholly due) to the action of the venous blood on the terminations of the vagi in the lungs, as the same phenomenon occurs when these nerves are cut; and, moreover, dyspnoea takes place if the venous blood is 526 RESPIRATION. [ch. xxxix. allowed to circulate through the brain alone, and not through the lungs at all. For instance, it ensues when localised venosity of the blood is produced in the brain by ligature of the carotid and vertebral arteries. But, as before stated, the normal activity of the respiratory centre is not automatic, it is reflex, and the principal afferent channel is the vagus. The way in which it works has been made out of recent years by Marckwald, Hering and Head. The following is a brief resume of Head's results. His method of recording the movements was by means of that convenient slip of the diaphragm which is found in rabbits (see p. 520). His method of dividing the vagus was by freezing it; he laid it across a copper wire, the end of which was placed in a freezing mixture. This method is free from the disadvantage which a cut with a knife or scissors possesses, namely, a stimulation at the moment of section. On dividing one vagus, respiration became slightly slower and deeper; on dividing the second nerve, this effect was much more marked. On exciting the central end of the divided nerve, inspiratory efforts increased until at last the diaphragm came to a standstill in the inspiratory position. But if a weak stimulus was em- ployed, the reverse was the case ; the expiratory efforts increased, inspiration becoming smaller and smaller, until at last the diaphragm stopped in the position of expiration. This result always follows stimulation of the superior laryngeal nerve. Most of these facts were known previously, but the interpreta- tion of them, in the light of further experiments immediately to be described, is the following : That there are in the vagus two sets of fibres, one which pro- duces an increased activity of the inspiratory part of the respira- tory centre, and the other an increased activity of the expiratory part of that centre. Stimulation of the first stops expiration and produces inspiration; stimulation of the second does the reverse. The question now is, What is it that normally produces this alternate stimulation of the two sets of fibres ? If we discover this we shall discover the prime moving cause in the alternation of the inspiratory and expiratory acts. It was sought and found in the alternate distension and contraction of the air-vesicles of the lungs where the vagus terminations are situated. In the first experiments positive ventilation was performed; that is, air was pumped repeatedly into the lungs, and so in- creased their normal distension; this was found to decrease the inspiratory contractions of the diaphragm, until at last they ceased CH. XXXIX.] APNOEA. 527 altogether, and the diaphragm stood still in the expiratory position (fig. 461, A). In the second series of experiments, negative ventilation was performed; that is, the air was pumped repeatedly out of the lungs, and a condition of collapse of the air-vesicles produced. This was found to increase the inspiratory contractions of the diaphragm, expiration became less and less, and at last the diaphragm assumed the position of inspiratory standstill (fig. 461, B). Distension of the air-vesicles therefore stimulates the fibres of Fig. 461.-Tracings of diaphragm. The upward movements of the tracings represent inspiration; the downward movements, expiration. A, result of positive, B, of negative ventilation. (After Head.) the vagus which stimulate the expiratory phase of respiration; collapse stimulates those which stimulate the inspiratory phase. Ordinary respiration is an alternate positive and negative ventilation, though not so excessive as in the experiments just described. Inspiration is positive ventilation, and so provides the nervous mechanism of respiration with a stimulus that leads to expiration. Expiration is a negative ventilation, and so provides the stimulus that leads to inspiration. Apnoea.-If positive and negative ventilation are used to- gether rapidly and alternately at a rate quicker than the respiratory rhythm, both inspiratory and expiratory processes are inhibited, and the respiration ceases for a short time. This follows naturally from the experiments previously described. This can be done on an animal with a pair of bellows fixed to a tube in the trachea; or voluntarily by oneself, taking a number of 528 RESPIRATION. [ch. xxxix. deep breaths rapidly. This condition, called apnoea, is not due, as at one time supposed, to over-oxygenation of the blood, but is produced reflexly. It is observed if inert gases, like nitrogen or hydrogen, are used instead of air. The pause, however, is shorter, as the blood becomes venous, and in a short time stimulates the respiratory centre to activity. Special Respiratory Acts. Coughing.-In the act of coughing there is first of all a deep inspiration, followed by an expiration; but the latter, instead of being easy and uninterrupted, as in normal breathing, is ob- structed, the glottis being momentarily closed by the approxima- tion of the vocal cords. The abdominal muscles, then strongly acting, push up the viscera against the diaphragm, and thus make pressure on the air in the lungs until its tension is sufficient to noisily open the vocal cords which oppose its outward passage. In this way considerable force is exercised, and mucus or any other matter that may need expulsion from the air-passages is quickly and sharply expelled by the outstreaming current of air. The act is a reflex one, the sensory surface which is excited being the mucous membrane of the larynx, and the superior laryngeal nerve is the afferent nerve ; stimulation of other parts of the respiratory mucous membrane will also produce cough, and the point of bifurcation of the trachea is specially sensitive. Other sensory surfaces may also act as the " signal surface " for a cough. Thus, a cold draught on the skin, or tickling the external auditory meatus, in some people will set up a cough. The question has been discussed whether such a thing as a stomach cough exists; it has not been produced experimentally, but there is no reason why irritation of the gastric mucous mem- brane, supplied as it is by the vagus, should not cause the reflex act of coughing. Sneezing.-The same remarks that apply to coughing, are almost exactly applicable to the act of sneezing; but, in this instance, the blast of air, on escaping from the lungs, is directed, by an instinctive contraction of the pillars of the fauces and descent of the soft palate, chiefly through the nose, and any offending matter is thence expelled. The signal surface is usually the nasal mucous membrane, but here, as in coughing, other causes (such as a bright light) will sometimes set the reflex going. Hiccough is an involuntary sudden contraction of the diaphragm CH. XXXIX.] CHEYNE-STOKES RESPIRATION. 529 causing an inspiration which is suddenly arrested by the closure of the glottis, causing a characteristic sound. It arises from gastric irritation. Snoring is due to vibration of the soft palate. Sobbing consists of a series of convulsive inspirations at the moment of which the glottis is partially closed. Sighing and Yawning are emotional forms of inspiration, the latter associated with stretching movements of jaws and limbs. They appear to be efforts of nature to correct by an extra deep inspiration, the venosity of the blood due to inactivity produced by'ennui or grief. Their contagious character is due to sympathy. Among abnormal disturbances of the nervous mechanism of respiration, the following diseases must be mentioned: laryn- gismus stridulus, asthma, and whooping cough. Cheyne-Stokes resynration is due to rhythmical activity of the respiratory centre. It reminds one somewhat of the Traube- Fig. 462.--Cheyne-Stokes respiration. Hering waves due to a similar rhythmical activity of the vaso- motor centre. It is seen in many nervous diseases and in fatty degeneration of the heart. A typical tracing of the condition is given above (fig. 462). It is seen to a slight extent during ordinary sleep, and is very marked in hibernating animals. The Effect of Respiration on the Circulation. As the heart, the aorta, and pulmonary vessels are situated in the air-tight thorax, they are exposed to a certain alteration of pressure when the capacity of the latter is increased in inspira- tion ; for although the expansion of the lungs tends to counter- balance this increase of area, it never does so entirely, since part of the pressure of the air which is drawn into the lungs through the trachea is expended in overcoming their elasticity. The amount thus used up increases as the lungs become more and more expanded, so that the pressure inside the thorax during inspiration, as far as the heart and great vessels are concerned, never quite equals that outside, and at the conclusion of inspira- tion is considerably less than the atmospheric pressure. It has 530 RESPIRATION. [ch. xxxix. been ascertained that the amount of the pressure used up in the way above described, varies from 5 or 7 mm. of mercury during the pause, to 30 mm. of mercury when the lungs are expanded at the end of a deep inspiration, so that it will be understood that the pressure to which the heart and great vessels are subjected diminishes as inspiration progresses, and at its minimum is less by 30 mm. than the normal pressure, 760 mm. of mercury. Kg. 463.-Diagram of an apparatus illustrating the eifect of inspiration upon the heart and great vessels within the thorax. I, the thorax at rest; II, during inspiration ; n, represents the diaphragm when relaxed; o', when contracted (it must be re- membered that this position is a mere diagram), i.e., when the capacity of the thorax is enlarged ; h, the heart; v, the veins entering it, and a. the aorta; rZ, lZ, the right and left lung; t, the trachea; m, mercurial manometer in connection with pleura. The increase in the capacity of the box representing the thorax is seen to dilate the heart as well as the lungs, and so to pump in blood through v, whereas the valve prevents reflex through a. The position of the mercury in m shows also the suction which is taking place. (Landois.) It will be understood from the accompanying diagram how, that if there were no lungs in the chest, if its capacity were increased, the effect of the increase would be expended in pumping blood into the heart from the veins. With the lungs placed as they are, during inspiration the pressure outside the heart and great vessels is diminished, and they have therefore a tendency to expand and to diminish the intra-vascular pressure. The diminution of pressure within the veins passing to the right auricle and within the right auricle itself, will draw the blood into the thorax, and so assist the circulation. This suction action is independent of the suction power of the diastole of the auricle about which we have previously spoken. The effect of sucking CH. xxxix.] RESPIRATION AND BLOOD-PRESSURE. 531 more blood into the right auricle will, cceteris paribus, increase the amount passing through the right ventricle, which also exerts a similar suction action, and through the lungs into the left auricle and ventricle, and thus into the aorta. This all tends to increase the blood-pressure. The effect of the diminished pressure upon the pulmonary vessels will also help towards the same end, i.e., an increased flow through the lungs, so that, as far as the heart and its veins are concerned, inspiration increases the blood- pressure in the arteries. The effect of inspiration upon the aorta and its branches within the thorax would be, however, the contrary ; for as the pressure outside is diminished, the vessels Fig. 464.-Comparison of blood-pressure curve with curve of intra-thoracic pressure. (To be read from left to right.) a is the curve of blood-pressure with its respiratory undu- lations, the slower beats on the descent being very marked; b is the curve of intra- thoracic pressure obtained by connecting one limb of a manometer with the pleural cavity. Inspiration begins at i and expiration at e. The intra-thoracic pressure rises very rapidly after the cessation of the inspiratory effort, and then slowly falls as the air issues from the chest; at the beginning of the inspiratory effort the fall becomes more rapid. (M. Foster.) would tend to expand, and thus to diminish the tension of the blood within them, but inasmuch as the large arteries are capable of little expansion beyond their natural calibre, the diminution of the arterial tension caused by this means would be insufficient to counteract the increase of blood-pressure produced by the effect of inspiration upon the veins of the chest, and the balance of the whole action would be in favour of an increase of blood-pressure during the inspiratory period. But if a blood-pressure tracing be taken at the same time that the respiratory movements are being recorded, it will be found that, although speaking generally, the arterial tension is increased during inspiration, the maximum of arterial tension does not correspond with the acme of inspira- tion (fig. 464). In fact, at the beginning of inspiration the 532 RESPIRATION. [ch. xxxix. pressure continues to fall, then gradually rises until the end of inspiration, and continues to do so for some time after expiration has commenced. As regards the effect of expiration, the capacity of the chest is diminished, and the intra-thoracic pressure returns to the normal, which is not exactly equal to the atmospheric pressure. The effect of this on the veins is to increase their extra-vascular and so their intra-vascular pressure, and to diminish the flow of blood into the left side of the heart, and with it the general blood-pressure, but this is almost exactly balanced by the neces- sary increase of arterial tension caused by the increase of the extra-vascular pressure of the aorta and large arteries, so that the arterial tension is not much affected during expiration either way. Thus, ordinary expiration does not produce a distinct obstruction to the circulation, as even when the expiration is at an end the intra-thoracic pressure is less than the extra-thoracic. The effect of violent expiratory efforts, however, has a distinct action in obstructing the current of blood through the lungs, as seen in the blueness of the face from congestion in straining ; this condition being produced by pressure on the small pulmonary vessels. We may summarize this mechanical effect of respiration on the blood-pressure therefore, and say that inspiration aids the circula- tion and so increases the arterial tension, and that although expi- ration does not materially aid the circulation, yet under ordinary conditions neither does it obstruct it. Under extraordinary con- ditions, however, as in violent expiration, the circulation is decidedly obstructed. We have seen, however, that there is no exact correspondence between the point of highest blood-pressure and the end of inspiration, and we must suppose that there are other mechanical factors, such, for example, as the effect of the abdominal move- ments, both in inspiration and in expiration, upon the arteries and veins within the abdomen and of the lower extremities, and the influence of the varying intra-thoracic pressure upon the pulmonary vessels, both of which ought to be taken into consideration. As regards the first of these, the effect during inspiration-as the cavity of the abdomen is diminished by the descent of the diaphragm-should be two-fold : on the one hand, blood would be sent upwards into the chest by compression of the vena cava inferior; on the other hand, the passage of blood downwards from the chest in the abdominal aorta, and upwards in the veins of the lower extremity, would be to a certain extent obstructed. In ordinary expiration all this would be reversed, but if the abdominal CH. xxxix.] TR A U BE- H E RIN G CURVES. 533 muscles are violently contracted, as in extraordinary expiration, the same effect would be produced as by inspiration. The effect of the varying intra-thoracic pressure which occurs during inspira- tion upon the pulmonary vessels is to produce an initial dilatation of Fig. 465.-Traube-Hering's curves. (To be read from left to right.) The curves 1, 2, 3, 4 and 5 are portions selected from one continuous tracing forming the record of a pro- longed observation, so that the several curves represent successive stages of the same experiment. Each curve is placed in its proper position relative to the base line, which is omitted ; the blood pressure rises in stages from 1 to 2, 3, and 4, but falls again in stage 5. Curve 1 is taken from a period when artificial respiration was being kept up, but the vagi having been divided, the pulsations on the ascent and descent of the undu- lations do not differ ; when artificial respiration ceased these undulations for a while disappeared, and the blood pressure rose steadily while the heart-beats became slower. Soon, as at 2, new undulations appeared; a little later, the blood-pressure was still rising, the heart-beats still slower, but the undulations still more obvious (3) ; still later (4), the pressure was still higher, but the heart-beats were quicker, and the undulations flatter, the pressure then began to fall rapidly (5), and continued to fall until some time after artificial respiration was resumed. (M. Foster.) both artery and veins, and this delays for a short time the passage of blood towards the left side of the heart, and the arterial pressure falls, but the fall of blood-pressure is soon followed by a steady rise, since the flow is increased by the initial dilatation of the vessels : the converse is the case with expiration. As, however, the pulmonary veins are more easily dilatable than the pulmonary 534 RESPIRATION. [ch. xxxix. artery, the greater distensibility increases the flow of blood as inspiration proceeds, whilst during expiration, except at its beginning, this property of theirs acts in the opposite direction, and diminishes the flow. Thus, at the beginning of inspiration the diminution of blood-pressure, which commenced during expira- tion, is continued, but after a time the diminution is succeeded by a steady rise ; the reverse is the case with expiration-at first a rise and then a fall. The effect of the nervous system in producing rhythmical alterations quite independent of the mechanically caused undula- tions of the blood-pressure is two-fold. In the first place the cardio-inhibitory centre is stimulated during the fall of blood- pressure, and produces a slower rate of heart-beat, which will be noticed in the tracing (fig. 464). The undulations during the decline of blood-pressure are therefore longer but less frequent. This effect disappears when, by section of the vagi, the effect of the centre is cut off from the heart. In the second place, the vaso-motor centre sends out rhythmical impulses, by which undula- tions of blood-pressure are produced, quite independent of the respiratory undulations. The capacity of this centre to produce such undulations is demonstrated by the existence of the Traube- Hering curves, which we have already studied (p. 473), but of which we give here an additional figure (fig. 465). It is possible that the normal respiratory undulations on a blood-pressure curve may in part at least be produced in a similar way. Asphyxia. Asphyxia may be produced in various ways : for example, by the prevention of the due entry of oxygen into the blood, either by direct obstruction of the trachea or other part of the respi- ratory passages, or by introducing instead of ordinary air a gas devoid of oxygen, or by interference with the due interchange of gases between the air and the blood. The symptoms of asphyxia may be divided into three groups, which correspond with the stages of the condition which are usually recognised; these are (i), the stage of exaggerated breathing ; (2), the stage of convulsions ; (3), the stage of ex- haustion. In the first stage the breathing becomes more rapid and at the same time deeper than usual, the inspirations at first being especially exaggerated and prolonged. The muscles of extra- ordinary inspiration are called into action, and the effort to respire is laboured and painful. This is soon followed by a CH. XXXIX.] ASPHYXIA. 535 similar increase in the expiratory efforts, which become exces- sively prolonged, being aided by all the muscles of extraordinary expiration. During this stage, which lasts a varying time, from a minute upwards, according as the deprivation of oxygen is sudden or gradual, the lips become blue, the eyes are prominent, and the expression intensely anxious. The prolonged respira- tions are accompanied by a distinctly audible sound; the muscles attached to the chest stand out as distinct cords. This stage includes the two conditions hyperpnoea (excessive breathing) and dyspnoea (difficult breathing) which follows later. It is due to the increasingly powerful stimulation of the respiratory centre by the increasingly venous blood. In the second stage, which is not marked by any distinct line of demarcation from the first, the violent expiratory efforts become convulsive, and then give way, in men and other warm- blooded animals, to general convulsions, which arise from the further stimulation of the centres. Spasms of the muscles of the body in general occur, and not of the respiratory muscles only. The convulsive stage is a short one, and lasts less than a minute. The third stage, or stage of exhaustion. In it the respirations all but cease, the spasms give way to flaccidity of the muscles, there is insensibility, the conjunctivae are insensitive and the pupils are widely dilated. Every now and then a prolonged sighing inspiration takes place, at longer and longer intervals, until they cease altogether, and death ensues. During this stage the pulse is scarcely to be felt, but the heart may beat for some seconds after respirations have quite ceased. The condition is due to the gradual paralysis of the respiratory centre by the prolonged action of the increasingly venous blood. As with the first stage, the duration of the second and third stages depends upon whether the manner of the deprivation of oxygen is sudden or gradual. The convulsive stage is short, lasting, it may be, only one minute. The third stage may last three minutes and upwards. The conditions of the vascular system in asphyxia are :-(i) More or less interference with the passage of the blood through the pulmonary blood-vessels; (2) Accumulation of blood in the right side of the heart and in the systemic veins; (3) Circulation of impure (non-aerated) blood in all parts of the body. After death from asphyxia it is found in the great majority of cases that the right side of the heart, the pulmonary arteries, and the systemic veins are gorged with dark, almost black, blood, and the left side of the heart, the pulmonary veins, and the arteries 536 RE SITRAT I OX. [ch. xxxix. are empty. The explanation of these appearances may be thus summarised : when respiration is stopped, venous blood at first Fig. 466.-The heart in the first stage of asphyxia. The left cavities are seen to be distended; the left ventricle partly overlaps the right. I. a., left auricle ; l.v., left ventricle ; a, aorta ; p.'J., pulmonary artery; p.v., pulmonary vein; r.a., right auricle ; r.v., right ventricle; v.c.d., descending vena cava; v.c.a., ascending vena cava. (Sir George Johnson.) passes freely through the lungs to the left heart, and so to the great arteries. When it reaches the arterioles either by its Fig.467.-The heart in the final stage of asphyxia. The letters have the same meaning as in fig. 466 ; in addition, p.c. represents the pulmonary capillaries. The right auricle and ventricle, and the pulmonary artery, are fully distended, while the left cavities of the heart and the aorta are nearly empty. (Sir George Johnson.) direct action upon their muscular tissue, or more probably through the medium of the vaso-motor centres, the arterioles -contract, particularly those of the splanchnic area, the blood- CH. XXXIX.] ASPHYXIA. 537 pressure rises, and the left side of the heart becomes distended. Although the arterioles are contracted, the highly venous blood is allowed to pass through them, and, favoured by the laboured respiratory movements, arrives at the right side of the heart. When it reaches the pulmonary arterioles it gives rise to the same constriction of them as of the systemic vessels. The obstruction to the circulation through the lungs thus produced causes a distended condition of the right heart and pulmonary artery, and, on the other hand, produces a greatly diminished blood-flow through the pulmonary veins into the left side of the heart, resulting after a time in its practical emptiness. So that in the third stage of asphyxia the heart gets into the condition in which it is found after death. The condition of the heart is illustrated by figs. 466 and 467. In the first and second stages of asphyxia the arterial blood- Fig'. 468.-Blood-pressure tracing during asphyxia. The tracing was taken by a manometer connected with the femoral artery of a dog under curare. Artificial respiration was discontinued at X. Both vagi had been previously divided. If the vagi are not /divided, the rise of pressure is much less, and the heart beats very slowly. This enables the heart to last longer, and is due to excitation of the cardio-inhibitory centre by venous blood. (Starling.) pressure continuously rises until it reaches a point far above the normal, and in the third stage blood-pressure falls rapidly. A tracing of the arterial pressure is shown in fig. 468. Effects of Breathing Gases other than the Atmosphere. The diminution of oxygen has a more direct influence in the production of asphyxia than the increased amount of carbonic acid. Indeed the fatal effect of a gradual accumulation of carbonic acid in the blood when a due supply of oxygen is maintained resembles rather the action of a narcotic poison than it does asphyxia. Then again we must carefully distinguish the asphyxiating effect of an insufficient supply of oxygen from the directly 538 RESPIRATION. [ch. xxxix. poisonous action of such gases as carbonic oxide, which is contained to a considerable amount in common coal-gas. The fatal effects often produced by this gas (as in accidents from burning charcoal stoves in small, close rooms), are due to its entering into combination with the haemoglobin of the blood- corpuscles and thus expelling the oxygen. Hydrogen may take the place of nitrogen if the oxygen is in the usual proportion with no marked ill effect. Sulphuretted hydrogen interferes with the oxygenation of blood. Nitrous oxide acts directly on the nervous system as a narcotic. Certain gases, such as carbon dioxide in more than a certain proportion ; sulphurous and other acid gases, ammonia, and chlorine produce spas- modic closure of the glottis, and are irrespirable. Alteration in the atmospheric pressure.-The normal condition of breathing is that the oxygen of the air breathed should be at the pressure of 4- of the atmosphere, viz., A of 760 mm. of mercury, or 152 mm., but it is found that life may be carried on by gradual diminution of the oxygen pressure to considerably less than one half of this, viz., to 76 mm., or partial pressure, which is reached at an altitude above 15,000 feet. Any pressure less than this may begin to produce alterations in the relations of the gases in the blood, and if an animal is subjected suddenly to a marked decrease of barometric pressure, and so of oxygen pressure (below 7 per cent.), it is thrown into convulsions, and it is found that the gases are set free in the blood-vessels, no doubt carbon dioxide and oxygen as well as nitrogen, although the latter is the only one of the three gases the presence of which in the vessels in death from this condition of affairs has been proved ; the others are said to be reabsorbed. Other derangements may precede this, e.g., bleeding from the nose, dyspnoea, and vascular derangement. On the other hand, the oxygen may be gradually increased to a considerable extent without marked effect, even to the extent of 8 or 10 atmospheres, but when the oxygen pressure is increased up to 20 atmospheres the animals experimented upon by Paul Bert died with severe tetanic convulsions. The alteration of pressure above or below a certain average affects primarily the gaseous interchange in the lungs, and then that in the tissues generally, but signs of dyspnoea may be produced as well either by cutting off the supply of blood to the medullary centres, or by warming the blood of the carotid arteries which supply them. The cause in the former case being the deprivation of oxygen and the accumulation of tbe carbon dioxide, and of the latter, the increased metabolism of the centre set up by the warmed blood. That considerable variations of pressure may occur without CH. XXXIX.] CHEMISTRY OF RESPIRATION. 539 producing ill effects, is due to the fact which we study more fully in the next section of this chapter, that the blood gases are mostly in a state of chemical combination not simple solution. Chemistry of Respiration. The atmospheric air does not actually penetrate beyond the largest bronchial tubes ; the gases which get into the smaller tubes and air-vesicles do so by diffusion. The most vigorous expiratory effort is unable to expel the alveolar air. This air and the blood in the capillaries are only separated by the thin capil- lary and alveolar walls. The blood parts with its excess of car- bonic acid and watery vapour to the alveolar air ; the blood at the same time receives from the alveolar air a supply of oxygen which renders it arterial. The intake of oxygen is the commencement, and the output of carbonic acid is the end of the series of changes known as respira- tion. The intermediate steps take place all over the body and constitute what is known as tissue-respiration. The oxygen which goes into the blood is held there in loose combination as oxy- haemoglobin. In the tissues this substance parts with its respiratory oxygen. The oxygen does not necessarily undergo immediate union with carbon to form carbonic acid, and with hydrogen to form water, but in most cases, as in muscle, is held in reserve by the tissue itself. Owing to this reserve oxygen, a muscle will contract in an atmosphere of pure nitrogen and yet give off carbonic acid; and a frog will live under the same condition and give off carbonic acid for several hours. Besides carbonic acid and water, certain other products of combustion are produced; those like urea and uric acid, which are the result of nitrogenous metabolism ultimately leave the body in the urine. The carbonic acid and a portion of the water find an outlet by the lungs. Inspired and Expired Air.-The composition of the in- spired or atmospheric air and the expired air may be compared in the following table :- - Inspired air. Expired air. Oxygen Nitrogen Carbonic acid Watery vapour . Temperature 20'96 vols. per cent. 79 0-04 „ variable 16'6 vols. per cent. 79 4'4 „ saturated that of body (36° C.) 540 RESPIRATION. [CH XXXIX. The nitrogen remains unchanged. The recently discovered gas, argon, is in the above table reckoned in with the nitrogen. It is, however, only present in minute quantities. The chief change is in the proportion of oxygen and carbonic acid. The loss of oxygen is about 5, the gain in carbonic acid about 4'5. If the inspired and expired airs are carefully measured at the same temperature and barometric pressure, the volume of expired air is thus rather less than that of the inspired. The conversion of oxygen into carbonic acid would not cause any change in the volume of the gas, for a molecule of oxygen (O„) would give rise to a molecule of carbonic acid (CO2) which would occupy the same volume. It must, however, be remembered that carbon is not the only element which is oxidised. Fats contain a number of atoms of hydrogen, which, during metabolism, are oxidised to form water ; a small amount of oxygen is also used in the formation of urea. Carbohydrates contain sufficient oxygen in their own molecules to oxidise their hydrogen; hence the apparent loss of oxygen is • least when a vegetable diet (that is, one consisting largely of starch and other carbohydrates) is taken, and greatest when much fat and proteid are eaten. The quotient 2 given °ft O2 absorbed is called the respiratory quotient. Normally it is = 0'9, but 5 it varies considerably with diet as just stated. It varies also with muscular exercise when the output of carbonic acid is much increased both absolutely and relatively to the amount of oxygen used up. The amount of respiratory interchange of gases is estimated by enclosing an animal in an air-tight chamber, except that there is a tube entering and another leaving it; by one tube oxygen or air can enter and is measured by a gas-meter as it passes in. The air is drawn through the chamber, and leaves it by the other tube; this air has been altered by the respiration of the animal, and in it the carbonic acid and water are estimated ; the carbonic acid is estimated by means of drawing the air through tubes containing a known amount of an alkali ; this combines with the carbonic acid and is increased in weight: the increase in weight giving the amount of carbonic acid; the alkali used in Regnault and Reiset's apparatus was potash ; Pettenkofer used baryta water ; Haldane recommends soda-lime. The water is similarly estimated in tubes containing pumice moistened with sulphuric acid. Ranke gives the following numbers from experiments made on a man, who was taking a mixed diet consisting of 100 grammes CH. XXXIX.] DIFFUSION OF GASES. 541 of proteid, 100 of fat, and 250 of carbohydrate in the 24 hours. The amount of oxygen absorbed in the same time was 666 grammes; of which 560 passed off as carbonic acid, 9 in urea, 19 as water formed from the hydrogen of the proteid, and 78 from that of the fat. Vierordt from a number of experiments on human beings gives the following numbers: the amount of oxygen absorbed in the twenty-four hours, 744 grammes ; this leads to the formation of 900 grammes of carbonic acid (this contains about half a pound of carbon) and 330 grammes of water. The respiratory interchange is lessened during sleep. It is especially small in the winter sleep of hibernating animals. Circumstances affecting the amount of carbonic acid excreted, (a) A ge a nd sex. In males the quantity increases with growth till the age of 30 ; at 50 it begins to diminish again. In females the decrease begins when menstruation ceases. In females the quantity exhaled is always less than in males of the same age. (b) Respiratory movements.-The quicker the respiration the smaller is the proportionate quantity of carbonic acid in each volume of expired air. The total quantity is, however, increased, not because more is formed in the tissues, but more is got rid of. The last portion of the expired air which comes from the more remote parts of the lungs is the richest in carbonic acid. (c) External temperature.-In cold-blooded animals, a rise in the external temperature causes a rise in their body temperature, accompanied with increased chemical changes, including the formation of a larger amount of carbonic acid. In warm-blooded animals, it is just the reverse ; in cold weather the temperature has to be kept at the normal level, and so increased combustion is necessary. (d) Food.-This produces an increase which usually comes on about an hour after a meal. (e) Exercise.-Moderate exercise causes an increase of about 30 to 40 per cent, in the amount excreted. With excessive work, the increase is still greater. Diffusion of Gases within the Lungs.-If two chambers containing a mixture of gases in unequal amount are connected together, a slow movement called diffusion takes place until the percentage amount of each gas in each chamber is the same. Let us suppose that one chamber contains a large quantity of oxygen and a small quantity of carbonic acid ; and the other a small quantity of oxygen and a large quantity of carbonic acid ; the oxygen moves from the first to the second, and the carbonic acid from the second to the first chamber. The pressure of a gas is proportional to the percentage amount in which it is present in a mixture. This is true for each gas in a mixture, the presence of the others making no difference. In the atmosphere for instance, the total barometric pressure 542 RESPIRATION. [ch. xxxix. is 760 mm. of mercury ; the amount of oxygen in the air is roughly one fifth, and the pressure it exercises is also one fifth of 760 ; the nitrogen accounting for the other four fifths. The carbonic acid is present in such small quantities that the pressure it exercises is only a fraction of a millimetre. In the alveolar air, how'ever, the carbonic acid is present in larger and the oxygen in smaller amount; and in the intermediate air passages there is an intermediate condition: hence as in the two chambers we first considered, oxygen diffuses down to the air vesicles, and carbonic acid from them. These slow movements of diffusion are assisted by the large draughts which are created in the upper respiratory tract by the respiratory movements of the chest. The actual figures giving the relationship between the oxygen and carbonic acid of the atmosphere and alveolar air are as follows :- Oxygen. Carbonic acid. Percentage. Pressure. Percentage. Pressure. Atmosphere .21 * 158 mm. Hg. . 0'04 t 0-38 mm. Hg. Alveolar air . 3-6 1 27-4 „ . 336 A 27 „ Gases of the Blood.-From 100 volumes of blood, about 60 volumes of gas can be removed by the mercurial air-pump. The average composition of this gas in dog's blood is :- Arterial blood. Venous blood. Oxygen ... 20 ... 8 to 12 Nitrogen . . . 1 to 2 . . . 1 to 2 Carbonic acid . . 40 ... 46 The nitrogen in the blood is simply dissolved from the air just as water would dissolve it; it has no physiological importance. The other two gases are present in much greater amount than can be explained by simple solution ; they are, in fact, chiefly present in loose chemical combinations. Less than one volume of the oxygen and about two of carbonic acid are present in simple solu- tion in the plasma. Oxygen in the Blood.-The amount of gas dissolved in a liquid varies with the pressure of the gas ; double the pressure and the amount of gas dissolved is doubled. Now this does not occur in the case of oxygen and blood ; very nearly the same amount of oxygen is dissolved whatever be the pressure. We have thus a proof that oxygen is not merely dissolved in the blood, but is in chemical union; and the fact that the oxygen of oxyhaemoglobin can be replaced by equivalent quantities of other gases, like carbonic oxide, is a further proof of the same state- CH. xxxix.] GASES OF THE BLOOD. 543 ment. The tension or partial pressure of oxygen in the air of the alveoli is less than that in the atmosphere, but greater than that in venous blood ; hence oxygen passes from the alveolar air into the blood; the oxygen immediately combines with the haemoglobin, and thus leaves the plasma free to absorb more oxygen ; and this goes on until the haemoglobin is entirely, or almost entirely, saturated with oxygen. The reverse change occurs in the tissues when the partial pressure of oxygen is lower than in the plasma, or in the lymph that bathes the tissue elements; the plasma parts with its oxygen to the lymph, the lymph to the tissues; the oxyhaemoglobin then undergoes dissociation to supply more oxygen to the plasma and lymph, and this in turn to the tissues once more. This goes on until the oxyhaemoglobin loses a great portion of its store of oxygen, but even in asphyxia it does not lose all. The avidity of the tissues for oxygen is shown by Ehrlich's experiments with methylene blue and similar pigments. Methy- lene blue is more stable than oxyhaemoglobin; but if it is injected into the circulation of a living animal, and the animal killed a few minutes later, the blood is found dark blue, but the organs colourless. On exposure to oxygen the organs become blue. In other words, the tissues have removed the oxygen from methylene blue to form a colourless reduction product; on exposure to the air this once more unites with oxygen to form methylene blue. Carbonic Acid in the Blood.-What has been said for oxygen holds good in the reverse direction for carbonic acid. Compounds are formed in the tissues where the tension of the gas is high : these pass into the lymph, then into the blood, and in the lungs the compounds undergo dissociation, carbonic acid passing into the alveolar air, where the tension of the gas is com- paratively low, though it is greater here than in the expired air. The relations of this gas and the compounds it forms are more complex than in the case of oxygen. If blood is divided into plasma and corpuscles, it will be found that both yield carbonic acid, but the yield from the plasma is the greater. If we place blood in a vacuum it bubbles, and gives out all its gases ; addi- tion of a weak acid causes no further liberation of carbonic acid. When plasma or serum is similarly treated the gas also comes off, but about 5 per cent, of the carbonic acid is fixed-that is, it requires the addition of some stronger acid, like phosphoric acid, to displace it. Fresh red corpuscles will, however, take the place of the phosphoric acid, and thus it has been surmised that oxy- haemoglobin has the properties of an acid. One hundred volumes of venous blood contain forty-six volumes 544 RESPIRATION'. [CH. XXXIX. of carbonic acid. Whether this is in solution or in chemical combination is determined by ascertaining the tension of the gas in the blood. One hundred volumes of blood plasma would dissolve fifty volumes of the gas at atmospheric pressure, if its solubility in plasma were equal to that in water. If, then, the carbonic acid were in a state of solution, its tension would be equal to that of the atmosphere, but it proves to be only equal to 5 per cent, of an atmosphere. This means that when venous blood is brought into an atmosphere containing 5 per cent, of carbonic acid, the blood neither gives off any carbonic acid nor takes up any from that atmosphere. Hence the remainder of the gas, 95 per cent., is in a condition of chemical combination. The chief compound appears to be sodium bicarbonate. The carbonic acid and phosphoric acid of the blood are in a state of constant struggle for the possession of the sodium. The salts formed by these two acids depend on their relative masses. If carbonic acid is in excess, we get sodium carbonate (Na,COH) and mono-sodium phosphate (NaH.,P(\); but if the carbonic acid is diminished, the phosphoric acid obtains the greater share of sodium to form disodium phosphate (Na2HPO4). In this way, as soon as the amount of free carbonic acid diminishes, as in the lungs, the amount of carbonic acid in combination also decreases; whereas in the tissues, where the tension of the gas is highest, a large amount is taken up into the blood, where it forms sodium bicarbonate. The tension of the carbonic acid in the tissues is high, but one cannot give exact figures; we can measure the tension of the gas in certain secretions : in the urine it is 9, in the bile 7 per cent. The tension in the cells themselves must be higher still. The following figures give the percentage amount and tension of the carbonic acid and the oxygen. They take into account only the small amounts of those gases which are free in the plasma and can therefore exercise tension ; the oxygen combined with haemoglobin and carbonic acid combined as carbonates and bicar- bonates are not reckoned :- Oxygen. Carbonic acid. Percentage. Pressure. Percentage. Pressure. Arterial blood . . 3'9 P 29'6 mm. Ilg. . 2 8 21 mm. Hg. Venous blood . . 2'9 22 ,, . 5'4 41 „ Alveolar air . . 3'6 27'4 ,. . 3'56 y 27 ,. The arrows indicate the way in which the gases pass. A comparison of the numbers of venous blood and alveolar air illustrate the fact that the gas passes in the direction of pressure, the oxygen passing to the venous blood, the carbonic acid to the CH XXXIX. ] TISSUE-RESPIRATION. 545 alveolar air. But the figures given for arterial blood show that the gaseous exchange goes beyond the establishment of equili- brium ; that in fact the carbonic acid has passed from where its tension is low to where its tension is higher, and the same is true for oxygen. It is quite possible that the figures may be erroneous, but until the error has been proved to be there, we can only explain this apparent reversal of a law of nature by supposing with Bohr, that the alveolar epithelium possesses the power of secreting carbonic acid and taking up oxygen, just as the cells of secreting glands are able to select certain materials from the blood and reject others. Tissue-Respiration.-Before the processes of respiration were fully understood the lungs were looked upon as the seat of combustion ; they were regarded as the stove for the rest of the body where effete material was brought by the venous blood to be burnt up. The impossibility of this was shown when it was pointed out that, if all the heat of the body was produced in the lungs, their temperature would be raised so high as to destroy them. Physiologists next transferred the seat of the combustion to the blood; but since then innumerable facts and experiments have shown that it is in the tissues themselves, and not in the blood, that combustion occurs. The methylene-blue experiments already described (p. 543) show this; and the following experiment is also quite conclusive. A frog can be kept alive for some time after salt solution is substituted for its blood. The metabolism goes on actively if the animal is kept in pure oxygen. The taking up of oxygen and giving out of carbonic acid must there- fore occur in the tissues, as the animal has no blood. Ventilation.-It is necessary to allude in conclusion to this very important practical outcome of our consideration of respiration. Some Continental observers have stated that certain noxious substances are ordinarily contained in expired air which are much more poisonous than carbonic acid, but researches in this country have failed to substantiate this. If precautions be taken by absolute cleanliness to prevent admixture of the air with exhala- tions from skin, teeth and clothes, the expired air only contains one noxious substance, and that is carbonic acid. Absolute cleanliness is however not the rule; and the air of rooms becomes stuffy when the amount of expired air in them is just so much as to raise the percentage of carbonic acid to o-i per cent. An adult gives off about o-6 cubic feet of carbonic 546 RESPIRATION. [ch. xxxix. acid per hour, and in order that the percentage may be kept below o'1 per cent., he must be supplied with between two and three thousand cubic feet of fresh air in that time ; and in order that the air may be renewed without giving rise to draughts, each adult should be allotted sufficient space in a room, at least 1,000 cubic feet. The Mercurial Air-Pump. The extraction of the gases from the blood is accomplished by means of a mercurial air-pump, of which there are many varieties, those of Ludwig, Alvergniat, Geissler, and Sprengel being the chief. The principle of action in all is the same. Ludwig's pump, which may be taken as a type, is repre- sented in fig. 469. It consists of two fixed glass globes, C and F. the upper one communicating by means of the stopcock. D, and a stout india-rubber tube with another glass globe. L. which can be raised or lowered by means of a pulley; it also communicates by means of a stopcock. B. and a bent glass tube. A, with a gas receiver (not represented in the figure), A, dipping into a bowl of mercury, so that the gas may be received over mercury. The lower globe, F. communicates with C by means of the stopcock. E. with I in which the blood is contained by the stopcock, G. and with a movable glass globe. JI. similar to L. by means of the stopcock, II. and the stout india-rubber tube, K. In order to work the pump. L and JI are filled with mercury, the blood from which the gases are to be extracted is placed in the bulb I. the stopcocks, II. E. D. and B. being open, and G closed. JZ is raised by means of the pulley until F is full of mercury, and the air is driven out. E is then closed, and E is raised so that C becomes full of mercury, and the air is driven off. B is then closed. On lowering L the mercury runs into it from C, and a vacuum is established in C. On opening E and lowering JI. a vacuum is similarly established in F ; if G be now opened, the blood in I will enter into ebullition, and the gases will pass off into F and 6*, and on raising M and then Z, the stopcock B being opened, the gas is driven through --1, and is received into the receiver over mercury. By repeating the experiment several times the whole of the gases of the specimen of blood is obtained, and may be estimated. The very simple air-pump devised by Leonard Hill will be. however, amply sufficient for most purposes. It consists of three glass bulbs (B.B.), which we will call the blood bulb ; it is closed above by a piece of tubing and a Fig-. 469.-Ludwig's Mercurial Pump. CH. XXXIX.] AIR-PUMPS. 547 clip, a ; this is connected by good india-rubber tubing to another bulb, d. Above d. however, there is a stopcock with two ways cut through it ; one by means of which B.B. and d may be connected, as in the figure ; and another seen in section, which unites d to the tube e, when the stopcock is turned through a right angle. In intermediate positions, the stopcock cuts off all communication from d to other parts of the apparatus ; d is connected by tubing to a receiver, R, which can be raised or lowered at will. At first the whole apparatus is filled with mer- cury, R being raised. Then, a being closed, R is lowered, and when it is more than the height of the baro- meter (30 inches) below the top of B.B. the mercury falls and leaves the blood bulb empty ; by lowering R still fur- ther// can also be rendered a vacuum. Fig. 470.-L. Hill's air-pump. Fig. 471.-Waller's apparatus for gas analysis. A few drops of mercury should be left behind in B.B. B.B. is then detached from the rest of the apparatus and weighed, the clips, a and 1). being tightly closed. Blood is then introduced into it by connecting the tube with the clip, a, on it to a cannula filled with blood inserted in an artery or vein of a living animal. Enough blood is withdrawn to fill about half of one of the bulbs. This is defibrinated by shaking it with the few drops of mercury left in the bulbs. It is then weighed again; the increase of weight gives the amount of blood which is being investigated. B.B. is then once more attached to the rest of the apparatus, hanging downwards, as in the side drawing in fig. 470, anti the blood gases boiled off ; these pass into d. which has been made a vacuum ; and then, by raising R again, the mercury rises in d. pushing the gases in front of it, through the tube, e (the stopcock being turned in the proper direction) into the eudiometer, E, filled with and placed over mercury. The gas can then be measured and analysed. Gas analysis.-There are many pieces of apparatus devised for this 548 THE CHEMICAL COMPOSITION OF THE BODY. [CH. XL. purpose. In physiology, however, we have generally to deal with only three gases, oxygen, nitrogen, and carbonic acid. Waller's modification of Zuntz's more complete apparatus will be found very useful in performing gas analysis, say, of the expired air : a 100 c.c. measuring-tube graduated in tenths of a cubic centimetre between 75 and 100 ; a filling bulb (A) and two gas pipettes are connected up as in the diagram. It is first charged with acidulated water up to the zero mark by raising the filling bulb, tap 1 being open. It is then filled with 100 c.c. of expired air, the filling bulb being lowered till the fluid in the tube has fallen to the 100 mark. Tap 1 is now closed. The amount of carbonic acid in the expired air is next ascertained ; tap 2 is opened, and the air is expelled into the gas pipette containing strong caustic potash solution by raising the filling bulb until the fluid has risen to the zero mark of the measuring tube. Tap 2 is closed, and the air left in the gas pipette for a few minutes, during which the carbonic acid is absorbed by the potash. Tap 2 is then opened and the air drawn back into the measuring tube by lowering the filling bulb. The volume of air (minus the carbonic acid) is read, the filling bulb being ad- justed so that its contents are at the same level as the fluid in the measuring tube. The amount of oxygen is next ascertained in a precisely similar manner by sending the air into the other gas pipette, which contains sticks of phosphorus in water, and measuring the loss of volume (due to absorption of oxygen) in the air when drawn back into the tube. CHAPTER XL. THE CHEMICAL COMPOSITION OF THE BODY. The body is built up of a large number of chemical elements, which are in most instances united together into compounds. The elements formed in the body are carbon, nitrogen, hydrogen, oxygen, sulphur, phosphorus, fluorine, chlorine, iodine, silicon, sodium, potassium, calcium, magnesium, lithium, iron, and occasionally traces of manganese, copper, and lead. Of these very few occur in the free state. Oxygen (to a small extent) and nitrogen are found dissolved in the blood ; hydrogen is formed by putrefaction in the alimentary canal. With some few exceptions such as these, the elements enumerated above are found combined with one another to form what are called compounds. The compounds, or, as they are generally termed in physio- logy, the proximate principles, found in the body are divided into- (1) Mineral or inorganic compounds. (2) Organic compounds, or compounds of carbon. The inorganic compounds present are water, various acids CH. XL.] PROXIMATE PRINCIPLES. 549 (such as hydrochloric acid in the gastric juice), ammonia (as in the urine), and numerous salts, such as calcium phosphate in bone, sodium chloride in blood and urine, and many others. The organic compounds are more numerous; they may be subdivided into- 1. Various groups of alcohols and organic acids, and their com- pounds, such as the fats and carbohydrates. 2. Various derivatives of ammonia, amides, amines, urea, etc. 3. Aromatic bodies, or derivatives of benzene. 4. "Proteids, the most important of all, and substances allied to proteids like the albuminoids, pigments, and ferments. Many of these substances we shall study with the blood, food, urine, etc. A more convenient practical method of grouping physiological proximate principles is the following:- Water. Salts-e.g. chlorides and phosphates of sodium and calcium. Inorg-anic Nitrogenous Proteids-e.g. albumin, myosin, casein. Albuminoids-e.g. gelatin, chondrin, nuclein. Simpler nitrogenous bodies - e.g. lecithin, creatine. Organic Fats-e.g. butter, fats of adipose tissue. Carbohydrates-e.g. sugar, starch. Simple organic bodies-e.g. cholesterin, lactic acids. N on-nitrogenous Many of the substances enumerated above only occur in small quantities. The most important are the inorganic substances, water and salts; and the organic substances, proteids, car- bohydrates, and fats. It is necessary in our subsequent study of the principles of chemical physiology that we should always keep in mind this simple classification; the subdivision of organic substances into proteids, fats, and carbohydrates forms the starting-point of chemical physiology. We will now proceed to consider the principal members of each group in turn. C arb ohy dr at e s. The carbohydrates are found chiefly in vegetable tissues, and many of them form important foods. Some carbohydrates are, however, found in or formed by the animal organism. The most important of these are glycogen, or animal starch; dextrose and lactose, or milk sugar. The carbohydrates may be conveniently defined as compounds 550 THE CHEMICAL COMPOSITION OF THE BODY. [ch. XL. of carbon, hydrogen, and oxygen, the two last-named elements being in the proportion in which they occur in water. They may be for the greater part arranged into three groups according to their empirical formulae. The names and formulae of these groups, and the most important members of each, are as follows :- i. Monosaccharides or Glucoses, C0H12Gg. 2. Disaccharides, Sucroses, or Saccharoses, 3. Polysaccharides or Amy- loses, (CoH1oO5)u. + Dextrose. - Levulose. 4- Galactose. + Cane sugar. + Lactose. + Maltose. + Starch. + Glycogen. + Dextrin. Cellulose. Gums. The + and - signs in the above list indicate that the substances to which they are prefixed are dextro- and levo-rotatory respec- tively as regards polarised light. The formulae given above are merely empirical; and there is no doubt that the quantity n in the starch group is variable and often large ; hence the name polysaccharides that Tollens gives to the group. Research has, moreover, shown that the glucoses are either aldehydes or ketones of hexatomic alcohols C6Hs(OH)c. Thus dextrose is the aldehyde of sorbite, levulose the ketone of mannite, and galactose the aldehyde of dulcite. The amyloses may be regarded as the anhydrides of the glucoses [nC6H12OG - nH.,0 = (C6H10O5)n]. The sucroses are condensed glucoses-i.e. they are formed by the combination of two molecules of glucose with the loss of one molecule of water (CGH12OG + CGH12OG - H2O = C12H22On) ; hence the term disaccharide. The following are the chief facts in relation to each of the principal carbohydrates :- Dextrose or Grape Sugar.-This carbohydrate is found in fruits, honey, and in minute quantities in the blood and numerous tissues, organs, and fluids of the body. It is the form of sugar found in large quantities in the blood and urine in the disease known as diabetes. Dextrose is soluble in hot and cold water and in alcohol. It is crystalline, but not so sweet as cane sugar. When heated with strong potash certain complex acids are formed which have a yellow or brown colour. This constitutes Moore's test for sugar. In alkaline solutions dextrose reduces salts of silver, bismuth, mercury, and copper. The reduction of cupric to cuprous salts constitutes Tronimer's test, which is performed as follows : put a CH. XL.] CARBOHYDRATES. 551 few drops of copper sulphate into a test-tube, then solution of dextrose, and then strong caustic potash. On adding the potash a precipitate is first formed which dissolves up forming a blue solution. On boiling this a yellow or red precipitate (cuprous hydrate or oxide) forms. On boiling a solution of dextrose with an alkaline solution of picric acid, a dark red opaque solution due to reduction to picramic acid is produced. Another important property of grape sugar is that under the influence of yeast it is converted into alcohol and carbonic acid (C6H12O6= 2C2H6O +2CO,). Dextrose may be estimated by the fermentation test, by the polarimeter, and by the use of Fehling's solution. The last method is the most important: it rests on the same principles as Trommer's test, and we shall study it in connection with diabetic urine. Levulose.-When cane sugar is treated with dilute mineral acids it undergoes a process known as inversion-i.e., it takes up water and is converted into equal parts of dextrose and levulose. The previously dextro-rotatory solution of cane sugar then becomes levo-rotatory, the levo-rotatory power of the levulose being greater than the dextro-rotatory power of the dextrose formed. Hence the term wiwrsm. Similar hydrolytic changes are produced by certain ferments, such as the invert ferment of the intestinal juice. Pure levulose can be crystallised, but so great is the difficulty of obtaining crystals of it that one of its names was uncrystallis- able sugar. Small quantities of levulose have been found in blood, urine, and muscle. It has been recommended as an article of diet in diabetes in place of ordinary sugar : in this disease it does not appeal' to have the harmful effect that other sugars produce. Levulose gives the same general reactions as dextrose. Galactose is formed by the action of dilute mineral acids or inverting ferments on lactose. It resembles dextrose in its action on polarised light, in reducing cupric salts in Trommer's test, and in being directly fermentable with yeast. When oxydised by means of nitric acid it yields an acid called muci' acid (C6H10O8), which is only slightly soluble in water. Dextrose when treated in this way yields an isomeric acid-i.e., an acid with the same empirical formula called saccharic acid, which is very soluble in water. Inosite, or muscle sugar, is found in muscle, kidney, liver, and other parts of the body in small quantities. It is also largely found in the vegetable kingdom. It is crystallisable, and has the same formula as the glucoses. It is, however, not a sugar 552 THE CHEMICAL COMPOSITION OE THE BODY. [ch. XL. and careful analysis has shown that it really belongs to the aromatic series. Cane Sugar is generally distributed in the vegetable king- dom, but especially in the juices of the sugar cane, beetroot, mallow, and sugar maple. It is a substance of great importance as a food. It undergoes inversion in the alimentary canal. It is crystalline, and dextro-rotatory. With Trommer's test it gives a blue solution, but no reduction occurs on boiling. After inver- sion it is, of course, strongly reducing Inversion may be accomplished by boiling with dilute mineral acids, or by means of inverting ferments such as that occurring in the intestinal juice. It then takes up water, and is split into equal parts of dextrose and levulose. C12H220„ + H20 = C8H1208 + C8H 08. [Cane sugar] [Dextrose] [Levulose] With yeast cane sugar is first inverted by means of a special soluble ferment secreted by the yeast cells, and then there is an alcoholic fermentation of the glucoses so formed. Lactose, or Milk Sugar, occurs in milk. It is occasionally found in the urine of women in the early days of lactation, or after weaning. It is crystallisable, dextro-rotatory, much less soluble in water than other sugars, and has only a slightly sweet taste. It gives Trommer's test, but when the reducing power is tested quantitatively by Fehling's solution it is found to be a less powerful reducing agent than dextrose, in the proportion of 7 to 10. When hydrolysed by the same agencies as mentioned in con- nection with cane sugar it takes up water and splits into dextrose and galactose. C,SHSSO + H2O = C6H12O + C8H12O8. [Lactose] [Dextrose] [Galactose] With yeast it is just inverted, and then alcohol is formed. This, however, occurs slowly. The lactic acid fermentation which occurs when milk turns sour is brought about by lactic acid micro-organisms, which are some- what similar to yeast cells. Putrefactive bacteria in the intestine bring about the same resrdt. The two stages of the lactic acid fermentation are represented in the following equations :- (i.) + H20 = 4C3H8O3. [Lactose] [Lactic acid] (2.) 4C3H803 = 2C+Hs0.2 + 4C02 + 4H2. [Lactic acid] [Butyric acid] CH. XL.] STARCH. 553 Maltose is the chief end product of the action of malt diastase on starch, and is also formed as an intermediate product in the action of dilute sulphuric acid in the same substance. It is also the chief sugar formed from starch by the diastatic ferments contained in the saliva and pancreatic juice. (An isomeric sugar called iso-maltose is also formed under the same circumstances.) It can be obtained in the form of acicular crystals, and is strongly dextro-rotatory. It gives Trommer's test; but its reducing power, as measured by Fehling's solution, is one-third less than that of dextrose. By prolonged boiling with water, or, more readily, by boiling with a dilute mineral acid, or by means of an inverting ferment, such as occurs in the intestinal juice, it is converted into dex- trose. C12H„20ll + H„O = 2CbH120„. [Maltose] [Dextrose] It undergoes readily the alcoholic fermentation. Phenyl hydrazine test.-The three important reducing sugars with which we have to deal in physiology are dextrose, lactose, and maltose. They may be distinguished by their rela- tive reducing powers on Fehling's solution, or by the characters of their osazones. The osazone is formed in each case by adding phenyl hydrazine hydrochloride, and sodium acetate, and boiling the mixture for half an hour. In each case the osazone is de- posited in the form of bright canary-coloured, needle-like crystals, usually in bunches, which differ in their crystalline form, melting- point, and solubilities. Starch is widely diffused through the vegetable kingdom. It occurs in nature in the form of mi- croscopic grains, varying in size and appearance, according to their source. Each consists of a central spot, round which more or less concentric en- velopes of starch proper or granulose alternate with layers of cellulose. Cel- lulose has very little digestive value, but starch is a most important food. Starch is insoluble in cold water: it forms an opalescent solution in boiling water, which if con- centrated gelatinises on cooling. Its most characteristic reaction is the blue colour it gives with iodine. On heating starch with mineral acids, dextrose is formed. By the action of diastatic ferments, maltose is the chief end product. In both cases dextrin is an intermediate stage in the process. Fig. 472.-Grains of potato starch. 554 THE CHEMICAL COMPOSITION OF THE BODY. [ch. xl. Before the formation of dextrin the starch solution loses its opalescence, a substance called soluble starch being formed. This, like native starch, gives a blue colour with iodine. Although the molecular weight of starch is unknown, the formula for soluble starch is probably 5(C12H20O10)20. Equations that represent the formation of sugars and dextrins from this are very complex, and are at present only hypothetical. Dextrin is the name given to the intermediate products in the hydration of starch, and two chief varieties are distinguished :- erythro-dextrin, which gives a reddish-brown colour with iodine ; and achroo-dextrin, which does not. It is readily soluble in water, but insoluble in alcohol and ether. It is gummy and amorphous. It does not give Trom- mer's test, nor does it ferment with yeast. It is dextro-rotatory. By hydrating agencies it is converted into glucose. Glycogen, or animal starch, is found in liver, muscle, and white blood-corpuscles. It is also abundant in all embryonic tissues. Glycogen is a white tasteless powder, soluble in water, but it forms, like starch, an opalescent solution. It is insoluble in alcohol and ether. It is dextro-rotatory. With Trommer's test it gives a blue solution, but no reduction occurs on boiling. With iodine it gives a reddish or port-wine colour, very similar to that given by dextrin. Dextrin may be distinguished from glycogen by (i) the fact that it gives a clear, not an opalescent, solution with water; and (2) it is not precipitated by basic lead acetate as glycogen is. It is, however, precipitated by basic lead acetate and ammonia. (2) Glycogen is precipitated by 60 per cent, of alcohol; the dextrins require 85 per cent, or more. Cellulose.-This is the colourless material of which the cell- walls and woody fibres of plants are composed. By treatment with strong mineral acids it is, like starch, converted into glu- cose, but with much greater difficulty. The various digestive ferments have little or no action on cellulose ; hence the necessity of boiling starch before it is taken as food. Boiling bursts the cellulose envelopes of the starch grains, and so allows the digestive juice to get at the starch proper. Cellulose is found in a few animals, as in the test or outer investment of the Tunicates. CH. XL.] THE FATS. 555 The Fats. Fat is found in small quantities in many animal tissues. It is, however, found in large quantities in three situations, viz., marrow, adipose tissue and milk. The contents of the fat cells of adipose tissue are fluid during life, the normal temperature of the body (36° C., or 99° F.) being considerably above the melting-point (25° C.) of the mixture of the fats found there. These fats are three in number, and are called jtxiZmzfwi, stearin, and olein. They differ from one another in chemical composition and in certain physical characters, such as melting-point and solubilities. Olein melts at 0° C., palmitin at 45° C., and stearin at 53-66° C. It is thus olein which holds the other two dissolved at the body temperature. Fats are all soluble in hot alcohol, ether, and chloroform, but insoluble in water. Chemical Constitution of the Fats.-The fats are com- pounds of fatty acids with glycerin, and may be termed gly- cerides. The term hydrocarbon, applied to them by some authors, is wholly incorrect. The fatty acids form a series of acids derived from the monatomic alcohols by oxidation. Thus, to take ordinary ethyl alcohol, C,HcO, the first stage in oxidation is the removal of two atoms of hydrogen to form aldehyde, C2H4O; on further oxidation an atom of oxygen is added to form acetic acid, C2H4O2. A similar acid can be obtained from all the other alcohols, thus:- From methyl alcohol CH3.H0, formic acid H.COOH is obtained. „ ethyl „ C2H5.H0, acetic ,, CH3.COOH „ ,. propyl „ C3H..HO, propionic „ C2H5.C00H „ „ butyl „ (AH".!™, butyric „ „ amyl „ C5HU.HO, valeric „ C4H9.C00H „ and so on. Or in general terms :- From the alcohol with formula CnH211+1.H0, the acid with formula is obtained. The sixteenth term of this series has the formula C15H31.COOH, and is called palmitic acid; the eighteenth has the formula C17H33.COOH, and is called stearic acid. Each acid, as will be seen, consists of a radicle, Cn_1H2n_1CO, united to hydroxyl (HO). Oleic acid, however, is not a member of this series, but belongs to a somewhat similar series known as the acrylic series, of which the general formula is 556 THE CHEMICAL COMPOSITION OF THE BODY. [ch. XL. Cn_1H2n_3.COOH. It is the eighteenth term of the series, and its formula is C17H.i3.COOH. Glycerin or Glycerol is a triatomic alcohol, C3H5(HO)3-i.e., three atoms of hydroxyl united to a radicle glyceryl (C3H5). The hydrogen in the hydroxyl atoms is replaceable by other organic radicles. As an example, take the radicle of acetic acid called acetyl (CH... CO). The following formulae represent the deriva- tives that can be obtained by replacing one, two, or all three hydroxyl hydrogen atoms in this way :- (OH C3HJ OH (oh [Glycerin] (OH C3H5 OH (o.ch3.co [Monoacetin] (OH C3H5J O.CH3.CO (o.ch co [Diacetin] f O.CH3.CO C3HJ O.CH3.CO (o.ch co [Triacetin] Triacetin is a type of a neutral fat; stearin, palmitin, and olein ought more properly to be called tristearin, tripalmitin, and tri- olein respectively. Each consists of glycerin in which the three atoms of hydrogen in the hydroxyls are replaced by radicles of the fatty acid. This is represented in the following formulae :- Acid. Radicle. Fat. Palmitic acid C15H31.COOH Palmityl C15H31.CO Palmitin C3H5(0C15H31.C0)3 Stearic acid C17H3..COOH Stearyl C17H35.CO Stearin C3H5(OC17H35.CO)3 Oleic acid C17H33.COOH Oleyl C17H33.CO Olein C3H5(0C17H33.C0)3 Decomposition Products of the Fats.-The fats split up into the substances out of which they are built up. Under the influence of superheated steam, mineral acids, and in the body by means of certain ferments (for instance, the fat- splitting ferment steapsin of the pancreatic juice), a fat combines with water and splits into glycerin and the fatty acid. The following equation represents what occurs in a fat, taking tripal- mitin as an example :- C3H5(O.C15H31CO)3 + 3H.O = C3H5(OH)3 + 3C15H31CO.OH [Palmitin-a fat] " [Glycerin] [Palmitic acid-a fatty acid] In the process of saponification much the same sort of reaction occurs, the final products being glycerin and a com- pound of the base with the fatty acid, which is called a soap. Suppose, for instance, that potassium hydrate is used; we get- C3H5(0.CI5H CO), + 3KHO = C3Hs(0H)3 + 3C15H31CO.OK [Palmitin-a fat] [Glycerin] [Potassium palmitate-a soap] Emulsification.-Another change that fats undergo in the body is very different from saponification. It is a physical rather CH. XL.] THE PROTEIDS. 557 than a chemical change; the fat is broken up into very small globules, such as are seen in the natural emulsion-milk. Lecithin (Ci2HstNPO9).-This is a very complex fat, which yields on decomposition not only glycerine and a fatty {stearic) acid, but phosphoric acid, and an alkaloid [N.(CH3)3C2Hc(.)2] called choline in addition. This substance is found to a great extent in the nervous system (see p. 170), and to a small extent in bile. Together with cholesterin, a crystallisable, monatomic alcohol (C27H45.HO), which we shall consider more at length in connection with the bile, it is found in small quantities in the protoplasm of all cells. The Proteids. The proteids are the most important substances that occur in animal and vegetable organisms ; none of the phenomena of life occur without their presence ; and though it is impossible to state positively that they occur as such in living protoplasm, they are invariably obtained by subjecting living structures to analytical processes. Proteids are highly complex and (for the most part) uncrystal- lisable compounds of carbon, hydrogen, oxygen, nitrogen, and sulphur occurring in a solid viscous condition or in solution in nearly all the liquids and solids of the body. The different members of the group present differences in chemical and physical properties. They all possess, however, certain common chemical .reactions, and are united by a close genetic relationship. The various proteids differ a good deal in elementary composi- tion. Hoppe-Seyler gives the following percentages :- C H N S 0 From 513 6-9 IV2 0-3 209 To 54-5 7-3 17-0 2-0 23-5 We are, however, not acquainted with the constitutional for- mula of proteid substances. There have been many theories on the subject, but practically all that is known with certainty is that many different substances may be obtained by the decompo- sition of proteids. How they are built up into the proteid mole- cule is unknown. The decompositions that occur in the body are, moreover, different from those which can be made to occur in the laboratory; hence the conclusion that living protoplasm differs somewhat from the non-living proteid material obtainable from it. (1) In the body. Carbonic acid, water, and urea are the chief 558 THE CHEMICAL COMPOSITION OF THE BODY. [ch. XL. final products. Glycocine, leucine, creatine, uric acid, Ac., are probably intermediate products. Carbohydrates (glycogen) and fats may also originate from proteids. (2) Outside the body. Various strong reagents break up proteids into ammonia, carbonic acid, amines, fatty acids, amido- acids like leucine, lysatine and glycocine, and aromatic compounds like tyrosine. Solubilities.--All proteids are insoluble in alcohol and ether. Some are soluble in water, others insoluble. Many of the latter are soluble in weak saline solutions. Some are insoluble, others soluble in concentrated saline solutions. It is on these varying solubilities that proteids are classified. All proteids are soluble with the aid of heat in concentrated mineral acids and alkalies. Such treatment, however, decomposes as well as dissolves the proteid. Proteids are also soluble in gastric and pancreatic juices; but here, again, they undergo a change, being converted into a hydrated variety of proteid called peptone. The intermediate substances formed in this process are called proteoses or albumoses. Commercial peptone contains a mixture of proteoses and true peptone. Heat Coagulation.-Most native proteids, like white of egg, are rendered insoluble when their solutions are heated. The temperature of heat coagulation differs in different proteids ; thus myosinogen and fibrinogen coagulate at 56° C., serum albumin and serum globulin at about 750 C. The proteids which are coagulated by heat come under two classes : the albumins and the globulins. These differ in solubility; the albumins are soluble in distilled water, the globulins require salts to hold them in solution. Indiffusibility.-The proteids (peptones excepted) belong to the class of substances called colloids by Thomas Graham ; that is, they pass with difficulty, or not at all, through animal mem- branes. In the construction of dialysers, vegetable parchment is largely used. Proteids may thus be separated from diffusible (crystalloid) substances like salts, but the process is a tedious one. If some serum or white of egg is placed in a dialyser and distilled water outside, the greater amount of the salts passes into the water through the membrane and are replaced by water ; the two pro- teids albumin and globulin remain inside; the globulin is how- ever precipitated, as the salts which previously kept it in solution are removed. Action, on Polarised Light.--All proteids are levo-rotatory, the amount of rotation varying with individual proteids. CH. XL.] THE PROTEIDS. 559 Colour Reactions.*-The principal colour reactions by which proteids are recognised are the following:- (1) The xantho-proteic reaction ; if a few drops of nitric acid are added to a solution of a proteid like white of egg, the result is a white precipitate; this and the surrounding- liquid become yellow on boiling and are turned orange by ammonia. The preliminary white precipitate is not given by some proteids like peptones; the colours, however, are always the same. (2) Millon's reaction. Millon's reagent is a mixture of mercuric and mercurous nitrate with excess of nitric acid. This gives a white precipitate with proteids which is turned brick-red on boiling. (3) Copper sulphate, or Piotrowski's test. A trace of copper sulphate and excess of strong caustic potash give with most proteids a violet solution. Proteoses and peptones, however, give a rose-red colour instead; this same colour is given by the substance called biuret; f hence the test is generally called the biuret reaction. Precipitants of Proteids.-Solutions of most proteids are precipitated by:- 1. Strong acids like nitric acid. 2. Picric acid. 3. Acetic acid and potassium ferrocyanide. 4. Acetic acid and excess of a neutral salt like sodium sulphate ; these are boiled with the proteid solution. 5. Salt of the heavy metals like copper sulphate, mercuric chloride, lead acetate, silver nitrate, Ac. 6. Tannin. 7. Alcohol. 8. Saturation with certain neutral salts such as ammonium sulphate. It is necessary that the words coagulation and precipitation should in connection with proteids be carefully distinguished. The term coagulation is used when an insoluble proteid (coagulated proteid) is formed from a soluble one. This may occur :- 1. When a proteid is heated-heat coagulation : * The first two colour reactions described depend on the presence in proteids of aromatic radicles. The third depends on the presence and arrangement of a CONH group. f Biuret is formed by heating solid urea ; ammonia passes off and leaves biuret, thus :- 2C0N„H4 - NH3 = C„O„N3H5 [Urea] [Ammonia] [Biuret] 560 THE CHEMICAL COMPOSITION OF THE BODY. [ch. xl. 2. Under the influence of a ferment; for instance, when a curd is formed in milk by rennet or a clot in shed blood by the fibrin ferment-ferment coagulation ; 3. When an insoluble precipitate is produced by the addition of certain reagents (nitric acid, picric acid, tannin, &c.). There are, however, other precipitants of proteids in which the precipitate formed is readily soluble in suitable reagents like saline solutions, and the proteid continues to show its typical reactions. Such precipitation is not coagulation. Such a preci- pitate is produced by saturation with ammonium sulphate. Certain proteids, called globulins, are more readily precipitated by such means than others. Thus, serum globulin is precipi- tated by half-saturation with ammonium sulphate. Full satura- tion with ammonium sulphate precipitates all proteids but peptone. The globulins are precipitated by certain salts, like sodium chloride and magnesium sulphate, which do not precipi- tate the albumins. The precipitation produced by alcohol is peculiar in that after a time it becomes a coagulation. Proteid freshly precipitated by alcohol is readily soluble in water or saline media; fyut after it has been allowed to stand some weeks under alcohol it becomes more and more insoluble. Albumins and globulins are most readily rendered insoluble by this method; albumoses and pep- tones are apparently never rendered insoluble by the action of alcohol. This fact is of value in the separation of these proteids from others. Classification. Both animal and vegetable proteids can be divided into the following six classes. We shall, however, be chiefly concerned with the animal proteids :- Class I. Albumins.-These are soluble in water, in dilute saline solutions, and in saturated solutions of sodium chloride and magnesium sulphate. They are, however, precipitated by satu- rating their solutions with ammonium sulphate. Their solutions are coagulated by heat, usually at 70-73° C. The following are instances :- (а) Serum albumin. Not precipitated by ether. (б) Egg albumin. Precipitated by ether. (c) Lact-albumin (see Milk). Class II. Globulins.-These are insoluble in water, soluble in dilute saline solutions, and insoluble in concentrated solutions of neutral salts like sodium chloride, magnesium sulphate, and CH. XL.] THE PROTEIDS. 561 ammonium sulphate. A globulin dissolved in a dilute saline solution may therefore be precipitated- 1. By removing the salt-by dialysis (see p. 558). 2. By increasing the amount of salt. The best salts to employ are ammonium sulphate (half saturation) or magnesium sulphate (complete saturation). The globulins are coagulated by heat; the temperature of heat coagulation varies considerably. The following are instances :- (а) Fibrinogen (б) Serum globulin (paraglobulin) in blood plasma. (c) Globin, the proteid constituent of haemoglobin. (cZ) Myosinogen in muscle. (e) Crystallin in the crystalline lens. If we compare together these two classes of proteids, the most important of the native proteids, we find that they all give the same general tests, that all are coagulated by heat, but that they differ in solubilities. This difference in solubility may be stated in tabular form as follows :- Reagent. Albumin. Globulin. Water soluble insoluble Dilute saline solution .... soluble soluble Saturated solution of magnesium sul- phate or sodium chloride . soluble insoluble Half-saturated solution of ammonium sulphate ...... soluble insoluble Saturated solution of ammonium sul- phate . . ■ . insoluble insoluble Class III. Albuminates are compounds of proteid with mineral substances. Thus, if a solution of copper sulphate is added to a solution of albumin, a precipitate of copper albuminate is obtained. Similarly, by the addition of other salts of the heavy metals, other metallic albuminates are obtainable. The albuminates which are obtained by the action of dilute acids and alkalies on either albumins or globulins are, however, of greater physiological interest, and it is to these we shall confine our attention. The general properties of the acid-albumin or syntonin, and the alkali-albumin, which are thereby respec- tively formed, are as follow : they are insoluble in pure water, but are soluble in either acid or alkali, and are precipitated by neutralisation unless disturbing influences like the presence of sodium phosphate are present. Like globulins, they are precipitated by saturation with such neutral salts as sodium 562 THE CHEMICAL COMPOSITION OF THE BODY. [ch. xl. chloride and magnesium sulphate. They are not coagulated by heat. A variety of alkali-albumin (probably a compound containing a large quantity of alkali) may be formed by adding strong potash to undiluted white of egg. The resulting jelly is called Lieber- kuhn's jelly. A similar jelly is formed by adding strong acetic acid to undiluted egg-white. Caseinogen, the chief proteid in milk, was at one time regarded as being a native alkali-albumin. It certainly is precipitated by acid, but we shall find that there are several reasons why it is no longer considered an alkali-albumin. Class IV. Nucleo-proteids.-Compounds of proteids with nuclein. They are found in the protoplasm of cells. Caseinogen of milk and vitellin of egg-yolk are similar substances. In physical characters they often closely simulate mucin; in fact, the substance called mucin in the bile is in some animals a nucleo-proteid. They may be distinguished from mucin by the fact that they yield on gastric digestion not only peptone but also an insoluble residue of nuclein which is soluble in alkalis, is precipitable by acetic acid from such a solution, and contains a high percentage (6-8) of phosphorus. Some of the nucleo-proteids also contain iron, and it is probable that the normal supply of iron to the body is contained in the nucleo-proteids, or hsematogens (Bunge), of plant and animal cells. The relationship of nucleo-proteids to the coagulation of the blood is described in the next chapter. Nucleo-proteids may be prepared from cellular structures like testis, thymus, kidney, &c. by two methods :- 1. Wooldridge's method.-The organ is minced and soaked in water for 24 hours. Acetic acid added to the aqueous extract precipitates the nucleo-proteid, or as Wooldridge called it tissue fibrinogen. 2. Sodium chloride method.-The minced organ is ground up in a mortar with solid sodium chloride ; the resulting viscous mass is poured into excess of distilled water, and the nucleo- proteids rises in strings to the top of the water. The solvent usually employed for a nucleo-proteid, whichever method it is prepared by, is a 1 per cent, solution of sodium carbonate. Class V. Proteoses Class VI. Peptones These products of digestion will be studied in connection with that subject. Class VII. Coagulated Proteids.- There are two sub- divisions of these :- CH. XL.] THE POLARIMETER. 