COLUMBIA UBRARIES OFFSITE HEALTH SCIENCES STANDARD ~ ~ ~" l|ll|llill{i|l| Hill nil nil HX64089223 QP34.P312 1916 Human physiology, RECAP i s ■^^ eP31- P3/2 mt\\t€xtydMmfotk College of ^fjpsiciang anb ^uvqtonsi Hibrarp G/fi of Dr. C- F, Ma-cDons-ldL HUMAN PHYSIOLOGY Digitized by tine Internet Arciiive in 2010 witii funding from Columbia University Libraries http://www.archive.org/details/humanphysiologyeOOpear HUMAN PHYSIOLOGY ESPECIALLY ADAPTED FOR DENTAL STUDENTS BY R. G. PEARCE, B.A., M.D., Assistant Professor of Physiology, University of Illinois AND J. J. R. MACLEOD, M.B., D.P.H., Professor of Physiology. Western Reserve University SECOND REVISED EDITION FIFTY-NINE ILLUSTRATIONS, INCLUDING TEN COLOR PLATES ST. LOUIS C. V. MOSBY COMPANY 1916 1 ^ ii Copyright, 1916, by the C. V. Mosby Company Press of The C. V. Mosby Company St. Louis PREFACE TO SECOND EDITION. The gratifying reception which the first edition of this book has met with and the kindly criticism of our colleagues have encouraged us to revise it carefully, with the particular object of presenting in simpler form some of the more difficult, and yet essential principles upon which the modern science of physiol- og;^^ depends. This has been done without changing the paging of the book, and therefore without in anywise altering its gen- eral nature. Although the book, as stated in the preface to the first edi- tion, has been specially written to meet the requirements of the student of dentistry, the principle has been constantly kept in mind that the physiology which he should learn is essentially the same as that required by the student of medicine. To em- phasize this point the title of the book has been somewhat altered in form. The outlines of experiments found in the appendix are ar- ranged to occupy about sixty hours of laboratory work. While the authors realize the impossibility of formulating experiments suitable to all laboratories, nevertheless they hope that these will be of some service. The authors are deeply indebted to Miss Achsa Parker and Dr. E. P. Carter for pointing out many of the errors that ap- peared in the first edition. R. G. Pearce. J. 'J. R. MACLEOD. PREFACE TO FIRST EDITION. A knowledge of the fundamentals of human physiologj' is essential in the training of the dental student, because physiology constitutes, along with anatomy, the basic science upon which all medical and surgical knowledge is founded ; and dentistry is a highly specialized department of surgical practice. To oper- ate on the teeth without knowing something about the physi- ology of the body as a whole, would reduce the dentist to the level of a craftsman who, although perhaps very highly skilled in his technical work, was yet quite ignorant of the nature of the machine upon a part of which his work had to be done. But there are also practical reasons why the dentist should be familiar with physiology, for good health, and not good looks alone, depends very largely on sound teeth. The neglect of this fact may cause disturbances in bodily functions to which, at first sight, the teeth may apparently bear very little relation- ship ; thus, extreme emaciation, with its consequent lowering of the normal resistance of the body towards disease and infection, is well-known to be frequently due to no other cause than some abnormal or pathological condition affecting the teeth ; and, on the other hand, this very condition itself may become intract- able to the most skilled dental treatment and hygiene, if meas- ures are not taken at the same time to improve the general health. Although it is" obviously beyond the province of the dentist to undertake the treatment of these general conditions, yet it is most important that he should be sufficiently familiar with the normal functioning of the human body to be able to recognize what is really at fault. A knowledge of the laws of nutrition and dietetics must therefore form a most important part of every course in dentistry, and these have received particular attention in this book. The physiology of the digestive system, of the circulation of , the blood and of the nervous system is scarcely less important. PREFACE. V The pain and shock produced by a dental operation may cause considerable disturbance in the action of the heart or in the dis- tribution of blood in the body, and this disturbance, especially in cases in which the heart and the blood vessels are diseased, may become so pronounced as to render a certain amount of medical skill necessary. Or if, to avoid such pain, it be deemed advisable to administer anesthesia, then must the dentist be constantly on his guard that no more than the proper amount of anesthetic is given, which he can do intelligently only by observing the condition of the nervous and circulatory systems. Besides knowing something, about the physiology of the body as a whole, the dentist must be particularly familiar with the local physiology of tlie mouth, such as the finely coordinated nervous mechanisms involved in the acts of mastication and swallowing and the secretion of saliva. He must understand the nature of the sensations of the teeth and buccal mucosa, and be on the lookout for any lesions of the cranial nerves that sup- ply the muscles and other tissues adjacent to the mouth cavity. The chemistry of the saliva has demanded special attention because of the very interesting scientific investigations which are being prosecuted regarding the nature of the undoubted relationship that exists between changes in the saliva and the in- cidence of dental caries. To adequately describe the present status of this work we have found it necessary to devote some space (in the second chapter) to a review of the main physico- chemical principles which may regulate the reaction and neu- tralizing power of saliva. , Whenever the occasion presented itself to do so, we have given a brief description of the general nature of the diseases in which dental involvement is possible. A few simple, but very instructive, laboratory demonstra- tions are described in an appendix at the close of the book. "We have* found that such demonstrations furnish an invaluable aid in the leaching of the subject. To facilitate a ch'ar understanding of the subject, diagrams have been used whenever necessary, and many of these have VI PEEPACE. been specially drawn for the work. To Prof. T. Wingate Todd and Mr. P. M. Spurney, the authors are deeply indebted for the valuable assistance which they gave in the preparation of these. R. G. Pearce. J. J. R. MACLEOD. CONTENTS. Chapter I. THE CHEMICAL BASIS OF THE CELL. Page The Scope of Physiology — The Physico-chemical Basis of Life — The Chemical Basis of Animal Tissues — Water — Proteins — Lipoids — Carbohydrates 17 Chapter II. THE INFLUENCE OF PHYSICO-CHEMICAL LAWS ON PHYSIOLOGICAL PROCESSES: ENZYMES. Properties of Crystalloids — Osmotic Phenomena in Cells — Reac- tion of Body Fluids — Colloids — General Nature of Enzymes or Ferments ' 26 Chapter III. DIGESTION: NECESSITY AND GENERAL NATURE. Digestion in the Mouth — The Salivary Glands — The Nerve Supply of the Salivary Glands^ — The Reflex Nerve Control of the Sali- vary Secretion — The Normal Stimulus for Salivary Secretion (Direct and Psychological) — General Functions of Saliva 37 Chapter IV. DIGESTION: THE CHEMISTRY OF SALIVA AND THE RELATIONSHIP OF SALIVA TO DENTAL CARIES. Organic and Inorganic Constituents — The Reaction of Saliva — The Method of Measurement of Neutralizing Power of Saliva— The Deposition of Tartar and Calculi 46 Chapter V. DIGESTION. Mastication — Deglutition or Swallowing — Vomiting 53 Viii CONTENTS, Chapter VI. DIGESTION: IN THE STOMACH. Page Mechanism of Secretion of Gastric Juice — The Active Constituents of Gastric Juice — The Movements of the Stomach — The Open- ing of the Pyloric Sphincter — Rate of Discharge of Food from the Stomach 60 Chapter VII. ' DIGESTION: IN THE INTESTINE. ^ Secretion of Bile and Pancreatic Juice — Functions and Composi- tion of Pancreatic Juice and Bile — Chemical Changes Produced by Intestinal Digestion — Bacterial Digestion in the Intestine — Products of Bacterial Digestion — Protection of Mucous Mem- brane of Intestine Against Autodigestion — Movements of the Intestines — The Absorption of Food — Resume of Actions of Digestive Enzymes 71 Chapter VIII. METABOLISM: ENERGY BALANCE. Introductory — General and Special Metabolism — Energy Balance- Caloric Value of Foods — Basal Heat Production — Influence of Food, Muscular Work, Atmosphere, and Size of Body 83 Chapter IX. METABOLISM: THE MATERIAL BALANCE OF THE BODY. Starvation-Nitrogen Balance — Protein Sparers — The Irreducible Protein Minimum — Varying Nutritive Values of Different Proteins 91 , Chapter X. THE SCIENCE OF DIETETICS. The Proper Amount of Nitrogen — Chittenden's Experiments — The Most Suitable Diet for Efficiency — Chemical Composition of the Common Foodstuffs 99 CONTENTS. IX Chapter XI. SPECIAL METABOLISM. Page Special Metabolism of Proteins — Urea — Ammonia — Creatinin — Purin Bodies — Relative Importance of Proteins, Fats and Carbohydrates in Metabolism 108 Chapter XII. SPECIAL METABOLISM. Metabolism of Fats — Metabolism of Carbohydrates — Metabolism of Inorganic Salts — Vitamines 115 Chapter XIII. THE DUCTLESS GLANDS. Introduction — Thyroid and Parathyroid Glands — Adrenal Glands^ Pituitary Gland — Spleen — Thymus Gland 124 Chapter XIV. ANIMAL HEAT AND FEVER. Animal Heat — Normal Temperature — Factors Concerned in Main- taining the Body Temperature — Regulation of Body Tempera- ture — Fever 134 Chapter XV. THE BLOOD. Introduction — Physical Properties — The Corpuscles — Erythrocytes — Haemoglobin — Enumeration of Blood Cells — The Origin of the Erythrocytes — The White Cells — Leucocytes — Lympho- cytes — Functions of the White Cells — The Blood Platelets — The Blood Plasma 140 Chapter XVI. THE BLOOD. The Defensive Mechanism of the Blood — Coagulation of the Blood — Antibodies in the Blood — The Process of Inflammation — Toxins — Antitoxins — Ehrlich's Side Chain Theory— Anaphy- laxis — Phagocytosis — Opsonins 147 X CONTENTS. Chapter XVII. THE LYMPH. Page Lymph Formation— Lymphagogues— Lymph Reabsorption— The Movement of Lymph 155 Chaptek XVIII. THE CIRCULATION. Introduction — The Heart — Anatomical Considerations — Physiologi- cal Properties of Heart Muscle — Character of Cardiac Con- traction — The Sequence of the Heart Beat — The Action of , Inorganic Salts on the Heart — The Vascular Mechanism of the Heart — Definition of Terms — Events of the Cardiac Cycle — The Heart Sounds — Diseases of the Cardiac Valves 159 Chapter XIX. THE CIRCULATION. The Blood Flow Through the Vessels — The Part the Heart Plays — The Part the Vessels Play — Arterial Blood Pressure — Factors That Maintain the Blood Pressure — Velocity of Blood Flow — The Return of the Blood to the Heart — Circulation Time — The Effect of the Circulation of the £lood Itself— The Pulsa- tile Acceleration of the Blood Flow — The Pulse — The Circula- tion in the Lungs 171 Chapter XX. THE CIRCULATION. The Influence of the Nervous System on the Circulation of the Blood — The Nervous Control of the Heart — The Cardiac Nerves — Accelerator Nerves — Inhibitory Nerves— Interrelation of Inhibitory and Accelerator Nerves — The Cardiac Center — The Cardiac Depressor Nerves — The Nervous Control of the Blood Vessels — Vasomotor Nerves — ^Vasoconstrictor Nerves — Vasodilator Nerves — Vasomotor Reflexes — The Effect of Grav- ity on the Circulation — Haemorrhage — Chemical Control of Circulation — Asphyxia — Nitrous Oxide — Cocain 184 CONTENTS. Xi Chapter XXI. THE RESPIRATION. Page Introduction — The Internal Respiration — Oxidation in the Tissues — Relation of Oxidative Process to Muscular Activity — Physi- cal Laws Governing Solution of Gases — Hsemoglobin — Rela- tion of Oxygen to Haemoglobin — The Mechanism of the Res- piratory Exchange — The Effect of Carbon Dioxide on Oxy- hsBmoglobin — The Exchange of Carbon Dioxide 197 Chapter XXII, THE RESPIRATION. The External Respiration — Structure of the Lungs — The Mechan- ism of the Respiratory Movements — The Part the Diaphragm Plays — The Part the Thorax Plays — The Movements of the Lungs — Respiratory Sounds — Effects of Respiration on the Circulation — Artificial Respiration — Volumes of Air Respired — Mechanism of Gaseous Exchange in Lungs 207 Chapter XXIII. THE respiration: The Nervous Control of the Respiration — Reflex Respiratory Move- ments — Chemical Control of the Respiration — The Effect of Changes in the Respired Air on the Respiration — Mountain Sickness — Ventilation — The Voice — Mechanism of the Voice — Speech 219 Chapter XXIV. THE FLUID EXCRETIONS. The Excretion of Urine — Composition of Urine — Organic Constitu- ents — Urea — Ammonia — Uric Acid — Creatinin — Inorganic Con- stituents — Abnormal Constituents — The Organs of Excretion — The Blood Supply of the Kidney — Nature of Urine Excretion — Micturition — The Secretions of the Skin — The Sweat Glands — The Sebaceous Glands — The Mammary Glands 229 Xii CONTENTS. Chapter XXV. THE NERVOUS SYSTEM. Page General Nature and Structure of the Nervous System in Different Groups of Animals — Fundamental Elements of the Reflex Arc — Integration of the Nervous System 239 Chapter XXVI. THE NERVOUS SYSTEM. Reflex Action— The Nerve Structures Involved in the Reflexes of the Higher Animals — The Receptors of Pain, Touch, Tempera- ture — Local Anesthesia and Analgesia — The Afferent Fiber — Choice of Paths on Entering Spinal Cord — The Nerve Center — The Efferent Neurone — Types of Reflexes — Spinal Shock — The Essential Characteristics of Reflex Action — Muscular Tone and Reciprocal Action of Muscles — Symptoms Due to Lesions Affecting the Reflexes 244 Chapter XXVII. THE NERVOUS SYSTEM. The Brain Stem — The General Course and Functions of the Cranial Nerves, Particularly of the Fifth and Seventh — Relationship of the Fifth Nerve to the Teeth and to Neuralgia — Referred Pain Through this Nerve — Sensitiveness of the Tooth — Tri- facial Neuralgia — Relationship of the Seventh Nerve to Bell's Paralysis 256 Chapter XXVIII. THE NERVOUS SYSTEM: THE BRAIN. Influence of the Brain on the Reflex Functions of the Spinal Cord — ^Functions of the Cerebrum — Cerebral Localization — Experi- mental and Clinical Observations — The Sensory Centers — The Mental Process — Aphasia — The Cerebellum — Relationship to Body Equilibrium — The Semicircular Canals — The Sympa- thetic Nervous System— General Characteristics — The_ Course of Some of the Most Important Pathways 267 CONTENTS. XUl Chapter XXIX. THE SPECIAL SENSES: VISION. Page Optical Apparatus of the Eye — Formation of Retinal Image — Changes in the Eye During Accommodation from Near Vision — The Function of the Pupil — Imperfections in the Optical System of the Eye — ^Long and Short-Sightedness — Astigma- tism, etc. — The Sensory Apparatus of the Eye — The Functions of the Retina — Blind Spot — Fovea Centralis — The Movements of the Eyeballs — Diplopia — Judgments of Vision — Color Vision —Color Blindness 279 Chapter XXX, THE SPECIAL. SENSES. Hearing — The Cochlea — How Sound Waves are Transmitted to this by Tympanic Membrane and Auditory Ossicles^Causes of Deafness — Taste — Nature of Receptors for Taste — The Location of the Four Fundamental Taste Sensations — Rela- tionship Between Chemical Structure and Taste — ^Association Between Taste, Common Sensation of Touch, and Smell — Action of Certain Drugs on Taste — Smell — Nature of the Re- ceptors of Smell (the Olfactory Epithelium) — Nature of Stimulus 291 Chapter XXXI. THE MUSCULAR SYSTEM. The General Properties of Muscular Tissues — Contractility — Irritability — The Simple Muscular Contraction — Tetanic Con- traction—Effect of Load— Elasticity of Muscle — Chemical Changes Accompanying Contraction — Rigor Mortis 300 Chapter XXXII. REPRODUCTION. Fertilization— The Accessory Phenomena of Reproduction in Man— Female Organs— Male Organs— Impregnation— Ovulation — Pregnancy — Birth • 303 APPENDIX. Fundamental Demonstrations in Physiology 309 ILLUSTRATIONS. Fig. Page 1. Dialyser 27 2. Cells of parotid gland showing zymogen granules 40 3. The nerve supply of the submaxillary gland 41 4. The changes which take place in the position of the root of the tongue, the soft palate, the epiglottis and the larynx during the second stage of swallowing 55 5 Diagrams of outline and position of stomach as indicated by skiagrams taken on man in erect position at intervals after swallowing food 61 6. Diagram of stomach showing miniature stomach separated from main stomach by a double layer of mucous membrane 62 7. Diagram of time it takes for a capsule containing bismuth to reach the various parts of the large intestine ■. . . . 80 8. Diagram of Atwater-Benedict Respiration Calorimeter 86 9. Dietetic chart (colored plate) 104 10. Cretin, 19 years old 126 11. Case of myxcedema 127 12. Before and after onset of acromegalis symptoms 132 13. Thomas-Zeiss Haemocytometer 142 14. Diagram of circulation (colored plate) 158 15. Position of the heart in the thorax 160 16. Generalized view of the vertebrate heart 161 17. Diagram of valves of heart 162 18. Dissection of heart to show auriculo-ventricular bundle 165 19. Relative pressure in auricle, ventricle and aorta 168 20. Diagram of experiment to show how a pulse comes to disap- pear when fluid flows through an elastic tube when there is resistance to the outflow 173 21. Apparatus for taking tracing of the blood pressure 174 22. Apparatus for measuring the arterial blood pressure in man. . 176 23. Jacquet Sphygmocardiograph 181 24. Pulse tracing made by sphygmograph 182 25. Effect of stimulating vagus and sympathetic nerves on the frog's heart 185 26. Tracings of arterial blood pressure ,.•♦. 186 27. Curve chart 203 28. Diagram of structure of lungs, showing larynx, bronchi, bronchioles and alveoli 207 29. The position of the lungs in the thorax 209 xiv ILLUSTRATIONS. XV Fig. Page 30. Hering's apparatus for demonstrating the action of the respir- atory pump 210 31. Diagram to show movement of diaphragm during respiration 211 32. Position to be adopted for effecting artificial respiration 215 33. Diagram of laryngoscope 22.5 34. Position of the glottis preliminary to the utterance of sound. . 226 35. Position of open glottis 226 36. The position of the tongue and lips during the utterance of the letters indicated 228 37. Diagram of the uriniferous tubules, the arteries and the veins of the kidney (colored plate) 232 38. Diagram of urinary system 236 39. Schema of simple reflex arc 240 40. Diagram of nervous system of segmented invertebrate 242 41. The simplest reflex arc in the spinal cord 244 42. Diagram of section of spinal cord, showing tracts 247 43. Reflex arc through the spinal cord, in v^^hich an intermediary neurone exists between the afferent and efferent neurones (colored plate) 247 44. Course of the pyramidal fibers from the cerebral cortex to the spinal cord (colored plate) 248 45. Under aspect of human brain 257 46. Vertical transverse section of human brain 258 47. Diagram of the dorsal aspect of the medulla and pons, show- ing the floor of the fourth ventricle with the nuclei of origin of the cranial nerves (colored plate) 260 48. Diagram to show areas of referred pain in distribution of fifth nerve due to affections of the various teeth (front view) (colored plate) 262 49. Diagram to show areas of referred pain in distribution of fifth nerve due to affections of the various teeth (side view) (colored plate) 264 50. Cortical centers in man 270 51. The semicircular canals of the ear, showing their arrange- ment in the three planes of space 276 52. Formation of image on retina 281 53. Section through the anterior portion of the eye 282 54. A, spherical aberration; B, chromatic aberration 285 55. Errors in refraction 286 56. Semidiagrammatic section through the right ear 292 57. Diagrammatic view of the organ of Corti (colored plate) 292 58. Tympanum of right side with the auditory ossicles in place.. 294 59. Showing course of taste fibers from tongue to brain 296 HUMAN PHYSIOLOGY CHAPTER I. THE CHEMICAL BASIS OF THE CELL. The Scope of Physiology. — Physiology is the study of the phenomena of living things, just as anatomy or morphology is a study of their structure. The study of anatomy is most logically pursued hy starting with the simplest organisms and gradually proceeding through the more complex forms until man is reached. Except for certain fundamental functions, such as nutrition, which are common to all cells, this method is not the most suitable one to pursue in physiology, because in the low- est organisms all of the functions are crowded together in a lim- ited number of cells — indeed, it may be in one single cell. It is easier to study a function when it is performed by a tissue or organ that has been set apart for this particular purpose than when it is performed by cells that do many other things. Another reason for paying more attention to the functions of higher rather than lower animals is that the knowledge which we acquire may be more directly applicable in explaining the functions of man, and therefore in enabling us more readily to detect and rectify any abnormalities. During the embryonic development of one of the higher ani- mals, a single cell, the ovum, produces numerous other cells, which become more and more collected into groups, in many of which the cells undergo very marked changes in shape and structure, or produce materials, such as the skeleton or teeth, which show no cell structure whatsoever. Thus we have formed the tissues and organs, each having some particular function of 17 18 HUMAN PHYSIOLOGY. its own, although certain functions remain which are common to all. In other words, as the organism becomes more and more complex, there comes to be a division of labor on the part of the cells that comprise it. The conditions are exactly like those which obtain in the development of a community of men. In primeval communities there is little division of labor, every indi- vidual makes his own clothes, hunts his own food, manufactures and uses his own implements of war, but as civilization begins to appear, certain individuals specialize as hunters and fighters, others as makers of clothing, others as artisans. Although, in its first stages, this division of labor may be far from absolute, for every member of the community must still fight and take part in the building of his hut, yet it soon tends to become more and more so, until, as in the civilized communities of this twentieth century of ours, specialization has become the order of the day. A good example of a one-celled animal is the amoeba, which is often found floating in stagnant water, and which consists of nothing more than a mass of tissue, or protoplasm, as it is called, and yet this apparently simple structure can move from place to place, it can pick up and incorporate with its own substance par- ticles of food with which it comes in contact, it can store up as granules certain of these foodstuffs, and get rid of others that it does not require ; it grows as a result of this incorporation, until at last it splits in two and each half repeats the cycle. In other words, this single cell shows all of the so-called attributes of life : movement, digestion and assimilation of food, growth and repro- duction. No one of these properties is necessarily confined to living structures alone, for some perfectly inanimate bodies may exhibit one or other of them, yet when all occur together, we consider the structure to be living. In the higher animals, these functions are performed by the so-called systems, such as the digestive, the circulatory, the res- piratory, the excretory, the motor, the nervous and the reproduc- tive, each system being composed of certain organs and tissues which are designed for the special purpose of carrying out some particular function or functions. One function, however, is com- mon to all of the organs and tissues, namely, that of nutrition, THE CHEMICAL BASIS OP THE CELL. 19 which includes the process by which the digested food is built up into the protoplasm of the cells, or assimilation, and that by which the resulting substances are broken down again, or disas- similation. It is by these processes that the energy of life is set free; the energy by which the tissues perform their functions, and which appears as body heat. Every cell in the animal body is therefore a seat of energy production, and at the same time each is a machine for converting this energy into some definite form of work. In this regard the animal machine differs from a steam engine, in which energy liberation occurs in the furnace, and conversion of this energy to movement occurs in the pis- tons. The furnace and the machinery of the animal body are located in the tissue cells, and the digestive, circulatory, respira- tory and excretory systems are provided for the purpose of transporting, to and from the living cells, the fuel (i. e., the food), along with the oxygen to burn it and the gases produced by its combustion. These processes of assimilation and disas- similation constitute the study of metabolism, the practical side of which is included in the science of nutrition. The Physico-Chemical Basis of Life. With the object of ascertaining to what extent the known laws of physics and chemistry can explain the fundamental processes that are common to all cells, we must make ourselves familiar, first of all, with the chemical and physical nature of the constitu- ents of the cell, and secondly with the physico-chemical laws which govern the reactions that take place between these con- stituents. The same laws will control the reactions which take place in the juices secreted by cells; for example, in the blood and in the secretions, such as the saliva. The Chemical Basis of Animal Tissues. — Certain substances are found in every living cell and in approximately equal quan- tities ; hence these may be considered the primary constituents of protoplasm. In general they consist of the proteins, lipoids, in- organic salts, water, and probably the carbohydrates. Protoplasm is the substance composed of these primary constituents. By its 20 HUMAN PHYSIOLOGY. activity the protoplasm produces the secondary constituents of the cell, which are not the same in all cells, and which include the granules of pigment or other material, the masses of glycogen, the globules of fat or the vesicles of fluid which are found em- bedded in the protoplasm. By whatever process we attempt to isolate its constituents, we of course kill the cell, so that we can never learn by analysis what may have been the real manner of union of these substances in the living condition. All we can find out is the nature of the building material after the structure (the cell) into which it is built has been pulled to pieces.. If the chemical process by which we disintegrate the cell is a very energetic one, for example, com- bustion, we always find the elements, carbon, hydrogen, nitrogen, oxygen, sulphur, phosphorus, sodium, potassium, calcium, chlo- rine, and usually traces of other elements, such as iodine, iron, etc. If the decomposition be less complete, definite chemical compounds are obtained, namely, water, proteins, lipoids, car- bohydrates, and the phosphates and chlorides of sodium, potas- sium and calcium. We shall proceed to consider briefly the main characteristics of each of these substances and their place in the animal economy. Water. — This is the principal constituent of active living organisms, and is the vehicle in which the absorbed foodstuffs and the excretory products are dissolved. It may be said indeed that protoplasm is essentially an aqueous solution, in which other substances of vast complexity are suspended. Water, on account of its very unique physical and chemical properties, is of prime importance in all physiological reactions. These properties are: its chemical inactivity at body temperatures; its great solvent power (it is the best known universal solvent) ; its specific heat, or capacity of absorbing heat ; and, depending on this, the large amount of heat which it takes to change water into a vapor — latent heat of steam. These last mentioned properties are made use of in the higher animals for regulating the body temperature. Of great importance in the maintenance of the chemical bal- ance of the body are the electric phenomena which attend the solution of certain substances in water. This will be discussed THE CHEMICAL BASIS OF THE CELL. 21 later in connection with ionization. Water has also a very great surface tension. It is this property which determines the height to which water will rise in plants and in the soil, and which no doubt plays a role in the processes of absorption going on in various parts of the animal body. Proteins. — The great importance of proteins in animal life is attested by the fact that they are absolutely indispensable in- gredients of food. An animal fed on food containing no protein will die nearly as soon as if food had been withheld altogether. Proteins are complex bodies composed of carbon, hydrogen, oxy- gen, nitrogen, and, in nearly all cases, sulphur. Some may con- tain in addition phosphorus, iron, iodine, or certain other elements. The proportions in which the above elements are found in different proteins do not vary so much as the differences in the chemical behavior of the proteins would lead us to expect. In general the percentage composition by weight is: Carbon 53 per cent Hydrogen 7 per cent Oxygen 22 per cent Nitrogen 16 per cent Sulphur .'