tntlieCttpotmetofotk ^°t^ ^ Digitized by tine Internet Archive in 2010 witii funding from Open Knowledge Commons (for the Medical Heritage Library project) http://www.archive.org/details/textbookofanimalOOmill "'""■ti'.<.'<:m.m A TEXT-BOOK OF ANIMAL PHYSIOLOG-T WITH INTRODUCTORY CHAPTERS ON GENERAL BIOLOOY AND A FULL TREATMENT OF REPRODUCTION FOR STUDENTS OF HUMAN AND COMPARATIVE (VETERINARY) MEDICINE AND OF GENERAL BIOLOGY UY WESLEY MILLS M. A., M. D., L. R. C. P. (ENG.) FBOFESSOR OF PHYSIOLOGY IN' MC GILL UNIVERSITY AND THE VETEniNARY COLLEGE MONTREAL WITH OVER FIVE HUNDRED ILLUSTRATIONS NEW Y O R K D . A P P L E T O N AND COMPANY LONDON: CAXTON HOUSE. PATERNOSTER SQUARE 1889 /Vl (9 2 Copyright, 1889, By D. APPLETON AND COMPANY. ®o tl}c iUcmorn of ROBERT PALMER HOWARD, M. D., LL. D., LATE DEAN AMI) PROFESSOR OF MEDICINE IN MC GILL UNIVERSITY, WHOSE TEACHING AND PRACTICE EVER TENDED TOWARDS THE RECOGNITION OF THE IMPORTANCE OF PHYSIOLOGY TO MEDICINE, AND WHOSE LIFE ILLUSTRATED WHAT IS LOFTY AND NOBLE IN HUMAN EXISTENCE, THIS WORK IS DEDICATED IN REVERENCE AND GRATITUDE. PREFACE The comparative method, the introduction of the teach- ings of embryology and of the welding principles of evolution as part of the essential structure of zoology, may be said to have completely revolutionized that science ; and there is scarcely a text-book treating of the subject, however element- ary, which has not been molded in accordance with these guiding lines of thought. So far as I am aware, this can not be said of a single book on the subject of physiology. Feeling, therefore, that the time had come for the appearance of a work which should attempt to do, in some degree at least, for physiology what has been so well done for morphol- ogy, the present task was undertaken. But there were other changes which it seemed desirable to make. I tliink any one who will examine the methods and reasoning of the physi- ology of the day will not fail, on close scrutiny, to recognize a tendency to speak of certain conclusions, for various organs (and functions), as though they applied to these organs in whatever group of animals found, or, at all events, for man, no matter what the species of the animal that liad been ex- perimented upon. For some years I have, in publications of my own original researches, strongly protested against such methods as illogical. I am wholly at a loss to understand how a work, built upon the most fragmentary and hetero- geneous evidence, derived from experiments on a few grf)ups of animals, or a certain amount of human (clinical or patho- logical evidence, can be fittingly termed a treatise on " human j>hy..siology." It will scarcely bo denied that conclusions such VI ANIMAL PHYSIOLOGY. as this method implies would not be tolerated in the sub'^ect of morphology. While in the present work what is strictly applicable to other animals and to man has not always been kept apart, an effort has been made throughout to be cautious in all the conclusions drawn — a state of mind warranted by the past history and the present tendencies of physiology. Until our laboratory methods become more perfected, the comparative method more extensively applied, and conclusions drawn from "experiments" modified by comparison with the results of clinical, pathological, and all other available sources of infor- mation, I feel convinced that we are called upon to teach cautiously and modestly. Treating, as we do in our books, each subject in a separate chapter, there is, as I know by observation, the greatest danger that the student may get the idea that each function of the body is discharged very much independently ; accordingly, there has been throughout a most persistent effort made to impress the necessity for ever remembering the absolute de- pendence of all parts. Unless this be thoroughly infused into a student, it is impossible that he can ever understand the wide world of natural objects, or the narrower one of un- natural (in a sense) organisms, as seen in the hospital ward. Recognizing how important it is to teach the young stu- dent to become an observer and an investigator in spirit and in some degree in fact, only such treatment of elaborate methods has been introduced as will enable him to form a general acquaintance with the modes in which laboratory work is carried on, while simple ways of verifying the essen- tial truths of physiology have been constantly brought before, him. As to how far these are actually carried out will de- pend not a little on the teacher. The student who learns thus to observe and to verify will not fail to apply the method in his future career, whatever that may be — whether medical or other — nor is he so likely to throw his physiology overboard as a useless cargo as soon as his primary examination has been passed. PREFACE. vii By frequently calling attention, as has been done tlirongli- out, to actually discovered or possible differences in function for different groups of animals, it is believed that the student will become possessed of a spirit of caution in drawing con- clusions that will fit him the better for the hospital ward in another respect, viz., that he will be prepared for those indi- vidual differences actually existing, and which seem to have been largely ignored in so many works on physiology, with the natural consequence that the student, not finding his physiology squaring with the facts of the clinique, and not being prepared for the situation, the result is disappointment and disgust, instead of the actual continuation of the study, especially as human physiology. With a view of widening the student's field of vision, sec- tions, under the heading " Special Considerations," have been introduced, which it is hoped will not fail to interest and stimulate. Most teachers of experience will welcome the summary with which each chapter concludes. In connection with no subject perhaps can the art of generalizing be better taught than with physiology, and to this end these brief synoptical sections will, it is thought, prove helpful. Systematic instruction in either macroscopic or microscopic anatomy has not been undertaken — in fact, can not be at- tempted, it is believed, except at the expense of physiology proper — in a work of moderate compass. At the same time attention has been called to those points which have a special bearing on each function, and a number of illustrations have been inserted with this object in view. The introduction of the subject of development at so early a stage is a departure that calls for a word of explanation. An attempt has been made to use embryological facts to throw light upon the different functions of the body, and especially their relations and interdependence. It therefore became necessary to treat the subject early. It is expected, however, that the student will return to it after reading the remaining chapters of the woik. yiii ANIMAL PHYSIOLOGY. As so large a proportion of those who enter upon the study of medicine begin their career without any adequate preparation in general biology, the subject, as presented in this work, will, let me hope, meet an actual need, and prove helpful in attaining a broad and sound view of the special doctrines of biology. It is scarcely necessary to remark that clinical and path- ological facts have not been introduced with the view of teaching either clinical medicine or pathology, but to indi- cate to the student how his physiology bears on his profes- sion, and how the above-mentioned subjects throw light upon physiology proper and lend interest to that subject. My aim has been to make the book, from first to last, educative ; and, retaining a vivid recollection of the severe strain put upon the memory of the medical student by our present method of crowding so much into at most four years of study, an attempt has been made to avoid overloading the book with mere facts or technical details, as well as to pre- sent the whole subject in as succinct a form as is compatible with clearness. Recognizing, too, the very shifting character of physiological theories, the latter have generally been pretty well kept apart from the actual facts. It is hoped that the abundance of the illustrations will prove more acceptable than would lengthy treatment of sub- jects in the text, for, if the matter of a book is to be digested and assimilated, either by the student of general biology or by the hard-worked medical student, it must not be bulky. The illustrations have been chosen from the best available sources, and the authorship of each one duly acknowledged in the body of the work. Several original diagrams, such as I find exceedingly useful in my own lectures, have been in- troduced. This book is really an embodiment of my own course of lectures, as given during the past two years more especially, and with the highest satisfaction, I think it may be said, to both students an.d teacher. I have unbounded confidence in the plan of the work, and PREFACE. , ix I ti'ust that its newness may excuse, to some degree, any shortcomings in the execution. Such a book has become a necessity to myself, and it is hoped will be welcomed by others. I trust the work may prove suitable, not only for the student of luiman medicine, but for the increasing num- ber of students of comparative or veterinary medicine, who may desire a broad basis for the study of disease in the various animals they are called upon to treat. I have en- deavored to make the work specially acceptable to the stu- dent of general biology. It only remains for me to crave the indulgence of all readers, and to thank my publishers, Messrs. D. Appleton & Co., for their uniform courtesy and the great pains they have taken to present the work in worthy form. Wesley Mills. Physiological Laboratory, McGill University', Montreal, September, 18S9. CONTENTS. PAGE General Biology 1 Introduction 1 Tabular statement of the subdivisions of Biology .... 4 The Cell 5 Animal and vegetable cells 5 Structure of cells 5 Cell-contents 7 The nucleus 7 Tissues 8 Summary 8 Unicellular Or(;anisms (Vegetable) 9 1. Yeast 9 Morphological 9 Chemical 10 Physiological 10 Conclusions 10 2. Protococcus 11 Morphological 11 Physiological 11 Conclusions 13 Unicellular Animals 13 The proteus animalcule 13 Morphological 13 Physiological 13 Conclusions 14 Parasitic Organisms 15 Fungi 15 Miicor niucodi) 1<> The Bacteria 18 Unkellular Animals with Differentiation of Structure ... 20 The Ijcil-animalcule 30 Structure -0 P^unctions 31 Multicellular Organisms 33 The fresh-water polyps , 33 The (Jell reconsioerko 36 The Animal Body — an epitomized account of the fmictions of u mainrnal . 27 I ^ii ANIMAL PHYSIOLOGY. PAGE Living and Lifeless Matter— General explanation and comparison of their properties °l Classification of the Animal Kingdom . 33 Tabular statement 35 Man's place in the animal kingdom 35 The Law op Periodicity or Rhythm in Nature— Explanations and illus- trations 36 The Law of Habit 40 Its foundation 40 Instincts '■ . • • • .41 The Origin of the Forms of Life 41 Arguments from : Morphology . . . ' 43 Embryology • .43 Mimicry 43 Rudimentary organs 43 Geographical distribution 45 Paleontology 45 Fossil and existing species . . . , 45 Progression 46 Domesticated animals . . 46 Summary 47 Reproduction . . .50 General 50 The ovum 54 The origin and development of the ovum 57 Changes in the ovum itself 59 The male cell .60 The origin of the spermatozoon 61 Fertilization of the ovum 63 Segmentation and subsequent changes . 63 The gastrula 66 The hen's egg 67 The origin of the fowl's egg • . . .68 Embryonic membranes of birds . . 72 The fcetal (embryonic) membranes of mammals 76 The placenta 80 The discoidal placenta 81 The metadiscoidal placenta 81 The zonary placenta 86 The diffuse placenta 86 The polycotyledonary placenta . .86 Microscopic structure of the placenta 87 Illustrations 87 Evolution 89 Summary 89 The Development of the Embryo Itself 90 Germ-layers 92 Origin of the vascular system .97 The growth of the embryo 102 CONTENTS. XUl ASONING Development of the Vascular System ix Vertebrates The later stages of the foetal circulation Development of the Urogenital System . The Physiological Aspects of Development Menstruation and ovulation .... The nutrition of the ovum .... The fffital circulation .... Parturition Changes in the circulation at birth Sexual coitus Organic Evolution reconsidered . Different theories criticised — new views The Chemical Constitution of the Animal Body Proximate principles . . . . , General characters of protcids Certain non-crystalline bodies The fats Peculiar fats Carbohydrates Nitrogenous metabolites . . . , Non-nitrogenous metabolites . Physiological Research and Physiological Re. The Blood Comparative • Corpuscles History of the blood-cells .... Chemical composition of the blood Composition of serum .... Composition of the corpuscles The quantity and distribution of the blood The coagulation of the blood . Clinical and pathological Summary The Contractile Tissues .... General Comparative Ciliary mr)vements The irritability of muscle and nerve Applications of the Graphic Method to the Physiology Chronographs and various kinds of apparatus A single muscular contraction Tetanic contraction The muscle-tone The strength of the stimulus. The changes in a muscle during contraction The elasticity of muscle .... The eloctrical phenomena of muscle Chemical changes in muscle . Thermal changes in tlie contracting muscle . Study of Muscle 171 PAGE 103 103 106 113 113 115 118 120 120 131 127 137 135 137 138 138 139 140 140 140 141 141 147 148 149 151 154 155 155 156 157 163 165 166 166 167 168 169 171 -174 178 183 184 185 186 187 188 193 195 XIV ANIMAL PHYSIOLOGY. PAaE The physiology of nerve ' 196 Eleetrotonus • 196 Pathological and clinical 198 Law of contraction 198 Electrical organs 199 Muscular work 199 Circumstances influencing the character of muscular and nervous activity 200 The influence of blood-supply and fatigue 200 Separation of muscle from the central nervous system .... 202 The influence of temperature 202 The intimate nature of muscular and nervous action . . . . 203 Unstriped muscle 204 General 204 Comparative 205 Special considerations 205 Functional variations 207 Summary of the physiology of muscle and nerve 208 The Nervous System — General Considerations 210 Experimental 212 Automatism 214 Conclusions 214 Nervous inhibition 215 The Circulation of the Blood . 216 General 216 The mammalian heart 217 Circulation in the mammal 221 The action of the mammalian heart 223 The velocity of the blood and blood-pressure 224 General 224 Comparative ■ 225 The circulation under the microscope 226 The characters of the blood-flow 227 Blood-pressure 228 The Heart '. . 232 The cardiac movements 232 The impulse of the heart . 233 Investigation of the heart-beat from within 234 The cardiac sounds 235 Causes of the sounds 236 Endo-cardiac pressures 238 The worlf of the heart ... 241 Variations in the cardiac pulsation 242 The pulse 244 Features of an arterial pulse-tracing 247 Venous pulse 251 Pathological 251 Comparative 251 The beat of the heart and its modifications 261 The nervous system in relation to the heart 261 Influence of the vagus nerve on the heart 265 CONTEXTS. XV PAGE Conclusions . . ' 269 The accelerator nerves of the heart 270 Human physiology 273 The heart in relation to blood-pressure 274 The influence of the quantity of blood 275 Conclusions 277 The capillaries 281 Special considerations 282 Pathological 282 Personal observations 283 Comparative 283 Evolution 285 Summary of the physiology of the circulation 286 DiGESTiox OF Food 290 Foodstufls. milk, etc 290 Embryologieal 295 Comparative 296 The digestive juices 306 Saliva and its action 306 Secretion of the different glands 307 Comparative 308 Pathological 308 Gastric juice 308 Bile . 311 General 311 Pigments 312 Digestive action 313 Comparative 314 Pancreatic secretion 314 Succus entericus 317 Comparative 319 Secretion as a physiological process 319 Secretion of the salivary glands 319 Secretion by the stomach 323 The secretion of bile and pancreatic juice 323 The nature of the act of secretion 326 Self-digestion of the digestive organs 329 Comparative 330 The movements of the digestive organs 331 Deglutition ' - . 332 CVtmparative 335 The movements of the stomach 335 Comparative 336 Pathological 336 The intestinal movements 337 Defecation 337 Vomiting 338 Com[)arative 339 Pathological 339 The removal of digestive jjroducts from the alimentary canal . . . 341 B ^^ ANIMAL PHYSIOLOGY. PAGE Lymph and chyle ^'*^ The movements of the lymph— comparative . . . . .342 Pathological ^^^ Fseces 353 Pathological ^^4 The changes produced in the food in the alimentary canal . ... 355 General ^^^ Comparative 357 Pathological 357 Special considerations 358 Various 358 Human physiology 363 Evolution 363 Summary • 364 The Respiratory System 365 General 365 Anatomical 368 The entrance and exit of air 369 The muscles of respiration 372 Types of respiration 373 Personal observation 374 Comparative . 375 The quantity of air respired 378 The respiratory rhythm 379 General 379 Pathological 379 Respiratory sounds 381 Comparison of the inspired and the expired air 381 Respiration in the blood 383 Haemoglobin and its derivatives 385 General 385 Blood-spectra 387 Comparative 389 The nitrogen and the carbon dioxide of the blood .... 389 Foreign gases and respiration 392 Respiration in the tissues 392 The nervous system in relation to respiration 393 Nerves and centers concerned 395 The influence of the condition of the blood on respiration . . . 397 The Cheyne-Stokes respiration 398 The effects of variations on the atmospheric pressure 399 The influence of respiration on the circulation 400 General 400 Comparative 402 The respiration and circulation in asphyxia 404 Pathological ' 406 Peculiar respiratory movements 406 Coughing, laughing, etc 406 Comparative 407 Special considerations 408 CONTENTS. ' xvii PAGE Pathological and chemical 408 Personal observation 408 Evolution 409 Summary of the physiology of respiration 410 Protective and Excretory Functions op the Skin 412 General 412 Comparative 413 The excretory function of the skin 415 Normal sweat 415 Pathological 415 Comparative — Respiration by the skin 415 Death from suppression of the functions of the skin .... 416 The excretion of perspiration 416 Experimental 416 Human physiology 417 Absorption by the skin 418 Comparative 418 Summary 418 Excretion by the Kidney 419 Anatomical 419 Comparative 419 Urine considered physically and chemically 422 Specific gravity 422 Color 423 Reaction 423 Quantity 423 Composition : Nitrogenous crystalline bodies 423 Non-nitrogenous organic bodies 424 Inorganic salts 424 Abnormal urine 425 Comparative 425 The secretion of urine 426 Methods of investigation 426 Theories of secretion 427 Nervous influence 428 Pathological 428 The expulsion of tirine 429 General 429 Facts of exitcrimont and of experience .....•• 429 Pathological 430 Comparative 430 Summary of urine and the functions of the kidneys 430 TiiK Metabolism ok the Body 431 (ieneral remarks 431 The metabolism of the liver 4.'}2 The glycogenic function 44^2 The uses of glycogen 434 Pathological 435 Metabolism of the spleen 436 Histological 436 XVlll ANIMAL PHYSIOLOGY. PAGE Chemical 437 Spleen curves 439 The nervous system in relation to the spleen . . . . . 439 The construction of fat 440 General and experimental 440 Histological 441 Changes in the cells of the mammary gland . . . . . . 443 Milk and colostrum 448 Nature of fat-formation 444 Pathological 445 Comparative 445 The metabolic processes concerned in the formation of urea, uric acid, hippuric acid, and allied bodies 446 General discussion 446 Pathological 448 Evolution 448 The study of the metabolic processes by other methods .... 449 Various tabular statements •. . . . 450 Starvation and its lessons ......... 450 Comparative 453 Diets . .453 Feeding experiments 454 General 454 Proteid metabolism 455 Nitrogenous equilibrium 456 Comparative 456 The effects of gelatine in the diet . 457 Fat and carbohydrates 457 Comparative 458 The effects of salts, water, etc., on the diet 458 Pathological 459 The energy of the animal body , . . 459 Tabular statements . 460 The sources of muscular energy 461 Animal heat , 461 General 461 Comparative 461 The regulation of temperature . . 464 Cold-blooded and warm-blooded animals compared .... 465 Theories of heat formation and heat regulation 466 Pathological 467 Special considerations , , , 467 Evolution 468 Hibernation 470 Daily variations in temperature in man and other mammals . . 470 The influence of the nervous system on metabolism (nutrition) . . . 471 Experimental facts 471 Discussion of their significance 473 General considerations, chemical and pathological .... 476 Summary of metabolism 476 CONTEXTS. XIX PAGE The Spixal Cord — General 480 General 480 Anatomical 482 The reflex functions of the spinal cord 484 General and experimental 484 Evolution and heredity 485 Inhibition of reflexes 485 Reflex time 480 The spinal cord as a conductor of imjjulses 487 Anatomical 488 Decussation 489 Pcthological 490 Paths of impulses 491 The automatic functions of the spinal cord 493 General 493 Spinal phenomena 493 Special considerations 495 Comparative 495 Evolution 49G Synoptical 497 The Braix 498 General and anatomical 498 Animals deprived of their cerebrum 500 Behavior of various animals and its significance 500 Have the semicircular canals a co-ordinary function ? . . . . 502 Experimental, etc 502 Discussion of the phenomena ........ 502 Forced movements 503 Functions of the cerebral convolutions 504 Comparative 505 Individual differences in brains . . . . . . . ■ . 518 The connection of one part of the brain with another .... 518 The cerebral cortex 521 Theories of different observers 522 The circulation in the brain 525 Sleep — hibernation — dreaming 52G Hypnotism — catalepsy — somnambulism 528 Pathological 530 Cerebral localization reconsidered 530 Illustrations of localization 535 Different methods criticised 535 Cerebral time 535 Functions of other portions of the brain 53G The corpus striatum and the optic thalamus 536 Corpora quadrigeniina 539 The cerebelbim 541 Pathological 541 Crura cerebri and and jjons Varolii 541 PallH.Iogical 542 iMedulla ol>lon„'ata 542 XX ANIMAL PHYSIOLOGY. PAGE Special considerations . ^43 Embryologieal . . . . • • • ... . • 542 Evolution 543 Synoptical 547 General Remarks on the Senses 548 Anatomical 548 General principles 550 The Skin as an Organ of Sense 551 General 551 Pathological 553 Pressure sensations 554 Thermal sensations 554 Tactile sensibility 555 The muscular sense 557 General 557 Pathological 557 Comparative 558 Synoptical 559 Vision 559 Physical 559 Anatomical 561 Embryologieal 562 Dioptrics of vision 563 Accommodation of the eye 563 Alterations in the size of the pupil 569 Phenomena and their explanations 570 Pathological 572 Optical imperfections of the eye 572 Spherical aberration 572 Astigmatism 573 Chromatic aberration 573 Entoptic phenomena 574 Anomalies of refraction 574 Visual sensations 576 General 576 Affections of the retina 578 The nature of the processes which originate visual impulses . . 580 The laws of retinal stimulation 581 The visual angle 582 Color sensations 583 Theories of color- vision 584 Color-blindness 585 Psychological aspects of vision 586 The visual field 586 Imperfections of visual perceptions as " irradiation," etc. . . . 587 Influence of the pigment of the macula lutea 589 After-images, etc 589 Misconceptions as to the comparative size of objects .... 590 Subjective phenomena 591 Co-ordination of the two eyes in vision 591 CONTENTS. xxi PAGE The visual axes 591 Ocular movements 592 The horopter 594 Estimation of the size and distance of objects 595 Solidity 595 Protective mechanisms of the eye 596 Special considerations 598 Comparative 597 Evolution 600 Pathological 602 Brief synopsis of the physiology of vision 602 Hearing 604 General 604 Anatomical 605 The membrana tympani 606 The auditory ossicles 607 Muscles of the middle ear 608 The Eustachian tube 609 Pathological 609 Auditory impulses 610 Auditory sensations, perceptions, judgments 615 General 615 Auditory judgment 615 Range of auditory discrimination 616 Special considerations 616 Comparative 616 Evolution 618 Synopsis of the physiology of hearing 620 The Sense of Smell i . . 620 Anatomical 620 General 631 Comparative 622 The Sense of Taste 623 Anatomical 623 General 623 Experimental 623 Pathological 625 Comparative 626 The Cerebro-Spinal System of Nerves 626 1. Spinal nerves 626 General 626 Exception 627 Additional experiments 627 Pathological 627 2. The cranial nerves 628 General 628 The motor-oculi, or third nerve 628 The trochlear, or fourth nerve 629 The al)diicenH, or sixth nerve 629 The facial, or seventh nerve . . . . 629 XXll ANIMAL PHYSIOLOGY. PAGE The trigeminus, or fifth nerve 630 The glosso-pharyngeal, or ninth nerve . . . . . • . 633 The pneumogastric, or tenth nerve 633 The spinal accessory, or eleventh nerve 685 The hypoglossal, or twelfth nerve 635 Relations of the cerebro-spinal and sympathetic systems .... 636 Eecent views on this subject . . 636 The Voice and Speech 639 Physical 639 Anatomical 640 Laryngoscopic observations 643 Voice-formation 643 The registers and the falsetto-voice .,.,.... 644 Pathological 646 Comparative 647 Speech 649 General 649 Formation of vowels and consonants 650 Whispering 650 Classification of consonants 651 Pathological . . . .651 Special considerations 652 Evolution 652 Summary 653 Locomotion 655 Anatomical 655 Mechanical 655 Standing 656 Walking 657 Running 659 Jumping 659 Hopping 659 Comparative : the gait of quadrupeds . . . . . . . 659 Evolution 662 Man considered physiologically at the Different Periods of his Existence 663 Size and growth 663 Digestive system 664 Circulatory and respiratory systems 664 Dentition 665 Nervous system 666 Puberty 666 The sexes 667 Old age 667 Comparative 668 Death 668 Appendix : Animal Chemistry . " 671 Index . . 691 AKIMAL PHYSIOLOGY. GENEEAL BIOLOGY. Introduction. Biology {/3lo<;, life ; koyos, a dissertation) is the science which treats of the nature of living things; and, since the properties of plants and animals can not be explained without some knowledge of their form, this science includes morphol- ogy {txop(f>r], form ; Aoyos, a dissertation) as well as physiology (^uo-is, nature ; Aoyos). Morphology describes the various forms of living things and their parts ; physiology, their action or function. General biology treats neither of animals nor plants exclu- sively. Its province is neither zoology nor botany ; but it at- tempts to define what is common to all living things. Its aim is to determine the properties of organic beings as such, rather than to classify or to give an exhaustive account of either ani- mals or plants. Manifestly, before this can be done, living things, both animal and vegetable, must be carefully compared, otherwise it would be impossible to recognize differences and resemblances ; in other words, to ascertain what they have in common. When only the highest animals and plants are contem- plated, the differences between them seem so vast that they appear to have, at first sight, nothing in common but that they are living : between a tree and a dog an infant can discrimi- nate; but there are microscopic forms of life that thus far defy the most learned to say whether they belong to the ani- mal or tlie vegetable world. As we descend in the organic series, the lines of distinction grow fainter, till they seem finally to all but disappear. But let us first inciuire : What are the determining charac- 1 2 ANIMAL PHYSIOLOGY. teristics of living things as sucli ? By what barriers are the animate and inanimate worlds separated? To decide this, falls within the province of general biology. Living things grow by interstitial additions of particles of matter derived from without and transformed into their own substance, while inanimate bodies increase in size by superfi- cial additions of matter over which they have no power of decomposition and recomposition so as to make them like themselves. Among lifeless objects, crystals approach near- est to living forms ; but the crystal builds itself up only from material in solution of the same chemical composition as itself. The chemical constitution of living objects is peculiar. Car- bon, hydrogen, oxygen, and nitrogen are combined into a very complex whole or molecule, as protein; and, when in com- bination with a large proportion of water, constitute the basis of all life, animal and vegetable, known as protoplasm. Only living things can manufacture this substance, or even protein. Again, in the very nature of the case, protoplasm is con- tinually wasting by a process of oxidation, and being built up from simpler chemical forms. Carbon dioxide is an invariable product of this waste and oxidation, while the rest of the car- bon, the hydrogen, oxygen, and nitrogen are given back to the inorganic kingdom in simpler forms of combination than those in which they exist in living beings. It will thus be evident that, while the flame of life continues to burn, there is constant chemical and physical change. Matter is being continuously taken from the world of things that are without life, trans- formed into living things, and then after a brief existence in that form returned to the source from which they were origi- nally derived. It is true, all animals require their food in or- ganized form — that is, they either feed on animal or plant forms ; but the latter derive their nourishment from the soil and the atmosphere, so that the above statement is a scientific truth. Another highly characteristic property of all living things is to be sought in their periodic changes and very limited dura- tion. Every animal and plant, no matter what its rank in the scale of existence, begins in a simple form, passes through a series of changes of varying degrees of complexity, and finally declines and dies ; which simply means that it rejoins the in- animate kingdom: it passes into another world to which it formerly belonged. Living things alone give rise to living things ; protoplasm GENERAL BIOLOGY. 3 alone can beget protoplasm ; cell begets cell. Omne aniTnal {anima, lite) ex ovo applies with a wide interpretation to all living forms. From what has been said it will appear that life is a condi- tion of ceaseless change. Many of the movements of the pro- toplasm composing the cell-units of which living beings are made are visible under the microscope ; their united effects are open to common observation — as, for example, in the move- ments of animals giving rise to locomotion we have the joint result of the movements of the protoplasm composing millions of muscle-cells. But, beyond the powers of any microscope that has been or probably ever will be invented, there are molecular movements, ceaseless as the flow of time itself. All the processes which make up the life-history of organisms involve this mo- lecular motion. The ebb and flow of the tide may symbolize the influx and efflux of the things that belong to the inanimate world, into and out of the things that live. It follows from this essential instability in living forms that life must involve a constant struggle against forces that tend to destroy it ; at best this contest is maintained successfully for but a few years in all the highest grades of being. So long as a certain equilibrium can be maintained, so long may life con- tinue and no longer. . The truths stated above will be illustrated in the simpler forms of plants and animals in the ensuing pages, and will be- come clearer as each chapter of this work is perused. They form the fundamental laws of general biology, and may be formulated as follows : 1. Living matter or protoplasm is characterized by its chem- ical composition, being made up of carbon, hydrogen, oxygen, and nitrogen, arranged into a very complex molecule. 2. Its universal and constant waste and its repair by inter- stitial formation of new matter similar to the old. 3. Its power to give rise to new forms similar to the parent ones by a process of division. 4. Its manifestation of periodic changes constituting devel- opment, decay, and death. Though there is little in relation to living beings which may not be appropriately set down under zoology or botany, it tends to breadth to have a science of general biology which deals with the properties of things simply as living, irrespective very much as to whether they belong to the realm of animals or plants. The relation of the sciences wlii(;h may be regarded ANIMAL PHYSIOLOGY. as subdivisions of general biology is well shown in the follow- ing table : * Anatomy. ^ The science of structure ; the term being usually ap- plied to the coarser and more obvious composition of plants or animals. Histology. Microscopical anatomy. The ultimate optical an- alysis of structure by the aid of the microscope ; separated from anatordy only as a matter of con- Biol- ogy. The science of liv- ing things ; i. e., of matter in the living state. Mor- phol- ogy. The science of form, struct- ure, etc. Essen- tially statical. Physi- ology. The science of action or func- tion. Essen- tially dynam- ical. Taxonomy. The classification of living things, based chiefly on phenomena of structure. Distribution. Considers the position of liv- ing things in space and time ; their distribution over the present face of the earth ; and their distri- bution and succession at former periods, as dis- played in fossil remains. Embryology. The science of development from the germ; includes many mixed problems pertaining both to mor- phology and physiology. At present largely mor- phological. Physiology. The special science of the functions of the individ- ual in health and in dis- ease ; hence including Pathology. Psychology. The science of mental phe- nomena. Sociology. The science of social life, i. e., the life of communi- ties, whether of men or of lower animals. Botany. The science of veg- etal living matter or plants. Zool- ogy. The science of animal living matter or ani- mals. Biol- ogy. The science of liv- ing things ; i. e., of matter in the living state. * Taken from the *' General Biology " of Sedgwick and Wilson. THE CELL. 5 THE CELL.* All living things, great and small, are composed of cells. Animals may be divided into those consisting of a single cell (Protozoa), and those made up of a multitude of cells (Metazoa) ; but in every case the animal begins as a single cell or ovum from which all the other cells, however different finally from one another either in form or function, are derived by processes of growth and division ; and, as will be seen later, the whole organism is at one period made up of cells practically alike in structure and behavior. The history of each individual animal or plant is the resultant of the conjoint histories of each of its cells, as that of a nation is, when complete, the story of the total outcome of the lives of the individuals composing it. It becomes, therefore, highly important that a clear notion of the characters of the cell be obtained at the outset; and this chapter will be devoted to presenting a general account of the cell. The cell, whether animal or vegetable, in its most complete form consists of a mass of viscid, semifluid, transparent sub- stance (protoplasm), a cell wall, and a more or less circular body (nucleus) situated generally centrally within ; in which, again, is found a similar structure (nucleolus). This description applies to both the vegetable and the ani- mal cell ; but the student will find that the greater proportion of animal cells have no cell wall, and that very few vegetable cells are without it. But there is this great difference between the animal and vegetable cell : the former never has a cellulose wall, while the latter rarely lacks such a covering. In every case the cell wall, whether in animal or vegetable cells, is of greater consistence than the rest of the cell. This is especially true of the vegetable cell. It is doubtful whether there are any cells without a nucleus, while not a few, especially when young and most active, pos- sess several. The circular form may be regarded as the typical form of both cells and nuclei, and their infinite variety in size and form may be considered as in great part the result of the action of mechanical forces, such as mutual pressure ; this is, of course, more es^jecially true of shape. Reduced to its great- est simplicity, then, the cell may be simply a mass of protoplasm with a nucleus. • The illustrations of the sections following will enable the student to form a generalized mental picture of the cell in all its parts. Q ANIMAL PHYSIOLOGY. It seems probable that the numerous researches of recent years and others now in progress will open up a new world of cell biology which will greatly advance our knowledge, espe- cially in the direction of increased depth and accuracy. Though many points are still in dispute, it may be safely said that the nucleus plays, in most cells, a role of the highest importance ; in fact, it seems as though we might regard the nucleus as the directive brain, so to speak, of the individual cell. It frequently happens that the behavior of the body of the cell is foreshadowed by that of the nucleus. Thus fre- Fig. 1. — Nuclear division. A-H, karyokinesis of a tissue-cell. A, nuclear reticulum in its or- dinary state. B, preparing for division ; the contour is less defined, and the fibers thicker and less intricate. C, wreath-stage; the chromatin is arranged in a complicated looping round the equator of the achromatin spindle. D, monaster-stage ; the chromatin now appears as centripetal equatorial V's, each of which should be represented as double. E, a migration of the half of each chromatin loop towards opposite poles of the spindle. F, diaster-stage ; the chromatin forms a star, round each pole of a spindle, each aster be- ing connected by strands of achromatin. G, daughter-wreath stage ; the newly formed nuclei are passing through their retrogressive development, which is completed in the resting stage, H. d-f, kar3'okinesis of an egg-cell, showing the smaller amount of chro- matin than in the tissue-cell. The stages d, e, /, correspond to D, E, F, respectively. The polar star at the end of the spindle is composed of protoplasm-granules of the cell itself, and must not be mistaken for the diaster (F). The coarse lines represent the chromatin, the fine Unes the achromatin, and the dotted lines cell-granules. (Chiefly modifled from Flemming.) X-Z, direct nuclear division in the cells of the embryonic integument of the Eui'opean scorpion. After Blochmann {Haddon). quently, if not always, division of the body of the nucleus pre- cedes that of the cell itself, and is of a most complicated char- acter (karyokinesis or mitosis). The cell wall is of subordinate importance in the processes of life, though of great value as a mechanical support to the protoplasm of the cell and the aggre- THE CELL. 7 gations of cells known as tissues. The greater part of a tree may be said to be made up of the thickened walls of the cells, and these are destitute of true vitality, unless of the lowest order ; while the really active, growing part of an old and large tree constitutes but a small and limited zone, as may be learned from the plates of a work on modern botany representing sec- tions of the wood. Animals, too, have their rigid parts, in the adult state espe- cially, resulting from the thickening of a part or the whole of the cell by a deposition usually of salts of lime, as in the case of the bones of animals. But in some cases, as in cartilage, the cell wall or capsule undergoes thickening and consolidation, and several may fuse together, constituting a matrix, which is also made up in part, possibly, of a secretion from the cell pro- toplasm. In the outer parts of the body of animals we have a great abundance of examples of thickening and hardening of cells. Very well known instances are the indurated patches of skin {epithelium) on the palms of the hands and elsewhere. It will be scarcely necessary to remark that in cells thus altered the mechanical has largely taken the place of the vital in function. This at once harmonizes with and explains what is a matter of common observation, that old men are less active — have less of life within them, in a word, than the young. Chemically, the cellulose wall of plant-cells consists of carbon, hydrogen, and oxygen, in the same relative proportion as exists in starch, though its properties are very different from those of that substance. Turning to cell contents, we find them everywhere made up of a clear, viscid substance, containing almost always granules of varying but very minute size, and differing in consistence, not only in different groups of cells, but often in the same cell, so that we can distinguish an outer portion {ectoplasm) and an inner more fluid and more granular region {endoplasm). The nucleus is a body with very clearly defined outline (in some cases limited by a membrane), through which an irregu- lar network of fibers extends that stains more deeply than any other part of the whole cell. Owing to the fact that it is so readily changed by the action of reagents, it is impossible to ascertain the exact chemical com- position of living protoplasm ; in consequence, wo can only infer its chemical structure, etc., from the examination of the dead substance. In general, it may be said that protoplasm belongs to the 3 ANIMAL PHYSIOLOGY. class of bodies known as proteids— that is, it consists chemically of carbon, hydrogen, a little sulphur, oxygen, and nitrogen, ar- ranged into a very complex and unstable molecule. This very instability seems to explain at once its adaptability for the manifestation of its nature as living matter, and at the same time the readiness with which it is modified by many circum- stances , so that it is possible to understand that life demands an incessant adaptation of internal to external conditions. It seems highly probable that protoplasm is not a single pro- teid substance, but a mixture of such ; or let us rather say, fur- nishes these when chemically examined and therefore dead. Very frequently, indeed generally, protoplasm contains other substances, as salts, fat, starch, chlorophyl, etc. From the fact that the nucleus stains differently from the cell contents, we may infer a difference between them, physical and especially chemical. It (nucleus) furnishes on analysis nu- clein, which contains the same elements as protoplasm (with the exception of sulphur) together with phosphorus. Nuclei have great resisting power to ordinary solvents and even the digest- ive juices. Inasmuch as all vital phenomena are associated with proto- plasm, it has been termed the "physical basis of life^^ (Hux- ley). Tissues. — A collection of cells performing a similar physio- logical action constitutes a tissue. Generally the cells are held together either by others with that sole function, or by cement material secreted by them- selves. An organ may consist of one or several tissues. Thus the stomach consists of muscular, serous, connective, and gland- ular tissues besides those constituting its blood-vessels, lym- phatics, and nerves. But all of the cells of each tissue have, speaking generally, the same function. The student is referred to works on general anatomy and histology for classifications and descriptions of the tissues. The statements of this chapter will find illustration in the pages immediately following, after which we shall return to the subject of the cell afresh. Summary. — The typical cell consists of a wall, protoplasmic contents, and a nucleus. The vegetable cell has a limiting membrane of cellulose. Cells undergo differentiation and may be united into groups forming tissues which serve one or more definite purposes. The chemical constitution of protoplasm is highly complex UNICELLULAR PLANTS. 9 and unstable. The nucleus plays a prominent part in the life- history of the cell, and seems to be essential to its perfect devel- opment and greatest physiological efficiency. UNICELLULAR PLANTS. Yeast (Torula, Saccharomyces CerevisicB). The essential part of the common substance, yeast, may be studied to advantage, as it affords a simple type of a vast group of organisms of profound interest to the student of physiology and medicine. To state, first, the main facts as ascertained by observation and experi- ment : Morphological. — The particles of which yeast is composed are cells of a circular or oval form, of an average diameter of about yoVo of ^^ inch. Each individual torula cell consists of a trans- parent homogeneous cov- ering (cellulose) and gran- ular semifluid contents (protoplasm). Within the latter there may be a space (vacuole) filled with more fluid contents. The various cells pro- duced by budding may remain united like strings of beads. Collections of masses composed of four or more subdivisions (as- cospores), which finally separate by rupture of the original cell wall, having thus become themselves inde XJendent cells, may be seen more rarely (endogenous division). Fig. 2. — Various stages in the development of brewer's yeast, seen, with the exception of the first in the series, with an ordinary high power (Zeiss, D. 4) of the microscope. The first is greatly magnified (Gundlach's t'j immersion lens). The second series of four represents stages in the division of a single cell ; and the third series a branching colony. Everywhere the light areas indicate vacuoles. Fio. 3.— The endogonidia (ascospore) phase of repro- duction— i. e., endogenous division. Fio. 4.— Further development of the forms represented in Fig. 3. IQ ANIMAL PHYSIOLOGY. The yeast-cell is now believed to possess a nucleus. Chemical. — When yeast is burned and the ashes analyzed, they are found to consist chiefly of salts of potassium, calcium, and magnesium. The elements of which yeast is composed are C, H, O, N, S, P, K, Mg, and Ca ; but chiefly the first four. Physiological. — If a little of the powder obtained by drying yeast at a temperature below blood-heat be added to a solution of sugar, and the latter be kept warm, bubbles of carbon di- oxide will be evolved, causing the mixture to become frothy ; and the fluid will acquire an alcoholic character {fermenta- ti07l). If the mixture be raised to the boiling-point, the process de- scribed at once ceases. It may be further noticed that in the fermenting saccharine solution there is a gradual increase of turbidity. All of these changes go on perfectly well in the total absence of sunlight. Yeast-cells are found to grow and reproduce abundantly in an artificial food solution consisting of a dilute solution of cer- tain salts, together with sugar. Conclusions. — What are the conclusions which may be legiti- mately drawn from the above facts ? That the essential part of yeast consists of cells of about the size of mammalian blood-corpuscles, but with a limiting wall of a substance different from the inclosed contents, which latter is composed chiefly of that substance common to all living things — protoplasm ; that like other cells they reproduce their kind, and in this instance by two methods : gemmation giving rise to the bead-like aggregations alluded to above ; and in- ternal division of the protoplasm {endogenous division). From the circumstances under which growth and reproduc- tion take place, it will be seen that the original protoplasm of the cells may increase its bulk or grow when supplied with suitable food, which is not, as will be learned later, the same in all respects as that on which green plants thrive ; and that this may occur in darkness. But it is to be especially noted that the protoplasm resulting from the action of the living cells is wholly different from any of the substances used as food. This power to construct protoplasm from inanimate and unorgan- ized materials, reproduction, and fermentation are all proper- ties characteristic of living organisms alone. It will be further observed that these changes all take place within narrow limits of temperature; or, to put the matter UNICELLULAR PLANTS. 11 more generally, that the life-history of this humble organism can only be unfolded under certain well-defined conditions. Protococcus {Protococcus pluvialis). The study of this one-celled plant will afford instructive comparison between the ordinary green plant and the colorless plants or fungi. Like Torula it is selected because of its simple nature, its abundance, and the ease with which it may be obtained, for it abounds in water-barrels, standing pools, drinking-troughs, etc. Morphological. — Protococcus consists of a structureless wall and viscid granular contents, i. e., of cellulose and protoplasm. The protoplasm may contain starch and a red or green color- ing matter {chloropliyl). It probably contains a nucleus. The cell is mostly globular in form. Fig. 5. Fig. G. 7tC Fig. Figs. 5 to 7 represent successive stages observed in the life-history of Protococci scraped from the bark of a tree. Fig. 5.— a group in the dried .state, illustrating method of division. Fig. 6. — One of the above after two days' immersion in water. Fig. 7.— Various phases in the later motile stage assumed by the above specimens. The nu- cleus is denoted by nc ; the cell wall by c.w ; and the coloring-matter by the dark spot. On the left of Fig. 7 an individual may be seen that is devoid of a cell wall. Physiological — It reproduces by division of the original cell (fission) into similar individuals, and by a process of budding and constriction ((jemmation) which is much rarer. Under the influence of sunlight it decomposes carbon dioxide (CO,), fix- ing the carbon and setting the oxygen free. It can flourish per- fectly in rain-vvator, which contains only carbon dioxide, salts of ammonium, and minute quantities of other soluble salts that may as dust have been blown into it. There is a motile form of this unicellular plant, and in this stage it moves through tlio fluid in which it lives by means of 12 ANIMAL PHYSIOLOGY. extensions of its protoplasm {cilia) through the cell wall; or the cell wall may disappear entirely. Finally, the motile form, withdrawing its cilia and clothing itself with a cellulose coat, becomes globular and passes into a quiescent state again. Much of this part of its history is common to lowly animal forms. Conclusions. — It will be seen that there is much in common in the life-history of Torula and Protococcus. By virtue of be- ing living protoplasm they transform unorganized material into their own substance ; and they grow and reproduce by analo- gous methods. But there are sharply defined differences. For the green plant sunlight is essential, in the presence of which its chloro- phyl prepares the atmosphere for animals by the removal of carbonic anhydride and the addition of oxygen, while for Torula neither this gas nor sunlight is essential. Moreover, the fungus {Torula) demands a higher kind of food, one more nearly related to the pabulum of animals ; and is absolutely independent of sunlight, if not actually injured by it ; not to mention the remarkable process of fermentation. UNICELLAR ANIMALS. The Proteus Animalcule {Amoeba). In order to illustrate animal life in its simpler form we choose the above-named creature, which is nearly as readily obtainable as Protococcus and often under the same circum- stances. Morphological. — Amoeha is a microscopic mass of transparent protoplasm, about the size of the largest of the colorless blood- corpuscles of cold-blooded animals, with a clearer, more con- sistent outer zone {ectosarc), (although without any proper cell wall), and a more fluid, granular inner part. A clear space {contractile vesicle, vacuole) makes its appearance at intervals in the ectosarc, which may disappear somewhat suddenly. This appearance and vanishing have suggested the term pulsating or contracting vesicle. Both a nucleus and nucleolus may be seen in Amoeba. At varying short periods certain parts of its body { pseudopodia) are thrust out and others withdrawn. Physiological. — Amoeba can not live on such food as proves adequate for either Protococcus or Torula, but requires, besides UNICELLAR ANIMALS. 13 inorganic and unorganized food, also organized matter in the form of a complex organic compound known as protein, which Fig. 9. Fig. 10. Fig. 11. Fig. 12. Fig. 13. Fig. 14. Fig. 15. Fig. 16. Figs. 8 to 15, represent successive phases in the life-history of an Amoeboid organism, kept under constant observation for three days ; Fig. 16 a similar organism encysted, which was a few hours later set free by the disintegration of the cyst. (All the figures are drawn under Zeiss, D. 3.) Fig. 8.— The locomotor phase ; the ectoplasm is seen protruding to form a pseudopodium, into which the endopla,sm pa.sses. Fig. 9.— a stage in the ingestive phase. A vegetable organism, fp, is undergoing intussus- ception. Fig. 10.— a portion of the creature represented in Fig. 9 after complete ingestion of the food- particle. Figs. H, 12.— Successive stages in the a.ssimilative and excretory processes. Fig. 12 repre- sents the organism some twenty hours later than as seen in Fig. 11. The undigested rem- nant* of the ingested organism are represented undergoing ejection (excretion) at fp, in Fig. 12. Figs. 1.3, \4. 15. represent successive stages in the reproductive process of the same individ- ual, observed two days later. It will be noticed (Fig. 13) that the nucleus divides first. In the above figures, vc, denotes the contracting vacuole ; nc, the nucleus ; ps, pseudopo- dium ; dt, diatom ; fp, food-particle. contains nitrogen in addition to carbon, hydrogen, and oxygen. In fact, Amrjiba can prey upon both plants and animals, and thus use up as food protoj^lasm itself. The pseudopodia serve the double purpose of organs of locomotion and prehension. This creature absorbs oxygen and evolves carbon dioxide. 14 ANIMAL PHYSIOLOGY. Inasmuch as any part of the body may serve for the admission, and possibly the digestion, of food and the ejection of the use- less remains, we are not able to define the functions of special parts. Amoeba exercises, however, some degree of choice as to what it accepts or rejects. The movements of the pseudopodia cease when the tempera- ture of the surrounding medium is raised or lowered beyond a certain point. It can, however, survive in a quiescent form greater depression than elevation of the temperature. Thus, iat 35° C, heat-rigor is induced ; at 40° to 45° C, death results ; but though all movement is arrested at the freezing-point of water, recovery ensues if the temperature be gradually raised. Its form is modified by electric shocks and chemical agents, as well as by variations in the temperature. At the present time it is not possible to define accurately the functions of the vacuoles found in any of the organisms thus far considered. It is worthy of note that Amoeba may spontaneously assume a spherical form, secrete a structureless covering, and remain in this condition for a variable period, reminding us of the similar behavior of Torula. Amoeba reproduces by fission, in which the nucleus takes a prominent if not a directive part, as seems likely it does in re- gard to all the functions of unicellular organisms. Conclusions. — It is evident that Amoeba is, in much of its be- havior, closely related to both colored and colorless one-celled plants. All of the three classes of organisms are composed of protoplasm ; each can construct protoplasm out of that which is very different from it ; each builds up the inanimate inorganic world into itself by virtue of that force which we call vital, but which in its essence we do not understand ; each multiplies by division of itself, and all can only live, move, and have their being under certain definite limitations. But even among forms of life so lowly as those we have been considering, the differences between the animal and vegetable worlds appear. Thus, Amoeba never has a cellulose wall, and can not subsist on inorganic food alone. The cellulose wall is not, however, invariably present in plants, though this is generally the case ; and there are animals (Ascidians) with a cellulose investment. Such are very exceptional cases. But the law that animals must have organized material {protein) as food is without ex- ception, and forms a broad line of distinction between the ani- mal and vegetable kingdoms. Amoeba will receive further consideration later; in the PARASITIC ORGANISMS. 15 mean time, we turn to tlie study of forms of life in many respects intermediate between plants and animals, and full of practical interest for mankind, on account of their relations to disease, as revealed by recent investigations. PARASITIC ORGANISMS. The Fungi. Molds {Penicillium Olaucum and Mucor Mucedo). Closely related to Torida physiologically, but of more com- plex structure, are the molds, of which we select for convenient study the common green mold {Penicillium), found growing in dark and moist places on bread and similar substances, and the white mold {Mucor), which grows readily on manure. The fungi originate in spores, which are essentially like Torula in structure, by a process of budding and longitudinal extension, resulting in the formation of transparent branches or tubules, filled with protoplasm and invested by cellulose walls, across which transverse partitions are found at regular intervals, and in which vacuoles are also visible. The spores, when growing thus in a liquid, give rise to up- ward branches {aerial hypha), and downward branches or root- lets {submerged hyplioe). These multitudinous branches inter- lace in every direction, forming an intricate felt-work, which supports the green powder (spores) which may be so easily shaken off from a growing mold. In certain cases the aerial hyphse terminate in tufts of branches, which, by transverse division, become split up into spores {Conidia), each of which is similar in structure to a yeast-cell. The green coloring matter of the fungi is not chlorophyl. The Conidia germinate under the same conditions as Torula. Mucor Mucedo. — The growth and development of this mold may be studied by simply inverting a glass tumbler over some horse-dung on a saucer, into which a very little water has been poured, and keeping the preparation in a warm place. Very soon whitish filaments, gradually getting stronger, ap- pear, and are finally topped by rounded heads or spore-cases {Sporangia). These filaments are the hypJue, similar in struct- ure to those of Penicillium. The spore-case is lil](;d with a multitude of oval bodies {spores), resulting from the subdivis- ion of the protof>lasm, which are finally released by the spore- 16 ANIMAL PHYSIOLOGY. Fig. 27, Fig. 2a PARASITIC ORGANISMS. j^ Figs. 17 to 38.— In the following figures, ha, denotes aerial hyphae ; sp, sporangium : zy, zy- gospore : ex. exosporium ; my, mycelium ; mc, mucilage ; cl, columella ; en, endogonidia. Fig. 1~.— Spore-bearing hyphae of Mucor. growing from horse-dung. Fig. 18. — The same, teased out with needles (A, 4). Figs. 19, 20, 21.— Successive stages in the development of the sporangium. P^G. 22. — Isolated spores of Mucor. Fig. 23. — Grerminating spores of the same mold. Fig. 24.— Successive stages in the germination of a single spore. Figs. 25, 26, 27.— Successive phases in the conjugative process of Mucor. Fig. 28. — Successive stages observed during ten hours in the growth of a conidiophore of Peni- cillium in an object-glass culture (D, 4). case becoming thinned to the point of rupture. The devel- opment of these spores takes place in substantially the same manner as those of Penicillium. Sporangia developing spores in this fashion by division of the protoplasm are termed asci, and the spores ascospores. So long as nourishment is abundant and the medium of growth fluid, this asexual method of reproduction is the only one ; but, under other circumstances, a mode of increase, known as conjugation, arises. Two adjacent hyphae enlarge at the ex- tremities into somewhat globular heads, bend over toward each other, and, meeting, their opposed faces become thinned, and the contents intermingle. The result of this union (zygosjjore) undergoes now certain further changes, the cellulose coat being separated into two — an outer, darker in color (exosporium), and an inner colorless one (endosporium). Under favoring circumstances these coats burst, and a branch sprouts forth from which a vertical tube arises that terminates in a sporangium, in which spores arise, as before de- scribed. It will be apparent that we have in Mucor the exem- plification of what is known in biology as " alternation of gen- erations"— that is, there is an intermediate generation be- tween the original form and that in which the original is again reached. Physiologically the molds closely resemble yeast, some of them, as Mucor, being capable of exciting a fermentation. The fungi are of special interest to the medical student, be- cause many forms of cutaneous disease are directly associated with their growth in the epithelium of the skin, as, for exam- ple, common ringworm ; and their great vitality, and the facil- ity with which their spores are widely dispersed, explain the highly contagious nature of such diseases. The media on which they flourish (feed) indicates their great physiological differ- ences in this particular from the green plants proper. They are closely related in not a f(!W respects to an important class of vegetable organisms, known as bacteria, to be considered forth- with, 2 18 ANIMAL PHYSIOLOGY. The Bacteria. The bacteria include numberless varieties of organisms of extreme minuteness, many of them visible only by the help of the most powerful lenses. Their size has been estimated at from 30^00 to To-ooT <^f ^'^ inch in diameter. They grow mostly in the longitudinal direction, and repro- duce by transverse division, forming spores from which new generations arise. Some of them have vibratile cilia, while the cause of the movements of others is quite unknown. As in many other lowly forms of life, there is a quiescent as well as an active stage. In this stage {zooglcea form) they Fig. 33. Fig. 32. Fig. 29.— Micrococcus, very like a spore, but usually much smaller. Fig. 30.— Bacterium. Fig. 31.— Bacillus. The central filament presented this segmented appearance as the result of I- %W°%^^? .?/ transverse division occurring during ten minutes' observation. i IG. d^.— bpinllum : various forms. The first two represent vibrio, which is possibly only a stage of spirillum. f j j Fig. 33.— a drop of the surface scum, showing a spirillum aggregate in the resting state. are surrounded by a gelatinous matter, probably secreted by themselves. PARASITIC ORGANISMS. 19 Bacteria grow and reproduce in Pasteur's solution, render- ing it opaque, as well as in almost all fluids that abound in proteid matter. That such fluids readily putrefy is owing to the presence of bacteria, the vital action of which suffices to break asunder complex chemical compounds and produce new ones. Some of the bacteria require oxygen, as Bacillus an- fhracis, while others do not, as the organism of putrefaction, Bacterium terino. Bacteria are not so sensitive to slight variations in tempera- ture as most other organisms. They can, many of them, with- stand freezing and high temperatures. All bacteria and all germs of bacteria are killed by boiling water, though the spores are much more resistant than the mature organisms themselves. Some spores can resist a dry heat of 140° C. The spores, like Torula and Protococcus, bear drying, with- out loss of vitality, for considerable periods. That different groups of bacteria have a somewhat different life-history is evident from the fact that the presence of one checks the other in the same fluid, and that successive swarms of different kinds may flourish where others have ceased to live. That these organisms are enemies of the constituent cells of the tissues of the highest mammals has now been abundantly demonstrated. That they interfere with the normal working of the organism in a great variety of ways is also clear ; and certain it is that the harm they do leads to aberration in cell- life, however that may be manifested. They rob the tissues of their nutriment and oxygen, and poison them by the products of the decompositions they produce. But apart from this, their very presence as foreign agents must hamper and derange the delicate mechanism of cell-life. These organisms seem to people the air, land, and waters with invisible hosts far more numerous than the forms of life we behold. Fortunately, they are not all dangerous to the higher forms of mammalian life ; but that a large proportion of the diseases which afflict both man and the domestic animals are directly caused, in the sense of being invariably associated with, the presence of such forms of life, is now beyond doubt. The facts stated above explain why that should be so ; why certain maladies should be infectious; how the germs of dis- ease may bo transported to a friend wrapped up in tin; fohls of a letter. Disease thus caused, it must not be forgotten, is an illustra- 20 ANIMAL PHYSIOLOGY. tion of the struggle for existence and tlie survival of the fittest. If the cells of an organism are mightier than the bacteria^ the latter are overwhelmed ; but if the bacteria are too great in numbers or more vigorous, the cells must yield ; the battle may waver — now dangerous disease, now improvement — but in the end the strongest in this, as in other instances, prevail. UNICELLULAR ANIMALS WITH DIFFERENTIATION OF STRUCTURE. The Bell-Animalcule (Voriicella). Amoeba is an example of a one-celled animal with little per- ceptible differentiation of structure or corresponding division of physiological labor. This is not, however, the case with all unicellular animals, and we proceed to study one of these with considerable development of both. The Bell - animalcule is found in both fresh and salt water, either single or in groups. It is anchored to some object by a rope-like stalk of clear pro- toplasm, that has a spiral appearance when contracted; and which, with a certain degree of regularity, shortens and length- ens alternately, suggesting that more definite movement (con- traction) of the form of protoplasm known as inuscle, to be studied later. The body of the creature is bell-shaped, hence its name ; the bell being provided with a thick everted lip (peristome), covered with bristle-like extensions of the protoplasm (cilia), which are in almost constant rhythmical motion. Covering the mouth of the bell is a lid, attached by a hinge of protoplasm to the body, which may be raised or lowered. A wide, funnel -like depres- sion ((Esophagus) leads into the softer substance within which it ends blindly. The outer part of the animal (cuticula) is denser and more transparent than any other part of the whole creature ; next to this is a portion more granular and of inter- mediate transparency between the external and innermost portions (cortical layer). Below the disk is a space (contractile vesicle) filled with a thin, clear fluid, which may be seen to enlarge slowly and then to collapse suddenly. When the Yorti- cella is feeding, these vesicles may contain food-particles, and in the former, apparently, digestion goes on. Such food vacu- oles (vesicles) may circulate up one side of the body of the ani- mal and down the other. Their exact significance is not known, but it would appear as if digestion went on within them ; and UNICELLULAR ANIMALS. 21 possibly th-e clear fluid with which they are filled may be a spe- cial secretion with solvent action on food. Fig. 37. Fig. 38. Fig. 39. Figs. 34 to 40.— In the following figures d. denotes disc ; p, peristome ; vc, contractile vacuole ; vf, food- vacuole ; rs, vestibule : cf, contractile fiber ; c, cyst ; nc, nucleus ; cl, cilium. Fig. 34.— a group of vorticellse showing the creature in various positions (A, 3). Fig. 35.— The sane, in the extended and in the retracted state. (Surface views.) Fig. 36.— Shows food-vacuoles ; one in the act of inges- tion. Fig. 37.— a vorticella, in which the process of multiplica- tion by fission is begun. Fig. 38. — the results of fission ; the production of two in- dividuals of unequal size. Fig. 39.— Illustration of reproduction by conjugation. Fig. 40.— An encysted vorticella. Fig. 35 Situated somewhat centrally is a horseshoe-shaped body, with well-defined edges, which stains more readily than the rest of the cell, indicating a different chemical composition ; and, from the prominent part it takes in the reproductive and other functions of the creature, it may be considered the nucleus {endoplasf). Multiplication of the species is either by gemmation or by fission. In the first case the nucleus divides and the fragments are transformed into locomotive germs; in the latter the entire animal, including the nucleus, divides longitudinally, each half becoming a similar complete, independent organism. Still an- 22 ANIMAL PHYSIOLOGY. other metliod of reproduction is known. A more or less globu- lar body encircled with a ring of cilia and of relatively small size may sometimes be seen attached to the usual form of Vorti- cella, with which it finally becomes blended into one mass. This seems to foreshadow the "' sexual conjugation " of higher forms, and is of great biological significance. Vorticella may pass into an encysted and quiescent stage for an indefinite period and again become active. The history of the Bell-animalcule is substantially that of a vast variety of one-celled organisms known as Infusoria, to which Amoeba itself belongs. It will be observed that the resemblance of this organism to Amoeba is very great ; it is, however, introduced here to illustrate an advance in differentiation of structure ; and to show how, with the latter, there is usually a physiological advance also, since there is additional functional progress or division of labor; but still the whole of the work is done with- in one cell. Amoeba and Vorticella are both factories in which all of the work is done in one room, but in the latter case the machinery is more complex than in the former ; there are cor- respondingly more processes, and each is performed with greater perfection. Thus, food in the case of the Bell-animalcule is swept into the gullet by the currents set up by the multitudes of vibrating arms around this opening and its immediate neigh- borhood ; the contractile vesicles play a more prominent part ; and the waste of undigested food is ejected at a more definite portion of the body, the floor of the oesophagus ; while all the movements of the animal are rhythmical to a degree not exem- plified in such simple forms as Amoeba; not to mention its various resources for multiplication and, therefore, for its perpetuation and permanence as a species. It, too, like all the unicellular organisms we have been considering, is susceptible of very wide distribution, being capable of retaining vitality in the dried state, so that these infusoria may be carried in vari- ous directions by winds in the form of microscopic dust. MULTICELLULAR ORGANISMS. The Fresh-Water Polyps {Hydra viridis j Hydra fusca). The comparison of an animal so simple in structure, though made up of many cells, as the Polyp, with the more complex organizations with which we shall have especially to deal, may MULTICELLULAR ORGANISMS. 23 be fitly undertaken at this stage. The Polyps are easily obtain- able from ponds in which they are found attached to various kinds of weeds. To the naked eye, they resemble translucent masses of jelly with a greenish or reddish tinge. They range in size from one quarter to one half an inch ; are of an elongated cylindrical form ; provided at the oral extremity with thread- like tentacles of considerable length, which are slowly moved about in all directions ; but they and the entire body may short- en rapidly into a globular mass. They are usually attached at the opposite (aboral) pole to some object, but may float free, or slowly crawl from place to place. It may be observed, under the microscope, that the tentacles now and then embrace some living object, convey it toward an opening (mouth) near their base, from which, from time to time, refuse material is cast out. It may be noticed, too, that a living object within the touch of these tentacles soon loses the power to struggle, which is owing to the peculiar cells (netUe -cells, urticating capsules, nemato- cysts) with which they are abundantly provided, and which se- crete a poisonous fluid that paralyzes prey. The mouth leads into a simple cavity {coelom) in which digestion proceeds. The green color in Hydra viridis, and the red color of Hydra fusca, is owing to the presence of clilorophyl, the function of which is not known. Hydra is structurally a sac, made up of two layers of cells, an outer {ectoderm) and an inner (endoderm) ; the tentacles being repetitions of the structure of the main body of the animal, and so hollow and composed of two cell layers. Speaking generally, the outer layer is devoted to obtaining information of the surroundings; the inner to the work of preparing nutriment, and probably, also, discharging waste matters, in which latter assistance is also received from the outer layer. As digestion takes place largely within the cells themselves, or is intracellular, we are reminded of Varticella and still more of Amoeba. There is in Hydra a general advance in development, but not very much in- dividual cell sx>ecialization. That of the urticating capsules is one of the best examples of such sj^ecialization in this creature. A Polyp is like a colony of Amoebae in which some division of labor (function) has taken place ; a sort of biological state in which every individual is nearly equal to his neighbor, but somewhat more advanced than those neighbors not members of the organization. But in one respect the Polyps show an enormous advance. Ordinarily wlien nourishment is abundant hydra multiplies by 24 ANIMAL PHYSIOLOGY. Fig. 43. MULTICELLULAR ORGANISMS. 25 Figs. 41 to -16.— In the following figures, ec, denotes ectoderm ; en, endoderm ; t, tentacle ; hp, hypostome ; /. foot ; ts, testes ; oi\ ovary ; ps. pseudopodium ; ec', larger ectoderm-cells ; ne\ larger nematocysts before inapture ; cp. Kleiueuberg's fibers ; c.l, supporting lamella ; cl, chloroph.yl-formfng bodies ; c, cilium Fig. 41.— The greeu h.ydra, at the maximum of contraction and elongation of its body. The creature is represented in the act of seizing a small crustacean (A, 2). Fig. 42.— Transverse section across the body of a hydra, in the digestive cavity of which a small crustacean is represented. Fig. 43. — The leading types of thread-cells, after liberation from the body (F. 3i. The cells are represented in the active and the resting conditions ; in the former all the parts are more distinctly seen in consequence of the necessary eversion. Fig. 44.— Small portion of a transverse .section across the body of a green hydra (D, 3). B^G. 45.— A large brown hydra bearmg at the same time buds produced asexually and sexual organs. Fig. 46.— Larger cells of the ectoderm isolated. Note the processes of the cells or Kleinen- berg's fibers. (F, 3.) AU of the cuts on pages, 9, 11, 13, 16, 18, 21 and 24, have been selected from Howes' "Atlas of Biology."' budding, and when cut into portions each may become a com- plete individual. However, under other circumstances, near the bases of the tentacles the body wall may protrude into little masses {testes), in which cells of peculiar formation {sperma- tozoa) arise, and are eventually set free and unite with a cell {ovum) formed in a similar protrusion of larger size {ovary). Here, then, is the first instance in which distinctly sexual re- production has been met in our studies of the lower forms of life. This is substantially the same process in Hydra as in mammals. But, as both male and female cells are produced by the same individual, the sexes are united {hermaphroditism) ; each is at once male and female. Any one watching the movements of a Polyp, and compar- ing it with those of a Bell-animalcule, will observe that the former are much less machine-like ; have greater range ; seem to be the result of a more deliberate choice ; are better adapted to the environment, and calculated to achieve higher ends. In the absence of a nervous system it is not easy to explain how one part moves in harmony with another, except by that process which seems to be of such wide application in nature, adapta- tion from habitual simultaneous effects on a protoplasm capable of responding to stimuli. When one process of an Amoeba is touched, it is likely to withdraw all. This we take to to be due to influences radiating through molecular movement to other parts ; the same j>rinciple of action may be extended to Hydra. The oftener any molecular movement is repeated, the more it tends to become organized into regularity, to become fixed in its mode of action ; and if we are not mistaken this is a funda- mental law throughout the entire world of living things, if not of all things animate and inanimate alike. To this law we shall return. But Hydra is a creature of but very limited specializations; there are neither organs of circulation, respiration, nor excretion. 