563 (а) Proteids in which coagulation has been produced by heat; they are insoluble in water, saline solutions, weak acids, and weak alkalis; soluble after prolonged boiling in concentrated mineral acids; dissolved by gastric and pancreatic juices, they give rise to peptones. (б) Proteids in which coagulation has been produced by ferments :- i. Fibrin (see Blood). ii. Myosin (see Muscle). iii. Casein (see Milk). The Polarimeter. This instrument is one by means of which the action of various substances on the plane of polarised light can be observed and measured. Most of the carbohydrates are dextro-rotatory. All the proteids are levo-rotatory. There are many varieties of the instrument; these can only be properly studied in a practical class, and all one can do here is to state briefly the principles on which they are constructed. Suppose one is shooting arrows at a fence made up of narrow vertical palings; suppose also that the arrows are flat like the laths of a Venetian blind. If the arrows are shot vertically they will pass easily through the gaps between the palings, but if they are shot horizontally they will be unable to pass through at all. This rough illustration will help us in understanding what is meant by polarised light. Ordinary light is produced by the undulations of Eether occurring in all directions at right angles to the path of propagation of the wave. Polarised light is produced by undulations in one plane only; we may compare it to our flat arrows. In a polarimeter, there is at one end of the instrument a Nicol's prism, which is made of Iceland spar. This polarises the light which passes through it; it is called the polariser. At the other end of the instrument is another called the analyser. Between the two is a tube which can be filled with fluid. If the analyser is parallel to the polariser, the light will pass through to the eye of the observer. But if the analyser is at right angles to the polariser, it is like the flat arrows hitting horizontally the vertical palings of the fence, and there is darkness. At inter- mediate angles there will be intermediate degrees of illumination. If the analyser and polariser are parallel and the intermediate tube filled with water, the light will pass as usual, because water 564 THE CHEMICAL COMPOSITION OF THE BODY. [ch. xl. has no action on the plane of polarised light. But if the water contains sugar or some ' optically active ' substance in solution, the plane is twisted in one direction or the other according as the substance is dextro- or levo-rotatory. The amount of rotation is measured by the number of angles. through which the analyser has to be turned in order to obtain the full illumination. This will vary with the length of the tube and the strength of the solution. Albuminoids The albuminoids are a group of substances which, though similar to the proteids in many particulars, differ from them in certain other points. The principal members of the group are the following :- Collagen, the substance of which the white fibres of connective tissue are composed. Some observers regard it as the anhydride of gelatin. In bone, it is often called ossein. Gelatin.-This substance is produced by boiling collagen with water. It possesses the peculiar property of setting into a jelly when a solution made with hot water cools. It gives most of the proteid colour tests. Many observers state, however, that it contains no sulphur. On digestion it is like proteid converted into peptone-like substances, and is readily absorbed. Though it will replace in diet a certain quantity of proteid, acting as what is called a ' proteid-sparing ' food, it cannot altogether take the place of proteid as a food. Animals fed on gelatin instead of proteid waste rapidly. Chondrin, the very similar substance obtained from hyaline cartilage is a mixture of gelatin with mucinoid materials. Mucin.-This is a widely distributed substance occurring in epithelial cells, or shed out by them (mucus, mucous glands, goblet cells); and in connective tissue, where it forms the chief constituent of the ground substance or intercellular material. There are several varieties of mucin, but all agree in the following points :- (а) Physical character. Viscid and tenacious. (б) Precipitability from solutions by acetic acid; they are soluble in dilute alkalis like lime water. (c) They are all compounds of a proteid with a carbohydrate called animal gum, which by treatment with dilute mineral acid can be hydrated into a reducing but non-fermentable sugar.* * Recent work has shown that by the use of somewhat elaborate methods, small quantities of a carbohydrate may be split off from various other proteids and albuminoids. CH. XL.] FERMENTS. 565 Elastin.-This is the substance of which the yellow or elastic fibres of connective tissue are composed. It is a very insoluble material. The sarcolemma of muscular fibres and certain base- ment membranes are very similar. Nuclein, the chief constituent of cell-nuclei. A similar substance is also found in milk and yolk of egg. Its physical characters are something like mucin, but it differs chemically in containing a high percentage of phosphorus. Nuclein, the chief constituent of which is a complex organic acid called nucleic acid, appears to be identical with the chromatin of histologists (see p. n). Keratin, or horny material, is the substance found in the sur- face layers of the epidermis, in hairs, nails, hoofs, and horns. It is very insoluble, and chiefly differs from proteids in its high percentage of sulphur. A similar substance, called neurokeratin, is found in neuroglia and nerve fibres. In this connection it is interesting to note that the epidermis and the nervous system are both formed from the same layer of the embryo-the epiblast. Ferments. The word fermentation was first applied to the change of sugar into alcohol and carbonic acid by means of yeast. The evolution of carbonic acid causes frothing and bubbling; hence the term ' fermenta- tion.' The agent yeast which produces this is called the ferment. Microscopic investiga- tion shows that yeast is composed of minute rapidly-growing unicellular organisms (torulse) belonging to the fungus group of plants. The souring of milk, the transformation of urea into ammonium carbonate in decom- posing urine, and the formation of vinegar (acetic acid) from alcohol, are produced by very similar organisms. The complex series of changes known as putrefaction which are accompanied by the formation of malodorous gases, and which are produced by the various forms of bacteria, also come into the same category. That the change or fermentation is produced by these organisms is shown by the fact that it occurs only when the organisms are present, and stops when they are removed or killed by a high temperature or by certain substances (carbolic acid, mercuric chloride, &c.) called antiseptics. Ffea^t73plaK p°rfocgt of budding. 566 THE CHEMICAL COMPOSITION OF THE BODY. [ch. xl. The ' germ theory ' of disease explains the infectious diseases by considering that the change in the system is of the nature of fermentation, and, like the others we have mentioned, produced by microbes; the transference of the bacteria or their spores from one person to another constitutes infection. The poisons pro- duced by the growing bacteria appear to be either alkaloidal (ptomaines) or proteid in nature. The existence of poisonous proteids is a very remarkable thing, as no chemical differences can be shown to exist between them and those which are not poisonous, but which are useful as foods. The most virulent poison in existence, namely snake poison, is a proteid of the proteose class. There is another class of chemical transformations which differ very considerably from all of these. They, however, resemble these fermentations in the fact that they occur independently of Fig. 474.-Types of micro-organisms, a, micrococci arranged singly ; in twos, diplococci- if all the micrococci at a were grouped together, they would be called staphylococci- and in fours, sarcinee ; &, micrococci, in chains streptococci; c and d, bacilli of various kinds, one is represented with flagellum; e, various forms of spirilla ; f, spores either free or in bacilli. any apparent change in the agents that produce them. The agents that produce them are not living organisms, but chemical substances, the result of the activity of living cells. The change of starch into sugar by the ptyalin of the saliva is an instance. Ferments may therefore be divided into two classes :- 1. The organised ferments-torulse, bacteria, &c. 2. The unorganised ferments or enzymes-like ptyalin. Each may be again subdivided according to the nature of the chemical change produced. In digestion, the study of which we shall be soon commencing, it is the unorganised ferments with the action of which we have chiefly to deal. CH. XL.] FERMENTS. 567 The unorganised ferments may be classified as follows :- (а) Amylolytic-those which change amyloses (starch, glycogen) into sugars. Examples : ptyalin, diastase, amylopsin. (б) Proteolytic-those which change proteids into proteoses and peptones. Examples : pepsin, trypsin. (c) Steatolytic-those which split fats into fatty acids and glycerine. An example, steapsin, is found in pancreatic juice. (cZ) Inversive-those which convert saccharoses (cane sugar, maltose, lactose) into glucose. Examples : invertin of intestinal juice and of yeast cells. (e) Coagulative-those which convert soluble into insoluble proteids. Examples: rennet, fibrin ferment, myosin ferment. Most ferment actions are hydrolytic-i.e., water is added to the material acted on, which then splits into new materials. This is seen by the following examples :- i. Conversion of cellulose into carbonic acid and marsh gas (methane) by putrefactive organisms. (C6H10O5)n + nH2O = 3nCO2 + snCH* [cellulose] [water] [carbonic acid] [methane] 2. Inversion of cane sugar by the unorganised ferment invertin :- c12H22on+H20=C6H12OG+C6H12O6 [cane sugar] [water] [dextrose] [levulose] A remarkable fact concerning the ferments is that the sub- stances they produce, in time put a stop to their activity; thus in the case of the organised ferments the alcohol produced by yeast, the phenol, cresol, &c., produced by putrefactive organisms from proteids, first stop the growth of, and ultimately kill the organisms which produce them. In the case of the unorganised ferments, the products of their activity hinder and finally stop their action, but on the removal of these products the ferments resume work. Ferments act best at a temperature of about 40° C. Their activity is stopped, but the ferments are not destroyed by cold ; it is stopped and the ferments killed by too great heat. A certain amount of moisture and oxygen is also necessary ; there are, however, certain micro-organisms that act without free oxygen, and are called anaerobic in contradistinction to those which require oxygen, and are called aerobic. 568 THE BLOOD. [ch. xli. The chemical nature of the enzymes or unorganised ferments is very difficult to investigate; they are substances that elude the grasp of the chemist to a great extent. So far, however, research has taught us that they are either proteid in nature or are substances closely allied to the proteids. CHAPTER ALL THE BLOOD. The blood is the fluid medium by means of which all the tissues of the body are directly or indirectly nourished; by means of it also such of the materials which result from the metabolism of the tissues as are of no further use in the economy, are carried to the excretory organs to be removed from the body. It is a somewhat viscid fluid, and in man and in all other vertebrate animals with the exception of two,* is red in colour. The exact shade of red is variable ; that taken from the arteries, from the left side of the heart and from the pulmonary veins is of a bright scarlet hue, that obtained from the systemic veins, from the right side of the heart, and from the pulmonary artery, is of a much darker colour. At first sight, the red colour appears to belong to the wThole mass of blood, but on further examination this is found not to be the case. In reality blood consists of an almost colourless fluid, called plasma or liquor sanguinis, in which are suspended numerous blood, corpuscles, which are, for the most part, coloured, and it is to their presence in the fluid that the red colour of the blood is due. Even when examined in very thin layers, blood is opaque, on account of the different refractive powers possessed by its two constituents, viz., the plasma and the corpuscles. On treatment with ether, water, and other reagents, however, it becomes trans- parent and assumes a lake colour, in consequence of the colouring matter of the corpuscles having been discharged into the plasma. The average specific gravity of blood at 150 C. (6o° F.), varies from 1055 to 1062. A rapid and useful method of estimating the specific gravity of blood was invented by Roy. Drops of blood are taken and allowed to fall into fluids of known specific gravity. * The amphiexus and the leptocephalu*. CH. XLI.J THE BLOOD. 569 When the drop neither rises nor sinks in the fluid, it is taken to be of the same specific gravity as that of the standard fluid. The reaction of blood is faintly alkaline and the taste saltish. Its temperature varies slightly, the average being 37'8° C. (100 F.). The blood stream is warmed by passing through the muscles, nerve centres, and glands, but is somewhat cooled on traversing the capillaries of the skin. Recently-drawn blood has a distinct odour, which in many cases is characteristic of the animal from which it has been taken. It may be further developed by adding to blood a mixture of equal parts of sulphuric acid and water. Quantity of the Blood.-The quantity of blood in any animal under normal conditions bears a fairly constant relation to the body-weight. The methods employed for estimating it are not so simple as might at first sight have been thought. For example, it would not be possible to get any accurate information on the point from the amount obtained by rapidly bleeding an animal to death, for then an indefinite quantity would remain in the vessels, as well as in the tissues ; nor, on the other hand, would it be possible to obtain a correct estimate by less rapid bleeding, as, since life would be more prolonged, time would be allowed for the passage into the blood of lymph from the lym- phatic vessels and from the tissues. In the former case, there- fore, we should under-estimate, and in the latter over-estimate the total amount of the blood. Of the several methods which have been employed, the most accurate is the following. A small quantity of blood is taken from an animal by venesection ; it is defibrinated and measured, and used to make standard solutions of blood. The animal is then rapidly bled to death, and the blood which escapes is collected. The blood-vessels are next washed out with saline solution until the washings are no longer coloured, and these are added to the previously-withdrawn blood; lastly the whole animal is finely minced with saline solution. The fluid obtained from the mincings is carefully filtered, and added to the diluted blood previously obtained, and the whole is measured. The next step in the process is the comparison of the colour of the diluted blood with that of standard solutions of blood and water of a known strength, until it is discovered to what standard solution the diluted blood corresponds. As the amount of blood in the corresponding standard solution is known, as well as the total quantity of diluted blood obtained from the animal, it is easy to calculate the absolute amount of blood which the latter contained, and to this is added the small amount which was withdrawn to make the standard solutions. This gives the total amount of 570 THE BLOOD. [ch. XLI. blood which the animal contained. It is contrasted with the weight of the animal, previously known. The result of many experiments shows that the quantity of blood in various animals averages to of the total body-weight. An estimate of the quantity in man which corresponded nearly with this proportion, has been more than once made from the following data. A criminal was weighed before and after decapi- tation ; the difference in the weight representing the quantity of blood which escaped. The blood-vessels of the head and trunk were then washed out by the injection of water, until the fluid which escaped had only a pale red or straw colour. This fluid was then also weighed ; and the amount of blood which it repre- sented was calculated by comparing the proportion of solid matter contained in it with that of the first blood which escaped on de- capitation. Two experiments of this kind gave precisely similar results. (Weber and Lehmann.) Coagulation of the Blood. One of the most characteristic properties which the blood possesses is that of clotting or coagulating. This phenomenon may be observed under the most favourable conditions in blood which has been drawn into an open vessel. In about two or three minutes, at the ordinary temperature of the air, the surface of the fluid is seen to become semi-solid or jelly-like, and this change takes place, in a minute or two afterwards at the sides of the vessel in which it is contained, and then extends throughout the entire mass. The time which is occupied in these changes is about eight or nine minutes. The solid mass is of exactly the same volume as the previously liquid blood, and adheres so closely to the sides of the containing vessel that if the latter be inverted none of its contents escape. The solid mass is the crassamentum or clot. If the clot is watched for a few minutes, drops of a light, straw-coloured fluid, the serum, may be seen to make their appearance on the surface, and, as they become more and more numerous, to run together, forming a complete superficial stratum above the solid clot. At the same time the fluid begins to tran- sude at the sides and at the under-surface of the clot, which in the course of an hour or two floats in the liquid. The first drops of serum appear on the surface about eleven or twelve minutes after the blood has been drawn; and the fluid continues to transude for from thirty-six to forty-eight hours. The clotting of blood is due to the development in it of a sub- stance called fibrin, which appears as a meshwork (fig. 475) of CH. XLI.] BLOOD-COAGULATION. 571 fine fibrils. This meshwork entangles and encloses within itself the blood corpuscles. The first clot formed, therefore, includes the whole of the constituents of the blood in an apparently solid mass, but soon the fibrinous meshwork begins to contract and the serum which does not belong to the clot is squeezed out. When the whole of the serum has transuded the clot is found to be smaller, but firmer and harder, as it is now made up chiefly of fibrin and blood corpuscles. Thus coagulation rearranges the constituents of the blood ; liquid blood being made up of plasma and blood corpuscles, and clotted blood of serum and clot. Fibrin is formed from the plasma, and may be obtained free from corpuscles when blood plasma is allowed to clot, the corpuscles Fig. 475.-Reticulum of fibrin, from a drop of human blood, after treatment with rosanilin. (Ranvier.) having previously been removed. It may be also obtained from blood by whipping it with a bunch of twigs ; the fibrin adheres to the twigs and entangles but few corpuscles. These may be removed by washing with water. Serum is plasma minus fibrin. The relation of plasma, serum, and clot can be seen at a glance in the following scheme of the constituents of the blood :- /Plasma Serum' Fibrin Blood- "Clot It may be roughly stated that in 100 parts by weight of blood 60-65 parts consist of plasma and 35-40 of corpuscles. 572 THE BLOOD. [ch. xli. The huffy coat is seen when blood coagulates slowly, as in horses' blood. The red corpuscles sink more rapidly than the white, and the upper stratum of the clot (buffy coat) consists mainly of fibrin and white corpuscles. Coagulation is hastened by- 1. A temperature a little over that of the body. 2. Contact with foreign matter. 3. Agitation. 4. Addition of calcium salts. Coagulation is hindered or prevented by- 1. A low temperature. In a vessel cooled by ice, coagulation may be prevented for an hour or more. 2. The addition of a large quantity of neutral salts like sodium sulphate or magnesium sulphate. 3. Contact -with the living vascular walls. 4. Contact with oil. 5. Addition of oxalates: these precipitate the calcium necessary for coagulation as insoluble calcium oxalate. 6. Injection of commercial peptone (which consists chiefly of proteoses) into the circulation of the living animal. 7. Addition of leech extract. This acts in virtue of a proteose it contains. The theory generally received which accounts best for the coagulation of the blood is that of Hammarsten, and it may be briefly stated as follows :- When blood is within the vessels one of the constituents of the plasma, a proteid of the globulin class called fibrinogen, exists in a soluble form. When the blood is shed the fibrinogen molecule is sjolit into two parts: one part is a globulin, which remains in solution, the other is the insoluble material fibrin. This change is brought about by the activity of a special unorgan- ised ferment called the fibrin-ferment. This ferment does not exist in healthy blood contained in healthy blood-vessels, but is one of the products of the disintegration of the white corpuscles and blood tablets that occurs when the blood leaves the vessels or comes into contact with foreign matter. To this it may be added, as the result of recent research, that a soluble calcium salt is essential for the effective action of the ferment; that the fibrin-ferment belongs to the class of nucleo- proteids ; that other nucleo-proteids (Wooldridge's tissue-fibrino- gens) obtained from most of the cellular organs of the body produce intravascular clotting when injected into the circulation of a living animal. It has been suggested that fibrin-ferment is CH. XLI.] THE PLASMA AND SERUM. 573 the agent by means of which the calcium and fibrinogen are united to form fibrin. The Plasma and. Serum. The liquid in which the corpuscles float may be obtained by employing one or other of the methods already described for Fig. 476.-Plan and section of centrifugal machine, a, an iron socket secured to top of table b ; c, a steel spindle carrying the turntable d, and turning freely in A- e, a flange round turntable d ; f f, shallow grooves on face of d, in which the test tubes are fixed by clamps go; h, a pulley fixed to end of spindle c, and turned by the cord k • 11 are two guide pulleys for cord k. (Gamgee.) preventing the blood from coagulating. The corpuscles being- heavy sink, and the supernatant plasma can then be removed by a pipette or siphon, or more thoroughly by the use of a centrifugal machine (see fig. 476). On counteracting the influence which has prevented the blood 574 THE BLOOD. [ch. xli. from coagulating, the plasma then itself coagulates. Thus plasma obtained by the use of cold clots on warming gently ; plasma which has been decalcified by the action of a soluble oxalate clots on the addition of a calcium salt; plasma obtained by the use of a strong solution of salt coagulates when this is diluted by the addition of water, the addition of fibrin-ferment being necessary in most cases; where coagulation occurs without the addition of fibrin-ferment, no doubt some is already present from the partial disintegration of the corpuscles which has already occurred. Pericardial and hydrocele fluid resemble pure plasma very closely in composition. As a rule, however, they contain few or no white corpuscles, and do not clot spontaneously, but after the addition of fibrin-ferment, or liquids like serum that contain fibrin-ferment, they always yield fibrin. Pure plasma may be obtained from horses' veins by what is known as the "living test-tube" experiment. If the jugular vein is ligatured in two places so as to include a quantity of blood within it, then removed from the animal and hung in a cool place, the blood will not clot for many hours. The corpuscles settle and the supernatant plasma can be removed with a pipette. The plasma is alkaline, yellowish in tint, and its specific gravity is about 1026 to 1029. 1000 parts of plasma contain:- Water 902'90 Solids 97'io Proteids : 1, yield of fibrin . . . 4'05 2, other proteids . . . . 78'84 Extractives (including fat) . . . 5'66 Inorganic salts §'55 In round numbers, plasma contains io per cent, of solids, of which 8 are proteid in nature. Serum contains the same three classes of constituents-pro- teids, extractives, and salts. The extractives and salts are the same in both liquids. The proteids are different, as is shown in the following table :- Proteids of Plasma. Fibrinogen. Serum globulin. Serum albumin. Proteids of Serum. Serum globulin. Serum albumin. Fibrin-ferment. The gases of plasma and serum are small quantities of oxygen, nitrogen, and carbonic acid. The greater part of the oxygen of the blood is combined in the red corpuscles with haemoglobin ; the carbonic acid is chiefly combined as carbonates. The gases CH. XLI.] PROTEIDS OF THE BLOOD. 575 of the blood have been already considered under Respiration (see p. 542). We may now study one by one the various constituents of the plasma and serum. A. Proteids. Fibrinogen.-This is the substance acted on by fibrin-ferment. It yields, under this action, an insoluble product called fibrin, and a soluble proteid of the globulin class. Fibrinogen is a globulin. It differs from serum globulin, and may be separated from it by the fact that half-saturation with sodium chloride precipitates it. It coagulates by heat at the low temperature of 56° C. Serum globulin and serum albumin.-These substances exhibit the usual differences already described between albumins and globu- lins (p. 561). Both are coagulated by heat at a little over 70° C. They may be separated by dialysis or the use of neutral salts. The readiest way to separate them is to add to the serum an equal volume of saturated solution of ammonium sulphate. This is equivalent to semi-saturation, and it precipitates the globulin. If magnesium sulphate is used as a precipitant of the globulin, it must be added in the form of crystals, and the mixture well shaken to insure complete saturation. Serum globulin was formerly called fibrinogdastin, because it was believed to take some share in fibrin formation. It is also called paraglobulin. It may be imperfectly precipitated by diluting serum with twenty times its volume of water, and then adding a trace of acetic acid or passing a stream of carbonic acid gas through the diluted serum. Fibrin-ferment.-Schmidt's method of preparing it is to take serum and add excess of alcohol. This precipitates all the proteids, fibrin-ferment included. After some weeks the alcohol is poured off; the serum globulin and serum albumin have been by this means rendered insoluble in water; an aqueous extract is, however, found to contain fibrin-ferment, which is not so easily coagulated by alcohol as the other proteids are. B. Extractives.- These are non-nitrogenous and nitrogenous. The non-nitrogenous are fats, soaps, cholesterin and sugar; the nitrogenous are urea (0'02 to 0'04 per cent.), and still smaller quantities of uric acid, creatine, creatinine, xanthine, and hypo- xanthine. C. Salts.- The most abundant salt is sodium chloride: it con- stitutes between 60 and 90 per cent, of the total mineral matter. Potassium chloride is present in much smaller amount. It consti- tutes about 4 per cent, of the total ash. The other salts are phosphates and sulphates. 576 THE BLOOD. [ch. xli. Schmidt gives the following table :- iooo parts of plasma yield- Mineral matter 8'550 Chlorine 3'64° S03 0-115 P205 0-191 Potassium 0323 Sodium 3'34T Calcium phosphate 0'311 Magnesium phosphate ... . . 0'222 The Blood-Corpuscles. There are two principal forms of corpuscles, the red and the white, or, as they are now frequently named, the coloured and the colourless. In the moist state, the red corpuscles form about 45 per cent, by weight of the whole mass of the blood. The propor- tion of colourless corpuscles is only as 1 to 500 or 600 of the coloured. Red or Coloured Corpuscles.-Human red blood-corpuscles are circular, biconcave discs with rounded edges, -3 2V0 inch *n diameter (7/z to 8p) and inch or about 2/x in thickness. When viewed singly they appear of a pale yellowish tinge ; the deep red colour which they give to the blood being observable in them only when they are seen en masse. They are composed of a colourless, structureless, and transparent filmy framework or stroma, infiltrated in all parts by a red colouring matter termed haemoglobin. The stroma is tough and elastic, so that, as the corpuscles circulate, they admit of changes of form, in adaptation to the vessels, and recover their natural shape as soon as they escape from compression. The colouring matter uniformly per- vades the stroma ; the consistency of the peripheral part of which is greater than that of the more central portions. This plays the part of a membrane in the processes of osmosis that occur when water and salt solution are added to the corpuscles. The red corpuscles have no nuclei j the unequal refraction of transmitted light gives the appearance of a central spot, brighter or darker than the border, according as it is viewed in or out of focus. Their specific gravity is about 1088. The corpuscles of all mammals with the exception of the camelidse are circular and biconcave. They are generally very nearly the size of human red corpuscles. They are smallest in the deer tribe and largest in the elephant. In the camelidro they are biconvex. In all mammals the corpuscles are non- nucleated, and in all other vertebrates (birds, reptiles, amphibia, and fish) the corpuscles are oval, biconvex, and nucleated (fig. CH. XLI.I RED BLOOD CORPUSCLES. 577 480) and larger than in mammals. They are largest of all in certain amphibians (aniphiuma, proteus). The red corpuscles are not all alike, for in almost every speci- men of blood may be also observed a certain number of corpuscles smaller than the rest. They are termed microcytes, or hoemato- blasts, and are probably immature corpuscles. A property of the red corpuscles, which is exaggerated in inflammatory blood, is a tendency to adhere together in rolls or columns (rouleaux), like piles of coins. These rolls quickly fasten together by their ends, and cluster; so that, when the blood is spread out thinly on a glass, they form an irregular network (fig. 477). Fig. 478.-Corpuscles of the frog. The central mass consists of nucleated coloured corpuscles. The other cor- puscles are two varieties of the colourless form. Fig. 477. - Red corpuscles in rouleaux. The white corpuscles are uncoloured. Action of Re-agents.-Considerable light has been thrown on the physical and chemical constitution of red blood-cells by studying the effects produced by mechanical means and by various re-agents ; the following is a brief .summary of these re-actions :- Water.-When water is added gradually to frog's blood, the oval disc- shaped corpuscles become spherical, and gradually discharge their haemo- globin, a pale, transparent stroma being left behind : human red blood-cells swell, change from a discoidal to a spheroidal form, and discharge their pigment, becoming quite transparent and all but invisible. Saline solution produces no appreciable effect on the red blood-cells of the frog. In the red blood-cells of man the discoid shape is exchanged for a spherical one, with spinous projections, like a horse-chestnut (fig. 479). Their tendency to run into rouleaux is prevented. Dilute acetic acid causes the nucleus of the red blood-cels in the frog to become more clearly defined ; if the action is prolonged, the nucleus becomes strongly granulated, and all the colouring matter seems to be concentrated in.it, the surrounding cell- Fig. 479.-Effect of saline so - lution (crena- tion). 578 THE BLOOD. [CH. XLI. Fig. 480.-The above illustration is. somewhat altered from a drawing by Gulliver, in the Proceed. Zool. Society, and exhibits the typical characters of the red blood-cells in the main divisions of the Vertebrata. The fractions are those of an inch, and represent the average diameter. In the case of the oval cells, only the long diameter is here given. It is remarkable, that although the size of the red blood-cells varies so much in the different classes of the vertebrate kingdom, that of the white corpuscles remains comparatively uniform, and thus they are, in some animals, larger, in others smaller, than the red corpuscles. substance and outline of the cell becoming almost invisible ; after a time -- i • the cells lose their colour altogether. The cells in the figure (fig. 481) represent the successive stages of the change. A similar loss of colour occurs in the red corpuscles of human blood, which, however, from the absence of nuclei, seem to disappear entirely. Dilute alkalis cause the red blood-cells to dissolve slowly, and finally to disappear. I Chloroform added to the red blood-cells of the frog causes them to part with their haemoglobin ; the stroma of the cells becomes gradually broken up. A similar effect is I produced ou the human-red blood-corpuscles.-- ... Fig. 481.-Effect of acetic acid. CH. XLI.J COLOURLESS BLOOD-CORPUSCLES. 579 Tannic, acid - When a 2 per cent, fresh,solution of tannic acid is applied to frog's blood it causes the appearance of a sharply- defined little knob, projecting from the free surface (Roberts' i the colouring matter becomes at the same time concentrated in the nucleus, which grows more distinct (fig. 482). A somewhat similar effect is produced on the human red blood-corpuscle, the colour- ing matter being discharged and coagulated as a little knob of htematin on the surface of the stroma. Boric acid.-A 2 per cent, solution applied to nucleated red blood-cells (frog) will cause the concentration of all the colouring matter in the nucleus ; the coloured body thus formed gradually quits its central position, and comes to be partly, sometimes entirely, protruded from the surface of the now colourless cell (fig. 483). The result of this experiment led Briicke to distinguish the coloured contents of the cell (zooid) from its colourless stroma (aecoid). When applied to the non-nucleated mammalian corpuscle its effect merely re- sembles that of other dilute acids. Heat.-The effect of heat up to 50°-60° C. (120°- 140° F.) is to cause the formation of a number of bud-like processes (fig. 484). Electricity causes the red blood-corpuscles to become crenated, and at length mulberry-like. Finally they re- cover their round form and become quite pale. The Colourless Corpuscles.-In human blood the white or colourless corpuscles or leucocytes are nearly spherical masses of granular protoplasm without cell-wall. In all cases one or more nuclei exist in each corpuscle. The size of the corpuscle varies, but averages 2"5Vcr an inch (iom) in diameter. In health, the proportion of white to red corpuscles, which, taking an average, is about 1 to 500 or 600, varies considerably even in the course of the same day. The variations appear to depend chiefly on the amount and probably also on the kind of food taken ; the number of leucocytes being very considerably increased by a meal, and diminished again on fasting. Also in young persons, during preg- nancy, and after great loss of blood, there is a larger proportion of colourless blood-corpuscles. In old age, on the other hand, their proportion is diminished. Varieties.-The colourless corpuscles present greater diversities of form than the red ones. Two chief varieties are to be seen in human blood; one of which contains a considerable number of coarse granules, and the other, which is paler and less granular, contains several nuclei united by fine threads. Fig. 482.-Effect of tannin. Fig. 483.-Effect of boric acid. Fig. 484.-Effect of heat. Fig. 485.-A. Three coloured blood- corpuscles. B. Three colourless blood - corpuscles acted on by acetic acid ; the nuclei are very clearly visible, x 900. 580 THE BLOOD. [ch. xli. According to their reactions to acid and neutral aniline dyes, they are called eosinophile (eosin being an acid stain) and neutrophile cells respectively ; the latter are the more numerous. It is the coarsely granular cells which are-eosinophile. In size the varia- tions are great, for in most specimens of blood it is possible to make out, in addition to the full-sized varieties, a number of smaller corpuscles, consisting of a large spherical nucleus sur- rounded by a variable amount of more or less granular proto- plasm. These small corpuscles are the undeveloped forms of the others, and are derived from the cells of the lymphatic glands ; they are called lymphocytes. A fourth variety of leu- cocyte is the hyaline corpuscle, in the protoplasm of which there aue no granules. They have a single nucleus. Very rarely basophile cells (?'.<"., having an affinity for basic aniline dyes like methylene blue) are fo,und. Amoeboid Movement.-The remarkable property of the colourless corpuscles of spontaneously changing their shape was first demonstrated by Wharton Jones in the blood of the skate. If a drop of blood is examined with a high power of the micro- scope, under conditions by which loss of moisture is prevented, and at the same time the temperature is maintained by a warm stage at about that of the body, 370 C. (98'5° F.), the colour- less corpuscles will be observed slowly to alter their shapes, and to send out processes at various parts of their circumfer- ence. The amoeboid movement, which can be demonstrated in human colourless blood-corpuscles, can be more readily seen in newt's blood. The full consideration of amoeboid movement is given on p. 12. An interesting variety of amoeboid movement is that which leads to the ingestion of foreign particles. This gives to the leucocytes their power of taking in and digesting bacilli (phagocytosis'). The multi-nucleated, finely granular corpuscles are the most vigorous phagocytes. The next figure illustrates this; the cells repre- sented, however, are not leucocytes, but the large amoeboid cells found in connective tissues, especially in inflamed parts. (See also p. 450). The process of emigration of the leucocytes is described on p. 449. Action of Reagents on the colourless corpuscles.- Water causes the corpuscles to swell and their nuclei to become apparent. Acetic acid (1 per cent.) has a similar action; it also causes the granules to aggregate round the nucleus (fig. 485). Dilute alkalis produce swelling and bursting of the corpuscles. CH. XLI.] THE BLOOD-PLATELETS. 581 The Blood Platelets. Besides the two principal varieties of blood corpuscles, a third kind has been described under the name blood-platelets (Blut- pliitcheri). These arc colourless disc-shaped or irregular bodies, much smaller than red corpuscles. Different views arc held as to their origin. At first they were regarded as immature red corpuscles; but this view is discarded. They may be disintegra- Healthy bacillus Healthy bacillus -Healthy bacillus -Partially digested bacillus Nucleus Bacillus in leucocyte .Partially digested leucocyte .Foreign matter Partially digested leucocyte Nuclei vacuolated Foreign matter Particles of foreign matter Particles of foreign matter Particles of foreign matter Leucocytes - Fig. 486.-Macrophages containing bacilli and other structures supposed to be under- going digestion. (Buffer.) tive products of white corpuscles; or it may be that they are merely a precipitate of nucleo-proteid which occurs when the plasma dies or is cooled. Enumeration of the Blood-Corpuscles. Several methods are employed for counting the blood-corpuscles, most of them depending upon the same principle, i.e., the dilution of a minute volume of blood with a given volume of a colourless solution similar in specific gravity to blood plasma, so that the size and shape of the cor- puscles is altered as little as possible. A minute quantity of the well- mixed solution is then taken, examined under the microscope, either in a flattened capillary tube (Malassez) or in a cell (Hayem & Nachet, Gowers) of known capacity, and the number of corpuscles in a measured length of the tube, or in a given area of the cell, is counted. 582 THE BLOOD. [CH. XLI. The length of the tube and the area of the cell are ascertained by means of a micrometer scale in the microscope ocular; or in the case of Gowers' modification, by the division of the cell area into squares of known size. Having ascertained the number of corpuscles in the diluted blood, it is easy to find out the number in a given volume of normal blood. Gowers' Hcemacytometer consists of a small pipette (a), which, when filled up to a mark on its stem, holds 995 cubic millimetres. It is furnished with an india-rubber tube and glass mouth-piece to facilitate filling and emptying ; a capillary tube (b) marked to hold 5 cubic millimetres, and also furnished with an india-rubber tube and mouth- piece; a small glass jar (d) in which the dilution of the blood is perfotmed; Fig. 487.-Heemacytometer. (Gowers.) a glass stirrer (e) for mixing the blood thoroughly; (f) a needle, the length of which can be regulated by a screw ; a brass stage plate (c) carrying a glass slide, on which is a cell one-fifth of a millimetre deep, and the bottom of which is divided into one-tenth millimetre squares. On the top of the cell rests the cover-glass, which is kept in its place by the pressure of two springs proceeding from the stage plate. A standard saline solution of sodium sulphate, or similar salt, of specific gravity 1025, is made, and 995 cubic millimetres are measured by means of the pipette into the glass jar, and with this five cubic millimetres of blood, obtained by pricking the finger with a needle, and measured in the capillary pipette (b), are thoroughly mixed by the glass stirring-rod. A drop of this diluted blood is then placed in the cell and covered with a cover-glass, which is fixed in position by means of the two lateral springs. The layer of diluted blood between the slide and cover-glass is 1 millimetre thick. The preparation is then examined under a microscope with a power of about 400 diameters, and focussed until the lines dividing the cell into squares are visible. After a short delay, the red corpuscles which have sunk to the bottoih of CH. XU,] DEVELOPMENT OF BLOOD-OOBTUSCLBS 583 the cell, and are resting on the squares, are counted in ten squares, and the number of white corpuscles noted. By adding together the numbers counted in ten (one-tenth millimetre) squares, and, as the blood has been diluted, multiplying by ten thousand, the number of corpuscles in one cubic millimetre of blood is obtained. The average number of- corpuscles per cubic millimetre of healthy blood, according to Vierordt and Weicker, is Fig. 488. 5,000,000 in adult men, and 4,500,000 in women ; this corresponds to an average of 50 and 45 corpuscles respectively per square of Gowers' instru- ment. A haemacytometer of another form, and one that is much used at the present time, is known as the Thoma-Zeiss haemacytometer. It consists of a carefully graduated pipette, in which the dilution of the blood is done ; this is so formed that the capillary stem has a capacity equalling one-hundredth of the ball above it. If the blood is drawn up in the capillary tube to the line marked 1 (fig. 489) the saline solution may afterwards be drawn up the stem to the line 101 ; in this way we have ioi parts, of which the blood forms 1. As the contents of the stem can be displaced unmixed we shall have in the mixtnre the proper dilution. The blood and the saline solution are well mixed by shaking the pipette, in the ball of which is, contained a small glass bead for the purpose of aiding the mixing. The other part of the instrument consists of a glass slide (fig. 488) upon which is mounted a covered disc, w,, accurately ruled so as to present one square millimetre divided into 400 squares of one-twentieth of a milli- metre each. The micrometer thus made is surrounded by another annular cell, <?, which has such a height-as to make the cell project exactly one-tenth millimetre beyond m. If a drop of the diluted blood be placed upon m, and c be covered with a perfectly flat cover- glass, the volume of the diluted blood above each of the squares of the micrometer, i.e., above each m, will be of a cubic millimetre. An average of ten or more squares are then taken, and this number multiplied by 4000 x 100 gives the number of corpuscles in a cubic millimetre of undiluted blood. Development of the Blood-Corpuscles. The first formed blood-corpuscles of the human embryo differ much in their general characters from those which belong to the later periods of intra-uterine, and to all periods of extra-uterine life. Their manner of origin is at first very simple. Surrounding the early embryo is a circular area, called the vascular area, in which the first rudiments of the blood-vessels and Fig. 489. Figs. 488 and 489.- Thoma-Zeiss Heemacytometer. 584 THE BLOOD. [ch. xli. blood-corpuscles arc developed. Here the nucleated embryonic cells of the mesoblast, from which the blood-vessels and corpuscles are to be formed, send out processes in various directions, and these, joining together, form an irregular meshwork. The nuclei increase in number, and collect chiefly in the larger masses of protoplasm, but partly also in the processes. These nuclei gather around them a certain amount of the protoplasm, and, becoming coloured, form the red blood corpuscles. The protoplasm of the cells and their branched network in which these corpuscles lie then become hollowed out into a system of canals enclosing fluid, in which the red nucleated corpuscles float. The corpuscles at Fig. 490.-Part of the netwoik of develop'ng b'ood-vcssc's in the vascular area of a guinea-pig. M, blood-corpuscles becoming fres in an enlarged and hollowed-out part of the network; a, process of protoplasm. (E. A. Schafer.) first are from about to yyVtr °f an * (IOM to i6g) in diameter, mostly spherical, and with granular contents, and a well-marked nucleus. Their nuclei, which are about - oVq of an inch (5/z) in diameter, arc central and circular. The corpuscles then strongly resemble the colourless corpuscles of the fully developed blood, but are coloured. They are capable of amoeboid movement and multiply by division. When, in the progress of embryonic development, the liver begins to be formed, the multiplication of blood-cells in the whole mass of blood ceases, and new blood-cells are produced by this organ, and also by the lymphatic glands, thymus and spleen. These arc at first colourless and nucleated, but afterwards acquire the ordinary blood-tinge, and resemble very much those of the first set. They also multiply by division. In whichever way produced, however, whether from the original formative cells of CH. xli.j DEVELOPMENT OF BLOOD-CORPUSCLES. 585 the embryo, or by the liver and the other organs mentioned above, these coloured nucleated cells begin very early in foetal life to be mingled with coloured ?ion-nucleated corpuscles resembling those of the adult, and at about the fourth or fifth month of embryonic existence arc completely replaced by them. Origin of the Matured Coloured Corpuscles.-The non- nucleated red corpuscles may possibly be derived from the nucle- ated, but in all probability are an entirely new formation. Their chief origin is :- From the medulla of bone.-It has been shown that coloured corpuscles are to a very large extent derived during adult life from the large pale cells in the red marrow pf bones, especially of the ribs (fig. 493). These cells become coloured from the forma- Fig. 491.-Development of red corpuscles in connective tissue cells. From the subcutaneous tissue of a new-born rat. A, a cell containing htemoglobin in a diffused form in the protoplasm ; A', one containing coloured globules of varying size and vacuoles; A", a cell filled with coloured globules of nearly uniform size; developing fat cells. (E. A. Schafer.) tion of hsemoglobin chiefly in one part of their protoplasm. This coloured part becomes separated from the rest of the cell and forms a red corpuscle, being at first cup-shaped, but soon taking on the normal appearance of the mature corpuscle. It is supposed that the protoplasm may grow again and form a number of red corpuscles in a similar way. From the tissue of the spleen.-It is probable that coloured as well as colourless corpuscles may be produced in the spleen. From Microcytes. - Hayem describes the small particles (Microcytes), previously mentioned as contained in the blood, and which he calls luematoblasts, as the precursors of the red corpuscles. They acquire colour, and enlarge to the normal size of red corpuscles. The belief that the red corpuscles are derived from the white which formerly prevailed has now been discarded. During foetal life and possibly in some animals, e.g. the rat, which arc born in an immature condition, for some little time 586 THE BLOOD. [CH. Xtl. after birth, the 'blood discs have been stated by Schafer to arise in the connective tissue cells in the following way. Small globules, of Varying size, of colouring matter arise in the proto- plasm of the cells, and the cells themselves become branched, their branches joining the branches of similar cells. The cells next become vacuolated, and the red globules are free in a cavity filled with fluid (fig. 492) ; by the extension of the cavity of the cells into their pro- cesses anastomosing vessels are produced, which ulti- mately join with the pre- viously existing vessels, and the globules, now having the size and appearance of the ordinary red corpuscles, are passed into the general circulation. This method of formation is called in- tracellular. Without doubt, the red corpuscles have, like all other parts of the organism, a tolerably defi- nite term of existence, and in a like manner die and waste away when the portion of work allotted to them has been performed. Neither the length of their life, however, nor the Fig. 492.-Further development of blood-cor- puscles in connective tissue cellsand trans- formation of the latter into capillary blood- vessels, a, an elongated cell with a cavity in the protoplasm occupied by fluid and by blood- corpuscles which are still globular a hoi- lowcell, the nucleus of which has multiplied. The new nuclei are arranged around the wall of the cavity, the corpuscles in which have now become discoid; c, shows the mode of union of a "hsemapoietic" cell, which, in this instance, contains only one corpuscle, with the prolongation (si) of a previously existing vessel; a and c, from the new-born rat; b, from the foetal sheep. (E. A. Schafer.) Fig. 493.-Coloured nucleated corpuscles, from the red marrow of the guinea-pig. (E. A. Schafer.) fashion of their decay lias been yet clearly made out. It is gene- rally believed that a certain number of the coloured corpuscles undergo disintegration in the spleen ; and indeed corpuscles in various degrees of degeneration have been observed in that organ. CH. XLI.] CHEMISTRY OF BLOOD-CORPUSCLES. 587 Origin of the White Corpuscles.-The hyaline corpuscles are derived from the lymphocytes which are formed in the lymph- atic glands, and enter the blood-stream by the thoracic duct. The finely granular leucocytes which are the most numerous white corpuscles in the blood originate either in the same way, or by cell division in the blood-stream itself. The eosinopliile corpuscles, which form about 5 per cent, of the total leucocytes in normal blood, are found in larger numbers, in the connective tissue in various parts of the body; they are found in special abundance in red marrow, in which at one time they were supposed to originate. But they do not seem to be exclu- sively formed here. Some look upon each eosinophile corpuscle as a little unicellular gland, and the mass of corpuscles as a migratory glandular tissue. Chemistry of the Blood-Corpuscles. The white blood corpuscles.-Our chemical knowledge of the white corpuscles is small. Their nucleus consists of nuclein, their cell-protoplasm yields proteids belonging to the globulin and nucleo-proteid groups. The nucleo-proteid obtained from them is probably the same thing as fibrin-ferment, or it may be the zymogen of that ferment, the addition of a calcium salt converting it into the ferment. The protoplasm of these cells often contains small quantities of fat and glycogen. The red blood corpuscles.-1000 parts of red corpuscles contain :- Water 688 parts. Solids /Organic 303'88 „ 8'12 „ One hundred parts of the dry organic matter contain- Proteid 5 to 12 parts. Haemoglobin . . . . . . . 86 to 94 „ Lecithin . . . . . . . r8 „ Cholesterin . . . . . . . cri „ P I The proteid present is identical with the nucleo-proteid of white corpuscles. The mineral matter consists chiefly of chlorides of potassium and sodium, and phosphates of calcium and magnesium. In man potassium chloride is more abundant than sodium chloride; this, however, does not hold good for all animals. Oxygen is contained in combination with the haemoglobin to form oxyhaemoglobin. The corpuscles also contain a certain amount of carbonic acid. 588 THE BLOOD. [ch. XLI. Haemoglobin and Oxyhaemoglobin. -The pigment is by far the most abundant and important of the constituents of the red corpuscles. It is a substance which gives the reactions of a proteid, but differs from other proteids in containing the element iron, and in being crystallisable. Haemoglobin and oxyhaemoglobin are both crystallisable; like other proteids they have enormously large molecules, and so are indiffusible. Though crystalline they are therefore not crystalloid in Graham's sense of that term (see p. 558). Blood pigment is, however, not the only crystallisablc proteid. Long ago crystals of proteid (globulin or vitellin) were observed in the aleurone grains of many seeds, and in the similar proteid occurring in the egg-yolk of some fishes and amphibians. By appropriate methods these have been separated and recrystallised. The crystals do not appear to be pure proteid, but compounds with some inorganic substance like lime or magnesia. Further, egg-albumin itself has been crystallised. If a solution of white of egg is diluted with half its volume of saturated solution of ammonium sulphate, the globulin present is precipitated, and is removed by filtration. The filtrate is now allowed to remain some days at the temperature of the air, and as it becomes more concentrated from evaporation, minute spheroidal globules of varying size, and finally minute needles, either aggregated or separate, make their appearance. These are composed of a compound of egg-albumin with ammonium sulphate. It exists in the blood in two conditions : in arterial blood it is combined loosely with oxygen, is of a bright red colour, and is called oxyhaemoglobin ; the other condition is the deoxygenated or reduced haemoglobin (better called simply haemoglobin). This is found in the blood after asphyxia. It also occurs in all venous blood-that is, blood which is returning to the heart after it has supplied the tissues with oxygen. Venous blood, however, always contains a considerable quantity of oxyhaemoglobin also. Haemo- globin is the oxygen-carrier of the body, and it may be called a respiratory pigment.* Crystals of oxyhaemoglobin may be obtained with readiness from the blood of such animals as the rat, guinea-pig, or dog ; with difticidty from other animals such as man, ape, and most of the common mammals. The following methods are the best :- 1. Mix a drop of defibrinated blood of the rat on a slide with a drop of water ; put on a cover glass ; in a few minutes the corpuscles are rendered colourless, and then the oxyhaemoglobin crystallises out from the solution so formed. 2. Microscopical specimens may also be made by Stein's * Iii the blood of invertebrate animals haemoglobin is sometimes found, but usually in the plasma, not in special corpuscles. Sometimes it is replaced by other respiratory pigments, such as the green one, chlorocruorin, found in certain worms, and the blue one, haemocyanin, found in many molluscs and Crustacea. Chlorocruorin contains iron ; haemocyanin contains copper. CH. xli.J BLOOD CRYSTALS. 589 method, which consists in using Canada balsam instead of water in the foregoing experiment. 3. On a larger scale, crystals may be obtained by mixing the blood with one-sixteenth of its volume of ether ; the corpuscles dissolve and the blood assumes a laky appearance. After a period varying from a few minutes to days, abundant crystals are deposited. In nearly all animals the crystals are rhombic prisms (fig. 494); but in the guinea-pig they are rhombic tetrahedia, or four-sided pyramids (fig.495); in the squirrel and hamster hexagonal plates (fig. 496). The crystals contain a varying amount of water of crystallization; this probably explains their different crystalline form and solubilities. Different observers have analysed haemo- globin. They find car- bon, hydrogen, nitrogen, oxygen, sulphur and iron. The percentage of iron is 0'4. The amounts of the other elements are variously given, but roughly they are the same as in the proteids. Hsematin.-On adding an acid or alkali to haemoglobin, it is broken up into two parts-a brown pigment called hoematin, which contains all the iron of the original substance, and a proteid of the globulin class called globin. Haematin is not crystallizable, it has the formula C68H70NsF2O10. Haematin shows Fig. 494.-Crystals of oxyhaemoglobin-prismatic, from human blood. Fig. 495.-Oxyhsvmoglobin crystals-tetrahedral, from blood of the guinea-pig. 590 THE BLOOD. [ch. xli>. different spectroscopic appearances in acid and alkaline solutions (see accompanying plate of spectra). Hsemochromogen is sometimes called reduced lucmatin ; it may be formed by adding a reducing agent like ammonium sulphide to an alkaline solution of hsematin. Its absorption spectrum shown on the accom- panying plate (No. 9), forms the best spectro- scopic test for blood pig- ment ; the suspected pigment is dissolved in potash and ammonium sulphide added. Very dilute specimens show the absorption bands, especially the one mid- way between D and E. Haemin is of great importance, as the ob- taining of this substance forms the best chemical test for blood. Haemin crystals, some- times called after their discoverer Teichmann's crystals, are composed of hydrochloride of hsematin. They may be prepared for microscopical examination by boiling a fragment of dried blood with a drop of glacial acetic acid on a slide; on cooling triclinic plates and prisms of a dark brown Co- lour, often in star-shaped clusters and with rounded angles (fig. 497), separate out. In the case of an old blood stain it is necessary to add a crystal of sodium chloride in addition. Fresh blood contains sufficient sodium chloride in itself. The action of the acetic acid is, (1) to split the haemoglobin into hsematin and globin; and (2) to evolve hydrochloric acid from the sodium chloride. The heematin unites with the hydro- chloric acid and thus haemin is formed. Haematoporphyrin is iron-free haematin ; it may be prepared . 1 , , - , ,. r , „ Fig. 495.-Hexagonal oxyhaemoglobin crystals, from blood of squirrel. ' (After Funke.) Fig. 497.-Haemin crystals. (Frey.) CH. XLI.J COMPOUNDS OF HAEMOGLOBIN. 591 by mixing blood with strong sulphuric acid ; the iron is taken out as ferrous sulphate. This substance is also found sometimes in nature ; it occurs in certain invertebrate pigments, and may also be found in certain forms of pathological urine. Even normal urine contains traces of it. It presents different spectro- scopic appearances according as it is dissolved in acid, neutral or alkaline media. The absorption spectrum figured (No. io) is that of acid haematoporphyrin. Haematoidin.-This substance is found in the form of yellowish crystals (fig. 498) in old blood extravasations, and is derived from the haemoglobin. Its crystalline form and the re- action it gives with fuming nitric acid shows it to be closely allied to Bilirubin, the chief colouring matter of the Bile, and on ana- lysis it is found to be identical with it. Like haematoporphyrin, haema- toidin is free from iron. These two substances are not identical (e.y., haematoidin shows no spectroscopic bands), they are pror bably isomeric. Fig. 498.-Heematoidih crystals. (Frey.) Compounds of Haemoglobin. Haemoglobin forms at least four compounds with gases :- With oxygen 1. Oxyhaemoglobin. 2. Methaemoglobin. With carbonic oxide. . . 3. Carbonic oxide haemoglobin. With nitric oxide . . .4- Nitric oxide haemoglobin. These compounds have similar crystalline forms; they each probably consist of a molecule of haemoglobin combined with one of the gas in question. They part with the combined gas somewhat readily ; they are arranged in order of stability in the above list, the least stable first. Oxyhaemoglobin is the compound that exists in arterial blood. Many of its properties have been already mentioned. The oxygen linked to the haemoglobin, which is removed by the tissues through which the blood circulates, may be called the respiratory oxygen of haemoglobin. The processes that occur in the lungs and tissues, resulting in the oxygenation and deoxy- genation respectively of the haemoglobin, may be imitated outside the body using either blood or pure solutions of haemoglobin. 592 THE BLOOD. [ch. xli. The respiratory oxygen can be removed, for example, in the Torricellian vacuum of a mercurial air-pump, or by passing a neutral gas like hydrogen through the blood, or by the use of reducing agents like ammonium .sulphide or Stokes' reagent.* The older observers estimated that i gramme of haemoglobin will combine with 1'6 c.c. of oxygen, f If any of these methods for reducing oxyhaemoglobin is used, the bright red (arterial) colour of oxyhaemoglobin changes to the purplish (venous) tint of haemoglobin. On once more allowing oxygen to come into contact with the haemoglobin, as by shaking the solution with the air, the bright arterial colour returns. These colour-changes may be more accurately studied with the spectroscope, and the constant position of the absorption bands seen constitutes an important test for blood pigment. It will be first necessary to describe briefly the instrument used. The Spectroscope.-When a ray of white light is passed through a prism, it is refracted or bent at each surface of the prism; the whole ray is, however, not equally bent, but it is split into its constituent colours, which may be allowed to fall on a screen. The band of colours beginning with the red, passing through orange, yellow, green, blue, and ending with violet, is called a spectrum : this is seen in nature in the rainbow. It may be obtained artificially by the glass prism or prisms of a spectro- scope. The spectrum of sunlight is interrupted by numerous dark lines crossing it vertically called Frauenhofer's lines. These are per- fectly constant in position and serve as landmarks in the spectrum. The more prominent are A, B, and C, in the red; D, in the yellow ; E, 6, and F, in the green; (I and H, in the violet. These lines are due to certain volatile substances in the solar atmosphere. If the light from burning sodium or its compounds be examined spectroscopically, it will be found to give a bright yellow line, or, rather, two bright yellow lines very close together. Potassium gives two bright red lines and one ••violet line ; and the other elements, when incandescent, give characteristic lines, but none so simple as sodium. If now the flame of a lamp be examined, it will be found to give a continuous spectrum like that of sunlight in the arrangement of its colours, but unlike it in the absence of dark lines ; but if the light from the lamp be * Stokes' reagent must always be freshly prepared ; it is a solution of ferrous sulphate to which a little tartaric acid has been added, and then ammonia till the reaction is alkaline. f Bohr has recently stated that oxyhaemoglobin is a mixture of several compounds of haemoglobin with different amounts of oxygen in each. BLOOD-SPECTRA COMPARED WITH SPECTRUM OF ARGAND-LAMP 1 Spectrum of Arf> and.-lamp with Fraunhofer's line's in position 2 Spectrum of Oxyhaemoglobin in diluted blood, 3 Spectrum of Reduced. Haemoglobin. 4- Spectrum of Carbonic oxide Hemoglobin. 5 Spectrum of Acid Haematin in ethenal solution. 6 Spectrum of Allraline Haematin 7 Spectrum of Chloroform extract of acidulated. Ox-bile. 8 Spectrum of Methaemoglobin. 9 Spectrum of Haemo chromo gen 10 Spectrum of Haematoporphynn Most of the above Spectra have been drawn from observations by M-WLepraik FCS. Paiaelsson iCcmp .Lcnlon.,Iith. CH. XLI. 1 the; spectroscope. 593 made to pass through sodium vapour > before it reaches the spectroscope, the bright yellow light will be found absent, and in its place a dark line, or, rather, two dark dines very close together, occupying the same position as the two bright lines of the sodium spectrum. The sodium vapour thus absorbs the same rays as those which it itself produces at a higher temperature. Thus the D line, as we term it in the solar spectrum, is due to the presence of sodium vapour in the solar atmosphere. The other dark lines are similarly accounted for by other elements. The large form of spectroscope (fig. 499) consists of a tube A, called the collimator, with a slit at the end S, and a convex lens at the end L. The latter makes the rays of light passing through Fig. 499.-Diagram of spectroscope. the slit from the source of light parallel: they fall on the prism P, and then the spectrum so formed is focussed by the telescope T. A third tube, not shown in the figure, carries a small trans- parent scale of wave-lengths, as in accurate observations the position of any point in the spectrum is given in the terms of the corresponding wave-lengths. If we now interpose between the source of light and the slit S a piece of coloured glass (H in fig. 499), or a solution of a coloured substance contained in a vessel with parallel sides (the haemato- scope of Herrmann), the spectrum is found to be no longer continuous, but is interrupted by a number of dark shadows, or absorption bands corresponding to the light absorbed by the coloured medium. Thus a solution of oxyhaemoglobin of a certain strength gives two bands between the D and E lines; haemoglobin gives only one; and other red solutions, though to the naked eye similar to oxyhaemoglobin, will give characteristic bands in other positions. 594 THE BLOOD. [ch. xli. A convenient form of small spectroscope is the direct vision spectroscope, in which, by an arrangement of alternating prisms of crown and flint glass, the spectrum is observed by the eye in the same line as the tube furnished with the slit-indeed slit and prisms are both contained in the same tube. In the examination of the spectrum of small coloured objects a combination of the microscope and direct vision spectroscope, called the micro-spectroscope, is used. The next figure illustrates a method of representing absorption spectra diagrammatically. The solution was examined in a layer 1 centimetre thick. The base line has on it at the proper dis- Fig. 500.-Graphic representations of the amount of absorption of light by solution of (I) oxyhaemoglobin, (II) of hsemoglobin, of different strengths. The shading indicates the amount of absorption of the spectrum ; the figures on the right border express percentages. (Rollett.) tances the chief Frauenhofer lines, and along the right-hand edges are percentages of the amount of oxyhaemoglobin present in I of haemoglobin in II. The width of the shadings at each level represents the position and amount of absorption corre- sponding to the percentages. The characteristic spectrum of oxyhaemoglobin, as it actually appears through the spectroscope, is seen in the accompanying plate (spectrum 2). There are two distinct absorption bands between the D and E lines ; the one nearest to D (the a band) being narrower, darker, and with better-defined edges than the other (the /3 band). As will be seen on looking at fig. 500, a solution of oxyhaemoglobin of concentration greater than 0'65 per cent, and less than 0'85 per cent, (examined in a cell of the usual thickness of 1 centimetre) gives one thick band over- lapping both D and E, and a stronger solution only lets the red CH. XLI.] COMPOUNDS OF HAEMOGLOBIN. 595 light through between C and D. A solution which gives the two characteristic bands must therefore be a dilute one. The one band (y band) of haemoglobin (spectrum 3) is not so well defined as the a or /3 bands. On dilution it fades rapidly, so that in a solution of such strength that both bands of oxyhaemoglobin would be quite distinct the single band of haemoglobin has dis- appeared from view. The oxyhaemoglobin bands can be dis- tinguished in a solution which contains only one part of the pigment to 10,000 of water, and even in more dilute solutions which seem to be colourless the a band is still visible. Methaemoglobin.-This may be produced artificially in various ways, as by adding potassium ferricyanide or amyl nitrite to blood, and as it also may occur in certain diseased conditions in the urine, it is of considerable practical importance. It can be crystallised, and is found to contain the same amount of oxygen as oxyhaemoglobin, only combined more firmly. The oxygen is not removable by the air-pump, nor by a stream of neutral gas like hydrogen. It can, however, by reducing agents like am- monium sulphide, be made to yield haemoglobin. Methaemoglobin is of a brownish-red colour, and gives a characteristic absorption band in the red between the C and D lines (spectrum 8). Carbonic oxide haemoglobin may be readily prepared by passing a stream of carbonic oxide through blood or through a solution of oxyhaemoglobin. It has a peculiar cherry-red colour. Its absorption spectrum is very like that of oxyhaemoglobin, but the two bands are slightly nearer the violet end of the spectrum (spectrum 4). Reducing agents, like ammonium sulphide, do not change it; the gas is more firmly combined than the oxygen in oxyhaemoglobin. CO-haemoglobin forms crystals like those of oxyhaemoglobin. It resists putrefaction for a very long time. Carbonic oxide is given off during the imperfect combustion of carbon such as occurs in charcoal stoves or during the explosions that occur in coal mines: it acts as a powerful poison by combining with the haemoglobin of the blood, and thus inter- fering with normal respiratory processes. The bright colour of the blood in both arteries and veins and its resistance to reducing- agents are in such cases characteristic. Nitric Oxide Haemoglobin.-When ammonia is added to blood, and then a stream of nitric oxide passed through it, this compound is formed. It may be obtained in crystals isomorphous with oxy- and CO-haemoglobin. It also has a similar spectrum. It is even more stable than CO-haemoglobin; it has no practical interest, but is only of theoretical importance as completing the series. 596 THE BI100D.. [ch. xli. Haemoglobin and its compounds also give an absorption band in the ultra- violet. This can be shown by using very dilute solutions and photographing the spectrum. HbO shows a band between G and H. In Hb, HbCO, and HbNO this is rather nearer G. Methaemoglobin and haematoporphyriu show similar bands (Gamgee). Recently C. Bohr has advanced the theory that haemoglobin forms a com- pound with carbonic acid. He considers that the union is, like oxyhaemo- globin, a dissociable one, and that dissociation leading to evolution of the gas takes place in the blood-vessels of the pulmonary alveoli. If this is really the case, haemoglobin appears to be not only an oxygen carrier but a carbonic-acid carrier. It has long been known that the red corpuscles contain carbonic acid, but it has always been supposed that this was nob in actual combination with the pigment ; and Bohr does not consider that the gas is united to the haemoglobin in the same way as oxygen is. Perhaps it Fig. 501.-Hsemoglobinometer of Dr. Gowers. may be united to the proteid globin rather than to the iron-containing constituent. The subject, however, cannot yet be considered settled. CO2-hsemoglobin, if it does exist, shows no spectroscopic differences from haemoglobin ; and no one disputes that there is, in addition to the carbonic acid of the corpuscles, a much larger amount dissolved in the plasma, chiefly in the form of carbonates. Estimation, of Haemoglobin.-The most exact method is by the estima- tion of the amount of iron (dry haemoglobin containing -42 per cent, of iron) in the ash of a given specimen of blood, but as this is a somewhat complicated process, various colorimetric methods have been proposed which, though not so exact, have the advantage of simplicity. Gowers's Hsemoglobinometer.-The apparatus consists of two glass tubes of the same size. One contains glycerine jelly tinted with carmine to a standard colour-viz. that of normal blood diluted 100 times with distilled water. The finger is pricked and 20 cubic millimetres of blood are measured out by the capillary pipette, B. This is blown out into the other tube and diluted with distilled water, added drop by drop from the pipette stopper of the bottle, A, until the tint of the diluted blood reaches the standard colour. This tube is graduated into 100 parts. If the tint, of the CH. XLI.] H2EM0GL0BIN0METER. 597 diluted blood is the same as the standard when the tube is filled up to the graduation 100, the quantity of oxyhaemoglobin in the blood is normal. If it has to be diluted more largely, the oxyhaemoglobin is in excess ; if to a smaller extent, it is less than normal. If the blood has, for instance, to be diluted up to the graduation 50, the amount of haemoglobin is only half what it ought to be-50 per cent, of the normal-and so for other percentages. Von Fleischl's Haemometer,-The apparatus (fig. 502) consists of a stand bearing a white reflecting surface (S) and a platform. Under the platform is a slot carrying a glass wedge stained red (K) and moved by a wheel (R). On the platform is a small cylindrical vessel divided vertically into two compartments, a and a'. ■Fill wifh a pipette the compartment a' over the wedge with distilled water, Fill q.bout a quarter pf the, other compartment (a) with distilled water. Fig. 502.-Fleischl's Hremoglobinometer. Prick the finger and fill the short capillary pipette provided with the instrument with blood. Dissolve this in the water in compartment «, and fill it up with distilled water. Having arranged the reflector (S) to throw light vertically through both compartments, look down through them, and move the wedge of glass by the milled head (T) until the colour of the two is identical. Read off the scale, which is so constructed as to give the percentage of haemoglobin. Dr. George Oliver's Method consists in examining a specimen of blood suitably diluted in a shallow white palette with a number of standard tests very carefully prepared by the use of Lovibond's coloured glasses. These standards are much better matches for blood in various degrees of dilution than in the other colorimetric methods. The yellow tint of diluted haemo- globin is very successfully imitated. Tests for Blood.-These may be gathered, from preceding descriptions. Briefly, they are microscopic, spectroscopic, and chemical. The best chemibal test is the formation of hsemin 598 THE ALIMENTARY CANAL. [ch. xlil crystals. The old test with tincture of guaiacum and hydrogen peroxide, the blood causing the red tincture to become green, is very untrustworthy, as it is also given by many othei' organic substances. In medico-legal cases it is often necessary to ascertain whether or not a red fluid or stain upon clothing is or is not blood. In any such case it is advisable not to rely upon one test only, but to try every means of detection at one's disposal. To discover whether it is blood or not is by no means a difficult problem, but to distinguish human blood from that of the common mam- mals is practically impossible. CHAPTER XLIL The alimentary canal consists of a long tube beginning at the mouth, and terminating at the anus. It comprises the mouth, pharynx, oesophagus, stomach, small intestine and large intestine. Opening into it are numerous glands which pour juices into it ; these bring about the digestion of the food as it passes along. Some of the glands like the gastric and intestinal glands are situated in the lining mucous membrane of the canal; others like the salivary glands, liver, and pancreas, are situated at a distance from the main canal, and pour their secretion into it by means of side tubes or ducts. The events that take place in the alimentary canal are, (i) digestion, that is the conversion of the food into soluble sub- stances; and (2) absorption, that is the passage of these soluble materials into the blood or lymph in the vessels of the wall of the canal. Digestion is a series of chemical actions produced by the digestive juices on the food. We shall therefore have to study the composition of the food as a preliminary to the consideration of their digestion. In addition to chemical processes, there are a number of mechanical actions such as mastication, deglutition, peristalsis, which we shall reserve for a separate chapter. In the present chapter we shall take the structure of the alimentary canal, reserving, however, a detailed study of the glands until we consider the action of their secretions. THE ALIMENTARY CANAL. CH. XLII.J THE MOUTH AND PHARYNX. 599 The Mouth This cavity is lined by a mucous membrane consisting of a corium of fibrous tissue with numerous patches of lymphoid tissue in it, especially in the posterior regions ; and an epithelium of the stratified variety closely resembling the epidermis. The surface layers, like those of the epidermis, are made of horny scales. Opening into the mouth are a large number of little mucous glands, and the salivary glands pour their secretion into the mouth also. The tongue (p. 291) and teeth (p. 70) have been previously studied. The Pharynx. That portion of the alimentary canal which intervenes between the month and the oesophagus is termed the Pharynx. It is constructed of a series of three mus- cles with striated fibres {constrictors), which are covered by a thin fascia ex- ternally, and are lined internally by a strong fascia (pharyngeal aponeurosis), on the inner aspect of which is areolar (submucous) tissue and mucous mem- brane, continuous with that of the mouth, and, as regards the part concerned in swallowing, is identical with it in general structure. The epithelium of this part of the pharynx, like that of the mouth, is stratified. The upper portion of the pharynx into which the nares open is lined with ciliated epithelium. The pharynx is well supplied with mucous glands. Between the anterior and posterior arches of the soft palate are ituated the Tonsils, one on each side. A tonsil consists of an elevation of the mucous membrane presenting 12 to 15 orifices, which lead into crypts or recesses, in the walls of which are placed nodules of lymphoid tissue (fig. 504). These nodules are enveloped in a less dense adenoid tissue which reaches the mucous surface. The surface is covered with stratified epithelium, and the corium may present rudimentary papillae formed of adenoid tissue. The tonsil is bounded beneath by a fibrous capsule (fig. 504, 4). Into the crypts open the ducts of numerous mucous glands. Fig. 503.-Lingual follicle or crypt, a, involution of mucous membrane with its papillae; ft, lymphoid tissue, with several lym- phoid nodules. (Frey.) 600 THE ALIMENTARY CANAL. [ch. xlii, The CEsophagus or Gullet. The (Esophagus or Gullet, the narrowest portion of the ali- mentary canal, is a muscular tube, nine or ten inches in length, 'which extends from the lower end of the pharynx to the cardiac orifice of the stomach. Structure,-The oesophagus is made up of three coats-viz., the outer muscular; the middle, submucous; and the inner, Tunica propria. Fig. 504.-Vertical section through,a crypt of the human tonsil. 1, entrance to the crypt • 2 and 3, the framework of adenoid tissue; 4, the enclosing fibrous tissue ; a and lymphoid nodules ; 5 and 6, hlood-vessels. (Stdhr.) mucous. The muscular coat is covered externally by a varying amount of loose fibrous tissue. It is composed of two layers of fibres, the outer being arranged longitudinally, and the inner circularly. At the upper part of the oesophagus this coat is made up principally of striated muscle fibres ; they are con- tinuous with the constrictor muscles of the pharynx; but lower down the unstriated fibres become more and more numerous, and towards the end of the tube form the entire coat. The muscular coat is connected with the mucous coat by a more or less developed layer of areolar tissue, which forms the submucous coat (fig. 505), in which is contained in the lower half or third of the tube many mucous glands, the ducts of which, passing through the mucous membrane, open on its surface. Separating this coat from the mucous membrane proper is a well-developed layer of CH. XLII.J (ESOPHAGUS AND STOMACH. 601 longitudinally arranged unstriated muscle, called the muscularis mucosce. The corium of mucous membrane is composed of fine connective tissue, which, towards the surface, is elevated into papillae. It is covered with a stratified epithelium, of which the Fig. 505.-Section of the mucous membrane of the oesophagus. most superficial layers are squamous. The epithelium is arranged upon a basement membrane. In newly-born children the coriuni exhibits, in many parts, the structure of lymphoid tissue (Klein). The Stomach. In man and those Mammalia which are provided with a single stomach, it consists of a dilatation of the alimentary canal placed between and continuous with the oesophagus, which enters its larger or cardiac end on the one hand, and the small intes- tine, which commences at its narrowed end or pylorus, on the other. It varies in shape and size according to its state of distension. 602 THE ALIMENTARY CANAL. [ch. XLII. Structure.-The stomach is composed of four coats, called respectively-(i) an external or peritoneal, (2) muscular, (3) sub- mucous, and (4) mucous coat; with blood-vessels, lymphatics, and nerves distributed in and be- tween them. (1) The peritoneal coat has the structure of serous membranes in general. (2) The muscular coat consists of three sepa- rate layers or sets of fibres, which, according to their several directions, are named the longitu- dinal, circular, and ob- lique. The longitudinal set are the most super- ficial : they are continuous with the longitudinal fibres of the oesophagus and spread out in a di- verging manner over the cardiac end and sides of the stomach. They ex- tend as far as the pylorus, being especially distinct at the lesser or upper curvature of the stomach, along which they pass in several strong bands. The next set, the circular or transverse fibres, is most abundant at the middle and in the pyloric portion of the organ, and form the chief part of the thick projecting ring of the pylorus. They are con- tinuous with the circular layer of the intestine. The deepest set of fibres is the oblique, con- tinuous with the circular muscular fibres of the oesophagus: they are comparatively few in number, and are found only at the cardiac Fig. 506.-From a vertical section through the mu- cous membrane of the cardiac end of stomach. Two glands are shown with a duct common to both, a, duct with columnar epithelium be- coming shorter as the cells are traced down- ward ; n, neck of gland tubes, with central and parietal cells ; b, fundus with curved ceecal ex- tremity-the parietal cells are not so numerous here. (Klein and Noble Smith.) CH. XLII.J STRUCTURE OF THE STOMACH. 603 portion of the stomach; they form a sphincter around the cardiac orifice. The muscular fibres of the stomach and of the intestinal canal are unstriated, being composed of elongated, spindle-shaped fibre-cells. (3) The submucous coat consists of loose areolar tissue, which connects the muscular coat to the mucous membrane. It con- tains blood-vessels and nerves; in the contracted state of the stomach, it is thrown into numerous, chiefly longitudinal, folds or rugae, which disappear when the organ is distended. (4) The mucous membrane is composed of a corium of fine connective tissue, which approaches closely in structure to adenoid tissue; this tissue supports the tubular glands of which the superficial and chief part of the mucous membrane is composed, and passing up between them assists in binding them together. The glands are separated from the rest of the mucous membrane by a very fine homogeneous base- ment membrane. The corium is covered with a layer of columnar epithelium, which passes down into the mouths of the glands. At the deepest part of the mucous membrane are two thin layers (circular and longitudinal) of unstriped muscular fibres, called the muscularis mucosa*, which separate the mucous membrane from the scanty submucous tissue. When examined with a lens, the internal or free surface of the stomach presents a peculiar honeycomb appearance, produced by shallow polygonal depressions, the diameter of which varies generally from to -gd-g-th of an inch (about 125/z); but near the pylorus is as much as of an inch (250/z). In the bottom of these little pits, and to some extent between them, minute openings are visible, which are the orifices of the ducts of perpendicularly arranged tubular glands (fig. 506), imbedded side by side in sets or bundles, on the surface of the mucous membrane, and composing nearly the whole structure. The glands of the mucous membrane are of two varieties, (a) Cardiac, (6) Pyloric. (a) Cardiac glands are found throughout the whole of the cardiac half and fundus of the stomach. They are arranged in groups of four or five, which are separated by a fine connective Fig. 507.