. 1 to 2 per cent The essential differences in the structure of the molecules of different proteins have been brought to light by studies of the products obtained by partially splitting up the molecule. We are able to do this by subjecting protein to the action of super- heated steam, or by boiling with acids or alkalies in various con- centrations, or by the action of the ferments of digestive juices or by bacteria. The cleavage produced by ferments or bacteria is much more discriminate than that brought about by strong chemical reagents ; that is to say, the chemical groupings are not so roughly torn asunder by the biological as by the chemical agencies. At jBrst the proteins break up into compounds still possessing many of the features of the protein molecule. These are the proteoses and peptones, which consist of aggregates of smaller 22 HUMAN PHYSIOLOGY. molecules, capable of being further resolved into simple crystal- line substances. These have been called the building stones of the protein molecule, and although they differ from one another in many respects, they have one feature in common, namely, that each consists of an organic acid having one or more of its hydro- gen atoms substituted by the radicle, NHg. Such substances are called amino todies or amino acids. For example, the formula of acetic acid is CH...COOH. If for one of the H atoms there is sub- stituted the NH„ group, we have CH0NH2COOH, which is amino acetic acid, or glycocoll. The same sort of substitution may take place, not alone in the simple organic acids containing one acid group, but also in those containing two acid groups, as in amino- succinic acid, COOH. CHgfNHJCOOH, or in acids containing the aromatic or benzene ring group, as in the case of tyrosine, CgH^OH. C2H3. NHoCOOH, or again there may be two amino acid groups present, as in the diamino acid, ornithin or diamino- valeric acid, C.H.CNHJXOOH. That the large and complex protein molecule is really built up out of these amino bodies has been very conclusively shown by Emil Fischer, who succeeded in causing two or more of them to become united to form a body called a polypeptid. When several amino bodies were thus synthesized, the polypeptid was found to possess many of the properties of peptones, which we have just stated are the earliest decomposition products of protein. Proteins differ from one another, not only in the nature of the amino bodies of which they are composed- (although certain of these are common to all proteins), but also in the manner in which the amino bodies are linked together. We shall see the practical value of knowing what are the amino bodies in a given protein when we come to the subject of dietetics (see p. 99). The proteins of the cell are classified into two groups. The first includes the simple proteins, such as egg and serum albumin ; and the second, the compound proteins, from which non-protein groups can be split off. As, primary cell constituents, the follow- ing simple and compound proteins are important: albumin, globulin, nucleoprotein, and the glycoproteins. They are all of the nature of colloidal substances (see p. 32), and therefore are THE CHEMICAL BASIS OF THE CELL. 23 either precipitated or coagulated when solutions containing them are boiled or have inorganic salts dissolved in them. Albumins are characterized chiefly by their great solubility in water. Three forms are of importance : egg albumin, lactal- bumin of milk, and serum albumin. Globulins occur principally in the muscle proteins, and are insoluble in Avater, but soluble in dilute neutral salt solutions. Many consider that the albumins and globulins are only nutri- tive materials out of which the protoplasm manufactures the compound proteins, these being the essential proteins of the cell. Nucleo proteins, both in quantity and in relation to their activ- ity, are probably the most important constituents of the cell. They have a very complex structure, and occur in many varieties. They consist of a combination between protein and a substance called nucleic acid, which, on being broken up by chemical means, yields phosphoric acid, a simple sugar called pentose, and nitrogenous substances known as purin bases, and pyrimidines. The purine bases are of great interest, because they are the ante- cedents in the body of uric acid, which, being relatively insoluble, may become deposited from the body fluids and cause gout or gravel. That it is possible to have an enormous variety of nucleo- proteins can be imagined when we consider that there exist differ- ent sort of purin bases, of carbohydrates, and of amino bodies. The nucleus of the cell contains a nucleoprotein which is particu- larly rich in purin bases and is often called nuclein. Phosphoproteins are compounds of phosphoric acid and simple proteins, without any nucleic acid. An example is the casein of milk (see p. 105). * Glycoproteins are compound of carbohydrates with proteins. The mucin of saliva is an example (see p. 46). Insoluble proteins resemble the coagulated proteins, and are left behind after the extraction of the other proteins from the cell. .''"I^ Lipoids. — These include all the substances composing a cell which are soluble in fat solvents. Besides fats and fatty acids, the most important of these substances are lecithin and choles- terol. 24 HUMAN PHYSIOLOGY, Lecithin is widely distributed in the animal body, and is very important in the metabolism and in the physical structure of the cell. It consists chemically of glycerine, fatty acid, phosphoric acid, and a nitrogenous base called cholin. Cholesterol is another widely distributed lipoid. It is not in reality a fatty body, but rather resembles the terpenes. Lecithin and cholesterol are abundant in brain tissue, in the envelopes of erythrocytes, and in bile. The fats exist mainly as secondary constituents of the cell, being deposited in very large amounts in certain of the connective tissue cells of the body, in bone marrow and in the omental tis- sues. Chemically, the tissue fats are of three kinds : olein, pal- mitin, and stearin, each having a distinctive melting point. They are compounds of the tri-valent alcohol, glycerine, and one of the higher fatty acids, oleic, palmitic, or stearic acid. Besides those that are present in the animal tissues, fats made up of glycerine combined with various lower members of the fatty acid series occur in such secretions as milk. In order to understand the influence which fats have on general metabolism, it is important to remember that they differ from the carbohydrates in contain- ing a very low percentage of oxygen and a relatively high per- centage of hydrogen and carbon. Thus, the empirical formula of palmitin is CgiHgsOg or C3lIg(Ci6H3i02)3, that of dextrose CeH^^.Oe? and of protein C^oHiiaNigOaaS. The Carbohydrates are also mainly secondary cell constitu- ents, although it is becoming more and more evident that they are also necessary as primary constituents. In general they may be defined chemically as consisting of the elements C, H, and 0, the latter two being present in the molecule in the same propor- tion as in water ; thus, the formula for dextrose is CJI^^^q. The basic carbohydrates are the simple sugars or monosac- charides, such as grape sugar or dextrose. When two molecules of monosaccharide become fused together with the elimination of a molecule of water (thus giving the formula Ci2H220ii)j a secondary sugar or disaccharide results. Cane sugar, lactose (or milk sugar) and maltose (or malt sugar) are examples. If sev- eral nonsaccharide molecules similarly fuse together, polysac- THE CHEMICAL BASIS OP THE CELL. 25 charides having the formula (CcHioOg)^^ are formed. These in- clude the dextrines or gums, glycogen or animal starch, the ordi- nary starches, and cellulose. Since so many molecules are fused together, it is not to be wondered at that there should be so many varieties of each of these classes of polysaccharides, for, as in the case of proteins, not only may the actual "building stones" of the molecule be different, but they may be built together in very diverse ways. The polysaccharides may be hydrolyzed (i. e., caused to take up water and split up) into disaccharides, and these into monosaccharides by boiling with acids or by the action of diastatic and inversive ferments (see p. 36). The following formulae illustrate these facts: 1. C(5Hj206=a monosaccharide (dextrose). 2. Ci2H2oOii::= a disaccharide (cane sugar) composed of: CeHoOe + CeHj.Oe— H.O. 3. {Q^^^O^)n = a polysaccharide (starch) composed of: n CgHjoOe — n HgO where n signifies that an indefinite number of molecules are involved in the reaction. CHAPTER 11. THE INFLUENCE OF PHYSICO-CHEMICAL LAWS ON PHYSIOLOGICAL PROCESSES: ENZYMES. Having learned of what materials the cell is composed, we may proceed to enquire into the chemical and physical reactions by which it performs its functions. The cell, erither of plants or of animals, may be considered as a chemical laboratory, in which are constantly going on reactions, that are guided, as to their direction and scope, by the physical conditions under which they occur. A study of the material outcome of these reactions constitutes the science of metabolism, to which special chapters are devoted further on. At present, however, we must briefly examine the physico-chemical conditions existing in the cell which may give the directive influence to the reactions. Why should certain cells, like those which line the intestine, absorb digested food and pass it on to the blood, whilst others, like those of the kidney, pick up the effete products from the blood and excrete them into the urine? We must ascertain whether these are processes depending on purely physico-chemical causes, or whether they are a function of the living protoplasm itself, a vital action, as we may call it. In general it may be said that the aim of most investigations of the activities of cells is to find a physico-chemical explanation for them, and it is one of the achievements of modern physiology that some should have been thus explainable. A large number, however, do not permit of such an explanatic-n, and this has induced certain investigators to believe that there are some animal functions which are strictly vital and can never be accounted for on a physical basis. The "physical" and the "vital schools" of physiologists are there- fore always with us. From the standpoint of physical chemistry, the cell may be considered as a collection of two classes of chemical substances, 26 CRYSTALLOIDS. 27 called crystalloids and colloids, dissolved in water, or in the lip- oids, or in each other, and surrounded by a membrane which is permeable towards certain substances but not towards others (semipermeable, as it is called). On a larger scale, the same gen- real conditions exist in all of the animal fluids, such as the blood, the lymph, the secretions and the excretions. We may therefore study the above laws with a view to applying them to both cells and body fluids. Properties of Crystalloids. — As their name implies, these form crystals under suitable conditions. When present in solu- tion they diffuse quickly thi-oughout the solution, and can readily Fig. 1. — Dlalyser made of lube of parchment paper suspended in a vessel of distilled water. The fluid to be dialysed is placed in the tube, and the distilled water must be frequently changed. pass through membranes, such as a piece of parchment, placed between the solution containing them and another solution. This process is called dialysis, and the apparatus used for observing it, a dialyser (see Fig. 1). Dialysis differs from filtration, the latter process consisting in the passage of fluids, and the sub- stances dissolved in them, through more or less pervious mem- branes as a result of differences of pressure on the two sides of the membrane. If instead of using a simple membrane, such as parchment, we choose one which does not permit the crystalloid itself to diffuse, but permits the solvent to do so — a semipermeable membrane, as it is called, — a very interesting property of dis- solved crystalloids comes to light, namely, their tendency to oc- 28 HUMAN PHYSIOLOGY. cupy more room in tjie solvent, that is, to cause dilution by at- tracting the solvent through the membrane. Cell membranes are semipermeable, but they are too small and delicate for most ex- perimental purposes. For this purpose we use an artificial mem- brane composed of a precipitate of copper ferrocyanide sup- ported in the pores of an unglazed clay vessel. If a solution of crystalloid — say, cane sugar — be placed in such a semipermeable membrane and this then submerged in water, it will be found that the cane sugar solution quickly increases in volume, or if expansion be impossible, a remarkably high pressure will be developed. This is called osmotic pressure, and it is a measure of the tendency of dissolved crystalloids to expand in the solvent. It has been found that the laws which govern osmotic pressure are identical with tliose governing the behavior of gases. There- fore, osmotic pressure ought to be proportional to the number of molecules of dissolved crystalloid. This is the case for the sugars, but it is not so for the saline crystalloids, such as the alkaline chlorides, nitrates, etc., for these cause a greater osmotic pres- sure than we should expect from their molecular weights. Why is this? The answer is revealed by observing the behavior of the two classes of crystalloids towards the electric current. So- lutions of sugars or urea do not conduct the current any better than water, whereas solutions of saline crystalloids conduct very readily. The former are therefore called non-electrolytes and the latter electrolytes. It has been found that the reason for this is that molecules of .electrolytes when they are dissolved break into parts called "ions," each ion being charged with electricity of a certain sign, i. e., positive or negative. When- ever an electric current is passed through the solution, the ions, hitherto distributed throughout the solution in pairs carrying electrical charges of opposite signs, now line themselves up so that the ions with one kind of charge form a chain across the solution along which that kind of electricity readily passes, and in so doing carries the ions with it. This splitting of electrolytes into ions is called dissociation or ionization. The ions which carry a charge of positive elec- tricity and which therefore travel towards the kathode or nega- CRYSTALLOIDS. 29 tive pole, (since unlike electricities attract each other) are called katJiions, and the negativelj' charged ions that travel to the anode, anions. Hydrogen and the metallic elements belong to the group of kathions; oxygen, the halogens and all acid groups, to the anions. These facts may be more clearly understood from the following equations : In water, or in a solution of a non-electrolyte, molecules of HoO or non-electrolyte may be represented as existing thus : H.O H2O H2O H.O H2O H2O H2O H2O H2O In a solution of an electrolyte, the molecules split into ions thus : Na+ CI- Na* CI" Na* Cl" Na^ CI- Na^ CI" Na^ Cl" Na^ CI- Na^ CI- Na^ CV When an electric current passes through a solution of an electrolyte, the ions arrange themselves thus : Kathode" Anode"^ Na^ Na^ Na^ CI" CI- CI" Na^ Na* Na^ . CI- Cb CI- Na^ Na^ Na^ CI- CI- Cl- To return to osmotic pressure, the ions influence this as if they were molecules, so that when we dissolve, say, sodium chloride in water, the osmotic pressure is almost twice what it should be, because every molecule has split into two ions. Osmotic Phenomena in Cells. — Over and over again we shall have to refer to these physico-chemical processes in explaining physiological phenomena. For the present it may make matters clearer if we consider how osmosis explains the behavior of cells when suspended in different solutions. The cell wall acts as a semipermeable membrane. Thus, if we examine red blood cor- puscles suspended in different saline solutions under the micro- scope, we shall observe that they shrink or crenate when the solu- 30 HUMAN PHYSIOLOGY. tions are strong, and expand and become globular in shape when these are weak. The shrinkage is due to diffusion of water out of the corpuscle and the swelling, to its diffusion in; that is to say, in the former case the osmotic pressure of the surrounding fluid is greater than that of the corpuscular contents and vice versa in the latter case. In this way we have a simple and con- venient method of comparing the relative osmotic pressure of dif- ferent solutions. "When the solution has a higher pressure, it is called hypertonic, when less, hypotonic, when the same, isotonic. It is evident that the body fluids must always be isotonic with the cell contents, and that we must be careful never to introduce fluids into the blood vessels that are not isotonic with the blood. A one per cent solution of common salt is almost isotonic with blood, and is accordingly used for intravenous or subcutaneous injections, or for washing out body cavities or surfaces lined with delicate membranes, such as the conjunctiva or nares. Reaction of Body Fluids. — Closely dependent upon these properties of ionization are the reactions which determine the acidity and alkalinity of the body fluids. When we speak of the degree of acidity or alkalinity of a solution in chemistry, we mean the amount of alkali or acid, respectively, which it is nec- essary to add in order that the solution may become neutral to- wards an indicator, such as litmus. This titrible reaction is how- ever a very different thing from the real strength of the acid or alkali; for example, we may have solutions of lactic and hydro- chloric acids that require the same amount of alkali to neutral- ize them, but the hydrochloric acid solution will have much more powerful acid properties (attack other substances, taste more acid, act much more powerfully as an antiseptic, etc.). The rea- son for the difference is the degree of ionization ; the strong acids ionize much more completely than the weak. As a result of this ionization, each molecule of the acid splits into H-ions and an ion composed of the remainder. To ascertain the real acidity we must therefore measure the concentration of H-ions. (These considerations also apply in the case of alkalies, only in this case OH-ions determine the degree of alkalinity.) This can be done accurately by measuring the speed at which certain chemical REACTION OF BODY FLUIDS. 31 processes proceed, that depend on the concentration of H-ions. The conversion of cane sugar into invert sugar is a good process to employ for measuring the speed of reaction. But even this refinement in technique does not enable us to measure the H-ion concentration — for now we must use this ex- pression when speaking of acidity or alkalinity — of such impor- tant fluids as hlood and saliva, in which there is an extremely low H-ion concentration. If either of these fluids be placed on litmus papers, the red litmus turns blue, but all that this signifies is that the litmus is a stronger acid than those present in blood or saliva, so that it decomposes the bases with which they were combined and changes the color. If we employ phenolphthalein, which is a much feebler acid, then blood serum reacts neutral and saliva often acid. There are two methods open to us for measuring the H-ion concentration in such cases : 1. The Hydrogen Electrode. — Place the fluid (e. g., blood serum or saliva), with a platinum electrode dipping into it, in a small vessel filled with hydrogen. Connect this hydrogen electrode, as it is called, with a standard calomel electrode by means of wires in the course of which are suitably arranged electrical instruments for the measurement of electromotive force. From the difference in the electromotive force which is found to exist between the hydrogen and calomel electrodes, we can calculate the H-ion concentration. This method is being employed for measuring the reaction of saliva in relationship to its influence on caries of the teeth. 2. The Use of StandardizcfL Indicators. — It has been found that different indicators change color at different H-ion concen- trations. By measuring the H-ion concentration — by the elec- trical method — of solutions containing known proportions of acid and alkaline salts (such as NaHoPO^ and Na2HP04 or NaHCO.^), and then observing their behavior with different in- dicators, it has been possible to evaluate the latter in terms of the H-ion concentration at which they change color. Expressing the results as the fraction of a normal solution of H-ion at which this change occurs, it has been found that paranitro-phenol turns 32 HUMAN PHYSIOLOGY. at about .000,001 (or 1x10-^), which is the H-ion concentration of pure water, and is therefore the most practical point to choose as indicating neutrality. Methyl red and rosolie acid also change color about this point. Phenolphthalein, on the other hand, changes color at a H-ion concentration of 1x10^^, i. e., it is very sensitive towards acids ; methyl orange changes at 1x10"*, i. e., it is relatively insensitive towards acids. The indicators which change color at about the H-ion' concen- trations found in animal fluids are therefore rosolie acid, para- nitrophenol and methyl red. By comparing the color produced by adding one of these indicators to the unknown fluid with the color obtained by adding the same indicator to a series of solu- tions containing varying but known H-ion concentrations, we can accurately tell the H-ion concentration of the unknown so- lution, for the H-ion concentration of the solution whose tint matches with that of the unknown is the H-ion concentration of the latter. The series of standard solutions is made by mixing varying proportions of acid and alkaline phosphates. Before leaving this subject, it is important to point out that the blood has an H-ion concentration which is practically the same as that of water, i. e., is as nearly neutral as it could be. It also has the power of maintaining this neutrality practically con- stant even when large amounts of acid or alkali are added to it. Although saliva and some other body fluids are not so nearly neutral as blood, yet they can also lock away much acid or alkali without materially changing the H-ion concentration. This property is due to the fact that the body fluids contain such salts as phosphates and carbonates, which exist as neutral and acid salts, and can change from the one state to the other without greatly altering the H-ion concentration, and yet, in so changing, can lock away or liberate H- or OH-ions. This has been called the "buffer" action, and is a most important factor in maintain- ing constant the neutrality of the animial body. Colloids. — These are substances which do not diffuse through membranes when they are dissolved. Thus if blood serum be placed in a dialyser which is surrounded by distilled water, all COLLOIDS. 