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OUTLINES OF THE 
 
 COMPARATIVE PHYSIOLOGY AND 
 
 MORPHOLOGY OF ANIMALS 
 
By Prof. JOSEPH LE CONTE. 
 
 THE COMPARATIVE PHYSIOLOGY 
 
 AND MORPHOLOGY OF ANIMALS. 
 
 Illustrated. i2mo. Cloth. 
 
 The work of Darwin on the derivation of species and 
 the descent of man awakened a new interest in the lower 
 animal i, and furnished additional evidence of their close 
 kinship with ourselves. A fresli field of study was thus 
 opened up, embracing the likenesses and differences of ac- 
 tion as well as structure found throughout the animal king- 
 dom. In this work Professor Le Conte gives us, in his 
 well-known clear and simp'e style and with the aid of 
 numtrjus illustrations, an interesting outline of these simi- 
 larit.es and variations of function as displayed among the 
 various classes of animals from the lowest to the highest, 
 man included. 
 
 RELIGION AND SCIENCE. 
 
 A Series of Sunday Lectures on the Rehiion of Natural and Re- 
 vealed Religion, or tne Truths revealtd in Nature and Scrip- 
 ture. i2mo. Clolh, $1.50. 
 
 ELEMENTS OF GEOLOGY. 
 
 A Text-Rook for Col'ege? and for the General Reader. With new 
 Plates, new Illustrations, new Matter, fully revised to date. 8vo. 
 Cloth, $4 00. 
 
 SIGHT. 
 
 An Exposition of the Principles of Morocular and Binocular Visior. 
 With Illustrations. Second edition. Ko. 31, International 
 Scientific Series. i2mo. Cloth, $1.50. 
 
 EVOLUTION AND ITS RELATION TO 
 RELIGIOUS THOUGHT. 
 
 Revised edition. i2ino. Cloth, $1.50. 
 
 D. APPLETON AND COMPANY, NEW YORK, 
 
OUTLINES OF 
 THE COMPARATIVE PHYSIOLOGY 
 AND MORPHOLOGY OF ANIMALS 
 
 BY 
 
 JOSEPH LE CONTE 
 
 AUTHOR OF 
 
 RELIGION AND SCIENCE ; ELEMENTS OF GEOLOGY ; SIGHT, 
 
 AN EXPOSITION OF THE PRINCIPLES OF MONOCULAR 
 
 AND BINOCULAR VISION ; EVOLUTION AND ITS 
 
 RELATION TO RELIGIOUS TH?)UGHT, ETC. 
 
 NEW YORK 
 
 D. APPLETON AND COMPANY 
 
 1900 
 
Copyright, 1899, 
 By D. APPLETON AND COMPANY. 
 
 fi?33 
 
PREFACE. 
 
 So many books have recently come out, and are still 
 coming out, on zoology and biology that it seems neces- 
 sary that 1 should say something of the reasons for 
 this one. 
 
 Nearly all the books now coming out are devoted, 
 and rightly so, to practical laboratory methods, and 
 especially to the study of selected types. This method, 
 first introduced by Rolleston and rendered popular by 
 Huxley, was a reaction against the barrenness of the old 
 text-book and lecture method. It was certainly timely 
 and necessary ; but there is danger that, like all reac- 
 tions, it may have, and indeed has already, gone too far. 
 Undoubtedly the teaching by types is indispensable in 
 the early part of the course, in order to introduce the 
 student into the true spirit and methods of science ; but 
 to continue it and "make it the main form of teaching 
 is a serious mistake." * There is serious danger that in 
 the attempt to explore thoroughly a few small spots 
 here and there in the field we lose sight of the general 
 connection of all parts to one another and to the whole 
 — that in the microscopic clearness but narrowness of 
 our knowledge we lose that general view of the whole 
 which alone gives significance to any knowledge. 
 
 Such a general view of the physiology and morphol- 
 ogy of the animal kingdom is, it seems to me, a great 
 
 * Lankester, Nature, Iviii, 25, 1S98. 
 
vi PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 want in our teaching of zoology to-day — a want which 
 is only now beginning to be recognized. It is just such 
 a general view which I have attempted to give in this 
 volume. The book is not intended to take the place of 
 books already in the field, but to supplement them. It 
 is intended to precede and accompany the special labo- 
 ratory courses of our high schools, colleges, and univer- 
 sities. It must itself be preceded by the type method in 
 the schools. 
 
 Some will think I have too much slighted the inver- 
 tebrates. I can only say that this was unavoidable if I 
 kept within the limits of a moderate-sized book. I have 
 given only what every intelligent person would like to 
 know. 
 
 Again, some will perhaps think that I dwell too 
 much relatively on certain functions — e.g., the sense of 
 sight and glycogeny. I can only answer that a perfectly 
 balanced treatise on any wide and complex subject is 
 well-nigh impossible, and I am not sure that it would be 
 best even if it were possible. A certain insistence on 
 points best known to and most thoroughly investigated 
 by the teacher — a certain hobby riding, if not carried 
 too far — is necessary to give life and interest to any 
 subject. 
 
 Some may object to the order of treatment — descen- 
 sive instead of ascensive. This, I believe, finds justifica- 
 tion in the fact that physiology, not morphology, is the 
 prominent point of view. This I explain fully in the 
 book (page 27). 
 
 The work is the final embodiment of a course of lec- 
 tures continued and compacted for many years, and 
 given in connection with and preparatory to the labora- 
 tory courses in zoology in the University of California. 
 
 Joseph Le Conte. 
 Berkeley, Cai.., October, iSgg. 
 
CONTENTS, 
 
 CHAPTER I. 
 
 INTRODUCTORY. — SOME GENERAL PRINCIPLES. 
 
 SECTION I. 
 
 Relation of the Three Kingdoms of Nature. 
 
 Living vs. IVonliving, i. — (i) Organization, (2) cellular structure, 
 (3) growth, 2. (4) Life history, (5) reproduction, (6) metabolism, 3. 
 
 Animals vs. Plants, i,. — (i) Sensation and volition, (2) nature of 
 food, 5. (3) The posession of a stomach, 6. (4) Waste and supply, 7. 
 
 SECTION II. 
 
 Definition of zoology, 8. Divisions of zoology : (i) comparative 
 anatomy, (2) comparative physiology, (3) comparative embryologv, (4) 
 taxonomy, 9 ; (5) descriptive zoology, '6) paleozoologv, (7) geographical 
 zoology, 10 ; this course consists mainly of second and first, 11. 
 
 SECTION III. 
 
 General Cellular Structure of Animals. 
 
 Definition of cell, animal vs. vegetal cell, 11. Size, softness, trans- 
 parency, differentiation, 12. 
 
 Tissues. 
 
 Definition, 12. Kinds : (i) Connective, 13 ; (2) cartilage, (3) bony, 
 15 ; (4) muscular, 17 ; (5) nervous, 19 ; (6) epithelial, 20. Law of 
 differentiation of cells, 22. 
 
 SECTION IV. 
 
 Organs and Functions of Animal Body. 
 
 Classification of function, 23. Animal functions defined, organic 
 functions defined, subdivisions, order of treatment, 24. 
 
 vii 
 
Vlll PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 PART I. 
 
 ORGANS AND FUNCTIONS OF ANIMAL LIFE. 
 
 Four systems of organs concerned, 26. How they co-operate, or- 
 der of treatment justified, 27. 
 
 CHAPTER L 
 
 NERVOUS SYSTEM. 
 
 Two subsystems (cerebro-spinal and ganglionic), cerebro-spinal sys- 
 tem of vertebrates, general plan of and subdivisions. 29. 
 
 SECTION I. 
 
 Brain of Man. 
 
 Skull, membranes, 30. Main parts of brain : (i) cerebrum, 31 ; 
 (2) cerebellum, (3) medulla and pons, (4) optic lobes, 32 ; (5) thalamus 
 and corpus, 33. Convolutions of cerebrum, of cerebellum, 34. Interior 
 structure, microscopic structure, 35. Embryonic development of brain, 
 37. Fore brain, mid brain, and hind brain, 39. Distinctive functions 
 of cerebruiTi 39 ; of cerebellum, of medulla, 40 ; optic lobes, thalamus 
 and corpus, 41. Localization of cerebral functions, 43. Dextrality, 45. 
 
 SECTION II. 
 
 Spinal Cord 0/ Man. 
 
 Envelopes, description of the cord, spinal nerves, 46. Section of 
 the cord, 47. General function as conductor and as center, 48. 
 
 SECTION III. 
 Nerves. 
 
 Cranial nerves, 49. (i) Olfactory, (2) optic, 50 ; (3, 4, and 6) 
 oculi motores, 52 ; (5) trigeminal, (7) facial, (8) auditory, (9) gustatory, 
 (10) vagus. 53 ; (11) spinal recurrent, (12) hypoglossal, general obsei^va- 
 tions on cranial nerves, 54. 
 
 Spinal nerves. Origin and distribution, 54. Structure of nerves, 
 function, 56. Mode of action illustrated, two subsystems, 57. Course 
 and termination of fibers, 58. General mode of action of whole, 59. 
 Course in reflex action, 60. Illustrated by telegraphy, 61. Applica- 
 tion to several cases, 62. Law of peripheral reference, 64. Nerve 
 force vs. electricity, 64. Function of spinal or reflex system, 66. 
 
 SECTION IV. 
 
 Ganglionic System. 
 
 Definition and description, 67. Principal plexuses, function of 
 ganglionic system, 69. 
 
CONTENTS. ix 
 
 SECTION V. 
 
 Comparative Physiology and Morphology of Nervous System. 
 
 Introductory. — Outline of the classification of animals, 70. 
 Comparative Morphology of the Vertebrate Nervous System. — Gen- 
 eral plan of structure, 72. 
 
 Brain 0/ Vertebrates. 
 
 (i) Variation in size, absolute, 72 ; and relative, 73. (2) Relative 
 amount of gray matter, 74. (3) Relative size of cerebrum, 76. Owen's 
 classification of mammals, 77. Pineal gland, 78. Embryonic and 
 taxonomic series compared, 79. (4) Relative size of frontal lobe, 
 82. Cephalization, 82. 
 
 SECTION VI. 
 
 Nervous System 0/ Invertebrates. 
 
 1. Articulata. — General plan of structure compared with vertebrates. 
 84. General plan of nervous system, 85. Functions of the several 
 ganglia, 86. Modifications in going down and up the scale, 87. 
 
 2. Mollusca, Sg. — General plan of nervous system, 90. 
 
 3. Radiata. — General plan of, 92. 
 
 4. Protozoa have no nervous system, 93. 
 
 CHAPTER II. 
 
 SENSE ORGANS. 
 
 SECTION I. 
 
 Introductory. 
 
 Relation of special sense to general sensibility, 94. Illustration of 
 the law of differentiation, 95. Gradations between the senses: (i) 
 In perception of vibrations, 96 ; (2) in kind of contact, (3) in objec- 
 tiveness, 97. Higher and lower senses, 98. 
 
 Sense 0/ Sight and its Organ, the Eye. 
 Primary divisions of the subject, 98. 
 
 SECTION II. 
 Eye 0/ Man — General Structure. 
 
 Shape, setting, 99. Muscles, 100. Coats of the ball, loi. Lin- 
 ings, contents or lenses, 102. 
 
 Forynation of the Image, 103. — Necessity of lenses, 104. Applica- 
 tion to the eye, 105. Proof of retinal images, ic6. 
 
 Comparison of the Eye and the Camera. — Defects of lenses and 
 their correction : (i ) Chromatism, 107 ; (2) aberration, 108. Accom- 
 modation, Helmholtz's theory of, 109. Adjustment for light, iii. 
 Structure of iris, 112. 
 
 Defects of the Eye as an Optical Instrument. — Normal sight — em- 
 metropy, myopy, 113. Hyperopy, presbyopy, 114. Astigmatism, 115. 
 
X PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 SECTION in. 
 
 The Retina and its Functions. 
 
 Structure of the retina, 117. Its layers, 118. Bacillary layer, dis- 
 tribution of the rods and cones, iig. Distinctive function of the lay- 
 ers, 120 ; of the rods and cones, 121. Visual purple, 121. 
 
 SECTION IV. 
 Perception of Space and 0/ Objects in Space. 
 
 First Law of Vision. Law of Spatial Reference of Retinal Impres- 
 sions, 122. — Comparison with other senses, 123. Illustrations of the 
 law : (i) Irritation of retina, (2) phosphenes, (3) muscse volitantes, 124. 
 (4) Purkinje's figures, (5) ocular spectra, 125 ; generalization, 126. 
 
 Second Law of Vision; Law of Direction, 126. — Comparison with 
 other senses, 128. Explanation of some visual phenomena : (i) erect 
 vision, explanation, 129. (2) Fovea and its spatial representative, 130 ; 
 minimum visibile, 131 ; compare with touch, 132. {3) Blind spot, 
 132 ; experimental proof, 133 ; spatial representative of, 134. 
 
 Color Perception and Color-blindness, 135. — Intensity vs. color, 
 primary colors, 136. Theory of color perception, general theory, 137. 
 Special theories, 138. Color-blindness, 139. Cause of coloi-blind- 
 ness, what the color-blind see, 140. Tests of color-blindness, 141. 
 
 SECTION V. 
 Binocular Vision. 
 
 Definition, 142. Single and double vision, 142. Experiments illus- 
 trating double vision, single vision, 143. Corresponding points, 144. 
 
 Third Law of Vision; Law of Corresponding Points. — Conditions 
 of single vision illustrated, 144. Iloropteric circle and horopter defined, 
 146. Relation of chiasm to corresponding points, 147. The two ad- 
 justments of the eyes, two kinds of corresponding points, 148. Two 
 fundamental laws of vision, 149. 
 
 Binocular perspective, experiments illustrating, 149. Limitation of 
 clear vision, 151. Different forms of perspective : (i) Aerial, 151 ; (2) 
 mathematical, (3) binocular, (4) focal, 152. 
 
 Judgments of Size and Distance. — Distance, size, 153. Form, gra- 
 dations of judgments, 155. 
 
 SECTION VI. 
 
 Comparative Physiology and Morphology of the Eye. 
 
 Vertebrates. 
 
 Mammals: Iris, pupil, tapetum, 156; fovea, 157. Birds: iris, 
 sclerotic bones, nictitating membrane, 157 ; fovea, 158. Reptiles, 158. 
 Fishes, binocular vision in vertebrates, 159. Chiasm in vertebrates, 
 position of the eyes, 160. Fovea, 161. 
 
CONTENTS. XI 
 
 Invertebrates. 
 
 JSIollusca : Cephalopods, gastropods, 163; acephala, 164. Arthro- 
 pods : Simple eye, 164; compound eyes of insects and crustaceans, 
 165 ; origin of compound eye, 167. 
 
 Evolution 0/ the Eye. 
 
 (i) Invertebrate eye, 167. Steps of evolution illustrated, 168. (2) 
 Vertebrate eye, 170. Several steps of evolution illustrated, 171. 
 Transition from the invertebrate to the vertebrate eye, 172. Further 
 evolution of the vertebrate eye, 173. 
 
 SECTION VII. 
 
 Sense 0/ Hearing and its Organ, the Ear. 
 
 Structure of human ear, exterior ear, mid-ear, 174. Ossicles, in- 
 terior ear, or labyrinth, 176. Bony labyrinth, membranous labyrinth, 
 178. Membranous cochlea, 179. Mode of action of the whole, dis- 
 tinctive functions of the parts, 181. 
 
 Comparative Morphology and Physiology 0/ the Ear. 
 
 Mammals, birds, reptiles, 183. Fishes, invertebrates, insects, 184. 
 Crustacea, mollusca, 185. Mosquito, lS6. Diagram illustrating suc- 
 cessive simplification, 1S7. 
 
 SECTION VIII. 
 
 Lower Senses. 
 
 Sense of Smell. — Nostril, iSS Smelling, igo. Comparative phys- 
 iology of smell, keenness of smell, how judged of, 191. Invertebrates, 
 
 193- 
 
 Settse of Taste. — Analysis of this sense, mixed with feeling, with 
 smell, 194. Examples of pure tastes, 195. Papilloe of the tongue, 
 196. Comparative physiology of taste, 197. 
 
 Sense of Touch — Analysis of this sense, mixture of many sensa- 
 tions, 198. General sensibility vs. special sen^e of touch, general or- 
 gan of this sense, 199. Special organ, 200. Minimum tactile, double 
 tactile images, 201. Comparative physiology of touch in vertebrates, 
 202 ; in invertebrates, 203. 
 
 SECTION IX. 
 
 The Voice and its Organ, the Larynx. 
 
 (i) The Simple Voice : Larynx, its position and relation, 204. 
 Structure, 206. Glottis and vocal cords, 207. Their action in vocali- 
 zation, 208. Muscles of the larynx and how they act, 209. (2) Song : 
 Larynx as a musical instrument, 210. (3) Speech, 211 : Vowel 
 sounds, 212. Consonants, 213. 
 
Xll PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 Comparative Physiology of Voice. 
 
 Mammals, 213. Birds, syrinx, 214. Structure and mode of action, 
 215. Reptiles, frogs, insects, 216. Grasshoppers, cicada, 217. 
 
 CHAPTER III. 
 
 MUSCULAR AND SKELETAL SYSTEMS. 
 
 SECTION I. 
 
 Muscular System. 
 
 Muscles, kinds of, 220. Voluntary muscle, 221. Structure of in- 
 voluntary muscle and its mode of action, 223. 
 
 SECTION II. 
 
 Skeletal System. 
 
 Defined, 223. Number of bones in man, joints, movable joints, 
 224. Examples of adaptation, 225. (i) Comparative morphology of 
 vertebral cohimn, 226. (2) Structure of shoulder joint and fore limb m 
 vertebrates, motion and locomotion, li?nb-»iotioii, 227. Power of mus- 
 cular contractions shown by examples, biceps, 228. Deltoid, gastroc- 
 nemius and soleus, 229. Locomotion, 230. Co-ordination of muscular 
 action, 231. 
 
 SECTION III. 
 Comparative Morphology and Physiology 0/ Muscle and Skeleton. 
 
 Vertebrates, invertebrates. Arthropods, 232. Relation of muscle 
 and skeleton, hinge motion, 233. Universal motion, 234. Worms, 
 mode of locomotion, 235. 
 
 Molhtsca : Acephala, 236 ; gastropods, cephalopods, 237. Caelen- 
 terata : Aledusce, 238. Protozoa : Infusoria, rhizopods, 239. 
 
 CHAPTER IV. 
 
 GENERAL LAWS OF ANIMAL STRUCTURE, OR GENERAL LAWS OF 
 MORPHOLOGY. 
 
 SECTION I. 
 
 Introductory. 
 
 Analogy vs. Homology, examples of each from animals, 242. 
 From plants, 243. Two fundamental ideas in homology, 246. Ho- 
 mology traceable only within the limits of primary divisions of the 
 animal kingdom, 247. 
 
CONTENTS. Xlll 
 
 SECTION II. / 
 
 Homology of Vertebrates. 
 
 I. General Plan of Structure, or General Homology. — General 
 characteristics of vertebrates : (i) Relation of muscle to skeleton, (2) 
 possession of backbone, (3) possession of two tioink cavities, (4) struc- 
 ture of head, (5) only two pairs of limbs, strong suggestion of common 
 origin, 249. 
 
 II. Special Homology. — Definition, the proof of common origin, best 
 shown in limbs, [a) Fore limbs of all classes compared, 250. Hind 
 limbs of different orders of mammals compared, 255. Plantigrade, 
 digitigrade, unguligrade, 256. Manus and pes, classification of ungu- 
 lates by foot structure, 257. Rudimentary and useless organs, 258. 
 Homology in other systems, 260. 
 
 III. Serial Homology. — Definition of, serial homology of the verte- 
 brate skeleton, 261. A vertebral segment, 262. Ov.en's archetype, 
 modifications in the series, 263. Origin of limbs, 264. Serial homol- 
 ogy of other systems, 265. 
 
 SECTION III. 
 Homology among Invertebrates. 
 
 1. Artictdata. — General plan of, 266. Serial homology, 267. So- 
 mite defined, repetition and modification of somites, 268. Illustrated 
 from crawfish, 269. Crab, modifications in going down the scale, 271. 
 In going up the scale, 272. Origin of insects' wings, 273. Law of 
 differentiation, homology of nervous system, 276. 
 
 2. Molhisca. — General plan, explanation of, 277. 
 
 3. Radiata. — General plan is radiated, 278. Comparison with 
 other types, 279. 
 
 4. Protozoa, 279. 
 General conclusions, 280. 
 
 PART II. 
 
 ORGANS AND FUNCTIONS OF ORGANIC LIFE. 
 
 CHAPTER I. 
 
 NUTRITIVE FUNCTIONS — METABOLISM. 
 
 Definition of metabolism, 283. Waste and its relation to life, to 
 work, to heat, 284. Necessity of food, 285. Necessity of waste- 
 removal, anabolism and katabolism, 286. Three divisions of the sub- 
 ject of this part, 287. 
 
XIV PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 CHAPTER II. 
 
 NUTRITION PROPER — ANABOLISM — FOOD PREPARATION — DIGESTIVE 
 SYSTEM. 
 
 SECTION I. 
 
 Food and its Uses. 
 
 Definition of food, kinds, 288. Uses, 289. Distinctive uses of 
 kinds, waste tissue used, 290. 
 
 Food Preparation. — The different steps of preparation, 291. 
 
 SECTION II. 
 Mouth Digestion in Vertebrates. 
 
 Salivary glands, 292 ; structure of, 293. Composition and use of 
 saliva, 294. Ferments defined, 295. 
 
 Comparative Physiology of Mouth Digestion in Vertebrates. — Teeth 
 in vertebrates, 295. Mammalian teeth, origin and development of, 
 296. Composition of teeth, kinds of teeth, 297. Variation in teeth : 
 in size, in relative number of the kinds, 298. Dental formula, 299. 
 Structure of molars, 300. Of herbivorous molars, origin of this struc- 
 ture, 302. Mouth armature of whales, 304. Homology of baleen 
 plates, birds, 305. Teeth of extinct birds, reptiles, fangs of serpents, 
 structure of, 30&. Origin of mammalian teeth, _/^j.//£j', kinds of teeth, 
 308. 
 
 SECTION III. 
 
 Stomach digestion — Chyvtiftcation — Peptonization. 
 
 Saccharization of food, 310. Stomach described, 311. Coats, me- 
 chanical work or chymification, 312. Chemical work or peptonization, 
 313. Composition and uses of gastric juice, effect of, on milk, ab- 
 sorption from stomach, 314. 
 
 Comparative Physiology of Stomach, 
 
 Ruminants, 315. Evolution of ruminant stomach, granivorous 
 birds, digestive apparatus of, 317. Evolution of this apparatus, 318. 
 
 SECTION IV. 
 In testinal Digestion — Chylification — Em ulsification . 
 
 Form and structure of intestines, 319. Relation to abdominal walls, 
 320. Mesentery, peritonceum, 321. Coats of the intestines, mechan- 
 ical work, 322. Chemical work, action of bile, 323. Of pancreatic 
 juice, emulsion defined, 324. Absorption, 325. Two modes of, gen- 
 eral course of each to the circulation, 326. Sangiiificatioii, '}2-j. Effect 
 of the liver and of the mesenteric glands, portal vein described, 328. 
 
 Modification of process of intestinal digestion in vertebrates, 329. 
 Ccccum in a rat, 330. 
 
CONTENTS. XV 
 
 SECTION V. 
 
 Digestive System in Invertebrates. 
 
 Arthropods. 
 
 /«j^<r/j.— Mouth parts of, 331. Of beetle, 332. Of grasshopper 
 serial homology of these parts, 333. Mouth parts of butterfly, 334. 
 Of bee, 335. Digestive apparatus of beetle, 336. Crustacea, mouth 
 parts of, 337. 
 
 Mollusca. — Digestive apparatus of, acephala, 338. Gastropoda, 339. 
 Radula, cephalopoda, 340. 
 
 Echinoderms.—ViTx.'sXxc'aXxw^ apparatus of echinus, 341. 
 
 Cw/enterata.—Mediisa:, 342. Lasso cells, 343. Polvps, structure, 
 344. Modes of digestion, /?wost;(Z, infusoria, 345. Rhi'zopods, 346. 
 
 CHAPTER III. 
 
 BLOOD SYSTEM. 
 
 SECTION I. 
 
 The Blood. 
 
 1. Globules. — Red globules, 347. Structure of, 348. White glob- 
 ules (leucocytes), blood plates, 349. 
 
 2. /"/ajww —Coagulation of blood, 350. Functions of Blood : Of 
 plasma, of red globules, 351 ; of leucocytes, 352. 
 
 Origin 0/ Blood.— (i) Of plasma, (2) of leucocytes, 352. (3) Of red 
 globules, 353. 
 
 Comparative Morphology of Blood. — (i) Mammalian blood, charac- 
 teristic of, (2) oviparous vertebrate blood, chaiacteristic of. 354. (3) 
 Higher invertebrate blood, characteristic of, (4) coelenterate blood, 
 (5) protozoa. Embryonic development of blood, 356. 
 
 SECTION II. 
 Respiratory Organs 0/ Vertebrates. 
 
 Lungs vs. gills. Lungs of man, ■}<i'?>. Structure, 359. Mechanics of 
 breathing, diaphragm, 361. Relation of pleura to lungs, 362. Costal 
 respiration, 363. Diaphragmatic or abdominal lespiration, 364. Cough- 
 ing, laughing, etc., 365. 
 
 Comparative Morphology of Vertebrate Respiration. — .\fammals, 
 birds, reptilt's, 2(>(). Tortoise, amnhibians, 367. Gill ropiration, f sites, 
 teleosts, 368. Meclianics of gill-breathing, 369. Variation in gills of 
 fishes, sharks, lampreys. 370. Classification of fishes by respiratory or- 
 gans, 372. Transition from gill breathing to lung breathing, 373. 
 Classification of amphibians, 374. 
 
XVI PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 SECTION III. 
 Blood Circulation — Vertebrates. 
 
 Circulation in man, 375. General course of, 376. Diagrams illus- 
 trating, 377, 378. Structure of the heart : valves, 380. Blood vessels : 
 (i) Arteries, (2) veins, 382 ; {3) capillaries, 383. 
 
 Comparative Alorphology of Blood System in Vertebrates. — Mam- 
 mals, birds, reptiles and amphibians, 385. Diagram of the course, 386. 
 Fishes, diagram of, 388. 
 
 Bearing of these Facts on Evolution. — (i) Heart structure, {2) aor- 
 tic arches and their relation to gill arches, 390. Diagram of origin of 
 aortic arches in birds, 394. In mammals, 395. Illustration of a funda- 
 mental law of evolution, 396. 
 
 SECTION IV. 
 Morphology of Respiratory and Circulatory System in Invertebrates. 
 
 Introductory, 397. Crustacea, respiratory organs of, mode of breath- 
 ing, 399. Circulation, diagram illustrating, 400. Mollusca : Acephala, 
 respiration, 401 ; circulation, 402 ; gastropods, 403 ; cephalopoda, 404. 
 Echinoderms, 404. Ccelenterata, protozoa, 406. Insects, why passed 
 over, 407. Blood system, 408. Respiratory system, 409. Air tubes, 
 410. Breathing, 411. 
 
 SECTION V. 
 
 Lymphatic or Absorbent System. 
 
 General description, distribution, 411. Structure, function, lym- 
 phatic glands, 413. Function, comparative morphology of lymphatic 
 system, 414. 
 
 CHAPTER IV. 
 
 KATABOLISM. 
 
 SECTION I. 
 Introductory, 415. Secretion vs. excretion, 416. 
 
 SECTION II. 
 
 Function of Respiration. 
 
 (i) Chemistry of respiration. 418. (2) Purpose of combustion, (3) 
 the fuels, 419. (4) How is force created? 420. Illustrative diagram, 
 422. Place of the combustion, 423. Relation of plants to animals in 
 the creation of animal force, 424. , 
 
CONTENTS. XVll 
 
 SECTION' III. 
 Kidneys and their Function. 
 
 Place and Form, 425. E.\cretory duct, 426. Pelvis of kidney, sec- 
 tion, minute structure, 427. Function, composition of urine, 429. 
 Comparison of kidneys and lungs, 430. Diagrams illustrating circula- 
 ti )n of C and O, and of circulation of elements of organic matter, 432. 
 Comparative Alorpholoi;}' of Kidneys : .Mammals, birds, reptiles, 433. 
 Insects, Crustacea, mollusca, 434. 
 
 SECTION IV. 
 The Skin and its Function. 
 
 Function, 435. Exhalation, excretion, structure of skin, 436. Su- 
 dorific glands, 437. Lungs, kidneys, and skin compared, 438. 
 
 Compa^-ative Morphology and Physiology of Skin. — General remarks, 
 438. Mammals, birds, reptiles, amphibians, ti.shes, 439. Insects, 
 crustaceans, mollusca, echinoderms, cct-lenterates, 440. 
 
 SECTION V 
 
 The Liver and its Function. 
 
 Position and structure of liver, 440. Its four systems of tubes, 441. 
 Threefold function, 442. Glycogeny, 443. Proof of, 444. Origin of 
 glycogen threefold, 445. The use of glycogen as fuel, 447. Diagram 
 illustrating the process of change, 448. Cause of diabetes, 449. Com- 
 parative morphology of liver, 450. 
 
 CHAPTER V. 
 
 TEGUMENTARY ORGANS — SKIN STRICTURES. 
 
 SECTION I. 
 
 Vertebrates. 
 
 Structure of vertebrate skin, various changes in epidermis, 452. 
 Importance in classification, hair, 453 Nails, claws, 454. Hoofs, 
 horns, 455. Feathers, 456. Structure of featliers, adaptation to flight, 
 457. Mode of formation of feathers, gradation to hairs, 458. Scales, 
 459. Classification of fishes by scales, 460. Reptile scales, rattle of 
 rattlesnake, 461. Turtle shell, 462. Mammalian shell. Endo.skeleton 
 and exoskeleton, 463. 
 
 SECTION II. 
 
 Invertebrates. 
 
 Insect shell, 463. Higher crustaceans, 464. Mollusca : Bivalves, 
 growth of shell, 464 ; gastropod, cephalopod, classification of mollusca 
 by shell, 465. Echinoderms, structure of the shell, 466. 
 
 Corals, 466. Structure of, 467. Structure and mode of formation 
 of the theca, 46S. Sponge, skeleton of, 469. Rhizopod shell, 470. 
 2 
 
XVill PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 CHAPTER VI. 
 
 GEOGRAPHICAL DISTRIBUTION OF ORGANISMS. 
 
 Definition of fauna and flora, 471. Illustration of harmonic rela- 
 tions of faunas, botanical temperature reL:ions, 472. Zoological tem- 
 perature regions, completer definition of temperature regions, range of 
 species, genera, etc., 474. Mode of grading of contiguous ranges, 475. 
 Effect of barriers, 475. 
 
 Continental iaunal regions, species usually distinct, 477. Excep- 
 tions, 478. Subdivisions of continental faunas, 479. Special cases : 
 (i) Australia, 479; (2) Madagascar, 480; (3) Galapagos, 481. River 
 mussels, 481. 
 
 Mai'ine Faunas. — Temperature regions of east coast of United 
 States, 481. Shore faunas, pelagic fauna, abyssal faunas, special cases, 
 482. 
 
 Primary Division of Land Faunas. — Wallace's divisions, 483. 
 Schedule of regions and provinces, subdivisions of the Nearctic, 484. 
 
 Theories of the Origin of Distribution of Organisms. — Old theory, 
 485. New theory, 487. Examples of explanation by new theory : 
 (l) Alpine species, (2) Australia, 4S8 ; (3) Africa, 489. Islands, kinds 
 of, (4) Madagascar, 490 ; (5) British Isles, (6) California coast islands, 
 491 ; (7) oceanic islands, 492. 
 
OUTLINES OF COMPARATIVE PHYSIOLOGY 
 AND MORPHOLOGY OF ANIMALS. 
 
 INTRODUCTORY. 
 
 SOME GENERAL PRINCIPLES. 
 SECTION I. 
 
 RELATIONS OF THE THREE KINGDOMS OF NATURE TO 
 ONE ANOTHER. 
 
 Nature is primarily divided into two l<ingdoms, 
 the living and the nonliving. The living kingdom is 
 subdivided into the animal and the plant kingdoms, 
 thus : 
 
 _ . . \ 3. Animal. 
 
 Living - „, 
 
 * / 2. Plant. 
 
 Nonliving 1 . Mineral. 
 
 We may, indeed, divide Nature into three kingdoms 
 — mineral, plant, and animal — as seen above; but, if so, 
 it is necessary to remember that the gap between the 
 organized and the unorganized, the living and the non- 
 living, is far greater than between the two divisions of 
 the living kingdom, i. e., between plants and animals. 
 The study of unorganized or nonliving Nature belongs 
 to physics and chemistry ; the study of organized or 
 living Nature belongs to biology. 
 
 Living vs. Nonliving. — Besides the essential proper- 
 ties belonging to all matter, living things have certain 
 
 I 
 
2 niVSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 additional and distinctive properties. These are all 
 connected with the endowment of life. But we know 
 nothing of life except by its properties. The same, 
 however, is true of all other forces of Nature. We 
 know not what they are, but only what they do, how 
 they behave. Life, like all other forces of Nature, is a 
 specific form of energy c\\7i.x'6.c:\.&x\zit'\ by a peculiar group 
 of phenomena, the subject-matter of a distinct science 
 — biology. What, then, are the peculiar phenomena 
 of life ? 
 
 1. Organization. — All living things are organized — 
 i. e., consist of different parts having different functions 
 and all co-operating for one given end, the conserva- 
 tion of the organism. It may at first seem that by this 
 definition a machine, say a steam engine, is living; but 
 there is this essential difference: the steam engine re- 
 quires an external force determining the co-operation, 
 while in living things the co-operation is self-determined. 
 
 2. Cellular Structure. — All living things are wholly 
 made up of cells, as completely as a brick building is 
 made up of bricks. Now there is an unorganized cellular 
 structure also — such, for example, as in foam, in soap- 
 suds, in vesicular lava, etc. But there is again this es- 
 sential difference : in unorganized cellular structure 
 there is simply a homogeneous mass full of hollow 
 spaces, while, on the contrary, in organized cellular 
 structure each cell is a living entity — the organized body 
 is a community of living cells. 
 
 3. Growth. — All living things grow. But do not 
 nonliving things grow also ? Take a saturated solution 
 of sugar, or alum, or salt. By evaporation crystals are 
 formed. Small at first, they grow from hour to hour, 
 from day to day, until they become as large as a walnut 
 or even as one's fist. But again, we find essential dif- 
 ferences in the kind of growth : i. The growth of the 
 
RELATIONS OK THE ITIREE KINCJDOMS. 
 
 3 
 
 crystal is by additions to the outside, layer by layer; 
 the growth of the living thing is by material taken into 
 every part of each cell — interstitial growth. 2. Again, 
 the growth of the crystal is by materials exactly like 
 itself already existing in the liquid, not made by the 
 crystal ; whereas the living thing makes the material of 
 its growth ; it manufactures material like itself out of mate- 
 rial wholly different from itself, and then uses it for groicth 
 — growth by assimilation. 
 
 4. Life History. — All living things pass through a 
 regular cycle of changes determined by forces within 
 itself. They are born, increase, culminate, decay, and 
 finally die. No such cycle of changes is observed in 
 nonliving things. Whatever changes they undergo are 
 accidental and the result of external causes. 
 
 5. Reproduction. — As living things have a definite 
 term of existence, they must reproduce their kind; 
 otherwise the organic kingdom would speedily pass 
 out of existence. This also is characteristic of living 
 things. 
 
 6. Waste and Supply — Metabolism.— Continual 
 change of the material composing the body in every 
 part is very characteristic of living things. The living 
 body has been compared to a whirlpool : the matter is 
 continually changing w^hile the form remains. In the 
 living body the change is in some sense self-determined. 
 This ceaseless internal change by waste and supply is 
 called metabolism. It is rapid in proportion as the life 
 is high. 
 
 We are concerned here only with living things. All 
 dead things and the sciences which concern them are 
 therefore put aside; but we must still limit our field. 
 We are not concerned with all living things, but only 
 with animals. We must therefore distinguish between 
 animals and plants. 
 
PHYSIOLO(;V AND MORPHOLOGY OF ANIMALS. 
 
 Fig. r. — Diagfram represent- 
 ing the differentiation of 
 animals and plants from 
 Protista. 
 
 Animals vs. Platits. — The distinctions between ani- 
 mals and plants are apparently so obvious that it may 
 seem useless to draw attention to them, but it is so only 
 to careless view and in compar- 
 ing the higher members of the 
 two kingdoms. As we descend 
 in the scale the two kingdoms 
 approach more and more, until 
 they absolutely come together — 
 in other words, the living king- 
 dom in its lowest members 
 consists of beings which are 
 both animals and plants, or else 
 neither. They are living things 
 7<.nthoiit further qualification. In 
 the present state of our knowl- 
 edge they may be claimed by either botany or zoology, 
 and it is proposed to call them Protista, or lowest living 
 beings. From these lowest beginnings the two kingdoms 
 separate more and more as we rise. Where they first 
 separate we call them Protozoa (first or lowest animals) 
 and Protophyta (first or lowest plants). Then follow the 
 more distinctive animals and plants, but the animals rise 
 the higher, as in Fig. i. 
 
 It is not so easy, then, to define the limits of the 
 animal kingdom. Popularly, perhaps, animals would be 
 defined as beings capable of motion; but this will not do, 
 for many plants also move under stimulus, as, for exam- 
 ple, the sensitive plant. Or perhaps locomotion is sup- 
 posed to be characteristic of animals ; but this also 
 fails, because many of the lower plants and the embryos 
 of some higher ones move about with such rapidity 
 that they can hardly be observed carefully under the 
 microscope. On the other hand, many animals some- 
 what high in the scale, such as ovsters, corals, etc., 
 
RELATIONS OF THE THREE KINGDOMS. 
 
 are incapable of locomotion. What, then, are the dif- 
 ferences ? 
 
 1. Sensation and Volition. — Doubtless conscious 
 sensation and voluntary motion are characteristic of 
 animals, but the difficulty is in applying the test. We 
 conclude, and probably rightly, that the motions of 
 plants are unconscious and involuntary. In the case 
 of any motion, if we could be certain that it was at- 
 tended with consciousness, we would rightly conclude 
 that the moving thing was an animal. But how are we 
 to know? Very many of the motions of animals, and 
 even motions within our own bodies, such as motions of 
 the heart, stomach, etc., are wholly unconscious and in- 
 voluntary. 
 
 2. Nature of the Food and the Relation to the 
 Mineral Kingdom. — Plants feed on the mineral king- 
 dom directly ; animals feed on plants or on one another. 
 The food of plants is mineral matter ; the food of animals 
 is organic matter made by plants. More specifically, 
 the food of plants is COg.HgO and XH3. These purely 
 mineral matters are taken 
 
 and decomposed. Some a*'""^/'?*''-? 
 
 parts are thrown away, and 
 the remainder are made 
 to combine into new sub- 
 stances not made elsewhere 
 — i. e., organic substances, 
 such as starch, sugar, cellu- 
 lose, and especially proto- 
 plasm. Thus all the or- 
 ganic substance in the world 
 is created by plants, under 
 the influence of sunlight. Animals, so far from creating, 
 are constantly destroying organic matter and resolving 
 it into its original components. Thus the relations of 
 
 <^f^r;;^i 
 
 Fig. 2. — Diagfrain illustrating: the 
 circulation of carbon and oxygen. 
 
(', I'HVSIULOGY AND MOUrilOLOGV OF ANliVlALS, 
 
 the two living kingdoms to the mineral kingdom aie the 
 converse of one another: plants making organic matter 
 from minerals, and animals destroying organic matter 
 and returning it again to the mineral kingdom. Limit- 
 ing our view now to one of these mineral plant-foods, 
 COo-. plants decompose COo, returning the oxygen to 
 the air and retaining the carbon to make with other ele- 
 ments organic matter; animals contrarily take carbon 
 from plants in tlTe form of organic matter of food, and 
 oxygen from the air in respiration, combine these, and 
 restore them to the air as COg. Thus there is a con- 
 tinual circulation of carbon and oxygen between these 
 three kingdoms, as shown in the diagram. Fig. 2. Thus 
 the plant kingdom is a necessary intermediary between 
 the mineral and the animal kingdoms. 
 
 This is probably the best and most philosophical dis- 
 tinction between the two kingdoms ; but even in this 
 there is a gradation as we go down the scale of life. In 
 any case we want an easier and more practical test. We 
 find this in 
 
 3. The Possession of a Stomach. — Animals have 
 stomachs, plants have not. This is really a philosoph- 
 ical distinction, because it is closely connected with the 
 nature of the food. The food of plants is mineral. 
 This mineral food exists in solution in water, or else in a 
 gaseous state in the atmosphere.. It is therefore already 
 in condition to be at once absorbed without further 
 preparation. It is thus absorbed by the surface of the 
 roots and of the leaves — external absorption. But the 
 food of animals, being organic matter, is usually in a 
 more or less solid condition, and can not be absorbed 
 until it is dissolved; and this requires time. Therefore 
 animals must have a reservoir in which the food is 
 stored until it is reduced to a liquid condition fit for 
 absorption — internal absorption. This reservoir is the 
 
RELATIONS OF THE THREE KINGDOMS. 7 
 
 Stomach. All animals therefore must have a stomach. 
 In the very lowest animals, however, this organ is ex- 
 temporized for use when wanted. A single cell, an al- 
 most microscopic spherule of gelatinous protoplasm, 
 meets its food, flows around it, takes it in, and di- 
 gests it (Fig. 156, p. 240). 
 
 Again, the food of plants is everyic/iere present. In 
 the form of solution it bathes the roots, in the form of 
 gases it bathes the surface of the leaves. The food- 
 taking is passive. Animals seek their food, and usually, 
 but not always, move about to gather it. This, again, re- 
 quires a reservoir in which to keep it while it is being 
 prepared for absorption. Both the kind of food and the 
 mode of taking it require a stomach. All these four — 
 viz., kind 0/ food, the possession of a stomach, power of 
 voluntary motion, and the seeking of food or desire — are 
 closely connected with one another; and the kind of 
 food — i. e., organic matter — is the basis of all. For this 
 necessitates appetite, therefore seeking of food, and this 
 locomotion and a reservoir to store ; therefore all are 
 characteristic of animals. 
 
 4. Waste and Supply. — Continual internal change, 
 as already seen, is coextensive with life. But this internal 
 change is far more rapid in animals than in plants. In 
 plants, supply is always in excess of waste, and therefore 
 plants grow as long as they live. In animals, on the 
 contrary, in early life supply is in excess of waste, in 
 maturity they balance and there is no growth, in age 
 waste is in excess. Again, as we shall see hereafter, the 
 whole of animal force is derived from waste, while in 
 plants only a small part is thus derived, the rest being 
 derived from sunlight. 
 
 We have now delimited our field of study from other 
 kingdoms of Nature — it is animals ; and our science from 
 other departments of science — it is zoology. But the 
 
8 PHYSIOLOGY AND MORPHOLOGY OF ANLMALS. 
 
 field of zoology is far too extensive; we can occupy but 
 a small part. 
 
 SECTION II. 
 
 DEFINITION OF ZOOLOGY AND THE SCOPE AND LIMITS 
 OF THIS COURSE. 
 
 Zoology is the science of animals. It embraces 
 every scientific question that may be raised concerning 
 animals — their form, structure, functions, habits, their 
 affinities, their distribution in time geologically and in 
 space geographically. The subject of this course is only 
 a part of zoology. 
 
 The most fundamental divisions of zoology may be 
 clearly brought out by considering an animal in differ- 
 ent ways. We may study the external form of the 
 whole and of each part; then cut into and dissect and 
 determine the form and structure of the internal organs 
 as far as the naked eye can see. Then with the micro- 
 scope we determine the minute structure of every tis- 
 sue and organ. All of this is anato?n}\ although the 
 naked-eye anatomy is often called morphology, and the 
 minute anatomy histology. All this can best be studied 
 in the dead animal. 
 
 Or, again, we may study the functions of the living 
 animal — i. e., the work that each part or organ performs, 
 and the manner in which they all co-operate for the 
 common life and happiness of the animal. This is 
 called physiology, and can be studied only in the living 
 animal. 
 
 But, again, we may study not only the living mature 
 animal, but the living growing animal. Commencing 
 with the yet, apparently, unorganized t%%, we may trace 
 
DEFINITION OF ZOOLOGY. g 
 
 the gradual development of each part and of the func- 
 tion which it performs to its completion in the mature 
 condition. This is embryology. 
 
 Now, if there were but one kind of animal in the 
 world, the study of that species from these three points of 
 view would still be inexhaustible. But the field of study 
 becomes far wider when we remember that there are an 
 almost infinite variety of kinds, of every degree of com- 
 plexity of organization, and that it is only or chiefly by 
 comparison of these kinds with one another that gen- 
 eral law's are reached. Thus each of these departments 
 must be made comparative before it can become truly sci- 
 entific. Thus, then, the three fundamental departments 
 of zoology thus far found are (i) comparative anatomy, 
 (2) comparative physiology, and (3) comparative embryology. 
 
 But the number and variety of animals is so great, 
 the material of science is so immense, that it is wholly 
 unmanageable and even bewildering unless arranged 
 and classified in an orderly way. Therefore animals 
 are divided and subdivided into groups, according to 
 their affinities or their differences and resemblances ; 
 those of the larger groups united with one another by 
 the most general characters, and separated from other 
 groups by the most profound differences, the smaller 
 groups being united by smaller but more numerous re- 
 semblances, and separated by less profound differences. 
 This is called (4) taxonomy, or classification. 
 
 But such orderly arrangement can not be made with- 
 out extensive knowledge of animals in all parts of the 
 world, such as no one man can individually acquire. 
 Therefore it is necessary to describe all the kinds of ani- 
 mals, each in its proper place in the orderly arrange- 
 ment. But this can be done only by the co-operation of 
 all zoologists in all parts of the world, and the publica- 
 tion of these results, so that each can use the work of 
 
lO PHYSIOLOGV AND MORPHOLOGV OF ANIMALS, 
 
 all. This is called (5) descriptive zoology. This consti- 
 tutes, along with taxonomy, systematic zoology. 
 
 But the scope of the subject is not yet exhausted. 
 The earth has been inhabited for millions of years, and 
 the animal forms have been changing during all this time 
 according to certain definite laws. The study of extinct 
 forms, and especially the laws of succession of forms, 
 constitutes another department, called {6) palceozoology. 
 
 Finally, the animals inhabiting different parts of the 
 earth are very different from one another. The causes 
 of these differences and their laws constitute another 
 absorbingly interesting department, for which no uni- 
 versally accepted name has yet been proposed. It has 
 been called chorology, but is usually spoken of as geo- 
 graphical distribution of animals, or (7) geographical zoology. 
 
 Thus, then, the main departments of zoology are: 
 
 1. Comparative anatomy or morphology. 
 
 2. Comparative physiology. 
 
 3. Comparative embryology. 
 
 4. Taxonomy, or classification 
 
 5. Descriptive zoology 
 
 6. Falfeozoology. 
 
 7. Geographical zoology. 
 
 Each one of these departments constitutes a field of 
 study sufficient to occupy the lifetime of any one. It 
 is evident, then, that we can not take up all these with 
 equal fullness. Instruction spread over so wide a field 
 must be far too meager. We select, then, as our cen- 
 tral subject the second one, comparative physiology of ani- 
 mals. For this, which deals with the phenomena of ani- 
 mal life, is certainly the department to which all others 
 are tributary. But it is impossible to understand function 
 without a knowledge of structure, with which it is as- 
 sociated. Therefore our subject will be physiology, and 
 so much anatomy or morphology as is necessary to ex- 
 
 (. systematic. 
 
GENERAL STRUCTURE OF ANIMALS. j i 
 
 plain the physiology, and often something more when 
 the morphology has an important bearing on classifica- 
 tion, and especially when it has an important bearing 
 on the question of evolution. Embryology we will touch 
 on only when it bears in an important way on the same 
 two subjects. Classification we shall not touch at all 
 except in the indirect way explained above. Some 
 scheme of classification, however, we must, of course, 
 assume as the necessary condition of study, for we can 
 deal with animals only /// groups. But this we put off 
 until we must have it. Lastly, if we find time we will 
 devote some pages to the laws of geographical distri- 
 bution of animals, as this has a most important bearing 
 on the question of evolution. 
 
 So much to define the scope and limits of our course. 
 It is limited (i) to the science of life, biology : (2) to 
 the science of animal life, zoology ; (3) in zoology it is 
 limited to comparative physiology mainly, but not entirely, 
 for function is indissolubly united with structure and 
 form. It may therefore be called comparative physiology 
 and morphology. 
 
 SECTION III. 
 
 GENERAL STRUCTURE OF ANIMALS. 
 
 We have already said that all or- 
 ganisms are composed of living cells. 
 A living cell consists of three parts, 
 viz., (i) a mass of semiliquid proto- 
 plasm, usually granulated, (2) a nu- 
 cleus of more solid matter, and (3) 
 a thin delimiting membrane (Fig. 3). 
 Cellular structure is coextensive with life, but the cells 
 of animals differ considerably from those of plants, and 
 are far less distinct for the'following reasons: 
 
12 PHYSTOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 1. Size. — Cells are nearly always microscopic in 
 size, but are far more minute in animals than in plants. 
 
 2. Softness. — In plants each cell is incased in a 
 firm shell of cellulose, so that in thin sections, such as 
 are used in microscopic examination, they retain their 
 form perfectly; while in animals, unless specially pre- 
 pared, they collapse and lose their cellular appearance. 
 
 3. Transparency. — The cells of animals, unless 
 specially prepared by staining, are so transparent that 
 their outlines are often detectable with difficulty. This 
 is far less true in plants. 
 
 4. More highly Differentiated. — But with proper 
 care all these difficulties may be overcome. The real 
 greatest difficulty is the differentiation of cell form, 
 which is much greater in animals than in plants. Cells 
 take on different forms in order to perform different 
 functions. Therefore as functions increase in number 
 and become more perfect the cells, take on more numer- 
 ous forms, and the forms differ more and more from one 
 another, and all from simple undifferentiated cells. Now 
 functions are more numerous and higher in animals than 
 in plants, and therefore the structure of animals, espe- 
 cially the higher animals, differs greatly from simple cellu- 
 lar structure ; so that in the mature condition of the high- 
 er animals the simple cellular structure may be entirely 
 lost. The universal cellular structure of animals there- 
 fore is best seen in the lowest animals and in the embry- 
 onic condition of the higher. 
 
 TISSUES. 
 
 A tissue may be defined as an aggregate of cells of 
 the same form and having the same function^ but differing 
 in form and function from the cells of other aggrega- 
 tions; different tissues therefore are different styles of cell 
 structure, each adapted to a peculiar function. The ani- 
 
GENERAL STRUCTURE OF ANIMALS. 
 
 13 
 
 mal body is made up of cells as completely as a brick 
 building is made up of bricks; but as we may have 
 different kinds of brickwork adapted for various pur- 
 poses, so we have various kinds of cell work, and these 
 constitute the different tissues. I'he kinds of tissues are 
 more numerous and more varied as the functions are 
 more numerous and higher, and consequently as we rise 
 in the scale of organization. Therefore they are more 
 numerous and varied in animals than in plants, and in 
 the higher than in the lower animals. It would be use- 
 less and confusing to speak of all the kinds of tissues 
 treated in special works on his- 
 tology. We shall treat of six 
 general kinds, although some of 
 these will be subdivided. 
 
 I. Connective. — This con- 
 sists of transparent white inter- 
 laced fibers or bands of fibers 
 running in all directions, forming 
 sometimes a loose mesh, some- 
 times a closely felted structure, in fig. 4.— Connective tissue. 
 which are found scattered spindle- 
 shaped nucleated cells with continuing branching fibers 
 (Fig. 4). It is called connective because it penetrates 
 and supports — it connects, and yet separates, all the 
 other tissues and organs of the body, forming a sort of 
 universal warp in which all other tissues are woven as a 
 woof determining the pattern of the fabric; so that if all 
 other tissues could be picked out and removed and this 
 one only remained, the whole form of the body with all 
 its organs would be retained in the connective tissue. 
 
 Examples. — In skinning the dead body of an animal, 
 as we pull the skin from the underlying flesh we observe 
 a white shining mesh connecting them. This is divided 
 with the knife in the act of skinning. This is the subcu- 
 
H 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS 
 
 taneous connective. It is so loose a mesh that water or 
 air may be pumped in so as to swell up the whole body. 
 General dropsy, in fact, arises from accumulation of 
 water in this tissue. When the skin is removed we may 
 dissect between and separate the several muscles. In 
 doing so we cut the connective between the muscles. 
 The same tissue in a denser form constitutes the invest- 
 ing membrane of the muscle. If we cut into the muscle 
 we find that every fiber is invested by the membranous 
 form, and separated from its neighbor fiber by the mesh 
 form of the same. In brief, we may say that this 
 tissue in its denser form invests every fiber, and in its 
 loose form separates and yet connects the fibers ; then 
 it emerges on the surface of the muscle in denser form, 
 investing and individuating it ; 
 and finally in loose form lies be- 
 tween the muscles, separating and 
 yet connecting them. The same 
 is true of all the organs of the 
 body. 
 
 Varieties. — The loose, meshlike 
 form is called areolar tissue, or 
 sometimes cellular tissue. The 
 denser form is called fibrous tissue. 
 By a little stretch of our defini- 
 tion, the skin, which consists of 
 closely felted, interlacing fibers, 
 may be regarded as an extreme 
 variety of connective. It is called 
 dermoid tissue. Scattered about 
 among the interlacing fibers, es- 
 pecially of the areolar variety, are found nucleated, 
 spindle-shaped cells with branching ^h^x^ —connective-tissue 
 cells. Also, the same variety is usually the place of de- 
 posit of so-called fat cells, which accumulate often in 
 
 Fig. 5. — Connective tissue 
 with fat cells. 
 
GENERAL STRUCTURE OF ANIMALS. 
 
 i; 
 
 Fig. 6. — Structure of cartilage. 
 
 large quantities (Fig. 5). This is sometimes spoken of 
 as adipose tissue, but it is not properly a tissue at all. 
 
 2. Cartilage. — This will be easily recognized under 
 its popular XidiVa^t, gristle. \\% firmness din<\ yet elasticity, 
 its white translucency, its smooth homogeneous surface 
 when cut, are familiar and 
 characteristic properties. If 
 a thin section be placed under 
 the microscope, it is at once 
 seen to consist of innumer- 
 able nucleated cells lying in 
 a structureless, semitranspar- 
 ent, hyaline mass (Fig. 6). The 
 cells are in clusters, evidently 
 formed by cell division. 
 
 Cartilage is the tissue used 
 in the animal body whenever a moderate degree of firm- 
 ness combined with elasticity is required. It therefore 
 caps the ends of the bones at the joints. The anterior 
 portions of the ribs are cartilage, so as to yield to respi- 
 ratory motions. The external ear consists of cartilage, 
 so as to retain its form and yet to be not liable to break. 
 The tip of the nose is of the same substance, and for the 
 same reason. 
 
 Varieties. — In higher animals there are two varieties, 
 viz., permanent cartilage, such as all those alreadv 
 mentioned, and temporary cartilage, which afterward 
 becomes bone. Hence cartilage is very abundant in 
 young animals. But the difference between these varie- 
 ties is too unimportant to detain us. 
 
 3. Bony Tissue. — Bone is the hardest tissue in the 
 body and is used wherever rigidity is required. It is 
 therefore in higher animals the material of the skeleton. 
 It consists of an organic tissue, a kind of connective, 
 hardened by a deposit in it of mineral matter, chiefly 
 
l6 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 lime phosphate. These two parts may be easily sepa- 
 rated. If bone be thoroughly burned, the organic mat- 
 ter is consumed and the white lime phosphate remains, 
 retaining the form, the structure, and the stiffness of 
 the bone. On the other hand, if a bone be immersed in 
 a weak acid — HCl — for several days, the lime phosphate 
 will be dissolved, leaving the organic tissue, also re- 
 taining the form and structure but not the stiffness of 
 the bone. It may now be tied in a knot. 
 
 Structure. — Looking closely on the surface of bone 
 one can see with the naked eye long channels, and on 
 
 
 Fig. 7.— Structure of bone : A, lonp:itudinal ; B, cross-section ; h. Haver- 
 sian canals. 
 
 cross-section pores like those seen in wood. In this re- 
 spect it differs from ivory, which has no such pores. 
 These are the Haversian canals (Fig. 7, Ji). They are, in 
 fact, blood vessels of the bone. This is as much as can 
 be seen with the naked eye. Under the microscope, in 
 addition, we see that the bony matter is arranged in con- 
 centric circles or cylinders about the canals, and that 
 
GENERAL STRUCTURE OF ANIMALS. 
 
 17 
 
 scattered numerously through the bony matter are black 
 cavities. These are the lacuiuc. Running from these in 
 all directions, like the legs of an insect, are very slender 
 tubes connecting the lacunae with one another and with 
 the canals. These are the canalicles (canaliculi). The 
 red blood circulates freely through the canals, but the 
 blood globules are too large to pass the 
 canalicles. Therefore only colorless blood 
 plasma reaches the lacunns. 
 
 As already said, bone is the material 
 of the skeleton of higher vertebrates, but 
 as we pass backward in the embryonic 
 series, or down in the animal series, car- 
 tilage replaces bone. In other words, 
 cartilage is the embryonic or imperfect 
 form of bone. 
 
 Origin. — Bone comes usually from car- 
 tilage by deposit of mineral matter ; but 
 bone may also be formed by deposit of 
 mineral matter in other tissues, as in 
 fibrous membranes and in skin. So we 
 have cartilage bones, membrane bones, 
 and skin bones. 
 
 Varieties. — Dentine is a denser kind of bone in which 
 the canals are wanting. This is the principal material 
 of teeth. Ivory is the finest example. Enamel is a still 
 denser variety of bone which covers the teeth of higher 
 vertebrates (Fig. 8). 
 
 We must bear in mind the distinction between a 
 bone as an organ and l>one as a tissue or material of 
 which the organ is composed. 
 
 4. Muscle Tissue. — We must also distinguish here 
 between a muscle as an organ and muscle as a tissue. A 
 muscle is an organ consisting of several tissues — for ex- 
 ample, connective, nervous tissue, etc. — but its charac- 
 
 FiG. 8. 
 Section of tooth. 
 
1 8 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 teristic tissue is the muscular. The characteristic prop- 
 erty of this tissue is contractility under stimulus of any 
 kind, but, normally, under the stimulus of nerve force. 
 
 Structure. — To the naked eye muscle tissue consists 
 of bundles or fascicles of fibers lying parallel to one an- 
 other; but under the microscope each fascicle is seen to 
 be composed of a multitude of cylindrical fibers trans- 
 versely striated, and under favorable 
 conditions these again are separable C 
 
 into still finer fibrilla3. Each fiber is 
 
 Fig. 9. — A, muscular fibers of voluntary muscle ; B, one fiber broken to 
 show its investing- sheath ; C, cells of involuntai-y muscle. 
 
 invested with a thin membrane of connective tissue (Fig. 
 9). When a muscle contracts the fibers are observed to 
 shorten and thicken (Fig. 9, A, B, C). , 
 
 Varieties. — There are two kinds of muscle, voluntary 
 and involuntary. The fibers of the one are transversely 
 striated or minutely wrinkled; the fibers of the other 
 are not thus wrinkled (Fig. 9, C). 
 
 Of all tissues, muscle is that one in which the original 
 cell structure is most obscured by modification for func- 
 tion. In perfectly formed striated muscle there is no ap- 
 
GENERAL STRUCTURE OF ANIMALS. 
 
 19 
 
 /Wfl 
 
 pearance of cell structure visible; but its essential cell 
 structure is seen in the embryonic development of muscle. 
 Fig. 10 shows the fibers of 
 the muscle of the embryonic 
 lieart of a monkey. The for- 
 mation of fibers by fusion of 
 nucleated --cells is evident. 
 W'e shall discuss this again in 
 connection with the physiol- 
 ogy of muscle as an organ. 
 
 5. Nerve Tissue. — This 
 is the highest and most won- 
 derful tissue of the animal 
 body ; not, however, so much 
 in structure (for it is perhaps 
 less specialized than muscle), 
 but in its function, ^^'ith it, 
 in some way imperfectly un- 
 derstood, is connected the Fig. 10.— Muscular fibers of the 
 J- • • heart of the embr\'o monkev. 
 
 transmission of impressions After Kent. 
 
 from the external world to 
 
 the consciousness, and from the will back again to the 
 
 e.Kternal world. With it also is connected sensation, 
 
 consciousness, will, thought, and all the higher faculties 
 
 of the mind. 
 
 Varieties. — There are, again, two kinds of tissue in 
 nerve tissue, \\z., gray g?anulcir and white fibrous. The 
 gray granular consists of nucleated cells of different 
 forms and sizes, apparently connected with one another 
 by interlacing fibers (Fig. 11). The white fibrous con- 
 sists of very slender parallel fibers, of great length, con- 
 nected with gray-matter cells. The characteristic func- 
 tion of the gray granular matter is the origination of 
 nerve force; the function of the white fibrous matter 
 is the transmission of the same. The one mav be com- 
 
20 PHYSIOLOGY AND MORPHOLOGY OF ANLMALS. 
 
 -M 
 
 1 
 
 
 pared to battery cells generating, the other to the wires 
 transmitting, electric energy. The one is found only in 
 the ncn'e centers, such as the brain, the spinal cord, etc. ; 
 
 the other also and most char- 
 acteristically in the nerves 
 proper. We will discuss this 
 more fully in connection with 
 the physiology of the nervous 
 system. 
 
 6. Epithelial Tissue. — 
 The whole surface of the ani- 
 mal body, both external and 
 internal, is covered with a 
 pavement of living nucleated 
 cells. These are called epithe- 
 lial cells. In the higher ver- 
 tebrates those on the outer 
 surface dry up, become more 
 or less indurated, and are 
 called epidermal. Those on 
 the interior surfaces or mu- 
 
 I \ * Si 
 
 W ^ 
 
 I 
 
 \^\ 
 
 " f 
 
 Fig. II.— \eiticai section of a cous membranes always re- 
 
 convolution of the brain, show- . • ..' • c- „t' „ a- 
 
 ing the cells of the gra.; gran- ^ani tneir soft, active condi- 
 uiar matter (gg ) giving out tion. They are constantly 
 
 fibers which go to form the . . . 
 
 'white fibrous matter (w/j be- dying, dissolving and passing 
 
 low. After Luys. c ^, 
 
 away as mucus of the mucous 
 surfaces, and as constantly being born and the pavement 
 renewed. It is the least modified of all the tissues. 
 
 Function. — Their function is perhaps the most impor- 
 tant in the body, viz., the absorption of matter (food) 
 from the e.xternal world into the interior of the body, 
 and the elimination of waste matter from the body into 
 the external world. In other words, the whole exchange 
 of matter between the exterior and interior is carried on 
 through this tissue. 
 
GENERAL STRUCTURE OF ANIMALS. 
 
 21 
 
 Varieties. — This pavement of living cells may be of 
 different patterns; most usually the cells are somewhat 
 rounded (cobble-stone pavement). Sometimes they are 
 flat, polygonal, and fitted together, like a tesselated pave- 
 ment. Sometimes they are elongated and set on end 
 
 Fig. 12. — Different forms of epithelial cells : A, rounded ; B, flat tesselated ; 
 C, columnar ; D, ciliated. 
 
 (columnar epithelium), like wooden block pavements. 
 Sometimes these living cells are provided with cilicne, 
 which are in continual waving motion and determine 
 currents on the surface in definite directions (ciliated 
 epithelium) (Fig. 12, A, B, C, D). 
 
 For convenience of reference we give a schedule of 
 the principal kinds and their varieties : 
 
22 I'HVSIOLOGN' AND MUKl'IKJLuGV UF ANIMALS. 
 
 ^ Areolar. 
 
 1. Connective ■] Fibrous. 
 
 ' Dermoid. 
 
 ( Permanent. 
 
 2. Cartilaije -, ,,, 
 
 * ( lemporary. 
 
 Bone proper. 
 Bone -! Dentine. 
 
 ( Enamel 
 
 ( Striated. 
 
 4. Muscle -^ X T , . ^ , 
 
 I Nonstriated. 
 
 \ Gray granular. 
 
 ^' ' I White fibrous. 
 
 r Rounded. 
 
 . , ,. Tesselated. 
 
 6. Epithelium < „ , 
 
 ' Columnar. 
 
 I Ciliated. 
 
 Thus, then, there are composing the animal body six 
 different kinds of tissues with their varieties, each with 
 a different function, and all co-operating to produce one 
 end, viz., the conservation of the life and happiness of 
 the organism. This is the type and expression of or- 
 ganization, but it is realized only in the higher animals. 
 As we go down the scale either in the animal series or in 
 the embryonic series, one tissue after another disap- 
 pears, first bone, then cartilage, then muscle, then nerve, 
 until only unmodified cell aggregate remains, and still 
 lower only a single unmodified cell remains. The cor- 
 responding functions merge into one another, and at 
 the same time become less and less perfect, until every 
 part performs, but very imperfectly, a// the functions of 
 life. Or, taking the reverse order, which is the order of 
 evolution : first there is only one cell performing all the 
 necessary functions of life, but very imperfectly; next 
 an aggregate of unmodified and therefore similar cells 
 
GENERAL STRUCTURE OF ANIMALS. 23 
 
 all performing similar, i. e., all the functions, but imper- 
 fectly. Then the process of differentiation commences 
 and proceeds. Some cells take on a special form 
 adapted to the performance of a special function, say 
 contraction, and aggregate to form a tissue, muscle. 
 Other cells take other forms and aggregate to form 
 other tissues adapted to perform other characteristic 
 functions, until finally in the mature condition of the 
 highest animals each kind of cell performs but one func- 
 tion, but performs it very perfectly. Thus a muscular 
 fiber can do nothing else but contract. A nerve cell 
 gives no other sign of life but feeling, etc. This whole 
 process of modification of form and limitation and per- 
 fecting of function, or division of labor, is called the 
 law of 'differentiation. It is the most fundamental and 
 universal law of evolution. 
 
 Observe here — and the same is true of all differentia- 
 tions — two ideas which must be kept distinct in the mind, 
 viz., (i) identity of plan or community of origin — in this 
 case cellular structure — and (2) adaptiir modification for 
 various functions. 
 
 So much for tissues. But physiology is concerned 
 with functions, and functions are usually and properly 
 treated in connection with organs, such as muscle, brain, 
 gland, etc. The body consists primarily of organs. 
 Thus cells aggregate to form tissues, tissues aggregate 
 to form organs, and organs aggregated form the animal 
 body. 
 
 SECTION IV. 
 
 ORGANS .AND FUXCnOXS OF THE ANIM.AL Bcmv. 
 
 Classification of Functions. — The functions of 
 the animal body are of two general kinds, viz., functions 
 of animal life and functions of vegetative or organic life. 
 
24 PHYSIOLOGY AND MORPHOLOGY OF ANLMALS. 
 
 or, more briefl}-, animal functions and organic functions. 
 The animal functions are those which are distinctive of 
 animals. The organic functions are those which are 
 possessed in common by animals and plants, and are 
 therefore coextensive with life. Thus an animal may 
 be regarded as a plant with certain higher and distinc- 
 tive functions superadded. So also man may be re- 
 garded as an animal with certain higher and distinctive 
 functions or faculties superadded. As the idea of ani- 
 mal life is realized in proportion as the distinctive ani- 
 mal functions predominate over the organic, so also 
 the idea of human life is realized only in proportion as 
 the distinctive human faculties predominate and control 
 the animal. 
 
 Now these superadded distinctive animal functions 
 ure all concerned with conscious action and reaction be- 
 tween the external world and the organism. They are 
 therefore divisible into sensation and consciousness 
 (action) on the one hand, and will and voluntary mo- 
 tion (reaction) on the other. These are, however, very 
 closely related, being both connected with the nervous 
 system. The organic functions — viz., those common to 
 all life — are subdivided into two more widely separated 
 groups, viz., the nutritive and reproductive. The nutritive 
 functions are all that assemblage of functions which 
 co-operate for the conservation of the life of the indi- 
 vidual ; the reproductive., all that assemblage of functions 
 which co-operate for the conservation of the kind, even 
 though the individual must die. 
 
 . . , r Sensation and consciousness. 
 
 ( Animal. ... J 
 
 Functions ) ( ^^^'^ ''^"^ voluntary motion. 
 
 i r\ ■ ( Nutritive functions. 
 
 ' Organic. . . j 
 
 I Reproductive functions. 
 
 Order of Treatment. — Of these groups we shall take 
 
 up animal functions first, because in the higher animals 
 
GENERAL STRUCTURE OF ANIMALS. 25 
 
 these functions dominate the others. These will form 
 the subject of Part I. In Part II we shall treat the 
 first subdivision of the organic functions, viz., the nutri- 
 tive functions, or those which have to do with the con- 
 servation of individual life. There ought to be a Part 
 III, treating of all that assemblage of organs and func- 
 tions concerned with the conservation of the race or 
 species; but this is so vast a subject that it would re- 
 quire a separate treatise. 
 
PART I. 
 
 ORGANS AND FUNCTIONS OF ANIMAL LIFE. 
 
 These must be treated under four groups or systems 
 of organs, viz. : i. Nervous system. 2. Sense organs. 
 3. Muscular system. 4- Skeletal system. These are 
 closely connected in the performance of the functions of 
 animal life. The general way in which they co-operate 
 is shown in the diagram (Fig. 13). 
 
 Fig. 13. — Diagram showing essential parts of an apparatus of exchange be- 
 tween the external world and consciousness : NC, nerve center ; J'c, sen- 
 sory cell ; s/\ sensory fiber ; SS, sensory surface ; inc, motor cell ; tn/, 
 motor fiber ; M, muscle. Arrowheads show the direction of transmis- 
 
 We have (i) an impression on a sense organ, SS\ (2) 
 a transmission imvard along a sensory fiber to a nerve 
 center N.C., say the brain ; (3) a change of some kind 
 in a sensory cell, s.c, which awakens conscious sensation ; 
 (4) an influence of some kind transferred by a connect- 
 ing fiber to a motor cell, m.c. ; (5) an impulse transmitted 
 outward along a motor fiber, w./., to a muscle, J/, and de- 
 26 
 
ORGANS AND FUNCTIONS OF ANIMAL LIFE. 2/ 
 
 termining (6) muscular cuntraction and motion and 
 changes in the external world. The skeleton acts as 
 levers to make the motion more rapid, precise, and ef- 
 fective. Thus the sense organs may be regarded as re- 
 ceptive organs of sensation and consciousness, and the mus- 
 cles and skeleton as executive organs of the 7vill, and 
 the whole as an instrument of action and reaction be- 
 tween the external and the internal world. 
 
 Therefore the necessary parts of an instrument of 
 communication between the outer and the inner world 
 are (i) two kinds of cells in the nerve center, viz., a 
 sensory cell and a motor cell with connecting fiber be- 
 tween ; (2) two kinds of transmitting fibers, the one 
 sensory, transmitting inward, the other motor, trans- 
 mitting outward ; and (3) two kinds of nerve-fiber end- 
 ings, one in a sensitive surface or a sense organ, the 
 other in a contractile tissue or muscle. Each cell, sen- 
 sory or motor, with its fiber and its ending is called a 
 neurone or neurocyte. The connection between a sensory 
 and a motor neurone, until recently, was supposed to be 
 continuous and permanent, as represented in the figure ; 
 but now it is believed to be by contact of branching pro- 
 cesses, and perhaps only during stimulation. This will 
 be explained more fully hereafter. 
 
 Of the four systems mentioned as concerned in ani- 
 mal functions, viz., nervous system, sense organs, mus- 
 cles, and skeleton, the fundamental one is the nervous. 
 The others may be regarded as appendages of this one. 
 We therefore take this first. 
 
 Order of Treatment. — There are two modes or 
 orders of taking up the subject of comparative physiol- 
 ogy and morphology. We may begin with the lowest 
 and go up the scale ; this is the order of evolution. 
 Or we mav begin with man and pass down the scale. 
 If our subject were mainlv morphology the former 
 
28 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 might possibly be the best, although even then, it is per- 
 haps doubtful; but in physiology there can be no doubt 
 that the latter is best. It is so for two reasons: first, 
 because we take hold at once of the interest of the 
 student, and, second, because functions are fully sepa- 
 rated and declared and therefore intelligible only in the 
 higher animals. As we go down the scale functions are 
 more and more merged into one another, and therefore 
 more and more indistinct, until in the lowest animals 
 each part performs all the functions, but \n so imperfect 
 a way that it is impossible to understand them unless 
 we have already studied them in their separated and 
 perfect form in the higher animals. " In higher animals 
 the functions rise to the surface; in lower animals they 
 are deeply buried. We grope in vain unless we find the 
 key by the study of the higher animals." * 
 
 Our plan, then, will be to take up each system of 
 organs first in man; then, running down the scale of 
 vertebrates^ show the modifications and simplifications 
 which we find there. Then we shall take the other de- 
 partments of the animal kingdom and treat them in the 
 same way, but much more cursorily. 
 
 * Foster, Nature, vol. Ivi, p. 437, 1897. 
 
CHAPTER I. 
 
 THE NERVOUS SYSTEM OF MAN. 
 
 The nervous- system of man, and indeed of all ver- 
 tebrates, may be divided into two subsystems, viz., the 
 cerebrospinal dir\6. ihe ganglionic. Their relations to one 
 another are shown in a subsequent figure (Fig. 38, 
 page 68). We put aside for the present the ganglionic 
 system. 
 
 THE CEREBRO-SPIN.AL SYSTEM. 
 
 General Plan. — The general plan of structure of 
 this system in man, and indeed in all 
 vertebrates, is simply expressed as a 
 continuous tract or axis of gray matter 
 extending nearly the whole length of 
 the body, from which run off in pairs 
 bundles of fibers (nerves) going to every 
 part of the body, as shown in the dia- 
 gram (Fig. 14). In the lower verte- 
 brates there is very little more than this, 
 but in the higher vertebrates, and espe- 
 cially in man, this simple plan is ob- 
 scured by the enormous development of 
 the anterior part as a brain, as shown 
 in the dotted outline. This continuous 
 tract is called the cerebro-spinal axis. 
 
 The cerebro-spinal axis may be 
 again subdivided into the brain and the 
 spinal cord \ so that the subject of the 
 
 Fig. 14. — Diagjam 
 showing the pen- 
 era! plan of struc- 
 ture of the verte- 
 brate nervous syS' 
 tem. 
 
 29 
 
30 
 
 I'lIVSlOLOGV AM) .MORl'lIOLOGV OF ANIMALS. 
 
 cerebro-spinal system may be treated under three heads 
 of (i) the brain^ (2) the spinal cord, and (3) the nerves. 
 The brain and spinal cord are centers, the nerves are 
 conductors. The first two contain gray matter as well 
 as white, the last white matter alone. The first two are 
 generators of nerve energy, the fibers of the third are 
 transmitters only. The former may be likened to bat- 
 tery cells, the latter to conducting wires. 
 
 SECTION I. 
 Brain of Afati. 
 
 We can give only such general description as is neces- 
 sary to explain physiology. 
 
 Skull. — The brain is inclosed in a bony bo.x consist- 
 ing of many pieces fitted together by sutures with inter- 
 locking teeth. The growth of the skull to accommo- 
 date the growing brain takes place along these sutures. 
 The sutures finally consolidate and the brain can grow 
 no more. The age of consolidation is later in the 
 higher races, and is probably also later in educated 
 men.* 
 
 Membranes. — Take off the skull, and beneath we 
 see the brain still enveloped by its membranes. These 
 are (i) the dura mater, a thick, strong, fibrous membrane. 
 This invests the brain and dips in and separates all the 
 great divisions of the brain, but not the convolutions. 
 It carries also the large blood vessels of the brain. 
 Beneath and more closely investing the brain, passing 
 between not only the larger but also the smaller di- 
 visions, and even dipping down between the convolu- 
 tions of the surface and carrying the smaller blood ves- 
 sels which penetrate the substance of the brain itself, there 
 
 ^ Gahon, Nature, vol, xxxviii, p. 14, 1888, 
 
THE NERVOUS SYSTEM OF MAN. 
 
 31 
 
 is seen (2) a more delicate membrane called the />ta mater. 
 Between these and uniting them is still a third mem- 
 brane (3) the arachnoid. It is intlammation of these 
 membranes rather than of the brain itself that consti- 
 tutes the more acute forms of brain disease attended 
 with violent delirium. 
 
 Main Parts of the Brain, i. Cerebrum. — Take 
 out the brain from the skull and place it on the table 
 
 Fig. 15. — Cerebrum seen from above. 
 
 and remove the membranes.* Looking down on it from 
 above we see nothing but a great hemispherical irregu- 
 larly convoluted mass — the cerebrum — divided along 
 the middle into two equal halves. These are the right 
 
 * In the absence of brain an Auzoux model will serve an excel- 
 lent purpose. 
 4 
 
32 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 and left cerebral hemispheres (Fig. 15). The trench that 
 divides them is about two inches deep and occupied by 
 an extension of the membranes. The two hemispheres 
 are connected below by a band about half an inch thick 
 — the corpus callosum. The cerebrum constitutes about 
 four fifths of the whole brain. 
 
 2. Cerebellum. — Lifting the cerebrum from behind, 
 the next most important part brought into view imme- 
 diately beneath the 
 hinder part of the 
 cerebrum is the cere- 
 bellum. This is also 
 a double organ like 
 
 f ■ /i W^ ^^%^ f f /"^yf the cerebrum, but the 
 
 t^ \,^u^ ^^^^ ^^^JMk^^l^ two halves are not 
 
 so deeply divided by 
 the membranes. The 
 peculiar leafiike ar- 
 rangement of the 
 convolutions will be 
 observed. A side 
 view of the brain 
 (Fig. 16) shows the 
 cerebellum beneath the under part of the cerebrum. 
 
 3. Medulla and Pons. — Lying beneath the cere- 
 bellum, as if this latter had grown out of it, is an en- 
 larged continuation of the spinal cord within the skull. 
 This is called the medulla. Beneath this again, with 
 fibers running across the medulla and connecting the 
 two sides of the cerebellum, is l\\& pons Varolii (bridge of 
 Varolius). This can only be seen, however, by turning 
 the brain over so as to see the under side (Fig. 17). 
 
 4. Optic Lobes. — Lifting the cerebrum from behind 
 still higher and looking farther forward, the optic lobes 
 are bronirht into view in front of the cerebellum. In 
 
 Fig. 16. — Side view of the brain : cr, cere- 
 brum ; cb^ cerebellum ; ;«, medulla ; s, fis- 
 sure of Sylvius ; r, fissure of Rolando. 
 
THE NERVOUS SYSTEM OF MAN. 
 
 33 
 
 human anatomy these are called the corpora quadri- 
 gemina, because they consist of two pairs of rounded emi- 
 nences, but in comparative anatomy they are called the 
 
 __i> 
 
 Fig. 17. — View of brain from below : cr, cerebrum : cb, cerebellum ; m, 
 medulla ; /, pons, showing the origins of the nerves ; ol, olfactory, and 
 f/, the optic nerves. 
 
 optic lobes, because connected with the sense of sight. 
 They are small and inconspicuous in the human brain, 
 but in the lower vertebrates they may be larger even 
 than the cerebrum. 
 
 5. Thalamus. — Lifting the hinder part of the cere- 
 brum still higher and looking as far forward as possible, 
 we see two pairs of much larger rounded masses. These 
 are the thalamus (the first pair) and the corpus striatum 
 (the second pair). We shall often speak of these to- 
 gether as the thalamus (Fig. 18). 
 
 It would appear, then, but will become far more evi- 
 dent presently, that the spinal cord enters the skull 
 
34 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 and runs along its base, forming successive swellings 
 and outgrowths in its course forward. First the me- 
 dulla with its outgrowth, the cerebellum, then the optic 
 
 lobes with their four swell- 
 ings atop, then the thala- 
 mus with its four swellings, 
 and lastly the cerebrum; 
 but this last has grown 
 backward and covered all 
 the other parts and thus 
 obscured the real structure 
 of the brain. This will come 
 out more clearly presently. 
 Convolutions of the 
 Brain. — The surface of the 
 cerebrum is diversified with 
 irregular folds (convolu- 
 tions) with deep trenches 
 between. Into the trenches 
 enter the pia mater, but not 
 the dura mater. There are 
 a few larger trenches called 
 fissures dividing the cere- 
 bral hemispheres into lobes. 
 The most conspicuous of these is the fissure of Sylvius 
 (Fig. i6, s). It commences well forward, runs backward 
 and upward, separating the posterior lobe from the rest 
 of the cerebrum. Another is the fissure of Rolando (Fig. 
 i6, r), which separates the rest of the cerebrum into two 
 parts. By these two fissures the cerebrum is divided 
 into three lobes — anterior, middle, and posterior, or 
 frontal, parietal, and occipital. 
 
 The cerebellum is convoluted also, but in a more 
 regular way, being deeply separated into parallel lami- 
 nae or leaves. All the other parts are smooth. 
 
 Fig. i8. — A cross-section of the lifted 
 cerebrum : Longitudinal section 
 of the cerebellum, showing optic 
 lobes, ol \ thalamus and corpora 
 striata, th. After Dalton. 
 
THE NERVOUS SYSTEM OF .MAN. 
 
 35 
 
 Interior Structure. — So much may be seen without 
 cutting, but on making section we find at once in all 
 parts the two kinds of nerve matter already spoken of 
 (page 19), viz., the gray granular and the white fibrous; 
 but the relative positions of these two kinds are different 
 in the different parts. In the two largest parts, viz., the 
 cerebrum and cerebellum, the gray matter is on the out- 
 side and the white fibrous matter on the inside. In these 
 the surface gray matter follows all the inequalities of 
 the surface convolutions — in fact, it is evident that the 
 convolutions are a device to increase the quantity of 
 gray matter. In the case of the cerebellum the com- 
 
 FiG. ig.- — Section of cerebellum showing; arbor vitae. 
 
 plexity of the infoldings of the surface gray matter is so 
 great as to give rise on section to the peculiar appear- 
 ance called arbor vitcc (Fig. 19). 
 
 In all the other parts mentioned, viz., the medulla, 
 the optic lobes, and the thalamus, the gray matter is in 
 the center and the white matter on the outside. 
 
 Microscopic Structure. — As already explained 
 (pages 19 and 27), the gray matter consists of cells of 
 various sizes and shapes, giving out fibers, some connect- 
 ing with other cells, and some going to form the white 
 
36 
 
 PHYSIOLUGY AND MORPHOLOGY OF ANIMALS 
 
 fibrous matter (Fig. 20). The white fibrous matter seems 
 to be made up wholly of slender fibers which come from 
 the cells of the gray matter. We may imagine these 
 
 fibers coming from the sur- 
 face gray-matter cells, con- 
 verging to form the white 
 matter of the brain, and then 
 passing out of the skull to 
 form the spinal cord, to be 
 distributed everywhere. Or, 
 conversely, we may conceive 
 fibers of the spinal cord com- 
 ing into the brain and diverg- 
 ing to end in the cells of the 
 surface gray matter. Then, 
 last of all, these gray-matter 
 cells send out each of them 
 fibers which connect with 
 those of other gray-matter 
 cells. Now it is probable that 
 such a cell with all its fibers, 
 both those connecting with 
 other cells and the long fiber 
 (axis cylinder) connecting 
 with other parts of the body, 
 together constitute one indi- 
 vidual cell. Such an individual element of nerve matter 
 is called a neurone. On this view the brain, and indeed 
 the whole nerve system, may be regarded as naught else 
 than a collection of neurones intricately connected. The 
 fibers connecting neurones are not simple, but branching 
 (dendrites), and the connection is not continuous, but 
 by contact. They do not unite, but only touch fingers 
 or interlace dendrites (Fig. 21).* 
 
 * Professor Turner, British Association Address, 1897. Mathi- 
 as-Duval, Rev. Sci., voL ix, p. 321, 1898. 
 
 Fig. 20. — \ ertical section of a 
 convolution of the brain, show- 
 ing the cells of the gray gran- 
 ular matter (g g ) giving out 
 fibers which go to form the 
 white fibrous matter {w / ) be- 
 low. After Luys. 
 
THE NERVOUS SYSTEM OF MAN. 
 
 37 
 
 Embryonic Development of Brain. — The funda- 
 mental fact that the brain may be regarded as an inter- 
 cranial continuation of the 
 spinal cord, with swellings 
 and outgrowths atop, is 
 made evident by its em- 
 bryonic development. The 
 following figures give the 
 stages of this develop- 
 ment. In the very early 
 stages the brain is a direct 
 
 continuation of the spinal Fig. 21.— Diagram showing the inter- 
 lacing of dendrites of neurones. 
 
 cord and consists of three 
 
 hollow swellings or vesicles. These are what afterward 
 
 become medulla (i), optic lobes (2), and thalamus (3). 
 
 ^'^ 
 
 We shall call these the hinJbrain, the midbrain, and the 
 forebrain (Fig. 22). The ne.xt step is the outgrowth of 
 the cerebrum [cr) and olfactory lobes {of) from the fore- 
 
 FiG. 23. 
 
3« 
 
 J'in SIOLOCV AND MORPHULUUV Of ANIMALS 
 
 brain (No. 3), the cerebellum (^Z') from the hindbrain 
 (No. i), and from the midbrain (No. 2) the formation 
 
 Fig. 24. 
 
 or outgrowth of the swellings characteristic of the optic 
 lobes (Fig. 23). The next step is that the outgrowths 
 from I and 3 — i. e., the cerebellum and cerebrum — in- 
 crease enormously. This is especially true of the cere- 
 brum, which, commencing as the foremost in the series, 
 grows forward, sidewise, and especially backward, cov- 
 ering first the thalamus (Fig. 24), then the optic lobes 
 (Fig. 55), and finally the cerebellum, and thus masks the 
 
 true structure of the brain (Fig. 26). The following 
 schedule gives the parts as brought out by embryology. 
 
THE MEKVOUS SYSTEM OF MAN. 39 
 
 The italicized are the basic parts, the others being out- 
 growths. 
 
 f Olfactory lobes. 
 
 Forebrain -| Cerebrum. 
 
 ' Thala7nus. 
 Midbrain Optic lobes. 
 
 ( Medulla. 
 Hindbrain j. Cerebellum. 
 
 ( Pons. 
 
 See also the strange upward and downward growths 
 from the thalamus. These are the pituitary (//) and the 
 pineal (/>//) glands. We shall speak of these again. 
 
 Fig. 26. 
 
 The Distinctive Functions of these Parts. — We 
 
 determine the functions of these several parts partly by 
 observation of injury or disease affecting them in case 
 of man, but mainly by removal of them in case of the 
 lower animals. 
 
 Cerebrum. — For example, if the cerebrum be removed 
 from the brain of pigeons, the bird continues to live, but 
 
40 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 remains in deep, comatose sleep, unconscious and inca- 
 pable of initiating any movement whatsoever. It stands; 
 if pushed, it will recover itself; if thrown into the air, 
 will fly a little way and alight, but lapses again into 
 coma. If food be put into the mouth, it swallows and 
 digests it, but does not voluntarily take it. Indeed, it 
 will starve in the presence of abundant food. From 
 these experiments it is believed that the cerebrum is the 
 seat of consciousness and conscious sensation, of voli- 
 tion and voluntary motion, and a fortiori oi all the still 
 higher functions, such as intelligence, etc. 
 
 Cerebellum. — If the cerebellum alone be removed, the 
 animal seems to be perfectly conscious, and tries to make 
 its usual movements of standing, walking, flying, etc., 
 but can not do so successfully. It can not stand or 
 walk steadily. It flutters, but can not fly. All its move- 
 ments are voluntary, but uncertain and staggering. It 
 is concluded, therefore, that the function of the cerebel- 
 lum is the co-ordination of muscular contraction. " It is 
 the reflex organ of equilibration."* In the acts of 
 standing, walking, flying, etc., very many muscles are 
 used. The contraction of these is initiated by the will, 
 whose seat is in the cerebrum, but they must be per- 
 fectly co-ordinated in order to accomplish any complex 
 act successfully. This is done by the cerebellum. The 
 staggering of drunkenness is the partial paralysis of the 
 cerebellum. 
 
 Medulla.— 'Y\\\'~, is the connecting link between the 
 other parts of the brain and the spinal cord, and through 
 the cord with the rest of the body. For this reason, then, 
 its importance is supreme. Again, the nerves control- 
 ling the most vital functions of the body, such as those 
 of the lungs and the heart, originate in the gray matter 
 
 * Rev. Sci., vol. viii, p. 503, 1897. 
 
THE NERVOUS SYSTEM OI- MAN. 
 
 41 
 
 of this part. It is therefore the part most immediately 
 necessary to Ufe. I'he gray matter of the medulla is 
 the center controlling autoviatically the most vital pro- 
 cesses of the body. Removal of this produces immedi- 
 ate death. 
 
 Optic Lobe. — This is probably the immediate con- 
 troller of the sense of sight; its destruction, therefore, 
 destroys that sense. The optic nerve, coming from the 
 eye, sends one root to the optic lobes and another to the 
 thalamus. The latter sends an influence to the visual area 
 of the cerebrum. We will exj^lain this more fully later. 
 
 Thdlamus and Corpus Striatum. — These ganglia are 
 undoubtedly very important and very necessary to life. 
 Their function is still ob- 
 scure, but from their con- 
 nection with the cerebrum 
 on the one hand, and the 
 rest of the body on the 
 other, they seem to be an 
 intermediary between these 
 two. Sense impressions 
 from surfaces and sense 
 organs, on their way to the 
 cerebrum, seem to pass 
 through the thalamus and 
 receive impulse from that 
 organ ; and nnpulses or 
 mandates from the cere- 
 brum on their way outward 
 to the muscles seem to pass 
 through the corpus stria- 
 tum and receive fresh impulse there. They are relay 
 batteries in the course of communication between brain 
 and body (Fig. 27). They are, moreover and especially, 
 centers of semiautomatic or habitual nun'ements. There are 
 
 Fig. 27. — Dia.PTam showing supposed 
 function of thalamus and corpus 
 striatum in relation to the cere- 
 brum : cs, cerebral senior}- ; cm, 
 motor ; csm, corpus striatum ; tJis, 
 sensory thalamus. 
 
4^ 
 
 rilVSIOLOGV ANT) MORPHOLOGY OF ANIMALS. 
 
 three kinds of movements in the animal bod}^ viz., vol- 
 untary, semivoluntary, and reflex. The cerebrum pre- 
 sides over the first — viz., the distinctly and consciously 
 
 voluntary actions — such as 
 movements undertaken for 
 the first time and requiring 
 full attention and distinct ef- 
 fort. On the other hand, the 
 medulla and spinal cord pre- 
 side over the purely auto- 
 matic or reflex movements 
 — movements wholly with- 
 drawn from consciousness 
 and will, like those of respi- 
 ration and of the heart. The 
 
 Fig. 28. — Illustratinsj function of ,1 1 .„ 4. -a 
 
 thalamus: ..sensory fiber ; .«, thalamus SCems tO preside 
 
 motor fiber; ///..sensory cell of Qver intermediate move- 
 
 thalarnus ; csm^ motor cell of 
 
 corpus striatum ; crs and crm, ments — i.e.. Semiautomatic or 
 
 sensory and motor cells of cere- ■< -u- . in ^u c .. 1 
 
 jjyyjjj ■' habitual, like those of stand- 
 
 ing, walking, flying, writing, 
 speaking, playing on a musical instrument, etc. All 
 these are acquired with some difficulty, the cerebrum 
 presiding, but gradually become easier and easier until 
 they require only the most general superintendence of 
 consciousness and will. If anything goes wrong, the 
 cerebrum takes control for a while and sets things right, 
 and again the movements lapse into semiautomatism. 
 It is as if the cerebrum gradually taught these under- 
 agents or employees — the thalamus and corpus striatum 
 — to do the work themselves, but under general super- 
 vision. To compare to an electric apparatus, it is as if 
 the sense impulse goes up to the cerebrum through a 
 sensory cell of the thalamus and comes back from the 
 cerebrum through a motor cell of the corpus to the 
 muscle, but a part of the current sJiort circuits from the 
 
THE NERVOUS SYSTEM OF MAN. 43 
 
 thalamus to the corpus and downward to the muscle. 
 This short circuiting becomes more and more perfect 
 until only a little overflow goes to the cerebrum, and 
 thus keeps it aware of what is going on. This view is 
 illustrated by the diagram (Fig. 28), in which s and m = 
 sensory and motor libers, i/is and cs/?i = sensory and 
 motor cells of thalamus and corpus striatum, and crs and 
 crm = similar cells in the cerebrum. The arrows show 
 the direction of the nerve current. 
 
 Localization of Cerebral Functions. — The cerebrum is 
 the highest ganglion of the brain, and therefore we 
 ought to expect there the greatest degree of differentia- 
 tion and localization of functions. The old phrenology 
 attempted to localiz-e the faculties of the mind ; but re- 
 cently there has arisen a new and more scientific though 
 far less ambitious attempt to localize not indeed the 
 faculties of the i/iinJ, but the functiofts of the cerebrum — 
 i. e., the areas of the cerebrum receiving and appreci- 
 ating sense impressions from, and the areas determining 
 and controlling the motions of, various parts of the 
 body. The conclusions arrived at in these investigations 
 are based almost wholly on experiments on the lower 
 animals, especially the monkey, although some of them 
 have been confirmed by observations on man in cases of 
 injury or diseases of the brain. 
 
 These investigations are as yet very imperfect, but 
 some reliable results have been attained. Fig. 29 gives 
 the best established areas. It must be remembered that 
 many of our movements are automatic or semiauto- 
 matic. These are presided over by the lower ganglia, 
 such as the thalamus, the optic lobes, or the medulla. 
 Take the sense of sight, for example. Many of our 
 sight impressions do not rise into distinct conscious- 
 ness, and yet appropriate actions may take place. 
 These are probably determined by the thalamus and 
 
44 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 optic lobes, into which the optic roots are seen to enter. 
 But in many cases we consciously observe and remem- 
 ber the impressions of sight — form mental images of 
 objects seen. In such cases the impression is sent on 
 from the thalamus to the cerebrum. The area to which 
 these impressions are sent — visual area — is situated in 
 
 yuph. 
 
 Fig. 29.- — Functional areas of the cerebrum : m, motor of the body ; j, sen- 
 sory of the body ; v^ visual ; olf, olfactory areas ; apli^ motor aphasia 
 (speech ; a.aph^ auditory aphasia; v.apli, visual aphasia (reading;; 
 g.aph, graphic aphasia (writing). 
 
 the posterior lobes and marked v. Similarly, auditory 
 areas are marked a.a, general sensation areas by s, gen- 
 eral motor areas by w, and olfactory areas by olf. 
 
 One of the most curious and interesting of these 
 discoveries is that of the speech area, aph. This, of 
 course, was discovered by observation on man — not 
 experiments on animals. It has been long observed 
 that there are cases in which a patient is perfectly intel- 
 ligent and knows what he wants to say, but can not say 
 it. Such an affection is called aphasia. In such cases it 
 is invariably found by post mortem that there is a lesion 
 of a particular convolution of the frontal lobe, especially 
 of the left side. 
 
THE NERVOUS SYSTEM OF MAN. 
 
 45 
 
 There are, however, many kinds of aphasia. The 
 one above mentioned is motor aphasia. But there is also 
 an auditory aphasia {a.ap/i), in which the patient can 
 speak but can not understand spoken words ; a visual 
 aphasia {v.aph), in which the patient can not read ; and, 
 finally, a graphic aphasia {g.aph), in which the patient 
 can not write. These are situated in different parts of 
 the brain and shown on Fig. 29.* 
 
 The higher operations of the mind, such as self- 
 consciousness, thought, moral sentiment, etc., which 
 the older phrenology sought to locate, are possibly not 
 localized at all, but involve the co-operative activity of 
 the whole brain, and such co-operative activity is prob- 
 ably controlled by special centers of association yet un- 
 known.f 
 
 Dextrality. — We have said that aphasia is an affec- 
 tion of a certain convolution of the frontal lobe, espe- 
 cially on the left side. This naturally leads one to draw 
 attention to the fact that the cerebral hemispheres- 
 control each the opposite side of the body. The fibers 
 from the gray matter of the cortex coming down cross 
 over to the other side. It would seem, therefore, that 
 the greater dexterity of the right side — right-sidedness 
 — is the result of the higher development of the left 
 cerebral hemisphere — left-brainedness. Dexterity is a 
 more perfect co-ordination of muscular motion. Now, 
 there is nothing in which this is more conspicuous than 
 in speech. 
 
 SECTION II. 
 Spinal Cord. 
 
 We have already said that the basal part of the brain 
 may be regarded as an intercranial continuation of the 
 
 * Duval, Rev. Sci., vol. xl, p. 769, 1S87. 
 
 f Turner, Brit. Assoc. Address, Nature, vol. Ivi, p. 525, 1897. 
 
46 PHYSIOLOGY AND MORPHOLOGY OF ANLMALS. 
 
 spinal cord, or, conversely, the spinal cord as an extra- 
 cranial continuation of the basal part of the brain. 
 But this extracranial part is so important that it must be 
 treated separately. 
 
 Envelopes. — Like the brain, it is incased and pro- 
 tected by a bony cover ; but in this case the bony cover 
 must be flexible, and is therefore jointed. This is the 
 jointed backbone so characteristic of vertebrate ani- 
 mals, and giving name to the department. Like the 
 brain, also, it is invested by membranes — an outer tough 
 fibrous and a thin vascular one. As in the brain, too, 
 it is the inflammation of the membranes which gives rise 
 to the acuter forms of disease, such as cerebro-spinal 
 meningitis. 
 