-Transverse section through lower part of cardiac glands of a cat. a, parietal cells; b, central cells; c. transverse section of capillaries. (Frey.) 604 THE ALIMENTARY CANAL. [ch. tissue. Two or three tubes open into one duct, which forms about a third of the whole length of the tube and opens on the surface. The ducts are lined with columnar epithelium. Of the gland tube proper, i.e. the part of the gland below the duct, the upper third is the neck and the rest the body. The' heck is narrower, than the body, and is lined with granular polyhedral ■■■> cells which are continuous with the columnar cells of the duct. Between these cells and the basement. mem>- brane of the tubes, are large oval or spherical cells, opaque or granular in appearance, with clear oval nuclei, bulging out the basement membrane; these cells are called parietal cells. They do not form a continuous layer. The body, which is broader than the neck and terminates in a blind extremity or fundus near the muscularis mucosae, is lined by cells continuous with the centra/;.cells of the neck, but longer, more columnar and more transparent. In this part are a few parietal cells of the same kind as in the neck (fig. 506). - >-r (6) Pyloric Glands.-These glands (fig. 508) have much longer ducts than the cardiac glands. Into each duct two or three tubes open by very short and narrow necks, and the body of each tube is branched, wavy, and convoluted. The lumen is large. The ducts are lined with columnar epithelium, and the neck and body with shorter and more granular cubical cells, which correspond with the central cells of the cardiac glands. As they approach the duodenum the pyloric glands become larger, more convoluted and more deeply situated. They are directly continuous with Brunner's glands in the duodenum. Lymphatics.-Lymphatic vessels surround the gland tubes to a greater or less extent. Towards the fundus of the cardial glands are found masses of lymphoid tissue, which may appear as distinct follicles, somewhat like the solitary glands of the small intestine. Fig. 508 .-Section showing the pyloric glands, s, free surface ; d, ducts of pyloric glands; n, neck of same; m, the gland alveoli ; mm, muscularis mucosse. (Klein and Noble Smith.) CH. XIJI.] THE INTESTINES. 605 - .Blood-vessels.-The bipod-vessels of the stomach,' which first break up in the sub-mucOus tissue, send branches upward between the closely packed glandular tubes, anastomosing around them by paeans of a fine capillary network, with oblong meshes. Con- tinuous with this deeper plexus, or prolonged upwards from it, is & more superficial network of larger capillaries, which branch densely around the orifices of the tubes, and form the framework on which are moulded the small elevated ridges of mu- cous membrane bounding the minute, polygonal pits before referred to. From this super- ficial network the veins chiefly take their origin. Thence pass- ing down between the tubes, with no very free connection with the deeper inter-tubular capillary plexus, they open finally into the venous network in the sub-mucous tissue (fig. 5°9)- Nerves.-The nerves of the stomach are derived from the pneumogastric and sympa- thetic, and form two plexuses, one in the sub-mucous and the other between the muscular layers. These plexuses are con- tinuous with those which occur in the same situations in the intestine, and which we shall again refer to there. Fig, 509.-Plan of the blood-vessels of the stomach, as they would be seen in a vertical section, v, arteries, passing up from the vessels of submucous coat; 6, capillaries branching between and around the tubes ; c, superficial plexus of capillaries occupying the ridges of the mucous membrane ; d, vein formed by the union of veins which, having collected the blood of the superficial capillary plexus, are seen passing down between the tubes. (Brinton.) The -Intestines. The Intestinal Canal is divided into two chief portions, named, from their differences in diameter, the small and large intestine- .(fig. 510). These are continuous with each other, and com- municate by means of an opening guarded by a valve, the ileo- ccecal valve, which allows the passage of the products of digestion from the small into the large bowel, but not, under ordinary .circumstances, in the opposite direction. The Small Intestine.-The Small Intestine, the average 606 THE ALIMENTARY CANAL. [ch. xlii. length of which in an adult is about twenty feet, has been divided, for convenience of description, into three portions, viz., the dttodenum, which extends for eight or ten inches beyond the pylorus; the jejunum, which forms two-fifths, and the ileum, which forms three-fifths of the rest of the canal. Structure.-The small intestine, like the stomach, is constructed Fig. 510. of four coats, viz., the serous, muscular, sub-mucous, and mucous. (1.) The serous coat is formed by the visceral layer of the peritoneum, and has the structure of serous membranes in general. (2.) The muscular coats consist of an internal circular and an external longitudinal layer : the former is usually considerably the thicker. Both alike consist of bundles of unstriped muscle CH. XLII.J THE INTESTINES. 607 supported by connective tissue. They are well provided with lymphatic vessels, which form a set distinct from those of the mucous membrane. Between the two muscular coats is a nerve-plexus (Auerbach's Fig. 511.-Horizontal section of a small fragment of the mucous membrane, including one entire crypt of Lieberkuhn and parts of several others. Fig. 512.-Auerbach's nerve-plexus in small intestine. The plexus consists of flbrillated substance, and is made up of trabeeulee of various thicknesses. Ganglion-cells are imbedded in the plexus, the whole of which is enclosed in a nucleated sheath. (Klein.) plexus) (fig. 512), similar in structure to Meissner's (in the sub- mucous tissue), but coarser and with more numerous ganglia. (3.) Between the mucous and muscular coats is the submucous coat, which consists of connective tissue, in which numerous blood- 608 THE ALIMENTARY CANAL. [ch. xlii, vessels and lymphatics ramify. A fine plexus, consisting, mainly of non-medullated nerve-fibres, Meissner's plexus, with ganglion cells at its nodes, occurs in the submucous tissue from the stqmach to the anus. (4.) The mucous membrane is the most important coat in relation to the function of digestion. The following structures, which enter into its composition, may now be successively described :-the valvulce conniventes ; the villi; and the glands. The general structure of the mucous membrane of the intestines resembles that of the stomach, and, like it, is lined on its inner surface by columnar epithelium. Adenoid tissue (fig. 511) enters largely into its construc- tion ; and on its deep surface is the mus- cularis mucosce (m m, fig. 517), the fibres of which are arranged in two layers : the outer longitudinal and the inner circular. Valvulce Conniventes. - The valvulse conniventes (fig. 513) commence in the duodenum, about one or two inches beyond the pylorus, and becoming larger and more numerous immediately beyond the entrance of the bile duct, continue thickly arranged and well developed throughout the jejunum ; then, gradually diminishing in size and number, they cease near the middle of the ileum. They are formed by a doubling inwards of the mucous membrane; the cres- centic, nearly circular, folds thus formed are arranged transversely to the axis of the intestine, but each individual fold seldom extends around more than | or -jj- of the bowel's circumference. Unlike the rugae in the oesophagus and stomach, they do not disappear on distension of the canal. Their function is to afford a largely increased surface for secretion and absorption. They are covered with villi. Villi.-The Villi (figs. 514, 515, and 516) are confined exclu- sively to the mucous membrane of the small intestine. They are minute vascular processes, from to of an inch ('5 to 3 mm.) in length, covering the surface of the mucous membrane, and giving it a peculiar velvety, fleecy appearance. Krause estimates them at fifty to ninety in number in a square line at the upper part of the small intestine, and at forty to seventy in the same area at the lower part. They vary in form even in the Fig. 513.-Piece of small in- testine (previously dis- tended and hardened by alcohol),laid opento show the normal position of the valvules conniventes. CH. XLII.J THE VILLI 609 same animal, and differ according as the lymphatic vessels or lacteals which they contain are empty or full; being usually, in the former case, flat and pointed at their summits, in the latter cylindrical. Each villus consists of a small projection of mucous mem- brane ; its interior consists of fine adenoid tissue, which forms the framework in which the other constituents are con- tained. The surface of the villus is clothed by columnar epithelium, which rests on a fine basement membrane ; while within this are found, reckoning from without inwards, blood-vessels, fibres of Fig. 514.-Vertical section of duode- num, showing a, villi; b, crypts of Lieberkuhn, and c, Brunner's glands in the submucosa s, with ducts, d ; muscularis mucosae, m; and circular muscular coat f. (Schofield.) Fig. 515.-Vertical section of a villus of the small intestine of a cat. a, striated border of the epithelium; b, columnar epithelium; c, goblet cells; d, central lymph-vessel; e, smooth muscular fibres; f, adenoid stroma of the villus in which lymph corpuscles lie. (Klein.) the muscularis mucosae, and a single lymphatic or lacteal vessel sometimes looped or branched (fig. 516). The epithelium is continuous with that lining the other parts of the mucous membrane. The cells are arranged with their long axis radiating from the surface of the villus (fig. 515), and their smaller ends resting on the basement membrane. The free surface of the epithelial cells of the villi, like that of the cells 610 THE ALIMENTARY CANAL. [ch. xlii. which cover the general surface of the mucous membrane, is covered by a fine border which exhibits very delicate striations, whence it derives its name, striated border. Beneath the basement membrane there is a rich supply of bloodrvessels. Two or more minute arteries are distributed within each villus; and from their capillaries, which form Fig. 516.-A. Villus of sheep. B. Villi of man. fSlightly altered from Teichmann.) a dense network, proceed one or two small veins, which pass out at the base of the villus. The layer of the muscularis mucosae in the villus forms a kind of thin hollow cone immediately around the central lacteal, and is, therefore, situate beneath the blood-vessels. It is instrumental in the propulsion of chyle along the lacteal. The lacteal vessel in each villus is the form of commencement of the lymphatic system of vessels in the intestines. It begins almost at the tip of the villus commonly by a dilated extremity. In the larger villi there may be two small lacteal vessels which join (fig. 516), or the lacteals may form a network in the villus. ch. xlil] INTESTINAL GLANDS. 611 Glands.-The glands are of two kinds :-viz., those of Lieber- kuhn and of Brunner. Peyer's patches and the solitary follicles are composed of lymphoid nodules. Though sometimes called glands, they form no external secretion. The glands or crypts of Lieberkuhn are tubular depressions of the intestinal mucous membrane, thickly distributed over the whole surface both of the large and small intestines. In the small intestine they are visible only with the aid of a lens; and their orifices appear as minute dots scattered between the villi. They are larger in the large intestine, and increase in size Fig. 517.-Transverse section through four crypts of Lieberkuhn from the large intestine of the pig. They are lined by columnar epithelial cells, the nuclei being placed in the outer part of the cells. The divisions between the cells are seen as lines radiating from l, the lumen of the crypt; g, epithelial cells, which have become transformed into goblet cells. X 350. (Klein and Noble Smith.) Fig. 518.-A gland of Lieberkuhn in longitudinal sec- tion. (Brinton.) the nearer they approach the anal end of the intestinal tube; and in the rectum their orifices may be visible to the naked eye. In length they vary from to of an inch. Each tubule (fig. 518) is constructed of a fine basement membrane, lined by a layer of columnar epithelium, many of the cells of which are goblet cells. Brunner's glands (fig. 514) are confined to the duodenum; they are most abundant and thickly set at its commencement, and diminish gradually as the duodenum advances. They are situated beneath the muscularis mucosae, imbedded in the submucous tissue; each gland is a branched and convoluted tube, lined with columnar epithelium. In structure they are very similar to the pyloric glands of the stomach, but they are more branched and convoluted and their ducts are longer. The duct of each gland 612 THE ALIMENTARY CANAL. [ch. xlii. passes through the muscularis mucosae, and opens on the surface of the mucous membrane. Peyer's patches are found in greatest abundance in the lower part of the ileum near to the ileo-caecal valve. They consist of aggregated groups of lymphoid nodules; they vary from one to three inches in length, and are about half-an-inch in width, chiefly of an oval form, their long axis parallel with that of the intestine. They are almost always placed opposite the attachment of the mesentery. When the lymphoid nodules occur singly, as they Fig. 519.-Agminate follicles, or Peyer's patch, in a state of distension. X 5. (Boehm.) often do both in small and large intestines, they are called solitary glands, or follicles. The Large Intestine.-The Large Intestine, which in an adult is from about 4 to 6 feet long, is subdivided for descriptive purposes into three portions, viz. :-the caecum, a short wide pouch, communicating with the lower end of the small intes- tine through an opening, guarded by the ileo-ccecal valve ; the colon, continuous with the caecum, which forms the principal part of the large intestine, and is divided into ascending, transverse, and descending portions ; and the rectum, which, after dilating at its lower part, again contracts, and immediately afterwards opens externally through the anus. Attached to the caecum is the small appendix vermiformis. Structure.-Like the small intestine, the large intestine is con- structed of four coats, viz., the serous, muscular, sub-mucous, and mucous. The serous coat has connected with it the small processes of peritoneum containing fat, called appendices epiploicae. The fibres of the muscular coat, like those of the small intestine, are arranged in two layers-the outer longitudinal, CH. XLII.J THE LARGE INTESTINE. 613 the inner circular. In the caecum and colon, the longitudinal fibres, besides being, as in the small intestine, thinly disposed in all parts of the wall of the bowel, are collected, for the most pait, into three strong bands, which, being shorter, from end to end, than the other coats of the intestine, hold the canal in folds, bounding intermediate sacculi. On the division of these bands, the intestine can be drawn out to its full length, and it then assumes an uniformly cylindrical form. In the rectum, the fasciculi Fig- 520.-Transverse section of injected Peyer's patch (from Kulliker). The drawing was taken from a preparation made by Frey : it represents the fine capillary-looped net- work spreading from the surrounding' blood-vessels into the interior of three of the lymphoid nodules from the intestine of the rabbit. of these longitudinal bands spread out and mingle with the other longitudinal fibres, forming with them a thicker layer of fibres than exists on any other part of the intestinal canal. The circular mus- cular fibres are spread over the whole surface of the bowel, but are somewhat more marked in the intervals between the sacculi. Towards the lower end of the rectum they become more numerous, and at the anus they form a strong band called the internal sphincter muscle. The mucous membrane of the large, like that of the small intestine, is lined throughout by columnar epithelium, but, unlike it, is quite destitute of villi, and is not projected in the form of valvulce conniventes. Its general microscopic structure otherwise 614 THE ALIMENTARY CANAL. [ch. xlii. resembles that of the small intestine : and it is bounded below by the muscularis mucosae. The arrangement of ganglia and nerve-fibres in the large intestine resembles that in the small. Glands.-The glands with which the large intestine is provided are simple tubular glands, or glands of Lieberkuhn; they re- semble those of the small intestine, but are somewhat larger and more numerous, and contain a very great number of goblet cells; nodules of adenoid or lymphoid tissue are most numerous in the caecum and vermiform appendix. They resemble in shape and structure the solitary glands of the small intestine. Peyer's patches are not found in the large intestine. Heo-caecal Valve.-The ileo-csecal valve is situate at the place of junction of the small with the large intestine, and guards against any reflux of the contents of the latter into the ileum. It is composed of two semilunar folds of mucous membrane. Each fold is formed by a doubling inwards of the mucous mem- brane, and is strengthened on the outside by some of the circular muscular fibres of the intestine, which are contained between the outer surfaces of the two layers of which each fold is composed. While the circular muscular fibres, however, of the bowel at the junction of the ileum with the caecum are contained between the outer opposed surfaces of the folds of mucous membrane which form the valve, the longitudinal muscular fibres and the peritoneum of the small and large intestine respectively are continuous with each other, without dipping in to follow the circular fibres and the mucous mem- brane. In this manner, therefore, the folding inwards of these two last-named structures is preserved, while on the other hand, by dividing the longitudinal muscular fibres and the peritoneum, the valve can be made to disappear, just as the constrictions between the sacculi of the large intestine can be made to dis- appear by performing a similar operation. The inner surface of the folds is smooth; the mucous membrane of the ileum being continuous with that of the caecum. That surface of each fold which looks towards the small intestine is covered with villi, while that which looks to the caecum has none. When the caecum is distended, the margins of the folds are stretched, and thus are brought into firm apposition one with the other. CH. XLIII.] FOOD. 615 CHAPTER XLIII FOOD. The chief chemical compounds or proximate principles in food are :- 1. Proteids 2. Carbohydrates . . . . 3. Fats -organic. 4. Water 5. Salts inorganic. In milk and in eggs, which form the exclusive food-stuffs of young animals, all varieties of these proximate principles are present in suitable proportions. Hence they are spoken of as perfect foods. Eggs, though a perfect food for the developing bird, contain too little carbohydrate for a mammal. In most vegetable foods carbohydrates are in excess, while in animal food, like meat, the proteids are predominant. In a suitable diet these should be mixed in proper proportions, which must vary for herbivorous and carnivorous animals. A healthy and suitable diet must possess the following cha- racters :- 1. It must contain the proper amount and proportion of the various proximate principles. 2. It must be adapted to the climate; to the age of the individual and to the amount of work done by him. 3. The food must contain not only the necessary amount of proximate principles, but these must be present in a digestible form. As an instance of this, many vegetables (peas, beans, lentils) contain even more proteid than beef or mutton, but are not so nutritious, as they are less digestible, much passing off in the faeces unused. The nutritive value of a diet depends chiefly on the amount of carbon and nitrogen it contains. A man doing a moderate amount of work will eliminate, chiefly from the lungs, in the form of carbonic acid, from 250 to 280 grammes of carbon per diem. During the same time he will eliminate, chiefly in the form of urea in the urine, about 15 to 18 grammes of nitrogen. These substances are derived from the metabolism of the tissues, and various forms of energy, work and heat being the chief, are simultaneously liberated. During muscular exercise the output of carbon greatly increases ; the increased excretion of nitrogen is not nearly so marked. Taking, then, the state of moderate 616 FOOD. [ch. xlhi. exercise, it is necessary that the waste of the tissues should be re- placed by fresh material in the form of food ; and the proportion of carbon to nitrogen should be the same as in the excretions : 250 to 15, or i6'6 to 1. The proportion of carbon to nitrogen in proteid is, however, 53 to 15, or 3'5 to 1. The extra supply of carbon must come from non-nitrogenous food-viz. fat and carbohydrate. Moleschott gives the following daily diet:- Proteid 120 grms. Fat 90 „ Carbohydrate 333 „ Ranke's diet closely resembles Moleschott's ; it is- Proteid 100 grms. Fat 100 „ Carbohydrate ..... 250 „ We shall have to return to the composition of diets again in our study of metabolism; and now we will proceed to consider the principal food-stuffs. Milk. Milk, which we have already spoken of as a perfect food, is only so for yonng children. For those who are older, it is so voluminous that unpleas- antly large quantities of it would have to be taken in the course of the day to ensure the proper supply of nitrogen and carbon. It also contains too little iron (Bunge) : hence children weaned late become ansemic. The microscope reveals that it consists of two parts : a clear fluid and a number of minute particles that float in it. These consist of minute oil glo- bules, varying in diameter from 0'0015 to 0'005 millimetre (fig. 521). The milk secreted during the first few days of lactation is called colostrum. It contains very little caseinogen, but large quantities of globulin instead. Microscopically, cells from the _ „ . , , , , r , Fig. 521.-Globules and molecules of cow's milk, x 400. CH. XLIII.] MILK. 617 acini of the mammary gland are seen, which contain fat globules in their interior; they are called colostrum corpuscles. Reaction and Specific Gravity.-The reaction of fresh cow's milk and of human milk is generally neutral or slightly alkaline. In carnivora the milk is acid. All milk readily turns acid or sour as the result of fermentative change, part of its lactose being transformed into lactic acid. The specific gravity of milk is usually ascertained with the hydrometer. That of normal cow's milk varies from 1028 to 1034. When the milk is skimmed the specific gravity rises, owing to the removal of the light con- stituent, the fat, to 1033 to 1037. In all cases the specific gravity of water, with which other substances are compared, is taken as 1000. Composition. - Frankland gives the following table, con- trasting the milk of women, ass, and cow :- - Woman. Ass. Cow. Proteids (chiefly caseinogen) . Butter (fat) . . . . Lactose Salts Per cent. 2'7 3'5 5-o 0'2 Per cent. 17 i'3 4'5 0'5 Per cent. 4'2 3'8 3'8 07 Hence, in feeding children on cow's milk, it will be necessary to dilute it, and add sugar to make it approximately equal to natural human milk. The Proteids of Milk. - The principal proteid in milk is called caseinogen ; it is precipitable by acids like acetic acid, and also like globulins, by saturation with magnesium sulphate, or half saturation with ammonium sulphate; it is coagulated by rennet to form casein. Cheese consists of casein with the entangled fat. The other proteid in milk is an albumin. It is present in small quantities only ; it differs in some of its properties (specific rotation, coagulation temperature, and solu- bilities) from serum-albumin ; it is called lact-albumin. The Coagulation of Milk.-When milk is allowed to stand the chief change that it undergoes is a conversion of part of its lactose into lactic acid. The acid so formed precipitates a portion of the caseinogen. This, however, must not be confounded with the formation of casein from caseinogen. Sometimes milk does undergo true coagulation. This is produced by certain bacterial growths that act like rennet. Rennet, however, is the agent usually employed for this purpose : it is a ferment secreted by 618 FOOD. [ch. xliii. the stomach, especially by sucking animals, and is generally obtained from the calf. The curd consists of the casein and entangled fat: the liquid residue called whey contains the sugar, salts, and albumin of the milk. There is also a small quantity of a new proteid called whey-protend, which differs from caseinogen by not being con- vertible into casein. It is produced by the decomposition of the caseinogen molecule during the process of curdling. The curd formed in human milk is more finely divided than that in cow's milk; and it is more digestible. In feeding children and invalids on cow's milk, the lumpy condition of the curd may be obviated by the addition of lime water or barley water to the milk. The addition of rennet produces coagulation in milk, provided that a sufficient amount of .calcium salts is present. In blood also the presence of calcium salts is necessary for coagulation. If the calcium salts are precipitated by the addition of potassium oxalate, rennet causes no formation of casein. Caseinogen is often compared to alkali-albumin. The latter, however, does not clot with rennet, and is, unlike caseinogen, readily soluble in acids. Caseinogen is not a globulin, though it is like globulins precipi- tated by neutral salts. It differs from a globulin in not being coagulated by heat. It is a nucleo-proteid ; a compound that is of a proteid, with the proteid-like but phosphorus rich material called nuclein. The Fats of Milk.-The chemical composition of the fat of milk (butter) is very like that of adipose tissue. It consists chiefly of palmitin, stearin, and olein. There are, however, smaller quantities of fats derived from fatty acids lower in the series, especially butyrin and caproin. The relation between these varies somewhat, but the proportion is roughly as follows:- Olein, ; palmitin, ; stearin, ; butyrin, caproin, and caprylin, ttt. The old statement that each fat globule is sur- rounded by a film of caseinogen is not now regarded as true by most authorities. Milk also contains small quantities of lecithin, a phosphorised fat; of cholesterin, an alcohol which resembles fat in its solubilities, and a yellow fatty pigment or lipochrome. Milk Sugar, or Lactose.-This is a saccharose (C12H22O11). Its properties have already been described in Chap. XL. The formation of lactic acid from lactose by the activity of certain fungoid growths gives rise to the souring of stale milk. By inverting ferments milk sugar is changed into dextrose and galactose. When yeast or similar fungi are added, as in the prepara- tion of koumiss, these sugars undergo the alcoholic fermentation. CH. XLIII.] THE MAMMARY GLANDS. 619 The Salts of Milk.-The chief salt present is calcium phos- phate ; a small quantity of magnesium phosphate is also present. The other salts are chiefly chlorides of sodium and potassium. The Mammary Glands. The mammary glands are composed of large divisions or lobes, and these are again divisible into lobules,-the lobules being composed of the con- voluted and dilated subdivisions of the main ducts held together by con- nective-tissue. Covering the general surface of the gland, with the exception Fig. 522.-Dissection of the lower half of the female mamma, during the period of lactation. |.-In the left-hand side of the dissected part the glandular lobes are exposed and partially unravelled; and on the right-hand side, the glandular substance has been removed to show the reticular loculi of the connective-tissue in which the glandular lobules are. placed : 1, upper part of the mamilla or nipple; 2, areola ; 3, subcutaneous masses of fat; 4, reticular loculi of the connective-tissue which support the glandular substance and contain the fatty masses ; 5, one of three lacti- ferous ducts shown passing towards the mamilla where they open; 6, one of the sinus laetei or reservoirs; 7, some of the glandular lobules which have been unravelled ; 7', others massed together. (Luschka.) of the nipple, is a considerable quantity of fat, itself lobulated by shea! hs and processes of areolar tissue (fig. 522) connected both with the skin in front and the gland behind ; the same bond of connection extends also from the under surface of the gland to the sheathing connective-tissue of the great pectoral muscle on which it lies. The main ducts of the gland, fifteen to twenty in number, called the lactiferous ducts, are formed by the union of the smaller (lobular) ducts, and open by small separate orifices through the nipple. At the points of junction of lobular ducts to form lactiferous ducts, and just before these enter the base of the nipple, the ducts are dilated ; and, during the period of active secretion by the gland, the dilatations 620 FOOD. [CH. XL1II. form reservoirs for the milk, which collects in and distends them. The walls of the gland-ducts are formed of areolar with some unstriped muscular tissue, and are lined internally by short columnar and near the nipple by flattened epithelium. The alveoli consist of a basement membrane of flattened cells lined by low columnar epithelium. The nipple is composed of areolar tissue, and contains unstriped muscular fibres. Blood-vessels are also freely supplied to it, so as to give it an erectile structure. On its surface are very sensitive papillfe ; and around it is a small area or areola of pink or dark-tinted skin, on which are to be seen small projections formed by minute secreting glands. Blood-vessels, nerves, and lymphatics are plentifully supplied to the mammary glands ; the calibre of the blood-vessels, as well as the size of the glands, varying very greatly under certain conditions, especially those of pregnancy and lactation. The alveoli of the glands during the secreting periods are found to be lined with very short columnar cells, with nuclei situated towards the centre Fig. 523.-Section of mammary gland of bitch, showing acini, lined with epithelial cells of a short columnar form. X 200. (V. D. Harris.) The edges of the cells towards the lumen may be irregular and jagged, and the remainder of the alveolus is filled up with the materials of the milk. During the intervals between the acts of discharge, the cells of the alveoli elongate towards the lumen, their nuclei divide, and in the part of the cells towards the lumen a collection of oil globules and of other materials takes place. The next stage is that the cells divide and the part of each towards the lumen containing a nucleus and the materials of the secretion, disintegrates and goes to form the solid part of the milk. The- cells also secrete water, salts, and milk sugar. The fat, &c., of milk are not simply picked out from the blood by the secreting cells, but these materials are formed by metabolic processes within the protoplasm of the cells. Tn the earlier days of lactation, epithelial cells only partially transformed are discharged in the secretion : these are termed colostrum corpuscles. It is stated that colostrum possesses a purgative action. During pregnancy the mammary glands undergo changes which are readily observable. They enlarge, become harder, and more distinctly lobulated : the veins on the surface become more prominent. The areola becomes enlarged and dusky, with projecting papillae ; the nipple too be- comes more prominent, and milk can be squeezed from the orifices of the ducts. This is a very gradual process, which commences about the time of conception, and progresses steadily during the whole period of gestation. In Ch. xliii.J EGGS AND MEAT. 621 the gland itself solid columns of cells bud off from the old alveoli to form new alveoli. But these solid columns after a while are converted into tubes by the central cells becoming fatty and being discharged as the colostrum corpuscles above mentioned. After the end of lactation, the mamma gradually returns to its origina size (involution'). The acini, in the early stages of involution, are lined with cells in all degrees of vacuolation. As involution proceeds the acini diminish considerably in size, and at length, instead of a mosaic of lining epithelial cells (twenty to thirty in each acinus), we have five or six nuclei (some with no surrounding protoplasm) lying in an irregular heap within the acinus. During the later stages of involution, large yellow granular cells are to be seen. As the acini diminish in size, the connective-tissue and fatty matter between them increase, and in some animals, when the gland is completely inactive it is found to consist of a thin film of glandular tissue overlying a thick cushion of fat. Many of the products of waste are carried off by the lymphatics. Eggs. In this country the eggs of hens and ducks are those particu- larly selected as food-stuffs. The chief constituent of the shell is calcium carbonate. The white is composed of a richly albuminous fluid enclosed in a network of firmer more fibrous material. The amount of solids is 13'3 per cent.; of this 12'2 is proteid in nature (egg-albumin, with small quantities of egg-globulin), and the remainder is made up of sugar (0'5 per cent.), traces of fats, lecithin and cholesterin, and o-6 per cent, of inorganic salts. The yolk is rich in food materials for the development of the future embryo. In it there are two varieties of yolk-spherules, one kind yellow and opaque (due to admixture with fat and a yellow lipochrome), and the other smaller, transparent and almost colourless : these are proteid in nature, consisting of the nucleo-proteid called vitellin. Small quantities of sugar, lecithin, cholesterin and inorganic salts are also present. The nutritive value of eggs is high, as they are so readily digestible ; but the more an egg is cooked the more insoluble do its proteid constituents become. Meat. This is composed of the muscular and connective (including adipose) tissues of certain animals. The food of some animals is not eaten; in some cases this is a matter of fashion, in others, owing to an unpleasant taste, such as the flesh of carnivora are said to have; and in other cases (e.g. the horse) because it is more lucrative to use the animal as a beast of burden. Meat is the most concentrated and most easily assimilable of nitro- 622 FOOD. [CH. XLIII. genous foods. It is our chief source of nitrogen ; its principal proteid is myosin. In addition to the extractives and salts con- tained in muscle, there is always a certain percentage of fat, even though all visible adipose tissue is dissected off. The fat-cells are placed between the muscular fibres, and the amount of fat so situated varies in different animals; it is particularly abundant in pork; hence the indigestibility of this form of flesh : the fat preventing the gastric juice from obtaining ready access to the muscular fibres. The following table gives the chief substances in some of the principal meats used as food:- Constituents. Ox. Calf. Pig. Horse. Fowl. Pike. Water . ' . 767 75'6 72'6 74'3 70'8 79'3 Solids . . . . 23'3 24'4 27'4 257 29'2 207 Proteids and gelatin* . 2OO 194 19'9 21'6 227 18-3 Fat 1'5 29 6'2 2'5 4'i 07 Carbohydrate 0'6 o-8 o-6 06 1'3 0'9 Salts . , . . 1'2 i'3 ri ro ri o-8 Flour. The best wheat flour is made from the interior of wheat grains, and contains the greater proportion of the starch of the grain and most of the proteid. Whole flour is made from the whole grain minus the husk, and thus contains not only the white interior but also the harder and browner outer portion of the grain. This outer region contains a somewhat larger proportion of the proteids of the grain. Whole flour contains i to 2 per cent, more proteid than the best white flour, but it has the disadvantage of being less readily digested. Brown flour contains a certain amount of bran in addition : it is still less digestible, but is useful as a mild laxative, the insoluble cellulose mechanically irritating the intestinal canal as it passes along. The best flour contains very little sugar. The presence of sugar indicates that germination has commenced in the grains. In the manufacture of malt from barley this is purposely allowed to go on. * The flesh of young animals is richer in gelatin than that of old : thus, 1000 parts of beef yield 6, of veal 50, parts of gelatin. CH. xliil] FLOUR AND BREAD. 623 When mixed with water, wheat flour forms a sticky, adhesive mass called dough. This is due to the formation of gluten, and the forms of grain poor in gluten cannot be made into dough (oats, rice, &c.). Gluten does not exist in the flour as such, but is formed on the addition of water from the pre-existing globulins in the flour. The following table contrasts the composition of some of the more important vegetable foods :- Constituents. Wheat. Barley. Oats. Rice. Lentils. Peas. Potatoes. Water 13'6 13'8 12'4 I3'i 12'5 148 76'0 Proteid . . . . 12'4 iri 10'4 79 24'8 237 2'0 Fat .... 1'4 2'2 5'2 o-9 i'9 I'6 0'2 Starch . . . . 67'9 64'9 57'8 76-5 54'8 49'3 20'6 Cellulose 2'5 5'3 11'2 o6 3-6 7'5 07 Mineral salts . . . i-8 27 3'0 ro 2'4 3'i ro We see from this table- 1. The great quantity of starch always present. 2. The small quantity of fat; that bread is generally eaten with butter is a popular recognition of this fact. 3. Proteid, except in potatoes, is pretty abundant, and espe- cially so in the pidses (lentils, peas, &c.). The proteid in the pulses is not gluten, but consists of vitellin and other globulin- like substances. In the mineral matters in vegetables, salts of potassium and magnesium are, as a rule, more abundant than those of sodium and calcium. Bread. Bread is made by cooking the dough of wheat flour mixed with yeast, salt, and flavouring materials. A ferment in the flour acts at the commencement of the baking, when the temperature of the oven is little over that of the body, and forms dextrin and sugar from the starch, and then the alcoholic fermentation, due to the action of the yeast, begins. The bubbles of carbonic acid, burrowing passages through the bread, make it light and spongy. This enables the digestive juices subsequently to soak into it readily and affect all parts of it. In the later stages of baking, the gas and alcohol are expelled from the bread, the yeast is killed, and a crust forms from the drying of the outer portions of the dough. 624 FOOD. Tch. xliti. White bread contains, in 100 parts, 7 to 10 of proteid, 55 of carbohydrates, one of fat, 2 of salts, and the rest water. Cooking of Food. The cooking of foods is a development of civilisation and serves many useful ends :- 1. It destroys all parasites and danger of infection. This relates not only to bacterial growths, but also to larger parasites, such as tapeworms and trichinae. 2. In the case of vegetable foods it breaks up the starch grains, bursting the cellulose and allowing the digestive juices to come into contact with the granulose. 3. In the case of animal foods it converts the insoluble collagen of the universally distributed connective tissues into the soluble gelatin. The loosening of the fibres is assisted by the formation of steam between them. By thus loosening the binding material, the more important elements of the food, such as muscular fibres, are rendered accessible to the gastric and other juices. Meat before it is cooked is generally kept a certain length of time to allow rigor mortis to pass off. Of the two chief methods of cooking, roasting and boiling, the former is the more economical, as by its means the meat is first surrounded with a coat of coagulated proteid on its exterior, which keeps in the juices to a great extent, letting little else escape but the dripping (fat). Whereas in boiling, unless both bouillon and bouilli are used, there is considerable waste. Cooking, especially boiling, renders the proteids more insoluble than they are in the raw state; but this is counterbalanced by the other advantages that cooking possesses. In making beef tea and similar extracts of meat it is necessary that the meat should be placed in cold water, and this is gradually and carefully warmed. In cooking a joint it is usual to put the meat into boiling water at once, so that the outer part is coagu- lated, and the loss of material minimised. An extremely important point in this connection is that beef tea and similar meat extracts should not be regarded as foods. They are valuable as pleasant stimulating drinks for invalids, but they contain very little of the nutritive material of the meat, their chief constituents, next to water, being the salts and extractives (creatine, creatinine, lactic acid, Ac.) of flesh. Soup contains the extractives of meat, a small proportiQn of CH. XLIII.j ACCESSORIES TO FOOD. 625 the proteids, and the principal part of the gelatin. The gelatin is usually increased by adding bones and fibrous tissue to the stock. It is the presence of this substance which causes the soup when cold to gelatinise. Accessories to Food. Among these must be placed alcohol, the value of which within moderate limits is not as a food but as a stimulant and aid to digestion; condiments (mustard, pepper, ginger, curry powder, &c.) which are stomachic stimulants, the abuse of which is followed by dyspeptic troubles ; and tea, coffee, cocoa, and similar drinks These are stimulants chiefly to the nervous system ; tea, coffee, mate (Paraguay), guarana (Brazil), cola nut (Central Africa), bush tea (South Africa) and a few other plants used in various countries all owe their chief property to an alkaloid called theine or caffeine (C8 H10 Nt 02) ; cocoa to the closely related alkaloid, theobromine (C7 H8 N4 O2) ; coca to cocaine. These alkaloids are all poisonous, and used in excess, even in the form of in- fusions of tea and coffee, produce over-excitement, loss of digestive power, and other disorders well known to physicians. Coffee differs from tea in being rich in aromatic matters; tea contains a bitter principle, tannin ; to avoid the injurious solution of too much tannin tea should only be allowed to infuse (draw) for a few minutes. Cocoa is a valuable food in addition to its stimu- lating properties; it contains about 50 per cent, of fat, and 12 per cent, of proteid. Green vegetables are taken as a palatable adjunct to other foods, rather than for their nutritive properties. Their potassium salts are, however, abundant. Cabbage, turnips, and asparagus contain 80 to 92 water, 1 to 2 proteid, 2 to 4 carbohydrates, and 1 to i'5 cellulose per cent. The small amount of nutriment in most green foods accounts for the large meals made by, and the vast capacity of the alimentary canal of, herbivorous animals. 626 SECRETING GLANDS. [CH. XLIV. CHAPTER XLIV. SECRETING GLANDS. Before passing on to the action of the digestive secretions on foods, it will be well to discuss some of the general aspects of the question, and the varieties of glands by means of which these substances are formed. It is the function of gland cells to produce by the metabolism of their protoplasm certain substances called secretions. These materials are of two kinds; viz., those which are employed for the purpose of serving some ulterior office in the economy, and those which are discharged from the body as useless or injurious. In the former case the separated materials are termed true secre- tions ; in the latter they are termed excretions. The secretions as a rule consist of substances which do not pre- exist in the same form in the blood, but require special cells and a process of elaboration for their formation, e.g., the liver cells for the formation of bile, the mammary gland-cells for the formation of milk. The excretions, on the other hand, commonly consist of substances which exist ready-formed in the blood, and are merely abstracted therefrom. If from any cause, such as extensive disease or extirpation of an excretory organ, the separation of an excretion is prevented, and an accumulation of it in the blood ensues, it frequently escapes through other organs, and may be detected in various fluids of the body. But this is never the case with secretions; for after the removal of the special organ by which each of them is manufactured, the secretion is no longer formed. The circumstances of their formation, and their final destina- tion, are, however, the only particulars in which secretions and excretions can be distinguished ; for, in general, the structure of the parts engaged in eliminating excretions is as complex as that of the parts concerned in the formation of secretions. And since the differences of the two processes of separation, corresponding with those in the several purposes and destinations of the fluids, are not yet ascertained, it will be sufficient to speak in general terms of the process. Every secreting apparatus consists essentially of a layer of secreting cells arranged round a central cavity; they take from the lymph which bathes them the necessary material and trans- form it into the secretion which they pour into the cavity. CH. XLIV.J SECRETING GLANDS. 627 The principal secreting organs are the following :-(1) the serous and synovial membranes ; (2) the mucous membranes with their special glands, e.y., the buccal, gastric and intestinal glands ; (3) the salivary glands and pancreas ; (4) the mammary glands ; (5) the liver; (6) the lacrimal gland; (7) the kidney and skin; and (8) the testes. Serous membranes.-We have already discussed the struc- ture of serous membranes (p. 379), and also the question whether the lymph is a true secretion (p. 486). Fig. 524.-Section of synovial membrane, a, epithelial covering of the elevations of the membrane ; &, underlying tissue containing fat and blood-vessels; c, ligament covered by the synovial membrane. (Cadiat.) The synovial membranes line the joints and the sheaths of tendons and ligaments with which we may include the synovial bursae. The contents of these sacs is called synovia; it lubricates the surfaces of the joint and so ensures an easy movement. Synovia is a rich lymph mucin; and it is this latter con- stituent which gives the gland its viscidity. It is thus a true secretion; and is formed by the epithelial cells which form an imperfect lining to the sac, and which are especially accumulated on the processes of the synovial fringes (fig. 524). A mucous membrane consists of two parts; the epithelium on its surface, and the coritim of connective tissue beneath. The 628 SECRETING GLANDS. [ch. xliv. epithelium generally rests on a basement membrane which is usually composed of clear flattened cells placed edge to edge. The name mucous is derived from the fact that these membranes all secrete mucin, the chief constituent of mucus ; this may be formed from the surface epithelium cells breaking down into goblet cells (see p. 29), or an analogous process may occur in the cells of little glands called mucous glands, situated more or less deeply under the epithelium, and opening on the surface by ducts. Many mucous membranes (e.g., that of the stomach) form other secretions as well. Mucous membranes line all those passages by which internal parts communicate with the exterior, and by which either matters are eliminated from the body or foreign substances taken into it The principal tracts are Gastro-pulmonary and Genito-urinary ; the former being sub-divided into the Digestive and Respiratory tracts. Secreting glands may be classified according to certain types which are the following :-1. The simple tubular gland (a, fig. 525), examples of which are furnished by the crypts of Lieberkuhn, in the intestinal wall. They are simple tubular depressions of the mucous membrane, the wall of which is formed of a basement membrane and is lined with secreting cells arranged as an epithelium. To the same class may be referred the elongated and tortuous sudoriferous glands. 2. The compound tubular glands (d, fig. 525), form another division. These consist of main gland-tubes, which divide and sub-divide. Each gland may be made up of the subdivisions of one or more main tubes. The ultimate subdivisions of the tubes are generally highly convoluted. They are formed of a basement- membrane, lined by epithelium of various forms. The larger tubes may have an outside coating of fibrous, areolar, or muscular tissue. The kidneys and testes are examples of this type. 3. The racemose glands are those in which a number of vesicles or acini are arranged in groups or lobules (c, fig. 525). The Meibomian follicles are examples of this kind of gland. Some glands are of a mixed character, combining some of the characters of the tubular with others of the racemose type; these are called tubulo-racemose or tubulo-acinous glands. These glands differ from each other only in secondary points of structure, but all have the same essential character in consisting of rounded groups of vesicles containing gland-cells, and opening by a common central cavity into minute ducts, which ducts in the large glands con- verge and unite to form larger and larger branches, and at length CH. XLIV.J SECRETING GLANDS. 629 open by one common trunk on a free surface. The larger racemose glands like the salivary glands are called compozmd race- mose glands. On internal secretions, see p. 489. Fig. 525.-Plans of extension of secreting membrane by inversion or recession in form of cavities, a, simple glands, viz., g, straight tube ; h, sac ; i, coiled tube, b, multilo- eular crypts; k, of tubular form; I, saccular, c, racemose, or saccular compound gland; m, entire gland, showing branched duct and lobular structure; n, a lobule, detached with 0, branch of duct proceeding from it. n, compound tubular gland. (Sharpey.) 630 SALIVA. [CH. XLV. CHAPTER XLV. SALIVA. The saliva is formed by three pairs of salivary glands, called the parotid, submaxillary, and sublingual glands. The Salivary Grlands. These are typical secreting glands. They are made up of lobules united by connective tissue. Each lobule is made of a group of tubulo-saccular alveoli or acini, from which a duct passes; this unites with other ducts to form larger and larger tubes, the main duct opening into the mouth. Each alveolus is surrounded by a plexus of capillaries; the lymph which exudes from these is in direct contact with the Fig. 526.-From a section through a salivary gland, a, serous or albuminous alveoli; b, intralobular duct cut transversely. (Klein and Noble Smith.) basement membrane that encloses the alveolus. The basement membrane is lined, by secreting cells which surround the central cavity or lumen. The basement membrane is thin in many places to allow the lymph more ready access to the secreting cells; it is continued along the ducts. The secreting epithelium is composed of a layer of polyhedral cells. The epithelium of the ducts is columnar, except where it passes into an alveolus where it is flattened. The columnar epithelium cells of the ducts exhibit striations in their outer part (see fig. 526); the inner zone of each cell is made of granular protoplasm. CH. XLV.] THE SALIVARY GLANDS. 631 The largest ducts have a wall of connective tissue outside the basement-membrane, and a few unstriated muscular fibres. The secreting cells differ according to the substance they secrete. In alveoli that secrete mucin (such as those in the dog's Vig*. 527--Section of sub-maxillary gland of dog. Showing gland-cells, b, and a duct, a, b, in section. (Kdlliker.) submaxillary, and some of the alveoli in the human submaxillary), the cells after treatment with water or alcohol are clear and swollen (fig. 528); this is the appearance they usually present in Fig. 528.-From a section through a mucous gland in a quiescent state. The alveoli are lined with transparent mucous cells, and outside these are the demilunes. (Heiden- hain.) sections of the organ. But if examined in their natural state by teasing a portion of the fresh gland in serum, they are seen to be occupied by large granules composed of a substance known as mucigen or mucinogen. When the gland is active, mucigen is transformed into mucin and discharged as a clear droplet of that substance into the lumen of the alveolus. Outside these are 632 SALIVA. [ch. xlv. smaller cells containing no mucigen; these marginal cells stain darkly, and generally form crescentic groups (crescents or demi- lunes of Gianuzzi) next to the basement membrane. They become in turn transformed into mucin-bearing cells, when the central cells have disintegrated. In those alveoli which do not secrete mucin, but a watery Fig. 529.-A part of a section through a mucous gland after prolonged electrical stimulrv tion. The alveoli are lined with small granular cells. (Lavdovski.) non-viscid saliva (parotid, and some of the alveoli of the human submaxillary), the cells are filled with small granules of albuminous nature. Such alveoli are called serous or albtiminous, Fig. 530.-Alveoli of parotid gland. A, before secretion ; B, in the first stage of secretion ; C, after prolonged secretion. (Langley.) to distinguish them from the other mucous alveoli we have just described. These yield to the secretion its ferment, ptyalin. The granular substance within the cell is the mother substance of the ferment (zymogen), not the ferment itself. It is converted into the ferment in the act of secretion. We shall study the question of zymogens more fully in connection with the gastric glands and the pancreas when by chemical methods they have been separated from the ferments. In the case of saliva we may CH. XLV.] SECRETORY NERVES. 633 term the zymogen, ptyalinogen provisionally, but it has never been satisfactorily separated chemically from ptyalin. After secretion, as after food or the administration of the drug- called pilocarpine, the cells shrink, they stain more readily, their nuclei become more conspicuous, and the outer part of each cell becomes clear and free from granules (fig. 530). The nerve-fibres which are derived from cranial and sym- pathetic nerves ramify between the gland cells, but have never actually been traced into them. These nerves control and regulate the secretion of saliva. The general truth concerning the existence of secretory nerves, we have already become acquainted with. The subject has been worked out most thoroughly in connection with the salivary glands, particularly the submaxillary gland in dog, rabbit, &c., which we will take first. The Submaxillary and Sublingual G-lands.-These glands receive two sets of nerve-fibres; namely, from the chorda tympani and the sympathetic. The chorda tympani is given off from the seventh cranial nerve in the region of the tympanum.* After quitting the temporal bone it passes downwards and forwards, and joins the lingual nerve, with which it is bound up for a short distance. On leaving the lingual nerve it traverses the svbmaxillary ganglion ; it then runs parallel to the duct of the gland, gives off a branch to the sublingual gland, and others to the tongue. The main nerve enters the hilus of the submaxillary gland, where it traverses a second ganglion concealed within the substance of the gland, and which may be called after its discoverer, Langley's ganglion. The sympathetic branches to these two glands are derived from the plexus around the facial artery, and accompany the arteries which supply the glands. Section of the nerves produces no immediate result; but after a few days an abundant secretion of thin watery saliva takes place; this is called paralytic secretion, and is produced either by the activity of the local nervous mechanism, which is then uncontrolled by impulses from the central nervous system; or else, it is a degenerative effect analogous to the fibrillar contrac- The Secretory Nerves of Salivary G-lands. * Though the chorda tympani is usually spoken of as a branch of the seventh nerve, it is probable that its fibres are really derived from the glosso-pharyngeal, which communicates with the facial in the tympanum. 634 SALIVA. [ch. xlv. tions which occur in degenerating muscles after severance of their nerves. If the operation is performed on one side, the glands of the opposite also show a similar condition, the thin saliva secreted there constituting the antilytic secretion. Stimulation of the peripheral end of the divided chorda tympani produces an abundant secretion of saliva, which is accompanied by vaso-dilatation (see p. 477). Stimulation of the peripheral end of the divided sympathetic causes a -scanty secretion of thick viscid saliva, accompanied by vaso-constriction. The abundant secretion of saliva, which follows stimulation of the chorda tympani, is not merely the result of a filtration of fluid from the blood-vessels, in consequence of the largely increased circulation through them. This is proved by the fact that, wrhen the main duct is obstructed, the pressure within may considerably exceed the blood pressure in the arteries, and also that when into the veins of the animal experimented upon, some atropine has been previously injected, stimulation of the peripheral end of the divided chorda produces all the vascular effects as before, without any secretion of saliva accompanying them. Again, if an animal's head is cut off, and the chorda be rapidly exposed and stimulated with an interrupted current, a secretion of saliva ensues for a short time, although the blood supply is necessarily absent. These experiments serve to prove that the chorda contains two sets of nerve fibres, one set (pasoalilatator) which, -when stimu- lated, causes the vessels to dilate ; while another set, which are paralysed by atropine, directly stimulate the cells themselves to activity, whereby they secrete and discharge the constituents of the saliva which they produce. On the other hand, the sym- pathetic fibres are also of two kinds, vaso-constrictor and secretory, the latter being paralysed by atropine. The chorda tympani nerve is, however, the principal nerve through which efferent impulses proceed from the central nervous system to excite the secretion of these glands. The function of the ganglia has been made out by Langley by the nicotine method (see p. 472). At one time the submaxillary ganglion was supposed to be the seat of reflex action for the secretion. This, however, is not the case. The ganglia are cell- stations on the course of the fibres to the submaxillary and sublingual glands. Nicotine applied locally has the power of paralysing nerve-cells, but not nerve-fibres. If the submaxillary ganglion is painted with nicotine, and the nerve stimulated on the central side of the ganglion, secretion from the submaxillary gland continues, but that from the sublingual gland ceases. The CH. XLV. SECRETORY NERVES. 635 paralysed nerve-cells in the ganglion act as blocks to the propaga- tion of the impulse, not to the submaxillary, but to the sublingual gland. The cell station for the submaxillary fibres is in Langley's ganglion. Parotid. G-land.-This gland also receives two sets of nerve- fibres analogous to those we have studied in connection with the Fig. 531.-Diagrammatic representation of the submaxillary gland of the dog with its nerves and blood-vessels. (This is not intended to illustrate the exact anatomical relations of the several structures.) sm. gid., the submaxillary gland into the duct (sm. d.) of which a cannula has been tied. The sublingual gland and duct and Langley's ganglion are not shown, n. I., n. I.', the lingual or gustatory nerve ; ch. t., ch. t.', the chorda tympani proceeding from the facial nerve, becoming con- joined with the lingual at n. I.', and afterwards diverging and passing to the gland along the duct; sm. gl., submaxillary ganglion with its roots; n. I., the lingual nerve proceeding to the tongue; a. car., the carotid artery, two branches of which, a. sm. a. and r. sm. p. pass to the anterior and posterior parts of the gland ; v. sm., the anterior and posterior veins from the gland ending in v.j., the jugular vein; v. sym., the conjoined vagus and sympathetic trunks ; gl. cer. s., the superior-cervical ganglion, two branches of which forming a plexus, a. f., over the facial artery are distributed (n. sym. sm.) along the two glandular arteries to the anterior and posterior portion of the gland. The arrows indicate the direction taken by the nervous impulses ; during reflex stimulations of the gland they ascend to the brain by the lingual and descend by the chorda tympani. (M. Foster.) submaxillary gland. The principal secretory nerve-fibres are glossopharyngeal in origin; the sympathetic is mainly vaso- constrictor, but in some animals does contain a few secretory fibres also. When secretory nerves are stimulated, the main results are secretion leading to a diminution of the granules in the cells. The accompanying vascular condition determines the quantity of saliva secreted. Electrical changes also accompany secretory 636 SALIVA. [ch. xlv. activity. A rise of temperature is stated by some to occur, but if this is the case it is very slight, so that some observers have not been able to detect it. Reflex Secretion.-Under ordinary circumstances the secre- tion of saliva is a reflex action. The principal afferent nerves are those of taste; but the smell, or sight of food will also'cause "the mouth to water ; " and under certain circumstances, as before vomiting, irritation of the stomach has a similar effect. These sensory nerves stimulate a centre in the medulla from which efferent secretory impulses are reflected along the secretory nerves (chorda tympani, Ac.) to the glands. Extirpation of the Salivary Glands.-These may be removed without any harmful effects in animals. The Saliva. The saliva is the first digestive juice to come in contact with the food. The secretions from the different glands differ some- what in composition, but they are mixed in the mouth, the secretion of the minute mucous glands of the mouth and a certain number of epithelial cells and debris being added to it. The so-called ' salivary corpuscles ' are derived from the glands themselves or from the tonsils. On microscopic examination of mixed saliva a few epithelial scales from the mouth and salivary corpuscles from the salivary glands are seen. The liquid is transparent, slightly opalescent, of slimy consistency, and may contain lumps of nearly pure mucin. On standing it becomes cloudy owing to the precipitation of calcium carbonate, the carbonic acid, which held it in solution as bicarbonate, escaping. Of the three forms of saliva which contribute to the mixture found in the mouth the sublingual is richest in solids (2'75 per cent.). The submaxillary saliva comes next (2'1 to 2'5 per cent.). When artificially obtained by stimulation of nerves in the dog the saliva obtained by stimulation of the sympathetic is richer in solids than that obtained by stimulation of the chorda tympani. The parotid saliva is poorest in total solids (0'3 to o'5 per cent.), and contains no mucin. Mixed saliva contains in man an average of about o'5 per cent, of solids : it is alkaline in reaction, due to the salts in it; and has a specific gravity of 1,002 to 1,006. The solid constituents dissolved in saliva may be classified thus: Organic a. Mucin : this may be precipitated by acetic acid. b. Ptyalin : an amylolytic ferment. e. Proteid : of the nature of a globulin. . d. Potassium sulphocyanide. CH. XLV. ] ACTION OF SALIVA. 637 e. Sodium chloride : the most abundant salt. f. Other salts : sodium carbonate, calcium phosphate and carbonate ; magnesium phosphate ; potassium chloride. Inorganic . The action of saliva is twofold, physical and chemical. The physical use of saliva consists in moistening the mucous membrane of the mouth, assisting the solution of soluble sub- stances in the food, and in virtue of its mucin lubricating the bolus of food to facilitate swallowing. The chemical action of saliva is due to its active principle, ptyalin. This substance belongs to the class of unorganised ferments, and to that special class of unorganised ferments which are called amylolytic (starch splitting) or diastatic (resembling- diastase, the similar ferment in germinating barley and other grains). The starch is first split into dextrin and maltose ; the dextrin is subsequently converted into maltose also : this occurs more quickly with erythro-dextrin, which gives a red colour with iodine, than in the other variety of dextrin called achroo-dextrin, which gives no colour with iodine. Brown and Morris give the following equation :- io(C6H10O5)n+4nH2O [starch] [water] = 4nC12H22O11 + (C6H10O5)n + (C6H10O5)n [maltose] [achroo-dextrin] [erythro-dextrin] Ptyalin acts in a similar way, but more slowly on glycogen : it has no action on cellulose ; hence it is inoperative on uncooked starch grains, in which the cellulose layers are intact. Ptyalin acts best at about the temperature of the body (35- 40°). It acts best in a neutral medium; a small amount of alkali makes but little difference; a very small amount of acid stops its activity. The conversion of starch into sugar by saliva in the stomach stops after 15 to 30 minutes, before any free acid appears ; the acid which is first poured out combining with the proteids in the food, and acid proteid retards salivary action. Free hydrochloric acid immediately destroys ptyalin, so that it does not resume work when the acid is neutralised in the duodenum. 638 THE GASTRIC JUICE. [CH. xlvi. CHAPTER XLVL THE GASTRIC JUICE. The juice secreted by the glands in the mucous membrane of the stomach varies in composition in the different regions, but the mixed gastric juice, as it may be termed, is a solution of a proteo- lytic ferment called pepsin in a saline solution, which also contains a little free hydrochloric acid. The gastric juice can be obtained during the life of an animal by means of a gastric fistula. Gastric fistulae have also been made in human beings, either by accidental injury or by surgical operations. The most celebrated case is that of Alexis St. Martin, a young Canadian, who received a musket wound in the abdomen in 1822. Observations made on him by Dr. Beaumont formed the starting-point for our correct knowledge of the physio- logy of the stomach and its secretion. We now make artificial gastric juice by mixing weak hydro- chloric acid (o'2 per cent.) with the glycerine extract of the stomach of a recently-killed animal. This artificial juice acts like the normal juice. Two kinds of glands are distinguished in the stomach, which differ from each other in their position, in the character of their epithelium, and in their secretion. Their structure will be found described on pp. 602 to 604. We may, however, repeat that the cardiac glands are those situated in the cardiac part of the stomach : their ducts are short, their tubules long in proportion. The latter are filled with polyhedral cells, only a small lumen being left; they are more closely granular than the corresponding- cells in the pyloric glands. They are called principal or central cells. Between them and the basement membrane of the tubule are other cells which stain readily with aniline dyes. They are called parietal or oxyntic cells. The pyloric glands, in the pyloric part of the stomach, have long ducts and short tubules lined with cubical granular cells. There are no parietal cells. The central cells of the cardiac glands and the cells of the pyloric glands are loaded with granules. During secretion they discharge their granules, those that remain being chiefly situated near the lumen, leaving in each cell a clear outer zone. These are the cells that secrete the pepsin. Like secreting cells gener- ally, they select certain materials from the lymph that bathes them ; these materials are worked up by the protoplasmic CH. XLVI.J THE GASTRIC GLANDS. 639 activity of the cells into the secretion, which is then discharged into the lumen of the gland. The most important substance in a digestive secretion is the ferment. In the case of the gastric juice this is pepsin. We can trace an intermediate step in this process by the presence of the granules. The granules are not, however, composed of pepsin, but of a mother-substance which is readily converted into pepsin. We shall find a similar ferment precursor in the cells of the pancreas, and the term zymogen is applied to these ferment precursors. The zymogen in the gastric cells is called pepsinogen. The rennet-ferment or rennin that causes the curdling of milk is distinct from pepsin, but is formed by the same cells. The parietal cells undergo merely a change of size during secretion, being at first somewhat enlarged and after secretion they shrink. They are also called oxyntic cells, because they secrete the hydrochloric acid of the juice. Heidenhain succeeded in making in one dog a cul-de-sac of the fundus, in another of the pyloric region of the stomach ; the former secreted a juice containing both acid and pepsin ; the latter, parietal cells being absent, secreted a viscid alkaline juice containing pepsin. The formation of a free acid from the alkaline blood and lymph is an important problem. There is no doubt that it is formed from the chlorides of the blood and lymph, and of the many theories advanced as to how this is done, Maly's is, on the whole, the most satisfactory. He considers that it originates by the interaction of the calcium chloride with the disodium hydrogen phosphate of the blood, thus :- 2Na2HPO1 + 3CaCl2 = Ca3(PO.1)2 + 4NaCl + 2HC1 [disodium hydrogen phosphate] [calcium chloride] [calcium phosphate] [sodium chloride] [hydro- chloric acid] or more simply by the interaction of sodium chloride and sodium di-hydrogen phosphate, as is shown in the following equation :- NaH2PO4 + NaCl = Na HPO4 + HOI [sodium di- hydrogen phosphate] [sodium chloride] [disodium hydrogen phosphate] [hydro- chloric acid] The sodium dihydrogen phosphate in the above equation is derived from the interaction of the disodium hydrogen phosphate and the carbonic acid of the blood, thus :- Na2HPO1 + C02 + H20 = NaHCO3 + NaH2PO4. But, as Professor Gamgee has pointed out, these reactions can 640 THE GASTRIC JUICE. [ch. XLvi. hardly be considered to occur in the blood generally, but rather in the oxyntic cells, which possess the necessary selective powers in reference to the saline constituents of the blood, and the hydro- chloric acid, as soon as it is- formed, passes into the secretion of the gland in consequence of its high power of diffusion. Composition of Gastric Juice. The following table gives the percentage composition of the gastric juice of man and the dog :- Constituents. Human. Dog- Water 99'44 97'30 Organic substances (chiefly pepsin) .... 032 171 HC1 002 030 CaClo o-oo6 o-o6 NaCl o-i4 0-25 KOI 0'05 0'1 1 NH4C1 - 0-05 Ca3(PO4)„ .... J 0-17 Mg3(PO4)2 .... 0'01 002 FePO ow8 One sees from this how much richer in all constituents the gastric juice of the dog is than that of man. Carnivorous animals have always a more powerful gastric juice than other animals ; they have more work for it to do ; but the great contrast seen in the table is, no doubt, partly due to the fact that the persons from whom it has been possible to collect gastric juice have been invalids. In the foregoing table one also sees the great preponderance of chlorides over other salts : apportioning the total chlorine to the various metals present, that which remains over must be combined with hydrogen to form the free hydrochloric acid of the juice. Pepsin stands apart from nearly all other ferments by requiring an acid medium in order that it may act. Probably a compound of the two substances called pepsin-hydrochloric acid is the really active agent. Other acids may take the place of hydrochloric acid, but none act so well. Lactic acid is often found in gastric juice : this is derived by fermentative processes from the food. Hydrochloric acid is absent in some diseases of the stomach ; the best colour tests for it are the following :- (a) Gunsberg's reagent consists of 2 parts of phloroglucinol, 1 part of vanillin, and 30 parts of rectified spirit. A drop of filtered gastric juice is evaporated with an equal quantity of the reagent. Red crystals form, or if CH. XLVI.] NERVES OF THE STOMACH. 641 much peptone is present, there will be a red paste. The reaction takes place with 1 part of hydrochloric acid in 10,000. The organic acids do not give the reaction. (Z») Tropaeolin test. Drops of a saturated solution of tropaeolin 00 in 94 per cent, methylated spirit are allowed to dry on a porcelain slab at 40° C. A drop of the fluid to be tested is placed on the tropaeolin drop, still at 40° C. ; and if hydrochloric acid is present, a violet spot is left when the fluid has evaporated. A drop of o'oob per cent, hydrochloric acid leaves a distinct mark. Lactic acid is generally detected by making an ethereal extract of the stomach contents, and evaporating the ether. If lactic acid is present it may be identified by the following way :- A solution of dilute ferric chloride and carbolic acid is made as follows :- 10 c.c. of a 4-per-cent. solution of carbolic acid. 20 c.c. of distilled water. 1 drop of the liquor ferri perchloridi of the British Pharmacopoeia. On mixing a solution containing a mere trace (up to I part in 10,000) of lactic acid with this violet solution, it is instantly turned yellow. Larger percentages of other acids (for instance, more than 0'2 per cent, of hydro- chloric acid) are necessary to decolorise the test solution. The Innervation of the Gastric Glands. As long ago as 1852 Bidder and Schmidt showed in a dog with a gastric fistula that the sight of food caused a secretion of gastric juice; and in 1878 Bichet showed that in a man with complete occlusion of the gullet, the act of mastication caused a copious flow of gastric juice. There can therefore have been no doubt that the glands'are under the control of the nervous system, but until quite recently all attempts to discover the secretory nerves of the stomach proved unsuccessful. Pawlow has solved the problem. He experimented on dogs : he first made a gastric fistula; and a few days later exposed the oesophagus, divided it, and sewed the two cut ends to the two corners of the wound in the neck. The animal was fed by means of the lower piece of the oesophagus; but any food taken by the mouth or any saliva secreted into the mouth was never allowed to enter the stomach, but fell out of the opening of the oesophagus in the neck. These animals were kept alive for months and soon accommodated themselves to their new conditions of life. If one of them was kept without food for a few hours, and then given a meal of meat, it devoured it with avidity though none ever reached the stomach. The effect of this sham feeding was a reflex and abundant flow of gastric juice, which commenced about five minutes after the beginning of the meal. If water, milk, or soup was given instead of meat no such secretion occurred. The same phenomena occurred when both splanchnics were 642 THE GASTRIC JUICE. [ch. XLvi. divided. It is therefore evident that the splanchnics are not the secretory nerves. But after division of both vagi (below the point of origin of the recurrent laryngeal nerves to avoid paralysis of the larynx), the reflex secretion ceased, though the dog went through the process of sham feeding with the same avidity as before. The vagi therefore contain the secretory fibres; this conclusion was confirmed by the experiment of stimulation. If the peri- pheral end of a divided vagus is stimulated, however, the usual result is stoppage of the heart; this difficulty was overcome by letting a few days elapse between the division of the nerves, and the experiment of stimulating them. During this fame the cardio- inhibitory fibres degenerated, and then stimulation of the nerve by induction shocks at intervals of one second called forth a flow of gastric juice, but always after a latent period of about five minutes. Pawlow's method enabled him to obtain a gastric juice free from any admixture with saliva or food. The main facts in relation to this pure juice are as follows It is clear and colourless ; it has a specific gravity of 1003 to 1006. It is feebly dextro-rotatory, gives no biuret reaction, but gives the ordinary proteid reactions. It contains from 0'4 to 0'6 per cent, of hydrochloric acid. It is strongly proteolytic and inverts cane sugar. When cooled to o° C. it deposits a fine precipitate of pepsin ; this settles in layers, and the layers first deposited contain most of the acid, which is loosely combined with and carried down by the pepsin. Pepsin is also precipitable by ammonium sulphate (Kiihne). Elementary analysis gave the following results :- Pepsin precipitated by cold- Carbon .... 5073 per cent. Hydrogen . . . . 7-23 „ Chlorine . . 101 to 1'17 „ Sulphur .... 0'98 „ Nitrogen . . . not estimated Oxygen . . . the remainder. Precipitated by Am„S04 50'37' 6-88 0'89 i'34 14'55 to 15 0 the remainder. Action of Gastric Juice. The principal action of the gastric juice consists in converting the proteids of the food into the diffusible peptones. In the case of milk this is preceded by the curdling due to rennet (see p. 617). There is a still further action-that is, the gastric juice is antiseptic; putrefactive processes do not normally occur in the stomach, and the organisms that produce such processes, many of which are swallowed with the food, are in great measure destroyed, and thus the body is protected from them. The acid is the agent in the juice that possesses this power. CH XLVI.J ACTION OF GASTRIC JUICE. 643 The formation of peptones is a process of hydration ; peptones may be formed by other hydrating agencies like super-heated steam and heating with dilute mineral acids. There are certain intermediate steps in this process : the intermediate substances are called pro-peptones or proteoses. The word ' proteose ' is the best to employ : it includes the albumoses (from albumin), globuloses (from globulin), vitelloses (from vitellin), &c. Similar substances are also formed from gelatin (gelatinoses) and elastin (elastoses). Another intermediate step in gastric digestion is called para- peptone : this is acid albumin or syntonin; it also, though with some difficulty, is converted into peptone. The products of digestion may be classified in various ways. It will be convenient to take albumin as our example, remembering that globulin, myosin, and all the other proteids form corresponding products. The products of digestion may be classified, according to their solubilities, as follows :- 1. Parapeptone . . . . . . . . Acid albumin. (a) Proto-albumose (i) Hetero-albumose The primary albumoses, f.e., - those which are formed first. 2. Propeptone (c) Deutero- albumose 3. Peptone. The primary albumoses are precipitated by saturation with magnesium sulphate or sodium chloride. Deutero-albumose is not; it is, however, precipitated by saturation with ammonium sulphate. Proto- and deutero-albumose are soluble in water; hetero-albumose is not; it requires salt to hold it in solution. But a more important classification may be called the physio- logical classification, and it is tiie following :- The albumin molecule may be considered to be made up of two parts called respectively hemi-albumin and anti-albumin. The former yields ultimately hemipeptone, the latter anti- peptone. The intermediate albumoses have similar names :- Albumin Hemi-albumin Anti-albumin Hemi-albumose Anti-albumose and acid albumin Hemipeptone Antipeptone. The hemipeptone differs from the antipeptone in the manner it is 644 THE GASTRIC JUICE. [ch. xlvi. affected by the prolonged action of the next digestive juice, the pancreatic secretion; hemipeptone yields leucine and tyrosine, antipeptone does not. This will be fully discussed in the next chapter. Peptones.-These are the final products of the action of gastric juice on native proteids. They are soluble in water, are not coagulated by heat, and are not precipitated by nitric acid, copper sulphate, ammonium sul- phate, and a number of other precipitants of proteids. They are precipitated but not coagulated by alcohol. They are also precipitated by tannin, picric acid, potassio-mercuric iodide, phospho-molybdic acid, and phospho-tungstic acid. They give the biuret reaction (rose-red with a trace of copper sulphate and caustic potash or soda). Peptone is readily diffusible through animal membranes. The utility of the formation of diffusible substances during digestion is obvious. Proteoses.-These are the intermediate products in the hydration of native proteids into peptones. They are not coagulated by heat; they are precipitated but not coagulated by alcohol : like peptone they give the biuret reaction. Variety of proteid Action of heat Action of alcohol Action of nitric acid Action of ammonium sulphate Action of copper sulphate and caustic potash Diffusi- bility. Albumin Coagulated Precipitated, then coagu- lated Precipitated in the cold ; not readily soluble on heating Precipitated by complete saturation Violet colour Nil Globulin Ditto. Ditto. Ditto. Precipitated by half satu- ration ; also precipitated by MgSOt Ditto. Ditto. Proteoses (albu- moses) Not coagu- lated Precipitated, but not co- agulated Precipitated in the cold; readily solu- ble on heat- ing ; the pre- cipitate re- appears on cooling* Precipitated by saturation Rose-red colour (biuret reaction) Slight Peptones Not coagu- lated Precipitated, but not co- agulated Not precipi- tated Not precipi- tated Rose-red colour (biuret reaction) Great * In the case of deutero-albumose this reaction only occurs in the presence of excess of salt. CH. XLVII.J DIGESTION IN THE INTESTINES. 645 They are precipitated by nitric acid, the precipitate being soluble on heating, and reappearing when the liquid cools. This last is a distinctive property of proteoses. They are slightly diffusible. The preceding table will give us at a glance the chief characters of peptones and proteoses in contrast with those of the native proteids, albumins, and globulins. We see that the main action of the gastric juice is upon the proteids of the food, converting them into more soluble and diffusible products. The fats are not chemically altered in the stomach; their proteid envelopes are, however, dissolved, and the solid fats are melted. Starch is unaffected ; but cane sugar is inverted. CHAPTER XLV1I. DIGESTION IN THE INTESTINES. Here we have to consider the action of pancreatic juice, of bile, and of the succus entericus. The Pancreas. This is a tubulo-racemose gland closely resembling the salivary glands in structure. The principal differences are that the alveoli or acini are more tubular in character ; the connective tissue be- tween them is looser, and in it are small groups of epithelium-like cells, which are supplied by a close net- work of capillaries (fig. 533). The secreting cells of the pancreas are polyhedral. When examined in the fresh condition, or in preparations preserved by osmic acid, their protoplasm is seen to be filled in the inner two-thirds with small granules ; but the outer third is left clear, and stains readily with reagents. Kg. 532.-Section of the pancreas of a dog during digestion, a, alveoli lined with cells, the outer zone of which is well stained with haematoxylin; d, duct lined with short cubical cells, x 350. (Klein and Noble Smith.) 646 DIGESTION IN THE INTESTINES. [ch. XLvii. During secretion the granules are discharged ; the clear zone consequently becomes wider, and the granular zone narrower. These granules indicate the presence of a zymogen which is Fig. 533.-Section of the pancreas of armadillo, showing alveoli and an islet of epithelium in the connective fissile. (V. D. Harris.) called trypsinogen; that is, the precursor of trypsin, the most important ferment of the pancreatic juice. In the centre of each acinus, spindle-shaped cells (centro-acinar cells) are often seen ; their function and origin are unknown. Composition and. Action of Pancreatic Jnice. The pancreatic juice may be obtained by a fistula in animals, a cannula being inserted into the main pancreatic duct; but as in the case of gastric juice, experiments on the pancreatic'secretion are usually performed with an artificial juice made by mixing a weak alkaline solution (1 per cent, sodium carbonate) with a glycerine extract of pancreas. The pancreas should be treated with dilute acid for a few hours before the glycerine is added. This ensures a conversion of the trypsinogen into trypsin. Quantitative analysis of human pancreatic juice gives the following results Water . . . 97'6 per cent. Organic solids. . 1 '8 ,, Inorganic salts . o'6 ,, The organic substances in pancreatic juice are- CH. XLVII.J PANCREATIC JUICE. 647 (а) Ferments. These are the most important both quantita- tively and functionally. They are four in number:- i. Trypsin, a proteolytic ferment. ii. Amylopsin or pancreatic diastase, an amylolytic ferment iii. Steapsin, a fat-splitting ferment. iv. A milk-curdling ferment. (б) A small amount of proteid matter, coagulable by heat. (c) Traces of leucine, tyrosine, xanthine, and soaps. The inorganic substances in pancreatic juice are- Sodium chloride, which is the most abundant, and smaller quantities of potassium chloride, and phosphates of sodium, calcium, and magnesium. The alkalinity of the juice is due to phosphates and carbonates, especially of sodium. The action of pancreatic juice, which is the most powerful and important of all the digestive juices, may be described under the headings of its four ferments. 1. Action of Trypsin.-Trypsin acts like pepsin, but with certain differences, which are as follows :- (а) It acts in an alkaline, pepsin in an acid medium. (б) It acts more rapidly than pepsin, but the same series of proteoses can be detected as intermediate products in the forma- tion of peptone. (c) An albuminate of the nature of alkali-albumin is formed in place of the acid-albumin of gastric digestion. (cZ) It acts more powerfully on certain albuminoids (such as elastin) which are difficult of digestion in gastric juice. (e) Acting on solid proteids like fibrin, it eats them away from the surface to the interior ; there is no preliminary swelling as in gastric digestion. (/)' Trypsin acts further than pepsin, on prolonged action partly decomposing the hemipeptone which has left the stomach into simpler products, of which the most important are leucine and tyrosine. It leaves the antipeptone unaffected. 2. Action of Amylopsin.-The conversion of starch into maltose is the most powerful and rapid of all the actions of the pancreatic juice. It is much more powerful than saliva, and will act even on unboiled starch. The absence of this ferment in the pancreatic juice of infants is an indication that milk, and not starch, is their natural diet. 3. Action on Fats.