33 the crystalloids will diffuse out of it, leaving the colloids, which consist mainly of proteins. The physical reason for this failure to diffuse is the large size of the molecules, in comparison with the small size of those of the crystalloids. By causing a beam of light to pass through a colloidal solution and holding a micro- scope at right angles to this beam, the colloidal particles become evident, just as particles of dust become evident in a beam of daylight in a darkened room. Filters can be made of unglazed porcelain impregnated with gelatin in which the pores are so very minute that colloids can not pass through them, though water and inorganic salts do so. When blood serum is filtered through such a filter, the filtrate contains no trace of protein. The colloidal molecules can very readily be caused to fuse together, thus forming aggregates of molecules which become so large that they either confer an opacity on the solution or actually form a precipitate. This fusing together of colloidal particles can be brought about either by adding certain neutral salts or by mixing with certain other colloids. The explanation of these results is as foltows: colloidal molecules carry either a positive or a negative electrical cliarge, and when this is neutralized, the colloidal molecules fuse together, i. e., become aggregated. This neutralization of elec- trical charge can be accomplished either by adding an electro- lyte, one of whose ions will supply the proper electrical charge, oi" by a colloid having an opposite charge. Thus the SO4 anion of NajSO^, in virtue of charges of negative electricity which it carries, will very readily precipitate such a colloid as colloidal iron (ferrum dialysatum, U. S. P.), which is charged with posi- tive electricity ; or again, this colloid itself will readily precipitate arsenious sulphide, another colloid carrying a negative charge. The physiological importance of these reactions lies in the fact that they probablj^ explain many of the peculiarities of behavior of mixtures of different animal fluids, such as toxins and anti- toxins (see p. 149). A property of colloids which is closely related to the above is that of adsorption. This means the tendency for dissolved sub- stances to become condensed or concentrated at the surface of 34 HUMAN PHYSIOLOGY. colloidal molecules. An example is the well known action of charcoal when shaken with colored solutions. It removes the pig- ment by adsorbing it. Adsorption is due to surface tension, which is the tension created at the surface between a solid and a liquid, or between a liquid and a gas. It is in virtue of surface tension that a raindrop assumes a more or less spherical shape. Since colloids exist as particles, there must be an enormous num- ber of surfaces throughout the solution, that is, an enormous sur- face tension. Now many substances, when in solution, have the power of decreasing the surface tension, and in doing so it has been found that they accumulate at the surface, that is to say, in a colloidal solution, at the surface of the colloidal molecules. The practical application of this is that it helps to explain the physical chemistry of the cell, the protoplasm of which is a col- loidal solution containing among other things proteins and lipoids. The lipoids depress the surface tension and therefore collect on the surface of the cell and form its supposed mem- brane, whilst the proteins exist in colloidal solution inside. It is possibly by their solvent action on lipoids that ether and chloro- form so disturb the condition of the nerve cells as to cause anes- thesia. A knowledge of colloidal chemistry is coming to be of great importance in physiology. General Nature of Enzymes or Ferments. To decompose proteins, fats or carbohydrates into simple mole- cules in the laboratory necessitates the use of powerful chemical or physico-chemical agencies. Thus, to decompose the protein molecule into amino bodies requires strong mineral acid and a high temperature. In the animal body similar processes occur readily at a comparatively low temperature and without the use of strong chemicals in the ordinary sense. The agencies which bring this about are the enzymes or ferments. These are all col- loidal substances (see p. 32), so that they are readily destroyed hy heat and are precipitated hy tlie same reagents as proteins. They are capable of acting in extremely small quantities. Thus, a few drops of saliva can convert large quantities of starch solu- tion into sugar. During their action, the enzymes do not them- ENZYMES. 35 selves undergo any permanent change, for even after they have been acting for a long time, they can still go on doing their work if fresh material be supplied upon which to act. These proper- ties are explained by the fact tliat they act catalytically, just as the oxides of nitrogen do in the manufacture of sulphuric acid. That is to say, they do not really contribute anything to a chemi- cal reaction, but merely serve as accelerators of reactions, which however would occur, though very slowly, in their absence. Thus, to take our example of starch again, if this were left for several j^ears in the presence of water, it would take up some of the water and split into several molecules of sugar (p. 34). The enz^nne ptyalin in saliva merely acts by hurrying up or accelerating the reaction so that it occurs in a few minutes. Enzymes differ from inorganic eatalysers in the remarkable specificity of their action, there being a special enzyme for prac- tically every chemical change that occurs in the animal hody. Thus, if we act on any of the sugars called disaccharides (cane sugar, lactose and maltose) with an inorganic caty lytic agent, such as hydrochloric acid, they will split up into their constitu- ent monosaccharide molecules, whereas in the body, each disac- charide requires a special or specific enzyme for itself. The en- zyme acting on one of them, in other words, will be absolutely inert towards the others. This specificity of action is explained by supposing that each substance to be acted on (called the sub- strat) is like a lock to open which the proper key (the enzyme) must be fitted. Enzymes are peculiarly sensitive towards the chemical condi- tion of the fluid in which they are acting, more particularly its reaction. Thus the enzyme of saliva acts best in neutral reaction, whereas the enzyme of gastric juice acts only in the presence of acid, and those of pancreatic juice, in the presence of alkali. Enzymes may unfold this action either inside or outside of the cells which produce them. Thus, the enzymes produced in the digestive tract act outside the gland cells, but the enzyme of the yeast cell acts in the cell itself and is never secreted. The former are called extracellular enzymes and the latter intracellular. The activities of intracellular enzymes are much more liable to be 36 HUMAN PHYSIOLOGY. interfered with bj unfavorable conditions than those of extra- cellular enzymes. This is because the former become inactive whenever anything occurs to destroy the protoplasm of the cell in which they act. The living protoplasm is necessary to bring the substrat in contact with them. On this account enzymes used to be classified into organized and unorganized. We know that there really is no difference in the enzyme itself ; the only differ- ence is with regard to the place of activity. The, cells that com- pose the tissues of animals perform their various chemical activi- ties in virtue of the intracellular enzymes which they contain. These are, therefore, the chemical reagents of the laboratory of life. After the animal dies, the intracellular enzymes may go on acting for a time and digest the cells from within. This is called autolysis. Enzymes are classified into groups according to the nature of the chemical action which they accelerate. Thus: Hydrolytic enzymes — cause large molecules to take up water and split into small molecules. (Most of the digestive enzymes belong to this class.) Oxidative enzymes (oxydases) — encourage oxidation. Deamidating — remove NHo group. Coagulative — convert soluble into insoluble proteins. Each group is further subdivided according to the nature of the substrat on which the enzymes act; e. g., hydrolytic enzymes are subdivided into amylolases^ — acting on starch; invertases — acting on disaccharides ; proteases — acting on proteins; ureases — acting on urea, etc. "When enzymes are repeatedly injected into the blood, or under certain other conditions, they have the power, like toxines, of producing antienzymes. As their name signifies, these are bodies which retard the action of enzymes. Thus, if sonle blood serum from an animal into which trypsin has been injected for some days previously be mixed with a trypsin solution, the mixture will digest protein very slowly, if at all, when compared with a mixture of the same amount of trypsin and protein (see also p. 78). CHAPTER III. DIGESTION. Necessity and General Nature of Digestion: Digestion in the Mouth. The never-ceasing process of combustion that goes on in the animal body, as well as the constant wear and tear of the tissues, makes it necessary that the supply of fuel and of building mate- rial be frequently renewed. For this purpose food is taken. This food is composed of fats and carbohydrates, which are mainly fuel materials, of inorganic salts and water, which are neces- sary to repair the worn tissues and of proteins which are both fuel and repair materials, and are therefore the most important of the organic foodstuffs. The blood transports the foodstuffs from the digestive canal to the tissues. In the digestive canal the foodstuffs are digested by hydrolyzing enzymes (see p. 36), which are furnished partly in the secretions of the digestive glands and partly from the numerous micro-organisms that swarm in the intestinal contents. The enzymes, as we have seen, are very discriminative in their action, for not only is the enzyme for protein without action on a fat or carbohydrate, but each of the different stages in protein break-down requires its own pe- culiar enzyme. It becomes necessary therefore that the enzymes be mixed with the food in proper sequence, and to render this possible the digestive canal is found to be divided into special compartments, such as the mouth, the stomach, the small intes- tines, etc., each provided with its own assortment of enzymes and with some mechanism by which the food, when it has been sufficiently digested, can be passed on to the next stage. Such correlation between the different stages of digestion necessitates the existence, in the different levels of the gastro- intestinal tract, of mechanisms which are specially developed to 37 38 HUMAN PHYSIOLOGY. bring about the right secretion at the right time. These mech- anisms are of two essentially different types, a nervous reflex control, and a chemical or "hormone" control. The nervous con- trol is exercised through a nerve center, which is called into ac- tivity by afferent stimuli proceeding from sensory nerve endings or receptors (see p. 244) that are especially sensitized so as to be stimulated by some property of food (its taste or smell, or its consistency or chemical nature). This type of control exists where prompt response of the glandular secretion is impor- tant, as in the mouth and in the early stages of digestion in the stomach. The hormone control consists in the action directly on the gland cells of substances which have been absorbed into the blood from the mucous membrane of the gastro-intestinal tract. The production of these substances depends upon the nature of the contents of the digestive tube. This is a more sluggish proc- ess of control than the nervous, but it is sufficient for the cor- relation of most of the digestive functions. These considerations point the way to the scheme which we must adopt in studying the process of digestion ; we must explain how each digestive juice comes to be secreted, what action it has on the foodstuffs, and what it is, after each stage in digestion is completed, that controls the movement onward of the food to the next stage. And when we have followed each foodstuff to its last stage in digestion, we may then proceed to study the means by which the digested foodstuffs are absorbed into the cir- culating fluids, and in what form they are carried to the tissues. On account of the varying nature of their food we find that the digestive system differs considerably in different groups of animals. In the omnivora, such as man, the digestive canal be- gins with the mouth cavity, in which the food is broken up me- chanically and is mixed with the saliva in sufficient amount to render it capable of being swallowed. The saliva, by containing starch-splitting ferment, also initiates the digestive process. The food is then carried by way of the oesoi^hagus to the stomach, in the near or cardiac end of which it collects and becomes gradually permeated by the acid gastric juice. It is then caught up, portion by portion, by the peristaltic waves of the SALIVARY SECRETION. 39 further or pyloric end of the stomach and, after being thor- oughly broken down by this movement and partially digested by the pepsin of gastric juice, is passed on in portions into the duodenum, where it meets with the secretions of the pancreas and liver. These secretions, acting along with auxiliary juices secreted by the intestine itself, ultimately bring most of it into a state suitable for absorption. What the digestive juices leave unacted on bacteria attack, especially in the cascum, so that by the time the food has gained the large intestine it has been di- gested as far as it can be. In its further slow movement along the large intestine the process of absorption of water proceeds rapidly. Disturbances in the digestive process may be due not only to possible inadequacy in the secretion of one or other of the diges- tive juices, but also to disturbances in the movem-ents of the digestive canal. Such disturbances wall not only prevent the forward movement of the food at the proper time, but, by failing to agitate the food, they will prevent its thorough admixture with the digestive juices, so that the enzymes which these con- tain will not become properly mixed with the food. / Digestion in the Mouth. Salivary Secretion. — In the mouth, besides its preparation for swallowing, by mastication, etc., the food, mainly on account of its taste and smell, stimulates sensory nerve endings which, by acting on nerve centers, set agoing several of the digestive secretions. The first of these is \he secretion of the salivary glands. On account of their ready accessibility to experimental investigation, very extended studies have been made of the sali- vary glands, and from these studies some of the most important physiological truths, concerning the nature of the nervous con- ti'ol of glands in general, have been drawn. Of the three salivary glands in man, the parotid secretes a watery saliva usually con- taining the enzyme, ptyalin, and the submaxillary and subling- ual secrete a sticky saliva containing mucin, usually along with some ptyalin. When the glands are not secreting, the cells that 40 HUMAN PHYSIOLOGY. compose them are engaged in preparing material to be secreted. By microscopical examination, this material is seen in the proto- plasm of the cells (Fig. 2) as granules, which are extremely small in the serous gland cells, but much larger in the mucous. In both types of gland the granules so crowd the cell that the nucleus becomes indistinct and the cell itself much swollen. After the gland has been active, the granules disappear, being evidently discharged from the cell into the duct of the gland. The granules are believed to represent the precursors of the ptyalin or mucin of saliva — hence their name of "zymogen" or "mother of ferment" granules — rather than these substances A. B. Fig. 2. — Cells of parotid gland showing zymogen granules : A, after pro- longed rest ; B, after a moderate secretion ; Cj after prolonged secretion. (Langley. ) themselves. Watery or saline extracts of the glands contain neither mucin nor ptyalin, nor does the addition of acetic acid to a mucous gland cause any precipitate of mucin; indeed, it has an entirely opposite action, it causes the granules to swell. The Nerve Supply of the Salivary Glands. — The nerve fibers supplying the glands are of the autonomic or visceral type (see p. 277), and they include sympathetic and cerebro-spinal fibers. The sympathetic fibers are derived from cells in the lateral horns of the spinal cord, from which they emerge by the upper three or four thoracic roots, and after ascending as meduUated fibers in the cervical sympathetic, terminate as synapses around the cells of the superior cervical ganglion. The axons of these cells proceed as non-meduUated post-ganglionic fibers along the near- est vessels to the respective glands. The cerebral autonomic SALIVARY SECRETION. 41 fibers arise from a center in the medulla and proceed to the glands by various routes; those to the submaxillary and sub- lingual glands in the chorda tympani, and those to the partoid by way of the tympanic branch of the glosso-pharyngeal. The ganglion cells connected with the cerebral fibers are situated more or less peripherally; in the case of the submaxillary they are embedded in the substance of the gland ; in the ease of the sublingual gland, in the connective tissue of the so-called submaxillary triangle, and in the case of the parotid, in the otic ganglion (Fig. 3). In both cerebral and sympathetic nerves there are two vari- eties of fibers, the one vasomotor, the other secretory. The for- o- Fig. 3. — The nerve supply of the submaxillary gland : Li, lingual nerve ; c. t., chorda tympani ; g. gland. Wharton's duct is ligated and it will be noticed that the chorda leaves the lingual nerve, just before this crosses the duct, thus forming the submaxillary triangle. (Claude Bernard.) mer, in the case of the cerebral nerves, are dilator in their action, but in the sympathetic they are constrictor. On account of the association of secretory and vasodilator fibers, in the cerebral nerves, stimulation leads to the secretion of large quantities of saliva, the amount of which, as well as its percentage of organic and inorganic constituents, varies with certain limits with the strength of the stimulus. Although secretory activities also be- come excited when the sympathetic nerve is stimulated, as is 42 HUMAN PHYSIOLOGY. revealed by histological examination of the gland, there is only a slight flow of saliva from the duet because of the concomitant curtailment of the blood supply. In so far as actual secretion of saliva is concerned, the net result of stimulation of either nerve is therefore dependent upon whether dilatation or constriction of the blood vessels of the gland occurs, and this might Jead us to conclude that the secretion is secondary to changes in the blood supply ; in other words, that it is unnecessary to assume the independent existence of specific secretory nerve impulses. That such secretory fibers do exist, however, is established by many facts. Two of these are: (1) The vessels still dilate but no secretion occurs after a certain amount of atropin has been allowed to act on the gland. This alkaloid paralyzes the secre- tory nerve fibers, but has no action on those concerned in. vaso- dilation. (2) If the secretions were merely the result of in- creased blood supply, in other words, were a filtrate from the blood, the pressure in the duct would at all times be less than that in the blood vessels; but this is not the case, for during stim- ulation of the cerebral nerves the duct pressure may rise far above that of the blood vessels. But it must never be lost sight of that although both kinds of fibers do exist, they are very closely associated in their action. The Reflex Nervous Control of Salivary Secretion. — The structural differences between the parotid and submaxillary glands suggest that their functions may not be the same; that their respective secretions must be required for different pur- poses. To put this supposition to the test, it becomes necessary to adopt some means by which the conditions calling forth the secretion of each gland may be separately studied. This can be accomplished by a small surgical operation in which the ducts ar-e transplanted so as to discharge through fistula in the cheek, the secretion being easily collected, by allowing it to flow into a funnel which is tied in place. * In general, two distinct types of stimuli may call forth secretion of one or other gland, namely: (1) direct stimulation of sensory nerve endings in the mouth, and (2) psychological stimuli in- volving more or less of an association of ideas. SALIVARY SECRETION. 43 Of the stimuli which cause secretion by acting on sensory nerve endings in the mouth, some ijifluence the parotid, others, the sub- maxillary gland, and ditiPerent stimuli produce different effects. Even for pure mechanical stimulation of the buccal mucosa, a marked degree of discrimination is shown; thus, smooth clean pebbles may be rolled around in the mouth and yet cause no saliva to be secreted, whereas dry sand will immediately cause the parotid to discharge enormous quantities of thin watery juice. Similarly dry bread crumbs invoke copious parotid secre- tion, bread itself having little effect; water, ice, etc., are inert, but if they contain a trace of acid an abundant secretion is in- stantly poured out. It is plain in all these cases that the pur- pose of the secretion is to assist in the removal or neutralization of the substance which is present in the mouth. The thick mucous secretion of the submaxillary and sublingual glands seems to depend more on the chemical nature of the food than on its mechanical state, boiled potatoes, hard boiled eggs, meat, etc., causing the secretion of a thick slimy saliva, which by coating the food assists swallowing. The relish for the food seems to be of little account in influencing the secretion of saliva, for noxious substances, or those that are acid, or very salty, call forth much more secretion than do savory morsels. Although mere mechani- cal stimulation is not in itself an adequate stimulus, yet move- ment of the lower jaw is (juite effective, as for example in chew- ing, or when the mouth is kept open, as by ai gag in a dental operation. The stimulus does not, however, require to be applied to the buccal mucosa itself; it may be psychic or associational, and liere again a remarkable discrimination is evident, although the response is not so predictable as when the stimulus is local. Thus, when dry bread or sand is shown to a dog to which previ- ously these substances have been given by mouth, salivation fol- lows, but this is not the case when moist bread or pebbles are offered. Appetite plays an important part in this psychic reflex, for when dry food is shown to a fasting, animal, salivation is marked, but may cause no secretion when it is offered to a well- fed animal. It is possible in this case, however, that there may 44 HUMAN PHYSIOLOGY. be inhibition of the glandular activities on account of the pres- ence of food products in the blood. Perhaps the most interesting fact of all is that even a fasting animal will after a time fail to salivate if he be repeatedly shown food which causes a secretion, but which he is not permitted to get. The response is immedi- ately established again, however, if some food, or indeed some other object, be placed in the mouth. A hungry animal will even salivate when he hears some sound which by previous experience he has learned to associate with feeding time. The psychic reflexes are evidently dependent upon an association of ideas (a nervous integration, see p. 242) ; they are conditioned reflexes, and are therefore the result of a certain degree of education. They are easily rendered ineffective by confusing the usual asso- ciations. General Functions of Saliva. — These observations indicate that a* very important function of the saliva is what we may call a mechanical one, namely, either to flood the mouth cavity with fluid and so to wash away objectionable objects in it, or to lubri- cate the food with mucin and so facilitate swallowing. The sol- vent action of saliva is also important for the act of tasting (see p. 295). Its cJiemical activities in many animals seem to be lim- ited to the neutralizing properties of the alkali which is present in it, but in man and the herbivora it also contains a certain amount of a diastatic enzyme, ptyalin, which can quickly con- vert cooked starches into dextrines and maltose. Even when this action is most pronounced, however — for it varies considerably in different individuals — it cannot proceed to any extent in the mouth cavity, partly on account of the short time food remains here, and partly because many starches, as in biscuits, are taken more or less in a raw state. In some animals, such as the dog, the saliva has no diastatic action whatever. Although there can therefore be little diastatic digestion in the mouth, a good deal may go on in the stomach, for the saliva that is swallowed along with the food does not become destroyed by the gastric juice until some thirty minutes after the food has gained the stomach. Although mastication of the food and its preparation for swallowing are undoubtedly the main functions of the mouth cav- SALIVARY SECRETION. 