 Description. — The spinal cord is a nearly cylindrical 
 white cord, about half an inch in diameter and eighteen 
 inches long. Like the brain, it is a double organ, divided 
 almost into two semicylinders by a cleft down the dorsal 
 and the ventral side. Thus it consists of two semicylin- 
 ders joined along the axis. Therefore, viewed from the 
 ventral side, we have two anterior columns, and, from the 
 dorsal side, two posterior columns. The posterior col- 
 umns carry sensory fibers; the anterior, motor fibers. 
 
 Spinal Nerves. — From the spinal cord proper there 
 go off thirty-one pairs of nerves; from the intercranial 
 continuation of the same there go off, in addition, twelve 
 pairs — making, in all, forty-three pairs of axial nerves. 
 The spinal or extracranial (but not the intercranial) 
 nerves have each two roots, which quickly unite to form 
 one nerve. One of these roots is connected with the 
 posterior or sensory column, and one with the anterior 
 or motor column of the cord. The posterior root has on 
 it a knot or ganglion (Fig. 30, a, b, and c). These nerves 
 pass out between the joints of the backbone and go to 
 be distributed to all parts of the body. This is the case 
 
THE NERVOUS SYSTEM OF MAN. 
 
 47 
 
 until we reach nearly to the sacrum, where the cord 
 splits up at once into nerves, but still in pairs, to form 
 the Cauda equincB — horsetail (Fig. 30, d). 
 
 Fig. 30. — Spinal cord : «, showing the membrane ; b, the two roots ; c, 
 transverse section showing the two roots ; </, cauda equinae. 
 
 Section. — On making a transverse section (Fig. 30, b), 
 it is at once seen that the two kinds of nerve matter are 
 found here also. But here, as in the intercranial basal 
 continuation (but not in the cerebrum and cerebellum), 
 the gray matter is within, and the white matter on the 
 outside inclosing it. We see also that the gray matter 
 has a peculiar form, found also in the medulla — viz.. 
 that of a semicircle on each side and a connecting band 
 between (Fig. 30, h). As this is only a section view, it 
 is evident that the gray matter on each side is in the 
 form of a plate scrolled outward and connected by a flat 
 5 
 
48 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 transverse plate. Tracing the fibers from the nerves, 
 we find that those from the posterior roots are seen to 
 enter the posterior horn of the gray matter; and those 
 of the anterior roots, into the anterior horn of the gray 
 matter (Fig. 31). The former enter, each, a sensory cell, 
 and through it communicates with a fiber going up the 
 posterior column toward the brain, and even, perhaps. 
 
 Fig. 31. — '"ross section of half the 
 spinal crd, showing: how srnsory 
 fibers, s, and motor fibers, f/i, 
 enter the gray matter. 
 
 Fig. ^2. — Cross and longitudinal 
 section, showing how sensory 
 fibers, s, and motor fibers, m, 
 communicate with cells of gray 
 matter and pass on. Arrows 
 show direction of transmission. 
 
 to the cerebral gray matter ; the latter similarly through 
 a motor cell and upward perhaps to the cerebrum. 
 This is shown in section (Fig. 32). 
 
 Geiural Functioti. — The general function of the cord, 
 therefore, is twofold : first, it is a cable — the biggest 
 cable— of conducting fibers from the gray matter of 
 the brain to all parts of the body; and, second, it is 
 also a center of gray matter from which issue fibers to 
 all parts. As a center, its distinctive function is reflex 
 or automatic. It is called reflex because an impression 
 coming from a sensitive surface to this center is reflected 
 
THE NERVOUS SYSTEM OF MAN. 
 
 49 
 
 back, like a bounding ball, to the appropriate muscle 
 without rising into consciousness at all. To.it is as- 
 signed the control of all those wholly unconscious and 
 involuntary movements — such as those of the heart, the 
 stomach, and the intestines — so necessary to the con- 
 tinuance of bodily life. 
 
 The gray matter of the cord is continuous with the 
 gray matter of the basal part of the brain, especially of 
 the medulla, but entirely separated from the e.xterior 
 gray matter of the cerebrum and cerebellum. The 
 function of the gray matter of the medulla, like that of 
 the spinal cord, is wholly reflex or automatic; that of 
 the thalamus, semiautomatic. Thus, speaking generally, 
 as we go headward and cerebrumward the function of 
 gray matter becomes higher, less automatic, more dis- 
 tinctly conscious, and voluntary. 
 
 SECTION III. 
 
 Nerves. 
 
 We have already said that there are forty-three pairs 
 of axial nerves — i. e., twelve intercranial and thirty-one 
 extracranial or spinal. 
 
 Cranial Nerves. — These all come from the inter- 
 cranial continuation of the axis at the base of the brain, 
 and not from the great outgrowths of the cerebrum and 
 cerebellum, unless we except one, the olfactory, coming 
 from the cerebrum. They all pass through holes in the 
 base of the skull, and are distributed, with two excep- 
 tions, to the head and face. Several of them are nerves 
 of special sense. The cranial nerves, on account of 
 their higher and more differentiated functions, have 
 all of them, in addition to their ordinal, also special 
 names. In the order of their position, beginning in 
 front, they are : 
 
50 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 First pair. — Olfactory = Special sense of smell. 
 
 Second pair. — Optic = Special sense of sight. 
 
 Third., fourth., and sixth pairs. — Motors of the eye. 
 
 Fifth pair. — Trigemini = Common sensory of the face. 
 
 Seventh pair. — Facial = Common motor of the face. 
 
 Eighth pair . — Auditory = Special sense of hearing. 
 
 Ninth pair. — Glosso-pharyngeal (gustatory) = Special 
 sense of taste. 
 
 Tenth pair. — Vagus or pneumogastric = Sensory and 
 motor of the heart, lungs, and stomach. 
 
 Eleventh pair. — Recurrens = Motor. 
 
 Tioelfth pair. — Hypoglossal = Motor of the tongue. 
 
 In Fig. ^Tf we give a diagram showing general distri- 
 bution of these nerves. The origins, as seen in Fig. 17, 
 page 2,2,, are near together. In this Fig. 33 the base of 
 the brain, and especially the medulla, are drawn out so 
 as to separate these origins. In Fig. 34 the medulla is a 
 little drawn out. Fig. 17, page 2,3, shows their natural 
 position. 
 
 Description. — i. The olfactory nerve, the only one 
 which comes from the cerebrum, is observed to pass for- 
 ward beneath the frontal lobe (Fig. 17), on the base of 
 the skull, until it reaches just above the nasal cavity 
 and on each side of the crista galli. There it throws 
 out a great number of small branches (Fig. t,?); through 
 the cribriform (colandered) plate, to be distributed to the 
 mucous membrane of the upper cavities of the nostrils. 
 Its function is to respond to the impression of odorifer- 
 ous vapors. 
 
 Even in man it is seen to swell a little at the end. 
 As we pass down the vertebrate scale, by the increasing 
 size of this swelling it loses entirely its character as a 
 nerve and becomes a great lobe — the olfactory lobe of 
 the brain. 
 
 2. Optic Nerve. — As already said (page 41), this arises 
 
52 
 
 PHYSIOLOGY AND MORPHOLOGY OP^ ANLMALS. 
 
 by two roots on each side, one from the optic lobe and 
 one from the thalamus. These quickly unite to form 
 one root on each side, and these join to form the 
 optic chiasm, which lies on the sella turcica (Turkish 
 seat). This is all within the skull (see Figs. 17 and ;^^). 
 The chiasm immediately separates again into two large 
 nerves, the optic nerves proper, which, piercing the skull 
 
 Fig. 34. — Diagram showing side view of brain and medulla somewhat 
 drawn out to separate the origin of the nerves. The last one (i s/>) is 
 the first spinal. 
 
 at the bottom of the eye sockets, pass forward to enter 
 the eyeball, and there form the retina. Its function is, 
 of course, to respond to impressions of light. 
 
 3, 4, and 6. Ocu/i Motores. — These we take together 
 because they all have a somewhat similar function, viz., 
 the movements of the eyeball. They come out from the 
 
THE NERVOUS SYSTEM OF MAN. 
 
 53 
 
 anterior part of the medulla, and are distributed to the 
 ocular muscles (Figs. ;^^ and 34). 
 
 5. Trigeminal. — This comes from the anterior portion 
 of the medulla, pierces the skull, and comes out on the 
 face on each side, just in front of the ear. It forms 
 there a ganglion or knob, and then divides into three 
 branches and is distributed to all parts of the face to 
 form the nerves of sensation of the face. It is a morbid 
 condition of this nerve which constitutes neuralgia of 
 the face, or tic douloureux. Fig. 34 shows how a branch 
 of this nerve goes to each tooth. Toothache also is a 
 painful affection of this nerve. 
 
 7. Facial. — This, also originating from the medulla, 
 comes out on the face near the ear and ramifies over 
 the whole face and head. It is the general motor nerve 
 of the face. It controls all the facial muscles, and 
 therefore gives emotional expression. Paralysis of the 
 face IS an affection of this nerve. 
 
 8. Auditive. — Coming also from the medulla in close 
 connection with the last, this does not come out on the 
 face at all, but passes immediately into the inner ear, 
 to be distributed there as the nerve of hearing. 
 
 9. Glossopharyngeal [Gustatory). — It is not quite cer- 
 tain what nerve is the gustatory, but the distribution of 
 this one to the back part of the tongue and adjacent 
 parts of the throat, where the gustatory sense chiefly 
 resides, makes it probable that this is it. The distribu- 
 tion is shown in Fig. ^i, page 51. 
 
 10. Vagus or Pneumogastric. — This large nerve comes 
 from the medulla, passes through the base of the skull 
 and down into the thoracic and abdominal cavities, and 
 is distributed to the lungs, the heart, and the stomach. 
 It reports their condition and wants and determines 
 their movements. It is therefore both sensory and 
 motor. 
 
54 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 11. Spinal Recurrent. — So called because, arising from 
 the spinal cord outside of the cranium, it passes upward 
 within the backbone, enters the skull, and again comes 
 out to be distributed to the muscles of the shoulder. It 
 is a motor nerve. 
 
 12. Hypoglossal. — Arising from the medulla, low 
 down, just before it becomes the cord, it passes out to 
 be distributed to the tongue and to become its motor 
 nerve. It therefore controls articulation. 
 
 General Observations on Cranial Nerves. — Observe (i) 
 they all except No. i come from the base of the brain 
 or intercranial continuation of the axis; (2) all except 
 I and 2 come from the medulla ; (3) all the special 
 senses are to be found here; (4) in most cases the sen- 
 sory and motor fibers are embodied in separate nerves, 
 in this regard differing from the spinal nerves, which 
 have each two roots, a sensory and a motor. 
 
 Spinal Nerves. — As already said, there are of these 
 thirty-one pairs, each with its two roots. Their dis- 
 tinctive functions are not so different as in the case of 
 the cranial, and they do not therefore need distinct 
 names. They are divided into four groups : cervical, 
 dorsal, lumbar, and sacral. There are eight cervical, 
 twelve dorsal, five lumbar, and six sacral. Those of each 
 group are numbered first, second, third, etc. (Fig. 35). 
 
 Distribution. — Most of these are distributed to adja- 
 cent parts of the body, but in the upper and lower por- 
 tion of the series several are united to form the great 
 limb nerves. Thus the fifth, sixth, seventh, and eighth 
 cervical and first dorsal form a plexus from which go the 
 nerves of the arm and hand, while the two last lumbars 
 and four of the sacrals form the plexus from which go 
 the great nerves which supply the leg and foot. In all 
 cases by division and subdivision the branches become 
 smaller and smaller until they pass beyond the power 
 
THE NERV'OUS SVSTEM OF MAN. 
 
 33 
 
 D-l 
 
 I^-.^ 
 
 ^ 'G. 35.— Diag:ram showing spinal nerves and their distribution : c, i, 2, 3, 
 etc., cervical ; (/, i, 2, 3, etc., dorsal ; /, i, 2, 3, etc., lumbar ; J, i, 2 3. 
 etc., sacral. After Flower. 
 
56 PHYSIOLOGY AND MORPHOLOGY OF ANLMALS. 
 
 of naked-eye vision, and finally terminate mainly in two 
 ways, viz., some in ?niiscuiar tissues and some in sefisitive 
 surfaces and sense organs. 
 
 Structure of Nerves. — A nerve is a bundle of 
 slender fibers of extreme fineness lying parallel and in- 
 vested by a membrane of fibrous tissue — neurolemma. 
 The size of the fibers varies from y^Vo ^'^ ^ oooo °^ ^"^ 
 inch, or even less. The coarsest are the motor fibers 
 and the finest the sensory fibers of the optic nerve. The 
 number in a nerve of Jg inch in diameter may be a mil- 
 lion or more. Each fiber consists of a central medullary 
 part and an investing sheath. Each fiber may be con- 
 tinuous from a cell in the central gray matter to its ter- 
 mination in the tissue, but this is probably not true of all. 
 The cerebral cells connect with the surface only through 
 a chain of several cells in the thalamus, the medulla, and 
 the spinal column. A branch of a nerve therefore con- 
 sists of a number of fibers separated and invested as 
 before, but without branching of the fibers themselves, 
 except at the extreme end where they may form dendrites. 
 Thus we may regard each fiber as continuous, one end 
 terminating in a central cell, the other in a tissue. 
 
 Function of Nerves. — Nerve fibers are of two 
 kinds, sensory and motor. The one transmits external 
 impressions inward to the nerve center (afferent), and may 
 or may not awaken consciousness; the other transmits 
 internal impulses on fraard (efferent), and determines mus- 
 cular contraction. These two kinds may lie side by side 
 in the same nerve undistinguishable from one another 
 except that the motor is usually larger. The termina- 
 tions of the one are centrally in a sensory cell of the 
 central gray matter, and peripherally by a peculiar end- 
 ing in a sensitive surface or a sense organ ; the termina- 
 tions of the others are centrally in a motor cell of the 
 central gray matter, and peripherally in a muscular fiber. 
 
THE NERVOUS SYSTEM OF MAN. 57 
 
 Every mechanism for action and reaction between 
 the organism and the external world must consist of 
 two kinds of central cells (a sensory and a motor), two 
 kinds of transmitting fibers (afferent and efferent), and 
 two kinds of peripheral terminations (a sensitive sur- 
 face or sense organ and a muscular or contractile tissue). 
 Fig- 36, repeated from page 26, is a diagram illustrating 
 this action and reaction. The manner in which the 
 whole acts is briefly as follows: Impression on a sen- 
 sitive surface or sense organ (S) is transmitted cen- 
 tripetally along a sensory fiber to the nerve center, 
 awakens response, which is transmitted centrifugally 
 
 ^S2 
 
 Fig. 36. — Diagram showing essential parts of an apparatus of exchange be- 
 tween the external world and consciousness : A'C, ner\-e center ; sc, sen- 
 sory cell ; sf, sensory 6ber ; SS, sensory surface ; mc, motor cell ; /«/", 
 motor fiber ; M, muscle. Arrowheads show the direction of transmis- 
 sion. 
 
 along a motor fiber, and determines muscular contrac- 
 tion, which produces motion. 
 
 Both these kinds of fibers are inclosed in the same 
 sheath in the case of spinal nerves, but are usually sepa- 
 rated and found in different nerves in the case of the 
 cranial nerves. Thus in the case of the cranial series 
 we speak of sensory and motor nerves, but in the spinal 
 series we can only speak of sensory and motor ^^ers. 
 
 The Two Subsystems. — The cerebro-spinal system may 
 be conveniently subdivided into two subsystems. By 
 function they may be called the conscio-voluntary and the 
 
58 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANLMALS. 
 
 reflex; by center they may be called cerebral ^\\^ spinal 
 or axial, for it includes the medulla as well as the cord. 
 The center of the one is the surface gray matter of the 
 cerebrum ; the center of the other the central gray matter 
 of the cord ■d.x\^ its continuation in the skull. Each sub- 
 system has its sensory or afferent and its motor or ef- 
 ferent fibers, but the two subsystems are so closely 
 connected that they may act as one. The spinal nerves 
 carry both kinds of fibers, which may act as belonging 
 to both systems. The cranial nerves usually carry but 
 one kind — i. e., either sensory or motor, acting for both 
 systems. 
 
 Course and Termination of Fibers. — A sensory fiber of 
 the cerebral system, beginning in a sensory cell of the 
 cerebral cortex, passes down a posterior column of the 
 cord, communicates with a sensory cell of a posterior 
 cornu, and, continuing, becomes a sensory fiber of the 
 reflex system as well as the cerebral system, and then 
 goes out by a posterior root of a spinal nerve to termi- 
 nate in a sensitive surface or a sense organ. A motor 
 fiber of the same system goes from a motor cell of the 
 cerebral cortex, down an anterior column of the cord, 
 communicates with a motor cell of the anterior cornu, 
 and continues as a motor fiber of both systems, to termi- 
 nate in a muscle. The sensory and motor cells, both of 
 the cerebrum and of the spinal cord, connect with one 
 another, so as to complete the circuit, of the cerebral 
 system in the one case, and of the spinal system in the 
 other. 
 
 Or, more explicitly, and tracing each impulse in the 
 direction of its transmission : .\ sensory fiber of the 
 cerebral system, commencing in a terminal on a sensi- 
 tive surface or in a sense organ, passes up a spinal 
 nerve, through a posterior root into a posterior cornu, 
 communicates there with a spinal sensory cell, then goes 
 
THE NERVOUS SYSTEM OF MAN. 
 
 59 
 
 C?7h 
 
 up a posterior column to the thalamus, communicating 
 with a sensory cell of that ganglion, and thence onward 
 to a sensory cell of the 
 cerebral cortex, awaken- 
 ing consciousness there ; 
 then the impression is 
 transferred to a motor cell 
 of the cerebral cortex, 
 which sends it on in the 
 form of will down through 
 a motor cell of the corpus 
 striatum, then down a fiber 
 of an anterior column of 
 the cord, and, after com- 
 municating with a spinal 
 motor cell, out by an an- 
 terior root and a spinal 
 nerve, to terminate in a 
 muscle and cause contrac- 
 tion there. 
 
 In the reflex system 
 the course is the same, ex- 
 cept that the impression 
 
 carried up by the sensory Fig. ,^7. Diagram of brain, thalamus- 
 
 . . ,, corpus, and a portion of spinal cord, 
 
 Xxh^X"" short Circuits across representing: course of transmission 
 
 f-^,^ »k„ ^,,:.,„i r.^^-. of nerve influence: cj, cerebral sen- 
 
 from the spinal sensory sory, and .;«, cerebral motor cells ; 
 
 cell to the spinal motor J/j, spinal sensory, and j/w, spinal 
 
 . motor cells ; jj, sensitive surface ; 
 
 cell without going up to m, muscle. The arrows show the 
 
 .1 u i 1 direction of transmission. 
 
 the cerebrum to awaken 
 
 consciousness there. The course is shown in the dia- 
 gram (Fig. 37). 
 
 General Mode of Action of the Whole. — Suppose each 
 sensory fiber to have its own terminal, its own spinal 
 sensory cell, and its own cerebral sensory cell, and each 
 motor fiber to have its own muscular fiber terminal, its 
 
6o PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 own spinal motor cell and cerebral motor cell, and the 
 sensory cells in the cerebrum and in the cord to com- 
 municate each with its corresponding motor cell. Thus, 
 every cerebral sensory cell would have its correspond- 
 ing terminal on the body surface, with a connecting fiber 
 between, passing through a corresponding spinal sen- 
 sory cell; and every cerebral motor cell its correspond- 
 ing spinal motor cell and muscular terminal with a con- 
 necting fiber between. Now touch a point on the body 
 surface, and a wave or current of influence is carried 
 along a sensory fiber through the cord, as already ex- 
 plained, to a cerebral sensory cell, awakening conscious- 
 ness there. Immediately, or perhaps only after delib- 
 eration, the influence is transferred by a connecting 
 fiber to a cerebral motor cell, awakening will, and by 
 It down the spinal cord by a motor fiber, and out to a 
 muscle determining appropriate motion. 
 
 See, then, all the phenomena in a case of simple re- 
 sponse to external impression : (i) Impression ; (2) trans- 
 mission inward ; (3) change in a sensory cell — conscious 
 sensation; (4) transmission to a motor cell; (5) change 
 in the motor cell — luill \ (6) transmission outward along 
 a motor fiber ; (7) contraction of a muscle. Metaphor- 
 ically we might say that we have here a complex instru- 
 ment of communication between the external world and 
 the conscious self, with the self playing on brain cells or 
 interior nerve terminals at one end, and the external world 
 playing on exterior nerve terminals at the other end. 
 
 In Reflex Action. — If the impression is on an interior 
 surface in a normal condition, the current of influence 
 on reaching the spinal sensory cell is transferred across 
 by short circuit to the corresponding spinal motor cell 
 and reflected immediately back along a corresponding 
 motor fiber to the appropriate muscle, without rising at 
 all into consciousness. Such is the case in impressions 
 
THE NERVOUS SYSTEM OF MAN. 6l 
 
 on the stomach, heart, etc. In other cases, as in swal- 
 lowing, sneezing, coughing, breathing, etc., the current 
 short circuits, indeed, and appropriate motion takes 
 place immediately by reflex, but sufficient overflow 
 reaches the cerebrum to produce consciousness. In 
 ordinary cases of impression on an external consciously 
 sensitive surface or sense organ, as already seen, the cur- 
 rent passes on without short circuit directly to the cere- 
 brum, and consciousness takes charge of the response; 
 but if the impression be painful, then the current short 
 circuits without waiting for the slower action of the 
 cerebral system. 
 
 For simplicity's sake we have represented the connec- 
 tion throughout as physical and continuous; but, as 
 already explained (Fig. 21, page 37, Fig. 28, page 42), 
 the connection between neurones is probably by touch- 
 ing fingers or interlacing dendrites. It has been sug- 
 gested that the fingerlike extensions are like pseudopods 
 of amoebie — that by extension and contraction they make 
 and break contact with one another. In the active wak- 
 ing state they elongate and make contact; in uncon- 
 sciousness, in coma, and in sleep they contract and break 
 contact. On this view disconnection of neurones is the 
 physical cause of sleep.* 
 
 Illustration by Telegraphy. — To enforce these princi- 
 ples still further and make them still clearer we make a 
 somewhat elaborate comparison with a system of teleg- 
 raphy. 
 
 Suppose, then, the Capitol at Washington represents 
 \.\\^ head. In it there is a great rotunda; this represents 
 the cerebrum. Suppose all about the walls a series of 
 alcoves; these shall be the convolutions. These are, say, 
 full of battery cells; these are the sensory and motor 
 
 * Mathias Duval, Rev. Sci., i.x, 321, 1898. 
 
62 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 cells of the cerebrum. From these battery cells there 
 go wires, converging to the hallway and forming there 
 a great cable of wires going out of the door ; these are 
 the white fibers converging and forming the w^(//^//c?, and 
 going out of the skull as the cord. Before going out, 
 however, certain wires are sent out from the cable to 
 all the offices in the building; these are the cranial 
 nerves going to the head and face, and especially to the 
 sense organs. The cable starts out now to communi- 
 cate with the whole country, but protected by an arch- 
 way ; this is the cord protected by the vertebral column. 
 As it goes, the cable gives out wires to adjacent and 
 even distant regions ; these are the spinal nerves. These 
 must go to every State, county, and city, and terminate 
 in intelligence offices and in executive or police offices; 
 these are the sense organs and the muscles. Suppose 
 also the alcoves are all named as States and the bat- 
 teries all numbered. 
 
 Now, suppose anything to occur in any place. The 
 intelligence office reports the fact to the head center. 
 The State, county, city, neighborhood, is at once known, 
 and the command immediately goes out to the execu- 
 tive office and determines appropriate action. 
 
 Application. — Let us now apply this idea and show 
 how it explains the j)henomena : 
 
 1. Cut the cord high up in the neck. The whole body 
 is paralyzed to both consciousness and volition, but not 
 to reflex function, for that is in the gray matter of the 
 cord, which we are not now considering. Prick the foot 
 and it will jerk, but the prick is unfelt and the jerk is in- 
 voluntary. Meanwhile all the parts of the face are unpar- 
 alyzed. The patient sees and speaks as usual, because 
 the nerves controlling these come out from the medulla. 
 
 2. Cut the cord in the middle of the back. Now the 
 upper parts of the body, including the arms, etc., feel 
 
THE NERVOUS SYSTEM OF MAN. 63 
 
 consciously and may be moved voluntarily; but the 
 whole lower portion, including the legs, is paralyzed 
 both to conscious sensation and to voluntary motion, 
 because these parts are cut off from the cerebral center. 
 But reflex movements remain. 
 
 3. Cut the posterior root of a spinal nerve. Now 
 all that part to which this nerve is distributed is para- 
 lyzed to sensation, but may be voluntarily moved. If, 
 on the contrary, the anterior root is cut instead of the 
 posterior, then the part is paralyzed to motion, though 
 not to sensation. If, finally, the nerve is cut below the 
 junction of the two roots, then the part to which the 
 nerve is distributed is paralyzed to both sensation and 
 motion. 
 
 4. Irritate a nerve in its course — say, by pinching it. 
 For example, pinch or strike the ulnar nerve, lying be- 
 tween the elbow joint and the inner condyle. We are 
 all familiar with the fact that we. feel pain in the little and 
 ring fingers, where this nerve is distributed. If it were not 
 for the skin covering the nerve, and which of course 
 has its own nerves of sensation — if the skin w'ere cut 
 away so as to bare the nerve and the nerve alone was 
 pinched, the only sensation we should feel would be in 
 the little and ring fingers and that side of the hand. 
 Why ? Because the nerves are distributed there. The 
 intelligence ofifices are there. Therefore at the head 
 center the painful intelligence seems to be reported from there. 
 How could it be otherwise ? 
 
 5. Cut a nerve, perhaps high up in the arm or leg. 
 Expose the ends. Pinch the end below the cut ; you 
 feel nothing. But pinch the end above the cut ; you 
 feel pain — but where ? Not at the place pinched, but 
 in the fingers or toes where the cut nerve is distributed 
 — i. e., where the nerve terminals, the intelligence offices, 
 are. In any telegraphic system, if a wire is cut and a 
 
64 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 message sent from the cut end, the head office at Wash- 
 ington could not but refer it to the place where this 
 wire ought to go. 
 
 Law of Peripheral Reference. — Thus we have 
 the law that an impulse received by the brain through a 
 nerve fiber is of necessity referred by the consciousness to the 
 peripheral extremity. . . . This explains the fact that in 
 the case of an amputated limb the patient still has a 
 sense of the presence of a foot or hand; and if the 
 nerve of the stump should become diseased, he will 
 often feel intense pain in foot or hand. 
 
 Nerve Force versus Electricity. — I have used 
 this comparison with a telegraphic system in order to 
 make the mode of action of the nervous system clear. 
 But we must not conclude, therefore, as many do, that 
 nerve force, and indeed life itself, is nothing but elec- 
 tricity. It becomes necessary, therefore, that we should 
 draw attention to some fundamental differences be- 
 tween these two forms of energy : 
 
 1. Wires lying in contact with one another in the 
 same bundle will not conduct true unless insulated. 
 Nerve fibers, on the contrary, conduct true although 
 lying in contact in the same sheath — in a moist condi- 
 tion, and therefore uninsulated. 
 
 2. Cut a wire and press the fresh-cut ends together 
 — they still conduct well. But a cut nerve pressed to- 
 gether utterly fails to conduct nerve influence. 
 
 3. In the case of an electric current there must be 
 a closed circuit. This is fundamental. If the circuit is 
 open anywhere there is no current and can not be. 
 Not so in the case of a nerve current. There is indeed 
 a sensory current and a return motor current. They 
 are connected, too, at the cerebral end, but certainly 
 not at the peripheral ends. Besides, there is often 
 current only one way — i. e., sensation without corre- 
 
THE NERVOUS SYSTEM OF MAN. 65 
 
 spending motion, or motion initiated without incitmg 
 sensation. 
 
 4. The velocity of electricity is always, like all 
 ethereal vibrations, inconceivably great; but the veloc- 
 ity of a nerve current has been measured and found to 
 be very moderate — only about one hundred feet a second. 
 In fact, the phenomena of transmission of nerve influence 
 would suggest an analogy with propagated chemical 
 change, such as combustion of a train of gunpowder 
 rather than electric current. 
 
 But it will be answered that "seeing is believing." 
 The electric organ of certain fishes, as the electric eel, 
 discharges powerful currents — sufficient, indeed, to kill 
 a man. These organs are connected with the brain by 
 very large nerves. The discharge of electricity is cer- 
 tainly under the control of the will. It is an act of 
 volition. The fish is exhausted by it as by any power- 
 ful effort. 
 
 At first sight this seems, indeed, demonstrative ; but 
 not so. All the forces of Nature, nerve force and life 
 force among the number, are correlated — i. e., are con- 
 vertible one into another. Now, the electric organ of 
 a fish constitutes an arrangement for converting nerve 
 force into electricity, precisely as a muscle is an arrange- 
 ment for converting nerve force into mechanical power. 
 ^Ve might as well say that nerve force is identical with 
 mechanical power as to say that it is naught else than 
 electricity. 
 
 The fact is, there are many different forms of force 
 in Nature, each producing a peculiar group of phenom- 
 ena, the study of which gives rise to a peculiar depart- 
 ment of science. Now the phenomena of nerve force 
 are so different from those of electricity that these two 
 are rightly called different /(?;'w.f of the universal energy, 
 although, indeed, they are transmutable into one another. 
 
66 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 Function of the Spinal or Reflex System. — As 
 
 already said, the function of this system is to preside 
 over and control all the routine work of the body — 
 work so constantly necessary that it can not be left to 
 the conscio-voluntary system, which is occupied with 
 other and higher work. Thus the beating of the heart, 
 the play of^the respiratory muscles, the movements of 
 the stomach and intestines, are under the control of this 
 system, which never sleeps night or day* The conscio- 
 voluntary system alone sleeps. The passage of control 
 from one system to the other is well seen in the act of 
 swallowing. The food is chewed, then gathered by the 
 tongue, then pressed back into the throat ; so much is 
 under control of the voluntary system. As soon as it 
 touches the throat it is seized by the involuntary sys- 
 tem and hurried on to the stomach. Nevertheless, there 
 are all gradations between reflex and conscio-voluntary 
 movements. The movements of stomach, intestines, and 
 heart are not only involuntary, but also, in health, un- 
 conscious; the acts of swallowing, sneezing, coughing, 
 are involuntary, but not unconscious; the act of breath- 
 ing is not only conscious, but also partly controlled by 
 volition. 
 
 When the conscio-voluntary system is in full activity 
 it takes possession of the consciously sensitive surfaces 
 and the voluntary muscles, so that the reflex system is in 
 abeyance except under conditions of extreme stimula- 
 tion or pain, in which case the reflex takes hold because 
 the conscio-voluntary is too slow. But when the conscio- 
 voluntary control is withdrawn, as in sleep, or paralyzed, 
 as in section of the cord or of a nerve, then the reflex is 
 far more active, as shown by the unconscious, involun- 
 tary movements of hand or foot on the least irritation. 
 
 Illustration by Telegraphy. — The system of telegraphy 
 already used to illustrate the action of the conscio-vol- 
 
THE NEk\OUS SYSTEM OF MAN. 67 
 
 untary system may be made to illustrate this also by 
 adding battery cells all along the cable within the arch- 
 way, and these also sending out wires to intelligence 
 offices and executive offices in every part of the country 
 and controlling all necessary routine business without 
 troubling the head center except in case of extreme 
 emergency. By some stretch of the imagination they 
 may be compared to state government. 
 
 SECTION IV. 
 Ganglionic System. 
 
 It will be remembered that we divided the whole 
 nervous system of vertebrates into two subsystems, 
 viz., the cerebro-spinal and the ganglionic. The latter 
 we put aside for the time. We now take it up, but very 
 briefly, because it is very imperfectly understood. 
 
 Definition. — Nerves are cylindrical bundles of fibers. 
 Every knot or swelling on these cylindrical strings con- 
 tains gray matter with cells and gives out the two kinds 
 of fibers terminating in the tissues. In a word, they are 
 little centers of force and are called ganglia. Now the 
 ganglionic system is so called because it consists en- 
 tirely of such small ganglia scattered about in the body 
 and connected by nerve strings. 
 
 Description. — The system consists (i) of a series of 
 ganglia on each side of the spinal column (nt)t in the 
 canal) the whole way from the base of the skull to the 
 end of the sacrum, one opposite each joint of the col- 
 umn (see Fig. 38,^^). (2) This series of ganglia is con- 
 nected throughout on each side by a nerve cord. The 
 two knotted cords thus formed are called the sympa- 
 thetic nerves (Fig. 38, // //). (3) From each spinal nerve 
 there goes off a small branch which connects with the 
 sympathetic nerve on each side, and thus the two sys- 
 
68 PHVSlULOGV AND MOKPIIOLOGV OF AMMALS. 
 
 ^\pl * 
 
 Fig. 38. — Diaf^ram showinp distribution of the {^aii; lionic system : g^ {jan- 
 glions ; «, sympathetic nerve ; csp, connectinj; spinal branch ; //, 1, 2, 3, 
 etc., plexuses. 
 
THE NERVOUS SYSTEM OF MAN. 69 
 
 terns, the axial and the ganghonic, are brought into re- 
 lation with one another (Fig. 38, en 01). (4) From the 
 sympathetic gangha on each side there go nerves to the 
 visceral region, where are performed the most impor- 
 tant functions. There the nerves from each side unite 
 to form plexuses or networks — i. e., the nerves cross one 
 another in every direction, uniting at the crossings and 
 forming ganglia there, and from these again come 
 smaller branches going to all the important viscera and 
 controlling their functions (Fig. 38,//). 
 
 The Plexuses. — Beginning above and going down- 
 ward, the principal plexuses are (i) the carotid and (2) 
 the pharyngeal, small plexuses with their ganglia con- 
 trolling the throat viscera (Fig. 38,// i and 2); (3) the 
 cardiac (plexus in the thorax) with its ganglia, control- 
 ling the action of the heart (Fig. 38,// 3); (4) in the 
 stomach region the epigastric or solar plexus with its 
 ganglia, controlling the action of the stomach, spleen, 
 and liver (Fig. 38,^/4); (5) the hypogastric plexus and 
 its ganglia, controlling the functions of the pelvic vis- 
 cera (Fig. 38, // 5). Into the cardiac and epigastric 
 plexus enter the branches of the pneumogastric nerve 
 from the medulla, and play an important part in the 
 control of the heart, lungs, and stomach. 
 
 Function. — The function of this system is obscure, 
 but certainly largely connected with the processes of nu- 
 trition, secretion, etc., or organic functions. Its func- 
 tion is doubtless also reflex, so far as the organs to 
 which its nerves are distributed are concerned, but 
 whether by its own fibers or by means of fibers derived 
 from the axial system is more doubtful. It seems to 
 control nutrition and secretion by controlling the blood 
 supply; and this is done by means of certain fibers — 
 vasomotor fibers — distributed to the cajiillary blood ves- 
 sels — vasomotor nerves. Cutting the vasomotor nerves 
 
70 
 
 PIIVSIOLOGV AND MORPHULOGV OF ANIMALS. 
 
 seems to paralyze the smaller blood vessels, which then 
 enlarge, become gorged with blood, and the part becomes 
 finally hot and inflamed. Stimulation of these nerves, 
 on the contrary, produces contraction of these blood 
 vessels and coolness and paleness of the part. Blushing, 
 on the one hand, and the paleness of terror, on the other, 
 are supposed to arise from opposite conditions of the 
 vasomotor nerves. 
 
 Illustration by Telegraphy. — If we must push the tele- 
 graphic illustration to include this system also, then it 
 may be compared to a municipal government control- 
 ling local affairs. 
 
 SECTION V. 
 
 COMPARATIVE PHYSIOLOGY AND MORPHOLOGY OF THE 
 NERVOUS SYSTEM. 
 
 Introductory — Outline of the Classification of Animals. 
 
 About to enter now on the comparative morphology 
 and physiology of the nervous system, it becomes neces- 
 sary to have in mind some scheme of classification of 
 the animal kingdom. A true classification is a compen- 
 dious expression of perfect knowledge, and would seem 
 therefore to come last of all. But some provisional 
 classification is a necessary condition of increase of 
 knowledge, because it is impossible to deal scientifically 
 with animals except in groups. Therefore our plan 
 will be to give a simple outline of such a classification 
 and to verify it or modify it as we proceed. There are 
 a great variety of classifications which have been pro- 
 posed, almost as many as the proposers. We select one 
 which is probably as good as any, and has, moreover, 
 the additional advantage of comparative simplicity, for 
 our main object is to be able to handle the material. 
 
NERVOUS SYSTEM OF VERTEBRATES. 
 
 71 
 
 The whole animal kingdom may be primarily divided 
 into seven groups called subkittgdoms or departments or 
 phyla. These are again each subdivided into classes, and 
 these latter into orders, families, genera, species, etc. In 
 the schedule given below we go no further than classes. 
 Orders will be referred to sometimes, but not often. 
 Even some classes are not used. 
 
 
 
 Metazoa. 
 
 
 
 Proto- 
 zoa. 
 
 
 Articl'l.^ta. 
 
 Mollusca. 
 
 Radiata. 
 
 
 Vertebrata. 
 
 Arthropoda. 
 
 Annelida. 
 
 Echino- 
 dermata. 
 
 Coelen- 
 terata. 
 
 Proto- 
 zoa. 
 
 Mammals 
 
 Birds 
 
 Reptiles 
 
 Amphibia 
 
 Fishes 
 
 Insects: 
 Arachnids 
 Myriapods 
 
 Crustacea 
 
 Annelids 
 
 Cephalopods 
 Gasteropods 
 Acephala 
 Brachiopods 
 
 Echinoids 
 Asteroids 
 Crinoids 
 Holothuri- 
 oids 
 
 Acalephae 
 Polyps 
 
 Infuso- 
 ria 
 
 Rhizo- 
 pods 
 
 These groups are not of equal value or significance, 
 as shown above. The whole animal kingdom may be 
 divided into two prime groups, viz., protozoa, or sim- 
 plest animals consisting of one cell only, and metazoa, or 
 animals consisting of an aggregate of more or less 
 differentiated cells. The metazoa, being higher, are 
 more differentiated, and therefore are divided into many 
 great departments. Again, I have linked together the 
 echinoderms and coelenterates under the name radiata, 
 as having a common radiated plan of structure; also the 
 arthropods and annelids, or segmented worms, under 
 the name articulata, as having a common jointed or 
 ringed plan of structure. We shall use these terms in 
 connection with the general laws of animal structure. I 
 take for granted that the student already has some gen- 
 eral knowledge of zoology. I give only such classifica- 
 tions and such names as I shall use in the comparison 
 that follows. 
 
n 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 COMPARATIVE MORPHOLOGY AND PHYSIOLOGY OF THE 
 VERTEBRATE NERVOUS SYSTEM. 
 
 The general plan of the nervous system is so pre- 
 cisely the same in all vertebrates that only the most 
 general statements are necessary in regard to this. In 
 all vertebrates, but in no other animals, we have both 
 an axial and a ganglionic system. In all vertebrates 
 the axial system consists of a continuous tract of gray 
 matter inclosed in white matter lying along the dorsal 
 aspect of the body, enlarged at the anterior end to form 
 a brain, and giving off nerves in pairs from one end to 
 the other (Fig. 14, page 29). In different vertebrates 
 the number of these pairs vary, being least in frogs and 
 toads, where there are only eighteen or twenty, and 
 greatest in some fishes, as the eels, where they may be 
 two hundred or more. Therefore the only part where 
 the differences are important enough to arrest our at- 
 tention in this rapid sketch is the brain. 
 
 THE BRAIN OF VERTEBRATES. 
 
 In running down the vertebrate scale there are three 
 important changes which take place in the brain : \. In 
 size, both absolute and relative. 2. In relative amount 
 of gray matter compared with white, as shown by the 
 complexity of the convolutions. 3. In the relative size 
 of the cerebrum as compared with the other ganglia of 
 the brain. Perhaps I may add : 4. In the relative size of 
 the frontal lobe compared with the other lobes of the 
 cerebrum, as shown by the position of the fissure of 
 Rolando. In all these respects the brain of man stands 
 pre-eminent. 
 
 I. Size {a) Absolute. — The brain of man weighs about 
 three pounds (forty-eight to fifty ounces). The heaviest 
 which have been weighed — viz., that of Cuvier, the great 
 
NERVOUS SYSTEM OF VERTEBRATES. 
 
 /J 
 
 comparative anatomist, and that of Turgenief, the great 
 novelist — were about four pounds. It varies slightly 
 in different races, being greater in the superior races, 
 but not so much greater as might have been expected. 
 There are only two animals that have larger brains than 
 man, viz., the elephant, whose brain is about eight 
 pounds, and the whale, whose brain is about five pounds. 
 The enormous size of these animals is sufificient reason. 
 [d) Size relative to the Body or to Rest of the Nervous 
 System. — This is far more significant than the last. The 
 brain of the highest animal of like size, viz., the gorilla 
 is only about one third that of man, viz., fifteen 
 ounces. Below this there is a constant decrease of rela- 
 tive size. This is shown in the following table. Of 
 course we only take averages of these various classes. 
 
 Classes. 
 
 Fishes 
 
 Reptiles 
 
 Birds 
 
 Mammals 
 
 Man 
 
 Brain 
 
 to nervous system. 
 
 ; 
 
 5 
 
 3 
 
 30 
 
 -.1= 4 
 : 5= ^ . 
 : 1= 5 times. 
 : 1= 3 times. 
 : 1=30 times. 
 
 There are some things in this table which require e.\- 
 planation. First, it is seen that there is no superiority 
 in reptiles over fishes in brain to body weight, but there 
 is in the relation to the rest of the nervous system. 
 .•\gain, it is seen that birds are apparently superior to 
 mammals. The reason of this is that, as a law, small 
 animals have larger brains proportionately than large 
 animals. Now birds, as a rule, are smaller animals than 
 mammals. Indeed, some of the smallest birds, such as 
 the humming bird and the kinglet, have actually larger 
 brains proportionately than man. The same is true of 
 the smallest mammals, such as the mouse. But there are 
 other things spoken of later, viz., fineness of organiza- 
 
74 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 tion, which determine intellect quite as much as or more 
 than size. Perhaps it might be well to say here, for the 
 
 comfort of those who 
 wear small-sized hats, 
 that brain power does 
 not depend on size 
 alone, any more than 
 bodily strength de- 
 pends on weight alone. 
 In both cases it is a 
 product of two factors, 
 viz., size and fineness 
 of organization, and 
 the latter is the more 
 important factor. 
 
 Brains of Extinct 
 Species. — It is a cu- 
 rious and significant 
 fact that in each of 
 these classes extinct 
 species are remarka- 
 ble for the smallness 
 of their brains. There 
 has been a gradual in- 
 
 FlG. 39.— ^, outline of the skull and brain Crease in the size of 
 
 cavityoflchthyomis victor I after Marsh) j^g brains of animals 
 seen from above, (tive sixths natural 
 
 size.) 5, outline of the skull and brain in each of these claSSeS 
 
 cavity of Sterna cantiaca ( after Gmelin), - »i, " fi t • «. 
 
 same view. (Natural size. ) <?/, olfactory trom tneir nrst iniro- 
 
 lobes; c, cerebral hemispheres; op, optic (Juction until nOW. To 
 lobes; cb, cerebellum. 
 
 give one example, the 
 extinct Cretaceous bird Ichthyornis was about the size 
 of a tern, but its brain was hardly one quarter as large 
 
 (Fig- 39)- 
 
 2. Relative Amount of Gray Matter.— The gray 
 matter is \.\\& generator, the white fibers only transmitters of 
 
NERVOUS SYSTEM OF VERTEBRATES. 
 
 75 
 
 Fig. 40. — Brain of Ornitho- 
 rhynchus. (Natural size. ) 
 ( After Parker. ) 
 
 nerve force. The former, therefore, is the higher. The 
 organization of the brain is tested by the relative amount 
 of gray matter. Comparing again 
 to electricity, the electro-motive 
 force varies as the gray matter. 
 Further, this relative amount, 
 other things being equal, is ex- 
 pressed by the number and depth 
 of the convolutions. Now, of all 
 animals the number and depth of 
 the convolutions of the cerebrum 
 is by far greatest in man. In 
 higher mammals, especially an- 
 thropoid apes, the convolutions 
 are well marked, but they become 
 less and less so as we go down 
 
 the mammalian scale, until they entirely disappear before 
 we reach the lowest mammals (Fig. 40). Therefore all 
 below mammals — i.e., all birds, reptiles, and fishes — have 
 smooth brains. 
 
 There is, however, a modifying cause here which 
 must not be neglected ; it is, that the brains of small 
 animals tend to smoothness irrespective of deficiency of 
 gray matter. The reason is that bulk varies as the cube, 
 while surface only as the square of the diameter, and 
 therefore a small sphere has proportionately a greater 
 surface than a large sphere. It follows from this that 
 the brain of a small animal, though smooth, may have 
 as much gray matter proportionately as the highly con- 
 voluted brain of a large animal. Thus all small mam- 
 mals have smooth brains, while large mammals have all 
 convoluted brains. The brain of an elephant, or even 
 of a whale, is wonderfully convoluted, almost as much 
 so as that of man. 
 
 As we go back in the embryonic series the brains of 
 
76 PHYSIOLOC^A' AND MORPHOLOGY OF ANIMALS. 
 
 mammals and of man become less and less convoluted, 
 until they become entirely smooth. The same is true 
 as we go back in the evolution series. Extinct mam- 
 mals have less and less convoluted brains as we go back 
 in time. All the earliest mammals had smooth brains. 
 
 3. Relative Size of Cerebrum. — The cerebrum is 
 confessedly the highest part of the brain. The relative 
 size of this is the best of all tests of position in the scale 
 of organization. Observe, then, that there are four 
 lobes used in this comparison, viz., the cerebellum, the 
 optic lobes, the cerebrum, and the olfactory lobes ; for 
 this last is an important lobe in all lower vertebrates. 
 
 Now in man, as alVeady seen, the cerebrum, growing 
 out from the thalamus, spreads forward, covering en- 
 tirely the olfactory lobes, and backward, covering first 
 
 Fig. 41. — Mammal brain. A, top view ; B, side view of the brain of a cat. 
 
 the optic lobes, then the cerebellum, until looking down 
 upon the brain we see nothing else. It covers and 
 dominates all. In monkeys, by a less backward prolon- 
 gation, the cerebellum begins to peep out behind In 
 the average mammals, such as the lion or the dog, the 
 olfactory lobes are exposed in front, and nearly the 
 whole of the cerebellum is uncovered behind (Fig. 41). 
 Still lower among mammals the cerebellum is wholly 
 uncovered and the ojnic lobes begin to appear, and all 
 the four lobes are seen in a series (Fig. 42). The brains 
 of extinct mammals are all of this low type. 
 
NERVOUS SYSTEM OF VERTEBRATES. 
 
 77 
 
 Owens Classification of Mammals. — Professor Owen 
 classified mammals by this test into four groups : i. 
 Archencephala, or ruling brain. In 
 this subclass he placed man alone. 
 2. Gyrencepkala, or convoluted 
 brains. In this he placed all the 
 larger mammals. 3. Lisscncephala, 
 or smooth brains. In this he 
 placed all the rodents and some 
 other small mammals. 4. Lyen- 
 ^^//d;/a, or separated brains. The 
 lowest mammals, such as insecti- 
 vores and marsupials, he placed 
 in this. This attempt is interest- 
 ing, but the classification can not 
 be regarded as natural, for the 
 earliest animals of all these sub- 
 classes, except one — man — have 
 smooth brains. 
 
 In the average bird (Fig. 43) not only is the olfac- 
 tory lobe wholly uncovered in front and the cerebellum 
 behind, but also the optic lobes are at least half un- 
 covered. 
 
 t _, 
 
 Fig. 42. — Brain of Das)rurus 
 ursinus, showing expo- 
 sure of the optic lobes, ol. 
 (From Owen.) 
 
 Fig. 43. — Bird brain : A, side view ; B, top view.* 
 
 In the reptile (Fig. 44) all the lobes are fully ex- 
 posed, and the brain becomes a succession of lobes in a 
 
 * In all these fii^ures'to 54: w, medulla; </', cerebellum; 01, 
 optic lobes ; cr, cerebrum ; of, olfactory lobes, 
 
78 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 linear series, but the cerebrum still maintains its pre- 
 eminence as the larjjest of the series. 
 
 Fig. 44. — Reptile brain : A, side view ; B, top view. 
 
 Finally, in fishes (Fig. 45) this pre-eminence of the 
 cerebrum is lost and the optic lobes are the largest. In 
 the very lowest fishes, such as the lampreys [Fetromy- 
 zon), there is scarcely any enlargement at the anterior 
 end, and in the lancelet {AmpJiioxus, Fig. 46) the cord 
 is continued into the head with no perceptible enlarge- 
 ment at all. 
 
 Fig. 45. — Fish brain : A, side view; B, top view. 
 
 In all these gradual changes by which the brain is re- 
 duced finally to a linear series of swellings, it is remark- 
 able to see how persistent are two little organs, the 
 functions of which are still doubtful, one below and one 
 
 V hi 
 
 Fig. 46. — Amphioxus : spc^ spinal cord. 
 
 above the thalamus, viz., the pituitary gland and the 
 pineal gland. These are seen in Figs. 48-53. The use 
 
NERVOUS SYSTEM OF VERTEBRATES. 
 
 79 
 
 of the former, if any, is unknown. The latter seems to 
 be a useless remnant of a once useful eye in the top of 
 the head. In some lizards it 
 still retains the structure of an 
 eye (Fig. 47). 
 
 Embryonic compared linth the 
 Taxonomic Series. — It is a most 
 significant fact that the brain 
 of the embryo of man passes 
 through all these stages: (i) 
 The earliest condition of the 
 human brain is seen in Fig. 48. 
 This can be compared with the 
 brain of only the very lowest 
 fishes. It may therefore be 
 called the subfish stage. It does 
 not yet contain a cerebrum. 
 (2) Then the cerebrum grows 
 out of the thalamus, but is yet inferior in size to the 
 optic lobes (Fig. 49). This may be called the average 
 fish stage. (3) Then the cerebrum grows until it is the 
 largest of the series, but covers nothing as yet (Fig. 
 50). This may be called the reptile stage. (4) Then it 
 begins to cover the optic lobes (Fig. 51). This corre- 
 
 FiG. 47. — Parietal eye of Hat- 
 teria (after Spencer) : /, lens ; 
 V, vitreous humor ; r, ret- 
 ina ; 0, optic nerve. 
 
 Fig. 48. — Sub-fish stage : ///, thalamus ; ol, optic lobe ; w, medulla. 
 
 sponds to the bird stage. (5) Then it covers the whole 
 of the optic lobe and encroaches on the olfactory lobe 
 7 
 
8o PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 in front and the cerebellum behind (Fig. 52). This 
 corresponds to the condition of the average mammal, 
 
 Fig. 49.— Fish stage. 
 
 and may be called the mammal stage. (6) Finally, it 
 grows forward, wholly covering the olfactory lobe, and 
 
 Fig. 50. — Reptile stage. 
 
 backward, wholly covering the cerebellum, and we have 
 the human stage (Fig. 53). 
 
 Fig. 51.— Bird stage. 
 
 Meanwhile several other changes are in progress : (i) 
 In proportion as the head is raised nearer and nearer to 
 a vertical position, the base of the brain, which was at 
 
NERVOUS SYSTEM OF VERTEBRATES. gl 
 
 first in direct line with the cord, begins to bend more 
 and more until it is at right angles in man ; (2) the cere- 
 
 FlG. 52. — Mammalian stage. 
 
 helium has been increasing in relative size; and (3) the 
 convolutions of both the cerebellum and the cerebrum 
 
 Fig. ^2- — Human stage. 
 
 have been becoming more and more complex. All these 
 changes are combined and re[)resented in Fig. 54. 
 
82 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 4- Relative Size of Frontal Lobe.— The highest 
 functions of the intellect are connected with the frontal 
 lobe of the cerebrum. This is marked off by the fis- 
 sure of Rolando. In man the portion anterior to this 
 fissure is large. In monkeys the fissure is plain, but 
 
 Man 
 
 Fig. 54. — Diagram showing all the stages in one. 
 
 the anterior lobe is much smaller, and increasingly so 
 as we go down the scale of monkeys. The fissure is 
 not certainly discernible at all below monkeys. 
 
 CEPHALIZATION. 
 
 This may be defined as headward development. Intro- 
 duced by Dana to express the gradual transfer of func- 
 tions headward by the modification of homologous organs 
 as we go up the scale of Crustacea (see page 268), we 
 here use it in a similar but a wider sense as the gradual 
 increasinfT dominance of higher over lower functions in 
 
NERVOUS SVSTExM OF VERTEBRATES. 83 
 
 the process of evolution and usually accompanied with 
 transfer headward. 
 
 In- the process of development, whether in the evolu- 
 tion series, or in the taxonomic series, or in the embry- 
 onic series, we observe the same order. Organisms are 
 at first unmodified cell-aggregates. From such aggre- 
 gates tissues performing different functions are differ- 
 entiated. From this time onward cephalization begins. 
 Among the tissues there is a gradually increasing domi- 
 nance of the highest, the controlling tissue, viz., the 
 nervous tissue. Then in the nervous tissue a gradually in- 
 creasing dominance of the highest part, viz., the braiji. 
 Then in the brain a gradually increasing dominance of 
 the highest ganglion, viz., the cerebrum. Then /';/ the 
 cerebrum a gradually increasing dominance of the high- 
 est substance, the surface gray matter, as shown by the 
 complexity of the convolutions. And, lastly, among the 
 convolutions a gradually increasing dominance of the high- 
 est, viz., those in the frontal lobe, as shown by the 
 position of the fissure of Rolando. In all there is an 
 increasing dominance of the higher over the lower, and 
 of the highest over all. This is everywhere the law of 
 evolution. 
 
 Shall it stop here ? Shall it not be carried forward 
 on a higher plane by the conscious effort of man ? Is 
 not all civilization, all culture, all education a voluntary 
 process of cephalization ? Here, also, there must pre- 
 vail the same law of progressive domination of the 
 higher over the lower, of the distinctively human over 
 the animal, of mind over body; and, in the mind, of the 
 higher faculties over the lower, the reflective over the 
 perceptive, and of the moral character over all. In all 
 your culture be sure that you strive to follow this law 
 of evolution. 
 
^4 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 SECTION VI. 
 
 Nervous System of Invertebrates. 
 
 The departments below the vertebiates we group 
 together under the general term invertebrates, not be- 
 cause they are more nearly related to one another than 
 they are to vertebrates, for this is not true, but because 
 we must treat them far more cursorily. 
 
 I. Articulata. Arthropods and Annelids. — The plan 
 of structure of these is widely different from that of 
 vertebrates. In these (i) the skeleton is on the out- 
 side, the muscles acting from within to produce motion 
 
 Fig. 55. — Diag;ram of section across an arthropod : b, blood system ; «, nerv- 
 ous system ; i, intestines. 
 
 and locomotion ; instead of being within, and muscles 
 working on the outside for the same purpose, as in the 
 vertebrates. (2) Of all departments these animals are 
 the most distinctly and most numerously jointed, and 
 therefore they are called articulata. (3) The skeleton 
 being on the outside, it forms a hollow tube or cavity 
 inclosing all the viscera and organs of the body (Fig. 55), 
 
NERVOUS SYSTEM OF INVERTEBRATES. 
 
 ,. vert 
 .b 
 
 while in vertebrates there is a separate cavity inclos- 
 ing and protecting the nervous centers (Fig. 56). (4) 
 In the vertebrates, as we have 
 seen, the nerve center con- 
 sists of a continuous tract of 
 gray matter lying along the 
 <^/.y<// aspect of the body, above 
 the visceral canal. In the ar- 
 ticulata, on the contrary, the 
 nerve center is a continuous 
 chain lying along the ventral 
 aspect of the body, below the 
 visceral canal. (5) In the case 
 of vertebrates, as already seen, 
 the nerve system is subdivided 
 into two subsystems, the axial 
 and the ganglionic. In the case 
 of the articulata there is but 
 one system, which probably per- 
 forms the functions of both, these subsystems not having 
 yet been differentiated. 
 
 It is difficult, indeed impossible, to conceive how the 
 vertebrate nervous system could have been evolved out 
 of that of the articulates. If vertebrates came as a 
 branch from the articulates, as many think, they must 
 have come off so low down that the distinctive plan of 
 neither was yet declared. 
 
 In treating of the plan of the nervous system we 
 shall take all the articulata together, as the plan is the 
 same in all, although most distinct in the arthropods. 
 
 General Plan of Articulate Nervous System. — To bring 
 this out clearly it is best to take an example from about 
 the middle of the series — as, for instance, a leech, or a 
 crayfish. In Fig. 57 we give a side view of the nervous 
 system of one of the lower Crustacea, and in Figs. 58 and 
 
 Fig. 56. — Cross section through 
 a fish : vs^ visceral system ; 
 vert, vertebra ; ^, bleed sys- 
 tem ; ns, nervous system. 
 
86 PHYSIOLOGY AND MORPHOLOGY OF ANL\L\LS. 
 
 59 a back view of that of a leech and of a crayfish. It 
 may be most simply described as consisting of a chain 
 of ganglia strung along the whole ventral aspect of the 
 body and below the alimentary canal, one to each joint, 
 connected by a single or double thread, and the most ante- 
 rior one — the a'sop/iagea/ gcingliou — being connected with 
 a large ganglion in the head, above the gullet — cephalic 
 ganglion — by two threads, one on each side of the gul- 
 
 bs a/s ns 
 
 Fig. 57. — Diagram of a crustacean ( Vibilia\ : ds, blood system ; a/s, alimen- 
 tary system ; ns, nervous system. 
 
 let, to form the (esophageal collar. It is a remarkable fact 
 that an oesophageal collar is found in nearly all inverte- 
 brates. 
 
 Functions of the Several Ganglia. — The cephalic gan- 
 glion seems to preside over the higher senses, viz., the 
 eyes and the antenncB. It is also the seat of conscious- 
 ness and volition and of whatever instinct or intelli- 
 gence the creature is possessed of, and, through its con- 
 nections with other ganglia, it dominates the whole body. 
 In a word, it corresponds in function to the cerebrum 
 
iNERVOUS SYSTEM OF INVERTEBRATES. 
 
 87 
 
 of vertebrates. The oesophageal ganglion presides over 
 the gathering and mastication of food, and, judging 
 from experiments on crustaceans, it seems also to co- 
 ordinate the motions of the _ 
 
 W 
 
 whole body. It may be said 
 to correspond in function 
 to the cerebellum of verte- 
 
 
 < 
 
 Fig. 58. — Diagram nf nervous sys- 
 tem of a leech, seen from above. 
 
 Fig. 59. — Nervous system of a cray- 
 fish, seen from above. 
 
 brates. The other ganglia preside each over its own 
 body-segment and corresponding limbs automatically, 
 but all under the conscio-voluntary control of the oeso- 
 phageal collar, and especially of the cephalic ganglion. 
 
 Modifications going down and up the Scale. — 
 Taking the above as the type, as we go down the scale 
 of articulates, the cephalic ganglion becomes smaller in 
 comparison with the others, and loses more and more its 
 dominance over them. The other ganglia become 
 more and more independent, and the movements more 
 and more automatic or reflex, until finally, in the lowest 
 
88 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 worms, the different segments become almost wholly 
 independent, and indeed almost like separate animals. 
 
 In going /// the scale, on the contrary, we find oppo- 
 site changes of two kinds, viz., centralization and cephali- 
 zation. Centralization reaches its highest degree in crabs 
 and spiders, where all the ganglia except the cephalic 
 are consolidated into one in the center of the cephalo- 
 thorax, and therefore in the vicinity of the stomach and 
 
 
 ..N.. \.-'^i 
 
 ir 
 
 .,-;^-r:-^ 
 
 U...'.V'.V.'Jii. '•■- -..-'l3'"""X'---' 
 
 ~ '-/V.;r.v.;;.-<r--'-''^V 
 
 4-'' ' '"'-^ 
 
 Fig. 6o. — Xen-ous system of a crab. 
 
 Fig. 6i. — Diarram of nerv- 
 ous system of a beetle. 
 
 locomotive organs (Fig. 6o). The cephalization reaches 
 its highest degree in the high insects, as bees, flies, beetles, 
 etc. (Fig. 6i). 
 
 Embryonic History of the Nervous System. — The same 
 changes are gone through in the embryonic history of 
 one of the higher insects, such as the butterfly. In the 
 caterpillar all the ganglia are separate, one to each seg- 
 ment. In the chrysalis the cephalic ganglion increases 
 in size, and three or four of the anterior body ganglia 
 are drawing closer together. In the butterfly the cephalic 
 ganglion is still larger, and four of the ventral ganglia, 
 together with the oesophageal, have united into a great 
 thoracic ganglion to control the powerful muscles moving 
 
NERVOUS SYSTEM OF INVERTEBRATES 
 
 89 
 
 the wings and legs, as well as to preside over the stomach, 
 etc. (Fig. 62). It is well to observe that the consolidation 
 
 -i^ 
 
 Fig. 62. — Diagram showing the embryonic development of a lepidopter : 
 a, caterpillar ; d, chrj'salis ; c, the perfect butterfly. 
 
 of the segments of the skeleton follows the same course 
 — goes hand in hand with that of the nervous system. 
 
 2. Mollusca. Comparison ivith Other Departments. — 
 The whole plan of structure of these is again different. 
 The vertebrates and arthropods both have a true loco- 
 motive skeleton, the one interior, the other exterior. In 
 both, also, but especially the latter, the skeleton con- 
 sists of segments repeated in a linear series. The mol- 
 lusks have no locomotive skeleton, but only a pro- 
 tective shell. The mollusks also have no segmented 
 
90 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 structure, no repetition of similar 
 parts in a linear series. Now it is 
 probable that the nervous system 
 controls the general structure of 
 the body. Thus while the nervous 
 system of vertebrates is a continu- 
 ous axis, and that of arthropods and 
 worms a string of ganglia running 
 through the body, in moUusks there 
 is no axial arrangement as in verte- 
 brates, nor a linear series of gan- 
 glion as in arthropods. 
 
 General Plan of the Ne7-vous Sys- 
 tem. — In these animals the nervous 
 system consists (i) of an eesophageal 
 collar, and (2) of ganglia irregular- 
 ly placed wherever important func- 
 tions, either nutritive or locomo- 
 tive, are situated. 
 
 Examples. — i. In bivalves (acephala), such as clams, 
 for instance, we have (Fig. 63) : (i) The oesophageal col- 
 lar, which presides over the mouth and head functions 
 (e. g., gathering of food and 
 whatever beginnings of intelli- 
 gence the creature may possess), 
 and also has general presidence 
 over conscious voluntary move- 
 ments, and therefore over other 
 ganglia; (2) a large visceral gan- 
 glion in the region of nutritive 
 and respiratory organs, to pre- 
 side over these ; and (3) a loco- 
 ^ ^ ^, , , motive or foot ganglion to con- 
 
 FlG. 64. — Nervous system of . , . . 
 
 an oyster : c, cephalic gan- trol locomotion. This is all. In 
 
 elion ; v, the visceral gan- , , i *. „ • 
 
 llion. the oyster the nervous system is 
 
 Fig. 63. — Nervous system 
 of a clam : eg, cephalic 
 ganglion ; pg^ pedal 
 ganglion ; vg^ visceral 
 ganglion. 
 
NERVOUS SYSTEM OF INVERTEBRATES. 
 
 9' 
 
 still simpler, since it lacks the locomotive ganglion 
 (Fig. 64). 
 
 2. In gastropods, as the snails (Fig. 65), we find, 
 (1) as usual, the cephalic ganglia and oesophageal col- 
 lar in which are lodged all the higher functions — con- 
 sciousness and volition, control of 
 voluntary movements, etc. ; (2) in 
 the foot or crawling disk a gan- 
 glion to preside over locomotion ; 
 (3) visceral ganglia in the visceral 
 region, appropriately placed. All 
 are, of course, connected with the 
 cephalic ganglion and dominated 
 by it. In the figure, however, only 
 the oesophageal collar is repre- 
 sented. 
 
 3. In cephalopods (squids, cut- 
 tlefish, etc.) (Fig. 66) we have (i) 
 a large ganglion in the head com- 
 pletely and closely encircling the 
 gullet, and forming a very per- 
 fect close-fitting oesophageal col- 
 lar. This is doubtless a combina- 
 tion of cephalic and oesophageal 
 ganglia. It controls the move- 
 ments of the jaws and arms, and 
 it presides over the higher senses of sight and hear- 
 ing, which are well developed in these animals. It is 
 the seat of conscious voluntary motion and of what- 
 ever higher faculties the creature may possess. (2) Be- 
 sides this there are a pair of locomotive ganglia in the 
 muscular mantle to control its contraction. (3) A large 
 visceral ganglion presiding over nutrition and respira- 
 tion. All of these are, of course, connected with the 
 head franelion. 
 
 Fig. 65. — Nervous system 
 of a gastropod : c, ce- 
 phalic ganglion ; e, oeso- 
 phageal ganglion. 
 
92 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 3. Radiata. — We include in these the echinoderms 
 and the coelenterates, as having the same general plan 
 of nervous system. Here again we have the whole plan 
 of structure of the animal different. We have again 
 ^segments of the body, not, however, repeated in a linear 
 
 Fig. 66. — Diagram of nervous system of a squid : eg, cephalic ganglion ; 
 oeff, oesophageal ganglion ; ?ng, mantle ganglion. 
 
 series, but in a circle about the stomach, like the seg- 
 ments of an orange about the pit or the spokes of a 
 wheel about the hub. Therefore we find that the nerv- 
 ous system follows the same plan (Fig. 67). 
 
 General Plan. — Take a starfish as an example. There 
 is a ganglion at the base of each arm, and therefore five 
 
 surrounding the mouth, con- 
 nected into a perfect oeso- 
 phageal collar. From these 
 ganglia there go nerves, one 
 to each arm, giving branches 
 to the arm and terminating at 
 the extremity in an eye spot. 
 In the medusae we have the 
 same perfect radiated struc- 
 ture, but the nervous system 
 has as yet no general center. 
 From the several eye spots on 
 the margin of the disk there go nerve cords a little way 
 toward the center of radiation, but they do not meet 
 one another in a common center. It is difficult to see 
 
 Fig. 67. — Diagram of the nerv 
 ous system of a starfish. 
 
NERVOUS SYSTEM OF INVERTEBRATES. 
 
 93 
 
 how such an animal can have a common consciousness 
 (Fig. 68). 
 
 4. Protozoa. — As these consist of a single cell and 
 not even a cell aggregate, and therefore can have no 
 tissue of any kind, they 
 have no nervous system. 
 Yet there is a general sen- 
 sibility or response to 
 stimulus even in these lit- 
 tle spherules of proto- 
 plasm, x^ll functions ai'e 
 performed, though imper- 
 fectly, by these living 
 drops of jelly. This is 
 the starting point from 
 which by evolution have 
 been differentiated all the tissues and organs and func- 
 tions found in higher departments. 
 
 Fig. 68. — Diagram of a medusa : n n, 
 nen'e ; m, mouth ; st, stomach ; rt, 
 radiating tubes. 
 
CHAPTER II. 
 
 SENSE ORGANS. 
 
 SECTION I. 
 Introductory. 
 
 A NERVOUS system, as seen, consists essentially of a 
 center and two kinds of fibers, one carrying impres- 
 sions from the external world to the center and pro- 
 ducing changes in consciousness, which we call sensation, 
 the other carrying impulses back from the center to the 
 external world and producing changes of phenomena 
 there. But to make these exchanges more efficient there 
 must be special receptive organs of sensation (these are 
 the sense organs) and special executive organs of the will 
 (these are the muscles). We have now to do only with 
 the sense organs. We shall speak of the muscles later. 
 
 Relation of Special Sense to General Sensibil- 
 ity. — The sensory fibers of the conscio-voluntary sys- 
 tem terminate peripherally in external sensitive surfaces, 
 and in the lowest animals everywhere alike — i. e., in the 
 skin. This gives only the existence of an external world, 
 but not \l% properties. This is the case in all the lowest 
 animals, and nothing more. But as we go up the animal 
 scale certain fibers are specialized to give knowledge of 
 certain properties, and certain other fibers to give knowl- 
 edge of other properties. Thus, for example, the fibers 
 of the first pair of cranial nerves are specialized to give 
 us cognizance of odors, and nothing else, and any kind of 
 stimulation of this nerve will produce a perception of 
 94 
 
SENSE ORGANS. 
 
 95 
 
 <^Wc\ 
 
 odor. The fibers of the second pair of cranial nerves are 
 specialized in such wise as to respond to impressions of 
 light, and nothing else, and any stimulation of this nerve, 
 whether by prick- 
 ing or by electric- 
 ity or otherwise, 
 gives rise to a 
 flash of light. The 
 eighth pair is so 
 specialized as to 
 respond to vibra- 
 tions of the air, 
 and any stimula- 
 tion of this nerve 
 gives rise to a sen- 
 sation of sound. 
 Similarlytheninth 
 pair is so special- 
 ized as to produce 
 the peculiar sen- 
 sation which we 
 call taste, and 
 nothing else, and 
 hence stimulation 
 of this nerve in 
 any way will pro- 
 duce this peculiar 
 sensation. This is 
 only one example 
 of the universal law of differentiation, but so admirable 
 a one that we stop a moment to dwell upon it. 
 
 Commencing with the lowest anim.als — protozoa — or 
 with the earliest embryonic condition of the higher, we 
 have first a single cell, and then an unmodified cell ag- 
 gregate. As we pass upward three fundamental systems 
 
 Uiz niociiyiecZ 
 Cell u tar Structure. 
 
 Fig. 69. 
 
96 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 separate, having each a characteristic fundamental func- 
 tion, viz., the epithelial or nutritive system, the blood 
 system, and the nervous system (Fig. 69). The func- 
 tion of the first is the exchange of matter with the ex- 
 ternal world — foreign commerce; of the second, ex- 
 change of matter between different parts of the body 
 — internal carrying trade ; of the third, the exchange of 
 intelligence, both foreign and interior. Putting aside 
 the other two (which, however, are also differentiated), 
 we take only the nervous system. In vertebrates this 
 again is differentiated into three subsystems, viz., the 
 conscio-voluntary, the reflex, and the ganglionic. Put- 
 ting aside again all but the conscio-voluntary, and tak- 
 ing here only the sensory fibers, these are again differ- 
 entiated into five kinds, viz., the five special senses. 
 Even these are probably further differentiated — i. e., dif- 
 ferent kinds of colors, tones, feelings, etc. — and per- 
 ceived by different fibers differently specialized. 
 
 Now as different as these kinds of sensation are from 
 one another, so different that they can not be conceived 
 the one in terms of another, yet they are all probably 
 but modifications of one another and all refinements of 
 the lowest — viz., feeling. It will be interesting, then, to 
 trace the gradations between them in several respects. 
 
 I. Vibrations. — Coarse vibrations are perceived by 
 the nerves of common sensation. We call them jars or 
 tremors. If these increase in number and decrease in 
 size until they lose their separateness — i. e., until there 
 are sixteen in a second, then they appear in conscious- 
 ness in another and entirely different form — as sound — 
 and we have a special nerve adapted to perceive these 
 more rapid vibrations. As the vibrations become more 
 and more rapid they are perceived as higher and higher 
 musical pitch, until they reach some thirty thousand to 
 forty thousand in a second, which is the acutest sound 
 
SENSE ORGANS. 
 
 97 
 
 we can hear. Beyond this the vibrations exist, but we 
 have no nerve specialized to perceive them. After a 
 long interval vibrations again appear in consciousness, 
 but in a new form — as light. This only takes place when 
 they reach a rapidity which can be taken on only by the 
 ethereal medium, viz., four hundred million millions in 
 a second. As they become still more rapid they appear 
 as different colors until they reach about eight hundred 
 million millions, when they again disappear from con- 
 sciousness. We know that they are there, for they show 
 themselves in other ways — e. g., by photography — but 
 we have no nerve specialized to perceive them. 
 
 2. Kind of Contact. — Feeling takes cognizance of 
 any kind of direct material contact, but especially of 
 solid contact. Taste must have liquid contact, and un- 
 less a substance is soluble it can not be tasted. In the 
 case of smell the contact must be more refined ; it must 
 ht gaseous, and unless a substance is volatile it can not 
 be smelled. In the case of hearing and sight there is no 
 direct contact at all of the sensible body, the impres- 
 sion being made by vibrations of a medium, the air in 
 case of hearing and the universal ether in case of sight. 
 All other senses are terrestrial ; sight alone is cosmical. 
 
 3. Objectiveness. — In the three so-called lower 
 senses, viz., feeling, taste, and smell, we are conscious 
 that the sensation is in us — i. e., subjective — although in . 
 smell we already refer the sensible body to a distance. 
 In hearing, the thing perceived no longer seems to be in 
 us, but in the sensible body — in yonder bell — though there 
 may be still a remnant of the subjective element. Final- 
 ly, in sight there is not a remnant of perception of any- 
 thing in us. The sensation, say of redness or blackness, 
 is completely externalized, objectified, referred outward 
 to the sensible body. To the untrained mind there is 
 not a suspicion of any change in the eye. 
 
98 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 Higher and Lower Senses. — The lower senses 
 are those in which contact is direct — viz., feeling, taste, 
 and smell. The higher senses are those in which the im- 
 pression is through the vibrations of a medium, viz., 
 hearing and sight. When the impression is indirect — i. e. 
 through a medium — besides the specialized nerve there 
 is also a mechanical instrument — acoustic in the case of 
 the ear and optic in the case of the eye — placed in front 
 of the specialized nerve in order to make the impression 
 stronger and more definite. The specialized nerve, to- 
 gether with the mechanical instrument, constitute the 
 sense organ. On these two higher senses, therefore, are 
 founded all our fine art and nearly all our science. 
 
 SENSE OF SIGHT AND ITS ORGAN, THE EYE. 
 
 The most specialized of all sensory fibers are those of 
 the optic nerve ; the most refined of aU mstruments is the 
 eye; the highest of all the senses is the sense of sight. 
 And yet we are apt to greatly overestimate what is 
 really given by this sense. The direct gifts of sight are 
 light, its intensity, its color, and its direction, nothing more. 
 All else, as size, distance, relief -form, etc., are judgments. 
 This will be explained hereafter. 
 
 Again, we must distinguish between light objective z.x\ A 
 light subjective. The one consists of vibrations of the ether 
 and exist independent of us ; the other is the peculiar sen- 
 sation produced in us or in some other percipient by these 
 vibrations. The one belongs to physics, the other to 
 physiology, or perhaps, some may claim, to psychology. 
 We are here concerned mainly with light as sensation ; 
 but since this is produced by vibrations, we shall be com- 
 pelled to say something of the physics of the subject also. 
 
 Primary Divisions of the Subject.— The phenom- 
 ena of vision may be divided into two primary groups: 
 monocular and binocular. Monocular visi(;n includes 
 
SENSE ORGANS. 
 
 99 
 
 all those phenomena which are essential and universal ; 
 binocular vision only certain additional phenomena 
 which are the result of the use of two eyes as one instru- 
 ment. We take up, first, the general phenomena char- 
 acterizing all vision — i. e., monocular vision. And as it 
 is impossible to understand these without a knowledge 
 of the structure of the eye, this must be our first subject. 
 
 SECTION II. 
 The Eye of Man — General Structure. 
 
 Shape, Setting, etc. — The eye is a nearly globular 
 organ, about one inch in diameter, a little more pro- 
 tuberant in front. Set in a deep conical socket, it occu- 
 pies only the anterior part; the posterior is filled with a 
 cushion of fat on w'hich it rests and rolls easily in all 
 directions. The exposed part may be 
 covered by the lids, which protect and 
 at the same time wipe and keep it clean 
 and bright. There is really no inter- 
 ruption of the skin here ; on the con- 
 trary, the skin passes over the lid, then 
 over its edge, then under the lid as mu- 
 cous membrane, then much short of the 
 equator of the ball it is reflected on to 
 the ball ((/) (Fig. 70), then over the 
 white of the eye, then over the clear 
 part (but here it is very closely adherent and is trans- 
 parent), and so on to be again reflected on to the lower 
 lid (at a') and out on to the face. By nice dissection it 
 is possible to separate it continuously, so as to leave the 
 ball behind the skin. Now all this tender portion lining 
 the lids and covering the front part of the ball is called 
 the conjunctiva. Ordinary inflammation of the eye is an 
 inflammation of this membrane. Inflammation of the 
 
 Fig. 70. 
 
lOO PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 eyeball itself is a much more serious affair. From this 
 arrangement it is evident that motes in the eye can not 
 go beyond easy removal, as it can not go beyond a a'. 
 Fig. 70. 
 
 Muscles. — The nimble movements of the eye are 
 effected by six muscles in each eye. Four of these, the 
 straight muscles {recti), arise near together at the bottom 
 of the conical socket (Fig. 71), come forward diverging, 
 and are attached to the ball a little in front of the 
 
 equator, one above 
 s??^^^! .1/ (superior rectus), 
 one below (inferior 
 rectus), one on the 
 outside (exterior 
 rectus), and one on 
 the inside (inte- 
 rior rectus). The 
 functions of these 
 are obvious. Each 
 turns the ball in the 
 direction of its pull. When we look upward the two 
 superior recti pull ; when we look downward, the two 
 inferior recti. When we look to the right, the external 
 rectus of the right eye and internal rectus of the left 
 eye pull, and vice versa when we look to the left. When 
 we look at a very near object, then the two interior recti 
 pull so as to converge the eyes on the object looked at. 
 But we can not contract the two exterior recti so as to 
 turn both eyes outward, nor can we turn one eye upward 
 and the other downward, because these movements can 
 not serve any useful purpose, and therefore have never 
 been learned. 
 
 There are two other muscles — the oblicjite. The supe- 
 rior oblique arises along with the recti at the bottom of 
 the socket, passes forward to the opening of the orbit, 
 
 Fig. 71. — Muscles of the eyeball : a, optic nerve ; 
 b, superior oblique muscle ; c, pulley ; d, in- 
 ferior oblique. The other four are the recti. 
 
SENSE ORGANS. lOI 
 
 then through a loop on the inside which acts Hke a pul- 
 ley, then back obliquely over the upper side of the ball, 
 to attach itself on the outside a little behind the equator. 
 Its pull being from the loop, it turns the eye downward 
 and outward and rotates it on its axis inward. The in- 
 ferior oblique arises from the lower part of the inside of 
 the orbital opening, runs under the eye obliquely back- 
 ward and outward across the equator, to attach itself on 
 the outside of the ball a little behind the equator. Its 
 action, therefore, is to turn the eye outward and up- 
 ward and rotate it on its axis outward. 
 
 The use of these oblique muscles is much more ob- 
 scure; their full explanation would carry us too far_ 
 For this we would refer the reader to the author's book 
 on Sight (pages ii and 12 and page 210). 
 
 In the normal condition, looking forward, the axes 
 of the eyes are either parallel or equally convergent, so 
 as to bring their axes together on the object looked at. 
 Any deviation from this position is quickly detected by 
 an observer as a squint or a cast. These malpositions 
 of the eyes are often, but not always, caused by too. 
 great action of some one of the muscles, and are cor- 
 rected by cutting the muscle and allowing it to attach 
 itself to a new point on the ball. 
 
 Coats of the Ball. — Take the ball out of the socket. 
 Dissect away the muscles. THe ball, except the front 
 part, is seen to be invested with a strong white coat of 
 fibrous tissue. This is the sclerotic. It gives form to 
 the eye and serves as attachment of the muscles. The 
 front or more protuberant part is covered with an 
 equally strong but perfectly transparent coat, appar- 
 ently continuous with the sclerotic. This is the cornea. 
 Its function is to retain the form of this part of the eye, 
 and at the same time to freely admit the light. Look- 
 ing through the transparent cornea, we see a little way 
 
102 PHVSIOLOCxY AND MORniOLOGY OF ANIMALS. 
 
 behind it a flat colored screen, the iris, in the center of 
 which is a round hole, the pupil, which in the living eye 
 
 is seen to expand 
 •? ^ or contract, accord- 
 
 ing to the intensity 
 of the light (F'ig. 
 72). The color of 
 the eye is the color 
 of the iris. The 
 pupil is black, as 
 any hole opening 
 in a dark room is 
 black. 
 
 Linings. — For 
 further examina- 
 tion dissection is 
 necessary. We thus 
 find that the sclerot- 
 ic part is lined with 
 
 Fig. 72. — Fection of the eye: O, optic nerve; tWO membranes. In 
 
 ^, sclerotic ; C'//, choroid ; i?, retina ; t', vitre- i- . . ■ 1 
 
 ous body; Cw, ciliary muscle; Cy, conjunc- direct COntact With 
 
 tiva; C, cornea ; /ir'is; /., lens; * aqueous ^| gclerotic is a 
 humor ; ■"■•^, ciliary body or zonule of Zinn. 
 
 dark brown, almost 
 black, and very vascular coat, the choroid. Its function 
 is to absorb the light as soon as it strikes. The choroid 
 lines the whole sclerotic as far forward as the outer 
 margin of the cornea. It is there split into two layers, 
 the anterior one, as it were, drawn together and thick- 
 ened, forms the iris. The posterior one, also drawn 
 together and plaited, forms the ciliary processes radiat- 
 ing about the back portion of the lens. The innermost 
 lining coat is the retina, the most important of all. This 
 is a deep cup-shaped expansion of the optic nerve. This 
 nerve enters the eve socket at the bottom, comes for- 
 ward through the middle of the fatty cushion, pierces 
 
SENSE ORGANS. I03 
 
 the sclerotic and choroid, and then spreads as a thin 
 translucent membrane nearly, but not quite, as far for- 
 ward as the choroid. 
 
 Contents. — The hollow globe thus described is filled 
 with materials as clear as finest glass. These are the 
 humors or lenses of the eye. They are three in num- 
 ber. The crystalline, or lens proper, is a clear, glassy, 
 double convex lens, one third of an inch in diameter 
 and one sixth of an inch in thickness, and somewhat firm 
 and elastic to the touch. It is placed just behind the iris 
 and in light contact with it. It is invested with a trans- 
 parent membrane, the capsule, which continues from its 
 margin outward as a curtain to attach itself all around 
 to the sclerotic a little behind the iris, and thus serves 
 to hold the lens in its place. The lens and the lens cur- 
 tain divides the interior of the eye into two unequal 
 parts. The smaller anterior part is filled with the aque- 
 ous humor, the larger posterior part with the vitreous 
 humor. The aqueous humor is as liquid as water, the 
 vitreous humor about the consistence of soft jelly. All 
 of these may be regarded as lenses. The crystalline, 
 with its double convexity, is the lens par excellence; the 
 aqueous, with its corneal surface, is also a powerful con- 
 vex lens; while the vitreous, on its anterior surface, is a 
 concave lens. 
 
 As already said, all of these are normally clear. 
 Opacity of the crystalline, which often comes with age, 
 constitutes what is called cataract, and produces blind- 
 ness. 
 
 FORMATION OF THE IMAGE. 
 
 Now the whole of what h.as been described is an 
 elaborate instrument to form an image on the sensitive 
 retina. If we ask, "Why an image?" the answer is. 
 Without an image we would perceive light, but not an ob- 
 
104 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 ject. Moreover, the image must be an exact facsimile of 
 the object, because what we see will be a facsimile of the 
 actual image. See, then, the. two very distinct parts of 
 the eye, viz., the specialized nerve (retina) and the optic 
 instrument for making an image. The one is necessary 
 for the perception of light, the other for the perception 
 of objects. 
 
 In order to bring out more clearly the distinctness 
 of these two parts I will use an illustration. Suppose 
 we eviscerate the eyeball — i. e., remove all its contents — 
 leaving only a deep cup-shaped cavity lined with the 
 retina, as can be, and indeed has been, done. Suppose, 
 further, that the retina could retain its healthy condition, 
 which, of course, is impossible but supposable. Then 
 I would undertake to have made a glass eye which, fitted 
 into the cup-shaped cavity, would see just as well as the 
 natural eye, and perhaps even a little better. 
 
 The Necessity of Lenses. — The image must be a 
 perfect facsimile of the object, because what we see will 
 be a perfect facsimile of the retinal image. If there is 
 710 image we will see no object. If the image is blurred 
 the object will seem blurred. If the image is clear and 
 sharp the object will be seen sharp in outline and clear 
 in all surface details. Now light passing through a 
 small hole will make an image (pinhole image), and 
 therefore a very small pupil would make an image on 
 the retina. But such an image is very imperfect. To 
 make a perfect image we must have a lens. The man- 
 ner in which a lens acts in producing an image is shown 
 in Fig. 73. It is seen that all the rays coming from 
 a point {a) of the object are bent (refracted) in such wise 
 as to meet one another at a point {a') of the image, all 
 the rays from the point b are gathered to one point b' , 
 and all from c to c\ and similarly for all other points of 
 the object. Thus for every radiant point of the object 
 
SENSE ORGANS. 
 
 105 
 
 there is a corresponding focal point in the image, and 
 therefore the image will be a perfect facsimile of the 
 object. 
 
 Observe, then, (i) the image is inverted. All images 
 made by lenses (dioptric images) are inverted. They 
 must be, because the central ray of the pencil from each 
 radiant passes straight through the lens withcnit bend- 
 ing, and therefore these central rays all cross one another 
 
 Fig. 73. 
 
 at a certain point in the lens called the nodal point. Ob- 
 serve again (2) that in order to have a sharp image the 
 receiving screen must be exactly at the focus of rays; 
 for nearer than this the rays have not yet come together 
 to a focal point ; farther than this they have already 
 crossed and spread out again. Observe (3) that the size 
 of the image will be to that of the object in the exact 
 proportion to their relative distances from the nodal point. 
 (4) Again, as the object comes nearer the lens the image 
 will be thrown farther back, while if the object recedes 
 from the lens the image will approach the lens. (5) It 
 is not every lens that will make a perfect image. It 
 must have a proper shape, and, moreover, it is found that 
 a system of several lenses is better than a single lens. 
 
 Application of these Principles to the Eye. — 
 Now the eye is an instrument consisting of a system of 
 lenses. The eye therefore forms its images of all ob- 
 jects presented to it. In Fig. 74 rays from the two 
 points A and B of an object .4 B are brought to focus on 
 
lo6 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 the retina at a' b', and so of all intermediate points. If 
 the retina is properly placed the image will be perfect. 
 If the retina be too far back or, what is the same, if 
 the lenses are too refractive, the image will fall short of 
 the retina a' // and will be blurred. This is the case in 
 the nearsighted. The object must be brought nearer, 
 so as to throw the image a little farther back. If the 
 retina be too near the lens or, which is the same, the 
 lenses too little refractive, the image will fall behind the 
 
 Fig. 74. — Diagram illustrating the formation of an image on the retina. 
 
 retina a" b" and will also be blurred. This is the case in 
 the old-sighted for near objects. The refraction must 
 be supplemented by glasses. These defects, however, 
 will be explained later. 
 
 The fundamental fact that the images of all external 
 objects are really formed on the retina may be shown 
 in many ways, (i) Take the dead eye of an ox. Re- 
 move the coats on the back part of the ball, and replace 
 them by a mica plate — an inverted image of the land- 
 scape is seen. (2) The eyeball of a white rabbit shows 
 it without mutilation, because in these albinos the scle- 
 rotic is more transparent and the black pigment of the 
 choroid is wanting. (3) The image may be seen in the 
 living eye by the use of the ophthalmoscope. 
 
 It is seen (Fig. 74) that the central rays of the pen- 
 cils cross one another at the nodal point. In the eye 
 the nodal point is a little behind the center of the lens. 
 
SENSE ORGANS. 
 
 107 
 
 The distance of this point from the retina is about six 
 tenths of an inch. Now when we remember that the 
 relative size of the object and image is exactly propor- 
 tioned to their relative distances from the nodal point, 
 we at once see how extremely minute the retinal images 
 of objects must be. 
 
 COMPARISON OF THE EYE AND CAMERA. 
 
 The purely instrumental character of the eye and its 
 mechanical perfection may be clearly brought out by a 
 comparison with the photographic camera. Take, then, 
 the dead eye and the dead camera — i. e., with only the 
 ground-glass plate in place. They are both optic instru- 
 ments for making an image. Look in at the back of the 
 camera and see the inverted image on the ground-glass 
 plate. Look in at the back of the eyeball and see also 
 the inverted images. Both are dark chambers, with a 
 lens in front to admit the light and make an image by 
 refraction ; both are lined within with black pigment, to 
 absorb the light and prevent reflection from side to side, 
 and so the spoiling of the image. But it is not every 
 lens that will make a perfect image. There are certain 
 defects in common lenses which must be cor- 
 rected to produce the best effects. These are 
 corrected in the best cameras and in all good 
 eyes. 
 
 I. Chromatism. — Li all simple lenses we 
 find that the images are bordered with colored 
 fringes — blue or orange. These mar the sharp- y 
 ness of the image. This defect is corrected in 
 optic instruments by a combination of lenses of different 
 curvatures and different refractive and dispersive pow- 
 ers. \\\ a fine telescope, for instance, a double convex 
 lens is combined with a plano-concave lens. The one is 
 crown glass, the other flint glass (Fig. 75). Such a com- 
 
I08 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 bination is achromatic. Now the eye also is corrected 
 for chromatism, otherwise all objects would appear 
 fringed with colors. The mode of correction also is the 
 same, for the eye also consists of several lenses differing 
 in curvature and in material. In fact, the structure of 
 the eye gave the first hint as to the proper mode of 
 correcting lenses — i. e., by combination. 
 
 2. Aberration. — In ordinary lenses (i. e., those 
 with spherical curvature) it is found that the marginal 
 rays are refracted too much for the central rays, and 
 therefore all the rays are not brought together to the 
 same focus. This may be partly remedied by cutting 
 off the marginal rays by a diaphragm, but, of course, 
 with great loss of light. But it can be completely reme- 
 died only by making the central part of 
 the lens more refractive. This can be done 
 either by graduating the density of the 
 matter of the lens from the margin to the 
 center, or else by graduating the curvature 
 from margin to center. The first method 
 art has found impossible to accomplish, 
 and therefore it adopts the second method. 
 Instead of a spherical curvature, it makes 
 
 an elliptical curvature, the axis of the ellipse being the 
 a.\is of the lens. In this way the best lenses are cor- 
 rected for aberration. 
 
 Now the eye also is corrected, for otherwise it could 
 not sharply define the objects it looks at. How is it cor- 
 rected ? It is probable that it uses both methods. The crys- 
 talline lens consists of concentric layers, becoming denser 
 and denser to the center (Fig. 76). Also the curvature 
 of the corneal surface is elliptical instead of spherical. 
 
 3. Adjustment for Distance. Focal Adjustment. 
 Accommodation. — We have seen that in order to have a 
 good image the receiving screen must be at the exactly 
 
 Fig. 76. — Sec- 
 tion showing 
 the structure 
 of the lens. 
 
SENSE ORGANS. IOq 
 
 proper place. Now if the images of all objects at all 
 distances were thrown to the same place, we might find 
 that place and fix the screen permanently there. But 
 we have already seen (page 105) that as the object is 
 farther away the image comes nearer the lens, and as 
 the object approaches the image recedes. Now there 
 are two ways of adjusting this : if the lens retains its 
 form then the screen must be moved back and forth to 
 the proper place, or else, if the screen be fixed, the lens 
 must change its form so as to throw the image on the 
 fixed screen — i. e., it must become more refractive as the 
 object comes nearer. The former is the method of the 
 camera and of nearly all optic instruments, such as the 
 opera glass, the field glass, etc. In these the tube is 
 drawn out so as to carry the screen back for near ob- 
 jects, and is pushed in so as to carry the screen nearer 
 the lens for distant objects. The microscope is an ex- 
 ception. Usually, when the object is brought near (as 
 
 /- 2^ — -iO^ 
 
 Fig. 77. — A, eye observed ; B, eye of observer ; C, section of candle flame ; 
 /, a distant point of sight, and n, a near point of sight. (After Helm- 
 
 holtz. I 
 
 in magnifying greatly) the lens is changed and the image 
 is thrown to the same place. 
 
 Now in the eye the adjustment for distance is per- 
 fect, for objects at all distances from five or six inches 
 to infinite distance ; for the moon or sun is seen perfectly 
 defined. How is it done ? It is done by changing the 
 
no PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 Fig. 78. 
 
 form of the lens. For the complete proof of this we are 
 
 indebted to Helmholtz. The experiment is shown in 
 
 F'g- 77> ii"* which A is the eye of the 
 
 • patient observed, B the eye of the ob- 
 server, and C a candle. In looking into 
 the eye A with an ophthalmoscope three 
 images of the candle are distinctly seen 
 (Fig. 78). One of these — the first {a) — is 
 the reflection from the corneal surface. 
 Thesecond {b), much fainter and smaller, 
 is from the anterior convex surface of the lens. Both of 
 these are upright. The third (^), still fainter and smaller, 
 is inverted, because it is reflected from the posterior sur- 
 face of the crystalline, which is concave. All this is ob- 
 served while the patient gazes at a distant point (/, 
 Fig. 77). Now tell the patient to look at a very near 
 point (;/), perhaps six inches from the eye. Immediately 
 the image {b, Fig. 78) is seen to change. It becomes 
 smaller, and changes its place in such wise as to show 
 that the anterior surface of the lens has become more 
 convex, has bulged out, and even pushed the iris out a 
 
 Fig. 79. — F^ lens adjusted to distant objects ; N, to near objects ; «, aqueous 
 humor ; d, ciliary muscle ; t", ciliary process. 
 
 little (see Fig. 79). Thus it seems certain that in accom- 
 modation of the eye to near vision the lens thickens and 
 becomes more refractive. But the question still remains, 
 How does it do this ? We are distinctly conscious of a 
 muscular strain. What muscle ? 
 
SENSE ORGANS. HI 
 
 This is not definitely settled, but we are again in- 
 debted to Helmholtz for the most probable view, viz., 
 that it is done by contraction of the ciliary muscle. W'e 
 have already mentioned (page 103) the lens capsule, its 
 continuation as a curtain outward all around, and its 
 attachment to the sclerotic a little behind the iris. Now 
 this curtain is taut, and therefore the capsule presses 
 gently on the elastic lens and flattens it. This is the 
 passive condition of the eye when it is accommodated 
 to distant objects. Now there is a muscular collar about 
 the iris, on the inside of the sclerotic, the fibers of w'hich, 
 arising from the outer margin of the iris, radiate out- 
 ward and backward, and, taking hold of the outer margin 
 of the lens curtain where it is attached to the sclerotic, 
 pulls it forward to where the circumference is less, and 
 therefore slackens its tautness and allows the elastic lens 
 to bulge. The amount of bulging is in proportion to 
 the slackening, which will be in proportion to the con- 
 traction, and this in proportion to the nearness of the 
 object. 
 
 See, then : the eye is more like the microscope, in 
 that it changes the lens rather than removes the screen. 
 But how much more perfect ! The microscope has its 
 four-inch lens, its two-inch lens, its one-inch, its half- 
 inch, its tenth-inch lens, and changes one for another 
 as the object is nearer. The eye has but one lens, 
 but it changes the form of its one lens so as to make it 
 a si.\-inch lens, a foot lens, a twenty-foot lens, a mile 
 lens, or a million-mile lens, for at all these distances it 
 makes a perfect image. 
 
 4- Adjustment for Light. — In both the camera and 
 the eye some contrivance is wanted to regulate the 
 amount of light admitted. In both, too, this is done by 
 diaphragms with holes of varying size. In the eye the 
 iris is the diaphragm and the pupil the hole. But in this 
 9 
 
112 PHYSIOLOGY AND MORPHOLOGY OF ANLMALS. 
 
 case the diaphragm is contractile and the pupil self- 
 regulating. 
 
 Structure of the Iris. — The iris, as already seen, is a 
 continuation of the choroid, which lines the sclerotic as 
 far forward as a little in front of the lens curtain, and then 
 is drawn together transversely to form the iris. This 
 part is thickened with muscular fibers. These are of 
 two kinds, circular and radiating, as shown in the figure 
 (Fig. 80). The circular fibers, by contracting, draw the 
 pupil together and make it small ; the radiating fibers 
 take hold on the margin of the pupil and pull it outward 
 
 in every direction and en- 
 large it. Or, perhaps better, 
 regard the circular fibers as 
 sensitive and actively con- 
 tractile and the radiating 
 
 FIG. 8o.-Showin| structure of ^^^^^ ^^ ^,^^^j^ ^^^ paSsively 
 
 contractile. When the circu- 
 lar fibers contract they draw up the pupil, stretching the 
 radiating fibers. When they relax, the radiating fibers 
 elastically contract and enlarge the pupil. Now, the 
 circular fibers are in sympathetic relation with thfe 
 retina in such wise that stimulation of the retina by 
 strong light reflexly causes the pupil to contract. As 
 the light decreases, the pupil expands to take in more 
 until, in the dark or in case of paralysis of the ret- 
 ina, the pupil expands until the iris becomes a slender 
 ring. 
 
 The hint has been taken here also by the instrument 
 maker. The iris diaphragm of the microscope is made 
 of thin overlapping plates of steel, which, by turning a 
 thumbscrew, slide toward or away from one another, 
 contracting or enlarging the opening between. It is 
 a beautiful contrivance, but far inferior to the liv- 
 ing iris. 
 
SENSE ORGANS. u 
 
 DEFECTS OF THE EYE AS AN INSTRUMENT. 
 
 We have shown the beauty of the eye as an instru- 
 ment by comparing it with the photographic camera. 
 But all eyes are not perfect. The defects of the eye are 
 indeed quite common, and apparently becoming more 
 and more common through abuse of this delicate organ, 
 especially in the schoolroom. In order to understand 
 these defects it is necessary to detine the normal eye. 
 