-The action of pancreatic juice on fats is a double one: it forms an emulsion, and it decomposes the fats into fatty acids and glycerin by means of its fat-splitting ferment steapsin. The fatty acids unite with the alkaline bases to form soaps (saponification). The chemistry of this is described on p. 556. 648 DIGESTION IN THE INTESTINES. [ch. xlvii. The fat-splitting power of pancreatic juice cannot be studied with a glycerine extract, as steapsin is not soluble in glycerine : eithei- the fresh juice or a watery extract of pancreas must be used. The formation of an emulsion may be studied in this way. Shake up olive oil and water together, and allow the mixture to stand; the finely divided oil globules soon separate from and float on the surface of the water; but if a colloid matter like albumin or gum is first mixed with the water, the oil separates more slowly. A more permanent emulsion is formed by an alkaline fluid, and especially when a small amount of free fatty acid is being continually liberated; the acid combines with the alkali to form a soap, which is stated to form a thin layer on the outside of each oil globule, and so prevents them running together again. Pancreatic fluid possesses all the necessary qualifications for forming an emulsion :- i. It is alkaline. ii. It is viscous from the presence of proteid. iii. It has the power of liberating free acids. 4. Milk-curdling Ferment.-The addition of pancreatic- extracts or pancreatic juice to milk causes clotting; but this action (which differs in some particulars from the clotting caused by rennet) can hardly ever be called into play, as the milk upon which the juice has to act has been already curdled by the rennin of the stomach. Intestinal digestion. The pancreatic juice does not act alone on the food in the intestines. There are, in addition, the bile, the succus intericus (secreted by the crypts of Lieberkuhn), and bacterial action to be considered. The bile, as we shall find, has little or no digestive action by itself, but combined with pancreatic juice it assists the latter in all its actions. This is true for the digestion of starch and of proteid, but most markedly so for the digestion of fat. Occlusion of the bile-duct by a gall-stone or by inflammation prevents bile entering the duodenum. Under these circumstances the feeces contain a large amount of undigested fat. The succus entericus appears to have to some extent the power of converting starch into sugar ; whether it acts on proteids or fats is very doubtful; its most important action is due to a ferment it contains called invertin, which inverts saccharoses- that is, it converts cane sugar and maltose into glucose. The original'use of the term "inversion" has been explained on p. 551. It may be extended to include the similar hydrolysis CH. XLVII.J SUCCUS ENTERICUS. 649 of other saccharoses, although there may be no formation of levorotatory substances. Succus entericus has been obtained free from other secretions by means of a fistula. Thiry's method is to cut the intestine across in two places ; the loop so cut out is still supplied with blood and nerves, as its mesentery is intact ; this loop is emptied, one end is sewn up, and the other stitched to the abdominal wound, and so a cul-de-sac from which the secretion can be collected is made. The continuity of the remainder of the intestine is Fig. 534.-Diagram of intestinal fistula. I., Thiry's method; II., Vella's method. A, abdominal wall; B, intestine with mesentery ; C, separated loop of intestine, with attached mesentery. restored by fastening together the upper and lower portions of the bowel from which the loop has been removed. Vella's method resembles Thiry's except that both ends of the loop are sutured to the wound in the abdomen. Fig. 534 illustrates the two methods. Bacterial action.-The gastric juice is an antiseptic; the pancreatic juice is not. A feebly-alkaline fluid like pancreatic juice is just the most suitable medium for bacteria to flourish in. Even in an artificial digestion the fluid is very soon putrid, unless special precautions to exclude or kill bacteria are taken. It is often difficult to say where pancreatic action ends and bacterial action begins, as many of the bacteria that grow in the intestinal contents, having reached that situation in spite of the gastric juice, act in the same way as the pancreatic juice. Some form, sugar from starch, others peptone, leucine, and tyrosine from proteids, while others, again, break up fats. There are, however, certain actions that are entirely due to these putrefactive organisms. i. On carbohydrates. The most frequent fermentation they set up is the lactic acid fermentation : this may go further and result in the formation of carbonic acid, hydrogen, and butyric acid (see p. 552). Cellulose is broken up into carbonic acid and 650 DIGESTION IN THE INTESTINES. [ch. xlvii. methane. This is the chief cause of the gases in the intestine, the amount of which is increased by vegetable food. ii. On fats. In addition to acting like steapsin, lower acids (valeric, butyric, &c.) are produced. The formation of acid products from fats and carbohydrates gives to the intestinal contents an acid reaction. Recent researches show that the contents of the intestine become acid much higher up than was formerly supposed. Organic acids do not, however, hinder pan- creatic digestion. iii. On proteids. Fatty acids and amido-acids, especially leu- cine and tyrosine, are produced; but these putrefactive organisms have a special action in addition, producing substances having an evil odour, like indole (C8H7N), skatole (C9H9N), and phenol (CcH6O). There are also gaseous products in some cases. If excessive, putrefactive processes are harmful; ' if within normal limits, they are useful, helping the pancreatic juice and, further, preventing the entrance into the body of poisonous products. It is possible that, in digestion, poisonous alkaloids are formed. Certainly this is so in one well-known case. Leci- thin, a material contained in small quantities in many foods, and in large quantities in egg-yolk and brain, is broken up by the pancreatic juice into glycerine, phosphoric acid, stearic acid, and an alkaloid called choline. We are, however, protected from the poisonous action of choline by the bacteria, which break it up into carbonic acid, methane, and ammonia. Leucine and Tyrosine. These two substances have been frequently mentioned in the preceding pages. As types of the decomposition products of proteids they are important, though probably only small quanti- ties are normally formed during digestion. They belong to the group of amido-acids. On p. 555, we have given a list of the fatty acids; if we replace one of the hydrogen atoms in a fatty acid by amidogen (NH2), we obtain what is called an amido-acid. Take acetic acid : its formula is C2H4O2; replace one H by NH2, and we get C2H3(NH2)O2, which is amido- acetic acid or glycocine. If we take caproic acid-a term a little higher in the series-its formula is C6H12O2; amido-caproic acid is C(.Hu(NH2)O2, which is also called leucine. Though empirically leucine may be regarded as amido-caproic acid as stated above, recent chemical analysis has shown that it should more properly be regarded as a-amido-isobutylacetic acid (CH3)2CH.CH2.CH(NH2)COOH. CH. XLVII.] NERVES OF THE PANCREAS. 651 Tyrosine is a little more complicated, as it is not only an amido-acid, but also contains an aromatic radicle. Propionic acid has the formula C3H6O2; amido-propionic acid is C3H5(NH2)O2, and is called alanine. If another H in this is replaced by oxy- phenyl (C6H4.OH), we get C3H4(C6Ht.OH)(NH2)O2, which is oxyphenyl-amido-propionic acid, or tyrosine. Leucine and tyrosine are both crystalline; the former crystallises in the form of spheroidal clumps of crystals, the latter in collections of fine silken needles. Secretory Nerves of the Pancreas It has been known since the work of Claude Bernard in 1856 that the introduction of ether into the stomach produces a reflex flow of pancreatic juice, but all attempts to discover the path of the nerve impulses failed until the recent work of Pawlow. The reason of the failure of previous workers is that the pancreas is remarkably sensitive to external conditions. If the pancreas is cooled or wounded during the process of making the fistula, or if sensory nerves are excited, or if anaesthesia is deep, the gland refuses to secrete. Pawlow discovered that the vagus contains the secretory nerves of the pancreas; he took care to avoid the sources of error just referred to. In the first place he stimulated the vagi below the origin of their cardiac branches ; in the second, the spinal cord was divided high up to prevent reflexes occurring from sensory nerves, and lastly, the operation of stimulating the nerve was done without an anaesthetic. In another series of experiments, he cut through one vagus in tb.e neck, and stimulated the peripheral end two or three days later, when the cardio-inhibitory fibres had degenerated : in this way he got rid of the heart stoppage which would have interfered with the normal condition of the animal. In all cases, the stimulation of the vagus produced an abundant flow of pancreatic juice, after a latent period of from fifteen seconds to two minutes. The stimulation applied to the nerve consisted of a slow series of shocks (either induction currents or mechanical blows) about once a second. By this means stimula- tion of vaso-constrictor nerves to the pancreas contained in the vagus is avoided. If the blood supply is diminished by stimula- tion of vaso-constrictor nerves, the secretion is stopped. 652 DIGESTION IN THE INTESTINES. [CH. xlvii. Extirpation of the Pancreas. Complete removal of the pancreas in animals and diseases of the pancreas in man produce a condition of diabetes, in addition to the loss of pancreatic action in the intestines. Grafting the pancreas from another animal into the abdomen of the animal from which the pancreas has been removed relieves the diabetic condition. How the pancreas acts other than in producing the pancreatic juice is not known. It must, however, have other functions related to the general metabolic phenomena of the body, which are disturbed by removal or disease of the gland. This is an illustration of a universal truth-viz., that each part of the body does not merely do its own special work, but is concerned in the great cycle of changes which is called general metabolism. Interference with any organ upsets not only its specific function, but causes disturbances through the body generally. The inter- dependence of the circulatory and respiratory systems is a well- known instance. Removal of the thyroid gland upsets the whole body, producing widespread changes known as myxoedema. Removal of the testis produces not only a loss of the spermatic secretion, but changes the whole growth and appearance of the animal. Removal of the greater part of the kidneys produces rapid wasting and the breaking down of the tissues to form an increased quantity of urea. The precise way in which these glands are related to the general body processes is, however, a subject of which we know as yet very little. The theory at present most in favour is that certain glands produce an internal secretion, which leaves them vid the lymph, and is then dis- tributed to minister to parts elsewhere. The question of the internal secretions of the thyroid and suprarenal capsules is discussed on p. 500. In the case of the pancreas, Professor Schafer has propounded the theory that its internal secretion, stoppage of which in some way leads to diabetes, is produced in the islets of epithelium-like cells scattered through the connective tissue of the organ (see fig. 533). CH. XLVIII.J THE LIVER. 653 CHAPTER XLVIII. THE LIVER. The Liver, the largest gland in the body, situated in the abdomen on the right side chiefly, is an extremely vascular organ, and receives its supply of blood from two distinct sources, viz., from the p>ortal ve^n and from the hepatic artery, while the blood is returned from it into the vena cava inferior by the hepatic veins. Its secretion, the bile, is conveyed. from it by the hepatic duct, either directly into the intestine, or, when digestion is not going on, into the cystic duct, and thence into the gall- bladder, where it accumulates until required. The portal vein, hepatic artery, and hepatic duct branch together throughout the liver, while the hepatic veins and their tributaries run by themselves. On the outside, the liver has an incomplete covering of peri- Fig. 535.-The under surface of the liver, o. b., gall-bladder ; n. u., common bile-duct; h. a., hepatic artery; v. p., portal vein; l. q., lobulus quadratus ; l. s., lobulus spigelii; n. c., lobulus caudatus; n. v., ductus venosus; u. v., umbilical vein. (Noble Smith.) toneum, and beneath this is a very fine coat of areolar tissue, con- tinuous over the whole surface of the organ. It is thickest where the peritoneum is absent, and is continuous on the general surface of the liver with the fine and, in the human subject, almost imperceptible areolar tissue investing the lobules. At the transverse fissure it is merged in the areolar investment called Glisson's capsule, which, surrounding the portal vein, hepatic 654 THE LIVER. Lch. xlviii. artery and hepatic duct, accompanies them in their branchings through the substance of the liver. Structure.-The liver is made up of small roundish or oval portions called lobules, each of which is about of an inch Fig. 536.-A. Liver-cells. B. Ditto, containing various-sized particles of fat. (about i mm.) in diameter, and composed of the liver cells, between which the blood-vessels and bile-vessels ramify. The hepatic cells (fig. 536), which form the glandular or secreting Fig. 537.-Longitudinal section of a portal canal, containing a portal vein, hepatic artery and hepatic duct, from the pig. p, branch of vena portae, situate in a portal canal formed amongst the lobules of the liver ; I, I, and giving off vaginal branches; there are also seen within the large portal vein numerous orifices of the smallest inter- lobular veins arising directly from it; a, hepatic artery; .cl, bile duct. x 5. (Kiernan.) part of the liver, are of a spheroidal form, somewhat polygonal from mutual pressure, about to inch (about to mm.) in diameter, possessing a nucleus, sometimes two. The cell-substance, composed of protoplasm, contains numerous fatty CH. XLVI1I.] THE LIVER. 655 Fig. 538.-Capillary network of the lobules of the rabbit's liver. The figure is taken from a very successful injection of the hepatic veins, made by Harting : it shows nearly the whole of two lobules, and parts of three others ; p, portal branches running in the interlobular spaces ; 7», hepatic veins penetrating and radiating from the centre of the lobules, x 45. (Kiilliker.) Fig. 539--Section of a portion of liver passing- longitudinally through, a considerable hepatic vein, from the pig. n, hepatic venous trunk, against which the sides of the lobules (Z) are applied ; X, h, Ti, sublobular hepatic veins, on which the bases of the lobules rest, and through the coats of which they are seen as polygonal figures; i, mouth of the intralobular veins, opening into the sublobular veins ; i', intralobular veins shown passing up the centre of some divided lobules ; I, I, cut surface of the liver; c, c, walls of the hepatic venous canal, formed by the polygonal bases of the lobules, x 5. (Kiernan.) 656 THE LIVER. [ch. xlviu. particles, as well as a variable amount of glycogen. The cells sometimes exhibit slow amceboid movements. They are held together by a very delicate sustentacular tissue, continuous with the interlobular connective tissue. To understand the distribution of the blood-vessels in the liver, it will be well to trace, first, the two blood-vessels and the duct which enter the organ on the under surface at the transverse fissure, viz., the portal vein, hepatic artery, and hepatic duct. As Fig. 540.--Portion of a lobule of liver, a, bile capillaries between liver-cells, the network in which is well seen ; b, blood capillaries. X 350. (Klein and Noble Smith.) before remarked, all' three run in company, and their appearance on longitudinal section is shown in fig. 537. Running together through the substance of the liver, they are contained in small channels called portal canals, their immediate investment being a sheath of areolar tissue continuous with Glisson's capsule. To take the distribution of the portal vein first:-In its course through the liver this vessel gives off small branches which divide and subdivide Lettveen the lobules surrounding them and limiting them, and from this circumstance called witer-lobular veins. From these small vessels a dense capillary network is prolonged into the substance of the lobule, and this network converges to a single small vein, occupying the centre of the lobule, and hence called mfra-lobular. This arrangement is well seen in fig. 538, which represents a transverse section of a lobule. The small mtfra-lobular veins discharge their contents into veins called s«6-lobular (A h h, fig. 539); while these again, by their union, form the main branches of the hepatic veins, which CH. XLVIII.] THE LIVER. 657 leave the posterior border of the liver to end by two or three principal trunks in the inferior vena cava, just before its passage through the diaphragm. The swi-lobular and hepatic veins, unlike the portal vein ail(l its companions, have little or no areolar tissue around them, and their coats being very thin, they form little more than mere channels in the liver substance which closely surrounds them. The hepatic artery, the chief function of which is to distribute blood for nutrition to Glisson's capsule, the walls of the ducts and blood-vessels, and other parts of the liver, is distributed in a very Fig. 541.-Hepatic cells and bile capillaries, from the liver of a child three months old. Both figures represent fragments of a section carried through the periphery of a lobule. The red corpuscles of the blood are recognized by their circular contour; vp, corresponds to an interlobular vein in immediate proximity with which are the epithelial cells of the biliary ducts, to which, at the lower part of the figures, the much larger hepatic cells suddenly succeed. (E. Hering.) similar manner to the portal vein, its blood being returned by small branches which pass into the capillary plexus of the lobules which connects the inter- and mtfra-lobular veins. The hepatic duct divides and sub-divides in a manner very like that of the portal vein and hepatic artery, the larger branches being lined by columnar, and the smaller by small polygonal epithelium. The bile-capillaries commence between the hepatic cells, and are bounded by a delicate membranous wall of their own. They are always bounded by hepatic cells on all sides, and are thus separated from the nearest blood-capillary by at least the breadth of one cell (figs. 540 and 541). To demonstrate the intercellular network of bile-capillaries, Chrzonszezewsky employed a method of natural injection. A saturated aqueous solution of sulph-indigotate of soda is introduced into the circulation of dogs and pigs by the jugular vein. The animals are killed an hour and a half afterwards, and the blood- 658 THE LIVER. [ch. XLVIII. vessels washed free from blood, or injected with gelatin stained with carmine. The bile-ducts are then seen filled with blue, and the blood-vessels with red material. If the animals are killed sooner than this, the pigment is found within the hepatic cells, thus demonstrating it was through their agency that the canals were filled. Pfliiger and Kupffer have since this shown that the relation between the hepatic cells and the bile-canaliculi is even more intimate, for they have demonstrated the existence of vacuoles in the cells communicating by minute intracellular channels with the adjoining bile-canaliculi (fig. 542). It is important to notice that the bile-canaliculi are always separated by at least a portion of a Fig. 542.-Sketches illustrating the mode of commencement of the bile-canaliculi within the liver-cells (Heidenhain, after Kupffer). A, rabbit'sliver, injected from hepatic duct with Berlin blue. The intercellular canaliculi give off minute twigs which penetrate into the liver-cells, and there terminate in vacuole-like enlargements. B, frog's liver naturally injected with sulph-indigotate of soda. A similar appearance is obtained, but the communicating twigs are ramified. cell from the nearest blood-capillaries, and that the formation of bile is no mere transudation from the blood or lymph. The liver- cells take certain materials from the lymph and elaborate the constituents of the bile, the bile-salts and the bile pigments. There can be no doubt that these substances are formed by the hepatic cells, for they are not found in the blood nor in any other organ or tissue; and after extirpation of the liver they do not accumulate in the blood. Intracellular canaliculi in the liver-cells are not unique. Recent research by Golgi's method has shown that in the salivary and gastric glands, and in the pancreas, there is a similar condition of affairs. The G-all-bladder (g, b, fig. 535) is a pyriform bag, attached CH. XLVIII.] BILE. 659 to the under surface of the liver, and supported also by the peri- toneum, which passes below it. The larger end or/wicfiis, projects beyond the front margin of the liver; while the smaller end contracts into the cystic duct. Structure.-The walls of the gall-bladder are constructed of three principal coats, (i) Externally (excepting that part which is in contact with the liver) is the serous coat, which has the same structure as the peritoneum with which it is continuous. Within this is (2) the fibrous or areolar coat, with which is mingled a considerable number of plain muscular fibres, both longitudinal and circular. (3) Internally the gall-bladder is lined by mucous membrane, and a layer of columnar epithelium. The surface of the mucous membrane presents to the naked eye a minutely honeycombed appearance from a number of tiny polygonal de- pressions with intervening ridges, by which its surface is mapped out. In the cystic duct the mucous membrane is raised up in the form of crescentic folds, which together appear like a spiral valve, and which assist the gall-bladder in retaining the bile during the interval of digestion. The gall-bladder and all the main biliary ducts are provided with mucous glands, which open on the internal surface. Functions of the Liver. The functions of the liver are connected with the general metabolism of the body; these are especially in connection with the metabolism of carbohydrates (glycogenic function); and in connection with the metabolism of nitrogenous material (forma- tion of urea and uric acid). This second function we shall discuss with the urine. The third function is the formation of bile, which must very largely be regarded as a subsidiary function, bile containing the waste products of the liver, the results of its other activities. This, however, it will be convenient to take first. Bile. Bile is the secretion of the liver which is poured into the duodenum ; it has been collected in living animals by means of a biliary fistula ; the same operation has occasionally been performed in human beings. After death the gall-bladder yields a good supply of bile which is more concentrated than that obtained from a fistula. Bile is being continuously poured into the intestine, but there 660 THE LIVER. [ch. xlviii. is an increased discharge immediately on the arrival of food in the duodenum; there is a second increase in secretion a few hours later. Though the chief blood supply of the liver is by a vein (the portal vein), the amount of blood in the liver varies with its needs, being increased during the periods of digestion. This is due to the fact that in the area from which the portal vein collects blood -stomach, intestine, spleen, and pancreas-the arterioles are all dilated, and the capillaries are thus gorged with blood. Further, the active peristalsis of the intestine and the pumping action of the spleen are additional factors in driving more blood onwards to the liver. The bile being secreted from the portal blood is secreted at much lower pressure than one finds in glands whose chief blood supply is arterial, such as the salivary glands. Heidenhain found that the pressure in the bile duct of the dog averaged 15 mm. of mercury, which is about double that in the portal vein. The second increase in the flow of bile-that which occurs some hours after the arrival of the semi-digested food (chyme) in the intestine-appears to be due to the effect of the digestive products carried by the blood to the liver, stimulating the hepatic cells to activity : this is supported by the fact that proteid food increases the quantity of bile secreted, whereas fatty food which is absorbed, not by the portal vein, but by the lacteals, has no such effect. The chemical processes by which the constituents of the bile are formed are obscure. We, however, know that the biliary pigment is produced by the decomposition of haemoglobin. Bili- rubin is, in fact, identical with the iron-free derivative of haemo- globin called haematoidin, which is found in the form of crystals in old blood-clots such as occur in the brain after cerebral haemorrhage (see p. 591). An injection of haemoglobin into the portal vein or of substances like water which liberate haemoglobin from the red blood corpuscles produces an increase of bile pigment. If the spleen takes any part in the elaboration of bile pigment, it does not proceed so far as to liberate haemoglobin from the corpuscles. No free haemo- globin is discoverable in the blood plasma in the splenic vein. The amount of bile secreted is differently estimated by different observers; the amount secreted daily in man varies from 500 c.c. to a litre (1,000 c.c.). The constituents of the bile are the bile salts proper (taurocholate and glycocholate of soda), the bile pigments (bili- rubin, biliverdin), a mucinoid substance, small quantities of fats, CH. XLVIII.J BILE. 661 soaps, cholesterin, lecithin, urea, and mineral salts, of which sodium chloride and the phosphates of iron, calcium, and magne- sium are the most important. Bile is a yellowish, reddish-brown, or green fluid, according to the relative preponderance of its two chief pigments. It has a musk-like odour, a bitter-sweet taste, and a neutral or faintly alkaline reaction. The specific gravity of human bile from the gall-bladder is 1026 to 1032 ; that from a fistula, 1010 to ion. The greater concentration of gall-bladder bile is partly but not wholly ex- plained by the addition to it from the walls of that cavity of the mucinoid material it secretes. The amount of solids in bladder bile is from 9 to 14 per cent., in fistula bile from 1-5 to 3 per cent. The following table shows that this low percentage of solids is almost entirely due to want of bile salts. This can be accounted for in the way first suggested by Schiff-that there is normally a bile circulation going on in the body, a large quantity of the bile salts that pass into the intestine being first split up, then reabsorbed and again secreted. Such a circulation would obviously be impossible in cases where all the bile is discharged to the exterior. The following table gives some important analyses of human bile :- Constituents. Fistula bile (healthy woman. Copeman and Winston). Fistula bile (ease of cancer. Yeo and Herroun). Normal bile (Frerichs). Sodium glycocholate Sodium taurocholate Cholesterin, lecithin, fat . Mucinoid material . Pigment .... Inorganic salts j- 0'6280 { 0'0990 O-I725 0'0725 0'4510 O'l65 0'055 O'O38 j- 0-148 0'878 j 9'14 118 2-98 0-78 Total solids Water (by difference) 1'4230 98-557O 1-284 98-716 14-08 85'92 100'0000 100'000 100'00 Bile Mucin.-There has been considerable diversity of opinion as to whether bile mucin is really mucin. The most recent work in Hammarsten's laboratory shows that differences occur in 662 THE LIVER. [ch. xlviii. different animals. Thus in the ox there is very little true mucin, but a great amount of nucleo-proteid; in human bile, on the other hand, there is very little if any nucleo-proteid : the mucinoid material present there is really mucin. The Bile Salts.-The bile contains the sodium salts of com- plex amido-acids called the bile acids. The two acids most frequently found are glycocholic and taurocholic acids. The former is the more abundant in the bile of man and herbivora ; the latter in carnivorous animals, like the dog. The most im- portant difference between the two acids is that taurocholic acid contains sulphur, glycocholic acid not. G-lyeocholic acid (C26H43NO6) is by the action of dilute acids and alkalis, and also in the intestine, hydrolysed and split into glycocine or amido-acetic acid and cholalic acid. C26HA3NO6 + H20 = c2h3no2 + c24h40o5 [glycocholic acid] [glycocine] [cholalic acid] The glycocholate of soda has the formula C.,GH4.,NaNOG. Taurocholic acid (C,GH43NO7S) similarly splits into taurine or amido-isethionic acid and cholalic acid. c2gh45no7s+h2o=c2h7no3s+c24h40o3 [taurocholic acid] [taurine] [cholalic acid] The taurocholate of soda has the formula C26H41NaNO7S. The colour reaction called Pettenkofer's reaction, is due to the presence of cholalic acid. Small quantities of cane sugar and strong sulphuric acid are added to the bile. The sulphuric acid acting on sugar forms a small quantity of a substance called furfuraldehyde, in addition to other products. The furfuralde- hyde gives a brilliant purple colour with cholalic acid. The Bile Pigments.-The two chief bile pigments are bili- rubin and biliverdin. Bile, which contains chiefly the former (such as dog's bile) is of a golden or orange-yellow colour, while the bile of many herbivora, which contains chiefly biliverdin, is either green or bluish green. Human bile is generally described as containing chiefly bilirubin, but there have been some cases described in which biliverdin was in excess. The bile pigments show no absorption bands with the spectroscope; their origin from the blood pigment has already been stated. Bilirubin has the formula C16H1SN.,O3: it is thus an iron-free derivative of haemoglobin. The iron is apparently stored up in the liver cells, perhaps for future use in the manufacture of new haemoglobin. The bile contains only a trace of iron. CH. XLVIII.] BILE 663 Biliverdin has the formula C16H18N2O4 (f.e. one atom of oxygen more than in bilirubin): it may occur as such in bile ; it may be formed by simply exposing red bile to the oxidising action of the atmosphere; or it may be formed as in Gmelin's test by the more vigorous oxidation produced by fuming nitric acid. Gmelin's test consists in a play of colours-green, blue, red, and finally yellow, produced by the oxidising action of fuming nitric acid (that is, nitric acid containing nitrous acid in solution). The end or yellow product is called choletelin, C10H18N2O6. Hydrobilirubin.-If a solution of bilirubin or biliverdin in dilute alkali is treated with sodium amalgam or allowed to putrefy, a brownish pigment is formed called hydrobilirubin, O32H44N4O7. With the spectroscope it shows a dark absorption band between 6 and F, and a fainter band in the region of the D line. The interest of this substance arises from the fact that many physiologists believe it is identical with stercobilin, the pigment of the faeces, and, according to some, with urobilin, one of the pig- ments of the urine. There are, however, certain differences (of solubility, spectroscopic appearances, <fcc.) which show that the three pigments, though probably closely related, are not abso- lutely identical. Cholesterin.-This substance is contained not only in bile, but very largely in nervous tissues. Like lecithin, it is an abundant constituent of the white substance of Schwann. It is found also in blood corpuscles. In bile it is normally present in small quantities only, but it may occur in excess, and form the concre- tions known as gallstones, which are generally more or less tinged with bilirubin. Though its solubilities remind one of a fat, cholesterin is not a fat. It is, in fact, chemically speaking, a monatomic alcohol. Its formula is C27H43.HO. From alcohol or ether containing water it crystallises in the form of rhombic tables, which contain one molecule of water of crystallisation : these are easily recognised under the microscope (see fig. 543). It gives the following. colour tests :- 1. With iodine and concentrated sulphuric acid the crystals give a play of red, blue, and green. 2. Heated with sulphuric acid and water (5:1) the edges of the crystals turn red. 3. A solution of cholesterin in chloroform, shaken with an equal amount of concentrated sulphuric acid, turns red, and ultimately purple, the subjacent acid acquiring a green fluorescence. 664 THE LIVER. [ch. xlviii. " The mode of origin of cholesterin in the body has not been clearly made out. Whether it is formed in the tissues generally, in the blood, or in the liver, is not known; nor has it been de- termined conclusively that it is derived from albuminous or nervous matter. It is also doubtful if we are to regard it as a waste substance of no use to the body, as its presence in the blood-corpuscles, in nervous matter, in the egg, and in vegetable grains, points to a possible func- tion of a histogenetic or tissue- forming character." (McKen- drick.) A substance called iso-chole- sterin, isomeric with ordinary cholesterin, is found in the fatty secretion of the skin (sebum); it is largely contained in the pre- paration called lanoline made from sheep's-wool fat. It does not give Salkowski's reaction with chloroform and sulphuric acid just described. The Uses of Bile.-One of the most remarkable facts con- cerning the bile is its apparently small use in the digestion of food. It is doubtless, to a large extent, excretory. Some state that it has a slight action on fats and carbohydrates, but it appears to be rather a coadjutor to the pancreatic juice (especially in the digestion of fat) than to have any independent digestive activity. Bile is said to be a natural antiseptic, lessening the putrefactive processes in the intestine. This is also very doubtful. Though the bile salts are weak antiseptics, the bile itself is readily putrescible, and the power it has of diminishing putrescence in the intestine is due chiefly to the fact that by increasing absorp- tion it lessens the amount of putrescible matter in the bowel. When the bile meets the chyme the turbidity of the latter is increased owing to the precipitation of4 unpeptonised proteid. This is an action due to the bile salts, and it has been surmised that this conversion of the chyme into a more viscid mass is to hinder somewhat its progress through the intestines; it clings to the intestinal wall, thus allowing absorption to take place. Bile is alkaline; it therefore assists in the pancreatic juice in neutralising the acid mixture that leaves the stomach. Bile assists in the absorption of fats, as we shall see in studying that subject. Fig.543.-Crystalline scales of cholesterin. CH. XLVIII.] JAUNDICE. 665 We have seen that fistula bile is poor in solids as compared with normal bile, and that this is explained on the supposition that the normal bile circulation is not occurring-the liver cannot excrete what it does not receive back from the intestine. Schiff was the first to show that if the bile is led back into the duo- denum, or even if the animal is fed on bile, the percentage of solids in the bile excreted is at once raised. It is on these ex- periments that the theory of a bile circulation is mainly founded. The bile circulation relates, however, chiefly, if not entirely, to the bile salts : they are found but sparingly in the faeces; they are only represented to a slight extent in the urine : hence it is calculated that seven-eighths of them are re-absorbed from the intestine. Small quantities of cholalic acid, taurine, and glycocine are found in the faeces ; the greater part of these products of the decomposition of the bile salts is taken by the portal vein to the liver, where they are once more synthetised into the bile salts. Some of the taurine is absorbed and excreted as tauro-carbamic acid in the urine. Some of the absorbed glycocine may be ex- creted as urea or uric acid. The cholesterin and mucus are found in the faeces; the pigment is changed into stercobilin, a substance like hydrobilirubin. Some of the stercobilin is absorbed, and leaves the body as the urinary pigment, urobilin. The bile-expelling mechanism must be carefully distin- guished from the bile-secreting action of the liver-cells. The bile is forced into the ducts, and ultimately into the duodenum, by the pressure of newly-formed bile pressing on that previously in the ducts, and this is assisted by the contraction of the plain muscular fibres of the larger ducts and gall-bladder, which occurs reflexly when the food enters the duodenum. In cases of obstruc- tion, as by a gall-stone, in the ducts, this action becomes excessive, and gives rise to the intense pain known as hepatic colic. Many so-called cholagogues (bile-drivers), like calomel, act on the bile-expelling mechanism and increase the peristalsis of the mus- cular tissue; they do not really cause an increased formation of bile. Jaundice.-The commonest form of jaundice is produced by obstruction in the bile ducts preventing the bile entering the intestine. A very small amount of obstruction, for instance, a plug of mucus produced in excess owing to inflammatory processes, will often be sufficient, as the bile is secreted at such low pressure. Under these circumstances, the freces are whitish or clay coloured, and the bile passing backwards into the lymph,* enters the blood * The absorption is by the lymph, because if jaundice be produced in an animal by ligature of the bile duct, it will cease when the thoracic duct is tied. 666 THE LIVER. [ch. xlviii. and is thus distributed over the body, causing a yellow tint in the skin and mucous membranes, and colours the urine deeply. In some cases of jaundice, however (e.g. produced by various poisons), there is no obvious obstruction; the causes of non- obstructive, or blood-jaundice, form a pathological problem of some interest. Until quite recently it was believed that the bile pigment was actually produced in the blood. But all recent work shows that the liver is the only place where production of bile occurs, and that in all cases of so-called non-obstructive jaundice, the bile is absorbed from the liver. There may be obstruction present in the smaller ducts, or the functions of the liver may be so upset that the bile passes into the lymph even when there is no obstruction. The Glycogenic Function of the Liver. The important fact that the liver normally forms sugar, or a substance readily convertible into it, was discovered by Claude Bernard in the following way : he fed a dog for seven days with food containing a large quantity of sugar and starch; and, as might be expected, found sugar in both the portal and hepatic blood. But when this dog was fed with meat only, to his sur- prise, sugar was still found in the blood of the hepatic veins. Repeated experiments gave invariably the same result; no sugar was found, under a meat diet, in the portal vein, if care were taken, by applying a ligature on it at the transverse fissure, to prevent reflux of blood from the hepatic venous system. Bernard found sugar also in the substance of the liver. It thus seemed certain that the liver formed sugar, even when, from the absence of saccharine and amyloid matters in the food, none could have been brought directly to it from the stomach or intestines. Bernard found, subsequently to the before-mentioned experi- ments, that a liver, removed from the body, and from which all sugar had been completely washed away by injecting a stream of water through its blood-vessels, after the lapse of a few hours, contained sugar in abundance. This post-mortem production of sugar was a fact which could only be explained on the supposition that the liver contained a substance readily convertible into sugar ; and this theory was proved correct by the discovery of a substance in the liver allied to starch, and now termed glycogen. We may believe that glycogen is first formed and stored in the liver cells, and that the sugar, when present, is the result of its transformation. Source of Glycogen.-Although the greatest amount of glycogen CH. XLVIlI.j GLYCOGEN. 667 is produced by the liver upon a diet of starch or sugar, a certain quantity is produced upon a proteid diet. It must, then, be pro- duced by protoplasmic activity within the cells. The glycogen when stored in the liver cells may readily be demonstrated in sections of liver containing it by its reaction (red or port-wine colour) with iodine, and moreover, when the hardened sections are so treated that the glycogen is dissolved out, the protoplasm of the cell is so vacuolated as to appear little more than a frame- work. There is no doubt that in the liver of a hibernating frog the amount of glycogen stored up in the outer parts of the liver cells is very considerable. Average Amount of Glycogen in the Liver of Dogs under various Diets (Pavy). Diet. Amount of Glycogen in Liver. Animal food . . . . . . .7'19 per cent. Animal food with sugar (about | lb. of sugar daily) 14-5 ,, Vegetable diet (potatoes, with bread or barley-meal) 17'23 ,, The dependence of the formation of glycogen on the kind of food taken is also well shown by the following results, obtained by the same experimenter :- Average Quantity of Glycogen found in the Liver of Rabbits after Fasting, and after a Diet of Starch and Sugar respectively. Average amount of Glycogen in Liver. After fasting for three days . . Practically absent. ,, diet of starch and grape-sugar . 15-4 per cent. ,, ,, cane-sugar . . .16'9 ,, Glycogen is also formed on a gelatin diet, but fats taken in as food do not increase its amount in the cells. The diet most favourable to the production of a large amount of glycogen is a mixed diet containing a large amount of carbo-hydrate, but with some proteid. Glycerin injected into the alimentary canal may also increase the glycogen of the liver. Destination of Glycogen.-'There are two chief theories as to the destination of hepatic glycogen. (1.) That the glycogen is converted into sugar during life by the agency of a ferment (liver diastase) also formed in the liver; and that the sugar is conveyed away by the blood of the hepatic veins, to undergo combustion in the tissues. (2.) That the conversion into sugar only occurs after death, and that during life no sugar exists in healthy livers; glycogen not undergoing this transformation. 668 THE LIVER. [ch. xlviii. The first view is that of Claude Bernard, and has been adopted by the majority of physiologists. The second view is that of Dr. Pavy: he denies that the livei1 is a sugar-forming organ, he regards it as a sugar-destroying organ; the sugar is stored as animal starch, but never again leaves the liver as sugar during life. He has been unable to find more sugar in the hepatic blood than in the portal blood. Other observers have found an increase in the sugar of the blood leaving the liver, but the estimation of sugar in a fluid rich in proteids, as is the blood, is a matter of great difficulty. Even if the increase is so small as hardly to be detected, it must be remembered that the whole blood of the body passes through the liver about twice a minute, so that a very small increase each time would mount up to a large total. Pavy further denies that the post-mortem formation of sugar from glycogen that occurs in an excised liver is a true picture of what occurs during life, but is due to a ferment which is only formed after death. During life, he regards the glycogen as a source of other substances, like fat and proteid. It is certainly a fact that increase of carbohydrate food leads to the formation of fat in the body and in the liver-cells. In support of the theory that glycogen may also lead to the formation of proteids, he has shown that these substances contain a carbohydrate radicle. The whole question is in a very unsettled state, and is under keen discussion at present. We may state, however, that the prevalent opinion is that the liver-cells may be able to convert part of the store of glycogen into fat, but that most of the glycogen leaves the liver as sugar, so justifying the name (literally, mother substance of sugar) given to it by Bernard. Carbohydrate metabolism is thus a series of hydrations and dehydrations before combustion finally occurs. Starch is first hydrated in the alimentary canal to form sugar. This passes to the liver, where it is dehydrated to form glycogen, or animal starch; and finally hydrated once more to pass to the tissues as sugar, where it undergoes combustion. Diabetes.-In certain disorders of hepatic metabolism, the glycogenic function is upset, and excess of sugar passes into the blood, leaving the body in the urine This may be due to an increased formation of sugar from glycogen, or to a diminished formation of glycogen from the sugar of the portal blood, according as either Bernard's or Pavy's view of the liver function is adopted. In many cases the diabetic condition may be removed by a close attention to diet; starchy and saccharine food must be rigidly abstained from. In other cases, which are much more serious, diet makes little CH. XLVIII.] DIABETES. 669 or no difference. Under these circumstances the sugar must come from the metabolism of the proteid constituents of protoplasm. The disease diabetes is not a single one; the term includes many pathological conditions, which all possess in common the symptom of excess of sugar in the blood and urine. A diabetic condition may be produced in animals artificially in several ways:- (1) By diabetic puncture.-Claude Bernard was the first to show that injury to the floor of the fourth ventricle in the region of the vaso-motor centre leads to glycosuria. The injury produces a disturbance of the vaso-motor mechanism, but the special result of diabetes cannot be regarded as purely vaso-motor in origin. This condition is of interest, because brain disease in man, especially in the region of the bulb, is frequently associated with glycosuria. (2) By extirpation of the pancreas.-This is alluded to on P- 652- (3) By administration of phloridzin.-Many poisons produce temporary glycosuria, but the most interesting and powerful of these is phloridzin. The diabetes produced is very intense. Phloridzin is a glucoside, but the sugar passed in the urine is too great to be accounted for by the small amount of sugar derivable from the drug. Besides that, phloretin, a derivative of phloridzin, free from sugar, produces the same results. Phloridzin produces diabetes in starved animals, or in those in which any carbohydrate store must have been got rid of by the previous administration of the same drug. Phloridzin-diabetes is therefore analogous to those intense forms of diabetes in man in which the sugar must be derived from protoplasmic metabolism. Acetonsemia.-Death in diabetic patients is usually preceded by deep coma, or unconsciousness. Some poison must be pro- duced that acts soporifically upon the brain. The breath and urine of these patients smell strongly of acetone ; hence the term acetonoemia. This apple-like smell should always suggest the possible onset of coma and death, but it is exceedingly doubtful whether acetone (which can certainly be detected in the urine) is the true poison; ethyl-diacetic acid, which accompanies, and is the source of the acetone, is regarded by some as the actual poison, but these substances, when introduced into the circulation artificially, do not cause serious symptoms. The actual poison is a matter of doubt; the idea most in vogue at present is that it is hydroxybutyric acid, which is discoverable in the blood and urine of the patients who die from so-called acetonsemia. 670 THE ABSORPTION OF FOOD. [CH. XLIX. CHAPTER XLIX. THE ABSORPTION OF FOOD. Food is digested in order that it may be absorbed. It is absorbed in order that it may be assimilated, that is, become an integral part of the living material of the body. The digested food thus diminishes in quantity as it passes along the alimentary canal, and the fteces contain the undigested or indigestible residue. In the mouth and oesophagus the thickness of the epithelium and the quick passage of the food through these parts reduce absorption to a minimum. Absorption takes place rapidly in the stomach : the small intestine with its folds and villi to increase its surface is, however, the great place for absorption; and although the villi are absent from the large intestine, absorption occurs there also, but to a less extent. Foods such as water and soluble salts like sodium chloride are absorbed unchanged. The organic foods are, however, consider- ably changed, colloid materials like starch and proteid being- converted respectively into the diffusible materials sugar and peptone. There are two channels of absorption, the blood-vessels (portal capillaries) and the lymphatic vessels or lacteals. Absorption, however, is no mere physical process of diffusion and filtration. We must also take into account the fact that the cells through which the absorbed substances pass are living, and in virtue of their vital activity not only select materials for absorption, but also change those substances while in contact with them. These cells are of two kinds-(i) the columnar epithelium that covers the surface ; and (2) the lymph cells in the lymphoid tissues beneath. It is now generally accepted that of the two the former, the columnar epithelium, is the more important. Absorption of Carbohydrates.-Though the sugar formed from starch by ptyalin and amylopsin is maltose, that found in the blood is glucose. Under normal circumstances little if any is absorbed by the lacteals. The glucose is formed from the maltose by the succus entericus, aided by the action of the epithelial cells through which it passes. Cane sugar and milk sugar are also converted into glucose before absorption. The carbohydrate food which enters the blood as glucose is CH. XLIX.J THE ABSORPTION OF FOOD. 671 taken to the liver, and there stored up in the form of glycogen- a reserve store of carbohydrate material for the future needs of the body. Glycogen, however, is found in animals who take no carbohydrate food. It must, then, be formed by the proto- plasmic activity of the liver cells from their proteid constituents. The glycogenic function of the liver is discussed in the chapter preceding this. Glucose is the only sugar from which the liver is capable of forming glycogen. If other carbohydrates like cane sugar or lactose are injected into the blood-stream direct, they are unaltered by the liver, and finally leave the body by the urine. Absorption of Proteids.-A certain amount of soluble proteid is absorbed unchanged. Thus, after taking a large number of eggs, egg albumin is found in the urine. Patients fed per rectum derive nourishment from proteid food, though proteo- lytic ferments are not present in this part of the intestine. Most proteid, however, is normally absorbed as peptone and proteose (albumose). Peptones and proteoses are absent from the blood under all circumstances, even from the portal blood during the most active digestion. In other words, during absorp- tion the epithelial cells change the products of proteolysis (peptones and proteoses) back once more into native proteids (albumin and globulin). The greater part of the proteid absorbed passes into the blood; a little into the lymph vessels also; but this undergoes the same change. When peptone (using the word to include the proteoses also) is injected into the blood stream, poisonous effects are produced, the coagulability of the blood is lessened, the blood pressure falls, secretion ceases, and in the dog 0'3 gramme of "peptone" per kilogramme of body weight is sufficient to kill the animal. The epithelial cells of the alimentary canal thus protect us from those poisonous effects by converting the harmful peptone into the useful albumin. Absorption of Fats.-The fats undergo in the intestine two changes: one a physical change (emulsification), the other a chemical change (saponification). The lymphatic vessels are the great channels for fat absorption, and their name, lacteals, is derived from the milk-like appearance of their contents {chyle} during the absorption of fat. The way in which the minute fat globules pass from the intestine into the lacteals has been the subject of much contro- versy. The course they take may be studied by killing animals at varying periods after a meal of fat, and making osmic acid 672 THE ABSORPTION OF FOOD. [ch. xlix. microscopic preparations of the villi. Figs. 544 and 545 illustrate the appearances observed by Professor Schafer. The columnar epithelium cells become first filled with fatty globules of varying size, which are generally larger near the free border. The globules pass down the cells, the larger ones breaking up into smaller ones during the journey \ they are then Kg. 544.-Section of the villus of a rat killed during fat absorption, ep, epithelium ; str, striated border ; c, lymph-cells ; c', lymph-cells in the epithelium; Z, central lacteal containing disintegrating lymph-corpuscles. (E. A. Schafer.) transferred to the amoeboid cells of the lymphoid tissue beneath : these ultimately penetrate into the central lacteal, where they either disintegrate or discharge their cargo into the lymph stream. The globules are by this time divided into immeasurably small ones, the molecular basis of chyle. The chyle enters the blood stream by the thoracic duct, and after an abundant fatty meal the blood plasma is quite milky; the fat droplets are so small that they circulate without hindrance through the capil- laries. The fat in the blood after a meal is eventually stored up in the connective-tissue cells of adipose tissue. It must, however, be borne in mind that the fat of the body is not exclusively de- rived from the fat of the food, but it may originate also both from proteid and from carbohydrate. CH. XLIX.J ABSORPTION OF FATS. 673 The great difficulty in fat absorption is how the fat first gets into the columnar epithelium : these cells will not take up other particles, and it appears certain that the epithelial cells do not in the higher animals protrude pseudopodia from their borders. This, however, does occur in the endoderm of some of the lower invertebrates. Recent research has shown that particles may be present in the epithelium and lymphoid cells while no fat is being absorbed. These particles are apparently protoplasmic in nature, as they stain with reagents that stain protoplasmic granules ; they however also stain darkly with osmic acid, and so are apt to be mistaken for fat. It also appears that the epithelial cells have the power of form- ing fat out of the fatty acids and glycerine into which they have been broken up in the intestine. Munk, who has performed a large number of experiments on the subject, considers that the splitting of fats into glycerine and fatty acids occurs to a much greater extent than is generally supposed; that these substances being soluble pass readily into the epithelium cells ; and that these cells perform the synthetic act of building them into fat once more, the fat so formed appearing in the form of small globules, surrounding or becoming mixed with the proto- plasmic granules that are ordinarily present. Another remark- able fact recently made out is that after feeding an animal on fatty acids the chyle contains fat. The necessary glycerine must have been formed by protoplasmic activity during absorption. Munk's views, however, require confirmation before they can be fully accepted. The role played by bile in aiding the absorption of fat must also be regarded as unsettled, but the chief fact in favour of the view that bile assists fat absorption is the following :-One of the chief properties of bile salts is the power they possess of reducing the surface tension of water when they pass into solution. It may be that this reduction of surface tension of the film of water between the oil globule and the epithelium cell enables the latter to take up the fat more readily. In cases of disease in which bile is absent from the intestines, Fig. 545.-Mucous membrane of frog's intes- tine during fat absorption, ep, epithe- lium ; str, striated border; C, lymph corpuscles; I, lacteal. (E. A. Schafer.) 674 THE MECHANICAL PROCESSES OF DIGESTION. [ch. l. a large proportion of the fat in the food passes into the faeces. The faeces are alkaline in reaction, and contain the following substances :- 1. Water : in health from 68 to 82 per cent.; in diarrhoea it is more abundant still. 2. Undigested food; that is, if food is taken in excess, some escapes the action of the digestive juices. On a moderate diet unaltered proteid is never found. 3. Indigestible constituents of the food : cellulose, keratin, mucin, chlorophyll, gums, resins, cholesterin. 4. Constituents digestible with difficulty: uncooked starch, tendons, elastin, various phosphates, and other salts of the alka- line earths. 5. Products of decomposition of the food : indole, skatole, phenol, acids such as fatty acids, lactic acid, &c. ; hsematin from haemoglobin; insoluble soaps like those of calcium and mag- nesium. 6. Bacteria of all sorts and debris from the intestinal wall; cells, nuclei, mucus, &c. 7. Bile residues: mucus, cholesterin, traces of bile acids and their products of decomposition, stercobilin from the bile pig- ment. The average quantity of solid faecal matter passed by the human adult per diem is 6 to 8 ounces. Meconium, is the name given to the greenish-black contents of the intestine of new-born children. It is chiefly concentrated bile, with debris from the intestinal wall. The pigment is a mixture of bilirubin and biliverdin, not stercobilin. CHAPTER L. THE MECHANICAL PROCESSES OF DIGESTION. Under this head we shall study the muscular movements of the alimentary canal, which have for their object the onward movements of the food, and its thorough admixture with the digestive juices. We shall therefore have to consider mastication, deglutition, the movements of the stomach and intestines, and the acts of defecation, and vomiting. CH. L.] MASTICATION AND DEGLUTITION. 675 Mastication. The act of chewing, or mastication, is performed by the biting and grinding movement of the lower range of teeth against the upper. The simultaneous movements of the tongue and cheeks assist partly by crushing the softer portions of the food against the hard palate and gums, and thus supplementing the action of the teeth, and partly by returning the morsels of food to the action of the teeth, again and again, as they are squeezed out from between them, until they have been sufficiently chewed. The act of mastication is much assisted by the saliva, and the intimate incorporation of this secretion with the food is called insalivation. Mastication is much more thoroughly performed by some animals than by others. Thus, dogs hardly chew their food at all, but the oesophagus is protected from abrasion by a thick coating of very viscid saliva which lubricates the pieces of rough food. In vegetable feeders, on the other hand, insalivation is a much more important process. This is especially so in the ruminants; in these animals, the grass, &c. taken, is hurriedly swallowed, and passes into the first compartment of their four-chambered stomach. Later on, it is returned to the mouth in small instal- ments for thorough mastication and insalivation; it is then once more swallowed and passes on to the digestive regions of the stomach. This is the act of rumination or " chewing the cud." In man, mastication is also an important process, and in people who have lost their teeth severe dyspepsia is often produced, and can be cured by a new set of teeth. Deglutition. When properly masticated, the food is transmitted in successive portions to the stomach by the act of deglutition or swallow- ing. This, for the purpose of description, may be divided into three acts. In the first, particles of food collected as a bolus are made to 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, (i.) The first act is voluntary, although it is usually performed unconsciously; the morsel of food, when sufficiently masticated-, being pressed between 676 THE MECHANICAL PROCESSES OF DIGESTION. [ch. l. the tongue and palate, by the agency of the muscles of the former, in such a manner as to force it back to the entrance of the pharynx. (2.) The second act is the most complicated, because the food must pass by the posterior orifice of the nose and the upper opening of the larynx without touching them. When it has been brought, by the first act, between the anterior arches of the palate, it is moved onwards by the movement of the tongue backwards, and by the muscles of the anterior arches contracting- on it and then behind it. The root of the tongue being retracted, and the larynx being raised with the pharynx and carried for- wards under the base of the tongue, the epiglottis is pressed over the upper opening of the larynx, and the morsel glides past it; the closure of the glottis being additionally secured by the simul- taneous contraction of its own muscles : so that, even when the epiglottis is destroyed, there is little danger of food or drink pass- ing into the larynx so long as its muscles can act freely. In man, and some other animals, the epiglottis is not drawn as a lid over the larynx during swallowing. At the same time, the raising of the soft palate, so that its posterior edge touches the back part of the pharynx, and the approximation of the sides of the posterior palatine arch, which move quickly inwards like side curtains, close the passage into the upper part of the pharynx and the posterior nares, and form an inclined plane, along the under surface of which the morsel descends ; then the- pharynx, raised up to receive it, in its turn contracts, and forces it onwards into the oesophagus. The passage of the bolus of food through the three constrictors of the pharynx is the last step in this stage. (3.) 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 or peristaltic contraction of the oesophagus, which is easily observable through the skin in long-necked animals like the swan. If we suppose the bolus to be at one particular place in the tube, it acts stimulatingly in the circular muscular fibres behind it, and inhibitingly on those in front; the contraction therefore- squeezes it into the dilated portion of the tube in front, where the same process is repeated, and this travels along the whole length of the tube. The second and third parts of the act of deglutition are in- voluntary. Nervous Mechanism.-The nerves engaged in the reflex act of deglutition are :-sensory, branches of the fifth cerebral supplying the soft palate; glosso-pharyngeal, supplying the tongue and OH. L.J DEGLUTITION. 677 pharynx; the superior laryngeal branch of the vagus, supplying the epiglottis and the glottis; while the motor fibres concerned are :-branches of the fifth, supplying part of the digastric and mylo-hyoid muscles, and the muscles of mastication ; the facial, supplying the levator palati; the glosso-pharyngeal, supplying the muscles of the pharynx; the vagus, supplying the muscles of the larynx through the inferior laryngeal branch, and the hypoglossal, the muscles of the tongue. The nerve-centre by which the muscles are harmonised in their action, is situated in the medulla oblongata. In the movements of the oesophagus, the ganglia con- tained in its walls, with the pneumo-gastrics, are the nerve- structures concerned. Division of both pneumogastric nerves gives rise to paralysis of the oesophagus and stomach, and firm contraction of the cardiac orifice. These nerves therefore nor- mally supply the oesophagus with motor, and the cardiac orifice with inhibitory fibres. If food is swallowed after these nerves are divided, it accumulates in the gullet and never reaches the stomach. In discussing peristalsis on previous occasions (p. 158), we have arrived at the conclusion that it is an inherent property of muscle rather than of nerve ; though normally it is controlled and influenced by nervous agency. This nervous control is especially marked in the oesophagus ; for if that tube is divided across, leaving the nerve branches intact, a wave of contraction will travel from one end to the other across the cut. Swallowing of Fluids.-We must next note that the swallow- ing both of food and drink is a muscular act, and can, therefore, take place in opposition to the force of gravity. Thus, horses and many other animals habitually drink up-hill, and the same feat can be performed by jugglers. Under ordinary circumstances, however, the swallowing of fluids is differently produced from what we have already de- scribed : the division of the act of deglutition into three stages is true for the swallowing of solids only. This has been shown by Kronecker. In swallowing liquids the two mylo-hyoid muscles form a diaphragm which pulls the root of the tongue upwards and back- wards ; the two hyo-glossi act with these, pulling the tongue backwards and downwards. The action of these four muscles resembles that of a force-pump projecting the mass of fluid down into the oesophagus ; it reaches the cardiac orifice with great speed, and the pharyngeal and oesophageal muscles do not contract on it at all, but are inhibited during the passage of the fluid through them. 678 THE MECHANICAL PROCESSES OF DIGESTION. [ch. l. This is proved in a striking way in cases of poisoning by corrosive substances like oil of vitriol; the mouth and tongue are scarred and burnt, but the pharynx and oesophagus escape serious injury, so rapidly does the fluid pass along them ; the cardiac orifice of the stomach is the next place to show the effects of the corrosive. There is, howrever, no hard-and-fast line between the swallowing of solids and fluids : the more liquid the food is, the more does the force-pump action just described manifest itself. Movements of the Stomach. The gastric fluid is assisted in accomplishing its share in digestion by the movements of the stomach. In granivorous birds, for example, the contraction of the strong muscular gizzard affords a necessary aid to digestion, by grinding and triturating the hard seeds which constitute their food. But in the stomachs of man and other Mammalia the movements of the muscular coat are too feeble to exercise any such mechanical force on the food; neither are they needed, for mastication has already done the mechanical work of a gizzard ; and experiments have demonstrated that substances are digested even enclosed in perforated tubes, and consequently protected from mechanical influence. The normal actions of the muscular fibres of the human stomach appear to have a three-fold purpose: (i) to adapt the stomach to the quantity of food in it, so that its walls may be in contact with the food on all sides, and, at the same time, may exercise a certain amount of compression upon it; (2) to keep the orifices of the stomach closed until the food is digested; and (3) to perform certain peristaltic movements, whereby the food, as it becomes chymified, is gradually propelled towards, and ultimately through, the pylorus. In accomplishing this latter end, the movements without doubt materially contribute towards effecting a thorough intermingling of the food and the gastric fluid. When digestion is not going on, the stomach is uniformly con- tracted, its orifices not more firmly than the rest of its walls; but, if examined shortly after the introduction of food, it is found closely encircling its contents, and its orifices are firmly closed like sphincters. The cardiac orifice, every time food is swallowed, opens to admit its passage to the stomach, and immediately again closes. The pyloric orifice, during the first part of gastric diges- tion, is usually so completely closed, that even when the stomach is separated from the intestines, none of its contents escape. CH. L.J MOVEMENTS OF THE STOMACH. 679 But towards the termination of the digestive process, the pylorus offers less resistance to the passage of substances from the stomach; first it yields to allow the successively digested portions to go through it; and then it allows the transit of even undigested substances. It appears that food, so soon as it enters the stomach, is subjected to a kind of peristaltic action of the muscular coat, whereby the digested portions are gradually moved towards the pylorus. The movements are observed to increase in rapidity as the process of chymification advances, and are continued until it is completed. The contraction of the fibres situated towards the pyloric end of the stomach seems to be more energetic and more decidedly peristaltic than those of the cardiac portion. Thus, it was found in the case of St. Martin, that when the bulb of the thermometer was placed about three inches from the pylorus, through the gastric fistula, it was tightly embraced from time to time, and drawn towards the pyloric orifice for a distance of three or four inches. The object of this movement appears to be, as just said, to carry the food towards the pylorus as fast as it is formed into chyme, and to propel the chyme into the duodenum; the un- digested portions of food being kept back until they are also reduced into chyme, or until all that is digestible has passed out. The action of these fibres is often seen in the contracted state of the pyloric portion of the stomach after death, when it alone is contracted and firm, while the cardiac portion forms a dilated sac. Sometimes, by a predominant action of strong circular fibres placed between the cardia and pylorus, the two portions, or ends as they are called, of the stomach, are partially separated from each other by a kind of hour-glass contraction. By means of the peristaltic action of the muscular coats of the stomach, not merely is chymified food gradually propelled through the pylorus, but a kind of double current is continually kept up among the contents of the stomach, the circumferential parts of the mass being gradually moved onward toward the pylorus by the con- traction of the muscular fibres, while the central portions are propelled in the opposite direction, namely towards the cardiac orifice; in this way is kept up a constant circulation of the contents of the viscus, highly conducive to their thorough ad- mixture with the gastric fluid and to their ready digestion. Under ordinary circumstances, three or four hours may be taken as the average time occupied by a digestion of the meal in the stomach. But the digestibility and quantity of the meal, and the state of body and mind of the individual, are important causes of variation. The pylorus usually opens for the first time 680 THE MECHANICAL PROCESSES OF DIGESTION. [ch. l. about twenty minutes after digestion begins; it, however, quickly closes again. The intervals between its openings diminish, and the periods during which it remains open increase, until towards the end of the time it is permanently open, and the chyme can pass freely into the duodenum. Influence of the Nervous System.-The normal move- Fig. 546.-Very diagrammatic representation of the nerves of the alimentary canal. Oe to Ret, the various parts of the alimentary canal from oesophagus to rectum : L. V, left vagus, ending on front of stomach; rl, recurrent laryngeal nerve, supplying upper part of oesophagus; R. V, right vagus, joining left vagus in oesophageal plexus : oe. pl., supplying the posterior part of stomach, and continues asR'V' to join the solar plexus, here represented by a single ganglion, and connected with the inferior- mesen- teric ganglion m.gl. ; a, branches from the solar plexus to stomach and small intestine, and from the mesenteric ganglia to the large intestine; Spl.maj., large splanchnic nerve, arising from the thoracic ganglia and rami communicantes; r.c, belonging to dorsal nerves from the 6th to the 9th (or 10th) ; Spl.min., small splanchnic nerve simi- larly from the 10th and nth dorsal nerves. These both join the solar plexus, and thence make their way to the alimentary canal; c.r., nerves from the ganglia, &c., belonging to rrth and 12th dorsal and rst and 2nd lumbar nerves, proceeding to the inferior mesenteric ganglia (or plexus), m.gl., and thence by the hypogastric nerve, n.hyp., and the hypogastric nerve, n.hyp., and the hypogastric plexus, pl.hyp., to the circular muscles of the rectum; l.r., nerves from the 2nd and 3rd sacral nerves, S. 2, S. 3 (nervi erigentes) proceeding by the hypogastric plexus to the longitudinal muscles of the rectum. (M. Foster.) ments of the stomach during gastric digestion do not appear to be so closely connected with the plexuses of nerves and ganglia contained in its walls as was formerly supposed. The action, however, appears to be set up by the presence of food within it. The stomach is, moreover, directly connected with the higher nerve-centres by means of branches of the vagi and of the splanchnic nerves through the solar plexus. CH. L.] VOMITING. 681 First as to the function of the vagi in connection with the gastric movements. Irritation of these nerves produces contrac- tion of the stomach, if digestion is proceeding; and conversely, peristaltic action is retarded, although it is not stopped, when they are divided. Secondly as to the other nerve-fibres, which reach the stomach and intestines througli the solar plexus. These fibres pass from the spinal cord in the anterior roots of the nerves from the sixth to the twelfth dorsal, passing in the splanchnic nerves to the solar plexus, and thence to the stomach. Stimulation of the splanchnics causes stoppage of the muscular movements as well as constriction of the blood-vessels. It seems probable that automatic peristaltic contraction is inherent in the muscular coat of the stomach, and that the central nervous system is only employed to regulate it by impulses passing down by the vagi or splanchnic nerves. The secretory nerves of gastric juice are treated on p. 641. Vomiting. The expulsion of the contents of the stomach in vomiting, like that of mucus or other matter from the lungs in coughing, is preceded by an inspiration ; the glottis is then closed, and imme- diately afterwards the abdominal muscles strongly act; but here -occurs the difference in the two actions. Instead of the vocal •cords yielding to the action of the abdominal muscles, they re- main tightly closed. Thus the diaphragm being unable to go up, forms an unyielding- surface against which the stomach can be pressed. At the same time the cardiac sphincter-muscle being- relaxed, and the orifice which it naturally guards being dilated, while the pylorus is closed, and the stomach itself also contract- ing, the action of the abdominal muscles expels the contents of the organ through the eesophagus, pharynx, and mouth. The reversed peristaltic action of the oesophagus probably increases the effect. It has been frequently stated that the stomach itself is quite passive during vomiting, and that the expulsion of its contents is •effected solely by the pressure exerted upon it when the capacity •of the abdomen is diminished by the contraction of the diaphragm, and subsequently of the abdominal muscles. The experiments and observations, however, which are supposed to confirm this statement, only show that the contraction of the abdominal muscles alone is sufficient to expel matters from an unresisting 682 THE MECHANICAL PROCESSES OF DIGESTION. [ch. l. bag through the oesophagus ; and that, under very abnormal circumstances, the stomach, by itself, cannot expel its contents. They by no means show that in ordinary vomiting the stomach is passive, for there are good reasons for believing the contrary. In some cases of violent vomiting the contents of the duodenum are passed by anti-peristalsis into the stomach, and are then vomited. Where there is obstruction to the intestine, as in strangulated hernia, the contents of all the small and even of the large in- testines may be vomited. Nervous mechanism.-Some few persons possess the power of vomiting at will, or the power may be acquired by effort and practice. But normally the action is a reflex one. The afferent nerves are principally the fifth, and glossopharyn- geal (as in vomiting produced by tickling the fauces), and the vagus (as in vomiting produced by gastric irritants) ; but vomiting may occur from stimulation of other sensory nerves, e.g. those from the kidney, uterus, testicle, &c. The centre may also be stimulated by impressions from the cerebrum and cerebellum,, producing so-called central vomiting occurring in diseases of those parts. The centre for vomiting is in the medulla oblongata, and coin- cides with the centres of the nerves concerned. The efferent (motor) impulses are carried by the vagi to the stomach, phrenics to the diaphragm, and various spinal nerves to the abdominal muscles. Emetics.-Some emetics produce vomiting by irritating the stomach; others, like tartar emetic, apomorphia, &c., by stimu- lating the vomiting centre. Movements of the Intestines. The movement of the intestines is peristaltic or vermicular, and is affected by the alternate contractions and dilatations of successive portions of the muscular coats. The contractions, which may commence at any point of the intestine, extend in a wave-like manner along the tube. They are exactly similar to what we have described in the oesophagus. In any given portion, the longitudinal muscular fibres contract first, or more than the circular ; they draw a portion of the intestine upwards, over the substance to be propelled, and then the circular fibres of the same portion contracting in succession from above downwards, press on the substance into the portion next below, in which at once the same succession of actions next ensues. These movements take CH. L.] INTESTINAL MOVEMENTS. 683 place slowly, and, in health, commonly give rise to no sensation; but they are perceptible when they are accelerated under the influence of any irritant. The movements of the intestines are sometimes retrograde ; and there is no hindrance to the backward movement of the contents of the small intestine, as in cases of violent vomiting just referred to. But almost complete security is afforded against the passage of the contents of the large into the small intestine by the ileo- csecal valve. Besides,-the orifice of communication between the ileum and caecum (at the borders of which orifice are the folds of mucous membrane which form the valve) is encircled with muscu- lar fibres, the contraction of which prevents the undue dilatation of the orifice. Proceeding from above downwards, the muscular fibres of the large intestine become, on the whole, stronger in direct proportion to the greater strength required for the onward moving of the faeces, which are gradually, owing to the absorption of water, becoming firmer. The greatest strength is in the rectum, at the termination of which the circular unstriped muscular fibres form a strong band called the internal sphincter ; while an external sphincter muscle with striped fibres is placed rather lower down, and more externally, and holds the orifice close by a constant slight tonic contraction. Nervous mechanism.-Experimental irritation of the brain or cord produces no evident or constant effect on the movements of the intestines during life; yet in consequence of certain mental conditions the movements are accelerated or retarded ; and in paraplegia the intestines appear after a time much weakened in their power, and costiveness, with a tympanitic condition, ensues. Stimulation of the pneumo-gastric nerves, if not too strong, induces genuine peristaltic movements of the intestines. Violent irritation stops the movements. As in the case of the oesophagus and stomach, the peristaltic movements of the intestines may be directly set up in the muscular fibres by the presence of food or chyme acting as the stimulus. Few or no movements occur when the intestines are empty. The intestines are connected with the central nervous system both by the vagi and by the splanchnic nerves, as well as by other branches of the sympathetic which come to them from the coeliac and other abdominal plexuses. The relations of these nerves respectively to the movements of the intestine and the secretions are the same as in the case of the stomach already treated of (see also fig. 546). Section of the sympathetic nerves going to any loop of intestine 684 THE MECHANICAL PROCESSES OF DIGESTION. [CH. L. produces not only vaso-dilatation, but a great increase of very watery succus entericus (paralytic secretion ?). The exact distribution of the sympathetic fibres, and the situation of their cell stations, is a complicated anatomical re- search, in which Langley and Anderson have obtained the most trustworthy results by the nicotine method (see p. 472). Duration of Intestinal Digestion.-The time occupied by the journey of a given portion of food from the stomach to the anus, varies considerably even in health, and on this account probably it is that such different opinions have been expressed in regard to the subject. About twelve hours are occupied by the journey of an ordinary meal through the small intestine, and twenty-four to thirty-six hours by the passage through the large bowel. Drugs given for relief of diarrhoea or constipation act in various ways ; some influence the amount of secretion and thus increase or diminish the fluidity of the intestinal contents ; others acting- on the muscular tissue or its nerves increase or diminish peristalsis. Defsecation. The act of the expulsion of faeces is in part due to an increased reflex peristaltic action of the lower part of the large intestine, namely of the sigmoid flexure and rectum, and in part to the action of the abdominal muscles. In the case of active voluntary efforts, there is usually, first an inspiration, as in the case of coughing, sneezing, and vomiting ; the glottis is then closed, and the diaphragm fixed. The abdominal muscles are contracted as in expiration; but as the glottis is closed, the whole of their pressure is exercised on the abdominal contents. The sphincter of the rectum being relaxed, the evacuation of its contents takes place accordingly; the effect being increased by the peristaltic action of the intestine. .Nervous Mechanism.-The anal sphincter muscle is normally in a state of tonic contraction. The nervous centre which governs this contraction is situated in the lumbar region of the spinal cord, inasmuch as in cases of division of the cord above this region the sphincter regains, after a time, to some extent the tonicity which is lost immediately after the operation. By an effort of the will, acting on the centre, the contraction may be relaxed oi' increased. Such voluntary control over the act is obviously impossible when the cord is divided. In ordinary cases the apparatus is set in action by the gradual accumula- tion of faeces in the sigmoid flexure and rectum, pressing by CH. LI.] THE URINARY APPARATUS. 685 the peristaltic action of these parts of the large intestine against the sphincter, and causing by reflex action its relaxation ; this sensory impulse acting through the brain and rcflexly through the spinal centre. At the same time that the sphincter is in- hibited or relaxed, impulses pass to the muscles of the lower intestine increasing their peristalsis, and to the abdominal muscles as well. CHAPTER LI. THE URINARY APPARATUS. This consists of the kidneys; from each a tube leads called the ureter to the bladder in which the urine is temporarily stored ; from the bladder a duct called the urethra leads to the exterior. The Kidneys are two in number, and are situated deeply in the lumbar region of the abdomen on either side of the spinal column behind the peritoneum. They correspond in position to the last two dorsal and two upper lumbar vertebrae; the right being slightly below the left in consequence of the position of the liver on the right side of the abdomen. They are about 4 inches long, 21 inches broad, and i| inch thick. The weight of each kidney is about oz. Structure.-The kidney is covered by a fibrous capsule, which is slightly attached by its inner surface to the proper substance of the organ by means of very fine bundles of areolar tissue and minute blood-vessels. From the healthy kidney, therefore, it may be easily torn off without injury to the subjacent cortical portion of the organ. At the hilus of the kidney, it becomes continuous with the external coat of the upper and dilated part of the ureter (fig. 547). On dividing the kidney into two equal parts by a section carried through its long convex border it is seen to be composed of two portions called respectively cortical and medullary ; the latter is composed of about a dozen conical bundles of urine tubes each bundle forming what is called a pyramid. The upper part of the ureter or duct of the organ, is dilated into the pelvis; and this, again, after separating into two or three principal divisions, is finally subdivided into still smaller portions, varying in number from about 8 to 12, called calyces. Each of these little calyces or cnps receives the pointed extremity or papilla of a pyramid. The 686 THE URINARY APPARATUS. [CH. LI. number of pyramids varies in different animals; in some there is only one. The kidney is a compound tubular gland, and both its cortical and medullary portions are composed of tubes, the tubuli uriniferi, which, by one extremity, in the cortical portion, commence around tufts of capillary blood-vessels, called Malpighian bodies, and, by the other, open through the papilla; into the pelvis of the kidney, and thus discharge the urine which flows through them. They are bound together by connective tissue. In the pyramids the tubes are straight-uniting to form Fig. 547.