26 ANIMAL PHYSIOLOGY. if we exclude the doubtful case of tlie thread-cells {urticating capsules). The animal breathes by the entire surface of the body ; nourishment passes from cell to cell, and waste is dis- charged into the water surrounding the creature from all cells, though probably not quite equally. All parts are not digestive, respiratory etc., to the same degree, and herein does it differ greatly from Amoeba or even Vorticella, though fuller knowl- edge will likely modify our views of the latter two and similar organisms in this regard. THE CELL EECONSIDERED, Having now studied certain one-celled plants and animals, and some very simple combinations of cells (molds, etc.), it will be profitable to endeavor to generalize the lessons these humble organisms convey ; for, as will be constantly seen in the study of the higher forms of life of which this work proposes to treat principally, the same laws operate as in the lowliest living creatures. The most complex organism is made up of tissues, which are but Cells and their products, as houses are made of bricks, mortar, wood, and a few other materials, however large or elaborate. The student of physiology who proceeds scientifically must endeavor, in investigating the functions of each organ, to learn the exact behavior of each cell as determined by its own inher- ent tendencies, and modified by the action of neighboring cells. The reason why the function of one organ differs from that of another is that its cells have departed in a special direction from those properties common to all cells, or have become func- tionally differentiated. But such a statement has no meaning unless it be well understood that cells have certain properties in common. This is one of the lessons imparted by the preceding studies which we now review. Briefly stated in language now extensively used in works on biology, the common properties of cells (protoplasm), whether animal or vegetable, whether con- stituting in themselves entire animals or plants, or forming the elements of tissues, are these : The collective chemical processes associated with the vital activities of cells are termed its meta- bolisTTi. Metabolism is constructive when more complex com- pounds are formed from simpler ones, as when the Protococcus- cell builds up its protoplasm out of the simple materials, found in rain-water, which make up its food. Metabolism is destruct- THE ANIMAL BODY. 27 ive when the reverse process takes place. The results of this process are eliminated as excreta, or useless and harmful prod- ucts. Since all the vital activities of cells can only be mani- fested when supplied with food, it follows that living organisms convert potential or possible energy into kinetic or actual en- ergy. When lifeless, immobile matter is taken in as food and, as a result, is converted by a process of assimilation into the protoplasm of the cell using it, we have an example of poten- tial being converted into actual energy, for one of the proper- ties of all protoplasm is its contractility. Assimilation implies, of course, the absorption of what is to be used, with rejection of waste matters. The movements of protoplasm of whatever kind, when due to a stimulus, are said to indicate irritability; while, if inde- pendent of any external source of excitation, they are denomi- nated automatic. Among agents that modify the action of all kinds of proto- plasm are heat, moisture, electricity, light, and others in great variety, both chemical and mechanical. It can not be too well remembered that living things are what they are, neither by virtue of their own organization alone nor through the action of their environment alone (else would they be in no sense dif- ferent from inanimate things), but because of the relation of the organization to the surroundings. Protoplasm, then, is contractile, irritable, automatic, absorp- tive, secretory (and excretory), metabolic, and reproductive. But when it is affirmed that these are the fundamental prop- erties of all protoplasm, the idea is not to be conveyed that cells exhibiting these properties are identical biologically. No two masses of protoplasm can be quite alike, else would there be no distinction in physiological demeanor — no individuality. Every cell, could we but behold its inner molecular mechanism, differs from its neighbor. When this difference reaches a certain de- gree in one direction, we have a manifest differentiation leading to physiological division of labor, which may now with advan- tage be treated in the following section. THE ANIMAL BODY. An animal, as we have learned, may be made up of a single cell in which each part x>erforms much the same work ; or, if there be differences in fuii(;ti()n, they are ill-defined as compared 28 ANIMAL PHYSIOLOGY. with those of higher animals. The condition of things in snch an animal as Amoeba may be compared to a civilized commu- nity in a very crude social condition. When each individual tries to perform every office for himself, he is at once carpenter, blacksmith, shoemaker, and much more, with the natural re- sult that he is not efficient in any one direction. A community may be judged in regard to its degree of advancement by the amount of division of labor existing within it. Thus is it with the animal body. We find in such a creature as the fresh-water Hydra, consisting of two layers of cells forming a simple sac, a slight amount of advancement on Amoeba. Its external surface no longer serves for inclosure of food, but it has the simplest form of mouth and tentacles. Each of the cells of the internal layer seems to act as a somewhat improved or specialized Amoe- ba, while in those of the outer layer we mark a beginning of those functions which taken collectively give the higher ani- mals information of the surrounding world. Looking to the existing state of things in the universe, it is I plain that an animal to attain to high ends must have powers of rapid locomotion, capacity to perceive what makes for its in- terest, and ability to utilize means to attain this when perceived. These considerations demand that an animal high in the scale of being should be provided with limbs sufficiently rigid to sup- port its weight, moved by strong muscles, which must act in harmony. But this implies abundance of nutriment duly pre- pared and regularly conveyed to the bones and muscles. All this would be useless unless there was a controlling and ener- gizing system capable both of being impressed and originating impressions. Such is found in the nerves and nerve-centers. Again, in order that this mechanism be kept in good running order, the waste of its own metabolism, which chokes and poi- sons, must be got rid of — hence the need of excretory apparatus. In order that the nervous system may get sufficient informa- tion of the world around, the surface of the body must be pro- vided with special message-receiving offices in the form of modified nerve-endings. In short, it is seen that an animal as high in the scale as a mammal must have muscular, osseous (and connective), digestive, circulatory, excretory, and nervous tissues ; and to these may be added certain forms of protective tissues, as hair, nails, etc. Assuming that the student has at least some general knowl- edge of the structure of these various tissues, we propose to tell in a simple way the whole physiological story in brief. THE ANIMAL BODY. 29 The blood is the source of all the nourishment of the organ- ism, including its oxygen supply, and is carried to every j^art of the body through elastic tubes which, continually branching and becoming gradually smaller, terminate in vessels of hair- like fineness in which the current is very slow — a condition per- m.itting that interchange between the cells surrounding them and the blood which may be compared to a process of barter, the cells taking nutriment and oxygen, and giving (excreting) in return carbonic anhydride. From these minute vessels the blood is conveyed back toward the source whence it came by similar elastic tubes which gradually increase in size and be- come fewer. The force which directly propels the blood in its onward course is a muscular pump, with both a forcing and suction action, though chiefly the former. The flow of blood is maintained constant owing to the resistance in the smaller tubes on the one hand and the elastic recoil of the larger tubes on the other; while in the returning vessels the column of blood is supported by elastic double gates which so close as to prevent reflux. The oxygen of the blood is carried in disks of microscopic size which give it up in proportion to the needs of the tissues past which they are carried. But in reality the tissues of the body are not nourished directly by the blood, but by a fluid derived from it and resem- bling it greatly in most particulars. This fluid bathes the tissue-cells on all sides. It also is taken up by tubes that convey it into the blood after it has passed through little fac- tories (lymphatic glands), in which it undergoes a regeneration. Since the tissues are impoverishing the blood by withdrawal of its constituents, and adding to it what is no longer useful, and is in reality poisonous, it becomes necessary that new material be added to it and the injurious components withdrawn. The former is accomplished by the absorption of the products of food digestion, and the addition of a fresh supply of oxygen derived from without, while the poisonous ingredients that have found their way into the blood are got rid of through processes that may be, in general, compared to those of a sew- age system of a very elaborate character. To explain this re- generation of the blood in somewhat more detail, we must first consider the fate of food from the time it enters the moutli till it leaves the tract of the body in which its preparation is carried on. The food is in the mouth submitted to the action of a series of cutting and grinding organs worked by powerful muscles ; 30 ANIMAL PHYSIOLOGY. mixed with a fluid wliich changes the starchy part of it into sugar, and prepares the whole to pass further on its course : when this has been accomplished, the food is grasped and squeezed and pushed along the tube, owing to the action of its own muscular cells, into a sac (stomach), in which it is rolled about and mixed with certain fluids, of peculiar chemical com- position derived from cells on its inner surface, which trans- form the proteid part of the food into a form susceptible of ready use (absorption). When this saccular organ has done its share of the work, the food is moved on by the action of the muscles of its walls into a very long portion of the tract in which, in addition to processes carried on in the mouth and stomach, there are others which transform the food into a condition in which it can pass into the blood. Thus, all of the food that is susceptible of changes of the kind described is acted upon somewhere in the long tract devoted to this task. But there is usually a remnant of indigestible material which is finally evacuated. How is the prepared material conveyed into the blood ? In part, directly through the walls of the minutest blood-vessels distributed throughout the length of this tube ; and in part through special vessels with appropriate cells covering them which act as minute porters {villi). The impure blood is carried periodically to an extensive sur- face, usually much folded, and there exposed in the hair-like tubes referred to before, and thus parts with its excess of car- bon dioxide and takes up fresh oxygen. But all the functions described do not go on in a fixed and invariable manner, but are modified somewhat according to circumstances. The for- cing-pump of the circulatory system does not always beat equally fast; the smaller blood-vessels are not always of the same size, but admit more or less blood to an organ according to its needs. This is all accomplished in obedience to the commands car- ried from the brain and spinal cord along the nerves. All movements of the limbs and other parts are executed in obe- dience to its behests ; and in order that these may be in accord- ance with the best interests of each particular organ and the whole animal, the nervous centers, which may be compared to the chief officers of, say, a telegraph or railway system, are in constant receipt of information by messages carried onward along the nerves. The command issuing is always related to the information arriving. All those parts commonly known as sense-organs — the eye, LIVING AND LIFELESS MATTER. 31 ear, nose, tongue, and the entire surface of the body — are faith- ful reporters of facts. They put the inner and outer worlds in communication, and without them all higher life at least must cease, for the organism, like a train directed by a conductor that disregards the danger-signals, must work its own destruction. Without going into further details, suffice it to say that the pro- cesses of the various cells are subordinated to the general good through the nervous system, and that susceptibility of proto- jDlasm to stimuli of a delicate kind which enables each cell to adapt to its surroundings, including the influence of remote as well as neighboring cells. Without this there could be no marked advance in organisms, no differentiation of a pro- nounced character, and so none of that physiological division of labor which will be inferred from our brief description of the functions of a mammal. The whole of physiology but illustrates this division of labor. It is hoped that the above account of the working of the animal body, brief as it is, may serve to show the connection of one part functionally with another, for it is much more impor- tant that this should be kept in mind throughout, than that all the details of any one function should be known. LIVING AND LIFELESS MATTER. In order to enable the student the better to realize the na- ture of living matter or protoplasm, and to render clearer the distinction between the forms that belong to the organic and inorganic worlds respectively, we shall make some com- parisons in detail which it is hoped may accomplish this ob- ject. A modern watch that keeps correct time must be regarded as a wonderful object, a marvelous triumph of human skill. That it has aroused the awe of savages, and been mistaken for a living being, is not surprising. But, admirable as is the result attained by the mechanism of a watch, it is, after all, composed of but a few metals, etc., chiefly in fact of two, brass and steel ; these are, however, made up into a great number of different parts, so adapted to one another as to work in unison and accomplish the desired object of indicating the time of day. Now, however well constructed the watch may be, there are waste, wear and tear, which will manifest themselves more and 32 ANIMAL PHYSIOLOGY. more, until finally tlie machine becomes worthless for the pur- pose of its construction. If this mechanism possessed the power of adapting from without foreign matter so as to con- struct it into steel and hrass and arrange this just when re- quired, it would imitate a living organism ; but this it can not do, nor is its waste chemically different from its component metals ; it does not break up brass and steel into something wholly different. In one particular it does closely resemble living things, in that it gradually deteriorates ; but the degra- dation of a living cell is the consequence of an actual change in its component parts, commonly a fatty degeneration. The one is a real transformation, the other mere wear. Had the watch the power to give rise to a new one like itself by any process, especially a process of division of itself into two parts, we should have a parallel with living forms ; but the watch can not even renew its own parts, much less give rise to a second mechanism like itself. Here, then, is a manifest dis- tinction between living and inanimate things. Suppose further that the watch was so constructed that, after the lapse of a certain time, it underwent a change in its inner machinery and perhaps its outer form, so as to be scarcely recognizable as the same ; and that as a result, instead of indi- cating the hours and minutes of a time-reckoning adapted to the inhabitants of our globe, it indicated time in a wholly dif- ferent way ; that after a series of such transformations it fell to pieces — took the original form of the metals from which it was constructed — we should then have in this succession of events a parallel with the development, decline, and death of living organisms. In another particular our illustration of a watch may serve a useful purpose. Suppose a watch to exist, the works of which are so concealed as to be quite inaccessible to our vision, so that all we know of it is that it has a mechanism which when in action we can hear, and the result of which we perceive in the movements of the hands on the face ; we should then be in the exact position in reference to the watch that we now are as re- gards the molecular movements of protoplasm. On the latter the entire behavior of living matter depends ; yet it is abso- lutely hidden from us. We know, too, that variations must be produced iu the mechanism of time-pieces by temperature, moisture, and other influences of the environment, resulting in altered action. The same, as will be shown in later chapters, occurs in protoplasm. CLASSIFICATION OF THE ANIMAL KINGDOM. 33 This, too, is primarily a molecular effect. If the works of watches were beyond observation, we should not be able to state exactly how the variations observed in different kinds, or even different individuals of the same kind occurred, though these differences might be of the most marked character, such as any one could recognize. Here once more we refer the differences to the mechanism. So is it with living beings: the ultimate molecular mechanism is unknown to us. Could we but render these molecular movements visible to our eyes, we should have a revelation of far greater scientific importance than that unfolded by the recent researches into those living forms of extreme minuteness that swarm every- where as dust in a sunbeam, and, as will be learned later, are often the source of deadly disease. Like the movements of the watch, the activities of protoplasm are ceaseless. A watch that will not run is, as such, worthless — it is mere metal — has under- gone an immense degradation in the scale of values; so proto- plasm is no longer protoplasm when its peculiar molecular movements cease ; it is at once degraded to the rank of dead matter. The student may observe that each of the four propositions, embodying the fundamental properties of living matter, stated in the preceding chapter, have been illustrated by the simile of a watch. Such an illustration is necessarily crude, but it helps one to realize the meaning of truths which gather force with each living form studied if regarded aright ; and it is upon the realization of truth that mental growth as well as practical efficiency depends. CLASSIFICATION OF THE ANIMAL KINGDOM. There are human beings so low in the scale as not to possess such general terms as tree, while they do employ names for dif- ferent kinds of trees. The use of such a word as " tree " im- ]jlies generalization, or the abstraction of a set of qualities from the tilings in which they reside, and making them the basis for the grouping of a multitude of objects by which we are sur- rounded. Manifestly without such a jjrocess knowledge must be vfry limited, and the world without significance; while in proportion as generalization may be safely widened, is our progress in the unification of knowledge toward which science is tending. But it also follow.s that without complete knowl- 8 34 ANIMAL PHYSIOLOGY. edge there can be no perfect classification of objects; hence, any classification must be regarded but as the temporary creed of science, to be modified with the extension of knowledge. As a matter of fact, this has been the history of all zoological and other systems of arrangement. The only purpose of grouping is to simplify and extend knowledge ; this being the case, it fol- lows that a method of grouping that accomplishes this has value, though the system may be artificial that is based on resemblances which, though real and constant, are associated with differences so numerous and radical that the total amount of likeness between objects thus grouped is often less than the difference. Such a system was that of Linnaeus, who classified plants according to the number of stamens, etc., they bore. Seeing that animals which resemble each other are of com- mon descent from some earlier form, to establish the line of de- scent is to determine in great part the classification. Much as- sistance in this direction is derived from embryology, or the history of the development of the individual ( ontogeny) ; so that it may be said that the ontogeny indicates, though it does not actually determine, the line of descent (phylogeny) ; and it is owing to the importance of this truth that naturalists have in recent years given so much attention to comparative embry- ology. It will be inferred that a natural system of classification must be based both on function and structure, though chiefly on the latter, since organs of very different origin may have a similar function ; or, to express this otherwise, homologous structures may not be analogous ; and homology gives the better basis for classification. To illustrate, the wing of a bat and a bird are both homologous and analogous; the wing of a butterfly is analogous but not homologous with these ; manifestly, to clas- sify bats and birds together would be better than to put birds and insects in the same group, thus leaving other points of re- lationship out of consideration. The broadest possible division of the animal kingdom is into groups, including respectively one-celled and many-celled forms — i. e., into Protozoa and Metazoa. As the wider the grouping the less are differences considered, it follows that the more subdivided the groups the more complete is the informa- tion conveyed : thus, to say that a dog is a metazoan is to con- vey a certain amount of information ; that it is a vertebrate, more ; that it is a mammal, a good deal more, because each of the latter terms includes the former. Animal Kiug- dom. CLASSIFICATION OF THE ANIMAL KINGDOM. 35 f Protozoa (amoeba, vorticella, etc.). j Coelenterata (sponges, jelly-fish, jjolyps, etc.). I Echinodermata (star-fish, sea-urchins, etc.). r Inverte- J Vermes (worms). brata. ] Arthropods (crabs, insects, spiders, etc.). Mollusca (oysters, snails, etc.). Molluscoidea (moss-like animals). [ Tunicata (ascidians). f Pisces (fishes). I Amphibia (frogs, menobranchus, etc.). t Vertebrata. -J Keptilia (snakes, turtles, etc.). I Aves (birds). [ Mammalia (domestic quadrupeds, etc.). The above classification (of Claus) is, like all such arrange- ments, but the expression of one out of many methods of view- ing the animal kingdom. For the details of classification and for the grounds of that we have presented, we refer the student to works on zoology ; but we advise those who are not familiar with this subject, when a technical term is used, to think of that animal belong- ing to the group in question with the structure of which they are best acquainted. Man's Place in the Animal Kingdom. It is no longer the custom with zoologists to place man in an entirely separate group by himself ; but he is classed with the primates, among which are also grouped the anthropoid apes (gorilla, chimpanzee, orang, and the gibbon), the monkeys of the Old and of the New World, and the lemurs. So great is the structural resemblance of man and the other primates that competent authorities declare that there is more difference be- tween the structure of the most widely separated members of the group than between certain of the anthr(^poid apes and man. The points of greatest resemblance between man and the anthropoid apes are the following : The same number of verte- bree ; the same general shape of the pelvis ; a brain distinguish- ing them from other mammals; and posture, being bipeds. The distinctive characters are size, rather than form of the brain, that of man being more than twice as large ; a relatively larger cranial base, by which, together with the greater size of the jaw.s, the face becomes prominent ; the earlier closure of the sutures of the cranium, arresting the growth of the brain ; more developed canine teeth and difference in the order of eruption of the permanent teeth ; the more i)osterior position of the foramen magnum ; the relative length of the limbs to 36 ANIMAL PHYSIOLOGY. each other and the rest of the body ; minor differences in the hands and feet, especially the greater freedom and power of apposition of the great-toe. But the greatest distinction between man and even his closest allies among the apes is to be found in the development to an incomparably higher degree of his intellectual and moral nature, corresponding to the differences in weight and structure of the human brain, and associated with the use of spoken and written language ; so that the experience of previous genera- tions is not only registered in the organism (heredity), but in a form more quickly available (books, etc.). The greatest structural difference between the races of men are referable to the cranium; but, since they all interbreed freely, they are to be considered varieties of one species. THE LAW OP PERIODICITY OR RHYTHM IN NATURE. The term rhythm to most minds suggests music, poetry, or dancing, in all of which it forms an essential part so simple, pronounced, and uncomplicated as to be recognized by all with ease. The regular division of music into bars, the recurrence of chords of the same notes at certain intervals, of forte and piano, seem to be demanded by the very nature of the human mind. The same applies to poetry. Even a child that can not under- stand the language used, or an adult listening to recitations in an unknown tongue, enjoys the flow and recurrences of the sounds. Dancing has in all ages met a want in human organi- zations, which is partly supplied in quieter moods by the regu- larity of the steps in walking and similar simple movements. But as rhythm runs through all the movements of animals, so is it also found in all literature and all art. Infinite variety wearies the mind, hence the fatigue felt by the sight-seer. Re- currence permits of repose, and gratifies an established taste or appetite. The mind delights in what it has once enjoyed, in repetition within limits. Repetition with variety is manifestly a condition of the growth and development of the mind. This seems to apply equally to the body, for every single function of each organism, however simple or complex it may be, exem- plifies this law of periodicity. The heart's action is rhythmical (beats) ; the blood flows in intermitting gushes from the central pump ; the to-and-fro movements of respiration are so regular THE LAW OF PERIODICITY OR RHYTHM IN NATURE. 37 that their cessation would arouse the attention of the least instructed ; food is demanded at regular intervals ; the juices of the digestive tract are poured out, not constantly but period- ically ; the movements by which the food is urged along its path are markedly rhythmic ; the chemical processes of the body wax and wane like the fires in a furnace, giving rise to regular augmentations of the temperature of the body at fixed hours of the day, with corresponding periods of greatest bodily activity and the reverse. This principle finds perfect illustration in the nervous sys- tem. The respiratory act of the higher animals is efi:ected through muscular movements dependent on regular waves of excitation reaching them along the nerves from the central cells which regularly discharge their forces along these channels. Were not the movements of the body periodic or rhythmical, instead of that harmony which now prevails, every muscular act would be a convulsion, though even in the movements of the latter there is a highly compounded rhythm, as a noise is made up of a variety of musical notes. The senses are subject to the same law. The eye ceases to see and the ear to hear and the hand to feel if continuously stimulated ; and doubtless in all art this law is unconsciously recognized. That ceases to be art which fails to provide for the alternate repose and excita- tion of the senses. The eye will not tolerate continuously one color, the ear a single sound. Why is a breeze on a warm day so refreshing ? The answer is obvious. Looking to the world of animate nature as a whole, it is noticed that i)lants have their period of sprouting, flowering, seeding, and decline ; animals are born, pass through various stages to maturity, diminish in vigor, and die. These events make epochs in the life-history of each species ; the recurrence of which is so constant that the agricultural and other arrange- ments even of savages are planned accordingly. That the in- dividuals of each animal group have a definite period of dura- tion is another manifestation of the same law. Superficial observation suffices to furnish facts which show that the same law of periodicity is being constantly exemplified in the world of inanimate things. The regular ebb and flow of the tides ; the rise and suT)sidence of rivers ; the storm and the calm; summer and winter; day and night — are all recurrent, none constant. Events apparently without any regularity, utterly l)eyond any law of recurrence, when sufficiently studied are found to 38 ANIMAL PHYSIOLOGY. fall under the same principle. Thus it took some time to learn that volcanic eruptions occurred with a very fair degree of regularity. In judging of this and all other rhythmical events it must be borne in mind that the time standard is for an irregularity that seems large, as in the instance just referred to, becomes small when considered in relation to the millions of years of geological time ; while in the case of music a trifling irregu- larity, judged by fractions of a second, can not be tolerated by the musical organization— which is equivalent to saying that the interval of departure from exact regularity seems large. As most of the rhythms of the universe are compounded of several, it follows that they may seem, until closely studied, very far from regular recurrences. This may be observed in the interference in the regularity of the tides themselves, the daily changes of which are subject to an increase and decrease twice in each month, owing to the influence of the sun and moon being then either coincident or antagonistic. In the functions of plants and animals, rhythms must be- come very greatly compounded, doubtless often beyond recog- nition. Among the best examples of rhythm in animals are daily sleep and winter sleep, or hibernation ; yet, amid sleep, dreams or recurrences of cerebral activity are common — that is, one rhythm (of activity) overlies another (of repose). In like man- ner many hibernating animals do not remain constantly in their dormant condition throughout the winter months, but have periods of wakefulness ; the active life recurs amid the life of functional repose. To return to the world of inanimate matter, we find that the crust of the earth itself is made of layers or strata the result of periods of elevation and depression, of denudation and deposi- tion, in recurring order. The same law is illustrated by the facts of the economic and other conditions of the social state of civilized men. Periods of depression alternate with periods of revival in commercial life. There are periods when many more marriages occur and many more children are born, corresponding with changes in the material conditions which influence men as well as other animals. Finally, and of special interest to the medical student, are THE LxVW OF PERIODICITY OR RHYTHM IN NATURE. 39 the laws of rhythm in disease. Certain fevers have their regu- lar periods of attack, as intermittent fever ; while all diseases have their periods of exacerbation, however invariable the symptoms may seem to be to the ordinary observer or even to the patient himself. Doubtless the fact that certain hereditary diseases do not appear in the offspring at once, but only at the age at which they were manifested in the parents, is owing to the same cause. Let us now examine more thoroughly into the real nature of this rhythm which pervades the entire universe. If a bow be drawn across a violin-string on which some small pieces of paper have been placed, these will be seen to fly off ; and if the largest string be experimented upon, it can be ob- served to be in rapid to-and-fro motion, known as vibration, which motion is perfectly regular, a definite number of move- ments occurring within a measured period of time ; in other words the motion is rhythmical. In strings of the finest size the motion is not visible, but we judge of its existence because of the result, which is in each instance a sound. Sound is to us, however, an affection of the nerve of hearing and the brain, owing to the vibrations of the ear caused by similar vibra- tions of the violin-strings. The movements of the nerves and nerve-cells are invisible and molecular, and we seem to be justified in regarding molecular movements as constant and associated with all the properties of matter ivhether living or dead. We see, then, that all things living and lifeless are in con- stant motion, visible or invisible ; there is no such thing in the universe as stable equilibrium. Change, ceaseless change, is written on all things; and, so far as we can judge, these changes, on the whole, tend to higher development. Neither rhythm, however, nor anything else, is perfect. Even the mo- tions of jjlanets are subject to perturbations or irregularities in their periodicity. This subject is plainly boundless in its scope. We have introduced it at this stage to prepare for its study in detail in dealing with each function of the animal body. If we are correct as to the universality of the law of rhythm, its importance in biology deserves fuller recognition than it has yet received in works on physiology ; it will, ac- cordingly, be frequently referred to in the future chapters of this book. 40 ANIMAL PHYSIOLOGY. THE LAW OF HABIT. Every one must have observed in himself and others the tendency to fall into set ways of doing certain things, in which will and clear purpose do not come prominently into view. Further observation shows that the lower animals exhibit this tendency, so that, for example, the habits of the horse or the dog may be an amusing reflection of those of the master. Trees are seen to bend permanently in the direction toward which the prevailing winds blow. The violin that has experienced the vibrations aroused by some master's hand acquires a potential musical capability not possessed by an instrument equally good originally, but the molecular movements of which never received such an educa- tion. It appears, then, that underlying what we call habit, there is some broad law not confined to living things ; indeed, the law of habit appears to be closely related to the law of rhythm we have already noticed. Certain it is that it is inseparable from all biological phenomena, though most manifest in those organ- isms provided with a nervous system, and in that system itself. What we usually call habit, however expressed, has its physical correlation in the nervous system. We may refer to it in this connection later : but the subject has relations so numerous and fundamental that it seems eminently proper to introduce it at this early stage, forming as it does one of those corner-stones of the biological building on which the superstructure must rest. When we seek to come to a final explanation of habit in this case, as in most others, in which the fundamental is involved, we are soon brought against a wall over which we are unable to climb, and through which no light comes to our intellects. We must simply believe, as the result of observation, that it is a law of matter, in all the forms manifested to us, to assume accustomed modes of behavior, perhaps we may say molecular movement, in obedience to inherent tendencies. But, to recog- nize this, throws a flood of light on what would be inexplicable, even in a minor degree. We can not explain gravitation in it- self ; but, assuming its universality, replaces chaos by order in our speculations on matter. Turning to living matter, we look for the origin of habit in the apparently universal principle that primary molecular movement in one direction renders that movement easier after- THE ORIGIN OP THE FORMS OF LIFE. 41 ward, and in proportion to the frequency of repetition ; which is equivalent to saying that functional activity facilitates func- tional activity. Once accepting this as of universal application in biology, we have an explanation of the origin, the compara- tive rigidity, and the necessity of habit. There must be a phys- ical basis or correlative of all mental and moral habits, as well as those that may be manifested during sleep, and so purely in- dependent of the will and consciousness. We are brought, in fact, to the habits of cells in considering those organs, and that combination of structures which makes up the complex individ- ual mammal. It is further apparent that if the cell can trans- mit its nature as altered by its experiences at all, then habits must be hereditary, which is known to be the case. Instincts seem to be but crystallized habits, the inherited results of ages of functional activity in certain well-defined directions. To a being with a highly developed moral nature like man, the law of habit is one of great, even fearful significance. We make to-day our to-morrow, and in the present we are deciding the future of others, as our present has been made for us in part by our ancestors. We shall not pursue the subject, which is of boundless extent, further now, but these somewhat general statements will be amplified and applied in future chapters. THE ORIGIN OF THE FORMS OF LIFE. It is a matter of common observation that animals originate from like kinds, and plants from forms resembling themselves ; while most carefully conducted experiments have failed to show that living matter can under any circumstances known to us arise from other than living matter. That in a former condition of the universe such may have been the case has not been disproved, and seems to be the logical outcome of the doctrine of evolution as applied to the universe generally. By evolution is meant the derivation of more complex and differentiated forms of matter from sirajjler and more homogene- ous ones. When this theory is applied to organized or living forms, it is termed organic evolution. There are two views of the origin of life: tlie one, tliat each distinct group of plants and animals was independently created ; while by " creation " is simply meant tliat they came into being in a manner we know 42 ANIMAL PHYSIOLOGY. not how, in obedience to the will of a First Cause. The other view is denominated the theory of descent with modification^ the theory of transmutation, organic evolution etc., which teaches that all the various forms of life have been derived from one or a few primordial forms in harmony with the recog- nized principles of heredity and variability. The most widely known and most favorably received exposition of this theory is that of Charles Darwin, so that his views will be first presented in the form of a hypothetical case. Assume that one of a group of living forms varies from its fellows in some particular, and mating with another that has similarly varied, leaves progeny inheriting this characteristic of the parents, that tends to be still further increased and rendered permanent by successive pairing with forms possessing this variation in form, color, or whatever it may be. We may suppose that the variations may be numerous, but are always small at the beginning. Since all animals and plants tend to multiply faster than the means of support, a competition for the means of subsistence arises, in which struggle the fittest, as judged by the circumstances, always is the most successful ; and if one must perish outright, it is the less fit. If any variation arises that is unfavorable in this contest, it will render the possessor a weaker competitor : hence it follows that only useful variations are preserved. The struggle for existence is, however, not alone for food, but for anything which may be an advantage to its possessor. One form of the contest is that which results from the rivalry of members of the same sex for the possession of the females ; and as the female chooses the strongest, most beautiful, most active, or the supreme in some respect, it follows that the best leave the greatest number of progeny. This has been termed sexual selection. In determining what forms shall survive, the presence of other plants or animals is quite as important as the abun- dance of food and the physical conditions, often more so. To illustrate this by an example : Certain kinds of clover are fer- tilized by the visits of the bumble-bee alone ; the numbers of bees existing at any one place depends on the abundance of the field-mice which destroy the nests of these insects ; the numbers of mice will depend on the abundance of creatures that prey on the mice, as hawks and owls ; these, again, on the creatures that specially destroy them, as foxes, etc. ; and so on, the chain of connections becoming more and more lengthy. If a certain proportion of forms varying similarly were sep- THE ORIGIN OF THE FORMS OF LIFE. 43 arated by auy great natural barrier, as a chain of lofty mount- ains or an intervening body of water of considerable extent, and so prevented from breeding with forms that did not vary, it is clear that there would be greater likelihood of their differ- ences being preserved and augmented up to the point of their greatest usefulness. We may now inquire whether such has actually been the course of events in nature. The evidence may be arranged under the follo^ving heads : 1. Morphology. — Briefly, there is much that is common to entire large groups of animals ; so great, indeed, are the resem- blances throughout the whole animal kingdom, that herein is found the strongest argument of all for the doctrine of descent. To illustrate by a single instance — fishes, reptiles, birds, and mammals possess in common a vertebral column bearing the same relationship to other parts of the animal. It is because of resemblances of this kind, as well as by their differences, that naturalists are enabled to classify animals. 2. Embryology. — In the stages through which animals pass in their development from the ovum to the adult, it is to be ob- served that the closer the resemblance of the mature organism in different groups, the more the embryos resemble one another. Up to a certain stage of development the similarity between groups of animals, widely separated in their post-embryonic life, is marked : thus the embryo of a reptile, a bird, and a mam- mal have much in common in their earlier stages. The embryo of the mammal passes through stages which represent condi- tions which are permanent in lower groups of animals, as for example that of the branchial arches, which are represented by the gills in fishes. It may be said that the developmental his- tory of the individual (ontogeny) is a brief recapitulation of the development of the species (phylogeny). Apart from the theory of descent, it does not seem possible to gather the true significance of such facts, which will become plainer after the study of the chapters on reproduction. 3. Mimicry may be cited as an instance of useful adapta- tion. Thus, certain beetles resemble bees and wasps, whicli lat- ter are protected by stings. It is believed that such groups of beetles as these arose by a species of selection ; those escajjing enemies which clianced to resemble dreaded insects most, so that birds wliich were accustomed to prey on beetles, yet feared bees, would likewise avoid the mimicking forms. 4. Rudimentary Organs. — Organs whicli were once functional 44 ANIMAL PHYSIOLOGY. Fig. 47. — Shows the embryos of four mammals in the three corresponding stages : of a hog (H), calf (C), rabbit (R), and a man (M). The conditions of the three different stages of devel- opment, which the three cross-rows (I, II, III) represent, are selected to correspond as exactly as possible. The first, or upper cross-row, I, represents a very early stage, with gill-openings, and without limbs. The second (middle) cross-row, II, shows a somewhat later stage, with the first rudiments of limbs, while the gill-openings are yet retained. The third (lowest) cross-row. III, shows a still later stage, with the limbs more developed and the gill-openings lost. The membranes atid appendages of the embryonic body (the amnion, yelk-sac, allantois) are omitted. The whole twelve figures are slightly magnified, the upper ones more than the lower. To facilitate the comparison, they are all reduced to nearly the same size in the cuts. All the embryos are seen from the left side : the head extremity is above, the tail extremity below ; the arched back turned to the right. The letters indicate the same parts in all the twelve figures, namely : v, fore-brain ; z, twixt- brain ; m, mid-brain ; h, hind-brain ; n, after-brain ; r, spinal marrow : e, nose ; a, eye ; o, ear ; fc, gill-arches ; g, heart ; w, vertebral colmnn ; /, fore-limbs ; b, hind-limbs ; s, tail. (After Haeekel.) THE ORIGIN OF THE FORMS OF LIFE. 45 in a more ancient form^ but serve no use in tlie creatures in which they are now found, have reached, it is thought, their rudimentary condition through long periods of comparative disuse, in many generations. Such are the rudimentary mus- cles of the ears of man, or the undeveloped incisor teeth found in the upper jaw of ruminants. 5. Geographical Distribution. — It can not be said that animals and plants are always found in the localities where they are best fitted to flourish. This has been well illustrated within the lifetime of the present generation, for the animals intro- duced into Australia have many of them so multiplied as to displace the forms native to that country. But, if we assume that migrations of animals and transmutations of species have taken place, this difficulty is in great part removed. 0. Paleontology. — The rocks bear record to the former exist- ence of a succession of related forms ; and, though all the in- termediate links that probably existed have not been found, the apparent discrepancy can be explained by the nature of the circumstances under which fossil forms are preserved ; and the " imperfection of the geological record." It is only in the sedimentary rocks arising from mud that fossils can be preserved, and those animals alone with hard parts are likely to leave a trace behind them ; while if these sedimentary rocks with their inclosed fossils should, owing to enormoiis pressure or heat be greatly changed (metamorphosed), all trace of fossils must disappear — so that the earliest forms of life, those that would most naturally, if preserved at all, be found in the most ancient rocks, are wanting, in consequence of the metamorphism which such formations have undergone. Moreover, our knowledge of the animal remains in the earth's crust is as yet very incomplete, though, the more it is explored, tlie more the evidence gathers force in favor of organic evolu- tion. But it must be remembered that those groups constitut- ing species ai-c in geological time intermediate links. 7. Fossil and Existing Species. — If the animals and plants now peopling the earth were entirely different from those that flour- ished in the past, the objections to the doctrine of descent would be greatly strengthened ; but when it is found that there is in some cases a scarcely broken succession of forms, great force is added to the arguments by which we are led to infer the con- nection of all forms with one another. To illustrate by a single instance: the existing group of horses, with a single toe to each foot, was preceded in geological 46 ANIMAL PHYSIOLOGY. time in America by forms with a greater number of toes, the latter increasing according to the antiquity of the group. Fig. 48.— Bones of the feet of the different genera of Equidce (after Marsh), a, foot of Oro- hippus (Eocene) ; b, foot of Anchitherium (Lower Miocene) ; c, foot of Hipparion (Plio- cene) ; d, foot of the recent genus Equus. These forms occur in succeeding geological formations. It is impossible to resist the conclusion that they are related gene- alogically (phylogenetically). 8. Progression. — Inasmuch as any form of specialization that would give an animal or plant an advantage in the struggle for existence would be preserved, and as in most cases when the competing forms are numerous such would be the case, it is possible to understand how the organisms that have appeared have tended, on the whole, toward a most pronounced pro- gression in the scale of existence. This is well illustrated in the history of civilization. Barbarous tribes give way be- fore civilized man with the numberless subdivisions of labor he institutes in the social organism. It enables greater num- bers to flourish as the competition is not so keen as if activities could be exercised in a few directions only. 9. Domesticated Animals. — Darwin studied our domestic ani- mals long and carefully, and drew many important conclusions from his researches. He was convinced that they had all been derived from a few wild representatives, in accordance with the principles of natural selection. Breeders have, both consciously and unconsciously, formed races of animals from stocks which the new groups have now supplanted ; while primitive man had tamed various species which he kept for food and to assist in the chase, or as beasts of burden. It is impossible to believe that all the different races of dogs have originated from dis- tinct wild stocks, for many of them have been formed within recent periods ; in fact, it is likely that to the jackal, wolf, and THE ORIGIN OF THE FORMS OF LIFE. 47 fox, must we look for the wild progenitors of our dogs. Dar- win concluded that, as man had only utilized the materials Na- ture provided in forming his races of domestic animals, he had availed himself of the variations that arose spontaneously, and increased and fixed them by breeding those possessing the same variation together, so the like had occurred without his aid in nature among wild forms. Evolutionists are divided as to the origin of man himself ; some, like Wallace, who are in accord with Darwin as to the ^ 3 Fio. 49.— Skeleton of hand or fore-foot of six mammals. I, man ; II, dog ; III, pig : FV, ox ; V, tapir : VI. horse, r, radius; it, ulna ; a, scaphoid ; b. semi-lunar ; c, triquetrum (cunei- form) ; d. trapezium ; e. trapezoid ; /, capitatum (unciform process) ; g, hamatum (unci- form bone) ; p. pisiform : 1, thumb ; 2. digit ; 3. middle finger ; 4. ring-finger ; 5, little finger. (After Gegenbaur.) origin of living forms in general, believe that the theory of natural selection does not suffice to account for the intellectual and moral nature of man. Wallace believes that man's body has been derived from lower forms, but that his higher nature is the result of some unknown law of accelerated development ; while Darwin, and those of his way of thinking, consider that man in his entire nature is but a grand development of powers existing in minor degree in the animals behnv him in the scale. Sumlnary. — Every group of animals and plants tends to in- crease in numbers in a geometrical progression, and must, if unchecked, overrun the earth. Every variety of animals and Y>lants imparts to its offspring a general resemblance to itself, but with minute variations from the original. The variations of oflfspring may be in any direction, and by accumulation ANIMAL PHYSIOLOGY. THE ORIGIN OF THE FORMS OF LIFE. 49 X ■^ Fio. 55.— a, chimpanzee ; 6, iiforilla ; c, orang ; d, negro. (Haeckel.) 50 ANIMAL PHYSIOLOGY. Fig. 56.— Head of a nose-ape iSeni- nopithecus nasicux) from Bor- neo. (After Brehm.) Fig. 57.— Head of Julia Pas- trana. (From a photo- graph by Hintze.) constitute fixed differences by which a new group is marked off. In the determination of the variations that persist, the law of survival of the fittest operates. EEPRODUCTION. As has been already noticed, protoplasm, in whatever form, after passing through certain stages in development, undergoes a decline, and finally dies and joins the world of unorganized matter ; so that the permanence of living things demands the constant formation of new individuals. Groups of animals and plants from time to time become extinct; but the lifetime of the species is always long compared with that of the individ- ual. Reproduction by division seems to arise from an exigency of a nutritive kind, best exemplified in the simpler organisms. When the total mass becomes too great to be supported by absorption of pabulum from without by the surface of the body, division of the organism must take place, or death ensues. It appears to be a matter of indifference how this is accom- plished, whether by fission, endogenous division, or gemmation, so long as separate portions of protoplasm result, capable of leading an independent existence. The very undifferentiated character of these simple forms prepares us to understand how each fragment may go through the same cycle of changes as the parent form. In such cases, speaking generally, a million individuals tell the same biological story as one ; yet these must exist as individuals, if at all, and not in one great united mass. But in the case of conjugation, which takes place some- times in the same groups as also multiply by division in its various forms, there is plainly an entirely new aspect of the REPRODUCTION". 51 case presented. We have already shown that no two cells, how- ever much alike they may seem as regards form and the cir- cumstances under which they exist, can have, in the nature of the case, precisely the same history, or be the subjects of ex- actly the same experiences. We have also pointed out that all these phenomena of cell-life are known to us only as adaptations of internal to external conditions ; for, though we may not be always able to trace this connection, the inference is justi- fiable, because there are no facts known to us that contradict such an assumption, while those that are within our knowledge bear out the generalization. We have already learned that liv- ing things are in a state of constant change, as indeed are all things ; we have observed a constant relation between certain changes in the environment, or sum total of the surrounding conditions, as, for example, temperature and the behavior of the protoplasm of plants and animals ; so that we must believe that any one form of protoplasm, however like another it may seem to our comparatively imperfect observation, is different in some respects from every other — as different, relatively, as two human beings living in the same community during the whole of their lives ; and in many cases as unlike as individuals of very different nationality and history. We are aware that when two such persons meet, provided the unlikeness is not so great as to prevent social intercourse, intercommunication may prove very instructive. Indeed, the latter grows out of the former ; our illustration is itself explained by the law we are endeavoring to make plain. It would appear, then, that con- tinuous division of protoplasm without external aid is not pos- sible ; but that the vigor necessary for this must in some way be imparted by a particle (cell) of similar, yet not wholly like, protoplasm. This seems to furnish an explanation of the neces- sity for the conjugation of living forms, and the differentiation of sex. Very frequently conjugation in the lowest animals and plants is followed by long periods when division is the prevail- ing method of reproduction. It is worthy of note, too, that when living forms conjugate, they both become quiescent for a longer or shorter time. It is as though a period of preparation preceded one of extraordinary activity. We can at present trace only a few of the ste{>s in this rejuvenation of life-stuff. Some of these have been already indicated, which, with others, will now be further studied in this division of our subject, both because reproduction throws so much light on cell-life, and be- '•ause it is so important for the understanding of the physio- 52 ANIMAL PHYSIOLOGY. logical behavior of tissues and organs. It may be said to be quite as important that the ancestral history of the cells of an organism be known as the history of the units composing a community. A, B, and C can be much better understood if we know something alike of the history of their race, their an- cestors, and their own past ; so is it with the study of any indi- vidual, animal, or group of animals or plants. Accordingly, embryology, or the history of the origin and development of tissues and organs, will occupy a prominent place in the va- rious chapters of this work. The student will, therefore, at the outset be furnished with a general account of the subject, while many details and applications of principles will be left for the chapters that treat of the functions of the various organs of animals. The more knowledge the student possesses of zo- ology the better, while this science will appear in a new light under the study of embryology. Animals are divisible, according to general structure, into Protozoa, or unicellular animals, and Metazoa, or multicellular forms — that is, animals composed of cell aggregates, tissues, or organs. Among the latter one form of reproduction appears for the first time in the animal kingdom, and becomes all but universal, though it is not the exclusive method ; for, as seen in Hydra, both this form of generation and the more primitive gemmation occur. It is known as sexual multiplication, which usually, though not invariably, involves conjugation of two un- like cells which may arise in the same or different individuals. That these cells, known as the male and female elements, the ovum and the spermatozoon, are not necessarily radically differ- ent, is clear from the fact that they may arise in the one individ- ual from the same tissue and be mingled together. These cells, however, like all others, tell a story of continual progressive differentiation corresponding to the advancing evolution of higher from lower forms. Thus liermaphroditisrii, or the coex- istence of organs for the production of male and of female cells in the same individual, is confined to invertebrates, among which it is rather the exception than the rule. Moreover, in such hermaphrodite forms the union of cells with greater differ- ence in experiences is provided for by the union of different in- dividuals, so that commonly the male cell of one individual unites with (fertilizes) the female cell of a different individual. It sometimes happens that among the invertebrates the cells produced in the female organs of generation possess the power of division, and continued development wholly independently of REPRODUCTION. 53 the access of any male cell {parthenogenesis) ; such, however, is almost never the exclusive method of increase for any group of animals, and is to be regarded as a retention of a more ancient method, or perhaps rather a reversion to a past biological con- dition. No instance of complete parthenogenesis is known among vertebrates, although in birds partial development of the egg may take place independently of the influence of the male sex. The best examples of parthenogenesis are to be found among insects and crustaceans. It is to be remembered that, while the cells which form the tissues of the body of an animal have become specialized to discharge one particular function, they have not wholly lost all others ; they do not remain characteristic amoeboids, as we may term cells closely resembling Amoeba in behavior, nor do they wholly forsake their ancestral habits. They all retain the power of reproduction by division, especially when young and most vigorous ; for tissues grow chiefly by the production of new cells rather than the enlargement of already mature ones. Cells wear out and must be replaced, which is effected by the processes already described for Amoeba and similar forms. Moreover, there is retained in the blood of animals an army of cells, true amoeboids, ever ready to hasten to repair tissues lost by injury. These are true remnants of an embryonic condition ; for at one period all the cells of the organism were of this undifferentiated, plastic character. But the cell (ovum) from which the individual in its entirety and with all its complexity arises mostly by the union with another cell {spermatozoon), must be considered as one that has remained unspecialized and retained, and perhaps increased its reproductive functions. They certainly have become more complex. The germ-cell may be considered unspecialized as regards other functions, but highly .specialized in the one direction of exceedingly great capacity for growth and complex division, if we take into ac- count the whole chain of results ; though in considering this it must be borne in mind that after a certain stage of division each individual cell repeats its ancestral history again ; that is to .say, it divides and gives rise to cells which progress in turn as well as multiply. From another point of view the ovum is a marvelous storehouse of energy, latent or potential, of course, but under proper conditions liberated in varied and unexpe(;ted forms of force. It is a sort of storeliouse of biological energy in the most concentrated form, the liberation of which in sim- pler forms gives rise to that complicated chain of events which 54: ANIMAL PHYSIOLOGY. is termed by the biologist development, but wMch. may be ex- pressed by the physiologist as the transformation of potential into kinetic energy, or the energy of motion. Viewed chemic- ally, it is the oft-repeated story of the production of forms, of greater stability and simplicity, from more unstable and com- plex ones, involving throughout the process of oxidation ; for it must ever be kept in mind that life and oxidation are concomi- tant and inseparable. The further study of reproduction in the concrete will render the meaning and force of many of the above statements clearer. The Ovum. The typical female cell, or ovum, consists of a mass of proto- plasm, usually globular in form, containing a nucleus and nu- cleolus. The ovum may or may not be invested by a membrane ; the protoplasm of the body of the cell is usually highly granular, and may have stored up within it a varying amount of proteid material (food-yelk), which has led to division of ova into classes, according to the manner of distribution of this nutri- tive reserve. It is either concentrated at one pole {telolecith- al) ; toward the center [centrolecitlial) ; or evenly distributed throughout {alecithal). Dur- ing development this material is converted by the agency of the cells of the young organ- ism {embryo) into active pro- toplasm ; in a word, they feed upon and assimilate or build up this food-stuff into their own substance, as Amoeba does with any proteid material it appropriates. The nucleus {germinal vesi- cle) is large and well-defined, and contains within itself a highly refractive nucleolus {germinal spot). These closely resemble in general the rest of the cell, but stain more deeply and are chemically different in that they contain nucleine {nucleoplasm, cliromatin). It will be observed that the ovum differs in no essential par- FiG. 58. — Semi-diagrammatic representation of a mammalian ovum (Schafer). Highly- magnified, zp, zona pellucida ; vi, vitel- lus ; gv, germinal vesicle ; gs, germinal spot. REPRODUCTION. 55 Fig. 59. — A human egg (much enlarged) from the ovary of a female. The whole egg is a simple spherical cell. The greater part of this cell is formed bj- the egg-yelk, by the gran- ular cell-substance (protoplasm), consisting of innumerable yelk-granules with a little inter-granular substance. In the upper part of the yelk lies the bright, globular, germ- vesicle, corresponding with the cell-kernel (dwc/eu.s). This contains a darker germ-spot, answering to the nucleolus. The globular yelk-mass is surrounded by a thick, light- colored egg-membrane (zona pellucida. or chorion). This is traversed by very numerous hair-like lines, radiating toward the central point of the ma.ss : these are the porous canals, through which, in the course of fertilization, the thread-shaped, active sperm-cells penetrate into the egg-yelk. (Haeckel.) ticular of structure from other cells. Its differences are hidden ones of molecular structure and functional behavior. In ac- cordance with the diverse circumstances under which ova mature and develop, certain variations in structure, mostly of the nature of additions, present themselves. Thus, ova may be naked, or provided with one or more coverings. In vertebrates there are usually two membranes around the protoplasm of the ovum : a delicate covering ( Vi- telline membrane), beneath which there is another, which is sieve-like from numerous perforations {zona radiata, or z. pellucida). The egg membrane may be impregnated with lime salts (shell). Between the membranes and the yelk there is a fluid albuminous substance secreted by the glands of the ovi- duct, or by other special glands, which jjrovide proteid nutri- ment in different x>hysical condition from that of the yolk. The general naked-eye apjjearances of the ovum may be learned from the examination of a hen's egg, which is one of 56 ANIMAL PHYSIOLOGY. the most complicated known, inasmuch as it is adapted for development outside of the body of the mother, and must, con- sequently, be capable of preserving its form and essential vital properties in a medium in which it is liable to undergo loss of water, protected as it now is with shell, etc., but which, at the ch.l Fig. 60.— Diagrammatic section of an unimpreffnated fowl's egg (Foster and Balfour, after Allen Thomson). 6i, blastoderm or cicatricula ; w. y, white yolk ; y. y, yellow yelk ; ch. I, chalaza ; i. s. ni, inner layer of shell membrane ; s. in, outer layer of shell membrane ; s, shell ; a. c. h, air-space ; w, the white of the egg ; v. t, vitelline membrane ; x, the denser albuminous layer lying next the vitelline membrane. same time, permits the entrance of oxygen and moisture, and conducts heat, all being essential for the development of the germ within this large food-mass. The shejl serves, evidently, chiefly for protection, since the eggs of serpents (snakes, turtles, etc.) are provided only with a very tough membranous cover- ing, this answering every purpose in eggs buried in sand or otherwise protected as theirs usually are. As the hen's egg is that most readily studied and most familiar, it may be well to describe it in somewhat further detail, as illustrated in the above figure, from the examination of which it will be ap- parent that the yelk itself is made up of a white and yellow portion distributed in alternating zones, and composed of cells of different microscopical appearances. The clear albumen is structureless. The relative distribution, and the nature of the accessory or non-essential parts of the hen's egg, will be understood when it is remembered that, after leaving its seat of origin, which will be presently described, the ovum passes along a tube (oviduct) REPRODUCTION. 57 Idv a movement imparted to it by the muscular walls of the latter, similar to that of the gullet during the swallowing of food ; that this tube is provided with glands which secrete in turn the albumen, the membrane (outer), the lime salts of the shell, etc. The twisted appearance of the rope-like structures (chalazce) at each end is owing to the spiral rotatory movement the egg has undergone in its descent. The air-chamber at the larger end is not present from the first, but results from evaporation of the fluids of the albumen and the entrance of atmos]oheric air after the egg is laid some time. The Origin and Development of the Ovum. Between that protrusion of cells which gives rise to the bud which develops directly into the new individual, and that which forms the ovary with- rS. TTE. PS. in which the ovum as a mod- ified cell arises, there is not in Hydra much difference at first to be observed. In the mammal, however, the ovary is a more complex etructure, though, relatively to many organs, still simple. It consists, in the main, of connective tissue supplied with vessels and nerves in- closing m(jdifications of that tissue (Graafian follicles) within which the ovum is matured. The ovum and the follicles arise from an inver- sion of epithelial cells, on a portion of the body cavity (fjerminal ridge), which give rise to the ovum itself, and the other cells surrounding it in the Graafian follicle. At first these inversions form tubules (egg-tubes) which lat- er bec(;nie broken up into iso- lated nests of cells, the ft^re-runners of the Graafian follicles. The Graafian follicle consists externally of a fibrous cai)sule Ei. Mp. • Fig. 61.— Section through portion of the ovary of mammal, illustrating mode of develop- ment of the Graafian follicles (Wieder- sheim). D, discus proligerus ; Ei, ripe ovum ; (t, follicular cells of germinal epithelium ; (I, blood-vessels : A', germinal vesicle (nucle- us) and germinal spot (nucleolus) : KE. ger- minal epithelium : Lf. Ifquor folliculi ; Afg, membrana or tunica granulosa, or follicular epithelium ; Mik zona pellucida : PS, in- grf)wths from tlie germinal ei)ithelium, ova- rian tubes, liy means of which some of the nests retain tlieir connection with the epithe- lium : .S, cavity which appears within the Graafian folli<;le ; .So, stroma of ovary ; Tf, theca follicidi or capsule ; U, primitive ova. When an ovum with its surrounding cells ha« becf)me se[)arated from the nest, it is known a« a Graafian follicle. 58 ANIMAL PHYSIOLOGY. {tunica fibrosa), in close relation to which is a layer of capillary blood-vessels {tunica vasculosa), the two together forming the Fig. 62.— Sagittal section of the ovary of an adult bitch (after Waldeyer). o. e, ovarian epi- thelium ; o. t, ovarian tubes ; y. f, younger follicles ; o. /, older follicle ; d. p, discus pro- ligerus, with the ovum ; e, epithelium of a second ovum in the same follicle ; /. c, fibrous coat of the follicle ; p. c, proper coat of the follicle ; e. /, epithehum of the follicle (mem- brana granulosa) : a. f. collapsed atrophied follicle ; b. v, blood-vessels ; c. t, cell-tubes of the parovarium, divided longitudinally and transversely ; t. d, tubular depression of the ovarian epithelium, in the tissue of the ovary ; b. e, beginning of the ovarian epithelium, close to the lower border of the ovary. general covering {tunica propria) for the more delicate and im- portant cells within. Lining the tunic is a layer of small, some- what cubical cells {meinbrana granulosa) , which at one part invest the ovum several layers deep {discus proligerus), while the remainder of the space is filled by a fluid {liquor foUiculi) probably either secreted by the cells themselves, or resulting from the disintegration of some of them, or both. REPRODUCTION. 59 In viewing a section of the ovary taken from a mammal at tlie breeding-season, ova and Graafian follicles may be seen in all stages of development — those, as a rule, nearest the surface being the least matured. The Graafian follicle appears to pass inward, to undergo growth and development and again retire toward the exterior, where it bursts, freeing the ovum, which is conducted to the site of its future development by appropriate mechanism to be described hereafter. Changes in the Ovum itself. — The series of transformations that take place in the ovum before and immediately after the access of the male element is, in the opinion of many biolo- gists, of the highest significance, as indicating the course evolu- tion has followed in the animal kingdom, as well as instructive in illustrating the behavior of nuclei generally. The germinal vesicle may acquire powers of slow movement (amoeboid), and the germinal spot disappear : the former passes to one surface (pole) of the ovum ; both these structures may undergo that peculiar form of rearrangement {karyoTxinesis) which may occur in the nuclei and nucleoli of other cells prior to division ; in other words, the ovum has features common to it and many other cells in that early stage which precedes the complicated transformations which constitute the future his- tory of the ovum. A portion of the changed nucleus {aster) with some of the protoplasm of the cell accumulates at one surface (pole), which ^^^m^ Fio. 63.— Formation of polar cells in a star-fish (Axt"to two asters ; F. formation of first pf>lar cells and withdrawal of remaininK part of nuclear spindle within the ovum : (J, surface view of livinf; fivum in the first polar cell ; H, com- pletion of second polar cell : I, a later stape, showinfj the remaining; internal half of the spindle in the form of two clear vesicles ; K. ovum with two polar cells and radial striae round female pronucleus, as seen in the living eu'tc 'K. F. H, and I from i)icric acid prepa- rations) ; L, expulsion of the first polar cell. (Iladdon.) is tf*rmed the upper pole because it is at this region that the epi- thelial cells will be ultimately developed, and is separated; this process is repeated. These bodies (polar cells, polar (jlohules, 60 ANIMAL PHYSIOLOGY. etc.), then, are simply expelled ; they take no part in the devel- opment of the ovum ; and their extrusion is to be regarded as a preparation for the progress of the cell, whether this event fol- lows or precedes the entrance of the male cell into the ovum. It is worthy of note that the ovum may become amoeboid in the region from which the polar globules are expelled. The remainder of the nucleus {female pronucleus) now passes inward to undergo further changes of undoubted im- portance, possibly those by virtue of which all the subsequent evolution of the ovum is determined. This brings us to the consideration of another cell destined to play a brief but im- portant role on the biological stage. The Male Cell {Spermatozoon). This cell, almost without exception, consists of a nucleus (head) and vibratile cilium. However, as indicating that the Fig. 64. -Spermatozoa (after Haddon). Not drawn to scale. 1, sponge ; 2, hydroid ; 3, nema- tode ; 4. cray-flsh ; 5. snaU ; 6, electric ray ; 7, salamander ; 8, horse ; 9, man. In many spermatozoa, as m Nos. 7 and 9, an extremely delicate vibratUe band is present. REPRODUCTION. 61 latter is not essential, spermatozoa without such, an appendage do occur. The obvious purpose of the cilium is to convey the male cell to the ovum through a fluid medium — either the water in which the ova are discharged in the case of most inverte- brates, or through the fluids that overspread the surfaces of the female generative organs. The Origin of the Spermatozoon.— The structures devoted to the production of male cells (testes), when reduced to their es- G5.— Spermatogenesis. A— H, isolated spcrm-ccllH of tlie rut, sliowiuj; the development of the spermatozoon and the T^ ^ Ji > :r^ -2 .^ as O J5 03 P= ^ Eq « a. wi_ "^ 3 .^ rl -tJ O) r-lH -s ^ a & s Pi:^ --s ^ js S c J3 H e 0 -H 2 m -5 to _g 'S o bt, s r- -c — s . be 3 ^1 CD 8 CD J3 0 bJD V ,-H "s "^ cS ':S o p g . o " bb O .?H _t> 1" .5^ 1 3 iff -1 CD" £ 1^ & 1 ft ^ 03 ■"^ a > '3 £ 'S'rS ;e~^ +^ 1 he o be CD H 3 ^ gj_a3 1^: 1? a 0 c o t4 ri O %' o of^ ? i 1 1 .s O 4^ > CD m ji CS J s cS t-' 3 s : tJ bCcS a .• CT ^^ E CO" "N s^ ^o bD >-i S fcb bb s O -2 . be •SPE .^«1 cS ;iS .„ o ^ fe =* j^ 1 r^ ^^ a e "^ c £^ ^ ^ -Si 'cS .2 S o c 03 3 O o a 3 ^ M .a 'fe ^ 0 s "a » Sk rT-S bp-T: ^ bJ) > m CD ? 3 o M --3 -~ ^ &• a ~ ^ ,_- be " ~ ° £ a CD =i « ^ 3 M TJ e ^ '^' ;3 5 a. . _ o _2 be .5 5 •r; tz^ ., benS fc ' " a> '« be .i'- to 3 "ol =5 -73 s 2 o bo I § T ^ CI ) ". be " --: S ^ OS £' rS CT -jS 3 > 'o I- "^ -S A O i-H C! « ® S ^ .be -S CD (— i CI, ^ A ■^ be I s, .-y m o ji ^ ^- is ri __ ■^ M -# m - a; c> fi* £ £ fe & g ... o '^ c '■X ^ :~ rt ? REPRODUCTION. 67 this region becoming more columnar {histological differentia- tion). This depression (i?M'agma^ion) deepens until a cavity is Fig. 70.