 Normal Sight— Emmetropy. — The normal eye in 
 2l passive state is prearranged for a perfect image of a dis- 
 tant object. The focus of parallel rays is on the retina. 
 For all nearer distances it accommodates itself by action 
 of the ciliary muscles until the object is as near as five 
 or six inches. Nearer than this it can not accommodate 
 itself to make a perfect image. Its range of distinct 
 vision, therefore, is from six inches to infinite distance. 
 This is the standard. Any considerable deviation from 
 this is a defect. The most common defects are niyopy\ 
 hyperopy, prcsbyopy, and astii^niatis)/!. 
 
 Myopy, Brachyopy — Nearsightedness. — This is 
 perhaps the most common of all defects of the eye, espe- 
 cially in large cities and in most advanced communities. 
 In the myopic eye the refractive power of the lenses of 
 the eye is too great for the position of the retina. The 
 focus of parallel rays when the eye is passive is not 
 on the retina, but in front of it. The rays must be di- 
 vergent to make a perfect image on the retina. There-, 
 fore distant objects can not be seen distinctly. The 
 object must be brought near to a certain limit before it 
 can be seen well. But within that limit it accommodates 
 itself like the normal eye. In the normal eye the range 
 of distinct vision is from infinite distance to six inches; 
 in the myopic eye the range is from a yard to four 
 inches, or a foot to three inches, or six inches to two 
 
114 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 inches, according to the degree of myopy. The fault 
 being too great refraction, the remedy is, of course, to 
 diminish the refraction by the use of concave glasses. 
 If these be so chosen that the focus of parallel rays is 
 on the retina when the eye is passive, so that distant 
 objects are seen distinctly ; then the eye accommodates 
 itself to all nearer objects, and behaves in all respects 
 like a normal eye. 
 
 Kyperopy— Oversightedness. — This is the oppo- 
 site of myopy. In the passive state the focus of parallel 
 rays is behind the retina. In this defect the refractive 
 power of the lenses is too small for the position of the 
 retina. The defect is far more common than generally 
 supposed. It often exists unknown to the |)atient or his 
 friends. Distant objects are seen perfectly well, because 
 a slight accommodation brings the focus on the retma. 
 But the eye is never passive unless in sleep. For this rea- 
 son it is often a distressing defect, producing headaches 
 and the like. Since the defect is a deficiency in refrac- 
 tive power, the obvious remedy is the use of slightly 
 conve.x glasses suited to the degree of deficiency. The 
 eye then functions exactly like a normal e3'e. 
 
 Presbyopy— Old-sightedness. — Both the preced- 
 ing are structnial defects; this is a functional defect. 
 The eye may be structurally normal — i. e., in a passive 
 state the focus of parallel rays is on the retina — but it 
 has lost the power of accommodating itself to divergent 
 rays. The patient sees well distant objects, but can not 
 see near objects well. In order to see near objects well 
 the eye must be re-enforced by convex glasses. But 
 the use cf glasses can not make the eye normal, as in 
 the other two defects, because it has lost the accommo- 
 dating power. Therefore the glasses are not worn ha- 
 bitually, as in the other two defects, but only in looking 
 at near objects — not in walking, but only in reading. 
 
SENSE ORGANS. II5 
 
 The term longsightedness or farsightedness is some- 
 times used to express this defect. It is a misnomer. 
 No eye can be longer-sighted than the young normal 
 eye. It can define perfectly the edge of the moon or of 
 the setting sun. Moreover, all eyes — the myopic and 
 hyperopic, as well as the normal — undergo the presby- 
 opic change with age ; but the myopic eye does not 
 thereby become normal, as many suppose. 
 
 Astigmatism — Dim-sightedness.— All other eyes 
 see distinctly at some distance, but the astigmatic eye 
 does not see distinctly at any distance. Hence the term 
 dim-sightedness. In all other eyes all the rays of light 
 issuing from a radiant point are brought to a ioczX point ; 
 in this one they are brought together to a focal line, or 
 rather to two focal lines, one farther than the other. 
 Hence the term astigmatism.* In all other eyes the 
 curve of the lenses, and therefore their refraction, is 
 equal in all directions. In this one the curve and the 
 refractive power /// and down is greater or less, usually 
 greater, than from side to side. The remedy is, of course, 
 the use of glasses which correct the unequal refraction. 
 For example, suppose the curve and the refractive power 
 from side to side is normal, but the curve and refractive 
 power up and down is too great, then the glasses should 
 have no curve horizontally, but should be concave ver- 
 tically — i. e., should be cylindrical concave glasses, with 
 the axes of cylinder horizontal. 
 
 The usual test for astigmatism is a large rectangular 
 cross, thus -\-. At a certain distance the astigmatic eye 
 sees the vertical line distinctly, but the horizontal line is 
 blurred. At a certain other distance the horizontal line 
 is distinct, but the vertical blurred. But at no distance 
 are they both distinct. 
 
 * Not a point. 
 
Il6 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 SECTION III. 
 
 The Retina and its Functions. 
 
 Thus far, as much as possible, we have confined our- 
 selves to the eye as an instrument for making an image, 
 and have compared it with the camera in order to show 
 the beauty of its adaptation for that purpose. But in 
 both the camera and the eye the image is only a means 
 to accomplish an end — to make a photogram in one case 
 and accomplish vision in the other. In both cases there 
 must be a sensitive screen to receive the image — the 
 
 Fig. 8i. — A view of the two eyes, with optic nerves: ck^ optic chiasm; 
 rr , nerve roots; n and «', right and left optic nerves. (After Helm- 
 hoitz. ; 
 
 iodized plate in the one, and the living retina in the 
 other. In both cases, too, the most wonderful changes 
 take place in these sensitive screens. Before we can un- 
 
SENSE ORGANS. 
 
 117 
 
 derstand the phenomena of vision we must know some- 
 thing of the general structure and function of the retina. 
 Structure of the Retina. — The second pair of cra- 
 nial nerves, as already seen, arise by fibers partly from 
 the optic lobes and partly from the thalamus. These 
 fibers unite to form the optic roots (;-, Fig. 81), which con- 
 verge and unite to form the chiasm {c/i). From the chiasm 
 there go out diverging the two optic nerves («), which 
 enter the eye sockets near the conical point, pass for- 
 
 FlG. 82. — Generalized section of retina, etc.: O, optic nen-e ; S, sclerotic; 
 ch, choroid ; R, retina ; /', baciUary layer ; g, r^anular and nuclear 
 layer ; /, fibrous layer ; \', vitreous humor ; c, central spot. 
 
 ward through the fatty cushion and between the recti 
 muscles, enter the eyeballs a little to the interior or 
 nasal side of the a.\is or south pole, pierce the sclerotic 
 and choroid, and spread to form the innermost lining 
 coat directly in contact with the vitreous humor. As a 
 thin, translucent coat it passes forward almost to the 
 attachment of the lens curtain, forming thus a deep cup- 
 shaped receptive plate (Fig. 33, p. 51). Its greatest 
 thickness at the bottom of the cup is one quarter milli- 
 metre or one one-hundredth inch, and thence thins out 
 to a feather edge on the forward margin of the cup. 
 
 Although so thin, its structure is very complex. In 
 a cross section under a low power of the microscope, it 
 
Il8 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 is seen to consist of three layers (Fig. 82) : (i) an inner 
 ox fibrous layer in contact with the vitreous body, con- 
 sisting wholly of in- 
 terlacing fibers of the 
 optic nerve; (2) of an 
 outermost layer — bacil- 
 lary layer — composed 
 entirely of rodlike 
 bodies set on end and 
 in contact with the 
 choroid; and (3) be- 
 tween these a middle 
 layer, consisting of 
 granules and larger 
 nucleated cells, and 
 therefore called the 
 agranular and nuclear 
 layer. 
 
 All three layers ex- 
 ist in all parts of the 
 retina except in two 
 small spots : (i) where 
 the optic nerve en- 
 ters there can be, 
 of course, only the 
 fibrous layer; (2) in 
 the center of the bot- 
 tom of the cup and in 
 the very axis of the 
 ball there is a small 
 
 Fig. 83. — Enlarged section of retina (after depression in which 
 Schultze) : A, general view; B, nervous v, cu i 
 elements; a, bacillary layer; b, interior the nDrouS layer IS en- 
 limit of this layer; c, external nuclear [Jrely, and the granu- 
 layer ; a, external granular layer ; e, m- -' ' ° 
 temal nuclear layer ;/, internal granular lar and nuclear layer 
 layer; ^, ganglioniclayer ; /i, fibrous layer, , . , 
 
 consisting of fibers of optic nerve. nearly entirely want- 
 
SENSE ORGANS. 1 19 
 
 ing. This is called the central spot on account of its 
 position, and the fovea on account of its depression. 
 
 But the importance of the retina is so great that it 
 must be studied more carefully under a higher magnifi- 
 cation. Fig. 83 is a highly magnified section. Concern- 
 ing the inner or fibrous layer nothing more is revealed. 
 The middle layer is seen to be very complex, consisting 
 of several granular layers and several layers of nucle- 
 ated cells and one layer 
 
 of very large ganglion- u»:^^,^ ^~ 
 
 ic cells. The functions ^^M^^^ ^M. 
 of these various layers 
 are not certainly known. 
 
 The bacillary layer is Fig. 84. — Pacillary layer, viewed from 
 
 the outside surface : A, appearance of 
 now seen to contain two usual surfac3; B, appearance of sur- 
 
 kinds of elements— the ^^"^, °P'^^ raised mar -in of central 
 
 spot ; C, surface of central spot. 
 
 one slenderer, longer, 
 
 and more rodlike, the other shorter, stouter, and more 
 conelike. The rods are about one fourteen-thousandth 
 of an inch (one five-hundred-and-sixtieth millimetre), 
 and the cones about one five-thousandth of an inch (one 
 two-hundredth millimetre) in diameter. The rods are 
 usually most numerous. Fig. 84 is a view of the ouier 
 surface, showing the larger cones surrounded by the 
 more num'=;rous rods. But the relative number of these 
 is not the same in all parts. 
 
 Distribution of the Rods and Cones. — On the 
 anterior margin of the retina there are no cones, but 
 only rods. As we approach the bottom of the retinal 
 cup the cones become more and more numerous, and at 
 the same time smaller until in the central spot or fovea 
 there are no rods, but only cones, and these have become 
 very small, only about one ten-thousandth of an inch 
 (one four-hundredth millimetre) in diameter (Fig. 84). 
 Further, it must be observed that the fibers of the fibrous 
 
120 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 layer — i. e., of the optic nerve — turn back through the 
 granular and nuclear layer and terminate in the rods 
 
 and cones (see Fig. 
 
 1 83). These, there- 
 fore, must be re- 
 garded as fiber ter- 
 minals of the optic 
 
 H nerve. It is proba- 
 ble that the connec- 
 tion between the 
 nuclear cells and the 
 
 4 rods and cones is by 
 means of dendrites; 
 and, furthermore, 
 that the dendrites of 
 
 a nuclear cell touch 
 those of several xodi%, 
 while the cells cor- 
 
 ^ responding to the 
 cones, especially 
 those of the fovea, 
 
 Fig. 85. — Diagram showing the n'.ode of con- . . , 
 
 nection between nucleated ce!ls and the rods COmmuniCate With 
 
 and cones: i, fibrous la^er ; 2, ganglionic ,^,,„i, ff^u-^r n^r 
 
 layer; 3, first granular layer; 4, first nuclear i""^'' lewer, pci- 
 
 layer; 5, second granular layer; 6, second hapS With only one 
 nuclear layer ; 7, bacillarv layer. 
 
 cone, as seen in the 
 accompanying figure (Fig. 85). The importance of this 
 will be seen hereafter (page 131). 
 
 The Distinctive Function of the Layers. — The 
 function of the fibrous layer is wholly transmissive. It 
 is made up of sensory fibers, which transmit impressions 
 on the retina to the brain. The function of the middle 
 layer is doubtless intermediary between the elements of 
 the bacillary layer and the fibers of the optic nerve. The 
 true receptive layer is the bacillary. This is proved (i) 
 by the fact that there is only one spot where this layer is 
 
SENSE ORGANS. 121 
 
 wanting — viz., where the optic nerve enters — and this 
 spot is blind ; and (2) by the fact that the central spot or 
 fovea is the most sensitive spot in the retina, and there 
 the fibrous layer is entirely, and the middle layer almost 
 entirely, wanting. In this spot the bacillary layer is al- 
 most directly exposed to the impression of light. Thus, 
 then, the fovea is the most highly organized spot of 
 the retina. It differs from other parts in three particu- 
 lars : I. The bacillary layer there consists only of cones. 
 
 2. The cones there are much smaller than elsewhere. 
 
 3. The bacillary layer is there almost directly exposed 
 to the influence of light. 
 
 The distinctive functions of the rods and the cones 
 will come up for discussion hereafter. Suffice it to say 
 now that the perception of color seems to reside in the 
 cones alone. 
 
 Visual Purple. — There has recently been found in 
 the outer or terminal ends of the rods, but not the cones, 
 a purplish red substance, which probably has an important 
 but imperfectly understood function in vision, and is 
 therefore called visual purple. It is bleached by light, 
 and again restored by darkness. Photographic images 
 (optograms) of objects may be taken on the purple retina 
 and by appropriate means may be fixed.* The discov- 
 ery of this substance naturally excited hopes that its 
 study would solve the mystery of sensation by reducing 
 it to a chemical process; but these hopes have not been 
 realized, for it is now known that the visual purple is 
 not present in all animals, nor does it exist in the cones, 
 and therefore is not present in the fovea, which is, nev- 
 ertheless, the most sensitive spot in the retina both to 
 form and color, though not to simple faint light. The 
 visual purple, therefore, is certainly not essential to the 
 perception of either light or color. 
 
 * Foster's Physiology, p. 1254.. 
 
122 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 There are, however, some facts concerning the occur- 
 rence of visual purple which throw light on its function.* 
 It is wanting in night-blind animals, such as snakes and 
 most birds, and is abundant in nocturnal animals, such 
 as most ruminants and all cats, and in owls among birds. 
 Its probable function is to give greater sensitiveness 
 to the impression of simple _/</////, diffused light, but not 
 to form and color, and is therefore found in the rods, 
 but not in the cones. It is easily destroyed by light 
 and re-formed in darkness, and is therefore specially 
 adapted to feeble light. Hence in very faint light, but 
 not in full light, at night, but not by day, we detect the 
 presence of an object (though not its form and color) by 
 indirect better than by direci vision. Direct vision is by 
 the cones only (because the image is then on the fovea) ; 
 indirect vision is by rods mostly, and these are made 
 specially sensitive by the presence of the visual purple. 
 This explains also the temporary night blindness of one 
 coming out from a brilliantly lighted room into the night. 
 The restoration of the night vision is the result of the 
 re-formation of the visual purple destroyed by the bril- 
 liant light. 
 
 SECTION IV. 
 
 Perception of Space and of Objects in Space, 
 
 There is a certain fundamental property of the retina, 
 optic nerve, and associated brain apparatus which must 
 now be explained, for it lies at the very basis of visual 
 phenomena. 
 
 The First Law of Vision. The Law of Exter- 
 nal Reference of Retinal Impressions. — An image 
 is formed on the retina, but we do not see the retinal 
 image. We do not see anything in the eye, but some- 
 
 * Parinaud, Rev. Scientifique, vol. iv, 134, 1895. 
 
SENSE ORGANS. 
 
 123 
 
 thing — the object — outside in space. The object, how- 
 ever, is the facsimile of the image. It is as if the retinal 
 image were [projected outward into space and appeared 
 there as an external image, which we interpret as an 
 object. I said as if. Really the retinal image is the 
 sign of the definiteness of the impression. For the 
 molecular changes in the retina are graduated in degree 
 and kind exactly as the light is graduated. The light 
 image is a sign of an invisible molecular image. It is 
 this perfectly definite impression, this invisible molecu- 
 lar image, which, by the brain or by the mind, is referred 
 outward into space and interpreted as an object. 
 
 This law is so fundamental that we stop a moment 
 to show that it is no new law specially enacted for the 
 sense of sight, but only an extreme modification of a 
 general law of sense-perception. 
 
 Comparison with Other Senses. — We have al- 
 ready seen that stimulation of any sensory nerve in its 
 course is referred by the brain to the /^/v/Z/dra/ extrem- 
 ity. If the ulnar or the sciatic nerve is pinched any- 
 where in its course, pain is felt not at the place, but in 
 the fingers or toes where the nerve is distributed. In 
 the case of an amputated leg, pinching the end of the 
 nerve in the stump causes pain in the toes where the 
 nerve is naturally distributed, even though there be no 
 longer any foot at all. In ordinary sensory nerves, 
 therefore, stimulation of any part of the nerve is referred 
 to the /'^/•//'//^/'a/ extremity. Now, the optic nerve differs 
 only in the fact that the impression is referred beyond \.\\^ 
 peripheral extremity and out into space. 
 
 This seems a great difference, but remember the grad- 
 ations already spoken of (|)age 96). In touch and taste 
 the reference is only to the peripheral extremity, because 
 the necessary condition of sensation is direct contact. In 
 smell we have indeed a sensation in the nose, but we 
 
124 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 already refer it to a distant body, because the condition 
 of sensation is air-borne particles. Finally, in hearing, 
 and especially in sight, we lose entirely the sense of con- 
 tact or local impression, and refer it wholly into space. 
 
 Illustrations of the Law. — We shall now try to 
 make the law clear by many illustrative experiments : 
 
 Experiment i. — If we could bare the retina and touch 
 its surface we would not feel it, but would see di flash of 
 light — Where ? In space and in a direction exactly oppo- 
 site, or at right angles to, the touched surface. If the 
 optic nerve be laid bare and pinched we would feel noth- 
 ing, but would see a flash of light in space opposite that 
 part of the retina where the pinched fibers are distributed. 
 Of course we can not deliberately make this experiment, 
 but the flash of light is observed in passing electricity 
 through the nerve, and also in cases of extirpation of 
 the eye at the moment of rupture of the optic nerve. 
 
 Experiment 2. Phosphenes. — Close the eyes, and 
 then press the finger into a corner of one of them. A 
 brilliant-colored circle is seen in the field of darkness 
 opposite the point pressed. These are called phos- 
 phenes. In my own case they are brilliant golden rings, 
 with steel-blue centers. They are caused by the inden- 
 tation of the sclerotic and, through it, the retina. But 
 any change whatever in the retina shows itself as an 
 appearance in space. 
 
 Experiment 3. Miiscce Volitantes. — Look at a white 
 wall, or better, a bright sky. Nearly all observers will see 
 specks or clouds or tangled threads in the bright field, 
 slowly gravitating downward. They are called " muscse 
 volitantes," or flying gnats. What are they ? They are 
 slight imperfections in the vitreous body. These less 
 transparent spots cast their shadows on the retinal bot- 
 tom. But any variation of the retinal surface shows itself 
 as an appearance in the field of view directly opposite. 
 
SENSE ORGANS. 
 
 125 
 
 Fig. 86. — Internal view of the 
 retina, showing the retinal 
 vessels ramifying; over the 
 surface, but avoiding the cen- 
 tral spot. (After Cleland.) 
 
 Experiment 4. Purkinjes Figjtres. — Darken the 
 room; close one eye, say the left ; hold a lighted candle 
 very near the open eye, three or four inches, and to the 
 right side, so that the retina is 
 strongly illuminated. Gaze on 
 the opposite wall until the field 
 of view becomes darkened by 
 excess of light. Now move the 
 candle about, back and forth, 
 up and down. Presently we set 
 a shadowy specter covering the 
 whole wall, like a great bodi- 
 less spider with branching legs, 
 or a spectral tree with leafless 
 branches. What is it? It is 
 an exact but greatly enlarged 
 image of the blood vessels of the retina (Fig. 86). These, 
 ramifying in the granular layer, and therefore in front 
 of the bacillary or receptive layer, cast their shadows on 
 the latter. But any change or variation of this layer is 
 seen as an appearance in space. 
 
 Experiment 5. Ocular Spectra — After-images. — Gaze 
 steadily at the setting sun a moment, and then turn 
 away and look at the wall, the sky, or at a distant build- 
 ing. A colored image follows the eye and is cast on 
 what it looks at. Why so ? The sun's image makes so 
 strong an impression on the retina that it is retained for 
 a considerable time; it makes a brand on the retina. 
 But every change or variation in the retina, whether 
 shadow or image or brand, shows itself as an appear- 
 ance in the field of view. 
 
 We have taken the extreme case of the sun, but any 
 bright object, such as a candle flame in a dark room or 
 a stained-glass window, will produce a similar effect. In 
 the case of bright colors, as in stained-glass windows, 
 
126 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 the after-image is of a complementary color. Red is seen 
 as green and vice versa; blue as orange-yellow and vice 
 versa. But to discuss these color phenomena would 
 carry us too far. 
 
 Generalization of these Facts. — We have seen 
 that all changes or variations of whatever kind in the 
 retina, whether images of objects, or shadows, or brands, 
 show themselves as appearances in space. TJicrefore 
 space itself as perceived by the eye is an externalization or out- 
 ward reference of retinal states. With the eyes open, the 
 field of vieiv crowded with objects is the externalization 
 of the stimulated retina crowded with images; with the 
 eyes shut, the field of darkness is still the externalization 
 of the unstimulated retina. The sense of space before 
 the eyes is ineradicable. We can not rid ourselves of it. 
 In the dark or with the eyes shut it is still there, as the 
 field of darkness. It is in front, not behind the head. 
 It is a positive appearance, which may be roughly out- 
 lined and may be described. It is not black, but rather 
 dark-gray or brown, with confused markings and cloud- 
 ings and sometimes colored spaces scattered through- 
 out. Thus we have an abiding sense of space as the ex- 
 ternal representative of the retina, even though we see 
 no object in it, precisely as we have an abiding sense of 
 a hand, even though we feel nothing with it. 
 
 The Second Law of Vision ; the Law of Direc- 
 tion. — The previous law asserts the perception of space 
 as the externalization of retinal states, and therefore the 
 reference of all retinal images to that space. This law 
 gives the direction of the external reference. In several 
 of the preceding experiments we have alluded to their 
 general direction. We come now to define it exactly. 
 
 The law may be given thus: Any impression on a 
 rod or cone of the bacillary layer is referred by the rod 
 or cone back into space end on — i.e., at right angles to 
 
SENSE ORGANS. 
 
 127 
 
 h\ 
 
 the retinal surface. Or it may be otherwise expressed 
 thus : The impression produced by Hght from any radiant 
 point in space is 
 referred back along 
 the ray line to the 
 place whence it 
 came. Either of 
 these formulae is 
 sufficient for gen- 
 eral statement, 
 but the former is 
 probably the more 
 exact. The lat- 
 ter, however, we «' 
 will use for illus- ^■ 
 tration. Thus we 
 have two con- 
 caves, the spatial 
 and the retinal ; 
 two worlds, a 
 macrocosm and a 
 microcosm ; and 
 these correspond 
 with one another 
 point for point, 
 and exchange 
 with one another by impression and by external refer- 
 ence along the lines connecting them. This is repre- 
 sented in Fig. 87. In this figure, of all the rays proceed- 
 ing from a radiant point and gathered by the lens to a 
 focal point on the retina, we take only the central ray, 
 for this represents the resultant impression. These cen- 
 tral rays cross one another at the nodal point «. If now 
 a, b, e, be three stars, then the light from a impresses 
 the retma at a', and is referred straight back along the 
 
 Fig. 87. 
 
 -Diagram representing corresponding 
 points, retinal and spatial. 
 
128 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 ray line to its proper place. IJght from b passes through 
 the nodal point n and strikes a rod or cone in the upper 
 part of the retina, b' , and is referred back along the ray- 
 line to b. Similarly light from e passes a little downward 
 through n and impresses the lower part of the retina, and 
 is referred back to its proper place below. Thus the 
 three stars will be seen in proper relative positions. Thus 
 the lower part of the retina corresponds to the upper 
 part of the field of view, and the upper part of the retina 
 to the lower part of the field ; the right side of the retina 
 to the left side of the field, and the left side of the retina 
 to the right side of the field ; and in each case with the 
 utmost exactness, point for point. Every rod and cone, 
 as it were, knows its own point in space, and refers its 
 impressions there. 
 
 Comparison with Other Senses.— Now, this is no 
 unique law peculiar to sight alone, but a general law of 
 sense-reference, though refined to the last degree in this 
 sense. In the case of any impression on a sensitive sur- 
 face the cause is referred in a general way along the line 
 of impression. Suppose we are captive, bound and blind- 
 folded, and surrounded on all sides by Apache Indians 
 and a target for their arrows, could we not tell roughly 
 the direction of each shooting Apache by the direction 
 of the punch of his arrow? Now, every radiant is shoot- 
 ing rays into the eye. Is it not natural that every im- 
 pinging ray should be referred back along the line of 
 its flight to the point whence it came ? Now, the retina 
 is specially and wonderfully organized to do this with 
 mathematical exactness. 
 
 These two laws, the law of spatial reference and the 
 law of direction, are fundamental. The one explains 
 why impressions made in ourselves are referred outward 
 into space. The retina, or the brain through the retina, 
 creates visible space. The other gives the direction of 
 
SENSE ORGANS. 
 
 129 
 
 such reference, the exact place of all objects and radiants 
 in space. Together they explain all the phenomena of 
 monocular vision except the perception of color. With 
 this exception, the whole science of monocular vision 
 is but an explication of these two laws. All that we 
 have further to do, therefore, is to take up several 
 important subjects and show how completely they are 
 explained by these laws. 
 
 I. ERECT VISION. 
 
 Statement of the Problem. — We have seen that 
 the retinal images are inverted. We have also seen that 
 these images are referred, as it were projected, outward 
 into space, and are seen there as external images, the 
 signs and facsimiles of the objects which produced them. 
 How is it, then, that objects are seen in their natural 
 positions — i.e., erect I This problem has puzzled think- 
 ers ever since the inverted retinal image became known. 
 
 True Solution. — But there is no mystery at all 
 about it, if we clearly understand the law of direction. 
 Most reasoners on the subject do not seem to perceive 
 that the problem of erectness of objects does not differ 
 at all from that of seeing objects in their right places. The 
 latter concerns the true position of objects, the former 
 the true position of radiants. Objects are composed 
 wholly of radiant and retinal images of corresponding 
 focal points. Now, if images are referred each back 
 along its ray line to its proper place in space, then also 
 the focal points of these images are similarly referred 
 each to its proper place. Now, as ray lines from radi- 
 ants cross one another at the nodal point and thus in- 
 vert the image, so the reference lines recrossat the same 
 point, and thus reinvert the image in the very act of external 
 reference. This is shown in Fig. 86, except that now an 
 object replaces the stars. It is evident that the two ends 
 
130 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 of the arrow must be seen, each in its proper place, and 
 therefore erect. Suppose we stand at night in the open 
 air beneath the star-lit sky. Is it any mystery that the 
 stars are seen each in its proper place ? Now, every ob- 
 ject consists of an infinite number of starlike radiants, 
 and each radiant is referred back along its ray line to 
 its proper place. 
 
 2. THE FOVEA AND ITS SPATIAL REPRESENTATION. 
 
 The fovea, or central spot, is directly in the axis of 
 the eye — the south pole of the globe. It differs from 
 other parts of the retina in several respects: i. The 
 fibrous layer and the larger part of the nuclear layer is 
 wanting, so that the bacillary layer is more directly 
 exposed than elsewhere to the direct action of light. 2. 
 This part of the bacillary layer consists of cones only. 
 3. The cones here are much smaller than elsewhere. 
 From the absence of the other layers this point is 
 depressed, hence the name fovea, a pit. It is evidently 
 the most highly organized part of the retina. 
 
 Its spatial representative is the spot we look at — the 
 point of sight, or rather the line of sight, and a small 
 area about it. When we look at anything the axis of the 
 eye is turned directly upon it, and the image of the thing 
 falls on the fovea. The point we look at and a small 
 area about it are seen distinctly. If we look steadily 
 at the point and at the same time observe our per- 
 ception of objects in other parts of the field of view, we 
 find that while their presence is plain enough, their ex- 
 act form and surface-detail are more and more imper- 
 fectly perceived as we go from the point of sight. This 
 is not the result of imperfect image, but of imperfect 
 perception of the image; not the result of an imperfect 
 instrument, but imperfect retinal response. We can not 
 have a better illustration of this than the act of reading. 
 
SENSE ORGANS. 
 
 131 
 
 We see perfectly the word we look at, but words right 
 and left are increasingly illegible, and therefore we are 
 compelled to run the eye along the line, so that the image 
 of every word falls successively on the fovea. The eye 
 is the most restless of organs. In looking at a scene we 
 sweep the point of sight about and gather up the results 
 in memory, and thus seem to see the whole scene dis- 
 tinctly. But really we see distinctly only a very small 
 area. The explanation of this is doubtless found in 
 retinal structure. As already seen (page 120) the cones 
 of the fovea are connected each with its own fiber, 
 whereas one fiber is connected with several rods. 
 
 Is this limitation of distinct vision a defect ? I think 
 not. Suppose we saw all parts of the field of view with 
 equal distinctness: it would be impossible to fi.\ thoughtful 
 attention on the thing looked at. But the development 
 of the higher faculties of the mind is conditioned on the 
 ability X.o fix the attetition. Thus there are three kinds 
 or grades of vision : i. Simple seeing, which may be un- 
 conscious and involuntary. 2. Looking, or the volun- 
 tary act of sight. 3. Observing, or the thoughtful act of 
 sight. This last is characteristic of man alone. 
 
 MinimufH Visibile. — What is the limit of sight as to 
 smallness ? We answer. There is nothing so small that 
 it can not be seen if there be light enough. A star is 
 the nearest to a mathematical point that we can well 
 conceive. We may magnify it three thousand diameters, 
 and still it is a point. And yet a star may be seen 
 plainly enough. The only sense in which there is a 
 minimum visibile at all is the smallest thing that can be 
 seen as a magnitude — the smallest distance between two 
 stars that they can be seen as two, the smallest distance 
 between two dots or two lines that they can still be 
 seen as two. This undoubtedly depends on the size of 
 the cones of the fovea. If the images fall on one cone. 
 
132 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 the two dots are seen as one; but if far enough apart to 
 fall on two cones, they are seen as two. Taking the 
 foveal cones as one seventy-five-hundredth of an inch 
 (one three-hundredth millimetre) in diameter, and the 
 nodal point as six tenths of an inch (fifteen millimetres) 
 from the fovea, and the point of sight as ten inches, the 
 niintmujn visibile ought to be about one four-hundred-and- 
 fiftieth of an inch (one eighteenth millimetre). This is 
 about the fact for good eyes. If the point of sight be 
 six inches, as it may be in young normal eyes, the mini- 
 mum will be one seven-hundred-and-fiftieth of an inch 
 (one thirtieth millimetre). 
 
 Comparison with Touch. — The only sense with 
 which we can make comparison in this regard is touch, 
 because these two are the only senses that take cogni- 
 zance of dimension. There is also a minimum tactile — 
 i. e., the smallest distance between two tactile impressions 
 in which they can be felt as tiuo. This varies greatly in 
 different parts of the body. 
 
 Experiment. — Take a pair of dividers, arm the points 
 with small shot or bits of cork, so as not to prick the 
 patient. Now try, on a blindfolded person, the distance 
 of separation between the points when these are felt as 
 two. It will be found that on the middle of the back 
 the separation must be two to three inches; on the out- 
 side of the forearm or back of the hand, about one half 
 or three quarters of an inch ; on the finger tips, about 
 one twelfth of an inch ; and on the tip of the tongue, 
 about one twenty-fifth of an inch, or one millimetre. In 
 the retina it is one seventy-five-hundredth of an inch. 
 
 3. BLIND SPOT. 
 
 We have already seen (page 118) that there is another 
 spot where all the layers of the retina are not present — 
 viz., just where the optic nerve enters the eye. As the 
 
SENSE ORGANS. 
 
 133 
 
 optic nerve consists of fibers and as it spreads these as 
 an innermost layer, all other layers must be absent here. 
 Now as the bacillary layer is the sensitive layer, it fol- 
 lows that the spot must be blind. 
 
 Experimental Proof of a Blind Spot. — Experi- 
 ment I. — Make the spots on a sheet of paper, a few 
 inches apart, thus : 
 
 a b 
 
 Shut the left eye and look with the right steadily at 
 the left figure, a, while the paper or page is slowly 
 brought nearer the face. At a certain distance — about 
 nine inches for the above figures — the right-hand figure, 
 b, will disappear, but on continuing the approach it 
 again reappears. The image of b has traveled across 
 the blind spot and come out on the other side. 
 
 Experiment 2. — Standing up, put a small coin on the 
 table and a finger of the left hand beside it. Now, shut- 
 ting the left eye and looking at the finger with the right 
 eye, move the finger slowly to the left and follow it with 
 the eye. At a certain point the coin disappears from view, 
 but reappears on continuing the movement of the finger. 
 
 Experiment 3. — Look at a bright star with one eye, 
 say the right. Now move the point of sight slowly and 
 steadily to the left, horizontally. At a certain point the 
 star will disappear, only, however, to reappear on con- 
 tinuing the movement of the point of sight in the 
 same direction. In this manner almost any object, if 
 not too large, may be made to disappear from view. A 
 man sitting at a distance of. say, one hundred feet may 
 be made to disappear. 
 
 Diagrammatic Illustration. — The condition under 
 which the disappearance occurs is represented in the 
 diagram (Fig. 88). The eyes, R and Z, are supposed to 
 
J 34 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 come nearer to the two objects, A and B, L being shut 
 and R looking across at A. The image of A falls all 
 the time on the fovea, a. The image of £ falls on the 
 inner or nasal side of the fovea at b, but not far enough 
 to reach the entrance of the 
 optic nerve, o (i?i) (i). But as 
 the face comes nearer the eye R 
 turns more and more, the image 
 of B falls first nea>- the place 
 (2), then on the place (3) of the 
 entrance of the optic nerve. 
 As R approaches still nearer 
 (4), the image of B has crossed 
 and appeared on the other or 
 nasal side of the optic entrance 
 and reappears. 
 
 Spatial Representative of 
 the Blind Spot.— Every part 
 of the retina has its representa- 
 tive in the field of view. There- 
 fore the blind spot has also. 
 Why, then, do we not see it ? 
 When both eyes are open, of 
 course we do not see it, because 
 we see with the one eye the spot 
 which represents the blind spot 
 of the other eye. There is no 
 place that represents the blind 
 spot of both eyes. But even 
 with one eye shut we see noth- 
 ing. In fact, the expectation of seeing such a repre- 
 sentative shows a misconception. The only true rep- 
 resentative of a blind spot must be an invisible spot. 
 It can not be differentiated from the rest of the field. 
 Nevertheless the place of the representative of the 
 
 Fig. 88. 
 
SENSE ORGANS. 
 
 135 
 
 blind spot can be perceived in the field of darkness. At 
 night in the dark — when the retina by long rest is very 
 sensitive — if the visual plane be lowered toward the feet, 
 and then the eyes be turned quickly and strongly to one 
 
 Fig. 89. — Diag:ram showing place of the invisible spots in the field of \ision. 
 The full lines show the eyes turned to the right ; the dotted lines the 
 same turned to the left. Ps = point of sight. 
 
 side or the other, two brilliant stars with dark centers 
 are seen to flash out for a moment in the dark field 
 (Fig. 89). The phenomenon is produced by a pull on the 
 optic nerves. The dark center is the spatial representa- 
 tive of the blind spot, and the brilliant radiating circle is 
 produced by irritation of the surrounding bacillary layer. 
 
 COLOR PERCF.PTION AND COLOR-BLINDNESS. 
 
 We have thus far treated of perception of light only 
 as intensity and direction. But another primary per- 
 ception is that of color. 
 
136 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 Intensity versus Color. — As there are two kinds of 
 perception of sound — viz., sound as simple sound or 
 noise, loud or faint, and sound as tone or pitch, high or 
 low, acute or grave — so there are two kinds of percep- 
 tion of light, viz., light as simple light, bright or faint, 
 and light as color. In both sound and light the one is a 
 question of quantity, the other of quality. In both cases 
 the one is a question of strength of vibration or wave- 
 height, the other of rate of vibration or wave length. 
 In both, too, there is a limit to the range of perception. 
 In the case of sound the range is great — viz., from the 
 lowest, sixteen per second, to the highest, some thirty to 
 forty thousand per second, or more than eleven octaves. 
 In the case of light it is very restricted, four hundred 
 million-million to nearly eight hundred million-million, 
 or about one octave. 
 
 Primary versus Mixed Colors. — Primary or pure 
 colorj are such as are simple sensations. Mixed or sec- 
 ondary colors are such as may be made by mixtures of 
 the primaries in various proportions. The former are 
 few, the latter almost infinite in number. Both primary 
 and secondary colors may be again mixed with black or 
 white, and give rise to an infinite number of shades of 
 each. 
 
 Primary, Colors. — There is much difference of 
 view as to which and how many colors should be called 
 primary. Brewster (and Newton before him) made 
 three — viz., red, yellow, and blue, rejecting green because 
 it can be made by mixing blue and yellow pigments. 
 Young, and after him Helmholtz and nearly all physicists, 
 make also three, but they are red, green, and violet or 
 blue approaching violet, rejecting yellow because a 
 mixture of spectral red and spectral green makes a kind 
 of yellow. From the purely physical point of view un- 
 doubtly Helmholtz and the physicists are right, and 
 
SENSE ORGANS. 1 37 
 
 Brewster wrong, for pigments are never pure colors. A 
 mixture of blue and yellow pigments makes green, be- 
 cause both of the components contain some green ; and 
 when they are mixed, the yellow and blue kill one 
 another, and the green of both comes out. 
 
 Hering differs from both the preceding. He makes 
 six primary colors — viz., white, black, red, yellow, green, 
 and blue. Furthermore, according to him, these con- 
 stitute three pairs of complementaries — viz., white and 
 black, red and green, yellow and blue. There is but one 
 objection that can be made to Hering's view — viz., his 
 inclusion of white and black. These should be put into 
 a different category — viz., that of shade instead of color, 
 of intensity or quantity instead of quality. Leaving out 
 these, Hering's four colors, or two pairs of complemen- 
 taries, are red and green, yellow and blue. Undoubt- 
 edly from the point of view of sensation, unplagued by 
 any physical considerations, Hering is right. As color- 
 sensations these are perfectly simple and wholly distinct, 
 and this is true of no other colors. Scarlet and orange 
 are plainly and visibly a mixture of red and yellow, 
 purple a mixture of blue and red, and even violet is a 
 blue with a glow of red. White and black are also in- 
 deed pure simple sensations as Hering maintains, but 
 color is not the proper word to express these sensations. 
 
 Theory of Color Perception ; General Theory. 
 — I. Color is a simple sensation and incapable of analy- 
 sis into any simpler elements. It must be, therefore^ 
 the result of retinal structure. 2. It is an endowment 
 of the cones and not of the rods. This is shown by the 
 fact that the distribution of color perception over the 
 surface of the retina is identical with the distribution 
 in number and fineness of the cones. In the fovea there 
 is nothing but cones, and these are very small, and the 
 color perception is therefore keenest at the point of 
 
138 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 sight and a small area about it. As we go outward from 
 the fovea in all directions, we find the cones are fewer 
 and larger, until there are none at all on the margins of 
 the retina. So, correspondingly, the perception of color 
 is more and more imperfect as we go from the point of 
 sight to the margins of the field of view, where it is 
 finally lost entirely. 3. There must be some response 
 of the retina characteristic of each color. We may im- 
 agine that different cones are adapted to vibrate respon- 
 sively — co-vibrate — with different colors. Or we may 
 imagine different substances in all the cones which are 
 photochemically affected each by a particular color. 
 This latter seems the more probable view. We shall 
 call such substances eolor-stibstances. Thus we have, say, 
 a red color-substance, meaning not that the substance 
 is red, but that it is photochemically affected by a certain 
 rate of vibration and produces the sensation of red. 
 
 Special Theories. — Applying this to the different 
 views as to primary colors, according to Helmholtz, there 
 are in the retinal cones three kinds of color-substance 
 which are responsive to three rates of vibration — viz., red, 
 green, and violet rays, respectively, and these give rise 
 to the corresponding color sensations. If two of them 
 are affected, they produce mixed colors. If all are 
 affected in certain proportions, we have white. Or, to 
 put it another way : pure colors affect only one, mixed 
 colors two or more, white light all in certain propor- 
 tions. According to Hering there are only two color- 
 substances (three, if we include white and black) ; the 
 one by opposite affections produces the complementaries 
 red and green, the other by opposite affections the 
 complementaries yellow and blue; and the essential na- 
 ture of complementariness, especially their mutual de- 
 structiveness, is the necessary result of these opposite 
 affections of the same substance. 
 
SENSE ORGANS. 
 
 139 
 
 Mrs. Franklin has recently brought forward a view 
 which deserves and has received much attention. She 
 thinks that color perception, like all other faculties, has 
 been gradually evolved. The steps were as follows: 
 First of all, in the early stages of evolution there was 
 but one color-substance in both the rods and the cones 
 This she calls gray color-substance, because it is photo- 
 chemically affected by, and gives rise to, the perception 
 of white and black and all shades between — i. e., grays. 
 At that time only white and shades, but not colors, were 
 perceived. Next, some of this substance in the cones, 
 but not in the rods, was differentiated into two color- 
 substances — viz., yellow and blue — which, separately af- 
 fected, give rise to these two colors respectively, but 
 simultaneously affected, to white and shades. Lastly, 
 one of these two — viz., yellow — was again differentiated 
 into red and green ; but these by simultaneous affection 
 give rise still to yellow. 
 
 COLOR-BLINDNESS. 
 
 Many people seem to discriminate colors imperfectly, 
 but only because they do not observe carefully. They 
 see colors perfectly well, but have not learned to name 
 them. This is not color-blindness. 
 
 What is Color-Blindness ? — The color-blind do 
 not see certain colors at all as colors, but only as shades. 
 To take one example : The commonest form of color- 
 blindness is that for the colors red and green. For such 
 a person the red berries and green leaves of a cherry 
 orchard, or the red carnations and the green lawn on 
 which they grow, look much alike, and neither of them 
 red or green, but gray. In a word, they look much as 
 a stereogram of the scene would look to an ordinary 
 person looking through the stereoscope ; for the iodized 
 plate is also blind for these colors. 
 
140 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 Cause of Color-Blindness. — Color-blindness is a 
 defect of retinal structure. In the case of the color- 
 blind one or more of the color substances are wanting. 
 In the red-green blind, for example, the red color-sub- 
 stance and the green color-substance of Helmholtz are 
 both wanting. Or, according to Hering's better view, the 
 one substance which, by opposite affections, produces 
 these complementaries, is wanting, and that is the rea- 
 son why these two are usually associated. Such persons 
 see yellow and blue perfectly well. According to Mrs. 
 Franklin, color-blindness is an example of atavism — 
 i. e., a reversion to a primitive condition. Total color- 
 blindness, which, though rare, sometimes occurs, is a 
 relapse to the earliest condition. There is only gray 
 substance in the retina. Red-green blindness is a re- 
 lapse to the second stage, in which some of the gray 
 substance has been differentiated into yellow and blue, 
 but the yellow has not been further differentiated; while 
 normal vision is the third or perfect stage, in which the 
 yellow has been further differentiated into red and green. 
 
 What the Color-Blind really See. — By the color- 
 blind////-^ colors are either seen correctly or not seen at 
 all as colors, but only as shades. The mixed colors they 
 always see incorrectly. Taking the commonest form of 
 color-blindness, the red-green blindness, the following 
 schedule shows what they see and why : 
 
 I. 
 
 See correctly. 
 
 a. White and black and all shades of the same — i. e., grays. 
 
 b. Yellotv and all shades of the same — i. e., browns. 
 
 c. Blue and all shades of the same — i. e., slate blues. 
 
 II. 
 
 Do not see at all as Colors. 
 a. Reds are seen as shades or grays. 
 , l>. Greens are seen as shades or grays. 
 
SENSE ORGANS. 
 
 141 
 
 III. 
 
 See incorrectly, 
 'a. Scarlet = red and yellow — i. e., gray and yellow = dark 
 brown. 
 
 b. Orange = red and yellow — i. e., gray and yellow = lighter 
 brown. 
 
 c. Bluish green = blue and green — i. e., blue and gray = slate- 
 blue. 
 
 d. Yellowish green = yellow and green — i. e., yellow and gray 
 = brown. 
 
 e. Purple = red and blue — i. e., gray and blue = slate-blue. 
 
 Tests. — It might seem that so striking a phenome- 
 non needs no test. Every one must know it. But this 
 is far from the fact. On the contrary, a man may be 
 color-blind unknown to himself and to his friends. He 
 may have observed some instances of curious confusion 
 of colors, but these are attributed to imperfect knowledge 
 of color names. In the case of persons in responsible 
 positions, such as locomotive-engine drivers, ship steers- 
 men, etc., where color signals are used, it is very impor- 
 tant that ability to see colors correctly should be tested. 
 The simplest test and one of the best is a box full of 
 skeins of yarn of all colors and shades, and several of 
 each. Such a box is placed before the person to be 
 tested, and he is directed to sort them and match the 
 colors. All normal-sighted people would match them 
 alike and correctly, but the color-blind make the most 
 extraordinary mistakes. Certain shades of red and green 
 and gray are put together as the same ; similarly certain 
 shades of scarlet and brown or purple and slate-blue. 
 
 By these tests the remarkable fact is brought out 
 that this defect is much more common in men than in 
 women. About one in every twenty-five men are more 
 or less color-blind, while among women hardly one in a 
 thousand is thus affected. 
 
142 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 SECTION V. 
 
 Binocular Vision. 
 
 All the phenomena thus far treated are essential to 
 vision. They would still be found if, like the Cyclops 
 Polyphemus, we had but one eye in the middle of the 
 forehead. But, in addition to these, there are certain 
 other phenomena which are wholly the result of the use 
 of two eyes as one instrument. These belong to binocular 
 vision. Observe, it is not the mere having of two eyes 
 which gives rise to these phenomena. We might have a 
 hundred eyes and have no binocular phenomena, for 
 each eye may act independently, as is the case in many 
 lower animals. The tivo eyes must act as one instrument. 
 
 The phenomena now about to be described are far 
 more illusory, more psychical, more difficult to be ob- 
 served. Although we are forming judgments based on 
 them every day of our lives, yet they usually drop out 
 of consciousness, and by many persons are recalled to 
 consciousness with difficulty. For this reason we shall 
 be compelled to treat them much more cursorily than 
 their importance deserves.* 
 
 SINGLE AND DOUBLE VISION. 
 
 Double Vision. — We have two eyes, two retinae, and 
 two fields of view — their spatial representatives — though 
 they indeed partly overlap and form a common field. 
 We have also two retinal images of each object, and two 
 external images, the spatial representatives of the two 
 retinal images. Why, then, do we not see everything 
 double? So indeed we often do, but without observing 
 it. It is necessary first of all to prove this. I do so by 
 some simple experiments. 
 
 * This subject is fully treated in my book Sight. 
 
SENSE ORGANS. 
 
 143 
 
 Experiment i. — Hold up the finger against the op- 
 posite wall or against the sky, and look not at the finger 
 but at the wall or sky. Two finders are seen, shadowy, 
 transparent, because they hide nothing; the place cov- 
 ered by each is seen by the other eye. While still look- 
 ing at the sky or wall, shut the right eye ; the left image 
 disappears. Shut the left eye ; the right image disappears. 
 Evidently the right image belongs to the left eye and 
 the left image to the right eye. Such are called heter- 
 onymously double images. 
 
 Experiment 2. — Hold the two forefingers, one be- 
 fore the other, directly in front — i. e., in the middle plane 
 of the head, and twelve to fifteen inches apart. Look 
 at the farther finger; the nearer one is double. Look at 
 the nearer finger ; the farther one is double. By shutting 
 alternately first one eye and then the other it will be found 
 that in the former case the images each belong to the 
 eye on the opposite side — i. e., are heteronymous, while in 
 the latter case they belong each respectively to the eye 
 on the same side. Such are called homonymous. 
 
 We might multiply experiments indefinitely, but these 
 are sufficient to show that we often see objects double. 
 They show more, viz., that when we look at an object 
 we see it single, but all objects beyond or this side of 
 the point of sight are doubled, but in opposite ways — 
 in the former case homonymously, in the latter heter- 
 onymously. This doubling of objects is evidently the 
 necessary result of the two retinal images. But the 
 questions occur : Why should we see objects single at 
 all ? What are the positions of the two retinal images 
 when objects are seen single ? 
 
 Single Vision. — Since there are two retinal images 
 of every object and two e.xternal images, their spatial 
 representatives, it is evident that single vision can only 
 take place when the two external images are superposed nwd. 
 
144 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 coincide perfectly ; and this takes place when the retinal 
 images fall on corresponding points of the two retinae. 
 It is necessary to define these exactly. 
 
 Corresponding Points. — Corresponding points are 
 points exactly similarly situated in the two retinae. The 
 foveae are, of course, /ar excellence corresponding points, 
 and all other corresponding points are symmetrically 
 arranged about these. If R and L (Fig. 90) represent 
 projections of the two retinae, and c c the centers of the 
 foveae, and vertical and horizontal lines be drawn 
 through the central spots, then points similarly situated 
 in reference to these — viz., e e', d d' — are corresponding 
 points. Or, suppose the two retinae be placed one on 
 
 Fig. 90. — Diagram showing corresponding halves of the retinae. 
 
 the Other in geometric coincidence, then the points — 
 the rods and cones — which coincide are corresponding 
 rods or cones. It follows that the two right or shaded 
 halves are corresponding halves, and similarly the two 
 left or unshaded halves — i. e., points similarly situated 
 in the two right halves or left halves — are correspond- 
 ing. But the two inner or nasal halves have no corre- 
 sponding points, nor have the two external or temporal 
 halves any correspondents. 
 
 The Third Law of Vision ; the Law of Corre- 
 sponding Points. — We restate now the conditions of 
 single vision as a law. IVlien the two retinal images of 
 
SENSE ORGANS. 
 
 H5 
 
 any object fall on corresponding points, then the external 
 images are thrown to the same place and are superposed 
 and seen single, but when the ttvo retinal images of an 
 object fall on non-corresponding points then the external 
 
 Fig. 91. 
 
 images are thrown to different places and are seen double. 
 Now it is at once seen why we see single what we look 
 at ; for then the axes of the two eyeballs are converged 
 on the object and the images fall on the central spots 
 or foveae, and these are par excellence corresponding 
 
146 PHYSIOLOGY AND MORPHOLOGY OF ANLMALS. 
 
 points. Why we see double all objects nearer and 
 farther off than the point of sight, and differently 
 double in the two cases, is shown in the diagram 
 (Fig. 91). Let A, B, and C be three objects in the 
 median plane, and the eyesi? and Z be fixed on A. The 
 images of A will fall on the central spots and be seen 
 single; but the images of B will fall on the two nasal 
 halves, b b\ but all points in these are non-correspondent 
 and therefore B will be seen double. Similarly C will 
 be seen double because its images fall on the two tem- 
 poral halves. The kind of doubling in each case may 
 be shown by referring all the external images to the 
 plane of sight, P P. It is then seen that the images 
 bb' oi B are homonymous, while the images c c' oi C are 
 heteronymous. That is, as we before found, objects 
 nearer than the point of sight are doubled heterony- 
 mously while objects farther than the point of sight are 
 doubled homonymously. 
 
 Horopteric Circle. — As already shown, objects be- 
 yond or on this side of the point of sight are seen double. 
 But how is it with points about the same distance, but 
 right or left, or above or below that point ? Take first 
 right and left. Let R and L (Fig. 92) be the two eyes 
 and A the point of sight. Draw a circle through A and 
 through the nodal points n n' . This is the horopteric 
 circle, or circle of single vision, of Miiller. For if the 
 eyes be fixed on A^ any object at that point will be seen 
 single because its images are on the central spots a a', 
 but at the same time B or any other point in the circle 
 will also be seen single because its images will fall on 
 bb\ which are obviously corresponding points. But this 
 is not true of any point B' in the plane P P. 
 
 Horopter. — We have taken points right and left. 
 If there be also points above and below seen single at 
 the same time, then there would be a surface of single 
 
SENSE ORGANS. 
 
 147 
 
 vision. Such a supposed surface of single vision with the 
 point of sight fixed \% cdiW&d the horopter. Whether there be 
 such a surface at all, and if there be, what is its form, 
 
 Fig. 92. — The horopteric circle of MuUer : R and L, two eyes ; n n\ point 
 of crossinf^ of ray lines — nodal point ; A, point of sight ; B, some other 
 point in the horopteric circle A nn \ a a', central spots; iia\ bb' , ret- 
 inal images of A and B. 
 
 are very complex and difificult questions which can not 
 be discussed here.* 
 
 The Relation of the Chiasm to Corresponding 
 Points. — The union of the optic nerves to form a 
 
 * They are fully discussed in author's book Sight. 
 
148 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 chiasm (Fig. 81, page 116) is undoubtedly related in 
 some way with the use of the two eyes as one instru- 
 ment, and therefore with the existence of corresponding 
 points. The fibers of the optic roots partly cross and 
 partly do not cross, as shown in the diagram (Fig. 93). 
 
 Fig. 93. — O 0\ optic roots ; NN\ optic nerves ; R and Z, sections of the 
 two eyes; c c' , central spots; nn', the nasal halves, and 1 1\ the tem- 
 poral halves, of the retinae. 
 
 Thus each root supplies both eyes, and conversely each eye 
 is controlled by both sides of the brain. The existence of a 
 true chiasm with fibers crossed in this peculiar way may 
 therefore be taken as evidence of the existence of corre- 
 spondinof points and the possession of binocular vision. 
 
 The Two Adjustments of the Eyes. — There are 
 two fundamental adjustments of the eyes in every act 
 of looking, viz., the focal adjustment, or accommoda- 
 tion, and the axial adjustment, or turning the axes so as 
 to converge on the object looked at. The one is neces- 
 sary for distinct vision, the other for single vision. Asso- 
 ciated with these, but far less important, is a third, viz., 
 pupillary contraction. 
 
 Two Kinds of Corresponding Points.— We have 
 already (page 127) spoken of corresponding points, ret- 
 
SENSE ORGANS. 
 
 149 
 
 inal and spatial. We have just explained the correspond- 
 ing points of the two retinae. Now we assert that the 
 corresponding points in the two retina have the same spatial 
 correspondent. So that there is a kind of triangular 
 correspondence between the two eyes and space. 
 
 The Two Fundamental Laws of Vision.— There 
 are also, as. we have seen, two fundamental laws of 
 vision — the law of direction and the law of corresponding 
 points. The one explains the apparent anomaly oi erect 
 vision with inverted retinal image, the other the appar- 
 ent anomaly of single vision with two retinal images. 
 The one is the fundamental law of monocular, the other 
 of binocular vision. We have seen how all the phenom- 
 ena of monocular vision flow logically from the one. 
 Now we proceed to show how all the phenomena of 
 binocular vision follow necessarily from the other. 
 There is, however, a third law underlying both and more 
 fundamental than either — viz., the law of outward or 
 spatial reference of all retinal states. 
 
 BINOCULAR PERSPECTIVE. 
 
 The law of external reference gives space. The law 
 of direction gives two dimensions of space — i. e., up and 
 down and from side to side. Now, the law of corre- 
 sponding points gives the third dimension of space — i. e., 
 depth or distance from the observer. The perception 
 of this third dimension, so far as it is dependent on the 
 use of the two eyes as one instrument, is our next sub- 
 ject. We begin again with experiments : 
 
 Experiment i. — We repeat that given on page 143, 
 but for another purpose. Place the two forefingers, one 
 before the other, in the median plane, and separated, say, 
 a foot from one another. We have already shown that 
 when we look at the nearer finger we see it single, but 
 the farther finger is doubled homonymously. When we 
 
I50 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 look at the farther finger we see that one single; but 
 now the nearer one is doubled heteronymously. Now 
 observe, further, that we are clearly conscious that it 
 requires more convergence, and therefore more effort, 
 to look at the nearer finger and see it single, and less 
 convergence and less effort of the ocular muscles to 
 look at the farther finger and see that .single. In 
 other words, we run the point of sight back and forth 
 from one finger to the other by greater and less con- 
 vergence, and thus acquire a distinct perception of dis- 
 tance between the two. It is literally a process of rapid 
 triangulation, with the interocular distance as the base 
 line. The same is true of all objects in space at differ- 
 ent distances if the distance of the nearer one be not 
 too great. 
 
 Experiment 2. — But single objects also occupy 
 depth of space. Take, therefore, next a rod, say a foot 
 long; hold in the median plane, a little below the hori- 
 zontal line, with the nearer end six to eight inches from 
 the face. Looked at with one eye, say the right, the 
 rod is seen projected thus / ; looked at with the left 
 
 eye, \ . Now, it is evident that these two images can 
 not combine. When we open both eyes and look at the 
 farther end, the nearer end is doubled heteronymously, 
 and we see the rod as an inverted V, with the open end 
 toward us, thus /K ; when we look at the nearer end, 
 the farther end is doubled homonymously, and we see a 
 V with the point toward us, thus \J ; when we look at 
 the middle, we see the two images cross in the middle to 
 make an X, thus V . Thus we run the point of sight 
 back and forth from one end to the other, by greater 
 and less convergence uniting each point looked at, and 
 acquire thus a distinct perception of the distance be- 
 
SENSE ORGANS. 151 
 
 tween the two ends. The same is true of all objects 
 occupying depth of space. 
 
 Thus, then, we may safely generalize : In viewing a 
 single object occupying considerable depth of space, or 
 a scene with objects one beyond the other, it is evident 
 that the retinal images of the object or of the scene in 
 the two eyes, and therefore the external images — their 
 spatial representatives — or the way the object or scene 
 looks to the two eyes, respectively, are different, because 
 taken from differe?it points of view. Therefore they can 
 not be united as a whole, but only in parts at a time. 
 When we look at the foreground, objects in the back- 
 ground are double ; when we look at the background 
 objects in the foreground are double. Thus we run the 
 the point of sight back and forth, uniting successively 
 different parts of the scene, and acquire thus a clear 
 perception of depth of space between. 
 
 Limitation of Clear Vision. — See, then, the ex- 
 treme limitation of distinct vision and of single vision. 
 As distinct vision is confined to a small area about the 
 point of sight, and we must therefore sweep about this 
 point and gather up the result in memory, even so 
 single vision is limited to the distance of the point of 
 sight, and we must run the point of sight back and forth, 
 uniting successively different parts o^ the scene, thus 
 probing space and gauging its depth, and gather up the 
 results in memory. 
 
 Different Forms of Perspective. — Of course, there 
 are other ways of judging of relative distance — other 
 forms of perspective. It may be well, therefore, to give 
 these, and very briefly compare them : 
 
 I. Aerial Perspective. — We judge of distance by the 
 color of the air through which we look. The atmos- 
 phere is not absolutely transparent, but bluish. Distant 
 objects, like mountains, are dimmer and bluer in pro- 
 
152 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 portion to their distance, and we judge of distance in 
 this way. 
 
 2. Mathematical Perspective. — The angular diameter 
 of objects, and therefore the size of the retinal image, is 
 mathematically proportioned to the distance. Therefore 
 objects seem smalMn proportion to their distance. Par- 
 allel lines, like railway tracks, converge, and houses on 
 the two sides of a street converge and grow smaller 
 with distance. We judge of distance quite accurately in 
 this way. 
 
 3. Binocular Perspective. — This, as already explained, 
 is a judgment of distance by running the point of sight 
 back and forth, successively uniting double images by 
 greater and less convergence, and thus gauging space. 
 
 4. Focal Perspective. — When with one eye we look at 
 a very near object, farther ones are dim, and vice versa. 
 We are aware of voluntary effort of accommodation 
 for distinct vision of near objects, and judge of relative 
 distance in this way also. 
 
 Distance at which these Operate. — Now, of these 
 four kinds, the focal operates for only about twenty 
 feet. Beyond this the accommodation is a vanishing 
 quantity. The binocular perspective operates for about 
 one quarter to one half mile. Beyond this it, too, be- 
 comes a vanishing quantity. The other two operate 
 without limit. 
 
 The painter can imitate the first and second, and much 
 of his art consists in skillfully introducing an appear- 
 ance of distance by dimming and bluing and making 
 smaller the objects in the background' of his picture. 
 The other two he can not imitate. The lack of focal 
 perspective is, however, of little importance, because 
 landscape pictures are usually viewed at a consider- 
 able distance. But the lack of binocular perspective 
 seriously interferes with the illusion which he seeks to 
 
SENSE ORGANS. 
 
 153 
 
 produce. Hence the perspective is far clearer wlien 
 the picture is looked at with one eye only. 
 
 JUDGMENTS OF SIZE AND DISTANCE. 
 
 The eye perceives at once direction up and down and 
 right and left, and therefore outline form and surface 
 contents, for this is a combination of directions. Thus 
 two dimensions of space — viz., angular diameter in all 
 directions — are given immediately. But this does not give 
 size, unless distance, or the tJiird dimension, is also 
 known. Now, this third dimension is not given in sense, 
 but is a. Judgment. The direct gifts of sight are light, its 
 intensity, its color, and its direction, and therefore also out- 
 line form. But size, distance, and solid form are judg- 
 ments based on these gifts. Moreover, size and distance 
 are closely correlated, so that a mistake in one will cause 
 a corresponding mistake in the other. 
 
 Distance. — We judge of distance by the various 
 forms of perspective already explained. Being a judg- 
 ment, we are liable to error. We often say "our senses 
 deceive us." Not so. We make false judgments on true 
 reports of the senses. 
 
 Size. — The size of an object is judged by lis angular 
 diameter, or size of its retinal image, multiplied by its 
 
 Fig. 94. 
 
 estimated distance. For example, in Fig. 94 the reti- 
 nal image a may be made by A or A' or A", and the ap- 
 parent size of the spatial correspondent will vary ac- 
 
154 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 cordingly. If we imagine it at A, it will look the size 
 of A, but if we imagine it at the distance ^4", it will seem 
 to be four times as large. If its real size and place is A", 
 and we imagine it to be at A, it will seem many times 
 too small. If, on the other hand, its real size and dis- 
 tance is represented asyi and we imagine it at A", it will 
 look many times too large. For example, if we hold up 
 a finger before one eye (the other being shut), very 
 near to the eye, say an inch, its image completely covers 
 a large building one hundred yards distant. Now, if we 
 imagined the finger one hundred yards distant, it would 
 look as large as the building. 
 
 The fact of dependence of apparent size on estimated 
 distance is well shown in the case of the sun and the moon. 
 We are accustomed to estimate the distance of terrestrial 
 objects, but have no means of judging of the distance of 
 celestial objects. Therefore different persons will differ 
 in the most extraordinary way about the apparent size of 
 the sun or the moon. Some will say that they look about 
 the size of a saucer, others the size of a dinner plate, 
 and others the size of the head of a barrel. There are 
 some extreme cases of persons who say they look about 
 the size of an orange, and others as big as a cart wheel. 
 
 The mathematical relation between apparent size and 
 estimated distance is well shown by spectral images. 
 Look at the setting sun steadily for a moment. The 
 image of the sun is branded on the retina so strongly 
 that the brand remains for some time. Now, every 
 change in the retina, whether it be image or shadow 
 or brand, is seen as something in the field of view. 
 With the sun brand still on the retina, look where we 
 will — on the wall, on the floor, on the sky — we see a spec- 
 tral image of the sun. Now as to the size. Look on a 
 sheet of paper two feet off; the image cast on the sheet 
 is about a quarter of an inch in diameter. Look at the 
 
SENSE ORGANS. I 55 
 
 wall twenty feet off ; the image is a little more than two 
 inches in diameter. Look at a building one hundred 
 feet off; the image is about ten inches in diameter. 
 
 Illustrations meet us on every side. In a fog objects 
 look large, because, being dim, they are supposed far- 
 ther off than they really are. In the exceptionally clear 
 atmosphere of Colorado or Nevada objects at first seem 
 smaller because they seem nearer than they are, and 
 they seem nearer because they are seen so plainly. 
 
 Form. — Outline form is a combination of directions 
 of radiants, and is therefore seen immediately. We are 
 not deceived. But solid form is always a judgment. We 
 judge sometimes by binocular perspective, sometimes by 
 shading produced by light. We may be deceived by 
 skillful shading of a picture, as in scene painting. 
 
 Gradations of Judgments. — There are all degrees 
 of complexity of judgments from simple gifts of sight 
 on the one hand to the most complex intellectual judg- 
 ments on the other, i. Light, its intensity, color, and 
 direction. These are direct gifts, are ultimate facts, and 
 therefore incapable of analysis. 2. Then come outline 
 form and surface contents. These are given immediately, 
 and therefore are not liable to deception, but are capable 
 of analysis into simpler elements — viz., a combination of 
 directions. 3. Next comes solid form, which is a judg- 
 ment, based partly on binocular perspective and partly 
 on the shading of light. Here, for the first time, we are 
 liable to deception. 4. Then come the complex judg- 
 ments of relative distance and size of objects in an ex- 
 tensive landscape. All of these judgments are so rapid 
 that they are usually not recognized as judgments at all. 
 I therefore call them z7V«a/ judgments. 5. These pass 
 by insensible gradations to the simpler intellectual judg- 
 ments, and these, in their turn, into the most complex 
 process of thought-work. 
 
156 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 SECTION VI. 
 Comparative Physiology and Morphology of the Eye. 
 
 VERTEBRATES. 
 
 Mammals. — The structure of the eye and the physi- 
 ology of vision in all mammals and, indeed, in all verte- 
 brates is substantially the same as that already given 
 for man, yet there are some points of difference worthy 
 of note. 
 
 Color. — The iris is, we have seen, a continuation of 
 the choroid coat. Normally and most usually, there- 
 fore, it has the dark, chocolate-brown color characteris- 
 tic of the pigment of that coat. Doubtless this is the 
 original and normal color of the human eye. The blue 
 and gray are the result of peculiar structure, together 
 with a deficiency in pigment. Nearly all mammals have 
 the normal brown color. In the cat tribe, however, as 
 is well known, it is brilliant yellow. 
 
 Pupil. — The form of the pupil is usually round, as in 
 man, but in the two most highly specialized and differ- 
 entiated orders — the cat tribe on the one hand, and the 
 grazing animals on the other — the pupil is greatly elon- 
 gated, vertically in the former and horizontally in the 
 latter. The vertical elongation is probably connected 
 with the habit of springing on its prey ; the horizontal 
 elongation, certainly with wide horizontal view, neces- 
 sary in grazing. This shape of the pupil, combmed with 
 the prominence of the eyes and their position on the 
 margin of a broad front, makes the view of these ani- 
 mals sweep the whole horizon without turning the head 
 or even the eyes. 
 
 Tapetum. — In many mammals, especially those of noc- 
 turnal habits, such as the cat tribe and ruminants, there 
 is found at the bottom of the retinal concave a large 
 
SENSE ORGANS. 
 
 157 
 
 patch, which has a bright, iridescent metallic luster. It 
 is called the tapetum. It is a modification of the cho7-oid 
 coat for the purpose of reflection of light. The use of 
 it is not well understood, but it is believed to double the 
 impression of feeble light by making it pass twice through 
 the retina. It is this that causes the shining of the eyes 
 in the dark, if a bright light is present. 
 
 Fovea. — There is in all mammals a central area, which 
 is a little more sensitive ; but a true fovea, with its three 
 characteristics (explained on page 121), is not found in 
 any mammal below man, except the anthropoid apes. 
 
 Birds. — The iris in birds is very various in color, 
 most commonly the normal brown, but sometimes yel- 
 low, as in birds of prey, sometimes scarlet-red (summer 
 duck), and ?,om.G.i\mes porcelain-w/iite (white-eyed vireo). 
 
 Sclerotic Bones. — In all birds and many reptiles 
 we find a series of bony plates in the front part of the 
 
 sclerotic and radiating 
 
 •5 ^ from the margin of the 
 
 iris. These are beveled 
 
 on the margins, and fit 
 
 Fig. 95. — Eye of an owl : on, optic 
 nerve ; c, cornea ; sb, sclerotic bones. 
 
 Fig. g6. — Sclerotic bones sepa- 
 rated and viewed in perspective. 
 
 together in such wise as to slide a little over one an- 
 other. By appropriate muscles these may be made to 
 squeeze the ball so as to adapt it to clear vision of very 
 near objects (Figs. 95 and 96). 
 
 Nictitating Membrane. — Birds have in the inner 
 corner of the eye a fold of the conjunctiva which may 
 
158 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 be drawn upward over the eye, wiping it and protecting it 
 from injury without entirely excluding the light, for it is 
 semitransparent (Fig. 97). A remnant of this membrane, 
 in useless condition, is found even in man. 
 
 Fovea. — Birds not only have a fovea, but in some there 
 are ^ wo in each eye. The most distinct of these is in the 
 axis of the eye, and therefore at the bottom of the retinal 
 concave. Now since the optic axes are not parallel, as 
 in man, but are widely divergent (Fig. too, page 162), 
 the side of the head must be turned toward an object in 
 
 Fig. 97. — Eye of a bird showing (nm) the nictitating membrane. 
 
 order that its image shall fall on this fovea. We will 
 speak of this again under binocular vision in vertebrates. 
 Reptiles. — These are in many ways similar to birds. 
 The sclerotic bones are found in lizards and turtles (Fig. 
 98), though not m crocodiles and snakes. In some rep- 
 tiles — e.g., in snakes — the lids are absent. The dry, 
 horny epidermis passes directly over the cornea of the 
 eye, and in skin-shedding comes off with the rest of the 
 epiderm. Also some lizards — e. g., chameleon and 
 phrynosoma— have a distinct fovea. 
 
SENSE ORGANS. 
 
 159 
 
 Fig. 98. ^Lizard's eye show- 
 ing' the sclerotic bones. 
 (After Wiedersheim. ) 
 
 Fishes. — In these the lids are wanting, the eyes be- 
 ing kept moist by the water. The lens of fishes is very 
 peculiar. It is perfectly spherical and much denser than 
 in land animals. Both of these 
 qualities give greater refractive 
 power. This is necessary on ac- 
 count of the medium in which 
 they live, for the refractive pow- 
 er of the eye is the difference 
 between that of the medium and 
 of the lenses. This is well illus- 
 trated in the case of the diver. 
 Even in the most transparent 
 water vision is very imperfect if 
 the eye is immersed. If the diver 
 wishes to see distinctly under water he must supplement 
 the refractive power of the eyes by strong double 
 convex lenses, or else by double concave air spectacles. 
 Such spectacles may be easily extemporized by putting 
 two watch glasses back to back and cementing imper- 
 meable paper about the margins. It is evident that 
 these would act precisely like two convex water-lenses 
 in air. 
 
 The ciliary muscles are wanting in fishes. They first 
 appear in amphibians — i. e., in the lowest land verte- 
 brates. Fishes, therefore, can not accommodate the eyes 
 for various distances by changing the form of the lens, 
 for it is already spherical. Their eyes are passively ad- 
 justed for near objects. They probably accommodate 
 for distant objects by drawing the lens back nearer to 
 the retina. 
 
 Binocular Vision in Vertebrates. — There are three 
 points of structure which throw light on this subject — 
 viz., (i) the optic chiasm, (2) the position of the optiC 
 axis, and (3) the fovea. 
 12 
 
l6o PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 Chiasm. — There is great diversity in the mode of 
 crossing of the optic nerves. In fishes they cross bod- 
 ily (Fig. 99, a), or else one pierces 
 the other (<^). In reptiles they form 
 a kind of basket-work {c and d). 
 Thus far there is a complete cross- 
 ing of fibers in a more or less com- 
 plex way. Each side of the brain 
 controls the opposite eye. But in 
 birds, and especially in mammals, 
 half of the fibers cross and half do 
 not {e), as shown more fully in Fig. 
 93, page 148. By this arrange- 
 ment each side of the brain sup- 
 plies both eyes, and each eye is 
 controlled by both sides of the 
 brain; and therefore the two eyes 
 co-operate as one instrument. This 
 arrangement is necessary to bin- 
 ocular vision. This, therefore, is 
 the only true chiasm. It is prob- 
 able, therefore, that no animals 
 below birds have binocular vi- 
 sion. This is confirmed by the po- 
 sition of the eyes, which is our 
 next point. 
 
 Position of the Eyes. — The position of the axes of 
 the eyes has an evident relation to binocular vision. In 
 man the two eyes are directly in front, with the axes 
 parallel in a passive state. From this state of parallelism 
 they may be easily converged on a near object. They 
 are therefore in the best possible position for binocular 
 vision. The same is true, and perhaps in equal degree, 
 in apes. But below this the eyes are wider and wider 
 apart, and set more and more on the side of the head. 
 
 Fig. 99. — Different modes 
 of crossing of the optic 
 nerves : a and b, fishes ; 
 c and d, reptiles ; e, 
 mammals. 
 
SENSE ORGANS. l6l 
 
 The difficulty of converging on a near point becomes 
 greater; the common field of view is more restricted, 
 until in fishes the eyes are completely on the side of 
 the head ; the optic axes diverge one hundred and eighty 
 degrees; convergence on a point is impossible ; each eye 
 has its own field of view, which do not overlap to make 
 a common field, and therefore they can not have binocu- 
 lar vision. 
 
 All mammals (except perhaps whales) probably en- 
 joy binocular vision in various degrees of perfection. 
 Birds also probably are similarly endowed (although 
 their eyes are so widely divergent), but this is by virtue 
 of a peculiar structure, to be spoken of under the next 
 head. 
 
 Fovea. — This is not only the most sensitive spot of 
 the retina, but it is the center about which the corre- 
 sponding points of the two retinae are symmetrically 
 arranged. It is undoubtedly necessary for binocular 
 vision in its highest perfection. Now, this pitlike spot is 
 found among mammals only in man and the anthropoid 
 apes. Mammals generally have indeed a central area 
 (which may become a tapetum), about the center of which 
 corresponding points are symmetrically arranged, but 
 no true fovea. It is probable that in them the advan- 
 tages of accurate observation of a single thing is sacri- 
 ficed to the much greater advantages of somewhat dis- 
 tinct vision over a wide field. 
 
 In birds the fovea again appears, and yet their optic 
 axes are so widely divergent as to make it impossible to 
 converge these axes on a point (see Fig. loo). Never- 
 theless, birds seem to have binocular vision, but this is 
 by virtue of atwther fovea. In other words, among all 
 animals birds are peculiar in having two foi^ece in each eye, 
 one monocular and the other binocular. The monocular 
 ones, aa\ are axial and are the more distinct ; the binoc- 
 
l62 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 ular ones, b b', are far removed from the axis on the 
 temporal side, and so situated that lines drawn through 
 
 them and through the pu- 
 pils are parallel. These 
 axes can be converged on 
 a given point, and doubt- 
 y less corresponding points 
 are symmetrically ar- 
 ranged about them and 
 not about the other. The 
 central or monocular 
 fovea is the most distinct, 
 and therefore the monoc- 
 ular vision is better than 
 the binocular. This is the 
 reason why birds — for ex- 
 
 FiG. ioo.-Secti,^rjbird's head (after ^^ple, the domestic fowl 
 
 Slonaker): T' T', monocular visual — in looking attentively 
 lines; vv\ binocular visual lines; 
 
 a a', d^', central and temporal foveae turn the head and look 
 
 respectively. ^j^^ ^^^ ^^^ 
 
 Below birds, except in some lizards, nothing like a 
 distinct fovea is found. 
 
 It seems certain, therefore, that binocular vision in 
 its most perfect form is found only in man and the higher 
 apes, and thence becomes gradually less and less per- 
 fect until it disappears entirely in . the lowest verte- 
 brates. It is almost needless to add that it is not found 
 at all in invertebrates. 
 
 INVERTEBRATES. 
 
 In all that follows we are compelled to be very brief, 
 touching only most salient points. Some of these points 
 will come up again under Evolution of the Eye. 
 
 We pass over the arthropods, because in most of 
 them the structure of the eye is so different from what 
 
SENSE ORGANS. 
 
 163 
 
 we have described in vertebrates that no comparison 
 can be instituted. In them we find a different ^/«^ of 
 instrument and not a mere modification and simplifica- 
 tion of that already studied. We shall come back to 
 
 these after completing 
 the comparison in the 
 case of other inverte- 
 brates. 
 
 Mollusca : Ceph- 
 alopods. — The higher 
 cephalopods, such as 
 the squid and cuttlefish, 
 have large eyes and by 
 far the most perfect 
 below vertebrates (Fig. 
 roi). Their instrumen- 
 tal structure is substan- 
 tially like that of ver- 
 
 FlG. 101. — Nervous system of an argonaut, 
 showing the eyes : eg-, cephalic ganglion ; 
 og, the optic ganglion ; mg, brg, vg, the 
 ganglia of the mantle, the branchise, and 
 the viscera, respectively ; E, eye. (After 
 Cuvier. ) 
 
 Fig. 102. — The eye 
 of a snail on the 
 end of the ten- 
 tacle, magnified. 
 
 tebrates and is fully as perfect as that of a fish. There 
 are, indeed, some very significant differences, especially 
 in the retina, but these will come in our discussion of the 
 evolution of the eye. 
 
 In the gastropods the lois is wanting, the vitreous 
 
164 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 humor being the only refractive medium. The eyes of 
 these are, of course, situated about the head, and often, 
 as in snails, on the ends or at the base of the tentacles 
 or so-called horns (Fig. 102). 
 
 The acephala, or bivalves, as the name indicates, are 
 without distinct head, and the eyes or eye-spots are 
 strung along on the margins of the mantle, as in pecten. 
 But in many lower acephala and in echinodermata and 
 coelenterata the lens or any kind of refracting image- 
 making instrument disappears, and the eye is reduced to 
 a deposit ol pigment to absorb the light and a specialized 
 nerve to respond to light. These are called eye-spots. 
 They differ from true eyes in not forming an image. 
 They perceive light, but not objects. They have the spe- 
 cialized nerve, but not the image-making instrument. 
 
 Arthropods. — We passed over these because out of 
 the direct line of evolution. We now return. 
 
 Many arthropods — for example, the spider — have 
 eyes on the same plan as other invertebrates, but usu- 
 ally very small. But the most characteristic eye of ar- 
 thropods is the compound eye of 
 insects and crustaceans. It is ne- 
 cessary, however, before describing 
 these, to say something of the si>?i- 
 ple eye of arthropods. 
 
 Simple Eye. — If a spider be ex- 
 amined with a hand lens, a number 
 ''^ of brilliant, gemlike spots are seen 
 
 Fig. 103. — Eyeof a spider : ° 
 
 z, lens; F, vitreous hu- on the front part of the cephalo- 
 TaV^^/opdc^rier^e'''" thorax. They are usually in groups 
 of four, six, and eight on each side. 
 These are the eyes. Though small, they are somewhat 
 perfect for invertebrate eyes, for we find a cornea, c 
 (Fig. 103), a lens, Z, a vitreous humor, F, a retina, r, 
 and an optic nerve, on. 
 
SENSE ORGANS. 
 
 165 
 
 Compound Eye. — The compound eye of insects and 
 crustaceans is very different. If we examine the head 
 of any insect, such as a fly, a dragon fly, a butterfly, or 
 a beetle, we find that it consists 
 largely of two great hemispherical 
 masses, often of brilliant metallic 
 luster, green, or purple, or yellow. 
 These are the two compound eyes 
 (Fig. 104). If their surface be ex- 
 amined with a hand lens, or, better, 
 if the outer transparent corneal por- 
 tion be removed and placed under Fig. 104.— Anterior part 
 
 . . of a dragon fly, show- 
 
 a microscope, we see that it consists ing the compound eyes. 
 
 of thousands (twenty-eight thou- 
 sand in the dragon fly) of transparent hexagonal plates 
 nicely fitted together (Fig. 105). Each plate covers a 
 hexagonal prism, which runs back to abut against the 
 convex surface of the optic ganglion, which acts as the 
 retina and connects in its turn through the optic nerve 
 with the cephalic ganglion. Each tube 
 is lined with pigment, which may be 
 likened to a choroid, and filled with a 
 transparent substance, which may be 
 likened to the vitreous humor. The 
 whole is covered with a hexagonal cor- 
 neal plate, which is thickened into a 
 kind of lens over each prism. One 
 prismatic element is called an ommatidium (Fig. 106). 
 
 Now see in a general way (for it is not well under- 
 stood) how vision is accomplished by this instrument. 
 Remember, the condition of distinct image is that each 
 radiant should impress its own focal point on the ret- 
 ina. Rays from several points must not mix (page 
 104). Now if an object, A B (Fig. 106), be placed before 
 such an eye, the central ray from each point, ABC, 
 
 .^^Ekf^^^ 
 
 W0Bi 
 
 Fig. 105. — A portion 
 of corneal surface 
 of the compound 
 eye magnified. 
 
1 66 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 Fig. io6. — Diagram section through 
 compound eye : C, cephalic ganghon. 
 
 passes down the corresponding tube and impresses its 
 own point on the retina, and thus forms the image. Rays 
 
 passing into other tubes 
 strike the pigmented 
 sides and are quenched, 
 and thus mixing is pre- 
 vented, but with great 
 loss of light. 
 
 Comparison with Si/n- 
 ple Eyes. — Comparing 
 now with normal eyes 
 of invertebrates, we see 
 great differences in sev- 
 eral respects, i. In or- 
 dinary eyes distinctness 
 is reached by bringing 
 all the rays from each 
 radiant to a single focal 
 point on the retina. In this method, on the contrary, 
 the same result is secured, but with great loss of light, 
 by allowing only the central rays from each radiant to 
 reach and impress the retina. 2. In all other eyes, ver- 
 tebrate or invertebrate, the image is inverted; in this, 
 on the contrary, it is erect (Fig. 106). Nevertheless, in 
 this case also, by the law of direction, the object is seen 
 erect. The reason is that in all other eyes the recipient 
 surface is concave, and therefore reinverts the image in 
 the act of external reference, while in this it is convex, 
 and does not reinvert the image. 3. In vertebrate eyes 
 a wide field is got by free motion of the eye in its 
 socket; but in compound eyes it is got by the sphericity 
 of the large surface. In Crustacea, where the sphericity 
 is less great, the eye is placed on the end of a movable 
 stalk. It is probable that the sight of the compound 
 eye is very imperfect except at short distance. 
 
SENSE ORGANS. 1 6/ 
 
 Origin of the Compound Eye. — The spider has many 
 very small simple eyes in two groups, one on each 
 side of the head. Now imagine the number greatly in- 
 creased, the size correspondingly diminished, and then 
 the whole group crowded together until by mutual 
 pressure they are squeezed and elongated into pris- 
 matic tubes, and we have a general idea of the prob- 
 able process of change. 
 
 EVOLUTION OF THE EYE. 
 
 The exquisite beauty of the mechanism of the eye 
 makes its evolution extremely interestmg ; but hereto- 
 fore it has seemed an insoluble mystery. Recently, how- 
 ever, much light has been thrown on the subject. 
 
 I. Invertebrate Eye. — General sensibility to light 
 is coextensive with life itself. But it is a law in biology 
 that any useful function will be gradually separated 
 from other functions, localized in an organ, and then im- 
 proved indefinitely. How did a light-perceiving organ 
 begin ? It probably began to be formed under the 
 stimulus of light itself, as follows : 
 
 (i) On the exposed epithelial surface certain spots 
 became pigmented, and thus more absorbent of light ; 
 the nerves to these spots became specialized to respond 
 to the light ; the epithelial cells of these spots became 
 slightly modified by elongation into rodlike form; and 
 already we have an eye-spot, the simplest beginnings of 
 an eye. Why this effect should occur only /// spots we 
 know not, any more than we know why freckles should 
 come in spots. Such eye-spots may occur anywhere in 
 exposed surfaces, but more commonly on the most 
 sensitive part, viz., the head, when there is a head. 
 This first step is found in very many lowest animals, 
 especially in lowest mollusks (Fig. 107, a). 
 
l68 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 (2) The next step is a slight saucerhke depression of 
 the pigmented spot with an increased pigmentation and 
 elongation of the cells. This step is found in the sword 
 shell {Solen), in which the eye spots are strung all along 
 the edge of the mantle as the only exposed part, for 
 these animals are headless (Fig. 107, b). 
 
 (3) In the next step the depression becomes deep, 
 cuplike. Evidently here there is a stronger impression 
 of light by reverberation in the hollow and consequent- 
 
 FiG. 107. — Diagram representing- the different stages in the evolution of the 
 invertebrate eye : a b c, eye-spots, no image ; d, pin-hole image ; e, sim- 
 ple lens image ; /", compound lens image ; r, retina ; on, optic nerve ; 
 V, vitreous humor ; /, lens ; cor, cornea. 
 
 ly a greater specialization of cells for response. This 
 step is found in the limpet or Patella, and the organ is 
 situated in the head, for this is a gastropod. Already 
 we begin to see in the pigmentary layer a choroid and 
 in the elongated rodlike cells a bacillary layer of a retina 
 (Fig. 107, c). 
 
 Thus far we have only eye-spots, not an eye proper ; 
 only a specialized layer of nerve terminals, not an im- 
 
SENSE ORGANS. 169 
 
 age-making instrument ; an organ perceiving light, but 
 not yet seeing objects. 
 
 (4) In the next step, which is found in the nautilus, the 
 cup-shaped depression is closed in above until it becomes 
 a hollow vesicle with only a pin-hole opening atop. Now 
 for the first time we have an image, an inverted image, 
 on what is now plainly a retina (Fig. 107, d^. Now for 
 the first time there is a perception not only of light, 
 but also objects. In a word, we have a true eye. But 
 the sight of objects is still imperfect, for it is only a 
 pin-hole image. 
 
 (5) In the next step the pin-hole opening closes, but 
 the point of closure remains transparent as a cornea, 
 and the cavity or vesicle thus formed (optic vesicle) is 
 filled by secretion with a transparent refractive sub- 
 stance which may be regarded as a vitreous humor. 
 We have now for the first time a lens image, but yet 
 only a simple lens image (Fig. 107, e). This is the case 
 in the snail and many other gastropods. 
 
 (6) Finally in the squid the last stage in this strange, 
 eventful history is found. In these there is a cutic- 
 ular ingrowth from the corneal surface which finally 
 separates as a crystalline lens (Fig. 107,/), and thus we 
 have a compound lens image. 
 
 That these are really the steps of evolution of the 
 eye is proved by the fact that in embryonic develop- 
 ment the squid's eye passes through all these stages. It 
 is first seen as a dark spot, then as a saucerlike de- 
 pression, then as a cup-shaped depression, then as a 
 hollow cavity with a pin-hole aperture; then the aper- 
 ture closes and the vesicle fills, and, lastly, the crys- 
 talline lens is formed by cuticular ingrowth from the 
 cornea. This is the most perfect eye found among 
 invertebrates. 
 
 In the invertebrate eye there is yet no chiasm (Fig. 
 
I/O 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 loi, page 163), nor is there any fovea. There are cer- 
 tainly no corresponding points of the two retinae, and 
 therefore no binocular vision ; also, as we shall see pres- 
 ently, no blind spot. 
 
 2. The Vertebrate Eye. — There are two essential 
 differences between the invertebrate and the vertebrate 
 eye. (i) In the former the nerve fibers terminate for- 
 ward in the posterior ends of the rods in the most 
 natural way, as in the case of nerve terminals, in all other 
 sense-organs. The bacillary layer is the innermost 
 layer of the retina, and exposed directly to the action 
 of light. In the vertebrate eye, on the contrary, the 
 bacillary layer is the outermost layer of the retina, and 
 therefore the fibers have to go forward beyond and 
 turn back and terminate in the anterior ends of the rods. 
 This is wholly exceptional not only among eyes, but 
 among special sense-organs. It is this course of the 
 fibers which makes a blind spot, and therefore the in- 
 vertebrate eye can not have a blind spot. 
 
 (2) In invertebrates the whole eye, both the retina 
 and the lenses, is made by infolding of an external epi- 
 thelial surface. In vertebrates, on the contrary, the 
 instrumental part, especially the crystalline lens, is made 
 in this way, but the retinal part is made, as embryonic 
 development shows, from the brain, by an outfolding of 
 the cerebral vesicle. 
 
 The steps of the development of the vertebrate eye 
 are briefly as follows: (i) The brain is developed as 
 three vesicles. The anterior one is the thalamus (Fig. 22, 
 page 37), which is the basal part of the cerebrum, and we 
 shall call this the cerebral vesicle. (2) From the cerebral 
 vesicle by outfolding is formed on each side the optic vesi- 
 cles {OV, Fig. 108, A), which become more and more con- 
 stricted off until they are connected only by a narrow 
 neck, which becomes the optic nerve (Fig. 108, B). 
 
SENSE ORGANS. 
 
 171 
 
 (3) Meanwhile the infolding from the epidermal sur- 
 face has formed the lens. (4) Then the optic vesicle 
 becomes folded back upon itself like a double nightcap, 
 so as to leave a large space between it and the lens. 
 The anterior or back-folded layer of the double night- 
 cap becomes the retina, and the posterior layer the 
 choroid (Fig. 108, C). (5) The two folds come in con- 
 
 FlG. 108. — Diagram representing different stages in the development of the 
 vertebrate eye : C V, cerebral vesicle ; O l', optic vesicle ; r, retina ; c/i, 
 choroid ; d, bacillary layer ; /, fibrous layer ; /, lens. 
 
 tact and the vesicle is obliterated. The space between 
 the concave retina and the lens is filled and forms the 
 vitreous humor, the whole becomes encysted by the 
 sclerotic, and the eye is finished. Now observe that 
 the cerebral vesicle and the optic vesicle are lined 
 with epithelium. When this is folded back the pos- 
 
172 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 terior or epithelial layer of the back-folded nightcap 
 becomes the bacillary layer of the retina, while the 
 anterior layer of the same is the fibrous layer of the 
 retina. 
 
 The difference between the vertebrate and inverte- 
 brate retina is great, but not so great as at first seems. 
 In both cases the bacillary layer is formed of epithelial 
 cells — in the one of exterior epithelium of skin, in the 
 other of interior epithelium of the brain. But the lining 
 epithelium of the brain is itself an infolded portion of 
 the epiderm (Fig. 108, A and B). 
 
 Transition from Invertebrate to Vertebrate 
 Eye. — If vertebrates came from some form of inverte- 
 brates, as undoubtedly they did, how was the vertebrate 
 eye evolved out of the invertebrate eye ? This is a very 
 difficult question, i. Of course, the vertebrate type of 
 eye must have branched off very low down and before 
 the invertebrate type was fully declared. 2. Much of the 
 difficulty has come of identifying the crystalline lens of 
 the vertebrate eye with the same of the invertebrate 
 eye. On the contrary, it corresponds to the whole eye 
 of the invertebrates. It is formed in the same way, 
 viz., by infolding of the epidermal surface, while the 
 lens of invertebrates is formed by cuticular ingrowth 
 from the corneal surface. The retina of the verte- 
 brate eye is something superadded to the whole eye of 
 invertebrates, the retinal part of the latter having been 
 aborted and modified to form the back part of the ver- 
 tebrate lens. 
 
 Thus much seems certain, but how the change came 
 about is obscure. We may imagine {a) some low form 
 of invertebrate with very imperfect invertebrate eye, the 
 infolded epiderm functioning as usual as retina, but this 
 very close to cephalic ganglion. The light stimulating 
 the cephalic ganglion might well provoke the formation 
 
SENSE ORGANS. 
 
 173 
 
 of anothe«r and better retina. This functions as retina, 
 while the whole invertebrate eye is transformed into a 
 lens. Or (d) some low form may have had a pigmented 
 spot on each side of the anterior part of the head. In 
 the formation of the brain and spinal cord by infolding 
 of epiderm these spots might well be carried into the 
 cerebral vesicle and thence into the optic vesicle and 
 become a retina. Meanwhile the lens was formed by in- 
 folding, as already explained. 
 
 Further Evolution of the Vertebrate Eye. — 
 However this may be, once the vertebrate plan is estab- 
 lished the process of improvement goes on again stead- 
 ily. In fishes the position of eyes on the side of the 
 head and the absence of true chiasm show that there 
 are as yet no corresponding points, and therefore no 
 binocular vision. The ciliary muscle is also wanting, 
 and the eye can not be accommodated to accurate 
 vision for various distances in the same way as in land 
 vertebrates. The eye is no better than that of the 
 squid. 
 
 But in land animals the lens becomes flattened to 
 double conve-x shape, and may now be accommodated 
 to different distances by action of the ciliary muscle. 
 A true chiasm is not formed, and therefore binocu- 
 lar vision is not evolved until we reach birds. Mean- 
 while the eyes are moved more and more to position 
 in front, with increasing capability to converge on a 
 given point ; corresponding points are established in 
 the retinae; binocular vision and judgments appertain- 
 ing thereto become possible and more and more per- 
 fect. Finally, there is added a fovea, and with it the 
 ability to fix undivided attention on the objects looked 
 at, and this, in its turn, is at least one necessary con- 
 dition of the evolution of the higher faculties of the 
 mind. 
 
174 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 SECTION VII. 
 Sense of Hearing and i/s Organ^ the Ear. 
 
 Sight and hearing are the two higher senses. In 
 these alone the impression of the sensible body is not on 
 the specialized nerve directly, but indirectly through the 
 vibrations of a medium. In these alone, therefore, in 
 addition to the specialized nerve and in front of it, there 
 is a mechanical instrument for making the impression 
 stronger and more definite. 
 
 The eye is undoubtedly the most refined mechanism 
 in the animal body, and yet its structure is more easily 
 explained than that of the ear. The structure of the ear 
 is not only very complex, but it is lodged in intricately 
 winding passages in the interior of the hardest bone in 
 the body. In these passages the branches of the eighth 
 pair of nerves are distributed and specialized to respond 
 to vibrations of the air. 
 
 Structure of the Human Ear. — The ear consists 
 of three general parts — the exterior, the middle, and the 
 interior ear. The first two are air-filled, the third is en- 
 tirely cut off from the air and is water-filled. The first 
 two are instrumental ; the third alone contains the spe- 
 cialized nerve (Fig. 109). 
 
 The exterior ear includes all that is visible from the 
 outside — i. e., as far as the membrane of the drum. It 
 consists of the conch and the meatus. The conch 
 collects the aerial vibrations, and the meatus carries 
 them to the membrane of the drum. The meatus se- 
 cretes a kind of wax — ear wax — which by accumu- 
 lation may cause partial deafness, but is easily re- 
 moved. 
 
 The mid-ear is a cavity just beyond the membrane 
 of the drum. It is about one third of an inch in diame- 
 
SENSE ORGANS. 
 
 175 
 
 ter in direction at right angles to the drumhead, and 
 three quarters of an inch in a direction up and down. 
 It is connected with the throat by a slender tube — the 
 Eustachian tube — and is therefore air-filled. The closure 
 of this tube by inflammation of the throat is a frequent 
 cause of partial deafness. By holding the nose and 
 blowing hard, air may be forced through the Eustachian 
 
 
 \-,'" ' 
 
 Fig. 109. — Simplified diagram representing a section through the ear ; the 
 cochlea is supposed to be unrolled : ma, meatus auditorius ; /, tympanic 
 membrane ; ni, in, st, the ossicles ; sc, semicircular canals ; c, cochlea; 
 scv, set, scala vestibuli and scala tympani ; eu, eustachian tube. The 
 shaded part represents bone. ( From Huxley. ) 
 
 tube into the drum, and cause sensible pressure on the 
 membrane of the drum. 
 
 This cavity is separated from the outer ear by the 
 membrana tympani, and from the inner ear by a bony 
 wall, in which are two openings closed with membrane, 
 viz., the foramen rotundum and foramen ovale. These 
 membranes act as counter-drumheads to the membrane 
 of the drum, 
 13 
 
1^6 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 Ossicles. — Running across from the membrane of 
 the drum to the foramen ovale is a chain of three little 
 bones, called the iiialleus (hammer), the incus (anvil), 
 
 St. in. m. 
 
 Fig. iio. — Ossicles, enlarged: st, stapes; in, incus; ;«, malleus. 
 
 and the stapes (stirrup). These are articulated together 
 and to the walls of the cavity in a somewhat intricate 
 way, but evidently contrived to carry vibrations of the 
 drumhead to the interior ear, for the malleus is attached 
 to the drumhead, the stapes to the membrane of the 
 foramen ovale, and the incus is intermediate. Sound 
 vibrations of the air cause corresponding vibrations of 
 the drumhead (/), and these are carried along the chain 
 of bones and shake the membrane of the oval opening, 
 and therefore the water filling the inner ear, where its 
 further effects will be given after the inner ear is de- 
 scribed. The actual shapes of these bones are shown 
 in Fig. 1 10. 
 
 Interior Ear, or Labyrinth. — The real receptive 
 part of the ear is here. All other parts are purely in- 
 strumental. It is called the labyrinth because of the 
 complex winding passages in which the branches of the 
 auditory nerve are distributed. The labyrinth may be 
 best described under two heads, viz., the bony labyrinth 
 and the membranous labyrinth. The bony labyrinth 
 consists of winding cavities in the solid bone ; the mem- 
 
SENSE ORGANS. 
 
 177 
 
 branous labj'rinth is a membranous apparatus lodged in 
 these cavities. Each of these two parts consists of 
 three parts, viz., the vestibule, the semicircular canals, and 
 the cocJilea. There is therefore a bony vestibule, semi- 
 circular canals, and cochlea, and a membranous vesti- 
 bule (vestibular sac), semicircular canals, and cochlea. 
 All these parts in the membranous apparatus have forms 
 similar to the bony cavity in which they are lodged, but 
 are much smaller, so that there is considerable space be- 
 tween the true receptive membranous part and the cav- 
 
 FlG. III. — Outer and middle ear seen in section, and the inner as a cast of 
 the bon)' labyrinth : nia, meatus auditorius ; 7nc, mastoid cells ; t, tym- 
 panic membrane ; ;«, malleus ; /', the incus ; z/, vestibule ; c , cochlea ; 
 sc^ semicircular canals ; aim, auditory nerve. ( After Cleland. ) 
 
 ity in which it is lodged. This space is filled with a 
 watery liquid called perilymph. The membranous ap- 
 paratus is also filled with a liquid called endolymph. In 
 the diagram (Fig. 109) the dotted parts represent the 
 membranous labyrinth. 
 