-Plan of a longitudinal section through the pelvis and substance of the right kidney, J : a, the cortical substance; b, b, broad part of the pyramids of Mal- pighi; c, c, the divisions of the pelvis named calyces, laid open; c', one of those unopened ; d, summit of the pyramids of papillae projecting into calyces; e, e, section of the narrow part of two pyra- mids near the calyces; p, pelvis or en- larged divisions of the ureter within the kidney; u, the ureter; s, the sinus; A, the hilus. Fig, 548.-a. Portion of a secreting tubule from the cortical sub- stance of the kidney, b. The epithelial or gland-cells, x 700 times. larger tubes as they descend through these from the cortical portion ; while in the latter region they spread out more irregularly, mid become much branched and convoluted. But in the boundary zone between cortex and medulla, small collections of straight tubes called medullary rays project into the cortical region. Tubuli Uriniferi.-The tubuli uriniferi (fig. 548) are composed of a basement membrane, lined internally by epithelium. They vary considerably in size in different parts of their course, but are, on an average, about of an inch mm.) in diameter, and are found to be made up of several distinct sections which differ from one another very markedly, both in situation and structure. CH. LI.] THE KIDNEY. 687 Each begins in the cortex as a dilatation called the Capsule of Bowman ; this encloses a tuft or glomerulus of capillaries called Fig. 549.-A diagram of the sections of uriniferous tubes. A, cortex limited externally by the capsule; a, subcapsular layer not containing Malpighian corpuscles; a', inner stratum of cortex, also without Malpighian capsules ; B, boundary layer; C, papillary part next the boundary layer; 1, Bowman's capsule of Malpighian corpuscle ; 2, neck of capsule; 3, first convoluted tubule; 4, spiral tubule; 5, descending limb of Henle's loop ; 6, the loop proper; 7, thick part of the ascending limb ; 8, spiral part of ascending limb; 9, narrow ascending limb in the medullary ray; 10, the zigzag tubule ; n, the second convoluted tubule; 12, the junctional tubule; 13, the collecting tubule of the medullary ray ; 14, the collecting tube of the boundary'layer ; 13, duct of Bellini. (Klein.) 688 THE URINARY APPARATUS. [ch. li. a Malpighian corpuscle. The tubule leaves the capsule by a neck, and then becomes convoluted (first convoluted tubule), but soon after becomes nearly straight or slightly spiral (spiral tubule) ; then rapidly narrowing it passes down into the medulla as the descending tubule of Henle; this turns round, forming a loop (loop of Henle), and passes up to the cortex again as the ascending tubule of Henle. It Fig. 550.-Transverse section of a deve- loping Malpighian capsule and tuft (human), x 300. From a foetus at about the fourth month ; a, flattened cells growing to form the capsule ; b, more rounded cells ; continuous with the above, reflected round c, and finally enveloping it; c, mass of embryonic cells which will later become developed into blood-vessels. (W. Pye.) Fig. 551.-Epithelial elements of a Malpi- ghian capsule and tuft, with the com- mencement of a urinary tubule showing the afferent and efferent vessel; a, layer of fiat epithelium forming the capsule ; b, similar, but rather larger epithelial cells, placed in the walls of the tube ; c, cells, covering the vessels of the capil- lary tuft; d, commencement of the tubule, somewhat narrower than the rest of it. (W. Pye.) then becomes larger and irregularly zigzag (zigzag tubule) and again convoluted (second convoluted tubule). Eventually it nar- rows into a junctional tubule, which joins a straight or collecting tubule. This passes straight through the medulla, where it joins with others to form one of the ducts of Bellini that open at the apex of the pyramid. These parts are all shown in fig. 549. The character of the epithelium that lines these several parts of the tubules is as follows :- In the capsule, the epithelium is flattened and reflected over the glomerulus. The way in which this takes place in process of development is shown in figs. 550 and 551. In the the epithelium is still flattened, but in some animals, like frogs, where the neck is longer, the epithelium is ciliated. CH. LI.] THE KIDNEY. 689 Fig. 552.-From a vertical section through the kidney of a dog-the capsule of which is supposed to be on the right, a, the capillaries of the Malpighian corpuscle, which are arranged in lobules ; n, neck of capsule; c, convoluted tubes cut in various directions ; b, zigzag tubule ; d, e, and /, are straight tubes in a medullary ray; d, collecting tube ; e, spiral tube ; /, narrow section of ascending limb, x 380. (Klein and Noble Smith.) Fig. 553.-Transverse section of a renal papilla; a, large tubes or ducts of Bellini; &, c, and d, smaller tubes of Henle ; e, f, blood capillaries, distinguished by their flatter epithelium. (Cadiat.) 690 THE URINARY APPARATUS. [ch. li. In the first convoluted and spiral tubules, it is thick and the cells show a fibrillated structure, except around the nucleus, where the protoplasm is granular. The cells inter- lock laterally and are diffi- cult to isolate. In some animals they are described as ciliated. In the narrow descending tubule of Henle and in the loop itself, the cells are clearand flattened and leave a considerable lumen; in the ascending limb they again become striated and nearly fill the tubule. In the zigzag and second convoluted tubules the fibrillations become even more marked. The junctional tubule has a large lumen, and is lined by clear flattened cells ; the collecting tubules and of Bellini are lined by clear cubical or colum- nar cells. Blood - vessels of Kidney. - The renal artery enters the kidney at the hilus, and divides into branches that pass towards the cortex, then turn over and form in- complete arches in the region between cortex and medulla. From these arches vessels pass to the surface which are called the interlobular arteries; they give off vessels at right angles, which are the afferent vessels of the glomeruli; a glomerulus is made up of capillaries as previously stated. From each, a smaller vessel (the efferent vessel of the Fig. 554.-Vascular supply of kidney, a, part of arterial arch; ft, interlobular artery; c, glomeru- lus ; d, efferent vessel passing to the medulla as false arteria recta; e, capillaries of cortex ; /, capillaries of medulla ; g, venous arch ; A, straight veins of medulla; j, vena stellula ; i, interlobular vein. (Cadiat.) CH. LI.] THE KIDNEY. 691 glomerulus') passes out, and like a portal vessel on a small scale, breaks up once more into capillaries which ramify between the Fig. 555.-Diagram showing the relation of the Malpighian body to the uriniferous ducts and blood-vessels, a, one of the interlobular arteries ; a', afferent artery passing into the glomerulus; c, capsule of the Malpighian body, forming the termination of and continuous with t, the uriniferous tube ; e', e', efferent vessels which subdivide in the plexus, p, surrounding the tube, and finally terminate in the branch of the renal vein e. (After Bowman.) Fig'. 556.-Malpighian corpuscle, injected through the renal artery with coloured gelatin ; a, glomerular vessels; b, capsule; c, anterior capsule; d, glomerular artery; e, efferent veins ; /, epithelium of tubes. (Cadiat.) convoluted tubules. These unite to form veins {interlobular veins) which accompany the interlobular arteries ; they pass to venous Y Y 2 692 THE URINARY APPARATUS. [CH. LI. arches, parallel to, but more complete than the corresponding arterial arches ; they ultimately unite to form the renal vein that leaves the hilus. These veins receive also others which have a stellate arrangement near the capsule (vence stelluloe). The medulla is supplied by pencils of fine straight arterioles which arise from the arterial arches. They are called arteries rectoe. The efferent vessels of the glomeruli nearest the medulla may also break up into similar vessels which are called false arteries rectos. The veins (venoe rectos') take a similar course and empty themselves into the venous arches. In the boundary zone groups of vasa recta alternate with groups of tubules, and give a striated appearance to this portion of the medulla. The Ureters.-The duct of each kidney, or ureter, is a tube about the size of a goose-quill, and from twelve to sixteen inches in length, which, continuous above with the pelvis, ends below by perforating obliquely the walls of the bladder, and opening on its internal surface. It is constructed of three coats : (a) an outer, fibrous coat; (6) a middle muscular coat, of which the fibres are unstriped, and arranged in three layers-the fibres of the central layer being circular, and those of the other two longitudinal in direction ; the outermost longitudinal layer is, however, present only in the lower part of the ureter; and (e) a mucous membrane continuous with that of the pelvis above, and of the urinary bladder below. It is composed of areolar tissue lined by transitional epithelium. The Urinary Bladder, which forms a receptacle for the temporary lodgment of the urine in the intervals of its expulsion from the body, is pyriform, its widest part, which is situate above and behind, being termed the fundus ; and the narrow constricted portion in front and below, by which it becomes continuous with the urethra, being called its cervix or neck. It is constructed of four coats,-serous, muscular, areolar or sw&wwzcotts, and mucous, (a.) The serous coat, which covers only the posterior and upper part of the bladder, has the same structure as that of the peritoneum, with which it is continuous. (6) The fibres of the muscular coat, wThich are un- striped, are arranged in three layers, of which the external and internal have a general longitudinal, and the middle layer a circular direction. The latter are especially developed around the cervix of the organ and form the sphincter vesicce. (c.) The areolar or submucous coat is constructed of connective tissue with a large proportion of elastic fibres, (d.) The mucous membrane is like that of the ureters. It is provided with mucous glands, which are most numerous near the neck of the bladder. CH. LI.] THE URETHRA. 693 The bladder is well provided with blood- and lymph-vessels, and with nerves. The latter are both medullated and non-medullated fibres, and consist of branches from the sacral plexus (spinal) and hypogastric plexus (sympathetic). Ganglion-cells are found, here and there, in the course of the nerve-fibres. The Urethra.-This occupies the centre of the corpus spongiosum in the male. As it passes through the prostate it is lined by transitional, but elsewhere by columnar epithelium, except near the orifice, where it is stratified like the epidermis with which it becomes continuous. The female urethra has Fig. 557.-Section of a small portion of the prostate, a, gland duct cut across obliquely; b, gland structure ; c, prostatic calculus. (Cadiat.) stratified epithelium throughout. The epithelium rests on a vascular corium, and this is covered by submucous tissue con- taining an inner longitudinal and an outer circular muscular layer. Outside this a plexus of veins passes insensibly into the surrounding erectile tissue. Into the urethra open a number of oblique recesses or lacunce, a number of small mucous glands (Glands of Littre), two com- pound racemose glands (Cowper's glands), the glands of the prostate, and the vas deferens. The prostate, which surrounds the commencement of the male urethra, is a muscular and glandular mass. Its glands are tubular and lined by columnar epithelium. The Functions of the Kidneys. The main function of the kidneys is to separate the urine from the blood. The true secreting part of the kidney is the glandular 694 THE URINARY APPARATUS. [ch. li. epithelium that lines the convoluted portions of the tubules ; there is in addition to this what is usually termed the filtering apparatus : we have already seen that the tufts of capillary blood- vessels called the Malpighian glomeruli are supplied with afferent vessels from the renal artery ; the efferent vessels that leave these have a smaller calibre, and thus there is high pressure in the Malpighian capillaries. Certain constituents of the blood, especially water and salts, pass through the thin walls of these vessels into the surrounding Bowman's capsule which forms the commencement of each renal tubule. Though the process which occurs here is generally spoken of as a filtration, yet it is no purely mechanical process, but the cells exercise a selective influence, and prevent the albuminous constituents of the blood from escaping. During the passage of the water which leaves the blood at the glomerulus through the rest of the renal tubule, it gains the constituents urea, urates, <fcc., which are poured into it by the secreting cells of the convoluted tubules. The term excretion is better than secretion as applied to the kidney, for the constituents of the urine are not actually formed in the kidney itself (as, for instance, the bile is formed in the liver), but they are formed elsewhere; the kidney is simply the place where they are picked out from the blood and eliminated from the body. The Nerves of the Kidney. Nerves.-The nerves of the kidney are derived from the renal plexus of each side. This consists of both medullated and non- medullated nerve-fibres, the former of varying size, and of nerve- cells. The renal plexus is derived from the solar plexus, par- ticularly from the semilunar ganglion. The renal plexus is thus indirectly connected with the vagi and with the splanchnic nerves. It is also directly connected with them by fibres which pass to them without first joining the solar plexus. Fibres from the anterior roots of the tenth, eleventh, twelfth, and thirteenth dorsal nerves in the dog also pass to the same plexus, either directly through the sympathetic chain or by first passing into the solar plexus. These nerves are vaso-motor in function ; we have at present no knowledge of true secretory nerves to the kidney; the amount of urine varies directly with the blood pressure in its capil- laries. Increase in the quantity of urine is caused by a rise of intra- capillary pressure. This may be produced by increasing the general blood pressure ; and this in turn may be produced in the following ways :- CH. LI.] NERVES OF THE KIDNEY. 695 1. By increase in the force or frequency of the heart beat. 2. By constriction of the arterioles of areas other than that of the kidney, as in cold weather, when the cutaneous capillaries are constricted.* . 3. By increase in the total contents of the vascular system, as after drinking large quantities of fluid. The blood pressure in the renal capillaries may also be increased locally by anything which leads to relaxation of the renal arterioles. Decrease in the quantity of urine is produced by the opposites in each case. If the renal nerves are divided, the renal arterioles are relaxed, and pressure in the renal capillaries is raised, so there is an increased flow of urine. This is accompanied by an increase in the volume of the kidney, as can be seen by the oncometer. Stimulation of the divided nerves produces a diminution in the amount of urine, and a shrinkage of the kidney due to a con- striction of its blood-vessels, f If the splanchnic nerves are experimented with instead of the renal, the effects are not so marked, as these nerves have a wide distribution, and section leads to vascular dilatation in the whole splanchnic area ; hence the increase in pressure in the renal capil- laries is not so noticeable. Puncture of the floor of the fourth ventricle in the neighbour- hood of the vaso-motor centre (close to the spot puncture of which produces glycosuria) leads to a relaxation of the renal arterioles and a consequent large increase of urine (polyuria). Section of the spinal cord just below the medulla causes a cessation of secretion of urine, because of the great fall of general blood pressure which occurs. If the animal is kept alive, however, blood pressure goes up after a time, owing to the action of subsidiary vaso-motor centres in the cord. When this has occurred stimulation of the peripheral end of the cut spinal cord again causes urinary secretion to stop, because the renal artery (like the other arteries of the body) is so constricted that the pressure in the renal capillaries becomes too low for secretion to occur. The Oncometer, which has just been referred to, is an instru- ment constructed on plethysmographic principles, by means of which the volume of the kidney is registered. The general characters of this instrument are described with diagrams on * The reciprocal action between skin and kidneys will be discussed more fully in the chapter on the skin. T The nerves also contain vaso-dilatator fibres, which are excited when a slow rate of stimulation is used (see p. 477). 696 THE URINARY APPARATUS. [ch. li. p. 478. The special form adapted for the kidney is shown in the next figure. Fig. 558.-Oncometers for kidneys of different sizes. It is found that the effect on the volume of the organ of dividing or stimulating nerves corresponds to blood pressure. A rise of blood pressure in the renal artery is produced by constriction of the renal arterioles ; this is accompanied by a fall of pressure in the renal capillaries, and a shrinkage of the kidney. Increase in the volume of the kidney is produced by the opposite circumstances. The accompanying figure (559) shows that in a kidney curve Fig. 559.-Curve taken by renal oncometer compared with that of ordinary blood pressure, a, Kidney curve; ft, blood-pressure curve. (Roy.) one gets a rise of volume due to each heart beat, and larger waves which accompany respiration. In many cases larger sweeping waves (Traube-Hering curves) are often shown as well. If a kidney curve is compared with a tracing of arterial pressure, it will be seen that the rise of arterial pressure coincides with a fall of the oncograph lever due to constriction of the renal vessels. CH. LT.] THE RENAL EPITHELIUM. 697 Diuretics are drugs which produce an increased flow of urine; they act in various ways, some by increasing the general blood pressure, others by acting locally upon the kidney (increasing its volume as measured by the oncometer); under this latter head are doubtless to be included some also which act on the renal epithelium rather than on the blood-vessels. Activity of the Renal Epithelium. The epithelium of the convoluted tubules has a structure which suggests from its resemblance to other forms of secreting epitheliums, that its function here also is secreting. This is confirmed by the manner in which the blood-vessels break up into capillaries around these tubules; and is further confirmed by experiments. Heidenhain has shown that if a substance (sodium sulph- indigotate), which ordinarily produces blue urine, is injected into the blood (after section of the medulla which causes lowering of the blood pressure in the renal glomeruli), that when the kidney is examined, the cells of the convoluted tubules (and of these alone) are stained with the substance, which is also found in the lumen of the tubules. This shows that the pig- ment at any rate is eliminated by the cells of the convoluted tubules, and that when by diminishing the blood-pressure, the filtration of urine ceases, the pigment remains in the convoluted tubes, and is not, as it would be under ordinary circumstances, swept away from them by the flushing of them by the watery part of urine derived from the glomeruli. It therefore is probable that the cells, if they excrete the pigment, excrete urea and other sub- stances also. But the proof is not absolute, for the pigment is a foreign substance. Urea is a very difficult substance to trace in this way because it does not leave any coloured trail behind it. In birds the place of urea is taken by uric acid, and the urates can be actually traced, because they are deposited as crystals, and can be seen in the cells and convoluted tubes much in the same way as Heidenhain's blue pigment. Another series of experiments, however, has proved the point for the case of urea. By using the kidney of the frog or newt, which has two distinct vascular supplies, one from the renal artery to the glomeruli, and the other from the renal-portal vein to the convoluted tubes, Nuss- baum has shown that certain substances, e.g. peptones and sugar, when injected into the blood, are eliminated by the glomeruli, 698 THE URINARY APPARATUS. [CH. LI. and so are not got rid of when the renal arteries are tied; whereas certain other substances, e.g. urea, when injected into the blood, arc eliminated by the convoluted tubes, even wThen the renal arteries have been tied. This evidence is very direct that urea is excreted by the convoluted tubes, and cannot be considered to be invalidated by the statement made by Adami that there is a slight anastomosis between the two sets of vessels. If the part of the cortex of the kidney which contains the glomeruli is removed, urea still continues to be formed. This is an additional proof that the excretion is performed by the portions of the convoluted tubules that remain. The Work done by the Kidney. Recent work by Starling, Hamburger, Dreser, and others has shown the great importance a proper study of osmosis in the body has in the understanding of many physiological facts. The subject is by no means a simple one, but the following account of its bearing on urinary secretion (abstracted from Star- ling) will not lead us into anything very abstruse. We have already seen that the urine is separated from the blood by a process which is not the simple one called filtration. This is further supported by the fact that it is possible to measure the work of the kidney, and it is found to be vastly greater than could be carried out by the intra-capillary blood pressure. The following facts will also teach us that reabsorption of water cannot, as Ludwig held, take place in the tubules. The measurement of the work done by the kidney depends upon a determination of the respective osmotic pressures of the urine and blood plasma. If a bladder containing strong salt solution is placed in a vessel of distilled water, water passes into the bladder by diffusion or osmosis, so that the bladder is swollen, and a manometer con- nected with its interior will show a rise of pressure (osmotic pressure). But the total rise of pressure cannot be measured in this way for two reasons : (i) because the salt diffuses out as the water diffuses in ; and (2) because the membrane of the bladder leaks; that is, permits of filtration when the pressure within it has attained a certain height. It is therefore necessary to use a membrane which will not allow salt to pass out either by osmosis or filtration, though it will let the water pass in. Such membranes are called semi-permeable membranes, and one of the best of these is ferrocyanide of copper. This may be made by taking a cell of porous earthenware and CH. LI.] THE WORK OF THE KIDNEY. 699 washing it out first with copper sulphate and then with potassium ferrocyanide. An insoluble precipitate of copper ferrocyanide is thus deposited in the pores of the earthenware. If such a cell is arranged as in fig. 560, and filled with a 1 per cent, solution of sodium chloride, water diffuses in till the pressure registered by the manometer reaches the enormous height of 5000 millim. of mercury. If the pressure in the cell is increased beyond this artificially, water will be pressed through the semi- permeable walls of the cell and the solution will become more concentrated. In other words, in order to make a solution of sodium chloride of greater concentration than 1 per cent., a pres- sure greater than 5000 mm. of mercury must be employed. It is, moreover, found that the osmotic pressures of various solutions depend merely on the number of molecules of any substance present; the nature of the substance makes no difference. The osmotic pressure is, in fact, equal to that which the dis- solved substance would exert if it occupied the same space in the form of a gas. Hence, if the osmotic pressures of blood plasma and urine are determined, the work done by the kidney cells in order to separate from the blood plasma a fluid with the osmotic pressure of the urine, can be estimated. The actual method of estimating osmotic pressures is not by means of a manometer as in the diagram, but by the depression of the freezing point that occurs when substances are dissolved in water. A full consideration of this method would, however, lead us too deeply into mathematical considerations. We may take some examples from Dreser's work. He took the case in which 200 c.c. of urine were excreted during a night; the blood plasma in this case had an osmotic pressure = 0'92 per cent, solution; while that of the urine was = 4/0 per cent, solution of sodium chloride. In this case the kidney had performed 37 kilogramme- metres of work. In another case of more concentrated urine Kg. 560.-A, outer vessel, con- taining distilled water; B, inner semi-permeable vessel containing 1 per cent, salt solution; M, mercurial manometer. (After Star- ling.) 700 THE URINARY APPARATUS. [ch. li. obtained from a cat previously deprived of water for three days the numbers were respectively ri and 8'o. The difference was equal to a pressure of 498 metres of water; so that the kidney had separated urine from the blood against a pressure of 49,800 grammes per square centimetre, a force about six times greater than the maximum force of voluntary muscle. Extirpation of the Kidneys. Extirpation of one kidney for various diseases (stone, etc.) is a by no means uncommon operation. It is not followed by any untoward result. The remaining kidney enlarges and does the work previously shared between the two. Extirpation of both kidneys is fatal ; the urea, etc., accumulate in the blood, and the animal dies in a condition of deep coma preceded by convulsions (uraemia). Ligature of both renal arteries practically amounts to the same thing as extirpation of the kidneys, and leads to the same result. If the ligature is released the kidney once more sets to work, but the urine secreted is then albuminous, owing to the epithelium having been impaired by being deprived for a time of its normal blood supply. Removal of one kidney, followed at a later period by removal of a half or two-thirds of the other leads in dogs, in which the operation has been performed by Bradford, to a surprising result. After the second operation the urine is increased in amount, and the quantity of urea is much greater than normal. This comes from a disintegration of the nitrogenous tissues; the animal wastes rapidly and dies in a few weeks. It is thus evident that the kidneys play an important part in nitrogenous metabolism apart from merely excreting waste substances. The exact explanation has still to be found, but it is probable that the kidney, like the pancreas and liver, and many ductless glands, forms an internal secretion (see pp. 500 and 652). The Passage of Urine into the Bladder. As each portion of urine is secreted it propels that which is already in the uriniferous tubes onwards into the pelvis of the kidney. 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 vesicce, i.e. of such fissures in the anterior or lower part of the walls of the abdomen, and of the CH. LI.] MICTURITION. 701 front wall of the bladder, as expose to view its hinder wall together with the orifices of the ureters. 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 raises up the little papilla through which the ureter opens, and then passes slowly through its orifice, which at once again closes like a sphincter. In the recumbent pos- ture, 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 aided by the peristaltic contractions of the ureters, and is increased in deep inspira- tion, or by straining, and in active exercise, and in fifteen or twenty minutes after a meal. The urine is prevented from regurgitation into the ureters by the mode in which these pass through the walls of the bladder, namely, by their lying for between half and three-quarters of an inch between the muscular and mucous coats before they turn rather abruptly forwards, and open through the latter into the interior of the bladder. Micturition. The contraction of the muscular walls of the bladder may by itself expel the urine with little or no help from other muscles. In so far, however, as it is a voluntary act, it is performed by means of the abdominal and other expiratory muscles, which in their contraction, press on the abdominal viscera, the diaphragm being fixed, and cause the expulsion of the contents of those whose sphincter muscles are at the same time relaxed. The muscular coat of the bladder co- operates, in micturition, by reflex involuntary action, with the abdominal muscles ; and the act is completed by the accelerator urince, which, as its name implies, quickens the stream, and expels the last drop of urine from the urethra. The act, so far as it is not directed by volition, is under the control of a nervous centre in the lumbar spinal cord, through which, as in the case of the similar centre for defeecation, the various muscles concerned are harmonized in their action. It is well known that the act may be reflexly induced, e.g. in children -who suffer from intestinal worms, or other such irritation. Generally the afferent impulse which calls into action the desire to micturate is excited by over distension of the bladder, or even by a few drops of urine passing into the urethra. This passes up to the lumbar centre and 702 THE URINE. [ch. lii. produces on the one hand inhibition of the sphincter and on the other hand contraction of the necessary muscles for the expulsion of the contents of the bladder. The tonic action of the lumbar centre can also be inhibited by the will. The bladder receives nerves from two sources :-(1) from the lower dorsal and upper lumbar nerves ; these fibres pass to the sympathetic chain, from here to the inferior mesenteric gang- lion, and ultimately reach the bladder by the hypogastric nerves. Stimulation of these nerves causes contraction of the circular fibres of the bladder including the sphincter; (2) from the second and third sacral nerves; these run to the bladder by the nervi erigentes. Stimulation of these nerves causes relaxation of the sphincter and contraction of the detrusor urinse. CHAPTER LII. THE URINE. Quantity.-A man of average weight and height passes from 1,400 to 1,600 c.c., or about 50 oz. daily. This contains about 50 grammes oz.) of solids. For analytical purposes it should be collected in a tall glass vessel capable of holding 3,000 c.c., which should have a smooth-edged neck accurately covered by a ground-glass plate to exclude dust and avoid evaporation. The vessel, moreover, should be graduated so that the amount may be easily read off. From the total quantity thus collected in the twenty-four hours, samples should be drawn off for examina- tion. Colour.-This is some shade of yellow which varies consider- ably in health with the concentration of the urine. It is due to a mixture of pigments ; of these urobilin is the one of which we have the most accurate knowledge. Urobilin has a reddish tint and is undoubtedly derived from the blood pigment, and, like bile pigment, is an iron-free derivative of haemoglobin. The theory usually accepted concerning its mode of origin is that bile pigment is in the intestines converted into stercobilin; that most of the stercobilin leaves the body with the faeces ; that some is reabsorbed and is excreted with the urine as urobilin. Both stercobilin and urobilin are very like the artificial reduction pro- CH. LII.J THE URINE. 703 duct of bilirubin called hydrobilirubin (see p. 663). Normal urine, however, contains very little urobilin. The actual body present is a chromogen or mother substance called urobilinogen, which by oxidation, for instance standing exposed to the air, is converted into the pigment proper. In certain diseased condi- tions the amount of urobilin is considerably increased. The yellow pigment of the urine called urochrome shows no absorption bands; its mode of origin in the body, and its relationship to urobilin, if any exists, are not known. Reaction.-The reaction of normal urine is acid. This is not due to free acid, as the uric and hippuric acids in the urine are combined as urates and hippurates respectively. The acidity is due to acid sodium phosphate. Under certain circumstances the urine becomes less acid and even alkaline; the most important of these are as follows :- 1. During digestion. Here there is a formation of free acid in the stomach, and a corresponding liberation of bases in the blood which passing into the urine diminish its acidity, or even render it alkaline. This is called fAe alkaline tide ; the opposite condi- tion, the acid tide, occurs after a fast-for instance, before breakfast. 2. In herbivorous animals and vegetarians. The food here contains excess of alkaline salts of acids like tartaric, citric, malic, &c. These acids are oxidised into carbonates, which passing into the urine give it an alkaline reaction. Specific Gravity.-This should be taken in a sample of the twenty-four hours' urine with a urinometer. The specific gravity varies inversely as the quantity of urine passed under normal conditions from 1015 to 1025. A specific gravity below 1010 should excite suspicion of hydruria; one over 1030, of a febrile condition, or of diabetes, a disease in which it may rise to 1050. The specific gravity has, however, been known to sink as low as 1002 (after large potations, urina potus), or to rise as high as 1035 (after great sweating) in perfectly healthy persons. Composition.-The following table gives the average amounts of the urinary constituents passed by a man in the twenty-four hours :- Water ........ 1500'00 grammes. Total solids 72'00 „ Urea 33'iS Uric acid C55 ,, Hippuric acid 0'40 „ Creatinine 0-91 „ Pigment and other organic substances . io'oo „ 704 THE URINE. [CH. lit. Sulphuric acid . . . 2OI grammes Phosphoric acid • 3'16 Chlorine .... • ■ 7'5° Ammonia . . . . 0'77 Potassium .... . . 2'50 •? Sodium . . . . . . 11-09 jj Calcium .... . . 0'26 Magnesium . . . . 0'21 The most abundant constituents of the urine are water, urea, and sodium chloride. In the foregoing table one must not be misled by seeing the names of the acids and metals separated. The acids and the bases are combined to form salts, such as urates, chlorides, sulphates, phosphates, &c. Urea. Urea, or Carbamide, CO(NH2)2, is isomeric (that is, has the same empirical, but not the same structural formula) with ammonium cyanate (NHJCNO, from which it was first prepared synthetically by Wohler in 1828. Since then it has been pre- pared synthetically in other ways. Wohler's observation derives interest from the fact that this was the first organic substance which was prepared synthetically by chemists. When crystallised out from the urine it is found to be readily soluble both in water and alcohol: it has a saltish taste, and is neutral to litmus paper. The form of its crystals is shown in fig. 561. When treated with nitric acid, nitrate of urea (CON2H4.HNO3) is formed; this crystallises in octahedra, lozenge-shaped tablets, or hexagons fig. 562); When treated with oxalic acid, flat or prismatic crystals of urea oxalate (CON2H4.H2C,O4 + H,O) are formed (fig. 563). These crystals may be readily obtained by adding excess of the respective acids to urine which has been concentrated to a third or a quarter of its bulk. Under the influence of an organised ferment, the torula or micrococcus urete, which grows readily in stale urine, urea takes up water, and is converted into ammonium carbonate Fig. 561.-Crystals of Urea. CH. LU. ] UREA. 705 [CON2Ht+2H2O = Hence the ammoniacal odour of putrid urine. By means of nitrous acid, urea is broken up into carbonic acid, water and nitrogen, CON2H4+2HN02 = CO2+3H2O + 2N2. The Fig. 562.-Crystals of Urea nitrate. Fig. 563.-Crystals of Urea oxalate. evolution of gas bubbles which takes place on the addition of fuming nitric acid may be used as a test for urea. Hypobromite of soda decomposes urea in the following way CON2Ht + 3NaBrO = CO2 + N2 + 2H2O + 3NaBr [urea] [sodium hypobromite] [carbonic acid] [nitrogen] [water] [sodium bromidi] This reaction is important, for on it one of the best methods for estimating urea depends. There have been various pieces of apparatus invented for rendering the analysis-easy ; but the one described below is the best. If the experiment is performed as directed, nitrogen is the only gas that comes off, the carbonic acid being absorbed by excess of soda. The amount of nitrogen is a measure of the amount of urea. Dupre's apparatus (fig. 564) consists of a bottle (A) united to a measuring tube by indiarubber tubing. The measuring tube (C) is placed within a cylinder of water (D), and can be raised and lowered at will. Measure 25 c.c. of alkaline solution of sodium hypobromite (made by mixing 2 c.c. of bromine with 23 c.c. of a 40 per cent, solution of caustic soda) into the bottle A. Measure 5 c.c. of urine into a small tube (B), and lower it care- fully, so that no urine spills, into the bottle. Close the bottle securely with a stopper perforated by a glass tube ; this glass tube (the bulb blown on this tube prevents froth from passing into the rest of the apparatus) is connected to the measuring tube by indiarubber tubing and a T-piece. The third limb of the T-piece is closed by a piece of indiarubber tubing and a pinch-cock, seen at the top of the figure. Open the pinch-cock and lower the measuring tube until the surface of the water with which the outer cylinder is filled is at the zero point of the graduation. Close the pinch-cock, and raise the measuring tube to ascertain if the apparatus is air-tight. Then lower it again. Tilt the bottle so as to upset the urine, and shake well for a minute or so. During this time there is an evolution of gas. Thpn immerse the bottle in a large beaker containing water of the same temperature as that in the cylinder. After two or three minutes raise the measuring tube until the surfaces of the water inside and outside it are at the same level. Read off 706 THE URINE. [ch. lii. the amount of gas (nitrogen) evolved. 35-4 c.c. of nitrogen are yielded by o'1 gramme of urea. From this the quantity of urea in the 5 c.c. of urine and the percentage of urea can be calculated. If the total urea passed in the twenty-four hours is to be ascertained, the twenty-four hours' urine must be carefully measured and thoroughly mixed. A sample is then taken from the total for analysis; and then, by a simple sum in proportion, the total amount of urea is ascertained. Sometimes the measuring tubes supplied with this apparatus are graduated in divisions corresponding to percentages of urea. Another method (Liebig's) of estimating urea in urine is the following :-Take 40 c c. of urine ; add to this 20 c.c. of baryta mixture (two volumes of barium hydrate and one of barium nitrate, both saturated in the cold). Filter off the precipitate of barium phosphate and. sulphate which is formed. Take 15 c.c. of the filtrate (this corresponds to 10 c.c. of urine) in a beaker. Run into it standard mercuric nitrate solution from a burette of such a strength that 1 c.c. exactly precipitates 0 01 gramme of urea as a compound with the formula (CON2H+)2Hg(NO3)2(HgO)3. The solution is run in until the precipitate ceases to form, and free mercuric nitrate is present in the mixture ; this can be detected by the yellow colour a drop of the mixture gives with a drop of saturated solution of sodium car- bonate on a white slab. The amount used from the burette can be read off. and the percentage of urea calculated. In another specimen of the same urine, the chlorides are then esti- mated. and 1 gramme of urea subtracted for every 1 -3 gramme of sodium chloride formed. The quantity of urea is somewhat variable, the chief cause of variation being the amount of proteid food in- gested. In a man in a state of equili- brium the quantity of urea secreted daily averages 33 grammes (500 grains). The normal percentage in human urine is 2 per cent. ; but this also varies, because the concentration of the urine varies considerably in health. In dogs it may be 10 per cent. The excretion of urea is usually at a maximum three hours after a meal, especially after a meal rich in proteids. The urea docs not come, however, direct from the food; the food must be first assimilated, and become part of the body before it can break down to form urea. An exception to this rule is to be found in the case of the amido-acids, especially leucine and lysatinine, which are formed in the intestinal canal Fig. 564.-Dupre's urea apparatus. CH. LU.] FORMATION OF UREA. 707 from proteids during digestion. These substances are carried to the liver, and there converted into urea; but only a very small fraction of the urea in the urine is formed in this way. Food increases the elimination of urea because it stimulates the tissues to increased activity ; their waste nitrogenous products are converted into urea, which, passing into the blood is directly excreted by the kidneys. The greater the amount of proteid food given, the more waste products do the tissues discharge from their proto- plasm, in order to make room for the new proteid which is built into its substance. Muscular exercise has little immediate effect on the amount of urea discharged. In very intense muscular work there is a slight immediate increase of urea, but this is quite insignificant when compared to the increase of work. This is strikingly different from what occurs in the case of carbonic acid ; the more the muscles work, the more carbonic acid do they send into the venous blood, which is rapidly discharged by the expired air. Recent carefid research has, however, shown that an increase of nitrogenous waste does occur on muscular exertion, but appears as urea in the urine to only a slight extent on the day of the work ; the major part being excreted during the next day. Where is Urea formed ?-The older authors considered that it was formed in the kidneys, just as they also erroneously thought that carbonic acid was formed in the lungs. Prevost and Dumas were the first to show that after complete extirpation of the kidneys the formation of urea goes on, and that it accu- mulates in the blood and tissues. Similarly, in those cases of disease in which the kidneys cease work, urea is still formed and accumulates. This condition is called urcemia (or urea in the blood), and unless the urea be discharged from the body the patient dies. There is no doubt, however, that it is not urea but some antecedent of urea that acts most poisonously, and is the cause of death. Where, then, is the seat of urea formation ? Nitrogenous waste occurs in all the living tissues, and the principal final result of this proteid metabolism is urea. It may not be that the formation of urea is perfected in each tissue, for if we look to the most abundant tissue, the muscular tissue, urea is absent, or nearly so. Yet there can be no doubt that the chief place from which urea ultimately comes is the muscular tissue. Some inter- mediate step occurs in the muscles ; the final steps occur elsewhere. In muscles we find a substance called creatine in fairly large quantities. If creatine is injected into the blood it is discharged as creatinin. But there is very little creatinine in normal 708 THE URINE. [ch. lit. urine ; what little there is can be nearly all accounted for by the creatine in the food ; the muscular creatine is discharged as urea ; in fact, urea can be artificially obtained from creatine in the laboratory. Similarly, other cellular organs, spleen, lymphatic glands, secreting glands, participate in the formation of urea; but the most important appears to be the liver : this is the organ where the final changes take place. The urea is then carried by the blood to the kidney, and is there excreted. The facts of experiment and of pathology point very strongly in support of the theory that urea is formed in the liver. The principal are the following :- 1. After removal of the liver in such animals as frogs, urea formation almost ceases, and ammonia is found in the urine instead. 2. When degenerative changes occur in the liver, as in cirrhosis of that organ, the urea formed is much lessened, and its place is taken by ammonia. In acute yellow atrophy urea is almost absent in the urine, and, again, there is considerable increase in the ammonia. In this disease leucine and tyrosine are also found in the urine ; undue stress should not be laid upon this latter fact, for the leucine and tyrosine doubtless originate in the intestine, and, escaping further decomposition in the degenerated liver, pass as such into the urine. We have to consider next the intermediate stages between proteid and urea. A few years ago Drechsel succeeded in arti- ficially producing urea from casein. More recent work has shown that this is true for other proteids also. If a proteid is decom- posed by hydrochloric acid, a little stannous chloride being added to prevent oxidation, a number of products are obtained such as ammonium salts, leucine, tyrosine, aspartic, and glutaminic acids. This was known before, so the chief interest centres round two new substances, precipitable by phosphotungstic acid. One of these is called lysine (C6HUN2O2, probably di-amido-caproic acid) : the other is called lysatine or lysatinine. This has the formula C6H13N3O2 or C6HUN3O, and is a homologue of either creatine (C4H9N3O2) or creatinine (CtH7N3O), according as the first or second formula is correct. Arguing from this analogy, Drechsel expected to be able to obtain urea from it, and his ex- pectation was confirmed by experiment. He took a silver com- pound of lysatine, boiled it with barium carbonate, and after twenty-five minutes' boiling obtained urea.* It is, however, ex- * Hedin has shown quite recently that lysatine is a mixture of lysine and another base called arginine (C8H14N402) ; it is fiom the arginine that the urea comes in the experiments just described. CH. LII.J FORMATION OF UREA. 709 tremely doubtful whether the chemical decompositions produced in laboratory experiments on proteids are comparable to those occurring in the body. Many physiologists consider that the amido-acids are intermediate stages in the metallic processes that lead to the formation of urea from proteids. We have already alluded to this question in relation to the creatine of muscle, and we are confronted with the difficulty that injection of creatine into the blood leads to an increase not of urea, but of creatinine in the urine. If creatine is an intermediate step, it must undergo some further change before it leaves the muscle. Other amido acids, such as glycocine (amido-acetic acid) and leucine (amido-caproic acid) are probably to be included in the same category. The facts upon which such a theory depends are (i) that the introduction of glycocine or leucine into the bowel, or into the circulation, leads to an increase of urea in the urine ; there is, however, no evidence that tyrosine acts in this way ; and (2) that amido-acids appear in the urine of patients suffering from acute yellow atrophy of the liver. Then, again, it is per- fectly true that, in the laboratory, urea can be obtained from creatine, and also from uric acid, but such experiments do not prove that creatine or uric acid are normally intermediate pro- ducts of urea formation in the body. Still, if we admit for the sake of argument that amido acids are normally intermediate stages in proteid metabolism, and glance at their formulae- Glycocine, C2H5NO.„ Leucine, C6H13NO9, Creatine, CjHgN3O9, -we see that the carbon atoms are more numerous than the nitrogen atoms. In urea, CON2HP the reverse is the case. The amido acids must therefore be split into simpler compounds, which unite with one another to form urea. Urea formation is thus, in part, synthetic. There have been various theories ad- vanced as to what these simpler compounds are. Some have considered that cyanate, others that carbamate, and others still that carbonate of ammonium is formed. Schroder's work proves that ammonium carbonate is one of the urea precursors, if not the principal one. The equation which represents the reaction is as follows :- (NH4)2CO8 - 2H20 = con2h1 [Ammonium carbonate] [Water] [Urea]. Schroder's principal experiment was this : a mixture of blood and 710 THE URINE. [ch. lit. ammonium carbonate was injected into the liver by the portal vein ; the blood leaving the liver by the hepatic vein was found to contain urea in great abundance. This does not occur when the same experiment is performed with any other organ of the body, so that Schroder's experiments also prove the great import- ance of the liver in urea formation. There is, however, no necessity to suppose that the formation of amido acids is a necessary preliminary to urea formation. The conversion of the leucine and lysatinine formed in the intestine into ammonium salts and then into urea does certainly occur, but this only accounts for quite an insignificant fraction of the urea in the urine. If this also occurs in tissue metabolism, we ought to find considerable quantities of leucine, glycocine, creatine, lysatinine, and such substances in the blood, leaving the various tissues and entering the liver ; but we do not. We do, however, constantly find ammonia which, after passing into the blood or lymph, has united with carbonic acid to form either carbonate or carbamate of ammonium. It is quite probable that the nitro- genous waste that leaves the muscles and other tissues is split oft' from them as ammonia, and not in the shape of large molecules of amido-acid, which are subsequently converted into ammonia. The experiments outside the body which most closely imitate those occurring within the body are those of Drechsel, in which he passed strong alternating currents through solutions of proteid-like materials. Such alternating currents are certainly absent in the body, but their effect, which is a rapidly changing series of small oxidations and reductions, are analogous to meta- bolic processes; under such circumstances the carbon atoms are burnt oft' as carbon dioxide, the nitrogen being split off in the form of ammonia, and by the union of these two substances am- monium carbonate is formed. Uric Acid. Uric Acid (CjNjH t03) is in mammals, next to urea, the medium by which the largest quantity of nitrogen is excreted from the body. It is, however, in birds and reptiles the principal nitro- genous constituent of their urine. It is not present in the free state, but is combined with bases to form urates. It may be obtained from human urine by adding 5 c.c. of hydrochloric acid to 100 c.c. of the urine, and allowing the mix- ture to stand for twelve to twenty-four hours. The crystals which form are deeply tinged with urinary pigment, and though by repeated solution in caustic soda or potash, and precipitation CH. LII.] URIC ACID. 711 by hydrochloric acid, they may be obtained fairly free from pig- ment, pure uric acid is more readily obtained from the solid urine of a serpent or bird, which consists principally of the acid ammonium urate. This is dissolved in soda, and then the addi- tion of hydrochloric acid produces as before the crystallisation of uric acid from the solution. The pure acid crystallises in colourless rectangular plates or prisms. In striking contrast to urea it is a most insoluble sub- stance, requiring for its solution 1,900 parts of hot and 15,000 parts of cold water. The forms which uric acid assumes when precipitated from human urine, either by the addition of hydro- chloric acid oi' in certain patho- logical processes, arc very various, the most frequent being the whetstone shape; there are also bundles of crystals resembling- sheaves, barrels, and dumb-bells (see fig. 565). The murexide test is the princi- pal test for uric acid. The test has received the name on account of the resemblance of the colour to the purple of the ancients, which was obtained from certain snails of the genus Murex. It is per- formed as follows : place a little uric acid or a urate in a capsule ; add a little dilute nitric acid and evaporate to dryness. A yellowish-red residue is left. Add a little ammonia carefully, and the residue turns violet; this is due to the formation of purpurate of ammonia. On the addition of potash the colour becomes bluer. Another reaction that uric acid undergoes (though it is not applicable as a test) is that on treatment with certain oxidising reagents urea and oxalic acid can be obtained from it. It is, however, doubtful whether a similar oxidation occurs in the normal metabolic processes of the body. Uric acid is dibasic, and thus there are two classes of urates- the normal urates and the acid urates. A normal urate is one in which two atoms of the hydrogen are replaced by two of a monad metal like sodium ; an acid urate is one in which only one atom of hydrogen is thus replaced. The formulae would be- Fig. 565.-Various forms of uric acid crystals. C5H4N4O3 - uric acid. C5H3NaNtO3 = acid sodium urate. C5H2Na2NtO3 = normal sodium urate. 712 THE URINE. [CH. LU. The acid sodium urate is the chief constituent of the pinkish deposit of urates, which often occurs in urine, and is called the lateritious deposit. The quantity of uric acid excreted by an adult varies from 7 to 10 grains (0'5 to 0'75 gramme) daily. The best method for determining the quantity of uric acid in the urine is that of Hopkins. Ammonium chloride in crystals is added to the urine until no more will dissolve. This saturation completely precipitates all the uric acid in the form of ammo- nium urate. After standing for two hours the precipitate is collected on a filter, washed with saturated solution of ammonium chloride, and then dissolved in weak alkali. From this solution the uric acid is precipitated by neutralising with hydrochloric acid. The precipitate of uric acid is collected on a weighed filter, dried, and weighed. Origin of Uric Acid.-Uric acid is not made by the kidneys. When the kidneys are removed uric acid continues to be formed and accumulates in the organs, especially in the liver and spleen. The liver has been removed from birds, and uric acid is then hardly formed at all, its place being taken by ammonia and lactic acid. It is therefore probable that ammonia and lactic acid are normally synthesised in the liver to form uric acid. The two conditions which lead to an increase of uric acid in the urine are- 1. Increase of meat diet and diminution of oxidation processes, such as occur in people with sedentary habits. 2. Increase of white corpuscles in the blood, especially in the disease known as leucocytlicemia. This latter fact is of great interest, as leucocytes contain large quantities of nuclein. Nuclein yields nitrogenous bases (adenine, C5H5N5 ; hypoxanthine, which are closely related to uric acid. Hippuric Acid. Hippuric Acid (C9HyNO3), combined with bases to form hippurates, is present in small quantities in human urine, but in large quantities in the urine of herbivora. This is due to the food of herbivora containing substances belonging to the aromatic group-the benzoic acid series. If benzoic acid is given to a man, it unites with glycocine with the elimination of a molecule of water, and is excreted as hippuric acid- CH,NH.CO.C6H5 C6H5.COOH+ | ' = | ' +H..0 COOH COOH [Benzoic acid] [Glycocine] [Hippuric acid] [Water], CH. LIT.] CREATININE. 713 This is a well-marked instance of synthesis carried out in the animal body, and experimental investigation shows that it is accomplished by the living cells of the kidney itself ; for if a mix- ture of glycocine, benzoic acid, and blood is injected through the kidney (or mixed with a minced kidney just removed from the body of an animal), their place is found to have been taken by hippuric acid. Creatinine. The creatinine in the urine is nearly all derived from the creatine contained in the meat of the food. There is, however, a small amount in the urine even during starvation : this possibly represents a small percentage of creatine from the muscles. The formation of creatinine from creatine is represented in the following equation :- C4H9N3O2 - H.,0 = C4H7N3O [Creatine] [Water] [Creatinine]. Creatine and creatinine are of considerable chemical interest, because urea can be obtained from them as one of their decompo- sition products in the laboratory; the equation which represents the formation of urea from creatine is as follows :- C4H9N3O2 + H2O == C3H7NO2 + CON2Hi [Creatine] [Water] [Sarcosine] [Urea]. The second substance formed is sarcosine. Sarcosine is methyl- glycocine-that is, amido-acetic acid in which one H is replaced by methyl (CH3) ch Znh.CH3 2XCOOH. It is, however, doubtful whether decompositions of this kind occur in the body. Creatinine with zinc chloride gives a characteristic crystalline precipitate (groups of fine needles) with composition C4H7N3O.ZnCl2. According to the recent researches of G. S. Johnson, urinary Fig. 566.-Crystals of hippuric acid. 714 THE URINE. [ch. lii. creatinine, though isomeric with the creatinine obtained artifici- ally from the creatine of flesh, differs from it in some of its properties, such as reducing power, solubility, and character of its gold salts. The reducing action of urinary creatinine has led to some confusion, for some physiologists have supposed that the reducing action on Fehling's solution and picric acid of normal urine is due to sugar, whereas it is really chiefly due to creatinine. The readiest way of separating creatinine from urine is the following - To the urine a twentieth of its volume of a saturated solution of sodium acetate is added, and then one-fourth of its volume of a saturated solution of mercuric chloride : this produces an imme- diate abundant precipitate of urates, sulphates, and phosphates, which is removed by filtration ; the filtrate is then allowed to stand for twenty-four hours, when the precipitation of a mercury salt of creatinine (C4H5HgN3OHCl)4(HgCl2)3+2H2O occurs in the form of minute spheres, quite typical on microscopic examination. This compound lends itself very well to quantitative analysis. It may be collected, dried, and weighed, and one-fifth of the weight found is creatinine. Creatinine may be obtained from it by suspending it in water, decomposing it with sulphuretted hydrogen, and filtering. The filtrate deposits creatinine hydro- chloride, from which Pb (OH)2 liberates creatinine. An important point in Johnson's process is that all the operations are carried out in the cold ; if heat is applied one obtains the creatinine of former writers, which has no reducing power. The Inorganic Constituents of Urine. The inorganic or mineral constituents of urine are chiefly chlorides, phosphates, sulphates, and carbonates ; the metals with which these are in combination are sodium, potassium, ammonium, calcium, and magnesium. The total amount of these salts varies from 9 to 25 grammes daily. The most abundant is sodium chloride, which averages in amount 10 to 13 grammes per diem. These substances are derived from two sources-first from the food, and secondly as the result of metabolic processes. The chlorides and most of the phosphates come from the food ; the sulphates and some of the phosphates, as a result of metabolism. The salts of the blood and of the urine are much the same, with the important exception that, whereas the blood contains only traces of sulphates, the urine contains abundance of these salts. The sulphates are derived from the changes that occur in the proteids of the body ; the nitrogen of proteids leaves the body as urea and uric acid ; the sulphur of the proteids is oxidised to CH. LU.] MINERAL SALTS IN URINE. 715 form sulphuric acid, which passes into the urine in the form of sulphates. The excretion of sulphates, moreover, runs parallel to that of urea. Chlorides.--The chief chloride is that of sodium. The inges- tion of sodium chloride is followed by its appearance in the urine, some on the same day, some on the next day. Some is decom- posed to form the hydrochloric acid of the gastric juice. The salt, in passing through the body, fulfils the useful office of stimulating metabolism and secretion. Sulphates.-The sulphates in the urine are principally those of potassium and sodium. They are derived from the metabolism of proteids in the body. Only the smallest trace enters the body with the food. Sulphates have an unpleasant bitter taste (for instance, Epsom salts) : hence we do not take food that contains them. The sulphates vary in amount from 1-5 to 3 grammes daily. In addition to these sulphates there is a small quantity, about one-tenth of the total sulphates, that are combined with organic radicles : these are known as ethereal sulphates, and they originate from putrefactive processes occurring in the intestine. The chief of these ethereal sulphates are phenol sulphate of potassium and indoxyl sulphate of potassium. The latter originates from the indole formed in the intestine, and as it yields indigo when treated with certain reagents it is sometimes called indican. It is very important to remember that the indican of urine is not the same thing as the indican of plants, which is a glucoside. Both yield indigo, but there the resemblance ceases. Carbonates.-Carbonates and bicarbonates of sodium, calcium, magnesium, and ammonium are only present in alkaline urine. They arise from the carbonates of the food, or from vegetable acids (malic, tartaric, &c.) in the food. They are, therefore, found in the urine of herbivora and vegetarians, whose urine is thus rendered alkaline. Urine containing carbonates becomes, like saliva, cloudy on standing, the precipitate consisting of cal- cium carbonate, and also phosphates. Phosphates.-Two classes of phosphates occur in normal urine :- (1) Alkaline phosphates-that is, phosphates of sodium (abun- dant) and potassium (scanty). (2) Earthy phosphates-that is, phosphates of calcium (abun- dant) and magnesium (scanty). The composition of the phosphates in urine is liable to variation. In acid urine the acidity is due to the acid salts. These are chiefly- 716 THE URINE. [ch. lit. Sodium dihydrogen phosphate, NaH2POt, and calcium dihydro- gen phosphate, Ca(H2PO4)2. In neutral urine, in addition, disodium hydrogen phosphate (Na2HPO4), calcium hydrogen phosphate, CaHPO4, and magnesium hydrogen phosphate, MgHPO4, are found. In alkaline urine there may be instead of, or in addition to the above, the normal phosphates of sodium, calcium, and magnesium [Na3POt,Ca3(POl).„ Mg3(PO4)J. The earthy phosphates are precipitated by rendering the urine alkaline by ammonia. In decomposing urine ammonia is formed from the urea : this also precipitates the earthy phosphates. The phosphates most frequently found in the white creamy precipitate which occurs in decomposing urine are- (1) Triple phosphate or ammonio - magnesium phosphate Fig-. 567.-Urinary sediment of triple phosphates (large prismatic crystals) and urate of ammonium, from urine which had undergone alkaline fer- mentation. Fig. 568.-Mucus deposited from urine. (NH1MgPOt + 6H2O). This crystallises in "coffin-lid" crystals (see tig. 567) or feathery stars. (2) Stellar phosphate, or calcium phosphate, which crystallises in star-like clusters of prisms. As a rule normal urine gives no precipitate when it is boiled; but sometimes neutral, alkaline, and occasionally faintly acid urines give a precipitate of calcium phosphate when boiled : this precipitate is amorphous, and is liable to be mistaken for albumin. It may be distinguished readily from albumin, as it is soluble in a few drops of acetic acid, whereas coagulated proteid does not dissolve. The phosphoric acid in the urine chiefly originates from the CH. LIT.] URINARY DEPOSITS. 717 phosphates of the food, but is partly a decomposition product of the phosphorised organic materials in the body, such as lecithin and nuclein. The amount of P2O5 in the twenty-four hours' urine varies from 2'5 to 3'5 grammes, of which the earthy phosphates contain about half (1 to 1'5 gr.). Tests foi* the Inorg-anic Salts of Urine. Chlorides.-Acidulate with nitric acid and add silver nitrate ; a white precipitate of silver chloride, soluble in ammonia, is produced. The object of acidulating with nitric acid is to prevent phosphates being precipitated by the silver nitrate. Sulphates.-Acidulate with hydrochloric acid, and add barium chloride. A white precipitate of barium sulphate is produced. Hydrochloric acid is again added first, to prevent precipitation of phosphates. Phosphates.-i. Add ammonia : a white flocculent precipitate of earthy (that is, calcium and magnesium) phosphates is produced. This becomes more apparent on standing. The alkaline (that is, sodium and potassium) phosphates remain in solution, ii. Mix another portion of urine with half its volume of nitric acid ; add ammonium molybdate, and boil. A yellow crystalline precipitate falls. This test is given by both classes of phosphates. Quantitative estimation of the salts is accomplished by the use of solutions of standard strength, which are run into the urine till the formation of a precipitate ceases. The standards are made of silver nitrate, barium chloride, and uranium nitrate or acetate for chlorides, sulphates and phosphates respectively. Urinary Deposits. The different substances that may occur in urinary deposits are formed elements and chemical substances. The formed, or anatomical elements may consist of blood corpuscles, pus, mucus, epithelium cells, spermatozoa, casts of the urinary tubules, fungi, and entozoa. All of these, with the exception of a small quantity of mucus, which forms a flocculent cloud in the urine, are pathological, and the microscope is chiefly employed in their detection. The chemical substances are uric acid, urates, calcium oxa- late, calcium carbonate, and phosphates. Rarer forms are leucine, tyrosine, xanthine, and cystine. We shall, however, here only consider the commoner deposits, and for their identification the microscope and chemical tests must both be employed. Deposit of Uric Acid.-This is a sandy reddish deposit resembling cayenne pepper. It may be recognised by its crystal- line form (fig. 565, p. 711) and the murexide reaction. The presence of these crystals generally indicates an increased formation of uric acid, and, if excessive, may lead to the formation of stones or calculi in the bladder. 718 THE URINE. [ch. lii. Deposit of Urates.-This is much commoner, and may, if the urine is concentrated, occur in normal urine when it cools. It is generally found in the concentrated urine of fevers ; and there appears to be a kind of fermentation, called the acid fermentation, which occurs in the urine after it has been passed, and which leads to the same result. The chief constituent of the deposit is the acid sodium urate, the formation of which from the normal sodium urate of the urine may be represented by the equation- 2C5H2Na2N4O3 + H.,0 + C02 = 2C5H3NaN4O3 + Na2CO3 [normal sodium urate] [water] [carbonic acid] [acid sodium urate] [sodium carbonate] This deposit may be recognised as follows:- 1. It has a pinkish colour; the pigment called uro-erythrin is Fig. 569.-Crystals of Calcium Oxalate. Fig. 570.-Crystals of Cystin, one of the pigments of the urine, but its relationship to the other urinary pigments is not known. 2. It dissolves upon warming the urine. 3. Microscopically it is usually amorphous, but crystalline forms similar to those depicted in fig. 567 may occur. Crystals of calcium oxalate may be mixed with this deposit (see fig. 569). Deposit of Calcium Oxalate.-This occurs in envelope crystals (octahedra) or dumb-bells. It is insoluble in ammonia, and in acetic acid. It is soluble with difficulty in hydrochloric acid. Deposit of Cystin.-Cystin (C6H12N2S2O.j) is recognised by its colourless six-sided crystals (fig. 570). These are rare: they CH. LII.J URINARY DEPOSITS. 719 occur only in acid urine, and they may form concretions or calculi. Cystinuria (cystin in the urine) is hereditary. Deposit of Phosphates.-These occur in alkaline urine. The urine may be alkaline when passed, due to fermentative changes occurring in the bladder. All urine, however, if exposed to the air (unless the air is perfectly pure, as on the top of a snow mountain), will in time become alkaline, owing to the growth of the micrococcus urece. This forms ammonium carbonate from the urea. CON2H4 + 2H2O = (NHt)2CO3. [urea] [water] [ammonium carbonate] The ammonia renders the urine alkaline and precipitates the earthy phosphates. The chief forms of phosphates that occur in urinary deposits are- 1. Calcium phosphate, Ca3(PO4)2; amorphous. 2. Triple or ammonio-magnesium phosphate, MgNHjPO^; coffin-lids and feathery stars (fig. 567). 3. Crystalline phosphate of calcium, CaHPOp in rosettes of prisms, in spherules, or in dumb-bells. 4. Magnesium phosphate, Mg8(POJ2 22H,O, occurs occa- sionally, and crystallises in long plates. All these phosphates are dissolved by acids, such as acetic acid, without effervescence. They do not dissolve on heating the urine; in fact, the amount of precipitate may be increased by heating. Very often neutral or alkaline urine will become cloudy when boiled; this may be due to albumin or to phosphates. It is very important to dis- tinguish between these two, as albuminuria is a serious condition. They may be distinguished by the use of acetic acid, which dis- solves phosphates but not albumin. A solution of ammonium carbonate (i-in-5) eats magnesium phosphate away at the edges ; it has no effect on the triple phos- phate. A phosphate of calcium (CaHPOt + 2H.,O) may occa- sionally be deposited in acid urine. Pus in urine is apt to be mistaken for phosphates, but can be distinguished by the microscope. Deposit of calcium carbonate, CaCO3, appears but rarely as whitish balls or biscuit-shaped bodies. It is commoner in the urine of herbivora. It dissolves in acetic or hydrochloric acid, with effervescence. The following is a summary of the chemical sediments that may occur in urine :- 720 THE URINE. [CH. LIL CHEMICAL SEDIMENTS IN URINE. In Acid Urine. Uric Acid.-Whetstone, dumb- bell, or sheaf-like aggregations of crystals deeply tinged by pigment. Urates. - Generally amorphous. The acid urate of sodium and of ammonium may sometimes occur in star-shaped clusters of needles or spheroidal clumps with projecting spines. Tinged brick-red. Soluble on warming. Calcium Oxalate.-Octahedra, so- called envelope crystals. Insoluble in acetic acid. Cystin.-Hexagonal plates. Rare. Leucine and Ty rosine.-Rare. Cal ci urn Phosp It ate, CaHPO4+ 2H..0.-Rare. In Alkaline Urine. Phosphates.-Calcium phosphate, Ca3(P04)2. Amorphous. Triple phosphate, MgNH4P04 + 6H.,0. Coffin-lids or feathery stars. Calcium hydrogen phosphate, CaHP04. Rosettes, spherules, or dumb-bells. Magnesium phosphate, MG3(P04)„ + 22H20. Long plates. All soluble in acetic acid without effervescence. Calcium Carbonate, CaC03.- Biscuit-shaped crystals. Soluble in acetic acid with effervescence. A m monbum Urate, C5H2(NH4).,.N403. - • Thorn-apple ' spherules. Leucine and Tyrosine.-Very rare. Pathological Urine. Under this head we shall briefly consider only those abnormal constituents which are most frequently met with. Proteids.-There is no proteid matter in normal urine,* and the most common cause of the appearance of albumin in the urine is disease of the kidney (Bright's disease). The term " albumin " is the one used by clinical observers. Properly speaking, it is a mixture of serum albumin and serum globulin. The best methods of testing for and estimating the albumin are the following :- («) Boil the top of a long column of urine in a test-tube. If the urine is acid, the albumin is coagulated. If the quantity of albumin is small, the cloudiness produced is readily seen, as the unboiled urine below it is clear. This is insoluble in a few drops of acetic acid, and so may be distinguished from phosphates. If the urine is alkaline, it should be first rendered acid with a little dilute acetic acid. (7?) Heller's Nitric-acid Test.-Pour some of the urine gently on to the surface of some nitric acid in a test-tube. A ring of white precipitate occurs at the junction of the two liquids. This test is used for small quantities of albumin. * This absolute statement is true for all practical purposes. Mbrner, however, has recently stated that a trace of proteid matter (serum albumin plus the proteid constituent of mucin) does occur in normal urine ; but the trace is negligeable, many hundreds of litres of urine having to be used to obtain an appreciable quantity. CH. LIT.] ALBUMIN IN URINE. 721 (c) Estimation of Albumin by Esbach's Albuminometer. - Esbach's reagent for precipitating the albumin is made by dissolving io grammes of picric acid and 20 grammes of citric acid in 800 or 900 c.c. of boiling water and then adding sufficient water to make up to a litre (1000 c.c.). The albuminometer is a test-tube graduated as shown in fig. 571. Pour the urine into the tube up to the mark U ; then the reagent up to the mark R. Close the tube with a cork, and to ensure complete mixture, tilt it to and fro a dozen times without shaking. Allow the corked tube to stand upright twenty-four hours ; then read off on the scale the height of the coagulum. The figures indicate grammes of dried albumin in a litre of urine. The percentage is obtained by dividing by 10. Thus, if the coagulum stands at 3, the amount of albumin is 3 grammes per litre, or 0'3 gr. in 100 c.c. If the sediment falls between any two figures, the distance |, J, or f from the upper or lower figure can be read off with sufficient accuracy. Thus, the surface of the sediment being midway between 3 and 4 would be read as 3'5. When the albumin is so abundant that the sediment is above 4, a more accurate result is obtained by first diluting the urine with one or two volumes of water, and then multiplying the resulting figure by 2 or 3, as the case may be. If the amount of albumin is less than o-5 per cent., it cannot be accurately estimated by this method. A condition called " peptonuria," or peptone in the urine, is observed in certain pathological states, es- pecially in diseases where there is a forma- tion of pus, and particularly if the pus is decomposed owing to the action of a bac- terial growth called staphylococcus ; one of the products of disintegration of pus cells appears to be peptone ; and this leaves the body by the urine. The term " peptone," however, includes the " proteoses." Indeed, in most, if not all, cases of so-called peptonuria, true peptone is absent. In the disease called " os- teomalacia " a proteose is also usually found in the urine. Sugar.-Normal urine contains no sugar, or so little that for clinical purposes it may be considered absent. It occurs in the disease called diabetes mellitus, which can be artificially produced by puncture of the medulla oblongata, oi' by extirpation of the pancreas. The disease as it occurs in man may be due to disordered metabolism of the liver, to disease of the pancreas, and to other not fully understood causes (see p. 668). The sugar present is dextrose. Lactose may occur in the urine of nursing mothers. Diabetic urine also contains hydroxy- butyric acid, and may contain or yield on distillation acetone and ethyl-diacetic acid. The methods usually adopted for detect- ing and estimating the sugar are as follows :- Fig. 571.-Esbach's albuminometer. 722 THE URINE. [ch. Lil. (a) The urine has generally a high specific gravity. (O The presence of sugar is shown by the reduction (yellow precipitate of cuprous oxide) that occurs on boiling with Fehling's solution. Fehling's solution is an alkaline solution of copper sulphate to which Rochelle salt has been added. The Rochelle salt (double tartrate of potash and soda) holds the cupric hydrate in solution. Fehling's solution should always be freshly prepared, as, on standing, racemic acid is formed from tire tartaric acid, and this substance itself reduces the cupric to cuprous oxide. Fehling's solution should, therefore, always be tested by boiling before it is used. If it remains clear on boiling, it is in good condition. (c) Picric Acid Test.--The work of Sir George Johnson and G. S. Johnson has shown the value of this reagent in detecting both albumin and sugar in the urine. The same reagent may be employed for the detection of both substances. The method of testing for albumin has been already studied with Esbach's tubes. To test for sugar do the following experiment. Take a drachm (about 4 c.c.) of diabetic urine ; add to it an equal volume of saturated aqueous solution of picric acid, and half the volume (i.e., 2 c.c.) of the liquor potass:® of the British Pharmacopoeia. Boil the mixture for about a minute, and it becomes so intensely dark red as to be opaque. Now do the same experiment with normal urine. An orange-red colour appears even in the cold, and is deepened by boiling, but it never becomes opaque, and so the urine for clinical purposes may be considered free from sugar. This reduction of picric acid by normal urine is due to creatinine (see p. 7Z4)- The reaction described may be used for quantitative purposes by means of Sir George Johnson's picro-saccharometer. (rf) Quantitative Determination of Sugar in Urine.-Fehling's solution is prepared as follows :-34'639 grammes of copper sulphate are dissolved in about 200 c.c. of distilled water ; 173 grammes of Rochelle salt are dissolved in 600 c.c. of a 14 per cent, solution of caustic soda. The two solutions are mixed and diluted to a litre. Ten c.c. of this solution are equivalent to O'°5 gramme of dextrose. Dilute 10 c.c. of this solution with about 40 c.c. of water, and boil it in a flask. Run into this from a burette the urine (which should be previously diluted with nine times its volume of distilled water) until the blue colour of the copper solution disappears-that is, till all the cupric hydrate is reduced. The mixture in the flask should be boiled after every addition. The quantity of diluted urine used from the burette contains 0'05 gramme of sugar. Calculate the percentage from this, re- membering that the urine has been diluted to ten times its original volume. It is somewhat difficult for the unpractised observer to determine accu- rately the exact point at which the blue disappears. The blue colour, if any remains, will be seen by holding the flask up to the light. Some prefer a white porcelain basin instead of a flask ; the blue can then be seen against the white of the basin. Pavy's modification of Fehling's solution is sometimes used. Here am- monia holds the copper in solution, and no precipitate forms on boiling with sugar, as ammonia holds the cuprous oxide in solution. The reduction is complete when the blue colour disappears ; 10 c.c. of Pavy's solution = 1 c.c. of. Fehling's solution = 0'005 grammes of dextrose. In some cases of diabetic urine where there is excess of ammonia-magnesic phosphate, the full reduction is not obtained with Fehling's solution, and when the quantity of sugar is small it may be missed. In such a case excess of soda or potash should be first added, the precipitated phosphates filtered off, and the filtrate after it has been well boiled may then be titrated with Fehling's solution. Fehling's test is not absolutely trustworthy. Often a normal urine will decolorise Fehlings solution, though seldom a red precipitate is formed. This appears to be due to excess of urates and creatinine. Another sub- stance called glycuronic acid (C6HloO7) is, however, very likely to be con- fused with sugar by Fehling's test; the cause of its appearance is sometimes CH. LII.J PATHOLOGICAL URINE. 723 the administration of drugs (chloral, camphor. &c.) ; but sometimes it appears independently of drug treatment. The cause of this is not known, but the condition has not the serious meaning one attaches to diabetes ; hence, for life assurance purposes, it is most necessary to confirm the presence of sugar by other tests. Then, too, in the condition called alcaptonuria, confusion may similarly arise. Alcapton is a substance which probably originates from tyrosine by an unusual form of metabolism. It gives the urine a brown tint, which darkens on exposure to the air. It is an aromatic substance, and the recent researches of Baumann and Wolkow have identified it with homogentisinic acid (C6H3.(0H)2CH2.C00H). (e) The best confirmatory test for sugar is the fermentation, test, which is performed as follows :- Half fill a test tube with the urine and add a little German yeast. Fill up the tube with mercury ; invert it in a basin of mercury, and leave it in a warm place for twenty-four hours. The sugar will undergo fermentation : carbonic acid gas accumulates in the tube, and the liquid no longer gives the tests for sugar, or only faintly, but gives those for alcohol instead. A control experiment should be made with yeast and water in another test- tube, as a small yield of carbonic acid is sometimes obtained from impurities in the yeast. Bile.-This occurs in jaundice. The urine is dark-brown, greenish, or in extreme cases almost black in colour. The most readily applied test is Gmelin's test for the bile pigments. Pettenkofer's test for the bile acids seldom succeeds in urine if the test is done in the ordinary way. The best method is to warm a thin film of urine and cane sugar solution in a flat porcelain dish. Then dip a glass rod in strong sulphuric acid, and draw it across the film. Its track is marked by a purplish line. Excess of urobilin should not be mistaken for bile pigment. Blood.-When haemorrhage occurs in any part of the urinary tract, blood appears in the urine. It is found in the acute stage of Bright's disease. If a large quantity is present, the urine is deep red. Microscopic examination then reveals the presence of blood corpuscles, and on spectroscopic examination the bands of oxyhaemoglobin are seen. If only a small quantity of blood is present, the secretion- especially if acid-has a characteristic reddish-brown colour, which physicians term " smoky." The blood pigment may, under certain circumstances, appear in the urine without the presence of any blood corpuscles at all. This is produced by a disintegration of the corpuscles occurring in the circulation, and the most frequent cause of this is a disease allied to ague, which is called paroxysmal hemoglobinuria. The pigment is in the condition of methaemoglobin mixed with more or less oxyhaemoglobin, and the spectroscope is the means used for identifying these substances. Pus occurs in the urine as the result of suppuration in any 724 THE SKIN. [ch. LIII. part of the urinary tract. It forms a white sediment resembling that of phosphates, and, indeed, is always mixed with phosphates. The pus corpuscles may, however, be seen with the microscope; their nuclei are rendered evident by treatment with 1 per cent, acetic acid, and the pus corpuscles are seen to resemble white blood corpuscles, which, in fact, they are in origin. Some of the proteid constituents of the pus cells-and the same is true for blood-pass into solution in the urine, so that the urine pipetted off from the surface of the deposit gives the tests for albumin. (hi the addition of liquor potassm to the deposit of pus cells, a ropy gelatinous mass is obtained. This is distinctive. Mucus treated in the same way is dissolved. CHAPTER LIII. THE SKIN. The skin is composed of two parts, epidermis, or cuticle, and dermis, or cutis vera. In connection with the skin we shall also have to consider the nails, the hairs with their sebaceous glands, and the sweat glands. The Epidermis is composed of a large number of layers of cells; it is a very thick stratified epithelium. The deeper layers are protoplasmic, and form the rete mucosum, or Malpighian layer ; the surface layers are hard and horny ; this horny layer is the thickest part of the epidermis, and is specially thick on the palms and soles, where it is subjected to most friction. The deepest layers of the Malpighian layer are columnar in shape ; the layers next to this are composed of polyhedral cells, which become flatter as they approach the horny layers. Between these cells arc fine intercellular passages, bridged across by fine protoplasmic processes, which pass from cell to cell; the channels between the cells serve for the passage of lymph. It is in the cells of the Malpighian layer that pigment granules are deposited in the coloured races. Between the horny layer and the Malpighian layer are two intermediate strata, in which the transformation of protoplasm into horny material (keratin) is taking place. In the first of CH. LUI.] THE SKIN. 725 these-that is, the one next to the Malpighian layer, the cells are flattened, and filled with large granules of eleidin, an inter- mediate substance in the formation of horp. This layer is called the stratum granulosum. Above this are several layers of clear, more rounded cells, which constitute the stratum lucidum; and above this the horny layer proper, many strata deep, begins. The cells become more Fig. 572.-Vertical section of the epidermis of the prepuce, a, stratum comeum, of very few layers, the stratum lucidum and stratum granulosum not being distinctly repre- sented ; 6, c, d, and e, the layers of the stratum Malpighii, a certain number of the cells in layers d and e showing signs of segmentation; it consists chiefly of prickle cells ; g, cells in cutis vera. (Cadiat.) and more scaly as they approach the surface, where they lose their nuclei and eventually become detached. The epidermis grows by a multiplication of the deepest layer of cells (fig. 572, e); the newly-formed cells push towards the surface those previously formed, in their progress undergoing the transformation into keratin. The epidermis has no blood-vessels; nerve-fibrils pass into its deepest layers, and ramify between the cells. The Dermis is composed of dense fibrous tissue, which be- comes looser and more reticular in its deeper part, where it passes 726 THE SKIN. [ch. Lin. by insensible degrees into the areolar and adipose tissue of the subcutaneous region. The denser superficial layer is very vascu- lar, and is covered with minute papillae; the epidermis is moulded over these, and in the palms and soles, where the papillae are largest and are disposed in rows, their presence is indicated by the well-known ridges on the surface. The papillae contain loops of capil- laries, and in some cases, especially in the palm of the hand and fingers, they contain tactile corpuscles (p. 285). Special capillary networks are distri- buted to the sweat-glands, sebaceous glands, and hair follicles. The deeper portions of the dermis in the scrotum, penis, and nipple, con- tain involuntary muscular tissue; there is also a bundle of muscular tissue attached to each hair follicle. The Nails are thickenings of the stratum lucidum. Each lies in a depression called the Led of the nail, the posterior part of which is overlapped by epidermis, and called the nail-groove. Fig. 573.