— Blastula and gastrula of amphio.xus iClaus, after Hatschek). A, blastula with flat- tened lower pole of larger cells ; B, comnieueing invagination ; C, gastrulation completed': the blastopore is still widely open, and one of the two hinder-pole mesoderm cells is seen at its ventral lip. The cilia of the epiblast cells are not represented. " formed (as when a hollow rubber ball is thrust in at one part till it meets the opposite wall), in consequence of which a two- layered embryo results, in which we recognize the primitive mouth {blastopore) and digestive cavity {archenferon), the outer layer {ectoderm) being usually separated from the inner {endoclerm) by the almost obliterated segmentation cavity. Such a form may be provided with cilia, be very actively loco- motive, and bear, consequently, the greatest resemblance to the permanent forms of some aquatic animals. The changes by which the segmented oosperm becomes a gastrula are not always so direct and simple as in the above- described case, but the behavior of the cells of the blastosphere may be hampered by a burden of relatively foreign matter, in the form of food-yelk, in certain instances ; so much so is this the case that distinct modes of gastrula formation may be rec- ognized as dependent on the quantity and arrangement of food- yelk. These we shall pass by as being somewhat too compli- cated for our purpose, and we return to the egg of the bird. The Hen's Egg. — By far the larger part of the hen's egg is made up of yelk ; but just beneath the vitelline membrane a small, circular, whitish body, aT)out four millimetres in diame- ter, which always floats uppermost in every portion of the egg, may be seen. This disk {blastoderm, cicatricula) in the fertilized egg presents an outer wliite rim {area opaca), within which is a transparent zone {area pellucida) , and most centrally a some- what elongated structure, which marks off the future being itself {emhryo). All of these parts together constitute that por- tion (hlasfoderrn) of the fowl's egg which is alone directly con- cernf;d in reproduction, all the rest serving for nutrition and 68 ANIMAL PHYSIOLOGY. protection. The appearance of relative opacity in some of the parts marked off as above is to be explained by thickening in the cell-layers of which they are composed. The Origin of the Fowl's Egg. — The ovary of a young but mature hen consists of a mass of connect- ive tissue {stroma), abundantly supplied with blood-vessels, from which hang the capsules which contain the ova in all stages of development, so that the whole suggests, but for the color, a bunch of grapes in an early stage. The ovum at first, in this case as in all others, a single cell, becomes com- plex by addition of other cells {dis- cus iDroligerus, etc.), which go to make up the yelk. All the other parts of the hen's Qgg are additions made to it, as explained before, in its passage down the oviduct. The original ovum remains as the blas- toderm, the segmentation of which may now be described briefly, its character being obvious from an examination of Fig. 72, which rep- resents a surface view of the seg- menting fertilized ovum {oosperm). A segmentation cavity appears early, and is bounded above by a single layer of epiblast cells and below by a single layer of primi- tive hypoblast cells, which latter is soon composed of several layers. Fig. 7i.-Femaie Reneiative organs of while the Segmentation cavity dis- the fo^^l (atter Dalton) A, ovarjs Q„„ocn-o B, Graafian follicle, from which the appwd.! b. ^^^J"^^^^^^ of The blastoderm of an unincu- o^itclSn'^wSdftreihSaSous ^^ted but fertilized egg consists afe"f™drT'thirrportSln of a layer of epiblastic cells, and which the fibrous shell membranes i^encatli this a mass of rouuded are produced ; G, fourth portion laid '^^'-'■^'^'^'■^ ^ ^ open, showing the egg completely ^ells, arranged irregularly and ly- formed with its calcareous shell ; H, ' => o j J canal through which the egg is ex- ing loosely in the yelk, constitut- REPRODUCTION. 69 Fig. 72.— Various stages in the segmentation of a fowl's egg (Kolliker). ing the primitive hypoblast. After incubation for a couple of hours, these cells become differentiated into a lower layer of flattened cells {hypoblast), with mesoblastic cells scattered be- tween the epiblast and hypoblast. It is noteworthy that, in the bird, segmentation will proceed up to a certain stage indepen- dently of the advent of the male cell, apparently indicating a tendency to parthenogenesis. FlO. 73.— Portion of section tiirough an uninoul)ated fowl's oosnenn (after Klein), a, epiblast W)mpo«efl of a single layer of columnar cells ; h, irregularly disposed lower layer cells of the primitive hypoblast ; r. larger formative ceils re.sting on white yelk ; /, arclienteron. The 8<^gmentation cavity lies between a and />, and is nearly obliterated. 70 ANIMAL PHYSIOLOGY. The fowl's ovum then belongs to the class, a portion of which alone segments and develops into the embryo (meroblastic) , in contradistinction to what happens in the mammalian ovum, the whole of which undergoes division (holoNasUc) ; a distinction which is, however, superficial rather than fundamental, for in reality in the fowl's egg the whole of the original ovum does Fig. 74. — Sections of ovum of a rabbit, illustrating formation of the plastodermic vesicle (after E. Van Beneden). A, B, C, D, are ova in successive stages of development, ep, zona pellu- cida ; ecf, ectomeres, or outer cells ; ent, entomeres, or inner cells. segment. This holoblastic character of the mammalian ovum and its resemblance to the segmentation of those invertebrate forms previously described may become apparent from an ex- amination of the accompanying figures. We shall return to the development of the mammalian ovum later ; in the mean time we present the main features of devel- opment in the bird. Remembering that the development of the embryo proper takes place within the pellucid area only, we point out that the area opaca gradually extends over the entire ovum, inclosing REPRODUCTION. n the je\k, so that the original disk which lay like a watch-glass on the rest of the ovum, has grown into a sphere. That portion of this area nearest the pellucid zone {area vasculosa) develops Fig. 75.— Diagrammatic transverse sections through a hypothetical mammal oosperm (Had- don). A. The yelk of the primitive mammalian oosperm is now lost. B. Later stage ; the non-embryonic epiblast has grown over the embryonic area to form the covering cells, ep, epiblast of embryo ; ep\ epiblast of yelk-sac ; hy, primitive hypoblast ; y. s, yelk-sac. or blastodermic vesicle. H blood-vessels that derive the food-supplies, which replenish the blood as it is exhausted, from the hypoblast of the area opaca. The first indications of future structural outlines in the embryo is the formation of the primitive streak^sjo. opaque band in the long diameter of the pellu- cid area, opaque in consequence of cell accummulation in that re- gion. Very soon a groove {primi- tive groove) extends throughout this band, which gradually occu- pies a more central position. The relative thickness of the several parts and the arrangement of cells may be gathered from Fig. 7(j. These structures are only tempo- rary, and those that replace them will be described subsequently. We have thus far spoken of cells as being arranged into epi- bla.st, hy[;oblast, and mesoblast. The origin of the first two has been sufficiently indicated. The mesoblast forms the interme/, folds of the amnion : pp. pleuroperitoneal cavitj' ; aw, cavity of the amnion ; «/, allantois ; a, position of the future anus ; /i, heart ; i, intestine ; vi, vitelline duct ; ys, yelk ; s, foregut ; »/i, position of the mouth ; rwe, mesentery. pleuro-peritoneal cavity, and hence consists of an outgrowth of mesoblast lined by hypoblast. The outer membrane of the allantois fuses with the subzo- nal (serous) membrane, and, with the latter extending beyond the yelk-sac, incloses the albumen of the egg in a space termed Fio. 82.— A. Diagrammatic longitudinal section through the egg of a fowl. B. Detail of por- tion of sai/i"! at a time when the allantt ouUt layer of amnion (serous membrane) ; I'/i. nin, ei)il)lastic ej)itlieliurn of inner layer of amniiin larnnion proper) : in., am, mesoblastic! layi-i- of latter ; ill. ejfjf-shell ; num. somatic mesoblast of outer layer of amnion ; v. in, vitelliin' membrane ; U-. jxtint where the mesoblastic tissue of the a\\a,uUAa fuses with that of tlie serous mem- brane. 76 ANIMAL PHYSIOLOGY. the placental sac by Duval, who has recently described this pro- cess. Villi, or tubular vascular outgrowths, spring from the lining of this sac and serve to convey the absorbed and prob- ably altered albumen to the embryo, in which process of vas- cular transport of nourishment the yelk-sac, that also abounds in blood-vessels as well as the allantois, takes part. The physiological import of the various structures above described will be considered more fully later. At this point a compari- son of the formation of the corresponding parts in mammals will be undertaken. The Fcetal (Embryonic) Membranes of Mammals. The differences between the development of the egg mem- branes of mammals and birds are chiefly such as result from Fib. 83. Fig. 84. Fig. 8.3. — Diagrammatic longitudinal section of oosperm of rabbit at an advanced stage of pregnancy (Kfilliker, after Bischoff ). a, amnion ; al, allantois with its blood-vessels ; e, embryo ; ds, yelk-sac ; ed, ed\ ed", hypoblastic epithelium of the 3'elk-sac and its stalk (umbilical vesicle and cord) ; fd, vascular mesoblastic membrane of the umbilical cord and vesicle ; p, placental villi formed by the allantois and subzonal membrane ; r, space filled with fluid between the amnion, the aUantois, and the yelk-sac ; st, sinus terminalis (marginal vitelline blood-vessel) ; it, urachus, or stalk of the allantois. Fig. 8-t. — Diagrammatic dorsal view of an embryo rabbit with its membranes at the stage of nine somites (Haddon, after Van Beneden and Julini. cd, allantois, showing from behind the tail fold of the embryo ; am. anterior border of true amnion ; a. v. area vaseulosa, the outer border of which indicates the farthest extension of the mesoblast ; bl, blastoderm, here consisting only of epiblast and hypoblast ; o. m. r, omphalo-mesenteric or vitelline veins ; p. am, proamnion ; pi, non-vascular epiblastic villi of the future placenta ; s. t, si- nus terminalis. the absence in the former of an egg-shell and its membranes, and of yelk and albumen. The mammalian ovum is inclosed by a zona radiata (zona pellucida) surrounding another very delicate covering (Fig. 58). The growth of the blastodermic vesicle (yelk-sac) is rapid. REPRODUCTION. 77 and, being filled with fluid, the zona is thinned and soon disap- pears. The germinal area alone is made up of three layers of cells (Fig. 104), the rest of the upper part of the oosperm being lined ■with epiblast and hypoblast, while the lower zone of the yelk- sac consists of epiblast only. Simple, non-vascular villi, serving to attach the embryo to the uterine walls, usually project from the epiblast of the subzonal membrane. In the rabbit they do not occur every- where, but only in that region of the epiblast beneath which the mesoblast does not extend, with the exception of a patch which soon appears and demarkates the site of the future placenta. The extension of the mesoblast takes place in every direction from the embryo except directly around the head ; but the two Fig. 85.— Diagrammatic median vertical longiturlinal section.s through embryo ral)l)it (Had- don. after Van Bi-neden and Julin). A. Section through embryo of Fig. H4. \i. Section tlirough f nibryf) of eleven days, al, allautois ; arn, amnion ; a. ins, anterior median plate of mi'soblast, formed by the junction of the anterior horns of the area opaca ; ti. pt, area placentalis ; a. r. area vasculosa ; ch, chorion ; cw, c(]Blom of embryo ; cw', extra-embry- onic portifin of the body-cavity ; eyj, epiblast ; h;/, hypoblast ; m. uusplit mesoblast ; o. «, oriflo- of amnion ; pi, |>lacenta ; jjrn, a, proamnion ; g. (, sinus terminalis ; v, epiblastic villi of blastodermic vesicle. expansions of the mesoblast which mark out this area extend for some di.stance in front of the head, and ultimately unite ; so that immediately in front of the head there is a circular region in wliich tlio Ijlastodei-m consists of epiblast and hypo- 78 ANIMAL PHYSIOLOGY. blast only, forming a cavity into which the anterior part of the embryo early projects (pro-amnion). The true amnion arises only from the posterior end of the embryo, and, extending over in a forward direction, meets the raised projection of the pro-amnion with which it fuses. The amniotic cavity becomes one with that space (extra-em- bryonic pleuro-peritoneal cavity) arising from the cleavage of A.pL Fig. 86.— Foetal envelopes of a rabbit embryo (Minot, after Van Beneden and Julin). Later stage than Fig. 85 B. The amnion has become fused with the blastoderm in front of the embryo, and its cavity is therefore continuous with the extra-embryonic portion of the body-cavity in front of the embryo. Al, allantois ; «m, amnion ; am', portion of the amnion united with the walls of the allantois ; A. pi, area placentalis ; Av, area vasculosa ; Ch, chorion ; Cce. coelom or body-cavity ; Cce", extra-embryonic portion of the body- cavity ; Coel, anterior portion of the same, produced by the fusion of the cavity of the amnion with that of the anterior portion of the area opaca ; Ec, epiblast ; En, alimentary canal of the embryo ; Ent, hypoblast ; PI, placenta ; pro. A, proamnion ; T, sinus ter- minalis ; V, villi of blastodermic vesicle ; Y, cavity of blastodermic vesicle. the mesoblast, which now advances beyond the head of the em- bryo and the pro-amnion. The pro-amnion by gradual atrophy gives place to the true amnion. At about the same period as these events are transpiring the vascular yelk-sac has become smaller, and the allantois with its abundant supply of blood-vessels is becoming more xjrominent, and extending between the amnion and subzonal membrane. The formation of the chorion marks an important step in the development of mammals in which it plays an important functional part. It is the result of the fusion of the allantois, which is highly vascular, with the subzonal membrane, the villi of which now become themselves vascular and more complex in other respects. An interesting resemblance to birds has been observed (by Osborn) in the opossum. When the allantois is small the REPRODUCTION. Y9 Fig. 87.— Embryo of dog, twenty-flve days old, opened on the ventral side. Chest and ven- tral walls have been removed, a, nose-pits ; 6, eyes : c, under-jaw (first gill-arch) ; d, second gill-arch; e.f,g, h, heart (e, right, f, left auricle; g, right, 7i, left ventricle); i, aorta (origin of); kk\ liver (in the middle between the two lobes is the cut yelk-vein); I, stomach ; tn, intestine ; n, yelk-sac ; o, primitive kidneys : p, allantois ; q, fore-limbs ; /i, hind-limbs. The crooked embryo has been stretched straight. (Haeckel, after Bischoff.) 'Ji-^-jp Fio. 88.— Dlajfram of an embryo showing the relations of the vascular allantoiR to the vllU of the chorion 'Cast generalized form. Fio. %. -Structure of placenta of a pig. Fio. 97.-«>f a cow. Fio. 'JH.—Or a fox. Fio. W,— Of a cat. Tint jjig possesses the simplest form of placenta yet known. The villi fit into depressions or crypts in the maternal uterine mucous membrane. The villi, consisting of a core of connective 88 ANIMAL PHYSIOLOGY. tissue, in whicli capillaries abound, are covered with, a flat epi- thelium; the maternal crypts correspond, being composed of a similar matrix, lined with epithelium and permeated by- capillary vessels, which constitute a plexus or mesh-work. It thus results that two layers of epithelium intervene between the maternal and f cetal capillaries. The arrangement is substantially the same in the diffuse and the cotyledonary placenta. In the deciduate placenta, naturally, there is greater compli- cation. In certain forms, as in the fox and cat, the maternal tis- sue shows a system of trabeculse assuming a meshed form, in which run dilated capillaries. These, whicli are covered with a somewhat columnar epithelium, are everywhere in contact with the foetal villi, which are themselves covered with a flat epithelium. 6. F, Fig. 100. Fig. 101. Fig 100.— Placenta of a sloth. Flat maternal epithelial cells shown in position on right side ; on left they are removed and dilated ; maternal vessel with its blood-corpuscles exposed. Fig. 101.— structure of human placenta ; ds, decidua serotina ; t, trabeculae of serotina passing to foetal villi ; ca, curling artery ; up, utero-placental vein ; .r, prolongation of maternal tissue on exterior of villus, outside cellular layer e'. which may represent either endothe- lium of maternal blood-vessels or delicate connective tissue of the serotina or both ; e' ma- ternal cells of the serotina. In the case of the sloth, with a more discoidal placenta, the dilatation of capillaries and the modification of epithelium are greater. In the placenta of the apes and of the human subject the most marked departure from simplicity is found. The maternal REPRODUCTION. 89 vessels are said to constitute large intercommunicating sinuses ; the villi may hang freely suspended in these sinuses, or be anchored to their walls by strands of tissue. There is believed to be only one layer of epithelial cells between the vessels of mother and foetus in the later stages of pregnancy. This, while closely investing the foetal vessels (capillaries), really belongs to the maternal structures. The significance of this general arrangement will be explained in the chapter on the physiological aspects of the subject. It remains to inquire into the relation of these forms to one another from a x>l^ylogenetic (derivative) point of view, or to trace the evolution of the placenta. Evolution. — Passing by the lowest mammals, in which the placental relations are as yet imperfectly understood, it seems clear that the simplest condition is found in the rodentia. Thus, in the rabbit, as has been described, both yelk-sac and allantois take a nutritive part ; ' but the latter remains small. In forms above the rodents, the allantois assumes more and more importance, becomes larger, and sooner or later predomi- nates over the yelk-sac. The discoidal, zonary, cotyledonary, etc., are plainly evolu- tions from the diffuse, for both differentiation of structure and integration of parts are evident. The human placenta seems to have arisen from the diffuse form; and it Avill be remem- bered that it is at one period represented by the chorion with its villi distributed universally. The resemblance in the embryonic membranes at any early stage in man and other mammals to those of birds certainly suggests an evolution of some kind, though exactly along what lines that has taken place it is difficult to determine with exact- ness ; however, as before remarked, nearly all the complications of the higher forms arise by concentration and fusion, on the one hand, and atrophy and disappearance of parts once functionally active, on the other. Summary. — The ovum is a typical cell ; unspecialized in most directions, but so specialized as to evolve from itself compli- cated structures of higher character. The segmentation of the ovum is usually preceded by fertilization, or the union of the nuclei of male and female cells, which is again preceded by the extrusion of polar globules. In the early changes of the ovum, including segmentation, periods of rest and activity alternate. The method of segmentation has relation to the quantity and arrangement of the food-yelk. Ova are divisible generally 90 ANIMAL PHYSIOLOGY. into completely segmenting (holoblastic), and those that under- go segmentation of only a part of their substance (meroblastic) ; but the processes are fundamentally the same. Provision is made for the nutrition, etc., of the ovum, when fertilized (oosperm) by the formation of yelk-sac and allan- tois; as development proceeds, one becomes more prominent than the other. The allantois may fuse with adjacent mem- branes and form at one part a condensed and hypertrophied chorion (placenta), with corresponding atrophy elsewhere. The arrangement of the placenta varies in different groups of ani- mals so constantly as to furnish a basis for classification. What- ever the variations in the structure of the placenta, it is always highly vascular ; its parts consist of villi fitting into crypts in the maternal uterine membrane — both the villi and the crypts being provided with capillaries supported by a connective-tissue matrix covered externally by epithelium. The placenta in its different forms would appear to have been evolved from the diffuse type. The peculiarities of the embryonic membranes in birds are owing to the presence of a large food-yelk, egg-shell, and egg- membranes ; but throughout, vertebrates follow in a common line of development, the differences which separate them into smaller and smaller groups appearing later and later. The same may be said of the animal kingdom as a whole. This seems to point clearly to a common origin with gradual diver- gence of type. THE DEVELOPMENT OF THE EMBRYO ITSELF. We now turn to the development of the body of the animal for which the structures we have been describing exist. It is important, however, to remember that the development of parts, though treated separately for the sake of convenience, really goes on together to a certain extent ; that new structures do not appear suddenly but gradually ; and that the same law apj^lies to the disappearance of organs which are being superseded by others. To represent this completely would require lengthy de- scriptions and an unlimited number of cuts ; but with the above caution it is hoped the student may be able to avoid erroneous conceptions, and form in his own mind that series of pictures which can not be well furnished in at least the space we have to devote to the subject. But, better than any abstract state- THE DEVELOPMENT OP THE EMBRYO ITSELF. 91 ments or pictorial representations, would be tlie examination of a setting of eggs day by day during their development under a Fio. 102.— Various stages in the development of the frog from the egg (after Howes). 1. The segmenting ovum, showing first cleavage furrow. 2. Section of the above at rig:ht angles to the furrow. 3. Same, on appearance of second furrow, viewed .slitjlitly from above. 4. The latter seen from beneath, ."j. The same, on appearance of first horizontal furrow. 6. The saniH. seen from above. 7. Longitudinal section of 0. 8 mid ti. Two jiliascs In segmentation, on appearance of fourth and fifth furrows. 10. Longitudinal vertical sct'tion at a slightly later stajje than the above. 11. Later stape. Ui)i)fr ])iKmciitc(l pole iliviiling more rapidlj- than lower. 12. Later phase of II. 18. fjoiigifuilinal vertical section of 12. 14. Segmenting ovum at bla.stopore stajje. 1.5. LonKitudinal vertical section of same. 13 and 1.5 x V) lall others x 5). Ki. Longitudinal vertical section of embryo at a stage later than 14 (1 x 10). 71c, nucleus; c. c. cleavage cavity ; »-;>, ejjiblast ; /. t, yelk-bearing lower-layer celts ; 01, blastopore : a/, archenteron (mid-gut) ; hh, hyijoblast ; mn, undiffer- entiated niesoblast ; ch, notf>chord ; 11. a, neural (cerebro-spinal) axis. ben. This is a very simple matter, and, while the making and mounting of sections from hardened specimens is valuable, it may require more time than the student can spare ; but it is neitlier so valuable nor so easily accomplislied as wliat we have indicated ; for, while the lack of sections made by the student 92 ANIMAL PHYSIOLOGY. may be made up in part by the exMbition to liim of a set of specimens permanently mounted or even by plates, nothing can, in our opinion, take the place of the examination of eggs as we have suggested. It prepares for the study of the development of the mammal, and exhibits the membranes in a simplicity, freshness, and beauty which impart a knowledge that only such direct contact with nature can supply. To proceed with great simplicity and very little apparatus, one requires but a forceps, a glass dish or two, a couple of watch-glasses, or a broad section-lifter (even a case-knife will answer), some water, containing just enough salt to be tasted, rendered lukewarm (blood-heat). Holding the egg longitudinally, crack it across the center transversely, gently and carefully pick away the shell and its membranes, when the blastoderm may be seen floating upward, as it always does. It should be well examined in position, using a hand lens, though this is not essential to getting a fair knowledge; in fact, if the examination goes no further. than the naked-eye appearances of a dozen eggs, selecting one every twenty-four hours during incubation, when opened and the shell and membranes well cleared away, such a knowledge will be supplied as can be obtained from no books or lectures how- ever good. It will be, of course, understood that the student approaches these examinations with some ideas gained from plates and previous reading. The latter will furnish a sort of biological pabulum on which he may feed till he can furnish for himself a more natural and therefore more healthful one. While these remarks apply with a certain degree of force to all the departments of physiology, they are of special importance to aid the constructive faculty in building up correct notions of the successive rapid transformations that occur in the de- velopment of a bird or mammal. Fig. 103 shows the embryo of the bird at a very early period, when already, however, some of the main outlines of structure are marked out. Development in the fowl is so rapid that a few days suffice to outline all the principal organs of the body. In the mammal the process is slower, but in the main takes place in the same fashion. As the result of long and patient observation, it is now set- tled that all the parts of the most complicated organism arise from the three-layered blastoderm previously figured ; every part may be traced back as arising in one or other of these lay- ers of cells — the epiblast, mesoblast, or hypoblast. It frequently THE DEVELOPMENT OP THE EMBRYO ITSELF. 93 aT/z happens that an organ is made up of cells derived from more than one layer. Structures may, accordingly, be classified as epiblastic, mesoblastic, or hypoblas- tic ; for, when two strata of cells unite in the formation of any part, one is always of subordinate impor- tance to the other : thus the digestive organs are made up of mesoblast as well as hypoblast, but the latter constitutes the essential secreting cell mechanism. As already indi- cated, the embryonic membranes are also derived from the same source. The epihlast gives rise to the skin and its appendages (hair, nails, feath- ers, etc.), the whole of the nervous system, and the chief parts of the or- gans of special sense. The mesoblast originates the skel- eton, all forms of connective tissue, including the framework of glands, the muscles, and the epithelial (en- dothelial) structures covering serous membranes. The hypoblast furnishes the se- creting cells of the digestive tract and its appendages — as the liver and pancreas — the lining epithelium of the lungs, and the cells of the secret- ing mucfjus membranes of their framework of bronchial tubes. It is difficult to overrate the im- portance of these morphological gen- eralizations for the physiologist ; for, once the origin of an organ is known, its functi(m and physiological rela- tions generally may be predicted with considerable certainty. We shall en- deavor to make this prominent in the futlire chapters of work. Being prepared with these generalizations, we continue our study of the develoxjment of the bird's embryo, Befon; tlie end Fig. 103.— Embryo fowl 3 mm. long, of about twenty -four hours, seen from , anterior umbilical sinus showing through the blastoderm. His divides the embryonic rudiment into a cen- tral trunk-zone, and a pair of lateral or parietal zones. this 94 ANIMAL PHYSIOLOGY. of the first twenty-four hours such, an appearance as that repre- sented in Fig. 104 is presented. Fig. 104.— Transverse section through the medullary groove and half the blastoderm of a chick of eighteen hours (Foster and Balfour). E, epiblast ; M, mesoblast ; H, hypoblast ; ni/, medullary fold ; mg. medullary groove ; c/i, notochord. The mounds of cells forming the medullary folds are seen coming in contact to form the medullary {neural) canal. Fig. 105.— Transverse section of embryo chick at end of first day (after Kolliker). M, meso- blast ; H, hjrpoblast ; ni, medullary plate ; E, epiblast : nigr, medullary groove ; nif, me- dullary told ; c/i, chorda dorsalis ; P, protovertebral plate ; dm, division of mesoblast. The notochord, marking out the future bony axis of the body, may also be seen during the first day as a well-marked linear extension, just beneath the medullary groove. The cleav- Fig. 106.— Transverse section of chick at end of second day (Kolliker). E, epiblast : H. hypo- blast ; e. m, external plate of mesoblast dividing (cleavage of mesoblast) ; ?n./, medullary fold ; m. g, medullary groove ; ao, aorta ; p, pleuroperitoneal cavity ; P, protovertebral plate. age of the mesoblast, resulting in the commencement of the formation of somatojjleure (body-fold) and the splanchnopleure (visceral fold), is als(3 an early and important event. These give rise between them to the pleuro-peritoneal cavity. The portions of mesoblast nearest the neural canal form masses {vertebral plates) distinct from the thinner outer ones {lateral plates). THE DEVELOPMENT OF THE ExMBRYO ITSELF. 95 op.v. m.b The vertebral plates, wlien distinctly marked off, as repre- sented in the figure, are termed the protovertehrce (mesohlast ic somites), and represent the future vertebrae and the voluntary- muscles of the trunk ; the former arising from the inner sub- divisions, and the latter from the outer {inusde-iilates). It will be understood that the pro- tovertebrae are the results of transverse division of the col- umns of mesoblast that formed the vertebral plates. Before the permanent verte- brae are formed, a reunion of the original protovertebrae takes place as one cartilaginous pillar, followed by a new segmentation midway between the original divisions. It thus appears that a large number of structures either ap- pear or are clearly outlined dur- ing the first day of incubation : the primitive streak, primitive groove, medullary plates and groove, the neural canal, the head-fold, the cleavage of the mesoblast, the protovertebrae, with traces of the amnion and area opaca. During the second day near- ly all the remaining important structures of the chick are marked out, while those that arose during the first day have progressed. Thus, the medullary folds close ; there is an increase in the number of protoverte- brae ; the formation of a tubular heart and the great blood-ves- sels ; the appearance of the Wolffian duct ; the progress of the head region ; the appearance of the three cerebral vesicles at the anterior extremity of the neural canal ; the subdivision of the first cerebral vesicle into the optic vesicles and the begin- nings of the cerebrum ; the auditory pit arising in the third cerebral vesicle (hind-brain); cranial flexure commences; both liead and tail folds become more distinct; the heart is not only Fig. 107.— Embryo of chick, between thirty and thirtj'-six hours, viewed from above as an opaque object (Foster and Balfour). /. 6, forebrain ; m. 6, midbrain ; h. b, hind-brain; op. v, optic vesicle; au. p, auditory pit ; o. /, vitelline vein ; p. v, niesoblastic somite : m. f. li^^' of func- tion of medullary fnlilsalidvc medullarv canal; s. r, sinus rh<:iml)c>i Blastoderm seen from below. Arteries made black. H, heart ; AA, second, third, and fourth aortic arches ; AO, dorsal aorta ; L. Of. A, left vitelline artery ; B. Of. A^ right vitelline artery ; S. T, sinus terminalis ; L. Of, left vitelline vein ; R. Of, right vitelline vein ; S. V, sinus venosus ; D. C, ductus Cuvieri ; S. Ca. V, superior cardinal or jugular vein ; V. Ca, inferior cardinal vein. num ; the latter into the large intestine and the cloaca. The lungs arise from the alimentary canal in front of the stomach ; from similar diverticula from the duodenum, the liver and pancreas originate. Changes in the protovertebrse and muscle- plates continue, while the WolfELan bodies are formed and the Wolffian duct modified. Up to the third day the embryo lies mouth downward, but now it comes to lie on its left side. See Fig. 109 with the ac- companying description, it being borne in mind that the view is from below, so that the right in the cut is the left in the em- THE DEVELOPMENT OF THE EMBRYO ITSELF. 101 Fig. 116. i-iu, 11:). Fio. 114. -Transverse section through lumbar region of an embryo at end of fourth day (Fos- ter an«l Balfour), nc. neural canal : pr. postf rinr root of spinal nerve with ganglion ; fir. anterior ro<>t ; A. G. C. anterior gray I'nluinn of siiinal curd : A. It'. C, anterior white column in course of formation : m. p. nnisi-li- plate : rh. nutocliord ; It'. /?, Wdllliim ridge ; A(J. dorsal aorta ; r. r. u. post^-rior cardinal vein ; II'. d, WultYlan duct : IT. /;. Wolffian IxxJy, consisting of tubules and Malpighian corpuscles ; g. e. germinal epithelium ; d, ali- mentary eanal ; .U. eorniiiein-jng mesentery : .SO, .soniatopleure ; SP, splanchnopleure ; V. bId-ves.s, pancreas. Fio. 11»). — Head of cliiek of third day. viewed sidewise as a transparent iibject iHu.vley*. la, cerebral heininjiheres ; Ih, vesicle of third ventricle: II, imd-brain : III, hind-brain; a, optic vesicle ; 7, na.sal pit : h. otic vesicle ; d, iiifundiiiulurn ; p, pineal body ; h, notochord; V, fifth nerve: VII, H»fventh nerve; VIII, united glossopharyngeal and i>neumoga8tric nerves. 1, 2, 3, 4, 5, the five visceral folds. 102 ANIMAL PHYSIOLOGY. bryo itself. Fig. 114 gives appearances furnislied by a vertical transverse section. The relations of the parts of the digestive tract and the mode of origin of the lungs may be learned from Fig. 115. Vir 117 — TTpad of chick of fourth day, viewed from below as an opaque object (Foster and Balfour) The neck is cut across between third and fourth visceral folds. C. H, cerebral hem°spl^eres i^ S, vesicle of third ventricle : Op. eyeball ; nf, naso-frontal process ;m, civity of moAth ; S. m, superior maxillary process of F. 1, the first visceral fold (mandibu- lar arch) ; F. 2, F. 3, second and third visceral arches ; N, nasal pit. An examination of the figures and subjoined descriptions must suffice to convey a general notion of the subsequent prog- G.Ph. VII. MP. Vxa 118 —Embryo at end of fourth day, seen as a transparent object (Foster and Balfour). Off cere^raThemilphere; F. B., for?4)rain, or vesicle of third ^^ntoc e tbalamencepha- lon) with pineal gland (Pii) projecting : 31. B, mid-bram ; Cb cerebellum ; II . P, fourtn ventricle- i lent- chs\ choroid slit f Cen. V. auditory vesicle; sm superior maxillary Drocess • IF '3^"ltc, first, second, etc., visceral folds ; F, fifth nerve; VII seventh nerve; & pl! glossopharyngeal ierve ; Pj,, pneumogastric. The distribution of these nerves is also iAchcated : ch. notochord ; Ht, heart ; MP. muscle-plates ; W wmg ; H. i;. ^'"id-limb. The amnion has been removed. Al, allantois protruding from cut end ot somatic stalk &S. DEVELOPMENT OF THE VASCULAR SYSTEM. 103 ress of tlie embryo. Special points will be considered, either in a separate chapter now, or deferred for treatment in the body of the work from time to time, as they seem to throw light upon the subjects under discussion. DEVELOPMENT OF THE VASCULAR SYSTEM IN VERTE- BRATES. This subject has been incidentally considered, but it is of such importance morphological, physiological, and pathological, as to deserve special treatment. In the earliest stages of the circulation of a vertebrate the arterial system is made up of a pair of arteries derived from the single hidbus arteriosus of the heart, which, after passing for- ward, bends round to the dorsal side of the pharynx, each giving off at right angles to the yelk-sac a vitelline artery ; the aortse unite dorsally, then again separate and become lost in the pos- terior end of the embryo. The so-called arches of the aorta are large branches in the anterior end of the embryo derived from the aorta itself. The venous system corresponding to the above is composed of anterior and posterior pairs of longitudinal (cardinal) veins, the former (jugular, cardinal) uniting with the posterior to form a common trunk {ductus Cuvieri) by which the venous blood is returned to the heart. The blood from the posterior part of the yelk-sac is collected by the vitelline veins, which terminate in the median sinus venosus. The Later Stages of the Foetal Circulation. — Corresponding to the number of visceral arches five pairs of aortic arches arise ; but they do not exist together, the first two having undergone more or less complete atrophy before the others appear. Figs. 119, 120 convey an idea of how the permanent forms (indicated by darker shading) stand related to the entire system of vessels in different groups of animals. Thus, in birds the right (fourth) aortic arch only remains in connection with the aorta, the left forming tlie subclavian artery, while the reverse occurs in mam- mals. The fifth arch (pulmonary) always supplies the lungs. The arrangement of the principal vessels in the bird, mam- mal, etc., is represented on jjage 104. In mammals the two primitive anterior abdominal {allantoic) veins develoi) early and unite in front with the vitelline ; but the right allantoic vein and the right vitelline veins soon disappear, while the long 104: ANIMAL PHYSIOLOGY. common trunk of the allantoic and vitelline veins {ductus veno- sus) passes through the liver, where it is said the ductus veno- FiG. 119.— Diagrams of the aortic arches of mammal CLandois and StirUng, after Rathke). 1. Arterial trunk with one pair of arches, and an indication where the second and third pairs will develop. 2. Ideal stage of five complete arches ; the fourth clefts are shown on the left side. 3. The two anterior pairs of arches have disappeared. 4. Transition to the final stage. A. aortic arch ; ad, dorsal aorta ; ax, subclavian or axillary artery ; Ce, ex- ternal carotid ; Ci. internal carotid : clB, ductus arteriosus BotalU ; P, pulmonary artery ; S, subclavian artery ; ta, truncus arteriosum ; v, vertebral artery. sus gives off and receives branches. The ductus venosus Aran- tii persists throughout life. (Compare the various figures illus- trating the circulation.) Fig. 120. — Diagram illustrating transformations of aortic arches in a lizard. A ; a snake, B ; a bird. C ; a manunal, D. Seen from below. (Haddon, after Rathke.) a. internal caro- tid ; h, external carotid ; c. common carotid. A. d, ductus Botalli between the third and fourth arches : e, right aortic arch ; /, subclavian ; gr, dorsal aorta : 7i, left aortic arch ; i, pulmonary artery : fc, rudiment of the ductus Botalli between the pulmonary artery and the aortic arches. B. d, right aortic arch : e, vertebral artery ; /, left aortic arch ; h. pulmonary artery : t, ductus Botalli of the latter. C. rf, origin of aorta ; e, fourth arch of the right side (root of dorsal aorta): /, right subclavian: g. dorsal aorta; h, left subclavian (fourth arch of the left side) ; j, pulmonary artery ; k and ?, right and left ductus BotaUi of the pulmonary arteries. D. d. origin of aorta : e. fourth arch of the left side (root of dorsal aorta); /, dorsal aorta: g, left vertebral artery: /;, left subclavian; i, right sub- clavian (fom-th arch of the right side) ; k, right vertebral arterj- ; U continuation of the right subclavian : m, pulmonary artery ; n, ductus BotalH of the latter (usually termed ductus arteriosus). DEVELOPMENT OF THE VASCULAR SYSTEM. 105 With the development of the phicenta the aHantoic circula- tion renders the vitelline subordinate, the vitelline and the larger mesenteric vein forming the portal. The portal vein at a later period joins one of the vence advehentes of the allantoic vein. At first the vena cava inferior and the ductus venosus enter the heart as a common trunk. The ductus venosus Arantii becomes a small branch of the vena cava. The allantoic vein is finally represented in its degenerated form as a solid cord {round ligament), the entire venous sup- ply of the liver being derived from the portal vein. The development of the heart has already been traced in the fowl up to a certain point. In the mammal its origin and early progress are similar, and its further history may be gathered from the following series of representations. In the fowl the heart shows the commencement of a division into a right and left half on the third day, and about the fourth week in man, from which fact alone some idea may be gained as to the relative rate of development. The division Fig. 122. Fig. 121. Fig. 121.— Development of the heart in the human embryo, from the fourth to the sixth week. A. Embryo of four weeks (K'<'ki-l, after Bischoff.) Fio. I 2.'i. Primitive kidney of a human embrvo. it. th<' uriiie-tiil)es of the j)riinilive kidney ; V. Wolffian duct: w'. upper end of the i.itter (MorgUKni's hydatid); in. Millb-riMi. duct; m'. ujipi.-r end of the latter (Fallopian liydutidi; y, hermaphrodite gland. i.Mli'i Kwbeil.) duct and also from an iiitormcjdiate cell-mass, from wliicli lat- ter the Malpighian bodies take rise. The tubes, at first not con- 108 ANIMAL PHYSIOLOGY. nected with, the duct, finally join it. This organ is continuous with the pronephros ; in fact, all three (pronephros, mesone- phros, and metanephros) may be regarded as largely continua- tions one of another. The metanephros, or kidney proper, arises from, mesoblast at the posterior part of the Wolffian body. The ureter origi- FiG. 126. — Section of the intermediate cell-mass of fourth day (Foster and Balfour, after Wal- deyer). 1 x 160. «i, mesentery ; L, somatopleure ; a', portion of the germinal epithelium from the duct of Bliiller is formed by involution ; a, thickened portion of the germinal epithelium, in which the primitive ova C and o are lying ; E, modified mesoblast which will form the stroma of the ovary ; WX, Wolffian body ; y. Wolffian duct. nates first from the hinder portion of the Wolffian duct. In the fowl the kidney tubules bud out from the ureter as rounded elevations. The ureter loses its connection with the Wolffian duct and opens independently into the cloaca. The following account will apply especially to the higher vertebrates : The segmental (archinephric) duct is divided horizontally into a dorsal or Wolffian (mesonephric) duct and a ventral or MuUerian duct. The Wolffian duct, as we have seen, develops into both ureter and kidney proper. To carry the subject somewhat further back, the epithelium lining the ccelom at one region becomes differentiated into col- umns or cells {germinal epithelium) which by involution into the underlying mesoblast forms a tubule extending from before backward and in close relation with the Wolffian duct, thus THE DEVELOPMENT OF THE UROGENITAL SYSTEM. 109 forming the Miillerian duct by the process of cleavage and separation referred to on preceding page. Fig. 127.— Diagrammatic representation of the genital organs of a human embryo previous to sexual distinction (Allen Thomson). W, Wolffian body; gc, genital cord; rn, Bliillerian duct ; ir. Wolffian duct ; ur/, urogenital sinus ; cp, clitoris or penis ; i. intestine ; cl, cloaca ; Is, part from which the scrotum or labia majora are developed ; ot, origin of the ovary or testicle respectively ; x, part of the Wolffian body developed later into the coni vasculosi : 3, ureter ; 4, bladder ; .5, urachus. The future of the Miillerian and Wolffian ducts varies ac- cording to the sex of the embryo. Fio. 128.— Diagram of the maiiiinalian type of male sexual organs Cafter Quaiii). Compare with Figs. 127. 129. C, Cowt>cr"s glanil of one side ; cp, cf)rpora cavernosa ])fnis, cut slmrt : f. caput (•i)iiiidymiH ; (/. {.'ulMTiiaculurri ; j, rectum ; hi, hydatid of MorgaKni, the pcrsisti'iit ant*frior end of the .Mljlji-riaii duct, the cf)njoint post^-rior end.s of which form Ihf iilrnis rnatMMilirius : //r, prostaU- gland ; h, Hcrotiirri ; hji, corpus spongiosum nrclhrd' ; /.testis (t, as in Fig. 129. 110 ANIMAL PHYSIOLOGY. In the male the Wolffian duct persists as the vas deferens ; in the female it remains as a rudiment in the region near the ovary (hydatid of Morgagni). In the female the Mtillerian duct becomes the oviduct and related parts (uterus and vagina) ; in the male it atrophies. One, usually the right, also atrophies in female birds. The sinus pocularis of the iDrostate is the rem- nant in the male of the fused tubes. The various forms of the generative apparatus derived from the Mtillerian ducts, as determined by different degrees of fu- sion, etc., of parts, may be learned from the accompanying figures. In both sexes the most posterior portion of the Wolffian duct gives rise to the metanephros, or what becomes the perma- FiG. 129. — Diagram of the mammalian type of female sexual organs (after Quain). The dotted lines in one figure indicate functional organs in the other. C, gland of Bartholin (Cowper's gland) ; c. c, corpus cavernosuni elitoridis ; clG, remains of the left Wolffian duct, which may persist as the duct of Gaertner ; /, abdominal opening of left Fallopian tube ; g, round ligament (corresponding to the gubernaculum) ; h, hymen ; i, rectum ; I, labium ; m, cut FaOopian tube (oviduct, or Miillerian duct) of the right side ; n, nynipha ; o, left ovary ; po. parovarium ; sc, vascular bulb or corpus spongiosum ; u, uterus ; v, vulva ; va, vagina ; W, scattered remains of Wolffian tubes (paroophoron) ; iv, cut end of van- ished right Wolffian duct ; 3, ureter ; 4, bladder passing below into the uretha ; 5, urachus, or remnant of stalk of aUantois. nent kidney and ureter ; in the male also to the vas deferens, testicle, vas aberrans, and seminal vesicle. The ovary has a similar origin to the testicle ; the germinal epithelium furnishing the cells, which are transformed into Graafian follicles, ova, etc., and the mesoblast the stroma in which these structures are imbedded. In the female the parovarium remains as the representative of the atrophied Wolffian body and duct. The bladder and urachus are both remnants of the formerly extensive aUantois. The final forms of the genito-urinary or- THE DEVELOPMENT OF THE UROGENITAL SYSTEM. m gans arise by differentiation, fusion, and atrophy: thus, the cloaca or common cavity of the genito-urinary ducts is divided by a septum (the perineum externally) into a genito-urinary and an intestinal (anal) part ; the j)enis in the male and the corresponding clitoris in the female appear in the region of the cloaca, as outgrowths which are followed by extension of folds of integument that become the scrotum in the one sex and the labia in the other. The urethra arises as a groove in the under surface of the penis, which becomes a canal. The original opening of the urethra was at the base of the penis. ALL ALL^ ^ CLi Fig. 130. Fig. 131. Fig. 1.32. Figs. 130 to 1.33.— Diagrams illustrating the evolution of the posterior passages (after Landoia and Stirling). Fig. 130. — Allantois continuous with rectum. Fig. 131.— Cloaca formed. Fig. 132.— Early condition in male, before the closure of the folds of the groove on the poste- rior sidf" of the penis. Fig. 1-3:5.— Early female condition. A, commencement of proctodceum ; ALL, allantois ; B, bladder ; C, penis ; CL, cloaca ; J/, Miillerian duct ; B, rectum : U, urethra ; S, vestibule ; SU, urogenital sinus ; V, vas deferens in Fig. 132, vagina in Fig. 133. In certain cases development of these parts is arrested at various stages, from which result abnormalities frequently re- quinng interference by the surgeon. The accounts of the previous chapters do not complete the history of development. Certain of the remaining subjects that are of special interest, from a physiological point of view, will be referred to again; and in the mean time we shall consider rather briefly some of the physiological problems of this subject to which scant reference has as yet been made. Though the pliysiology of reproduction is introduced here, so that ties of natural connection may not be severed, it may very well be omitted by the student who is dealing with embry- 112 ANIMAL PHYSIOLOGY. A Fig. 134.— "Various forms of mammalian uteri. A. Ornithorhynchus. B. Didelphys dorsigera. C. Phalangista vulpina. D. Double uterus and vagina ; human anomaly. E. Lepus cuni- culus (rabbit), utei-us duplex. F. Uterus bicornis. G. Utenis bipartitus. H. Uterus simplex (human), o, anus ; cl, cloaca ; o. d, oviduct ; o. t, os tincae (os uteri) ; ov, ovary ; r, rectum ; s, vaginal septum ; u. b, urinary bladder ; ur, ureter ; ur. o, orifice of same ; us, m'ogenital sinus ; ut, uterus ; v, vagina ; v. c, vaginal caecum (Haddon). ology for tlie first time, and in any case slionld be read again after the other functions of the body have been studied. The Physiological Aspects of Development. According to that law of rhythm which, as we have seen, prevails throughout the world of animated nature, there are periods of growth and progress, of quietude and arrest of devel- opment ; and in vertebrates one of the most pronounced epochs — in fact, the most marked of all — is that by which the young organism, through a series of rapid stages, attains to sexual maturity. While the growth and development of the generative or- gans share to the greatest degree in this progress, other parts of the body and the entire being participate. So great is the change that it is common to indicate, in the case of the human subject, the developed organism by a new name— the " boy " becomes the " man," the " girl " the "woman.'' Relatively this is by far the most rapid and general of all the transformations the organism undergoes during its extra-uter- ine life. In this the entire. body takes part, but very unequally. The increase in stature is not proportionate to the increase in weight, and the latter is not so great as the change in form. The modifications of the organism are localized and yet affect the whole being. The outlines become more rounded ; the pel- THE DEVELOPMENT OF THE UROGENITAL SYSTEM. II3 vis in females alters in shape ; not only do the generative organs themselves rapidly undergo increased development, but certain related glands (mammse) participate ; hair appears in certain regions of the body ; the larynx, especially in the male, under- goes enlargement and changes in the relative size of parts, re- sulting in an alteration of voice (breaking of the voice), etc. — all in conformity with that excess of nutritive energy which marks this biological epoch. Correlated with these physical changes are others belonging to the intellectual and moral (psychic) nature equally impor- tant, and, accordingly, the future being depends largely on the full and unwarped developments of these few years. Sexual maturity, or the capacity to furnish ripe sexual ele- ments (cells), is from the biological standpoint the most impor- tant result of the onset of that period termed, as regards the human species, puberty. The age at which this epoch is reached varies with race, sex, climate, and the moral influences which envelop the indi- vidual. In temperate regions and with European races i3u- berty is reached at from about the thirteenth to the eighteenth year in the female, and rather later in the male, in whom de- velopment generally is somewhat slower. Menstruation and Ovulation. In all vertebrates, at periods recurring with great regu- larity, the generative organs of the female manifest unusual activity. This is characterized by increased vascularity of the ovary and adjacent parts ; with other changes dependent on this, and that heightened nerve influence which, in the verte- brate, seems to be insejiarable from all important functional changes. Ovulation is the maturation and discharge of ova from the Graafian follicles. The latter, reaching the exterior zone of the ovary, becf^ming distended and thinned, burst ex- ternally and thus free the ovum. The follicles being very vas- cular at this j^eriod, blood escapes, owing to this rupture, into the emptied capsule and clots; and as a result of organization and subsequent degeneration undergoes a certain series of changes dependent on the condition of the ovary and adjacent parts, which varies according as the ovum has been fertilized or not. When fertilization occuirs the Graafian follicle under- goes changes of a more marked and lasting character, becom- ing a true corpus luteum of pregnancy. 8 114 ANIMAL PHYSIOLOGY. Tlie ovum, in the fowl is fertilized in tlie upper part of tlie o^T.duct ; in the mammal mostly in this region also, as is shown by the site of the embryos in those groups of animals with a two-horned uterus, and the occasional occurrence of tubal preg- nancy in woman. But this is not, in the human subject at least, invariably the site of impregnation. After the ovum has been set free, as above described, it is conveyed into the ovi- duct (Fallopian tube), though exactly how is still a matter of dispute : some holding that the current produced by the action of the ciliated cells of the Fallopian tube suffices ; others that the o^nim is grasped by the fimbriated extremity of the tube as part of a co-ordinated act. It is likely, as in so many other instances, that both -sdews are correct but partial ; that is to say, both these methods are employed. The columnar ciliated cells, lining the oviduct, act so as to produce a current in the direction of the uterus, thus assisting the ovum in its passage toward its final resting place. Menstmation. — As a part of the general acti^dty occurring at this time, the uterus manifests certain changes, chiefly in its internal mucous lining, in which thickening and increased Fig. 135.— Diagram of the human uterus just before menstruation. The shaded por- tion represents the mucous membrane (Hart and Barbour, after J. Williams). Fig. 136. — Uterus after menstraation has just ceased. The cavity of the body of the uterus is supposed to have been deprived of mucous membrane (J. Williams). vascularity are prominent. A flow of blood from the uterus in the form of a gentle oozing follows ; and as the superficial THE DEVELOPMENT OF THE UROGENITAL SYSTEM. 115 parts of the mucous lining of the uterus undergo softening and fatty degeneration, they are thrown off and renewed at these periods {catamenia, menses, etc.), provided pregnancy does not take place. In mammals below man, in their nat- ural state, pregnancy does almost invariably take place at such times, hence this exalted activity of the mucous coat of the uterus, in preparation for the reception and nutrition of the ovum, is not often in vain. In the human subject the menses appear monthly ; pregnancy may or may not occur, and consequently -there may be waste of nature's forces; though there is a certain amount of evidence that menstruation does not wholly represent a loss ; but that it is largely of that char- acter among a certain class of women is only too evident. As can be readily understood, the catamenial flow may take place prior to, during, or after the rupture of the egg-capsule. As the uterus is well supplied with glands, during this period of increased functional activity of its lining membrane, mucus in considerable excess over the usual quantity is dis- charged ; and this phase of activity is continued should preg- nancy occur. All the parts of the generative organs are supplied with muscular tissue, and with nerves as well as blood-vessels, so that it is possible to understand how, by the influence of nerve- centers, the various events of ovulation, menstruation, and those that follow when pregnancy takes place, form a related series, very regular in their succession, though little prominent in the consciousness of the individual animal when normal. The Nutrition of the Ovum (oosperm). This will be best understood if it be remembered that the ovum is a cell, undifferentiated in most directions, and thus a sort of amoeboid organism. In the fowl it is known that the cells of the primitive germ devour, amoeba-like, the yelk-cells, while in the mammalian oviduct the ovum is surrounded by abundance of proteid, which is doubtless utilized in a somewhat similar fashion, as also in the uterus itself, until the embryonic membranes have formed. To speak of the ovum being nour- ished by diffusion, and es])ecially by osmosis, is an unnecessary assumption, and, as we believe, at variance Avith fundamental principles ; for we doubt much whether any vital process is one of pure osmosis. As soon as the yelk-sac and allantois have been formed, nutriment is derived in great part through 116 ANIMAL PHYSIOLOGY. the vessel- walls, which, it will be remembered, are differentia- ted from the cells of the mesoblast, and, it may well be as- sumed, have not at this early stage entirely lost their amoeboid character. The blood-vessels certainly have a respiratory func- tion, and suffice, till the more complicated villi are formed. The latter structures are in the main similar in build to the villi of the alimentary tract, and are adapted to being sur- rounded by similar structures of maternal origin. Both the maternal crypts and the fcetal villi are, though complementary in shape, all but identical in minute structure- in most in- stances. In each case the blood-vessels are covered superfi- cially by cells which we can not help thinking are essential in nutrition. The villi are both nutritive and respiratory. It is no more difficult to understand their function than that of the cells of the endoderm of a polyp, or the epithelial coverings of lungs or gills. Experiment proves that there is a respiratory interchange of gases between the maternal and fcetal blood which nowhere mingle physically. The same law holds in the respiration of the foetus as in the mammals. Oxygen passes to the region where there is least of it, and likewise carbonic anhydride. If the mother be asphyxiated so is the foetus, and indeed more rapidly than if its own umbilical vessels be tied, for the mater- nal blood in the first instance abstracts the oxygen from that of the foetus when the tension of this gas becomes lower in the maternal than in the foetal blood ; the usual course of affairs is reversed, and the mother satisfies the oxygen hunger of her own blood and tissues by withdrawing that which she recently supplied to the foetus. It will be seen, then, that the embryo is from the first a parasite. This explains that exhaustion which pregnancy, and especially a series of gestations, entails. True, nature usually for the time meets the demand by an excess of nutritive energy : hence many persons are never so vigorous in appearance as when in this condition ; often, however, to be fol- lowed by corresponding emaciation and senescence. The full and frequent respirations, the bounding pulse, are succeeded by reverse conditions ; action and reaction are alike present in the animate and inanimate worlds. Moreover, it falls to the parent to eliminate not only the waste of its own organism but that of the foetus ; and not infrequently in the human subject the over- wrought excretory organs, especially the kidneys, fail, entailing disastrous consequences. The digestive functions of the embryo are naturally inact- THE DEVELOPMENT OP THE UROGENITAL SYSTEM. 117 ire, the blood being supplied with all its needful constituents through the placenta by a much shorter process ; indeed, the placental nutritive functions, so far as the fcetus is concerned may be compared with the removal of already digested material from the alimentary canal, though of course only in a general way. During foetal life the digestive glands are developing, and at the time of birth all the digestive juices are secreted in an efficient condition, though only relatively so, necessitating a special liquid food (milk) in a form in which all the constituents of a normal diet are provided, easy of digestion. frL~th^^f^ «T'°1'^'^ embryos from the second to the fifteenth week (natural size) seen from the left side, the arched back turned t«jward the right. (Priucii)all\- after Ecker^ 5lil""^Ar'"i'i7'' o? I'' days ; in, of 3 weeks ; JV, of 4 weeks V, of 5 weeks • V^^ weeka ; V U, of 7 weeks ; Vlll, of 8 weeks ; XU, of 12 weeks ; XV, of 15 weeks ' Bile, inspissated and mixed with the dead and cast-off epi- thelium of the alimentary tract, is abundant in the intestine at birth in the human subject ; but bile is to be regarded perhaps rather in the light of an excretion than as a digestive fluid. The .skin and kidneys, thougli not functionless, are rendered unnecessary in great part by the fact that waste can be and is withdrawn by the placenta, which proves to be a nutritive, re- 118 ANIMAL PHYSIOLOGY. spiratory, and excretory organ ; it is in itself a sort of abstract and brief chronicle of the whole physiological story in foetal life. All of the foetal organs, especially the muscles, abound in an animal starch (glycogen), which in some way, not well under- stood, forms a reserve fund of nutritive energy which is pretty well used up in the earlier months of pregnancy. We may suppose that the amoeboid cells — all the undifferentiated cells of the body — feed on it in primitive fashion ; and it will not be forgotten that the older the cells become, the more do they depart from the simpler habits of their earlier, cruder existence ; hence the disappearance of this substance in the later months of foetal life. In one respect the foetus closely resembles the adult: it draws the pabulum for all its various tissues from blood which itself may be regarded as the first completed tissue. We are, accordingly, led to inquire how this river of life is distributed ; in a word, into the nature of the foetal circulation. Foetal Circulation. — The blood leaves the placenta by the um- bilical vein, reaches the ' inferior vena cava, either directly (by the ductus venosus), or, after first passing to the liver (by the vencE advehentes, and returning by the venm revehentes), and proceeds, mingled with the blood returning from the lower ex- tremities, to the right auricle. This blood, though far from being as arterial in character as the blood after birth, is the best that reaches the heart or any part of the organism. After arriving at the right auricle, being dammed back by the Eus- tachian valve, it avoids the right ventricle, and shoots on into the left auricle, passing thence into the left ventricle, from which it is sent into the aorta, and is then carried by the great trunks of this arch to the head and upper extremities. The blood returning from these parts passes into the right auricle, then to the corresponding ventricle and thence into the pul- monary artery; but, finding the branches of this vessel un- opened, it takes the line of least resistance through the ductus arteriosus into the aortic arch beyond the point where its great branches emerge. It will be seen that the blood going to the head and upper parts of the body is greatly more valuable as nutritive pabulum than the rest, especially in the quantity of oxygen it contains ; that the blood of the foetus, at best, is rela- tively ill-supplied with this vital essential ; and as a result we find the upper (anterior in quadrupeds) parts of the foetus best developed, and a decided resemblance between the mammalian foetus functionally and the adult forms of reptiles and kindred THE DEVELOPMENT OF THE UROGENITAL SYSTEM. 119 groups of the lower vertebrates. But this condition is well enough adapted to the general ends to be attained at this pe- Pulmonary Arf. Foramen Ovale Husiachian Valve. ... Right Auric. - Vent. Opening. Branches of the Uriihilical Vein, to the Liver. Bladder. Pulmonary Art. ■■ Left Auricle. .. Left Aicric. - Vent. Opening. Hepatic Vein. Ductus Venostts. Internal Iliac Arteries. Fin. 138.-Diaj^am of the foetal circulation (Flint). 120 ANIMAL PHYSIOLOGY. riod— the nourishment of structures on the way to a higher path of progress. As embryonic maturity is being reached, preparation is made for a new form of existence ; so it is found that the Eustachian valve is less prominent and the foramen ovale smaller. Parturition. ■ All the efforts that have hitherto been made to determine the exact cause of the result of that series of events which make up parturition have failed. This has probably been owing to an attempt at too simple a solution. The foetus lies surrounded (protected) by fluid contained in the amniotic sac. For its expul- sion there is required, on the one hand, a dilatation of the uter- ine opening {os uteri), and, on the other, a vis a tergo. The lat- ter is furnished by the contractions of the uterus itself, aided by the simultaneous action of the abdominal muscles. Through- out the greater part of gestation the uterus experiences some- what rhythmical contractions, feeble as compared with the final ones which lead to expulsion of the foetus, but to be regard- ed as of the same character. With the growth and functional development of other organs, the placenta becomes of less con- sequence, and a fatty degeneration sets in, most marked at the periphery, usually where it is thinnest and of least use. It does not seem rational to believe that the onset of labor is referable to any one cause, as has been so often taught ; but rather that it is the final issue to a series of processes long existing and grad- ually, though at last rapidly, reaching that climax which seems like a vital storm. The law of rhythm affects the nervous sys- tem as others, and upon this depends the direction and co-ordi- nation of those many activities which make up parturition. We have seen that throughout the whole of foeta^l life changes in one part are accompanied by corresponding changes in oth- ers ; and in the final chapter of this history it is not to be ex- pected that this connection should be severed, though it is not at present possible to give the evolution of this process with any more than a general approach to probable correctness. Changes in the Circulation after Birth. When the new-born mammal takes the first breath, effected by the harmonious action of the respiratory muscles, excited to action by stimuli reaching them from the nerve-center (or THE DEVELOPMENT OF THE UROGENITAL SYSTEM. 121 centers) which preside over respiration, owing to its being roused into action by the lack of its accustomed supply of oxygen, the hitherto solid lungs are expanded ; the pulmonary vessels are rendered permeable, hence the blood now takes the path of least resistance along them, as it formerly did through the ductus arteriosus. The latter, from lack of use, atrophies in most instances. The blood, returning to the left auricle of the heart from the lungs in increased volume, so raises the pressure in this chamber that the stream that formerly flowed through the foramen ovale from the right auricle is opposed by a force equal to its own, if not greater, and hence passes by an easier route into the right ventricle. The fold that tends to close the foramen ovale grows gradually over the latter, so that it usually ceases to exist in a few days after birth. At birth, ligature of the umbilical cord cuts off the placental circulation ; hence the ductus venosus atrophies and becomes a mere ligament. The placenta, being now a foreign body in the uterus, is ex- pelled, and this organ, by the contractions of its walls, closes the ruptured and gaping vessels, thus providing against hsemor- rhage. Coitus between the Sexes. In all the higher vertebrates congress of the sexes is essential to bring the male sexual product into contact with the ovum. F'lO. 139.— Sfctif)n of erfM.-tile tissuf ff'adiat). ri, traJieculse of connective tissue, with elastic flbere, and bundleH of plain niuHCular tissue (c) ; b, venous spaces (Scliiifcn. 122 ANIMAL PHYSIOLOGY. Erection of the penis results from the conveyance of an excess of blood to the organ, owing to dilation of its arteries, and the retention of this blood within its caverns. The structure of the penis is peculiar, and, for the details of the anatomy of both the male and female generative organs, the student is referred to works on this subject ; suffice it to say that it consists of erectile tissue, the chief characteristic of which is the opening of the capillaries into cavernous venous spaces {sinuses) from which the veinlets arise ; with such an arrangment the circulation must be very slow — the inflow being greatly in excess of the outflow — apart altogether from the compressive action of certain muscles connected with the organ. As previously explained, the spermatozoa originate in the seminal tubes, from which they find their way to the Fig. 140.— Section of parts of three seminiferous tubules of the rat (Schafer). a, with the spermatozoa least advanced in development ; b, more a,dvanced ; c, containing fuUy de- veloped spermatozoa. Between the tubules are seen strands of interstitial cells, with blood-vessels and lymph-spaces. seminal vesicles or receptacles for semen till required to be discharged. The spermatozoa as they mature are forced on by fresh additions from behind and by the action of the ciliated cells of the epididymis, together with the wave-like (peristaltic) action of the vas deferens. Discharge of semen during coitus is effected by more vigorous peristaltic action of the vas defer- ens and the seminal vesicles, followed by a similar rhythmical action of the bulbo-cavernosus and ischio-cavernosus muscles, by which the fluid is forcibly ejaculated. Semen itself, though composed essentially of spermatozoa. THE DEVELOPMENT OF THE UROGENITAL SYSTEM. 123 is mixed with the secretions of the vas deferens, of the seminal vesicles, of Cowper's glands, and of the prostate. Chemically it is neutral or alkaline in reaction, highly albuminous, and contains nuclein, lecithen, cholesterin, fats, and salts. The movements of the male cell, owing to the action of the tail (cilium), suflBce of themselves to convey them to the ovi- FlG. 141. — Left broad ligament. Fallopian tube, ovary, and parovarium in the human subject (Henle). «, uterus ; i, isthmus of Fallopian tube ; a, ampulla ; /, fimbriated end of the tube, with the parovarium to its right ; o. ovary ; o. I, ovarian ligament. ducts : but there is little doubt that during or after sexual con- gress there is in the female, even in the human subject, at least Fio. 142.— UteruH and ovaries of the sow, 8emi-dia(|rammatic (after Dalton). o, ovary ; H, Fallopian tube ; h, horn of the uterus ; b, bfxiy of the uterus ; v, vagina. in many cases, a retrograde peristalsis of the uterus and ovi- ducts which would tend to overcome the results of the activity 124: ANIMAL PHYSIOLOGY. of the ciliated cells lining the oviduct. It is known that the male cell can survive in the female organs of generation for several days, a fact not difficult to understand, from the method of nutrition of the female cell (ovum) ; for we may suppose that both elements are not a little alike, as they are both slightly modified amoeboid organisms. Nervous Mechanism. — Incidental reference has been made to the directing influence of the nervous system over the events of reproduction ; especially their subordination one to another to bring about the general result. These may now be consid- ered in greater detail. Most of the processes in which the nervous system takes part are of the nature of reflexes, or the result of the automa- ticity (independent action) of the nerve-centerSj, increased by some afferent (ingoing) impressions along a nerve-path. It is not always possible to estimate the exact share each factor takes, which must be highly variable. Certain experiments have assisted in making the matter clear. It has been found that, if in a female dog, the spinal cord be divided when the animal is still a puppy, menstruation and impregnation may occur. If the same experiment be performed on a male dog, erection of the penis and ejaculation of semen may be caused by stimulation of the penis. As the section of the cord has left the hinder part of the animal's body severed from the brain, the creature is, of course, unconscious of anything happening in all the parts below the section, of whatever nature. If the nervi erigentes (from the lower part of the spinal cord) be stimulated, the penis is erected ; and if they be cut, this act be- comes impossible, either refiexly by experiment or otherwise. Seminal emissions, it is well known, may occur during sleep, and may be associated, either as result or cause, with voluptu- ous dreams. Putting all these facts together, it seems reason- able to conclude that the lower part of the sx)inal cord contains the nervous machinery requisite to initiate those influences (im- pulses) which, passing along the nerves to the generative or- gans, excite and regulate the processes which take place in them. In these, vascular changes, as we have seen, always play a prominent part. Usually we can recognize some afferent influence, either from the brain (psychical), from the surface — at all events from without that part of the nervous system (center) which functions directly in the various sexual processes. It is com- mon to speak of a number of sexual centers — as the erection THi! DEVELOPMENT OF THE UROGENITAL SYSTEM. 125 center, the ejaeulatory center, etc. — but we much doubt whether there is such sharp division of physiological labor as these terms imply, and they are liable to lead to misconception ; ac- cordingly, in the present state of our knowledge, we prefer to speak of the sexual center, using even that term in a somewhat broad sense. The effects of stimulation of the sexual organs are not con- fined to the parts themselves, but the ingoing impulses set up radiating outgoing ones, which affect widely remote areas of the body, as is evident, especially in the vascular changes ; the central current of nerve influence breaks up into many streams as a result of the rapid and extensive rise of the outflowing current, which breaks over ordinary barriers, and takes paths which are not properly its own. Bearing this fact in mind, the chemical composition of semen, so rich in proteid and other material valuable from a nutritive point of view, and consid- ering how the sexual appetites may engross the mind, it is not difficult to understand that nothing so quickly disorganizes the whole man, physical, mental, and moral, as sexual excesses, whether by the use of the organs in a natural way, or from masturbation. Nature has protected the lower animals by the strong bar- rier of instinct, so that habitual sexual excess is with them an impossibility, since the females do not permit of the approaches of the male except during the rutting period, which occurs only at stated, comparatively distant periods in most of the higher mammals. When man keeps his sexual functions in subjection to his higher nature, they likewise tend to advance his whole development. Summary. — Certain changes, commencing with the ripening of ova, followed by their discharge and conveyance into the uterus, accompanied by simultaneous and subsequent modifica- tions of the uterine mucous membrane, constitute, when preg- nancy occurs, an unbroken chain of biological events, though usually described separately for the sake of convenience. When impregnation does not result, there is a retrogression in the uterus (menstruation) and a return to general quiescence in all the reproductive organs. Parturition is to be regarded as the climax of a variety of rhythmic occurrences which have been gradually gathering head for a long period. The changes which take place in the placenta of a degenerative character fit it for being cast off, and may render this structure to some extent a foreign b(Kly })(ifove 120 ANIMAL PHYSIOLOGY. it and the foetus are finally expelled, so that these changes may constitute one of a number of exciting causes of the increased uterine action of parturition. But it is important to regard the whole of the occurrences of pregnancy as a connected series of processes co-ordinated by the central nervous system so as to accomplish one great end, the development of a new individual. The nutrition of the ovum in its earliest stages is effected by means in harmony with its nature as an amoeboid organism ; nutrition by the cells of blood-vessels is similar, while that by villi may be compared to what takes place through the agency of similar structures in the alimentary canal of the adult mam- mal. The circulation of the foetus puts it on a par physiologically with the lower vertebrates. Before birth there is a gradual though somewhat rapid preparation, resulting in changes which speedily culminate after birth on the establishment of the permanent condition of the circulation of extra-uterine life. The blood of the foetus (as in the adult) is the great store- house of nutriment and the common receptacle of all waste products ; these latter are in the main transferred to the moth- er's blood indirectly in the placenta ; in a similar way nutri- ment is imported from the mother's blood to that of the foetus. The placenta takes the place of digestive, respiratory, and ex- cretory organs. Coitus is essential to bring the male and female elements together in the higher vertebrates. The erection of the penis is owing to vascular changes taking place in an organ composed of erectile tissue ; ejaculation of semen is the result of the peri- staltic action of the various parts of the sexual tract, aided by rhythmical action of certain striped muscles. The spermatozoa, which are unicellular, flagellated (ciliated) cells, make up the es- sential part of semen ; though the latter is complicated by the addition of the secretions of several glands in connection with the seminal tract. Though competent by their own movements of reaching the ovum in the oviduct, it is probable that the uterus and oviduct experience peristaltic actions in a direction toward the ovary, at least in a number of mammals. The lower part of the spinal cord is the seat in the higher mammals of a sexual center or collection of cells that receives afferent impulses and sends out efferent impulses to the sexual organs. This, like all the lower centers, is under the control of the higher centers in the brain, so that its action may be either initiated or inhibited by the cerebrum. ORGANIC EVOLUTION RECONSIDERED. 