1^8 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 Fig. 
 
 -Section of the cochlea showing the 
 winding cavity. 
 
 Bony Labyrinth. — The vestibule is a hollow space 
 about the size of a wheat grain. It is called the vesti- 
 bule because it is 
 the general hall into 
 which open all the 
 winding passages. 
 It is separated from 
 the tympanic cavity 
 by a wall in which 
 is the membrane- 
 closed oval open- 
 ing. Out of this 
 hall go out and 
 return again three 
 slender tubes — the 
 semicircular canals. 
 Two of these unite 
 at one end and enter by a common opening, so that 
 there are five openings into the vestibule instead of 
 six. At one end of each canal there is a flasklike en- 
 largement. 
 
 The bony cochlea is a spiral cavity like a spiral stair- 
 way, winding about a central pillar two and a half 
 times and growing smaller to the end (Fig. 112). The 
 name is taken from its resemblance to the shell of a 
 snail. 
 
 Membranous Labyrinth. — In the bony labyrin- 
 thine cavity just described is lodged the membranous 
 parts of the same names. The membranous vestibule, 
 or vestibular sac, within the cavity of the bony vestibule 
 is a small sac from which go and return the three mem- 
 branous semicircular canals, each with its flask-shaped 
 enlargement at one end, called the ampulla. The audi- 
 tory nerves are distributed on the vestibular sac and on 
 the ampulLne, the fibers terminating directly on the inte- 
 
SENSE ORGANS. 
 
 179 
 
 rior surface (Fig. 113). In the vestibular sac and at- 
 tached to hairlike nerve terminals there are several little 
 sandlike grains of carbonate of lime (otoliths). By mo- 
 tion of the endolymph these are shaken and affect the 
 hairlike nerve terminals, which thus become delicate 
 perceivers of the slightest movements caused by vibra- 
 tion. Again, in the ampullae the nerve fibers terminate 
 in little stififish hairs projecting from the walls toward 
 the center like the hairs in a mule's ears (Fig. 114). 
 Vibratory shakings of the endolymph, passing up from 
 the vestibular sac through the ampullae into the semi- 
 
 's c 
 
 Fig. 113. — vs, vestibule sac; sc, membranous semicircular canals; a/fi, am- 
 pulla ; n, nerve ; J, sacculus. 
 
 circular canals and back to the vestibular sac, set these 
 hairs trembling, and hence they also become delicate 
 perceivers of slight vibration. 
 
 Membranous Cochlea. — We have compared the 
 bony cochlea to a hollow stairway winding about a 
 central pillar ; now the membranous cochlea may be 
 compared to the s/air in this stairway, running across 
 
l8o PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 from a bony ledge on the pillar to the outer wall of the 
 stairway, and dividing it into two semicylindrical spiral 
 hollows, one above and one below, but not quite reach- 
 ing the extreme end ; so that vibration might run spirally 
 around in the upper way, over at the top, and down 
 spirally by the lower way. These two ways are called 
 the one scalavestibuli, because it opens into the cavity of 
 the bony vestibule, and the other the scala tympani, be- 
 cause it abuts against the tympanum, being separated 
 from it only by the membrane of the foramen rotundum. 
 The whole bony cavity of the cochlea is of course filled 
 with perilymph. 
 
 We have spoken as if there were but one membrane 
 separating the scala vestibuli from the scala tympani, 
 but really there are two, and these are separated by a 
 little space which is filled with endolymph. This space 
 is called the scala media and connects through the saccu- 
 
 lus with the vestibular 
 sac. Now in the scala 
 media are found a great 
 number of stififish rods 
 running from the cen- 
 tral pillar to the outer 
 wall, like stair rods, and 
 of diminishing length to 
 
 Fig. 114.— Sectian through an ampulla, the very end. These are 
 showing a branch of the auditory the rods of Corti. There 
 nerve, n, and the sensitive hairs, a a. 
 
 are several thousands 
 of them. A branch of the auditory nerve runs up the 
 central pillar and sends its fibers into the scala media, 
 and the rods of Corti are supposed to be the percipient 
 terminals of these fibers, as are the rods and cones of 
 the retina terminals of the fibers of the optic nerve. 
 Here again, therefore, we have a most delicate arrange- 
 ment for perceiving the slightest vibratory movement 
 
SENSE ORGANS. l8l 
 
 of the lymph. All this membranous apparatus is con- 
 nected throughout (Fig. 115). 
 
 Mode of Action of the Whole.— Sound vibrations 
 of the air are gathered by the conch, carried by the 
 meatus to the drumhead, and through the chain of 
 
 Fig. 115. — The whole membranous labyrinth: I's, vestibular sac; s, the 
 sacculus ; ?>tsc, memVjranous semicircular canals ; mc, the membranous 
 cochlea. (After Cleland.) 
 
 bones to the sfaj^cs ; the shaking of the stapes communi- 
 cates a vibratory motion to the perilymph, and this to 
 the endolymph of the vestibular sac. The shaking of 
 this causes the otoliths to agitate the nerve terminals 
 exposed on the interior of the sac. The vibratory move- 
 ment now divides in several branches. Three of these 
 go up through the semicircular canals, shaking the hairs 
 of the ampullse in which nerve fibers terminate. Still 
 another branch runs spirally up the scala vestibuli over 
 at the extreme end and down spirally by the scala 
 tympani. These vibrations are communicated to the 
 endolymph of the scala media and impress the rods of 
 Corti. 
 
 The Distinctive Functions of these Parts. — It 
 is believed that there is a distinctive function of each of 
 these several parts. The vestibular sac with its otoliths 
 seems especially adapted to perceive the slightest sound 
 as soutid or noise, while the cochlea with its rods of 
 graduated lengths seems specially adapted to the percep- 
 tion of sound as tone or pitchy and therefore for the per- 
 ception of music. These rods might well be supposed 
 
1 82 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 to respond each to a special rate of vibration, somewhat 
 as a note on a violin falling on all the strings of a piano 
 only one will respond, viz., that one which vibrates in 
 unison — co-vibrates — with the given note. 
 
 The semicircular canals with their ampullae and 
 terminal hairs may concur with the vestibular sac and 
 otoliths in perceiving sound as noise, but they are also 
 supposed to have another function. 
 
 We have not yet drawn attention to the remarkable 
 fact that these three canals are set in three rectangular 
 plains, one vertical fore and aft, one vertical from side 
 to side, and the third horizontal (Fig. ii6, page 183). 
 The least movement of the head back and forth, as in 
 nodding, would move the water in the vertical fore-and- 
 aft canal and would be perceived by the hairs of its 
 ampulla. Similarly movements of the head from side to 
 side, as in wagging, would be perceived by the ampulla 
 of the canal set in the vertical transverse plain ; while 
 rotation of the head on the spinal column, as in the sign 
 of negation, would be perceived by the ampulla of the 
 horizontal canal. In other words, these canals with 
 their ampulla are a most delicate indicator of the 
 position and movement of the head, and therefore 
 necessary for maintaining the equi/ibriufn of the body. 
 Surely this is a fundamental and most important func- 
 tion. Many experiments seem to substantiate this 
 view.* 
 
 The perception oi direction, which is so mathematically 
 exact in the case of the eye, is extremely inexact in the 
 case of the ear. It is this inexactness which is utilized 
 by the ventriloquist in producing his deceptions. 
 
 * Some would go further and say this is the sole function, and 
 others still further and say the only organ of hearing is the 
 cochlea. This, however, seems improbable. 
 
SENSE ORGANS. 
 
 183 
 
 COMPARATIVE MORPHOLOGY AND PHYSIOLOGY 
 OF THE EAR. 
 
 In no Other organ do we find so regular a simplifica- 
 tion in descending the scale of animals. 
 
 In mammals the structure and function of the ear are 
 almost exactly what we described in man. The only 
 important differences are the greater size and efficiency 
 of the external ear as gatherers 
 of sound waves, and the movable- 
 ness of the ear by the use of ap- 
 propriate muscles by which ani- 
 mals perceive direction better 
 than we. These muscles exist 
 even in man, but in a rudimentary 
 and therefore useless condition. 
 The hearing of most mammals is 
 keener than that of man, as they 
 rely much on this sense for their 
 safety. 
 
 Birds. — The first important 
 simplification is found in birds. 
 In the exterior ear the conch is 
 entirely wanting and the meatus 
 is very shallow, so that the mem- 
 brane of the drum is very near the surface of the head. 
 In the mid-ear the chain of bones is reduced substan- 
 tially to one, the columella (which represents the stapes 
 and probably the malleus), and the tympanic cavity is 
 broadly connected with the throat instead of by a slen- 
 der Eustachian tube. In the interior ear we find the 
 cochlea much shorter and uncoiled {¥\g. 116). 
 
 Reptiles. — In reptiles the exterior ear is gone; the 
 membrane of the drum is at the surface, covered with 
 skin and often with muscle. The. mid-ear is very similar 
 
 Fig. 116. — Interior ear of a 
 bird, showing cochlea {c) 
 uncoiled. The semicircu- 
 lar canals in three rec- 
 tangular planes are also 
 shown. (From Parker.) 
 
l84 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 to that of birds, but the interior ear has lost, or nearly 
 lost, the cochlea. 
 
 In amphibians the whole of the mid-ear as well as 
 the exterior ear is lost ; only the interior ear remains, 
 
 A 
 
 B 
 
 ■ty 
 
 auv gc 
 
 Fig. 117. — A, tibia of a grasshopper {Meconcfnia)^ with auditory organ. 
 B, section ot the same enlarged : t\\ tympanic membrane ; auv^ audi- 
 tory vesicle ; gc^ ganglionic cell. (From Packard. ) 
 
 and this is reduced to vestibule and semicircular canals, 
 the cochlea being wanting. 
 
 In fishes we have still the vestibular sac and mem- 
 branous semicircular canals, but the bone has not grown 
 completely about these so as to make bony cavities of 
 similar shape; nor are the cavities of the ear cut off 
 from the brain cavity. 
 
 Finally in invertebrates the ear is reduced to a vestibu- 
 lar sac and otoliths. These, therefore, are the most fun- 
 damental and necessary parts of an organ of hearing. 
 
 The hearing organs of invertebrates, however, are 
 much diversified in form and position. In insects they 
 are found sometimes in the first joint of the abdomen, as 
 in some grasshoppers ; sometimes in the lower joint 
 (tibia) of the leg, as in other grasshoppers; and probably 
 sometimes in the antennae. Insects certainly hear, for 
 they produce sounds which are intended to be heard. 
 In all the cases above mentioned there is a hollow re- 
 
SENSE ORGANS. 185 
 
 verberatory cavity, with a tense membrane, ty (which 
 may be compared to a membrana tympani, or, better, 
 with the membrane of the foramen ovale), a vestibular 
 sac, ain\ containing otoliths (Fig. 117). In crustaceans \s 
 found a similar organ, sometimes on the anterior lower 
 surface of the cephalothorax, as in crabs, and sometimes 
 on the basal joint of the antennae, as in lobsters. 
 
 Mollusca. — In cephalopod mollusks the hearing organ 
 is in the head just below the brain, as a cavity in the 
 cartilage filled with endolymph and containing otoliths 
 (Fig. 118). \\\ gastropods 
 it is a capsule of con- 
 densed connective tissue 
 lined with epithelium, 
 filled with liquid and con- 
 taining otoliths, situated 
 just below the oesophag- 
 eal ganglion.* In aceph- 
 ala a similar capsule has 
 been found at the base of 
 
 Fig. 118. — Section throuf'h the head of 
 the gills which is sup- a squid, showing the auditon- organ: 
 
 nncpH to hnvP a Qimilar w, vestibular sac and otoliths ; eg, 
 
 posea to nave a similar cephalic ganglion ; e, the eye. 
 
 function. 
 
 Below this a hearing organ has not been found. 
 Whether there be a nerve specialized for hearing is not 
 known, as we can judge only by the existence of some 
 apparatus like a capsule and otoliths. 
 
 Thus far the most essential part of the ear is the 
 vestibular sac with its otoliths. But in spiders and 
 certain insects there is found another type of hear- 
 ing organs which may be compared not to the vestibular 
 sac, but to the hairs of the ampullae. In spiders, on the 
 feelers are found cup-shaped hollows, from the bottom 
 
 * Nat., iv, 51S ; Arch, des Sci., xliv, 261. 1872. 
 
1 86 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 of which arise a fine tapering hair, into which runs a 
 nerve fiber. The vibrations of the air reverberated in the 
 hollow determines corresponding vibrations of the hair 
 and affect the nerve. 
 
 This style of hearing organ reaches its highest per- 
 fection in the mosquito, especially in the male mosquito.* 
 
 Fig. 119. — Head of a mosquito. The two lower and longer antenna are 
 feelers, the two upper ones with radiating hairs are auditory. ( After 
 Mayer. ) 
 
 Fig. 119 represents the head of the male mosquito with 
 its compound eyes and auditory organs. There are two 
 kinds of antennae : the one (the lower in the figure) are 
 feelers or organs of touch, the other (the upper) are 
 organs of hearing. As in the spider, these come from 
 the bottom of hollow cups, which probably act as resona- 
 tors, increasing the air vibrations; and the long hair- 
 like, many-branching antennae respond by co-vibrations. 
 A nerve runs up each antenna and sends a fiber to every 
 branch. Surely this is an admirable arrangement for 
 responding to aerial vibrations. 
 
 But, according to the investigations of Mayer and 
 
 Mayer, Am. Jour., viii, 81, 1874 ; Arch, des Sci., li, 263, 1874. 
 
SENSE ORGANS. 1 87 
 
 Johnston, it is also admirably adapted to appreciate 
 both musical tone and direction. If a mosquito be fixed 
 under the microscope and a sound be made on a violin, 
 among the many hairs of all lengths only a few are 
 observed to vibrate in response, viz., those which by 
 length are adapted to co-vibrate. Again, it was observed 
 that in making the sound in different parts of the room, 
 of the hairs pointing in all directions only those vibrated 
 strongly which were at right angles to the direction of 
 the sound. They probably perceive direction much bet- 
 ter than we do.* 
 
 Leaving out, however, these last contrivances as out 
 of the direct line of evolution, and regarding the ves- 
 tibular sac and otoliths as the simplest form of hearing 
 organ, the simplification as we go down the scale is 
 very regular and may in a general way be expressed by 
 the following diagram, which, with the legend, will be 
 readily understood and requires no further explanation : 
 
 Cla.-<scs 
 
 Outer 
 
 Middle 
 
 Inner Ear \ 
 
 Mammals 
 
 Birds 
 
 Reptiles 
 
 Amphibians 
 
 Fishes 
 
 Invertebrates 
 
 Conch 1 Meatus 
 
 Tympm. 1 Osaicles 
 
 Bony 
 V sc c 
 
 Membranous 
 
 - -)f X--- 
 
 1. 
 
 -Jf- 
 
 
 IX::: 
 
 --4-— )<---* -- 
 
 ZTt: 
 
 Tt 
 
 
 
 
 
 
 
 
 
 
 ^ 
 
 Fig. I20. — Diagram showing the g:radual simplification of the hearing organ 
 as we go down the scale. The stars represent the presence of the parts 
 named above. Middle ear, i, 2, .-5, ossicles; inner ear, F, .ST, C, bony 
 vestibule, semicircular canals, and cochlea ; f^', S'C, C, membranous 
 same parts. The dotted continuations of the cochlea line mean that 
 rudiments of cochlea are found in some reptiles. 
 
 * Observe here that the greatest effect is produced when the 
 sound-waves strike the sensitive hairs broadside, and becomes 
 nothing when it strikes end on. In the case of sight the very re- 
 
1 88 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 Therefore in the evolution of the organic kingdom 
 the most necessary and first evolved part of a hearing 
 organ was the vestibular sac and otoliths. These are 
 found in the higher invertebrates. Then in fishes there 
 were added the semicircular canals. Next the bone grew 
 about these parts, so as to shut them off from the brain 
 cavity, and at the same time inclosed them in such wise 
 as to form cavities of shape similar to the organs them- 
 selves. This is found in the amphibians. Then the mid- 
 ear was added, with its tympanic membrane and at least 
 one ossicle. This is the case in reptiles. Then in birds 
 a cochlea was added and a shallow meatus. Finally, in 
 mammals the cochlea was greatly elongated, and there- 
 fore coiled, more links were added to the chain of ossi- 
 cles, the whole apparatus was sunk deeper into the head 
 and with a longer meatus, and the conch was added. 
 
 SECTION VIII. 
 
 Lower Senses. 
 
 The three lower senses will be more rapidly dis- 
 patched, because the impression in these is directly on 
 the specialized nerve without the intermediation of an 
 instrument. We have therefore only the specialized 
 nerve and terminals to deal with, and the affections of 
 these are so inscrutable that we can have little to say. 
 
 SENSE OF SMELL AND ITS ORGAN, THE NOSTRIL. 
 
 The nostril in man is a quadrangular cavity, passing 
 directly backward from the face to the throat (Fig. 121). 
 It is covered in front by the overhanging nose, but in 
 
 verse is true — i. e., the effect is greatest when the rods are struck 
 e7id on. The reason of the difference is that sound waves are 
 waves of longitudinal vibration, while light waves are waves of 
 transverse vibration. 
 
SENSE ORGANS. 
 
 189 
 
 Fig. 121. — Vertical section through the cav- 
 ity of the nostril : rw, roof of mouth ; 
 opl, orbital plate ; cpl^ cribriform plate. 
 
 the skeleton it is largely, though not entirely, exposed. 
 It is bounded on each side by parts of the maxillary 
 
 bone, and the orbital 
 plates of the ethmoid, 
 opl\ above, it is sepa- 
 rated from the brain 
 cavity by a thin plate 
 perforated with many 
 holes — the cribriform 
 or colander plate of 
 the ethmoid, cpl — and 
 below, from the mouth 
 cavity, by the palatal 
 plates, which form the 
 roof of the mouth and 
 floor of the nasal cav- 
 ity, rm. It is divided 
 into two symmetric halves (right and left nostril) by 
 a bony septum, the 
 vomer, which is con- 
 tinued into the car- 
 tilaginous septum of 
 the nose. Each nos- 
 tril is again divided 
 by the turbinated or 
 scroll bones into an 
 upper and lower part, 
 which differ in struc- 
 ture and function. 
 The lower part is 
 comparatively sim- 
 ple, the upper part 
 complex ; the one is 
 lined with the ordinary epithelium of the alimentary 
 passages, the other by a peculiar smooth epithelium. 
 
 Fig. 122. — \'ertical fore-and-aft section through 
 one nostril, showing the olfactorj' nerve 
 coming down from the olfactory lobe and 
 the nerve of feeling coming from behind. 
 
IQO 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 The former is supplied by a branch of the fifth pair of 
 nerves (Fig. 122, //), which are the nerves of common 
 sensation for the face; the latter is supplied by the first 
 pair, or olfactory nerve. The former lies along the floor 
 of the cavity and passes directly to the throat; the lat- 
 ter lies above the general passage to the throat and 
 lungs. The function of the former is breathing, of the 
 latter is smelling. Each of these have connections with 
 the other cavities, the former with cavities in the cheek 
 bones, the latter with cavities m the frontal bones over 
 the brows. Inflammation sometimes extends from the 
 nostrils into these cavities. 
 
 Smelling. — The olfactory lobes rest directly on the 
 cribriform plate of the ethmoid, which here forms a part 
 of the floor of the 
 cranium. It sends 
 out a large number 
 of nerves through 
 the holes of the 
 colander directly 
 down and into the 
 nostrils, and these 
 ramify over its 
 upper chambers 
 and their terminal 
 bulbs are exposed 
 on the surface (Fig. 
 122). Particles of 
 odoriferous sub- 
 stances are carried by the air current, and, coming in 
 contact with the olfactory terminals, produce the sen- 
 sation we call smell. The peculiar form of the terminals 
 sensitive to odors is shown in Fig. 123, 
 
 Any sensation may be involuntary or voluntary. 
 Thus seeing may be involuntary, but looking is voluntary. 
 
 Fig. 123. — Nerve terminals of the olfactory 
 nerves : olfc, olfactory cells ; olfnf, olfac- 
 tory nerve fibers ; epc, epithelial cells. 
 
SENSE ORGANS. 
 
 191 
 
 Hearing may be involuntary, but listening is voluntary. 
 So smelling may be involuntary, but sniffing is a volun- 
 tary act of smelling. In sniffing we draw in the air 
 suddenly and then stop it. The air is thus forced 
 more thoroughly into the upper chambers. 
 
 Odoriferous particles being air-borne, it is evident 
 that a substance can not be smelled unless it is volatile. 
 The amount of matter in the air which may be detected 
 by this sense is so infinitesimally small that it can not 
 be estimated. No chemical test can compare with it in 
 delicacy. 
 
 COMPARATIVE PHYSIOLOGY OF SMELL. 
 
 We judge of the delicacy of this sense in other ani- 
 mals in three ways — viz., by the size of the olfactory 
 lobes, by the complexity of the olfactory surfaces, and 
 by the habits of the animal. Judging by either or all of 
 these, there can be no doubt of the enormous superiority 
 of mammals over man. This is especially true of car- 
 nivores, which hunt by smell, and of herbivores, which 
 detect danger by the same sense. Referring to Fig. 41 
 (page 76), we see at once the great size of the olfactory 
 lobes in mammals. Making a transverse section through 
 the nostrils of a dog, a horse, or a cow, we observe at 
 once the great complexity of its chambers. The same 
 superiority is brought out still more strongly by obser- 
 vation of habits. Think of the keenness of the smell of 
 a dog, who follows his master's tracks an hour after he 
 has passed ; or of a hound tracking a deer ; or, again, of 
 a deer sniffing the air and detecting the hunter a mile 
 away. It is probable that among mammals and lower 
 vertebrates generally smell, not sight, is the most impor- 
 tant and most cbjective sense. In passing through a 
 strange country u<e take ocular notes, and may return 
 the same way by the use of these. A dog under simi- 
 14 
 
192 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 lar circumstances takes olfactory notes for the same 
 purpose. 
 
 Judging in the same way, birds are doubtless supe- 
 rior to man, though greatly inferior to mammals. It is 
 commonly supposed that vultures are especially distin- 
 guished for keenness of scent; but experiments of Au- 
 dubon and Bachman show that they are inferior in this 
 respect to dogs. A stuffed deer attracted vultures 
 from the clouds, but dogs paid no attention to it. But 
 a real carcass concealed from view was quickly discov- 
 ered by dogs, while circling vultures did not detect it. 
 
 In all the lower land vertebrates, such as reptiles and 
 amphibians, the olfactory lobes form an important lobe 
 of the brain, and their smell is 
 probably correspondingly devel- 
 oped, but no observations have 
 been made to test it. We there- 
 fore pass on. 
 
 Thus far vertebrates are air- 
 breathing, the odoriferous parti- 
 cles are air-borne, and the smell- 
 ing organs are therefore con- 
 nected with the breathing passages. 
 But fishes breathe water, not air, 
 by gills, not nostrils. Smells with 
 them are therefore water-borne, 
 not air-borne. The organs in 
 fishes are two deep pits near the 
 end of the snout, in the usual 
 position of nostrils, but do not 
 yet open into the throat. The interior of these pits are 
 plicated in a complex manner (Fig. 250, page 369), so 
 as to increase the surface of contact with odorous par- 
 ticles, and large nerves from the olfactory lobes are dis- 
 tributed in them. In sharks, those bloodhounds of the 
 
 Fig. 124. — Brain of a shark : 
 w, medulla ; cb, cerebel- 
 lum ; o, optic lobes ; cr, 
 cerebrum ; <?/, olfactory 
 lobes ; wc, capsule of the 
 olfactory nerve. ( From 
 Gegenbaur. ) 
 
SENSE ORGANS. 
 
 193 
 
 sea, the olfactory pits are particularly complex, and the 
 olfactory lobes are of enormous size, often as large or 
 even larger than the cerebrum (Fig. 124). 
 
 Invertebrates. — That many invertebrates, especially 
 insects, have a keen sense of smell is clearly evidenced 
 by the fact of their being attracted by odors, as blow- 
 flies by putrid flesh or butterflies by the fragrance of 
 flowers. But where the organs of this sense are situated 
 is not certainly known. The reason why it is so difficult 
 to determine is that there is no instrument connected 
 with the sense by which we can know it. It is probable, 
 however, that in the case of insects it is situated in the 
 antennae. Insects are air-breathing. The breathing 
 tubes, as we shall see hereafter, penetrate every part of 
 the body, but especially pass into and to the end of all 
 branches in the antennae. In spiders, too, the organ of 
 smell, as well as of hearing, is supposed to have been 
 found in the feelers.* 
 
 In still lower invertebrates the organs of smell have 
 not been certainly detected. 
 
 SENSE OF TASTE AND ITS ORGAN, THE TONGUE. 
 
 What we usually call taste is a complex sensation, a 
 mixture of several sensations. It is impossible to dis- 
 cuss the subject scientifically without analysis. There 
 is usually a mixture of three sensations belonging to as 
 many different kinds of nerves — viz., common sensation, 
 smell, and taste proper. 
 
 Examples of mixture of gustation with common sen- 
 sation are numerous. The same batch of dough may 
 be so mixed and baked that it shall be heavy and stick 
 to the teeth in chewing, or may be light and spongy. 
 The one we call disagreeable, the other agreeable, to the 
 
 * Dahl, An. and Mag. Nat. Hist., xiv, 329, 1SS4. 
 
194 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANLMALS. 
 
 taste. The true taste is exactly the same in both ; the 
 real difference is in the /<?^/ of the alimentary bolus as it 
 is chewed and moved about in the mouth. The same 
 parcel of rice may be cooked thoroughly and yet in such 
 wise that each grain shall stand in separate and self- 
 reliant individuality, or in such wise that all individual- 
 ity is lost in a common socialistic mush. We all know 
 the entire difference in what we call the taste. But really 
 it is a difference in the feel only. There is very little 
 taste of any kind in rice, but what there is is exactly 
 the same in the two cases. 
 
 It is far more difficult to separate taste and smell, 
 and yet even popular language has taken note of the 
 difference. It is embodied in the two words savor and 
 flavor. Savors are tastes, flavors are smells. \Ve have 
 heard of "salt that had lost its savor,"* but never its 
 flavor, for it has none to lose. It is necessary to re- 
 member, then, that what we call flavors are not tastes at 
 all, but smells. They are affections of the olfactory 
 nerves, not the gustatory. They are all, therefore, vola- 
 tile substances, essential oils, compound ethers, etc. 
 They can all be smelled without eating. 
 
 Examples. — Coffee or tea, so far as taste in the ordi- 
 nary sense is concerned, has two principles : a bitter 
 astringent principle, which is a taste, and 2. flavor, which 
 is a smell. This last is due to a volatile substance 
 which may be all driven off by long boiling. In a 
 broiled steak the slight saltiness is nearly all that affects 
 the gustatory nerve. Its pleasant flavor is an affection 
 of the olfactory nerve entirely. It can be enjoyed with- 
 out tasting at all. The same is true of all fruits. In a 
 
 * The salt used by the poor of Palestine is said to have been 
 very impure — in fact, a sort of salty earth, from which the salt was 
 easily washed out. 
 
SENSE ORGANS. 
 
 195 
 
 strawberry we have, first, the pleasant combination of 
 acid and sweet. This is taste proper. Then there is 
 also the peculiar flavor characteristic of that fruit, which, 
 of course, is a smell. In wine — e. g., champagne — we 
 have first the pungency of the CO,. This is common 
 sensation. Then the pleasant combination of acid and 
 sweet. This is taste proper. And last, the character- 
 istic flavor, aroma, or bouquet. This is a smell. 
 
 Examples of Tastes. — As thus limited and defined, 
 the tastes are few in number, while the flavors are al- 
 most infinite. They are: (i) Siceet. Pure white sugar 
 has no flavor, and therefore no smell. Brown sugar has, 
 because it has a smell. (2) Sour. Pure lime juice, or 
 citric or tartaric acid, has no flavor, but a very intense 
 taste, but vinegar has a flavor, because it has also a 
 smell ; it is volatile. (3) Bitter. Quinine, or morphine, 
 or strychnine has a most intense taste but no smell, and 
 therefore no flavor. (4) Salty. In the other three there 
 is scarcely any variety at all in each, but in saltiness 
 there is much variety, although the taste of chloride of 
 sodium is the type. There can hardly be said to be any 
 other pure tastes besides these four. 
 
 The proof of the essential difference between tastes 
 and flavors is found in the fact that a bad cold, if it 
 aft'ects the upper chambers of the nostrils, is said to de- 
 stroy the taste. It does not destroy the taste of sugar, 
 or lemon juice, or quinine, or salt. It destroys the sense 
 of smell, and therefore the appreciation of flavors. If 
 we hold the nose, flavors are almost destroyed ; not 
 entirely, however, because some will come in by the 
 back door — i. e., by the throat into the nose. 
 
 If it be asked, If this be so, why are we not satisfied 
 with smelling ? the answer is plain. Not only is the flavor 
 stronger when taken as food, because it then is taken in 
 both ways, front door and back door, but mainly because 
 
igS PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 there is in that case a combination of pleasant sensations. 
 For example, what we call the taste of a ripe peach is 
 first the delicious softness and juiciness. This is a 
 feeliug. Then the agreeable combination of acid and 
 sw^eet. This is taste proper. And last, the whole is 
 
 glorified and crowned 
 by the exquisite fla- 
 vor, which, as already 
 shown, is a smell. 
 
 Organ of Taste. 
 — As thus limited, the 
 organ of taste is the 
 tongue, especially the 
 back part of the tongue 
 and the adjacent parts 
 of the throat, and still 
 more especially certain 
 large papillae in that 
 part of the tongue (Fig. 
 125). There are two 
 kinds of papillae on the 
 tongue — one kind large 
 and flat, mushroom- 
 like,* the other small 
 and conical. Those of 
 the one kind are found 
 only on the back part 
 of the tongue and are 
 
 Fig. 125. — Interior of mouth, showings the 
 papillae of the tongue and the distribu- 
 tion of the nerves of taste : /, fungiform 
 papilla. (After Huxley.) 
 
 taste papillae; those of the other are found in every part, 
 but mainly near the tip, and are tactile papillae. There 
 are three kinds of nerves distributed to this nimble little 
 organ : (i) A branch of the fifth pair (Fig. t^t,, 5, page 51). 
 This gives common sensation. It is distributed to all 
 
 * These are again subdivided into fungiform and circumvallate. 
 
SENSE ORGANS. 
 
 197 
 
 parts of the tongue, but more and more toward the tip. 
 (2) The glosso-pharyngeal (Fig. 33, p). This also is dis- 
 tributed to all parts, but mainly to the back part and 
 
 adjacent parts of the throat. 
 This is supposed to be the 
 special nerve of taste. (3) 
 
 Fig. 126. — A fungfiform papilla, showing; the taste bulbs (/6) of a rabbit : 
 A, magnified ; B, highly magnified, i^ After Tuckerman.) 
 
 The hypoglossal (Fig. ;^^, 12). This is a motor nerve, and 
 presides over the movements of the tongue. Through 
 these three it becomes a tactile organ, a tasting organ, 
 and a talking organ. The manner in which the nerves 
 terminate in the taste papillae is shown in Fig. 125. The 
 flat fungoid papillae are eminently adapted by shape to 
 retain liquids in contact with the taste bulbs. 
 
 COMPARATIVE PHYSIOLOGY OF TASTE. 
 
 There are only two ways in which we can judge of 
 the keenness of taste in lower animals, viz., by the or- 
 ganization of the tongue — i. e., its softness, and espe- 
 cially the development of its papillag, and by observation 
 of the habits of the animal. Judging in these ways, 
 there is probably little difference in this regard between 
 man and mammals. The main difference is that man 
 much more than mammals takes food for the enjoyment 
 
igS PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 of its taste. The urgency of appetite in mammals for- 
 bids the leisurely enjoyrnent of taste as such. 
 
 Birds have no teeth. They do not masticate, but 
 bolt their food. In many cases, too, their food, as, for 
 example, seeds and the like, is hard and tasteless. It is 
 probable, therefore, that their sense of taste is more 
 imperfect than that of mammals. 
 
 Reptiles, atnphibiaiis, and fishes all swallow their food 
 without mastication. They have teeth ; but they are 
 prehensile, not masticatory. We know little of their 
 sense of taste, but it is probably feeble. 
 
 Of taste among invertebrates we know nothing, except 
 in the case of insects. These are doubtless attracted to 
 food mainly by smell; but sweets — sugar, honey, nectar 
 of flowers — are sought and enjoyed by bees, ants, 
 butterflies, and flies. 
 
 THE SENSE OF TOUCH AND ITS ORGANS. 
 
 Here, again, and even more than in the case of taste, 
 scientific discussion is impossible without analysis. The 
 word feeling includes very many distinct sensations. 
 For example: lay the hand on the table, palm upward. 
 (i) Lay a card across the finger tips; we have a sense or 
 feeling of contact. (2) Instead of a card let it be a ten- 
 pound weight; we now have in addition a feeling of 
 pressure. (3) Let the weight be fifty pounds; we have 
 now in addition a feeling of pain. (4) Let the weight 
 be hot or cold ; we have now a corresponding feeling of 
 heat or cold. (5) Now lift the hand from the table; 
 in addition to all the preceding, we have a feeling of 
 weight or resistance to our muscular effort — we feel the 
 heft. Thus, then, there are many kinds of sensations in- 
 cluded in the word feeling. These are so different that 
 they are probably perceived by different specialized 
 nerves. It is almost certain, according to recent obser- 
 
SENSE ORGANS. 
 
 199 
 
 vations,* that different nerve fibers take cognizance of 
 contact, pain, heat, cold, and muscular resistance (Fig. 
 68, page 95). In physics cold is a negative term, a mere 
 absence of heat ; but in physiology, as we all know, it 
 is a v^xy positive sensation. 
 
 Now, it is impossible to take up all the sensations in 
 detail. We take only that which is the most universal 
 and fundamental, viz., contact (pressure being only a 
 stronger contact). This, together with the muscular 
 sense of resistance, gives us externality, or the existence 
 of the external world. 
 
 Again, it is necessary to distinguish between general 
 sensibility and special sense of touch. The former, to- 
 gether with the so-called muscular sense, gives the exist- 
 ence of the external world ; the latter may be regarded 
 as the same, specially organized to give definite knowl- 
 edge of some of its properties, such as shape, hardness, 
 roughness, etc. These two kinds of sense of contact are 
 by no means developed in the same degree. The con- 
 junctiva of the eye is exquisitely sensitive to contact, 
 but it does not appreciate the properties of the touching 
 body as does the finger tips or the tongue tip. The same 
 difference, it will be remembered, we found in the retina. 
 Mere sensitiveness to light is keenest a little way from 
 the central spot, but this spot alone is specially organized 
 to give us accurate knowledge of shape, color, etc. 
 
 General Organ of Touch. — The general organ 
 for perception of contact is the skin and portions of the 
 mucous membranes near the outlets of the passages, 
 especially the mouth. This is for all contact. Besides, 
 there are certain portions of the skin specially organized 
 as organs of touch, such as the hands and the parts about 
 the mouth. 
 
 * Sci., vii, 151, rSS6, and 459, 1886. 
 
200 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 General Structure of the Skin. — The skin con- 
 sists of two parts — dermis and epidermis. The dermis 
 consists of fibers crossing one another in all directions, 
 and, as it were, felted together. It is very strong and 
 very highly organized, full of blood vessels and nerves. 
 The epidermis contains no blood vessels or nerves, and 
 is not organized at all. It consists wholly of epithelial 
 cells; living nucleated cells in contact with the dermis, 
 
 Fig. 127. — Section throucfh the epidermis (ep) and the dermis {d^ : «, nerves 
 of touch terminating- in tactile corpuscles. 
 
 but becoming more lifeless and flatter as we go from the 
 dermis to the surface, where they continually pass off as 
 scales or scarf. The nerve fibers come to the epidermis, 
 but do not penetrate it. They terminate near some of 
 the cells of the lowest layer (Fig. 127). 
 
 Special Organ of Touch. — Wherever the skin is 
 specially organized for touch it is thrown into ridges, 
 as on the finger tips, or else rises into papillae, as on 
 the tongue (Fig. 128). In the case of ridges there is a 
 row of tactile corpuscles or bulbs in each ridge; in the 
 
SENSE ORGANS. 
 
 20 1 
 
 case of papill?e each one contains a tactile bulb. A sen- 
 sory fiber enters and terminates in each bulb. These 
 
 are therefore end organs or 
 terminals. The epidermis 
 proper is not sensitive; it 
 only protects the sensitive 
 dermis from too rude con- 
 tact. 
 
 Minimum Tactile. — 
 7 We have already alluded 
 to this subject in compar- 
 ing sight with touch (page 
 T^ o T^ .-, ■ . 132). We repeat it here in 
 
 Fig. 128. — Tactile corpuscles : A, from . 
 
 the finger of a man; H, from the itS proper Connection. If 
 skin of a bird; n. nerves. (After ■ r :• -j t, j 
 
 Wiedersheim.) a pair 01 dividers be opened 
 
 widely and the skin be 
 touched with the points (blunted a little so that they do 
 not prick) we feel two distinct impressions. If we now 
 bring the points nearer and nearer together, repeating 
 the experiment until we feel but one impression, the 
 nearest distance apart that we can still 
 feel two impressions is called the mini- 
 mum tactile. It is a measure of capacity 
 to give definite knowledge by touch. 
 It differs greatly in different parts of 
 the skin. On the middle of the back 
 it is about three inches, on the arm or 
 back of the hand about one half to 
 three quarters of an inch, on the finger 
 tips about one twelfth of an inch, on 
 the tip of the tongue about one twenty- 
 fourth of an inch, or one millimetre. 
 
 Double Tactile Images. — There 
 is here also a kind of double images comparable to the 
 double images of sight. If the middle finger be crossed 
 
 Fig. 129. 
 
202 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 over the forefinger, and a small object, like a bullet or 
 a pea, be rolled beneath the crossed tips, Hvo objects are 
 distinctly felt (Fig. 129). It is because the impressions 
 are on unaccustomed or non-corresponding points of the 
 fingers. 
 
 COMPARATIVE PHYSIOLOGY OF TOUCH. 
 
 In our rapid survey in passing down the scale, it is 
 necessary to keep in mind the distinction between gen- 
 eral sensibility and special sense. They are indeed 
 almost in inverse proportions. 
 
 Mammals. — Man differs from mam?iials chiefly in the 
 use of the hand as an organ of touch. Indeed the fore 
 limbs are liberated from the function of support and 
 progression expressly for this purpose. In other mam- 
 mals, with the exception of apes, the special tact organ 
 is localized elsewhere, generally in the prehensile organ 
 wherever that may be, most usually about the mouth, 
 as the lips, the snout, the tongue. In such cases we 
 may often find a strong development of papillre, as on 
 the nose of the dog or on the sole of the foot of all un- 
 hoofed animals. 
 
 Jn birds the beak is an exquisitely sensitive organ of 
 touch. Sensory nerves are abundantly distributed at its 
 base and beneath the horny sheath, exactly as in the 
 case of our finger nails, which are also delicate organs 
 of touch. The strong papillae on the feet of birds show 
 that they are good organs of touch. The general sensi- 
 bility of birds is probably inferior to that of mammals 
 on account of the thick covering of feathers. 
 
 Reptiles are all of them more or less covered with 
 dry horny or bony scales which must diminish their gen- 
 eral sensibility, nor have they any well-marked organ 
 specially organized for touch. They are probably, 
 poorly endowed both in general and special sensibility. 
 
SENSE ORGANS. 
 
 203 
 
 In amphibians, so far as concerns general sensibility, 
 we have the extreme opposite condition — i. e., a moist, 
 active, sensitive skin — but there are no special organs 
 of touch. 
 
 Fishes are probably similar to amphibians in regard 
 to general sensibility. They also have a moist, sensi- 
 tive, mucous surface, and therefore general sensibility 
 well developed. In addition, many of them have special 
 organs of touch (feelers) about the mouth. 
 
 In arthropods we have again the other extreme. They 
 all have a hard skeletal coat of mail on the outside, 
 which almost entirely cuts off general sensibility; but to 
 compensate they are endowed with very delicate special 
 organs of touch, as, for example, the long antennae of 
 insects and crustaceans. 
 
 In ftiollusca we pass again to the other extreme. A 
 universal characteristic of mollusca is that they are 
 everywhere, except when inclosed in shell, covered with 
 a soft, active, mucous surface, which is endowed with 
 great sensibility. Many also, in addition, have good 
 tactile organs, such as the grasping arms of cephalopods 
 and the so-called horns or feelers of snails and other 
 gastropods. 
 
 In echiiwderms the body is again usually incased in 
 immovable shell, as in echinus; but again we find com- 
 pensation in their long, delicate tentacles, which are 
 feelers as well as locomotive organs. 
 
 Again the pendulum swings back in ca/e/iferates 
 (medusje and polyps), where we find again the soft, 
 active mucous surface sensitively responsive to contact. 
 Their long tentacles also doubtless act as touch organs, 
 though perhaps imperfectly. 
 
 Finally, m protozoa we find only general sensibility of 
 the lowest grade, to what extent conscious we can not 
 tell. From this lowest form of response to external 
 
204 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 Stimulus have been differentiated by evolution not only 
 the different forms of feeling, such as heat and cold, 
 pain, and touch proper, but also all the higher and more 
 special forms of sense. 
 
 SECTION IX. 
 The Voice and tfs Organ, the Larynx. 
 
 The voice is not one of the senses, nor is the larynx 
 a sense organ, but its close relation with the sense of 
 hearing makes this the proper place to take it up. 
 
 There are three kinds of voice — viz., the call or cry, 
 the song, and speech. The first is the simple voice, the 
 second the harmonically modulated voice, the third the in- 
 telligently articulated \o\q.^. The first is common to all or 
 nearly all air-breathing vertebrates ; the second is pecul- 
 iar to man and perhaps birds; the third is characteristic 
 of man alone, although an imitated speech may be 
 taught by man to some birds. We take first 
 
 I. SIMPLE VOICE. 
 
 The Larynx. — Its Position and Relation. — The organ 
 of the voice is the larynx. There are two pipes leading 
 from the throat into the cavity of the trunk — the gullet 
 or ce sop hag us, and the windpipe or trachea. The one 
 leads into the stomach, the other into the lungs ; the one 
 is the passage for food, the other for air in breathing. 
 The trachea is in front, and may be felt with the hand, 
 for it is hard, being kept open by a series of bony or 
 cartilaginous rings, so that the air passes through with- 
 out resistance. The gullet is a soft, extensible pipe, 
 collapsed when not occupied by food. Crowning the 
 trachea and opening into the throat, just behind the root 
 of the tongue, is the larynx. Nov/, it will be seen (Fig. 
 
SENSE ORGANS. 
 
 20S 
 
 130), that the passage of the food from the mouth to the 
 stomach and of the air from the nostrils to the lungs 
 cross one another in the throat. Therefore there must 
 be a valve which shall close the larynx when the food is 
 passing, otherwise the food, especially liquids, would fall 
 into the larynx in swallow'ing. This would produce great 
 irritation and pain. This valve is the epiglottis. When 
 we swallow, the larynx is drawn up by certain muscles 
 with great force toward the roof of the throat. In this 
 position the food in passing presses down the epiglottis 
 and closes perfectly the open- 
 ing of the larynx. The rela- M^^id 
 
 tion of the epiglottis to the 
 larynx is seen in Fig. 131. 
 
 Fig. I •50.— Section of head, .•show- 
 ing- the relations of the air pas- 
 sage and the food passage : g, 
 gullet ; tr, trachea ; rr, the point 
 of crossing. 
 
 cr 
 
 Fig. \%\. — Side view of larynx : hy, 
 hyoid ; epg. epiijlottis ; ///, thy- 
 roid ; cr, cricoid ; a, arj-tenoid. 
 The different parts are seen in 
 transparency. 
 
 At the junction of the epiglottis with the larynx there 
 are a kind of cords (false vocal cords), which are sup- 
 posed to have some function in modifying the voice. 
 
 Experiment. — Put your finger on the Adam's apple 
 (larynx), and try to hold it down while you swallow. You 
 will find it impossible. It rises in spite of your effort. 
 
2o6 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 Structure.* — Aside from the epiglottis, the larynx 
 proper consists of four cartilages, the cricoid, the thyroid^ 
 and the two arytenoids. The cricoid is a cartilaginous ring. 
 
 Fig. 132. — Cartilages of the lar>'nx : A, cricoid ; B, thyroid ; C, arytenoid, 
 with the vocal cord (fc) attached ; a, crico-thyroid articulation. 
 
 which sits directly on the top of the trachea. It is narrow 
 in front and very wide behind (Fig. 132, A). The thyroid is 
 the largest cartilage. Its singular irregular form, which 
 can hardly be described in words, is seen in Fig. 132, B. 
 It is bent on itself in the form of a V, opening back- 
 ward, and the point of the V forms the point of the 
 Adam's apple in front. The arytenoids are two trian- 
 gular cartilages in the form seen in Fig. 132, C. These 
 four pieces are put together in such wise that the lower 
 posterior horns of the thyroid are articulated, i.e., mov- 
 ably attached to the sides of the cricoid low down {a), so 
 that the wide gap between the two arms of the thyroid 
 V, \% partly filled up by the cricoid. Now the two aryte- 
 noids sit directly on the top of the cricoid (Figs. 131 
 and 134) between the legs of the thyroid, and thus fill 
 up the wide space more fully. This is the skeleton or 
 framework. The rest of the larynx is made up of 
 muscles, except the most important part of all, which we 
 now proceed to describe. 
 
 * There should be a large model for demonstration of these 
 parts. 
 
SENSE ORGANS. 
 
 207 
 
 The Glottis and its Vocal Cords. — The cavity 
 of the larynx is divided into an upper and lower cham- 
 ber by a transverse partition — the glottis. In the glottis 
 there is a fore-and-aft opening — the rima glottidis or chink 
 of the glottis. This chink is 
 bounded on each side by a ^-^ . ^.A)L 
 
 Fig. 133. — View of the larynx from 
 above : ar, the arytenoids ; cr^ 
 the cricoid ; c, vocal cords with 
 the rima or opening between. 
 
 Fig. 134. — View from behind : cr, 
 cricoid ; th, thyroid ; a, aryte- 
 noid ; ep, epiglottis ; hy, hyoid 
 bones. 
 
 firm tendinous cord — the vocal cords. These cords are at- 
 tached in front to the V point of the thyroid, and behind 
 to the two arytenoids (Fig. 133, also Fig. 131). The 
 tension of these cords and the size of the chink or open- 
 ing between them varies very much under different con- 
 ditions. This is determined by observations with the 
 laryngoscope, (i) In quiet breathing the chink is wide 
 open, the cords lax, and the breath comes and goes 
 noiselessly (Fig. 135, A). (2) In aspiration (sighing) 
 the opening remains much the same, but the breath is 
 driven through with a rushing sound. The position of 
 the cords is somewhat as seen in Fig. 133. (3) In 
 making a vocal sound three changes are observed, 
 viz., the vocal cords are brought nearer together., they be- 
 15 
 
2o8 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 come tense, and the edges are observed to vibrate. (4) 
 If the vocal sound is high-pitched, the chink becomes 
 very narrow, the cords very tense, and the vibration 
 caused by air driven through very rapid (Fig. 135, B). 
 (5) In the highest soprano head notes the cords are still 
 
 Fig. 135. — Glottis as seen with the laryngoscope : A, in simple breathing : 
 B and C, in singing ; D, in straining. 
 
 tenser, more pressed together, so that the air is driven 
 through only a small opening in the middle, and the 
 vibration is, of course, still more rapid (Fig. 135, C). 
 (6) Fmally, in violent straining or strong muscular 
 effort the glottis closes absolutely air-tight (Fig. 135, D). 
 We first fill the lungs, then close the glottis, so as to fix 
 
SENSE ORGANS. 209 
 
 the chest as a fulcrum for the action of the great 
 muscles of the trunk and limbs. 
 
 MUSCLES OF THE LARYNX. 
 
 The muscles by which these changes are accom- 
 plished are numerous, some tightening, some loosening 
 the cords, some closing the chink. The following are 
 the main ones : 
 
 Crico-thyroids Censors. 
 
 Crico-arytenoids, posterior ) 
 
 Thyro-arytenoids } Relaxers. 
 
 Inter-arytenoids ' Closers. 
 
 Crico-arytenoids, lateral ) 
 
 Of the tensors, the crico-thyroid, as seen in Fig. 130 
 arising from the cricoid, takes hold of the thyroid and 
 pulls it downward, and its upper part forward. The 
 crico-arytenoids, arising from the cricoid behind, take 
 hold of the arytenoids and pull them backivard. These 
 may be seen in Fig. 134, being indicated by the dotted 
 spaces. These two, the one pulling the angle of the 
 thyroid forivard and the other pulling the arytenoids 
 backward, stretch the vocal cords. These, therefore, 
 are the stretchers or tensors of the cords. 
 
 The relaxers must pull the thyroid and the arytenoids 
 toward one another. This is done by the thyro-aryte- 
 noids, which run fore and aft within the larynx from 
 the thyroid to the arytenoids just outside of the vocal 
 cords. They are seen in Fig. 133. 
 
 T\it closers are of two kinds — one, the inter-arytenoid, 
 runs from arytenoid to arytenoid and brings these to- 
 gether ; the other, the lateral crico-arytenoids, rotate 
 the arytenoids in such wise as to bring together the for- 
 ward projecting points to which the cords are attached. 
 These can not be well shown except on a model. 
 
210 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 Application. — In quiet breathing all the muscles 
 are relaxed ; the opening is wide, and the breath comes 
 and goes quietly. In aspiration the condition of the 
 laryngeal muscles is much the same, but the breath is 
 driven more strongly by the respiratory muscles so as 
 to make a rushing noise, but not a vocal sound. In mak- 
 ing a vocal sound the tensors and the closers are brought 
 into action, the cords are made more and more tense and 
 pressed closer and closer together, and the breath is 
 driven through with greater and greater velocity, produ- 
 cing more and more rapid vibration in proportion as the 
 pitch of the voice is higher. The cavities of the mouth 
 and nose above and of the trachea and bronchi below 
 act as resonators to increase the volume and modify the 
 character of the sound. 
 
 2. SONG. 
 
 We have spoken thus far only of the simple voice. 
 Singing is only the skillful modulation of the voice ac- 
 cording to the laws of harmony. This is done by skill- 
 ful use of the vocal muscle in producing a pure sound, 
 and a skillful changing of the play of these muscles so 
 as to modulate the pitch, guided by the ear, and, lastly, a 
 skillful modification of the resonant cavities of the throat, 
 mouth, and nose. We all know the wonderful result. 
 
 The Larynx as a Musical Instrument. — But what kind 
 of instrument is the larynx ? To what shall we compare 
 it ? Some have compared it to a wind instrument, espe- 
 cially a tongued instrument, like an organ pipe, or a 
 clarinet, some to a bird-call. But the favorite com- 
 parison is with the stringed instrument, as is shown by 
 the term vocal cords. But the least reflection is sufficient 
 to show that the comparison is not true. The vocal 
 cords are only about three quarters of an inch long in 
 the male and half an inch in the female. Now, since 
 
SENSE ORGANS. 211 
 
 Strings must be tense in order to vibrate elastically at 
 all, and since, further, other things being equal, they 
 make higher pitch in proportion as they are shorter, it 
 is evident that strings of any such length as this, if 
 tense enough to vibrate at all, could only produce an 
 inconceivably high note. But see the range of the voice i 
 To what, then, shall we compare it ? 
 
 It is strange that no one has thought to compare it to 
 an ordinary horn — a stage horn, for example, or, better, 
 a French horn. In this instrument the sound is modu- 
 lated exactly as in the larynx — viz., by the tension and 
 the pressing together of the lips of the performer. The 
 edges of the rima glottidis ought to be called the vocal 
 lips, as indeed they are, and not the vocal cords, which 
 they are not in any sense. The analogy between the 
 two instruments is perfect. The performer on the horn 
 presses his lips together tighter, and makes them tenser 
 and the opening between them smaller in proportion as 
 he desires a higher note. He then drives the air between 
 the tense lips so as to set their edges in vibration ; this 
 vibration, by alternate partial closing and opening of the 
 aperture, gives rise to successive jets or pulses of the 
 out-driven breath, and this in its turn gives correspond- 
 ing pulses to the air in the sounding cavity of the horn. 
 Precisely the same, as we have seen, takes place in the 
 larynx. The only wonder is that so small an instrument 
 as the larynx and the mouth cavity should be capable 
 of such marvelous effects. 
 
 3. SPEECH. 
 
 Of course the subject of speech concerns other sci- 
 ences besides physiology. But the mechanism of the pro- 
 duction of the various sounds used in speech belongs to 
 physiology alone. We need no apology, therefore, for 
 taking it up briefly. 
 
2 12 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 There are two kinds of speech — viz., vocal and whis- 
 pered. The one is the articulation of the voice, the 
 other of the aspiratioti. 
 
 Speech may be defined as a succession of vowel 
 sounds interrupted and separated by consonants. Vowels 
 are modifications of the voice. Consonants are the modes 
 of interruption. There are two kinds of modifications of 
 the voice — viz., modification of pitch, high or low, and 
 modification of the timbre or quality of the voice. The 
 former is done in the larynx, as already explained; the 
 latter is done by changes in the mouth cavity. The vowels 
 are modifications in timbre, not in pitch. The larynx 
 has nothing to do with it. We have a good illustration 
 of what we mean by timbre in the sounds of different 
 musical instruments. The same musical note may be 
 made on the flute, the clarionet, the violin, or the bugle, 
 but how different is the quality of the sound in each case ! 
 
 Voicels. — We give a series of seven vowels in such 
 order as to show and easily describe the changes in the 
 mouth cavity, thus: e, a, ah, au, d, oo, ii. (i) Bring the 
 teeth near together, retract the lips a little, bring the 
 tongue forward until it nearly touches the teeth, and 
 then make a sound with the larynx; the sound is the 
 long ee, and can not be anything else. (2) Other things 
 remaining the same, separate the teeth a little more and 
 draw back the tongue a little, and make a sound of the 
 same pitch ; the sound now appears as a \w fate, and can 
 not be anything else. (3) Open the mouth much wider, 
 draw back the tongue still more, and again make a 
 sound of the same pitch; it comes out now as a in far, 
 and it can not be anything else. (4) Separate the jaws 
 as much as possible, draw back the tongue as far as 
 possible, but bring the lips a little nearer together in 
 front ; and the same note now becomes au in awe. (5) 
 Now bring the jaws again a little more together and the 
 
SENSE ORGANS. 
 
 2T3 
 
 tongue not quite so much retracted, the lips drawn more 
 together, and a little protruded, and the same note be- 
 comes (?, as in lo ! (6) Bring the jaws still more together 
 and the tongue a very little more to the front, and the lips 
 more drawn together and more protruded, and the same 
 note now becomes oo as in tool. (7) Finally, with all parts 
 remaining as in the last, bring the tip of the tongue for- 
 ward as in the first position, as in making ee., and the 
 same note now becomes //, or the French //, or the Ger- 
 man il with the Umlaut. 
 
 Consonants. — Articulation is the breaking of the voice 
 into segments. The vowels are the segments, the con- 
 sonants the modes of breaking, or interruption. The 
 interruption may be complete, as in b, p, d, t, k, and g 
 hard, or may be incomplete, as in s,f, /, r, etc. The in- 
 terruption may be by the lips, as in /, b, in, or between the 
 tongue and teeth, as in /, d, n, or between the tongue and 
 roof of the mouth, as in k and 
 g hard. Again, every one of 
 these may be non-vocalized or 
 vocalized, so that we may make 
 two parallel series of conso- 
 nants, the terms of which cor- 
 respond each to each, only dif- 
 fering in the fact that one is 
 vocal and the other not. Since 
 
 N^on-vocal . Vocal. 
 
 P b 
 
 t d 
 
 k g hard 
 
 s 2 
 
 / V 
 
 ch J ^"^ S soft 
 
 sh j French 
 
 /// th soft 
 
 whispered speech is articulation of the aspiration, it is 
 easy to see why it is difificult to distinguish the corre- 
 sponding terms of this series in whispering. 
 
 There are other modes of classifying consonants, 
 but our object is only to bring out principles. 
 
 COMPARATIVE PHYSIOLOGY OF THE VOICE. 
 
 Mammals. — The structure of the larynx and the mode 
 of making a voice is precisely the same in mammals as 
 
214 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 in man. The only difference is that by constant use in 
 modulating the voice in speech and in song the larynx 
 of man is much more flexible. 
 
 Birds. — Next to man, birds have the greatest power 
 of modulating the voice, for many of them sing and 
 some may be taught an imperfect speech. 
 But both the speech and the song of 
 birds have an entirely different signifi- 
 cance from that of man. This, however, 
 belongs to psychology, not physiology.* 
 We should expect, then, that the 
 larynx of birds would be highly devel- 
 oped. On the contrary, it is far inferior 
 to that of mammals (Fig. 136). But the 
 larynx is not the organ of song in birds. 
 The larynx is used only in the simpler 
 and harsher sounds, such as cries of 
 pain, distress, or anger, and perhaps the 
 simple chirp. Their singing organ is an- 
 other organ — the syrinx. 
 Syrinx. — The bird, then, has two organs of voice, the 
 larynx and the syrinx. The larynx is in the usual place 
 at the top of the trachea and opening into the throat ; 
 the syrinx is at the lower end oi the trachea. It is made 
 up of the enlarged lower rings of the trachea and upper 
 rings of the two bronchi. Fig. 137, A, B, C, are different 
 views of this organ. Observe (i) that the rings of the 
 bronchi in this part are only a little more than half rings, 
 and the bronchi are completed on the inner side looking 
 toward one another by a tense membrane, which acts as a 
 resonator. (2) On transverse section (Fig. 138) we see 
 transverse floors across the openings of the bronchi into 
 the trachea and a true rima glottidis bounded by vocal 
 
 Fig. 136.— Larynx 
 of a bird. 
 
 From Animals to Man, Monist, vi, p. 356, i5 
 
SENSE ORGANS. 
 
 215 
 
 cords in each. (3) Rising up from the fore-and-aft car- 
 tilage formed by the union of the two bronchi, observe 
 a tense membrane with scythe-like edge — semilunar 
 membrane (Fig. 137, C). 
 
 Mode of Action. — The several rings of this apparatus 
 are movable on one another. By appropriate muscles 
 the resonating membrane may be made more or less 
 tense — the vocal cords may be stretched and the lips of 
 the rima pressed together so as to vibrate with various 
 degrees of rapidity and give rise to various notes when 
 the air is driven through them. The cavity of the 
 syrinx, the tense membrane of the bronchi, and the 
 whole cavity of the trachea above act as resonators, in- 
 
 B 
 A 
 
 Fig. 137. — Syrinx : A, front view ; B, side view ; C, section through the 
 lower part of the trachea and between the legs of the bronchi, showing- 
 the resonating membrane on the inner side of one bronchus : rm, reso- 
 nating membrane ; ««, semilunar membrane. 
 
 creasing the volume of the sound. The semilunar mem- 
 brane is found only in the best singers, and is supposed 
 to produce the trilling so characteristic of some birds. 
 
2i6 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 Reptiles. — As a class these are silent, although a few 
 do make sounds intended as calls. 
 
 In amphibians, however, especially in frogs, we have 
 again animals abundantly vocal. Their vocal organs are 
 indeed very imperfect, but their 
 lungs, which are hollow sacs 
 capable of great distention, act 
 as powerful resonant cavities, 
 giving considerable volume to 
 their voice. 
 
 Fishes, being gill-breathers or 
 water-breathers, can have no 
 
 voice in a proper sense. Probably some of them do 
 make audible sounds, but not vocal. 
 
 Arthropods ; Insects. — True voice is confined to air- 
 breathing vertebrates, and is connected with respiratory 
 
 passages; but if we extend 
 the term to any sounds 
 intended to be heard by 
 mates, then we may include 
 
 Fig. 138. — Transverse section 
 just above the bronchi : vc, 
 vocal cords. 
 
 Fig. 139. — A, sonant organ of an orthopter : rnt, resonating membrane ; 
 r, rasp. B, the same, magnified. 
 
 insects also among natural musicians. If birds are the 
 vocalists, then are insects the instrumentalists of Nature. 
 
SENSE ORGANS. 
 
 217 
 
 We all know the cheerful chirp of the ^'■cricket on the 
 hearth," the insistent, contradictory, answering cry of 
 the katydid, the deafening clatter of the cicada. The 
 organs by which these noises are made are very various 
 and interesting. Most of these sonant insects belong to 
 the grasshopper order (orthopter). In these the anterior 
 pair of wings are somewhat hard, with strong stiff ribs 
 and tense membrane between. Sometimes the hind leg 
 is rubbed against ridges on the stiff edges of the front 
 wings. These wirelike ribs are especially stiff in the 
 overlapping parts on the back. Sometimes these parts 
 in the two wings are rubbed together with a rapid vibra- 
 tory motion, the stiff membrane between them acting as 
 resonators. Often a kind of rasp 
 is added to produce more effect 
 
 (Fig. 139)- 
 
 But the most elaborate con- 
 trivance is found in the cicada, a 
 homopter. Fig. 140, A, is the un- 
 
 vcp 
 
 Fig. 140. — A, view of the under side of a cicada (natural size) ; the legs are 
 removed on one side : vcp, ventral cover plate. B, section a little en- 
 larged : vm, vertical membrane ; w, muscle ; dr, drumhead ; t'r, ventral 
 resonator. (After Lloyd-Morgan.) 
 
 der side of the insect, natural size. Lifting up the ven- 
 tral cover plates, vcp, a tense membrane is disclosed (w). 
 This is a resonant membrane. Beyond this there is an 
 enormous cavity, almost filling the whole body, and di- 
 vided into two by a thin vertical septum (zv//, Fig. 140, B). 
 
2i8 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 This cavity is closed below by the membrane already 
 spoken of, vr; but above on each side it is closed by a 
 very tense membrane like a drumhead (Fig. 140, B, dr). 
 This drumhead is stiffened by radiating ribs which are a 
 little convex outward. A slender tendon coming from 
 a muscle, m, in the cavity is attached to the center of 
 radiation of the ribs. By the action of the muscle the 
 stiff convex drumhead is drawn in with a clack, and 
 again springs out with a clack when the muscle is re- 
 laxed. The muscle contracts and relaxes with a rapid 
 vibratory motion, and this gives rise to the characteristic 
 clattering noise of these creatures. 
 
 It is hardly necessary to say that the humming and 
 buzzing sounds so common in insects while flying are 
 due wholly to the rapid vibration of the wings. 
 
 Animals in other and lower departments are not 
 known to make sounds intended to be heard. 
 
CHAPTER III. 
 
 MUSCULAR AND SKELETAL SYSTEMS. 
 
 We have seen (page 26) that four systems are con- 
 cerned with the distinctive functions of animal life, viz., 
 the nervous system, the sense organs, the muscular sys- 
 tem, and the skeletal system. We have finished the first 
 two. We now take up the second two. 
 
 The object of both these is to produce motion. But 
 motion is coextensive with life, and therefore not pecul- 
 iar to animals. What is really characteristic of animals, 
 except the very lowest, is the use of a peculiar apparatus 
 of nerve and muscle to give greater efficiency to the 
 motion. In the lowest animals we have only general 
 sensibility and general contractility. As we rise in the scale 
 nerve and muscle are introduced, but not yet skeleton. 
 The muscle acts directly on the body to give motion and 
 locomotion. Only in animals somewhat advanced in the 
 scale the skeleton is introduced to give greater velocity 
 and precision to the motion. 
 
 SECTION I. 
 Muscular System. 
 
 We have already explained the /m/c^ called muscular. 
 Its one property is that it contracts under stimulus of 
 any kind. Now, a muscle, as an organ, is composed of 
 an aggregation of several tissues, of which the muscular 
 is most abundant and characteristic. But besides mus- 
 
 219 
 
220 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 cular tissue it has connective tissue to web it together, 
 nerves to stimulate it, and blood vessels to nourish it. 
 
 Kinds. — There are two kinds of muscle — voluntary 
 and involuntary — differing from one another in many 
 respects, {a) The one is found on the exterior^ the other 
 in the interior of the body. (/->) The one is red, the other 
 is white, [c) The fibers of the one are transversely stri- 
 ated, of the other are non-striated, {d) The nerve supply 
 of the one is largely from the conscio-voluntary system, 
 
 of the other from the reflex 
 system, {e) In the voluntary 
 muscle the fibers are massed 
 into a distinct organ, having 
 a distinct name, and all co- 
 operate through a tendon to 
 produce one motion ; while 
 the involuntary muscle exists 
 in sheets of parallel fibers, 
 surrounding hollow organs 
 (stomach, intestines, bladder, 
 etc.), has no tendon, and the 
 fibers do not contract co- 
 operatively, but consecutively, 
 by a contraction propagated 
 from fiber to fiber. (/) The 
 Fig. 141.— Muscular fibers of the voluntary muscles contract 
 jAaerlentr'*""^" "'°"''^^' quickly and powerfully, the 
 involuntary slowly and fee- 
 bly, {g) Lastly, the voluntary are attached to the skele- 
 ton, while the involuntary are not attached to the skele- 
 ton, but surround hollow organs. 
 
 There are some muscles, howe»ver, which are inter- 
 mediate. The most striking case is the heart. The 
 muscle of the heart is red, transversely striated, and 
 contracts powerfully, and yet it is /V/voluntary, consists 
 
MUSCULAR AND SKELETAL SYSTEMS. 22 1 
 
 of parallel fibers, and surrounds a hollow organ without 
 skeletal attachment. 
 
 A muscular fiber is apparently evenly cylindrical, 
 without any evidence of cellular origin. But in embry- 
 onic development it is seen to be formed by a coales- 
 cence of elongated nucleated cells. This is well seen 
 in the fibers of the embryonic heart of the monkey 
 (Fig. 141). 
 
 Voluntary Muscle. — Form. — The typical form of a 
 voluntary muscle is seen in the muscles of the limbs — 
 e.g., the biceps — which is shown in Fig. 146, page 228. 
 It is attached to the skeleton at both ends. The nearer 
 and more fixed point is called the origin, the farther and 
 more movable point, the insertion. Between the two 
 the largest and most contractile part is called the belly. 
 The fibers all unite to forrn the tendon, by which it is 
 attached to the skeleton. This is the type, but there is 
 considerable variation. Sometimes the fibers are con- 
 vergent. This is mainly in muscles connecting the limbs 
 with the trunk, as in the deltoid, the pectoral, etc. 
 Sometimes the fibers are nearly parallel, as in the mas- 
 seters. 
 
 Structure. — x\ voluntary muscle is a definite mass in- 
 vested by a thin fibrous membrane — sheath. If cut into we 
 find it made up of bundles of fibers — fasciculi {Y'\g. 142, A). 
 These are conspicuous in cured meat, such as corned 
 beef. They constitute the grain of the flesh. These, 
 too, are invested with a thin membrane of fibrous tissue. 
 These bundles are in their turn composed of fibers lying 
 parallel to one another in the bundle. Each fiber is 
 also invested with a very thin sheath of fibrous tissue. 
 The fibers themselves are supposed by some to be com- 
 posed of smaller fibrillar, but this is doubtful. Fig. 142, B, 
 represents a single fiber, broken and twisted, showing 
 the sheath. We may regard the whole muscle as pene- 
 
222 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 trated and webbed together in all its parts with con- 
 nective tissue. In a condensed form it invests each 
 fiber ; in a loose form it lies between and connects them. 
 In a condensed form it again invests the bundles; in a 
 loose form it lies between and con- 
 nects these also. Finally, it emerges C 
 on the surface, and in a condensed 
 
 Fig. 142. — A, a fascicle of muscular fibers of voluntary muscle ; B, one 
 fiber broken to show its investing sheath ; C, cells of involuntary muscle. 
 
 form invests the whole muscle in a strong sheath, and 
 in a loose form lies between the different muscles, con- 
 necting yet separating them. Add to this the blood 
 vessels to nourish and nerves abundantly distributed to 
 stimulate to contraction, and we have a good general 
 idea of the organ we call a muscle. 
 
 The tendon consists of all the sheaths of the muscle, 
 of the bundles, and of the fibers united and continuing, 
 and also of the transformed fibers themselves. It is 
 therefore essentially fibrous in structure. It is the 
 strongest — /. <?., has the greatest tensile strength of any 
 organic substance known. 
 
 The only function of a muscle is to contract. This, 
 
MUSCULAR AND SKELETAL SYSTEMS. 223 
 
 indeed, is the only sign of its life. Any stimulus — me- 
 chanical, chemical, or electrical — may cause it to con- 
 tract, but the normal physiological stimulus is the nerve 
 influence, whatever that may be. When a muscle con- 
 tracts it shortens, thickens, and hardens. The power 
 with which it pulls in shortening is almost incredible. 
 
 Involuntary Muscle. — We have already sufficient- 
 ly characterized this in contrasting it with the voluntary. 
 Good examples of these are found in the muscular coats 
 of the stomach, intestines, and bladder. Every one is 
 familiar with this type in the white muscular substance 
 of tripe. 
 
 No comparative physiology of this system is neces- 
 sary, as the function, structure, and mode of action of 
 muscle is much the same in all animals, as far as the 
 tissue can be traced. Striation of muscular fiber is a 
 sign of great activity, and is therefore found only in 
 animals of considerable energy. It is conspicuous in 
 vertebrates, and among invertebrates in arthropods, 
 especially insects. As we go down the animal scale 
 muscular fiber is found as low as the coelenterates — 
 medusae and polyps. In protozoa it is replaced by 
 general contractility of protoplasm. 
 
 SECTION n. 
 
 Skeletal Systetn. 
 
 The skeleton in vertebrates is usually knie, because 
 this is the most rigid of the tissues. In the lower fishes, 
 however, and in the embryonic condition of all verte- 
 brates cartilage takes the place of bone. 
 
 Bone is tissue hardened by deposit of lime salts, 
 
 mainly phosphate. Several kinds of tissue take on this 
 
 change. Thus, as to origin, we may have cartilage bone, 
 
 tendon bone, and skin bone. Most of the true skeleton 
 
 16 
 
224 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 consists of cartilage bone, but the kneecap and several 
 small bones in the joints are tendon bones, while the 
 teeth and scutes are examples of skin bones. Bones 
 are covered with a strong membrane called the peri- 
 osteum. 
 
 Number of Bones in Man. — The number of bones 
 in the human skeleton is variously estimated from a little 
 over two hundred to about two hundred and fifty, the dif- 
 ference being the result of difference of view as to what 
 ought to be included. Some include the teeth, and some 
 do not, because these are gum structures. Some include 
 the sesamoid or joint bones, some do not. Agam many 
 pieces are separate in the embryo, and become consoli- 
 dated later. These two hundred or more pieces are ar- 
 ticulated together into a complex structure, which is 
 moved by the muscles. 
 
 Joints. — The articulations or joints are of two gen- 
 eral kinds, _/?.w^ and movable. In fixed joints the pieces 
 may be cemented together with cartilage {symphyses), 
 as the hip bones with the sacrum, or they may be inter- 
 digitated or dovetailed {suture), as in the bones of the 
 skull. The movable joints are also of two kinds — the 
 hinge Joint, when motion in one plane is required, as in 
 the knee, elbow, etc., and the ball-and-socket Joint, where 
 universal motion is required, as in the shoulder and hip. 
 Only the movable joints concern us here. 
 
 Movable Joints. — The bones in a movable joint 
 are {a) enlarged at the ends where they come together, 
 so as to make a firmer contact, {b) They are covered 
 with cartilage for perfect smoothness and elasticity, {c) 
 The joint is inclosed in a capsule of strong fibrous tis- 
 sue to prevent displacement, and at the same time ex- 
 clude the air. {d) The closed cavity thus formed is 
 lined with a serous membrane, which secretes a smooth, 
 glairy, lubricating fluid — the synovia or joint juice. 
 
MUSCULAR AND SKELETAL SYSTEMS. 
 
 225 
 
 All of these characters, intended for easy motion and 
 to prevent dislocation, are common to all movable joints; 
 but to these in the case of 
 hinge joints are added (<?) 
 two strong ligaments, one 
 on each side, which hold 
 
 Fig. 143. — Diagrammatic view of 
 the knee joint. The dotted lines 
 show the position of the capsule 
 and of the lateral lij^aments ; the 
 black lines show the crossed liga- 
 ments. 
 
 Fig. 144. — Section through the hip 
 joint, showing the capsule (cl) 
 and the round ligament (rl). 
 
 the parts together and yet allow free motion in one 
 plane. These are the lateral ligametits. They are found 
 in all hinge joints. In addition to these (/), in the 
 knee joint there are two crossed ligaments within the 
 joint itself, as shown in Fig. 143. 
 
 In ball-and-socket joints we have, of course, d:, b, c,d ; 
 but in the case of the hip joint, in addition to these, 
 there is a short, strong, round ligament running from 
 the bottom of the deep socket to the top of the ball, as 
 shown in the diagram (Fig. 144). 
 
 SOME EXAMPLES OF ADAPTATION. 
 
 The articulated skeleton together forms a really 
 wonderful contrivance. Some examples of adaptation 
 may be mentioned. I select such as lead to interesting 
 comparisons with other vertebrates. 
 
226 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 I. Spinal Column. — Observe (i) the double or S curva- 
 ture. This is peculiar to man, and the result of the 
 erect attitude. In this position it acts as a spring to 
 prevent shocks to the brain in falling, leaping, etc. Ob- 
 serve (2) the intervertebral substance. This is an elas- 
 tic cushion, of half an inch thickness, between all the 
 vertebrae, and also acts to prevent concussion of the 
 brain. It is, however, not peculiar to man. 
 
 Comparative Morphology of the Column. — The manner 
 of articulation of the vertebrre is very characteristic of 
 
 the different classes of ver- 
 tebrates. In mammals the 
 vertebrae are short seg- 
 ments of a cylinder with 
 flat faces and intervertebral 
 substance between (Fig. 
 145, a^. In reptiles the ar- 
 ticulation is by ball-and- 
 socket joint. The faces of 
 each vertebra are one of 
 them concave and the other 
 convex, and these fit over 
 one another, b. In some 
 reptiles the hollow face 
 looks forward (procoelian)^ 
 in others backward (opisthocoelian). In fishes the seg- 
 ments are double concave (amphicoelian), with double 
 convex intervertebral substance between,^. Some early 
 reptiles were like fishes in this regard. In the lowest 
 fishes there is a continuous unsegmented cord (noto- 
 chord), d. This is also the early condition of all ver- 
 tebrates, whether in the embryonic development or in 
 the evolution of vertebrates. This early notochord is 
 afterward changed into a vertebral column by ossifica- 
 tion in segments. These characteristics of the verte- 
 
 ffO Q Q Q 
 
 Fig. 145. — Diagfram showing the 
 characteristics of the vertebral 
 column in mammals (a), reptiles 
 ((5), fishes (c), and lowest verte- 
 brates (d). 
 
MUSCULAR AND SKELETAL SYSTEMS. 227 
 
 bral column in different classes are very important in 
 geology. 
 
 2. Structure of Shoulder Joint and Fore Limb. — There 
 is a progressive change from man to the more specialized 
 mammals in the position of the shoulder joint, its mov- 
 ableness, the presence and movableness of the tivo bones 
 of the forearm, and the free use of the hand. All these 
 are strictly correlated, and all reach their highest point 
 in man, because all are in him connected with the erect 
 attitude and the consequent liberation of the fore limbs 
 from the function of support and locomotion for higher 
 uses. In man the shoulder joint and the arms are on 
 the side of the body, being kept wide apart by the clavi- 
 cle, and the motion of the arm is the extreme of free- 
 dom. For this freedom firmness of the joint is sacrificed. 
 The two bones of the forearm roll the one on the other, 
 carrying the hand with it, and the hand has the most 
 perfect capacity for grasping. Now, as we go down the 
 scale of mammals, the collar bone disappears, the fore 
 limbs are brought together in front for support, the two 
 bones of the forearm become less and less movable on 
 one another, and the paw loses its power of grasping, un- 
 til, fiinally, in the hoofed animals the extreme is reached ; 
 the limbs are brought closer together in front and used 
 only for support, and therefore restricted in motion ; 
 the two bones of the forearm are consolidated into one, 
 and therefore lose entirely all rotary motion ; the paw 
 is no longer a paw, much less a hand, but a hoof, wholly 
 incapable of grasping. 
 
 Motion and Locomotion. — We have explained the 
 function of muscle and of skeleton. We must now 
 show how they co-operate to produce motion and loco- 
 motion. 
 
 I. Limb Motion. — Remember, then, that a muscle has 
 two skeletal attachments — viz., the origin and insertion. 
 
228 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 Between these there is always a joint. When the muscle 
 contracts, its two attachments are brought nearer to- 
 gether, and the joint bends. Remember, again, that a 
 muscle can only /////, not push, and therefore motion 
 in two directions can only be effected by two opposing 
 muscles. Again, it must be borne in mind that in ani- 
 mal mechanism power is nearly always sacrificed \.o vetoc- 
 //v, because swiftness is more valuable in the struggle for 
 life than slow dead strength. Therefore, of the 
 different orders of levers, the second is rarely, 
 almost never used, and in the first order the 
 fulcrum is so placed that power is sacrificed to 
 velocity. 
 
 Examples of the first order are very numer- 
 ous. The action of the triceps on the point of 
 the elbow in straightening the arm and of the 
 gastrocnemius on the heel bone in bringing 
 down the toes (Fig. 148) 
 
 j-^ are excellent examples. 
 
 Examples of the third or- 
 ) der are equally numerous. 
 We again take two : i . Ac- 
 tion of biceps, pulling on its insertion, and flexing the 
 elbow (Fig. 146). 2. The action of the deltoid on its 
 insertion, in raising the arm (Fig. 147). 
 
 The prodigious force of muscular contraction may 
 be easily calculated from these examples. Take the 
 case of the biceps in bending the arm at the elbow. I 
 suppose any one with average muscular vigor can hold 
 fifty pounds in the hand with the elbow joint at right 
 angles, as in Fig. 146. In such case, taking the distance 
 of the insertion of the tendon, /', from the fulcrum,/, as 
 one inch, and from the fulcrum to the weight, W, one foot, 
 the pull of the muscle on the insertion at P necessary 
 to hold up the weight would be 50 X 12 = 600 pounds. 
 
 Fig. 146. — Diagram showing 
 the power of the biceps in 
 flexing the arm. 
 
Muscular and skeletal systems. 
 
 229 
 
 Or take the case of the deltoid holding a weight at 
 arm's length (Fig. 147). I suppose a man of good mus- 
 cular vigor will hold at arm's length a weight of thirty 
 
 'fl C~" ■ " -^ 
 
 :2^ 
 
 Fig. 147. — Diagram showing; the power of the deltoid in raising ,^, 
 the arm. 
 
 pounds. Now, in order to do so, the deltoid taking hold 
 at four inches from the fulcrum, /, and the weight held 
 at two feet from the same, if it pulled directly upward it 
 would have to pull with a force of 30 X 6 = 180 pounds. 
 But it pulls at a small angle, and we must 
 multiply this again by at least four — i. e., 
 180 X 4 = 720 pounds. 
 
 Or take one more — viz., the case of the 
 
 gastrocnemius and soleus muscles (the calf 
 
 the leg) lifting the heel. I suppose any 
 
 erson of ordinary weight and vigor can 
 
 e another person of average weight, 
 
 one hundred and fifty pounds, on his 
 
 back, making altogether, say, three hundred 
 
 nds, and, standing on one foot, rise to 
 
 06. Let us see what the strain on the 
 
 tendo-Achillis is in doing so. 
 
 Taking the distance from the 
 
 fulcrum (2, Fig. 148) to the 
 
 insertion of the tendon, /, as 
 
 one inch, and the distance 
 
 from the same to the ball of 
 
 the toes, j, as si.x inches, then we have the proportion /, 2 
 
 = one inch : .2, j = 6 inches : : 300 pounds : x, and x =: 
 
 1,800 pounds. Or if some one objects (as has been often 
 
 Fig. 14H. — Diaf;;ram showing the 
 power of the gastrocnemius and 
 soleus muscles in tiptoeing. 
 
230 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 objected) that j must be taken as the fixed point or 
 fulcrum, and therefore we must treat the question as 
 falling under the second order of levers — i. e., the power 
 pulling upon the lever /, j> = 7 inches, and the weight 
 pushing down on the lever 2, j = 6 inches, then we must 
 remember that the muscle pulls the body downward, 
 adding to the weight exactly as much as it pulls the 
 heel upward. Therefore, under this conception, we must 
 put the proportion thus — 7 : 6 :: 300 -\- x : x. yx = 6x -\- 
 1,800 — or X = 1,800. 
 
 It has been determined by experiment that one square 
 inch of muscle will contract with a power equal to 
 about one hundred to one hundred and twenty pounds. 
 Why, then, does not the muscle break ? For dead 
 muscle can stand no such tensile strain as this. The 
 answer is : It would break if it were passive, and the 
 force was external to itself, but it is the attraction be- 
 tween the molecules of the muscle itself that develops 
 the pull ; attraction can not produce separation. Muscles 
 do break sometimes, but always from irregular contrac- 
 tion — i. e., one part contracts while another part does 
 not, and is therefore subject to tensile strain, just as the 
 tendon is. 
 
 2. Locomotion. — In limb motion the origin or body 
 end of the muscle is fixed, and the insertion or limb end 
 moves; but these are interchangeable. If we fix the 
 limb, then the body moves. Thus, for example, if we 
 hold up the hands above the head and bring into action 
 the great muscles about the armpit, and also the biceps, 
 the elbow is brought down to the side and the fist to 
 the chin ; but if we fix the hands by taking hold of a 
 bar and bring into action the satne muscles, the body 
 rises until the chin goes over the bar. Now locomotion 
 is nothing more than limb motion reacting against the 
 ground in walking, running, leaping, against the water 
 
MUSCULAR AND SKELETAL SYSTEMS. 23 1 
 
 in swimming, and against the air in flying. For instance, 
 let a steam engine be lifted from the rails and steam be 
 put on ; we have only wheel motion. But while thus 
 working set the engine on the track, and wheel motion 
 is changed into locomotion. Or lift a cyclist above the 
 ground and let him work his pedals ; we have now 
 ' limb motion and wheel motion. But set him on the 
 ground, and away he shoots, scorching the ground as he 
 passes; wheel motion is converted into locomotion. Or, 
 again, take the sprinter, hang him up in the air, and let 
 him set his running muscles into action. We have, of 
 course, only extravagant limb motion. But while this 
 is going on, if we set him down on the earth, instantly 
 limb motion is converted into rapid locomotion. There 
 is therefore no new principle involved, and no further 
 discussion required. 
 
 But there is still one point which must be mentioned 
 — viz., the exquisite co-ordination of action of many mus- 
 cles required in nearly all our motions. For example, 
 in the simple act of standing there are probably at least 
 one hundred muscles in perfect co-ordinate action to 
 maintain the equilibrium. It is so easy and so instinct- 
 ive that we are unconscious of the constant play of 
 many muscles. If so in standing, how much more in 
 walking, running, leaping, swimming, flying ! So won- 
 derful, indeed, is this co-ordination that it could not be 
 learned in a lifetime if it were not largely inherited. A 
 calf newly born will stand on its feet and walk. It has 
 not learned to do so, but has inherited the capacity. A 
 chick newly hatched will walk and use its eyes correctly 
 and peck its food. A wild bird's egg may be taken 
 from the nest, hatched in an incubator, and reared in a 
 cage until the young bird is well feathered, until nerves 
 and muscles are sufficiently developed. If then it be 
 carried out and thrown m the air it will at once flv awav 
 
232 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 with ease. Even a child, when the proper time comes — 
 i. e., when the nervous and muscular systems are suffi- 
 ciently developed — will learn to walk in a week. Even 
 in the child a large portion of this capacity of co-ordina- 
 tion is inherited, though far less than in animals. The 
 whole sum of capacity in all animals is partly inherited 
 and partly individually acquired. In animals, and al- 
 most in proportion as they are lower in the scale, the 
 inherited part is large in proportion to the acquired 
 part. In man the reverse is true. 
 
 SECTION III. 
 
 Coviparative Morphology and Physiology of Muscle ajid 
 Skeleton. 
 
 VERTEBRATES. 
 
 So far as vertebrates are concerned, the function of 
 muscle and skeleton is almost identical with that already 
 explained in man, although there is great variation in 
 the structure by means of which function is carried out. 
 But this will be brought out in a separate chapter on 
 the laws of animal structure in relation to the origin of 
 organic forms by evolution. We pass, therefore, directly 
 on to the invertebrates. 
 
 INVERTEBRATES. 
 
 The function of motion in invertebrates is so infi- 
 nitely various that all that is possible in this work is to 
 give some characteristic examples of widely different 
 modes. The most interesting of these is that of ar- 
 thropods. 
 
 ARTHROPODS. 
 
 We all know the intense muscular activity of ar- 
 thropods, especially insects — the arrowy swiftness of 
 
MUSCULAR AND SKELETAL SYSTExMS. 
 
 233 
 
 the flight of many flies; the prodigious leaps of fleas, 
 three hundred times their own length ; the enormous 
 masses, twenty times their own weight, dragged by ants, 
 etc. ; and yet the relation of muscle to skeleton, and 
 therefore the mechanism of motion and locomotion, is 
 wholly different from that of vertebrates. In verte- 
 brates we have an internal skeleton and the muscles act- 
 ing on it from the outside; in the case of arthropods we 
 have an external skeleton and the muscles acting on it 
 from the inside. The whole animal is inclosed in a skele- 
 tal coat of mail. 
 The body is a hol- 
 low, jointed barrel A 
 inclosing the vis- 
 cera, and the limbs 
 are hollow pipes 
 filled with muscle. 
 The manner in 
 which this ar- 
 rangement is used 
 for limb motion is 
 shown in the fol- 
 lowing figures: Fig. 149, A and B, represents the joints 
 of a stovepipe beveled a little on the two opposite sides 
 so that when fitted, the one in the other, there is a small 
 vacant space between. Now if the interfitted parts, a a, 
 be riveted together, and strings, ;// ///, within the pipe be 
 attached to the beveled margins, we have a perfect hinge 
 joint, and pulling on one string or the other produces 
 motion in two directions in one plane. Now the mechan- 
 ism for limb motion in all arthropods is like this, except 
 that we have ligaments instead of rivets, and muscle 
 and tendon instead of strings pulled by hand. Fig. 150 
 shows four joints of the limb of a crab or lobster 
 and the manner in which the muscles bend the limbs. 
 
 Fig. 149. — Diagram showing; mode of action of 
 muscle and skeleton in an arthropod : a, the 
 joint ; w, the muscle. 
 
234 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 It is evident that with a hollow skeleton and muscles 
 within, only hinge joints can be formed. A ball and 
 socket is impossible. How, then, is universal motion 
 effected ? This is done by two hmge joints moving in 
 planes at right angles to one another, as in the diagram, 
 Fig. 149. If we examine the leg of any arthropod, say 
 
 Fig. 150. — Four joints of the limb of a crustacean cut so as to show the 
 muscles (mm) and their attachments. 
 
 a lobster, we shall find that the consecutive joints are 
 hinged alternately in planes at right angles to one 
 another (Fig. 150). 
 
 Now of these two different modes of relation of 
 skeleton and muscle, which is the best ? We have al- 
 ready seen the intense locomotive activity of insects. 
 Many writers hastily conclude that the nervous and 
 muscular activity of insects is far greater than that of 
 vertebrates, or else that their mechanism is superior. 
 This is probably a mistake. The superior locomotive 
 activity of insects is simply the result oi small size. It is 
 evident that, other things being equal, the contractile 
 power of a muscle varies as its cross section — i. e., as the 
 square of its diameter. But the weight to be moved — 
 i. e., the weight of the body — varies as the cube of the 
 diameter. Therefore, as the size of the animal in- 
 creases, its weight increases faster than its muscular 
 power. Therefore more and more of the whole energy 
 is used up for support of weight, and less and less is 
 left over for locomotion, until, if the animal is large 
 enough, the whole power is used up for support, and 
 none is left over for locomotion. There is therefore a 
 
MUSCULAR AND SKELETAL SYSTEMS. 235 
 
 limit to the size of a walking animal, and a much lower 
 limit to the size of a flying animal. Contrarily, as an 
 animal becomes smaller, the muscular energy per unit 
 section remaining the same, the weight decreasing 
 faster than the power, less and less proportion is neces- 
 sary for support of weight, and more and more is left 
 over for activity of all kinds. This is the true reason 
 why small animals seem so much more vivacious than 
 large animals. 
 
 WORMS. 
 
 In these there is no skeleton, but the muscles act 
 directly on the body to produce motion and locomotion. 
 We have, therefore, in these an entirely different mode 
 of action. 
 
 Take an earthworm as a good example. In these 
 there are two kinds of muscular fibers, viz., the longitu- 
 dittal 3.n6. the )'ing fibers. The longitudinal fibers, acting 
 all together, shorten the body ; acting on one side or an- 
 other, bend the body to the corresponding side. The 
 ring fibers constrict the body, and, acting all together, 
 elongate it. But the most conspicuous peculiarity of 
 all the fibers, both longitudinal and ring, is their //-<7/<z- 
 gated ?iCt\on. Watch an earthworm in locomotion. We 
 observe a wave of constriction — i. e., contraction of ring 
 fibers — running forward, advancing each part consecu- 
 tively until it reaches the head, which then advances and 
 takes hold ; and then begins a wave of contraction of 
 longitudinal fibers, running backward and bringing up 
 successively the parts of the body toward the head. 
 Usually several such waves chase each other along the 
 body. This kind of motion is so characteristic of worms 
 that it may be called vermicular. It is found in all ani- 
 mals, even the highest vertebrates, in the involuntary 
 muscles, especially those of the stomach and intestines. 
 
236 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 It is there wholly automatic. It is probably at least 
 semi-automatic in the locomotion of worms. 
 
 The locomotion of all wormlike animals, as, for ex- 
 ample, caterpillars, etc., is similar to that described. 
 
 MOLLUSCA. 
 
 Acephala. — In oysters there is only one conspicuous 
 motion, viz., that of closing the valves. This is effected 
 by a large muscle running transversely from valve to 
 valve. When this is cut, as in opening an oyster, the 
 valves fall apart. The purple spot seen on the interior 
 of an oyster shell is the place of attachment of this mus- 
 cle, and the firmer, sometimes tough, portion of the 
 oyster is the muscle itself. When the muscle relaxes, 
 the valves open by means of an elastic substance in the 
 hinge of the valves; when it contracts, the valves are 
 closed with great force. 
 
 In clams (Fig. 151) there are two of these transverse 
 muscles and an additional locomotive organ, the foot, 
 
 Fig. 151. — Mactra with one valve removed, showing the anterior iattt) and 
 the posterior \pm) shell muscles, and the foot (/"). (From Gegenbaur.) 
 
 connected with the two transverse or shell muscles. 
 This organ contains longitudinal fibers for retraction, 
 and also oblique and transverse fibers for constriction 
 and consequent protrusion. It is used for locomotion 
 and also for burrowing in the mud. In locomotion it is 
 protruded, takes hold, and then is retracted, and thus 
 draws the body forward. 
 
MUSCULAR AND SKELETAL SYSTEMS. 
 
 237 
 
 Fig. 152. — Snail walking. 
 
 Gastropoda. — In snails (Fig. 152) and slugs the loco- 
 motive organ ox foot is an elongated flat disk, lying along 
 the ground, which consists of muscular fibers running in 
 all directions, some transverse, some oblique, and some 
 longitudinal. The longitudinal fibers shorten, the ob- 
 lique and trans- 
 verse constrict 
 and lengthen. 
 A snail or slug 
 seems to glide 
 slowly and con- 
 tinuously along 
 without appar- 
 ent mechanism. 
 If watched care- 
 fully, however, 
 waves of con- 
 striction and ad- 
 vance chase one another from one end of the foot to 
 the other. In the case of those having a shell, as the 
 snail, muscular fibers from the foot are attached to the 
 interior of the shell, whereby the foot is drawn into the 
 shell. Of course there are also small muscles, ring fibers, 
 and longitudinal fibers for moving the tentacles. 
 
 Cephalopoda. — Confining ourselves to locomotion, 
 we find here, again, a new kind. The squid and cuttle- 
 fish are invested with a thick, hollow, muscular mantle, 
 within which are inclosed all the viscera, but with con- 
 siderable space between filled with sea water. Just be- 
 neath the head protrudes a conical tube [sipho/i), valvu- 
 larly connected with the hollow in such wise that when 
 the mantle contracts the water is forced through the 
 siphon with great force, and the animal is shot back- 
 ward with great speed. Besides this rapid locomotion 
 for escape, there is another in both the sc]uid and cuttle- 
 
238 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 fish, but different in the two cases. The squid is pro- 
 vided with a horizontal, arrowhead-shaped caudal fin 
 (not shown in the diagram), which is not flapped from 
 
 Fig. 153. — Diagram showing; general structure of a squid : mc, the mantle 
 cavity ; j, the siphon ; g, gills ; ib^ ink bag ; ish^ internal shell ; e^ eye ; 
 oe, oesophagus ; k, kidney. The arrows show the direction of currents. 
 
 side to side, as in fish, nor up and down, in the man- 
 ner of whales, but locomotion is effected by waves propa- 
 gated backward or forward — in the one case giving 
 
 rise to gentle forward, in 
 the other to gentle back- 
 ward motion. These grace- 
 \^-^ ful movements may be 
 7^ watched in an aquarium. 
 The short-bodied cut- 
 tlefish, on the contrary, 
 uses its long, flexible, mus- 
 cular arms for crawling on 
 
 Fig. 154.— Diagram of a medusa : ««, the bottom, or even for 
 nerve ; tn. mouth ; st, stomach \ rt. , • , • 1 
 
 radiating tubes. climbing Up rOCks. 
 
 CCELENTERATA. 
 
 Medusae. — As we are taking only the most strik- 
 ingly different modes, we pass over the echinoderms 
 and take next the acalephs or jnedusce. The transparent 
 
MUSCULAR AiMJ SKELETAL SYSTEMS. 
 
 239 
 
 character and the saucerlike (Fig. 154) or bell-shaped 
 form and graceful movements of these beautiful crea- 
 tures are well known. Their locomotive apparatus 
 consists of fine muscular fibers, arranged in circular 
 and radiating manner on the interior of the saucer or 
 bell. When these fibers contract, the saucer or bell 
 is drawn together, the water expelled, and the animal 
 driven in the contrary direction. When the fibers relax, 
 the somewhat firm, gelatinous mass again expands by 
 its own elasticity to its original form and size. 
 
 PROTOZOA. 
 
 Infusoria. — in these, again, we have a wholly differ- 
 ent mode, VIZ., ciliary Jiiotion and locomotion. If the ani- 
 mal be fixed, as a, then the incessant lashing of micro- 
 scopic cilia, situated mainly 
 about the mouth, creates 
 whirling currents, which lead 
 down to the throat, and thus 
 contribute to alimentation. 
 Whatever is suitable as food 
 goes down ; what is not, is 
 rejected and carried away 
 by the same current. But 
 if the body is free, b, the 
 same ciliary motion, react- 
 ing against the water, produces the most vivacious loco- 
 motion. Under the microscope it is seen to whirl and 
 dart about in every direction without visible means; 
 but with the higher power, especially when the motion 
 is slower, the incessant lashing of the cilia is visible. 
 
 Rhizopods. — Here we have motion, as well as all 
 other functions, reduced to simplest terms, and find its 
 origin in general contractility of protoplasm. The struc- 
 tureless, or almost structureless, mass of living jelly con- 
 17 
 
 Fig. 155. — Infusoria: a, an at- 
 tached form ; b, a free form. 
 
240 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANLMALS. 
 
 tracts itself in any part and in any direction, putting out 
 here or there a fingerlike projection or a hairlike thread, 
 
 and again withdrawing and 
 absorbing it into its sub- 
 stance (motion) ; or such a 
 projection may take hold of 
 some movable body, and then 
 
 A B 
 
 Fig. 156. — Amoeba proteus (after Leidy) : A, with the pseudopods ex- 
 tended ; B, the same, with pseudopods withdrawn ; fb^ food bodies ; 
 ex, excrement discharged. 
 
 by retraction the movable body is drawn toward and 
 absorbed as food into the animal (prehension) ; or, final- 
 ly, if the point taken hold of is fixed, then the creature 
 is moved and absorbed into the fingerlike projection 
 (locomotion). 
 
CHAPTER IV. 
 
 GENERAL LAWS OF ANIMAL STRUCTURE, OR GENERAL 
 LAWS OF MORPHOLOGY, OR PHILOSOPHICAL ANATOMY. 
 
 SECTION I. 
 
 Ititroductory. 
 
 This is a subject of fascinating interest in many 
 ways, but especially on account of its bearing on the 
 theory of the origin of organic forms by evolution. It 
 is for this reason mainly that we shall treat it some- 
 what fully. 
 
 I find it necessary, however, to make some prelimi- 
 nary definitions of terms. 
 
 Analogy versus Homology. — Parts or organs in 
 different animals are said to be analogous when they 
 have a similar form, and especially when they perform 
 a similar function. Contrarily, parts or organs of dif- 
 ferent animals are said to be homologous when, however 
 different their general apearance and however different 
 their functions, they can be shown to have a common 
 origin — to be, in fact, the same part, only modified in 
 order to perform different functions. The difference 
 between these can be best shown by examples ; and the 
 ideas involved in these terms lie at the basis of all I 
 shall say. We will give examples from plants, as well 
 as animals. 
 