-Vertical section of skin. A. Sebaceous gland opening into hair follicle. B. Muscu- lar fibres. C. Sudoriferous or sweat-gland. D. Subcutaneous fat. E. Fundus of hair-follicle, with hair-papillre. (Klein.) CH. LUI.] THE HAIRS. 727 The dermis beneath is beset with longitudinal ridges instead of papillae;. these are very vascular; in the lunula, however, the crescent at the base of the nail, there are papillae, and this part, is not so vascular. The Hairs are epidermal growths, contained in pits called Fig. 574.-Surface of a white hair, magnified. 160 diameters. The wavy lines mark the upper or free edges of the cortical scales. B, separated scales, magnified 350 diameters. (Kolliker.) hair follicles. The part within the follicle is called the root of the hair. The main substance of the hair is composed of pigmented horny fibrous material, in reality long fibrillated cells. It is Fig. 575-'-Longitudinal section of a hair follicle, a and b, external root-sheath; c, internal root-sheath ; d, fibrous layer of the hair; e, medulla; /, hair papilla; g, blood-vessels of the hair-papilla; A, dermic coat. (Cadiat.) covered by a layer of scales imbricated upwards (hair cuticle). In many hairs the centre is occupied by a medulla, formed of rounded cells containing eleidin granules. Minute- air-bubbles 728 THE SKIN. [CH. LIII. may be present in both medulla and fibrous layer, and cause the hair to look white by reflected light. The grey hair of old age, however, is produced by a loss of pigment. The root is enlarged at its extremity into a knob, into which projects a vascular papilla from the true skin. The hair follicle consists of two parts, one continuous with the epidermis, called the root-sheath, the other continuous with the Fig. 576.-Transverse section of a hair and hair-follicle made below the opening of the sebaceous gland, a, medulla or pith of the hair; b, fibrous layer; c, cuticle; d, Huxley's layer; e, Henle's layer of internal root-sheath; f and g, layers of external root-sheath, outside of g is the basement membrane; h, fibrous coat of hair sac; i, vessels. (Cadiat.) dermis, called the dermic coat. The two are separated by a basement membrane called the hyaline layer of the follicle. The root-sheath consists of an outer layer of cells like the Malpighian layer of the epidermis, with which it is directly continuous (outer root-sheath), and of an inner horny layer (inner root-sheath), continuous with the horny layer of the epidermis. The inner root-sheath consists of three- layers, the outermost being com- posed of long, non-nucleated cells (Henle's layer), the next of squarish nucleated cells (Huxley's layer), and the third is a cuticle of scales, imbricated downwards, which fit over the scales of the cuticle of the hair itself. CH. LIU.] THE SKIN-GLANDS. 729 A small bundle of plain muscular fibres is attached to each follicle (fig. 573). When it contracts, as under the influence of cold, or certain emotions such as fear, the hair is erected and the whole skin is roughened (" goose skin "). The nerves supplying these muscles are called pilo-motor nerves. The sebaceous glands (figs. 573 and 577) are small saccular Tig. 577.-Sebaceous gland from human skin. (Klein and Noble Smith.) glands, with ducts opening into the upper portion of the hair follicles. The secreting cells become charged with fatty matter, which is discharged into the lumen of the saccules owing to the disintegration of the cells. The secretion, sebum, contains isocholesterin in addition to fatty matter. It acts as a lubricant to the hairs The sweat-glands are abundant over the whole human skin, but are most numerous where hairs are absent on the palms and soles. Each consists of a coiled tube in the deepest part of the dermis ; the duct from which passes up through the dermis, and by a corkscrew-like canal through the epidermis to the surface. 730 THE SKIN. [cp. LIU. The, secreting tube is lined by one or two layers of cubical or columnar cells; outside this is a layer of longitudinally arranged muscular fibres, and then a basement-membrane. The duct is of similar structure, except that there is usually but one layer of cubical cells, and muscular fibres are absent; Kg. 578.-Terminal tubules of sudoriferous or sweat-glands, cut in various directions from the skin of the pig's ear. (V. D. Harris.) the passage through the epidermis has no proper wall ; it is merely a channel excavated between the epidermal cells. The ceruminous glands of the ear are modified sweat-glands. The Functions of the Skin. Protection.-The skin acts as a protective organ, not only by mechanically covering and so defending internal structures from external violence, but more particularly in virtue of its being an organ of sensation (see p. 282 et seq.). Heat Regulation.-See Chapter LV. Respiration.-A small amount of respiratory interchange of gases occurs through the skin, but in thick-skinned animals this is very small. In man, the carbonic acid exhaled by the skin is about j-g-Q- to of that which passes from the lungs. But in thin-skinned animals, like frogs, cutaneous respiration is very important; after the removal of the lungs of a frog/ the respiratory interchange through the skin is sufficient to keep the animal alive, the amount of carbonic acid formed being about half as .much as when the lungs are present (Bischoff). Absorption.-This also is an unimportant function; but the CH. LIU.] THE SWEAT. 731 skin will in a small measure absorb materials placed in contact with it; thus in some cases infants 'who will not take cod-liver oil by the mouth, can yet be dosed with it by rubbing it into the skin. Many ointments also are absorbed, and thus general effects produced by local inunction. Secretion.-The secretions of the skin are two in number. The sebum is the natural lubricant of the hairs. The sweat is an excretion. The secretion of sweat is an important function of the skin, and we will therefore discuss it at greater length. The Sweat. Physiology of the Secretion of Sweat.-We have seen that the sweat-glands are most abundant in man on the palms and soles, and here the greatest amount of perspiration occurs. Different animals vary a good deal in the amount of sweat they secrete, and in the place where the secretion is most abundant. Thus the ox perspires less than the horse and sheep ; perspiration is absent from rats, rabbits, and goats; pigs perspire mostly on the snout; dogs and cats on the pads of the feet. As long as the secretion is small in amount, it is evaporated from the surface at once; this is called insensible perspiration. As soon as the secretion is increased or evaporation prevented, drops appear on the surface of the skin. This is known as sensible perspiration. The relation of these two varies with the temperature of the air, the drier and hotter the air, the greater being the proportion of insensible to sensible perspiration. In round numbers the total amount of sweat secreted by a man is two pounds in the twenty-four hours. The amount of secretion is influenced by two sets of nerves: (i) the vaso-motor nerves ; an increase in the size of the skin-vessels, leading to increased, a constriction of the vessels to diminished, perspiration. There are also special secretory fibres, stimulation of which causes a secretion even when the circulation is suspended, as in a recently amputated limb. These fibres are paralysed by atropine. They are contained in the same nerve-trunks as the vaso-motor nerves, as are also the nerve-fibres which supply the plain muscular fibres of the sweat-glands which act during the expulsion of the secretion. The secretory nerves for the lower limbs issue from the spinal cord by the last two or three dorsal and first two or four lumbar nerves (in the cat) ; they pass to the abdominal sympathetic and thence to the sciatic nerve. They 732 THE SKIN. [ch. lux. are controlled by a centre in the upper lumbar region of the cord; those for the upper limbs leave the cord by the fifth and sixth cervical roots and ultimately pass to the ulnar and median nerves; they are controlled by a centre in the cervical enlargement of the cord. The secretory fibres for the head pass in the cervical sympathetic, and in some branches of the fifth cranial nerves. These subsidiary centres are dominated by one in the medulla oblongata (Adamkiewicz). These facts have been obtained by experiments on animals (cat, horse). The sweat-centres may be excited directly by venous blood, as in asphyxia ; or by over-heated blood (over 450 C.); or by certain drugs (see further) ; or reflexly by stimulation of the crural and peroneal nerves. Nervous diseases are often accompanied with disordered sweat- ing ; thus unilateral perspiration is seen in some cases of hemiplegia; degeneration of the anterior nerve-cells of the cord may cause stoppage of the secretion. It is sometimes increased in paralysed limbs. The changes that occur in the secreting cells have been investigated by Renaut in the horse. When charged they are clear and swollen, the nucleus being situated near their attached ends; when discharged they are smaller, granular, and their nucleus is more central. The sweat, like the urine, must be regarded as an excretion, the secreting cells eliminating substances formed elsewhere. Composition of the Sweat.-Sweat may be obtained in abundant quantities by placing the animal or man in a closed hot-air bath, or from a limb by enclosing it in a vessel made air- tight with an elastic bandage. Thus obtained it is mixed with epidermal scales and a small quantity of fatty matter from the sebaceous glands. The continual shedding of epidermal scales is in reality an excretion. Keratin, of which they are chiefly composed, is rich in sulphur, and, consequently, this is one means by which sulphur is removed from the body. The reaction of sweat is acid, and the acidity, as in the urine, is due to acid sodium phosphate. In profuse sweating, however, the secretion usually becomes alkaline or neutral. It has a peculiar and characteristic odour, which varies in different parts of the body, and is due to volatile fatty acids ; its taste is saltish, its specific gravity about 1005. In round numbers the percentage of solids is 1*2, of which o'g is organic matter. The following table is a compilation from several analyses :- CH. LIII.] THE SWEAT. 733 Water . 98 88 per cent. Solids . .112 ,, Salts . . 0'57 NaCl . . o-22 to 0'33 Other salts . o-i8 ,, (alkaline sulphates, phosphates, lactates, and potassium chlo- ride) Fats . .0 41 (including fatty acids and iso- cholesterin) Epithelium. 0'17 „ Urea . . o-o8 The salts are in kind and relative quantity very like those of the urine. Funke was unable to find any urea, but most other observers agree on the presence of a minute quantity. It appears to become quickly transformed into ammonium carbonate. The proteid which is present, is probably derived from the epithelial cells of the epidermis, sweat-glands, and sebaceous glands, which are suspended in the excretion ; but in the horse there is albumin actually in solution in the sweat. Abnormal, Unusual, or Pathological Conditions of the Sweat.-Drugs.-Certain drugs (sudorifics) favour sweating, e.g. pilocarpine, Calabar bean, strychnine, picrotoxine, muscarine, nicotine, camphor, ammonia. Others diminish the secretion, e.g. atropine and morphine in large doses. Large quantities of water, by raising the blood-pressure, increase the perspiration. Some substances introduced into the body reappear in the sweat, e.g. benzoic, tartaric, and succinic acids readily, quinine and iodine with more difficulty. Compounds of arsenic and mercury behave similarly. Diseases.-Cystin has been found in some cases, dextrose in diabetic patients ; bile-pigment in those with jaundice (as evi- denced by the staining of the clothes); indigo in a peculiar condition known as chromidrosis ; blood or hsematin derivatives in red sweat; albumin in the sweat of acute rheumatism, which is often very acid; urates and calcium oxalate in gout; lactic acid in puerperal fever, and occasionally in rickets and scrofula. Kidney Diseases.-The relation of the secretion of the skin to that of the kidneys is a very close one. Thus copious secretions of urine, or watery evacuations from the alimentary canal, coincide with dryness of the skin ; abundant perspiration and scanty urine generally go together. In the condition known as uraemia, when the kidneys secrete little or no urine, the percentage of urea rises in the sweat ; the sputum and the saliva also contain urea under those circumstances. The clear indication for the physician in such cases is to stimulate the skin to action by hot-air baths and 734 GENERAL METABOLISM. [ch. liv. pilocarpine, and the alimentary canal by means of purgatives. In some of these cases the skin secretes urea so abundantly that when the sweat dries on the body, the patient is covered with a coating of urea crystals. Varnishing the Skin.-By covering the skin of such an animal as a rabbit with an impermeable varnish, the temperature is reduced, a peculiar train of symptoms set up, and ultimately the animal dies. If, however, cooling be prevented by keeping such an animal in warm, cotton-wool, it lives longer. Varnishing the human skin does not seem to be dangerous. Many explanations have been offered to explain the peculiar condition observed in animals ; retention of the sweat would hardly do it; the blood is not found post mortem to contain any abnormal substance, nor is it poisonous when transfused into another animal. Cutaneous respiration is so slight in mammals that stoppage of this function cannot be supposed to cause death. The animal, in fact, dies of cold; the normal function of the skin in regulating temperature is interfered with by injury to its vaso-motor nerves, and it is only animals with delicate skins which are thus affected. CHAPTER LIV. GENERAL METAIOLISM. The word metabolism has been often employed in the preceding chapters, and, as there explained, it is used to express the sum total of the chemical exchanges that occur in living tissues. The chemical changes have been considered separately under the headings Alimentation, Excretion, Respiration, &c. We have now to put our knowledge together, and consider these subjects in their relation to one another. The living body is always giving off by the lungs, kidneys, and skin the products of its combustion, and is thus always tending . to lose weight. This loss is compensated for by the intake of food and of oxygen. For the material it loses, it receives in exchange fresh substances. If, as in a normal adult, the income ..is exactly equal to the expenditure, the body-weight remains constant. If, as in a growing child, the income exceeds the expenditure, the body gains weight; and if, as in febrile condi- CH. liv.] GENERAL METABOLISM. 735 tions, or during starvation, the expenditure exceeds the income, the body wastes. The first act in the many steps which constitute metabolism is the taking of food, the next digestion of that food, the third absorption, and the fourth assimilation. In connection with these subjects, it is important to note the necessity for a mixed diet, and the relative and absolute quantities of the various proximate principles which are most advantageous. Assimilation is a subject which is exceedingly difficult to describe; it is the act of the living tissues in selecting, appropriating, and making part of themselves the substances brought to them by the nutrient blood-stream from the lungs on the one hand, and the alimentary canal on the other. The chemical processes involved in some of these transactions have been already dwelt on in connection with the functions of the liver and other secreting organs, but even there our information on the subject is limited; much more is this the case in connection with other tissues. Assimilation, or the building up of the living tissues, may, to use Gaskell's expression, be spoken of as anabolic. Supposing the body to remain in the condition produced by these anabolic processes, what is its composition ? A glance through the chapters on the cell, the blood, the tissues, and the organs will Convince the inquirer that different parts of the body have very different compositions ; still, speaking of the body as a whole, Volckmann and Bischoff state that it contains 64 per cent, of water, 16 of proteids (including gelatin), 14 of fat, 5 of salt, and 1 of carbohydrates. The carbohydrates are thus the smallest constituent of the body; they are the glycogen of the liver and muscles, and small quantities of dextrose in various parts. The most important, because the most abundant of the tissues of the body, is the muscular tissue. Muscle forms about 42 per cent, of the body-weight,* and contains, in round numbers, 75 per cent, of water and 21 per cent, of proteids : thus about half the proteid material and of the water of the body exist in its muscles. The body, however, does not remain in this stable condition; even while nutrition is occurring, destructive changes are taking place simultaneously; each cell may be considered to be in a state of unstable equilibrium, undergoing anabolic, or constructive * The following is in round numbers the percentage proportion of the different structural elements of the body: skeleton, 16 ; muscles,'42 ; fat, 18 ; viscera, 9 ; skin, 8 ; brain, 2 ; blood, 5. 736 GENERAL METABOLISM. [ch. liv. processes, on the one hand, and destructive, or katabolic, processes on the other. The katabolic series of phenomena commences with combustion; the union of oxygen with carbon to form carbonic acid, with hydrogen to form water, with nitrogen, carbon, and hydrogen to form urea, uric acid, creatinine, and other less important substances of the same nature. The formation of these last-mentioned substances, the nitrogenous metabolites, is, how- ever, as previously pointed out, partly synthetical. The discharge of these products of destructive metabolism by the expired air, the urine, the sweat, and fseces is what constitutes excretion ; excretion is the final act in the metabolic round, and the com- position of the various excretions have been considered in some of the later chapters of this book. An examination of the intake (food and oxygen) and of the output (excretion) of the body can be readily made; much more readily, it need hardly be said, than an examination of the inter- mediate steps in the process. A contrast between the two can be made by means of a balance-sheet. A familiar comparison may be drawn between the affairs of the animal body and those of a commercial company. At the end of the year the company presents a report in which its income and its expenditure are contrasted on two sides of a balance-sheet. This sheet is a summary of the monetary affairs of the undertaking ; it gives few details, it gives none of the intermediate steps of the manner in which the property has been employed. This is given in the preliminary parts of the report, or may be entered into by still further examining the books of the company. In the parts of this book that precede this chapter I have endeavoured to give an account of the various transactions that occur in the body. I now propose to wind up by presenting a balance-sheet. Those who wish still further to investigate the affairs of the body may do so by the careful study of works on physiology ; still, text-books and monographs, however good, will teach one only a small amount; the rest is to be learnt by practical study and research ; and we may compare physiologists to the accountants of a commercial enterprise, who examine into the details of its working. Sometimes, in business undertakings, a deficit or some other error is discovered, and it may be that the source of the mistake is only found after careful search. Under these conditions, the accountants should be compared to physicians, who discover that something is wrong in the working of the animal body ; and their object should be to discover where, in the metabolic cycle, the mistake has occurred, and subsequently endeavour to rectify it. CH. LIV.) EXCHANGE OF MATERIAL. 737 The construction of balance-sheets for the human and animal body may be summed up in the German word Stoffwechsel, or " exchange of material." A large number of investigators have applied themselves to this task, and from the large mass of material published, it is only possible to select a few typical examples. The subject has been worked out specially by the Munich school, under the lead of Pettenkofer and Voit. The necessary data for the construction of such tables are :- (1) The weight of the animal before, during, and after the experiment. (2) The quantity and composition of-its food. (3) The amount of oxygen absorbed during respiration. (4) The quantity and composition of urine, faeces, sweat, and expired air. (5) The amount of work done, and the amount of heat developed. (The subject of animal heat will be considered in the next chapter.) Water is determined by subtracting the amount of water in- gested as food from the quantity lost by bowels, urine, lungs, and skin. The difference is a measure of the katabolism of hydrogen. Nitrogen.-The nitrogen is derived from proteids and albumi- noids, and appears chiefly in the urine as urea and uric acid. Minute quantities are eliminated as similar compounds in sweat and faeces. From the amount of nitrogen so found, the amount of proteids which have undergone katabolism is calculated. Proteids contain, roughly, 16 per cent, of nitrogen ; so 1 part of nitrogen is equivalent to 6'3 parts of proteid ; or 1 gramme of nitrogen to 30 grammes of flesh. Fat.-Subtract the carbon in the metabolised proteid (proteid contains 54 per cent, of carbon) from the total carbon eliminated by lungs, skin, bowels, and kidney, and the difference represents fat that has undergone metabolism. Fat contains 76'5 per cent, of carbon ; hence the carbon, which represents fat, multiplied by i'3, gives the amount of katabolised fat. The Discharge of Carbon. The influence of food on the rate of discharge of carbonic acid is immediate. The increase after each meal, which may amount to 20 per cent., reaches its maximum in about one or two hours. This effect is most marked when the diet consists largely of carbohydrates. About 95 per cent, of the carbon discharged leaves the organism 738 GENERAL METABOLISM. [ch. liv. as carbonic acid. The total insensible loss ( = carbonic acid + water given oft'-oxygen absorbed) amounts in man to about 25 grammes per hour. Of this total hourly discharge of carbonic acid, less than 0'5 per cent, is cutaneous. The hourly discharge of carbonic acid in a man at rest is about 32 grammes, the weight of oxygen absorbed being 25 to 28 grammes in the same time. The hourly discharge of watery vapour is about 20 grammes. As a volume of carbonic acid (C02) contains the same weight of oxygen as an equal volume of oxygen (O2), it is obvious that, if all the absorbed oxygen were discharged as carbonic acid, the ' respiratory quotient' (by volume) = C°2 exPired WOuld be equal 0., absorbed to 1. This, however, is not the case, the volume of oxygen absorbed being in excess of the carbonic acid discharged. In animals fed exclusively on carbohydrates (this would only be possible for a short time) equality is approached. The excess of oxygen is greatest when the diet consists largely of fats. On a mixed diet, comprising 100 grammes of proteid, 100 of fat, and 250 of carbohydrates, with a carbonic acid discharge of 770 grammes daily, and a daily assumption of 666 grammes of oxygen, 5 60 grammes of the oxygen are discharged in the carbonic acid, about 9 in urea, and 97 grammes in the form of water (of which 78 grammes are formed from the hydrogen of the fat); the respiratory quotient is then 0'84. In hibernation the respiratory quotient sinks lower than in any other known con- dition (often less than 0'5), for the animal then lives almost entirely on its own fat. The discharge of carbonic acid is in- creased by muscular work, and the respiratory quotient also rises. Diminution of the surrounding temperature causes increased dis- charge of carbonic acid. (These points are all discussed more fully in Chapter XIX.) The Discharge of Nitrogen In man the minimum daily allowance of nitrogen is 15 grammes, or 0-02 per cent, of the body-weight; in the carnivora about o 1 pei cent.; in the ox, as an instance of a herbivorous animal, o 005 per cent. In certain races of mankind coolies) the nitrogen requirement is less than in Europeans. The reason why this is so is not understood. Some recent experiments by Hirschfeld have shown that for a short time nitrogenous equilibrium can be maintained on a smaller daily supply of nitrogen than 15 grammes. But experi- CH. LIV.J EXCHANGE OF MATERIAL. 739 ments extended over a longer time have shown that sooner or later the body begins to waste if the 15 grammes daily are not supplied in the food. In an animal fed exclusively on flesh the discharge of nitrogen at first increases pari passu with the absorption of proteid, the absorption of oxygen being proportionately increased at the same time. The animal, however, gains weight from increase of fat, the proteid being split into what is called a nitrogenous moiety, which is burnt toff, and a non-nitrogenous moiety which is con- verted into fat. The discharge of nitrogen is not immediately or markedly influenced by muscular work (see p. 707) ; the increased com- bustion that occurs in working as compared with resting muscles falls chiefly on their non-nitrogenous constituents. Balance of Income and Discharge in Health. In Chapter XLIII. tables are given of adequate diets ; these will in our balance-sheets represent the source of income; the other side of the balance-sheet, the expenditure, consists of the excretions. Exchange of Material on an Adequate Diet (Ranke's table).* Income. Expenditure. Foods. Nitrogen. Carbon. Excretions. Nitrogen. Carbon. Proteid . 100 gr. i5'5 gr- 53'0 gr. Urea . 3'1'5 gr- 1 6'16 Fat . 100 .. 00 .. 79 0 - Uric acid 0-5 .. 1 144 Carbohy- Fteces II 10-84 drates. 250 .. 0'0 ,, 93'0 - Respiration(CO„) 0'0 208'OO i5'5 » 225-0 „ 15-5 225-00 In man the discharge of nitrogen per kilo, of body-weight is o'2i gramme, and of carbon 3'03 grammes, the quotient C N = 14'5. In carnivorous animals, which, according to Bidder * The above table was constructed from data derived from the observa- tions of Prof. Ranke on himself. Though made many years ago, they still serve as typical and standard examples of metabolic balance-sheets. 740 GENERAL METABOLISM. [CH. L1V. c and Schmidt, use 1-4 N and 6'2 C per kilo, per diem, = 4-4. C In the human being on a flesh diet = 5-2, the exchange thus approaching the condition of the carnivora. This is illustrated by the following balance-sheet (Ranke) :- Income. Expenditure. Nitrogen. Carbon. - Nitrogen. Carbon. Food . Disintegration of tissues . . 62-3 gr. 279 6 45'9 62-3 325-5 Discharged by excretion Retained in store 44'0 183 62'3 26 30 62-5 325-0 The details of the above experiment may be given as illus- trating the method of working out a problem in exchange of material: 1832 grammes of meat used as food yielded 3-4 per cent, of nitrogen, i.e. 62'3 gr., and 12'5 per cent, of carbon, i.e. 229-3 gr.; 7° gr- fat added to the food yielded 72 per cent, of carbon, i.e. 50-3 gr. : 229-3 + 50-3 = 279-6 = total carbon in food. During the same period 86-3 gr. of urea were discharged, containing 46'6 per cent., i.e. 40-4 gr. of nitrogen, and 20 per cent., i.e. 17-3 gr. of carbon, to which must be added 2 gr. of uric acid, containing 33 per cent., i.e. o-66 gr. of nitrogen, and 35 per cent., i.e. 0-7 gr. of carbon. Further, 2-9 gr. of nitrogen and 14 gr. of carbon were discharged in the feeces, and 231 gr. of carbon as carbonic acid in the expired air. Hence the total discharge of nitrogen = 40-4 + 0-66-|-2-9 = 44 gr., and the total discharge of carbon =17-3 + 0- 7 + 14+231 = 263 gr. Deducting the quantity of nitrogen discharged from that taken in, 18-3 gr. must have been retained in the body, as 108 gr. of proteid, and consequently 53 per cent, of that weight = 62-5 gr. of carbon, were also retained. Comparing the quantity of carbon disposed of in the twenty-four hours with the quantity introduced as food, we find the latter is in excess by 45-9 gr., which must have been derived from the disintegration of the fat of the body. Another table of exchange of material on adequate diet may be quoted from the work of Pettenkofer and Voit. This takes into account the elimination of water as well as of carbon and nitrogen. In the first experiment the man did no work. CH. L1V.J INANITION 741 Income. Expenditure. Food. Nitrogen. Carbon. Excre- tions. Nitrogen. Carbon. Water. Proteid. 137 gr. Urine . 174 127 1279 Fat . 117 „ Carbohy- | I9'5 3i5'5 Fteces . Lungs . 21 14'5 248'6 83 828 drate. 352 - - - Water .2016 ,, - - i9'5 275-8 2190 Here the body was in nitrogenous equilibrium, and it elimi- nated more water than it took in by 174 grammes, this being- derived from oxidation of hydrogen. It stored 39-7 grammes of carbon, which is equivalent to 5 2 grammes of fat. The next table gives the results of an experiment on the same man on the same diet, but who did active muscular work during the day :- Expenditure. Nitrogen. Carbon. Water. Urine . • 17'4 12'6 1194 F seces . . 21 14'5 94 Lungs. - 309'2 1412 - i9'5 3363 2700 It is important to notice that the discharge of nitrogen was unaltered, while that of both carbon and hydrogen was increased. Inanition or Starvation. The income from without is, under these circumstances, nil; expenditure still goes on, as a result of the disintegration of the tissues; the amount of disintegration is measured by the dis- charges in the manner already described. The following table from Ranke's experiment on himself represents the exchange for a period of twenty-four hours, twenty-four hours having elapsed since the last meal. Disintegration of tissue. Expenditure. ' - Nitrogen. Carbon. - Nitrogen. Carbon. Proteid. 50 gr. 7'8 26'5 Urea . 17 gr. }78 3'4 Fat . 199-6 „ 0'0 157'5 Uric acid o-2 „ - - Respiration(Cd„') 0'0 180-6 7'8 184-0 - - 7'8 1840 742 GENERAL METABOLISM. [ch. liv. The discharge of nitrogen per kilo, of body-weight was reduced C to o'1, being 23-5. In carnivorous animals : in prolonged C inanition, the discharge of nitrogen per kilo, is 0'9 and y = 6'6. During starvation the man or animal gradually loses weight, the temperature, after a preliminary rise, sinks ; the functions get weaker by degrees, and ultimately death ensues, the total weight lost being from 0'3 to 0'5 of the original body-weight. The age of the animal influences the time at which death occurs, old animals withstanding the effects of hunger better than young ones. This statement was originally made by Hippocrates, and was borne out by the experiments of Martigny and Chossat. Young animals lose weight more quickly, and die after a smaller loss of weight, than old ones. The excretion of nitrogen falls quickly at the commencement of an experiment; it reaches a minimum which remains constant for several days; it then rises when the fat of the animal has been used up, and then quickly falls with the onset of symptoms of approaching death. The sulphates and phosphates in the urine show approximately the same series of changes. The discharge of carbonic acid and the intake of oxygen fall, but not so quickly as the body loses weight; it is not until quite the last stages that these are small in proportion to one another. The faeces become smaller and smaller in quantity until no discharge from the rectum occurs at all. The amount of bile secreted also falls; but bile is found in the gall-bladder and intestine after death. Taking the total loss of weight as 100, the loss due to that of individual organs may be stated as follows (Voit) :- Bone . . .5'4 Muscle . . . 42'2 Liver . . .4'8 Kidneys . . . o-6 Spleen . . . o'6 Pancreas . . o-i Lungs . . . 0'3 Heart . . . o-o Testes . . . oi Intestine , . 2'o Brain and cord . ou Skin and hair . . 8'8 Fat . . . 26'2 Blood . • • 3'7 Other parts . . 5'0 Some organs thus lose but little weight; the loss of weight is greatest in the muscles, fat, skin, liver, and blood. Of the muscles, the great pectoral muscles waste most. Death may be delayed somewhat by artificial warmth, but ultimately occurs from asthenia, sometimes accompanied by convulsions. CH. LIV.J VARIOUS DIETS. 743 Exchange of Material with various Diets. The reasons why a mixed diet is necessary have been already explained (p. 616). Numerous experiments have, however, been made in the study of metabolism on abnormal diets. Feeding with meat.-As the chief solid in meat is proteid, one must take either too much nitrogen or too little carbon. The principle that underlies Banting's method of treating obesity is to give meat almost exclusively : the individual then derives the additional supply of carbon necessary for com- bustion from his own adipose tissue. We have already seen that this may be and often is counteracted by the laying on of fat which comes from the non-nitrogenous moiety of the proteid. Feeding with fat.-If an animal receives fat only, the nitrogenous excreta are derived from the disintegration of tissue without any corresponding supply of nitrogen being supplied in exchange in the food. When fat only is given, or a large excess of fat exists in the food, the respiratory quotient falls. F. Hofmann fed a dog on a mixture of a large amount of fat and a small amount of proteid. After death the quantity of fat found in the body was such that only a small part could have been derived from the proteid, the greater amount being directly derived from the fat of the food. The animal, moreover, lays on fat in which palmitin, stearin, and olein are mixed in a definite proportion : this proportion is often different in the fat of the food. In. addition to this an animal will fatten (laying on fat with its usual composition) on fatty food, such as spermaceti, which contains no glycerides. Feeding with carbohydrates.-The respiratory quotient approaches unity when carbohydrates alone are taken. So far as regards nitrogen the animal is in a state of inanition, as when fat alone is taken. If given in combina- tion with other foods, both carbohydrates and fat act foods. The following table is from Pettenkofer and Voit, and illustrates what happens in a dog on a mixed diet of flesh and carbohydrates. Cn OC.Cn OOGn 4 4 4- -L OOOOOOOOQ ooooooooooo Flesh. Food. 1 GJ M G> I M GJ I to OsGj • -4 -4 m o -J 1 43 to \D 1 4- 1 O OOO Starch. 1 1 1 1 »1 11 1 1 1 Sugar. | 1 Ch Ch I O to -4 Fat. 10 M M Cn 4 ChGn (ji 4* Gi 4* m to m O 4. Uj h 'O G; «O m to 43 Cn 00-4 o 4 Gj OxGj m Amount of pro- teid decomposed calculated from urea excreted. Changes in the body. ++++111+111 GJ •-» M tO H G; tO O G) 4 h Gj 43 m tO H 7l IO 'J Q Cj 4 ChGJ m Proteid gained or lost by the body. Gj H Gi H h G- tJ M ChGJ 4 4 4 00 Ch 4- N « Q 4 0*0 to 43 to 4- O COO Am mnt of carbohydrates decomposed. +++ ++ +++ | | Ch Ch 1 ON* From fat of food. rt- IIII11li'11 Gn 00 Lost from the body. | „ Derived from is » £ a 0.56 1 1 S " food other than fat. Even when the diet consists wholly of carbohydrates, fat is laid on ; the fat laid on when meat and starch are both present in the food comes partly from the proteid and partly from the carbohydrate of the food. When no carbohydrate is given at. all, as in the last experiment, the nitrogenous 744 GENERAL METABOLISM. [ch. liv. metabolism is raised. Carbohydrate food is thus when given with other foods both fat-sparing and proteid-sparing. The formation of fat from carbohydrates was first observed in pigs by Lawes and Gilbert, and has since been confirmed by numerous investigators. One of the most important instances of the carbohydrate origin of fat is the formation of bee's-wax. A chemical link between carbohydrates and fats is the fact that butyric acid is obtainable from starches and sugars. Instances of the formation of fat from proteids are (T) the laying on of fat in carnivorous animals ; (2) the formation of adipocere, a wax-like material which forms in the muscles of corpses buried in damp soil, or allowed to remain in water ; (3) the gradually increasing quantity of fat in old cheeses. The most striking examples of the formation of fat by intracellular metabolic processes is seen in fatty degeneration, and in that special form of this degeneration that occurs in the formation of milk. The blood contains a mere trace of fat, so milk formation is no mere filtration process. The food may, as in the case of cows, contain little or no fat. Feeding with gelatin.-A diet containing gelatin alone will not support life. This fact is somewhat remarkable when one considers the closely allied chemical nature of gelatin and proteids. When gelatin alone is given the body wastes, and the urea excreted is diminished as in inanition. If an enormous, amount of gelatin is given the urea increases. Gelatin, however, like carbohydrates and fats, appears to be a " proteid-sparing " food, and if given mixed with proteids seems to protect the proteids from oxidation. Gelatin can thus be substituted for a part of the proteid in the' food. Feeding with peptones.-In the present day, when artificially digested foods are so much employed, it is of great importance that their nutritive value should be known. Here experimental and clinical evidence coincide in a most favourable way in relation to their nutritive value. Peptone and albumoses thus have the same nutritive value as meat, this result contrasting with the loss of nitrogen and body-weight when gelatin is employed. Effect of Varying- External Conditions on Exchange of Material. Effect of atmosgjheric temperature.-In warm-blooded animals the effect of a low surrounding temperature is to increase katabolism, or combustion in the body ; the body loses more heat, and therefore more must be produced to keep the animal's temperature within normal limits. The effect of a rise of atmospheric temperature is the reverse. In cold-blooded animals, i.e., animals whose temperature varies with that of the surrounding atmosphere, a rise or fall of the latter is accompanied respectively with a rise or fall of combustion in the body. Alterations of body-temperature.-If the changes of the externa] tempera- ture are so great as to cause a rise (as in steam-baths) or a fall (as in hibernation) of body-temperature, the metabolic changes are increased and decreased respectively as in cold-blooded animals. Effect of removal of blood from the body.-The chief effect of a removal of blood from the body is the speedy formation of new blood-corpuscles. The intake of oxygen and discharge of carbonic acid are lessened, and the output of urea is increased. The menstrual flow and epistaxis in strong, healthy people cause no alteration in exchange of material. CH. LIV.J METABOLISM IX DISEASE. 745 Exchange of Material in Diseases. Fever.-Fever is a condition in which the temperature of the body is raised above the normal, and the degree to which it is raised is a measure of the intensity of the febrile condition. A rise of temperature may be produced either by increased produc- tion of heat, due to the increase of katabolic processes in the body, or to a diminished loss of heat from the body. A mere increase in the production of heat does not necessarily produce fever. By administering an excess of food, combustion is increased in the body; but in the healthy individual this does not produce a rise of temperature, because pari passu with the increased pro- duction, there is increased loss of heat. Similarly, diminution in the loss of heat, such as occurs on a hot as compared with a cold day, does not produce fever, because the production of heat within the body is correspondingly diminished. In fever there is increased production of heat, as is seen by the study of exchange •of material; the intake of food is, as a rule, very small; the discharge of nitrogen and carbon results from the disintegration of tissues, which, as compared with that in simple inanition, is large ; the tissues are said to be in a labile condition, that is, they are easily broken down. In most febrile states, the skin is dry, the sweat-glands, like most of the secreting organs of the body, being comparatively inactive, and so the discharge of heat is lessened. The skin may, however, sometimes be bathed in perspiration, and yet high fever be present. The essential cause of the high temperature is neither increased formation nor dimi- nished discharge of heat, but an interference with the reflex mechanism, which in health operates so as to equalise the two. Increased nitrogenous metabolism in fever has been observed in pneumonia, in pyeemic and in other febrile conditions. Ringer showed the correspondence in temperature and output of nitro- gen very clearly in intermittent fever (ague). What is known as the epicritical increase of urea is the greatly increased secretion of urea that occurs at the commencement of the defervescence of a fever. It is probably not due to an in- creased formation of urea, but to the removal of urea which has accumulated, owing to the fact that the kidneys have been acting sluggishly during the height of the fever. Increased output of carbonic acid also occurs in fever. Other changes noted in fever are a rapid loss of the liver glycogen, a lessening of chlorides in the urine, and often an in- crease of the urobilin in the urine. 746 GENERAL METABOLISM. [ch. liv. The following table illustrates exchange of material in fever, no food being taken :- Income. Expenditure. Disintegration of tissue. Nitrogen. Carbon. Excretions. Nitrogen Carbon. Proteid. i2ogr. . 18-6 63'6 Urea and uric Fat, 2057 gr. 00 157'4 acid, 40 gr. 18'6 8'3 - - Respiration i8-6 221'0 (C02), 780 gr. 0'0 2127 i8'6 221 0 This table should be compared with that on p. 741. Diabetes mellitus.-In addition to the presence of sugar in the urine in this disease, the most marked symptoms are intense thirst and ravenous hunger. As a rule, diabetic patients digest their food well. The thirst is an indication of the necessity of replacing the large quantities of water lost by the kidneys ; the hunger, that of replacing the great waste of tissues that occurs. For not only does the urine contain sugar, but, in addition, a great excess of urea and uric acid. The carbonic acid output is somewhat smaller than in health. In health the carbohydrates, after assimilation, give rise, by oxidation, to carbonic acid; in diabetes, all the carbohydrates do not undergo this change, but pass as sugar into the urine. Not that all the sugar of the urine is derived from carbohydrates, for many diabetics continue to pass large quantities when all carbohydrate food is withheld ; under these circumstances, it must be derived from the destruc- tion of proteid matter (see also pp. 668, 721). Luxus Consumption. In former portions of this book we have insisted on the fact that the food does not undergo combustion, or katabolic changes, until after it is assimilated, that is, until after it has become an integral part of the tissues. Formerly the blood was supposed to be the seat of oxidation ; but the reasons why this view is not held now have been already given. When a student is first con- fronted with balance-sheets, representing metabolic exchanges, it is at first a little difficult for him to grasp the fact, that although the amount of nitrogen and carbon ingested is equal to the CH. LIV.] LUXUS CONSUMPTION. 747 amount of the same elements which are eliminated, yet the eliminated carbon and hydrogen are not derived from the food direct, but from the tissues already formed; the food becomes assimilated and takes the place of the tissues thus disintegrated. Let us suppose we have a tube open at both ends and filled with a row of marbles; if an extra marble is pushed in at one end, a marble falls out at the other ; if two marbles are introduced in- stead of one, there is an output of two at the other end; if a dozen, or any larger number be substituted, there is always a corresponding exit of the same number at the other end of the tube. This very rough illustration may perhaps assist in the comprehension of the metabolic exchanges. The difficulty just alluded to, which a student feels, was also felt by the physiologists who first studied metabolism; and Voit formulated a theory, of which the following is the gist : All pro- teid taken into the alimentary canal appears to affect proteid metabolism in two ways ; on the one hand, it excites rapid dis- integration of proteids, giving rise to an immediate increase of urea ; on the other hand, it serves to maintain the more regular proteid metabolism continually taking place in the body, and so contributes to the normal regular discharge of urea. It has been, therefore, supposed that the proteid which plays the first of these two parts is not really built up into the tissues, does not be- come living tissue, but undergoes the changes that give rise to urea, somewhere outside the actual living substance. The pro- teids are therefore divided into ' tissue-proteids,'which are actually built up into living substance, and ' floating or circulating pro- teids,' which are not thus built up, but by their metabolism outside the living substance set free energy in the form of heat only. It was at this time erroneously supposed that the exclusive use of proteid food was to supply proteid tissue elements, and that vital manifestations other than heat had their origin in proteid metabolism, the metabolism of fats and carbohydrates giving rise to heat only. Hence, when it was first surmised that a certain proportion of proteids underwent metabolism, which gave rise to heat only, this appeared to be a wasteful expendi- ture of precious material, and the metabolism of this portion of food was spoken of as a 'luxus consumption,' a wasteful consump- tion. There were many deductions from this general theory to explain particular points ; of these two may be mentioned : (i) In inanition, the urea discharged for the first few days is much greater than it is subsequently : this was supposed to be due to the fact that in the first few days all the floating capital was consumed ; (2) the effect of feeding with a mixture of gelatin 748 GENERAL METABOLISM. [ch. liv. and proteid was supposed to be due to the fact that gelatin was able to replace ' floating proteid,' but not 'tissue proteid.' This theory of Voit's, ingenious and plausible at first sight, has met with but little general acceptance, because so many observed facts are incompatible with it. Professor Michael Foster writes as follows : ' The evidence we have tends to show that in muscle (taking it as an instance of a tissue) there exists a framework of what we may call more distinctly living substance, whose metabolism, though high in quality, does not give rise to massive discharges of energy, and that the interstices, so to speak, of this framework are occupied by various kinds of material related in different degrees to this framework, and therefore deserving to be spoken of as more or less living, the chief part of the energy set free coming directly from the metabolism of some or other of this material. Both frame- work and intercalated material undergo metabolism, and have in different degrees their anabolic and katabolic changes ; both are concerned in the life of the organism, but one more directly than the other. We can, moreover, recognise no sharp break between the intercalated material and the lymph which bathes it; hence such phrases as " tissue proteid " and " floating proteid " are undesirable if they are understood to imply a sharp line of de- marcation between the " tissue " and the blood or lymph, though useful as indicating two different lines or degrees of metabolism.' Professor Burdon-Sanderson writes as follows : ' The production of urea and other nitrogenous metabolites is exclusively a func- tion of " living material "; and this process is carried on in the organism with an activity which is dependent on the activity of tlie living substance itself, and on the quantity of material sup- plied to it. No evidence at present exists in favour of a " luxus consumption" of proteid.' Professor Hoppe-Seyler, after stating that he can make out no clear distinction between the two varieties of proteid from Voit's own writings, proceeds as follows : ' Voit states that the circu- lating proteid is no other than that which is dissolved in the tissue juice, which is derived from the lymph-stream, and ultimately from the circulating blood. He (Voit) further says : "As soon as the proteid of the blood-plasma leaves the blood-vessels, and circulates among the tissue elements themselves, it is then the proteid of the nutrient fluid or circulating proteid. It is no longer proteid of the blood-plasma, nor yet is it the proteid of the lymph-stream." The place where Voit situates his circulating proteid is beyond the ken of the anatomist; it is in a mysterious space between tissue- ch. i.v.J ANIMAL HEAT. 749 elements, blood-vessels, and lymph-vessels; the chemist meets with equal difficulties, as there is apparently no chemical differ- ence between tissue proteid and circulating proteid. I can, therefore, arrive at no other conclusion than that these terms are not only useless, but unscientific, and are the outcome of speculations in a region where there is as yet no positive know- ledge. These criticisms on Voit's theories do not, however, by any means, lessen the importance and high value of the immense amount of practical research carried on by Voit and his pupils.' 1 have placed Professor Foster's view first because it takes into account certain facts which tend to show that there are degrees in metabolism. The most important of these is the formation of amido-acids in the intestine. The fate of tyrosine is uncertain ; but it is an undoubted fact that by feeding an animal on leucine, the urea is increased. The transformation of leucine into urea occurs in the liver. It can hardly be supposed that leucine becomes to any great extent an integral part of the living frame- work of the liver cells, but like other extractives, and like aromatic compounds absorbed from the alimentary canal, it be- comes a part of what Foster terms the intercalated material. Here it undergoes the final change, and is ultimately and appa- rently very rapidly discharged in the urine. Dr. Sheridan Lea discussing the probable role of the amido-acids in the animal economy, compares it to the part played by the salts of the food. Neither salts nor extractives simply pass into the urine without fulfilling a useful purpose on their way; but the exact and specific use of each, whether on the synthetic or analytic side of metabolic phenomena, must be the subject of renewed research. CHAPTER LV. ANIMAL HEAT. Among the most important results of the chemical processes we sum up under the term metabolism, is the production of heat. Heat, like mechanical work, is the result of the katabolic side of metabolic processes; the result, or accompaniment, that is to say, of the formation of carbonic acid, water, urea, and other ex- creted products. 750 ANIMAL HEAT. [ch. lv. As regards temperature, animals may be divided into two great classes:- (1) Warm-blooded or homoiothermal animals, or those which have an almost constant temperature. This class includes mammals and birds. (2) Cold-blooded or poikilothermal animals, or those whose temperature varies with that of the surrounding medium, being, always, however, a degree, or a fraction of a degree, above that of the medium. This class includes reptiles, amphibians, fish, and probably most invertebrates. The temperature of a man in health varies but slightly, being between 36,5° and 37'5° C. (98° to 990 F.). Most mam- mals have approximately the same temperature : horse, donkey, ox, 37'5° to 38°; dog, cat, 38'5° to 390; sheep, rabbit, 38° to 39'5° ■, mouse, 40° C. Birds have a higher temperature, about 420 C. The temperature varies a little in different parts of the body, that of the interior being greater than that of the surface; the blood coming from the liver when oxidation is very active is warmer than that of the general circulation ; the blood becomes rather cooler in its passage through the lungs. The temperature also shows slight diurnal variations, reaching a maximum about 3 p.m. (37'5° C.) and a minimum about 3 a.m. (36'8° C.); that is, at a time when the functions of the body are least active. If, however, the habits of a man be altered, and he sleeps in the day, working during the night, the times of the maximum and minimum temperatures are also inverted. Inani- tion causes the temperature to fall, and just at the onset of death may be below 30° C. Active muscular exercise raises the tem- perature temporarily by about 0-5° to i° C. Diseases may cause the temperature to vary considerably, especially those which we term febrile (see p. 745). Although certain mechanical actions, such as friction, due to movements of various kinds, may contribute a minute share in the production of heat in the body, yet we have no knowledge as to the actual amount thus generated. The great source of heat is, as already stated, chemical action, especially oxidation. Any given oxidation will always produce the same amount of heat. Thus, if we oxidise a gramme of carbon, a known amount of heat is produced, whether the element be free or in a chemical compound. The following figures show the approximate number of heat-units produced by the combustion of one gramme of the following substances. A heat-unit, or calorie, is the amount of heat necessary to raise the temperature of one gramme of water i° C. CH. LV.J PHYSIOLOGICAL HEAT VALUE 751 Hydrogen .... 3450 Carbon8100 Urea . . . . 2205 Albumin . . . . 4998 Fat . . . . . 9069 Cane sugar . . . . 3348 Starch .... 3898 It is, however, most important to remember that the 'physio- logical heat-value ' of a food may be different from the ' physical heat-value,' i.e., the amount of heat produced by combustion in the body may be different from that produced when the same amount of the same food is burnt in a calorimeter. This is especially the case with the proteids, because they do not undergo complete combustion in the body, for each gramme of proteid yields a third of a gramme of urea, which has a considerable heat- value of its own. Thus albumin, -which, by complete combustion, yields 4998 heat-units, has a physiological heat-value=4998 minus one-third of the heat-value of urea (2205)2=4998 - 735 = 4263. Of the heat produced in the body, it is estimated by Helmholtz that about 7 per cent, is represented by external mechanical work, and that of the remainder about four-fifths are discharged by radiation and evaporation from the skin, and the remaining fifth by the lungs and excreta. The following table exhibits the relation between the produc- tion and discharge of heat in twenty-four hours in the human organism at rest, estimated in calories.* The table conveniently takes the form of a balance-sheet in which production and dis- charge of heat are compared ; to keep the body-temperature normal these must be equal. The basis of the table in the left- hand (income) side is the same as Ranke's adequate diet (see p. 616 and p. 739) :- Production of heat. Consumption of Calories. Proteid (100 gr.) 100 x 4263 = 426,300 Fat (100 gr.) . too x 9069 = 906.900 Carbohydrates (250 gr) .250x3898 = 974.500 2,307,700 Discharge of heat. Calories. Warming water in food, 2'6 kilos, x 25° C. = 65,000 Warming air in respiration, 16 kilos, x 250 x 0-24 = 96.000 Evaporation in lungs, 630 gr. x 582= 366.660 Radiation and evapora- tion at surface. . = 1,780,040 2.307,700 * The calorie we are taking is sometimes called the small calorie ; by some the word calorie is used to denote the amount of heat necessary to raise one kilogramme of water i° C. 752 ANIMAL HEAT. [ch. lv. The figures under the heading Production are obtained by multiplying the weight of food by its physiological heat-value. The figures on the other side of the balance-sheet are obtained as follows : The water in the food is reckoned as weighing 2'6 kilos. This is supposed to be at the temperature of the air taken as i2° C.; it has to be raised to the temperature of the body, 370 C., that is through 250 C. Hence the weight of water multiplied by 25 gives the number of calories expended in heating it. The weight of air is taken as weighing 16 kilos. ; this also has to be raised 250 C., and so to be multiplied by 25 ; it has further to be multiplied by the relative heat of air (0'24). The 630 grammes of water evaporated in the lungs has to be multiplied by the potential or latent heat of steam at 370 C. (582); the portion of heat lost by radiation and evaporation from the skin constitutes about four-fifths of the whole, and is obtained by deducting the three previous amounts from the total. This table does not take into account the small quantities of heat lost with urine and faeces. It need hardly be remarked that the above is a mere illustrative experiment. Changes in the diet, in the atmospheric temperature, in the temperature of the food taken, in the activity of the sweat-glands, in the amount of moisture in the atmosphere, and in the amount of work done would considerably alter the above figures. Calorimetry.-Calorimeters employed in chemical operations are not suitable for experiments on living animals. An animal surrounded by ice or mercury, the melting and expansion of which respectively are measures of the amount of heat evolved, would be under such abnormal conditions that the results would be valueless. The apparatus often employed is the water calorimeter. This was first used by Lavoisier, and his apparatus as modified by Dulong is shown in fig. 578. The animal is placed in a metal chamber, surrounded by a water-jacket. There are also tubes for the entrance and exit of the inspired and expired gases respec- tively. The heat given out by the animal warms the water in the jacket, and is measured by the rise of temperature observed in the water, of which the volume is also known. The air which passes out from the chamber goes through a long spiral tube, passing through the water-jacket, and thus the heat is abstracted from it and not lost. Air-calorimeters are now, however, generally used. The follow- ing is an outline sketch of the one which has been most used in this country. It consists of two precisely similar chambers made of thin CH. LV.J CALORIMETERS. 753 sheet copper. Each chamber has two walls between which is an air space; and the outer is covered by a thick casing of felt (F) to prevent fluctuations in the temperature of the surroundings from affecting the air in the air-space. The chambers are made perfectly air-tight, except for the ventilating tubes AA, A'A'. Fig. 579.-Dulong's calorimeter: C, calorimeter, consisting of a vessel of cold water in which the chamber holding the animal is placed ; G', gasometer from which air is ex- pelled by a stream of water. The air enters the respiratory chamber. G, gasometer receiving the gases of expiration and the excess of air. t, t', thermometers ; «, a wheel for agitating the water. Observe the delivery-tube on the left is much twisted in the water-chamber, so as to give off its heat to the surrounding water. (From McKendrick's " Physiology.") By means of these, the chambers are filled with perfectly dry air before the experiment is commenced. Leading from each air- space is a tube ; the twTo tubes are connected to the two limbs of Fig. 580.-Air calorimeter of Haldane, Hale White and Washbourn. a manometer (M) shaped as in the figure, and containing oil of erigeron. The action of the calorimeter is as follows :-In one chamber,1 the animal, the heat production of which is to be ascertained, is 754 ANIMAL HEAT. [ch. lv. placed within the cage C. In the other, hydrogen is burnt (H). Both chambers are shut, the tubes AA, A'A' being clamped. The heat given off from the animal warms its chamber, and thus increases the pressure of the air in the air-space between the two copper walls of the chamber. This would lead to movement of the fluid in the manometer, but that the heat given off by the burning of the hydrogen increases at the same time the pressure in the air-space between the walls of its chamber. This latter increase of pressure tends to make the fluid in the manometer move in the other direction. If the fluid in the manometer remains stationary, the amount of heat given off by the animal is equal to that produced by the burning hydrogen ; and during an experiment the fluid in the manometer is kept stationary by turning the hydrogen flame up and down. The amount of hydrogen burnt is estimated by the amount of water formed, and the heat of combustion of hydrogen being known, it is perfectly easy to calculate the calories produced, which equal those given off by the animal. Regulation of the Temperature of Warm-blooded Animals. We have seen that heat is produced by combustion processes, and lost in various ways. In order to maintain a normal temperature, both sides of the balance-sheet must be equal. This equalisation may be produced by the production of heat, adapting itself to variations in discharge, or by the discharge of heat adapting itself to variations in production, or lastly, and more probably, both sets of processes may adapt themselves mutually to one another. We have, therefore, to consider (i) regulation by variations in loss and (2) regulation by variations in production. Regulation by Variations in Loss.-The two means of loss susceptible of any amount of variation are the lungs and the skin. The more air that passes in and out of the lungs, the greater will be the loss in warming the expired air and in evaporating the water of respiration. In such animals, as the dog, which perspire but little, respiration is a most important means of regulating the temperature ; and in these animals a close connection is observed between the production of heat and the respiratory activity. The panting of a dog when overheated is a familiar instance of this. A dog also, under the same circumstances, puts out its tongue, and loses heat from the evaporation that occurs from its surface. The great regulator, however, is undoubtedly the skin, and this has a double action. In the first place, it regulates the loss of heat by its vaso-motor mechanism; the more blood passing CH. LV.] REGULATION OF TEMPERATURE. 755 through the skin, the greater will be the loss of heat by con- duction, radiation, and evaporation. Conversely, the loss of heat is diminished by anything that lessens the amount of blood in the skin, such as constriction of the cutaneous vessels, or dilatation of the splanchnic vascular area. In the second place, the special nerves of the sweat-glands are called into action. Familiar instances of the combined action of these two sets of nerves are the redden- ing of the skin and sweating that occurs after severe exercise, on a hot day, or in a hot-air or vapour bath, and the pallor of the skin and absence of sensible perspiration on the application of cold to the body. Regulation by Variations in Production.-The rate of produc- tion of heat in a living body, as determined by calorimetry, depends on a variety of circumstances. It varies in different kinds of animals. The general rate of katabolism of a man is greater than that of a dog, and of a dog greater than that of a rabbit. Probably every species has a specific coefficient, and every individual a personal coefficient of heat production, which is the expression of the inborn qualities proper to the living substance of the species and individual. Another factor is the proportion of the bulk of the animal to its surface area, the struggle for existence raising the specific coefficient of the animals in which the ratio is high. Other important considerations are the relation of the intake of food to metabolic processes, and the amount of muscular work which is performed. These various influences are themselves regulated by the nervous system, and physiologists have long suspected that afferent impulses arising in the skin or elsewhere may, through the central nervous system, originate efferent impulses, the effect of which would be to increase or diminish the metabolism of the muscles and other organs, and by that means increase or diminish respectively the amount of heat there generated. That such a metabolic or thermogenic nervous mechanism does exist in warm-blooded animals is supported by the following experimental evidence :- (1) Though in cold-blooded animals, a rise or fall of the surrounding temperature causes respectively a rise and fall of their metabolic activity, in a warm-blooded animal the effect is just the reverse. Warmth from the exterior demands a diminished, production of heat in the interior, and vice versa. (2) That this is due to a reflex nervous impulse is supported by the fact that a warm-blooded animal, when poisoned by curare no longer manifests its normal behaviour to external heat and cold but is affected in the same way as a cold-blooded animal. Section of the medulla produces the same effects, as the nerve-channels, 756 THE REPRODUCTIVE ORGANS. [ch. lvi. by which the impulses travel, are severed. When curare is given, the reflex chain is broken at its muscular end, the poison exerting its influence on the end-plates, and causing a diminution of the chemical tonus of the muscles. The centre of this thermotaxic reflex mechanism must be situated somewhere above the spinal cord; according to some observers, in the neighbourhood of the optic thalamus. (3) Various injuries caused by accident, or purposely produced by puncture, or cautery, or electrical stimulation of limited portions of the more central portions of the brain, may give rise to great increase of temperature, not accompanied by other marked symptoms. We thus see that the nervous system is intimately associated with the regulation of the temperature of the body. There is at least one-there may be several centres associated in this action. The centres receive afferent impulses from without; they send out efferent impulses by at least three sets of nerves: (1) the vaso- motor nerves, (2) the secretory nerves of the sweat-glands, (3) trophic or nutritional nerves. The first two sets of nerves, the vaso-motor and the secretory, affect the regulation of temperature on the side of discharge; the third set on the side of production. CHAPTER LVI. THE REPRODUCTIVE ORGANS. The reproductive organs consist in the male of the two testes which produce spermatozoa, and the ducts which lead from them, and in the female of the two ovaries which produce ova, the Fallopian tubes or oviducts, the uterus, and the vagina. Male Organs. The testis is enclosed in a serous membrane called the tunica vaginalis, originally a part of the peritoneum. When the testis descends into the scrotum it carries with it this part of the peri- toneum, which then is entirely cut off from the remainder of that serous membrane. The external covering of the testicle itself is a strong fibrous capsule, called, on account of its white appear- ance, the tunica albuginea. Passing from its inner surface' are a CH. LVI.J THE TESTES. 757 number of septa or trabeculae, which divide the organ imperfectly into lobules. On the posterior aspect of the organ the capsule is greatly thickened, and forms a mass of fibrous tissue called the Corpus High- morianum (body of Highmore) or medias- tinum testis. Attached to this is a much convoluted tube, which forms a mass called Fig. 581.-Flan of a vertical section of the testicle, showing the arrange- ment of the ducts. The true length and diameter of the ducts have been disregarded, a a, tubuli seminiferi coiled up in the separate lobes; &, tubuli recti; c, rete testis ; <f, vasa efferentia ending in the coni vascu- losi; I, e, g, convoluted canal of the epididymis ; h, vas deferens; /, section of the back part of the tunica albuginea; i, i, fibrous processes running between the lobes; s, me- diastinum. Fig. 582.-Section of a tubule of the testicle of a rat, to show the formation of the spermatozoa. a, spermatozoa ; 5, seminal cells; c, sustentacu- lar cells, to which the spermatozoa are ad- herent ; d, basement membrane; e, connective tissue. (Cadiat.) Fig. 583.-From a section of the testis of dog, showing portions of seminal tubes. A, seminal epithelial cells, and numerous small cells loosely arranged ; B, the small cells or spermatoblasts converted into spermatozoa ; 0, groups of these in a further stage of development. (Klein.) the epididymis. This receives the ducts of the testis, and is prolonged into a thick walled tube, the vas deferens, by which the semen passes to the urethra. 758 THE REPRODUCTIVE ORGANS. [ch. lvi. The testis is itself composed of convoluted tubes. Each of these commences near the tunica albuginea, and terminates after join- ing with others in a straight tubule, which passes into the body of Highmore, where it forms a network (rete testis') by communicating by branches with those of other straight tubules. From the rete about twenty efferent ducts (yasa efferentid) arise, which become Fig- 584- Section of the epididymis of a dog-The tube is cut in several places, both transversely and obliquely; it is seen to be lined by a ciliated epithelium, the nuclei of which are well shown, c, connective tissue. (Schofield.) convoluted to form the coni vasculosi, and then pass into the tube of the epididymis. The convoluted or seminiferous tubules have the following- structure : each is formed externally of a thick basement mem- brane, consisting of several layers of flattened cells. Next comes the lining epithelium of clear cubical cells, a few of which show karyokinetic figures in their nuclei, indicating they are about to divide. Some of these cells are longer than the rest, and project into the cavity of the tube, where they form a connection with groups of developing spermatozoa. They are called sustentacular cells. Next to the lining epithelium is a zone of larger cells, two or CH. LVI.J THE TESTES. 759 three deep. These are called spermatogenic cells; the nuclei of nearly all of these show karyokinetic figures. Most internal of all are a large number of small cells with circular nuclei. They are called spermatoblasts. In other tubules the spermatoblasts may be seen in various stages developing into spermatozoa ; they become elongated : their nucleus is at one end, and from the other a tail-like process grows; groups of the young spermatozoa apply their heads to the sustentacular cells, from which they Fig. 585.-Dissection of the base of the bladder and prostate gland, showing the vesiculee seminales and vasa deferentia. a, lower surface of the bladder at the place of re- flexion of the peritoneum ; J, the part above covered by the peritoneum ; f, left vas deferens, ending in e, the ejaculatory duct; the vas deferens has been divided near i, and all except the vesical portion has been taken away; s, left vesicula seminalis j oining the same duct; s s, the right vas deferens and right vesicula seminalis, which has been unravelled ; p, under side of the prostate gland; m, part of the urethra ; u u, the ureters (cut short), the right one turned aside. (Haller.) derive nutriment; their tails project into the lumen ; they even- tually become free. The straight tubules consist of basement membrane and lining cubical epithelium only. The interstitial connective tissue of the testis is loose, and contains numerous lymphatic clefts. Lying in it accompanying the blood-vessels are strands of polyhedral epithelial cells, of a yellowish colour (interstitial cells). The tubules of the rete testis are lined by cubical epithelium ; the basement membrane is absent. 760 THE REPRODUCTIVE ORGANS. [ch. lvi. The vasa efferentia, coni vasculosi, and epididymis are lined by columnar cells, with very long cilia. There is a good deal of muscular tissue in their walls. The vas deferens consists of a muscular wall (outer layer longi- tudinal, middle circular, inner longitudinal), lined by a mucous membrane, the inner surface of which is covered by columnar epithelium. The vesicidae seminales are outgrowths of the vas deferens. Each is a much convoluted, branched, and sacculated tube of structure similar to that of the vas deferens, except that the wall is thinner. The penis is composed of cavernous tissue covered by skin. Fig. 586.--Erectile tissue of the human penis, a, fibrous trabecula; with their ordinary capillaries; b, section of the venous sinuses ; c, muscular tissue. (Cadiat.) The cavernous tissue is collected into three tracts, the two corpora cavernosa and the corpus spongiosum in the middle line inferiorly. All these are enclosed in a capsule of fibrous and plain muscular tissue ; the septa which are continued in from these, form the boundaries of the cavernous venous spaces of the tissue. The arteries run in the septa; the capillaries open into the venous spaces. The arteries are often called helicine, as in injected specimens they form twisted loops projecting into the cavernous spaces (see also p. 453). The structure of the urethra and prostate are described on p. 693. The Spermatozoa, suspended in a richly albuminous fluid, constitute the semen. Each spermatozoon is composed of three parts, a head, a middle part, and a tail. The head varies in shape in different animals, but in man it is oval, and pointed anteriorly. CH. LVI.] SPERMATOZOA. 761 The middle piece is short and cylindrical, with a spiral fibre pass- ing round it. The tail is long, tapering, and vibratile ; its action resembles that of a cilium, and gives to the spermatozoon its power of locomotion. The end piece of the tail is described by Retzius as distinct from the rest, and in some animals is divided into two or three fibrils. In some animals (newts, salamanders, &c.) a fine filament or membranous expansion is attached to the tail in a spiral manner Fig. 587.-Spermatic fila- ments from the human vas deferens. 1, magni- fied 300 diameters; 2, magnified 800 diameters; a. from the side ; b, from above. (From Kolliker.) Fig. 588.-Spermatozoa. 1, of sala- mander ; 2, human. (H. Gibbes.) (fig. 588). A similar appearance has been described by some obser- vers in mammalian spermatozoa. The spermatozoa are formed from the small spermotoblasts of the third or innermost layer of the seminiferous tubules; these originate from the spermatogenic cells of the second layer, and these from the lining cubical epithelium. When a lining cell divides into two, one becomes a spermatogenic cell, the other becomes elongated to form a sustentacular cell. In the conversion of a spermatoblast into a spermatozoon, the nucleus forms the head ; the tail develops as a fine filament within the protoplasm, from which it subsequently grows out; it is connected to the nucleus from the first. The greater part of the protoplasm drops off (seminal granules); the remainder forms the middle piece of the spermatozoon. 762 THE REPRODUCTIVE ORGANS. [ch. lvi- Female Organs. The Ovary is a solid organ composed of fibrous tissue (stroma), containing near its attachment to the broad ligament a number of plain muscular fibres. It is covered with a layer of short columnar cells (germinal epithelium), which may be seen, especially in young animals, dipping down into the stroma. The interstitial connective tissue contains a number of epithelial polyhedral yellow cells, like the interstitial cells of the testis. When cut across, the surface part of the stroma is seen to be Fig. 589.-Diagrammatic view of the uterus and its appendages, as seen from behind. The uterus and upper part of the vagina have been laid open by removing the posterior wall; the Fallopian tube, round ligament, and ovarian ligament have been cut short, and the broad ligament removed on the left side ; w, the upper part of the uterus ; c, the cervix opposite the os internum ; the triangular shape of the uterine cavity is shown, and the dilatation of the cervical cavity with the rugae termed arbor vitae ; v, upjier part of the vagina ; od, Fallopian tube or oviduct; the narrow communication of its cavity with that of the cornu of the uterus on each side is seen ; i, round liga- ment; to, ligament of the ovary; o, ovary; i, -nude outer part of the right Fallopian tube ; fi, its fimbriated extremity; po, parovarium; h, one of the hydatids frequently found, connected with the broad ligament. J. (Allen Thomson.) crowded with a number of rounded cells (przWfwe ova or ovigerms), and vesicles of different sizes are also visible. These are called the Graafian follicles. The smallest are near the sur- face of the organ ; the larger ones are deeper, though they extend to the surface as they grow. A Graafian follicle has a proper wall formed from the stroma; it contains within it an ovum formed from one of the primitive ova, and it is lined by epithelium. At first there is simply one layer of epithelium cells ; this lines the follicle and covers the ovum ; later there are two layers, one lining the follicle, and the other covering CH. LVI.] THE OVARY. 763 the ovum, but the two are close together. A viscid fluid collects between the two layers, and as the follicle increases in size separates them. Fig. 590.-View of a section of the ovary of the cat. 1, outer covering and free border of the ovary; 1', attached border; 2, the ovarian stroma, presenting a fibrous and vascular structure ; 3, granular substance lying external to the fibrous stroma ; 4, blood- vessels ; 5, ovigerms in their earliest stages occupying a part of the granular layer near the surface ; 6, ovigerms which have begun to enlarge and to pass more deeply into the ovary ; 7, ovigerms round which the Graafian follicle and tunica granulosa are now formed, and which have passed somewhat deeper into the ovary and are surrounded by the fibrous stroma ; 8, more advanced Graafian follicle with the ovum imbedded in the layer of cells constituting the proligerous disc ; 9, the most advanced follicle containing the ovum, &c.; 9', a follicle from which the ovum has accidentally escaped; 10, corpus luteum. f-. (Schron.) Fig-. 591.-Section of the ovary of a cat. A, germinal epithelium ; B, immature Graafian follicle ; C, stroma of ovary ; D, vitelline membrane containing the ovum ; E, Graafian follicle showing lining cells; F, follicle from which the ovum has fallen out. (V. D. Harris.) 764 THE REPRODUCTIVE ORGANS. [ch. lvi. The cells in each layer multiply, so that they are arranged in several strata. The lining epithelium of the follicle is then called the membrana granulosa, and the heaped mass of cells around the ovum, the discus proligerus. The fluid increases in quantity, the follicle becomes tenser and tenser, and finally it reaches the surface of the organ and bursts; the ovum is thus set free, and is seized by the fringed ends of the Fallopian tube and thence passes to the uterus. The bursting of a Graafian follicle usually occurs about the time of menstruation. After the bursting of a Graafian follicle, it is filled up with what is known as a corpus luteum. This is derived from the wall of the follicle, and consists of columns of yellow cells developed from the yellow interstitial cells previously mentioned; it con- tains a blood clot in its centre. These cells multiply, and their strands get folded and converge to a central strand of connective tissue; between the columns there are septa of connective tissue with blood-vessels. The corpus luteum after a time gradually disappears; but if pregnancy supervenes it becomes larger and more persistent. The following table gives the chief facts in the life history of the ordinary corpus luteum of menstruation, compared with that of pregnancy :- Corpus Luteum of Men- struation. Corpus Luteum of Preg- nancy. At the end of three weeks. One month . Two months . Six months . Nine months. Three-quarters of an inch in diameter ; central clot reddish ; convoluted wall pale. Smaller; convoluted wall bright yellow ; clot still reddish. Reduced to the condi- tion of an insignifi- cant cicatrix. . Absent. Larger ; convoluted wall bright yellow ; clot still reddish. Seven-eighths of an inch in dia- meter ; convoluted wall bright yellow ; clot perfectly de- colorised. Still as large as at end of second month ; clot fibrinous ; convo- luted wall paler. One half an inch in diameter ; central clot converted into a radiating cicatrix; the external wall tolerably thick and con- voluted, but without any bright yellow colour. Some of the Graafian follicles never burst; they attain a certain degree of maturity, then atrophy and disappear. An ovum is a large spheroidal cell surrounded by a trans- CH. LVI.] THE OVARY. 765 parent striated membrane called the vitelline membrane, or zona pellucida. The protoplasm is filled with large fatty and albu- Fig. 592.-Corpora lutea of different periods. B, corpus luteum of about the sixth week after impregnation, showing its plicated form at that period. 1, substance of the ovary; 2, substance of the corpus luteum; 3, a greyish coagulum in its cavity. (Paterson.) A, corpus luteum two days after delivery; D, in the twelfth week after delivery. (Montgomery.) Nucleus or germinal vesicle. Nucleolus or germinal spot. Space left by retraction of protoplasm. Protoplasm containing yolk spherules. Vitelline membrane. Fig. 593.-A human ovum. (Cadiat.) Fig. 594.-Germinal epithelium of the surface of the ovary of five days' chick, a, small ovoblasts ; b, larger ovoblasts. (Cadiat.) minous granules (yolk s}->herules), except in the part around the nucleus, which is comparatively free from these granules. It 766 THE REPRODUCTIVE ORGANS. Lch. lvi. contains a nucleus which has the usual structure of nuclei; there is generally one very well-marked nucleolus. The nucleus and nucleolus are still often called by their old names, germinal vesicle and germinal spot respectively. The ova and the epithelium of the Graafian follicles are developed from the germinal epithelium which in the embryo forms a thick layer over the ovary; cords of these cells, solid in some animals, tubular in others, grow down into the stroma, and in time these are broken up into nests by ingrowths of the stroma. Fig. 595.-A. Diagram of uterus just before menstruation ; the shaded portion represents the thickened mucous membrane. B. Diagram of uterus when menstruation has just ceased, showing the cavity of the uterus deprived of mucous membrane. C. Diagram of uterus a week after the menstrual flux has ceased: the shaded portion represents renewed mucous membrane. (J. Williams.) Each nest represents a primitive Graafian follicle. In this, one cell in particular becomes enlarged to form the ovum; the remainder form the epithelium of the follicle. The Fallopian tubes have externally a serous coat derived from the peritoneum, then a muscular coat (longitudinal fibres outside, circular inside), and most internally a very vascular mucous membrane thrown into longitudinal folds, and covered with ciliated epithelium. The uterus consists of the same three layers. The muscular CH. LVI.J THE UTERUS. 767 coat is, however, very thick and is made up of two strata imper- fectly separated by connective tissue and blood-vessels. Of these the thinner outer division is the true muscular coat, the fibres of which are arranged partly longitudinally, partly circularly. The inner division is very thick ; its fibres run chiefly in a circular direction; the extremities of the uterine glands extend into its internal surface. It is in fact a much hypertrophied muscularis mucosae. The mucous membrane is thick, and consists of a corium of soft connective tissue, lined with ciliated epithelium ; this is continued Kg. 596.-Section of the lining membrane of a human uterus at the period of commencing pregnancy showing the arrangement and other peculiarities of the glands, d, d, d, with their orifices, a, a, a, on the internal surface of the organ. Twice the natural size. down into long tubular glands which have as a rule a convoluted course. In the cervix the glands are shorter. Near the os uteri the epithelium becomes stratified ; stratified epithelium also lines the vagina. At each menstrual period, the greater part of the mucous membrane of the body of the uterus disintegrates and is shed ; this with some blood which escapes from the ruptured capillaries constitutes the menstrual flow. This is followed by a rapid renewal of the mucous membrane. But if gestation takes place, the new mucous membrane is much thicker than that which is usually formed. The glands are correspondingly long. This thick membrane is then called the decidua. For a description of the mammary glands see p. 619. 