127 ORGANIC EVOLUTION RECONSIDERED. The study of reproduction has prepared the student for the comprehension of certain views of the origin of the forms of life which could not be as profitably considered before. While the great majority of biologists are convinced that there has been a gradual evolution of more complex organisms from simpler ones, and while most believe that Darwin's the- ory furnishes some of the elements of a solution of the problem as to how this has occurred, many still feel that the whole ex- planation was not furnished by that great naturalist. Accordingly, we shall notice very briefly a few of the more important contributions to this subject since Darwin's views were published. In America, under the influence of the writings of Cope and Hyatt, a school of evolutionists has been formed, holding doc- trines that constitute a modification of those announced in cruder form by Lamarck, hence termed neo-Lamarckianism. These authors have imported consciousness into the list of factors of organic evolution and given it a prominent place. They regard consciousness as a fundamental property of proto- plasm ; it determines effort and the direction that activity shall take : thus hunger leads to migration, and brings the creature under a new set of conditions which influence its nature. A certain proportion of the changes an animal undergoes are at- tributed to the direct influence of surrounding conditions (en- vironment), but the larger number are owing to efforts involv- ing the greater use of some parts than others, which tends to Vjecorae habitual. This is the explanation neo-Lamarckianism offers for the origin of variations. It is assumed that the re- sults of use or disuse of parts is inherited, so that the gain or loss is not transient with the individual, but remains with the group. This theory also refers the loss or preservation of certain structures to " acceleration " or " retardation " of growth ; thus, if the growth of gills were greatly and progressively retarded during embryonic life, they might become only rudimentary, and this would furnish an explanation of the origin of rudiment- ary organs, tliough it is clear that use and effort could not di- rectly explain such acceleration or retardation. It is further a fact, which this theory does not explain, that all variations of structure produced by use are not inherited. 128 ANIMAL PHYSIOLOGY. Weismann, in fact, denies that peculiarities acquired dur- ing the lifetime of the adult are passed dn to offspring. This writer believes that we must seek in Amoeba, as the ancestral representative of the ovum, for the clew to the laws of heredity. The Amoeba must divide or cease to exist as a group form — hence the segmentation of the ovum ; this is but the inherited tendency to divide. What the individual becomes is determined entirely by the ovum, the v^hole of which does not develop into the new being, but a part is laid aside in reserve as the future ovum. Any variations that show themselves in future indi- viduals are such as arise from the variations of the ovum itself. According to this writer, it is as natural for the offspring to resemble the parent (heredity) in the higher groups of animals as that one Amoeba should resemble another, and for the same reason. Weismann has also attempted to explain the necessity and the significance of the extrusion of polar globules. The first polar globule is expelled from all ova, even those that can de- velop independent of a male cell (parthenogenetic). This rep- resents that part of the original ovum which determines its peculiarities of form, etc. (ovogenetic idioplasm) ; while the second polar globule is one half of the nucleus of the mature ovum ready to enter upon development, if fertilized. When the latter takes place, it is joined by the corresponding nuclear substance of the male cell to form the segmentation nucleas. It is this substance (germ-plasma) which determines exactly what line of development, to the minutest details, the ovum shall follow. In the course of time the nucleus would thus come to represent many generations of united plasmas. There must be a limit to this, from the physical necessities of the case ; hence the expulsion of a second polar globule, which also is a provision against parthenogenesis, for in some cases the plasma of the nucleus has the power, without the accession of any male plasma, to segment and develop the mature animal. But in any case there is a great advantage in the union of the two plasmas with their diverse experiences ; hence sexual rejjro- duction, though the most costly apparently, is in reality the most economical for Nature in the end, for higher results are reached, and it seems, in fact, that this lies at the very foun- dation of organic progress. The theory of Brooks may be regarded as eclectic, being a combination of that of Weismann and Darwin more particu- larly, with entirely new additions by himself. ORGANIC EVOLUTION RECONSIDERED. 129 Darwin believed that every part of the body gave off " gem- mules/' or very minute bodies, which were collected into the ovum, and thus the ovum came to be a sort of abstract of the whole body — hence the resemblance of offspring to parents, since the development of the ovum was but that of the gem- mules. Some of the gemmules might remain latent for genera- tions, and then develop ; hence that resemblance often seen to ancestors more remote than the parents (reversion). This is a very brief account of Darwin's hypothesis of pangenesis. This writer, however, never accounted for variations. He spoke of variations as " spontaneous," meaning, not that they were supernatural, but that it was not possible to assign them to a definite cause. To account for variation has naturally been the aim of later writers. How neo-Lamarckianism does this has been already considered. We now give the views of Brooks on this and other points in connection with organic evolution. This thinker, like Weismann, looks to the fertilized ovum for an explanation of the main facts ; but Brooks refers the origin of variations to the influence of the male cell. This is, of course, a pure hypothesis, but it is in harmony with many facts which were in need of explanation. It had been noticed by Darwin that variations of all kinds were most apt to arise upon alteration in the conditions under which an animal lived. Brooks also believes in gemmules, but does not think they are given off from all parts equally or at all times, but that they are derived from those parts most affected by the change of surroundings ; and since this would influence parts much when for the worse, variation would coincide with suf- fering or need ; hence those very parts would vary, and so pre- pare for adaptation, just when this was most called for by the nature of the case. But the male sexual element, it has been shovrn, is more liable to variation than the ovum ; hence the ex- planation of what Brooks believes to be a fact, that it is the sperm-cell that generally is responsible for variation, since it chiefly collects the gemmules. The author of this theory points to parthenogenetic forms being less variable, as evidence of the truth of his view. To introduce a male cell is to impart vast numbers of new gem- mules, and thus induce variability. This hypothesis would ex- plain why the female represents what is most fundamental and ancient in the history of psychological development, and the male what is associated with enterjjriso — in a word, the female preserves, the male originates, in the widest sense. 9 230 ANIMAL PHYSIOLOGY. Vines lias stated that the equivalent of parthenogenesis takes place in the male cell in plants. Though this may be an objection to the universality of application of Brooks's theory, it does not seem to us to be fatal to it as a whole. As has been pointed out, in a previous chapter, Darwin held that the differences that caused ultimately the formation of new groups of living forms were the result of extremely slow accumulation of variations, at first very minute. He every- where insists upon this. But, unquestionably, it is just here that the greatest difficulty is to be encountered in the Darwin- ian account of evolution. The chances against the loss of the variation by breeding with forms that did not possess it seem to be numerous, hence various theories have been proposed to lessen the difficulty. Mivart introduced the doctrine of extraordinary births, be- lieving that variations were often sudden and pronounced. That they were so occasionally Darwin himself admitted ; but he considered a theory like that of Mivart as a surrender, a resort to an explanation that verged in its character on the introduction of the supernatural itself. A view that has attracted much attention and caused a great deal of controversy, is that of Romanes, which was intro- duced in part to meet the difficulty just referred to ; and to lessen the further one arising from the infertility of species with one another, as compared with the perfect fertility of varieties. It has often been noticed that, though the difference anatomically between varieties might be greater than between species, the above law as to fertility still held. Such a fact calls for ex- planation ; hence Romanes has proposed his theory of " physi- ological selection" (segregation, isolation). If it be admitted that some change may take place in the sexual organs of two forms so that the members of one are fertile with each other while those of the other are not, it will at once appear that they are as much isolated physiologically as if separated by an ocean. That such does take place is an assumption based on the great tendency in the reproductive organs to change ; and it is claimed that, if this assumption be granted, that the main difficulty of Darwin's theory will be removed, for the '" swamp- ing " action of intercrossing forms that vary slightly, or one of them not at all, in the given direction, will not occur. Romanes believes that forms that vary are fertile inter se, but not with the parent forms, which would meet the case fairly well. Cer- tain it is that species are not generally fertile with one an- ORGANIC EVOLUTION RECONSIDERED. 131 other while varieties are so invariably ; and it is this that, in the opinion of Romanes and many others, has never been ade- quately explained. Admitting that the theories of Romanes, Brooks, and Weis- mann have advanced us on the way to more complete views of the mode of origin of the forms of the organic world, it must still be felt that all theories yet propounded fall short of being entirely satisfactory. It seems to us unfortunate that the sub- ject has not received more attention from physiologists, as without doubt the final solution must come through that sci- ence which deals with the properties rather than the forms of protoplasm ; or, in other words, the fundamental principles underlying organic evolution are physiological. But, in the unraveling of a subject of such extreme complexity, all sci- ences must probably contribute their quota to make up the truth, as many rays of different colors compounded form white light. As with other theories of the inductive sciences, none can be more than temporary ; there must be constant modifi- cation to meet increasing knowledge. Conscious that any views we ourselves advance must sooner or later be modified as all others, even if acceptable now, we venture to lay before the reader the opinions we have formed upon this subject as the result of considerable thought. All vital phenomena may be regarded as the resultant of the action of external conditions and internal tendencies. Amid the constant change which life involves we recognize two things : the tendency to retain old modes of behavior, and the tendency to modification or variation. Since those impulses originally bestowed on matter when it became living, must, in order to prevail against the forces from without, which tend to destroy it, have considerable potency, the tendency to modi- fication is naturally and necessarily less than to permanence of form and function. From these principles it follows that when an Amoeba or kindred organism divides after a longer or shorter period, it is not in reality the same in all respects as when its existence began, thougli we may be quite unable to detect the changes ; and when two infusorians con jugate, the one brings to the other protophism flifTerent in molecular behavior, of necessity, from having had diffr-rent experiences. We attach great importance to these principles, as they seem to us to lie at the root of the whole matter. What has been said of these lower but inde- ]^32 ANIMAL PHYSIOLOGY. pendent forms of life applies to the higher. All organisms are made up of cells or aggregations of cells and their products. For the present we may disregard the latter. When a muscle- cell by division gives rise to a new cell, the latter is not identi- cally the same in every particular as the parent cell was origi- nally. It is what its parent has become by virtue of those experiences it has had as a muscle-cell per se, and as a member of a populous biological community, of the complexities of which we can scarcely conceive. Now, as a body at rest may remain so, or may move in a certain direction according as the forces acting upon it exactly counterbalance one another, or produce a resultant effect in the direction in which the body moves, so in the case of he- redity, whether a certain quality in the parent appears in the offspring, depends on whether this quality is neutralized, aug- mented, or otherwise modified by any corresponding quality in the other parent, or by some opposite quality, taken in connec- tion with the direct influence of the environment during devel- opment. This assumption explains among other things why acquired peculiarities (the results of accident, habit, etc.) may or may not be inherited. These are not usually inherited because, as is to be expect- ed, those forces of the organism which have been gathering head for ages are naturally not easily turned aside. Again, we urge, heredity must be more pronounced than variation. The ovum and sperm-cell, like all other cells of the body, are microcosms representing the whole to a certain extent in themselves — that is to say, cell A is what it is by reason of what all the other millions of its fellows in the biological republic are ; so that it is possible to understand why sexual cells repre- sent, embody, and repeat the whole biological story, though it is not yet possible to indicate exactly how they more than others have this power. This falls under the laws of speciali- zation and the physiological division of labor ; but along what paths they have reached this we can not determine. Strong evidence is furnished for the above views by the his- tory of disease. Scar-tissue, for example, continues to repro- duce itself as such ; like produces like, though in this instance the like is in the first instance a departure from the normal. Gout is well known to be a hereditary disease ; not only so, but it arises in the offspring at about the same age as in the parent, which is equivalent to saying that in the rhythmical life of ORGANIC EVOLUTION RECONSIDERED. 133 certain cells a period is reached when they display the behav- ior, physiologically, of their parents. Yet gout is a disease that can be traced to peculiar habits of living and may be eventually escaped by radical changes in this respect — that is to say, the behavior of the cells leading to gout can be induced and can be altered ; gout is hereditary, yet eradicable. Just as gout may be set up by formation of certain modes of action of the cells of the body, so may a mode of behavior, in the nervous system, for example, become organized or fixed, be- come a habit, and so be transmitted to offspring. It will pass to the descendants or not according to the principles already noticed. If so fixed in the individual in which it arises as to predominate over more ancient methods of cell behavior, and not neutralized by the strength of the normal physiological ac- tion of the corresponding parts in the other parent, it will reap- pear. We can never determine whether this is so or not before- hand ; hence the fact that it is impossible, especially in the case of man, whose vital processes are so modified by his psychic life, to predict whether acquired variations shall become heredi- tary ; hence also the irregularity which characterizes heredity in such cases ; they may reappear in offspring or they may not. In viewing heredity and modification it is impossible to get a true insight into the matter without taking into the account both original natural tendencies of living matter and the influ- ence of environment. Wg only know of vital manifestations in some environment ; and, so far as our experience goes, life is impossible apart from the influence of surroundings. With these general principles to guide us, we shall attempt a brief examination of the leading theories of organic evolution. First of all, Spencer seems to be correct in regarding evolu- tion as universal, and organic evolution but one part of a whole. No one who looks at the facts presented in every field of nature can doubt that struggle (opposition, action and reac- tion) is universal, and that in the organic world the fittest to a given environment survives. But Darwin has probably fixed his attention too closely on this principle and attempted to ex- plain too much by it, as well as failed to see that there are other deeper facts underlying it. Variation, which this author scarcely attempted to explain, seems to us to be the natural re- sult of the very conditions under which living things have an existence. Stable equilibriiim is an idea incompatible with our fundamental conceptions of life. Altered function implies al- tered molecular action, which sometimes leads to appreciable 134 ANIMAL PHYSIOLOGY. structural change. From our conceptions of the nature of liv- ing matter, it naturally follows that variation should be great- est, as has been observed, under the greatest alteration in the surroundings. We are but very imperfectly acquainted as yet with the conditions under which life existed in the earlier epochs of the earth's history. Of late, deep-sea soundings and arctic explo- rations have brought surprising facts to light, showing that living matter can exist under a greater variety of conditions than was previously supposed. Thus it turns out that light is not an essential for life everywhere. We think these recent revelations of unexpected facts should make us cautious in as- suming that life always manifested itself under conditions closely similar to those we know. Variation may at one period have been more sudden and marked than Darwin supposes; and there does seem to be room for such a conception as the " extraordinary births " of Mivart implies ; though we would not have it understood that we think Darwin's view of slow modi- fication inadequate to produce a new species ; we simply vent- ure to think that he was not justified in insisting so strongly that this was the only method of Nature ; or, to put it more justly for the great author of the " Origin of Species," with the facts that have accumulated since his time he would scarcely be warranted in maintaining so rigidily his conviction that new forms arose almost exclusively by the slow process he has so ably described. As there must be all degrees in consciousness, we do not deny that it may be logical to assume some dim spark of this quality in all protoplasm, as Cope insists ; and that it plays a part in determining action and growth there seems to be no doubt. But is it not more philosophical to regard conscious- ness and all allied qualities as correlatives, and underlaid by a molecular constitution with which it is associated as other qual- ities ? It is unduly exalted in the neo-Lamarckian philosophy. We must allow a great deal to use and effort, doubtless, and they explain the origin of variations up to a certain point, but the solution is only partial. Variations must arise as we have attempted to explain, and use and disuse are only two of the factors amid many. Correlated growth, or the changes in one part induced by changes in another, is a principle which, though recognized by Darwin, Cope, and others, has not, we think, re- ceived the attention it deserves. To the mind of the physiolo- gist, all changes must be correlated with others. THE CHEMICAL CONSTITUTION OF THE ANIMAL BODY. 135 This principle has played a great part in the development of man, as we shall show later. Weismann's theories have called attention to the ovum in a new and valuable way, though he seems to have given too ex- clusive attention to the nucleus {gevni-plasma) in itself and out of relation to the influence of the countless cells that make up the body and must be constantly determining modi- fications of the generative organs and the sexual cells them- selves ; so that Brooks's explanation, by adding a new factor, or, at least, presenting a new aspect of the case, was called for and seems to be warranted on the general principle that advance in protoplasmic life is dependent on new experiences, and that the male cell represents a little world of the concen- trated experiences gathered during the lifetime of the or- ganism that produced it. But we must consider the whole doctrine of gemmules as a crude and entirely unnecessary hypothesis. In what sense has the line that evolution has taken been predetermined ? In the sense that all things in the universe are unstable, are undergoing change, leading to new forms and qualities of such a character that they result in a gradual prog- ress toward what our minds can not but consider higher mani- festations of being. The secondary methods according to which this takes place constitute the laws of nature, and as we learn from the progress of science are very numerous. The unity of nature is a real- ity toward which our conceptions are constantly leading us. Evolution is a necessity of living matter (indeed, all matter) as we view it. THE CHEMICAL CONSTITUTION OF THE ANIMAL BODY. One visiting the ruins of a vast and elaborate building, which had been thoroughly pulled to pieces, would get an amount of information relative to the original structure and uses of the various parts of the edifice largely in proportion to his familiarity with architecture and the various trades which make that art a practical success. The study of the ch(!mistry of the animal body is illustrated by such a case. Any attempt to determine the exact chemical composition of living inatter mu.st result in its destruction; and the amcmnt of information conveyed by the examination of the chemical ruins, so to speak. 136 ANIMAL PHYSIOLOGY. will depend a great deal on tlie knowledge already possessed of ch.emical and vital processes. It is in all probability true tbat the nature of any vital pro- cess is at all events closely bound up with the chemical changes involved ; but we must not go too far in this direction. We are not yet prepared to say that life is only the manifestation of certain chemical and physical processes, meaning thereby such chemistry and physics as are known to us ; nor are we prepared to go the length of those who regard life as but the equivalent of some other force or forces ; as electricity may be considered as the transformed representative of so much heat and vice versa. It may be so, but we do not consider that this view is warranted in the present state of our knowledge. On the other hand, vital phenomena, when our investiga- tions are pushed far enough, always seem to be closely asso- ciated with chemical action ; hence the importance to the stu- dent of physiology of a sound knowledge of chemical princi- ples. We think the most satisfactory method of studying the functions of an organ will be found to be that which takes into consideration the totality of the operations of which it is the seat, together with its structure and chemical composition; hence we shall treat chemical details in the chapters devoted to special physiology, and here give only such an outline as will bring before the view the chemical composition of the body in its main outlines ; and even many of these will gather a signifi- cance, as the study of physiology progresses, that they can not possibly have at the present. Fewer than one third of the chemical elements enter into the composition of the mammalian body ; in fact, the great bulk of the organism is composed of carbon, hydrogen, nitro- gen, and oxygen; sodium, potassium, magnesium, calcium, sulphur, phosphorus, chlorine, iron, fluorine, silicon, though occurring in very small quantity, seem to be indispensable to the living body ; while certain others are evidently only pres- ent as foreign bodies or impurities to be thrown out sooner or later. It need scarcely be said that the elements do not occur as such in the living body, but in combination form- ing salts, which latter are usually united with albuminous compounds. As previously mentioned, the various parts which make up the entire body of an animal are composed of living matter in very different degrees ; hence we find in such parts as the bones abundance of salts, relative to the proportion of proteid matter ; a condition demanded by that rigidity without THE CHEMICAL CONSTITUTION OP THE ANIMAL BODY. I37 whicli an internal skeleton would be useless, a defect well illus- trated by that disease of the bones known as rickets, in which the lime-salts are insufficient. It is manifest that there may be a very great variety of classifications of the compounds found in the animal body according as we regard it from a chemical, physical, or physiological point of view, or combine many aspects in one whole. The latter is, of course, the most correct and profitable method, and as such is impossible at this stage of the student's progress ; we shall simply present him with the following outline, which will be found both simjjle and com- prehensive.* The subject of Animal Chemistry will be found treated in detail in the Appendix. CHEMICAL CONSTITUTION OF THE BODY. Such food as supplies energy directly must contain carbon compounds. Living matter or protoplasm always contains nitrogenous carbon compounds. In consequence, C, H, O, IST, are the elements found in great- est abundance in the body. The elements S and P are associated with the nitrogenous carbon compounds ; they also form metallic sulphates and phos- XJhates. CI and F form salts with the alkaline metals Na, K, and the earthy metals Ca and Mg. Fe is found in luemoglohin and its derivatives. Protoplasm, when submitted to chemical examination, is killed. It is then found to consist of proteids, fats, carbohy- drates, salines, and extractives. It is probable that when living it has a very complex mole> cule consisting of C, H, O, N, S, and P chiefly. PROXIMATE PRINCIPLES. 1 Or anic j ^^^ ^'*^*'^^^°"^- ] Certain crystalline bodies. ° ■ I ,-,. TVT -i. S Carbohydrates. ( (b) Non-mtrogenous. •) xj^ ^ . „ , .1 Mineral salts. 2. Inorganic. ^.^ I Water Salts. — In general, the salts of sodium are more characteris- tic of animal tissues and those of potassium of vegetable tissues. * Takf-ri from Iho iiuthor's "Outlines of Lectures on Physiology," W. Dry.sdalo & Co., Moritr«;ul. 138 ANIMAL PHYSIOLOGY. N"a CI is more abundant in the fluids of animals ; K and phosphates more abundant in the tissues. Earthy salts are most abundant in the harder tissues. The salts are probably not much, if at all, changed in their passage through the body. In some cases there is a change from acid to neutral or alkaline. The salts are essential to preserve the balance of the nutri- tive processes. Their absence leads to disease, e. g., scurvy. GENERAL CHARACTERISTICS OF PROTEIDS. They are the chief constituents of most living tissues, in- cluding blood and lymph. The molecule consists of a great number of atoms (complex constitution), and is formed of the elements C, H, 'N, O, S, and P. All proteids are amorphous. All are non-diffusible, the peptones excepted. They are soluble in strong acids and alkalies, with change of properties or constitution. In general, they are coagulated by alcohol, ether, and heating. Coagulated proteids are soluble only in strong acids and alkalies. Classification and Distinguishing Characters of Proteids. 1. Native albumins : Serum albumin ; Qgg albumin ; soluble in water. 2. Derived albumins {albuminates) : Acid and alkali albu- min ; casein ; soluble in dilute acids and alkalies, insoluble in water. ISTot precipitated by boiling. 3. Olohidins : Globulin (globin) ; paraglobulin ; myosin ; fibrinogen. Soluble in dilute saline solutions, and precipitated by stronger saline solutions. 4. Peptones : Soluble in water ; diffusible through animal membranes ; not precipitated by acids, alkalies, or heat. De- rived from the digestion (peptic, pancreatic) of all proteids. 5. Fibrin: Insoluble in water and dilute saline solutions. Soluble, but not readily, in strong saline solutions and in dilute acids and alkalies. CERTAIN NON-CRYSTALLINE BODIES. The following bodies are allied to proteids, but are not the equivalents of the latter in the food. THE CHEMICAL CONSTITUTION OF THE ANIMAL BODY. I39 They are all composed of C, H, IST, O. Chondrin, gelatin, ceratin have, in addition, S. Cliondrin : The organic basis of cartilage. Its solutions set into a firm jelly on cooling. Gelatin : The organic basis of bone, teeth, tendon, etc. Its solutions set (glue) on cooling. Elastin : The basis of elastic tissue. Its solutions do not set jelly-like (gelatinize). Mucin : From the secretion of mucous membranes ; precipi- tated by acetic acid, and insoluble in excess. Keratin : Derived from hair, nails, epidermis, horn, feathers. Highly insoluble. Xuclein : Derived from the nuclei of cells. Not digested by pepsin ; contains P but no S. THE FATS. The fats are hydrocarbons ; are less oxidized than the carbo- hydrates; are inflammable; possess latent energy in a high degree. Chemically, the neutral fats are glycerides or ethers of the fatty acids, i. e., the acid radicles of the fatty acids of the oleic and acetic series replace the exchangeable atoms of H in the triatomic alcohol glycerine, e. g. : Glycerine. Palmitic acid. Glycerine tripalmitate or palmitin. I OH HO.OC.C.5H3, ( O.CO.CuHa. C3H5 ' OH + HO.OC.CsHs, = C3H5 ] O.CO.CsHa, -I- 3H,0 ) OH HO.OC.C,5H3, ( O.CO.CisHs, A soajy is formed by the action of caustic alkalies on fats, e. g. : Tripalmitin. Potassium palmitate. The soap may be decomposed by a strong acid into a fatty acid and glycerine, e. g. : C,5H3,.CO,K + HCl = CsHai.CO.H + KCl. Potassium palmitate. Palmitic acid. The/a/.v are insoluble in water, but soluble in hot alcohol, ether, chloroform, etc. The alkaline soaps are soluble in water. 140 ANIMAL PHYSIOLOGY. Most animal fats are mixtures of several kinds in varying proportion ; hence the melting-point for the fat of each species of animal is different. PECULIAR FATS. Lecithin, Protagon, Cerebrin : They consist of C, H, N, O, and the first two of P in addi- tion. They occur in the nervous tissues. CAEBOHYDRATES. General formula, Cm (H20)„. 1. The Sugars : Dextrose, or grape-sugar, CeHiaOe + H2O readily undergoes alcoholic fermentation; less readily lactic fermentation. Lactose, milk-sugar, Ci2H220n + H2O ; susceptible of the lactic acid fermentation. Inosit, or muscle-sugar, C6H12O6 + 2H2O; capable of the lac- tic fermentation. Maltose, C12H22O11 + H2O, capable of the alcoholic fermenta- tion. The chief sugar of the digestive process. All the above are much less sweet and soluble than ordinary cane-sugar. 2. The Starches : Glycogen, CeHioOs, convertible into dex- trose. Occurs abundantly in many foetal tissues and in the liver, especially of the adult animal. Dextrin, CeHioOe, convertible into dextrose. Soluble in water ; intermediate between starch and dextrose ; a product of digestion. Pathological: Grape-sugar occurs in the urine in diabetes mellitus. Certain substances formed within the body may be regarded as chiefly waste-products, the result of metabolism or tissue- changes. They are divisible into nitrogenous metabolites and non- nitrogenous metabolites. Nitrogenous Metabolites. 1. Urea, uric acid and compounds, kreatinin, xanthin, hypo- xanthin (sarkin), hippuric acid, all occurring in urine. 2. Leucin, tyrosin, taurocholic, and glycocholic acids, which occur in the digestive tract. PHYSIOLOGICAL RESEARCH, PHYSIOLOGICAL REASONING. 141 3. Kreatin, constantly found in muscle, and a few others of less constant occurrence. The above consists of C, H, N, O. Taurocholic acid contains also S. The molecule in most instances is complex. Non- Nitrogenous Metabolites. These occur in small quantity, and some of them are secreted in an altered form. They include lactic and sarcolactic acid, oxalic acid, succinic acid, etc. PHYSIOLOGICAL RESEARCH AND PHYSIOLOGICAL REASONING. We propose in this chapter to examine into the methods employed in physiological investigation and teaching, and the character of conclusions arrived at by physiologists as depend- ent on a certain method of reasoning. The first step toward a legitimate conclusion in any one of the inductive sciences to which physiology belongs is the col- lection of facts which are to constitute the foundation on which the inference is to be based. If there be any error in these, a correct conclusion can not be drawn by any reliable logical process. On the other hand, facts may abound in thou- sands and yet the correct conclusion never be reached, because the method of interpretation is faulty, which is equivalent to saying that the process of inference is either incomplete or in- correct. The conclusions of the ancients in regard to nature were usually faulty from errors in both these directions; they neither had the requisite facts, nor did they correctly interpret those with which they were conversant. Let us first examine into the methods employed by modern physiologists, and determine in how far they are reliable. First, there is the method of direct observation, in which no appara- tus whatever or only the simplest kind is employed ; thus, the student may count his own respirations, feel his own heart- beats, count his pulse, and do a very great deal more that will be pointed out hereafter ; or he may examine in like manner an- other fellow-being or (me of the lower animals. This method is simple, easy of application, and is that usually employed by the physician even at the present day, especially in private 1^2 ANIMAL PHYSIOLOGY. practice. The value of the results obviously depends on the reliability of the observer in two respects : First, as to the ac- curacy, extent, and delicacy of his perceptions ; and, secondly, on the inferences based on these sense-observations. Much must depend on practice— that is to say, the education of the senses. The hand may become a most delicate instrument of observation ; the eye may learn to see what it once could not ; the ear to detect and discriminate what is quite beyond the uncultured hearing of the many. But it is one of the most convincing evidences of man's superiority that in every field of observation he has risen above the lower animals, some of which by their unaided senses naturally excel him. So in this science, instruments have opened up mines of facts that must have otherwise remained hidden; they have, as it were, provided man with additional senses, so much have the natural powers of those he already possessed been sharp- ened. But the chief value of the results reached by instruments consists in the fact that the movements of the living body can be registered ; i. e., the great characteristic of modern physiol- ogy is the extensive employment of the graphic method, which has been most largely developed by the distinguished French experimenter Marey. Usually the movements of the point of lever are impressed on a smoked surface, either of glazed paper or glass, and rendered permanent by a coating of some material applied in solution and drying quickly, as shellac in alcohol. The surface on which the tracing is written may be stationary, though this is rarely the case, as the object is to get a succession of records for comparison ; hence the most used form of writing surface is a cylinder which may be raised or lowered, and which is moved around regularly by some sort of clock-work. It follows that the lever-point, which is moved by the physiological effect, describes curves of varying complexity. That tracings of this or any other character should be of any value for the purposes of physiology, they must be susceptible of relative measurement both for time and space. This can be accomplished only when there is a known base-line or abscissa from which the lever begins its rise, and a time record which is usually in seconds or portions of a second. The first is easily obtained by simply allowing the lever to write a straight line before the physiological effect proper is recorded. Time inter- vals are usually indicated by the interruptions of an electric current, or by the vibrations of a tuning-fork, a pen or writer PHYSIOLOGICAL RESEARCH, PHYSIOLOGICAL REASONING. I43 of some kind being iu each instance attached to the apparatus so as to record its movements. As levers, in proportion to their length, exaggerate all the movements imparted to them, a constant process of correction must be carried on in the mind in reading the records of the graphic method, as in interpreting the field of view presented by the microscope. The student is especially warned to carry on this process, otherwise highly distorted views of the reality will become fixed in his own mind ; and certainly a condition of ignorance is to be preferred to such false knowledge as this may become. But it is likewise apparent that movements that would without such mechanism be quite unrecognized may be rendered visible and utilized for inference. There is another source of possible misconception in the use of the graphic method. The lever is sometimes used to record the movements of a column of fluid (manometer. Fig. 207), as water or mercury, the inertia of which is considerable, so that the record is not that of the lever as affected by the physiological (tissue) movement, but that move- ment conveyed through a fluid of the kind indicated. Again, all points, however delicate, write with some friction, and the question always arises. In how far is that friction sufiicient to be a source of inaccuracy in the record ? When organs are di- rectly connected with levers or apparatus in mechanical rela- tion with them, one must be sure that the natural action of the organ under investigation is in no way modified by this con- nection. From these remarks it will be obvious that in the graphic method physiologists possess a means of investigation at once valuable and liable to mislead. Already electricity has been extensively used in the researches of physiologists, and it is to this and the employment of photography that we look in the near future for methods that are less open to the objections we have noticed. However important the methods of physiology, the results are vastly more so. We next notice, then, the progress from methods and observations to inferences, which we shall en- deavor to make clear by certain cases of a hypothetical charac- ter. Proceeding from the brain and entering the substance of the heart, there is in vertebrates a nerve known as the vagus. Suppose tliat, on stimulating this nerve by eh-ctricity in a rab- bit, tlie hf;art ceases to beat, what is tlie legitimate inference ? Apparently that the effect has been due to the action of tlio 14:4 ANIMAL PHYSIOLOGY. nerve on the heart, an action excited by tlie use of electricity. This does not, however, according to the principles of a rigid logic, follow. The heart may have ceased beating from some cause wholly unconnected with this experiment, or from the electric current escaping along the nerve and affecting some nervous mechanism within the heart, which is not a part of the vagus nerve ; or it may have been due to the action of the cur- rent on the muscular tissue of the heart directly, or in some other way. But suppose that invariably, whenever this experiment is repeated, the one result (arrest of the beat) follows, then it is clear that the vagus nerve is in some way a factor in the causa- tion. Now, if it could be ascertained that certain branches of the nerve were distributed to the heart-muscle directly, and that stimulation of these gave rise to arrest of the cardiac pulsation, then would it be highly probable, though not certain, that there was in the first instance no intermediate mechanism; while this inference would become still more probable if in hearts totally without any such nervous apparatus whatever, such a result followed on stimulation of the vagus. Suppose, further, that the application of some drug or poison to the heart pro- vided with special nervous elements besides the vagus termi- nals prevented the effect before noticed on stimulating the vagus, while a like result followed under similar circumstances in those forms of heart unprovided with such nervous struct- ures, there would be additional evidence in favor of the view that the result we are considering was due solely to some action of the vagus nerve ; while, if arrest of the heart followed in the first case but not in the second, and this result were invariable, there would be roused the suspicion that the action of the vagus was not direct, but through the nervous structures with- in the heart other than vagus endings. And if, again, there were a portion of the rabbit's heart to which there were distributed this intrinsic nervous supply, which on stimulation directly was arrested in its pulsation, it would be still more probable that the effect in the first instance we have considered was due to these structures, and only indirectly to the vagus. But be it observed, in all these cases there is only probability. The con- clusions of physiology never rise above probability, though this may be so strong as to be practically equal in value to absolute certainty. Would it be correct, from any or all the experi- ments we have supposed to have been made, to assert that the vagus was the arresting (inhibitory) nerve of the heart ? All hearts thus far examined have much in common in structure PHYSIOLOGICAL RESEAECH, PHYSIOLOGICAL REASONING. I45 and function, and in so far is the above generalization probable. Such a statement would, however, be far from that degree of probability which is possible, and should therefore not be ac- cepted till more evidence has been gathered. The mere resem- blance in form and general function does not suffice to meet the demands of a critical logic. Such a statement as the above would not necessarily apply to the hearts of all vertebrates or even all rabbits, if the experiments had been conducted on one animal alone, for the result might be owing to a mere idiosyncrasy of the rabbit under observation. The further we depart from the grouj) of animals to which the creature under experiment be- longs, the less is the probability that our generalizations for the one class will apply to another. It will, therefore, be seen that wide generalizations can not be made with that amount of certainty which is attainable until experiments shall have be- come very numerous and widely extended. A really broad and sound physiology can only be constructed when this science has become much more comparative — that is, extended to many more groups and sub-groups of animals than at present. To attempt to generalize for the heart, the kidney, the liver, etc., when only the dog, cat, rabbit, and frog, have been made as a rule the subjects of experiment, except for the groups of animals to which the above belong, is not only hazardous but positively illogical ; while to denominate conclusions based on such experiments, even when supplemented by the teachings of disease, " human physiology " is, in the writer's opinion, a wholly unwarrantable proceeding. It is this conviction which has had much to do with this book being written; to the introduction of the comparative element ; and the separation so frequently in form as well as in reality of facts and inferences. A genuine human physi- ology, with the exact nature and value of the inferences clearly stated, is yet to be written ; and it seems not only judicious, but demanded as a matter of candor and honesty, to state at the outset to the student what we feel able to teach confidently, and what must be jjresented as feebly probable or barely pos- sible Human physiology projxjr must of necessity be accumulated slowly. Much may be, indeed must be, inferred from the ex- l^eriments disease is making; still, certain forms of accident or surgical operation provide the opportunity to investigate the liuinan body in health or in a moderately near approach to that condition. Close self-observation under a variety of condi- 10 146 ANIMAL PHYSIOLOGY. tions, so precisely defined as to meet the demands of science, may be made by the intelligent student. Much of this might be verified in the case of other healthy persons. Some of it is in certain respects of more value than any experiments that can be made upon the lower animals, for the latter can not communicate to us their sensations; in their case all our in- formation must be derived from the use of our own senses, mostly unaided by any reports of theirs. It is not possible during any experiment, especially any one in which vivisection is employed, to observe the animal under conditions that are strictly normal, for, by the very nature of the case, we have rendered it abnormal. We must in all such instances draw conclusions with corresponding caution. It will be understood that the expression "conclusive experi- ment," as applied to such a case, is only approximately correct. At the present time it is very common to experiment upon organs disconnected, either anatomically or physiologically (functionally), from the rest of the body to a greater or less extent. This is termed the isolated method. It has the advan- tage of being more simple, and permits of the study of certain points apart from others — one factor being considered inde- pendently of the rest in the physiological total. But, in draw- ing conclusions, it is very important in such a case not to forget the premises. There is manifest danger of making the gener- alization wider than the facts warrant. It is only when such experiments are supplemented by a great many others, and when judged in connection with the action of the organ under consideration, as it is infl.uenced by other organs, that such re- sults can be of great value in building up a normal physiology. To know, for example, that the isolated heart behaves in a cer- tain manner is not useless information, but its value depends entirely on the conclusions drawn from it, especially as to what it is conceived as teaching of the functions of the heart as it beats within the body of. an animal while it walks, or flies, or swims, in carrying out the purpose of its being. We have incidentally alluded to the teaching of disease. " Disease " is but a name for disordered function. One viewing a piece of machinery for the first time in improper action might draw conclusions with comparative safety, provided he had a knowledge of the correct action of similar machines. Our ex- perience gives us a certain knowledge of the functions of our own bodies. By ordinary observation and by experiment on other animals we get additional data, which, taken with the THE BLOOD. 147 disordered action resulting from gross or molecular injury (disease), gives a basis for certain conclusions as to the normal functions of the human body or those of lower animals. This information is especially valuable in the case of man, since he can report with a fair degree of reliability, in most diseased conditions, his own sensations. It is hoped that this brief treatment of the methods and logic of physiology will suffice for the present. Throughout the work they will be illustrated in every chapter, though not always with distinct references to the nature of the intellectual process followed. Summary. — There are two methods of physiological observa- tion, the direct and the indirect. The first is the simplest, and is valuable in proportion to the accuracy and delicacy and range of the observer ; the latter implies the use of apparatus, and is more complex, more extended, more delicate, and precise. It is usually employed with the graphic method, which has the advantage of recording and thus preserving movements which correspond with more or less exactness to the movements of tissues or organs. It is valuable, but liable to errors in record- ing and in interpretation. The logic of physiology is that of the inductive sciences. It proceeds from the special to the general. The conclusions of physiology never pass beyond extreme probability, which, in some cases, is practically equal to certainty. It is especially important not to make generalizations that are too wide. THE BLOOD. It is a matter of common observation that the loss of the whole, or a very large part, of the blood of the body entails death ; while an abundant haemorrhage, or blood-disease in any of its forms, causes great general weakness. The student of embryology is led to inquire as to the neces- sity for the very early appearance and the rapid development of the blood-vascular system so prominent in all vertebrates. An examination of the means of transit of the blood, as already intimated, reveals a complicated system of tubes dis- tributed to every organ and tissue of the body. These facts would lead one to suppose that the blood must have a ti-an- -cendent importance in the economy, and such, upon tlie most minute investigation, proves to be the case. The blood has 148 ANIMAL PHYSIOLOGY. been aptly compared to an internal world for the tissues, an- swering to the external world for the organism as a whole. This fluid is the great storehouse containing all that the most exacting cell can demand ; and, further, is the temporary receptacle of all the waste that the most busy cell requires to discharge. Should such a life-stream cease to flow, the whole vital machinery must stop — death must ensue. Comparative.— It-will prove more scientific and generally sat- isfactory to regard the blood as a tissue having a fluid and flowing matrix, in which float cellular elements or corpuscles — a view of the subject that is less startling when it is remem- bered that the greater part of the protoplasm which makes up the other tissues of the body is of a semifluid consistence. In all animals possessing blood, the matrix is a clear, usually more or less colored fluid. Among invertebrates the color may be pronounced : thus, in cephalopods and some crustaceans it is blue, but in most groups of animals and all vertebrates the matrix is either colorless or more commonly of some slight tinge of yellow. Invertebrates with few exceptions possess only colorless corpuscles, but all vertebrates have colored cells which invariably outnumber the other variety, and display forms and sizes which are sufficiently constant to be characteristic. In all groups below mam- mals the colored corpus- cles are oval, mostly bi- convex, and nucleated during all periods of the animal's existence ; in mammals they are cir- cular biconcave disks (except in the camel tribe, the corpuscles of which are oval), and in post-embryonic life with- out a nucleus ; nor do they possess a cell-wall. The red cells vary in size in different groups and sub-groups of animals, being smaller the higher the place the animal occupies, as a general rule : thus, they are very large in vertebrates below mammals, in some cases being almost Fig. 143.— Leucocytes of human blood, showing amce- boid movements (Landois). These movements are not normalli^ in the blood-vessels so marked as pic- tured here, so that the figure represents an ex- treme case. THE BLOOD. 149 Fig. 144.— Photograph of colored corpuscles of frog. 1 X 370. (After Flint.) visible to tlie unaided eye, while in the whole class of mam- mals they are very minute ; their numbers also in this group are vastly greater than in others lower in the scale. The average size in man is s^o inch ("0077 mm.) and the number in a cubic mil- limetre of the blood about 5,000,000 for the male and 500,000 less for the female, which would furnish about 250,000,000,000 in a pound of blood. It will be under- stood that averages only are spoken of, as all kinds of variations occur, some of which will be referred to later, and their significance explained. Under the microscope the blood of vertebrates is seen to owe its color to the cells chiefly, and, so far as the red goes, almost wholly. Corpuscles when seen singly are never of the deep red, however, of the blood as a whole, but rather a yellowish red, the tinge varying some- what with the class of ani- mals from which the spec- imen has been taken. Certain other morpho- logical elements found in mammalian blood deserve l)rief mention, though their significance is as yet a mat- ter of much dispute : 1. The blood - plates Wflorwi ciiskK, which are many of them arranged ( plaqUBS, hrpuBcle8 from human subject (Funke). A few colorles.s cftrpuscles are seen among the 150 ANIMAL PHYSIOLOGY. time, aggregations of very minute granules {elementary gran- ules) may be seen. These are supposed to represent the disin- tegrating protoplasm of the corpuscles. s •0 /> Fig. 146. — Blood-plaques and their derivatives (Landois, after Bizzozero and Laker). 1, red blood-corpuscles on the flat ; 2, from the side ; 3, unchanged blood- plaques ; 4, lymph- corpuscle surrounded with blood-plaques ; 5, blood-plaques variously altered ; 6, lymph- corpuscle with two masses of fused blood-plaques and threads of fibrin ; 7, group of blood-plaques fused or run together ; 8, similar small mass of partially dissolved blood- plaques with fibrils of fibrin. The pale or colorless corpuscles are very few in number in m.ammals compared with the red, there being on the average only about 1 in 400 to 600, though they become much more numerous after a meal. They are granular in appearance, and possess one or more nuclei, which are not, however, readily seen in all cases without the use of reagents. They are character- ized by greater size, a globular form, the lack of pigment, and the tendency to amoeboid movements, which latter may be ex- aggerated in disordered conditions of the blood, or when the blood is withdrawn and observed under artificial conditions. It will be understood that these cells (leucocytes) are not con- fined to the blood, but abound in lymph and other fluids. They are the representatives of the primitive cells of the em- bryo, as is shown by their tendency (like ova) to throw out processes, develop into higher forms, etc. In behavior they strongly suggest Amceba and kindred forms. We may, then, say that in all invertebrates the blood, when it exists, consists of a plasma (licjuor sanguinis), in which float the cellular elements which are colorless; and that in verte- brates in addition there are colored cells which are always nu- cleated at some period of their existence. The colorless cells THE BLOOD. 151 are globular masses of protoplasm, containing one or more nuclei, and with the general character of amoeboid organisms. The History of the Blood-Cells. We have already seen that the blood and the vessels in which it flows have a common origin in the mesoblastic cells of the embryo chick ; the same applies to mammals and lower groups. The main facts may be grouped under two head- ings: 1. Development of the blood-corpuscles during embry- onic life. 2. Development of the corpuscles in ]30st-embryonic life. In the bird and the mammal, cells of the mesoblast in the area opaca give off processes which unite; later they become hollowed out (vacuolated), and thus form capillaries. At the same time the nuclei of these cells multiply ( pr-o- l if e rate), gather small por- tions of the protoplasm of the main cells about them, become colored, and thus form the nucleated corpus- cles of the embryo. This, or a similar process, is known to occur in some animals (rat) after birth ; but in the human f cetus there is a grad- ual decline in the number of nucleated cells found free in the blood, and at birth they Fjg 147.-Surface view from below of a small por- ' •' tion of posterior end of pellucid^area of a cmck are very rare, which is prob- ably the case with most mammals. While the origin of the red cells, as above descfiljed, may be regarded as the earliest and most general, it is not their exclusive source. When the liver has been formed this organ seems to carry on a development begun in tlie spleen, for the nucleated but as yet colorless cells formed in the spleen seem to become pig- mented in the liver. There is also evidence that colored corjjuscles may arise by endogenous formation in the lymphatic glands. of thirty-six hours, 1 x 4()0 (Foster and Bal- four), h. c. lilood-corpuscles ; a, nuclei, which subsequently become nuclei of cells forming walls of Ijlood-vessels ; p. \)r. protoplasmic processes, containing nuclei with large nu- cleoli, >i. 152 ANIMAL PHYSIOLOGY. There is no doubt that the greater number of the non-nucle- ated corpuscles are derived from the nucleated forms. The post-embryonic development of colored corpuscles is naturally less understood from the greater difl&culties attend- ® ® m ® Fig. 148. Fig. 149. a 3 1 © ^ f©c^ Fig. 151. Fig. 152. Fig. 148.— Cell elements of red marrow, n, large granular marrow cells ; &, smaller, more vesicular cells ; c, free nuclei, or small lymphoid cells, some of which may be even sur- rounded with a delicate rim of protoplasm ; d, nucleated red corpuscles of the bone marrow. Fig. 149. — Nucleated red cells of marrow, iOustrating mode of development into the ordinary non-nucleated red corpuscles, a, common forms of the colored nucleated cells of red mar- row ; 6, 1, 2, 3, gradual disappearance of the nucleus ; c, large non-nucleated red corpuscle resembling 2 and 3 of b, in all respects save in the absence of any trace of nucleus. Fig. 150.— Nucleated red corpuscles, illustrating the migration of the nucleus from the cell, a process not unfrequently seen in the red marrow. Fig. 151. — Blood of embryo of four months, a, 1, 2, 3, 4, nucleated red corpuscles. In 4 the same granular disintegrated appearance of the nucleus as is noted in marrow cells, b, 1, microcyte ; 2, megalocyte ; 3, ordinary red corpuscle. Fig. 152.— From spleen. 1, blood-plaques, colorless and varying a little in size ; 2, two micro- cytes of a deep-red color ; 3, two ordinary red corpuscles ; 4, a solid, translucent, lymphoid cell or free nucleus. (Figs. 148-152, after Osier.) ing its investigation. The following may be regarded as a summary of the chief facts or rather opinions on this subject : 1. From the colorless cells ; though, whether the nucleus disappears, or remains to form the chief part of the cell and become pigmented, is undetermined. 2. From peculiar cells of the red marrow of the bones (head, trunk, etc.), though there is also some doubt as to whether the THE BLOOD. I53 nuclei of these cells remain or not ; but as all grades of transi- tion forms have been found in the bone-marrow ; since anaemia occurs in disease of bones; since the bone-marrow has been found in an unusually active condition after haemorrhage and imder other circumstances demanding a rapid replacement of lost cells — there seems to be little room for doubt that in the adult the red marrow of the bones is the chief site of the devel- opment of red corpuscles. It is not, however, the only one, for under peculiar stress of need even the lymphatic glands pro- duce red cells, and the latter have been seen to be budded off from the spleen in a young animal (kid). The colorless cells of the blood first arise as migrated undif- ferentiated remnants of the early embryonic cell colonies. That they remain such is seen by their physiological behavior, to be considered a little later. Afterward they are chiefly produced from a peculiar form of connective tissue known as leucocy- tenic, and which is gathered into organs, the chief function of which (lymphatic glands) is to produce these cells, though this tissue is rather widely distributed in the mammalian body in other forms than these. . Smninary. — The student may, with considerable certainty, consider the colorless corpuscle of the blood as the most primi- tive ; the red, derived either from the white or some form of more specialized cell ; the nucleated, as the earlier and more youthful form of the colored corpuscle, which may in some groups of vertebrates be replaced by a more specialized (or de- graded ?) non-nucleated form mostly derived directly from the former ; that in the first instance the blood-vessels and blood arise simultaneously in the mesoblastic embryonic tissue ; that such an origin may exist after birth, either normally in some mammals or under unusual functional need ; that the red marrow is the chief birthplace of colored cells in adult life ; that the spleen, liver, lymphatic glands, and other tissues of similar structure contribute in a less degree to the develop- ment of tlie red corpuscles ; and that the last mentioned organs are the chief producers of the colorless amoeboid blood-cells. Finally, it is well to remember that Nature's resources in this, as in many other cases, are numerous, and that her mode of procedure is not invariable ; and tliat, if one road to an end is blocked, another is taken. The Decline and Death of the Blood-Cells. — Th(! blood -corpuscles, like oUu-v cf^lls, li;ivc a limited durat ion, with the usual chai)ters in a biological history oi rise, maturity, and decay. There is 154 ANIMAL PHYSIOLOGY. reason to believe tliat the red cells do not live longer than a few weeks at most. The red cells, in various degrees of disor- ganization, have been seen within the white cells {pliagocytes) , and the related cells of the spleen, liver, bone-marrow, etc. In fact, these cells, by virtue of retained ancestral {amoeboid) quali- ties, have devonred the weakened, dying red cells. It seems to be a case of survival of the fittest. It is further known that abundance of pigment containing iron is found in both spleen and liver ; and there seems to be no good reason for doubting that the various pigments of the secretions of the body {urine, hile, etc.) are derived from the universal pigment of the blood. These coloring matters, then, are to be regarded as the excreta in the first instance of cells behaving like amoeboids, and later as the elaborations of certain others in the kidney and else- v^^here, the special function of which is to get rid of waste products. The birth-rate and the death-rate of the blood- cells must be in close relation to each other in health ; and some of the gravest disturbances arise from decided changes in the normal projiortions of the cells {anoe.mia, leucocythe- mia). Both the red and white corpuscles show, like all other cells of the organism, alterations corresponding to changes in the surrounding conditions. The blood may be withdrawn and its cells more readily observed than those of laost tissues ; so that the study of the influence of temperature, feeding of the leuco- cytes, and the action of reagents in both classes of cells is both of practical importance and theoretic interest, and will well re- pay the student for the outlay in time and labor, if attention is directed chiefly to the results and the lessons they convey, and not, as too commonly happens, principally to the methods of manipulation. The Chemical Composition of the Blood, — Blood has a decided but faint alkaline reaction, owing chiefly to the presence of sodium biphosphate (N"a2HP04), a saline taste, and a faint odor characteristic of the animal group to which it belongs, owing probably to volatile fatty acids. The specific gravity of blood varies between 1045 and 1075, with a mean of 1055 ; the spe- cific gravity of the corpuscles being about 1105 and of the plasma 1027. This difference explains the sinking of the cor- puscles in blood withdrawn from the vessels and kept quiet. Much the same difiiculties are encountered in attempts at the exact determination of the chemical composition of the blood, as in the case of other living tissues. Plasma alters its phys- THE BLOOD. I55 ical and its chemical composition, to what extent is not exactly known, when removed from the body. Composition of Serum. — The liuid remaining after coagulation of the blood can, of course, be examined chemically with con- siderable thoroughness and confidence. By far the greater part of serum consists of water; thus, it has been estimated that of 100 parts the following statement will represent fairly well the proportional comj)osition : Water 90 parts ; Proteids 8 to 9 " Salines, fats, and extractives (small in quantity and not readily obtained free) 1 to 2 parts. The proteids are made up of two substances which can be distinguished by solubility, temperature at which coagulation occurs, etc., known as paraglohulin and serum-albumen, and which may exist in equal amount. It is not possible, of course, to say whether these substances exist as such in the living blood-plasma or not. The fats are very variable in quantity in serum, depend- ing on a corresponding variability in the plasma, in which they would be naturally found in greatest abundance after a meal. They exist as neutral stearin, palmitin, olein, and as soaps. The principal extractives found are urea, creatin, and allied bodies, sugar, and lactic acid. Serum in most animals contains more of sodium salts than the corpuscles, while the latter in man and some other mammals contain a preponderating quan- tity of potassium compounds. The princi^jal salts of serum are sodium chloride, sodium bi- carbonate, sodium sulphate, and phosphate in smaller quantity, as also of calcium and magnesium phosphate, with rather more of potassium chloride. It is highly probable that this proportion also represents moderately well the composition of plasma, which is, of course, from a ])]iysiological point f)f view, the important matter. The Composition of the Corpuscles. — Taken together, the differ- ent forms of blood-cells make up from one tliird to nearly one half the weight of the blood, and of this the red corpuscles may be considered as constituting nearly the whole. The coloi'less cells are known to contain fats and glycogen, which, with salts, we may boli(;ve exist in the living cells, and, in addition t«> the proteids, into which protoplasm resolves it- 156 ANIMAL PHYSIOLOGY. self upon the disorganization that constitutes its dying, lecithin, protagon, and other extractives^ The prominent chemical fact connected with the red corpus- cles is their being composed in great part of a peculiar colored proteid compound containing iron. This will be fully considered later ; but, in the mean time, we may state that the haemoglobin is itself infiltrated into the meshes or framework (stroma) of the corpuscle, which latter seems to be composed of a member of the globulin class, so well characterized by solubility in weak saline solutions. The following tabular statement represents the relative pro- portions in 100 parts of the dried organic matter of the red cor- puscles : Hsemoglobin 90*54 Proteids 8-67 Lecithin 0'54 Cholesterin 0*25 100-00 The quantity of salts is very small, less than one per cent (inorganic). So much for the results of our analyses ; but when we con- sider the part the blood plays in the economy of the body, it must appear that, since the life-work of every cell expresses it- self through this fluid, both as to what it removes and what it adds, the blood can not for any two successive moments be of precisely the same composition ; yet the departures from a nor- mal standard must be kept within very narrow limits, other- wise derangement or possibly death results. We think that, before we have concluded the study of the various organs of the body, it will appear to the student, as it does to the writer, that it is highly probable that there are great numbers of com- pounds in the blood, either of a character unknown as yet to our chemistry, or in such small quantity that they elude detec- tion by our methods ; and we may add that we believe the same holds for all the fluids of the body. The complexity of vital processes is great beyond our comprehension. It must be especially borne in mind that all the pabulum for every cell, however varied its needs, can be derived from the blood alone ; or, as we shall show presently, strictly speak- ing from the lymph, a sort of middle-man between the blood and the tissues. The Quantity and the Distribution of the Blood. — Any attempt THE BLOOD. 15'j' to estimate the total quantity of blood in the body of an animal by bleeding is highly fallacious for various reasons. It is im- possible to withdraw all the blood from the vessels by merely opening even the largest of them, and, if it were, the original quantity would be augmented by fluid absorbed into them dur- ing the very act. No method has as yet been devised that is free from objection, hence the conclusions arrived at as to the total quantity of blood are not in accord ; and in the nature of the case no accurate estimate can be made, but about one thir- teenth to one fourteenth may be taken as a fair average ; so that in a man of one hundred and forty pounds weight there should be about ten pounds of blood ; but, of course, this will vary with every hour of the day and will be greatest after a meal. As an example of the methods referred to, we give Welck- er's, which is briefly as follows : The animal is bled to death from the carotid; a sample of the defibrinated blood (1 cc.) is saturated with carbon monoxide (CO), which gives a perma- nent red color ; this diluted with 500 cc. of water furnishes a standard sample. The blood-vessels of the animal are washed out with a "G per cent solution of common salt, but the out- flowing stream is colorless ; to this is added the fluid obtained by chopping up the tissues of the animal, steeping, washing out, and pressing. The whole is diluted to give the color of the standard solution, from which the amount of blood in this mixt- ure may be calculated, since every 500 cc. answers to 1 cc. of blood ; the blood obtained by bleeding can, of course, be accu- rately measured. It would be slightly more accurate to make the diluted blood of the animal operated upon the standard without treat- ment with carbon monoxide. Such a method, though the best yet devised, is open to ob- jection also, as will occur to most readers. The relative quantities of blood in different parts of the body have been estimated to be as follows : Liver one fourth. Skeletal muscles " " Heart, lungs, large arteries, and veins. " " Other structures " " The significance of this distribution will appear later. The Coagulation of the Blood. — When blood is removed from it.s accu.storiied channels, it undergoes a marked chemical and ]>hysical change, termed clotting or coagulation. In the case of most vertebrates, almost as soon as the blood leaves the ves- 158 ANIMAL PHYSIOLOGY. sels it begins to thicken, and gradually acquires a consistence that may be compared to that of jelly, so that it can no longer be poured from the containing vessel. Though some have rec- ognized different stages as distinct, and named them, we think that an unprejudiced observer might fail to see that there were any well-marked appearances occurring invariably at the same moment, or with resting stages in the process, as with the development of ova. After coagulation has reduced the blood to a condition in which it is no longer diffluent, minute drops of a thin fluid gradually show themselves, exuding from the main mass, faintly colored, but never red, if the vessel in which the clot has formed has been kept quiet so that the red corpuscles have not been disturbed ; and later it may be noticed that the main mass is beginning to sink in the center {cupping) ; and in the blood of certain animals, as the horse, which clots slowly, the upper part of the coagulum {crassainentum) appears of a lighter color, owing, as microscopic examination shows, to the relative fewness of red corpuscles. This is the buff y-coat, or, as it occurs in inflammatory conditions of the blood, was termed by older writers, the crusta phlogistica. It is to be distinguished from the lighter red of certain parts of a clot, often the result of greater exposure to the air and more complete oxidation in consequence. The white blood-cells, being lighter than the red, are also more abundant in the upper part of the clot (buffy- coaf). If the coagulation of a drop of blood withdrawn from one's own finger be watched under the microscope, the red cor- puscles may be seen to run into heaps, like rows of coins lying against each other {rouleaux, Fig. 145), and threads of the greatest fineness are observed to radiate throughout the mass, gradually increasing in number, and, at last, including the whole in a meshwork which slowly contracts. It is the forma- tion of this fibrin which is the essential factor in clotting ; the inclusion of the blood-cells and the extrusion of the serum naturally resulting from its formation and contraction. The great mass of every clot consists, however, of corpus- cles ; the quantity of fibrin, though variable, not amounting to more usually than about '2 per cent in mammals. The forma- tion of the clot does not occupy more than a few minutes (two to seven) in most mammals, including man, but its contraction lasts a very considerable time, so that serum may continue to exude from the clot for hours. It is thus seen that, instead of the plasma and corpuscles of the blood as it exists within the THE BLOOD. 159 living body, coagulation has resulted in the formation of two new products — serum and fibrin — differing both physically and chemically. These facts may be put in tabular form thus : Blood as it flows j Liquor sanguinis (plasma), in the vessels. { Corpuscles. Blood after co- j Coagulum | corpuscles. agulation. j ^^ '^ { Serum. As fibrin may be seen to arise in the form of threads, under the microscope, in coagulating blood, and since no trace of it in any form has been detected in the plasma, and the process can be accounted for otherwise, it seems unjustifiable to assume that fibrin exists preformed in the blood, or arises in any way prior to actual coagulation. Fibrin belongs to the class of bodies known as proteids, and can be distinguished from the other subdivisions of this group of substances by certain chemical as well as physical charac- teristics. It is insoluble in water and in solutions of sodium chloride ; insoluble in hydrochloric acid, though it swells in this menstruum. It may be whipped out from the freshly shed blood by a bundle of twigs, wires, or other similar arrangement present- ing a considerable extent of surface ; and when washed free from red blood-cells presents itself as a white, stringy, tough substance, admirably adapted to retain anything entangled in its meshes. If fibrin does not exist in the plasma, or does not arise directly as such in the clot, it must have some antecedents already existing as its immediate factors in the plasma, either before or after it is shed. We .shall here present certain facts, and examine the conclu- sions drawn from them afterward : 1. Blood may be prevented from coagulating by receiving it in a solution of a neutral salt {magnesium sidphafe, etc.), and upon certain chemical treatment precipitate a body which may be obtained by additional manipulation as a white, flaky sub- stance, that may be shown not to be fibrin, but which will 'lot and so give rise to this body. Such is the plasmine of Denis. 2. By treatment of plasma with solid sodium chloride, two bodies with different coagulating points, but belonging to the same gronp of proteids ((jlohulins, soluble in saline solutions), may be obtained, denominated panujlubulin and fibriiiuycn re- -pectively. 160 ANIMAL PHYSIOLOGY. 3. Paraglobulin may be obtained from serum, also, and fibrin- ogen from certain fluids occurring normally (jje?'/car(f[a7, jj?f?/- ral, etc.) or abnormally {hydrocele fluid). ■4. Serum added to these fluids sometimes induces coagula- tion. 5. Coagulation may occur spontaneously in the above-men- tioned fluids when removed from the natural seat of their for- mation. 6. A preparation, made by extracting serum or the -whipped (defibrinated) blood added to specimens of certain fluids when they do not coagulate spontaneously, as hydrocele fluid, often induces speedy clotting. 7. This extract {fibrin-fennenf) loses its properties on boil- ing, and a very small quantity sufiices in most cases to induce the result. For these and other reasons this agent has been classed among bodies known as uiiorganized ferments, y^'hicli are distinguished by the following properties : Thej^ exert their influence only under well-defined circum- stances, among which is a certain narrow range of tempera- ture, about blood-heat, being most favorable for their action. They do not seem to enter themselves into the resulting prod- uct, but act from without as it were (catalytic action), hence a very small quantity sufiices to effect the result. In all cases they are destroyed by boiling, though, they bear exposure for a limited period to a freezing temperature. The conclusions drawn from the above statements are these : 1. Coagulation results from the action of a fibrin -ferment on fibrinogen and paraglobulin. 2. Coagulation results from the action of a fibrin-ferment on fibrinogen alone. 