 Examples : Animals. — i. The 7C'ing of a bird and the 
 wing of a butterfly are analogous organs. They have a 
 similar flat form, adapting to a similar function — viz., 
 
 241 
 
242 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 that of flying. But they are not at all homologous. 
 They have not a common origin ; the one could not 
 have, and did not, come from the other. In a word, 
 they are not at all the same thing. But the wing of a 
 bird and the wing of a bat are homologous parts. Not 
 only so, but both are homologous with the pectoral fin 
 of a fish, the fore limb of a reptile or a mammal, and 
 the arm and hand of a man ; for all these, though now 
 so different in form and function, can be shown to have 
 a common origin by descent — to be, in fact, the same 
 thing, only modified for various purposes. 
 
 2. The /u»gs of man and the gills of a fish are analo- 
 gous parts — i. e., they perform the same function in the 
 animal economy — viz., the aeration of the blood. But they 
 are not homologous. By no possibility could one have 
 come by modification out of the other. What, then, in 
 the fish is the organ homologous with the lung of man ? 
 It is the air bladder. This is the organ which by modifi- 
 cation became the lung of air-breathing animals. The 
 proof of this is complete, for all the steps of the gradual 
 change can be traced. This is shown as follows : 
 
 {a) In most typical fishes the air bladder is wholly 
 isolated and used only as a float. In such cases it is 
 colorless. But in some fishes it is connected by a slen- 
 der tube with the throat, and doubtless the contained 
 air is renewed from that source. In still other fishes it 
 is not only connected with the throat, but air is regu- 
 larly and voluntarily taken in. In such cases it is vas- 
 cular and therefore reddish in color. This is the case in 
 the gar pike [Lepidosteiis). This fish may be observed 
 to come at intervals to the surface and gulp down air, 
 which is again afterward expelled in bubbles. The gill 
 breathing is to some extent supplemented by air breath- 
 ing. Finally, in a few of the most reptilian fishes [Cerato- 
 diis and lepidosiren) the air bladder is not only vascular, 
 
GENERAL LAWS OF ANIMAL STRUCTURE. 243 
 
 but cellular, like the lung of a frog. It is really a lung, 
 and these animals breathe partly by gills as a fish, and 
 partly by lungs as an amphibian reptile. It is an am- 
 phibian fish. 
 
 [^) Now, the condition last described is exactly that 
 of the lowest amphibians and the early or larval condi- 
 tion of a// amphibians. In fact, in the development of 
 the individual, in the individual life history of an am- 
 phibian, we have all the stages of change of the respira- 
 tory organs from the complete gill-breathing stage to the 
 complete lung-breathing stage. The very young tad- 
 pole breathes wholly by gills, like a fish ; the frog wholly 
 bv lungs, like a reptile or a mammal. How did the 
 change take place? By modification of the gills? No 
 — but dy the development of another organ in the position of 
 the air bladder of a fish. At first, as already said, the 
 breathing is wholly by gills, like a fish. Then by the de- 
 velopment of an organ in the position of the air bladder 
 of the fish and connected with the throat it begins to 
 breathe also by a commencing lung. It now breathes 
 equally by gills and by lungs, both water and air. This 
 is the permanent condition of the highest fishes (lung 
 fishes) and of the lowest amphibians (the Ferenni- 
 branchiata), such as the siren, etc. From this time onward 
 the gills decrease and the lungs increase, until in the 
 adult the gills disappear and the breathing is wholly by 
 lungs. The first argument [a) is strongly presumptive; 
 the second [b) is demonstrative. 
 
 It is unnecessary to give further examples from ani- 
 mals, as all that we shall have to say in this chapter will 
 be a continuous illustration of homology ; but a few 
 illustrations from plants will show the universality of the 
 principle. 
 
 (i) A rhizome or rootstock — such as that of a flag, 
 or of calamus, or of a fern — is analogous to a root, for 
 
244 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 it has the underground position of a root, and probably 
 performs some of the functions of a root. But it is 
 homologous with a stem — it is, in fact, an underground 
 stem, for it has the structure of a stem, it bears leaves 
 like a stem, and in the axils of those leaves come buds, 
 making new shoots. Moreover, in many plants, as in 
 palms and in ferns, all gradations can be traced even 
 in the same family from the underground, through the 
 prostrate and inclined, to the upright position. 
 
 (2) The so-called leaf of a cactus is no doubt analogous 
 to a leaf. It is flat and green, and performs the func- 
 tions of a leaf — viz., the assimilation of plant food. But 
 it is homologous with a stem. It is, in fact, a stem modi- 
 fied in shape and color to perform the function usually 
 performed by leaves. This is proved by its structure — 
 viz., pith, wood, and bark, with medullary rays connect- 
 ing the pith and bark ; and also by the gradations 
 among cactuses between the flat, leaflike form and the 
 cylindrical, stemlike form. Where, then, are the true 
 homologues of the leaves ? We find them in the spines. 
 These are abortive leaves modified as defensive organs. 
 They have the spiral arrangement of leaves, and in 
 their axils come the buds which form new shoots. 
 
 3. One more example : The acacias — of which there 
 are in California about twenty species, introduced from 
 Australia — are by appearance easily divided into two 
 groups, viz., the feathcr-lcaved acacias and the j-/;«//^- 
 /?d!Z^^^ acacias. These are so different in general appear- 
 ance, that the mere popular observer would probably 
 put them not only in different genera, but even in differ- 
 ent families. But doubtless the botanists are right in 
 putting them all in the same genus, for they are really 
 so closely allied that the same individual may pass from 
 one form to the other. 
 
 The fact is, the so-called leaf of the simple-leaved 
 
GENERAL LAWS OF ANIMAL STRUCTURE. 
 
 245 
 
 acacias is not homologically a leaf at all, but a leaf- 
 stem, broadened and flattened to perform the function 
 of a leaf. Such an organ, functionally a leaf but homolo- 
 gous with a leaf-stem, is called by botanists a phyllode. 
 Its true nature is shown in the development of a simple- 
 
 FlG. 157. — K branch of young; acacia, showing change from one form of 
 leaf to the other : <7, ^, c, d^ .successive stages of change ; /y, leaf stalk 
 which gradually changes into the blade in c, d, and e. 
 
 leaved acacia from a seedling. At first it bears only 
 feather leaves, then imperfect feather leaves with flat- 
 tened stems, then broadly flattened stems with a mere 
 remnant of featherlike leaflets, and finally the leaflets 
 are gone and only the broad leaf-stems remain. All these 
 forms may often be seen in the same spray (Fig. 157). 
 
246 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 After these illustrations, we come back to define 
 again these terms. 
 
 Analogy is founded on fu?iction. Homology on com- 
 moti origin by descent. Parts of most diverse origin 
 may be modified to perform the same function, and 
 therefore assume a general resemblance. This is anal- 
 ogy. Parts of the same origin — really the same parts in 
 different species — by modification for different functions 
 may become so different that they are no longer easily 
 recognized as the same part. This is homology. In 
 the one case the parts seem like the same and behave 
 like the same, but are really very different ; in the other 
 they are really the same part, but they seem to the su- 
 perficial observer to be very different both in appear- 
 ance and in behavior. In the one case there is a superfi- 
 cial resemblance produced by functions easily observed, 
 and therefore determining popular names; in the other 
 there is a deep-seated resemblance shown by essential 
 structure and structural relations, but more or less ob- 
 scured by adaptation to different functions. This deep- 
 seated resemblance can only be found by wide com- 
 parison in the animal series and in the embryonic series, 
 for it is in this way only that we find the steps of grada- 
 tion connecting. 
 
 There are therefore two ideas underlying homology, 
 viz., common descent and adaptive modificatio7i. Things 
 having common origin by descent may be so modified 
 to adapt them to different functions as to conceal their 
 common origin. It is the duty of the morphologist to 
 trace out the evidences of common descent in spite of the 
 obscurations of adaptive modification. 
 
 The idea of homology or common origin with obscura- 
 tions by adaptive modification, lies at the very basis of 
 biology and must be universal. We have therefore an ex- 
 cellent e.xample of it in essential cell structure charac- 
 
GENERAL LAWS OF ANIMAL STRUCTURE. 247 
 
 teristic of all living things. We have already given this 
 illustration on page 22. All tissues have a common 
 origin in unmodified cell structure. But in the differ- 
 entiated tissues of the higher animals this common 
 origin is so obscured by adaptive modification for func- 
 tion that it can not be made out except by extensive 
 comparison in the animal scale and in the embryonic 
 scale. Now the object of this chapter is to trace the 
 homologies or the common origin in animal structure in 
 spite of the obscurations produced by modification of 
 parts adapting it to various functions^ and of the whole 
 organism adapting it to various places in the economy of 
 Nature. 
 
 But the question occurs : How far can we trace 
 homology ? Analogy can be traced throughout the ani- 
 mal kingdom, because it is based on function. Doubt- 
 less, also, homology or common descent must exist 
 throughout, but it is not traceable with certainty. The 
 obscurations of adaptive modification completely oblit- 
 erate the evidences of common origin except in animals 
 not too far separated in character. How far, then, can 
 we satisfactorily trace it? ^^'e answer: Only through 
 the primary divisions or departments. And, conversely 
 from the morphological or evolutional point of view, this 
 ability to trace common origin is the true ground of pri- 
 mary divisions. From this point of view Agassiz's four 
 primary branches from the protozoan trunk finds much 
 justification. 
 
 Thus, then, we would say, the structure of all 7'erte- 
 brates is exactly what it would be if they all came by 
 descent with modification from one primal vertebrate; 
 and therefore we must believe they did so come. All 
 articulates — i. e., arthropods and annulates — for like rea- 
 son, came from a primal form of articulate by adaptive 
 modification. Similarlv all mollusca came bv descent 
 
248 THYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 with modification from some primal form of mollusk; 
 and radiates (including echinoderms and ccelenterates) 
 from some primal form of radiate animal. 
 
 That these prime divisions also have a common 
 origin is shown by their common cellular structure, and 
 
 also by the fact that the first 
 steps in embryonic devel- 
 opment is the same in all. 
 But this common origin is 
 not shown in the completed 
 structure. 
 
 From this point of view, 
 at least four main branches 
 of the tree of life came from 
 t\\^ protozoan trunk and grew 
 in different directions, di- 
 verging more and more, and 
 giving off each subordinate 
 branches (classes, orders, families, etc.) ; but the point of 
 union with the protozoan trunk and with one another is, 
 as it were, hidden from view beneath the common ground 
 of cellular structure. But the subordinate branches of 
 each primary branch can be traced throughout. Dia- 
 gram Fig. 158 roughly expresses what we mean. 
 
 Fig. 158.^ — Diagram showing sup- 
 posed common origin of the dif- 
 ferent phyla. 
 
 SECTION II. 
 
 Homology of Vertebrates. 
 
 The thesis to be established is that all vertebrates 
 have come from some primal form of vertebrate by 
 modification. This is best shown in the skeleton, and 
 we shall confine ourselves mainly, though not entirely, 
 to this system. The common origin is shown, first, 
 in the 
 
GENERAL LAWS OF ANIMAL STRUCTURE. 249 
 
 ns 
 
 vert 
 
 b 
 
 I. GENERAL PLAN OF STRUCTURE GENERAL 
 
 HOMOLOGY. 
 
 1. All vertebrates, and no other animals, have an 
 interior skeleton, with the muscles acting on it from the 
 outside to produce motion and locomotion. 
 
 2. In all vertebrates, and in no other animals, the 
 axis of this skeleton is a jointed backbone or vertebral 
 column. It is this that gives name to this department. 
 They are vertebrates or backboned animals. 
 
 3. In all vertebrates, and in no other, this axis in- 
 closes two cavities : one, the neural cavity, //, is above, to 
 protect the nervous centers; 
 and the other, the visceral cav- 
 ity, ?', below to inclose the vis- 
 ceral centers (Fig. 159). 
 
 4. In all vertebrates, and 
 in no other, the head may be 
 regarded as a coalescence of 
 several vertebral joints.* 
 
 5. In nearly all vertebrates, 
 and in no other, there are 
 two, and only two, pairs of 
 limbs. The exceptions to this 
 law fall under two categories 
 — viz., those like the lowest 
 fishes, which represent the 
 condition before limbs were 
 added to the skeleton, and 
 those like snakes and like some lizards and amphibians, 
 in which the limbs have been lost. 
 
 This general common plan of skeletal structure 
 strongly suggests a common origin. 
 
 Fig. 159. — Cross section through 
 a fish : vs, visceral system ; 
 verf, vertebra ; b, blood sys- 
 tem ; ?is, nervous system. 
 
 * Some do not hold this view of the origin of the head. 
 
250 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 II. SPECIAL HOMOLOGY. 
 
 But not only do we find a common general plan sug- 
 gesting common origin, but, when examined thought- 
 fully, we find all vertebrate skeletons exactly corre- 
 sponding, part for part, bone for bone, only modified by 
 necessary adaptation for various functions. The modi- 
 fications may be so great as to obscure the essential 
 identity, but extensive study of comparative anatomy 
 and embryology reveal the missing links in the continu- 
 ous chain of change. The evidence here is demon- 
 strative. 
 
 Limbs. — To show this we take the case of the 
 limbs, partly because their structure is better known to 
 common observation, but mainly because we have in 
 them the two ideas of common origin and adaptive 
 modification, both equally illustrated. The modifica- 
 tions here have been great, but not so great as to 
 wholly conceal the common origin. 
 
 (a) Fore Limbs. — For fore limbs we take man's as 
 the type or term of comparison because it is best known, 
 and also because it is really far less modified than many 
 others; although in this regard alone that of reptiles 
 would be the best. 
 
 See, then, the fore limbs of various classes of verte- 
 brates (Figs. 160, 161, 162). The same parts are similarly 
 lettered in all, so that the legend sufficiently explains 
 the corresponding parts. Moreover, dotted lines are 
 drawn through the most important corresponding parts 
 to make the comparison more easy. But it is necessary 
 to say something more in the way of explanation 
 concerning several of these parts. 
 
 I. Shoulder Girdle. — This consists of three important 
 bones — viz., the blade (scapula), the clavicle, and the cora- 
 coid. The type is seen in the reptile (Fig. 162, A) which 
 
GENERAL LAWS OF ANLMAL STRUCTURE. 
 
 251 
 
 may be regarded as the original form of the land animal. 
 In these all the parts are large. The coracoid is as large 
 as the blade, and is joined firmly to the sternum, making 
 
 3 V 
 
 rt 3 
 « o 
 
 tJ5 c 
 
 O 4) 
 
 .. o 
 E u, 
 
 P ? 
 
 
 < -v 
 
 thus a firm girdle for the attachment of the fore limb. 
 In birds also the coracoid is a large bone firmly united 
 to the breastbone (sternum). These are types of shoul- 
 der girdle. In man the coracoid is reduced to a small 
 
252 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 process of the blade. In most mammals the coracoid is 
 still more reduced, and the clavicle is wanting alto- 
 gether ; but all the steps of its gradual obsolescence 
 may be picked up by extensive comparison. 
 
 2. Elbow Joint. — In man, in monkeys, and in some 
 other mammals, and in all reptiles, the whole limb is 
 free of the body and the elbow joint halfway down the 
 limb. This is undoubtedly the original and typical con- 
 dition, but in all highly specialized mammals, by the 
 shortening of the humerus, the elbow is drawn up on 
 
 Fig. 161. — A, fore limb of bat ; B, bird ; C, archasopteryx ; D, pterodactyl. 
 (Lettered as in previous figure ; grouped from various sources. ) 
 
 the side of the body so that the limb is free only from 
 the elbow downward. 
 
 3. In man, in monkeys, and many other mammals, 
 and in all birds and reptiles, there are two bones in the 
 forearm. But in all the more specialized mammals 
 these are reduced to one., although the remnant of the 
 other is always found forming the point of the elbow 
 
GENERAL LAWS OF ANIMAL STRUCTURE. 253 
 
 (see figure of horse, Fig. 160, E). All the gradations are 
 easily found. 
 
 4. JFrist Joint. — In man (when he comes down on all 
 fours), in monkeys, in bears, and some other mammals, 
 
 A BCD 
 
 Fig. 162. — A, fore limb of turtle ; B, mole ; C, whale ; D, fish. 
 
 and in all reptiles., the wrist joint comes down to the 
 ground — the tread is on the whole hand. This was the 
 original tread of early land animals; but in all the 
 more specialized mammals, and especially in hoofed ani- 
 mals, the tread is on the toes, and the wrist is elevated 
 far above the ground, and in hoofed animals, as the 
 horse and the cow, is usually, but erroneously, called 
 the knee. 
 
 5. In man, in many mammals, and in all reptiles, 
 there are five palm bones (metacarpals) and five fingers, 
 and this was undoubtedly the original and typical num- 
 ber ; but in all the more specialized mammals the num- 
 ber of toes is reduced in some, as the carnivores, to 
 four, in some hoofed animals to three (rhinoceros), in 
 some to two, as in ruminants, and in the horse to one. 
 The palm bones in ruminants are reduced to one (canon 
 
254 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 bone) and in the horse also to one, but with two rudi- 
 ments of others (splints). But in all cases we can trace 
 all the steps of gradation. This is well shown in rumi- 
 nants. In the deer, besides the two functional toes, 
 there are two rudiments — hoofed and supplied with 
 bones — but useless; in the cow these are further re- 
 duced to mere wartlike rudiments; in the goat and 
 sheep they disappear entirely. 
 
 6. We need hardly call attention to the extreme 
 modification of the hand in the bird's wing, of the whole 
 fore limb in the whale and in the mole, and especially in 
 this last to the elongation of the point of the elbow to 
 increase its power of digging (Fig. 162, B) and to the 
 still more extreme modification in the case of the pec- 
 toral fin of fishes, and the enormous elongation of one 
 finger of the hand in the extinct flying reptile — the 
 pterosaurs of the Mesozoic (Fig. 161, D). 
 
 It is well to observe, too, how the same part may be 
 differently modified for the same function. This is well 
 illustrated by the different devices used for flight in 
 birds, in mammals (bat), and in reptiles (pterodactyls). In 
 the bird the hand is shortened and consolidated, and the 
 flat plane is formed by the addition of quill feathers. In 
 the bat the hand is elongated, and a web is stretched be- 
 tween the greatly lengthened palm bones and fingers, 
 leaving only one finger, the thumb, free and clawed. In 
 the flying reptile one finger is enormously elongated and 
 strengthened, and the web is stretched from the point of 
 this to the hind limb, leaving the three other fingers 
 free and clawed. It is a most significant fact, however, 
 that these several devices are not accomplished at once, 
 but by a gradual process through successive generations. 
 In the earliest bird (the Archaopteryx) (Fig. 161, C) of 
 the Jurassic the finger bones are not consolidated nor 
 feathered, but are all four free and clawed. 
 
GENERAL LAWS OE ANIMAL STRUCTURE. 255 
 
 {/>) Hind Limbs. — 1 lie hind limbs are less specialized 
 than the fore limbs. We have therefore contented our- 
 
 selves with illustrations from mammals alone, as thefee 
 
 are the most specialized of vertebrates (Fig. 163, A, B, 
 
 C, D, E). We have again drawn dotted lines through 
 i3 
 
256 PHYSIOLOGY AND MORPHOLOGY OF ANLMALS. 
 
 the important corresponding parts. We draw special 
 attention to the following points: 
 
 1. Hip Girdle. — This is particularly strong in all bi- 
 peds, and therefore in man, in birds, in kangaroos, and 
 in the extinct biped reptiles (dinosaurs). The hip 
 girdles are not represented in the figure. 
 
 2. Position of the Knee. — In man, in monkeys, and 
 bears among mammals, and in all reptiles and amphibians, 
 the whole hind limb is free of the body and the knee is 
 halfway down the limb. This is undoubtedly the origi- 
 nal and normal condition in land vertebrates ; but in 
 the more specialized mammals, such as carnivores and 
 herbivores, especially the ungulates or hoofed animals, 
 the knee is high up on the side of the body, in the mid- 
 dle of the so-called thigh, and the limb is free of the body 
 only from the knee down. 
 
 3. Position of the Heel. — -In man, in monkeys, in the 
 bear, and several other mammals, and in all reptiles and 
 amphibians, the tread is on the whole foot — i. e., heel 
 down. This is undoubtedly the original and normal 
 tread of the primal land vertebrate. But in all the more 
 specialized mammals the heel is lifted high in the air — 
 in the horse fifteen to eighteen inches above ground — 
 and the tread is on the toes only. Therefore land verte- 
 brates, as to their tread, may be divided into two 
 groups, \\z., platttigrade and digitigrade. Man, monkeys, 
 bears, and some other mammals, and all reptiles and 
 amphibians, are plantigrade, but all the more specialized 
 and swifter mammals and all birds are digitigrade — 
 tread only on their toes. 
 
 Again, in mammals there are two degrees of digiti- 
 gradeness. The carnivores and all other clawed digiti- 
 grades and all birds tread on the whole length of the 
 toes to the ball, while the hoofed mammals (ungulates) 
 tread only on the tip of the last joint of the toes. The 
 
GENERAL LAWS OF ANLMAL STRUCTURE. 257 
 
 one treads tiptoe, the other on the tip of the toe. 
 These may be called unguligrade. This becomes the 
 more striking when, as in the horse, the tread is on the 
 tip of the last joint of the one toe. We look with won- 
 der and delight at the danseuse pirouetting on the tip of 
 one toe. The horse is doing this all the time. 
 
 4. Maims and Pes. — There are two senses in which 
 we may use the term foot, (i) We may use it as that 
 part on which the animal treads. This is the functional 
 or analogical sense. Or (2) we may use it as that part 
 which corresponds to the foot of man, monkey, bear, 
 
 Fig. 164. — Restoration of Rhamphorhynchus phyllurus (after Marsh). 
 One seventh natural size. 
 
 reptile — the original foot. This is the morphological or 
 homological sense. It is in this latter sense that we use 
 it in comparative anatomy. In this sense the horse's 
 foot is eighteen inches long, and the hand of the largest 
 flying dragon [Fteranodon ingens) was fully seven or eight 
 feet long (Fig. 164). In order, however, not to violate too 
 flagrantly common usage, comparative anatomists use the 
 Latin terms manus d,x\di pes to signify all that corresponds 
 to the hand and foot of man and plantigrade animals. 
 
 5. Classification of Ungulates by Foot Structure. — The 
 ungulates or hoofed animals are divided by foot struc- 
 
258 PHYSIOLOGY AND MORPHOLOGY OF ANLMALS. 
 
 ture into two groups, viz., artiodactyle or even-toed, and 
 perissodactyle or odd-toed. The artiodactyles may have 
 four toes, as in the hippopotamus and the hog, or two 
 toes, as in ruminants. The perissodactyles may have 
 three toes, as in the rhinoceros and tapir, or one toe only, 
 as in the horse and zebra. Both of these groups have 
 come from a five-toed ancestor by successive dropping 
 of toes. There is also a regular order in the dropping. 
 In the five-toed ancestor the two groups were not yet 
 differentiated. The first toe to drop was the thumb, 
 leaving the remaining four toes all functional, as in the 
 hippopotamus. Then, if evolution is on the artiodac- 
 tyle line, the two side toes gradually dwindle and shorten 
 up, as in the hog, and finally disappear, as in ruminants, 
 leaving only two greatly enlarged toes, which correspond 
 to the middle and ring fingers of man. But if, on the 
 other hand, the animal is on the perissodactyle line, 
 then, of the four toes, the little finger first dwindles, as 
 in the fore foot of the tapir, and then disappears, leaving 
 three. Now, of these three, the side toes dwindle and 
 shorten up and finally disappear, leaving only the great- 
 ly enlarged middle toe, as in the horse. 
 
 6. Rudimentary Organs ; Useless Orgatis. — All through 
 the animal kingdom, especially in the more specialized 
 forms of mammals, we find rudimentary and often 
 wholly useless organs. These are evidently remnants 
 of once useful organs, which have dwindled by disuse, 
 but have not yet entirely disappeared. Examples meet us 
 on every side, (a) The horse's toes are reduced to one, but 
 not at once. It was a gradual process, every step of which 
 may be traced. The first representative of the horse 
 in the line of its descent had five toes on the fore foot, 
 then, in later times, four, then three. Three-toed horses 
 continued a long time, the two side toes meanwhile 
 dwindling and shortening up and finally passing away. 
 
GENERAL LAWS OF ANL\LVL STRUCTURE. 259 
 b e d e J 9 
 
 Equus : Qua- 
 ternary and 
 Recent. 
 
 Orohippus : 
 Eocene. 
 
 Fig. i6";.— Diagram illustrating gradual changes in the horse family : 
 Throughout a is fore foot ; t>, hind foot ; c, forearm ; d, shank ; e, molar 
 on side view ; / and g, grinding suiface of upper and lower molars. 
 (After Marsh.) 
 
26o PHYSIOLOGY AND MORPHOLOGY OF ANLMALS. 
 
 But the two palm bones to which they were attached 
 still remain as useless splints to attest the original 
 three-toed condition. These and other changes are 
 shown in diagram, Fig. 165. (d) In ruminants the toes 
 are reduced to two, but we have already shown (page 
 253) that useless remnants of the other two are still 
 found in most ruminants, showing their original four-toed 
 condition, (c) Dewclaws in the dog and many other 
 mammals reveal an original five-toed condition, (d) The 
 whale is a very specialized mammal, and therefore full of 
 useless remnants. Whales have no hair, but rudiments of 
 hair in the skin show that their ancestors were hairy. 
 They have no hind limbs, but rudiments are found buried 
 in the flesh, and therefore useless. The baleen whales 
 have no teeth, but rudiments of teeth are found buried 
 in the jawbone, and are never cut. (e) We have already 
 said (page 249) that snakes have lost their limbs by disuse. 
 This is true, for rudiments of limbs are found buried in the 
 flesh of some (the python), and, of course, useless. (/) 
 Even in man, although he is far less specialized than most 
 mammals, rudiments are found. Among these may be 
 mentioned the muscles for moving the ears and for mov- 
 ing the scalp. Rudiments must be regarded as demon- 
 strative proof of the derivative origin of organic forms. 
 
 Whole Skeleton. — We have taken only the limbs 
 as best illustrating the principle, but the same is true of 
 the whole skeleton. The skeleton, as a whole, is in all 
 vertebrates the same machine, but modified in all its parts 
 to adapt it for various purposes — viz., as a swimming 
 machine, a walking and running machine, or a flying ma- 
 chine, and all without essential change of plan. 
 
 Other Systems. — The whole argument for deriva- 
 tive origin of organic forms is based on the equal bal- 
 ance of the two ideas — essential identity and adaptive 
 modification. If the modification be too great, the essen- 
 
GENERAL LAWS OF ANIMAL STRUCTURE. 261 
 
 tial identity and derivative origin of parts may be ob- 
 scured or even obliterated. If, on the other hand, it be 
 too small, then the identity may have come in some 
 other way than by common origin — may have been made 
 out of hand at once. Now, this equal balance is found 
 in the skeleton, and especially in the limbs. In the »ius- 
 cular system the adaptive modification is too extreme ; it 
 obscures the derivative origin. In the case of the vis- 
 ceral system^ on the contrary, there is scarcely enough 
 modification to make any evidence. Next to the skele- 
 tal, the best evidences are found in the nervous system. 
 Here the essential identity of parts in all vertebrates, 
 and yet their modification in each class, is very striking, 
 especially in the brain. We have already explained this 
 (pages 76-78). In the case of the muscular system the 
 modification in passing from the fish to the land verte- 
 brates is so extreme that all hope of homology seems 
 vain. But in land vertebrates, from the amphibia (frogs, 
 etc.) to man, it may probably be traced by careful study, 
 but this has not been attempted, except in a fragmentary 
 way. Undoubtedly a rich field is open here. 
 
 in. SERIAL HOMOLOGY. 
 
 There is an evident correspondence in the several 
 parts of the fore and hind limbs. The hip girdle corre- 
 sponds with the shoulder girdle, and bone for bone, 
 although the parts are more consolidated in the former; 
 the femur corresponds to the humerus ; the two bones 
 of the leg to the two bones of the forearm, each to each ; 
 the seven bones of the ankle to the eight bones of the 
 wrist, two of the former having been consolidated into 
 one; the five bones of the instep (metatarsal) to the five 
 bones of the palm (metacarpal) and the fourteen bones 
 of the toes to the fourteen bones of the fingers. This 
 introduces us to the idea of a serial repetition of similar 
 
262 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 v.b 
 
 parts — i. e., of serial homology. In general terms as ap- 
 plied to the whole skeleton, it may be stated thus: As 
 the whole vertebrate kingdom is made up of a repeti- 
 tion of individuals constructed on the same plan, but 
 modified according to the place and function in the verte- 
 brate scale, so each individual vertebrate is made up of a 
 repetition of segments, similar in plan but modified ac- 
 cording to its place and function in the series of segments. 
 This is, of course, best brought out in the skeleton. 
 
 Take any vertebrate, such as a fish or a man. Make 
 a cross section of the body and look at the end. What 
 we see is shown diagrammatically in Fig. i66. The sec- 
 tion of the skeleton consists 
 of three parts — centrum, c, an 
 arch above (neural arch, n^ 
 surrounding nervous centers, 
 and an arch below (visceral 
 arch, v) surrounding the vis- 
 ceral cavity. The whole is 
 enveloped in muscle and skin, 
 with which we have no con- 
 cern now. The three parts 
 named above constitute one 
 segment of the skeletal axis, 
 and may be called a vertebra. 
 Now the whole skeletal axis 
 may be regarded as made up 
 of a repetition of such skeletal 
 segments or vertebra modi- 
 fied according to place and function in the series (Fig. 
 167). The centrums repeated form the vertebral col- 
 umn, the neural arches repeated constitute the neural 
 canal, in which are lodged the nervous centers, and the 
 visceral arches repeated make the visceral cavity. But 
 these segments are modified according to the place in 
 
 Fig. 166. — Cross section throug-h 
 a fish : v^ visceral system ; 
 c, vertebra ; b. blood system ; 
 «, nervous system. 
 
GENERAL LAWS OF ANIMAL STRUCTURE. 263 
 
 the series, sometimes by enlargement, sometimes by 
 diminution and even disappearance, sometimes by con- 
 solidation of several together. But throughout these 
 
 Fig. 167. — Owen's archetypal vertebrate, showing the successive segments 
 but slightly modified. The olfactory capsule, the eye, and the ear are 
 represented detached. 
 
 modifications the essential identity, although obscured, 
 is still discernible. 
 
 For example: In the dorsal region of the higher 
 vertebrates the visceral arches are greatly enlarged to 
 contain the thoracic viscera. Going forward, in the neck 
 region in man and most vertebrates the visceral arches 
 are wanting in order to give greater freedom of motion, 
 but not in a// vertebrates, nor in all stages of develop- 
 ment. Ribs are borne by all the vertebrse up to the 
 head in fishes, in serpents, and also in the embryos of 
 mammals, and even of man. Going still forward, accord- 
 ing to one view, the head is composed of several vertebra's 
 consolidated and greatly modified (see Fig. 167). Ac- 
 cording to this view, the basilar portions of the occipital 
 and sphenoid, and the ethmoid are the centrums of three 
 successive vertebras, the neural arches of which, greatly 
 enlarged, form the skull, while the diminished and much 
 modified visceral arches make up the face. There is, 
 however, some doubt whether the vertebral theory of 
 the skull is true in this strict sense; and, if so, of how 
 many vertebrae it consists. 
 
 Going now backward, in the lumbar region, again, the 
 ribs are wanting in man and mammals, but not in fishes, 
 
264 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 nor in serpents, nor in tailed amphibians, nor in the em- 
 bryos of mammals, nor even in the embryo of man. So 
 that their absence is only an extreme term of modifica- 
 tion, all the steps of which may be found. Going still 
 back, the sacrum may be regarded as five to six vertebrae 
 with their bodies — neural arches and visceral arches — all 
 consolidated. Indeed they are distinct, and have their 
 ribs in many lower vertebrates and in the embryos of 
 mammals and man.* 
 
 Signification of Limbs. — It will be observed that 
 in this scheme we have left out the limbs. This seems 
 a very great omission, but this is so only in the higher 
 vertebrates. The primal vertebrates were probably limb- 
 less. And in all the earliest vertebrates and in the 
 lowest vertebrates to-day limbs are comparatively in- 
 significant appendages, which may be left out in any 
 general scheme of skeletal structure. It is probable 
 that limbs ca/i not be brought into the original plan of 
 homologous segments, but have been added afterward 
 as a sort of afterthought. 
 
 Origin of Limbs. — Professor Owen f thought to 
 bring limbs into this scheme of repeated segments by 
 making them appendages to the visceral arches, and 
 this view is expressed in his figure of the archetypal ver- 
 tebrate (Fig. 167), where small appendages are seen on 
 every arch. He believed that the shoulder girdle and 
 hip girdle were formed by consolidation of several vis- 
 ceral arches, and that the limbs were greatly enlarged 
 appendages to these arches, the appendages to the 
 other arches being still rudimentary or wanting. But 
 this view, though very suggestive, is not now generally 
 
 * Cervical, lumbar, and sacral ribs are found in the embryo of 
 mammals and of man. Wiedersheim, p. 51. 
 
 f Owen, " Homologies of Vertebrate Skeletons " and " Signifi- 
 cation of Limbs." 
 
GENERAL LAWS OF ANL\L\L STRUCTURE. 265 
 
 accepted. The more probable view is that the same fold 
 of the skin which formed the lower unpaired fin (anal 
 fin) divided as it came forward into two lateral folds 
 (Fig. 168, A), that these by local enlargements became 
 the paired fins, anterior and posterior (Fig. 168, B), and 
 by addition of skeletal framework were developed into 
 
 Fig. 168. — Diagrams showing probable origin of limbs : A, earliest stage, 
 showing lateral folds ; B, later stage, after paired fins were formed (from 
 Wiedersheiraj ; C, the formation of limbs from fins (from Parker). 
 
 limbs (Fig. 168, C) and finally became connected with 
 the skeletal axis for more effective action. The subject 
 is, however, still obscure. 
 
 Serial Homology of Other Systems. — Other sys- 
 tems show segmented structure, but not so clearly. 
 Next to the skeletal in this regard comes the nervous 
 system. Thus we may conceive the cerebro-spinal axis 
 as a series of ganglia (fused, however, into a continuous 
 tract) corresponding to the vertebrae, and giving off each 
 a pair of nerves; and, according to the vertebral theory 
 
266 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 of the head, the successive lobes of the brain (cerebel- 
 lum, optic lobes, cerebrum, and olfactory lobes) corre- 
 spond to the successive cranial vertebrae. 
 
 The muscular system shows segmentation in a marked 
 degree in fishes and some lizards. The flakes of a fish's 
 flesh correspond to the vertebrae, and we have here lit- 
 erally body segments {somites). See also the so-called 
 glass snake — really a limbless lizard. Under a sharp 
 blow the tail breaks into segments right through be- 
 tween scales, flesh flakes, and vertebrae. 
 
 In the visceral system the segmentation is not dis- 
 cernible. 
 
 As a general law, the number of segments and their 
 similarity is greater as we go down the scale of verte- 
 brates, and, contrarily, as we go up the scale adaptive 
 modification obscures more and more the homology. 
 
 SECTION III. 
 
 Invertebrates. 
 
 I. ARTICULATA. 
 
 Under Articulata we include arthropods and annelids 
 or worms (see schedule, page 7 1), because, from the homo- 
 logical point of view (though not from any other), they 
 are best united. What we propose to prove is that the 
 structure of all these animals is exactly such as it would 
 be if they all came by descent, with modifications, from 
 one primal articulate animal; and, therefore, that we 
 must conclude that they did all so come. 
 
 General Plan. — The general plan of structure is the 
 same throughout, but wholly different from that of ver- 
 tebrates. I. The skeleton is external, instead of internal. 
 2. The nervous system lies along the ventral, instead of 
 the dorsal, aspect of the body. 3. The body consists of 
 one cavity, instead of two. 
 
GENERAL LAWS OF ANIMAL STRUCTURE. 267 
 
 Of these two positions of the skeleton, which is best ? 
 For motion and locomotion they are probably equally 
 good ; but the external skeleton has the great advantage 
 of being protective as well as locomotive, while the in- 
 ternal skeleton has the much greater advantage of leav- 
 ing the surface sensitive to external impressions, and 
 therefore an inlet to knowledge of external nature, 
 and thus gives greater capacity for higher development. 
 
 So much for the general plan. Now the details of 
 the homology. 
 
 Serial Homology. — In the case of vertebrates we 
 took up first special homology, because this is by far the 
 
 Fig. 169. — Diag:ram section across an arthropod, showing the inclosing 
 skeleton ring and a pair of jointed appendages : //, nervous center ; 
 V, viscera ; d, blood system. 
 
 most distinct. In the case of Articidata we take up only 
 serial homology, because not only is this the most dis- 
 tinct, but it includes the other also; for we may regard 
 each articulate animal as consisting of a series of seg- 
 ments essentially similar, but modified according to its 
 place and function in the series, and then the structure 
 thus formed is again modified according to the place in 
 
268 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 the scale of Articulata, to make the infinite variety of 
 forms constituting this department of animals. 
 
 Our object, then, is to show that the skeleton of an 
 arthropod consists of naught else than a series of similar 
 segments, modified according to the place and function 
 in the series. 
 
 Take any articulate animal, say an arthropod, like an 
 insect or crustacean. Make a cross section and view it on 
 end (Fig. 169). We see an external bony ring inclosing 
 everything, and attached to it a pair of jointed append- 
 ages, right and left. Now, the whole skeleton is made up 
 of such rings and appendages, repeated and modified. The 
 repetition of the rings gives rise to a hollow, many-jointed 
 cylinder or barrel, and the repetition of the appendages 
 to a continuous row of such on each side. Each ring, 
 with its pair of appendages, is called a somite, or body 
 segment. Such is the simple idea or archetype of an ar- 
 ticulate animal ; but, in fact, these ideal rings and append- 
 ages are very variously modified for function. Some 
 are modified for swimming appendages, some for walk- 
 ing appendages or limbs, some as food-gathering append- 
 ages (jawfeet), some as biting appendages (jaws), some 
 as sense appendages (eyes, ears, feelers), but all made 
 on the same plan, only modified. The mianner of modifi- 
 cation is sometimes by enlargement, sometimes by dimi- 
 nution and even disappearance, sometimes by coalescence 
 and consolidation of several into one piece. 
 
 For example, take one from about the middle of the 
 scale — say, a crawfish or a lobster. This animal consists 
 of about twenty-one rings and pairs of appendages (Fig. 
 170). In the tail region or abdomen the seven rings 
 are all separate and but little modified, but their jointed 
 appendages are greatly modified for swimming, D' ; 
 those of the last joint but one are enlarged and flat- 
 tened, and, together with the flattened last joint, which 
 
Fig. 
 
 1 70. -External anatomy of the lobster, r After Kingsley.^ 
 
270 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 is without appendages, form the powerful flipper at the 
 end of the tail for backward darting. Going thence 
 forward, we come to the cephalo- 
 thorax. Here many joints in their 
 dorsal parts are consolidated into 
 V a carapace, and their ringed struc- 
 .1 ture is lost, but their entire dis- 
 ^ tinctness is evident in the lower 
 crustaceans and in the embryos 
 of the higher. But observe that 
 it is only the dorsal parts that are 
 ^ consolidated. In the ventral parts 
 
 1 the separate rings are distinct, 
 ^ and each has its own pair of 
 ^ jointed appendages greatly en- 
 
 I - larged for walking,pj\f\f\f\ 
 % These are the five pairs of limbs. 
 a. Going still forward, we find next 
 ^ three or four pairs modified for 
 ^ the gathering of food. These 
 rt are called maxillipeds^ or jawfeet, 
 £ md. ind"^, nuP . The modification is 
 ^ < not so great but that their resem- 
 1^ y blance to the limbs is obvious. 
 " Next come two or three pairs more 
 
 2 modified, so as to adapt them for 
 biting. These are the maxil/a;, or 
 jaws, mx. Next come two pairs 
 of highly modified, greatly elon- 
 
 ^ gated, and many-jointed append- 
 ages, which are organs of touch 
 and hearing. They are sense ap- 
 pendages. Lastly, a pair of joint- 
 
 j; ed appendages, on the ends of 
 Qp J c^ which are placed the eyes. Some 
 
GENERAL LAWS OF ANLMAL STRUCTURE. 
 
 271 
 
 doubt whether these belong to the same category as the 
 other appendages, but they are usually so regarded. In 
 Fig. 171 we give the whole series of appendages in the 
 crawfish : 
 
 The crab is much more modiiied, and the consolida- 
 tion more complete. The tail seems to be absent, but 
 is really only diminished in size and bent under and con- 
 cealed beneath the body ; but in the embryo the tail is 
 similar to that of the crawfish (Fig. 172). The maxilli- 
 
 FiG. 172. — Development of Carcinus moenas : A, zoa>a stage ; B, megalopa 
 stage ; C, final state. (After Couch.) 
 
 peds and sense appendages are also much reduced in size, 
 but they are all present. 
 
 Going down the Scale. — We have taken a case 
 
 from about the middle of the articulate scale, because 
 there the essential identity and adaptive modification 
 are evenly balanced and both conspicuous. But the evi- 
 dence is completed by going down and up the scale. In 
 the lower crustaceans the rings are all separate and the 
 appendages less and less modified (Fig. 173). A series 
 19 
 
2^2 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 of appendages of a still lower form is given in Fig. 174. 
 In this case the appendages are too much alike to be 
 easily classified. Often, too — e. g., in the Liniulus — the 
 appendages which in the higher crustaceans are walking 
 appendages become swimmers, while the maxillipeds be- 
 come walkers. Finally, in the lowest crustaceans and, 
 still better, in the centipede and in some marine worms, 
 the rings and appendages are greatly increased in num- 
 ber, and are so similar that there is not modification 
 
 Fig. 173. — Vibilia, an amphibod crustacean. (After Milne Edwards.) 
 
 enough to furnish argument for homology. There is 
 only a slight modification of the head and tail joints 
 (see Fig. 175). Indeed, some marine worms multiply 
 by self-division. In such cases at the point of division 
 some of the rings are consolidated and appendages 
 modified to form a new /lead and a new tail (Fig. 176), 
 
 Going up the Scale. — Going up the scale to in- 
 sects the modification is greater, but the elements — viz., 
 rings and appendages — are the same. An insect consists 
 ideally of about sixteen or seventeen segments and ap- 
 
GENERAL LAWS OF ANLNLXL STRUCTURE. 
 
 -/i 
 
 --< 
 
 pendages (Fig. 177). In the abdomen the rings are 
 perfect and movable, although the appendages are 
 wanting; but they are present in the -^ 
 
 larval or caterpillar state. The thorax '^^ 
 
 is a consolidation of three rings, each ^^^ 
 
 with its pair of appendages greatly en- 
 larged for walking (legs). The head 
 consists of three or four consolidated 
 segments with appendages much modi- 
 fied — the first pair into antennae, the 
 second pair into mandibles, and the 
 third pair into maxillae and maxillary 
 appendages or feelers, and the fourth 
 pair into labium and labial append- 
 ages. 
 
 Origin of Insects' Wings.— The 
 wings of insects are not homologous 
 with legs or other appendages. There 
 is some doubt as to their origin, but it 
 is most probable that they are modifi- 
 cations of the trachi-branchise of the 
 larvae of aquatic forms. If so, insects 
 sprang from aquatic species. 
 
 That insects are higher than crusta- 
 ceans is shown by the distinctness of the 
 head. The ring-series in crustaceans 
 are grouped into two regions, the ab- 
 domen and the cephalothorax, the head 
 being undistinguishable from the tho- 
 rax. The same is true of spiders and 
 scorpions. But in insects we have three 
 distinct groups — the head, the thorax, 
 and the abdomen. A distinct movable 
 head is always a sign of the dominance 
 of head functions. 
 
 £^ 
 
274 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 The higher position of insects is further shown by the 
 number of their legs or appendages used for locomotion. 
 It is a law in biology that a great number of parts, simi- 
 
 FiG. 175. — Lithobius forcipatus. (After Carpenter.) 
 
 lar in form and function (vegetative repetition), indicates 
 a low position in the scale of organisms. As we go up 
 the scale the number of parts used for one function be- 
 comes less, and their efficiency becomes correspondingly 
 greater. Legs are an admirable illustration of this law. 
 In marine worms and in the lowest Crustacea there is an 
 indefinite but very great number of similar legs. As we 
 rise among Crustacea the number becomes definite, and 
 countable as legs, when there are fourteen or seven pairs. 
 These are called tetradecapods. In the higher Crusta- 
 cea — crabs, crawfish, etc. — they are reduced to five pairs. 
 
 Fig. 176. — Syllis prolifera. 
 
 These are therefore called decapods. In spiders and scor- 
 pions there are only four pairs. These might be called 
 octopod insects. In true insects they are reduced to three 
 
GENERAL LAWS OF ANIMAL STRUCTURE. 275 
 
 pairs, and these are called hexapods. If it be allowable 
 to pass from one department to another, we might go 
 
 (XtvteiiUV;^ 
 
 TvbvaxX- p^^^^ tm^m. <P% 
 
 / 
 
 <=fr=-sa 
 
 •^ab^Lovnew, 
 
 eutvic JceitttiO'^S, 
 
 f Q 
 
 SteTKVvU ^ lo-^^^-P- K 
 
 Fig. 177. — External anatomy of Caloptenus spretus, the head and thorax 
 disjointed : ///, uropatagium ; y, furcula ; c, cercus. (Drawn by J. T. 
 Kingsley. ) 
 
2-6 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 on and say that in vertebrates the limbs are reduced to 
 two pairs, and these are therefore quadrupeds. This is as 
 far as reduction can go for highest locomotive efficiency 
 on land. In birds locomotive efficiency on land is sacri- 
 ficed for flight; and in man it is sacrificed for efficiency 
 of another and still higher kind, and the reduction goes 
 on to one pair, the other pair common to vertebrates 
 being set free for higher uses as wings and hands. 
 
 Law of Differentiation. — We have already seen 
 (page 23) that cells, commencing in the lowest animals 
 and in the earliest embryonic condition, all alike, and per- 
 forming each all the functions of life but very imper- 
 fectly, as we rise in the scale become differentiated in 
 form and specialized in function, each doing its own 
 work and doing it much better. So now we find the 
 same law in the segments. In the earliest geological 
 times, and in the lowest animals now, we find all the 
 segments alike and performing similar functions, but 
 imperfectly. But as we go up the scale the segments 
 and their appendages are more and more differentiated 
 in form and function, and by this division of labor the 
 functions are better performed. The same law may be 
 carried still a step further. Regarding a whole depart- 
 ment, such as the vertebrata or articulata, as composed 
 of a repetition of organisms made on the same plan ; 
 then in early geological times there was doubtless far 
 more similarity than now. In the process of evolution 
 these somewhat similar organisms were differentiated, 
 each kind for its own place in the economy of Nature, 
 and specially fitted for that place through the survival 
 of the fittest. This law of differentiation, therefore, 
 is the most fundamental and all-pervasive law of evo- 
 lution. 
 
 Nervous System. — We have confined ourselves thus 
 far to the skeletal system. Next to this, as illustrat- 
 
GENERAL LAWS OF ANIMAL STRUCTURE. 
 
 277 
 
 ing serial repetition, would come the nervous system. 
 Ideally the nervous system of articulata consists of a 
 series of ganglia, one to each somite or body segment 
 and presiding over that segment, but sympathetically 
 connected by a continuous thread with one another, and 
 all with the cephalic ganglion. But as we pass up the 
 scale, modification and consolidation, differentiation and 
 specialization, proceed pari passu, as already shown on 
 page 87. 
 
 2. ISIOLLUSCA. 
 
 Here, again, we find an entirely different general plan 
 of structure. In vertebrates we have an internal skele- 
 ton, the axis of which consists of segments modified ac- 
 cording to place and function. In articulates we have 
 an external skeleton still more obviously segmented and 
 modified. In moUusca alone, among all the great de- 
 partments, we have no segmentation, no serial repetition 
 of ideally similar parts. This want of repeated seg- 
 ments is its most distinctive peculiarity. In mollusca, 
 also, we have no true — i. e., locomotive — skeleton at all, 
 either internal or external, but only a protective shell. 
 
 General Character. — The general characters of this 
 type are mostly negative: i. A want of a true locomo- 
 tive skeleton. 2. A complete want of segmentation of 
 any system. 3. The presence of a soft, mucous, sensi- 
 tive surface wherever not covered with shell. 
 
 The most important of these is the complete absence 
 of segmentation. It follows from this that there can be 
 no serial homology. There is, of course, a special ho- 
 mology — i. e., a homology of part with part in different 
 orders and classes of mollusks, but it is obscure and 
 little studied. I shall not attempt its exposition. 
 
 ]]'hy there is no Segmentation in this Type. — It is seen 
 that homology, especially serial homology, is mostly lim- 
 ited to the skeleton and nervous system — i. e., to the or- 
 
2/8 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 gans concerned with the distinctive anivial life. Animal 
 Ufe and activity has to do specially with action and reac- 
 tion between the animal and its environment. The limbs 
 especially take hold on the environment and are modi- 
 fied by it. The vegetative organs, on the contrary, are 
 little affected by the environment. Segmented structure 
 is peculiarly adapted for diversified modification, showing 
 homology. Now, in articulates the distinctive animal 
 functions are very highly developed in proportion to the 
 vegetative, and these are therefore the most perfectly 
 segmented of all animals. In mollusks, on the contrary, 
 the vegetative functions are very highly developed in 
 proportion to the distinctively animal, and these do not 
 lend themselves readily to homology. The vertebrates 
 seem in this regard to combine the characters of both. 
 
 3. RADIATA. 
 
 In these, including echinoderms and coelenterates, 
 we again find an entirely different type of structure. We 
 find, indeed, segmentation again, but of an entirely dif- 
 ferent kind. All these animals have an essentially radi- 
 ated structure. Like the plugs of an orange arranged 
 about the central pith, or like the spokes and fellies of 
 a wheel about the central hub, so are the several seg- 
 ments of a radiated animal symmetrically arranged about 
 its central mouth and stomach, and all the organs of the 
 body are repeated in each segment. Thus, in a starfish, 
 for example, the mouth and the stomach are in the cen- 
 ter and surrounded by the nerve centers (oesophageal 
 collar) and by the blood centers. From this center go 
 to each arm or segment a branch of the stomach, a 
 branch of the nervous system from a corresponding gan- 
 glion of the collar, and a great branch of the blood sys- 
 tem. Each arm contains also its own equal i^2iX\. of the 
 respiratory system and the reproductive system. The 
 
GENERAL LAWS OF ANIMAL STRUCTURE. 
 
 279 
 
 radial arrangement of the nervous system is shown in 
 Fig 67, page 92. The other systems will be illustrated 
 hereafter. 
 
 Comparison of this with Other Types. — Com- 
 paring this with other types, in both vertebrates and 
 articulates we have segments repeated in a linear series, 
 and therefore an anterior d^nd posterior extremity; but in 
 this we have segments repeated in a circular series, and 
 therefore no beginning and no end, no anterior and no 
 posterior extremity. Again, in all other types, includ- 
 ing mollusca, we have all the organs, especially those of 
 animal life, repeated on the two sides of a median plane 
 — i. e., bilateral symmetry ; in this one we have repetition 
 of organs about a central column — i. e., radial symme- 
 try. In the highest radiates alone are found the distinct 
 beginnings of a bilateral symmetry. 
 
 In this also, as in the other two segmented types, the 
 completeness of the segmentation, the number of re- 
 peated parts, and their similarity are greatest in the 
 lower part of the scale ; and as we rise the segments 
 become more and more dissimilar by modification for 
 various functions. But the obscuration by modification 
 is less conspicuous in this type, because, taken as a whole, 
 the animals of this are less highly organized. 
 
 4. PROTOZOA. 
 
 Among these, of course, there is as yet no distinct 
 plan of structure (for they consist of a single cell), and 
 structure such as we have been speaking of is produced 
 by differentiation of the elements of a cell aggregate. 
 These lowest animals may be regarded as the living 
 stuff out of which the different types were constructed — 
 the trunk from which diverged the four great branches 
 (F'ig. 158, page 248). There is no room for homology 
 here. 
 
28o PHYSIOLOGY AND MORPHOLOGY OF ANLMALS. 
 
 GENERAL CONCLUSIONS. 
 
 The general conclusions of this chapter may be 
 briefly summarized in several propositions. 
 
 I. There are at least four very distinct plans of 
 structure among animals through which homology may 
 be more or less clearly traced, but beyond which it can 
 not be distinctly traced, although it doubtless exists. 
 These are the vertebrata, articulata, mollusca, and radi- 
 ata. The characteristic plan of vertebrates is that of 
 an internal skeleton, the axis of which consists of seg- 
 ments ideally similar, but modified according to the place 
 and function in the series of segments. The animal 
 thus formed is again modified according to its place in 
 the scale of vertebrates. 
 
 The characteristic plan of the articulata is that of an 
 external skeleton, composed of segments and pairs of 
 appendages ideally similar, but modified according to 
 the place in the series. The annnal thus formed is 
 again modified according to its place in the scale of 
 articulata. 
 
 The characteristic plan of the mollusca is the total 
 want of a true locomotive skeleton, and especially the 
 entire absence of segmentation. There is therefore no 
 room for serial homology. Even special homology is 
 more obscure in this than in other departments. Nev- 
 ertheless, there is enough to assure us that these also 
 came by modification from some primal form of mol- 
 lusk. 
 
 The characteristic plan of radiata is that of similar 
 segments arranged symmetrically about a central mouth 
 and stomach. In vertebrates and articulates we have 
 segments repeatecf in a linear series ; in radiates in a cir- 
 cular series. In all other types we have dilateral symme- 
 try ; in this we have radial symmetry. 
 
GENERAL LAWS OF AMMAL STRUCTURE. 28 1 
 
 2. In all plans the ideal similarity of the repeated 
 parts in the animal and of the repeated animals in the 
 department is clearest in the lower part of the scale, and 
 adaptive modification for function becomes more con- 
 spicuous as we rise, until finally the essential identity 
 may be wholly obscured by adaptive modification. This 
 is the law of progressive differentiation so universal in 
 Nature. 
 
 3. All these phenomena may be completely explained 
 by the origin of organic forms by derivation — i. e., by 
 " descent with modifications " ; in a word, by the theory 
 of evolution, and can not be e.xplained in any other way. 
 
PART II. 
 
 ORGANS AND FUNCTIONS OF ORGANIC LIFE. 
 
 We have explained (page 24) that all the functions 
 of the animal body fall into two groups — one, distinctive 
 of animals, and therefore called the functions of animal 
 life; the other, possessed in common with plants, and 
 called functions of vegetative or organic life. We have 
 now treated of the distinctively animal functions. We 
 come now to treat of the functions of organic life and 
 their organs. 
 
 These are again subdivided into two very distinct 
 groups — viz., the nutritive and the reproductive — the 
 one including all that assemblage of functions which con- 
 tribute to the conservation of the individual life; the 
 other, all that assemblage of functions which insures the 
 continuance of the species. We reproduce here, with 
 slight addition, the schedule already used on page 24: 
 
 f . ( Sensation and consciousness. 
 
 Ani- Functions of animal life.. - ^^ v..- j 1 ^ 
 
 I I { Volition and voluntary motion. 
 
 body i \ Nutritive -^ Nutrition proper. 
 
 [ Functions of vegetative life < { Elimination. 
 
 ( Reproductive 
 
 2S2 
 
CHAPTER I. 
 
 NUTRITIVE FUNCTIONS — METABOLISM, OR WASTE AND 
 SUPPLY. 
 
 Coextensive with life and lying at the very basis 
 of all life phenomena is a continuous change by waste 
 and supply of the material of which the body is com- 
 posed. This whole process of change is called metabo- 
 lism, ox transformation. It is not only coextensive with 
 life, but it may be said to be life itself. It consists 
 necessarily of two parts — viz., an ascensive change, by 
 which new tissue is formed from crude material, and a 
 descensive change, by which old tissue is decomposed 
 and eliminated from the body. The former is called 
 anabolisni, the latter katabolism, or waste. This latter is 
 apparently the active, initiative agent of the whole pro- 
 cess. 
 
 "Waste. — The process of waste is so fundamental 
 that it must be thoroughly illustrated. 
 
 I. Suppose, then, we had a pair of scales of enormous 
 size, and one of you (hearers or readers) were lying in a 
 comfortable position in one pan and a weight for perfect 
 counterpoise in the other. I shall suppose you at per- 
 fect rest physically and peace mentally, and, as contrib- 
 uting to this condition, perhaps smoking a cigar. The 
 equilibrium would not continue indefinitely; if the scales 
 were delicate, not even for a minute. On the contrary, 
 even while we watch the experiment, your side of the 
 
 283 
 
284 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 scale goes up. The body is consuviing like the cigar — 
 literally burning up and passing away as invisible gas 
 through the nostrils. Probably about two pounds is 
 thus consumed in twenty-four hours. This process of 
 waste is more rapid in higher than in lower animals. It 
 is continuous with life and in proportion to its grade. 
 This may be expressed by the formula: 
 
 I. Waste cc Life. 
 
 2. We have taken the case of absolute rest of body 
 and mind. But suppose next that the subject of this 
 experiment is in violent activity, physical, mental, or 
 moral, perhaps dancing a hornpipe, perhaps wrestling 
 with a mathematical problem, perhaps in intense anx- 
 iety, remorse, or grief. Under these conditions the wast- 
 ing, as measured by pounds, is more rapid. This is ex- 
 pressed by formula 2 : 
 
 2. Waste oc Work. 
 
 3. We will next suppose the subject to be inclosed in 
 a large calorimeter, or instrument for measuring heathy 
 melting of ice. In such case there would be a constant 
 stream of water from the instrument, indicating a con- 
 stant evolution of heat generated by the combustion of 
 waste. Furthermore, the formation of water, and there- 
 fore the generation of heat, would be in proportion to 
 the grade of life and the amount of activity. All this is 
 expressed by formula 3 : 
 
 3. Heat oc Waste oc Life a Work. 
 
 4. Supply. — Now it is evident that this can not go on 
 continuously without a contrary process, otherwise the 
 body would completely consume itself and exhale in 
 smoke, like the cigar. There must be a supply exactly 
 proportioned to the 7vaste. It is this that creates the neces- 
 sity iox food. Food, therefore, must be proportioned to 
 
NUTRITIVE FUNCTIONS. 285 
 
 waste, and therefore to grade of life and intensity of 
 work. This is expressed in formula 4: 
 
 4. Food oc IWiste oc Life and Work. 
 
 Therefore life in a scale pan would show a continual 
 oscillation of level — i. e., of weight of body. In adults, 
 whose supply is only equal to waste, the average weight 
 is maintained ; in a child the supply is a little greater 
 than the waste, and the average weight increases. 
 
 Observe that the most fundamental of these two op- 
 posite processes is the waste. This is continuous with 
 life and apparently its cause; the other (supply) is oc- 
 casional. The waste goes on continuously, whether 
 there be supply or not, as long as life lasts. The sup- 
 ply may be regarded as a secondary consequence of 
 the waste. 
 
 Illustrations. — i. Thus the living animal body may be 
 compared to a burning lamp — ever consuming and ever 
 resupplied. In both cases there is waste and supply, 
 and in both cases there is continual oscillation of weight. 
 In both cases heat is produced by the consumption of 
 material. Moreover, the amount of heat produced by 
 the burning of a pound of material is substantially the 
 same in the two cases, only in the one case the heat is 
 concentrated on a given point and compressed into a 
 short time, and is therefore intense, while in the other it 
 is spread over the whole body and stretched over twenty- 
 four hours, and is therefore less intense at one time and 
 place. 
 
 2. Again, the living body may be compared to a 
 temple on which are constantly engaged two opposite 
 forces — the one tearing down, the other repairing; the 
 one rtVstructive, the other r^//structive ; the more rapid 
 the destruction, the more active the construction. These 
 two go on with varying success until at last the de- 
 
286 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 structive forces prevail and the struggle terminates in 
 death. 
 
 3. Or, again, the living body may be compared to a 
 pool, with its inlet and outlet. The form remains the 
 same, but the matter is continually changing. The more 
 rapid the change, the quicker and fresher is the water. 
 
 Waste Removal. — It is seen, then, that one necessity 
 arising from waste is food. But there is another and 
 far more urgent necessity — viz., the quick removal of 
 7vaste from the body. The reason is that the waste is 
 poisonous to the blood. Food-taking may be delayed for 
 a day or several days, or even perhaps for forty days; 
 but waste removal, suspended for five to ten minutes, 
 destroys life. 
 
 For the removal of waste there are two main pipes — 
 viz., the lungs and the kidneys. In the one case the re- 
 moval is by combustion, in the other by solution. In the 
 one the final product is gaseous, in the other liquid. 
 By far the largest amount — seven eighths of the whole* 
 — is removed by the lungs, and the urgency of this re- 
 moval is also the greatest. Stop the removal by the 
 lungs, and death occurs in five minutes; stop the elimi- 
 nation by the kidneys, and death occurs by blood 
 poisoning in about forty-eight hours. 
 
 Thus, to summarize, there are going on continually 
 in the living body two opposite processes— the one gath- 
 ering and constructive, the other destructive and removing; 
 the one ascensive from food to tissue, the other descensive 
 from tissue to ivaste removed. The one is called anabo- 
 lism, the other katabolistn. The one is nutrition proper, 
 the other is decomposition and elimination. The latter — 
 i. e., katabolism — is that which is most closely connected 
 with life; it is that which starts the whole process, that 
 
 * Berthelot, Rev. Sci., viii, 134, 1S97. 
 
NUTRITIVE FUNCTIONS. 287 
 
 which generates life force itself. Even the force neces- 
 sary for anabolism seems to be generated by katabolism. 
 The truth of this proposition we are not yet prepared to 
 prove, but will do so hereafter. 
 
 The whole subject of the nutritive functions — i. e., 
 all the functions concerned in the conservation of the 
 life and health of the individual — divides itself naturally 
 into three parts, viz., ascensive, distributive, and descensive. 
 The first is anabolic, the second intermediate, and the 
 third katobolic. The first includes all the processes 
 from crude food to finished tissue ; the third, all the pro- 
 cesses from perfect tissue through all its changes by 
 decomposition to final elimination from the body as 
 waste ; the second, or intermediate, all the processes 
 whereby food is distributed to all parts of the body and 
 also all waste is distributed, each kind to its appropriate 
 organ of elimination. Briefly, they may be called food 
 preparation, food and waste distribution, and waste re- 
 moval. Each is concerned with a distinct system of or- 
 gans — the first with the digestive system, the second with 
 the blood system, and the third with the excretory system. 
 
CHAPTER II. 
 
 NUTRITION PROPER ANABOLISM FOOD PREPARATION 
 
 — DIGESTIVE SYSTEM. 
 
 This includes all the changes from crude /^<?^ to fin- 
 ished tissue. 
 
 SECTION I. 
 Food : its Kinds and Uses. 
 
 Definition. — The word food is used in a wider and a 
 narrower sense. In the wider sense it includes all sub- 
 stances the ingestion of which is necessary to life. In 
 this sense it includes water and air and many salts. In 
 a narrower sense it means such substances as are used 
 for tissue building and iov force making. It is in the nar- 
 row sense that we shall use it here. 
 
 Kinds. — In this sense there are three kinds of food, 
 viz., albuminoids, amyloids, and/a/i-. The first are com- 
 posed of C, H, O, N, and sometimes a little P and S, and 
 are therefore called quaternary compounds, or often ni- 
 trogenous compounds; the second, of C, H, and O, the 
 two latter in proportions forming water, and are there- 
 fore called carbohydrates ; and the third, also of C, H, 
 and O, but the C in excess and the O in deficit. The 
 last two are called ternary compounds. Examples of 
 the first are found in albumen (white of egg), fibrin (lean 
 meat), gluten of wheat and other grains, legumin of 
 peas, beans, etc., and protoplasm, or living substance of 
 288 
 
NUTRITION PROPER. 
 
 289 
 
 animals and plants. Examples of the second are the 
 sugars and starches, and of the third all the animal and 
 vegetable fats and oils. The following schedule ex- 
 presses most of these facts : 
 
 Name. 
 
 Food 
 
 f Albuminoids 
 ■I Amyloids. . . . 
 I Fats 
 
 Composition. 
 
 j C, H, O, N, etc., Quater- 
 ( nai-y 
 
 C, H, O, Ternary 
 
 + - 
 
 C, H, O, Ternary 
 
 Examples. 
 
 {Fibrin, albumen, 
 protein, gluten, 
 protoplasm. 
 Starches and sugars 
 
 Fats and oils. 
 
 Milk. — The material prepared by Nature as the food 
 of the young of mammals must contain all these, .\lbu- 
 minoids are represented by the casein, or curd ; amyloids, 
 by the milk-sugar; and fats by the butter. 
 
 Uses of Food. — The uses of food are twofold, viz., 
 (i) for tissue-building — i.e., repair of waste in the adult 
 and repair and growth in the young; {2) iov force and 
 heat making by combustion, or, w*e may say briefly, tissue 
 food and/?/^/ food. This is expressed by schedule: 
 
 Food 
 
 Tissue. 
 Fuel. . , 
 
 I Repair. 
 ( Growth. 
 j Force. 
 / Heat. 
 
 We say fuel for force and /leat, but the real ob- 
 ject in the animal body, as in the steam engine, is 
 force, although, in both, heat is a necessary concomi- 
 tant. In the case of the body it is sometimes an in- 
 different concomitant, as in a moderate temperature ; 
 sometimes a very comfortable concomitant, as in cold 
 weather; and sometimes a distressing concomitant, as 
 in very hot weather. We will explain this more fully 
 hereafter. 
 
290 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANLMALS. 
 
 Distinctive Uses of the Kinds. — The albuminoids 
 are used mainly for tissue-building, and they alone can 
 be used for this purpose, for the tissues are themselves 
 albuminoid. But whatever of albuminoids is left over 
 from tissue-building is used for fuel also — i. e., for force 
 and heat making. The amyloids and fats can be used 
 only for force and heat, for these do not contain the 
 nitrogen necessary for the formation of tissue. Thus 
 an animal fed on amyloids and fats alone can not con- 
 tinue to live indelinitely for want of the necessary repair 
 of the tissues. This is called nitrogen starvatio?i. There- 
 fore tissue-making can be done only by albuminoids, 
 but force and heat making by all three kinds. 
 
 Illustrations. — Carnivores fed on lean meats con- 
 sume only albuminoids. They use first whatever is 
 necessary for repair of tissues, and whatever is left 
 over is burned for force and heat. Hertjivores take 
 albuminoids in, relatively, small quantity; their food is 
 mostly amyloids. They probably use the whole of their 
 albuminoids for repair, ^and their amyloids for force and 
 heat. Alan is an oninivore, but, except the livers on rice 
 or potatoes, he probably takes more albuminoid than 
 is necessary for repair. What is left over, which we 
 shall call albuminoid excess, he uses for force and heat. 
 All the amyloids and fats are used for the latter 
 purpose. 
 
 Waste Tissue. — But the waste is not wasted. This 
 also is burned as fuel. But since in the adult the ^vaste 
 is exactly equal to the repair, it is evident that the 
 equivalent of the whole* albuminoid food is burned as 
 fuel for force. But since amyloid and fats are also 
 burned, it is evident that the equivalent of the whole 
 food is burned for force and heat. 
 
 * Except an incombustible part, as explained hereafter. 
 
NUTRITION PROPER. 
 
 PREPARATION OF FOOD. 
 
 291 
 
 Food must he /prepared bei ore it can be absorbed into 
 the blood, because it is nearly always solid and must 
 be reduced to a liquid condition before it can be taken 
 up — it must be able to soak* through membranes. The 
 food of plants is already dissolved, as gases in the air 
 bathing the leaves, or as liquids bathing the roots. It 
 is therefore absorbed at once without further prepara- 
 tion. It is already prepared by Nature. But in animals 
 there must be a reservoir in which the solid food is 
 stored and dissolved. This reservoir is the stomach, and 
 the process of solution is digestion. But if we compare 
 the lower with the higher animals, we find, in accordance 
 with the law of differentiation, a gradually increasing 
 complexity in the process. In the lowest protozoa — 
 amoeba — the living protoplasm flows around the prey, 
 dissolves, and appropriates it at once into the tissues. 
 The captured prey passes at o>ie step from crude food to 
 living tissue, but the tissue thus summarily made is a 
 poor article. In the highest animals, on the contrary, this 
 simple process is differentiated into many consecutive 
 processes, and the finished article of tissue is far more 
 perfect. In man and the higher animals the steps are : 
 
 Mechanical process, i Ciiemical process. 
 
 1. Mouth digestion Insalivation. 1 Saccharization. 
 
 2. Stomach digestion Chymification. I Peptonization. 
 
 3. Intestinal digestion Chylification. ' Emulsification. 
 
 4. Absorption By capillaries and lacteals. 
 
 5. Sanguification By liver and mesenteric glands. 
 
 6. Circulation I 
 
 7. Assimilation [ 
 
 * We say soak, but it is necessary to remember that absorp- 
 tion is not a purely ///_;'j7V(7/ process, for it is selective. 
 
292 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 Our plan will be to take up each of these and carry 
 the process through the vertebrates ; and then in the 
 invertebrates to take up the whole process together in 
 each department. 
 
 SECTION II. 
 
 Mouth Digestion in Vertebrates. 
 
 This includes the gathering (prehension), the mastica- 
 tion, and the insalivation of food. Prehension in man, 
 monkeys, and some other mammals is done by the hands 
 or paws. In some, as the elephant, by the snout, but 
 
 in most vertebrates 
 directly by the 
 teeth. The object 
 oi mastication is trit- 
 uration of the food 
 for more perfect 
 insalivation, and is 
 done by the teeth. 
 We shall have 
 much to say about 
 this process in con- 
 nection with the 
 comparative mor- 
 phology of the 
 teeth and physiol- 
 ogy of mouth di- 
 gestion. For the 
 present we dwell 
 only on the process of insalivation and its effect on 
 digestion of the food. 
 
 Salivary Glands. — There are three pairs of these, 
 viz., the parotids, p (Fig. 178), on the side of the face 
 just below and a little in front of the ear; the submax- 
 
 FiG. 17S. — Lower jaw on right side and some 
 adjacent parts cut away so as to show the 
 salivary glands : p, parotid ; sm, submaxil- 
 lary ; i-/, sublingual. (After Cleland.) 
 
NUTRITION PROPER. 
 
 293 
 
 illaries, sm, just within the angle of the lower jaw on 
 each side; and the sublinguals, si, just behind the chin 
 on each side. Every gland has its excretory duct. That 
 of the parotids runs forward and opens into the mouth 
 between the cheek and the upper jaw teeth on each 
 side; that of the submaxillaries runs upward and opens 
 on each side of the back part of the tongue; while that 
 of the sublingual opens on each side of the frenum near 
 the tip of the tongue. 
 
 Structure. — Imagine a slender tube the size of a 
 knitting needle branching and rebranching to capillary 
 fineness, each capillary branch terminating in a saccule, 
 
 ep 
 
 Fig. 179. — Structure of a gland : i, 2, 3, 4, different stages in the process of 
 infolding of the epithelial surface, ep ; 4, fully formed gland. 
 
 the whole, even to the minutest branch and saccule, 
 lined with a pavement of living nucleated cells, an ex- 
 tension of the epithelium which lines the mouth cavity, 
 then the whole system of tubes webbed together by 
 loose connective tissue and invested with a membrane 
 of fibrous, or condensed connective tissue, and we have 
 a tolerably good general idea of the structure of a sali- 
 vary gland, which may indeed be taken as a type of a 
 
294 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 secreting gland. It is evidently only a device to bring 
 as large a surface as possible of epithelial cells into a 
 small space; for the secretion is a product of epithe- 
 lial action. Fig. 179 shows the different stages of in- 
 folding. 
 
 Hxcitant. — Any stimulation of the tongue and in- 
 terior of the mouth, even mechanical, will excite the 
 secretion, but especially any strong taste. Even the 
 sight of food or the idea of food will often cause it to 
 flow. The first effect of excitement is the rush of blood 
 to the gland, and then follows a flow of secretion. Evi- 
 dently the secretion is manufactured out of materials in 
 the blood. This preliminary rush of blood and conse- 
 quent swelling of the gland is the cause of the pain pro- 
 duced by food or even the sight of food in mumps, which 
 is a disease of the parotid gland. 
 
 Composition and Use of Saliva. — It is easy to 
 collect the salivary secretion in considerable quantity, 
 and thus to determine its composition. In a horse the 
 duct of the parotid gland runs just beneath the skin 
 over the broad, flat surface of the jaw and may be easily 
 taken up, a metallic tube introduced and turned out- 
 ward. If, now, food be given, or even a sheaf of hay be 
 shown, immediately the liquid secretion begins to pour 
 from the tube, and continues to pour as long as the food 
 is masticated. In this way half a pint or a pint of the 
 clear liquid may be collected. 
 
 Saliva is a watery liquid, consisting mainly of mucus 
 (water and broken-down epithelial cells), but containing 
 a peculiar ferment called //jy///;;, which is its active prin- 
 ciple ; its composition and its chemical properties seem to 
 be identical with diastase of sprouting seeds. Like dias- 
 tase, it changes the insoluble forms of amyloids (starch) 
 into the soluble forms (sugar). It does so by hydra- 
 tion of the starch. 
 
NUTRITION PROPER. 295 
 
 Starch. Water. .Sugar (glucose). 
 
 QH.0O3 + H,0 = QH„0, 
 
 Its function, then, is the digestion of amyloids. This, 
 of course, takes time. Starches, therefore, are dissolved 
 in the stomach, although the digestive juice is made in 
 the mouth. Ptyalin is far more important in herbivores 
 than in carnivores. 
 
 Ferments. — Ferments are of two general kinds — viz., 
 those, like yeast, that contain living microbes which de- 
 termine decomposition in the fermentmg substance, and 
 those, like diastase, that contain no microbes and deter- 
 mine change, but not decomposition. All the digestive 
 ferments are of this latter kind. They are called en- 
 zymes. 
 
 After mastication and thorough insalivation the food 
 is gathered into a bolus, pressed by the tongue into 
 the throat, and swallotved — i. e., it is there seized by 
 the involuntary muscles and hurried into the stomach. 
 There we leave it for the present to take up the 
 
 COMP.ARATIVE PHYSIOLOGY OF MOUTH DIGESTION IN 
 VERTEBRATES. 
 
 The chemical process of saccharization is precise- 
 ly the same in all vertebrates and probably in all 
 animals. The only important variation is in the me- 
 chanical processes of food-taking and mastication, espe- 
 cially the latter. This brings us to the important 
 subject of 
 
 Teeth invertebrates. — We take up this somewhat 
 fully on account of its important bearing on classifica- 
 tion, especially of mammals. The character of the teeth 
 is determined by the food, and the nature of the food 
 determines the habits, and therefore the whole structure 
 of the animal. All the parts of an animal are in har- 
 monic relation with one another. The keynote of this 
 
296 PHYSIOLOGY AND MORPHOLOGY OF ANLMALS. 
 
 complex harmony is the teeth. Next to the teeth, foot 
 structure is most important in classification. 
 
 Mammalian Teeth. — i. Origin and Development. — In 
 speaking of the skeleton we said (page 224) that teeth 
 do not belong to the true internal skeleton, but are an 
 
 Fig. 180. — I, 2, 3, 4, 5, 6, 7, showing successive stages in the development 
 of a human incisor : //, tooth pulp ; ts, tooth sac. 
 
 epidermal structure, or, more specifically, an epithelial, 
 a gum structure. The evidence of this is found in their 
 embryonic development. The following figures (Fig. 
 
NUTRITION PROPER. 297 
 
 180, I, 2, 3, etc.) show the embryonic development of 
 teeth in man. The same process is true of all mammals. 
 At first there is a little papilla on the gum, which is 
 the pulp of the future tooth (i). Then this is sunk more 
 and more into the gum, to become the future socket (2). 
 Then the pulp begins to secrete the tooth, and the socket 
 closes up above, forming the tooth (sac 3, 4, and 5), and 
 the tooth is now entirely inclosed in the gum and in the 
 jawbone. Then by continued growth it breaks through 
 the jaw and the gum and the tooth is cut, and thence 
 continues to grow to its full size (6 and 7). But some 
 teeth — milk teeth — are shed and replaced by permanent. 
 Are these also gum structures ? Yes. It is seen that 
 the tooth sac has a saccule on each side. One of these 
 continues to develop, and a tooth is formed in it. This 
 grows, and finally pushes out the first tooth and takes 
 .its place (5, 6, 7). In some rare cases the other saccule 
 also forms a tooth, which may develop. It is in this 
 way that we account for those rare cases of a third set 
 of teeth. But what is exceptional in man is the rule 
 in many reptiles and in sharks, as we shall see here- 
 after. 
 
 2. Composition of Teeth. — Mammalian teeth consist usu- 
 ally of three kinds of substance — viz., dentine, enamel, and 
 cement. The dentine is a denser kind of bone, already 
 described, and forms the principal part ; the enamel is a 
 still denser variety, and covers the crown or exposed 
 part ; and the whole is covered and the inequalities 
 filled up with cement, which is a more structureless 
 variety than either. The cement is almost wanting in 
 many teeth, as man's, and in such cases is quickly worn 
 off and disappears, but is an important part of the more 
 specialized teeth of many mammals. 
 
 3. Kinds of Teeth. — There are four kinds of teeth in 
 the jaws of mammals — viz., (i) the incisors, or front 
 
298 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 teeth, (2) the canines (tusks of carnivores, eye teeth of 
 man), (3) //rwtp/rtrj-, or deciduous molars or bicuspids of 
 man, and (4) the permanent or true molars. The func- 
 tion of the first is the cutting off of morsels of food, of 
 the second is seizing and holding the prey. The jaw teeth 
 — i. e., the molars and premolars — are the true masticat- 
 ing teeth. This grouping of the teeth into kinds having 
 different functions is very characteristic of mammals. 
 
 4. Variation of the Teeth. — The teeth of different or- 
 ders and families of mammals vary in relative size and 
 relative number of the kinds, and in the structure of the 
 molars. 
 
 (a) Relative Size. — In man all the teeth are of similar 
 size, forming thus a continuous even arch; but in the 
 more specialized mammals some of the teeth may be 
 enormously developed — for example, the canines in car- 
 nivores, and especially the walrus; the incisors in ro- 
 dents, and especially in proboscidians (elephants). In 
 cases of enormous development, such as the incisors of 
 rodents and the tusks of the walrus, the boar, and the 
 elephant, the pulp is permanent, and the tooth grows 
 continuously. Such teeth have a hollow at the base. 
 Thus, teeth are sometimes of definite and sometimes of 
 indefinite growth. 
 
 [b) N' umber ofi Teeth and Relative Number of the Kinds. 
 — The normal number of mammalian teeth seems to be 
 forty-four, and any less number must be regarded as the 
 result of gradual loss; precisely as in the case of toes 
 of less number than the normal five. But the whole 
 number, and especially the relative number of the sev- 
 eral kinds, vary in a way which is very characteristic 
 of the different orders and families of mammals. This 
 introduces the subject of the dental formula, which is a 
 compendious way of expressing the number of different 
 kinds of teeth in mammals, very necessary in descrip- 
 
NUTRITION PROPER. 
 
 299 
 
 tion of mammals, and therefore universally used by 
 naturalists. A number of these are given below : 
 
 Type 
 
 i -^"^ ■ 
 3—3 ' 
 
 c, 
 
 I — I 
 I — I ' 
 
 pm., 
 
 4—4. 
 4—4' 
 
 m. 
 
 ■3-3 ''■ 
 
 Man 
 
 ■ 2—2 
 ' 2—2 ' 
 
 c, 
 
 I — I 
 
 I — I ' 
 
 pm., 
 
 2 — 2 
 2—2' 
 
 m., 
 
 ■P=- 
 
 Bear 
 
 J 3 . 
 1., ^ , 
 
 c. 
 
 I 
 I ' 
 
 pm.. 
 
 4 . 
 4 
 
 m., 
 
 ^ I =- 
 
 CaU 
 
 J 3 . 
 ' 3 ' 
 
 c, 
 
 I 
 I 
 
 pm., 
 
 3 . 
 2 ' 
 
 m., 
 
 I 
 
 
 
 
 c, 
 
 
 I ' 
 
 pm., 
 
 3 . 
 3 
 
 m., 
 
 .-f=3. 
 
 
 3 
 
 Rodent 
 
 2 
 
 c, 
 
 
 
 
 pm., 
 
 3 . 
 2 ' 
 
 m., 
 
 3 -"8 
 
 
 ' I ' 
 
 3 
 
 Sloth , 
 
 
 ' 
 
 c, 
 
 I 
 I ' 
 
 
 
 m., 
 
 ^ -13 
 
 
 
 
 3 
 
 00 00 
 
 Ant-eater 1., ; c., ; = o. 
 
 00 00 
 
 Ornithorhynchus O. 
 
 To explain : In the type mammal, probably in all the 
 early Tertiary mammals and in the most generalized 
 mammals, like the hog 7io7v (Fig. 183), there are forty- 
 four teeth in all, of which twelve are incisors — i. e., three 
 
 on each side above and below, 1. = ; four are ca- 
 
 3 — Z 
 
 nines — i. e., one on each side above and below, c. = 
 
 ; sixteen are premolars — i.e., four on each side 
 
 I — I 
 
 4 — 4 
 above and below, pm. = ; and twelve are molars 
 
 4 — 4 
 
 •7 — 2 
 
 — i. e., three on each side above and below, m. = —. 
 
 But, as the bilateral symmetry is always perfect in the 
 teeth, there is no necessity to express but one side — i. e., 
 all the teeth as seen from one side, as in all the formulae, 
 except the first two. 
 
 The normal number, we have said, is forty-four. 
 
300 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 This was probably the number in the early Tertiary 
 mammals ; any less number is the result of gradual abor- 
 tion. Thus ruminants have no front teeth in the upper 
 jaw, but the rudiments of these buried in the jawbone 
 unborn show that they came from animals having a full 
 set. The same is true, as already explained (page 260), 
 of whales, and also of edentates, etc. 
 
 (c) Structure of Molars. — Molars are the masticatory 
 teeth, and are therefore, more than all others, subject 
 
 to variation accord- 
 ing to the character 
 of the food. In this 
 regard there are three 
 main kinds, viz., om- 
 nivorous., carnivorous, 
 and herbivorous. Om- 
 nivorous molars are 
 simply tuberculated, 
 and are equally adapt- 
 ed to all kinds of 
 food, but not specially and perfectly adapted to any one 
 kind. Such are the teeth of man, of monkeys, of bears, 
 and the hog (Figs. 181, 182, 183). 
 
 Fig. 181. — View of teeth of the right side 
 of the upper jaw of man. 
 
 Fig. 182.— Teeth of the right side of the upper jaw of a monkey. 
 
 Carnivorous molars are specially adapted for flesh 
 eating. They only crush2iX\6. divide the food sufficiently 
 for swallowing, but do not grind or triturate it. This is 
 
NUTRITION PROPER. 
 
 301 
 
 not necessary, because thorough insalivation is not re- 
 quired for flesh-food (Fig. 184). 
 
 Herbivorous molars are by far the most specialized 
 and complex, because their food requires the most com- 
 plete trituration and insalivation (Fig. 185). 
 
 Fig. 183. — Teeth of the right side of the upper jaw of a hog. 
 
 Undoubtedly the primal mammal was omnivorous 
 and had simple tuberculated molars. From this gener- 
 
 FiG. 184. — Side view of the upper jaw of a dog. 
 
 alized form, as time went on, mammals were specialized 
 in two main directions — the one more and more adapted 
 
 Fig. 185. — Face view of the upper jaw of a sheep. 
 
 to flesh eating, the other to herb eating. The extreme 
 forms are represented now by the cat tribe on the one 
 hand and the ruminants on the other. In the meanwhile 
 
302 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 an intermediate type continued in a more generalized 
 form, including man. The diagram shows these facts: 
 
 /Carnivorous. 
 
 Omnivorousc . . . Monkey, man, etc. 
 ^ Herbivorous. 
 
 Structure of Herbivorous Molars. — The most 
 interesting examples of masticatory teeth are found 
 in herbivores. In these we have veritable upper and 
 nether millstones. Fig. i86 is a face view of a molar 
 of a horse. The double lines are enamel, the shaded 
 spaces dentine, and the unshaded cement. On account 
 of its greater hardness, the enamel stands out as ridges 
 above the softer dentine and cement, and continues to 
 do so however much the tooth wears. 
 
 Fig. i86. — Grinding; face of 
 a horse's molar. 
 
 Fig. 187. — Incisor of a horse : 
 A, vertical ; B, cross section. 
 
 Origin of this Structure. — The dentine is secreted 
 by the tooth pulp, the enamel by the membranes of the 
 tooth sac. Therefore the tooth sac must have followed 
 the enamel in all its windings. The complexity of 
 structure is therefore the result of tnfoidi/igs of the sac 
 on the side and down- dippings of the same from above. 
 The cement is afterward formed on the outside, and, as 
 it were, poured over all, filling up the inequalities. A 
 
NUTRITION PROPER. 
 
 303 
 
 simple case of the down-dipping is seen in the front 
 teeth of the horse. In this case the down-dipping (Fig. 
 187, A) determines on cross section a concentric arrange- 
 ment of the enamel and cement (Fig. 187, B), which suc- 
 cessively disappear as, by use, the tooth wears to lower 
 and lower level [a l>, c d, ef), first 
 the cement, and then the enamel, 
 until only the dentine remains. 
 These changes occur first in the 
 middle front teeth, and then in the 
 side teeth. On this fact is founded 
 the mode of estimating the age of 
 horses by the teeth. 
 
 In the horse and cow the ar- 
 rangement of the enamel plates 
 among the dentine and cement is 
 adapted for side-to-side grinding, 
 but some mammals have fore-and- 
 aft grinding. In these the enamel 
 
 plates are transverse. This is well seen in rodents (Fig. 
 188), and especially in the elephant. The molar of an 
 
 Fig. 188.— The grinding 
 face of a molar of a paca 
 ( Caelos^enys) of South 
 America, enlarged. 
 
 Fig. 189. — Elephant's molar : A, showing side and grinding face ; B, section 
 showing the plates ; s .f, original surface ; a b, worn to a face as in A. 
 
304 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 elephant is the most efficient fore-and-aft grinder imagi- 
 nable. The whole tooth, as seen (Fig. 189), is composed 
 of narrow transverse islands of dentine, surrounded by 
 ridges or plates of enamel separated by cement. The 
 section shows how^ this has been formed. Imagine a 
 mass of dentine from which spring many thin plates of 
 the same, each plate sheathed with enamel, and then 
 cement poured over the w'hole, and finally the tooth 
 subjected to wear. 
 
 Mouth Armature of Whales. — Many cetaceans — 
 e. g., the sperm whales, porpoises, etc. — have teeth, but 
 these are conical, prehensile, not masticatory, teeth. 
 But the baleen (whalebone) whales have no teeth, but 
 in their stead have the most efficient food-taking ap- 
 paratus known. These animals have enormous heads 
 
 A B 
 
 Fig. 190. — Head of a whale : A, side view ; B, section. 
 
 (nearly half the whole body), fifteen to twenty feet long 
 and ten to fifteen feet deep, and nearly the whole of this 
 great head is mouth. This huge cavern is largely occu- 
 pied with whalebone plates (Fig. 190). 
 
 These horny plates, hundreds in number, are at- 
 tached above to the roof of the mouth, hang down, 
 and are split up into fibers at their edges, so that 
 the open mouth is like a moss-roofed cavern. The 
 hollow space beneath is filled up by the enormous 
 tongue. 
 
NUTRITION PROPER. 
 
 305 
 
 Mode of Feeding. — The baleen whales feed on 
 squids, cuttlefish, medusae, and small crustaceans, which 
 exist in enormous numbers in arctic seas, usually near 
 the surface. The whale rushes forward at great speed 
 with mouth open, so the water pours like a torrent into 
 the mouth and out at the sides between the plates. All 
 the surface animals are caught on the mossy roof and 
 sides. When a sufficient quantity is gathered the mouth 
 is closed, the superfluous water is spouted through the 
 blowhole (nostril), and the prey is swept up by the 
 tongue and swallowed. 
 
 Homology of Baleen Plates. — These plates are a 
 substitute for, not a modification of, teeth. They are 
 therefore analogous, but not homologous with teeth. As 
 already explained, these 
 whales have rudiments 
 of teeth, which are nev- 
 er cut. What, then, are 
 the plates homologous 
 with ? They are prob- 
 ably extreme modifica- 
 tions of gu^ii ridges — 
 such, e. g., as those found 
 on the mouth-roof of a 
 horse. If each of such 
 ridges produced a down- 
 ward growth of horny 
 tissue we should have 
 something like the ba- 
 leen plates of the whale. 
 
 Birds. — Existing 
 birds have no teeth, yet 
 in the embryo of some 
 
 birds rudimentary teeth are found which are never 
 developed. This is, of course, strong presumptive evi- 
 
 FiG. 195. — Ichthyornis victor, x J^. 
 (^Restored by Marsh.) 
 
3o6 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 dence that birds once had teeth, but they have gradually- 
 dwindled and passed away, because another apparatus 
 — viz., a horny beak — was used in its place. This pre- 
 sumption is confirmed by the finding of Jurassic and 
 Cretaceous birds with the mouth full of teeth — true sock- 
 eted \.^&\.\i (Fig. 191). These, however, were conical, pre- 
 hensile, not masticatory teeth. 
 
 Reptiles. — There is great variety among reptiles in 
 this regard. This was to be expected, for they are the 
 ancestors of both birds and mammals. Some, as the tur- 
 tles, have no teeth, but, like birds, a horny, nipping beak 
 in their place. Some, like serpents, have teeth not only 
 in the jaws, but on other bones of the mouth, as, e. g., 
 the palatal. But in all reptiles which have teeth these 
 are conical, prehensile, and not masticatory teeth. Also, 
 with the exception of crocodilians, the teeth are not sock- 
 eted. They are formed in a fold of the gum, and after- 
 ward fixed to, but not sunk into and inclosed by the 
 jawbone. Among extinct reptiles socketed teeth were 
 more common. In serpents the teeth all point back- 
 ward. This is necessary in swallowing large prey, which, 
 by a peculiar movableness of the bones of the head, are 
 thus dragged by main force down the throat. 
 
 Fangs of Serpents. — These are worthy of brief 
 notice as an example of admirable adaptive modification. 
 First, observe that the canal in the tooth which conveys 
 the poison does not go to the end, for that would inter- 
 fere with the keenness of the point, but comes out a lit- 
 tle short of the extreme end. The same device is used 
 in the subcutaneous injector of the surgeon. Observe, 
 second, that the formation of the poison canal is not in 
 violation of the ordinary structure of teeth, but a curi- 
 ous modification of it. It is not along the tooth cavity, 
 but is really outside of the tooth. Fig. 192, a, represents 
 a section of a flat tooth. Suppose such a tooth bent 
 
NUTRITION PROPER 
 
 upward (^ and c) until the 
 edges meet above and are sol- 
 dered ; // is the poison tube. 
 That this peculiar structure 
 was gradually formed is 
 shown by the fact that all 
 the steps of the process may pf- 
 be found among living ser- 
 pents — viz., teeth slightly 
 grooved, deeply grooved, 
 and tubulated. All degrees 
 of virulence of the poison 
 may be found. The poison 
 gland is probably an extreme 
 modification of a salivary 
 gland, and the poison of 
 saliva. The saliva is slightly 
 poisonous in many animals. 
 
 Observe again, t/ii'rd, that 
 the fang is formed in a fold 
 in the gum, and at first loose, 
 but afterward fi.xed to the jawbone. Also that in the 
 fold there are many subordinate folds, in each of which 
 
 Fig. 192. — A longitudinal section 
 of the fang of a serpent : the 
 cross sections show mode of 
 formation of the poison tube ; 
 //■, poison tube ; tc, tooth cavity ; 
 /jf, poison gland. 
 
 'm 
 
 Fig. 193. -Head of a rattlesnake, showing^ several fangs in different stages 
 of development. The arrow shows the poison tube ; m, ma.xillary bone. 
 
3o8 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 is formed a tooth, and thus there is a magazine of teeth 
 of all sizes, which are successively brought forward and 
 attached as the old tooth drops off (Fig. 193). 
 
 Origin of Mammalian Teeth. — A characteristic 
 of mammalian teeth is that they are differentiated into 
 three groups, distinct in form and in function, viz., incisors 
 for cutting, canines for seizing, and molars and premolars 
 for masticating. The complex structure of these last 
 is especially significant. Now in some early extinct rep- 
 tiles of the Permian and Triassic periods, immediately 
 before the appearance of mammals, the teeth were already 
 
 Fig. 194. — Head of Cynog-nathus, a Triassic reptile, showing teeth similar 
 to those of mammals. (From Woodward.) 
 
 differentiated into the three groups, and the jaw teeth 
 were already become molariform (Fig. 194). Indeed, a 
 complete series may be traced from the simple conical 
 prehensile teeth of ordinary reptiles to the most com- 
 plex grinders of ruminants. It is certain, therefore, that 
 mammalian teeth have come by gradual modification 
 from simple prehensile teeth of reptiles. 
 
 Fishes. — The teeth of fishes are of three kinds — viz., 
 conical^ lancet-shaped^ q.y\<^ pavement teeth. The conical are 
 the commonest; they are prehensile only. The lancet- 
 shaped teeth are very characteristic of sharks. They are 
 
NUTRITION PROPER. 
 
 309 
 
 interesting as examples of magazines of teeth of all sizes, 
 and a successive dropping of old and a coming forward 
 of new to their place. In a shark's jaw there is a 
 magazine of many 
 hundreds of teeth, 
 growing smaller as 
 we pass inward 
 from the edge of 
 the jaw (Fig. 195). 
 Only the large 
 teeth of the outer 
 rows are in use 
 at one time ; but 
 there is a contin- 
 ual growing outward of the gum, carrying the teeth with 
 it in the direction of the arrow. Sharks' teeth are very 
 interesting in another respect — viz., as showing their 
 
 Fig. 195. — Section of lower jaw of a shark, show- 
 ing the magazine of teeth. The arrows show 
 the direction of replacement. 
 
 Fig. 196. — Jaw of Port Jackson shark Fig. 197. — Jaw of a skate (Hylo- 
 (Cestraceon)^ showing pavement of bates), showing tesselated pave- 
 
 rounded teeth. (Owen.) ment of teeth. (From Owen.) 
 
 homology with scales, which are, of course, a skin struc- 
 ture. Every gradation from one to the other can be 
 traced over the edge of the jaw of some sharks. 
 
 Pavement teeth also may be of several kinds, notably 
 what might be called cobble-stone pavement (Fig. 196) 
 
310 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 and tessellated pavement (Fig. 197), such as are found in 
 skates. Pavement teeth are used for crushing shells 
 before swallowing. They are interesting as the sim- 
 plest form of teeth, and as showing their origin from 
 the gum. They are a secretion on the surface of the 
 gum, and afterward fixed to the subjacent bone. 
 
 SECTION III. 
 
 Stomach Digestion ; Chymification ; Peptonization. 
 
 As already seen, after mastication and insalivation, 
 the food is gathered by the tongue into a bolus and 
 pressed into the throat (so much is voluntary) ; then 
 seized by the involuntary muscles and rushed down 
 through the gullet (oesophagus) into the stomach, to 
 undergo there the second stage of preparation. The 
 oesophagus is a muscular tube about ten inches long and 
 nearly an inch in diameter. The muscular fibers are 
 mostly circular or ring fibers. We have already said 
 that a characteristic of involuntary muscles is a con- 
 secutive contraction of fibers, producing propagated 
 waves of contraction in one direction. Such a wave of 
 contraction propagated downward carries the food be- 
 fore it to the stomach. It is a strong, water-tight con- 
 traction, as shown by the fact that long-necked animals, 
 like the horse, swallow water upward from the ground 
 in drinking. The normal direction of waves is down- 
 ward or stomachward. These are called peristaltic. 
 Sometimes — abnormally in man, as in vomiting, but nor- 
 mally in ruminants, as in bringing up the cud — the waves 
 may run in the contrary direction. These are called 
 antiperistaltic. 
 
 Saccharization of the Food. — This belongs to 
 mouth digestion, for the saliva is the digestive juice for 
 
NUTRITION PROPER. 
 
 ^11 
 
 starch. If the food remained long enough in the mouth 
 the saccharization would take place there, but in fact 
 it takes place in the stomach, although the stomach has 
 really nothing to do with the process. On the contrary, 
 the gastric juice by its acidity rather checks the saccha- 
 rization of starch, so that this change takes place mainly 
 in the early stages of stomach digestion before gastric 
 juice is yet formed in large quantities. 
 
 The Stomach. — The. positio?i is just below the dia- 
 phragm and behind the triangular space in front called 
 
 Fig. 198. Stomach of man ; a section showing form and interior surface : 
 oe, cesophagus ; /, pylorus ; bd, bile duct ; gbl, gall bladder ; ps, pan- 
 creas. 
 
 the pit of the stomach, but a little more to the left side. 
 Its shape js seen in Fig. 198. Of its two openings, that 
 leading into the oesophagus is called the cardiac, and 
 that leading into the inte'^tino'^ \\\e pylorif orifice. 
 
312 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 Coats. — The stomach consists of three coats — an 
 investing serous coat, very smooth, thin, and tough; a 
 middle muscular coat ; and a lining mucous coat. The mus- 
 cular coat is by far the thickest, so that the organ may 
 be called a hollow muscle, invested with serous mem- 
 brane and Imed with mucous membrane. The function 
 of the serous coat is to give toughness and easy gliding 
 without friction over other organs in the abdomen. The 
 function of the muscular coat is to do the mechanical work, 
 and of the mucous coat the chemical work of digestion. 
 
 Mechanical Work ; Chymification. — The me- 
 chanical work of mixing the food with the gastric juice 
 is done by the muscular coat. This consists of several 
 sheets of parallel fibers running in different directions, 
 longitudinal, transverse or ring, and oblique fibers. 
 Under the contraction of these the stomach is seen to 
 squirm, and especially to transmit light waves of annu- 
 lar contraction, chasing one another from the cardiac to 
 the pyloric end. By these contractions the food is gen- 
 tly urged along the. greater curvature to the pylorus and 
 back along the lesser curvature to the cardiac orifice, 
 and so on repeatedly until the digestion is complete. 
 