768 DEVELOPMENT. [CH. LVHi CHAPTER LVIL DEVELOPMENT. The description of the origin and formation of the tissues and organs constitutes the portion of biological science known as embryology. This subject is a large one, and many books are written which have for their exclusive object its elucidation. All one can possibly attempt in a physiological text-book is the merest outline of the principal facts of development. In our descriptions throughout we shall endeavour to speak of the development of the mammal principally ; it will not be possible to do so altogether, for many of the facts which are believed to be true of the mammal (man included) have only been actually seen in the lower animals. That they occur in the higher animals too is a matter of inference. It will, however, add interest to the subject to draw some of our descriptions from the lower animals ; for the scientific dis- cussion of embryology must always start from a wide survey of the whole animal kingdom. Without entering into any argu- ments in relation to the Darwinian theory of evolution, it will be sufficient to state in general terms that the series of changes which occur in the ejnbryological history of the highest animals, form a compressed picture of the changes which have taken place in their historical development from lower types of living creatures. The Ovum. The human ovum is like that of other mammals, a small spherical body about T-A=- to -j-l-y inch in diameter ; it is an organised animal cell consisting of a mass of protoplasm enclosing a nucleus. The protoplasm, however, also contains a granular material, vitelline or yolk substance which nourishes the proto- plasmic part. The way in which ova are formed in the ovary is described in the chapter preceding this. In many animals, as in birds, most of the changes from ovum to adult take place, not within the body of the parent, but outside of it. The egg can therefore derive no nutriment from the mother; and, as one would expect, the yolk substance is much more abundant. It is due to this fact that the eggs of birds, reptiles, and fishes are so much larger than the mammalian ovum. Each CH. LVJI.J THE POLAR GLOBULES. 769 however, is still essentially a single cell, much bulged by yolk material. The yellow part of the hen's egg which is alone com- parable to the mammalian ovum has upon it a whitish speck or cicatricula about inch in diameter; in this the nucleus or germinal vesicle is imbedded; it is here and in the surrounding protoplasm that cell division and growth goes on, the rest of the yolk serving to nourish this part. Ova in which the whole cell takes part in cell division are called holoblastic; but those like the hen's egg, in which only a part, the cicatricula just alluded to, divides and subdivides, are termed meroblastic. There are, however, gradations between the two extremes. The structure of the mammalian ovum has already been given (p. 765). The surrounding zona pellucida is perforated, at any rate in some animals, by a small hole called the micropyle, which enables a spermatozoon to enter. Some observers have described a second more delicate membrane within the zona pellucida. Changes in the Ovum previous to Fecundation. The most important change is the disappearance of the greater part of the nucleus ; simultaneously with this two particles called the polar or directing globules appear on the surface of the ovum, between it and the zona pellucida. The relation between these two phenomena, and many of the facts relating to the impregnation of, and early changes in the ovum were first made out by Oscar Hertwug, and Ed. v. Beneden, in the ova of sea-urchins and other echinoderms; they have been verified in many of the higher animals; and, though they have never been actually seen in the human ovum, there is practically no doubt that they occur there too. In the case of the sea-urchin, the problem is a comparatively easy one to study. Sea-water containing the elements of each sex can be obtained ; the changes w'atched, and finally the two specimens can be mixed and impregnation watched. In the ovum, the nucleus travels to the surface of the ovum, loses its investing membrane, and undergoes the changes associated with karyokinesis, the spindle lying horizontally under the outer circumference of the cell. At each end of the spindle an attraction sphere is situated; the granules of the protoplasm take a radial arrangement around the two attraction spheres. The spindle then becomes vertical, and the nucleus divides into two; the upper daughter nucleus with a thin investment of protoplasm is extruded from the body of the cell. The other daughter 770 DEVELOPMENT. (ch. lvii. nucleus remains within the ovum ; it proceeds to repeat the process, and a second polar globule is extruded, so that only a quarter of the original nucleus remains within the ovum; this is then called the female pronude,us, and it travels towards the centre of the ovum. Simultaneously the ovum shrinks, not only from the loss of the polar globules; but also from a shedding out of liquid which collects between the ovum and the zona pellucida and is called the fluid. In this, later on, spermatozoa which have penetrated the zona pellucida, may be seen swimming. Impregnation. We now have a somewhat shrunken ovum possessing a female pronucleus ; for some time, this continues to have the radiating arrangement of granules around it. Impregnation or fertilization consists in the embedding of the head of one spermatozoon in the protoplasm of the ovum; the tail is lost, and the head has around it the same star-like arrangement of the protoplasmic granules of the ovum which we described around the female pronucleus. It is now called the male pronucleus; it travels to the female pro- nucleus, and in some animals may for a time leave a distinct groove marking its pathway. Having reached the female pro- nucleus, it fuses with it and forms an ordinary nucleus. The whole cell so produced is then often called the blastosphere. A great deal of discussion has taken place as to the meaning of the discharge of the polar globules. Fertilization consists in bringing to the ovum of a certain amount of germinal plasma from another individual or male, and Weismann assumes that it is necessary that the ovum, prior to development, should get rid both of its old histogenetic plasma, and of so much germinal plasma (z.e. matter endowed with heredity) as may be brought to it by the spermatozoon. This is effected by the extrusion of the two polar globules. The second polar globule is the one which contains the germinal plasma, or the hereditary male element. In those animals which can reproduce their species for many generations without the intervention of a fresh male (partheno- genesis), the second polar globule is not extruded. Fig. 597 represents a fertilized ovum, and if its appearance is compared with that of the ovarian ovum (fig. 593), there is not much anatomical distinction to be noticed between the twro. The fertilized ovum is rather smaller, as it has discharged the polar globules and the perivitelline fluid, and its nucleus is composed of male and female elements ; but how great is the physiological CH. LVII.J SEGMENTATION. 771 difference between the two, for the fertilized ovum now is a new individual, though in a very rudimentary condition. These processes all take place in the Fallopian tube of the mammal as Zona pellucida -peri-uitelline liquid -Polar globules Fig. 507.-The fertilized ovum, or blastosphere. the ovum travels towards the uterus, and then the process of segmentation or cell division begins. Segmentation. The ovum first divides into two cells, then each of these into two again, and so on ; until at last it consists of a mulberry-like mass of little cells, all still enclosed within the zona pellucida. The polar globules are soon lost to view. Cell division is always accompanied by the usual karyokinetic changes in the nucleus. On cutting a section through the embryo at this stage, it is found to consist of a single layer of cells arranged around a central cavity filled with fluid shed out from the cells. This is called the blastula stage or the stage of the unilaminar (one layered) blastoderm. The cells by mutual compression become columnar in shape, and soon the cells are arranged two layers deep; the formation of the second layer within the first differs in different animals. In amphioxus and many of the invertebrates, it is formed by a process of invagination; that is, a portion of the surface layer is gradually pushed in until it completes a second layer within the first; the orifice of invagination is called the blastopore, and corresponds to a primitive mouth opening into a primitive alimentary cavity surrounded by the inner layer of cells. In mammals, the cells which are going to form the second layer take up a central position from the very start, and the outer cells by multiplying more quickly than the inner ones grow round and 772 DEVELOPMENT. [ch. lvii. enclose them. This is the gastrula stage or the stage of the bilaminar blastoderm. Then a third layer is formed between the other two, and thus we arrive at the stage of the trilaminar blastoderm. The three layers are called from within outwards the epiblast, mesoblast, and hypoblast, or the ectoderm, mesoderm, and endoderm. We must next study the way in which the mesoblast is formed between the other two layers. When the outer Fig. 599.-Impregnated egg, with commencement of formation of embryo ; showing the area ger- minativa or embryonic spot, the area pellucida, and the primitive groove and streak. (Dalton.) surface of the ovum is ■ viewed from above, a streak or shadow is visible; this occurs first at its posterioi end, and it gradually ex- tends towards the anterior end. It is due to a thicken- ing of the epiblast; the cells of which multiply so that they are several layers deep. This appearance is called the primitive streak. This is soon marked by a narrow groove along its centre, which is called the primitive groove. If we cut a transverse section through the ovum at this period, we have the appearance presented in the next figure. The actual preparation is from a chick's egg ; the cicatricula has divided into a number of cells, and these are arranged in two Fig. 598.-Diagrams of the various stages of cleavage of the ovum. (Dalton.) CH. LVII.] THE BLASTODERM. 773 layers, epiblast and hypoblast, but these instead of surrounding the whole ovum lie spread out on the surface of the yolk. This part of the embryo is subsequently pinched off from the yolk-sac. The area which actually gives rise to the embryo is called the germinal disc, and the area opaca in the middle, seen at the left Fig. 600.-Vertical, section of area pellucida and area opaca (left extremity of figure) of blastoderm of a fresh-laid egg. S, epiblast; D, hypoblast; M, large "formative cells," filled with yolk granules, derived from the hypoblast; A, the white yolk immediately underlying the segmentation cavity. (Stricker.) extremity of the figure, is the opacity produced by the primitive streak, and this is seen to be caused by the proliferatioh of epiblastic cells, so that at this point they come into close contact with the hypoblast. Fig. 601 shows a rather later stage ; this mass of cells, chiefly Fig. 601.-Transverse section through embryo chick (26 hours). a, epiblast; b, mesoblast; c, hypoblast; d, mass of cells at primitive streak ; e, primitive groove. (Klein.) epiblastic (cT), gives origin to the intermediate layer or meso- blast (6) which grows between the other two layers. The mesoblast, however, is not exclusively epiblastic in origin, for some of the cells in the mass d, fig. 601, are doubtless hypoblastic. Moreover, certain large ' formative cells ' seen in fig. 600 M and in the next figure, originate from hypoblast and wander into the middle layer, and it is these cells which give origin to the connective-tissues and blood-vessels. 774 DEVELOPMENT. [ch. lvii. The three layers of the blastoderm show from the first distinctive characters ; the epiblast and hypoblast presenting the appearance of epithelium, whereas the mesoblast is composed of cells which are not arranged close together, and many of them are branched. The primitive streak and groove are evanescent structures; they indicate the longitudinal axis of the embryo, but they are Fig. 602.-Vertical section of blastoderm of chick (1st day of incubation). <S, epiblast, consisting of short columnar cells ; D, hypoblast, consisting of a single layer of flattened cells; M, " formative cells." They are seen on the right of the figure, passing in between the epiblast and hypoblast to contribute to the mesoblast; A, white yolk granules. Many of the large "formative cells" are seen containing these granules. (Stricker.) soon replaced by a new and larger groove. This is formed by two new thickenings of epiblast which rise up like walls on each side of the middle line ; they are united together in front, and they extend backwards, enclosing and then (fig. 603) obliterating the primitive groove. These two walls are called the medullary plates or the dorsal ridges; the valley between them is called the medullary groove ; this is the first appearance of the central nervous system. They approach one another, and meet in the middle line, and so convert the medullary groove into a canal. Fig. 604 shows this diagram- matically in transverse section. The epiblastic cells which line the neural canal are, by the union of the two dorsal ridges, entirely cut off from the surface epiblast; these cells multiply and the tube of epiblast gets much thicker; anteriorly it forms the brain and its cavity the cerebral ventricles; behind this it forms the spinal cord with its central canal. The nerves grow out from brain and cord at a later date. Fig. 603.-Embryonic area of a rabbit's ovum (7 days), pr., primitive streak and groove; m.g., medullary groove ; <Z.r., dorsal ridge. (After Kolliker.) CH. LVII.J THE NEURAL GROOVE. 775 The same diagram shows that the mesoblast is collected into large masses on each side of the neural canal; these are called the protovertebrce; a rod of cells has been also pinched off from the Fig. 604.-Diagram of transverse section through an embryo before the closing-in of the medullary groove, m, cells of epiblast lining the medullary groove which will form the spinal cord ; h, epiblast; d, hypoblast; ch, noto-chord ; u, protovertebra; sp, mesoblast; w, edge of dorsal ridge, folding over medullary groove. (Kolliker.) hypoblast, and is seen in transverse section (cA); it is called the notochord. Fig. 605 shows a surface view of the embryo at rather a later Fig. 605.-Embryo of dog. The neural groove, a, is not yet closed, and at its upper or cephalic end presents three dilatations, b, which correspond to the three divisions or vesicles of the brain. At its lower extremity the groove presents a lancet-shaped dilata- tion (sinus rhomboidalis), c. The margins of the groove consist of clear pellucid nerve- substance. Along the bottom of the groove is observed a faint-streak, which is the chorda dorsalis, d, Dorsal ridges. (Bischoff.) stage. The union of the dorsal ridges takes place first about the neck of the future embryo; they soon after unite over the region of the head, while the closing in of the groove progresses much more slowly towards the hinder extremity of the embryo. The 776 DEVELOPMENT. [ch. lvh. medullary groove is by no means of uniform diameter throughout, and even before the dorsal laminae have united over it, is seen to be dilated at the anterior extremity and obscurely divided by constric- tions into the three primary cerebral vesicles. The part from which the spinal cord is formed is of nearly uniform calibre, while towards the .posterior extremity is a lozenge- shaped dilatation, which is the last part to close in. The notochord can be seen under- neath the neural canal. The thickenings of the mesoblast called, the pfrotovertebrw are not continuous longitudinal structures like the neural canal and notochord, but consist of a number of quadri- lateral masses situated down each side of the neural canal. They are seen in fig. 606, which also ■shows the primitive heart, and other structures to be described later on. The neural groove has by this time ' been quite closed in. A transverse section through the embryo at this or a rather later stage (fig'. 607) shows the points already mentioned, but there is also seen a splitting of the general meso- blast into two layers. One adheres to the epiblast and is called the parietal mesoblast, or somatopleur ; the other adheres to the hypoblast and is called the visceral mesoblast or splanchnopleur; the space be- tween them is the body-cavity, coelom or pleuro-peritoneal cavity ; it is subdivided subsequently into the cavities of the pleurae, pericardium and peritoneum. ■ Head and Tail Folds. Body- cavity .-Every vertebrate animal consists essentially of a longitudinal axis (vertebral column) with Fig. 606.-Embryo ehick (36 hours), viewed from beneath as a transpa- rent object (magnified), pl, outline of pellucid area; FB, fore-brain, or first cerebral vesicle : from its sides project op, the optiq vesicles; SO, backward limit of somatopleur fold, " tucked in " under head; a, head-fold of true amnion; a', re- flected layer of amnion, sometimes termed " false amnion; •" sp, back- ward limit of splanchnopleur folds, along which run the omphalomesa- raic veins uniting to form h, the heart, which is continued forwards into ba, the bulbus arteriosus; d, the fore-gut, lying behind the heart, and having a wide crescentic open- ing between the splanchnopleur folds; IIB, hind-brain ; MB, mid- brain; pv, protovertebrae lying behind the fore-gut; me, line of junction of medullary folds and of notochord ; ch, front end of noto- chord ; vpl, vertebral plates; />?-, the primitive groove at its caudal end. (Foster and Balfour.) CH. LVII.] HEAD AND TAIL FOLDS. 777 a neural canal above it, and a body-cavity (containing the alimentary canal) beneath. We have seen how the earliest rudiments of the central axis and the neural canal are formed; we must now consider how the g. 607.-Transverse section through dorsal region of embryo chick (45 hrs.). One half of the section is represented ; if completed it would extend as far to the left as to the right of the line of the medullary canal (Jfc). A, epiblast; C, hypoblast, consisting of a single layer of flattened cells ; Me, medullary canal; Pc, protovertebra ; Wd, Wolffian duct; So, somatopleur; Sp, splanchnopleur; pp, pleuro-peritoneal cavity ; ch, noto- chord ; ao, dorsal aorta, containing blood-cells; v, blood-vessels of the yolk-sac. (Foster and Balfour.) general body-cavity is developed. In the earliest stages the embryo lies flat on the surface of the yolk, and is not clearly marked off from the rest of the blastoderm; but gradually the head-fold, a Fig. 608.-Diagrammatic longitudinal section through the axis of an embryo. The head- fold has commenced, but the tail-fold has not yet appeared. FSo, fold of the somato- pleur ; Fsp, fold of the splanchnopleur; the line of reference, Fso, lies outside the embryo in the " moat," which marks off the overhanging head from the amnion ; D, inside the embryo, is that part which is to become the fore-gut; Fso and Fsp, are both parts of the head-fold, and travel to the left of the figure as development proceeds ; pp, space between somatopleur and splanchnopleur, pleuro-peritoneal cavity; km, commencing head-fold of amnion ; NC, neural canal; Ch, notochord ; Ht, heart; A, B, C, epiblast, mesoblast, hypoblast. (Foster and Balfour.) crescentic depression (with its concavity backwards), is formed in the blastoderm, limiting the head of the embryo; the blastoderm is tucked in under the head, which thus comes to project above the general surface of the membrane : a similar tucking in 778 DEVELOPMENT. [ch. lvii. of blastoderm takes place at the caudal extremity, and thus the head- and tail-folds are formed. Similar depressions mark off the embryo laterally, until it is completely surrounded by a sort of moat which it overhangs on all sides, and which clearly defines it from the yolk. It will be understood that in mammals the yolk-sac is comparatively small. This moat runs in further and further all round beneath the overhanging embryo, till the latter comes to resemble a canoe turned upside-down, the ends and middle being, as it were, decked in by the folding or tucking in of the blastoderm, while on the ventral surface there is still a large communication with the yolk, corresponding to the ivell or undecked portion of the canoe. This communication between the embryo and the yolk is gra- dually contracted by the further tucking in of the blastoderm from all sides, till it becomes narrowed down, as by an invisible con- stricting band, to a mere pedicle which passes out of the body of the embryo at the point of the future umbilicus. The downwardly folded portions of blastoderm are termed the visceral plates. Thus we see that the body-cavity is formed by the downward folding of the visceral plates, just as the neural cavity is pro- duced by the upward growth of the dorsal laminae, the difference being that, in the visceral or ventral laminae, all three layers of the blastoderm are concerned. The folding in of the splanchnopleur, lined by hypoblast, pinches off a portion of the yolk-sac, inclosing it in the body-cavity. This forms the rudiment of the alimentary canal, which at this period ends blindly towards the head and tail, while in the centre it communicates at first freely, and then by a narrow tube with the yolk-sac. The cavity within the hypoblast thus becomes divided into two portions which communicate through the vitelline duct; the portion within the body gives rise, as above stated, to the diges- tive canal, and that outside the body remains for some time as the umbilical vesicle (fig. 609, v). The hypoblast forming the epithelium of the intestine is continuous with the lining mem- brane of the umbilical vesicle, while the visceral plate of the mesoblast is continuous with the outer layer of the umbilical vesicle. The above details will be clear on reference to the accompany- ing diagrams, some of which, however, allude to structures we have not as yet touched upon. We may here mention three CH. LVII.] LAYERS OF THE BLASTODERM. 779 other terms that are employed. The part of the primitive alimentary canal enclosed by the head-fold is called the fore-gut ; that enclosed by the tail-fold is called the hind-gut ; the remainder is called the mid-gut. We have now seen the way in which a distinct embryo with Fig 609.-Diagrammatic section showing the relation in a mammal between the primitive alimentary canal and the membranes of the ovum. The stage represented in this diagram corresponds to that of the fifteenth or seventeenth day in the human embryo, previous to the expansion of the allantois ; c, the villous chorion ; a, the amnion ; a', the place of convergence of the amnion and reflexion of the false amnion a" a", or outer or corneousdayer; e, the head and trunk of the embryo, comprising the primitive vertebrae and cerebro-spinal axis ; i, i, the simple alimentary canal in its upper and lower portions. Immediately beneath the right hand i is seen the foetal heart, lying in the anterior part of the pleuro-peritoneal cavity ; v, the yolk-sac or umbilical vesicle; vi, the vitello-intestinal opening; ». the allantois connected by a pedicle with the anal portion of thetalimentary canal. (Quain.) foreshadowings of the future organs is formed. In subsequent sections we shall have to study the way in which each set of organs is elaborated from these primitive structures. We may conclude this section by giving a list of the organs which are formed from the several primary embryonic layers:- i. From. Epiblast.-a. The epidermis and its appendages. b. The nervous system, both central and peripheral. c. The epithelial structures of the sense-organs. d. The epithelium of the mouth, the enamel of the teeth. 780 DEVELOPMENT. [ch. lvii. e. The epithelium of the nasal passages. /. The epithelium of the glands opening on the skin and into the mouth, and nasal passages. ff. The muscular fibres of the sweat-glands. 2. From Mesoblast.-a. The skeleton and all the connective- tissues of the body. b. All the muscles of the body except those of the sweat- glands. c. The vascular system, including the lymphatics, serous mem- branes, and spleen. d. The urinary and generative organs, except the epithelium of the bladder and urethra. The Somatopleur forms the osseous, fibrous, and muscular tissues of the body-wall including the true skin. The Splanchnopleur forms the fibrous and muscular walls of the alimentary canal, the vascular system, and the urino-genital organs. 3. From Hypoblast.-a. The epithelium of the alimentary canal from the back of the mouth to the anus, and that of all the glands which open into this part of the alimentary tube. b. The epithelium of the respiratory cavity. c. The epithelium of the Eustachian tube and tympanum. d. The epithelium lining the vesicles of the thyroid. e. The epithelial nests of the thymus. f. The epithelium of the bladder and urethra. The Foetal Membranes. This subject will be best understood by taking a view of the uterus and its contents after the formation of all the membranes. We can then pass to the way in which the several membranes are formed. The uterus, the muscular walls of which are hypertrophied, is lined by a greatly thickened mucous membrane, which is called the decidua, because after the delivery of the child it comes away from the uterus with the other membranes. The decidua is divided into three parts; the lining of the uterine cavity is called the decidua vera (dv) ; a continuation of this reflected over the foetus and its membranes is called the decidua reflexa (dr) •, the portion of the decidua vera which is situated within the line of attachment of the decidua reflexa is called the decidua serotina (ds). These membranes are maternal in origin. Within the decidua reflexa are situated the foetal membranes ; the outer- CH. LVII.J THE FCETAL MEMBRANES. 781 most of these is called the chorion ; at first this is covered with villi containing blood-vessels ; the villi dip into the surrounding decidua, but soon all of them atrophy and disappear, except those that dip into the decidua serotina, where they become greatly enlarged. The chorion is really formed by a fusion of two foetal mem- branes ; the false amnion and the allantois ; the allantois begins as an outgrowth from the hind-gut; the mesoblast which covers it becomes developed into blood-vessels, and thus the false amnion to which it becomes adherent is vascularised; the main Fig. 61cl-Diagrammatic view of a vertical transverse section of the uterus at the seventh or eighth week of pregnancy, c, c, c', cavity of uterus, which becomes the cavity of the decidua, opening at c, c, the cornua, into the Fallopian tubes, and at d into the cavity of the cervix, which is closed by a plug of mucus ; d v, decidua vera; d r decidua reflexa, with the sparser villi imbedded in its substance; d s, decidua serotina' involving the more developed chorionic villi of the commencing placenta. The foetus is seen lying in the amniotic sac; passing up from the umbilicus is seen the umbilical cord, and its vessels passing to their distribution in the villi of the chorion ; also the pedicle of the yolk-sac, which lies in the cavity between the amnion and chorion (Allen Thomson.) vessels in. the stalk of the allantois convey blood to and fro between the foetus and the placenta. The placenta is formed 782 DEVELOPMENT. [CH. LVII. of partly maternal tissue (decidua serotina); partly of foetal tissue (chorion). Within the chorion is another foetal mem- brane, which is attached to the ventral wall of the embryo ; it is called the amnion. This forms a sheath to the allantoic stalk or umbilical cord, and is then reflected over the rest of the embryo. In the umbilical cord is seen the remains of the yolk- Fig. 611.-Diagram of an early stage of the formation of the human placenta. <z, embryo ; ft, amnion ; c, placental vessels ; d, decidua reflexa ; e, chorion ; /, placental villi; g, mucous membrane or decidua vera. (Cadiat.) sac or umbilical vesicle. The amnion is filled with amniotic fluid in which the foetus floats, and is thus protected from external violence. The os uteri is closed by a ping of mucus. Fig. 611 is a rather more diagrammatic view of the same structures in outline, the villi over the general surface of the chorion having disappeared. Development of the Decidua. The ovum which has been fertilised in the Fallopian tube usually arrives in the uterus in the condition of the trilaminar blastoderm. It is larger than the undeveloped ovum, but still it is extremely small. It arrives in the uterus from which the mucous membrane was removed in the preceding menstrual flow, and the new mucous lining (decidua) which is in progress of growth is thicker, more pulpy, and has longer glands than it would have had if fertilisation had not occurred. In this the ovum is speedily imbedded, usually near the fundus of the uterus ; the mucous membrane CH. LVII.] THE FOETAL MEMBRANES. 783 grows over the little ovum and encloses it, and so the decidua reflexa is formed ; the decidua serotina is that part of the decidua vera which intervenes between the ovum and the uterine wall within the circle of attachment of the decidua reflexa. With the subsequent growth of the ovum, the decidua reflexa expands also, encroaching more and more on the uterine cavity, and ultimately coming into contact with the decidua vera, with which it blends, The glands of the decidua were at one time supposed to receive the villous outgrowths of the chorion. It has since been shown that these grow into the substance between the glands. The glands, however, furnish a secretion called uterine milk, which assists the nourishment of the embryo previous to the establish- ment of the placental circulation. Later on the glands are obliterated. The decidua serotina is the part which undergoes most change ; an irregular spongy tissue is formed in this situation; and the spaces in the spongework are filled with blood ; the spongework of vascular spaces is incompletely divided into what are called cotyledons by fibrous bands ; and each cotyledon receives a much hypertrophied chorionic villus. It is this conjunction of chorionic villi with decidual tissue that makes up the placenta, which at full term is seven or eight inches across and weighs nearly a pound. The placenta is the situation where the foetus receives its nutriment and its oxygen. There is no direct communication between the vascular systems of the mother and foetus. The sinuses of the placenta are filled with maternal blood from the uterine arteries; the uterine veins carry it away; but in these blood-spaces the tufts of foetal blood-vessels are hanging. Oxygen and nutriment pass through the walls of the foetal blood-vessels from the maternal to the foetal blood, and carbonic acid, urea and other waste products pass in the contrary direction. The foetal blood leaves the foetus by the two umbilical arteries, which are the terminal branches of the foetal aorta; these pass in the stalk of the allantois to the placenta, and after undergoing oxygenation in the placental tufts, it returns by the umbilical vein to the foetus once more. Development of the Foetal Membranes. The Yolk-sac.-We have already considered the way in which the body of the embryo is pinched off from the yolk-sac. 784 DEVELOPMENT. [ch. lvii. Numerous blood-vessels are developed in its wall, and the nutriment thus absorbed from it passes to the foetal heart by two veins called the omphalo-mesenteric veins. The blood-vessels are first formed at the circumference of a clear area surrounding the embryo; and the place where they are situated is called the vascular area. This is shown about the natural size in the hen's egg in fig. 612. In birds the yolk-sac affords nutriment till the end of incuba- tion, and the omphalo-mesenteric vessels are developed to a corre- sponding degree; but in mammalia the office of the umbilical vesicle Fig. 612.-Diagram showing vas- cular area in the chick, a, area pellucida ; b, area vasculosa; c, area vitellina. Fig. 613. Fig. 614. Kg. 615. Figs. 613 614, and 615.-Diagrams showing three successive stages of development. Trans- verse vertical sections. The yolk-sac, ys, is seen progressively diminishing in size. In the embryo itself the medullary canal and notochord are seen in section, a', in middle figure, the alimentary canal, becoming pinched off from the yolk-sac; a', in lower figure, alimentary canal completely closed ; a, in last two figures, amnion ; ac, cavity of amnion filled with amniotic fluid ; pp, space between amnion and chorion continuous with the pleuro-peritoneal cavity inside the body; vt, vitelline membrane, or zona pellucida ; ys, yolk-sac, or umbilical vesicle. (Foster and Balfour.) CH. LVII.J THE FCETAL MEMBRANES. 785 ceases at a very early period, as the quantity of the yolk is small, and the embryo soon becomes independent of it by the connections it forms with the parent. Moreover, in birds, as the sac is emptied, it is gradually drawn into the abdomen through the umbilical opening, which then closes over it : but in mammalia it always remains on the outside ; and as it is emptied it contracts (fig. 615), shrivels up, and together with the part of its duct external to the abdomen, is detached and disappears, either before or at the termination of intra-uterine life, the period of its disappearance varying in different orders of mammalia. When blood-vessels begin to be developed, they ramify largely over the walls of the umbilical vesicle, and are actively concerned in absorbing its contents and conveying them away for the nutrition of the embryo. At an early stage of development of the foetus, and some time before the completion of the changes which have been just described, two important structures, called respectively the amnion and the allantois, begin to be formed. Amnion.-The amnion is produced as follows :-Beyond the head-and tail-folds before described (p. 776), the somatopleur coated by epiblast, is raised into folds, which grow up, arching over the embryo, not only anteriorly and posteriorly but also laterally, and all converging towards one point over its dorsal surface (fig. 617). The folds not only come into contact but coalesce. The inner of the two layers forms the true amnion ; it is composed externally of mesoblast and is lined by epiblast; the outer layer is termed the false amnion ; it is composed externally of epiblast and is lined by mesoblast. It. coalesces with the inner surface of the remains of the original vitelline membrane or zona pellucida. The cavity between the true amnion and the external surface of the embryo becomes a closed space, termed the amniotic cavity (ac, fig. 615). At first, the amnion closely invests the embryo, but it becomes gradually distended with fluid (liquor amnii), which, as pregnancy advances, reaches a considerable quantity. This fluid consists of water containing small quantities of albumin, urea, and salts. Its chief function during gestation appears to be the mechanical one of affording equal support to the embryo on all sides, and of protecting it as far as possible from the effects of blows and other injuries to the abdomen of the mother. It is an exudation from both foetal and maternal blood; the urea in it comes from the foetal urine, which is passed into it in the later months of pregnancy. 786 DEVELOPMENT. [ch. lvii. On referring to figs. 613, 614 and 615, it will be obvious that the cavity outside the amnion, between it and the false amnion, is continuous with the pleuro-peritoneal cavity at the umbilicus. This cavity is not entirely obliterated even at birth, and contains a small quantity of fluid, which is discharged during parturition either before, or at the same time as the amnio- tic fluid. Allantois. - Into this space the allantois sprouts out, its forma- tion commencing during the development of the amnion. Growing out from the hinder portion of the in- testinal canal (c, fig. 617), with which it communi- cates, the allantois is at first a solid pear-shaped mass of splanch- nopleur; it becomes vesicular by the projection into it of a hollow outgrowth of hypoblast. The hypoblast, however, does not extend very far. The mesoblastic part of the allantois very soon becomes vascular, and insinuates itself between the amniotic folds, just de- scribed. It unites with the outer of the two folds (false amnion), which has itself, as before said, become one with the re- mains of external investing membrane of the egg. As it grows, the allantois be- comes exceedingly vascular; in birds it envelopes the whole embryo-taking up vessels to the outer investing membrane of the egg, and lining the inner surface of the shell with a vascular membrane, by these means affording an extensive surface in which the blood may be aerated. In the human subject and in other mam- malia, the vessels carried by the allan- tois are ultimately distributed only to a special part of the false amnion, where, by interlacement with the vascular system of the mother, the placenta is developed. Fig. 616.-Human em- bryo of fifth week with umbilical vesi- cle ; about natural size. (Dalton.) The human umbilical ve- sicle never exceeds the size of a small pea. Fig. 617.-Diagram of fecundated egg. a, umbilical vesicle; b, amniotic cavity; c, allantois. (Dalton.] Fig. 618.-Fecundated egg with allantois nearly complete, a, inner layer of amniotic fold; b, outer layer of ditto; c, point where the amniotic folds come in contact. The allantois is seen pene- trating between the outer and inner layers of the amniotic folds. This figure, which represents only the amniotic folds and the parts within them, should be com- pared with figs. 596, 597, in which will be found the structures external to these folds. (Dalton.) CH. LVII.J THE FCETAL MEMBRANES. 787 In mammalia, as the visceral lamina? close in the abdominal cavity, the allantois is thereby divided at the umbilicus into two portions ; the outer part, extending from the umbilicus to the chorion, soon shrivelling ; while the inner part remaining in the abdomen, is in part converted into the urinary bladder ; the portion of the inner part not so converted, extends from the bladder to the Fig. 619. Fig. 620. Figs. 619 and 620.-«, chorion with villi. The villi are shown to be best developed in the part of the chorion to which the allantois is extending; this portion ultimately be- comes the placenta; b, space between the true and false amnion ; c, amniotic cavity ; d, situation of the intestine, showing its connection with the umbilical vesicle; e, um- bilical vesicle ; f, situation of heart and vessels ; g, allantois. umbilicus, under the name of the urachus. After birth the um- bilical cord, and with it the external and shrivelled portion of the allantois, are cast off at the umbilicus, while the urachus remains as an impervious cord stretched from the top of the urinary bladder to the umbilicus, in the middle line of the body, imme- diately beneath the parietal layer of the peritoneum. It must not be supposed that the phenomena which have been successively described, occur in any regular order one after another. On the contrary, the development of one part is going on side by side with that of another. The Chorion.-This is formed by the fusion of three membranes, namely, the original vitelline membrane, the outer layer of the amniotic fold (false amnion), and the allantois which supplies it with blood-vessels. Very soon after its formation, its outer surface is beset with fine processes, chorionic villi (a, figs. 619, 620), which give it a rough and shaggy appearance. At first only cellular in structure, these little outgrowths subsequently become vascular by the development in them of loops of capillaries (fig. 621); and the 788 DEVELOPMENT. [ch. lvii. latter at length form the minute extremities of the blood-vessels which are conducted from the foetus to the chorion by the allantois. The function of the villi of the chorion is evidently the absorption of nutrient matter for the foetus ; and this is probably supplied to them at first from the fluid matter, secreted by the glands of the uterus in which they are soaked. Soon, however, the foetal vessels of the villi come into more intimate relation with the vessels of the uterus. The part at which this relation between the vessels of the foetus and those of the parent ensues, is not, however, over the whole surface of the chorion; for, although all the villi become vascular, yet they become indistinct or disappear except atone part where they are greatly developed, and by their branching give rise, with the vessels of the uterus, to the formation of the placenta. The structure and functions of the placenta, however, we have already described in connection with the decidua. The umbilical cord is composed of the following parts:-(i.) Ex- ternally, a layer of the amnion, re- flected over it from the umbilicus. (2.) The umbilical vesicle or yolk-sac with its duct and ap- pertaining omphalo-mesenteric blood-vessels. (3.) The remains of the allantois, and continuous with it the urachus. (4.) The umbilical vessels, two arteries and one vein, which ultimately form the greater part of the cord. These are embedded in a jelly- like connective tissue called the Whartonian jelly. The After-birth.-In parturition, the pressure of the uterine and abdominal walls upon the uterine contents, and especially on the amniotic fluids, causes a bulging of the membranes (combined deciduae, chorion and amnion) through the os uteri. When the membranes are ruptured the fluid escapes, and then the foetus is expelled. Later contractions of the uterus detach the placenta from the uterine wall, and this is in turn expelled ; the separation extends around the decidua lining the rest of the uterus, and, turned inside out, this follows the placenta, carrying with it the other membranes. This constitutes the after-birth. The sever- ance of the umbilical cord should not be done until some minutes after the birth of the child, or it is deprived of a good deal of the blood which is subsequently squeezed out of the placenta into it. Fig. 621. CH. LVII.J THE protovertebra:. 789 Development of the Framework of the Body. The notochord, is a primitive vertebral column which, unlike the true vertebral column that replaces it, is a single rod. In ampbioxus and the lampreys, however, it remains in the adult as a permanent skeletal support. In structure it closely resembles cellular cartilage (p. 57), and is enclosed in a sheath. It is composed of a very insoluble proteid-like matter, which is, however, not collagen. Collagen, and gelatin which is formed from it by boiling, are characteristic of true connective tissues; these are formed from mesoblast; the notochord is hypoblastic. It contains also, like all embryonic tissues, a large quantity of glycogen. Its place is ultimately occupied by the vertebral bodies, but traces of it are found even in the adult in the centre of the intervertebral discs. The protovertebrse or protovertebral somites form the vertebra) and other structures as well. Each divides vertically into two parts, an inner and an outer. It is the inner division that forms the vertebra; the outer division is called the musculo- cutaneous plate, and it is continued into the general mesoblast which divides into the splanchnopleur and somatopleur with the pleuro-peritoneal cavity between them. The inner portion of each pair of protovertebra grows around the notochord and in time almost obliterates it; this forms the body of the vertebra ; it also grows around the primitive spinal cord, and so forms the neural arch of the vertebra. This part of the protovertebra is more distinctly separated from the other segments of the protovertebral column from the first, and so allows the spinal nerves which are sprouting out from the spinal cord to leave the spinal cord for the body walls. At first, all these parts are composed of protoplasmic embryonic cells, but as development progresses the cells become specialised in function and structure, some becoming cartilage cells, others muscular fibres, &c. At a later date still the cartilaginous vertebra are replaced by bone. The vertebra do not exactly correspond in their position to the protovertebra ; each vertebra is developed from the contiguous halves of two protovertebra. The original segmentation of the protovertebra disappears, and a fresh subdivision occurs in such a way that the intervertebral disc is developed opposite the centre of each protovertebra. In the musculo-cutaneous plate are developed the muscles and true skin of the body wall, and the ribs. 790 DEVELOPMENT. [ch. lvii. While these changes have been going on, the ventral walls of the embryo have been formed by the downgrowth of the cephalic fold in the head region, the caudal fold at the tail end of the animal, and the two lateral or umbilical folds which grow last and enclose the thoracic and abdominal organs. The embryo also undergoes certain changes in form and attitude; in the first place torsion takes place; this is more marked in birds and reptiles than in mammals ; by this term one means that the embryo no longer lies ventral surface downwards facing the yolk-sac, but turns slightly over so that the left side is lowermost; in birds the embryo may turn through a quarter of a Fig. 622.-A human embryo of the fourth week, 3J lines in length.-1, the chorion; 3, part of the amnion ; 4, umbilical vesicle with its long pedicle passing into the abdomen; 7, the heart; 8, the liver; 9, the visceral arch destined to form the lower jaw, beneath which are two other visceral arches separated by the branchial clefts ; 10, rudiment of the upper extremity; n, that of the lower extremity; 12, the umbilical cord; 15, the eye ; r6, the ear ; 17, cerebral hemispheres ; r8, optic lobes, corpora quadrigemina. (Muller.) circle. Then the vertebral column becomes curved, but the chief bend is known as the principal cephalic flexure. This occurs at the anterior end of the notochord ; it is a strong angular flexion in the region of the mid-brain, which is subsequently the position of the sella turcica. In connection with this must be mentioned the development of the pituitary body which occupies the sella turcica in the adult. Lt is formed by the meeting of two out-growths, one from the foetal brain, which grows downward, and the other from the epiblast of the buccal cavity, which grows up towards it. The surrounding mesoblast also takes part in its formation. The connection of the first process with the brain becomes narrowed, and persists as the infundibulum, while that of the other process with the buccal cavity disappears completely. CH. LVII.] FORMATION OF LIMBS AND HEAD. 791 The Limbs.-The muscles of the body developed from the lateral extensions of the protovertebrse are, at first, like the vertebra?, arranged in successive segments or myotomes. This is very well seen in the ringed condition of the muscles in such simple vertebrates as amphioxus. Even in fishes, where the limbs are not in a high state of development, the muscular segments are well seen. They are seen also in man in the intercostal muscles, and in the abdominal region are indicated by the transverse septa across the rectus abdominis, but here, as in other mammals, this simple metameric segmentation is masked by the great develop- ment of the large muscles which attach the limbs to the four corners of the trunk. The limbs are lateral extensions of segments or somites in certain situations. They consist of parietal mesoblast covered by epiblast. At first there is simply a bud, but this grows, and in time divides into three segments, arm, fore-arm, and hand in the upper limb ; thigh, leg, and foot in the lower limb. The hand and foot then give rise to buds corresponding to the digits. Each limb is connected to a limb girdle. The epiblast here, as else- where, forms the epidermis; the true skin, subcutaneous tissues, muscles, blood-vessels, and cartilages (subsequently replaced by bone) are formed by differentiation from the mesoblast. In further development the positions of the limbs become shifted by rotation, so that the anterior (radial) border of the upper limb becomes outermost, and the anterior (tibial) border of the lower limb becomes internal. Formation of the Head. In the formation of the head, a number of elements are con- cerned. There is first the notochord, which extends as far forwards as the dorsum sella? ; this, however, as in the vertebral column, is transitory, and as soon replaced by a primitive cartila- ginous cranium developed from the mesoblast around it, as the vertebrae are developed from the protovertebrae. This forms the base of the skull. The roof or cranial vault is formed by mem- brane bones, that is, bones not preceded by cartilage ; sense capsules which form around prolongations of the brain, and the visceral arches and slits contribute to the formation of that part of the head which is called the face. The mesoblast, which continues up the protovertebrae into the head region on each side of the notochord, is not separated into parts corresponding to vertebrae. Cartilage is formed in it; two cartilaginous bars, one on each side of the notochord, are called the parachordal cartilages, 792 DEVELOPMENT. [ch. lvii. and two other bars embracing the pituitary body situated in front of these are called the trabeculae cranii (fig. 623, a). These unite in front, and with the parachordal cartilages behind to form a continuous mass (basilar cartilage), which completely invests the notochord posteriorly (fig. 623, b). The parachordal part of this represents the basi-occipital and basi-sphenoid; the prechordal part represents the pre-sphenoid and ethmoid portions ; posteriorly and at the sides, cartilaginous plates grow over the cerebral vesicles, but this only occurs to a small extent in mammals. In these animals the occipital region alone is roofed in by cartilage; the rest of the cranial vault being formed of membrane bones. Anteriorly the united trabecuke form the ethnoid cartilage and the nasal septum, and enclose the nasal pits externally. From the sides of the pre-sphenoid, the lesser wings or orbito- Fig'. 623.-Diagrams of the cartilaginous cranium. A, first stage. Ch, notochord; Tr, trabeculae cranii; P.ch., parachordal cartilages; P, situation of pituitary body; N, E, O, situations of olfactory, visual, and auditory organs. B, later stage. Il, basilar cartilages; S, nasal septum and ethmoidal cartilages ; Eth, ECi, prolongations of ethmoid around olfactory organ, completing the nasal capsule ; A, E, O, Ch, Tr, P, as before. (After Wiedersheim.) sphenoids containing the optic foramina arc developed, and from 1 he sides of the basi-sphenoid the greater wings or alisphenoids A cartilaginous capsule invests the auditory vesicle and becomes connected to the parachordal cartilage on each side. It is called the periotic capsule; within this bony centres are formed, and the bone constitutes the petrous and mastoid portions of the tem- poral bone. The Visceral Clefts and Arches.-In all vertebrata there is at one period of development a series of slits in the neck region; these are formed as inpushings from the exterior, and open into the anterior end of the alimentary canal. These are six in number, but in man the two hinder ones rapidly disappear; the first enlarges and forms the mouth ; and at the sides of this the CH. LVII.] VISCERAL CLEFTS AND ARCHES. 793 eyes are formed by depressions of the skin which meet the optic vesicles, outgrowths from the brain. The nasal pits take origin as two simple depressions, the primary olfactory or nasal pits; these become connected to the first visceral slit or cleft. The second slit, which corresponds to the spiracle of fishes, becomes the external auditory meatus and the Eustachian tube. The remainder, which correspond to the gill slits of fishes, entirely close up in mammals, and no gills are developed on their margins. The anterior border of each cleft forms a fold or lip, the branchial or visceral fold. The posterior border of the last cleft is also formed into a fold, so that there are four clefts and five folds, but the three most anterior are far more prominent than the others, and of these the second is the most conspicuous. The first fold nearly meets its fellow in the middle line, the second less nearly, and the others in order still less so. The first arch Fig. 624.-a. Magnified view from before of tiie Lead and neck of a human embryo of about three weeks (from Ecker). - 1, anterior cerebral vesicle or cerebrum; 2, middle ditto ; 3, middle or fronto-nasal process; 4, superior maxillary process; 5, eye ; 6, inferior maxillary process, or first visceral arch, and below it the first cleft; 7, 8, 9, second, third, and fourth arches and clefts, b. Anterior view of the head of a human foetus of about the fifth week (from Ecker, as before, fig. IV.). 1, 2, 3, 5' the same parts as in a ; 4, the external nasal or lateral frontal process; 6, the superior maxillary process; 7, the lower jaw ; x, the tongue ; 8, first postoral cleft becoming the meatus auditorius externus. gives oft' a branch from its front edge, which passes forwards to meet its fellow, but these offshoots do not quite meet, being- separated by a process which grows downwards from the head. Between the branches, or maxillary processes, and the main first fold is the cavity of the mouth. The branches represent the superior maxilla, and the main folds the mandible or lower jaw. The central process, which grows down, is the fronto-nasal process. From or in connection with these arches the following parts are developed The first arch (mandibular) contains a cartilaginous rod 794 DEVELOPMENT. [ch. LVII. (Meckel's cartilage), around the distal end of which the lower jaw is developed, while the malleus is ossified from the proximal end. When the maxillary processes on the two sides fail partially or completely to unite in the middle line, the well-known condition termed cleft-palate results. When the integument of the face presents a similar deficiency, we have the deformity known as Fig. 625.-Embryo chick (4th day), viewed as a transparent object, lying- on its left side (magnified). C H, cerebral hemispheres ; F B, fore-brain or vesicle of third ventricle, with Pn, pineal gland projecting from its summit; M B, mid-brain ; C b, cerebellum ; IV, V, fourth ventricle; L, lens ; c h s, choroidal slit; Gen. V, auditory vesicle; s m, superior maxillary process; iF, zF, &c., first, second, third, and fourth visceral folds ; V, fifth nerve, sending one branch (ophthalmic) to the eye, and another to the first visceral arch; FZZ, seventh nerve, passing to the second visceral arch ; G. Ph, glosso- pharyngeal nerve, passing to the third visceral arch ; P g, pneumogastric nerve, pass- ing towards the fourth visceral arch ; i v, investing mass ; c h, notochord; its front end cannot be seen in the living embryo, and it does not end as shown in the figure but takes a sudden bend downwards, and then terminates in a point; lit, heart seen through the walls of the chest; MP', muscle-plates ; TV, wing, showing commen pin p- differentiation of segments, corresponding to arm, forearm, and hand ; 8 S, somatic stalk ; Al, allantois; II L, hind-limb, as yet a shapeless bud, showing no differentia- tion. Beneath it is seen the curved tail. (Foster and Balfour.) hare-lip. Though these two deformities frequently co-exist, they arc by no means always necessarily associated. The upper part of the face in the middle line is developed from the frontal-nasal process (a, 3, fig. 624). From the second arch are developed the incus, stapes,* and stapedius muscle, the styloid process of the temporal bone, the stylo-hyoid, ligament, and the smaller cornu of the hyoid bone. * The origin of the ear ossicles given in the text is only one of five or six different views that have been advanced by different observers. CH. LVII.] DEVELOPMENT OF THE HEART. 795 From the third visceral arch, the greater cornu and l)ody of the hyoid bone arise. In man and other mammalia the other arches disappear. They occupy the position where the neck is afterwards developed. A distinct connection is traceable between these visceral arches and certain cranial nerves : the trigeminal, the facial, the glosso- pharyngeal, and the vagus. The ophthalmic division of the trigeminal supplies the fronto-nasal process; the superior and inferior maxillary divisions supply the maxillary and mandibular arches respectively. The facial nerve distributes one branch (chorda tympani) to the first visceral arch, and others to the second visceral arch. Thus it divides, enclosing the cleft next behind the mouth. Similarly, the glosso-pharyngeal divides to enclose the third visceral cleft, its lingual branch being distributed to the second, and its pharyngeal branch to the third arch. The vagus, too, sends a branch (pharyngeal) along the next arch, and in fishes gives off paired branches, which divide to enclose the remaining branchial clefts. Development of the Vascular System. We have already mentioned that at an early stage in develop- ment the area vasculosa makes its appearance in the part of the yolk-sac which is separated from the body of the embryo by a clear space (see fig. 612). This is produced by mesoblastic cells becoming hollow, filled with blood, and uniting to form embryonic capillaries (see p. 584). These vessels converge to two trunks, one on each side, which are called the omphalo-mesenteric or vitelline veins, and these lead to the embryonic heart. The heart is developed in the splanchnopleur, by a folding off of the pleuroperitoneal cavity. It appears beneath the posterior end of the fore-gut. Its first appearance, however, is as two tubes, one on each side of the fore-gut. This is shown in outline in the next diagram (fig. 626). It will be seen that the medullary groove is enlarged anteriorly, and the primary optic vesicles are growing from the first cerebral enlargement. Eight pairs of pro to vertebrae are seen; and on either side of the head the primitive tubular heart (H) is seen. If we look at a rather later stage, shown in transverse section in the next figure (fig. 627), we see the two tubular hearts cut across, and approaching one another beneath the alimentary canal, which is being cut off from the yolk-sac. 796 DEVELOPMENT. [ch. lvii. Fig. 628 shows how the two primitive tubes have coalesced to form one (H) beneath the anterioi1 end of the alimentary canal, which is in this region quite cut off from the yolk-sac. On each Fig. 626. Babbit embryo of the ninth day, seen from the surface. (After Kolliker.) Fig. 627.-Diagrammatic section of embryo. F, epiblast; So, somatopleur; Sp, splanch- nopleur; P.P.,thepleuro-peritoneal cavi- ty between them ; If, heart ; .V. C., neural cord; N, notochord; Pi-., protovertebra ; A.C., alimentary canal; K.S., yolk-sac. side of the .notochord are seen two smaller tubes (A A) in section ; these are the transverse sections of the primitive aortse. Fig. 628.-Diagrammatic transverse section of embryo. N, notochord ; A.C.,. alimentary canal; H, heart; Y.S., yolk-sac ; P.P., pleuro-peritoneal cavity. The heart so formed presents the appearance depicted in fig. CH. LVII.J DEVELOPMENT OF THE HEART. 797 629 this is viewed from below, so that the heart receiving the two omphalo-mesenteric veins at its hinder end is seen above the ali- mentary canal, the yolk-sac having- been removed. The heart gives off anteriorly the primitive aorta, which soon divides into two; these pass round the blind anterior end of the ali- mentary canal, and then pass back along its dorsal aspect on each side of the notochord, as seen in trans- verse section in fig. 628. The heart is at first a simple tube, but soon is divided into a longitudinal series of chambers which contract in the order named from before backwards: (1) the sinus venosus, where the veins enter; (2) the auricle, which in Fig. 629.-Embryo chick (36 hours), viewed from beneath as a trans- parent object (magnified). Fig. 630.-Heart of the chick at the 45th, 65th, and 85th hours of incubation, i, the venous trunks; 2, the auricle; 3, the ventricle; 4, the bulbus arteriosus. (Allen Thomson.) mammals fuses with the sinus to form a single chamber; in fishes and amphibians sinus and auricle are distinct; then comes (3) the ventricle, and (4) the commencement of the aorta, which is called the aortic bulb. Later on the heart is twisted upon itself in the way represented in fig. 630, so that the auricle gets on the top of the ventricle, and the latter cavity increases in relative strength and size. Fig. 631 represents the primitive heart and vessels in outline. The omphalo-mesenteric veins (1,1) enter the auricle (2) ; then come the ventricle (3) and aortic bulb (4); the two primitive aorta) arising from this pass forwards and then turn backwards over the end of the fore-gut, and join together to form the dorsal aorta (6) lower down ; the big branches (7, 7) which it gives off are the two omphalo-mesenteric arteries to the yolk-sac. The smaller allantoic or umbilical arteries (8, 8) pass to the 798 DEVELOPMENT. [ch. lvii. allantois, the circulation in which begins soon, and this replaces the circulation from the yolk-sac, which in mammals is very in- significant. The umbilical arteries are the terminal branches of Fig. 632.-Diagram of young embryo and its vessels, showing course of circu- lation in the umbilical vesicle ; and also that of the allantois (near the caudal extremity), which is just com- mencing. (Dalton.) Fig. 631.-Early stage of em- bryonic heart and blood- vessels. the foetal aorta; the common iliac arteries to the lower limbs arise later, when the limbs begin to form. Fig. 633.-Diagram of embryo and its vessels at a later stage, showing the second cir- culation. The pharynx, oesophagus, and intestinal canal have become further developed, and the mesenteric arteries have enlarged, while the umbilical vesicle and its vascular branches are very much reduced in size. The large umbilical arteries are seen passing out in the placenta. (Dalton.) CH. LVilC! THE FCETAL CIRCULATION. 799 The replacement of the circulation in the yolk-sac, or umbilical vesicle, by the allantoic or placental circulation is illustrated by the next two diagrams (figs. 632, 633). As the body develops new arteries and new veins form, and the heart becomes more complicated. The ventricle is divided into two (right and left) by a septum ; the bulb divides into two, and one division leads from the right ventricle to the pulmonary artery; the other leads as the main aorta from the left. The auricles are also divided into two (right and left), but the com- plete separation of the two auricles does not take place till after birth. Before birth, as we shall see when describing the later foetal circulation, some of the blood which enters the right auricle passes into the left auricle by a wide opening called the foramen o vale. The pulmonary artery leads direct into the aorta; the branches to the lungs are small and unimportant : it is not till the child is born, and begins to use its lungs, that the arteries to them assume importance ; then also the communication with the aorta is closed, and remains as a cord, called the ductus arteriosus. These changes will be grasped better if we look at the two next diagrams (figs. 634, 635). The heart is in a rather more advanced stage than in fig. 631 ; it is beginning to get a twist which is bringing the ventricle, now increasing in size, into its subsequent position ; but it is seen that instead of two simple arches uniting to form a dorsal aorta, there are now five. These correspond to the gill arteries of fishes, but in mammals never break up into capillaries, as in a fish's gills. They, however, run in the visceral arches, between the visceral clefts. In amphibia, three pairs persist through life. In reptiles the fourth pair remains throughout life as the per- manent right and left aorta ; in birds the right one remains as the permanent aorta, curving over the right bronchus instead of the left as in mammals. In mammals the left fourth aortic arch develops into the per- manent aorta, the right one remaining as the subclavian artery of that side. Thus the subclavian artery on the right side cor- responds to the aortic arch on the left, and this homology is further confirmed by the fact that the recurrent laryngeal nerve hooks under the subclavian on the right side, and the aortic arch on the left. The fifth arch disappears on the right side, but on the left forms the pulmonary artery. The distal end of this arch originally opens into the descending aorta, and this communication (which is permanent throughout life in many reptiles on both sides of the 800 DEVELOPMENT. [CH. LVII. body) remains throughout foetal life under the name of the ductus arteriosus : the branches of the pulmonary artery, to the right and left lung, are very small, and most of the blood which is forced into the pulmonary artery passes through the wide ductus arteriosus into the descending aorta. The first and second arches soon dis- appear, but the third arches and portions of the aortic roots remain as the carotid arteries (see fig. 635). As the umbilical vesicle dwindles in size, the portion of the omphalo-mesenteric arteries outside the body gra- dually disappears, the part inside the body remaining as the mesenteric arteries. Meanwhile with the growth Fig. 635.-Diagram of the aortic arches in a mammal, showing transformations which give rise to the permanent arterial vessels. .4, primitive arterial stem or aortic bulb, now divided into A, the ascending part of the aortic arch, and p, the pulmonary; a a', right and left aortic roots; A', de- scending aorta; i, 2, 3, 4, 5, the five primitive aortic or branchial arches; Z, ZZ, ZZZ, IV, the four branchial clefts which, for the sake of clearness, have been omitted on the right side. The per- manent systemic vessels are deeply, the pulmonary arteries lightly, shaded ; the parts of the primitive arches which are transitory are simply outlined ; c, placed between the permanent common carotid arteries ; c e, external carotid arteries ; <; i, internal carotid arteries; s, right subclavian, rising from the right aortic root beyond the fifth arch ; v, right verte- bral from the same, opposite the fourth arch; i>' s', left vertebral and. subclavian arteries rising together from the left, or permanent aortic root, opposite the fourth arch ; p, pulmonary arteries rising toge- ther from the left fifth arch ; d, outer or back part of left fifth arch, forming ductus arteriosus ; p n, p n', right and left pneumogastric nerves descending in front of aortic arch, with their recurrent branches represented diagrammatical]y as passing behind, to illustrate the rela- tions of these nerves respectively to th j right subclavian artery (4) and the arch of the aorta and ductus arteriosus (Z). (Allen Thomson, after Rathke.) Fig. 634.-Embryonic heart and vessels. O, veins; Au, auricle; V, ventricle ; B, bulb ; 1, 2, 3, 4, 5, primitive aortic arches. of the allantois two new arteries (wmfo'ZzcaZ) appear, and rapidly increase in size till they are the largest branches of the aorta; CH. LVII.] THE F(ETAL CIRCULATION. 801 they are for a long time considerably larger than the external iliacs which supply the comparatively small hind-limbs. Veins.-The earliest veins to appear in the foetus are the omphalo- mesenteric or vitelline, which return the blood from the yolk-sac to the developing auricle. As soon as the placenta with its umbilical veins is developed, these unite with the omphalo-mesenteric, and thus the blood which reaches the auricle comes partly from the yolk-sac and partly from the placenta. The right omphalo- mesenteric and the right umbilical veins soon disappear, and the united left omphalo-mesenteric and umbilical veins pass through the developing liver on the way to the auricle. Two sets of Fig. 636.-Diagrams illustrating the development of veins about the liver. B, d, c, ducts of Cuvier, right and left; c a, right and left cardinal veins ; o, left omphalo-mesenteric vein ; o', right omphalo-mesenteric vein, shrivelled up; u u', umbilical veins, of which the right one, has disappeared. Between the venae cardinale is seen the outline of the rudimentary liver with its venae hepaticae advehentes, and revehentes. D, later stage ; I, ductus venosus ; I', hepatic veins; c i, vena cava inferior ; P, portal vein; P' P', venae advehentes ; m, mesenteric veins. (Kolliker.) vessels make their appearance in connection with the liver (ve/ice advehentes, and revehentes}, both opening into the united omphalo- mesenteric and umbilical veins, in such a way that a portion of the venous blood traversing the latter is diverted into the developing liver, and, having passed through its capillaries, returns to the um- bilical vein through the venae revehentes at a point nearer the heart (see fig. 636). The portion of vein between the afferent and efferent veins of the liver is called the ductus venosus. The venae advehentes become the right and left branches of the portal vein, the venae revehentes become the hepatic veins, which open just at the junction of the ductus venosus with another large vein (vena cava inferior), which is now being developed. The mesenteric portion of the omphalo-mesenteric vein returning blood from the developing intestines remains as the mesenteric vein, which by its union with the splenic vein, forms the portal. Thus the foetal liver is supplied with venous blood from two 802 DEVELOPMENT. Lch. lvii sources, through the umbilical and portal vein respectively. At birth the circulation through the umbilical vein of course com- pletely ceases and the vessel begins at once to dwindle, so that now the only venous supply of the liver is through the portal vein. Another system of veins which makes their appearance early consist of two short transverse veins (ducts of Cuvier) which open into the right auricle on either side; each is formed by the union of an anterior cardinal, afterwards forming a jugular, vein, collect- ing blood from the head and neck, and a posterior cardinal vein which returns the blood from the Wolffian bodies, the vertebral column, and the parieties of the trunk. This arrangement persists Fig. 637.-Diagrams illustrating the development of the great veins, d c, ducts of Cuvier ; 7, jugular veins; h, hepatic veins; c, cardinal veins ; s, subclavian vein; j i, internal jugular vein ; j e, external jugular vein ; a z, azygos vein; c i, inferior vena cava ; r, renal veins ; i I, iliac veins ; h ij, hypogastric veins. (Gegenbaur.) throughout life in fishes, but in mammals the following trans- formations occur. As the kidneys are developing a new vein appears (vena cava inferior), formed by the junction of their efferent veins. It receives branches from the legs (iliac) and increases rapidly in size as they grow : further up it receives the hepatic veins, which by now have lost their original opening into the ductus venosus. The heart gradually descends into the thorax, causing the ducts of Cuvier to become oblique instead of transverse. As the fore- limbs develop, the subclavian veins are formed. CH. LVII.j THE FCETAL CIRCULATION. 803 A transverse communicating trunk now unites the two primi- tive jugular veins, and gradually increases, while the left duct of Cuvier becomes almost entirely obliterated (all its blood passing by the communicating trunk to the right side) (fig. 637, c. d.). The right primitive jugular vein remains as the right innominate vein, while the communicating branch forms the left innominate. The right duct of Cuvier becomes the superior vena cava. The remnant of the left duct of Cuvier generally remains as a fibrous Hepatic Veins Left Common Iliac Ext. Iliac Sciatic- Fig. 638. band, running obliquely down to the coronary vein, which is really the proximal part of the left duct of Cuvier. In front of the root of the left lung, another relic may be found in the form of the so-called vestigial fold of Marshall, which is a fold of peri- cardium running in the same direction. In some of the lower mammals, such as the rat, the left ductus Cuvieri remains as a left superior cava. Meanwhile, a transverse branch carries across most of the blood of the left posterior cardinal vein into the right; and by this union the great azygos vein is formed. 804 DEVELOPMENT. [_CH. LVII- The upper portions of the left posterior cardinal vein remain as the left superior intercostal and vena azygos minor. The azygos veins receive the intercostal veins as shown in the diagrams. These views of the origins of the veins are chiefly derived from the observations of Rathke and Gegenbaur. They have been generally accepted by embryologists. Hochstetter, however, has more recently stated that some modification of these views is necessary. According to him, the right common iliac vein and the portion of the inferior vena cava below the renal vein are parts of the right cardinal vein, and the greater part of rhe left common iliac is the transverse iliac vein, a vein which grows across at a lower level than the transverse azygos. According to these views, the lower part of diagram 637D would have to be altered as in fig. 638. Circulation of Blood in the Fcetus. The circulation of blood in the foetus differs considerably from that of the adult. It will be well, perhaps, to begin its descrip- tion by tracing the course of the blood, which, after being carried out to the placenta by the two umbilical arteries, has returned, cleansed and replenished, to the foetus by the umbilical vein. It is at first conveyed to the under surface of the liver, and there the stream is divided,-a part of the blood passing straight on to the inferior vena cava, through a venous canal called the ductus venosus, while the remainder passes into the portal vein, and reaches the inferior vena cava only after circulating through the liver. Whether, however, by the direct route through the ductus venosus or by the roundabout way through the liver,-all the blood which is returned from the placenta by the umbilical vein reaches the inferior vena cava at last, and is carried by it to the right auricle of the heart, into which cavity is also pouring the blood that has circulated in the head and neck and arms, and has been brought to the auricle by the superior vena cava. It might be naturally expected that the two streams of blood would be mingled in the right auricle, but such is not the case, or only to a slight extent. The blood from the superioi' vena cava,-the less pure fluid of the two-passes almost exclusively into the right ventricle, through the auriculo-ventricular opening, just as it does in the adult; while the blood of the inferior vena cava is directed by the fold of the living membrane of the heart, called the Eusta- chian valve, through the foramen ovale into the left auricle, whence it passes into the left ventricle, and out of this into the aorta, and thence to all the body, but chiefly to the head and neck. The blood of the superior vena cava, which, as before said, passes into CH. LVII. ] THE FCETAL CIRCULATION. 805 the right ventricle, is sent out thence in small amount through the pulmonary artery to the lungs, and thence to the left auricle, its in the adult. The greater part, however, does not go to the Fig. 639.-Diagram of the Foetal Circulation. lungs, but instead, passes through a canal, the ductus arteriosus, leading from the pulmonary artery into the aorta just below the origin of the three great vessels which supply the upper parts of the body ; and there meeting that part of the blood of the inferior vena cava which has not gone into these large vessels, it is dis- 806 DEVELOPMENT. I CH. LVII tributed with it to the trunk and other parts,-a portion passing out by way of the two umbilical arteries to the placenta. From the placenta it is returned by the umbilical vein to the under surface of the liver, from which the description started. Changes after Birth.-Immediately after birth the foramen ovale begins to close, and so do the ductus arteriosus and ductus venosus, as well as the umbilical vessels ; the closure is com- pleted in a few days, so that the two streams of blood which arrive at the right auricle by the superior and inferior vena cava respectively, thenceforth mingle in this cavity of the heart, and passing into the right ventricle, go by way of the pulmonary artery to the lungs, and through these, after purification, to the left auricle and ventricle, to be distributed by the aorta to the body generally. Development of the Nervous System. The central nervous system originates from the thickened walls of the medullary groove, which by the [meeting of the dorsal ridges is converted into the medullary canal. These walls are composed entirely of epiblast. The anterior part of this mass of epiblast becomes the brain, the rest of it the spinal cord ; the canal itself is seen in the adult as the ventricles of the brain and central canal of the spinal cord. The nerves are formed of epiblast too, they are outgrowths from masses of cells called neuroblasts, the primitive nerve-cells. In the case, however, of the olfactory and optic nerves we have not to deal with solid outgrowths, but with hollow' protrusions from the brain, which become solid at a later stage. The Spinal Cord.-The cavity formed by the closure of the neural canal soon becomes a cleft running from before backwards. It is bounded at first by columnar epithelium; these cells afterwards become ciliated; on their exterior is a homogeneous basement membrane. The wall soon becomes thicker, and the basement membrane is thus separated further and further from the central canal. This increase in thickness is due in part to the increase in length of the columnar cells : in part to the appearance of new cells. The inner part of the columnar lining retains its palisade- like character, and forms ultimately the lining epithelium of the central canal. The cells are called spongioblasts. The external ends of the cells break up into a reticulum called the niyelo- spongium, and this is limited by the basement membrane at the circumference. The myelospongium forms the neuroglia. CH. LVII.J DEVELOPMENT OF SPINAL CORD. 807 Between the inner ends of the spongioblasts (fig. 640, S) numerous rounded cells called germinal cells (G) next appear. These rapidly divide, and so form neuroblasts (N). The neuro- blasts are pear-shaped; each has a large oval nucleus, and its tapering stalk is directed towards the outer surface of the cord ; Fig. 640.-Inner ends of spongioblasts (S), with germinal cells (G) between them. N N, neuroblasts which have resulted from the division of a germinal cell; M, myelo- spongium formed by the branching outer ends of the spongioblasts. (After His.) the process ultimately pierces the basement membrane (fig. 641). These are the primitive nerve cells ; their processes are the axis cylinder processes which grow out as nerve fibres. The nerve Fig. 641.-Three neuroblasts, each with a nerve fibre process, growing out beyond the basement membrane of the embryonic spinal cord. (After His.) fibres are first without sheaths; the formation of the sheaths comes later (see p. 105). The neuroblasts collect into groups, one of which, especially large, is at the situation of the future anterior horn, and their processes pass out of the cord as the beginnings of the anterior roots (fig. 642). The somewhat oblique coursing of these fibres before they leave the cord forms the beginning of the anterior white column. The posterior white columns simultaneously begin to appear on each side of the narrow dorsal part of the canal. They are formed by the posterior roots entering the cord. As the cornua of grey matter grow out from the central mass, 808 DEVELOPMENT. [ch. lvii. the fissures of the cord begin to appear. The anterior or ventral fissure is simply a cleft between the enlarging lateral valves of the cord. The posterior fissure is formed by the closure of the dorsal portion of the neural canal which meets an ingrowth of connective tissue from the exterior. The characteristic cylindrical form of the cord is attained by the development of the lateral columns, which are formed by the processes from neuroblasts in the brain growing down the sides of the cord, and these become medullated at a iFig. 642.-Section of spinal cord of a four weeks human embryo. The posterior roots are continued within the cord into a small longitudinal bundle, which is the rudiment of the posterior white column. The anterior roots are formed by the convergence of the processes of the neuroblasts. The latter, along with the elongated cells of the myelo- spongium, compose the grey matter. (His.) later period. The membranes and blood-vessels aye formed from mesoblast. Up to the fourth month the cord and vertebral canal increase in length pari passu, but after that, the vertebral canal grows faster, so that at birth the coccygeal end of the cord is opposite the third lumbar, and in the adult opposite the first lumbar vertebra. This gives an obliquity to the lower nerve roots, which together with they'z'Zwwz terminate form the cauda equina. The Nerves.-These grow from the spinal cord; the origin of the anterior roots we have already considered. The posterior roots are formed in the following way. CH. LVII.j DEVELOPMENT OF THE BRAIN. 809 Along the dorsal aspect of the primitive cord, a crest of epiblast appears and is called the neural crest. Special enlarge- ments of this appear opposite the middle of each pair of proto- vertebrae ; these grow downwards on each side, and their attachment to the cord is then entirely lost. These little islands of epiblast contain numerous neuroblasts; each forms a spinal ganglion, and the neuroblasts within it become the cells of that Fig. 643.-a, Bipolar cell from spinal ganglion of a 4'- weeks embryo (after''His). n nucleus ; the arrows indicate the direction in which the nerve processes grow, one to the spinal cord, the other to the periphery, b, a cell from a spinal ganglion of the adult; the two processes have coalesced to form a T-shaped junction. ganglion. Two processes grow from each cell ; one directed towards the spinal cord, where it contributes to the formation of the posterior white column, and ultimately arborises around the cells of the grey matter at a higher level. The other grows to the periphery. The two processes become blended in the first part of their course and so the T-shaped junction is formed (fig. 643) The Brain.-The histological details of the formation of the epithelium of the ventricles from spongioblasts, of neuroglia from the myelospongium, of nerve cells from neuroblasts, and of the nerve fibres of the white matter and of the nerves as the out- growths from the neuroblasts, are all essentially the same, as already described in connection with the spinal cord. But the grosser anatomical details differ. The front portion of the medullary canal is almost from the first widened out and divided into three vesicles. From the anterior vesicle the two primary optic vesicles are budded off laterally : their further history will be traced in the next section. 810 DEVELOPMENT. [ch. lvii. Somewhat later the same vesicle divides into two, and thus the prosencephalon and thalamencephalon are formed. In the walls of the posterior (third) cerebral vesicle, a thicken- ing appears (rudimentary cerebellum) which becomes separated from the rest of the vesicle by a deep inflection. At this time there are two chief curvatures of the brain (fig 644). (1.) A sharp bend of the whole cerebral mass downwards round the end of the notochord, by which the anterior vesicle, Fig. 644.-Early stages in development of human brain (magnified), i, 2, 3, are from an embryo about seven weeks old ; 4, about three months old. m, middle cerebral vesicle (mesencephalon); c, cerebellum ; m 0, medidla oblongata ; i, thalamencephalon ; A, hemispheres; i', infundibulum; fig. 3 shows the several curves which occur in the course of development; fig. 4 is a lateral view, showing the great enlargement of the cerebral hemispheres which have covered in the thalami, leaving the optic lobes, m, (Kblliker.) N.B.-In fig. 2 the line i terminates in the right hemisphere; it ought to be continued nto the thalamencephalon. which was the highest of the three, is bent downwards, and the middle one conies to occupy the highest position. (2.) A sharp bend, with the convexity forwards, which runs in from behind beneath the rudimentary cerebellum separating it from the medulla. Thus, five fundamental parts of the foetal brain may be distin- guished, which, together with the parts developed from them, may be presented in the following tabular view :- CH. LVII.J DEVELOPMENT OF THE BRAIN. 811 Table of Parts developed from Fundamental Parts of Brain. Anterior end of third ventricle, foramen of Monro, lateral ven- tricles, cerebral hemispheres, corpora striata, corpus callosum, fornix, lateral ventricles, olfac- tory bulb. I. Anterior Primary Vesicle. First Secondary Vesicle, I Prosencephalon, or Fore-brain. Second Secondary Vesi- cle or Thalamen- cephalon, or Twixt-' brain. Thalami optici, pineal gland, part of pituitary body, third ven- tricle, optic nerve and retina, infundibulum. II. Middle Primary Vesicle. Third Secondary Vesicle, Mesencephalon, or Mid-brain. Corpora quadrigemina, crura cerebri, aqueduct of Sylvius. Fourth Secondary Vesi- cle, Epencephalon, or Hind-brain. Fifth Secondary Vesicle, Metencephalon, or After-brain. Cerebellum and Pons. III Posterior 111. rostenoi rimaiy Vesicle. Fourth ven- tricle. Medulla oblon- gata. The cerebral hemispheres grow rapidly upwards and backwards, while from their inferior surface the olfactory bulbs are budded off. The middle cerebral vesicle (mesencephalon) for some time is the most prominent part of the fmtal brain, and in fishes, Fig. 645.-Side view of foetal brain at six months, showing commencement of formation of the principal fissures and convolutions. F, frontal lobe; P, parietal; 0, occipital; T, temporal; a a a, commencing frontal convolutions; a, Sylvian fissure; s', its anterior division ; c, within it the central lobe or island of Keil; r, fissure of Rolando p, perpendicular fissure. (R. Wagner.) amphibia, and reptiles, it remains uncovered through life as the optic lobes. But in birds the growth of the cerebral hemispheres thrusts the optic lobes down laterally, and in mammalia com- pletely overlaps them. In the lower mammalia the backward growth of the hemi- spheres ceases, but in the higher groups, such as the monkeys 812 DEVELOPMENT. [ch. lvii. and man, they grow still further back, until they completely cover in the cerebellum, so that on looking down on the brain from above, the cerebellum is quite concealed from view. The surface of the hemispheres is at first quite smooth, but as early as the third month the great Sylvian fissure begins to be formed (fig- 645)- The next to appear is the parieto-oceipital fissure ; these two great fissures, unlike the rest of the sulci, are formed by a curving round of the whole cerebral mass. In the sixth month the fissure of Rolando appears : from this time till the end of foetal life the brain grows rapidly in size, and the convolutions appear in quick succession ; first the great pri- mary ones are sketched out, then the secondary ones. The Fig. 646.