45 ity, another exists which is of very great importance for proper digestion; this is the stimulation of the taste nerve endings, and, for foods with a flavor, of those of the olfactory nerve in the posterior nares. Such stimulation not only gratifies the appetite, but it serves as the adequate stimulus to set agoing the secretion of the gastric juice. Without any relish for food, digestion as a whole materially suffers, and for this reason unpalatable food is always more or less indigestible. Recent investigations point to another function of the saliva. Pepsin, a ferment which is important in the digestion of the proteins and which is found in the juice secreted by the glands of the stomach, is readily absorbed by starch when in the col- loidal state as it is generally eaten. • In this condition the fer- ment is not free to act upon the proteins and digestion is de- layed. If the saliva be allowed to partially digest the starch into sugar before the food reaches the stomach, the colloidal state is changed by the action of the ptyalin of the saliva, and ab- sorption of the ferment does not occur. CHAPTER IV. DIGESTION (Cont'd). The Chemistry of Saliva and the Relationship of Saliva to Dental Caries. A knowledge of the composition and chemical properties of saliva is of great importance because of the undoubted etiologi- cal relationship which exists between this secretion and dental caries. Mixed saliva when freshly secreted is a watery, more or less opalescent and sticky fluid, often containing smalL masses of mucin, but on standing it becomes cloudy because of precipi- tation of calcium carbonate. Its specific gravity is 1002-1006, and it contains about 0.05 per cent of solids. The saliva from the sublingual and submaxillary glands is very much richer in solids than that from the parotid. The parotid saliva also differs from that of the other glands in containing no mucin, although it is often rich in ferment. The solid constituents, with some of their properties, are as follows: ' Glycoprotein (mucin) : precipitated by acid. Other proteins: coagulated by heat. Organic. . .^ Ptyalin: a starch-splitting enzyme. Potassium sulphoeyanide : gives a red color with ferric chloride. Sodium chloride : ) give a precipitate with sil- Potassium chloride : j ver nitrate. Calcium bicarbonate : in fresh saliva. Calcium carbonate : precipitated in saliva after Inorganic . ietetic chart, showing ttie percentage amounts of llie vai'ious liroximate pr-incii)les Hndicated iiy the shiided ;irc:is) :in0 70 SO *fO WO Fu 27 Ordinate.s — Percentage .saturation of haemoglobin with oxygen. Abscissae — Tension of oxygen in mm. of meicury. Curve A — Degree of saturation of pure htemoglobin .solutions at varying pressures. Curve B — Modification of degree of saturation caused by presence of salts in the blood. Curve C — Effect of 20 mm. CO^ pressure on above solution. Cui-ve IJ — The saturation curve in normal blood at 40 mm. carbon dioxide pressure. 204 HUMAN PHYSIOLOGY. other factors besides oxygen pressure. These are: (1) tempera- ture, (2) the presence of inorganic salts, and (3) carbon dioxide or other weak acids in the blood. If haemoglobin be dissolved in a saline solution containing the same concentration of inorganic salts as is found in blood, it will take up oxygen in a manner somewhat similar to blood under like oxygen pressures. The similarity will become perfect if the saline solutions of hemo- globin be subjected to the same pressure of carbon dioxide as that present in the sample of blood, that is, provided the temperature is the same in the two cases. Of the curves shown in Fig. 27, a represents the degree of dissociation of oxygen from pure oxy- hasmoglobin solution at varying oxygen pressures ; h, the modifi- cation in the degree of the association produced by the presence of the salts of the blood; c, the effect of carbon dioxide on the oxygen content of the haemoglobin in a saline solution; and d is the dissociation curve of normal blood with a carbon dioxide tension of 40 mm. The effect of carbon dioxide is of special interest. It is seen that the greater the concentration of carbon dioxide, the more readily is the oxygen dissociated from the oxyhaemoglobin. Thus, at an oxygen pressure of 20 mm. of mercury, the amount of oxyhsemoglobin formed is 67.5 per cent of the total haemoglobin at a carbon dioxide pressure of 5 mm., whereas at a pressure of 40 mm. of carbon dioxide the amount of oxyhaemoglobin is only 29.5 per cent. Inasmuch as the amount of carbon dioxide is con- stantly changing in arterial and venous blood, the presence of this gas would seem to be an important factor in the control of the oxidation or the dissociation of the hsemoglobin compounds. At any rate, it would help to account for the ease with which oxygen is broken from the oxyhaemoglobin molecule in the capil- laries which are imbedded in the tissues where the carbon dioxide is formed, and its pressure is correspondingly high. The Mechanism of the Respiratory Exchange. — The oxygen in the alveoli or air passages of the lungs comprises about 14 to 15 per cent of the total air, and exerts on the cells of the respira- tory epithelium a pressure of about 100 mm. mercury, more or less. Venous blood when it reaches the lungs contains about 50 RESPIKATOBY EXCHANGE. 205 per cent less oxj^gen than does arterial blood, and can take up from 6 to 8 c. c. of oxygen for every one hundred e. c. of blood. Haemoglobin solutions are almost completely saturated with oxy- gen at pressures of oxygen much less than 100 mm. of mercury. There are, therefore, very favorable conditions in the lungs for haemoglobin to take up oxygen from the air. It must be under- stood, however, that the haemoglobin does not obtain oxygen di- rectly from the air. The haemoglobin is held in the blood corpus- cles which are floating in the blood plasma. Between the plasma and the. air in the lungs lie two thin membranes, the capillary wall and the wall lining the air sac of the lung. The oxygen must first be dissolved by the fluid in the lung epithelium; from this the cells of the capillary walls take oxygen, and the plasma in turn takes the oxygen from the capillary cells. The plasma loses the oxygen thus obtained because the hsemoglobin is very greedy for oxygen. There is accordingly a difference in the oxygen pressure in the plasma of the capillaries of the lungs sufficient to account for the absorption of oxygen by the haemoglobin of the blood. The blood leaving the lungs is delivered into the left heart, from which it is distributed over the body. Since oxidation takes place within the tissue cells, oxygen is being continually called for, and the lymph surrounding the cells must continually gain a fresh supply of oxygen from the plasma of the blood. This re- duces the tension of oxygen in the plasma and causes an evolution of oxygen from the dxyhsemoglobin, which is taken up by the plasma to be passed on to the lymph and then on to the cell. There is thus a descending scale of pressure or tension of oxygen from the air of the lungs, where its pressure may amount to 100 mm. of mercury, until it reaches the tissue elements, where the pressure may be considered zero. Under ordinary conditions the circulation is fast enough to prohibit the complete reduction of the oxyhaemoglobin. In case it is not, or in case the oxygen sup- ply is short, the phenomena of asphyxia develop (see p. 195). Effect of Carbon Dioxide on Oxyh.^ 2. Destruction of the anterior horn cells not only causes absence of reflex action, but is followed by marked atrophy of the affected muscles. It has been supposed that this points to a so-called trophic influence of these nerve cells, that is to say, a power of influencing nutrition. Such changes occur in infan- tile paralysis (poliomyelitis anterior). 3. Stimulation of the above fibers may cause exaggeration of the reflexes, as in the earlier irritative stages of neuritis, in tumors pressing on the nerve roots, or when the membranes of the cord become inflamed, as in meningitis. 4. Removal of impulses coming from the cerebrum by way of the pyramidal tracts causes exaggerated reflexes. Such occur in paralysis of both sides of the body in paraplegia, and on the paralyzed side in hemiplegia. In a paraplegic patient the weakest stimulus applied to the skin of the paralyzed portion of the body will call forth a wide- spread and much exaggerated reflex contraction. CHAPTER XXVII. THE NERVOUS SYSTEM (Cont'd). The Brain Stem and the Cranial Nerves. The Brain Stem. — The medulla, the pons Varolii, and the mid- brain (Figs. 45 and 46), compose the brain stem, which is really an upward extension of the grey matter, and of certain of the columns of the spinal cord, into the base of the brain with special nerve centers and especially large bundles of inter-connecting nerve fibers superadded. It is because of the crossing in various directions of these bundles of fibers that the structure of the medulla, pons and mesencephalon is so difficult to understand. The grey matter, as in the spinal cord, lies deep and the fibers are superficial. Of the latter, the pyramids and fillet, already de- scribed, are the most important, and their direction is longi- tudinal. The most prominent of the connecting or commisural nerve bundles are the upper, middle and lower peduncles of the cerebellum,, or small brain, which, it will be remembered, lies over and at the side of the pons varolii and midbrain. The lower peduncles spring from the medulla and connect the spinal cord with the cerebellum. They form the lower edges of the fourth ventricle. The middle peduncles enter the sides of the pons, in which they cross at right angles with the pyramidal fibers (p. 248). They connect the cerebellum of one side with the cerebrum of the opposite side. The superior peduncles join the encephalon just under the posterior corpora quadrigemina, and the fibers composing them decussate to the other side to be- come connected with certain of the so-called basal ganglia. The dasal ganglia are the optic thalamus and the corpora stri- ata, two large collections of nerve cells protruding into the third and lateral ventricles of the brain and having the internal capsule between them (see p. 248). The nerve cells composing these ganglia receive impulses from nerve fibers arriving at them both 256 THE BRAIN STEM. 257 from below (coming from the spinal cord) or from above (com- ing from the cerebrum). They then transmit these impulses along their own nerve fibers, which ma}^ run to various other Fig. 45. — Under aspect of human brain. In the center line from below upward.s are seen a section of the upper end of the spinal cord, and the medulla oblongata (»i), with certain of the cranial nerves* (as numbered). In front of this is the pons (}}}, with the large fifth nerve arising from it, and the middle peduncles of the cerebellum (M. Ped) running into the cere- bellum (A). The rounder bodies anterior to the pons are the corpora quad- rigemina (C'q), at the sides of which are the crura cerebri and the origins of the third and fourth nerves. The optic and olfactory nerves are in front. The under surfaces of the cerebrum (.Cb) and cerebellum (A) constitute the remainder of the drawing. (From a i)reparation by I'. M. Spurney. ) 258 HUMAN PHYSIOLOGY, parts of the brain. The optic thalamus, as its name signifies, is intimately associated with the optic nerves. Another important collection of nerve cells occurs in the corpora quadrigemina. These exist as four rounded swellings, two on either side, just where the superior peduncles of the cere- bellum come together. Their nerve cells serve as distributing centers for visual and auditory impulses, carried to them through tracts of nerve fibers connected with the optic and auditory Fig. 46. — Vertical transverse section of human brain. Below is a section of the pons (P) showing the fibers which connect the brain stem and cere- brum radiating up through the internal capsule (/C), which is bounded mesially by the optic thalmus (T), and laterally by the corpus striatum (i). The third (III-V) and lateral ventricles (.LV) of the brain are seen in the center (black). The thicltness of the grey matter and the infolding of the surfaces, as convolutions, should be noted. (From a preparation by P. M. Spurney. ) nerves. The corpora quadrigemina are usually more developed in the brain of the lower animals than in that of man. The branial Nerves. — On account of the introduction of the new structures described above there is no regularity in the THE CRANIAL NERVES. 259 arrangement of the grey matter in the brain stem as there is in the cord. Instead of forming horns, the grey matter is scat- tered in colonies or nuclei, many of which are centers for the fibers of the cranial nerves. Some of these fibers are, of course, afferent and some efferent. Since many of the cranial nerves are connected with the nose, mouth and teeth, it is im- portant for us to learn something concerning the location of their centers and the general function of the nerves. There are twelve pairs of cranial nerves, and the last ten of these originate from the grey matter of the medulla, pons or midbrain. The following list indicates the general functions of the nerves: 1. Olfactory. 2. Optic. 3. Oculo motor. 4. Trochlear. 6. Abducens. 5. Trigeminal. 7. Facial. 8. Auditory. 9. Glosso-pharyn- geal. 10. Vagus. 11. Spinal accessory. 12. Hypoglossal. nerve of smell, nerve of sight. nerves to the mus- cles of the eyeball. sensory nerve of face, main motor nerve of face muscles, nerve of hearing and of semicircular canals, motor nerve of phar- ynx, sensory nerve of taste, efferent and afferent nerve to various viscera, mainly blends with vagus motor nerve for tongue muscles It is important to note that, like the spin the cranial nerves are composed of two roots. arises from fore- brain. arises from fore- brain. arise from midbrain. arises mainly in pons, arises in pons and medulla, arises in pons. arises mainly in medulla. arises in medulla. arises with vagus except spinal por- tion, which extends down into spinal cord. arises in medulla. al nerves, many of motor and sensory, 260, HUMAN PHYSIOLOGY. each having its own center. This fact justifies the statement which we have already made that the brain stem is really an up- ward prolongation of the spinal cord, and just as we saw that each posterior root of the spinal cord is characterized by pos- sessing a ganglion, so also is there a ganglion in the sensory divisions of the cranial nerves. This ganglion, however, is often difficult to find. The nerve cells which compose it unite with the fibers of the sensory root by a T-shaped junction, and the fibers terminate by synapsis around the cells of the sensory nuclei. The ganglion of the fifth nerve is the Gasserian. Those for the eighth are the ganglia found in the cochlea and internal auditory meatus (Scarpa's ganglion). The ganglia of the ninth and tenth nerves are situated along the course of the nerves. The approximate position of the various ganglia will be best learned by consultatipn of the accompanying diagram (Fig. 47). ■In -the brain stem there are three sensory or afferent nuclei, a long, combined one for the ninth, tenth and eleventh nerves, ex- tending practically from the upper to the lower limits of the medulla, one for the eighth in the center of the pons, and a very long one for the fifth, extending from near the upper limit of the pons down into the spinal cord. The motor or efferent nuclei for the third, fourth, sixth and twelfth nerves are com- posed of cells shaped like those of the anterior horn of the spinal cord. They lie near the middle line and extend throughout the whole length of medulla and pons. The motor nuclei of the fifth, seventh, ninth, tenth and eleventh lie outside the above. It is important that the following functions of these nerves be studied by dental students : The Third Nerve. — The third nerve controls: (1) the mus- cles of accommodation inside the eye; (2) all of those which are attached to the outside of the eyeball, except the muscle which moves it out (external rectus), and the one which rotates it down and out (the superior oblique) ; and (3) the elevator muscle of the eyelids (levator palpebrse). When the third nerve is paralyzed, the symptoms are therefore: (1) drooping of the eyelid (ptosis) so that the chin is tilted upward when the pa- tient looks at anything; (2) inability to see clearly unless when fig. 47. — Diagram of the dorsa! aspect of the medulla and pons showing the floor of the fourth ventric'.e with the nuclei of origin of the cranial nerves. (After Sherrington.) The sensory nuclei are colored red and are numbered on the left of the diagram, the motor, blue and numbered on the right. The peduncles of the cerebellum — 8. (superior), M. (middle), and /.. (inferior), are shown cut across. CO., corpora quidrigimina. The above nuclei are of course present on both sides. THE CRANIAL NERVES. 261 objects are at a distance (long sight) ; (3) squint of the eye so that it is directed outward and domiward. Such a paralysis of the eye is sometimes accompanied by a partial hemiplegia (see p. 271) of the opposite side of the body, thus idicating that some destructive lesion (haemorrhage, de- structive tumour) exists on one side of the midbrain, so that it involves the nucleus of origin of the third nerve and also the P3^ramidal fibers lying near. Since the fibers of the third nerve do not cross to the opposite side, but those of the pyramids do (see p. 243), we get a crossed or alternating paralysis. Some- times only one part of the third nerve may be paralyzed, for example, that portion going to the muscles of accommodation. The Fourth and Sixth Nerves. — The fourth and sixth nerves supply the two extra-ocular muscles not supplied by the third, viz., the superior oblique (fourth) and the external rectus (sixth), respectively. The Fifth Nerve.— -The fifth nerve is the largest of the cranial nerves, and is a representative mixed nerve. It supplies the teeth. The motor hranch runs to the muscles of mastica- tion, the tensor nmsele of the palate, the mylohyoid muscle (in the floor of the mouth) and the anterior belly of the digastric. These last two mentioned muscles pull the hyoid bone and there- fore the root of the tongue upward and forward during the act of swallowing. Both mastication and swallowing are seriously impaired when this nerve is paralyzed. The sensory fibers are connected with the receptors for all the conmion sensations of the head and face. As already explained, they are connected with the nerve cells of the Gasserian ganglion, which is lodged in a depression near the apex of the petrous portion of the temporal bone. Shortly after leaving this ganglion, the nerve divides into three branches: (1) Upper or ophthalmic, carry- ing the sensory nerve fibers for the conjunctiva, the mucous membrane of the nasal fossae, and the skin of the eyebrow, fore- head and nose. (2) Middle or superior maxillary, supplying the meninges, the lower eyelid, the skin of the side of the nose and upper lip and all the teeth and gums of the upper jaw. (3) Inferior maxillary, supplying the teeth and gums of the lowei* 262 HUMAN PHYSIOLOGY. jaw, the skin of the temple and external ear, the lower part of the face and the lower lip. Eelationship OB' THE FiFTH Nerve TO THE Teeth. — In any in- flammatory condition of the teeth, the terminations of the sen- sory fibers become stimulated, causing ' extreme pain. This is toothache. The relationship of the fifth nerve to the teeth ex- plains why disturbance in the latter should often cause the pain to be referred not to the tooth that is involved, but to some skin area on the face. This is called referred pain. The skin areas corresponding to the different teeth have been worked out by Head, and are indicated in the accompanying diagrams (Figs. 48 and 49). Not only may the pain be referred to the skin area, but this itself may become hypersensitive. There is, moreover, in each area usually a maximal spot at which the pain and ten- derness are most marked. The sensory nerve endings in the teeth are all of the nature of pain receptors; there are no temperature or tactile receptors, these latter sensations being particularly developed in the tongue and lips (see p. 244). The pain receptors of the teeth, like those of the cornea, react practically in full intensity to every strength of stimulus. This explains why a small degree of irritation, as that due to caries, may cause as painful a toothache as an in- tense irritation. As we have already explained, the purpose of painful or nocuous sensation is protective, causing, for example, withdrawal of the irritated portion of the body or some move- ment of offense (see p. 251). In the case of the teeth it serves as a warning that something must be done to arrest whatever condition is causing it. The enamel and cement are devoid of nerve endings, which, however, are very abundant in the pulp, and probably also in the dental tubules (Mummery). An inert, sensationless exterior covering, a highly sensitive center, and between these a moderately sensitive tissue, describes the sensi- tiveness of a tooth. The sensitiveness of the pulp is so great as to suggest that it is partly of the nature of a highly specialized noci-receptor, just as the taste buds and olfactory epithelium- are specialized receptors for taste and smell. The sensitiveness of the teeth diminishes with advancing age. Fionlo-nasal area lary incisors). (maxil- Naso-labial area (maxillary canine and first premolar). The points of maximum intensity Maxillary area (maxillary second premolar and first molar). Mental area (mandibular incisors, canine and first jiremolar). are ringed. Kijr. 48. — Diagram to show areas of referred pain in distriljution of fifth nerve due to affections of the various teeth (Front view). ( Krom drawing by T. Wingate Todd.) THE CRANIAL NERVES. 263 The fifth nerve is very commonly the seat of neuralgia, which may affect one or all of its branches. This is called "tic douloureux" or tri-facial neuralgia. The attacks come in spasms, and besides the excruciating pain, there is often twitch- ing of the muscles or flushing of the skin of the face. Pressure at the points where the branches of the nerve come out of the skull, as at the supra or infra-orbital notches, is usually espe- cially painful in tic. An unhealthy condition of the teeth is often responsible for the symptoms, but if dental treatment and general medical care do not remove the neuralgia, it is usually advisable to ffut out a portion of the nerve or even to remove the entire Gasserian ganglion. Sometimes the fifth nerve becomes paralyzed, causing anes- thesia involving the area of its distribution. Tingling, numb- ness or neuralgic pains often precede the anesthesia. Since the conjunctiva loses its sensitiveness, particles of dust, etc., are not removed from the eye by the tears so that tliey set up inflam- mation, which may develop and cause ulceration of the cornea. For the same reason, or perhaps because the nerve independently controls the nutrition of tissues, the gums and cheeks may be- come ulcerated and the teeth loosened. Partial loss of taste and inability to smell pungent vapors, which act on sensory nerves, are also common symptoms. The Seventh Nerve. — The seventh nerve is purely motor in function. All the facial muscles, except those concerned in mastication, the platysma of the neck, the posterior belly of the digastric and one of the muscles of the middle ear (the sta- pedius) are supplied by it. On account of its tortuous course the seventh nerve is peculiarly liable to inflammation and com- pression. Thus tumors or inflammation located at the base of the brain may involve that portion running between the upper end of the medulla "oblongata and the internal auditory meatus, where the nerve enters the aqueduct of Fallopius. In this region it -is likely to become involved when there is disease cf the internal ear or mastoid sinus (mastoiditis). After its exit from the skull (by the stylomastoid foramen) its close association with the parotid gland renders it liable to be involved in eel- 264 HUMAN PHYSIOLOGY. lulitis of this gland, and on account of its superficial position, it may be injured by blows on the side of the head. Quite com- monly the seventh nerve becomes the seat of inflammation after exposure to a draught, as by sitting at an open window. The paralysis is almost always one-sided. The eyelid on the affected side cannot be properly closed, a chink remains and the eyeball becomes rotated upward, thus showing the sclera. On smiling or showing the teeth the mouth is drawn up on the healthy side, causing a triangular opening because the lips do not become separated on the paralyzed side. Articulation is difficult and such acts as whistling and blowing are impossible. Because of paralysis of the buccinator muscle, food collects between the cheek and gums. The distortion of the face is much more pro- nounced in old, than in young persons; indeed in the ease of the latter the paralysis may be overlooked until speaking or laughing is attempted. The Eighth or Auditory Nerve. — The eighth or auditory nerve is composed of two branches, the one called cochlear, con- nected with the organ of Corti (see p. 291), which collects sound waves, and the other, called vestibular, with the semicircular canals which, by the movements of the fluid contained in them, record changes in the position of the head (see p. 276). Both branches, being sensory, are connected with ganglia situated in or near the internal ear (ganglion spirale for the cochlear di- vision and ganglion of Scarpa for the vestibular). Paralysis of the auditory nerve causes a degree of deafness wliich is more profound than that due to disease of the middle ear, for in the latter case a tuning fork can be heard when the end of it is applied to the skull or is held in the teeth, which is not the case when the nerve is diseased. When the eighth nerve becomes irritated (as by inflammation of the ear, or a general condition such as migraine, epilepsy, etc.), various kinds of sounds are heard. This is called tinnitus. It is not infrequently followed by deafness. The Ninth or Glosso-pharyngeal Nerve. — This nerve is partly motor and partly sensory. The motor fibers supply the muscles of the pharynx and most of those of the soft palate. 