 The pylorus (gate keeper) is a strong collection of 
 circular fibers at the outgoing orifice of the stomach. 
 During digestion it is completely closed. After two to 
 five hours, depending on the nature of the food and the 
 digestive power of the stomach, the digested food is 
 allowed to pass the pylorus; but if any undigested por- 
 tions appear the pylorus closes, and sends it on its way 
 round again until in a proper condition. But sometimes, 
 by repeated application, the gate keeper is, as it were, 
 teased into compliance, and even undigested matter may be 
 allowed to pass, and may create trouble farther on. 
 The final result of this process is a grayish, slightly acid, 
 semiliquid mass, about the consistence and somewhat 
 
NUTRITION PROPER. 
 
 13 
 
 the appearance of pea soup, and called chyme. This is 
 passed on to the intestines, to undergo the next stage. 
 
 Chemical Work ; Peptonization. — The gastric 
 juice is elaborated by the whole mucous membrane of 
 the stomach, but especially by the peptic glands. 
 Glands, as already explained (page 294), are a device for 
 increasing the extent of the epithelial surface. In this 
 case the device is of the simplest sort. The interior of 
 the stomach is thickly strewed with deep///i- lined with 
 
 Fig. 199. — A, interior of stomach, showing the openings of the peptic 
 glands ; B, section through the walls. 
 
 epithelium (Fig. 199). Some of these pitlike tubes are 
 branched, but only simply. The epithelial surface is 
 thus many times greater than the interior surface of 
 the stomach. The gastric juice is secreted mainly in 
 these pits, both simple and branched. As soon as the 
 food touches the stomach the mucous membrane be- 
 comes engorged with blood and reddened, and the gas- 
 tric secretion commences. 
 
314 PHYSIOLOGY AND MORPHOLOGY OF ANLMALS. 
 
 Composition and Uses of Gastric Juice. — Acci- 
 dental woundings of the stomach have afforded means 
 of collecting the gastric juice of man in considerable 
 quantity ; that of animals has been collected in vivisec- 
 tion experiments. The secretion, therefore, has been 
 analyzed. It consists of mucus, with a little free acid, 
 hydrochloric and lactic, and a peculiar ferment, an en- 
 zyme called/<f/j-/;;, which has the property of dissolving 
 albuminoids. This it does by hydration. The soluble 
 forms of albuminoids thus formed are called peptones, 
 and the process of change peptonization. In some 
 cases of weak digestion pepsin made from a calf's stom- 
 ach may be used as medicine with good effect. 
 
 In both saccharization and peptonization the process 
 is purely chemical, and takes place just as well in a warm 
 flask as in the stomach. The true vital process is the 
 formation of the ferment, not the change effected by it 
 on the food. 
 
 Effect on Milk. — The effect of pepsin on milk is 
 very characteristic. It first curdles, and then dissolves 
 it. The albuminoid — casein — in milk is in a liquid state 
 and apparently suitable for direct absorption. But albu- 
 minoids in their natural state are unstable and liable 
 to pass into a solid. Therefore the casein is first solidi- 
 fied and then changed into peptones, in which state it is 
 no longer liable to solidification. Advantage is taken 
 of this property of curdling milk in the manufacture of 
 cheese. Fresh milk is treated with a small quantity of 
 an extract of rennet (calf stomach) ; the quantity used 
 is sufficient to curdle, but not enough to dissolve the 
 casein. 
 
 Absorption. — Water, alcohol, perhaps to some ex- 
 tent sugar, may be taken up directly by the capillaries 
 of the stomach into the blood. But absorption is the 
 special function of the intestines. The chyme is there- 
 
NUTRITION PROPER. 
 
 315 
 
 fore passed on into the intestines to undergo the third 
 stage of food preparation. There we leave it for the 
 present while we take up the 
 
 Comparative Physiology of the Stomach. — The 
 chemical process of digestion is the same, and the appa- 
 ratus nearly the same in all vertebrates. There are 
 only two modifications sufficiently important to arrest 
 our attention — viz., that of ruminant mammals and that 
 of granivorous birds. 
 
 Ruminants. — The stomach of ruminants (Fig. 200) 
 is very complex, and the whole digestive process very 
 elaborate. The stomach consists of four parts — viz. : 
 
 Fig. 200. — Stomach of a sheep, partly cut open so as to show the interior: 
 oe, oesophagus ; v, valvular opening from the oesophagus to the omasum. 
 
 (i) the rumefi or paunch; (2) the reticulum or honey- 
 comb ; (3) the psalferiutn or omasum (psalter, or book, 
 or manyplies) ; and (4) the abomasutn or rennet. The 
 rumen or paunch (i) is of immense size, and its func- 
 tion is to store the half-chewed food and soften it by 
 maceration. It is thick and muscular, and constitutes 
 what is called the tripe. The reticulum (2) may be re- 
 garded as an appendage of the paunch, and its function 
 is probably to prepare a macerating liquid and perhaps 
 also to make up the cud-balls. It is full of deep pits, 
 
3i6 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 like a honeycomb. The psalter- or manyplies (3), as its 
 name indicates, has its mucous membrane thrown into 
 many wide and thin folds, like the leaves of a book. It 
 is probable that the digestion of starch (saccharization) 
 takes place here, and is all the more complete because 
 the acid gastric juice is not secreted until we reach the 
 last compartment. Finally, the abomasum or rennet (4) 
 is the true and final digestive stomach, where pepsin is 
 formed and peptonization takes place. 
 
 We see, then, that several functions which are com- 
 bined in the same organ in most animals are here dif- 
 ferentiated with more perfect results. The reason of 
 this is found in the nature of the food and the habits of 
 the animals. The food consists of grasses and herbage, 
 in which the amount of nutritious matter is so small that 
 they must take a very large quantity, and therefore the 
 stomach must be correspondingly large. But again, on 
 account of the small percentage of nutritious matter, 
 the food must be very thoroughly triturated and the di- 
 gestive process very complete so as to extract it all. 
 We have already seen how well their teeth are adapted 
 to thorough trituration; we see also how the stomach is 
 adapted to perfect digestion. But perfect trituration of 
 so large an amount of food would take much time, and 
 these animals are timid and preyed upon by carnivores. 
 Therefore they are compelled to take their food rapidly, 
 imperfectly chewing and hastily packing it away in the 
 paunch, where it is soaked and softened. Then at their 
 leisure they lie in some concealed place and naiiinate, or 
 chew the cud. Every one must have observed the pro- 
 cess in domestic animals, and perhaps envied their pla- 
 cidity of mind. If we observe closely we see the chew- 
 ing stop a while; the bolus goes down the gullet by 
 peristalsis, then another comes up by antiperistalsis, 
 and the chewing recommences and continues until the 
 
NUTRITION PROPER. 
 
 317 
 
 cud is reduced to a fine, smooth paste, and again swal- 
 lowed. 
 
 Now, when the imperfectly chewed food is swallowed 
 the first time it finds a broad open way to the paunch ; 
 but when, after perfect chewing, it is swallowed the sec- 
 ond time the powerful muscles about the oesophageal 
 orifice of the stomach, by a reflex action little under- 
 stood, contract in such wise as to bring the orifice of the 
 oesophagus directly into contact with the opening into the 
 manyplies, and the food passes into this compartment. 
 
 Evolution of Ruminant Stomach. — We have al- 
 ready seen that the teeth of ruminants were only gradu- 
 ally developed in geologic times from a simple tubercu- 
 lated structure into the complex grinders which we now 
 find. The same is true of the 
 complex stomach, but the evi- 
 dence is less complete, because 
 the stomachs of extinct animals 
 are not preserved. Nevertheless, \ P 
 
 some stages are still found in ex- 
 isting animals. In all mammals. Fig. 201.— Stomach of a 
 
 , . ... , . , rat : r, cardiac ; p, py- 
 
 even m man, there is a slight dif- lork portion, 
 ference of function in the cardiac 
 
 and pyloric ends of the stomach. In many, as the horse, 
 there is a strong line of demarcation between them. In 
 others, as rodents (Fig. 201), there is a strong hourglass 
 contraction between; but nowhere is the differentiation 
 so marked as in ruminants. 
 
 Granivorous Birds. — The food of grain-eating 
 birds is hard and requires thorough trituration, yet birds 
 have no teeth, and therefore mastication and insaliva- 
 tion can not take place in the mouth. Their food is 
 swallowed whole and insalivated in the crop, and masti- 
 cated in the gizzard. Fig. 202 gives the whole appara- 
 tus. It consists of three parts — viz., (i) the crop {inglu- 
 
3i8 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 vies), a reservoir for storing and softening the food by the 
 saliva; (2) the, prove ntriculus, v{\\\ch. furnishes the peptic 
 juice; and (3) the gizzard, or ventricidus, or gigerium, 
 which triturates the food and mixes it thoroughly with 
 saliva and gastric juice furnished in the parts above. The 
 
 gizzard consists of two very 
 powerful muscles, provided 
 
 Fig. 202. — Digestive apparatus of a Fig. ■2o\. — Tiie gizzard of a goose 
 
 granivorous bird : cr, crop ; pv, cut open : tp, triturating pad. 
 
 proventriculus ; g, gizzard; /, in- (After Owen.) 
 testine. 
 
 each with a cushion or pad, which rub against one an- 
 other (Fig. 203). The cavity is small and lined with a 
 hard, almost horny skin. Gravel is taken as grinders 
 to this mill, and renewed as required. The whole pro- 
 cess is briefly as follows: The food is first stored and 
 insalivated and softened in the crop. The crop acts as 
 a hopper to the gizzard mill, dropping little by little as 
 required. The digestive juice secreted by the proven- 
 triculus is also added little by little as required. The 
 triturated and digested material tiiially passes on to the 
 intestines. 
 
 Evolution of this Apparatus. — This elaborate ap- 
 paratus is most perfect in grain-eating birds ; but all 
 grades approaching it may be found in birds from the 
 simple thin sac of the flesh-eating to the powerful mill 
 of the grain-eating, 
 
NUTRITION PROPER. 
 
 319 
 
 SECTION IV. 
 Intestinal Digestion : Chylification ; Emiilsification. 
 
 Intestines ; Form, Structure, and Relations to 
 the Abdominal Cavity.— I'he intestines consist of a 
 long, slender tube — in man about thirty-five feet long 
 and one inch to two inches in diameter. It is divided 
 into two very distinct parts — viz., the small and large 
 intestines. These differ in size, the small being about 
 one inch, the large two inches in diameter. They differ 
 also in appearance, the first being smooth, cylindrical, 
 the second puckered and sacculated by a strong mus- 
 
 FiG. 204. — The junction of the small and large intestines : si, small intes- 
 tines ; //, large mtestines ; cae, caecum ; vap, vermiform appendage. 
 
 cular band (Fig. 204). The true process of digestion is 
 substantially completed in the former. The function of 
 the latter is not well understood, but both the charac- 
 teristic color and odor of excrements are taken on here. 
 Also the peculiar shape of the balls m the horse or pel- 
 lets of sheep and goats are given here. 
 
 The two parts do not grade continuously the one 
 into the other. On the contrary, the small open into the 
 
320 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 large on one side a little way from the end by a valve 
 (ileocsecal valve; Fig. 204). Each of these two parts are 
 again subdivided into three, as shown in the schedule: 
 
 i Duodenum. t Caecum. 
 
 Jejunum. Large -s Colon. 
 
 Ileum. ' Rectum. 
 
 The duodenum is the part next the stomach, seen in 
 Fig. 198, into which are poured the digestive juices 
 from the liver and pancreas. It is the largest and 
 shortest part of the small intestines, being about ten 
 inches long and two inches in diameter. The jejunum 
 and ileum grade completely into one another both in 
 structure and function. Of the large intestines, the 
 caecum is the somewhat enlarged blind extremity into 
 the side of which the ileum opens (Fig. 204) by the ileo- 
 caecal valve. Attached to the blind extremity there is 
 a curious, wormlike appendage, which by inflammation 
 gives rise to the grave disease (appendicitis) which is 
 now attracting so much attention. The rectum is the 
 last or lower part, about six inches long, and cylindrical 
 in form, opening through the anus. The colon is the 
 sacculated part between the caecum on the one hand 
 and the rectum on the other, and constitutes the prin- 
 cipal part of the large intestines. The caecum is in the 
 lower right-hand part of the abdomen. The colon runs 
 from it upward on the right side to the region of the 
 stomach, then across to the left, and then down the left 
 side to the rectum. 
 
 Relations to the Abdominal Walls.— So long 
 and so slender a tube must be so held in place that it be 
 not tangled ; and also it must be in easy reach of the 
 great vessels which supply it with blood, and which take 
 away the digested food. This is done by a thin, trans- 
 parent membrane which is attached by one edge to the 
 
NUTRITION PROPER. 
 
 321 
 
 backbone, and by the other to the whole length of the 
 intestine, following all its complex windings. This is 
 called the mesentery (Fig. 205). From this arrangement 
 
 Fig. 205. — Small intestines attached to the mesentery. 
 
 
 it follows that the intestines are nowhere more than six 
 inches away from the great vessels lying along the 
 backbone. 
 
 PeritoncButn. — It may seem paradoxical, but is never- 
 theless in some sense true, that the intestines, and in- 
 deed all the abdominal viscera, are outside the abdomi- 
 nal cavity. The whole abdominal cavity {vc, Fig. 206) 
 is lined with smooth, shining, serous membrane called 
 the peritonecum {per). This, on reaching the backbone, 
 is reflected forward as a double membrane, the mesen- 
 tery, and then over the intestine as its investing coat. 
 If this serous membrane could be dissected off com- 
 pletely it would form a complete sac without opening. 
 
322 PHYSIOLOGY AND MORPHOLOGY OF ANLMALS. 
 
 What we mean by saying that the intestines are outside 
 of the peritoneal cavity is shown in Fig. 206. 
 
 Fig. 206. — Diag;ram showing the relation of the intestines to the abdominal 
 cavity : z', intestine ; vc, visceral cavity ; per, peritonaeum ; nt, muscular 
 coat ; £■/, epithelial coat ; sp, spinal column ; ao, aorta ; spc, spinal cord. 
 (After Wiedersheim. i 
 
 Coats of the Intestines. — Like the stomach, the 
 intestines have three coats — the outer, peritoneal ; the 
 middle, mtiscular; and the inner, epithelial. In other 
 words, it is a hollow, muscular tube, invested with the 
 peritoneal coat and lined with epithelial or mucous mem- 
 brane. The peritoneal coat gives smoothness, the mus- 
 cular coat does the mechanical work, and the epithelial 
 the chemical work of digestion. 
 
 Mechanical Work. — The muscular coat consists 
 mainly of two sheets of parallel fibers. In the outer 
 one the fibers run lengthwise, in the inner ringwise. 
 Under the contraction of these the intestines may be 
 seen to squirm wormlike from side to side, and light 
 waves of contraction may be seen running always in 
 one direction — downward. This is the peristaltic action 
 of the intestines. The waves must be light, otherwise 
 
NUTRITION PROPER. 
 
 323 
 
 the food would be too much hurried along its way. 
 Slowly, therefore, the digested food is urged along. In 
 the meantime the absorbents are taking up the liquid 
 parts. The percentage of liquids decreases, until finally 
 only solid, indigestible parts remain. This is finally 
 pushed through the ileocoa^al valve (Fig. 204) into the 
 large intestines, and the digestion is substantially done. 
 There are changes of an obscure kind which go on there, 
 but these are too little known to detain us. 
 
 Chemical Work. — The digestive juices of intesti- 
 nal digestion are three — viz., the bile, the pancreatic 
 juice, and the intestinal secretion. The general effect 
 of these is to produce a milky liquid called chyle. 
 
 [ci) The bile is secreted by the liver, and during diges- 
 tion is poured out in large quantities into the duodenum 
 a little way below the stomach through a common duct 
 made up of the union of the bile duct with the pancre- 
 atic duct (Fig. 198). The digestive effect of the bile is 
 manifold, i. It will be remembered that the chyme is 
 acid, and that acidity is unfavorable to the sacchariza- 
 tion of starch. Thus it happens that some starch may 
 escape solution in the stomach. Now, the bile is alka- 
 line, and therefore neutralizes the acidity of the chyme, 
 and thus revives the activity of the ptyalin on the 
 starches. 2. It will be remembered, again, that of the 
 three kinds of food we have had digestive juices for 
 two, viz., ptyalin for starches and pepsin for albumi- 
 noids; but the fats have not yet been touched. Now, 
 the alkalinity of bile partially saponifies the fats, and 
 thus prepares them for emulsification by the pancre- 
 atic and intestinal juices. 3. It is found, too, that in 
 order to be absorbed easily a liquid must be either 
 neutral or a little alkaline. Thus the bile, by neutral- 
 izing the acidity of the chyme, prepares it for easy 
 absorption. 
 
324 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 [b) The pancreas [siveetbread) lies just below the 
 stomach, and its excretory duct unites with the bile duct 
 to open by a common duct into the duodenum. Its 
 structure is similar to that already described in the sali- 
 vary gland. Its secretion is perhaps the most impor- 
 tant of all the digestive juices. It performs the func- 
 tion of all previously mentioned, and supplements them 
 all. It saccharizes starch, like the saliva. It peptonizes 
 albuminoids, like the gastric juice. It is slightly alka- 
 line, like the bile, and it is a powerful emulsifier of fat, 
 like the intestinal mucus. These properties are the 
 result of several ferments, among which may be men- 
 tioned paucreatin or aiiiylopsin and trypsin, the former a 
 solvent of starch and the latter of albuminoids. What- 
 ever of albuminoids or amyloids escape digestion in the 
 stomach are dissolved here.* It also forms still another 
 ferment which splits fats into glycerin and fatty acids. f 
 In addition to all these the pancreas has still other 
 functions, which will be discussed later. Suffice it to 
 say now that it apparently delivers a peculiar ferment 
 directly to the blood. 
 
 {c) The intestinal secretion probably has other diges- 
 tive properties little known, but certainly by its slimy 
 viscidity it is very efficient in the emulsification of fats. 
 
 Emulsion. — We have said that chyle is a milky 
 liquid. Its whiteness is wholly due to emulsified fats. 
 We explain this as follows: Ice is transparent, but break 
 it up into fine particles like snow and it is intensely 
 white. Glass is transparent, but grind it to fine powder 
 and it is white. Water is transparent, but spray and 
 foam are white. And so in all cases of whiteness. Oils 
 
 * Recently shown (Archives des Sciences, iv, 490, 1897) that the 
 spleen furnishes a product necessary to the formation of trypsin. 
 f Am. Nat. xxxi, 1040, 1897. 
 
NUTRITION PROPER. 
 
 325 
 
 and fats are transparent, but broken up into microscopic 
 globules they are also white. Now, what is called an 
 emulsion consists of millions of microscopic globules of 
 oil swimming in a transparent liquid, but, in order that 
 the globules do not run together and unite, the liquid 
 must be viscid. 
 
 Experiment. — Take a small bottle, fill it partly with 
 a viscid liquid like gum water or mucus, and then pour 
 in a little oil of some kind, as turpentine or sweet oil. 
 We see them as two equally transparent layers one atop 
 the other. Now, putting the thumb on the mouth of 
 the bottle, shake it violently. The result is an intensely 
 white fluid, which, examined by microscope, shows noth- 
 ing but transparent globules floating in a transparent 
 liquid. This is an emulsion. Milk is also an emulsion, 
 and its whiteness is due to the same cause. Chyle is an 
 emulsion, and its whiteness is due to the presence of 
 microscopic fat globules. If there be no fat in the food, 
 the chyle will be transparent. In the process of diges- 
 tion the fats and oils are broken up by the constant 
 pressing and kneading ac- 
 tion of the intestines in the 
 presence of a viscid liquid. 
 
 Absorption. — The 
 three kinds of food are 
 now all become absorb- 
 able. The starches have 
 become sugars, the albu- 
 minoids peptones, and the 
 fats emulsions. The food 
 is now ready for absorp- 
 tion. But for rapid ab- 
 sorption we must have a 
 large surface. This con- 
 dition is supplied partly 
 
 Fig. 207. — Interior of the intestines, 
 showing the valvula; conniventes. 
 
326 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANLMALS. 
 
 Fig. 208. — Two of the villi, greatly 
 enlarged ; one showing; blood cap- 
 illaries, the other, the lacteals : 
 ep, epithelium. 
 
 by the great length of the intestines, still more by the 
 many folds of the mucous membrane {T.'alviilce conniventes) 
 (Fig. 207), but most of all by the villi. These micro- 
 scopic hairlike projections 
 cover the surface as thick- 
 ly as the pile of velvet or 
 the tufts of Brussels car- 
 pet. They give a velvety 
 feel to the mucous mem- 
 brane (Fig. 208). Each 
 villus contains two kinds 
 of absorbent vessels — viz., 
 capillary blood vessels and 
 lacteals. These, though 
 separated in the figure, 
 are both of them in each 
 villus. 
 Two Modes of Absorption. — The food is ab- 
 sorbed in two ways — viz., by blood capillaries and by lac- 
 teals. The blood capillaries contain a circulati?ig current^ 
 and therefore take up the food and carry it along with 
 the blood. The lacteals, on the contrary, are purely 
 absorbent, and they end in blind, fingerlike extremities, 
 and therefore suck up the liquid food. Whatever of food 
 is absorbed by the stomach is wholly by blood capil- 
 laries. But the intestines are specially organized for 
 absorption in both ways. The food is divided between 
 these two modes. The sugars are absorbed mainly by 
 the capillaries, the fats by the lacteals, while the pep- 
 tones are divided between them. 
 
 Course of Each to the General Circulation. — 
 That taken up by the capillaries is carried to the por- 
 tal vein ; thence it passes through the liver, and by the 
 hepatic vein it reaches the vena cava ascendens and by 
 it is carried to the heart, and is thence distributed every- 
 
NUTRITION PROPER. 
 
 327 
 
 where. That taken up by the lacleals is carried by the 
 lacteal or lymphatic vessels of the mesentery through 
 the mesenteric glands into the receptaculum chyli, and 
 thence up to the thoracic duct, the lymphatic trunk 
 lying along the backbone on the left side (Fig. 209). 
 This duct rises to the collar bone, then turns downward 
 and empties into the angle formed by the junction of 
 
 01/ c 
 
 \J|lMi)t::ii!J 
 
 Fig. 209. — A seg^nent of the intestines ('«'\ shownng; the lacteal vessels pass- 
 ing along the mesentery im \, through the mesenteric glands \mgl), and 
 into the thoracic duct {thd) : avc, ciscending vena cava. 
 
 the subclavian vein from the left arm and the jugular 
 coming from the head, and thence it passes by the vena 
 cava descendens directly to the heart, to be distributed 
 to the whole body. This will be explained more fully 
 hereafter. 
 
 Sanguificatioti. 
 
 In the course of each of these to the general circu- 
 lation there are certain changes which assimilate it to 
 
328 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 the character of the blood, (i) We have said that that 
 which is taken up by the capillaries passes into the vena 
 porta. Now this is a quite unique and exceptional vein. 
 All other veins ramify at one end only; this at both 
 ends. All other veins, after receiving their supply by 
 the capillaries from the tissues, empty by an open mouth 
 into the general circulation. This one receives its sup- 
 ply from the capillaries of the intestines, the stomach, 
 and the spleen, and then, instead of emptying mto the 
 vena cava, close at hand, goes to the liver, to be again 
 distributed hy inverse capillary ramification through that 
 organ, and gathered a second time by the capillaries of 
 the hepatic vein, and so delivered to the general circu- 
 lation. Thus there is a mesenteric artery, but no corre- 
 sponding mesenteric vein. There is a splenic artery, but 
 no splenic vein. The vena porta takes the blood of both 
 these, and carries it to the liver, to be again distributed 
 there. This is shown in the diagram (Fig. 210). Now, 
 during the passage of the digested food through the 
 liver it undergoes an important change, preparing it for 
 immediate use in the generation of force and heat. In 
 other words, it is sanguified — i. e., made into material 
 suitable for circulation in the blood. The nature of this 
 important change we will explain more fully when we 
 come to speak of the functions of the liver (page 446). 
 
 That part of the dissolved food which is taken up by 
 the lacteals, as we have seen, is carried through the mes- 
 enteric glands before reaching the thoracic duct. In 
 these glands an important change takes place. After 
 passing through these it is no longer mere dissolved 
 food — i. e., peptones and fats — but apparently a living 
 fluid like the blood, although not yet red. Before pass- 
 ing through the glands it is noncoagulable ; after pass- 
 ing it is coagulable, like the blood. Before passing it 
 contains no globules except fat globules. After passing 
 
NUTRITION PROPER. 
 
 329 
 
 it contains also living nucleated cells like the white cor- 
 puscles of the blood, contributed to it by the glands. 
 In a word, before passing, it is chyle ; after passing, it is 
 
 Fig. 210. — Diagram showing the distribution of the blood through the liver. 
 The arrows sliow the direction of the current. 
 
 at least partly converted into blood. The change is 
 completed in the blood itself. 
 
 The last stage — i. e., the actual assimilation into tis- 
 sue — will be spoken of in connection with the circulation. 
 
 MODIFICATION OF THE PROCESS OF INTESTINAL 
 DIGESTION IN VERTEBRATES. 
 
 There is little to be said on this subject, because the 
 modification of the process, as already given, is unimpor- 
 tant. The whole process of digestion — mouth digestion, 
 stomach digestion, and intestinal digestion — is simpler in 
 carnivores and more elaborate in herbivores. Man in 
 
330 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 this regard, as in so many others, may be regarded as a 
 generalized mean. The intestinal canal is shorter in car- 
 nivores and much longer in herbivores than m man. It 
 may be well, however, to mention one or two interesting 
 points. The csecum, which in man is but a slight en- 
 largement of the blind end of the large intestine (see 
 Fig. 204), is greatly enlarged in the horse, and in the 
 
 Fig. 211. — The alimentary canal of a rat : sf, stomach ; ca, cacum. 
 (From Owen. ) 
 
 rat is indeed a sort of second stomach as big as the true 
 stomach (Fig. 211). The most interesting thing in this 
 connection is the origin of the strange wormlike appen- 
 dix. It is probably the remnant of the shriveling of the 
 extreme end of a much longer caecum. It is an example 
 of a useless remnant of a once useful organ. 
 
NUTRITION PROPER. 
 
 331 
 
 One more example: In fishes the whole digestive 
 process is very simple, and the intestines are correspond- 
 ingly short. But in the shark, although externally the 
 intestine is almost a straight tube running through the 
 body, yet iutcmally its surface of absorption is made very 
 large by means of a curious spiral valve. This gives 
 the peculiar spiral marking on the dung of these and 
 of some other lower vertebrates, both living and extinct, 
 for fossil dung of sharks and reptiles is not uncommon. 
 
 SECTION V. 
 Digestive Systeiti in Invertebrates. 
 
 General Remarks. — i. As already explained, we 
 shall pursue a different plan here. Instead of following 
 each stage of digestion through the different classes, we 
 shall run through the whole digestive process in each 
 class as taken up. 
 
 2. The complexity, and especially the diversity, 
 among invertebrates is so great that if we took them up 
 with anything like the fullness that we have taken up 
 vertebrates, our object — viz., to make a small volume, 
 giving only an outline of the most interesting points — 
 would be defeated. We will, therefore, only give a few 
 very striking examples from different departments. 
 
 ARTHROPODS. 
 
 Insects : Mouth Parts. — Insects are wonderfully 
 specialized animals. This is especially true of their mouth 
 parts. From this point of view insects may be divided 
 into two groups — viz., the biting and the sucking in- 
 sects, or the 7uaudibulate and the haustellate. The former 
 include the orthopters (grasshoppers), the neuropters 
 (dragon flies), and the coleopters (beetles). The latter 
 include the lepidopters (butterflies and moths), the dip- 
 
332 
 
 o PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 ters (flies), and the hymenopters (bees and ants), al- 
 though these last are intermediate. Of these two kinds 
 of mouth parts the former is undoubtedly the original 
 and normal, of which the latter must be regarded as an 
 extreme modification, for the mandibulate form alone 
 is found in the larval state (e. g., caterpillars) and in 
 early geological times. 
 
 Normal Mouth Parts. — Taking, then, the orthop- 
 ter, or grasshopper, or else a beetle, as example, there 
 
 are six movable mouth 
 parts; but as four of 
 these are in pairs, we 
 may say there are four 
 kinds. These are the 
 labruni (so-called upper 
 lip), the two mandibles., 
 the two m axil Ice, with 
 their jointed append- 
 ages, and the labium 
 (lower lip), with its 
 jointed appendages. 
 Sometimes there is an 
 elongation of this last 
 called the tongue. 
 These are seen in place 
 in the beetle (Fig. 212), 
 and the same parts in a 
 grasshopper separated 
 to display their forms 
 and position in Fig. 213. Looking directly at the mouth 
 of the grasshopper or beetle, their relative position and 
 direction of motion is shown in the diagram (Fig. 214). 
 The food is gathered by the maxillary and labial palpi, 
 is placed between the jaws (mandibles and maxillae), 
 pressed together between the upper and lower lips, and 
 
 Fig. 212. — Head of a beetle seen from 
 behind : lb, labium ; Ip, labial palpi ; 
 m VI, mandibles ; mx, maxillae ; mp, 
 maxillary palpi ; /br, labrum. 
 
NUTRITION PROl'ER. 
 
 333 
 
 divided and masticated by the jaws working toward one 
 another laterally (not vertically, as in vertebrates). 
 
 mxp\- 
 
 .'y nwcp 
 
 Fig. 213. — The mouth parts of a grasshopper enlarged : Ibr, labrum ; m m, 
 mandibles; /d, labium; //>, labial palpi; /.tongue; tnx rnx, maxillae; 
 mxp tnxp, maxillary palpi. 
 
 Serial Homology of these Parts.— As already 
 said (page 273), the insect head consists of four segments 
 with their paired appendages. The labrum is an exten- 
 sion downward of the first segment, and the antenniT are 
 the jointed appendages; the mandibles are the paired 
 appendages of the second segment; the maxilhx, the 
 paired appendages of the third segment ; while the la- 
 bium and its paired appendages belong to the fourth 
 segment (Fig. 215). 
 
334 
 
 PHYSIOLOGY AND MORPHOLOGY OP^ ANIMALS. 
 
 Special Homolog'y. — In suctorial insects the mouth 
 parts are an extreme adaptive modification — so extreme 
 in some cases that their homology with those of masti- 
 
 JliT 
 
 mx- 
 
 ib 
 
 Fig. 214. — Diagram showing; the 
 relative positions and direction of 
 motion in mastication. The arrows 
 show the direction of motion. 
 
 Fig. 215. — Side view of the head of 
 a gfrasshopper : e, eye ; other let- 
 ters are tlie same as in Figs. 212, 
 213, and 214. 
 
 Fig. 216. — Head of a butterfly seen 
 from in front. The letters show 
 the same parts as in previous fig- 
 ures. 
 
 catory insects is somewhat doubtful. We give what 
 seems the most probable view in two extreme forms: 
 
 Butterfly. — In this case the labrum, Ibr, and the man- 
 dibles, m m (Fig. 216), are rudimentary and useless. 
 The maxillae, vix 7!ix, are enormously elongated into 
 muscular hollow semicylinders, which, when put together, 
 form a long, flexible, hollow sucking tube. This is usu- 
 ally carried coiled up like a watch spring, but is uncoiled 
 and straightened when used as a proboscis for sounding 
 the nectar tubes of flowers. 
 
NUTRITION PROPER. 
 
 33: 
 
 Bees are a good example of intermediate form (Fig. 
 217). In these the labrum and mandibles are as usual, 
 but the maxillae and labium, with their appendages, are 
 
 Fig. 217.— Head of a bee seen from Fig. 218. — A. enlarged view of a bee's 
 
 behind. The labrum is not seen. tong:ue ; r, inner tube ; B, section 
 
 The other parts are lettered as be- more enlarged, showing its struc- 
 
 fore ; t, tongue. ture. 
 
 greatly elongated to form a sucking organ. This is 
 especially true of that projection of the labium called 
 the tongue, /, which is modified into an apparatus of 
 marvelously complex structure. This is shown in Fig. 
 218, in which A shows the outer tube with the inner tube 
 23 
 
336 PHYSIOLOGY AND MORPHOLOGY OF ANIiMALS. 
 
 pulled out; B, a section of the whole when the inner 
 tube is in place. The bee, therefore, both bites and 
 sucks. If the nectary tube of a flower is too long to 
 reach the nectar with the sucker, he bites the nectary 
 and reaches the honey more directly. 
 
 Digestive Apparatus. — Fig. 219 shows the whole 
 digestive apparatus of a beetle. In Fig. 220 we give 
 
 -nit 
 
 Fig. 219. — Dii^estive apparatus of a bee- 
 tle : m, mandibles ; a, antenna ; ce, 
 oesophagus ; c, crop ; prz', proventricu- 
 lus ; s/, stomach ; dd, biliary ducts ; d, 
 duodenum ; i, intestines ; ;-, rectum ; 
 ag-/, anal gland. 
 
 Fig. 220. — Digestive system 
 of a Belostoma : /t, head ; 
 ce, oesophagus : sg, salivary 
 gland ; sr, salivarj' recepta- 
 cle ; s^, stomach ; mi, Mal- 
 pighian tubes ; ccc, caecum. 
 (After Packard.) 
 
NUTRITION PROPER. 
 
 337 
 
 also the same in another insect [Belostoma) to show the 
 variations, and especially to show the salivary glands, 
 not shown in the previous figure. The distinctive func- 
 tions of these several parts are somewhat doubtful. 
 The most probable view is given in the legends. 
 
 We have given the simplest case of a carnivorous bee- 
 tle. In many insects, especially the herbivorous, like the 
 grasshopper (orthopter), the digestive apparatus is much 
 more complex and the 
 intestines much longer 
 and more convoluted. 
 
 It is well to remark 
 that what we have called 
 the biliary or Malpighi- 
 an tubes are also, appar- 
 ently, uriniferous tubes 
 as well. These two func- 
 tions are not yet well 
 differentiated. The sig- 
 nificance of this will ap- 
 pear hereafter. 
 
 Crustaceans: 
 Mouth Organs. — We 
 have already seen (page 
 268) that these are all 
 modified appendages of 
 the anterior somites. 
 Four pairs, called max- 
 illipeds, are food gather- 
 ers, and two pairs, max- 
 illae and mandibles, di- 
 vide and chew the food 
 (see Fig. 170). 
 
 Stomach. — The stomach is situated in the anterior 
 part of the cephalothorax, immediately above the 
 
 Fig. 221. — Lobster with the carapace 
 taken off : st, stomach ; dotted tube, 
 ii, intestines; Z, liver; H, heart. 
 
338 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 mouth and connected therewith by a short oesophagus. 
 If we take off the carapace of a lobster by dividing the 
 attachments with the sternum, F'ig. 221 will represent 
 what we see. Immediately beneath the carapace is the 
 heart, H, with the blood vessels running fore and aft. 
 We have nothing to do with these now. We will speak 
 of them later. In front and beneath the heart is seen 
 the large stomach, st, with the straight intestine, /, run- 
 ning backward beneath the great artery. Behind the 
 stomach, on each side of the anterior part of the intes- 
 tine, is seen the large liver, Z, composed of many tubules 
 opening into the intestine. The stomach is very mus- 
 cular, and if opened is seen to contain what might be 
 called sto?nach teeth — i. e., powerful triturating organs 
 composed of chitin and armed with prickles of the same. 
 
 MOLLUSCA. 
 
 We take one example from each class. 
 Acephala. — Among these we take the Mactra (Fig. 
 222). In all acephala the food-taking is wholly involun- 
 
 Fig. 222. — Mactra with one valve removed, showing the anterior iatn) and 
 the posterior (prn) shell muscles, and the foot (/"). (From Gegenbaur.) 
 
 tary. The clam buries itself in mud with the mouth 
 downward and the siphon upward, just reaching the 
 water. By ciliary action, currents are created which 
 pass down one tube of the siphon, through the gills, 
 contributing thus to respiration ; then to the mouth, 
 contributing thus to alimentation; then back again 
 
NUTRITION PROPER. 
 
 339 
 
 through the gills and out by the other tube of the siphon, 
 carrying with it refuse or excretions of all kinds. The 
 most remarkable thing about the digestive apparatus is 
 the enormous liver (nearly the whole of the dark part of 
 
 Fig. 223. — \'^ertical longitudinal section of Anodonta : pm, a»i, posterior- 
 anterior adductor muscles ; m, mouth ; st, stomach ; cce, csecum ; Hy 
 heart ; /", foot ; G, gills ; ma, mantle. 
 
 an oyster or a clam is liver), through which the long and 
 convoluted intestine winds, receiving in its course the 
 biliary secretion from many openings, and then, strange 
 to say, passing through the heart on its way backward 
 to the vent at the si- 
 phon. The stomach 
 and the winding course 
 of the intestine is shown 
 in Fig. 223. 
 
 Gastropoda.— For 
 an example of these 
 take a snail. Gastro- 
 pods are much more 
 highly organized than 
 the acephala. They 
 have a distinct head 
 
 Fig. 224. — Section through snout of a car- 
 nivorous gastropod showing the radu- 
 la, r, in place : A', lingual cartilage ; 
 tnp/i, muscle of pharynx ; m, mouth ; 
 £p, oesophagus. (After Lang.) 
 
 and their food-taking is voluntary. The mouth is armed 
 with transversely ridged chitinous plates, which have 
 
340 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 been called jaws, and a ribbon of chitin, called radula, 
 thickly set with sharp teeth, by which they rasp their 
 vegetable food. In carnivorous conchs the radula is 
 used for boring round holes through the shells of other 
 
 species for the purpose 
 of sucking their juices 
 (Figs. 224 and 225). The 
 intestine, as in acephala, 
 winds through the liver, 
 and in shelled forms turns 
 forward to discharge at 
 the opening of the shell. 
 Here, in fact, in the snail 
 we find four tubes open- 
 ing: (i) Of course, the 
 mouth ; (2) the intestinal 
 opening (anus) ; (3) the 
 genital opening ; 
 and (4) the respir- 
 atory opening. 
 
 Cephalopoda. 
 — For this class 
 we take the squid 
 {Sepia). Squids 
 take their food 
 by means of the 
 powerful muscu- 
 lar arms surrounding the mouth (Fig. 226) and divide 
 it with their powerful parrotlike beak (Fig. 227), which 
 moves up and down like the jaws of vertebrates, not 
 laterally, like those of arthropods. The digestive tube 
 is very simple, the long oesophagus passing into the 
 large stomach and the intestines coming thencefor- 
 ward to discharge in front, so that the debris is carried 
 away by the water currents of respiration. These ani- 
 
 FiG. 225. — Different forms of the teeth of 
 carnivorous gastropods. 
 
NUTRITION PROPER. 
 
 341 
 
 mals are the most highly organized among mollusca. 
 All the organs contributive to digestion, such as salivary 
 glands, liver, and perhaps pancreas, are found. 
 
 Fig. 226, — Diagram of digestive system of a squid : eg, cephalic ganglion ; 
 ceg, oesophageal ganglion ; ce, oesophagus ; st, stomach siphon ; mg, 
 mantle ganglion ; vg, visceral ganglion ; H, heart ; G, gills. 
 
 ECHINODERMS. 
 
 In the echinus, or sea chestnut, we have perhaps the 
 most elaborate jaw structure to be found in the whole 
 animal kingdom — viz., the so-called " lantern of Aris- 
 
 FiG. 227. — Jaws of a squid : 
 «/, upper jaw ; /, lower jaw. 
 
 Fig. 228. — The masticating appa- 
 ratus of an echinus. 
 
 totle " (Fig. 228). This consists of five three-sided hol- 
 low prismatic pieces surrounding the oesophagus and 
 fitted together so as to make a somewhat conical whole. 
 Through each prismatic piece there runs, as through a 
 
342 
 
 PHYSIOLOGY AND MORPHOLO(;Y OF ANIMALS. 
 
 sheath, a long, slender, slightly curved rod, sharp and 
 enameled at the point. These prismatic pieces with 
 their sharp teeth are worked by powerful muscles, so 
 as to move radially to and from the center. The 
 food is taken by curious nipping appendages (pedi- 
 cellarife) and handed on to the mouth and divided 
 and chewed by the lantern apparatus. The stomach is 
 immediately above the lantern ; the intestine is much 
 convoluted, and emerges by opening in the center of the 
 radiated upper surface of the animal. 
 
 CCF.LENTERATES. 
 
 Thus far we have found intestinal as well as stom- 
 achal digestion. The food preparation is carried to 
 the condition of chyle. In all such cases there is a blood 
 system distinct from the digestive system, and into 
 which the chyle is absorbed, to become afterward 
 changed into blood. In all animals spoken of thus far 
 blood and digested food are distinct from one another. 
 The digested food is absorbed and changed into blood 
 (sanguified). But now we find a great change in this re- 
 gard. In the coelenterates, and below, there is no longer 
 any intestine as distinct from the stomach. The diges- 
 tion goes only as far as chyme-making. There is no 
 longer any distinction between the digestive system and 
 the blood system, nor between digested food and blood. 
 The digested food is the only nutrient fluid. We take 
 as an excellent example of coelenterates the 
 
 Medusa, or Jellyfish, or Sea-Blubber. — These 
 beautiful, almost transparent, saucerlike or bell-shaped 
 animals (Fig. 229), with their long trailing tentacles and 
 graceful movements, are the delight of the intelligent 
 seashore visitor, and especially of the naturalist. 
 
 How do they take their food ? The movement of 
 their tentacles is far too slow for this purpose. They 
 
NUTRITION PROl'KR. 
 
 343 
 
 Fig. 22g. — Diagram of a medusa: nn, 
 nen.-e; w, mouth; si, stomach; rt, 
 radiating tubes. 
 
 take it by means of what have been called nettle cells, 
 or stinging cells, or thread cells (nematocysts), or, by 
 Agassiz, most appropri- 
 ately, /asso cells. They 
 occur in clusters, espe- 
 cially on the tentacles. 
 Their sha[)e, like an elec- 
 tric lamp, is seen in Fig. 
 230. A. Examined with 
 the microscope, they are 
 seen to contain — like an 
 electric lamp — a fine 
 thread coiled within. If 
 the animal be irritated 
 the long thread flashes out like lightning (Fig. 230, B) 
 and its extremity pierces, discharges a poison, and par- 
 alyzes its prey, which is 
 then slowly brought to 
 the mouth and swallow'ed. 
 The effect of these sting- 
 ing cells, with their in- 
 visible threads charged 
 with poison, is so power- 
 ful that handling these 
 animals will produce pain- 
 ful inflammation of the 
 hands. 
 
 The mouth is the open- 
 ing at the end of the pro- 
 boscis (Fig. 229), and the 
 stomach — often called the 
 oesophagus — the hollow 
 proboscis itself. The food 
 is taken by the mouth, and retained in the stomach until 
 digested. Whatever is indigestible is rejected through 
 
 F'iG. 230. — Lasso cell : A, in passive 
 state ; B, with the thread discharg- 
 ing. 
 
344 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 the mouth, and the digested food is then circulated by 
 ciliary currents through all the canals of the body (chy- 
 miferous canals), and thence directly absorbed into the 
 tissues. 
 
 Polyp. — Another excellent example is the polyp, or 
 actinia, or sea anemone. The well-known appearance of 
 
 Fig. 231. — Simplified figure of an actinia. 
 
 this animal is shown in Fig. 231, and the interior struc- 
 ture in Fig. 232. Imagine a short, hollow, fleshy cyl- 
 inder (body cavity), with a disk below (the foot disk) 
 and a disk above (the mouth disk). The upper disk is 
 
 Fig. 232. — Ideal section, vertical and horizontal, showing structure : /, ten- 
 tacles ; J, stomach ; //, partitions. 
 
NUTRITION PROPER. 
 
 345 
 
 surrounded with hollow tentacles opening into the body 
 cavity, and in its center is the mouth, opening into the 
 stomach or gullet, the lower end of which opens into the 
 general cavity. The stomach does not hang free, but is 
 held firmly in its place by partitions running from the 
 outer wall to the stomach, and dividing the general 
 cavity into many triangular apartments. Many parti- 
 tions do not reach the stomach. Below the stomach the 
 partitions end in free scythelike margins, so that all the 
 triangular apartments are in communication with one 
 another. 
 
 Process of Digestion. — The food is taken, as in 
 medusae, by thread cells, is put into the mouth and 
 swallowed, and, partly at least, digested in the stomach. 
 Whatever is indigestible is thrown back to the sea 
 through the mouth. The partly digested food is then 
 dropped into the general cavity, its digestion completed, 
 and then circulated by ciliary currents throughout the 
 whole interior cavity even to the extremity of the hol- 
 low tentacles, and directly absorbed and appropriated by 
 the tissues. 
 
 PROTOZO.\. 
 
 Here, again, we find a great step downward. Thus far 
 we have found a circulating fluid, although in the coelen- 
 terates it is not blood but digested food, and circulated 
 not by the mechanical action of a heart, but by ciliary 
 currents. But now we find no circulating fluid of any 
 kind. The food is digested and at once appropriated 
 by the living protoplasm. But even in protozoa we find 
 two grades. In the Infusoria (Fig. 233) the mouth is 
 surrounded by cilia, and particles of all kinds are 
 brought there by ciliary currents. If any are suitable 
 for food they are carried down into the stomach ; if 
 not, they are rejected and whirled away by the same 
 current. In RJiizopods there is neither mouth nor stom- 
 
346 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 ach as a permanent organ. Food touching the body any- 
 where is ingulfed. The semifluid protoplasm flows around 
 
 It, takes it in, and digests it. 
 Whatever is indigestible is 
 thrown out again, and the di- 
 gested part at once appropri- 
 
 M 
 
 ,-s? 
 
 
 ") 
 
 Chr 
 
 Fig. 233. — Paramcecum. The ar- 
 rows show the course of the food 
 until discharged. 
 
 Fig. 234. — Araoebaproteus:y^, food 
 bodies ; cv, contractile vesicle ; 
 K, nucleus. (After Leidy.; 
 
 ated and assimilated (Fig. 234); see also Fig. 156, page 
 240). These lowest animals have neither tissues nor 
 organs of any kind, but extemporize these as wanted — 
 whether locomotive organs, prehensile organs, mouth, 
 or stomach. 
 
 It is from such low beginnings that, it is believed, all 
 the types of animal structure have been gradually differ- 
 entiated by a process of evolution through all geological 
 times. 
 
CHAPTER III. 
 
 BLOOD SYSTEM. 
 
 Returning again to man, we have now carried the 
 food to the blood. We therefore take next the blood 
 system. But first of all we must say something about 
 the blood itself. 
 
 SECTION I. 
 The Blood. 
 
 Blood is an intensely red fluid, of a slightly viscid 
 feel and a faintly nauseous smell and taste. To the 
 naked eye it is quite homogeneous, but under the micro- 
 scope it is easily seen to consist of a multitude of solid 
 red particles or globules floating in an almost colorless 
 liquid. The liquid part is called plasma. The color is 
 wholly due to the globules. The quantity of this fluid 
 in a healthy man is about one and a half gallon. 
 
 r. The Globules. — These are of two, possibly three 
 kinds, viz. : (a) the red globules (Fig. 235) ; {b) the white 
 or colorless globules ; and, doubtfully, (r) the blood plates. 
 Of these the red are far the most numerous and conspic- 
 uous, and are therefore taken first. 
 
 {a) Red Globules. — These in size are about ■g-s'oTr '"*^h 
 (tIt ni'llin^etre) in diameter. In shape they are flattened 
 circular disks, a little depressed in the middle, their 
 thickness being about one quarter the diameter of the 
 disk. Their immense number may be shown thus : 
 From the size given above it is evident that it would 
 
 347 
 
348 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 take ten million to cover a square inch ; but on account 
 of their flatness it would take more than twelve thou- 
 sand to make a pile an inch high. Thus a cubic inch of 
 blood globules would contain one hundred and twenty 
 thousand million globules. Now it has been estimated 
 that the actual number in a cubic inch of blood is about 
 seventy thousand millions. Therefore considerably more 
 than half of the blood consists of these globules. 
 
 Behavior of Blood. — If we watch fresh-drawn blood 
 with a microscope under a glass cover we observe that 
 the disks run together and pile on one another like a 
 
 pile of coins thrown down 
 (Fig. 235). This is the re- 
 sult of coagulation. If a 
 little water on the finger is 
 touched to the side of the 
 cover, so as to be drawn 
 in and mingled with the 
 blood, then the globules 
 become round like mar- 
 bles, and roll around freely 
 in the current. They have 
 imbibed the water and be- 
 come swollen. If the fluid 
 in which they float should 
 dry away ever so little, 
 they shrivel and become 
 irregular in shape (Fig. 235 ;'). The same change takes 
 place if a strong solution of salt be added. The salt 
 draws water out of the globules and shrivels them. But 
 if a weak solution of salt be added it prevents coagu- 
 lation, and at the same time does not alter the shape. 
 The disklike shape may be thus examined at leisure. 
 
 Structure. — The red globules are not cells, but solid 
 masses, a little softer in the center. 
 
 Fig. 235. — Blood globules : r rr, red 
 globules ; / /, white globules or 
 leucocytes. 
 
BLOOD SYSTEM. 
 
 349 
 
 (i>) White Globules {^Leiicocytes). — These are much fewer 
 in number, being on an average only about one in eight 
 hundred. They differ from the red in color, being color- 
 less ; in size, being a little greater in diameter ; in shape, 
 
 7 ^/ v::^^/ 
 
 Fig. 236. — Different forms of leucocytes. 
 
 being normally spherical instead of disk-shaped ; in 
 structure, being distinctly nucleated j and, last and most 
 important of all, in being living cells, endowed with 
 amoiboid movement — in fact, in having all the properties 
 and capacities of living protozoa, crawling about like 
 living things. Some of the shapes which they take on 
 are shown in Fig. 236. 
 
 (c) Blood Plates. — These are very much smaller than 
 either of the others. They are difficult of demonstra- 
 tion, and their function is doubtful. In fact, they are 
 believed by some investigators to be not true blood 
 elements at all, but only the disintegrated remains 
 of wornout white corpuscles. This is still under dis- 
 cussion. 
 
 Chemical Composition. — The globules of blood 
 seem to be composed of an albuminoid stroma, colored 
 in the case of the red, with another peculiar albuminoid 
 substance — hczmoglobin. This substance contains a nota- 
 ble quantity of iron, and has a remarkable relation to 
 oxygen. It readily takes up o.xygen, and as readily 
 gives it up again, and in doing so changes color. In 
 an oxidized condition (oxyhaemoglobin) it is intensely 
 scarlet red. In the deoxidized condition it is darkpur- 
 plish red. 
 
350 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 2. Plasma. — The liquid in which the globules swim 
 (plasma) is a solution of fibrin^ albumen, and salts, with 
 often a trace of sugar and fats. 
 
 Coagulation of Blood, — If blood be drawn and 
 allowed to stand it coagulates. This is produced by the 
 solidification of the fibrin. All albuminoids pass more 
 or less easily from the liquid to the solid state. Albu- 
 men, or white of tgg, is solidified by heat at i6o°, or by 
 alcohol ; casein of milk is solidified by acid and heat at 
 70° to 80° or less. Fibrin solidifies much more easily; 
 exposed to air or touching any foreign body, it solidifies 
 at once. The causes and conditions of the solidification 
 of fibrin are still under discussion. 
 
 If blood be drawn and allowed to stand, as already 
 said, it coagulates first into a tremulous jelly by the 
 solidification of the fibrin. If it continues to stand, 
 gradually the fibrin contracts more and more firmly, and 
 a liquid is squeezed out. The whole mass thus separates 
 into two parts, clot and serum. The clot consists of 
 the solidified fibrin and the entangled globules. The 
 serum consists of a solution of albumen and salts, with 
 perhaps a trace of sugar. This is shown in the follow- 
 ing schedule : 
 
 Blood 
 Fresh. 
 
 Globules 
 
 ^ Red. 
 
 Coagulated. 
 
 '/ \Vhite. 
 
 \ Clot. 
 
 r Fibrin. 
 
 ) 
 
 Albumen. 
 
 1 
 
 1 
 
 ' Salts. 
 Sugar, a trace. 
 Fat, a trace. 
 
 1 
 
 I' Serum. 
 
 1 
 
 J 
 
 Plasma, solution of. . 
 
 In fever blood coagulates more slowly. In coagu- 
 lation the globules have time to settle a little before 
 the solidification of the fibrin, leaving thus the char- 
 acteristic buff coat of fevers on the top. If salt be added 
 
BLOOD SYSTEM. 
 
 351 
 
 to fresh blood it prevents coagulation, and the globules 
 will settle to the bottom. If freshly drawn blood be 
 stirred or whipped with a bundle of twigs or wires, the 
 whole of the fibrin coagulates on the wires or twigs as 
 fleshy strings, and may be withdrawn. The defibrinated 
 blood will no longer coagulate. This is important, be- 
 cause it is necessary sometimes to transfuse the blood 
 of one person into the veins of another, or even the 
 blood of an animal into the veins of a man. If so, 
 then the blood so transfused must not be liable to 
 coagulation. 
 
 Functions of the Blood. — i. Plasma. — The plasma 
 may be regarded as essentially the finished result of 
 albuminoid food, although it contains in small quantities 
 many other substances for use or for elimination by ex- 
 cretory organs. It is essentially the peptones, sangui- 
 fied, vitalized, and ready for use. Its fundamental 
 function, then, is tissue building — i. e., repair and growth 
 of tissue. Its peculiar property of easy change from 
 liquid to solid form is eminently adapted for this pur- 
 pose. Its liquid condition is necessary for circulation ; 
 its solid condition is necessary for making tissue. Fur- 
 ther than this, however, the whole process of assimila- 
 tion, or change from blood to tissue, is still enveloped in 
 mystery. 
 
 2. Red Globules. — The function of these is better un- 
 derstood than any other part of the blood. They are 
 undoubtedly carriers of oxygen from the air to the tis- 
 sues for combustion of their waste. This it does by vir- 
 tue of the property of haemoglobin, already mentioned. 
 This substance takes oxygen from the air in respiration, 
 becomes oxidized as oxyhaemoglobin, is carried to the 
 tissues, and there gives up its oxygen for the combus- 
 tion of waste. It is easily seen, therefore, why an abun- 
 dance of red globules is necessary for health and vigor. 
 24 
 
352 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 3. White Globules. — The function of the white globules 
 is much more obscure. In some way little understood 
 they seem to vitalize the blood, for, as already seen, 
 they are really living protozoa. It will be remembered 
 (page 329) that as soon as these enter the chyle in the 
 mesenteric glands this fluid becomes endowed with the 
 property of coagulation. Again, it is believed that these 
 globules act as scavengers of the blood, removing mi- 
 crobes and other offending substances by inclosing and 
 devouring them after the manner of rhizopods (page 
 346). Again, as already seen, these corpuscles crawl 
 about and change their shape like amoeba. When the 
 blood stagnates, as in inflammation, they actually squirm 
 themselves through the thin walls of the capillaries, and 
 wander about in the tissues; and, in case of breaking 
 down of the tissues and the formation of pus, they prob- 
 ably become the characteristic pus corpuscles. 
 
 Origin and Life History of Blood. — i. Plasma, as 
 already explained, is essentially the sanguified and vital- 
 ized peptones. Sugar is also taken up into the blood, 
 and in so far as it remains there is a constituent of the 
 plasma ; but normally there is only a trace of sugar in 
 the blond of \.\\& general circulation, because it is quickly 
 burned up. We shall speak of this more fully in another 
 place. 
 
 2. White Globules, or Leucocytes. — We have already 
 seen that these are formed in the mesenteric glands; 
 but these are only lymphatic glands situated in the mes- 
 enterv. Leucocytes are formed in all lymphatic glands 
 wherever situated and at all times, whether chyle be 
 passing through them or not. They are also formed in 
 the spleen, for the blood of the splenic vein before it 
 enters the vena porta is ten to twenty times richer in 
 leucocytes than that of the general circulation. It 
 seems certain, then, that the leucocytes are formed in 
 
BLOOD SYSTEM. 353 
 
 the lymphatic glands and in the spleen, and that the 
 blood receives them while passing through these organs. 
 How they disappear is not certainly known. Possibly 
 they break up into so-called blood plates. 
 
 3. Red Globules. — These are constantly changing ; 
 they have, like all living things, a delinite life history; 
 they are born, mature, decay, and die. If the body is 
 depleted of blood it quickly recovers its supply. If the 
 blood of another animal be transfused into the veins of 
 a man, at first two kinds of globules may be seen in the 
 blood, but as time goes on the animal globules decrease 
 and the human increase until only the human remain. 
 The red globules are therefore being renewed all the 
 time. Where do they come from ? At one time they 
 were thought to be transformed leucocytes; but this is 
 probably not true. It is now believed that they are 
 formed in the red marrow of the bones, and perhaps also 
 in the spleen, by the division of certain large cells ob- 
 served there. This is their birthplace. Their active life 
 is in the circulation, doing their work of o.xygen carrv- 
 ing. Their death place is probably the liver and the 
 spleen. 
 
 Briefly, then : 
 
 Leucocytes are born in the lymphatics and the spleen. 
 
 Red globules are born in the bones and the spleen. 
 
 Red globules die m the liver and the spleen. 
 
 It would seem, then, that the spleen has many and 
 important functions, but that it shares all these with 
 other organs. May not this explain the singular fact 
 that, although so important, yet it has been extirpated 
 in the dog without immediate serious injury to health. 
 
 Now the blood goes everywhere, touches every tissue 
 and every cell, (i) It carries food, and thus becomes 
 the all-nourisher. (2) It takes up all waste and carries 
 it to the appropriate organ of elimination, and thus be- 
 
354 PHYSIOLOGY AND MORPHOLOGY OP' ANIMALS. 
 
 comes the all-purifier. (3) It takes oxygen to every 
 tissue to consume waste and food by combustion, and 
 thus generates heat and force, and thus becomes the 
 all-warmer and energizer. (4) It acts also as a reservoir, 
 especially for food. The food taken to-day is not used 
 to-day, but the blood is drawn upon for heat and force 
 and tissue repair and again resupplied. The. blood is 
 like a lake in irrigation or a bank in currency. (5) To 
 a much less extent it is also a reservoir for waste; to a 
 less extent because the urgency of waste removal is 
 much greater. 
 
 Comparative Morphology of Blood. — In all ver- 
 tebrates we have two kinds of globules, the red and the 
 white. The white, or leucocytes, are substantially simi- 
 lar in all ; but the red differ in size, shape, and structure 
 in the different classes. 
 
 1. Mammalian Blood. — The blood of all mammals 
 (Fig. 237) is substantially similar to that of man, already 
 described. The red corpuscles (globules) are all small 
 circular .,no7i- nucleated (X\sks. The only exception in shape 
 is in the camel, in which they are elliptic instead of cir- 
 cular, but otherwise they are as stated above. In size 
 they bear no apparent relation to the size of the animal ; 
 those of a mouse, on the one hand, and of an elephant, 
 on the other, not differing in any marked degree from 
 that of man. Even expert microscopists are in doubt 
 whether the blood of a dog can be certainly distin- 
 guished from that of a man. 
 
 All mammals (except monotremes) are viviparous or 
 young-bearing. Therefore this style of blood may be 
 called viviparous vertebrate blood. 
 
 2. Birds, Reptiles., Amphibians, and Fishes. — All other 
 classes of vertebrates — viz., birds, reptiles, amphibians, 
 and fishes — have blood globules differing in size, shape, 
 and structure from those of mammals, but similar to one 
 
BLOOD SYSTEM. 
 
 >s 
 
 another in s/iajyi" and structure, though varying in size. 
 In shape they are all elliptic; in structure they are nucle- 
 ated j in size they vary from nearly the size found in 
 mammals in certain fishes to very many times that size 
 in some amphibians. The largest are found in some 
 
 Mai 
 
 Bird 
 
 tep 
 
 Ol O 6 
 
 7ish 
 
 Fig. 237. — Blood of different classes of vertebrates, showing; comparative 
 sizes. All are equally mag:nified : /«, man ; el, elephant ; ms, mouse ; 
 W7, musk deer ; lib, humming:-bird ; /, pigeon : ost, ostrich ; sn, snake ; 
 /, toad ; //', proteus ; /, perch ; pk, pike ; sh^ shark. 
 
 tailed amphibians, such as \.\\^ proteus, in which they are 
 ten times the diameter or one hundred times the surface 
 area of those of man (Fig. 237). 
 
 All these classes of vertebrates are egg-laying. 
 Therefore blood containing this style of red corpuscles 
 may be called oinparous vertebrate blood. 
 
 All vertebrate blood (unless we except that of the 
 
356 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 amphioxus, which is very doubtfully a vertebrate) is red. 
 Below this — i. e., in invertebrates — the blood is not red.* 
 
 3. Higher Invertebrates. — Among these we include 
 arthropods, worms, mollusca, and echinoderms — i. e., 
 
 all above coelenterates ; in other 
 words, all that have a blood sys- 
 tem and a true blood at all. In 
 these the red corpuscles are 
 wanting; only white corpuscles 
 are found (Fig. 238). In these, 
 Fig. 238.— Blood corpuscles therefore, the blood is colorless 
 
 of a crustacean. (After „,.,., , i, 
 
 Gegenbaur. ) or nearly so. 1 his kind we shall 
 
 call mvertebrate blood. 
 
 4. Civlenterates. — These include medusae and polyps. 
 In these, as already said (page 342), we have no true 
 blood differentiated from digested food, nor blood sys- 
 tem differentiated from the digestive system. The di- 
 gested food carried by ciliary circulation is the nutrient 
 fluid. This, therefore, may be cMtd food blood, or coelen- 
 terate blood. 
 
 5. Protozoa. — In these there is no circulating fluid of 
 any kind. 
 
 The following schedule briefly expresses these facts : 
 
 „, ] r I. Mammalian ) ^^ , , , , 
 
 lilood \ . . J- Blood — red, non-nucleated, 
 
 red. ■ viviparous. \ 
 
 (2. Oviparous vertebrate blood — red, nucleated — birds, rep- 
 tiles, amphibians, and fishes. 
 Blood / 3. Invertebrate blood — white, nucleated — arthropods, 
 color- \ worms, mollusca, echinoderms. 
 
 less. ' 4. Ccelenterate blood — white, no globules, food blood — 
 medusas, and polyps. 
 5. Protozoa — no circulating fluid. 
 
 Embryonic Development of Blood. — Some of the 
 above phases are found in the embryonic development 
 
 * Some worms have a kind of red "blood. 
 
BLOOD SYSTEM. 
 
 JO/ 
 
 of mammals, and even of man. (i) In the earliest stages 
 of egg development — i. e., before the organs are formed 
 — there is, of course, no circulating fluid of any kind, 
 no blood or blood system. This corresponds remotely 
 to the lowest cell-aggregate animals in which there is 
 yet no circulation. (2) The next thing observed is the 
 liberation of nucleated tissue-cells along certain lines and 
 the oscillatory movement of the liberated cells along 
 these lines. This is the beginning of blood vessels and 
 of a blood which has colorless nucleated cells like the blood 
 of invertebrates. A heart is added later, by the develop- 
 ment of a part of the vascular system. (3) The nucleated 
 corpuscles become reddened, and we have now nucleated 
 red corpuscles like the blood of the lower vertebrates. 
 (4) The small non-nucleated round disks begin to appear 
 among the others, which gradually disappear, and we 
 have true mammalian blood. 
 
 It would seem now that in logical order we ought to 
 take up the circulatory system. But the course of the 
 blood is so wholly determined by its distribution through 
 the respiratory organs that it would be impossible to 
 understand that course, especially its comparative 
 morphology, without a previous knowledge of the 
 organs. These, therefore, must be first taken up. On 
 the other hand, the function of respiration is wholly a 
 katabolic process and the most important of all these 
 processes, and must be treated along with these in the 
 fourth chapter. Therefore, our plan will be to take up, 
 first, the morphology of the respiratory organs in vertebrates, 
 then the morphology of the circulatory system, also in 
 vertebrates ; then the morphology of circulation and res- 
 piration together in the invertebrates; and, finally, the 
 physiology or function of respiration — which is the same 
 in all animals. This, of course, will be treated with the 
 
358 PHYSIOLOGY AND MORPHOLOGY OF ANLMALS. 
 
 katabolic processes, as the most important of all of them. 
 All that is necessary now in regard to the respiratory 
 function is only to remember that the general object of 
 respiration is the aeration of the blood, and thereby the 
 exchange of its COg for oxygen of air. 
 
 SECTION II. 
 
 Respiratory Organs among Vertebrates. 
 
 Respiratory organs are of two general kinds, viz., 
 lungs and gills. The one kind is adapted for air breath- 
 ing, the other for water breath- 
 ing. The general plan of the 
 one is a complexly /;;-folded epi- 
 thelial surface ; of the other of 
 a complexly <?«/-folded epithe- 
 lial surface (Fig. 239). In both 
 cases alike the purpose is to ex- 
 pose as large a surface as pos- 
 sible to the oxygen of the air or 
 the water. The two kinds are 
 not homologous ; the one is not 
 transformed into, but, as already 
 shown (page 242), substituted for 
 the other. We take first lungs, and, of course, first of 
 all the lung-s of man. 
 
 Fig. 239. — General plan of 
 structure, A, of lungs ; 
 B, of grills. 
 
 I. LUNG RESPIRATION. 
 
 The Lungs of Man. — The lungs are by far the 
 
 largest organ in the body, although in tveight they are 
 greatly exceeded by the liver. Their great volume is, 
 of course, due to the contained air. Their grayish-pur- 
 ple color and soft, elastic feel is well known. They are 
 a double organ, being divided into a right and left lung 
 by the mediastinum. 
 
BLOOD SYSTEM. 
 
 359 
 
 Structure. — We have already said (page 204) that 
 two tubes go down from the throat into the body cavity 
 — the posterior one, the gullet, to the stomach, and the 
 anterior one, the windpipe, or trachea, to the lungs. 
 The one is soft and flaccid, but muscular; the other is 
 a firm, open tube, being kept open by a series of carti- 
 laginous rings. This 
 is necessary in order 
 that the air may come 
 and go with the least 
 resistance possible. 
 The trachea is capped 
 above by the larynx, 
 as already explained 
 (page 204). After a 
 course of about five 
 inches it divides into 
 two great branches, 
 one to each lung, 
 called the bronchi. 
 These are also ringed. 
 The primary bronchi 
 are subdivided into 
 
 secondary bronchi. Fig. 240. — Diafjram showing the general 
 J , . . structure of the lungs of man : L, larynx ; 
 
 and these again into 7-^, trachea; br, bronchi. 
 
 tertiary, and so on 
 
 until the tubes become of capillary fineness and corre- 
 spondingly numerous. This is shown diagrammatically 
 in Fig. 240. They finally terminate in minute cells or 
 cellulated cells (Fig. 241). The ringed structure con- 
 tinues, except in the smallest subdivisions and the ter- 
 minal cells. The terminal cells are y^ to ^^^ of an inch 
 {\ to \ millimetre) in diameter, and their number has 
 been estimated as 600,000,000. Now conceive this mass 
 of finely divided tubes and terminal cells, ///^\/ through- 
 
360 PHYSIOLOGY AND MORPHOLOGY OF ANLMALS. 
 
 out with epithelial membrane — an extension of the mu- 
 cous membrane of the mouth and throat — webbed to- 
 gether with loose connective tissue and invested with 
 serous membrane, and we shall have a sufficiently clear 
 idea of the general structure of the lungs. 
 
 The obvious purpose of all this is to expose as much 
 surface as possible to the oxygen of the air. The 
 minuter the ramification, the smaller the terminal cells; 
 or the finer the sponge, the 
 larger will be the surface. 
 The area of the epithelial 
 
 Fig. 241. — Termination of a capil- 
 lary bronchus : a, bronchus ; 6, 
 cell ; c, cellule. 
 
 Fig. 242. — Showing; the capillary 
 blood vessels (b) ramifying on 
 the surface of the cells (a). 
 
 surface exposed in the lungs has been variously esti- 
 mated from four hundred to fifteen hundred square feet. 
 Now beneath this extensive epithelial surface the blood 
 vessels ramify most minutely and exchange with the air 
 COo for oxygen (Fig. 242). 
 
 But to make this exchange effective both the air and 
 the blood must be in constant circulation. When the 
 blood has discharged its COg and taken in its supply of 
 O it must get out of the way for other blood to do the 
 same. Similarly, when the air has given up its O and 
 taken in its supply of COo, it must move on and give 
 
BLOOD SYSTEM. 
 
 361 
 
 place for other air. In the case of the blood the circu- 
 lation is kept up by the mechanical action of the licart ; 
 in the case of the air, by the mechanical action of 
 breathing. 
 
 Mechanics of Breathing. — The body cavity is 
 divided by a thin transverse partition into an upper 
 chamber, the thorax, and a lower chamber, the abdomen. 
 In the one is found the lungs and heart, in the other the 
 stomach, spleen, liver, intestines, kidneys, etc. The dia- 
 phragm is not 3l plane, or it could not be used for respira- 
 tion. It is deeply concave below, or domelike. It is also 
 a muscular partition, the fibers radiating from a clear 
 membranous space in the middle — the skylight of this 
 dome — in all directions and taking hold of the walls 
 of the body cavity all around. Contraction of these 
 fibers brings down the dome and flattens it. The arch 
 of the dome is filled below by the stomach and spleen 
 on the left and the liver on the right. These in their 
 turn rest on the mass of convoluted intestines, and the 
 whole is supported by the abdominal walls. Above the 
 diaphragm the concave lower surface of the lungs rests 
 directly on the upper convex surface of the dome. 
 Although in contact with the viscera above and below, 
 the diaphragm is not united with them. 
 
 Now, the thoracic, like the abdominal, cavity is lined 
 with a smooth serous membrane. In the abdomen it is 
 called the peritonceum, in the thorax the pleura. Like 
 the abdominal, so the thoracic cavity is a closed cavity — 
 i. e., the pleura lines the ribs, etc., and is then reflected 
 to form the investing membrane of the lungs and heart. 
 If it could be successfully dissected off it would form a 
 continuous bag without a hole in it. The manner in 
 which the pleura lines the thorax, is then reflected over 
 the lungs as its investing membrane, and then between 
 the two lungs as the mediastinum, is shown in Figs. 243 
 
362 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 and 244. For greater clearness there is a space between 
 the lungs and the walls. In reality they are in contact. 
 
 This description is necessary to make clear the me- 
 chanics of breathing, which we now proceed to explain. 
 
 Breathing consists in alternate expansion and con- 
 traction of the thoracic cavity by means of the respira- 
 
 FiG. 243. — Vertical section through 
 the lungs, showing the relation 
 to the pleura : /r, trachea ; b7\ 
 bronchi ; cpl, costal pleura ; ///, 
 pulmonic pleura. 
 
 Fig. 244. — Horizontal section through 
 the thorax : /r, trachea ; H, heart ; 
 pc, pericardium ; ///, pulmonic 
 pleura ; c//, costal pleura. 
 
 tory muscles. The lungs are perfectly passive in the 
 operation. To illustrate : Suppose we take a hand 
 bellows and arrange a bladder within so as to connect 
 with the nozzle, with no opening into the bellows, but 
 only into the bladder, through the nozzle. Now, on 
 expanding the bellows the air rushes in through the 
 nozzle into the bladder and expands it, but no air enters 
 the cavity of the bellows. On shutting up the bellows 
 the air is again squeezed out from the bladder through 
 the nozzle. In all these movements the exterior sur- 
 face of the bladder and the interior surface of the bel- 
 lows never break contact. But if there be an opening on 
 the side of the bellows the air will rush in there and not 
 fill the bladder. 
 
 Application to Breathing. — So the lungs are 
 placed in the thorax as the bladder in the bellows. If 
 
BLOOD SYSTEM. 
 
 363 
 
 the thorax expands, air rushes in through the nostrils 
 into the lungs, expanding them and keeping them in con- 
 tact with the ribs. If the thorax contracts, it squeezes 
 out the air through the nostrils. But if there be an 
 opening into the thorax (by wound), then, on expand- 
 ing the thorax, the air rushes in there, gets into the cav- 
 ity instead of into the lungs, and the lungs do not fill. 
 
 Mode of expanding and contracting the 
 Thorax. — rhis is done in two ways — viz., by the ribs 
 and by the diaphragm. There are therefore two kinds 
 of respiration — viz., costal or thoracic, and diaphrag- 
 matic or abdominal. 
 
 Costal Respiration. — The ribs do not run straight 
 around the chest horizontally, but slope downward all 
 the way. The simple lifting of the ribs, therefore, 
 increases the whole transverse section of the chest. 
 Now, between the ribs 
 and connecting them there 
 are two sheets of muscu- 
 lar fibers, external and in- 
 ternal. The fibers of the 
 external sheet, ex, run 
 obliquely downward and 
 forward, those of the in- 
 
 ■ iTlt 
 
 ,i,X. 
 
 Fig. 245. — Showing two ribs 
 and the intercostals be- 
 tween : ex, external, and 
 /'«/, internal sheet. 
 
 Fig. 246. — Diagram illustrating action of 
 the intercostal muscles : a a, fiber of 
 exterior sheet when passive ; a a\ 
 when contracted, b' b' a fiber of inte- 
 rior sheet when stretched, and b b 
 when contracted. 
 
 terior sheet, int, downward and backward (Fig. 245). 
 The external sheet raises the ribs and expands the chest, 
 the internal sheet pulls down the ribs and contracts the 
 
364 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 chest. Why this must be so is shown in diagram, Fig. 
 246. If I, 2 and 3, 4 be two ribs connected with the 
 backbone behind and the breastbone in front, and there- 
 fore compelled to nwi'e together and in the natural slop- 
 ing position, and if a a represent one fiber of the exter- 
 nal sheet, it is evident that if this fiber contracts it will 
 elevate both ribs, because, although it pulls one up and 
 the other down, yet the leverage on 3, 4, by which it 
 pulls up, is greater than that on i, 2, by which it pulls 
 down ; and since they must move together, the pair will 
 move up. But upward movement expands the chest. Or, 
 put it another way : Any position of the ribs will be 
 assumed which shortens the fiber a a to a' a' . For the 
 same reason contraction of the fibers of the interior 
 sheet {b' b') will depress the ribs and contract the chest, 
 because here the downward pulling acts with the greater 
 leverage on i, 2'. Therefore thoracic or costal respira- 
 tion is accomplished by alternate contraction of the ex- 
 terior and interior intercostal muscles. The one pro- 
 duces /aspiration, the other (?jcpiration. 
 
 Diaphragmatic or Abdominal Respiration.— This 
 is accomplished by the alternate contraction of the dia- 
 phragm and the abdominal muscles. When the muscular 
 fibers of the diaphragm contract, the high arch is brought 
 down and the dome flattens. This increases the vertical 
 diameter of the thorax as the intercostal contraction 
 does the transverse diameter. But the descent of the dia- 
 phragm presses on the stomach and liver, and these again 
 on the intestines on which they rest; and these in turn 
 press outward on the abdominal wall, which therefore pro- 
 trudes. This is /;;spiration. The stretched abdominal 
 muscles reacting, by contraction, press on the intestines, 
 and these upward on the stomach and liver, and these 
 in turn lift the dome of the diaphragm and press on the 
 lungs, squeezing out the air. This is i?Apiration. 
 
BLOOD SYSTEM. 
 
 365 
 
 Fig. 247. — Diagrams showing; the different kinds 
 of respiration : A, pure thoracic ; H, pure ab- 
 dominal ; C, mixed. 1 he dotted lines repre- 
 sent the expanded condition. 
 
 These two kinds of respiration may be considered, 
 and indeed may be used separately. Fig. 247, A, is 
 a diagram repre- 
 senting pure tho- 
 racic, and Fig. 
 247, B, pure ab- 
 dominal respira- 
 tion. But the two 
 are usually com- 
 bined, and the 
 whole body cavity 
 enlarges and con- 
 tracts as in Fig. 
 247, C. In labored 
 respiration the 
 
 thoracic predominates; in quiet respiration, and espe- 
 cially in sleep, the abdominal predominates. In men 
 there is a slight predominance of the abdominal, in 
 women of the thoracic. 
 
 Thus, then, the /V/spiratory muscles are the ^.rternal 
 intercostals and the diaphragm; the expiratory mus- 
 cles are the internal intercostals and the abdominal 
 muscles. 
 
 Coughirjg and sneezing are violent convulsive actions 
 of the ^.vpiratorv muscles. It is a little singular that in 
 popular literature, and even in many school te.\t-books 
 of physiology, these should be so often referred to con- 
 vulsive action of the diaphragm. Of course, this is im- 
 possible since the diaphragm is an /V/spiratory muscle, 
 and coughing and sneezing are expulsive efforts. Every- 
 body knows that constant e.xcessive coughing will pro- 
 duce soreness of the abdominal muscles. Hiccough is a 
 spasmodic action of the diaphragm, and laughter a spas- 
 modic alternate action of the inspiratory and e.xpiratory 
 muscles, with the latter predominating. 
 
366 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATE RESPIRA- 
 TION. 
 
 Mammals. — The respiratory organs and the mode 
 of breathing among mammals do not differ from those 
 of man. All mammals have a diaphragm, and therefore 
 both kinds of respiration. On account of the horizontal 
 position of the body, of course the body cavity is divided 
 into an anterior and posterior instead of an upper and 
 lower chamber. 
 
 Birds. — In birds we find large lungs of minutely 
 spongy structure, and therefore a perfect aeration of the 
 blood. They are, therefore, hot-blooded animals. Yet 
 birds have no diaphragm. This, however, is of little dis- 
 advantage, as their thorax is large and abdomen small, 
 and their thoracic respiration very perfect. It is interest- 
 ing to note, however, that some birds, sijch as the os- 
 trich, seem to have the beginnings of a diaphragm. In 
 these, certain muscular fibers arise from the backbone 
 and lowest ribs behind and take hold on the lung and 
 pull it downward, and undoubtedly assist in breathing. 
 
 Reptiles. — Reptiles have no diaphragm ; but since 
 their ribs surround the whole body cavity to the hips. 
 
 Fig. 248.— Skeleton of a lizard (Ne/oderma), showing how the ribs surround 
 the whole body cavity. 
 
 there seems to be no use for a diaphragm in them (Fig. 
 248). Probably the shortening of the rib series, in order 
 to give greater freedom to the loin created the necessity 
 
BLOOD SYSTEM. 
 
 367 
 
 of a diaphragm in mammals. The aeration of the blood 
 of reptiles, however, is very imperfect, for reptiles are 
 cold-blooded. This, however, is due not so much to 
 imperfect respiration as to the coarseness of the spongy 
 structure of the lungs, and therefore the smaller surface 
 of contact of blood with air. It is due, also — as we shall 
 see hereafter — to the fact that only a part of the blood 
 is exposed to the air. 
 
 Tortoise. — There is one order of reptile the me- 
 chanics of whose respiration is entirely peculiar — viz., the 
 tortoise. Like other reptiles, they have no diaphragm, 
 and therefore can not have this kind of breathing, but 
 neither can they have costal breathing, for their ribs are 
 immovably consolidated with the shell. The problem, 
 then, is how to expand the body cavity. As might be 
 expected, therefore, the breathing of tortoises and turtles 
 is exceptional. It is effected partly by muscular sheets 
 which arise from the shell and pass over the viscera, 
 including the lungs, and by contraction compress them 
 and force out the air; and partly (especially in land 
 tortoises) by movements of the shoulder and hip girdles. 
 In most vertebrates the shoulder girdle is movable, but 
 the hip girdle is fixed; but in ioxioxsts both girdles are 
 movable. In inspiration the shoulder girdle is drawn 
 forward and the hip girdle backward, and the body 
 cavity is thus enlarged. In expiration there are con- 
 trary movements — i. e., backward of the shoulder girdle 
 and forward of the hip girdle, and the body cavity is 
 contracted.* 
 
 Amphibians. — The respiration of these, e. g., the 
 frog, is far inferior to even that of reptiles, (i) The 
 lung is no sponge at all, but only a sac. There is no 
 trachea and very short bronchi. The glottis opens from 
 
 * Charbonnel-Salle, An. des. Sci. Nat., vol. xv, art. 6, 1883. 
 25 
 
368 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 the throat almost directly into two large sacs ; only 
 each sac is sacculated on the interior, so as to increase 
 its surface (Fig. 249). The aeration of the blood, there- 
 fore, is still more imperfect than in reptiles. (2) Again 
 amphibians not only have no diaphragm, but they have 
 no ribs. The mechanics of their breathing is entirely 
 different from all previously men- 
 tioned. It is not diaphragmatic nor 
 yet costal. It is a throat 1 espiration. 
 The submaxillary space, or space be- 
 tween the branches of the lower jaw, 
 is large. This space is pressed down 
 by the hyoid or tongue bone, and 
 thus draws air through the nostrils 
 and fills the throat. The nostrils are 
 then closed, the throat is brought up, 
 and the air is forced down into the 
 lung sacs. This, of course, swells 
 the walls of the abdomen, which, re- 
 acting by contraction, drive out the air again through 
 the nostrils. 
 
 This is the lowest form of lung respiration and 
 grades into gill respiration. In fact, all amphibians in 
 their early larval life breathe by gills, and only after- 
 ward by lungs. This, therefore, leads us naturally to 
 gill respiration. 
 
 Fig. 249. — One lung- of 
 a frog : br, bronchus. 
 
 2. GILL RESPIRATION. 
 
 The most perfect form of gills, or branchicE, is found 
 in the ordinary typical fishes (teleosts). We take this as 
 a type. 
 
 Take any typical fish, such as a perch or a salmon 
 (Fig. 250). We observe on each side of the head an 
 opening extending downward and forward to beneath 
 the chin. It is covered with a movable flap, the oper- 
 
BLOOD SYSTEM. 
 
 369 
 
 culum or gill cover. Lift this and look in; we see sev- 
 eral rows of red gill fringes, between which there are 
 openings into the throat — gill slits. Look next into the 
 throat; we see several cartilaginous diXQ)c\^%, gill arches, 
 with gill slits between. On these arches are fixed the 
 gill fringes, extending backward and outward under 
 
 Fig. 250. — Head of a fish with parts cut away so as to show the brain, spinal 
 cord, and the gills : cb, cerebellum ; op^ optic lobes ; cr, cerebrum ; olfl, 
 olfactory lobes ; olforg, olfactory org;an. rhe opercle is removed and 
 the gill fringes partly removed, so as to show the four gill arches. 
 
 ,the opercle. The fringes are thin, flat plates fixed to 
 the gill arches, like the teeth of a fine comb to its stem. 
 They are very thin, very numerous, and arranged in two 
 rows (Fig. 251). The purpose is to make as much sur- 
 face as possible. They are intensely red, because the 
 blood is profusely distributed in them. The blood passes 
 along each arch, and is thence distributed on the fringes. 
 Mechanics of Breathing. — The mouth is opened 
 and the throat enlarged and filled with water. By shut- 
 ting the mouth and contracting the throat the water is 
 
70 PHYSIOLOGY AND MORPHOLOGY OF ANLMALS. 
 
 370 
 
 forced through the gill slits. The current, quickened 
 by bringing down the opercles, runs between all the teeth 
 of the fringes and aerates the blood 
 there. This series of movements is 
 continually repeated. 
 
 The aeration of the blood in gill 
 breathers is by means of the //r^ oxy- 
 gen dissolved in the water. If water 
 be boiled so as to drive out all the 
 dissolved gases, it will no longer sup- 
 port life. If the gills of fishes be 
 kept moist and the fringes separated, 
 life out of water may be maintained 
 for some time. Some fishes have the 
 means of keeping their gills moist. 
 Such fishes often leave the water and 
 crawl about on land for hours. 
 
 Variation of Gills among 
 Fishes. — ^Ve have taken teleosts as 
 a type. Gills of other fishes may be 
 regarded as modifications of this 
 type. In sharks (Fig. 252) there are 
 five gill slits in the throat and five 
 corresponding separate gill openings 
 on the sides of the head, but not covered by an opcrcle. 
 There are cartilaginous plates between the gill openings, 
 and the throat slits, and on these are fixed the gill 
 fringes. The breathing is similar to that already de- 
 scribed in teleosts, except in regard to the movements 
 of the opercle. 
 
 In the lamprey (petromyzont, Fig. 253) we have 
 seven holes in the throat and corresponding holes on 
 the side of the neck, connected each by a pouch. In this 
 pouch are arranged the fringes. The breathing is the 
 same (Fig. 254). 
 
 Fig. 251. — Transverse 
 section of a gill arch 
 GA, showing a pair 
 of fringe plates ; A, 
 artery ; F, vein ; a, 
 arteriole ; w, veinlet. 
 
BLOOD SYSTEM. 
 
 In the lowest of all fishes, if fish it may be called, 
 VIZ., the lancelet (amphioxus), the enormously large 
 
 Fig. 252. — Anterior portion of a shark (Carcharias), showing; the five gill 
 openings. 
 
 throat has many — twenty or more — slits, edged with im- 
 perfect fringes (Fig. 255). 
 
 Going up now the other way, \n ganoids, such as the 
 garfish or bony pike [Lepidosteus) of our American fresh 
 waters, and in the Polyptenis of the Nile, we have an 
 opercle and gills like the teleosts, but gill breathing is 
 
 Fig. 253. — Petromyzon marius, showing the seven branchial openings. 
 (After Cuvier. ) 
 
 supplemented by a little air breathing by means of air 
 taken into a vascular air bladder (Fig. 256). 
 
 Finally, in the most reptilian of all fishes, such as the 
 protopterus of Africa, the lepidosiren of South America, 
 
372 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 Jat.o. 
 
 and the ceratodus of 
 Australia, besides their 
 gill breathing by organs 
 of the pattern of the tele- 
 osts, we have nostrils 
 opening into the throat 
 and very good lungs, al- 
 most as good as some 
 amphibians, and there- 
 fore both lung breath- 
 ing and gill breathing 
 in about equal propor- 
 tions. These are there- 
 fore called /«/7^ fishes. 
 
 Classification of 
 Fishes by Respira- 
 tory Organs. — The 
 form of the respiratory 
 organs is so characteris- 
 tic of the various orders 
 of fishes that Huxley 
 has made it the basis of 
 classification. Accord- 
 ing to Huxley, fishes 
 may be divided into six 
 orders — viz., (i) Dipnoi, 
 or nostril breathers, rep- 
 resented by the protopterus, lepidosiren, etc. These 
 breathe equally by lungs and gills. (2) Ganoids, so called 
 V nc 
 
 ^-'^\f\\Sj 
 
 Fig. 254. — Anterior portion of a lamprey 
 with parts cut away so as to show 
 the structure of the gill pouches : mt 
 op, interior opening ; ext op, exte- 
 rior Opening- ; r s, respiratory sac. 
 
 Fig. 255. 
 
 -Amphioxus : spc, spinal cord ; nc, notochord ; dv, dorsal vessel ; 
 st, stomach ; aa, aortic arches or gill arches. 
 
BLOOD SYSTEM. 
 
 373 
 
 Lei,--- "^'^iyj^" *.' 
 
 from their bony, enameled scales, which form a com 
 plete external armor. The}' supplement their gill breath 
 ing by a little air breathing. 
 They were abundant in early 
 geological times, but are now 
 represented by the Lepidosteus, 
 Folypterus, etc. (3) Teieosfs 
 (perfect bone), or ordinary 
 bony fishes. They breathe 
 wholly by gills, but in some 
 the air bladder opens into the 
 throat. (4) Elas7)iobraiichs 
 (plate gills), represented by 
 sharks, skates, rays, etc. (5) 
 Marsipobranchs (pouch gills), 
 represented by the lampreys 
 or conger eels. And (6) the 
 pharyngobranchs (throat gills), 
 represented only by that strange creature the amphioxus 
 or lancelet. The schedule will express this classifica- 
 tion in brief space : 
 
 1. Dipnoi — lung fishes. 
 
 2. Ganoids — armored fishes. 
 
 3. Teleosts — typical fishes. 
 
 4. Elasmobranchs — sharks, skates, etc. 
 
 5. Marsipobranchs — lampreys. 
 
 6. Pharyngobranchs — lancelet. 
 
 J ^^l'' *->' V-'"^: i 
 
 Fig. 256.— a portion of the in- 
 terior of the air bladder of a 
 Lepidosteus i enlarged 1, show- 
 ing its cellular structure. 
 
 TRANSITION FROM GILL BREATHING TO LUNG 
 BREATHING. 
 
 We have seen pure gill breathing in teleosts and 
 lower fishes. We saw pure lung breathing attained in 
 the higher amphibians, as frogs, etc. Now, in the higher 
 fishes and the lower amphibians we find every gradation 
 between — not, indeed, by transformation of the one into 
 
374 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 the other, but by siibstitutioii of the one for the other. 
 Some steps we have already seen in the higher fishes, 
 the others we find in the lower amphibians. In Gaiioids 
 we saw a little supplementing of gill breathing by the 
 taking of air into a vascular air bladder. This is the 
 beginning of lung breathing. In Dipnoi wt saw an equal 
 gill breathing and lung breathing. Now, in the lowest 
 amphibians, such as the siren and others, we find but 
 little advance on the Dipnoi. These breathe both by 
 
 gills and by lungs 
 /'^ all their lives. 
 
 They do not lose 
 their gills. They 
 are called peretmi- 
 branchs. The nec- 
 turus and the siren 
 are examples of 
 these (Fig. 257). 
 In the next higher 
 amphibians the gill 
 breathing is confined to their early life. They drop 
 their gills in maturity. These are called cadiicibranchs. 
 The salamanders and water dogs are of this kind. In 
 the highest amphibians, such as frogs and toads, the 
 changes are greater. These lose their gills somewhat 
 earlier, and also lose their tails, and are therefore called 
 anura, or tailless. Therefore amphibians may be classi- 
 fied thus : 
 
 Afiura — tailless, such as frogs and toads. 
 
 j-T J , . -1 1 \ Caducibranchs. 
 Urodela — tailed, - , , 
 
 I Perenmbranchs. 
 
 Here there is one great distinction between am- 
 phibians and reptiles. Amphibians all breathe by gills 
 during some period of their lives, and some always; 
 reptiles never. 
 
 Fig. 257. — Head and gills of Necturus. 
 
BLOOD SYSTEM. 3-3 
 
 Again, fishes are all throat breathers. They must be, 
 for they must force the water outward through their 
 gills. Now, amphibians also have gills, at least in early 
 life, and therefore must be throat breathers, at least in 
 early life. When they become lung breathers they 
 retain the throat method, and now force air down to the 
 lungs, instead of water out through the gills. Reptiles 
 never have this mode of breathing. 
 
 Amphibians were once regarded as an order of 
 reptiles, but now they are recognized as not only a 
 different class, but as more nearly related to fishes than 
 to reptiles. 
 
 SECTION III. 
 Blood Ci'rcn/att'on — / 'ertebrates. 
 
 Having given the general morphology of respira- 
 tory organs among vertebrates, we are now prepared 
 to give the course of circulation, and show how it is 
 controlled by the necessity of aeration. We take first 
 mammals, and, as usual, man as the type. 
 
 Circulation in Man. — Circulation means a going 
 round in a circle and a coming back to the starting- 
 point. A machine for circulation implies (i) a pumping 
 organ and a system of pipes. In the animal body the 
 pumping organ is the heart and the pipes are the 
 blood vessels. It implies (2) two kinds of pipes, 
 one carrying away, efferent ; the other bringing back, 
 afferent. In the animal body these are called arteries 
 and veins. There is, however, a third kind, connecting 
 these with one another, called the capillaries. The 
 arteries carry to the tissues, the capillaries among the 
 tissues, and the veins back again /;'<?/;/ the tissues. The 
 arteries carry blood to the work, the veins bring it 
 back after the work, but the work itself is done in the 
 capillaries. (3) In a good machine the pump must have 
 
3;6 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 two chambers — a receiving chamber and a pumping 
 chamber. This latter chamber must be strong, for it 
 urges the whole circulation. In the animal body the 
 receiving chamber is called the auricle and the pumping 
 chamber the ventricle. 
 
 (4) But there are two distinct objects to be subserved 
 in animal circulation — viz., the nutrition of the tissues 
 and carrying away of waste, on the one hand, and aera- 
 tion of the blood on the other. In the highest animals 
 these two are wholly differentiated and jealously kept 
 separate. There are, therefore, two complete circula- 
 tions and two hearts, each with its two chambers (auricle 
 and ventricle) and its three kinds of blood vessels (ar- 
 teries, veins, and capillaries). But for convenience the 
 two hearts are put together and form a/i^^/r-chambered 
 heart, but separated by a dead wall between. The blood 
 starts out from the heart on its journey in one circula- 
 tion and comes back ; then starts again on the other 
 circulation and comes back, and so on continuously. 
 The first is called the greater or systemic circulation, and 
 its function is to nourish the tissues and carry away 
 their waste. The second is the lesser ox pulmofiic circu- 
 lation, and its function is to aerate the blood or ex- 
 change the CO2 of the blood for O of the air. Of the two 
 hearts, that on the left side is the systemic and that on 
 the right the pulmonic. Through each of these circula- 
 tions the whole blood passes, and so jealously are they 
 kept separate that although the whole of the blood 
 passes through the lungs in the pulmonic circulation, 
 yet none of it is used for the nutrition of the lungs, but 
 this work is done by a small branch from the systemic 
 circulation. 
 
 General Course of the Circulation. — Fig. 258 
 gives a very generalized idea of this course. The ar- 
 rows show the course of the current. Commencing at 
 
BLOOD SYSTEM. 
 
 377 
 
 the left ventricle, the blood is thrown into the great 
 arterial trunk, called the aorta. This forms an arch — 
 the aortic arch — from the top of which go branches to 
 the upper part of the trunk, arms, 
 and head, while the main part 
 turns downward, running along 
 the backbone, branching and re- 
 branching to supply the viscera 
 and lower trunk and limbs. It 
 left the heart bright blood ; in 
 the tissues it gives up O and 
 takes CO2 and becomes dark 
 blood, and in this condition is 
 brought back — that from above 
 by the vena cava descendens, 
 that from below by the vena cava 
 ascendens — to the right auricle. 
 The right auricle contracts and 
 throws it into the right ventri- 
 cle, which in turn throws it 
 through the pulmonic artery, to 
 be distributed through the capil- 
 laries of the lungs, where it dis- 
 charges its CO2 and retakes O, 
 and becomes again bright blood, 
 left auricle and to the left ventricle, to commence again 
 another round. 
 
 Such is a most generalized formula. Fig. 259 sho\.s 
 it still very diagrammatically, but yet a little more in 
 detail, especially as to the great branches supplying the 
 liver, spleen, the intestines, and the kidneys, and the 
 corresponding veins to make up the vena cava ascendens. 
 Mark here the portal vein already spoken of (page 328) 
 and its singularity. 
 
 In both these diagrams the arterial trunks are on one 
 
 Fig. 258. — Diagp-am showing 
 {general course of the cir- 
 culation : upeXy upper ex- 
 tremities and head ; A.v, 
 lower extremities ; black 
 = dark blood. 
 
 Thence it goes to the 
 
378 PHYSIOLOGY AND MORPHOLOGY OF ANLMALS. 
 
 Sp 
 
 side and the venous trunks on the other. This is merely 
 for convenience of diagrammatic representation. Real- 
 ly they lie close together 
 along the backbone, as if 
 the diagram were folded 
 along the middle backward. 
 Again, of course, the aorta 
 in its lower part divides 
 into two branches to sup- 
 ply the legs, and similarly 
 the great branches going 
 upward divide to supply the 
 arms and head. These are 
 not represented. 
 
 For pure purposes of 
 physiology the formula ex- 
 pressed in these diagrams 
 is sufficient. But there is 
 so peculiar a significance in 
 relation to evolution in the 
 great oiitgoi/ig (not incom- 
 tn ing) vessels of the heart 
 that a more particular ac- 
 count of these is necessary. 
 The aorta coming out of 
 the left ventricle (Fig. 260), 
 as already seen, makes an 
 arch to the left and goes 
 down to form the abdomi- 
 nal aorta. From the top 
 
 Fig. 259. — Diapram showing^ course of the arch it sends off up- 
 of circulation, more in detail : T , , , , mi 
 
 and r, tissues ; //, hepatic artery ; Ward three branches. 1 he 
 
 sp splenic artery ; m, mesenteric ^ ^ is On the right side of 
 artery ; pv, portal vem ; 7\ renal '^ 
 
 artery ; i^, heart ; Z, lungs. The the arch (left in the dia- 
 arrows show the course of the . rr^, • ■ j- -j 
 
 circulation. (After Daiton.) gram). This again divides 
 
 
BLOOD SYSTEM. 
 
 379 
 
 into two branches, one (carotid) going to the right side 
 of the head and brain, the other (right subclavian) 
 going to the right arm. The ne.xt in order coming from 
 the arch is the left 
 carotid. It goes to 
 the left side of the 
 head and brain, 
 while the third and 
 last bends to the 
 left as the left sub- 
 clavian and goes to 
 the left arm. In the 
 right heart all that 
 is necessary to p(Mnt 
 out is the great out- 
 going vessel. The 
 pulmonary artery 
 arches to the left to 
 go to the left lung, 
 but sends back a 
 
 branch of equal size to supply the right lung. The sig- 
 nificance of all of these special arrangements will be 
 seen later. 
 
 The change from the bright blood to the dark blood 
 takes place in the capillaries of the tissues ; the change 
 back again to bright blood in the capillaries of the 
 lungs. Therefore in the systemic circulation the arteries 
 carry bright blood and the veins dark blood, while in 
 the pulmonic circulation the reverse is the case. All the 
 blood going from the lungs to the heart and from the 
 heart to the tissues is bright blood, and all the blood 
 from the tissues to the heart and from the heart to the 
 lungs is dark blood. Or, all the blood that ever visits 
 the left heart is bright and all the blood that ever visits 
 the right heart is dark. It is common to speak of the 
 
 Fig. 260. — The heart and the gjeat vessels, in- 
 coming and outgoing^. The right heart and 
 its vessels are shaded. 
 