3. Denis plasmine is made up of fibrinogen and paraglobulin. From observations, microscopic and other, it has been con- cluded that the corpuscles play an important part in coagula- tion by furnishing the fibrin-ferment ; but tbe greatest diver- sity of opinion prevails as to which one of the morphological elements of the blood furnishes the ferment, for each one of them has been advocated as the exclusive source of this fer- ment by different observers. The above conclusions do not seem to us to follow neces- sarily from the premises. It might be true that a solution of fibrinogen, on ha^•ing fibrin-ferment added to it, would clot, and yet it would not follow that such was the process of coagula- tion in the blood itself. All specimens of hj^drocele fluid, and similar ones not spontaneously coagulable, do not clot when THE BLOOD, 161 fibrin-ferment is added. Moreover, fibrin-ferment has not been isolated as an absolutely distinct chemical individual, free from all impurities. Because fibrinogen and paraglobulin give rise, under certain circumstances (it is asserted), to fibrin, and since plasmine acts likewise, it does not follow that plasmine contains these bodies. Further, it is stated that in the blood of crustaceans the clot arises from the corpuscles chiefly, which run together and blend into a homogeneous mass. The fibrin so called in such a case differs not a little chemically, it could probably be shown, if our tests were delicate enough to discover it, from that which is denominated fibrin in other cases. " Fibrin-ferment " seems to have been used to cover much ignorance and unnecessary invention, as we shall endeavor to show later on ; and we can not but regard the reasoning in regard to the coagulation of the blood as evidence of an erroneous interpretation of certain facts on the one hand, and a large oversight of additional facts on the other hand. In the mean time we turn to certain well-known phenomena which bear a clear interpretation : 1. The blood remains fluid in the vessels for some time after the death of an animal ; clots first in the larger vessels, and keeps fluid longest in the smaller veins. 2. The blood in the heart of a cold-blooded animal, as that of the frog or turtle, which will beat for days after the animal itself is dead, maintains its fluidity, but clots at once on removal. 3. The blood inclosed in a large vein removed be- tween ligatures does not coagulate for many hours (twenty- four to forty-eight). There are also facts of an opposite nature, thus : 1. When blood passes from a blood-vessel into one of the cavities of the body, it clots as if shed externally. 2. If a ligature be passed tightly around an artery so as to rupture the elastic coat, co- agulation ensues at the site of the ligature. 3. A similar clotting results when the inner coat of a blood-vessel is dis- eased, as in the case of roughening of the valves of the heart from inflammation, or the changes that give rise to aneurism of an artery. 4. A wire, thread, or other like foreign body, introduced into a vein, is speedily covered with fibrin. These facts, and others of like character, have been inter- preted as indicating that the living tissues of the blood-vessel or heart in some way ];revent coagulation, but as to details there is difference of opinion. Some believe that the fi])rin-ferment (essential to coagulation, according to their view) is formed by 11 162 ANIMAL PHYSIOLOGY. the corpuscles constantly, but in tlie above cases and during life is not effective because at once removed by the vessel walls ; while others are of opinion that the living cells composing these walls prevent the formation of the ferment. Even when injected into the blood-vessels, fibrin-ferment does not induce Coagulation, nor does the constant death of the blood-cells, supposed thus to give rise to this substance, cause clotting. But the truth is, there is no necessity for all these somewhat artificial views, which seem to us to smack more of the labora- tory than of nature. We would explain the whole matter somewhat thus : What the blood is in chemical composition and other properties from moment to moment is the result of the complicated interaction of all the various cells and tissues of the body. Any one of these, departing from its normal behavior, at once affects the blood ; but health implies a constant effort toward a certain equilibrium, never actually reached but always being striven after by the whole organism. The blood can no more maintain its vital equilibrium, or exist as a living tissue out of its usual environment, than any other tissue. But the exact circum- stances under which it may become disorganized, or die, are legion ; hence, it is not likely that the blood always clots in the same way in all groups of animals, or even in the same group. The normal disorganization or death of the tissue re- sults in clotting ; but there may be death without clotting, as when the blood is frozen, in various diseases, etc. To say that fibrin is formed during coagulation expresses in a crude way a certain fact, or rather the resultant of many facts. To explain : When gunpowder and certain other ex- plosives are decomposed, the result is the production of cer- tain gases. If we knew these gases and their mode of com- position but in the vaguest way, we should be in much the same position as we are in regard to the coagulation of the blood. There is no difficulty in understanding why the blood does not clot in the vessels after death so long as they live, nor why it does coagulate upon foreign bodies introduced into the blood- stream. So long as it exists under the very conditions under which it began its being, there is no reason why the blood should become disorganized (clot). It would be marvelous if it did clot, for then we could not understand how it could ever have been developed as a tissue at all. It is just as reasonable THE BLOOD. 1(33 to ask why does not a muscle-cell become rigid (clot) in the body during life. Probably in no field in physiology has so much work been done with so little profit as in the one we are now discussing ; and, as we venture to think, owing to a misconception of the real nature of the problem. We can understand the practical im- portance of determining what circumstances favor coagulation or retard it, both within the vessels and without them ; but from a theoretical point of view the subject has been exalted out of all proportion to its importance; and we should not have dwelt, so long upon it, or burdened the student with sop^ie of the theories we have stated, except in deference to the views held by so many physiologists. It is not surprising that, looking at the subject with a dis- torted mental perspective, one theory should have replaced an- other with such rapidity. It is, however, of practical impor- tance to the medical student to remember some of the factors that hasten or retard, as the case may be, the coagulation of the blood. Coagulation is favored by gentle movement, contact with foreign bodies, a temperature of about 38° to 40° C, addi- tion of a small quantity of water, free access of oxygen, etc. The process is retarded by a low temperature, addition of abundance of neutral salts, extract of the mouth of the leech, peptone, much water, alkalies, and many other substances. The excess of carbonic anhydride and diminution of oxygen, seem to be the cause of the slower coagulation of venous blood, hence the blood long remains fluid in animals asphyxiated. A little reflection suffices to explain the action of most of the fac- tors enumerated. Any cause which hastens the disintegration of the blood-cells must accelerate coagulation ; chemical changes underlie the changes in this as in all other cases of vital action. Slowing of the blood-stream to any appreciable extent likewise favors clotting, hence the explanation- of the success of the treatment of aneurisms by jjressure. It is i)lain that in all such cases the normal relations between the blood and the tis- sues are disturbed, and, when this reaches a certain pcnnt, death (coagulation) ensues, as with any other tissue. Clinical and Pathological — The changes in the blood that cliaraclcrize certain abnormal states are highly instructive. If blf>od from an animal be injected into the veins of one of an- other species, the death of tlie latter often results, owing to non- adaptation to the blood already in the ve.s.sels, and to the tissues of the creature generally. The corpuscles V^ ak up — the change 164 ANIMAL PHYSIOLOGY. of conditions has been too great. Deficiency in the quantity of the blood as a whole {oligcemia) causes serious change in the functions of the body ; but that a haemorrhage of considerable extent can be so quickly recovered from by many persons, speaks much for the recuperative power of the blood-forming tissues. Various kinds of disturbances in these blood-forming organs result in either deficiency or excess of the blood-cells, and in some cases the appearance of unusual forms of corpuscles. AncBmia may arise from a deficiency either in the numbers or the quality of the red cells ; they may be too few, deficient Fig. 157. Fig. 153.— Outlines of red corpuscles in a case of profound ansemia. 1, 1, normal corpuscles ; 2, large red corpuscle — megalocyte ; 3, 3, very irregular forms— poikilocytes ; 4, very small, deep-red corpuscles — microcytes. Fig. 154.— Origin of microcytes from red corpuscles by process of budding and fission. Speci- men from red marrow. Fig. 155.— Nucleated red blood-corpuscles from blood in case of leukaemia. Fig. 156. — Corpuscles containing red blood-corpuscles. 1, from blood of child at term ; 2, from blood of a leuksemic patient. Fig. 157.— a, 1, 2, 3, spleen-cells containing red blood-corpuscles. 6, from marrow ; 1, ceU con- taining nine red corpuscles ; 2, cell with reddish granular pigment ; 3, fusiform cell con- taining a single red corpuscle, c, connective-tissue corpuscle from subcutaneous tissue of young rat, showing the intracellular development of red blood-corpuscles. (Figs. 15:3-157, after Osier.) in size, or lacking in the normal quantity of haemoglobin. In one form {pernicious ancBmia), which often proves fatal, a variety of forms of the red blood-cells may appear in the blood- stream ; some may be very small, some larger than usual, others THE BLOOD. 165 nucleated, etc. Again, the white cells may be so multiplied that the blood may bear in extreme cases a resemblance to milk. In these cases there has been found associated an unusual condition of the bone-marrow, the lymphatic glands, the spleen, and, some have thought, of other parts. The excessive action of these organs results in the production and discharge into the blood-current of cells that are immature and embryonic in character. This seems to us an examjole of a reversion to an earlier condition. It is instructive also in that the facts point to a possible seat of origin of the cells in the adult, and, taken in connection with other facts, we may say, to their normal source. These blood-producing organs, having too much to do in disease, do their work badly — it is incom- plete. Although the evidence, from experiment, to show that the nervous system in mammals, and especially in man, has an in- fluence over the formation and fate of the blood generally, is scanty, there can be little doubt that such is the case, when we take into account instances that frequently fall under the notice of physicians. Certain forms of anaemia have followed so di- rectly upon emotional shocks, excessiA^e mental work and worry, as to leave no uncertainty of a connection between these and the changes in the blood ; and the former must, of course, have acted chiefly if not solely through the nervous system. It will thus be apparent that the facts of disease are in har- mony with the views we have been enforcing in regard to the blood, which we may now briefly recapitulate. Summary. — Blood may be regarded as a tissue, with a fluid matrix, in which float cell-contents. Like other tissues, it has its phases of development, including origin, maturity, and death. The colorless cells of the blood may be considered as original undiff'erentiated embryo cells, which retain their primi- tive character ; the non-nucleated red cells of the adult are the mature form of nucleated cells that in the first instance are colorless, and arise from a variety of tissues, and which in certain diseases do not mature, but remain, as they originally were at first, nucleated. When the red cells are no longer fitted to discharge their functions, they are in some instances taken up by amoeboid organisms (cells) of the spleen, liver, etc. The chief function of the red corpuscles is to convey oxy- gen ; of the white, to develop as required into some more differ- '•ijtiated form of tissue, act as porters of food -material, and ^QQ ANIMAL PHYSIOLOGY. probably to take up the work of many other kinds of cells when the needs of the economy demand it. The fluid matrix or plasma furnishes the lymph by which the tissues are direct- ly nourished, and serves as a means of transport for the cells of the blood. The chemical composition of the blood is highly complex, in accordance with the function it discharges as the reservoir whence the varied needs of the tissues are supplied ; and the immediate receptacle (together with the lymph) of the entire waste of the body ; but the greater number of substances exist in very minute quantities. The blood must be maintained of a certain composition, varying only within narrow limits, in order that neither the other tissues nor itself may suffer. The normal disorganization of the blood results in coagula- tion, by which a substance, proteid in nature, known as fibrin, is formed, the antecedents of which are probably very variable throughout the animal kingdom, and are likely so even in the same group of animals, under different circumstances ; and a substa.nce abounding in proteids (as does also plasma), known as serum, squeezed from the clot by the contracting fibrin. It represents the altered plasma. Certain well-known inorganic salts enter into the composi- tion of the blood — ^both plasma and corpuscles — but the princi- pal constituent of the red corpuscles is a pigmented, ferrugi- nous proteid capable of crystallization, termed haemoglobin. It is respiratory in function. THE CONTEACTILE TISSUES. That contractility, which is a fundamental property in some degree of all protoplasm, becoming pronounced and definite, giving rise to movements the character of which can be pre- dicted with certainty once the form of the tissue is known, finds its highest manifestation in muscular tissue. Very briefly, this tissue is made up of cells which may be either elongated, fusiform, nucleated, finely striated lengthwise, but non-striped transversely, united by a homogeneous cement substance, the whole constituting non-striped or involuntary muscle ; or, long nucleated fibers transversely striped, covered with an elastic sheath of extreme thinness, bound together into small bundles by a delicate connective tissue, these again into larger ones, till what is commonly known as a " muscle ^^ THE CONTRACTILE TISSUES. 167 is formed. This, in the higher vertebrates, ends in tough, inelastic extremities suitable for attachment to the levers it may be required to move (bones). Fio. 158. Fig. 159. Fig. 1.58.— Muscular fibers from the urinary bladder of the human subject. 1 x 200. (Sappey.) 1, 1, 1, nuclei ; 2, 2, 2, borders of some of the fibers : 3, 3, isolated fibers ; 4, 4, two fibers joined together at 5. Fig. 1.59.— Muscular fibers from the aorta of the calf. 1 x 200. (Sappey.) 1, 1, fibers joined with each other ; 2, 2, 2, isolated fibers. Comparative. — The lowest animal forms possess the power of movement, which, as we have seen in Amoeba, is a result rather of a groping after food ; and takes place in a direction it is im- possible to predict, though no doubt regulated by laws definite enough, if our knowledge were equal to the task of defining them. Those ciliary movements among the infusorians, connected with locomotion and the capture of food, are examples of a pro- toplasmic rhythm of wonderful beauty and simplicity. Muscular tissue proper first appears in the Codenteruta, but not as a wholly independent tissue in all cases. In many ffjelenterates colls exist, the low- •T part of which alone forms a delicate muscular fibei', wliilo the superficial portion (inijoblast), composing the body of the cell, may be ciliated and is not contractile in any s])ecial ■'■»=-"?»».'}■ Fig. 100.— Myoblasts of a jelly-flsh, the 3/e- duHU Aurclia (Claus). IQQ ANIMAL PHYSIOLOGY. sense. The non-striped muscle-cells are most abundant among tlie invertebrates, though found in the viscera and a few other parts of vertebrates. This form is plainly the simpler and more primitive. The voluntary muscles are of the striped variety in articulates and some other invertebrate groups and in all vertebrates ; and there seems to be some relation between the size of the muscle-fiber and the functional power of the tissue — the finer they are and the better supplied with bloody two constant relations, the greater the contractility. Whether a single smooth muscle-cell, a striped fiber {cell), or a collection of the latter {muscle) be observed, the invariable result of contraction is a change of shape which is perfectly definite, the long diameter of the cell or muscle becoming shorter, and the short diameter longer. Ciliary Movements. — This subject has been already considered briefly in connection with some of the lower forms of life pre- sented for study. It is to be noted that there is a gradual replacement of this form of action by that of muscle as we ascend the animal scale ; it is, however, retained even in the highest animals in the discharge of functions analogous to those it fulfills in the invertebrates. Thus, in Vorticella, we saw that the ciliary movements of the peristome caused currents that carried in all sorts of parti- cles, including food. In a creature so high in the scale as the frog we find the alimentary tract ciliated ; and in man himself a portion of the respiratory tract is provided with ciliated cells concerned with assisting gaseous interchange, a matter of the highest importance to the well-being of the mammal. As be- fore indicated, ciliated cells are found in the female generative organs, where they play a part already explained. It is a matter of no little significance from an evolutionary point of view, that ciliated cells are more widely distributed in the foetus than in the adult human subject. As would be expected, the movements of cilia are affected by a variety of circumstances and reagents : thus, they are quick- ened by bile, acids, alkalies, alcohol, elevation of temperature up to about 40° C, etc. ; retarded by cold, carbonic anhydride, ether, chloroform, etc. In some cases their action may be arrested and re-estab- lished by treatment with reagents, or it may recommence with- out such assistance. All this seems to point to ciliary action as falling under the laws governing the movements of protoplasm THE CONTRACTILE TISSUES. 169 in general. It is important to bear in mind that ciliary action may go on in the cells of a tissue completely isolated from the animal to which it belongs, and though influenced, as just ex- plained, by the surroundings, that the movement is essentially automatic, that is, independent of any special stimulus, in which respect it differs a good deal from voluntary muscle, which usually, if not always, contracts only when stimulated. The lines along which the evolution of the contractile tissues has proceeded from the indefinite outflowings and withdraw- als of the substance of Amoeba up to the highly specialized movements of a striped muscle-cell are not all clearly marked out ; but even the few facts mentioned above suffice to show gradation, intermediate forms. A similar law is involved in the muscular contractility manifested by cells with other func- tions. The automatic (self-originated, independent largely of a stimulus) rhythm suggestive of ciliary movement, more manifest in the earlier developed smooth muscle than in the voluntary striped muscle of higher vertebrates, indicating further by the regularity with which certain organs act in which this smooth muscular tissue is predominant, a relation- ship to ciliary movement something in common as to origin — in a word, an evo- lution. And if this be borne in mind, we believe many facts will appear in a new light, and be invested with a breadth of meaning they would not otherwise possess. The Irritability of Muscle and Nerve. — An animal, as a frog, deprived of its brain, will remain motion- less till its tissues have died, unless the animal be in some way stimulated. If a muscle be isolated from the body with the nerve to which it belongs, it will also remain passive ; but, if an electric current be passed into it, if it be pricked, pinched, touched with a liot l>ody or with certain chemical reagents. Fig. 101.— Nodes of Ranvier and lines of Froniann iRanvier). A. Intercostal nerve of the niinise, treated with silver nitrate. B. Nerve-fiber from the sciatic nerve of a full-ffrown rabbit. A, node of I-Janvier ; Jl/, medullary substance rendered transparent }>y the action of glycerine; C'Y. axis- cylinder presi-nting tlie lines of Fi'omaim, which are very distinct near the node. The lines are less marked at a distance from the node. 170 ANIMAL PHYSIOLOGY, 4_> contraction ensues ; the same happening if the nerve be thus treated instead of the muscle. The changes in the muscle and the nerve will be seen later to have much in common ; the mus- cle alone^ however, contracts, und ergoes a visible change of form. Now, the agent causing this is a stimulus, and, as we have seen, may be mechanical, chemical, thermal, electrical, or nerv- ous. As both nerve and muscle are capable of being functionally af- fected by a stimulus, they are said to be irrita- hle ; and, since muscle does not contract with- out a stimulus, it is said to be non-automatic. Now, since muscle is supplied with nerves as well as blood - vessels, which end in a peculiar way beneath the muscle- covering {sarcolemma) which is seen extending to the terminal plate, where ■;■,-, -Htp vpvv ^mlisstflnr.p of it disappears ; 4, termiSal plate situated beneath the ^^ ^^^ ^^^ ^ ^^^ UStanoe Oi the protoplasm (end- plates), it might be that when muscle seemed to be stimulated, as above indicated, the responsive contraction was really due to the excited nerve terminals ; and thus has arisen the question. Is muscle of itself really irritable ? What has been said as to the origin of muscular tissue Fig. 163.— Mode of termination of the motor nerves (Flint, after Rouget). A. Primitive fasciculus of the thyrohyoid muscle of the human subject, and its nerve-tube : 1, 1, primitive muscular fasciculus ; 2, nerve-tube ; 3, medullary substance of the tube, sarcolemma— that is to say, between it and the ele- mentary flbrillee ; 5, 5, sarcolemma. B. Primitive fasciculus of the intercostal muscle of the lizard, in which a nerve-tube terminates : 1, 1, sheath of the nerve-tube ; 2, nucleus of the sheath ; 3, 3, sarco- lemma becoming continuous with the sheath ; 4, medullary substance of the nerve-tube, ceasing abruptly at the site of the terminal plate ; 5, 5, ter- minal plate ; 6, 6, nuclei of the plate ; 7, 7, granular substance which forms the principal element of the terminal plate and which is continuous with the axis-cylinder ; 8, 8, undulations of the sarcolemma reproducing those of the fibrillae ; 9, 9, nuclei of the sarcolemma. Fig. 163.— Intraflbrillar terminations of the motor nerve in striated muscle, stained with gold chloride (Landois). points very strongly to an affirmative answer, though it does not follow that a property once possessed in the lower forms of APPLICATIOXS OF THE GRAPHIC METHOD. 171 a tissue may not be lost in the higher ; hence the resort to ex- periments which have long been thought to settle the matter : 1. The curare experiment may be thus performed : Lift up the sciatic nerve of a frog, and ligature the whole limb (ex- clusive of the nerve) so that no blood may reach the muscles ; then inject curare, which paralyzes nerves but not muscles, into the general circulation through the posterior lymi^h-sac. On stimulating the sciatic nerve the muscles of the leg beneath the ligature contract, while no contraction of the muscles of the opposite leg follows from stimulation of its sciatic nerve. In the latter case the curare has reached the nerve terminals through the blood ; in the former, these were left uninfluenced by the poison. If, now, the muscle itself be directly stimulated in the latter case, contraction follows, from which it is con- cluded that curare has destroyed the functional capacity of the nerve {terminals), but not of the muscle. 2. Stimulation of those parts of muscles in which no nervous terminations have been found, as the lower part of the sartorius muscle in the frog, is followed by contraction. 3. Certain substances (as ammonia), when applied directly to the muscle, cause contraction, but are not capable of pro- ducing this effect when applied to the nerve. From these and various other facts it may be concluded that muscle possesses independent irritability. APPLICATIONS OF THE GRAPHIC METHOD TO THE STUDY OF MUSCLE PHYSIOLOGY. It is impossible to study the physiology of muscle to the I)est advantage without .the employment of the graphic method ; and, on the other hand, no tissue is so well adapted for investi- gation by tlie isolated method — i. e., apart from the animal to which it actually belongs— as muscle ; hence the convenience of introducing at an early period our study of the physiology of contractile tissue and illustrations of the graphic method, the general principles of which have already been considered. The descriptions in the text will be brief, and the student is recommended to examine the figures and accompanying ex- planations with some care. Chronographs, Revolving Cylinders, etc. — Fig. 164 represents one of the (jiirlio.st Uji-uih (A apparatus for the measurement of brief intervals of time, consisting of a simple mechanism for pro- 172 ANIMAL PHYSIOLOGY. ducing the movement of a cylinder, which may be covered with smoked paper, or otherwise prepared to receive impressions made upon it by a point and capa- ble of being raised or lowered, and its movements regulated. The cylinder is ruled vertically into a certain number of spaces, so that, if its rate of revolution is known and is constant (very im- portant), the length of time of any event recorded on the sen- sitive surface may be accurately known. This whole apparatus may be considered a chrono- graph in a rough form. But a tuning-fork is the most reliable form of chronograph, provided it can be kept in con- stant action so long as required ; and is provided with a recording apparatus that does not cause enough friction to interfere with its vibrations. Fig. 166 illustrates one ar- rangement that answers these conditions fairly well. The marker, or chronograph, in the more limited sense, is kept in automatic action by the fork interrupting the current from a battery at a certain definite rate answeiing to its own proper note. Fig. 164. — Original chronometer, devised by Thomas Young, for measuring minute portions of time (after McKendrick). a, cylinder revolving on vertical axis ; 6, weight acting as motive power ; c, d, small balls for regulating the velocity of the cylinder ; e, marker recording a line on cylinder. C. J, I. } b. Fig. 165.— Myographic tracing, such as is obtained when the cylinder on which it is written does not revolve during the contraction of the muscle (after McKendrick). Marey's chronograph, which is represented at h above, and in more detail below, in Fig. 167, consists of two electro-magnets armed with keepers, between which is the writer, which has a APPLICATIONS OF THE GRAPHIC METHOD. 173 little mass of steel attached to it, the whole working in unison with the tuning-fork, so that an interruption of the current Fig. 166. — Marey's chronograph as applied to revolving cj'linder (after McKendrick). a. gal- vanic element ; 6, wooden stand bearing tuning-fork (tVo hundred vibrations per second); c. electro-magnet between limbs of tuning-fork ; d. e, positions for tuning-forks of one hun- dred and fifty vibrations per second ; /, tuning-fork lying loose, which may be applied to d : f/. revolving cylinder ; h, electric chronograph kept in vibration synchronous with the tuning-fork interrupter. The current working the electro-magnet from a is interrupted at i. Foucault's regulator is seen over the clock-work of the cylinder, a little to the right of g. implies a like change of position of the writing-style, which is always kept in contact with the recording surface. Fio. 167.— Side view of Marey's chronograph (after McKendrick). a, a, coils of wire ; ft, b, keepers of electro-magnets : c. vibrating style fixed to the steel plate e ; rl, binding screws for attachment of wires ; + from interrupting tuning-fork ; - to tlie battery. Fig. 177 shows the arrangements for recording a single niuschi contraction, and Fig. 178 the cliaracter of the tracing ol^tained. A musclo-nervo preparation, which usually consists of the gastrocnemius of tlie frog witli the sciatic nerve attached, 174 ANIMAL PHYSIOLOGY. clamped by a portion of the femur cut off with the muscle, is made, on stimulation, to raise a weighted lever which is at- tached to a point writing on a cylinder moved by some sort of clock-work. In this case the cylinder is kept stationary dur- ing the contraction of the mus- cle ; hence the records appear as straight vertical lines. Fig. 1G8.— Muscle-nerve preparation, showing JTor recording mOVemeuts of gastrocnemius muscle, sciatic nerve, and n , ,-t portion of femur of frog, for attachment great rapidity, SO that the in- to a vise (after Rosenthal). , i n , n ^ tervals between them may be apparent, such an apparatus as is figured below (Fig. 169) an- swers well, the vibrations of a tuning-fork being written on a Fig. 169.— Spring myograph of Du Bois-Reymond (after Rosenthal). The arrangements for registering various details are similar to those for pendulum myograph (Fig. 177). blackened glass plate, shot before a chronograph by releasing a spring. Several records may be made successively by more compli- cated arrangements, as will be explained by another figure later. The Appakatus used for the Stimulation of Muscle. It is not only important that there should be accurate and delicate methods of recording muscular contractions, but that APPLICATIONS OF THE GRAPHIC METHODo 175 and tliere be equally exact methods of applying, regulatin measuring the stimulus that induces the contraction. Fig. 170 gives a representation of the inductorium of Du Bois-Reymond, by which either a single brief stimulation or a Fig. 170. — Du Bois-Reymond's inductorium (after Rosenthal), i, secondary coil ; c, primary coil ; b, electro-magnet ; h, armature of hammer ; /, small movable screw. The current from battery, ascending metal pillar, passes along hammer, and by screw gets into primary coil, thus inducing current in secondary coil. By connection between primary coil and wires around soft iron of 6, iron Itecomes a magnet, hammer is attracted from screw /, and current thus broken ; but when this occurs, soft iron ceases to be a magnet neces- sarily, and, hammer springing back, the whole course of events is repeated. This may occur several hundred times in a second. The above may be clearer from diagram, Fig. 171. By sliding secondary coil up and down, strength of induced current can be giaduated. series of such repeated with great regularity and frequency may be effected. The apjjaratus consists essentially of a pr.i- I'Ui. )71.— diagrammatic representation of the u'orking of Fig. 170 (after Rosenthal) 176 ANIMAL PHYSIOLOGY. mary coil, secondary coil, magnetic interrupter, and a scale to determine the relative strength of the current employed. The instrument is put into action by one or more of the various well-known galvanic cells, of which Daniell's are suitable for most experiments. Fig. 173. Fig. 172. Fig. 173. — Pfliiger's myograph. The muscle may be fixed to the vise C in the moist-chamber, the vise connecting with the lever E E, the point of which touches the plate of smoked glass G. The lever is held in equipoise by H. When weights are placed in scale-pan F, the lever writes the degree of extension effected (after Rosenthal). Fig. 173.— Tetanizing key of Du Bois-Reymond (after Rosenthal). Wires may be attached at b and c. When d is down the current is " short-circuited," i. e., does not pass through the wires, but direct from c through d to t», or the reverse, since b, c, d are of metal, and, on account of their greater cross-section, conduct so much more readily than the wires, a is an insulating plate of ebonite. This form of key is adapted for attachment to a table, etc. The access to, or exclusion of the current from, the induc- torium is effected by some of the forms of keys, a specimen of which is illustrated in Fig. 173. The moist chamber, or some other means of preventing the drying of the preparation, which would soon result in impaired action, followed by death, is essential. A moist chamber con- sists essentially of an inclosed cavity, in which is placed some wet blotting-paper, etc., and is usually made with glass sides. The air in such a chamber must remain saturated with moist- ure. APPLICATIONS OP THE GRAPHIC METHOD. 177 A good knowledge of the subject of electricity is especially valuable to tlie student of physiology. But there are a few ele- mentary facts it is absolutely necessary to bear in mind : 1. An induced current exists only at the moment of making or break- ing a primary (battery) current. 2. At the moment of making, the induced current is in the opposite direction to that of the primary current, and the reverse at breaking. 3. The strength of the induced current varies with the strength of the primary current. 4. The more removed the secondary coil from the primary the weaker the current (induced) becomes. The clock-work mechanism and its associated parts, as seen in Fig. 174, on the right, is usually termed a myograph. Fio. 17-1. —Arrangement of apparatus for transmission of muscular movement by tambours (after McKendrick). a, galvanic element ; b, primary coil ; e, secondary coil of inducto- rium ; d, metronome for interrupting primary circuit when induction current is sent to electrodes Ic ; h. forceps for femur ; the muscle, which is not here represented, is attached to the receiving tambour y, by which movement is transmitted to recording tambour e, which writes on cylinder/. Instead of muscular or other movements being communi- cated directly to levers, the contact may be througli columns Fifi. 17.1.— Tambfjur of Marey (after McKendrick). a, metallic case ; h, thin India-rubber mem- brane; r. thin disk of aluminium siipporting lever rf, a Bmall portion of which only Is repre- wnti-d ; '-. wn-w for placing Kujijxirt of lever vi-rticully over c ; f, metallic tube communi- cating with cavity of tambour t(^r attachiiieut to an India-rubber tul>e. 178 ANIMAL PHYSIOLOGY. of air, which, it will be apparent, must be capable of comin-iiiii- cating very slight changes if the apparatus responds readily to the alterations in volume of the inclosed air. Fig. 175 represents a Marey's tambour, which consists essen- tially of a rigid metallic case provided with an elastic top, to which a lever is attached, the whole being brought into com- munication with a column of air in an elastic tube. The work- ing of such a mechanism will be evident from Figs. 174 and 176. Fig. 176.— Tambours of Marey arranged for transmission of movement (after McKendrick). a, receiving tambour ; 6, india-rubber tube ; c, registering tambour ; d, spiral of wire, owing to elasticity of which, when tension is removed from a, the lever ascends. The greatest danger in the use of such apparatus is not fric- tion but oscillation, so that it is possible that the original move- ment may not be expressed alone or simply exaggerated, but also complicated by additions, for which the apparatus itself is responsible. Apparatus of this kind is not usually employed much for experiments with muscle ; such an arrangement is, however, shown in Fig. 174, in which all will be seen — a metronome, the pendulum of which, by dipping into cups containing mercury, makes the circuit. Such or a simple clock may be utilized for indicating the longer intervals of time, as seconds. A Single Simple Muscular Contraction. Experimental Facts. — The phases in a single twitch or muscu- lar contraction may be studied by means of the pendulum APPLICATIONS OF THE GRAPHIC METHOD. (2 1T9 Fio. 177.— Diajframmatie representation of the pendtilum myoj^raph. The smoked-plass plate, A. Kwin«H with a pendulum, li. Before an experiment is commeneed the pendulum is rainefl up to the rif^ht and kept in position l)y tlie tooth, tt, eati-hinR on tlie spring eat eh, /). On denresMinK tlie cateh, h, the ((lass plate lieinK set free swiiiffs into the new iHisiliun indi- caletl hy the dott»-d lines, and Is held tliere \>y the tooth, ii\ meeting the ealeli, /<'. In the eourw-of its swintc the tf)oth, «.eomin« into eontaet with I lie project iri(j steel rod, c, knocks it to one side. irit/» the pr>sition indicated l)y the dotted line, c'. The rod. c, is in electric continuity with the wire, .c. of the primary cidl of an inductlou machine. In like manner ISO ANIMAL PHYSIOLOGY. the screw, d, is in electric continuity with the wire, y, of the same primarr coil. The screw, d. and the rod, c. are provided with platinum points, and both are insulated by means of the ebonite block, the same animal. , Tetanic Contraction. It is well known that a weight may be held by the out- : Stretched arm with apparently XJerfect steadiness for a few seconds, but that presently the arm begins to tremble or vi- brate, and soon the weight must be dropped. The arm was maintained in its position by the joint contraction of several muscles, the action of which might be described (traced) by a writer attached to the hand and recording on a moving sur- face. Such a record would indicate roughly what had hap- pened ; but the exact nature of a muscular contraction in such a case can best be learned by laying bare a single muscle, say in the thigh of a frog, and arranging the experiment so that a graphic record shall be made. Using the apparatus previously described (Fig. 177), a second induction shock may be sent into the muscle before the effect Fig. 180.— Tracing of a double muscular contraction (Foster). A second induction shock was sent into muscle when it had so far completed its contraction as is indicated by beginning of second rise. Dotted line indicates what the curve would have been but for this. APPLICATIONS OF THE GRAPHIC METHOD. 183 of the first has passed away, the result depending on the phase of the contraction, during which the stimuhis acts on the mus- cle. Thus, if a second shock be applied during the latent pe- riod, no visible change in the nature of the muscle-curve can be seen ; but if during one of the other phases of contraction, a re- sult like that figured below (Fig. 180) follows. If a series of such shocks be sent into the muscle before its contraction pe- riod is over, a succession of curves may be superposed on one Fig. 181.— Curve of imperfect tetanic contraction (Foster). Uppermost tracing indicates con- tractions of muscle ; intermediate, when the shocks were given ; lower, time-markings of intervals of one second. Curve to be read, like others, from left to right, and illustrates at the end a '" contraction remainder." another, to the total height of which, however, there is a limit, no matter what the strength of the stimulus used. If the stimuli follow each other with a certain rapidity, such a tracing as that represented in Fig. 181 is obtained ; and if the rapidity of the stimulation exceeds a certain rate, the result is that seen in Fig. 182. Fia. 182.— Curve of complete tetanic contraction (Foster). It is possible to see in these tracings a genetic relation, the second figure being evidently derivable from the first, and the third from the second, by the fusion of all the curves into one straight line. If a muscle, isolated as wo have described, be watched dur- ing the period that it is writing the second and the third 184 ANIMAL PHYSIOLOGY. tracing, it may be observed that, during that corresponding to the former, though it is shortened, it does not remain equally so throughout, while during the writing of the third tracing, there is no variation in its condition appreciable by the eye. What has happened is this : The muscle during the condition figured in the second tracing has periods of alternating contraction and partial relaxation, but during the third case the latter phase has been apparently omitted — the muscle remains in continuous contraction. In reality this is not the case unless we are mis- taken as to the meaning of the muscle-sound. The Muscle Tone. — There are a number of experimental facts from which important conclusions have been drawn, to which attention is now directed : 1. It has been found that a sound may be heard in a still room when one brings the muscles of mastication into action by biting hard; or listens over a contracting biceps with a stethoscope, etc. 2. When the wires of a telephone (communicator) are con- nected with a muscle, a sound is heard during the contraction of the muscle. From these facts it was concluded that a muscle when con- tracting gives rise to a sound ; that tetanus, as the form of con- traction we are describing is called, is essentially vibratory in character, which seems to answer to the graphic representations from a muscle when in tetanic contraction, and is in harmony with the case to which we called attention at the commence- ment of this subdivision of the subject. The note heard cor- responded, in the case of an isolated muscle, to the number of stimulations per second ; while for muscles made to contract by the will the note was always the same, answering to about forty vibrations per second ; but as forty stimuli are not re- quired within this period of time to induce tetanus, it was thought that this note was probably the harmonic of a lower one answering to twenty vibrations in a second. It has been recently shown that a very much smaller num- ber of vibrations of the muscle can give rise to an audible sound, so that the explanation it would seem must now be modified ; and it is likely that some peculiarities of the ear itself must be taken into the account in the explanation. In making the observations referred to above (in 1), the student will find it very important to be on his guard against sources of error, especially with the use of a stethoscope. We may safely conclude that, at all events, most of the mus- APPLICATIONS OF THE GRAPHIC METHOD. 185 cular contractions occurring within the living body are tetanic — i. e., the muscle is in a condition of shortening, with only very brief and slight phases of relaxation ; and that a comparatively small number of individual contractions suffice for tetanus when caused by the action of the central nervous system; though, as proved by experiments on muscle removed from the body, they may be enormously increased. While a few stimu- lations per second suffice to cause tetanus, it will also persist though thousands be employed. The Strength of the StimuluB. — We have assumed that in the cases of contraction thus far considered the stimulus was ade- quate to produce the full amount of contraction, or as much as could be obtained. Such a contraction and such a stimulus are spoken of as maximal; but the stimulus might fall a little short of this, and is then termed sub-maximal ; or it may be re- garded from the point of view of being the least that will cause a contraction, and is then the minimal stimulus. It is important to note that any sudden change in an electric current will act as an excitant to muscular contraction, but that very considerable changes in the strength of the current if made gradually do not react on the muscle. It sometimes hap- pens that a sudden onward push of the secondary coil of an induction-machine will produce either a tetanus (though the terminal wires or electrodes were arranged for a single induc- tion shock) or what is known as a supermaximal contraction — i. e., one in excess of what could be obtained by more gradual advances, which have no effect usually after a certain maxi- mum of contraction is reached. This, we think, a matter of considerable practical importance, and shall refer to its signifi- cance in a later chapter. Since the opening or closing of a key which makes or breaks the current really implies a very great change in the strength of the current affected suddenly — that is in fact from 0 to some -I- quantity or the reverse — we find that usually the most marked contractions occur only at these times, and this holds, whether the current be slowly or rapidly made and broken (inter- rupted). The nerve being the natural means of conveying a stimu- lus, it is easy to understand how the contraction happens to follow most perfectly and with less strength of stimulus when this structure is excited. 186 ANIMAL PHYSIOLOGY. The Changes in a Muscle during Contraction. Though the change in form is very great during the con- traction of a muscle, the change in bulk is almost inappreci- able, amounting to a diminution of not more -than about xoVo of the volume. In fact, according to the latest investigator, there is no diminution whatever. A series of levers may be laid on a muscle or the columns of air in a series of Marey's tambours may be influenced by the contracting muscle, and from some such apparatus a graphic record like that seen in Fig. 183 may be obtained. It is to be observed that the contraction passes along the muscle in the form of a wave, the size and speed of which are Fig. 183.— Tracing of the propagation of the muscular wave. Chronographic tracing, one hundred vibrations per second underneath (Marey). susceptible of measurement. For the frog the wave-length is estimated at from 200 to 400 mm., and the velocity at about 3 to 4 metres per second. It is probably rather greater in the muscles of mammals and greater under the more natural conditions of the muscle in the intact living body. But since the fibers of striped muscle are of very limited length (30 to 40 mm.), it would seem that a contraction origi- nating in one fiber must be capable of initiating a similar action in its neighbor ; and, as the ends of the fibers lie in con- tact, it is easy to understand how the wave of contraction spreads. Normally, the contraction must pass from about the center of the muscle-cell where the nerve terminates in the end-plate. The microscopic changes occurring in contracting muscle are not well understood. The living muscle of a beetle's thigh when placed under a microscope may be seen in contraction — a sight of the most striking nature, reminding one of a billowy, tempestuous sea, and by the use of reagents the waves of con- traction may be fixed. It may be stated that the parts distinct before remain so APPLICATIONS OP THE GRAPHIC METHOD. 187 during contraction, and that all parts of the muscle-substance seem to share in the changes of form involved. The Elasticity of Muscle. In proportion as bodies tend to resume their original form when altered by mechanical force are they elastic, and the ex- tent to which they do this marks the limit of their elasticity. If a muscle (best one with bundles of fibers of about equal length and parallel arrange- ment) be stretched by a weight attached to one end, it will, on removal of the extending force, return to its original length ; and if a series of weights which differ by a common increment be applied in succession and the degrees of extensions compared, as may be done by the graphic method, it will be appar- ent that the increase in the extension does not exactly correspond with the increment in the weight, but is proportionally less. With an inorganic body, as a watch-spring, this is not the case. Further, the recoil of the muscle after the removal of the weight is not perfect for all weights ; but within certain narrow limits this is the case, i. e., the elasticity of muscle, though slight (for it is easily over-extended), is perfect. When once a muscle is over-ex- tended, so weighted that it can not reach its original length almost at once, it is very slow to recover, which exxjlains the well-known duration of the effects of sprains, no doubt owing to some profound molecular change fig. i84.-du Bois-Rey associated with the stretching. tiie"study^*rL'iasUc The tracings below show at a glance the ptT^'"'' liosinlhait difference between the elasticity of muscle It'tL-ia-dto muscie'^s and of f;i-dinary bodies. L'elTs. ""''"*''' '''"' It is a curious fact that a muscle during the act of contraction is more extensible than when passive ; a disadvantage from a purely X'hysical jjoint of view, but proba- bly a real advantage as tending to obviate sprain by prevent- ing too sudden an application of the extending force. 188 ANIMAL PHYSIOLOGY. It will he borne in mind that the limbs are held together as by elastic bands slightly on the stretch, owing to the elasticity Fig. 185.— Illustrations of the difference in elasticity of inanimate and living matter (after Yeo). 1. Shows graphically behavior of a steel spring under equal increments of weight. 2. A similar tracing obtained from an India-rubber band. 3. The same from a frog's muscle. Note that the extension decreases with equal increments of weight, and that the muscle fails to return to the original position (abscissa) after removal of the weight. of the mnscles. Now, as seen in many tracings of muscular contraction, there is a tendency to imperfect relaxation after contraction — the contraction remainder or elastic after-effect, which can be overcome by gentle traction. In the living body, the weight of the limbs and the action of the stretched muscles on the side of the limb opposite to that on which the muscles in actual contraction are situated, combine to make the action of the muscle more perfect by overcoming this tendency to im- perfect relaxation, which is probably less marked, independent of these considerations, in the living body. This elasticity of living muscles, which is completely lost on death, is a fair measure of their state of health or organic perfection. Hence that hard (elastic recoil) feeling of the muscles in young and vigorous persons, especially athletes, in whom muscle is brought to the highest degree of perfection. This property is then essentially the outcome of vitality, which is in a word the foundation of the differences noted be- tween the elasticity of inorganic and organic bodies. A mus- cle, the nutrition of which is suffering from whatever cause, whether deficient blood-supply, fatigue, or actual disease, is deficient in elasticity. We Avisli to emphasize these relations, for we consider it very important to avoid regarding vital phe- nomena in the light of physics merely, which the employment of the graphic method (and indeed all methods by which we re- move living things out of their normal relations) fosters. Electrical Phenomena of Muscle. — Certain pieces of apparatus APPLICATIOXS OF THE GRAPHIC METHOD. 189 not as yet referred to are required to demonstrate the electrical condition of muscle. The galvanometer suitable for physio- logical experiments is one having very many coils of extreme- ly fine wire, and so adapted to indicate the presence of currents of slight intensity. In order that it may be ascertained definitely that the cur- rents that deflect the galvanometer needle do not originate out- side of the muscle itself, non-polarizable electrodes very care- fully made must be used, for the contact of ordinary metallic electrodes with living tissues suffices of itself to generate an electric current, as may be simply illustrated to one's self by placing two coins, one silver and the other copper, in contact with the upper and under surfaces of the tongue respectively, and meeting in front ; a peculiar taste results from the current excited. The construction of the non-polarizable electrodes common- ly employed, and as arranged for use, is diagrammatically rep- resented below (Fig. 180). Assuming the apparatus for the detection of electrical cur- rent in muscle to be in working order, a muscle from one of Fig. 1W5.— Non-polarizable electrodes of Du Bois-Beymond (after Rosenthal!. At c. clay tip, moLstened with saline solution, is laid on muscle. Glass cylinder a is filled with strong sf>lution of zinc sulphate, a good conductor, by which current is conveyed to amalgamated zinc plate 6, and thence to galvanometer. the cold-blooded animals, prepared as rapidly and carefully as possible^ avoiding all contact with foreign bodies, is cut across the ends transversely, and placed on pads of bibulous yjaper moistened with physiological (•0()--75 per cent) saline solution. The non-polarizable electrodes connected with the galvanome- ter are brought in contact with the muscle. What results depends on the parts of the muscle that touch the electrodes, and is represented diagramatically in Fig. 1S7. It will be observed that the diagram indicates that between no current and the strongest obtainable there are all shades of 190 ANIMAL PHYSIOLOGY. strength, according to the parts of the muscle connected by the electrodes. The strongest is that resulting when the superfi- FiG. 187.— Representation of electrical currents in a muscle-rhombus (after Rosenthal). cial equator and the transverse center are connected ; and it is found that the nearer these points are approached the stronger the current becomes, as is indicated by the greater extent of swing of the galvanometer needle. In connection with these sur- prising phenomena, one naturally inquires whether such a mus- cle-current, for such it must be, is natural or artificial. Does such exist in a living muscle in its position in the body, or has the injury done to a muscle in its preparation by section, re- moval from the usual conditions of nutrition, and such like changes, been the cause of the current ? After much investigation, by some of the ablest physiolo- gists of the day, different answers are returned to these queries. Du Bois-Reymond maintains that such currents are natural, and may be obtained from muscle contracting in situ ; while Hermann and others believe that such a current is owing to the injury done by the section, and that the current from the equator to the poles of the section is due to the fact that the injured part is negative to the uninjured region. It is a fact that if the current be led off from an exposed muscle prior to section, it is relatively very weak. Further, the electrodes placed on the uninjured ventricle of an animal's APPLICATIONS OF THE GRAPHIC METHOD. 191 heart convey no current to the galvanometer ; but after section, as in the case of a skeletal muscle, the usual result follows. All observers, however, are agreed that a current is produced during contraction. Those not believing in that just referred to above (" current of rest "), term this one the " current of action "" ; while the other school names it the negative variation of the current of rest, inasmuch as the galvanometer needle swings in the opposite direction indicating, as they say, a diminution in the original current. The presence of this undisputed current can be made evident by a simple experiment, without the use of any of the elabo- rate apparatus noticed above. Let two frog's limbs, with the Fio. 188.— Arranwenient of parts to show secondary eotitra(;tion in muscle (after Rosenthal). Fig. 189.— The same when the primary cause is in nerve (after Roseutlial). nerves belonging to them, be prepared in good condition and arranged as in Fig. 188, so that the nerve of A rests along the thigh of B. On stimulating the nerve of B, the muscular effect in this limb is answered by a similar one in A. That this is not necessarily due to escape of the current upon tlie nerve of A, may be shown by putting a ligature around the nerve of B below the point of application of the current and moistening it so as to allow of th(! free passage of the current. In such cas(> stimu- lation of the nerve o^ B givt^s wliolly negative results, })ecause the ligature has destroyed physiological (molecular) continuity, though it does not prevent the passage of the current. More- 292 ANIMAL PHYSIOLOGY. over, tlie result may be obtained by other than electrical stimnli. The explanation of these phenomena of the "rheoscopic frog" (physiological rheoscope) is simply that the electrical condition of B has been suddenly changed by the passage of the current into the nerve, and that this difference of electrical condition (potential) between the muscle of B and A's nerve suffices to stimulate the muscle of A (one is in fact + and the other — ) ; hence the stimulus and the contraction, the nature of which in A is the same as that in B — i. e., a single twitch in B gives rise to the same in A, and a tetanic contraction to a tetanic contraction. Plainly the contraction of A must be due to a current in B, hence the proof that a current actually exists during the contraction of a muscle. It may be noted that a mere prick of B will arouse in it a contraction which is fol- lowed by the same result as before in A, so that in this we can exclude the original stimulating current altogether as a pos- sible source of fallacy, as stated above. But one of the most striking proofs that there is a current of action (or negative variation), is obtained by placing the nerve of such a prepara- tion as that represented in B on a contracting mammalian heart ; with each systole there is a spasm of the frog's leg. It is important to note that the electric current of muscle, however viewed, is an event of the latent period. It is asso- ciated with the chemical and all the other molecular changes of which the actual contraction is but the outward and visible sign ; and since the currents of rest have an appreciable dura- tion, wane with the vitality of the tissue, and wholly disappear at death, they must be associated with the fundamental facts of organic life ; for it is to be remembered that electrical cur- rents are not confined to muscle, but have been detected in the developing embryo, and even in vegetable protoplasm. Though the evidence is not yet complete, it seems likely that electrical phenomena may prove to be associated with (we designedly avoid any more definite expression) all vital phenomena. Chemical Changes in Muscle. — In an animal, at a variable period after death, the muscles become rigid, producing that stiffness {rigor mortis) so characteristic of a recent cadaver. The subject can be studied in some of its aspects to great advantage in an isolated individual muscle. Three changes in a muscle that has passed into death rigor are constant and pronounced. The living muscle, either alka- line or neutral in reaction, has become decidedly acid; an APPLICATIONS OF THE GRAPHIC METHOD. 19a abundance of carbonic anhydride is suddenly given off ; and myosin, a specific proteid, lias been formed. That these phe- nomena have some indissoluble connection with each other so far as the first two at least are concerned, while not absolutely certain, seems probable, as will be learned shortly. It will be borne in mind that muscle-fibers are tubes con- taining semifluid protoplasm, and that a coagulation of the latter must give rise to general rigor. This protoplasmic sub- stance can be extracted at a low temperature from the muscles of the frog, and, as the temperature rises coagulates like blood, giving rise to a clot (myosin) and muscle-serum, a fluid not very unlike the serum of blood. This myosin can also be extracted from dead rigid mus- cles by ammonium choride, etc. It resembles the globulins generally, but is less soluble in saline solutions than the globu- lin of blood (paraglobulin) ; is less tough than fibrin ; has a very low coagulating point (55° to 60° C.) ; and is somewhat jelly-like in appearance. The clotting of blood and of muscle is thus analogous, myosin answering to fibrin, and there being a serum in each case, both processes marking the permanent disorganization of the tissue. The reaction seems to be due to the formation of a kind of lactic acid, probably sarolactic ; though whether due to excessive production of this acid, on the death of the muscle, which for some reason does not remain free in the living muscle, or whether sarcolactic acid arises as a new product, is uncertain. It is certain that the acid reaction of dead muscle is not owing to carbonic acid, for the reddened litmus does not change color on drying. That a muscle in action does use up oxygen and give off carbonic anhydride can be definitely proved ; though it is equally clear that the life of a muscle is not dependent on a constant supply of oxygen as is that of the individual, for a muscle can live, even contract long and vigorously, in an atmos- phere free from this gas, as in nitrogen. From the suddenness of the increase of carbonic anhydride, the onset of death and rigor mortis has been compared to an explosion. After this the muscle becomes greatly changed physically: its elasticity and translucency are lost; there is absence of muscle-currents ; it is wholly unirritable, is less extensible — it is, as before stated, firmer — it is dead. But these fundamental plienomena, the increase of carbonic anhydride and the acid reaction, are observable after prolonged 18 194 ANIMAL PHYSIOLOGY. tetanus. It was, therefore— putting all the facts together that we now refer to and others, not forgetting that a muscle is always respiring^ inhaling oxygen, and exhaling carbonic an- hydride— not unreasonable to conclude that normal tetanus and rigor mortis were but exaggerated conditions of a natural state. The coagulation of the muscle protoplasm {plasma), giving rise to myosin, was, however, a serious obstacle to the adoption of this view. But it has very recently been urged with great plausibility that an old view is correct, viz., that rigor mortis (contracture) is the last act of muscle-life ; it is, in fact, a prolonged tetanus or contracture, ending in most cases, though not all, in coagulation of the myosin. This state can be induced and recovered from in favorable cases by cutting ^off the blood from a part by ligature, and later readmitting it to the starving region. It has been suggested that the prod- ucts of the muscle-waste, usually washed away by the blood- stream, in such an experiment and after death, collect and act as a. stimulant to the muscle, causing it to remain in permanent contraction. Th€ other constituents of dead muscle and their relative properties may be learned from the following table (Von Bibra) : Water 744*5 Solids: Myosin, elastic substance, etc., in- soluble in water. 155'4 Soluble proteids .* . 19'3 Gelatin 207 Extractives and salts 37'1 Fats 23-0 255 5—255-5 Total 1,000 Among the extractives of muscle very important is creatin ('2 to "3 per cent), a nitrogenous crystalline body. Certain allied forms, as xanthin, hypoxanthin (sarkin), karnin, taurin and uric acid, are also found. Glycogen (animal starch), very abundant in all the tissues, including the muscles of the embryo, is found in small quantity in the muscles of the adult ; and in the heart-muscle a peculiar sugar {inosit) is present. It is, of course, very difficult to say to what extent the bodies known as extractives exist in living muscle, though that glyco- gen, fats, and certain salts are normally present admits of little doubt. APPLICATIONS OF THE GRAPHIC METHOD. 195 There is a coloring matter in muscle, more abundant in the red muscles of certain animals than the pale, allied to heemo- globin, if not identical with that body. It may be stated as a fact, the exact significance of which is unknown, that during contraction the extractives soluble in water decrease, while those soluble in alcohol increase. It will, however, be very plain, from what has been stated in this section, that life processes and chemical changes are closely associated, and to realize this is worth much to the student of Nature. Thermal Changes in the Contracting Muscle. Since very marked chemical changes accompany muscular contraction, it might be expected that there would be some modification in temperature, and probably in the direction of elevation. Experiment proves this to be the case. If a ther- mometer finelj" graduated be kept among the muscles of the limb of a mammal during the contractions that follow the stimulation of the main nerve, a decided rise of temi^erature may be noted during the prolonged tetanus that may be thus originated. True, during the contraction of a set of muscles under such circumstances, there is a possible fallacy, from the excess of blood going to the parts owing to dilatation of the blood-vessels, which it would be necessary to exclude — i. e., we must either ascertain that such does not take place, or take it into account as a factor in the causation of the rise of tempera- ture. However, by using a delicate thermopyle, a muscle to which no blood passes may be shown to grow warmer during contraction. But why should a muscle when at rest, as may be shown, maintain a certain temperature, unless chemical changes are constantly taking place ? As already stated, such is the case, and the rise on passing into tetanus is simply an expression of increased chemical action. What is the nature of the combustion originating this heat ? Are certain crude materials withdrawn from the blood and burned up directly in the muscle-substance ; or is the muscle itself continuously building up and tearing down its own sub- stance, all of which implies oxidation ? All attempts to explain the facts apart from the latter view have been unsuccessful, and we are forced to conclude that such is the synoptical statement of the life-history of muscle. 196 ANIMAL PHYSIOLOGY. No machine known to ns resembles muscle except super- ficially. The steam-engine changes fuel into heat and mechani- cal motion, but there the resemblance ends. Muscle changes its food, or fuel, not directly into either heat or motion, but into itself ; yet as a machine it is more effective than the steam- engine, for more work and less heat are the outcome of its activity than is the case with the steam-engine. Ci ft c The Physiology of Nerve. Muscle and nerve are constantly associated functionally, and have so much in common that it becomes desirable to study them together. Much that has been established for muscle holds equally well for nerve ; and the latter, though apparently wholly different in structure at first sight, is really not so. Nerve has its protoplasmic part (axis-cylinder), which is the essential structure, its protective sheaths, and its nuclei (nerve- corpuscles) As already indicated, a nerve possesses irritability, and, since a muscle does not respond to an electric current sent through a nerve except when there is a sudden change in the strength of the current, it becomes interesting to learn why this should be the case. Experimental. — In Fig. 190 are shown ■\ R = rest ; C = contraction. Electrical Organs. — Electrical properties can be manifested by a large number of fishes ; and the subject is of special theoretical interest. It is now established that the development of electrical organs points to their being specially modified muscles — tissues, in fact, in which the contractile substance has disap- peared and the nervous elements become predominant and peculiar. No work is done, but the whole of the chemical energy is represented by electricity. Functionally an electric organ (which usually is some form of cell, on the walls of which nerves are distributed, in- closing a gelatinous substance, the whole being very suggestive of a galvanic battery) closely re- sembles a muscle-nerve prepara- tion or its equivalent in the nor- mal body. The electric organs ex- perience fatigue ; have a latent period; their discharge is tetanic (interrupted) ; is excited by me- chanical, thermal, or electrical stimuli ; and the effectiveness of the organs is heightened by elevation of temperature, and the reverse by cooling, etc. Fig. 192.— The electric - fish torpedo, dis- sected to show electric apparatus (Huxley), h, branchiae ; c, brain ; e, electric organ; assage of a nervous impulse and to study electrical phe- nomena generally to some extent. Putting all that is known together, it would appear that, without referring to minor dif- ferences which unquestionably exist, the muscle and nerve physiology of man corresponds pretty closely with that of one of the highest mammals, and, as compared with the lower ver- tebrates, his muscles and nerves possess an irritability of a very exalted type, with, however, a corresponding loss or dif- ference in other directions. 208 ANIMAL PHYSIOLOGY. Summary of the Physiology of Muscle and Nerve. — The move- ments of a muscle are distinguished from those of other forms of protoplasm by their marked definiteness and limitation. The contraction of a muscle-fiber (cell) results in an increase in its short transverse diameter, and a diminution of its long diameter, without appreciable change in its total bulk. Muscle and nerve are not automatic, but are irritable. Though muscle normally receives its stimulus through a nerve, it possesses independent irritability. Stimuli may be mechanical, chemical, thermal, electrical, and in the case of muscle, nervous ; and to be effective they must be applied suddenly and last for a brief but appreciable time. Electrical stimulation, especially, is only effective when there is a sudden change in the force or direction of the cur- rents. This applies to both muscle and nerve. A muscular contraction consists of three phases : the latent period, the period of rising, and the period of falling energy, or of contraction and relaxation. When the phase of relaxation is minimal and that of con- traction approaches continuity, a tetanus results. The contrac- tions of the muscles in situ are tetanic, and are accompanied by a low sound, evidence in itself of their vibratory character. The prolonged contraction of a muscle leads to fatigue; owing in part, at least, to the accumulation of waste-products within the muscle which depress its energies. This is a necessary consequence of the fact that all proto- plasmic activity is accompanied by chemical change, and that some of these processes result in the formation of products which are hurtful and are usually rapidly expelled. Muscular contraction is accompanied by chemical changes, in which the formation of carbon dioxide, and some substance that causes an acid reaction to take the place of an alkaline or neutral one. Since free oxygen is not required for the act of contraction, but is still used up by a contracting muscle, it may be assumed that the oxygen that plays a part in actual con- traction is intra-molecular. Chemical changes are inseparable from the vital processes of all protoplasm, and the phenomena of muscle show that they are constantly in operation, but exalted during ordinary contraction and that tetanic condition which precedes and may end in coagulation of muscle plasma and the formation of myosin. The latter is a result of the disorganization of muscle, and has points of resemblance to the coagulation of the blood. APPLICATIONS OF THE GRAPHIC METHOD. 209 The contraction of a muscle, and the passage of a nervous impulse, are accompanied by electrical changes. Whether cur- rents exist in uninjured muscle and nerve is a matter of con- troversy. All physiologists agree that they exist in muscle (and nerve) during functional activity. This electrical condi- tion is termed the " negative variation " by those believing in currents of rest, and the " current of action " by those holding opposite opinions. The current is of momentary duration, and is manifested during the latent period of muscle, in which also the chemical changes take place ; so that a muscular contrac- tion must be regarded as the outcome of the events of the latent period, which is, therefore, tliough the shortest, the most important of the phases of a muscular contraction. During the passage of a constant (polarizing) current from a battery through a nerve, it undergoes a change in its irrita- bility and shows a variation in the electro-motive force of the ordinary nerve-current (electrotonus). This fact is of thera- peutic importance. The electrical phenomena of nerve are alto- gether more prominent than the chemical, the reverse of which is true of muscle. The activity of a muscle (and nerve proba- bly) is accompanied by the generation of heat, an exaltation of which takes place during muscular contraction. Rigor mortis causes an increase in temperature and the chemical interchanges which accompany the other phenomena. A muscle may also become rigid by passing into rigor caloris. Living muscle is translucent, alkaline or neutral in reaction, and elastic ; dead muscle, opaque, acid in reaction, and devoid of elasticity, but firmer than living muscle, owing to coagula- tion of the muscle-plasma. Dead nerve undergoes similar changes. The elasticity of muscle is restricted but perfect within its own limits. It differs from that of inorganic bodies in that the increments of extension are not directly proportional to the in- crements of the weight. When overstretched, muscle does not return to its original length (loss of elasticity), lience the serious nature of sprains. It is important to regard muscular elasticity as an expres- sion of vital properties. The work done by a muscle is ascertained by multiplying the load lifted by the height; and the capacity of an individual muscle will vary with its hiugth, the arrangement of its libers, and the area of its cross-secti(jn (i. e., on the number of fibers). The work done may be regarded as a function of the resist- 14 210 ANIMAL PHYSIOLOGY. ance (load)^ as the contraction is also a function of the stimulus. The separation of a muscle from its nerve by section of the lat- ter leads to certain changes^ most rapid in the nerve;, which show that the two are so related that prolonged independent vitality of the muscle is impossible, and make it highly proba- ble that muscle is constantly receiving some beneficial stimulus from nerve^ which is exalted and manifest when contraction takes place. The study of the development of the electrical cells of cer- tain fishes shows that they are greatly modified muscles in which contractility, etc., has been exchanged for a very decided exaltation of electrical properties. It is likely, though not demonstrated, that all forms of protoplasm undergo electrical changes — that these, in fact, like chemical phenomena, are vital constants. The phases of the contraction of smooth muscular tissue are all of longer duration ; the contraction- wave passes in different directions, and may spread into cells devoid of nerves, which we think not unlikely also to be the case, though less so, for all forms of muscle. The smooth muscle-cell must be regarded as a more primi- tive, less specialized, form of tissue. Variations in all the phe- nomena of muscle with the animal and the circumstances are clear and impressive. Finally, muscle illustrates an evolution of structure and function, and the law of rhythm. THE NEEVOUS SYSTEM.— GENERAL CONSIDERATIONS. Since in the higher vertebrates the nervous system is domi- nant, regulating apparently every process in the organism, it will be well before proceeding further to treat of some of its functions in a general way to a greater extent than we have yet done. Manifestly it must be highly important that an animal shall be able to place itself so in relation to its surroundings that it may adapt itself to them. Prominent among these adaptations are certain movements by which food is secured and dangers avoided. The movements having a central origin, a peripheral mechanism of some kind must exist so as to place the centers in connection with the outer world. Passing by the evolution of the nervous system for the present, it is found that in verte- brates generally there is externally a modification of the epi- THE NERVOUS SYSTEM— GENERAL CONSIDERATIONS. 211 thelial covering of the body (end-organ) iu wliich a nerve ter- minates, which latter may be traced to a cell or cells removed from the surface [cenier), and from which in most cases other nerves proceed. The nervous system, we may remind the student, consists in vertebrates of centers in which nerve-cells abound, united by nerve-fibers and by the most delicate form of connective tissue known, in connection with which there are incased strands of protoplasm or nerves as outgrowths. The main centers are, of course, aggregated in the brain and spinal cord. It is possible to conceive of the work of a nervous system carried on by a single cell and an afferent and efferent nerve ; but inasmuch as such an arrangement would imply that the central cell should act the part of both receiving and origi- nating impulses (except it were a mere conductor, in which case there would be no advantage whatever in the existence of a cell at all), according to the principle of the physiological division of labor, we might expect that there would be at least two cen- tral cells — one to receive and the other to transmit impulses — or at least that there should be some specialization among the central cells ; and we shall have good reason later to believe that this has reached a surprising degree in the highest ani- mals. Moreover^ it would be a great advantage if the termination of the ingoing (afferent) nerve should not lie exposed on the surface, but be protected by some form of cell that had also the power to transmit to it the impressions received from without, in a form suitable to the nature of the nerve and the needs of the organism. So that a complete mechanism in its simplest form would furnish : 1. A peripheral cell or nerve end-organ. 3. An affer- ent or sensory nerve. 