'J'emporal area ( max ilia ry second premolar). MandiVjular area (maxillary second and third premo- lars). Hyoid area (mandibular sec- ond premolar ; first and - second molars). S u p e r i o r laryngeal area (mandibular third molar). The points of maximum intensity are ringed. Fig. 49. — Diagram to show areas of referred pain in distribution of fifth nerve due to affections of the various teeth (Side view). (From drawing by T. Wingate Todd.) THE CRANIAL NERVES. 265 Tlie sensory fibers carry impulses of common sensation and of taste from the root of the tongue, the neighboring portions of the pharynx, the tonsils, the soft palate, and the pillars of the fauces. This nerve does not commonly become the seat of local lesions. The Tenth or Vagus Nerve. — This is the main cerebrospinal nerve supplying the viscera and it is both motor and sensory in function. We shall see later that the nerves to the viscera belong to the so-called autonomic system, which is distinguished from the somatic by two main facts, one anatomical and one functional. Tlie anatomical difference is that every nerve fiber becomes connected through synapses with nerve cells located peripherally (i. e., near the end of the nerve), and the axons of the cells continue tlie impulse on to the structure ; the functional difference is that the autonomic fibers, as their name indicates, control automatically-acting or involuntary functions instead of voluntary movements, as is the case with the ordinary or somatic cerebrospinal nerve fibers. The most important of the vagus autonomic fibers run to the heart (see p. 185), the a?sophagus (p. 57), the stomach (p. 60) and the intestines (p. 79). The vagus also contains afferent fibers which have their cell stations in ganglia situated in the trunk of the nerve. These fibers carry sensory impulses par- ticularly from the larynx and lungs (p. 219). Further details regarding the functions controlled by the vagus are fully given in the references indicated above. When the vagus nerve, or its center, is the seat of paralysis, swallowing is seriously in- terfered with, and food is liable to pass into the larynx and cause pneumonia. Various forms of paralysis of the vocal cords may also result from paralysis of the vagus. The Eleventh or Spinal Accessory Nerve. — The eleventh or spinal accessory is entirely an efferent nerve, one part of it, the accessory, being derived from the same column of nerve cells as the vagus and being really a part of this nerve; the other arises from the cells of the anterior horn of the spinal cord in the upper cervical region atifl supplies the trapezius and sterno-mastoid muscles. 266 HUMAN PHYSIOLOGY. The Twelfth or Hypoglossal Nerve. — The twelfth or hypo- glossal nerve is entirely efferent, being the motor nerve of the tongue muscles and of most of the muscles attached to the hyoid bone. When it is paralyzed, as in bulbar paralysis, swallowing of food becomes impossible, the tongue cannot be protruded and soon atrophies because of the removal of the trophic in- fluence of the nerve cells. Rarely the paralysis is unilateral, but this is because of lesions higher up in the nervous system than the medulla and so situated that they destroy the con- nection of the fibers which run from the higher motor centers in the cerebrum to the hypoglossal nucleus. Such lesions neces- sarily involve fibers of -the same type running to the nerve cells of the spinal cord, so that hemiplegia (p. 248) accompanies and is on the same side as the tongue paralysis. "When a patient with such a lesion attempts to put out the tongue, it is directed towards the affected side but it shows no atrophy. CHAPTER XXVIII. THE NERVOUS SYSTEM (Cont'd). The Brain. The first question which naturally arises is, what influence does the brain have on the reflex movements produced through the spinal cord? These influences may be summarized as fol- lows: 1. The brain enables the animal to will that a particular movement shall or shall not take place, irrespective of the stimu- lation of spinal reflexes. Much of this influence of the brain is of course voluntary in nature, but some of it is subconscious or involuntary. In general it may be ,said that the cerebrum, through the pyramidal tracts, usually exercises a damping or inhibitory influence on the spinal reflexes. It is for this reason that the reflex response to a certain stimulus is usually much more pronounced in a spinal, as compared with a normal animal. For example, it is impossible to bring about the scratch reflex in many normal dogs, whereas it is always present in spinal animals. In man this restraining influence of the pyramidal tracts on spinal reflexes is very evident in the case of knee-jerk, which, it will be remembered, is the extension of the leg which occurs when the stretched patellar tendon is tapped. Ordinarily the kick is moderate in degree, but in patients whose pyramidal tracts are diseased, as in spastic paraplegia, it becomes very pronounced. 2. The brain, being the receiving station for the projicient sensations (p. 279), sight, hearing and smell, adds greatly to the number of afferent pathways by which reflex actions can be excited. 3. Since in higher animals all the afferent impulses usually 267 268 HUMAN PHYSIOLOGY. travel through the brain (p. 248), many nerve centers become more or less involved in the reflex actions, so that a much higher degree of co-ordination than that seen in a spinal animal attends the muscular response. For example, some of these afferent impulses reach the cerebellum, whose function, as we shall see, is to strengthen some impulses and weaken others, so that a more perfect movement results. 4. The animal becomes conscious not only of the nature and place of application of the sensory stimulus itself, but of the degree to which it has moved its muscles in response. The Functions of the Cerebrum. The complicated movements, such as those involved in the scratch reflex, which we have seen that a spinal animal can carry out in the paralyzed region after shock has passed away, become more and more numerous and complicated as the higher centers are left in connection Math the spinal cord. That is to say, the higher up in the cerebrospinal axis the section is made, the more capable does the part of the animal below the section be- come to peform complicated niovements. The important centers in the medulla, pons and mesencephalon add their influence to those of the spinal cord itself, so that integration becomes more comprehensive. If the cut is made above the level of the pons, in other words, if the cerebral hemispheres alone be discon- nected from the rest of the cerebrospinal axis — decerebration, as it is called — we obtain an animal possessing all the reflex actions that are necessary for its bare existence, although it is of course incapable of feeling or, if the basal ganglion be also destroyed, of seeing or hearing. It becomes a mere automaton : it breathes, the blood circulation is normal, it can walk or run . or swim, it swallows food if the reflex act of swallowing be stimulated by placing the food in the mouth, but it has not the sense to take food itself even when this is placed near it. All the mental processes are absent; it has no memory, no volition, no likes and dislikes. By seeing that it takes food, it has been possible to keep such a decerebrated dog alive for eighteen months, and the lower we descend in the animal scale, the easier THE FUNCTIONS OF THE CEREBRUM. 269 it becomes to perform the operation and to keep the animal alive. In higher animals, such as monkeys, however, life is impossible without the cerebrum, thus supporting the conclusion, which we have already drawn (see p. 243), that the cerebrum comes to be a necessary part of every reflex action in the higher animals. Cerebral Localization. — The various functions of the cere- brum are located in different portions of it. This localization of cerebral functions has been very extensively studied during recent years, partly by experimental work on the higher mam- malia and partly by clinical studies bn man. Careful observa- tions are made of the behavior of the various functions of tlie animal either after removal or destruction of a portion of the cerebrum, or during its stimulation by the electric current. Im- portant additions to our knowledge of cerebral localization are also being made by correlating the symptoms observed in insane persons with the lesions wliicli are revealed by post-mortem examination. It has been found that there are roughly three areas on the cerebrum with distinct and separate functions (Fig. 50). I. In the portions of the cerebrum which lie in front of tlie ascending frontal convolutions — prefrontal region — are located the centers of the intellect (thought, ideation, memory, etc.). This part of the cerebrum is accordingly by far the best de- veloped in man; it is much less so in the apes and monkeys, becomes insignificant in the dog, and still more so in the rabbit. It has been destroyed by accident in man with the result that all the higher mental powers vanished. II. The next portion includes roughly the region of the cerebrum bordering upo2i the Rolandic fissure (i. e., the ascend- ing frontal and ascending parietal convolutions). Here are located the highest centers for the movements of the various parts of the body. Microscopic examination of the grey matter reveals tlie presence of large triangular nerve cells, which com- municate by synapses (sec p. 241) with the afferent fibers that carry the sensory impulses, whose course from the posterior spinal roots we have already traced (p. 246). From each of these cells an efferent fiber runs to join the pyramidal tract 270 HUMAN PHYSIOLOGY, (p. 248), and thus connect with the anterior horn cells of the spinal cord. In the Rolandic area, as it is called, is therefore situated the cerebral link in the chain of neurones (see p. 249) through which the ordinary movements of the body take place. Such movements may be set. agoing, either by stimulation of the Rolandic nerve cells through afferent fibers — a pure reflex — or by impulses coming to them from the centers of volition situated Fig. 50. — Cortical centers in man. Of the three sliaded areas bordering on the Rolandic fissure (,Rol.), the most anterior is the precentral associational area, the middle one is the motor area (the position of the body areas are indicated on it), and the most posterior is the sensory area, to the cells of which the fillet fibers proceed. The centers for seeing and hearing are also shown. The unshaded portion in front of the Rolandic area is the precentral ; the portions behind, the parietal and temperosphenoidal. in the prefrontal convolutions. Or, again, the nerve cell, at the same time that it receives a sensory impulse coming up from the spinal cord, may receive one from the prefrontal convolu- tions which may either interdict or greatly modify the reflex response. Every possible muscular group in the body has a center of its own in the Rolandic area, the determination of the exact location of these centers being one of the achievements of modern medical science. Thus, if we stimulate with a finely ' THE FUNCTIONS OF THE CEREBRUM. 271 graded electric stimulus, say, the center of the thumb, it -will be found that the thumb undergoes a sIqw, purposeful, co-ordi- nated movement ; and so on for every other center. Or, if in- stead of stimulating, we cut away one of the centers and allow the animal to recover from the immediate effects of the opera- tion, it will be found that all the more finely co-ordinated move- ments of the corresponding part of the body have disappeared, although gross reflex movements may be possible, because the spinal reflexes are still intact. If the entire Rolandic area on one side is removed, the muscles of the opposite side of the bod}^, except those of the trunk, become completely paralyzed for some time, after which, however, particularly in the case of young animals, the paralysis becomes recovered from, thus in- dicating that some other portions of the brain have assumed the function of the destroyed centers. If the stimulus- is a very strong one, the movements do not remain conflued to the cor- responding muscle group, but they spread on to neighboring groups until ultimately the whole extremity or perhaps even all the muscles of that side of the body are involved. These experimental results find their exact counterpart in clinical experience. Thus when some center becomes irritated by pressure on it of some tumor growing in the membranes of the brain (meningeal tumor), or by a piece of bone, as in de- pressed fracture of the' skull, or by blood clot, convulsive at- tacks (known as Jacksonian epilepsy) are common. The first sign of such an attack is usually some peculiar sensation (aura) affecting the part of the body which corresponds to the irritated area; the muscles of this part begin to twitch and more muscles are involved, until ultimately all those of the cor- responding half of the body become contracted. There is, how- ever, no loss of consciousness, as there is in true' epilepsy. The evident cause of these symptoms has clearly indicated the proper treatment for such cases, namely, surgical removal of the cause of irritation. For this purpose a very careful study is first of all made of the exact group of muscles in which the convulsions originate; the location of the area on the cerebrum is thus ascertained and a trephine hole is made in the correspond- 272 HITMAN PHYSIOLOGY. ing part of the cranium and through this hole the tumor or blood clot is removed. III. These so-called motor areas are of course also sensory areas in the sense that the afferent stimuli which come up from the spinal eord run to them. They are really sensori-motor centers. For some of the more highly specialized proficient sensations, such as vision and hearing (see p. 279), there are, however, special centers. These, along with an extensive field of associational or junctional grey matter, constitute the third main division of the cerebral cortex and occupy the greater part of the parietal, the temporosphenoidal and the occipital lobes. The visual is the most definite of these centers. Thus if the occipital lobe be removed or destroyed by disease on one side, the corresponding half of each retina becomes blind. It is by studying the exact nature of the involvement of vision in such eases that the physician is able to locate the position of a tumor, etc. The center for Jiearing is in the temporosphenoidal lobe, but its location is not very definite. It will be seen, however, that the visual and auditory centers take up but a small part of this third division of the cerebrum, the most of it being occupied by associational areas. The nerve cells of these areas do not, like those of the motor and sensory centers, send fibers which run as pyramidal or optic fibers to some lower nerve center, but only to other cerebral centers, which they serve to link together. They are specialized to serve as junction points for all the receiving and discharging centers of the cerebrum, so that all actions may be properly correlated or integrated. These junctional centers thus perform the great function of adapting every action of the entire animal to some definite purjDose. Together with the nerve cells in the prefrontal areas, the associational cells represent the highest development of cerebral integration, so that we find the areas in which they lie becoming more and more pronounced, the higher we ascend the animal scale. The Mental Process. — The impression received by the visual center when a young animal looks for the first time at, say a THE FUNCTIONS OF THE CERF^RUM. 273 bell, becomes stored away in nerve cells lying in or close to that center, and when the bell is moved sound memories are likewise stored in the auditory center. At first these remain as isolated memory impressions and the animal is unable to associate the sight with the sound of the bell- But later, with repetition, the visual and the auditory centers become linked together, through nerve cells and fibers which occupy the associational areas, so that the invocation of one memory is followed by association with others. It is evident that the intricacy of this interlace- ment of different centers will, in large part, determine the in- tellectual development of the animal, and the possibility of his learning to judge of all the consequences that must follow every impression which he receives or every act which he performs. In man these associational areas are very poorly developed at the time of birth, so that the human infant can perform but a few acts for itself. Everything has to be learned, and the learning process goes hand in hand with development of the associational areas, which proceeds through many years. On the other hand, most of the lower animals are born with the associational areas already laid down and capable of very little further increase, so that, although much more able than the human infant to fend for itself at birth, the lower animal does not afterwards develop mentally to the same extent. The practical application of these facts concerning the func- tions of different areas of the cerebrum is in the study of mental diseases. To serve as an example we may take aphasia. This means inability to interpret sights or sounds or to express the thoughts in language. In the former variety — called sensory aphasia — ^the patient can see or hear perfectly well, but fails to recognize that he has seen or heard the object before. He fails to recognize a printed word (word blindness) or to in- terpret it when spoken (word deafness). The lesion responsible for this condition is located in the associational areas and not in the centers themselves. In the other variety, called motor aphasia, the patient understands the meaning of sounds or sights, of spoken or written words, but is unable to express his thoughts or impressions in language. The lesion in this case in- 274 HUMAN PHYSIOLOGY. volves some of the centers concerned in the higher control of the muscles which are used in speech, and very commonly it is situated in the left side of the cerebrum. In all three forms of aphasia there is more or less decrease in the mental powers. Cerebellum. The afferent impulses set up by stimulation of the nerves of the skin in a spinal animal, and due therefore to changes in the environment, after entering the spinal cord travel to the various centers in the cord. Although complicated movements may result (e.g., the scratch reflex), there is an entire absence of the power of maintaining bodily equilibrium, and the animal cannot stand because the muscles are not kept in the degree of tone which is necessary to keep the joints properly stiffened. A similar inability to maintain the center of gravity of the body results from removal of the cerebellum, or small brain, which it will be remembered is situated dorsal to the medulla and pons, with which it is connected by three peduncles. The cerebellum consists of two lateral hemispheres and a median lobe called the vermis. The remarkable infolding of the grey matter which composes its surface, and the large number of nuclei which lie embedded in its central white matter are struc- tural peculiarities of the cerebellum. The immediate results of removal of the cerebellum consist in extreme restlessness and inco-ordination of movements. The animal is constantly throwing itself about in so violent a man- ner that unless controlled it may dash itself to death. Gradually the excitement gets less, until after several weeks all that is noticed is that there is a' condition of muscular weakness and tremor, and difficulty in maintaining the body equilibrium. Quite similar symptoms occur when the cerebellum is diseased in man (as by the growth of a tumor) j the condition being called cerebellar ataxia, and being characterized by the uncer- tain gait which is like that of a drunken man. These observations indicate that the function of the cerebellum is to harmonize the actions of the various muscular groups, so THE FUNCTIONS OP THE CEREBELLUM. 275 that any disturbance in the center of gravity of the body may be subconsciously rectified by appropriate action of the various muscular groups. It evidently represents the nerve center hav- ing supreme control over other nerve centers, so that these may not bring about such movements as would disturb the equiii- hrimn of the animal. In order that the cerebellum may perform this function it must, however, be informed of two things. In the first place, it must know the existing state of contraction of the muscles and the tightness of the various tendons that pull upon the joints, and in the second, it must know the exact position of the center of gravity of the body. Information of the condition of the muscles and tendons is supplied through the nerves of muscle sense, which run in every muscle nerve and are connected in the muscles with peculiar -sensory nerve terminations called muscle spindles. When the muscles contract, or the tendons are put on the stretch, these spindles are compressed and sensory or afferent stimuli pass up the nerves of muscle sense, enter the cord by the pos- terior roots and reach the cerebellum by way of the lateral col- umns (see p. 249). Information regarding the center of gravity of the body is supplied through the vestibular division of the eighth nerve, which, it will be recalled, is connected with the semicircular can- als and vestibule. In these structures are membranous tubes or sacs containing a sensory organ (called the crista or macula acoustica), which consists essentially of groups of columnar cells furnished with very fine hair-like processes at their free ends and connected at the other end with the fibers of the eighth nerve. The hair-like processes float in the fluid which is con- tained in the membranous canals or sacs. This fluid does not, however, completely fill these structures, so that it moves when- ever the head is moved. This movement affects the hair-like processes and thus sets up nerve impulses which are carried to the cerebellum. To make the hair cells of this receiving apparatus capable of responding to every possible movement of the bead, it is, 276 HUMAN PHYSIOLOGY. however, evident that there must be some definite arrangement of the tubes. This is provided for in the disposition of the semi- circular canals in three planes, namely, a horizontal and two vertical (Fig. 51). Taken together the three canals form a struc- ture which looks somewhat like a chair, the horizontal canals being the seat of the chair and the two vertical canals joining together to form its back and arms. The back of each chair is directed inwards so that they are back to back. At one end of each canal is a swelling, the ampulla, in which the sensory nerve Fig. 51. — The semicircular canals of the ear, showing their arrangement In the three planes of space. (From Howell's Physiology.) apparatus above described is located. It is evident that when the head is moved in any direction the fluid in some of these canals will be set in motion. It is this movement of the fluid which stimulates the hair cells. That this is really the function of the semicircular canals is proved by the fact that if they are irritated or destroyed, grave disturbances occur in the bodily movements. This is what occurs in Meniere's disease, in which attacks of giddiness, often severe enough to cause the patient to fall, and accompanied by extreme nausea, are the chief symp- toms, the lesion being a chronic inflammation involving the THE SYMPATHETIC NERVOUS SYSTEM. 