38o PHYSIOLOGY AND MORPHOLOGY OF ANLMALS. 
 
 bright blood as arterial and the dark blood as venous. 
 This is true only of the systemic circulation. The very 
 reverse is true of the pulmonic circulation. 
 
 Structure of the Heart : The Valves.— We have 
 
 given the course of the circulation ; but the question 
 arises, How is it maintained continuously in the same 
 direction ? This is done in the same way as it is done 
 in machinery — viz., by a system of valves v^\i\c\\ prevents 
 its going in the other direction. 
 
 The valves of the heart are of two kinds, which may 
 be called curtain valves and pocket valves (semilunar 
 valves). The former are between the two chambers of 
 
 one heart, to prevent 
 py^ ^^^^^^ the blood from going 
 
 back to the auricle; 
 the latter are placed 
 at the outlet of the 
 ventricles into the 
 great outgoing arter- 
 ies, i. e., at the base 
 of the aorta and the 
 pulmonary artery to 
 prevent the blood dis- 
 charged into these ar- 
 teries from falling 
 back into the heart 
 when the ventricle ex- 
 pands again. The cur- 
 tain valves I so call because they are thin membranous 
 curtains between the two chambers of the heart and 
 opening always into the ventricle. From the interior of 
 the wall of the ventricle there go cords (heart strings), 
 which are attached to the edges of the curtain, so that 
 while these open easily into the ventricle, allowing blood 
 to pass (Fig. 261, A), yet if blood attempts to pass back 
 
 Fig. 261. — Left heart. A, auricle contracted, 
 ventricle expanded. B, ventricle con- 
 tracted, auricle expanded : /z', pulmo- 
 nary vein ; aii, auricle ; z', ventricle ; ao, 
 aorta. 
 
BLOOD SYSTEM. 
 
 381 
 
 Fig. 262. — Aorta cut open 
 and spread out to show 
 the semilunar valves. 
 
 into the auricle they flap back, pressing against one an- 
 other, closing the way, and are held in place by the 
 cords* (Fig. 261, B). The valve between the right 
 auricle and ventricle is called the tricuspid, because it 
 has three cusps. That in the left 
 heart is called the bicuspid, as hav- 
 ing only two cusps. 
 
 The semilunar, or pocket valves 
 are at the base of the aorta and 
 of the pulmonary artery, three in 
 each, of crescentic shape like shal- 
 low pockets, which, when filled, 
 press against each other from 
 three sides and completely close 
 the artery. When the ventricle 
 contracts, the blood rushes into 
 the artery, the valves pressing 
 close against the wall; but as 
 
 soon as the ventricle relaxes, the pockets fill, press 
 against each other, and close the gate (Fig. 262). 
 
 We now follow again the course of the blood, show- 
 ing how the valves work. Blood, dark from the tissues, 
 enters the right auricle. The auricle contracts, the cur- 
 tains flap wide open into the ventricle, and the blood 
 enters freely. The ventricle contracts, the blood shuts 
 the curtains, they are held in place by the cords, and 
 the blood is forced into the pulmonic artery. When the 
 ventricle relaxes, in order to fill itself again, the pocket 
 valves fill and close the way. The action on the other 
 side of the heart is the same. The blood coming from 
 the lungs into the auricle is thrown into the ventricle and 
 the door closed behind. It is therefore forced upward 
 into the aorta and again the door closed behind. It is 
 
 * To appreciate the action of these they must be examined on 
 a heart or a good model. 
 
382 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 the flapping and closing of these valves, both curtain 
 and pocket, that make the characteristic sounds of the 
 beating heart, and it is the inflammation and thickening 
 of them that constitute the commonest and gravest dis- 
 ease of the heart. 
 
 Blood Vessels. — i. Arteries. — In the dissection of 
 a body the arteries can be at once distinguished from 
 the veins by the fact that they retain their form as pipes, 
 while the thinner veins collapse. 
 
 Structure. — They consist of three coats : an outer thin, 
 very tough fibrous coat; a middle elastic, muscular, or 
 proper coat ; and an inner lining epithelial coat. The 
 outer coat gives toughness, the middle gives elasticity 
 and firmness, the inner is in some way necessary to the 
 life and integrity of the blood. In tying an artery the 
 surgeon draws the thread until he feels the cutting of 
 the middle and interior coats, and then secures the knot. 
 In aneurism the inner and middle coats are broken 
 and the tough outer coat is extended into a pulsating 
 tumor. The firmness of the middle coat is necessary 
 to maintain an open passage for the blood under power- 
 ful action of the heart, and by its elasticity it yields to 
 the impulse of the heart, and then, contracting, it carries 
 forward the blood in its course. It is this propagated 
 wave of swelling and contraction that constitutes the 
 pulse. From the great aortic trunk, of course, the 
 arteries branch and rebranch until they become of capil- 
 lary fineness in all the tissues, and finally grade into the 
 capillaries proper. The blood does not ooze through 
 the arterial walls ; these pipes do not leak. The nutri- 
 tion of the tissues is the function of the capillaries alone. 
 
 2. Veins. — The veins are much larger than the arteries, 
 and yet far less conspicuous, because they collapse when 
 empty. In structure they are similar to the arteries. 
 They, too, have three coats, but the middle coat is much 
 
BLOOD SYSTEM. 
 
 383 
 
 thinner. The veins are found in two positions; (1) 
 deep-seated and accompanying the arteries, and (2) 
 superficial or subcutaneous. It is the subcutaneous 
 which show bluish through the skin, especially in 
 blondes. The greater size and number of the veins is 
 necessary because the blood current is sluggish in them 
 as compared with the arteries. 
 
 Valves. — The comparative sluggishness of the current 
 is also the reason for the existence of semilunar valves 
 in veins. These are not in triplets, as at the opening 
 of the great arterial trunks into the heart. They are 
 
 B 
 
 Fig. 263. — Vein : A, cut open so as 
 to show the valves, v ; B, section 
 through the valves. 
 
 Fig. 264. — a, arteriole ; v, veinlet ; 
 Cy capillary network. The ar- 
 rows show the course. 
 
 scattered along their course irregularly and singly. 
 They can not, therefore, arrest, but only retard the 
 backward flow of the blood. The knotted appearance 
 of the subcutaneous veins when gorged with blood (as 
 when the arm is corded) is due to the filling of these 
 valves (Fig. 263). 
 
 3. Capillaries. — The blood system is a closed system of 
 pipes. There is no discharge of the blood from the 
 arteries into the tissues and taking up of it from the 
 26 
 
384 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 tissues by the veins ; on the contrary, the arterial 
 pipes are continuous through the capillaries with the 
 veins. Yet the capillaries are quite distinct from both 
 in several respects: i. In the mode of branching. The 
 arteries branch and rebranch tissneward, growing ever 
 smaller and more numerous; the veins run together 
 heartivard, growing ever larger and fewer ; the capil- 
 laries branch and run together again, without change of 
 size and without definite direction, forming a fine «<?/- 
 work (Fig. 264). 2. /// the course of the current. The 
 course of the current in arteries and in veins are alike 
 fixed. In capillaries it may go in any direction., although 
 it works deviously toward the veins. 3. hi structure. 
 The walls of the capillaries are of extreme thinness, and 
 therefore permeable to the blood plasm, and in case of 
 engorgement even to the leucocytes. 4. In function. 
 The function of the arteries is to carry the blood, without 
 loss by leakage, to the tissues; the function of the veins is 
 to bring it back without loss to the heart ; the function 
 of the capillaries is to allow the blood to exude into the 
 tissues. In any good system of irrigation the pipes car- 
 rying the water to the soil and bringing it back from 
 the soil must be /w/^rzw/zi', but those running among 
 the soil must be permeable. The blood system is such 
 a system of pipes. So jealously is this function of 
 nutrition restricted to the capillaries that even the 
 arteries and veins themselves are not nourished by the 
 blood that flows through them, but must have their own 
 arterioles, veinlets, and capillaries (vasa vasorum) for 
 that purpose. In the capillaries, then, the nutritive 
 matters of the plasma and the oxygen of the red glob- 
 ules exude into the tissues (exosmose), and the waste 
 matters of the tissues and the COg transude into the 
 blood (endosmose). But the exact nature of this pro- 
 cess is yet obscure. 
 
BLOOD SYSTEM. 385 
 
 COMPARATIVE MORPHOLOGY OF THE BLOOD SYSTEM IN 
 VERTEBRATES. 
 
 Mammals. — In all essential features the blood sys- 
 tem of mammals is exactly the same as that of man, 
 already described. All mammals have a four-chambered 
 heart — i. e., a double heart with no communication be- 
 tween except through the capillaries. They all, there- 
 fore, have a complete double circulation, with the whole 
 blood passing through both. 
 
 Birds. — Birds have also a double heart and complete 
 double circulation. There is only one thing worthy of 
 note, and that only on account of its significance in the 
 evolution of birds. It is that the aortic arch turns to 
 the right instead of to the left, as in mammals. 
 
 Reptiles and Amphibians. — The first important 
 variation from the model already given is found in these. 
 The heart of the reptile and the amphibian is three- 
 chambered instead of four-chambered. If the heart of 
 mammals and birds may be compared to two tenements, 
 each with two chambers, joined together, but with dead 
 wall between, then the heart of reptiles and amphibians 
 may be compared to one tenement with a suite of three 
 rooms. With such a structure it is impossible to have a 
 complete double circulation. The pulmonic circulation 
 is only a branch of the systemic, and therefore only a 
 part of the blood passes through the lungs to be 
 aerated, and therefore also not pure oxygenated, but 
 more or less mixed blood goes to the tissues. 
 
 Course of Circulation. — Fig. 265 is a schematic 
 diagram showing the course of circulation in reptiles. 
 The heart, as is seen, consists of two auricles and one 
 ventricle. The blood is thrown out of the ventricle into 
 the aorta, which makes an arch, but there divides and 
 sends a large branch to the lungs, while the main branch 
 
386 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANLMALS. 
 
 goes to the tissues above and below. That which goes 
 to the tissues comes back to the right auricle as dark, 
 and that which goes to the lungs to the left auricle as 
 bright blood. The auricles con- 
 tract and empty their contents 
 into the common ventricle as 
 mixed blood, which is again 
 thrown into the aorta, to be 
 again divided between the tis- 
 sues and the lungs. 
 
 There is a double reason why 
 these animals are cold-blooded : 
 (i) We have already seen that 
 the lungs in them are a coarse 
 sponge, and therefore the sur- 
 face of exposure of the blood 
 to the air is small ; (2) and now 
 we see that not the whole but 
 only a part of the blood is oxi- 
 dized in each round of the cir- 
 culation. Mixed blood goes to 
 the tissues. The metabolic process — i. e., waste and 
 supply — is less active, and therefore the heat is less. 
 
 We have given in Fig. 265 a schematic diagram illus- 
 trating the general principle. This is sufficient for 
 physiology but not for morphology, and especially not 
 for evolution. The actual course is far more complex, 
 and it differs also a little in different reptiles. We can 
 not take all the cases. We take the typical case of the 
 lizard. 
 
 The lizard (Fig. 266) has three aortic arches on each 
 side — six in all. What conceivable use can there be for 
 six aortic aches ? The blood from two of these — the 
 lower one on each side — goes to the lungs to be aerated, 
 while that of the other four, after sending a branch to 
 
 Fig. 265. — Diagram showing 
 the structure of the heart 
 and the general course of 
 the circulation in reptiles 
 and amphibians. 
 
BLOOD SYSTEM. 
 
 38/ 
 
 the head, unite to form the abdominal aorta which goes 
 to supply the viscera and lower portion of the trunk and 
 limbs. 
 
 I have taken the lizard as a type both of the three- 
 chambered heart and of the structure of the aortic arches. 
 But there is considerable variation among reptiles in 
 both of these, and these 
 variations show transitions 
 such as one would expect 
 on the theory of derivative 
 origin of organic forms. 
 For example, the perfect 
 three-chambered heart is 
 general among reptiles, as 
 in lizards and tortoises, 
 and also in all amphibians 
 — e. g., in frogs, toads, etc. ; 
 but in serpents there is an 
 imperfect four-chambered 
 heart. The ventricle has 
 been partly but not com- 
 pletely divided. In the 
 crocodile there is a cotn- 
 plete four-chambered heart, 
 but still the blood is mixed 
 in the course of the circu- 
 lation, though not in the 
 heart. Also the arches are 
 more or less modified from 
 the type given above. 
 
 Fishes. — Fishes have 
 chambered heart, and yet the whole of the blood passes 
 through the gills to be aerated, and therefore pure blood 
 only goes to the tissues. Fig. 267 is a schematic dia- 
 gram showing the general course of the circulation in a 
 
 Fig, 266. — Showing heart and out- 
 going blood vessels of a lizard. 
 The arrows show the course of 
 the blood. (After Owen.) 
 
 a Still simpler, viz., a t7vo- 
 
388 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 teleost fish. The blood from the single ventricle, v (dark 
 blood), is thrown into the aorta, which then divides into 
 three or four arches on each side, in order to pass through 
 the gill arches, to be distributed 
 in the gill fringes and aerated 
 there. From the gill fringes it 
 is gathered as bright blood, and 
 passes on to unite into the ab- 
 dominal aorta ivithout going back 
 to the heart, and is then distrib- 
 uted to the tissues. Its course 
 is shown by the arrows. 
 
 I'his is schematic. The ac- 
 tual course is shown in Fig. 268, 
 which shows also the change in 
 the blood in passing through 
 the gill fringes. As to variation 
 from this type, it is sufficient to 
 say that in sharks there are five 
 arches, in lampreys seven, and 
 in lancelets tw-enty or more on 
 each side. On the other hand, 
 the Dipnoi or lung fishes, lepido- 
 siren, and protopterus, like the reptiles, have only three 
 on each side. 
 
 Several interesting questions occur here, (i) We 
 have seen that fishes have a single heart. Which is it ? 
 Physiologically it is a pulmonic heart, for nothing but 
 dark blood visits it. But morphologically it doubtless 
 corresponds to the whole double heart of mammals. (2) 
 Is the circulation of reptiles or that of fishes the better? 
 Physiologically, in some respects the one, in some re- 
 spects the other is the better. The reptile has the ad- 
 vantage of air breathing, which aerates the blood more 
 efficiently than water breathing, but, on the other hand, 
 
 Fig. 267. — Diagram showing 
 structure of the heart and 
 course of circulation in 
 fishes. 
 
BLOOD SYSTEM. 
 
 389 
 
 the fish has the advantage of aerating the whole of the 
 blood, and therefore sending only aerated blood to the 
 tissues, while in reptiles only a part of the blood is 
 aerated, and therefore mixed blood goes to the tissues. 
 Per contra, however, the fish has again the disadvan- 
 tage of the blood not coming back from the gills to the 
 heart to receive fresh impulse, so that the heart has to 
 do the double duty of sending the blood by one impulse 
 
 Fig. 268. — A, heart and gill arches of a fish ; B, one arch with fringe (after 
 Owen) ; //, the heart. Dark blood is shaded. 
 
 through two capillary circulations. In a morphological 
 point of view the reptile is undoubtedly the higher form 
 of circulation, for it is a transition to the still higher 
 forms. The three-chambered heart is a transition from 
 the two-chambered heart of the fish to the four-cham- 
 bered heart of birds and mammals, and what might be 
 called the one-and-one-half circulation of reptiles a tran- 
 sition from the single circulation of the fish to the double 
 circulation of the bird and mammal. 
 
390 
 
 PHYSIOLOGY Ax\D MORPHOLOGY OF ANLMALS. 
 
 BEARING OF SOME OF THESE FACTS ON EVOLUTION. 
 
 1. Heart Structure. — There is abundant evidence, 
 both in the taxouomic series (animal scale) and the 
 embryonic series, that the heart has been gradually 
 formed by a process of evolution, for nearly all the 
 stages may be traced from the simplest pulsating organ 
 to the complex four-chambered valvulated structure of 
 the higher animals. The steps are briefly as follows: 
 
 (i) First there is a simple pulsating dorsal vessel. This 
 stage is found in many invertebrates, and even among 
 vertebrates in the amphioxus. 
 
 (2) Then comes a one-chambered heart. This is 
 found in many invertebrates. 
 
 (3) Then comes the two-chambered heart of many 
 invertebrates, and of fishes among vertebrates. 
 
 (4) Then the three-chambered heart of the typical 
 reptile and amphibian. 
 
 (5) Then the imperfect four-chambered heart of ser- 
 pents, the two sides of the heart still connected. 
 
 (6) Then the perfect four-chambered heart of croco- 
 dilians ; but even yet a connection (ductus arteriosus) by 
 which the blood to the tissues is mixed, and therefore 
 the circulation is not completely double. 
 
 (7) The final step is the perfect four-chambered heart 
 and complete double circulation of birds and mammals. 
 
 Nearly all these steps are found also in the embry- 
 onic development of mammals and even of man. 
 
 2. Origin of Aortic Arches. — But by far the most 
 interesting question in this regard is the origin and 
 meaning of aortic arches. 
 
 We have already drawn attention to the fact that the 
 aortic arch in the bird turns to the right, and not to the 
 left, as in mammals. This shows that mammals did not 
 come from birds by modification, for the intermediate 
 
BLUOD SYSTEM. 
 
 391 
 
 Stages from a right-turning to a left-turning arch would 
 be unsuitable. We will show that they both came from 
 reptiles, but by different routes. 
 
 Again we have drawn attention to the strange fact 
 that in reptiles (e. g., the lizard) there are three arches 
 on each side — six in all. Why six arches ? Surely, one 
 arch IS enough : for birds and mammals have but one, 
 and they have the most perfect circulation of all. The 
 key to the mystery is found in the circulation of fishes. 
 Fishes have three or four or five arches on each side, but 
 the reason in their case is obvious. They are the gill 
 arches. They are absolutely necessary for the aeration 
 of the blood. If the reptiles came by modification from 
 fishes, then the explanation of their numerous arches is 
 plain. They are the rcvmants of ancestral gill arches. 
 Some of the highest of fishes, the Dipnoi, have only 
 three arches on each side like the lizard. These fishes 
 also breathe by lungs as well as by gills. Now, sup- 
 pose in successive generations the lungs of Dipnoi to 
 increase in efficiency, and the gills to dwindle and dry 
 up, it is evident that exactly the structure found in the 
 lizard would remain. 
 
 That exactly this did take place in the history of the 
 evolution of vertebrates is proved by the fact that every 
 step is found now among fishes, amphibians, and reptiles, 
 and furthermore that the same change takes place now 
 in the individual history of every one of the higher am- 
 phibians, as, for example, the frog. We have already 
 traced the successive steps, through teleosts, ganoids, 
 Dipnoi, perennibranchiate amphibians, caducibranchiate 
 amphibians, to reptiles. We wish now to trace the same 
 change in the individual development of a frog. 
 
 The frog, as we all know, is at first a tadpole ; without 
 feet, swimming only by the tail; without lungs or air 
 breathing — in fact, breathing only water by gills. In this 
 
392 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 state it is essentially a fish. Now this fishlike gill-breath- 
 ing animal changes into a lung-breathing land animal, 
 
 Fig. 270. 
 
 Figs. 269, 270. — Diagrams showing the change of the course of blood in the 
 development of a frog. Fig. 269, the tadpole stacre. Fig. 270, the ma- 
 ture condition: //, heart; G G' G \ external gills; ff g' g' , internal 
 gills ; c c, connecting branches in the tadpole ; //, pulmonary branches. 
 
 and in doing so the gill arches are changed into aortic 
 arches before our eyes. The process is a little more 
 
BLOOD SYSTEM. 
 
 393 
 
 1 
 
 ^^^/^ 
 
 complex than I have represented, because the amphibian 
 has external as well as internal gills. But the same was 
 doubtless true in the evolution of reptiles. How, then, 
 does the change take place ? Fig. 269 represents the cir- 
 culation of the embryo 
 or tadpole, and Fig. 270 
 the adult. In the embryo 
 (Fig. 269), although all 
 the blood goes through 
 the gills, yet note rudi- 
 mentary vessels to the 
 rudimentary lungs, not 
 yet used, and also rudi- 
 mentary vessels connect- 
 ing gill arch to gill arch. 
 Again note in the adult 
 (Fig. 270) remnants of 
 dried-up gill vessels. The 
 letters are the same in 
 the two figures, and the 
 arrows show the direc- 
 tion of the current. 
 
 We have thus proved Fig. 271.— ideal diagram representing 
 
 the origin of the aortic rIXT''"" ^'''''' "'*'''• ^^^''' 
 
 arches of the lizard. 
 
 Now, the same is true of all aortic arches, even those of 
 man himself. 
 
 Birds and Mammals. — In birds and mammals the 
 modification has become so great as to obscure the 
 homology. The proposition to be proved is that the 
 great outgoing vessels of the heart are the modified rem- 
 nants of gill arches. Indeed, this is obvious enough if 
 we take the early embryonic condition of a bird or 
 mammal. The early mammalian, and even human, 
 embryo has gill slits, several of them on each side of 
 
394 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 the neck, and gill arches through which the blood 
 passes, forming, in fact, several aortic arches on each 
 
 side. But afterward 
 these are modified into 
 the outgoing vessels 
 of the heart, and their 
 original arch form is 
 retained only by one. 
 The manner in which 
 the change takes place 
 is shown in Figs. 271, 
 272, and 273. Fig. 271 
 shows in a schematic 
 way the primitive aor- 
 tic or gill arches (al- 
 ({, most exactly repre- 
 sented now by sharks), 
 and Figs. 272 and 273 
 the same as it is modi- 
 fied for a bird and 
 mammal respectively. 
 Finally, Fig. 274 is the 
 mammalian heart slightly modified to suggest the homol- 
 ogy of the several parts. 
 
 To explain: The gill arches are five in number on 
 each side, as in a shark (Fig. 271). The two upper pairs 
 are soon aborted, even before leaving the class of fishes, 
 and only three on each side remain to be accounted for. 
 These are the three found in the lizard, and inherited 
 with modifications in birds and mammals. These are 
 therefore the only ones we have to deal with. Fig. 272 
 shows the modification of this formula in birds. It is 
 seen that the first arch on each side becomes the two 
 pulmonic arteries, one going to each lung, precisely as in 
 the lizard and in the frog. Of the next pair of arches, 
 
 Fig. 272. — Modified for bird. 
 
BLOOD SYSTEM. 
 
 395 
 
 that on the right (left of the figure) becomes the aorta, 
 and that on the left the subclavian on that side. The 
 subclavian on the other side is a branch of the aorta. 
 The next pair became 
 the carotids. They are 
 already so in the lizard 
 and in the frog (see Fig. 
 270). 
 
 In the mammals the 
 modification is a little 
 different. It is seen 
 (Fig. 273) that of the 
 ^rst pa.ir of arches, that 
 on the left is the pul- 
 monary artery, which in 
 this case supplies both 
 lungs, while that on the 
 right is aborted. Of the 
 second pair, that on the 
 left (right of the figure) 
 becomes the aorta, while 
 that on the right the 
 subclavian on that side. The third pair, as in birds, and 
 as already in reptiles and amphibians, becomes the carot- 
 ids. In Fig. 274 we give a figure of the heart, with its 
 outgoing vessels a little modified, to suggest their ho- 
 mology, and numbered so as to show the corresponding 
 parts. 
 
 We see now that the only difference between bird 
 and mammal in the aorta and other outgoing vessels of 
 the heart is that among the various arches different ones 
 have been selected to be retained in the arch form as 
 aorta and for pulmonary artery and for subclavian. 
 In both cases — i. e., in both birds and mammals — the 
 steps of the change may be traced in the embryo. 
 
 Fig. 273. — Modified for mammal. 
 
396 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 Illustration of a Fwidatnental Law of Evolution. — It is a 
 fundamental law of evolution that t\\Q phylogenic or evo- 
 lution series is repeated more or less perfectly in the 
 ontogenic or embryonic series, and often also in the taxo- 
 iwviic or classification series. An admirable illustration 
 of this IS found in the life history of amphibians. 
 
 We have already seen that the frog in its embryonic 
 development is at first a legless, gill-breathing animal. 
 
 Fig. 274. — Diagram of mammalian heart: a, aorta; /, pulmonary artery; 
 jc, s c\ subclavium on each side ; c, c\ carotids on each side. The 
 numerals give the ordinal numbers of the arches as in the previous figures. 
 
 If it stopped there it would be classed as a fish ; but it 
 goes on. It next gets itself legs and an imperfect lung, 
 and breathes now both air and water. If it stopped here 
 it would be classed as a perennibranch ; but it goes on. 
 It next loses its gills and improves its lungs and breathes 
 air only, but retains the tail. If it stopped now it would 
 
BLOOD SYSTEM. 
 
 397 
 
 be classed as a caducibranch ; but it still presses on. 
 Finally, it loses its tail and reaches the highest order of 
 amphibia — the Anura. Of course the caducibranchs 
 and perennibranchs pass through similar stages, but 
 stop earlier. So much for the similarity of the onto- 
 genic and taxonomic series. 
 
 Now, there can be no doubt that the phylogenic series 
 is also similar, that the amphibians were evolved from 
 the dipnoan fishes, and that in the course of geologic 
 times they passed through the same stages — i. e., peren- 
 nibranch, caducibranch, and became anurous only in 
 Tertiary times. Thus the three series are similar. All 
 these facts are expressed in the following diagram : 
 
 Anura — ► = 
 Caducibranch - 
 Pereaaihrancb ■ 
 Fish 
 
 Ontogenic 
 
 -*Fish~^Perennibrancb^^c:aducibraBcb ->■ Anura ' 
 ■i- Ss 3 4 
 
 -Fisb—* Perennibrancb 
 
 Fig. 275. 
 
 SECTION IV. 
 
 Morphology of Respiratory and Circulatory System in 
 Invertebrates. 
 
 Some General Introductory Remarks. — (1) In 
 
 the case of vertebrates we gave first the morphology of 
 the respiratory organs and then of the circulatory organs 
 separately, but in the case of the invertebrates we shall 
 take these together in each class treated. 
 
 (2) In the vertebrates the blood system is a system 
 of closed pipes, continuous and without either discharge 
 or intake, e.xcept by exosmose and endosmose. In nearly 
 
398 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 all invertebrates there are blood-sinuses or reservoirs 
 in various parts of the body, into which the blood is 
 discharged, and from which it is again taken up. 
 
 (3) In vertebrates the oxidized blood is bright red 
 and the deoxidized blood dark purple-red, and this, 
 
 therefore, is the conven- 
 tional mode of repre- 
 senting these two kinds 
 of blood in colored dia- 
 gram. In default of col- 
 or we have used unshad- 
 ed and shaded spaces to 
 represent the same. We 
 continue to use the same 
 purely conventionally, 
 although in the case of 
 invertebrates there is no 
 such marked change in 
 color or shade. 
 
 (4) The diversity 
 among invertebrates is 
 so extreme that it is im- 
 possible to do more than 
 select a few striking ex- 
 amples from each de- 
 partment. 
 
 (5) In this selection 
 we pass over insects for 
 the present to come back 
 to them. The reason is 
 
 Fig. 276. — Lobster with the carapace 
 taken off : st, stomach ; dotted tube, 
 it, intestines; Z, liver; //, heart; 
 G, gills ; GG, green gland. 
 
 the same which induced us to do so in giving the com- 
 parative morphology of the eye (page 162) — viz., that in 
 respiration and circulation insects are entirely peculiar, 
 and are outside of the direct line of evolution, or of 
 increasing complication as we go up. 
 
BLOOD SYSTEM. 
 
 399 
 
 Arthropods : Crustacea. — As an example of the 
 department of arthropods, therefore, we select crusta- 
 ceans. 
 
 Respiratory Organs. — Take a crab, lobster, or crawfish, 
 and remove the carapace or dorsal shell. Directly 
 exposed on each side and occupying a large part of the 
 upper surface are seen 
 
 a great number of taper- 
 ing, finely lamellated or 
 tufted organs, Fig. 276, 
 G G. These are the 
 gills. They are not 7vith- 
 in the body cavity, but 
 wholly outside., in special 
 respiratory chambers, 
 opening by a large cleft 
 on each side of the shell 
 (Fig. 277). The gills 
 are, some of them, con- 
 nected each with a limb 
 or a maxilliped, and are 
 indeed appendages of 
 these, and some with the 
 thoracic walls. The fine 
 mosslike structure is a 
 device for producing as 
 large a surface as pos- 
 sible of exposure of the 
 blood to the aerated 
 water. 
 
 Fig. 277. — Transverse section throug;h a 
 crawfish, showing the g^lls : gcli, gill 
 chamber ; //, heart ; /, liver ; »', intes- 
 tine ; vs., blood sinus. The arrows 
 show the course of the blood. 
 
 Breathing. — The exchange of water is effected partly 
 involuntarily by ciliary currents, partly in some by the 
 action of the maxillipeds, and partly — i. e., when in active 
 locomotion — by the movements of the limbs, and there- 
 fore of their appendages, the gills. 
 27 
 
400 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 In all animals violent motion causes increased respi- 
 ration, but the mechanism may be different. In verte- 
 
 FiG. 278. — Side view of the circulatory and respiratory system of a crawfish,: 
 h, heart ; gg, the gills. The arrows show the course of the blood. 
 
 brates increased motion causes, of course, increased 
 waste in the blood. This in time stimulates the respira- 
 tory centers (medulla), and determines through the 
 pneumogastric nerve increased action of the respiratory 
 muscles. In crustaceans the increased motion produces 
 increased respiration not only through the mediation of 
 the nervous system, but also directly by the motion of 
 the gills themselves. 
 
 Circulation. — In the dorsal region behind the middle, 
 immediately beneath the carapace and between the points 
 
 of the gills, is seen 
 
 *r^^^^u ^. — =^..^ the heart [H, Fig. 
 
 276), a large or- 
 gan whose pulsa- 
 tions throw the 
 ^_^__ blood fore and 
 
 f,^ r>- 1 • .u 1 f ^ft through the 
 
 riG. 279. — Diagram showing the general course of * 
 
 the circulation in crustaceans: G, gills; H, large dorsal arte- 
 heart. ■ / \ 1 
 
 ries (« a) to the 
 
 tissues. Another artery goes from the heart downward 
 
 to form the sternal artery, and thence to the limbs and 
 
 lower part of the body (Fig. 277). From the tissues the 
 
BLOOD SYSTEM. 
 
 401 
 
 blood is gathered into large reservoirs (blood sinuses), 
 one on each side, running along near the base of the 
 limbs. From these sinuses the blood is again taken up 
 by the veins, to be distributed to the gills (Fig. 278) and 
 oxidized, and thence to the heart, to be again distributed 
 to the tissues. Fig. 279 is a schematic diagram showing 
 the general course. 
 
 It is plain that the heart in this case is physiologic- 
 ally a systemic heart, since it contains only oxidized blood, 
 and throws it to the tissues. 
 
 MOLLUSCA. 
 
 Acephala, or Bivalves. — Take as an example the 
 river clam {^Anodonta). Fig. 280 is a longitudinal, and 
 Fig. 281 is a transverse section. The gills, two on each 
 
 Fig. 280. — Vertical longitudinal section of Anodonta : ptn, am, p)osterior- 
 anterior adductor muscles ; w, mouth ; st, stomach ; c<e, cascum ; //, 
 heart ; /, foot ; G, gills ; ma, mantle. 
 
 side, are cellulated sacs, very much divided, to produce 
 large surface of contact with the aerated water. 
 
 Breathing. — Ciliary currents pass from behind for- 
 ward through the gills, determining oxidation of the 
 blood, and thenceforward to the mouth for alimenta- 
 tion, as already explained (page 338), and then backward 
 
402 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 to carry away excretions. In siphonated bivalves, as 
 already seen, the currents pass down one siphon and out 
 the other. 
 
 Circulation : Heart. — In the longitudinal section (Fig. 
 280) H is the heart, through which runs the intestine, i, 
 on its way to the vent. The heart consists of one ven- 
 tricle, and usually of two 
 M auricles. The relation of 
 
 the heart to the gills is seen 
 in Fig. 282. The blood 
 from the ventricle is thrown 
 
 Fig. 281. — Transverse section of 
 Anodonta : v, ventricle ; au^ auri- 
 cle ; vs, venous sinus ; ;«, mantle ; 
 br, branchiae or gills ; y, foot ; /^, 
 kidneys. The arrows show the 
 course of the circulation. 
 
 Fig. 282. — Diagram of heart and 
 branchiae of a bivalve, viewed 
 from above : v, ventricle ; au, 
 auricle ; br, branchia. 
 
 fore and aft to the tissues, thence gathered from capil- 
 laries by veinlets and emptied into several sinuses scat- 
 tered in different parts of the body. From these it is 
 taken up by the branchial vessels, which distribute it to 
 the gills, where it is aerated, and thence by branchial 
 veins to the auricles on each side and to the ventricle, 
 which again throws it in the tissues. A schematic dia- 
 gram which represents this would be similar to that used 
 for crustaceans (Fig. 279), except for the addition here 
 of the auricle, but the details of the circulation are, of 
 
BLOOD SYSTEM. 
 
 403 
 
 course, quite different. Like the crustacean, also, the 
 heart is a systemic heart — i. e., aerated blood fills the 
 heart and is distributed to the tissues. 
 
 Gastropods, Univalves. — Take, for example, a 
 snail. These are air breathers. As already said (page 
 340), there are four openings of the body in front. 
 These are (i) the mouth, (2) the genital opening in the 
 immediate vicinity, (3) the vent, and (4) the pulmonic 
 opening beneath the shell on the right side. We are 
 concerned now only with this last. It opens into a sac 
 immediately beneath the anterior upper portion of the 
 shell (pulmonic sac), on the interior of which are pro- 
 fusely distributed capillary blood vessels. 
 
 Breathing. — The change of air seems to be effected 
 by a muscular arched membrane just beneath the lung 
 sac, which may be compared to a diaphragm. The con- 
 traction of this membrane lowers the arch, expands the 
 lung sac, and draws in the air. This contraction, of 
 course, compresses the viscera, 
 on which it rests. On the relaxa- 
 tion of the membrane the natural 
 elasticity of the compressed vis- 
 cera lifts the arch and expels 
 the air. 
 
 Circulation. — The heart has 
 three chambers, two auricles and 
 a ventricle. Contraction of the 
 ventricle throws the blood to the 
 tissues, whence it is gathered into 
 
 the sinuses, and thence it is again taken up and dis- 
 tributed over the inner surface of the lung sac, where 
 it is aerated and then returned to the heart, to be dis- 
 tributed again to the tissues as aerated blood. 
 
 We have taken the higher air-breathing forms of 
 snails and slugs, but in water breathers, of course, we 
 
 Fig. 283. — Diagram of heart 
 and branchiae of a Gastro- 
 pod : v\ ventricle ; au, au- 
 ricle ; br, branchiae. 
 
404 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 have gills instead of lung sacs and water breathing in- 
 stead of air breathing. In Fig. 283 we show in diagram 
 the relation of heart and gills in these. 
 
 In many lower forms [Nudibra/ichiafa) the gills con- 
 sist of external tufted projections from the skin, waving 
 in the water. 
 
 Cephalopoda. — These are the highest of mollusca. 
 They are always water breathers, but their gills are 
 quite complex and efficient. The gills of these animals 
 lie on each side, in the dorsal region, and the breathing 
 is by the contraction of the hol- 
 low water-filled muscular man- 
 tle, which throws out the water 
 through the siphon (see Fig. 226, 
 page 341), and its own elasticity 
 restores again its form and draws 
 Fig. 284.^Diagram of heart j,-, ^^e fresh water. 
 
 and gills of dibranch Ceph- 
 
 alopod : z;, ventricle ; au. Circulation. — In dibranchs 
 
 auricle ; br. branchia;. , ■ , , , ^ 11 11 
 
 (two-gilled) or naked cephalo- 
 
 pods, such as the squid and cuttlefish, there are two auri- 
 cles, one to each gill; in tetrabranchs (four-gilled) or 
 shelled cephalopods, such as the nautilus, there are four 
 auricles. In all there is but one ventricle. The relation 
 of the heart to the gills is seen in Fig. 284. 
 
 ECHINODERMS. 
 
 As an example of these we take the starfish {As- 
 terias) . 
 
 Respiration. — The respiration of these is performed 
 in two ways: i. The whole body is hollow and the body 
 cavity is only partially filled with the viscera, leaving a 
 large perivisceral space filled with water. Contraction 
 of the whole body presses out the water through a mul- 
 titude of pores (Fig. 285, a «), while fresh water is drawn 
 HI by the restoration of the body form. The conta-ned 
 
BLOOD SYSTEM. 
 
 405 
 
 water is stirred about by ciliary currents and the blood 
 in all the viscera aerated. 2. The second method is by 
 
 TTi ^^^p^y..^y 
 
 Fig. 2S5. — Section through one arm of a starfish : si, stomach ; >«, mouth ; 
 cp, caecal pouches ; a/iip, ampullae ; i/, tube feet ; cr, circular vessel or 
 heart from which go the blood vessels ; a a, openings into the perivis- 
 ceral cavity. 
 
 the vesicles of the ambulacra. On the underside of each 
 of the five arms there is a longitudinal space perforated 
 with rows of holes (ambulacra), through which project 
 hollow tentacles, which are used for walking (tube feet). 
 Each hollow tentacle is connected with a vesicle within 
 (ambulacral vesicle, or ampul- 
 la), so that corresponding with 
 the rows of ambulacral tenta- 
 cles on the outside there are 
 rows of ambulacral vesicles 
 within (Fig. 285 and Fig. 286). 
 These vesicles are connected 
 with the aquiferous system, 
 characteristic of echinoderms, 
 in such wise that they can be 
 filled with water and again par- 
 tially emptied and the water 
 changed. 
 
 Circulation and Aeration. — The heart in these is a pul- 
 sating organ, extending from a vascular ring under the 
 dorsal skin above to another vascular ring surrounding 
 
 Fig. 286. — Transverse section of 
 an arm of a starfish : amp, 
 ampulla ; tj\ tube feet. (Aft- 
 er Parker.) 
 
4o6 PHYSIOLUGY AND MORPHOLOGY OF ANIMALS, 
 
 the mouth below (cv, Fig. 285). From these rings go 
 vessels to the end of each arm, which distribute blood 
 to all the viscera. This blood is aerated on the spot by 
 the water in the perivisceral cavity. Moreover, a por- 
 tion of the blood is specially aerated by distribution on 
 the ambulacral vesicles. 
 
 In addition to these two methods the highest echino- 
 derms, such as the Echinus, have tufted external gills 
 attached around the mouth. 
 
 CCELENTERATA. 
 
 Thus far we have a distinct blood system. The three 
 systems — food system, blood system, and respiratory 
 system — are well differentiated. But in the coelenterates 
 these three are not yet completely separated. The di- 
 gested food serves as blood, and the digestive system as 
 blood system. Neither is there any respiratory system 
 distinct from either, for the aeration of the tissues takes 
 place through the contact of water on the outside and 
 ciliary circulation of the food mixed with water on the 
 interior surface. 
 
 Taking the polyp (actinia) as example, as already 
 shown (Fig. 232, page 344), food taken into the stomach, 
 s, is retained by pyloric contraction until digested, then 
 dropped into the general cavity, and mixed with fresh 
 sea water. The mixture is stirred about by ciliary cur- 
 rents and directly aerates the tissues. Also the tissues 
 are similarly aerated by contact with sea water on the 
 whole exterior surface. 
 
 PROTOZOA. 
 
 Finally, in the protozoa there is no circulation of any 
 kind, nor is there any interior aeration, but the living 
 protoplasm is directly aerated only by contact of water 
 with the external surface. 
 
BLOOD SYSTEM. 
 
 407 
 
 This kind of aeration is sufficient for animals so low 
 in the scale of life, and especially of so small size, for in 
 these small animals the surface is large in proportion 
 to the bulk. But with increasing size, since bulk in- 
 creases as the cube while surface only as the square of 
 diameter, it is evident that the same degree of aeration 
 can not be effected without some device to increase 
 the surface of contact — i. e., a respiratory organ ; and 
 this must be still further increased when the organiza- 
 tion is higher as well as the bulk greater. And then, 
 last of all, for greater efficiency the whole is relegated 
 to an ititernal surface. 
 
 The same is true of the nutritive system proper — i. e., 
 food system and blood system. In a small body suf- 
 ficient food may be taken directly by a simple surface, 
 external or internal ; but in a large body the absorbing 
 surface is not great enough, and many parts are too far 
 away from the absorbing surface. There must be a sys- 
 tem of vessels to carry nutriment to distant parts. This 
 is the blood system. To illustrate : In a small island no 
 elaborate system of internal carrying trade is necessary, 
 for all parts are near the coast where products are 
 delivered. But as the island becomes larger, and espe- 
 cially as the commercial life becomes higher, the inter- 
 nal carrying trade becomes more elaborate. 
 
 INSECTS. 
 
 It will be remembered that we passed over these be- 
 cause the whole plan of their circulation and respiration 
 is entirely different and wholly out of the line of gradual 
 simplification which we otherwise find. In all other 
 classes the respiration controls the course of the blood, 
 and was taken up first ; but in insects the blood system 
 controls the character of the air system, and therefore 
 the blood system must be taken up first. 
 
4o8 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 
 Blood System. — Insects are very highly organized 
 animals, and yet their blood system is very simple and 
 incomplete, far more so than in mollusca or even echino- 
 derms. 
 
 Along the dorsal aspect of the body, immediately be- 
 neath the chitinous shell, there is a long, valvulated, 
 pulsating vessel — dorsal vessel (Fig. 287). This may be 
 called the heart. The valvules are so arranged as to 
 direct the course of the blood continualiy/(9;-7("«;-^. This 
 dorsal vessel divides into several arterial branches at its 
 anterior end, and receives several venous branches at 
 its posterior end. The anterior branches discharge into 
 the tissues forward, while the posterior branches suck 
 in from the tissues behind. This is apparently all that 
 there is of true vessels. In verte- 
 brates the blood system is a closed 
 system of pipes. In the higher in- 
 vertebrates there are indeed a 
 number of reservoirs scattered 
 about the body ; but still the blood 
 
 ^..., . , ,.„,,. system is essentially a vascular sys- 
 1^ ■',','• ^-'-■>-f \<-- 's'T -^ 
 
 ]'<:< {] A'l'i tem. But in insects there are no 
 ▼ '■'," ~'->-'/iv"*''' ',<■'■ 
 
 vessels at all except the large ves- 
 sels of the heart, but the tissues 
 are everywhere full of minute in- 
 tratissue spaces {laciincc) connected 
 with one another in all directions. 
 Now, by the continuous discharge 
 of blood in front and the sucking 
 up of blood behind, it is evident 
 that the blood must work round 
 among the tissues without definite channels, but in a 
 general way backward, to be taken in again into the 
 heart and forced forward. This, therefore, is called lacu- 
 nary circulation. There are also probably valvular open- 
 
 Fig. 287. — Uiagram show- 
 ing the heart and general 
 course of the circulation 
 in insects. 
 
BLOOD SVSTEM. 
 
 409 
 
 ^ 
 
 \ 
 
 ings on the sides of the heart by which blood may be 
 taken in. The diagram (Fig. 287) is an attempt to rep- 
 resent schematically the general course of the circula- 
 tion. The dotted lines represent the lacunary circula- 
 tion without definite vessels. 
 
 The difference between this lacunary circulation and 
 a true vascular circulation may be illustrated by an irri- 
 gation system. The circula- 
 tion of vertebrates may be 
 compared to a pump and a sys- 
 tem of pipes closed through- 
 out and impermeable until the 
 soil to be irrigated is reached 
 and there permeable. After 
 use the overplus of liquid is 
 again gathered into imperme- 
 able pipes and returned to the 
 pump to be again used. The 
 circulatory system of an in- 
 sect, on the contrary, is like 
 a pump with short pipes dis- 
 charging on the soil in front 
 and sucking up from the soil 
 behind. But the soil, instead 
 of being penetrated in all di- 
 rections by pipes which con- 
 fine and guide the currents, is 
 
 covered with little pools connected by channels. Under 
 these conditions the water would work around in an in- 
 definite way, and be sucked up behind, to be used again. 
 
 Respiratory System. — Now, it is this peculiar 
 mode of circulation that compels the very exceptional 
 kind of respiratory apparatus. Insects are active, and 
 somewhat warm-blooded animals, and therefore require 
 a perfect aeration of the blood ; but with a lacunary cir- 
 
 FlG. 288. — Tracheal system of an 
 insect : sp sp, spiracles ; s s, 
 air sacs. 
 
4IO PHYSIOLOGY AND MORPHOLOGY OF ANLMALS. 
 
 culation this is impossible in any localized or ga.n. In a 
 pipe system the whole of the blood may be made to go 
 through an organ localized in some part, but in a lacu- 
 nary system a local organ can receive only its small 
 share of the blood. Evidently, then, if the blood can 
 not go to the air, there is nothing left but that the air 
 must go to the blood. This is exactly what it does. The 
 air, by means of ramifying tubes, is carried to every 
 part of the body and aerates the blood on the spot 
 everywhere. 
 
 Air Tubes; Trachece. — On the margins of the dorsal 
 part of the chitinous shell are openings, one on each side 
 of each movable joint. These openings (spiracles) con- 
 nect by a short 
 tube with a long 
 lateral tube extend- 
 ing on each side 
 the whole length 
 of the body (Fig. 
 288). The lat- 
 eral tubes connect 
 across the body 
 with one another 
 by several trans- 
 verse tubes; and 
 from the lateral tubes and their transverse connections 
 go branches and sub-branches until the minute capil- 
 lary branches touch every cell of every tissue. All these 
 tubes are kept open by a spiral thread in their interior 
 (Fig. 289). 
 
 Now, as in vertebrates, in proportion to the vitality 
 of a part is the minuteness of the distribution of the 
 capillary blood vessels, so in insects, in proportion to 
 the vitality of any part is the minuteness of the distri- 
 bution of the air tubes. We have given the most com- 
 
 FlG. 2S 
 
 -Tracheas enlarged to show the spiral 
 structure. 
 
BLOOD SYSTEM. 
 
 411 
 
 mon arrangement, but there is some variation in this 
 regard. 
 
 Breathing. — If vve watch a hornet or wasp at rest, 
 we will observe a back-and-forth movement, an alter- 
 nate lengthening and shortening, of the abdomen. This 
 enlargement and contraction of the body cavity draws 
 in and expels air. We at once, therefore, see why oil is 
 so fatal to insects. It covers the spiracles with a film and 
 thus suffocates the insect. 
 
 SECTION V. 
 
 Lympliatic or Absorbent System. 
 
 Besides the blood system, there is another system of 
 vessels penetrating the tissues everywhere, which may 
 be regarded as supplementary to the blood system. It 
 carries not blood, but a clear liquid called lymph, and is 
 therefore called the lymphatic system. It is not a circu- 
 latory system, but purely an absorbent or drainage sys- 
 tem. We have already seen those of the intestines — 
 viz., the lacteals (page 326) ; but they are not confined 
 to the intestines, but occur everywhere. Nor is their 
 function even in the intestines confined to the absorp- 
 tion of food ; they absorb many other things. They are 
 far less understood, both as to their distribution and as 
 to function, than the blood system, for they are diffi- 
 cult to see, as they carry colorless liquid; and they are 
 difficult to inject on account of their valvular structure. 
 We treat them, therefore, very briefly. 
 
 General Description. — They begin by blind ex- 
 tremities in all the tissues, but especially in the ab- 
 dominal viscera, forming a capillary network (Fig. 290), 
 and therefore increasing but little in size until they 
 reach the larger trunks. The great emptying trunks are 
 (i) the thoracic duct on the left side of the backbone. 
 
412 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 This we have already seen as the trunk of the lacteals, 
 but it is also the trunk of much else. This is the largest. 
 (2) Another smaller 
 duct similarly situ- 
 ated on the }ight 
 side. These two 
 empty into the cir- 
 culation similarly — 
 
 Fig. 290. — Lymphatics 
 of the arm : agl^ axil- 
 lary glands. 
 
 Fig. 291. — Diagram of the lymphatic system : rch^ 
 receptacle of chyle ; tlid, thoracic duct ; th'd\ 
 right thoracic duct. The arrows show the 
 course of the blood in the jugular and sub- 
 clavian veins. 
 
 the one, as already explained, into the angle of junction 
 of the left subclavian and left jugular veins, the other at 
 
BLOOD SYSTEM. 
 
 413 
 
 the junction of the right subclavian and right jugular 
 veins. The former (left thoracic duct) drains the whole 
 of the viscera, the whole of the lower limbs and the left 
 side of the trunk, the left arm, and the left side of the 
 head, while the latter (right thoracic) drains only the right 
 side of the trunk, the right arm, and right side of the 
 head. Fig. 291 is a diagram showing this. 
 
 Structure. — The valvular structure is everywhere 
 conspicuous, and gives a beaded appearance to these 
 vessels. The valves open toward the 
 main trunks, and therefore toward the 
 heart, and tend to prevent backward 
 movement. This is the more neces- 
 sary as there is no impelling heart, the 
 force of motion being only a suction 
 force at the extremities. Doubtless 
 movements of the body and pressure 
 of the muscles tend to force on the con- 
 tained fluid (Fig. 292). 
 
 Function. — Their function seems 
 to be : I. The. absorption of waste. 2. The 
 absorption of excess of plasma exuded 
 from the capillaries into the tissues. 
 These two are going on everywhere and 
 at all times. 3. In the case of those of 
 the intestines (lacteals), in addition to 
 these two and at particular times, also 
 the absorption of the digested food. In 
 all these functions they are assisted 
 by the capillary blood vessels. 
 
 Lymphatic Glands. — In the 
 course of the lymphatic vessels everywhere, but espe- 
 cially the bend of the joints, as in the bend of the elbow 
 and knee, in armpit and groin, and in the spaces be- 
 tween the swelling muscles, as on the side of the neck, 
 
 Fig. 292. — Enlarged 
 section of a lym- 
 phatic vessel, show- 
 ing valves, V. 
 
414 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 and still more especially in the visceral cavities, are 
 found glands. We have already spoken of those of the 
 mesentery in connection with the lacteals. 
 
 Function of the Glands. — The functions of these 
 glands are (i) to furnish leucocytes to the blood. We have 
 already spoken of this function in connection with the 
 mesenteric glands, but now add that all the lymphatic 
 glands have this important function. (2) They also, as 
 already seen in the case of the mesenteric glands, seem 
 to confer upon the lymph, and upon the blood, perhaps 
 through the presence of the leucocytes, the property of 
 coagulation. (3) They seem also to have the power to 
 arrest poisons on their way to the blood. The lymphatic 
 vessels seem to absorb everything. They absorb also 
 poisons. Such absorbed poisons are arrested in the 
 glands, producing inflammation and suppuration espe- 
 cially m the armpit and groin. The poison is thus elimi- 
 nated, and the patient saved. They suffer vicariously 
 to save the body. 
 
 Comparative Morphology of the Lymphatic 
 System. — This system is found only in vertebrates. The 
 distinctive functions of the two systems — blood system 
 and lymph system — have not yet been differentiated in 
 invertebrates. The blood system performs the func- 
 tions of both. 
 
 In lower vertebrates the lymphatic vessels become 
 much more distinct than in man; and in amphibians — 
 e. g., in frogs — we find even propelling organs, lymphatic 
 hearts. Two of these are found in the sacral region and 
 two it! the scapular region. They are also found in 
 some birds, especially in the embryo of birds. These 
 vessels empty into the blood system in various places. 
 
 The \ym\i\\2iUc glands seem to appear first in birds, or 
 perhaps in Crocodilia. 
 
 Mammals in this regard are in all respects like man. 
 
CHAPTER IV. 
 
 KATABOLISM. 
 SECTION I. 
 Introductory. 
 
 Thus far we have treated oi food preparation and dis- 
 tributioti. Now we take up tissue decomposition and waste 
 elimination. Thus far the processes are ascensive and 
 distributive; those now to be discussed are descensive 
 and eliminative. In a word, thus far we have had to do 
 with anabolism. Now we take up katabolism. 
 
 Introductory. — Exchange of matter with the exter- 
 nal world can take place only through an external sur- 
 face or an infolding of an external surface. Foreign 
 commerce can take place only through a coast line or an 
 infolded coast line or bay. Many of the so-called inte- 
 rior surfaces, such as the stomach, the intestines, the 
 lungs, the bladder, etc., are examples of such infoldings 
 of the exterior surfaces. Real interior surfaces — i.e., 
 surfaces which have no connection with the external 
 world — are found in the cavities of the blood system and 
 of the nervous system (brain), and also in the pleural 
 and peritoneal cavities. We repeat, then, that exchange 
 with the external world can take place only through an 
 external surface or an infolding of the same. 
 
 \x\ plants it takes place on a directly external surface 
 — on an exposed coast line. Air containing food bathes 
 the surface of the leaves, and water containing food the 
 2S 415 
 
4i6 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 surface of the roots. But in all atiimals a part at least 
 of the exchange is through an infolded surface, or a bay 
 or harbor. In the lowest animals (^Protozoa), absorp- 
 tion or imports are through an infolded surface or bay, 
 ih^ stomach j but elimination or exports are still through 
 a directly external surface, an exposed coast line. In 
 the highest animals all exchange, both imports and ex- 
 ports, are through an infolded surface. 
 
 In going up the scale of life there are three progres- 
 sive changes in this regard, i. The relegation of more 
 and more of exchange to an infolded surface or bay 
 until a// is thus relegated. 2. The gradual differentiation 
 of the several kinds of functions, both absorptive and 
 eliminative (which in the lowest animals are performed 
 in every part alike), and their localization, each in 
 its own separate place. 3. The increasing complexity 
 of the infolding until it becomes almost inconceivably 
 great. If the simple infolding may be compared to a 
 bay or harbor, then the infolding of this again may be 
 compared to the slips and docks of the harbor. 
 
 Now, every such surface through which exchange 
 takes place, as already said (page 20), no matter how 
 complex, is covered with a pavement of living nucleated 
 epithelial cells, through the agency of which the ex- 
 change takes place. Such a complexly infolded surface, 
 covered with epithelial cells webbed together by con- 
 nective tissue and invested and isolated by fibrous or 
 serous membrane, constitutes an organ absorptive or 
 eliminative. The extreme complexity is especially char- 
 acteristic of eliminative organs, and among these the most 
 complex of all is the lungs. 
 
 Secretion versus Excretion. — Now, elimination 
 and eliminative organs are of two kinds — viz., secretions 
 and excretions, secretory organs and excretory organs. 
 In the former the products do noi pre-exist in the blood, 
 
K ATA 150 L ISM. 
 
 41/ 
 
 but are iiiaiiufactui cd out of blood and used in the 
 economy of the animal mainly in the preparation of 
 food. In the latter the products pre-exist in the blood, 
 are poisonous to the blood, and must be removed. In 
 the former there is first manufacture and then elimina- 
 tion of a useful product. In the latter there is simple 
 elimination of a hurtful product. Salivary glands, pep- 
 tic glands, pancreas, and mammary glands are examples 
 of secretory organs. The lungs and the kidneys are 
 the best examples of excretory organs. The liver is pe- 
 culiar and of a mixed character. The pure secretory 
 organs connected with the process of food preparation 
 we have already discussed. They are concerned with 
 anabolic processes. We are now concerned with the 
 purely eliminative or excretory organs, for these belong 
 to katabolism. The liver, being mixed in its functions, 
 will be taken up later. By far the most important of all 
 the katabolic processes is that of respiration. 
 
 SECTION II. 
 
 Function of Respiration. 
 
 We have already given the morphology of the respira- 
 tory organs, because this could not be separated from an 
 account of the circulation. But the physiology of respi- 
 ration — i e., its function in animal economy, its relation 
 to katabolism — is the same in all animals and must be 
 taken now. 
 
 We have already seen (page 358) that the general 
 purpose of respiration is the aeration of the blood, or, 
 more specifically, the exchange of CO2 of the blood for 
 oxygen of the air. Thus much it was necessary to assume 
 in order to understand the course of the circulation and 
 the changes in the blood in that course. We must now 
 explain the essential nature of the function and its neces- 
 
41 8 PHYSIOLOGY AND MORPHOLOGY OF ANLMALS. 
 
 sity. For greater clearness we throw the explanation 
 into the form of several propositions. 
 
 t. The Chemistry of Respiration. — The essential 
 chemical nature of respiration is an oxidation of food and 
 waste in the blood. Now, oxidation is a process of com- 
 bustion. Respiration is, therefore, a burning of food 
 and waste ; more specifically, it is a process of union of O 
 of the air with C and H of the blood, and the formation 
 of CO2 and HgO, which are returned to the air. The 
 fouling of the air by respiration is due mainly to the 
 accumulation of the eliminated CO3. 
 
 Proof. — The proof that oxygen is consumed and CO2 
 eliminated in respiration is so familiar that it is unneces- 
 sary to give it except in bare outline, (a) Thrust a 
 lighted taper into a gallon jar, and it burns freely; but 
 blow into the jar through a tube reaching to the bottom 
 until the jar is filled with the expired air only, and again 
 introduce the taper; it is immediately extinguished. 
 {b) Fill a test tube with clear limewater and blow through 
 the water with a glass tube for some time. The lime- 
 water becomes milky with precipitated lime, carbonate 
 {c) Take an equal bulk of pure air and of expired air 
 and compare their composition by analysis. It will be 
 found that in the expired air a certain volume of oxygen 
 has disappeared and been replaced by an equal or nearly 
 equal volume of CO2. So much for the COo. Now for 
 the HgO. (^) Breathe on cold glass. Immediately the 
 clear glass is clouded with deposited water. 
 
 Now, the union of oxygen with carbon and hydrogen 
 — i. e., the combustion of carbon and hydrogen, always 
 produces heat. This, then, is the source of animal heat, and 
 the difference between warm-blooded and cold-blooded 
 animals is simply a difference in rate of combustion. In 
 warm-blooded animals, as birds and mammals, the whole 
 of the blood is oxidized, the surface of exposure is larger, 
 
KATABOLISM. 
 
 419 
 
 the internal fires burn fiercely, and therefore the tem- 
 perature of the blood is high and independent of the 
 temperature of the exterior medium. In cold-bloocJed 
 animals, such as reptiles, amphibians, and fishes among 
 vertebrates, and in nearly all invertebrates, the fires 
 burn so low that the temperature of the blood fol- 
 lows somewhat closely that of the exterior medium. 
 Every stage of gradation, however, may be traced be- 
 tween them. 
 
 2. The Purpose of the Combustion. — The pur- 
 pose of the combustion is threefold: {a) To remove 
 waste. The waste of tissues is highly poisonous and 
 must be quickly removed ; most of it is removed in res- 
 piration by burning it up and changing it into gaseous 
 CO2 and HgO vapor, which are readily exhaled from the 
 lungs. {U) A still more fundamental and necessary pur- 
 pose of combustion is \.\\& generation of force. This, in- 
 deed, is the origin of the vital force, as it is that of the 
 force of the steam engine, (r) A third purpose is the 
 generation of heat — i. e., animal heat. This, however, is 
 of secondary importance. In the animal machine, as in 
 the steam engine, the real purpose is force, and the heat 
 is a necessary concomitant. In the animal body the heat 
 is sometimes a comfortable (in cold weather), sometimes 
 an indifferent, and sometimes a distressing (in hot 
 weather) concomitant. But we can not get the force 
 without the heat. It is worthy of note, however, that 
 from this point of view the animal body is a far more 
 efificient machine than any engine ever constructed — 
 i.e., of the heat of combustion, a far greater proportion 
 is converted into force. 
 
 3. The Fuel. — Again, the fuel used in combustion 
 is of three general kinds — viz., the amyloids and fats, the 
 albuminoid excess, and the 7vaste. {a) The amyloids and 
 fats consist only of C, H, and O, and therefore these 
 
A20 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 burn into CO3 and H2O without residue, and are elimi- 
 nated by the lungs alone. They do not form tissue. 
 They are used for fuel only, {b) Albuminoid excess — 
 i. e., albuminoid over and above what is necessary for 
 tissue building, both repair and growth, {c) Waste tissue. 
 These last two — i. e., albuminoid food excess and waste 
 — consist of C, H, O, and N, and often a small quantity 
 of sulphur and phosphorus. They are not wholly com- 
 bustible into CO2 and 
 H2O, and wholly elim- 
 inable by the lungs. In 
 CQtKO^ ^'^^^-XZ^iorea burning they leave an 
 Fig. 293.-Diagram showing the splitting incombustible residue 
 
 of albuminoids in a combustible and tO be eliminated by the 
 incombustible portion. ,„, 
 
 kidneys. 1 hey are, as 
 it were, j^/// into two parts, a combustible and an incom- 
 bustible. The combustible, consisting of the larger por- 
 tion of the C, H, and O, is eliminated by the lungs, but a 
 portion of the C, H, and O, together with the whole of 
 the N, is eliminated as urea by the kidneys. This is 
 diagrammatically represented by the formula (Fig. 293), 
 in which the line a b represents the line of splitting. 
 Therefore these two organs, the lungs and the kidneys, 
 are complementary to one another. They divide the 
 albuminoid food excess and the waste between them. 
 But the amyloids and fats are disposed of only by the 
 lungs. 
 
 We have seen that the three kinds of fuel used are (i) 
 amyloids and fats; (2) albuminoid excess; and (3) waste. 
 Now, since in the mature body the repair just balances 
 the waste., it is evident the fuel burned is exactly equiva- 
 lent to the whole of the food. 
 
 4. But the question occurs: (i) How is force created 
 by combustion ? and especially (2) How can force enough 
 be generated not only to do the work of repair and main- 
 
KATABOLISM. 42 1 
 
 tenance, but also for growth and activities of all kinds ? 
 These are obviously the questions most fundamental in 
 physiology. 
 
 In answer to the first question we may say that in 
 the passing of matter from a more complex to a simpler 
 condition, from a more unstable to a stabler condition, 
 as in combustion or in organic decomposition, force or 
 energy is liberated which is converted partly into heat 
 and partly also into other forms of energy, mechanical 
 or vital. To illustrate : Matter on a high plane (organic 
 matter) running down (katabolism) to a lower plane 
 (COo and HoO) generates force to raise (anabolism) other 
 matter (food) from a lower to a higher plane (tissue). 
 Thus while di large part oi the force oi plant life (viz., the 
 creation of organic matter) is derived from the sun in 
 the form of light, the n'hole of the force of animal life is 
 generated by the katabolic process going on in the 
 body. 
 
 But it will be objected that a certain amount of mat- 
 ter running down can do no more than raise the same 
 amount of matter the same height. The whole force 
 of u<aste is consumed in repair and nothing is left over 
 for the other activities of the body. This objection is 
 embodied in the second question. The answer to it 
 is (i) that in case of waste tissue, the running down 
 is to a much lower plane (COg and HoO) than that 
 from which the lifting took place (food). Therefore 
 the running down of say one pound of tissue to CO3 
 and H3O will easily lift one pound of albuminoid food 
 to the plane of living tissue and leave much force over 
 for activities of all kinds. (2) There is also much 
 food — viz., all the amyloids and fats and all the albu- 
 minoids in excess of that necessary for repair — that 
 runs down and generates force without expending its 
 force in liftinjr at all. 
 
422 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 I have constructed the following diagram to illus- 
 trate this process (Fig. 294) : 
 
 Animal Life 
 
 „ , ( Albuminoids 
 r. Food 
 
 lAviylafds i&fats_ 
 
 Fig. 294. — Diagram illustrating the generation of animal heat and 
 animal force. 
 
 We have represented here three planes raised one 
 above the other, viz., (i) the plane of the mineral king- 
 dom, (2) the plane of food, and (3) the plane of living 
 tissue. The plane of food is subdivided into a lower and 
 a higher. The lower form of food — viz., the amyloids and 
 fats — is at once burned without residue; the whole of it 
 runs down to the mineral kingdom COo and HgO * and 
 generates a corresponding amount of force. The higher 
 form of food, albuminoids, if in excess of the necessities 
 of repair and growth, also runs down, but in its down- 
 ward course is split (see Fig. 293 on page 420) into (a) a 
 combustible portion which is burned to COg and HgO, 
 and eliminated by the lungs, and {/>) an incombustible 
 portion which is eliminated on a little higher plane as 
 urea by the kidneys. Another portion of albuminoid 
 food is raised to the plane of living tissue for repair 
 and growth, and after remaining on that plane and 
 playing its part there for a time, also runs down as 
 
 * In the diagram we have used only CO9, because this really 
 gives the amount of force, the H and the O being already in the 
 food in proportions forming HjO, and therefore supplying no 
 heat or force. 
 
KATABOLISM. 
 
 423 
 
 waste to be disposed of in a similar way — i. e., partly by 
 the lungs and partly by the kidneys. The whole process 
 may be likened to a current in a siphon : the shorter arm 
 is anabolism, the longer arm and the one which de- 
 ter tiii ties the w/ioie current is kafabolism. 
 
 5. We have said that respiration is a process of com- 
 bustion in which oxygen of the air unites with C and H 
 of the blood and produces COo and HoO. Now exactly 
 the same takes place in the burning of oil. The material 
 is the same (C and H) ; the process is the same (union 
 with oxygen of the air) ; and the product, chemical and 
 physical, is the same — viz., COg and HoO and heat. 
 Moreover, it is certain that, estimating the whole force 
 produced in terms of heat, the amount of heat is the 
 same in the two cases. But in the case of the animal 
 body the heat is spread over a larger space and over a 
 longer time, and is therefore less intense. 
 
 6. Another important question is, Where does the 
 combustion take place ? The old view was that it takes 
 place in the lungs, and that this organ is the furnace of 
 the animal machine, that circulation brings the fuel and 
 respiration brings the air, and that the fuel is burned 
 at once then and there. It is now known that this is 
 not the fact. The blood brings the products of com- 
 bustion (CO2) to the lungs to be eliminated. Respira- 
 tion brings a fresh supply of oxygen to be taken by the 
 blood to the place of combustion. This place of com- 
 bustion is in the capillaries in contact with the tissues. 
 Yet neither are the decomposing tissues burned at once, 
 but circulate in the blood and undergo changes there 
 before they are finally consumed into COo. These 
 changes are very obscure and little understood. Some 
 of them are spoken of later. 
 
 Thus the blood is a reservoir for many things. It is 
 a reservoir for oxygen, which circulates in it until used. 
 
424 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 3 Animals 
 
 It is a reservoir for COg until eliminated by the lungs. 
 It is a reservoir (or food, which is drawn upon and re- 
 supplied by digestion and absorption. It is a reservoir 
 for wasie tissue until it is finally prepared for combus- 
 tion. In a word, it is a reservoir both for material and 
 for force. It acts like a fly wheel in the animal ma- 
 chine. 
 
 7. The Relation of Plants to Animals in regard 
 to the Creation of Animal Force.— It is seen from 
 the diagram (Fig. 294) that the anabolic /i/f, deter- 
 mined by the kat- 
 
 -> -~^ abolic descent, de- 
 
 gins more than half- 
 7vay up the scale of 
 existence. Other- 
 wise there could 
 be generated only 
 force enough to 
 make the lift and 
 maintain the tis- 
 sues, and none 
 would be left over 
 for activities of all 
 kinds. The ques- 
 tion then occurs, 
 How does matter 
 attain this half- 
 way plane ? The 
 answer is: It is 
 raised tothat plane 
 by sunlight acting on the green leaves of plants. Ani- 
 mals must feed on organic matter, but they have no 
 power to make it. It is prepared ready for them by 
 plants. The reason is, animals are wholly dependent on 
 katabolic processes within themselves for the generation 
 
 Fig. 295. — Diagram to show the forces of circula- 
 tion of organic matter. 
 
KATABOLISM. 425 
 
 of vital energy, and therefore the katabolic descent must 
 go far below the plane from which material is drawn by 
 anabolism. Plants, on the contrary, lake much energy, 
 viz., that expended in making organic matter, directly 
 from the sun ; an energy generated by a katabolic pro- 
 cess, true — viz., decomposition of CO2 — but determined 
 by an extertial force, sunlight. So that in the eternal 
 circulation of matter from the plane of minerals to that 
 of animal life and back again to the plane of minerals 
 the anabolic ascent begins in plants and completes itself 
 in animals; but the katabolic descent completes itself at 
 once in animal katabolism. So that the whole circula- 
 tion may be represented by an endless chain, as in the 
 adjoining figure (295). This circulation must be driven 
 in part by a force external to the chain. That force is 
 the sun. It is evident that animal force in activity, in 
 excess of maintenance of tissues, is the equivalent of 
 the excess of katabolism over anabolism — of the excess 
 {2, i) of the katabolic branch of the siphon. But this 
 is exactly equal to the lift by the sun (/, 2). Therefore 
 animal force or activity is equivalent to sun force in 
 making organic matter. 
 
 SECTION III. 
 T/:c fCt'dfieys : The Organ and its Fit net /on. 
 
 The Organ. — We have already said that the lungs 
 and kidneys are the two great organs for elimination of 
 the products of katabolism. They are also correlative, 
 since they divide between them the final disposal of the 
 albuminoid food and the waste. The kidneys, therefore, 
 must be our next subject. 
 
 Place and Form. — The kidneys are in the abdomi- 
 nal cavity, in the hollow on each side of the lumbar ver- 
 tebree, and just below the roots of the diaphragm. The 
 
426 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 vca 
 
 shape is very like a kidney bean, and they lie with their 
 concave sides toward each other. The blood supply 
 
 is large in propor- 
 tion to the size of 
 the organ, because, 
 as in the case of 
 the lungs, the sup- 
 ply is not mainly 
 for nutrition of the 
 organ, but for puri- 
 fication of the blood 
 (Fig. 296). The 
 blood brought back 
 from the kidneys 
 by the renal vein 
 is the purest blood 
 in the body, for it 
 is brought to the 
 kidneys as bright 
 blood purified of 
 CO3 in the lungs, 
 but - still contain- 
 ing urea ; but now 
 
 Fig. 296. — Showing form and position of the . . .^,.. 
 
 kidneys: ao, aorta; vca, vena cava ascen- it IS purincd Ot ItS 
 dens; ra, renal artery; rv^ renal vein; tt, , _„ • -.l virln^vc 
 
 ureters ; <5A bladder. urea in ttie kidneys 
 
 and returned to the 
 vena cava ascendens. There, however, it is, of course, 
 again mixed with impure blood from other tissues. 
 
 Excretory Duct. — The excretory ducts (the ure- 
 ters) are peculiar. By a trumpet-shaped mouth each 
 ureter grasps the deeply concave part (the pelvis) of 
 the kidney so as to receive the excretions; then passes 
 down on each side as a tube about the size of a crow 
 quill and about fifteen inches long and enters the blad- 
 der on each side below by a valvular opening, which 
 
KATABOLISM. 
 
 effectually prevents regurgitation. The secretion of the 
 kidneys drips, little by little, continually. It accumu- 
 lates in the bladder, and is thence from time to time 
 voided through the urethra. 
 
 Pelvis of the Kidney. — If the ureter be cut away, 
 we expose a deep concavity called the pelvis. Its whole 
 interior is covered with 
 mammillary protuber- 
 ances, like the ends of 
 the fingers put together. 
 These are the ends of 
 the cones. In the living 
 animal (chloroformed) 
 we may see liquid oozing 
 from innumerable pores 
 on this surface, collect 
 into drops, and run 
 down. These pores are 
 the openings of the uri- 
 niferous tubules. 
 
 Section. — By longi- 
 tudinal section through 
 the pelvis (Fig. 297) it 
 is at once seen that there 
 are two parts of the kid- 
 ney, differing in color 
 and structure — an inner 
 portion, of lighter color 
 
 and radiated structure, and an outer portion, darker and 
 nonradiated. The former is the tnedi/I/ary and the latter 
 the cortical portion. The excretion takes place mainly 
 in the cortical portion ; the medullary mainly transmits 
 it to the pelvis. 
 
 Minute Structure. — Examined with a microscope, 
 the medullary portion is seen to consist of straight 
 
 Fig. 297. — Section of the kidney showing 
 structure : P, pelvis ; u, ureter ; /«, 
 medullary, and c, cortical portion ; a a, 
 arteries. 
 
428 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 tubes, branching and becoming more numerous, but the 
 branches lie" parallel, and give rise to a radiating ar- 
 rangement. In the cortical portion, on the contrary, 
 the tubes are much convoluted, and finally terminate each 
 in a vesicle, which is filled with a tuft of capillary blood 
 vessels, looping round and connecting with an arteriole 
 
 o/i the one hand ax\d with a 
 veinlet on the other (Fig. 
 298). Blood vessels are 
 
 Fig. 298. — Uriniferous tubules : m, 
 Malpighian corpuscles ; gl, glom- 
 erules. 
 
 Fig. 299. — Uriniferous tubule : ttt, 
 with its Malpighian corpuscle and 
 glomerule ; a, arteriole ; i\ veinlet. 
 
 also abundantly distributed among the tubules. These 
 terminal vesicles are the Malpighian corpuscles, and the 
 contained vascular tufts glomendes. 
 
 Now, as in all eliminative organs, so here the whole 
 is lined throughout with epithelial cells (Fig. 299). The 
 epithelial membrane passes from the surface up the ure- 
 thra into and lines the bladder; from the bladder it 
 passes up the ureters and covers the pelvis. It then 
 passes up and lines the tubules and their terminal vesi- 
 
KATABOLISM. 
 
 429 
 
 cles. In fact, the kidney is little else than a mass of 
 such tubes, lined with epithelium, webbed together with 
 connective tissue, and invested with peritoneal mem- 
 brane, and then the whole liberally supplied with nerves 
 and blood vessels. The epithelial surface thus produced 
 is enormously great. 
 
 Function. — The function of the kidneys is apparent- 
 ly twofold — the ont physical^ the other vital. The one is 
 a purely physical and therefore nonselective filtration, the 
 other is a selective excretion of a particular product of 
 katabolism — viz., urea. The one is probably through the 
 capillary tufts of the terminal vesicles, and apparently 
 not by the agency of the ordinary epithelial cells (cells 
 here are squamous epithelium) ; the other, as in the case 
 of all the secretions and excretions, is by the agency of 
 the living nucleated cells lining the tubes. These are 
 glandular epithelium. By the former are eliminated from 
 the blood superabundant water and salts of all kinds, 
 normal or accidental. By the latter is removed a char- 
 acteristic product of katabolism — urea. This is the spe- 
 cial characteristic function of the kidneys. 
 
 Composition of Urine. — The secretion of the kid- 
 neys is a solution of urea (sometimes partly replaced by 
 uric acid) and of various salts — viz., sulphates, phos- 
 phates, chlorides and carbonates of potash, soda, and 
 lime. The most important parts are the urea (or its 
 equivalent, uric acid), together with the sulphur and 
 phosphorus oi Vat. sulphates and phosphates. The chlo- 
 rides and carbonates are constantly present in the blood, 
 and their excess eliminated by the kidneys. Urea and 
 to some extent also the sulphates and phosphates are 
 products of katabolism. The essential function is the 
 elimination of N and a little S and P. Of the elements of 
 albuminoids — viz., C, H, O, N, and S and P, as already 
 explained — the larger part of the C, H, and O are re- 
 
430 nivsiOLOGY and morphology of animals. 
 
 moved through the lungs as COg and HgO ; but the whole 
 of the N, P, and S, together with enough C, H, and O to 
 form with the N the substance urea, are removed by 
 the kidneys. The composition of urea is CH4N2O. 
 
 I Urea 
 c w f A r Chlor. 1 
 
 Urine = Solution of -. and I „ , 
 
 ,, ,^ I Garb. f 1 -rr 
 
 { Salts = - . of -; K 
 
 Sulph. 
 
 t Phos. 
 
 Na 
 
 K 
 
 Ca 
 
 The manner in which albuminoids, both food excess 
 and waste, are divided between lungs and kidneys is 
 shown in Fig. 300, already given, but repeated here 
 slightly modified. The amyloids and fats are not elimi- 
 nated by the kid- 
 .^ neys, but only the 
 
 lungs. 
 
 It is evident, 
 lungs^ ^"-^ ~XZ^Z^::^^ n.Lincys then, that the quan- 
 
 FlG. 300. — Diagram showing the division of tity of urea elimi- 
 albuminoids between the lungs and the kid- 
 neys, nated is in propor- 
 tion to the quantity 
 of albuminoid food, and is therefore greater in carnivores 
 than in herbivores, and also in proportion to the waste, 
 and therefore to the work done or degree of activity. 
 The estimation of the urea excreted is therefore the 
 most convenient mode of measuring tuaste, and is con- 
 stantly used for this purpose. 
 
 When urea decomposes in the presence of water it is 
 wholly converted into ammonium carbonate. 
 
 CH^XoO + 2H2O = (H4N)oC03 
 urea -(- water = am. carb. 
 
 Comparison of Lungs and Kidneys. — The com- 
 parison between these two complementary organs is in- 
 teresting in many respects. 
 
KATABOI.ISM. 
 
 431 
 
 1. We have alluded (page 419) to the importance of 
 the quick removal of the products of katabolism. Stop 
 respiration, and death by blood poisoning takes place in 
 five to ten minutes. If the function of the kidneys stops, 
 death by blood poisoning (uraemia) occurs in twenty-four 
 to forty-eight hours. See, then, the much greater im- 
 portance of the lungs. 
 
 2. It is on account of this supreme importance that 
 in the higher animals the whole of the blood passes 
 through the lungs, that there is complete double cir- 
 culation, and that blood completely purified of COg goes 
 to the tissues. On the contrary, only a portion of the 
 blood passes through the kidneys; the renal circula- 
 tion is only a branch of the systemic circulation, and 
 therefore only mixed blood, so far as urea purification 
 is concerned, goes to the tissues. But it will be remem- 
 bered that the same is true of the pulmonic circulation 
 in reptiles and amphibians. 
 
 3. Of foods, amyloids and fats are wholly removed 
 by the lungs, while the albuminoids are divided between 
 the lungs and the kidneys in the manner already ex- 
 plained. Of the elements of organic matter, the larger 
 part of the C, H, and O is removed by the lungs, but 
 the whole of the N, S, and P by the kidneys. 
 
 4. The lungs not only eliminate C'Oo and HgO, but 
 take in O for combustion. The function of the kidneys, 
 on the contrary, is purely eliminative. The eliminated 
 product in case of the lungs is the result of oxidation ; 
 in the case of the kidneys, of decomposition. 
 
 5. The circulation of the elements of organic matter 
 
 back to the atmosphere is through both of these organs. 
 
 The return of C in the form of COo is through the lungs ; 
 
 the return of N in the form of ammonia is through the 
 
 kidneys as urea, which, as we have seen, is quickly 
 
 changed into carbonate of ammonia. Thus all the ele- 
 29 
 
432 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 ments of living organisms taken from the atmosphere 
 and embodied for a brief time are again returned, and 
 the same matter is worked over and over again by an 
 eternal circulation. The circulation of C and O through 
 the atmosphere, plants, and animals back to the atmos- 
 phere is represented in diagram (Fig. 301, repeated from 
 page 5). It is seen that C is taken in the form of CO2 
 from the atmosphere by plants, is decomposed, the C 
 fixed in organic matter, and oxygen returned to the air. 
 The C is taken from plants by animals as food, is then 
 combined with O from air taken in respiration, and re- 
 turned to the air in its original form as COg, and so on 
 continually. 
 
 The circulation of all the elements of organic matter 
 between the organic kingdom and the atmosphere is 
 represented by the diagram (Fig. 302). The food of 
 plants consists of CO3, HgO, and NH3. These are taken 
 
 ^tffoi^^e^^ 
 
 '^limaM 
 
 P\a-n\S 
 
 Fig. 301. -Diagram illustrating the 
 circulation of carbon and oxy- 
 sen. 
 
 Fig. 302. — Diagram showing the 
 circulation of the elements of or- 
 ganic matter. 
 
 from the atmosphere and embodied in organic form of 
 both plants and animals. In their death these are again 
 returned to the atmosphere as COo, HgO, and NH3, to 
 be again taken as before. Thus the same small quan- 
 
KATABOLISM. 
 
 433 
 
 tity of CO2 and NH3 are embodied and disembodied 
 many times in the history of the organic kingdom. 
 
 COMPARATIVE MORPHOLOGY OF THE KIDNEYS. 
 
 Vertebrates : Mammals. — Little need be said on 
 the mammahan kidney. The organ has a similar posi- 
 tion in all mammals, and in most a similar shape and 
 structure to that of man. 
 In the ox the surface is 
 mammillated as if there 
 were a commencing sepa- 
 ration of the cones. This 
 separation is realized in 
 some living water mam- 
 mals, such as the otter 
 and the porpoise (Fig. 303). 
 But in these cases the dif- 
 ference is not significant. 
 Each cone consists of a 
 medullary and a cortical 
 portion, and they all dis- 
 charge into one pelvis and 
 ureter. 
 
 Birds. — The first important change in this, as in so 
 many other characters, is found in birds. In these (i) 
 the kidneys are not yet clearly differentiated into two 
 parts, a cortical and a medullary, having different func- 
 tions. (2) In these there is no bladder, but the ureters 
 empty into a cloaca or enlargement of the rectum just 
 within the vent. 
 
 Reptiles and Amphibians (i. e., all cold-blooded 
 land vertebrates) have a cloaca \ but some — e. g., snakes 
 and lizards — like birds, have no bladder, the ureters 
 emptying directly into the cloaca; while others — e. g., 
 tortoises and frogs — have a bladder. In these cases the 
 
 Fig. 303. — Section of the kidney of a 
 porpoise, showing its structure : 
 tt, ureter. ( From Owen. ) 
 
434 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 ureters empty into the bladder and the tirethra into the 
 cloaca. 
 
 The diversity in structure in the different orders of 
 fishes is so great that their description would carry us 
 beyond the scope of this work. 
 
 Invertebrates : Insects. — The essential character 
 of the kidneys — i. e., its significance in katabolism — is the 
 excretion of urea. This is therefore the true test of an 
 organ corresponding functionally to the kidneys. Now, 
 there is an extremely delicate chemical test of urea — viz., 
 the splendid//^;;/!'/*? produced by certain reagents. It is 
 wholly by this test that we are able to determine the renal 
 organ in invertebrates. In this way it is determined 
 that the so-called biliary tubes of insects (Fig. 219, bd, 
 page 336) are also their kidney — i. e., they excrete urea. 
 
 In crustaceans the so-cdAX^d, green glands (Fig. 221, 
 SSi V^Z^ 337)> which are large glands near the base of the 
 antennae and very liberally supplied with blood, are the 
 kidneys. But it is very noteworthy that in addition to a 
 peculiar form of uric acid (carcinuric acid) there is 
 secreted also leucomaines* which is the first product of 
 albuminoid waste in higher animals, and which in them 
 is split into a combustible portion, for use as fuel, and 
 an incombustible portion excreted as urea (pages 420 
 and 430). In crustaceans some of these substances is 
 excreted unutilized, and thus wasted. This is evidence 
 of low organization. 
 
 MoUusca. — By similar tests renal organs have been 
 detected in different classes of mollusca — viz., the 
 " spongy bodies " of cephalopods, the organ of Bojanus 
 of acephala situated at the base of the gills (Fig. 281, 
 page 402), and the lamellar gland near the pulmonic 
 sac, or the branchiae of gastropods. 
 
 * Marchal, Rev. Sci., li, 178, 1S93. 
 
KATABOI-ISM. 
 
 435 
 
 Below these the renal organ has not been detected. 
 Probably this function has not yet been differentiated 
 from others. 
 
 Observe, then : (i) Only a little way down the verte- 
 brate scale [birds) the distinction of cortex and medulla 
 is lost. (2) Only a little farther down the animal scale 
 {insects) the functions of liver and kidneys are merged, 
 although they are found separate even lower down. 
 (3) A little lower down [cn/s/acea) the function of the 
 kidneys is incompletely performed — i. e., a part of the 
 waste escapes decomposition and is thus wasted by not 
 being utilized as fuel. (4) A little lower down — i. e., 
 below mollusks — this organ has not been certainly 
 found. Probably the function is not yet differentiated. 
 
 SECTION IV. 
 T/ic Skin and its Function. 
 
 We have spoken of the lungs and kidneys as the two 
 great organs which share the elimination of katabolic 
 products between them. But there is still a third, the 
 skin, which, though less important, must not be neg- 
 lected. 
 
 Function. — All organs and functions are sympa- 
 thetically related to one another, so that the importance 
 of healthy action of an organ is not to be measured by 
 that of its more obvious and distinctive function. The 
 distinctive function of the skin is t/ie moderation of the 
 blood heat produced by continual combustion, by the 
 elimination of excess of water, and its evaporation on an 
 exposed surface. The elimination of water it shares with 
 the kidneys and is complementary with it, but it is 
 peculiar in the elimination of water in such wise as 
 thereby to cool the blood. 
 
 But elimination of water by the skin, as also by the 
 
4^6 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 kidneys, is of two kinds — physical and vital. The for- 
 mer is exhalation, the latter excretion. The one is always 
 insensible, the other is sensible — sweat. 
 
 Exhalation. — Water in a porous earthenware jar or 
 in a canvas bag, if hung up in the air in a hot, dry 
 climate, keeps cool, and even grows cooler. The water 
 soaks through and evaporates on the surface, but never 
 drips. So the tissues of the body are all permeable and 
 filled with water. Evaporation from the surface calls 
 the water to the surface as fast as it is evaporated. It 
 can not drip as sweat, because it is called there only 
 by drying. It is for this reason that in very arid re- 
 gions the suffering from heat is not very great, and 
 sunstrokes are unknown, although the thermometer in 
 the shade may run up to iio° to 120° F. Of course the 
 water which comes out and evaporates in this physical 
 way is nearly pure water. 
 
 Excretion. — But besides this physical exhalation, 
 which would go on even if there were no special organs, 
 there is also the excretion of srveat. This is the product 
 of the sweat glands. It is not called out by evaporation, 
 but as if it were pushed by the specific activity of the 
 glands, and may exude in such quantity as to roll down 
 and drip from the skin. This also spreads and evap- 
 orates, and thus cools the surface. This is not pure 
 water like the other, but water containing many salts 
 with a little oil and a little CO, and a trace of urea. 
 
 Structure of the Skin.— The skin, as already ex- 
 plained, is composed of two parts — viz., the dermis ox trtie 
 skin and the epidermis. The dermis is a mass of felted 
 interlacing fibers. It is very strong, highly organized, 
 and full of blood vessels and nerves. The epidermis, 
 which alone concerns us here, is continuous with the 
 epithelium, and, as to its truly living parts, similarly 
 constituted. There is, however, this difference: The 
 
KATABOLISM. 
 
 437 
 
 epithelium consists of only a few layers of living cells, 
 which are constantly dying, shedding, dissolving, and 
 forming the slimy mucus which covers the interior sur- 
 face and again renew- 
 ing ; while in the epi- 
 dermis the lower layer 
 in contact with the 
 dermis consists also 
 of living nucleated 
 cells, which are also 
 constantly dying and 
 renewing; but the dy- 
 ing cells, instead of 
 dissolving and form- 
 ing slime and leav- 
 ing the surface still 
 composed of living 
 cells, gradually dry 
 
 up, mummify, flatten Fig. 304.— Section through the skin, show- 
 mnrp and mnrp anrl '"^ its structure: ep, epidermis, and w, 
 
 more ana more, ana its Malpighian layer ; a-, dermis ; j-^, sweat 
 
 finally pass off as gland ; jo', sweat duct ; j/, sense papills ; 
 
 //, hair ; bx\ blood vessels. 
 
 scales of the scarf 
 
 skin. Thus, besides the lower layer [Malpighian layer) 
 of living cells, there are many layers in various stages 
 of dying and mummification. This mummified part is 
 the cuticle. The color of the skin is in the lower living 
 layer. A blister is a lifting of the epiderm from the 
 derm and an accumulation of lymph beneath. 
 
 Sudorific Glands. — Scattered in great numbers over 
 the surface of the skin are found pores formed by the 
 infolding of the living or Malpighian layer of the epi- 
 derm, forming tubes (sJ, Fig. 304) opening on the sur- 
 face. They pass through the dermis into the subcutane- 
 ous connective tissue, and are there convoluted into a 
 pellet, sg. These are the sudorific glands. The num- 
 
^^S PiiVSlULOGY AND MORPHOLOGY OF ANIMALS. 
 
 ber of the pores has been variously estimated at from 
 five hundred to twenty-eight hundred to the square inch, 
 and from one and a half million to seven millions on 
 the whole surface of the body. The aggregate length 
 of the tubes has been estimated at twenty-eight miles.* 
 Like all infolded surfaces, the tubes are lined throughout 
 with living epithelial cells, which preside directly over 
 the excretion. Blood vessels furnishing the materials 
 are distributed over the exterior surface of the tubes. 
 
 It is quite possible that the number and aggregate 
 length of the uriniferous tubes may be as great as that 
 of the sudoriferous, but in the one case these are com- 
 pacted into an organ (the kidney), and the excretion 
 gathered into a reservoir (the bladder), while in the 
 other they are spread over the wide surface of the skin. 
 The reason is obvious. The purpose of cooling the 
 blood by evaporation could only be subserved by a large 
 surface of direct exposure. 
 
 Lungs, Kidneys, and Skin compared. — AH three 
 of these eliminate COg, HgO, and urea. But the distinc- 
 tive duty of the lungs is the elimination of COg, that of 
 the kidneys urea, and that of the skin 7vater in such wise 
 as to cool the blood. In each case the elimination of 
 the other two products is subsidiary and, as it were, ac- 
 cidental. Nevertheless, we ought not to be surprised to 
 find a mingling of these functions more and more as we 
 go down the scale. 
 
 COMPARATIVE MORPHOLOGY AND PHYSIOLOGY OF 
 THE SKIN. 
 
 Genera Remarks. — i. If the skin be dry, harsh, 
 and without glands, it is a sign either that cooling of 
 the blood is not necessary, as in cold-blooded animals, 
 
 * Nature, 1, 257, 1894. 
 
KATABOLISM. 4-50 
 
 or else that this function is performed in some other way. 
 On the other hand, if the skin be soft, moist, and mu- 
 cous, it IS a sign that it is very active, and performs 
 many functions which have yet been but partially differ- 
 entiated and relegated to an infolded surface. 
 
 2. The function of cooling the blood by evapora- 
 tion can only e.xist in air-breathing and land-inhabiting 
 animals. 
 
 Mammals. — The structure and function of the skin 
 in mammals are similar to the same in man; yet there 
 are many mammals that are nonsw^eating. Such, in 
 many cases at least, are panting animals. The most 
 familiar illustration of this is the dog. The dog is 2ivery 
 hot-blooded animal, and yet in it there is no visible 
 sweat, although there is, of course, exhalation. The 
 cooling of the blood is largely through the lungs by pant- 
 ing. A dog pants not because he is tired and wants 
 more o.xygen, but because he is hot. By panting he fans 
 his lungs. 
 
 Birds are also very hot-blooded, and in hot weather 
 they also supplement the cooling of the blood through 
 the skin by panting. 
 
 Reptiles have dry, scaly, inactive skin. But little or 
 no cooling of the blood is required in them, because the 
 internal fires burn low; they are cold-blooded. 
 
 Amphibians have the extreme opposite condition. 
 They have soft, moist, mucous, and very active skins, 
 not, however, because they require cooling of the blood, 
 for they are cold-blooded and live mostly in the water, 
 but because many other functions are to some extent 
 performed by the skin — for example, respiration and 
 even the absorption of food. 
 
 Fishes live in water, and there can be no evapora- 
 tion from the skin ; also, like amphibians, the skin is soft 
 and slimy and active. 
 
440 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 Insects are more or less warm-blooded and require 
 blood cooling, yet they are covered with a dry, hard, 
 skeletal coat, which cuts off entirely any evaporation. 
 The function of blood cooling is undoubtedly performed 
 by the tracheae, which are admirably adapted for this 
 purpose. 
 
 Crustaceans are similarly covered and have no air 
 tubes, for they live in water, but they are again cold- 
 blooded. 
 
 Mollusca, again, go to the other extreme. The skin 
 of these is soft, mucous, and very active, and doubtless 
 performs many functions and supplements many other 
 organs. 
 
 In Echinoderms, again, we find the hard shell and 
 inactive surface. 
 
 In Coelenterates we return again to the soft and 
 active condition, which is found also in most Protozoa. 
 
 SECTION V. 
 The Live?- and its Fiinction. 
 
 We have now treated of the eliminative organs con- 
 nected with anabolism (secretory) and those connected 
 with katabolisvi (excretory) ; but there are still some 
 whose functions are of a mixed character, and therefore 
 put off to the last. Chief among these is the liver. 
 
 1. The Organ: Position and Structure.— The 
 liver, together with the stomach and the spleen, fill up 
 the concavity of the diaphragm, the liver being chiefly 
 on the right and the stomach and spleen on the left. 
 Its color, shape, and size are well known, and need not 
 detain us, as they have no special relation to function. 
 In structure it differs from other glands in not being en- 
 tirely or chiefly an aggregate of excretory tubes. On 
 
KATABOLISM. 
 
 44' 
 
 the contrary, it consists mainly of certain peculiar, very 
 solid, nucleated cells called liver cells (Fig. 305). 
 
 Four Systems of Tubes. — An eliminating organ 
 or gland has usually three systems of tubes or vessels: 
 
 (1) The artery and its branches, carrymg blood to the 
 work. (2) The vein and its branches, carrying back the 
 blood when the work is done ; these two connect 
 through the capillaries. (3) A system of excretory or 
 secretory tubes, carrying away the product of manufac- 
 ture. But the liver hdiS four systems of tubes ramifying 
 through its mass of liver cells : (i) The hepatic artery, 
 
 (2) the hepatic vein, (3) \.\\t. portal vein, and (4) the biliary 
 ducts. The one which 
 
 is peculiar is the portal 
 vein. We have already 
 (page 328) drawn atten- 
 tion to the peculiar- 
 ity of this vein. It is 
 the vein corresponding 
 to the mesenteric and 
 splenic arteries. But, in- 
 stead of emptying its 
 blood, like all normal 
 
 veins, into the vena cava, Fig. -^o^. — .Microscopic structure of the 
 , 11- liver : /c, liver cells ; etc, epithelial 
 
 close at hand, it goes to ceils; bd, biliary duct. 
 
 the liver, to be ramified 
 
 by capillaries through its substance, and only after doing 
 so carries its blood to the hepatic vein and thence to the 
 vena cava and the general circulation. So that instead 
 of two systems of vascular pipes, arteries, and veins, 
 connecting with one another through the capillaries, we 
 have three systems of such pipes — viz., hepatic artery, 
 portal vein, and hepatic vein — all connected continu- 
 ously by capillary circulation, so that water injected 
 into any one of these trunks will flow out of the other 
 
442 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANLMALS. 
 
 two. Besides these tubes, of course, there are the bile 
 ducts (Fig. 306) ; but these, like all excretory tubes, end 
 in blind extremities (Fig. 305). As we shall see pres- 
 ently, the products of the liver are bile and sugar. Of 
 these, the bile is carried away by the biliary ducts into 
 
 Fig. 306. — Diagram showing the distribution of the blood through the liver. 
 The arrows show the direction of the current. 
 
 the intestines, while the sugar is delivered to the blood 
 and carried into the general circulation by the hepatic 
 veifi. 
 
 2. Function : Two General Kinds of Glands. — 
 Glands are of two general kinds — ducted and ductless. 
 The one delivers its products by a duct to the surface, 
 the other delivers its product directly to the blood with- 
 out a duct. We have already seen two examples of 
 these in the lymphatic glands and the spleen, and have 
 stated (page 324) that the pancreas is also partly so. 
 
KATABOLISM. 
 
 443 
 
 But there are several others — viz., the thymus, the thy- 
 roid, the suprarenal, etc. Their functions are very ob- 
 scure, but very important. Only recently has attention 
 been strongly drawn to them. Their products have been 
 called internal secretions. 
 
 Again, the ducted glands, as already explained, are of 
 two kinds — secretory and excretory. The one manu- 
 factures useful products out of the blood, the other 
 eliminates hurtful products of katabolism from the 
 blood. 
 
 Now, the liver belongs to all three kinds. It manu- 
 factures two things — viz., bile and sugar. As a ducted 
 gland it delivers its bile by a duct into the intestines. 
 As a ductless gland it delivers its sugar directly to the 
 blood. Again, the bile is both a secretion used in the 
 digestive process and an excretion, purifying the blood 
 of the hurtful katabolic products. 
 
 Therefore the function of the liver is threefold: (i) 
 The manufacture of sugar, (2) the manufacture of bile 
 as a secretion, and (3) the elimination of bile as an excre- 
 tion. This last is probably connected with the destruc- 
 tion of the red blood-globules in the liver. We have 
 already discussed the bile as a digestive secretion. We 
 have also already spoken of the liver as the cemetery of 
 red globules, and said all that was necessary in the very 
 imperfect state of knowledge on this subject. All that 
 remains is the discussion of the liver as a manufactory of 
 sugar. This introduces the very important subject of 
 glycogeny. 
 
 Glycogeny and its Relation to Vital Force and 
 Vital Heat. — If we examine the blood of the hepatic 
 vein we always find a notable quantity of sugar. 
 Whence comes it ? It is at first natural to suppose that 
 it comes from the digested food, the sugar of which, we 
 have already seen, is taken up by the capillaries of the 
 
444 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 portal vein and carried to the liver. But, no. It may be 
 shown to be formed in the liver itself. The following 
 e.xperiments show it : 
 
 1. An animal may be fed on flesh alone (which we 
 know contains no sugar-making substance) for weeks 
 continuously, and yet sugar will be found in the hepatic 
 vein. Or else an animal may be starved for weeks, and 
 still sugar will be found in the hepatic vein. Evidently 
 it must be made in the liver. 
 