3. Two or more central cells. 4. An efferent nerve, usually connected with — 5. A muscle or other form of cell, the action of which may be modified by the out- going nerve, or, as we should prefer to say, by the central nerv- ous cells through the efferent nerve. The advantages of the principal cells being within and protected are obvious. When, then, an im^jression made on the perifjlieral cell is carrieulse of 81, the time occupied in the completion of the course of the blood from and to the heart would be f4- = 3 ; i. e,, the circulation is completed three times in one minute, or its period is twenty seconds; and it is to be well borne in mind that by far the greater part of this time is occupied in travers- ing the capillaries. 226 ANIMAL PHYSIOLOGY. The Circulation under the Microscope. There are few pictures more instructive and impressive than a view of the circulation of the blood under the microscope. It is well to have similar preparations^ one under a low power and another under a magnification of 300i:o 500 diameters. With the former a ^ew of arterioles, veins, and capillaries may be Fig. 205.— Portion of the web of a frog's foot as seen under a low magnifying power, showing the blood-vessels, and in one corner the pigment-spots (after Huxley), a, small arteries (arterioles) ; v, spiall veins. The smaller vessels are the capillaries. The course of the blood is indicated by arrows. obtained at once. Many different parts of animals may be used, as the web of the frog's foot, its tongue, lung, or mesentery ; the gill or tail of a small fish, tadpole, etc. The relative size of the vessels ; the speed of the blood-flow ; the greater velocity of the central part of the stream ; the aggre- gation of colorless corpuscles at the sides of the vessels, and the occasional passage of one through a capillary wall, when the exposure has lasted some time ; the crowding of the red cells ; their plasticity ; the small size of some of the capillaries, barely THE CIRCULATION OF THE BLOOD. 227 allowing the corpuscles to be squeezed through ; the changes in the velocit)^ of the current, especially in the capillaries ; its pos- sible arrest or retrocession ; the velocity in one so much greater than in its neighbor, without very obvious cause — all this and much more forms, as we have said, a remarkable lesson for the thinking student. This, like all microscopic views, especially if motion is represented, has its fallacies. It is to be remem- Fio. 206. — Circulation in the web of the frog's foot (Wagner). F, venous trunk composed of the three principal branches (v, v, v), covered with a plexus of smaller vessels. The whole is dotted over with pigment masses. bered that the movements are all magnified, or else one is apt to suppose the capillary circulation extremely rapid, whereas it is like that of the most sluggish part of a stream, and very irregular. The Characters op the Blood-Flow. If an artery be opened, the blood is seen to flow from it in a constant stream, with periodic exaggerations, which, it is found, answer to the heart-beats ; in the case of veins and capillaries the flow is also constant, but shows none of the spurting of the arterial stream, nor has the cardiac beat appar- ently an equal modifying effect upon it. We have already explained why the flow should be constant, though it would 1)0 well to be clearer as to the peripheral re- sistance. The anKJunt of friction from linings so smooth as 228 ANIMAL PHYSIOLOGY. those of the blood-vessels can not be considerable. Whence, then, arises that friction which keeps the arterial vessels always distended by its backward influence ? The microscopic study of the circulation helps to answer this question. The plas- ticity of the corpuscles and of the vessel walls themselves must be taken into account, in consequence of which a drag- ging influence is exerted whenever the corpuscles touch the wall, which must constantly happen with vast numbers of them in the smallest vessels and especially in the capillaries. The arrangement of capillaries into a mesh-work, must also, in consequence of so many angles, be a source of much friction. The action of the corpuscles on one another may be com- pared to a crowd of people hurrying along a narrow passage — the obstruction comes from interaction of a variety of forces, owing to the crowd itself rather than the nature of the thor- oughfare. We must set down a great deal to the influence of the corpuscles on one another, as they are carried along, accord- ing to mechanical principles ; but, as we shall see later, other and more subtile factors play a part in the capillary circulation. Owing to the peripheral resistance and the pumping force of the heart, the arteries become distended, so that, during cardiac diastole, their recoil, owing to the closure of the semilunar valves, forces on the blood in a steady stream. It follows, then, that the main force of the heart is spent in distending the arteries, and that the immediate propelling force of the circu- lation is the elasticity of the arteries in which the heart stores up the energy of its systole for the moment, Blood-Pressure. Keeping in mind our schematic representation of the circu- lation, we should expect that the blood must exercise a certain pressure everywhere throughout the vascular system ; that this blood-pressure would be highest in the heart itself ; considera- ble in the whole arterial system, though gradually diminishing toward the capillaries, in which it would be feeble ; lower still in the smaller veins ; and at its minimum where the great veins enter the heart. Actual experiments confirm the truth of these views ; and, as the subject is one of considerable importance, we shall direct attention to the methods of estimating and re- cording an animal's blood-pressure. First of all, the well-known fact that, when an artery is cut, the issuing stream spurts a certain distance, as when a water- THE CIRCULATIOX OF THE BLOOD. 229 main, fed from an elevated reservoir, bursts, or a hydrant is opened, is itself a proof of the existence of blood-pressure, and is a crude measure of tlie amount of the pressure. One of the simplest and most impressive ways of demon- strating blood-pressure is to connect the carotid, femoral, or other large artery of an animal by means of a small glass tube (drawn out in a peculiar manner to favor insertion and reten- tion by ligature in the vessel), known as a cannula, by rubber tubing, with a long glass rod of bore approaching that of the artery opened, into which the blood is allowed to flow through the above-mentioned connections, while it is maintained in a vertical position. To prevent the rapid coagulation of the blood in such ex- periments, it is customary to fill the cannula and other tubes to a certain extent, at least, with a solution of some salt that tends to retard coagulation, such as sodium carbonate or bicar- bonate, magnesium sulphate, etc. If other connections are made in a similar way with smaller arteries and veins, it may be seen that the height of the respective columns, representing the blood-pressure, varies in each and in accordance with ex- pectations. While all the essential facts of blood-pressure and many others may be illustrated by the above simple methods, it is inadequate when exact measurements are to be made or the results to be recorded for permanent preservation ; hence appa- ratus of a somewhat elaborate kind has been devised to accom- plish these purposes. The graphic methods are substantially those already ex- plained in connection with the physiology of muscle; but, since it is often desirable to maintain blood-pressure experi- ments for a considerable time, instead of a single cylinder, a series so connected as to provide a practically endless roll of paper (Fig, 208) is employed. When, in tlie sort of experiments referred to above, the height of the fluid used in the glass tube to prevent coagula- tion just suffices to prevent outflow from the artery into the connections, we have, of course, in this a measure of the blood- pressure ; however, it is convenient in most instances to use mercury, contained in a glass tube bent in the form of a U, for a measure, as shown in the subjoined illustration. It is also desirable, in order to prevent outflow of the blood into the apparatus, to get up a pressure in the U-tube or manomc^ter as near as may be equal U) that of the animal to be employed in 230 ANIMAL PHYSIOLOGY. Fig. 207.— Apparatus used in making a blood-pressure experiment (after Foster), pb^ pressure- bottle, elevated so as to raise the pressure several inches of mercury, as seen in the ma- nometer (m) below. It contains a saturated solution of sodium carbonate ; r.t. rubber tune connecting the pb with the leaden tube ; U, tube made of lead, so as to be pliable, Jft have rigid walls ; s.c.a, stop-cock, the top of which is removable, to allow escape of bubbles ot a§ : », the pen, writing on the roll of paper, r. The former floats on the mercury ; .w. the manoiiieter, the shaded portion of the bent tube denoting the mercury, the rest is fUled with a fluid unfavorable to the coagulation of the blood, and derived from the pressure- THE CmCULATIOX OF THE BLOOD. 231 bottle ; ca, the carotid, in which is placed the canula, and below the latter a forceps, which may be removed when the blood-pressure is to be actuallj' measured. The registration of the height, variation, etc., of blood-pressure, is best made on a continuous roll of paper, as seen in Fig 208. the experiment. This may be effected in a variety of ways, one of the most convenient of which is by means of a vessel containing some saturated sodium carbonate or similar solu- tion in connection with the manometer. It is important that the pressure should express itself as directly and truthfully on the mercury of the manometer as possible, hence the employment of a tube with rigid walls, yet capable of being bent readily in different directions for the sake of convenience. Mercury, on account of its inertia, is not free from objec- tion ; and when very delicate variations in the blood-pressure — e. g., feeble pulse-beats — are to be indicated, it fails to express them, in which case other fluids may be employed. Fio. 208.— Large kj-mograph, with continuous roll of paper (Foster). The clock-work ma- chinery unroU.s the paper from the roll C, carries it smoothly over the cylinder B, and then wind.s'it up int<^) the roll A. Two electro-magnetic markers are seen in position recording Intervals of time on the moving roll of paper. A manometer may be fixed in any con- venient position. It will be noted that when an ordinary cannula is used, in- serted as it is lengthwise into the blood-vessel, the i)ressure recorded is not that on the side of the vessel into which it is inserted as when an- piece is used, but of the vessel, of which the one in question is a branch. The blood-pressure, in the main arterial trunk for example, must depend largely on the force of the heart-beat ; consequently it would be expected, and it 232 ANIMAL PHYSIOLOGY. is actually found, that the pressure varies for different animals, size having, of course, in most instances a relation to the result. It has been estimated that in the carotid of the horse the arte- rial pressure is 150 to 200 mm. of mercury, of the dog 100 to 175, of the rabbit 50 to 90. Man's blood-pressure is not known, but is probably high, we may suppose not less than 150 to 200 mm. After the fact that there is a certain considerable blood- pressure, the other most important one to notice is that this blood-pressure is constantly varying during the experiment, and, as we shall give reason to believe, in the normal animal ; and to these variations and their causes we shall presently turn our attention. THE HEART. The heart, being one of the great centers of life, to speak figuratively, it demands an unusually close study. The Cardiac Movements. There is no special difficulty in ascertaining the outlines of the heart by means of percussion on either the dead or the living subject. Quite otherwise is it with the changes in form which accompany cardiac action. Attempts have been made to ascertain the alterations in position of the heart with respect to other parts, and especially its own alterations in shape dur- ing a systole, the chest being unopened, by the use of needles thrust into its substance through the thoracic walls ; but the results have proved fallacious. Again, casts have been made of the heart after death, in a condition of moderate extension, prior to rigor mortis ; and also when contracted by a hardening fluid. These methods, like all others as yet employed, are open to serious objections. Following the rapidly beating heart of the mammal with the eye produces uncertainty and confusion of mind. We look to instantaneous photography to furnish a possible way out of the difficulty. It may be very confidently said that the mode of contrac- tion of the hearts of different groups of vertebrates is variable, though it seems highly probable that the divergences for mam- mals are slight. The most that can be certainly affirmed of the mammalian heart is, that during contraction of the ventri- cles they become more conical ; that the long diameter is not appreciably altered ; that the antero - posterior diameter is THE CIRCULATION OP THE BLOOD. 233 lengthened ; and that the left ventricle at least turns on its own axis from left to right. This latter may be distinctly made out by the eye in watching the heart in the opened chest. The Impulse of the Heart. When one places his hand over the region of the heart in man and other mammals, he experiences a sense of pressure varying with the part touched, and from moment to moment. Instruments constructed to convey this movement to recording levers also teach that certain movements of the chest wall cor- respond with the propagation of the pulse, and therefore to the systole of the heart. It can be recognized, whether the hand or an instrument be used, that all parts of the chest wall over the heart are not equally raised at the one instant. If the beat- ing heart be held in the hand, it will be noticed that during systole there is a sudden hardening. The relation of the apex to the chest wall is variable for different mammals, and with different positions of the body in man. As a result of the investigation which this subject has re- ceived, it may be inferred that the sudden tension of the heart, owing to the ventricle contracting over its fluid contents, causes in those cases in which during diastole the ventricle lies against the chest wall, a sense of pressure beneath the hand, which is usually accompanied by a visible movement upward in some part of the thoracic wall, and downward in adjacent parts. The exact characters of the cardiac impulse are very variable with different human subjects. The term " apex-beat " is fre- quently employed instead of cardiac impulse, on the assump- tion that the apex of the heart is brought into sudden contact with the thoracic walls from which it is supposed to recede during diastole. But, in some positions of the body at all ev^ents in a certain proportion of cases, the apex of the heart lies against the chest wall during diastole, so that in these instances certainly such a view would not be wholly correct. But we would not deny that in some subjects there may be a genuine knock of the apex against the walls of the chest during the ventricular systole. It will not be forgotten that the heart lies in a pericardial sac, moistened with a small quantity of allmminous fluid ; and that by this sac the organ is tethered to the walls of tin; chest by its mediastinal fastenings; so that in receding from the chest wall the latter may be drawn after it ; though this might 234 ANIMAL PHYSIOLOGY. also follow from the intercostal muscles being simply unsup- ported when the heart recedes. Investigation of the Heart-Beat from within. By the use of apparatus introduced within the heart of the mammal and reporting those changes susceptible of graphic record, certain tracings have been obtained about the details of Fig. 209.— Marey's cardiac sound which may be used to explore the chambers of the heart (after Foster), a, is made of rubber stretched over a wire framework, with metallic supports above and below ; 6, is a long tube. which there are uncertainty and disagreement, though they seem to establish the nature of the main features of the cardiac beat clearly enough. An interpretation of such traciijgs in the Right auricle. Right ventricle. Cardiac impulse. Fig. 210.— Simultaneous tracings from the interior of the right auricle, from the interior of the right ventricle, and of the cardiac impulse, in the horse (after Chauveau and Marey). Tracings to be read from left to right, and the references above are in the order from top to bottom. A complete cardiac cycle is included between the thick vertical lines I and II. The thin vertical lines indicate tenths of a second. The gradual rise of pressure within the ventricle (middle tracing) during diastole, the sudden rise with the systole, its maintenance with oscillations for an appreciable time, its sudden fall, etc., are all well shown. There is disagreement as to the exact meaning of the minor curves in the larger ones. light of our general and special knowledge warrants the fol- lowing statement. THE CIRCULATION OF THE BLOOD. 235 1. Both auricular and ventricular systole are sudden, but the latter is of very much greater diiration. 2. While the chest wall feels the ventricular systole, the au- riculo-ventricular valves shield the auricle from its shock. 3. During diastole in both chambers the pressure rises gradually from the inflow of blood ; and the auricular contrac- tion produces a brief, decided, though but slight rise of press- ure in the ventricles. 4. The onset of the ventricular systole is rapid, its maximum pressure suddenly reached, and its duration considerable. The relations of these various events, their duration, and the corresponding movements of the chest wall, may be learned by a study of the above tracing which the student will find worthy of his close attention. The Cardiac Sounds. Two sounds, differing in pitch, duration, and intensity, may be heard over the heart, when the chest is opened and the heart listened to by means of a stethoscope. These sounds may also be heard, and present the same characters when the heart is auscultated through the chest wall ; hence the cardiac im- pulse can take no essential part in their production. The sounds are thought to be fairly well represented, so far as the human heart is concerned, by the syllables lub, chqj; the first sound being longer, louder, lower-pitched, and " boom- ing " in quality ; the second short, sharp, and high-pitched. In the exposed heart, the first sound is heard most distinct- ly over the base of the organ or a little below it ; while the sec- ond is communicated most distinctly over the roots of the great vessels — that is to say, both sounds are heard best oyer the auriculo- ventricular and semilunar valves respectively. When the chest wall intervenes between the heart and the ear, it is found that the second sound is usually heard most distinctly over the second costal cartilage on the right ; and the first in the fifth costal interspace where the heart's impulse is also often most distinct. In these situations the arch of the aorta in the one case, and the ventricular walls in the other, are close to the situations referred to during the cardiac systole; hence it is inferred that, though the sounds do not originate directly beneath these spots, they are best propagated to the chest wall at these points. There are, however, individual differences, owing to a va- 236 ANIMAL PHYSIOLOGY. riety of causes, which it is not always possible to explain fully in each case, but owing doubtless in great part to variations in the anatomical relations. The Causes of the Sounds of the Heart. — There is general agree- ment in the view that the second sound is owing to the closure of the semilunar valves of the aortic and pulmonary vessels ; the former, owing to their greater tension inconsequence of the higher blood-pressure in the aorta, taking much the larger share in the production of the sound, as may be ascertained by listen- ing over these vessels in the exposed heart. When these valves are hooked back, the second sound disappears, so that there can be no doubt that they bear some important relation to the cau- sation of the sound. In regard to the first sound of the heart the greatest diver- sity of opinion has prevailed and still continues to exist. The following among other views have been advocated by physi- ologists : 1. The first sound is caused by the tension and vibration of the auriculo-ventricular valves. 2. The first sound is owing to the contractions of the large mass of muscle composing the ventricles. 3. The sound is directly traceable to eddies in the blood. In favor of the first view it was argued that by agreement the second sound was valvular, and why not the first ? — And again that malformations of the valves gave rise to " murmurs " (" bruits"), which either obscured or replaced the true sound. The second opinion was supported by the fact that the larger the heart the more powerful the sound ; that when the blood was cut off from the heart by ligature of the vessels success- ively, the sound could still be heard ; that with fatty degenera- tion of the muscle-fibers of the heart, it had been found that the sound was weak — and similar arguments. Recently it has been contended very strongly that the first sound may be heard by a double stethoscope placed over an ex- cised, bloodless, mammalian heart, or even ventricle, while it still beats. The third opinion was less vigorously upheld, but certain experiments and physical phenomena were pointed to in sup- port of it. Against the arguments adduced above it may be stated that the first sound may be conceived as overpowered by a bruit without being replaced by it in the proper sense of the word. It is well known that the cardiac muscle is peculiar, occupying THE CIRCULATIOX OF THE BLOOD. 237 in structure a position intermediate between the striped vol- untary fibers and tlie smooth muscle-cells. Numerous investi- gations have shown that the heart is not susceptible of true Fig. 2n. Fig. 212. Fig. 211.— Microscopic appearances of fibers from the heart. The cross-striae, divisions (branching), and junctures are visible (Landois). Fig. 212.— Muscular fiber-cells from the heart. (.1 x 425.) a, line of juncture between two cells ; 6, c, branching cells. tetanic contraction, certainly not the heart of the mammal ; so that it is customary to term the cardiac contraction peristaltic. If this view be correct, how could there be a sound produced by muscular contraction alone ? To this it has been replied that the sudden tension of the ventricular wall when tightened over the blood may give rise to vibrations that account for the sound ; and recent investigations have shown that the vibrations that give rise to the sound emitted by a contracting skeletal muscle may be fewer than was once supposed. The statement that a sound may be heard from the excised ventricle under the circumstances above mentioned has not been denied ; but its source has been traced to the action of the heart wall against the stethoscope — i. e., some believe the sound to be, in this case, of extrinsic origin. Most physicians would be very loath to abandon the view that the valves are always to be taken into serious account as a factor in the causation of the sound. But, looking at the whole question broadly, is it not unrea- sonable to exphiin the sound resulting from such a complex act as the contraction of the heart and what it implies in the light of any single factor ? That such narrow and exclusive views should liave been })ro[)agatod, even by eminent j)hysi()logists, should admonisli tlie student to receive witli great caution ex- 238 ANIMAL PHYSIOLOGY. planations of the working of complex organs, based on a single experiment, observation, or argument of any kind. The view we recommend the student to adopt in the light of our present knowledge is, that the first sound is the result of several causative factors, prominent among which are the sud- den tension of the auriculo-ventricular valves, and the contrac- tion of the cardiac muscle, not leaving out of the account the possible and probable influence of the blood itself through eddies or otherwise ; nor would we ridicule the idea that in some cases, at all events, the sound may be modified in quality and intensity by the shock given to the chest wall during sys- tole. Endo-Cardiac Pressures. Bearing in mind the relative extent of the pulmonary and systemic portions of the circulation, we should suppose that the resistance to be overcome in opening the aortic valves and lifting the column of blood that keeps them pressed together, would be much greater in the left ventricle than in the right ; or, in other words, that the intra-ventricular pressure of the left side of the heart would greatly exceed that of the right, and this is confirmed by actual experiment. By means of an instrument known as the maximum and minimum manometer, the highest and lowest pressure within any chamber of the heart may be learned approximately. As a specimen measurement it may be stated that it has been found that in a dog the greatest pressure was 140 mm. of mer- cury for the left ventricle, for the right only 60, and for the right auricle 20. But it is also found — a matter not quite so obvious — that a minimum pressure proportionate to the maxi- mum may exist in all the chambers of the heart ; and the press- ure may fall below that of the atmosphere, or be negative. By the same method it was found that in a dog the negative pressure varied between —52 and —20 mm. of mercury for the left ven- tricle and —17 to —16 mm. for the right, with —12 to —7 mm. for the right auricle. As will be shown later, part of this diminished pressure is due to the effect of the respiratory movements ; and, indeed, more recent experiments seem to show that ordinarily, with the heart beating with its usual rate and force, the nega- tive pressure or suck from its own action is comparatively slight. The discussion of the cause of this negative pressure, like the related subject of the cause of the heart's diastole, has given rise to much difference of opinion. THE CIRCULATION OP THE BLOOD. 239 Some find it difficult to understand how the heart after sys- tole may regain its original form apart from the assistance of diastolic muscles, which are assumed to act so as to antagonize those causing systole. Others think the elasticity of the heart's muscle sufficient of itself to account for the organ's return to its original form. But there is surely a misconception involved in both of these views. If small portions of the heart of the frog, tortoise, or other cold-blooded animal, just removed from the body, be observed under a microscope it will be seen that they alternately con- tract and relax. Now, it is only necessary to sujjpose that the relaxation of the heart is complete after each systole, to under- stand how even an empty heart regains its diastolic form. That there should be a negative pressure in, say, the left ventricle, follows naturally enough from the fact that not only are the contents of the ventricle expelled with great sudden- ness, but that its walls remain (see Figs. 210 and 214) pressed together for a considerable portion of the time occupied by the whole systole ; so that in relaxation it follows that there must FlO. 213.— Diajrram showing Die relative lieitclif, of the hlood-pressure in (lifTereiif ])art8 of the vfttwjiilar svHtem (after Yeo). /(, heart ; ii, art-erioles ; v, Ktnall veins ; ,(, arterieH ; c, cap- illarieH ; I , liirge veins : //. V, representing Uio zero-line, i. e., atrnoHjilieric pressure ; the >)loulsations \Hir minute (after Gamgee) : 244 ANIMAL PHYSIOLOGY. SPECIES. Adult. Youth. Old age. Horse 36- 40 46- 50 45- 50 70- 80 70- 80 90-100 120-140 60- 72 65- 75 60- 70 85- 95 100-110 110-120 120-140 32- 88 Ass and irnile 55- 60 Ox 40- 45 Sheep and goat 55- 60 Pig' 55- 60 i^^fe • Doff 60- 70 Cat 100-120 The variations with, age, for the horse and the ox, are as fol- lows, according to Kreutzer : Horse. At birth 100-120 When 14 days old 80-96 When 3 months old 68-76 When 6 months old 64-72 When 1 year old 48- 56 When 2 years old 40-48 When 3 years old 38-48 When 4 years old 38-50 When aged 32-40 Ox. At birth 92-132 When 4-5 days old 100-120 When 14 days old 68 When 4-6 weeks old 64 When 6-12 months old 56-68 For the young cow 46 For the four-year-old ox 40 The Pulse. Naturally the intermittent action of the heart gives rise to corresponding phenomena in the elastic tubes into which it may be said to be continued, for it is very desirable to keep in mind the complete continuity of the vascular system. The following phenomena are easy of observation : When a finger-tip is laid on any artery, an interrupted pressure is felt ; if the vessel be laid bare (or observed in an old man), it may be seen to be moved in its bed forward and upward ; the press- ure is less the farther the artery from the heart ; if the vessel be opened, blood flows from it continuously, but in spurts ; if one finger be laid on the carotid and another on a distant ves- sel, as one of the arteries of the foot, it may be observed (thou^ it is not easy, from difficulty in attending to two events hap- pening so very close together) that the beat in the nearer ves- sel precedes by a slight interval that in the more distant. Investigating the latter phenomenon with instruments, it is found that an appreciable interval, depending on the distance apart of the points observed, intervenes. What is the explanation of these facts ? The student may get at this by a few additional observa- tions that can be easily made. THE CIRCULATION OF THE BLOOD. 245 If water be seut through a long elastic tube (so coiled that points near and remote may be felt at the same time) by a bulb syringe, imitating the heart, and against a resistance made by drawing out a glass tube to a fine point and inserting it into the terminal end of the rubber tube, an intermittent pressure like that occurring in the artery may be observed ; and further Fig. 215. — Marej-'s apparatus for showing the mode in which the pulse is propagated in the arteries. B, a rubber pump, with valves to prevent regurgitation. The working of the apparatus will be apparent from the inspection of the figure. that it does not occur at precisely the same moment at the two points tested. Information more exact, though possibly open to error, may be obtained by the use of more elaborate apparatus, and the graphic method. Fig. 210 gives an idea of the main features of the pulse-trac- ings of an arterial scheme or arrangement of tubes in supposed imitation of the conditions existing in the vascular system of the mammalian body. Attention is especially directed to the abrupt ascent, the more gradual descent, and the secondary waves, which are either waves of oscillation or reflex waves. It may also be noticed that the rise is later as the part of the tube at which it occurs is more distant from the pump ; also that it gets gradually less in height and at the same time that all the secondary waves are diminished or totally disap- pear; and with the exception of the latter these results hold g<^)od of the pulse in the arteries of a living animal. By measurement it has been ascertained that in man the puLse- wave travels at the rate of from five to ten metres per sec- ond, being of course very variable in velocity. It would seem that the more rigid the arteries the m<^re rapid the rate, for in 246 ANIMAL PHYSIOLOGY. children with their more elastic arteries the speed is slower ; and the same principle is supposed to explain the higher veloci- jn 0.20 < Fig. 316.— Pulse-curves described by a series of sphygmographic levers placed 20 cm. apart along an elastic tube into which fluid is forced by the sudden stroke of a pump. The arrows indicate the onward and the reflected waves. The gradual flattening and total or partial extinction of the waves are noteworthy (after Marey). ty noticed in the arteries of the lower extremities. But with such a speed as even five metres a second it is evident that with a systole of moderate duration (say '3 second) the most distant arteriole will have been reached by the pulse-wave before that systole is completed. It is known that the blood- current at its swiftest has no such speed as this, never perhaps exceeding in man half a metre per second, so that the pulse and the blood-current must be two totally distinct things. THE CIRCULATIOX OF THE BLOOD. 247 The student may very simply illustrate this matter for him- self. By tapping sharply against a pipe through which a stream is flowing slowly and. quietly, a wave may be seen to arise and pass with considerable velocity along the moving water, and with a speed far in excess of the rapidity of the main current. When the left ventricle throws its six ounces of blood into vessels already full to distention, there must be consider- able concussion in consequence of the rapid and forcible nature of the cardiac systole, and this gives rise to a wave in the blood which, as it passes along its surface, causes each part of every artery in succession to respond by an elevation above the gen- eral level, and it is this which the finger feels when laid upon an artery. That there is considerable distention of the arterial system with each pulse may be realized in various ways, as by watch- ing and feeling an artery laid bare in its course, or in very thin or very old people, and by noticing the jerking of one leg crossed over the other, by which method in fact the pulse-rate may be ascertained. And that not only the whole body but the entire room in which a person sits is thrown into ^'ibration by the heart's beat, may be learned by the use of a telescope to observe objects in the room, which may thus be seen to be in motion. Features of an Arterial Pulse-Tracing. — In order to judge of the nature of arterial tracings, it is important that the circum- stances under which they are obtained should be known. The movements of the vessel wall in most mammals suit- Fio. 217.— Mar^-y's improved sphypmoj?raph arranKfl for taking a tracing. ^, stwl spring; H. first l<-v»-r ; C, »Titintf-lt;vt-r ; C. it.s free writintf end : I), screw for WxnantR h m con- ta/t with C: fi. slide with sinokea|)er ; //. clortal anastomosis ; il' . ileal ; np, splenic ; du', duodenal ; I. int, lieno-intestliial ; y, gas- trie ; J), portal ; /y", left lung ; jnd, pulmonary ; m. cu, niusculo-cutaueous ; br, brachial. '256 ANIMAL PHYSIOLOGY. Passing on to the vertebrates, in the lowest group, the fishes, the heart consists of two chambers, an auricle and a ventricle, the latter being supplemented by an extension (buTbus arterio- sus) pulsatile in certain species; and an examination of the course of the circulation will show that the heart is through- out venous, the blood being oxidized in the gills after leaving the former. Among the amphibians, represented by the frog, there are two auricles separated by an almost complete septum, and one Fig. 233. Fig 233. Fig. 232. — The frog's heart, seen from the front, the aortic arches of the left side having been removed. (1 x 4.) ca, carotid ; c. gl, carotid gland ; ao, aorta ; au', right auricle ; au". left auricle ; pr. c, vena cava superior ; p^. c. vena cava inferior ; p. cu, pulmo-cutaneous trunk ; ^^, truncus arteriosus ; v, ventricle (Howes). Fig. 233.— The same, seen from behind, the sinus venosus having been opened up to show the slnu-auricular valves. (1 x 4.) p. v. pulmonary vein ; s.v, sinus venosus ; va", sinu-au- ricular valve. Other lettering as in Fig. 232 (Howes). ventricle characterized by a spongy arrangement of the mus- cle-fibers of its walls. In the reptiles the division between the auricles is complete, and there is one ventricle which shows imperfect subdivisions. In the crocodile, however, the heart consists of four per- fectly divided chambers. Of the two aortic arches, one arises together with the pulmonary artery from the right ventricle, and, as it crosses over, the left communicates with it by a small opening, so that, although the arterial and the venous blood are completely separated in the heart, they intermingle outside of this organ. In birds the circulatory system is substantially the same as in mammals ; but in all vertebrate forms below birds the blood distributed to the tissues is imperfectly oxidized or is partially venous. THE CIRCULATION OF THE BLOOD. 257 As an example of the influence of valves and of blood-press- ure on the distribution of the blood we may take the case of the turtle, in which the subject has been most carefully studied. Fig. 2:U. Fig. :i35. Fig. 234.— The heart, dissected from the front, the ventral wall and one of the aurieulo-ven- tricular valves having been removed. (1 x 6.) The rod, passin;^ from the ventricle into the i)ylangiuni, shows the course taken by the blood flowing into the carotid and aortic trunks, sy, synangium ; p. v\ aperture of entry of pulmonary vein ; va'\ sinu-auricular valve ; «. .v, inter-auricular septum ; ra'. aiiriculo-ventricular valve ; va. s. semi-lunar valves : jjij. pylangium : ra. I. longitudinal valve (septum) of pylangium ; p. cu'. point of origin of pulmn-cutaneous trunk (Ho\v('.«.) Fig. 23.5. —Heart and arteries of a reptile (hoai. r, right, and I, left auricle ; c, carotid artery; ra. right aortic arch ; la, left aortic arch ; p, pulmonary artery ; rv, right vena cava ; Iv, left vena cava superior ; vi. vena cava inferior. The arrows indicate the course of the circulation i after Gegeubaur). The structure of the heart and the relations of its main ves- sels, etc., will probably be sufficiently clear upon an examina- tion of the accompanying figures and the descriptions beneath them. The right and left auricles pour their blood, kept somewhat apart by valves, into the eavum venosum. Two arterial arches arise from the right-hand part of this region, while the pulmonary artery is a branch carrying off blood to the lungs from the cnvum puhnnup. No vessels arise from the f-avum arteriosum. Since the blood flows in the direction of least resistance when the ventricle contracts, the venous blood of the cavum venosum passes on into the pulmonary artery in which the pressure is, of course, lower tlian in tlic aortic arches, but, as the systoh; f:ontinues, the arterial ldo(jd of the cavum arterio- 17 258 ANIMAL PHYSIOLOGY. sum crowds on the venous blood, and passes itself with, some of the darker blood into the aortic vessels, in which the arrange- BAih Fig. 236. Fig. 237. Fig. 236.— Heart and arteries of a turtle (Chelydrd). rp, right pulmonary, and Zp, left pul- monary artery ; other letters the same signification as in the previous figure (after Gegen- baur). Fig. 237. — Heart of a turtle {Chelone midas). A. Drawing from nature, the ventral face of the ventricle being laid open. B. Diagram explanatory of the circulation. Everywhere the arrows indicate the course the blood takes. R. A., L. A., right and left auricles. V, the right, w', the left median auriculo-ventricular valves. C. v, cavum venosum. C. p, cavum pulmonale, a, the incomplete septum which divides the cavum pulmonale from the rest of the cavity of the ventricle. P. A, pulmonary artery. R. Ao, L. Ao, right and left aortse (after Huxley). ment of the valves assists materially. Note that, as the systole advances, the imperfect septum between the cavum pulmonum and cavum venosum approaches the back of the heart wall, and thus tends to shut off the cavum pulmone from the purer blood. As a result of the entire arrangement, the least oxidized blood passes to the lungs, and the most aerated to the head and anterior parts of the animal. In the frog and other creatures, with three imperfectly sepa- rated heart cavities, a similar result is attained. The resemblances in such cases to the foetal conditions in mammals, including man, will be apparent, and it is especially THE CIRCULATION OF THE BLOOD. 259 to be observed that in the case of the foetus and these lower groups of vertebrates the brain and anterior parts — that is, the most important portions of the animal functionally, the parts on which the rest depend for their well-being (since the brain is the seat of all the main directive centers) — are fed with the best blood the organism possesses, a fact which probably explains in part the relatively large size of these portions of the body early in foetal life and throughout its duration. We now urge upon the student the importance of making some observations for himself upon the heart of the frog, tur- tle, snake, fish, or other of the cold-blooded animals. At- tention should be given chiefly to the functions of the heart, though to do this intelligently it must be preceded by some study of the anatomy of the organ. It will be understood that any directions we may give for the manipulative part of the work will be of the simplest kind, and rather suggestive of the general method of procedure than intended to illustrate the best methods. In reality, it is better for exact investigation of the heart that no anaesthetic be given, and an animal may be rendered insensible by a sudden blow upon the head, which, as we shall show later, may be painless. However, it will be, upon the whole, perhaps, best that the animal be given a few whiffs of ether beneath some (glass) vessel, and as soon as it becomes insensible, to withdraw the anaesthetic, remove or crush the head (brain), so that throughout the investigation there may be neither interference with the heart from this organ nor any doubt about the animal's insensibility. It is well to open the abdomen a little below the heart, so that the latter may be exposed, with its pericardium intact, when the relations of the heart to the surrounding parts may be noticed. What strikes every observer is the sluggish action of the hearts of these animals — a great advantage in attempting to estimate roughly the relative time occupied by the systole and diastole of tlie different chambers; the peculiar vermiform nature of the contraction ; the changes of color dependent on the degree to which any chamber is filled with blood; and many of those minor details important in making u]) a total general impression, but not readily (expressed in words." After the animal has been bled, the heart's action may still be profitably studi(jd ; and, finally, it may be learned that the 260 ANIMAL PHYSIOLOGY. heart will pulsate when removed, either entire or after being divided into sections. In another specimen it would be desirable to allow the heart, to be kept bathed in serum or physiological saline solu- tion, to beat as long as it will, and to note the various phases of irregularity, weakening, and cessation of action in its dif- ferent parts. It is also highly instructive to observe the eif ect of ligating oif certain of the chambers from the rest of the organ. Any one who makes a few such observations will be pre- pared to comprehend readily any of the experiments on the hearts of the cold-blooded animals, and w^U be able, especially if he has followed out earlier recommendations as to the study of the heart of the mammal, to form a mental picture of what is transpiring within his own breast, which is one of the most desirable accomplishments — in fact, the best test of real knowl- edge. Whatever ground for differences of opinion there may be as to the extent to which the phenomena we have as yet been describing are mechanical in their nature, all are agreed that such explanations are insufficient when applied to the facts with which we have yet to deal. They, at all events, can be regarded only as the result of vitality. When one reflects upon the vicissitudes through which an animal must pass daily and hourly, necessitating either that they be met by modified action of the organs of the body or that the destruction of the organism ensue, it becomes clear that the varying nutritive needs of each part must be met by changes in the circulatory system. These changes may affect any part of the entire arrangement, and it rarely happens, as will appear, that one part is modified without a correspond- ing one, very frequently of a different kind, taking place in some other. What these various correlated modifications are, and how they are brought about, we shall now attempt to describe, and it will greatly assist in the comprehension of the whole if the student will endeavor to keep a clear mental pict- ure of the parts before his mind throughout, using the figures and verbal descriptions only to assist in the construction of such a mental image. We shall begin with the vital pump — the heart. THE CIRCULATION OF THE BLOOD. 261 The Beat of the Heart and its Modifications. As lias been already noted, the cardiac muscle lias features peculiar to itself, and occupies histologically an intermediate place between the plain and the striped muscle-cells, and that the contraction of the heart is also intermediate in character, and is best seen in those forms of the organ which are some- what tubular and beat slowly. But the contraction, though peristaltic, is more rapid than is usually the case in other organs with the smooth form of muscle-fiber. The heart behaves under a stimulus in a peculiar manner. The effect of a single induction shock depends on the phase of contraction in which the heart is at the moment of its applica- tion. Thus at the commencement of a systole there is no visi- ble effect, while beats of unusual character result at other times. But tetanus can not be induced by any form or method of stimulation. The latent period of cardiac muscle is long. In a heart at rest a single stimulus (as the prick of a needle) usually calls forth but one contraction. The Nervous System in Relation to the Heart. The attempts to determine just why the heart beats at all, and especially the share taken by the nervous system, if any direct one, are beset with great difficulty ; though, as we shall attempt to show later, this subject also has been cramped within too narrow limits, and hence regarded in a false light. Till comparatively recently the frog's heart alone received much attention, if we except those of certain well-known mam- mals. In the heart of the frog there are ganglion-cells in vari- ous parts, especially numerous in the sinus venosus (or expan- sion of the great veins where they meet the auricles) ; also in the auricles, more especially in the septum (ganglia of Remak), while they are absent from the greater part of the ventricle, though found in the auriculo-ventricular groove (ganglia of Bidder). Recently it has been found that ganglion-cells occur in the ventricles of warm-blood animals. In the hearts of the dog, calf, sheep, and pig, which are those lately subjected to investi- gation, it is found tliat the nerve-cells do not occur near the apex of the ventricles, but mainly in the middle and basal por- tions, being most abundant in the anterior and ])osterior inter- ventricular furrows and in the left ventricle. But there are 262 ANIMAL PHYSIOLOGY. differences for each group of animals; thus, these ganglion- cells are most abundant, so far as the mammals as yet inves- tigated are concerned, in the ventricles of the pig, and least so in those of the dog. In the cat they are also scanty. Ganglion- cells occur in the auricles, and are especially abundant near the terminations of the great veins. It has long been known that the heart of a frog removed from the body will pulsate for hours, especially if fed with serum, blood, or similar fluids ; and that it may be divided in almost any conceivable way, even when teased up into minute particles, and still continue to beat. The apex, however, when separated does not beat. Yet even this quiescent apex may be set pulsating if tied upon the end of a tube, through which it may be fed under pressure. We may here point out that the whole heart or a part of it may be made to describe its action by the graphic method in various ways, the principles underlying which are either that the heart pulls upon a recording lever (lifts it) acts against the fluid of a manometer ; or, inclosed in a vessel containing oil or similar fluid, moves a piston in a cylinder. It has also long been known that a ligature drawn around the sinus venosus (in the frog) at its junction with the auricles stopped the heart for a certain period, and this experiment (of Stannius) was thought to demonstrate that the heart was ar- rested because the nervous impulses proceeding to the ganglion- cells along the cardiac nerves or ganglia of this region were cut off by the ligature ; in other words, the heart ceased to beat because the outside machinery on which the action of the inner depended was suddenly disconnected. Other explanations have been offered of this fact. Within the last few years great light has been thrown upon the whole subject of cardiac physiology in consequence of in- vestigators having studied the hearts of various cold-blooded animals and of several invertebrates. The hearts of the Clie- lonians (tortoises, turtles) have received special attention, and their investigation has been fruitful of results, to the general outcome of which, as well as those accruing from recent com- parative studies as a whole, we can alone refer. Very briefly, the following are some of the main facts : 1. In all cold-blooded animals the order in which the sub- divisions of the heart cease to pulsate when kept under the same conditions is invariable, viz., ventricle, auricles, sinus. 2. The sinus and auricles, when separated by section, liga- THE CIRCULATION OF THE BLOOD. 263 ture, or otherwise, either together or singly, continue to beat, whether amply provided with or surrounded by blood. 3. The ventricle thus separated displays less tendency to beat independent of some stimulus (as feeding under pressure), though a very weak one usually suffices — i. e., its tendency to spontaneous rhythm is less marked than is the case with the other parts of the heart. These remarks apply to the hearts of Chelonians — fishes, snakes, and some other cold-blooded animals. 4. In certain fishes (skate, ray, shark) the beat may be re- versed by stimulation, as a prick of the ventricle. This is accomplished with more difficulty in other cold-blooded animals, and still more so in the mammal. 5. In certain invertebrates, notably the Poulpe (Octopus), a careful search has revealed no nerve-cells, yet their hearts con- tinue to beat when their nerves are severed, on section of parts of the organ, etc. 6. A strip of the muscle from the ventricle of the tortoise, when placed in a moist chamber and a current of electricity passed through it for some hours, will commence to pulsate and continue to do so after the current lias been withdrawn ; and this holds when the strip is wholly free from nerve-cells. From the above facts certain inferences have been drawn : 1. It has been concluded that the sinus is the originator and director of the movements of the rest of the heart. 2. That this is owing to the ganglia in its walls. While all recognize the importance of the sinus, some physiologists hold to the gangli- onic influence as essential to the heart-beart, still ; while others, influenced by the facts mentioned above, are disposed to regard them as of very doubtful importance — at all events, as origina- tors of the movements of the heart. The tendency now seems to be to attach undue imj^ortance to the spontaneous contractility of the heart-muscle ; for it by no means follows Logically that, because a muscle treated by electricity, when cut off from the usual nerve influence that we ]>elieve is being constantly exerted on the heart like other or- gans, will contract and continue to do so in the absence of the stimulus, it does so normally; or, because some hearts beat in the absence of nerve-cells, that therefore nerve-cells are of no account in any case. Such views, when pressed to the extreme, lead to as na?-rf)\v conceptions as those they are intended to re- place Taking into account the facts mentioned and others wo have 264 ANIMAL PHYSIOLOGY. not space to enumerate, we submit the following as a safe view to entertain of the beat of the heart in the light of our present knowledge : Recent investigations show clearly that there are great dif- ferences in the hearts of animals of diverse groups, so that it is not possible to speak of "the heart" as though our remarks applied equally to this organ in all groups of animals. It must be admitted that our understanding of the hearts of the cold-blood animals is greater than of the mammalian heart ; while, so far as exact or experimental knowledge is concerned, the human heart is the least understood of all, though there is evidence of a pathological and clinical kind and subjective experience on which to base conclusions possessing a certain value; but it is clear to those who have devoted attention to (Comparative physiology that the more this subject is extended the better prepared we shall be for taking a broad and sound view of the physiology of the human heart and man's other organs. Whatever may be said of the invertebrates, among which greater simplicity of mechanism doubtless jjrevails, there can be no doubt that the execution of a cardiac cycle of the heart in all vertebrates, and especially in the higher, is a very com- plex process from the number of the factors involved, their in- teraction, and their normal variation with circumstances ; and we must therefore be suspicious of any theory of excessive sim- plicity in this as well as other parts of physiology. We submit, then, the following as a safe provisional view of the causation of the heart-beat : 1. The factors entering into the causation of the heart-beat of all vertebrates as yet examined are : (a) A tendency to spon- taneous contraction of the muscle-cells composing the organ ; (5) intra-cardiac blood-pressure ; (c) condition of nutrition as determined directly by the nervous supply of the organ and in- directly by the blood. 2. The tendency to spontaneous contraction of muscle-cells is most marked in the oldest parts of the heart (e. g., sinus), ancestrally (phylogenetically) considered. 3. Intra-cardiac pressure exercises an influence in determin- ing the origin of pulsation in probably all hearts, though like other factors its influence varies with the animal group. In the mollusk (and allied forms) and in the fish it seems to be the controlling factor. 4. We must recognize the power one cell has to excite when THE CIRCULATION OF THE BLOOD. 265 in action neighboring heart-cells to contraction. The ability that one protoplasmic cell-mass has to initiate in others, under certain circumstances, like conditions with its own, is worthy of more serious consideration in health and disease than it has yet received. 5. The influence of the cardiac nerves becomes more pro- nounced as we ascend the animal scale. Their share in the heart's beat will be considered later. 6. Apparently in all hearts there is a functional connection leading to a regular sequence of beat in the difit'erent parts, in which the sinus or its representatives (the terminations of great veins in the heart) always takes the initiative. One part hav- ing contracted, the others must necessarily follow ; hence the rapid onset of the ventricular after the auricular contraction in tlie mammal, and the long wave of contraction that seems to pass evenly over the whole organ in cold-blooded animals. The basis of all these factors is to be sought finally in the natural contractility of protoplasm. A heart in its most devel- oped form still retains, so to speak, the inherited but modified Amoeba in its every cell. Whether the intrinsic nerve -cells of the heart take any share directly in the cardiac beat must be considered as yet undetermined. Possibly they do modify motor impulses from nerves, while again it may be that they have an influence over nutritive processes only. The subject requires further study, both anatomical and physiological. Influence of the Vagus Nerve upon the Heart. The principal facts in this connection may be stated as fol- lows, and apply to all the animals thus far examined : L In all cases the action of the heart is modified by stimu- lation of the medulla oblongata or the vagus nerve, 2. The modification may consist in prompt arrest of thfj heart, in slowing, in enfeeblement of the beat, or a combination of the two latter effects. 3. After the application of the stimulation there is a latent period before the effect is manifest, and the latter may outlast the stimulation by a considerable period. 4. In most animals the sinus venosus and auricles are af- fected >>efore the ventricles, and the vagus may influence these parts when it is powerless over the ventricle. 5. After vagus inhibition, the action of the heart is (almost 266 ANIMAL PHYSIOLOGY. unexceptionally) different, the precise result being variable, but generally the beat is both accelerated and increased in force. FiGf. 238.— Inhibition of frog's heart by stimulation of the vagus nerve. To be read from right to left. The contractions of the ventricle are registered by a simple lever resting on it. The interrupted current was thrown in at a. Note that one beat occurred before arrest (latent period), and that when standstill of the heart did take place it lasted for a consider- able period (Foster). We may say that the working capacity of the heart is tem- porarily increased. 6. The improvement in the efficiency of the heart is in pro- portion to its previous working power, and in cases when the stimulation Vagufi. Fig. 239.— Effects of vagus stimulation, illustrated by a form of sphygmographic curve derived • from the carotid of a rabbit (Foster). action is feeble and irregular (abnormal) it might be said to be in proportion to its needs. This is a very important law that deserves to receive a general recognition. 7. Section of both vagi nerves results in histological altera- tions in the heart's structure, chiefly fatty degeneration, which must, of course, impair its working capacity and expose it to rupture or other accidents under the frequently recurring strains of life. THE CIRCULATION OF THE BLOOD. 267 8. In the cold-blooded animals the heart may be kept at a standstill by vagus stimulation till it dies, a period of hours (one case of six hours reported for the sea-turtle). 9. Certain drugs (as atropine), applied directly to the heart, or injected into the blood, prevent the usual action of the vagus. 10. During vagus arrest the heart substance undergoes a change, resulting in an unusual dilatation of the organ. This may be witnessed whether the heart contains blood or not. 11. The heart may be arrested by direct stimulation, espe- cially of the sinus, and at the points at which the electrodes are applied there is apparently a temporary paralysis. The same alteration in the beat may be noticed, as when the main trunk of the vagus is stimulated. 12. The heart may be inhibited through stimulation of vari- ous parts of the body, both of the surface and internal organs (reflex inhibition), 13. One vagus being divided, stimulation of its upper end may cause arrest of the heart. 14. Stimulation of a small part of the medulla oblongata will produce the same result, provided one or both vagi be intact. 15. Section of both vagi in some animals (the dog notably) increases the rate of the cardiac beat. The result of section of one pneumogastric nerve is variable. The heart's rhythm is usually to some extent quickened. 16. During vagus inhibition from any cause in mammals and many other animals, the heart responds to a single stimu- lus, as the prick of a needle, by at least one beat. An observer studying for himself the behavior of the heart in several groups of animals with an open mind, for the purpose of observing all he can rather than proving or disproving some one point, becomes strongly impressed with the variety in unity that runs through cardiac physiology, including the influence of nerve- cells (centers) through nerves; for it will not be forgotten that normally nerves originate nothing, being conductors only, so that when the vagus is stimulated by us we are at the most but imitating in a rough way the work of central nerve-cells. We can only mention a few points to illustrate this. In the frog a succession of light taps, or a single sharp one (" Klopfversuch" of Ooltz), will usually arnjst tlie heart re- flexly; though sometimes it is V(;ry diflicult to acconii)lish. But in the fisli the ease with which tlio heart may be reflexly 268 ANIMAL PHYSIOLOGY. inhibited by gentle stimulation of almost any portion of the animal is wonderful. Again, in some animals the vagus arrests R. Vagus. Heart. Brain above Medulla. Cardio-inhibitory Center in Medulla Oblongata. -Afferent Nerve. Outlying Area with its Nerves. Fig. 840.— Diagram of the inhibitory mechanism of the heart. The arrows indicate in all cases the path the nervous impulses take. I. Path of afferent impulses from the heart itself, n. Path from parts of the brain above (or anterior to) the vaso-motor center. A similar one might, of course, be mapped out along the spinal cord. III. Path from some peripheral region. The downward arrows indicate the course of efferent impulses, which probably usually pass by both vagi. the heart for only a brief period, when it breaks away into its usual (but increased) action. In the fish, menobranchus, and probably other animals, the irritability of some subdivision of the heart is lost during the vagus inhibition — i. e., it does not respond to a mechanical stimulus. There is usually a certain order in which the heart recom- mences after inhibition (viz., sinus, auricles, ventricles) ; but there are variations in this, also, for different animals. It is THE CIRCULATION OF THE BLOOD. 269 also a fact that in most of the cold-blooded animals the right vagus is more efficient than the left, owing, we think, not to the nerves themselves so much as to their manner of distribution in the heart — the greater portion of the driving part of the organ, so to speak, being supplied by the right nerve ; for, when even a small part of the heart is arrested, it may be overcome by the action of a larger portion of the same, or a more domi- nant region (the sinus mostly). Conclusions. — The inferences from the facts stated in the above paragraphs are these : 1. There is in the medulla a col- lection of cells (center) which can generate impulses that reach the heart by the vagi nerves and influence its muscular tissue, though whether directly or through the intermediation of nerve-cells in its substance is uncertain. It may possibly be in both ways. 2. This center (cardio-inhibitory) may be influ- enced reflexly by influences ascending by a variety x>f nerves from the periphery, including paths in the brain itself, as shown by the influence of emotions or the behavior of the heart. 3. The cardio-inhibitory center is the agent, in part, through which the rhythm of the heart is adapted to the needs of the body. 4. The arrest, on direct stimulation of the heart, is owing to the effect produced on the terminal fibers of the vagi, as shown by the dilation, etc., corresponding to what takes place when the trunk of the nerve or the center is stimu- lated. 5. The quickening of the heart, following section of the vagi, seems to show that in some animals the inhibitory center exercises a constant regulative influence over the rhythm of the heart, 6. The irritability and dilatability of the cardiac tissue may be greatly modified during vagus inhibition. Some- times this is evident before the rhythm itself is appreciably altered. 7. The heart-muscle has a latent period, like other kinds of muscle ; and cardiac effects, when initiated, last a variable jjeriod. There are many other obvious conclusions, which the stu- dent will draw for himself. But a question arises in regard to the significance of the f-ardiac arrest under these circumstances, and the altered action that follows. The fact that, when the heart is severed from the central nervous system by section of its nerves, profound changes in the minute structure of its cells ensue, points un- mistakably to some nutritive influence that must have operated through the vagi nerves. That stimulation of the vagus re- stores regularity of rhythm and strengthens the beat of the 270 ANIMAL PHYSIOLOGY. failing heart, is also very suggestive. That many disorders of the heart are coincident with periods of mental anguish or worry, and that in certain cases of severe mental application the heart's rhythm has become very slow, also point to influ- ences of a central origin as greatly affecting the life-processes of this organ. It has been shown that the vagus nerve in some cold-blooded animals, as is probable also in the higher vertebrates, consists of two sets of fibers — those which are inhibitory jproi^er and those which are not, but belong to the sympathetic system. Separate stimulation of the former favors nutritive pro- cesses, is preservative; of the latter, destructive. This has been expressed by saying that the former favors constructive (anabolic) metabolism ; the latter destructive (katabolic) me- tabolism. It is assumed that all the metabolism of the body may be represented as made up of katabolic following anabolic processes. Whether such a view of metabolism expresses any more than a sort of general tendency of the chemistry of the body is doubtful. It is a very simple representation of what in all probability is extremely complex ; and if it be implied that throughout the body certain steps are always taken upward in construction to be always afterwards followed by certain down- ward destructive changes, we must reject it as too rigid and artificial a representation of natural processes. We think, however, that, upon all the evidence, pathological and clinical as well as physiological, the student may believe that the vagus nerve, like the other nerves of the body, accord- ing to our own theory, exercises a constant beneficial, guiding — let us say determining — influence over the metabolism of the organ it supplies ; and we here suggest that, if this view were applied to the origin and course of cardiac disease, it would result in a gain to the science and art of medicine. The Accelerator (Augmentor) Nerves op the Heart. It has been known for many years that in the dog, cat, rab- bit, and some other mammals, there were nerves proceeding from certain of the ganglia of the sympathetic chain high up, stimulation of which led to an acceleration of the heart-beat. Very recently these nerves have been traced in a number of cold-blooded animals, and the whole subject placed on a broader and sounder basis. THE CIRCULATION OF THE BLOOD. 271 There are variations in the distribntion of these nerves for different groups of animals, but it will suffice if we indicate their course in a general way, without special reference to the variations for each animal group : 1. These nerves emerge from the spinal cord (upper dorsal region), and proceed upward before being distributed to the heart, 2. They may leave for their cardiac destination either at (a) the first thoracic (or basal cardiac ganglion, as it might be named in this case), (b) the in- ferior cervical ganglion, (c) the annulus of Vieussens, or {d) the middle cervical ganglion. 2 n Middle Cervical Ganglion. Spinal Cord. Accelerator Center in Medulla. Superior Cervical Ganglion. 3 Q- 1 — Inferior Cervical Ganglion. Region of First Rib. Accelerator Nerves. Heart. Fio. 211.— Diagram to illustrate the origin, course, etc., of accelerator impulses. It will be understood that this is inU'tided to iiidi<;ate the general plan, and not preciselv what takes pla<'e in any one atiiinal. Thus, while the accelrd or tlu; various parts mentioned al)()ve, or nerve brandies from tliem, gave rise either to ac^celcsration of 272 ANIMAL PHYSIOLOGY. the cardiac beat or augmentation of its force^ or to both, as is commonly the case. In every instance the work of the heart is increased, so that they may be called more appropriately augmentor nerves; and their effect may be more evident on one part of the heart, as regards increase of the force of the beat, than on another. They require for their fullest effect a rather strong and con- tinuous stimulation (interrupted current), and the augmenta- tion outlasts the stimulus a considerable period. The same law applies to them as to the vagus nerve, viz., that the result is inversely proportional to the rhythm of the heart at the period of stimulation ; a slow-beating heart will be more augmented proportionally than a rapidly-pulsating organ. It is noticeable that after one or more experiments the heart often falls into an irregular or weakened action quite the re- verse of what ensues when the vagus is stimulated. But it has also been observed that certain of the vagus fibers on stimula- tion give rise to a like result. Further, it is found that the electrical condition of the heart is different, according as the inhibitory or other fibers of the heart are stimulated. The latter fact seemed to point strongly to a fundamental difference in their effect on cardiac metabo- lism ; hence it is proposed to speak of the vagus as a vago- sympathetic nerve, containing inhibitory fibers proper and sympathetic or motor fibers to be classed with the nerves that were formerly known as " accelerators," and to be compared in their action to the ordinary motor nerves of voluntary muscles. Indeed, these conceptions will probably give rise to a broader view of the whole nervous system, especially as regards the relations of the nerves themselves. Certainly the augmentor nerves to which we are now refer- ring exhaust the heart, lead it to expend its nutritive capital, and leave it worse than before. One can understand the ad- vantage in the heart having a double supply of nerve-fibers with opposite action ; and it is worthy of special note in this connection that, when the vagus (vago-sympathetic) is stimu- lated at the same time as the augmentors, the inhibitory effect, preservative of nutritive resources, prevails. It will be seen that the heart may be made to do increased work in three ways : Firstly, the relaxation of a normal inhibi- tory control through the vagus nerve by the cardio-inhibitory center; secondly, through the sympathetic (motor) fibers in THE CIRCULATION OF THE BLOOD. 273 the vagus itself ; and, finally, through fibers with similar action in the sympathetic system, usually so called. The share taken by these factors is certainly variable in dif- ferent species of animals, and it is likely that this is true of the same animals on difl^erent occasions. It is also conceivable, and indeed probable, that they act together at times, the inhibi- tory action being diminished and the augmentor influence in- creased. Human Physiology. — Of the three cardiac nerves — superior, middle, and inferior — the strongest, which is the middle one, passes from the inferior cervical ganglion to the middle, from which it proceeds to the heart, and the inferior, may be re- garded as the chief augmentor cardiac nerves. That man's pneumogastric contains inhibitory fibers is evi- dent from the experiment of Czermak, who, by pressing a bony tumor in his neck against his vagus nerve, could arrest his heart. Another individual could arrest his heart-beat at will, and if not through the vagus, how ? We are probably all aware of alterations in the rhythm of the heart from emotions. During a period of intense, brief, sympathetic anxiety, as in watching two competitors during a severe struggle for supremacy, a change in the rhythm of the heart, amounting, it may be, to momentary arrest, may be observed. Enough has been said, we trust, to show that the nerves of the heart can no longer be regarded merely as the reins for bridling the cardiac steed; but that all the phenomena of accel- eration, slowing, or other changes of rhythm, are only the out- ward evidences of profound vital changes accompanied by cor- responding chemical and electrical effects. If tliese views be correct, nervous influence must play no small part in the causa- tion and modification of disordered conditions ; and we would extend such a view to all the organs of the body, and especially in the case of man. The heart's rhythm can, however, be modified in other ways than we have as yet described. Th(;ugh an isolated heart, fed by serum or some artificial nutritive fluid, may beat well for a time, it is liable to peri- odic interruptions, which are probably owing to its imperfect nutrition. Many drugs greatly modify the heart-beat; but, in attempt- ing to exjjlain how the result is accomplished, the difficulty is in UMi-av<'ling tluj i)art each anatomical element plays in the total result. Does the drug act on the muscular tissue, the 18 2T4 ANIiVlAL PHYSIOLOGY. nerve terminals, or the ganglia; or does it affect the heart through the central nervous system ? Resort to comparative physiology is important in such cases, if only to foster caution and avoid narrow views. The Heart in Relation to Blood- Pressure. It is plain that all the other conditions throughout the cir- culatory system remaining the same, an increase in either the force or the frequency of the heart-beat must raise the blood-press- ure. But, if the pressure were generally raised when the heart beats rapidly, it would fare ill with the aged, the elasticity of their arteries being usually greatly impaired. As a matter of fact any marked rise of pressure that would thus occur is pre- vented as a rule, and in different ways, as will be seen ; but, so far as the heart is concerned, its beat is usually the weaker the more rapid it is, so that the cardiac rhythm and the blood- pressure are in inverse proportion to each other. By what method is the heart's action tempered to the condi- tions prevailing at the time in the other parts of the vascular system ? The matter is complex. It is possible to conceive that there is a local nervous apparatus which regulates the beat of the heart according to the intra-cardiac pressure, which latter again will depend on conditions outside of the heart itself — the arte- rial pressure, in fact. It is possible to understand that, apart from any nervous elements at all, the cardiac cells regulate their own action in obedience to the impressions made upon them. But, inasmuch as the heart is not regulated perfectly in the mammal according to the blood -pressure, when the vagi nerves are cut, and considering the dominance of the central nervous system, it does not seem likely that it should resign the con- trol of so important a matter. Experiment bears this out. There is some evidence for believing that not only may the vagus itself act as an afferent sensory nerve, but that the de- pressor nerve, to be shortly referred to more particularly, is also such a sensory nerve. However, such a view does not exclude previously men- tioned factors, and there can be little doubt that in forms below mammals the muscular tissue is to some degree self -regulative ; and it is not likely that this quality is wholly lost even in the highest mammals. THE CIRCULATION OP THE BLOOD. 275 V^y The effect of vagus stimulation on the blood-pressure is always very marked, as would be supposed. To examine an extreme case, suppose the heart arrested for a few seconds, the elastic recoil of the arteries continues to maintain for a time the blood-pressure, though there is, of course, an immediate and pronounced fall. And it may be remarked, by-the-way, that in cases of fainting, when the heart ceases to beat, or beats in the feeblest man- ner, the importance of this arterial elas- ticity as a force, maintaining the circulation for sev- eral seconds at least, is of great importance. As seen in the tracing, the beats, when the heart commences its aC- Fig. 242. —Tracing from a rabbit, showing the influence of car- diac inhibition on blood-pressure. The fall in this case tion ao"ain tell on ^^^ very rapid, owing to sudden cessation of the heart- ^ ' . beat. The relative emptiness of the vessels accounts for the comparatively the pecuUar character of the curve of rising blood-pressure slack' walls of the arteries, distending them greatly, and this may be made evident by the sphygmograph as well as the manometer ; indeed, may be evident to the finger, the pulse resembling in some features that following excessive loss of blood. If the heart has been merely slowed, or its pulsation weak- ened, the effects will of course be less marked. The Quantity of Blood. — The blood-pressure may also be augmented, the cardiac frequency remaining the same, by the quantity of blood ejected from the ventricles, which again depends on the quantity entering them, a factor determined by the condition of the ves.sels, and to this we shall presently turn. In consequence of changes in different X)arts of the system by way of compensation, results follow in an animal which might not have been anticipated. Thus, ble(;ding, unless to a dangerous extreme, does not lower the blood -pressure except temy)orarily. It is estimated that the body can adapt itself to a loss of as much as 3 per cent of the body-weight. The adaptation is probably not through absorption chiefly, 276 ANIMAL PHYSIOLOGY. but through, constriction of the vessels by the vaso-motor nerves. Again, an injection of fluid into the blood does not cause an appreciable rise of blood-pressure, so long as the nervous sys- tem is intact ; but, if by section of the spinal cord the vaso- motor influences are cut off, then a rise may take place to the extent of 2 to 3 per cent of the body-weight, the extra quan- tity of fluid seeming to be accommodated in the capillaries and smaller veins. These facts are highly significant in illustrat- ing the adaptive power of the circulatory system (protective in its nature), and are of practical importance in the treatment of disease. We think the benefit that sometimes follows bleeding has not as yet received an adequate explanation, but we shall not attempt to tackle the problem now. Changes in the circulation depend on variations in the size of the blood-vessels. It is important in considering this subject to have clear no- tions of the structure of the blood-vessels. It will be borne in mind that, while muscular elements are perhaps not wholly lacking in any of the arteries, they are most abundant in the smallest, the arterioles, which by their variations in size are best fitted to determine the quantity of blood reaching any organ. It is well known that nerves derived chiefly from the sympathetic system pass to blood-vessels, though their exact mode of termination is obscure. We may now examine into the nature of certain facts, which may be stated briefly thus : 1. In certain vascular areas of some vertebrates, as in the vessels of the ear of the rabbit and this animal's saphena artery, rhythmical variations in the size of the small arteries may be observed ; also in the veins of the bat's wing and of the fins of certain fishes (e. g., caudal vein of the eel), as well as in certain arteries of some groups of the cold-blooded animals. 