277 semicircular canals. It is believed by some that the constant movements of the fluid in the semicircular canals is the cause of sea sickness. The unusual nature of these movements causes confusion in the impressions transmitted to the cerebellum from the canals, but after a while the cerebellum may become accus- tomed to them and the sea sickness passes away. The Sympathetic Nervous System. Along with the vagus and one or two less prominent cere- brospinal nerves, the sympathetic constitutes the autonomic nervous system, so-called because it has to do with the innerva- tion of automatically acting structures, such as the viscera, the glands and the blood vessels. The characteristic structural fea- ture of the nerves of this system is that they are connected with nerve ganglia located outside the central nervous system. In these ganglia the nerve fibers run to nerve cells, around which they form synapses, thus permitting the nerve impulse to pass on to the cell, which then transmits it to its destination along its own axon (see p. 241). Before arriving at the ganglion in which the synapsis is formed, the fibers are called pregan- glionic; after they leave, they are called postganglionic. A preganglionic fiber may run through several ganglia before it becomes changed to a postganglionic fiber. In the case of the vagus and other cerebral autonomic nerves, the ganglia are often situated, as in the heart (see p. 185), at the end of the nerve, but in the case of the sympathetic itself, they are more numerous, and are mainly situated at the sides of the vertebral column, where, together with the connecting fibers, they form a chain — the sympathetic chain — which can easily be seen on opening the thorax and displacing the heart and lungs. Two fine branches connect each of the spinal nerves with the corresponding sympathetic ganglion. It is through one of these branches that the sympathetic chain receives its fibers from the spinal cord. Through the other, fibers run from the ganglion to the spinal nerve. Some of the sympathetic ganglia are situated at a distance from the spinal cord ; the ganglia which compose the solar and hypogastric plexuses are examples. 278 - HUMAN PHYSIOLOGY. In the thorax, the uppermost ganglion is very large and is called the stellate ganglion. Its postganglionic fibers constitute the vasomotor nerves of the blood vessels of the anterior ex- tremity, and the sympathetic fibers to the heart. Some pregan- glionic fibers run through the stellate ganglion to pass up the neck as the cervical sympathetic, their cell station being in the superior cervical ganglion. They act on the pupil (dilating it), on the salivary glands (causing vasoconstriction and stimulating glandular changes), and on the blood vessels of the head, face and mucosa of the inside of the mouth. From about the fifth dorsal vertebra downwards, branches run from the sympathetic chain on each side to become collected into a large nerve called the great splanchnic, which passes down by the pillars of the diaphragm into the abdomen and runs to the ganglia of the coeliac plexus. This nerve supplies all of the blood vessels of the intestines and other abdominal viscera. Its action on these vessels has already been described (see p. 191). It also carries nerve impulses for the control of the move- ments of the stomach and intestines and for some .of the digestive glands. In the abdomen the sympathetic chain gives off branches, which form the pelvic nerves and supply the blood vessels of the lower extremity. It is important to note that the connections between the sympathetic system and the cerebrospinal axis are limited to the spinal nerve roots between the second thoracic and the second lumbar. The results which follow stimulation of the sympathetic system are exactly like those which are pro- duced by injections of adrenalin (see p. 130). CHAPTER XXIX. THE SPECIAL SENSES, The sensory nerve terminations, or afferent receptors, that are scattered over the skin are affected by stimuli which come in actual contact with the surface of the body. In order that the stimuli transmitted from a distance, such as those of light, sound and smell, or the projicient sensations as they are called, may be appreciated by the nervous system, specifically designed or- gans, called the organs of special sense, are required. These organs collect the stimuli in such a way as to cause them to act effectively on receptors which have been especially adapted to react to them. Although not really a projicient sensation, taste is conven- iently considered along with the above. Vision. Light is due to vibration of the ethereal particles that oc- cupy space. The vibrations occur at right angles to the rays of light, and these travel at high velocity in straight lines from the source of the light. The rate of vibration of the rays is not always the same, and on this difference depends the color of the light, red light vibrating much slower, and its waves being accordingly much longer, than those of violet light. The termi- nations of the optic nerve, the retina, have been specially developed to receive the light waves. But in order that a comprehensive picture of everything that is to be seen may be projected on the retina, an optical apparatus, consisting of the cornea and lens, is situated in front of it. The retina and the optical apparatus are built into a globe — the eyeball — which, pivoting on the attachment of the optic nerve, can be so moved that images from different parts of the field of vision may be 279 280 HUMAN PHYSIOLOGY. focused in turn on the retina. These movements are effected by the so-called ocular muscles. There are, therefore, three functions involved in the act of seeing: (1) That of the retina, in reacting to light. (2) That of the cornea, etc., in focusing the light. (3) That of the ocular muscles, in moving the eyeball. The Optical Apparatus of the Eye. It will readily be seen that the eye , is constructed on much the same principle as a photographic camera, the retina being like the sensitive plate. There is, however, an important dif- ference in the manner by which objects at varying distances are brought to a focus on the sensitive surface in these two cases : in the camera, it is done by adjusting the distance between the lens and the focusing screen; in the eye, it is done by varying the convexity of the lens. In order to understand how the optical apparatus works, it is necessary to know something about the refraction of light. "When a ray of light passes from one medium to another, it be- comes bent or refracted. When it passes from air to water or glass, for example, it becomes refracted so that the angle which the refracted ray makes with the perpendicular to the surface is less than that of the entering ray. In other words, the ray becomes bent towards the perpendicular. The greater the dif- ference in density between the two media, the greater is the' difference between the two angles. A figure expressing the ratio between these two angles is called the index of refraction. If the ray of light leaves the denser medium by a surface which is parallel with that by which it entered (as in passing through a pane of glass), it will be refracted back to its old direction, but if, as in a prism, it leaves the denser medium by a surface which forms an angle with that by which it entered, the original refraction will be exaggerated. If two prisms be placed with their broad ends together, parallel rays of light coming from a certain direction will be bent so that, on leaving the prisms, they meet somewhere behind them. Two prisms so arranged are virtually the same as a biconvex lens. It is plain that the VISION. 281 focusing power of such a lens will depend on two things : first, its index of refraction, and, secondly, the curvature of its sur- faces. A considerable part of the actual refraction of the rays which enter the eye is accomplished at the curved surface of the cornea, a smaller degree of refraction taking place at the lens itself. The reason for this is that the refractive index from air to cornea is much greater than that between the lens and the humors of the eye in which the lens is suspended, these humors and the cornea having very much the same refractive indices. The entering rays are, therefore, refracted at two places in the eye, namely, at the anterior surface of the cornea and on passing through the lens. Fig. 52. — Formation of image on retina. O.A. is the optic axis. Accommodation of the Eye for Near Vision. — When the eye is at rest, its optical system is of such a strength that parallel rays, i. e., rays that are reflected from objects at a distance, are brought to a focus exactly on the retina. The picture thus formed is, however, upside down for the same reason that it is so on the screen of a camera (Fig. 52). When the object looked at is so near that the rays reflected from it are divergent when they enter the eye, it becomes necessary, if the image is still to be focused on the retina, that some adjustment take place in the optical system of the eye. This could happen in one of two ways, either by lengthening the distance between the lens and the retina (the method used in a camera), or by in- creasing the convexity of the lens. The former process cannot 282 HUMAN PHYSIOLOGY. occur in the eye, but the second is rendered possible by bulging of the anterior surface of tJie lens. There are several ways by which this bulging of the lens can be proven to occur. Thus, if the eye of a person who is looking at some distant object be inspected from the side of the head, that is to say, in profile, it is easy to note the exact position of the iris, which, with the 'pupil in its center, hangs as a circular curtain just in front of the lens (Fig. 53). If the person is now told to regard some Fig. 53.- — Section through the anterior portion of the eye: C, the cornea; I, the iris (note the circular muscular fibers cut across at the margin) ; L, the lens ; Cij the ciliary process ; S, the suspensory ligament ; 8cl, the scler- otic or outer protective coat of the eye. (From a preparation by P. M. Spur- ney. ) object held close to him, it will be seen that the iris is pushed forward nearer to the cornea. That this is really due to a bulg- ing of the anterior surface of the lens can be shown by placing a candle to one side and a little in front of the head and then, from the other side, viewing the images of the candle flame which are cast on the eye. It will be seen that one image occurs at the anterior surface of the cornea, and another, less distinct, at the anterior surface of the lens. This image from the lens VISION. 283 will be seen to move forward — that is to say, closer to the image at the cornea — when the person shifts his gaze from a distant to a near object. By using optical apparatus for measuring the size of the images, the degree to which the convexity of the lens has increased, as a result of the bulging, can be accurately measured. This change in the convexity of the lens depends on the fact that it is composed of a ball of transparent elastic material, which is kept more or less flattened antero-posteriorly because of its being slung in a capsule w^hich compresses it. The edges of the capsule are attached to a fine ligament (the suspensory liga- ment), which runs backwards and outwards to become inserted into the ciliar}^ processes (Fig. 53). These processes exist as thickenings of the anterior portion of the choroid, or pigment coat of the eye, and they can be moved forwards by the action of a small fan-shaped muscle, called the ciliary muscle, which at its narrow^ end originates in the corneo-seleral junction, and runs back to be attached, by its wide end, to the ciliary pro- cesses. When this muscle is at rest, the ciliary processes lie at such a distance from the edges of the lens that the suspensory ligament is put on the stretch. When the ciliary muscle con- tracts, it pulls the ciliary processes forward, thus slackening the suspensory ligament and removing the tension on the capsule of the lens, with the result that the latter bulges because of its elasticity. The ability of the lens to become accommodated for near vision depends, therefore, first, on the elasticity of the lens, and secondly, on the action of the ciliary muscle. Inter- ference with either of these renders accommodation faulty. For example, the lens, along with the other elastic tissues of the body [e. g., the arteries (p. 175)], becomes less elastic in old age, thus accounting for the "long-sightedness" (or presbyopia) which ordinai'ily develops at this time. Paralysis of the ciliary muscle produces the same effect in even more marked degree, which explains the utter inability to bring about any accommo- dation after treating the eye with atropin, which is given for this purpose before testing the vision in order to find out the strength of lenses required to correct for errors in refraction. 284 HUMAN PHYSIOLOGY. The Function of the Pupil. — ^Every optical instrument con- tains a so-called diaphragm, which is a black curtain having a central aperture whose diameter can be altered to any required size. The object of this is to prevent all unnecessary rays of light from entering the optical instrument, thus materially in- creasing the distinctness of the image. In the eye, this function is performed by the iris with the pupil in its center. The size of the pupil is altered by the action of two sets of muscle fibers in the iris. One of these runs in a circular manner around the inner edge of the iris; by contracting it causes constriction of the pupil, an event which occurs, along with the bulging of the lens, during accommodation for near vision. The other layer of fibers runs in a radial manner, and by contracting causes dila- tation of the pupil. This occurs in partial darkness, or when the eye is at rest (although not during sleep). The circular fibers are supplied by the third nerve, and the radial fibers by the sympathetic. Under ordinary conditions both muscles are in a state of tonic contraction (see p. 253), so that the actual size of the pupil at any moment is the balance between two opposing muscular forces. This renders its adjustment in size very sensi- tive. For example, it can become dilated either by stimulation of the sympathetic (which occurs when any irritative tumor affects the cervical sympathetic nerve), or by paralysis of the third nerve (as by giving atropin). Conversely, constriction of the pupil may be the result of stimulation of the third nerve (as by a tumor at the base of the brain) or paralysis of the sympa- thetic. These local conditions acting on the afferent nerves to either pupil are not nearly so often called into play as conditions acting reflexly on both eyes at the same time. Certain of the afferent impulses which call these reflexes into play travel by the optic rrerve to the nerve centers for the pupil, such for example as the stimulus set up by light falling on the retina. The afferent pathway concerned in the contraction of the pupil, which occurs in accommodation, must, on the other hand, be a different one because in the disease locomotor ataxia (see p, 254), the pupil contracts on accommodation, but does not VISION. 285 do so when light is thrown into the eyes. The nerve centers for the pupil are very sensitive to general nervous conditions, thus accounting for the dilatation of the pupil which occurs during fright or other emotions, or pain. The pupils are contracted in the early stages of asphyxia or anesthesia, as in the early stages of nitrous oxide administration, but they become dilated when the anesthesia or asphyxia becomes profound. Their condition helps to serve as a gauge of the depth of anesthesia. Imperfections of Vision. — The optical system of the eye is not perfect. Some of these imperfections exist in every eye, whilst others are only occasional. The errors in every eye are those known as spherical and chromatic aberration. Spherical Fig. 54. — A, spherical aberration. The rays which strike the margins of the lens are brought to a focus before those striking near the center. B, Chromatic aberration. The ray of white light (W) is dissociated by the lens into the spectral colors, of which those at the red end {R) are not brought to a focus so soon as those at the violet end (V). aberration (Fig. 54), occurs because the edges of the lens have a higher refractive power than the center, so that the image on the retina is surrounded by a halo of overfocused rays. Chro- matic aberration is due to the fact that white light, on passing through the lens, suffers some decomposition into its constituent colored rays (the rainbow colors), of which certain ones (viz., those towards the violet end of the spectrum) come to a focus sooner than others (viz., those towards the red end), thus creat- ing a colored edge on the focused image. These errors are greatly minimized, although not entirely removed, by the pupil, which cuts out the peripheral rays. The occasional errors are long-sightedness or hypermetropia. 286 HUMAN PHYSIOLOGY. short-sightedness or myopia, and astigmatism (Fig. 55). Hyper- metropia is due to the eyeball being too short so that the focus of the image is behind the retina. The error is corrected by prescribing convex glasses. Myopia is due to the opposite con- Fig. 55. — Errors in refraction: E shows the formation of the image on the retina in the normal or emmetropic eye ; H shows the condition in long- sight, or hypermetropia, where the eyeball is too short ; M shows the condi- tion in short-sight, or myopia, where the eyeball is too long. ditiou, that is, the eyeball is too long, so that the focus occurs in front of it. Concave glasses correct it. Astigmatism is due to the lens or cornea being of unequal curvature in its different VISION. 287 meridians. This causes the rays of light in one plane to be brought to a focus before those in other planes, so that the two hands of a clock, when they are at right angles to each other, cannot be seen distinctly at the same iiistant, although they can be successively focused. A certain amount of astigmatism exists in every eye, but when it becomes extreme, it is necessary to correct it by prescribing glasses which are astigmatic in the opposite meridian to that of the eye. Such glasses are called cjdindrical. Astigmatism may occur along with either myopia or hyper- metropia, and when any of these errors is only slight in degree, the patient may be able, by efforts of accommodation, to over- come the defect. The strain thus thrown on the ciliary muscle is, however, quite commonly the cause of severe headache. The correction of the errors should never be left to untrained per- sons, but a proper oculist should be consulted, since it is usually necessary to give atropin so that the accommodation may be paralyzed and the exact extent of the error measured. The use of improper glasses may aggravate the defect of vision and do much more harm than good. The Sensory Apparatus of the Eye. The Functions of the Retina. — The image which is formed on the retina by the optical system of the eye sets up nerve im- pulses which travel by the optic nerve to the visual center in the occipital lobes of the cerebrum (see p. 272), where they are interpreted. Microscopic examination of the retina has shown that it consists of several layers of structures, the innermost being of fine nerve fibers which arise from an adjacent layer of large nerve cells, and the outermost of peculiar rod or cone- shaped cells, called the rods and cones. Between the layer of large cells and the layer of rods and cones are several layers composed of other nerve cells and of interlacements of the pro- cesses of cells and nerve fibers. The rods and cones are the .structures acted on by light, the other layers of the retina being for the purpose of connecting the rods and cones with the large nerve cells from which the fibers of the innermost layer arise. 288 HUMAN" PHYSIOLOGY. The fibers all converge to the optic disc, which is a little to the inside of the posterior pole of the eyeball. At this point the fibers of the nerve fiber layer bend backwards at right angles and run into the optic nerve, thus crowding out the other layers and causing the existence of a Hind spot, which can be readily demonstrated by closing one eye, say the left, and with the other regarding the letter B in the next line. Although the S is also distinctly visible in most positions, yet if the book be moved towards and away from the eye, the S will become in- visible at a certain distance corresponding to that at which the rays from it are impinging upon the blind spot. As we alter the distance of the book from the eye, the line of vision, or visual axis, being fixed on the B, the image of the S travels from side to side across the inner or nasal half of the retina, and at a certain position strikes the optic disc. Ordinarily we are unaware of the blind spot, partly because we have two eyes and, the blind spot being towards the nasal side of each retina, the image of an object does not fall on it in both eyes at the same time ; and partly because we have learned to disregard it. The area or extent of the blind spot may become so increased, as by excessive smoking, that it becomes noticeable. At another portion of the retina called the fovea centralis, all the layers become thinned out except that of the rods and cones, especially the cones. This, as we should expect, is by far the most sensitive portion of the retina, and is indeed the portion on which we cause the image to be focused when we desire to see an object clearly. The remainder of the retina is only suffi- ciently sensitive to give us a general impression of what we are looking at. Thus when we view a landscape, we can see only a small portion clearly at one time, although we have a general impression of the whole. The portion which we see clearly is that which is focused on the fovea, and we keep moving our eyes in all directions so that every part of the landscape may in turn be properly seen. We see with the fovea what the rest of the retina informs us there is to be seen. VISION. 289 The Movements of the Eyeballs. — In order that we may be enabled to move our eyes so as to see objects in different posi- tions in the visual field, the eyeballs are provided with six little muscles, four recti and two obliques. These muscles are in- nervated by the third, fourth and sixth nerves (see p. 259). The images in the two eyes cannot of course fall on anatomically identical parts of the retinae, but they fall on parts that are physiologically identical. Thus, an object, say on the right of the field of vision, will cause an image to fall on the nasal side of the right retina and on the temporal side of the left retina. We do not, however, see two objects because by experience we have come to learn that these are corresponding points on the retinae. When an object is brought near to the eye, the two eyeballs must converge so as to bring the visual axes on to the corresponding points. This convergence of the eyeballs con- stitutes the third change occurring in the eyes during accom- modation for near vision, the other two being, as we have seen, bulging of the lens and contraction of the pupil. It is interest- ing that these three changes are controlled by the third nerve. If anything happens to throw one of the images on to some other portion of one retina, double vision is the result. This condition of diplopia, as it is called, can be brought about, vol- untarily, by pressing on one eyeball at the edge of the eye, or it may occur as a result of paralysis or incoordinate action of one or more of the ocular muscles. This occurs in certain in- toxications, as, for example, that produced by alcohol. Just as in the case of errors of refraction, e. g., astigmatism, slight degrees of diplopia may cause symptoms that are more distressing than when marked diplopia exists, because we try to correct for slight errors and the effort causes pain (headache) and fatigue, whereas with extreme errors we do not try to correct but, in.stead, we learn to disregard entirely the image in one eye. Whenever the incoordination of ocular movement is per- manent, as when due to shortening of one of the muscles, it is called strabismus. This condition is usually congenital, and can often be rectified by a surgical operation. Judgments of Vision. — Besides these purely physiological 290 HUMAN PHYSIOLOGY. problems of vision, there are many others of a physio-psycho- logical nature. Such for example are the visual judgments of size, distance, solidity, and color. Judgments of size and dis- tance are dependent on: (1) the size of the retinal image, (2) the effort of accommodation necessary to obtain sharp defini- tion, and (3) the amount of haze which appears to surround the object. Judgment of solidity depends on the fact that the images produced on the two retinae are not exactly from the same point of view; they are like the two photographs of a stereoscopic picture. The brain on receiving these two slightly different pictures fuses them into one, but judges the solidity of the object from the differences in the two pictures. Judgment of color, or color vision, forms a subject of great complexity. It apparently depends on the existence in the re- tina of three varieties of cones, one variety for each of the three primary colors. The primary colors are red, green and violet; and by mixing them on the retina in equal proportions (as by rotating a disc or top on which they are painted as sectors) a sensation of white results ; by using other proportions, any of the other colors of the spectrum may be produced. When one of these primary color receptors is absent from the retina, color blindness exists. Thus if the red or the green receptors are absent, the patient cannot distinguish between red and green lights. Such persons cannot be employed in railway or nautical work. CHAPTER XXX. THE SPECIAL SENSES (Cont'd). Hearing-. Like light, sound travels in waves, but not as transverse waves of the ether that fills space, but as longitudinal waves of con- densation and rarefaction of the atmosphere itself. The magni- tude of these waves is much greater and their rate of trans- mission much slower than the waves of light; therefore we see the fiash of a gun long before we hear its sound. The several qualities of sound, such as pitch, loudness and quality or timber, depend respectively on the frequency, the magnitude and the contour of the waves. Sound waves are not appreciated by the ordinary nerve receptors but only by those of the cochlear division of the eighth nerve. These are connected, in the cochlea of the internal ear, with a highly specialized receptor capable of converting the sound waves into nerve impulses. The cochlea consists of a bony tube wound two and one-half times as a spiral around a central column, up the center of which runs the end of the cochlear nerve. A longitudinal section of the cochlea (Fig. 56), therefore shows us this spiral tube in sec- tion at several places, and it is noticed that there projects into it from the central column a ledge of bone having a C-shaped free margin. From the lower lip of the C, a membrane called the basilar membrane, stretches across the tube, which it thus divides into two canals, of which the upper is again divided into two by another membrane running from the upper surface of the bony ledge. The basilar membrane is a very important part of the mechan- ism for reacting to sound waves. Resting on it is a peculiar struc- ture called the Organ of Corti (Fig. 57), which in transverse sec- tions of the cochlear canal is seen to be composed of two rows of long epithelial cells set up on end like the rafters of a roof, with 291 292 HUMAN PHYSIOLOGY. shorter ' ' hair ' ' cells leaning up against them, particularly on the side away from the central column. The sound waves, which act on the basilar membrane, are transmitted to the fluid, which fills the uppermost of the three divisions of the cochlear tube (see Fig. 56), through a membrane covering an oval-shaped opening (the oval window) in the bony partition separating the internal from the middle ear. After reaching the apex of the cochlea they pass through a small aperture in the basilar membrane into the lowest Fig. 56. — Semidiagrammatic section through the right ear (Czermak) : G, external auditory meatus ; T, membrana tympani ; P, tj^mpanic cavity or middle ear with the auditory ossicles stretching across it and the Eustachian tube (JS) entering it; o, oval window; r, round window; B, semicircular canals ; 8, cochlea ; Vt, upper canal of cochlea ; Pt, lower canal of cochlea. (From Howell's Physiology.) canal, down which they travel to lose themselves against the mem- brane covering another opening (the round window) situated near the oval window in the same partition of bone. As they pass along these canals the waves cause the basilar membrane to move or vibrate. The vibration affects the cells of the Organ of Corti, and so sets up nerve impulses which are transmitted to the coch- lear nerve by means of nerve fibers which connect with each of the main cells of the Organ. A fine membrane (called Tec- Fig. 57. — Diagrammatic view of the organ of Corti (Testut) : D. basilar membrane : A, B. inner and outer rods of Corti ; «, 6', G," hair cells ; 7, T, supporting cflls. (From Howell's Physiology.) HEARING. 