 2. Let the liver be taken from a recently dead animal 
 and laid on the table. If now water be injected into 
 any one of the trunk vessels, the liquid running out of 
 the other two, if tested, will, of course, show sugar ; con- 
 tinue the current until only pure water runs out, pure 
 both of blood and of sugar. Let the liver stand a while 
 — say an hour. If now the current be started again, the 
 issuing water will again show sugar. This may be re- 
 peated many times with similar result until finally no 
 sugar comes out. The material out of which sugar is 
 made is exhausted. The sugar, therefore, is evidently 
 made from some insoluble or nearly insoluble substance, 
 which is continually being changed into soluble sugar 
 and washed out. 
 
 3. If a liver be kept a considerable time the sugar 
 accumulates until the liver becomes perceptibly sweetish 
 to the taste, even when cooked. 
 
 Now, the substance from which sugar is formed has 
 been isolated and its properties and composition deter- 
 mined. It is a colorless, tasteless, odorless, nearly in- 
 soluble substance, having the composition of starch — 
 CgHiyOg. It is indeed a kind of animal starch. It exists 
 in the liver in large quantities — two per cent, seven per 
 cent, and even fifteen per cent of the w^hole weight. 
 Like starch or dextrine, but more readily than either, it 
 is changed into sugar by enzymes. There is an enzyme 
 
KATABOLISM. ^- 
 
 for this purpose in the liver cells. The process of 
 change is, of course, a hydration, exactly like that which 
 saccharizes starch — viz., QHiuOj + HgO = QH,oOe. 
 This substance is called glycogen, or the sugar-maker. 
 It has the same equivalent composition as starch and dex- 
 trine, but almost certamly a different molecular structure, 
 and therefore slightly different properties. For exam- 
 ple, it is more easily changed into sugar, and it gives a 
 different reaction with iodine. The sugar formed from 
 it, though it has the same equivalent composition as 
 glucose or intestinal sugar, has also probably a differ- 
 ent molecular structure, and therefore also slightly dif- 
 erent properties. It is apparently more easily oxidized 
 into COg and HgO. It is better, therefore, to call it 
 liver sugar or hepatose. 
 
 The formation of this sugar is a pure chemical not a 
 vital process, for it takes place in the dead as well as in 
 the living liver. The true vital process is the formation 
 of the glycogen, not the sugar. 
 
 Such are the undoubted facts. But the question oc- 
 curs. Whence comes the glycogen and what is its pur- 
 pose ? There are certain other facts which throw light 
 on this question. 
 
 1. The amount of amyloid food — say, potatoes and 
 rice — which is consumed by a man in a day, or even at a 
 single meal, may be two pounds. The whole of this is 
 changed into sugar, and, if carried at once into the blood, 
 would make that liquid as sweet as sirup. But, on the 
 contrary, the quantity of sugar in the general circula- 
 tion is very small — only a trace, ^^'hat becomes of it ? 
 
 2. It will be remembered that the whole of the sugar 
 is taken up by the capillaries of the portal vein, and car- 
 ried to be distributed through the liver before reaching 
 the general circulation. 
 
 Now, putting these facts together, it is evident that 
 
446 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 the large quantity of sugar carried by the portal vein to 
 the liver is detained there by dehydration into the insolu- 
 ble form of glycogen, and then slowly and continuously 
 rehydrated into the soluble form of liver sugar, and de- 
 livered little by little to the blood as the necessities of 
 combustion require. 
 
 Observe the double change. The sugar from the in- 
 testines is ^/^'hydrated only to be /rhydrated. The rea- 
 son is twofold : (i) It must be stored 2,x\.di delivered little 
 by little, because sugar in the blood in large quantity is 
 hurtful. Among other hurtful effects, see the cataract 
 and blindness of diabetic patients. 
 
 (2) Liver sugar is a more easily oxidizable sugar, a 
 more combustible fuel, than glucose. Glucose will cir- 
 culate in the blood for a long time until it is finally ex- 
 creted through the kidneys in diabetic patients. But 
 liver sugar seems to burn up almost as soon as it touches 
 the oxidized blood. 
 
 We have, then, one source of glycogen — viz., the glu- 
 cose taken up from the intestines and dehydrated in the 
 liver; but this can not be the only source, for flesh-fed 
 animals also make liver sugar. Therefore glycogen 
 must be made from albuminoids also. How it is made 
 is more uncertain. I believe it is made as follows:* 
 Remember that albuminoid food excess is split into a 
 combustible carbohydrate part and a nitrogenous incom- 
 bustible part ; the one eliminated by the lungs, the other 
 by the kidneys. Now I believe that the place of this 
 splitting is the liver, the combustible carbohydrate 
 formed \s glycogen, and the incombustible part is urea.f 
 
 But this is not yet enough. There must be still a 
 
 * See writer's views, Am. jnur., xv, (jg, 1878, and xi.x, 25, 
 1880. 
 
 f Urea is proved to be formed in the liver (Nature, Ivii, 395, 
 1S98). 
 
KATABOLISM. 
 
 447 
 
 third source, for starving animals still continue to make 
 liver sugar. Glycogen must also be made from waste 
 tissue. Remember, again, that waste also is split some- 
 where into a combustible and an incombustible part, the 
 one eliminated by the lungs as CO2 and HgO, the other 
 by the kidneys as urea. Now, again, is it not probable 
 that l\\t place of splitting is the liver, and the combusti- 
 ble portion is glycogen ? The splitting has long been 
 known. My view is that the place is the liver, the com- 
 bustible product glycogen, and the incombustible iirea. 
 
 Therefore, according to my view, there are three 
 sources of glycogen : (i) The whole of the amyloids de- 
 hydrated and detained in the liver, (2) the combustible 
 part of the albuminoid food excess, and (3) the combus- 
 tible part of the waste tissue. But since in adults waste 
 is equal to repair, this is equivalent to the whole of the 
 amyloids and the whole of the combustible part of the 
 albuminoid food, or the whole combustible food. 
 
 But there are also exactly the same three sources of 
 vital force. Therefore the whole purpose of this function 
 of the liver is the preparation of fuel, and the only fuel used 
 in the animal body is glycogen. The liver prepares the fuel, 
 the lung burns it, the kidney removes the incombustible 
 residue, or ash. The only food not taken into account 
 here is the fats. How this is burned, whether directly, 
 or whether it also, as is most probable,* is changed into 
 glycogen, is not certainly known. f 
 
 Strong confirmation of this view, so far as waste is 
 concerned, is brought out by some experiments of 
 Schiff. \ If the trunk vessels of the liver of a dog (Fig. 
 306, page 442) be ligated so that the blood can not trav- 
 
 * Chittenden, Nature, Iv, 303, 1897. 
 
 f Berthelot, Rev. Sci., viii, 129, 1897. Chittenden, Sci., v, 
 517, 1897. 
 
 % Arch, des Sci., Iviii, 293, March, 1877. 
 30 
 
448 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 erse that organ, the animal speedily falls into deep 
 coma and dies in a halt hour. Frogs bear well the liga- 
 tion of the liver, because katabolic processes are very 
 slow in these cold-blooded animals; but if a few drops 
 of the blood of a dog dead of ligated liver be injected 
 into a frog's veins it speedily dies if its liver be ligated j 
 but if its liver be not ligated it is not har fried at all. Evi- 
 dently, therefore, there is formed by katabolism a viru- 
 lently poisonous substance, which is decomposed and ren- 
 dered innocuous in the liver. This is certain. Now, my 
 view is that the manner in which this substance is de- 
 composed is by splitting into glycogen and urea. 
 
 The exact process of change of waste and of albu- 
 minoids generally in katabolism is very obscure and im- 
 perfectly known. Probably it is complex and contains 
 many steps. But the poisonous results of albuminoid 
 
 Animal Life 
 
 Fig. 307. — Diarjram i.lus.r.t^n^j the r;cnerr.'.ion of animal heat and 
 animal force, and the products formed in the process. 
 
 descensive change are called in a general way leiico- 
 maines. Now, the propositi'n is that leucomaines are 
 decomposed in the liver in the manner already explained. 
 After this explanation we now repeat the diagram on 
 page 422, with some additions, and are prepared to ex- 
 plain it more fully (Fig. 307). 
 
 This figure, in addition to the general process by 
 which vital force is generated, gives also some details: 
 (i) that amyloids, and probably fats also, are changed 
 
KATABOLISM. 
 
 449 
 
 into glycogen before turning into COg ; (2) that albu- 
 minoid excess and waste are changed into leucomaines 
 before splitting, and, when split, glycogen is the com- 
 bustible result. 
 
 If this view be true, then glycogeny is indeed a most 
 fundamental function, and its failure must sap the vital- 
 ity of the body in a marked degree. I believe that this 
 is shown in glycosuria or diabetes. 
 
 Cause of Diabetes. — This very grave and obscure 
 disease, in which sugar in large quantities is excreted by 
 the kidneys, is marked by extreme failure of vitality. 
 At first it was supposed that the kidney was at fault, 
 but, on the contrary, it does all it can to help the patient. 
 Sugar in large quantity in the blood is hurtful. The 
 kidneys remove it. Then it was supposed that the lungs 
 were at fault. The lungs, it was said, failed to take in 
 oxygen enough to burn up the sugar, and therefore it 
 must be excreted by the kidney. But not so, for in 
 these cases the blood seems to be sufficiently oxidized. 
 Then it was supposed that the fault lay in the liver, 
 which was thought to make too much sugar, more than 
 the lungs could burn. According to my view, the liver 
 is indeed in fault, but not in that way. Not by too 
 much sugar-making, but by too little glycogen-making. The 
 sugar from the intestines is not arrested and dehy- 
 drated in the liver, but passes right through and floods 
 the general circulation, and therefore must be removed 
 by the kidneys. Probably also the leucomaines are not 
 split as promptly as they ought to be, and remain to 
 poison the blood. There is therefore an inadequate 
 supply of liver sugar, which is the necessary fuel of the 
 body. It is easily seen, then, why this disease is char- 
 acterized by low vitality. 
 
 Recently another cause has been assigned for this 
 disease — a cause very important in relation to the whole 
 
450 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 subject of glycogeny — by Chavaux and Lepine.* Ac- 
 cording to these authors, the pancreas, like the liver, is 
 both a ducted and a ductless gland. As a ducted gland 
 it pours its digestive secretion into the intestines; as a 
 ductless gland it pours into the blood a peculiar fer- 
 ment, which has the property of changing liver sugar in 
 such wise that it is more readily combustible, and which 
 is therefore called glycolytic ferment. With the failure 
 of this function of the pancreas the sugar is not burned, 
 and must therefore be excreted by the kidneys. 
 
 It seems almost certain, then, that the cause of dia- 
 betes is either the failure of the liver to form glycogen 
 (by dehydration and arrestation of sugar in the case of 
 amyloids, and by splitting in the case of albuminoids), 
 or else it is the failure of the pancreas to secrete a 
 glycolytic ferment necessary for combustion of sugar 
 in the blood. 
 
 COMPARATIVE MORPHOLOGY AND PHYSIOLOGY OF 
 THE LIVER. 
 
 We have already spoken briefly of the liver in con- 
 nection with the digestive organs and functions of dif- 
 ferent animals, and little more need be said here. Its 
 position, shape, and even color is similar in all verte- 
 brates, and its function is doubtless the same in all. It 
 remains a very large and important organ in inverte- 
 brates even down very low in the scale. It is nearly 
 always distinguishable by its dark color. 
 
 As to glycogeny, this function is still performed by 
 the liver, yet glycogen is found somewhat widely dif- 
 fused in the tissues of many invertebrates, especially 
 mollusks. It is significant that the same is true of the 
 embryos of higher animals. This is exactly in accord- 
 
 * Rev. Sci., li, 376, 1893. 
 
KATABOLISM. 
 
 45' 
 
 ance with the law of differentiation, speciaUzation, and 
 localization of functions as we rise in the scale. Both 
 in the embryo-nic series and in the taxonomic series 
 functions are at first performed everywhere and by all 
 the tissues ; and as we rise in the scale they are more 
 and more separated, localized, and perfected. This one 
 has been gradually more and more localized in the liver. 
 
CHAPTER V. 
 
 TEGUMENTARY ORGANS SKIN STRUCTURES. 
 
 SECTION I. 
 Vertebrates. 
 
 It will be remembered that the whole surface, both 
 external and internal (by infolding of the external), is 
 covered with a pavement of living nucleated cells, and 
 that all exchange by absorption or elimination is by the 
 agency of these cells. When these are very active the 
 surface is soft and covered with a slimy mucus by the 
 continual decay and solution of dead cells, and is 
 called epithelium. When less active the old cells accumu- 
 late in many layers and dry up, mummify, and flatten, 
 and finally pass away as scales. This is called the 
 epidermis. 
 
 The epiderm, therefore, is said to consist of two 
 layers — viz., a layer of living nucleated cells in direct 
 contact with the dermis and called the mucous or Mal- 
 pighian layer, and a layer of accumulated cells in all 
 stages of dying and flattening and mummification, and 
 called the ctiticular layer. The color of the skin, whether 
 blond or brunette, or brown or black, is determined by 
 the amount of pigment in the mucous layer. The 
 cuticular layer, in the act of drying up, may harden in 
 various degrees. 
 
 For example, if the epithelial cells only mummify 
 they form the ordinary cuticle. If they simply harden, 
 452 
 
TEGUMENTARV ORGANS— SKIN STRUCTURES. 
 
 453 
 
 they form horn, as in hair, nails, claws, hoofs, horns, 
 feathers, scales, whalebone, etc. If they calcify or ossify 
 by deposit of lime car- 
 bonate or phosphate, 
 then they form bony 
 scales, scutes, shell, 
 etc. 
 
 Importance in 
 Classification. — As 
 the skin comes directly 
 in contact with and is 
 modified by the envi- 
 ronment, these struc- 
 tures are very charac- 
 teristic of the various 
 classes, orders, and 
 families of animals, 
 and are therefore very 
 important in classifi- 
 cation. We have al- 
 ready shown that teeth 
 are skin structures, 
 and their importance 
 in classification is uni- 
 versally acknowl- 
 edged. The structures 
 about to be described 
 are hardly less so. We 
 give only the most im- 
 portant. 
 
 Hair. — True hair is entirely characteristic of mammals. 
 It is always formed in a follicle or infolded socket, in 
 which the epithelial cells multiply more rapidly than 
 elsewhere and at the same time harden to the condition 
 of horn (Fig. 308). Hairs, therefore, grow by successive 
 
 ■o'-'O K'^^-^. 
 
 Fig. 308. — Section of skin shewing; hair 
 follicle : pap, papilla ; ml, .Malpighian 
 layer ; rsh, root sheath ; t/, dermis ; sgl, 
 sebaceous gland ; f , cuticle ; cort, corti- 
 cal, and med., medullary part of the hair. 
 
454 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 additions to the bottom, pushing outward, not by addi- 
 tions to the top Uke a tree. The follicle in which it is 
 formed acts as a mold, determining its shape. The 
 quills of the porcupine are only large hairs. 
 
 Nails. — If we tear off a finger nail, we find beneath 
 an exquisitely tender surface of dermis covered with a 
 layer of very active epithelium. At the base of the nail 
 is a pocketlike infolding, where the epithelium is espe- 
 cially active. Over the whole surface the epithelial cells 
 harden into horn, but this process is especially active 
 in the pocket (Fig. 309). Therefore the nail, by its 
 more rapid formation in the pocket at its base, is pushed 
 forward continuously, and, if not worn away or pared 
 away, will grow indefinitely. In some countries, as in 
 
 Fig. 309. — Section of the end of the 
 finger, showing how the nail is 
 formed : c, cuticle ; ml, Malpighi- 
 an layer ; hc^ the horny cuticle ; 
 ^, the bone. 
 
 Fig. 310. — Section through terminal 
 joint : clcr, claw core ; he, horny 
 cuticle ; ml, Malpighian layer ; 
 c, unhardened cuticle. 
 
 Japan, they are sometimes protected and become enor- 
 mously long, as a badge of a leisure class. 
 
 Claws. — These differ from nails only in the fact that 
 they grow on all sides instead of one side of a peculiarly 
 shaped terminal joint of a finger or toe. The bone of 
 the terminal joint is the claw core and determines the 
 shape of the claw. The core is covered with dermis and 
 with an active layer of epithelium, which hardens into 
 
TEGUMENTARY (ORGANS-SKIN STRUCTURES. 
 
 455 
 
 horn. At the base there is an infolded pocket in which 
 growth is more active, so that the claw is pushed for- 
 ward in proportion as it is worn off by use (Fig. 310). 
 
 Hoofs. — These, again, differ from claws only by the 
 size and shape of the terminal joint on which thev are 
 molded. This terminal joint is 
 the hoof core or coffin bone. We 
 have here the same pocketlike 
 infolding, and the forward-push- 
 ing growth in proportion to 
 wear (Fig. 311). If wearing is 
 prevented by shoeing they must 
 be trimmed from time to time. 
 
 The horny armature of the 
 terminal joints of the toes is 
 characteristic of land vertebrates 
 — i. e., reptiles, birds, and mam- 
 mals. They are absent even in 
 amphibians on account of their 
 greater alliance to fishes. Furthermore, mammals, by 
 the character of this armature, are divided into ungulates 
 (hoofed) and unguiculates (clawed). 
 
 The original generalized form of this armature was 
 apparently of flattened shape on the dorsal side of the 
 toe somewhat like that of man. This was the case in 
 some of the earliest mammals, as, for example, the 
 Phenacodus, and has been retained by apes and by man. 
 From this generalized form have been differentiated 
 claws on the one hand and hoofs on the other. 
 
 Horns. — These are almost characteristic of rumi- 
 nants. Paired horns on frontal bones are entirely so at 
 present, although not so in early geological times. Again, 
 frontal horns are of two kir.ds — viz., solid horns, as in 
 the Cervidce (deer family), and hollow horns, as in the 
 Bovidce and Ovidie — e. g., o.\, sheep, goats, etc. 
 
 Fig. 311. — The joints of the 
 toe of a horse. The parts 
 are similar to the last two 
 figures. 
 
456 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 Fig. 312. — Section throug-h 
 a cow's horn : c, cuticle ; 
 hc^ horny cuticle ; w//, Mal- 
 pighian layer ; //cr, horn 
 core ; d^ dermis. 
 
 In hollow horns (Fig. 312) we have first a conical 
 projection of the frontal bone, covered as usual with 
 dermis, and this in its turn with active epithelium hard- 
 ening into horn. About the base 
 there is, as usual, an infolding in 
 which the growth of horn is espe- 
 cially rapid. Successive layers 
 are formed one within another 
 precisely as in claws. The hol- 
 low horns are permanent. 
 
 Solid horns, as in the deer, elk, 
 etc., are bosses which grow out 
 from the two frontal bones with 
 great rapidity, and are at first 
 covered with skin and fine hair 
 (so-called velvet). In this condition it is very vascular. 
 In a little while — a month or so — the blood gradually 
 withdraws, the skin dies and is rubbed off, and the antler 
 is really dead though firmly attached to the skull. At 
 the end of the year a separation takes place at the 
 skull, the antler drops, and the skin of the skull grows 
 over and covers the wound. Soon growth begins again 
 at the same place and a new pair of antlers is formed, to 
 pass again through the same annual cycle of changes. 
 
 In comparing the antler of a deer with the horn of 
 an ox or sheep it is evident that the mature antler of 
 the former corresponds with the horn core of the latter, 
 while the horny exterior of the latter corresponds to the 
 skin and velvet of the former. 
 
 Thus, then, the ruminants are divided into two groups, 
 the hollow-horned and the solid-horned. The horns of 
 the former are permanent, those of the latter are decidu- 
 ous. The two grade into one another in the antelopes. 
 
 Feathers. — This most wonderful of all skin struc- 
 tures is whollv characteristic of birds. 
 
TEGUMENTARY ORGANS— SKIN STRUCTURES. 
 
 457 
 
 Structure. — An ordinary quill feather is a marvel 
 of lightness, strength, and elasticity. The quill is hollow 
 — a form which gives greatest strength for the same weight 
 
 (pf sk p.v 
 
 Fig. 313.— Section throuj^h the successive feathers of a bird's wing, showing 
 the mode of overlap : s/i, shaft ; av, anterior, and pv, posterior vane. 
 
 A.A ,,,)il 
 
 of material. The s/ia/l may be regarded as a hollow 
 tube filled within with lightest bracing, with the horny 
 envelope thickest on the back, where the strain comes in 
 flight. On each side of the shaft is the 7a/ie. It will be 
 observed that the two parts of the vane are not equal, 
 the backward 07'erlaJ>/>ed pa.rt,J> v, being much the broader. 
 The effect of this 
 is to close up the 
 feathers into a 
 solid plane in the 
 downward blow 
 of the wing, while 
 it opens the feath- 
 ers and lets the 
 air through in the sh-^ 
 upstroke (Fig. 
 313). x\gain, the 
 vane consists of 
 barbs, the shafts of 
 which are broad 
 and therefore 
 strong in a verti- 
 cal direction, and 
 
 on these barbs again are a vanelet on each side com- 
 posed of barbulcs, and finally the barbules are hooked 
 together by little elastic hooks (Fig. 314). In Fig. 315 
 
 Fig. 314. — Portion of a shaft ish'^ and vane on 
 one side, showing; b) the barbs and yb' ) the 
 barbules with their hooks. 
 
458 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 Fig. 315. — A magnified view of one barb (b b), 
 with its barbules (b' } armed with hooks. 
 
 we give a magnified view of one barb with its barbules 
 and hooks. The object of this structure is to make an 
 impermeable, light, and elastic plane. If these delicate 
 
 parts are disarranged 
 or ruffled, they are 
 easily rearranged, 
 and hooked together 
 again by preening. 
 So much for the de- 
 vice for making an 
 impermeable plane. 
 Now observe how 
 the plane is set on the fore limb. It is not set on both 
 sides, but only on the backward side of the extended 
 arm. The downward stroke of the wing therefore tips 
 up the wing plane behind, so that the same stroke sus- 
 tains and also drives forward. In Fig. 316, which is a 
 cross section of the wing plane, if i, 3 represent the whole 
 air pressure in the down- 
 stroke, then I, 2 will be ^^ 
 the force which sustains 
 the bird, and 3, 2 that 
 which drives it forward. 
 How Formed. — 
 We have seen that all 
 these structures thus far 
 are formed in or on a 
 mold, which determines their shape, and from which they 
 are pushed out. Now, the same is true of feathers also. 
 All this complex structure is secreted in an equally com- 
 plex mold, from which it is necessarily pushed out, and 
 the mold corresponding successively destroyed. 
 
 Gradation to Hairs. — We have described the quill 
 feathers of the wings and tail, for these are most com- 
 plex and characteristic, but all gradations may be traced 
 
 Fig. 316.— Cross section of the wing 
 plane of a bird, showing its action in 
 flying. 
 
TEGUMENTARV ORGANS— SKIN STRUCTURES. 
 
 459 
 
 Fig. 317. — Diagfram of an ostrich plume, con- 
 sistinj^ of shaft (s/i i, barbs, and barbules, but 
 no hooks. 
 
 through the body feathers, down, and plumes, to simple 
 hairs. The plumes of the ostrich have the structure 
 of feathers, except 
 they have no hook- 
 lets, and therefore 
 the barbules do 
 not cohere (Fig. 
 317). Plumes like 
 those of the heron, 
 egret, etc., have no 
 barbules; they are 
 essentially slender, 
 branching hairs 
 (Fig. 318). Again, 
 about the beaks of 
 many birds we find 
 simple hairs (Fig. 
 319). This would 
 seem to indicate a 
 close relation in 
 this regard be- 
 tween birds and 
 mammals. But 
 birds undoubtedly 
 came from reptiles, 
 
 and therefore feathers are probably some modifica- 
 tion of scales. But the gradations here have not been 
 found. 
 
 Scales are characteristic of reptiles and fishes, espe- 
 cially the latter. 
 
 Fish Scales. — We take these as the type. They cover 
 the whole body. They are formed much like nails — 
 i.e., on the surface of the mucous layer, and especially 
 in pocketlike infoldings. The manner in which from 
 those pockets they grow backward, shingling over one 
 
 Fig. 318. — Diagram of an egret plume, consist- 
 ing only of shaft and barbs. 
 
 Fig. 319. — Hair about the beak of a bird, con- 
 sisting of shaft only. 
 
460 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 another, is shown in the figure (320), which is taken with 
 some simplification from Owen. 
 
 Classification of Fishes by Scales. — Agassiz di- 
 vided fishes by the character of their scales into four 
 
 Fig. 320. — Section through skin of a fish: d, dermis; tnl, Malpighian 
 layer ; sc, scale, v From Owen. ) 
 
 orders — viz., the Ctenoids, the Cycloids, the Ganoids, and 
 the Placoids. Ctenoid scales are pectinate on their pos- 
 terior or exposed margin (Fig. 321, A). Cycloid scales 
 are smooth and rounded on this margin (Fig. 321, B). 
 Ganoid scales are bony, enameled, and usually rhom- 
 
 D 
 
 Fig. 321.— Fish scales, illustrating Agassiz's classification. 
 
 boidal, close fitting, and not shingling (Fig. 321, C). 
 Placoid scales are very small, with outsticking sharp 
 points (Fig. 321, D). The first two are horny, the last 
 
TEGUMENTARY ORGANS— SKIN STRUCTURES. 
 
 461 
 
 two are bony. The Ctenoids are spine-rayed fishes, like 
 the perch, etc. ; the Cycloids are soft-rayed fishes, Hke 
 the cod ; the Ganoids are sturgeons and bony pikes or 
 gars; iht Piacoids are sharks, skates, and rays. If we 
 make the Ctenoids and Cycloids subdivisions of teleosts ox 
 ordinary typical fishes, then the classification is a good 
 one as far as it goes, and has done good service in geol- 
 ogy? for it is the scales which, together with the teeth, 
 are most apt to be preserved. 
 
 Reptile Scales. — We have taken fish scales as the type ; 
 but scales are found also in reptiles, and even in mam- 
 mals. In reptiles they may be horny, as in snakes and 
 lizards, or bony scutes, as in Crocodilia, or a combina- 
 
 C^d 
 
 Fig. 322. — Diagram showing- strrcture and mode of formation of a rattle- 
 snake's rattle: abc, horny cuticle of last joints of tlie vertebra [V\\ 
 a' h' c\ that of last year slipped back and caught ; ad" c , and 
 a" b'" c"\ are still earlier cuticles. 
 
 tion of those two, as in the shells of tortoises and tur- 
 tles. Scutes differ from scales proper in involving the 
 dermis as well as the epidermis. 
 
 In snakes the horny, scaly cuticle is shed every year, 
 and a new one is formed by the mucous layer. This an- 
 nual skin shedding gives rise in an interesting way to 
 the rattle of the rattlesnake. The few last joints of the 
 vertebral column (a, b, c) are enlarged and consolidated 
 into an irregular mass, and covered with a horny scale 
 thicker than elsewhere on the body (Fig. 322). With 
 the skin shedding this loosens, and is shed like the 
 rest of the skin, but, being elastic, it slips back and 
 catches on the neck of the next joint. In the next 
 
462 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 year's skin shedding the horny covering of this part 
 slips back again, and is caught in the same way, and so 
 on. Thus is formed a series of molds of the three con- 
 solidated joints loosely caught together. Every year 
 adds another to the chain, and, if not broken or worn 
 off, the number indicates the number of skin sheddings, 
 and therefore the age of the snake. 
 
 Turtle Shell. — Turtles are incased in an immovable 
 shell. The dorsal part is called the carapace, the ventral 
 part tht plastron. Where, then, is the jointed backbone 
 characteristic of vertebrates ? Their structure seems to 
 violate the vertebrate plan. But not so. It is only an 
 extreme modification of that plan, and is an admirable 
 illustration of adaptive modification underlying homology 
 (page 246). If we look into the interior of the carapace 
 of a complete skeleton of a turtle we see a continuous 
 series of vertebrne, consolidated with one another and 
 with the shell in the region of the trunk, but movable 
 in the neck and tail. The shell of a turtle or tortoise is 
 indeed a very complex structure, consisting of three 
 parts — skeletal, dermal, and epidertnal. The spinous pro- 
 cesses of the vertebrae expand at the top into broad flat 
 plates, which unite with one another to form the ridge ox 
 central row of plates of the carapace. The ribs also 
 expand into broad plates, which, uniting, form the slop- 
 ing under-roof on each side. Then the dermis over all 
 this is ossified into bony plates, which unite with the 
 skeleton proper to form the main part of the carapace. 
 Lastly, the epidermis completes the structure by forming 
 the horny covering over all. This last is the so-called 
 tortoise shell so much prized. Between this and the bony 
 shell, of course, there is a Malpighian layer, which by 
 transformation forms the horny layer. Similarly the 
 plastron is formed by the union of dermal bony plates 
 with the ventral ribs (like those of an alligator), and 
 
TEGUMENTARY ORGANS— SKIN STRTCTURES. 
 
 463 
 
 covered with epidermal horn. In both the carapace and 
 the plastron the epidermal horny plates do not corre- 
 spond with the bony plates beneath, but break joints with 
 them. 
 
 Mammalian Shell. — At the present time shells are 
 found only in the armadillos and pangolins, but in early 
 times — Quaternary — armored mammals were numerous 
 and of great size. 
 
 Endoskeleton and Exoskeleton. — Thus, even in mam- 
 mals and much more in reptiles, we begin to have the 
 distinction between an exterior shell and interior skele- 
 ton, or exoskeleton and endoskeleton. I n vertebrates the exo- 
 skeleton is purely protective^ but ni invertebrates, which 
 have no endoskeleton, it becomes locomotive as well as 
 protective. This brings us naturally to the invertebrates. 
 
 SECTKJX II. 
 Invertebrates. 
 ARTHROPODS. 
 
 Insects. — Arthropods are all covered with an effi- 
 cient exoskeleton, both protective and locomotive. In 
 insects it is composed of chitin (a partly calcified horny 
 substance). As this is usually rigid and unyielding and 
 is not shed, insects can not grow after they have once 
 put on this coat of mail. Therefore the whole growth 
 must take place in the soft larval condition. Neither 
 butterflies, nor beetles, nor flies, nor bees and ants, etc., 
 grow. They finish their growth in the form of cater- 
 pillar or grub. Many extremely beautiful and curious 
 appendages are found on insects in the form of elab- 
 orately sculptured scales, and the splendid colors of 
 insects are mainly due to these. Such are the scales 
 which give color to butterflies, and the gorgeous iri- 
 descent green-gold hues to some beetles. 
 31 
 
464 PHYSIOLOOV AND MORPHOLOGY OF ANIMALS. 
 
 The Higher Crustaceans are equally incased in 
 an unyielding shell more calcified than that of insects; 
 and yet they gr on.'. This is possible, however, only by a 
 periodic shedding of the shell, which leaves them for a 
 time almost helpless for want of a rigid skeleton until a 
 new one is formed by deposit of carbonate of lime in the 
 skin. 
 
 In lower crustaceans the shell is chitinous, like that 
 of insects. Thus crustaceans have been divided by their 
 shell substance into a lower group [Ento»tostraca), in- 
 sect-shelled or chitinous-shelled, and a higher group 
 {^Malacosiraca), mollusk-shelled or calcareous-shelled. 
 
 MOLLUSKS. 
 
 These are par excelletice shell-covered animals, and 
 their classification is largely based on the character of 
 their shells. 
 
 Acephala have two shells, a right and a left, hinged 
 along the back — bivalves. Gastropods have but one, usu- 
 ally much coiled shell — 
 univalves. Cephalopods, 
 when they have a shell 
 at all, are distinguished 
 by their many-chambered 
 structure. The animal 
 lives only in the large 
 outer chamber. All the 
 other chambers are 
 closed and filled with 
 air, and a slender mem- 
 branous tube — the siphicncle — runs from the animal 
 through all these chambers, but not opening into them 
 (Fig. 325)- 
 
 Growth of Shell. — The shell is a calcareous secre- 
 tion by the skin of the mantle. In bivalves (see Figs. 280 
 
 Fig. 323. — Surface view of a bivalve, 
 showing lines of growth or successive 
 sizes of the shell. 
 
TEGUMENTARV ORGANS— SKIN STRUCTURES. 465 
 
 Fig. 324. — Section along the line a b oi 
 previous figure, showing structure. 
 
 and 281, pages 401 and 402) the mantle covers the inte- 
 rior of the shell to the very edge, and forms the shell in 
 its epidermic cells, lay- 
 er by layer, each on the 
 inside of the last, and 
 extending a little be- 
 yond it, by the growth 
 of the animal. The 
 successive growths are 
 easily seen on the out- 
 side of the shell (Fig. 
 
 323; on section. Fig. 324). If a small object like a 
 coin be slipped between the mantle and the shell it will 
 soon be covered by a secretion from the mantle, and 
 finally inclosed in the thickness of the shell. 
 
 The subdivisions of the Acephala are seen in the 
 shells. The Monomyaria have one muscular impression 
 
 on the shell, as in 
 the oyster ; the Dhn- 
 yaria have two mus- 
 cular impressions, as 
 in the clam or the 
 river-mussel, etc. 
 
 The gastropod 
 shell grows in a sim- 
 ilar way, and the 
 successiveshell mar- 
 gins can generally 
 be recognized, and 
 often form conspicu- 
 ous ornaments on 
 the shell. 
 Cephalopods live, as already said, in the larger outer 
 chamber, all the others being empty and connected with 
 one another in the living animal by the siphuncle (Fig. 
 
 Fig. 325.— Section through a nautilus shell: 
 ch, living chamber ; ch\ empty chamber : 
 si si, siphuncle ; s s, septa. A new septum 
 to be formed and an extension of the outer 
 chamber is shown by the dotted lines. 
 
466 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 325). As the animal grows, the extenduig mantle adds 
 to the length and size of the outer chamber until finally 
 the animal withdraws from contact with the last cham- 
 ber wall and secretes another partition, and thus adds 
 another to the series of empty chambers. Cephalopods 
 are classified into two groups — viz., the naked, like 
 squids and cuttlefish, and the shelled, like the nautilus; 
 the former have two gills (dibranchs), the latter four 
 gills (tetrabranchs). 
 
 The exquisite beauty and variegated iridescent luster 
 of molluscous shells when polished is the result of the 
 superposition of extremely thin, translucent plates and 
 the interference of light thus produced. The play of 
 color becomes more splendid if the plates are corru- 
 gated, as in the abalone [Haliofiis). 
 
 ECHINODERMS. 
 
 The shell of a sea urchin is a wonderful structure 
 when examined under a microscope. Imagine a calca- 
 reous shell composed of several layers, each layer a 
 reticulation of calcareous fibers, the several layers sepa- 
 rated from one another by calcareous pillars, but the 
 openings or mesh of the several layers not coincidiiig — 
 and we have a structure which is a marvel of lightness 
 and strength combined with perfect permeability. This 
 structure is easily seen under the microscope, and the 
 skin structures of echinoderms generally furnish some 
 of the most beautiful of microscopic preparations, and 
 are very characteristic of the several orders. 
 
 CORALS. 
 
 The deposits of coralline limestone in corals are some- 
 what peculiar, yet, as they are formed by epidermal cells, 
 like epithelial cells, and therefore are of similar origin, 
 
TEGUMENTARV ORCIANS— SKIN STRUCTURES. 
 
 467 
 
 •they may well be treated under this head. Everywhere 
 the deposit is in the epidermis; for even the partitions 
 are infoldings of the epiderm. 
 
 Fig. 326. — Simplified figure of an actinia. 
 
 We recall, then, the structure of the soft polyp, with 
 its radiating partitions, and reproduce the figure show- 
 ing it (Fig. 326). Now, in corals, the structure is the 
 
 Fig. 327.— Ideal section, vertical and horizontal, showing structure : /, ten- 
 tacles ; s, stomach ; //, partitions. 
 
 same, except that deposits of lime carbonate are formed 
 in the lower part— i. e., in the foot disk, the walls, and 
 the partitions to more than midway up the animal, 
 but leaving the upper part of the wall, partitions, and 
 
468 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 the tentacles soft and free (Fig. 327). When the coral • 
 teems to disappear, as in a cell, it is simply the with- 
 drawal of the soft up- 
 per parts into the lower 
 calcareous part or the- 
 ca. In the dead coral 
 the organic matter dis- 
 appears and leaves the 
 cuplike theca showing 
 the radiated structure 
 
 Fig 328.-^. stonypartof a single coral; f ^^e animal (Fig. 328, 
 b, section of same, showing structure. \ tj o > 
 
 a and b). 
 Theca. — Only recently has the origin of the theca 
 been well understood. It was formerly supposed that 
 the coral limestone was deposited in 
 the substance of the tissues, and there- 
 fore constituted a sort of endoskeleton. 
 But now it is known to be a true exo- 
 skeleton or an epidermal 
 structure. The mode of 
 
 formation seems to be as I' WSSSSM^^!S^^MWm^^ 
 
 follows: (i) The basal 
 
 Fig. 329. — Basal plate or foot disk 
 of a larva of coral, showing the 
 commencing laniellae of the theca. 
 (After Sedgwick. j 
 
 Fig. 330.— Diagram showing struc- 
 ture of the theca ; the white in 
 section is coral limestone : Bp^ 
 basal plate ; Tli, theca ; ep, epi- 
 theca ; ec^ ectoderm or epiderm ; 
 en^ endoderm ; 7", tentacles ; ae, 
 oesophagus ; ?«, mesentery ; ;«/", 
 mesenteric filaments. (After 
 Sedgwick. ) 
 
TEGUMENTARY ORGANS— SKIN STRUCTURES. 
 
 469 
 
 plate forms radiating upgrowths by the infolding from 
 below of the epiderm and deposit of calcareous matter 
 in the folds (Fig. 329); (2) the outer margins of the 
 radiating calcareous septa unite to form the outer wall 
 of the theca; (3) the body wall by downfolding grows 
 over this outer wall, inclosing and covering it and add- 
 ing to it by calcareous deposit, and thus completing the 
 outer surface (Fig. 330). 
 
 SPONGES. 
 
 The skeletal deposits in sponges also are formed by 
 the agency of unmodified cells, and therefore may be 
 brought under this head. What we call a spotige is the 
 horny skeleton of the animal of the same name, and 
 deposited within its tissues in the most intricate way. 
 
 Fig. 331. — Shells of living foraminifera : A, textularia variabilis; B, pene- 
 roplis planatus ; C, rotalia concamerata. (After Williamson.) The 
 figures are greatly enlarged. 
 
 The glass sponge, such as the Venus's flower-basket — 
 Euplectella — with its beautiful and intricate mesh of 
 glass fibers, is similarly produced by a deposit of silica 
 in the tissues of the animal. The classification of 
 sponges is based on the character of ' the skeleton. 
 There are horny sponges, glass sponges, and calcareous 
 sponges. 
 
470 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 RHIZOPODS. 
 
 Many rhizopods have no hard parts, but others, 
 especially Foraminifera and Radiolaria, form calcareous 
 or siliceous skeletons of exquisite beauty and com- 
 plexity. They form the most beautiful objects under 
 the microscope. These skeletons are also very charac- 
 teristic of the various orders (Fig. 331). 
 
CHAPTER VI. 
 
 GEOGRAPHICAL DISTRIBUTION OF ORGANISMS. 
 
 This subject is not directly connected with physi- 
 ology, although it is with morphology, but its extreme 
 interest and importance in connection with evolution 
 justifies its treatment with some fullness. 
 
 It is well known that the observant traveler from 
 one continent to another — as, for example, to take an 
 extreme case, from Europe or the United States to 
 Australia — finds all the animals and plants entirely dif- 
 ferent from those he has been accustomed to see at 
 home. The same is true in less degree in going from 
 America to Europe, or even from the Atlantic to the 
 Pacific side of our own continent. Until comparatively 
 recently the facts of this distribution of species were a 
 mere chaotic mass without a guiding principle. The 
 theory of evolution has brought law and order into this 
 chaos. All we can do here is to give a bare outline of the 
 general laws of this distribution and their explanation. 
 
 Fauna and Flora. — In popular language fauna 
 and flora means the group of animals and plants inhabit- 
 ing any place; but in scientific language it is a natural 
 group of organisms differing from other natural groups 
 and separated from them by geographical boundaries, or 
 by temperature or climate, or physical conditions of 
 some sort, and in harmonic relations with one another 
 and with the environment. 
 
 471 
 
472 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 Illustrations of Harmonic Relations. — The com- 
 plexity of the harmonic relations of all members of a 
 group of organisms is such that if one element is altered 
 a wave of change, often of the most unexpected kind, 
 is propagated through the group until a new harmonic 
 relation is established. It was Darwin who first drew 
 attention to the relation of cats to the flourishing of 
 clover. The fertilization of clover flowers is dependent 
 on the presence of bumble bees, but the nests of these are 
 destroyed hy field mice, and field mice are destroyed by 
 cats. Professor Morgan somewhat humorously extends 
 the complex relation by adding that cats are cherished by 
 old maids. Thus the presence of old maids is favorable 
 to the growth of clover. The most unexpected results 
 often come from interference with these natural rela- 
 tions. Farmers to protect crops destroy birds, and in- 
 sects injurious to crops increase. Sportsmen introduce 
 English rabbits into Australia and New Zealand, and 
 the governments of those countries have spent millions 
 of pounds in vain attempts to destroy them. 
 
 In a word, every species in order to continue to exist 
 must be in harmonic relation with the environment both 
 physical and organic. Now, the physical environment 
 consists of soil, climate, and geographical barriers. 
 Among these the simplest and the most universal is 
 temperature. We will therefore speak first of temperature 
 regio7is. And among organisms the simplest in their re- 
 lations are plants. Therefore we speak first of all of 
 
 Botanical Temperature Regions. — As we travel 
 from equator to pole we pass through successive zones 
 of temperature ranging from 80° to 0° F. These zones 
 are characterized predominantly by different groups of 
 plants. We have first a region of palms and tree ferns, 
 corresponding with the intertropical zone; then a region 
 of evergreen hard-wood trees corresponding with the 
 
GEOGRAPHICAL DISTRIBUTION OF ORGANISMS. 
 
 473 
 
 warm temperate zone; then a region of deciduous hard- 
 wood trees, corresponding with the full temperate zone; 
 then a region of conifers and birches, corresponding to 
 the cold temperate or subarctic zone; then a treeless 
 region, corresponding to the arctic or circumpolar zone; 
 and finally a plantless or nearly plantless region, occupied 
 by the polar ice cap (Fig. 332). 
 
 Qualification. — i. By the terms region of palms, 
 region of pines, etc., we mean only that these kinds of 
 trees by their abundance give character to the aspect of 
 field and forest. 
 
 2. We have drawn lines separating these regions, but 
 in fact they shade completely into one another. 
 
 3. We have drawn 
 the separating lines 
 regular, like lines of 
 latitude, but, in fact, 
 they are irregular, 
 like isothermal lines. 
 
 Regions in Al- 
 titude. — These re- 
 gions being tempera- 
 ture regions, similar 
 regions are found in 
 ascending mountains 
 (Fig. 332). A moun- 
 tain in the tropics if 
 it reaches perpetual 
 snow will contain all 
 of them, while moun- 
 tains in higher lati- 
 tude only the higher 
 portions of the se- 
 ries. For example, in the Peruvian Andes we have all 
 these regions successively encircling the mountain — 
 
 Fig. 332. — Diagram showing temperature 
 zones in latitude and corresponding zones 
 in altitude : i, tropical ; 2, temperate ; 3, 
 subarctic ; 4, arctic ; 5, perpetual snow, 
 a, b, c, mountams in tropical, temperate, 
 and subarctic regions respectively. 
 
474 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 i. e., a region of palms at the base, a region of hard-wood 
 trees higher up, then a region of pines, a treeless region, 
 and a plantless region or perpetual snow. In the sierra, 
 which is in the temperate zone, the region of palms is 
 wanting, but all the others are present. To two thou- 
 sand feet or more hard-wood trees predominate, then 
 pines and spruces up to ten thousand to twelve thousand, 
 then shrubs and flowers, and finally perpetual snow. 
 
 Zoological Temperature Regions.— Thus far 
 we have spoken only of plants, because their relations 
 are more simple and easily brought out on account of 
 their being fixed to the soil. But the same laws govern 
 animal species also. Animals also have their tempera- 
 ture regions. For example, the great cats, the hyenas, 
 the great pachyderms, the camels, the ostriches, parrots, 
 toucans, the reef-building corals, etc., are tropical, while 
 the polar bear, the civets, martens, seals, walruses, and 
 whales are predominantly arctic. 
 
 COMPLETER DEFINITION OF TEMPERATURE REGIONS. 
 
 I. The range of any organic form is the extent or 
 area of its distribution. It is most restricted for spe- 
 cies ; is greater for genera, because when a species 
 ceases the genus may be continued by other species of 
 the same genus. It is still greater for families, because 
 when a genus ceases the family may still continue as 
 other genera of the same family, etc. The range is ex- 
 tensive in proportion to the largeness of the taxonomic 
 group. For example, we have said that the range of 
 the conifers in the sierra was from two thousand to ten 
 thousand feet. This is the range of the order, but no 
 species extend so far. If we take the genus Pinus we 
 have first the digger pine (P. salnniana), then the yellow 
 pine {P. ponderosa), then the sugar pine {P. Lamberti- 
 
GEOGRAPHICAL DISTRIBUTION OF ORGANISMS. 
 
 475 
 
 ana), then the tamarack pine {J\ contorta), then the /'. 
 flex Hi Sy etc. 
 
 2. The ranges of contiguous species shade insensibly 
 into one another by overlapping interpenetration and 
 co-existence on the margins. 
 
 Each species is most abundant and vigorous about 
 the middle of its range, and becomes less and less numer- 
 ous and vigorous on the margins until it ceases and 
 gives place to some other species. Thus, if a a' (Fig. 
 
 333) be the range of species A, and^<^' of species -/5, 
 then the rising and declining curves represent the rela- 
 tive abundance in different parts of their ranges, and 
 a! b their overlap or area of coexistence on the margins. 
 
 3. But species do not usually grade into other species, 
 which take their place, in specific characters. There is 
 not usually any evidence of transmutation of one spe- 
 cies into another. One species seems to \i& replaced by, 
 not transmuted into, another species. The change is 
 usually by substitution^ not by transmutation. I say usu- 
 ally because sometimes such gradation in specific char- 
 acters is found. 
 
 Illustrations. — We take for illustration only two ex- 
 amples : (i) The sequoia, or big tree, and redwood exist 
 to-day only in California; one (the redwood) confined 
 to the coast range, and the other (the big tree) to the 
 Sierra. Now, in commencing south, wherever found 
 these trees are perfect in all their specific characters of 
 bark, wood, leaf, and fruit. They remain substantially 
 unchanged throughout their range, and stop at their 
 
476 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 northern limit still unmistakably the same. They do 
 not, and apparently can not, change their specific char- 
 acters. They die first. (2) Take another example, the 
 sweet gum (liquidambar) of the eastern coast. This 
 remarkably distinct form ranges from Florida to the 
 borders of the Great Lakes. Throughout all this wide 
 range it is precisely the same unmistakable species, 
 characterized by its peculiar starred leaf, winged twigs, 
 spinous burr, and fragrant gum. On the limits of its 
 range it does not change into any other species, but sim- 
 ply dies out and is replaced by others. 
 
 These are illustrations of what has been called ^^per- 
 manence of specific form." It is as if species originated, no 
 matter how — say, at once by creation — in their present 
 form, somewhere in the region where we find them, and 
 thence spread in all directions as far as physical condi- 
 tions and struggle for life with other species would allow, 
 interlocking there with other contesting species and co- 
 existing with them on the common border. 
 
 4. Barriers. — Faunas and floras shade gradually 
 into one another when limited only by temperature con- 
 ditions ; but if there should be an impassable physical 
 barrier, like an east and west mountain range, or a des- 
 ert or sea, then the fauna and flora on each side of such 
 barrier will differ greatly and without any shading. Thus 
 the species north and south of the Himalaya, or north 
 and south of the Sahara, differ conspicuously and tren- 
 chantly. It is again as z/ each group of organisms had 
 originated or been created at once out of hand just as 
 they are and where we find them, and spread as far as 
 they could, but could not mingle with the next contigu- 
 ous group on account of the intervening barrier. 
 
 5. North and South of the Equator.— Again, 
 there are temperature zones south of the equator as well 
 as north ; but none of the species found in temperate or 
 
GEOGRAPHICAL DISTRIBUTION OF ORGANISMS. 
 
 477 
 
 arctic zones north are the same as those found in similar 
 zones south. It is, again, as //each group was created 
 as it is and where we find it, and prevented from pass- 
 ing to similar zone, north or south, by the torrid tem- 
 perature intervening. 
 
 CONTINENTAL FAUNAL REGIONS. 
 
 Thus far we have considered onlv temperature con 
 ditions; now we take up other limiting conditions. 
 
 If continuous 
 land existed all 
 around the earth, 
 then, barring des- 
 ert regions, there 
 is no reason to 
 doubt that species 
 would range all 
 around, and there 
 would be a strict- 
 ly zonal arrange- 
 ment of species de- 
 termined by tem- 
 perature alone. 
 But the continents 
 are widely and im- 
 passably separat- 
 ed by oceans. 
 Therefore the species on different continents are wholly 
 different. It is, again, as if each continent had been 
 populated by its own inhabitants, suited to its climate, 
 just as we find it now, and had not been able to cross 
 the ocean barrier and mingle. 
 
 If we take the facts in detail the case becomes still 
 stronger. Let Fig. 334 represent a north polar projec- 
 tion of the earth. The five zones are represented by 
 
 Fig. 3-?4. — Polar projection of the earth : i, 
 tropical ; 2, temperate ; 3, subarctic ; 4, arc- 
 tic ; 5, polar regions. 
 
478 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 corresponding numerals. We may leave out No. 5, as 
 this is unknown and largely uninhabited. In No. 4 the 
 fauna is practically the same all around, because of the 
 close approximation of the lands of the two continents 
 here and the easy communication over the solid ice. 
 
 But in Nos. 3 and 2, which include the United States 
 and Europe, we find the species are substantially all dif- 
 ferent. Even many of the genera and some families 
 are peculiar to each continent. Some species, indeed, 
 are representative or resembling species, but not identi- 
 cal. To this general statement there are some excep- 
 tions, but these are of the kind which prove the rule, 
 or rather the principle on which the rule is founded. 
 
 Exceptions. — I. Hardy or widely migrating species, such 
 as geese and ducks. These are the same on the two 
 continents, because they range also into No. 4, and thence 
 go down on either continent. 
 
 2. Introduced species, such as all the useful plants and 
 domesticated animals and all the noxious weeds and 
 animal pests — flies, rats, etc. — which follow the footsteps 
 of civilization. We not only find these on both conti- 
 nents, but they often do as well, or even better in their 
 new homes than in their native places. They were not 
 in those new homes before only because they could not 
 get there. 
 
 3. Alpine Species. — It is a curious fact that animals 
 and plants inhabiting the tops of high mountains in Eu- 
 rope and in the United States are extremely similar, and 
 even sometimes identical, even though so widely and 
 impassably separated. The explanation of this will come 
 up later. 
 
 In No. I, or tropic zone, the difference is still greater; 
 not only all the species in South America and Africa are 
 wholly different, but many whole families are entirely 
 peculiar to one or the other continent. For example, 
 
GEOGRAPHICAL DISTRIBUTION OF ORGANISMS. 470 
 
 the monkeys are found in both, but the tailless monkeys 
 are peculiar to the Old World, while the prehensile 
 tailed monkeys are peculiar to America. The great 
 pachyderms are peculiar to the Old, while the sloths are 
 peculiar to the New. Among birds the great family of 
 humming birds, containing over four hundred species, 
 are peculiar to America, while the true ostriches are con- 
 fined to Africa. Among plants, the great family of cac- 
 tuses, with its innumerable species, is peculiar to 
 America. 
 
 Nos. 2 and 3, in the southern hemisphere, continue 
 quite distinct in South America and Africa, because these 
 are still widely separated. There are some evidences, 
 however, derived entirely from their faunas and floras, 
 that they were once more appro.ximated than now. 
 
 SUIU)lVISIO.\S OF CONTINENT.AL F.\UNAS. 
 
 We have already spoken of subdivisions of these de- 
 termined by temperature, and in a more marked manner 
 by east and west barriers, such as mountain chains, etc. 
 But they are divided also by north and south barriers. 
 Thus the United States fauna is divided in a very marked 
 way by north and south mountain chains into distinct 
 faunal regions. By the Appalachian range the division 
 is not very marked, because the chain is not very high 
 nor very long, but the Rocky Mountain chain, running 
 the length of the continent, and, together with the inter- 
 mountain deserts, more than one thousand miles wide, 
 form an insuperable barrier to most species of animals 
 and plants. So that California is a very distinct faunal 
 and floral region. Similarly, but in less degree, the 
 Ural separates a European from an Asiatic fauna and 
 flora. 
 
 Special Cases. — i. Australia. — We have thus far 
 spoken only of the two great continental masses, east- 
 32 
 
48o PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 ern and western, but there is another land mass of con- 
 tinental proportions — viz., Australia. This we take as 
 our first example of special cases. It is the most isolated 
 of continents, and the most distinct of all faunal regions. 
 All the animals and plants there are widely different 
 from those of any other region. To show the greatness 
 of this difference we take up more particularly one class 
 only — viz., the mammals. 
 
 Mammals are divided into three subclasses : Euthe- 
 ria, or ordinary typical mammals, found everywhere ex- 
 cept Australia ; Metatheria, or marsupials ; and Proto- 
 theria, or monotremes. Eutheria are perfect young-bear- 
 ers ; the Prototheria are perfect egg-layers ; the Meta- 
 theria are intermediate. The young of these are born 
 in a very imperfectly organized condition, and embry- 
 onic development is finished in the marsupium or 
 pouch. Now, there are about one hundred and fifty 
 species of mammals in Australia, all of which are mar- 
 supials and monotremes. Moreover, monotremes are 
 found nowhere else, and of marsupials only a few opos- 
 sums are found elsewhere, viz., in America, North and 
 South. The only exception to this sweeping differ- 
 ence is the existence of a few bats and a few rats 
 — animals especially liable to dispersal by means of 
 floating logs, etc., and therefore to accidental intro- 
 duction. 
 
 2. Madagascar. — This very large island off the east 
 coast of Africa is separated from the latter by the wide 
 and deep Mozambique Channel. Next to Australia, it is 
 perhaps the most distinct of all faunal regions. It may 
 be called the home of the lemurs, and has besides many 
 curious forms. All the mammals are peculiar, except 
 such as have been introduced. But it is well to remem- 
 ber that whatever distant resemblances they have are 
 mostly with those of South Africa. 
 
GEOGRAPHICAL DISTRIBUTION OK ORGANISMS. 
 
 4<Si 
 
 3. Galapagos. — These islands, off the west coast of 
 South America, strongly attracted the attention of Dar- 
 win during his voyage on the Beagle on account of the 
 singularity of their fauna. They have been visited by 
 many other naturalists for the same reason. There are 
 no mammals or amphibians, and the species of other 
 groups are all peculiar. Among them are found many 
 very large lizards and several species of gigantic land 
 tortoises. These islands are separated from South Amer- 
 ica by deep water. 
 
 4. River Mussels. — We might multiply examples in- 
 definitely. We take one more. In the Altamaha River, 
 Georgia, among other shells found there is one, Unio 
 spinosus, with needlelike spines an inch and a half long. 
 This species is not found elsewhere on the face of the 
 earth. How did it get there ? 
 
 In all these cases it would seem as if the groups of 
 animals and plants had been made and put there where 
 we find them, and are not found elsewhere because they 
 could not get away. 
 
 Marine Faunas. — Thus far we have spoken only 
 of land organisms, but the same laws are true in less 
 degree for marine species. 
 
 Temperature Regions.— Here, also, of course, we 
 have temperature regions, and consequent gradations by 
 change of species as we go north or south. Here, also, 
 at certain places we may have more abrupt changes. 
 For example, on the east coast of the United States we 
 have two abrupt changes of coast fauna, one at Cape 
 Cod and the other at Cape Hatteras. Scarcely a single 
 species passes from north to south of these points, or 
 vice versa. The cause is found in the marine currents off 
 the coast. The warm Gulf waters coming through the 
 straits of Florida hug the coast and warm the shore 
 waters as far as Cape Hatteras (<^ F'ig. 3.^6), and then 
 
482 
 
 PHYSIOLaOY AND MORPHOLOGY OF ANIMALS. 
 
 leave the coast. The icy-cold waters coming out of 
 Baffin's Bay hug the New England coast as far as Cape 
 Cod, b, giving it its peculiarly harsh climate, and then 
 disappear and become a deep current. Thus the sub- 
 tropical fauna is carried beyond its limits to Cape Hat- 
 teras, while the arctic shore fauna is carried beyond its 
 natural limits as far south as Cape Cod, making three 
 sharply defined shore faunas. 
 
 Continental Shore Faunas. — The richest marine 
 faunas are along the shores of continents. The deep sea 
 is an impassable barrier to these. Therefore the faunas 
 along the two shores, European and American, of the 
 Atlantic are almost wholly different. The continent 
 itself is, of course, a still more effectual barrier, and 
 therefore our Atlantic shore species and Pacific shore 
 species are wholly different. 
 
 Pelagic Fauna. — Many marine species swim or 
 fioat freely in the open sea, and are much more widely 
 diffused, being limited by temperature alone. These 
 are called pelagic species. 
 
 Abyssal Fauna. — Again, certain species live only 
 at very great depths. These are abyssal species. 
 
 Special Cases. — There are special cases here also 
 — that is., the species found along the shores of iso- 
 lated islands, as about Australia, Madagascar, Gala- 
 pagos, etc. 
 
 Primary Divisions of Land Faunas.— Thus, then, 
 organic forms are limited in range in every direction by 
 many kinds of physical conditions. They are limited 
 north and south by temperature, and in all directions by 
 barriers such as mountain chains, deserts, and oceans. 
 Besides these, climate and soils limit especially plants, 
 and these limit animals. Thus the whole earth is di- 
 vided into a few great \)x'\vr^^xy fan nal regions, and these 
 are subdivided into prmnnces, etc. Many schemes of 
 
GEOGKAI'IIICAL DISTRIBUTION OF ORGANISMS. 483 
 
 primary and secondary divisions have been proposed. 
 I give that which is most generally adopted — viz., that of 
 Mr. Sclater and Mr. Wallace. According to this scheme 
 there are six primary regions, each subdivided into four 
 provinces. The primary regions are : i. Palearctic, in- 
 cluding the whole of the Old World north of Sahara and 
 the Himalayas. 2. Ethiopian, including Africa south of 
 Sahara. 3. Oriental, including Asia south of the Hima- 
 
 FiG. 335. — Map of the world, showing the six primary regions of Mr. 
 Wallace. 
 
 layas, together with the adjacent islands, Ceylon, Java, 
 Borneo, Philippines, etc. 4. Australian, including Aus- 
 tralia, New Guinea, New Zealand, and Polynesia. 5. 
 Neotropic, including South America, Central America, 
 and the Antilles. 6. iVearctic, including all North Amer- 
 ica north of Mexico. These regions are shown on map 
 (Fig. 335)- 
 
 These are each subdivided into four provinces, as 
 shown in the following sched4jle; but we will give more 
 particularly only those of our own Nearctic region. 
 
484 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 r I. European. 
 
 1. Palearctic J 2* Mediterranean. 
 
 • 3. Siberian. 
 [ 4. Manchurian. 
 f I. Eastern. 
 
 2. Ethiopian j -■ Western. 
 
 I 3. Soutliem. 
 ( 4. Malagasian. 
 r I. Indian. 
 
 3. Oriental | 2. Ceylonese. 
 
 'l 3. Indo-Chinese. 
 [ 4. Indo-Malayan. 
 C I. Austro-Malayan. 
 
 4. Australian 1 2. Australian 
 
 1 3. New Zealandian. 
 [ 4. Polynesian. 
 C I. Chilian. 
 
 5. Neotropic J 2. Brazilian. 
 
 1 3. Mexican. 
 
 I 
 
 [ 4. Antillean. 
 
 [ I. Californian. 
 
 6. Nearctic J -■ Rocky Mountain. 
 
 I 3. Alleghanian. 
 [ 4. Canadian. 
 
 The subdivisions of the Nearctic are given in Fig. 
 336. They are : i. Ca///<^r«/<T, including the Pacific bor- 
 der from Vancouver Island to the borders of Mexico. 
 
 2. Rocky Mountains, including all the mountains and 
 desert region, and extending into the Mexican plateau. 
 
 3. Alleghanian, including all the United States east of the 
 plains and south of the Great Lakes. 4. Canadian, in- 
 cluding all north of i, 2, and 3. 
 
 Primary Divisions of Marine Faunas. — Sclater 
 makes of these also six. But in the present imperfect 
 state of knowledge the simpler classification proposed 
 by Gill seems preferable. Gill divides marine faunas 
 into three great realms: i. North Polar or Arctalian. 2. 
 Tropicalian. 3. South Polar, or Notalian. 
 
GEOGRAPHICAL DISTRIBUTION OF ORGANISMS. 485 
 
 Theories of the Origin of the Distribution of 
 Organisms.— We have given in outline the tacts. The 
 question is, " How came it so ? " Before the advent of 
 the theory of evokition the most rational theory was 
 based on the ideas oi permanence of specific types and centers 
 of specific origin, as if each species was made in its pres- 
 ent form and its present place, and thence spread as far 
 
 Fig. 336. 
 
 as it could without changing its character. It did not 
 deny some variation, but always within certain limits. 
 If the centre of a circle represents a specific type, then 
 radii will represent the variation in all directions, and 
 the circumference the limit of variations. Some species 
 are more and some less variable ; the circle may be 
 larger or smaller, but in all there is a limiting line be- 
 yond which it is impossible to go without destroying the 
 
486 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 species. Or, again, permanence may be compared to a 
 right cylinder standing on end. It may lean in any 
 direction to a limit and right itself, but pressed too far it 
 is overthrown, and the species is destroyed. Within these 
 limits species are made according to a plan or type at 
 once, and these continued by generation unchanged. 
 They are, as it were, struck from the same die, until the 
 die is broken or worn out and another made. Thus the 
 process goes on by an alternation of supernatural and 
 natural means. The origin was supernatural, the con- 
 tinuance by natural process of generation. The mak- 
 ing of dies was supernatural, the coinage was natural. 
 
 This old theory, as already shown, explains many of 
 the phenomena given above, hut not all. For example : 
 I. If each species were made especially for a certain 
 place and environment, then it ought to be more per- 
 fectly adapted to that place than any other, but, on the 
 contrary, introduced species of ten flourish better in their new 
 than in their old homes. 2. Again, if species are made each 
 in its own place and spread as far as they can and wher- 
 ever they can — as indeed they do — then the amount of 
 difference between faunas of different places ought to 
 be in strict proportion to the impassableness of the bar- 
 riers between. This is indeed largely true, but not the 
 whole truth. There is another element which is left out — 
 viz., the element of time. The difference is proportioned 
 to the impassableness of the barrier and the time since the 
 barrer was set up: This element of time connects the 
 subject with the idea of evolution of organic forms 
 throughout all geological time. In a word, the old 
 theory was well enough for the present condition of 
 things, though not perfect even there, but fails entirely 
 to connect present faunas and their distribution with 
 those of previous times. The study of the present alone 
 is but a flash-light view of the world, and therefore the 
 
GEOGRArillCAl. DISTRIBUTION OF ORGANISMS. 487 
 
 world seems to stand still, and organic forms seem to be 
 perma)ie?it. Geology alone shows us the world in con- 
 tinuous motion, in continuous change by evolution. 
 
 In the ne7>.> and now universally adopted view there 
 are four principles to be borne in mind: i. The origin 
 of organic forms "by descent with modifications" — i.e., 
 by evolution — and the steady forward march of evolu- 
 tion everywhere and through all time. If this were all, 
 there would be no geographical diversity at all. 2. In 
 different places, with different environments and isolated 
 from one another, evolution took different directions, and 
 faunas became more and more different as long as the 
 isolation continued. If this had been all, geographical 
 diversity would by this time have been far more extreme 
 than we find it anywhere. 3. But from time to time, at 
 long intervals in geological history, there occurred wide- 
 spread changes in physical geography and climate by 
 which barriers were removed and new barriers set up. 
 The result was wide migration and mingling and con- 
 flict of faunas, and consequently more rapid evolution, 
 but at the same time a decrease of geographical diver- 
 sity. 4. A re-isolation in new localities, and a commenc- 
 ing process of divergence which is still going on. 
 
 Now, the last of these periods of great changes, cli- 
 matic and geographic, and of extensive migrations and 
 minglings and conflict of faunas, and therefore of rapid 
 evolution, was the glacial epoch, or ice age. It is evi- 
 dent, then, that the geographic and climatic changes of 
 the glacial epoch furnish the key to the present distri- 
 bution of species, and, conversely, the present geo- 
 graphic distribution of species furnishes a key to the 
 direction of migrations of that time. 
 
 A full discussion of this interesting subject should 
 be preceded by a course in geology, but an outline may 
 be brought out by means of illustrative examples. 
 
488 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 1. Alpine Species. — We have already spoken of the 
 remarkable fact that alpine species of plants and of in- 
 sects are very similar (though not usually identical) in 
 Europe and America, although so widely separated and 
 completely isolated. The key to the explanation is 
 found in the additional fact that they are both similar 
 to arctic species, and the explanation of both is found in 
 the migrations of the glacial epoch. 
 
 We have already seen that arctic species are the 
 same on the two continents, because they are circum- 
 polar and in substantial connection all around. Now, 
 as the glacial cold came on, the ice sheet advanced south- 
 ward, driving the arctic species before it on both conti- 
 nents, until they invaded and occupied all America to 
 the Gulf and all Europe to the Mediterranean. As the 
 ice sheet retreated, arctic species followed it, step by step, 
 back to their present arctic home. But many individ- 
 uals sought arctic conditions by retreating up the slopes 
 of mountains, and were left stranded there on both con- 
 tinents. Since then they have been changed somewhat 
 in different directions by isolation, but the time has not 
 been long enough for great divergence. There is some 
 difference, but not so great as the wide separation would 
 lead us to expect. 
 
 2. Australia. — Of all known faunas and floras, this is 
 the most distinct. It is so because longest isolated. We 
 have seen that all its mammals are nonplacentals — i. e., 
 marsupials and monotremes — except a few introduced 
 accidentally or by man. Furthermore, that monotremes 
 are found nowhere else, and marsupials nowhere else, 
 except a few opossums in America. But this has not 
 always been so. In Jurassic (middle geological) times 
 the earth was everywhere abundantly inhabited by mar- 
 supials and monotremes, but not by eutheres or typical 
 mammals. These last were introduced subsequently in 
 
GEOGRAPHICAL DISTRIBUTION OF ORGANISMS. 489 
 
 the Tertiary. Therefore we conclude that Australia 
 was connected with other continents during middle geo- 
 logical times, and, in common with other lands, was 
 inhabited by metatheres and prototheres, but that before 
 or about the beginning of the Tertiary it was separated, 
 and has remained so ever since. Therefore, when, by 
 struggle, migrations, and conflicts on the great theater 
 of evolution Arctogcea (Eurasia and North America), 
 eutheres were evolved at the beginning of the Tertiary, 
 Australia was already isolated, and they never got there. 
 
 3. Africa. — The fauna of Africa south of Sahara is 
 very distinct, though less so than that of Australia. The 
 mammals of Africa are of two groups: (i) A group of 
 large, powerful animals somewhat like the Eurasians, but 
 especially like the Pliocene Eurasians. (2) A group of 
 small animals of low organization (mostly insectivores 
 and lemurs) very peculiar to Africa, but more like those 
 of Madagascar than any other. These latter we shall 
 call indigenes or natives, the former group we will call 
 invaders. 
 
 Explanation. — In preglacial times Africa south of 
 Sahara was isolated, and inhabited only by the indigenes. 
 Then came the glacial elevation, opening Africa to mi- 
 grations from the north, and the glacial cold, driving 
 Pliocene mammals southward into Africa, where they 
 were shut up by subsequent changes in physical geog- 
 raphy. Then came the struggle between the invaders 
 and the natives. Both were sorely changed, the in- 
 vaders mainly by the new environment, the natives 
 mainly by the struggle for life. The final result was 
 the mixture of the two groups, but the invaders greatly 
 predominated. (See Wallace's Geographical Distribu- 
 tion of Animals.) 
 
 Island Faunas. — We have spoken thus far of conti- 
 nental faunas, but island faunas are peculiarly interest- 
 
^go PHYSTOLOGV AND MORPHOLOGY OF ANIMALS. 
 
 ing. Islands are of two kinds — continental islands and 
 oceanic islands. Continental islands are outliers of con- 
 tinents, separated from them only by subsidence. They 
 have a geological structure similar to the mother conti- 
 nent. Oceanic islands are those which have no connec- 
 tion with any continent, but have been built up from the 
 ocean bed by volcanic eruption in comparatively recent 
 geological times. The fauna of continental islands is 
 always related to that of the mother continent, differing 
 from it in proportion to the width of ocean separating 
 and the ti/zie of separation. The fauna of oceanic is- 
 lands have no such evident relation to any continent. 
 As examples of the former group we have Ceylon, Java, 
 Borneo, Sumatra, Japan, etc., as appendages of Asia; 
 Madagascar, of Africa ; the British Isles, of Europe ; the 
 West Indies, of North America, etc. Of oceanic islands 
 we have as examples the Bermudas and Azores in the 
 Atlantic, and all the Polynesian Islands in the Pacific. 
 
 4. Madagascar. — We have already seen that the 
 mammals of Madagascar differ greatly from those of 
 any other country, but that their nearest alliance is with 
 those of Africa. We now add that this alliance, how- 
 ever, is only with those we called the indigenes, and not 
 at all with the invaders. 
 
 Explanatio7i. — In preglacial times, when Africa was 
 isolated from the rest of the world, Madagascar was 
 connected with it, and both were inhabited by the in- 
 digenes. Before the glacial epoch, and therefore before 
 the invasion of Pliocene mammals, Madagascar was sep- 
 arated from the continent by subsidence, and these iso- 
 lated indigenes were spared the invasion and conflict. 
 Since that time divergence has gone on until now the 
 fauna is very peculiar. In the fauna of Madagascar we 
 probably have a nearer approach to the original inhab- 
 itants of both than we now have in the African indige- 
 
GEOGRAPIIICAL DISTRIBUTION OP' ORGANISMS. 
 
 491 
 
 nous group. For although both have changed with time, 
 yet the African, more than the Malagasian, because of 
 the struggle which the former alone suffered. 
 
 5. British Isles. — The fauna of the British Isles is 
 substantially the same as that of the European conti- 
 nent, but there are some very significant differences, 
 (i) It is i 2.x poorer in species, and this is especially true 
 of Ireland. For example, of mammals — Europe has 
 ninety species, England forty, and Ireland only twentv- 
 two. Of reptiles and amphibians Europe has twenty- 
 two species, England thirteen, and Ireland four. (2) In 
 many cases there is a difference, but not sufificient to 
 make species; the differences are varietal instead of 
 specific. Such are the facts. 
 
 Explanation. — In preglacial times the British Isles 
 were a part of the continent and inhabited by the same 
 animals. In glacial times they were covered by the ice 
 sheet and all animals were destroyed or driven south- 
 ward. After glacial times and the withdrawal of the 
 ice sheet they became again a part of the continent and 
 were recolonized from Europe. But the time of con- 
 nection with Europe after the withdrawal of the ice 
 sheet was too short for complete colonization, especially 
 of extreme Ireland. Some species had not colonized at 
 all and more had not yet reached Ireland, when by sub- 
 sidence the islands were cut off from the continent, and 
 at the same time Ireland from England. This entirely 
 explains the comparative poverty of the fauna. Now 
 the slight differences. The time has been too short and 
 isolation too imperfect for great divergence. Diver- 
 gence has commenced, but has gone only to varietal and 
 not to specific differences. 
 
 6. California Coast Islands. — Heretofore I have 
 spoken almost wholly of faunas. In this case I will 
 deal with the flora. But the principles are the same. 
 
492 
 
 PHYSIOLOGY AND MORl'HOLOGY OF ANIMALS. 
 
 The large islands off the coast of southern California 
 have all the characters of continental islands. They 
 have been separated from California in comparatively 
 recent times. The flora of these islands is very peculiar. 
 Out of three hundred species described, at least fifty 
 are found nowhere else. The others are similar to 
 those in California. But in the California flora it is 
 necessary to distinguish two groups — viz., a distinctively 
 Californian group and a group which is more widely 
 diffused. The former I shall call indigenes or natives; 
 the latter are probably invaders. Now, it is the distinc- 
 tively Californian group only that is found on the islands. 
 These are the facts. Now the 
 
 Explanation. — Before the glacial epoch the islands 
 were a part of the mainland and all had a common flora 
 — viz., the group I have called indigenes. Then came 
 the separation by subsidence and the isolation of many 
 indigenes on the islands. Then came the glacial cold 
 and the invasion of California by a northern flora, the 
 struggle between invaders and natives, the destruction 
 of some native species, and the modification of both na- 
 tives and invaders by new environment and by conflict. 
 The final result was the California flora of to-day. The 
 island flora was spared the invasion and the conflict. 
 It has been changed less than the native Californian. 
 We have in them a nearer approach to the preglacial 
 flora of both. 
 
 7. Oceanic Islands and their Fauna. — Oceanic islands 
 are built up from the sea bottom mostly by volcanic 
 action, and have never had any connection with a conti- 
 nent. We see these forming now. When first formed 
 they are, of course, uninhabited. They receive their 
 species, animals and plants, as gifts from the sea — mere 
 floating waifs brought by waves and currents. Their 
 fauna and flora are always peculiar because isolated, 
 
GEOGRAPHICAL DISTRIBUTION OF ORGANISMS. 403 
 
 but with affinities connecting with several different lands. 
 The predominance of affinities will depend partly on 
 proximity and partly on the direction of winds and cur- 
 rents. Mammals (except bats) and amphibians are en- 
 tirely wanting (unless introduced by man) because these 
 are not apt to be drifted on logs, as are some reptiles. 
 
INDEX 
 
 Abyssal fauna, 482. 
 
 Anabolism, 286. 
 
 Analogy versus homology, 241- 
 24S. 
 
 Anatomy, definition of, 8 ; phil- 
 osophical, 241-281. 
 
 Animal functions, classification 
 of, 23-25. 
 
 Animal life, organs and func- 
 tions of, 26-2S. 
 
 Animals and plants, distinction 
 between, 4. 
 
 Animals, general structure of, 
 11-23. 
 
 Animal structure, general laws 
 of, 241-281. 
 
 Aortic arches, origin of, 390. 
 
 Arteries, 3S2. 
 
 Astigmatism, 115. 
 
 Binocular perspective, 149. 
 
 Blind spot, 132. 
 
 Blood, globules of, 347-349, 351, 
 352, 353 ; plates of, 349 ; chem- 
 ical composition of, 349 ; 
 plasma, 350 ; coagulation of, 
 350 ; functions of, 351 ; origin 
 and history of, 352 ; compar- 
 ative morphology of, 354 ; em- 
 bryonic development of, 356 ; 
 33 
 
 circulation of, in man, 375- 
 380 ; in reptiles and amphibi- 
 ans, 385 ; in arthropods, 400 ; 
 in bivalves, 402 ; in gastro- 
 pods, 403 ; in cephalopods, 
 404 ; in echinoderms, 405 ; in 
 insects, 408. 
 
 Blood system of mammals, 385 ; 
 of birds, 385 ; of reptiles and 
 amphibians, 385 ; of fishes, 
 387 ; of arthropods, 399 ; 
 of bivalves, 401 ; of gastro- 
 pods, 403 : of cephalopods, 
 404 ; of echinoderms, 404 ; of 
 coelenterates, 406 ; of proto- 
 zoans, 408 ; of insects, 408. 
 
 Blood vessels, 382. 
 
 Botanical temperature regions, 
 472. 
 
 Brachyopy, 113. 
 
 Brain of man, 30, 72 ; convolu- 
 tions of, 34 ; interior structure 
 of, 35 ; microscopic structure 
 of, 35 ; embryonic develop- 
 ment of, 37 ; functions of 
 parts of, 39-45- 
 
 Brain of vertebrates, 72. 
 
 Brain, relative size of, 73, 74. 
 
 Breathing, mechanics of, 361, 
 369 ; transition from gill to 
 495 
 
496 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 lung, 373 ; of arthropods, 399 ; 
 of bivalves, 401 ; of gastro- 
 pods, 403 ; of echinoderms, 
 404 ; of insects, 411. 
 
 Capillaries, 3S3. 
 
 Cephalization, 82. 
 
 Cerebellum, 32, 40. 
 
 Cerebro-spinal system, 29. 
 
 Cerebrum, 31, 39; relative size 
 of, 76. - 
 
 Chromatism, 107. 
 
 Circulation of blood in man, 
 375-380; in reptiles and am- 
 phibians, 385 ; in arthropods, 
 400 ; in bivalves, 402 ; in gas- 
 tropods, 403 ; in cephalopods, 
 404; in echinoderms, 405 ; in 
 insects, 40S. 
 
 Classification, definition of, 9. 
 
 Classification of animals, out- 
 line of, 70, 71. 
 
 Classification of fishes by scales, 
 460. 
 
 Claws, 454. 
 
 Coats of the eyeball, loi. 
 
 Color-blindness, 139-141. 
 
 Color perception, 137. 
 
 Colors, primary, 136. 
 
 Contact, kinds of, 97. 
 
 Continental faunal regions, 
 477-4S1. 
 
 Contents of the eye, 103. 
 
 Corals, 466-469. 
 
 Corpus striatum, 41. 
 
 Cranial nerves. 46, 54. 
 
 Dental formulae, 299. 
 Diabetes, cause of, 449. 
 Digestion, mouth, 292-310 ; 
 
 stomach, 310-318 ; intestinal, 
 
 319-331- 
 
 Digestive system in inverte- 
 brates, 331-346. 
 
 Dim-sightedness, 115. 
 
 Distribution of organisms, the- 
 ories of the origin of, 485- 
 489. 
 
 Ear, human, structure of the, 
 174-181 ; mode of action of 
 the whole, 181 ; functions of 
 parts of, 181, 1S2 ; of birds, 
 1S3 ; of reptiles, 183 ; of fishes, 
 184 ; of mollusks, 185 ; evo- 
 lution of the, 187, 188. 
 
 Elbow joint, 252. 
 
 Embryology, definition of, 9. 
 
 Emmetropy, 113. 
 
 Eye of mammals, 156 ; of birds, 
 157 ; of reptiles, i|i8 ; of fishes, 
 159 ; of cephalopods, 163 ; of 
 arthropods, 164. 
 
 Eye of man, its shape, setting, 
 etc., 99; muscles of, 100; 
 coats of the ball, loi ; linings, 
 103 ; formation of image in, 
 103 ; necessity of lenses in, 
 104; application of principles 
 to, 105 ; compared with cam- 
 era, 10.7 ; chromatism in, 107 ; 
 aberration in, 108 ; adjust- 
 ment of, for distance, 108 ; ad- 
 justment of, for light. III ; 
 structure of the iris in, 112; 
 normal sight of, 113; defects 
 in sight of, 113-115 ; retina 
 of, 116-122. 
 
 Fauna and flora, 471. 
 
 Feathers, structure of, 457 ; 
 gradation of, to hairs, 458. 
 
 Fishes, classification of, by re- 
 spiratory organs, 372, 373. 
 
INDEX. 
 
 497 
 
 Food, definition of, 288 ; kinds 
 of, 288 ; milk as, 289 ; uses 
 of, 289 ; distinctive uses of 
 the kinds of, 290 ; preparation 
 of, 291 ; saccharization of, 
 310; peptonization of, 313; 
 chylification of, 323 ; emulsi- 
 fication of, 324. 
 
 Forearm, 252. 
 
 Fore limbs, various figures of, 
 
 251-253- 
 Fovea, 130. 
 Frontal lobe, relative size of, 82. 
 
 Ganglia, functions, of, 86. 
 Ganglionic system, 67-70. 
 General principles, 1-28. 
 Glottis and its vocal cords, 207. 
 Glycogeny and its relation to 
 vital force and vital heat, 443- 
 
 449- 
 Gray matter, relative amount 
 
 of, 74. 75- 
 
 Hair, 453. 
 
 Hand and foot bones, 253. 
 
 Hand, modifications of, 254. 
 
 Harmonic relations, illustra- 
 tions of, 472. 
 
 Heart, structure of the, 380. 390. 
 
 Heel, position of the, 256. 
 
 Hind limbs, 255. 
 
 Hip girdle, 256. 
 
 Histology, definition of, 8. 
 
 Homology of vertebrates, 248- 
 266. 
 
 Hoofs, 455. 
 
 Horns, 455. 
 
 Horopter, 146. 
 
 Hyperopy, 114. 
 
 Island faunas, 489-493. 
 
 Joints, 224. 
 
 Katabolism, 2S6, 415-451. 
 
 Kidneys, place and form of, 425 ; 
 excretory ducts of, 426 ; pelvis 
 of, 427 ; minute structure of, 
 427 ; function of, 429 ; com- 
 pared with lungs, 430; in 
 mammals, birds, reptiles, and 
 amphibians, 433 ; in insects 
 and mollusks, 434. 
 
 Knee, position of the, 256. 
 
 Land faunas, primary divisions 
 of, 482-484. 
 
 Larynx, 204 ; structure of, 206 ; 
 muscles of, 209 ; as a musical 
 instrument, 210. 
 
 Law of peripheral reference, 64. 
 
 Limb motion, 227-230. 
 
 Limbs, signification of, 264 ; 
 origin of, 264. 
 
 Linings of the eye, 102. 
 
 Liver, position and structure of, 
 440-442 ; function of, 442. 
 
 Locomotion, 230-232. 
 
 Lungs of man, 358 ; structure of, 
 359 ; application of, to breath- 
 ing, 362 ; of birds, 366 ; of 
 amphibians, 367 ; compared 
 with kidneys, 430-433. 
 
 Lymphatic glands, function of, 
 414. 
 
 Lymphatic system, 411-414. 
 
 Manus and pes, 257. 
 Marine faunas, 482. 
 Medulla, 32, 40. 
 Membranes, 30. 
 Mollusca, structure of, 277. 
 Morphology, definition of, 8 ; 
 general laws of, 241-281. 
 
498 
 
 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 
 
 Motion and locomotion, 227-232. 
 
 Muscle and skeleton, compara- 
 tive morphology and physi- 
 ology of, 232-240. 
 
 Muscles of the eye, 100. 
 
 Muscle, voluntary, 221 ; invol- 
 untary, 223. 
 
 Muscular system, 219-223. 
 
 Myopy, 113. 
 
 Nails, 454. 
 
 Nearsightedness, 113. 
 
 Nerve force versus electricity, 
 64. 
 
 Nerves, 49-65 ; structure of, 56 ; 
 function of, 56. 
 
 Nervous system of man, 29. 
 
 Nervous system of vertebrates, 
 70-83 ; of invertebrates, 84- 
 93 ; of arthropods and anne- 
 lids, 84; of mollusks, 89; of 
 radiates, 92 ; of protozoans, 
 
 93- 
 Number of bones in man, 224. 
 Nutritive functions, 283-287. 
 
 Old-sightedness, 114. 
 
 Optic lobes, 32, 41. 
 
 Organs, rudimentary and use- 
 less, 258. 
 
 Oversightedness, 114. 
 
 Owen's classification of mam- 
 mals, 77. 
 
 Pelagic fauna, 482. 
 Perspective, forms of, 151. 
 Physiology, definition of, 8. 
 Plexuses, 69. 
 Pons Varolii, 32. 
 Presbyopy, 114. 
 Protozoa, structure of, 279. 
 
 Radiata, structure of, 278. 
 
 Relation of plants to animals in 
 regard to creation of animal 
 force, 424. 
 
 Respiration, costal, 363 ; dia- 
 phragmatic or abdominal, 364 ; 
 of mammals, 366 ; of birds, 
 
 366 ; of reptiles, 366 ; of the 
 tortoise, 367 ; of amphibians, 
 
 367 ; of fishes, 368 ; function 
 of, in animal economy, 417 ; 
 chemistry of, 41S ; purpose of 
 combustion in, 419. 
 
 Respiratory organs, 358, 399, 
 
 403, 404, 409. 
 Retina and its functions, 116- 
 
 122. 
 
 Saliva, composition and use of, 
 294. 
 
 Salivary glands, 292 ; structure 
 of, 293 ; excitation of, 294. 
 
 Scales, 460; of reptiles, 461. 
 
 Secretion versus excretion, 416. 
 
 Serial homology of vertebrates, 
 261-266 ; of arthropods and 
 annelids, 267-277. 
 
 Shells, turtle, 462 ; mammalian, 
 463 ; of insects, 463 ; of crusta- 
 ceans, 464 ; of mollusks, 464- 
 466 ; of echinoderms, 466. 
 
 Shoulder girdle, 250. 
 
 Shoulder joint and fore limb, 
 227. 
 
 Sight, sense of, 98. 
 
 Size and distance, judgments of, 
 
 153- 
 
 Skeletal system, 223-232. 
 
 Skin, function of, 435 ; struc- 
 ture of, 436-438 ; comparative 
 morphology and physiology 
 
INDEX. 
 
 499 
 
 of, 438-440 ; of vertebrates, 
 452 ; importance of, in classifi- 
 cation, 453. 
 
 Smell, sense of, and its organ, 
 188-193. 
 
 Special sense, relation of, to gen- 
 eral sensibility, 94-96. 
 
 Spinal column, 226. 
 
 Spinal cord, 45-49. 
 
 Spinal nerves, 46, 54. 
 
 Spinal or reflex system, function 
 of, 66. 
 
 Sponges, skeletal deposits in, 
 469. 
 
 Structure of vertebrates, general 
 plan of, 249. 
 
 Sudorific glands, 437 
 
 Taste, sense of, and its organ, 
 193-198. 
 
 Taxonomy, definition of, 9. 
 
 Teeth in vertebrates, 295-310 ; 
 mammalian, 296; composition 
 of, 297 ; kinds of, 297 ; varia- 
 tion of, 298 ; relative size of, 
 298 ; number of, and relative 
 number of kinds of, 298 ; mo- 
 lar, structure of, 300 ; origin 
 of structure of herbivorous, 
 302 ; of whales, 304 ; of birds, 
 305 ; of reptiles, 305 ; origin 
 of mammalian, 30S ; of fishes, 
 308. 
 
 Temperature regions, definition 
 of, 474-477- 
 
 Thalamus, 33, 41. 
 
 Theca, 468. 
 
 Three kingdoms of Nature, rela- 
 tion of the, 1-8. 
 
 Touch, sense of, and its organs, 
 19S-204. 
 
 Ungulates, classification of, by 
 
 foot structure, 257. 
 Urine, composition of, 429, 430. 
 
 Veins, 382. 
 
 Vibrations, perception of, g6. 
 
 Vision, 122-155 ; first law of, 
 122 ; second law of, 126 ; third 
 law of, 144; two fundamental 
 laws of, 149; erect, 129; bin- 
 ocular, 142 ; double, 142 ; sin- 
 gle, 143 ; limitation of, 151. 
 
 Visual purple, 121. 
 
 Voice, 204-218; simple, 204; 
 singing, 210; speaking, 211; 
 comparative physiology of, 
 
 213 ; in birds, 214 ; in reptiles, 
 
 214 ; in fishes, 214 ; in arthro- 
 pods, 216. 
 
 Waste tissue, 290, 420. 
 Whales, mouth armature of, 
 304 ; mode of feeding of, 305. 
 
 Wrist joint, 253. 
 
 Zoological temperature regions, 
 
 474. 
 Zoology, definition of, 8-10 ; de- 
 partments of, 10. 
 
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 that may last as long as the present Government. . . . We read the pages without 
 finding one dull, sometimes in reluctant agreement, sometimes with amused content, 
 sometimes with angry indignation." — London Saturday Review. 
 
 " Herr Nordau's book fills a void, not merely in the systems of Lombroso, as he 
 says, but in all existing systems of Knglish and American criticism with which we are 
 acquainted. It is not literary criticism, puie and simple, though it is not lacking in 
 literary qualities of a high order, but it is something which has long been needed. . . . 
 A great book, which every thoughtful lover of art and literature and every serious 
 student of sociology and morality should read carefully and ponder slowly and wisely." 
 — Richard Henry Stoddard, in the Mail and Express. 
 
 "'ihe book is one of more than ordinary- interest Nothing just like it has ever 
 been written. Agree or disagree with its conclusions, wholly or in p.irt, no one can fail 
 to recoijnize the force of its argument and the timeliness of its injunctions," — Chicago 
 Eziening fast. 
 
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 EN I US AND DEGENERATION. A Study in 
 Psychology. By Dr. William Hirsch. Translated from the 
 second edition of the German work. Uniform with " Degen- 
 eration." Large 8vo. Cloth, $3.50. 
 
 "The first intelligent, rational, and scientific study of a great subject ... In the 
 development of his argument Dr. Hirsch frequently finds it necessary to attack the 
 positii>ns assumed by Nordau and Lombroso, his two leading adversaries. . . . Only 
 calm and sober reason endure. Dr. Hirsch possesses that calmnefs and sobriety. His 
 work will find a permanent place among the authorities of science." — N. \ . Herald. 
 
 "Dr. Hirsch's researches .ire intended to biing the reader to the conviction that 
 'no psychological meaning can be attached to the word genius. ' . . . While all men of 
 genius liave common traits, they are not traits characteristic of genius: they .ire such 
 as are possessed by other men, and more or less by all men. . . . Dr. Hirsch believes 
 that most of the great men, both of art and of science, were misunderstood by their 
 contemporaries, and were only appreciated after they were dead." — .^liss J. L. Gilder, 
 in the Sunday li 'or Id. 
 
 " ' Genius and Degeneration ' ought to be read by every man and woman who pro- 
 fesses to keep in touch with modern thought. It is deeply interesting and so fiiU ot 
 information that by intellectual readers it will be seized upon with avidity."— BuJ^aU 
 Commercial. 
 
 D. APPLETON AND COMPANY. NEW YORK. 
 
D. APPLETON & CO.'S PUBLICATIONS. 
 
 p OP ULAR ASTRO NO M Y. A General Description 
 -^ of the Heavens. By Camille Flammarion. Translated 
 from the French by J. Ellard Gore. With 3 Plates and 
 283 Illustrations. 8vo. Cloth, S4.50. 
 
 "The fullest and most elaborate compendium of popular knowledge of astron- 
 omy. . . . The book might reasonably be pronounced the most desirable of its kind." 
 —New York Sun. 
 
 "M Flammarion has produced a work that charms while it interests. He has 
 classified ^istronomy so perfectly that any person of ordinary intelligence may learn 
 from his book prartically all the men in the observatories know."' — New York Times. 
 
 "Flammarion talks, and his conversation is free from those teclinical expressions 
 which make the obscure style more obscure. He treats the most abstruse problems in 
 such a faslii ,n that you see through tliem more clearly than you ever thought it pos- 
 sible to do without years of study."— iV?w York He' aid. 
 
 " While the tianslator has done excellent work, he hds also added largely to the 
 value of the book by his carefully prepared notes, in which he brings every astro- 
 nomical theme down 10 date." — Chicago Iiiier-Ocean. 
 
 "The l)ook is one of extreme interest, and to our mind far surpasses in fascination 
 any novel that was ever written." — London Literary World. 
 
 /1STR0N0MY WITH AN OPERA-GLASS. A 
 
 -^^^ Popular Introduction to the Study of the Starry Heavens with 
 the Simplest of Optical Instruments. By Garrett P. Ser- 
 viss. Svo. Cloth, $1.50. 
 
 " The glimpses he allows to be seen of far-stretching vistas opening out on every 
 side of his modest course of observation help to fix the attention of the neghgent, and 
 lighten the toil of the painstaking student. . . . Mr. Serviss writes with freshness and 
 vivacity." — London Saturday Review. 
 
 " By its aid thousands of people who have resigned themselves to the ignorance in 
 which they were left at school, by our wretched system of teaching by the book only, 
 will thank Mr. Serviss for the suggestions he has so well carried out."— New York 
 Times. 
 
 " We are glad to welcome this popular introduction to the study of the heavens. 
 There couUl hardly be a more pleasant road to astronomical knowledge than it 
 affords. ... A child may understand the text, which reads more like a collection of 
 anecdotes than anything else, but this does not mar its scientific vA:\e."—Natnre. 
 
 "Mr. Garrett P. Serviss's book, 'Astronomy with an Opera Glass,' offers us an 
 admirable handbook and guide m the cultivation of this noble esthetic discipline (the 
 study of the stars)." — New York Hojne Journal. 
 
 " The book should belong to every family Whrary."— Boston Home Journal. 
 
 New York : D. APPLETON & CO., 72 Fifth Avenue. 
 
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 WORKS BY ARABELLA B. BUCKLEY (MRS. FISHER). 
 
 n^HE FAIRY-LAND OF SCI FACE. With 74 
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 " Deserves to take a permanent place in the literature of youth." — Londnn Times. 
 
 " So interesting that, h:iving on.;c opened the book, wc do not know how to Icavt 
 iff reading." —Saturday Revieu,. 
 
 TROUGH MAGIC GLASSES, and other Lectures. 
 A Sequel to " The Fairy- Land of Science." Illustrated. 
 i2mo. Cloth, $1.50. 
 
 CONTENTS. 
 The Magician's Chamber by Mooniight. A n Hour with the Sun. 
 Magic Classes and Hoiv to Use Them. An Evening with the Stars. 
 Fairy Rings and How Tluy are Made. Little Beings from a Miniature Ocean. 
 The Lije-liistory rf Lichens and Mosses. The Dartn.oor Ponies. 
 Tlu History of a Lava-Stream. 1 he Magician's Dream ^Ancient Days. 
 
 T IFE AND HER CHLLDREN .- Glimpses of Ani- 
 J-^^ vtal Life front the Amceba to the Insects. With over loo Illus- 
 trations. i2mo. Cloth, gilt, $1.50. 
 "The work forms i. charminc; iurnduciion ro the study of zoology— the science of 
 living things — wliich. we trust, will find its way into many hands."— iVa/7//v. 
 
 TA DINNERS IN LIFE'S RACE; or, I he Greai 
 ' ' BackbDned Family. With numerous Illustralions. i2mo. 
 Cloth, gilt, Si-50. 
 
 " We can conceive of no better eift book than this volume. Miss Buckley has spared 
 no oains to incorporate in her book the latest results of scientific research, llie I'lu^- 
 trations in the book deserve the highest praise — they are numerous, accurate, and 
 Striking." — Spectator. 
 
 SHORT HISTORY OF NATURAL SCI- 
 ENCE ; and of the Progress of Discovery from the Time of 
 the Greeks to the Present Time. New edition, revised and re- 
 arranged. With 77 Illustrations. i2mo. Cloth, S2.00. 
 
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 minds a filler and clearer conception ol ' the promises, the achievements, and the claims 
 "A. science." " — 'Journal of Science. 
 
 ORAL TEACHINGS OF SCIENCE. i2mo. 
 Cloth, 75 cents. 
 
 " A I'ttle book that proves, with excellent clearness and force, how many and strik- 
 ing are the monl lessons suggested hy t'le study of tlie life history of the p'ant or bird, 
 bcasl or xasaci." - London Saturday liemew. 
 
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 New York: D. APPLETON & CO.. 72 Fifth Avenue. 
 
D. APPLETON & CO.'S PUBLICATIONS. 
 piONEERS OF SCIENCE IN AMERICA. 
 
 Sketches of their Lives and Scientific Work. Edited and re- 
 vised by William Jav Youmans, M. D. With Portraits. 
 8vo. Cloth, I4.00. 
 Impelled solely by an enthusiastic love of Nature, and neither asking 
 nor receiving outside aid, these early workers opened the way and initiated 
 the movement through which American science has reached its present com- 
 manding position. This book gives some account of these men, their early 
 struggles, their scientific labors, and, whenever possible, something of their 
 personal characteristics. This information, often very difficult to obtain, has 
 been collected from a great variety of sources, with the utmost care to secure 
 accuracy. It is presented in a series of sketches, some fifty in all, each with 
 a single exception accompanied with a well-authenticated portrait. 
 
 " Fills a place that needed filling, and is likely to be widely read." — N. Y. Sum. 
 
 " It is certainly a useful and convenient volume, and readable too, if we judge cor- 
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 to us that the handy volume is specially to be commended for setting in just historical 
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 known." — Afeiu "i'ork Evening Post. 
 
 " A wonderfully interesting volume. Many a young man will find it fascinating. 
 The compilation of the book is a work well done, well worth the doing." — Philadelphia 
 Press. 
 
 " One of the most valuable books which we have received." — Boston Advertiser. 
 
 " A book of no little educational value. ... An extremely valuable work of refer- 
 ence.' ' — Boston Beacon. 
 
 " A valuable handbook for those whose work runs on these same lines, and is likely 
 to prove of lasting interest to those for whom ' les documents humai • ' are second only 
 to history in importance — nay, are a vital part of history." — Boston Transcript. 
 
 " A biographical history of science in America, noteworthy for its completeness and 
 scope. . . . All of the sketches are excellently prepared and unusually interesting." — 
 Chicago Record. 
 
 "One of the most valuable contributions to American literature recently made. . . , 
 The pleasing st\ le in which these sketches are written, the plans taken 10 secure ac- 
 curacy, and the information conveyed, combine to give them great value and interest. 
 No better or more inspiring reading c mid be placed in the hands of an intelligent and 
 aspiring young man." — New York Christian ll^ork. 
 
 " A book whose interest and value are not for to-day or to-morrow, but for indefinite 
 time." — Rochester Herald. 
 
 " It is difficult to imagine a reader of ordinary intelligence who would not be enter- 
 tained by the book. . . . Conciseness, exactness, urbanity of tone, and interestingness 
 are the four qualities which chiefly impress the reader of these sketches." — Buffalo 
 Express. 
 
 " Full of interesting and valuable matter."— 7"A* Churchman. 
 
 New York : D. APPLETON & CO., 72 Fifth Avenue. 
 
Date Due 
 
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