2. Under the microscope the arterioles of various parts of the frog, including those of the muscles, may be seen to vary apparently spontaneously, and may through stimulation be made to depart widely from their usual size. 3. Section of a large number of nerves is followed by red- dening of the parts to which they are distributed. This is well seen when the cervical sympathetic of the rabbit is divided ; the ear becomes redder, owing to obvious dilatation of its blood- vessels ; and warmer, owing to the increased quantity of blood in it, etc. It has also been noticed in cases of paralysis, and THE CIRCULATION OF THE BLOOD. 277 especially in gunshot and other wounds involving nerves, that vaso-motor effects have followed. 4. Section of certain nerves, as the nervi erigentes of the penis, is not followed by dilatation ; but these nerves and the chorda tympani supplying the salivary gland are examples of so-called vaso-dilators, inasmuch as their stimulation gives rise to enlargement of the caliber of the arterioles in their area of distribution. 5. On the other hand, such a nerve as the cervical sympa- thetic, as may be readily shown in the rabbit, when its periph- eral end is stimulated, gives rise to constriction, and hence is termed a vaso-consiricior. 6. When, however, the divided sciatic nerve is stimulated peripherally, the result may be either constriction or dilata- tion. 7. When the spinal cord of an animal is divided across, there is vascular dilatation of all the parts below the section (loss of arterial tone) ; but in time the vessels return to their usual size (restoration of arterial tone). 8. On destruction of a certain minute portion of the medulla oblongata, there is a general loss of arterial tone. This area (center) extends in the rabbit from a short distance below the corpora quadrigemina (1 to 2 mm.) to within 4 to 5 of the calamus scriptorius, as ascertained by the effects on the vessels of cutting away the medulla in thin transverse sections. At the spot indicated there is a collection of large multipolar nerve-cells (antero-lateral nucleus of Clarke). Conclusions. — 1. There are vaso-motor nerves of two kinds — vaso-constrictors and vaso-dilators — which may exist in nerve- trunks either alone or mingled. Examples of the former are found in the cervical sympa- thetic, splanchnic, etc., of the latter in the chorda tympani, nerves of the muscles and nervi erigentes (from the first, second, and third sacral nerves), while the sciatic seems to contain both. 2. Impulses are constantly passing from the medullary vaso-motor center along the nerves to the blood-vessels, hence their dilatation after section of the nerves. The nerves are traceable to the spinal cord, and in some part of their course run, as a rule, in the sympathetic system. '.i. Impulses j>ass at intervals to the areas of (listri})utif)n of vaso-dilators along these nerves, the effect of which is to dilate the vessels through their influence, as in other cases, on the muscular coat. 278 ANIMAL PHYSIOLOGY. It is stated that in course of time the vessels of the rabbit's ear regain their tone, notwithstanding that the influence of the Spinal Cord Vaso-motor Center in \/'^\ / Medulla. Depressor Nerve. Efferent Vaso-raotor Nerve. Outlying Vascular Area. Afferent Nerve from Periphery. Fig. 243. — Diagram of nervous vasomotor mechanism. I. Course of afferent impulses from the heart itself along the depressor nerve. II. Course from some other part of the brain. ni. Course from some peripheral region along a nerve joining the spinal cord. The effer- ent impulses are represented as passing to a vascular area, reduced for the sake of sim- plicity to a single arteriole. central nervous system has been cut off by section of the vaso- motor nerves. To explain this result, a local nervous mechanism has been assumed to exist, though not demonstrated either anatomically or physiologically. Interesting experiments have lately shown that both in mammals and cold-blooded animals the effect on the blood-vessels varies with the intensity and character of the stimulus, and not only with the group of animals tested, but even with the same individuals at different periods during the experiment ; and we take the opportunity to renew our expres- sion of opinion with this fresh evidence that the laws of physi- ology can not be laid down in the rigid way that has prevailed THE CIRCULATION OF THE BLOOD. 279 to so large an extent up to the present time ; but that our widen- ing experience shows (what ought to have been expected) that the greatest allowance mast be made for group if not individ- ual variations everywhere. There is also evidence to show that the mode of stimulation in experimental cases causes the result to vary. From such facts as are stated in paragraph seven, it is inferred that there are vaso-motor centers in the si3inal cord which are usually subordinated to the main center in the me- dulla, but which in the absence of the control of the chief cen- ter in the medulla assume an independent regulating influence. A local vaso-motor mechanism does not seem to us neces- sary to explain the changes which the blood-vessels undergo, and should not be adopted as an article of physiological faith till demonstrated to exist. If we assume that the independent contractility of muscle-cells is retained in the blood-vessels, and that, when freed from the influence of the central nervous system, which becomes more and more dominant as we ascend the animal scale, there is a reversion to an ancestral condition, a new light is thrown upon the facts. It is a case of old habits gaining sway when the check-rein of nervous influence is re- moved ; and, as we shall show from time to time, this law applies to every organ of the body. Moreover, not to go beyond the vascular system, this independent rhythmic activity is seen in the isolated sections of the pulsatile veins of the bat's wing, devoid, so far as we know, of nervous cells. Such facts lend some color to the view that, after distention of the vessels by the cardiac systole, the return to their previous size is aided by rhythmical contractions of the muscle-cells. Let us now consider certain other well-known experimental facts : 1. There is a nerve with variable origin, course, etc., in dif- ferent mammals, but in the rabbit given off from either the vagus, the superior laryngeal, or by a branch from each, which, running near the sympathetic nerve and the carotid artery, reaches the heart, to which it is distributed. This is known as the deprefisor nerve. 2. The vagi nerves having been divided, stimulation of the central end of the cut depressor nerve is followed by a fall in blood-prf!ssure, which may not be accompanied by any altera- tion in the cardiac rliythm. 3. This effect may in great part be prevented if the splancli- nic nerves be divided previous to stimulation of the depressor. 4. If the splanchnic area (region of the main abdominal 280 ANIMAL PHYSIOLOGY. viscera) be inspected during the fall in blood-pressure, it may- be noticed that there is vascular fullness under these circum- stances. These results are interpreted as being due to afferent im- pulses ascending the depressor, acting on the vaso-motor center^ Fig. 244. — Curve of blood-pressure resulting from stimulation of the central end of the de- pressor nerve. To be read from right to left. T indicates the rate at which the recording surface moved, the intervals denoting seconds. At C the current was thrown into the nerve, and shut off at O. The result appears after a period of latency, and outlasts the stimulus (Foster). and interfering with (inhibiting) the outflow of efferent, con- strictive, or tonic impulses, which start from the vaso-motor center, descend the cord, and find their way to the organs of the region in question, in consequence of which the mus- cular coats of the arterioles relax, more blood flows to this area which is very large, and the general blood-pressure is lowered. Again, if the central end of one of the main nerves — e. g., sciatic — be stimulated, a marked change in the blood-pressure results, but whether in the direction of rise or fall seems to depend upon the condition of the central nervous system, for, with the animal under the influence of chloral, there is a fall ; if under urari, a rise. It is not to be supposed that the change in any of these cases is confined to any one vascular area invariably, but that it is this or that, according to the nerve stimulated, the condi- tion of the centers, and a number of other circumstances. Moreover, it is important to bear in mind that with a fall of blood-pressure in one region there may be a corresponding rise in another. With these considerations in mind, it will be ap- parent that the changes in the vascular system during the THE CIRCULATION OF THE BLOOD. 281 course of a single hour are of the most complex and variable character. Though special attention has been drawn to such rhyth- mical variations as may be witnessed in the rabbit's ear, bat's wing, etc., there can be little doubt that changes as marked, though possibly less distinctly rhythmical, are constantly tak- ing place in the vertebrate body, and especially in that of man, with his complex emotional nature and the many vicissitudes of modern civilized life. The frequent changes in color in the faces of certain people are in this connection suggestive, though we hope we have made it clear that these vascular modifica- tions are dependent chiefly on centripetal influences from every quarter, though actually brought about by centrifugal im- pulses. Whether there is a rhythm obscured by minor rhythms, owing to an independent or automatic action of the vaso-motor center, though not improbable, must be regarded as undeter- mined as yet. The question of the distribution of vaso-motor nerves to veins is also one to which a definite answer can not be given. The Capillaries. The cells of which the capillaries are composed have a con- tractility of their own, and hence the caliber of the capillaries is not determined merely by the arterial pressure or any similar mechanical effect. Certain abnormal conditions, induced in these vessels by the application of irritants, cause changes in the blood-flow, which can not be explained apart from the vitality of the ves- sels themselves. Watched through the microscope under such circumstances, the blood-corpuscles no longer pursue their usual course in the mid-stream, but seem to be generally distributed and to hug the walls, one result of which is a slowing of the stream, wholly independent of events taking place in other vessels. It is thus seen that in this Ci^ndition {stasis) the capillaries have an in- dependent influence essentially vital. We say independent, for it is still an open question whether nerves are distributed to capillaries or not. That inflammation, in which also the walls undergo such serious changes that white and even red l)lood- cells may pass through thom {diapedesis), is not uninfluenced by the nervous system, possiljly induced through it in certain cases, if not all, seems more than i)iobjible. 282 ANIMAL PHYSIOLOGY. But when we consider the lymphatic system new light will, it is hoped, be thrown upon the subject of the nature and the influences which modify the capillaries. One thing will be clear from what has been said, that even normally the capil- laries must exert an influence of the nature of a resistance, owing to their peculiar vital properties ; and, as we have already intimated, such considerations should not be excluded from any conclusions we may draw in regard to tubes that are made up of living cells, whether arteries, veins, or capillaries, though manifestly the applicability to capillaries with their less modified or more primitive structure is stronger. It has now become clear that the circulation may be modi- fied either centrally or peripherally ; that a change is never purely local, but is correlated with other changes ; that the whole is, in the higher animals, directly under the dominion of the central nervous system ; and that it is through this part chiefly that harmony in the vascular as in other sys- tems and with other systems is established. To have ade- quately grasped this conception is worth more than a knowl- edge of all the details. Special Considerations. Pathological. — Changes may take place either in the sub- stance of the cardiac muscles, in the valves, or in the blood-ves- sels, of a nature unfavorable to the welfare of the body. Some of these have been incidentally referred to already. Hypertrophy, or an increase in the tissue of the heart, is generally dependent on increased resistance, either within or without the heart, in the region of the arterioles or capillaries. Imperfections of the aortic valves may permit of regurgitation of blood, entailing an extra effort if it is to be expelled in addi- tion to the usual quantity, which again leads to hypertrophy ; but this is often succeeded by dilatation of the chambers of the heart one after the other, and a host of evils growing out of this, largely dependent on imperfect venous circulation, and increased venous pressure. And it may be here noticed that arterial and venous pressures are, as a general rule, in inverse proportion to each other. If the quantity of blood in the ventricle, in consequence of regurgitation, should prove to be greater than it can lift (eject), the heart ceases to beat in diastole ; hence some of the sudden deaths from disease of the aortic valves. THE CIRCULATION OP THE BLOOD. 283 As a result of fatty, or other forms of degeneration, the heart may suddenly rupture under strains. Actual experiment on the arteries of animals recently dead, including men, shows that the elasticity of the arteries of even adult mammals is as perfect as that of the vessels of the child, so that man ranks lower than other animals in this respect. After middle life the loss of arterial elasticity is consider- able and progressive. The arteries may undergo a degenera- tion from fatty changes or deposit of lime ; such vessels are, of course, liable to rupture ; hence one of the frequent modes of death among old persons is from paralysis traceable to rupture of vessels in the brain. These and other changes also cause the heart more work, and may lead to hypertrophy. Even in young persons the strain of a prolonged athletic career may entail hypertrophy or some other form of heart-disease. We mention such facts as these to show the more clearly how important is balance and the power of ready adaptation in all parts of the circulation to the maintenance of a healthy condition of body. The heart is itself nourished through the coronary arteries ; so that morbid alterations in these vessels cause, if not sudden and painful death, at least nutritive changes in the heart-sub- stance, which may lead to a dramatic end or to a slow impair- ment of cardiac power, etc. Personal Observation. — The circulation is one of those depart- ments of physiology in which the student may verify much upon his own person. The cardiac impulse, the heart's sounds (with a double stethoscope), the pulse — its nature and changes with cir- cumstances, the venous circulation, and many other subjects, are all easy of observation, and after a little practice without liability of causing those aberrations due to the attention being drawn to one's self. The observations need not, of course, be confined to the stu- dent's own person ; it is, however, very important that the nor- mal should be known before the observer is introduced to cases of disease. Frequent comparison of the natural and the dis- eased condition renders physiology, pathology, and clinical medicine much good service. We again urge upon the student to try to form increasingly vivid and correct mental pictures of the cirr-iilation under its many changes. Comparative. — An interesting arrangement of blood-vessels, known as a rete mirabile, occurs in every main group of verte- 284 ANIMAL PHYSIOLOGY. brates. An artery breaks up into a great number of vessels of nearly the same size, wbicli terminate, abruptly and without capillaries, in another arterial trunk. Fig. 245.— iJeie mirabile of sheep, seen in profile (after Chauveau). The larger rete is in eon nection with the encephahc arteries ; the smaller, the ophthalmic. The large artery is the carotid. They are found in a variety of situations, as on the carotid and vertebrate arteries of animals that naturally feed from the ground for long periods together, as the ruminants ; in the Fig. 246.— Section of a lymphatic rete mirabile, from the popliteal space (after Chauveau'). a, a, afferent vessels ; b, 6, efferent vessels. The whole very strongly suggests a crude form of lymphatic gland. THE CIRCULATION OF THE BLOOD. 285 sloth, that hangs from trees ; in the legs of swans, geese, etc. ; in the horse's foot, in which the arteries break up into many small divisions. It has been suggested that these ar- rangements permit of a supply of arterial blood being maintained without congestion of the parts. Very marked tortuosity of vessels, as in the seal, the carotid of which is said to be forty times as long as the space it trav- erses, in all probability serves the same purpose. Evolution. — The com- parative sketch we have given of the vascular sys- tem will doubtless sug- gest a gradual evolution. We observe throughout a dependence and resem- blance which we think can not be otherwise ex- plained. The similarity of the foetal circulation in the mammal to the permanent circu- lation of lower groups has much meaning. Even in the high- est form of heart the original pulsatile tube is not lost. The great veins still contract in the mammal; the sinus venosus is probably the result of blending and expansion. The later differentiations of the parts of the heart are clearly related to the adaptation to altered surroundings. Such is seen in the foetal heart and circulation, and has probably been the deter- mining cause of the forms which the circulatory organs have assumed. It is a fact that the part of the heart that survives the long- est under adverse conditions is that which bears the stamp of greatest ancestral antiquity. It (the sinus venosus) may not bo less under nervous control, but it certainly is least depend- ent on the nervous system, and has the greatest automaticity. It is surely fortunate for man that this part of the reptilian heart is represented in his own. In cases of fainting, partial drowning, or other instances of impending death, this part, with Fig. 247. -Veins of the foot of the horse (after Chau- veau). 286 ANIMAL PHYSIOLOGY. the auricles it may be, continues to beat when the ventricles have ceased ; and we have learned that so long as these parts are functionally active there is a greater probability that the quiescent regions may recommence. Activity begets activity, in cardiac muscle-cells at least. How are these facts to be explained apart from evolution ? The law of rhythm in organic nature finds some of its most evident exemplifications in the circulation. Most of the rhythms are compound, one being blended with or superim- posed on another. Even the apparent irregularities of the nor- mal heart are rhythmical, such as the very marked slowing and other changes accompanying expiration, especially in some animals. We trust we have made it evident that the greatest allow- ance must be made for the animal group, and some even for the individual, in estimating any one of the factors of the cir- culation. We know a good deal at present of cardiac physiol- ogy, but we do not know a physiology of " the heart " in the sense in which we understand that term to have been used till recently — i. e., we are not in a position to state the laws that apply to all forms of heart. Summary of the Physiology of the Circulation. — In the mammal the circulatory apparatus forms a closed system consisting of a central pump or heart, arteries, capillaries, and veins. All the parts of the vascular system are elastic, but this property is most developed in the arteries. Since the tissue-lymph is prepared from the blood in the capillaries, it may be said that the whole circulatory system exists for these vessels. As a result of the action of an intermittent pump on elastic vessels against peripheral resistance, in consequence of which the arteries are always kept more than full (distended), the flow through the capillaries and veins is constant — a very great advantage, enabling the capillaries to accomplish their work of feeding the ever-hungry tissues. While physical forces play a very prominent part in the circulation of the blood, vital ones must not be ignored. They lie at the foundation of the whole, here as elsewhere, and must be taken into the account in every explanation. As a consequence of the anatomical, physical, and vital char- acters of the circulatory system, it follows that the velocity of the blood is greatest in the arteries, least in the capillaries, and intermediate in the veins. THE CIRCULATION OF THE BLOOD. 287 The veins with their valves, their superficial position and thinner walls, make up a set of conditions favoring the onflow of the blood, especially under muscular exercise. In the mammal tlie circulatory system, by reason of its con- nections with the digestive, respiratory, and lymphatic systems, and in a lesser degree with all parts of the body, especially the glandular organs, maintains at once the usefulness and the fit- ness of the blood. The arterioles, by virtue of their highly developed muscular coat, are enabled to regulate the blood-supply to every part, in obedience to the nervous system. The blood exercises a certain pressure on the walls of all parts of the vascular system, which is greatest in the heart it- self, high in the arteries, lower in the capillaries, and lowest in the veins, in the largest of which it may be less than the atmos- pheric pressure, or negative. The heart in the mammal consists of four perfectly separated chambers, each upper and each lower pair working synchronously, intermixture of arterial and venous blood being prevented by septa and interference in working by valves. The heart is a force-pump chiefly, but, to some extent, a suction-pump also, though its power as such purely from its own action and independent of the respiratory movements of the chest is slight under ordinary circumstances. In consequence of the lesser resistance in the pulmonary divis- ion of the circulation, the blood-pressure within the heart is much less in the right than in the left ventricle — a fact in har- mony with and causative of the greater thickness of the walls of the latter ; for in the foetus, in which the conditions are dif- ferent, this distinction does not hold. The ventricles usually completely empty themselves of blood and maintain their systolic contraction even after this has been effected. The contraction of the heart, which really begins in the great veins near their junction with the auricles (that do not fully empty themselves), is at once followed up by the auricular and ventricular contraction, the whole constitu- ting one long peristaltic wave. Then follows the cardiac pause, which is of longer duration than the entire systole. When the heart contracts it hardens, owing to closing on a non-compressible fluid dammed back within its walls by resist- ance a fronte. At the same time the hand y^laced on the chest- walls over the heart is sensible of the cardiac impulse, owing to what has just Ixien mentioned. The systole of the chambers of the heart gives rise to a first and a second sound, so called, 288 ANIMAL PHYSIOLOGY. caused by several events combined, in whicb, however, the ten- sion of the valves must take a prominent share. The work of the heart is dependent on the quantity of blood it ejects and the pressure against which it acts. The pulse is an elevation of the arterial wall, occurring with each heart-beat, in conse- quence of the passage of a wave over the general blood-stream. There is a distention of the entire arterial system in every di- rection. The pulse travels with extreme velocity as compared with the blood-current. The heart-beat varies in force, fre- quency, duration, etc., and with age, sex, posture, and numer- ous other circumstances. The whole of the circulatory system is regulated by the cen- tral nervous system through nerves. There is in the medulla oblongata a small collection of nerve-cells making up the cardio-inhibitory center. This center, with varying degrees of constancy, depending on the group of animals and the needs of the organism, sends forth impulses (which modify the beat of the heart in force and frequency) through the vagi nerves. There are nerves of the sympathetic system with a center in the cervical spinal cord, and possibly another in the medulla, which are capable of originating either an acceleration of the heart-rhythm or an increase of the force of the beat, or both together, known as accelerators or augmentors. In the verte- brates thus far examined the vagus is in reality a vago-sympa- thetic nerve, containing inhibitory fibers proper, and sympa- thetic, accelerator, or motor fibers. The inhibitory fibers can arrest, slow, or weaken the cardiac beat; the sympathetic accelerate it or augment its force. When both are stimulated together, the inhibitory prevail. . These nerves, as also the accelerators, exercise a profound influence upon the nutrition of the heart, and effect its electri- cal condition when stimulated, and we may believe when influ- enced by their own centers. The inhibitory fibers tend to preserve and restore cardiac energy ; the sympathetic, whether in the vagus or as the aug- mentors, the reverse. The vagus nerve (and probably the de- pressor) acts as an afferent, cardiac sensory nerve reporting on the intra - cardiac pressure, etc., and so enabling the vaso- motor and cardio-inhibitory centers, which are, it would seem, capable of related and harmonious action to act for the general good. The arterioles must be conceived as undergoing very fre- quent changes of caliber. They are governed by the vase- THE CIRCULATION OF THE BLOOD. 289 motor center, situated in the medulla, and possibly certain sub- ordinate centers in the spinal cord, through vaso-motor nerves. These are (a) vaso-constrictors, which maintain a constant but variable degree of contraction of the muscle-cells of the vessels ; (6) vaso-dilators, which are not in constant functional activity ; and (c) mixed nerves, with both kinds. An inherited tendency to rhythmical contraction throughout the entire vascular sys- tem, including the vessels, must be taken into account. The depressor nerve acts by lessening the tonic contraction of (dilating) the vessels of the splanchnic area especially. It is important to remember that all the changes of the vascular system, so long as the nervous system is intact — i. e., so long as an animal is normal — are correlated ; and that the action of such nerves as the depressor is to be taken rather as an example of how some of these changes are brought about, mere chapters in an incomplete but voluminous history, if we could but write it all. The changes in blood-pressure, by the addition or removal of a considerable quantity of blood, are slight, owing to the sort of adaptation referred to above, effected through the nervous system. Finally, the capillary circulation, when studied microscopically, and especially in disordered con- ditions, shows clearly that the vital properties of these vessels have an important share in determining the character of the circulation in themselves directly and elsewhere indirectly. The study of the circulation in other groups shows that below birds the arterial and venous blood undergoes mixture somewhere, usually in the heart, but that in all the vertebrates the best blood is invariably that which passes to the head and upper regions of the body. The deficiencies in the heart, owing to the imperfections of valves, septa, etc., are in part counter- acted in some groups by pressure relations, the blood always flowing in the direction of least resistance, so that the above- mentioned result is achieved. Caj)illaries are wanting in most of the invertebrates, the blood flowing from the arteries into spaces (sinuses) in tlie tis- sues. It is to be noted that a modified blood (lymph) is also found in the interspaces of the cells of organs. Indeed, the circulatoi-y system of lower forms is in many respects analogous to the lymj^hatic system of higher ones. 19 290 ANIMAL PHYSIOLOGY. DIGESTION OF FOOD. The processes of digestion may be considered as having for their end the preparation of food for entrance into the blood. This is in part attained when the insoluble parts have been rendered soluble. At this stage it becomes necessary to inquire as to what constitutes food or a food. Inasmuch as animals, unlike plants, derive none of their food from the atmosphere, it is manifest that what they take in by the mouth must contain every chemical element, in some form, that enters into the composition of the body. But actual experience demonstrates that the food of animals must, if we except certain salts, be in organized form — i. e., it must approximate to the condition of the tissues of the body in a large degree. Plants, in fact, are necessary to animals in working up the elements of the earth and air into form suit- able for them. Foodstuffs are divisible into : I. Organic. 1. Nitrogenous. (a.) Albumins. (6.) Albuminoids (as gelatine). 2. Non-nitrogenous. (a.) Carbohydrates (sugars, starches). (Z>.) Fats. II. Inorganic. 1. Water, 2. Salts. Animals may derive the whole of their food from the bodies of other animals (carnivora) ; from vegetable matter exclusively {herbivora) ; or from a mixture of the animal and vegetable, as in the case of the pig, bear, and man himself {omnivora). It has been found by feeding experiments, carried out mostly on dogs, that animals die when they lack any one of the con- stituents of food, though they live longer on the nitrogenous than any other kind. In some instances, as when fed on gela- tine and water, or sugar and water, the animals died almost as soon as if they had been wholly deprived of food. But it has also been observed that some animals will all but starve rather than eat certain kinds of food, though chemically sufficient. DIGESTION OF POOD. 291 We must thus recognize something more in an animal than merely the mechanical and chemical processes which suffice to accomplish digestion in the laboratory. A food must be not only sufficient from the chemical and physical point of view, but be capable of being acted on by the digestive juices, and of such a nature as to suit the particular animal that eats it. To illustrate, bones may be masticated and readily digested by a hyena, but not by an ox or by man, though they meet the conditions of a food in containing all the requisite constituents. Further, the food that one man digests readily is scarcely digesti- ble at all by another ; and it is within the experience of every one that a frequent change of diet is absolutely necessary. Since all mammals, for a considerable period of their exist- ence, feed upon milk exclusively, this must represent a perfect or typical food. It will be worth while to examine the compo- sition of milk. The various substances composing it, and their relative proportions for different animals, may be seen from the following table, which is based on a total of 1,000 parts : CONSTITUENTS. Hnmau. Cow. Goat. Ass. Water 889 08 857-05 863-58 910-24 Casein Albumin Butter 39-24 26-66 43-64 1-38 j 48-28 } 5-76 43-05 40-37 5-48 33-60 12-99 43-57 40-04 6-22 I 20-18 12-56 Milk-sugar . . Salts [ 57-02 Total solids 110-92 142-95 136-43 89-76 The fact that human milk is poorer in proteids and fats especially is of practical importance, for, when cow's milk is sub- stituted in the feeding of infants, it should be diluted, and sugar and cream added if the normal proportions of mother's milk are to be retained. 1. The proteids of milk are : (a.) An albumin very like serum-albumin. (b.) Casein, normally in suspension, in the form of extremely minute particles, which contributes to the opacity of milk. It can be removed by filtration through porcelain ; and pre- cipitated or coagulated by acids and by rennet, an extract of the mucous membrane of the calf's stomach. After this coagu- lation, whey, a fluid more or loss clear, separates, which con- tains the salts and sugar of milk and most of tlie water. Much of the fat is entangled with the casein. 292 ANIMAL PHYSIOLOGY. Casein, with some fat, makes up the greater part of cheese. 2. Fats. — Milk is an emnlsion — i. e., contains fat suspended in a fine state of division. The globules, which vary greatly in size, are surrounded by an envelope of proteid matter. This covering is broken up by churning, allowing the fatty globules to run together and form butter. Butter consists chiefly of olein, palmitin, and stearin, but contains in smaller quantity a variety of other fats. The ran- cidity of butter is due to the presence of free fatty acids, espe- cially butyric. The fat of milk usually rises to the surface as cream when milk is allowed to stand. 3. Milk-sugar, which is converted into lactic acid, probably by the agency of some form of micro-organism, thus furnish- ing acid sufficient to cause the precipitation or coagulation of the casein. Milk-sugar. Lactic acid. CgHiaOe = 2C3H8O3 Milk, when fresh, should be neutral or faintly alkaline. 4. Salts (and other extractives), consisting of phosphates of calcium, potassium, and magnesium, potassium chloride, with traces of iron and other substances. It can be readily understood why children fed on milk rarely suffer from that deficiency of calcium salts in the bones leading to rickets, so common in ill-fed children. It thus appears that milk contains all the constituents requisite for the building up of the healthy mammalian body ; and experiments prove that these exist in proper proportions and in a readily digestible form. The author has found that a large number of animals, into the usual food of which, in the adult form, milk does not enter, like most of our wild mammals, as well as most birds, will not only take milk but soon learn to like it, and thrive well upon it. Since the embryo chick lives upon the egg, it might have been supposed that eggs would form excellent food for adult animals, and common experience proves this to be the case ; while chemical analysis shows that they, like milk, con- tain all the necessary food constituents. Meat (muscle, with fat chiefly) is also, of course, a valuable food, abounding in proteids. Cereals contain starch in large proportion, but also a mixture of proteids. Green vegetables contain little actual nu- tritive material, but are useful in furnishing salts and special substances, as certain compounds of sulphur which, in some ill- understood way, act beneficially on the metabolism of the body.' DIGESTION OF FOOD. 293 They also seem to stimulate the flow of healthy digestive fluids. Condiments act chiefly, perhaps, in the latter way. Tea, coffee, etc., contain alkaloids, which it is likely have a conservative effect on tissue waste, but we really know very little as to how it is that they prove so beneficial. Though they are recognized to have a powerful effect on the nervous system as stimulants, nevertheless it would be erroneous to suppose that their action was confined to this alone. Animal Foods. Explanation of the signs. Beef. Pork. Fowl. Fixh. Egg. Cou-'s milh. Human milk. Proteids. Albuminoids. N-free org. bodies. Salts. i5 ! 5 1-5 3' U'.5 ' i-Hl^.f] 1.3 73.5 HHo.o 1110.4 Wheaten-bread. Peas. Rice. Potatoes. WhiU Turnip. CauHflt/wer. lieer. Vegetable Foods. Explanation of the signs. Digestible Non-digestible Salts. N-free organ bodies. m 90.5 90 90 FiQ. 348 (Landois). 0.5 1' 0.5 Hm^ 31II«-' The accompanying diagrams will serve to represent to the eye the relative proportions of the food-essentials in various kinds of articles of diet. 294 ANIMAL PHYSIOLOGY. It is plain that if, in the digestive tract, foods are changed in solubility and actual chemical constitution, this must have Fig. 349.— Alimentary canal of embryo while the rudimentary mid-gut is stiU in continuity with yelk-sac (Kolliker, after BischofC). A. View from .before, a, pharyngeal plates ; 6, pharynx \ c, c, diverticula forming the lungs ; d, stomach ; /, diverticula of liver ; g. membrane torn from yelk-sac ; h, hind-gut. B. Longitudinal section, a, diverticulum of a lung ; 6, stomach ; c, liver ; d, yelk-sac. been brought about by chemical agencies. That food is broken up at the very commencement of the alimentary tract is a matter of common observation; and that there should be a gradual movement of the food from one part of the canal to Fig. 250. Fig. 251. Fig. 250.— Diagram of alimentary canal of chick at fourth day (Foster and Balfour, after Gotte). Ig, diverticulum of one lung ; St, stomach ; I, liver ; p, pancreas. Fig. 251.— Position of various parts of alimentary canal at different stages. A. Embryo of five weeks. B. Of eight weeks. C. Of ten weeks (Allen Thompson). I, pharynx with the lungs ; .s, stomach ; i", small intestine ; i\ large intestine ; gr, genital duct ; u, bladder ; cl, cloaca ; c, csecimi ; vi, ductus vitello-intestinalis ; si, urogenital sinus ; v, yelk-sac. DIGESTION OF FOOD. 295 another, where a different fluid is secreted, would be expected. As a matter of fact, mechanical and chemical forces play a large part in the actual preparation of the food for absorption. Behind these lie, of course, the vital properties of the glands, which prepare the active fluids from the blood, so that a study of digestion naturally divides itself into the consideration of — B^G. 252. — Ammothea pycnogonides, a marine animal (after Quatrefages). ce, cesophagus ; o, antennae ; s, stomach, with prolongations into antennae and limbs (I). 1. The digestive juices; 2. The secretory processes ; and, 3. The muscular and nervous mechanism by which the food is carried from one part of the digestive tract to another, and the waste matte? finally expelled. :2i.^^^ Fio. 253.— Ixjngitudinal vertical section of body of leech, Hirudo niedicinalis (After Leuckart). a, mrtutb : Ij, h, h, Hacculation.s of alimentary canal ; c, anus ; d, terminal sucker ; e, cere- bral ganglia ; /, /', chain of post-u^sr^phageal ganglia ; r/, g, g, segmental organs. Embryological. — Tlic alimentary tract, as we have seen, is fornic'i by ;iii infolding of tlu; splanchnopleure, and, according as the growth is more or less marked, does the canal become 296 ANIMAL PHYSIOLOGY. iortuous or remain somewliat straight. The alimentary tract of a mammal passes through stages of development which cor- respond with the permanent form of other groups of verte- brates, according to a general law of evolution. Inasmuch as the embryonic gut is formed of mesoblast and hypoblast, it is easy -to understand why the developed tract should so invaria- bly consist of glandular structures and muscular tissue dis- posed in a certain regular arrangement. The fact that all the Fig. 254. — Portion of a jelly-flsh, the Medusa Aurelia, showing gastro-vascular canals radi- ating from central stomach and terminating in a circular marginal canal (after Romanes). All these are shaded very dark ; the light spaces indicate artificial sections. Inasmuch as these canals as well as the stomach must contain some sea-water, and since their contents represent the whole of the nutritive fluid (answering to the blood. l.ymph, and chyle of higher forms), we have both anatomically and physiologically a very crude or undifferen- tiated condition in such animals, and one of great interest from an evolutionary point of view. organs that pour digestive juices into the alimentary tract are outgrowths from it serves to explain why there should remain a physiological connection with an anatomical isolation. The general resemblance of the epithelium throughout, even in parts widely separated, also becomes clear, as well as many otter points we can not now refer to in detail, to one who realizes the significance of the laws of descent (evolution). Comparative. — Amoeba ingests and digests apparently by every part of its body ; though exact studies have shown that it neither accepts nor retains without considerable power of DIGESTION OP FOOD. 2y: discrimination ; and it is also possible that some sort of digest- ive fluid may be secreted from the part of the body with which the food-particles come in contact. It has been shown, too, that there are differences in the digestive capacity of closely allied forms among Infiisorians. The ciliated Infusorians have a permanent mouth, which may also serve as an anus ; or, there may be an anus, though , usually less distinct from the rest of the body than the mouth. Among the Coelenterates inira-cellular digestion is found. Certain cells of the endoderm (as in Hydra) take up food-parti- Fio. '2T)T).—\ jflly-fish, the Mcduna TJninnrodium (aft^r Allman). Note the lotiR proljoscif; ^rnouth) \ftu\\nv; iir> to th«* Ktoinach, frnm which radiate the pastro-vasciilar canals. A fKirtion of the b»-ll haw Ix-eri removed, shfiwinj; the generative arranged around the diKCHtive organs. Most of the tentacles are turned up. ch'K Ama'})a-like, digest them, and thus ])rovicle material for other cells as well as thcmselvf^s, in a form suitable for assimi- lation. This is a beginning of that differentiation of function 298 ANIMAL PHYSIOLOGY. which is carried so far among the higher vertebrates. But, as recent investigations have shown, such intra-cellular digestion exists to some extent in the alimentary canal of the highest members of the vertebrate groap (see page 345). The means for grasping and triturating food among in- vertebrates are very complicated and varied, as are also those adapted for sucking the juices of prey. Examples to hand are to be found in the crab, crayfish, spider, grasshopper, beetle, Fig. 256.— Diagram illustrating arrangement of intestine, nervous system, etc., in common snail. Helix (after Huxley), m, mouth ; t, tooth ; od, odontophore ; g, gullet ; c, crop ; s, stomach ; r, rectum ; a, anus ; r. s, renal sac ; /i, heart ; I, lung (modified pallial cham- ber) ; 11, its external aperture ; em, thick edge of mantle united with sides of body : /, foot ; cpg, cerebral, pedal, and parieto-splancnnic ganglia aggregated round gullet. etc., on the one hand, and the butterfly, house-fly, leech, etc., on the other. The digestive system of the earth-worm has been studied with some care. It illustrates a sort of extra-corporeal diges- tion, in that it secretes a fluid from the mouth which seems 'to act both chemically and mechanically on the starch-grains of the leaves on which it feeds. It is provided with an organ in which, as with birds, small stones are found, so that the imperfections of its mouth are compensated for by this gizzard which triturates the food. Its calciferous glands supply the alkaline fluids necessary to neutralize the humus acids of de- caying leaves, for intestinal digestion only proceeds in an alka- line medium. The gastric mill of a crab (Fig. 228) is a provision of ob- vious value in so voracious a creature. DIGESTION OF FOOD. 299 Pal > ^ =5 - 2 m 5« Before passing on to higher groups, it will be well to bear in mind that the digestive organs are to be regarded as the out- come both of he- redity and adap- tation to circum- stances. We find parts of the in- testine, e, g., re- tained in some animals in whose economy they seem to serve little if any good purpose, as the vermiform ap- pendix of man. Adaptation has been illustrated in the lifetime of a single indi- vidual in a re- markable man- ner ; thus, a sea- gull, by being fed on grain, has had its stomach, nat- urally thin and soft-walled, con- verted into a muscular giz- zard. Since diges- tion is a process in which the mechanical and chemical are both involved, and tlni food of animuls differs so widely, great variety in the alimentary tract, both ana- tomical and pliysiologicul, must be (jxpected. Vegetable food must usually be eat(;n in miu-h larger bulk to furnish the needed elements; hence the great length of intestine habitually §*s g I °. s s a O S o GO .^ d g 5. ; 3 t-. a"S eg Q m C :3ci_j J- .N ^^ h- ;3 ^ ~ , S^3.« ex) Cii =-« 57,, a! c a "g a S 5 j. §^ O m 5 eS !-tc •So 75 a.-r o O ,5 t> .« 3 £;•!= g^vj oi ..£_; a cs-^ _ .5 O) 0) ai'T;.^ O r/i ■^o^.S £-5 = 2 ^ k„'-5 < a C) cj . "'^ ^•- . . 3 ft; > a o Qj^ 55 0) _H = :''^^^|a'J t; •• = N a ao o ci > — N o -M £ 0/ ij' ^T, 1) »: O C 83"""^ = " 5.P. oOoj>fcS JH515 ■i=S^6 o o 2 bl So ^ E •- ' c! "^ 'o a 1:; ■.- " a . . en . m »t ? fcj 5 '*' '^.'^ ^2 » £ <•, OOa'^t-OirH 300 ANIMAL PHYSIOLOGY. found in herbiivorons animals, associated often with, a capacious and chambered stomach, furnishing a larger laboratory in Fig. 258.— The viscera of a rabbit as seen upon simply opening the cavities of the thorax and abdomen without any further dissection. A, cavity of the thorax, pleural cavity on either side ; B, diaphragm ; C, ventricles of the heart ; D, auricles ; E, pulmonary artery ; F", aorta ; G, lungs collapsed, and occupying only back part of chest ; H, lateral portions of pleural membranes ; /, cartilage at the end of sternum (ensiform cartilage) ; K, portion of the waU of body left between thorax and abdomen ; a, cut ends of the ribs ; L, the liver, in this case lying more to the left than to the right of the body ; M, the stomach, a large part of the greater curvature being shown ; N, duodenum ; O, small intestine ; P, the caecum, so largely developed in this and other herbivorous animals ; Q, the large intestine. (Huxley.) which Nature may carry on her processes. To illustrate, the stomach of the ruminants consists of four parts {rumen, reticu- lum, omasum (psalterium,), dbomasum). The food when cropped is immediately swallowed ; so that the paunch {rumen) is a mere storehouse in which it is softened, though but little changed otherwise ; and it would seem that real gastric di- DIGESTION OF POOD. 301 gestioii is almost confined to the last division, which may be compared to the simple stomach of the Carnivora or of man ; and, before the food reaches this region, it has been thoroughly masticated and mixed with saliva. Fig. 259.— Stomach, ii.n i lu^. iiit(«.tine, etc (aft<'r Sapjx-y) 1, anterior surface of liver; 2. (^all-bluiM'i , ^ ition of diaphraKiii : 4, postciior '-uiface of stomach; 5, lobas Rpiffelii of liver . 0, uelidc axis , 7, coionary arter\ of stoiiiath ; 8, splenic art«ry ; 9. spleen: 10. pancreas, 11, isupcnor nn-st-ntcriu vmnt-ln ; 12, Juoclenuni ; 13, upper ex- tremity of small intestine ; 14, lower end of ileum ; 1.5, 1.5, mesentery ; 16, cajcum ; 17, apjjendix vermiformis : 18, ascending: colon ; 19, 19, transverse colon ; 20, descending colon ; 21, sijrmoid flexure of colon ; 22, rectum ; 23, urinary bladder. The reticulum is especially adapted for holding water, which may serve a good purpose in moistening and thinning the con- tents of the stomach. In the camels and llamas a portion of the stomach is made up of pouches, which can be closed with sphincter muschis, and thus shut off tlie water-supply in s(?p- arate tanks, as it w(^re. The stomach of th(! horse is small, though the; intestine, especially the large gut, is (;aj)acious. Tlif stomach is divisible into a cardiac region, of a light color internally, and lined witli epithelium, like that of the 302 ANIMAL PHYSIOLOGY. Fig. 260.— a. Stomach of sheep. B. Stomach of musk-deer, ce, oesophagus ; Rn, rumen ; Bet, reticulum ; Ps, psalterium ; A, Ab, abomasum ; Du, duodenum ; Py. pylorus (Huxley). Fig. 261.— Stomach of horse (after ChauveauV A, cardiac extremity of oesophagus; B, pyloric ring. DIGESTION OF FOOD. 303 oesophagus, and a redder pyloric area, in which the greater part of the digestive process goes on. Fic;. 262.- Stomach of dog (after C'liauvi-aui. . I , oesophagus ; S, pylorus. The mouth parts, even in some of the higher vertebrates, as the Carnii'ora, serve a prehensile rather than a digestive pur- pose. This is well seen in the dog, that bolts his food ; but in this and allied groups of mammals gastric digestion is very active. Fig. 2C3.— General and lateral view of dog"H teeth (aft»'r (!hauvcaii). 304 ANIMAL PHYSIOLOGY. The teeth, as triturating organs find their highest develop- ment in ruminants, the combined side-to-side and forward-and- backward motion of the jaws rendering them very effective. Fig. 265. Fig. 264. Fig. 264.— Dentition of inferior jaw of horse (after Chauveau). Fig. 265. — Inferior maxilla of man (after Sappey). Alveolar border ; /, incisor teeth ; c, canine teeth ; 6, bicuspid teeth ; m, molars. In Carnivora the teeth serve for grasping and tearing, while in the Insectivora the tongue, as also in certain birds (wood- peckers), is an important organ for securing food. It is to be noted, too, that, while the horse crops grass by biting it off, the ox uses the tongue, as well as the teeth and lips, to secure the mouthful. DIGESTION OP FOOD. 305 Fio. 2WJ. — dcncral view of (lij."'stivp apparatiiH of fowl faflcr Chnuvcaui. 1, tnnpiie ; 2, pharynx: 3. first. iKirtion of , wcfiiid portion of sopliii(;iis : fi, Buccentric v«ntricl(? (provgous to the secretion of sweat in tins foot of ;i recently (h^ad animal, under stimulation of tlicf sciatic 21 322 ANIMAL PHYSIOLOGY. nerve. 5. The character of the saliva secreted varies with the nerve stimulated, so that it seems likely that the nervous centers normally in the intact animal regulate the quality of the saliva through the degree to which one or the other kind of nerves is called into action. 6. Secretion of saliva may be induced reflexly by experiment, and such is probably the normal course of events. 7. The action of the medullary center may be inhibited by the cerebrum (emotions). Some have located a center in the cerebral cortex (taste cen- ter), to which it is assumed impulses first travel from the tongue and which then rouses the proper secreting centers in the medulla into activity. It seems more likely that the corti- cal center, if there be one, completes the physiological processes by which taste sensations are elaborated. From the influence of drugs (atropin and its antagonist pilocarpin) it is plain that the gland can be affected through the blood, though whether wholly by direct action on the cen- ter, on any local nervous mechanism or directly on the cells, is as yet undetermined. It is found that pilocarpin can act long after section of the nerves. This does not, however, prove that in the intact animal such is the usual modus operandi of this or other drugs, any more than the so-called paralytic secretion after the section of nerves proves that the latter are not con- cerned in secretion. We look upon paralytic secretion as the work of the cells when gone wrong — passed from under the dominion of the nerve-centers. Secretion is a part of the natural life-processes of gland-cells — we may say a series in the long chain of pro- cesses which are indispensable for the health of these cells. They must be either secreting cells, or have no place in the nat- ural order of things. It is to be especially noted that the secre- tion of saliva continues when the pressure in the ducts of the gland is greater than that of the blood in its vessels or even of the carotid ; so that it seems possible that over-importance has been attached to blood - pressure in secretory processes generally. It may, then, be safely assumed that formation of saliva re- sults in consequence of the natural activity of certain cells, the processes of which are correlated and harmonized by the nerv- ous system ; their activity being accompanied by an abundant supply of blood. The actual outpouring of saliva depends usu- ally on the establishment of a nervous reflex arc. The other glands have been less carefully studied, but the parotid is DIGESTION OP FOOD. 323 known to have a double nervous supply from the cerebro- spinal and the sympathetic systems. It would appear that, as the vaso-motor changes run paral- lel with the secretory ones, the vaso-motor and the proper secretory centers act in concert, as we have seen holds of the former and the respiratory center. But it is to our own mind very doubtful whether the doctrine of so sharp a demarkation of independent centers, prominently recognized in the physi- ology of the day, will be that ultimately accepted. Secretion by the Stomach. — The mucous membrane of St. Mar- tin's stomach was observed to be pale in the intervals of diges- tion, but flushed when secreting, which resembled sweating, so far as the flow of the fluid is concerned. When the man was irritated, the gastric membrane became pale, and secretion was lessened or arrested, and it is a common experience that emo- tions may help, hinder, or even render aberrant the digestive processes. While the evidence is thus clear that gastric secretion is regulated by the nervous system, the way in which this is accomplished is very obscure. We know little of either the centers or nerves concerned, and what we do know helps but doubtfully to an understanding of the matter, if, indeed, it does not actually confuse and jDuzzle. Digestion can proceed in a fashion after section of the nerves going to the stomach, though this has little force as an argu- ment against nerve influence. We may conclude the subject by stating that, while the influence of the nervous system over gastric secretion is undoubted as a fact, the method is not understood ; and the same remark applies to the secreting activity of the Ywcv and pancreas. The Secretion of Bile and Pancreatic Juice. — When the contents of the stomach have reached the orifice of the discharging bile- duct, a large flow of the biliary secretion takes place, probably as the result of the emptying of the gall-bladder by the con- traction of its walls and those of its ducts. This is probably a reflex act, and the augmented flow of bile when digestiwi is proceeding is also to be traced chiefly to nervous influences reaching the gland, though by what nerves or under the gov- ernment of what part of the nervous centers is unknown. Very similar statements apply to the secretion of the pancre- atic glands, though this is not constant, as in the case of bile — at all events, in most animals. It is known that after food lias been taken there is a sudden 324 ANIMAL PHYSIOLOGY. increase in tlie quantity of bile secreted, followed by a sudden diminution, then a more gradual rise, with a subsequent fall. Almost the same holds for the pancreas. Fig. 274.— Diagram to show influence of food in secretion of pancreatic juice (after N. O. Bern- stein). Ttie abscissae represent hours after taking food ; ordinates amount in cubic centi- grammes of secretion in ten minutes. Food was taken at B and C. This diagram very nearly also represents the secretion of bile. It seems impossible to explain these facts, especially the first rapid discharge of fluid apart from the direct influence of the nervous system. Upon the whole, the evidence seems to show that the press- ure in the bile-ducts is greater than in the veins that unite to make up the portal system; but there are difficulties in the investigation of such and kindred subjects as regards the liver, owing to its peculiar vascular supply. It will be borne in mind that the liver in mammals consists of a mass of blood-vessels, between the meshes of which are packed innumerable cells, and that around the latter meander the bile capillaries ; that the portal vein breaks up into the interlobular, from which capil- laries arise, that terminate in the central intralobular veins, which make up the hepatic veinlets or terminate in these vessels But the structure is complicated by the branches of the hepatic artery, which, as arterioles and capillaries, enters to some extent into the formation of the lobular vessels. It is remarkable that the cells of the liver are so similar, considering the complicated functions they appear to discharge. DIGESTION OP FOOD. 325 A question of interest, though difficult to answer, is the extent to which the various constituents of bile are manufact- ured in the liver. Taurin, for example, is present in some of Fig. 275.— Lobules of liver, interlobular vessels, and intralobular veins (Sappey). 1, 1, 1, 1, 3, 4, lobules : 2. 2. 2. 2. intralobular veins injected with white ; 5, 5, 5, 5, 5, intralobular vessels filled with a dark injection. the tissues, but whether this is used in the manufacture of taurocholic acid or whether the latter is made entirely anew, and possibly by a method in which taurin never ap- pears as such, is an open question. It is highly prob- able that a portion of the bile poured into the intes- tine is absorbed either as such or after i)artial decom- position, the products to be used in some way in the economy and presumably in the construction of bile by the liver. There arc many facts, including some patho- logical phenomena, that point clearly to the forma- tion of the pigments of bile from hemoglobin in some of its stages of degeneration. Pathological. — When tlic Fio 276.— Portion of transvei se section of hepatic lobnle of rabbit: inaKnIflfd 4(H) diam.-ters (Koliiker). I>, h. h, rai)illary lilood vessels ; 7, J/, '/. capillary l)ile-du<;ts ; /, Z, L liver-cells. liver fails to act either from de- 326 ANIMAL PHYSIOLOGY. rangemeiit of its cells primarily or owing to obstruction to the outflow of bile leading to reabsorption by the liver, bile acids and bile pigments appear in the urine or may stain the tissues, indicating their presence in excess in the blood. This action of one gland (kidneys) for another is highly suggestive, and especially important to bear in mind in medical practice, both in treatment and prognosis. The chances of re- covery when only one excreting gland is diseased are much greater evidently than when several are involved. Such facts as we have cited show, moreover, that there are certain common fundamental principles underlying secretion everywhere — a statement which will be soon more fully illustrated. The Nature of the Act of Secretion. We are now about to consider some investigations, more particularly their results, which are of extraordinary interest. The secreting cells of the salivary, the pancreatic glands, and the stomach have been studied by a combination of histo- logical and, more strictly, physiological methods, to which we shall now refer. Specimens of these glands, both before and after prolonged secretion, under stimulation of these nerves, were hardened, stained, and sections prepared. As was to be expected, the results were not entirely satisfactory under these methods ; however, the pancreas of a living rabbit has been viewed with the microscope in its natural condition ; and by this plan, especially when supplemented by the more involved and artificial method first referred to, results have been reached Fig. 277.— Portion of pancreas of rabbit (after Kiihne and Lea). A represents gland at rest ; B, during secretion. which may be ranked among the greatest triumphs of modern physiology. DIGESTION OP FOOD. 327 Some of these we now proceed to state briefly. To begin with the pancreas, it has been shown that, when the gland is not secreting — i. e., not discharging its prepared fluid — or dur- ing the so-called resting stage, the appearances are strikingly- different from what they are during activity. The cell pre- sents during rest an inner granular zone and an outer clearer zone, which stains more readily, and is relatively small in size. The lumen of the alveolus is almost obliterated, and the in- dividual cells very indistinct. After a period of secreting activity, the lumen is easily perceived, the granules have dis- appeared in great part, the cells as a whole are smaller, and have a clear appearance throughout. Coincident with the changes in the gland's cells it is to be noticed that more blood passes through it, owing to dilatation of the arterioles. \\ ,-n<^ B Fig. 278.— Section of mucous gland (after Lavdowsky). In A, gland at rest ; in B, after secreting for some time. Again, the course of the changes in the salivary glands, whether of the mucous or serous variety, is very similar. In the mucous gland in the resting stage the cells are large, and hold much clear matter in the interspaces of the cell network ; and, as this does not stain readily, it can not be ordinary protoplasm. This, when the gland is stimulated through its nerves, disappears, leaving the containing cells smaller. It has become mucin, and may itself be called mucinoyen. It is to be noted that, as the cells become more protoplasmic, less burdened with the products of their activity, the nucleus becomes more prominent, suggestive of its having a probable directive influence over these manufacturing processes. Substantially the same chain of events has been established for the serous salivary glands and the stomach, so that we may safely generalize upon these well-established facts. 328 ANIMAL PHYSIOLOGY. It seems clear that a series of clianges constructive and, from one point of view, destructive, following the former are con- FiG. 279.— Changes in parotid (serous) gland during secretion (after Langlej'). A, during rest ; B, after moderate, C, after prolonged stimulation. Figures partly diagrammatic. stantly going on in the glands of the digestive organs. Proto- plasm under nerve influence constructs a certain substance, which is an antecedent of the final product, which we term a ferment. It is now customary to speak of these changes as constructive (anabolic) and destructive (katabolic), though we have already pointed out (page 270) that this view is, at best, only one way of looking at the matter, and we doubt if it may. not be cramping and misleading. We must also urge caution in regard to the conception to be associated with the use of the terms " resting " and " active " stage. It is not to be forgotten that strictly in living cells there is no absolute rest — such means death ; but, if these terms be understood as denoting but degrees of activity, they need not mislead. It is also more than probable that in certain of the glands, or in some animals, the processes go on simultane- ously : the protoplasm being renewed, the zymogen, or mother- ferment, being formed, and the latter converted into actual fer- ment, all at the same time. It has been pointed out that chorda saliva is usually more watery than that secreted under stimulation of the sympathetic. When atropine is injected there is no discharge whatever, not- withstanding that the usual vascular dilatation follows, from which it is clear that the water is actually secreted. The nature of secretion is now tolerably clear as a whole ; though it is to be remembered that this account is but general, and that there are many minor differences for each gland and variations that can scarcely be denominated minor for different animals. Evidently no theory of filtration, no process depend- ing solely on blood-pressure, will apply here. And if in this, the best-studied case, mechanical theories of vital processes utterly fail, why attempt to fasten them upon other glands, as DIGESTION OF FOOD. 329 the kidneys and the lungs, or, indeed, apply such crude concep- tions to the subtle processes of living protoplasm anywhere or in any form ? It is somewhat remarkable that an extract of a perfectly fresh pancreas is not proteolytic ; yet the gland yields such an extract when it has stood some hours or been treated with a weak acid. These facts, together with the microscopic appear- ances, suggested that there is formed a forerunner to the actual ferment — a zymogen, or mother-ferment, which at the moment of discharge of the completed secretion is converted into the actual ferment. We might, therefore, speak of a pepsinogen, typsinogen, etc., and, though there may be a cessation in the series of processes, and no doubt there is in some animals, this may not be the case in all or in all glands. Secretion by the Stomach. — The glands of the stomach differ in most animals in the cardiac and pyloric regions. In those of the former zone, both central, columnar, and parietal (ovoid) cells are to be recognized. It was thought that possibly the latter were concerned in the secretion of the acid of the stomach, but this is by no means certain. Possibly these, like the demilune cells of the pancreas, may be the progenitors of the central (chief) cells. The latter certainly secrete pepsin, and probably also rennet. Mucus is secreted by the cells lining the neck of glands and covering the raucous membrane inter- vening between their mouths. The production of hydrochloric acid by any act of secretion is not believed in by all writers, some holding that it is derived from decomposition of sodium chloride, possibly by lactic acid. So simple an origin is not probable, not being in keeping with what we know of chemical processes within the animal body. Self-Digestion of the Digestive Organs. — It has been found, both in man and other mammals, that when death follows in a healthy subject while gastric digestion is in active progress and the body is kept warm, a part of the stomach itself and often adjacent organs are digested, and the question is con- stantly being raised, Why does not the stomach digest itself during life ? To this it has been answered that the gastric juice is constantly being neutralized by the alkaline blood ; and, again, that the very vitality of a tissue gives it the neces- sary resisting powers, a view contradicted by an experiment which is conclusive. If the legs of a living frog be allowed to hang against the inner walls of the stomach of a mammal when gastric digestion is going on, they will be digested. 330 ANIMAL PHYSIOLOGY. Tlie first view (the alkalinity of the blood) would not suffice to explain why the pancreas, the secretion of which acts best in an alkaline medium, should not be digested. It seems to us there is a good deal of misconception about the facts of the case. Observation on St. Martin shows that the secretion of gastric juice runs parallel with the need of it, as dependent on the introduction of food, its quantity, quality, etc. Now, there can be little doubt that, if the stomach were abundantly bathed when empty with a large quantity of its own acid secretion, it would suffer to some extent at least. But this is never the case ; the juice is carried off and mixed with the food. This food is in constant motion and doubtless the inner portions of the cells, which may be regarded as the discharging region, while the outer (next the blood capillaries, the chief manufacturing region of the digestive ferment) are frequently renewed. Such considerations, though they seem to have been some- what left out of the case, do not go to the bottom of the matter. Amoeba and kindred organisms do not digest them- selves. Some believe that the little pulsatile vacuoles of the Infusorians are a sort of temporary digestive cavities. But, to one who sees in the light of evolution, it must be clear that a structure could not have been evolved that would be self -destructive. The difficulty here is that which lies at the very basis of all life. We might ask. Why do living things live, since they are constantly threatened with destruction from within as from without ? Why do not the liver, kidney, and other glands that secrete noxious substances, poison themselves ? We can not in detail explain these things; but we wish to make it clear that the difficulty as regards the stomach is not peculiar to that gland, and that even from the ordinary point of view it has been exaggerated. Comparative. — More careful examination of the stomachs of some mammals has revealed the fact that in several animals, in which the stomach appears to be simple, it is in reality compound. There are different grades, however, which may be regarded as transition forms between the true simple stomach and that highly compound form of the organ met with in the ruminants. It has been shown recently that the stomach of the hog has an oesophageal dilatation; and that the entire organ may be divided into several zones with different kinds of glandular DIGESTION OF FOOD. 331 opifhelium, etc. These portions differ in digestive power, in the characteristics of the fluid secreted, and other details be- yond those which a superficial examination of this organ would lead one to suspect. The stomach of the horse represents a more advanced form of compound stomach than that of the hog, which is not evi- dent, however, until its glandular structure is examined closely. The entire left portion of the stomach represents an oesophageal dilata- tion lined with an ejjithelium that closely resembles that of the oesoph- agus, and with little if any digest- ive function. It thus appears that the stomach of the horse is in reali- ty smaller, as a true digestive gland, than it seems, so that a great part of the work of digestion must be done in the intestine ; though in this animal, if the food be retained long as it is in the hog, which is not, however, the general opinion as regards the stomach of the horse, salivary digestion may continue for a considerable period after the food has left the mouth. The secretion of mucus by the stomach in herbivora is abundant. Fig. 280.— Interior of horse's stomach (after Chauveau). A, left sac ; B, right sac ; C, duodenal dilatation. The Movements of the Digestive Organs. As with other parts of the body, so in the alimentary tract, the slower kind of movement is carried out by plain muscu- lar fibers ; and the movements, as a whole, belong to the class known as peristaltic ; in fact, it is only at the l)eginning of the digestive tract that voluntary (striped) muscle is to be found and to a limited extent in the part next to this — i. e., in the oesophagus. Teeth in the highly organized mammal are remarkable in being to the least degree living structures of any in the entire animal, thus being in marked contrast to other organs. The enamel covering their exposed surfaces is the hardest of all the ti.ssues and is necessarily of low vitality. We have already alluded to the difference in the teeth of different animals, and their relation to customary food and digestive functions. In fact, it is clear that the teeth and all the parts of the digestive 332 ANIMAL PHYSIOLOGY. system are correlated to one another. The compound stomach of the ruminants, with its slow digestion of a bulky mass of food, which must be softened and thoroughly masticated be- fore the digestive juices can attack it successfully, harmonizes with the powerful jaws, strong muscles of mastication, and grinding teeth ; and all these in marked contrast with the teeth of a carnivorous animal with its simple but highly effect- ive stomach. Compare figures in earlier pages. Mastication in man is of that intermediate character befit- ting an omnivorous animal. The jaws have a lateral and forward-and-backward movement, as well as a vertical one, though the latter is predominant. The upper jaw is like a fixed millstone, against which the lower jaw works as a nether millstone. The elevation of the jaw is effected by the mas- seter, temporal, and internal pterygoid muscles ; depressed by the mylohyoid and geniohyoid, though principally by the di- gastric. The jaw is advanced by the external pterygoids; unilateral contraction of these muscles also produces lateral movement of the inferior maxilla, which is retracted by the more horizontal fibers of the temporal. The cheeks and tongue likewise take part in preparing the food for the work of the stomach, nor must the lips be over- looked even in man. The importance of these parts is well illustrated by the imperfect mastication, etc., when there is paralysis of the muscles of which they are formed. Even when there is loss of sensation only, the work of the mouth is done in a clumsy way, showing the importance of common sensation, as well as the muscular sense. Nervous Supply. — The muscles of the tongue are governed by the hypoglossal nerve ; the other muscles of mastication chiefly by the fifth. The afferent nerves are branches of the fifth and glosso-pharyngeal. It is, of course, important that the food should be rolled about and thoroughly mixed with saliva (in- salivation). Deglutition. — The transportation of the food from the mouth to the stomach involves a series of co-ordinated muscular acts of a complicated character, by which difficulties are overcome with marvelous success. It will be remembered that the respiratory and digestive tracts are both developed from a common simple tube — a fact which makes the close anatomical relation between these two physiologically distinct systems intelligible ; but it also involves •difficulties and dangers. It is well known that a small quantity DIGESTION OF FOOD. 333 of food or drink entering the windjjipe produces a perfect storm of excitement in the respiratory system. The food, there- '^iS.-J Fig. 281.— Cavities of mouth and pharynx, etc. (after Sappey). Section, in median Hne, of face and superior portion of neck, designed to show the mouth in its relations to the nasal fossa-, pharynx, and larj'nx: 1, sphenoidal sinuses; 2, internal orifice of Eustachian tube; 3. palatine arch ; 4. velum pendulum palati ; 5, anterior pillar of soft palate ; 0, po.sterior pillar of soft palate ; 7, tonsil ; 8. lingual portion of cavity of pharynx ; 0, epiglottis ; 10, section of hyoid bone ; 11, laryngeal portion of cavity of pharynx ; 12, cavity of larynx. fore, when it reaches the oesophagus, must be kept, on the one hand, from entering the nasal, and, on the other, the laryngeal openings. This is accomplished as follows : When the food has been gathered into a bolus on the back of the tongue, the tip of this organ is pressed against the hard palate, by which the ma.ss is prevented from passing forward, and, at the same time, f<.»rced back into the pharynx, the soft palate being raised and the edges of the x>illars of the fauces made to approach the uvula, which fills up the gap remaining, so that the posterior nares are closed and an inclined plane provided, over which the morsel glides. The after-result is said to depend on the siz«j of the bolus. When considerable, the constrictors of the 334 ANIMAL PHYSIOLOGY. pharynx seize it and press it on into tlie gullet ; when the mor- sel is small or liquid is swallowed, it is rapidly propelled on- ward by the tongue, the oesophagus and pharynx being largely passive at the time, though contracting slowly afterward ; at the same time the larynx as a whole is raised, the epiglottis pressed down, chiefly by the meeting of the tongue and itself, while its cushion lies over the rima glottidis, which is closed or all but closed by the action of the sphincter muscles of the larynx, so that the food passes over and by this avenue of life, not only closed but covered by the glottic lid. The latter is not so essential as might be supposed, for persons in whom it was absent have been known to swallow fairly well. The ascent of the larynx any one may feel for himself ; and the be- havior of the pharynx and larynx, especially the latter, may be viewed by the laryngoscope. The grip of the pharyngeal muscles and the oesophagus may be made clear by attaching a piece of food (meat) to a string and allowing it to be partially swallowed. The upward movement of food under the action of the con- strictors of the pharynx is anticipated by the closure of the passage by the palato-glossi of the anterior pillars of the fauces. The circular muscular fibers of the gullet are probably the most important in squeezing on the food by a peristaltic move- ment, passing progressively over the whole tube, though the longitudinal also take part in swallowing, perhaps, by steady- ing the organ. Swallowing will take place in an animal so long as the medulla oblongata remains intact ; and the center seems to lie higher than that for respiration, as the latter act is possible when, from slicing away the medulla, the former is not. An- encephalous monsters lacking the cerebrum can swallow, suck, and breathe. Food placed in the pharynx of animals when unconscious is swallowed, proving that volition is not essential to the act ; but our own consciousness declares that the first stage, or the removal of the food from the mouth to the pharynx, is volun- tary. "When we seem to swallow voluntarily there is in reality a stimulus applied to the fauces, in the absence of food and drink, either by the back of the tongue or by a little saliva. It thus appears that deglutition is an act in the main reflex, though initiated by volition. The afferent nerves concerned are usually the glosso-pharyngeal, some branches of the fifth. DIGESTION OP FOOD. 335 and of the vagus. The efferent nerves are those of the numer- ous muscles concerned. When food hats reached the gullet, it is, of course, no longer under the control of the will. Section of the vagus or stimulation of this nerve modifies the action of the oesophagus, though it is known that contrac- tions may be excited in the excised organ ; but no doubt nor- mally the movements of the gullet arise in response to natural nerve stimulation. Comparative. — That swallowing is independent of gravity is evident from the fact that long-necked animals (horse, giraffe) can and do usually swallow with the head and neck down, so that the fluid is rolled up an inclined plane. The peristaltic nature of the contractions of the gullet can also be well seen in such animals. In the frog the gullet, as well as the mouth, is lined with ciliated epithelium, so that in a recently killed animal one may watch a slice of moistened cork disappear from the mouth, to be found shortly afterward in the stomach. The rate of the descent is surprising — in fact, the movement is plainly visible to the unaided eye. The Movements of the Stomach. — The stomach of mammals, including man, is provided with three layers of muscular fibers : 1. External longitudinal, a continuation of those of the oesopha- gus. 2. Middle circular. 