-Longitudinal section of the primary optic vesicle in the chick, magnified (from Remak).-A, from an emhryo of sixty-five hours ; B, a few hours later ; C, of the , fourth day ; c, the corneous layer or epidermis, presenting in A the open depression for the lens, which is closed in B and C ; I, the lens follicle and lens ; pr, the primary optic vesicle ; in A and B, the pedicle is shown; in C, the section being to the side of the pedicle, the latter is not shown; v, the secondary ocular vesicle and [vitreous humour. commissures of the brain (anterior, middle, and posterior), and the corpus callosum, are developed by the growth of fibres across the middle line. The Hippocampus major is formed by the folding in of the grey matter from the exterior into the lateral ventricles. The Eye.-Soon after the first three cerebral vesicles have become distinct from each other, the anterior one sends out a lateral vesicle from each side (primary optic vesicle), which grows out towards the free surface, its cavity communicating with that of the cerebral vesicle through the canal in its pedicle. It remains connected to the thalamencephalon. It is soon met and invaginated by an in-growing process from the epiblast of the surface (fig. 646). This process of the epiblast is at first a depres- sion, which ultimately becomes closed in at the edges so as to produce a hollow ball, which is thus completely severed from the epidermis with which it was originally continuous. From this CH. LVII.j DEVELOPMENT OF THE EYE. 813 hollow ball the crystalline lens is developed. The way in which this occurs has been described in a previous chapter under the head of structure of the lens (see p. 334). By the in-growth of the lens the anterior wall of the primary optic vesicle is forced back nearly into contact with the posterior, and thus the primary optic vesicle is almost obliterated. The cells in the anterior wall are much longer than those of the posterior wall; from the Fig. 648.-Transverse vertical section of the eyeball of a human embryo of four weeks. The anterior half of the section is represented : pr, the remains of the cavity of the primary optic vesicle ; p, the inner part of the outer layer forming the retinal pigment; r, the thickened inner part giving rise to the colum- nar and other structures of the retina ; v, the commencing vitreous humour within the secondary optic vesicle ; v', the ocular cleft through which the loop of the central blood- vessel, a, projects from below ; I, the lens with a central cavity. X 100. (Kolliker.) Fig. 647.-Diagrammatic sketch of a vertical longitudinal section through the eyeball of a human foetus of four weeks. The section is a little to the side, so as to avoid passing through the ocular cleft; c, the cuticle where it becomes later the corneal epithe- lium ; I, the lens ; op, optic nerve formed by the pedicle of the primary optic vesicle ; primary medullary cavity or optic vesi- cle ; p, the pigment layer of the retina; r, the inner wall forming the retina proper ; vs, secondary optic vesicle containing the rudiment of the vitreous humour, x 100. (Kolliker.) former all the layers of the retina are developed, except the layer of pigment cells which is formed from the latter. The cup-shaped hollow in which the lens is now lodged is termed the secondary optic vesicle : its walls grow up all round, leaving, however, a slit where it meets the lens. Through this slit, termed the choroidal fissure, a process of mesoblast containing numerous blood-vessels projects, and occupies the cavity of the secondary optic vesicle behind the lens, filling it with vitreous humour and furnishing the lens capsule and the capsulo-pupillary membrane. This process in mammals projects, not only into the secondary optic vesicle, but also into the pedicle of the primary optic vesicle invaginating it for some distance from beneath, and thus carrying up the arteria centralis retinae into its permanent position in the centre of the optic nerve. 814 DEVELOPMENT. [ch. lvii. This invagination of the optic nerve does not occur in birds, and consequently no arteria centralis retinge exists in them. But they possess an important permanent relic of the original protru- sion of the mesoblast through the choroidal fissure, in the pecten, while a remnant of the same fissure sometimes occurs in man under the name coloboma iridis. The cavity of the primary optic vesicle becomes completely obliterated, and the rods and cones get into apposition with the pigment layer of the retina. The inner segments of the rods are the first formed, then the outer. Fig. 649.-Blood-vessels of the capsulo-pupillary membrane of a new-born kitten, magnified. The drawing is taken from a preparation injected by Tiersch, and shows in the central part the convergence of the net-work of vessels in the pupillary membrane. (Kiilliker.) The cavity of its pedicle disappears and the solid optic nerve is formed. Meanwhile the cavity which existed in the centre of the primitive lens becomes filled up by the growth of fibres from its posterior wall. The epithelium of the cornea is developed from the epiblast, while the corneal tissue proper is derived from the mesoblast which intervenes between the epiblast and the primi- tive lens which was originally continuous with it. The sclerotic coat is developed round the eye-ball from the general mesoblast in which it is embedded. The choroid is developed from the meso- blast on the outside of the optic cup, and the iris by the growing forwards of the anterior edge of the optic cup. The ciliary processes arise from the hypertrophy of the edge of the optic cup which forms folds into which the choroidal mesoblast grows and in which blood-vessels and pigment-cells develop. The iris is formed rather late, as a circular septum projecting inwards, from the fore part of the choroid, between the lens and CH. LVII.J DEVELOPMENT OF SENSE ORGANS. 815 the cornea. In the eye of the foetus of mammalia, the pupil is closed by a delicate membrane, the membrana pupillaris, which forms the front portion of a highly vascular membrane that, in the foetus, surrounds the lens, and is named the membrana capsulo- })up>illaris (fig. 635). It is supplied with blood by a branch of the arteria centralis retinae, which, passing forwards to the back of the lens, there subdivides. It is obliterated in the adult, and is then called the canal of Stilling. The membrana capsulo-pupillaris withers and disappears in the human subject a short time before birth. The eyelids of the human subject and mammiferous animals, like those of birds, are first developed in the form of a ring. They then extend over the globe of the eye until they meet and become firmly agglutinated to each other. But before birth, or in the carnivora after birth, they separate. The Ear.-Very early in the development of the embryo a depression or ingrowth of the epiblast occurs on each side of the head which deepens and soon becomes a closed follicle. This primary otic vesicle, which closely corresponds in its formation to the lens follicle in the eye, sinks down to some distance from the free surface; from it are developed the epithelial lining of the membranous labyrinth of the internal ear, consisting of the vesti- bule and its semicircular canals and the scala media of the cochlea. The surrounding mesoblast gives rise to the various fibrous bony and cartilaginous parts which complete and enclose this mem- branous labyrinth, the bony semicircular canals, the walls of the cochlea with its scala vestibuli and scala tympani. The auditory nerve is gradually differentiated and grows towards the internal ear. The Eustachian tube, the cavity of the tympanum, and the external auditory passage, are the remains of the first post-oral cleft. The membrana tympani divides the cavity of this cleft into an internal space, the tympanum, and the external meatus. The mucous membrane of the mouth, which is prolonged in the form of a diverticulum through the Eustachian tube into the tympanum, and the external cutaneous system come into relation with each other at this point; the'two membranes being separated only by the proper membrane of the tympanum. The pinna or external ear is developed from a process of integument in the neighbourhood of the first and second visceral arches, and probably corresponds to the gill-cover (operculum) in fishes. The Nose.-The nose originates like the eye and ear in a de- pression of the superficial epiblast at each side of the fronto-nasal process (primary olfactory pit), which is at first completely 816 DEVELOPMENT [ch. lvii. separated from the cavity of the mouth, and gradually extends backwards and downwards till it opens into the mouth. The outer angles of the fronto-nasal process, uniting with the maxillary process on each side, convert what was at first a groove into a closed canal. The olfactory nerve which meets this is, like the optic nerve, primarily a hollow process of the brain. Development of the Alimentary Canal. The alimentary canal in the earliest stages of its development consists of three distinct parts-the fore and hind gut ending blindly at each end of the body, and a middle segment which Fig1. 650.-Outlines of the form and position of the alimentary canal in successive stages of its development. A, alimentary canal, &c., in an embryo of four weeks ; B, at six weeks; C, at eight weeks ; D, at ten weeks ; I, the primitive lungs connected with the pharynx; s, the stomach ; d, duodenum ; i, the small intestine ; i', the large ; c, the caecum and vermiform appendage ; r, the rectum ; cl, in A, the cloaca ; a, in B, the anus distinct from si, the sinus uro-genitalis ; v, the yolk-sac ; vi, the vitello-intestinal duct; u, the urinary bladder and urachus leading to the allantois ; g, genital ducts. (Allen Thomson.) communicates freely on its ventral surface with the cavity of the yolk-sac through the vitelline or omphalo-mesenteric duct. From the fore-gut are formed the pharynx, oesophagus, and stomach; from the hind-gut, the lower end of the colon and the rectum. The mouth is developed by an involution of the epiblast between the maxillary and mandibular processes, which becomes deeper and deeper till it reaches the blind end of the fore-gut, and CH. lvii.] DEVELOPMENT OF ALIMENTARY CANAL. 817 at length communicates freely with the pharynx by the absorption of the partition between the two. At the other end of the alimentary canal the anus is formed in a precisely similar way by an involution from the free surface, which at length opens into the hind-gut. When the depression from the free surface does not reach the intestine, the condition known as imperforate anus results. A similar condition may exist at the other end of the alimentary canal from the failure of the involution which forms the mouth, to meet the fore-gut. The middle portion of the digestive canal becomes more and more closed in, till its originally wide communication with the yolk-sac becomes narrowed down to a small duct (vitelline). This duct Fig. 651.-Lobules of the parotid, with the salivary ducts, in the embryo of the sheep, at a more advanced stage. usually completely disappears in the adult, but occasionally7the proximal portion remains as a diverticulum from the intestine. Sometimes a fibrous cord attaching some part of the intestine to the umbilicus, remains to represent the vitelline duct. Such a cord has been known to cause in after-life, strangulation of the bowel and death. The alimentary canal lies in the form of a straight tube close beneath the vertebral column, but it gradually becomes divided into its special parts, stomach, small intestine, and large intestine (fig. 650), and at the same time comes to be suspended in the 818 DEVELOPMENT. [CH. LV1I. abdominal cavity by means of a lengthening mesentery formed from the splanchnopleur which attaches it to the vertebral column. The stomach originally has the same direction as the rest of the canal; its cardiac extremity being superior, its pylorus inferior. The changes of position which the alimentary canal undergoes may be readily gathered from the accompanying figures (fig. 650). Pancreas and Salivary G-lands.-The principal glands in con- nection with the intestinal canal are the salivary glands, pancreas, and the liver. In mammalia, each salivary gland first appears as a simple canal with bud-like processes (fig. 651), lying in a mass of mesoblast, and communicating with the cavity of the mouth. As the development of the gland advances, the canal becomes more and more ramified. The pancreas is developed exactly in the same way, but its cells are derived from the Fig. 652.-Diagram of part of digestive tract of a chick (4th day). The black line represents hypoblast, the outer shading mesoblast. I g, lung diverticulum with expanded end forming primary lung-vesicle ; S t, stomach ; Z, two hepatic diverticula, with their terminations united by solid rows of hypoblast cells ; p, diverticulum of the pancreas with the vesicular diverticula coming from it. (Gotte.) hypoblast lining the intestine, while those of the salivary glands are formed from the epiblast lining the mouth. In both cases the blood-vessels and connective tissues are formed from the mesoblast into which the glandular structure grows. The Liver.-The liver is developed by the protrusion of a part of the walls of the fore-gut, in the form of two conical hollow branches (figs. 652, 653). The inner portion of the cones consists of a number of solid cylindrical masses of cells, derived from the hypoblast, which become gradually hollowed by the formation of the hepatic ducts, and among which blood-vessels are rapidly CH. LVII.J DEVELOPMENT OF THE LUNGS. 819 developed. The secreting cells of the organ and the lining epithelium of the ducts are derived from the hypoblast, the con- nective-tissue and vessels from the mesoblast. The gall-bladder is developed as a diverticulum from the hepatic duct. The spleen and lymg>hatic glands are developed from the meso- blast : the thyroid originates also from the hypoblast; it grows as diverticula from the fore-gut, opposite the second and also opposite Fig. 653.-Rudiments of the liver on the intestine of a chick at the fifth day of incubation. 1, heart; 2, intestine; 3, diverticulum of the intestine in which the liver (4) is deve- loped ; 5, part of the mucous layer of the germinal membrane. (Muller.) the fourth visceral arches. The hypoblastic cells form the lining epithelium of the vesicles; the stroma of the gland is formed by the surrounding mesoblast. The thymus is formed in a similar way opposite the third and fourth visceral arches. These hypo- blastic cells form the nests called the corpuscles of Hassall; the lymphoid tissue by which they are invaded and ultimately sur- rounded is mesoblastic. Development of the Respiratory Apparatus. The Lungs, at their first development, appear as small tuber- cles or diverticula from the abdominal surface of the oesophagus. The two diverticula at first open directly into the oesophagus, but as they grow, a separate tube (the future trachea) is formed at their point of fusion, opening into the oesophagus on its anterior surface. These primary diverticula of the hypoblast of the ali- mentary canal send off secondary branches into the surrounding mesoblast, and these again give off tertiary branches, forming the air-cells. Thus we have the lungs formed : the epithelium lining the air-cells, bronchi, and trachea being derived from the hypo- blast, and all the rest of the lung-tissue, nerves, lymphatics, and 820 DEVELOPMENT. [ch. lvii. blood-vessels, cartilaginous rings, and muscular fibres of the bronchi from the mesoblast. The diaphragm is early developed as a partition of mesoblast Fig. 654 illustrates the development of the respiratory organs, a, is the oesophagus of a chick on the fourth day of incubation, with the rudiments of the trachea on the lung of the left side, viewed laterally ; 1, the inferior wall of the oesophagus; 2, the upper portion of the same tube ; 3, the rudimentary lung ; 4, the stomach ; b, is the same object seen from below, so that both lungs are visible, c, shows the tongue and respiratory organs of the embryo of a horse; 1, the tongue; 2, the'larynx ; y, the trachea; 4, the lungs, viewed from the upper side. (After Rathke.) J dividing the original pleuroperitoneal cavity into thoracic and abdominal serous cavities. Development of the G-emto-urinary Apparatus In the early stage of the development of the urino-genital organs, the most striking thing seen is their resemblance to the Fig. 655.-Diagram of transverse section of embiyo dogfish. On the right of the middle line, A B, the primitive segmental tube (A') is seen in transverse section ; on the left side a later stage is represented; it here forms a well marked projection into the pleuro-peritoneal cavity, and the section is represented as passing through the trumpet-shaped opening of the tube into that cavity (A"). segmental organs, or nephridia of worms. The subject was first worked out by Balfour in the elasmobranch fishes; we may there- fore first describe what he found here, and then pass on to what occurs in mammals. CH. LVII.J THE WOLFFIAN BODIES. 821 In the preceding diagram (fig. 655) we have a transverse section through the embryo in which the structures represented will be familiar from our previous studies. About the fifth segment a thickening in the mesoblast occurs, which grows backwards as a solid column of cells, this becomes hollow, and is seen in transverse section at A'; later on the hollow extends at one part into the pleuro-peritoneal cavity by a trumpet-shaped opening, and this is seen cut through at A". This duct may be termed the archinephros. The prominence created by this duct grows into the pleuro-peritoneal cavity; and a number of convoluted tubes, one in each segment, open into the duct, which soon splits into two longitudinally; one division, the pronephros or Mullerian duct (fig. 656, M), has the original opening into the body cavity; the other convoluted tubes open into the other division of the tube ; they become united together by connective tissue, and form a solid organ called the Wolffian body, or mesonephros. The duct is called the mesonephric, or Wolffian duct (fig. 656, W). The two ducts open into the cloaca which also receives the hinder opening of the ali- mentary canal. The tubules of the Wolffian body become more convoluted and form the tubules of the head-kidney; some of their original openings into the peritoneal cavity can be traced, however, even in the adult. From the lower end of the Wolffian duct a protrusion or growth takes place, and this also becomes hollow, and a number of segmental tubes develop and form with it an organ similar to the Wolffian body ; this is called the metanephros, and it forms the hind kidney, which represents the true kidney in the higher vertebrates; the metanephric duct becomes the ureter. It is represented at K, in fig. 657. In the female the Mullerian ducts become the oviducts, and, where they join, the uterus. In the male they disappear. The head or Wolffian kidney, and the hind or true kidney both execute renal functions in both sexes; but in the male, the Wolffian tubules apply themselves to the testis and constitute its efferent ducts ; the main Wolffian duct becoming the vas deferens. Fig. 656. - Diagram representing the splitting of the arehinephros into Mullerian (M) and Wolffian (W) ducts. 822 DEVELOPMENT [CH. LVII. Thus in fishes and amphibians, the semen passes through tubules which are also renal in function. In the higher vertebrates the subject has been chiefly studied in Fig. 657.-Diagram showing the relations of the female (the left-hand figure $ ) and of the male (the right-hand figure <J) reproductive organs to the general plan (the middle figure) of these organs in the higher vertebrata (including man). Cl, cloaca; 7?, rectum ; B I, urinary bladder ; IT, ureter; K, kidney ; U h, urethra ; G, genital gland, ovary, or testis; W, Wolffian body ; IF d, Wolffian duct; M, Mullerian duct; P s t, prostate gland ; G p, Cowper's gland ; G sp, corpus spongiosum; C c, corpus cavernosum. In the female.-V, vagina ; IT t, uterus; F p, Fallop'an tube ; G t, Gaertner's duct; P v, parovarium ; A, anus; C c, C s p, clitoris. In the male.-C sp, Ge, penis ; IT t, uterus masculinis ; V s, vesicula seminalis ; F d, vas deferens. (Huxley.) the chick, but most of the facts have been confirmed in the mammal. The archinephros which is first formed becomes the Wolffian duct, and the segmental tubules, which are rather more numerous than one to each segment, get bound into the Wolffian body. The Mullerian duct is not split off from this, but is formed separately by a longitudinal folding in of the pleuro-peritoneal cavity ; the hind or true kidney is formed in both sexes as before by a growth backwards from the Wolffian duct. The Wolffian bodies, or temporary kidneys, as they may be termed, give place at an early period in the human foetus to their CH. LVII.] THE WOLFFIAN BODIES. 823 Fig. 658.-Transverse section of embryo chick (third day), mr, rudimentary spinal cord ; the priffiitive central canal has become constricted in the middle ; ch, notochord ; u w h, primordial vertebral mass ; m, muscle-plate; dr, df, hypoblast and visceral layer of mesoblast lining groove, which is not yet closed in to form the intestines ; a o, one of the primitive aortee ; u n, Wolfflan body ; u n g, Wolfflan duct; v c, vena cardiualis; h, epiblast; hp, somatopleur and its reflection to form a J', amniotic fold ; p, pleuro-peritoneal cavity. (Kolliker.) Fig. 659.-Section of intermediate cell-mass on the fourth day. m, mesentery ; L, somato- pleur; a, germinal epithelium, from which z, the duct of Muller, becomes involuted ; n, thickened part of germinal epithelium in which the primitive ova 0 and 0, are lying ; E, modified mesoblast, which will form the stroma of the ovary; JJW, Wolfflan body ; y, Wolffian duct, x 160. (Waldeyer.) 824 DEVELOPMENT. [CH. LVII. successors, the permanent kidneys, which are developed behind them. Each diminishes rapidly in size, and loses all renal functions. In the male it is developed into the vasa efferentia, coni vasculosi, and globus major of the epididymis; and thus a direct connection between the secreting part of the testicle and its duct is brought about. The Wolffian ducts persist in the male, and are developed to form the body and globus minor of the epididymis, the vas deferens, and ejaculatory duct on each side; the vesiculse seminales form diverticula from their lower part. In the female a small relic of the Wolffian body persists as the joarovarz'wm, a functionless collection of tubules lined with ciliated epithelium near the ovary ; in the male a similar relic is termed Fig. 660.-Diagram of two-horned uterus. The body of the uterus (U) is formed by the fusion of the two Mullerian ducts, the ununited portions of which form the oviducts, Fallopian tubes or horns of the uterus (0,0); V, vagina. the organ of Giraldes. The lower end of the Wolffian duct remains in the female as the duct of Gaertner, which descends towards, and is lost upon, the anterior wall of the vagina. The Fallopian tubes, the uterus, and the vagina are developed from the Mullerian ducts. The two Mullerian ducts are united below into a single cord, called the genital cord, and from this are developed the vagina, as well as the cervix and the lower portion of the body of the uterus; while the ununited portion of the duct on each side forms the upper part of the uterus, and the Fallopian tube. In certain cases of arrested or abnormal development, these portions of the Mullerian ducts may not become fused together at their lower extremities, and there is left CH. LVII.] DEVELOPMENT OF GENERATIVE ORGANS. 825 a cleft or horned condition of the upper part of the uterus re- sembling a condition which is permanent in certain of the lower animals (see fig. 660). In the male, the Mullerian ducts have no special function, and are but slightly developed. The hydatid of Morgagni is the remnant of the upper part of the Mullerian duct. The small prostatic pouch, uterus masculinus, or sinus pocularis, forms the atrophied remnant of the distal end of the genital cord, and is, of course, therefore, the homologue, in the male, of the vagina and uterus in the female. We must now pass to the development of the ovary and testis. Between the Wolftian body and the mesentery, the mesoblast covering the ridge produced by the projecting Wolffian body, is converted into a thick epithelium called the germ epithelium (see fig. 659). From this the reproductive gland (ovary or testis as the case may be) is developed. The outline of the manner in which the ovary is formed is described in Chapter LVI. (p. 766) ; the testis is formed in a similar way, only the downgrowths of cells which become nests of cells to form ova and germinal epithelium in the female, become hollowed out as seminiferous tubules in the male. For some time it is impossible to determine whether an ovary or testis will be developed; gradually however the special characters belonging to one of them appear, and in either case the organ soon begins to assume a relatively lower position in the body; the ovaries are thus ultimately placed in the pelvis ; while towards the end of foetal existence the testicles descend into the scrotum, the testicle entering the internal inguinal ring in the seventh month of foetal life, and completing its descent through the inguinal canal and external ring into the scrotum by the end of the eighth month. A pouch of peritoneum, the processus vaginalis, precedes it in its descent, and ultimately forms the tunica vaginalis or serous covering of the organ ; the com- munication between the tunica vaginalis and the cavity of the peritoneum is closed only a short time before birth. In its descent, the testicle or ovary of course retains the blood-vessels, nerves, and lymphatics, which were supplied to it while in the lumbar region, and which accompany it as it assumes a lower position in the body. Hence the explanation of the otherwise strange fact of the origin of these parts at so considerable a distance from the organ to which they are distributed. Descent of the Testicles into the Scrotum.-The means by which the descent of the testicles into the scrotum is effected are not fully and exactly known. It was formerly believed that a mem- 826 DEVELOPMENT. [ch. LVII. branous and partly muscular cord, called the guhernaculum testis, which extends while the testicle is yet high in the abdomen, from its lower part, through the abdominal wall (in the situation of the inguinal canal) to the front of the pubes and lower part of the scrotum, was the agent by the contraction of which the descent was effected. It is now generally thought, however, that such is not the case, and that the descent of the testicle and ovary is rather the result of a general process of development in these and neighbouring parts, the tendency of which is to pro- Fig. 661.-Diagram of the Wolffian bodies, Mullerian ducts and adjacent,parts previous to sexual distinction, as seen from before, sr, the supra-renal bodies ; r, the kidneys ; ot, common blastema of ovaries or testicles ; W, Wolffian bodies; w, Wolffian ducts ; mm, Mullerian ducts; a c, genital cord ; uq, sinus urogenitalis; ), intestine : cl, cloaca. (Allen Thomson.) duce this change in the relative position of these organs. In other words, the descent is not the result of a mere mechanical action, by which the organ is dragged down to a lower position, but rather one change out of many which attend the gradual development and rearrangement of these organs. The homologue, in the female, of the gubernaculum testis is a structure called the round ligament of the uterus, which extends through the inguinal canal, from the outer and upper part of CH. LVII.] DEVELOPMENT OF GENERATIVE ORGANS. 827 the uterus to the subcutaneous tissue in front of the symphysis pubis. At a very early stage of foetal life, the Wolffian ducts, ureters, and Mullerian ducts, open into a receptacle formed by the lower end of the allantois, or rudimentary bladder; and as this com- municates with the lower extremity of the intestine, there is for the time a common receptacle or cloaca for all these parts, which opens to the exterior of the body through a part corresponding with the future anus, an arrangement which is permanent in reptiles, birds, and some of the lower mammalia. In the human foetus, however, the intestinal portion of the cloaca is cut off from that which belongs to the urinary and generative organs ; a separate passage or canal to the exterior of the body, belonging to these parts, being called the sinus uro-genitalis. Subsequently, this canal is divided, by a process of division extending from before backwards or from above downwards, into a ' pars urinaria ' and a ' pars genitalis.' The former, continuous with the urachus, is converted into the urinary bladder. The external parts of generation are at first the same in both sexes. The opening of the genito-urinary apparatus is, in both sexes, bounded by two folds of skin, whilst in front of it there is formed a penis-like body surmounted by the glans, with a cleft or furrow along its under surface. The borders of the furrows diverge posteriorly, running at the sides of the genito-urinary orifice internally to the cutaneous folds just mentioned. In the female, this body becoming retracted; forms the clitoris, and the margins of 'the furrow on its under surface are converted into the nymphse, or labite minora, the labia majora pudendte being constituted by the great cutaneous folds. In the male foetus, the margins of the furrow at the under surface of the penis unite at about the fourteenth week, and form that part of the urethra which is included in the penis. The large cutaneous folds form the scrotum, and later (in the eighth month of develop- ment), receive the testicles, which descend into them from the abdominal cavity. Sometimes the urethra is not closed, and the deformity called hypospadias then results. The appearance of hermaphroditism may, in these cases, be increased by the reten- tion of the testes within the abdomen. The supra-renal capsules originate in a mass of mesoblast just above the kidneys ; soon after their first appearance they are very much larger than the kidneys (see fig. 661), but by the more rapid growth of the latter this relation is soon reversed. INDEX. A. Abdominal muscles, action of in respira- tion, 516 Abducens nerve, 216 centre, 217 Aberration, chromatic, 352 spherical, 351 Absorption, of carbohydrates, 670 fats, 671 food, 670 proteids, 671 by the skin, 730 Accelerator nerves, 160 Accommodation of eye, 346 defects of, 350 .Acetonaemia, 669 Acetyl, 556 Achroo-dextrin, 554, 637 Acids in gastric j uice, 640 Acid-albumin, 501, 643 properties of, 561 Acini of secreting glands, 628 Acrylic series, 555 Adamantoblasts, 80 Adductors and sphincters, 316 Adenoid or lymphoid tissue, 51 in intestines, 614 Adipose tissue, 47. See Fat. development, 49 situations of, 47 structure of, ib. uses, 49 vessels and nerves, ib. Afferent nerves, 161 After-birth, 788 Age, influence on capacity of respira- tion, 522 Air, atmospheric, composition of, 539 breathing, 521 complemental, ib. reserve, ib. residual, ib.' tidal, ib. Amphioxus. Air-continued. changes by breathing, 540 quantity breathed, 521 transmission of sonorous vibrations through, 309 undulations of, conducted by external ear, ib. Air-cells, 511 Air-tubes. See Bronchi. Alanine, 651 Albumin, 558, 560 acid, 561 alkali, ib. chemical composition of, 557 egg, 560 lact-, ib. serum, ib. of blood, 575 Albuminates, 561 Albuminoids, 564 Albuminous substances, 557 action of gastric fluid on, 644 Albumoses, 558 Alcohol as an accessory to food, 625 Alimentary canal, 598 et seq. development of, 816 Alkali-albumin, 561 properties of, ib. Allantois, development of, 786 Amido-acetic acid, 650 Ammonia, cyanate of, isomeric with urea, 704 urate of, 712 Amnesia, 326 Amnion, development of, 785 fluid of, 782, 785 Amoebae, 6 Amoeboid movements, 580 cells, 6 colourless corpuscles, 580 cornea-cells, 329 protoplasm, 12, 106 Tran descantia, 14 Amphioxus, circulatory system of, 404 830 INDEX. Amyloids. Amyloids or Starches, 553 action of pancreas and intestinal glands, 647 of saliva on, 637 Amylopsin, action of, 647 Anabolic phenomena, 375, 735 Anacrotic pulse, 446 Anelectrotonus, 177 Angulus opticus seu visorius, 730 Animal cell, structure of, 8 et seq. Animal heat. See Heat and Tempera- ture. Ankle-clonus, 252 Antero-lateral ascending tract, 205 Antero-lateral descending tract, 204 Antihelix, 301 Antitragus, ib. Aphasia, 326 Aphemia, ib. Apnoea. 527 Appendices epiploicre, 612 Appendix veriniformis, ib. Aquseductus cochleae, 305 Aqueduct of Sylvius, 214 Aqueous humour, 341 Arachnoid membrane, 184 Arches, visceral, 792 Archinephros, 821 Area germinativa, 772 opaca, 773 pellueida, 772 vasculosa, 784. 795 Areolar tissue, 38 development of, 43 Arginine, 708 Arteria centralis retime, 813, 815 Arterial tension in asphyxia, 537 Arteries, 387 bronchial, 512 circulation in, 437 velocity of, ib. coronary, 412 distribution, 387 elasticity, 441 muscularity, ib. nerves of, 390 nervous system, influence of, 475 pressure of blood in asphyxia, 537 pulse. See Pulse. renal, 700 rhythmic contraction, 442 et seq. structure, 388 et seq. umbilical, 788 velocity of blood in, 437 Articulate sounds, classification of, vowels and consonants, 325 Arytenoid cartilages, 315 effect of approximation, 317 movements of, ib. Arytenoid muscle, 317 Ascending tubule of Henle, 688 Asphyxia, 534 causes of death in, 535 Blastema. Asphyxia-continued. symptoms, 534 Association fibres, 240 Astigmatism, 351 Atmospheric air, 538. See Air. composition of, 539 pressure in relation to respiration, 538 Attraction sphere, 12 Auditory canal, 309 et seq. function, iA Auditory area, 266 Auditory nerve, 217, 306 distribution, ib. origin. 217 Auerbach's plexus, 607 Auricles of heart. See Heart. Auricular diastole, 406 systole, 406, 407 Auriculo-ventricular valves. See Heart valves. Axis-cylinder of nerve-fibre, 99 B. Bacterial action on intestinal digestion, 649 Bacterium lactis, 552 Basement-membranes, 51, 611 Batteries and keys, 112 Daniell cell, io. Bicuspid valve, 383 Bidder's ganglion, 429 Bile, 659 absorption by lymph, 665 analyses of human, 661 capillaries, 657 characters of, 661 constituents of, 660, 661 digestive properties, 648, 664 doubtful antiseptic power, 664 influence of fasting on secretion, 742 mixture with chyme, 664 mucin, 661 pigments, 662 process of secretion, 659 quantity secreted, 660 salts, 662 secretion and flow, 660 specific gravity, 661 uses, 664 Bile-expelling mechanism, 665 Bilirubin, 660, 662 Biliverdin, 663 Binocular vision, 367 Bipolar nerve-cells, 187 Birth, changes after, 806 Biuret test, 559 Bladder, urinary. See Urinary Bladder. Blastema. See Protoplasm. INDEX 831 Blastoderm. Blastoderm, bilaminar, 771 trilaminar, 772 unilaminar, ib. Blastcpore, 771 Blastosphere, 770 Blind spot. 355 Blocking, 430 Blood, 82, 568 arterial and venous, difference be- tween, 386 huffy coat, 573 carbonic acid in, 543 circulation of, 400 et seq. in the foetus, 804 local peculiarities of, 451 schema of, 403 coagulation, 82, 570 et seq. colour, 82, 568 colouring matter, 588 relation to that of bile, 591 corpuscles or cells of, 82, 576. See Blood-corpuscles. red, 576 white, 579 crystals, 588 development, 583 extractive matters, 575 fatty matters, ib. fibrin, 570 , separation of, 571 gases of, 542 haemoglobin, 576, 588 odour or halitus of, 569 oxygen in, 542 oxyhaemoglobin, 588 plasma, 568, 573 " quantity, 369 reaction, ib. salts, 575 serum of, 573 specific gravity, 568 splenic, 493 structural composition, 576 taste, 569 temperature, ib. tests for, 597 venous, 386 Blood-corpuscles, red, 82, 576 action of reagents on, 577 composition of, 587 development, 583 intracellular, 586 disintegration and removal, 493 method of counting, 581 origin of matured, 585 rouleaux, 577 specific gravity, 576 stroma, ib. tendency to adhere, 577 varieties, 576 vertebrate, various, 578 Blood-corpuscles, white, 579 amoeboid movements of, 580 Branchial. Blood-corpuscles, white-continued. composition of, 587 derivation of, ib. formation of, in spleen, 493, 585 locomotion, 579 varieties, ib. Blood-crystals, 588 Blood-platelets, 581 Blood-pressure, 454 et seq. in capillaries, 462 in veins, ib. action of respiratory movements on, 451 schema to illustrate, 455, 457 Blood-vessels, circulation in, 437 elasticity of, 440 of eyeball, 341 in intestines, 610 of kidney, 690 of stomach, 605 influence of nervous system on, 467 Body-cavity, 776 Bone, 58 eanaliculi, 60 cancellous, 58 chemical composition, ib. compact, ib. development, 63 et seq. growth, 69 Haversian canals, 61 lacunae, ib. lamellae, 62 marrow, 59 medullary canal, 58 microscopic structure, 60 ossification in cartilage, 64 periosteum and nutrient blood-vessels, 59 structure, 58 et seq. Brain. See Bulb, Cerebellum, Cere- brum, Pons, etc. capillaries of, 451 of child, 243 circulation of blood in, 451 convolutions, 242 development, 809 of dog, 260 in foetus, 211 grey matter, 185 lobes, 244 of lunatic, 271 membranes of, 184 monkey, 261 motor areas, 262 orang, 243 quantity of blood in, 451 sensori-motor areas, 267 sensory areas, 266 vertebrate (section), 211 ventricles, 209 white matter, 185 Branchial clefts, 793 832 INDEX. Bread. Bread as food, 623 Breathing. See Kespiration. Bronchi, arrangement and structure of, 507 Bronchial arteries and veins, 512 Brownian movement, 106 Brunner's glands, 611 Buffy coat, formation of, 572 Bulb, pons and mid-brain, 212 anterior aspect, ib. et seq. internal structure, 221 et seq. posterior aspect, 214 Bulbus arteriosus, 797 Burdach's column, 214, 222 Burd on-Sanderson's stethograph, 518 Bursee, synovial, 627 Butyric acid, 552, 649 C. Caffeine, 625 Calcification of bone, 66 Calcium carbonate, 58 in urine, 719 fluoride, 58 oxalate in urine, 718 phosphate, 58 Calorimeters, 752, 733 Calyces of the kidneys, 685 Canal, alimentary. See Stomach, Intes- tine, etc. external auditory, 300 function of, 309 spiral, of cochlea, 306 Canaliculi of bone, 60 Canal of Schlemm, 335 of Petit, 341 of Stilling, 815 Canals, semicircular, of ear, 273 development of, 815 Cancellous tissue of bone, 58 Cane sugar, 552 Capacity of chest, vital, 521 Capillaries, 394 bile, 657 circulation in, 438, 447 velocity of, 438 development, 795 diameter, 394 form, 395 ' influence on circulation, 447 network of, 395 number, 396 passage of corpuscles through walls of, 449 pressure in, 463 resistance to flow of blood in, 447 still layer in, ib. size, 395 structure of, 394 Celis. Capsule of Bowman, 687 of Glisson, 656 Capsules, Malpighian, 686 Carbamide. See Urea. Carbohydrates, 549 absorption of, 670 Carbonates in urine, 715 Carbonic acid in atmosphere, 539 in blood, 543 effect of, 543 increase of in breathed air, 539 in lungs, 342 Carbonic oxide haemoglobin, 595 Cardiac cycle, 406 Cardiac glands, 603, 638 Cardiac orifice of stomach, action of, 681 sphincter of, ib. relaxation in vomiting, ib. Cardiac sympathetic, 425 Cardiogram from human heart, 415 Cardiographs, 413 et seq. Cardio-inhibitory centre, 534 Carotid gland, 504 Cartilage, 53 articular, 53, 54 cellular, 57 chondrin obtained from, 55 classification, 53 costal, 33, 54 development, 56 elastic, 53, 56 fibrous, 55, 36. See Fibro-cartilage. hyaline, 53' matrix, ib. ossification, 64 perichondrium of, 54 structure, 53 temporary, 54 transitional, ib. varieties, 53 Cartilages of larynx, 314 Casein, 617. See Milk. Caseinogen, 562, 617 Cauda equina, 808 Caudate nucleus, 233 Cavity of reserve, 81 Cell division, 16 Cells, 5 amoeboid, 6 blood. See Blood-corpuscles, cartilage, 33 et seq. ciliated, 30 connective tissue, 39 definition of, 6 epithelium, 27. See Epithelium, fission, 16 formative, 773 gustatory, 294 hepatic, 654 nerve, 187 olfactory, 298 parietal, 604, 638, 639 INDEX. 833 Cells. Cells-continued. pigment, 106 structure, 9 et seq. varieties, 26 et seq. vegetable, 6, 13 distinctions from animal cells, 6 et seq. Cells of Purkinje, 229 Cellular cartilage. See Cartilage. Cellulose, 554 Cement of teeth, 74, 77 Centres, nervous, &c. See Nerve- centres. of ossification, 64 Centrifugal machine, 573 nerve-tibres, 160 Centripetal nerve-fibres, 161 Centro-acinar cells, 646 Cerebellar ataxy, 271 Cerebellum, 228 effects of removal or disease, 270 equilibration, 271 functions of, 269 et seq. grey matter, 189, 211, 229 hemi-extirpation, results of, 270 semicircular canals, 272 extirpation of, 275 sensory impulses, 271 structure of, 228 Cerebral cortex, 237 histological structure, ib. Cerebral hemispheres. See Cerebrum. Cerebral nerves, origin of, 215 et seq. See under names of nerves. Cere bro-spinal fluid, 210 Cerebro-spinal axis, 184 Cerebro-spinal nervous system, 240 See Brain, Spinal Cord, etc. Cerebrum, its structure, 231 convolutions of, 242 et seq. crura of, 209 degeneration tracts after injury of .Rolandic area, 268 development, 809 effects of injury, 259 removal, 256, 258 external capsule, 235 functions of, 255 et seq. early notions, ib. grey matter, 189, 232 motor areas, 259, 262 in relation to speech, 326 internal capsule, 235 localisation of functions, 256 sensory areas, 259 stimulation, ib. structure, 231 et seq. white matter, 235 Ceruminous glands of ear, 730 Chambers of the eye, 341 Chauveau's dromograph, 438 Chemical composition of the human body, 548 et seq. K.P. Cocaine. Chest, expansion in inspiration, 516 Cheyne-Stokes' respiration, 529 Chlorides in urine, 715 Cholagogues, 665 Cholalic acid, 662 Cholesterin, 99, «7, 663 Choletelin, 663 Choline, 557 Chondrin, 564 Chorda tympani, 633 effects of stimulation of divided, 634 Chordal tendineae. See Heart. Chorion, 787 Choroid coat of eye, 328 blood-vessels, 332 development, 814 structure, 332 Choroidal fissure, 813 Chromatic aberration, 352 Chromophanes, 365 Chromoplasm, 10 Chyle, 399, 482, 671 molecular basis of, ib. Chyme, 660 Cilia, 30 Ciliary epithelium, ib. function of, 31 Ciliary motion, ib. nature of, 32 Ciliary muscles, 333 action of in adaptation to distances, 347 Ciliary processes, 333 Circulation of blood, 377 action of heart, 378 in brain, 451 capillaries, 447 course of, 386 et seq. erectile structures, 453 influence of respiration on, 529 peculiarities of, in different parts, 451 pulmonary, 513 systemic, 387 in veins, 390 velocity of, 439 Circulatory system, 377 et seq. Circumvallate papillae, 293 Claustrum, 234 Cleft-palate, cause of, 794 Clefts, visceral, 792 Clerk-Maxwell's experiment, 360 Clitoris, development of, 827 Cloaca, ib. Clonus, 128 Clot or coagulum of blood. See Coagulation. Coagulated proteids, 562 Coagulation of blood, 82, 570 et seq. conditions affecting, 572 theories of, ib. of milk, 617 Cocaine, 625 834 INDEX. Coccygeal gland. Coccygeal gland, 504 Cochlea of the ear, 305 theories in connection with, 312 Coelom, 776 Cohnheim, areas of, 87 Cold spots, 290 Collagen, 40, 38, Colloids, 558 Colostrum, 616 corpuscles, 617, 620 Colour-blindness, 364 Colour sensations, 360 theories of, 362 Colours, optical phenomena of, 360 et seq. Columnar epithelium, 27 Comma tract, 204 Commissural fibres, 240 Complemental air, 521 Complementary colours, 362 Compound tubular glands, 628 Conception, 278 Condiments, 625 Conducting paths in cord, 247 Coni vasculosi, 758, 760 Conjunctiva, 328 Connective tissues, 37 classification, 37 corpuscles, 40 fibrous, 44 general structure of, 38 jelly-like, 52 retiform, 50 varieties, 37 Contractility of muscle, 106 Contraction of pupil, 349 Convolutions, cerebral, 242 et seq. Co-ordination of muscular movements, 130 Cooking, effect of, 624 Copper sulphate, or Piotrowski's test, 559 Corona radiata, 235 Cord, spinal. See Spinal Cord. Corium, 627 Cornea, 328 corpuscles, 33 c nerves, ib. structure, 330 Corneo-scleral junction, 334 Corona radiata, 235 Corpora Arantii, 385 quadrigemina, 227 Corpus callosum, 231, 232 dentatum of cerebellum, 229 of olivary body, ib. Highmorianum, 757 luteum, 764 of human female, 765 of menstruation and pregnancy compared, 764 striatum, 233 Dental. Corpuscles of blood, 82, 576. See Blood-corpuscles. Corpuscles of Herbst, 283 Corti's rods, 307 et seq. office of, 312 Coughing, mechanism of, 528 Cowper's glands, 693 Cranial nerves, 215 et seq. Crassamentum, 570 Creatine, 707 Creatinine, 713 Crescents of Gianuzzi, 632 Crico-arytenoid muscles, 316, 317 Cricoid cartilages, 314 Crista acustica, 274 Crosses of Ranvier, 100 Crossed pyramidal tract, 203 Crura cerebelli, 228 cerebri, 209 Crusta, 227 Crusta petrosa, 77 Crypts of Lieberkuhn, 611 Crystallin, 334 Crystalline lens, 333 in relation to vision at different distances, 346 Crystalloids, 558 Cupula, 274 Curdling ferments, 618 Currents of action, constant, 113 induced, 114 nerve, no Cuticle. See Epidermis, Epithelium. Cutis vera, 725 Cystic duct, 653 Cystin in urine, 718 D. Daltonism, 364 Daniell's battery, 112, 113 Decidua, 780 development of, 782 menstrualis, 767 reflexa, 780 serotina, ib. vera, ib. Decussation of fibres in medulla ob- longata, 223 in spinal cord, 247 of optic nerves, 370 Defaecation, mechanism of, 684 influence of spinal cord on, ib. Degeneration method, 164, 167, 199 Deglutition. See Swallowing. Dental germ, 78 papilla, ib. INDEX. 835 Dentine. Dentine, 74 formation of, 79 Depressor nerve, 475 Dermis, 725 Descemet's membrane, 331 Descending tubule of llenle, 688 Deutero-albumose, 643 Development, 768 adipose tissue, 49 alimentary canal, 816 allantois, 786 arteries, 797 blood-vessels, ib. brain, 809 decidua, 782 ear, 815 extremities, 790 eye, 812 face, 791 foetal membranes, 780 genito-urinary apparatus, 820 et seq. head, 791 heart, 795 limbs, 791 liver, 818 lungs, 819 medulla oblongata, 810 muscle, 95 nerves, 808 nervous system, 806 nose, 815 organs of sense, 812 ovum, 768 pancreas, 818 pituitary body, 790 respiratory apparatus, 819 salivary glands, 818 spinal cord, 806 teeth, 77 vascular system, 795 veins, 801 visceral arches and clefts, 792 of Wolffian bodies, urinary apparatus and sexual organs, 820 Dextrin, 554 Dextrose, 550 in urine, 721 tests for determining, 722 Diabetes, 668, 746 artificial production in animals, 669 Diapedesis of blood-corpuscles, 449 Diaphragm. See Inspiration, &c. development, 820 Diastase of liver, 667 Diastole of heart, 406 Dicrotic pulse, 446 Diet, 739 et seq. nutritive value, 615 Digestion, duration of intestinal, 684 in the intestines, 645 et seq. mechanical processes of, 674 et seq. See Gastric fluid, Food, Stomach. Emetics. Dilatator pupilhe, T.T.T. Diplopia, 367 Direct cerebellar tract, 204 pyramidal tract, ib. Disaccharides, 550 Discus proligerus, 764 Diuretics, 697 Dobie's line, 88 Dorsal ridges, 774 Double vision, 367 Dromograph, Chauveau's, 438 Drugs, action of, 684 Ductless glands, 488 et seq. Ducts of Bellini, 688 of Cuvier, 802 Ductus arteriosus, 799, 805 closure of, 806 venosus, 804 closure of, 806 Dudgeon's sphygmograph, 444 Dulong's calorimeter, 753 Duodenum, 606 Dura mater, 184 Dyspnoea, 525 E. Ear, 300 bones or ossicles of, 303 function of, 310 development of, 815 external, 300 function of, 309 internal, 303 function of, 309 middle, 302 function of, 310 Ectoderm, 772 Efferent nerves, 161 Eggs as food, 615, 621 Elastic cartilage, 53, 56 fibres, 44 tissue, 46 Elastin, 40, 565 Electrical nerves, 161 Electricity, action on blood-corpuscles, 579 in muscle, 180 nerve, ib. Electrodes, non-polarisable, 142 Electrometer, Lippmann's capillary, 143 Electrotonus, 171 Eleidin, 725 Elementary substances in the human body, 548 Embryo, 768 et seq. See Development. Embryological method, 199 Embryonic heart and blood-vessels, 798 Embryonic spot, 772 Emetics, 682 836 INDEX. Emulsification. Emulsification, 556 Enamel of teeth, 76 formation of, 79 Enamel organ, 79 Enchylema, 9 End-bulbs, 283 End-plates, motorial, 102 Endocardiac pressure, 417 Endocardium, 379 Endoderm, 772 Endolymph, 273, 304 Endomysium, 85 Endothelium, 26 distinctive characters, ib. germinating, 25 Epencephalon, 212, 811 Epiblast, 22, 772 organs formed from, 779 Epidermis, 724 Epididymis, 757, 760 Epiglottis, 319, 320 Epimysium, 85 Epithelium, 23 chemistry of, 36 ciliated, 30 cogged, 35 columnar, 27 compound, 24 cubical, 27 germinal, 762 goblet-shaped, 29 nutrition of, 36 pavement, 26 renal, 697 simple, 24 spheroidal, 27 stratified, 34 transitional, 33 Erectile structures, circulation in, 453 Erection, ib. cause of, ib. influence of muscular tissue in, ib. Ergograph, 153 Erythro-dextnn, 554 Esbach's albuminometer, 721 Eustachian tube, 302 function of, 310 Exchange of material, 737 in diseases, 743 with various diets, 743 Excitability of tissues, 105 Exercise, effects of, on temperature of body, 750 Expiration, 517 force of expiratory act, 522 influence of, on circulation, 532 mechanism of, 517 muscles concerned in, ib. relative duration of, 520 External capsule, 235 sphincter muscle, 683 Extremities, development of, 790 Eye, 327 Fibres. Eye-continued. action of drugs on pupil, 354 adaptation of vision at different dis- tances, 346 et seq. blood-vessels, 341 causes of dilatation and contraction of pupil, 354 development of, 812 optical apparatus of, 342 defects in, 330 refractive media of, 342 resemblance to camera, ib. Eyeball, blood-vessels of, 341 muscles influencing movement, 366 Eyelids, 328 development of, 815 Eyes, simultaneous action of in vision, 367. F. Face, development of, 791 Facial nerve, 217 effects of paralysis of, ib. origin, ib. relation of, to expression, ib. Faeces, composition of, 674 quantity of, ib. Fallopian tubes, 766 development of, 824 Falsetto voice, 323 Faradisation, 127 Far-point, 351 Fasting, influence on secretion of bile, 742 Fat. See Adipose tissue. action of bile on, 664 of pancreatic secretion, 647 situations where found, 47 uses of, 49 Fats, 555 absorption of, 671 action of pancreatic juice on, 647 chemical constitution, ib. decomposition products, 556 emulsification, ib. of milk, 618 saponification, ib. Fatty acids, 555 Female generative organs, 762 Fenestra ovalis, 304 rotunda, 305 action of, 311 Ferment coagulation, 560 Ferments, 565, 639 classification of, 566 in pancreatic juice, 647 Fibres of Muller, 336 of Remak, 102 INDEX. 837 Fibrin. Fibrin, 57° ferment, 572, 575 formation of, 571 Fibrinogen, 82, 572, 575 Fibrinoplastin, 575 Fibro-cartilage, 55 classification, ib. development, 56 white, 55 yellow, 56 Fibrous tissue, 44 white, ib. yellow, 46 Fick's spring-kymograph, 462, 463 Fifth nerve, 217 Fillet, 226 Filum terminale, 194 Fleischl's haemoglobinometer, 597 Flesh of animals, 615 Flour as food, 622 Fluids, swallowing, 677 Fluoride of calcium, 58 Focal distance, 346 Foetal membranes, 780 development of, 783 Foetus, circulation in, 804 communication with mother, 788 Follicles, Graafian. See Graafian Vesi- cles. Food, 615 absorption of, 670 et seq. accessories to, 625 cooking of, 624 digestibility of articles of, 615 value dependent on, ib. of man, 616 too little, 741 proximate principles in, 615 vegetable, 615, 623 Foramen ovale, 799 Foramen of Magendie, 210 Fore-gut, 779 Formative cells, 774 Formic acid, 555 Fornix, 234 Fourth cranial nerve, 216 Fovea centralis, 360 Fundus of eye, 357 of urinary bladder, 692 Frontal-nasal process, 794 Funiculus solitarius, 218, 224 Furfur aldehyde, 662 Fuscin granules, 366 G. Galactose, 551 Gall-bladder, 658 structure, 659 Glycosuria. Galvanism, 139 Galvanometers. 140 Ganglia. See Nerve-centres. Ganglion spirale, 309 Gas analysis, 547 Gases, extraction from blood, 542 in blood, 542 in the lungs, 541 of plasma and serum, 574 Gastric glands, 638 innervation of, 641 Gastric juice, 638 acids in, 640 test for, ib. action of, on food, 642. See 678. artificial, 638 bacterial action, 649 composition of, 640 pepsin of, 639, 640 secretion of, 640 influence of nervous system on, 641 Gelatin, 40, 564 as a constituent of food, 622 Genito-urinary apparatus, development of, 820 Generative organs of the female, 762 of the male, 756 Gerlach's network, 197 Germinal cells, 807 disc, 773 epithelium, 762, 765 Germinal spot, 766 vesicle, ib. Giant cells, 59 Glands. See names of different. Gland, prostate, 693 Glisson's capsule, 656 Globulins, 558, distinctions from albumin, 558. Glosso-pharyngeal nerve, 218 communications of, ib. functions, 219 motor filaments, 218 a nerve of common sensation and of taste, 219 Glottis, movements of, 323 Glucose in liver, 666 test for, 554 Glycerides, 555 Glycerin or Glycerol, 556 Glycocholic acid, 662 Glycocine, 650, 662 Glycogen, 554, 666 characters, 554 destination of, 667 preparation, ib. quantity formed, ib. source of, 666 variation with diet, 667 Glycosuria, 668 838 INDEX. Gmelin. Gmelin's test, 663 Goblet cells, 29, 628 Goll's column, 214, 222 Gowers' haemacytometer, 582 haemoglobinometer, 596 Graafian vesicles, 762 formation and development of, 762 et seq. relation of ovum to, 763 rupture of, changes following, 764 et seq. Grandry, corpuscles of, 286 Granular layers of retina, 337 Grape-sugar. See Dextrose. Grey matter of cerebellum, 229 of cerebrum, 232 of crura cerebri, 211 of medulla oblongata, 214, 221, 223 of pons Varolii, 214 of spinal cord, 185, 196 Groove, primitive, 772 Growth of bone, 69 Gubernaculum testis, 826 Gullet, 600 H. Haemacytometers, 582, 583 Haemadromometer, Volkmann's, 439 Heematin, 589 Haematoblasts, 577, 585 Heematochometer, Vierordt's, 438 Haematoidin, 591 Hematoporphyrin, 590 Haem-autograph, 447 Haemin, 590 Haemochromogen, ib. Haemoglobin, 83, 576, 588 et seq. analysis of, 589 compounds of, 591 distribution, 588 Haemoglobinometer, 596, 597 Haemoglobinuria, paroxysmal, 723 Hair-follicles, 727 Hairs, ib. structure of, ib. Hamulus, 306 Hassall, concentric corpuscles of, 496 Haversian canals, 61 Head, development of, 791 Head and tail folds, 776. Hearing, anatomy of organ of, 300 et seq. influence of external ear on, 309 of middle ear, 310 physiology of, 309 range of, 313 See Sound, Vibrations, etc. Helmholtz. Heart, 378 et seq. action of, accelerated, 425 force of, 421 frequency of, ib. inhibited, 424 work of, 422 auricles of, 379, 406 capacity, 382 chambers, 379 chordae tendineae of, 384 columnse carneae of, ib. course of blood in, 386 cycle, 406 development, 795 endocardiac pressure, 416 endocardium, 379, 383 force, 421 frog's, 404, 405, 433 nerves of, 426 ganglia of, 425 influence of drugs, 427 of pneumogastric nerve, 423 of sympathetic nerve, 425 innervation, 422 intracardiac nerves, 428 investing sac, 378 muscular fibres of, 93 musculi papillares, 384 nervous system, influence on, 422 pericardium, 378 physiology, 406 et seq. reflex inhibition, 427 situation, 378 size and weight, 382 sounds of, 410 causes, 411 structure of, 383 valves, ib. auriculo-ventricular, 382 function of, 408 semilunar, 385 structure, ib. function of, 409 ventricles, their action, 379, 382 work of, 422 Heat, animal. See Temperature influence of nervous system, 755 of various circumstances on, 754 et seq. losses by radiation, etc., 752 variations of, 750 Heat spots, 290 Heat-value of food, 751 Height, relation to respiratory capacity, 522 Helicotrema, 307 Helix of ear, 301 Heller's nitric-acid test, 720 Helmholtz's induction coil, 117 myograph, 118 phakoscope, 348 INDEX. 839 Hemi-albumin. Hemi-albumin, 643 albumose, ib. peptone, ib. Hemiplegia, 259 Hemisection of spinal cord, 205 Hemispheres, Cerebral. See Cerebrum. Hensen's line, 89 Hepatic cells, 654 colic, 665 Herbst, corpuscles of, 283 Hering's theory of colour, 364 Hetero-albumose, 643 Hiccough, mechanism of, 528 Hill's air-pump, 547 Hind-gut, 779 Hipp uric acid, 712 Horopter, 369, 370 Holoblastic ova, 769 Hurthle's manometer, 420, 463 Hyaline cartilage, 53 Hyaloplasm, 9 Hydrobilirubin, 663, 703 Hypermetropia, 351 Hypoblast, 22, 772 organs formed from, 780 Hypoglossal nerve, 219 distribution, 220 origin, 219 I. Ileo-csecal valve, 605, 614 Ileum, 606 Image, formation of, on retina, 344 Impregnation of ovum, 770 Inanition or starvation, 741 Incus, 303 development of, 794 Indican, 715 Indigo, ib. Induction coil, 114 et seq. current, 114 Infundibulum, 511 Inhibitory influence of pneumogastric nerve, 424 Inhibitory nerves, 161 Inogen, 152, 156 Inorganic compounds in body, 548 salts in protoplasm, 9 Inosite, 551 Insalivation, 675 Inspiration, 513 elastic resistance overcome by, 514 expansion of chest in, 516 extraordinary, ib. force employed in, 522 mechanism of, 513 et seq. • Instruments for demonstrating muscular action, 112 et seq. Katelectrotonus. Intercellular material, 39 passage, 511 Intercentral nerves, 163 Intercostal muscles, action in inspira- tion, 516 et seq. in expiration, 517 Intermittent pulse, 442 Internal capsule, 234, 235 importance of, 235 Internal sphincter muscle, 613, 683 Interstitial cells, 759 Intestinal juice, 648, 684 Intestines, 605 action of drugs, 684 digestion in, 645, 648 duration of, 684 development of, 817 large, 612 glands, 614 structure, 612 movements, 682 nervous mechanism, 683 small, 605 glands, 611 structure, 606 Inversion, 551, 648 Invertin, 648 Involuntary muscles, 84 (see 157 et seq.) structure of, 84 Iris, 333 development of, 814 functions of, 353 reflex actions of, 354 Irradiation, 352 Irritability of tissues, 105 Iso-cholesterin, 664 Iso-maltose, 553 J. Jacksonian epilepsy, 261 Jacobson's nerve, 218 Jaundice, 665 Jejunum, 606 Jelly of Wharton, 43 Jelly-like connective tissue, 52 Juice, gastric, 638 pancreatic, 646 K. Karyokinesis, 17 et seq. phases of, 21 Katabolic phenomena, 375, 736 Katelectrotonus, 177 840 INDEX. Keratin. Keratin, 37, 565, 724 Key, DuBois Reymond's, 113 Kidneys, 685 blood-vessels of, how distributed, 690 calyces, 685 capillaries of, 690 development of, 824 extirpation of, 700 function of, 693. See Urine. Malpighian corpuscles of, 688 nerves, 694 structure, 685 tubules of, 688 et seq. weight, 685 work done by, 698 Knee-jerk, 252 Kbnig's apparatus for obtaining flame- pictures of musical notes, 324 Krause's membrane, 88 Kronecker's perfusion cannula, 433 Kymograph, Fick's spring, 462, 463 Ludwig's, 459 tracings, 462, 464 L. Labia externa and interna, development of, 827 Labyrinth of the ear. See Ear. Lacrimal gland, 328 Lact-albumin, 617 Lacteals, 398, 609 fermentation, 552 Lactiferous ducts, 619 Lactose, 552,618, 721 Lamina cribrosa, 336 Laminae viscerales or ventrales, 778 Langley's ganglion, 633 nicotine method, 472, 634, 684 Large intestine. See Intestine. Laryngoscope, 319 Larynx, anatomy of, 314 cartilages of, ib. mucous membrane, 318 muscles of, 316 nerves of, 318 vocal cords, 314, 320 movements of, 321 Lateral sclerosis, 251 Lateritious deposit, 712 Lecithin, 99, 557 Lens, crystalline, 333 Lenticular nucleus, 233 Leucin, 650 Leucocytes. See Blood corpuscles (white). Levulose, 531 Lieberkuhn's glands, 611, 614 jelly, 562 Lysine. Limbs, development of, 791 Lippmann's capillary electrometer, 143 Liquor amnii, 785 Liquor sanguinis, or plasma, 82, 568 Liver, 653 blood-vessels of, 656 capillaries of, 657 cells of, 654 circulation in, 657 development of, 818 functions of, 659 glycogenic function of, 666 secretion of. See Bile. structure, 654 sugar formed by, 666 (see 668) supply of blood to, 653, 660 Locality, sense of, 287 Locomotor ataxy, 248 Loop of Henle, 688 Ludwig's air-pump, 546 kymograph, 439 Stromuhr, 437 Lungs, 509 blood-supply, 512 capillaries of, 511 cells of, 511 changes of air in, 539 circulation in, 512 coverings of, 509 development of, 819 diffusion of gases within the, 541 lobes of, 510 lobules of, ib. lymphatics, 512 muscular tissue of, 511 nerves, 513 nutrition of, 512 position of, 504 structure of, 509 Luxus consumption, 746 Lymph, 397, 481 composition of, 481 current of, 485 formation of, 486 Lymph capillaries, 397 origin of, 399 structure, ib. Lymph-hearts, structure and action of, 485 relation of, to spinal cord, 486 Lymphatic glands, 399, 482 development, 819 Lymphatic vessels, 397 of arteries and veins, 394 communication with blood-vessels, 397 structure of, 399 Lymphocytes, 482 Lymphoid or retiform tissue, 50. See Adenoid Tissue. Lysatine or Lysatinine, 708 Lysine, ib. INDEX. 841 MUCOUS MEMBRANE. Mercurial kymograph, 459 Meroblastic ova, 769 Mesencephalon, 212, 811 Mesoblast, 22, 772 organs formed from, 780 Mesocephalon, 811 Mesoderm, 772 Mesonephros, 821 Metabolic balance-sheets, 739 et seq. Metabolism, 375 general, 734 et seq. Metanephros, 821 Metencephalon, 212, 811 Methaemoglobin, 595 Microcytes, 577 Micro-organisms, types of, 566 Micropyle, 769 Micturition, 701 Middle ear. See Tympanum. Mid-gut, 779 Milk, as food, 616 chemical composition, 617 coagulation of, ib. fats of, 618 proteids of, 617 reaction and specific gravity, ib. salts of, 619 secretion of, 616 Milk-curdling ferments, 648 Milk-globules, 616 Milk-sugar, 552, 618 properties of, 5S2 Milk-teeth, 70 et seq. Millon's re-agent and test, 559 Mitral cells, 299 Modiolus, 305 Molars. See Teeth. Molecular layers, 337, 338 Moleschott's diet table, 616 Monaster stage of karyokinesis, 18 Monoplegia, 259 Moore's test for sugar, 550 Morphological development, 22 Mosso's ergograph, 153 Motor areas, 259, 262 Motor impulses, transmission of in cord, 247 nerve-fibres, 98 Motor nerves, 160 Motor oculi nerve, 216 origin of, ib. Mouth, 599 Movements of intestines, 682 protoplasm, 12, 106 Mucigen or Mucinogen, 631 Mucic acid, 551 Mucin, 36, 564 Mucous membrane, 627 basement membrane of, 628 epithehum-cells of, ib. See Epithe- lium. digestive tract, 628 gastro-pulmonary tract, ib. M. Macula, 274 lutea, 335 Male organs of generation, 756 Male sexual functions, 760 Malleus, 303 Malpighian bodies or corpuscles of kid- ney, 686. See Kidney. corpuscles of spleen, 492 Maltose, 553 Mammary glands, 619 evolution, 620 involution, 621 lactation, 620 structure, 619 Mandibular arch, 793 Manometer, Hurthle's, 420, 463 Marey's sphygmograph, 443 tambour, 129, 415 Mastication, 675 Mastoid cells, 302 Meat as food, 621 Meatus of ear, 306 Meconium, 672 Medulla oblongata, 209, 212 et seq. columns of, 213 decussation of fibres, 223 fibres of, how distributed, 213 grey matter in, 210 pyramids of, anterior, 213 posterior, 214 structure of, 221 Medullary groove, 774 plates, ib. Meibomian follicles, 628 Meissner's plexus, 608 Melanin, 366 Membrana capsulo-pupillaris, 815 decidua, 780 granulosa, 764 development of into corpus luteum, ib. hyaloidea, 341 limitans externa, 338 interna, 336 propria or basement membrane. See Basement Membrane. pupillaris, 815 tectoria, 308 action of, 312 tympani, 301, 302 Membrane of the brain and spinal cord, 184 vitelline, 765 Membranes, mucous. See Mucous Membranes. Membranes, serous. See Serous Mem- branes. Membranous labyrinth, 305. See Ear. Menstruation, 764, 767 coincident with discharge of ova, 767 corpus luteum of, 764 842 INDEX. MUCOUS MEMBRANE. Mucous membrane-continued. genito-urinary tract, 628 gland-cells of, iJ. of intestines, 608, 613 of stomach, 603 of uterus, changes of in pregnancy, 767 respiratory tract, 628 Mucus deposited from urine, 716 Muller's fibres, 336 Mullerian duct, 821 Murexide test, 711 Multipolar nerve-cells, 187 Muscle, in blood-vessels of, 93 cardiac, 93 changes in form, when it contracts, 112 et seq. chemical composition of, 155 clot, 155 columns, 87 contractility, 107 curves, 119, 122, 124 development, 95 dynamometer, 137 elasticity, 131 electrical phenomena of, 139, 180 extensibility of, 131 et seq. curves of, 133, 134 fatigue, effect of, 124, 153 curves, 124 Hensen's line, 89 involuntary, 84 (see 157 et seq.) irritability of, 108 nerves of, 93 plain, 94 plasma, 155 red, 93 response to stimuli, 109 et seq. rigor, 154, 159 sarcolemma, 86 serum, 155 shape, changes in, 128 skeletal, 85 sound, developed in contraction of, 128 spindle, 96 stimuli, 109 striated, structure of, 86 et seq. tetanus, 127 tonus, 136, 159 twitch, 122 voluntary, 85 (see 157 et seq.) wave, 125 work of, 136 Muscular action, conditions of, 138 Muscular contraction, ill, 123 effect of two successive stimuli, 126 of more than two stimuli, 127 voluntary tetanus, 128 Muscular fibres, development, 95 plain, 84 transversely striated, ib. Nerves. Muscular force, 136 irritability, 108 sense, 290 . tissue, 84 et seq. Muscularis mucosae, 507, 601, 603, 610 Musical sounds, 323 Myeloplaxes, 59 Myelospongium, 806 Myograph, 118 pendulum, 120 spring, ib. transmission, 129 Myopia, or short-sight, 351 Myosin, 155 Myosinogen, 156 N. Nails, 726 Nasal cavities in relation to smell, 297 et seq. Nasmyth's membrane, 74 Near point, 349 Nerve-cells, varieties of, 187 Nerve-centres, 183 et seq. See Cere- bellum, Cerebrum, tec. ano-spinal, 684 cardio-inhibitory, 534 cilio-spinal, 255 defaecation, 684 deglutition, 677 erection, 255 micturition, ib. parturition, ib. respiratory, 524 secretion of saliva, 633 speech, 265 vaso-motor, 255, 468 Nerve-corpuscles, 187 et seq. bipolar, 187 polar, ib. Nerve epithelium, 279 Nerves, 97 accelerator, 160 action of stimuli on, 109, 702 afferent, 97, r6i axis-cylinder of, 99 cells, 98, 187 centrifugal, 160 centripetal, 161 cerebro-spinal, 240 changes in, during activity, 168 classification, 160 cranial. See Cerebral Nerves, degeneration of, 163 reaction, 181 direction of a nerve impulse, 169 efferent, 97, 160 electrical, 161 stimulation of, 180 fibres, 98 development of, 104 INDEX. 843 Nerves. N erves-con tinued. functions of, 163 funiculi of, 101 grey, 98 inhibitory, 161 intercentral, 163 intracardiac, 428 irritability of, 105 laws of conduction, 160 etseq. medullary sheath, 99 medullated, 98 motor, 160 termination of, 102 olfactory, 298 nodes of Ranvier, 99 non-medullated, 98 plexuses of, 102 secretory, 161 section of, 163 size of, 101 spinal. See Spinal Nerves, splanchnic, stimulation of, 524 stimulation of cut, 163,524 structure, 98 sympathetic, influence on heart, 425 taste, 294 terminations of, in corpuscles of Golgi, 286 in corpuscles of Grandry, ib. in corpuscles of Herbst, 283 in end-bulbs, ib. in motorial end-plates, 102 in networks or plexuses, 287 in Pacinian corpuscles, 282 in touch-corpuscles, 285 trophic, 161, 375 velocity of nerve-impulse, 168 Nervous force, velocity of, ib. Nervous system, cerebro-spinal, 184, 240 development, 806 et seq. sympathetic, 425 Nervous tissues, chemistry of, 170 Neural crest, 809 Neuroblasts, 807 Neuroglia, 186, 196 Neurokeratin, 99, 186 Nitric oxide hsemoglobin, 595 Nitrogen eliminated in the form of urea, 615 Nodal point, 342 Nodes of Ranvier, 99 Nose. See Smell. development of, 815 Notochord, 775, 789 Nuclear layers, 337, 338 sap or matrix, 10 Nucleic acid, 565 Nuclein, 11, 565, 618 Nuclei pontis, 225 Nucleo-proteids, 562 Nucleoli, 10 Ovum. Nucleus of animal cell, 6, 9 et seq. chemical composition, 11 division, 16 staining of, 10 structure, 10 Nucleus ambiguus, 218, 224 0. Odontoblasts, 78 Odontogen, 79 Odours, 300 (Esophagus, development, 816 structure of, 600 Oleaginous principles, 555 Oleic acid, ib. Olein, ib. Olfactory bulb, 299 cells, 298 nerves, 216, 298 tract, 299 Olivary body, 214, 224 Omphalo-mesenteric veins, 784, 795 Oncograph, Roy's, 478, 479 Oncometer, Roy's, 479, 494, 696 Ophthalmoscope, 357 et seq. Optic nerve, 216 decussation of fibres in, 370 development of, 813 Optic thalamus, 233 vesicle, primary, 809 Optical angle, 344 apparatus of eye, 342 Optogram, 365 Ora serrata of retina, 335 Organ of Corti, 307 of Giraldes, 824 Organic compounds in body, 549 Organised ferments, 566 Organs of sense, development of, 812 Osseous labyrinth, 304 Ossicles of the ear, 303 action of, 310 Ossification, 64 et seq. Osteoblasts, 64, 68 Osteoclasts, 68 Ovary, 762 development of, 766 Graafian vesicles in, 762 Oviduct, or Fallopian tube, 766 Ovigerms, 762 Ovum, 22, 768 action of seminal fluid on, 770 et seq. changes of, in ovary, 769 previous to fecundation, 771 cleaving of yolk, 771 fertilised, ib. formation of, 766 germinal vesicle and spot of, 765 et seq. 844 INDEX. Ovum. Ovum-continued. impregnation of, 770 segmentation, 771 structure of, 768 in mammals, 765 subsequent to cleavage, 771, et seq. unimpregnated, 768 Oxygen in the blood, 542, 548 Oxyhaemoglobin, 83, 388 et seq., 391 spectrum of, 594 P. Pacchionian bodies, 183 Pacinian corpuscles, 282 Pain, 279 (see 248, 267) Palmitic acid, 555 Palmitin, ib. Pancreas, 645 development of, 818 extirpation of, 652 (see 669) functions of, 652 secretory nerves of, 651 structure, 645 Pancreatic juice, 646 action on fats, 647 composition and action, 646 Papillae of the kidney, 689 of skin, distribution of, 726 of tongue, 293 Parachordal cartilages, 791 Paraglobulin, 575 Parapeptone, 643 Parietal cells, 639 mesoblast, 776 Parotid gland, 635 Paroxysmal haemoglobinuria, 723 Pars ciliaris retinae, 341 Par vagum. See Pneumogastric nerve. Pathological urine, 720 Pavy's views as to the liver being a sugar-forming organ, 668 Pawlow's method for obtaining pure gastric juice, 642, 651 Pelvis of the kidney, 686 Pendulum myograph, 120 Penis, 760 structure, ib. Pepsin, 638 Pepsinogen, 639 Pepsin-hydrochloric acid, 640 Peptones, 558, 643, 664 characters of, 644 Peptonuria, 721 Perception, 278 Perfusion cannula, Kronecker's, 433 Pericardium, 378 Perichondrium of cartilage, 54 Perilymph, or fluid of labyrinth of ear, 2 73, 304 Pressor nerves. Perimeter, 359 Perimysium, 85 Peristaltic movements of intestines, 682 of involuntary muscle, 157, 158 (see 677) of stomach, 678 Perivitelline fluid, 770 Permanent teeth. See Teeth. Perspiration, cutaneous, 731 insensible and sensible, fi. ordinary constituents of, 732 Pettenkofer's reaction, 662 Peyer's patches, 612 Pfliiger's law, 177, 182 Phagocytes, 450 Phakoscope, Helmholtz's, 348 Pharynx, 599 action of in swallowing, ib. development, 816 Phenyl hydrazine test, 553 Phloridzin-diabetes, 669 Phosphates in urine, 715, 719 Phrenograph, 520 Physiological methods, 3 Physiological rheoscope, 148, 158 Pia mater, 184 Pigment cells of retina, 106 Pineal gland, 503 Piotrowski's reaction, 559 Pituitary body, 503 development, 790 Placenta, maternal, 781, 782 foetal, 786 Plasma of blood, 82, 568, 573 gases of, 574 Plethysmograph, 478 Schafer's, 435 Pleura, 509 Pleuro-peritoneal cavity, 776 Plexus, terminal, 286 Pneumogastric nerve, 218 distribution of, 219 influence on action of heat, 424 gastric digestion, 681 respiration, 526 mixed function of, 219 origin, ib. Poggendorf's rheocord, 173 Pohl's commutator, 172 Polar globules, 769 function of, 770 Polarimeter, 563 Polysaccharides, 550 Pons Varolii, 209, 213 grey matter in, 210 Portal circulation, 387 Portal vein, 656. See Liver. Pregnancy, corpus luteinn of, 764 Presbyopia, 353 Pressor nerves, 474 INDEX. 845 Pressure. Pressure, sense of, 289 Pressure-measurers, 455 Primitive groove, 772 streak, ib. Primitive nerve-sheath, or Schwann's sheath, 98 Processus vaginalis, 825 Projection fibres, 240 Pronephros, 821 Pro-nucleus, female, 770 male, ib. Propeptone, 643 Prosencephalon, 212, 810 Prostate gland, 693 Proteids, 357 absorption of, 671 action on polarized light, 558 of blood, 575 classification, 560 coagulated, 562 colour reactions, 559 composition, 557 indiifusibility of, 558 precipitants of, 559 solubilities, 558 Proteoses, 558, 644 characters of, 6 [ -] Proto-albumose, 643 Protoplasm, 6, 8 chemical structure, 9 irritability of, 15 movements, 12, 106 Proto-vertebrae, 775, 776, 789 Pseudopodia, 13 Pseudoscope, 374 Pseudo-stomata, 394 Ptosis, 367 Ptyalin, 032 Ptyalinogen, 633 Pulmonary artery, 799 Pulse, arterial, 442 et seq. Purkinje's cells, 229 fibres, 95 figures, 356 Pyloric glands, 604, 638 Pyramidal tracts, 203 et seq. Pyramids of medulla oblongata, 213, 214 R. Racemose glands, 628 Ranke's metabolic balance-sheet, 739 Reduced eye, 343 Reflex arc, 251 Reflex actions, 247 inhibition of, 250 Retina. Reflex actions-continued. in frog, 249, 256 in man, 251 superficial, ib. tendon, 252 of nerves, 162, 191 of spinal cord, 249 et seq. Refractive media of eye, 342 Refraction, laws of, ib. Regions of body. See Frontispiece. Remak, fibres of, 102 Remak's ganglion, 429 Renal circulation, 387 epithelium, activity of, 697 Rennet, 618 Reproductive organs, 756 et seq. Requisites of diet, 739 Reserve air, 521 Residual air, ib. Respiration, 504 abdominal type, 516 alteration in atmospheric pressure, 539 breathing or tidal air, 521 chemistry of, 339 effect on circulation, 529 gases in relation to, 537, 541 influence of nervous system, 534 mechanism of, 513 et seq. nervous, 524 movements, 513 of vocal cords in, 321 quantity of air changed, 321 Respiratory acts, special, 528 apparatus, 505 development of, 819 capacity of chest, 521 circumstances affecting, 522 movements of glottis, 520 methods of recording, 517 rate, 522 relation to pulse rate, ib. size of animal, ib. relation to will, 524 et seq. muscles, 513 et seq. nerve-centre, 524 rhythm, 320 sounds, ib. Restiform bodies, 224, 228 Rete testis, 758 Reticulum, 8 Retiform tissue, 50 Retina, 335 blind spot, 355 blood-vessels, 341 duration of impression on, 356 of after-sensations, ib. excitation of, 333 focal distance of, 346 fovea centralis, 333 functions of, 355 image on, how formed distinctly 344 846 INDEX. Retina. Retin a-continued. layers, 336 ora serrata, 335 pigment-cells, 106, 107 pigments of, 365 in relation to single vision, 367 structure of, 335 visual purple, 365 Rheocord, 172 Poggendorf 's, 173 Rheoscope, physiological, 148, 158 Rheoscopic frog, 148 Rheotome, 145 Rhythmicality of movement, 107 Rigor mortis, 154 affects all classes of muscles, 154, 159 phenomena and causes of, 159 Ritter's tetanus, 179 Rods and cones, 338, 339 Rolandic area, 262 injury of, 268 Roy's oncograph, 478, 479 oncometer, 479, 494 tonometer, 434 Rumination, 675 S. Saccharic acid, 551 Saccharoses, 552 Saccule, 306 Saliva, 630 action of, 637 composition, 636 process of secretion, ii. reflex secretion, 636 secretion following stimulation of nerves, 634 Salivary glands, 630 development of, 818 extirpation of, 636 influence of nervous system, 634 secretory nerves of, 633 effect of section of, ib. structure, 630 Sanderson's cardiograph, 414 Sanson's images, 347 Saponification, Sarcolemma, 86 Sarcomeres, 89 Sarcoplasm, 87 Sarcostyles, ib. Schafer's heart plethysinograph, 435 Schemer's experiment, 349 Schematic eye, 343 Sclerotic, 328 development of, 814 Sebaceous glands, 729 Secreting glands, 626 et seq. classification of, 628 Secreting membranes. (See Mucous and Serous membranes. Snoring. Secretory nerves, 161 of salivary glands, 633 effect of section of, ib. Segmentation of cells, 771 in chick, 773 ovum, 771 Semen, 760 filaments or spermatozoa, ib. Semicircular canals of ear, 273 development of, 815 Semilunar valves. See Heart valves. Seminiferous tubules, 758 Sensation, 277 et seq. conception, 278 homologous stimuli, 281 nerves of, 161 of pain, 279 perception, 278 subjective, 281 tactile, 279 Sensori-motor area, 267 Sensory areas in cerebral cortex, 259 Sensory impressions, conduction of by spinal cord, 247 in brain, 266, 267 Serous membranes, 627 Serum, albumin, C60, S7S of blood, 570, 573 globulin, 575 Seventh cerebral nerve, 217 Sexual organs in the female, 762 in the male, 756 Sighing, mechanism of, 529 Sight. See Vision. Simple tubular glands, 628 Sinuses of Valsalva, 385 Sinus uro-genitalis, 827 Sixth cerebral nerve, 216 Skeleton. See Frontispiece. Skin, 724 absorption by, 730 dermis, 725 epidermis of, 724 functions of, 730 papillae of, 726 respiration, 730 rete mucosum of, 724 sebaceous glands of, 729 secretions, 731 sensory nerves of, 524 sweat, 731 sweat-glands, 729 varnishing the, 734 Small intestine, 605 et seq. Smell, sense of, 297 anatomy of regions, 297 delicacy of sense of, 300 tests for, 300 varies in different animals, 297 Smith's perimeter, 361 Sneezing, mechanism of, 528 Snoring, mechanism of, 529 INDEX. 847 Soap. Soap, 556 Sobbing, 529 Solitary glands. See Peyer's. Somatopleur, 776 Sonorous vibrations, how communicated in ear, 309 et seq, in air and in water, ib. See Sound. Sound, conduction of by ear, 309 heart, 410 production of, 323 Spaces of Fontana, 334 Speaking, mechanism of, 325 Special senses, 282 et seq. Spectroscope, 592 et seq. Speech, 314, 325 defects of, 326 Speech centre, 265 Spermatoblasts, 759 Spermatogenic cells, ib. Spermatozoa, 760 form and structure of, iA Spherical aberration, 351 correction of, 332 Sphincter ani. See Defaecation. pupillee, 333 Sphygmographs, 443, 444 tracings, 445 et seq. Sphygmoscope, Anderson Stuart's, 457, 458 Spinal accessory nerve, 218 functions of, 219 origin, ib. Spinal cord, 194 canal of, ib. centres in, 255 a collection of nervous centres, ib. columns of, 195 commissures of, ib. conduction of impressions by, 247 et seq. course of fibres in, 200 development of, 806 fissures and furrows of, 195 functions of, 247 et seq. of columns, 204 hemisection, 205, 247 injuries of, 247, 251 membranes of, 184 morbid irritability of, 253 nerves of, 200 reflex action of, 249 et seq. inhibition of, 250 in frog, 249, 256 in man, 251 superficial, ib. regions of, 206 special centres in, 255 structure of, 194 et seq. tracts, 195, 203, 247 white matter, 183, 196 tracts in, 198 grey matter, 185, 196 cells in, 197 Substantia nigra. Spinal nerves, 165 origin of, 165 et seq. functions of roots of, 165 physiology of, 166 Spindle-shaped cells, 646 Spirem, 18 Spirometer, 521 Splanchnopleur, 776 Spleen, 490 development, 819 functions, 492 Malpighian corpuscles of, 492 pulp, 490 structure of, 490 trabeculae of, ib. Spongioblasts, 337, 806 Spongioplasm, 9 Spot, germinal, 766 Stannius' experiment, 430 Stapedius muscle, development of, 794 Stapes, 303 development of, 794 Starch, 553 Starvation, 741 eifects of, 742 Steapsin, 647 Stearic acid, 555 Stearin, ib. Stercobilin, 663, 702 Stethographs, 518, 519 Stimulants as accessories to food, 625 Stimuli, varieties of, 109 St. Martin, Alexis, case of, 679 Stomach, 601 blood-vessels, 605 development, 816 digestion in, 642 glands, 603 lymphatics, 604 movements, 678 influence of nervous system on, 680 mucous membrane, 603 muscular coat, 602 nerves, 605 peritoneal coat, 602 secretion of. See Gastric juice, submucous coat, 603 structure, 602 Stomata, 25 Stratum granulosum, 725 . intermedium of Hannover, 80 lucidum, ib. Striated muscle. See Muscle. Stroma, 762 Stromuhr, Ludwig's, 437 Structure of cells, 9 Stuart's sphygmoscope, 457, 458 Submaxillary gland of dog, 635 Submaxillary and sublingual glands, 633 Substantia gelatinosa of Rolando, 222, 223 Substantia nigra, 227 848 INDEX. Subthalamic area. Subthalamic area, 235 Succus entericus, 648, 684 functions of, 648 Sugar. See Dextrose. Sulphates in urine, 715 Superior laryngeal nerve, effects of stimulation of cut, 524 Superior olivary nucleus, 225 Suprarenal capsules, 500 development of, 827 function of, 502 structure, 500 Sustentacular cells, 758 Swallowing, 675 centre, 677 fluids, 677 nerves engaged, 676 Sweat glands. See Skin. Synovial fluid, secretion of, 627 membranes, 627 Syntonin, $61 Systemic circulation. See Circulation. Systole of heart, 406 T. Tables of diet, 739 et seq. Tactile end organs, 282 sensibility, 266, 279 variations in, 288 Taste, sense of, 291 classification of, 296 connection with smell, 291 delicacy of, 297 nerves of, 294 Taste-buds, ib. Taurine, 662 Taurocholic acid, ib. Teeth, 70 development, 77 eruption, times of, 71 structure of, 72 et seq. temporary and permanent, 70 et seq. Tegmentum, 226 Temperature, 749 average of body, 750 changes of, effects of, 750 et seq. circumstances modifying, 754 of cold-blooded and warm-blooded animals, 750 in disease, io. loss of, 754 maintenance of, 750 of Mammalia, birds, etc., ib. regulation of, 754 se(i- sensation of variation of, 290. See Heat. Tendon-reflexes, 252 Tension, arterial, 537 Tubes, Fallopian. Tensor tympani muscle, 303 action of, 311 Testicle, 756 development, 825 descent of, ib. structure of, 756 et seq. Tetanus, 127 Ritter's, 179 voluntary, 128, 158 Thalamencephalon, 212, 810 Thalami optici, 233 Theine, 625 Theobromine, ib. Thoma-Zeiss hemacytometer, 583 Thoracic duct. 398 Thymus gland, 495 development, 819 function of, 498 structure, 495 Thyro-arytenoid muscles, 317 Thyroid cartilage, 314 Thyroid gland, 498 development of, 819 function of, 499 structure, ib. Timbre of voice, 323 Tissue-respiration, 539, 545 Tongue, 291 action in deglutition, 675 epithelium of, 295 papillse of, 293 parts most sensitive to taste, 294, 295 structure of, 291 Tonometer, Roy's, 434 Tonsils, 599 Tonus, 136, 253 Torsion, 790 Touch, 282 et seq. muscular sense, 290 sense of locality, 287 of pressure, 289 of temperature, 200 tactile end organs, 282 Touch-corpuscles, 285 Trabeculae cranii, 792 Trachea, 305 Tract of Gowers, 205 of Lissauer, 205 Tracts in the spinal cord, 203, 247 . Transmission myograph, 129 Traube-Hering's curves, 473, 533, 696 Tricuspid valve, 383 Trigeminal nerve, 217 function, ib. origin of, ib. Trochlear nerve, 216 origin of, ib. Trommer's test, 550 Trophic nerves, influence of, 375 Trypsin, action of, 647 Tubercle of Rolando, 222 Tubes, Fallopian. See Fallopian tubes. INDEX. 849 Tubuli seminiferi. Tubuli seminiferi, 758 uniferi, 686 et seq. Tubulo-racemose or tubulo - acinous glands, 628 Tunica albuginea of testicle, 756 dartos, 94 propria, 273 vaginalis, 776 vasculosa, 332 Tympanum or middle ear, 302 development of, 815 membrane of, 302 structure of, ib. Tyrosin, 651 U. Umbilical arteries, 788, 800 cord, 782, 788 vesicle, 778, 782 Unicellular organisms, 6 Unipolar nerve cells, 187 Unorganised ferments, 566 Urachus, 787 Uraemia, 733 Urate of sodium, 718 Urea, 704 apparatus for estimating quantity, 705, 706 chemical composition of, 704 formation of, 707 identical with cyanide of ammonium, 704 quantity, 706 Ureters, 692 Urethra, 693 Uric acid, 7IO condition in which it exists in urine, 712 deposit of, 717 forms in which it is deposited, 710, 717 origin of, 712 presence in the spleen, 493 proportionate quantity of, 712 tests, 711 variations in quantity, 712 Urina potus, 703 Urinary apparatus, 685 et seq. Urinary bladder, 692 development, 827 nerves, 693 structure, 692 Urinary deposits, 717 et seq. Urine, 702 analysis of, 702 bile in, 723 blood in, ib. Vaso-motor nerves. U rin e-con tinued. chemical composition, 703 sediments in, 720 colour, 702 cystin in, 718 effect of blood-pressure on, 694 expulsion, 700 flow into bladder, ib. hippuric acid in, 712 inorganic constituents, 714 mineral salts in, ib. mucus in, 716 pathological, 720 physical characters 702 pigments, ib. pus in, 723 quantity, 702 reaction of, 703 in different animals, ib. made alkaline by diet, ib saline matter, 714 solids, 702 specific gravity of, 703 variations of, ib. sugar in, 721 el seq. tests for estimating, 722 tests for inorganic salts of, 717 urates, 718 urea, 704 uric acid in, 710 Urobilin, 663, 702 Urochrome, 703 Uterine milk, 783 Uterus, 766 change of mucous membrane of, 767 et seq. development of in pregnancy, ib. follicular glands of, ib. structure, ib. Utricle, 306 V. Vagina, development of, 825 Vagus nerve. See Pneumogastric. Vagus pneumonia, 377 Valves of heart, 383. See Heart valves. Valvulae conniventes, 608 Vas deferens, 757, 760 Vasa efferentia of testicle, 758, 760 Vasavasorum, 390 Vascular system in asphyxia, 535 development of, 795 Vaso-constrictor nerves, 469 Vaso-dilatator nerves, 471 Vaso-motor nerves, distribution of, 469 effect of section, 468 et seq. experiments on, 476 influence upon blood-pressure, 473 850 INDEX. Vaso-motor nerve-centre. Vaso-motor nerve-centre, 534 nervous system, 467 et seq. reflex action, 474 Vegetables as food, 615, 623, 625 Vegetable cells, 6 protoplasmic movement in, 13, 14 Veins, 390 cardinal, 802 circulation in, 450 et seq. velocity of, 439 collateral circulation in, 390 development, 801 distribution, 390 pressure in, 462 rhythmical action in, 451 structure of, 391 umbilical, 788 valves of, 391 velocity of blood in, 439 Velocity of blood in arteries, 437 in capillaries, 438 in veins, 439 of circulation, ib. of nervous force, 168 Venae advehentes, 801 revehentes, ib. Ventilation, 543 Ventral cerebellar tract, 205 Ventricles of heart. See Heart. Ventricular diastole, 407 systole, 406, 407 Ventriloquism, 324 Veratrine, effect of, on muscular con- traction, 125 Vermicular movement of intestines, 682 Vertebrae, development of, 789 Vesicle, germinal, 766 Vesiculae seminales, 760 structure, ib. Vestigial fold of Marshall, 803 Vibrations, conveyance of to auditory nerve, 309 et seq. Vierordt's hsematochometer, 438 Villi in chorion, 787 function of, 788 of intestines, 608 Visceral clefts and arches, development of, 792 connection with cranial nerves, 795 Visceral mesoblast, 776 Visceral plates, 778 Vision, ib. angle of, 344 at different distances, adaptation of eye to, 346 et seq. corpora quadrigemina, the principal nerve-centres of, 227 correction of aberration, 352 et seq. of inversion of image, 372 defects of, 351 et seq. distinctness of, how secured, 374 et seq. Work of heart. Vision-continued. duration of sensation in, 356 estimation of the form of objects, 373. of their size, 372 focal distance of, 346 impaired by lesion of fifth nerve, 376 single, with two eyes, 367 et seq.. Visual area, 266 judgments, 372 Visual purple, 339, 365 Vitellin, 621 Vitelline duct, 816 membrane, 765 spheres, ib. Vitreous humour, 328, 341 Vocal cords, 314, 320 action of in respiratory actions, 321 approximation of, effect on height of note, ib. vibrations of, cause voice, 321, 323 Voice, 314, 323 range of, 324 Volkmann's haemadromometer, 438 Voluntary muscle, 85 nerves of, 93 Voluntary tetanus, 128 Vomiting, 681 action of stomach in, ib. centre, 682 nerve actions in, ib. voluntary and acquired, ib. Vowels and consonants, 325 W. Wallerian degeneration method, 164, 167, 199 Waller's apparatus for gas analysis, 547, 548 Water hammer pulse, 442 Wave of blood causing the pulse, ib. velocity of, ib. Weber's paradox, 135 W eight, influence on capacity of respi- ration, 522 Whey proteid, 618 White corpuscles. See Blood-corpuscles, white; and Lymph-corpuscles. White fibro-cartilage. See Fibro-car- tilage, white. fibrous tissue, 44 Wolffian bodies, 820 et sea. duct, 821 Work of heart, 422 INDEX. 851 X. Xanthine, 493, 497 Xantho-proteic reaction, 559 Y. Yawning, 529 Yellow elastic fibre, 46 fibro-cartilage, 56 spot of Sommering, 335 Zymogen. Yolk-sac, 782, 783 et seq. Yolk-spherules, 765 Young-Helmholtz theory, 363 Z. Zona pellucida, 765 Zonule of Zinn, 341 Zymogen, 632, 639 THE END. BRADBURY, AGNEW, & CO. T.D., PRINTERS, WIHTEFRIARS AND TONBRIDGE.