293 tonal) rests on the tops of the hair cells, and by rubbing on them when they move, this membrane augments the action of the basilar membrane. We must now consider how the sound waves are brought from the outside to the oval window. The pinna of the ear col- lects the sound waves from the outside and directs them into the external auditory canal, at the inner end of which they strike the drum of the ear or tympanic membrane. This membrane is stretched loosely in an oblique direction across the canal, and is composed partly of fibers which radiate to the edge of the membrane from the handle of the malleus, a process of one of the auditory ossicles, to which it is attached. Because of these properties, the tympanic membrane, unlike an ordinary drum, is capable of vibrating to a great variety of notes, and the vibrations cause the handle of the malleus to move in and out. Between the tympanic membrane and the cochlea is the middle ear, or tympanum, consisting of a cavity across which stretches the auditory ossicles composed of three small bones, the malleus, the incus and the stapes. Besides the long process or handle already described, the malleus consists of a rounded head sit- uated above and forming a saddle-shaped articulation with the head of the incus and a short process which runs from just be- low the head to the anterior M^all of the tympanum. The incus is somewhat like a bicuspid tooth, the malleus articulating with the crown, and having two fangs, a short one passing backward and a long one vertically downwards. This process, at its lower end, suddenly bends inwards to form a ball and socket joint with a stirrup-shaped bone (the stapes), the foot piece of which is oval in shape and fits into the oval window already mentioned. The ossicles act together as a bent lever, the axis of rotation passing through the short process of the malleus in front and the short process of the incus behind. If perpendiculars be drawn from this axis to the tips of the handle of the malleus and the long process of the incus, i\ will be found that the latter is only two-thirds the length of the former (Fig. 58). The amplitude of movement at the stapes will therefore be only two-thirds of that at the center of the tympanic membrane, but one and one- 294 HUMAN PHYSIOLOGY. half times stronger. The increase in force with which the movements of the tympanic membrane are conveyed to the oval window is still further magnified by the fact that the latter is only one-twentieth the size of the former. It is by these move- ments at the oval window that waves are set up in the fluid occupying the uppermost membranous tube of the cochlea and thus acting on the basilar membrane. The tympanic cavity or Fig. 58. — Tympanum of right side with the auditory ossicles in place (Mor- ris) : 1, incus (like bicuspid tooth) with one process (.?) attached to wall of tympanum and the other running downwards to articulate at 9 and 8, the stapes; 16, head of malleus attached to tympanic membrane. (From How- ell's Physiology.) tympanum across which the chain of ossicles stretches is kept at atmospheric pressure by the Eustacliian tube, which connects it with the posterior nares. Deafness may he due to tlie folloiving causes: 1. Rupture of the tympanic membrane. 2. Ankylosis or stiffening of the joints between the ossicles THE SENSE OF TASTE. 295 and the ligaments which hold them in place in the tympanic cavity. Flexibility of the joints between the ossicles prevents sudden jars at the oval window, for the joint between the mal- leus and incus, being saddle-shaped, unlocks whenever abnormal or excessive movements are transmitted to the malleus. 3. Blocking of the Eustachian tube. This is quite com- monly a result of aden&ids or it may be due simply to a catarrh of the tube. The result of the block is that the pressure on the tympanic cavity falls below that of the atmosphere because of absorption of oxygen into the blood, and the tympanic mem- brane bulges inwards and becomes stretched so that it cannot vibrate properly to the sound waves. The deafness in this case is easily removed by reopening the Eustachian tube by forcing air into it. This can be done by attaching a large syringe bulb to one nostril, closing the other nostril, and while the patient is swallowing a mouthful of water, suddenly compressing the bulb. The auditory distress which is experienced by a person on going into compressed air (as into a caisson) is also due to dis- turbance in the tympanic pressure, for it takes a few moments before this reaches that on the outside. Blowing the nose usually removes the distress. In all these conditions, the patient hears perfectly when a tuning fork is applied to the skull or teeth. This is because the sound vibrations are then transmitted to the cochlea through the bones of the head. When the cochlea is diseased, however, the tuning fork cannot be heard either when it is sounded in the air or when it is applied to the skull or teeth. The Sense of Taste. Scattered over the mucous membrane of the tongue and buccal cavity, and extending back into the pharynx and even into the larynx, are the receptors of taste, or taste huds. They are most numerous in the grooves around the circumvallate papillae at the root of the tongue, and in the fungiform papillas. Each taste bud is composed of a mass of fusiform cells packed like a barrel filled with staves. The staves in the center project as hairs beyond those on the outside, and it is evidently by action 296 HUMAN PHYSIOLOGY. on these hairs that certain dissolved substances set up a stimulus of taste. This stimulus is then conveyed by fine nerve fibers which arborize around the taste cells, to the chorda tympani and lingual nerves in the anterior portion of the tongue and the glossopharyngeal in the posterior part. Through these nerves the sensations are carried to the combined afferent nucleus of the fifth and ninth nerves in the medulla oblongata (see Fig. 59). eTros.svu(«r'|ia(il\s imVivor Fig. 59. — Schema to show the course of the taste fibers from tongue to brain (Gushing). The dotted lines represent the course as indicated by Cush- ing's observations. The full black lines indicate another path by which the impulses may reach the brain. (From Howell's Physiology.) Substances cannot be tasted unless they are in solution, thus, quinine powder is tasteless. One of the functions of saliva is to bring substances into solution in order that they may be tasted. There are four fundamental taste sensations : sweet, saline, bitter and sour or acid. The ability to distinguish each of these tastes is not evenly distributed over the tongue, but occurs in definite areas. These can be mapped out by applying solutions, THE SENSE OF TASTE. 297 possessing one or another of these qualities, by means of a fine camel-hair brush, to different portions of the tongue previously dried somewhat with a towel. Bitter taste is absent from all parts of the tongue except the base, hence a mouthful of a weak solution of quinine sulphate has practically no taste until it is swallowed, when however it tastes intensely bitter. Sweet and sour tastes are most acute at the tip and sides of the tongue. Saline taste is more evenly distributed. This location of taste sensations is not a hard and fast one, for neighboring taste buds in, say, the bitter area at the root of the tongue may appreciate different tastes; thus, if a solution containing quinine and sugar be applied to one papilla, it may taste sweet, whereas when applied to a neighboring one, it tastes bitter. With weak solutions one taste may neutralize another; thus the addition of a small amount of salt to a weak sugar solu- tion may remove its sweet taste. This neutralization of one taste by another does not occur when the solutions are stronger ; thus a mixture of acid and sugar, as in lemonade, causes stimu- lation of both "acid" and "sweet" taste buds. The stimula- tion of one kind of taste bud may cause other taste buds to be- come more acutely sensitive, which explains the sweetish taste of water after washing out the mouth with a solution of salt. Attempts have been made to correlate the chemical structure of organic substances with the taste which thej^ excite, but with little success. Thus pure proteins have very little taste, whereas half-digested protein is intensely bitter; on the other hand, the pure amino acids, which form a large proportion of the de- composition products in such a digest, are' sweet. In the case of acids and alkalies, however, it has been established that the acid taste is due to the H-ion and the alkaline to the OH-ion. Some acids, such as acetic, taste more acid than we should expect from their degree of dissociation into H-ions. This is because of their power of penetration into the cells of the taste buds. When platinum terminals from a battery are applied to the tongue, the positive pole tastes alkaline and the negative acid, because OH-ions accumulate at the former and H-ions at the latter. The Assocl\tion op Taste, Touch and Smell. — The four 298 HUMAN PHYSIOLOGY. fundamental tastes do not nearly represent all the tastes and flavors with which we are familiar. The relish of an appetizing meal, the piquancy of condiments, the bouquet of a fine wine, would remain unappreciated were there no other nerve receptors than those described above. Two other types of nerve, receptors are involved, namely, (1) those of common sensation, as in the case of acids, which add an astringent character to the sour taste, and, (2) those of smell, as in wines and flavored foods. The importance of the sense of smell in '' tasting" explains the loss of this ability during nasal catarrh or cold in the head. Under such conditions an apple and an onion may taste alike. Certain drugs when applied to the tongue affect taste sensa- tions in different degrees. Thus cocaine first of all paralyzes the receptors of common sensation so that pain is no longer felt and an acid loses all of its astringent qualities and merely tastes sour. A little later the bitter taste also disappears, then salt, then sour, but the saline taste remains even after the cocaine has developed its full effect. Another interesting drug acting on the taste sensations, is a substance present in the leaves of Gymnema sylvestre. When these leaves are chewed, the sweet and bitter tastes are absent, those of acid and of salt and ordi- nary sensation (astringency, etc.) being, however, unaffected. The Sense of Smell. In man the sense of smell is very feeble when compared with that of the lower animals, and it is of very unequal development in different individuals. It is, moreover, readily fatigued, as is the experience of every one who has been compelled to live in stuffy rooms. The receptors are represented by the columnar epithelium of the superior and middle turbinate bones and the adjacent parts of the nasal septum. This epithelium is composed of large columnar cells, each cell being connected with a nerve fiber which is one of the branches of a fusiform bipolar nerve cell lying im- mediately beneath the epithelium. The second branch of each nerve cell runs through the cribiform plate to join the olfactory bulb. After making connections with nerve cells here, the path- way is continued along the olfactory tract to the hippocampal THE SENSE OF SMELL. 299 region of the brain. As we would expect, this portion of the brain is highly developed in those animals having a very acute sense of smell. The olfactory epithelium is kept constantly moist with fluid, and substances cannot be smelled unless the odorous particles which they give off become dissolved in this fluid. These odor- ous particles diffuse into the upper nares from the air currents which, with each respiration, are passing backwards and for- wards along the lower nasal passages. There is no actual move- ment of air over the olfactory epithelium. Nature of Stimulus. — It is impossible to state just exactly what it is that emanates from an odorous body to excite the ol- factory sense. All we can say is that it does not require to be present in more than the merest trace in the air in order to un- fold its action. Thus even in the case of man, with his undevel- oped sense of smell, 0.000,000,000,04 of a gramme of mercaptan, suspended in a liter of air, can be smelled, and in the case of the dog, the dilution may no doubt be many thousand times greater. The sense of smell is the most important of the projicient sensa- tions in certain aquatic animals, and is very closely associated with the sexual functions of the animal. Just as in the case of taste, certain substances owe their peculiar odors to simultane- ous stimulation of the olfactory epithelium and the receptors of common sensation. Thus the pungency of acids, of ammonia, chlorine, etc., is due to stimulation of the endings of the fifth nerve. Attempts have been made to classify odors, as has been done for tastes, but with no success. CHAPTER XXXI. THE MUSCULAR SYSTEM. The General Properties of Muscular Tissues. — The intimate nature of the physical changes taking place during the contrac- tion of a muscle are not understood, and the histological changes which occur have had various interpretations put on them. For a discussion of these a textbook on histology should be consulted. The physiological property which distinguishes muscular tis- sue from other forms of tissue is that of contractility. It is to this property that the forcible shortening of the muscles which produces movements is due. The shortening occurs in the long axis of the muscle and is accompanied by a compensatory thick- ening in the transverse diameter, which keeps the bulk of the muscle constant. After the period of active contraction the muscle remains in the contracted position unless it be pulled back into extension by some force. No isolated muscle can actively expand ; it can only do so passively. Muscle does not possess the property of initiating the contraction. This depends on the ner- vous system acting on another property of muscle, namely, its irritability , that is, the ability of the muscle to react very quickly to a stimulus. The amount of stimulus which it requires is very small compared with the reaction brought about in the muscle. A muscle can be stimulated in other ways than through its nerve, namely, by mechanical, thermal, electrical, and chemical stimuli applied directly to it. By using these artificial stimuli on muscles excised from the body the properties of muscular contraction can be studied. A record of the contraction of a muscle of a frog may be made by excising it and attaching. one end to a suitable clamp and the other end to a light lever the opposite end of which is arranged to trace on smoked paper placed on a rapidly revolving drum. If such a muscle be electrically excited, it will record its- con- traction as a curve on the smoked surface of the paper, and show 300 THE MUSCULAR SYSTEM. 301 a number of interesting details as to the properties of contracting muscles. The muscle does not begin to contract at the exact moment that the stimulus is applied. A very short latent period (.01 sec.) elapses between the stimulus and the beginning of the contrac- tion. During this time the muscle is undergoing some internal change which must precede the contraction. The period of active contraction is relatively short (.04 sec.) and the period of relax- ation somewhat longer (.05 sec). The ordinary movements of the body cannot obviously be of the nature of a single muscular contraction, for they much exceed one-tenth of a second in dura- tion. They are in fact produced by a prolonged contraction of muscles caused by the fusions of several single contractions. This is known as tetanic cont7'actio7i, smd it can easily be produced in the muscle preparation described above by giving it a series of electrical stimuli from an induction coil. If the stimuli be properly timed, a contraction curve somewhat higher and showing no relaxation phase will be produced. When the ex- citation is discontinued, the muscle returns to its normal length. The amount of load which the muscle lifts has a peculiar effect. Up to a certain point an increase in the load increases the effi- ciency of the muscle and the muscle will actually perform more work with a moderate load than with no load at all. After a certain load is reached, the efficiency of the muscle begins to diminish and further increase of the load decreases the work accomplished by the muscle. The principle involved here is made use of by fork and shovel manufacturers, who are careful to make their implements carry the load best suited to develop the maximal efficiency of the muscles of a normal average man. Al- lowing the laborer to choose his own shovel is not always the best for the laborer or for his employer. Another interesting fact is that a contracted muscle is more elastic than a relaxed muscle. Equal weights attached to a con- tracted and to a relaxed muscle will produce a greater elonga- tion in the contracted than in the relaxed muscle. It is this prop- erty which protects the muscle from sudden rupture when at- tempts are made to lift loads that are too heavy. 302 HUMAN PHYSIOLOGY. The Chemical Changes Which Accompany Muscular Contrac- tion are concerned in the liberation of energy by the oxidation of the organic foodstuffs and the converting of this energy into museular energy. Just how this change is brought about is not known. During muscular activity a great amount of oxygen is required and a large amount of carbon dioxide is given off. It is very interesting, however, to know that the maximal exchange of these gases does not actually accompany but follows the mus- cular activity, thus indicating that a muscle becomes charged with energy, so to speak, during rest and discharges itself in much the same manner as a storage battery during a period of activity. If a muscle be made to contract till it becomes fatigued, a large amount of sarco-lactic acid accumulates in the tissue. This poisons the muscle and makes it unable to contract. If this be washed out with saline, the muscle will again contract for a time. Rigor mortis, or the rigidity which comes on after death, may be due to the development of sarco-lactic acid in the tissues be- cause they have become deprived of oxygen. CHAPTER XXXII. REPRODUCTION. The most important function of an animal 's life is the produc- tion of a new individual which in all peculiarities of function and structure is essentially like the parent. The fundamental prob- lems of the process of reproduction which are of physiological importance, are those of fertilization and heredity. Fertiliza tion consists in the union of two parent cells to produce a new cell which is endowed with the power of growth and subdivision. Heredity refers to the phenomenon which directs the cell thus fertilized to develop into an individual like its parents. Since up to the present time most of our knowledge of these processes is based on anatomical data, we will discuss them very briefly and will pay more attention to what we may term the accessory phenomena of reproduction, which are of more practi- cal interest at present. Reproduction in the unicellular animals is a simple process. The parent cell divides exactly in halves and two daughter cells are produced. In the multicellular animals this type of repro- duction is impossible and the process is delegated to a portion of the animal's body known as the reproductive system. This system in man includes the specialized tissues which produce the cells or eggs from which the new individual develops, and the accessory organs which are concerned in providing favorable conditions for the development of these cells. Fertilization. — A very simple type of fertilization is seen in unicellular animals, which ordinarily reproduce by simple divi- sion. After a series of simple divisions the cell becomes unable to develop more cells until after it has united with another cell to form one large cell. This process is termed conjugation. In higher forms, the development of the egg is always preceded by the phenomenon of fertilization, which is somewhat similar to 303 304 HUMAN PHYSIOLOGY. that of conjugation in lower forms. In this process, cells of two types are concerned, the male, or sperm cell, or spermatozoon, and the female cell or ovum. The spermatozoon has the ability to move and to penetrate the ovum. The nuclear elements of both cells unite to form a new nucleus, which is then capable of undergoing a long series of subdivisions. In changes which pre- cede fertilization, the nuclear material originally present in both male and female cells is reduced, and when the cells fuse, the re- sulting nucleus contains a normal quantity of nuclear material. The Accessory Phenomena of Reproduction in Man. — The beginning of the active sexual life in man is between the ages of fourteen and sixteen, and is called the age of puberty. In both boys and girls the whole body shows a marked develop- ment at this time. The growth of hair on the pubic regions and arm pits, and on the face of boys, the deepening of the male voice, and the development of the breasts in the female, are all accompanying phenomena of the development of puberty. In females this age is marked by the onset of menstruation, which consists of a periodic flow of mucus and blood from the uterus. The flow lasts from four to five days, and recurs with great regu- larity about every four weeks. In males fully formed seminal fluid, containing live sperm cells, appears. The Female Organs of Reproduction. — These are the ovaries, oviducts, uterus and the vagina. The ovaries are paired bodies lying in the lower part of the abdominal cavity and held in posi- tion by the broad ligament. The cells from which the ova de- velop are imbedded in the fibrous tissue of the ovary. A number of these cells, better developed than their fellows, and surrounded by a layer of cells, which form a sort of follicle, lie near the sur- face of the ovary. These are the Graafian follicles, in which the ova develop till they are ripe, when they are extruded into the abdominal cavity by rupture of the follicle. In very close appo- sition to the ovaries is a tube, the oviduct, which leads to the uterus. The outer end of this tube is fimbriated, and it is fur- nished with cilia, the movements of which cause currents in the fluids of the abdominal cavity, and which direct the ova dis- REPRODUCTION. 