3. Internal oblique. The latter are •^Jtzi^mVrh i' Fio . ywj. -Human Htoma<.-h fafter Sttppcy). 1, o-soiihaffiis : 2. circular flbf-rs at wsophageal «ix'nln(f : 3, :j, circiilur flbtM-s at leswr curvatiirn ; 4, 4, cin-ular fihi-rs at the |)yli»riis ; 5, 5, ei-8 have been removed to show the subjacent circular fibers. 336 ANIMAL PHYSIOLOGY. the least perfect, viewed as an investing coat. The pyloric end of the stomach is iDest supplied with muscles ; where also there is a thick muscular ring or sphincter, as compared with which the cardiac sphincter is weak and ill-developed. The movements of the stomach begin shortly after a meal has been taken, and, as shown by observations on St. Martin, continue for hours, not constantly, but periodically. The effect of the conjoint action of the different sets of muscular fibers is to move the food from the cardiac toward the pyloric end of the stomach, along the greater curvature and back by the lesser curvature, while there is also, probably, a series of in-and-out currents to and from the center of the food-mass. The quantity of food is constantly being lessened by the removal of digested portions, either by the blood-vessels of the organ or by its passing through the pyloric sphincter. The empty stomach is quiescent and contracted, its mucous membrane being thrown into folds. The movements of the stomach may be regarded as reflex, the presence of food being an exciting cause, though probably not the only one ; and so largely automatic is the central mech- anism concerned, that but a feeble stimulus suffices to arouse them, especially at the accustomed time. Of the paths of the impulses, either afferent or efferent, little is known. Certain effects follow section or stimulation of the vagi or splanchnics, but these can not be predicted with certainty, or the exact relation of events indicated. It is said that the movements of the stomach cease, even when it is full, during sleep, from which it is argued that gas- tric movements do normally depend on the influence of the nervous system. However, the subject is too obscure at pres- ent for further discussion. Comparative. — Recent investigations on the stomach of the pig indicate that in this animal the contents of the two ends of the stomach may long remain but little mingled ; and such is certainly the case in this organ among ruminants. Pathological. — Distention of the stomach, either from excess of food or gas arising from fermentative changes, or by secre- tion from the blood, may cause, by upward pressure on the diaphragm, etc., uneasiness from hampered respiration^ and ir- regularity of the heart, possibly, also, in part traceable to the physical interference with its movements. After great and prolonged distention there may be weakened digestion for a considerable interval. It seems not improbable that this is to DIGESTION OF FOOD. 337 be explained, not alone by the impaired elasticity (vitality) of the muscular tissue, but also by defective secreting power. It is not necessary to impress the lesson such facts convey. The Intestinal Movements. — The circular fibers i:>lay a much more important part than the longitudinal, being, in fact, much more developed. It is also to be remembered that nerves in the form of plexuses (of Auerbach and Meissner) abound in its walls. Normally the movement, slowly progressive, with occasional lialtings, is from above downward, stopping at the ileo-csecal valve ; the movements of the large gut being apparently mostly independent. Movements may be excited by external or internal stimula- tion, and may be regarded as reflex ; in which, however, the tendency for the central cells to discharge themselves is so great (automatic) that only a feeble stimulus is required, the normal one being the presence of food. It is noticeable in a recently killed animal, or in one in the last stages of asphyxia, that the intestines contract vigorously. Whether this is due to the action of blood overcharged with carbonic anhydride and deficient in oxygen on the centers pre- siding over the movements, on the nerves in the intestinal walls, or on the mugcle-cells directly, is not wholly clear, but it is probable that all of these may enter into the result. The vagus nerve, when stimulated, gives rise to movements of the intestines, while the splanchnic seems to have the reverse efl'ect ; but the cerebrum itself has an influence over the movements of the gut, as is plain from the diarrhoea traceable to unusual fear or anxiety. There is little to add in regard to the move- ments of the large intestine. They are, no doubt, of consider- able importance in animals in which it is extensive. Normally they Ijogin at tlie ileo-ciecal valve. Defecation. — The removal of the waste matter from the ali- mentary tract is a complicated process, in which both smooth and striped muscle, the spinal cord, and the brain take part. Defecation may take place during the unconsciousness of sleep or of disease, and so be wholly independent of the will ; but, as we well know, this is not usually the case. Against ac- cidental discharge of faeces there is a provision in the sphinc- ter ani, the tone of which is lost when the lower part of the spinal cord is destroyed. We are conscious of being able, by an effort of will, to prevent the relaxation of the sphincter or to increase its holding jjower, though the latter is probably almost 338 ANIMAL PHYSIOLOGY. wholly due to the action of extrinsic muscles ; at all events any- one may convince himself that the latter may be made to take a great part in preventing fsecal discharge, though whether the ione of the sphincter can be increased or not by volition it is difficult to say. What happens during an ordinary act of defecation, is about as follows : After a long inspiration the glottis is closed ; the diaphragm, which has descended, remains low, affording, with the obstructed laryngeal outlet, a firm basis of support for the action of the abdominal muscles, which, bearing on the intes- tine, forces on their contents, which, before the act has been called for, have been lodged mostly in the large intestine ; at the same time the sphincter ani is relaxed and peristaltic move- ments accompany and in some instances precede the action of the abdominal muscles. The latter may contract vigorously on a full gut without success in the absence of the intestinal peri- stalsis, as too many cases of obstinate constipation bear witness. Like deglutition, and unlike vomiting, there is usually both a voluntary and involuntary part to the act. Though the will, through the cerebrum, can inhibit defeca- tion, it is likely that it does so through the influence of the cerebrum on some center in the cord ; for in a dog, the lumbar cord of which has been divided from the dorsal, the act is, like micturition, erection of the penis, and others which are under the control of the will, still possible, though, of course, performed entirely unconsciously. Vomiting. — If we consult our own consciousness and observe to the best of our ability, supplementing information thus gained by observations on others and on the lower animals, it will become apparent that vomiting implies a series of co-ordi- nated movements, into which volition does not enter either necessarily or habitually. There is usually a preceding nausea, with a temporary flow of saliva to excess. The act is initiated by a deep inspiration, followed by closure of the glottis. Whether the glottis is closed during or prior to the entrance of air is a matter of disagreement. At all events, the dia- phragm descends and remains fixed, the lower ribs being re- tracted. The abdominal muscles then acting against this sup- port, force out the contents of the stomach, in which they are assisted by the essential relaxation of the cardiac sphincter, the shortening of the oesophagus by its longitudinal fibers, and the extension and straightening of the neck, together with the open- ing of the mouth. DIGESTION OF FOOD. 339 As the expulsive effort takes place, it is accompanied by an expiratory act which tends to keep the egesta out of the larynx and carry them onward, though it may also contribute to over- come the resistance of the elevated soft palate, which serves to protect the nasal passages. The stomach and oesophagus are not wholly passive, though their part is not so important in the adult as might be inferred from observing vomiting in infants, the peristalsis of these organs apparently sufficing in them to empty the stomach. Retching may be very violent and yet ineffectual when the cardiac sphincter is not fully relaxed. The pyloric outlet is usually closed, though in severe and long- continued vomiting bile is often ejected, which must have reached the stomach through the pylorus. Comparative. — The ease with which some animals vomit in comparison with others is extraordinary, as in carnivora like our dogs and cats : a matter of importance to an animal ac- customed in the wild state to eat entire carcasses of animals — hair, bones, etc., included. The readiness with which an animal vomits depends in great part on the conformation and relations of the parts of its digest- ive tract. The horse vomits with difliculty — its stomach and its car- diac opening being small and peculiar in shape (Figs. 261 and 280), while its oesophagus is long. The stomach of the human being during infantile life is less pouched than in the adult, which in part explains the ease with which infants vomit. But the matter is complex; much depends on the proper co-ordinations being made, and, this being well or ill accom- plished, accounts for the variations in the ease with which dif- ferent persons vomit. Pathological. — Vomiting may arise from the presence of renal or biliary culculi (reflex action) ; from disease of the cerebrum or the medulla : from obstruction in the pyloric region or in the intestines: from emotions; from revived unpleasant m(!n- tal associations; from nauseous tastes, etc. It may be ques- tionable whether some of these are properly termed " patho- logical." Pyro.sis is due to the anti-peristaltic action of the stomach and fiisophagus alone, so that it is a sort of partial vomiting and allied to the regurgitation of special secretions, as from the crops ()\' [)igeons, or of food from the stomachs of ruminants. We have known cases in which anti-peristalsis was confined to •340 ANIMAL PHYSIOLOGY. the pharynx alone. Some persons seem to have acquired the power of regurgitating food and masticating it afresh. The excessive vomiting following obstruction of the bowels is comparable to the unusual action of the heart, ureter, blad- der, etc., when there is hindrance to the outflow. As we have already explained for the heart, we regard this as the resump- tion of a power of independent action seen in ancestral forms and marked when the nervous system is no longer exercising its usual control and direction. Not that this or similar be- havior may not result from excessive stimulation, leading to unusual central nervous discharge, but it certainly does happen independently of the nervous system, and may be witnessed in the hearts of cold-blooded animals when all their nerves are divided. Similarly, the habit of regurgitating the food is intelligible in the light of evolution. The fact that mammals are descended from lower forms in which unstriped muscle-cells go to form organs that have a rhythmically contractile function, renders it clear why this function may become, as in ruminants, spe- cialized in certain parts of the digestive tract ; why carnivora should vomit readily, and why human subjects should learn to regurgitate food. There is, so to speak, a latent inherited ca- pacity which may be developed into actual function. Apart from this it is difficult to understand such cases at all. The vomiting center is usually located in the medulla, and is represented as working in concert with the respiratory center. But when we consider that there is usually an increased flow of saliva and other phenomena involving additional central nervous influence, we see reason to believe in co-ordinated action implying the use of parts of the central nervous system not so closely connected anatomically as the respiratory and vomiting centers are assumed to be. Indeed, as we before indicated, it does not seem probable that the doctrine of centers in its present form, especially with such precise limitations, both anatomically and physiologically, will continue to be maintained. We seem to have been over- looking the connection of parts while occupied with defining their limits. It is not, however, yet possible to substitute other explanations that shall be wholly satisfactory ; and we make these remarks to keep the student expectant of progress, for, as a distinguished exponent of science has said, " When Science adopts a [rigid] creed, she commits suicide." We do not know the part taken, if any, by the splanchnic DIGESTION OP FOOD. 34I or other nerves of the sympathetic system ; but, from the fact that discharge of the gastric contents is impossible when the vagi are cut, it is likely that the efferent impulses, determining the relaxation of the cardiac sphincter, descend by these nerves, while the chorda tympani is concerned, of course, in the secre- tion of saliva. But it will be clear, from the facts of the case, that many nerves, both afferent and efferent, are concerned ; and it is more than likely that our explanations of the entire process are quite inadequate to unravel its real complexity. Therapeutics. — The evidence from the use of drugs seems to emphasize the last statement. At all events, emetics act in a variety of ways, and differently in different animals. The Removal op Digested Products from the Aliment- ary Canal. The glands of the stomach are simply secretive, and all ab- sorption from this organ is either by blood-vessels directly or by lymphatics ; at least, such is the ordinary view of the sub- ject— whether it is not too narrow a one remains to be seen. It is imjjortant to remember that the intestinal mucous membrane is supplied not only with secreting glands but lym- phatic tissue, in the form of the solitary and agminated glands (Peyer's patches) and thickly studded with villi, giving the small gut that velvety appearance appreciable even by the naked eye. It will not be forgotten that the capillaries of the digestive organs terminate in the veins of the portal system, and that the blood from these parts is conducted through the liver before it reaches the general circulation. The lymphatics of these organs form a part of the general lymphatic system of the body ; but the peculiar way in which absorj>tion is effected by villi, and the fact that the lymphatics of the intestine, etc., at one time (fasting) contain ordinary lymph and at another (after meals) the products of digestion, imparts to them a physiological character of their own. Absorption will be the better understood if we treat now of lymph and chyle and the lymph vascular system, which were jjurposely postponed till the present; though its connection with the vascular system is as close and important as with the digestive organs. The lymphatic system, as a whole, more closely resembles the venous than the artcsrial vessels. "Wo may speak (jf lym- 342 ANIMAL PHYSIOLOGY. phatic capillaries, which are, in essential points of structure, like the arterial capillaries ; while the larger vessels may be compared to veins, though thinner, being provided with valves and having very numerous anastomoses. These lymphatic capillaries begin in spaces between the tissue-cells, from which they take up the effete lymph. It is interesting to note that there are also perivascular lymphatics, the ex- istence of which again shows how close is the relation between the blood vascular and lym- phatic systems, and as we would suppose, and as is actually found to be the case, between the contents of each. Lymph and Chyle. — If one compares the mes- entery in a kitten, when fasting, with the same part in an animal that was killed some hours after a full meal of milk, it may be seen that the formerly clear lines indicating the course of the lymphatics and ending in glands have in the latter case become whitish (hence their name, lacteals), owing to the absorption of the emulsified fat of the milk. Fig. 283.— Valves ot lymphatics (Sappey). Fig. 284.— Origin of lymphatics (after Landois). I. From central tendon of diaphragm of rabbit (semi-diagrammatic) ; s, lymph-canals communicating by X with lymphatic vessel L ; A, origin of lymphatic by union of lymph-canals ; E, E, endothelium. U. Perivas- cular canal. DIGESTION OP FOOD. 343 Microscopic examination shows the chyle tu contain (when coagulated) librin, many leucocytes, a few developing red cor- puscles, an abundance of fat in the form both of very minute oil-globules and particles smaller still. Fig. 285.— Epithelium from duodenum of rab- bit, two hours after having been fed with melted butter (Funke) Fig. 286.— VilU filled with fat, from small intestine of an executed criminal, one hour after death ( Funke). There are also present fatty acids, soaps small in quantity as compared with the neutral fats, also a little cholesterin and lecithin. But chyle varies very widely even in the same animal at different times. To the above mu.st be added proteids (fibrin, serum-albumin, and globulin) ; extractives (sugar, urea, leu- cin) ; and salts in which sodium chloride is abundant. The composition of lymph is so similar to that of chyle, and both to blood, that lymph might, with a fair degree of ac- curacy, be regarded as blood without its red corpuscles, and chyle as lymph with much neu- tral fat in a very fine state of divisirjn. The Movements of the Lymph — comparative.— In some fishes, .some birds, and amphibians, there are lymph hearts. In the frog there are two axillary and two sacral lymph hearts. The latter are, especially, easily seen, and there is no doubt that they are under the control of tlie nervous system. Fig. 287.— Chyle taken and thoracic duct from the lacteala f a criminal exe- cuted during digestion (Funke). Shows leucocytes and excessively fine granules of fatty emulsion. 344 ANIMAL PHYSIOLOGY. In the mammals no such special helps for the propulsion of lymph exist. There is little doubt that the blood - pressure is always higher than the lymph-pressure, and when the blood-vessels Fig. 288.— Thoracic duct (Mascagni) 1, thoracic duct , 2, great lymphatic duct ; 3, recep- taculum chyli ; 4, curve of thoracic duct ]ust before it empties mto the venous system. are dilated the fluid within the perivascular lymph-channels is likely compressed ; muscular exercise must act on the lymph- channels as on veins, both being provided w,ith valves, though themselves readily compressible ; the inspiratory efforts, espe- cially when forcible, assist in two ways : by the compressing effect of the respiratory muscles, and by the aspirating effect of the negative pressure within the thorax, producing a similar aspirating effect within the great veins, into which the large lymphatic trunks empty. The latter are provided at this point with valves, so that there is no back-flow ; and, with the posi- tive pressure within the large lymphatic trunks (thoracic duct, etc.), the physical conditions are favorable to the outflow of lymph or chyle. DIGESTION OP FOOD. 345 Onr knowledge of the nature of the passage of the chyle from the intestines into the blood is now clearer than it was till recently, though still incomplete. The exact structure of a villus is to be carefully considered. If we assume that the muscular cells in its structure have a rhythmically contractile function, the blind terminal portion of the lacteal inclosed within the villus must, after being emptied, act as a suction-pump to some extent ; at all events, the conditions as to pressure would be favorable to inflow of any material, especially fluid without the lacteal. The great difficvilty hitherto was to understand how the fat found its Kir;. 28t>.— Lymphatic vessels and plands (Sappey). 1, upper extremity of thoracic duct, pass- iDK behind the inU^rnal jugular vein ; 2. ofjening of thoracic duct into internal jugular and left Hubclavian vein. Lymphatic glands are seen in (;ourse of vessels. way tlirough the villus into tlie Ijlood, for, that most of it passes in this direction there is little doubt. It is now known that leucocytes (amneboids, phagocytes) migrate fnmi within the villus outward, and may even reach its surffice ; that they take uj) (eat) fat-particles fiom the 346 ANIMAL PHYSIOLOGY. Fig. 290.— Stomach, intestine, and mesentery, with mesenteric blood-vessels and lacteals (slightly reduced from a figure in the original work of Asellius, published in 1628) (after Flint). A, A, A, A, A, mesenteric arteries and veins ; B, B, B, B, B, B, B, B, B, B, lacteals ; C, C, C, C, mesentery ; D, D, stomach ; E, pyloric portion of stomach ; F, duodenum ; G", O, G, jejunum ; H, H, H, H, H, ileum ; I, artery and vein on fundus of stomach ; K, portion of omentum. epitlieliuin of the villus, and, indepen(iently themselves, carry them inward, reach the central lacteal and break up, thus releas- ing the fat. How the fat gets into the covering epithelium is DIGESTION OF FOOD. 347 not yet so fully known — possibly by a simi- lar inceptive process ; nor is it ascertained ■what constructive or other chemical pro- cesses they may perform ; though it is not at all likely that the work of the amoeboid cells is confined to the transport of fat alone, but that other matters are also thus removed inward to the lacteal. Experimental. — If two frogs under the influence of urari, to remove the effect of muscular movements, be placed under ob- servation, the one having its brain and spinal cord destroyed, the other intact, in both the aorta divided across, and normal saline solution injected into the posterior lymi^h-sac (beneath the skin of the back), it will be found, on suspending the two by the lower jaw, that, in the frog with the nerve - centers uninjured, abundance of saline fluid is taken up from the dor- sal sac and expelled through the aorta, but in the other case none, the heart remaining all but empty. Fig. ;iyi.— Intestinal villus (after Leydig). a, a, a, epithelial covering ; b, b, capillary network ; c, c, longitudinal mus- cular fibers ; d, lacteal. ITio. 292,— A. Villi )f man, showiiij^ blood-veasels and lacteals ; (!h(iuvi'(iiii Villus of sherp Caften" 348 ANIMAL PHYSIOLOGY. Different interpretations have been put upon this experi- ment. Some point to it as clear proof of the influence of the str Fig. 293.— a. Section of villus of rat killed during fat absorption (Schafer). ep, epithelium ; str, striated border ; c, lymph-cells : c', lymph-cells in epithelium ; I, central lacteal con- taining disintegrating corpuscles. B. Mucous membrane of frog's intestine during fat absorption (Schafer). ep, epithelium ; s^r, striated border; C, lymph-corpuscles; i, lacteal. nervous system directly ; to others it seems that the failure of absorption is owing to the greatly dilated condition of the blood-vessels, consequent upon the loss of arterial tone, the blood remaining in the veins, and the circulation being, in fact, practically arrested. It certainly can not be claimed that the first conclusion necessarily follows from the experiment; the second may be a partial explanation of the failure of absorp- tion; but, when a multitude of other facts are taken into account, there seems little reason to doubt that so important a process as absorption can not fail to be regulated by the nerv- ous centers. The danger of founding any important conclu- sion on a single experiment is very great. Again, if the leg of a frog, exclusive of the nerves, be liga- tured, the limb will be found to swell rapidly if placed in water, which is not true of a dead limb. This is adduced as evidence for the independence of the absorptive process and the circula- tion; and, since section of the sciatic nerve is said to arrest absorption, such an experiment, taken together with the two DIGESTION OF FOOD. 349 preAions ones, points in the direction of the control of this process by the nervous system. But if the views we hold of the absolute dependence, especially in the higher animals, of all "s^tal processes on the nervous system are correct, it fol- lows, as a matter of course, that absorption in living tissues, which we do not regard as wholly explicable by any physical process, but as bound up with all the functions of cell-life, must be dependent on that connection we are endeavoring to emphasize between one tissue and another, and especially the dominating tissue, the nervous system. There are two points that are very far from being deter- mined : the one the fate of the products of digestion ; the other the exact limit to which digestion is carried. How much — e. g., of proteid matter — does actually undergo conversion into peptone ; how much is converted into leucin and tyrosin ; or, again, what proportion of the albuminous matters are dealt with as such by the intestine without conversion into peptone at all, either as soluble proteid or in the form of solid particles ? 1, It is generally believed that soluble sugars are absorbed, usually after conversion into maltose or glucose, by the capil- laries of the stomach and intestine. 2. There is some positive evidence of the presence of fats, soaps, and sugars in unusual amount after a meal in the portal vein, which implies removal from the intestinal contents by the capillaries, though, so far as experiment goes, the fat is chiefly in the form of soaps. Certain experiments have been made b^^ ligating the pyloric end of the stomach, by introducing a cannula into the thoracic duct, so as to continually remove its contents, etc. But we are surprised that serious conclusions should have been drawn under such circumstances, seeing that the natural conditions are so altered. What we wish to get at in physiology is the normal function of parts, and not the possible results after our inter- ference. Under such circumstances the phenomena may have a suggestive but certainly can not have a conclusive value. It is a very striking fact that little peptone (none, according to some observers) can be detected even in the portal blood. True it is, the circulation is rapid and constant, and a small quantity might e.s(;ape detection, yet a considerable amount be removed from the intestine in the space of a few hours by the capillaries alone, Pej^tone is not found in the cojitents of the thoracic duct. Recent investigations have thrown a new light on p(!ptone. 350 ANIMAL PHYSIOLOGY. It is now known that there are several kinds of peptones, a disclosure for which we were not unprepared, considering our imperfect knowledge of proteids in general ; but there have been other developments which, on the supposition that the peptone of the alimentary canal is freely absorbed as such, are startling enough. It has been shown that these peptones, at least as prepared by artificial digestion, have three efi^ects when injected in quantity into the blood of an animal : They produce narcosis ; they retard or prevent coagulation of the blood ; they lower blood-pressure. The first effect may be dependent in whole or in part on the third. But, inasmuch as the venom of poisonous reptiles, according to recent investigations, is essentially proteid in nature, it is plain that we must exercise great caution in drawing conclu- sions in regard to the physiological effects of proteid bodies, so long as our knowledge of their exact chemical composition is so imperfect. That the chemist can make out no great differ- ence between peptones prepared in the laboratory and the di- gestive tract, or even between these and snake- venom, though they haye such different effects when injected into the blood, is clear proof of how much we have yet to learn of these bodies. But we introduce these considerations here rather to show that it is by no means likely that any great quantity of pep- tones passes into the blood as such at any one time. It has been recently suggested that peptone is converted into globulin in the liver. But what proof is there of this ? And already we have credited the liver with a large share of work. For a considerable period it has been customary to use the terms endosmosis and diffusion in connection with the func- tions of the alimentary canal, and especially the intestinal tract, as if this thin- walled but complicated organ, or rather collec- tion of organs, were little more, so far as absorption is con- cerned, than a moist membrane, leaving the process of the re- moval of digested food products to be explained almost wholly on physical principles. From such views we dissent. We believe they are opposed to what we know of living tissue everywhere, and are not sup- ported by the special facts of digestion. When certain foreign bodies (as purgatives) are introduced into the blood or the ali- mentary canal, that diffusion takes place, according to physical laws, may indicate the manner in which the intestine can act ; but even admitting that under such circumstances physical DIGESTION OF FOOD. 351 principles actually do explain the whole, which we do not grant, it would by no means follow that such was the natural behav- ior of this organ in the discharge of its ordinary functions. When we consider that the blood tends to maintain an equi- librium, it must be evident that the removal of substances from the alimentary canal, unless there is to be excessive activity of the excretory organs and waste of energy both by them and the digestive tract, must in some degree depend on the demand for the products of digestion by the tissues. That there is to some extent a corrective action of the excretory organs always going on is no doubt true, and that it may in cases of emergency be great is also true ; but that this is minimized in ways too complex for us to follow in every detail is equally true. Diges- tion waits on appetite, and the latter is an expression of the needs of the tissues. We believe it is literally true that in a healthy organism the rate and character of digestion and of the removal of prepared products are largely dependent on the condition of the tissues of the body. Why is digestion more perfect in overfed individuals after a short fast ? The whole matter is very complex, but w:e think it is infinitely better to admit ignorance than attempt to ex- plain by principles that do violence to our fundamental con- ceptions of life processes. To introduce " ferments " to explain so many obscure points in physiology, as the conversion of jjeptone in the blood, for example, is taking refuge in a way that does no credit to science. Without denying that endosmosis, etc., may play a part in the vital processes we are considering, we believe a truer view of the whole matter will be ultimately reached. In the mean time we think it best to express our belief that we are ignorant of the real nature of absorption in great part ; but we think that, if the alimentary tract were regarded as doing for the digested food (chyle, etc.) some such work as certain other glands do for the blood, we would be on the way to a truer con- (;eption of the real nature of the processes. It would then be possible to understand that proteids either in the form of soluble or insoluble substances, including pep- tone, might be taken in hand and converted by a true vital pro(;ess into the constituents of the blood. If we were to regard the kidney as manufacturing useful instead of harmful pn^ducts, the resemblance in behavior would in many points b<; parallel. We have seen that mechanical ex]>lajiHtions of llic functions of tin- kidiu-y have failed, and 352 ANIMAL PHYSIOLOGY. that it must be regarded even in those parts that eliminate most water as a genuine secreting mechanism. We wish to present a somewhat truer conception of the lymph that is separated from the capillaries and bathes the tissues. We would regard its separation as a true secretion, and not a mere diffusion dependent wholly on blood-pressure. The mere ligature of a vein does not suffice to cause an excess of diffusion, but the vaso-motor nerves have been shown to be concerned. The effusions that result from pathological pro- cesses do not correspond with the lymph — that is, the nutrient material — provided by the capillaries for the tissues. These vessels are more than mere carriers ; they are secretors — in a sense they are glands. We have seen that in the foetus they function both as respiratory and nutrient organs in the allan- tois and yelk-sac, and, in our opinion, they never wholly lose this function. The kind of lymph that bathes a tissue, we believe, depends on its nature and its condition "at the time, so that, as we view tissue-lymph, it is not a mere effusion with which the tissues, for which it is provided, have nothing to do. The differences may be beyond our chemistry to determine, but to assume that all lymph poured out is alike is too crude a conception to meet the facts of the case. Glands, too, it will be remembered, derive their materials, like all other tissues, not directly from the blood, but from the lymph. We believe that the cells of the capillaries, like all others, are influenced by the nervous system, notwithstanding that nerves have not been traced terminating in them. It is to be borne in mind that the lymph, like the blood, receives tissue waste-products — in fact, it is very important to realize that the lymph is, in the first instance, a sort of better blood — an improved, selected material, so far as any tissue is concerned, which becomes gradually deteriorated (see Fig. 329). We have not the space to give all the reasons on which the opinions expressed above are founded ; but, if the student has become imbued with the principles that pervade this work thus far, he will be prepared for the attitude we have taken, and sympathize with our departures from the mechanical (physical) physiology. We think it would be a great gain for physiology if the use of the term " absorption," as applied to the alimentary tract, were given up altogether, as it is sure to lead to the substitu- DKJESTIOX OF FOOD. 353 tion of the gross conceptions of physical processes instead of the subtle though at present rather indefinite ideas of vital processes. We prefer ignorance to narrow, artificial, and erro- neous views. Pathological. — Under certain circumstances, of which one is obstruction to the venous circulation or the lymphatics, fluid may be poured out or effused into the neighboring tissues or the serous cavities. This is of very variable composition, but always contains enough salts and proteids to remind one of the blood. Such fluids are often spoken of as " lymph," though the resemblance to normal tissue-lymph is but of the crudest kind ; and the condition of the vessels when it is secreted, if such a term is here appropriate, is not to be compared to the natural separation of the normal lymph — in fact, were this not so, it would be like the latter, which it is not. When such effusions take place they are in themselves evidence of altered (and not merely increased) function. The Faeces. — The faeces may be regarded in at least a three- fold aspect. They contain undigested and indigestible rem- nants, the ferments and certain decomposition products of the digestive fluids, and true excretory matters. In carnivorous and omnivorous animals, including man, the undigested materials are those that have escaped the action of the secretions — such as starch and fats — together with those substances that the digestive juices are powerless to attack, as horny matter, hairs, elastic tissue, etc. In vegetable feeders a larger proportion of chlorophyl, cel- lulose, and starch will, of course, be found. These, naturally, are variable with the individual, the spe- cies, and the vigor of the digestive organs at the time. Besides the above, certain products are to be detected in the f£eces plainly traceable to the digestive fluids, and showing that tliey have undergone chemical decomposition in the ali- mentary tract, such as cholalic acid, altered coloring-matters like urobilin, derivable probably from bilirubin ; also choles- terine, fatty acids, insolulile soaps (calcium, magnesium), to- gether with ferments, having the properties of pepsin anle ? To reply tliat, in the one case, the digestive fluids are poured out and in the other not, is to go little below 362 , ANIMAL PHYSIOLOGY. the surface, for one asks the reason of this, if it be a fact, as it no doubt is. When we look further into the peculiarities of digestion, etc., we recognize the influence of race as such, and in the race and the individual that obtrusive though ill-under- stood fact — the force of habit, operative here as elsewhere. And there can be little doubt that the habits of a people, as to food eaten and digestive peculiarities established, become or- ganized, fixed, and transmitted to posterity. It is probably in this way that, in the course of the evolu- tion of the various groups of animals, they have come to vary so much in their choice of diet and in their digestive processes, did we but know them thoroughly as they are ; for to assume that even the digestion of mammals can be summed up in the simple way now prevalent seems to us too broad an assump- tion. The field is very wide, and as yet but little explored. Human Physiology. — The study of Alexis St. Martin has fur- nished probably the best example of genuine human physiology to be found, and has yielded a harvest rich in results. We suggest to the student that self-observation, without interfering with the natural processes, may lead to valuable knowledge ; for, though it may lack some of the precision of laboratory experiments, it will prove in many respects more instructive, suggestive, and impressive, and have a bearing on medical practice that will make it telling. Not that we would be understood now or at any time as depreciating laboratory experiments ; but we wish to point out from time to time how much may be learned in ways that are simple, inexpensive, and consume but little time. The latu of rhythm is illustrated, both in health and disease, in striking ways in the digestive tract. An individual long accustomed to eat at a certain hour of the day will experience at that time not only hunger, but other sensations, probably referable to secretion of a certain quantity of the digestive juices and to the movements that usually accompany the pres- ence of food in the alimentary tract. Some persons find their digestion disordered by a change in the hours of meals. It is well known that defecation at periods fixed, even within a few minutes, has become an established habit with hosts of people ; and the same is to a degree true of dogs, etc., kept in confinement, that are taught cleanly habits, and encouraged therein by regular attention to their needs. Now and then a case of what is very similar to regurgita- tion of food in ruminants is to be found among human beings. DIGESTION OF FOOD. 3(33 This is traceable to habit, which is bound up with the law of rhythm or periodic increased and diminished activity. Indeed, every one sufficiently observant may notice in him- self instances of the application of this law in the economy of his own digestive organs. This tendency is imjiortant in preserving energy for higher ends, for such is the result of the operation of this law every- where. The law of correlation, or mutual dependence, is well illus- trated in the series of organs composing the alimentary tract. The condition of the stomach has its counterpart in the rest of the tract : thus, when St. Martin had a disordered stomach, the epithelium of his tongue showed corresponding changes. We have already referred to the fact that one part may do extra work to make up for the deficiencies of another. It is confidently asserted of late that, in the case of persons long unable to take food by the mouth, nutritive substances given by enemata find their way up to the duodenum by anti- peristalsis. Here, then, is an example of an acquired adaptive arrangement under the stress of circumstances. It can not be too much impressed on the mind that in the complicated body of the mammal the work of any one organ is constantly varying with the changes elsewhere. It is this mutual dependence and adaptation — an old doctrine, too much left out of sight in modern physiology — which makes the at- tempt to coinpleiely unravel vital processes well-nigh hopeless; though each accumulating true observation gives a Ijetter in- sight into this kaleidoscopic mechanism. We have not attempted to make any statements as to the quantity of the various secretions discharged. This is large, douljtless, but much is prol)ably reabsorbed, either altered or unaltered, and used over again. In the case of fistulm the con- ditions are so unnatural that any conclusions as to the normal quantity from tlie data tliey afford must be liighly unsatisfac- tory. Moreover, the quantity must be very variable, accord- ing to th(? law we are now considering. It is well known that dry ffjod pr(;v(jkes a more abundant discharge of saliva, and this is doubtless but one example of many other relations be- tween the cliaractf-r of the food and the quantity of secretion provided. Evolution. — We have from time to time eitlier distinctly point(;d out or hinted at the evolutionary implications of tlio facts of this department of physiology. The structure of the 3^4 ANIMAL PHYSIOLOGY. digestive organs, plainly indicating a rising scale of complexity with greater and greater differentiation of function, is, beyond question, an evidence of evolution. The law of natural selection and the law of adaptation, giving rise to new forms, have both operated, we may believe, from what can be observed going on around us and in our- selves. The occurrence of transitional forms, as in the epi- thelium of the digestive tract of the frog, is also in harmony with the conception of a progressive evolution of structure and function. But the limits of space will not permit of the enumeration of details. Summary. — A very brief resume of the subject of digestion will probably suffice. Food is either organic or inorganic and comprises proteids, fats, carbohydrates, salts, and water ; and each of these must enter into the diet of all known animals. They must also be in a form that is digestible. Digestion is the reduction of food to a form such that it may be further dealt with by the aliment- ary tract prior to being introduced into the blood (absorption). This is effected in different parts of the tract, the various con- stituents of food being differently modified, according to the secretions there provided, etc. The digestive juices contain essentially ferments which act only under definite conditions of chemical reaction, temperature, etc. The changes wrought in the food are the following : starches are converted into sugars, proteids into peptones, and fats into fatty acids, soaps, and emulsion ; which alterations are effected by ptyalin and amylopsin, pepsin and trypsin, and bile and pancreatic steapsin, respectively. Outside the mucous membrane containing the glands are muscular coats, serving to bring about the movements of the food along the digestive tract and to expel the faeces, the circu- lar fibers being the more important. These movements and the processes of secretion and so-called absorption are under the control of the nervous system. The preparation of the digestive secretions involves a series of changes in the epithelial cells concerned, which can be dis- tinctly traced, and take place in response to nervous stimula- tion. These we regard as inseparably bound up with the healthy life of the cell. To be natural, it must secrete. The blood-vessels of the stomach and intestine and the villi of the latter receive the digested food for further elaboration THE KESPIKATORY SYSTEM. 365 (absorption). The undigested remnant of food and the excre- tions of the intestine make up the faeces, the latter being ex- pelled by a series of co-ordinated muscular movements essen- tially reflex in origin. THE RESPIRATORY SYSTEM. In the mammal the breathing organs are lodged in a closed cavity, separated by a muscular partition from that in which the digestive and certain other organs are contained. This FlO. ifl^T). -LuriKH, anU-rior vnw iSapix-i). 1, upper lohr of left lungf; 2, lower lobo; :i. fissure; 4, notch corrfrHfKiridinK t^> afx-x of heart ; T>, pericardium ; (!. ujjper lobe of rlKht liine ; 7, middle lol»e : H. lower lof>e ; !l. flBHiire ; 10, fissure ; 11, diaphraKin ; 12, anterior mediasti- num ; 13, thyroid xlaiid ; M, middle cervical aponeurosis ; If), process i>t attachment of mediastinum to pericardium : 1<5, 16, seventh ribs ; 17, 17, tranaversales muscles ; IH, linea alba. 366 ANIMAL PHYSIOLOGY. thoracic cliainher may be said to be reserved for circulatory and respiratory organs which, we again point out, are so related that they really form parts of one system. The mammal's blood requires so much aeration (ventilation) that the lungs are very large and the respiratory system has become greatly specialized. We no longer find the skin or ali- mentary canal taking any large share in the process ; and the lungs and the mechanisms by which they are made to move the gases with which the blood and tissues are concerned become very complicated. Fig. 296.— Bronchia and lungs, posterior view (Sappey). 1,1, summit of lungs ; 2, 2, base of lungs ; 3, trachea ; 4, right bronchus ; 5, division to upper lobe of lung : 6, division to lower lobe ; 7, left bronchus ; 8, division to upper lobe ; 9, division to lower lobe ; 10, left branch of pulmonary artery ; 11, right branch ; 12, left auricle of heart ; 13, left superior pulmonary vein ; 14, left inferior pulmonary vein ; 15. right superior pulmonary vein ; 16, right inferior pulmonary vein ; 17, inferior vena cava : 18. left ventricle of heart ; 19, right ventricle. Our studies of muscle physiology should have made clear the fact that tissue-life implies the constant consumption of oxygen and discharge of carbonic anhydride, and that the pro- cesses which give rise to this are going on at a rapid rate ; so that the demands of the animal for oxygen constantly may be readily understood if one assumes, what can be shown, though less readily than in the case of muscle, that all the tissues are constantly craving, as it were, for this essential oxygen — well called " vital air." THE RESPIRATORY SYSTEM, 367 Respiration may, then, be regarded from a physical and chemical point of view, though in this as in other instances we Fig. 297.— Trachea and branchial tubes (Sappey). 1, 2, larynx ; 3, 3, trachea ; 4. bifurcation of trachea : :>. rifcht bronchus ; G. left bronchus ; 7, bronchial division to upper lobe or right lunK ; H. division to middle lobe ; 9, division to lower lobe ; 10. division to upper lobe of left XnuK : 11. division to lower lobe ; 12, 12, 12, 12, ultimate ramifications of bronchia; 13. 13. 13. 13, luntfs. represented in contour ; 14, 14, summit of lungs ; 15, 15, base of lungs. must be on our guard against regarding physiological processes as ever purely j^hysical or purely chemical. The respiratory {irocess in the mammal, unlike the frog, consists of an active and a (largely) passive phase. The air is not pumped into the lungs, but sucked in. So great is the complexity of the lungs in the mammal, that the frog's lung (whicli may be rcsadily understood by blowing it up by inserting a small pipe in the glottic opf;ning of the animal and then ligaturing the distended organ) may be c;ompared to a single infundibulum of the mam- malian lung. AsHuniing that the student is somewhat conversant with the 368 ANIMAL PHYSIOLOGY. coarse and fine anatomy of the respiratory organs, we call at- tention to the physiological aspects of some points. The lungs represent a membranous expansion of great extent, lined with flattened cells and supporting innumerable capillary blood-ves- sels. The air is admitted to the complicated foldings of this membrane by tubes which remain, throughout the greater part of their extent, open, being composed of cartilaginous rings, completed by soft tissues, of which plain muscle-cells form an Fig. 298.— Mold of a terminal bronchus and a group of air-cells moderately distended by- injection, from the human subject (Robin). important part, serving to maintain a tonic resistance against pulmonary and bronchial pressure, as well as serving to aid in the act of coughing, etc., so important in expelling foreign bodies or preventing their ingress. The bronchial tubes are lined with a mucous membrane, kept moist by the secretions of its glands, and covered with ciliated epithelium, as are also the nasal passages, which by the outward currents they create, favor diffusion of gases, and removal of excess of mucus. The thoracic walls and the lungs themselves are covered with a tough but thin membrane lined with flattened cells, which secrete a small quantity of fluid. THE RESPIRATORY SYSTEM. 369 that serves to maintain the surrounding parts in a moist con- dition, thus lessening friction. The importance of this ar- FiG. 299.— Section of the parenchyma of the human lung. Injected through the pulmonary- artery (Schulze). a, a, c, c, walls of the air-cells ; 6, small arterial branch. rangement is well seen when, in consequence of inflammation of this pleura, it becomes dry, giving rise during each respira- tory movement to a friction-sound and a painful sensation. It will not be forgotten that this membrane extends over the diai)hragm, and that, in consequence of the lungs completely filling all the space (not occupied by other organs) during every position of the chest- walls, the costal and pulmonary pleural surfaces are in constant contact. By far the greater part of the lung-substance consists of elastic tissue, thus adapting the principal respiratory organs to that amount of distention and recoil to which they are ceaselessly subjected during the en- tire lif(;time of the animal. The Entrance and Exit of Air. Since the lungs fill uj) so completely the thoracic cavity, manifestly any change in the size of the latter must lead to an increase or diminuticjn in the quantity of air they contain. Since the air within the respiratory organs is being constantly 24 370 ANIMAL PHYSIOLOGY. robbed of its oxygen, and rendered impure by the addition of carbonic dioxide, the former must be renewed and the latter expelled ; and, as mere diffu- sion takes place too slowly to accomplish this in the mam- mal, this process is assisted by the nervous system set- ting certain muscles at work to alter the size of the chest cavity. Because of the ribs being placed obliquely, it fol- lows that their elevation will result in the enlargement of the thoracic cavity in the an- tero-posterior diameter ; and, as the chest, in consequence, gets wider from above down- ward, also in the transverse diameter ; which is more- over assisted by the eversion of the lower borders of the ribs ; and, if the convexity of the diaphragm were dimin- ished by its contraction and consequent descent, it would follow that the chest would be in- creased in the vertical diameter also. All these events, favor- able to the entrance of air, actually take place through agencies we must now consider. The student is recommended to look into the insertion, etc., of the muscles concerned, to which we can only briefly refer. The act of inspiration commences by the fixation of the uppermost ribs, beginning with the first two, by means of the scaleni muscles, this act being followed up by the contraction of the external intercostals, leading to the elevation of the other ribs ; at the same time, the arch of the diaphragm de- scends in consequence of the contraction of its various mus- cular bundles. Under these circumstances, the air from with- out must rush in, or a vacuum be formed in the thoracic cavity ; and, since there is free access for the air through the glottic opening, the lungs are of necessity expanded. This in- going air has had to overcome the elastic resistance of the lungs, which amounts to about 5 millimetres of mercury in man, as ascertained by tying a manometer in the windpipe of Fig. 300.— Dia,s:ram illustrating elevation of ribs in inspiration (Beclard). Tlie dark lines rep- resent the ribs, sternum, and costal cartilages in inspiration. THE RESPIRATORY SYSTEM. 371 a dead subject, and then opening the thorax to equalize the inside and outside pressures, when the lungs at once collapse and the manometer shows a rise of the mercury to the ex- tent indicated above. To this we must add the influence of the tonic contraction of the bronchial muscles before re- ferred to, though this is prob- ably not very great. That there are variations of intrapulmonary pressure may be ascertained by con- necting a manometer with one nostril — the other being closed — or with the windpipe. The mercury shows a negative pressure with each inspirato- ry, and a positive with each expiratory act. This may amount to from 30 to 70 mil- limetres with strong inspira- tion, and 60 to 100 in forcible expiration. When inspiration ceases, the elastic recoil of the rib carti- lages and the ribs themselves, and of the sternum, the weight Fig. 301. — Diagrammatic representation of action of diaphragm in inspiration (Her- mann). Vertical section tiirough second rib on right side. The broken and dotted lines show the amount of the descent of the diaphragm in ordinary and in deep inspira- tion. Fio. 302.— Apparatus U> illustrat/- relations of intraf horacic and external pressures (after H»-aunis). A glans liell-iar is jirovidcd with a liK'lii slopi.cr, through vvhi<,'li i)ns.scKa branch- ing glaH.H tube (ltt>-d with a pair of elastii; l)agH ri-pr<'scriliiig luiig.M. Tlic bottom of the jiii' i.s flowed \iy rubber membrane n-preHentiiig diaphragm. A mercMt\' iriiiiiomctiT iiidi('ate,s the diff>T<-iii;e in prfssure within and without tlie Iw^ll-Jar. In Iffi-lmml figure it will be Heen that thew; pr<;HKureH are e<]iiiil ; in right (irmpirationi, tlie e.vtcriiiil pn-ssurf iH cntj.sid- •rably greater. At one part (>>) an elaHtic munds are caused in part by the friction of the air, though they are probably complex, several factors entering into their causation. Comparison op the Inspired and Expired Air. The changes that take i)lace in the air respired may be briefly stated as follows : 382 ANIMAL PHYSIOLOGY. 1. Whatever the condition of the inspired air, that expired is about saturated with aqueous vapor — i, e., it contains all that it is capable of holding at the existing temperature. 2. The temperature of the expired air is about that of the blood itself, so that if the air is very cold when breathed, the body loses a great deal of its heat in warming it. The expired air of the nasal passages is slightly warmer than that of the mouth. 3. Experiment shows that the expired air is really dimin- ished in volume to the extent of from one fortieth to one fiftieth of the whole. Since two volumes of carbonic anhydride require for their composition two volumes of oxygen, if the amount of the former gas expired be not equal to the amount of oxygen inspired, some of the latter must have been used to form other CO combinations. -~7^, amounting to rather less than 1, is called the respiratory coefficient, 4. The difference between inspired and expired air may be gathered from the following : Inspired air. Expired air . Oxygen. Nitrogen. Carbonic dioxide. 20-810 79-150 0-040 16-033 79-587 4-380 From which the most important conclusions to be drawn are, that the expired air is poorer in oxygen to the extent of 4 to 5 per cent, and richer in carbonic anhydride to somewhat less than this amount. From experiment it has been ascertained that the amount of carbonic dioxide is for the average man 800 grammes (406 litres, equivalent to !^18'1 grammes carbon) daily, the oxygen actually used for the same period being 700 grammes. But the variations in such cases are very great, so that these num- bers must not be interpreted too rigidly. Experience proves that, while chemists often work in laboratories in which the percentage of carbonic anhydride (from chemical decomposi- tions) reaches 5 per cent, an ordinary room in which the amount of this gas reaches 1 per cent is entirely unfit for occupation. This is not because of the amount of the carbon dioxide pres- ent, but of other impurities which seem to be excreted in pro- portion to the amount of this gas, so that the latter may be taken as a measure of these poisons. What these are is as yet almost entirely unknown, but that they are poisons is beyond doubt. Small effete particles of THE RESPIRATORY SYSTEM. 383 once-living protoplasm are carried out with the breath, but these other substances are got rid of from the blood by a vital process of secretion (excretion), we must believe ; which shows that the lungs to some degree play the part of glands, and that their whole action is not to be explained as if they were merely moistened bladders acting in accordance with ordinary physical laws. An estimation of the amount of atmospheric air required may be calculated from data already given. Thus, assuming that a man gives up at each breath 4 per cent of carbon dioxide to the 500 cc. of tidal air he expires, and breathes, say, seventeen times a minute, we get for the amount of air thus charged in one hour to the extent of 1 per cent : 500 X 4 X i: X60 = 2,040,000 cc, or 2,040 litres. But if the air is to be contaminated to the extent of only ^ per cent of carbonic anhydride, the amount should equal at least 2,040 X 10 hourly. Respiration in the Blood. It may be noticed that arterial blood kept in a confined space grows gradually darker in color, and that the original bright scarlet hue may be restored by shaking it up with air. When the blood has passed through the capillaries and reached the veins, the color has changed to a sort of purple, character- istic of venous blood. Putting these two facts together, we are led to suspect that the change has been caused in some way by oxygen. Exact experiments with an appropriate form of blood- pump show that from one hundred volumes of blood, whether arterial or venous, about sixty volumes of gas may be obtained ; that this gas consists chiefly of oxygen and carbonic anhydride, but that the proportions of each jjresent depends upon whether the blood is arterial or venous. The following table will make this clear : Arterial blood. Venous blood.. Oxygen. Carbonic anhydride. Nitrogen. 20 40 1-3 8-12 46 1-2 from 100 volumes of blood at 0° C. and 760 millimetre pressure. Arterial blood, then, contains 8 to 12 per cent more oxygen and about C par cent more carbonic dioxide than venous blood. It is not, of course, true, as is sometimes supposcid, tliat arterial 384 ANIMAL PHYSIOLOGY. blood is " pure blood " in the sense that it contains no carbonic anhydride, as in reality it always carries a large percentage of this gas. Fig. 315.— Diagrammatic illustration of Ludwig's mercurial gas-pump. A and B are two glass globes, connected by strong India-rubber tubes, with two similar glass globes, A' and B'. A is further connected by means of the stop-cock c with the receiver C, containing the blood (or other fluid) to be analyzed ; and B, by means of the stop-cock d and tube e with the receiver Z), for receiving the gases. A and B are also connected with each other by means of the stop-cocks / and g, the latter being so arranged that B also communicates with B' by the passage g'. A' and B' being full of mercury, and the cocks fc, /, g and d being open, but c and g' closed, on raising A' by means of the pulley p the mercury of A' fills A, driving out the air contained in it into B, and so out through e. When the mercury has risen above g, f is closed ; and g' being opened, B' is in turn raised till B is completely filled with mercury, all the air previously in it being driven out through e. Upon closing d and lowering B', the whole of the mercury in B falls into B', and a vacuum consequently is established in B. On closing g' but opening g, /, and Ic. and lowering A', a vacuum is similarly established jn A and in the junction between A and B. If the cock c be now opened, the gases of the blood in C escape into the vacuum of A and B. By raising A' after the closure of c and opening of d, the gases so set free are driven from A into B, and by the raising of B' from B through e into the receiver D, standing over mercury. (After Foster.) The Conditions under which the Gases exist in the Blood. — If a fluid, as water, be exposed to a mixture of gases which it can THE RESPIRATORY SYSTEM, 335 absorb under pressure, it is found that the amount taken up depends on the quantity of the particuhir gas present independ- ent of the presence or quantity of the others ; thus, if water be exposed to a mixture of oxygen and nitrogen, the quantity of oxygen absorbed will be the same as if no nitrogen were present — i. e,, the absorption of a gas varies with the iJartial pressure of that gas in the atmosphere to which it is exposed. But whether blood, deprived of its gases, be thus exposed to oxygen under pressure, or whether the attempt be made to remove this gas from arterial blood, it is found that the above- stated law does not apply. When blood is f)laced under the exhaustion-pump, at first very little oxygen is given off ; then, when the pressure is con- siderably reduced, the gas is suddenly liberated in large quan- tity, and after this comparatively little. A precisely analogous course of events takes place when blood deprived of its oxygen is submitted to this gas under pressure. On the other hand, if these experiments be made with serum, absorption follows according to the law of pressures. Evidently, then, if the oxy- gen is merely dissolved in the blood, such solution is peculiar, and we shall xjresently see that this supposition is neither neces- sary nor reasonable. HEMOGLOBIN AND ITS DERIVATIVES. Haemoglobin constitutes about y'V of the corpuscles, and, though amorphous in the living blood-cells, may be obtained in crystals, the form of which varies with the animal ; in- deed, in many animals this substance crystallizes spontane- ously on the death of the red cells. It is unique among albu- minous compounds in being the only one found in the animal body that is susceptible of crystallization. Its estimated com- position is : Carbon 53-85 Hydrogen 7-33 Nitrogen 1 G'17 Oxygen 21-84 Iron '43 8ul{)liur -39 together witli 3 to 4 per cent of water of crystallization. The formula assigned is: CgooHasoOnsNiMFeSg. The molecular constitution is not known, and the above formula is merely an approximation, wliich will, however, servo to convey an idea 2.-5 386 ANIMAL PHYSIOLOGY. of the great complexity of this compound. The presence of iron seems to be of great importance. If not the essential respiratory constituent, cer- tainly the administration of this metal in some form proves very valuable when the blood is deficient in hsemoglobin. This substance can be recognized most certainly by the spectroscope. The ap- pearances vary with the strength of the solution, and, as this test for blood (haemoglobin) is of much practical importance, it will be necessary to dwell a little upon the subject ; though, after a student has once rec- ognized clearly the differ- ences of the spectrum ap- pearances, he has a sort of knowledge that no verbal description can convey. This is easily acquired. One only needs a small, flat-sided bot- tle and a pocket - spectro- scope. Filling the bottle half -full of water, and getting the spectroscope so focused that the Fraunhofer lines appear distinctly, blood, blood-stained serum, a solution of hsemoglobin-crystals, or the essential sub- stance in any form of dilute solution, may be added drop by drop till changes in the spectrum in the form of dark bands appear. By gradually increasing the quantity, appearances like those figured below may be observed, though, of course, much will depend on the thickness of the layer of fluid as to the quantity to be added before a particular band comes into view. When wishing to be precise, we speak of the most highly oxidized form of haemoglobin as oxy-hgemoglobin (0-H), and the reduced form as haemoglobin simply, or reduced hsemo- globin (H). By a comparison of the spectra it will be seen that the bands Fig. 316. — Crystallized haemoglobin (Gautier). a. b, crystals f i cm venous blood of man ; c, from blood of cat ; d, of Guinea-pig ; e, of marmot ; /, of squirrel. THE RESPIRATORY SYSTEM. 387 of oxy-h£emoglobin lie between the D and E lines; that the left band near D is always the most definite in outline and the -= § £ S te most pronounced in every respect except breadth; that it is in weak solutions the first to appear, and tlie last to disappear on 388 ANIMAL PHYSIOLOGY. reduction ; that there are two instances in which, there may be a single band from haemoglobin — in the one case when the solu- tion is very dilute and when it is very concentrated. These need never be mistaken for each other nor for the band of re- duced hsemoglobin. The latter is a hazy broad band with com- paratively indistinct outlines, and darkest in the middle. It will be further noticed that in all these instances, apart from the bands, the spectrum is otherwise modified at each end, so that the darker the more centrally placed characteristic bands, the more is the light at the same time cut oft* at each end of the spectrum. If, now, to a specimen showing the two bands of oxy-hsemo- globin distinctly a few drops of ammonium sulphide or other reducing agent be added, a change in the color of the solution will result, and the single hazy band characteristic of hsemo- globin will appear. It is not to be supposed, however, that venous blood gives this spectrum. Even after asphyxia it will be difficult to see this band, for usually some of the- oxy-haemoglobin remains reduced ; but it is worthy of note, as showing that the appear- ances are normal, that the blood, viewed through thin tissues when actually circulating, whether arterial or venous, gives the spectrum of oxy-hsemoglobin. At the same time there can be no doubt that the changes in color which the blood under- goes in passing through the capillaries is due chiefly to loss of oxygen, as evidenced by the experiments before referred to ; and the reason that the two bands are always to be seen in venous blood is simply that enough oxy-haemoglobin remains to give the two-band spectrum which prevails over that of (reduced) hsemoglobin. We are thus led by many paths to the important conclusion that the red corpuscles are oxygen-carriers, and, though this may not be and probably is not their only func- tion, it is without doubt their principal one. Of their oxygen they are being constantly relieved by the tissues ; hence the necessity of a circulation of the blood from a respiratory point of view. There are other gases that can replace oxygen and form compounds with hsemoglobin ; hence we have CO-hsemoglobin and NO-hsemoglobin, which in turn are replaced by oxygen with no little difficulty — a fact which explains why carbonic oxide is so fatal when respired, and, as it is a constituent of illuminat- ing gas, the cause of the death of those inhaling the latter is often not far to seek. Blood may, in fact, be saturated with THE RESPIRATORY SYSTEM. 389 carbonic oxide by allowing illuminating gas to pass through it, when a change of color to a cherry red may be observed, and which will remain in spite of prolonged shaking up with air or attempts at reduction with the usual reagents. Haemoglobin may be resolved into a proteid (globin) not well understood, and hcemaiiii. This happens when the blood is boiled (perhaps also in certain cases of lightning-stroke), and when strong acids are added. Hsematin is soluble in dilute acids and alkalies, and has then characteristic spectra. Alkaline hsematin may be re- duced ; and, as the iron can be separated, resulting in a change of color to brownish red, after which there are no longer any reducing effects, it would seem that the oxygen-carrying power and iron are associated. This iron-free hsematin is named hceniatopnrphyrin or luemaioin.. Hmmin is hydrochlorate of hsematin (Teichmann's crystals), and may be formed by adding glacial acetic acid and common salt to blood, dried blood-clot, etc., and heating to boiling. This is one of the best tests for blood, valuable in medico-legal and other cases. When oxy-hsemoglobin stands exposed to the air, or when diffused in urine, it changes color and becomes, in fact, another substance — methcemoglohin, irreducible by other gases (CO, etc.), and not surrendering its oxygen in vacuo, though giving it up to ammonium sulphide, becoming again oxy-hsemoglobin, when shaken up with atmospheric air. Its spectrum differs from that of oxy-hsemoglobin in that it has a band in the red end of the spectrum between the C and D lines. Hcematoidin is some- times found in the body as a remnant of old blood-clots. It is probably closely allied to if not identical with the hiliruhin of bile. Comparative. — While haemoglobin is the respiratory agent in all the groups of vertebrates, this is not true of the inverte- brates. Red blood-cells have as yet been found in but a few species, though haemoglobin does exist in the blood plasma of several groups, to one of which the earth-worm and several other annelif tubule. Comparative. — Among the lowest forms, the Infusori- ans and CcpJenterates, ex- cretory organs have not been definitely traced. In tlie Ferm^.s-, organs known as nephridia (segmental or- gans, see Figs. 253, 257) are , . ^ ° 1 , / , \ I- Fio. 324.-Vertioal section of kidney (after Sap- BUppOSed to act the part OI peyj. l, l, a, a, 3, 3, a. 4, 4, 4, 4, nyramids of , , , . , . /. 1 • MalpiKhi : 5, 5, 5, 5, 5, f), apicfs of pyramidH, the kidney m some lashlOn. surrounded by calices ; (1, (!, coliiiimH of Ber- mv 1 i>i Ml tin ; 7, pelvis of kidney ; 8, upper extremity of lhe.se are long, often coiled urvusr. 420 ANIMAL PHYSIOLOGY. tubes lined with cells, and with an internal, cilated, funnel- shaped extremity opening into the body cavity. In such crus- FiG. 325.— structure of kidney (after Landois). I. Blood-vessels and tubes (semi-diagi'am- matic). A. Capillaries of cortical substance. B. Capillaries of medullary substance. 1, artery penetrating Malpighian body ; 2, vein enierging from a Malpighian body ; R- arteriolcB rectse ; C, ven» rectte ; V, V, Interlobular veins ; S, stellate veins ; I, I, cap. sules of Miiller ; X, X, convoluted tubes ; T, T, T, tubes of Henle ; N, N, N, N, communi- cating tubes ; O, O, straight tubes ; O, opening into pelvis of kidney. II. Malpighian body. A, artery ; jB, vein ; C, capillaries ; K, epithelium of capsule ; H, beginning of convoluted tube. III. Rodded cells from convoluted tube. 1, view from surface ; 2, side view ((?, granular zone). IV. Cells lining tubes of Henle. V. Cells lining communicating tubes. VI. Section of straight tube. EXCRETIOX BY THE KIDNEY. 421 taceans as tlie crayfish the green gland is supposed to repre- sent a kidney. It does not open into the body cavity like the preceding and the following form of the organ. It is well sup- plied with capillaries. The organ of Bojanus (Fig. 30G) is the Fio. 326.— Blood-vessels of Malpighian hoflies and convoluted tubes of kidney (after Sappey). 1, 1. Malpit'hian bodies surround.-d by eajwiiles ; 2, 2, 2, convoluted tubes connected with MaJpi^'inan bodies ; :i. arlcry braiK-lilii^' to ko to Malj)iKhian bodies ; 4, 4, 4, branches of Z'X'^/ ' ^1 ^' MaljiiKhian bodies from which a portion of cai)sules has been removed ; <■ 7, ( vessels passing out of Malpighian bodies ; «, vessel, branches of which (9) pass to capillary plexus (10^ main excretory channel in many groups of mollusks. In in- sects the long, coiled Malj>ighian tubules, which open into the intestine, are believed to secrete both bile and uric .acid. Among vertebrates, till the reptiles are reached, the kidney is a persistent Wolffian body, hence its more sini])l(' form. 422 ANIMAL PHYSIOLOGY. >C In most fishes the kidney is a very elongated organ, though in the lowest it consists of little more than tubules, coiling but slightly, ending by one extrem- ity in a glomerulus and by the other opening into a long com- mon efferent tube or duct. The glomerulus is, however, pecul- iar to the vertebrate kidney. The graded complexity in ar- rangement, etc., of the tubes is ^-S represented well in the figure below. It is a significant fact that the kidney of the human subject is lobulated in the em- bryo, which condition is persist- ent in some mammals (rumi- nants, etc.). As the lungs are the organs employed especially for the elimination of carbonic anhy- dride, so the kidneys are above all others the excretors of the nitrogenous waste products of the body chiefly in the form of uric acid or urea. Before treat- ing of secretion by the kidney it will be well to examine into the physical and chemical prop- erties of urine with some detail, especially on account of its great importance in the diagnosis of disease. >P Fig. 327.— Diagrammatic representation of distribution of tubules of kidney (after Huxley). C. cortical region ; B, bound- ary zone, containing large part of Hen- le's loops : P, papillary zone, in which are the main outflow tubules. Urine considered Physically and Chemically. Urine is naturally a fluid of very variable composition, espe- cially regarded quantitatively — a fact to be borne in mind in considering all statements of the constitution of this fluid. Specific Gravity. — Urine must needs be heavier than water, on account of the large varietj'' of solids it contains. The average specific gravity of the urine for the twenty-four hours is 1015 to 1030. It. is lowest in the morning and varies greatly with the quantity and kind of food eaten, the activity of the lungs and especially of the skin, with emotions, etc. EXCRETION BY THE KIDNEY. 423 Color. — A light straw color, which is also very variable, being increased in depth either by the presence of an excess of pigment or a diminution of water. There are probably several pigments, among which occur urobilin, derived probably from bile pigment ; urochrome, becoming red on oxidation ; and indican, wliich may be oxidized to indigo. The reaction of human urine is acid, owing to acid salts, espe- cially acid sodium phosphate (NaH2P04). There is usually but a trifling quantity, if any, of free acid in the urine when secreted. The acidity diminishes after meals, and the urine may be neutral or alkaline when the food is wholly vegetable, or unduly acid when the diet is entirely fleshy. ftuantity. — Usually about 1,500 c.c. or from 50 to 52 ounces (two pints) in twenty-four hours. This is, of course, like the specific gravity, highly variable, and frequently they run par- allel with each other. The following tabular statement will prove useful for refer- ence : Quantitative Estimation of the Constituents of the Urine for Twenty-four Hours {after Parkes). By an average man of 65 kilos. Per 1 kilo of body weight. Water Total solids Grammes. 1500-000 72-000 33-180 •555 •400 •910 10-000 2-012 3-1(54 7-000 •770 2-500 11-090 •200 •207 Grammes. 23-000 1-1000 Urea ; Uric acid -5000 •0084 Hippuric aoid •0060 Creatinin •0140 Pigment, etc Sulphuric acid -1510 -0305 Phosphoric acid Chlorine •0480 •1260 Ammonia Potassium Sodium Calcium .... Magnesium Attention is directed more particularly to the preponderance among the solids of urea, and sodium chloride, for the latter is the form in whicli a large part of tlu; scxlium reappears. We may say that in round numbers about 35 grammes or 500 grains {2 to ."> per cf'iit) of urea are excreted daily. Nitrogenous Crystalline Bodies. — Tliese are the derivatives of the metaljolistn of llie bo