305 chargf^d from the follicle into the oviduct. The uterus is a pear- shaped organ with muscular walls. It is about 7 cm. in length, and consists of an upper dilated portion, called the fundus, and a lower constricted portion, called the cervix. The cervix opens by a small aperture into the vagina, which is a membranous canal about 10 cm. long extending to the vaginal outlet at the external genitalia. The Male Organs of Generation are the testicles, vas deferens, seminal vesicles, the penis, the prostate gland, and a number of small glands along the uretlira. The testicles consist of two parts, a portion of which is cellular and is concerned in the development of the spermatozoa; and a portion called the epididymis, containing the lower portion of the very long and convoluted duct, the vas deferens. This duct connect.' the testicles with the seminal vesicles, which lie at the base of lie bladder and in close relation to the prostate gland. The seminal vesicles are united by a short duct with the urethra, which is the outlet for the excretions of both the kidney and the testicles. The spermatozoa are developed in the testicles and find their way to the seminal vesicles through the vas deferens. On their way they become mixed with a number of fluid secretions, the chief of which are derived from the seminal vesicles of the pros- tate gland and of the glands of Cowper. The resulting mixture is the seminal fluid. Impregnation. — The seminal fluid containing the spermatozoa is deposited in the vagina during coitus. Attracted by the acid reaction of the secretions of the uterus or under an unknown in- fluence, the spermatozoa soon enter the uterine cavity through its opening into the vagina, and find their way to the oviduct, where they remain waiting for the ovum to appear. Ovulation. — At about the time of a menstrual period an ovum . is discharged from a ripened Graafian follicle and finds its way into the oviduct by way of the fimbriated extremity of. the tube, down which it is conducted to the uterus. It is a debated ques- tion as to wljat the exact relation between menstruation and ovu- 306 HUMAN PHYSIOLOGY. lation may be. Whether ovulation precedes or follows menstrua- tion is not known, but the weight of evidence favors the belief that menstruation serves to prepare the uterine walls for the reception of the fertilized ovum should one be discharged. In animals there are periods, called the rutting period, during which impregnation of the ovum with the spermatozoon is pos- sible. Preceding this period there occurs a swelling of the exter- nal genitalia and some discharge of mucus. This period probably corresponds to the menstrual period in woman, for there is much evidence to show that impregnation occurs most frequently fol- lowing the menses. Menstruation ceases during pregnancy and is generally absent during the period of lactation. It ceases altogether between the ages of about forty-five and fifty. After this time, which is known as the climacteric period, a woman is no longer capable of bearing children. The union of the spermatozoon and the ovum usually occurs in the oviduct. If the ovum is not fertilized it is cast off. If it is fertilized, a considerable thickening of the uterine mucous membrane takes place from the proliferation of its cells. When the ovum reaches the uterus, it becomes imbedded in the mucous membrane of the fundus of the uterus. This mucous membrane is very vascular and soon becomes fused with the outer layer of the ovum. Pregnancy. — At first the ovum receives its nourishment directly from the mucous membrane of the uterus, but as the ovum develops and becomes what we term an embryo, the part lying next to the uterine mucosa becomes very vascular ; a similar process takes place in the uterine mucosa directly in contact with the embryo. By this process the placenta is formed, the organ through which the embryo obtains nourishment from the mother. The vascular system of the embryo is, however, entirely sepa- rate from the maternal vessels, and the blood of the mother never directly enters the embryo. The interchange between the two must be effected through the cells covering the vessels of the uterine and foetal portions of the placenta. In other words, the embryo may be said to live a parasitic yet entirely independent REPRODUCTION. 307 life, since through its placental vessels it exchanges its effete products for the oxygen and nourishment contained in the mother's blood. Birth. — "While the ovum is being developed into a human being bj division of the original cell of the fertilized ovum, the uterus becomes very much enlarged, and its walls increase in size by the growth of muscular tissue. At the end of approximately 280 days from tlte date of impregnation of the ovum, the devel- opment is complete and birth takes place. This consists in the expulsion of the foetus by muscular contractions of the uterus. Directly the child is born, the placenta begins to separate from the uterine wall and is soon expelled. The child deprived of its placental nourishment must now begin an independent life. It must take in its own oxygen and give off carbon dioxide by its respiratory organs. It must take its food through the alimentary canal, and excrete its waste products through its kidneys. APPENDIX. Since the amount of time devoted to laboratory instruction and the general equipment of the physiological laboratories in our dental schools varies greatly, detailed laboratory outlines for an experimental course in physiology are out of place. The following is a summary of the experiments in experimental physiology which is given at the pres- ent time to the dental classes in the University of Illinois. It is pre- sented here in hope that it may help others in planning a course of like character. The exercises occupy thirty periods of about two hours each. "When possible, the experiments are done by the individual students; the more diflBcult experiments are given as demonstrations. EXERCISE No. 1. This period is devoted to a study of the chemical nature of the pro- teins. The solubility of egg white in distilled water and in salt solu- tion is tested. The coagulation of albumin by heat, and its precipita- tion by the addition of neutral salts, is shown. The common chemical tests for the proteins are made. The alterations which take place in the chemical and physical properties of protein when partially digested are illustrated by doing the above experiments on solutions of peptone in place of egg white. EXERCISE No. 2. The physical and chemical properties of the fats, carbohydrates and inorganic salts found in the body, are studied. EXERCISE No. 3. The physical chemical properties of water which are of interest from a physiological standpoint, as for example, its solvent action, specific heat, and latent heat of vaporization, are compared with the same prop- erties of other solvents, such as oil, ether, acid and alkali. EXERCISE No. 4. This consists of a demonstration of experiments which illustrate the phenomena of surface tension, osmosis, dialysis and ionization. EXERCISE No. 5. The well known experiment showing the nervous mechanism involved in the secretion of the saliva is made upon a dog. 309 310 HUMAN PHYSIOLOGY. EXERCISE No. 6. In this exercise each student tests his own saliva for its organic and inorganic constituents. The digestive power of the saliva is estimated by determining the length of time that it takes to change a solution of starch into sugar. The determination of the neutralizing power of the saliva is determined by Marshall's method (see page 49). EXERCISE No. 7. The acidity and the digestive power of gastric juice are determined in this exercise. As gastric juice is difficult to obtain, a very good arti- ficial juice can be prepared by dissolving commercial pepsin in 0.4% hydrochloric acid. EXERCISE No. 8. The peristaltic muscular movements of the stomach are studied by means of a strip of muscle obtained from the pyloric end of a frog's stomach. The muscle strip is attached to the end of a light lever which traces on the smoked drum of a kymographion. EXERCISE No. 9. The secretion of pancreatic juice in a dog following the intravenous injection of the harmone, secretin, is demonstrated. The pendulous and peristaltic movements of the small intestines are also observed. EXERCISE No. 10. Each student determines the number of red blood corpuscles per cubic millimeter of blood in another student's blood, by use of the hsemocytometer. EXERCISE No. 11. The white sells in each cubic millimeter of blood are determined as in the case of the red cells. EXERCISE No. 12. The number of white cells in a cubic millimeter of blood obtained from patients suffering from infections of the mouth, is determined. EXERCISE No. 13. This period is used for demonstrating the physical and chemical phe- nomena of the coagulation of the blood. EXERCISE No. 14. The circulation of the blood in the mesentery of the frog's intestines is studied under the microscope. EXERCISE No. 15. The principles of the circulation as shown by the Lombard's Heart Model are demonstrated. APPENDIX. 311 EXERCISE No. 16. This period is devoted to a study of the physiological properties of the turtle's heart muscle. A record of the heart beat is obtained by attaching the heart to a light lever which records its movements on the revolving drum of a kymographion. EXERCISE No. 17. A demonstration upon a dog, showing the factors which maintain the blood pressure, occupies this period. Tracings are made of the blood pressure and the heart beat by means of the mercury manometer and the kymographion. Attention is called to the part which the heart beat and the peri- phereal resistance of the blood vessels play in the maintaining of the circulation of the blood. EXERCISE No. 18. The nervous regulation of the heart beat is graphically shown by de- termining the effect which section and subsequent stimulation of the peripheral ends of the sympathetic fibers to the heart, and the peri- pheral end of the vagus nerves have upon the heart beat and the blood pressure. The method of recording these observations is the same as in the previous experiment. EXERCISE No. 19. The vaso-constrictor fibers to the ear in the cervical sympathetic nerve of the rabbit are demonstrated. In the same animal the action of the cardiac depressor nerve upon the heart beat and the blood pres- sure is sh6wn. EXERCISE No. 20. The effect of sensory nerve stimulation upon the respiration and cir- culation is studied. The central end of the fifth nerve in an anaesthe- tized dog is stimulated with an electric current, while tracings of the heart beat, blood pressure, and the respiration are being taken upon a kymographion. Likewise the effect of stimulating the sensory fibers of the sciatic nerve (by burning the paw of a deeply anaesthetized dog) upon the blood pressure, etc., can be shown. The effects of asphyxia, haemorrhage and gravity on the circulation and respiration is also dem- onstrated. EXERCISE No. 21. In this experiment the blood pressure is recorded as in the former experiments. The changes in the volume of the kidney are determined by means of the oncometer. The effect of stimulation of the splanchnic nerve upon the kidney volume and the blood pressure and the effect which follows the injection of adrenalin, are compared. 312 HUMAN PHYSIOLOGY. EXERCISE No. 22. The students, using each other as subjects, determine the tidal, com- plemental, and supplemental air and the vital capacity of their lungs by means of the spirometer. The various forms of artificial respira- tion are tested out and the most effective type determined by compar- ing the respiratory exchange as measured by the spirometer or a gas meter. A graphic record of the respiratory movements is made. EXERCISE No. 23. An analysis of the atmospheric and expired air is made and compared. EXERCISE No. 24. The blood pressure of each student is determined by the ausculatory and palpation methods. Both systolic and diastolic pressures are taken. The effect upon the blood pressure, of running up and down stairs, is determined. EXERCISE No. 25. The student is taught to recognize the heart and the respiratory sounds by the use of the stethoscope. EXERCISE No. 26. The effect of the administration of nitrous oxide upon the blood pres- sure and heart beat is demonstrated. The various stages of the anaes- thesia are indicated. The experiment is terminated by showing the effect upon the blood pressure and the heart of injecting a small and a large dose of cocain into the animal. EXERCISE No. 27. This period is devoted to the study of the physical and chemical prop- erties of normal and pathological urine. EXERCISE No. 28. This is a demonstration experiment of the physiological properties of the skeletal or voluntary muscle. EXERCISE No. 29. This exercise is a demonstration of the reflexes which can be elicited in the spinal frog. The common reflexes which can be elicited in man are shown and their physiological significance pointed out. EXERCISE No. 30. This period is devoted to some of the simple experiments on the spe- cial senses. INDEX. Abducens or sixth nerve, 261 Aberration, cliromatic, 285 spherical, 285 Absorption, 80 Accelerator nerves of heart, 184 Accommodation, 281 mechanism, 283 pupil in, 284 Acidity, 30 of gastric juice, 64 of saliva, 48 Acromegaly, 131 Addison's disease and adrenals, 129 Adrenalin, 130 Adrenals (suprarenal capsules), 129 Adsorption, 33 Afferent nerve paths, 245 Albumin, 22 Albuminuria, 232 Amino bodies, 22 Amoeba, 18 Ammonia, 108 in urine, 230 Amylopsin, 74 Anesthesia, 245 Analgesia, 245 Anaphylaxis, 151 Animal heat. 134 Antibodies in blood, 148 Antienzymes, 36, 77 Antipyretics, 138 Autithrombin, 148 Antitoxin, 130 Apex beat of heart, 162 Aphasia, 273 Appetite, 43, 60 Arterial blood pressure, 173 Asphyxia, 195 Assimilation (see Metabolism) Association areas of cerebrum, 272 fibers of cerebrum, 272 Associative memory, 273 Asthma, 222 Astigmatism, 286 Atmosphere and metabolism, 88 Auditory areas of cerebrum, 272 Auditory ossicles, 293 Augmentor nerves of heart, 184 Auricle, 167 Auriculo-ventricular valves, 159 Auscultation of lungs, 213 Autonomic nervous system, 277 Bacterial digestion, 66, 76 Beat of heart, 161 Beef tea, 107 Beri-Beri, 121 Bile, 71 Binocular vision, 289 Bladder, urinary, 235 Blind spot, 288 Blood, 140 coagulation of, 147 functions of, 140 gases of, 201 microscopic characters of, 140 physical properties of, 140 plates, 145 plasma, 145 Blood corpuscles, 140 enumeration of, 141 source of, 143 Blood flow, rate of, 179 Blood pressure, 173 Bleed vessels, nervous control of 189 Body fat, source of, 115 Brain, 256 Bread, 105 Breathing,, mechanism of, 209 Bright's disease, 232 Bundle of His, 165 Butter, 106 Calcium, 120 Calcium salts and coagulation of blood, 148 Calorimeter, 85 Calory, 84 Capacity of lungs, 216 313 314 INDEX. Carbohydrates, 24 food values of, 84 metabolism of, 106 relative metabolic importance, 113 Carbon dioxide: effect of oxyhsemoglobin, 202- 206 mechanism of exchange, 205 production of, 197 Cardiac cycle, events of, 167 Cardiac muscle, 163 Cardiac depressor nerve, 187 Centers, vascular-nervous, 187 Cerebellum, 274 Cereals, 105 Cerebrum, 268 function of, in modifying re- flexes, 270 localization in, 269 relation to receptor system, 269 sensory areas, 272 Cheese, 106 Chemical composition of body, 19 Chemistry, of bile, 73 of foods, 104 of gastric juice, 64 of pancreatic juice, 71 of saliva, 46 of urine, 229 Childbirth, 307 Cholesterol, 24 Chordae tendineae, 163 Chyme, 68 Ciliary muscle, 283 Circulation, 180 diagram of, 159 influence of arteries, 172; of cocain, 196; of gravity, 194; of haemorrhage, 194; of ner- vous system, 184; of nitrous oxide, 195; of respiratory movements, 183 pulmonary, 182 renal, 233 time of, 179 venous, 178 Circulatory system, anatomy, 159 Circumvallate papillae, 295 Clothing, 136 Climate, effect of temperature, 137 Coagulation of blood, 147, 148 Cocain, 196 Colloids, 32 Complemental air, 216 Condiments, 107 Cones of retina, 287 Consciousness, 268 Consonants, 227 Contraction of muscle, 300 tetanic contraction, 301 Co-ordination, function of cere- bellum, 274 Cord, spinal, 245 Cords, vocal, 225 Cornea, 282 Corpora quadrigemina, 257 Corpuscles of blood, 141 Corti, organ of, 291 Coughing, 214 Cranial nerves, 259 Creatinin, 230 Cretinism, 126 Cream, 106 Crying, 214 Crystalloids, 27 Cystine, 112 Deglutition, 55 Dentrite, 241 Determination of blood pressure, 174 Diabetes, 117 Dialysis, 27 Diaphragm, relation to breathing, 210 Diastole of heart, 167 Diastolic blood pressure, 174 Dietetics, 99 Diet, suitability of, 102 fundamentals of, 103 Digestion: bacterial-intestine, 76 of cellulose, 76 necessity of, 37 in intestine, 71-75 in mouth, 39 in stomach, 60 object of, 37 resume of digestive ferments, 82 Direct pyramidal tract, 248 Disaccharides, 24 Ductless glands, 124 Dyspnea, 221 Efferent nerve paths, 250 Eggs, 106 Electrolytes, 28 Enamel, action of saliva on, 51 Energy balance (see Metabolism) INDEX. 315 Enterokinase, 74 Enzymes, 34 Erepsin, 75 Erythrocytes, 141 Eustachian tube, 294 Excreta, endogenous and exogen- ous, 112 Excretion, from lungs, 206 renal, 229 Exercise, muscular, and metabol- ism, 114 Exogenous excreta, 112 Expiration, 209 Expired air, composition of, 218 Eye {see Vision) Fat, chemical composition of, 24 food value of, 84 of body, source of, 115 structure of, 24 relative metabolic importance of, 113 metabolism of, 115 Ferments, 34 Fertilization, 303 Fetus, nutrition of, 306 Fever, 137 Fibrin, source of, 147 Fibrinogen, 147 Flavor, 307 Foods, common composition of, 104 Fovea centralis, 288 Gall bladder, 71 stones, 74 Ganglia, 241 s'pinal, 242, 277 sympathetic, 242, 277 Ganglion, definition of, 241 Gasserian, 261 semilunar, 191, 278 Gas, absorption of, by liquid, 199 partial pressure of, 199 Gases of blood, 201 Gas exchanges, in lungs, 217 in tissues, 198 Gasserian ganglion, 261 Gastric digestion, 6;> Gastric juice, constituents of, 64 Gastric secretion, control of, 61 Giantism, 131 '"'.lands, ductless, 124 gastric, 60 mammary, 238 pancreatic, 71 salivary, 39 sebaceous, 238 of skin, 236 sweat, 236 thyroid, 125 Globulin, 23 Glomerulus, 232 Glottis, 225 Gluten, 104 Glycogen, 116 Glycoprotein, 23 Glycosuria, 116 Goiter, 128 Graafian follicle, 304 Growth, curve of, 97 Hair-eells of cochlea, 242 Hoptophore group, 150 Hearing, 292 Heart, anatomy of, 160 augmentor nerves of, 184 heart block, 165 cavities of, 160 change in form of, 161 contractions, maximal, 163 influence of salts on, 166 inhibitory center of, 187 inhibitory nerves of, 185 nerves of, 184 passage of beat over, 164 pace-maker of, 164 physiological peculiarities of. 163 position of, 160 refractory period of, 160 rhythmic action of, 163 sounds of, 169 valves of, 162 vascular mechanism of, 166 work of, 172 Heart valves, 162 Heat, animal, sources of, 134 value of foodstuffs, 85 Hematin, 141 Hemorrhage, 194 Hemoglobin, 141 absorption of oxygen by, 201 chemical nature of, 141 estimation of, 141 influence of acid, 202 of carbon dioxide, 202 Hiccough, 214 Hippuric acid, 112 Hormones, 38, 124 316 INDEX. Hunger, 81 Hydrogen ions, 30 measurement of, 31 Hydrogen electrode, 31 Hydrochloric acid in gastric juice, 64 Hyperglycsemia, 116 Hypothyroidism, 128 Hyperthyroidism, 128 Immunity, Ehrlich's theory of, 150 specific nature of, 151 Immunization, 151 Impregnation, 305 Infection-resisting mechanism, 150 Inflammation, 149 Inhibitory nerves of heart, 185 Inorganic salts, metabolism, 119 Inspiration, 209 Internal capsule, 248 Internal secretion, 125 Intestinal digestion, 75 Intestinal juice, 75 Intestine, large, movements of, 79 Intestine, small, movements of, 78 Ions, 28 Ionization, 28 Iron, 120 Katabolic processes, 84 Kephalin, 148 Kidney, blood flow through, 233 blood supply of, 233 minute structure of, 232 nerve of, 233 Knee jerk, 251 Lactation, 238 Lacteals, 155 Lecithin, 24 Lens, crystalline, 283 Leucocytes, movements of, 144 function of, 145 Lipase, in gastric juice, 67 in pancreatic juice, 74 Lipoids, 23 Liver, excretory function of, 70 glycogenetic function of, 117 Localizing power of retina, 289 Locomotor ataxia, 254 Lungs, changes of blood in, 217 movements of, 213 Lymph, movements of, 157 formation of, 156 glands, 158 relation of, to blood, 155 resorption of, 157 vessels, 157 Lymphagogues, 156 Lymphocytes, 144 Lymph nodes, 158 Maintenance food, 99 Malpighian capsule, 232 Malpighian pyramids of kidney, 232 Mammary gland, 238 Mastication, 53 saliva and, 54 Material balance of body, 91 Measurement of arterial pressure, 175 Meat, 106 extract, 107 Menstruation, 304 Mental process, 273 Metabolism, general, 83 influence of atmosphere, 87 muscular work, 87 surface area, 87 basal heat production, 87 specific dynamic action, 87 Metabolism, special, 108 carbohydrates, 116 fats, 115 inorganic salts, 119 proteins, 108 Middle ear, 292 Milk, composition of, 105 Micturition, 236. Monosaccharides, 24 Motor area of cortex, 269 Mountain sickness, 222 Mouth, digestion in, 47 Muscles, 300 Muscle sense, 275 Muscular elasticity, 301 Muscular energy, source of, 199 Muscular tone, 253 Muscular work, expenditure of energy, 99 Myopia, 286 Myxoedema, 127 Nausea, 58 Nerve: abdueens, 261 auditory, 264 cranial, 259 depressor, 187 INDEX. 317 facial, 263 glossopharyngeal, 264 hypoglossal, 266 inferior maxillary, 261 oculomotor, 260 olfactory, 298 phrenic, 219 sciatic, 219 spinal accessory, 265 trigeminal, 261 vagus, 265 Nerve impulse, 239 Nerve paths, afferent, 246 efferent, 250 method of tracing, 245 Nerve plexus, 240 Nerve system, 239 sympathetic, 277 Neurones, intermediary, 247 Nitrogen equilibrium, 94 balance sheet, 94 Nucleoprotein, 22 Nutrition (see Metabolism) Nutrition of embryo, 306 Nutritive value of foods, 104 Obesity, treatment of, 95 Oculomotor nerve, 260 Opsonins, 153 Optical defects, 285 Optic thalami, 247 Organ of Corti, 291 Osmosis, 28 Osmotic pressure, 28 Oviduct, 304 Ovulation, 305 Ovum, 304 Oxidase, 198 Oxidation, in tissues, 198 as source of animal heat, 199 Oxygen, absorption of, by blood, 201 Oxyhsemoglobin, effect of COn on, 205 Pain, 245 Pancreatic juice, 71 composition of, 74 Pancreatic secretin, 72 Pancreatic secretion, control of, 71 Paralysis, 255 Parathyroids, 125 Pepsin, 64 Pepsinogen, 64 Peptone, 21 Peristalsis, 79 Perspiration, 237 Phagocytosis, 152 Physico-chemical laws, 26 Physiological division of labor, 18 Physiological properties, 18 Physiological systems, 19 Pituitary body, 131 Platelets, or plaques, of blood, 145 Plasma, blood, 145 Pneumogastric nerves, 265 Polypeptide, 22 Pons Varolii, 246 Postsphygmic period, 167 Potassium sulphocyanide in sa- liva, 47 Precipitins, 151 Pregnancy, 306 Presbyopia, 286 Pressure, arterial, 175 intrathoracic, 211 osmotic, 28 Presphygmic period, 167 Properties of body, physical and physiological, 20 Proteins, chemical composition of, 21 compound, 22 insoluble, 23 irreducible minimum, 96 nutritive value of, 94 relative metabolic importance of, 113 requirement of body for, 100 simple, 22 sparers of, 95 subdivisions of Proteose, 21 Protoplasm, composition of, 19 primary constituents of, 19 secondary constituents of, 20 Ptyalin, 47 Puberty, 304 Pulmonary circulation, 182 Pulse, use of, in diagnosis, 180 tracings, 180 wave, 121 Purin bodies, 110 Pyloric sphincter, control of, 68 Pylorus, 67 Pyramidal tracts, 248 Range of voice, 226 Rate of blood flow, 179 318 INDEX. Reaction of blood, 32 of body fluids, 30 Reason, faculty of, 273 Reciprocal inhibition, 254 Receptors, 151, 244 Red blood corpuscles, 141 Reflex animal, characteristics of, compared with normal, 251 Reflex arcs, 240 Reflex action, 252 Reflex paths, 244 Reflex time, 250 Reflexes, function of spinal cord in, 250 types of, 250 Renal secretion, 232 Reproduction, sexual, 303 Reproductory organs, accessory: female, 304 male, 305 Residual air, 216 Respiration, 197 artificial, 214 control of, 221 external, 207 influence of, on circulation, 213 internal, 197 nerves of, 219 volume of air in, 225 Respiratory center, 219 exchange, 204 movements, 211 organs, 207 quotient, 91, 216 reflex, 219 sounds, 213 Rickets, 120 Rolando, fissure of, 269 Roots of spinal nerves, 246 Saliva, character of, 46 dental caries and, 51 function of, 44 neutralizing power of, 49 reaction of, 48 tartar formation and, 51 Salivary calculi, 52 Salivary glands, 39 nerve supply of, 40 nervous control of, 42 secretion of, 39 Scratch reflex, 251 Salt hunger, 120 Sea sickness, 277 Sebaceous glands, 258 Secretin, gastric, 63 pancreatic, 72 Secretion: control of, 38 gastric, control of, 61 milk, 238 pancreatic, control of, 71 salivary, control of, 42 sebaceous, 238 Secretory process: hormone control of, 38 nervous control of, 38 Semicircular canals, bony, 275 Semilunar ganglion, 191 Semilunar valves, 160, 162 Semipermeable membrane, 27 Sensory areas of cortex, 272 Shivering, 138 Shock, 193 Sight, 279 Skin, function of, 236 Smell, 297 Sneezing, 214 Solutions, isotonic, 30 hypertonic, 30 hypotonic, 30 Sound, loudness of, 226 Sounds of heart, 169 Special senses, 279 Specific dynamic action of foods, 87 Tartar, 52 Taste, 296 Taste-buds, 296 Tectorial membrane, 292 Teeth and fifth nerve, 262 Temperature of body, 134 Temperature, effect of, on mus- cular contraction, 135 Temperature sensation zero, 245 Temperature sense, 245 Temperature, bodily, regulation of, 135 Tetany, 128 Thorax, contents of, 207 movements of, in respiration, 211 Thrombin, 148 Thrombogen, 148 Thymus, 133 Thyroid gland, 125 Tidal air, 215 Touch, sensations of, 244 Toxins, bacterial, 149 INDEX. 319 Toxophores, 150 Trigeminal nerve, 261 Trypsin, 74 Trypsinogen, 74 Urea, 108, 230 Uric acid, 110, 230 Urinary organs, 232 Urinary salts, nitrogen, 108 Urine, ammonia, 108 excretion of, 229 nature of excretory process, 233 Vagus nerve, action of, on heart, 183 Valves of heart, 162, 170 Varicose veins, 179 ^ Vasoconstrictor nerves, 190 Vasodilator center, 187 Vasodilator nerves, 191 Vasomotor tone, 194 Veins, blood in, 178 Velocity of blood, 177 Ventilation, 223 Vision, 279 color, 290 stereoscopic, 290 Visual defects, 284 treatment of, 285 Vital capacity, 216 Vitamines, 121 Vocal cords, false, 224 ' relation of, to pitch, 225 Voice> 224 Vomiting. 58 Vowels, 227 Water, proportion of, in body, 20 physiological properties of, 20 Wheat flour, 104 White blood-corpuscles, 144 Xanthin bodies, 110 Yawning, 214 COLUMBIA UNIVERSITY as provided by the rule, Ztu,u ''='*<= °f borrowing, range„.e„t wiL tlL'uLria'nlnXe." "' ''''''' - C2e(63e)M50 QP34 Pearce P312 1916